Humidity and Hot Melt Pressure Sensitive Adhesives

According to the PSTC standard test condition, appendix A, standard conditions are a temperature of 23 ± 1 degree C and a relative humidity (RH) of 50 ± 2% at that temperature, allowing for short term fluctuation of 23 ± 2 degrees C and RH as 50 ± 5%.

The above standard test condition is rather difficult to achieve in some hot and humid counties, such as those countries in southern Asia and China. As a result, pressure sensitive adhesion test is commonly carried out at 25 ± 2 degrees C and RH of 65 ± 5%.

In many of the previous articles, the effect of test temperature on adhesion performance has been widely discussed. The rheological property, such as G’ and Tan delta, of HMPSA (Hot Melt Pressure Sensitive Adhesive) is affected by test temperatures and adhesion performance varies as temperature fluctuates.

Humidity is also a factor that may affect pressure sensitive adhesion performance. This is especially apparent for water based PSAs containing moisture sensitive surfactant. However, humidity is not a concern for those hydrophobic SBC (Styrenic Block Copolymer)-based HMPSAs. Many adhesion performance variations found in HMPSAs have resulted from different humidity environments. Most of those findings actually resulted from those moisture sensitive substrates instead of the HMPSA itself. Some moisture absorbable materials, such as paper, polyamides, polyesters, polyurethanes, etc., may alter their surface energies when they are exposed at different percentages of humidity and therefore result in different bonding strengths of a fixed HMPSA. For those pretty humid surfaces, very often, HMPSA is not able to intimately contact the surface of substrates. An invisible micro layer of water film may exist in between HMPSA and bonded substrates that will greatly deteriorate bonding performance.

In summary, most SBC-based HMPSAs are not moisture sensitive hydrophobic materials and will not change their own properties under different humidity environment. In practice, for tapes and label applications, HMPSAs can not form satisfactory bonding on humid backing materials during coating process. Similarly, HMPSA tapes and labels can not create appreciable bonding on humid substrates. Both situations noted above will cause performance variations under different humidity conditions.


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