PROCESS FOR THE PREPARATION OF A MODIFIED AMINO-FORMALDEHYDE RESIN; MODIFIED AM1NO-FORMALDEHYDE RESIN THUS OBTAINABLE, AND USE

THEREOF

The invention relates to a process for the preparation of a modified amino-formaldehyde resin. Furthermore, the invention relates to a modified amino- formaldehyde resin. Moreover, the invention relates to the use of the modified amino- formaldehyde resin as cross-linker in coatings, in particular powder coatings.

It is well-known to prepare hexa(alkoxymethyl)melamines such as hexa(methoxymethyl)melamine (HMMM) or hexa(butoxymethyl)melamine (HBMM), and to use HMMM or HBMM or mixtures thereof as a cross-linking agent in coatings such as for polyester powders coatings. The disadvantage associated with the use of HMMM and/or HBMM, however, is that they have a low glass transition temperature (Tg) of about -60 ⁰ C. When HMMM and/or HBMM are used in an effective amount - typically 10 to 20 wt.% of a coating composition - the T ₉ of an HMMM- or HBMM- containing powder coating composition is lowered to undesirably low values, lying often below room temperature.

It is the objective of the present invention to reduce or even eliminate the aforementioned disadvantage. The objective is achieved by a modified amino-formaldehyde resin, obtainable by a process for the preparation of a modified amino-formaldehyde resin comprising: a) a mixing step, in which a liquid phase, an amino-formaldehyde resin and free formaldehyde are brought together to form a mixture, whereby the amino- formaldehyde resin has a molar formaldehyde to (NH ₂ ) ₂ ratio (F/(NH ₂ ) ₂ ratio) lying between 0.5 and 3 and has a weight average molecular weight lying between 500 and 50,000, and whereby the overall molar F/(NH ₂ ) ₂ ratio in the mixture is higher than the molar F/(NH ₂ ) ₂ ratio of the amino-formaldehyde resin and lies between 2 and 6; b) an F-binding step, in which the mixture is brought to conditions whereby at least part of the free formaldehyde reacts with the amino-formaldehyde resin to form a formaldehyde-enriched amino-formaldehyde resin, such that the F/(NH ₂ ) ₂ ratio of the formaldehyde-enriched amino-formaldehyde resin is at least 1 higher than that of the amino-formaldehyde resin;

c) an alkylation step, in which an alcohol and an acid having a pKa lower than 6 are added to the mixture, the mixture then being brought to conditions whereby at least part of the alcohol reacts with the formaldehyde-enriched amino-formaldehyde resin to form the modified amino-formaldehyde resin, whereby steps a) and b) may be executed either consecutively or simultaneously.

The advantage of the modified amino-formaldehyde resin according to the invention is that it can have a higher or even substantially higher T ₉ than HMMM or HBMM. Depending on the T ₉ , the modified amino-formaldehyde resin according to the invention may at room temperature still be a liquid or it may be a solid. The modified amino-formaldehyde resin according to the invention is suitable as cross- linking agent in coating compositions, in particular in powder coating compositions.

US 3,488,350 discloses a process for preparing high purity polyalkyl ethers of polymethylol melamine compounds comprising reacting a polymethylol melamine with a quantity of a monohydric aliphatic alcohol having from 1 to 4 carbon atoms in the presence of an acid cation exchange resin and isolating the melamine derivative therefrom, wherein the number of methylol groups in the melamine starting compounds is at least three and the number of moles of the alcohol actually reacted with said polymethylol compound is at least three. The starting compounds do not comprise a melamine-formaldehyde resin. US 2,998,411 discloses a process comprising dissolving melamine in a substantially non-alcoholic aqueous solution of formaldehyde at a mol ratio of at least 6.5:1 formaldehyde to melamine, respectively, at a pH between 7 and 11 and correspondingly at a temperature between about 75 ⁰ C and 35 ⁰ C, with constant stirring until the solution clears, while adjusting the water content to a minimum of 60% by weight of the total charge, maintaining the temperature between 75°C and 35 ⁰ C, while stirring the reaction mixture, while the reaction continues, with movement through the reaction medium in an otherwise unagitated dispersion, raising the pH to between 8 and 11 upon exotherm subsidence, in order to inhibit temperature decline, until the reaction product has substantially completely precipitated out of solution and separating the solids from the supernatant liquid; this is then followed by alkylation. The starting compounds do not comprise a melamine-formaldehyde resin.

