IE42669B1 - Heat-curable melamine-formaldehyde resins and process for their preparation - Google Patents

Abstract

1508103 Heat - curable melamineformaldehyde resin CASSELLA FARBWERKE MAINKUR AG 26 March 1976 [29 March 1975] 12236/76 Heading C3R [Also in Division B5] A heat-curable melamine-formaldehyde resin contains as latent curing agent 0À01 to 2% by weight, relative to solid resin, of a polyphosphoric acid alkyl ester prepared by reacting P2O5 at 30‹ to 120‹ C. with N mols per mol of P2O5 of a dialkyl ether and/or m mols per mol of P2O5 of an alkanol, n+m being at least 0À5 and the alkyl chain length of the alkyl ether and/or alkanol being 1, 2 or 3 carbon atoms, until a sample of the reaction product is completely soluble at 60‹ to 100‹ C. in a C1-4 alkanol or aliphatic chlorohydrocarbon. The melamineformaldehyde resin may have been prepared in the presence of an alkanol, and other modifiers may be included in the precondensation or added subsequently. It is preferred that the melamine-formaldehyde resin be diluted with water and the polyphosphoric acid ester added thereto in the form of a solution in water or a water-miscible organic solvent. The resulting solutions may be used to impregnate papers or fabrics for the manufacture of decorative laminates or for coating wood-based materials.

Description

The present invention relates to a heat-curable melamine-formaldehyde resin, characterised in that it contains, as the latent curing agent, 0.01 to 2% by weight, relative to solid resin, of a polyphosphoric acid alkyl ester which has been prepared by reaction of phosphorus pentoxide with n mols/mol of PjOg a dialkyl ether and/or m mols/mol of an aLkanol, the sum of n and m being at least 0.5, the alkyl chain length of the alkyl ether and/or of the alkanol being 1 to 3 carbon atoms, and the reaction being continued until a sample of the reaction product is completely soluble at 60° to 100°C in an alkanoi or an aliphatic chlorohydrocarbon having 1 to 4 carbon atoms; this resin exhibits a technically very advantageous curing behaviour coupled with good stability on storage.

The industrially important melamine-formaldehydes are prepared by condensation of melamine (which may be a substituted melamine) and formaldehyde. They include those which are obtained by co-condensation of known modifying components or by subsequent addition of known O modifiers. Examples of known modifiers are o/p-toluenesulphonamide, amidosulphonic acid and its Balts, caprolactam, glucose, sorbitol, ethylene glycol, di-ethylene glycol, pentaerythritol, sucrose, methylene-bis-formamide, methylene-bis-acetamide, carbamates, such as methyl 2 6 6 9 carbamate and methoxyethyl carbamate, and salts of maleamic acid or fumaramic acid.

The reaction between the melamine and formaldehyde is only taken to the point that the products still leinain soluble and fusible. As soon as this state is reached, the condensation is discontinued, for example by cooling and adjusting the reaction mixture to a weakly alkaline pH value. The incompletely condensed products thus produced (melamine-formaldehyde precondensates) are used in the form of their aqueous solutions, especially as impregnating resins for the laminates industry and for the surface finishing of chipboard, as well as for the preparation of compression moulding compositions.

Solutions of melamine-formaldehyde impregnating resins are used to impregnate papers or fabrics which are employed for the manufacture of decorative laminates or for coating of wood-based materials, particularly chipboard or wood fibre-board.

To prepare compression moulding compositions, the melamine-formaldehyde precondensate is mixed with fillers such as cellulose or wood flour. From these compression moulding compositions, mouldings of all kinds such as housings, operating knobs, electrical switches and many others are prepared by pressing in heated moulds.

In the course of processing, the soluble and fusible melamine-to-formaldeh; de precondensates are converted to infusible and insoluble products. In this process, which is described as curing, extensive crosslinking of the precondensates occurs. However, the rate of this cross- 3 42669 linking reaction is too low for technological processes, even at the elevated processing temperatures, and must therefore be accelerated by addition of so-called curing agents. Compounds which react acid and/or eliminate acid > are used as curing agents.

