Urethane-Modified Alkyd Resins

Summary

Alkyd resins are synthetic resins widely used in the paints and coatings industry due to their superior performance, ease of application, and versatile properties. Alkyd resins are modified in various ways.

These modifications can be carried out by blending with various additives or by subjecting the resin to reaction with different chemicals, enabling the synthesis of resins with desired properties.

Alkyd resins can be modified with styrene, vinyl, toluene, methyl methacrylate, acrylonitrile, phenolics, benzoic acid and its derivatives, epoxy, silicones, and isocyanates.

In this review, general information on urethane-modified alkyd resins obtained through modification reactions with isocyanates is presented, and studies in the literature are briefly summarized.

Alkyd Resins

Alkyd resins, widely used as a binder component in paint formulations, are polyesters modified with oils or fatty acids. They are obtained through polycondensation of dibasic acids with polyhydric alcohols in environments containing oils and fatty acids. The polyester chain in the alkyd structure develops the hardness of the resin, while the presence of oils and fatty acids in the structure improves flexibility, adhesion, and solubility in solvents. At the same time, oils and fatty acids also increase the pigment-wetting property of the resin. In oxidative drying, the presence of unsaturated groups in the structure improves the drying properties of the paint film [Yürekli, 1995]. Alkyd resins, compared to other synthetic surface coatings, are the most widely used resin type in paint and varnish production. The main reasons for this are that they show high stability during storage, are cost-effective, and the modifications required to achieve desired product properties can be easily carried out. By applying different modifications, products can be obtained ranging from hard and fast-drying film properties to slow-drying and high-flexibility film properties [Tuna, 2011].

Modification of Alkyd Resins

The applications of alkyd resins obtained as a result of condensation reactions between polyalcohols and polyfunctional acids are limited. For this reason, alkyd resins are modified with other resins / oils / simpler alcohols and acids. The alkyd resins with the widest application are oil-modified alkyds. Since the use of such modified resins is very widespread, over time the term "oil-modified" was dropped, and only the term alkyd is used for this type of resin. Non-oil-modified, pure polyester-based coating materials used for this purpose are also called oil-free alkyd or saturated polyester [Tahmaz, 2013]. Alkyd resins can be modified by blending with various additives or by subjecting them to reaction with different chemicals, enabling the synthesis of resins with desired properties. Alkyd resins can be modified with styrene, vinyl, toluene, methyl methacrylate, acrylonitrile, phenolics, benzoic acid, epoxy, silicones, and isocyanates [Saçak, 1998]. Today, there are numerous studies in the literature on epoxy, vinyl, acrylate, urethane, styrene, phenolic, and silicone-modified alkyds, and research in this area continues [Akgün et al., 2016].

Polyurethane Resins

Polyurethanes are polymers synthesized from diols and diisocyanates and are also known as carbamates. For example, crystalline polyurethane, produced under the trade name Perlon U, is a polymer of 1,4-butanediol and hexamethylene diisocyanate [Saçak, 2002]. In polyurethane production, reaction between diisocyanates and compounds containing at least two hydroxyl groups is involved. Polyurethane systems consist of two components, one isocyanate and the other polyol. As polyols, compounds containing hydroxyl groups can be used, for example polyesters, polyethers, and glycols. The polyurethane synthesis reaction using 2,4-toluene diisocyanate (TDI) and ethylene glycol (EG) as starting materials is presented schematically in Figure 1 [Yürekli, 1995; Paksoy, 1999].

The isocyanate group-containing compound used in the reaction can be difunctional or polyfunctional, and of aliphatic, aromatic, or cycloaliphatic character.

Depending on the properties of the two starting components used (polyol and isocyanate), final products are obtained in a wide range varying from highly elastic products to extremely rigid products. As the amount of hydroxyl groups in the polyol increases, the degree of cross-linking will increase and a denser network structure will form.

Thus, harder and more chemically resistant paint films will be obtained.

