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.

Aromatic Polyamine Curing Time

As assessed by curing time, the relative reactivity in descending order of three commonly used aromatic amine hardeners is metaphenylenediamine (MPD), diaminodiphenylmethane (MDA), diaminodiphenyl sulphone (DDS). MPD can be considered nearly twice as last as MDA; but it is difficult to assign relative values; since end point of curing is not a precise measurement, curing time is a somewhat arbitrary quantity.

Example of the adhesive strength obtainable at different temperatures with two aromatic polyamines compared with the aliphatic polyamine, tri ethylenetetramine are shown in the figure below:

Aromatic Polyamine Hardeners

Interest in aromatic polyamide as hardeners is largely confined to diprimary amines. Most of these are solids, and two that have assumed importance are meta-phenilenediamine and 4,4-diaminodiphenylmethane (methylene dianiline). The later is nowadays an important hardener for adhesives, but it is interesting to note that when originally reported it was described as giving such low bond strength that is could hardly be considered for adhesive use. The reason may have been that the aromatic amines in general give rather brittle cured resins which, if tested by a method that involves much bonding, can appear to have low adhesive strength. The sulfur homologue of this hardener, 4,4-diaminodiphenyl sulfone, has more recently also become fairly widely used, it has an advantage in being of low toxidity-indeed it is administered orally in the treatment of leprosy.

The aromatic polyamine hardeners are of much higher molecular weight, relative to their functionally, than the aliphatic ones and therefore the quantity added to the resin is quite large, sometimes one half the weight, (some of the aromatic amine hardeners are also much more expensive that the resin).

Generally speaking, the aromatic polyamine hardeners, unlike most aliphatic ones, are not very hygroscopic. Under normal circumstances they require heat for curing, preferably a temperature considerably above 100 ^oC. However, some aromatic diamines. In particular 4,4-diaminodiphenylmethane can be cured at a much lower temperature- even at room temperature –by dibutyl phthalate. This addition reduces hot strength, but other desirable properties of the aromatic amine adhesive systems are largely retained. A small amount of a monocarboxylic acid such as acetic acid will also accelerate the curing of aromatic polyamine hardeners.

Room temperature curing is claimed as one of the advantages of certain hydrogenated aromatic amines. Some of these amine (presumably not those capable of curing at room temperature) are said to produce B stage resins, and some, for example, hydrogenated meta-phenylenediamine and 4,4-diaminodiphenilmethane, with a fairly high proportion of their structure converted to cycloaliphatic, have the great advantage of being liquids.

Polyamines Hardeners

This polyamine hardener can use of alkylene series. The amount of the alkylene series of polyamines which needs to be added is usually between 6 and 12 % of the weight of the resin depending on its epoxies content. Diethylenetriamine and triethylenetetramine react vigorously exothermally, a shortcoming that limits the amount of adhesive it is advisable to mix at one time unless steps are taken to dissipate the heat. These amines are hygroscopic, a property that should be borne in mind when working in conditions of high relative humidity. They are also liable to cause skin irritation and dermatitis in some persons, an effect that may be lessened to some extent by using an N-hydroxyalkyl derivative of these polyamines, such as N,N-bis (hydroxyethyl) diethylenetriamine.




Aliphatic amines react to form adhesives that five good strength with a variety of adherents including most metals, glass, wood and mainly plastics. In spite of the fact that certain types are cold curing, all give a higher bond strength if hot cured or post cured, for example, at a temperature of 80oC - 100 oC. In some cases hot curing increases hot strength even more than room temperature strength, but in making comparisons it is important not to overlook the fact that subjecting the glued joint to heat in the course of the test may be equivalent to deliberate post-curing.

Example of the bond strength obtainable with two aliphatic polyamines is given in the table. The effect of curing temperature is well emphasized, even in the case of the highly reactive triethylenetetramine. But a factor that also affects adhesive strength is the exothermic temperature rises (if any) that has been reached before the glue is applied.

