IE50813B1 - Inorganic foam - Google Patents

Abstract

This invention relates to inorganic foams and in particular to rigid foams comprising a clay mineral. In particular the invention relates to inorganic foam materials comprising a plurality of foam 5 prills derived from one or more layer minerals and a method for making such products; uses of the inorganic foam products; and an intermediate product for use in the production of inorganic foam products. The layer minerals (layer-silicate minerals) are 10 naturally-occurring forms of silica and are phyllosilicate materials comprising silicate sheets and having a layer-structure. Included within the term layer minerals are, for example, vermiculite, kaolinite and other clay minerals, montmorillonite, sepiolite, 15 attapulgite, illite and saponite

Description

This invention relates to inorganic foams and in particular to rigid foams comprising a clay mineral. In particular the invention relates to inorganic foam materials comprising a plurality of foam prills derived from one or more layer minerals and a method for making such products; uses of the inorganic foam products; and an intermediate product for use in the production of inorganic foam products.

The layer minerals (layer-silicate minerals) are naturally-occurring forms of silica and are phyllosilicate materials comprising silicate sheets and having a layer-structure. Included within the term layer minerals are, for example, vermiculite, kaolinite and other clay minerals, montmorillonite, sepiolite, attapulgite, illite and saponite.

The clay minerals occur in clays as particles Of the order of a few microns diameter which are aggregates or agglomerates of small crystalline units of the mineral of sub-micron size. Kaolin-type clay is essentially an aggregation of book-shaped units of sheets of the clay mineral kaolinite; it is to be understood that as used herein the term kaolinite includes kaolin-type clays, ball clays, fire clays and China clays in which kaolin minerals occur in nature although such clays may not comprise pure kaolinite. Fire clays are a mixture of kaolinite and illite.

The layer minerals are well known and some at least are used extensively in industry. Kaolinite and kaolin-containing clays are used extensively in numerous industries, for example in the ceramics industries (the major usage) for the manufacture of white-ware, porcelain and refractories, and as a filler for paper, paints, adhesives, plastics and rubbers. Vermiculite is used, commonly in heat-exfoliated form (exfoliated vermiculite) as loose-fill insulation material, in bound form as slabstock or boardstock for insulation and fire protection applications, and in Agricultural applications. Delaminated vermiculite, by which is meant vermiculite which has been delaminated by chemical treatment followed by swelling in water and milling or grinding, has been proposed for use in making sheet-like materials or papers, as a coating material for substrates and for making rigid inorganic foam products for insulation and fire protection applications. Delaminated vermiculite foam and uses thereof are described, for example, in our United States Patent Specification No 4 130 637.

Montmorillonite is used extensively in industry as a 50Q13 filler for paper, adhesives and paints. Sepiolite is used extensively in the ceramics industry.

Rigid materials made from kaolinite, e.g whiteware porcelain and refractories, are dense, brittle materials produced by processes involving a firing or sintering operation. Although kaolinite itself is a poor conductor of heat, the high-density rigid materials hitherto produced from it do not exhibit good insulation properties. Because of their high density (and hence heaviness), brittleness and unexceptional insulation properties, products made from kaolin containing clays are not used to any significant extent in thermal-insulation or fire-protection applications. Rigid materials made from heat-exfoliated vermiculite tend to be dense (heavy), rather brittle materials and whilst they are used in industry for the fire-protection of structural steelwork they are not used extensively as insulation materials. Rigid foam materials made from delaminated (as opposed to heat exfoliated) vermiculite are lightweight and exhibit good fire protection and insulation properties, but are difficult to make in large sizes. Because such products tend to crack and deform extensively upon drying, they are difficlt to produce in the form of slabstock or boardstock in sizes greater than about 30 cm square and 3 cm thickness.

The present invention resides in the discovery of a low-density product form of layer minerals which is both lightweight and exhibits good heat-insulation and fire protection properties, and is readily produced in the form of slabstock or boardstock of large sizes, for example up to 3 m x 1 m x 10 cm thickness.

According to a first embodiment of the invention there is provided a rigid inorganic foam product, preferably having a density of less than 0.4g/ml and more preferably less than 0.2g/ml, comprising prills of one or more layer minerals, each prill being of cellular structure.

By the term prills as used throughout this specification we mean products produced by shaping a gasified suspension of a layer mineral to form particles, beads, pieces or small lumps of foam having an essentially continuous cellular structure in which the walls of the cells are constituted by the layer mineral(s) particles, although the term is not intended as implying any particular size, shape or configuration of the pieces of foam. Typically, and as a guide only, the prills will be.cylindrical or essentially spherical pieces of foam of maximum dimension below about 5 mm, for example from 0.5 to 5 mm.

As will be described more fully hereinafter, the inorganic foam products are made by assembling the prills of cellular structure into desired product forms such as slabstock or boardstock, such that the products have an essentially cellular structure although the true cellular structure may not be continuous throughout the product. It is to be understood that the term rigid inorganic foam product as used throughout this specification includes such products wherein the cellular structure is not truly continuous; thus for example the term includes products wherein the prills are bound together by means of an adhesive or by mutual 508 13 attraction and voids exist between prills within the product structure.

The density of the foam products of the invention is normally below 0.25 g/ml, and may be as low as 0.06g/ml for especially lightweight products.

Typically, the products will have a density in the range of 0.08g/ml to 0.15g/ml.

By the term rigid foam as applied to the prills we mean a material having structural integrity which is a two-phase dispersion of gas in a solid matrix which is an essentially continuous cellular structure, and by the term rigid inorganic foam as applied to the prills we mean a rigid foam which is essentially made of inorganic material, though the presence of small amounts of organic materials as impurity in the layer mineral(s) or by deliberate addition (for example an organic surfactant used in production of the foam as described hereinafter) is not excluded. Moreover, by the term rigid inorganic foam product as applied to the products comprising an assembly of prills, we do not exclude the presence of a small amount, e.g up to 20% of an organic material present in the prills or added deliberately for example as a binder for uniting the prills into a self25 supporting structure.

