GB2167757A - Epoxy-cured toluene formaldehyde-phenol resin varnish and products prepared therewith - Google Patents

Abstract

Varnishes particularly useful for high grade glasscloth laminates can be made from a solution of a toluene- formaldehyde-phenol resin together with specified amounts of an epoxy resin and hexamine.

Description

SPECIFICATION Epoxy-cured Toluene Formaldehyde-Phenol Resin Varnish and Products Prepared Therewith This invention relates to an epoxy-cured toluene formaldehyde-phenol (TFP) resin varnish and to products made therewith. In particular, but not exclusively, the varnish is for use in the preparation of glass fibre laminates.

There is a requirement in industry for a resin varnish for use in the preparation of glass fibre laminates (sometimes called glass cloth laminates) which are suitable for use at temperatures up to about 180 C. One resin system which is presently available for these purposes is a petroleum-based resin marketed under the trade name "Xylok 237" (Xylok is a registered trade mark). Since "Xylok 237" is produced from petroleum-derived chemicals its long term availability cannot be assured. It is therefore desirable to produce an alternative material from coal-based feedstocks.

Toluene, a by-product of coal carbonisation and liquefaction processes, appeared to be a suitable feedstockfrom which a phenolated resin precursor could be prepared for employment in the preparation of glass fibre laminates. It was found that glass fibre laminates, particularly those of thick section, prepared using TFP resins and conventionally cured, blistered and cracked severely during curing or post-curing unless the composition of the resin and the conditions of the preparation were extremely closely controlled.

Our co-pending application No. 2,087,414 is directed to a novel method of preparing glass fibre laminates from epoxy-cured naphthalene-formaldehyde phenol resins, and it has now surprisingly been found that epoxy-cured TFP resins show desirable properties also.

It is therefore an object of the present invention to provide a TFP resin varnish for use in the preparation of glass fibre laminates, particularly those of thick section, which will overcome at least in part the disadvantages of presently used systems.

According to the present invention, there is provided a resin varnish, for use in the preparation of glass fibre laminates, comprising a solution of 100 parts of a TFP resin having a combined oxygen content of about 8% and a combined phenol content of about 40 to 50%, from 60 to 200 parts of an epoxy resin having a epoxy equivalent of about 120 and up to 15 and preferably about 10 parts of hexemthylene tetramine (hexamine). The TFP resin suitably has a number average molecular weight of 500 to 1200. Preferably, the TFP resin is prepared as described in British published Patent Application No. 2,087,414 or 2,043,662.

All parts and percentages in the specification are by weight unless specifically stated to be otherwise.

Preferably there are from 80 to 120 parts of the epoxy resin, which is conveniently a cyclo-aliphatic epoxy resin. A suitable resin is 3,4 epoxycyclohexylmethyl-(3,4-epoxy cyclohexane carboxylate.

The solvent may be a light aliphatic alcohol, such as iso-propyl alcohol (IPA) or a mixture of aliphatic materials, such as industrial methylated spirit (IMS). Preferably, the varnish comprises about 50% solvent, although this may need to be adjusted to provide a laminating solution of required viscosity.

Conveniently, the varnish is provided as a two pack system, the first pack containing a solution of the epoxy resin and hexamine, and the second pack containing a solution of the TFP resin. Preferably, the packs are made up so that, on mixing equal quantities from each pack, the appropriate mixture is provided.

It is envisaged that the varnish will be of particular but not exclusive use in the preparation of thick glass fibre laminates for use as metal clad laminates or printed circuit boards.

Conveniently, glass fibre laminates are made by impregnating glasscloths with the varnish, pre-curing the individual impregnated cloths and stacking the impregnated cloths. Stacks of cloth are put under pressure and the stacks are heated to cure the resin component. It has been found that such laminates, irrespective of their thickness, do not crack or blister during fabrication and do not need to be post-cured.

The present invention also includes laminates made using the varnish described above.

The present invention will now be described by way of example only with reference to four laminate preparations.

A toluene-formaldehyde condensation resin was made by charging water (3.4 kg) and 77% sulphuric acid (18.7 kg) to a stirred reactor. The temperature of the acid was raised to about 35"C and paraform '87' (28.2 kg) was added. The reactor was heated to about 60"C and toluene (25 kg) was added (Molar ratios: formaldehyde to toluene=3: 1; sulphuric acid to toluene=0.54: 1). The reactor was heated to 90"C and the toluene/water azeotrope allowed to reflux in a vertical condenser. The temperature of the reaction mixture rose during the reaction to about 102"C and was held at this temperature for 3 hours. The reactor was then cooled, the aqueous layer separated off and the condensation resin water-washed until substantially neutral.The condensation resin has an oxygen content of 11% and a number average molecular weight of 440.

