Curing and cross-linking

The vast majority of rubbers are cured (or vulcanised) and a wide range of curing systems can be used, such as the following  

All of these systems have the shared characteristic of a strong curing exotherm due to the wide range of chemical reactions that accompany the cross-linking process. This exotherm. 

Comment CCA7 0 C, 40 nL pans Project no. J0316 Sample weight 12.910 mg Data colleted 22/08/2003 01 39 20 

Although it is desirable to carryout these analyses using sealed pans (see also Section 6.4.6 below), to ensure that no volatile breakdown products can escape and produce an endotherm in the heat capacity curve work can be carried out using the standard semi-open pans if the objective is to identify unknowns, determine the onset temperature of cure, and produce comparative data on a range of closely related samples. 

There are some cases in which peroxides are used in plastic compounds to produce a cross-linked matrix. A classic example of this is in the production of polyethylene cable covering materials. These types of systems can be studied in the same ways, and with the same considerations, as those that apply to rubbers. 

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Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

Silicone They have excellent heat resistance up to 260°C (500°F), chemical resistance, good electricals, compatible with human body tissues, etc. and a high cost. There are the room temperature vulcanizing (RTV) types that cure and cross-link at ambient temperatures, catalyzed by moisture in the air. It is a good sealant and excellent for making flexible molds for casting. It is widely used for human implants. 

A critical requirement for obtaining engineering properties from a rubbery material is its existence in a network structure. Charles Goodyear s discovery of vulcanization changed natural rubber from a material that became sticky when hot and brittle when cold into a material that could be used over a wide range of conditions. Basically, he had found a way to chemically connect the individual polymer chains into a three-dimensional network. Chains that previously could flow past one another under stress now had only limited extensibility, which allowed for the support of considerable stress and retraction upon release of the stress. The terms vulcanization, rubber cure, and cross-linking all refer to the same general phenomenon. 

Dynamic mechanical methods may be also used to characterize time-dependent changes in the elastic modulus during polymer curing and cross-linking, from which... 

Phenolic resin n. Any of a wide range of thermosetting resins made by reacting a phenol with an aldehyde, followed by curing and cross-linking. 

Modifying Processing Characteristics Curing and Cross-linking... 

Process NMR is used for chemicals (free/bound moisture, viscosity, activity, loading efficiency in powders, catalysts, liquids, detergents, pigments) and polymers (density, crystallinity, rubber and copolymer content, dispersion of fillers, melt properties, finish content, extent of cure and cross-linking, content of solubles, plasticisers, moisture, etc.). Process NMR is fully operational in the polymer industry, both as on-line units [202] which provide virtually continuous process feedback control as well as off-line and laboratory units for checks of the various processes [198]. The use of NMR for advanced process control has reduced the need for frequent wet tests, has reduced off-spec materials and has improved product transition times. 

Actual cases of adhesive recycling are still very rare because the amount of adhesive used in a product is much smaller than that of the adherend materials. In addition, adhesives consist usually of various kinds of materials. On the other hand, removing an adhesive layer from substrate is very difficult. Resolving obtained adhesive wastes by any solvent is also not easy because they are fully cured and cross-linked. However, the needs of sealant recycling are recently becoming important because the total amount of its use is huge. Thus, we should start to consider seriously the method to recycle sealant wastes. 

In order to be suitable for the synthesis of ceramics, the precursors should have a high molecular weight in order to avoid volatilization of low molecular components, appropriate rheological properties and solubility for the shaping process as well as latent reactivity (presence of functional groups) for the curing and cross-linking step. 

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