The foam product of the present invention is made by a process involving incorporating a gas in a suspension (or dispersion) of a layer mineral in a liquid medium and a further feature of the invention resides in a process for the production of a rigid inorganic foam of cellular structure comprising one or more layer minerals which involves gasification of a suspension of one or more layer minerals in a liquid medium containing a surface active agent to form a stable wet foam or froth and removal of at least part of the liquid medium from the froth.

By the term stable wet foam or froth we mean a gasified suspension which does not collapse upon standing or upon removal of liquid from it, and in particular which upon standing does not collapse (no substantial reduction in foam-height) within a period of 10 minutes. As is discussed in more detail hereinafter, the stability of the gasified suspension is dependent mainly upon the particular surface active agent used to form it and we have found that whilst some surface active agents, for example fatty amines and saponin, enable a froth to be produced the resulting froth is not stable and collapses within a few minutes; production of such an unstable gasified suspension is not included within the scope of the present invention.

As stated, the rigid foams are made by a process involving gasification of a suspension of one or more layer minerals and removal of the liquid medium from the resulting froth. The foam can be produced in the form of prills by dividing the gasified suspension or froth into droplets or wet particles before removal of the liquid medium. Division of the gasified suspension or froth into particles or wet droplets may be effected in a variety of ways, for example by spraying the froth through a nozzle or other orifice, extrusion of the froth through orifices in a belt or any other known technique for dividing suspensions into droplet or particle form. The wet particles or droplets should be at least partially dried before they have the opportunity to re-combine. Dry or partially dry prills 3 ίί may be produced using a spray-drying apparatus. Partially dry prills may be further dried by heating them under conditions whereby they are prevented from combining, for example in single layers or agitated beds such as a fluid bed. Prills may also be formed by shaping the froth into fibre-like lengths, drying it, and chopping the dry or partially dried material.

The density of the rigid foams produced by the process of the invention may be varied in several different ways, for example by incorporating different amounts of gas into the suspension, by using blowing agents and by varying the solids content of the suspension. The solids content of the suspension affects the viscosity of the suspension, as also do the particular surface active agents used and the temperature at which gasification is effected, but in general increasing the solids content of the suspension results in an increase in the density of the foam produced from the suspension. Typically, the solids content of the suspension will be from 10% to 60% by weight of the suspension, preferably from 20% to 40% by weight. A deflocculating agent, for example sodium tripolyphosphate may be added to enable suspensions of high solids content to be produced.

The suspension of the layer mineral will usually be aqueous and in particular will be a suspension or dispersion of the layer mineral particles in water, preferably distilled or de-ion.ised water. Layer minerals are generally readily suspended or dispersed in water to form suspensions exhibiting colloidal properties.

The liquid medium of the suspension may if desired be a mixture of water and a water-miscible solvent such as an alcohol. If desired, however, the liquid may be an organic liquid. In converting the suspension into a froth and thereafter into a rigid foam, it is necessary to incorporate a surface active agent in the suspension and this agent will normally be added to the water prior to or during formation of the suspension. It is to be understood that in the case of the layer mineral vermiculite, delamination of the mineral may result in a surface active agent being incorporated in the delaminated material and a separate agent may not be required. In addition to a surface active agent, other agents such as fillers, compressive strength improvers, water-stability improvers and deflocculating agents may be incorporated in the suspension prior to, during or after production of the suspension.

Any surface active agent may be used which upon gasification of the suspension results in a wet foam or froth which is stable, by which is meant does not collapse upon standing for a period of at least 10 minutes or upon removal from it of the liquid medium. Anionic, non-ionic or cationic surface active agents may be used provided they result in a stable froth. The suitability of a surface active agent for use in the process is thus readily determined by simple experiment merely involving determination of whether the agent enables a wet foam or froth to be created from a suspension of say 30* solids content and if so whether the froth is stable. As a guide, a wet foam or froth which upon standing does not collapse (e.g. no substantial reduction in foam - height is observed) S0813 within a period of 10 minutes, preferably within a period of 1 hour, will in general be suitable for drying to yield a rigid foam according to the invention. For purposes of the test, the surface active agent under test may be used in any desired amount, or at various concentrations provided that it does not flocculate the foam; in general a large amount of the agent, for example 2% by weight of the solution, will provide an indication in an initial test as to whether the agent is worth further testing.

Surface active agents which can be used at low concentrations are preferred, though this is not critical. It has been observed that a surface active agent which provides a stable foam from a suspension of one layer mineral may not provide· a foam of comparable z stability from a suspension of another layer mineral or of mixed minerals such as a mixture of kaolinite and delaminated vermiculite. Equally, a surface active agent which fails to provide a stable foam from a suspension of one layer mineral may nevertheless provide a stable foam from a suspension of another layer mineral or of mixed minerals. For example the surface active agent n-butyl ammonium chloride does not yield a particularly stable foam from a suspension of kaolinite alone but does yield a stable foam from a suspension of delaminated vermiculite or of a 50:50 mixture by weight of kaolinite and delaminated vermiculite. This must be borne in mind when testing the suitability of a surface active agent, i.e. the test should preferably be performed using the actual suspension it is desired to gasify and later dry to a rigid foam.

In respect of the surface active agent it has been observed also that those surface active agents which most readily produce a foam or froth do not necessarily produce the most stable froth. In fact we have found that, in general, surface active agents which produce a froth only with some difficulty (e.g. after prolonged whisking of the suspension) tend to produce the more stable froths. The ease with which a surface active agent enables a froth to be created is not, however, conclusive evidence as to the suitability of that agent for use in the process of the present invention and it is to be understood that the invention is not limited to agents of low foaming characteristics.

