GB1589744A

GB1589744A - Resin impregnation of a fibrous cellulose sheet

Info

Publication number: GB1589744A
Application number: GB34620/77A
Authority: GB
Original assignee: Dynamit Nobel AG
Current assignee: Dynamit Nobel AG
Priority date: 1976-08-20
Filing date: 1977-08-17
Publication date: 1981-05-20
Also published as: AT362937B; FR2362002B1; DE2637494A1; NL7709217A; BE857949A; FR2362002A1; JPS618774B2; SE7709314L; IT1115918B; DE2637494C2; ATA597377A; JPS5325674A

Description

(54) RESIN IMPREGNATION OF A FIBROUS CELLULOSE SHEET (71) We, DYNAMIT NOBEL AKTIENGESELLSCHAFT, a German Company of 521 Troisdorf bez Koln, Postfach 1209, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of treating a sheet of fibrous material with resin.

Laminated plastics sheets are conventionally produced by impregnating a fibrous, for example cellulose-based, material with a binding resin, generally a phenolic and/or epoxy resin, which if required may contain a plasticiser and/or a fire-retarding agent. The impregnated material is then dried, thus pre-condensing the resin; thereafter several layers of this pre-condensed material are placed one on top of the other and hardened under the application of heat and pressure, e.g. in a platen press, to form the laminated and moulded plastics sheet. As the fibrous material, it is preferred to use paper in the form of sheets, fibre layers or even fleeces, or webs of, for example, cotton, to form laminated plastics sheets for electrical insulating materials. By way of example, to form carriers for printed circuits, individual layers of a hot-setting adhesive can be coated on the laminated plastics sheet and a metal foil, preferably a copper foil, applied thereto in the same operation as that in which the laminated sheet is moulded.

When laminated plastics sheets, in particular impregnated hard paper sheets are to be used in electrical engineering, for example in radio and television sets, it is important that the hard paper sheets have very good properties of electrical insulation, high mechanical durability, and are suitable for processing into printed circuits.

For electrically insulating materials the following requirements should generally be met: a) Insulation resistance between the plugs after 4 days storage at 40"C and 92% relative humidity: 10'Q (DIN 53 482). b) Electrolytic corrosive effect on metals after 96 hours at 40"C and 92% relative humidity: characteristic value equal to or less than AB 1.6 (DIN 40 802). c) Break-down resistance (gradient method according to NEMA) after pre-treatment for 48 hours at 500C: water storage equal to or greater than 15 KV (NEMA LJ 1/10). d) Water absorption after 24 hours at 230C: < 40 mg (DIN 7 735). e) Ability to be cut by hammer shears at room temperature: cutting edges smooth and free from tears (own method). f) Ability to be punched at room temperature: characteristic value equal to or less than 2.5 (DIN 53 488).

When the laminated sheet is to be widely used it is desirable for sheets having the properties mentioned above to be produced by a simple manufacturing process. The production process should also ideally not have a detrimental effect on the environment.

Conventionally, the impregnation of cellulose based fibrous sheets takes place according to two different processes, these being one stage and two stage processes respectively.

In the one stage process the sheet is impregnated with a synthetic resin solution which may also contain a plasticiser and a fire-retarding agent. The synthetic resin may be a phenol and/or cresol-resol or also an epoxy resin. Depending on the type of resin used, the solvents used may be either alcoholic, generally methanolic, or acetonic. In addition, water from the resin production process may be present as a solvent. In a single stage impregnating process of this type, the sheet first passes through a bath of the impregnating solution. After excess resin has been squeezed out or scraped off, the coated sheet passes immediately into a drying channel where first of all the solvent is removed and then the precondensation of the resin takes place. As it leaves the drying channel the impregnated material is either wound onto rollers or immediately cut into mouldable prepregs.

Although the one stage impregnating process is economical, the laminate sheets obtained do not simultaneously fulfil all the requirements a) to f) above of electrical insulators. For example, when using impregnating solutions of a phenol-cresol resin, which can be plasticised to a limited extent only and which contains a relatively large number of low molecular weight components, requirements a), b) and c) can be met, but not the capability for cold cutting e) and also in many cases not the low water absorption d). While it is possible to achieve a compromise in the case of the requirement for cold punching f) by means of the selection of the best resin plasticisers, it is not possible to meet the cutting requirement e) at 23 C, but only after heating.

