Resins and additives for powder coatings and alkyd paints, based on renewable resources.
Keywords Alkyd resins, Renewable resources, Reactive diluents, Powder-coating resins, Cobalt-free drying catalysts, Iron, Reducing agents
Faced with current crude oil price hikes, the emerging economies of China, Brazil, and India, and the limited success in finding new vast crude oil reservoirs, the perception is growing among the masses that the end of the cheap oil era has arrived, and that prices for crude oil and transportation fuels are much more likely to increase than to decrease in the coming years and decades. This, combined with the desire to become less dependent on the countries of the Middle East with regard to feedstocks and the desire for more sustainable technology, drives society to develop alternative sources for energy and chemical feedstocks. At the same time, prices of renewable feedstocks have dropped continuously over the last few decades as a result of increased yields of biomass per acre. (1) Consequently there is in all respects, a growing opportunity to price effectively use of renewable feedstocks in nonfood products such as paints and coatings.
Many items in our everyday life are coatings, with the coatings having either a protective, signal, or decorative function. In addition to their aesthetic function, coatings obviously aim at enhancing the durability of a product. In addition, the increased sustainability of coatings is required today as well, with demand increasing for coatings with properties such as "solvent-free," "easy-to-apply," "recyclable," and "produces less waste." As long as such coatings contain resins and additives that are based on undepletable, renewable resources, they will contribute to enhanced sustainability.
In the "Biobased Products" Department of Wageningen University and Research Centre, various research programs are being carried out in co-operation with universities and industries that aim to develop both durable and sustainable coatings based on renewable resources. Current research projects cover the four principal components of coatings (i.e., binders, pigments/fillers, additives, and solvents). With regard to binders, products have been obtained that are suitable as paper coatings, floor coverings, high-solid alkyd resins, or resins for water-based systems. These products are based on new fatty acid or carbohydrate technology using combinations of carbohydrate and fatty acid technology, (2), (3) or are based on microbial polymers (e.g., polyhydroxyalkanoates or polypeptides). (4) Microbial production of solvents and the production of reactive diluents is also under study. (5) Patented alternative technology in the area of additives is under development, leading to cobalt-free drying catalysts for alkyd-based paints and inks. (6) In contrast, the development of renewable crosslinkers for power coatings has only recently begun.
Towards sustainable paint and coating resins
To develop safer working conditions for those who produce or apply paint, and to reduce the environmental burden, there has been a drive over the last few decades toward the development of coating technologies using less, or even no, volatile organic solvents. This has led to the development of powder coatings, as well as UV-curing and waterborne paint technologies. Solvent-free powder-coating systems have already become a commodity and are especially suited for the industrial coating of metal surfaces. New and improved technologies will make powder-coating systems also increasingly more suitable for protecting wood-based products. (7)
Next to the industrial use of solvent-free systems, the use of solvent-or waterborne paints that are applied by brush will remain very important. Most likely, alkyd resins and alkyd resin-based paints will also remain of utmost importance for the decorative coatings industry. Currently, alkyd resins are being produced annually in quantities of approximately 400,000 tons in Western Europe. Traditionally, alkyd paints are organic and solvent-based, using up to 50% of volatile organic carbon-based (VOC) solvents in a paint formulation. These types of coatings are especially popular because of their ease of application and high gloss characterisctics. New, existing, and anticipated legislation aiming at reducing VOC levels is the current driving force for the coatings industry, especially for decorative paints. Manufacturers of solvent-based paints have to respond to these forces, which will lead to three main options:
--Switch to water-based systems, making use of alternatives to alkyd technology
--Switch to water-based alkyd systems
--Switch to high-solid systems containing less or no VOCs
Another environmental issue connected with the use of current alkyd resin-based paints is the use of cobalt-based drying catalysts for this type of paint. There are indications that cobalt aerosols are potentially carcinogenic. (8) Consequently, there is a driving force to move away from cobalt-based drying catalysts and toward effective and more environmentally acceptable drying catalysts.
In this paper, activities in the development of renewable-based powder coating resins, high-solid alkyd resins, reactive diluents, and cobalt-free drying catalysts are described.
Results and discussion
Powder coating resins and crosslinkers
Whereas alkyd resins have low melting points and glass transition temperatures ([T.sub.g]), powder coating resins need to have a sufficiently high glass transition temperature in order to provide good storage stability and processing ability. The resins should have [T.sub.g] values above 45[degrees]C.
To synthesize renewable-based powder coating resins, combinations derivatives can be used. To create systems with a high [T.sub.g], the polyster backbone should be rigid, suggesting the use of rigid carbohydrate derivatives in combination with short chain diacids.
