GB1585659A - Plaster compositions - Google Patents

Description

(54) PLASTER COMPOSITIONS (71) We, SURFACE DEVELOPMENT LIMITED, a British company, of 65 Stafford Road, Wallington, Surrey, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to calcium sulphate plaster compositions.

Expanded polystyrene beads are commonly used as loose fill insulation or formed into lightweight insulation building panels. These panels tend to have poor fire-resistant properties; in particular when the polystyrene is subjected to excessive heat the material melts and flows. When the polystyrene is in this liquid state and in contact with flame a fire can rapidly spread from area to area, at the same time giving off suffocating noxious fumes.

Also special apparatus is required for the production of the expanded polystyrene in panel form. Furthermore the panels have very limited mechanical strength.

According to this invention we provide a set plaster product in which expanded polystyrene beads are coated with and adhered together by calcium sulphate plaster such that the interstices between the beads are not completely filled, at least some of the plaster being alpha-hemihydrate plaster. Such a product, which may conveniently be in the form of a panel may be made for example by setting a fluid plaster/bead mixture in a mould or in situ. The beads in the products of the invention are completely encapsulated by the plaster but a low plaster to bead ratio is used so that the interstices between the beads are not completely filled.

It has been found in practice that panels of the invention can be formulated to have relatively high compressive strength and to have good head- ,sound- and fire-resistance together with anti-condensation properties. The polystyrene may melt at the point on the surface to which heat is applied, but closed cells remain in the plaster and continue to provide good insulation against the penetration of heat.

These closed cells also reduce the likelihood of noxious gases being given off.

Formed polystyrene beads have the advantage, as compared with a number of other fillers, such as pumice that they are resilient when mixed with a fluid plaster composition and do not collapse. The closed cellular structure of the beads is thus retained and products of reproducible insulation properties and lightweight densities may be made. Of course, the more closed cells there are in a rigid insulant the lower its thermal conductivity.

Products according to the invention can be used for cores of composite building panels, insulated partitioning, roof and floor panelling.

It has been found that panels according to the invention have a further advantage over conventional expanded polystyrene panels in that surfacing fabrics and coatings can be adhered to their surfaces using solvent- as well as aqueousbased adhesive compositions. Also when products according to the invention are used as a roof lining no breakdown occurs when attaching roofing felt or applying hot asphalt.

Although beta-hemihydrate plasters can be present they have the disadvantage or relatively low strength both wet and dry and a relatively long drying time, because of the excess water needed to give the plaster a workable consistency. The long natural drying time is particularly disadvantageous for factory-made products since production line considerations require the use of expensive forced drying equipment and facilities. Due to the relatively low strength of the set plaster, handling of low density beta-plaster-based panels without damage is difficult.

The invention therefore uses alpha-hemihydrate plasters because of their greater strengths and lower water requirements. Alphaplasters also have higher densities and thus a far slower moisture absorbtion rate than betaplasters. Furthermore their compressive strengths after absorbing moisture can still be greater than the compressive strengths of dry beta-plasters. The reasons for the differences in compressive strengths and other properties between alpha and beta types of plasters are fully described in Patent Specification No: 1 390 360. It is however envisaged that a mixture of alpha- and beta-plasters may be used for cost reasons. Products according to the invention can be produced with high impact resistance and compressive strengths in relation to their densities.

Very little expensive equipment is required to produce the preferred products of the invention and because so little residual water is retained in the preferred cast products they can easily be handled in, say, 10-20 minutes without damage, irrespective of dimensional size.

The provision of forced drying conditions is unnecessary. Similarly with in situ applications special drying is normally not required.

When more water resistance is required for a product together with higher impact resistance and compressive strength, a resin emulsion can be mixed with the plaster, for example, as described in Patent Specification No: 1 390 360. However for the purposes of the present invention it has been found that the resin solids can be further reduced to 2% of the plaster solids which proportion will give some of the improvements already mentioned over the non-modified plaster mixes. The resin addition also improves the adhesion of the set plaster to the polystrene beads. The fluidizing of the alpha-plaster mix by the resin emulsion as described in Patent Specification No: 1 390 360 enables a thin, dense and high strength coating to be evenly distributed over the entire surface area of the polystyrene beads.This permits higher proportions of beads relative to plaster, making it possible to achieve densities down to 8 Ibs per cu. ft. When using the plaster compositions as described in the Patent Specification No: 1 390 360 all the various additives as specified therein can be used, i.e. plasticisers, solvents, calciferous mineral powders, silicaceous aggregates, defoamers, cork, glass fibre, etc. It is however desirable to omit the highly water sensitive substances that are mentioned such as the derivatives of cellulose ethers, as these adversely affect the final water resistance and reduce the set dry strengths of the products.

The relative amounts of the plaster and the beads can be varied as desired but sufficient plaster should be present to evenly encapsulate all the beads when the plaster sets.

In the following Examples 1 to 8, the fluid plaster composition (premixed with resin emulsion if used) is mixed with the polystyrene beads in a tumble mixer and the mixture is subsequently cast. The plaster does not completely fill the interstices between the beads in the product. All percentages are by weight.

EXAMPLE 1: Water 22% Alpha hemihydrate plaster (Crystacel) 71% Polystyrene beads (size 2-3 mm) 7% Approximate dry density: 17 Ibs per cu ft.

