WO2005095712A1

WO2005095712A1 - Wallpaper

Info

Publication number: WO2005095712A1
Application number: PCT/FI2005/050082
Authority: WO
Inventors: Maija Pitkänen; Jukka LÄHTEENMÄKI; Veli-Matti Kaukonen; Tarja Turkki
Original assignee: M-Real Oyj
Priority date: 2004-03-30
Filing date: 2005-03-15
Publication date: 2005-10-13
Also published as: FI20045113A0; FI121969B; FI20045113A

Abstract

The invention relates to a wallpaper comprising a wood-fibre based base paper with a first and a second surface, and a barrier layer on the side of the first surface of the base paper. The wallpaper comprises a hydrophilic layer between the first surface and the barrier surface, or on the surface of the barrier layer.

Description

Wallpaper

The invention relates to a wallpaper comprising a wood-fibre based base paper with a first and a second surface, and a barrier layer on the side of the first surface of the base paper.

This application relates to a wallpaper which is suitable for use in such a way that an adhesive, normally a size for wallpaper, is applied onto the wall to be papered, and the dry wallpaper is placed onto the adhe- sive layer. This wallpapering method is called "paste-the-wall".

It is a problem of wood-fibre based wallpapers that they expand in the width direction of the web when the paper is moistened by the adhesive. The wet expansion in the width direction is a result of the fibre orientation of the web. In the web, the fibres tend to be oriented in the length direction of the web. When wetted, the wood fibres swell in the cross direction of the fibre, wherein the web is expanded by moisture in the width direction of the web. The wet expansion of wood-fibre based base papers is typically in the order of 1.5 to 2 %.

At present, wallpapering is normally performed by using a butt joint, wherein the effects of the wet expansion in the width direction become easily visible. As a result of the expansion, the papering result becomes poor; the butt joints bulge and wrinkles appear in the wall- paper. Contrary to the conventional papering method, in which the adhesive is applied onto the back of the wallpaper and the wallpaper is fixed onto the wall first after a given wetting time (10 to 15 minutes), wherein the wet expansion of the web in the width direction has already taken place before the fixing onto the wall, there is no wetting time in the past-the-wall papering method. Therefore, this papering technique requires a web with almost no wet expansion, because the wetting of the wallpaper and the subsequent wet expansion and simultaneous fixing onto the wall easily result in the above-mentioned problems in the papering result.

Attempts have been made to solve the problem of wet expansion by using, as the base material for the wallpaper, a non-woven fabric which comprises man-made fibres and an adhesive, such as latex. It is true that a wet expansion of about 0.2 % can be achieved with products based on a non-woven fabric, but in some respects, however, the non- woven fabric is a poor alternative. It has poor opacity, printing onto the base with an uneven surface is difficult (missing points), and recycling of the non-woven fibres comprising man-made fibres and a latex adhesive between them is difficult. The broke formed in the process of manufacturing the base paper cannot be recirculated in the manufacturing process, but it is dumped.

The wet expansion tendency of the wood-fibre based wallpaper can be reduced by treating the base paper with a hydrofobic adhesive or, for example, with cross-linking chemicals (by cross-linking the fibres), but even if combined, these treatments do not give a satisfactory result. Moreover, if the adhesive used for fixing the wallpaper has a high pH value (11 or higher) and a low surface tension, the hydrofobicity cannot prevent the fast wetting of the fibres. The hydrofobic layer will be destroyed. Consequently, special demands are set on the surface treatment layers of the base paper by the low surface tension of the size for the wallpaper.

According to the experience of the applicant, the wet dimensional stability of the wood-fibre based wallpaper cannot be achieved by any single treatment. The dimensional stability required by the paste-the-wall papering method requires a combination of more than one layer. In addition to the dimensional stability, the wallpaper must be movable on the wall during the papering (positioning) and subsequently it must have sufficient adhesion to various wall materials (stability on the wall). Problems are also involved in approaches with several layers. For example, in experiments with several porous and sealing coating layers under the barrier layer, good dimensional stability was achieved with a high content of latex in the coating, but the wallpaper was thus too easy to remove from the wall. The coating layer did not adhere properly to the base paper nor to the wall.