In DE 1 595 432 a process is disclosed for the preparation of a solvent-free etherified methylolmelamine condensate in powder form, wherein 1 mol melamine is condensed in a weakly acidic medium at reflux temperature with 3 to 6 mol of formaldehyde in the presence of methanol, ethanol, isopropanol or a mixture thereof,

until a few drops of the reaction mixture cause turbidity in a 65 vol.% aqueous ethanol solution, the neutralised reaction mixture then being poured into water, the so obtained precipitate separated in the usual fashion and dried at a temperature below 50 ⁰ C. The starting compounds do not comprise a melamine-formaldehyde resin. In CH206611 a process is disclosed for the preparation of a resin, whereby melamine, formaldehyde and isopropanol are made to react with each other until resin-forming has taken place. The starting compounds do not comprise a melamine-formaldehyde resin.

The modified amino-formaldehyde resin according to the invention is obtainable by a process. Said process comprises a mixing step a) in which a liquid phase, an amino-formaldehyde resin and free formaldehyde are brought together to form a mixture.

As liquid phase, any liquid compound or mixture of liquid compounds is suitable provided that an amino-formaldehyde resin and free formaldehyde can dissolve in it, and that it can be a liquid under the conditions where steps b) and c) are performed. It may be helpful or necessary to execute the process according to the invention under increased pressure so as to prevent evaporation of the liquid phase. Examples of suitable liquid phases are water, various alcohols, or mixtures of water and one or more alcohols. Preferably the liquid phase comprises or even consists essentially of water, isopropanol, butanol or mixtures thereof.

In the process according to the invention, the liquid phase is brought together with an amino-formaldehyde resin, whereby the amino-formaldehyde resin has a molar formaldehyde to (NH ₂ ) ₂ ratio (F/(NH ₂ ) ₂ ratio) lying between 0.5 and 3. Such resins are known per se from for example Kunststoff Handbuch, Vol. 10 Duroplaste (Becker, Braun; Carl Hanser Verlag 1988), where in chapter 1.2.3 the preparation of melamine resins is disclosed. Examples of widely-used amino-formaldehyde resins are melamine-formaldehyde resins, urea-formaldehyde resins, or melamine-urea- formaldehyde resins.

The molar F/(NH ₂ ) ₂ ratio of the amino-formaldehyde resin should be at least 0.5, so as to enable proper resin formation. Preferably, the molar F/(NH ₂ ) ₂ ratio is preferably at least so high that essentially all amino compounds are bridged to at least one other amino compound. More preferably, the molar F/(NH ₂ ) ₂ ratio of the amino-formaldehyde resin is at least 0.7 or 0.9, most preferably at least 1.0. On the other hand, the molar F/(NH ₂ ) ₂ ratio of the amino-formaldehyde resin should remain below 3 so that significant HMMM formation during the process according to the

- A -

invention is avoided; preferably, the molar F/(NH ₂ ) ₂ ratio lies below 2.8 or 2.6, more preferably below 2.4 or 2.2, most preferably below 2.0 or 1.8. An amino compound is herein defined as a compound having at least one -NHR or -NH ₂ group, attached to an electron-withdrawing atom or group or to a carbon atom that itself is attached to an electron-withdrawing atom or group. Examples of electron-withdrawing atoms are oxygen, nitrogen, and sulphur. R can in principle be any organic group. Examples of suitable amino compounds are urea, guanidine, triazines such as melamine, or mixtures of these amino compounds.

