HK40000699A - Method for manufacturing plasterboards - Google Patents

Description

Method for producing plasterboards

The present invention relates to a process for the preparation of gypsum board, more precisely gypsum board containing activated carbon. The invention also relates to gypsum boards containing activated carbon, and to the use thereof for reducing the content of volatile organic compounds in buildings.

Volatile organic Compounds (COV) are carbon and hydrogen based chemicals that exist in the air in a gaseous state. European union directive No.2010/75 on 11/24/2010 defines them as any organic compound having a vapor pressure of 0.01kPa or higher at a temperature of 293.15K or having a corresponding volatility under specific use conditions. They include chemicals of various nature, such as alkanes, alkenes, such as terpenes, alkynes, alcohols, aldehydes, such as formaldehyde, acetaldehyde and heptanal, ketones, ethers, such as glycol ethers, aromatic hydrocarbons, such as benzene and toluene, or halogenated hydrocarbons, such as tetrachloroethylene and dichlorobenzene. COV is present in most paints, building materials, solvents, detergents and fuels, also in resins, varnishes or adhesives for furniture or appliances, or also in cigarette smoke. These COVs are present in the ambient air of buildings and, even if their content is low, they can be long-term disturbing to people who come into contact with them and may even affect their health. In particular, some COVs can cause allergic reactions, respiratory problems, nausea or headache.

In recent years, the proportion of COV emitted by the above materials has been greatly reduced due to stricter regulations. However, alternative materials with little or no COV emissions typically have higher costs and lower performance levels.

While efforts are being made to control COV emissions, different methods have therefore been proposed which allow reducing the COV content in the ambient air. Therefore, it has been proposed to incorporate adsorbents, such as activated carbon, into building materials.

Gypsum board is a board that includes a layer of gypsum between two facing sheets, typically made of paperboard. Industrially, the process for making gypsum board comprises three main steps: forming, solidifying and drying. During the step of shaping the gypsum board, the mixing batch is continuously prepared in a mixer using gypsum, water, and other specific ingredients for adjusting the properties of the mixing batch and/or the final product. It is particularly known to add directly a foaming agent or foam to reduce the density of the gypsum board. The mixed batch is then continuously poured onto a first facing sheet conveyed by a conveyor belt towards an extruder for forming the panel. After the edge fold of the first facing sheet, a second facing sheet is introduced at the extruder location. The extruder lays the second facing sheet flat on the mixing batch, smoothes the surface and reduces the thickness of the gypsum board to the desired value. In order to improve the mechanical properties of the gypsum board, it is also known to form a denser gypsum layer on one face and optionally on the edges of the gypsum board. To this end, upstream of pouring the main mix, a layer of a denser first mix (called roll coat) is cast on a first facing sheet and shaped, which then forms a second layer (called the body of the gypsum board).

Activated carbon is very difficult to use on an industrial scale, in particular due to its high specific surface area and hydrophobicity. Furthermore, the processability problems associated with the incorporation of activated carbon in gypsum plasterboards thus have a non-negligible effect on the properties and quality of the resulting gypsum plasterboards. In particular, the applicant has surprisingly noticed that activated carbon, in particular powdered activated carbon, interferes with the action of the foaming agent, which can undesirably increase the density of the gypsum board. Furthermore, the activated carbon is not uniformly dispersed in the mixing batch, which can cause problems in controlling the amount of activated carbon introduced and affect the effectiveness of the panel in terms of COV adsorption. The present invention provides a process for the preparation of gypsum boards which allows to overcome at least one of the difficulties posed on an industrial scale by the addition of activated carbon.

Accordingly, one aspect of the present invention is directed to a method for making gypsum board comprising a board body and a roll coated layer, wherein powdered activated carbon is introduced into a mixing batch used to form the roll coated layer, and the mixing batch used to form the board body does not include activated carbon. This is because the mixing batch used to form the body of the board has a foam-like appearance to reduce the weight of the gypsum board. The introduction of activated carbon into such a mixing batch can have the effect of destroying the foam. The roll coat layer itself has a density greater than that of the plate body. Thus, the presence of foam in the mixing batch used to form the roll coat is undesirable. Thus, the introduction of activated carbon into the mixing batch used to form the roll coat layer does not result in an undesirable increase in the weight of the gypsum board. In contrast, the defoaming effect of activated carbon can be advantageously used to remove foam that may be present in a mixing batch for roll coating cladding.

