Pressure sensitive adhesion performances (for tape and label applications) often change during long-term storage after hot melt pressure sensitive adhesives (HMPSA) are coated on a specific face stock such as paper, film, non-woven, or cloth. Most formulators and users believe that the change of performance is primarily caused by the aging of adhesive during manufacturing, processing, and/or storage. Besides the aging effect, in fact, one of the major causes results from the mating of HMPSA and face stock that is used.
Most Styrenic Block Copolymer (SBC) based HMPSAs contain a certain percentage of low molecular weight tackifiers and mineral oils. These low molecular weight species are homogeneously dispersed within the SBC matrix before coating. Once HMPSAs are coated onto certain face stocks, some molecular motions of those low molecular weight species will occur. These low molecular weight species within both adhesive and face stock can penetrate into each other. That is, low molecular weight tackifiers and oils may cross the interface and penetrate into the matrix of face stock. Simultaneously, if any low molecular weight materials exist in the face stock, they can also cross the boundary and diffuse into adhesive. The degree of migration or penetration is greatly dependent upon the molecular weight and polarity of those low molecular weight species. It is also dependent upon the porosity of the face stock used. More migration may occur if molecular weight is lower; the compatibility of those mobile ingredients is higher; and the porosity of face stock is greater. Once migration occurs – the formulation of original HMPSA is changed and adhesion performance is also affected.
The degree of migration can be easily pre-determined by the variation of the adhesive’s rheology. When a test specimen is purposely heat aged at an elevated temperature for a period of time, e.g. 80℃ for 24 hours, the rheology of adhesive may be changed if migration happened. Generally, when the mineral oil of a HMPSA migrates to face stock, the glass transition temperature (Tg) of adhesive is raised. This is because that the percentage of oil (low Tg ingredient) is reduced in the formulation. On the other hand, when a low molecular weight material is extracted out from face stock and becomes a part of HMPSA, the Tg of adhesive is also altered depending upon the Tg of the extracted materials. Besides the variation of Tg, other rheological properties are also affected by the migration of low molecular weight species.
Adhesive formulators may make efforts to improve the aging resistance of HMPSA; unfortunately, those efforts will never resolve the problem resulting from migration between adhesive and face stock. To completely avoid this undesired phenomenon: apply an inert priming material on the coating side of face stock.
For more information call or email Pierce Covert,
Glue Machinery Corporation