GB2264114A

GB2264114A - Gypsum products

Info

Publication number: GB2264114A
Application number: GB9127436A
Authority: GB
Inventors: Jonathan Lincoln Brown; Peter Lyddon Vickery
Original assignee: TRICORN TECHNOLOGY Ltd
Current assignee: TRICORN TECHNOLOGY Ltd
Priority date: 1991-12-27
Filing date: 1991-12-27
Publication date: 1993-08-18
Anticipated expiration: 2011-12-27
Also published as: GB9127436D0; GB2264114B

Abstract

A method of producing a gypsum product, comprises the steps of: (1) mixing together calcium sulphate hemi-hydrate or other gypsum precursor, water and polymer binders, wherein the polymer binders comprise, per 100 parts by weight of the final gypsum, the following components:- (A) 1 to 50 parts by weight of an acrylic polymer, (B) 1 to 10 parts by weight of a fluorine resin, preferably polytetrafluoroethylene, (C) 1 to 10 parts by weight of a melamine formaldehyde polymer, and (D) 1 to 10 parts by weight of an urea formaldehyde polymer; and (2) allowing the mixture to cure. du

Description

"Gvpsum Products and Method of Manufacture Thereof" This invention relates to gypsum products and to a method of manufacture thereof. By the term "gypsum products" is meant products wherein at least 50% by weight of the product is formed of gypsum.

It has been previously proposed to provide gypsum products which have been modified by incorporation of one or more polymers therein for improving the properties thereof. For example, in GB-A-1390360 there are disclosed gypsum products which have been modified by various polymers including polymers of ethylene unsaturated monomers, and include vinyl resins, and polymers of acrylates and methacrylates, particularly alkyl esters of acrylic and methacrylic acids in which the alkyl group has up to twelve carbon atoms. Other resin emulsions include polyvinylacetate emulsions, polystyrene emulsions and combinations of polymers or copolymers.

The disadvantages of these polymers is that they are prone to degradation by weathering or ageing, giving rise to softening, water penetration, staining, yellowing, cracking, chalking, blistering and/or flaking.

It is an object of the present invention to provide an improved gypsum product, and a method of manufacturing such product.

According to one aspect of the present invention, there is provided a gypsum product (as defined herein) including gypsum and polymer binders, wherein said polymer binders comprise, per 100 parts by weight of gypsum, the following components: (A) 10 to 50 parts by weight of an acrylic polymer, (B) 1 to 10 parts by weight of a fluorine resin, preferably polytetrafluoroethylene (PTFE), (C) l to 10 parts by weight of a melamine formaldehyde polymer, and (D) 1 to 10 parts by weight of an urea formaldehyde polymer.

According to another aspect of the present invention, there is provided a method of producing a gypsum product as defined in the last preceding paragraph, comprising the steps of: (1) mixing together calcium sulphate hemi-hydrate or other gypsum precursor, water and polymer binders, wherein the polymer binders comprise, per 100 parts by weight of the final gypsum, the following components: (A) 1 to 50 parts by weight of an acrylic polymer, (B) 1 to 10 parts by weight of a fluorine resin, preferably polytetrafluoroethylene, (C) 1 to 10 parts by weight of a melamine formaldehyde polymer, and (D) 1 to 10 parts by weight of an urea formaldehyde polymer; and (2) allowing the mixture to cure.

If a particularly waterproof gypsum product is required, the method may further comprise the steps of, after allowing the mixture to cure, applying a layer of component (B) to the surface of the cured mixture, and heating the layer to a temperature of 180 Celsius. The layer may be applied before the mixture is fully cured, and may, for example, be applied by spraying.

The preferred gypsum is calcium sulphate alpha hemi-hydrate plaster. One advantage of this gypsum over the beta hemi-hydrate plaster is that a much lower water to plaster ratio is required to form a workable mix.

Alpha hemi-hydrate plaster requires 30 to 45 cc of water to 100 grams, whereas beta hemi-hydrate plaster requires 50 to 80 cc of water to 100 grams to form a workable slurry.

It is preferred for the polymer binder components A and B to be introduced in aqueous emulsion form so that the water in the aqueous emulsion is available for reaction with the gypsum precursor.

It is preferred for the polymer binder components C and D to be mixed in powder form with the gypsum precursor before being mixed with the other components.

