Core-shell rubber

In polyesters, most of the attention has been directed towards poly(butylene terephthalate) (PBT), but recent work has focused on seeing whether the same concept holds good for toughening poly(ethylene terephthalate) (PET). PET is chemically similar to PBT, but has a higher melting point and slower rate of crystallization. Because of this, toughening systems used in PBT may thermally degrade in PET. 

An alternative approach is to incorporate reactive functional groups Into the elastomer, producing an in-situ graft copolymer. This reduces interfacial tension, improving dispersion in processing, and improves the adhesion of the rubber to the thermoplastic In the solid state. A maleated polystyrene/poly(ethylene-co-butylene)/polystyrene triblock copolymer (SEES) has been used successfully to toughen polyamides and polyesters. 

It has been found that as little as 1 % SEBS-g-M A in PET increases the fracture strain by more than 10 times. The graft copolymer acts as an emulsifier to decrease interfacial tension and reduces the tendency of dispersed particles to coalesce, while promoting adhesion between the phases in the blend. 

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Fig. 2. Morphology model of a core-shell, rubber-toughened epoxy adhesive.

Figure 14.9 Effect of various impact modifiers (25wt%) on the notched Izod impact strength of recycled PET (as moulded and annealed at 150°C for 16 h) E-GMA, glycidyl-methacrylate-functionalized ethylene copolymer E-EA-GMA, ethylene-ethyl acrylate-glycidyl methacrylate (72/20/8) terpolymer E-EA, ethylene-ethyl acrylate EPR, ethylene propylene rubber MA-GPR, maleic anhydride grafted ethylene propylene rubber MBS, poly(methyl methacrylate)-g-poly(butadiene/styrene) BuA-C/S, poly(butyl acrylate-g-poly(methyl methacrylate) core/shell rubber. Data taken from Akkapeddi etal. [26]...

In Secs. 13.2-13.3 the principles of toughening of thermosets by rubber particles, and the role of morphologies, interfacial adhesion, composition, and structural parameters on the toughening effect are analyzed. Section 13.4 is devoted to the use of initially miscible thermoplastics for toughening purposes. The effect of core-shell rubber particles is discussed in Sec. 13.5 and, in Sec. 13.6, miscellaneous ways of toughening thermosets (liquid crystals, hybrid composites, etc.), are analyzed. 

Core-shell rubber (CSR) particles are prepared by emulsion polymerization, and typically exhibit two or more alternating rubbery and glassy spherical layers (Lovell 1996 Chapter 8). These core-shell particles are widely used in thermoplastics, especially in acrylic materials (Lovell, 1996), and have also been used to modify thermosets, such as epoxies, cyanates, vinyl ester resins, etc. (Becu et al., 1995). 

A variation of the HIPS process uses diblock polybutadiene-polystyrene rubbers to produce core-shell rubber particles with polystyrene cores and thin polybutadiene shells. The small particle size of 0.1 to 0.4 pm is less than optimum for toughening but provides a high-gloss material. 

A variant of rubber toughening involves the use of preformed core-shell rubbers comprising a highly cross-linked polybutadiene core with a grafted shell of a vinylic polymer. In this case, the particles are small, typically ca. 0.1 pm, and thus have little effect on the observed viscosity of the epoxy. One of the principal advantages of this over simple rubber toughening is the ability to produce predetermined controllable morphology in the cured polymer. " ... 

Figure 1. DN-4FB plane-strain damage zone of epoxy that has been modified with core-shell rubber. Top ROM image obtained under Nomarski interference contrast. Bottom TEM image of the croids in the plane-strain region. In both images the crack propagates from the upper right to the upper left.

Essentially all the literature reviewed in the preceding paragraph emphasized identification rather than control of rubber-phase distribution. Recently, our laboratory conducted a series of investigations to identify and control rubber-phase distribution in several binary blends by using functionalized core-shell rubber. We were able to control butyl acrylate core-shell rubber in PC, PET, or both phases in PC-PET-rubber blends by functionalizing the shell structure of the core-shell rubber with glycidyl methacylate monomer units (16). 

MBS-MA rubber KCA-503, a similar type of core-shell rubber, except the shell contains methyl methacrylate (MMA), methacrylic acid (MAA), and maleic anhydride (MA) monomer units (Kureha Chemical Co., Japan). 

This study demonstrated that the final destination of the added core-shell rubber particles, in PC, PA, or both, in the PC-PA binary blend can be controlled by properly selecting the chemical structure of the shell in the core-shell rubber. The unreactive MBS rubber tends to reside in the PC phase and near the vicinity of the PC-PA interface. The reactive MBS-MA rubber can have a chemical reaction with PA end groups and can therefore be retained within the PA phase. High-molecular-weight bisphenol A epoxy resin has proved to be an efficient compatibilizer for PC-PA blends. Rubber-toughening of the PC-PA blend in which PC is the matrix is much more effective than with blends in which PA is the matrix. 

Acrylate core-shell rubbers (PMMA-g-BuA) Weatherability, 53 69 192 1041... 

In order to improve both the thickness sensitivity and low temperature sensitivity of the impact strength, polycarbonate has been blended with a variety of low T, elastomeric impact modifiers. More important among these are the core-shell rubbers like PMMA-g-polybutadiene, PMMA-g-SBR (MBS), PMMA-g-n-butylacrylate (acrylic core-shell), all normally composed of 0.1/core particles. These modifiers improve both the thick-section (6.4 mm) and low temperature notched Izod impact properties of polycarbonate [Witman, 1981 Neuray and Ott, 1981 Bussink et al., 1977] (Figures 15.13 and 15.14). The blends are of course opaque. Impact modified polycarbonate also shows better retention of impact strength with heat-aging. 

All the commercial PBT/PC and PET/PC blends also contain typically 10-20 wt% of an additional elastomeric impact modifier. The exact nature and the content of the impact modifier is kept proprietary and often forms the basis for a particular blend patent. Typically core-shell rubbers such as polymethylmethacrylate grafted butadiene-styrene rubber (MBS) or an all acrylic core-shell rubber such as poly (MMA-g-n-BuA) are used [Nakamura, 1975 Chung, 1985]. ABS (with high polybutadiene content S 50%) or ASA... 

Cavitation was also identified as an active mechanism in systems where a mbberlike phase (particles or interphase) is susceptible to implode under the effect of the hydrostatic stress induced by the applied tension. Fond (36) has recently revisited the critical conditions under which this form of damage becomes energetically favorable. In the case of core-shell rubber-toughened PMMA, he ascribed the extensive whitening under tension at room temperature to the profuse formation of voids in the mbber shell of the toughening particles. 

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