PROGRESS ‘%rn&

Progress in Organic Coatings 27 (1996) 45-53

Renewable resources in coatings technology: a review

Johannes T.P. Derksen, F. Petrus Cuperus, Peter Kolster Agrotechnological Research Institute (ATO-DLO), PO Box 17, 6700 AA Wageningen, Netherlands

Received 17 October 1994; accepted 22 June 1995

Abstract

In recent years an increasing interest is observed in the development of more environment friendly paints and coatings. This paper discusses advances in the use of renewable resources in formulations for various types of coatings. In particular, current research on the application of plant proteins and vegetable oils in coatings systems is addressed. In ongoing plant protein re- search at ATO-DLO corn, but particularly wheat gluten, was modified chemically to obtain aqueous protein dispersions that have excellent film-forming characteristics and strong adhesion to various surfaces. In particular, wheat gluten films have very interesting mechanical properties, such as an extensibility over 600%. Gas and moisture permeabilities were found to be easily adjustable by changing the exact formulation of the protein dispersion. Durability and water resistance of the coatings can be tailored by, for example, varying the degree of crosslinking of the protein binder. Based on the observed characteristics of the modified protein binders the development of novel, organic solvent-free paints and coatings appears to be well within reach. Regarding vegetable oil-based binders, research at ATO-DLO and elsewhere includes the application of oils from conventional as well as new oilseed crops. A very interesting new vegetable oil originates from such crops as Euphorbia lagascae and Vernonia galamensis, which have high contents (>60%) of an epoxy fatty acid (9c,12,13 epoxy-octadecenoic acid or vernolic acid) that can be used as a reactive diluent. Another interesting new oil is derived from Calendula officinalis, or ‘marigold’. This oil contains >63% of a Cl8 conjugated triene fatty acid (8t,lOt,l2c-octadecatrienoic acid or calendic acid) like that in tung oil. Current research is focused on the film-forming abilities of these oils and of chemical derivatives of these oils, in particular in emulsion systems.

Keywords: Renewable resources; Technology

1. Introduction

Agricultural raw materials precede petrochemicals by millenia in non-food applications. Vegetable oils, for instance, have been used for illumination and lubricat- ing purposes as well as for coatings and paints for many centuries before an abundant and cheap supply of mineral oil became available for a wide range of products [l]. This has resulted in a steady decline in the use of renewable resources in paints and coatings indus- tries as well as in other non-food fields. As an illustra- tion, the total volume of fats and oils used in drying oil products is now less than a third of the volume in 1950. Realizing that the total volume of resins in paints has increased substantially since that time, the reduction in the share of vegetable oil-based products becomes even more striking [2].

0300-9440/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI 0300-9440(95)00518-J

However, in the past few years consumer’s and industrial interest in environmentally friendlier paints and coatings has been growing tremendously. This trend has been spurred not only by the realization that the supply of fossil resources is inherently finite, but also by a growing concern for environmental issues, such as volatile organic solvent emissions and recycling or waste disposal problems at the end of a resin’s economic lifetime. Furthermore, developments in or- ganic chemistry and fundamental knowledge on the physics and chemistry of paints and coatings enabled some problems encountered before in vegetable oil- based products to be solved. This resulted in the devel- opment of coatings formulations with much improved performance that are based on renewable resources.

The above has, on the one hand, led to a further reduction in the use of organic solvents in paint sys-

46 J.T.P. Derksen ef al. 1 Progress in Organic Coatings 27 (1996) 45-53

terns, through the development of, for example, new water-based paint formulations and high-solids sys- tems. On the other hand, it has also led to an in- creased interest in the use of renewable resources, i.e. derived from agriculture, in paints and coatings for- mulations.

The present paper presents an overview of recent developments and perspectives on the application of renewable resources in coatings technology. The use of proteins as biopolymer binders and of vegetable oils as binder constituents in coatings formulations is dealt with successively. The paper also discusses some recent results of our institute on the identification and appli- cation of new seed oils as well as on protein-based coatings.

2. Protein-based coatings

2.1. Introduction

Proteins are polymers of amino acids with a wide range of chain lengths, from about 50 amino acids (for instance the hormone insulin) to large complexes of more than over 100 000 amino acids (wheat gluten proteins). Two main groups of proteins can be distin- guished: firstly, proteins showing a physiological func- tion, such as enzymes and hormones and secondly, structural or storage proteins such as collagen (precur- sor of gelatin) and pea seed proteins.

