The invention relates to salts that promote the curing of formaldehyde-containing resins, especially those having low molar ratios of formaldehyde to co- monomer.

In recent years there has been rising concern over toxic formaldehyde emission from products containing formaldehyde resins, such as, for example, particleboards bonded with urea-formaldehyde (UF) resins. As a result, formaldehyde emission standards have become increasingly strict. In Europe there now exist three European

Emission Classes, El, E2 and E3, as shown in Table 1 below:

Table 1

Total Extractable Formaldehyde * (mg/lOOg dry board)

< 10

> 10 to 30 > 30 to 60

* determined according to European Standard EN 120. (European Committee for Standardisation, Brussels, January 1984.) (See also the Dangerous Substances Regulations (26th February 1986) and West German Regulations On The Emission Of Formaldehyde From Wood-based Panel Products May 1987.)

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West Germany is an example of one country that has already introduced legislation requiring that particleboards comply with the El Emission Standard. That standard, however, presents resin manufacturers with particular difficulties. These difficulties are exacer¬ bated when the product produced contains a high propor¬ tion of resin, as may be the case, for example, with moulded chipboard.

In the case of UF resins, for example, whereas resins with the lowest traditional formaldehyde:urea

(F:U) molar ratios, of about 1.3:1, fulfil the E2 stan¬ dard, it has been necessary to develop new resins with F:U molar ratios of as little as 1.05:1 in order to meet the El Standard. Such resins, however, do not cure easily because of the decreased availability of formal¬ dehyde for reaction to form the cross-linked chains.

Traditional curing agents for formaldehyde resins are either acids or substances that liberate acids upon mixing with the resin, both being used in aqueous solution. Generally no other solvent is used. Typical acid curing agents include inorganic acids such as hydrochloric, nitric, phosphoric and sulphuric acids, and organic acids such as formic acid, oxalic acid and sulphonic acids, for example para-toluene sulphonic acid. Typical acid-liberating curing agents include the amine and ammonium salts of such acids, such as, for example, triethylamine sulphate and ammonium nitrate, phosphate,

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chloride, sulphate and sulphamate.

The acid curing agents provide a very rapid rate of cure with the result that aqueous resin compositions incorporating such curing agents suffer from the problem of premature thickening and gelation, and hence have only a short usable pot life. In the case of the ammonium or other salts, acid is liberated by reaction with free formaldehyde already present in the resin and with formaldehyde generated subsequently during curing, the acid being liberated by the following mechanism:

4 NHj + 6 HCHO > N ₄ (CH ₂ ) ₆ + 4 H ⁺ + 6 H ₂ 0

Since the curing reaction is dependent on acid- liberation, the acid-liberating curing agents have a slower rate of cure. Thus, ammonium salts, for example, are widely used as curing agents, to provide resin compositions with a higher ratio of pot-life to curing time than acid curing agents.

In the case of low-formaldehyde resins, however, not only is there the problem that there is less formaldehyde for cross-linking, but also the traditional ammonium salts are less able to catalyse the cross-linking reac¬ tions, because their catalytic activity depends on the presence of formaldehyde.

It is an object of this invention to provide a curing agent that is suitable for curing those formaldehyde-containing resins that contain low levels of formaldehyde.

We have found that the use of (i) an ammonium salt

together with (ii) a metal salt that is a Group II or Group III salt or a zinc or iron salt is especially useful for curing those resins having a ratio of formal¬ dehyde to comonomer of no more than about 1.1:1. We have found that the combination of an ammonium salt and an aluminium salt, (e.g. ammonium chloride or sulphate and aluminium chloride or sulphate) is especially preferred, the combination of aluminium sulphate and either ammonium chloride or ammonium sulphate being especially effective, and that in the presence of urea these salts can be combined into a single aqueous solution that remains stable over an extended period of time.

Accordingly, the present invention provides an aqueous batch solution for curing a formaldehyde- containing resin containing ammonium, aluminium and sulphate ions, and urea or a urea derivative.