Accordingly, the present invention relates to a process for making gypsum board comprising activated carbon, the process comprising:

-preparing a main mix batch free of activated carbon;

-preparing a second mixed batch comprising powdered activated carbon;

-providing a first facing sheet;

-pouring a second mixed batch onto the first facing sheet, the second mixed batch forming a first gypsum layer;

-pouring a main mixing batch on the first layer, the main mixing batch forming a second gypsum layer;

-providing a second facing sheet over the first and second plies; and

-shaping the gypsum board;

wherein the first layer has a density greater than the first layer. Since the first layer is denser than the second layer, it is understood that the second mixing batch also has a higher density than the main mixing batch. In other words, the second mixing batch has less foaming properties than the main mixing batch. For this purpose, the main mixing batch contains a blowing agent. The blowing agent is generally introduced into the main mixing batch in the form of a foam obtained from water and blowing agent. The main mix batch typically contains 2 to 10 parts of foam obtained from a mixture of water and a blowing agent. The density of the second mixing batch is typically at least 5% higher relative to the main mixing batch, preferably at most 40% higher, for example 15% to 25% higher relative to the main mixing batch. The main mixing batch typically has a density of from 0.6 to 1.5, preferably from 0.8 to 1.3. The second mixing batch typically has a density of 1.0 to 2.3, preferably 1.3 to 2.0.

In the sense of the present invention, the term "gypsum" generally denotes set gypsum, i.e. calcium sulphate dihydrate (CaSO)₄·2H₂O), and unset gypsum, calcium sulfate hemihydrate (CaSO)₄)·½H₂O). The expression "gypsum board" denotes a finished product formed, for example, from set gypsum, but also gypsum boards in a manufacturing process in which the board is not set completely. However, in some cases the term "gypsum" will be understood within the strict meaning, i.e. to mean calcium sulphate hemihydrate. This is evident, for example, when the term "gypsum" is used, from the starting materials used to prepare the mixing batch. Likewise, when referring to amounts relative to the dry weight of gypsum, the latter is considered to be its calcium sulfate hemihydrate form.

The thickness of the gypsum board is generally from 6 to 25mm, preferably from 10 to 15 mm. The first gypsum layer is a roll-coated layer. The second gypsum layer forms the body of the gypsum board. Within the meaning of the present invention, "roll-coated cover layer" means a gypsum layer having a small thickness, which has a density greater than that of the layer forming the main body of the gypsum board. In the sense of the present invention, the "body" of the gypsum board denotes a layer of gypsum whose thickness is at least equal to half the thickness of the gypsum board. The first gypsum layer typically has a thickness of less than 4mm, for example 0.1 to 4mm, preferably 0.8 to 3 mm. The thickness of the second gypsum layer is typically 6 to 25mm, preferably 10 to 15 mm. Density of the first gypsum layerTypically at least 5% higher relative to the second layer and preferably at most 40% higher, for example 15 to 25% higher relative to the second layer. The density of the first layer is typically 0.8 to 1.5, preferably 1.0 to 1.2. The second layer typically has a density of 0.4 to 1.5, preferably 0.6 to 1.0. In the present invention, the density of the body is defined in a conventional manner, i.e. as the bulk mass of said body and the bulk mass of the water (equal to 1000 kg/m)³) The ratio of.

The activated carbon according to the present invention is preferably a powdered activated carbon. This is because, at the same specific surface area, powdered carbon has a greater COV absorption efficiency than granular activated carbon when it is incorporated into gypsum board. Furthermore, although granular activated carbon has a lower defoaming effect than powdered activated carbon and allows a gypsum board having a satisfactory density to be obtained, it is not advantageous to introduce granular activated carbon into the board body. This is because the applicant has surprisingly noted that when activated carbon is incorporated in the roll coat in powder form, the content of activated carbon in the gypsum board can be significantly reduced, while the efficiency of the board in absorbing COV is significantly improved. In the present invention, when it is mentioned that the mixing batch or layer does not contain activated carbon, this means that the mixing batch or layer does not contain activated carbon, whatever its form, in particular powdered or granular activated carbon. Therefore, the powdered activated carbon according to the present invention generally has an average particle size of 1 to 100 μm, preferably 10 to 50 μm. Average particle size according to the standard ASTM-D-5158-93 using an air-jet sieve "Standard Test Method for Determination of the Particle Size of Powdered Activated Carbon by Air Jet Sieving"measure. It typically has at least 100m²A/g, preferably of at least 250m²Per g, more preferably at least 500m²A sum of at most 1500m²In g, even 2000m²G, even 2500m²Specific surface area in g. Specific surface area according to standard ISO 9277: 2010, measurement was performed with nitrogen by using the BET method.