Alternatively, all four polymer binder components A to D may be introduced in aqueous emulsion form.

The drawback of previously proposed polymer binders for gypsum is that they fail to impart adequate water and weather resistance. These are important particularly where the gypsum products are intended to have outdoor applications. Gypsum is particularly prone to water and weather erosion because of its layer lattice structure where the calcium and sulphate ions together make up the individual layers which are linked by hydrogen bonds between the water molecules and the oxygen atoms of the sulphate ions. Exposure of the gypsum products to water causes the hydrogen bonding to break down with a consequent loss of structural integrity.

For this invention, the use of at least one of the polymer binder components in aqueous emulsion form is considered to be particularly advantageous because, during curing, the gypsum extracts water from the emulsion and causes polymer films to form around the bridging water molecules in the gypsum lattice structure. This imparts a greatly increased water resistance and strength.

The component (A) which contains acrylic polymer is preferably one or more thermosetting acrylic resins, having high molecular weight. For example, the component (A) may comprise monomers of mixed acrylic polymers such as those supplied by Macphersons Polymers Limited under their Batch Reference Number SR 804. The emulsion containing component (A) has a viscosity of preferably 300 to 400 cp (Brookfield), a particle size of preferably less than 1 pm and a pH of preferably 2 to 3. The solids content of the aqueous emulsion is preferably 44% to 46% by weight. The preferred Tg of the emulsion is 15C to 20 Celsius with a stabilising anionic charge. The component (A) is present so as to improve the water resistance of the final product by preventing water from entering the pores in the structure.Excess water in the mix being subjected to curing or setting allows the acrylic component, because of its small particle size, to migrate to the outer surface, thereby sealing the product against ingress of moisture and forming a transparent film on the outer surface thereof. The component (A) is also resistant to W and IR radiation.

The component (B) containing a fluorine resin tends to be evenly dispersed throughout the mix, and to migrate to the surface of the product to impart a highly water-repellent surface, and also improves the stability of the product against degradation by W and IR radiation.

The component (B) is preferably included in the mix in the form of an aqueous dispersion or emulsion having a viscosity of 10 to 20 mPAS. The solids content of the aqueous emulsion is preferably 50% to 56% by weight. The average particle size is preferably less than 1 micron.

The dispersion is preferably weakly acidic, and preferably has a specific gravity of between 1.3 to 1.4 grams per cubic centimetre. The fluorine resin may include PTFE, and a suitable example of a fluorine resin containinq PTFE

is HostafloniTFB X 7100 D which is manufactured by Hoechst L of Germany.

Component (B) is not used as a binder component, instead it is used as a water and weather resistance improver. With the fluorine resin temperatures in excess of 170C Celsius are required to achieve true film formation. Coagulation of the PTFE does occur over an extended period. The PTFE molecules have an extremely low compressive strength. By incorporating PTFE into the material small movements caused by temperature changes and exposure to W radiation can be accommodated without damaging the integrity of the structure.

If polymer components in true solution forms are used, i.e. those containing solvents and oils, then the oils can inhibit the cure of the alpha hemi-hydrate plaster. In these instances the gypsum becomes a filler for the polymer mix rather than an inherent structural medium. Ultimate tensile strength is diminished and the expansion of the gypsum during cure is lost.

It is the expansion of the alpha hemi-hydrate gypsum plaster during cure that enables a superb finish to be achieved.

The component (C) containing melamine formaldehyde polymer is preferably .,added in powder form, and is preferably mixed with gypsum powder before any water or emulsion is added. Component (C) may comprise, for example, BL434 from BIP Chemicals Limited. The component (C) has a tendency to migrate towards the centre of the mixture as the polymer therein cures, thus serving to reinforce and strengthen the main bulk of the product.

The component (D) containing urea formaldehyde polymer is preferably incorporated into the mix in powder form and is preferably mixed with gypsum powder before any water or emulsion is added. An example of a suitable urea formaldehyde polymer is W436 from BIP Chemicals Limited.

During curing of the product, the urea formaldehyde polymer has a tendency to migrate towards the outside of the mixture and assists in providing, with the melamine formaldehyde component (C), a more even distribution of strengthening binder polymer throughout the mixture.