We have found that in the absence of urea the combination of an ammonium salt and an aluminium salt, at least one of which is a sulphate, can be used in only a limited concentration in aqueous solution, because the presence of sulphate ions leads to the formation of the complex salt ammonium aluminium sulphate, NH4Al(S0 ₄ )2.12H2θ, which is relatively insoluble. Anhydrous aluminium sulphate, Al2(S0 ₄ ) ₃ , has a solubility in water at 0°C of 31.3g and at 100°C of 98.lg, each expressed in terms of g per 100ml of solu¬ tion. Ammonium sulphate has a solubility at the same

- 5 -

temperatures of 70.6g and 103.8g per 100ml, respectively. However, the solubility of the complex salt ammonium aluminium sulphate, NH A1(S0 ₄ ) ₂ •12H ₂ 0, is only 15.Og per 100ml solution at 20°C, or effectively 8.0g per lOO l solution when expressed in terms of the anhydrous salt. The simple mixing of concentrated solutions of the two components results in the immediate precipitation of the much less soluble mixed salt. Such precipitation causes considerable problems. For example, it would not be possible to deliver the curing agent solution from jet sprays as is common during large-scale board manufacture, as the sprays would become blocked.

If, alternatively, the amount of complex salt used were restricted to the maximum soluble concentration (effectively 8g/100ml in terms of the anhydrous salt, for a solution at 20 ^β C) , it would be necessary for a larger volume of water to be present in order to provide a sufficient molar quantity of the individual aluminium and ammonium salts for curing. In the case of particleboard manufacture, the increased volume of water would give rise to longer pressing times and a reduction in mechani¬ cal properties of the resin-bonded product. Although the wood chips could be dried prior to incorporation in the resin binder, the product would still be likely to have inferior mechanical properties.

We have found that by the use of urea the solubility of the complex salt can be substantially improved, even at very low temperatures. For instance a mixture of log

- 6 -

ammonium sulphate, lOg aluminium sulphate tetradeca- hydrate and 80g water would not remain in solution even when kept at 40°C: substantial quantities of crystalline material were present. However, when half the quantity of the water above was replaced by an equal quantity of urea, the solution remained completely stable for long periods with no trace of precipitation even at temperatures, as low as 2°C. If the solution were to be kept at 25°C only 25g of the water would require replace- ent by urea.

Thus, the present invention enables the above- mentioned problems to be substantially overcome by means of the addition of urea to the solution so as to increase the solubility of the complex salt and allow the curing agent solution to be used in the desired higher concentration.

French Patent Specification No. 2268044 is concerned with the pot life of adhesives to which a hardener has been applied and with the difficulties of adjusting the hardener/adhesive ratio to compensate. It discloses a method of controlling pot life by adding to the resin two hardener components in aqueous solution, the first component being an ammonium or Group la or Group Ila metal salt and the second component being a copper, aluminium or iron salt. Although one or both of the hardener components may contain urea to enhance the strength of the bond formed between the components to which the adhesives is applied, those components are kept

separately before adding to the resin, as opposed to being kept in a pre-prepared combined solution, so that there is no disclosure of preparing an aqueous curing batch containing the components of the present invention. Furthermore, FR 2268044 is concerned only with the hardening of traditional urea formaldehyde resins so there is no appreciation of the particular problems associated with low formaldehyde resins and, since that specification does not combine ammonium, aluminium and sulphate ions into an aqueous curing batch, nor is there any appreciation of the problems of preparing an aqueous batch containing those particular ions.

The present invention also provides an ammonium salt and an aluminium salt, at least one of which is a sul- phate, for curing a formaldehyde-containing resin, wherein the salts are used as an aqueous batch solution containing urea or a urea derivative that improves the solubility of the complex salt, ammonium aluminium sulphate. If desired, the salts may be supplied to the resin in the form of one or more batches.

The present invention also provides a closable vessel, especially a closed vessel, containing an aqueous batch of the invention. The invention also provides a process for curing a formaldehyde-containing resin, wherein there is used an aqueous batch of the invention.

