NO144596B - PROCEDURE FOR LAMINATE MANUFACTURING. - Google Patents

Description

The invention relates to a method for the production of a laminate, whereby a paper or fiber material impregnated with a melamine-formaldehyde resin containing an inorganic water-soluble silicate in an amount corresponding to 0.2 - 1.4 weight-% SiC^ calculated on the weight of the entire resin solution is pressed by for-

high temperature and high pressure on a suitable substrate.

It is known that laminates can be produced by a paper impregnated with an aminoplast resin or another fiber mat-

rial is pressed at elevated temperature and pressure on a substrate. According to the method most used to date, an impregnated and dried paper is placed on a suitable base,

in most cases a chipboard, after which the whole thing is placed in a press. The press is closed, temperature and pressure are increased to 125-175 o C resp. 10-50 kg/cm 2 , temperature and pressure are maintained for 3-6 minutes under these conditions, after which the produced laminate is allowed to cool under pressure in the press.

According to a more recent method, the whole consisting of the base layer and impregnated paper is brought into a heated press, pressed for 0.5 to 2 minutes at 125-200°C and 15-50 kg/cm<2> into a laminate , after which the laminate is removed from the press and cooled without pressure. This method is known as the short stroke method.

The aminoplast resins used for impregnating the paper are melamine-formaldehyde resins which, if desired, may contain one or more modifiers which serve to improve the storage capacity of the resin solution or the surface properties of the laminate.

It is also known to use melamine-formaldehyde resin solutions which contain a relatively small amount of a water-insoluble inorganic silicate. At very low silicate contents, the addition of the silicate serves to improve the stability of the resin solution and, at somewhat higher silicate contents, also produces an improvement in the surface properties of the laminate, particularly of laminates obtained using the short-stroke method, see Norwegian patent no. 134 749.

A disadvantage of such impregnation resin solutions is that the water-soluble silicate in the melamine-formaldehyde impregnation resin solution must be added taking into account special devices and preferably in the form of a stabilized strongly acidic silicate solution.

A disadvantage of the melamine-formaldehyde resins is generally their relatively high price.

However, the cheap urea formaldehyde resins and urea melamine formaldehyde resins cannot be used as impregnation resins for the cover layer or the decoration layer for the production of laminates with a high quality surface. The use of such resins leads to laminates with a leaky surface and a very poor chemical durability.

According to the invention, a high-quality laminate can be produced

by impregnating the paper or fiber material with a urea-modified melamine-formaldehyde resin solution with a urea-

content between 4 and 50% by weight calculated on the weight of urea combined

but with melamine and a molar ratio of formaldehyde per three to urea or melamine belonging to Nt^ groups between 1:1 and 2.5:1.

The invention has several advantages. Firstly, according to the invention, laminates with a high-quality surface are produced, whereby one starts from relatively cheap urea-modified melamine-formaldehyde resin solutions which were hitherto completely unsuitable for this purpose.

Furthermore, the urea-modified melamine-formaldehyde resin to be used allows water-soluble silicate to be added easily and without special precautions, while a silicate solution must be added very carefully to a melamine-formaldehyde resin solution in order to

avoid a flocculation. Third, the shelf life or service life of the catalyzed silicate-containing urea-modified resin is better than that of a silicate-containing pure melamine formaldehyde resin.

It is noted that it is indeed known from the German specification no. 2,136,942 that a wearable hard bellows which can be glued to a substrate can be cured from a mixture of an aqueous silicate solution and an aminoplast resin solution impregnated paper at elevated temperature and pressure. The surface is then towed and provided with a cover layer. However, the literature contains no indication that a silicate- and urea-modified formaldehyde resin can be used to impregnate paper which can be immediately processed into the cover layer in a laminate.

When paper is impregnated by adding a modified melamine-formaldehyde resin with added urea and pressing the impregnated paper, inferior quality laminates are obtained. In particular, the density of the surface and the chemical resistance of such laminates are insufficient.

For this reason, urea-modified melamine-formaldehyde resins are not used for the production of laminates. When, however, for the manufacture according to the invention of the cover layer of a laminate, one starts from a silicate modified through the addition of a silicate to ureamelamine formaldehyde resin, a high-quality laminate is obtained. The addition of the silicate thus not only causes a difference in degree, but

a qualitative difference and conquers a new area of application for urea-modified melamine-formaldehyde resins.

