Tecbond hot melt adhesives (Glue Sticks) are complex adhesives based on EVA, Polyamide, Acrylic, Polyurethane (PUR), Polypropylene & Polyolefin polymers, tackifying resins, rheological agents, stabilisers, waxes and in some cases fillers.
The key properties for the Tecbond adhesives are the open time, the setting time, the viscosity, the mechanical properties and the adhesion. The choice of the base polymer affects adhesion, temperature resistance, chemical resistance, flexibility and cost.
When heat is applied, the Tecbond hot melt adhesive liquifies. When liquid the Tecbond adhesive wets the components to be bonded and develops adhesion with these Substrates. However, a Tecbond glue stick in its liquid form cannot transfer bond strength; consequently the adhesive has little cohesion. However, after cooling the Tecbond hot melt adhesive becomes a solid (mostly Amorphous) with very high cohesion. The built-up cohesion provides the adhesive strength between the substrates.
Tackifying resins give the immediate hot-tack of the glue stick and are mainly responsible for the adhesion. These are mostly low-molecular compounds (compared to base polymers).
Wax thins the adhesive, so it can be applied and waxes can be used to significantly shorten open time and speed up the set time. Anti-oxidants help preserve the adhesive at high temperatures and prevent the adhesive degrading when being applied.
The properties of the bond are not only influenced by the adhesive components used, but also by those of the substrates. Although high performance bonds are now possible on many substrates with Tecbond hot melt adhesives, it should be remembered that final bond strength will vary depending upon the following criteria:
1. Temperature resistance
Hot melts are thermoplastic and consequently become more brittle at low temperatures and softer at higher temperatures. Due to the thermoplastic nature of Tecbond hot melts, the strength of a bond will vary with both the amount and rate of load at different temperatures.
2. Amount of adhesive applied
Bond strength will vary with the amount of adhesive applied. Larger amounts of adhesive cover a larger area and so increase overall strength. Additionally, larger amounts of adhesive conserve heat and consequently the adhesive tends to be at a higher temperature when parts are brought together. This also increases bond strength.
3. Time taken to assemble
Once hot melt is applied, it starts to cool. If the assembly is not completed quickly enough, then the bond strength will suffer as the adhesive will have cooled to a point where a strong bond cannot be achieved. This is known as a cold bond and although initial results look acceptable, the bond can deteriorate over a 24-hour period to an unacceptable level and fail. The maximum open time of a hot melt adhesive is dependent upon the temperature of application, the amount & configuration of the adhesive applied, the type of substrate, its temperature, and the ambient conditions. For maximum bond strength it is important to get the components together as soon as practically possible after the adhesive has been applied.
Compression applied to the substrates while the adhesive is liquid is very important and helps generate a strong bond. In all, there is no one single factor that makes a Tecbond glue stick, stick. The effects of all the various points covered above, all have equal parts to play in creating the perfect bond.
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