By effecting the one stage process with solutions of elastified impregnating resins, such as cresol- or phenol-resols modified with wood oil or modified epoxy resins, it is possible to produce sheets having a good cold cutting capability and cold punching capability by the optimal selection of the raw materials; however this does not simultaneously succeed in meeting the electrical requirements a), b). c) and d), because there are insufficient low-molecular weight components present. The single stage impregnation process using resins modified with wood oil or modified epoxy resins causes particular difficulties because there is only a very limited compatibility with the aqueous alcoholic phenol-resols which are required in order to fulfil requirements a) to d). The separation which occurs because of the limited compatibility hinders the production of laminated sheets having good layer bonding.

In the two stage process the sheets are first immersed in a pre-impregnating solution and subsequently dried. Following this the sheets are impregnated, in a second impregnating process, with another synthetic resin solution and then dried for a second time. The paper sheet therefore passes through four successive process operations.

The pre-impregnation is usually effected with phenol-resol solutions which generally do not contain flame-retarding agents or plasticisers. This low-molecular weight preimpregnating resin is able rapidly to soak the paper sheet and thoroughly to impregnate the cellulose structure.

The synthetic resin solutions used generally contain from 10 to 20% by weight of phenol-resol solids and from 90 to 80% by weight of part alcoholic, part aqueous solvent.

The pre-impregnating operation ensures an efficient impregnation with low molecular weight components and therefore leads to fulfillment of requirements a) to d) but results in a deterioration in the properties e) and f). The ensuing subsequent impregnating operation is carried out with the relevant desired coating resin, either cresol-resol or epoxy resin. The coating resin prepared is made as elastic as possible in order to aid the achievement of the requirement of cold cutting capability e) and of cold punching capability f) in the finished sheets.

The two-stagc process has the following commercial and technical disadvantages.

1) The cold cutting capability and cold punching capability cannot be so well adjusted as with the one stage impregnating and drying process.

2) It is necessary in the two stage process for the paper sheet to be passed through an impregnating machine twice; bccause of this the efficicncy of the impregnation is more than halved and the costs are consequently doubled.

In order to obtain a resin coat, it is necessary to remove the solvents by drying, after the first impregnating operation. The exhaust gases from the drier consequently have a high concentration of solvents. Accordingly there is a higher consumption of solvents, an increased energy requirement for the drying operation, and also the need for a large ifter-burning plant to remove the solvents from the exhaust flow.

According to the present invention. there is provided a method of treating a sheet of fibrous cellulose material. which comprises (i) contacting the sheet, in a first operation, with a first resin solution in an amount such that from 4 to 2() parts by weight of solid resin per 100 parts by weight of sheet penetrates the sheet, the resin solution comprising from 5 to 25% by weight of solid resin and from 20 to 50% by weight of water; (ii) impregnating the sheet, in a second operation, with a second resin solution, the second resin solution being a solution of (a) a phenol resin modified by the presence therein of structural units derived from a fatty acid or a fatty acid ester, (b) a cresol resin modified by the presence therein of structural units derived from a fatty acid or a fatty acid ester, (c) a water-free epoxy resin, or (d) an elastic resin containing up to 100 monomer units; and (iii) drying the impregnated sheet; there being no drying operation intermediate the first and second operations.

The treated sheet may then be subjected to heat and pressure to mould and harden the same. Several sheets may be heated and pressed to form a multilayer hardened laminate.

The fibrous material employed in the method of the invention may be for example of natural cellulose or cellulose altered by chemical means, such as paper, fleeces, fabric, wefts or fibre layers.

The weights of the sheets employed are preferably from 60 to 240, more preferably 90 to 180 g/m2.

It is preferred that the amount of resin of the first resin solution which penetrates the sheet is from 4 to 16, more preferably from 4 to 14, most preferably 4 to 12, parts by weight of solid resin per 100 parts by weight of dry sheet. Surprisingly, it has been found that these relatively small amounts of resin are sufficient to achieve an almost complete soaking of the sheet in a continuous process employing short treatment times of approximately 5 to 10 seconds. It has also been found that these relatively small amounts of penetrating resin, in combination with the resin applied in the second operation, are generally sufficient to produce treated sheets which can be moulded into a laminate which fulfils the previously mentioned requirements a) to d) and which can also be cut and punched when cold.

