WO2001068758A1

WO2001068758A1 - Formulation for making polyurethane foams

Info

Publication number: WO2001068758A1
Application number: PCT/EP2001/003126
Authority: WO
Inventors: Rene Aerts; Jan Hermen Hendrik Meurs; Ingeborg Vaesen
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2000-03-17
Filing date: 2001-03-16
Publication date: 2001-09-20
Also published as: AU2001258290A1

Abstract

Formulation comprising a polyol component, an organotin catalyst and from 0.001 to 8 parts by weight, per 100 parts by weight of the polyol component, of at least one ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid.

Description

FORMULATION FOR MAKING POLYURETHANE FOAMS

The present invention relates to formulation suitable for making polyurethane foams and to a process for preparing such foam.

Tin-containing or organotin catalysts are widely used for preparing polyurethane foams. Examples of such organotin catalysts include (i) tin salts of organic acids, such as stannous acetate, stannous octoate and stannous oleate; (ii) dialkyltin salts of carboxylic acids, such as dibutyltin diacetate, dibutyltin dilaureate, dibutyltin maleate, dilauryltin diacetate, dioctyltin diacetate, dibutyltin-bis ( 4-methylamino- benzoate) , dibuytyltindilaurylmercaptide and dibutyltin- bis ( 6-methylaminocaproate) ; (iii) trialkyltin hydroxides, such as trimethyltin hydroxide, tributyltin hydroxide and trioctyltin hydroxide; (iv) dialkyltin oxides, such as dibutyltin oxide, dioctyltin oxide and dilauryltin oxide; (v) dialkyltin dialkoxides, such as dibutyltin-bis (iso- propoxide) and dibutyltin-bis (2-dimethylamino pentylate) ; and (vi) dialkyltin dichlorides, such as dibutyltin dichloride and dioctyltin dichloride. Of all these organotin catalysts dibutyltindilaurate and stannous octoate are most frequently applied.

When an organotin catalyst is used it is suitably deactivated once the polyurethane foam has formed, because otherwise it may cause foam degradation. In order to deactivate organotin catalysts a halogen-containing fire retardant, often a halogenated phosphate ester, is commonly used. During the curing stage of the foam such halogenated phosphate ester liberates traces of acid (generally considered to be hydrochloric acid, hydro- bromic acid and/or phosphoric acid) which deactivate the organotin catalyst. However, the use of halogenated compounds like the halogenated phosphate esters more and more becomes undesired in some applications for environmental reasons, so that there is a need to find alternatives for these compounds.

It is an object of the present invention to find an alternative for said halogen-containing fire retardants and in particular for the halogenated phosphate esters, which alternative should result in foams having at least the same quality as the conventional foams. Consequently, such alternative should also be capable of effectively deactivating any organotin catalyst present in the foam.

It was found that this could be realised by using a specific type of halogen-free ester in stead of the halogen-containing fire retardants with halogenated phosphate esters as most representative group of compounds .

It is known from US-A-5, 464 , 560 to include a poly- carboxylic acid in an isocyanate-reactive composition (suitably a polyol) to be used in the production of HCFC- blown foams. The polycarboxylic acid is disclosed to reduce the decomposition of HCFCs during the foaming process, thereby increasing the effectiveness of that blowing agent. However, US-A-5, 464 , 560 is completely silent about the use of organic esters of polycarboxylic acids as well as about the possibility of replacing halogen-containing fire retardants by such esters and the deactivating effect these esters have on organotin catalysts . US-A-3, 892, 685 relates to a process for the production of flame-resistant polyurethane foam in which a polymeric polyol is reacted with an aromatic poly- isocyanate in a foam-forming reaction mixture which contains (a) a substance normally effective as a catalyst for the polymerisation of tolylene diisocyanate, especially sodium hydroxide or sodium carbonate, as foam modifier, and (b) a precursor of a sulphonic acid, for example an ester of an alcohol having 1-6 carbon atoms, as an anti-ageing additive for the foam. US-A-3, 892 , 685 does not contain information on formulations containing organotin catalyst.

