2 сентября, 2015 
Malyar For a given polyester formulation, the properties of the final compound are a function of its condensation (e. g., carboxyl and hydroxyl group concentration), viscosity and molecular weight distribution, and the structural features of the three-dimensional network obtained after free-radical copolymerization. An increase in the molecular weight of an unsaturated polyester improves its hardness, tensile and flexural strength, and its heat distortion temperature (HDT) until a plateau value is reached. Carboxyl end groups impart higher viscosities and better physical properties to polyesters than do hydroxyl end groups.
Generally, both the physical and chemical properties of a polyester are affected by the ratio and type of the acid and diol components and of the copolymerizable monomer. To this effect higher proportions of maleic anhydride lead to a higher density of crosslinking and thus greater hardness and heat resistance of the cured resin. Conversely, phthalic anhydride is the most common reagent used to decrease the density of crosslinking, increasing the flexural strength. Equally, a variety of glycol can be used to obtain different resin properties. Propylene and ethylene glycols, diethylene glycol, and neopentyl glycol are commonly used.
Vinyl monomers are usually added to the polyester resin as solvents of the unsaturated polyester; this is to decrease viscosity within manageable limits, as well as to function as cross-linking reagents. Styrene is the vinyl monomer most commonly used. Thus the degree of cross-linking can be controlled not only by modifying the concentration of unsaturated acid residues in the resin backbone, but also by changing the proportion of vinyl monomer added to the resin. The length of the cross-links can be controlled to a certain extent by modifying the concentration and type of vinyl monomer used.
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