Elastomeric particles

Mechanical Properties. Properties of typical grades of PBT, either as unfiUed neat resin, glass-fiber fiUed, and FR-grades, are set out in Table 8. This table also includes impact-modified grades which incorporate dispersions of elastomeric particles inside the semicrystalHne polyester matrix. These dispersions act as effective toughening agents which greatly improve impact properties. The mechanisms are not fiiUy understood in all cases. The subject has been discussed in detail (171) and the particular case of impact-modified polyesters such as PBT has also been discussed (172,173). 

To achieve low stress embedding material, low modulus material such as siUcones (elastomers or gels) and polyurethanes are usually used. Soft-domain elastomeric particles are usually incorporated into the hard (high modulus) materials such as epoxies and polyimides to reduce the stress of embedding materials. With the addition of the perfect particle size, distribution, and loading of soft domain particles, low stress epoxy mol ding compounds have been developed as excellent embedding materials for electronic appHcations. 

The mechanical properties of pure polymeric materials are often inadequate for particular applications, and to overcome this problem these materials may be reinforced in some way. The most common method is to include a substantial amount of a rigid filler or fillers, generally as finely divided powder, or as rods or fibres. For certain materials, elastomeric particles may be used, and these have the effect of reducing brittleness. 

Thermoplastic polymers, such as poly(styrene) may be filled with soft elastomeric particles in order to improve their impact resistance. The elastomer of choice is usually butadiene-styrene, and the presence of common chemical groups in the matrix and the filler leads to improved adhesion between them. In a typical filled system, the presence of elastomeric particles at a level of 50% by volume improves the impact strength of a brittle glassy polymer by a factor of between 5 and 10. 

Cavitation in the rubber particles of PS/high-impact PS (HIPS) was also identified as a heterogeneous nucleation site, using batch-foam processing [15, 16]. The experimentally observed cell densities as a function of the temperature, the rubber (HIPS) concentration, the rubber particle size, and saturation pressure were found to be in good agreement with the proposed nucleation model. Similar nucleation mechanisms of elastomeric particles were claimed for acrylic and di-olefinic latex particles in various thermoplastics [17, 18]. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness but without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a amine- or carboxyl-terminated acrylonitrile butadiene copolymer (ATBN and CTBN). 

For electrostatic and steric stabilization, the particles can be viewed effectively as colloids consisting of a soft and deformable corona surrounding a rigid core. Colloidal particles with bulk elastomeric properties are also available. These particles, which are generally of submicron size, are developed and used as reinforcement additives to improve the Impact resistance of various polymer matrices [28-30]. The rubber of choice is often a styrene/butadiene copolymer. The presence of chemical groups at the matrix-filler interface leads to improved adhesion between them. Typically, the addition of about 30% by volume of these elastomeric particles increases the impact strength of a brittle glassy polymer like polystyrene by up to a factor of 10. For some applications, particles with more complex architecture have been... 

Figure 1.2. Izod impact strength at room temperature as a function of diameter of elastomeric particles in methylmethacrylate-butadiene-styrene copolymer used for toughening polyvinylchloride resin [after Bertelo and Mori, 1994].

In many cases, toughening of a brittle polymer can be achieved by introduction of stiffness heterogeneity, viz. incorporation of an elastomer, immiscible polymer, sohd particles, gas bubbles (i.e., foaming or micro-foaming), etc. However, the size and concentration of these heterogeneities should be optimized. For most thermoplastic s the optimum diameter of the dispersed elastomeric particle is d < 3 jj.m and its volume fraction ... 

MBS with controlled size of the elastomeric particles transparent Copolymer of vinylchloride, alkyl acrylate, and vinyhdene chloride Butadiene-styrene-methylacrylate-ethylacrylate Core-shell crosslinked ABS with grafted onto it PMMA shell PB-grafted with MM A, styrene and vinyl acetate Poly(butadiene-co-butyl acrylate-co-styrene)... 

It is a common practice to toughen brittle resins by addition of elastomeric particles. The effectiveness of the process depends on their diameter and concentration. It has been found that at constant concentration of the toughening agent its effectiveness, i.e., the plot of toughness vs. particle diameter, follows a bell-shape curve, defining the optimum particle diameter. As shown in Figure 4.17 the optimum does not change with concentration. 

By contrast with brittle resins where is independent of concentration, in pseudo-ductile one these two variables are related — when the concentration of the toughening agent decreases the elastomeric particle size must be reduced. In other words, in the latter systems it is the distance between the elastomeric particles that seems to control the fracture mechanism. Again, there are resins showing intermediate behavior between these two limits. 

In PO blends the preferred compatibilizer has been EPR, while in styrenic blends SBS or SEES maintain high visibility in spite of the price. Both compatibilizers can also improve the impact strength. However, excessive amount of elastomer can lower the modulus and strength of the alloy, thus the elastomeric particles size should be optimized. 

The low-speed stress-strain dependence for PS and HIPS is shown in Figure 12.6. These data well illustrate the change induced by incorporation of elastomeric particles into PS matrix. As shown,... 

Incorporation of elastomeric particles improves nucleation of gas bubbles, hence it stabilizes the foaming process, reduces bubble size and the final foam density. 

One reason advanced to account for this improvement is as follows. Impact strength is esssentially a function of how readily cracks can be propagated within the matrix. When elastomeric particles are present as filler, they stretch as the crack passes by,... 

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