The amount used of the surface active agent may vary within wide limits, depending for example upon the solids content of the suspension, the particular layer mineral and surface active agent, the particular gasification technique employed and the temperature of gasification. As a guide the amount of surface active agent will typically be from 0.1% to 5% by weight based on the weight of the layer mineral in the suspension to be gasified. Since the surface active agent remains in the rigid foam upon removal of liquid from the froth and its presence in the rigid foam is undesirable, we prefer to use the minimum possible amount of surface active agent consistent with production of a stable froth which does not collapse upon removal of liquid from it.

Gasification (or frothing) of the suspension may be effected in a variety of ways, for example by release of gas or vapour in the suspension or by mechanically entraining a gas in the suspension by rapid agitation of the suspension. The gas will normally be one which is inert to the suspension for example air, nitrogen, carbon dioxide, a hydrocarbon or a chlorofluorocarbon. Mechanical entrainment of the gas in the suspension can be achieved, for example, by rapid churning, beating or whisking of the suspension.

Release of gas or vapour in the suspension can be achieved by heating the suspension, preferably rapidly, to release bubbles of the gasified liquid medium (steam where the liquid medium is aqueous) or to release bubbles of the vapour of a substance (blowing agent) deliberately incorporated in the suspension as a source of vapour for gasification of the foam. The blowing agent may be, for example, a hydrocarbon, a chlorocarbon, a fluorocarbon, a chlorofluorocarbon or a source of carbon dioxide. The suspension can be gasified by subjecting it to electromagnetic radiation having a frequency in the range of from lO^Hz to 1O12Hz.

Production of the suspension, and gasification of the suspension in the cases where gasification does not involve a heating step, conveniently can be carried out at room temperature, though higher or lower temperatures may be employed if desired.

The removal of the liquid medium from the gasified suspension in the formation of the prills will normally be mainly by evaporation usually induced by heating the shaped gasified suspension. The rate of removal of liquid from the froth can be controlled, for example by controlling the temperature of the froth or by use of a drying vessel having provision for humidity control, such that rapid drying of the foam is avoided. If desired the wet foam, after shaping, may be allowed to stand at room temperature for a prolonged period, for example several days, to allow the foam to dry out and attain a rigid structure. Normally, however, the froth will be heated, after shaping, to remove the liquid medium. Control of the drying conditions is in general not important in the production of prills and rapid drying is possible for example at temperatures above 90*C and up to 2OO’C or even higher.

Rigid inorganic foam prills comprising layer minerals tend to be soft and of low compressive strength. Depending upon the particular layer mineral, the strength of the foams can be improved by incorporating therein a compressive strength improver and/or by heating the dry foam to sinter it. Incorporation in mixed-mineral foams, e.g. by incorporation in the suspension prior to gasification, of vermiculite lamellae (delaminated vermiculite) results in general in an increase in the compressive strength of the foam. Except in the case of wholly vermiculite foams, strong foams are obtained by sintering the dry, rigid foam prills obtained by drying the shaped gasified suspension, for example by heating the dry foam prills at a temperature of up to 1000‘C or even higher. Sintering of the foam may result in densification of the foam but the sintered foam retains a cellular structure and remains a lightweight material. Sintering of mixed vermiculite/other mineral foams may result in an increase or a decrease or very little change in the density of the foams, depending upon the proportion of vermiculite in the foam and the S0813 weight of material lost upon heating the foam at sintering temperatures.

Sintered layer mineral foam prills are included within the scope of the present invention, as is their process of production.

Unsintered rigid foams comprising layer minerals exhibit little resistance to degradation by liquid water and we prefer to subject the foams to a treatment to improve their water stability. For instance, the foams can be water-proofed by incorporating a silicone polymer precursor therein and subsequently creating within the foam acidic conditions under which polymerisation of the precursor occurs with the formation of a silicone polymer in the foam. For example, sodium methyl siliconate can be incorporated in an aqueous suspension of kaolinite prior to or during gasification of the suspension and the resulting foam whilst still wet can be treated with an acidic gas such as carbon dioxide gas to create the acid conditions necessary for polymerisation of the siliconate to yield a silicone polymer. Instead of treating the foam with an acidic gas during the drying of the rigid foam in the foam-making process, the foam may be fully dried and then subsequently wetted out with water to the desired extent. If desired, instead of a deliberate treatment with an acidic gas, the wet foam may be allowed to stand in air for a prolonged period whereupon carbon dioxide in the air will be absorbed to provide the necessary acidic conditions in the foam. Sintered foams, where a silicone polymer incorporated before sintering would be destroyed, can be water-proofed with a silicone polymer after sintering .

The relative proportions of the layer minerals in suspensions of mixed minerals, and hence in the resulting rigid foams, may vary within wide limits, depending for example upon the compressive strength and thermal insulation properties required in the rigid foam. The foams may comprise, for example, kaolinite or a kaolin-containing clay and vermiculite in the relative proportions of from 90:10 to 10:90 by weight. In general, increasing the relative proportion of vermiculite lamallae in the rigid foam results in an increase in the compressive strength of the rigid foam but also an increase in the thermal insulation coefficient (K-value) of the rigid foam.

Rigid foams of cellular structure comprising lamallae of vermiculite and their production by gasification of a suspension of vermiculite lamellae to form a froth, extrusion of the froth and removal of the liquid medium from the froth are described in United States Patent Specification No 4130687 in which the production of suspensions of vermiculite lamellae is also described.

The rigid mixed-mineral foams of the present invention containing delaminated vermiculite are conveniently derived from a suspension of vermiculite lamallae by incorporating another layer mineral in the suspension prior to gasification thereof- As is described in United States Patent Specification No. 130 687, the suspension of vermiculite lamellae will usually contain a surface active agent such as n-butyl ammonium chloride used in production of the suspension so that incorporation of another layer mineral in the suspension provides both the suspension and the surface active agent necessary for the production of rigid foams as described herein.