GB-B-1094489 relates to a cellular fire-retardant polyurethane foam in which part of the conventional polyol reactant is replaced with a first phosphorus compound having at least 2 active hydrogen atoms per molecule, preferably propoxylated dibutylpyrophosphoric acid, said first phosphorus compound being present in an amount of at least 5 %wt of the polyol, and a second phosphorus compound consisting of at least one trialkyl- phosphate or trialkylphosphonate, preferably dimethyl methyl phosphonate, in an amount of from 10 to 90 %wt, of the total of said first and second phosphorus compounds. In the exemplified formulations, the phosphorus compounds are present in amounts of at least 9 parts by weight per 100 parts by weight of polyol. GB-B-1094489 contains no information on the effect of the phosphorus compounds on the organotin catalyst. Furthermore, the pyrolysis of esters of phosphorus oxo-acids having beta-hydrogens usually starts at 160-200 °C. These esters undergo olefin elimination upon pyrolysis.

Accordingly, the present invention relates to a formulation comprising a polyol component, an organotin catalyst and from 0.001 to 8 parts by weight, per 100 parts by weight of the polyol component, of at least one ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, suitably between 100 and 150 °C, thereby releasing the acid.

The formulation of the present invention can be used for preparing rigid, flexible and semi-flexible poly- urethane foams as well as polyisocyanurate foams and polyurethane urea foams. It will be appreciated that the exact type of foam will be determined by the type of polyol and type and amount of polyisocyanate used. It is within the normal competence of the person skilled in the art to make these variations. The formulation of the present invention, however, was found to be particularly suitable for preparing flexible foams and more particularly high resilience foams. The type of polyol component used will depend on the type of foam envisaged. In general, the polyol component may comprise a polyester polyol, a polyether polyol or a combination thereof. Of these, the use of polyether polyol is preferred. In general any polyester polyol or polyether polyol having at least two hydroxyl groups available to react with a polyisocyanate may be used. Suitable polyols have a molecular weight of from 150 to 20,000, preferably from 200 to 10,000, more preferably from 300 to 8000, and contain at least two, preferably 2 to 8 and most preferably 2 to 6 hydroxyl groups. The term "polyether polyol" as used in this connection refers to polyols comprising poly (alkylene oxide) chains. The term "polyester polyol" refers to polyols comprising ester bondings in the polymer chain. One way of preparing such polyols is, for instance, reacting a polycarboxylic acid or carboxylic acid anhydride with a polyhydroxy compound. The term "molecular weight" as used throughout this specification refers to number average molecular weight. Suitable polyester polyols include polyester polyols, which are typically produced from organic dicarboxylic acids or derivatives thereof (like anhydrides and esters) having from 2 to 12, preferably 4 to 6, carbon atoms and polyfunctional alcohols having from 2 to 12, preferably 2 to 5, carbon atoms. Suitable dicarboxylic acids include both aliphatic and aromatic acids like adipic acid, phthalic acid, fumaric acid and terephthalic acid, while the most preferred polyfunctional alcohols are diols like ethanediol, 1, -butanediol and diethylene glycol. One category of suitable polyester polyols are the polyester polyols produced from phthalic anhydride and diethylene glycol. Another category uses either the heavy residue of the production of dimethyl terephthalate or scraps of recycled polyethylene terephthalate (PET) as the feedstock.

Polyether polyols or poly (oxyalkylene) polyols can be obtained by methods known in the art. Typically, methods for preparing polyether polyols involve reacting a starting compound having a plurality of active hydrogen atoms with propylene oxide, optionally together with one or more other alkylene oxides like ethylene oxide or butylene oxide, in the presence of a suitable alkoxylation catalyst. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 8 hydroxyl groups. Examples of such alcohols are glycols, glycerol, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, mannitol, etc. Suitable alkoxylation catalysts are known in the art and include strong bases like potassium hydroxide and dimetal cyanide complex (DMC) catalysts.

