GB2153362A - Foamable thermoset resins - Google Patents

Abstract

A composition for producing foamed articles of a thermoset resin curable by a free radical catalyst contains a foaming system comprising a carboxylic acid and an alkali or alkaline earth metal carbonate or bicarbonate which are in a non-aqueous phase.

Description

SPECIFICATION Unsaturated polyester foams The foaming of unsaturated polyester resins is well known and several methods have been proposed which may be summarised as: (a) mechanical, i.e. by whipping air into the resin, (b) the incorporation into the resin of low boiling point organic liquids which expand on vaporisation due to the exotherm when the resin is cured, (c) the use of azo compounds which decompose at elevated temperatures; some of these compounds generate free radicals capable of curing the polyester, (d) the generation of gas by chemical reaction between two additives e.g. carbonate and acid.

Of the latter the carbonate/bicarbonate/acid systems are by far the most numerous and were first described in US 2,461,761 which used sodium bicarbonate, water and acetic acid together with a metal stearate as a foam stabiliser in a hot cured composition.

Since that time many variations on the theme have been described e.g. GB 652,770: GB 1,066,436: US 4,028,289: US 4,119,583: US 4,122,047: GB 2,019,313A: GB 2,031,900A: FR 1,233,525: FR 1,254,449: FR 1,468,237: DE 1,230,561: DE 2,533,922: Belgian 828,221: Spanish 294,999: Canadian 939,100.

As is well known, the curing of a thermosetting resin may include a gelling stage followed by a post cure stage, or alternatively, especially at relatively higher temperatures, curing may be a single stage reaction. Hereinafter, the term "curing" refers to a reaction which results in at least gelling.

During the manufacture of foamed thermosetting resins such as unsaturated polyesters two processes occur-foaming and curing. If the timing of these is not correct then poor foams result. For example if the curing exotherm occurs before foaming is complete high density foams of poor structure i.e. having large blow holes are obtained. On the other hand if the foaming is completed and curing then takes place foam stability becomes a problem and collapse of the foam can occur giving high density regions.

The abovementioned specifications describe processes in which curing is effected by a free radical catalyst system and foaming is effected by an acid carbonate or bicarbonate foaming system. All of these processes include the use of water in the foaming system which allows the acid to dissociate and react with the carbonate or bicarbonate and liberate carbon dioxide. This causes the resin to foam before cure is completed.

Only one system does not mention the use of water and that is GB 1,416,822 which describes the use of an acid organophosphate in conjunction with carbonates or bicarbonates of alkali metals, alkaline earth metals, iron, cobalt or nickel in a hot cured composition.

Organophosphates tend, however, to be relatively expensive.

We have now found that if an acid selected from certain solid organic carboxylic acids is dispersed in the resin and mixed with an alkali, or alkaline earth metal, carbonate or bicarbonate no water is required to achieve adequate foaming at elevated temperatures.

This means that both components of the foaming system can be incorporated into a liquid resin to obtain storage stable systems which may then be used with relatively inexpensive and readily available foaming agents and without the need of mixing further ingredients.

The present invention provides a composition for producing an at least partially foamed article of a thermoset resin, which composition comprises a thermosetting resin curable by a free radical catalyst and a foaming system, which foaming system comprises a carboxylic acid and an alkali, or alkaline earth metal, carbonate or bicarbonate, which said acid and carbonate or bicarbonate are in a non-aqueous phase.

Foaming and curing of the composition may take place in a mould.

It is necessary that during the moulding process the temperature of the moulding composition at some time prior to the conversion from a liquid to a solid, exceeds the melting point of the organic acid incorporated into the moulding composition. The source of this heat may be from the heated mould into which the moulding composition is placed or from the exotherm which occurs during the curing reaction. When the compositions of the invention are used and cured in a closed mould at ambient temperature using the exotherm to initiate foaming, the caboxylic acid of the foaming system should have a melting point below the exothermic temperature of the gelling reaction. Such ambient temperature curing may result in moulded products which have a foamed core and an unfoamed skin.

The organic carboxylic acids which work particularly well in the invention are those having melting points between 70"C and 200"C preferably between 100"C and 160"C and which also have a pK of 1.0 x 10-4 (first dissociation) or above. Such acids include citric, tartaric, maleic, itaconic, succinic and crotonic.

The amount employed will depend on the amount of foaming desired but may be varied between 0.2 and 10% of the total weight of the composition, preferably between 0.5 and 5%.

The acids may be used in pure form or in some cases coated forms normally used in the food industry.

The co-reactant may be an alkali carbonate or bicarbonate or an alkaline earth metal carbonate or bicarbonate and examples of the coreactant are sodium bicarbonate, calcium carbonate (natural or synthetic), magnesium carbonate, calcium magnesium carbonate (dolomite), or mixtures thereof and preferably has a mean particle size of 200 mesh or finer.

Typically the coreactant may pass a 200 or 300 mesh. Besides acting as coreactant this material may also function as a nucleating agent and in the case of, for example, calcium carbonate or dolomite acts as a filler if present in excess.

The curing of the polyester resins may be accomplished by means of free radical initiators at temperatures of up to 200"C, preferably from 120-180"C. Suitable initiators include amongst others benzoyl peroxide, tbutyl perbenzoate and I , 1 -di-t-butylperoxy 3,3, 5-trimethyl cyclohexane. Alternatively ambient temperature cure systems e.g. methyl ethyl ketone peroxide/cobalt octoate or benzoyl peroxide/tertiary amines may be used provided they case a high exoterm.

Other constituents that may be incorporated are release agents such as zinc stearate, foam stabilisers such as silicone oils, pigments and reinforcing fibres such as glass, carbon or Kevlar(g).