The first operation, or penetrating process, of the method serves to apply to the sheet a fixed amount of resin solution which is less than that corresponding to the absorptive capacity of the sheet, for example paper, used. Thus in carrying out the method, in order to control the amount of first (penetrating) resin solution applied, apportioning devices are used; this contrasts with the known two stage processes in which the first stage, the pre-impregnation, is effected by impregnating the sheet until it is saturated with the resin.

A suitable contacting of the first resin solution with the sheet can be provided in a variety of ways in the case of a vertically or horizontally running sheet, for example by means of transfer rollers with controllable rotational speed or by means of so-called reverse roll coaters; the amount of first resin solution contacted with the sheet may additionally or alternatively be determined by the appropriate selection of the type and concentration of the resin solution.

The first resin solutions should preferably be incompatible, or only compatible in special cases, with the second resin solutions used in the second (impregnation) operation.

Preferably, the first resin- solution comprises an aqueous ketone and/or an aqueous alcohol having a comparatively low resin content. A high water content of from 20 to 50% by weight is preferred. First resin solutions with from 7 to 20% by weight solid resin, from 25 to 40% by weight water and from 25 to 75%, preferably from 40 to 73%, by weight non-aqeuous solvents relative to the total weight of the resin solution are preferably used in the method of the invention.

With regard to the incomplete saturation of cellulose webs, first resin solutions containing at least 8 %, preferably at least 10 % by weight of solid resin are preferred in order to achieve the desired degree of penetration.

The resins of the first (penetrating) solutions are preferably resins, for example phenol-resol resins, consisting of not more than four monomer units.

As non-aqueous components of the liquid phase of the solutions there may be used, for example, ketones such as acetone or methyl ethyl ketone, alcohols such as methanol, ethanol or isopropanol, glycols and glycol ethers, such as methyl glycol or ethylene glycol monomethyl ether, dimethyl formamide, as well as other solvents which are compatible with water.

Surprisingly, it has been found that despite the high water content of the preferred first resin solutions, sheets penetrated with such solutions are compatible with the second resin solutions used in the second operation according to the invention to impregnate the sheet without the aqueous solvents previously being dried off. This compatability has been found to be imparted even if the second (impregnating) solution is not itself compatible with the first (penetrating) solution.

The second operation or immersion process takes place immediately after the first operation or penetrating process, i.e. with no intermediate drying operation. lt has been found that a paper sheet soaked by the penetrating resin will generally receive large applications of impregnating resin. In this case some 100 to 150 parts by weight of solid resin, derived from both the first and second operations, relative to 100 parts by weight of sheet, can be absorbed without the solvents of the first resin solution being previously dried out. It has also been found that the impregnating solution is not diluted by taking up the solvents of the penetrating solution even where the second operation is performed by immersion of the penetrated sheet in the second resin solution for a relatively long period.

In the conventional two stage process, if the subsequent impregnating solution is applied immediately after immersion of the sheet in the pre-impregnating solution without intermediate drying, there results the disadvantage that on the one hand the paper sheet saturated with the pre-impregnating resin is not able to absorb sufficient quantities. of the impregnating resin to obtain the desired resin content of 120% or more in the end product, and on the other hand during the impregnating stage, the concentration of the solvents of the pre-impregnating resin solution and the solid resin concentration in the impregnation bath would be reduced because said solvents would be taken up by said bath. The present invention avoids this latter disadvantage because although the amount of first resin solution used to contact the sheet is sufficient to soak the sheet, i.e. to pre-impregnate it with a predetermined amount of resin, the sheet is not saturated and so the second resin solution is able to impregnate the penetrated web in the second operation.

By virtue of the soaking action without saturation of the first operation, i.e. by virtue of the only partial impregnation of the sheet with the resin. the second (immersion) operation may be carried out using second resin solutions of higher solid matter content. Whereas the achievement of the requirements a) to d) in the product is substantially determined by the conditions of the first operation, the properties of the product as regards the ability to be cut and punched when cold are substantially determined by the selection of the second resin solution in which the penetrated sheet is immersed. For this reason a very well modified elastic resin is preferably used as the second (impregnating) resin, for example in the form of a phenol- or cresol-resol resin modified with wood oil or of a modified epoxy resin.