On the other hand, the wallpaper must not be too impermeable either, because water vapour must be allowed to exit also outwards when the size dries on the wall. These requirements set limits on the solutions available for the prevention of wet expansion. Furthermore, the selection of the treating agents is limited, for example, by strong yellowing of some substances or combinations of substances, or by factors of prod- uct safety.

The wallpaper according to the invention is characterized in that the wallpaper comprises a hydrophilic layer between the first surface and the barrier layer, or on the surface of the barrier layer.

The wallpaper according to the invention comprises a wood-fibre based base paper with a first and a second surface. The first surface of the base paper comes against the wall, and the second surface comprises the printing and/or other patterning of the wallpaper. The base paper may comprise one or two layers. Preferably, the base paper does not comprise man-made fibres, but the fibre portion of the base paper comprises mechanical and/or chemical pulp.

After the wallpaper according to the invention has been installed on the wall, an adhesive layer which has been applied onto the wall before the placement of the wallpaper is left between the paper and the wall. Consequently, in the present application, no layer to be applied onto the wallpaper refers to the adhesive layer, because the actual adhesive is applied onto the wall. In addition to the layers presented in this appli- cation, the wallpaper may also comprise an adhesive layer if the adhesive is to be included in the product. In such a case, it would not be necessary to apply the adhesive onto the wall but the adhesive could be, for example, merely wetted to provide a glueing effect.

The property of no wet expansion is achieved by applying two different technologies of functional surface treatment. Before these surface treatments at the manufacturing stage, the base paper may be strongly hydrofobically glued in the wet end of the paper machine. The idea of the invention is that the functionality of the barrier layer is improved by means of a hydrophilous layer. When applied alone, said layer cannot prevent the wet expansion to a sufficient degree, but when applied onto or under the barrier layer, the wallpaper can be made non wet expansive, or the wet expansion can be sufficiently reduced. The treatment layers of the wallpaper according to the invention are formed by using known techniques for coating paper, primarily by spreading.

The first alternative is to apply a barrier layer onto the first surface of the base paper to slow down the penetration of water contained in the adhesive into the base paper. In the barrier layer, a variety of substances can be used which have the capacity to prevent the penetration of water or water vapour, even though the selection of the sub- stances is limited by e.g. the yellowing tendency of certain substances or combinations of substances, or by factors of product safety. The barrier layer can be, for example, styrene acrylate, polyacrylate, styrene butadiene, polyvinyl acetate, or ethyl vinyl acetate. The barrier layer can also be a mixture of several substances.

A hydrophilic layer is formed onto the surface of this barrier layer which is capable of absorbing water included in the adhesive under alkaline conditions. However, under neutral conditions, the hydrophilic layer can be dried onto the surface of the barrier layer at the manufacturing stage. The hydrophilic layer may be, for example, a cross-linked polyvinyl alcohol hydrogel. The polyvinyl alcohol can be cross-linked, for example, by a treatment with boric acid. The polyvinyl alcohol also improves the adhesion of the wallpaper to the wall. A base paper manufactured according to this embodiment of the invention has a wet expansion rate of 0 % when the quantity of barrier layer applied is 10 to 13 g/m² and the quantity of hydrophilic layer applied is 13 to 17 g/m².

The polyvinyl alcohol to be used in the hydrophilic layer can be selected from fully hydrolyzed, superhydrolyzed or ready cross-linked, fully hydrolyzed or superhydrolyzed polyvinyl alcohols. Fully hydrolyzed refers to the hydrolysis degree of 99 to 100 %. The degree of hydrolysis refers to the quantity of alkaline solution added in the preparation of polyvinylalcohol from polyvinyl acetate by saponification. A drop in the degree of hydrolysis refers to an increase in the hydrophilicity of the polyvinyl alcohol. The hydrophilicity can be reduced by controlled thermal treatment by increasing the degree of crystallinity of the polyvinyl alcohol or by cross-linking the polyvinyl alcohol e.g. with boric acid, borax, glyoxal, urea, or melamine resin.

The polyvinyl alcohol may also be ready cross-linked. Ready cross- linked polyvinyl alcohols may have been treated with borax or boric acid. The treatment with borax or boric acid increases the viscosity and the tackiness/adhesion properties of the treatment solution. As a result of the treatment, there is less penetration of the polyvinyl alcohol into the paper, wherein the PVA remains on the surface of the paper and gives an even layer thickness.

