Rubber phase, discrete

With high shear mixing equipment such as the Banbury or continuous mixer, the use of block polymers with a number average molecular weight (Mw) below 150,000 did not give the expected improvement in impact over polystyrene. This anomalous behavior occurred because the particle size of the discrete rubber phase averaged less than 0.2 pm compared with an average of 1 pm for blends with optimum impact obtained in the solution blends. 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Styrene monomer and a styrene/butadiene copolymer are fed to the first reaction zone. The polymerization is initiated either thermally or chemically. Many chemical initiators are available such as ferf-butyl peroxybenzoate and ferf-butyl peracetate. Conditions are established to prevent a phase inversion or the formation of discrete rubber particles in the first reaction zone. The conversion in the first reaction zone should be 5-12%. An important function of the first reaction zone is to provide an opportunity for grafting of the styrene monomer to the elastomer (8). 

Overall, the relatively brittle polyurethane resin matrix was transformed by the incorporation of a discrete rubber particle phase into a semiductile material... 

If a linear rubber is used as a feedstock for the mass process (85), the rubber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete rubber particles are formed. This is referred to as phase inversion since the continuous phase shifts from rubber to SAN. Grafting of some of the SAN onto the rubber particles occurs as in the emulsion process. Typically, the mass-produced rubber particles are larger (0.5 to 5 JJ.ni) than those of emulsion-based ABS (0.1 to 1 Jim) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to facilitate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extruders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Thus it follows that to obtain HIPS with high physico-mechanical properties independent of the manner of its obtaining, it is necessary to create conditions for forming discrete particles of rubber phase, which are at most filled with polystyrene occlusions and which have an optimal external diameter as well as possessing siifficient connection with polystyrene matrix due to the intermediate adhesion layer. As shown by the authors (25) the thickness of this layer essentially depends on the number and size of polystyrene occlusions inside the rubber particles. 

The grafting is accomplished in the commercial mass polymerization process by polymerizing styrene in the presence of a dissolved rubber. Dissolving the elastomer in the styrene monomer before polymerization produces HIPS grades. Since the two polymer solutions are incompatible, the styrene-rubber system phase separates very early in conversion. Polystyrene forms the continuous phase, with the rubber phase existing as discrete particles having occlusions of polystyrene. Different production techniques and formulations allow the rubber phase to be tailored to a wide range of properties. Typically ... 

As a means to improve the rubber utilization, a bulk/suspension process evolved, whereby polybutadiene rubber was dissolved in styrene monomer and polymerized in bulk beyond phase inversion before being dropped into suspension. The HIPS produced this way had two distinct advantages over the compounded version styrene to rubber grafting and discrete rubber spheres or particles uniformly dispersed in a polystyrene matrix. This improved the impact strength dramatically per unit of rubber and gave better processing stability, because the rubber phase was dispersed instead of being co-continuous with the polystyrene. 

After phase inversion, the dispersed phase is the rubber phase. This is the first time discrete rubber particles are present in the reaction mixture. The particle size in the final product is an important parameter to optimize the physical properties. To be successful in the manufacturing of mass ABS, it is necessary to understand and control which parameters can be used to control the final rubber particle size. 

Styrenic block copolymers derive their useful properties from their ability to form distinct styrene (hard phase) and diene (rubber phase) domains, with well defined morphologies. To achieve this requires an unusual degree of control over the polymerization. The polymerization must yield discrete blocks of a uniform and controlled size, and the interface between the blocks must be sharp. This is best achieved by so-called living polymerization. For a polymerization to be classified as truly living, it is generally accepted that it must meet several criteria [3] ... 

In rubber-modified polystyrenes, the rubber is dispersed in the polystyrene matrix in the form of discrete particles. The two-phase nature of rubber-modified polystyrene was first suggested by Buchdahl and Nielsen [47] based on data on dynamic mechanical properties obtained with a torsion pendulum. The existence of two prominent loss peaks led to this conclusion, one at low temperatures which is due to the a relaxation of the rubber (e.g. 193 K for polybutadiene) and one at high temperatures which is due to the a relaxation of the matrix (e.g. 373 K for polystyrene). Later, microscopy provided proof of the existence of the rubber phase as a discrete dispersed phase in polystyrene [48]. 

The program can be used in several ways to model the geometry of real materials. In the modeling of a blend of two rubber-toughened plastic components, the discrete-phase volume and rubber particle-size distribution of each blend component would have to be known. Files containing the actual diameters of discrete-phase spheres would also be required. Finally, the rubber-phase volume in each blend component would have to be multiplied by the components weight fraction in the blend. This assumes that both components have the same continuous phase and, therefore, no volume change when blended. 

In general, high impact polystyrenes are multiphase systems consisting of a continuous rigid polystyrene phase and discrete rubber particles 0.5-lOjU in diameter. The incorporated rubber particles are crosslinked and contain grafted polystyrene, and their inner structure is determined by the manufacturing process and can vary considerably. The principle polystyrene structures have been described in detail (I, 2). 

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

Latex compound maturation is the period when the latex compound is stored after mixing, prior to use in the production line. After maturation the latex compound is mixed with a sulfur dispersion to crosslink the rubber molecules to improve their properties. The latex compounds are agitated for a maturation period of 1-7 days, depending on the nature of the production process and on the scale of the mixing operation. During maturation, crosslinking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex and air bubbles introduced in the mixing rise to the surface. 

The rubber phase is discrete and distributed within the polyamide matrix. Agglomeration of particles may result from application of modifiers under manufacture conditions. 

Impact polystyrene contains polybutadiene added to reduce brittleness. The polybutadiene is usually dispersed as a discrete phase in a continuous polystyrene matrix. Polystyrene can be grafted onto rubber particles, which assures good adhesion between the phases. 

Block copolymers of polystyrene with rubbery polymers are made by polymerizing styrene in the presence of an unsaturated rubber such as 1,4 polybutadiene or polystyrene co-butadiene. Some of the growing polystyrene chains incorporate vinyl groups from the rubbers to create block copolymers of the type shown in Fig. 21.4. The combination of incompatible hard polystyrene blocks and soft rubber blocks creates a material in which the different molecular blocks segregate into discrete phases. The chemical composition and lengths of the block controls the phase morphology. When polystyrene dominates, the rubber particles form... 

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