Known curing agents of this type are ammonium salts or amine salts, for example ammonium chloride, ammonium thiocyanate, ethanolamine hydrochloride or strong organic acids such as p-toluenesulphonic acid. These known curing .0 agents suffer from important disadvantages. Thus, if free acids or salts which have a strong acid reaction are used, the resulting pot lives are relatively low, which greatly interferes with processing. Numerous attempts have therefore already been made to find curing agents which only develop their activity at elevated temperatures, such as are used when processing the aminoplasts. Such curing agents are described as latent curing agents. The known salts of strongly basic amines, used as curing agents, already exhibit a certain latency but this still leaves unsatisfied some important practical requirements.

The above-mentioned disadvantages of the known curing agents manifest themselves in a more acute form when the melamine-formaldehyde resins are to be processed by modern methods such as, for example, the quick-cycle process.

In this case, the use of very powerful curing agents is extremely critical if the curing times in the press are to be as short as possible, since the danger of forming a heat haze on the lower face of the sheet, through the latter resting on the hot press plate for a long time, is very - 4 42G69 great. If weaker known curing agents or smaller amounts of known strong curing agents are used, the press time becomes so long that it falls within an uneconomical range.

A further considerable danger when using strong curing agents is that of overcuring of the melamineformaldehyde resins, which can lead to a considerable reduction in the elasticity of the cured product, and to crack formation. There is therefore an urgent need for heat-curable melamine-formaldehyde resins which, whilst having an adequste pot life, give perfect curing even with short press times and which do not exhibit the disadvantages of the resins to which known curing agents have been added, such as the danger of over-curing, heat haze formation or damage to the moulds.

Surprisingly, it has now been found that a heatcurable melamine-formaldehyde resin which contains, as the latent curing agent, 0.01 to 2% by weight, based on solid resin, of polyphosphoric acid alkyl esters, and which has been prepared by reaction of phosphorus pentoxide with n mols/mol of P2O5 of a dialkyl ether and/or m mols/mol of P2°5of an alkanol, the sum of n and m being at least 0.5, the alkyl chain length of the alkyl ether and/or of the alkanol being 1 to 3 carbon atoms, and the reaction being continued until a sample of the reaction product is completely soluble at 60° to 100°C in an alkanol or an aliphatic chlorohydrocarbon having 1 to 4 carbon atoms, does not exhibit the disadvantages of known heat-curable melamine-formaldehyde resins.

The polyphosphoric acid esters to be used, according 43869 to the invention, as latent curing agents are not single chemical compounds, but mixtures of different compounds, of cyclic or linear structure, which contain the structural element -POP- at least once. Reaction products of phosphorus pentoxide with dialkyl ethers have been investigated by M. Calvin (JACS 87, 591 (1965)), whose investigations suggest that the polyphosphoric acid esters obtained by reacting PgO^ diethyl ether are mixtures of compounds of the following formulae X to TV (the symbol Et in which represents the ethyl radical, CgH^); the composition of the mixtures depends on certain reaction conditions, namely the molar ratio of the reactants, the reaction time and possibly also the reaction temperature, and the nature and proportion of any organic solvent which may be added.

EtO I 0<£-P— 0\ OEt P—>0 EtO I -P — i EtO P— 0 ii T P--OEt \ V OEt II OEt EtO—P / ' 0 01 T.

P-0- P ' \ · OET III 0 OEt 0 EtO AΛ .. OEt ' P 0 P · 0 P - 0 P EtO ψ 4- OEt 0 OEt 0 IV Depending on the composition of the polyphosphoric acid esters, the latter, in the solvent-free form, are mobile or viscous oils or rubbery or plastic-like compositions, which all have the characteristic that they are soluble in organic solvents, such as lower alkanols, for example in isopropanol, or halogenohydrocarbons, for example chloroform, methylene chloride and ethylene chloride. If the reaction of phosphorus pentoxide with lower alkanols is carried out, it is to be assumed that, as in the case of the reaction of alkanols with polyphosphoric acid esters which have been produced from phosphorus pentoxide and dialkyl ethers, products are formed which differ from the polyphosphoric acid esters formed by reaction of phosphorus pentoxide with dialkyl ethers, in that in these products a part of the alkoxy groups is replaced by hydroxyl groups.

