Improved oil-based paint

Abstract: An oil-based artist paint composition is disclosed in which a plasticizer is included that acts to circumvent embrittlement of a dried paint film on ageing. The invention has particular application to artists' paint compositions that are linseed oil based. A preferred plasticizer is butyl polyacrylate. In a preferred embodiment, alkyd resins are included in the composition to reduce the tendency of paint films to shrink on ageing and "creep" when used in very heavy impasto applications.

Description: FIELD OF THE INVENTION This invention relates to artist paints and more particularly to an improved linseed oil-based artist paint and a medium therefor. BACKGROUND TO THE INVENTION Traditional artist paints are oil-based wherein the oil is generally linseed oil. Such paints have been in use for at least the past three centuries and continue to retain popularity. Oil-based paints are often used in conjunction with a medium which allows the artist to adjust the rheology of the paint and adjust the pigment/binder ratio. It has long been recognised that whilst linseed oil-based artist paints have many desirable properties, in use they suffer from two defects. The first defect is a tendency to yellow. This has been found to be controllable within acceptable limits by a careful formulation. The second and most serious defect is that of an uncontrollable tendency to become brittle with age. It is understood that this embrittlement is caused by continued polymerisation or the linseed oil, a process which cannot be arrested at any desired stage of "cure". This defect is particularly acute in both large and thickly painted oil paintings. In order to circumvent these defects, the prior art has provided acrylic artist paints. It is notable that acrylic paints have achieved considerable popularity and that these paints do not suffer from the two defects of linseed oil based artist paints. However, acrylic paint does not possess the "feel" of traditional oil-based artist paints. For that reason these paints are not generally used by "serious" artists. Other attempts at meeting the embrittlement defect have been based on discovering formulations that were used by the various "old masters". None of these attempts, to the present inventor's knowledge, has proved satisfactory. Two other problems encountered in the use of artists' oil paints are shrinkage and "creep". Ageing oil paint films shrink as they age, and at a time in their ageing cycle when they are already too brittle to accommodate such movement without cracking. "Creep" results, for example, in very heavy impasto applications of paint which often shrink and move about while the underlayers are still wet, causing the surface to crinkle. SUMMARY OF THE INVENTION The present inventor has recognised that there exists a need for a linseed oil or other oil based artist paint that will not only have the right "feel" but will not embrittle on ageing, or suffer from shrinkage or where applicable, "creep". To meet the need to reduce embrittlement, the inventor proposes the introduction of an agent or agents into the paint that will act to plasticise the oil. It follows that mediums for use with such a paint are also required and hence such mediums will also incorporate the requisite oil plasticiser. Accordingly, a first aspect of the present invention provides an oil-based artist paint composition characterised in that the composition includes an oil selected from linseed oil, sunflower oil and safflower oil and an effective amount of a plasticiser for the oil, wherein the ratio of plasticiser to oil is no more than 20% plasticiser to 80% oil, provided, however, that the plasticiser is not monomeric. In a second aspect, the present invention further provides a medium composition for use with an oil-based artist paint, the oil being selected from linseed oil, sunflower oil and safflower oil, characterised in that the composition includes a solvent; at least one alkyd resin which is thixotropic and is present in an amount effective to aid through drying of the oil in the paint; and a compound effective to plasticise the paint in an amount such that when the medium is used with the paint, the weight ratio of the plasticiser compound to oil will be no more than 20% plasticiser to 80% oil, provided, however, that the plasticiser is not monomeric. It will be recognized by those skilled in the art that the mediums of the invention may also be used with a traditional linseed oil-based artist paint, to impart the beneficial properties of the invention thereto. Although the present invention is useful for oil-based paints in which the oil is selected from sunflower, safflower and linseed oils. Particularly preferred is linseed oil where the colour of the paint composition will not be adversely affected by the yellowing or darkening of the linseed oil with time. In those paint compositions that are light in colour, sunflower oil or safflowor oil are preferred. The present inventor believes that a selection of plasticiser and inclusion at an appropriate level in the paint and/or medium will produce paintings, the paint film of which when aged will contain sufficient soft polymer to ensure that film integrity and a degree of pliability are maintained. Many soft polymers are potentially satisfactory for use in the inventive compositions however, in selecting suitable polymers, it will be recognised that the polymer must have the following properties: 1) A great degree of flexibility which remains unaltered by aging indoors. It is noted that the greater the flexibility, the smaller the addition required. 