Accelerated swelling

Mixtures of liquid vinyl monomers and polymer powders serve as a basis for a special group of engineering materials. Vinyl acetate and acrylonitrile monomers are injected into polymer powders to accelerate swelling and gel formation. Styrene is added to improved the molding characteristics of reactive mixtures. Polystyrene, poly(vinyl acetate), poly(vinyl chloride), etc. are the most commonly used polymer powders. 

Dmg loading can be accompHshed by dispersion or adsorption. In dispersed systems, a dmg is blended into a polymer by mechanical means, such as a kneader. The viscosity of the polymer, and the size and concentration of the dmg, need to be optimized to minimize aggregates. Dmgs can also be absorbed by equiUbrating a polymer in a dmg solution. The absorption rate can be accelerated by introducing an appropriate solvent to swell the polymer. AH solvents would then have to be removed. 

Accelerators may be added to improve the physical properties of the polymer when needed. Where high modulus or low oil swell is required, thiocarbanihde is the preferred accelerator, with a cure time of 60 min at 100°C. Tetraethyl thiuram disulfide and sodium dibutyl dithiocarbamate are preferred for high tensile strength and cured at 121°C. 

However, there are a significant number of cases, sometimes estimated as 19% of fractures, where repair does not occur in a reasonable amount of time. The problems are associated primarily with severe injury, infection, arthritis, or biochemical abnormalities. A very common cause, known as the compartment syndrome, is related to severe swelling pressure on the blood vessels that limits blood access to the muscles. In many of these cases, electrical stimulation has been shown to be effective in accelerating repair. 

When the effect of a liquid is purely physical (e.g., due to absorption and swelling) and it is possible to continue the immersion until equilibrium absorption is reached, then actually no acceleration is involved. If chemical reactions are taking place, including the effect of temperature, the situation is similar to that for heat ageing and generally exposures will be needed at a series of temperatures and, perhaps, concentrations of the test liquid. 

The case of liquids is simpler than weathering in that it is essentially a two agent situation, liquid and temperature. As described in Sections 4.12.3 and 6.6.1, the rate of attack may be governed by the rate of diffusion of the liquid into the material, which may be slow in relation to the time scale of an accelerated test. Also, it is necessary to consider that there may be physical change (swelling) of the polymer as well as chemical degradation. 

Auto-acceleration was observed in the homopolymerization of methacrylic acid solutions over limited concentration ranges in methanol and in water. Perhaps under such conditions swelling of the polymer favors monomer diffusion leading to a larger amount of pre-oriented structures III. Alternatively, a monomer-solvent complex may arise which favors a pre-oriented structure and thus, may be responsible for the onset of a matrix effect (9). 

These results indicate that the hydroxide ion is an accelerating factor of its own right. In the next section we present some evidence that suggests that one of the reasons hydroxide ion is a delaminating agent is that it causes unusual swelling of some adhesives. 

Which of the two steps that occur in the solution process, (a) swelling and (b) dispersion of the polymer particles, can be accelerated by agitation ... 

Swelling of dry GPC beads takes up to several hours and may be accelerated by heating (cf Table 3.2). Take care that sufficient solvent is available during swelling (as a rule of thumb twice the expected volume). 

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