Accelerators for the Cure of Epoxy Resins

Epoxy resins can be cured by acid anhydrides and the application of heat. Typical of the anhydrides used are those of phthalic, hexahydrophthalic, succinic, dodecenyl succinic, maleic, the adduct of methyl cyclopentadiene and maleic, methyl tetrahydrophthalic and trimellitic acid and 3,3', 4,4'-benzophenone tetracarboxylic dianhydride. The anhydride ring must open for cure to occur and this can be accelerated or caused to happen at lower temperatures by the addition of small quantities of proton containing compounds e.g. amines. Typical conventional cure accelerators used in epoxy/anhydride systems are benzyldimethylamine, 2,4,6,-tris(dimethylaminomethyl)phenol, 2-ethyl-4-methylimidazole ##STR1## and stannous octoate.

Epoxy resin will also react with carboxylic acids at elevated temperature and this fact is made use of in powder coatings where solid epoxy resins are mixed with solid polyester resins which contain free carboxyl groups either at the ends of chains or on side chains and heated to effect cure. In this application accelerators are often used and amongst those described in the literature are choline chloride (EP-A-0010805) and heterocyclics e.g. imidazole and 2-methylimidazole (GB 1516077 and GB 1558303).

We now find that benzotriazoles including those with aromatic ring substitution behave as cure accelerators or catalysts for the reaction between compounds containing epoxy groups and carboxylic acid compounds, such as compounds which are in the form of polyesters with free carboxyl groups at the ends of chains or on branched chains.

Preferably, such benzotriazoles have the formula: ##STR2## where R is absent or is lower alkyl such as C.sub.1-4 alkyl, e.g. CH.sub.3.

The group R may assume any position on the benzene nucleus, and a mixture of isomers may be used.

In particular excellent powder coatings can be obtained from epoxide resins based on bisphenol A such as Epikote.RTM. 1055 or Araldite.RTM. GT 7004 or epoxy-containing compounds such as triglycidyl isocyanurate (e.g. Araldite.RTM. PT 810). Other types of epoxy resins can be used e.g. those based on novolaks having e.g. a melting point above 50.degree. C.

Typical polyester resins that can be used are those based on a symmetrical glycol e.g. neopentyl glycol and one or more aromatic dicarboxylic acids e.g. isophthalic acid or terephthalic acid, the acid content optionally additionally being provided by an aliphatic mono or dicarboxylic acid e.g. pelargonic acid, adipic acid or trimellitic anhydride. The polyester should preferably have an acid value of 30-100 mg KOH/g, a melting point of 50.degree.-70.degree. C. and a melt viscosity of 10-14 poise at 200.degree. C. (as measured using an I.C.I. Cone and Plate Viscometer).

The powder coatings can include in addition to epoxy, polyester and cure accelerator, pigments to obtain the desired colour, flow control agents and light stabilisers.

Preferred reactions embodying the invention will now be described with reference to the following Examples, which include a comparison between the performance of an accelerator used in a reaction embodying the invention and reactions carried out with conventional accelerators.

A polyester was prepared from 5.3 mole neopentyl glycol and 4.3 mole terephthalic acid reacted at 200.degree.-240.degree. C. to an acid value less than 10 mg KOH/g. 1 mole of trimellitic anhydride was added and further reaction carried out at 190.degree.-200.degree. C. until an acid value of 80-90 mg KOH/g was reached. On cooling, a solid resin was obtained which could be easily powdered.

A series of powder coatings were made using:

After dry blending, the constituents were passed through an extruder at 100.degree. C. and ground to a powder. Gel times of the various formulations were determined at 155.degree. C. using the "stroke gel time" on a hot plate (B.S. 2782 Method 835D: 1980). The results, shown in FIG. 1, indicate that benzotriazole is a cure accelerator and that it is even more efficient than choline or imidazole.

to a temperature of 200.degree.-240.degree. C. until an acid value of below 10 mg KOH/g was obtained. The water was present to allow a slurry to be formed so that the reaction mixture could be stirred. 141 g of trimellitic anhydride was then added and further condensation carried out until an acid value of 80.+-.5 mg KOH/g had been reached.

The solid polyester had a melting point of 57.degree. C. and a melt viscosity of approximately 20 p at 200.degree. C.

A powder coating was prepared from the above resin by powdering and dry blending the following materials:

The mixture was then extruded at a melt temperature of 120.degree.-125.degree. C. and the cooled product ground to give a powder paint with a particle size of below 100.mu.. It can be electrostatically sprayed and cured at a temperature of 200.degree. C. for 10 minutes.

It is desirable to reduce the temperature and duration of cure to save energy costs and this can be done by incorporating a cure accelerator. The stroke gel time is a measure of the time needed to cure. The results are shown in Table I.

These figures again show the greater efficiency of benzotriazole as a cure accelerator as compared with the conventional accelerators choline chloride and imidazole.

A polyester resin was prepared by condensing the following at 200.degree.-240.degree. C. until an acid value below 20 mg KOH/g was obtained.

Mixtures of 90 pbw of the above polyester, 10 pbw Araldite.RTM. PT810 (triglycidyl isocyanurate), 50 pbw titanium dioxide pigment, 1 pbw flow control agent and 0, 0.1% and 0.3% benzotriazole were made, extruded and powdered.

The stroke gel times of these combinations are shown in Table II.

A polyester resin was prepared by heating:

to a temperature of 200.degree.-240.degree. C. until an acid value below 10 mg KOH/g was obtained. The water was present to allow a slurry to be formed so that the reaction mixture could be stirred. 141 g of trimellitic anhydride was then added and further condensation carried out until an acid value of 80.+-.5 mg KOH/g had been reached.

The resin was mixed with an equal quantity of Araldite.RTM. GT 7004 epoxy resin and powdered.

To two portions of this mixture 0.5% by weight of tolyltriazole (Sandoz TT-100) and benzotriazole were respectively added. Gel times were determined at 155.degree. C. and 180.degree. C. using the stroke gel time on a hot plate (B.S. 2782: Method 835D).

It can be seen that the gel time is greatly reduced when using benzotriazole or tolyltriazole in a reaction in accordance with the invention as cure accelerator.