Difunctional curing agents

Effects of Curing Agent Type. Epoxide-Cured Propellant. Carboxyl-terminated polybutadiene is a linear, difunctional molecule that requires the use of a polyfunctional crosslinker to achieve a gel. The crosslinkers used in most epoxide-cured propellants are summarized in Table IV and consist of Epon X-801, ERLA-0510, or Epotuf. DER-332, a high-purity diepoxide that exhibits a minimum of side reactions in the presence of the ammonium perchlorate oxidizer, can be used to provide chain extension for further modification of the mechanical properties. A typical study to adjust and optimize the crosslinker level and compensate for side reactions and achieve the best balance of uniaxial tensile properties for a CTPB propellant is shown in Table V. These results are characteristic of epoxide-cured propellants at this solids level and show the effects of curing agent type and plasticizer level on the mechanical properties of propellants. 

Most reactive diluents are mono- or difunctional. They are made by reacting epichloro-hydrin with an alcohol, a phenol, or a polyol to produce a mono- or polyglycidyl ether resin. However, there are some nonepoxy diluents that are used as well. These nonepoxy diluents generally react with the curing agent or other functional groups in the epoxy chain. 

The properties of a cured epoxy network depend primarily on two factors nature of the resin and nature of the curing agent. The functionality of an epoxy resin plays an important part in determining the thermomechanical properties. The properties of epoxy resins of various functionalities cured with DETDA are presented in Table 3.4. Multifunctional epoxies exhibit a higher T compared with the difunctional epoxy when cured with the same hardener. This is due to the increase in crosslink density as a result of an increase in epoxy functionality and the formation of a tighter network. This significantly reduces the free volume of the network, leading to an increase in the T. ... 

Epoxides can be simply difunctional or polyfunctional. The most widely used version is the difunctional epoxy type diglycidyl ether of bisphenol A, with (n) from 0.2 to 12 (formula 1 in Figure 6.17) which can be used with different types of curing agents, (i.e., various amines). Epoxidised novolaks (formula 2 in Figure 6.17) have multi-... 

The useful Bronsted acid type curing agents for the epoxy resins are polyfunctional phenols, polyfunctional organic acids, and anhydrides of mono- and difunctional organic acids. 

In practice, difunctional acids and diphenols are little used as curing agents for epoxy resins. They are, however, often used in combination with a second, more highly functional or efficient curing agent (novolac resins or fatty trimer acids). 

Elastomers with chemically saturated polymer backbones cannot be crosslinked with sulfur and so require alternate curing agents. The most widely used of these are the peroxides, which can also be used with unsatmated elastomers. Metal oxides or difunctional compounds are used, as well, in special cases. 

Nonsulfur Vulcanizing Agents. Many high performance specialty elastomers do not contain diene moieties in their molecular structure and therefore cannot be sulfur-cured. These elastomers require cross-linking agents capable of reacting with the specific functional group(s) contained by the specific elastomer. Some common nonsulfur curatives include peroxides, difunctional resins, and metal oxides. 

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