It may sound odd to discuss the topic of “How to make a consistent hot melt pressure sensitive adhesive” if one has a good formulation in hand. Although products may look the same – they are not identical in size, shape, and weight. Molecular weight (MW), molecular weight distribution (MWD), branching, networking, etc. of each polymerized batch are not always the same. Although HMPSA production does not involve any chemical reaction during mixing, it is simply a blending of several different but compatible ingredients.
Typical HMPSAs are composed of a SBC (Styrenic Block Copolymer), tackifiers, mineral oil, and antioxidants. If formulators and producers carefully read the COA (Certificate of Analysis) of each received ingredient, they may be surprised at the significant deviations between batches or lots received. For example, the specification of MFR (Melt Flow Rate) at 190C for a typical SIS (Styrene-Isoprene-Styrene block copolymer) is 8.0. But the range of this specific prime grade SIS may be indicated as 8 +/- 3, or 5-11. What does this mean? If adhesive manufacturers happen to receive two lots of SIS having different FMR at 5 and 11, they may produce two lots of finished products showing viscosity deviations at +/- 50%. Are these products acceptable by manufacturers and users? Certainly, the finished HMPSAs are far out of the spec but may work. Can the adhesive manufacturers make a claim to their supplier? No, because they are within the spec. So, how to make HMPSAs having a very consistent lot-to-lot deviation becomes a very tough job.
Below are two recommended approaches to make consistent HMPSAs.
- Always purchase raw materials with very tight specifications. Very often, they are more expensive. For instance, an adhesive manufacturer can request a SIS supplier to only provide a deviation of FMR less than 0.5. Then, the finished products should result in smaller deviation between lots.
- If the raw material supplier is not capable of offering very tight spec at all times due to their limited synthesis technology then adhesive manufacturers should have their own capabilities of adjusting formulations without sacrificing customer requested performances. For example, if the FMR of the incoming SIS is higher, i.e. the molecular weight is lower, than the typical spec, formulators may increase a small amount of SIS and reduce a small amount of oil to bring the cohesion, viscosity, etc. back to the target range. On the contrary, when the FMR is lower than typical spec, formulators may decrease a small amount of SIS and add some more oil. Certainly, besides the above simple guidelines, there are so many ways to modify formulations in order to bring the final performances back to targeted range without sacrificing the quality of the final product. This is certainly a very tough job but a must skill for all HMPSA manufacturers.
A reliable and technically skilled HMPSA manufacturer must modify or alter their formulations from lot-to-lot according to their incoming raw materials.
For more information call or email Pierce Covert,
Glue Machinery Corporation