To assist the impregnating action of the second operation an unmodified phenol- or cresol-resol resin, preferably from 5 to 25%, more preferably from 10 to 20% may be added to the modified elastic resin.

In a preferred embodiment the solid resin content of the second resin solution is generally from 40 to 80, more preferably from 45 to 70, and most preferably from 50 to 60 % by weight based on the resin solution.

The impregnating resins may be anhydrous or of low water content, for example containing up to 10%, preferably up to 5% by weight of water, these proportions having no effect on the action of the resin solution. In the case where epoxy resins are used, these are anhydrous or of very low water content.

It is preferred that the resin of the second resin solution is of relatively high molecular weight. Typical high molecular weight resins are for example the oil-modified phenol-resol Dynamit Nobel T 323 and the unmodified resin Dynamit Nobel T3V which have polymer chains of generally up to some 30 to 40 monomer units, and frequently up to some 80 to 100 monomer units (according to gel chromatographic analysis). If desired it is possible to use solutions having higher degrees of condensation and resin concentrations. Suitable modifying means are. for example. wood oil, caster oil and esters of higher fatty acids.

The non-aqueous solvents in the second resin solutions may be entirely conventional, for example those mentioned previouslv as suitable for the first resin solutions. The proportion of solvent in the second solution is preferably from 20 to 6() C/? by weight based on the total weight of the solution.

If it is desired to form a flame resistant end product the first and/or second resin solutions may contain a flame retarding agent. They may also contain a plasticiser.

Adipic acid esters, phthalic acid esters and phenoxy acetals, such as diphenoxy ethyl formal, and in particular phosphoric acid ester, such as diphenyl cresyl phosphate or tricresyl phosphate are particularly suitable as plasticisers. The amount of plasticiser included preferably amounts to up to 10%, more preferably up to 5% by weight of the resin solution.

Flame retarding agents are preferably included in amounts of up to 10% by weight of the resin solution. Brominated phenyl ether has proved particularly suitable as a flame retarding agent.

The impregnated sheet may be dried in conventional manner, for example in a heating tunnel at from 160 to 180 C.

Prepregs cut from the sheet may then be moulded in a hot press, preferably at a pressure of from 80 to 120 Kp/cm2 at a preferred temperature of from 150 to 180 C for, for example, from 40 to 90 minutes.

Products formed from sheets treated by the method according to the invention generally achieve the quality of laminates produced by the known two stage process, but, of course, the capacity of the drying plant is doubled.

It is a particular advantage of the method of the invention that the amount of non-aqueous solvents required may be reduced. The recovery of these solvents from the exhaust gases of the drying plant is therefore simplified, or it may be effected with the plant working at a lower capacity.

The desired, important purity of the exhaust air is therefore easily achieved.

If the solvent vapours are burned, the capacity of the kilns can also be reduced and an improved purity of the exhaust air can be achieved respectively.

'I he following Examples, of which Examples 4. 5, 9 and 1() are by way of comparison, serve to illustrate the invention Examples 1 to 10 The laminates listed in the following Table were produced in the following manner: A sheet of cotton-linters-paper was unwound from its roll and conducted over a transfer roller which partially embraced it. The rotational speed of the transfer roller was adjusted so that its peripheral speed was approximately 50% greater than the speed of travel of the sheet. The amount of resin applied to the sheet was adjustable by the regulation of this peripheral speed. The underside of the transfer roller was immersed in a tank containing (in Examples 1 to 3 and 6 to 8) the penetrating (first) resin solution, the composition of which was kept constant by means of a circulation pump.

The fixed amount of penetrating resin thus applied to the sheet is indicated in the Table under section I as "lacquer coat", calculated as parts by weight of solid resin per 100 parts by weight of the sheet.

Approximately 1 to 2 m after leaving the penetrating (first) solution rollers the penetrated sheet was carried, whilst still in a moist condition, i.e. without intermediate drying, into a subsequent impregnation plant. Here, an impregnation, listed under Section II in the Table, was carried out by immersion. Alternatively, it is possible to apply the same amount of lacquer coat to produce a laminate with the same property values by means of, for example doctor blades, coating, squeeze rollers or spraying.

