Reactive Diluents or Monomers

The neat polyester is highly viscons (2-80 Pa-s) and very difficult to process. Addition of monomer rednces the viscosity abont 10-times (0.2-0.5 Pa-s) and makes the resin easy to process. Polyester resin offers better processability compared with other thermoset resins (e.g., epoxy). In general, the mole ratio of polyester to monomer is maintained in the range 1.9 to 2.4. Further increase in styrene content reduces 

The main problem of styrene is high emission, which restricts applications in closed compartments (e.g., naval application). Styrene emission is a health hazard (threshold value of 50 ppm). The need to rednce the volatile organic compound (VOC) is not only expressed in government regulations, but also equally and persuasively by various environmental concerns. The evaporation of styrene can be reduced by the addition of waxes or pyrogenic silicic acid with hydrophilic and hydrophobic end groups. However, such additives may affect the interfacial properties of composites made out of polyester resins. Some styrene-less UPE resin formulations using diacrylate monomers have been reported [71]. However, mostly styrene is used as a reactive diluent in UPE resins. 

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