Friday, August 29, 2008

Anhydride Hardeners

The use of organic polycarboxylic anhydrides to cure bisphenol resins has proved to be a very important step in the development of epoxy resins.

The mechanism of the reaction between anhydrides and epoxy resins has been satisfied by Fish and Hofman, who have shown that, predominantly, the curing involves first the reaction of an anhydride group with a hydroxyl group of resin to form a monoester. This leave a free carboxyl group which reacts with an epoxy group to form a diester and a new hydroxyl group. Simultaneously an epoxy group react with a hydroxyl group to form an ether and a secondary hydroxyl group. Thus the cured resin contains diester and other linkage, and monoester, anhydride and hydroxyl group. An important point is that the curing temperature affects the extent to which these different reactions take place and, as a result, the properties of the cured resin.

Considered overall, the most important anhydride used for curing bisphenol. A resin is phtalic anhydride, but in adhesive applications this anhydride is not widely used. It gives slightly lower adhesive strength to metal than for example dicyandiamide, but has a somewhat milder curing cycle. Some of the more valuable anhydride hardeners are those that are liquid at room temperature, such as the aliphatic dodecylsuccinic anhydride and the cycloaliphatic methyl-endomethylenetetrahydrophtalic anhydride (methyl “Nadic” anhydride). Certain others in the form of mixtures, especially in the form of eutetic mixture, have the advantage of low melting points, for example tetrahydrophtalic anhydride (m.p. 102oC) and hexahydrophtalic anhydride (m.p. 35 oC). On the other hand, some anhydride, while forming adhesives with high hot-strength, have themselves very high melting points, making them difficult to dissolve, even if the resin is brought to high temperature. Examples of these are hexachloro-endomethylene-tetrahydrophtalic anhydride, more commonly called chlorendic anhydride or HET-anhydride, (m.p. 283oC0, and pyromellitic dianhydride (m.p. 286 oC). But here also the inconvenience of a high melting point can in some cases be lessened by forming eutectic mixtures with other anhydrides, and this has been especially considered in the case of chlorendic anhydride.

The amount information available on the hot-strength of anhydride cured adhesive is small, but an indication of the behavior of glued joint at elevated temperatures can be obtained from measurements of Deflection Temperature under load, commonly called the heat distortion temperature. This property has been specially studied in the case of a number of anhydrides, and while anhydride with high functionalities produce somewhat brittle adhesives, they do at the same time give higher hot strength, in this respect they are similar to the aromatic diamines.

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