The second mixing batch typically contains 0.1 to 10 wt%, preferably 0.5 to 3 wt% powdered activated carbon. The second mixing batch preferably comprises activated carbon only in powder form. In particular, the second mixed batch preferably does not contain granular activated carbon. In a particular embodiment, the preparation of the second mixing batch preferably comprises preparing a pre-mixture of powdered activated carbon and fluidizing agent in water and adding said pre-mixture to the mixing batch. This is because the applicant has noted that the premix of powdered activated carbon with a fluidizing agent makes it possible to obtain a sufficiently stable suspension, which allows to properly meter the activated carbon, in particular in a continuous industrial process, and to distribute it more uniformly in the obtained mixed batch and therefore in the gypsum board produced. The premix typically comprises 1 to 25 wt%, preferably 5 to 15 wt% powdered activated carbon. It generally comprises at least 0.01%, preferably from 0.05 to 5%, more preferably from 0.1 to 3% by weight of fluidizing agent. The fluidizing agent can be chosen in particular from polycarboxylates, in particular polycarboxylate ethers, sulfonated polynaphthalenes (salts of sulfonated polycondensates of naphthalene and formaldehyde), lignosulfonates, sulfonated melamine resins (salts of sulfonated polycondensates of melamine and formaldehyde) and polyacrylates commonly used for the preparation of gypsum boards. As examples of fluidising agents, mention may be made of those sold by BASF under the name GLENIUM @, those sold by Bozzetto under the name FLUBE @, those sold by Chryso under the name chrysofol @, those sold by Sika under the name Viscocrete @, those sold by BASF under the name Melment @orthose sold by Mapei @ pefluld @. The fluidizing agent is preferably selected from sulfonated polynaphthalenes and sulfonated melamine-formaldehyde resins, more preferably from sulfonated polynaphthalenes.

The master mix batch typically comprises, for 100 parts by weight of gypsum:

-50 to 200 parts of water;

-from 2 to 10 parts of a foam obtained from a mixture of water and a foaming agent, such as alkyl sulphate, optionally mixed with alkyl ether sulphate; and

0.1 to 1 part of a setting accelerator, for example calcium sulphate hydrate or potassium sulphate.

It may include processing additives that allow for adjustment of the properties of the mixing batch, as well as other functional agents that allow for modification of the properties of the final gypsum board. The processing additives known to the person skilled in the art may be, inter alia, binders, retarders, accelerators, fluidizing agents or thickeners. The functional agents, which are also well known to those skilled in the art, may be biocides, water repellents, flame retardants or reinforcing agents. For example, the master mix batch may comprise one or more of the following components:

from 0.1 to 15 parts of a binder, for example poly (vinyl acetate), poly (vinyl alcohol), starch, in particular starch pretreated with acid or pregelatinized, dextrin or vegetable flour, in particular wheat flour or corn flour;

-0.001 to 10 parts of a biocide, such as a carbamate, e.g. 3-iodoprop-2-yn-1-ylbutyl carbamate, or a pyrothione complex;

0.1 to 10 parts of at least one water repellent, such as a silicone, polysiloxane or wax;

-0.1 to 20 parts of at least one flame retardant, such as vermiculite, silica, in particular micron-sized silica, or clay; and/or

0.1 to 20 parts of at least one reinforcing agent, such as polymer fibers, mineral fibers, in particular glass fibers or vegetable fibers.

As described above, the composition of the second mixing batch is as described above for the main mixing batch, except that it contains no foam and it contains powdered activated carbon, as described above. The main mixing batch and the second mixing batch may be prepared independently. In this case, no foam was added to the second mixing batch during the preparation. However, the second mixing batch is preferably obtained from the main mixing batch. In this case, a portion of the main mixing batch is taken out and the foam must be broken to prepare a second mixing batch. This is because the second mixing batch must have less foaming, preferably non-foaming, properties than the main mixing batch to allow for the formation of a roller coated layer that is denser than the panel body. In order to break up the foam present in the main mix batch, it is often necessary to add a defoamer thereto. However, the process according to the invention makes it possible to benefit from the defoaming properties of powdered activated carbon while reducing the amount of defoaming agent required, even without adding any defoaming agent other than powdered activated carbon in the main mixing batch portion taken out to prepare the second mixing batch. In a particular embodiment, the method according to the invention therefore comprises:

