PROCESS FOR PRODUCING WATER RESISTANT PLASTERBOARDS WITH FIBER CEMENT POWDER

TECHNICAL FIELD

[0001] The present invention concerns a process for producing water resistance plasterboards formed of a gypsum core sandwiched between two facers. The plasterboards obtained by the process of the present invention have enhanced water resistance and use cured fiber cement powder. The invention also concerns the use of fiber cement powder with poly methyl hydrogen siloxane.

BACKGROUND OF THE INVENTION

[0002] Fiber cement (FC) material is a composite material typically comprising cement, cellulose fibers, and at least one of silica sand, synthetic fibers and fillers. It is widely used in construction and can take the form of a plurality of products, such as for example corrugated sheets for roofs, small sheets for tiles (slates), sheets for sidings, cladding, boards, etc.

[0003] Cured fiber cement waste material has a chemical composition similar to the corresponding fresh fiber cement products from which the waste is derived, and therefore could, and ideally should, be recycled and reused. However, recycling waste material from cured fiber cement products, e.g. finished products being out of spec, demolition and/or construction waste and alike, remains a major challenge. Reusing fiber cement waste for various new purposes remains however the main, if not the only, approach to avoid disposal of large fiber cement waste streams.

[0004] Plasterboards are plates particularly useful for albeit not restrict to the building industry, comprising a core made of gypsum with various additives in minor amounts, sandwiched between bottom and top facers, generally made of paper and I or glass mat. They can be produced continuously on long production lines and cut to the desired lengths and dried in a drier.

[0005] A slurry of calcined calcium sulphate hemihydrate (or stucco) in water with the desired additives, such as accelerators, fibrous reinforcements, and the like, is continuously dispensed onto a bottom facer moving on a conveyor. The thickness of the layer of slurry on the bottom facer is controlled and a top facer is laid on top of a free surface of the slurry, such as to form a sandwich structure with a core formed by the slurry sandwiched between bottom and top facers. The calcined stucco in the core is allowed to undergo a hydration reaction to form a setting plasterboard with calcium sulphate hemihydrate (CaSO4. ¹ /2H2O) being progressively replaced by calcium sulphate dihydrate (CaSO4.2H2O) as the hydration reaction proceeds.

[0006] Once the core has set to a reasonably hard structure, the continuous setting plasterboard is cut to a desired length, prior to being moved into a drier to complete the hydration reaction and remove any excess water present in the core. [0007] According to the final application, some plasterboard requires enhanced water resistance as it is known that regular gypsum plasterboard can lose its strength when exposed to high amount of moisture.

[0008] In order to improve the moisture resistance of a gypsum board, silicone and in particular poly methyl hydrogen siloxane (PHMS) is well known as water repellent agent. Increasing the pH of the slurry enhances the PHMS effect.

[0009] EP1112986 indeed describes the use PHMS and Portland cement to produce water resistant plasterboard. The hydrogen methyl siloxane used is cross-linked to form a silicone network in the material. The polymerization of siloxane to silicone requires water as a reagent. During the drying of a board, free water is removed, thereby reducing the water available for the reaction. Therefore, unreacted siloxane will not continue the cross-linking reaction after the board is completely dried. If the cross linking reaction is too slow, and the processing time of the board too short, the siloxane will not polymerize. Difficulties will also arise if the reaction occurs too quickly. If the siloxane begins to crosslink during its feeding into the slurry, a gel type silicone will form which is difficult to further uniformly disperse into the whole slurry, and hence the matrix of material. Additionally, if the polymerization reaction is completed before the rehydration of the hemihydrate is completed, the surface of the hemihydrate crystals will be blocked from contacting water needed for continuation of rehydration, causing a decrease in the strength of the material. Control of the cross-linking time to match the process time is an important factor. The amount of Portland cement as catalyst to the curing is however limited to 2 % relative to stucco because higher amount would be dangerous specifically in the case of a plasterboard produced not by a cake filtering but by an online process wherein the plasterboards enter in a dryer after few minutes.