The weight-averaged molecular weight of the amino-formaldehyde resin is preferably at least such that it equals the molecular weight of a resin molecule formed after at least two molecules of the amino compound or compounds as comprised in the starting materials to prepare the amino-formaldehyde resin have reacted. More preferably, the molecular weight is at least equal to that of a resin molecule comprising the reacted form of three or even four molecules of the amino compound comprised in the starting materials. Without committing to theory, it is thought that an increase of molecular weight of the amino-formaldehyde resin may lead to an increase of the T ₉ of the modified amino-formaldehyde resin ultimately obtained according to the invention. It is preferred that the molecular weight of the amino- formaldehyde resin is such that a resin molecule comprises on average at most 100, more preferably at most 50 or 40 of the amino compounds as comprised in the starting materials. In a preferred embodiment of the invention, the amino-formaldehyde resin has a weight-average molecular weight (Mw) lying between 500 or 1 ,000 and 50,000; more preferably the Mw lies between 2,000, 3,000 or 5,000 and 45,000. If the amino- formaldehyde resin is a melamine-formaldehyde (MF) or melamine-urea-formaldehyde (MUF) resin and the liquid phase with which it will be brought into contact with in step a) is water, it is preferred that the resin has reacted at least to such a molecular weight that the cloud point at O ⁰ C has been reached. As is known, the cloud point of a MF or MUF resin at a temperature T ₀ is reached during its preparation when a drop of resin, added to a large amount of water at T _C °C, no longer dissolves but shows turbidity. On the other hand, it is preferred that the resin has not reacted beyond the cloud point at the boiling temperature of the mixture, i.e. when a dispersion of solid resin particles in the mixture is formed. This boiling temperature will be not far removed from 100 ⁰ C when working at atmospheric pressure, or it may be significantly higher than that if the process according to the invention is carried out at elevated pressure. If the amino-formaldehyde resin is a melamine-formaldehyde (MF) or

melamine-urea-formaldehyde (MUF) resin and the liquid phase with which it will be brought into contact with in step a) is not water, then a different type of cloud point measurement could be implemented: instead of adding a drop of resin to water, a drop of resin is added to white spirit (CAS number 8052-41-3). A similar evaluation criterion applies: the white spirit cloud point at T ₀ is reached when a drop of resin, added to a large amount of white spirit at T ₀ ⁰ C, no longer dissolves but shows turbidity. It is preferred that the resin has reacted at least to such a molecular weight that the white spirit cloud point at T ₀ = 0 ⁰ C has been reached, but not beyond the cloud point at T ₀ = boiling point of the white spirit at the prevailing pressure. The boiling point of white spirit lies - as is known - between 130 ⁰ C and 23O ⁰ C at atmospheric pressure.

The resin may be an essentially dry material or it may be a solution or dispersion, e.g. in aqueous phase, when it is brought into contact with the liquid phase.

In the process according to the invention, the liquid phase and the amino-formaldehyde resin are brought into contact with each other and with free formaldehyde to form a mixture. Free formaldehyde means herein either formaldehyde itself or a compound that can release formaldehyde at the conditions prevailing in step a) or step b), such as for example paraformaldehyde or trioxane. The free formaldehyde can be brought into the mixture as such, or in the form of a solution. In practice, aqueous solutions of formaldehyde are very commonly used and are suitable for use in the process according to the invention. As a result of the free formaldehyde addition, the overall molar F/(NH ₂ ) ₂ ratio of the mixture should increase. In a preferred embodiment according to the invention, no amino compound is added in this step or in any subsequent step of the process according to the invention. Thus, the molar F/(NH ₂ ) ₂ ratio of the mixture will be higher than the molar F/(NH ₂ ) ₂ ratio of the amino- formaldehyde resin. According to the invention, an amount of free formaldehyde should be added in such a way that the overall molar F/(NH ₂ ) ₂ ratio of the mixture lies between 2 and 6. Preferably, the overall molar F/(NH ₂ ) ₂ ratio of the mixture is increased in the mixing step to at least 2.3, 2.6, 2.9, 3.2, or 3.5. Preferably, the overall molar F/(NH ₂ ) ₂ ratio of the mixture is increased in the mixing step to at most 5.5, 5.0, 4.5, 4.0, or 3.8. In a preferred embodiment, the amount of free formaldehyde as added to the mixture is such that the overall molar F/(NH ₂ ) ₂ ratio of the mixture is at least 0.2 or 0.4 higher than that of the amino-formaldehyde resin. More preferably, the said ratio increases by at least 0.8 or 1.0; most preferably, the said ratio increases by at least 1.5, 0, 2.5 or 3, or even by at least 3.5, 4.0, 4.5 or 5.0. It was found that a higher increase of the overall