Because of the scale on which they are produced, only proteins belonging to the latter group are of interest for commodity applications. These proteins may originate from animal products, such as milk (casein), hides and connective tissues (gelatin) and hen’s eggs (egg-white proteins), and from plant prod- ucts such as seeds (soy and (corn) gluten proteins). These proteins will be referred to hereafter as ‘indus- trial proteins’. Industrial proteins are traditionally used in the feed and food industry. In the food indus- try, industrial proteins are used because of their func- tional properties. Well known examples of their properties are emulsifying (e.g. casein), foaming (e.g. egg white), gelling (gelatin), and viscoelastic properties (wheat gluten proteins).

These and other functional properties (e.g. adhe- sion) of proteins have also been exploited in non-food applications. In most of these applications, animal proteins have been used. The use of proteins such as blood proteins, casein and gelatin in adhesives goes back over many centuries [3]. Another example of a well-established non-food application of proteins is the use of gelatin in photographic emulsions, which is based on the unique gelling properties of this protein. Casein has been used in paper coatings, paints, plas- tics and in leather finishes [4-61.

2.2. Historical overview

Development of non-food applications of plant proteins has also been studied. In the thirties, industrial non-food applications of soy proteins were developed in the framework of the ‘chemurgic movement’, which aimed at the development of new industrial feedstocks from agronomic raw materials [7]. Products such as plastics, fibers, plywood adhesives and paper coatings were developed. In several of these non-food applications, soy proteins were not successful due to the competition from petrochemicals.

As a result of the rise of petrochemicals, proteins and agricultural feedstocks in general as a feedstock for the non-food sector were replaced by synthetic polymers. There are some exceptions, such as the use of gelatin in photographic emulsions and, to some extent, soy proteins in paper coatings and casein in adhesives. It is estimated that in the USA about 25 000-50 000 tons of soy proteins are used in paper coatings [7]. An important reason for this substitution of protein by petrochemicals has been the lower price of the latter, but also differences in performance have been important in this respect. Since World War II, there has been an enormous increase in the knowledge of the production and adjustment of synthetic polymers. Hence, chemical industry is able to produce tailor-made products that can meet high industrial demands, both in terms of performance and, last but not least, stability of characteristics over time.

For the following reasons, there are now new opportunities for proteins to regain the non-food market, hence to substitute synthetic polymers in specific applications. Firstly, knowledge of protein technology and chemistry has increased considerably in the last decades. This includes aspects such as isolation and characterization of proteins, elucidation of their structures, structure-function relationships, and modifications. Secondly, in many cases the price of proteins is now comparable or even lower than that of synthetic polymers (with the exception of commodity plastics). In this respect, plant proteins are a more attractive feed stock for non-food applications than animal proteins because of the differences in price. For instance, wheat gluten in a factor of 2 to 3 cheaper than casein. Thirdly, there is an increasing demand both from industries and consumers for the substitution of synthetic polymers by environmentally friendly polymers from renewable resources. This development is sup- ported by governments, both in Europe and the USA.

2.3. Wheat gluten coatings

In developing non-food applications of proteins, various proteins such as soy protein, corn gluten, wheat gluten and pea proteins are being studied. The research

J.T.P. Derksen et al. / Progress in Organic Coatings 27 (1996) 45-53

on the replacement of synthetic polymers will be illus- trated by the research on wheat gluten proteins. About 300 000 tons of gluten are produced worldwide each year. Based on its unique functional properties, wheat gluten can be distinguished from other industrial proteins. Examples are its insolubility in water, adhe- sive/cohesive properties, viscoelastic behavior, film forming properties and barrier properties for water vapor and gases. These properties have been exploited in the development of edible coatings based on wheat gluten [8,9].

These properties of wheat gluten can also be ex- ploited in the non-food sector [lo], which is an impor- tant aim of our research on wheat gluten. The insolubility and film-forming properties of gluten are for instance important for the development of a gluten- based replacer of synthetic binders. Techniques have been developed for the production of water-borne binder dispersions based on wheat gluten. These sus- pensions show good film forming properties and the resulting coating has a strong adhesion to various substrates. Examples will be given of the polymeric properties of gluten and the adjustment of properties of the gluten coatings.

Wheat gluten shows, like other amorphous poly- mers, a glass transition temperature (T,). Below the Tg, gluten films are brittle. To obtain rubbery gluten coat- ings, the addition of plasticizers is required. In Fig. 1, the effect of two plasticizers is shown.

