WO2004000906A1

WO2004000906A1 - Low emission tin catalysts

Info

Publication number: WO2004000906A1
Application number: PCT/EP2003/006265
Authority: WO
Inventors: Oliver Schumacher
Original assignee: Crompton GmbH
Current assignee: Lanxess Organometallics GmbH
Priority date: 2002-06-21
Filing date: 2003-06-13
Publication date: 2003-12-31
Anticipated expiration: 2004-12-21
Also published as: EP1375546A1; JP2005530016A; ZA200409958B; DE60320416D1; KR20050012286A; CA2490252A1; EP1532186B1; AU2003242702A1; TW200400210A; ATE392445T1; US20060111516A1; MXPA04012883A; EP1532186A1; BR0312164A

Abstract

Plastic articles with low emission obtainable by polymerization, condensation, and/or cross-linking reaction including the use of metal catalysts wherein said metal catalyst has a low emissivity and is an organotin compound of the general formula R2SnX2 wherein R is a C1-C8-hydrocarbyl, X is a carboxylate group with 14-20 carbon atoms having at least one olefinic double bond. Moreover, the invention relates to the use of an organotin compound in the manufacture of plastic articles with low emissivity of said organotin compound.

Description

LOW EMISSION TIN CATALYSTS

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention is related to the use of simple tin catalysts for the

manufacturing of polyurethane foams with significantly reduced emission. More

particularly, the present invention is related to the use of dialkyltin dicarboxylates

₂SnX₂ which are derived from carboxylic acids with particularly low emissivity,

but provide high activity for catalyzing the reaction of isocyanates with polyols and are highly compatible with the components of typical polyurethane formulations.

2. Description of Related Art

Tin compounds are well known as very effective catalysts for the manufacturing of polyurethanes, silicones, and polyesters.

Polyurethanes are basically manufactured by reaction of isocyanates with polyols. Commonly used isocyanates are either aromatic or aliphatic di- or polyiso-

cyanates, commonly used polyols are either polyetherpolyols or polyesterpolyols.

Polyurethanes derived from aliphatic isocyanates have the general advantage of a better light stability than polyurethanes derived from aromatic isocyanates. Aliphatic isocyanates are generally less reactive than aromatic isocyanates and hence require

particularly strong catalysts; typically organotin catalysts are used, either alone or in

combination with other catalysts.

Polymers, and in particular polyurethanes are of increasing importance in the

manufacturing of modern car interiors. E.g. US 5656677 teaches the use of polyure- thane foams derived from aliphatic isocyanates for the manufacturing of light stable car interiors.

A general problem connected with the use of plastics in car interiors is the emission of volatile organic compounds at elevated temperatures; said volatile organic compounds may form condensate films on the car windows, reducing the visual transparency and thereby causing the so called "fogging effect". The emissivity ("fogging") properties of a plastic material are determined either by the amount (by weight) of condensate formed under defined conditions, or by the loss of transparency caused by this condensate on a glass sheet. Modern plastic materials are formulations of different base materials and additives, which can separately or in combination contribute to the fogging. Several efforts have been undertaken to reduce the fogging from plastic materials by optimising base materials and additives. In manufacturing of polyurethanes, e.g., major achievements have already been made by the introduction of purified polyes- terpolyols (with reduced contents of volatile cyclic esters), and by the elimination of volatile antioxidant additives (see e.g.: EP 1153951 to Bayer; DE 19611670 to BASF; G. Baatz , S. Franyutti, Paper 9, UTECH '94 Conference,.1994, The Hague). Facing increasingly tight regulations and consumer demands, further reductions of emission levels are required. After elimination of the previous main contributors, further improvement has to target the so-far neglected minor additives. Among said additives, particularly urethane catalysts contribute to the fogging.

Common catalysts for the urethane reaction are tertiary amines, stannous tin compounds, dialkyltin compounds, and compounds of other metals. The mentioned classes of catalysts may contribute to fogging either because of their own volatility (e.g. amines), or by formation of volatile reaction products or degradation products.

Attempts have been reported to reduce the fogging properties of said catalysts: using

catalysts which are reactive with isocyanates can lead to firm fixation of those catalysts in the polymer matrix and thereby reduce fogging. Examples for isocy-

anate-reactive amines are given e.g. in EP0799821 (and in the literature cited there).

