CA1184000A

CA1184000A - Expandable thermoplastic polymer composition containing thermally decomposable carbonates

Info

Publication number: CA1184000A
Application number: CA000223820A
Authority: CA
Inventors: Kenneth T. Collington; Rishi R. Puri; Peter J. Gibson
Original assignee: Fisons Ltd
Current assignee: Fisons Ltd
Priority date: 1974-04-03
Filing date: 1975-04-03
Publication date: 1985-03-12
Also published as: IT1034815B; NO750830L; FR2266718A1; JPS50140575A; AR207773A1; DK140975A; IE41220L; GB1466427A; CH609996A5; BE827088A; ZA751491B; SE7503788L; ES436232A1; FR2266718B1; DE2513033A1; NL7503471A; IE41220B1; AU7922975A

Abstract

ABSTRACT

The present invention relates to a process for expanding thermoplastic polymer compositions notably those containing polymers having processing temperatures in excess of 200°C and/or containing an ingredient which is susceptible to ammonia, characterised in that as the blowing agent certain carbonates (notably zinc carbonate) are used. Preferably a nucleating agent and a weak acid are present to aid cell formation.
The invention also includes expandable polymer compositions and blowing agent formulations containing the carbonates.

Description

01/~ 3 ~'~W~

~ le p~esent invention relates to a process fDr e~panding thermopl~stic polyrners ~TId to a con~position for use there~l.
~ Je have discovered that zinc carbonates ~ld sertain other carbonates D~y be used as blo~ing agen~s for the expansion of high ten~erature processing thermoplastic pol~2rs.
Accordingly, ~he present in~ention provides a process or expanding.a ther~oplastic polymer which co~prises heat~ng a composition containing the polymer and a the~nally deComposable blowing agent so as to cause decomposition of the blowing agent characterised in that as primary blowing agent there is used ¦ zinc and/or cadmium carbonate.
The teh~ primary blowing agen~ is used herein to deno~e that the blowing agent pro~ides at least 50% by ~olwne oE the gas evolved upon heating the composition. The primary blowing a~ent is desirably present in at least 70%, preferably 90% or m~re, by weight of the total blowing agent. It is also preferred ~hat the con~osition to be expanded be substantially free from strong acids (e.g. contaln less than 5~0, preferably less than 2~o~ w/w of acid) and that the blowing agent be cubstantially free ti~e~ contain less than 10~) of an an~noJlia evolving organic blowing agent.
Accordingly~ the prcsent in^~ention also p~ovides a process ~or expanding a the~noplastic pol~ner which comprises heating, to a temperature below the degradation teTnperature o the polymer, but in excess of t~le softening point of the 02/~/183 3 ~ @~
.

thermoplastic polymer and o the m s tu decomposition temperaturç of the blo~ing agent and preferably in excess of ~20C, a composition preferably containing less than 5% added strong acids and less than 10% ~mmonia e~-olviIIg organic blo~ing agen~s which comp~.si~ion contains a thermoplastic polymer and as blowing agent a compound s~lected from particulate zinc carbon?te, lead carbonat~, ca~nium carbonate and/or lithium carbonate.
The inYention also provides a storage stable exp~ldable thermoplastic polymer composition comprising a thermoplastic polymer, no~ably ~ne hav~lg a processing temperature in excess of 200C, cmd as primary blowing agent a compound selected from zinc carl;onate cmd/or cadmium carbonate. Desirably, the blowing agent in the con~osition contains less than 20% of other blowing ! 15 agents, notably a~nonia evolving organic blowing agents. Also in place of or in addition to the ~inc and cadmium carbonates, lead and/or lithil~ carbonates may be used.
~ Ic term s~rong acid as used herein denotes c~n acid which has a pKA value of less than 3 ~or any one o its ncutralisation points. ~le decon~osition temperatllre cf the carbonate blowing agent is the tem~er2ture at which a gas phase, i.e. water vapour cmd/or carbon dioxide, is evolved, and is not l;mitrd to the temperature at which the C~3 anion therein is decomposed.
The carbonate blowing agents for present use are often 03/~

