Epoxy resin Etherification

Xu, L.S. Schlup, J.R. (1998). Etherification versus amine addition during epoxy resin amine cure An in situ study using near-infrared spectroscopy. Journal of Applied Polymer Science, Vol.67, No.5, 0anuary 1998), pp. 895-90, ISSN 0021-8995. 

Uses Mfg. of phenolic and thermosetting resins, furan polymers, urea-formaldehyde resins wetting agent foundry sand binders corrosion-resist, resins intermediate for esterification and etherification paints solvent, plasticizer for phenolic resins solvent for dyes and resins nonreactive epoxy resin diluent vise, reducer, cure promoter, and carrier in amine-cured epoxy resins gel retarder in casein/protein glues polymer sealants/cements synthetic flavoring agent in foods and pharmaceuticals cosmetics ingred. in food-pkg. adhesives... 

Different compounds such as epoxides and allyl chloride are used in this important modification of phenolic resins. Epoxides are one key component for phenolic resin modification by an etherification reaction. Technological important reactions are the conversion of novolacs into epoxy novolacs and the ctosslinking of phenolics with epoxy resins. Recently, hydroxymethyl-group-containing phenolics have been converted into epoxides. 

The package stability of alcohol-soluble resole resins and their compatibility with epoxy resins can be improved by partial conversion of the methylol groups to ethers. Allyl ethers have been used with epoxy resins in interior can coatings. Low molecular weight butyl ethers are used with acid catalysts to cross-link epoxy resins and other hydroxy-substituted resins, by etherification and transetherification reactions (148). 

The mechanism of anhydride cure is complex and controversial because of the possibility of several competing reactions. The imcatalyzed reaction of epoxy resins with acid anhydrides proceeds slowly even at 200°C both esterification and etherification occur. Secondary alcohols from the epoxy backbone react with the anhydride to give a half ester, which in turn reacts with an epoxy group to give the diester. A competing reaction is etherification of an epoxy with a secondary alcohol, either on the resin backbone or that formed during the esterification, resulting in a -hydroxy ether. It has been reported that etherification is a probable reaction since only 0.85 eqiuvalents of anhydrides are required to obtain optimiun cross-linked density and cured properties (103). 

Cyclic anhydrides of carboxylic acid are also used as cross-linkers for epoxy resins. Curing is initiated by reaction of a hydroxyl with an anhydride to yield a half ester and a carboxylic acid group. The newly formed carboxylic acid will in turn react with the epoxy to generate an ester and a new hydroxyl group. These complementary reactions result in a cross-linked network. Etherification reactions... 

The reaction mechanisms between the epoxy group and the carboxyl and hydroxyl groups have been studied and three different reaetions - two esterifications and one etherification -are identified [21]. The results of DSC, FTIR and reaction mechanisms indicate that the cin-ing process between the epoxy resin and GMAEVC can be explained by condensation esterification (carboxyl-hydroxyl reaction), addition esterification (carboxyl-epoxy reaction) and etherification (hydroxyl-epoxy reaction) from lower temperature to higher temperature. 

This result is consistent with the study of Barral et al. that showed the relationship between the structure and the mechanical properties of the epoxy matrix as a function of the degree of cure [38]. It explained that etherification reactions are important in the highest temperature treatment of epoxy resin and influenced the mechanical properties of the matrix. The higher activation energy for a polymer matrix cured at high temperature is attributed to the higher crosslink density of the network, which diminishes with the availability of molecular sized holes in the polymer structure. 

The three possible main reactions during cure of an epoxy resin with an amine - (/) primary amine-epoxy addition, (//) secondary amine-epoxy addition, (///) etherification (from Chiao, L., (1990) Macromolecules 23 1286). 

An alternative way to obtain end-group stabilized homopolymers is the etherification or total acetalization of the semi-acetal end groups. Alkylating agents, such as dimethyl sulfate or ortho-formic acid ester, epoxy alkanes, such as ethylene oxide or propylene oxide, and acetals in the presence of Lewis acids, or cation exchange resins are suitable. End-group etherified homopolymers have the additional advantage of alkali stability over esterified polymers, known also in copolymers blocked by comonomer components. 

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