KevinMayer

Kevin M

=A review of Alkyd, Alkyd Emulsion, and Alkyd-Acrylic hybrid Coatings=

Abstract:
With increasing environmental restrictions of volatile solvent use as well as a shift towards health conscious alternatives, emulsions of alkyd polyester resins as well as acrylic emulsions have gained significant traction over the more traditional oil and solvent based coating systems as the primary binder. These emulsion systems are not without limitations, however, and their shortcomings still make solvent based binders a preferential choice, especially in industrial application where film strength and permeability can have significant effects on coating lifespan. Research in monomer choice and emulsifier is being done to improve the properties of the films formed from the emulsion systems [1,2,3].

Introduction:
Alkyd polyester resins have been gaining popularity as a push towards more renewable methods has been sweeping though industry. Alkyd polyester resins can be based from plant derived fatty acids such as oleic, linoleic, and ricinoleic acid to name a few. Because of the renewable feedstock options for alkyds they are gaining popularity. In alkyd polyester resins the typical polyol monomer block is, but is not limited to, glycerol. Glycerol is already present in the typical oils used and allows for its use via trans-esterification with excess glycerol. Pentaerythritol is also used to offer a slightly higher degree of linkage. At its core, alkyds are simply polyesters. The reaction is between the polyol, a dibasic acid, and the long chain fatty acid like those mentioned above. Reaction conditions and reagent ratios can be adjusted to achieve the desired properties [4]. Traditionally, alkyd polyester coatings are oil based and require the use of volatile organic solvents to control the viscosity of the alkyd resin. These types of systems are typically used in industrial application where odor during application is not a high concern. Changing legislation as well as green efforts are driving a market for water based paints. Emulsions of the alkyd polyester resin allow for the replacement of volatile organic solvents with water. Alkyd polyester emulsion paints offer easier application for at home do-it-yourself type uses. The full uptake of alkyd polyester emulsion technology has been hindered by the impact in performance the emulsion has over higher performing oil based alkyd resins. Shelf life and water sensitivity are two aspects of impacted performance in alkyd emulsions [1]. Attempts have been made to close the performance gap between oil based and emulsion based alkyds. New emulsifiers have been investigated to improve emulsion stability as well as increase water sensitivity [1]. Improving the cross linking of the alkyd polyester during the curing process has been studied using acrylate modified fatty acids [5]. Hybrid alkyd polyester and acrylic solutions have also been explored [6]. Studies in the catalyst drier and partitioning have also been studied [1,7].

Synthesis, Curing, and Modification:
In order to better understand the solutions being studied to improve alkyd emulsion performance it is important to understand how traditional alkyd polyester resins are synthesized and how they cure. As mentioned, alkyd resins are polyesters. The typical blocks of the polymer are linoleic acid (I), glycerol (II), and phthalic anhydride (IV). It is possible to use other oil derived fatty acids, polyols, and dibasic acids. The fatty acid and polyol are reacted in a ratio of 1:3 in order to favor the formation of monoester polyol molecules (III).

Stock triglyceride oil can be also used as a starting point with the addition of excess glycerol in order to transesterificate in order to have a preferred monoglyceride. The reaction can be done under vacuum or utilizing other methods to remove water during the process. The addition of the phthalic anhydride or other dibasic acid is the last addition of monomer. Once the reaction is complete a dibasic acid is added. The reaction is continued until the desired parameters of the polyester are met. The polymer chain is built in blocks of repeating A and B units until either have been exhausted in supply. Typically the parameter of interest is the acid value, a value derived from the amount of potassium hydroxide needed to neutralize the polyester. The acid values of a completed resin that are out of specification can have adverse affects on the emulsion stability when using ionic surfactants and may require neutralization. The acid value is used as a measurement of reaction completion. Lower acid values correlate to product formation as the fatty acid is consumed and the ester is formed. To drive the reaction to a low acid value, the reaction is done either under vacuum or solvent blanket and azeotropic/solvent extraction distillation. Typical acid values for alkyd polyester resins are around 7. When using non ionic surfactants, such as ethoxylated fatty alcohols, the acid value has negligible effect on choosing the HLB of the surfactant [1,4,8].