molar F/(NH ₂ ) ₂ ratio of the mixture leads to further improved properties of the end product - i.e. the modified amino-formaldehyde resin - in particular relating to its efficiency as cross-linking agent. The amount of free formaldehyde is preferably such that at least 40 mol%, more preferably at least 50, 60, 70, 80, 90 mol%, or even essentially all -NH functionalities in the -NHR or -NH ₂ groups as present in the amino- formaldehyde resin can be converted into -N-CH ₂ -OH groups in the F-binding step b) to be discussed below.

In F-binding step b) of the process according to the invention, the mixture is brought to conditions whereby at least part of the free formaldehyde reacts with the amino-formaldehyde resin to form a formaldehyde-enriched amino- formaldehyde resin. It was found that the said reaction will in many cases occur spontaneously; this is typically at least the case if the mixture comprises water as the liquid phase and the temperature lies between room temperature and the boiling point of the mixture. In such a case, and representing a preferred embodiment of the process according to the invention, steps a) and b) according to the invention will occur simultaneously or almost simultaneously. Should the F-binding step b) not occur spontaneously, the skilled person can find suitable conditions for implementing this step by varying temperature, pH, and pressure: a higher temperature (possibly under increased pressure in order to prevent boiling) will speed up the F-binding step, as well as a (further) removal of the pH from neutral. The F-binding step according to the invention should be carried out in such a fashion that the molar F/(NH ₂ ) ₂ ratio of the formaldehyde-enriched amino-formaldehyde resin is at least 0.5 higher than that of the amino-formaldehyde resin; preferably, the said increase in molar F/(NH ₂ ) ₂ ratio is at least 1.0, 1.5, 2, 2.5 or even 3. In a preferred embodiment, the F-binding step is carried out in such a fashion that the molar F/(NH ₂ ) ₂ ratio of the formaldehyde-enriched amino- formaldehyde resin lies between 3.5 and 6.0, more preferably between 4.0 and 6.0, most preferably between 4.5 or 5.0 and 5.9 or 6.0.

In alkylation step c) of the process according to the invention, the mixture comprising the formaldehyde-enriched amino-formaldehyde resin is brought into contact with an alcohol and an acid having a pKa lower than 6, the mixture then being brought to conditions whereby at least part of the alcohol reacts with the formaldehyde-enriched amino-formaldehyde resin. With this reaction, the modified amino-formaldehyde resin according to the invention is formed.

The mixture is brought into contact with an alcohol. Preferably, an alcohol or mixture thereof is chosen so that is at least partly miscible with the liquid

phase. More preferably, the alcohol or mixture of alcohols is chosen such that it is miscible with the liquid phase in all ratios. In a preferred embodiment of the process according to the invention, water is the liquid phase and isopropanol is the alcohol. In an alternative embodiment, the liquid phase already comprises the alcohol as needed in alkylation step c), in which case it may not be necessary anymore to add any alcohol during this step. Preferably within this embodiment, the liquid phase is a water/isopropanol mixture and no alcohol is added to the mixture. Examples of other suitable alcohols that are to be added in step c) and/or are comprised within the liquid phase are: isopropanol, n-propanol, isobutanol, 2-butanol, and ethanol. In alkylation step c) of the process according to the invention, an acid having a pKa lower than 6 is added to the mixture. It was found that an acidic environment is often required in order to enable the formation of the modified amino- aldehyde resin from the formaldehyde-enriched amino-formaldehyde resin. Preferably, the acid to be added has a pKa of at most 5 or 4; more preferably, the acid to be added has a pKa of at most 3 or 2. Hydrochloric acid (HCI) is an example of a suitable acid. When adding the acid, it is preferred to add the acid in such an amount that a pH between 0 and 7 is reached, preferably between 1 and 6 or between 1.5 and 5.5. In general, the speed of alkylation step c) will increase when the pH is lower.

