WO2007111501A1

WO2007111501A1 - Process for the preparation of an aminoplast resin

Info

Publication number: WO2007111501A1
Application number: PCT/NL2007/000088
Authority: WO
Inventors: Lars Evers; Jozef Maria Johannes Mattheij; Josef Weiss; Leonardus Hubertus Maria Augustus
Original assignee: Dsm Ip Assets B.V.
Priority date: 2006-03-29
Filing date: 2007-03-29
Publication date: 2007-10-04
Also published as: WO2007111501A8; AT503516A1

Abstract

The invention describes a condensation process for the preparation of a melamine-urea-formaldehyde condensate by reacting a reaction batch comprising melamine, formaldehyde and urea in an aqueous alkaline phase at a temperature between 75°C and the boiling point of the reaction batch, wherein the melamine being added exclusively or predominantly in a first stage and the urea exclusively or predominantly in a second stage. In the first stage, only melamine is present or melamine is present at least in twice the molar proportion of urea, and the pH of the first stage is between 7.5 and 10, and, in the second stage, the molar ratio of urea to melamine is between 0.3 and 1.5 and the molar ratio of formaldehyde to -(NH2)2 is between 0.7 and 1.5, and the pH in the second stage is between 7.0 and 9.0.

Description

PROCESS FOR THE PREPARATION OF AN AMINOPLAST RESIN

The present invention relates to a condensation process for the preparation of a phenol-free aminoplast resin based on reacting an aldehyde, a (substituted) urea and a 1 ,3,5-triazine compound. The present invention specifically relates to a condensation process for the preparation of a phenol-free melamine-urea- formaldehyde condensate by reacting melamine, formaldehyde and urea in an aqueous alkaline phase at a temperature between 75 ⁰C and the boiling point of the reaction batch. The present invention also relates to a phenol-free aminoplast resin based on a 1 ,3,5-triazine compound, a (substituted) urea, and an aldehyde. It specifically relates to a phenol-free aminoplast resin based on melamine, formaldehyde and urea, as well as to a process for the production of a phenol-free laminate, comprising at least one substrate layer impregnated with an impregnating resin comprising an aminoplast resin. Such a laminate comprises in particular at least one core layer, and at least one further layer, in particular a decorative layer. In such a process the core layer is arranged on an outer surface of the at least one substrate layer, wherein the substrate layer is impregnated with the aminoplast resin, the individual layers being stacked one on top of the other, after which this stack is pressed under pressure at elevated temperature to give a laminate. The present invention further relates to a phenol-free laminate comprising at least one aminoplast resin- impregnated substrate layer, in particular at least one core layer, and at least one further layer which is arranged on a surface of the substrate layer and being bonded thereto. In addition, the present invention also relates to the use of such an aminoplast resin for the production of laminates. According to DIN EN 438, decorative laminated sheets are also designated as decorative laminates. They usually have a multilayer structure comprising layers of fiber webs, such as, in particular, paper, which are impregnated with curable resins. In order to produce the decorative laminates from these fiber webs, they are usually pressed at an elevated temperature (i.e. above room temperature) and under pressure (i.e. above atmospheric pressure).

In the case of the decorative laminates known to date, two resin types are usually used, namely a melamine based resin for the surface layer and a phenol based resin for the core layers. It is also possible to use a so-called kraft paper for the core layers, whereas the surface layers usually consist of a special decorative paper which is provided with special colors by printing. The disadvantage of these decorative laminates is that they have a greater or lesser proportion of phenol based resins. This is not only associated with a strong intrinsic odor of phenol, but very often traces of toxicologically unacceptable phenolic monomers, which can be released from these laminates, are also detectable. In addition, the thermal recycling of these laminates is also problematic, since toxic substances may be formed from these resins on incineration, owing to thermal secondary reactions.