The use of the polymer components (A), (C) and (D) as identified above provides an optimised arrangement where the natural expansion of the gypsum during curing is balanced by the tendency of the polymers to shrink upon curing and to migrate to various regions of the body, thereby resulting in a very stable and strong structure.

The gypsum cures and expands first.

The particle size of the polymer components is important. It is preferred for the particle size of the urea formaldehyde polymer in component (D) to be smaller than that of the melamine formaldehyde polymer in component (C). The migration of the urea formaldehyde particles towards the outer surface of the mix tends to trap the melamine formaldehyde within the mix which then cures.

Research by BIP Chemicals Limited, completed several years ago, compared gypsum plaster with urea formaldehyde as the sole polymer additive and gypsum plaster with melamine formaldehyde as the sole polymer additive. The urea formaldehyde mix cured more slowly than the melamine formaldehyde mix and the urea formaldehyde tended to migrate towards the outer surfaces, whilst the melamine formaldehyde tended to migrate towards the core of the mix.

The component (A) is an oxidatively drying binder. There are two aspects of film formation with oxidatively drying polymers which need to be considered in aqueous polymer binder systems: (i) Mere extraction of water and any neutralising agents is sufficient to produce dried particles.

(ii) Oxidative cross-linking by oxygen absorption.

Both forms of skinning are irreversible - a fact that needs to be taken into account for storage of the emulsion and in the method of mixing the product The first stage in the drying of the emulsion film involves the extraction of the water, in part, from the hydrolisation of the gypsum plaster, with the remainder by residual evaporation. However, whereas in film formation of a solution the binder is distributed completely homogeneously, the component (A) is an emulsion where individual particles will be in surface contact.

These particles will start the film formation process in advance of those particles that remain wet.

Research from various sources including ICI, Hoechst and Akzo Chemicals shows that the more homogeneously any film fuses, the more pronounced are its binder related characteristics.

Control over the curing process is essential to achieve the homogeneous film formation of polymers in components (A), (C) and (D). Film formation should not commence until after the hydrolisation of the gypsum plaster is virtually complete. The addition of component (D) slows the polymer curing process of both component (A) and component (C) to where they occur simultaneously with component (D) forming an homogeneous whole.

As far as the relative proportions of the polymer components is concerned, it is preferred for the weight ratio of the component (A) to that of the component (B) to be in the range of 4:1 to 10:1, the most preferable ratio being about 8:1. The weight ratio of component (C) to component (D) is preferably in the range of 1:1 to 3:1.

More preferably, the components (C) and (D) are present in equal quantities by weight.

The mix preferably further contains aqueous ammonia as a neutralising agent and also as an antifoaming agent. In this respect the use of aqueous ammonia is preferred to the use of alternative neutralising agents such as triethylamine and dimethylethanolamine, as ammonia corresponds to various clean air legislation (Category III in Germany) and is more volatile than triethylamine.

Ammonia is also preferred to anti-foaming agents as most of these contain either soaps or oils that expose particles of component (A) to oxidisation by adversely affecting the water meniscus membrane. The aqueous ammonia solution is preferably employed in the form of a 9.5% concentration of ammonia, preferably in distilled water, so that the content of ammonia in the mix is 0.5 to 1.5 parts by weight per 100 parts by weight of gypsum.

The mix may further include alum as a catalyst for the melamine and urea formaldehyde resins, typically in an amount of one part by weight per 100 parts by weight of gypsum.

The mix may also include sodium citrate.

The mix and therefore the resultant gypsum products may further include aggregates and/or natural or synthetic pigments to impart a range of different properties, finishes and/or colours to the resultant products. For example, aggregates may be included to produce finishes with the appearance of Bath stone or natural slate. The products may contain up to about 30% by weight of such aggregates.

Where a high degree of resistance to water and UV and IR radiation is not required, for example for internal use, the weight ratio of polymer to plaster may be reduced to as low as 1:10. Such low loadings of polymer may be suitable for use in loud speaker cabinets where materials having good sound absorption properties are often required.

The maximum weight ratio of polymer to gypsum is generally about 1:1.

The mix may further include glass fibres in chopped strand, tissue, random mat, woven and/or non woven reinforcing fabrics, thus making an excellent substitute for glass-reinforced concrete.

The mix may further include other reinforcing materials such as carbon fibres, nylon and pre-preg reinforcing fabrics to produce true composite materials.

