A random coil is a polymer conformation where the monomers are arranged at random. Many simple polymers such as polyethylene inhabit only this conformation, more complex polymers with varying chemical groups attached to its backbone, such as proteins, self-assemble into well defined structures.
Proteins, segments of proteins, and peptides that lack secondary structure inhabit the random coil conformation. In random coil, the only fixed relationship between amino acids is that between adjacent residues through the peptide bond. The conformation's name is derived from the idea that, in the absence of specific, stabilizing interactions, the polypeptide backbone will sample all possible conformations randomly. This is actually not the case, since the ensemble will be energy-weighted, with lower-energy conformations being present more frequently. For this reason, the term "statistical coil" is occasionally preferred.
The random coil conformation can be detected using spectroscopic techniques. The arrangement of the amide planes results in a distinctive signal by circular dichroism. By nuclear magnetic resonance (NMR), the chemical shift of amino acids in the random coil conformation is well known. Deviations from these values often indicates the presence of secondary structure and thus the absence of random coil. Furthermore, signals in multidimensional NMR experiment that indicate stable, non-local amino acid interactions are absent for polypeptides in the random coil conformation. Likewise in the images produced in crystallography experiments, pieces of random coil appear simply as an absence of "electron density" or contrast. The random coil state for any polypeptide chain can be attained through Denaturing of the system.
There is evidence that proteins perhaps never are truly random coils even when denatured (Shortle et al.).
Random flight model
By looking at the polymer as a freely jointed chain, one can look at each chain as doing random walk or more accurately constrained random walk and it can be shown that number of polymers with a any distance l between the ends of the chain will follow a normal distribution.
Monday, August 25, 2008
Sunday, August 24, 2008
Isomer
In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Many isomers share similar if not identical properties in most chemical contexts.
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and the isomers
Propan-1-ol (n-propyl alcohol; left) Propan-2-ol (isopropyl alcohol; right)
Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxl group the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and the isomers
Propan-1-ol (n-propyl alcohol; left) Propan-2-ol (isopropyl alcohol; right)
Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second. It can be readily shown that the number of possible isomers increases rapidly as the number of atoms increase; for example the next largest alcohol, named butanol (C4H10O), has five different isomers.
In the example above it should also be noted that in both isomers all the bonds are single bonds; there is no type of bond that appears in one isomer and not in the other. Also the number of bonds is the same. From the structures of the two molecules it could be deduced that their chemical stabilities are liable to be identical or nearly so.
There is, however, another isomer of C3H8O which has significantly different properties: methyl ethyl ether:
Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxl group the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
Saturday, August 23, 2008
Self-assembly
Self-assembly is the fundamental principle which generates structural organization on all scales from molecules to galaxies। It is defined as reversible processes in which pre-existing parts or disordered components of a preexisting system form structures of patterns. Self-assembly can be classified as either static or dynamic. Static self-assembly is when the ordered state occurs when the system is in equilibrium and does not dissipate energy. Dynamic self-assembly is when the ordered state requires dissipation of energy. Examples of self-assembling system include weather patterns, solar systems, histogenesis and self-assembled monolayers. The most well-studied subfield of self-assembly is molecular self-assembly, but in recent years it has been demonstrated that self-assembly is possible with micro and milimeterscale structures lying in the interface between two liquids.
Molecular self-assembly
Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly, although in some books and articles the term self-assembly refers only to intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure (secondary and tertiary structure). An example of intramolecular self-assembly is protein folding. Intermolecular self-assembly is the ability of molecules to form supramolecular assemblies (quarternary structure). A simple example is the formation of a micelle by surfactant molecules in solution.
Self-assembly can occur spontaneously in nature, for example in cells (such as the self-assembly of the lipid bilayer membrane) and other biological systems, as well as in human engineered systems. It usually results in the increase in internal organization of the system.
Also, self-assembly is a manufacturing method used to construct things at the nanometre-scale. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly the final (desired) structure is 'encoded' in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a 'bottom-up' manufacturing technique, as compared to lithography being a 'top-down' technique. The synthesis of molecules for self-assembly often involves a chemical process called convergent synthesis. Microchips of the future might be made by molecular self-assembly. An example of self-assembly in nature is the way that hydrophilic and hydrophobic interactions cause cell membranes to self assemble.
Molecular self-assembly
Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly, although in some books and articles the term self-assembly refers only to intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure (secondary and tertiary structure). An example of intramolecular self-assembly is protein folding. Intermolecular self-assembly is the ability of molecules to form supramolecular assemblies (quarternary structure). A simple example is the formation of a micelle by surfactant molecules in solution.
Self-assembly can occur spontaneously in nature, for example in cells (such as the self-assembly of the lipid bilayer membrane) and other biological systems, as well as in human engineered systems. It usually results in the increase in internal organization of the system.
Also, self-assembly is a manufacturing method used to construct things at the nanometre-scale. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly the final (desired) structure is 'encoded' in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a 'bottom-up' manufacturing technique, as compared to lithography being a 'top-down' technique. The synthesis of molecules for self-assembly often involves a chemical process called convergent synthesis. Microchips of the future might be made by molecular self-assembly. An example of self-assembly in nature is the way that hydrophilic and hydrophobic interactions cause cell membranes to self assemble.
Self-assembly
Self-assembly is the fundamental principle which generates structural organization on all scales from molecules to galaxies। It is defined as reversible processes in which pre-existing parts or disordered components of a preexisting system form structures of patterns. Self-assembly can be classified as either static or dynamic. Static self-assembly is when the ordered state occurs when the system is in equilibrium and does not dissipate energy. Dynamic self-assembly is when the ordered state requires dissipation of energy. Examples of self-assembling system include weather patterns, solar systems, histogenesis and self-assembled monolayers. The most well-studied subfield of self-assembly is molecular self-assembly, but in recent years it has been demonstrated that self-assembly is possible with micro and milimeterscale structures lying in the interface between two liquids.
Molecular self-assembly
Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly, although in some books and articles the term self-assembly refers only to intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure (secondary and tertiary structure). An example of intramolecular self-assembly is protein folding. Intermolecular self-assembly is the ability of molecules to form supramolecular assemblies (quarternary structure). A simple example is the formation of a micelle by surfactant molecules in solution.
Self-assembly can occur spontaneously in nature, for example in cells (such as the self-assembly of the lipid bilayer membrane) and other biological systems, as well as in human engineered systems. It usually results in the increase in internal organization of the system.
Also, self-assembly is a manufacturing method used to construct things at the nanometre-scale. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly the final (desired) structure is 'encoded' in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a 'bottom-up' manufacturing technique, as compared to lithography being a 'top-down' technique. The synthesis of molecules for self-assembly often involves a chemical process called convergent synthesis. Microchips of the future might be made by molecular self-assembly. An example of self-assembly in nature is the way that hydrophilic and hydrophobic interactions cause cell membranes to self assemble.
Molecular self-assembly
Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly, although in some books and articles the term self-assembly refers only to intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure (secondary and tertiary structure). An example of intramolecular self-assembly is protein folding. Intermolecular self-assembly is the ability of molecules to form supramolecular assemblies (quarternary structure). A simple example is the formation of a micelle by surfactant molecules in solution.
Self-assembly can occur spontaneously in nature, for example in cells (such as the self-assembly of the lipid bilayer membrane) and other biological systems, as well as in human engineered systems. It usually results in the increase in internal organization of the system.
Also, self-assembly is a manufacturing method used to construct things at the nanometre-scale. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly the final (desired) structure is 'encoded' in the shape and properties of the molecules that are used, as compared to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter. Self-assembly is thus referred to as a 'bottom-up' manufacturing technique, as compared to lithography being a 'top-down' technique. The synthesis of molecules for self-assembly often involves a chemical process called convergent synthesis. Microchips of the future might be made by molecular self-assembly. An example of self-assembly in nature is the way that hydrophilic and hydrophobic interactions cause cell membranes to self assemble.
Thursday, August 21, 2008
Molecule
In science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of two or more atoms joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O). Abstractly, a single atom may be considered a molecule, as it is when referred to collectively with molecules of multiple atoms, but in practice the use of the word molecule is usually confined to chemical compounds, of multiple atoms.
A substance that consists of molecules is a molecular substance or molecular compound।
A substance that consists of molecules is a molecular substance or molecular compound।
Figure 1. 3D (left and center) and 2D (right) representations of the terpenoid, atisane . In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.
Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes.
A property of molecules is the integer ratio of the elements that constitute the compound, the empirical formula. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers.
Chemical formula on the other hand reflects the exact number of atoms that compose a molecule. The molecular mass is calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a 12C isotope atom.
Molecules have fixed equilibrium geometries—bond lengths and angles—that are dictated by the laws of quantum mechanics. A pure substance is composed of molecules with the same geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.
Monomer
In chemistry, a monomer (from Greek mono "one" and meros "part") is a small molecule that may become chemically bonded to other monomers to form a polymer.
Examples of monomers are hydrocarbons such as the alkane, alkene, and alene (homologous) series. Other hydrocarbon monomers such as styrene and ethene form polymers to make plastics like polystyrene and polyethene.
Amino acids are natural monomers, and polymerize to form proteins. Glucose monomers can also polymerize to form starches, amilopectins and glycogen polymers. The polymerization reaction is known as a dehydration or condensation reaction (due to the formation of water (H2O) as one of the products) where a hydrogen atom and a hydroxyl (-OH) group are lost to form H2O and an oxygen molecule bonds between each monomer unit.
Note that polymers built from monomers can also be called dimers, trimers, tetramers, pentamers, octamers, 20-mers, etc. if they have 2, 3, 4, 5, 8, or 20 monomer units, respectively. Any number of these monomer units may be indicated by the appropriate prefix, eg, decamer, being a 10-unit monomer chain or polymer. Larger numbers are often stated in English in lieu of Greek.
Examples of monomers are hydrocarbons such as the alkane, alkene, and alene (homologous) series. Other hydrocarbon monomers such as styrene and ethene form polymers to make plastics like polystyrene and polyethene.
Amino acids are natural monomers, and polymerize to form proteins. Glucose monomers can also polymerize to form starches, amilopectins and glycogen polymers. The polymerization reaction is known as a dehydration or condensation reaction (due to the formation of water (H2O) as one of the products) where a hydrogen atom and a hydroxyl (-OH) group are lost to form H2O and an oxygen molecule bonds between each monomer unit.
