GB2095678A - Epoxy-cured NFP resin varnish and products prepared therewith - Google Patents

Abstract

This invention relates to an epoxy-cured naphthalene- formaldehyde-phenol (NFP) resin varnish for use in the preparation of glass fibre laminates, especially those of thick section. The invention also includes laminates made with the varnish. The varnish comprises a solution of 100 parts of an NFP resin having molecular weight of 400, oxygen content of 8% and combined phenol content of 40%, from 60 to 200 parts of an epoxy resin of epoxy equivalent of 120 and up to 15 parts of hexamine. The varnish is preferably supplied as a two pack system for use in the preparation of thick section glass fibre laminates for use as printed circuit boards.

Description

SPECIFICATION Epoxy-cured naphthalene-formaldehyde-phenol resin varnish and products prepared therewith This invention relates to an epoxy-cured naphthalene-formaldehyde-phenol (NFP) 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 which are suitable for use at temperatures up to about 1 800C. 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.

Naphthalene, a by-product of coal carbonisation and liquefaction processes, appeared to be a suitable feedstock from 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 NFP 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 closely controlled. Our co-pending application Nd. 8,009,948 is directed to a novel method of preparing glass fibre laminates from conventionally cured NFP resins have carefully selected properties.The necessity of closely controlling the composition of the NFP resin and the conditions for preparing laminates limits the application of the invention described in our co-pending application.

It is therefore an object of the present invention to provide an NFP 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 an NFP resin having 3 number average molecular weight of about 400, a combined oxygen content of about 8% and a combined phenol content of about 40%, from 60 to 200 parts of an epoxy resin having an epoxy equivalent of about 120 and up to 1 5 and preferably about 10 parts of hexamethylene tetramine (hexamine).

All parts and percentages in this specification and the appended claims 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 cycloaliphatic 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 (I MS). Preferably, the varnish comprises about 50% solvent, although this will 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 NFP 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.

Conveniqntly, glass fibre laminates are made by impregnating glasscloths with the varnish, precuring 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 thicknesses, do not crack or blister during fabrication and do not need to be post-cured. They are thus superior to most conventionally cured NFP/glasscloth laminates.

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 naphthalene-formaldehyde resin was prepared by mixing paraformaldehyde (a polymer of formaldehyde and water containing about 13% water), naphthalene and an acid catalyst in a stirred reaction vessel. (Molar ratios:-- formaldehyde to naphthalene::2.5:1 and acid catalyst to naphthalene::1 .8:1). The mixture was maintained at 100-1 050C for 42 h, after which time the product naphthalene-formaldehyde resin was isolated. The naphthalene-formaldehyde resin has a number average molecular weight of 470 and an oxygen content of 8.9%.

The naphthalene-formaldehyde resin was phenolated as follows. Phenol (3.32 kg) was placed in a flask and heated under reflux. Paratoluene sulphonic acid (6 g) was stirred into the phenol. When the temperature of the phenol reached 1000C, naphthalene-formaldehyde resin (2 kg) dissolved in toluene was added dropwise so that the exotherm did not exceed 50C. The resin addition took 2 h, after which the mixture was maintained at about 1 000C for a further 1 h. The toluene was removed by distillation under a nitrogen purge and free phenol was removed by steam distillation. The product NFP resin had the following properties.

Number average molecular weight = 380 Free phenol content = 0.3% Combined phenol content = 42% Oxygen content = 8.6% Four examples of NFP resin varnishes according to the invention were made up using the NFP 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 III 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 40 g of varnish poured 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 h after which they were pre-cured in an air-circulating oven at 1 700C.

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 kg/cm2 in an hydraulic press and heated to 1 750C for 1 hr to cure the resin component. The pressure was immediately released after the 1 h curing time.

None of the laminates blistered or cracked during their preparation or after treatment at 1 800C for 250 h. The laminates were tested to determine their cross-breaking strength immediately after their preparation and after treatment at 1 800C for 250 h. The results obtained are given in Table 1, and are there compared with glasscloth laminates made using "Xylok 237".

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

TABLE 1

I 11 Ill IV "XYLOK 237" NFP Resin 75 75 75 60 Two Solvent 45 55 55 40 Samples Hexamine 6 7.5 6 6 Water 8.4 10.5 8.4 8.4 Epoxy Resin 47 47 47 63 Cross-breaking Strength 562 610 630 667 574 518 (Initial) (N/mm2) 562 610 630 667 574 518 Cross-breaking Strength 419 507 573 655 N.D. 481 after 250 hat 1800C (N.D. = Not Determined).

Claims (10)

1. A resin varnish, for use in the preparation of glass fibre laminates, comprising a solution of 100 parts of a naphthalene-formaldehyde-phenol resin having a number average molecular weight of about 400, a combined oxygen of about 8% and a combined phenol content of about 40%, from 60 to 200 parts of an epoxy resin having an epoxy equivalent of about 120 and up to 1 5 parts of hexamine.

2. A varnish according to claim 1, comprising about 10 parts of hexamine.

3. A varnish according to either one of claims 1 and 2, wherein there are from 80 to 120 parts of the epoxy resin.

4. A varnish according to any one of the preceding claims, wherein the epoxy resin is a cycloaliphatic epoxy resin.

5. A varnish according to any one of the preceding claims, wherein the solvent is a lower aliphatic alcohol or a mixture of lower aliphatic alcohols.

6. A varnish according to any one of the preceding claims, comprising about 50% solvent.

7. A varnish according to any one of the preceding claims, wherein the varnish is provided as a two pack system, the first pack comprising a solution of the epoxy resin and the hexamine, and the second pack comprising a solution of the NFP resin.

8. A resin varnish for use in the preparation of glass fibre laminates, substantially as hereinbefore described.

9. A glass fibre laminate comprising a cured resin varnish according to any one of the preceding claims.

10. A glass fibre laminate, substantially as hereinbefore described.