GB2036044A

GB2036044A - Phenolic resin blends

Info

Publication number: GB2036044A
Application number: GB7846341A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1978-11-28
Filing date: 1978-11-28
Publication date: 1980-06-25
Also published as: GB2036044B

Abstract

A curable resin blend comprises, by weight, (A) 1 part of an aromatic hydrocarbonformaldehyde-phenol (AFP) resin (preferably a toluene- formaldehyde-phenol resin) and (B) 0.5 to 4, preferablyl to 3, parts of a phenolic resin (preferably made using phenol itself). Use: solutions of the blend are used to impregnate paper or cotton sheets for laminating.

Description

SPECIFICATION Phenolic resin blends This invention relates to phenolic resin blends, and in particular to blends of phenolic resins with aromatic hydrocarbon-formaldehyde-phenol (AFP) resins.

Phenolic resins, either in the resol or novolak form, are made by reacting a phenol, generally phenol itself, with an aldehyde, generally formaldehyde, to form a resin. Novolak resins are cured to a crosslinked product by heating with a curing agent, such as hexamethylene tetramine (hexamine). Resol resins may be cured by heating alone, although a small amount of acid catalyst may be added if desired.

The uncured resins in either form are used as lacquers, varnishes or adhesives, and such use is well known.

AFP resins, in which the phenol is generally phenol itself, are also well known and are used in comparable ways for many of the purposes for which phenolic resins are used. In particular the properties of both these types of resin are such that they are suitable for use in the manufacture of components used in the electrical and electronics industries, for instance as electrically insulating cements, in the preparation of laminated cotton or paper circuit boards, etc.

It is known that laminates made from cresol-based phenolic resins have better electrical properties than do laminates made from ordinary phenolic resins. However, cresol is difficult to obtain, and so substitutes for cresol resins have been sought. One such substitute is disclosed in British Patent No.

1187 307 to Sterling Moulding Materials Limited. This discloses a process for the production of a modified phenolic resin, which process comprises condensing together under alkaline conditions a phenol, a styrenated phenol and an aldehyde. Although laminates made from the modified phenolic resin made by this process have good electrical properties, they are expensive because of the cost of providing the styrenated phenol.

It is an object of the present invention to provide a further substitute for cresol-based phenolic resins which is cheaper than other substitutes and is made from easily available materials.

The present invention is based on the discovery that a blend of a phenolic resin (either in the resol or novolak form) and an AFP resin can be used to make a cured paper or cotton laminate which has electrical properties that are unexpectedly better than those of a laminate made from either of the resins separately. The properties are also unexpectedly better than the properties that would have been predicted for a laminate made from the blend. The improvement in properties of the laminate is particularly noticeable in respect of the electrical properties and resistance to water ingress.

Therefore, according to a first aspect of the present invention, there is provided a curable resin blend comprising one part by weight of an AFP resin and from 0.5 to 4, preferably 1 to 3 parts by weight of a phenolic resin.

The phenolic resin may be in the novolak or resol form.

A novolak phenolic resin may be prepared by reacting together one part by weight of a phenol and from 0.6 to 1 parts by weight of an aldehyde. Preferably the phenol is phenol itself, although the resin may be made from a combination of any phenol and an aldehyde, as is already known. The reaction is preferably carried out in the presence of an acid catalyst, for example oxalic acid, present in an amount of from 0.5 to 2.0% of the weight of the phenol. At the end of the reaction, excess reactants may, if desired, be removed, conveniently by distillation. The novolak resin preferably has a melting point (capillary tube method) of from 30 to 11000, a gel time (as a 50% w/w solution in industrial methylated pints (IMS) when measured at 1 300C) of from 5 to 50 minutes and a viscosity (as a 50% w/w solution in IMS at 250C) of from 5 to 250 cP.

A resol phenolic resin may be prepared by reacting one mole of a phenol with from 1.1 to 4, preferably 1.25 to 1.4, moles of formaldehyde in the presence of an alkaline catalyst such as sodium hydroxide or, preferably, ammonium hydroxide. The reaction is carried out at a temperature of from 60 to 11000, preferably from 80 to 900C. At the end of the reaction the product is purified by evaporation under vacuum, typically at a pressure of about 200 mm Hg, at a temperature low enough so that the resol resin does not further condense. The resol resin may be a liquid at room temperature or a solid having a melting point (capillary tube method) of from 30 to 800C. The resol resin preferably has a gel time of from 3-40 minutes (at 1300C as a 50% w/w solution is IMS).A similar solution should have a viscosity of from 5 to 250 cP at 250C.

In the AFP resin, the aromatic hydrocarbon may be a mono- or binuclear aromatic hydrocarbon, and is preferably alkylated. Suitable hydrocarbons include toluene, xylenes, ethylbenzenes, polyalkyl benzenes, naphthalene, and methyl-, ethyl- and polyalkylnaphthalenes. Preferably the aromatic hydrocarbon is either toluene or naphthalene.