In order to avoid this, a phenol-free decorative laminate which is composed of a plurality of substrate materials which are impregnated with an phenol- free impregnating resin, and adhesively bonded to one another and cured, was proposed in EP-1 ,391,478-A. According to this EP-A, the impregnating resin used is a phenol-free urea-melamine-formaldehyde resin which is processed exclusively in an aqueous reaction medium and to which a polymer dispersion is added. In a first stage, a precondensate of urea, melamine and formaldehyde is prepared in the presence of a not very reactive catalyst in a pH range between 7.0 and 8.5, and in a temperature range between 75 ⁰C and 95 ⁰C and, in a second stage, a stronger acidic catalyst is added together with urea, melamine and an aqueous polymer dispersion to the precondensate of the first stage, in a pH range between 4.0 and 6.0 and at a temperature between 75 ⁰C and 95 ⁰C and, after the end of the reaction time, the prepared impregnating resin is adjusted to a pH between 8.0 and 8.5 by addition of alkali. The polymer dispersion serves for flexibilizing the impregnating resin, and dispersions of acrylate or methacrylate or vinyl acetate homopolymers, styrene- acrylate, styrene-methacrylate, or polybutadiene-styrene copolymers are added in amounts between 0.5 and 10 wt.% for this purpose. Phenol-free aminoplast resins for the production of laminates are also disclosed, for example, in DE-40,18,766-A. A process for the preparation of urea- melamine-formaldehyde resin solutions is described therein, NaOH and/or triethanolamine and/or NH₄OH being used as curing agents and the condensation being carried out in a plurality of stages. At first a formaldehyde solution comprising from 5 to 11.5 % by mass of methanol is adjusted to a pH between 7.5 and 8.1 by addition of triethanolamine or NaOH or NH₄OH, and then to a pH between 8.7 and 10, in particular between 8.7 and 9.3, by further addition of NaOH. Thereafter, melamine and urea are being added in a molar ratio from 1 :1.3 to 2 at a temperature below 323 K, in particular between 308 K and 318 K, in amounts such that a molar ratio of the amino compounds to formaldehyde between 1 :1.5 and 1.9 results. The condensation is carried out at a temperature between 363 K and 373 K up to a water compatibility between 1 :1 and 2, the pH in the first 15 to 25 minutes of the condensation being decreased from 9.3-8.7 initially to 8.2-8.0 by stepwise or continuous addition of NaOH, and the condensation being ended at a pH between 8.0 and 7.5. DE-40,18,765-A furthermore discloses the production of Tollable, decorative laminates consisting of a plurality of paper webs which are impregnated with thermosetting resins and are pressed under pressure at a temperature greater than 423 K. The paper webs are impregnated using urea-melamine-formaldehyde condensate solutions having a molar ratio of urea to melamine between 2 and 1.3:1; and the molar ratio of the amino compounds to formaldehyde being between 1 :1.5 and 1.9. Triethanolamine and/or NaOH or NH₄OH are used as catalyst. The impregnating resin solution, optionally modified, can be adjusted as desired with known curing agents based on acidic salts or acids, or with latent curing agents to the gelling times required for further processing, the coating for the decorative paper being betweeni 10 and 160 wt.% and that for the core paper being between 70 and 110 wt.%, relative to the amount of paper, and the pressing of the paper webs being effected continuously.

It is an object of the present invention to provide a process for the preparation of an aminoplast resin, as well as to provide a corresponding resin, in particular for use in laminates, which is simpler and more economical to be carried out, the laminates to be produced therefrom having properties which at least approximately correspond to those of laminates from the prior art.

This object is achieved in a process for the preparation of an aminoplast resin, in which the 1 ,3,5-triazine compound is added exclusively or predominantly in a first stage together with at least part of the aldehyde, and the (substituted) urea exclusively or predominantly in a second stage, wherein in the first stage only the 1 ,3,5-triazine compound is present or the 1 ,3,5-triazine compound is present at least in twice the molar proportion of the (substituted) urea, and wherein the pH of the first stage is kept in a range between 7.5 and 10, and wherein in the second stage the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound is kept between 0.3 and 1.5 and the molar ratio of the aldehyde to -(NH₂)₂ is kept between 0.7 and 1.5, and the pH in the second stage is kept constant in a range between 7.0 and 9.0. The pH-values referred to are to be taken at the conditions of the condensation, especially the temperature condition. More preferred, the pH of the first stage is between 8.5 and 9.5. Another preference is for the pH of the second stage being between 7.5 and 8.5. Preferably, the object of the invention is achieved in a process in which melamine is added exclusively or predominantly in a first stage together with at least part of the formaldehyde, and urea exclusively or predominantly in a second stage, wherein in the first stage only melamine is present or melamine is present at least in twice the molar proportion of the urea, and wherein the pH of the first stage is kept in a range between 7.5 and 10, and wherein in the second stage, the molar ratio of urea to melamine is kept between 0.3 and 1.5 and the molar ratio of formaldehyde to -(NH₂)₂ is kept between 0.7 and 1.5, and the pH in the second stage is kept constant in a range between 7.0 and 9.0. The pH-values referred to are to be taken at the conditions of the condensation, especially the temperature condition. Also here, the preference for the pH in the first stage is between 8.5 and 9.5; the preference for the pH in the second stage is between 7.5 and 8.5. The object of the invention is also achieved by an aminoplast resin in which the proportion of the 1 ,3,5-triazine compound is greater than the proportion of the (substituted) urea, and also by the use of this aminoplast resin for the production of phenol-free laminates, and also in phenol-free laminates. An advantage thereby is that, because the amount of the 1 ,3,5-triazine compound, more specifically melamine, is greater in comparison with processes according to the prior art, it is possible to dispense with the further addition of a polymer dispersion described in the prior art for flexibilizing the impregnating resin. Thus, not only is an additional step omitted, but the costs of this additional polymer dispersion can also be saved so that, depending on the proportion of the 1 ,3,5-triazine compound, a corresponding cost benefit can be generated thereby. By adjusting the pH of the reaction batch in the first stage, before or during the heating thereof, in the range between 7.5 and 10, the addition and subsequent polycondensation of the 1 ,3,5- triazine compound during the first stage solution polycondensation in the aqueous aldehyde solution can be improved, i.e. the solubility of the 1 ,3,5-triazine compound in this solution can be increased. In addition, it has been found that the mechanical properties and the stability to hydrolysis of the resin composition used here is improved. In the case of thin CPL (continuously pressed laminate) and HPL (high- pressure laminate) with a thickness up to 2 mm, it is also possible to realize lower production costs through shorter production times, in particular pressing times, and the possibility of preparing decorative and core layers, in particular decorative and core papers, on the same unit without postcombustion of exhaust air and intermediate cleaning. In the case of HPUs having layer thicknesses between 2 mm and 40 mm (so- called compact boards), advantages are achieved with regard to a very wide range of design possibilities via the core, i.e. not necessarily the decorative layer, owing to the transparency of the applied resin and as a result thereof of the laminate, as well as shorter pressing times, which are associated with a corresponding gain in capacity and a more homogeneous and more environmentally friendly laminate. Examples of 1 ,3,5-triazine compounds that can be used according to the invention are 1 ,3,5-triazines such as melamine, ammeline, ammelide, cyanuric acid, 2-ureidomelamine, melam, melem, melon, melamine salts such as for instance melamine cyanurate, melamine phosphate, dimelamine pyrophosphate or melamine polyphosphate and functionalized melamines, such as for instance hexamethoxymethyl melamine or acrylate-functionalized melamine. However, the invention is not limited to these1,3,5-triazine compounds. The preferred 1 ,3,5-triazine compounds are melamine, melam, melem, melon, or a combination thereof, melamine being particularly preferred.