According to one of the aspects according to the invention, when impregnating paper or another fiber material that is processed into a non-further transformable laminate, a urea-modified melamine-formaldehyde resin with a molar ratio F/3 NI^ of formaldehyde to urea together with melamine between 1:1 and 2 .5 : 1, preferably between 1.2 : 1 and 2.0 : 1, and especially between 1.5 : 1

and 1.8:1. The pH of such a non-catalyzed resin solution can be between 8.5 and 10.0, preferably between 9.0 and 9.5. One

such a resin solution "generally has a storage capacity of a few weeks. The calculated content of solids generally varies between 5 3 and 58 percent by weight. The resin solution can, if desired, also contain common modifiers such as caprolactam, sugar, toluenesulfonamide, glycols or glycol ether.

Such resin solutions can also be obtained by allowing melamine and formaldehyde to react in the usual way in an aqueous solution, if desired in the presence of one or more modifiers. The molar ratio formaldehyde : melamine should therefore be less than 3.5 : 1 and preferably lie between 2 : 1 and 3 : 1.

When the condensation has gone far enough, i.e. at a water tolerance between 2 and 6, urea is added, after which the condensation reaction can be allowed to continue for a short time or the resin solution can be cooled. The pH of the resin solution that occurs in this way is set in most cases between 9.0 and 9.5.

The resin solution can also be obtained in other ways, e.g. whereby a melamine formaldehyde resin solution is mixed with a urea formaldehyde resin solution.

The impregnation resin solution to be used according to the invention can contain between 4 and 50 weight percent urea calculated on the total weight of urea and melamine. With higher urea content, laminates with a somewhat poorer surface are obtained. The urea content can be very low, but then the economic advantage is also small. In view of the price of the resin solution and the properties of the laminate, a resin solution with a urea content of between 25 and 40 percent by weight is preferably used.

A further aspect according to the invention is aimed more specifically at the production of laminates that can be later reshaped, i.e. laminates that can be bent after curing or otherwise reshaped during heating without the surface properties returning.

It has already been proposed to produce later reshapeable laminates using a melamine-formaldehyde resin solution with a lower molar ratio between formaldehyde and melamine, preferably between 1.0:1 and 1.5:1 (see DT-AS 1,629,592). In practice, however, it has been shown that it is extremely difficult to prepare impregnation resin solutions with a formaldehyde/melamine ratio below 1.5:1, and this is due to the poor solubility of the melamine in the reaction medium.

Soker has found that urea-modified melamine-formaldehyde resin solutions with an effective F/3-NH2 ratio between 1.0:1 and 1.5:1 can easily be produced. The resins can be processed into laminates which are very easily later reformable, even better than when using a pure melamine-formaldehyde resin solution with the same F/S-NH^ ratio produced laminates. Unfortunately, the surface properties such as porosity and chemical resistance of the laminates, both before and after the design, fall short of the requirements.

However, if a water-insoluble inorganic silicate is added to the impregnation resin solution, the surface properties of the laminate are greatly improved without reducing the subsequent reshaping ability.

The third aspect according to the invention relates to a method for the production of laminates that can later be reformed, characterized in that a urea-modified melamine-formaldehyde impregnation resin solution is used with a molar ratio of formaldehyde per three of NH2~groups belonging to melamine or urea between 1 : 1 and 1.5 : 1, a content of 4 to 20 weight percent urea calculated on the combined weight of urea and melamine and a content of water-soluble inorganic silicate corresponding to a Si0 2~content in the solution between 0.2 and 1.4% by weight.

As it is difficult to produce melamine-formaldehyde resin solutions with an F/3-NH2 ratio far below 1.5, at least an amount of urea corresponding to a good 4 weight percent calculated on urea together with melamine is necessary to bring the ratio below 1.5:1. Larger amount of urea makes it possible to start the resin production with a higher formaldehyde/melamine ratio and/or to reach a lower final value of the F/3-NH2 ratio. The cost price of the resin becomes lower and the subsequent reformability increases with increasing urea content, but the surface properties are then reduced despite the presence of the silicate.

A lowering of the F/ ratio improves the later formability, but reduces the surface properties by prohibiting the amount of urea that must be added. The optimal F/3-NH2 ratio is between 1.2 : log 1.4 : 1.

The impregnation resin solution is prepared by first melamine and formaldehyde being condensed in the usual way at an F/3-NH2~ ratio of approx. 2 : 1 and 1.5 : 1, urea is added and these substances, if desired, are condensed once more for a short time.

The following applies to both forms of application of the method according to the invention.

