GB2167073A

GB2167073A - Soluble alkali silicate based moulding and coating compositions

Info

Publication number: GB2167073A
Application number: GB08428330A
Authority: GB
Inventors: Ronald Powell
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-11-09
Filing date: 1984-11-09
Publication date: 1986-05-21
Also published as: GB8428330D0; GB2167073B

Abstract

A moulding or coating composition comprising an intimate mixture of a minor amount of synthetic resin latex and a pre-gelled mixture of soluble alkali silicate and a hardening agent therefor, and a process for preparing such a composition.

Description

SPECIFICATION Soluble alkali silicate based moulding and coating compositions The present invention relates to soluble alkali silicate compositions, and especially those having fire resistant and flame retarding properties both in the form of moulding compositions for forming solid shaped articles and as coating compositions for coating inorganic materials such as vermiculate, mica, cement, alkali silicate and like materials.

In recent years substitute materials for asbestos have been sought on an increasing scale. Such materials are invariably inorganic in nature since they are required to possess at least the desirable properties of asbestos which in essence derive from the inorganic nature of that material. A number of substitutes have been produced such as boards composed principally of mica, vermiculate, alkali silicates and similar materials. Such materials however, whilst having satisfactory properties in other respects suffer from the drawback that known organic based coating materials cannot be used to produce satisfactory decorative surfaces on such materials especially when the coating composition is also required to be flame proof.

Known organic and synthetic organic coating materials have been found to disintegrate into flakes or where they retain a continuous surface are easily stripped from the substrate surface making it impossible to cut or saw panels carrying such coatings without stripping the coating.

It has been proposed to use soluble alkali silicates together with a hardenerto produce unshaped refractories, inorganic paints, inorganic adhesive materials and inorganic moulding materials. A hardener suggested for such applications is aluminium triphosphate. This material reacts with alkali silicates to form a solid hardened material by slow gel formation. Such materials however require special techniques which must be followed for successful use. Although such materials are used in large quantities therefore they do not lend themselves to general application by relatively unskilled operatives and users.

In the field of inorganic coatings, the necessity for long shelf life leads to drying or curing times which are often too lengthy for convenience or high volume production without specialised equipment.

It is an object of the present invention to provide soluble alkali silicate based compositions which reduce or eliminate the above difficulties.

The invention is predicated upon the surprising discovery that if a pre-gelled mixture of soluble alkali silicate and a hardening agent such as aluminium triphosphate is intimately mixed with a minor amount of synthetic resin latex, the resulting composition can be utilized for coating (e.g. by spray coating), or for moulding (e.g. vacuum forming) using conventional equipment to form surface coatings and shaped articles which have excellent properties such as water-resistance, durability, and in the case of coatings adhesion to surfaces such as vermiculate, mica, cement etc.

According to the present invention there is provided a composition comprising an intimate mixture of a minor amount of synthetic resin latex and a pre-gelled mixture of soluble alkali silicate and a hardening agent therefor.

The composition may also include inorganic pigments (such as titanium dioxide or powdered slate) and/or other inert filler materials (such as talc) provided they are neutral or alkaline materials. By inert is meant a material which does not chemically react with any of the other ingredients of the composition.

Any of the synthetic polymers and copolymers which can be formed into latexes may be used the choice of polymer or copolymer depending on the end use of the product. Where flame-proof or retarding properties are required the polymer is preferably one which itself has such properties such as polymers of vinyl chloride or vinylidine chloride and copolymers thereof with another unsaturated monomer such as acrylic monomers, although other polymers not having flame retarding properties such as acrylic polymers may be used in conjunction with a flame retarding agent. In the absence of the synthetic resin latex the hardened composition whether in moulded form or applied as coating exhibits crazing or cracking. It has been found that the synthetic resin latex acts as a binder to prevent this happening and the minimum proportion of synthetic resin is determined by this factor.The synthetic resin latex is present in a minor amount relative to the pre-gelled mixture of soluble alkali silicate and hardening agent.

