Limitations of Conventional Hot Melt Adhesives and Hot Melt Pressure Sensitive Adhesives

Hot-melt adhesives are useful and versatile, but they don't resist heat or plasticizers. September 2, 2006

This article was provided by Pierce Covert of Glue Machinery Corporation.

Plus: Options to conventional hot melt adhesives

Conventional hot melt adhesive (HMA) and hot melt pressure sensitive adhesive (HMPSA) are 100% solid thermoplastic materials. They do not contain any volatile organic compounds (V. O. C.). Therefore, they are safe during production, transportation, storage, and application. Most importantly, they are environmentally friendly and are not harmful to human health. Because of the fast set feature, HMA is the best candidate among all types of adhesive for high speed production lines. They have been widely used in various industries, such as packaging, bookbinding, wood-working, D.I.Y., tapes and labels, hygiene, automobile, medical, electronic, construction, etc.

Although conventional HMA and HMPSA have several advantages, they can not replace cross-linkable solvent and water borne adhesives in some application markets. HMA and HMPSA have two major shortcomings. First, they are thermoplastic compounds, which melt-flow or creep when the surrounded temperature is close to their flow point, typically less than 200. Therefore, they are not suitable for high temperature applications. Secondly, almost all HMA and HMPSA formulations contain portions of low molecular weight tackifying resins, which are organic solvent and/or plasticizer soluble ingredients. A typical case is the bonding of HMA or HMPSA onto Poly-Vinyl-Chloride (PVC). Almost all HMAs and HMPSAs fail to bond onto heavily plasticized PVC objects. PVC alone is a rigid material, which is very easy to bond by any adhesive due to its high surface energy. However, when a HMA or a HMPSA is used to bond onto plasticized PVCs, it will be gradually softened by the plasticizer-phthalates. The adhesive composition and adhesion performance are therefore gradually changed the passing of time.

How to improve the heat and plasticizer resistances of HMA and HMPSA? Instead of constantly improving the heat and plasticizer resistances for those conventional HMAs and HMPSAs, many new technologies have been introduced to the adhesive industry in the recent years. Two major successful technologies are radiation (e.g. some Kraton elastomers are UV and/or E-beam curable) and moisture (e.g. reactive Polyurethane, PUR) curable HMAs. Once adhesives are cured or cross-linked, their molecular weights become infinite. They will therefore offer high temperature and plasticizer resistance.

This article was provided by Pierce Covert of Glue Machinery Corporation.