GB2237282A

GB2237282A - Moulding elastomers using heat-destructible polystyrene core

Info

Publication number: GB2237282A
Application number: GB8924046A
Authority: GB
Inventors: Antoon Peeters
Original assignee: Dow Corning SA
Current assignee: Dow Silicones Belgium SPRL
Priority date: 1989-10-25
Filing date: 1989-10-25
Publication date: 1991-05-01
Also published as: BE1003949A3; GB8924046D0

Abstract

The specification describes and claims a method of making an inflatable moulding element comprising a layer of elastomeric material suitable for use in a process in which the inflatable element is located within an outer mould, a curable resin is moulded between the outer mould and the inflatable element with the inflatable element containing fluid under pressure which may be subsequently released to permit recovery of the inflatable element. The method comprises the steps of (i) procuring a support comprising a mass of polystyrene foam; (ii) working the support to shape it into a foam mass of dimensions and configuration conforming to those required of the interior of the elastic inflatable element; (iii) applying to the surface of the support a coating of a silicone composition capable of curing to an elastomeric condition at a temperature less than 140 DEG C and preferably at room temperature; (iv) curing the composition to convert the coating into said layer of elastomeric material and (v) heating the support to a temperature sufficient to substantially reduce the volume of the polystyrene mass, e.g. 150 DEG C.

Description

MAKING INFLATABLE ELEMENTS This invention is concerned with making inflatable elements.

Inflatable elements of elastomeric materials are known and used for a variety of purposes. Inflatable elements in the form of bags are used in the manufacture of parts in which a curable resin is moulded between an outer mould and an inner mould provided by an inflatable bag which contains fluid under pressure, which fluid may be released after the moulding operation to enable de-moulding of the moulded part.

Various proposals have been made for the making of inflatable elements of elastomeric material. However, it has proved difficult to produce repeatedly by a simple manufacturing technique inflatable elements, e.g. bags which are intended to have, when inflated, a specific geometrical regular or irregular shape including corners, for example a cube.

We have now found that one may make bags of elastomeric material simply and easily by use of a technique which employs a polystyrene foam support and a curable elastomeric material coated onto the support.

The invention provides in one of its aspects a method of making an inflatable element comprising a layer of elastomeric material comprising the steps of (i) procuring a support comprising a mass of polystyrene foam, (ii) applying to the surface of the support a continuous coating of a composition capable of curing at a temperature of less than 140 C, (iii) curing the composition to convert the coating into said layer of elastomeric material, and (iv) heating the support to a temperature sufficient to reduce the volume of the polystyrene mass.

The polystyrene foam used in a method according to the invention is a rigid, brittle material which may be cut, carved, abraded or otherwise shaped into a desired configuration, for example using simple hand tools. Provision of sharp corners, undercuts and indents is comparatively simple with this material. The shaped polystyrene foam provides a sufficiently stiff and inflexible support for the layer of elastomeric material as it cures, and subsequently is caused to shrink or collapse by heating to an extent sufficient to collapse the foam e.g. by heating to a temperature of about 150 C to 180 C.

In a method according to the invention the composition may be applied as a solution, emulsion or solvent free material, for example by dipping spraying, trowelling or other convenient method. The composition may be applied in a single step or may be applied as several coatings one on top of another before or after curing of the previously applied coating. If desired a reinforcement layer of a fabric or other suitable material may be incorporated in the coating. In order to ensure that the completed element is of desired outer shape and dimensions, it is preferred that the composition employed does not flow significantly after application and prior to cure of the composition.

Accordingly it is preferred that the composition is at least somewhat thixotropic which is to say that it has a viscosity which, when measured using a Brookfield viscosimeter, shows a difference in viscosity of a factor from 3 to 5 when measured at shear rates differing by a factor of 2. Preferably the composition has a viscosity under pressure which is sufficiently low to enable ease of application (e.g. in the range 5,000 to 1nrn25-l 50,000 mm2s ) but which has a viscosity when not under pressure which is sufficiently high to retain the configuration of the layer until it is cured (e.g. in the range 10,000 to mm2s -l 400,000 mm2s ).The composition is applied to form a continuous coating, interrupted only by desired perforations for inflation or deflation purposes, and the coating is preferably of uniform thickness irrespective of the shape of the support.