On leaving the impregnation plant the sheet was passed directly into a drying channel and, after the solvents had been dried off, was polymerized on the desired flow of resin.

In the case of comparison Examples 4 and 9 there were no penetration and impregnation operations. Instead the entire amount of resin was applied to the sheet in one operation according to the one stage immersion impregnation process.

The resin solutions used and the amounts applied are as listed in section II of the Table.

In Comparison Examples 5 and 10 the application of the pre-impregnating or penetrating resin was carried out by immersion with subsequent heating in the drying tunnel. Following this the subsequent impregnation was carried out by immersion, under the conditions listed in section II of the Table, followed by another drying operation.

For better comparison the amount of resin applied in the comparison Examples was the same as that applied in each of Examples 1 to 3 and 6 to 8.

The resin used in the initial penetrating operation of Examples 1 to 3 and 6 to 8 was a low molecular weight aqueous phenol resol. Typical low molecular weight resins of this type contain oligomeric particles of up to 3 to 4 monomer units (according to gel chromatographic analysis), for example resin 2447 of Bakelite, a low-molecular weight aqueous phenolic resin with approximately 15% free phenol and approximately 3% free formaldehyde (the word "Bakelite" is a registered Trade Mark).

The penetrating resin which was used in the first (penetration) operation of Comparison Examples 5 and 10 was the same as the impregnating resin, Dynamit Nobel T323, which was used in the second (impregnation) operation of Examples 1, 2 and 3 according to the invention and comparison Examples 4 and 5. This was a cresol resol modified with wood oil, having a solid matter content of 50% dissolved in an acetone/isopropanol solvent.

The impregnating resin used in Examples 6 to 10 was Bakelite 0139, a brominated epoxy resin modified with fatty acid esters. To the 150g of epoxy resin there was added 15 g of a paste made of 710/0 by weight Sb2O3 and 29% by weight of the same epoxy resin.

Examples 11 and 12 Examples 1 and 2 were repeated, with the exceptions that a bleached sulfate paper of density 120g/m2 was used as the sheet and resin solutions with 58% and 65% by weight respectively of methanol instead of acetone were employed. The amounts of resin used were the same, but the application was effected by means of doctor blades. In the impregnating solution used in the impregnation operation tricresyl phosphate was used as plasticiser. The drying operation took place at 1700C.

The measured characteristic values of the laminates produced in Example 11 and 12 corresponded to those of Examples 1 and 2 respectively.

In order to produce a 1.5mm thick laminate, 8 prepregs were used in each case, and as indicated below these were moulded to form a laminated plastics sheet. At the same time a copper foil coated with hot setting adhesive was applied to the prepegs to form a 1.5mm thick laminate coated with copper on one side.

In the Table, properties a) to f) of Examples 1 to 3 according to the invention are compared with Comparison Examples 4 and 5; Examples 6 to 8 are compared with Comparison Examples 9 and 10. Examples l to 3 according to the invention show basically better physical properties a) to e) than the laminate of Comparison Example 4 which was also produced by a process which included only one drying operation. In contrast to that of Comparison Example 5 the laminates of Examples 1 to 3 produced according to the invention show improved capacity for machining by hammer shears. The other physical properties a) to d) of the laminate of Example 5 are, though, up to the required standard.

Similar advantages are seen in the case of Examples 6, 7 and 8 produced according to the invention, compared with Comparison Example 10.

In order to measure the properties listed in the Table the webs were cut and in each case 8 prepegs were moulded to form a laminate 1.5mm thick. The moulding temperature was 170 C, the moulding pressure 1()() bar and the cooling time 60 minutes; the moulding time was 6() minutes for Examples 1 to 5 and 100 minutes for Examples 6 to 10.

The prelimin;iry treatment of the laminates for the tests according to a) and b) took place in a conditioning cabinet for 96 hours at 40 C and 92% humidity, and for the tests according to c) and d) by immersion in water for 48 hours at 50 C, there being no preliminary treatment for the tests according to e) and f).

