CA1231703A

CA1231703A - Grease composition

Info

Publication number: CA1231703A
Application number: CA000465412A
Authority: CA
Inventors: John P. Doner; Andrew G. Horodysky; John A. Keller, Jr.
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1984-02-06
Filing date: 1984-10-15
Publication date: 1988-01-19
Also published as: EP0151859A2; EP0151859A3; DE3485789D1; EP0151859B1; BR8406567A; DE3485789T2; AU3483484A; AU575596B2; JPS60166399A; NZ209963A; JPH0694557B2

Abstract

GREASE COMPOSITION

ABSTRACT

Grease compositions, wherein the grease is thickened with a metal hydroxy-containing soap grease thickener are provided. Other essential ingredients of the compositions include borated hydroxy-hydrocarbyl esters and phosphorus and sulfur moieties.

Description

~33~1L7~

F-2876(2678) - 1 -GREASE COMPOSITION

The invention is concerned with a novel group of oompositions.
It more p~rticularly relates to a grease oomposition comprising oil, hydr~xy-containlng soap thickener and boratèd hydroxy-containing esters and optionally containLng phosphorus and sulfur moieties.
Alcohols are well kncwn for their lubricity properties when formulatel into lubricating oils and for their water scavenging characteristics when blended into fuels. The use of vicinal hydroxyl-containing alkyl carboxylates s~ch as glycerol monooleate have also found widespread use as lubricity additives. U.S. Patent 2,788,326 discloses some of the esters suitable for the pres~nt invention, such as glycerol ~Dnooleate, as munor components of lubricating oil oompositions. U.S. Patent 3,235,498 discloses the same ester as an additive to other oils. U.S. Patent 2,443,578 teaches est~rs wherein the fr e hy~roxyl is found in the acid portion, as for example, in tartaric acid.
U.S. Patent Nos. 2,798,083, 2,820,014, 3,115,519, 3,282,917 and 3,309,318, as well as an article by R.R. Barnes et al entitled "Synthetic Ester Lubricants" in Lubrication Engineering, August, 1975, pp. 454-457, teach lubricants prepared by polyhydric alo~hols and acids oontaimng n~ hy~ro~yls other than those asscciated with the acid functionO
In accardance with the invention, there is provided an improved grease compDsitlon oontaining a major proportion of a grease and a minor amDunt of a oo~pound prepared by reacting a hydrccarbyl ester of ~he form~la R(OOOR )n wherein R and R are hy~rccarbyl groups, or hy~roxyhydrocarbyl groups, oDntaming 1 ~o 40 carbon atoms, preferably 8 to 20 carbon ~kZ3 1~3 .