Such rigid carbohydrate derivatives can be obtained by the dehydration of sugar alcohols. The use of isosorbide, in this respect, is of special interest. The use of isosorbide has already been given some attention in polyester synthesis, because it is easily obtained from sorbitol. Terephthalic acid-based polyesters containing isosorbide have been described, (9) as well as polyurethanes and poly(ester carbonate)s. Coating, and especially fiber, application of polymers containing isosorbide have been described in the literature, including polyesters cured with styrene. (10) Powder coating applications using isosorbide are not specifically mentioned or described. Isosorbide can be obtained from sorbitol, which is commercially produced from starch through hydrolysis and subsequently hydrogenating the glucose into sorbitol (see Fig. 1). In Europe, over 400,000 tons of sorbitol are produced by companies such as Cerestar, Roquette and Tate & Lyle.
[FIGURE 1 OMITTED]
A series of renewable isosorbide-based copolyesters were synthesized by direct esterification of isosorbide with short-chain diacids such as succinic acid, using Ti[(OBu).sub.4] as a catelyst. Next to isosorbide, varying amounts of other diols such as 2,3-butanediol, 1,3-propanediol (potentially obtained by fermentation of sugars) and/or neopentyl glycol were built in (see Fig. 2). Reaction temperatures of 150-240[degrees]C and further polycondensation under reduced pressure (P < 5 mbar) led to polyesters with number-average molecular weights of a few thousand Daltons, and polydispersities between 1.6 and 2.0.
[FIGURE 2 OMITTED]
Figure 2 shows the effect of incorporating isosorbide on the [T.sub.g] values of the resulting terpolyesters. Upon incorporation of 60-80% of isosorbide, the [T.sub.g] is high enough to insure a good processing and storage stability of the polyester resin. It was also found that incorporation of other diols had a beneficial effect on the color of the resin. The introduction of small amounts of trifunctional acids (citric acid) or triols (trimethylol propane) in the resins resulted in improved coating properties.
Polyesters with either carboxyl or hydroxyl end groups were cured using different commercially available curing agents. Hydroxyl-functionalized resins were cured with (blocked) isocyanate groups containing petrochemical-based crosslinkers, whereas acidfunctionalized resins were cured with crosslinkers having epoxy groups (TGIC--triglycidyl isocyanurate, Aldrich Chemicals) or activated hydroxyl groups (Primide XL-552; N, N, N' N'-tetrakis (2-hydroxyethyl) adipamide, EMS (Chemie). Depending on the specific crosslinker used and the functionality of the resin, films were obtained that can withstand full impact testing and show good-to-excellent solvent resistance (see Tabel 1).
Table 1 shows that, after crosslinking, interesting coating properties can be obtained with these renewable-based polyester resins and conventional petrochemical-based crosslinkers. All films showed Konig hardnesses values above 200 s. Good solvent resistance and impact resistance could be obtained. The best performing crosslinker (TGIC), nonetheless, is also the least desired from a health point of view. Therefore, to develop crosslinkers with an increased sustainability profile and less negative impact on health and environmental issues, it is desirable to evaluate renewablebased crosslinking agents.
Epoxidized linseed oil has already been known for several years as a bio-based crosslinker for powder coating resins, (11) but although it effectively crosslinks numerous powder coating formulations and is commercially applied to some extent, its application scope is limited because it has a tendency to severely lower the [T.sub.g] of powder coating formulations, and thereby has a negative effect on storage ability and the processing window.
Table 1: Coating properties for renewable polyesters crosslinked with conventional crosslinkers Resin Curing agent Impact test Solvent resistance (a) functionality (1 kg, 100 cm) OH-functional Linear VB - - Linear DN + +/- Branched DN + + Branched VB +/- + COOH-functional Branched TGIC + ++ Branched P55 +/- + Note: Coatings were prepared by applying a film (polyester and crosslinker dissolved in N-methylpyrrolidone) (NMP)of approximately 250 -[micro]m thickness onto an aluminum panel. After drying, the films were cured at 200[degrees]C during 30 min under nitrogen. Branched OH- functional resins contain small amounts of triols. Reverse impact test was carried out according to ASTM D 2794. DN = Desmodur N3600 (hexamethylene diisocyanate-based polyisocyanate, Bayer), VB = Vestagon B1530 (polyisocyanate based on isophorone diisocyanate, Bayer), P552 = Primid XL-552. (a) Acetone rub test.
The authors, therefore, recently started to explore the potential of using more rigid renewable building blocks to produce tri- or tetra-functional crosslinkers suitable for both hydroxy- acid acid-functionalized resins.
High-solid alkyd resins based on renewable resources
Conventional air-drying paints usually utilize alkyd oligomers as binders. These alkyds used in coating formulations are suitable for applications at ambient temperature and are synthesized by a polycondensation reaction of polycarboxylic acids, polyhydric alcohols, and unsaturated fatty acids or oils (see Fig. 3). Highly suitable polycarboxylic acids are, for instance, phthalic acid anhydride, terephthalic acid anhydride, trimellitic acid, adipic acid, and maleic acid.