The mix may further include either a water based

epoxy resin such as AlpolltlWP 4726B and a compatible

hardener such as Beckoox r EH 623 both manufactured bv

Hoechst, or a normal epoxy resin such as EpiresdER8 and a

special water miscifying hardener such as EpilinkiDP 660 as produced by Akzo Chemicals Limited, to impart properties suitable for boat construction.

The present invention will now be described in further detail in the following example: EXAMPLE A mix is produced by blending together, utilising a high shear mixer such as a paint stirrer attached to an electric drill, 100 parts by weight of

an (?k) alpha calcium sulphate hemi-hydrate, such as Crystacal made by British Gypsum Plc, with 3 parts by weight of a melamine formaldehyde powder as manufactured by BIP Chemicals Limited and designated B1434, and 3 parts by weight of a urea formaldehyde powder also supplied by BIP Chemicals Limited under the designation W436. These powders should be thoroughly blended in an oversized plastic container and form a dry powder mix.

A separate mix is produced, again using a high shear mixer and an oversized plastic container, using 50 parts by weight of mixed acrylic monomers as made by Macphersons Polymers Limited and currently code-named SR 804, 3 parts by weight of a polytetraflouroethylene dispersion such as Hostaflon TFB X 7100 D as made by Hoechst UK Chemicals. At the same time, 10 parts by weight of a 1 w/w % alum solution, 10 parts of a 2 w/w % aqueous solution of sodium citrate and 2 parts by weight of a 9.6 w/w % aqueous ammonia solution are included in the mix. This wet polymer mix is then thoroughly homogenised before being added to-the dry powder mix.

The combined wet and dry mixes are thoroughly homogenised, using the same shear mixing techniques, before being poured into a suitable mould. Within one hour, the mix cures sufficiently to allow demoulding and handling. 98% of water extraction is achieved within 72 hours whilst the remaining 2% water can take up to six months to evaporate, depending upon ambient temperatures.

A moulded panels thus formed was found to withstand an extreme lap test involving causing fast flowing water to pass over half of the panel for one week.

Claims

1. A method of producing a gypsum product (as defined herein), comprising the steps of: (1) mixing together calcium sulphate hemi-hydrate or other gypsum precursor, water and polymer binders, wherein the polymer binders comprise, per 100 parts by weight of the final gypsum, the following components: (A) 1 to 50 parts by weight of an acrylic polymer, (B) 1 to 10 parts by weight of a fluorine resin, (C) 1 to 10 parts by weight of a melamine formaldehyde polymer, and (D) 1 to 10 parts by weight of an urea formaldehyde polymer, and (2) allowing the mixture to cure.

2. A method as claimed in Claim 1, which further comprises the steps of, after allowing the mixture to cure, applying a layer of component (B) to the surface of the cured mixture, and heating the layer to a temperature of 1800 Celsius.

3. A method as claimed in Claim 2, wherein the layer is applied before the mixture is fully cured.

4. A method as claimed in Claim 2 or 3, wherein the layer is applied by spraying.

5. A method as claimed in any preceding claim, wherein the gypsum is calcium sulphate alpha hemi-hydrate plaster.

6. A method as claimed in any preceding claim, wherein the polymer binder components A and B are introduced in aqueous emulsion form so that the water in the aqueous emulsion is available for reaction with the gypsum precursor.

7. A method as claimed in any preceding claim, wherein the polymer binder components C and D are mixed in powder form with the gypsum precursor before being mixed with the other components.

8. A method as claimed in any of Claims 1 to 6, wherein all four polymer binder components A to D are introduced in aqueous emulsion form.

9. A method as claimed in any preceding claim, wherein at least one of the polymer binder components is introduced in aqueous emulsion form.

10. A method as claimed in any preceding claim, wherein the component (A) which contains acrylic polymer is one or more thermosetting acrylic resins, having high molecular weight.

11. A method as claimed in Claim 10, wherein the component (A) comprises monomers of mixed acrylic polymers such as those supplied by Macphersons Polymers Limited under their Batch Reference Number SR 804.

12, A method as claimed in any preceding claim, wherein component A is introduced in aqueous emulsion form.

13. A method as claimed in Claim 12, wherein the emulsion containing component (A) has a viscosity of 300 to 400 cp (Brookfield).