Essentially the synthetic resin latex is dispersed as binder throughout a matrix formed by the soluble alkali silicate/hardener combination. The amount of synthetic resin latex should not be such (e.g. a major amount) that the hardened composition is in effect a dispersion of the soluble alkali silicate/hardener gel throughout a matrix formed of the synthetic resin since this wouid give rise to some of the very problems the composition of the invention sets out to prevent. In general the amount of synthetic resin (as represented by the solids content of the latex) is small compared to the silicate/hardener combination. Thus a weight ratio of synthetic resin to silicate/hardener of about 1:8 has been found to produce satisfactory bonding. Larger amounts of synthetic resin may be used however if desired.

The amount of hardener required depends on the nature of the hardener- i.e. its effectiveness. Some hardeners such as hydrochloric acid or magnesium carbonate are rapid acting and relatively small amounts are required to form the initial gel. Such hardeners are more useful in moulding compositions than in coating compositions. The preferred hardener is aluminium triphosphate which is a kind of solid acid having the formula:-

O 0 0 II i1 II HO-P-O-P-O-P-O-H I I O ThOl y Al3+ 2 H20 (crystal water) This material has been found to have no oral toxicity and no skin irritation.Aluminium triphosphate reacts with soluble sodium silicate as follows: Na2O.xSiO2 + H2AIP30,o r Na2AlP3010 + H20.xSiO2 The time necessary to initially form a gel after addition of the hardener to the soluble alkali silicate decreases as the amount of hardener used is increased. When aluminium triphosphate is used as the hardener, amounts of from about 3 parts by weight to about 8 parts by weight of triphosphate per 100 parts by weight of soluble alkali silicate will give initial gelling times of from 12 to 3 hours. The higher the content of hardener the shorter the useful working life (i.e. pot life) of the composition will become. The amount of hardener included should therefore be chosen to provide a convenient initial gelling time consistent with an adequate pot life for the circumstances under which the composition is to be used.In general the amount of hardener incorporated can be greater in compositions intended for moulding than in those intended for use as coating compositions.

Hitherto, inorganic coating compositions based on soluble alkali silicate have relied on the gelling of the soluble silicate to achieve adequate drying for a second coat to be applied over a first coat. This has meant that drying times are relatively long, i.e. several hours. The compositions of the invention, in contrast provide touch-dry coatings after only about 5 to 10 minutes of applying the composition so that several coatings can be applied in short periods of time. This property lends itself to spray coating techniques especially when several coatings need to be applied to achieve a desired final thickness of coating.

Any of the commercially available aqueous dispersions of finely divided synthetic resin may be used, especially those formulated for paint manufacture, may be used as the synthetic resin latex. Such products will already contain dispersing agents and other conventional additives normally included. Additional additives may be incorporated in the composition of the invention to provide special properties if desired.

Thus it has been found advantageous to include a wetting agent when adding the hardenerto the soluble alkali silicate. Furthermore, slip promoting and surface protection agents such as micronised polypropylene wax (particle size 1-10 microns with average particle size of 3-4 microns) may advantageously be added to the composition of the invention. About 3.5 parts by weight of micronised polypropylene wax per 100 parts of soluble alkali silicate have been found advantageous although greater or lesser amounts may be incorporated if desired. Thus amounts of from about 0.1% by weight to about 0.5% by weight based on the total weight of composition provide slip promoting effects although an amount up to 5% by weight can be used if special matting surface effects are desired.

The soluble alkali silicate is used in the form normally available which are aqueous solutions. Thus aqueous sodium silicate of specific gravity 1.398 is typical of the commercially available material (sometimes referred to as water glass). The weight or volume amounts of soluble alkali silicate stated herein refer to the aqueous materials which are commercially available.

Whilst the coating compositions of the invention lend themselves to application by spray coating techniques they may also be applied by brush coating if desired.

