The Viscosity of Hot Melt Adhesive
Most hot melt adhesive users believe that the value of “viscosity” is an important parameter to various application techniques. The truth is that viscosity in hot melt manufacturing is simply an indicator to determine the consistency between produced batches or lots. In other words, viscosity is simply useful for quality assurance and quality control. Viscosity variances are the easiest way to determine that your adhesives are not the same product.
The definition of viscosity is: “A measure of the ability of a liquid to resist flow”. The unit of viscosity is in poise, centipoises, or mPas. It is also defined as the ratio of shear stress and strain rate. The viscosity of water is 1 cps.
Most hot melt adhesives’ viscosity is determined with a rotational shearing viscometer accompanied with a temperature controlled heating unit. The viscosity of hot melt adhesives is greatly affected by temperature and shear rate. Generally, viscosity is lowered at higher temperatures and rates. During evaluation, the viscosity of hot melt adhesive is gradually reduced with elevated temperature. This is because the polymeric molecular chains, starting with random coils, are gradually disentangled and orientated upon rotational shearing. When most polymeric molecular chains are stretched out and orientated, the viscosity values are leveled off and recorded as the viscosity of hot melt adhesive.
In real world applications, none of the hot melt adhesives are rotationally pre-sheared as those determined in the lab, so viscosity actually has minimal effect on the application.
The ease of application through your hot melt machinery is actually affected by the viscoelasticity of your adhesive instead of simple viscosity. An adhesive with more elasticity will be relatively less deformable and harder to extrude, swirl, spray or coat than products with little or no elasticity. Therefore, to determine the correct application techniques, you need to determine the viscoelasticity of the hot melt adhesive in advance. This is typically done with a rheometer and is an involved test.
For wide range applications of hot melt adhesive, we recommend that you ask your hot melt producer to ensure that the viscosity is consistent from batch to batch. Furthermore, to make sure that your adhesive quality is consistent, you may want to purchase a viscometer and perform simple viscosity tests yourself.
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