Lightweight plasterboard and plaster slurry composition used for its manufacture

One subject of the invention is a novel lightweight board or a novel board having better mechanical compression properties, and also a plaster slurry composition used in its manufacture. The invention also provides a method for manufacturing such a board and use of a specific starch in the manufacture of a plasterboard.

Plasterboards have been known for many years. These boards generally comprise two sheets or facings, generally made of paper or paperboard, between which a plaster core is sandwiched. The facings have a certain tensile strength whereas the core has a certain compressive strength.

Furthermore, it is sought to lighten the plasterboards, while still having good mechanical properties. A critical aspect is the compressive strength of the core of the board. A method is therefore sought for preparing a plasterboard with a core having a higher compressive strength for densities that are identical to current densities, which would allow the board strengths to be obtained for lower densities.

One feature that has been the subject of numerous studies is the adhesion of the paper onto the plaster core of the board. In order to improve the adhesion, starch is generally added to the plaster slurry and during drying of the board the starch migrates towards the paper and concentrates at the core/facing interface. This is obtained by selecting a starch that has a particular rheology as a function of the temperature.

FR-A-1 429 406 and EP-A-0 172 773 describe the use of natural or modified starches for manufacturing boards with a paper facing, having an improved core/paper adhesion.

EP-A-0 936 201 describes the use of starch mixtures that are modified with respect to the long-term strength of mixtures of cement-type material, including plaster. The objective in this document is to control the setting and the slump value of the mixed slurry without there being any effect on the long-term strengths.

EP-A-0 807 612 describes the use of cationic starch in plaster slurries, which has a nitrogen content of less than 0.15% and a solubility greater tan 50%. The starch is presented as a thickener for the plaster slurry.

EP-A-0 172 773 describes the use of starch having a very precise particle size, as an adhesive in the core for the facings. The advantage presented by the specific particle size is a better migration of the starch towards the core/facing interface.

SU-A-1 100 264 describes the use of alkyl sulphates and fibres for increasing the shock resistance and impact strength. The presence of starch is optional.

Thus, the starch in the plasterboards is very generally used only at the core/facing interface.

US 2004/0 045 481 and U.S. Pat. No. 6,783,587 describe a plaster slurry for a lightweight board comprising hemihydrate, starch, foam and water, and in which the starch represents from 1.5 to 3% by weight relative to the weight of the hemihydrate and the W/P ratio is between 0.7 and 0.95. It is indicated that the resulting boards have a reduced density. The starch used is any starch, but the example uses an acid-modified starch, available from Staley Manufacturing. The starch from this document is of the thin-boiling type.

US 2003/0 084 980 describes a plaster slurry for a lightweight board comprising hemihydrate, acid-modified starch, a starch crosslinking agent and water, the pH of the slurry being around 9 to 11. The acid-modified starch is of the same type as that mentioned in documents US 2004/0 045 481 and U.S. Pat. No. 6,783,587. It is indicated that the starch used has a lower gelatinization temperature, which facilitates its migration towards the core/facing interface, while the crosslinking makes it possible to prevent the starch from completely migrating out of the core. It is furthermore indicated that the starch also has a use as a binder in the core of the board. It is indicated that the acid-modified starches have gelatinization temperatures typically of 70 to 77° C., for example 72° C. In the cooked form, in the board at the time it passes into the dryer, the starch is said to be thin boiling, that is to say that it is fluid, in the form of a syrup, but it turns to a gel when the temperature decreases. In the invention that is the subject of the patent, increasing the pH value of the plaster slurry reduces the gelatinization temperature to values around 60-66° C., typically 61° C. In the cooked form, the acid-modified starch thus treated (i.e. thus crosslinked) is solid, that is to say that it is more viscous than a thick-boiling starch, and a syneresis effect (separation of the water and of a solid from a gel) appears. In this document, increasing the pH within a given range is necessary in order to obtain the crosslinking and the associated effect.

US 2005/0 126 437 describes the use, for increasing the strength, of a modified starch that is insoluble in the plaster slurry during mixing, but which is dissolved when the temperature increases. The starch is modified, for example, by hydroxyalkylation or acetylation.

Nothing in the prior art describes or suggests the invention, which satisfies the requirement of providing a lightweight board having good mechanical characteristics.

The addition of a certain type of starch to the core of the board makes it possible to increase the compressive strength by up to 30%, or even 50%, the starch having a particular rheological profile.

The invention therefore provides a plaster slurry composition comprising plaster, water and starch, said starch having a Brookfield viscosity of less than 60 cps at a temperature below 60° C. and a Brookfield viscosity greater than 10,000 cps at a temperature of 70° C.

According to one embodiment, the starch has a Brookfield viscosity of less than 20 cps at a temperature of 60° C.

According to one embodiment, the starch has a Brookfield viscosity of greater than 20,000 cps, preferably greater than 30,000 cps, at a temperature of 70° C.

According to one embodiment, the starch has a Brookfield viscosity of greater than 60,000 cps at a temperature of 80° C.

According to one embodiment, the starch is of the non-retrograding type.