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.