TABLE UNIT Theoretical Examples Phenolic resin laminates Examples Epoxy resin laminates value of the Comparison Examples of the Comparison Examples invention invention 1 2 3 4 5 6 7 8 9 10 a) Internal resistance # (X 1010) # 1010 4 0.5 6 0.03 6 8 1 20 0.05 9 b) Electrolytic Characteristic # 1 6 1.4 1.6 1.4 1.8 1.4 1.6 1.2 1.8 1.4 corrosion value c) Break-down kV > 20 > 40 25 > 45 14 > 40 > 40 > 30 > 50 12 > 40 resistance d) Water absorption mg # 40 21 28 20 30 18 20 25 28 35 17 e) Cutting capability C tear- 23 23 23 23 45 23 23 23 23 23 by shears free f) Punching capability C # 2.5 23 23 23 23 23 23 23 23 23 23 Raw Paper Weight g/m2 - 120 120 120 120 120 120 I) Penetration operation Phenol resol resin % by weight - 15 10 20 - * 15 10 20 - * Acetone " - 58 65 50 - * 58 65 50 - * Water " - 27 25 30 - * 27 25 30 - * Lacquer coat (perm.) g/100 g - 9 6 12 - 18 12 6 12 - 18 Drying temperature C - - - - - 170 - - - - 170 Drying time minutes - - - - - N5 - - - - N5 II)Impregnation operation Modified phenol resol parts by wt. - 110@ 110 110 - - Phenol resol " - 20 20 20 - - phenyl cresyl phosphate " - 25 25 25 - - Modified epoxy resin " - - - - 150 150 150 Sb2O3-paste " - - - - 15 15 15 Lacquer coat (perm.I+II) g/100 g - 140 140 140 140 140 140 Drying temperature C 170 170 170 170 170 170 Drying time minutes - #5 #5 #5 #5 #5 #5 *The penetrating resin used in Examples 5 and 10 is identical with the impregnating resin of Examples 1 to 5.

Claims

Wl-IAT WE CLAIM IS:

1. A method of treating a sheet of fibrous cellulose material, which comprises (i) contacting the sheet, in a first operation, with a first resin solution in an amount such that from 4 to 20 parts by weight of solid resin per 100 parts by weight of sheet penetrates the sheet, the resin solution comprising from 5 to 25% by weight of solid resin and from 20 to 50% by weight of water; (ii) impregnating the sheet, in a second operation, with a second resin solution, the second resin solution being a solution of (a) a phenol resin modified by the presence therein of structural units derived from a fatty acid or a fatty acid ester, (b) a crcsol resin modified by the presence therein of structural units derived from a fatty acid or a fatty acid ester, (c) a water-free epoxy resin. or (d) an elastic resin containing up to 100 monomer units: and (iii) drying the impregnated sheet; there being no drying operation intermediate the first and second operations.

2. A method according to claim 1, wherein the first operation from 4 to 16 parts by weight of solid resin per 100 parts by weight of sheet penetrates the sheet.

3. A method according to claim 2, wherein in the first operation from 4 to 14 parts by weight of solid resin per 1()() parts by weight of sheet penetrates the sheet.

4. A method according to claim 4, wherein in the first operation from 4 to 12 parts by weight of solid resin per 100 parts by weight of sheet penetrates the sheet.

5. A method according to any of the preceding claims, wherein the resin of the first resin solution consists of not more than four monomer units.

6. A method according to claim 5, wherein said resin is a phenol-resol resin.

7. A method according to any of the preceding claims, wherein the first resin solution contains from 7 to 20% by weight of solid resin.

8. A method according to any of the preceding claims, wherein the liquid phase of the first resin solution comprises an aqueous ketone.

9. A method according to claim 8, wherein the ketone is acetone or methylethyl ketone.

10. A method according to any of claims 1 to 7, wherein the liquid phase of the first resin solution comprises an aqueous alcohol.

11. A method according to claim 1(), wherein the alcohol is methanol, ethanol or isopropanol.

12. A method according to any of claims 8 to 11, wherein the first resin solution contains from 25 to 75'S, by weight of ketone or alcohol.

13. A method according to claim 12, wherein the first resin solution contains from 40 to 73% by weight of ketone or alcohol.

14. A method according to any of claims 1 to 7, wherein the liquid phase of the first resin solution comprises an aqueous glycol, an aqueous glycol ether or aqueous dimethylformamide.