Preferably foams containing vermiculite contain an agent for improving the compressive strength and water stability of the foams. The enhancement of the compressive strength and water stability of vermiculite foams by incorporation of a compressive strength improver which is a solid particulate material having a basic reaction in water is described in European Patent No. 9310. As is described in the European Patent, the preferred compressive strength and water stability improver is particulate magnesium oxide, and we prefer to incorporate particulate magnesium oxide in the vermiculite or mixed-mineral (containing vermiculite) foams of the present invention. As described hereinbefore, the compressive strength of the mixed mineral foams can alternatively be enhanced by sintering the foam.

The suspensions used to make the foams can be dried as also can similar suspensions free from surface active agents, for example by spray-drying, to yield a corresponding free-flowing, dry-powder material. Such materials are readily re-dispersed in a liquid medium, particularly water, to form a suspension suitable for conversion into a rigid foam by the process described herein. In the case of dry-powder materials containing magnesium oxide, it is preferred to dry a suspension free from magnesium oxide and to add dry magnesium oxide to the dried suspension. It will be appreciated that the dry powders may also be produced by mixing dry-powder comprising vermiculite lamellae with another powdered layer mineral (optionally mixed with sodium tripolyphosphate) and optionally with dry, powdered magnesium oxide.

The rigid foam products provided by the invention, whether made wholly of one layer mineral or comprising mixed minerals, for example kaolinite and vermiculite, are heat-resistant and heat-insulating materials which are useful in a wide variety of fireprotection and thermal insulation applications. The products may be produced as slab stock or board stock for use in subsequent fabrication processes, for example for formation of laminates with sheets of a wide variety of materials such as wood, veneers, asbestos, mica, plastics, vermiculite board (foamed or made from heat-exfoliated vermiculite granules), glass fibre scrim impregnated with vermiculite and polymers. Such laminates from useful decorative construction panels for the building industry. Slab stock may be used directly, without lamination to another material, for example for cladding wood, cement or steel construction elements to provide a fire-protection barrier and heat-insulation layer around the elements, and as roofing boards, lining boards and ceiling tiles.

The rigid foams may be subjected to high temperatures, for example up to 1OOOC for prolonged periods without disintegration although prolonged exposure to high temperatures results in embrittlement of the products. Press-moulding of the surface of the rigid foams after or during drying thereof produces a smooth surface which may be sculptured for decorative effect if desired.

The rigid foams, if desired in the form of a laminate with another material, may be used in firedoors or fire-barrier partitions. In the form of prills they may be used as loose-filling for cavities, voids and the like.

The prills of foam may be glued together to form the desired products. A variety of inorganic and organic (but preferably inorganic) adhesives may be used to glue the prills together to make slab stock, to form laminates from the slab stock or to apply slab stock as a coating or cladding to substrates such as wood, cement and steel construction elements. Slab stock of thickness up to 10 cm or more can be produced by cementing together prills of dry foam. Examples of inorganic binders which may be used are phosphoric acid, aqueous solutions of phosphates and silicates, cements and plasters. Examples of organic binders which may be used are aqueous emulsions of vinyl and vinylidene polymers and copolymers.

Prills of the foams, especially those comprising wholly or partially delaminated vermiculite (vermiculite lamellae) can be dry-pressed into products having structural integrity without the need to employ an adhesive or binder. Preferably products obtained by dry-pressing prills are in the form of laminates wherein the prill product is faced with or sandwiched between layers, for example of paper or sheet. Whilst products may be formed by dry-pressing the prills we prefer to moisten or dampen the prills before pressing them into products.

The wet foam (i.e. the gasified suspension), and the suspension prior to gasification may be used as a binder in cementing together rigid foam prills.

For use as an adhesive, we prefer that the wet foam or suspension should comprise or contain vermiculite lamellae and preferably an appreciable proportion of vermiculite lamellale, for example at least 50% by weight of the total solids content of the wet foam or suspension.

According to one embodiment of the invention for making prill products, there is provided a process for the production of shaped articles from prills of rigid inorganic foam comprising one or more layer minerals, which process comprises applying a solution containing phosphate ions or silicate ions to the prills and drying the resulting wet prills whilst they are retained in the desired shape.

The solution containing phosphate ions may be a phosphoric acid or a solution of a phosphate salt. Organic and inorganic phosphates may be used, including complex phosphates, although since the shaped article produced by the process is desirably wholly or at least essentially inorganic we prefer to use inorganic phosphates or a phosphoric acid. The preferred solution containing phosphate ions is orthophosphoric acid. Sodium silicate solution is the preferred solution containing silicate ions.

In carrying out this process prills of foam are assembled into the shape of .the desired product, for example a slab or board, and whilst in this shape are dried in the presence of a solution containing phosphate or silicate ions such.that after drying the prills are glued together and the shaped article formed from them has structural integrity. The solution may be applied to the individual prills before the latter are assembled into the desired shape or the solution may be applied to the assembled prills whilst they are retained in the desired configuration to produce a shaped article. Alternatively the solution may be applied to the prills, before or after assembly of the prills into the desired shape, by creating on the prills a dry coating of a phosphoric acid, phosphate salt or silicate and subsequently wetting the coated prills to create the solution containing phosphate or silicate ions.

It is preferred to apply the solution to the individual prills prior to assembling the prills into the desired shape. It is especially preferred to produce prills having thereon a dry coating of a phosphoric acid, phosphate salt or silicate and which are readily stored and transported, and which simply require wetting by a fabricator of shaped articles before or after assembly of the prills into the desired shaped articles. It is usually more convenient in practice to wet the individual prills prior to assembly into desired shapes than to wet the prills after assembly, and it is also easier in this way to control the amount of solution applied to the individual prills and to ensure a uniform concentration of the solution throughout an assembly of the prills.

