BACKGROUND ART The present invention relates to a liner for use with plasterboard or "wallboard". The term"plasterboard"will, for simplicity, be used throughout the specification to refer to a building material which is formed from any cementitious slurry resulting in a panel of indefinite length which will be then cut to the required sizes.

Any discussion of the prior art. throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

Previous attempts have been made to create a plasterboard liner which does not require any decorative treatment after installation, or which is easy to paint or plaster to a high quality finish. Some of these liners have a coating applied to their exterior in order to modify their appearance or other properties.

A problem with such coatings is providing a liner with sufficiently low short term water absorption characteristics which is also porous enough to steam to allow sufficiently high drying rates during the plasterboard manufacturing process. Insufficient steam porosity may lead to blistering or

delamination of the coating or the paper liner, or may require drying at lower temperatures with an associated increase in dryer residence time. This may in turn force a decrease in the rate of production of the plasterboard.

US Patent No. 5945198 discloses a porous coating which may be applied to the face of the plasterboard on the plasterboard line. The porosity of the coating is enhanced through addition of soy protein, with the coating weight ranging from 14-25 g/m2. Use of this method may increase the cost and complexity of converting from the use of an uncoated liner to using a coated liner due to the changes necessary to the plasterboard manufacturing line to allow the coating to be applied to the liner as the plasterboard is manufactured.

US Patent No. 6105325 discloses a base paper with an upper web comprising white cellulose fibres, a mineral filler of light colour and a pigment layer comprising a mineral filler of light colour, preferably white, and a binder suitable for processing in a conventional plasterboard manufacturing facility.

Complementary fillers and jointing compounds are also disclosed to be used in combination with the plasterboard to provide a system with similar brightness and water absorption characteristics, in order to eliminate at least one preparation step in the decorating process. The use of such a complementary system may cause an increase in costs in ensuring that the relevant properties of each component are sufficiently similar.

International Publication WO 99/57371 discloses a plasterboard made with coated paper liner which is intended to be used without any further decorative coating. This may limit the colour options available to the end user.

OBJECT OF THE INVENTION It is an object of a preferred embodiment of the invention to provide a plasterboard liner, a plasterboard including such a liner, a method of manufacturing a plasterboard liner, or a method of manufacturing a plasterboard, which will overcome or ameliorate problems with such plasterboards, plasterboard liners and methods at present, or at least one which will provide the public with a useful choice.

Further objects of the present invention may become apparent from the following description, which is given by way of example only.

DISCLOSURE OF THE INVENTION According to one aspect of the present invention there is provided a plasterboard liner including a base paper coated with a coating, the coating including ; between substantially 20%-27% by dry weight latex, the latex selected from SBR latex and SAC latex ; between 45%-55% by dry weight of one or more fillers, the one or more fillers in combination having particle size 68%-81% below 2 microns, at least 99% of said one or more fillers having particle size below 10 microns; between 18%-23% by dry weight of one or more further fillers having particle size less than 1 micron; the balance of the coating comprising one or more further fillers, pigments and/or other materials suitable for inclusion in a plasterboard liner coating.

Preferably, said latex may be SBR latex.

Preferably, the plasterboard may include at least one pigment, wherein the pigment volume concentration of said coating is between substantially 50% and 57%.

Preferably, the plasterboard liner may include at least one pigment and at least one binder, wherein the dry pigment to binder ratio is between substantially 2.68 : 1 and 3.35 : 1.

Preferably, said base paper may contain recycled paper.

Preferably, said base paper may include crude fibre at or near its surface.

Preferably, said base paper may include a surface size at its surface to reduce the absorption of said coating.

Preferably, said base paper may weigh between 1 60g/m2 and 200 g/m2.

Preferably, said base paper may have a Gurley air permeance of between substantially 40 seconds and 70 seconds.

Preferably, said SBR latex may be a carboxylated latex.

Preferably, said coating may be applied to said base paper at a weight of between 6 g/m2 and 25 g/m2.

Preferably, said coating may be applied at a weight of substantially 12 glum2.