Bf~OO

con~ercially availabl~ ~l the form of so called basic carbonates and for convenience the term carbonate is used herein and in the claims to cleno-te ~here the context permits both the ca~b~nate and the basic carbonate. The carbonates ~or present use may be S used in the fonn of hydrates thereof, 3~nC03.2~n~.31~20, although with certain pol~ners (notably polycarbonates) it may be necessar7r to use substantially allh,vdrc,us materials. If desired mixtures of carbonates may be used, although it is preferred to use one ccarbonate ~notably zinc car~ona~e). It is also preferred that the carbonate provides at least 90~0 (prefera~ly substantially lOOQQ~ by ~eight of the primary blo.ring agen~ in the polymer composition; although the carbonates may be used in conjunction with other non-a~nonia blowing agents, e.g.
with carbonates such as potassiem carbonate which have a lower 5 decomposition temperature.
e blowing agent for ~resent use is particulate and pre~erably has a particle size of less than 100 micrometres, notably less than 30 micrometres. It is particularly preferred that the particles have a si~e r~nge of from 0.1 to 20 micrometres. These size ranges are conveniently measured using sieving techniques.
The carbonate blowing agents for pres~lt use decompose to give of car'~on dioxi,de. They do not produce ~nonia as do many currently commercially available blowing agents nor do they give rise to strongly acidic or basic residues in the expanded 0~/~/183 _ 5 _ the~loplastic polymcr. The carbnnate blow~.g agents for presen~
use thus find especial use in the exl,c~lSiQn of polymer con~ositions ~hich contain ingredients l~hich are adversely affected by ~Immonia. The ingredient may be the thermoplas~ic polymer itself, e.g. a polycarbonate, a polyphenylene sulphide, a polysulphone, a polyc~mide, a polyester or a styrene homopolymer copolyrner, blend or alloy, all being polylners whose phys;cal proper~ies are adversely affected by ammonia and, ~.o a lesser extent water. Alternatively, the ingreclient rnay be a fibre reinforcement in the composition, notably one which has been given an ar~nonia susceptible surface coating (e.g. a polysiloxane coating) to irnprove bonding between the fibre and the thermop]astic poiymer; or a filier, e.g. asbestos, which is susceptible to a~nonia. The carbonates for present use are also of ~alue in that they do not forrn ~ases or residues which seriously attack the equipment or moulds in ~]lich the polymer compositions ~re-to be expanded. The carbonate blowing agents for present use thus also find use in the expansion of a wide range o high ternperature processed polymer cornpositions ~hich - 20 are not susceptible to ammonia.
The thern~plastic polymers for present use include those which require processing at temperatures in excess of 200C and include ~olycarbonates; polysulphonc~; polyesters (for exc~mple, polytercphthalates); polyamides tfor example~ polyadipamides e.g.
nylon); polyacetals; polyphenylene oxides and sulphidcs; polystyrenes;
poly(acryloni~riletbutadiene/styrene); poly~styrcne/acrylonitrile);
. _ 5 _ '. . .

~ 0~ /1S~3 - 6 - ~l~fl~
.

polyole~ins (for example, high density polyethyl~ne or polypropylene) and ~luorinated and Pr chlorinated ethylen~
polyrners (for example, PT~E and chlorinated ethylene). The polymers may be fibre-reinforced, for exa]nple with glass, asbestos, carbon o~ boron fibres. I-f desired blends, al]oys or copol~ners of thermoplastic polymers may be used.
The polymer composition will typically contain from 0.05 to 5~5 e.g. 0.2 to 3~0, by ~ight, of the dry carbonate blowing agent based on the weight of the thermoplas~ic polymer. lYhere a polymer premix is made, as described below, this may contain from 10 to 5~5 e.g. 20 to 30%, by weight o~ the carbonace blowing agent based on the weight of the polymer.
I~e yol~ner composition may contain ingredients other than the polymer and blowing agent; for ex~mple, other carbonate blowing agents; dyes; pigments; other polyJners; fill~rs;
~ntioxidants; extenders or fire-retardants, In order to improve the cell structure in the expanded polymer, we prefer that a nuclecting agent be present during the decomposition of the blowing agent. The term nucleating agent is used herein to denotc a ~a~erial which aids ormation of gas bubbles within the plastic polymer composition during expansion of the composition.
Suitable nucleating agents include particulate solids, notably soli.ds with mean particle sizes o less than 100 micrometres; an~
liquids which are imrniscible in any liquid phase within the plastic ~5 or molten polymer system. The nucleating agent may be provided hy a separate in~redient which is incor~orated into the polymer composition ~it]l or a~er the blowing agent Al~e m atively, the nucleating agent may be provided by one or more of the ingredienks already present in the polyrner cor~osition into l~hich the blowing agent is to be incorporated Thus, suitable nucledting agen~s may be provided by pigments and/or illers in the polymsr con~osition and/or by a ll~uid comi)ollent SUCil ~S a wetting agent or me~al soap present in the polymer cor~osition as a lubricant. However, it will usually be preferred to incol~ora~e a solid ~ucleating agent in admixture with the blowing agent. Suitable solid nucleating agents include magnesium and aluminium silicates (for example, talc, or mica); clays (e.g. attapulgite clay, china clay or ball clay); fumed oxides te.g. ~umed silica); or magnesiurn oxides.
Pre~erably, the solid nucleating agent has a mean particle size of less than lO0 micrometres, typically less than ~0 micrometres, preferably less than l micromstre. It is also within the scope of the present invention to use as nucleating agent a material which is solid whell mixed with the b;owing agent, but which rnelts in situ during expansi.on of the polyrner co~position. Such materials include waxes and/or orgc~nic acids. The use of organic acids is preferred, since these often assist in re~llatirlg the decompositi.on of the blowing agent as well ~s, in some cases, providing the nucleating agent. The acicls for present use are characterised in being weak organic carl~oxylic acicls; i.e. all their neutralisation points have a pKA greater than 5, typically 07/~ 3 - 8 - 1 ~ 8~L~