Examples for isocyanate-reactive dialkyltin catalysts are given e.g. in EP0417605. A

general drawback of such isocyanate-reactive catalysts is their reduced catalytic

activity. Also, reaction with the isocyanate and incorporation into the polymer

matrix changes the polymer properties. A useful polyurethane catalyst must have high activity for the urethane reac¬

tion, and a sufficiently high selectivity for the urethane reaction over undesired side reactions. Furthermore, it should be storage stable, readily soluble in and compatible with the polyols and or the isocyanates, and best be liquid at ambient temperature.

Dialkyltin compounds are well known for their strong catalytic power in

polyurethane reactions, and are often indispensable in order to achieve the required

material properties. Particularly useful are dialkyltin dicarboxylates. Among the dialkyltin dicarboxylate polyurethane catalysts, dimethyltin dicarboxylates are the

strongest.

The most common carboxylate types for dialkyltin dicarboxylate catalysts

are acetate, 2-ethylhexanoate, neodecanoate, and laurate. All dialkyltin carboxylates

containing these carboxylate types contribute to fogging, not only by their own volatility, but particularly by the volatility of their degradation products, the most

important being the corresponding carboxylic acids.

It can be reasonably expected that dialkyltin dicarboxylates derived from car- carboxylic acids with longer alkyl chain than lauric acid would contribute less to

fogging.

When simply the length of the carboxylate alkyl chain of a dialkyltin car¬

boxylate is further increased (e.g. to saturated Cι₃ - Cπ), one significant drawback

is a decrease in the catalytic activity. Even more important drawbacks are the higher

melting points ( e.g. dimethyltin dimyristate approx. 70°C, dimethyltin dipalmitate

approx. 80°C), the limited solubility in the typical main components of polyurethane

formulations (i.e. polyols and/or isocyanates), and the limited compatibility with

said main components.

Certain dialkyltin dicarboxylates having 13 or more carbon atoms and at

least one olefmic double bond in the carboxylate alkyl chain are liquid at ambient

temperature. Example are oleates, ricinoleates, linolates, and linoleates of di¬

methyltin and dibutyltin.

E.g., dimethyltin dioleate has been described as a heat stabiliser for PVC. No

reference to polyurethane catalysis was made. Furthermore, GB 1250498 teaches the

use of a "basic dimethyltin oleate" as a curing catalyst for silicone rubbers. Said "

basic dimethyltin oleate" is described as a "Harada complex" R₂SnA₂*R₂SnO;

according to the modern state-of-the-art, it would be called l, ,3,3'-tetramethyl-

1 ,3-oleoyloxo- 1 ,3 ,2-stannoxane.

E.g., dibutyltin dioleate has been described as a heat stabiliser for PVC, as

solvent extraction agent for arsenate ions, as catalyst for esterifications, as catalyst

for curing of silicones and as catalyst for curing of electrodeposition coatings. One

publication (R. V. Russo, J. Cell. Plast. Y2, (1976), 203) reported comparative

testing of dibutyltin dioleate as polyurethane foam catalyst, but said article teaches that dibutyltin dioleate is a particularly poor catalyst. No reference to emissivity was made.

E.g., use of dioctyltin diricinoleate has been reported as a polyurethane gela¬

tion catalyst, having reduced toxicity (US4332927 to Caschem). No reference to

emissivity was made.

SUMMARY OF THE INVENTION

The present invention is directed to low emission organotin compounds of the general formula

R₂SnX₂ wherein R is -Cs-hydrocarbyl, preferred are methyl and butyl, particularly

preferred is methyl. X is a carboxylate group with 14-20 carbon atoms having at

least one olefinic double bond, optionally substituted; preferred are oleate, rici-

noleate, linoleate and linolenate; particularly preferred is oleate.