Preferably, said coating may include; between substantially 0% and 2% Dispersant by dry weight; between substantially 0% and 0. 1% pH Stabiliser by dry weight; between substantially 0% and 0.05% Biocide Controller by dry weight; between substantially 20% and 60% Clay by dry weight; between substantially 0% and 30% Calcium Carbonate by dry weight; between substantially 0% and 30% Titanium Dioxide by dry weight ; between substantially 10% and 20% Calcined Clay by dry weight; between substantially 0% and 2% CMC Thickener by dry weight; between substantially 20% and 27% SBR Latex by dry weight; between substantially 0.04% and 1% Defoamer by dry weight; and between substantially 0% and 1% Calcium Stearate by dry weight.

Preferably, said coating may include; substantially 0.19% Dispersant by dry weight; substantially 0.07% pH Stabiliser by dry weight; substantially 0.02% Biocide Controller by dry weight; substantially 33.12% Clay by dry weight; substantially 22.04% Titanium Dioxide by dry weight; substantially 18.37% Calcined Clay by dry weight; substantially 0.83% CMC Thickener by dry weight; substantially 25.32% SBR Latex by dry weight; and substantially 0. 05% Defoamer by dry weight; Preferably, at least a portion of said Titanium Dioxide may be replaced with inert fillers of similar particle size, absorptive properties and volume.

According to a second aspect of the present invention there is provided a plasterboard including a cementitious core and at least one face lined by the plasterboard liner of any one of the 15 preceding paragraphs.

According to a third aspect of the present invention there is provided a method of manufacturing a plasterboard liner, the method including coating a base paper with a coating including ; between substantially 20%-27% by dry weight latex, the latex selected from SBR latex and SAC latex ; 'between 45%-55% by dry weight of one or more fillers, the one or more fillers in combination having particle size 68%-81 % below 2 microns, at least 99% of said one or more fillers having particle size below 10 microns ; 'between 18%-23% by dry weight of one or more further fillers having particle size less than 1 micron; the balance of the coating comprising one or more further fillers, pigments and/or other materials suitable for inclusion in a plasterboard liner coating; the method further including subjecting said coated liner to a drying process until said coating has substantially set.

Preferably, said latex may be SBR latex.

Preferably, said coating may include at least one pigment and the pigment volume concentration of said coating may be between substantially 50% and 57%.

Preferably, said coating may include at least one pigment and at least one binder, and the dry pigment to binder ratio may be between substantially 2.68 : 1 and 3.35 : 1

Preferably, said base paper may contain recycled paper.

Preferably, said base paper may include crude fibre at or near its surface.

Preferably, said base paper may include a surface size at its surface to reduce the absorption of said coating.

Preferably, said base paper may weigh between 160 g/m2 and 200 9lu2.

Preferably, said base paper may have a Gurley air permeance of between substantially 40 seconds and 70 seconds.

Preferably, said SBR latex may be a carboxylated latex.

Preferably, said coating may be applied to said base paper at a weight of between 6 g/m2 and 25 g/m2.

Preferably, said coating may be applied at a weight of substantially 12 9lu2.

Preferably, said coating may include ; between substantially 0% and 2% Dispersant by dry weight; between substantially 0% and 0. 1% pH Stabiliser by dry weight; between substantially 0% and 0.05% Biocide Controller by dry weight; between substantially 20% and 60% Clay by dry weight; between substantially 0% and 30% Calcium Carbonate by dry weight; between substantially 0% and 30% Titanium Dioxide by dry weight;

between substantially 10% and 20% Calcined Clay by dry weight; between substantially 0% and 2% CMC Thickener by dry weight; between substantially 20% and 27% SBR Latex by dry weight; between substantially 0.04% and 1 % Defoamer by dry weight; and between substantially 0% and 1 % Calcium Stearate by dry weight.

Preferably, said coating may include ; substantially 0.19% Dispersant by dry weight; substantially 0.07% pH Stabiliser by dry weight; substantially 0.02% Biocide Controller by dry weight; substantially 33.12% Clay by dry weight; substantially 22.04% Titanium Dioxide by dry weight; substantially 18.37% Calcined Clay by dry weight; substantially 0.83% CMC Thickener by dry weight; substantially 25.32% SBR Latex by dry weight; and substantially 0.05% Defoamer by dry weight; Preferably, at least a portion of said Titanium Dioxide may be replaced with inert fillers of similar particle size, absorptive properties and volume.