For biopolymers such as wheat gluten, water is a powerful plasticizer. An increase in water content of one per cent results in a decrease in Tg of 7.5 “C. The addition of an additional plasticizer, such as glycerol, results in a further decrease in T, of the gluten binder [lo]. There is a good correlation between the measured Tg and the Tg calculated by the Couchman-Karasz equation [l 11. This equation predicts the T, of mixtures based on the properties of the pure constituents. Also other plasticizers, including plasticizers insoluble in wa- ter, were shown to be effective.

Tg (9 loo3

water content (w/w %) ??0% glycerol A 10% glycerol

Fig. 1. Effect of water and glycerol on the TB of gluten. The symbols represent the measured values. The lines represent the Ts of gluten with 0 or 10% glycerol, calculated from the Couchman-Karasz equation.

stress (MPa) 6,

strain (%) I

concentration of plasticizer (%) -stress + strain

Fig. 2. The mechanical properties (stress and strain) of gluten coat- ings vs. the amount of glycerol.

The mechanical properties of gluten coating as a function of the plasticizer concentration are shown in Fig. 2 [lo]. At a plasticizer concentration of 30%, the extensibility of the gluten coatings is about 600%. A further increase in the amount of plasticizer does not result in an additional increase in extensibility. An increase in extensibility accompanies a decrease in strength of the coating. By modifications of the gluten proteins, such as crosslinking, this reduction in strength can be compensated.

In Fig. 3 the effect of hydrophobic additives on the water vapor permeability of gluten coatings is shown [lo]. The permeability decreases proportionally with the amount of additive, and can therefore be adjusted towards specific requirements for an application. The permeability of the coating with 20% additive is com- parable to that of commercially applied low density polyethene foil.

As shown, coatings from gluten binders have inter- esting barrier and mechanical properties. Furthermore, the binder and resulting coating is insoluble in water. By including additives, such as hydrophobic substances, it is possible to adjust the properties of the coatings towards specific requirements. By modifications that are not permitted in food applications, such as crosslinking and grafting, it is possible to increase the range of properties of the gluten binder and resulting coating. It has already been shown that by modifications the water

‘watervapor permeability (g)

-Cl%

+ 7.5% -a- 10% - 12.5% * 15% + 20%

time (hrs)

Fig. 3. The effect of hydrophobic additives on the water vapor permeability of gluten coatings.

48 J.T.P. Derksen et al. /Progress in Organic Coatings 27 (1996) 45-53

FoH a-linolenic acid (Qc,l2c,l5c-octadecatrienoic acid)

elaeostearic acid (9~1 lt,l3bxtadacatrienoic acid)

-OH catendic acid (8t,lOt,12c-cctadecatrienoic acid)

-T--TOH ricinoleic acid (12hydroxy-Qc-octadecanoic acid)

-x---TOH lesquerolic acid (14hydroxy-1 lc-eicosenoic acid)

-OH dimorphscolic acid (Qhydrory-lM.l2t-octadecadienoic acid)

OH

vemolic acid (12,13epoxy-Sxctadienoic acid)

Fig. 4. Selected fatty acids for paint applications.

adsorption of gluten binders, which in the past has been an important draw-back of proteins in non-food appli- cations; can be reduced significantly. Furthermore, it has been shown that by acylation of gluten, it is possi- ble to improve the suitability of gluten as a co-binder in papercoatings [ 121.

3. Vegetable oil-based coatings

3.1. Introduction

The limited number of available oil crops, with concurrent limited variability in fatty acid composition, has spurred industrial interest in the development of new crops, which are optimized for specific applica- tions. These new crops include crops that contain a higher percentage of a desirable fatty acid and crops that contain unique, unusual fatty acids. For non-food applications, oleochemical as well as fine chemicals industries have expressed their interest in new fatty acids with unusual properties and functionalities, since current sources contain no more than approximately 10 different types of fatty acid. Such unusual fatty acids (Fig. 4) could, on the one hand, replace raw materials from petrochemical origins with renewable resources,

and, on the other hand, expand the existing range of raw materials available and potentially lead to novel products. Moreover, consumer products made from renewable resources may also carry an appealing envi- ronment-friendly or ‘green’ label.

Below, the current and past use of conventional fats and oils in the paints and coatings industry as well as the potential use of ‘new’ seed oils in these applications is discussed.