The polyether polyol may a rigid polyol or a flexible polyol. Rigid polyols suitably have a molecular weight in the range of from 200 to 1500, preferably 300 to 1000, a functionality in the range of from 2.5 to 8, preferably from 3 to 6, and a hydroxyl value in the range of from 200 to 600, preferably 200 to 500. The rigid polyol is most suitably based on propylene oxide as the only alkylene oxide, although ethylene oxide may in addition be used in amounts up to 40% by weight based on total alkylene oxide used. Preferably, however, a flexible polyether polyol is used. Such flexible polyether polyol is based on propylene oxide, optionally in combination with ethylene oxide, and suitably has a molecular weight of from 2500 to 7000, a hydroxyl value of from 20 to 100 mg KOH/g, preferably 25 to 75 mg KOH/g, an ethylene oxide content in the range of from 0 to 80 wt%, preferably 5 to 50 wt%, based on total weight of the polyol and a functionality of at least 2, preferably of from 2.5 to 4. The polyol component may also comprise a polymer polyol as the sole polyol or in addition to another polyether polyol or polyester polyol. In general, a polymer polyol is a dispersion of a solid polymer in a liquid base polyol. Such systems are well known in the art. Examples of such polymer polyol systems and methods for their preparation are disclosed in, for instance, European patent specifications Nos . 0,076,491; 0,343,907 and 0,495,551. Polyurea or polyurethane polymers are also known to be useful as the dispersed polymer in polymer polyols.

As the base polyol a flexible polyol as described hereinbefore, or a mixture of such flexible polyols, may be used.

The polymer dispersed in the base polyol, may in principle be any such polymer known to be applicable for this purpose. Thus, suitable polymers include the polymers based on ethylenically unsaturated monomers and particularly polymers of vinyl aromatic hydrocarbons, like styrene, alpha-methyl styrene, methyl styrene and various other alkyl-substituted styrenes. Of these, the use of styrene is preferred. The vinyl aromatic monomer may be used alone or in combination with other ethylenically unsaturated monomers, such as acrylo- nitrile, methacrylonitrile, vinylidene chloride, various acrylates and conjugated dienes like 1, 3-butadiene and isoprene. Preferred polymers, however, are polystyrene and styrene-acrylonitrile (SAN) copolymers . Another suitable class of polymers are the polyurea and polyurethane polymers. Particularly the condensation products of polyhydric alcohol amines and aromatic diisocyanates are very useful in this respect. A very much preferred polymer is the condensation product of triethanol amine and toluene diisocyanate (TDI) . The dispersed polymer is preferably present in an amount of from 5 to 40% by weight based on total weight of polymer polyol. In case the polymer is polystyrene or SAN polymer, preferred amounts are between 10 and 35% by weight, whilst in case of polyurea polymers and condensation products of polyhydric alcohol amines and aromatic diisocyanates the preferred amount of polymer is between 5 and 20% by weight.

It will be understood that the polyol component may comprise one or a mixture of two or more of the polyol types described hereinbefore. The ester is the reaction product of an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, suitably between 100 and 150 °C, thereby releasing the acid. A mixture of two or more of such esters may also be used. Without wishing to be bound by any particular theory it is believed that the acid, once released, will deactivate or neutralise any urethane catalyst and/or amines present in the curing foam, in particular those urethane catalysts and amines which are stable at the curing temperatures and which may cause degradation of the polyurethane foam. As indicated hereinbefore, especially organotin catalysts may cause such foam degradation and should be deactivated. Typical temperatures reached during curing of the foam are between 80 and 165 °C, while for safety reasons temperatures between 100 and 150 °C are preferred. Accordingly, during the curing of the foam, when the most of the urethane linkages have been formed, the ester will decompose and release the acid. The ester should not decompose and release the acid at a temperature below 80 °C, because at that temperature the tin catalyst and/or amines are still catalysing the foam formation and hence should not be deactivated. The ester should also not be so stable that it will only starts decomposing and releasing the acid at a temperature above 165 °C, since such high curing temperatures are unsafe and should be avoided.

