WO2007074332A2

WO2007074332A2 - Coating compositions and reactive diluents therefor

Info

Publication number: WO2007074332A2
Application number: PCT/GB2006/004836
Authority: WO
Inventors: Hanamanthsa Shankarsa Bevinakatti; Johny Denis Grade
Original assignee: Croda International Plc; Unichema Chemie Bv
Priority date: 2005-12-24
Filing date: 2006-12-21
Publication date: 2007-07-05
Also published as: GB0526453D0; WO2007074332A3

Abstract

A coating compositions are disclosed which have at least one binder, for example an alkyd resin, and a reactive diluent. The reactive diluent is obtained by reacting a polyglycerol with a polyunsaturated fatty acid. A particularly preferred reactive diluent is obtainable by reacting diglycerol and/or triglycerol with linoleic acid. In preferred embodiments, the polyunsaturated acids are derived from natural oils.

Description

Coating Compositions and Reactive Diluents Therefor

The present invention relates to coating compositions and reactive diluents therefor. In particular, the present invention relates to coating compositions such as paints, varnishes and wood stains containing reactive diluents prepared from polyglycerols and polyunsaturated fatty acids and to such reactive diluents per se.

Coating compositions based on alkyd resins are in very wide spread use owing to the ease of applicability of such compositions to various substrates and to subsequent adhesion, protective properties and aesthetic appearance of the polymeric film formed from such compositions on drying. In many applications, the compositions air dry as the alkyd resins are auto-oxidisable in air.

For safety and environmental reasons, there is a significant drive to reduce or eliminate the use of volatile organic solvents in such coating compositions, which solvents evaporate off during the drying (auto-oxidation) process. Accordingly, attempts have been made to increase the solids content of such compositions, ie to reduce the solvent content, or to reduce or eliminate the volatile organic solvent component by introducing water-based formulations. An alternative approach has been to consider the use of reactive diluents. Such reactive diluents act as solvents for the alkyd resins but are subsequently incorporated into the polymeric network formed as the composition dries, ie the reactive diluent is essentially non-volatile or at most of low volatility thus enabling the presence of volatile organic solvents in the composition to be at least significantly reduced if not eliminated entirely. Examples of the use of such reactive diluents in alkyd resin coating compositions may be found in EP-A-0685543, US 4689266, US 4798859, US 5064892, US 2006/0240194, WO 03/064548 and WO 05/052070. As can be seen from these disclosures of coating compositions incorporating reactive diluents, there are strong environmental reasons for preparing such reactive diluents from materials derived from renewable resources.

Clearly, an important factor in selecting suitable reactive diluents is the ability of the diluent to function as a solvent for the alkyd resin components, ie to have a suitable viscosity to impart the correct viscosity range to the coating composition for its subsequent use, whilst having a low volatility. A factor that affects the viscosity of reactive diluents based on fatty acid esters of polyols as described in the aforementioned prior disclosures is the number of free OH groups remaining in the molecules following esterification. It is an object of the present invention to provide coating compositions containing reactive diluents and to provide reactive diluents for use in such compositions in which the number of free OH groups is minimised. More especially, it is an object to provide such reactive diluents from renewable resources. According to a first aspect of the present invention, a coating composition comprises at least one binder and a reactive diluent therefor, said reactive diluent being obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

According to a second aspect of the present invention, the use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester compound obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

A third aspect of the invention comprises a polyunsaturated fatty acid polyester of a polyglycerol which is obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

The Applicants have found that the reaction of the polyunsaturated fatty acid and the polyglycerol (or derivatives thereof) results primarily in the formation of polyunsaturated fatty acid esters of the polyglycerol in which the presence of OH groups is minimised. Such polyglycerol polyunsaturated fatty acid esters are particularly useful as reactive diluents in accordance with the invention.

In particularly preferred embodiments of the present invention, the reactive diluent has a free OH group hydroxyl value of not more than 50 mgKOH/g, more especially not more than 30 mgKOH/g. It is especially preferred that the reactive diluent has a free OH group hydroxyl value of not more than 20 mgKOH/g, more preferably not more than 15 mgKOH/g and more especially not more than 10 mgKOH/g. In typical examples, the reactive diluent has a free OH group hydroxyl value of around 5 mgKOH/g or less. It is to be noted that, in measuring hydroxyl values, the resultant figure is a combination of residual acid OH groups and residual alcohol OH groups. Accordingly, in this specification, the expression "free OH group hydroxyl value" is the difference between the total hydroxyl value and the acid value (which is also expressed in mgKOH/g).

