Most hot melt users anticipate achieving a high heat resistance with a “high softening point” hot melt adhesive. Softening point, like the viscosity measurement, is simply an indicator used to evaluate if adhesives are consistent between batches or lots.
- Softening point is not heat resistance and customers must carefully evaluate their adhesive where temperature resistance is required.
- Softening point is the temperature at which a material softens sufficiently to allow significant flow under a small stress. Most hot melt adhesives are measured by a Ring and Ball apparatus according to the ASTM D-2398 Test Method. Softening point is heating rate dependent. If the heating unit cannot provide a consistent heating rate, the determined softening point may be varied. The faster the heating rate – the higher the softening point observed.
For example, any APO-based hot melt adhesives may present very high softening points and yet they actually do not resist flowing in the real world applications. This is because most APO-based adhesives do not have sufficient cohesion to resist both inter and intra-molecular flow upon heating.
To evaluate the heat resistance, one must perform the following tests: shear adhesion fail temperature (SAFT), peel adhesion fail temperature (PAFT), or holding power (shear) at a fixed high temperature.
For more information call or email Pierce Covert,
Glue Machinery Corporation