[FIGURE 3 OMITTED]
The polyalcohols commonly used in the alkyd synthesis are, among others: (di)pentaerythritol, glycerol, ethylene glycol, trimethyllolpropane, and neopentylgylcol. Whereas the fatty acid or oil component is derived from renewable resources, the, majority of the aforementioned polyacids, and polyalcohols are alkyd building blocks derived from petrochemical resources.
Conventional synthesis of alkyd resins leads to polycondensates with a rather high polydispersity, in which case the high molecular-weight part contributes to a high intrinsic viscosity, and the low molecular-weight part has a negative impact on drying properties. To obtain high-solid paints, it is important to decrease the intrinsic viscosity of the alkyd resin. This might be achieved by developing an alkyd oligomer with a suitable average molecular mass and a narrow molecular-weight distribution (low polydispersity).
Alkyd resins completely based on renewable resources have thus far been the subject of a few studies. Hoechst's patent (12) describes waterborne alkyd emulsions based on renewable resources. The patent describes the use of sorbitol (easily derived from starch) as the polyhydric alcohol and the use of succinic acid anhydride as the polycarboxylic acid. Hajek (13) described the use of sorbitol and xylitol in alkyd resin synthesis, whereas, Bagchi and Malakar (14) described the partial replacement of conventional polyols (i.e., glycerol and pentaerythritol) by sorbitol.
A US patent assigned to the Research Corporation in New York, (15) discloses oil-modified sucrose resins obtainable by reaction of a partial esterified sucrose ester with a cyclic dicarboxylic dicarboxylic anhydride and a diepoxide.
Although this work mentions alkyd resins based in renewables, it is not specifically aimed at deriving high-solids alkyd paints based on resins with a low intrinsic viscosity. Within the framework of a research project, together with paint producer SigmaKalon, an attempt was made to develop alkyd resins with low intrinsic viscosity. To achieve this, the authors explored the potential of two types of different renewable carbohydrate-derived resources such as sucrose and inulin.
Sucrose, in particular, is an interesting starting material because it is produced worldwide from crops such as sugar beet and sugar cane at over 140 million tons per annum, and at prices below those for common synthetic polyols used in alkyd-polyester resin synthesis.
Synthesis of alkyd resins
Conventional alkyd resins are synthesized by mixing the appropriate amount of polyhydric alcohol, polycarboxylic acid (anhydride), and fatty acid or oil. Subsequently, a polycondensation reaction at an elevated temperature is carried out (usually at about 250[degree]C), with the use of an appropriate catalyst, resulting in a polycondensate mixture with a relatively high polydispersity. This method is not directly applicable to carbohydrates such as sucrose and inulin because these carbohydrates decompose at such high temperatures, and are not easily miscible with fatty acid methyl esters or oils.
An alternative synthesis procedure was developed by dissolving nonmodified or partially acetylated) sucrose or inulin in DMAA (n, N-dimethylacetamide), adding sunflower oil or methyl linoleate, and reacting the mixture at 140[degree]C. Catalysts such as potassium carbonate or lithium hydroxide were found to be the most effective.
Derivatization of inulin with unsaturated fatty acid methyl esters or oils directly results in products with alkyd-type characteristics. In the case of sucrose, the use of renewable dicarboxylic acids (or polyucarboxylic acids) is required in addition to sucrose and fatty acid methyl esters to synthesize oligomers. Examples of such renewable dicarboxylic acids are dimethyl suberate, dimethyl sebacate, or dimerized (un)saturated fatty acids. In the case of sucrose, these result in structures of alkyd resins, as shown in Fig. 4. This type of alkyd resin was synthesized by using either sucrose or sucrose acetate as the starting material.
[FIGURE 4 OMITTED]
Characterization of alkyd resins
After synthesis and a work-up, the characteristics of the alkyd resins were determined by techniques such as nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatorgraphy (GPC). The hydroxyl value, acid value, and viscosity of the resins were also determined. Table 2 shows the characteristics of a typical sucrose-based alkyd resin at varying ratios of dimethyl adipate/methyl linoleate. Some effects of the type and amount of crosslinked on properties such as molecular weight, polydispersity, OH-value, and acid value were found, but from the limited data obtained, no clear relation could be established.
Table 2: Influence of the ratio crosslinker dimethyladipate/methyllinoleate on the resin properties Ratio dimethyl GPC analysis Hydroxyl Acid Viscosity adipate/methyl value value (dPa.5) linoleate (a) Min Polydispersity 2.4/5.6 4.2 X 1.6 54 11 18 [10.sup.3] 4.8/5.6 7.7 X 6.9 41.8 9.6 123 [10.sup.3] 5.6/5.2 9.7 X 21.4 n.d. 3.2 56 [10.sup.3] (27% solvent) 7.0/4.5 4.2 X Gelation [10.sup.3] (a) Interesterification.