Note that polymers built from monomers can also be called dimers, trimers, tetramers, pentamers, octamers, 20-mers, etc. if they have 2, 3, 4, 5, 8, or 20 monomer units, respectively. Any number of these monomer units may be indicated by the appropriate prefix, eg, decamer, being a 10-unit monomer chain or polymer. Larger numbers are often stated in English in lieu of Greek.
Wednesday, August 20, 2008
Catalyst
A catalyst (Greek: καταλύτης, catalytis) is a substance that accelerates the rate of a chemical reaction, at some temperature, but without itself being transformed or consumed by the reaction (see also catalysis). A catalyst participates in the reaction but is neither a chemical reactant nor a chemical product.
In some rare situations, one may describe an atomic nucleus as a catalyst in a nuclear reaction (see, for example, the CNO cycle). More generally, one may sometimes call anything which accelerates a reaction without itself being consumed or transformed a catalyst. This article will focus on chemical catalysts.
Catalysts enable reactions to occur much faster or at lower temperatures because of changes that they induce in the reactants. Catalysts provide an alternative pathway, with a lower activation energy, for a reaction to proceed. This means that catalysts reduce the amount of energy needed to start a chemical reaction. Molecules that would not have had the energy to react or that have such low energies that they probably would have taken a long time to react are able to react in the presence of a catalyst. Thus, more molecules that need to gain less energy to react will go through the chemical reaction.
The two main categories of catalysts are heterogeneous and homogeneous catalysts. Heterogeneous catalysts are present in different phases from the reactants in the reaction they are catalysing, whereas homogenous catalysts are in the same phase. A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed. Bonds in the substrate become weakened sufficiently for new products to be created. The bonds between the products and the catalyst are weaker, so the products are released.
Homogenous catalysts generally react with one or more reactants to form a chemical intermediate that subsequently reacts to form the final reaction product, in the process regenerating the catalyst. The following is a typical catalytic reaction scheme, where C represents the catalyst:
A + C → AC (1)
B + AC → AB + C (2)
Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:
A + B + C → AB + C
the catalyst is neither consumed nor produced. Enzymes are biocatalysts. Use of "catalyst" in a broader cultural sense is in rough analogy to the sense described here.
Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene. Probably the best-known catalytic reaction is the Haber process for ammonia synthesis, where ordinary iron is used as a catalyst. Catalytic converters break down some of the nastier byproducts of automobile exhaust. They are made from platinum and rhodium.
Although catalysts will change rate of a reaction, they will not shift an equilibrium reaction. This is because a catalyst affects the rate of reaction equally in both the forward and reverse directions.
In some rare situations, one may describe an atomic nucleus as a catalyst in a nuclear reaction (see, for example, the CNO cycle). More generally, one may sometimes call anything which accelerates a reaction without itself being consumed or transformed a catalyst. This article will focus on chemical catalysts.
Catalysts enable reactions to occur much faster or at lower temperatures because of changes that they induce in the reactants. Catalysts provide an alternative pathway, with a lower activation energy, for a reaction to proceed. This means that catalysts reduce the amount of energy needed to start a chemical reaction. Molecules that would not have had the energy to react or that have such low energies that they probably would have taken a long time to react are able to react in the presence of a catalyst. Thus, more molecules that need to gain less energy to react will go through the chemical reaction.
The two main categories of catalysts are heterogeneous and homogeneous catalysts. Heterogeneous catalysts are present in different phases from the reactants in the reaction they are catalysing, whereas homogenous catalysts are in the same phase. A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed. Bonds in the substrate become weakened sufficiently for new products to be created. The bonds between the products and the catalyst are weaker, so the products are released.
Homogenous catalysts generally react with one or more reactants to form a chemical intermediate that subsequently reacts to form the final reaction product, in the process regenerating the catalyst. The following is a typical catalytic reaction scheme, where C represents the catalyst:
A + C → AC (1)
B + AC → AB + C (2)
Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:
A + B + C → AB + C
the catalyst is neither consumed nor produced. Enzymes are biocatalysts. Use of "catalyst" in a broader cultural sense is in rough analogy to the sense described here.
Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene. Probably the best-known catalytic reaction is the Haber process for ammonia synthesis, where ordinary iron is used as a catalyst. Catalytic converters break down some of the nastier byproducts of automobile exhaust. They are made from platinum and rhodium.
Although catalysts will change rate of a reaction, they will not shift an equilibrium reaction. This is because a catalyst affects the rate of reaction equally in both the forward and reverse directions.
Tuesday, August 19, 2008
Polymers
Intermolecular forces
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogens in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole। The attractive forces between polyethylene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogens in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole। The attractive forces between polyethylene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
Branching
During the propagation of polymer chains, branching can occur। In radical polymerization, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
During the propagation of polymer chains, branching can occur। In radical polymerization, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
Stereoregularity
Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.
In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.
Syndiotactic substituent groups alternate regularly in opposite directions। because of this regularity, syndiotactic chains can position themselves close to each, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.
Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.
In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.
Syndiotactic substituent groups alternate regularly in opposite directions। because of this regularity, syndiotactic chains can position themselves close to each, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.
Copolymerization
Copolymerization is polymerization with two or more different monomers. Already mentioned are the twenty amino acid monomers that make up protein chains. Copolymerization of different monomers can result in varied properties of polymers, just as different amino acids result in different shapes of proteins. For example, copolymerising ethene with small amounts of hex-1-ene is one way to form linear low density polyethene (LLDPE) (See Polyethylene). The C4 branches resulting from the hexene lower the density and prevent such large crystalline regions within the polymer as in HDPE. This means that LLDPE can withstand strong tearing forces whilst remaining flexible.
The following image shows a specific type of copolymerization called a step-growth polymerization, or condensation polymerization। In this particular polymerization a small molecule is released upon polymerization. In the following reaction scheme, water is given off and nylon is formed. The type of nylon (name and properties) are governed by the R and R' groups in the monomers used.
Copolymerization is polymerization with two or more different monomers. Already mentioned are the twenty amino acid monomers that make up protein chains. Copolymerization of different monomers can result in varied properties of polymers, just as different amino acids result in different shapes of proteins. For example, copolymerising ethene with small amounts of hex-1-ene is one way to form linear low density polyethene (LLDPE) (See Polyethylene). The C4 branches resulting from the hexene lower the density and prevent such large crystalline regions within the polymer as in HDPE. This means that LLDPE can withstand strong tearing forces whilst remaining flexible.
The following image shows a specific type of copolymerization called a step-growth polymerization, or condensation polymerization। In this particular polymerization a small molecule is released upon polymerization. In the following reaction scheme, water is given off and nylon is formed. The type of nylon (name and properties) are governed by the R and R' groups in the monomers used.
Polymer characterization
A variety of laboratory techniques are used to determine the properties of polymers. Techniques such as wide angle xray scattering, small angle xray scattering, and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the number average molecular weight, weight average molecular weight, and polydispersity. FTIR is used to determine composition. Thermal properties such as the glass transition temperature and melting point can be determined by differential scanning calorimetry and dynamic mechanical analysis. Thermal degradation followed by analysis of the fragments is one more technique for determining the possible structure of the polymer.
Monday, August 18, 2008
Polymer
A polymer is a long, repeating chain of atoms, formed through the linkage of many molecules called monomers। The monomers can be identical, or they can have one or more substituted chemical groups. These differences between monomers can affect properties such as solubility, flexibility, or strength. In proteins, these differences can give the polymer the ability to preferentially adopt one conformation over another, as opposed to adopting a random coil . Although most polymers are organic (based on carbon chains), there are also many inorganic polymers.
The term polymer covers a large, diverse group of molecules, including substances from proteins to high-strength kevlar fibres. A key feature that distinguishes polymers from other large molecules is the repetition of units of atoms (monomers) in their chains. This occurs during polymerization, in which many monomer molecules link to each other. For example, the formation of polyethene involves thousands of ethene molecules bonding together to form a chain of repeating -CH2- units:
The term polymer covers a large, diverse group of molecules, including substances from proteins to high-strength kevlar fibres. A key feature that distinguishes polymers from other large molecules is the repetition of units of atoms (monomers) in their chains. This occurs during polymerization, in which many monomer molecules link to each other. For example, the formation of polyethene involves thousands of ethene molecules bonding together to form a chain of repeating -CH2- units:
Polymers are often named in terms of their monomer units, for example polyethylene is represented by:
Because polymers are distinguished by their constituent monomers, polymer chains within a substance are often not of equal length. This is unlike other molecules in which every atom is acounted for, each molecule having a set molecular mass. Differing chain lengths occur because polymer chains terminate during polymerization after random intervals of chain lengthening (propagation).
Proteins are polymers of amino acids. From a dozen to some hundred of the (about) twenty different monomers form the chain, the sequence of monomers determining the shape and activity of the final protein. But there are active regions, surrounded by, as is believed now (Aug 2003), structural regions, whose sole role is to expose the active region(s) (there may be more than one on a given protein). So the absolute sequence of amino acids is not important, as long as the active regions are expressed (being accessible from the outside) properly. Also, whereas the formation of polyethylene occurs spontaneously given the right conditions, the manufacture of biopolymers such as proteins and nucleic acids requires the help of catalysts (substances that facilitate or accelerate reactions.) Since the 1950s, catalysts have also revolutionised the development of synthetic polymers. By allowing more careful control over polymerization reactions, polymers with new properties, such as the ability to emit coloured light, have been manufactured.
Saturday, August 16, 2008
Different Terms Related To Fiber and Fabric:
Filament:-
1. A fiber of indefinite length, such as filament acetate, rayon, nylon, May be miles long.
2. A single strand of rayon spinning solution as it is exuded from a spinneret orifice and coagulated in an acid bath or other medium; also true of other manmade filaments.
3. The single unit which is extruded by a silkworm in spinning its cocoon. Actually the silkworm makes two filaments at the one time, and they are cemented or glued together by the sericin, or silkgum, exuded by the silkworm in the action. Filaments are then spun into yarn.
Denier: –
A weight-per--unit-length measure of any linear material. Officially, it is the number of unit weights of 0.05 grams per 450-meter length. This is numerically equal to the weight in grams of 9.000 meters of the material. Denier is a direct numbering system in which the low numbers represent the finer sizes and the higher numbers the coarser sizes. In the U.S. the denier is used for numbering filament yarns (except glass), man-made fiber staple (but not spun yarns), and tow. In most countries outside the U.S. the denier system has been replaced by the tex system.
Yarn: –
A generic term for an assemblage of fibers or filaments, either natural or man-made, twisted together to form a continuous strand which can be used for weaving, knitting, plaiting, braiding, of the manufacture of lace, or otherwise made into a textile material.