14. A method as claimed in Claim 12 or 13, wherein the emulsion containing component A has a particle size of less than 1 ssm.

15. A method as claimed in any of Claims 12 to 14, wherein the emulsion containing component A has a pH of 2 to 3.

16. A method as claimed in any of Claims 12 to 15, wherein the solids content of the aqueous emulsion is 44% to 46% by weight.

17. A method as claimed in any of Claims 12 to 16, wherein the preferred Tg of the emulsion is 15 to 200 Celsius with a stabilising anionic charge.

18. A method as claimed in any preceding claim, wherein component (A) is resistant to W and IR radiation.

19. A method as claimed in any preceding claim, wherein component (B) is included in the mix in the form of an aqueous dispersion or emulsion.

20. A method as claimed in Claim 19, wherein the aqueous dispersion or emulsion has a viscosity of 10 to 20 mPAS.

21. A method as claimed in Claim 19 or 20, wherein the aqueous dispersion or emulsion has a solids content of 50% to 56% by weight.

22. A method as claimed in any of Claims 19 to 21, wherein average particle size of the aqueous dispersion or emulsion is less than 1 micron.

23. A method as claimed in any of Claims 19 to 22, wherein the aqueous dispersion or emulsion is weakly acidic.

24. A method as claimed in any of Claims 19 to 23, wherein the aqueous dispersion or emulsion has a specific gravity of between 1.3 and 1.4 grams per cubic centimetre.

25. A method as claimed in any preceding claim, wherein the fluorine resin includes PTFE.

26. A method as claimed in any preceding claim, wherein component (C) comprises BL434 from BIP Chemicals Limited.

27. A method as claimed in any preceding claim, wherein component (D) is W436 from BIP Chemicals Limited.

28. A method as claimed in any preceding claim, wherein the particle size of the urea formaldehyde polymer in component (D) is smaller than that of the melamine formaldehyde polymer in component (C).

29. A method as claimed in any preceding claim, wherein the weight ratio of the component (A) to that of the component (B) is in the range of 4:1 to 10:1.

30. A method as claimed in Claim 29, wherein the weight ratio is about 8:1.

31. A method as claimed in any preceding claim, wherein the weight ratio of component (C) to component (D) is in the range of 1:1 to 3:1.

32. A method as claimed in Claim 31, wherein the components (C) and (D) are present in equal quantities by weight.

33. A method as claimed in any preceding claim, wherein the mix further contains aqueous ammonia as a neutralising agent and also as an antifoaming agent.

34. A method as claimed in Claim 33, wherein the aqueous ammonia solution is employed in the form of a 9.5% concentration of ammonia, so that the content of ammonia in the mix is 0.5 to 1.5 parts by weight per 100 parts by weight of gypsum.

35. A method as claimed in any preceding claim, wherein the mix further includes alum as a catalyst for the melamine and urea formaldehyde resins.

36. A method as claimed in Claim 35, wherein the mix includes alum in an amount of one part by weight per 100 parts by weight of gypsum.

37. A method as claimed in any preceding claim, wherein the mix also includes sodium citrate.

38. A method as claimed in any preceding claim, wherein the mix and therefore the resultant gypsum product further include aggregates.

39. A method as claimed in any preceding claim, wherein the mix further includes natural or synthetic pigments.

40. A method as claimed in any preceding claim, wherein the weight ratio of polymer to gypsum is between 1:10 and 1:1.

41 A method as claimed in any preceding claim, wherein the mix further includes glass fibres in reinforcing fabrics.

42. A method as claimed in any preceding claim, wherein the mix further includes a water based epoxy resin such as Alpollt WP 4726B and a compatible hardener such as Beckopox EH 623 both manufactured by Hoechst.

43. A method as claimed in any of Claims 1 to 41, wherein the mix further includes a normal epoxy resin such as Epires ER8 and a special water miscifying hardener such as Epilink DP 660 as produced by Akzo Chemicals Limited.

44. A gypsum product (as defined herein) including gypsum and polymer binders, wherein said polymer binders comprise, per 100 parts by weight of gypsum, the following components: (A) 10 to 50 parts by weight of an acrylic polymer, (B) 1 to 10 parts by weight of a fluorine resin, (C) 1 to 10 parts by weight of a melamine formaldehyde polymer, and (D) 1 to 10 parts by weight of an urea formaldehyde polymer.