The polyphosphoric acid alkyl esters to be employed, according to the invention, as latent curing agents, are prepared by reaction of phosphorus pentoxide with n mols, per mol of P2°5' of a dialkyl ether and/or m mols, per mol of I?2O5' an alkanol, with the sum n + m being at least 0.5. The reaction is continued until a sample of the reaction product is completely soluble at 60° to 1OO°C in an alkanol or an aliphatic chlorohydrocarbon having 1 to 4 carbon atoms. Preferably, the polyphosphoric acid esters which are obtained by reaction of phosphorus pentoxide with n mols, per mol of p2°5' of a dialkyl ether and/or m mols, - 7 42669 per mol of P2°5' an alkanol, with the sum n + m being between 1 and 4, are employed as latent curing agents for the melamine-formaldehyde resins according to the invention.

The reaction can in principle be carried out without additional organic solvent. It can, however, also be carried out in an excess of the liquid reactants, namely ether and/or alkanol, or in an inert organic solvent, preferably a chlorohydrocarbon, especially chloroform, ethylene chloride or methylene chloride. It is preferred to carry out the reaction in an inert organic solvent, since this, inter alia, offers the technical advantage that the completeness of the reaction can be recognised, extremely simply, from the fact that the phosphorus pentoxide dissolves completely and a homogeneous reaction phase is obtained.

The reaction is usually carried out at temperatures of between 30 and 120°C. If, as for example in the case of the reaction of phosphorus pentoxide with dimethyl ether, the process is carried out in a closed, pressure-tight system, temperatures of between 60 and 100°C are preferred for the reaction. If the reaction is carried out without the use of superatmospheric pressure, it is preferred to work at the boiling point of the reaction mixture, under reflux.

With a view to shortening the reaction time, the dialkyl ether and/or the alkanol can be used in a substantial excess. In that case, as soon as the reaction product gives a clear solution in the organic solvent, the excess of the liquid reactants is distilled off in vacuo.

The heat-curable melamine-formaldehyde resins according to the invention are prepared by condensing melamine optionally together with modifiers, such as o/p-toluenesulphonamide, amidosulphonic acid or its salts, sorbitol or caprolactam, with the formaldehyde, and optionally a lower alkanol, in a manner which is in itself known (compare Ullmanns Enzyklop'adie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), 4th edition, volume 7, 1974, page 403 to 424) at temperatures between 80 and 1OO°C, preferably between 90 and 95°C, the reaction time being so chosen that the resulting precondensates can be diluted with water in ratios ranging from 1:4 to 1:0.1, preferably 1:3 to 1:1; 0.01 to 2% by weight, preferably 0.05 to 1% by weight, relative to solid resin, of the polyphosphoric acid esters described above are then added to the precondensate, and for reasons of greater simplicity of handling the polyphosphoric acid esters are used in the form of 50% strength solutions in water or in a watermiscible organic solvent, such as methanol, dimethylformamide, dioxane or, preferably, isopropanol. The heatcurable melamine-formaldehyde resins according to the invention have good stability at room temperature, because of the good latency of the polyphosphoric acid esters used as curing agents, so that sufficient time is available for processing the resins. When processing the new melamine-formaldehyde resins according to the invention at the customary pres:; temperatures, good curing, and a perfect pore-free surface are achieved, and at the same time the press time is shortened. Damage to the processing moulds and press plates, crack formation due to over-curing and heat haze, are not found.

The degree of curing indicated in the examples which follow was determined by the known Kiton test. In this, > the degree of curing is indicated in accordance with a sixstage scale, with scale 1 denoting very good curing and stage 6 very poor curing. The percentages indicated in the examples are percentages by weight.

Example 1 .0 a) 94 kg of 39% strength aqueous formaldehyde solution, 77 kg of melamine, 4.5 kg of sodium amidoaulphonate (in the form of a 40% strength aqueous solution), 14.3 kg of methanol and 0.4 kg of 2 N sodium hydroxide solution were condensed at 9O-95°C until the product could .5 be diluted with water in the ratio of Is2. Thereafter, 1 of water and 15 kg of (crude) methylene-bis-formamide in the form of a 50% strength aqueous solution, and, as a latent curing agent, 990 ml of the 50% strength solution of a polyphosphoric acid methyl ester prepared according to paragraph b of the present example, were added to the cooled solution from the condensation reaction. A decorό ative paper weighing 110 g/m was impregnated in the resulting solution to a resin content of about 58% and wae subsequently dried to a residual moisture content of 5-6% (determined by weighing a sample before and after 5 minutes' drying at 160°C). The paper was pressed onto a chipboard, weighing about 700 kg/m2, in a single-daylight press under the following conditions· - 10 4 2 6 6 9 Contact time until the full press pressure is reached: about 5 seconds.

