Polystyrene rubber particles

Crosslinking Reactions in High Impact Polystyrene Rubber Particles... 

Impact polystyrene contains polybutadiene added to reduce brittleness. The polybutadiene is usually dispersed as a discrete phase in a continuous polystyrene matrix. Polystyrene can be grafted onto rubber particles, which assures good adhesion between the phases. 

The rubber particles should not be so small that they are completely embedded in a craze. It is interesting to note that in high-impact polystyrene crazes tend to be about 2 p.m thick and the optimum particle sizes observed as a result of experience are quoted in the range 1-10 p.m. For ABS the figures are about 0.5 p.m and 0.1-l.Op.m respectively. 

Addition of rubber particles of 30% to 100% by weight to cement with a grain size of approximately 40 to 60 mesh (0.4 to 0.25 mm) will produce a lightweight cement. The addition of rubber particles also creates a low permeability. The compositions are advantageous for cementing zones subjected to extreme dynamic stresses such as perforation zones and the junctions of branches in a multi-sidetrack well. Recycled, expanded polystyrene lowers the density of a hydraulic cement formulation and is an environmentally friendly solution for downcycling waste materials. 

Block copolymers of polystyrene with rubbery polymers are made by polymerizing styrene in the presence of an unsaturated rubber such as 1,4 polybutadiene or polystyrene co-butadiene. Some of the growing polystyrene chains incorporate vinyl groups from the rubbers to create block copolymers of the type shown in Fig. 21.4. The combination of incompatible hard polystyrene blocks and soft rubber blocks creates a material in which the different molecular blocks segregate into discrete phases. The chemical composition and lengths of the block controls the phase morphology. When polystyrene dominates, the rubber particles form... 

We most often encounter polystyrene in one of three forms, each of which displays characteristic properties. In its pure solid state, polystyrene is a hard, brittle material. When toughened with rubber particles, it can absorb significant mechanical energy prior to failure. Lastly, in its foamed state, it is versatile, light weight thermal insulator. 

Rubber toughened polystyrene is widely used in electronic and kitchen appliances. This type of application requires a good balance of stiffness, impact resistance, and ready coloration. Telephones, which are frequently dropped, are an excellent example of the benefits of rubber toughened polystyrene. The high surface gloss that we desire is obtained by minimizing the size of the rubber particles. Larger items, such as canoes, can be thermoformed from extruded sheet. 

Transition from liquid behavior to solid behavior has been reported with fine particle suspensions with increased filler content in both Newtonian and non-Newtonian liquids. Industrially important classes are rubber-modified polymer melts (small rubber particles embedded in a polymer melt), e.g. ABS (acrylo-nitrile-butadiene-styrene) or HIPS (high-impact polystyrene) and fiber-reinforced polymers. Another interesting suspension is present in plasticized polyvinylchloride (PVC) at low temperatures, when suspended PVC particles are formed in the melt [96], The transition becomes evident in the following... 

The preferred morphology of these rubber modified amorphous thermoplastics is the distribution of distinct rubber particles unfilled or filled in an isotropic matrix of the basic polymer. This was shown to be the case for rubber modified polystyrene and for ABS-type polymers. 

Experimental Evidence. Morphology. Figure 3 (33) shows in phase contrast microscopy the development of crack or craze patterns around rubber particles in a toughened polystyrene. The lack of dependence of crack inclination on direction of stress is especially marked in this micrograph, and can be explained only by reference to dynamic branching rather than to crack or craze nucleation by stress raisers. Schmitt and Keskkula refer to the lines as craze cracks and cracks. ... 

The rubber particle size in the final product increases several fold if the prepolymerization is carried out in the presence of a dilute aqueous solution of an alkane sulfonate or polyvinyl alcohol in place of pure water. The addition of a surface-active agent converts the coarsely dispersed oil-water mixture—obtained as above in the presence of pure water—into an oil-in-water emulsion. In this case even prolonged stirring during prepolymerization does not decrease the rubber particle size appreciably in the final product. The stabilization of the droplets of the organic phase in water by the emulsifier obviously impedes or prevents agitation within the polymeric phase. Figure 1 shows the influence of these three prepolymerization methods (under otherwise equal reaction conditions) on the dispersion of rubber particles in polystyrene. 

As explained above the rubber particle size in the final product is a measure for the rate of agitation—under otherwise equal reaction conditions—within the rubber-polystyrene-styrene solution during prepolymerization. Figure 1 shows that agitation is least effective if the organic... 

Figure I. Interference phase contrast micrographs of rubber particles in polystyrene. Prepared by prepolymerization (A) in bulk, (B) in the presence of water, and (C) in an o/w emulsion.

A good dispersion of rubber particles appears to favor the nucleation and growth of a large number of thick crazes uniformly distributed in the polystyrene matrix. This is believed to be an efficient source of energy absorption for the material under mechanical loading. The concepts of stress field overlap and critical volume of stress concentration zone for craze initiation were introduced to explain the observed mechanical behavior of HIPS. 

Examples of particle sizing using photon correlation spectroscopy are presented to demonstrate the straight forward uses and difficulties with the technique. Examples include NBS SRM 1691, a crosslinked polystyrene-divinylbenzene, a fractionated sample of polyvinylchloride, "dusty" samples of reference latex, a bimodal mixture of reference latices, a bimodal mixture of rubber particles, and a 2 1 aggregate. 

In general, rubber particles in all these classes are non-porous and compact. An electron micrograph of a polystyrene-rubber blend (Figure 3) can illustrate the general feature of the disperse phase. The adhesion... 

The rubber particles were examined with an electron microscope after the sample was treated with osmium tetroxide (27). The micrograph (Figure 7) clearly indicates the porous nature of the rubber phase and the occlusion of polystyrene. We therefore classify this type of rubber phase as filled graft rubber. Since grafting takes place before and after the rubber chain is coiled, therefore, for this case, the monomer is grafted onto the rubber both within and without the rubber phase. Polybutadiene is thus made more compatible to the polymer matrix surrounding the rubber phase and the polymer filling the rubber phase. Here we have an... 

The sample was prepared by osmium tetroxide technique (magnification, 8670 X), dark phase = rubber particles, white spots in the particles are occluded polystyrene... 

The major weaknesses of polystyrene are brittleness, and softening in hot water. Brittleness is remedied by dissolving 2-10 percent of rubber in styrene monomer before polymerization, producing high-impact styrene (HIPS), in which 10- xm rubber particles improve impact strength by an order of magnitude, with some sacrifice of other mechanical properties and transparency this accounts for more than half of the total polystyrene market. 

There are some additional applications of the theory which are presently under investigation. These are the effects of drawing on fibers for which the three-dimensional theory with transverse symmetry is applicable and the toughening mechanism in high impact polystyrene for which the flaw spectrum may be viewed as caused by the size, orientation, and spacing distributions of the rubber particles. 

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