3.2. Conventional oilseed crops

In the past many seed oils have been applied in various coatings formulations. In the 1950s the most common plant oil in trade sales paint formulations was linseed oil with a share of 50% [2]. Since then not only the total volume of fats and oils used in drying oil products has declined, also the relative position of linseed oil has slowly declined to less than 30% of the plant oil used. Simultaneously the share of soybean oil increased such that now soybean oil is the predominant oil used in this area. The use of soybean fatty acids in ‘soybean-modified’ alkyds is obviously a contributing factor to this.

Table 1 gives an overview of a number of major plant oils that find use in coatings formulations, to- gether with some of their characteristics. The origin and characteristics of the oils mentioned will be discussed hereafter.

3.2.1. Drying oils

3.2.1.1. Linseed oil Linseed oil is obtained, typically by mechanical

expelling, from the seeds of the flax plant (Linum usitatissimum). This oilseed crop is grown in many parts of the world, including those with a temperate climate. The oil has a high content of a-linolenic acid (Table l), giving rise to a high iodine value (I.V. 175-204, depending on, e.g., cultivar and climatic conditions during the growing season). This high degree of unsaturation renders linseed oil very susceptible to autoxidation and polymerization, resulting in crosslinked and tough films upon exposure to air. For this reason linseed oil has been used for centuries as a chief ingredient in paints and varnishes.

Crude linseed oil is often treated to increase its suitability as a binder. An important first step is the refining of the oil, in which process undesirables such as gums (sometimes called ‘break’), which consist mainly of lecithins, and free fatty acids are removed. This is usually achieved through a combination of physical and chemical treatments, such as heating, acid or alkali addition, water washing, bleaching and vacuum or heat drying. The exact refining protocol depends on whether the oil will be used for, e.g., varnishes or as a grinding

J.T.P. Derksen et al. / Progress in Organic Coatings 27 (1996) 45-53 49

Table 1 Plant oils currently used in coatings industries

Plant oil Source Major fatty acids

Linseed oil Linum usitatissimum linolenic/linoleic Tung oil Aleurites spp. elaeostearic/oleic Perilla oil Perilla frutescens linolenic/hnoleic Oiticica oil Licania rigida licanic/elaeostearic Soybean oil Glycine max linoleic/oleic Safflower oil Carthamus tinctorius linoleic/oleic Tall oil Pinewood pulping fatty acids/rosin acids Castor oil Ricinus communis ricinoleic/linoleic Dehydrated Castor Ricinus communis conjugated FAs/ricinoleic

FA content (% of total)

40/35 19/l 1 43137 7617 55128 59131 50/40 9014 80/10

Iodine value (I.V.)

175-204 155-175 192-208 179-218 125-140 140-150

82-88 135

Specific gravity (g/ml 25 “C)

0.931-0.936 0.939-0.943 0.932-0.935 0.96660.969 0.923-0.929 0.925-0.928

0.958-0.969

medium for pigments. The obtained refined oil may then be heat-bodied. In this process the double bonds of the unsaturated fatty acids in the oil are oxidized and to some extent polymerized. This leads to an increase in the viscosity of the oil and a decrease in drying time. Heat-bodying may be accomplished in several ways, each with its specific applications.

Boiled oil is obtained by heating oil in a vessel in the presence of air and zirconium, manganese or cobalt driers. The oil is kept at a temperature of, typically, 137.5 “C until its specific gravity reaches 0.942 at 15.6 “C. This process results in an oil with a viscosity of 80-120 centipoise and a drying time of 12-20 h, whereas raw linseed oil has a viscosity of 40 centipoise and a drying time of 2-4 days. Boiled oil can be added to both oil-based and alkyd paints to improve flow and ease of brushing.

Blown oil can be prepared in a similar fashion as boiled oil, except that no metal driers are present. Blown oil has a somewhat longer drying time than boiled oil (24-36 h) but a higher viscosity (typically more than 3 poise). It can be used in highly pigmented systems to improve flow and leveling properties in undercoats and wall paints.

Stand oil is prepared by allowing the oil to thicken at a relatively high temperature in the absence of metal driers. Usually the oil is kept at temperatures around 290 “C for 6 h or as long as it takes to reach the desired viscosity. The thickening can proceed under exposure to air or, preferably, to an inert gas, which produces a paler oil. The viscosity of stand oil can be as high as 200 poise. Linseed stand oil can be used in, e.g., lithographic varnishes, as enamel oils (mixed with tung oils) and as a leveling agent in undercoats.