Prills of a layer mineral foam having thereon a dry coating of a phosphoric acid, a phosphate salt or a silicate are provided according to a preferred feature of the invention. Such coated prills are readily prepared by applying a phosphoric acid or a solution of a phosphate salt or silicate to the prills and drying them by evaporation of the liquid medium from the coating under conditions whereby gluing together of the prills is avoided; re-wetting of the dry, coated prills recreates the solution containing phosphate or silicate ions on the prills. Drying of the prills may be, for example, by fluid-bed drying.

The amount of the solution containing phosphate or silicate ions and the concentration of ions in the solution applied to the prills may vary within wide limits but affect the physical properties of shaped articles formed from the prills. In particular the amount of solution applied and the concentration of the solution affects the density of the shaped products; in general increasing the amount of a particular solution applied to the prills will result in an increase in the density of shaped products made from the prills and likewise increasing the concentration of ions in the solution will result in an increase in the density of shaped products made from the prills.

Another physical property of the products which may be affected by the amount and concentration of the solution applied to the prills, at least in the case of phosphate solutions, is the strength of the products.

We have observed that as the amount of phosphate ions (at least the amount in the surface region of the prills) increases there exists a peak in the strength of the product formed from the prills and that increasing the amount beyond that which provides the peak, either by increasing the amount of solution applied or the concentration of the solution, tends to result in a decrease in the strength of the products.

The amount of solution applied to the prills may depend to some extent upon the method by which the solution is applied, but for a particular application technique the optimum combination of amount of solution applied and solution concentration is readily determined by simple trial and experiment. Application of the solution to the prills may be by any convenient technique, for example immersion, brushing or roller coating, but by far the preferred application technique is spraying. Spraying has the advantage of most readily enabling control to be exercised over the amount of solution applied to the prills and in particular enables the prills to be surface coated with minimum impregnation of the prill structure by the solution.

Layer mineral foam prills are generally highly porous structures which readily absorb liquids and unless steps are taken to avoid it any solution applied to the prills will rapidly penetrate the structure to the interior of the prills. This is undesirable in the present invention from the standpoints of the density of products made from the prills and the thermal conductivity properties of such products. A spraying technique operated in a controlled manner to apply the minimum amount of solution necessary to surface-coat the prills is therefore preferred.

In addition to applying the minimum amount of solution required to surface-coat the prills, we prefer to employ fairly dilute solutions of the phosphoric acid or phosphate or silicate salt so as to again restrict the amount of phosphoric acid, phosphate or silicate applied to the prills. As a guide we prefer to employ solutions of concentration from 5% to 20% by weight, especially solutions of concentration from 7% to 15% by weight. In the embodiment of the invention in which dry, coated prills are wetted out with water for formation into products, the amount of water added will normally be about 60-70% by weight of the prills.

The wet prills, either as individual prills or assembled into shaped products, may be allowed to dry at ambient (room) temperature but usually will be heated to increase the rate of drying. The temperature employed is not critical and may be up to several hundred *C if desired. In general the wet prills will be heated at about the boiling point of the liquid to be removed when drying shaped products made of the wet prills, for example at about 3O*C to 110‘C where the liquid is water, but for drying individual prills there may be an advantage in employing higher temperatures, for example up to 60 The products may be produced by gasifying a suspension of vermiculite lamellae to form a froth, shaping the froth, and removing the liquid from the froth under conditions such that the rigid foam obtained is in the form of prills or a product form which can be converted to prills. An example of a suitable product form for conversion into prills by chopping e.g. using a gas jet is a fibre-like extrusion of froth which may be chopped into prills before drying or dried and then chopped into prills. A wide variety of direct prill-forming techniques may be employed, for example spray-drying and belt-extrusion in which the froth is caused to pass through holes in a belt to form droplets which are dislodged after drying.

The chemical delamination of vermiculite to produce suspensions, usually aqueous suspensions, of vermiculite lamellae suitable for conversion into rigid vermiculite-foam prills is known; delamination processes are described for example in British Patent Specifications Nos 1,016,385; 1,076,786; and 1,119,305; and by Baumeister and Hahn in Micron 7 247 (1976). Prills of foam made from the suspensions produced by any of the known processes may be employed in the present invention.

For use in the preparation of foam prills of or containing vermiculite it is preferred to employ suspensions of vermiculite lamellae which have been wet-classified to remove substantially all particles of size greater than 50 microns, preferably 20 microns and which contain a high proportion, for example 40% to 60% by weight, of lamellae of size below 5 microns.

The strength of the shaped products of the invention and particularly their flexural strength can be improved by laminating the layer of bonded prills with a surface layer of a flexible sheet material such as paper (e.g. Kraft paper or vermiculite paper), glass fibre scrim impregnated with vermiculite or metal strip or foil. Such a facing layer or facing layers may be applied by conventional laminating techniques to pre-formed foam articles but for convenience the facing layer(s) is applied during production of the foam article. Thus, for example, slabs or boards can be produced by laying wet phosphate or silicate-coated prills down between layers of a facing material, slightly compressing the assembly, and drying the prills to form a laminate of foam core with integral facing layers.

Slab stock (or board stock) comprising rigid foam prills bound in a cellular matrix are another feature of the present invention, including the following materials: (i) Prills of foam made of one layer mineral in a cellular matrix comprising the same or another layer mineral, (ii) Mixtures of foam prills of different layer minerals in a cellular matrix comprising one or more layer minerals, (iii) Prills of kaolinite foam in a cellular matrix comprising vermiculite, (iv) Prills of vermiculite foam in a cellular matrix comprising vermiculite or kaolinite, or both (v) Prills of kaolinite foam in a cellular matrix comprising kaolinite, and (vi, Prills of kaolinite/vermiculite foam in a cellular matrix comprising vermiculite or kaolinite or a mixture of kaolinite and vermiculite.