The ester suitably is an ester obtained by reacting an alcohol and a carboxylic acid or a mineral acid. The acid released will be the same acid as used to prepare the ester and should be sufficiently strong to be capable of deactivating the (organotin) catalyst and/or amine. Typically, the pKa of the acid is less than 6. Preferably, the acid is selected from a monocarboxylic acid, a dicarboxylic acid, a polycarboxylic acid (i.e. tri- and higher carboxylic acids), phosphoric acid, sulphuric acid, sulphonic acid or a mixture of two or more of these.

In general, suitable carboxylic acids are aliphatic, unsaturated or saturated, acids and aromatic acids and suitably contain from 2 to 10 carbon atoms, more suitably from 2 to 5 carbon atoms, and at least one, suitably one to four and most suitably one or two, carboxylic acids groups. Examples of suitable monocarboxylic acids include formic acid, acetic acid, pyruvic acid and lactic acid. Examples of suitable dicarboxylic acids include oxalic acid, maleic acid, fumaric acid, citric acid, malonic acid, methylmalonic acid (1, 1-ethanedicarboxylic acid), adipic acid, butanedioic acid, tartaric acid and phthalic acid. An example of a tricarboxylic acid is 1, 2, 3-propane tricarboxylic acid. Examples of suitable mineral acids are phosphoric acid, sulphuric acid and sulphonic acid. The other building block of the ester, the alcohol, may be any compound containing at least one hydroxy group which is capable of esterifying with an acid. Preferably, it should also be such an alcohol, which upon decomposition of the ester and release of the acid, gives a compound which is volatile at the curing temperature of between 80 and 165 °C. Suitably, the alcohol is an unsubstituted or substituted aliphatic mono-alcohol containing from 2 to 10 carbon atoms, preferably from 3 to 6 carbon atoms. Specific examples include ethanol, propanol, isopropanol, 1-butanol, 2-butanol, tert- butanol, 1, 1-dimethyl-l-propanol (tert-amyl alcohol) and 1, 1-dimethyl-l-butanol . However, the alcohol may also be a polymeric chain containing a terminal hydroxyl group which can form an ester with one the acids listed above, which ester subsequently decomposes between 80 and 165 °C, thereby releasing the acid. Examples of very suitable esters include di (tert- butyl) oxalate, di (tert-amyl) oxalate, tert-butyl pyruvate, tert-amyl pyruvate, tert-butyl formiate, tert- amyl formiate, tri (isopropyl ) phosphate, tri (isopropyl) sulphate, tri (isopropyl ) sulphonate and mixtures of two or more of these esters. Such mixtures may contain the esters in all possible weight ratios.

The ester is suitably used in an amount of from 0.01 to 5 parts by weight per 100 parts by weight of the polyol component (php) , preferably 0.05 to 3.5 php and more preferably from 0.1 to 2 php.

Additionally, it was observed that the final products obtained from formulations containing an ester according to the present invention contained less tributyl tin and dibutyl tin. This is advantageous as these compounds are considered potentially harmful. In addition to the polyol component and the ester the formulation may contain ingredients which are normally used in polyol formulations for preparing polyurethane foams. Accordingly, the present invention also relates to a formulation comprising

(a) a polyol component,

(b) an ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid, which ester is present in an amount of from 0.001 to 8 parts by weight, per 100 parts by weight of polyol,

(c) one or more organotin catalysts,

(d) one or more blowing agents, and

(e) usual auxiliaries. The polyol component and the ester are as described hereinbefore. The ester is suitably used in an amount of from 0.01 to 5 php, more suitably 0.05 to 3.5 php and most suitably 0.1 to 2 php.