F~2876(2678~ - 2 -atoms~ at least one of R or Rl being a hydroxyhydrocarbyl group, and n is 1 to 5, with a boron compound which may be a metaborate or similar boron source, boric acid, boric oxide or an alkyl borate of the formula (R20)xB(OH)y wherein x is 1 to 3, y is O to 2, their sum being 3, and R2 is an alkyl group containing from 1 to 6 carbon atoms, the improvement comprising employing at least 15% by weight of a hydroxy-containing soap thickener to prepare the grease. The presence of phosphorus and sulfur moieties provides an even higher dropping point. Hydrocarbyl and hydroxyhydrocarbyl include alkyl, aryl, aralkyl, alkaryl and cycloalkyl groups. Preferably the ester is overborated, meaning the presence in the product of more than a stoichiometric amount of boron.
The esters may contain from l to 5 hydroxy groups in the molecule. They may all be attached to either R or Rl or they may be attached to R and Rl in varying proportions. Further, the hydroxy groups can be at any position or positions along the chain of R or Rl. It will be understood that the esters contain at least one free hydroxy group prior to reaction with the boron compound.
The disclosed esters can be made by methods well known in the art. In general, they are made by reacting the desired acid or acyl halide and alcohol at temperatures and for -~imes one skilled in the art can easily select. To give an ester having a free hydroxy group, eithe~ in R or Rl, reactant proportions are chosen accordingly, as one:s~illed in the art will understand. Suitable hydroxy-containing esters-prior to boration include trimethylolpropane monooleate, trimethylolpropane dioleate, trimethylolpropane monostearate, trimethylolpropane distearate, trimethylolpropane monopelargonate, trimethylo1propane monooleate monopelargonate, pentaerythrit~l monooleate, pentaerythritol dioleate, pentaerythritol trioleate, pentaeryt.hritol monoisostearate, pentaerythritol monooleate ~ 3~.7t)3 F-2876~2678) - 3 -monopelargonate, p~ntaerythritol monoisostearate, monopelargonate or monohexanoate, pentaerythritol tripelargonate, glycerol monooleate, glycerol dioleate, glycerol monostearate, glycerol monoricinoleate, glycerol monoisostearate, glycerol nopelargonate, glycerol monohexadecanoate, sorbitan monooleate, sorbitan dioleate, sorbitan monostearate, sorbitan distearate, sorbitan monopelargonate, dipentaerythritol monooleate, dipentaerythritol monooleate, dipentaerythritol dioleate, dipentaerythritol trioleate, ethyleneglycol monooleate, ethyleneglycol monostearate, ethyleneglycol monoisostearate, diethyleneglycol monooleate, diethyleneglycol monopelargonate, diethyleneglycol monoisostearate and triethyleneglycol m~r~oleate, methyl hydrD~ystearate and ethyl hydroxystearate, or mixtures of the above.
The borated esters o~ the invention can be made by reacting an ester as defined above with a boron compound, such as boric oxide, a metaborate, boric acid, an alkyl borate or mixtures thereof. The resulting products are primarily borate esters, but other possible products present are the products of reaction between ester dimers, or higher oligomers, and a boron compound to form the corresponding borate esters.
As noted hereinabove, the boron compound used is boric acid, boric oxide or an alkyl borate, preferably boric acid. The alkyl borates include the mono-, di- and trialkyl borates, such as the mono-, di- and trimethyl borates, mono-~ di- and triethyl borates, mono-, di- and tripropyl borates9 mono-, di- and tributyl borates, mono-, di- and tripentyl borates and mono-, di- and trihexyl borates.
The reaction to form the borate ester can be carried out at from 80C to 260C, preferably from 110C to 180C. The ~emperature chosen will depend for the most part on the particular reactants and on whether or not a solvent is used. Reaction pressures can be vacuum, atmospheric, or positive pressure. In carrying out this reaction, it is preferable that quantities of reactants be chosen such that the molar ratio of ester to boron compound be from 1 to 4, preferably from 1 to 2, or more preferably, of about 1. The ester may ,~
.~ .

~;~3:~703 F-2876(2678) - 4 -be reacted with an excess of the borating species to form a borate ester containing from 0.1% by weight of boron to more than 10% of boron.
While atmospheric pressure is generally preferred, the reaction can be advantageously run at from 1 to 5 atmospheres.
Furthermore, where conditions warrant it, a solvent may be used. In genéral, any relatively non-polar, unreactive solvent can be used, including benzene, toluene, xylene and 1,4-dioxane. Other hydrocarbon and alcoholic soluents, which include propanol, butanol, and hexamethyleneglycol can be used. Mixtures of alcoholic and hydrocarbon solvents can be used also.
The reaction time is not critical. Thus, any phase of the process can be carried out in from 1 to 20 hours.
A narrow class of thickening agents is used to make the grease of this invention. The thickening agents contain at least a portion of alkali metal, alkaline earth metal or amine soaps of hydroxyl-containing fatty acids, fatty glycerides and fatty esters having from 12 to 30 carbon atoms per molecule. The metals are typified by sodium, lithium, calcium and barium. Preferred is lithium. Preferred members among these acids and fatty materials are 12-hydroxystearic acid and glycerides containing 12-hydroxystearates, 14-hydroxystearic acid, 16-hydroxystearic acid and 6-hydroxystearic acid.
The entire amount of thickener need not be derived from the aforementioned members. Significant benefit can be attained using as little thereof as 15% by weight of the total thickener. A
complementary amount, such as up to 85% by weight of a wide variety Or thickening agents, can be used in the grease of this invention.
Included among the other useful thickening agents are alkali and alkaline earth metal soaps of methyl-12-hydroxystearate, diesters of a C4 to C12 dicarboxylic acid and tall oil fatty acids. Other alkali or alkaline earth metal fatty acids containing from 12 to 30 carbon atoms and no free hydroxyl may be used. These include soaps of stearic and oleic acids.