The phenol is preferably phenol itself, but may also be a cresol, another polyhydric phenol, xylenol, or another alkylated phenol.

Preferably the AFP resin is prepared in two stages. In the first stage one part by weight of the aromatic hydrocarbon is reacted with from 1.5 to 4.5 parts by weight of formaldehyde in the presence of a catalyst, which may be a protonic acid or a Lewis acid. Advantageously, the catalyst is 55% sulphuric acid, and is present at a level of about 0.6 parts by weight per part of aromatic hydrocarbon.

The unreacted reactants may be removed after reaction, and the catalyst washed out of the product.

The product, if derived from a mono nuclear hydrocarbon such as toluene, should have a molecular weight of about 450, an oxygen content of from 8 to 15% by weight, and a viscosity of about 500 to 1,000 cP at 250C. If a binuclear hydrocarbon is used, these figures will be altered accordingly; as is known in the art.

In the second stage of the preparation of the AFP resin approximately 1 0 parts by weight of the product obtained above, dissolved in toluene or another suitable solvent, is dripped into about 1 0 parts by weight of the phenol. The phenol should contain about 0.5% by weight of an acid catalyst, for instance para-toluene sulphonic acid. The resin is dripped in over a period of about 3 hours and the reaction is maintained at a temperature of about 850C. Unreacted components, such as toluene and phenol, and water produced during the reaction may be removed, conveniently by distillation, to give a yellow-brown AFP resin having a molecular weight of about 460, a combined phenol content of about 40% and an oxygen content of about 12%. (These figures relate to a mono-nuclear aromatic hydrocarbon derived resin.Some of the figures will alter for a binuclear hydrocarbon as is known in the art).

The phenolic and AFP resins may then be dissolved in a solvent, for example IMS, and mixed in the appropriate proportions to give a blend according to the invention. If the phenolic resin is in the novolak form, from lotto 12% by weight of the combined resins of a curing agent, such as hexamine will need to be added to the blend to enable it to cure. If the phenolic resin is in the resol form it will not be necessary to add a curing agent, although it may be desirable to add up to 5% of an acid to catalyse the curing.

The blends may be used to form laminates. For instance dried cotton or paper sheets are impregnated with the dissolved blend and heated at a temperature of about 1 300C to bring the resin to the B-stage. The sheets are then stacked, pressurised and heated to about 1 750C to form a cured laminate.

According to a second aspect of the present invention there is provided an artefact, for example a paper of cottom laminate, comprising a cured resin blend according t6 the first aspect of the invention.

The invention will now be described by way of example only, and hereinafter all parts and percentages are by weight unless otherwise specified.

a) Toluene/Formaldehyde/Phenol Resin.

2 parts of formaldehyde, in the form of paraformaldehyde (87% formaldehyde, 13% water) and 0.6 parts of 55% aqueous sulphuric acid are placed in a flask and one part of toluene is added thereto after the formaldehyde/acid mixture has equilibrated. The resulting mixture is heated with stirring under reflux for 3 hours. At the end of the resinification period toluene is added to the flask to dissolve the resin. The aqueous layer which is formed during the reaction is separated from the resin solution which is then washed to remove the catalyst. The resin solvent may be removed, if desired for instance by distillation. This leaves about one part of a yellow liquid toluene-formaldehyde condensation resin having a molecular weight of about 400.

1 11 parts of the toluene-formaldehyde resin prepared above are dissolved in toluene and are dripped over a period of about 1 hour into a flask containing 8 parts of phenol and 0.5% (by weight of the phenol) of para-toluene sulphonic acid. The reaction is allowed to continue for about 1 5 minutes after the resin has been totally added. The reaction product is then purified by distilling off residual phenol and the toluene solvent while a stream of air is blown through the product. This gives a final brown toluene-formaldehyde-phenol resin having a molecular weight of 480, a viscosity (as a 50% w/w solution in IMS at 250C) of 70 cP and a softening point of 540 C.

b) Phenolic Resin Novolak.

1 part of phenol and 0.8 parts of formaldehyde are mixed together with 1.5% w/w (based on the weight of the phenol) of oxalic acid, and are heated with stirring under reflux for one hour. Excess reactants are then distilled off to give a yellow phenolic novolak having a molecular weight of 440, a free phenol content of 7% and a softening point of 720C.

c) Phenolic Resin Resol.

1 part of phenol and 1.3 parts of formaldehyde are mixed together with 1.5% (based on the weight of the phenol) ammonium hydroxide and are heated to 850C for one hour. Vacuum (200 mm Hg) is then applied to remove excess reactants and water produced during the reaction. The product is a liquid resol phenolic resin having a molecular weight of about 480, a gelation time (as a 50% w/w solution in IMS at 1 300C) of 20 minutes and a viscosity (as a 50% w/w solution in IMS at 250C) of 100 cP.

d) Blended Products.