Included in the large number of other aliphatic amines that are suitable as hardeners are amino ethers such as butanediol bis(aminopropyl) ether. There is evidence that some of these ethers cure more completely at low temperatures than the alkylenepolyamines.

Particularly valuable amine curing agents which contain both primary and secondary amino groups and also amido groups are the polyamides known as the ‘Versamids’. These polyamides are made be reacting a dimerized drying oil acid and a polyamines, particularly dilinoleic acid and ethylenediamine, and they react with epoxy resins to form versatile adhesives. The most useful Versamid hardener is those that are liquids as distinct from resinous solids, especially Versamid 115. In cases where the resultant viscosity of the adhesive is too high, solvent mixtures of the lower alcohol and hydrocarbons may be used a diluents.

Aliphatic Polyamine Hardeners

The importance of the group of aliphatic polyamine hardeners with primary amine groups lies in the fact these are the hardeners used in adhesive systems capable of curing at normal or slightly elevated temperatures. The most important ones are the homologous alkylene polyamines, especially diethylenetriamine and triethylenetetramine.

The chemical mechanism involved in curing bisphenol A resin with amines (and other compounds) has been discussed by Narracott and by bruin, and been reviewed by Smith. It can be said that, at least in part, a primary amine on reacting with an epoxide becomes progressively a secondary amine and a tertiary amine; since, however, a tertiary amine is known to catalyse polymerization of epoxide groups themselves, it would appear that the curing reaction need not involve crosslinking via molecule of hardener entirely.




Indeed for amines containing both a primary and tertiary amine group, for instance the N,N-dialkyl-1,3-propylene diamines (dialkylaminopropylamines), a curing mechanism involving polymerization of some the epoxide groups may be deliberately invoked by incorporating such a quantity of amine that there are insufficient amino hydrogen to react with all the epoxide groups, curing being completed, usually at elevated temperature, by catalytic polymerization of the remaining epoxide groups. This same mechanism can operate if instead of an amine containing both primary and tertiary groups, a mixture of two amines, one containing only tertiary and the other containing only primary or primary and secondary groups, is used. Again fewer amino hydrogens than are required to react with all the epoxy groups are introduced.

In the alkaline series of polyamines, as the length of the molecule increases the hardener becomes less reactive, thus diethylenetriamine is faster curing that triethylenetetramine, and ethylenediamine is faster still, in fact too fast for most work, and not convenient to use anyway. At the other end of the series, tetraethylenepentamine needs a slightly raised temperature in order to cure satisfactorily. Certain hydroxyic compounds may have an accelerating cure satisfactorily. Certain hydroxylic compounds may have an accelerating effect on curing, for example alcohol and water, but clearly the latter is undesirable, and the former may be.

Other Factor Curing of Bisphenol A

A separate factor that can also have an important effect on the end-product is the temperature of curing; in certain cases the reaction between resin and hardener will not take place at all, or only to a limited extent, unless heat is applied. At the same time complete cure, that is to say theoretically complete cross linking, is virtually impossible because of strict restrictions in the growing macromolecule.

Theoretically, a stoichiometric and therefore calculable amount of hardener is required, but the amount giving optimum properties may be greater or less than this. To achieve the optimum in one respect may mean downgrading in another, hence the “correct” amount of hardener is an arbitrary quantity that depends to some extent on what is required of the cured polymer; this is in some ways an advantage.

The range of chemical substances that react and harden bisphenol A resins, as distinct from those that catalyze homopolymerisation, is wide, and consist primarily of compounds containing active and alkalies, are numerous and include a number of other synthetic resins, such as melamine, urea and phenolic resins. In adhesive applications the most valuable hardeners and phenolic resins.

In the polyamine class both aromatic and aliphatic are important, in the anhydride class both aromatic and aliphatic are important, in the anhydride class aromatic and cycloaliphatic are more important than aliphatic. Another class of hardener that has a limited importance in adhesive comprises the boron triflouride complexes. Each class of hardener is relation to the curing of bisphenol A resin will now be discussed separately.