These compounds can be used as low emission catalysts in all fields of applications where organotin compounds are known to be useful as catalysts. Such fields

include, but are not limited to, catalysis of esterification and transesterification

reactions, condensation curing of RTV LT silicones, curing of cataphoretic electrode- position coatings, deblocking of blocked isocyanates, and, especially, curing of the

synthesis of polyurethanes by the reaction of isocyanates with polyols. Advantageous is particularly the low emissivity, but high activity for catalyzing the reaction

of isocyanates with polyol, and high compatibility with the typical components of polyurethane formulations. The invention is further directed to the use of said organotin compounds_. as

catalysts for the production of low emission polyurethanes, particularly for use in car

interiors. Especially preferred is the use of said catalysts for the production of

polyurethanes derived from aliphatic isocyanates. The inventive catalysts can be used alone or in combination with other catalysts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to low emission dialkyltin dicarboxylates of

the general formula

R₂SnX₂

R is a CrCrhydrocarbyl group. Typically, R is an aliphatic, saturated, unbranched,

and not further substituted alkyl group, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl,. Preferred alkyl groups are methyl, butyl, and octyl. Particularly preferred is methyl.

X is a carboxylate group derived from a carboxylic acid of the type

R'-COOH wherein R' is a Cπ- g-hydrocarbyl group having one or more olefinic double bonds. Typically, R' is an aliphatic and unbranched alkenyl group; it may be further

substituted, e.g., with one or more hydroxy groups. The alkenyl group maybe

present in the cis-form, or in the trans-form, or a mixtures of both forms.

Preferred carboxylate groups are oleate, ricinoleate, linoleate and linolenate.

Particularly preferred is oleate.

Said low emission dialkyltin dicarboxylates can be synthesised from commercially

available raw materials using standard synthesis methods for dialkyltin dicarboxy- lates; e.g. by reaction of dialkyltin oxides with carboxylic acids, or by reaction of dialkyltin dichlorides with alkali carboxylates, or by reaction of dialkyltin dichlo-

rides with carboxylic acids and bases, etc.

Said low emission dialkyltin dicarboxylates are liquid at room temperature, or melt

at a low temperature slightly above room temperature. They are well soluble or

mixable with polyetheφolyols and/or polyesteφolyols, which are widely used in the manufacturing of polyurethanes. Several of said low emission dialkyltin dicarboxy¬

lates are soluble in aliphatic and/or aromatic isocyanates, which are widely used in

the manufacturing of polyurethanes. When dissolved, they have an excellent compatibility with said polyols and isocyanates, and do not precipitate from solution when stored at ambient temperature. When dissolved in isocyanates, they do not promote the formation of isocyanurates (isocyanate trimers), which is a common

side-reaction of several other organotin catalysts.

Said low emission dialkyltin dicarboxylates have only very low volatility, and when degraded by hydrolysis, alcoholysis, acidolysis, or related reactions, the degradation

products formed therefrom also have only very low volatility.

Said low emission dialkyltin dicarboxylates have a high but at least sufficient catalytic activity for catalyzing the reaction of isocyanates with alcohols to form urethanes.

The present invention is further directed to the use of said low emission dial¬

kyltin dicarboxylates as catalysts for the production of low emission polyurethanes or polysilicones. Said low emission polyurethanes or polysilicones produced by to the use_of

said low emission dialkyltin dicarboxylates may appear in any form generally

applicable to polyurethanes or polysilicones, as foams (rigid, flexible, high resiliency, integral, microcellular), RIM, RRTM, elastomers, coatings, etc.

By use of said low emission dialkyltin dicarboxylate catalysts, any general

type of low emission polyurethane or polysilicone may be produced: foams (rigid, flexible, high resiliency, integral, microcellular ...), RTJVI, RRTM, elastomers, coatings, etc.

Preferred low emission polyurethanes are polyurethane foams.

Also preferred low emission polyurethanes are light stable polyurethanes de¬

rived from aliphatic isocyanates.

In manufacturing of low emission polyurethanes, the inventive low emission

dialkyltin dicarboxylate catalysts can be used either alone or in combination with

other catalysts. Especially, the well know synergy of dialkyltin compounds with tertiary amines in the catalysis of the urethane reaction may be used to enhance the

catalytic power of the inventive dialkyltin dicarboxylate catalysts. Also, in the production of water blown foam, tertiary amine catalysts may be used to speed and

direct the reaction of isocyanates with water. Examples of further common catalysts

which may be used together with the inventive catalysts include metals compounds

of stannous tin, Ti, Pb, Hg, Bi, Fe, Ni ...

