The performance requirements for HMPSAs are more complicated than those for EVA and APO-based HMAs. In addition to basic physical properties, such as viscosity and softening point, some pressure sensitive “adhesion performances” should be evaluated. All equipment may honestly reveal certain values determined under different geometries and conditions. No equipment can actually guide formulators or adhesion scientists in the appropriate direction to manipulate the formulation. Trial and error remains the most common way to deal with formulation if those determined values do not meet desired target.
Following are basic equipment and instruments required for a fully functional HMPSA laboratory.
- Viscometer and Thermosel – to determine melt viscosity at various temperatures.
- Ring and Ball Softening Point Apparatus including a heating unit – to evaluate softening point.
Sample Specimen Preparation:
- Hot melt lab coater and laminator – to make testing specimens.
- Specimen roller – to laminate test specimens on test panels.
- Standard test panels – to use as reference substrates for various adhesion tests.
- Tack tester- to determine the initial stickiness without additional pressure.
- Peel tester- to evaluate separation forces under various conditions: dwell time, angle, rate, temperature, thickness, etc.
- Holding power tester- to find the failure time of a specimen upon shearing geometry with different loads.
- SAFT (Shear Adhesion Fail Temperature) microprocessor controlled oven- to determine the failure temperature of a specimen under a constant heating rate, typically at 2-3 degrees/min.
- Force convection oven- to perform high temperature holding power test and sample aging.
- Rheometer- to evaluate the viscoelasticity of HMPSAs under various scanning conditions: strain, time, frequency, temperature, etc.
- Hot melt lab mixer- to make small amounts of lab testing samples.
- Hot melt pilot mixer (optional but recommended) – to produce experimental samples for customerfs evaluation.
Since the 1980fs, rheology has become an extremely powerful formulating tool, which can precisely determine and reproduce the viscoelasticity of HMPSAs. Moreover, formulators can also apply those determined values to correlate many physical and adhesion properties. If rheological windows or targeting viscoelastic values for certain HMPSAs are able to be precisely preset, then, a rheometer becomes a very powerful tool that can guide formulators to modify formulations and move the viscoelasticity into the targeting windows. To evaluate viscoelasticity of HMPSAs, a rheometer and mechanical dynamic analyzer are important pieces of equipment.
Another methodology is based on a more scientifically statistical technology, named “DOX” (Design of Experiment). Formulators can utilize this methodology to zoom in their targeting formulation in order to locate the optimum composition which may impart desired adhesion performances. Without knowing the complicated characteristics of polymeric materials, inter/intra molecular compatibility and networking, the mechanism of adhesion, and fracture mechanics, the DOX is simply another more scientific trail and error process. It is an extremely time consuming method of searching for the optimum composition of desired HMPSAs.