2) Non-yellowing. 3) Compatible with raw linseed oil and solvents that are mixed with linseed oil. 4) Unaffected by any of the constituents of the linseed oil per se (acid free). DESCRIPTION OF THE INVENTION In the paint composition of the present invention, two polymers that have been found to perform satisfactorily as plasticisers are butyl polyacrylate and polyvinyl chloride isobutylether resin. These are available from BASF under the trade names Acronal 4F and Laroflex MP25, respectively. Acronal 4F is most preferred since the Laroflex is somewhat viscous. Additionally, it is a soft resin (copolymer) rather than a plasticiser per se so that substantial amounts are need to achieve the effect of the Acronal. Other polymeric plasticisers that are operable are, for example, epoxidized oils such as Paraplex G62, available from Rohm & Haas. The common monomeric plasticisers are not recommended as over a time frame of say ten years, volatilisation would lead to their loss, thereby resulting in embrittlement and shrinkage of the oil paint film. It would appear that a ratio of up to 20% polymer to 80% oil, particularly linseed oil, is possible without noticeably altering the "feel" of the paint. It is to be expected that the desired improvement in film characteristics should be achieved with polymer ratios of 7-20%. To meet the shrinkage and "creep" problems a non-yellowing thermoplastic resin may be included in the inventive paint composition, preferably with at least one alkyd resin. Desirably, the at least one alkyd resin will be thixotropic and will act to aid in the through drying of the oil and maintain "balance" in handling properties. A second alkyd resin may be advantageously included to aid in the through drying of the oil. One non-yellowing thermoplastic resin that has been found suitable is Laropal A81, which is available from BASF. Two alkyd resins that have been found suitable are Jordathix and Jordasol, both of which are available from Ashland Chemicals. These are long oil alkyd resins. Without wishing to be bound by theory, the present inventor believes that the presence of an alkyd resin triggers the drying process with the oils used. By selection and combination of two alkyd resins and by appropriately formulating the other components of a paint composition of the invention, the characteristics of the paint will be such that its handling properties will appear entirely conventional. Additionally, the slower drying sunflower or safflower oils can be made to dry at much the same speed as linseed oil. In order to more fully appreciate the nature of the invention, several examples of typical paint compositions of the invention are set out below. The examples show the formulation of a number of different pigment types: titanium white burnt umber red oxide pthalo blue Burnt umber is an inherently thixotropic pigment with a high oil demand. Red oxide is a characteristically strong inorganic pigment requiring an intermediate level of extender. Pthalo blue is an organic pigment with a requirement for a high level of extender. In addition, two extenders are exemplified, kaolin with a high oil demand and calcite with a low oil demand. EXAMPLE 1 Titanium White Ingredient Parts by weight Pigment 300 Laropal (50%) 200 Jordathix (40%) 130 Jordasol (70%) 100 Sunflower oil 160 Swelling clay 80 Calcite 460 Acronal 4F 80 EXAMPLE 2 Red Oxide Ingredient Parts by weight Pigment 200 Laropal (50%) 270 Linseed oil 220 Jordathix (40%) 120 Jordasol (70%) 50 Acronal 4F 80 Kaolin 250 Swelling clay 100 EXAMPLE 3 Burnt Umber Ingredient Parts by weight Pigment 375 Laropal (50%) 250 Jordathix (40%) 120 Jordasol (70%) 50 Acronal 4F 80 Linseed oil 250 Swelling clay 70 Calcite 250 EXAMPLE 4 Beta Pthalo Blue Ingredient Parts by weight Pigment 60 Laropal (50%) 281 Jordathix (40%) 100 Jordasol (70%) 50 Linseed oil 200 Swelling clay 85 Acronal 4F 80 Kaolin clay 300 To each of these compositions may be added other ingredients well known in the act such as dispersants and viscosity modifiers. Further ingredients well known in the art such as drying agents, will be included in the paint compositions at a level appropriate to the compositions. To prepare Examples 1-3, the swelling clay is dispersed in the Laropal, Jordathix and Jordasol. The requisite pigment and oil is then added followed by the Acronal 4F and the calcite or kaoline. Drying agents and other minor ingredients are then mixed into the composition as appropriate. To prepare Example 4, a premix of the pigment is made with the Laropal. The premix is added to the remaining ingredients after they have been suitably mixed. Drying agents and other minor ingredients may be mixed into the composition as appropriate. It should be noted that the ratio of the liquid components of these