As the amount of hydroxyl groups decreases, a more widely spaced network structure will develop, resulting in more elastic, softer paint films that are less resistant to chemicals and atmospheric corrosion.

Through the selection of the isocyanate component and the appropriate combination of both components with each other, paint films with all desired properties such as drying characteristics, hardness, elasticity, abrasion resistance, atmospheric corrosion, and chemical resistance can be obtained [Yürekli, 1995; Paksoy, 1999].

Polyurethanes, due to their properties such as hardness, high abrasion resistance, and high chemical resistance, are widely used not only in paint and varnish systems but also in fiber, foam, sheet, and adhesive production. At the same time, various applications in mechanical, thermal, and sound insulation are also encountered [Kent, 1992].

Urethane-Modified Alkyd Resins

Urethane alkyds, also called uralkyds or urethane oils, are alkyds modified with diisocyanates (typically TDI). TDI partially or completely replaces phthalic anhydride used in alkyd preparation [Wicks et al., 1992]. The main two advantages of uralkyds are better abrasion resistance and better hydrolysis resistance than alkyd coatings. Their disadvantages are, especially when TDI is used, yellowing of the films over time and cost slightly higher than alkyds. When uralkyds are made with aliphatic diisocyanates, yellowing over time is reduced but this increases cost. Furthermore, uralkyds are widely used as binders in construction paints [Wicks et al., 1992].

The urethane-modified alkyd synthesis reaction is presented schematically in Figure 2. Isocyanates have very high activity and react with hydroxyl groups in the alkyd resin at room temperature to form urethane (carbamate) (NHCOO) bonds, creating high molecular weight products.

In alkyd production, polyols or polyol mixtures containing primary hydroxyl groups must be used. The activity of primary hydroxyl groups is higher than that of secondary hydroxyl groups.

Urethane-modified alkyds have advantages such as fast drying characteristics, less susceptibility to moisture, high chemical resistance, producing a highly glossy film on wood surfaces, high solubility in conventional solvents except low molecular weight alcohols, compatibility with other films.

On the other hand, high tendency to yellowing, consequently unsuitability for light-colored and white paints, weak color retention and brush application properties, and low flowability are among their disadvantages [Yürekli, 1995].

 

Studies on Urethane-Modified Alkyd Resins

In a study conducted by Güner and colleagues, sunflower oil-based urethane oils were obtained using three different types of isocyanate components.

The isocyanate components used were; (I) toluene diisocyanate (TDI), (II) hexamethylene diisocyanate (HMDI), and (III) poly(1,4 butanediol) toluene 2,4 diisocyanate (PBTDI) terminal prepolymer. Polymers were prepared at four different isocyanate/oil ratios.

Sunflower oil and linseed oil-based alkyd resin samples were also prepared for comparison purposes. Results showed that the film properties and viscosity values of urethane oils depend on the type and amount of isocyanate.

Increasing the isocyanate content in the urethane oil caused high viscosity. The PBTDI-based samples prepared for comparison and having the same oil content showed the highest viscosity value. Polymer viscosities dramatically affected some film properties.

For example, high polymer viscosity caused shorter drying times. When the alkali and water resistance of urethane oils and alkyd resins were compared, better results were obtained depending on the structure of the urethane oils.

On the other hand, alkyd resins and TDI-based polymers showed the best hardness properties [Güner et al., 2002].

Ling and colleagues, in a study they conducted, synthesized new poly(alkyd-urethane)s obtained from vegetable oils and examined their properties.

In the study, the preparation of poly(alkyd-urethane)s was carried out in three stages: (I) preparation of monoglycerides, (II) preparation of alkyd diols, and (III) synthesis of poly(alkyd-urethane) (PAU). The triglycerides of palm, soybean, and sunflower oils were converted to their monoglycerides through a glycolysis process.

