Epoxides, curing agents

Epoxidized phenol novolak and cresol novolak are the most common curing agents. The composition of the resin and hardener system is optimized for each specific appHcation eg, incorporating phenol novolaks in the matrix resin can iacrease cure speed. 

In the organic chemicals industry, H2O2 is used in the production of epoxides, propylene oxide, and caprolactones for PVC stabilizers and polyurethanes, in the manufacture of organic peroxy compounds for use as polymerization initiators and curing agents, and in the synthesis of fine chemicals such as hydroquinone, pharmaceuticals (e.g. cephalosporin) and food products (e.g. tartaric acid). 

Aliphatic polyamines, amine adducts and polyamides react with epoxide resins at normal temperatures to give complexes with outstanding chemical resistance. Paints based on this type of reaction must be supplied in two separate containers, one containing the epoxide resin and the other the curing agent , the two being mixed in prescribed proportions immediately before use. 

Few non.chemists know exactly what an epoxide is. but practically everyone has used an "epoxy glue for household repairs or an epoxy resin for a protective coating. Epoxy resins and adhesives generally consist of two components that are mixed just prior to use. One component is a liquid "prepolymer/ and the second is a "curing agent" that reacts with the prepolymer and causes it to solidify. 

When the epoxide is to be used, a basic curing agent such as a tertiary amine, R3N, is added to cause the individual prepolymer chains to link together. This "cross-linking" of chains is simply a base-catalyzed epoxide... 

The final resin product is obtained by reacting (curing or crosslinking) the above di-epoxide with acid anhydrides or polyamines. The curing agents (sometimes incorrectly called catalysts) react with the three-membered epoxide rings to produce a highly crosslinked polymer. 

A cement slurry additive consisting of methylcellulose, melamine-formaldehyde resin, and trioxane has been proposed for better bonding of cement to the casing string [20]. Bisphenol-A epoxide resins, with amine-based curing agents, sand filler, and a mixture of n-butanol and dimethyl benzene as a diluent, have been proposed as additives to increase adhesion properties of cement [572]. 

These low molecular weight compounds with epoxy end groups are cross-linked by adding a curing agent such as ethylenediamine. Primary amines react with epoxides to form tertiary amines and branches. 

Crosslinking of epoxy plastics through the hydroxyl groups of the repeat unit is used for prepolymers with low epoxide group contents. The most common curing agent is phthalic anhydride although other acid anhydrides such as tetrahydrophthalic, nadic methyl, and chloroendic anhydrides are used in specialty applications ... 

The curing process of epoxies is mainly dependent on the reactivity characteristics, the structure, and the functional group of resin [61,65], The basic chemical reactions, catalyzed by hydroxyl group (OH), between the epoxide group and the curing agent are [194] ... 

Curing Agents for Carboxyl-Terminated Polybutadiene Prepolymers. The types of curing agents used to prepare binders for CTPB propellants are the same as those for PBAN or PBAA. The bifunctionality of CTPB, however, requires that part of the curing agents be polyfunctional to provide for the formation of the tridimensional network. Almost without exception, the polyfunctional aziridines and epoxides used with CTPB undergo side reactions in the presence of ammonium perchlorate, which affects the binder network formation. Kinetic studies conducted with model compounds have established the nature and extent of the cure interference by these side reactions. The types and properties of some of the crosslinkers and chain extenders used to prepare solid propellants are summarized in Table IV. 

The problems associated with the multifunctional curing agents for CTPB and the resultant aging behavior of the cured polymers have led to a practical solution for curing binders and propellants—i.e., using mixed aziridines or a mixture of an aziridine and an epoxide. Such mixtures, when appropriately balanced, usually provide satisfactory mechanical behavior and high temperature stability. In dual curing systems such as MAPO and BITA or MAPO and a suitable multifunctional epoxide,... 

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. 

Propellant Aging. Three structurally different chemicals and mixtures of these materials have been used to cure CTPB propellants. These are MAPO, other aziridines which do not contain the P—N bond, and epoxides. As stated in the discussion of curing agents, the aging behavior of CTPB propellants prepared with these materials is distinctly different, owing to the behavior of these compounds and their reaction products in the presence of ammonium perchlorate and at elevated temperatures. 

Mixed aziridine or mixed epoxide-MAPO cures have been the practical solution to problems encountered with single curing agents. The mechanical properties of mixed aziridine or mixed epoxide—aziridine-cured propellants show less change on aging than those of propellants cured with MAPO or BITA alone. It appears that the BITA or the multifunctional epoxide (ERLA-0510 or Epon X-801) in the mixed curing systems provides stable crosslinks and a slight amount of postcure, which results... 

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