EP4630386A1

EP4630386A1 - Prime coating of gypsum fiber boards

Info

Publication number: EP4630386A1
Application number: EP22840021.4A
Authority: EP
Inventors: Jürgen BURCZECK
Original assignee: Knauf Gips KG
Current assignee: Knauf Gips KG
Priority date: 2022-12-09
Filing date: 2022-12-09
Publication date: 2025-10-15
Also published as: WO2024120602A8; WO2024120602A1; MX2025006244A; AR131286A1

Abstract

A process for prime coating of gypsum fiber boards comprising the steps of: a) providing a gypsum fiber board b) applying a first primer comprising a siloxane c) applying a second primer comprising an alkaline compound and water.

Description

Prime coating of Gypsum Fiber Boards

The present invention relates to a process for prime coating of gypsum fiber boards.

Gypsum fiber boards are prepared from mixtures of fibers (typically cellulose fibers) and p- calcium sulfate hemihydrate. A typical production method comprises the step of spreading the fibers and the calcium sulfate hemihydrate on a continuously moving conveyor belt. Water is added before the board is formed, e.g. pressed into a strand that is cut into a plurality of separate pieces and subsequently dried.

After drying, the board is typically grinded to achieve planar surfaces and remove calcium hemihydrate formed on the board’s outside surfaces during drying. Subsequently, the board’s main surfaces are hydrophobized by applying, for example, waterglass or siliconates or a mixture thereof. This treatment is necessary to reduce the hygroscopic properties of the gypsum fiber board, because gypsum is not waterproof, gypsum fiber boards do not have a protective encasement and partly due to the hygroscopic cellulose fibers included. Furthermore, the hydrophobization provides board surfaces with low and homogeneous absorption properties. Homogeneous absorption properties are a precondition for the achievement of high-quality aesthetic finishes as for example paint, wallpaper or surfacing in general. Additionally, remaining grinding dust is fixed to the board during this step.

Gypsum fiber boards have some advantages over typical plasterboards where a gypsum core is encased in a liner, for example a paper liner or a non-woven fiber mat. Gypsum fiber boards have a higher mechanical stability than plasterboards; they are more resistant to humidity and less susceptible to mold formation.

Because the fiber boards are made of gypsum, they shrink when strongly heated, for example in a fire. The shrinkage is due to the evaporation of water bound in the gypsum crystals. The gypsum dehydrates and this dehydration results in cracks and ultimately in the breakdown of the structural integrity of the board. Due to the shrinkage gypsum fiber boards become brittle. Parts of the boards can tear away from the sub construction. This is especially problematic if the boards are part of a wall or a ceiling.

Thus, fire stability of gypsum fiber boards is a very important quality requirement apart from a basic water resistance.

It is known to include - in addition to cellulose fibers - glass fibers into the gypsum fiber board because they enhance the fire stability. As described above, gypsum fiber boards become increasingly brittle when under fire for a prolonged time. Cellulose fibers included into the fiber board burn or disintegrate when exposed to high temperatures. Glass fibers are more temperature resistant than cellulose fibers and thus provide cohesion even when the fiber board starts to become brittle in the initial phases of the dehydration. However, glass fibers are more expensive than cellulose fibers.

There is still a need to provide gypsum fiber boards having a good fire resistance while avoiding or at least reducing the amount of glass fibers in the boards.

It is the object of the present invention to provide a process for producing gypsum fiber boards that have a low water absorptivity and at the same time a suitable fire resistance at an economically reasonable price.

Another object of the invention is to provide gypsum fiber boards with an increased fire resistance.

The inventive process for prime coating of gypsum fiber boards comprises the steps of: a) providing a gypsum fiber board b) applying a first primer comprising a siloxane c) applying a second primer comprising an alkaline compound and water.

The term primer with respect to this invention means a coating that enhances the water resistance of a gypsum fiber board and prepares the board for additional coatings. Potentially present grinding dust is fixed to the board’s surface and the impact resistance of the surface is increased by the primer. Additionally, the primer prepares the surface for the application of diverse kinds of finishes as for example tiles or lacquers, in that it enhances the tensile bond strength and provides a homogeneously sucking surface property. According to the invention the primer (coating) also improves the fire-resistance additionally to the water-resistance.

The process of the present invention starts with a typical gypsum fiber board having cellulose fibers embedded therein. The surfaces can be grinded to equalize gypsum fiber board, provide a predetermined thickness and remove superficially re-calcined areas.