Before addition to the soluble alkali silicate the aluminium triphosphate is preferably vigorously mixed with water and wetting agent and the mixture allowed to stand for several hours. The water/triphosphate mixture is then slowly added to the soluble alkali silicate with stirring and the mixture allowed to gel. The formation of the gel may take 3 to 12 hours depending on the amount of aluminium triphosphate included.

The synthetic resin latex is preferably adjusted to neutral to alkaline pH before being mixed with the pre-gelled soluble alkali silicate. The adjustment may be made by adding an aqueous alkali preferably ammonia. It has also been found advantageous to add a surface active agent such as ethoxylated nonylphenol to the latex with the aqueous alkali.

The pigment, filler and other inert additives are preferably added and mixed with the latex before the latex is mixed with the pre-gelled alkali silicate. Preferably the pigment and other additives are vigorously premixed with water including a wetting agent before being mixed with the pH adjusted latex.

Finally the pre-gelled soluble alkali silicate should be slowly stirred and the pH adjusted latex containing the other additives slowly added with continued stirring. The final composition should be allowed to stand for a short period, e.g. about an hour before use.

The invention will be further illustrated by the following Examples.

Example 1 50 50 ml of water, 10 my of Polywet (a proprietory wetting agent) and 10 g. of aluminium triphosphate were mixed in a high speed mixer and allowed to stand for 12 hours. The mixture was then slowly added to 200 ml of aqueous sodium silicate of specific gravity 1.398 with mild agitation (as by slow stirring). After 5 hours the mixture had formed into a gel.

ii.50 ml of water, 35 gm. of titanium dioxide, 40 g. of talc, 10 ml of Polywet and 10 g. of Lanco 1778 (a proprietory micronised polypropylene wax of particle size 1-10 microns and average particle size 3-4 microns) were mixed together in a high speed mixture for 10 minutes.

iii. 50 ml of Haloflex 208 (a proprietory vinyl acrylic copolymer having a non-volatile content w/w of 56-58%, a pH of 1-2 a density of 1.19kg/land a viscosity of 20 mPa.s) 10 ml of NP 30 (a proprietory detergent composed essentially of ethoxylated nonylphenol) and 4 ml of ammonia 35% NH 37 were mixed together to form a mixture having a pH of 10-12. The mixture produced in ii. above was then slowly mixed with the pH adjusted latex.

iv. The gel formed in i. above was slowly stirred and the mixtures formed in iii. slowly added with continued stirring. After standing for 1 hour the composition was ready for use as a coating composition.

Example 2 The composition prepared according to Example 1 was applied to Monolux board to provide 860 gm per sq. metre of composition. The coating was touch dry in ten minutes. Monolux board is a laminar board formed from a calcium silicate matrix resin forced with cellulose fibre and is free from asbestos and mineral and synthetic fibres).

A sample 15.24 cm square of the coated board was subjected to testing for 500 hours in a humidity cabinet at 60"C. The test is reyarded as equivalent to 3 years normal exposure. Little or no deterioration of the surface coating occurred except for a slight yellowing in colour. The coated board was found to be abrasion resistant and to be resistant to staining by inks.

Claims

1. A composition comprising an intimate mixture of a minor amount of synthetic resin latex, and a pre-gelled mixture of soluble alkali silicate and a hardening agenttherefor.

2. A composition as claimed in claim 1 wherein the synthetic resin latex comprises a fire-proof or retardant material.

3. A composition as claimed in any of the preceding claims wherein the synthetic resin latex comprises at least one of vinyl chloride polymers, vinylidine chloride polymers, acrylic polymers, or copolymers thereof.

4. A composition as claimed in any of the preceding claims wherein the hardener comprises at least one of hydrochloric acid, magnesium carbonate and aluminium triphosphate.