Characteristics of paint formulations
Table 3 shows come favorable properties of the paint formulations derived from the sucrose-based alkyd resins such as fast through drying and also at lower temperatures, very good leveling, and good compatibility with (standard) color pastes. Gloss retention, determined via QUV/A measurements, can be regarded as an indication for durability. It was found that the gloss retention (see Fig. 5) of the best performing systems equals that of commercial systems suitable for outdoor applications.
[FIGURE 5 OMITTED]
Table 3: Paint formulation containing a sucrose-based alkyd resin (OVN 299); methyllinoleate was used as the fatty acid component Test OVN-299 Commercial paint reference Low shear viscosity (dPa.s) 9.4 High shear viscosity (dPa.s) 4.8 Solids content (% w/w) 80.2 Solids content (% w/w) 65.3 VOC (g/L) 262 Specific weight (g/mL) 1.27 Whiteness 1 day (Cie) 76.9 71.2 Whiteness 15 day (Cie) 78.0 73.0 Yellowing [NH.sub.4]OH ([DELTA]Cie) 77.9/48.4 72.5/50.9 Yellowing 5 days 50[degrees]C ([DELTA]Cie) 78.6/67.0 73.7/64.3 Yellowing 1 day 80[degrees]C ([DELTA]Cie) 78.5/54.6 73.4/55.8 Drying recorder 23[degrees] C/50% RH + 0 Drying recorder 5[degrees]C/90% RH 0/+ 0 Gloss 20[degrees]/60[degrees] 1 day 75.1/85.3 71.2/87.0 Gloss 20[degrees]/60[degrees] 1 day 62.1/80.5 47.2/78.4 Water sensitivity 15 days 1 h/8 h +/- +/- Leveling ++ ++ Hiding power + + Paste compatibility (standard paste) + + Wrinkling BA60 0 0 Elasticity (Erichsen mm) 7.5 7.7 Konig hardness 1 day(s) 16.6 12.6 Konig hardness 15 day(s) 22.4 25.2 Scratch resistance - +/-
In summary, it can be concluded that high-solid alkyd paints can be formulated containing the sucrose-based alkyd resins, and that the paints possess favorable properties. Given the attractive price of sucrose as a starting material, it might be expected that more solvent-free routes to sucrose-based alkyds will offer a commercial perspective.
High-solid alkyd resins completely based on renewable resources could result (except for pigments and fillers) in completely renewable-based paints, in which case the additives and solvents could also be based on renewable resources. Attractive solvents for high-solid alkyd paints consist of so-called reactive diluents: upon drying, these solvents react with the alkyd resins and therefore are nonvolatile.
Reactive diluents partly based on renewables have been described before. WO 97/02326, for example, discloses the use of octadienyl maleates, fumarates, and succinates as reactive diluents in high-slid paints. (16) EP 0 685 543 A2 describes the use of alkyl esters derived from the acid of a unsaturated vegetable oil in a coating composition containing an alkyd and a reactive diluent. (17)
Reactive diluents that are based on combinations of unsaturated fatty acids and carbohydrates, however, have not to our knowledge been systematically studied. Within the framework of the authors' research, they explored reactive diluents based on sorbitol, methyl-glucoside, sucrose, and 1-kestose.
All these components showed the properties of reactive diluents. The best performance was found for those reactive diluents that consisted of at least two saccharide units (e.g., sucrose). These reactive diluents had the best overall performance with regard to cutting behavior, compatibility with commercial binders and pastes, and drying performance. Their drying performance equals that of a commercial reactive diluent with a much higher molecular mass. Reactive diluents based on monosaccharides such as sorbitol or methylglucoside showed some negative influence on drying properties. Table 4 shows the behavior of a sucrose-based reactive diluent [sucrose octalinoleate (SOL)] in a commercial paint formulation.
Table 4: Test results of an alkyd-based paint formulation containing a sucrose-based reactive diluent and a commercial reference reactive diluent (RT = room temperature, RH = relaative humidity). Water sensitivity, Leveling, Hiding power, Wrinkling and QUV-A are relative measurements with 0 = excellent performance and 5 = very poor performance. Test Formulation Formulation base don based on commercial sucrose reactive octalinoleate diluent reactive diluent Viscosity (dPa.s) 13.1 11.4 ICI-viscosity 6.8 6.6 Solids content (% w/w) 83.3 83.3 VOC (g/L) 222 222 Whiteness 73.0 72.2 Yellowing N[H.sub.4]OH -29.9 -33.8 Yellowing 5 days 5 [50[degrees]C -12.6 -14.6 Yellowing 1 day 80[degrees]C -8.75 -19.0 Drying recorder RT 0 0 Drying recorder 5[degrees]C/90% RH 0 0/- Gloss 87.5 88.7 Water sensitivity 1 day 0 0 Leveling 1 1 Hiding power 1 1 Color strength 98.7 97.0 Wrinkling 0 0 QUV-A 0 0 Adhesion on old alkyd layers Pass Pass Adhesion on bare spruce Pass Pass Erichsen (mm) 8.6 8.3
The results of Table 5 demonstrate that the overall performance of the sucrose-based reactive diluent in the paint formulation is very comparable with that of a commercial reactive diluent with three times the molecular mass. Viscosity is a little lower and gloss somewhat higher compared with the paint formulation containing the commercial reference reactive diluent.
