Increasing concern about volatile organic compound emissions, embodied in building standards like LEED and Greenguard, are encouraging the uptake of high-solids adhesives. The solids content of an adhesive formulation includes the proportion of base polymer, inorganic filler and non-volatile additives. Non-solids parts are made up of solvent or water, and must generally be eliminated before the adhesive cures. This requires energy, and the solvent can often pose a threat to health, which is why minimizing these parts of the formulation is desirable. As well as the performance of the bonded assemblies, the amount of solids in an adhesive system strongly influences the behaviour of the adhesive during application. As a consequence of the low energy needed for curing, high solids content will produce rapidly-curing adhesives. High solids adhesives normally have high viscosities, which reduces both their spreading and penetration into their substrates if too high.
Most elastomers used in high-solids adhesives are produced by emulsion polymerisation, where the polymer solids content by volume typically ranges from 40 to 55 percent. Here higher polymer contents are desirable as they increase the output of the reactor, and makes product transport more efficient. Latices produced by emulsion polymerisation have lower viscosities at a given solid content, making them easier to handle than polymers produced by other processes. The polymer solid content at which the viscosity becomes significant depends to a very large extent on the particle size distribution, generally remaining fairly low for solid volume contents up to 55 percent. Above this limit the viscosity can increase extremely rapidly. Fillers are widely used to improve physical properties of polymers, increasing modulus and hardness, and reducing creep and distortion at elevated temperature, although the increased modulus in turn reduces tack. Adding filler also has the advantage in many cases of reducing the cost of the final formulation. Consequently filler content in high-solids adhesives can vary from almost none to the dominant proportion depending upon the specification of its end use. It also provides a means by which to raise solids contents even above the heights achieved by emulsion polymerisation. In many conventional formulations, filler is added post-polymerisation. As adhesive usage changes shifting specifications may demand that filler loading increases beyond levels at which it can be homogeneously incorporated into a particular backbone polymer. Consequently conversion to other polymers that can take higher loadings may then be one option. One example of this was in carpet-backing adhesives during the late 1980s. Filler loadings increased to a level that ethylene vinyl acetate copolymer latex adhesives, which had been in use since the 1970s, were unable to support without separation. However the styrene-butadiene rubber based adhesives that replaced them initially suffered from an unpleasant ammonia smell.
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