Yarn Standards: –
These standards for the major yarns are given below. It should be noted that yarn counts or numbers are written by number and this is followed by the letter "s." The apostrophe is not necessarily used between the number and the "s."
1. A. 840 yards in one pound of a 1s cotton yarn. (Boston or New England)
B. 560 yards in one pound of a 1s worsted yarn (two-thirds the number used for
the cotton standard of 840 yards in one pound of 1s).
C. 1,600 yards in one pound of a 1s run woolen yarn (Boston or New England)
D. 300 yards in one pound of a 1s cut woolen yarn (Philadelphia system)
E. 300 yards in one pound of a 1s linen yarn.
F. 840 yards in one pound of a 1s spun silk yarn, in the single ply.
G. 4,464,528 yards in one pound of a Number One denier filament yarn: used to
figure silk and man-made filament yarns.
2. Approximate Highest Counts of Yarn Spun for the Natural Fibers: On a per pound
basis, these approximations for commercial purpose follow:
A. Cotton: 140s x 840 standards, gives 117,600 yards.
B. Cut Wool: 30s x 300 standards, gives 9.000 yards.
C. Run Wool: 10s x 1,600 standard, gives 16,000 yards.
D. Worsted: 70s x 560 standard, gives 39,200 yards.
Taslan:–
A registered trademark. A textured yarn that is different from spun yarn or continuous filament yarn in that it is made on a bulking process developed by DuPont. Its hand, loftiness, covering power, and yarn texture are such that these properties are permanent and do not require special handling or care. The method may be applied to any thermoplastic fiber.
Denim:–
This basic cotton cloth? First brought to America by Columbus almost 500 years ago as the sails on the Santa Maria? is rugged, tough, and serviceable. It is easily recognized by its traditional indigo-blue color warp and gray or molted white filling, and its left hand twill on the face. Coarse single yarns are used mostly, but today many versions are available for the fashion world. A two-up and one-down or a three-up and one-down twill may be used in the weave construction. Long considered the most popular fabric for work clothes and army uniforms, denim today has won great fashion significance in dress goods for women's and men's wear, a wide range of sportswear, and even evening wear. It is estimated last year American textile mills consumed some 1,500,000 bales of cotton to produce denim which was known centuries ago as "Serge de Nimes" from its early origins in Nimes, France.
Dobby Loom: –
A type of loom on which small, geometric figures can be woven in a regular pattern. Originally this type of loom needed a "dobby boy" who sat on the top of the loom and drew up warp threads to form a pattern. Now the weaving is done entirely by machine. This loom differs from a plain loom in that it may have up to thirty-two harnesses and a pattern chain. Is expensive weaving.
Double Knit: -
A Circular knit fabric knitted via double stitch on a double needle frame to provide a double thickness. It is the same on both sides. Today, most double knits are made of 150 denier polyester although many lighter weight versions are now being made using finer denier yarns and blends of filament and spun yarns.
Dyeing of Textiles: –
Dyeing: The process of applying color to fiber stock, yarn or fabric; there may or may not be thorough penetration of the colorant into the fibers or yarns. Major methods of dyeing follow:
Bale Dyeing: A low cost method to dye cotton cloth. The material is sent, without scouring or singeing, through a cold water bath where the sized warp yarn has affinity for the dye. With the natural wax not removed from the filling yarn the dye will not be absorbed by this filling. Imitation chambray and comparable fabrics are often dyed in this way.
Batik Dyeing: One of the oldest forms known to man; originated in Java. Portions of the fabric are coated with wax so that only the un-waxed areas will take on the dye matter. The operation may be repeated several times and several colors may be used for the rather bizarre effects. Motifs show a melage, mottled or streaked effect. Imitated in machine printing.
Beam Dyeing: The warp is dyed prior to weaving. It is wound onto a perforated beam and the dye is forced through the perforations thereby saturating the yarn with color.
Burl or Speck Dyeing: Done mostly on woolens and worsteds, colored specks and blemishes are covered by the use of special colored inks which come in many colors and shades. This is a hand operation.
Chain Dyeing: Used when yarns and cloths are low in tensile strength. Several cuts or pieces of cloth are tacked end-to-end and run through in a continuous chain in the dye liquor. Affords high production.
Cross Dyeing: Varied color effects are obtained in the one dyebath for a cloth which contains fibers with varying affinities for the dye used. For example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6, 6 a light blue shade, and have no affinity for polyester thereby leaving the polyester area unscathed or white. it is a very popular method.
Jig Dyeing: This is done in a jig, vat, beck or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dyebath until the desired shade is achieved.
Package Dyeing: Yarns are dyed while on cones, cakes, cheeses or in the conventional or standard layout or set-up.
Piece Dyeing: The dyeing of fabric in the cut, bolt or piece form it follows the weaving of the goods and provides a single color for the material, such as a blue serge, a green organdy.
Random Dyeing: Coloring only certain designated portions of yarn. There are three ways of doing this type of coloring:
Skeins may be tightly tied in two or more places and dyed at one side of the tie with one color and at the other side with another one.
Color may be printed onto the skeins which are spread out on the blanket fabric of the printing machine.
Cones or packages of yarn on hollow spindles may be arranged to form channels through which the yarn, by means of an air-operated punch, and the dyestuff are drawn through three holes by suction. The yarn in the immediate area of the punch absorbs the dye and the random effects are thereby attained.
Raw Stock Dyeing: Dyeing of fiber stock precedes spinning of the yarn. Dyeing follows the degreasing of the wool fibers and dyeing of the stock.
Resist Dyeing: Treating yarn or cloth so that in any subsequent dyeing operation the treated portions resist the dye and do not absorb it at all.
Solution Dyeing: Also called dope dyeing and spun dyeing, the pigment or color is bonded-in in the solution and is picked up as the filaments are being formed in the liquor. Cellulosic and noncellulosic fibers are dyed to perfection by this method. Colors are bright, clear, clean, and fast.
Stock Dyed: Fibers are dyed after degreasing and drying and preceding, blending, oiling, mixing, carding (combing in worsted yarn manufacturer), and spinning of the yarn. Ideal for woolen and worsted fibers since it affords an endless array of color, cast, shade, tone or hue in the fabric.
Top Dyed: Often referred to as Vigoureux Printing, it is the dyeing or printing of worsted top or sliver in a rather loose formation of combed, parallel fibers. Precedes the spinning of the yarn and affords a host of colors, cats, and shades.
Union Dyed: The coloring of two or more different textile fibers in the one dyebath to provide different colors simultaneously or dyeing the fabric in a single shade, usually the latter.
Vat Dyed: Cloth dyed by the use of vat dyes which are obtained through oxidation. Very fast in all respects. Vat dyeing may be considered to be a misnomer since fabric colored with these dyes are piece dyed in the conventional manner.
Williams Unit: An open-width dyeing machine invented by the late S.H.. Williams of GAF Corporation, New York City. The fabric passes up and down over rollers in the dyeing bath. Widely used for dyeing, washing, pretreating and aftertreating.
Yarn Dyeing: yarn which has been dyed prior to the weaving of the goods; follows spinning of the yarn. May be done in either total immersion of the yarn.
Dyes, Types of: -
Acid Dyes: Being water soluble they are applied directly with an acid, such as sulphuric acid. Bright colors do not stand up too well in colorfastness when wet-treated; fair to poor in washing, good in dry cleaning and lightfastness. Used on wool, worsted, acrylics, and nylon.
Acid-Milling Dyes: Ideal for coloring wool, worsted, acrylics, modacrylics, nylon, and spandex fibers. Also used in printing. Good in drycleaning, excellent resistance to light.
Acid-premetalized dyes: Used on wool, acrylics, and nylon, they rate excellent to fastness in drycleaning, perspiration, and washing. Much used for carpeting, suiting fabrics, and upholstery.
Alizarine Dyes: Originally natural dyestuff. They are now synthetic dyes. Used on cotton and wool, they are resistant to sunlight and washing. Considerable use for apparel fabrics.
Azoic or Naphthol dyes: Also called Ice colors, Insoluble Azos, and Ingrain Colors. Rated good to laundering and washing. Ideal for decorative fabrics, draperies, dress goods, sportswear.
Basic Dyes: The first of the many groups of synthetic dyes. Sir William Perkin discovered them in 1856. Used on cotton, paper, and wool. Provide brilliant shades but rated poor to fair to light and washfastness; color resistance is only fair.
Chrome dyes: See Mordant Dyes Below. Developed Dyes: Used on cotton and rayon and other man-made fibers; on the latter when developed from disperse dye bases. Lightfastness rated from poor to good. See Mordant Dyes below.
Direct Dyes: Also known as Application or Commercial Dyes. Applied to cellulosic fibers such as cotton and rayon. Lightfastness rated from poor to excellent, and because of solubility are not rated highly in washfastness.
Disperse Dyes: Also known as Azo or Anthraquinone Dyes, they are ideal on nylon, acrylics, modacrylics, and polyesters?either dyed or printed. These dyes have a great many applications in the industry. Colorfastness is rated poor to very good for light and washing, depending on the fiber used. Ingrain: See Azoic or Naphthol Dyes.
Mineral Colors: Actually they are not true dyes but are precipitated oxides or insoluble salts of chromium, iron, lead or manganese. Dull in appearance these colors are much used to color awnings and comparable fabrics.
Mordant Dyes: A mordant is a substance used in dyeing to apply or fix coloring matter to a fiber, yarn or fabric, especially a metallic compound such as an oxide which combines with the fiber and organic dye and forms an insoluble color compound or take in the fiber. Also known as Mordant-Acid Dyes or Chrome Dyes, they are closely related to Acid Dyes. Results are dull when compared with those from acid dyes. Exceptionally fast on wool and other animal fibers. Much used as well on carpeting, nylon, and silk.
Neutral-premetalized Acid Dyes: Much used on wool and other protein fibers, acrylics, modacrylics, and nylon and ideal for use in blends. Much used in coloring apparel. Fair to excellent to light, good to excellent in washing, excellent in drycleaning.
Oxidation Bases: One of these bases is aniline dye which is formed in the fiber by oxidation. Ideal for coloring fur, sheepskins, and pile cloths, as well as in dyeing cotton for use in a wide range of fabrics. Finds much application in printgoods.