~23$7(!3 F-287~(267~) - 5 -Manufacture of the thickening agents can be done in a variety of grease-making equipment such as open kettles at reduced, atmospheric, or positive pressures; higher pressure reactîon chambers which may be operated as high as 180 psig, dr continuous manufacturing equipment. The temperature range from the bulk grease under manufacture may range from 15C ~60F) to 238C (460F).
Other thickening agents include salt and salt-soap complexes as calcium stearate-acetate (U.S. Patent No. 2,197,263), barium stearate acetate (U.S. Patent No. 2,564,561), calcium, stearate-caprylate-acetate complexes (U.S. Patent No. 2,999,065), calcium caprylate-acetate (U.S. Patent No. 2,999,065), and calcium salts and soaps of low-, intermediate and high-molecular weight acids and of nut oil acids.
Another group of thickening agents comprises substituted ureas, phthalocyamines, indanthrene, pigments such as perylimides, pyromellitdiimides, and ammeline, as well as certain hydrophobic clays. These thickening agents can be prepared from clays which are initially hydrophilic in character, but which have been converted into a hydrophobic condition by the introduction of long-chain hydrocarbon radicals into the surface of the clay prior to their use as a component of a grease composition, as, for example, by being subjected to a preliminary treatment with an organic cationic surface active agent, such as an onium compound. Typical onium compounds are tetraalkylammonium chlorides, such as dimethyl dioctadecyl ammonium chloride, dimethyl dibenzyl ammonium chloride and mixtures thereof.
This method of conversion is well known to those skilled in the art.
The third member(s) that may be present in the grease composition are the phosphorus and sulfur moieties. Both of these can be present in the same molecule, such as in a metal or non-metal phosphorodithioate of the formula _ _ "
(R30)2 PZ n M

~2~3~7~3 F-2876t2678) - 6 -wherein R3 is a hydrocarbyl group containing 3 to 18 carbon atoms, M
is preferably a metal, but may be a non-metal, such as one of those mentioned hereinbelow, n is the valence of M and Z is oxygen or sulfur, at least one Z being sulfur.
In this compound, R3 is preferably an alkyl group and may be a propyl, butyl, pentyl, hexyl, octyl, decyl, dodecyl, tetradecyl or octadecyl group, including those derived from isopropanol, propanol, butanol, isobutanol, sec-butanol, 4-methyl-2-pentanol,

2-ethylhexanol, oleyl alcohol, and mixtures thereof. Further included are alkaryl groups such as butylphenyl, octylphenyl, nonylphenyl and dodecylphenyl groups.
The metals embraced by M include those in Groups IA, IIA, IIB
and VIII of the Periodic Table. Some that may be men~ioned are lithium, sodium, calcium, zinc, cadmium, silver, gold and molybdenum.
Non-metallic ions include organic groups derived from vinyl esters such as vinyl acetate, vinyl ethers such as butyl vinyl ether and epoxides such as propylene oxide and 1,2-epoxydodecane. The non-metallic ions may also be derived from nitrogenous compounds such as those derived from hydrocarbyl amines and diamines, including oleylamine and N-oleyl-1,3-propylenediamine and such as the imidazolines and oxazolines.
The phosphorus and sulfur can also be supplied from the combination of t~o separate compounds, such as the combination of (1) a dihydrocarbyl phosphite having 2 to 10 carbon atoms in each hydrocarbyl group or mixtures of phosphites and (2) a sulfide such as sulfurized isobutylene, dibenzyl disulfide, sulfurized terpenes, phosphorodithionyl disulfide and sulfurized jojoba oil. The phosphites include the dibutyl, dihexyl, dioctyl, didecyl and similar phosphites. Phosphate esters containing 4 to 20 carbon atoms in each hydrocarbyl group, such as tributyl phosphate~ tridecyl phosphate, tricresyl phosphate and mixtures of such phosphates, can also be used.

