Emulsion polymerization toughening

We have considerable latitude when it comes to choosing the chemical composition of rubber toughened polystyrene. Suitable unsaturated rubbers include styrene-butadiene copolymers, cis 1,4 polybutadiene, and ethylene-propylene-diene copolymers. Acrylonitrile-butadiene-styrene is a more complex type of block copolymer. It is made by swelling polybutadiene with styrene and acrylonitrile, then initiating copolymerization. This typically takes place in an emulsion polymerization process. 

A blend was prepared by dissolving a rubber material in styrene and polymerizing the system. The blend contains not only rubber and polystyrene (PS), but also a graft polymer because of the attachment of short polystyrene side chains to the rubber molecules. The toughness of this material was markedly improved compared to that of the unmodified PS. A technology based on bulk polymerization [26] has been widely used the concentrated emulsion polymerization method employed by us, however, allows one to obtain rubber toughened latexes. 

For each type of toughening particle, the latex obtained from emulsion polymerization was coagulated by addition to magnesium sulphate solution to yield loose aggregates of the particles. These aggregates were isolated by filtration, washed thoroughly with water, and then dried at 70 °C. 

Lovell PA, Pierre D. Rubber-toughened plastics. In Lovell PA, El-Aasser MS, editors. Emulsion Polymerization and Emulsion Polymers. England John Wiley Sons 1997. p 657. 

The essence of rubber-toughening a brittle plastics material is to disperse a rubber as discrete micron, or submicron, size particles in the polymer to be toughened, typically using the rubber at volume fractions in the range 0.05-0.20 [1-3]. Emulsion polymerization provides one means of preparing rubbery particles for this purpose, and this chapter gives an overview of the use of emulsion polymer particles for toughening of important plastics materials. 

Figure 19 Transmission electron micrographs of thin sections cut from ABS materials. The rubbery domains appear darker, (a) An ABS material prepared by bulk polymerization showing the characteristic salami-like morphology of the toughening particl which contain several SAN domains within the rubber, (b) ABS materials prepared by emulsion polymerization showing cote-shell partides (i) widi and (ii) widiout subinclusions. (Reproduced with permission from ref. 29.)...

Toughening particles prepared by emulsion polymerization of butadiene and aciylonitrile were used in an early study of pre-formed particles [95]. However, although the particle dispersion in the epoxy resin was reasonable, the increase in fracture energy was not as high as that achieved using CTBN. Another early study [96] described use of commercial ABS toughening particles (see... 

The impact modifiers or toughening agents described above are all hnear polymers or segments. More recently, there has been some interest in the use of Core-Shell tougheners. These materials consist of a glassy shell grafted by seeded emulsion polymerization onto an elastomeric core. 

Recently, Wills et al. [16,17] proposed a novel agglomeration method. Core/shell impact modifiers, useful in toughening polyfvinyl chloride) and other thermoplastics, are prepared at small particle size by an emulsion polymerization process, agglomerated by the novel technique which has little effect on emulsion solid, are further encapsulated by a final shell, and are isolated by spray drying or coagulation. The isolated particles are readily redispersed to their original particle size in the matrix polymer. 

It is intended to toughen poly(methyl methacrylate) (PMMA) by the addition of particles of rubbery poly[(n-butyl acrylate)-co-styrene] made by emulsion polymerization. In order to retain the good optical clarity of the PMMA it is necessary to match the refractive indices, n, of the PMMA and copolymer particles. Estimate the relative proportions of the two comonomers required in the particles given that at room temperature the refractive index of PMMA is 1.489 and n is 1.466 for poly(n-butyl acrylate) and 1.591 for polystyrene. Would you expect the material to retain its clarity if the temperature were changed from ambient ... 

The emulsion free-radical polymerization carried out in different steps ensures a precise control of the particle size and particle size distribution. The particle diameter can be adjusted between 100 nm and 1000 nm, with a low polydispersity (generally less than 1.1) (Chapter 8). Rubber particles with sizes lower than 100 nm are ineffective for toughening purposes (Sec. 13.3.2b). 

Concentrated Emulsion Pathway to Toughened Polymeric Latexes... 

In what follows, a number of methods developed by our group to prepare toughened polymeric composites, which involve both physical and chemical modifications and are based on the concentrated emulsion pathway, are presented [27-29]. 

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