Лако-красочные материалы — производство

Epichlorhydrin is capable of reacting with hydroxyl groups, with the elimination of hydro­chloric acid. The most widely used epoxy resins are the family of products produced by the reaction between epichlorhydrin and bisphenol A.

Commercially useful grades are relatively low-molecular-weight products in which n ranges from 0 to about 4. When n is between 0 and 1, the product is a liquid, and this is the most useful product for adhesive applications. As n increases, the product moves toward a brittle solid. Solid grades find application principally in paints. Regardless of molecular weight, the resulting resin has two epoxy groups per molecule. Resins of greater functionality can be produced from polyols having more than two hydroxyl groups per molecule. Thus phenol novolac resins, having the general structure can be reacted with epichlorhydrin to produce epoxy novolac resins.

These products may have much greater functionality, although stearic considera­tions limit the useful size of the molecule. Because of their higher functionality, epoxy novolacs have greater cross-link density, generally yielding better temperature resistance at the expense of increased brittleness. They are thus seldom used on their own, but make useful modifiers of the properties of DGEBA resins.

Other products that may be epoxidized in this way include dihydric and trihydric phenols, aliphatic polyols such as glycerol, and simple alcohols such as butanol or allyl alcohol. These products, especially the monofunctional glycidyl ethers, are used at rela­tively low percentages to modify properties of DGEBA resins, particularly to achieve lower viscosities.

Epoxy groups may also be produced by oxidation of olefinic unsaturation within animal and vegetable oils. The resulting products have too low a functionality for use as resins in their own right, but are added to DGEBA resins to introduce a measure of flexibility. Major manufacturers of epoxy resins include Shell, Dow and CIBA-GEIGY.

The resulting epoxy resin is capable of reacting with various products, or itself, to form a solid, infusible product of considerable strength. The fact that these reactions generally occur without the production of low-molecular-weight by-products means that shrinkage during cure is negligible. This reduces stresses in the cured structure, contribut­ing to the strength of the cross-linked matrix and eliminating the need for sophisticated clamping techniques.

The two cross-linking reactions are external, by reaction of the oxirane group with active hydrogen, and internal, by homopolymerization through the oxirane oxygen. The former is typical of cross-linking by hardeners and the latter of catalyzed cross-linking. Both hardeners and catalysts are referred to as curing agents. The classic epoxy curing mechanism is illustrated by the reaction between a primary amine and an epoxy group:

This product can react with an additional epoxy group to continue the cross-linking process.

OH

^CH2-CH

>CH„-CH 2 OH

This reaction is characteristic of hardeners having active hydrogens available, including amines, amides, and mercaptans. The reaction is catalyzed by hydroxyl groups, especially phenolic hydroxyls and tertiary amines. Because of the bulk of the substituent groups involved, steric factors have a major influence on the reaction rate. Thus low-molecular — weight hardeners tend to react more vigorously and produce more cross-linked structures, while hardeners of high molecular weight tend to react more sluggishly. Hardeners may
thus be selected to produce highly exothermic reactions or reactions that take place only under the influence of external heat. Similarly, DGEBA resins having both epoxy groups at the ends of the molecule will react more readily with hardeners than will epoxy novolacs or other types of epoxies in which one or more of the functional groups may be hindered by the rest of the molecule.

Homopolymerization occurs readily in the presence of catalysts, especially at elevated temperatures.

Again this reaction is accelerated by hydroxyl groups or tertiary amines. This is also the predominant reaction with anhydrides. In fact, reactions with resin and hardener or catalyst are very much more complex than these idealized reactions, and both reactions as well as a number of side reactions probably occur to varying extents in any cross-linking mechanism. Major suppliers of curing agents include Anchor Chemicals, Dow, Shell, and Cray Valley Products.