Recently, progress in hotmelt adhesives has allowed many product assembly applications to benefit from the efficiencies of hotmelts.
Although performance bonds are now possible on many substrates, it should be noted that bond strength will vary depending upon the following factors:
- Temperature resistance
Hotmelts are thermoplastic and, therefore, become more brittle at low temperatures and softer at higher temperatures. The technical data sheets provide heat resistance and glass transition figures (tg), but it must be recognised that these are laboratory tests and that the results are only a guide. As a result of the thermoplastic nature of hotmelts, the strength of a bond will vary with both the amount and rate of load at different temperatures.
- Amount of adhesive applied
The strength of a bond varies with the amount of adhesive applied. Larger amounts of adhesive cover a greater area and increase overall bond strength. Also, larger amounts of adhesive conserve heat and the adhesive tends to be a higher temperature when parts are brought together and increases bond strength.
- Time taken to assemble
Once the hotmelt adhesive is applied, it starts to cool. If the assembly is not finished fast enough, then the bond strength will reduce 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 early results look acceptable, the bond can deteriorate over a 24-hour period to an unacceptable level.
In general, potential users of hotmelt adhesive for applications that require semi-structural bonds should perform their own tests to satisfy themselves that the adhesive will meet their requirements in the various conditions the finished product may encounter. They should also ensure adequate controls are in place to ensure that the amount of adhesive and time taken for assembly is adequately controlled.
Fospat Industrial will be pleased to make recommendations and offer guidance, but because we cannot control the application, the quantity of adhesive or variations in substrates, it is not possible for us to provide specific performance guarantees. In all cases we would urge the user to conduct their own tests to ensure that the recommended adhesive meets their requirements.