15. A method according to any of the preceding claims, wherein the first resin solution contains from 25 to 40% by weight of water.

16. A method according to any of the preceding claims, wherein the second resin solution contains from 40 to 80% by weight of solid resin.

17. A method according to claim 16, wherein the second resin solution contains from 45 to 70% by weight of solid resin.

18. A method according to claim 17, wherein the second resin solution contains from 50 to 60% by weight of solid resin.

19. A method according to any of the preceding claims, wherein the resin of the second resin solution is a resin which has been modified with wood oil or castor oil.

2(). A method according to any of the preceding claims, wherein the resin component of the second resin solution includes from S to 25'S. by weight of a phenol-resol or cresol-resol resin.

21. A method according to claim 20, wherein the resin component of the second resin solution includes from 10 to 20% of a phenol-resol or cresol-resol resin.

22. A method according to any of the preceding claims, wherein the liquid phase of the second resin solution contains not more than 10% by weight of water.

23. A method according to claim 22, wherein the liquid phase of the second resin solution contains not more than 5% by weight of water.

24. A method according to any of the preceding claims, wherein the liquid phase of the second resin solution comprises a ketone.

25. A method according to claim 24, wherein the ketone is acetone or methyl ethyl ketone.

26. A method according to any of claims 1 to 23, wherein the liquid phase of the second resin solution comprises -in alcohol.

27. A method according to claim 26, wherein the alcohol is methanol, ethanol or isopropanol.

28. A method according to any of claims l to 23, wherein the liquid phase of the second resin solution comprises a glycol, a glycol ether or dimethylformamide.

29. A method according to any of claims 24 to 28, wherein the second resin solution contains from 20 to 60% by weight of the ketone, alcohol, glycol, glycol ether or dimethyl formamide.

30. A method according to any of the preceding claims, wherein the first resin solution additionally includes a plasticiser.

31. A method according to any of the preceding claims, wherein the second resin solution additionally includes a plasticiser.

32. A method according to claim 30 or 31, wherein the solution includes up to 10% by weight of the plasticiser.

33. A method according to claim 32, wherein the solution includes up to 5% by weight of the plasticiser.

34. A method according to any of claims 30 to 33, wherein the plasticiser is a phosphoric acid ester.

35. A method according to claim 34, wherein the plasticiser is tricresyl phosphate or diphenyl cresyl phosphate.

36. A method according to any of the preceding claims, wherein the first resin solution additionally includes a flame retarding agent.

37. A method according to any of the preceding claims, wherein the second resin solution additionally includes a flame retarding agent.

38. A method according to claim 36 or 37, wherein the solution includes up to 10% by weight of the flame retarding agent.

39. A method according to claim 36, 37 or 38, wherein the flame retarding agent is a brominated phenyl ether.

40. A method according to any of the preceding claims, wherein impregnation with the first resin solution is carried out by means of a transfer roller or a reverse roll coater.

41. A method according to any of the preceding claims, wherein the impregnated web is dried at a temperature of from 160 to 1800C.

42. A method according to any of the preceding claims, wherein the sheet is a sheet of paper.

43. A method according to any of claims 1 to 41, wherein the sheet is a sheet of cotton.

44. A method according to claim 1, substantially as hereinbefore described in any of Examples 1 to 3, 6 to 8, 11 and 12.

45. A sheet whenever treated by the method according to any of the preceding claims.

46. A method of forming a moulding, which comprises subjecting a treated sheet according to claim 45 to heat and pressure.

47. A method according to claim 46, which comprises using several treated sheets according to claim 45 to form a moulding in the form of a multi-layer hardened laminate.

48. A method according to claim 46 or 47, wherein the sheet is heated at a temperature of from 150 to 1800C.

49. A method according to claim 46, 47 or 48, wherein the sheet is pressed at a pressure of from 80 to 120 kp/cm .

50. A method according to any of claims 46 to 49, wherein a metal foil is simultaneously adhered to the or one of the treated sheets to form a moulding in the form of a laminate having the metal foil as a surface.

51. A method according to claim 50, wherein the metal foil is a copper foil.

52. A moulding whenever formed by the method according to any of claims 46 to 51.