A disadvantage of linseed oil and other drying oils as binders especially in formulations of light-colored coatings is its tendency to yellow with age. This is mainly due to their relatively high content of linolenic acid. This property may be reduced by admixing other, semi-drying vegetable oils. However, this may also negatively affect drying time, so an optimum balance must be found.

3.2.1.2. Tung oil Tung oil or (Chinese) wood oil is recovered from

the nuts of the tree Aleurites fordii or A. montana (oil content approx. 18%). These trees were originally grown in South-East Asia, but lately crops have also been established in Malawi, Argentina and the USA. The oil contains over 70% of the unusual fatty acid elaeostearic acid (SC, 11 t, 13t-octadecatrienoic acid) (Fig. 4). This fatty acid contains three double bonds that are conjugated rather than methylene-interrupted as in linolenic acid. This feature causes the oil to dry much faster than linseed oil, but also makes the film more UV-Vis-light sensitive, leading to, for example, more rigid films. Coatings based on tung oil have a high resistance against water penetration and saponification. In contrast to linseed oil films, the air-dried tung oil film has a characteristic ‘frosted’ appearance. This property can be exploited in so-called ‘wrinkle finishes’. If this effect is not desired the oil has to be subjected to heat-bodying, analogous to linseed oil. A tung stand oil, for instance, can be prepared by heating the oil to 280 “C. However, the increase in viscosity is much faster than for linseed oil. This may even result in an irreversible ‘gelling’ of the oil that can be prevented by a careful control of the heat-bodying conditions. It can also be prevented by the addition of other (semi) drying oils or rosin acids (from tall oil) during the bodying process.

Tung oil is frequently used in combination with other compounds such as phenolic resins, coumarone resins and ester gums. Applications include air-drying finishes, marine spar varnishes, in aluminum paints and in primers for alkaline surfaces (plasters, concrete) [13,14].

3.2.1.3. Castor oil Castor oil, obtained from the Castor nut (Ricinus

communis) (oil content approx. 50%), is an unusual oil in that it contains a very high amount (900/) of ricinoleic acid: a hydroxy moiety-containing fatty acid (12-hydroxy-9c-octadecenoic acid) (Fig. 4). It is at

50 J.T.P. Derksen et al. / Progress in Organic Coatings 27 (1996) 45-53

present the only commercially available source of natural hydroxylated triglycerides. Although castor oil in itself has no drying properties, it is an interesting oil for the coating industry. This is to some extent due to the fact that its hydroxyl functionality allows the oil to be used, for example, in polyurethane coatings. Also the hydroxyl functionality imparts a high viscosity, stability and polar solvent (alcohol) miscibility to this oil, making it useful as viscosity modifiers, plasticizers and wetting agents in various applications [15- 171.

The versatility of castor oil can be further enhanced by a catalytic, high-temperature conversion of this oil to dehydrated castor oil. In this process the ricinoleic acid in castor oil is converted to partially conjugated fatty acids, rendering oil drying properties that are intermediate to linseed and tung oil. Dehydrated castor oil yields pale, non-yellowing films with outstanding color retention properties that are exploited in vehicles for gloss lithographic inks, metal decorating inks as well as in air-drying and stoving alkyd resins. It is seldomly used as an oil in air-drying varnishes or paints, since after drying it retains a characteristic ‘after-tack’ for some time [13,14].

3.2.1.4. Other drying oils A number of minor oilseed crops produce drying oils

that have been evaluated in coatings systems. These include such crops as perilla (Perillu frutescens) and oiticica (Licania riguda).

Perilla oil is recovered from the seeds of this crop, which carry about 38% oil, and has been used in the past as a strong drying oil. It has a fatty acid composition that resembles linseed oil but has a higher degree of unsaturation. As its higher iodine value (I.V. = 192-208) already suggests, this oil dries faster than linseed oil under comparable conditions [14].

Oiticica oil contains a high content of licanic acid (4-keto-9c, 11 t, 13t-octadecatrienoic acid) together with some elaeostearic acid. It is extracted from the nuts of the brasilian oiticica tree, which contain 55-63% oil on a dry weight basis. Just like tung oil, oiticica oil contains a large percentage of conjugated fatty acids. In reactivity and other properties oiticica oil is, therefore, very similar to tung oil [14].

Despite their applicability in coatings formulations both perilla and oiticica oil have been replaced to a large extent by synthetic binders. This is to a large degree due to their limited availability and the uncertain stability of their supply.