In alkylation step c) of the process according to the invention, the mixture comprising the formaldehyde-enriched amino-aldehyde resin, the alcohol and the acid is brought to conditions whereby at least part of the alcohol reacts with the formaldehyde-enriched amino-aldehyde resin in an alkylation reaction. Typical examples of such conditions are, besides the already given pH indications: a temperature lying between 50 ⁰ C and 110 ⁰ C, and a pressure lying between 0.01 and 2 MPa. It can be beneficial to select temperature and pressure in the previous steps a) and b) such that these parameters do not need to be altered for step c). Preferably, step c) is maintained for such an amount of time that at least 25% of all amino- formaldehyde bonds have undergone alkylation. More preferably, at least 35, 50, 65, or even at least 80% or essentially all of the amino-formaldehyde bonds have undergone alkylation.

In a preferred embodiment of the process according to the invention, the alkylation step c) is followed by a pH correction step d) in which the pH of the mixture is increased, to between 7 and 14. The advantage of implementing pH correction step d) is that the chance that self-condensation of the modified amino- formaldehyde resin according to the invention occurs is reduced or even eliminated. As

a rule of thumb, the risk of self-condensation is (further) reduced with as the pH to which the mixture is brought is increased. Preferably, the pH of the mixture is brought to between 8 or 9 and 13.

Alkylation step c) is, optionally after implementing pH correction step d), preferably followed by isolation step e), in which the mixture is combined with a sufficient amount of water to let the modified amino-formaldehyde resin precipitate, followed by isolation of the solid modified amino-formaldehyde resin. The water preferably has a temperature between O ⁰ C and 23°C, more preferably between 5°C and the T ₉ of the resin - that is, if the T ₉ of the resin lies above 5°C. Since the modified amino-formaldehyde resin according to the invention is usually not soluble in water to any significant extent, the feeding of the mixture to water subsequent to step c) or d) will lead to its precipitation. In order to achieve said precipitation, a certain amount of water must be present. Typically, this amount will be at least as much as the total amount of alcohol added to the mixture in steps a) and/or c). Preferably, this amount lies between 5 and 50 times the amount of alcohol added to the mixture in steps a) and/or c).

The precipitation of the modified amino-formaldehyde resin enables its separation from the mixture. The said separation can be carried out by any suitable means, by for example using a filter or a centrifuge. In a preferred embodiment according to the invention, the isolation step e) does not only comprise the said separation from the mixture but also - and subsequent to the separation - one, two, or more washing steps. In such a washing step, the modified amino-formaldehyde resin is brought into contact with a washing liquid. A suitable washing liquid is a liquid in which the modified amino-formaldehyde resin does not dissolve, and which is not an alcohol as used in any one of the preceding steps. Preferably, the washing liquid is miscible with the alcohols as used in previous steps in all ratios, thereby allowing complete removal of the said alcohols. In a preferred embodiment, the alcohol as used in previous process step c) is isopropanol and the washing liquid is water. The contacting of the modified amino-formaldehyde resin with the washing liquid is followed by its separation from the washing liquid. Preferably, the number of washing steps is limited to 5, more preferably 4. The advantage of the washing step is that the alcohol or alcohols as used in steps a) and/or c) are essentially removed from the modified amino-formaldehyde resin, thereby preventing undesirable physical and/or chemical side effects. An example of such an undesirable side effect is a T ₉ depressing effect, which could prevent obtaining the modified amino-formaldehyde resin according to the

invention as powder.

It may also be useful to perform one or more precipitation steps on the modified amino-formaldehyde resin according to the invention. In a precipitation step, the resin is first dissolved in a suitable solvent such as for example acetone, followed by precipitation in preferably water. The advantage of a precipitation step is that the amount of any impurities can be reduced. In order to be able to execute the precipitation, it is desirable - also in general for all modified amino-formaldehyde resins according to the invention - that the resin is in ungelled form; this means that the resin can be dissolved in a suitable solvent or resin. Following the separation step and washing and/or precipitation step or steps, it may be beneficial to dry the modified amino-formaldehyde resin, for example under vacuum and preferably at a temperature below its T ₉ , such as for example at about 30 ⁰ C.