Press time: 38 seconds.

Press temperature: lower face 140°C, upper faca 142°C (temperature of the object), press pressure 20 kp/cm2. After releasing the board whilst hot, no lines due to overcuring (heat haze) were found on its lower face. The coated chipboard produced showed a perfect pore-free surface. The cure corresponded to stage 2 of the sixstage Kiton scale. After a heat treatment for 17 hours at 80°C no crack formation was observable. b) 200 g of phosphorus pentoxide and 800 ml of chloroform are initially introduced into an autoclave, g of dimethyl ether are metered in, whilst stirring, and the mixture is stirred for 10 hours at 60°C. A sample of the reaction product shows that a homogeneous liquid phase has been produced. The clear colourless solution is evaporated, 260 g of a viscous oil remain, and are dissolved in 260 g of isopropanol. This 50% strength solution is miscible with water in all proportions and is employed as a latent curing agent in the preparation of the resin according to the invention.

Example 2 (Comparative example) In place of the polyphosphoric acid methyl ester, 930 ml of a 20% strength aqueous ammonium chloride solution are added to a melamine resin solution prepared according to Example la, and the mixture is processed under the same conditions to give a coated chipboard. After release from V the press, a surface severely impaired by heat haze is obtained on the lower face of the board. The cure corresponds to stage 2 of the six-stage Kiton scale. The surfaces of both sides are pore-free, but after a heat treatment of 17 hours at 70°C, cracks are observed.

Example 3 a) 94 kg of a 39% strength aqueous formaldehyde solution, 77 kg of melamine, 4.5 kg of sodium amidosulphonate (in the form of a 40% strength aqueous solution), ) 14.3 kg of methanol and 0.41 kg of 2 N sodium hydroxide solution were condensed at 9O-95°C until the product could be diluted with water in the ratio of Is2. Thereafter 35 1 of water, 15 kg of (crude) methylene-bis-formamide in the form of a 50% strength aqueous solution and 127 ml of a 50% strength solution of polyphosphoric acid methyl ester in isopropanol, prepared according to paragraph of the present example, were added to the cooled resin solution.

A printed (linen pattern) decorative paper weighing about 110 g/cm is impregnated in this solution. The resin content was about 56.5% (relative to the final weight of the paper) and the residual moisture content was 5.6-5.9% (5 minutes/160°C)« The papers were pressed onto a chipboard of mean raw density 720 kg/m , using a temperature, of the object, of 135°C, a heating time of 3 minutes, a cooling time of 4-5 minutes, depending on the cooling water temperature, and a press pressure of 20 kp/cm .

Press assembly: Heating platen Asbestos pad, about 1,200 g/m 22669 Press plate, high gloss chromed Decorative paper, as described Chipboard Decorative paper, as described Press plate, as above ' 2 Asbestos pad, about 1,200 g/m Carrier plate Heating platen After release from the press at about 85-95°C, a coated chipboard is obtained, which has a perfect pore-free surface corresponding to stage 2 of a Kiton test. After a heat treatment carried out for 17 hours at 80°C, no crack formation was observable. b) 2 kg of methanol are added dropwise, over the course of 3 hours to a suspension of 2 kg of phosphorus pentoxide in 4 1 of chloroform whilst stirring at the reflux temperature. The clear solution thereby produced is concentrated until the internal temperature is 120°C. 3,900 g of a colourless oil which is readily soluble in lower alkanols remains; this oil is dissolved in isopropanol to give a 50% strength solution. The solution thus obtained can be employed as a latent curing agent in accordance with section a).