[0010] US2014/0178 624 describes a method for making a water-resistant gypsum board comprising a hydroxide with siloxane. In order to minimise the production of hydrogen, part of the hydroxide is replaced by siliconate.

[0011] Looking for recycling waste of FC, it has been surprisingly found that the use of powder of fiber cement board could be used in a large amount with a limited amount of hydrogen produced and an increase of the water repellence resulting not only from an increase of pH was observed.

[0012] As the correct dosage of fresh cement is very delicate to reach, most of the time the use of cement brings more disadvantages than advantages with regard to the expected benefits. Cement is so reactive with silicone that a slight overdosing releases a very high amount of gas (H2) in a short period of time. This has two main negative consequences: one is potential safety issue related to H2 gas and the other is a quality issue as the released gas can destroy the bond between the paper and the gypsum core. Also, the quick reaction of silicone may have also at least one negative consequence which is the too quick hydrofugation of the board in the process. Water is then expelled from the board, interfering with the hydration process and humidity balance of the core in the wet end part of the process where water is critical. [0013] The use of fiber cement powder circumvents these negative consequences as the dosage is more easily controlled. Overdosing cured fiber cement powder is much less critical than it is for cement. In addition, the hydrophobization of the boards produced is higher than the one expected with the solely an increase of pH.

SUMMARY OF THE INVENTION

[0014] The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a process for producing water resistant plasterboards comprising the following steps,

• feeding a gypsum slurry (1g) comprising stucco, water, alkyl hydrogeno polysiloxane, cured fiber cement powder and additives onto a bottom facer (21) laid on a conveyor (7) in motion,

• applying a top facer (22) onto a free surface of the gypsum slurry to form a sandwich structure with a core (1 c) made of the gypsum slurry sandwiched between the top and bottom facers (21 , 22),

• setting by hydration the gypsum slurry (1g) forming the core (1 c) to form a setting plasterboard on the conveyor,

• cutting the setting plasterboard to desired dimensions to yield cut plasterboards (1 cp),

• drying the cut plasterboards in a drying station (11) to yield the plasterboards (1), wherein the cured fiber cement powder in an amount from 0.5 to 10 wt%, preferably 2 to 8, most preferably 3 to 5 wt.% relative to stucco.

[0015] In a preferred embodiment, the gypsum slurry comprises 100 parts of stucco and between 60 and 90 parts of water.

[0016] In a preferred embodiment the alkyl hydrogeno polysiloxane is polymethylhydrogensiloxane (PMHS), the amount of PHMS is between 0.05 and 2 wt.%, most preferably between 0.1 and 1 wt. % relative to stucco.

[0017] The fiber cement can be air cured or autoclaved cured. In a preferred embodiment the cured fiber cement powder is autoclaved.

[0018] In a preferred embodiment, the pH of the slurry is between 9 and 11.5. The pH is measured according the testing method ME-A5-005.

[0019] The cured fiber cement can have different granulometry according the waste it comes from. In a preferred embodiment, the fiber cement powder has a d50 below 100 pm, preferably below 50 pm, most preferably below 40pm.

[0020] In a preferred embodiment, the slurry does not comprise fly ash nor added clay.

[0021] The invention also relates plasterboards obtained by the process described here above. [0022] The invention also relates to the use of cured fiber cement with PHMS to provide water resistant plasterboard having a water absorption according to the Norm EN520-2009 below 5%

BRIEF DESCRIPTION OF THE FIGURE

Figure 1 shows a side view of an example of production line configured for carrying out the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns a process for producing water resistant plasterboards (1) comprising the following steps,

• feeding a plaster slurry (1 g) comprising stucco, water, alkyl hydrogeno polysiloxane, cured fiber cement powder and additives onto a bottom facer (21) laid on a conveyor (7) in motion,

• applying a top facer (22) onto a free surface of the gypsum slurry to form a sandwich structure with a core (1 c) made of the gypsum slurry sandwiched between the top and bottom facers (21 , 22),

• setting by hydration the gypsum slurry (1g) forming the core (1 c) to form a setting plasterboard on the conveyor,

• cutting the setting plasterboard to desired dimensions to yield cut plasterboards (1 cp),

• drying the cut plasterboards in a drying station (11) to yield the plasterboards (1),

The cured fiber cement powder is added in an amount from 0.5 to 10 wt%, preferably 2 to 8, most preferably 3 to 5 wt.% relative to stucco.