As indicated earlier, the invention relates to the above-described process for the preparation of a modified amino-formaldehyde resin, as well as to the modified amino-formaldehyde resin itself as obtainable by the said process. Closely interrelated with this, the invention also relates to a modified amino-formaldehyde resin, comprising a compound or a mixture of compounds according to formula (I):

(I) wherein:

• A is the core of an amino compound A-(-NH ₂ ) _x ;

• x is at least 2;

• B is H, CH ₂ OR ¹ , a group according to formula (III), or a group according to formula (IV), whereby: o R ¹ is H or a group according to formula (II) wherein R ² , R ³ , R ⁴ , and R ⁵ are H or a C ₁ -C ₄ alkyl:

R ³

R ² C -H

-H

R ⁴ - -H

R ⁵

(H) o formula (III) is:

(III) o formula (IV) is:

(IV) whereby:

• at least 30% or 40%, preferably 50%, 60%, ore even 70% or 80% of all B groups are CH ₂ OR ¹ in which R ¹ is a group according to formula (II);

• the resin as a whole comprises at most 1.5 or 1.3, preferably at most 1.0, 0.8, 0.7 or even at most 0.6, 0.5 or 0.4 -NH groups per 'A' moiety, and

• the weight-average molecular weight of the resin lies between 500 and 10,000; preferably, the said weight-average molecular weight of the resin lies between 750 and 9,500, more preferably between 1 ,000 and 9,000.

In formula (I) above, A indicates the core of an amino compound A-(-NH ₂ ) _x . A is thus that part of the chemical formula of an amino compound that remains when all, -NHR and -NH ₂ groups are removed. Examples of suitable amino compounds A-(-NH ₂ ) _x are melamine, urea, melam, and melem; preferably, the amino compound is melamine, urea of a mixture thereof. On the level of a molecule, x is an integer and should be at least 2 in order to enable resin formation. However, x can also

be viewed from the point of view of the resin as a whole. Then, x is the averaged value over all amino compounds present in the resin and need not be an integer. In a preferred embodiment of this modified amino-formaldehyde resin according to the invention, A-(-NH ₂ ) _x is melamine; x is 3; and R ² , R ³ , R ⁴ , and R ⁵ are all H. In order to obtain the modified amino-formaldehyde resin, comprising a compound or a mixture of compounds according to formula (I), the process according to the invention as disclosed above can be advantageously used. Care should be taken to ensure that the resin as a whole comprises at most 1.5 -NH group per 'A' moiety; this can be achieved for example by ensuring that F-binding step b) is executed during a sufficiently long time period or under optimised conditions of temperature or pH, and/or by ensuring that the molar F/(NH ₂ ) ₂ ratio of the mixture is sufficiently high. Care should also be taken to ensure that at least 30% of all B groups are CH ₂ OR ¹ in which R ¹ is a group according to formula (II); this can be achieved for example by ensuring that alkylation step c) executed during a sufficiently long time period or under optimised conditions of temperature or pH. Moreover, care should be taken to ensure that the weight-average molecular weight of the resin lies between 500 and 10,000; this can be achieved for example by choosing an adequate weight-average molecular weight of the amino-formaldehyde resin as used in step a).

The invention furthermore also relates to the use of the modified amino-formaldehyde resin according to the invention as cross-linking agent in a coating composition, in particular in a powder coating composition. Consequently, the invention also relates to a coating composition, preferably a powder coating composition, comprising a modified amino-formaldehyde resin according to the invention; moreover, the invention also relates to a coating as obtainable by curing of a coating composition according to the invention.

As is known, a coating composition typically comprises a coating resin, a cross-linking agent and optionally other compounds such as pigments, catalysts, stabilizers, flowing agents and other compounds. The coating composition is then subsequently cured so as to obtain a coating. The modified amino-formaldehyde resin according to the invention may be used as sole cross-linking agent; typical amounts as used in coating compositions are 5 - 40 wt.%, preferably 10 - 30 wt.%. This has the advantage, in particular in powder coating compositions, that the amount as used is hardly or even not at all limited by considerations relating to the T ₉ such as is the case with HMMM and HBMM but rather can be chosen so as to obtain a desired (high) cross-linking result. The modified amino-formaldehyde resin according to the

invention may also be used in combination with another cross-linking agent such as a limited amount of the aforementioned HMMM, HBMM or others.