Products with comparably advantageous curing agent properties to those of the product described in paragraph 3b are obtained if instead of the methanol used above one of the following mixtures is employed; 1.0 kg of methanol and 1.0 kg of ethanol 1.5 kg of methanol and 0.5 kg of ethanol 1.8 kg of methanol and 0.3 kg of propanol 1.8 kg of methanol and 0.3 kg of isopropanol Example 4 (Comparative example) In place of polyphosphoric acid methyl ester, 2,460 ml of a 50% strength aqueous solution of diethanolamine acetate ate added as the curing agent to a resin solution prepared according to Example 3a, and pressing is carried out in accordance with Example 3. After release from the press, the curing stage obtained is 4-5, and the surface obtained is pore-free. After the heat treatment, no cracks are observed. The example shows that even with a twentyfold amount of a known curing agent, a lower degree of condensation than that in Example 3 is achieved.

Example 5 a) 100 kg of melamine, 152.5 kg of a 39% strength aqueous formaldehyde solution, 20 kg of methanol, 5 kg of sorbitol, 2 kg of caprolactam and 2 kg of 2 N sodium hydroxide solution are condensed at pH 8.5-9.5 and 90-95°C, whilst constantlychecking the pH value, until the product can be diluted with water in a ratio of 1:2.0.

After the solution has cooled, 25 1 of water and 173 ml of a 50% strength solution of polyphosphoric acid ethyl ester in isopropanol are added thereto. An overlay paper weighing about 30g/m and a printed decorative paper weighing about 110 g/m are impregnated in this resin solution. The resin treatment was carried out to give the following: Overlay paper, resin content: 74%, residual moisture content: 7% - 14 4 2 66 9 Decorative paper, resin content: 45%, residual moisture content: 5.5%.

The impregnated papers were pressed together with kraft papers impregnated with phenolic resin, in a multidaylight press.

Press assembly: heating platen kraft paper pad, 1,250 g/m press plate impregnated overlay paper impregnated decorative paper 5 kraft papers impregnated with phenolic resin, resin content: 35-37%, residual moisture content 5% phenolic resin-impregnated kraft paper, resin content: 37%, residual moisture content 7‘;' release papers phenolic paper, as described phenolic j apers, as described decorative paper overlay paper press plate paper pad consisting of kraft paper, , 2 1,000 g/m carrier plate heating platen The press time was 6 minutes, the cooling time 4-5 minutes, the press temperature (of the object) 140°C and the press i33S9 , pressure 80 kp/cm . After release from the press, a perfect pore-free surface is obtained. The Kiton test corresponds to stage 1-2. After a heat treatment of the laminate at 80°C for 20 hours in accordance with DIN 16,926 or DIN Draft 53,799, item 4.7.1.2, no crack formation is observed. b) 2 kg of phosphorus pentoxide, 1.4 kg of diethyl ether and 4 1 of chloroform are heated to the boil under a reflux'condenser, whilst stirring, whereupon the phosphorus 3 pentoxide dissolves completely over the course of about 40 hours. As soon as complete solution has occurred, the chloroform is distilled off, 3.4 kg Of a colourless oil which is soluble in lower alkanols, such as methanol, ethanol, propanol or isopropanol, remain. A 50% strength 5 solution is prepared by dissolving the product in 3.4 kg of isopropanol and is used as a latent curing agent in accordance with paragraph a) of the present example. if the same batch is reacted in an autoclave at 100 to 120°C, the reaction time is reduced to 10 hours. The properties of the product thereby obtained correspond to those of the product prepared at the reflux temperature. c) 2 kg of phosphorus pentoxide and 4 1 of chloroform are heated to the boil under a reflux condenser, whilst stirring. 1.7 kg of ethanol are added dropwise to the 3 boiling suspension over the course of 3 hours. Stirring is allowed to continue until the phosphorus pentoxide has dissolved completely, and the chloroform is distilled off. The colourless oil which remains is completely soluble in lower alkanols, suoh as methanol, ethanol or propanol, and - 16 its properties correspond to those of the polyphosphoric acid ethyl ester prepared in Example 5b. d) 2 kg of phosphorus pentoxide are suspended in 4 1 of ethylene chloride and the suspension is heated to the boil under a reflux condenser, whilst stirring. A mixture of 1 kg of ethanol and 0.6 kg of diethyl ether is added dropwise over the course of 3 hours to the boiling suspension. Stirring is allowed to continue for six hours, in the course of which the phosphorus pentoxide dissolves completely.