In a first step, a first primer that comprises a siloxane is applied to the surfaces of the board (at least one surface). Preferred siloxanes are H-siloxanes (definition see below), alkyl siloxanes and mixtures thereof. These siloxanes may be applied in pure form or as dispersion in water. Alkyl siloxanes may be selected from methyl and ethyl siloxanes or combinations thereof. A preferred siloxane has at least one Si-H bond.

A typically amount of siloxane applied per square meter of gypsum fiber board is 150 to 400 g or 150 to 300 g of siloxane. If a dispersion of siloxane and water is used, the applied amounts of the dispersion are higher because the amount of 150 to 400 g/m² refers to the siloxane only.

In a next step, a second primer is applied to the first primer coating, the second primer comprising an alkaline compound and usually water. This second primer has a high pH. Preferably, the pH of the second primer is at least 10, preferably at least 1 1 or at least 13. The higher the pH the faster the polymerization of the siloxane. The second primer improves the polymerization of the siloxane and thus, provides for a better hydrophobization of the board’s surfaces at a given amount of applied siloxane. The alkaline compound preferably is an alkaline hydroxide, usually KOH. The alkaline compound can also be a waterglass or a combination of an alkaline hydroxide and waterglass.

The second primer may additionally comprise a silicate. Mixtures of alkali hydroxides and silicates are also known as waterglass which can be produced from quartz sand and alkali carbonates. The addition of a silicate, e.g. waterglass, may additionally improve the surface hardness of the gypsum fiber board.

Additionally, the second primer may comprise a siliconate. The siliconate may further improve the hydrophobization.

This second primer may for example be a mixture that is conventionally used as the only primer in gypsum fiber board production on the condition that it is alkaline to super alkaline. The first and/or the second primer may be in the form of an aqueous dispersion.

The first and/or the second primer may be applied by spray-coating, dipping, brushing or combinations thereof.

Depending on the amount of water used in the primer, it might be useful to apply a heating step to dry the primed gypsum fiber board. A typical way to effect the heating is by the use of a dryer oven or an infrared lamp or any combination thereof. Temperatures above the water evaporation temperature are suitable, typically between 120 to 150 °C. On the other hand, temperatures should not be too high for too long in order to avoid re-calcination of the set gypsum in the gypsum fiber boards. A person skilled in the art will know how to select suitable temperatures for the drying step.

Siloxanes are compounds comprising a Si-O-Si bond. Siloxanes are often oligomeric or polymeric and typically follow the formula H(OSiR2)nOH or, for cyclic compounds, (OSiR2)n where R is H or alkyl. A typical compound is polydimethyl siloxane or poly (methylhydrogen) siloxane. A H-siloxane is a siloxane having at least one H-Si-bond. Preferably, most to all R are hydrogens.

Without wishing to be bound by theory, it is believed that the alkaline second primer reacts with the first primer to form siloxides of the formula RsSiOM, wherein R is as mentioned above, and M typically is hydrogen.

The invention is further explained by the following non-limiting examples.

Examples

Materials

Primer BRB: BRB Silicon Oil HY43, polydimethylhydrogensiloxane, commercially available from BRB International, Netherlands

Primer BS46: Wacker Silres BS46, water emulsion of polymethylhydrogensiloxane, 50 wt.%, commercially available from Wacker Chemie AG, Germany

Primer WG: waterglass mixed with siliconate, KOH and water, KOH content: 1 ,6 wt.-% with respect to the total mass of the primer

Primer KOH: solution with 2.5 wt.% KOH Primer WS: solution having 8.25 wt.% water glass, here KOH

Reference samples 1 , 5 and 10 are gypsum fiber boards treated only with the primer WG, being the internal state of the art to be matched in terms of water resistance.

Sample Preparation

Test samples 1 to 4 were cut from typical gypsum fiber boards produced on a board line. Gypsum fiber boards were produced according to standard procedure including drying at about 150°C for several hours, for example about 4 h (drying time strongly depends on the thickness of the board), and equalizing the board’s main surfaces by grinding. All gypsum fiber boards used for the tests described here were devoid of glass fibers and contained only cellulose fibers. The samples cut out of the boards had the size of 140 x 140 x 12,5 mm (length, width, height). First and/or second primer were applied on both, front and back side surfaces, by brush. Samples were weighed before and after each individual coating was applied. The samples were dried to weight constancy subsequently in a laboratory oven. The weight difference is denoted as the “applied quantity” of the primer in Tab. 1. The applied quantity is the sum of the coatings on front and back side of the sample. Samples 1 to 4 were produced on the same day.

