Core-shell modifiers

Above its glass transition temperature, shear processes dominate in sPS as was shown earlier. In Figure 19.15, the TEM images of deformed sPS modified with the core-shell modifier prepared using the microsuspension method are reproduced. At the surface of the specimen highly oriented rubber particle are discernible without voiding. In the inner part of the specimen, however, cavi-tated highly oriented particles have been formed. Crazes are not seen in the... 

Composition (type of polymeric components). The base polymer (which is to be modified) may be an amorphous polymer [e.g., polystyrene (PS), styrene-acrylonitrile copolymer, polycarbonate, or poly(vinyl chloride)], a semicrystalline polymer [e.g., polyamide (PA) or polypropylene (PP)], or a thermoset resin (e.g., epoxy resin). The modifier may be a rubber-like elastomer (e.g., polybutadiene, ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, or ethylene-propylene-diene copolymer), a core-shell modifier, or another polymer. Even smaller amounts of a compatibilizer, such as a copolymer, are sometimes added as a third component to control the morphology. 

Figure 6.10. Models of different types of core-shell modifiers (a) traditional core/shell (b) IPN core/shell (c) blend of separate core/shell and (d) multilayered core/shell. Possible polymer compositions are polybutadiene copolymer as polymer A, acrylic copolymer as polymer B, and SAN as polymer C.

Lu Lu, F., Kausch, H.-H., Cantwell, W. J., Fischer, M. The effect of crosslink density on the fracture toughness of core-shell modified epoxy resins. J. Mater. Sci. Lett. 15 (1996) 1018-1021. 

Fig. 6.10 Models of different types of core-shell modifiers (a) traditional core/ shell, (b) IPN core/shell,...

As mentioned, PVC can be toughened by MBS. It is also possible to use methacrylate-butyl acrylate or methacrylate/polybutadiene core-shell modifiers, octyl acrylate-styrene terpolymer, ABS, MABS, EVA or chlorinated polyethylene. 

Sederel, L.C., Mooney, J. and Weese, R.H., Super Tough Blends based on LV Nylon and an Acrylic Core/Shell Modifier, Proceedings of the European Symposium on Polymer Blends, May 25-27, 1987, Strasbourg (France). 

EPDM and EPR are used to modify polyolefins, primarily in the automotive industry. The largest volume is in automotive PP bumpers. This application is gradually being replaced by impact-resistant polymers produced by metallocene technology, providing better performance and economics. DuPont Dow and Exxon are leading producers in North America. When used with plastics such as nylon, PET, and PBT, the EPDM and EPR are often modified with a functionalized monomer to allow them to react with the plastic. Additionally, the shell of a core-shell modifier can also be modified to include a reactive group. Suppliers of fimctionalized modifiers include Rohm Haas, AtoFina, Shell, and Exxon. 

Core-shell modifiers often have an outer shell of PMMA copolymer, which is miscible with, or can be wetted by, several engineering polymers. The result is improved adhesion to the matrices and a better dispersion of the rubbery component. SAN, PVC, PBT, PET, PET-PBT blends, and even blends containing immiscible components (such as ABS, HIPS and PC), can be toughened by adding about 20% of these modifiers. 

Figure 2 Schematic architecture of a core/shell modifier,...

Figure 3 Transmission electron micrographs of (a) a functionalized bulk rubber and (b) a functionalized core/shell modifier, dispersed in nylon 6. (20 wt% loading in both cases.)...

Figure 5 Dependence of the impact strength on temperature for a nylon 6 matrix toughened with 20wt% of a core/shell modifier.

Impact modifiers are added to many formulations. As the name implies, they impart toughness to the polymer article or film. Many of them are butadiene copolymers that disperse in the polymer matrix. One type - the so-called core shell modifiers - has a rubbery core surrounded by a harder acrylate layer. They have been compared to an egg soft on the inside and hard on the outside. The outer shell also has some adhesion to the matrix so that the modifier can be dispersed. Other impact modifiers include methacrylate-butadiene-styrene copolymers or EPDM, ethylene-propylene-diene monomer copolymers. Acrylonitrile-butadiene-styrene (ABS) and ethylene-vinyl acetate (EVA) are also used. 

PA6 PMMA core-shell modifier PMMA shell-acrylic or butadiene core impact modifiers with SMA addition yield impact toughening of PA6 SMA, miscible with PMMA, facilitates rubber particle dispersion in PA6 matrix 42... 

Epoxy PnBA-PMMA core-sheU modifier PnBA-PMMA core-shell modifier with PMMA containing epoxy groups showed Kk improvement with optimum interparticle distance of400 nm. 44... 

Fig. 14-7 Influence of temperature and core-shell modifier addition on the impact performance of PVC.

Common examples of toughened engineering resins include polycarbonate and polyesters. Unlike most other engineering resins, polycarbonate has some miscibiH-ty with PM MA, and traditional core-shell modifiers can significantly enhance the impact performance (Fig. 14-12). 

Fig. 14-14 Morphology of impact modified two-phase PC-PBT blend. The blend contains 40 % PBT, 50 % PC and 10 % core-shell modifier. The coreshell modifier tends to reside in the polymer phase it Is most compatible with, in this case, PC.

Impact modification of nylons is generally achieved through the incorporation of reactive groups in the rubber [139, 140]. Since these compositions react with the nylon during the melt processing, the conditions and compositions must be carefully controlled to prevent undesired increases in melt viscosity. While core-shell modifiers have been applied successfully to these systems [141, 142], the most common commercial approach is the use of olefin-based elastomers grafted with functional monomers as maleic anhydride. 

R. Berardino, Toughening of Epoxy resins Using Core-Shell Modifiers,... 

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