In a method according to the invention, the composition may be selected from those capable of curing at a temperature of less than 1400C and is preferably a composition capable of curing at 700C or less, more preferably at room temperature.

Silicone compositions are especially preferred which contain an organosilicone compound capable of crosslinking with or without the presence of crosslinking agents. Such crosslinking may be performed at elevated or at ambient temperatures. Preferably the composition is a room temperature vulcanisable silicone compound capable of curing within a few minutes to provide the elastomeric layer.Suitable compositions typically comprise one or more organopolysiloxanes present in any fluid form, from a freely flowing liquid to a highly viscous substance, and comprises two or more units of the general formula

and units

wherein Q denotes a monovalent hydrocarbon or substituted hydrocarbon group having no more than 8 carbon atoms, Q' denotes a hydroxyl group or an organic group having olefinic unsaturation, e.g. a vinyl or allyl group, at least 80% of the remaining silicon-bonded substituents being methyl, a has a value of 1 or 2 and b has a value of 0, 1, 2 or 3. Those organopolysiloxanes having silicon-bonded hydroxyl groups may be crosslinked to elastomers by the addition of a crosslinking agent and a condensation catalyst. In such compounds the organopolysiloxane is generally a polydiorganosiloxane having terminal silanol groups.The crosslinking agent may be, for example, a silane having groups which are capable of condensation with silanol groups or of hydrolysis to provide groups capable of condensation with silanol groups, for example a polyalkoxysilane or an alkyl polysilicate e.g. methyl trimethoxysilane or ethyl polysilicate, or it may be an alkylhydrogen polysiloxane e.g. a polymethylhydrogensiloxane. A variety of catalysts may be employed the organic metal compounds e.g.

stannous octoate, dibutyltin dilaurate, alkyl titanates and titanium chelates being illustrative of these. Such elastomerforming compounds are well known in the art and have been described in for example British Patents Nos. 841 825, 844 128, 957 255 and 962 061. The crosslinking agent employed in those compositions which comprise an organopolysiloxane having at least two silicon-bonded groups having olefinic unsaturation, may be an organosilicon compound having at least two siliconbonded hydrogen atoms, any remaining silicon-bonded substituents being monovalent hydrocarbon groups having no more than 8 carbon atoms, preferably being methyl groups. In such compositions a catalyst is employed which is e.g. a platinum compound or complex which promotes the reaction between olefinic groups and silicon-bonded hydrogen groups.The platinum catalyst may be, for example, chloroplatinic acid, platinum acetylacetonate, complexes of platinous halides with unsaturated compounds such as ethylene, propylene, organovinylsiloxanes and styrene, hexamethyl-diplatinum and Pt(CN)3. Such compositions may be preferred because no by-products are produced during the crosslinking reaction. In addition little or no alteration in size occurs during crosslinking. This may permit a more accurate manufacture of the bags with respect to their shape and size.

Compositions of this type are also well known in the art (see for example British Patent Specifications Nos. 1 090 122, 1 141 868 and 1 409 223) and are commercially available.

The elastomer forming composition may also comprise other ingredients, for example fillers, extenders and colorants.

In a method according to the invention, the composition is cured to convert the coating to an elastomeric layer, for example under room conditions or by heating in an oven to a temperature less than that at which the polystyrene foam loses its size and shape. In a preferred method according to the invention the composition is cured by aging for a short time at room temperature and then further cured by placing the support and the partially cured layer of composition thereon in an oven heated to about 80 C. Subsequently the support and its cured layer of elastomeric material is heated, for example in an oven to an extent sufficient to ensure that the foam collapses or melts to an extent such that the foam does not interfere with the intended use of the element.

By use of a method according to the invention one may prepare inflatable elastomeric elements, for example bags of silicone elastomers, which elements comprise a layer of cured elastomeric material of desired thickness and outer shape. The process is simple to carry out and provides consistently reliable products.