Examples of aldehydes that can be used are (para-)formaldehyde, acetaldehyde, crotonaldehyde, benzaldehyde and cinnamaldehyde, or combinations hereof, formaldehyde being particularly preferred.

Examples of (substituted) ureas that can be used are (di-)alkyl-, aryl- or allyl substituted urea (like 1-butylurea, 1-allylurea, 1.1-dimethylurea, or 1-phenylurea), urea being particularly preferred. Hereinafter the specification will focus on the preferred embodiment of the invention, i.e. the preparation of an aminoplast resin based on melamine, urea, and formaldehyde. The teachings therein are equally applicable for the more generic form of the invention, i.e. the preparation of an aminoplast resin based on a 1,3,5- triazine compound, a (substituted) urea, and an aldehyde. In certain embodiments of the present invention, the proportion of melamine in the second stage, based on the total amount of melamine and urea, is selected from 50 to 98 wt.%; more preferred from 60 to 90 wt.%, even more preferred from 70 to 80 wt.%.

In order to further improve the properties (hydrolytic and mechanical stability) of the laminates produced using the resin of the present invention, the molar ratio of urea to melamine in the first reaction step (first stage) is chosen to be less than 0.5:1 , preferably less than 0.2:1 , more preferred less than 0.05:1. Particularly preferably, no urea is added in the first stage.

For establishing a balanced relationship between brittleness and deformability of the resin and of the products produced therefrom, it is advantageous that in the first stage the molar ratio of melamine and optionally urea to formaldehyde is between 1.5:1 and 3.0:1; preferably between 2.0:1 and 2.5:1; particularly preferably between 0.95:1 and 1.25:1. It is thus possible to adjust the hardness on the basis of the number of linkage points of the melamine molecules, i.e. the degree to which the macromolecule is crosslinked. If more formaldehyde is used there is the danger that the resin will become too brittle. Too little formaldehyde results in certain circumstances in a resin which is too soft.

In order to permit a rapid conversion in combination with constant quality of the resin to be formed, the reaction batch in the first stage is kept at the elevated temperature for a time between 5 and 60 minutes, in particular between 5 and 30 minutes, more preferred between 10 and 20 minutes.

After the formation of the resin in the first stage, optionally a further proportion of melamine, urea and optionally formaldehyde are added in a second stage to the reaction batch resulting from the first stage, the molar ratio of urea to melamine thereafter being altogether not more than 1.5 and/or the molar ratio of formaldehyde to all amino groups -(NH₂)₂ present in free or reacted form being between 0.7 and 1.5. The viscosity of the solution or of the subsequent impregnating resin can thus be appropriately influenced and hence also the processibility of the resin in the preparation of laminates. The molar ratio of urea to melamine in this second stage is not more than 1.0, preferably between 0.3 and 0.5. The molar ratio of formaldehyde to all amino groups [-(NH₂)₂. present in free or reacted form is between 1.1 and 1.4.

