Patent Description:
Calcium sulphate-based products are widely used in the construction of buildings, for example, to form internal partitions (using wallboard, also known as dry wall, gypsum board or plaster board) and ceilings or to encase ducts (e.g. ventilation ducts) within buildings.

Calcium sulphate-based products such as wallboard are typically formed by drying an aqueous slurry of the hemihydrate of calcium sulphate (CaSO<NUM>. ¼ H<NUM>O), also known as calcined gypsum or stucco, between two sheets of lining paper or fibreglass matting. As the slurry dries and the calcined gypsum is hydrated, a hard, rigid core of gypsum (calcium sulphate dihydrate - (CaSO<NUM>. <NUM><NUM>O)) sandwiched between the lining sheets/mats is formed.

When wallboard or ceiling tiles are exposed to high temperatures such as those experienced in a building fire or those experienced by wallboards used for encasing ducts carrying high temperature fluids, the water of crystallization contained within the gypsum is driven off to yield the anhydrite of calcium sulphate. Initially, this has the advantage that heat transfer across the wallboard/ceiling tile is reduced thus helping to contain the heat emanating from ducting or generated during a building fire. However, at temperatures around <NUM>-<NUM>, the initially formed Alll phase anhydrite (also known as γ-CaSO<NUM> or "soluble" anhydrite) converts to the All phase (or "insoluble" anhydrite) and this phase change results in shrinkage of the wallboard/tile i.e. a loss of dimensional stability. This shrinkage often causes the wallboards to pull away from their supporting structures. This is obviously undesirable. It can leave ducts exposed to high temperatures. Furthermore, in situations where wallboard is used for internal partitions and a fire breaks out, shrinkage can leaves gaps exposing rooms adjacent to the fire source to the effects of the heat/fire. Gaps also allow ingress of oxygen into the fire source thus fuelling the fire and negating the effects of any fire doors.

At higher temperatures (in excess of <NUM>), the insoluble anhydrite goes on to sinter resulting in large reductions in wallboard volume. This results in extreme shrinkage which eventually causes collapse of the internal walls/ceilings/duct casings as they are no longer held by their supporting structures.

Efforts have been made to improve the heat resistance of calcium sulphate-based products such as wallboard in an attempt to reduce shrinkage.

It is known e.g. from <CIT> to use micro silica as an additive in the gypsum core of wallboard to reduce shrinkage. However, this additive only has an effect at temperatures greater than <NUM> i.e. it does not resist the shrinkage of the board at lower temperatures and linear shrinkage of more than <NUM>% is still seen as temperatures around <NUM>.

It is known from <CIT> and <CIT> to add sodium trimetaphosphate (STMP), sodium hexametaphosphate (SHMP) or ammonium polyphosphate (APP) to a calcium sulphate wallboard core to improve strength, sag resistance and shrinkage during drying. No effect of these additives on shrinkage during exposure to high temperatures is recorded.

<CIT> discloses an ablative type heat protecting material containing calcium sulphate hemihydrate and a hydrated salt. A number of hydrated salts are used including magnesium nitrate hexahydrate (used in an amount of 40wt% based on the weight of dry ingredients). The time taken for heat transfer across the heat ablative material was recorded. No mention is made of any shrinkage resistance properties of the hydrated salts.

<CIT> proposes a refractory gypsum hardened product including gypsum as a principal material and one or more of aluminium oxide or aluminium compounds convertible into alumina at high temperatures in an amount of ≥<NUM>. % expressed in terms of the alumina in the gypsum.

Calcium sulphate-based products are also used to cast metal or glass objects. Calcium sulphate moulds are heated to <NUM>-<NUM> prior to being filled with molten metal/glass. It is important to control high temperature shrinkage of such calcium sulphate-based moulds to ensure that the moulds do not leak and to ensure that the cast metal/glass products are not warped.

A preferred aim of the present invention is to provide an improved heat resistant calcium sulphate-based product having reduced shrinkage after heat exposure e.g. when in contact with ducting, during a building fire or during casting of metal products. Such an improved heat resistant product may have particular use as a building product e.g. wallboard or panels for forming internal partitions in buildings, ceiling tiles, wallboard or panels for encasing ventilation/smoke extraction ducting, joint filler materials for joining wallboard/panels/tiles or for moulds for use in metal/glass product casting.

Accordingly, in a first aspect, the present invention provides a calcium sulphate-based product as set out in claim <NUM>.

In a second (non-claimed) aspect, the present disclosure provides a calcium sulphate-based product comprising gypsum and a shrinkage resistance additive, wherein the product is formed from drying an aqueous slurry containing calcined gypsum and said shrinkage resistance additive, the shrinkage resistance additive being a metal nitrate, hydroxide, acetate or sulphate.

