Rubber-phase distribution

In HIPS, desirably the PS is the continuous phase including a discontinuous phase of rubber particles. The size and distribution of the rubber particles in the continuous PS phase can affect the properties of the HIPS. In blends of PS with other materials, the distribution of the noncontinuous phase in the continuous poly(styrene) phase is often similarly important (2). The impact strength of HIPS can go up to sevenfold of that of general purpose PS. 

Lednicky F et al. (1991) Silicone rubber-hydrogel composites as polymeric biomaterials. Ill An investigation of phase distribution by scanning electron microscopy. Biomaterials 12(9) 848—852... 

Figure 7. Transmission electron micrograph of an OsO stained sample containing 22% rubber, showing the widely distributed rubber phase. The size of the rubber domains is on the order of a few hundred angstroms. The light region in the micrograph is the brittle polyester-epoxy phase domains. Magnification 60,000x.

Essentially all the literature reviewed in the preceding paragraph emphasized identification rather than control of rubber-phase distribution. Recently, our laboratory conducted a series of investigations to identify and control rubber-phase distribution in several binary blends by using functionalized core-shell rubber. We were able to control butyl acrylate core-shell rubber in PC, PET, or both phases in PC-PET-rubber blends by functionalizing the shell structure of the core-shell rubber with glycidyl methacylate monomer units (16). 

According to more recent theories, the toughness of high impact polystyrene is caused by flow and energy dissipation processes in the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and in the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution in impact polystyrene. Stress maxima create zones of lower density, called crazes (3), in which the polystyrene molecules are extended parallel to the direction of stress. Macroscopi-cally craze formation appears as whitening the flow processes result in irreversible deformation (cold flow). 

The RCP part of the mixture is designed to have ethylene contents on the order of 40-65% ethylene and is termed the rubber phase. The rubber phase can be mechanically blended into the ICP by mixing rubber and HPP in an extruder or it can be polymerized in situ in a two-reactor system. The HPP is made in the first reactor and the HPP with active catalyst still in it is conveyed into a second reactor where a mixture of ethylene and propylene monomer is polymerized in the voids and interstices of the HPP polymer powder particle. The amount of rubber phase that is blended into the HPP by mechanical or reactor methods is determined by the level of impact resistance needed. The impact resistance of the ICP product is determined not only by its rubber content but also by the size, shape, and distribution of the rubber particles throughout the ICP product. Reactor products usually give better impact resistance at a given rubber level for this reason. 

Figure 8 shows the SEM images with a low level of strain (50%). It is clear that even with a low-strain level defects are initiated in the sulfur cured system with the formation of large cracks at the boundary layer between the two phases. However, in the peroxide cured system the mechanism of crack initiation is very different. In the latter case the NR-LDPE interface is not the site for crack initiation. In this case, stress due to externally applied strains is distributed throughout the matrix by formation of fine crazes. Furthermore, such crazes are developed in the continuous rubber matrix in a direction... 

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