Rubbery particles, size distribution

A threshold of interfacial adhesion between both phases is needed to (a) promote the cavitation mechanism and (b) activate the crack-bridging mechanism. For rubbery particles, the former contributes much more than the latter to the total fracture energy. Adhesion is achieved by the use of functionalized rubbers that become covalently bonded to the matrix. Higher toughness values have been reported by the use of functionalized rubbers (Kinloch, 1989 Huang et al., 1993b). However, these experimental results also reflect the effect of other changes (particle size distribution,... 

Assessment of Particle Size Distribution and Spatial Dispersion of Rubbery Phase in a Toughened Plastic... 

At a low rubber content, these four formulations form two-phase thermosets with a continuous rigid DGEBA phase (matrix) and randomly distributed, small spherical rubbery particles (triglyceride) that are several micrometers in diameter and have a unimodal particle-size distribution (Figure 1). 

Microstructural features such as volume fraction of rubbery particles and the particle size distribution will depend on the initial resin/modifier compatibility and the cure scheme. From the processor s standpoint, the blend needs to be stable and produce the desired morphology upon cure consistently. Ideally, one would like the liquid rubber modifier and the unreacted resin to form a miscible blend at room temperature. Cure conditions will then control rubbery particle formation. Factors that affect phase separation include cure temperature and time to gelation, which is a function of temperature and catalyst system. 

W. A. Lees ° has pointed out the difficulties of achieving the optimum particle size distribution of the precipitating rubbery phase. Moser and Slowik have addressed this problem by using preformed rubbery domains of the type used in impact resistant plastics. Suitable impact improvers are described as core/shell polymers which have a crosslinked acrylic or butadiene-based elastomer graft-linked to an outer rigid thermoplastic polymer. 

The morphology of the rubber-modified polystyrenes system involves some complex aspects, such as particle size, size distribution, occlusions of polystyrene inside the rubber phase, interfacial bonding between the rubbery particles and the brittle matrix, etc. Many authors have observed that some of the most important factors in controlling the mechanical properties of HIPS and ABS are rubber particle size [49], volume fraction of the rubbery phase (rubber + occluded polystyrene) [50,51] and the degree of graft [52]. Grafting occurs during the polymerization of styrene when some of the free radicals react with the rubber... 

Often, very small rubber particles or modifier particles are used to enhance the toughness, for instance, of PMMA or PP at lower temperatures. Figure 5.12 shows schematically one example with PBA core shell particles they consist of a hard core of PMMA (diameter about 180 nm) and a rubbery shell of poly(bu-tyl acrylate-co-styrene) (PBA) (approximately 40 nm thick). An outer PMMA shell increases compatibility between particles and matrix. The particles were preformed and possess spherical shapes with a narrow size distribution. Under load, the plastic deformation starts in the particles with cavitation and fibrillation of the rubbery shell. The second step is deformation in highly stressed zones between the particles in the form of crazes or homogeneous yielding. 

Uniform distribution of stress is more likely rather than that of strain unlike the conventional concept of uniform strain for rubber elasticity. Especially for inhomogeneous polymer blend, stress tends to be uniform among domains. For rupture, the degree of uniformity in stress is a determining factor. Active filler enhances strength of rubber, and the rubbery dispersion improves impact strength of plastics when the particle size is very small. 

It is known that a small amount of discrete rubbery particles with an average size of several microns, randomly distributed in a glassy, brittle epoxy resin, dissipates part of the impact energy thus improving crack and impact resistance without deterioration of other properties of the initial epoxy resins. 

Chloroprene polymerizes thermally to yield a true rubbery solid with a characteristic liquid-like structure. Stretching produces crystallinity and the expected X-ray pattern for an oriented fiber. A detailed mechanistic analysis yielded the multiple structures that appear during the reaction. Another synthetic pathway leads to polychloroprene latex. Characterization of these particles with the ultracentrifuge yielded a highly peaked distribution with a mean radius near 0.06 micron. Because of the very small particle size compared with natural mbber latex, this material can be used for many applications requiring minute particles. The detailed analysis and keen structural insight set a standard that is rarely met, even today. 

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