The ethylene chloride is then distilled off, whereupon a colourless oil is obtained, which is completely soluble in lower alkanols, such as methanol, ethanol or propanol, and exhibits an excellent activity as a latent curing agent if it is employed in accordance with Example 5a.

Products which also have a very good activity as latent curing agents are obtained if the mixture of ethanol and diethyl ether used in Example 5d is replaced by the following mixtures: 500 g of ethanol and 1,000 g of diethyl ether 300 g of ethanol and 1,200 g of diethyl ether 1,200 g of etharol and 500 g of diethyl ether Example 6 (Corparative example) In place of the curing agent employed in Example 5, 3,450 ml of a 50% str 2 63 9 Example 7 (Comparative example) In place of the curing agent employed in Example 5, 173 ml of a 50% strength aqueous solution of ethanolamine hydrochloride are added to the resin solution prepared according to Example 5, and the mixture is further processed as in Example 5. The laminate produced has a pore-free surface and a curing stage of 2, but shows distinct crack formation after a heat treatment which was carried· out for LO 20 hours at 80°C.

Example 8 100 ml of methanol are added dropwise over the course of 2 hours, whilst stirring, to a suspension, boiling under a reflux condenser, of 50 g of phosphorus pentoxide in 300 L5 ml of methylene chloride. In the course thereof, the phosphorus pentoxide dissolves completely. The methylene chloride and the excess methanol are distilled off. 96 g of an oil which is completely soluble in lower alkanols remain. The product can be employed as a latent curing agent in accordance with one of Examples 1, 3 or 5.‘ It is advantageously employed, as described above, in the form of a dilute solution in an alkanol, Possible solvents or diluents are methanol, ethanol, propanol, ethylene glycol or diethylene glycol or mixtures of these.

If the 100 ml of methanol used in Example 8 are replaced by 150 ml of ethanol, and in other respects the procedure described in Example 8 is followed, a product having substantially similar technological properties to those of the product described in Example 8 is obtained. - 18 The Kiton test is carried out on laminates as follows: Half of the material to be tested is introduced for minutes into a boiling solution of the following composition: 1 1 of water, 5 ml of concentrated sulphuric 5 acid and 1 ml of a 2% strength aqueous solution of Kiton Fast Red 2 BL (C.I. Acid Red 45). Thereafter, the degree of colouration is compared with a six-stage scale according to which stage 1 shows no colouration and stage 6 shows a substantial colouration. Stage 1 denotes perfect curing and stage 6 is to be regarded as inadequate curing.

Xn the case of coated chipboard, the Kiton test is carried out as follows: ml of a solution composed of: 20 ml of concentrated sulphuric acid and 20 ml of a 2% strength aqueous solution of Kiton Fast Red 2 BL (C.I. Acid Red 45) is applied to the surface to be tested, and covered with a watchglass. After hours,the degree of colouration is compared with the abovementioned six-stage scale.

The heat treatment of coated chipboard was carried out in accordance with DIN Draft 68,765 of January 1971, item 3.3, and DIN Draft 53,799 of September 1973, item

Claims (22)

1. , A heat-curable melamine-formaldehyde resin which contains a latent curing agent comprising 0.01 to 2% by weight, relative to solid resin, of a polyphosphoric acid 5 alkyl ester prepared by reacting P 2 °g a ^ 30 to 120°C with n mols per mol of ? 2 °5 of a dialkyl ether and/or m mols per mol of P 2 °5 ° f an alkanol, n + m being at least 0.5 and the alkyl chain length of the alkyl ether and/or alkanol being 1, 2 or 3'carbon atoms, until a sample of the reaction LO product is completely soluble at 60° to 100° C in an alkanol or an aliphatic chlorohydrocarbon having 1 to 4 carbon atoms

2. A resin according to claim 1 which contains 0.05 to 1% weight, relative to solid resin, of the polyphosphoric acid alkyl ester. .5

3. A resin according to claim 1 or 2 which contains a polyphosphoric acid alkyl ester prepared by reacting P 2 °5 with n mols per mol of Ρ 2 Ο 5 of a dialkyl ether and/or m mols per mol of Ρ 2 θ5 an alkanol, n +'m being 1 to 4.