According to a fourth aspect of the present invention there is provided a method of manufacturing a plasterboard including the steps of manufacturing a plasterboard liner using the method of any one of the fifteen immediately preceding paragraphs, the method further including providing said plasterboard liner with a cementitious core.

Preferably, the method may include the step of subjecting said coated liner and said cementitious core to a drying process until said cementitious core has substantially set and said coating has substantially formed a homogeneous film.

Preferably, the method may include the step of drying said plasterboard in a multistage conventional plasterboard dryer with an average drying rate in a first zone of up to substantially 250 g/m2/minute.

According to a further aspect of the present invention a plasterboard liner and/or a method of manufacturing a plasterboard liner are substantially as herein described.

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description given by way of example of possible embodiments of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION The paper liner of the present invention may be manufactured using equipment common to the production of paper plasterboard liner. The base paper may have a standard"Ivory layer"exterior finish applied to one side, as may be applied to the exterior of a liner on the working side of a plasterboard of the prior art.

The paper may be of a lower grade pulp than would normally be required for a premium plasterboard due to the tendency of the coating to fill and smooth over any surface imperfections. Preferably paper containing recycled paper and/or having a crude fibre near its surface may be used. The potential reduction in cost of the base paper may at least partially offset the cost of coating the liner.

The base paper may weigh between 160g/m2 and 200 g/m2 and may have a Gurley air permeance of between substantially 40s and substantially 70s.

A coating may be applied to the finished side by a standard air knife at a weight of between 6g/m2 and 25 g/m2, but most preferably at a weight of around 12 g/m2. The coating may then be dried at substantially 85°C for substantially 3 seconds.

After coating the paper may be rewound and cut into widths suitable for use in plasterboard manufacture.

The advantages of the present invention may be obtained by a coating which includes between 20% and 27% latex of either the Styrene Butadiene (SBR) type or the Styrene Acrylic (SAC) type, and between 45%-55% by dry weight of one or more fillers, the one or more fillers in combination having particle size 68%-81% below 2 microns. At least 99% of the particles in said one or more fillers should have a size of below 10 microns.

The coating should also include between 18%-23% by dry weight of one or more further fillers having a particle size of less than 1 micron.

The sub 2 micron filler particles may come from a single filler or a plurality of fillers. At least 99% of the total fillers added to the coating should be below 10 microns in particle size.

All percentages are expressed as percent dry weight unless otherwise stated or unless the context clearly requires otherwise.

The balance of the coating, if any additional materials are included, may include one or more further fillers, pigments and/or other materials suitable for inclusion in a plasterboard liner coating. These additional materials may also include, for example, defoamers, stabilisers and thickeners.

The latex used provides compatibility with cellulose substrates such as paper, as well as being compatible with many of the solvents used in interior paints. In a preferred embodiment the SBR latex may be carboxylated latex.

Those skilled in the art will appreciate that pigments having an appreciable proportion of particles in the sub 1 micron range, such as Titanium Dioxide for example, may be considered to be fillers for the purposes of their contribution to the total proportion of sub 1 micron particles in the coating, although they may also provide pigmentation to the coating.

Subject to the requirement to provide the correct proportions of sub 2 micron filler particles and sub 1 micron filler particles, preferred ranges for the constituents of the coating may be: between 0% and 2% Dispersant by dry weight; between 0% and 0. 1% Stabiliser by dry weight; between 0% and 0.05% Biocide Controller by dry weight; between 20% and 60% Clay by dry weight ; between 0% and 30% Calcium Carbonate by dry weight; between 0% and 30% Titanium Dioxide by dry weight; between 10% and 20% Calcined Clay by dry weight; between 0% and 2% CMC Thickener by dry weight; between 20% and 27% SBR Latex by dry weight; between 0.04% and 1 % Defoamer by dry weight; between 0. 1% and 1% Calcium Stearate by dry weight;

It is preferred that the pigment volume concentration should be between 50% and 57% and that the dry pigment to binder ratio is between substantially 2.68 : 1 and 3.35 : 1.

