Low-profile polyesters

The addition of SMC and BMC to reinforced polyesters has changed the basic mechanical property picture very little. The same is true for low profile polyesters. The mechanical performance is at the same general level that is attained with the older wet systems, such as preform and mat, and premix molding. 

The development of low profile polyester resin adds an additional dimension to SMC and BMC. The post-molding finishing steps with FRP have been a strong deterrent in high appearance requirement applications. The low profile resins utilize thermoplastic additives to obtain... 

E A Davies. Non Shrink or low Profile Polyester Moulding Compounds for the Automotive Industry. 7th International Reinforced Plastics Conference, Brighton, UK 1970. 

The tendency of thick sections of DMC structural parts to crack has been overcome by using low-profile polyester resins (or low-shrink resins). These are prepared by making a blend of a thermoplastic (e.g., acrylic) polymer-styrene system with a polyester-styrene system. Moldings of this blend cured at elevated temperatures exhibit negligible shrinkage and minimal warpage and have very smooth surface, to which paint may be applied with very little pretreatment. 

Figure 7. Pressure profiles in function of die length for differential pulling velocities. Polyester-glass fiber system, low profile polyester. Experimental points of Sumerak (Ref 11) and model results (Ref 23). Note Volumetric fibercontent Vz=0.508, 1.016, and 1.524x10" m/s.

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. 

Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are the dominant materials used in automotive applications. These composites of unsaturated polyester resin, fillers and fiberglass have advantages of high stiffness, heat resistance and low coefficient of expansion. Coupled with low creep resistance, which is a distinct advantage over thermoplastic competition, and low-profile additives, which can yield Class A surfaces, these materials are well suited for applications from exterior body panels to under the hood components. 

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. 

Again, as in many other fields covered in the book, modified epoxies are the most studied systems (toughened epoxies for adhesive coatings and composites). But also rubber-modified phenolics and low-profile unsaturated polyesters for sheet and bulk molding compounds have been extensively studied. 

DMTA is a very interesting tool for characterizing heterogeneous materials in which domains of distinct Tg values coexist. The most interesting cases involve modified thermosets of different types (see Chapter 8). Examples are the use of rubbers (e.g., liquid polybutadiene and random copolymers), or thermoplastics (e.g., polyethersulphone or polyetherimide in epoxy matrices or poly(vinyl acetate) in unsaturated polyesters), as impact modifier (epoxies), or low-profile additives (polyesters). The modifier-rich phase may be characterized by the presence of a new a peak (Fig. 11.10). But on occasions there may be superposition of peaks and the presence of the modifier cannot be easily detected by these techniques. If part of the added polymer is soluble in the thermoset matrix, its eventual plasticizing effect can be determined from the corresponding matrix Tg depletion, and the... 

Poly(vinyl acetate) (PVAc) is very often used for low-profile applications. At low PVAc contents, the continuous matrix is a polyester network with PVAc inclusions. Increasing the PVAc amount leads first to a bicon-tinuous structure, and then to a phase-inverted system (Chapter 8). The low-profile action is observed in the concentration range where bicontinuous structures are formed (Pascault and Williams, 2000). However, the fracture energy attains a maximum value for lower PVAc concentrations (Bucknall et al., 1991). 

This moldable material primarily consists of TS polyester resin, glass fiber reinforcement, and filler. Additional ingredients, such as low-profile additives, cure initiators, thickeners, and mold-release agents are used to enhance the performance or processing of the material. As with any material, such as metallics and plastics, SMC can be formulated in-house or by compounders to meet performance requirements of a particular application such as tensile properties or Class A surface finish. Varying the type and percentage of the composition will result in variations in mechanical properties and processability. 

Low profile plastics are added to reduce shrinkage during cure. They are normally thermoplastics that include polyvinyl acetates, polymethyl methacrylate, and copolymers with other acrylate, vinyl chloride-vinyl acetate copolymers, polyurethane, polystyrene, polycaprolactone, cellulose acetate butyrate, saturated polyester, and styrene butadiene copolymers. More details about the low profile additive (LPA) mechanism are published in the literature. ... 

Some new types of S-B block copolymers are now being used as low-shrink/low-profile additives in sheet molding compounds made from unsaturated polyesters. 

PolyeCL has an ability to blend with several other polymers and can be used as an adhesive for polymer films of the same or different composition, as an excellent low-profile additive for fiber-glass-reinforced polyesters and as a pigment dispersant. It improves the impact strength and crack resistance of some thermoplastic polymers. 

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. 

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