[0023] The expression "water resistant" plasterboard should be understood to mean the ability of a plasterboard to limit the uptake of water by the plasterboard, while still retaining the dimensional stability and mechanical integrity of the structural element in question.

[0024] According to the norm NF EN520+A1 ,2009 the total water absorption of a plasterboard should be below 5%.

[0025] Gypsum slurry comprises stucco or calcined gypsum in the form of calcium sulphate hemihydrate (CaSO4. ¹ /2H2O), water, and additives, including an accelerator, retarder, fibre reinforcement, fluidizer, starch. In order to obtain stucco, natural or synthetic gypsum may be used. Natural gypsum may be obtained from gypsum rock or gypsum sand. Synthetic gypsum typically originates from flue gas desulfurization (FGD) or phosphoric acid production. The stucco comprises more than 70 wt% of p calcium sulphate hemihydrate, preferably more than 80%, most preferably more than 90 wt% relative to the stucco weight.

[0026] The water repellent additive is polysiloxane hydrogeno methyl (PHMS), in the form of an oligomer (several tens or more of siloxane units), in the form of an oil or of an aqueous emulsion. [0027] The cured fiber cement material powder is typically a waste material which is either originated from comminuted cured fiber cement waste or directly produced in the process of fiber cement board when the board is sanded for example.

[0028] Cured fiber cement powder for use in plasterboard manufacturing process, preferably, though not necessarily, is autoclaved-cured fiber cement powder. In the alternative, the cured fiber cement product may be air-cured fiber cement powder.

[0029] The cured fiber cement powder particles of the invention have a particle size distribution, which is similar to the particle size distribution of a cementitious binder material (e.g. cement) or a siliceous source (e.g. sand or quartz) or a filler material (e.g. CaC03). In particular embodiments, the produced cured fiber cement powder particles are characterized by a particle size distribution, which is similar to the particle size distribution of cement. In particular embodiments, the produced cured fiber cement powder particles are characterized by a particle size distribution, which is similar to the particle size distribution of a siliceous source.

[0030] An example of a chemical composition in w% of one of the autoclaved cured fiber cement is the following (in (m-%)

Tablel [0031] The particle size can be expressed by d50. The term "d50" in the sense of the present invention is a measure for the average particle size and is defined as follows: 50% (by number) of the particles (e.g. grains) in the corresponding sample have a size which is equal or smaller than the given d50 value. The term "particle size" especially represents the diameter of a sphere whose volume is identical to that of the particle under consideration having an arbitrary shape. The particle sizes referred to herein can be measured using Laser Diffraction Spectrometry (LOS). The particle size distribution was measured via laser beam diffraction in dry dispersion at 3 bar with a Malvern MasterSizer 2000.

CONTINUOUS PRODUCTION LINE

[0032] A production line for the continuous production of plasterboards suitable for the present invention is schematically illustrated in Figure 1 . It comprises a roll of bottom facer (21) configured for continuously feeding a conveyor (7) with the bottom facer (21). A slurry dispensing unit (3) is positioned above the bottom facer (21) as it is transported by the conveyor (7) and is configured for pouring a gypsum slurry (1 g) onto an inner face of the bottom facer (21), which faces upwards. A levelling blade (5) or roller is arranged downstream of the slurry feeding unit (3) to control the thickness of the layer of slurry deposited on the bottom facer (21). A roll of top facer is positioned above the conveyor, downstream of the levelling blade (5) and is configured for applying the top facer (22) on the free surface of the gypsum slurry (1g) to form a sandwich structure with a core (1 c) made of the gypsum slurry sandwiched between the top and bottom facers (21 , 22).