Table 5: The effect of incubation time on the Fe/ImAsA6p systems as a drying catalyst for high gloss white paint based on Setal 16 LV WS 70 (Nuplex Resins) Drier % of active Incubation Dust free Tack-free drier time (days) drying (h) drying (h) Fe/IM/AsA6p 1/4/1 0.07 Fe 12 5.06 3.8 Fe/IM/AsA6p 1/4/2 0.07 Fe 12 2.0 4.9 Fe/IM/AsA6p 1/4/3 0.07 Fe 12 8.6 4.2 Fe/IM/AsA6p 1/4/4 0.07 Fe 12 - - Co + ExKin 2 0.07 Co 12 3.3 4.2 Mn 0.07 Mn 12 0.66 6 Mn 0.07 Mn 12 0.66 4.76 Fe/IM/AsA6p 1/4/1 0.07 Fe 18 0.7 2.66 Fe/IM/AsA6p 1/4/2 0.07 Fe 18 0.63 0.43 Fe/IM/AsA6p 1/4/3 0.07 Fe 18 0.6 1.53 Fe/IM/AsA6p 1/4/4 0.07 Fe 18 1.16 1.53 Co + ExKin 2 0.07 Co 12 3.3 4.23 Mn 0.07 Mn 12 0.66 6 Mn 0.07 Mn 12 0.66 4.76 Drier Total drying Hardness Skin Whiteness time (h) (s/K) 5 formation Index days Fe/IM/AsA6p 1/4/1 10.8 56 N.O. 80.6 Fe/IM/AsA6p 1/4/2 11.3 56.4 O. 80.8 Fe/IM/AsA6p 1/4/3 17.3 68.6 O. 81.5 Fe/IM/AsA6p 1/4/4 >20 59.7 N.O. 81.2 Co + ExKin 2 14.1 73 N.O. 82.8 Mn 12.5 52 O. 83 Mn 9.9 51.3 O. 82.1 Fe/IM/AsA6p 1/4/1 4.5 73.6 N.O. 80.6 Fe/IM/AsA6p 1/4/2 2.2 80.6 O. 80.8 Fe/IM/AsA6p 1/4/3 5.8 85 O. 81.5 Fe/IM/AsA6p 1/4/4 6.6 80 N.O. 81.2 Co + ExKin 2 14.1 73 N.O. 82.8 Mn 12.5 49 O. 83 Mn 9.9 51.3 O. 82.1 As Co-based catalyst Nuodex WEB 6 (obtained from Elementis, formerly Sasol Servo Delden) was used. NO = not observed. O = observed. Exkin; antiskinning agent.
Cobalt-free drying catalysts
Next to volatile organic solvents, there are other environmental issues related with the use of alkyd-based paints. A quick drying of alkyd paints is of enormous commercial importance. Common solvent-borne alkyd paints contain, besides the main constituents, alkyd resins (binders), pigments and solvents, also small amounts of cobalt-based driers (e.g.. cobalt-2-ethylhexanoate). The cobalt salts increase the oxidative crosslinking rate of the unsaturated fatty acids that are present as constituents of alkyd resins (see Fig. 6).
[FIGURE 6 OMITTED]
As a result of the fatty acid oxidation during air drying, hydroperoxides are formed. Their decomposition into free radicals, catalyzed by metal ions, gives rise to crosslinking reactions, leading to the hardening of the film Despite their important role in the drying process of alkyd paints, cobalt driers, however, are potentially carcinogenic in aerosol form (shown in animal testing). Therefore, finding substitutes for this cobalt-based catalyst is of high importance to the alkyd-producing and using industry.
Several substitutes for cobalt-based drying agents are in development. Most of these are based on either manganese (18) or vanadium salts. (19) Manganese-bipyridine complexes have especially been promoted as alternatives. (18) Although these alternatives can be applied in specific systems, their overall performance does not match up to that of cobalt-based driers; the paint films usually remain too soft, and manganese can have a negative impact on the film color. Bipyridine is potentially harmful as well. More recent research has been carried out on other manganese-based catalysts. (20), (21)
An innovative biomimetic approach was being followed to develop alternative cobalt-free drying catalysts using combinations of iron salts and reducing agents that are know to oxidize unsaturated fatty acids in nature. This concept has been translated and adopted for usage in both waterborne and solvent-based alkyd system; the reducing agents evaluated in this study, namely ascorbic acid and ascorbylpalmitate, are both derived from renewable resources.