The paster solids on setting take up chemically approximately 75% of the water in the plaster mix. The set composition can therefore be cast and moved within 15 minutes without damage to the product. The product requires no subsequent forced drying.

EXAMPLE 2: Water 19% Styrene/butyl acrylate (2:3) copolymer emulsion (51% resin solids) 3% Alpha hemihydrate plaster (Crystacel) 69% Polystyrene beads (size 2-3 mm) 9% Approximate dry density: 12 Ibs per cu. ft.

The fluidizing of the plaster without using more water than Example 1 gives a lower density product since the loading of the polystyrene beads in the plaster is increased. Approximately the same relatively high initial wet strength (which is required for handling after casting) is retained.

EXAMPLE 3: Water 23% Alpha hemihydrate plaster (Crystacel) 72% Polystyrene beads (size 2-3 mm) 5% Approximate dry density: 24 Ibs per cu. ft.

EXAMPLE 4: Water 21% Styrene/methyl acrylate (3:2) copolymer emulsion (51% resin solids) 3% Alpha hemihydrate plaster (Herculite) 70% Polystyrene beads (size 2-3 mm) 6% Approximate dry density: 18 Ibs per cu. ft.

EXAMPLE 5: Water 23% Alpha hemihydrate plaster (Herculite) 74% Polystyrene beads (size 2-3 mm) 3% Approximate dry density: 38 Ibs per cu. ft.

EXAMPLE 6: Water 18% Vinyl/acrylic (3:2) copolymer emulsion (51% resin solids)-Vinacryl 5.5% Alpha hemihydrate plaster (Herculite) 72% Polystyrene beads (size 2-3 mm) 4.5% Approximate dry density 27 lbs per cu. ft.

EXAMPLE 7: Water 5.5% Styrene/butyl acrylate (2:3) copolymer emulsion (51% resin solids) 16.5% Alpha hemihydrate plaster (Crystacel) 69% Polystyrene beads (size 2-3 mm) 9% Approximate dry density: 14 lbs per cu. ft.

As compared with Example 2 where the water content is 31% of the plaster solids, the water content of this composition is 20% which is equivalent to the water of crystallisation of the set plaster solids. Therefore the compressive strength and water resistance of the set product is extremely high in relation to its relatively low density.

EXAMPLE 8: Water 5.5% Vinacryl emulsion (51% resin solids) 18% Alpha hemihydrate plaster (Herculite) 72% Polystyrene beads (2-3 mm) 4.5% Approximate dry density: 30 Ibs per cu. ft.

Again the product has a very high compressive strength and water resistance compared to that of Example 6 for the same reason as set out for Example 7.

Although the resin modification plaster compositions as set out in Patent Specification No: 1 390 360 improve the water resistance and compressive strengths of the plaster/polystyrene bead mixes, this improvement is not always required when the products of this invention are to be used in constantly dry conditions such as for fireproof doors and interior partitions.

The use of the resin modified plaster composition wffl be important when the products are to be used as exterior wall and roof claddings etc. The most important factor in this context is that the insulation properties of the cladding will otherwise be impaired if moisture is allowed to be absorbed by the plaster matrix without the resin modification, as a result of the very high thermal conductivity of water.

WHAT WE CLAIM IS: 1. A set plaster product in which expanded polystyrene beads are coated with and adhered together by calcium sulphate plaster such that the interstices between the beads are not completely filled, at least some of the plaster being alpha-hemihydrate plaster.

2. A product according to Claim 1 wherein the plaster is wholly alpha-hemihydrate plaster.

3.A product according to claim 1, wherein the plaster is a mixture of alpha- and beta-hemihydrate plasters.

4. A product according to any preceding claim, wherein the plaster incorporates a resin.

5. A product according to claim 4, which has been obtained by mixing expanded polystyrene beads with a fluid plaster composition as claimed in any of claims 1-10, 12, 25 or 27 of Patent No. 1 390 360, and thereafter allowing the mixture to set.

6. A product according to any preceding claim which has been cast in mould.

7. A product according to claim 1, prepared substantially as described in any of the foregoing Examples.

8. A building panel comprising a product according to any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. are to be used in constantly dry conditions such as for fireproof doors and interior partitions. The use of the resin modified plaster composition wffl be important when the products are to be used as exterior wall and roof claddings etc. The most important factor in this context is that the insulation properties of the cladding will otherwise be impaired if moisture is allowed to be absorbed by the plaster matrix without the resin modification, as a result of the very high thermal conductivity of water. WHAT WE CLAIM IS:

1. A set plaster product in which expanded polystyrene beads are coated with and adhered together by calcium sulphate plaster such that the interstices between the beads are not completely filled, at least some of the plaster being alpha-hemihydrate plaster.

2. A product according to Claim 1 wherein the plaster is wholly alpha-hemihydrate plaster.

3.A product according to claim 1, wherein the plaster is a mixture of alpha- and beta-hemihydrate plasters.

4. A product according to any preceding claim, wherein the plaster incorporates a resin.

5. A product according to claim 4, which has been obtained by mixing expanded polystyrene beads with a fluid plaster composition as claimed in any of claims 1-10, 12, 25 or 27 of Patent No. 1 390 360, and thereafter allowing the mixture to set.

6. A product according to any preceding claim which has been cast in mould.

7. A product according to claim 1, prepared substantially as described in any of the foregoing Examples.

8. A building panel comprising a product according to any preceding claim.