3.2.2. Semi-drying oils

3.2.2.1. Soybean Together with protein, soybean oil is the main

product of the crop soybean (Glycine max), which is cultivated on a large scale in, e.g., the USA and

Europe. The oil is extracted with hexane from the flaked beans, which contain 15- 18% oil on a dry weight basis. Before further use the crude oil must be refined, in particular to remove the relatively high amount of gums (lecithin, or phospholipids). Being a good surfactant, soybean lecithin finds application in the coatings industry as pigment-wetting and flocculation-control agents. The oil itself contains predominantly linoleic and oleic acids (I.V. 125-140) and dries about three times slower than linseed oil under the same conditions. Since it contains only little linolenic acid it can be mixed with drying oils such as linseed or tung oil to improve the yellowing characteristics. However, a quantitatively more important use of soybean oil is the incorporation of its fatty acids in (‘soy modified’ or ‘linoleic rich’) alkyd resins for non-yellowing white paints.

3.2.2.2. Safflower Safflower oil, obtained from the thistle Curthamus

tinctorius, is very similar to soybean oil, although with a higher content of linoleic acid and somewhat less saturated fatty acids. It has, therefore, a slightly higher iodine value than soybean oil (I.V. = 140-150) and is a little faster drying. Just like soybean oil it has excellent non-yellowing properties and is for this reason also used to produce light-colored, non-yellowing alkyd paints.

3.2.2.3. Other oils Tall oil is an atypical oil since it is not obtained from

oil-bearing seeds or fruits like other vegetable oils. Tall oil is a by-product of the paper industry, where it is recovered from the ‘black liquor’ resulting from the Kraft (sulfate) pulping of coniferous woods. The obtained dark-colored crude tall oil is not composed of pure triglycerides, like other vegetable oils, but is rather a mixture of fatty acids, rosin acids and unsaponifiable matter (e.g., sterols, waxes, hydrocarbons) in a ratio of 5 : 4: 1. The fatty acid fraction can be enriched by fractional distillation to a product that is known as TOFA (tall oil fatty acids), which contains mainly oleic and linoleic acid. It is chiefly used for the production of alkyd resins and dimer acids. The rosin acid-rich fraction is converted to drier metal salts (‘tallates’) or incorporated into alkyds [ 13,141’.

Although not a vegetable oil, fish oil is interesting to the paints and coatings industries since it has good drying properties. In particular sardine and menhaden oils possess drying properties that can be further enhanced by a segregation, i.e. a solvent separation process, in which the fraction of saturated fatty acids is decreased. The resulting fraction contains a large proportion of unsaturated fatty acids with long chains (C20-C22) and a high number of isolated double bonds (4-6). Fish oil acids can be used in preparing

J.T.P. Derksen et al. / Progress in Organic Coatings 27 (1996) 45-53

Table 2 Selected potential oilseed crops (after Refs. [24-261)

51

Oilseed crop Seed yield Oil content (tons/ha) (% dry weight)

Major fatty acid FA content (% of total)

I.V. Specific gravity (g/ml 25 “C)

Crambe abyssinica (crambe) 2.5-3.5 26-39 Limnanthes alba (meadowfoam) 0.5-1.0 17-29 Dimorphotheca pluvialis (cape marigold) 1.2-1.7 18-26 Lesquerella fendleri (lesquerella) 1.6-2.3 23-29 Calendula ojjicinalis (marigold) 1.552.5 19-24 Euphorbia lagascae (spurge) 1.0-1.5 45-52 Vernonia (vernonia) spp. 1.5-2.0 31-42

erucic 55560 112 0.910 very long chain 95+ 114 0.905 dimorphecolic 58-65 167 0.905 lesquerolic 51-53 calendic acid 58-63 242 0.940 vernolic 59965 102 0.955 vernolic 68-75

alkyd and urethane resins. Its sometimes fishy odor is hard to remove and may in certain cases be objectionable [ 131.

3.3. New seed oils

Table 2 presents a selection of seven vegetable oil- bearing plant species that are currently being evaluated as potential oilseed crops in Europe. These oilseed plants were selected for reasons of agronomic feasibility as well as of industrial interest and market opportuni- ties and result from a wide range of plant species screened, both in Europe [18-211 and in the USA [22,23]. The seed oils from the plants selected all have features that make them ‘unusual’, when compared to existing oilseed crops.

