No abstract

No abstract

SYNOPSISThe rheological behavior of blends of natural rubber (NR) and styrene-butadiene rubber (SBR) latices has been studied with reference to the effects of blend ratio, shear rate, surface-active agents (casein and sodium carboxymethyl cellulose), and temperature. When the SBR content was less than 50%, the viscosities of the blends appeared to be a nonadditive function of the viscosities of the constituent homopolymers; i.e., a positive deviation was observed. This was due to the structural buildup of the SBR domains. The SBR domains underwent agglomeration and consequently so-called microflocculation took place. The viscosities of all the blends were found to decrease with increase of temperature and shear rate. The increase in temperature and shear rate marginally weakened the structural buildup as evidenced by the lowering of viscosity. As the SBR content in the system increased, the pseudoplasticity of the blend increased. Even in the presence of surface-active agents the blends showed composition-dependent positive deviation. However, surface-active agents marginally reduced the extent of structural buildup by reducing the microflocculation behavior of SBR domains. 0 1995 John Wiley

SYNOPSISThe effect of prevulcanization on the rheological behavior of natural rubber (NR), styrene butadiene rubber (SBR) latices, and their blends was studied with special reference to shear rate, blend ratio, vulcanizing systems, prevulcanization time, and accelerator systems. The NR latex showed a sharp increase in viscosity with increase in prevulcanization time due to high extent of crosslinking. However, SBR latex showed marginal effect on viscosity with prevulcanization time due to its low dry rubber content and low degree of unsaturation. Blends showed variations in viscosity according to the change in composition. The use of a single accelerator was found to have marked influence on the viscosity of the blends compared with a combination of accelerators. Swelling experiments were carried out in order to determine the crosslink density of the blends. The viscosity changes have been correlated with the crosslinking density of the latices and their blends. 0 1996 John Wiley & Sons, Inc.

This article focuses mainly on the effect of maturation time on the rheological behavior of unvulcanized natural rubber (NR) -styrene butadiene rubber (SBR) latex blends. Viscosity shows a composition-dependent behavior with maturation time. It was found that there is a marginal decrease in viscosity for all the systems with maturation time except for the 70/30 NR-SBR blend. In this blend, there is a sharp decrease in viscosity with maturation time. This is associated with the exchange of stabilizers with one another until an equilibrium is reached; that is, all the particles of the blend are stabilized with random mixture of stabilizers. The structural build up observed in 70/30 NR-SBR blend was found to be diminished as the maturation time increases. At equilibrium, there is no further exchange of stabilizers. The behavior of this blend has been explained with the help of a schematic model. The effects of blend ratio and surface active agents on the viscosity were also studied. In addition, the time-dependent flow behavior of prevulcanized latex blends was evaluated as a function of vulcanizing systems and prevulcanization time. There is a regular increase in viscosity with prevulcanization time. However, after 3 h, the viscosity of almost all blends levels off, indicating that the curing reaction is complete within this time. Finally, the morphological changes occurred during film formation of the blends were studied using scanning electron microscopy.

No abstract

In order to collect isocyanate in both aerosol and vapor forms, we have developed a dual-membrane sampler which features a moist porous bed, backed up by an impermeable membrane. A polyester-polyurethane foam (PUF) was selected to function as the porous bed, and it was evaluated for its ability to resist solvent RUDZlNSKl ET AL. degradation and swelling while permitting rapid air penetration. Natural rubber (NR) and styrene-butadience rubber (SBR) membranes were prepared to serve as backup membranes. These were also evaluated for their solvent compatibility and air-penetration characteristics. A polyester-polyurethane foam backed up by a NR membrane prepared by an efficient vulcanization procedure was selected as the best combination for development of the isocyanate sampler. Recovery studies using the hexamethylenebased polyisocyanate, biuret, indicate better than 80% recovery of polyisocyanate after application to the PUF.

No abstract