During or after the indicated further addition of melamine (optionally), urea and formaldehyde (optionally) in the second stage, the reaction batch is adjusted to a pH of 7.0-9.0. Preferably this pH is lower than the pH in the first stage, but still in the alkaline range, i.e. not in the acidic range, as described in the prior art. There is therefore no further condensation of the precondensate in the acidic range, with the result that costs with regard to the chemicals used for adjusting the pH are saved, and also the conversion of the precondensate into the final aminoplast resin is also positively influenced. By avoiding neutralization into the acidic range, an excessive increase in the salt load, which leads to an increase in the consumption of latent curing agent owing to the buffer effect (i.e. by sodium formate), is also avoided.

The pH in the second stage is between 7.0 and 9.0, preferably between 7.5 and 8.5, more preferred between 7.9 and 8.2. The adjustment of the pH is preferably carried out at a temperature which is at least approximately 90 ⁰C, a certain variation in this temperature value being possible.

The second phase of the condensation, with the further addition of melamine (optionally), urea and (optionally) formaldehyde, as well as the adjustment of the pH of the reaction batch is effected at a temperature which is between 75 ⁰C and the boiling point of the reaction batch, preferably between 85 ^CC and 95 ⁰C. This second condensation phase is carried out until the resulting resin has a water tolerance (water compatibility) between 1 :0.1 and 1 :10. This has the result that the aminoplast resin prepared by the process according to the invention can be used not only for laminates for indoor applications, but also for outdoor applications.

Preferably, the second phase condensation is carried out until the value of the water tolerance of the resin is between 1 :0.75 and 1 :4.0; more preferably between 1:1.5 and 1:3.

After the second phase, the reaction batch can be adjusted to a pH which is between 8.5, preferably 8.7, and 9.5, preferably 9.0, in order thus to increase the shelf life of the final aminoplast resin.

The first and second stage of the condensation can be performed batch wise, and also continuously. In the first case one reaction vessel would suffice; in the second case two or more reaction vessels in series are applied. The pressure in the first and second stage can be selected freely, to choice. It can be atmospheric or also super atmospheric, with a preference for atmospheric conditions.

In an embodiment of the present process for the production of a phenol-free laminate, at least one curing agent or a latent curing agent or a curing agent mixture and/or water can be added to the impregnating resin prior to impregnation of the substrate layer(s) of the laminate, in order to positively influence the reaction time, i.e. the curing time. For further acceleration of the curing time or in order to positively influence the reactivity of the curing agent or of the curing agent mixture, it is possible also to add further reagents, such as, for example, wetting agents, release agents, modifiers, such as polyalcohols, sulfonamides, etc., as is known in the art for preparing laminates.

It is particularly preferred if the curing agent is added in an amount between 0.2 and 2 wt.%, based on the total amount of the mixture. The respective amount to be added depends on the product to be produced (CPL or HPL), on the respective technology for the production of the desired products after the preparation of the resin, and on the curing agent or curing agent mixture used. For example, from 0.8 to 1.2 wt.% of curing agent may be added.

For a better understanding of the invention, it is described in more detail with reference to the following process sequence, wherein it is to be understood that the scope of the invention is not being limited herewith. It should be stated at the outset that individual features or combinations of features from the different Examples described can also represent solutions which are independent, inventive or according to the invention.

All data regarding value ranges in the present description are to be understood as meaning that they include any desired part-ranges and all part-ranges therefrom. For example, the statement "from 1 to 10" is to be understood as meaning that all part-ranges starting from the lower limit 1 and the upper limit 10 are included, i.e. all part-ranges begin with a lower limit of 1 or greater and end at an upper limit of

10 or less, e.g. from 1 to 1.7 or from 3.2 to 8.1 or from 5.5 to 10.

The preparation of the aminoplast resin is effected via a precondensate. For this purpose, formalin (= formaldehyde in water), water and optionally modifiers such as monoethylene glycol, diethylene glycol, sulphonamides, sugars like sorbitol, or the like, are initially introduced and the pH is adjusted to 7.5 to

10, more preferred to 8.5 to 9.5, in particular 8.8 to 9.2. Thereafter, in an aqueous phase, melamine, formaldehyde and optionally urea are reacted with one another. According to the invention, the molar ratio of urea to melamine in the first stage is equal or less than 0.5. The molar ratio of formaldehyde to melamine and optionally urea in this stage is between 1 :1.5 and 1 :3.0, preferably between 1 :2.0 and 1 :2.5.

This reaction batch is subsequently heated to a temperature between

75 ⁰C and the boiling point of the reaction batch, preferably between 85 ⁰C and 95 ⁰C, this temperature being kept in this range for a time span of from 5 to 60 minutes, preferably from 5 to 30 minutes. The precondensate is formed thereby.

The final condensate is prepared from this precondensate by further addition of (optionally) melamine, urea and (optionally) formaldehyde, the molar ratio of the further addition of urea to melamine being not more than 1.5, preferably between 0.3 and 0.5, and the molar ratio of formaldehyde to -(NH₂^ is between 0.7 and 1.5, preferably between 1.1 and 1.4. The pH is adjusted to a value between 7.0 and 9.0, more preferred to a value between 7.5 and 8.5.