In a particularly preferred embodiment of the present invention, the reactive diluent is obtainable by the reaction of only said at least one polyglycerol and said at least one polyunsaturated fatty acid or alkyl ester thereof, optionally with not more than 30 wt%, more especially not more than 20 wt% and particularly not more than 10 wt% of polyglycerol is replaced in the reaction mixture by such other polyols. Preferably, the reaction is carried out substantially in the absence of any other polyols.

In another particularly preferred embodiment of the present invention, the polyunsaturated fatty acid polyester of a polyglycerol is obtainable by the reaction of only said at least one polyglycerol and said at least one polyunsaturated fatty acid or alkyl ester thereof , optionally with not more than 30 wt%, more especially not more than 20 wt% and particularly not more than 10 wt% of polyglycerol is replaced in the reaction mixture by such other polyols. Preferably, the reaction is carried out substantially in the absence of any other polyols. The term "polyglycerol" as used in this specification means diglycerol, triglycerol, tetraglycerol, pentaglycerol, hexaglycerol, heptaglycerol, octaglycerol, nonaglycerol and decaglycerol and/or mixtures thereof. The polyglycerols used to obtain reactive diluents and fatty acid esters of polyglycerols in accordance with the present invention are unsubstituted polyglycerols, ie the polyglycerols do not have any substituents substituted for or in any of their hydroxyl groups.

In preferred embodiments, the polyglycerol used in the reaction to form the reactive diluent of the present invention is selected from diglycerol, triglycerol, tetraglycerol, pentaglycerol, hexaglycerol and heptaglycerol and/or mixtures thereof; more preferably from diglycerol, triglycerol, tetraglycerol and pentaglycerol and/or mixtures thereof; and especially from diglycerol, triglycerol and tetraglycerol and/or mixtures thereof. In a particularly preferred embodiment of the invention, the polyglycerol used in the reaction to form the reactive diluent of the present invention is selected from diglycerol and/or triglycerol.

If desired, minor proportions of other polyols and/or mixtures of other polyols may be included in the reaction mixture from which the reactive diluent according to the invention is formed. Preferably, not more than 50 wt% of polyglycerol is replaced in the reaction mixture by such other polyols. More preferably, not more than 30 wt%, more especially not more than 20 wt% and particularly not more than 10 wt% of polyglycerol is replaced in the reaction mixture by such other polyols. Such other polyols are preferably selected from C₃ to C₁₀ polyols, preferably aliphatic, and having a minimum of three hydroxyl groups. Preferably, such other polyols have at least two primary hydroxyl groups and, optionally, at least one secondary hydroxyl group. Preferred additional polyols are selected from C₃ to C₈ polyols, more especially C₃ to C₆ polyols. Suitable additional polyols may be selected from: a) alditols, etherified derivatives thereof and cyclitols preferably selected from C₃ to C₆ alditols but may include dimeric and higher anhydrides thereof, , C₄ polyols such as threitol and erythritol, C₅ polyols such as ribitol, arabitol and xylitol and hexitols, especially sorbitol or mannitol, but including sorbitan and inositols; b) alkyl, especially C₁ to C₄ alkyl, glycosides, more particularly glycosides of mono- and disaccharides; and c) polymethylol alkanes such as neopentyl glycol, trimethylol ethane, trimethylol propane, pentaerythritol and dimers and trimers thereof and partial esters thereof.

Suitable polyunsaturated fatty acids used to form the reactive diluent of the present invention are the semi-drying or drying polyunsaturated fatty acids or mixtures thereof that are ethylenically unsaturated, whether conjugated or non-conjugated. More particularly, the acids are selected from Ci₈ to C₂₂ polyunsaturated fatty acids and may be linoleic, linolenic, arachindonic, eicosatetraenoic, eicosapentaenoic and docasahexaenoic acids or mixtures thereof. More especially, the acids are selected from C₁₈ to C₂O polyunsaturated fatty acids or mixtures thereof. In a particularly preferred embodiment, the polyunsaturated fatty acid is linoleic acid.