The invention is of especial application where the

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formaldehyde-containing resin is a urea-formaldehyde resin having a ratio of formaldehyde to comonomer of no more than 1.1:1, more especially substantially 1.05:1 or below, each ratio calculated on the total quantity of urea present in the resin and the curing agent system.

The invention further provides a process for curing a formaldehyde-containing resin having a ratio of formaldehyde to comonomer of no more than 1.1:1, wherein there is used as curing agent a combination of (i) an ammonium salt and (ii) a metal salt that is a Group II or Group III salt or a zinc or iron salt.

If desired two or more ammonium salts and/or two or more of the specified metal salts may be used.

Preferably, the ratio by weight of ammonium salt: metal salt, more especially aluminium salt, is from 0.2:1 to 1.8:1, more especially substantially 1:1. With a resin having a low level of formaldehyde available for reaction with the ammonium salt, the proportion of ammonium salt should generally not be too high, and preferably should not exceed 1.2:1.

The ammonium salt may be, for example, ammonium chloride or ammonium sulphate.

The metal salt is preferably a salt of a strong acid, such as, for example, a chloride, nitrate or sulphate. Preferred metal salts are, not only aluminium, but also zinc or magnesium salts, although the aluminium salt, such as, for example, aluminium chloride or aluminium sulphate, is especially preferred. An iron

salt, such as, for example, ferric chloride may also be used, but is less preferred. These metal salts, which are water-soluble, form only weak bases upon reaction during curing, which bases do not adversely affect the rate of curing. The invention also contemplates, in place of or together with the or one of the metal salts specified, the use of a transition metal having similar aqueous solubility which forms a weakly basic hydroxide. In contrast to the ammonium salts, which are able to promote curing at room temperature, the above-defined metal salts only undergo reaction so as to liberate free acid at elevated temperatures. Hydrolysis of magnesium chloride, for example, according to the reaction: MgCl ₂ + 2 H ₂ 0 > Mg(OH) ₂ + 2 HCl does not occur below about 100°C.

Suitable formaldehyde-containing resins include urea resins, e.g. urea formaldehyde resins, melamine resins and phenolic resins, including their co-condensed products, for example urea/melamine and urea/melamine/phenol resins, and mixtures of two or more such resins.

The formaldehyde-containing resin may be used, for example, to bond water-penetrable cellulosic particles, fibres or sheets, more especially to bond particleboard, for example chipboard, wafer board, medium density fibre board and oriented strandboard. Use for bonding particleboards that comply with the El Standard should especially be mentioned.

- 10 -

The invention further provides a resin-bonded product, for example extruded, flat-pressed or moulded particleboard, when produced by a process of the invention. The amount of resin present in the product can vary and may be, for example, 7 to 8 %, or up to 10 %, by weight of the wood component. Amounts substantially larger than those may, however, also be used; for example, moulded chipboard may be prepared using 4 to 5 times that amount of.resin.

Ultimately, the curing agent salts will form part of an aqueous binder comprising the resin, water, the particulate or fibrous material and one or more other additives such as, for example, alkali metal salts, for example sodium chloride, lubricants, waxes, adhesion promoters and scavengers for formaldehyde; the latter are compounds that are able to react with formaldehyde so as to minimize residual free formaldehyde, and include compounds such as, for example, ammonia and urea; these formaldehyde-scavengers are preferably added when the binder is added to the particulate or fibrous material.

The manner of application of the ammonium and metal salts, and of the resin and any other additives, will depend on the type of product being produced. The salts may be supplied either separately or together, in either case in powder or, more usually, in liquid form.