As the silicate-containing resin solution only has a limited storage capacity at a silicate content above 0.2% SiO2, the silicate is added briefly for processing the resin solution. The silicate can easily be added to the resin solution in the form of an aqueous solution.

By using a concentrated silicate solution, the formation of gel-like flocks is not excluded, so that a silicate solution containing 10 to 20% by weight SiO2 is preferably used.

During the addition of the silicate, the pH in the resin solution can rise to approx. 11. Before, during and after the silicate addition, the resin solution is normally catalysed for further processing by setting the pH between 7 and 9, especially between 8.5 and 9. These resulting catalyzed resin solutions according to the invention have a storage capacity of at least 24 hours at 20 °C. The acid catalyst can be added before, during and after the addition of the silicate solution to the resin solution.

If the silicate is added to a pure melamine-formaldehyde resin solution, the risk of flocculation is much greater, so that the silicate is preferably added in the form of a stabilized acidic solution. The storage capacity of catalyzed silicate-containing pure melamine-formaldehyde resin solutions with the same silicate content is in most cases considerably less.

As used silicates according to the invention, the inorganic water-soluble silicates come into consideration, such as lithium silicates, sodium silicates, potassium silicates and quaternary ammonium silicates.

Very good results are achieved with aqueous solutions of sodium silicate (also called water glass). In general, so much of a silicate solution is added that the SiC content of the resin solution amounts to 0.2 to 1.2 percent by weight, in particular 0.4 to 1 percent by weight and preferably 0.4 to 0.8 percent by weight.

The resin solutions according to the invention are used to impregnate paper or other fiber material, after which the impregnated material is dried and processed into laminates. The impregnation is carried out in the usual way. The resin content in the dried and impregnated materials can vary between 50 and 60 percent by weight. The percentage content of volatile material can be between 5 and 7.5% by weight, preferably between 6 and 6.5% by weight.

The impregnated and dried material can be processed in the usual way into a laminate, either according to the short-stroke method whereby the material is briefly pressed and cooled outside the press, or according to the usual method whereby the laminate is cooled in the press. The invention is illustrated through the following examples.

Example 1

A urea-modified resin solution was prepared by mixing 2.5 kg of 30% formalin with a pH of 9.3, 23.6 kg of melamine, 8.7 kg of water and 0.6 kg of aqueous sodium silicate solution {S±0^^ ^ 2Q ~ ^,3) is heated to 95-96°C and that 12.8 kg of urea was added at the end of the condensation reaction. After cooling, the pH of the resin solution is adjusted to approx. 9.5 when dimethylaminoethanol is added.

The resin solution is divided into three portions, each of which is catalyzed and modified in the following manner. White decorative paper (quality 90 g/m 2 ) is impregnated with the resin solutions thus formed. After drying, the decorative paper is pressed onto a chipboard according to the short-stroke process with a pressing time of 60 seconds and at a temperature in the press tin of approx. 155°C. The laminate surface is tested using a chiton dye test for curing, using a shoe polish test for surface density and by heating in air to 100°C (16 hours) for hairline crack resistance. The results are summarized in the table.

A. The resin solution A is prepared on the basis of the resin solution described above, with 19 g of triethylene glycol monoethyl ether added per kg of resin solution and the B time is set with toluenesulfonic acid in 99 seconds. The B time is an indication of the reactivity of the resin solution and is the time that a resin solution needs in a fused rudder heated to 140°C to become cloudy. The resin solution A has a total SiO2 content of 0.05% and falls outside the scope of the invention.

B. The resin solution B is prepared using 19 g of triethylene glycol monoethyl ether and 40 g of an 18% sodium silicate solution (SiO^Na^ = 3.3) per kg of resin solution is added while stirring and that the B time is then set with toluenesulfonic acid to 99 seconds. The total Si02 content amounts to 0.6%.

C. The resin solution C is prepared as the resin solution below

stirring, an acidic stabilized silicate solution is added to a total SiO2 content of 0.6% and the B time is set to 95 seconds. The added solution contains toluenesulfonic acid, triethylene glycol monoethyl ether and sodium silicate (SiO^Na^ = 3.3) and occurs as described in Dutch patent application No. 72.10.402.

The properties of the laminates obtained using resins A, B and C are as follows:

The assessment is 1 = excellent, 2 = good, 3 = fairly good, 4 = bad, 5 = very bad.

<x>) The Kiton experiment also shows that with longer pressing times, which precedes a strong color change, the chemical surface resistance is too poor for the degree of curing to be assessed on the basis of this experiment.

xx)

Not in accordance with the invention.