Another alternative is that the first surface of the base paper is first provided with a water absorbent hydrophilic layer and, on the surface of the hydrophilic layer, a barrier layer. The function of the different layers is the same as above, and also the substances used for the formation of the layers are the same. This embodiment, in which the barrier layer is in contact with the adhesive, has the particular advantage that the wallpaper can be easily moved on the wall; in other words, the placement of the wallpaper on the wall becomes easier. The wallpaper according to the second alternative has a wet expansion rate of 0.13 to 0.34 %, when the quantity of hydrophilic layer applied is 9 g/m² and the quantity of barrier layer applied is 9 to 11 g/m².

In a third embodiment according to the invention, the functional preconditions of the barrier layer are improved by compressing the base before the application of the barrier layer. The compression of the base prevents the penetration of the barrier polymer into the base, wherein the result is a more even barrier layer on the surface. In this third embodiment, a coating comprising a filler with a polyvinyl alcohol, such as a kaoline coating, is first applied onto the first surface of the wallpaper to seal the surface, and the barrier layer is applied onto the same. When a filler, such as kaoline or talc, is used together with the polyvinyl alcohol, the quantity of the polyvinyl alcohol can be reduced, and the runnability and drying are improved by the addition of the filler. The polyvinyl alcohol constitutes the hydrophilic portion of the layer. A filler in a plate-like form, such as kaoline, is best suitable; preferably, it has a form factor of at least 70. With a low coating content (8 g/m²) and a barrier layer (10 g/m²) applied onto it, a product with no wet expansion was achieved. When the hydrophilic layer comprising kaoline and poly- vinyl alcohol, and the barrier layer were further combined with a cross- linking treatment (e.g. with the chemical Fixapret) of the layer comprising polyvinyl alcohol, the quantity of the barrier layer could be reduced by 2 g/m² (from 10 to 8 g/m²).

According to one embodiment of the invention, the filler, such as kaoline or talc, can also be used in the barrier layer.

In all the surface treatment solutions according to the invention, it is advantageous that the base paper is hydrofobically well sized, for example by resin, AKD (alkyl ketene dimer) or ASA (alkenyl succinic anhydride) adherence in the wet end of the paper machine.

Example 1.

An experiment was made by forming various barrier and hydrophilic layers on that side of the base paper which comes against the wall during papering. The agent used for forming the barrier layer was poly- acrylate (Cellfob 18 by Cellkem Oy, Finland). The hydrophilic layer used was cross-linked polyvinyl alcohol (Celanese Celvol MM-14, Noviant) or a mixture of polyvinyl alcohol, kaoline and resin (69/29/6 Kuraray PVA 105 (supplied by Algol) / Capim NP (supplied by Imerys Minerals) / Cellcoat 6040 (supplied by Cellkem)). Celvol MM-14 has been treated with boric acid. Cellcoat 6040 is a polymeric aqueous resin solution containing reactive hydroxyl groups and containing 18 weight-% of glyoxal as a cross-linking agent.

The solution used in the experiment was prepared from cross-linked polyvinyl alcohol (Celvol MM-14) as follows: The powder was mixed into water at a temperature of about 25°C. The temperature was raised by heating with steam to 91 °C and maintained for 30 min. The temperature had to be kept below 96°C all the time. The mixture of polyvinyl alcohol, kaoline and resin was prepared as follows: The polyvinyl alcohol powder was mixed into water at a temperature of about 25°C. The temperature was raised by heating with steam to 91 °C and was maintained for 110 min. Kaoline and resin were added into the above-mentioned solution by continuously stirring after the temperature of the solution had been reduced / cooled down to about 30°C.

The base paper quality used was Cresta D2 (M-Real, Finland). The base paper had a basis weight of 120 g/m². The base paper quality contains two layers, of which the first layer coming to the side of the wall consists of mechanical pulp, and the second layer consists of chemical pulp. The surface of the second layer is coated with a double coating layer.

A barrier layer and a hydrophilic layer were formed by coating with a groove application rod. The formulae used are tabulated in tables 1 and 2, and the test results obtained are tabulated in tables 3 and 4.

In addition to the laboratory results, a wallpapering experiment was made onto a surface of a plasterboard, using a starch-based size (Solvite ES, Henkel).

