Shrinkage polyester resin

Dicylopentadiene Resins. Dicyclopentadiene (DCPD) can be used as a reactive component in polyester resins in two distinct reactions with maleic anhydride (7). The addition reaction of maleic anhydride in the presence of an equivalent of water produces a dicyclopentadiene acid maleate that can condense with ethylene or diethylene glycol to form low molecular weight, highly reactive resins. These resins, introduced commercially in 1980, have largely displaced OfXv o-phthahc resins in marine apphcations because of beneficial shrinkage properties that reduce surface profile. The inherent low viscosity of these polymers also allows for the use of high levels of fillers, such as alumina tfihydrate, to extend the resin-enhancing, fiame-retardant properties for apphcation in bathtub products (Table 4). 

Poly(methyl methacrylate) and poly(vinyl acetate) precipitate from the resin solution as it cures. This mechanism offsets the contraction in volume as the polyester resin cross-links, resulting in a nonshrinking thermoset. Other polymer additives such as poly(butylene adipate) provide similar shrinkage... 

Polyester resin systems and, more recently, acrylic resins are also used. Polyester resin-based systems have a tendency to shrinkage during and after application and application is very critical. 

Acrylic resin systems developed in Germany are similar to polyester resins but, by careful formulation, the problems due to shrinkage have been largely overcome. The acrylic resin-based systems are currently based on highly flammable materials (flash point 10°C), which can present hazards during laying. However, there are systems available that can take foot traffic 2-3 hours after application and full service conditions within 24 hours, even at very low temperatures. 

Polyester resin floor toppings, similar in performance to the epoxy toppings, have been used but, as indicated earlier, polyester systems tend to shrink and, without careful formulation and laying, shrinkage stresses with polyester resin systems can develop at the interface between the topping and the concrete substrate. Coupled with the additional stresses due to the differences in their coefficients to thermal expansion, this can cause failure at the surface of the concrete substrate." ... 

P.O.36 is also used to color unsaturated polyester resins. Both transparent and opaque samples exhibit a lightfastness in these media that equals step 7 on the Blue Scale. The pigment does not affect the shrinkage of the plastic. 

Low-profile additives, which control shrinkage, have emerged as a distinct science and class of additive. Unsaturated polyester resins, as do all thermosetting polymers, shrink when cured. Low-profile additives are a major class of additives used to control shrinkage, which vastly improves surface quality. This science is credited with the opening of automotive markets where surface quality is of prime importance. In exterior automotive body panels, Class A surfaces are required for market acceptance. 

Low-profile additives are generally materials such as poly (vinyl acetate), polystyrene, polyethylene or polycarbonate. During the unsaturated polyester cure cycle, the low-profile additives separate into a second phase, which expand to counteract the shrinkage of the curing unsaturated polyester resin. Material development and the science of low-profile additives have helped create substantial markets for unsaturated polyesters. Their use in automotive markets, where Class A show room quality surfaces is a requirement, is an example of this. 

Bucknall, C.B.. Partridge, I.K. and Phillips, M.J. (1991). Mechanics of shrinkage control in polyester resins containing low-profile additives. Polymer 32. 636-640. 

Examples used in practice are polyester and epoxy casting resins, both as two-component systems. The epoxies are more expensive than the polyester resins, but they show less shrinkage the shrinkage, expressed as the volume difference before and after curing, amounts to 1 to 6 % for epoxies, and to 5 to 10 % for polyesters. 

Alkyds. Alkyd resins consist of a combination of unsaturated polyester resins, a monomer, and fillers. Alkyd compounds generally contain glass fiber filler, but they may also include clay, calcium carbonate, alumina, and other fillers. Alkyds have good heat, chemical, and water resistance, and they have good arc resistance and electrical properties. Alkyds are easy to mold and economical to use. Postmolding shrinkage is small. Then-greatest limitation is extremes of temperature (above 175°C) and humidity. 

In spite of the fact that the rheological behavior during c ire of the Ashlemd polyester resin looks very similar to that of the Dow vinyl ester resin, the Ashlcuid polyester resin is found to be more reactive them the Dow vinyl ester resin, with the same formulation emd initiator system used in this study. Some iii Kjrtemt differences in the rheological responses are reflected on (1) the values of (2) the time at which shrinkage begins to occur when the fluid is at rest emd (3) the slope of the -( ii- 22 versus cure time curve. 

Polyvinylacetate (PVAc) and vinyl acetate-acryUc copolymers (VAc-A), thermoplastic polyurethanes, polyethylene, polystyrene and polycap-rolactone are some of the candidates for low-profile shrinkage additives to SMC and BMC. PVAc and VAc copolymers are the most widely used thermoplastic additives. Typically a low-profile SMC recipe contains about 15% unsaturated polyester resin, 8% thermoplastic additive, 50% calcium carbonate and 27% glass fiber. 

More investigations are necessary to formulate flexible polyester resins that produce a nontacky surface after air irradiation. Flexibility is important in order to reduce the radial and tangential shrinkages. No air inhibition is important in obtaining a good surface consolidation. 

Fig. A shows the effects of CoOc and MEKPO contents on setting shrinkage of polyester resin concrete. The setting shrinkage increases with rising contents of CoOc and MEKPO. In particular, the shrinkage is remarkably affected by MEKPO (catalyst) content rather than CoOc (accelerator) content.

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