Pigment Colors: Insoluble in water and the color has to be fixed onto the fiber by use of resinous-binders insolubilized by a curing treatment at high temperatures. Used mostly on cotton and acetate, rayon and some other man-made fibers, these dyes, generally speaking, color, by dyeing or printing, just about all types of fibers and blends. Light and medium shades of sailcloths and many types of dress goods use Pigment Colors. Good to excellent in lightfastness but may be poor in cocking or rubbing.
Reactive Dyes: They actually bond-in the colorant. Provide bright colors on cottons and can dye acrylics, nylon, silk, and wool, and blends of these fibers. Also used for printing cotton fabrics. Rated good to very good to light and washing, but fugitive to chlorine-base bleaches.
Sulphur Dyes: They do not provide bright shades to any marked degree and their fastness properties have to be developed in the chemical inertness and insolubility in water. There are now also soluble forms of these dyes on the market. Used to color heavy cottons, knitwear in medium to full shades and readily dye stock, yarn, and piecegoods. weak in sunlight except for deep shades where fastness is good. Fugitive to chlorine-base bleaches.
Vat and Vat-Soluble Dyes: These are the fastest dyes known to man; insoluble in water and made soluble by chemical reduction. They actually bond-in the colorant and are the most resistant of any types of light, drycleaning, sunlight, and washing. Has many applications on cotton, rayon, polyesters, etc. Vat dyes are used to color awnings, bed linens, decorative fabrics, outerwear, sportswear, toweling, workclothes.
Twill Weave: -
Identified by the diagonal lines in the goods. It is one of the three basic weaves, the others being plain and satin. All weaves, either simple, elaborate or complex, are derived from these three weaves. Most twills are 45 degrees in angle. Steep twills are made from angles of 63, 70, 75 degrees while reclining twills use angles of 27, 20 and 15 degrees. Right-hand twilled clothes include cassimere, cavalry twill, covert, elastique, gabardine, serge, tackle twill, tricotin, tweed, whipcord. Left-hand twills include denim, galatea, jean cloth, some drill and twill cloth, and some ticking fabrics.
Tricot: –
· A type of warp knitted fabric which has a thin texture since it is made from very fine yarn. The French verb tricoter, means "to knit." The fabric is made on one, two, or three bar frames. It is knitted flat and made on spring-beard needles and has from one to four warps or thread systems which are mounted in a stationary position. Industry Standard Machine is 28 needles to the inch or a 28 gauge (gg).
· "Stocking-net" as applied to a warp-knitted fabric irrespective of the motif ; often refers to a flat knitted cloth since it is not tubular. The meaning, however, is not to be constructed to imply a flat-machine knit fabric.
· A French serge lining fabric made on a 20-inch width.
· A fine woven worsted made on the tricot weave which presents fine break lines in the filling direction. This chainbreak effect fabric is dyed all popular shades, has a high, compact texture, and is a good material to use in tailoring. Gives excellent wear in the better type of tailored garments for women. See warp knitting.
Teflon:-
Registered trademark of DuPont for its tetrafluoroethylene polymer fiber, announced in 1953. Type TFE is the polytetrafluoroethylene and it is not thermoplastic. Type FEP is fluorinated ethylene and is thermoplastic. The fiber is of smooth, circular surface and the cross-section is round. Teflon is ecru, brown or tan in color cast but bleaches to white in strong oxidizing mineral acids. It does not absorb moisture and is the most non-wettable fiber known to man. It possesses excellent thermal stability is very tough and strong, and also chemical resistant. It remains inert to all chemicals except to molten alkali metals, hot fluorine gas or chlorine trifluoride under pressure. It is used in filters, felts, gaskets, diaphragm fabrics, electrical tape, pump-packing, et al.
Terry Cloth: -
This cloth has uncut loops on both sides of the fabric. Woven on a dobby loom with Terry arrangement, various sizes of yarns are used in the construction. Terry is also made on a Jacquard loom to form interesting motifs. It may be yarn-dyed in different colors to form attractive patterns. It is bleached, piece-dyed, and even printed for beachwear and bathrobes, etc. Also called Turkish toweling. Today knitted Terry is very popular in fashion.
Terry Toweling: –
These are classified according to weave or design:
1. Cam-Woven, Plain Terry?Plain border.
2. Dobby-Woven?Simple pattern in the border, or all over. Border design include rope and corduroy borders.
3. Jacquard-Woven?Those which have rather elaborate allover motifs, or names woven into the goods.
4. Mitcheline?This border type has a heavy, distinct, raised or embossed border effect, formed by a stout colored filling yarn; the roving is used sometimes instead of a yarn to obtain the effect. Most of this fabric is made on Jacquard looms.
5. Texture-Designed?This is made on either a dobby or a jacquard loom. It has an allover, raised, and recessed motif. The athletic-rib towel, which has raised terry stripes with alternating plain ground stripes, is in this classification; also known as corduroy toweling. These are classified according to type:
All-White Plain?This has a plain border, white or colors. It also implies fancy-woven, colored border toweling.
Bath Mat?This is a heavy type of Terry made for bath mats. Coarse ply are used to provide bulkiness, strength, and the weight necessary to give the fabric body and substance.
Jacquard Reversible Allover?this features colored pile on side with white pile effect on the other side. The borders are plain or fancy. The interchanging white and colored loops form a contrasting motif on each side of the material.
Pastel-Color Plain?IT is made with dyed filling yarns, white pile yarns.
Oxford:–
Soft, somewhat porous, and rather stout cotton shirting given a silk-like luster finish. Made on small repeat basket weaves, the fabric soils easily because of the soft, bulky filling used in the goods. The cloth comes in all white or may have stripes with small geometric designs between these stripes. Now is made from spun rayon, acetate, and other manmade fibers. Oxford also means a woolen or worsted fabric which has a grayish cast made from a combination of black and white yarns or by use of dyed gray yarn.
1. A fiber of indefinite length, such as filament acetate, rayon, nylon, May be miles long.
2. A single strand of rayon spinning solution as it is exuded from a spinneret orifice and coagulated in an acid bath or other medium; also true of other manmade filaments.
3. The single unit which is extruded by a silkworm in spinning its cocoon. Actually the silkworm makes two filaments at the one time, and they are cemented or glued together by the sericin, or silkgum, exuded by the silkworm in the action. Filaments are then spun into yarn.
Denier: –
A weight-per--unit-length measure of any linear material. Officially, it is the number of unit weights of 0.05 grams per 450-meter length. This is numerically equal to the weight in grams of 9.000 meters of the material. Denier is a direct numbering system in which the low numbers represent the finer sizes and the higher numbers the coarser sizes. In the U.S. the denier is used for numbering filament yarns (except glass), man-made fiber staple (but not spun yarns), and tow. In most countries outside the U.S. the denier system has been replaced by the tex system.
Yarn: –
A generic term for an assemblage of fibers or filaments, either natural or man-made, twisted together to form a continuous strand which can be used for weaving, knitting, plaiting, braiding, of the manufacture of lace, or otherwise made into a textile material.
Yarn Standards: –
These standards for the major yarns are given below. It should be noted that yarn counts or numbers are written by number and this is followed by the letter "s." The apostrophe is not necessarily used between the number and the "s."
1. A. 840 yards in one pound of a 1s cotton yarn. (Boston or New England)
B. 560 yards in one pound of a 1s worsted yarn (two-thirds the number used for
the cotton standard of 840 yards in one pound of 1s).
C. 1,600 yards in one pound of a 1s run woolen yarn (Boston or New England)
D. 300 yards in one pound of a 1s cut woolen yarn (Philadelphia system)
E. 300 yards in one pound of a 1s linen yarn.
F. 840 yards in one pound of a 1s spun silk yarn, in the single ply.
G. 4,464,528 yards in one pound of a Number One denier filament yarn: used to
figure silk and man-made filament yarns.
2. Approximate Highest Counts of Yarn Spun for the Natural Fibers: On a per pound
basis, these approximations for commercial purpose follow:
A. Cotton: 140s x 840 standards, gives 117,600 yards.
B. Cut Wool: 30s x 300 standards, gives 9.000 yards.
C. Run Wool: 10s x 1,600 standard, gives 16,000 yards.
D. Worsted: 70s x 560 standard, gives 39,200 yards.
Taslan:–
A registered trademark. A textured yarn that is different from spun yarn or continuous filament yarn in that it is made on a bulking process developed by DuPont. Its hand, loftiness, covering power, and yarn texture are such that these properties are permanent and do not require special handling or care. The method may be applied to any thermoplastic fiber.
Denim:–
This basic cotton cloth? First brought to America by Columbus almost 500 years ago as the sails on the Santa Maria? is rugged, tough, and serviceable. It is easily recognized by its traditional indigo-blue color warp and gray or molted white filling, and its left hand twill on the face. Coarse single yarns are used mostly, but today many versions are available for the fashion world. A two-up and one-down or a three-up and one-down twill may be used in the weave construction. Long considered the most popular fabric for work clothes and army uniforms, denim today has won great fashion significance in dress goods for women's and men's wear, a wide range of sportswear, and even evening wear. It is estimated last year American textile mills consumed some 1,500,000 bales of cotton to produce denim which was known centuries ago as "Serge de Nimes" from its early origins in Nimes, France.
Dobby Loom: –
A type of loom on which small, geometric figures can be woven in a regular pattern. Originally this type of loom needed a "dobby boy" who sat on the top of the loom and drew up warp threads to form a pattern. Now the weaving is done entirely by machine. This loom differs from a plain loom in that it may have up to thirty-two harnesses and a pattern chain. Is expensive weaving.
Double Knit: -
A Circular knit fabric knitted via double stitch on a double needle frame to provide a double thickness. It is the same on both sides. Today, most double knits are made of 150 denier polyester although many lighter weight versions are now being made using finer denier yarns and blends of filament and spun yarns.
Dyeing of Textiles: –
Dyeing: The process of applying color to fiber stock, yarn or fabric; there may or may not be thorough penetration of the colorant into the fibers or yarns. Major methods of dyeing follow:
Bale Dyeing: A low cost method to dye cotton cloth. The material is sent, without scouring or singeing, through a cold water bath where the sized warp yarn has affinity for the dye. With the natural wax not removed from the filling yarn the dye will not be absorbed by this filling. Imitation chambray and comparable fabrics are often dyed in this way.
Batik Dyeing: One of the oldest forms known to man; originated in Java. Portions of the fabric are coated with wax so that only the un-waxed areas will take on the dye matter. The operation may be repeated several times and several colors may be used for the rather bizarre effects. Motifs show a melage, mottled or streaked effect. Imitated in machine printing.