-preparing a main mix batch free of activated carbon;

-removing a portion of the main mixing batch;

-adding powdered activated carbon to the withdrawn part of the main mixing batch to obtain a second mixing batch;

-providing a first facing sheet;

-pouring a second mixed batch onto the first facing sheet, the second mixed batch forming a first gypsum layer;

-pouring a main mixing batch on the first layer, the main mixing batch forming a second gypsum layer;

-providing a second facing sheet on said first and second plies; and

-shaping the gypsum board;

wherein the density of the first layer is higher than the density of the first layer. Preferably, no defoaming agent is added to the withdrawal portion of the main mixing batch (for obtaining the second mixing batch) except for the powdered activated carbon.

Fig. 1 schematically shows an example of an industrial production line allowing the implementation of the method according to the invention. The main-mix batch is continuously prepared in the main mixer 4. A part of the main mixing batch is taken out in the main mixer 4 and sent to the second mixer 2 through a pipe and mixed with the powdered activated carbon to obtain a second mixing batch. The first facing sheet 1 (the side visible when the board is installed), which normally forms the front side of the gypsum board, is carried by a conveyor belt. A second mixing batch is continuously poured from the second mixer 2 onto the first facing sheet 1 and formed into a first homogenous layer by a set of rollers 3. This first layer will produce a roll coated overlay. The main mix batch is poured from the main mixer 4 onto the first layer at a predetermined distance that allows the first layer to solidify sufficiently. The second facing sheet 5 is introduced above the main mixing batch at the main extruder 6 position. A ribbon of gypsum having the desired thickness is extruded through the main extruder 6, and the main extruder 6 distributes the main mix batch to form the layer that will produce the body of the panel. After the gypsum tape has been transported for a suitable period of time (to set the gypsum sufficiently), the gypsum tape is cut to the desired size by the rotating knife 7. The board is then typically turned over to prevent damage to the front of the board and transported to a series of classifying dryers 8(une srie chlorirs a tage 8) to complete the setting of the gypsum and remove excess water before packaging and transport to a storage area 9.

The invention also relates to a gypsum board comprising powdered activated carbon obtained by the process as defined above. More precisely, the invention relates to a gypsum board comprising a first gypsum layer and a second gypsum layer, characterized in that the first layer is denser than the second layer; the first layer comprises powdered activated carbon; the second layer does not contain activated carbon. The gypsum board according to the invention typically comprises 0.01 to 2%, preferably 0.05 to 1% by weight of activated carbon, relative to the dry weight of the gypsum. In particular, the first layer typically contains 0.2 to 10%, preferably 1 to 5% by dry weight of powdered activated carbon, relative to the dry weight of gypsum. The first layer preferably comprises activated carbon only in powder form. In particular, it preferably does not contain granular activated carbon. The thickness and density of the first and second layers are preferably as described above.

The invention also relates to the use of a plasterboard as described above for reducing the COV content in the air inside a building. The COV is preferably selected from formaldehyde, hexanal, benzene, toluene, tetrachloroethylene, 1, 2-dichlorobenzene and undecane.

The invention is illustrated by means of the following non-limiting examples.

Example 1

By only oneThe reference plate P0 formed of a body layer having a thickness of 12.5mm was made of a material containing 100 parts by weight of gypsum (CaSO 4.1/2H)₂O), 75 parts water, 4 parts foam (obtained from water containing 1% by weight sodium lauryl sulfate), 5 parts starch, 3 parts fluidizing agent and 0.5 part coagulant. The different ingredients were weighed and mixed for 1 minute at 1600 revolutions per minute using a mixer and a deflocculating type paddle to obtain a homogeneous paste. A foam having the desired density was prepared and mixed in a stirrer at 250 revolutions per minute using a paddle to obtain a main mix batch. A first cardboard sheet was placed in a mould measuring 20 x 25cm and having a thickness of 12.5 mm. The main mix batch was poured onto a first cardboard and leveled at the thickness of the mold. A second paperboard sheet is placed over the main mixing batch and the mold is closed to maintain pressure against the sheet. After the board hardened, the board was demolded and dried in a drying oven at 40 ℃ for 24 hours.