~ L23~7~;3 F-2876(2678) - 7 -In summary, it is essential to the practice of this invention,in which greases having improved dropping points are obtained, that at least the first two of the above-mentioned ingredients be formulated into the composition. Thus, first, with respect to the preparation of the grease, the thickener will have at least 15% by weight of a metal or non-metal hydroxyl-containing soap therein, the total thickener being from 3% to 20% by weight of the grease composition. Secondly, there will be added to the grease from 0.01% to 10% by weight, preferably 0.1% to 2%, of a borated ester, in which such ester has been reacted with preferably at least an equimolar amount of boron compoiund; and, thirdly, the composition may have therein from 0.01%
to 10% by weight, preferably from 0.2% to 2% by weight, of phosphorus-and sulfur-containing compounds or a mixture of two or more compounds which separately supply the phosphorus and sulfur moieties. If separate compounds are used, an amount of the mixture equivalent to the above concentration levels is used to supply desired amounts of phosphorus and sulfur.
It was noted that, when the hydroxy-containing thickener was used with the borated ester, the dropping point of the grease was consistently unexpectedly higher than with a grease from the same grease vehicle and the same borated ester, but with a different thickener, such as a non-hydroxy-containing thickener. Thus, the broad invention is to a grease composition comprising the two components mentioned.
In general, the reaction products of the present invention may be employed in any amount which is effective for imparting the desired degree of friction reduction, antiwear activity, antioxidant activity, high temperature stability or antirust activity. In many applications, however, the borated esters and the phosphorus- and/or sulfur-containing compound(s) are effectively employed in combined amounts from 0.02% to 20% by weight, and preferably from 0.2% to 4% of the total weight of the composition.

~ Z3~703 F-2876(267B) - 8 -The greases of the present invention can be made from eithera mineral oil or a synthetic oil, or mixtures thereof. In general, mineral oils, both paraffinic, naphthenic and mixtures thereof, may be of any suitable lubricating viscosity range, as for example, from 45 SSU at 37.8C to 6000 SSU at 37.8C, and preferably from 50 to 250 SSU
at 99C. These oils may have viscosity indexes ranging to 100 or higher. Viscosity indexes from 70 to 95 are preferred. The average molecular weights of these oils may range from 250 to 800. In making the grease, the lubricating oil from which it is prepared is generally employed in an amount sufficient to balance the total grease composition, after accounting for the desired quantity of the thickening agent, and other additive components to be included in the grease formulation.
In instances where synthetic oils are desired, in preference to mineral oils, various compounds of this type may be successfully utilized. Typical synthetic vehicles include polyisobutylene, polybutenes, hydrogenated polydecenes, polypropylene glycol, polyethylene glycol, trimethylol propane esters, neopentyl and pentaerythritol esters, di(2-ethylhexyl) sebacate, di(2-ethylhexyl) adipate, dibutyl phthalate, fluorocarbons, silicate esters, silanes9 esters of-phosphorus-containing acids, liquid ureas, ferrocene derivatives, hydrogenated synthetic oils, chain-type polyphenyls, siloxanes and silicones (polysiloxanes), alkyl-substituted diphenyl ethers typified by a butyl-substituted bis~p-phenoxy phenyl) ether, ~phenoxy phenylethers.
The metallic soap grease compositions containing one or more of the borated hydroxy-containing ester and, optionally, one or more of the sulfur and phosphorus combinations described herein provide advantages in increased dropping point, improved grease consistency properties, antirust characteristics and potential antifatigue, antiwear and antioxidant benefits unavailable in any of the prior greases now known. The grease composition of this invention is unique in that it can be preferably manufactured by the admixture of additive

3~7i~33 F-2876(2678) - 9 -quantities of the alcohol borates to the fully formed soap grease after completion o~ saponification.
Ths following Examples will present illustrations of the invention.

Glycerol monooleate (115 g), consisting of a mixture of about 60% of glycerol monooleate and about 40% of glycerol dioleate, xylene (38 g) and butanol (122 g) were charged to a reactor equipped with a heater, an agitator and a condenser. The reactor contents were heated to 50C, and boric acid (100 g) was charged to the ester and solvent.
The reaction mixture was heated to 204C and was held there for abollt 3 hours until water evolution ceased. The reactor contents were cooled to 155C and vacuum (to less than 2 mm of Hg) was applied to remove the solvents. The crude product was further cooled to 105C
and it was then filtered through diatomaceous earth to yield a viscous, amber liquid.

