Hot Melt Adhesive - High and Low Temperature Resistance
Many hot melt adhesive producers and users are under the impression that hot melt adhesive with high temperature and low temperature resistances are not possible. They consider these two properties conflicting and difficult to produce. In reality, this perception is completely incorrect and without any scientific support.
Most thermoplastic elastomers used for making hot melt adhesives possess both rubbery and plastic features. These polymeric materials normally exhibit two significant transition zones: a glass transition zone from glassy state to rubbery plateau for the rubbery domain, and a softening zone from rubbery plateau to fluid (or melt flow) state for the plastic domain. These two transition zones are independent of each other and are tailored by the polymer’s molecular structure, such as molecular weight, molecular weight distribution, volume fraction of rubber-plastic components, and other factors. Moreover, the blend of thermoplastic elastomers with various tackifying resins can also greatly affect the status of glass transition and softening zones.
The high temperature and low temperature resistances of hot melt adhesives are relevant to the status of both transition zones. For a styrene-isoprene-styrene (SIS) block copolymer-based hot melt pressure sensitive adhesive, the high temperature resistance can be correlated with the flow point, where the styrene (plastic) predominate phase is softened from a rubbery state to a fluid state. And the low temperature resistance, independent of the styrene predominate phase, is related to the glass transition temperature of isoprene (rubber) predominate phase.
In real world applications, not every hot melt adhesive manufacturers requires both high temperature and low temperature resistances. Most hot melt adhesives are developed or designed as general purposed products with specific adhesion performances at room temperature. The heat resistance of hot melt adhesives can only be improved to certain extents by introducing some specific reinforcement materials such as an aromatic hydrocarbon resin.
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