To determine the correct application techniques – determine the viscoelasticity of hot melt adhesive. 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. The ease of application through your hot melt machinery is actually affected by the viscoelasticity of your adhesive instead of simple viscosity. In real world applications none of the hot melt adhesives are pre-sheared rotationally as those determined in the lab – so viscosity actually has minimal effect on the application. 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. This is because the polymeric molecular chains, starting with random coils, are gradually disentangled and orientated upon rotational shearing. During evaluation, the viscosity of hot melt adhesive is gradually reduced with elevated temperature. Viscosity is lowered at higher temperatures and rates. The viscosity of hot melt adhesives is greatly affected by temperature and shear rate. Most hot melt adhesive viscosities are determined with a rotational shearing viscometer accompanied with a temperature controlled heating unit.

It is also defined as the ratio of shear stress and strain rate. The unit of viscosity is in poise, centipoises, or mPas. The definition of viscosity is: “A measure of the ability of a liquid to resist flow”. Viscosity variances are the easiest way to determine that your adhesive is not the same product. In other words, viscosity is simply useful for quality assurance (QA) and quality control (QC). The truth is that viscosity in hot melt manufacturing is simply an indicator to determine the consistency between produced batches or lots. Most hot melt adhesive users believe that the value of “viscosity” is an important parameter to various application techniques.

Test viscosity