A panel P1 formed from only one body ply having a thickness of 12.5mm was prepared in the same manner as the panel P0, but with the difference that the master mix batch contained 2.8% by weight of Envirocarb 207C marketed under the trademark Envirocarb @ 207C having a thickness of 1100m²Specific surface area per gram, granular activated Carbon (CAG) of average particle size of about 2.4 mm. The amount of activated carbon in panel P1 was about 210 grams per square meter.

A plate P2 formed from only one body layer of thickness 12.5mm was prepared in the same manner as the plate P1, except that the activated carbon used was powdered activated Carbon (CAP) sold under the trade designation Pulsorb 208CP and having a specific surface area of 1200 m/g and an average particle size of about 30 μm. The amount of activated carbon in panel P2 was about 210 grams per square meter.

A panel P3 formed from an 11.5mm bulk layer and a 1mm thick roll coat was prepared using the same primary mix batch for the bulk layer and secondary mix batch for the roll coat as panel P0. A second mixing batch was prepared by adding 35 wt% Pulsorb 208CP powdered activated Carbon (CAP) to the same mixing batch as the main mixing batch. A first cardboard sheet was placed in a mould measuring 20 x 25cm and having a thickness of 12.5 mm. The second mixed batch was poured onto the first cardboard and leveled at a thickness of about 1 mm. When the second mixing batch is sufficiently solidified, the main mixing batch is poured over the second mixing batch and leveled across the thickness of the mold. A second paperboard sheet is placed over the main mixing batch and the mold is closed to maintain pressure against the sheet. After the board had hardened, the board was demolded and dried in a drying oven at 40 ℃ for 24 hours. The amount of activated carbon in the panel was about 210 grams per square meter.

The densities of the panels P0 to P3 are listed in the table below. The plates P1 and P2, in which activated carbon has been introduced into the plate body, have a higher density than the reference plate P0. It is expected that activated carbon hinders the action of the blowing agent, which has the effect of breaking up the foam and increasing the density of the gypsum board. This effect is particularly evident for the plate P2 containing powdered activated carbon. In contrast, panel P3 had a density comparable to reference panel P0.

TABLE 1

	P0	P1	P2	P3
					Density of	0.72	0.76	1	0.72

To avoid an undesirable increase in the density of the gypsum board, activated carbon must therefore be used in granular form in the body of the board or in powder form in the roll coating.

Example 2

Panel a, formed from only one bulk layer comprising granulated activated Carbon (CAG) having a thickness of 12.5mm, was prepared in the same manner as panel P1, except that the mixed batch contained 1 wt% activated carbon instead of 2.8%. The amount of activated carbon in the panel was about 76 grams per square meter.

Panel B, formed of a 11.5mm bulk layer and a 1mm thick roll coated layer comprising powdered activated Carbon (CAP), was prepared in the same manner as panel P3, except that the second mixed batch contained 1% activated carbon instead of 35%. The amount of activated carbon in the panel was about 6 grams per square meter.

The reaction mixture is then processed according to standard ISO 1600-24: 2009 the ability of each panel to capture COV was evaluated during the test at a temperature of 23 ℃ ± 2 ℃ and a relative humidity of 50% ± 5%. Load factor of board is set to 1 m/m³The air renewal rate is 0.5vol.h^-1Benzene concentration was set to 5. mu.g.m^-3The toluene concentration was set at 36. mu.g.m^-3The heptanal concentration was set to 19. mu.g.m^-3The tetrachloroethylene concentration was set at 6. mu.g.m^-3Dichlorobenzene concentration was set at 5.5. mu.g.m^-3And the concentration of undecane was set to 18. mu.g.m^-3. The percentage reduction in COV content considered for each plate is shown in table 2.

TABLE 2

	A	B
			Type and amount of activated carbon in the panel	CAG 76g/m (in main body)	CAP 6g/m (in roller coating)
Benzene and its derivatives	50	85
			Toluene	34	65
Heptanal	54	64
			Tetrachloroethylene	45	76
Dichlorobenzene	50	74
			Undecane	40	69

At a constant specific surface area, plate B is more effective in trapping COV, although the amount of activated carbon in plate a is much higher. In addition to preventing an undesirable increase in the density of the panel (see example 1), the introduction of powdered activated carbon into the roll coated layer makes it possible to increase the collection efficiency of COV, while significantly reducing the amount of activated carbon required compared to the use of granular activated carbon in the panel body.