3.3.1. Very long chain fatty acid-rich seed oils Crambe abyssinica and Limnanthes alba (or meadow-

foam) seed oils contain large amounts of very long chain fatty acids. Crambe abyssinica contains up to 60% erucic acid (A 13c-docosenoic acid), exclusively posi- tioned on the 1,3-positions of its triglycerides. The presence of erucic acid makes this crop an alternative for high erucic acid rapeseed (HEAR), but has the advantage of a consistently higher erucic acid content. Currently, the major product of erucic acid is eru- camide, that is used as a surface-active additive in coatings production and as an anti-block or slip agent. Many other applications are foreseen for erucic acid and its hydrogenated derivative behenic acid, in fields such as detergents, lubricants and cosmetics, but also in coatings industries. Examples for the latter are found in recently issued patents on their use in hot-melt ink jet printing inks and in printing ribbons [27]. Limnanthes alba contains over 95% fatty acids of the C20 and C22 type, 63% being A5- and Al 1-eicosenoic acids [25]. Limnanthes alba seed oil already finds use as a base oil in cosmetics but also has a large potential market in lubricants. It is a non-drying oil and has therefore limited use as a binder in coatings formulations. How- ever, derivatives from this oil, such as estolides, are tested in inks and paints as, e.g., viscosity modifiers.

3.3.2. New hydroxy fatty acid seed oils Dimorphotheca pluvialis (or cape marigold) seed oil

contains more than 60% of a hydroxydiene fatty acid: dimorphecolic acid (A9-hydroxy-lOt,l2t-octa- decadienoic acid) (Fig. 4). In contrast to the more familiar hydroxy fatty acid ricinoleic acid (A12-hy- droxy-9c-octadecenoic acid) from castor oil, or to the newer hydroxy fatty acid lesquerolic acid (A14-hy- droxy-14c-eicosenoic acid) from Lesquerella spp., di- morphecolic acid contains a conjugated diene moiety, cc-positioned with respect to the hydroxyl functionality (see Fig. 4.). This feature renders this fatty acid much more reactive than the two other hydroxy fatty acids mentioned. This reactivity may be exploited to obtain innovative oleochemicals, but also requires careful seed and oil handling and processing [25,28].

Like castor oil, it was found that lesquerella oil is also non-drying. Despite its two conjugated double bonds dimorphotheca oil also is a non-drying oil. This is probably due to the fact that the double bonds are positioned in an all-trans configuration. Also the pres- ence of an allylic hydroxyl moiety may contribute to this behavior [29]. However, it was found that after a prolonged period of time in the presence of cobalt driers dimorphotheca oil did dry to a wrinkled, frosted film [29]. This property, similar to tung and oiticica oil, suggests that dimorphotheca oil is dehydrated to conju- gated di-and trienes prior to peroxidation and drying. However, it is expected that chemical dehydration of dimorphotheca oil and lesquerella oil produces oils that are similar in drying properties to tung oil and linseed oil or dehydrated castor oil, respectively.

Apart from producing dehydrated oils, dimor- photheca oil and lesquerella oil can also be utilized in coatings system in which the presence of a hydroxyl group is exploited. An interesting example for such a system is the production of urethane resins based on these new oils plus, e.g., polyols or rosin esters [29].

3.3.3. New drying oils Calendula oficinalis (or common marigold) contains

fatty acids with conjugated triene functionality (calendic acid or A8t, lot, 12c-octadecatrienoic acid) (Fig. 4) very

52 J.T.P. Derksen et al. / Progress in Organic Coatings 27 (1996) 45-53

similar to the elaeostearic acid in tung oil from Aleuritis fordii. This oil presents an alternative source of these fatty acids, and can be utilized in the same applications as tung oil. The film-forming properties and film char- acteristics are expected to be very comparable to those of tung oil.

3.3.4. Natural epoxidized seed oils Euphorbia lagascae (or spurge) yields a natural epox-

idized oil that contains 60-65% vernolic acid (A12,13- epoxy-9c-octadecenoic acid) (Fig. 4) [25]. This fatty acid also occurs to an even higher extent (72-78%) in Vernonia galamensis and other Vernonia species, which are currently under investigation in the USA [30]. Ap- plications for this fatty acid are similar to those for epoxidized soybean oil, such as in stabilizers and plasti- cizers in polyvinylchloride production, but can also be found in paints and coatings industries.