Beam Dyeing: The warp is dyed prior to weaving. It is wound onto a perforated beam and the dye is forced through the perforations thereby saturating the yarn with color.
Burl or Speck Dyeing: Done mostly on woolens and worsteds, colored specks and blemishes are covered by the use of special colored inks which come in many colors and shades. This is a hand operation.
Chain Dyeing: Used when yarns and cloths are low in tensile strength. Several cuts or pieces of cloth are tacked end-to-end and run through in a continuous chain in the dye liquor. Affords high production.
Cross Dyeing: Varied color effects are obtained in the one dyebath for a cloth which contains fibers with varying affinities for the dye used. For example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6, 6 a light blue shade, and have no affinity for polyester thereby leaving the polyester area unscathed or white. it is a very popular method.
Jig Dyeing: This is done in a jig, vat, beck or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dyebath until the desired shade is achieved.
Package Dyeing: Yarns are dyed while on cones, cakes, cheeses or in the conventional or standard layout or set-up.
Piece Dyeing: The dyeing of fabric in the cut, bolt or piece form it follows the weaving of the goods and provides a single color for the material, such as a blue serge, a green organdy.
Random Dyeing: Coloring only certain designated portions of yarn. There are three ways of doing this type of coloring:
Skeins may be tightly tied in two or more places and dyed at one side of the tie with one color and at the other side with another one.
Color may be printed onto the skeins which are spread out on the blanket fabric of the printing machine.
Cones or packages of yarn on hollow spindles may be arranged to form channels through which the yarn, by means of an air-operated punch, and the dyestuff are drawn through three holes by suction. The yarn in the immediate area of the punch absorbs the dye and the random effects are thereby attained.
Raw Stock Dyeing: Dyeing of fiber stock precedes spinning of the yarn. Dyeing follows the degreasing of the wool fibers and dyeing of the stock.
Resist Dyeing: Treating yarn or cloth so that in any subsequent dyeing operation the treated portions resist the dye and do not absorb it at all.
Solution Dyeing: Also called dope dyeing and spun dyeing, the pigment or color is bonded-in in the solution and is picked up as the filaments are being formed in the liquor. Cellulosic and noncellulosic fibers are dyed to perfection by this method. Colors are bright, clear, clean, and fast.
Stock Dyed: Fibers are dyed after degreasing and drying and preceding, blending, oiling, mixing, carding (combing in worsted yarn manufacturer), and spinning of the yarn. Ideal for woolen and worsted fibers since it affords an endless array of color, cast, shade, tone or hue in the fabric.
Top Dyed: Often referred to as Vigoureux Printing, it is the dyeing or printing of worsted top or sliver in a rather loose formation of combed, parallel fibers. Precedes the spinning of the yarn and affords a host of colors, cats, and shades.
Union Dyed: The coloring of two or more different textile fibers in the one dyebath to provide different colors simultaneously or dyeing the fabric in a single shade, usually the latter.
Vat Dyed: Cloth dyed by the use of vat dyes which are obtained through oxidation. Very fast in all respects. Vat dyeing may be considered to be a misnomer since fabric colored with these dyes are piece dyed in the conventional manner.
Williams Unit: An open-width dyeing machine invented by the late S.H.. Williams of GAF Corporation, New York City. The fabric passes up and down over rollers in the dyeing bath. Widely used for dyeing, washing, pretreating and aftertreating.
Yarn Dyeing: yarn which has been dyed prior to the weaving of the goods; follows spinning of the yarn. May be done in either total immersion of the yarn.
Dyes, Types of: -
Acid Dyes: Being water soluble they are applied directly with an acid, such as sulphuric acid. Bright colors do not stand up too well in colorfastness when wet-treated; fair to poor in washing, good in dry cleaning and lightfastness. Used on wool, worsted, acrylics, and nylon.
Acid-Milling Dyes: Ideal for coloring wool, worsted, acrylics, modacrylics, nylon, and spandex fibers. Also used in printing. Good in drycleaning, excellent resistance to light.
Acid-premetalized dyes: Used on wool, acrylics, and nylon, they rate excellent to fastness in drycleaning, perspiration, and washing. Much used for carpeting, suiting fabrics, and upholstery.
Alizarine Dyes: Originally natural dyestuff. They are now synthetic dyes. Used on cotton and wool, they are resistant to sunlight and washing. Considerable use for apparel fabrics.
Azoic or Naphthol dyes: Also called Ice colors, Insoluble Azos, and Ingrain Colors. Rated good to laundering and washing. Ideal for decorative fabrics, draperies, dress goods, sportswear.
Basic Dyes: The first of the many groups of synthetic dyes. Sir William Perkin discovered them in 1856. Used on cotton, paper, and wool. Provide brilliant shades but rated poor to fair to light and washfastness; color resistance is only fair.
Chrome dyes: See Mordant Dyes Below. Developed Dyes: Used on cotton and rayon and other man-made fibers; on the latter when developed from disperse dye bases. Lightfastness rated from poor to good. See Mordant Dyes below.
Direct Dyes: Also known as Application or Commercial Dyes. Applied to cellulosic fibers such as cotton and rayon. Lightfastness rated from poor to excellent, and because of solubility are not rated highly in washfastness.
Disperse Dyes: Also known as Azo or Anthraquinone Dyes, they are ideal on nylon, acrylics, modacrylics, and polyesters?either dyed or printed. These dyes have a great many applications in the industry. Colorfastness is rated poor to very good for light and washing, depending on the fiber used. Ingrain: See Azoic or Naphthol Dyes.
Mineral Colors: Actually they are not true dyes but are precipitated oxides or insoluble salts of chromium, iron, lead or manganese. Dull in appearance these colors are much used to color awnings and comparable fabrics.
Mordant Dyes: A mordant is a substance used in dyeing to apply or fix coloring matter to a fiber, yarn or fabric, especially a metallic compound such as an oxide which combines with the fiber and organic dye and forms an insoluble color compound or take in the fiber. Also known as Mordant-Acid Dyes or Chrome Dyes, they are closely related to Acid Dyes. Results are dull when compared with those from acid dyes. Exceptionally fast on wool and other animal fibers. Much used as well on carpeting, nylon, and silk.
Neutral-premetalized Acid Dyes: Much used on wool and other protein fibers, acrylics, modacrylics, and nylon and ideal for use in blends. Much used in coloring apparel. Fair to excellent to light, good to excellent in washing, excellent in drycleaning.
Oxidation Bases: One of these bases is aniline dye which is formed in the fiber by oxidation. Ideal for coloring fur, sheepskins, and pile cloths, as well as in dyeing cotton for use in a wide range of fabrics. Finds much application in printgoods.
Pigment Colors: Insoluble in water and the color has to be fixed onto the fiber by use of resinous-binders insolubilized by a curing treatment at high temperatures. Used mostly on cotton and acetate, rayon and some other man-made fibers, these dyes, generally speaking, color, by dyeing or printing, just about all types of fibers and blends. Light and medium shades of sailcloths and many types of dress goods use Pigment Colors. Good to excellent in lightfastness but may be poor in cocking or rubbing.
Reactive Dyes: They actually bond-in the colorant. Provide bright colors on cottons and can dye acrylics, nylon, silk, and wool, and blends of these fibers. Also used for printing cotton fabrics. Rated good to very good to light and washing, but fugitive to chlorine-base bleaches.
Sulphur Dyes: They do not provide bright shades to any marked degree and their fastness properties have to be developed in the chemical inertness and insolubility in water. There are now also soluble forms of these dyes on the market. Used to color heavy cottons, knitwear in medium to full shades and readily dye stock, yarn, and piecegoods. weak in sunlight except for deep shades where fastness is good. Fugitive to chlorine-base bleaches.
Vat and Vat-Soluble Dyes: These are the fastest dyes known to man; insoluble in water and made soluble by chemical reduction. They actually bond-in the colorant and are the most resistant of any types of light, drycleaning, sunlight, and washing. Has many applications on cotton, rayon, polyesters, etc. Vat dyes are used to color awnings, bed linens, decorative fabrics, outerwear, sportswear, toweling, workclothes.
Twill Weave: -
Identified by the diagonal lines in the goods. It is one of the three basic weaves, the others being plain and satin. All weaves, either simple, elaborate or complex, are derived from these three weaves. Most twills are 45 degrees in angle. Steep twills are made from angles of 63, 70, 75 degrees while reclining twills use angles of 27, 20 and 15 degrees. Right-hand twilled clothes include cassimere, cavalry twill, covert, elastique, gabardine, serge, tackle twill, tricotin, tweed, whipcord. Left-hand twills include denim, galatea, jean cloth, some drill and twill cloth, and some ticking fabrics.
Tricot: –
· A type of warp knitted fabric which has a thin texture since it is made from very fine yarn. The French verb tricoter, means "to knit." The fabric is made on one, two, or three bar frames. It is knitted flat and made on spring-beard needles and has from one to four warps or thread systems which are mounted in a stationary position. Industry Standard Machine is 28 needles to the inch or a 28 gauge (gg).
· "Stocking-net" as applied to a warp-knitted fabric irrespective of the motif ; often refers to a flat knitted cloth since it is not tubular. The meaning, however, is not to be constructed to imply a flat-machine knit fabric.
· A French serge lining fabric made on a 20-inch width.
· A fine woven worsted made on the tricot weave which presents fine break lines in the filling direction. This chainbreak effect fabric is dyed all popular shades, has a high, compact texture, and is a good material to use in tailoring. Gives excellent wear in the better type of tailored garments for women. See warp knitting.
Teflon:-
Registered trademark of DuPont for its tetrafluoroethylene polymer fiber, announced in 1953. Type TFE is the polytetrafluoroethylene and it is not thermoplastic. Type FEP is fluorinated ethylene and is thermoplastic. The fiber is of smooth, circular surface and the cross-section is round. Teflon is ecru, brown or tan in color cast but bleaches to white in strong oxidizing mineral acids. It does not absorb moisture and is the most non-wettable fiber known to man. It possesses excellent thermal stability is very tough and strong, and also chemical resistant. It remains inert to all chemicals except to molten alkali metals, hot fluorine gas or chlorine trifluoride under pressure. It is used in filters, felts, gaskets, diaphragm fabrics, electrical tape, pump-packing, et al.
Terry Cloth: -
This cloth has uncut loops on both sides of the fabric. Woven on a dobby loom with Terry arrangement, various sizes of yarns are used in the construction. Terry is also made on a Jacquard loom to form interesting motifs. It may be yarn-dyed in different colors to form attractive patterns. It is bleached, piece-dyed, and even printed for beachwear and bathrobes, etc. Also called Turkish toweling. Today knitted Terry is very popular in fashion.
