Hot melt adhesives formulating

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.