Euphorbia lagascae and Vernonia spp. oils are very interesting for use in organic coatings. A major reason for this is that, in contrast to epoxidized soybean and linseed oils, these oils have a relatively low viscosity (approx. 101 centipoise at 25 “C [25]). Its pourability is also very good, even below 0 “C [30,31]. A very interest- ing application for such low viscosity epoxy functional- ity-containing oils is as reactive diluent in paints. In such an application the oil functions, on the one hand, as a solvent, making dispersion or solubilization of the formulation with volatile organic solvents superfluous, while, on the other hand, the oil reacts with other components in the formulation to form an integral part of the dried coating. It has been calculated that a 10 wt.% addition of vernonia oil per gallon of paint would reduce volatiles by as much as 160 million pounds per year across the USA [32]. Other applica- tions for Euphorbia lagascae and Vernonia spp. oils include coatings relying on the formation of interpene- trating polymer networks [33-351 and flexible baked coatings on metals [31]. The latter coatings show excel- lent flexibility, adhesion to substrate, chemical resis- tance, cohesive film properties and resistance to chipping [30].

3.4. Vegetable oil-based, water-borne emulsion coatings

The major advantage of water-borne emulsion coat- ings is the reduction in volatile organic compounds emission upon drying of the film. In the past, research has been focused on the emulsification behavior of pure linseed oil [36]. This has, however, not yet led to marketable products. Recently a joint project between our Institute and the University of Technology of End- hoven, Netherlands, has started to investigate the film forming properties of emulsion formulations that con- tain prepolymerized and pre-oxidized vegetable oils. This research aims at the development of emulsion

paints that are exclusively based on renewable re- sources.

3.5. Vegetable oil-based printing inks

An interesting new development that has come off the ground in recent years is the application of soybean oil in printing inks [37-391. Apart from the replacement of petroleum based resins by renewable resources, soy- bean oil-based vehicles has considerable advantages in the production of colored inks. The exceptionally light color of the vehicle allows substantially reduced pig- ment levels compared to conventional ink formulations [37]. So far, a large range of formulations, including 75 black and 25 colored, have been tested [38]. At present, various formulations are already employed by the American Newspaper Publishers Association in com- mercial lithographic and letterpress newsprint applica- tions [39]. Other advantages of vegetable oil-based inks include the formulation of inks over a broader range of viscosity, low rub-off characteristics, a reduction in emission of volatile organic compounds and decreased health hazards in ‘ink mist’ inhalation in the working environment: a problem sometimes encountered around high speed, rotary letterpress machines [39]. In Europe, other vegetable oils, such as rapeseed are also currently evaluated for use in, especially, cold-set offset ink for- mulations [39].

Apart from their use as a binder or diluent in printing inks, vegetable oils have also been successfully used as a washing solvent to remove excess printing ink and as a cleaning agent for printing rollers [l]. It was found that, in contrast to organic solvents, vegetable oils do not cause swelling of the rubber coatings on the rollers in the printing machinery. These applications are clearly not only interesting from an environmental, but also from a performance point of view.

4. Conclusions and perspectives

Since the advent of alkyd-type paints and of syn- thetic polymers for latex or vinyl and acrylic resin- based emulsions for surface coatings, the use of linseed oil in coating formulations has declined significantly. However, lately a revival of interest in linseed oil-based paints is observed. This is due to the user’s and con- sumer’s perception of this type of binder as environ- ment-friendly as well as to the industries interest in renewable resources as alternatives for petrochemical feedstocks of an inherently finite supply.

Many new paint and ink formulations appear each year. In patent literature one can observe an increasing application of renewable resources. In particular hy- droxyl- or epoxy-functionalized fatty acids are very versatile in their applications (for a few examples of recently issued patents see Refs. [40-431).

J.T.P. Derksen et al. /Progress in Organic Coatings 27 (1996) 45-53 53

Furthermore, because of the inherent polymer prop- erties of proteins, their price, and the possibilities to adjust the technological properties of these biopoly- mers, there are very good perspectives for the use of proteins in non-food applications. The possibilities within the coatings areas are only now beginning to be recognized.

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Crops, VCH, Weinheim, Germany, 1994, p. 73. i15] R. Vignolo and F. Naughton, INFORM, 2 (1991) 692. [16] F.C. Naughton, J. Am. Oil Chem. Sot., 51 (1974) 65. [17] K.T. Achaya, J. Am. Oil Chem. Sot., 48(1971) 758. [I81 W. Hondelmann and W. Radatz, Fette, Se@, Anstrichm., 84

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[26] J.T.P. Derksen, B.G. Muuse and F.P. Cuperus, in D.J. Murphy (ed.), Designer Oil Crops, VCH, Weinheim, Germany, 1994, p. 253.

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