The second condensation stage is carried out at a temperature between 75 ⁰C and the boiling point of this reaction batch, the pH is between 7.0 and 9.0, specifically between 7.5 and 8.5, preferably between 7.9 and 8.2, until the desired water compatibility is reached.

Compliance with these conditions permits good condensation of urea, melamine, and formaldehyde.

After reaching the desired water compatibility at 25 ⁰C of between 1 :0.75 and 1 :4, preferably between 1:1.5 and 1 :3, the reaction is stopped and the pH is adjusted to a value between 8.5 and 9.5, preferably between 8.7 to 9.0, and cooling is effected to a temperature between 15 ⁰C and 25 ⁰C in order to obtain a good shelf life.

The preparation of the precondensate in the first stage can be effected by customary procedures which are known to the person skilled in the art in this area. The person skilled in the art is therefore referred to the relevant literature. For example, melamine and optionally urea can be added to a formalin solution, i.e. an aqueous solution of formaldehyde. This formalin solution may have a concentration between 35 % and 55 %.

In this first step, for the formation of the precondensate, the molar ratio of urea to melamine should be equal or less than 0.5, with the result that the properties of the end product, i.e. of the aminoplast resin, are improved. This molar ratio of urea to melamine in this stage is preferably equal to or less than 0.2, more preferred equal or less than 0.05, particularly preferably this molar ratio is 0, i.e. no urea is added in the first stage. The adjustment of the pH to a value between 8.5 and 9.5, where the stated numerical value of the pH is established at the prevailing temperature of the reaction batch, can be effected by the addition of an acid or a base, such as, for example, NaOH, NH₄OH or the like. This too is known to the person skilled in the art on the basis of his general technical knowledge. Generally, however, a base is to be added in the case of the chosen batch. It is also possible to use buffer systems to keep the pH in the desired range.

The adjustment of the pH can be effected after the preparation of the reaction batch, i.e. the addition of the corresponding amounts of melamine, formaldehyde and optionally urea, or simultaneously therewith; all before the heating to the reaction temperature is effected.

In a preferred embodiment of the invention, the pH is adjusted to a value between 8.8 and 9.2, with the result that positive properties of the end product, such as, for example, better shelf life of the resin, are obtained.

For monitoring or establishing the reaction temperature, it is possible to use apparatuses known to the person skilled in the art on the basis of his general technical knowledge, such as, for example, a thermostated reaction vessel. For example, the wall of this reaction vessel can be appropriately thermostated. Heat exchangers can also be used in this context.

Within the indicated temperature range, the reaction of the starting materials to give the desired precondensate now takes place. A temperature between 85 °C and 95 ⁰C is preferably established. In this first step of the preparation of the condensate, this temperature is maintained for 5 minutes to 30 minutes, preferably from 6 to 20 minutes; more preferred from 7 to 18 minutes; even more preferred from 10 to 15 minutes. For the further, second phase, condensation to give the final aminoplast resin, melamine (optionally), urea and formaldehyde (optionally) are added to the precondensate in the aqueous phase. The molar ratio of urea to melamine should be not more than 1.5, preferably not more than 1.4 or 1.3 or 1.2 or 1.1 or 1.0 or 0.9 or 0.8 or 0.7 or 0.6. With regard to the result to be achieved, a molar ratio of urea to melamine which is between 0.3 and 0.5 is particularly preferred.

The molar ratio of formaldehyde to -(NH₂)₂ groups in this second stage should be between 0.7 and 1.5. In the context of the invention, the expression -(NH₂)₂ groups is to be understood as meaning that amino groups, in particular of urea and melamine, are referred to therewith, independently of whether these are present in free or in reacted form. This ratio of formaldehyde to -(NH₂)₂ groups is preferably between 1.1 and 1.4.

For the further condensation, the pH is adjusted to 7.5 to 8.5, preferably to 7.9 to 8.2, more preferred from 7.9 to 8.0, the temperature of the second phase being kept at a value selected from 75 ⁰C to the boiling point of the reaction batch, in particular between 85 ⁰C to 95 ⁰C. It should be noted that the pH and the temperature are kept in the stated ranges and a readjustment is optionally required. As already mentioned, corresponding devices or chemicals, i.e. acids or bases, can be chosen according to needs for monitoring and readjusting the pH if necessary. The temperature, too, can be kept in the desired range by heating or cooling, once again depending on needs. The more exactly these values are maintained, particularly preferably both the temperature and the pH during the reaction varying only in a narrow part-range which is selected from the respective larger range, the better the properties that can be achieved for the final resin. The monitoring of these values leads to a resin which has improved properties, such as, for example, stability to hydrolysis and/or improved mechanical properties. These properties of the resin are transferred during the further processing thereof to give the end product, i.e. for example the laminate, to the latter so that it is therefore advantageous to ensure an appropriate reaction procedure in this stage itself.