4. A resin according to claim 1, 2 or 3, which 0 contains a polyphosphoric acid alkyl ester prepared by reacting P 2 °5 with the respective dialkyl ether and/or alkanol in the presence of an organic solvent.

5. A resin according to any of claims 1 to 4, which contains a polyphosphoric acid alkyl ester prepared by 5 reacting Ρ,,Ο,. with the respective dialkyl ether and/or alkanol in the presence of an organic solvent constituted by an excess of the liquid ether and/or alkanol.

6. A resin according to any of claims 1 to 4, which contains a polyphosphoric acid alkyl ester prepared by reacting P 2°5 with fc he respective dialkyl ether and/or alkanol in the presence of an organic solvent constituted by a chlorohydrocarbon.

7. A resin according to any of claims 1 to 6, which contains a polyphosphoric acid alkyl ester prepared by reacting P 2 °5 the respective dialkyl ether and/or alkanol at 60° to 100°c.

8. A resin according to any of claims 1 to 7, which contains a polyphosphoric acid alkyl ester prepared by reacting Ρ 2 θ5 with the respective dialkyl ether and/or alkanol at the boiling point of the reaction mixture.

9. Process for preparing a heat-curable melamineformaldehyde resin by precondensing melamine with formaldehyde and, optionally, an alkanol, and adding a latent curing agent, in which the latent curing agent added comprises 0.01 to 2% by weight, relative to solid resin, of a polyphosphoric acid alkyl ester prepared by reacting PjOg at 30 to 120°C with n mols per mol of Ρ 2 0^ of a dialkyl ether and/or m mols per mol of P 2 O 5 of an alkanol, n + m being at least 0.5 and the alkyl chain length of the alkyl ether and/or alkanol being 1, 2 or 3 carbon atoms, until a sample of the reaction product is completely soluble at 60 to 100°C, in an alkanol or an aliphatic chlorohydrocarbon having 1 to 4 carbon atoms.

10. Process according to claim 9 wherein one or more modifiers is included in the precordensation or added subsequently.

11. Process according to claim 9 or 10, in which 0.05 to 1% by weight, relative to solid resin, of the poly- 21 4 2SS9 phosphoric acid alkyl ester is added.

12. Process according to claim 9, 10 or 11, in which the polyphosphoric acid alkyl ester added is one prepared by reacting p 2°5 2. rools per mol of P^O^ a dialkyl > ether and/or m mols per mol of ρ 2°5 an a l^ an °l< n + m being 1 to 4.

13. Process according to claim 9, 10, 11 or 12, in which the polyphosphoric acid alkyl ester added is one prepared by reacting Ρ 2 θ5 '•th the respective dialkyl ether LO and/or alkanol in the presence of an organic solventi

14. Process according to any Of claims 9 to 13, in which the polyphosphoric acid alkyl ester added is one prepared by reacting P 2 °5 with the respective dialkyl ether and/or alkanol in the presence of an organic solvent .5 constituted by an excess of the liquid ether and/or alkanol

15. Process according to any of claims 9 to 13, in which the polyphosphoric acid alkyl ester added is one prepared by reacting Ρ 2 0^ with the respective dialkyl ether and/or alkanol in the presence of an organic solvent !0 constituted by a chlorohydrocarbon.

16. Process according to any of claims 9 to 15, in which the polyphosphoric acid alkyl ester added is one prepared by reacting p 20g with the respective dialkyl ether and/or alkanol at 60 to 100°G. 15

17. Process according to any of claims 9 to 16, in which the polyphosphoric acid alkyl ester added is one prepared by reacting Ρ 2 θ 5 with the respective dialkyl ether and/or alkanol at the boiling point of the reaction mixture.

18. Process according to any of claims 9 to 17, in which the polyphosphoric acid alkyl ester is added in the form of a solution in an alkanol having 1 to 4 carbon atoms.

19. A resin according to claim 1 wherein the latent curing agent is one prepared by a procedure substantially 5 as described in Example 1, 3, 5 or 8.

20. Process according to claim 9 substantially as described in Example 1, 3, 5 or 8.

21. A heat-curable melamine-formaldehyde resin prepared by a process according to any of claims 9 to 18 10 or claim 20.

22. A heat-curable melamine-formaldehyde resin obtained by heat-curing a resin according to any of claims 1 to 8 or claim 18 or 20.