The condensation resin was then subjected to distillation by raising the mass temperature to 110 to .115 C and using steam at about 130"C. Finally, the pressure is reduced to about 20 mmHg for 30 minutes, to produce an intermediate resin as the residue.

The intermediate resin was phenolated with phenol itself as follows: Phenol (22.5 kg) and 100% para-toluene sulphonic acid (165 kg) were charged to a heated reactor fitted with a reflux condenser. When the reactor temperature reached 85"C, the intermediate resin (27.5 kg), maintained at about 50"C to keep the resin mobile, was slowly dripped into the reactor. The resin addition took approximately 1 hour. The temperature of the mixture rose to about 1 18"C during the addition. After all the resin had been added, the mixture was heated to about 1 400C and superheated steam at about 1 50"C was passed through it to raise the softening point of the phenolated resin.When the softening point of the resin was 1054C (determined by the Ring and Ball method) the phenolated resin was allowed to cool and was dried. The phenolated resin had the properties given below: Softening point 105"C Free Phenol Content 0.9% Combined Phenol Content 48% Oxygen Content 8.3% Number average molecular weight 1050 Four examples of TFP resin varnishes according to the invention were made up using the TFP resin. The composition of each varnish is shown in Table 1 below. All quantities are given in grams. In each case the epoxy resin was 3,4-epoxycyclohexylmethyl-(3,4-epoxy) cyclohexane carboxylate having an epoxy equivalent of 120. In Examples I and IV the solvent was IMS, and in Examples II and Ill it was IPA.

Each of the varnishes was used to prepare glass fibre laminates as follows: Five sheets, each of 304 mm square, were cut from a roll of glasscloth. Each sheet was placed on a glass plate and about 409 of varnish pour onto it. A hand roller was then drawn across the sheet several times to ensure even impregnation of the fabric and to remove excess varnish. The impregnated sheets were suspended freely in air and dried for about 1 hr after which they were pre-cured in an air-circulating oven at 170 C. Each sheet was then cut into four equal squares and the twenty sheets thus obtained were stacked on top of one another between steel plates covered with aluminium foil. The stacked sheets were then subjected to a pressure of 94 kgtsm2 in an hydraulic press and heated to 175for 1 hr to cure the resin component.The pressure was immediately released afterthe 1 her curing time.

None of the laminates blistered or cracked during their preparation or after treatment at 180"C for 250h.

The laminates were tested to determine their cross-breaking strength immediately after their preparation and after treatment at 180"C for 250h. The results obtained are given in Table I and are there compared with similar glasscloth laminates made using "Xylok 237".

It can be seen from this that the coal-based laminates made from the epoxy-cured TFP resin varnish of the present invention are at least equivalent to those made from the petroleum-based "Xylok 237" resin.

TABLE 1

II Ill IV "XYLOK 237" TFP resin 75 75 75 60 Solvent 50 55 55 40 Two Hexamine 6 7.5 6 6 Samples Water 8.4 10.5 8.4 8.4 Epoxy Resin 47 47 47 63 Cross-breaking Strength (Initial) (N/mm2) 551 580 610 605 574 518 Cross-breaking Strength after250 h at 180 C 415 530 510 650 N.D. 481 (N.D.=Not Determined).

Claims (9)

1.A A resin varnish comprising a solution of 100 parts by weight of a TFP resin having a combined oxygen content of about 8% and a combined phenol content of about 40 to 50%, from 60 to 200 parts of an epoxy resin having an epoxy equivalent of about 120 and hexamine in a proportion up to 15 parts, in a solvent.

2. A resin varnish according to claim 1, wherein the proportion of hexamine is up to 10 parts.

3. A resin varnish according to claim 1 or 2, wherein the TFP resin has a number average molecular weight of 500 to 1200.

4. A resin varnish according to any one of the preceding claims, wherein the epoxy resin is present in an amount from 80to 120 parts.

5. A resin varnish according to any one of the preceding claims, wherein the varnish comprises about 50% of solvent.

6. A resin varnish according to any one of the preceding claims, wherein the solvent is a light aliphatic alcohol or a mixture of aliphatic material.

7. A resin varnish according to claim 1, substantially as hereinbefore described.

8. A laminate made using a varnish according to any one of the preceding claims.

9. A glasscloth laminate according to claim 8.