[0033] A cutting unit (9) is positioned at a cutter distance (L) from the slurry dispensing unit (3) and is configured for cutting the continuous setting plasterboard into panels of specific length referred to as cut plasterboards (1 cp). The cut plasterboards (1 cp) are then driven into a drying station (11

[0034] The hydration reaction transforming the calcium hemihydrate to gypsum according to the reaction CaSO4. ¹ /2H2O + 1 % H2O —> CaSO4.2H2O is almost finished (around 97 % of hydration) when the board get to the drying station (11).

[0035] The drying station (11) is positioned downstream of the cutting unit (9). It is configured for drying the cut plasterboards (1 cp) during a drying period sufficient for gently evaporating any excess water, which was required to form the initial slurry. After the plasterboards (1) have dried, they are trimmed and stacked to form pallets ready for use (not shown).

[0036] The water-repellence of PHMS is enhanced when the silicon compound cures or cross linked. However, this accompanied by significant evolution of hydrogen, which is particularly dangerous if this hydrogen is confined in the dryer.

[0037] The invention is further described in and illustrated by the following Examples. The Examples are not to be construed as limiting the invention in anyway. [0038] EXAMPLES

[0039] A series of experiments were carried out using stucco and FC powders.

Table 2

PHMS used in the examples is Wacker SILRES® BS 94.

All the wt % are expressed relative to the stucco weight.

[0040] Production of HYDROGEN

In order to assess the potential release of hydrogen when using fiber cement, a test was conducted using a calcimeter instrument. The cured FC powder and PHMS were brought into contact with each other, in water at 20°C in the reactor, in the same relative proportions than in recipe of Table 5. The evolution of hydrogen is monitored over time, at a controlled temperature of 20° C.

The total volume of hydrogen evolved after three hours is measured.

This volume is then converted into an equivalent gas volume released for 1 sqm of 12.5mm board.

Tables

The H2 volume produced is lower than the one obtained with a mixture with CaO being an major component of cement.

[0041] Water intake on 4x 4 x 16 Prism samples;

First tests were carried out by preparing prism samples using two different kinds of stucco, same amount of PHMS (0.74wt%) and different amounts of FC powder. When qualifying a plasterboard water resistant, the target value for water uptake after 2hr immersion of water resistant plasterboard is below 5%. The immersion tests were performed according to the standard EN520+A1 ; said standard is followed unless specified otherwise.

Table 4

[0042] The FC powder has a positive impact on the hydrophobization of silicone in gypsum prism formulations, irrespective to the gypsum origin. Depending on the gypsum source, the performance of PHMS alone varies greatly. [0043] Water uptake on lab miniboards (316mm x 316 mm x 12.5 mm)

Lab miniboards having a thickness of 12,5mm are made by mixing the ingredients as set out in table

5 into a gypsum slurry. The slurry was mixed using a suitable mixer such as a propeller type mixer and was poured into a mould and allowed to hydrate and set between two facers. After setting the mould was removed and the miniboard was dried in a heated air circulating drying kiln. In this preparation, the time sequence for the mixing, pouring and drying mimics the one of the industrial process. The sample surface size is 1/10 sqm.

Tables

[0044] The water uptake done according to the norm NF EN520-A+ 2009 has also been measured after 24 and 48 hrs of full immersion in order to get more insight on the long-term behaviour of the formulations tested.

Table 6

The results confirm the positive effect of PHMS/FC powder observed on the prism samples.

[0045] Table 7 demonstrates that an increase of pH is not solely responsible of the improvement of the water resistance of the plasterboard.

Table?

[0046] Ref 2 shows an improvement of the water resistance of the plasterboard obtained by increasing the pH. According to the invention sample 1 shows better water resistance than ref2 with a lower pH. The inventor believes that in addition to the pH effect, the FC powder provides surface adhesion allowing a better spread of the PHMS in the slurry.

[0047]

REF DESCRIPTION

1 plasterboard

1 c core

1cp Cut plasterboard

1g Gypsum slurry

I cp cut plasterboards

3 Slurry dispensing unit

5 Levelling blade

7 conveyor

9 Cutting unit

I I Drying station

21 Bottom facer

22 Top facer

30 Cooling station