Hot Melt Pressure Sensitive Adhesives (HMPSAs) Used for Labels

A tremendous amount of HMPSAs are used for making self stick labels. Although both solvent and water borne PSAs are also used in these application markets, HMPSAs offer the following advantages.

1. Environmental friendly and healthy – HMPSA is a PSA with 100% solids. It does not contain any harmful volatile organic compounds (VOC). Although most water-based PSAs do not contain any VOC, they include about 40% of water. Extra efforts are required to clean containers, coating equipment, and waste water treatment.
2. Fast coating – Since HMPSA has 100% solids, no drying oven is required for the adhesive coating line. HMPSA can cool and solidify immediately after coating. Consequently, it can be coated at very high speed up to 1500 feet per minute or even higher on a very short coating line.
3. High tack performance – Most general purpose HMPSAs impart relatively high tack versus their solvent and water based counterparts. This unique feature results from the narrower molecular weight distribution of the mid block, e.g. the Isoprene of SIS (Styrene-Isoprene-Styrene Block Copolymer). High tack performance is the most important adhesion performance for self stick labels.
4. Low shipping and storage cost – Unlike typical solvent and water based PSAs which contain about 60% solvent and 40% water respectively. The packaging size of HMPSA is relatively small. Therefore, the cost of shipping and storage is cheaper than its counterparts.
5. Safety – HMPSA does not have any VOC. Besides environmental concern, it is not flammable under any rigorous storage environment.
6. Versatile – HMPSAs are formulated adhesives. The formulation can be easily adjusted and modified to achieve very diverse adhesion performance.

The major weakness of conventional HMPSAs is its relatively low heat resistance and plasticizer resistance. To significantly improve the above weakness, radiation curable HMPSA is an ideal alternative. However, the cost of both UV and E-beam curable HMPSA is much higher than those conventional HMPSAs.

Beside processibility, following are commonly requested performances.

1. A balance of peel, tack and holding power
   There is no need to offer all excellent adhesion performances for general purpose labels. Tack is the most important performance that allows labels to grasp on various substrates easily. The peel force should be high enough to tear the used face stock. Holding power or shear force is not critical for most label applications.
2. Easy converting
   HMPSA is more elastic and tackier than solvent and water based PSAs. Consequently, they are more difficult to die-cut, strip, and guillotine. In addition to modifing adhesive formulations, converting processes sometimes require a certain degree of adjustment. Moreover, in order to improve converting performance significantly, it is recommended that each applied HMPSA should match with a release paper having appropriate release force.
3. Specific performances
   Besides room temperature peel, tack, and hold, HMPSAs sometimes require specific performances, such as high and low temperature resistance; bonding to low surface energy substrates, and etc. How to achieve these specific adhesion performances are discussed on separate articles.