Epoxide resins cross-linking reactions

The presence of the C=C in unsaturated fatty acids is employed as the reaction sites in the formation of cross-linking. The networked structure can be achieved by a functional reaction of the C=C to enable use in high-molecular-weight products. The properties of the epoxidized resin-based composites are reliant on the extent of cross-linking reaction with the materials. The higher the cross-link density, the better is the mechanical and thermal properties. 

In comparison to the polyester resins, epoxides have limited and specialist use, and cost at least twice as much. However, they have superior toughness, will operate at slightly higher temperatures, have excellent adhesion to many substrates and are particularly resistant to alkaline environments. In particular, the chain extension and cross-linking reactions do not normally... 

AH of the amine hydrogens are replaced when MDA or PMDA reacts with epoxides to form amine based polyols. These polyols can be used in reactions with isocyanates to form urethanes or with additional epoxide to form cross-linked thermo set resins. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

The typical protocol of the catalytic Sharpless epoxidation requires 5-10 mol% of Ti(OiPr)4, which is chelated in situ by an equimolar amount of enantiomerically pure dimethyl or diethyl tartrate (15). The reaction proceeds in anhydrous CH2CI2, and a 4A zeolite is added to make the reaction catalytic. An early attempt by Farrall et al. (30) to anchor the catalyst was carried out with a tartrate monomer covalently attached to a cross-linked polystyrene resin. The enantiomeric excess (ee) values obtained (about 60%) were significantly lower than in the homogeneous reaction, for which values greater than 90% ee have been regularly achieved. 

Epoxy resins, which are used as adhesives, are also thermoset polymers that form by cross-linking when the two components of the resin are mixed. One component is a low-molecular-mass linear polymer formed by the reaction of the conjugate base of bisphenol A with epichlorohydrin. The nucleophilic oxygens of the phenolate dianion can either displace the chlorine or open the epoxide ring of epichlorohydrin. A slight excess of epichlorohydrin is used to keep these polymer chains short and to ensure that the linear molecules have epoxide groups at their ends. 

In polyaddition, cross-linking occurs by means of chain extension. The majority of ambient-cured construction products are based on this type of reaction, where in situ polymerisation occurs after the epoxy resin base and the curing agent are mixed. The curing agent causes the epoxide or hydroxyl groups to react (Ellis, 1994). 

There are two options for the other component of an epoxy resin system. Use of mono- or di-anhydrides as curing agents, usually catalyzed by a tertiary amine, causes reactions with the residual secondary hydroxyls in the repeating unit of the prepolymer forming esters and free carboxylic acids. The carboxylic acids formed also react with the epoxide end groups forming cross-links and further free secondary hydroxyl groups. Maleic anhydride, phthalic anhydride, or pyromellitic dianhydride are suitable for this process (Eq. 21.27). 

Under this heading come a wide variety of finishes based on the epoxy resins. These resins involve in their preparation, and later in their cross-linking, the reactions of the epoxide ring, which are described in Chapter 4. A brief revision of these reactions would be worthwhile before proceeding further. 

Epoxide resins contain the characteristic oxirane structure which can be converted to cross-linked structures in what is known as the curing or hardening reaction. More than 85% of the world production of epoxide resins consists of the bisphenol-A-diglycidyl ether, also known as 2,2-bis(p-glycidyloxyphenyl)propane, which has the idealized structure... 

---