The oxidative and crosslinking ability of iron salt combination with reducing agents was studied for model system consisting of either neat ethyl linoleate or methyl linoleate, in the case of diluted aqueous emulsions. (22) (See Fig. 7)
[FIGURE 7 OMITTED]
For both systems, it was found that the combination of iron salts and ascorbic acid palmitate (AsA6p) as the reducing agent was, depending on the ratio of iron to reducing agent, capable of generating hydroperoxides. In neat ethyl linoleate (EL), however, crosslinking is also induced (23) (see Fig. 8). Ascorbyl palmitate appeared to be more suitable than ascorbic acid, especially for nonaqueous systems.
[FIGURE 8 OMITTED]
From Fig. 8 it is clear that within the first 100 h, Fe-eh (Fe-ethylhexanoate) induces almost no crosslinking at all at room temperature, whereas most crosslinking occurs at an AsA6p/Fe-eh ratio of 2:1.
To understand better the mode of action of the Febased catalysts, both the structure of Fe-eh and that of the system formed upon addition of AsA6p were studied using a wide array of different techniques. Using ElectroSpray ionization mass spectrometry (ESI-MS), it was found (24) that Fe-eh, in solution, consists of trinuclear oxocentered Fe (III) clusters of formula [[Fe [(III).sub.3] ([mu].sub.3]-O)[(eh).sub.6].sup.+]] (see Fig.9). Moss-bauer's studies revealed that the trinuclear iron was not symmetric as two equilvalent Fe (III) sites and one unique Fe (III) site were identified. Upon addion of AsA6p to the Fe-eh complex, a new species was formed that was a mixed valence complex with a similar iron core.
[FIGURE 9 OMITTED]
Structural details of these complexes are not yet fully known and attempts to crystallize the complexes have so far failed. From NMR measurements it can nonetheless be concluded that the AsA6p most probably is coordinating to the iron via the C3 (and C4)-OH group to draw up a mechanism through which the combination AsA6p/Fe-eh most likely oxidizes unsaturated fatty acids such as ethyl linoleate (see Fig. 10). The ability of AsA6p to reduce Fe (III) to Fe (II) drives the catalytic cycle.
[FIGURE 10 OMITTED]
Besides model systems, the AsA6p/Fe-eh combination was also further evaluated in different waterborne and solventborne (commercial) varnish and paint formulation. Form these evaluations, a rather complex pattern appears. The effectiveness of the Fe/AsA6p system as a drying catalyst was tested in a high gloss white paint based on Setal 16 LV WS-70 (linseed fatty acid-based alkyd resin) and good results were ontained. Results were further improved by the addition of Imidazole (Im) as a ligand. (25) The results in Table 6 show that the catalyst is not immediately effective after mixing iron and ascorbic acid palmitate, but that it need some standing or incubation time after mixing to become active.
Table 6: Drying times of a DSM waterborne (WB) paint based on Uradil AZ 554 Z-50 (based on special processed soy bean oil) Catalyst Drying Stages Composition Concentration Basic Ripped film Surface (% Fe, w/w) trace film trace (end; h) (end; h) (end; start total dry; h) No catalyst 0.15 <10.7 Nuodex WEB Co 6DP 0.08 0.17 1.1 >10.7 Cobalt-drier (225 [micro]l) Fe[[gluc].sub.2] 0.04 0.15 0.23 4.6 [[Hasc].sub.1] Fe[[EDTA].sub.0.5] 0.04 0.17 0.23 4.0 [[Hasc].sub.1] Fe[[citrate].sub.0.67] 0.04 0.17 -- 2.0 [[Hasc].sub.1]
After an incubation time of 14 days, interestingly in the absence of auxiliary driers, the drying activity is higher than that of cobalt-based drying catalysts. Hardness development is comparable with cobalt and better than for managanese. Another very intersting feature is that the system, at various Fe/AsA6p ratios, does not show skin formation. This implies that the system will either not need antiskinning agents or will need much less than cobalt-based systems.
Similar results, with respect to drying time and hardness development, were found upon using a varnish based on Uralac AD 142 W-50 (based on linsead oil). Very interesting results in regard to drying times and hardness development were also obtained using solvenborne alkyed paints based on specially processed soy bean oil. As conventional solventborne alkyd paints will gradually be substituted by either high-solid alkyds of alkyd emulsions. these systems were also evaluated.
Few experiments so far have been carried out on high-solid paints, but the results show a less efficient drying than in the case of conventional alkyds. To date it is unknown whether this results from a decreased compatibility of the drier with these systems, from the different structure of the alkyd resins in such paints, or because of other factors.