examples may be considerably varied. For example, to achieve a satisfactory rheology, the thixotropic Jordathix and low viscosity Laropal solutions, at high concentration, are "balanced out". The Acronal and Jordasol which both have high viscosity need to be compensated for by the Laropal and Jordathix. In the medium composition of the present invention, polymers that have been found to be satisfactory are polyvinyl chloride isobutyl other copolymer (Laroflex MP25), butyl polyacrylate (Acronal 4F) and expoxidised oil (available from Rohm and Haas under the trade name Paraplex G62). It is noted that prior art mediums are commonly oleo resinous blends in a solvent, having 20-40% solids, according to the artist's preference. The proportion of oil to resin determines the oiliness of the "feel", the higher the proportion of oil, the oiler, less tacky is the "feel". In a medium composition of the present invention the plasticising polymer may be used in place of part or all of the resinous component in an oleo resinous blend. It is noted that Laroflex MP25 may be used without an oil component. By way of example, a typical medium composition comprises polyvinyl chloride isobutyl ether copolymer in a solvent, having approximately 30% solids. The formulation is prepared by methods well known in the art. The medium composition of the present invention is suitable for use with oil-based paints. Alkyd resins are used where paint is to be applied thickly. Without the addition of alkyd resins, where paint is applied thickly the paint film will dry on the surface while remaining soft underneath. Movement of the soft underlayer may cause wrinkling of the surface. Inclusion of an alkyd resin allows the paint to set after application. In order to better understand the nature of the medium composition of the invention, set out below are three examples. EXAMPLE 1 "FAT GEL" Ingredient Parts by weight Resin Solids Jordasol 400 280 Jordathix 200 80 Turpentine 100 Acronal 4F (10% solids) 36 This is a syrupy medium for glazing at 51% solids. EXAMPLE 2 "HEAVY GEL" Ingredient Parts by weight Resin Solids Jordasol 600 420 Jordathix 200 80 Laropal (50%) 400 200 Swelling clay 160 Acronal 4F 80 80 and drying agents This composition has a paste consistency suitable for impasto techniques and a solids content of 60.9%. EXAMPLE 3 "LEAN LIQUID GEL" Ingredient Parts by weight Jordathix 300 Jordasol 300 Turpentine 450 Acronal 4F 10% 33 and drying agents This is a main medium used to dilute paint and has a solids content or 33%. Although the invention has been described with reference to certain preferred embodiments and examples, the person skilled in the art will appreciate that numerous variations and modifications may be made to the invention.

Claims: An oil-based artist paint composition characterised in that the composition includes an oil selected from linseed oil, sunflower oil and safflower oil and an effective amount of a plasticiser for the oil, wherein the ratio of plasticiser to oil is no more than 20% plasticiser to 80% oil, provided, however, that the plasticiser is not monomeric. A paint composition as in claim 1 wherein the oil is linseed oil. A paint composition as in claim 2 wherein the plasticiser is selected from the group consisting of acrylate polymers, soft resin copolymers and epoxidized oils. A paint composition as in claim 3, wherein the plasticiser ratio is from 7 to 20%. A paint composition as in claim 4, wherein the plasticiser is butyl polyacrylate or polyvinylchlorideisobutylether. A paint composition as in claim 5, which includes a non-yellowing thermoplastic resin. A paint composition as in claim 6, which includes at least one alkyd resin. A paint composition as in claim 7, wherein the at least one alkyd resin is thixotropic and functions to aid through drying of the oil. A paint composition as in claim 8, which includes a second alkyd resin which functions to aid through drying of the oil. A paint composition as in claim 10, wherein the plasticiser is butyl polyacrylate. A medium composition for use with an oil-based artist paint, the oil being selected from linseed oil, sunflower oil and safflower oil, characterised in that the composition includes a solvent; at least one alkyd resin which is thixotropic and is present in a amount effective to aid through drying of the oil in the paint; and a compound effective to plasticise the paint in an amount such that when the medium is used with the paint, the weight ratio of the plasticiser compound to oil will be no more than 20% plasticiser to 80% oil, provided, however, that the plasticiser is not monomeric. A medium composition as in claim 11, wherein the plasticiser is an epoxidised oil or an acrylate polymer. A medium composition as in claim 11 or claim 12, which includes a second alkyd resin which functions to aid through drying of the oil. A medium composition as in claim 13, wherein the plasticiser is butyl polyacrylate. A medium composition as in claim 14, which includes a non-yellowing thermoplastic resin.

Inventors: COBB JAMES CAMPBELL, COBB, JAMES CAMPBELL
Applicants: CHROMA ACRYLICS NSW LTD, CHROMA ACRYLICS (NSW) LTD
Application year: 1990
Main classification: C09D5
IPC codes: C09D5