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In order to obtain adequate strength and heat resistance from a nonreactive formulation, hot melt adhesives generally require some component to separate out into a dispersed but interconnected hard phase network on cooling. For adhesives based on styrene block copolymers, the hard phase consists of glassy styrene domains. For adhesive based on ethylene copolymers, waxes, or olefinic copolymers, the hard phase consists of organic crystallites (Figure 1).
Figure 1: Crystalline and amorphous regions in a polymer structure
Figure 2 illustrates the mechanical spectrum of amorphous and crystalline polymers. The glassy region is indicative of a brittle polymer; in the flow region the polymer is in its melt form and lacks strength. The rubbery region is indicative of high strength and viscoelastic deformation to absorb stress and impact.
Figure 2: Mechanical spectrum of two hot melt polymers1
Because hot melt adhesives are applied in melt form and achieve their bond strength on resolidification on cooling, there are two important physical properties: glass transition temperature and melt temperature. These will vary significantly depending on the type of base polymer used and the additives or modifiers present in the formulation. Generally, most hot melt adhesives and sealants have a glass transition temperature, Tg, below room temperature. The melt temperature, Tm, should be low enough to conveniently provide for application but not so high as to result in a safety or fire issue. Most hot melt adhesives have a Tm so that typical application temperatures are 150-200°C, and they become fully solid at temperatures below 80°C. The viscosity of the hot melt at elevated temperature is also an important criterion. The hot melt must be capable of flow to be applied and wet the substrate, but it must not have such a low viscosity so as to flow out of the processing equipment or joint. The viscosity in the melt form will also determine how the product can be applied (Figure 3).
Figure 3: Viscosity of molten hot-melt adhesives as a function of temperature
Viscosity of hot melt adhesives or sealants is generally not measured in centipoises. The viscosity at elevated temperature is usually present as a melt flow index (MFI), which is the weight of polymer that can be extruded from a nozzle at a given temperature, pressure, and time. Polymers with a high MFI have low viscosity at elevated temperatures. The melt flow index test method is specified in ASTM D1238. The melt viscosity of most polymers is an exponential function of the molecular weight, but the relationship is not simple. The hot melt system must achieve a relatively low viscosity when in the molten state to achieve wetting, but it must not cool too rapidly or it will not have time to completely wet the molecular roughness of the substrate. Fast cooling can be the result of applying the hot melt to a substrate with high thermal conductivity (e.g., metals) or polymers, which crystallize rapidly. The problem of achieving high bond strength with metals can be resolved by preheating the metal prior to applying the adhesive. This will provide a longer time for the adhesive to be in the melt form and wet the substrate. The problem of rapid crystallization with certain polymers must be balanced against the higher cohesive strength that usually results from these systems.