The laminate obtained through pressing in the usual way has approximately the same properties.

Example II

A urea-modified resin solution is prepared by adjusting 29.1 kg of 30% formalin to a pH of 9.3 with 0.2 kg of aqueous sodium silicate solution (18% by weight SiO2, SiO 2/^2*3 = 3.3) and heating together with 9 kg of water and 25.3 kg of melamine to a temperature of approx. 95°C until the resulting resin solution has reached a water dilution degree of approx. 2.0 (at 20°C).

At the end of the condensation reaction, 1.3 kg of urea is added.

In this way, the resin solution A, F/3 NI^ = 1.35 is obtained, which contains 4.9 weight percent urea calculated on melamine together with urea.

In a similar way, a urea-modified melamine formaldehyde solution is produced, whereby one starts from 22.4 kg of melamine with the addition of 3.4 kg of urea. The resulting discharge (discharge B) has an F/S-NI^ ratio of 1.35 and contains 13.1% by weight of urea.

To each of solutions A and B, 38 g of sodium silicate solution (18 weight percent SiO2, Si0 2/Na2O = 3.3) per kg of resin solution, after which the B time is set to 100 seconds with the addition of toluenesulfonic acid5 The pH is then approx. 7.7 (resin solutions A' and B' respectively). The B time is an indication of the reactivity of the resin solution and is the time that a resin solution heated to 140°C in a fused rod needs to become cloudy.

For comparison, a part of the mixture B to which no silicate has been added is similarly brought to a B time of approx. 100 seconds when adding toluenesulfonic acid (resin solution C). Likewise, for comparison, a regulation is followed which is often used for the production of resin solutions for post-formable laminates. According to this regulation, a melamine-formaldehyde resin solution (F/3 NH2 = 1.8) is modified with a condensate of toluenesulfonamide and formaldehyde and set with toluenesulfonic acid at a B-time of 240 seconds (pH = 9.0)

(resin solution D).

All the catalyzed resin solutions are used for the impregnation of white decorative paper (quality 90 g/m ) which, after drying in the usual way with a laminate temperature of a maximum of 148°C and after cooling in the press, is pressed onto 6 layers of a standard-quality post-formable core paper impregnated with a phenolic hair pix.

The resulting laminates are then examined. The degree of curing and chemical resistance are examined using the known chiton dye tests. Post-formability is tested as the laminate is locally heated to approx. 140°C by infrared radiation and bent with a radius of 9 mm, after which the bent surface is inspected for cracks and loss of gloss.

The results are presented in the table where the rating goes from 1 (excellent, no staining, no scratches) to 5 (very poor).

Claims (6)

1. Method for producing a laminate, whereby a paper or fiber material impregnated with a melamine-formaldehyde resin containing an inorganic water-soluble silicate in an amount corresponding to 0.2 - 1.4% by weight SiC^ calculated on the weight of the entire resin solution is pressed at elevated temperature and elevated pressure onto a suitable substrate, characterized by that the paper or fiber material is impregnated with a urea-modified melamine-formaldehyde resin solution with a urea content of between 4 and 50% by weight calculated on the weight of urea together with melamine and a molar ratio of formaldehyde per three to urea or melamine belonging NH^ groups between 1:1 and 2.5:1. 2. Method according to claim 1, characterized in that the paper or fiber material is impregnated with a resin solution with a silicate content of between 0.4 and 0.8 weight percent SiC^. 3. Method according to claims 1 and 2, characterized by starting from a material that is impregnated with a silicate-containing "resin solution with a urea content of between 25 and 40 percent by weight calculated on the weight of urea and melamine, and with a molar ratio of formaldehyde per three NHg groups between 1.2 : 1 and 2.0 : 1. 4. Method according to claim 3, characterized in that one starts from a material consisting of a silicate-containing resin solution with a molar ratio of formaldehyde per three NH^ groups between 1.5 : log 1.8 : 1. 5. Method according to claims 1 and 2, characterized in that paper or another fibrous material is used which is impregnated with a silicate-containing resin solution with a molar ratio of formaldehyde per three to urea and melamine-related NH^ groups between 1 : 1 and 1.5 : 1, where the resin contains 4 to 20 weight percent urea calculated on the total weight of urea and melamine. 6. Method according to claim 5,... characterized in that the resin solution used has a ratio of formaldehyde per three Nl^ groups between 1.2 : 1 and 1.4 : 1.