Crosslinking premature

The third impregnation method, which is often referred to as injection or reaction-injection molding (RIM) pultrusion, employs a different kind of die. Unimpregnated reinforcement is guided into a narrow opening in the die, which widens into a cavity further downstream where the resin is injected under pressure (see Fig. 11.2). The cavity, which is maintained at a temperature that ensures that the resin does not start crosslinking prematurely,... 

A breaker an enzyme (at T<140°F), strong oxidizing agent, or an acid, is used to depolymerize polysaccharides and break crosslinks such that viscosity declines at a controlled rate so that the proppant may be deposited in the fracture. Too rapid proppant dropout would cause a premature "sand-out" which prevents future extension of the fracture. Peroxydisulfates are the most frequently used breakers. Less reactive organic peroxides may be preferred for high temperature formations (85). 

The mode of polymerisation and crosslinking for coatings is very similar to that of bulk polymers. An important requirement is that premature polymerisation should not take place before application. In the presence of activators, e.g., cobalt naphthenate, many paints on exposure to air polymerise by radical oxidation resulting in crosslinked structures. Stepwise growth polymerisation, e.g., urethanes, is promoted by heat therefore storage at high temperatures (>50°C) should be avoided. 

Inaba et al. prepared a series of model styrene/butyl acrylate copolymer latexes with glass transition temperatures at room temperature. The functional monomer 2-(3-isopropenylphenyl)-2-methylethylisocyanate (TMI) was used as monomer/crosslinking agent for further film formation. A small amount of methacrylic acid was introduced in some formulations in order to enhance the crosslinking reaction. A redox initiation system was used to reduce premature crosslinking during the polymerization [82]. 

The process engineer would like materials that have unlimited shelf life (warehouse storage before use), pot life (working time after the reactive components are mixed), and process working time in general (resistance to premature crosslinking between cycles, in dead spots, and during down time). In fact, ideally, one would like a one-part system, which means that a mixture of all the reactants would be stable indefinitely. All these requirements spell low reactivity. 

In con3)ariscn with MAM copolymers there is less likely to be premature crosslinking. Crosslinking of MBA copolymers may... 

The second method should be feasible, at least for small and medium-sized siloxanes, as the reactivity of co-alkenes and methacrylates towards hydrosilylation differs by a factor of at least 10, thus minimizing premature crosslinks via hydrosilylation. 

Relationships between microvoid heterogeneity and physical properties in crosslinked elastomers, poly-(isobutylene-/7-methylstyrene-p-bromomethylstyrene) (PIB-PMS/BrPMS) terpolymers, were identified by a 3D-NMR imaging study. Three-dimensional reconstruction of the sample images reveals that the voids are spherically shaped. The experimental results indicate that high microvoid density in cured elastomers leads to crack initiation and accelerated crack growth, thereby resulting in premature mechanism failure of the materials. 

The butadiene acrylonitrile rubbers have clearly been successful to a considerable extent in improving the toughness characteristics of epoxy networks. However, since the butadiene component of the elastomer contains unsaturation, it would appear to be a site for premature thermal and/or oxidative instability. One would imagine that excessive crosslinking could take place with time which would detract from the otherwise desirable improvements accomplished with these structures. 

The effect of the addition of small concentrations of the premature vulcanization inhibitor (PVI), N-(cyclohexylthio)phthalimide, is also given by Figure 7.11. This retarder (Coran and Kerwood, 1970) is frequently used to independently control scorch resistance with little effect on the rate of crosslinking (Coran, 1978). Before the development of N-(cyclohexylthio)phthalimide as a PVI, acidic retarders like salicylic acid, acetylsaUcylic acid, phthaUc anhydride, and benzoic acid were used. These additives improved scorch resistance but also gave greatly reduced rates of crosslink formation after the delay. Another retarder of the past was N-nitrosodiphenylamine, which is less active and not now used because of toxicological concerns. 

---