Subsequently, monoglycerides derived from different oils were subjected to reaction with phthalic anhydride at a monoglyceride/phthalic anhydride ratio of 2/1 to obtain new polyols named alkyd diols. The polyols were treated with 4,4'methylenediphenyl diisocyanate (MDI) for the production of five new poly(alkyd-urethane) (PAU)s, thus obtaining palm oil-based (POPAU), soybean oil-based (SOPAU), sunflower oil-based (SFPAU), palm-soybean oil-based (POSOPAU), and palm-sunflower oil-based (POSFPAU) poly(alkyd-urethane)s. The synthesis of monoglycerides, alkyd diols, and polyurethane alkyds was confirmed by FTIR, H-NMR, C-NMR spectroscopy, and their morphologies were examined by SEM. Additionally, viscosity, solubility, iodine value, gel content, drying time, thermogravimetric analysis (TGA), adhesion, impact resistance, pencil hardness, chemical resistance, and water resistance tests were performed. Palm oil-based PAU showed good thermal stability with only 5% mass loss at 270°C in a nitrogen atmosphere. Sunflower oil-based PAU showed superior drying characteristics. Overall, all poly(alkyd-urethane)s showed good mechanical performance and chemical resistance [Ling et al., 2014].

In a study conducted by Saravari and Praditvatanakit, urethane alkyds based on castor oil/jatropha oil mixture were prepared and their properties were examined. In the study, castor oil was subjected to interesterification reaction with jatropha oil, and the resulting product was then treated with TDI to obtain urethane alkyd.

The castor oil-jatropha oil-based urethane alkyd prepared showed lower viscosity, hardness, and molecular weight values compared to commercial urethane alkyds and higher drying times, but the film properties were found to be largely similar. Excellent impact resistance was achieved, obtaining highly flexible and high impact-resistant films. They also showed excellent acid and water resistance [Saravari and Praditvatanakit, 2013]. Velayutham and colleagues, in a study they conducted, synthesized and characterized polyurethane (PUR) coatings derived from polyols synthesized with glycerol, phthalic anhydride, and oleic acid. In the study, 99.5% purity oleic acid and glycerol, phthalic anhydride, toluene diisocyanate (80/20; 2,4-TDI/2,6-TDI), and toluene as solvent were used. In the study, work was carried out in two stages: (I) polyester polyol synthesis and (II) polyurethane prepolymer synthesis, thus synthesizing acid polyols Alk28, Alk40, and Alk65 containing 28%, 40%, and 65% oleic acid respectively. The polyols were subjected to reaction with an aromatic isocyanate (TDI) to form PUR coating. The effect of the oleic acid content of the polyols and various NCO/OH ratios on the physical properties of the PUR coating was examined. The properties of PUR coatings such as drying time, adhesion, pencil hardness, solvent resistance, and corrosion/chemical resistance were examined. Coatings obtained from polyol Alk28 showed the best general properties, followed by Alk40. PUR coatings from Alk65 are softer. As the NCO/OH ratio of the coatings increased, there were increases in the mechanical and anti-corrosive properties of all coatings.

Overall, as a result of these studies, it was observed that the design of formulations with desired performance and the use of these materials as an effective surface coating component is promising [Velayutham et al., 2009].

Athawale and Nimbalkar, in a study they conducted, synthesized emulsifiable air-drying urethane alkyd. In the synthesis of water-based polyurethane dispersion, "acrylamido tertiary butane sulfonic acid (ATBS)" was used in place of dimethyl propionic acid (DMPA). In the study, linseed oil, fish oil, triethylamine, ethylene diamine, glycerol, phthalic anhydride, maleic anhydride, N-methyl-2-pyrrolidone, dibutyl tin dilaurate, azobisisobutyronitrile, dimethylol propionic acid, and isophorone diisocyanate (IPDI) were also used. In the study, work was carried out in three stages: (I) oil-modified polyester synthesis, (II) polyurethane dispersion synthesis, (III) synthesis of acrylate-modified aqueous polyurethane dispersions. Urethane alkyds characterized by FTIR analysis, particle size analysis, viscosity, H-NMR, thermogravimetric analysis (TGA), and film properties (drying time, adhesion, flexibility, pencil hardness, scratch hardness, impact resistance, chemical/solvent resistance, etc.) were determined.