In production of a polyurethane, the inventive low emission dialkyltin dicar¬

boxylate catalysts can be either added prior to the reaction to the polyol component,

or to the isocyanate component, or it can be admixed with other additives to form a master blend, or it can be added directly to the reaction mixture. The isocyanates commonly used in the production of polyurethanes are well

know to those skilled in the art. Examples include TDI (toluene diisocyanate,

typically mixtures of the para-isomer, and the ortho-isomer), MDI (4,4'-

diphenylmethane diisocyanate), polymeric MDI, IPDI (isophorone diisocyanate),

HDI (hexamethylene diisocyanate). The isocyanates are either used as such, or can

also be used in a blocked form; when used in a blocked form, the blocking agent has

to be cleaved of the isocyanate shortly before or during the processing.

The polyols commonly used in the production of polyurethanes are also well known to those skilled in the art. The most important classes are polyesteφolyols

and polyetheφolyols, which are basically polyester resp. polyether chains, termi¬

nated and optionally further substituted with isocyanate-reactive hydroxyl groups. E.g., the most commonly used polyetheφolyols are derived from ethylen oxide

and/or propylene oxide.

The polyurethane may contain further additives (like blowing agents, foam

stabilisers, chain extenders, flame retardants, fillers, pigments etc.), known to those

skilled in the art.

The present invention is further directed to the use of said low emission

polyurethanes for use in car interiors.

The advantages and the important features of the present invention will be

more apparent from the following examples.

EXAMPLES

Glossary: Polyol 1 is a 3500 MW polyether polyol, (OH-No. approx. 38) available from Elastogran as Lupranol 3032.

Polyol 2 is a 4700 MW polyether polyol (OH-No. approx. 36), available from Shell

Chemicals as Caradol ET 36-17.

Polyol 3 is a 6000 MW polyether polyol (OH-No. approx. 28), available from DOW

Chemicals as Voranol CP 6001.

Polyol 4 is a 6000 MW polyether polyol (OH-No. approx. 32-35), available from

DOW Chemicals as Voranol CP 1421.

Isocyanate 1 is toluene diisocyanate (TDI, mixture of 80% para, and 20% ortho).

Isocyanate 2 is Isophorone diisocyanate (LPDi). Isocyanate 3 is 4,4'-Diphenylmethane diisocyanate (MDI 2447).

Foam stabiliser 1 is a silicone, available from Crompton Coφ. as Niax RS-171.

Amine cocatalyst 1 is Diethanolamine (DEOA).

Amine cocatalyst 2 is blend of bis(dimethylaminoethyl)ether and dipropylene glycol, available from Crompton Coφ. as Niax A-l.

Example 1

Preparation of the Catalysts 1 a) Preparation of dimethyltin dioleate from dimethyltin dichloride

Into a 3 -neck glass flask, equipped with a mechanical stirrer, thermometer,

, dropping funnel, and pH glass electrode, were placed 44 g of dimethyltindichloride

(0.2 mol) and 44 g of water. The mixture was stirred until the dimethyltindichloride

is completely dissolved. 113 g of oleic acid (0.4 mol) were added and the mixture

was heated to 60°C.

An aqueous NaOH solution (35.5% by weight) was placed into the dropping funnel. While stirring, the NaOH solution was slowly added to the reaction mixture.

NaOH addition was stopped when a pH of approx. 6 had been reached.

The mixture was heated to approx. 80 °C, then the stirrer was stopped and the phases allowed to settle.

The phases were separated and the lower (aqueous) phase discarded. The or¬

ganic phase was dried in a rotary evaporator at approx. 80°C / 1 mbar, and subse¬

quently further dried with Na2SO4. Finally, 1% of Celite (a filter aid) were added and the product was filtered.

Yield: 136.6 g of dimethyltin dioleate (96.0% of theor.). The product was a

clear yellow liquid, and contained 15.8% Sn (theor. 16.7%), and 0.0% Cl (theor.

0.0%).

1 b) Preparation of dimethyltin dioleate from dimethyltin oxide Into a 3 -neck glass flask, equipped with a mechanical stirrer, thermometer,

and a vacuum connector, were placed 57.7g of dimethyltin oxide (0.35 mol) and

197.6 g of oleic acid (0.7 mol). While stirring, the mixture was heated to 40°C, and a vacuum of 10 mbar was applied. During 1 hour the temperature was slowly risen

to 70°C, and was subsequently held for another hour. Subsequently a vacuum of 1

mbar was applied, and the reaction mixture was further stirred for 1 more hour.