Samples 5 to 8 were produced on another day and were cut from boards produced on a board line trial according to standard procedure including drying and equalizing by grinding. The hydrophobic coating was applied in amounts as indicated in Tab. 1. Coating WG is the standard hydrophobization of the board line used. If an additional coating was applied or no Coating WG, individual boards were removed from the line and the individual coating applied if applicable. After the application of the coating, the boards were re-inserted onto the line, either before or after the hydrophobization station, depending on whether an additional Coating WG was applied or not. Test samples, the size of 140 x 140 x 12,5 mm (length, width, height), were cut from the boards.

Samples 10 to 12 are from line trials on yet another day. Samples of 140 x 140 x 18 mm (length, width, height) were cut from the finally processed boards and the respective coatings (Tab. 1 ) applied as described above. Samples 13 to 21 are from line trials on yet another day. Samples of 140 x 140 x 12.5 mm (length, width, height) were cut from the finally processed boards and the respective coatings applied as described above. Tab. 1

Reference sample

Water-Resistance

A surface water absorption test according to DIN EN 15283-2:2009, 5.8 was conducted with the dried, coated board samples. Please note that the sizes of samples differed from the size described in DIN EN 15283-2:2009, 5.8.3, actual sizes as described above. However, the procedure described in DIN EN 15283-2:2009, 5.8 was closely followed for the determination of the surface water absorption of the samples: The samples were dried at 23°C until constant weight was reached. The dry weight of each sample was recorded. Samples were placed in a Cobb device, the ring filled with water of 23°C was placed on the sample surface and remained there for 30 or 60 min., as applicable. Subsequently, the samples were taken out of the Cobb device, water clinging to the surfaces was dried off and immediately thereafter the samples were weighed again.

The weight difference is recorded in Tab. 1 , normalized to 1 m² of gypsum fiber board after 30 or 60 min of exposure.

Fire-Resistance

Samples were placed in a special metallic holder with path transducer, positioned in a furnace and heated from room temperature to 945 °C according to a standard heating curve (DIN EN 13501 -2: 2016-12) for 60 min, then kept constant for another 60 min at 945 °C. All samples expand for a certain time before they started to shrink, see Figs. 1 and 2. Each value represented in Tab. 1 is the mean value of two replicates.

As described above, samples 1 , 5 and 10 are reference samples from current production. These samples are coated with a mixture of waterglass, siliconate and KOH. The water absorption according to DIN 15283-2: 2009, 4.8 has to be below 300 g/m² in order to qualify as GF-W1 , meaning gypsum fiber boards with reduced water absorption. The reference samples easily comply, having water absorption values of less than 105 g/m². However, when exposed to high temperatures, the total shrinkage of the samples is high, see discussion below.

Samples 4, 8 and 9 show a water absorption that is close to the reference samples 1 and 5, respectively. The water absorption of samples 2, 3, 6 and 7 was a magnitude higher than the references and higher than 300 g/m². Without wanting to be bound by theory, the inventors believe that the difference can be attributed to the missing polymerization of the silicones applied as coatings. Only samples that were treated with KOH (contained in coating WG) before or after the application of the silicone coating show good water absorption properties. Samples 7 and 9 were produced on the same day and treated with comparable amounts of BRB coating. Sample 7 was dried at high temperatures to improve polymerization, whereas Sample 9 was treated with a subsequent WG coating comprising KOH, for polymerization. The difference in water absorption is striking: Sample 7 absorbed about 18 times the amount of water per m² board than Sample 9. Polymerization via change of pH to higher values seems to be clearly more effective than by temperature - at least in the temperature range typically used for drying gypsum fiber boards.

Samples 4 and 9 differ by the application order of the coatings. Sample 4 was first treated with WG coating and then with BRB coating, whereas the treatment of Sample 9 was reversed. Additionally, Sample 9 was treated with a higher amount of silicon oil than Sample 4. Both samples showed a comparable water absorption, the one of sample 9 being slightly lower. This indicates that the application order of the coatings is of low relevance.

The samples 1 1 and 12 were similar and even better in water absorption than the reference sample 10.

The samples 13 to 21 all have similar amounts of siloxane coating BRB but different alkaline compounds for enabling polymerization of the siloxane. KOH, waterglass solution (WS), and the waterglass-siliconate mixture (WG) result in sufficient polymerization and thus, low water absorption in the Cobb test.