In order that the invention may become more clear there follows a description which is to be read with the accompanying drawings of one example method which is illustrative of the invention.

In the drawings: Figure 1 is a view of a support (10); Figure 2 is a view of the support and a coating of a curable composition, and Figure 3 is a view of the element produced which is partially broken away to show residue of the support.

In carrying out the illustrative method a cuboid block (130mm x 170 mm x llOmm) of polystyrene of density 0.025 g/cc was cut from a slab of the polystyrene to provide the block (10). A layer (12) 2mm thick of a room temperature vulcanisable silicone composition was applied to the surface of the support (10). The composition comprised a mixture of 467 parts (all parts recited are by weight) of an cc,o dihydroxy polydimethylsiloxane having a viscosity of about 4,000 mm2s 1, 281 parts of an a,## dihydroxy polydimethylsiloxane having a viscosity of about 13,500 mm2s 1, 184 parts of fume silica, 63 parts of methyl trimethoxysilane and 5 parts of tetrabutyl titanate. The composition was spread over the surface with a trowel to provide a uniform coating all over the surface of the support, with the exception of a small circular area (14) intended to permit inflation and deflation of the element produced. The composi- tion was allowed to cure at room temperature for 18 hours and then placed in an oven heated to 8000 for 60 minutes to convert the coating to an elastomeric layer. The assembly of support and elastomeric layer was placed in an oven heated to 1500C for 100 minutes. It was found that the elastomeric layer remained intact but the polystyrene support disintegrated leaving a small deposit (16) of polystyrene and some beads thereof within the bag.

Claims

1. A method of making an element comprising a layer of elastomeric material comprising the steps of (i) procuring a support comprising a mass of polystyrene foam, (ii) applying to the surface of the support a continuous coating of a composition capable of curing at a tempe rature of less than 140 C, (iii) curing the composition to convert the coating into said layer of elastomeric material, (iv) heating the support to a temperature sufficient to reduce the volume of the polystryrene mass.

2. A method according to Claim 1 wherein the composition comprises a room temperature curable silicone composition.

3. A method according to Claim 2 wherein the coating is permitted to cure at ambient temperature and is then heated to a temperature of about 80 C.

4. A method according to Claim 1 wherein the support is heated in step (iv) to a temperature of about 150 C.

Amendments to the claims have been filed as follows CLAIMS 1. A method of making an inflatable moulding element comprising a layer of elastomeric material suitable for use in a process in which the inflatable element is located within an outer mould, a curable resin is moulded between the outer mould and the inflatable element with the inflatable element containing fluid under pressure which may be subsequently released to permit recovery of the inflatable element, the method comprising the steps of (i) procuring a support comprising a mass of polystyrene foam; (ii) working the support to shape it into a foam mass of dimensions and configuration conforming to those required of the interior of the elastic inflatable element;; (iii) applying to the surface of the support a coating of a silicone composition capable of curing to an elastomeric condition at a temperature less than 140 C; (iv) curing the composition to convert the coating into said layer of elastomeric material and (v) heating the support to a temperature sufficient to substantially reduce the volume of the polystryrene mass.

4. A method according to Claim 1 wherein the support is heated in step (v) to a temperature of about 150 C.

5. A method according to Claim 1 wherein the inflatable moulding element is in the form of a cube.

6. The use of a method comprising the steps of (i) procuring a support comprising a mass of polystyrene foam; (ii) working the support to shape it into a foam mass of dimensions and configuration conforming to those required of the interior of the elastic inflatable element; (iii) applying to the surface of the support a coating of a silicone composition capable of curing to an elastomeric condition at a temperature less than 1400C; ; (iv) curing the composition to convert the coating into said layer of elastomeric material and (v) heating the support to a temperature sufficient to substantially reduce the volume of the polystryrene mass for making an inflatable moulding element comprising a layer of elastomeric material suitable for use in a process in which the inflatable element is located within an outer mould, a curable resin is moulded between the outer mould and the inflatable element with the inflatable element containing fluid under pressure which may be subsequently released to permit recovery of the inflatable element.