ATBS-based polyurethane dispersions showed better thermal, chemical, and coating properties compared to those prepared with DMPA anionomer [Athawale and Nimbalkar, 2010].

In a study conducted by Athawale and Kolekar, hydroxyl-terminated alkyds synthesized using castor oil, glycerol, and different dibasic acids were used to develop both uralkyd and polybutyl methacrylate (PBMA) interpenetrating polymer network (IPN) structures. In the study, both uralkyd homopolymer structures containing dimethyl terephthalate (UA-D) and phthalic anhydride (UA-P) were synthesized, and also IPN structures containing UA-D/PBMA and UA-P/PBMA in weight ratios of 40-60% and 60-40% were synthesized. As uralkyd concentration in the IPN increased, a gradual increase in swelling, density, and elongation was observed along with a decrease in hardness in all IPNs. Swelling is relatively more pronounced in toluene and methyl ethyl ketone (MEK) compared to water. The study was carried out in three stages: (I) synthesis of hydroxyl-terminated castor oil alkyds, (II) synthesis of uralkyd networks (UA), and (III) synthesis of interpenetrating polymer networks. Glass transition temperature measurements proved the formation of interpenetrating structures. The structure and phase morphology of the IPN were characterized using SEM and DSC. Phase separation was observed in both IPN structures. The UA-P/PBMA IPN structure was found to be more compatible in terms of improving mechanical properties. The interpenetration of PBMA with uralkyd increased the solvent resistance of the IPN system. The thermal properties of the IPN structures are independent of the acidic portion in the urethane alkyd backbone.

IPN systems demonstrated good solvent resistance, hardness, elongation (flexibility), tensile strength, and excellent general film properties that would make them potentially usable in the coating and rubber industries [Athawale and Kolekar, 2000].

Bakare and colleagues, in a study they conducted, synthesized and characterized polyurethanes based on rubber seed oil (rubber seed oil-RSO). Thus, new bio-based polyurethanes were synthesized from RSO oil, which is a renewable resource. For the synthesis of the urethane-based prepolymer, two different isocyanates were used along with RSO monoglyceride; hexamethylene diisocyanate (HMDI) and toluene diisocyanate (TDI). The properties of the resulting polyurethane were found to depend on the type of diisocyanate used and the diisocyanate/RSO monoglyceride molar ratio. The degree of cross-linking of the polyurethanes after curing was evaluated along with their swelling behavior. HMDI-based networks at an NCO/OH molar ratio of 1.5 showed better cross-linking than TDI-based ones. The thermal properties of the samples were determined by thermogravimetric analysis, and two and three decomposition stages were observed in aliphatic and aromatic-based RSO-based polyurethanes, respectively. The highest thermal stability was obtained in the aliphatic isocyanate-based RSO polyurethane [Bakare et al., 2008].

Conclusion

Today, in the paint industry, modifications can be made to improve the film properties of alkyd resins used as binders. There are currently numerous research articles on epoxy, vinyl, acrylate, urethane, styrene, phenolic, and silicone-modified alkyds. In this review study, general information on urethane-modified alkyd resins and articles in the literature are briefly summarized. Summary information on the articles is presented in Table 1. Chemical Engineering M.S. Ferda Civan (Doctoral Student) / Department of Chemical Engineering - Istanbul University Research Assistant Demet Özaltun (Doctoral Student) / Department of Chemical Engineering - Istanbul University Assoc. Prof. Dr. Işıl Acar / Department of Chemical Engineering - Istanbul University Prof. Dr. Gamze Güçlü / Department of Chemical Engineering - Istanbul University

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