The present invention further provides the use of a composition which comprises

- 11 -

(i) one or more ammonium salts and

(ii) one or more metal salts selected from Group II and

Group III salts and zinc and iron salts, for curing a formaldehyde-containing resin having a ratio of formaldehyde to comonomer of no more than 1.1:1. As mentioned above, when component (ii) is an aluminium salt and the hardener-component contains sulphate ions, urea is desirably present, to provide a suitably concentrated stable aqueous solution. The present invention also provides the use, for curing a formaldehyde-containing resin having a ratio of formaldehyde to comonomer of no more than 1.1:1, of a combined preparation which comprises as separate components for simultaneous or sequential addition to the resin

(i) one or more ammonium salts and

(ii) one or more metal salts selected from Group II and Group III salts and zinc and iron salts. The salts may each be added to the resin binder immediately prior to curing, or may be pre-mixed and added together, for example as a batch solution. Alternatively, they may, for example, be mixed with the particulate or fibrous material, which is then dried prior to the application of the resin and other additive(s). Methods for the production of resin- bonded products are well known and are described in the literature; see, for example, the CIBA Information Manual No. BP.6a, June 1970.

- 12 -

If desired, urea or other formaldehyde-scavenger may be present as a separate component or present with the separate ammonium, and/or metal salts or, if desired or - in the case of urea - if required, with a mixture of the salts.

Thus, urea may, for example, be added to the resin- binder mixture prior to the addition of the salts, or later, prior to or during the addition of water, or may be mixed with either or both of the salts or with a mixture of the salts prior to their being mixed with the resin.

As mentioned above, where an aqueous solution batch containing ammonium, aluminium and sulphate ions is made up, urea should be present to provide a suitably con- centrated stable solution.

The present invention further provides a process for improving the water-solubility of complex salts, for example alums, more especially ammonium aluminium sulphate, by the incorporation of urea in the solution. The present invention also provides solutions of such salts in the form of aqueous liquids of higher con¬ centration than would be normally expected.

For many applications (and the use of these materials to promote the curing of certain resinous materials is only one such application) salts need to be dissolved in solvents in order to provide easier handling or dispensing, and to ensure more intimate mixing with other components used in formulations. Aqueous solutions

- 13 -

have many advantages in that the solvent, water, is readily available, cheap, and non-flammable.

Many salts have good or even excellent solubility in water and if necessary can be used at high concentration. However, in other cases it is impossible to achieve the desired concentration at the required temperature of use or storage, because of the poor inherent solubility of the salt in water. In other cases where it is desired to use a combination of two or more salts the solubility of the combination is often inferior to that of the individual components. We have found that by the use of urea the solubility of certain salts can be substantially ^" improved.

Aluminium salts, for example, often have very good solubility in water especially when simple salts such as aluminium chloride or sulphate are used. However, as mentioned above, when aluminium salts are to be used in combination with other salts with which they can form complex salts, e.g. ammonium salts, difficulties often occur because the complex salt has only relatively poor water solubility.

The mechanism of action of the urea is not entirely clear. Its action may be at least partly attributable to breaking of hydrogen bonds. Accordingly, the use of urea derivatives or other hydrogen-bond-breaking compounds is also contemplated. Possible examples include formamide, dimethylformamide, acetamide, acrylamide, polyacrylamide and 4- or 5-hydroxypropyl urea.

- 14 -

Urea is a preferred hydrogen-bond-breaking compound, however, especially in the field of curing agents for formaldehyde-containing resins, because it may also act as a scavenger for formaldehyde. In that case the amount of urea present when the salts are present in solution must, of course, be sufficient to improve the water-solubility sufficiently so as to provide a curing agent solution with long-term stability, (i.e. preferably stable for more than six weeks) . A suitable urea:water weight ratio may be, for example, 1.5:1 to 1:4, and a ratio of more than 1:1.5, advantageously about 1:1, should especially be mentioned.

In one embodiment of the present invention a curing agent solution comprises ammonium chloride or sulphate and aluminium sulphate in a weight ratio of approximately 1:1, and urea and water in a weight ratio of approxi¬ mately 1:1. The ratio of the weight of combined salts to the weight of urea and water may be, for example, 1:2.5 to 1:3, e.g. 1:2.7. The solution may be added to a resin in a weight ratio of curing agent solution:resin of, for example, 1:25 to 3:25.

