Curing dispersion

Table I shows the effect of crosslinker chain length and concentration on mechanical properties. Contrary to expectations, both flexural and impact properties increased slightly with increasing ethylene dimethacrylate (EDMA) content up to 10%. The crosslinker had a relatively slight effect on the stiffness and brittleness of the cured dispersions up to 15 vol %. Triethylene glycol dimethacrylate (TEDMA) behaved similarly but had a less favorable effect on impact strength. The 5 to 10% improvements in all mechanical properties at the 10% EDMA level compared with uncrosslinked dispersion might well offer a substantial improvement in functionality and durability under bioconditions if combined with other dispersion technology (1,2).

Produces acetic acid, requires 50% RH 7 days to cure. Non-curing dispersion - becomes adhesive as solvent evaporates. 

This article, the third instalment of a seven-part series, examines vulcanised properties of rubber, including hardness, stress-strain properties and uniaxial tension. Tensile testing merits considerable attention because it is done so extensively in the rabber industry. Results from tensile tests are sensitive to manufacturing errors such as state of cure, dispersion of ingredients and presence of foreign material, and these limitations should be recognised. Tensile properties of mbber demonstrate little if any relation to serviceability of most rabber products. 26 refs. 

Urea.—Forma.IdehydeResins. Cellular urea—formaldehyde resins can be prepared in the following manner an aqueous solution containing surfactant and catalyst is made into a low density, fine-celled foam by dispersing air into it mechanically. A second aqueous solution consisting of partially cured urea—formaldehyde resin is then mixed into the foam by mechanical agitation. The catalyst in the initial foam causes the dispersed resin to cure in the cellular state. The resultant hardened foam is dried at elevated temperatures. Densities as low as 8 kg/m can be obtained by this method (117). 

Moleculady mixed composites of montmorillonite clay and polyimide which have a higher resistance to gas permeation and a lower coefficient of thermal expansion than ordinary polyimides have been produced (60). These polyimide hybrids were synthesized using montmorillonite intercalated with the ammonium salt of dodecylamine. When polymerized in the presence of dimethyl acetamide and polyamic acid, the resulting dispersion was cast onto glass plates and cured. The cured films were as transparent as polyimide. 

The quantity of water is two to three times the weight of the hides. The salt from the cure dissolves in the water and the reverse of the curing takes place. The water is drawn into the hides by osmotic forces. The concentration of the salt solution is about 3-5 g/lOO mL. At this concentration some of the soluble proteins disperse. The soak water removes the salt, some proteins, some loose fat, blood, dirt, and manure. 

Because the heat distortion temperature of cured epoxy resins (qv) increases with the functionality of the curing agents, pyromellitic dianhydride is used to cross-link epoxy resins for elevated temperature service. The dianhydride may be added as a dispersion of micropulverized powder in liquid epoxy resin or as a glycol adduct (158). Such epoxies may be used as an insulating layer in printed circuit boards to improve heat resistance (159). Other uses include inhibition of corrosion (160,161), hot melt traffic paints (162), azo pigments (163), adhesives (164), and photoresist compounds (165). 

Water Dispersions. Polysulftde products are offered as aqueous dispersions (Thiokol WD-6). These are useful for applyiag protective coatings to line fuel tanks, and for concrete, wood, and ia some cases fabrics, felt, leather (qv), and paper (qv). It has been found that a stable emulsion can be made that contains both LP and manganese oxide curing agent. The emulsion can be thinned and appHed as a spray coating. After it is appHed, water evaporates and the LP cures to form a soHd mbber (13). 

The processing methods for siHcone mbber are similar to those used in the natural mbber industry (59,369—371). Polymer gum stock and fillers are compounded in a dough or Banbury-type mixer. Catalysts are added and additional compounding is completed on water-cooled roU mills. For small batches, the entire process can be carried out on a two-roU mill. Heat-cured siHcone mbber is commercially available as gum stock, reinforced gum, partially filled gum, uncatalyzed compounds, dispersions, and catalyzed compounds. The latter is ready for use without additional processing. Before being used, sihcone mbber is often freshened, ie, the compound is freshly worked on a mbber mill until it is a smooth continuous sheet. The freshening process eliminates the stmcturing problems associated with polymer—filler interactions. 

The compression set of sihcone mbber is similar to organic types of mbber at low (0—50°C) temperatures, ranging from 5 to 15% (380). Above 50°C, sihcone mbber is superior, but compression set increases with time and temperature. Sihcone mbber is more tear-sensitive than butyl mbber, and the degree of sensitivity is a function of filler size and dispersion, cross-link density, and curing conditions. The electrical properties of sihcone mbber are generally superior to organic mbbers and are retained over a temperature range from —50 to 250°C (51). Typical electrical values for a heat-cured sihcone mbber are shown in Table 9. 

Water-dispersible resins contain carboxyhc groups which are neutralized using base or amine compounds. This solubilizes the resin in solution and also promotes pigment wetting. Film formation occurs by the evaporation of volatiles foUowed by cross-linking through ambient cure oxidative reactions or elevated temperature reactions. Solvents, most commonly glycol ethers, are used to promote film formation and improve film quahty. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

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