5. A composition as claimed in claim 4wherein the hardener comprises aluminium triphosphate.

6. A composition as claimed in claim 5 wherein the aluminium triphosphate is present in amounts of 3 parts by weight to 8 parts by weight per 100 parts by weight of the soluble alkali silicate.

7. A composition as claimed in any of the preceding claims wherein the soluble alkali silicate comprises aqueous sodium silicate solution.

8. A composition as claimed in claim 7 wherein the aqueous sodium silicate solution has a specific gravity of 1398.

9. A composition as claimed in any of the preceding claims wherein the ratio of the weight of synthetic resin latex to the combined weight of soluble alkali silicate and hardening agent is 1: 8.

10. A composition as claimed in any of the preceding claims further comprising an inorganic pigment.

11. A composition as claimed in claim 10 wherein the inorganic pigment comprises at least one oF titanium dioxide and powdered slate.

12. A composition as claimed in any of the preceding claims further comprising a neutral or alkaline filler material.

13. A composition as claimed in claim 12 wherein the filler material is talcum powder.

14. A composition as claimed in any of the preceding claims further comprising a dispersing agent.

15. A composition as claimed in any of the preceding claims further comprising a wetting agent.

16. A composition as claimed in any of the preceding claims, further comprising a slip-promoting and surface-protection agent.

17. A composition as claimed in claim 16, wherein the slip-promoting and surface-protection agent is micronised polypropylene wax.

18. A composition as claimed in claim 17, wherein the particle size of the micronised polypropylene wax is from 1 to 10 microns, and the average particle size is from 3 to 4 microns.

19. A composition as claimed in either of claims 17 and 18, wherein the micronised polypropylene wax is present in amounts of from 0.1% by weight to 5.0% by weight based on the total weight of the composition.

20. A composition as claimed in claim 19, wherein the micronised polypropylene wax is present in amounts of from 0.1% by weight to 0.5% by weight based on the total weight of the composition.

21. A process for preparing a moulding or coating composition comprising adding a soluble alkali silicate and a hardening agent, allowing the resulting mixture to gel, and intimately mixing a minor proportion of synthetic resin latex therethrough to form a dispersion of said latex throughout a matrix formed by the gelled mixture of soluble alkali silicate and hardening agent.

22. A process as claimed in claim 21, wherein the hardener comprises aluminium triphosphate.

23. A process as claimed in claim 22, further comprising vigorously mixing the hardener with water and a wetting agent and allowing the mixture to stand before adding the mixture to the soluble alkali silicate with stirring.

24. A process as claimed in any of claims 21 to 23, further comprising adjusting the pH of the synthetic resin latex to neutral or alkaline, before mixing the synthetic resin latex with the gelled mixture of hardener and soluble alkali silicate.

25. A process as claimed in claim 24, wherein the pH is adjusted by adding an aqueous alkali to the synthetic resin latex.

26. A process as claimed in claim 25, wherein the aqueous alkali is ammonia.

27. A process as claimed in either of claims 25 or 26, further comprising addition of a surface active agent to the synthetic resin latex with the aqueous alkali.

28. A process as claimed in claim 27, wherein the surface active agent is ethoxylated nonylphenol.

29. A process as claimed in any of claims 21 to 28, further comprising adding and mixing inert additives to the synthetic resin latex before the synthetic resin latex is added to the gelled mixture of hardener and soluble alkali silicate.

30. A process as claimed in claim 29, wherein the inert additives comprise pigments and fillers.

31. A process as claimed in either of claims 29 or 30, further comprising mixing the inert additives with water and a wetting agent before mixing the additive with the synthetic resin latex.

32. A process as claimed in any of claims 21 to 31, wherein the synthetic resin latex is slowly added to the gelled mixture of hardener and soluble alkali silicate with continued stirring.

33. A process as claimed in any of claims 21 to 32, wherein the final composition stands for an hour after manufacture.

34. A composition substantially as hereinbefore described with reference to the accompanying Examples.

35. A process substantially as hereinbefore described with reference to the accompanying Examples.