For waterborne alkyd emulsions, the system was found to be very effective. It was found that the system could be made more effective with regard to the results previously obtained by additional water-soluble metal ion-coordinating ligands. This way, very short drying times were obtained for Uradil AZ 554 Z-50 (based on special processed soy bean oil) emulsions (see Table 6).
From the results of Tables 5 and 6, it can be concluded that the combination of iron-ascorbic acid (palmitate), in the presence or absence of additional ligands, can be a powerful system for the drying of alkyd paints. Drying capacity is dependent on the ratio of Fe to ascorbic acid palmitate, and the drying capacity obtained so far is larger for specific systems (e.g., solvent-based or water-based alkyds containing special processed soy bean oil) than other systems (e.g., high solids). Hardness development is improved compared to manganese-based systems. More research will be needed to develop iron-based systems that are more widely applicable and also show effectiveness in high-solid systems.
The results shown in this paper clearly indicate that both resins and additives for non or low-VOC paint formulations can be based on those raw materials that are the least disputed from an environmental point of view (i.e., renewable resources). High-solid paints based on alkvds containing sucrose--the world's most produced pure chemical--with a performance equal to modern high-solid paints were obtained. Lower molecular-weight sucrose Tatty esters were found to be excellent reactive diluents. It may be expected that sucrose-based alkyds will become highly attractive from a commercial point of view once effective synthesis procedures, starting from the free fatty acids or without the use of polar solvents, have been established.
Very promising results have also been obtained with regard to powder coating resins based on isosorbide. Further research is needed to determine and broaden the application scope.
With regard to renewable-based additives, the alternative iron-based catalysts demonstrate their potential for use in important types of alkyd paints. More research will be needed to broaden and fine tune their applcation scope.
Given the continued interest in developing more sustainable technology and the fact that the prices of fossil feedstocks are destined to increase, the coming decades inevitable will seen an increase in renewablebased coatings and paints components, combining economics with unique properties.
Acknowledgments Research and development work on the high-solid alkyd resins and reactive diluents was carried out in a joint cooperation project between WUR/BbP and SigmaKalon. Research on the iron-based drying catalyst was financially supported in part within the framework of the Dutch IOP Environmental Technology/Heavy Metals program. The research on polyester resins forms part of the research program of the Dutch Polymer Institute (DPI), Project Number 451. We thank DSM Resins for supplying alkyd emulsions and Elementis (formerly Sasol Servo) for supplying Co- and Mn-based drying catalysts.
(1.) Soetaers, W, Proceedings from the Renewables and Biorefineries Conference, Gent, 19-21 September 2005
(2.) Oostveen. EA, Weijnen, JW. van Haveren. J. Gillard, M. "Polysaccharide Esters and their Use as Binders in Coatings." WO Patent 03064477 to SigmaKalon, 30 January 2002
(3.) Oostveen. EA, Weijnen. JW. van Haveren, J, Gillard, M, "Air-drying Paint Compositions Comprising Carbohydrate-based Polyesters." WO Patent 03064498 to SigmaKalon. 30 January 2002
(4.) Buisman, GJH, Cuperus, FP, Weusthuis, RA. Eggink. G. "Poly (3-hydroxy alkanoat) Paint and Method for the Preparation Thereof." US Patent 6024784. 26 October 1998.
(5.) Oostveen, EA, Weijnen. JW, van Haveren, J, Gillard, M, "Reactive Diluents and Coatings Comprising Them." WO Patent 03064548 to SigmaKalon, 30 January 2002
(6.) Oostveen. EA, van Haveren. J, Micciche, F, van der Linde, R. "Drier for Air-drying Coatings." WO Patent 03093384, 5 May 2003.
