Bimodal rubber particle distribution

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... 

Baumgartner, E. Hofmann, J. Jung, R.H. Moors, R. ABS Molding Materials having a Bimodal Rubber Particle Size Distribution. U.S. Patent 5,434,218, Jul 18, 1995 BASF. 

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)...

Chen, T. K. Jan, Y. H., Fracture Mechanism of Toughened Epoxy Resin with Bimodal Rubber-Particle Size Distribution. J. Mater. Sci. 1992, 27,111-121. 

Takahashi, J. Watanabe, H. Nakamoto, J. Arakawa, K. Todo, M., In-Situ Polymerization and Properties of Methyl Methacrylate-Butadiene-Styrene Resin with Bimodal Rubber Particle Size Distributions. Polym. J. 2006, 38,... 

The result of the diameter-dependent modulus is an increased stress concentration and increased tendency to craze initiation with increasing particle diameter. This result is demonstrated in the micrograph of an ABS polymer in Figure 7, which shows the preferred craze formation at the largest rubber particles. Therefore, toughened materials with a broad particle-diameter distribution or a bimodal diameter distribution often show preferred craze initiation at the largest particles, which has the disadvantage of reduced effectiveness (23). The maximum formation of crazes appears in a material with rubber particles of optimum diameter, Dopt and a small diameter distribution. 

Earlier Rowe and co-workers (37) had discussed improved toughness of cured epoxy polymers having a bimodal distribution of rubber particles embedded in the matrix—especially in light of failure mechanisms and what they suggest or mean. Similar models were addressed by Bascom and Hunston (38,39) and Ting (40,41) to show that such toughness improvements are carried over to the adhesive joint or to the fiber-reinforced composite. 

Okamoto, Y. Miyagi, H. Kakugo, M. Takahashi, K., Impact Improvement Mechanism of HIPS with Bimodal Distribution of Rubber Particle Size. Macromolecules 1991, 24, 5639-5644. 

Arakawa, K. Mada, T. Takahashi, J. Todo, M. Ooka, S., Effect of Rubber Particle Size on the Impact Tensile Fracture Behavior of MBS Resin with a Bimodal Particle Size Distribution. J. Mater. Sci. 2007,42,8700-8706. 

Li, D. Peng, J. Zhai, M. Qiao, J. Zhang, X. Wei, G., Novel Methods for Synthesis of High-Impact Polystyrene with Bimodal Distribution of Rubber Particle Size. J. Appl. Polym. Sci. 2008,109, 2071-2075. 

There is an optimum rubber particle size for toughening, dependent on the polymer being upgraded. If crazing is to be promoted, ABS requires a low particle size of around 500 mn to 1 micron, whereas HIPS needs 2 to 4 microns, and can benefit from even larger particles. Bimodal particle size distributions may be beneficial in some instances to facilitate two energyabsorbing mechanisms at the same time. 

The rubber phase size and size distribution is also affected by the manufacturing process. Typically, the size of the rubber phase averages 200-400 nm for resin produced by an emnlsion process and 1000-2000 nm for resin produced by mass pol5unerization. The size distribution of the rubber particles can be very broad, narrow monomodal, or bimodal. The dependence of the impact toughness of ABS on rubber phase particle size and size distribution can be of a complex nature because of the interactions with the graft interface. A maximum impact is reported (1) to occur for emulsion ABS at a mean rubber particle size of about 300 nm for a matrix SAN containing 25% AN. 

The particle size restrictions for toughening PS depend on the mechanical properties of the rubber particle. For a HIPS rubber particle, with a Young s modulus of 333 MPa, the minimum size is 0.8 mm, while for a pure PB particle, with a Young s modulus of 47 MPa, the critical size is 0.44 mm (27). The stiffer the particle, the larger it must be. HIPS systems with bimodal particle size distributions can also be effective in toughening (28), but the bimodal particle size distribution is generally detrimental to optical clarity. 

It is believed that a bimodal, or a broad distribution of rubber particles resulting from such rubber mixtures, helps in the craze or crack blunting because the larger particles and the small particles provide resistance to crack initiation through increased local shear deformation of the matrix. 

ABS compositions with bimodal particle size distributions of the grafted rubber can be prepared by emulsion graft polymerization techniques. The preparation of ABS types by emulsion polymerization consists in brief of (13) ... 

Examples of size distribution functions are shown ill Figs. 1.4 and 1.5. Figure 1.4 shows number distributions of commercially produced silica particles in terms of the fraction of particles in the,size range around dp, dN/N d dp) = na(,dp)fNxs where is the total particle concentration. The total particle surface area corresponding to each size distribution is shown. Commercial silica manufactured by the oxidation of SiCU is used as a filler (additive) in rubber. Both coordinate axes in Fig. 1.4 are linear, and the area under each curve should be normalized to unity. A bimodal volume distribution with a minimum near a particle size of 1 is shown in Fig. 1.5. Distributions of this type are often observed for atmospheric aerosols (Chapter 13) the volume of aerosol material per unit volume of gas above and below a micron is about the same as shown by the area under the curve. Bimodal distributions are also often observed in aerosols from industrial sources as discus.sed below. 

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