The vacuum was broken, and the reaction mixture was allowed to cool to

room temperature. Finally, 1% of Celite (a filter aid) were added and the product

was filtered.

Yield: 245.8 g of dimethyltin dioleate (98.7% of theor.). The product con¬

tained 16.5% Sn (theor. 16.7%). It was a clear yellow liquid, having a viscosity of 100 mPa*s. It was miscible

with Polyols 1, 2, and 3. It was readily soluble in Isocyanates 1, 2, and 3.

A 1% solution (by weight) of the product in Isocyanate 2 was prepared and

stored at 25°C. After 3 weeks the solution was still clear, no solid material had formed and the infrared spectrum of the solution did not show the carbonyl band of

an isocyanurate.

1 c-g ) Preparation of further dialkyltin dicarboxylates

Following the procedure described in example 1 b, the following materials were synthesised (see table 1):

to Table 1

Example 2

Catalyst Activity Tests

Viscosity Measurement in Elastomers based on aromatic Isocyanates (TDI)

80 g of Polyol 1 were placed at room temperature into a dry 100 mL wide-

neck glass bottle. 0.0002 mol of the respective organotin catalyst were added. The mixture was stirred for 2 minutes to dissolve the catalyst.

0.036 mol of Isocyanate 1 were added, and the mixture stirred for 2 more

minutes. The bottle was then placed under a Brookfield rotary viscosimeter. The raw

mixture had a Brookfield viscosity of « 1 Pa*s. Sample temperature and viscosity were recorded until the mixture became too viscous for further measurement (> 25

Pa*s). In each experiment, the time of isocyanate addition to the polyol considered as the start of the reaction (t = 0 min). Results are summarised in table 2.

Table 2

Example 3

Catalyst Activity Tests

Viscosity Measurement in Elastomers based on aromatic Isocyanates (TDI) Example 2 was repeated with the difference that after mixing of all components at room temperature the glass bottle was immersed in an oil heating bath. The oil bath was heated at a nearly constant rate from room temperature to 100°C, and was than held at this temperature (heating to 100°C takes typically approx. 20 minutes). In each experiment, the time of isocyanate addition to the polyol consid- ered as the start of the reaction (t = 0 min). Results are summarised in table 3.

Table 3

Example 4

Viscosimetric Catalyst Activity Tests

Elastomer based on aliphatic Isocyanate(IPDI) 75 g of Polyol 2 were placed at room temperature into a dry 100 mL wide-

neck glass bottle.

In a dry glass flask, 0.00016 mol of the respective organotin catalyst and 5,6 g of Isocyanate 2 are mixed by stirring.

The isocyanate/catalyst mixture is added to the polyol at room temperature, and the mixture stirred for 2 more minutes. The glass bottle was immersed in an oil heating bath, placed under a Brookfield rotary viscosimeter. The oil bath was heated

at a nearly constant rate from room temperature to 100°C, and was than held at this temperature (heating to 100°C takes typically approx. 20 minutes). The raw mixture had a Brookfield viscosity of « 1 Pa*s. Sample temperature and viscosity were recorded until the mixture became too viscous for further measurement (> 25 Pa*s).

In each experiment, the time of isocyanate addition to the polyol considered as the

start of the reaction (t = 0 min). Results are summarised in table 4.

Example 5

Preparation of Polyurethane Foams

Water blown Foams from Polyether Polyols and aromatic Isocyanate (MDI)

100 g of Polyol 3 and 2 g of Polyol 4 were mixed and placed into a cardboard cup.

A master blend was made of 3.6 g of water, 0.5 g of Foam stabiliser 1, 0,6 g of Amine

cocatalyst 1, and 0.15 g of Amine cocatalyst 2. The blend was added to the polyol and mixed.

0.5 g of the resp. organotin catalyst was added to the mixture and the mixture was stirred for 2 minutes.

61.8 g of Isocyanate 3 (index 100) were quickly added to the mixture. The mixture was

stirred for 10 seconds, and than poured into a cardboard box.

A polyurethane foam formed and expanded. Cream time and rise time of the foam were recorded.

Results are summarised in Table 5.

Table 5

Example 6

Determination of the Fogging of Catalysts by Gravimetry

A dry, clean round piece of aluminum foil (diameter 103 mm, thickness 0.03 mm) was

weighed. 5 g of the resp. liquid organotin catalyst and 0.5 g of water were placed onto the bottom of a dry and clean glass beaker (inner diameter 80 mm, outer diameter 90 mm).

