Hot melt adhesives (HMAs) and hot melt pressure sensitive adhesives (HMPSAs) have been widely used in manufacturing for over 40 years. Almost every industry including packaging, bookbinding, woodworking, hygiene, construction, automotive, electronics, shoemaking, textile lamination, product assembly, tapes and labels use hot melt adhesives extensively. What are these materials?
HMA is a 100% solid adhesive which is applied in a molten state to achieve flow and wetting. HMA relies on cooling to a solid to give a serviceable bond. HMAs generally remain as thermoplastics after application.
A HMPSA pressure sensitive hot melt is a HMA which retains the ability to form a serviceable bond under light pressure at room temperature. Pressure sensitive adhesives are very tacky and have unlimited open time – meaning that they can bond to another substrate at most any time. HMPSAs are commonly used to manufacture pressure sensitive tapes and labels.
HMA can be categorized into two major families: non-formulated and formulated HMAs. Non formulated HMAs are intentionally synthesized as serviceable adhesives without further modifications by other materials such as tackifiers. Typical non-formulated HMAs are Poly-Esters (PET), Poly-Amides (PA), Poly-Urethanes (PU), and Poly-Olefins. They offer appreciable “hot tack” or the ability of hot melt adhesives to hold substrates together prior to solidification or set or bonding strength while they are heated and bonded at elevated temperatures.
Formulated HMAs are composed of thermoplastic elastomers, tackifiers and other ingredients. Unlike those non-formulated HMAs, these basic thermoplastic elastomers alone are not tacky at either room temperature or elevated temperatures. Three commonly used basic thermoplastic elastomers are Styrenic Block Copolymers (SBCs), Ethylene Vinyl-Acetates (EVAs), and Amorphous Poly-Olefins (APOs). These thermoplastic elastomers are modified by various types of tackifiers (natural and synthetic resins) which generate diverse adhesion performances according to specific market needs.
Most HMAs are normally based on EVAs. These products exhibit relatively short open time (typically less than 10 seconds) and fast set speed. Only very minimal tack is detectable on the adhesive surface at room temperature. HMPSAs are primarily based on SBCs. They are permanently tacky at room temperature and offer good bonding strength under a light finger pressure. APO-based HMAs offer very long open time after they are applied and cooled from their molten stage. However; they are not permanently open and will lose most of the surface tack once they are completely set. This unique characteristic is very useful for those bonding processes requiring long open time but low surface tack after bonding. The low residual surface tack will avoid future contamination at the edge of those bonding areas.
What is the perfect HMA and/or HMPSA? In reality, there is no such perfect product. All adhesives must be designed or formulated according to actual needs. How do we select an appropriate HMA or HMPSA for actual needs? Before one can select an optimum product for a specific application, both the end use adhesion performances and application techniques must be clearly defined. More discussions for each individual market’s requirements will be addressed in future articles.