S to 7. Suitable acids includc C10 20 fatty acids, e.g. palmiLic, oleic, lauric, stearic and n~rlstic acids; and aromatic acids.
T~ie nucleating agent is suitably present in from 0 to 50~, pree-rably from 5 to 305, by ~eight of the carbonate blowing ager.t.
present or to be ~corporated into thc polymer composition. lYhere the nucleating agellt is no~ an ~cid) it is pre~erred to incorpora-te up ~o 50%, preferal-].y from 5 to 30~, by weigll~ ~based on the carbonate blowing agent thereLn) of a weak aci.d as descrlbed above .into ~he compositi.on being exp~ldccl.
Mixtures of sol.id pa.rticulate carbonate blowing agent ~nd nucleating agent are novel and the invention therefore also provicles a ~articulate composi~ion compri.si.ng zinc carbona~c, lead carbonate, aclmium carbonate and/or lithi~ carbonate in admixture with a solid nu.leating agent, notably fumed oxides~
silicates or clays (e.g. ~gnesium oxi.de, umed silica a~d/or talc). The particul.ate composition comprises from 5 to 50 parts ~f nucleating agent per 100 parts by weight of the carbonate. It is also prc~erred that the particulate compositi.on contain from 5 to 50 parts o a weak acid a.s described above per 100 parts of weig}lt of the carbona~e. I desired, this particulate composition may also include solid diluents (~or example, other metal carbonates), liquid diluents (Eor example paraffins), lubricating agents (or example, metal stearates such as n~gnesium stearate), antioxidants and the like. Since the carbonate blowing agents Eor present use do not require the use of a strong acid during 0~/~/lS3 ex~ansion of the polymcr mto t~hich they are incorporated, the blowing agen~s can be fonnulated as storage stable mixtures for direct incorporation into the polymer as a single co~onent at the time of use.
The polymer compositions containing the thermoplastic pol)~ner, the carbonate blowing agent and the ot]ler ingredients (~lere present) may he~ formulated using conventional techniques, e.g. to give granular, po~der, emulsion or liquid solution formulations. l~us, the compos:itions may be made by dry mixing the ingrediellts or, more pre~erably ~y dispersing the carbonate in a liquid carrier and mixing the dispersion into the pGlymer.
If desired, a solid diluent or carrier may be mixed with the carbonate blowing agent to aid ~miform distribution thereof throughout the polymer. }lowever, a particularly preferred method is to mix the carbonate blowing agent optionally ith cther ingredients (notably a solid mlcleating agent) I~Tit:h only par~ of the thermoplastic polymer or another poly~er compa~ible therel~-ith, ko form a concentrated premix which is tllen mixed with ~urther ~olymer for use.
Thc pol~ner compositionscontaining ~he carb~na~e blowing agent are expanded and moulded in the conventional manner. Thus, thc cor,lposition may bc ~ormed into a sheet by extruding, casting, calenderlng or spreading as a powder blend. If desired, the layer m~y ba formed on a backing member such as a resinous material, impregnated felt, coated paper and tha li~e. The layer ~ 9 _ - 1 0 -- ' may optionally be overcoated with a protective plastics layer.
Alternatively, the composition may be moulded by any of the conventional injection moulding, blow moulding or other moulding techniques to form a hollow or solid article.
The composition is heated in any of the usual ways, such as in hot air ovens or by infra red heaters, preferably to at least 220C, to decompose the blowing agent and cause expansion of the composition. The optimum temperature for decomposition of the blowing agent will vary with the carbonate used and the other components of the composition. Thus, for zinc carbonate, decomposition temperatures of 220 to 300C are generally suitable. The time during which the heating is carried out will of course depend on the temperature and the degree of decomposi-tion required.
The process of the invention will now be illustrated by the following Examples in which all parts and percentages are by weight unless stated otherwise:
Example 1 ~a) Gra,nular polycarbonate resin of intrinsic viscosity 0.495 (available as and known by the trade mark LEXAN 900) was tumble mixed with 0.~% blowing agent consisti.ng of a mixture of 85%
basic zinc carbonate (mean particle size 15 micrometres) having the general formula ZnCO3.2~nO.3H20, and 15% talc. The, mixture was ~ed to an injection moulding machine f'itted with a fixed 25 steel mould. Barrel temperatures were 280 (hopper)/280/285/
285C (nozzle). The total cycle (injection and cooling) time was 70 seconds. The moulding had a density of................ O.