Various aspects of the present invention will be illustrated by means of the following Example, without being limited thereto.

Example I

Preparation of a melamine-formaldehyde resin

A reactor was heated to 50 ⁰ C; 99 gram of a 50.5 wt.% solution of formaldehyde (F) in water was put into the reactor. The pH was adjusted to 9.3 with sodium hydroxide. Then, 100 gram melamine and 74 gram water were added. The overall F/(NH ₂ ) ₂ ratio was 1.4. The reactor contents were brought to 5O ⁰ C, and then heated further to 95°C so that the reaction to form a melamine-formaldehyde resin could start. The resin-forming reaction was continued until the cloud point at 2O ⁰ C was reached. Then the resin was cooled to 70 ⁰ C.

Preparation of the formaldehyde-enriched amino-formaldehyde resin

To the resin, as cooled to 70°C, was slowly added 185 gram of a 50.5 wt.% solution of formaldehyde in water, having a pH 9.3. The addition of formaldehyde brought the overall molar F/(NH ₂ ) ₂ ratio to 4. Under the circumstances as described, the formaldehyde reacted almost immediately with the resin, whereby the formaldehyde-enriched amino-formaldehyde resin was formed. The reactor contents were subsequently cooled to 50°C; the contents were then removed from the reactor and allowed to cool further, whereby a solid matter was formed. The cooled formaldehyde-enriched amino-formaldehyde resin was left overnight, then subsequently milled to a powder.

Preparation of the modified amino-formaldehyde resin

The milled formaldehyde-enriched amino-formaldehyde resin was washed twice with methanol, in order to dry the resin, followed with a drying step during two hours at 45°C under vacuum. This drying step is not essential according to the invention; it was done with a view on easy handling. A reactor was filled with 50 gram isopropanol, 20 gram of water, and 20 gram of the milled formaldehyde-enriched amino-formaldehyde resin under stirring. The contents were heated by placing the reactor in an oil batch of 95 ⁰ C, and the pH was brought to 5.5 with HCI. At 82.3°C, an

isopropanol/water vapour mixture started to boil off. As is known, water and isopropanol form an azeotropic mixture. The amount of fluid as boiled off was replenished with pure isopropanol; this was done as long as necessary to ensure that the amount of water in the vapour mixture was reduced to only 1 wt.%; this took 10 hours. During this time, in total 84.9 more grams of isopropanol than the amount of boiled-off vapour were added. The reactor contents were brought to a pH of 11.5 with 'dry' KOH (in this case, KOH in methanol). 104 gram of a mixture of modified amino- formaldehyde resin and isopropanol was obtained; this mixture was first filtered to remove any precipitated impurities such as salts, and then concentrated in a rotating film evaporator to 48 gram. The 48 grams were put in 250 ml of water, leading to precipitation. The solid product was filtered using a paper filter, then dissolved in 100 ml acetone. The clear solution was then put into about 250 ml water, leading to precipitation. The modified amino-formaldehyde resin was then collected through filtration.

Evaluation

A differential scanning calorimetry (DSC) test was done on the modified amino-formaldehyde resin. The T ₉ was determined to be 42.3°C (mid-point DSC). The DSC measurement also enabled to determine the self-condensation temperature, at 151.5°C (onset) / 186°C (peak). These values prove that the modified amino-formaldehyde resin according to the invention had a much higher T ₉ than HMMM, and also that it is stable until relatively high temperatures; this enables a use under typical conditions in which cross-linkers in coatings, in particular powder coatings, are used. In for example powder coatings, it is common to mix all ingredients prior to the curing of the coating at temperatures of about 120 ⁰ C.