(7.) Schmidt, T, "Powder Coating Medium-density Fibre Board: A New Approach to Success." Powder Coating, June 2004, www.pcoating.com (websitc checked 4/9/06)
(8.) Bucher, JR, Hailey. JR, Roycroft, JR, Haseman, JK, Sills, RC, Grumlein, SL, Mellick, Chou, BJ, "Inhalation Toxicity and Carcinogenicity Studies of Cobalt Sulphate." Toxicol. Sci., 49 56 (1999)
(9.) Witteler, HB. Charbonneau, LF, Garo. K, Johnson, RE, "Polyesters Including Isosorbide as a Co-monomer and Methods of Making Same." US Patent application 595 59 066 to HNA Holding, 23rd April 1998
(10.) James, WR, Mott, A, Frederick, JV, "Unsaturated Polyesters for Coating Applications." Patent GB 92 77 86 to Howard of Ilford Ltd. 14 July 1958
(11.) Overeem, A, Buisman, GHJ, Derksen, JTP, Cuperus. FP. Molhoek, L, Grisnich, W, Goemans, C, "Seed Oil Rich in Linolenic acid as Renewable Feedstock for Environmentally-Friendly Crosslinkers in Powder Coatings." Indus. Crops Products. 10 157-165 (1999)
(12.) Zoller, J, Merten, G. Urbano, E, Gobec, M, "Wassrige Alkydharzemulsionen aus Nachwachsenden Rohstoffe." EP 0741 175 A2 to Hoechst AG, 2 May 1995
(13.) Hajek, K, "Verwendung von Sorbit and Xylit zure Synthese von Alkydharzen." Farbe Lacke, 83 798-804 (1977)
(14.) Bagchi, D, Malakar. RK, "Sorbitol as an Alkyd Ingredient." J. Coat. Technol., 58 (740) 51-57 (1986)
(15.) Faulkner. RN, "Resinous Reaction Product of a Sucrose Partial Ester. A Cyclic Acid Anhydride and a Diepoxide." US Patent 3 870 664 10 January 1974
(16.) Deans, H. Gracey, BP, Hallet, C, Hodgson, PKG. Klaasen. RP, "Reactive Diluents." WO Patent 9702326 to BP Chem Ltd., 30 June 1995
(17.) Zonjee, T. "Resin Composition Comaining an Alkyd Resin and a Reactive Diluent." EP 0685543 A2 to DSM NV. 23 May 1995
(18.) Bieleman, JH, "Progress in the Development of Cobalt-free Drier Systems." Macromol. Symp., 187 811-821 (2002)
(19.) http://www.borchers.com/index.php?page ID = 21 (site checked 4/9/06)
(20.) Gorkum, RV, Bouwman, E, Reedijk, J,"Fact Autooxidation of Ethyl Linoleate Catalysed by Mn[(AcAc).sub.3] and Bipryidine: A Possible Drying Catalyst for Alkyd Paints." Inorg. Chem., 43 (8) 2456-2458 (2004)
(21.) Oyman, ZO. Ming, W, Micciche, F. Oostveen, E. van Haveren, J. van der Linde, R, "A Promising Environmentally-friendly Manganese-based Catalyst for Alkyd Emulsion Coatings." Polymer, 45 7431-7436 (2004)
(22.) Micciche, F, van Haveren, J, Oostveen, E, Laven. J, Ming, W, Oyman, ZO, van der Linde, R, "Oxidation of Methyl Linoleate in Micellar Solutions Induces by the Combination Iron (II)/Ascorbic Acid and Iron(II)/[H.sub.2][O.sub.2]." Arch. Biochem. Biophys., 443 45-52 (2005)
(23.) Micciche. F, van Haveren, J. Oostveen. E, Ming, W. van der Linder, R, "Oxidation and Oligomerization of Ethyllinoleate Under the Influene of the Combination of Ascorbic Acid 6-palmitate/iron-2-ethylhexanoate." Appl. Catal. A. 297 (2) 174-181 (2006)
(24.) Micciche, F, Straten. MA, Ming, W, Oostveen, E., van Haveren, J, van der Linde. R, Reedijk, J. "Identification of Mixed-valence Metal Clusters in Drier Solutions for Alkyd based-paints by Electrospray Ionization Mass Spectrometry (ESI-MS)." Int. J. Mass Spectrom., 246 80-83 (2005)
(25.) Micciche, F, Oostveen, E, van Haveren, J, van der Linde, R. "The Oostveen, E, van Haveren, J, van der Linde, R. "The Combination of Reducing Agents/Iron as Environmentally Friendlier Alternatives for Co-based Driers in the Drying of Alkyd Paints." Prog. Org. Coat. 53 99-105 (2005)
Presented at the XXVIII FATIPEC Congress, in Hungary, June 2006.
J. van Haveren, E, A. Oostveen,
A. E. Frissen
Department Biobased Products, Wageningen University and Research Centrum, Bornsesteeg 59, PO Box 17, Wageningen 6700 MA, The Netherlands
Fundamentals of Advanced Materials, Technical University of Delft, Kluyverweg 1, Delft 2629 HS, The Netherlands
B. A. J. Noordover, C. E. Koning, R. A. T. M. van Benthem Eindhoven University of Technology, Den Dolech 2, Eindhoven 5600 MB, The Netherlands
J. G. J. Weijnen
SigmaKalon Research, Oceanenweg 2, Amsterdam 1047 BB, The Netherlands
J. van Haveren, B. A. J. Noordover, A. E. Frissen Dutch Polymer Institute (DPI), PO Box 902, Eindhoven 5600 AX, The Netherlands
E. A. Oostveen
Givaudan Nederland BV, Nijverheidsweg 60, 3770 AK Barneveld, The Netherlands
|Printer friendly Cite/link Email Feedback|
|Author:||van Haveren, J.; Oostveen, E.A.; Micciche, F.; Noordover, B.A.J.; Koning, C.E.; van Benthem, R.A.T.M|
|Date:||Jun 1, 2007|
|Previous Article:||A new pigment for smart anticorrosive coatings.|
|Next Article:||Waterborne silicone-organic hybrid coatings for exterior applications.|