Terry Toweling: –
These are classified according to weave or design:
1. Cam-Woven, Plain Terry?Plain border.
2. Dobby-Woven?Simple pattern in the border, or all over. Border design include rope and corduroy borders.
3. Jacquard-Woven?Those which have rather elaborate allover motifs, or names woven into the goods.
4. Mitcheline?This border type has a heavy, distinct, raised or embossed border effect, formed by a stout colored filling yarn; the roving is used sometimes instead of a yarn to obtain the effect. Most of this fabric is made on Jacquard looms.
5. Texture-Designed?This is made on either a dobby or a jacquard loom. It has an allover, raised, and recessed motif. The athletic-rib towel, which has raised terry stripes with alternating plain ground stripes, is in this classification; also known as corduroy toweling. These are classified according to type:
All-White Plain?This has a plain border, white or colors. It also implies fancy-woven, colored border toweling.
Bath Mat?This is a heavy type of Terry made for bath mats. Coarse ply are used to provide bulkiness, strength, and the weight necessary to give the fabric body and substance.
Jacquard Reversible Allover?this features colored pile on side with white pile effect on the other side. The borders are plain or fancy. The interchanging white and colored loops form a contrasting motif on each side of the material.
Pastel-Color Plain?IT is made with dyed filling yarns, white pile yarns.
Oxford:–
Soft, somewhat porous, and rather stout cotton shirting given a silk-like luster finish. Made on small repeat basket weaves, the fabric soils easily because of the soft, bulky filling used in the goods. The cloth comes in all white or may have stripes with small geometric designs between these stripes. Now is made from spun rayon, acetate, and other manmade fibers. Oxford also means a woolen or worsted fabric which has a grayish cast made from a combination of black and white yarns or by use of dyed gray yarn.
Thursday, August 14, 2008
Surface Preparation
Good surface preparation is crucial to ensuring sufficient bond strength and reliable performance of bonded joints, particularly under hostile service or manufacturing conditions. Unsatisfactory surface preparation will result in premature and unpredictable bond failure at the adhesive/adherend interface either in service or at some further stage of the bonding process. Surface preparation is recognized as a most critical step in the adhesive bonding process and considerable effort may be expended in optimizing the surface treatment.
The purposes of surface treatments are to:
1. Remove contaminants that may interfere with bond formation;
2. Remove weak surface layers;
3. Produce a surface morphology that enhances the surface area available
for bonding and/or allows mechanical keying;
4. Chemically modify the surface to increase surface energy and
chemical compatibility with the adhesive.
The selection of surface treatment is largely dependent on the substrate, the required strength and durability of the joint and economic considerations (such as costs and time involved in preparation). Surface treatment processes often consist of a series of different steps. Surface treatments can be classified as either passive or active.
Passive surface treatments (e.g. solvent washing and mechanical abrasion) clean the surface, remove weakly attached surface layers and alter the surface topography without altering the surface chemistry. Active surface treatments (e.g. corona discharge or plasma treatment) alter the surface chemistry (i.e. introduction of functional groups). After completion of the surface preparation process, the adherends should be handled and stored carefully in order to prevent surface contamination prior to bonding. It is normally advisable that bonding be performed immediately following surface treatment to maximize performance.
It is normally important that the process of surface preparation only affects the chemistry and morphology of a thin surface layer of the adherend(s) and does not alter the mechanical and physical properties of the underlying substrate. There are many procedures available for engineering surfaces [e.g. 16, 20–24] but comparatively few for materials used non-engineering applications. Advice is usually sought on surface preparation from the adhesive manufacturer. Surface preparation procedures may often require potentially hazardous or environmentally damaging chemicals. All preparation should be carried out to COSHH specifications.
The purposes of surface treatments are to:
1. Remove contaminants that may interfere with bond formation;
2. Remove weak surface layers;
3. Produce a surface morphology that enhances the surface area available
for bonding and/or allows mechanical keying;
4. Chemically modify the surface to increase surface energy and
chemical compatibility with the adhesive.
The selection of surface treatment is largely dependent on the substrate, the required strength and durability of the joint and economic considerations (such as costs and time involved in preparation). Surface treatment processes often consist of a series of different steps. Surface treatments can be classified as either passive or active.
Passive surface treatments (e.g. solvent washing and mechanical abrasion) clean the surface, remove weakly attached surface layers and alter the surface topography without altering the surface chemistry. Active surface treatments (e.g. corona discharge or plasma treatment) alter the surface chemistry (i.e. introduction of functional groups). After completion of the surface preparation process, the adherends should be handled and stored carefully in order to prevent surface contamination prior to bonding. It is normally advisable that bonding be performed immediately following surface treatment to maximize performance.
It is normally important that the process of surface preparation only affects the chemistry and morphology of a thin surface layer of the adherend(s) and does not alter the mechanical and physical properties of the underlying substrate. There are many procedures available for engineering surfaces [e.g. 16, 20–24] but comparatively few for materials used non-engineering applications. Advice is usually sought on surface preparation from the adhesive manufacturer. Surface preparation procedures may often require potentially hazardous or environmentally damaging chemicals. All preparation should be carried out to COSHH specifications.
Wednesday, August 13, 2008
Adhesives
The bonding of hot melt adhesives was a major focus of this study. Hot melt adhesives
are thermoplastic polymer based compounds. They are applied as molten liquids,
which increase in viscosity as they cool before freezing and becoming more rigid. Two
common base polymers are ethylene vinyl acetate (EVA) copolymers and polyethylene
(PE) homopolymers, with metallocene based hot melts now entering the market. A
tackifier resin is added to achieve good hot tack, waxes to reduce viscosity (viscosity
must be low at application temperature to allow good substrate wetting, but not too low
to allow excessive spreading) and control setting speed, and stabilisers to prevent
charring.
A typical formulation for a hot melt packaging adhesive would be:
1. Tackifier resin 35-50%
2. Polymer 25-35%
3. Wax 20-30%.
Recently, pressure sensitive hot melt adhesives, which have permanent tack, have been
introduced in packaging and garments. A further trend is the introduction of low
process temperature (cool running) adhesives that can be dispensed around 90C-
100C, allowing more energy efficient and safer processes than traditional systems
where dispense temperatures are typically over 150C. Hot melt adhesives are
formulated for many different purposes with properties that are specific to the market
application but there are certain adhesive attributes that are common:
1. Low cost Clean running (absence of stringing or dripping)
2. Long pot life (thermal stability – resistance to viscosity change and charring)
3. Low temperature application – mainly desirable to reduce worker exposure to
fumes and lessen the burn hazard
4. Wide temperature application window
5. Taint free – food packaging applications
6. Generates substrate failure – fibre tear is a universal measure of satisfactory
bond performance in the packaging industry
Several hot melt adhesive grades were supplied by National Starch and Chemical,
representing the broad application categories used in the packaging industry.
1. General purpose
2. Deep freeze
3. Heat/creep resistant
4. Difficult substrates
5. Low temperature application
6. Pressure sensitive
Most of the hot melt adhesives were supplied as bags of solid pellets from which sub-
samples could be taken. However, the pressure sensitive adhesives were supplied in 1
kg blocks of soft, tacky solid present a particular challenge for testing. In normal
production use the whole block would be melted in the tank of the dispensing machine
before application but for testing smaller quantities are desired. It is extremely difficult
to separate small pieces of adhesive from the block due to the high tack of the adhesive
– it tends to re-bond as it is cut. Cooling in a deep freeze (to increase rigidity and
reduce tack) and heating in an oven to over 100 C (to make it more liquid) made little
difference to the ease of cutting of the particular adhesives studied.
are thermoplastic polymer based compounds. They are applied as molten liquids,
which increase in viscosity as they cool before freezing and becoming more rigid. Two
common base polymers are ethylene vinyl acetate (EVA) copolymers and polyethylene
(PE) homopolymers, with metallocene based hot melts now entering the market. A
tackifier resin is added to achieve good hot tack, waxes to reduce viscosity (viscosity
must be low at application temperature to allow good substrate wetting, but not too low
to allow excessive spreading) and control setting speed, and stabilisers to prevent
charring.
A typical formulation for a hot melt packaging adhesive would be:
1. Tackifier resin 35-50%
2. Polymer 25-35%
3. Wax 20-30%.
Recently, pressure sensitive hot melt adhesives, which have permanent tack, have been
introduced in packaging and garments. A further trend is the introduction of low
process temperature (cool running) adhesives that can be dispensed around 90C-
100C, allowing more energy efficient and safer processes than traditional systems
where dispense temperatures are typically over 150C. Hot melt adhesives are
formulated for many different purposes with properties that are specific to the market
application but there are certain adhesive attributes that are common:
1. Low cost Clean running (absence of stringing or dripping)
2. Long pot life (thermal stability – resistance to viscosity change and charring)
3. Low temperature application – mainly desirable to reduce worker exposure to
fumes and lessen the burn hazard
4. Wide temperature application window
5. Taint free – food packaging applications
6. Generates substrate failure – fibre tear is a universal measure of satisfactory
bond performance in the packaging industry
Several hot melt adhesive grades were supplied by National Starch and Chemical,
representing the broad application categories used in the packaging industry.
1. General purpose
2. Deep freeze
3. Heat/creep resistant
4. Difficult substrates
5. Low temperature application
6. Pressure sensitive
Most of the hot melt adhesives were supplied as bags of solid pellets from which sub-
samples could be taken. However, the pressure sensitive adhesives were supplied in 1
kg blocks of soft, tacky solid present a particular challenge for testing. In normal
production use the whole block would be melted in the tank of the dispensing machine
before application but for testing smaller quantities are desired. It is extremely difficult
to separate small pieces of adhesive from the block due to the high tack of the adhesive
– it tends to re-bond as it is cut. Cooling in a deep freeze (to increase rigidity and
reduce tack) and heating in an oven to over 100 C (to make it more liquid) made little
difference to the ease of cutting of the particular adhesives studied.
Tuesday, August 12, 2008
Monday, August 11, 2008
Applications and Advantages Of Plasma Treatment
Plasma advantages
Following advantages are reached by using this technique:
1. Environmental friendly technique: because of the low energy consumption, the fact
it is a dry technique (no additional drying step), no waste disposal problem and
disposal cost.
