Styrene acrylonitrile, brittleness

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

In the late 1940s, the demand for styrene homopolymers (PS) and styrene-acrylonitrile copolymers (SAN) was drastically reduced due to their inherent brittleness. Thus, the interest was shifted to multiphase high-impact polystyrene (HIPS) and rubber-modified SAN (ABS). In principle, both HIPS and ABS can be manufactured by either bulk or emulsion techniques. However, in actual practice, HIPS is made only by the bulk process, whereas ABS is produced by both methods [132,133]. 

Copolymerization allows the synthesis of an almost unlimited number of different products by variations in the nature and relative amounts of the two monomer units in the copolymer product. A prime example of the versatility of the copolymerization process is the case of polystyrene. More than 11 billion pounds per year of polystyrene products are produced annually in the United States. Only about one-third of the total is styrene homopolymer. Polystyrene is a brittle plastic with low impact strength and low solvent resistance (Sec. 3-14b). Copolymerization as well as blending greatly increase the usefulness of polystyrene. Styrene copolymers and blends of copolymers are useful not only as plastics but also as elastomers. Thus copolymerization of styrene with acrylonitrile leads to increased impact and solvent resistance, while copolymerization with 1,3-butadiene leads to elastomeric properties. Combinations of styrene, acrylonitrile, and 1,3-butadiene improve all three properties simultaneously. This and other technological applications of copolymerization are discussed further in Sec. 6-8. 

A typical unsaturated unreinforced polyester resin has an extremely low notched Izod impact strength. The addition of fibrous glass can change this extremely brittle material into a high impact strength composite. The same phenomenon occurs with some brittle thermoplastics, such as polystyrene and styrene—acrylonitrile. 

We have just discussed several methods for improving the mechanical properties of polymers. In addition to these techniques, one could think about synthesizing copolymers of styrene and less brittle monomer(s). Actually, we have already seen that this approach has been used with considerable success (see Chapter 5 and Table 5-2). Styrene-acrylonitrile (SAN) copolymers and acrylonitrile-butadiene-styrene (ABS) terpolymers have excellent impact strength. Although sometimes copolymerization is a viable option, oftentimes a completely different approach is called for. Let s see how. 

Wettability of Elastomers and Copolymers. The wettability of elastomers (37, 38) in terms of critical surface tension was reported previously. The elastomers commonly used for the reinforcement of brittle polymers are polybutadiene, styrene-butadiene random and block copolymers, and butadiene-acrylonitrile rubber. Critical surface tensions for several typical elastomers are 31 dyne/cm. for "Diene rubber, 33 dyne/cm. for both GR-S1006 rubber and styrene-butadiene block copolymer (25 75) and 37 dyne/cm. for butadiene-acrylonitrile rubber, ( Paracril BJLT nitrile rubber). The copolymerization of butadiene with a relatively polar monomer—e.g., styrene or acrylonitrile—generally results in an increase in critical surface tension. The increase in polarity is also reflected in the increase in the solubility parameter (34,39, 40) and in the increase of glass temperature (40). We also noted a similar increase in critical surface tensions of styrene-acrylonitrile copolymers with the... 

Homogeneous single-phase polyblends are very rare. Liquid-liquid phase separation of optically homogeneous polyblends of a styrene/acrylonitrile copolymer with poly (methyl methacrylate) has been studied by L. P. McMaster. A quantitative test method of the dynamic mechanical properties of multiphase polymer systems was developed by L. Bohn. He was able to demonstrate the correlation between shear modulus and gel volume of brittle polymers... 

Figure 4.18. Optimum elastomeric drop diameter vs. entanglement density of the matrix resin for brittle resins (polystyrene, styrene-acrylonitrile and polymethylmethacrylate), and for semi-ductile resins (polyvinylchloride and polycarbonate).

Brittle Polystyrene, acrylics, styrene acrylonitrile, highly reinforced material... 

The r-PC/r-ABS blends may also include PMMA as a third component (Laverty et al. 1996 Rybnicek et al. 2005). In this case, the chemical composition of the styrene-acrylonitrile matrix of ABS was found to have a dramatic effect on the morphology, while lubricant additives of ABS are adverse to the chemical stability. However, all the compositions rich in PMMA exhibited inferior mechanical performance (Laverty et al. 1996). A more detailed mechanical study (Rybnicek et al. 2005) demonstrated that the transition from ductile elastoplastic behavior to brittle linear elastic fracture behavior occurs when the PMMA is above 10 wt%. 

The method for obtaining an impact-resistant resin composition by introducing a rubber component into hard and brittle thermoplastic resin such as polystyrene, polymethyl methacrylate, styrene-acrylonitrile copolymer, polyvinyl chloride, or the like has been well known for many years. 

These blends with LDPE were weak and brittle. Adding chlorinated polyethylene, or PE grafted with 26% methyl methacrylate, greatly reduced domain size and increased ultimate elongation [31,107]. Adding hydrogenated polybutadiene-b-styrene-acrylonitrile-b-polybutadiene reduced domain size, increased interfacial adhesion, and improved mechanical properties [204b]. 

ABS is a copolymer in which a rubbery polybutadiene phase is dispersed in a rigid but brittle styrene-acrylonitrile (SAN) phase. The presence of rubber particles imparts flexibility and impact strength to the material even at low temperatures. Due to its excellent strength, ABS pipes are also available whose pipe walls are made of foamed core rather than being solid walls, which decreases weight without compromising physical properties. The major use of ABS pipes is in drain, waste, and vent (DWV) applications. 

Polystyrenes are available at different levels of impact strength from brittle to tough and ductile. Copolymers and terpolymers such as styrene-acrylonitrile (SAN) and acrylonitrile butadiene styrene (ABS) have been developed as extensions of the styrene range. SAN is at the low impact end but suitable for packaging and tableware while ABS is a tough, exceptionally durable material relatively unaffected by hostile environments. 

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

An important class of copolymers made by chain copolymerisation is graft copolymers, synthesized in order to toughen brittle materials through inclusion of a rubber phase. Examples are the cases of styrenic copolymers called "HIPS" for High-Impact Polystyrene and ABS for Acrylonitrile-Butadiene-Styrene. Both are synthesized in two steps. 

Copolymers. Mixtures of two or more different bifunctional monomers can undergo additional polymerization to form copolymers. Why copolymerize Well, polymers have different properties that depend on their composition, molecular weight, branching, crystallinity, etc. Many copolymers have been developed to combine the best features of each monomer. For example, polystyrene is low cost and clear, but it is also brittle with no toughness. It needs internal plasticization. By copolymerizing styrene with a small amount of acrylonitrile or butadiene, the impact and toughness properties are dramatically improved. 

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