Production of products having prills of foam embedded in a cellular matrix involves incorporating pre-formed prills into a gasified suspension or froth of a layer mineral and drying the resulting prillfilled froth. The prills can be incorporated in the froth by gently stirring them into the froth and the filled froth may be shaped e.g. by extrusion into the desired product form and dried. Alternatively the prills may be assembled into the desired shape in, say, a mould, and the froth may be forced into the assembled prills by the application of pressure or drawn into the prills by the application of suction, e.g. in a vacuumforming technique. The pressing of prills into a preformed layer of froth to form boards or slab stock is also possible, though in general such a technique tends to result in collapse of the prills and/or the froth, leading to a denser product than is obtainable by other techniques. The amount of froth used may vary within wide limits but will usually be just sufficient to completely fill the voids between the packed prills, for example about an equal weight of froth after drying based on the weight of the prills.

The invention is illustrated by the following Examples .

EXAMPLES 1 to 3 In these examples the vermiculite prills employed were made by the following general procedure: Prill-Formation: An aqueous suspension of vermiculite lamellae obtained by swelling vermiculite using consecutive treatments with refluxing salt solution, refluxing n-butylammonium chloride solution and water was milled and wet-classified by removal of all particles greater than 50 microns. The suspension was gasified by heating in an Oakes Mixer or Kenwood Food Mixer (Oakes' and. 'Kenwood' are trademarks) to form a froth and magnesium oxide powder (10% by weight based on the vermiculite) was incorporated during the gasification operation.

The wet froth was cast immediately onto a perforated 'Melinex' belt, ('Melinex' is a trademark) the froth passing through the holes in the belt and forming 'beads' on the underside of the belt. The beads were allowed to cure and partially dry for a few minutes before being dislodged from the belt by scraping. The beads were then oven-dried on trays to provide prills of dry, rigid foam for fabrication into products. By varying the concentration of vermiculite in the suspension employed, prills of foam of various density were obtained. The prills were of roughly cylindrical shape and of average dimensions 2-3 mm diameter and 3-5mm length. They had a uniform cellular structure.

EXAMPLE 1 Vermiculite foam prills (20 g) of density 112 kg/m3 were stirred carefully with an aqueous solution (66.5 g) of concentrated phosphoric acid (5 g) in deionised water. The thus-moistened prills were spread on a flat drying tray and oven-dried at 60'C for 16 hours. Any agglomerates of the prills were broken up by hand and any fine dust was removed by sieving.

Dry, phosphate-coated prills (8 g) were mixed thoroughly with deionised water (16 g) and the moistened prills were lightly compacted into two cylindrical tubes lined with ’Melinex’ plastic of diameter 4.35 cm and height 2.0 cm using a knife spatula. The flat top and bottom surfaces of the prill assembly were skimmed with the spatula to produce a smooth finish and the tubes were heated in an oven at 150’C for 4 hours.

The tubes were removed from the oven and the foam cylinders were removed from the tubes and immediately their compressive strengths (10% compression)· were determined using a Houndsfield Tensoroeter. The article had a 20% by weight loading of phosphate binding agent and its density (mean of the two samples) was 206 kg/m2 and compressive strength 274.8 KN/m2.

EXAMPLE 2 Articles were made as described in Example 1 except that they had a 10% by weight loading of the phosphate binding agent instead of a 20% loading; this 10% loading was achieved by mixing 20 g of the foam prills with a solution (62.5 g) of concentrated orthophosphoric acid (2.22 g) in deionised water.

The article had a density (mean of the two samples) of 154 kg/m2 and a compressive strength of 126.4 KN/m2.

EXAMPLE 3 Vermiculite foam prills (50 g) of density 104 kg/m2 were coated with orthophosphoric acid using a laboratory-scale fluid bed drier, (Model ’FBD/L72 by PR Engineering Ltd). To coat the prills they were placed in a fluidised cylindrical bed of height 30 cm and diameter 13 cm held in the head of the fluid bed drier and were heated at 140°C. 2.5 m orthophosphoric acid (12 ml) was sprayed onto the fluidised prills using a Delavan” air atomizing syphon nozzle (model 30610-1) at a delivery rate of 0.22 cm2/second.

A prill loading of 2.5% by weight orthophosphoric acid was thus achieved.

The dry, coated prills (8 g) were formed into cylindrical articles and tested as is described in Example 1. The finished articles had a density (average for the two samples) of 127 Kg/m2 and a compressive strength of 386 KN/m2.

EXAMPLES 4-6 Dry, rigid ball clay foams were produced by the method generally described in Examples 1-3 from the following ball clay suspensions.

Example Ball Clay(g) Water (cm3) Forafac (g) Beating time (mins) .13 Hymod”/AT(10O) 200 2 20 14 BSK/L1371) 200 2.4 20 15 Hycast/VC (100) 200 2 20 Hymod and Hycast are trademarks. Forafac is a trademark.

HYMOD/AT is a ball clay of Dorset origin available from English China Clay.

HYCAST’VC is a ball clay of Devon origin available from English China Clay.

BSK/L is a ball clay of North Devon origin available from Watson Blake.

Deionised water and Forafac 1157 were employed.

The wet foams were converted into dry, rigid-foam prills by the belt-extrusion technique described for making vermiculite foam prills in Examples 1-3. In each case, handleable prills of cellular structure were obtained.

In a still further series of experiments, the wet foams were made into prills of cellular structure in a spray-drying apparatus.

The prills from all six experiments were sintered at 1O5O’C and in each case the cellular structure of the prill was retained.

