Polybutadiene dispersions

The polybutadiene dispersions used are customarily copolymers based on butadiene, styrene and acrylonitrile. Their advantage is the substantial flexibility in thick layers, as required in the finishing of split leathers for example. These systems are typically crossHnked using zinc oxide. As the hiding component, polybutadiene dispersions are also used in combination with polyacrylate dispersions. The double bonds in the polymer make polybutadiene dispersions susceptible to oxidative aging (Hght, heat) and sensitive to heavy metals. 

Depending on the polyol component used, there are polyetherurethanes and poly-esterurethanes. With regard to the isocyanate component, a distinction is made between aromatic and ahphatic monomers. The somewhat less costly aromatic systems, however, do not meet the extreme aging resistance requirements of automotive leathers, for example. 

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PMMA/ABS In blends of PMMA and styrene-acrylonitrile (SAN) copolymers, there is a critical composition of the SAN, at approximately 28 wt % acrylonitrile, that marks the transformation fix>m miscibility to immiscibility with PMMA. Confirmation of this behavior is provided by the curves presented in Figure la for blends involving SAN containing 25 wt % and 30 wt % AN, respectively, obtained by conventional d.s.c. The matrix of ABS would be expected to behave in a similar manner. However, the presence of the strongly scattering polybutadiene dispersed phase in the ABS excludes optical scattering methods for the determination of phase... 

The simplest method of polyblending involves equipment such as rolls or extruders, which can effect the mechanical blending of the two polymeric components in the molten state (Matsuo, 1968). High-impact polystyrene (HiPS) is an important example of a polyblend made by this technique. Such materials commonly contain 5-20 % of rubber, usually polybutadiene, dispersed in a polystyrene matrix. As shown in Figure 3.1, electron microscopy studies on specimens stained with osmium tetroxide reveal well-defined, irregular rubber particles (1-10 fim in diameter) dispersed in the polystyrene matrix. The elastomer domains appear dark because the osmium tetroxide stains the elastomer preferentially (see Section 2.4). 

Fig. 4.15 Interfacial tension coefficient as a function of diblock copolymer in the minor phase. System matrix, polybutadiene dispersed phase (pendant drop technique), polystyrene with styrene-b-butadiene diblock copolymer (Data Anastasiadis et al. 1988). Dotted and solid lines were computed from Eqs. 4.29 and 4.30, respectively...

About 180 000 tons annum of binders are estimated to be used in leather finishing world-wide. Of that, about 60 % are polyacrylate and polybutadiene dispersions and about 12 % polyurethane dispersions (Fig. 11-6). At about 20 %, the solvent-containing lacquers still account for a relatively large share today. However, this share will in future decrease further in favor of polyurethane dispersions. 

The requisite hiding performance is achieved through the partial use of the polybutadiene dispersion. Furniture leathers are softer than automotive leathers. Embossing is accordingly done under less pressure. Since the fastness requirements are lower, the top coat is less crosshnked and the appHed amount is lower. For aesthetic reasons, the leather is briefly plated after the appHcation of the top coat. 

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. 

M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

ABS plastic, a polymer consisting of polybutadiene spheroids is dispersed in a continuous phase of poly(styrene—acrylonitrile). The chromic acid attacks the polybutadiene at a much higher rate than the continuous phase. This gives an excellent microroughened surface with superior metal-to-plastic bond strength. A typical recommended formulation consists of 20 vol % sulfuric acid, 420 g/L chromic acid, and 0.1—1.0% of a fluorocarbon wetting agent. The plastic is treated with this formulation for 6—10 min at 60—65°C. 

The particle size of the dispersed phase depends upon the viscosity of the elastomer-monomer solution. Preferably the molecular weight of the polybutadiene elastomer should be around 2 x 10 and should have reasonable branching to reduce cold flow. Furthermore, the microstructure of the elastomer provides an important contribution toward the low-temperature impact behavior of the final product. It should also be emphasized that the use of EPDM rubber [136] or acrylate rubber [137] may provide improved weatherability. It has been observed that with an increase in agitator speed the mean diameter of the dispersed phase (D) decreases, which subsequently levels out at high shear [138-141]. However, reagglomeration may occur in the case of bulk... 

Fig. 10. Concentration dependence of a modulus in the region of low-frequency plateau (i.e. yield stress , measured by a dynamic modulus). Dispersion medium poly (butadiene) with M = 1.35 x 105 (7), silicone oil (2) polybutadiene with M = 1 x I04 (3). The points are taken from Ref. [6], The straight line through these points is drawn by the author of the present paper. In the original work the points are connected by a curve in another manner...

Fig. B8.2.2. Logarithmic plot of the correlation time versus T — Tg for DIPHANT dispersed in polybutadiene (PB), polyisoprene (PI) and poly(dimethylsiloxane) (PDMS). The broken lines are the best fits with the WLF equation (reproduced with permission from Bokobza and Monneriea ).

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