The invention is illustrated by the following Examples, in which all percentages are by weight. Example 1 In this Example, the catalytic activity of a curing agent solution of the invention, comprising an ammonium and an aluminium salt (Solution C) , was compared with that of solutions of the same amounts of the individual

- 15 -

salts (Solutions A and B) .

The curing agent solutions A, B, and C were prepared according to the compositions given in Table 2 below. Urea was also present so as to prevent the precipitation of the complex salt ammonium aluminium sulphate. The solutions were each added to a urea-formaldehyde resin of F:U molar ratio 1.05:1 in amounts of 5 % and 10 % (relative to the weight of the resin) . Six drops of each mixture were transferred to a 100mm by 10mm test-tube, which was then heated in a bath of boiling water. The gelation time was recorded for each of the six resin mixtures, and is given in Table 2 below. b

Curing agent Solution A, containing only an ammonium salt, gave the longest gelation time and did not improve the curing rate when present in a greater amount.

Solution B provided a faster curing rate, which rate decreased with an increase in volume of solution added. Solution C, which contained the same amounts of ammonium chloride and aluminium sulphate as Solutions A and B, respectively, provided the fastest curing rate.

Example 2

This Example illustrates how the solubility in an aqueous solution of ammonium aluminium sulphate increases when the hydrogen-bond-breaking compound urea is added to the solution..

Referring to Table 3 below, five aqueous solutions, A to E, each comprising an ammonium and an aluminium salt, were prepared in the quantities indicated. In each of solutions B,C and E, a significant amount of water was replaced by urea. Each solution was heated to a suffi¬ cient temperature to cause complete dissolution of the components. The stability of each solution was then monitored at three different temperatures.

Table 3 Component

Ammonium Sulphate Ammonium Chloride Water Urea

- 17 -

Curing Agent Stability at different temperatures Solution 2 20 40

A * * immediate crystallisation

B * 6 weeks *

C > 6 weeks > 6 weeks *

D * immediate crystallisation

E * > 6 weeks * * not tested

Referring first to solutions A, B and C, each of which contains 10% (by weight) aluminium sulphate tetradecahydrate and 10 % ammonium sulphate, it will be noted that ammonium aluminium sulphate precipitates from Solution A, in which the solvent is pure water, even at the highest temperature of 40 ^β C. In Solutions B and C, however, the solubility of the complex salt improves with increasing % by weight of urea present in the aqueous solution. A similar result is found for Solutions D and E, in which the ammonium sulphate is replaced with ammonium chloride.

Example 3

In this Example, two particleboards, A and B, were prepared from wood chips using a low formaldehyde content UF resin as a binder. Board A was cured using a curing

- 18 -

agent comprising an aqueous solution of ammonium chloride and aluminium sulphate, in which the hydrogen-bond- breaking compound, urea, was also present. Board B was cured using a traditional ammonium salt curing agent. Board A

25.1 kg of a UF resin having a F:U molar ratio of 1.05:1, and solids content of 65.0%, was mixed with 1.25 kg of an aqueous curing agent solution consisting of 13.33% ammonium chloride, 13.33% aluminium sulphate tetradecahydrate, 36.67% urea, the remainder being water, and with 3.15 kg wax emulsion, and 70.5 kg wood chips. (The aqueous curing agent solution had been kept for 2 weeks at 10°C to 20 ^β C, prior to use, and had remained a clear solution in which there was no precipitate.) A moulded particleboard was formed which was pressed for 2% minutes at 160°C. Board B

A similar board was made using 24.5 kg of the same UF resin, which was mixed with 2.48 kg of a curing agent solution containing 20% ammonium chloride, 3.12 kg wax emulsion and 70.0 kg wood chips. The board was pressed for 2 minutes at 160°C.

The boards were conditioned for 1 week at 68°F (20°C) and the total extractable formaldehyde determined using the iodometric method.

The results were as follows:-

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Total extractable formaldehyde ⁽ mg/100 g dry board) Board A 4.8

Board B 7.6

Although both boards met the El classification. Board A contained the least residual free formaldehyde and was also found to have the better mechanical properties.