EXAMPLE 7 7.04 Kg of ball clay EWVA, 12.9 Kg of de-ionised water and 169 ml of Forafac 1157 (0.6% on the clay) were mixed to form a slurry of 35% solids content. The slurry was whisked in a Kenwood Chef Food mixer for 20 minutes and the resulting stable wet foam was converted into prills of foam in a conventional spray-drying apparatus. The resulting prills were sintered at 1150*C for 5 minutes. The wet foam had a density of 256 Kg/m3 and the sintered prills had a density of 150 Kg/m3. EXAMPLE 8 Sepiolite (38 g) was mixed with de-ionised water (162 g) and Forafac 1157 (0.06 g - 0.4% on the clay) in a Kenwood Chef Food Mixer for 15 minutes to form a stable wet foam. The wet foam, of density 195 Kg/m3, 50813' was converted into prills of cellular stucture by the belt-extrusion technique described in Examples 1-3. The prills were sintered at 1050*C for 5 minutes and the sintered prills had a density of 58 Kg/m3.

Example 9 18.9% delaminated vermiculite slurry in deionised water (116 g) was mixed with de-ionised water (78 g) and sodium tripolyphosphate (0.5 g). Light Grade Kaolin clay, ex BDH (67 g) was added to the mixture which was then whisked in a Kenwood Chef mixed for about 15 minutes. Light Grade magnesium oxide powder, ex DBH (3.7 g) was added and the mixture was whisked to disperse the powder. The wet foam was made into prills by the belt-extrusion method of Examples 1-3, and the prills were sintered at 1500°C for 10 minutes. The density of the sintered prills was 238 Kg/m3.

Example 10 Montmorillonite (50 g) was dispersed in deionised water (338 g) and 18.3% delaminated vermiculite slurry (137 g) was added to the dispersion, followed by Forafac 1157 (3 g). The mixture was whisked in a Kenwood Chef Mixer for 1 hour to produce a stable wet foam. The wet foam was converted into dry prills of cellular structure by the belt-extrusion method of Examples 1-3. The prills, dried at 90°C, had a density of 108 Kg/m3.

Example 11 Sodium Montmorillonite (50 g - Wyoming bentonite) and Kaolin clay (50 g) were mixed with de30 ionised water (450 g) in a Kenwood Chef Mixer until the montmorillonite had been thoroughly dispersed.

Forafac 1157 (6 g) was added and the mixture was whisked for 1 hour at maximum speed to produce a stable wet foam. Prills of dry foam made from the wet foam by belt-extrusion as in Examples 1-3 and then sintered at 1050*C for 10 minutes had a density of 118 Kg/m3.

Example 12 Sodium MontmorilIonite (Wyoming bentonite - 50 g) and de-ionised water (450 g) were stirred in a Kenwood Chef Mixer until the montmorillonite was thoroughly dispersed. Forafac 1157 (6 g) was added and the mixture was beaten with a whisk attachment at maximum speed setting for about 1 hour to produce a stable wet foam. Prills were made from the wet foam by the beltextrusion method of Examples 1-3 and sintered at 1000‘C for 10 minutes. The sintered prills had a density of 110 Kg/m3.

Example 13 A board measuring 15 cm x 15 cm x 2.5 cm was made from approximately spherical, dry prills of delaminated vermiculite foam of diameter 3 mm prepared as described in Examples 1-3. The prills had a true density of 65 Kg/m3 and a packing density of 45 Kg/m3. The dry prills ( 22 g) were placed in a steel mould of size 15 cm x 15 cm x 2.5 cm and a force of 15 Kg/m3 was applied using a steel plate to compress the assembly of prills. Pressure was applied until the volume of the assembly of prills was reduced by about one half. The product was -a board of essentially cellular structure and had a density of 90 Kg/m3, a flexural strength of 30 KN/m2 and a compressive strength of 100 KN/m2. The board was pushed from the mould and its two major faces were coated with a 35% aqueous solution of sodium silicate. Glass fibre scrim of weight 50 g/m3 was pressed onto the coated surfaces and the resulting laminate was dried in an oven for 2 hours. The laminate had a density of 95 Kg/m3, 50Θ13 a flexural strength of 300 KN/m2 and a compressive strength of 150 KN/m2. The thermal conductivity of the laminate measured according to BS 874 was 0.056.

Example 14 A board measuring 15 cm x 15 cm x 2.5 cm was prepared as described in Example 13 except that the prills were moistened with de-ionised water (44 g) before being placed in the mould. The resulting board had an essentially cellular structure and its density was 90 Kg/m3. The flexural strength of the board was KN/m2 and its compressive strength was 110 KN/m2. The board was laminated with glass fibre scrim as described in Example 13 and the dry laminate had a density of 95 Kg/m3, a flexural strength of 400 KN/m2 and a compressive strength of 130 KN/m2.

Example 15 A composite board containing dry prills made from vermiculite foam and a matrix of vermiculite foam was made as follows.

Vermiculite foam was generated by beating a 20 wt % suspension of delaminated vermiculite. 250 g of suspension was beaten for 5 minutes using a Kenwood mixer at 60 revolutions/minute. The resulting foam was then mixed with 50 g of prills made from foamed vermiculite and stirred with a large mechanical stirrer at 5 revolutions/minute. The vermiculite prills had a density of 70 Kg/m3, a compressive strength of 100 KN/m2 and a diameter of 3 mm.

The resulting mixture was smoothed into a metal tray measuring 25 cm x 25 cm x 2.5 cm, dried at room temperature for 24 hours and then at 50°C for 2 days.

The resulting block had a compressive strength of 200 KN/m2, a flexural strength of 350 KN/M2 and a density of 95 Kg/m3. Thermal conductivity of the board was 0.059 W/mk at 20*C.

Example 16 A composite board containing dry prills made 5 from foamed Kaolin and a matrix of vermiculite foam was made as follows.

Vermiculite foam was generated as described in Example 15. The resulting foam was then mixed with 50 g of prills made of foamed kaolin. The prills had a density of 65 Kg/m3, a compressive strength of 90 KN/m2 and a diameter of 3 mm.