In a third aspect, the present invention provides a method of forming a calcium sulphate-based product as set out in claim <NUM>.

In a fourth (non-claimed) aspect, the present invention provides the use of a metal nitrate, hydroxide, acetate or sulphate as an additive in a gypsum matrix for reducing shrinkage in a calcium sulphate-based product during heat exposure.

In a fifth aspect, the present invention provides a calcium sulphate-based composition for use in forming a calcium sulphate-based product by drying an aqueous slurry of the calcium sulphate-based composition as set out in claim <NUM>.

The inventors have found that including an acetate of magnesium, an acetate of zinc, or an acetate of iron in a calcium sulphate based product e.g. the gypsum core of a wallboard reduces shrinkage of the wallboard when the board is exposed to high temperatures. Unlike micro silica which only has an effect above <NUM>, the metal nitrate, hydroxide, acetate or sulphate begins to have an effect around <NUM> where it undergoes an endothermic decomposition (to yield oxides, oxygen and nitrogen oxides) and thus acts as a heat sink. The acetate of magnesium, acetate of zinc, or acetate of iron also acts to increase the temperature at which the transition from the soluble to insoluble calcium sulphate anhydrite occurs thus allowing the product to resist the shrinkage arising from the phase change until higher temperatures (greater than <NUM>) are reached. The inventors have found that a metal-rich layer forms at the surface of the calcium sulphate based product and it is believed that this metal-rich layer protects the calcium sulphate anhydrite and delays the transition until higher temperatures. Products, compositions, and methods according to the present invention utilise acetates of magnesium, zinc or iron.

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

The additive is magnesium acetate , zinc acetate or iron acetate. Preferably, it is magnesium acetate. Magnesium acetate is hygroscopic and typically exists as magnesium acetate tetrahydrate, Mg(C<NUM>H<NUM>O<NUM>)<NUM>•<NUM><NUM>O.

The calcium sulphate-based product is formed from drying an aqueous slurry containing calcined gypsum (or stucco) and the metal acetate anti-shrinkage additive.

The metal acetate is present in the slurry and in the calcium sulphate-based composition in an amount greater than or equal to <NUM> wt% or greater than or equal to <NUM> wt% or greater or equal to than <NUM> wt% or greater than or equal to <NUM> wt% or greater than or equal to <NUM> wt% (based on the amount of acetate /calcined gypsum in the slurry/composition).

The metal acetate is present in the slurry and in the calcium sulphate-based composition in an amount equal to or less than <NUM> wt% or equal to or less than <NUM> wt% or equal to or less than 30wt% (based on the amount of nitrate, hydroxide, acetate or sulphate /calcined gypsum in the slurry/composition).

In preferred embodiments, the metal acetate is present in the slurry/composition in an amount between <NUM> wt% and less than <NUM> wt%.

The metal acetate is present in the resulting calcium sulphate-based product in an amount greater than or equal to <NUM> wt% or greater than or equal to <NUM> wt% or greater or equal to than <NUM> wt% or greater than or equal to <NUM> wt% or greater than or equal to <NUM> wt% (based on the amount of acetate /gypsum in the product).

The metal acetate is present in the calcium sulphate-based product in an amount equal to or less than <NUM> wt% or equal to or less than <NUM> wt% or equal to or less than <NUM> wt% (based on the amount of acetate /gypsum in the product).

In preferred embodiments, the metal acetate is present in the calcium sulphate-based product in an amount between <NUM> wt% and less than <NUM> wt%.

The term calcined gypsum (or stucco) is intended to refer predominantly to calcium sulphate hemihydrate (CaSO<NUM>. <NUM>/<NUM><NUM><NUM>) but may also encompass any other calcium sulphate compound having a lower bound water content than calcium sulphate dihydrate (e.g. calcium sulphate anhydrite).

The term "gypsum" is intended to refer predominantly to calcium sulphate dihydrate (CaSO<NUM>. <NUM><NUM><NUM>).

In some embodiments, the calcined gypsum is present in the slurry and in the calciumsulphate-based composition in an amount of <NUM> - <NUM> wt% (based on the amount of nitrate, hydroxide, acetate or sulphate /calcined gypsum in the slurry/composition). More preferably, it is present in an amount from <NUM> to 70wt% or <NUM> to <NUM> wt%.