The second phase reaction is continued to a certain water compatibility and then stopped. Water tolerance is the amount of water in milliliters which can be added per gram of resin at 25 ⁰C without said resin becoming opaque. It provides an indication of the degree of condensation of the resin. The reaction is preferably carried out until the value of the water compatibility is between 1 :0.1 and 1 :10, preferably between 1 :0.5 and 1 :5, more preferred between from 1 :0.75 and 1:4 or 1 :3 or 1:2.

As soon as the desired water compatibility was reached, the pH can be adjusted to a value between 8.5 and 9.5. Although this step is not absolutely essential, in particular not required immediately after or soon after the reaction, it has the advantage that the resin remains stable for longer if the temperature after the reaction is not kept in a range between 0 ⁰C and to 30 ⁰C.

For the production of a laminate from the aminoplast resin according to the invention, a curing agent and optionally water and additives, such as, for example, wetting agents or release agents according to the prior art, can be added to said aminoplast resin. At least one substrate layer is subsequently impregnated with this mixture, a plurality of substrate layers preferably being used. For example, kraft papers as known from the prior art are suitable as the substrate layer, where this is not to be understood as being limiting. Such substrate layers for the use of laminates, in particular decorative laminates, such as, for example, so-called HPL (high-pressure laminate) or CPL (continuous pressure laminate) and LPL (low pressure laminate) according to DIN EN 438 part 1 are known to the person skilled in the art who is active in this area and said person skilled in the art will choose these known materials if required or exchange them for the kraft paper. In general, all adsorptive materials, in particular papers or wood, or generally fiber materials can be used as substrate layers. These substrate layers can form the so-called core layers in the final laminate. Optionally, a drying process for the substrate layers can take place after the impregnation, in order to evaporate a certain proportion of solvent.

A stack is subsequently formed from the individual substrate layers and optionally at least one decorative layer additionally to be arranged, by placing them one on top of the other, it being possible for the decorative layer to be a printed layer, in particular a resin-impregnated one, for example a paper layer. It is also possible to arrange at least one further layer of a resin, in particular comprising a transparent resin, above the decorative layer.

As known per se, this stack is subsequently pressed at elevated temperature and elevated pressure, as known to the person skilled in the art, to give the final laminate.

The laminate thus produced exhibits balanced properties, the process according to the invention achieving the advantage that the formation of the laminate, i.e. the curing, takes place within a short time in comparison with the formation of laminates from phenol based resins. Furthermore, laminates can be produced in a large number of different colors using the resin according to the invention, since the aminoplast resin according to the present invention is colorless.

In principle, any curing agent which is known to the person skilled in the art in this area can be used as the curing agent. For example it is possible to use para-toluenesulfonic acid as the curing agent. The amount of the curing agent used can be selected from a range between 0.2 and 2 wt.%. Optionally, curing agent mixtures comprising two or more curing agents may also be used.

The principle of the production of laminates is described in

"Kunststoff Handbuch, 10-Duroplaste [Plastics Handbook, 10-Thermosetting Plastics], W. Becker, D. Braun, 1988, Karl Hanser Verlag" and the person skilled in the art is therefore referred to this literature, the content of which in this context is hereby incorporated by reference. It should be noted that, if more than three layers are used, it is possible to make a distinction between so-called core layers and outer layers, it being possible in each case for an outer layer to be arranged on opposite surfaces of the core layer(s). Of course, it is possible to provide only one of these outer layers. In a preferred embodiment, at least a third or at least half or at least three quarters of the core layers is/are impregnated with the aminoplast resin according to the present invention. In other words, it is possible in the case of such laminates to treat at least a major part of the substrate layers with the aminoplast resin, for example also where mixed resin laminates are produced. It is possible, in combination with the aminoplast resin according to the present invention, to impregnate further substrate layers with other formaldehyde based resins or other impregnating resins. For example, urethane and/or acrylate based layers can be used as outer layers.

The preferred temperature for the production of the laminate is between 130 ⁰C and 175 ⁰C (e.g. for HPL), in particular between 140 ⁰C and 160 ⁰C. The curing time is dependent on the chosen temperature, the required time being shorter the higher the temperature. The number of individual layers, the amount of resin and the amount of curing agent(s) are important for determining the required time.

For example, from 20 s to 25 s are required at a mold temperature of 190 °C for a CPL structure having four layers, of which two layers are impregnated with the resin according to the present invention and form the core, in the case of a thickness of from

0.5 mm to 0.6 mm. For an HPL structure having 35 layers and 5 mm thickness, about

20 minutes are required at 140 ⁰C.

In general, the required time is shorter than the required time for the production of continuous laminates (CPL) from the prior art, which are impregnated with phenol-based resins. A corresponding economic advantage is achievable thereby with the present invention.