Same as Example 1, except that the following ratios by weight were used to give an excess of boron:
7.55 pounds glycerol monooleate/pound boric acid 1.13 pounds butanol/pound boric acid 0.17 pound xylene/pound boric acid A lithium hydroxystearate grease thickener was prepared by saponification of a mixture containing 12-hydroxystearic acid (8%) and the glyceride thereof ~9%) with lithium hydroxide in a mineral oil vehicle at 177~C in a closed contactor. After depressuring and dehydration of the thickener in an open kettle, sufficient mineral oil was added to reduce the thickener con~ent to 9.0%. After cooling ~L23~
F-2876(2678) - 10 -to 99C, a typical grease additive package, consisting of an amine antioxidant, phenolic antioxident, metallic dithiophospate, sulfur-containing metal deactivator and nitrogen containing antirust additives, was added. This produced a base grease for evaluating the effect of borated hydroxyl-containing esters on grease dropping point and other properties.

To the grease of Fxample 3 was added 2 wt. % of the borated glycerol monooleate of Example 1. The addition was accomplished in a laboratory grease blender from 110C to 116C.

Same as Example 4, except that 2 wt. % of the borated glycerol monooleate of Example 2 was used.

Prepared like the grease of Example 3, except that the mineral oil was thickened with the lithium soap of a 50/S0 (wt) mixture of stearic and palmitic acids.

The results, obtained using the ASTM D-2265-78 Test, are summarized in Table I.

3~7103 F-2876(2678) Table I
D2265 Dropping Sample Point, C.
Base Grease A (containing amine antioxidant, phenolic (Example 3~ antioxidant, 1.5 wt.% of zinc dialkyl phosphorodithioate, (where alkyl is derived from a mixture of C3 to C6 primary alcohols), and sulfur~
containing metal deactivator and nitrogen containing antirust additives. 202 Example 4 - Base grease A + 2.0 wt. % borated glycerol monooleate 240 Exam.ple 5 - Base grease A + 2.0 wt. % borated glycerol monooleate (1/3 excess boron) 290 Base Grease B
(Example 6) Base grease thickened with Lithium and with lithium soap of a 50/50 (wt) mixture of stearic and palmitic acids 209 Example 7 - 50 wt. % of base grease A used in Examples

4 and 5 and 50 wt. % of base grease B of Example`6 190 Example 8 - Base grease of Example 6 containing 2 wt. % of the borated glycerol monooleate of Example 1 207 Example 8 clearly shows no dropping point improvement when the borated ester is added to a non-hydroxy-containing carboxylate soap thickened grease.

Claims

CLAIMS:

1. An improved grease composition comprising a major proportion of a grease, from 0.01% to 10% by weight of a reaction product made by reacting an ester of the formula R(COOR1)n wherein n is 1 to 5 and R and R1 are hydrocarbyl or hydroxy-hydrocarbyl groups containing 1 to 20 carbon atoms, at least one of R
and R1 being a hydroxyhydrocarbyl group, with a boron compound, and at least 15% by weight of a hydroxy-containing soap thickener.

2. The composition of Claim 1 additionally containing from 0.01% to 10% by weight of a phosphorus and sulfur compound or a mixture of phosphorus-containing and sulfur-containing compounds to supply an equivalent amount of phosphorus and sulfur.

3. The composition of Claim 1 wherein the thickener is an alkali metal, alkaline earth metal or amine soap of a hydroxy-containing fatty acid, fatty glyceride or fatty ester containing 12 to 30 carbon atoms.

4. The composition of Claim 3 wherein the metal is sodium, lithium, calcium or barium.

5. The composition of Claim 3 wherein the thickener is derived from 12-hydroxystearic acid, 14-hydroxystearic acid, 16-hydroxystearic acid, 6-hydroxystearic acid or the glyceride or ester thereof.

6. The composition of Claim 1 wherein R is an alkyl, aryl, aralkyl, alkaryl or cycloalkyl group or a hydroxy-substituted member thereof.

7. The composition of Claim 6 wherein the ester is trimethylolpropane monooleate, trimethylolpropane dioleate, trimethylolpropane monostearate, trimethylolpropane distearate, trimethylolpropane monopelargonate, trimethylolpropane monooleate monopelargonate, pentaerythritol monooleate, pentaerythritol dioleate, pentaerythritol trioleate, pentaerythritol monoisostearate, pentaerythritol monooleate monopelargonate, pentaerythritol monoisostearate, monopelargonate or monohexanoate, pentaerythritol tripelargonate, glycerol monooleate, glycerol dioleate, glycerol monostearate, glycerol monoricinoleate, glycerol monoisostearate, glycerol monopelargonate, glycerol monohexadecanoate, sorbitan monooleate, sorbitan dioleate, sorbitan monostearate, sorbitan distearate, sorbitan monopelargonate, dipentaerythritol monooleate, dipentaerythritol monooleate, dipentaerythritol dioleate, dipentaerythritol trioleate, ethyleneglycol monooleate, ethyleneglycol monostearate, ethyleneglycol monoisostearate, diethyleneglycol monooleate, diethyleneglycol monopelargonate, diethyleneglycol monoisostearate and triethyleneglycol monooleate,methyl hydroxystearate or ethyl hydroxystearate.