Examples of suitable organotin catalysts have already been given in the introductory part of this specification. Preferred organotin catalysts are dibutyl- tindilaurate and stannous octoate, with dibutyl- tindilaurate being most preferred. An extensive list of further suitable polyurethane catalysts is, for instance, given in US-5, 011 , 908. A preferred additional catalyst is an amine, especially a tertiary amine, catalyst. Suitable amine catalysts include an amine group substituted by at least two optionally-substituted, preferably unsubstituted, lower alkyl groups which may be the same or different, but are preferably the same. A lower alkyl group may have up to 8, preferably up to 6, more preferably up to 4, carbon atoms, with methyl and ethyl groups being especially preferred. A tertiary amine catalyst may be selected from bis (2 , 2 ' -dimethylamino) - ethyl ether, trimethylamine, triethylamine, triethylene- diamine, dimethylethanolamine, N, N' , N' -dimethylamino- propylhexahydrotriazine and N, N-dimethylcyclohexylamine . Examples of commercially available tertiary amine catalysts are those sold under the trade names NIAX, TEGOAMIN, JEFFCAT and DABCO (NIAX, TEGOAMIN, JEFFCAT and DABCO are trademarks). Within the polyurethane catalysts and even within the tertiary amine catalysts a distinction can be made between gellation catalysts and blowing catalysts. Gellation catalysts are catalysts which predominantly promote the gellation of the foaming mixture, i.e. which promote the reaction between polyols and polyisocyanate. Blowing catalysts predominantly promote the NCO/H2O reaction, whereby carbon dioxide is released which causes the blowing to occur. Each organotin catalyst is usually present in an amount of from 0.01 to 5 php, preferably from 0.05 to 2.5 php.

As the blowing agent water is suitably used. It is typically used in an amount of from 1 to 5 php and serves as a (chemical) blowing agent. Namely, water reacts with isocyanate groups according to the well known NCO/H2O reaction, thereby releasing carbon dioxide which causes the blowing to occur. In addition to water a conventional blowing agent may be used. Such conventional blowing agents include (liquid) carbon dioxide, halogenated hydrocarbons (HCFC's and HFC's), aliphatic alkanes (e.g. pentane, isopentane) and alicyclic alkanes (e.g. cyclo- pentane) . If used at all, the additional blowing agent will be typically be used in an amount of from about 0.1 to 15 php in case of halogenated hydrocarbons, aliphatic alkanes, alicyclic alkanes and liquid carbon dioxide.

In addition, other well known auxiliaries, such as cross-linking agents, fire retardants, foam stabilisers (surfactants) and fillers may also be used in their usual amounts .

The use of cross-linking agents in the production of polyurethane foams is well known. Polyfunctional glycol amines are known to be useful for this purpose. The polyfunctional glycol amine which is most frequently used is diethanolamine, often abbreviated as DEOA. If used at all, the cross-linking agent is applied in amounts up to 3.0 php, for example from 0.2 to 1.5 php. Organosilicone surfactants are most conventionally applied as foam stabilisers in polyurethane production. A large variety of such organosilicone surfactants is commercially available. Usually, such foam stabiliser is used in an amount of up to 5% by weight based on the reaction mixture of polyol combination and polyisocyanate component .

An important advantage of the present invention is that the use of halogen-containing fire retardants can be avoided. Hence, it is preferred that the formulation is free of such fire retardants.

In a further aspect the present invention relates to a process for the preparation of a polyurethane foam, which process comprises reacting (i) a polyol component, (ii) an ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid, which ester is present in an amount of from 0.001 to 8 parts by weight, per 100 parts by weight of polyol, (iii) one or more blowing agents, (iv) auxiliaries and (v) a polyisocyanate component in the presence of (vi) one or more organotin catalysts. It is preferred that at least one of the catalysts used is an organotin catalyst. Preferred organotin catalysts for this purpose are stannous octoate and in particular dibutyltindilaurate . A preferred process according to the present invention is a process for the preparation of a polyurethane foam, which process comprises mixing a formulation as described above with a polyisocyanate component and allowing the foaming reaction to proceed to completion.