2. Operator friendly technique: no chemical products, gasses etc.
3. Qualitative and full controllable process: all parameters are controlled by the
unit and quality control possible by print-out and data-logging
4. Effective treatment: higher degree of activation, longer shelf-life than
alternative methods as corona and flaming
5. No substrate damage or bulk property changes
6. Different processes can run in the same unit
7. No limit to substrate geometries : small and large, simple or complex, parts or
textiles are possible
All these advantages make it part of the future techniques for surface preparation and modification.
Plasma applications
The different possible treatments and there applications are presented below.
Plasma can be used and industrial systems are on the market for following substrates:
1. Small parts like hubs or balloons up to very huge and complex substrates
2. Fibers, non woven, woven, paper
3. Plastic foils
4. Metal and ceramic parts
Plasma can be used for following goals:
Adhesion promotion:
All polymers as for example PP, PE, PA, POM, Teflon, have a low to medium surface energy. Therefore they are difficult to glue, paint, coat etc. By using oxygen plasma it is possible to get them in a condition that they obtain the best possible contact with the glue or coating. This chemical/physical process has formed a surface with an optimal number of bonding sites at the surface. Through this it is possible to attach the glue or coating to the surface without non-contact zones as bubbles or unreachable areas in holes. The surface is as such asking for contact with the liquid. By using plasma the surface energy is increased for example for a propylene from 29 dynes/cm to 72 dynes/cm which is about the value of full water contact. This activation process is used were a high quality bonding has to be achieved. Gluing of catheters and balloon catheters, gluing of the needle in the hub of syringes, dialysis filters, and other medical parts.
Hydrophilic properties:
Another and specially developed activation process can be used to make the surface hydrophilic. This permanently hydrophilic character is used to give woven, and non woven textiles the capability to be used as blood filter or filtering membranes for specific applications. Applications are micro filtration systems based on these textiles or capillaries: blood filters, dialysis filter systems...
Oleophobicity and Hydrophobicity:
By using semi-continuous textile treaters it is possible to plasma polymerise the surface of non woven and other textiles so that they become hydrophobic of nature. A lot of industrial users are looking to replace their conventional techniques or improve the final result by using plasma technology. Applications are oleophobic or hydrophobic treatment of paper, tissues and filter elements.
Biocompatibility:
Activation of surfaces for cell growth or protein bonding is another industrial application. In this case it is possible to prepare Petry disks and micro titre plates for laboratory experiments or drug production purposes. This can also be applied on implants to increase the acceptation by the human body. The surface will be open to adhere a blood compatible layer. This improves biocompatibility. Typical applications are vascular grafts, lenses, drug delivery implants… Surfaces can also be modified to decrease the bonding of proteins or blood compatibility. For example in food industry or in pharmaceutical industry.
Cross linking:
Some surfaces have to be made more chemical or scratch resistant. These typical surface conditions can be obtained by using plasma to reform the surface by cross linking the surface bindings. Used for catheters, medical instruments, contact lenses,
Following advantages are reached by using this technique:
1. Environmental friendly technique: because of the low energy consumption, the fact
it is a dry technique (no additional drying step), no waste disposal problem and
disposal cost.
2. Operator friendly technique: no chemical products, gasses etc.
3. Qualitative and full controllable process: all parameters are controlled by the
unit and quality control possible by print-out and data-logging
4. Effective treatment: higher degree of activation, longer shelf-life than
alternative methods as corona and flaming
5. No substrate damage or bulk property changes
6. Different processes can run in the same unit
7. No limit to substrate geometries : small and large, simple or complex, parts or
textiles are possible
All these advantages make it part of the future techniques for surface preparation and modification.
Plasma applications
The different possible treatments and there applications are presented below.
Plasma can be used and industrial systems are on the market for following substrates:
1. Small parts like hubs or balloons up to very huge and complex substrates
2. Fibers, non woven, woven, paper
3. Plastic foils
4. Metal and ceramic parts
Plasma can be used for following goals:
Adhesion promotion:
All polymers as for example PP, PE, PA, POM, Teflon, have a low to medium surface energy. Therefore they are difficult to glue, paint, coat etc. By using oxygen plasma it is possible to get them in a condition that they obtain the best possible contact with the glue or coating. This chemical/physical process has formed a surface with an optimal number of bonding sites at the surface. Through this it is possible to attach the glue or coating to the surface without non-contact zones as bubbles or unreachable areas in holes. The surface is as such asking for contact with the liquid. By using plasma the surface energy is increased for example for a propylene from 29 dynes/cm to 72 dynes/cm which is about the value of full water contact. This activation process is used were a high quality bonding has to be achieved. Gluing of catheters and balloon catheters, gluing of the needle in the hub of syringes, dialysis filters, and other medical parts.
Hydrophilic properties:
Another and specially developed activation process can be used to make the surface hydrophilic. This permanently hydrophilic character is used to give woven, and non woven textiles the capability to be used as blood filter or filtering membranes for specific applications. Applications are micro filtration systems based on these textiles or capillaries: blood filters, dialysis filter systems...
Oleophobicity and Hydrophobicity:
By using semi-continuous textile treaters it is possible to plasma polymerise the surface of non woven and other textiles so that they become hydrophobic of nature. A lot of industrial users are looking to replace their conventional techniques or improve the final result by using plasma technology. Applications are oleophobic or hydrophobic treatment of paper, tissues and filter elements.
Biocompatibility:
Activation of surfaces for cell growth or protein bonding is another industrial application. In this case it is possible to prepare Petry disks and micro titre plates for laboratory experiments or drug production purposes. This can also be applied on implants to increase the acceptation by the human body. The surface will be open to adhere a blood compatible layer. This improves biocompatibility. Typical applications are vascular grafts, lenses, drug delivery implants… Surfaces can also be modified to decrease the bonding of proteins or blood compatibility. For example in food industry or in pharmaceutical industry.
Cross linking:
Some surfaces have to be made more chemical or scratch resistant. These typical surface conditions can be obtained by using plasma to reform the surface by cross linking the surface bindings. Used for catheters, medical instruments, contact lenses,
Sunday, August 10, 2008
Action of contact angle in surface wettability
The surface tension, or more accurately the liquid-air interfacial tension, of a liquid adhesive is a key property for bond formation. Surface tension arises from the attractive forces of molecules at the surface of a liquid. It is the force required to break the “film” formed at the surface. Surface tension is often expressed in dynes/cm (a surface tension of 1 dyne/cm is equivalent to 
This part of this Guide describes the pendant drop method for determining the surface tension of hot melt adhesives specifically at elevated temperatures above 100 C।
Surface tension is important in bonding. Interfacial tension of the adhesive and the surface energy of the substrate determine the wetting of the adhesive on the surface, good wetting promotes intimate contact and
higher adhesion. The penetration of the liquid into craters and pores in rough and porous substrate, which enhances bond strength through mechanical ‘keying’, is a surface tension driven process. Surface tension is one of the properties controlling the stability of adhesive jets or sprays during dispensing; it may be an important parameter in effects such as stringing or drop break-up.
Surface tension depends on the interfacial intermolecular forces and can be split into
contributions from non-polar (e.g. van der Waals) and polar (e.g. hydrogen bonding)
components। The polar components can be further broken into electron acceptor or electron donor components (or Lewis acid/base components). Polar molecules have varying proportions of acceptor/donor components and in many cases one component will be much more dominant. Water is fairly unusual in having both strong acceptor and strong donor properties.
The surface tension of a liquid will influence how that liquid (adhesive) will wet a solid
(adherend)। Force balance or equilibrium at the solid-liquid boundary is given by Young’s equation (1) for contact angles greater than zero (see Figure 1):
The lower the contact angle, the Greater the tendency for the liquid to wet the solid, until complete wetting occurs (contact angle Thita= 0, cos Thita= 1). For complete wetting to occur the surface tension of the liquid should be less than or equal to the critical surface tension of the सुब्स्त्रते। Large contact angles are associated with poor wettability. Thus surface tension potentially plays a significant role in the dispensing and spreading of hot melt adhesives.
This part of this Guide describes the pendant drop method for determining the surface tension of hot melt adhesives specifically at elevated temperatures above 100 C।
Saturday, August 9, 2008
Durable Water Repellent (DWR)
Durable Water Repellent (DWR) finishes are hydrophobic coatings applied to fabrics to make them water-resistant by causing water to bead up and roll off fabrics, rather than soaking into them. They are often used in conjunction with certain waterproof/breathable fabrics to prevent the outer layer of a laminated waterproof/breathable from becoming saturated with water. Maintaining the DWR is critical to maintaining the breathable nature of waterproof/breathable outerwear.
Despite the name, durable water repellent finishes tend to wear off and may need to be re-applied from time to time. The application method will probably make a big difference in the DWR you choose to use. Some, like the Nikwax boot finish, are thick pastes that are applied by hand while the vast majority can be poured or sprayed on. Many are applied by adding to a regular wash cycle in a washing machine.
Common brands of DWR finishes include:
Nikwax (uses patented wax-based elastomer - non fluoropolymer)
Granger's (fluoropolymer base)
Tectron (fluoropolymer base)
ReviveX (no information available, most likely industry standard; fluoropolymer base)
Deluge (fluoropolymer base)
Often the best way to choose a good DWR is to visit an outdoor gear store such as REI or Eastern Mountian Sports and talk to a sales rep who has tried a number of different finishes.
WARNING: While fluoropolymers offer a high level of water repellency, as well as a high degree of oil and stain repellency, everyday users should be aware that fluoropolymer compounds can degrade into PFOA's. PFOA's are being reviewed by the E.P.A. (United States Environmental Protection Agency] to determine their potential for human carcinogenicity. One of the only exceptions to this is the above-mentioned Nikwax, which uses a patented, environmentally-safe wax elastomer.
Despite the name, durable water repellent finishes tend to wear off and may need to be re-applied from time to time. The application method will probably make a big difference in the DWR you choose to use. Some, like the Nikwax boot finish, are thick pastes that are applied by hand while the vast majority can be poured or sprayed on. Many are applied by adding to a regular wash cycle in a washing machine.
Common brands of DWR finishes include:
Nikwax (uses patented wax-based elastomer - non fluoropolymer)
Granger's (fluoropolymer base)
Tectron (fluoropolymer base)
ReviveX (no information available, most likely industry standard; fluoropolymer base)
Deluge (fluoropolymer base)
Often the best way to choose a good DWR is to visit an outdoor gear store such as REI or Eastern Mountian Sports and talk to a sales rep who has tried a number of different finishes.
