Rubber particle distribution, modified

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. 

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. 

Other rubber systems have been commercially successful. Styrene block copolymers yield a HIPS product with a small particle size and provide high gloss. A mixed rubber system consisting of styrene-butadiene block rubber and/or ethylene-propylene diene modified (EPDM) rubber can be blended with the polybutadiene to form bimodal rubber particle size distribution for a... 

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... 

Figure 2B shows the SEM image of the blend modified with MBS-MA, where the domain size of the PC phase is comparable to that of the unmodified one (Figure 1A). The MBS-MA rubber particles are evenly distributed within the PA phase see Figure 6), and so the PC phase makes direct contact with the PA phase. Therefore, the interfacial tensions of this blend modified...

Fig. 35. Dependence of fracture energy on the modifier composition (CTBN 1300 X 9 = carboxyl-tenninated acrylonitrile, acrylic acid and butadiene rubber with 18% acrylonitrile and 2% acrylic acid contents CTBN 1300x 13 = carboxyl-terminated acrylonitrile, butadiene rubber with 26% acrylonitrile content) (Reprinted from Journal of Materials Science, 27, T.K. Chen, Y.H. Jan, Fracture mechanism of toughened epoxy resin with bimodal rubber-particle size distribution, 111-121, Copyright (1992), with kind permission from Chapman Hall, London, UK)...

Because of its inherent brittleness, polystyrene homopolymer itself has limited application in blends. However, its impact-modified version, viz., HIPS, is more widely used. HIPS itself is a reactor-made multiphase system with 5-13 % polybutadiene ( cis -rich) dispersed as discrete particles in the polystyrene phase, with an optimum particle size of mean diameter of 2.5 pm. The rubber in HIPS is chemically grafted to some extent to the polystyrene. The effective volume of the rubber dispersion is actually increased through the occlusion of some polystyrene. To optimize the impact strength, the rubber particle size (>2.5 pm) and the distribution is normally controlled by the agitation and the proper choice of other process conditions during the polymerization. The property improvements in HIPS, viz., increased impact strength and ductility, are accompanied by the loss in clarity and a decrease in the tensile strength and modulus compared to the unmodified polystyrene. 

The sophisticated application of the mechanistic and thermodynamic principles which have been worked out extensively over the last 10 years is really the basis of these technologies. From simple compounding of rubbers in hard polymers we have come to actual control over the mechanical behaviour as required by the foreseen application. For single polymer based systems this often leads to a specific combination of impact modifying additions. A simple case being the use of a combination of small and large rubber particles, socalled Binodal distribution. 

Pig. 8. General mechanisms of enhancing the toughness of structurally modified polymers-group II mechanisms (in the boxes the crosshatched areas are inorganic particles, fibers or weak (rubber-like) particles distributed in a bulk matrix polymer [Pg.4723]

Rubber particle-size distribution is usually measured with a Coulter coimter or directly from electron photomicrographs (110,343) the latter also gives details of particle morphology. Rheological studies are made with a modified tensile tester with capillary rheometer (ASTM D1238-79) or with the powerful Rheometrics mechanical spectrometer (344). For product specifications, a simple melt-fiow test is used (ASTM D1238 condition G) with a measurement of the heat-distortion... 

Multiphase or multicomponent polymers can clearly be more complex structurally than single phase materials, for there is the distribution of the various phases to describe as well as their internal structure. Most polymer blends, block and graft copolymers and interpenetrating networks are multiphase systems. A major commercial set of multiphase polymer systems are the toughened, high impact or impact modified polymers. These are combinations of polymers with dispersed elastomer (rubber) particles in a continuous matrix. Most commonly the matrix is a glassy amorphous thermoplastic, but it can also be crystalline or a thermoset. The impact modified materials may be blends, block or graft copolymers or even all of these at once. 

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