Polyisocyanates that may be used are those conventionally applied in the production of polyurethane foams. Useful polyisocyanates should contain at least two isocyanate groups and include both aliphatic (usually alkylene) and aromatic di-, tri-, tetra- and higher isocyanates known in the art to be suitably applied in the production of flexible polyurethane foams . Mixtures of two or more of such aliphatic and/or aromatic poly- isocyanates may also be applied. Examples of suitable polyisocyanates, include 2,4-toluene diisocyanate (2,4-TDI), 2,6-TDI, mixtures of 2,4-TDI and 2,6-TDI, 1, 5-naphthene diisocyanate, 2 , 4-methoxyphenyl diisocyanate, 4 , 4 ' -diphenylmethane diisocyanate (MDI), 4,4'- biphenylene diisocyanate, 3, 3 ' -dimethoxy-4 , 4 ' - biphenylene diisocyanate, 3, 3 ' -dimethyl-4 , 4 ' -biphenylene diisocyanate and 3, 3 ' -dimethyl-4 , 4 ' -diphenylmethane diisocyanate, 4 , 4 ' , 4 "-triphenylmethane triisocyanate, 2, 4, 6-toluene triisocyanate, 4 , 4 ' -dimethyl-2, 2 ' , 5, 5 ' - diphenylmethane tetraisocyanate, polymethylene-poly- phenylene polyisocyanate, carbodiimide modified isocyanates, MDI prepolymers and mixtures of two or more of these. Polymeric MDI, a mixture of polyisocyanates with MDI as the main component, may also be used. For the purpose of the present invention the use of MDI or a modified MDI is preferred. These are commercially available .

The polyisocyanate is used in such amount that the isocyanate index (i.e. the equivalence ratio of isocyanate groups to hydroxyl groups) is in the range of from 80 to 130, suitably 90 to 120.

Finally, the invention also relates to a polyurethane foam obtainable by a process as described above and to shaped articles comprising this foam. Preferably the foam is a flexible polyurethane foam prepared via slabstock techniques, but flexible moulded foams or rigid foams could also be prepared. Most preferably no halogen- containing fire retardant is used, so that the polyurethane foam is halogen-free.

The invention is illustrated by the following examples without limiting the scope of the invention to these particular embodiments.

In the Examples the following materials were used: Polyol A a polymer polyol containing 18 wt% dispersed polystyrene particles, the base polyol having a molecular weight of 4700, a hydroxyl value of 30 mg KOH/g, a functionality of 3 and an ethylene oxide (tipped) content of 14 wt%.

NIAX Al 70% bis-dimethylaminoethylether in dipropylene glycol (amine catalyst).

DABCO 33LV 33 wt% solution of triethylenediamine in dipropylene glycol (amine catalyst) .

B8681 TEGOSTAB B8681, a silicone surfactant (TEGOSTAB is a trademark) .

DBTDL dibutyltindilaurate (organotin catalyst) . TMCP tris (monochloro-isopropyl) phosphate (liquid fire retardant) .

DEOA diethanolamine . di-tBuOx di (tert-butyl) oxalate. di-tAmOx di (tert-amyl) oxalate. tBuPyr tert-butyl pyruvate.

TDI-80 a blend of 80% weight 2,4-isomer and 20% weight 2,6-isomer of toluene di-isocyanate . Example 1

A foam was prepared by a standard hand-mixing technique as follows. Polyol, water, silicone surfactant, di (tert-butyl) oxalate and amine catalysts were pre- blended in a 800 ml beaker for 30 seconds. The organotin catalyst (DBTDL) was subsequently added and mixed for an additional 10 seconds. Finally, the toluene di-isocyanate was added and stirring was continued for a further 8 seconds . The reaction mixture was then poured into a cardboard box of dimensions 30 x 20 x 15 cm and the foaming reaction was allowed to proceed until completion.