WARNING: While fluoropolymers offer a high level of water repellency, as well as a high degree of oil and stain repellency, everyday users should be aware that fluoropolymer compounds can degrade into PFOA's. PFOA's are being reviewed by the E.P.A. (United States Environmental Protection Agency] to determine their potential for human carcinogenicity. One of the only exceptions to this is the above-mentioned Nikwax, which uses a patented, environmentally-safe wax elastomer.
Saturday, August 2, 2008
The 4th And 5th State Of Matter
The fourth state of matter
The fourth state of matter is plasma. Plasma is an ionized gas, a gas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist. In effect a plasma is a cloud of protons, neutrons and electrons where all the electrons have come loose from their respective molecules and atoms, giving the plasma the ability to act as a whole rather than as a bunch of atoms. Plasmas are the most common state of matter in the universe comprising more than 99% of our visible universe and most of that not visible. Plasma occurs naturally and makes up the stuff of our sun, the core of stars and occurs in quasars, x-ray beam emitting pulsars, and supernovas. On earth, plasma is naturally occurring in flames, lightning and the auroras. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless.
The Fifth State Of Matter -- Bose Einstein
The collapse of the atoms into a single quantum state is known as Bose condensation or Bose-Einstein condensation is now considered a 5th state of matter.
Recently, scientists have discovered the Bose-Einstein condensate, which can be thought of as the opposite of a plasma. It occurs at ultra-low temperature, close to the point that the atoms are not moving at all. A Bose-Einstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nK). --Under such conditions, a large fraction of the atoms collapse into the lowest quantum state, producing a superfluid. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein.
The fourth state of matter is plasma. Plasma is an ionized gas, a gas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist. In effect a plasma is a cloud of protons, neutrons and electrons where all the electrons have come loose from their respective molecules and atoms, giving the plasma the ability to act as a whole rather than as a bunch of atoms. Plasmas are the most common state of matter in the universe comprising more than 99% of our visible universe and most of that not visible. Plasma occurs naturally and makes up the stuff of our sun, the core of stars and occurs in quasars, x-ray beam emitting pulsars, and supernovas. On earth, plasma is naturally occurring in flames, lightning and the auroras. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless.
The Fifth State Of Matter -- Bose Einstein
The collapse of the atoms into a single quantum state is known as Bose condensation or Bose-Einstein condensation is now considered a 5th state of matter.
Recently, scientists have discovered the Bose-Einstein condensate, which can be thought of as the opposite of a plasma. It occurs at ultra-low temperature, close to the point that the atoms are not moving at all. A Bose-Einstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nK). --Under such conditions, a large fraction of the atoms collapse into the lowest quantum state, producing a superfluid. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein.
Friday, August 1, 2008
Controlled Plasma Etching
Over the past thirty years, plasma, the fourth
state of matter, has become a very useful
means of removing small quantities of
material from a variety of substrates quickly
and efficiently. Plasma processes have been
used in many highly sensitive integrated
circuit packaging and optoelectronic
applications to precisely remove specific
materials from sample surfaces. The theory
of chemical etch plasmas, some typical
advanced technology applications, and how
to control the plasma etch process for these
applications will be discussed.
THE CHEMICAL PLASMA
The room-temperature gas plasmas
employed are typically generated in a
vacuum chamber. To generate plasma, the
chamber is pumped to a pre-set base
pressure, process gas is introduced, and a
radio frequency (RF) electromagnetic field is
applied to the electrodes in the chamber
producing a glow-discharge plasma. In the
plasma, many different gaseous species are
produced. These species include ions, free
radicals, electrons, photons, neutrals, and
reaction by-products such as ozone. These
species make a highly active, lowtemperature
plasma that can etch material
selectively and quickly.
Controlling this type of plasma for effective
etching is a balancing exercise. The proper
amount of ions and free radicals (the
species that do most of the work) in and
around the area to be etched must balanced
with the RF power input into the system.
Generally, the amount of ions and free
radicals is governed by the process
pressure, thus making process pressure a
very important process parameter. RF
power input, on the other hand, must be
selected so that the highest etch rate is
produced without over etching or
inadvertently damaging other materials or
substrates. The process time parameter
must also be selected with care because it,
like power and pressure, can greatly effect
the outcome of the process cycle.
One significant advantage that chemical
plasmas have over other types of plasma is
their natural selectivity. Selectivity, in this
sense, is defined as a chemical’s propensity
to react with one substance rather than
another. In plasma, this is extremely useful
characteristic. It provides the opportunity to
tailor the plasma etching process to the
substance of interest and not cause
unwanted etching to other substances which
can be in close proximity.
Reactive Ion Etching (RIE) is a type of
chemical plasma. It is characterized not
only by the parameters and characteristics
mentioned above, but also RIE is extremely
directional and anisotropic. It has many
applications, this Application Note discusses
applications specific to semiconductor and
optoelectronic processing and packaging.
APPLICATIONS
Photoresist Removal:
Photoresist removal has two plasma
process applications. The first application is
uniform removal of small quantities of resist
over the entire surface of a wafer. This is
known as "descum." In this case, etch rate
should be moderate, and a low-reactivity
process gas, such as O2, is used. RF power
should be kept low as well. To increase the
uniformity of the descum operation,
operating pressure must remain relatively
high and usually from 600 mTorr to 1000
mTorr. At low power and high pressure, a
very isotropic and uniform distribution of ions
exists thus achieving a highly uniform,
moderately rapid etching operation.
The other plasma application for photoresist
removal is etching features from patterned
photoresist. In this case, the etching
operation must be fast and especially
anisotropic. The isotropic etch must
produce very vertical wall features with no
undercutting. As a rule, highly reactive
process gases or gas combinations such as
CF4 or a mixture of CF4 and O2 are used.
To make the plasma as anisotropic as
possible, pressure is lower than the descum
process discussed above, while RF power is
increased. The decreased pressure (usually
in the 100 to 200 mTorr range) provides the
anisotropic nature of the plasma, and the
increased power compensates for the lack
of ions to increase the etch rate. The side
effects of this operation are over etching and
non-uniformity. Process time will manage
any over etching issues, but increased
uniformity will require a rather close balance
of power and pressure. This power and
pressure balance will be material and
substrate-geometry specific and may take a
small amount of process development to
achieve satisfactory results.
Glass and Glass-like Compound Etching
In many ways, etching glass or glass-like
substances like SiO2, Si3N4, and singlecrystal
silicon is similar to photoresist
etching. The biggest difference is process
gas selection. Glass is a very non-reactive
or stable substance. Consequently, highly
reactive process gases like CF4 and SF6 are
employed. Analogous to photoresist
removal, the degree of anisotropy and
uniformity can be controlled largely by
pressure and power using the gases
mentioned. However, unlike photoresist
removal, etch rate will vary widely due to the
fact that glass is an amorphous substance
that can vary widely in composition. Care
must be taken when developing etching
recipes not to damage valuable parts by
inadvertent over etching. Applications for
this type of etch include fused-silica optical
fiber etching and etching BPSG, SiO2 and
Si3N4 in IC failure analysis operations.
Polymer Etching:
Etching polymers can be exceedingly
challenging or exceedingly simple. The
challenge springs from the fact that
polymeric substances are widely varied in
their make up. For instance, polypropylene
encompasses several hundred different
compounds of polymer, all of which conform
to the characteristics innate to
polypropylene. The slightest change in
plasticizer or UV stabilizer makes
developing a single, all-encompassing
etching recipe nearly impossible because
there is usually no common starting point.
The technique for etching polymers is the
use of a mixture of process gases. Oxygen
and tetrafluoromethane (CF4), when mixed
together for use in plasma etching, create
the oxyfluoride ion (OF-). The oxyfluoride
ion is a powerful etching agent for polymeric
substances. This ion is particularly adept at
cutting the carbon-carbon molecular bonds
in the polymer backbone and removing the
molecule quickly.
One application of polymer etching is hole
boring in polyamide when the polyamide is
sandwiched between two, conductive sheets
of metal. The holes are easily made using a
ratio 80% oxygen and 20% CF4 at low
pressure and high power. Time will depend
on the polyamide make up and the depth,
but low pressure will insure clean, straight
sidewalls in the hole. Selectivity will insure
that only the polymer inside the whole is
etched. Due to the selectivity of the etching
process, the metal around should be
unharmed by the etching operation.
Another application is the etching of
polyamide in bulk form such as off a wafer.
This, again, is similar to etching photoresist
in that the process pressure is set relatively
high for good uniformity, and the power is
increased to speed etching rate. In addition,
this operation is like etching glass because
the composition of the polyamide can be
varied, thus causing unpredictable etch
rates. Generally, a good starting point for
process recipe development is high
pressure and moderate power.
A final application in the discussion of
polymer etch is optical fiber cladding
etching. Optical fibers consist of a fused
silica core, which is approximately 100 μm
thick surrounded by polyurethane cladding
usually another 125 μm thick. The
application here is to etch the cladding
completely off a specific section of the fiber
expose the fused silica core without
damaging it. To uniformly treat all the fiber
optic strand, an especially isotropic plasma
is needed. Thus, pressure should be near
500 mTorr with high power. Time is the
most important parameter in this application,
because, even at 90% O2 and 10 % CF4, the
CF4 can damage the silica core and diminish
strand pull strength. The fiber optic stand
must be etched only long enough to remove
the cladding.
Bleedout Removal:
During epoxy dispensing operations, the
amount of dispensed epoxy is excessive or
the substrate material causes a small
quantity of the epoxy to wet out over its
surface. This problem is particularly critical
when subsequent wire bonding is required.
The epoxy residue can contaminate wire
bond pads causing poor bond strength or
wire bond "pull-up" which is the complete
failure of the wire bond. To remove the
epoxy contamination, an argon, argon and
oxygen, or argon and hydrogen plasma is
used around the 200mTorr to 250 mTorr
pressure range. Argon is an inert gas, but it
can be highly effective in removal of the
epoxy bleedout through pure bombardment
using Ar+ ions. The bombardment cleans
the surfaces of the wire bond sites by
ablating the epoxy and leaving a pristine
metal surface behind. The addition of a
chemically reactive agent such as oxygen or
hydrogen simply increases the reaction rate.
The amount of reactive gas can vary, but
usually, no more than 30% by volume is
added. There is one caveat however:
addition of oxygen can cause oxidation of
silver-filled epoxies thus turning them black.
This oxidation is nothing more than the
surface tarnishing of the silver in the epoxy,
and it does not affect the epoxy’s ability to
conduct heat or electricity.
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