Anhydride curing agents, epoxy resins

By contrast with tertiary amines used in catalytic quantities, primary and secondary amines or acid anhydrides may be used to bring about the cure of epoxy resins by reaction in stoichiometric proportions. A typical amine curing agent used at this level is diaminodiphenylmethane (DDM), which reacts with an individual epoxy-group in the way shown in Reaction 4.17. 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

HHPA is generally used in a concentration between 55 and 80 pph depending on the nature of the epoxy resin. The viscosity is generally about 200 cP at 40°C when mixed with a DGEBA epoxy resin. A typical cure schedule for a 0.5 to 2 percent BDMA catalyzed system is 2 h at 80°C plus 1 h at 200°C. Typical of all the anhydride curing agents, the cured epoxy will demonstrate high heat distortion temperatures and excellent chemical resistance. 

Since the reaction between an epoxy resin and an acid anhydride curing agent also produces an ester linkage, anhydride cured epoxies have poorer hydrolytic stability than do... 

Liquid epoxy resins are stable for long periods at room temperature. Resins that are cut with diluent or solvent should be stored in tight containers to prevent loss of volatiles. Also, certain resins and curing agents (e.g., anhydride curing agents) can be degraded by exposure... 

To improve high temperature stability over amine cured systems and to give better physical and electrical properties above their heat distortion temperatures, it has been general practice in epoxy resin systems to use anhydride curing agents with DGEBA epoxy resins (8 ). Most anhydride formulations require elevated-temperature cures with the ultimate properties dependent on postcures at temperatures of 150 C or higher. 

Carboxylic acid and anhydride curing agents are used to a lesser extent in aerospace applications as compared to the amines which have mechanical properties and cure conditions that can be tailored to a wider variety of specific applications. Anhydrides tend to be somewhat brittle but offer useful service as high as 250 °C with novolac-type epoxy resins. In addition, the aliphatic dicarboxylic acid anhydrides give tough and sometimes flexible properties which are useful in encapsulation applications. 

Lindride. [Lindau Chems] Anhydride curing agent to epoxy resins. 

Properties of a composition consisting of a standard bisphenol A/ECH epoxy resin with 20% cyanoethylated ACEC (XLVI) and an acid anhydride curing agent exhibit a considerably elevated heat resistance. 

Aliphatic and cycloahphatic epoxy resins have less tendency to yellow and can even be used as a basis for epoxy gelcoats. Such gelcoats are not widely available due to difficulties in application and adhesion to the substrate, as well as high costs. Use of non-aromatic anhydride curing agents such as hexahydrophthalic anhydride will produce further improvements in colour stability and these systems are suitable for high temperature curing applications. 

Fluoro-anhydride Curing Agent for Heavily Fluorinated Epoxy Resins," in Adhesion Science and Technology, L. H. Lee, Editor, 429 Plenum, New York (1975). 

Flame Retardants. Flame retardance can be built into the epoxy resin by use of tetrabromobisphenol A or anhydride curing agents containing phosphorus or halogen. It can also be helped by nonreactive additives such as alumina trihydrate or organo-halogens + antimony oxide. 

Acid anhydride curing agents, especially the liquid anhydrides, are particularly important for epoxy resins. They cure at elevated temperatures but offer long pot life, good electrical properties, and heat resistance. Typical of these agents are phthalic anhydride and nadic methyl anhydride. The high-temperature properties of resin systans cured with these materials are better than those of resin systems cured with... 

Paints, varnishes and lacquers based on epoxy resins are used in various industrial applications because of their strength and durability. Two-component epoxy paints that cure at room temperature need a hardener added before their use. One-component epoxy paints that are heat-cured contain a hardener which can be activated only by heating. Polyfunctional aliphatic amines, aromatic amines, solid polyamides and anhydrides can be used as curing agents. Epoxy-ester-resin paints are formed by reacting epoxy resin with... 

The rate of cure of epoxy resins with tertiary amines depends primarily upon the extent to which the nitrogen is sterically blocked. The homopolymerization reaction depends on the temperature as well as the concentration and type of tertiary amine. Benzyldimethylamine (BDMA) and TDMAMP are mainly used as accelerators for other curing agents, in the cining of anhydride-and dicyandiamide-based systems. Other tertiary amine catalysts include 1,4-diazabicyclo(2,2,2)octane (DABCO) and diazabicycloundecene (DBU). 

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