Example 3

Tests of stabilization of activated carbon in aqueous suspension have been carried out using different surfactants (comparative examples C1 to C5) and using fluidizing agents for the preparation of gypsum boards (examples I1 to I3 according to the invention). FL1 and FL2 are sulfonated polynaphthalene type fluidizers, and FL3 is a sulfonated melamine resin type fluidizer. The premix was prepared using an electric stirrer-type mixer at 700 rpm.

The stability of the suspension was evaluated by visual inspection: the premix was placed in a transparent container and visually observed at regular time intervals. The stability of the suspension was measured by recording the height of the column (clear supernatant). The results summarized in table 3 are expressed as the percentage of the height of the water column (clear supernatant) relative to the total height of the suspension.

TABLE 3

^(a)Stepanol EHS marketed by Stepan

^(b)Triton-X100 marketed by SPI

^(c)n-dodecyl- β -D-maltoside

^(d)3- [ (3-Cholamidopropyl) dimethylamino group]A propanesulfonic acid salt.

The fluidizing agent allows powdered activated carbon to be stabilized more effectively than conventional surfactants. Thus, the preparation of a premix of powdered activated carbon with a fluidizing agent in an aqueous suspension makes it possible to better control the amount of activated carbon added, in particular in a continuous industrial process. This also allows for a more uniform distribution of the activated carbon in the roll coat and thus allows for better gypsum board to COV capture efficiency and better consistency between gypsum boards. Finally, the use of a fluidizing agent already used for the preparation of the mixing batch has the advantage of avoiding the use of other auxiliaries which may alter the properties of the mixing batch and/or disrupt the preparation process.

Claims (16)

1. A method of making gypsum board comprising activated carbon, the method comprising:

-preparing a main mix batch comprising a blowing agent and no activated carbon;

-preparing a second mixed batch comprising powdered activated carbon;

-providing a first facing sheet;

-pouring a second mixed batch onto the first facing sheet, the second mixed batch forming a first gypsum layer;

-pouring a main mixing batch on the first layer, the main mixing batch forming a second gypsum layer;

-providing a second facing sheet on said first and second plies; and

-shaping the gypsum board;

wherein the second mixing batch has a higher density than the main mixing batch.

2. The method of claim 1 wherein the second layer has a thickness at least equal to one-half the thickness of the gypsum board.

3. The method of any of claims 1 and 2, wherein the first layer has a thickness of less than 4mm and the second layer has a thickness of 6 to 25 mm.

4. The method of any one of claims 1 to 3, wherein the second mixing batch has a density at least 5% higher than the main mixing batch.

5. The method of any one of claims 1 to 4, wherein the primary mixing batch material has a density of 0.8 to 1.3 and the secondary mixing batch material has a density of 1.3 to 2.0.

6. The method of any one of claims 1 to 5, wherein the main mix batch comprises, for 100 parts by weight of gypsum:

-50 to 200 parts of water;

-2 to 10 parts of a foam obtained from a mixture of water and a blowing agent; and

-0.1 to 1 part accelerator.

7. The method as claimed in any one of claims 1 to 6, wherein the powdered activated carbon has a particle size of 100-2500m²Specific surface area per gram, average particle size of 1-100 μmAnd (4) size.

8. The method of any one of claims 1 to 7, wherein the second mixing batch is obtained from a main mixing batch.

9. The method of any of claims 1-8, wherein the second mixed batch comprises 0.1 to 10 wt.% powdered activated carbon.

10. The method of any of claims 1-9, wherein the preparing of the second mixing batch comprises preparing a pre-mixture of powdered activated carbon and a fluidizing agent in water, and adding the pre-mixture to the mixing batch.

11. The method of claim 10, wherein the premix comprises 1 to 25 wt.% powdered activated carbon.

12. The method of any of claims 9 or 11, wherein the premix comprises at least 0.01 wt.% of a fluidizing agent.

13. A method according to any one of claims 9 to 12, wherein the fluidising agent is selected from the group consisting of polycarboxylates, sulphonated polynaphthalenes, lignosulphonates, sulphonated melamine resins and polyacrylates.

14. The method of any one of claims 1-13, wherein the second mixing batch is free of a defoamer.

15. Gypsum board obtainable by a process as defined in any one of claims 1 to 14.

16. Use of a gypsum board as defined in claim 12 for reducing the amount of volatile organic compounds in the air in a building.