The foam was allowed to cure and its mechanical properties (hardness, tensile strength and elongation) were determined. The foam was also placed in an oven for 2 hours at 155 °C and its properties were determined again afterwards .

The composition of the foaming formulation and the results are indicated in table 1.

The CLD 40% hardness (i.e. the pressure required to achieve a 40% compression load deflection (CLD)) was determined in accordance with DIN 53577. Elongation and tensile strength were determined in accordance with ASTM 3574. Example 2-4 Example 1 was repeated except that di (tert-amyl ) oxalate, tert-amyl pyruvate or a mixture of di (tert- butyl) oxalate and tert-amyl pyruvate (10:90 w/w) were used as the ester instead of di (tert-butyl ) oxalate.

The results are indicated in Table 1. Comparative Examples 1-3

Example 1 was repeated except that in stead of the oxalate either TMCP (C-l and C-3) or no other compound (C-2) was added.

The compositions of the foaming formulations and the results are indicated in table 1. From table 1 it can be seen that before oven treatment the foams prepared from formulations containing the carboxylate esters have similar mechanical properties as the foams based on formulations containing the halogenated phosphate esters (C-1 and C-3) or free of any fire retardant (C-2) . However, after the oven treatment the foams of comparative examples C-1, C-2 and C-3 have clearly suffered from a deterioration of mechanical properties, whereas the mechanical properties of the foams illustrating the invention have not suffered at all from the over treatment and have even slightly improved.

Claims

C L A I S

1. Formulation comprising a polyol component, an organotin catalyst and from 0.001 to 8 parts by weight, per 100 parts by weight of the polyol component, of at least one ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid.

2. Formulation as claimed in claim 1, wherein the polyol component comprises a polyester polyol, a polyether polyol or a combination thereof. 3. Formulation as claimed in claim 1 or 2 , wherein the polyol component comprises a polymer polyol. . Formulation as claimed in any one of the preceding claims, wherein the ester decomposes at a temperature between 100 and 150 °C. 5. Formulation as claimed in any one of the preceding claims, wherein the ester is an ester of an alcohol and an acid selected from a monocarboxylic acid, a dicarboxylic acid, a polycarboxylic acid, phosphoric acid, sulphuric acid, sulphonic acid and mixtures of two or more of these.

6. Formulation as claimed in claim 5, wherein the alcohol is tert-amyl alcohol, tert-butyl alcohol or isopropyl alcohol and the acid is a monocarboxylic acid or a dicarboxylic acid. 7. Formulation comprising

(a) a polyol component,

(b) an ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid, which ester is present in an amount of from 0.001 to 8 parts by weight, per 100 parts by weight of polyol, (c) one or more organotin catalysts,

(d) one or more blowing agents, and

(e) usual auxiliaries.

8. Formulation as claimed in claim 7, wherein the ester is present in an amount of from 0.01 to 5 parts by weight per 100 parts by weight of polyol component (a) .

9. Process for the preparation of a polyurethane foam, which process comprises reacting (i) a polyol component,

(ii) an ester obtained by reacting an acid and an alcohol, which ester decomposes at a temperature between 80 and 165 °C, thereby releasing the acid, which ester is present in an amount of from 0.001 to 8 parts by weight, per 100 parts by weight of polyol, (iii) one or more blowing agents, (iv) auxiliaries and (v) a polyisocyanate component in the presence of (vi) one or more organotin catalysts .

10. Process for the preparation of a polyurethane foam, which process comprises mixing a formulation as claimed in claim 7 or 8 with a polyisocyanate component and allowing the foaming reaction to proceed to completion.

11. Polyurethane foam obtainable by a process as claimed in claim 9 or 10.

12. Shaped article comprising the polyurethane foam of claim 11.