In some embodiments, the gypsum is present in the calcium sulphate-based product an amount of <NUM> - <NUM> wt% (based on the amount of acetate /gypsum in the product). More preferably, it is present in an amount from <NUM> to <NUM> wt% or <NUM> to <NUM> wt%.

Preferably, the product e.g. the gypsum core of the product contains no clinker i.e. no product produced by sintering limestone and alumina-silicate.

The term "calcium sulphate-based product" may include building products such as wallboards (with or without liners) (with or without fibrous reinforcement), tiles (e.g. ceiling tiles), duct encasement panels, joint filler materials (e.g. for joining adjacent wallboards/tiles/panels etc.), plaster compositions and moulds for casting metal products.

The term "calcium sulphate-based" will be readily understood as meaning that the product comprises gypsum as a major component i.e. that gypsum is the largest single component in terms of wt% of the product. The term may mean that the product comprises gypsum in <NUM> wt%, <NUM> wt%, <NUM> wt%, <NUM> wt%, <NUM> wt%, <NUM> wt% or greater based on the total weight of the product.

The calcium sulphate-based product may be a composite product e.g. it may be a wallboard having a gypsum matrix core (containing the shrinkage resistance additive) sandwiched between two liners (e.g. paper liners or fibreglass matting).

In some embodiments, the calcium sulphate-based product contains substantially no inorganic fibres e.g. no glass or asbestos fibres. The present inventors have found that the addition of a combination of a clay additive and a metal salt can help maintain strength and structural integrity after heating even in the absence of a fibrous network.

However, in some embodiments, the calcium sulphate-based product may contain inorganic fibres (e.g. glass fibres) and/or matting (e.g. glass matting) as this may help improve strength of the product prior to heating.

The calcium sulphate-based product may contain additives such as accelerators, retarders, foaming/anti-foaming agents, fluidisers etc.. The accelerators may be, for example, freshly ground gypsum having an additive of sugar or surfactant. Such accelerators may include Ground Mineral NANSA (GMN), heat resistant accelerator (HRA) and ball milled accelerator (BMA). Alternatively, the accelerator may be a chemical additive such as aluminium sulphate, zinc sulphate or potassium sulphate. In certain cases, a mixture of accelerators may be used, e.g. GMN in combination with a sulphate accelerator. As a further alternative, ultrasound may be used to accelerate the setting rate of the slurry, e.g. as described in <CIT>.

<FIG> shows a graph of linear shrinkage for a control sample and inventive samples during heating to <NUM>.

The following examples are given by way of illustration only. Example <NUM> is according to the invention.

<NUM> of calcined gypsum was added to <NUM> of water at <NUM>. This was mixed by hand for <NUM> seconds and the resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>). The sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium nitrate hexahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of calcined gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium nitrate hexahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of calcined gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium nitrate hexahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of aluminium nitrate nonahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of calcined gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of aluminium nitrate nonahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of zinc nitrate hexahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of calcined gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of zinc nitrate hexahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of iron (III) nitrate nonahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of calcined gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of iron (III) nitrate nonahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of potassium nitrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of copper nitrate tetrahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of calcium nitrate tetrahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM> wt% nitrate based on weight of dry ingredients/containing <NUM> wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium hydroxide was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM>. 1wt% hydroxide based on weight of dry ingredients/containing <NUM>. 8wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium acetate tetrahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM>. 1wt% acetate based on weight of dry ingredients/containing <NUM>. 8wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

<NUM> of magnesium sulphate heptahydrate was added to <NUM> of water at <NUM>. <NUM> of calcined gypsum was added to the solution and the resulting slurry (containing <NUM>. 1wt% sulphate based on weight of dry ingredients/containing <NUM>. 8wt% based on weight of gypsum) was blended by hand for <NUM> seconds to form a slurry. The resulting slurry was poured into a cylindrical silicone mould (of height <NUM> and diameter <NUM>) and the sample was transferred to an oven at <NUM> and left to dry overnight (at least <NUM> hours).

A summary of all sample formulations is shown in Table <NUM> below.

The linear shrinkage of the samples was measured using a Netzsch dilatometer with a ceramic rod attached to a linear displacement transducer having a resolution of <NUM>. The samples were supported by other ceramic rods and the heated in a furnace to <NUM> at a rate of <NUM>/min. The results are shown in <FIG> and <NUM> and Table <NUM> below.

Claim 1:
A calcium sulphate-based product, wherein the product is a building material, and wherein the product comprises gypsum and a shrinkage resistance additive provided in an amount between <NUM> and <NUM> wt%, based on the weight of gypsum and additive, wherein the shrinkage resistance additive is an acetate of magnesium, an acetate of zinc, or an acetate of iron.