The laminate can of course be produced batchwise in presses, but continuous production of web-like laminates which are appropriately cut to size for their use is also preferably possible. The time for the production of the laminates is preferably from 3 sec to 5 min, particularly preferably from 5 sec to 3 min. In particular, this time may be in the range from 10 sec to 15 sec, or from 10 sec to 20 sec. The pressure used for the formation of the laminate is selected from a range between

2.5 MPa and 7 MPa.

Examples and comparative experiments are given below for further clarification of the present invention, once again reference being made to the fact that these Examples do not denote any limitation at all for the present invention.

Examples:

2000 kg (15.9 Kmol) of melamine, 384 kg (6.4 Kmol) of urea and 3048 kg of 37 % strength formalin solution (37.6 Kmol formaldehyde) in about 1000 kg of water were used for the preparation of the resin. It was possible to prepare 3450 kg of resin therewith. A 50% strength NaOH solution was used for setting the pH.

After the formalin solution was introduced into a suitable reaction vessel, the pH was adjusted to 9.0. Thereafter, melamine was added and the reaction batch was heated to 90 ⁰C until the melamine has dissolved, which lasted for 20 minutes. The urea is then added while the pH was corrected to 8.1 ; the temperature was maintained at 90 ⁰C. After turbidity of the batch had occurred as the reaction had proceeded for 65 minutes, the pH was adjusted to 9.2 and cooling to room temperature was effected. CPL thin laminates were produced using the resin and were - 1 -

compared with a commercially available phenol based resin. The following Table shows the test results.

CPL structure according to the invention: overlay 28 g (impregnated with a commercial melamine-formaldehyde resin)/decoration 80 g (impregnated with the same commercial melamine-formaldehyde resin)/core 155 g (impregnated with a resin according to the present invention)/kraft paper 100 g.

For the phenolic laminate, a phenol-based resin (Perasit 3472) was used as core resin. Otherwise, the structure corresponds to that with the resin according to the invention. The formaldehyde emission was furthermore measured (according to EN 717-2), with the following measured values in [mg/h m²]:

Accordingly, the resin according to the invention, i.e. the CPL produced therefrom, can therefore also be designated as equally formaldehyde-free as the phenol-based resin.

The Examples show possible embodiments of the aminoplast resin or of the laminate, and it should be noted at this point that the present invention is not limited to the specially described embodiments thereof but rather various combinations of the individual embodiments with one another are also possible and this possibility for variation is within the skill of the person skilled in the art who is active in this technical area, owing to the teachings of the present invention with regard to the technical action. All conceivable embodiments which are possible by combinations of individual details of the embodiments shown and described are therefore also included within the scope of protection.

Claims

1. A condensation process for the preparation of a phenol-free aminoplast resin by reacting an aldehyde, a (substituted) urea and a 1 ,3,5-triazine compound in an aqueous alkaline phase at a temperature between 75 ⁰C and the boiling point of the reaction batch, wherein the 1 ,3,5-triazine compound is added exclusively or predominantly in a first stage together with at least part of the aldehyde, and the (substituted) urea exclusively or predominantly in a second stage, wherein, in the first stage, only the 1 ,3,5-triazine compound is present or the 1 ,3,5-triazine compound is present at least in twice the molar proportion of the (substituted) urea, and the pH of the first stage is between 7.5 and 10 and, in the second stage, the molar ratio of the (substituted) urea to the 1 ,3,5- triazine compound is between 0.3 and 1.5 and the molar ratio of the aldehyde to -(NH₂)₂ is between 0.7 and 1.5, and the pH in the second stage is between 7.0 and 9.0.

2. The process according to claim 1 , wherein the 1 ,3,5-triazine compound is melamine, the (substituted) urea is urea, and the aldehyde is formaldehyde.

3. The process according to anyone of claims 1-2, wherein the pH in the first stage is between 8.5 and 9.5.

4. The process according to anyone of claims 1-3, wherein the pH in the second stage is between 7.5 and 8.5.

5. The process according to anyone of claims 1-4, wherein the proportion of the 1,3,5-triazine compound in the second stage, based on the total amount of the 1 ,3,5-triazine compound and the (substituted) urea, is between 50 and 98 wt.%.

6. The process according to claim 5, wherein the proportion of the 1 ,3,5-triazine compound in the second stage, based on the total amount of the 1 ,3,5-triazine compound and the (substituted) urea, is between 60 and 90 wt.%.

7. The process according to anyone of claims 5-6, wherein the proportion of the 1 ,3,5-triazine compound in the second stage, based on the total amount of the

1,3,5-triazine compound and the (substituted) urea, is between 70 and 80 wt.%.

8. The process according to anyone of claims 1-7, wherein the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound in the first stage is less than 0.5:1.

9. The process according to claim 8, wherein the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound in the first stage is less than 0.2:1.

10. The process according to anyone of claims 8-9, wherein the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound in the first stage is less than 0.05:1.

11. The process according to anyone of claims 8-10, wherein no (substituted) urea is added in the first stage.

12. The process according to anyone of claims 1-11 , wherein the molar ratio of the 1 ,3,5-triazine compound and optionally (substituted) urea to aldehyde in the first stage is between 1.5:1 and of 3.0:1.