. ~

10~ 3 , 11 0.~5g/cm3, an irl~r~lsic viscosity of 0.~50 ~md a ~liform Eine cell st mcture with a s~looth surface. The moulding had an impact strength o. 4.835 Kg/m as mcasurcd on 6.3cm disc cut from the moulding using the falling weight met}lod with a ball of 2.5an diameter. The surface was free from discolouratiorl, indicating a minimu~. of degradation of the polym~r.
~b) By ~ay oE comparison, the e~ample was repeated using azodicarbonamide (an ammonia evolving blowing agent) in ~lace of the zinc ca-rbonate The cell structure of the product was ~ood but the intrinsic viscosity was about 0.2 (mdicating severe degradation). The p}lysical properties of tlle foam were msatisfactory~ and the polymeT was badly discoloured, confinning that polymer was badly degraded.
~ ple 2 Granular poly(acrylonitrile/butadiene/styrene), available from Sterling ~hulding ~aterials ~rade B 30~, a~nsity 1.16Jo~ was ~umble mixed Wit}l 0.8~ of the blo~ing agent composition descri~ed iTi Ex~nple l(a). The mixture was moulded as in Example l(a) with barrel temperatures o~ 225/260/260/260 and a cycle time of 70 seconds. ~e product moulding had a dens;ty o 0.75~/cm3, a ~miform ~ine ce]l structure and a stain-free sur-~ace.
r~plc ~
~ .
(a) ~x ~le 2 l~a~ repeated using 1.1~ of the blowing agent composition described in F~ample l(a). The procluct moulding had a density of 0.69g/cm , a fine cell structure and a surface free - 11 , ll/A/183 - 12 ~ 3a~ q3 from discolouration.
(b) By W.ly o~ comparison ~he ex~l~le was repeated using azodicarbonamide in plac~ of the ~inc carbonateO The mouldings had a fine cell st~lcture but sufferred from dark brown staining.
Example 4 Example 2 ~as re~eated Usillg 0.8~ zinc carbonate as the blowing agent. The product moulding had a densi~y of 0.82~/Gn3, a good surface and cell s~ructure, although the cell s-t~ucture ~as inferior to that obtained in ~xamples 2 and 3(a)~
F~ample 5 a) Ex~nple 2 ~as repeated using granular 45% asbestos fibre reinforced polypropylene, barrel temperatures of 20Q/Z3S/235/
235 and a cycle tilne of 66 seconds. The product had a density reduction of 30~, a uniform fine cell structure ~Id a slnooth sta.;n-free surface.
b) By ~ay of comparison the example was repea-ted using azodicarbonamide in place of the zinc carbonate. The mouldings had a fine cell structure but were stained black.
Fxample 6 ~x~nple 2 ~as repeated using gra~ular 20~ glass fibre reinforced polypropylene, barrel ~emperatures of 200/235/
235/235C and a oycle time o~ 66 seconds. l~e product had a ~ensity rcduction of 30~, a ~iform fine cell structure and a smooth stain-free surface.

