Low-profile additive

Cellulose esters, especially acetate propionate and acetate butyrate mixed esters, have found limited use in a wide variety of specialty appHcations such as in nonfogging optical sheeting (171), low profile additives to improve the surface characteristics of sheet-molding (SMC) compounds and hulk-molding (BMC) compounds (172,173), and controlled dmg release via encapsulation (174). 

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. 

TP polymer typically low-shrinkage additive (PE, PS, etc.) or low-profile additive (PVAc, etc.) 20 parts... 

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... 

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. 

Polarite. CC Int L] Surface-treated minerals low-profile additives for low and zero>shrink DMC and BMC moldings, high gloss prods. 

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. 

Bucknall, C., Davies, P., and Partridge, I., Phase separation in styrenated polyester resin containing a polyjvinyl acetate) low profile additive. Polymer. 26, January (1985). 

Huang, Y.J. and Chen, L.D. 1997. Effects of chemical composition and structure of unsaturated polyester resins on the miscibility, sample morphology and mechanical properties of styrene/unsaturated polyester/low-profile additive ternary systems. 1. Miscibility and cured sample morphology. Polymer 39 6631-6641. 

Shrinkage Any filler will decrease shrinkage the most commonly-used fillers are silica, clay, calcium carbonate, alumina talc, powdered metals, and lithium aluminum silicate. Low-shrink or low-profile additives are used in TS systems such as SMCs and BMCs to minimize shrinkage during and after molding, and improve the surface appearance of a molding. 

In the laboratory, styrene can be prepared by the decarboxylation of cinnamic acid, as shown in Reaction 1, using dry distillation. However, styrene is produced commercially from ethylene and benzene, two basic ingiedienis of the petrochemical industry. With electrophilic addition of ethylene to benzene, a mixture of ethyl benzene and diethylbenzene is obtained as own in Reaction 2. The dehydrogenation of these benzene derivatives produces slyrene and divinylbenzene, respectively (Reaction 3). A detailed synthesis of styrene is described by Berthelot et al (6). As mentioned earlier, styrene is an important monomer in many industrial polymers. Additionally, divinylbenzene which is produced as a by-product is an effective crosslinker for ion-exchange resins, polystyrene-based supported reagents and catalysts, and low profile additive in a number of liquid molding resin systems. 

As indicated earlier, the materials may be either in the form of compounds or resins and fibres separately. Sheet moulding compound (SMC), dough moulding compound (DMC) and bulk moulding compound (BMC) use polyester resin plus filler, catalyst, pigment, low profile additive, etc. 

Dicyclopentadiene (DCPD) with or without a saturated polyester as a low smoke low profile additive can pass various building fire tests such as the German Chimney test or the British and French spread of flame tests at the highest or next but one classifications. Likewise an unsaturated polyester blended with methyl methacrylate can achieve such levels with either 120 phr of APP, 225 phr ATH and 25 phr APP, or with 300 phr of ATH. Similar formulations also pass German and US railway tests with 100 phr of ATH and 10 phr APP. Not only are these formulations highly flame retardant but they also fulfil smoke density and toxicity requirements as severe as those demanded by the aircraft industry. 

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