Three major pressure sensitive adhesion performances are determined and reported for all hot melt pressure sensitive adhesives (HMPSAs). They are peel (typically 180°-peel), tack, and shear (or holding power). Sometimes, the heat resistance, Shear Adhesion Fail Temperature (SAFT), and cold resistance are also requested for specific applications. These standard test methods are available form ASTM, PSTC, TLMI, FINAT, and AFREA, etc. Although most lab technicians may follow test procedures according to those test methods, test results vary from time to time, person to person, and lab to lab.
Below are some typical factors affecting test results:
- The face stock used
Face stocks may absorb low molecular weight ingredients from adhesive, or release low molecular weight components to adhesive. As a result, the formulation of Hot Melt Pressure Sensitive Adhesive is changed after contacting the face stock. Adhesion performance is also altered.
- Face stock thickness
The thickness of face stock will cause different radius of curvature upon peeling a specimen away from a test panel. This geometric variation will affect test results. The energy required to peel face stocks with different thicknesses is also varied.
- The release liner used
Non-cured silicone residues may migrate from liner to adhesive surface, which is one of the major cause lowering PSA performances.
- Adhesive thickness
Adhesion performances are affected by the thickness of tested adhesive. Normally, the thicker the adhesive film, the higher the peel force and tack, but the lower the shear force is.
- Uneven coating surface
A poor control of adhesive coating thickness will cause test results to vary.
- Sample size
Test specimen must be cut precisely into specific size for each individual testing. Otherwise, the test results will not be consistent.
- Panel cleaning
Cleaning the test panel is very tedious but critical to obtain reproducible test results. Besides silicone migration, this is another major cause affecting adhesion performances.
- Lamination of test specimen and test panel
A poor lamination of test specimen and test panel may create some bubbles or cavities in between. These defects will lower the test performances.
- Environmental condition: temperature and humidity
It is difficult to maintain a constant temperature year round. If temperature in a room is unstable, the test result will be varied. Most Hot Melt Pressure Sensitive Adhesives are not sensitive to humidity.
There are other factors that may affect test results of Hot Melt Pressure Sensitive Adhesives in the lab. It is not feasible and realistic to request perfect reproducible data for the same Hot Melt Pressure Sensitive Adhesives all the time. A standard deviation of +/- 15% for those test results should be accepted.