- 12 ~

Example 7 Example 4 was repeated using granular 20% asbestos ribre reinforced polypropylene, barrel temperatures of 200/235 /235~/
235C and a cycle time of 66 seconds. The product had a density reduction of about 30%, a good cell structure and a stain-free surface.
Example 8 Granular nylon 6 (available as and known by the trade mark 'Akrulon'; Grade 2 Special~, density 1.2g/cm3 was tumble mixed with 1% of the blowing agent composition described in Example l(aj.
The mixture was then moulded as in Example 1 using barrel temperatures oE 275/270/270/270C and a cycle time of ~5 seconds. The moulding had a density of 0.75g/cm3, a uniform fine cell structure and a smooth stain-free surface.
Example 9 Polystyrene (available as and known by the trade mark 'Shell S173') was tumble mixed with 0.8~ of the blowing agent composition of Example l(a). The mixture was then moulded as in Example l(a) using barrel temperatures of 190/220/220/220C
and a cycle time of 70 seconds. The moulding had a density reduction of 23% and a uniform very fine cell structure.
EXample 10 Granular polycarbonate resin (availahle as and known by the trade mark MaKrolon 3200) of intrinsic viscosity 0.531 was tumble mixed with 0.3~ o-F blowing agent as shown in the table, and injection moulded at barrel...................... ~

13/.~]83 tcmperatures of 280/280/~85/285~C. The blo~ing agent c.omposition c~nd the impac~ strength and intrinsic viscosity cla~a is tabulated be].ow. ~le mouldings were 16~n square plaques lcm thick. I~he impact streng~hs were n!casure~ on 6.3cm discs cut from the moulclings by the fallin~ wei~ht method using a ball of diameter 2.5CDI, ~ ~ VISCOSIIY ~I~NGI~I
. _ _~ . .
Basic Zinc Talc ~cid _ Kg/metre Carbonate (~) (~o) (~o) . . .... . ___ . .. . , . ~..... I
~ 0.469 5.25 5 palmitic 0.4~2 6,12 15 palmitic 0.492 6.64 10 lauric 0.479 7.ll 10 stearic 0.476 6.18 .15 10 trimesic 0.469 6.35 Ex~mpl _ A polylner premix suitable ~or use as the blowing agent in the polymer o~ Exan~le 2 was prepared by dry mixing a powdered pol.yethylene having a ].ow melting point ~70 parts) and 30 parts of a mixture of zinc carbonate ~75~0) talc (l5~) and lauric acid (10~ e mixt-lrc was tllen extruded at 110C and the extrudate chopped into pellets. 'I~IQ process o~ ~xænple 2 was repeated using the pellets to pro~ide the blowing agent.

l~/A/183 Example 12 ~ thermoplastic composition was prepared by tumble blending together high impact polystyrene granules (available as and known by the trade mark Shell S173 styrene) with 1% w/w o~ a blowing composition comprising a mixture oE basic zinc carbonate (75~, talc (15%) and lauric acid (10%). The blend was injection moulded with a temperature pro~ile of 180/220/230/230C, and injection time of 1 second and a mould cooling time of 60 seconds. Mouldings with uniform cell structure and good sur~ace were obtained.
Example 13 A thermoplastic composition was prepared with polypropylene (available as and known by the trade mark Carlona K571~ as in Example 12 and was injection moulded with a temperature profile of 220/240/240/240C using a one minute cycle.
Example 14 A thermoplastic composition was prepared with high dsnsity polyethylene (available as and known by the trade mark Rigidex 50) as per Example 12 and was injection moulded with a temperature profile o~ 220/235/235/235C using a one minute cycle.
Example 15 A thermoplastic composition was prepared by tumble blending together Noryl (trade mark)(modi~ied PPO) FN215 granules with 1% w/w o~ blowing agent consisting of basic zinc carbonate (85%) and talc (15%). The blend was injection moulded with a temperature profile of 260/275/275/275C using a one minute cycle.

- 16 ~ 3 Example 16 (a) A thermoplastic composition was prepared by tumble blending together pre-dried polycarbonate (available as and known by ~he trade marks ~akrolon 3000 and Lexan FL900) with 0.3~ w/w of the blowing agent of Example 15 and injection moulded using a temperature profile of 285/295~295/295C. ~ 2% drop in intrinsic viscosity value was recorded.
(b) Under the same condition, azodicarbonamide produced a 50% drop in intrinsic viscosity value.
Example 17 A flame retardant grade of polypropylene was tumble mixed with 0.8% w/w of the blowing agent of Example 15 and injection moulded with a temperature profile of 210/235/235/235C and a 1 minute cycle.
Example 18 A thermoplastic composition was prepared by tumble blending together polyethersulphone (available as and known by the trade mark 200P from ICI) with 0O7% w/w of the blowing agent of Example 15 and injection moulded with a temperature profile of 20 330/330/330/350C and a cycle time of 50 seconds.
Trihydrazino~riazine, a blowing agent which decomposes to release nitrogen and ammonia, gave mouldings with cracks in them.
Example 19 The process of Example 1 was repeated using 0.5% of a mixture 25 of basic cadmium carbonate (Cd CO3.2CdO.3H2O, 85%) and talc (15%).

