A Deep Understanding of Hot Melt Adhesive
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?
In fact, hot melt adhesive is a 100% solid adhesive which is applied in a molten state to achieve flow and wetting. Hot melt adhesive relies on cooling to a solid to give a serviceable bond. Hot melt adhesives generally remain as thermoplastics after application.
A hot melt pressure sensitive adhesive is a hot melt adhesive 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. Hot melt pressure sensitive adhesives are commonly used to manufacture pressure sensitive tapes and labels.
Hot melt adhesive can be categorized into two major families: non-formulated and formulated hot melt adhesives. Non formulated hot melt adhesives are intentionally synthesized as serviceable adhesives without further modifications by other materials such as tackifiers. On the other hand, typical non-formulated hot melt adhesives are polyesters (PET), polyamides (PA), polyurethanes (PU), and polyolefins. 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 hot melt adhesives are composed of thermoplastic elastomers, tackifiers and other ingredients. Unlike those non-formulated hot melt adhesives, 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 polyolefins (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 Hot Melt Adhesive 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. While hot melt pressure sensitive adhesives are primarily based on SBCs. They are permanently tacky at room temperature and offer good bonding strength under a light finger pressure. APO-based hot melt adhesives 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.
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