The results of Tables 3 and 4 show that the best results in the wet expansion are achieved if there is a hydrophilic layer between the barrier layer of polyacrylate and the base paper, the hydrophilic layer being either of a cross-linked polyvinyl alcohol hydrogel or a mixture of polyvinyl alcohol, kaoline and resin (samples N8, N9, N12, N13).

The following abbreviations are used in the tables 3 and 4: us = upper side, rs = reverse side, tc = treating conditions, norm = normal, spil = spilled, air = in the air, WVTR = water vapour transmission rate. Table 1. Compositions of the surface coating layers of samples Ref. 1 and N1 to N5. The hydrophilic layer is against the wall, i.e. in contact with the size.

Table 2. Compositions of the surface coating layers of samples Ref. 2 and N6 to N13. The barrier layer is against the wall, i.e. in contact with the size.

Table 3. Test results of samples Ref. 1 and N1 to N5. The hydrophilic layer is against the wall.

Table 4. Test results of samples Ref. 2 and N6 to N15. The barrier layer is against the wall.

Table 4. Test results of samples Ref. 2 and N6 to N15 (continued from previous page).

Example 2.

Different alternatives for a hydrophilic layer were tested under laboratory conditions. The wet expansion was measured from the samples.

Table 5. Wet expansion values of treated paper.

Celvol 103 is a fully hydrolyzed polyvinyl alcohol with a low molecular weight. Celvol 125 is a superhydrolyzed polyvinyl alcohol with a low molecular weight. Celvol 165 is a superhydrolyzed polyvinyl alcohol with a high molecular weight. Celvol 350 is a fully hydrolyzed polyvinyl alcohol with a high molecular weight. Celvol MM-14 is a ready cross- linked polyvinyl alcohol like Celvol MM-51 , but it has a higher molecular weight than Celvol MM-14. The Celvol products are produced by Celanese. The Celvol products which were not ready cross-linked were treated with boric acid to provide cross-linking. The treatment was carried out as follows: 300 g of 15 % polyvinyl alcohol solution were weighed and mixed with 150 g of 3 % boric acid.

The wet expansion values were measured after 5 minutes from seizing. In spite of the low wet expansion values, all the samples blistered after the fixing onto the base; in other words, the mere hydrophilic layer does not work alone.

Example 3.

Experiments were further made with the Celvol 103 and Celvol 350 products described in connection with Example 2, with kaoline. The ratio between polyvinyl alcohol and kaoline was 50:50. The polyvinyl alcohol solution contained 10 weight-% of boric acid calculated from the quantity of polyvinyl alcohol. Tests were made with kaolines having different form factors, ND 2591 and a so-called delaminated test pigment having a high form factor. The size was Glutolin "Normal", whose pH was adjusted to > 11.

The combination of Celvol 103 and ND 2591 gave the wet expansion values of 0.4 to 0.6 % with the coating quantities of 12.3 and 16.0 g/m², respectively. The combination of Celvol 350 and delaminated test kao- line gave the wet expansion values of 0.3 to 0.4 % with the coating quantities of 12.2 and 17.4 g/m², respectively.

Also in this case, the samples blistered after being attached to the substrate.

The invention is not restricted to the description above, but it may vary within the scope of the claims.

Claims

Claims:

1. Wallpaper comprising a wood-fibre based base paper with a first and a second surface, and a barrier layer on the side of the first side of the base paper, characterized in that the wallpaper comprises a hydrophilic layer between the first surface and the barrier layer, or on the surface of the barrier layer.

2. The wallpaper according to claim 1 , characterized in that the barrier layer comprises a barrier-layer forming latex, such as styrene acrylate, polyacrylate, styrene butadiene, polyvinyl acetate, or ethyl vinyl acetate.

3. The wallpaper according to claim 1 or 2, characterized in that the hydrophilic layer comprises a polyvinyl alcohol.

4. The wallpaper according to claim 3, characterized in that the hydrophilic layer comprises a cross-linked polyvinyl alcohol hydrogel.

5. The wallpaper according to claim 3, characterized in that the hydrophilic layer comprises a filler, preferably plate-like kaoline.

6. The wallpaper according to any of the preceding claims, character- ized in that the barrier layer comprises a filler, preferably plate-like kaoline.