T}IC process o Example 2 was re~,eated except ~hat the blowin(J agent consisted o:f basic ~inc carbonate (85~) and potassi~n calbonate (] 5~) . In this case the product had a 5 slightly fir~er cell structure than in Ex~ le 2.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for expanding a thermoplastic polymer which comprises heating a composition comprising a thermoplastic polymer, and a blowing agent so as to cause thermal decomposition of the blowing agent characterised in that as primary blowing agent there is used zinc carbonate and/or cadmium carbonate with the proviso that where an acid is present in the composition the acid has a pKa value greater than 3 for each of its neutralization points.

2. A process as claimed in claim 1 in which there is also present a nucleating agent consisting of a particulate solid with a mean particle size of less than 100 micrometres.

3. A process as claimed in claim 1 characterised in that less than 20% by weight of the blowing agent is provided by ammonia evolving organic blowing agent.

4. A process as claimed in claim 3 in which the composition is heated to at least 200°C and the primary blowing agent provides more than 90% of the total active blowing agent.

5. A process as claimed in claim 2 characterised in that the nucleating agent is selected from a pigment, a filler, a magnesium silicate, an aluminium silicate, a clay, a fumed oxide, fumed silica and magnesium oxide.

6. A process as claimed in claim 5 in which the nucleating agent is present in up to 50% by weight of the carbonate.

7. A process for expanding an ammonia susceptible thermo-plastic polymer composition characterised in that it comprises incorporating into the composition as primary blowing agent zinc carbonate in an amount of from 0.05 to 5% by weight, with the proviso that where the composition contains an acid, this acid has a pKa value greater than 3 for each of its neutraliza-tion points; and heating the composition to at least 200°C
whereby decomposition of the primary blowing agent is caused.

8. A process as claimed in claim 4 in which the thermo-plastic polymer has a processing temperature of at least 200°C, contains less than 20% of ammonia evolving organic blowing agents and in which decomposition of the primary blowing agent is caused during extrusion or moulding of the composition.

9. An expanded thermoplastic polymer characterised in that it contains thermal decomposition products of a carbonate selected from zinc carbonate and/or cadmium carbonate and is substantially free from zinc or cadmium salts of an acid having a pKa value less than 3 for each of its neutralization points and from the thermal decomposition products of an ammonia evolving organic blowing agent.

10. A storage stable expandable thermoplastic polymer composition characterised in that it comprises a thermoplastic polymer and as primary blowing agent a compound selected from zinc carbonate and/or cadmium carbonate.

11. A storage stable expandable thermoplastic polymer composition as claimed in claim 10 characterised in that where the zinc and/or cadmium carbonate blowing agent is used in admixture with another blowing agent such other blowing agent provides less than 20% by weight an ammonia evolving organic blowing agent in the mixture of blowing agents.

12. A storage stable blowing agent composition suitable for use in the expansion of a thermoplastic polymer, characterised in that it comprises a mixture of a compound selected from zinc carbonate and/or cadmium carbonate with a nucleating agent.

13. A storage stable composition as claimed in claim 10 in which the thermoplastic polymer has a processing temperature greater than 200°C and the nucleating agent is present in from 5 to 50 parts per 100 parts of carbonate.

14. A storage stable blowing agent composition suitable for use as the primary blowing agent for a thermoplastic polymer, which composition is characterised in that it comprises a particulate mixture of zinc carbonate and one or more of magnesium oxide, silica and/or talc.

15. A storage stable blowing agent composition as claimed in either of claims 12 or 14 characterised in that it also contains a polymer, the carbonate being present in from 10 to 50% by weight of the polymer.

16. A process for expanding a thermoplastic polymer except a polyamide which comprises heating a composition comprising a thermoplastic polymer and zinc carbonate as the main blowing agent so as to cause thermal decomposition of the blowing agent.

17. An expandable composition comprising a thermoplastic polymer except a polyamide and zinc carbonate as the main blowing agent.