Example Il

Preparation of a melamine-formaldehyde resin A reactor was heated to 5O ⁰ C. 441 gram of a 29.7 wt.% solution of formaldehyde (F) in water was put into the reactor. The pH was adjusted to 9.3 with sodium hydroxide. Then, 262.5 gram melamine and 12 gram water were added. The overall F/(NH ₂ ) ₂ ratio was 1.4 (F/M molar ratio was thus 2.1). The reactor contents were brought to 50 ⁰ C, and then heated further to 95°C so that the reaction to form a melamine-formaldehyde resin could start. The resin-forming reaction was continued for

2.5 hours, until the content of reactor turned bluish and somewhat hazy, indicative of initiation of the formation of very small particles. Then the resin was cooled to 7O ⁰ C.

Preparation of the formaldehyde-enriched amino-formaldehyde resin To the resin, as cooled to 7O ⁰ C, was slowly added 441 gram of a 50 wt.% solution of formaldehyde in water, having a pH 9.3. The addition of formaldehyde brought the overall molar F/(NH ₂ ) ₂ ratio to 4 (F/M molar ratio was 6). Under the circumstances as described, the formaldehyde reacted almost immediately with the resin, whereby the formaldehyde-enriched amino-formaldehyde resin was formed. The reactor contents were subsequently cooled to 5O ⁰ C; the contents were then removed from the reactor and allowed to cool further, whereby a solid matter was formed; this indicates that the water as present was adsorbed to the formaldehyde-enriched amino- formaldehyde resin. The cooled formaldehyde-enriched amino-formaldehyde resin was subsequently milled to a powder.

Preparation of the modified amino-formaldehyde resin

A reactor was filled with 160 gram isopropanol, then heated to 75 ⁰ C. 100 gram of the milled formaldehyde-enriched amino-formaldehyde resin was added under stirring. Then, 3.5 ml of 37 wt.% HCI was added. The whole mixture was allowed to react at 75°C until a clear solution was obtained. This indicated that the modified amino-formaldehyde resin was being prepared. Then, the clear solution was cooled to room temperature and allowed to stand overnight while being stirred. The pH was subsequently increased with sodium hydroxide to pH 8; the solution remained clear. The solution was then put into about 1 litre water under stirring. The modified amino- formaldehyde resin precipitated, and was subsequently filtered out using a paper filter. The modified amino-formaldehyde resin was subsequently washed four times with water; each time, isolation was done by means of filtering. Finally, the modified amino- formaldehyde resin was dried in a vacuum oven at 30 ⁰ C and obtained as a white powder.

Use of the modified amino-formaldehyde resin as cross-linker

20 parts of the modified amino-formaldehyde resin as prepared above, 80 parts of Uralac™ P6504 powder (an OH-functional saturated carboxylated polyester coating resin; supplier: DSM Resins), 0.5 part of a catalysing compound consisting of 1 part paratoluene sulfonic acid and 3 parts AEPD (2-amino-2-ethyl-1 ,3-

propanediol), 0.75 part of benzoin as anti-foaming agent, 50 parts titanium white (Kronos™ 2160), and 1.5 part of Resiflow™ PV5 as flowing agent (supplier: Worlee- Chemie) were fed to a twin-screw extruder and extruded at 100°C. The extrudate was milled, and subsequently sieved so as to obtain a powder coating composition having no particles bigger than 90 μm.

A layer of the powder coating composition, which had a T ₉ lying above room temperature, was sprayed electrostatically onto an aluminium sheet. The layer had a thickness of about 35 μm. In order to investigate at what temperature a curing of the coating could be achieved - including cross-linking as achieved by the modified amino-formaldehyde resin according to the invention - the powder-covered aluminium sheet was put into a gradient oven during 10 minutes. The oven had a gradient from 100 ⁰ C to 200 ⁰ C.

The aluminium sheet as taken out of the gradient oven was put to the acetone double rub test. In this test, a cloth is drenched in acetone. The cloth is then rubbed to and fro over the coating. One to and fro movement counts as one double rub. In executing the test, it is counted how many times the cloth can be swiped over the coating until the coating has dissolved, whereby the requirement for satisfactory performance is achieved if the coating withstands at least 100 double rubs. This requirement was met at curing temperatures of 14O ⁰ C and higher, thereby proving that sufficient curing of the powder coating composition had indeed been achieved at those temperatures.