Although pure forms of the polyunsaturated acids may be used, in a preferred form of the invention, the polyunsaturated fatty acids are derived from natural or synthetic oils in which the preferred acids are a major component, other acids being present in minor amounts. The raw oils themselves may be used; preferably, however, the polyunsaturated acids are the distilled fractions obtained from the raw oils. Examples of suitable natural oils include but are not limited to safflower oil, tall oil, calendula oil, corn oil, rapeseed oil, peanut oil, soya bean oil, tung oil, linseed oil, sardine oil, herring oil, sesame oil, olive oil, dehydrated castor oil, tallow oil, sunflower oil, cottonseed oil and mixtures thereof. In the preferred embodiment when the polyunsaturated fatty acid is linoleic acid, the linoleic acid is preferably derived from sunflower oil or soya bean oil. In preferred embodiments of the invention, distilled fatty acid fractions derived from such oils are used to make the reactive diluent of the present invention, such distilled fractions having at least 50 wt% polyunsaturated fatty acids, the majority of the remaining balance being long chain saturated fatty acids, preferably no more than about 20 wt%, and mono-unsaturated fatty acids, preferably no more than about 30 wt%. Typically, the long chain saturated fatty acids are C₁₄ to C₂₀ or greater fatty acids and the mono-unsaturated fatty acid is Ci₈ mono-unsaturated fatty acid.

Such polyunsaturated fatty acids are auto-oxidisable similarly to alkyd resins and the distilled fatty acid fractions used in the reactive diluent of the present invention preferably have an iodine number of at least 100, more preferably of at least 120 and preferably up to around 140 expressed in terms of the number of centigrams of iodine per gram of fatty acids (eg ASTM test method D-1959).

If alkyl esters of the polyunsaturated fatty acids are used, preferably it is a Ci to C₆ alkyl ester, more preferably it is a Ci to C₄ alkyl ester and even more preferably is a C₁ to C₂ alkyl ester. Mixtures esters may also be used.

In the polyunsaturated fatty acid polyesters of the present invention, the ratio of polyglycerol : polyunsaturated fatty acid used in the reaction is preferably in the range 1 : n+1.5 to n+2.5, more preferably in the ratio of 1 : n+2, where n is the degree of polymerisation of the polyglycerol. Preferred reactive diluents in accordance with the present invention are obtainable by reacting diglycerol and/or triglycerol with linoleic acid. It will be appreciated that, when the linoleic acid is derived from natural oils, other acid esters may also be present in the final product.

In particular, in preferred reactive diluents, ie polyunsaturated fatty acid esters, in accordance with the present invention, on average at least 50 wt% of said fatty acid ester groups are polyunsaturated fatty acid ester groups and the remaining fatty acid ester groups are on average not more than 20 wt% saturated fatty acid ester groups and not more than 30 wt% monounsaturated fatty acid ester groups.

Preferably, reactive diluents of the present invention have viscosity of less than less than 0.2 Pa s at 25°C, more particularly less than 0.1 Pa s at 25°C and more especially less than 0.09 Pa s at 25°C. In preferred embodiments of the invention, the reactive diluents have a viscosity of less than 0.06 Pa s at 25°C and more especially less than 0.05 Pa s at 25°C.

In a preferred embodiment of the invention, a coating composition comprises at least one binder and a reactive diluent therefor, said reactive diluent being obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.2 Pa s at 25°C. In an especially preferred embodiment of the invention, a coating composition comprises at least one binder and a reactive diluent therefor, said reactive diluent being obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.1 Pa s at 25°C.

In another preferred embodiment of the invention, the use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester compound obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.2 Pa s at 25°C.

In an especially preferred embodiment of the invention, the use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester compound obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.1 Pa s at 25^CC.

In another preferred embodiment of the invention, a polyunsaturated fatty acid polyester of a polyglycerol which is obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester, said polyester having a viscosity of less than less than 0.2 Pa s at 25°C. In an especially preferred embodiment of the invention, a polyunsaturated fatty acid polyester of a polyglycerol which is obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester, said polyester having a viscosity of less than less than 0.1 Pa s at 25°C.