A typical formulation for the coating may be as follows ; Dispersant 0. 19% pH Stabiliser 0.07% Biocide Controller 0.02% Clay 33.12% Titanium Dioxide 22.04% Calcined Clay 18.37% CMC Thickener 0.83% SBR Latex 25. 31% Defoamer 0.05% The balance of the coating may be substantially water.

The preferred formulation above may provide a suitable rheology for air knife application of the coating. The proportions may be varied to provide suitable rheology for other application methods which are known to the art.

The method of adjustment of the rheology will be known to those skilled in the art and is therefore not discussed further herein. A suitable surface size may be used to reduce absorption of the coating by the base paper as required.

The use of Titanium Dioxide in the above formulation is to provide suitable brightness to the coating for markets demanding such qualities. A proportion of the Titanium Dioxide may be replaced by alternative

fillers/pigments of similar particle size, absorptive properties and volume, for markets with different brightness and/or cost requirement.

Plasterboard may be manufactured using the liner of the present invention to directly replace a paper liner of the prior art. Dryer temperatures and residence times may be very similar to those needed for uncoated liner, and may be optimised for a particular plasterboard composition by means of routine experiment. Typically a slight increase in dryer temperature over that used for uncoated liner may be necessary if the production rate is to be kept substantially the same.

In particular, drying may be carried out in a multistage conventional plasterboard dryer with average drying rates in the first zone of up to approximately 250 glm2/minute. The coating may remain well bonded at these drying rates and with zone temperatures of up to 250°C.

It has been surprisingly found by experiment that coating a paper plasterboard liner with a coating in accordance with the present invention may decrease the short term moisture absorption characteristics of the liner while at the same time providing sufficient steam porosity to allow manufacture of the plasterboard using dryer temperatures and residence times similar to those used in the production of gypsum plasterboard with liners of the prior art.

In particular, paper liner made in accordance with the present invention may have a 30 second Cobb test value of around 11-18 g/m2 compared to around 20-22 g/m2 for the same paper without the coating. It has also been surprisingly found that the value given by the Cobb test may decrease even further after the coated paper has been processed through the plasterboard manufacturing process. The applicant believes that subjecting the coated paper liner to the high temperature conditions experienced in the drying

process of gypsum plasterboard manufacture may assist the coating in forming a film which is even less permeable to water. After this process, a 30 second Cobb test of some samples has been surprisingly found to give results as low as 4 gin2.

Cobb testing may also be performed over 60 seconds in order to represent the timeframe in which improved properties are believed to be perceived by joint treatment and painting tradespeople. When tested over 60 seconds, the plasterboard liner of the present invention may yield results of approximately 19 to 22 g/m2. After processing, a plasterboard with the liner of the present invention may yield 60 second Cobb results of approximately 9 g/m2.

It is believed that the present invention may provide an optimum balance of particle sizes in the constituents to allow steam to pass through the molecular structure of the coating while hindering the progress of liquid water.

The fine particle size of the fillers may also result in high surface area and high absorption of resins which may increase the steam porosity of the composition.

The decrease in short term water absorption of the liner may increase the"holdout"of jointing compounds and paint applied to the liner, that is, the wet edge of the jointing compound or paint may take longer to dry. A similar effect may be noticed when plastering the paper. This increased"holdout" may make painting and plastering of the plasterboard easier since a greater time is available to perfect the appearance of the jointing compound or decorative coating before it dries or sets.

In addition, the undecorated paper may be easier to wipe clean and may be resistant to on-site moisture, In particular, plasterboard manufactured with the liner of the present invention may exhibit increased resistance to sag

since the moisture uptake into the paper liner may be decreased. Some of the properties of the coated liner may also be brought closer to those of-the finishing compound used in assembling the plasterboard in use, which may assist in providing a more uniform or monolithic appearance to the finished and painted surface.

The paper liner and the face surface of the plasterboard manufactured from it may be compatible with commonly used solvent borne decorative paints incorporating common solvents such as mineral turpentine. They may also be compatible with and provide good adhesion with commonly used waterborne decorative paints.

It will be apparent to those skilled in the art that the present invention may provide a plasterboard liner with improved moisture"holdout"qualities and adequate steam porosity which is economic to manufacture.

Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the claims.