Sample 3 was coated only with a siloxane solution, but no second primer was applied. In order to substitute for the chemical catalysator the drying time was prolonged to 20 min.. However, Cobb test results reveal that the substitution was not sufficient to compensate for the missing catalyst.

Figures 1 and 2 show the length change of Samples 1 to 4 and 10 to 12 over time, when subjected to heating. During the first about 25 to 50 minutes all samples show an expansion phase. Presumably, during this phase the crystal water of the gypsum becomes gaseous, inflates the sample and slowly diffuses out of the sample body. Thereby, the sample body shrinks back to its original size. When the crystal water has vanished, the now more or less calcined gypsum starts to sinter. During this phase the sample body shrinks. After 120 min all samples are shorter than at the beginning.

Important key points in this development are the total shrinkage after 120 min and the time after which the sample body actually begins to shrink below its original size. These key points are markedly different in the individual samples.

Total shrinkage of the sample is relevant because in case of fire gypsum fiber boards will shrink accordingly and thereby are prone to rip off from their sub-construction. Depending on where the board is placed in a building such a separation of board from the sub-construction is very dangerous, for example, if the board is a wall or ceiling board. Ripping off of the subconstruction in this case means that parts of the boards may drop down and hurt people. Further, missing parts of a wall cladding open up gaps for the fire to break through to an adjacent room. Thus, total shrinkage should be as low as possible after a defined time of heating.

The time during which the board is in the expansion phase is relevant because during this phase the construction is usually safe. During this time frame temperatures opposite of side of the construction which is under fire only marginally exceed 100°C. If and as long as water vapor diffuses from the boards, the vaporization cools the sub-construction and thereby prevents it from weakening or even melting. The longer this time-period the more time remains for the rescue of people and extinguishing measures.

Reference Sample 1 exhibits the highest overall shrinkage and the earliest start of the sintering phase. Thus, reference Sample 1 has good water-resistance, but the fire-resistance is not sufficient.

Sample 2 has the longest expansion phase (about 50 min) and the lowest encountered total shrinkage of 1%. Fire-resistance is excellent, but the water absorption is intolerable.

Sample 3 has a moderate total shrinkage and takes relatively long before actual shrinkage starts. However, the water absorption is higher than allowed for the classification GF-W1 .

Sample 4 has a water absorption in the range of Sample 1 and the second-best fire-resistance in terms of the overall shrinkage and the on-set time of shrinkage.

Sample 10, a reference sample, showed a total shrinkage of 5,6%, see Fig. 2. Samples 1 1 and 12 that were coated first with Coating BRB and then with Coating WG show a total shrinkage of 3,8 and 2,2, respectively. Additionally, both samples, 12 and 13, revealed a clearly longer expansion phase (about 45 min compared to 30 min) than the reference Sample 10. Thus, samples 12 and 13 have better water-resistance and better fire-resistance than reference Sample 10.

Claims

1 . A process for prime coating of gypsum fiber boards comprising the steps of: a) providing a gypsum fiber board b) applying a first primer comprising a siloxane c) applying a second primer comprising an alkaline compound and water.

2. The process according to claim 1 , wherein the siloxane is a H-siloxane, an alkyl siloxane or a mixture thereof.

3. The process according to claim 1 or 2, wherein the alkaline compound is an alkaline hydroxide, preferably KOH or waterglass.

4. The process according to claim 1 to 3, wherein the second primer additionally comprises silicate.

5. The process according to any one of claims 1 to 4, wherein the second primer additionally comprises a siliconate.

6. The process according to any one of claims 1 to 5, further comprising the step of d) heating the gypsum fiber board. . The process according to any one of claims 1 to 6, wherein the first or the second primer is an aqueous dispersion. . The process according to any one of claims 1 to 7, wherein the first or the second primer is applied by spray-coating, dipping, or brushing. The process according to any one of claims 1 to 8, wherein the amount of siloxane applied with the first primer is 150 to 400 g per square meter of gypsum fiber board. A gypsum fiber board obtainable by the method of any one of claims 1 to 9. A gypsum fiber board comprising a coating of a cross-linked siloxane obtained by applying a first coat of a siloxane and a second coat of an alkaline compound. Use of siloxanes as a primer for hydrophobic and fire-resistant coatings of gypsum fiber boards Use according to claim 12, wherein the siloxane is a H-siloxane.