In particularly preferred embodiments of the invention, the reactive diluent or polyester is obtained by the reaction of a polyglycerol selected from the group consisting of diglycerol, triglycerol and tetraglycerol and mixtures thereof, especially diglycerol and triglycerol and mixtures thereof with at least one polyunsaturated fatty acid or alkyl ester, especially linoleic acid, more especially linoleic acid as sourced in sunflower oil and/or soya bean oil, the reactive diluent or polyester having a viscosity of less than less than 0.1 Pa s at 25°C, more especially less than 0.6 Pa s at 25°C.

Preferably, the reactive diluent according to the invention is made by reacting polyglycerol(s) with the polyunsaturated fatty acid(s) in the presence of acid and/or base catalysts, eg phosphorous acid, sodium hydroxide. The reaction mixture is heated to a temperature of about 17O⁰C to 240⁰C, more preferably 180°C to 240⁰C, and typically about 22O⁰C, preferably under vacuum. Depending on the composition of the starting materials, the esterification reaction may be direct, inter- or trans- esterification or a combination thereof.

The reaction is terminated after the acid value of the reaction mixture has dropped to a suitable level, typically less than 50 mgKOH/g, more preferably less than 30 mgKOH/g and more particularly less than 10 mgKOH/g.

The binders of the coating compositions of the present invention are preferably auto- oxidisable alkyd resins. As is well known in the art, alkyd resins are in general the reaction product of the esterification of polyhydric alcohols with polybasic acids (or their anhydrides) and fatty acids (or glycerol esters thereof) and need not to be further described herein. The properties of the alkyd resins are primarily determined by the nature and the ratios of the alcohols and acids used and by the degree of condensation. Suitable alkyd resins include long oil and medium oil resins eg derived from 45 to 70 wt. % of fatty acids. The binder may also be selected from (meth)acrylic-modified alkyd resins, vinylated alkyd resins, styrene alkyd resins, polyurethane alkyd resins, silicone alkyd resins, polyamide modified alkyds and epoxy esters.

The coating composition according to the present invention preferably has a binder to reactive diluent ratio in the range from 98 to 50 : 2 to 50 parts by weight, more preferably from 95 to 50 : 5 to 40 and more especially from 90 to 70 : 10 to 30 parts by weight.

The coating compositions according to the present invention may be formulated by any conventional mixing techniques including, as appropriate, dispersing and grinding with a liquid carrier and at least one drier and optionally pigments, filler/extenders and other conventional additives. Although the aim of using reactive diluents in coating compositions is to reduce or eliminate the use of organic solvents as liquid carriers, it may be necessary in some formulations of the coating compositions to include some organic solvent. Examples of suitable solvents include aliphatic, cycloaliphatic and aromatic hydrocarbons, alcohol ethers, alcohol ether esters, alcohol esters and mixtures thereof. Typical examples of such solvents are: butyl glycol butyl diglycol isoparafins propylene glycol mono methyl ether dipropylene glycol mono methyl ether white spirits including de-aromatised and higher flash point white spirits.

Mixtures of solvents may also be used. The solvent is typically present at about 10 to about 20 wt% and preferably about 10 to about 15 wt%. The auto-oxidation process that polymerises the alkyd resin compositions may be accelerated by the inclusion of driers, for example the commercial material available under the trade name Nuodex, preferably incorporated in the range 1% to 10% by weight based on the binder. Examples of suitable driers are salts of (cyclo)aliphatic, natural or synthetic acids, such as, for example, linoleic acid, naphthenic acid, 2-ethyl-hexanoic acid and neodecanoic acid of various metal like cobalt, manganese, iron, lead, zirconium, strontium, aluminium, calcium, barium, bismuth, zinc, lithium and potassium. To control the catalytic activity of the driers and to preclude untimely oxidation of the alkyd resin and reactive diluent, an anti- skinning agent such as a ketoxime, typically methyl ethyl ketoxime, n-butyl ketoxime or cyclohexane ketoxime, may be present in the coating composition.

Other additives to the coating composition according to the present invention may include rheology modifiers, wetting and compability agents, flow and levelling agents, extenders, anti-sagging agents, anti-oxidants, bactericides, fungicides, insecticides, UV absorbers, flame retardants and other additives that are well known in the art.

Coating compositions according to the present invention will optionally comprise one or more inorganic or organic, transparent or non-transparent pigments. Typically, such pigments may be selected from titanium dioxide, iron oxides, carbon black, mixed metal oxides, mono- and di- azo pigments, copper phthalocyanines and anthraquinones.