A silicone rubber ring was fitted to the neck of the beaker, the aluminum foil was

placed on top of it, and covered with a glass sheet (110 x 110 x 3 mm). The beaker was hang

into a thermostated glycerol heating bath in such a way, that the glass sheet was 60 mm above

the glycerol level. An aluminum cooling block (connected to another thermostat) was placed onto the glass sheet. For 16 hours, a glycerol bath temperature of 100°C, and a cooling block temperature of 21°C was maintained. Subsequently, the aluminum foil was placed into a

dessicator and kept there for 1 hour at room temperature over silica.

The aluminum foil was then weighed again, and the weight difference (in mg) was recorded as mg of fogging condensate.

Results are summarised in Table 6.

Table 6

Example 7

Determination of the Fogging of Polyurethane Foams by Gravimetry

Foams based on Polyether Polyols and aromatic Isocyanate (MDI)

The foam samples prepared in Example 5 were cut into round disks (each 80 mm in

diameter, and 10 g of weight). Example 6 was repeated with the difference, that instead of 5 g of the resp. liquid organotin catalyst and 0.5 g of water, now the resp. foam disks were placed onto the bottom of the glass beaker.

Results are summarised in Table 7.

Table 7

In view of the many changes and modifications that can be made without departing

from principles underlying the invention, reference should be made to the appended claims for an understanding of the scope of the protection to be afforded the invention.

Claims

Plastic articles with low emission obtainable by polymerization, condensation, and/or cross-linking reaction including the use of metal catalysts wherein said metal catalyst has a low emissivity and is an organotin compound of the general formula

R₂SnX₂

wherein R is a CrCs-hydrocarbyl, X is a carboxylate group with 14-20 carbon atoms having at least one olefmic double bond with the proviso that

a) said plastic article is not made of polyvinyl chloride including dimethyltin dioleate or dibutyltin dioleate as heat stabilizer,

b) said organotin compound is not dibutyltin dioleate and said plastic article is not a polyurethane foam,

c) said organotin compound is not dibutyltin diricinoleate and said plastic article is not a polyurethane,

d) said organotin compound is not dibutyltin dioleate and said plastic article is not polyester or cured silicone.

2. Plastic article according to claim 1, wherein in said organotin compound R is an aliphatic, saturated alkyl group.

3. Plastic article according to claim 2, wherein in said organotin compound at least one alkyl group is methyl, butyl, or octyl.

4. Plastic article according to anyone of claims 1 to 3, wherein in said organotin compound X is a carboxylate group derived from a carboxylic acid of the type

R'-COOH

wherein R is a C₁₃-Ci₉-hydrocarbyl group having one or more olefinic double bonds.

5. Plastic article according to claim 4, wherein said olefinic double bonds are isolated double bonds.

6. Plastic article according to claim 4 or 5, wherein R' is an aliphatic, substituted or unsubstituted alkenyl group.

7. Plastic article according to anyone of the preceding claims, wherein in said organotin compound said hydrocarbyl and/or carboxylate group is a linear group.

8. Plastic article according to anyone of the preceding claims, wherein in said organotin compound the carboxylate group is selected from:

oleate, ricinoleate, linoleate and linoleate.

9. Plastic article according to anyone of the preceding claims, wherein said organotin compound is liquid at room temperature (20-25°C).

10. Plastic article according to anyone of the preceding claims, wherein said plastic article is made of polyurethane or polysilicone.

11. Plastic article according to anyone of the preceding claims, wherein said plastic article is a foamed article.

12. Plastic article of claim 11, wherein the polyurethane foam is derived from aliphatic isocyanate.

13. Use of an organotin compound according to anyone of claims 1 to 10 in the manufacture of plastic articles with low emissivity of said organotin compound. i

14. The use of claim 13, wherein said plastic article is a foamed article.

15. The use of claim 13 or 14, wherein said plastic article is made of polyurethane or polysilicone.

16. The use of any of claims 13 to 15 wherein said organotin compound is dioctyltin dicarboxylate.

17. The use of claim 16 wherein the carboxylate group is selected from:

oleate, ricinoleate, linoleate and linoleate.

18. The use of claim 17 wherein the organotin compound is dioctyl dioleate.