The resulting block had a compressive strength of 110KN/m3, a flexural strength of 250 KN/m2 and a density of 85 Kg/m3. The thermal conductivity of the block was 0.045 W/mk at 20*C.

S08 1 3

Claims (5)

1. A rigid inorganic foam product (as hereinbefore defined) comprising prills (as hereinbefore defined) of foam of one or more layer minerals (as hereinbefore 5 defined), each prill being of cellular structure.

2. A rigid inorganic foam product as claimed in Claim 1 having a density of less than 0.4 g/ml.

3. A rigid foam product as claimed in Claim 2 having a density of less than 0.2 g/ml. XO

4. A foam product as claimed in Claim 1, 2 or 3 wherein the prills of foam are made of delaminated vermiculite . 5. A foam product as claimed in Claim 1, 2 or 3 wherein the prills of foam are made of kaolinite or a 15 kaolin-containing clay. 6. A foam product as claimed in Claim 1, 2 or 3 wherein the prills of foam are made of montmorillonite. 7. A foam product as claimed in Claim 1, 2 or 3 2o wherein the prills of foam are made of sepiolite. 8. A foam product as claimed in any one of the preceding claims comprising a mixture of prills of foam of different layer minerals. 9. A foam product as claimed in any one of the 25 preceding claims wherein the prills of foam are glued together by means of an adhesive. 10. A foam product as claimed in Claim 9 wherein the adhesive is phosphoric acid or a phosphate. 11. A foam product as claimed in Claim 9 wherain the 30 adhesive is sodium silicate. 12. A foam product as claimed in claim 9 wherein the adhesive is an organic binder. I 50813 13. A foam product as claimed in any one of Claims 1 to 9 wherein the prills of foam are embedded in a cellular matrix comprising one or more layer minerals (as hereinbefore defined). 14. A foam product as claimed in Claim 13 wherein the cellular matrix comprises delaminated vermiculite. 15. A rigid inorganic foam as claimed in any one of the preceding claims which is in the form of a laminate with one or more layers of a non-foamed material. 16. A process for the production of a rigid inorganic foam product (as hereinbefore defined) which comprises gasification of a suspension of one or more layer minerals (as hereinbefore defined) in a liquid medium containing a surface active agent to form a stable wet foam or froth (as hereinbefore defined), division of the wet foam or froth into droplets or fibre-like extrusions, removal of at least part of the liquid medium from the droplets or extrusions and chopping the extrusions to form prills of foam and forming of the prills into a foam product. 17. A process for the production of a rigid inorganic foam in the form of prills (as hereinbefore defined) which comprises gasification of a suspension of one or more layer minerals (as hereinbefore defined) in a liquid medium to form a stable wet foam or froth (as hereinbefore defined), shaping of the wet foam or froth into droplets or fibre-like extrusions and removal of at least part of the liquid medium from the droplets or extrusions followed by chopping of the extrusions. 18. A process as claimed in Claim 16 or 17 wherein the suspension comprises delaminated vermiculite and a compressive strength and water stability improver for the resulting foam is incorporated in the suspension. 5 0 8 13 19. A process as claimed in Claim 18 wherein the compressive strength and water stability improver is particulate magnesium oxide. 20. A process as claimed in Claim 16 or 17 wherein 5 the suspension comprises a layer mineral other than delaminated vermiculite and the resulting rigid inorganic foam prills or products are sintered to improve their compressive strength and water stability . 10 21. A process as claimed in Claim 20 wherein the foam is heated at a temperature of up to 1200C to sinter it. 22. Rigid inorganic foam prills (as hereinbefore defined) having a cellular structure and comprising one 15 or more layer minerals (as hereinbefore defined). 23. Rigid inorganic foam prills as claimed in Claim 22 comprising delaminated vermiculite. 24. Rigid inorganic foam prills as claimed in Claim 22 comprising kaolinite or kaolin-containing 2Θ cla V25. Rigid inorganic foam prills as claimed in Claim 22 comprising montmorillonite. 26. Rigid inorganic foam prills as claimed in Claim 22 comprising sepiolite. 25 27. Rigid inorganic foam prills as claimed in Claim 22 comprising delaminated vermiculite and kaolinite. 28. Rigid inorganic foam prills as claimed in any one of claims 22 to 27 coated with a binding agent or 30 adhesive. 29. Rigid inorganic foam prills as claimed in any one of Claims 22 to 28 having a density of less than 0.4 g/ml. 30. Rigid inorganic prills as claimed in Claim 29 having a density of less than 0.2 g/ml. 31. Use of a rigid inorganic foam product or prills as claimed in any one of Claims 1 to 15 and 22 to 30 as an insulating material. 32. Use of a rigid inorganic foam product or prills as claimed in any one of claims 1 to 15 and 22 to 30 as a fire-protection material. 33. A process for the production of a rigid inorganic foam product (as hereinbefore defined) which comprises assembling prills (as hereinbefore defined) of one or more layer minerals (as hereinbefore defined) and having a cellular structure into a desired product shape and consolidating the assembly of prills to form the foam product. 34. A process as claimed in claim 33 wherein consolidation of the assembly of prills is by applying pressure to the assembly. 35. A process as claimed in claim 34 wherein the prills are moistened prior to applying pressure to the assembly. 36. A process as claimed in claim 34 or 35 wherein the prills are moistened prior to assembly into a desired product shape. 37. A process as claimed in claim 33 wherein consolidation of the assembly of prills is achieved by means of a binding agent. 38. A process as claimed in claim 37 wherein the binding agent is an inorganic binder. 39. A process as claimed in claim 37 wherein the binding agent is an organic binder. 40. A process for the production of a rigid inorganic foam product (as hereinbefore defined) which comprises incorporating prills (as hereinbefore defined) of one or more layer minerals (as hereinbefore

5. Defined) and having a cellular structure into a gasified suspension of one or more layer minerals in a liquid medium and removing at least part of the liquid medium from the gasified suspension.