Hot Melt Adhesives
This is just a short note about how hot melt adhesives (HMA) fit into the world of PSA. The use of the term "HMA" can be very confusing. Hopefully this short pages clears up some of that confusion.
What is a section on HMA (Hot Melt Adhesives) doing here? The answer is that the majority of HMA are PSA that happen to be delivered via an extrusion process. This doesn't necessarily have to be the case. The polymers in the hot formulation have some chance to intermingle with those in the substrate and therefore boost the "true" adhesion from the typical 40mJ/m2 of more typical PSA. Undoubtedly there are some HMA which are "real" adhesives so the worries of G' and G'' don't apply. But the working assumption of many practitioners is that HMA are PSA, with, perhaps, a little more extra "true" adhesion giving a larger margin of safety for those formulations that require stronger PSA-style performance over a wider range of speeds/temperatures.
Same rules apply
Because HMA are extruded (in a real extruder/coater or in a hot melt gun) a lot of thinking goes in to the melt viscosities and thermal stabilities of the formulations. This is necessary, but can remove attention from the G' and G'' considerations of a good PSA. Or to put it another way, an otherwise marvellous tackifier for the PSA system might be ruled out (wisely or unwisely) because the melt properties are seen as being unsuitable. If a company has always been comfortable with an MFI (Melt Flow Index) of 5 at 190°C then it takes a lot of courage to go with a MFI of 50 which might entail running the extruder at an unusually low temperature.
So the formulator has a doubly difficult job: getting the PSA right (which we know to be hard) and getting the extrusion parameters right (also hard).
But what is the alternative? Usually it's a lot of trial and error which, if it involves a trial on a production line, can cost far more in time and money than getting all the required measurements (G', G'', Tg etc.) in the lab.