These coating compositions according to the present invention are typically used, for coating substrates such as wood, plastics, leather, textiles, glass, ceramic or metals. The coating compositions according to the present invention may be applied to such substrates by any known methods, such as spraying, brushing, flooding, casting, dipping and rolling.

The present invention will now be further described by way of illustration only with reference to the following examples.

In this specification, the following test methods have been used. Acid Value

The acid value was determined using ASTM D1980-87. Hvdroxyl Value

The hydroxyl value was determined using ASTM D1957-86. Colour

Colour was determined using ASTM D1544-89.

Viscosity

The viscosity was determined using ISO 2555.

Molecular Weight Molecular weight was determined by gel permeation chromatography (GPC) using a Viscotek Evolution system consisting of a solvent pump, vacuum degasser, autosampler, column oven and refractive index detector. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1ml/min, along with a set of two ViscoGel (TSK GMHHR) columns. Column and detector temperature was maintained at 40⁰C. Polystyrene standards were used for determination of molecular weight information.

Drying Time

The drying time was determined using a drying time recorder from Byk-Gardner. Kόnio Hardness The Kδnig hardness was determined using DIN/ISO 1522.

Yellowing Index The yellowing Index was determined using ASTM E313 96.

Gloss was determined using DIN/ISO 2813. Example 1

Samples 1 to 3 of polyglycerol esters, suitable as reactive diluents according to the invention, were prepared.

In the preparation of Sample 1, diglycerol (trade name "Polyglycerin 801" from

Sakamoto) (20.Og, 0.1205mol), fatty acid (134.94g, 0.482 mol) and phosphorous acid (0.1g, 1 mol % based on diglycerol) were charged to a 250ml round bottomed flask fitted with a propeller stirrer, side-arm water condenser and collection flask, nitrogen sparge and thermometer (thermocouple), the flask being mounted on an isomantle.

In the preparation of Sample 2, tetraglycerol (trade name "Polyglycerin 310" from Sakamoto) (27.Og, 0.086M), fatty acid (132.4g, 0.473M), phosphorous acid (0.11g, 1.6M% based on tetraglycerol), and 50% sodium hydroxide solution (0.27g, 4M% based on tetraglycerol) were charged to a round bottom flask as described for Sample 1.

In the preparation of Sample 3, decaglycerol (trade name "Polyglycerin 750" from Sakamoto) (32.Og, 0.042M), fatty acid (118.2g, 0.42M), phosphorous acid (0.06g, 1.6M% based on decaglycerol), and 50% sodium hydroxide solution (0.14g, 4M% based on decaglycerol) were charged to a round bottom flask as described for Sample 1.

In the preparation of all of the Samples, the fatty acid used was a distilled sunflower fatty acid fraction available from Uniqema under the trade mark PRIFAC 8960.

For each Sample, the reaction mixture was heated slowly to 240⁰C using the isomantle under stirring (300 rpm). The temperature was held at 24O⁰C until the acid value of the reaction mix was <10mgKOH/g. The reaction was then stopped by switching off the isomantle and the product cooled to <100°C after which it was filtered under reduced pressure, using a 110mm Buchner funnel fitted with a Whatman No 1 filter paper and dicalite filter aid to remove catalyst residues.

The Samples had the properties shown in Table 1 below. Table 1

Example 2

Long oil alkyd resin formulations (Formulations 1 to 3) based on Samples 1 , 2 and 3 as described in Example 1 were made using the components shown in Table 2. The long oil alkyd resin has the composition shown in Table 3. Table 2

The formulations were tested for drying time, Konig hardness and yellowing. The results are shown in Table 4.

Table 4

The above results show that the reactive diluents can be used to formulate an alkyd based coating which give comparable results to traditional based solvent alkyd formulations.

Claims

1. A coating composition comprises at least one binder and a reactive diluent therefor, said reactive diluent being obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

2. A coating composition according to claim 1 in which said at least one polyol is selected from diglycerol, triglycerol and tetraglycerol and/or mixtures thereof.

3. A coating composition according to claim 1 or claim 2 in which said at least one polyol is selected from diglycerol and/or triglycerol.

4. A coating composition according to any one of the preceding claims in which, in the reaction from which said reactive diluent is obtainable, optionally minor proportions of other polyols are included.

5. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid is a semi-drying or drying polyunsaturated fatty acids or mixtures thereof that are ethylenically unsaturated, whether conjugated or non-conjugated.

6. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid is selected from C₁₈ to C₂₂ polyunsaturated fatty acids.

7. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid is selected from linoleic, linolenic, arachindonic, eicosatetraenoic, eicosapentaenoic and docasahexaenoic acids or mixtures thereof.

8. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid is selected from Ci₈ to C₂o polyunsaturated fatty acids or mixtures thereof.

9. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid is linoleic acid.

10. A coating composition according to any one of the preceding claims in which said at least one polyunsaturated fatty acid, or mixtures of such acids, is derived from natural or synthetic oils in which the acid or acids are a major component.

11. A coating composition according to claim 10 in which the natural oil is selected from safflower, tall oil, calendula oil, corn oil, rapeseed oil, peanut oil, soya bean oil, tung oil, linseed oil, sardine oil, herring oil, sesame oil, olive oil, dehydrated castor oil, tallow oil, sunflower oil, cottonseed oil and mixtures thereof.

12. A coating composition according to claim 10 or claim 11 in which the natural oil is sunflower oil or soya bean oil.

13. A coating composition according to any one of the preceding claims in which the reactive diluent has a free OH group hydroxyl value of not more than 50 mgKOH/g, more preferably not more than 30 mgKOH/g.

14. A coating composition according to claim 13 in which the reactive diluent has a free OH group hydroxyl value of not more than 20 mgKOH/g, more preferably not more than 15 mgKOH/g and more especially not more than 10 mgKOH/g.

15. A coating composition according to claim 14 in which the reactive diluent has a free OH group hydroxyl value of around 5 mgKOH/g or less.

16. A coating composition according to any one of the preceding claims in which the reactive diluent has a viscosity of less than 0.2 Pa s at 25°C.

17. A coating composition according to claim 16 in which the reactive diluent has a viscosity of less than 0.1 Pa s at 25°C and particularly less than 0.09 Pa s at 25°C.

18. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester compound obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

19. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 18 in which said at least one polyol is selected from diglycerol, triglycerol and tetraglycerol and/or mixtures thereof.

20. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid ester according to claim 19 in which said at least one polyol is selected from diglycerol and/or triglycerol.

21. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid ester according to any one of claims 18 to 20 in which, in the reaction from which said reactive diluent is obtainable, optionally minor proportions of other polyols are included.

22. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 21 in which said at least one polyunsaturated fatty acid is a semi-drying or drying polyunsaturated fatty acid,, or mixtures of such acids, that is ethylenically unsaturated, whether conjugated or non-conjugated.

23. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 22 in which said at least one polyunsaturated fatty acid is selected from C_1S to C₂₂ polyunsaturated fatty acids.

24. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 23 in which said at least one polyunsaturated fatty acid is selected from linoleic, linolenic, arachindonic, eicosatetraenoic, eicosapentaenoic and docasahexaenoic acids or mixtures thereof.

25. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 24 in which said at least one polyunsaturated fatty acid is selected from C₁₈ to C₂₀ polyunsaturated fatty acids or mixtures thereof.

26. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 25 in which said at least one polyunsaturated fatty acid is linoleic acid.

27. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 26 in which said at least one polyunsaturated fatty acid, or mixtures of such acids, is derived from natural or synthetic oils in which the acid or acids are a major component.

28. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 27 in which the natural oil is selected from safflower, tall oil, calendula oil, corn oil, rapeseed oil, peanut oil, soya bean oil, tung oil, linseed oil, sardine oil, herring oil, sesame oil, olive oil, dehydrated castor oil, tallow oil, sunflower oil, cottonseed oil and mixtures thereof.

29. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 27 or claim 28 in which the natural oil is sunflower oil or soya bean oil.

30. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 29 in which the reactive diluent has a free OH group hydroxyl value of not more than 50 mgKOH/g, more preferably not more than 30 mgKOH/g.

31. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 30 in which the reactive diluent has a free OH group hydroxyl value of not more than 20 mgKOH/g, more preferably not more than 15 mgKOH/g and more especially not more than 10 mgKOH/g.

32. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 31 in which the reactive diluent has a free OH group hydroxyl value of around 5 mgKOH/g or less.

33. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to any one of claims 18 to 32 in which the reactive diluent has a viscosity of less than 0.2 Pa s at 25°C.

34. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester according to claim 33 in which the reactive diluent has a viscosity of less than 0.1 Pa s at 25°C and particularly less than 0.09 Pa s at 25⁰C.

35. A polyunsaturated fatty acid polyester which is obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof.

36. A polyunsaturated fatty acid ester according to claim 35 in which said at least one polyglycerol is selected from diglycerol, triglycerol and tetraglycerol and/or mixtures thereof.

37. A polyunsaturated fatty acid ester according to claim 36 in which said at least one polyol is selected from diglycerol and/or triglycerol.

38. A polyunsaturated fatty acid ester according to any one of claims 35 to 37 in which, in the reaction from which said reactive diluent is obtainable, optionally minor proportions of other polyols are included.

39. A polyunsaturated fatty acid ester according to any one of claims 35 to 38 in which said at least one polyunsaturated fatty acid is a semi-drying or drying polyunsaturated fatty acid, or mixtures of such acids, that is ethylenically unsaturated, whether conjugated or non-conjugated.

40. A polyunsaturated fatty acid ester according to any one of claims 35 to 39 in which said at least one polyunsaturated fatty acid is selected from C₁₈ to C₂₂ polyunsaturated fatty acids.

41. A polyunsaturated fatty acid ester according to any one of claims 35 to 40 in which said at least one polyunsaturated fatty acid is selected from linoleic, linolenic, arachindonic, eicosatetraenoic, eicosapentaenoic and docasahexaenoic acids or mixtures thereof.

42. A polyunsaturated fatty acid ester according to any one of claims 35 to 41 in which said at least one polyunsaturated fatty acid is selected from C₁₈ to C₂o polyunsaturated fatty acids or mixtures thereof.

43. A polyunsaturated fatty acid ester according to any one of claims 35 to 42 in which said at least one polyunsaturated fatty acid is linoleic acid.

44. A polyunsaturated fatty acid ester according to any one of claims 35 to 43 in which said at least one polyunsaturated fatty acid, or mixtures of such acids, is derived from natural or synthetic oils in which the acid or acids are a major component.

45. A polyunsaturated fatty acid ester according to claim 44 in which the natural oil is selected from safflower, tall oil, calendula oil, corn oil, rapeseed oil, peanut oil, soya bean oil, tung oil, linseed oil, sardine oil, herring oil, sesame oil, olive oil, dehydrated castor oil, tallow oil, sunflower oil, cottonseed oil and mixtures thereof.

46. A polyunsaturated fatty acid ester according to claim 43 or claim 44 in which the natural oil is sunflower oil or soya bean oil.

47. A polyunsaturated fatty acid polyester according to any one of claims 35 to 46 the reactive diluent has a free OH group hydroxy! value of not more than 50 mgKOH/g, more preferably not more than 30 mgKOH/g.

48. A polyunsaturated fatty acid polyester according to claim 47 in which the reactive diluent has a free OH group hydroxyl value of not more than 20 mgKOH/g, more preferably not more than 15 mgKOH/g and more especially not more than 10 mgKOH/g.

49. A polyunsaturated fatty acid polyester according to claim 48 in which the reactive diluent has a free OH group hydroxyl value of around 5 mgKOH/g or less.

50. A polyunsaturated fatty acid polyester according to any one of claims 35 to 49 in which the reactive diluent has a viscosity of less than 0.2 Pa s at 25°C.

51. A polyunsaturated fatty acid polyester according to claim 50 in which the reactive diluent has a viscosity of less than 0.1 Pa s at 25°C and particularly less than 0.09 Pa s at 25°C.

52. A coating composition comprises at least one binder and a reactive diluent therefor, said reactive diluent being obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.1 Pa s at 25°C.

53. The use as a reactive diluent in a coating composition of a polyunsaturated fatty acid polyester compound obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said reactive diluent having a viscosity of less than less than 0.1 Pa s at 25°C.

54. A polyunsaturated fatty acid polyester of a polyglycerol which is obtainable by the reaction of at least one polyglycerol with at least one polyunsaturated fatty acid or alkyl ester thereof, said polyester having a viscosity of less than less than 0.1 Pa s at 25°C.