Rubber particle size, graft

The graft rubber particle sizes for these MBAS and SA impact polymers are nearly equivalent hence they can be compared directly. Other variables which may affect strength properties such as the craze size of the resin matrix, the interphase adhesion, molecular weights and... 

Most ABS is made by emulsion polymerization. A polybutadiene or nitrile rubber latex is prepared, and styrene plus acrylonitrile are grafted upon the elastomer in emulsion. The effect of rubber particle size in ABS graft copolymer on physical properties is the subject Chapter 22 by C. F. Parsons and E. L. Suck. Methyl methacrylate was substituted for acrylonitrile in ABS by R. D. Deanin and co-workers. They found a better thermoprocessability, lighter color, and better ultraviolet light stability. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the rubber phase or solution of die acrylonitrile in die aqueous phase (72). In emulsion systems, rubber particle size may also influence graft structure so that die number of graft chains per unit of rubber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-shell vs "wart-like" morphologies) of die grafted copolymer on die rubber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of die initiator by die polybutadiene have been described (75,76). 

In this process uncrosslinked rubber is dissolved in a mixture of the monomers and solvent(s). This solution is pumped into the first reactor which is connected to a series of reactors. The polymerization is started by increasing the temperature, eventually in the presence of an initiator. Most of the rubber grafting and particle sizing happen early in the process. Chain transfer agent level, initiator (type/amount) and shear have a great influence in this stage. Crosslinking of the rubber particles occurs later in the process. The final step is the removal of residual monomer and solvent. 

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... 

Rubber particle size is extremely important to make an optimized impact product. Particles that are both too small and too large cause a loss of impact strength. The ability to form stable particles of optimum size depends on the graft that functions as an oil in oil emulsion. This might better be referred to as an emulsion of two incompatible organic phases. To size the rubber particles, shearing agitation must be provided. If it is not provided, phase inversion does not occur, and a cross-linked continuous phase that produces gel is the result. 

In heterophase polymeric materials such as rubber modified polystyrene or acrylonitrile-butadiene-styrene (ABS) resins, outstanding mechanical properties can be obtained only by regulating the dispersed rubber particle size and by achieving adhesion between the rubber and the resin phase. This can usually be achieved by adding block or graft copolymers, or by their formation in situ, as in industry. 

Historically, a number of different impact-modification technologies have been used. These include various maleated oleflnic rubber such as EB, EP, and so on SBS bromi-nated isobutylene-para-methyl styrene elastomers produced by Exxon and many others. Dow s metallocene-based ethylene-a-olefln elastomers were found to be very effective as well. The rheology of toughened nylon 6,6 is usually directly related to the maleic anhydride graft level of the impact modifier. Rubber particle size averages of greater than 0.25 pm and less than 0.5 pm are required to achieve the required balance of mechanical performance. Optimum particle size varies with the percentage of rubber. 

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 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. 

Similarly, when a mixture of maleated EPR (0.7% MA) and non-reactive EPR was blended at a total of 20% rubber loading in PA6, a maximum in impact improvement could be achieved only with a high content (>70%) of the maleated EPR in the rubber mixture. Thus the degree to which a reactive (maleated) rubber can be diluted with a non-reactive rubber seems to depend on the maleic anhydride content of the reactive rubber. In each of these cases, both the degree of grafting and the consequent rubber particle size seem to reach the optimiun requirement for the toughening efficiency at a particular ratio of the mixtures of reactive and non-reactive rubbers. 

ABS is also produced via bulk polymerization, with similar characteristics as noted above for HIPS. However, commercial ABS is primarily produced via emulsion polymerization, where SAN is polymerized in the presence of prepolymerized rubber (PB or SBR) emulsions. SAN is grafted to the rubber particles and also crosshnks the rubber particles, providing melt processing particle size stability. The advantage of emulsion polymerization versus bulk polymerization involves the ability to attain lower particle size and thus improved gloss of the injection molded or extruded articles of mamrfacture. ABS can tolerate lower rubber particle size than HIPS without loss of impact strength. In addition to PB and SBR, EPDM and acrylate rubbers have also been employed in the impact modification of PS and SAN. The primary advantage of these rubber modifiers involves the improved weatherabihty and thermal (oxidative) stability. Styrene-maleic anhydride (SMA) copolymers have been impact modified with similar rubber and processes as HIPS and ABS. 

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