High impact polystyrene HIPS crazing

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

An analogous microstructure can be produced in amorphous polymers by crazing them. Figure 2b shows the similarity of the craze structure to the crystalline morphology. We have reported that extensively crazed high-impact polystyrene (HIPS) exhibits hard elastic behavior, as the loading cycle illustrated in Figure 3 shows [10,11]. Hence, hard elastic behavior is associated with a bulk-microfibril composite structure... 

The modifier forms a particulate dispersion in the polymer phase, and under stress, crazing develops around modifier particles. Moreover, the addition of a compatibilizer improves particle dispersion and interfacial adhesion, resulting in engaged toughness. Rubber modified polystyreue, high-impact polystyrene (HIPS), or polyphenylene oxide can be used as a modifier. The compatibilizer can be a triblock copolymer of styrene-ethylene-propylene. The copolymer can even be random. 

The incorporation of rubber particles into a brittle polymer has a profound effect upon the mechanical properties as shown from the stress-strain curves in Fig. 5.66. This can be seen in Fig. 5.66(a) for high-impact polystyrene (HIPS) which is a blend of polystyrene and polybutadiene. The stress-strain curve for polystyrene shows brittle behaviour, whereas the inclusion of the rubbery phase causes the material to undergo yield and the sample to deform plastically to about 40% strain before eventually fracturing. The plastic deformation is accompanied by stress-whitening whereby the necked region becomes white in appearance during deformation. As will be explained later, this is due to the formation of a large number of crazes around the rubber particles in the material. 

Figure 3.25 In situ deformation of a rubber-toughened polymer (HIPS - high impact polystyrene) (a) overview of the area under load and (b) area in front of a crack tip (in micrograph (a) at the bottom) with rubber particles (gray) in a matrix (black) with crazes (bright) 2-pm-thick deformed section, deformation direction see arrow, in lOOOkV HEM.

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