The invention may be used with a wide range of systems curable by free radical catalysts such as unsaturated polyester resins, vinyl ester resins and urethane acrylates. Such resins usually consist of an unsaturated polymer such as the unsaturated polyester, vinyl ester or urethane acrylate dissolved in or admixed with a monomer copolymerizable therewith, for example styrene or diallyl phthalate.

The following Examples illustrate the invention.

Example 7 To 100 p.b.w. CRYSTIC(g) 198 (an orthophthalic acid based polyester) were added 40 p.b.w. calcium carbonate (passing 300 mesh), 5.0 p.b.w. powdered citric acid, 1.0 p.b.w.

Dow 1 93 (a silicone foam stabiliser) and 5.0 p.b.w. zinc stearate and the additives dispersed under a high shear mixer. This mix remains stable for at least 4 weeks. To 100 p.b.w. of this premix 1.0 p.b.w. benzoyl peroxide (50%) was added and mixed in and the mixture was injected into a closed mould at 1 20'C so that the mould was one third filled.

Within 5 minutes the resin mix had foamed and gel led to yield a block of foamed, cured polyester three times the volume of the original liquid.

Example 2 To 100 p.b.w. CRYSTIC 199 (a heat resistant isophthalic acid based polyester) were added 40 p.b.w. dolomite (a mixed calcium/magnesium carbonate) passing 300 mesh, 5.0 p.b.w. powdered tartaric acid, 1.0 p.b.w. Dow 1 93 (a silicone oil foam stabiliser) and 5.0 p.b.w. zinc stearate and these were dispersed by high shear mixing. To 100 p.b.w. of the above premix 1.0 p b.w. t-butyl perbenzoate was added and mixed in and the mixture was injected into a closed mould at 180"C so that the mould was one third filled.

Within 5 minutes the resin had foamed and gelled to yield a block of foamed resin three times the volume of the original liquid.

Example 3 To 100 p b.w. CRYSTICO D4364 (a fire retardant polyester resin) were added 35 p.b.w. FRF80 (alumina hydrate), 5 p.b.w.

maleic acid, 5 p.b.w. sodium hydrogen carbonate, 5 p.b.w. zinc stearate and 1 p.b.w. Dow 193. These were dispersed by high shear mixing.

To 100 p.b.w. of the above premix were added 10 p.b.w. of XGS 1 730 (" glass fibre from TBA) and 1 p.b.w. t-butyl-perbenzoate. These were mixed together and then heated in an open top mould at 150C where the mix increased in volume 2-fold and gelled to yield a reinforced self extinguishing foam.

Example 4 To 100 p.b.w. of polyester resin (containing 20% diallyl phthalate) which had a melting point of 80"C were mixed (at 100 C), 30 p.b.w. Norwegian talc, 5 p.b.w. sodium hydrogen carbonate, 5 p.b.w. maleic acid, 5 p.b.w. zinc stearate and 1.4 p.b.w. di-cumyl peroxide. This was injected into a closed mould at 180"C so that it was half full. This mix expanded to fill the mould and gelled to form a block of foamed polyester, half the density of the original mix.

Claims (16)

1. A composition for producing an at least partially foamed article of a thermoset resin, which composition comprises a thermosetting resin curable by a free radical catalyst and a foaming system, which foaming system comprises a carboxylic acid and an alkali, or alkaline earth metal, carbonate or bicarbonate, which said acid and carbonate or bicarbonate are in a non-aqueous phase.

2. A composition according to claim 1, wherein the thermosetting resin is an unsaturated polyester, vinyl ester or urethane acrylate resin.

3. A composition according to claim 1 or claim 2, wherein the said acid and carbonate or bicarbonate are each in solid form.

4. A composition according to claim 1, claim 2 or claim 3, wherein the said acid has a melting point of from 70 to 200"C inclusive and a pK value (first dissociation) of at least 1.0X 10-4.

5. A composition according to claim 4, wherein the said acid is citric, tartaric, maleic, itaconic, succinic or crotonic acid.

6. A composition according to any one of the preceding claims, wherein the said acid is present in an amount of from 0.2 to 10% of the total weight of the composition.

7. A composition according to any one of the preceding claims, wherein the said carbonate or bicarbonate is sodium bicarbonate, calcium carbonate, magnesium carbonate, calcium magnesium carbonate or a mixture of any of these with one another.

8. A composition according to claim 7, wherein the said carbonate or bicarbonate has a mean particle size no greater than 200 mesh.

9. A composition according to any one of the preceding claims substantially as herein described and exemplified.

10. A method of producing on at least partially foamed article of a thermoset resin comprising admixing a thermosetting resin with a free radical catalyst capable of initiating curing of the thermosetting resin and a foaming system capable of foaming the thermosetting resin, which foaming system comprises a carboxylic acid and an alkali, or alkaline earth metal, carbonate or bicarbonate, which said acid and carbonate or bicarbonate are in a non-aqueous phase, and curing and foaming the thermosetting resin.

11. A method according to claim 10, wherein curing is effected at ambient temperature and the said acid melts at a temperature below the exothermic temperature of the gelling reaction.

12. A method according to claim 10, wherein curing is effected by heating the composition to a temperature of up to 200"C.

1 3. A method according to claim 12, wherein curing is effected by heating the composition to a temperature of from 120-180"C inclusive.

14. A method according to any one of claims 10 to 13, which is carried out in a mould.

1 5. A method according to any one of claims 10 to 14, substantially as herein described and exemplified.

16. An at least partially foamed article whenever produced from a composition of claim 1.