13. The process according to claim 12, wherein the molar ratio of the 1 ,3,5- triazine compound and optionally (substituted) urea to aldehyde in the first stage is between 2.0:1 and 2.5:1.

14. The process according to anyone of claims 1-13, wherein the reaction batch in the first stage is kept at the condensation temperature for a time between

5 minutes and 30 minutes.

15. The process according to claim 11 , wherein the reaction batch in the first stage is kept at the condensation temperature for a time between 10 minutes and 20 minutes.

16. The process according to anyone of claims 1-12, wherein the molar ratio of (substituted) urea to the 1 ,3,5-triazine compound in the second stage is not more than 1.0.

17. The process according to claim 16, wherein the molar ratio of the (substituted) urea to the 1,3,5-triazine compound in the second stage is between 0.3 and 0.5.

18. The process according to anyone of claims 1-17, wherein the molar ratio of the aldehyde to -(NH₂)₂ in the second stage is between 0.7, preferably 1.1 , and 1.5, preferably 1.4.

19. The process according to anyone of claims 1-18, wherein, in the second stage, the pH is between 7.9 and 8.2.

20. The process according to anyone of claims 1-19, wherein, in the second stage, the reaction batch is reacted until the resulting resin has a water compatibility between 1 :0.1 and 1 :10.

21. The process according to claim 20, wherein the water compatibility is between 1 :0.75 and 1 :4.0.

22. The process according to anyone of claims 20-21 , wherein the water compatibility is between 1 :1.5 and 1 :3.

23. The process according to anyone of claims 1-22, wherein, after the second stage, the pH of the reaction batch is adjusted to a value between 8.5, preferably 8.7, and 9.5, preferably 9.0.

24. Phenol-free aminoplast resin formed in a process according to anyone of claims 1 to 20, wherein the proportion of the 1 ,3,5-triazine compound, based on the total amount of the 1 ,3,5-triazine compound and the (substituted) urea, is between 50 and 98 wt.%.

25. The aminoplast resin according to claim 24, wherein the proportion is between 60 and 90 wt.%.

26. The aminoplast resin according to anyone of claims 24-25, wherein the proportion is between 70 and 80 wt.%.

27. The aminoplast resin according to anyone of claims 24-26, wherein the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound is less than 1.5:1.

28. The aminoplast resin according to claim 27, wherein the molar ratio of the (substituted) urea to the 1 ,3,5-triazine compound is less than 0.5:1.

29. The aminoplast resin according to anyone of claims 27-28, wherein the molar ratio of (substituted) urea to the 1 ,3,5-triazine compound is less than 0.3:1.

30. The aminoplast resin according to anyone of claims 24-29, wherein the molar ratio of the aldehyde to -(NH₂)₂ groups (from the (substituted) urea and the 1 ,3,5-triazine compound) is between 0.7 and 1.5.

31. The aminoplast resin according to claim 30, wherein the molar ratio is between 1.1 and 1.4.

32. The use of the aminoplast resin according to anyone of claims 24-31 or obtainable by the process according to anyone of claims 1-20 for the production of laminates, in particular high-pressure laminates.

33. A process for the production of a phenol-free laminate, in particular high- pressure laminate, wherein said laminate comprising at least one substrate layer, in particular a core layer, is impregnated with an impregnating resin comprising an aminoplast resin, and wherein said laminate comprises at least one further layer, in particular a decorative layer, wherein said further layer is arranged on an outer surface of the at least one substrate layer, wherein the substrate layer, impregnated with the aminoplast resin according to anyone of claims 24-31 or obtainable by the process according to anyone of claims 1-23, - -

and the other layers are stacked one on top of the other, after which this stack is pressed under pressure at elevated temperature to give the laminate.

34. The process according to claim 31 , wherein at least one curing agent or a latent curing agent or a mixture of curing agents are added to the substrate impregnating aminoplast resin before the impregnation of the substrate layer.

35. The process according to claim 34, wherein the at least one curing agent is added in an amount between 0.2 and 2 wt.%, based on the total amount of the 1 ,3,5-triazine compound, the (substituted) urea and the aldehyde.

36. The process according to anyone of claims 1-23, the aminoplast resin according to anyone of claims 24-31 , the use according to claim 32, and the process according to anyone of claims 33-35, wherein the 1 ,3,5-triazine compound is melamine, the (substituted) urea is urea, and the aldehyde is formaldehyde.

37. A phenol-free laminate, in particular high-pressure laminate, comprising at least one aminoplast resin-impregnated substrate layer, in particular at least one core layer, and at least one further layer which is arranged on a surface of the substrate layer and is bonded thereto, wherein said laminate is produced by a process according to anyone of claims 30-32 using an aminoplast resin according to anyone of claims 21-29 or obtainable by the process according to anyone of claims 1-20.