8. The composition of Claim 1 wherein the boron compound is selected from the group consisting of a metaborate, boric acid, boric acid, mono-, di- and trimethyl borate, mono-, di- and triethyl borate, mono-, di- and tripropyl borate, mono-, di- and tributyl borate, mono-, di- and tripentyl borate and mono-, di- and trihexyl borate.

9. The composition of Claim 2 wherein the phosphorus and sulfur moieties are supplied by a phosphorothioate of the formula wherein R3 is a hydrocarbyl group containing 3 to 18 carbon atoms, M
is a metal or non-metal, n is the valence of M and Z is oxygen or sulfur, at least one of which is sulfur.

10. The composition of Claim 9 wherein R3 is an alkyl or alkaryl group.

11. The composition of Claim 10 wherein R3 is a propyl, butyl, pentyl, hexyl, octyl, dodecyl, tetradecyl, octadecyl, butylphenyl, octylphenyl, nonylphenyl, dodecylphenyl or oleyl group or mixtures thereof.

12. The composition of Claim 11 wherein R3 is derived from isopropanol, butanol, isobutanol, sec-butanol, 4-methyl-2-pentanol, 2-ethylhexanol or mixtures thereof.

13. The composition of Claim 9 wherein M is a metal from Group IA, IIA, IIB or VIII of the Periodic Table.

14. The composition of Claim 13 wherein the metal is lithium, sodium, molybdenum, calcium, zinc, cadmium, silver or gold.

15. The composition of Claim 9 wherein M is derived from vinyl acetate, butyl vinyl ether, propylene oxide, 1,2-epoxydodecane or nitrogenous compounds.

16. The composition of Claim 1 wherein the phosphorus and sulfur moieties are supplied by a combination of (1) a dihydrocarbyl phosphite having 2 to 6 carbon atoms in each hydrocarbyl group, mixtures of such phosphites, or a phosphate ester having 4 to 20 carbon atoms in each hydrocarbyl group and (2) a sulfide selected from sulfurized isobutylene, dibenzyl disulfide, sulfurized terpenes, phosphorodithionyl disulfide and sulfurized jojoba oil.

17. The composition of Claim 16 wherein the phosphite is a dibutyl, dihexyl, dioctyl or didecyl phosphite or mixtures thereof.

18. The composition of Claim 16 wherein the phosphate ester is a tributyl, tridecyl or tricresyl phosphate or mixtures thereof.

19. The composition of Claim 1 wherein the ester is glycerol monooleate, the boron compound is boric acid, the thickener is lithium 12-hydroxystearate and the phosphorus and sulfur moieties are from zinc dialkyl phosphorodithioate, where alkyl is a mixture of alkyls containing 3 to 6 carbon atoms.

20. The composition of Claim 1 wherein the grease vehicle is a mineral oil, a synthetic oil or a mixture thereof.

21. A method of improving the dropping point of a grease composition comprising a major proportion of a grease and from 0.01%
to 10% by weight of a reaction product made by reacting an ester of the formula R(COOR1)n wherein n is 1 to 5 and R and R1 are hydrocarbyl or hydroxy-hydrocarbyl groups, with a boron compound, the method comprising thickening the grease with a thickener containing at least 15% by weight of a hydroxy-containing thickener.

22. The method of Claim 21 in which the grease additionally contains from 0.01% to 10% by weight of a phosphorus and sulfur compound or a mixture of phosphorus-containing and sulfur-containing compounds to supply an equivalent amount of phosphorus and sulfur.

23. The composition of Claim 1 wherein the boron compound is selected from a metaborate, boric acid, boric oxide and an alkyl borate of the formula (R2O)xB(OH)y wherein x is 1 to 3 and y is 0 to 2, their sum being 3, and R2 is an alkyl group having 1 to 6 carbon atoms.