Each resin was then dissolved in IMS to product a 50% solution. These solutions were used either separately or mixed in varying proportions to form cotton or paper laminates. If the blend contained resol phenolic resin no curing agent was added but about 1% (based on the weight of the resin) of an acid catalyst was added. In all other cases the blends contained 10 or 12% (based on the total weight of the resin) hexamine.

Laminates were made in the following way. Paper or cotton sheets were air dried and separately impregnated with the resin blend solution. The separate sheets were placed in an oven at 130 C for 10 minutes, causing the blended resin to gel to the Stage. The sheets were piled on top of each other, usually with ten or twelve sheets in each laminate, and were heated at 1 750C under a pressure of about 7 x 106 Pa for up to 90 minutes to complete the curing of the resin. The laminates were cooled while still under pressure.When the laminates were cool, the pressure was released, and the electrical properties of the laminates were tested according to BS 2782:1970. The results of these tests are shown in the table below.

Phenolic Hexamine Resin content Water Breakdown Laminate T.F Phenol Phenolic Resin (% based) of laminate Absorbtion Dielectric Power Voltage Material Resin (Parts) Resin (Parts) (on resin) (%) % Constant Factor (KV/mm) Paper 1 - - 12 33.9 17.3 4.64 0.108 28.6 " - N 1 10 52.9 7.9 6.33 0.166 16.5 " 1 N 1 10 44.1 12.6 4.16 0.060 27.8 " 1 N 3 10 45.5 9.1 3.99 0.054 32.0 Cotton 1 - - 12 32.4 6.8 4.50 0.160 31.2 " - N 1 10 64.6 1.0 6.57 0.096 13.6 " 1 N 1 10 52.5 1.3 4.23 0.030 16.0 " 1 N 3 10 60.3 0.9 4.70 0.040 20.3 Paper - R 1 - 50.0 3.0 4.65 0.047 21.3 " 1 R 3 - 47.0 2.4 3.90 0.022 27.7 Cotton - R 1 - 61.0 N.D. 6.67 0.140 11.8 " 1 R 3 - 50.0 0.8 6.73 0.082 8.6

All parts and percentages by weight. T - Toluene F - Formaldehyde R - Resol N - Novolak The table compares the properties of the laminates made from the phenolic and AFP resins alone with those made from blends according to the invention. It can be seen from the table that the blends produce laminates having much better all round electrical properties. In general the power factor, dielectric constant and water absorption of the laminates are desirably low and the breakdown voltage desirably high, although individually, in some cases, the unblended resins provide a single better property. The invention therefore provides an improved composition for use in making electrical laminates. Moreover since these blends may be made cheaply from commonly available materials, they are economically more useful than previously used compositions which may have better properties.

Claims

1. A curable resin blend comprising one part by weight of an AFP resin and from 0.5 to 4 parts by weight of a phenolic resin.

2. A blend according to claim 1, comprising from 1 to 3 parts by weight of the phenolic resin.

3. A blend according to either claim 1 or 2, wherein the phenolic resin is in the resol form.

4. A blend according to either claim 1 or 2, wherein the phenolic resin is in the novolak form.

5. A blend according to claim 4, and including from 10 to 1 2% of a curing agent.

6. A blend according to claim 5, wherein the curing agent is hexamine.

7. A blend according to any one of claims 4 to 6, wherein the phenolic novolak is made by reacting together 1 part by weight of a phenol and from 0.6 to 1 parts by weight of formaldehyde.

8. A blend according to claim 7, in which the novolak is prepared in the presence of an acid catalyst.

9. A blend according to claim 3, wherein the phenolic resol is made by reacting together one mole of phenol and from 1.1 to 4 moles of formaldehyde.

10. A blend according to claim 9, wherein the phenolic resol is made in the presence of a basic catalyst.

11. A blend according to any one of the preceding claims, wherein the AFP is made by reacting together about 1 part by weight of a phenol and 1 part by weight of an aromatic hydrocarbonformaldehyde resin, in the presence of an acid catalyst.

1 2. A blend according to claim 11 , wherein the aromatic hydrocarbon-formaldehyde resin is made by reacting together one part by weight of the aromatic hydrocarbon and from 1.5 to 4.5 parts by weight of formaldehyde in the presence of an acid catalyst.

13. A curable resin blend, substantially as hereinbefore described.

14. An artefact comprising a cured resin blend according to any one of the preceding claims.

1 5. A paper laminate comprising a cured resin blend according to any one of claims 1 to 1 3.

1 6. A cotton laminate comprising a cured resin blend according to any one of claims 1 to 1 3.