A completed alkyd acquires its film strength through oxidative curing. The fatty acids used typically contain oils that have dienes separated by an allylic CH2. This creates a site where hydrogen abstraction can be favored and result in the formation of a radical (VI). This radical is able to react with oxygen in the air and produce hydroperoxide and conjugated dienes. The layout of the chain also allows for Diels-Alder and radical polymerization as other types of cross linking between chains. Alkyd polyester resins rely on metal catalysts to break down the hydroperoxide into alkoxy and peroxy radicals. The recombination of the various radicals can produce peroxy, ether, or carbon-carbon links between the chains. The degree of cross linking determines the film strength properties. The oxidation reaction chain propagates the same way one would expect a chain reaction free radical polymerization to propagate. They chain terminate in the same manner too.

The formation of the radical begins with the abstraction of a hydrogen in the diene system from the presence of pre-existing radicals. This newly formed radical is able to begin a chain reaction with other diene sites in the resin. This can result in carbon to carbon cross linking of the chains. Also, depending on the arrangement of the dienes, Diels-Alder adducts can be present. This also can link the chains of the alkyd polyester resin together. Typically, the carbon radical reacts with oxygen in the air to form hydroperoxides. As stated above, metal catalysis decompose the hydroperoxides into peroxide radicals and alkoxy radicals. These then chain terminate with other alkyd resin chains to form ether or peroxide links [7,9,10,11].

Studies have been done using metal catalyst drying agents in order to better enhance the rate of curing as well as improve film strength. The solubility of the catalyst in the emulsion of alkyds has been studied to determine if water soluble or alkyd phase soluble catalysts would perform better or answer why alkyd emulsions cure slower than alkyd polyester resins in organic solvent. Studies have also been done using reactive emulsifiers. The emulsifiers used contain functionality that would be able to react with the alkyd polyester during the autoxidation curing process. This prevents surfactant migration to the surface. In the case of catalyst partitioning, ideally the catalyst for the alkyd polyester autoxiation would be best employed in the alkyd phase versus being soluble in the aqueous phase. Depending on the dryer used the solubility of the system will change with pH. Typical hydrophobic cobalt driers are found to be concentrated more in the alkyd phase under pH conditions around and above 7. Calcium metal dryers were also seen following this solubility trend, however, zirconium driers seemed to be pH independent. Choosing a pH as well as ideal emulsifier for the system will affect the solubility of the catalyst. Choosing a system to concentrate the catalyst in the alkyd phase will improve dry time [1,2]. Removing cobalt and other metal driers from alkyd polyester formulations has also been a concern since VOC regulations limit the amount of volatiles that can be put into a formulation. Oxidative curing of alkyd polyester resins can also be significantly slow at low temperatures. Low molecular weight alkyds, such as short oil alkyds, require high temperature baking in order to cure. Cobalt replacement driers aid in the curing of lower molecular weight alkyds without the need for higher temperature curing. Using driers with thiol-ene functionality allows for curing without the need of metal catalysts to decompose the peroxide formed during typical alkyd polyester oxidative curing. The process is similar to traditional metal catalyzed curing, except the thiol radical formed in the formation of the peroxide is reactive enough to break down the peroxide into a reactive alkoxy radical. The thiol-ene can either be added as a component soluble in the emulsified portion or as part of the alkyd fatty acid linker backbone as a substituent on pentaerythritol to further enhance cross linking [3].

Using acrylate modified alkyd polyester resins has also been studied for enhancing the curing time of the film. Acrylic esters can form Diels-Alder products with the oleate and linoleate chains in the glyceride. The addition of these Diels-Alder adducts increase cross linking which in turn translates to faster curing times, stronger films, and higher glass transition temperatures. Changing the type of acrylate can also impart film surface properties. Fluorinated acrylates can improve water resistance of the film [5].

Other types of alkyd-acrylate hybrids are acrylic emulsions that are polymerized in the presence of an alkyd polyester resin. These types of systems can also be created through the combination of the two emulsions after the polymerization phase. Acrylic emulsion coatings have the benefit of fast drying times but lack the film strength of an alkyd polyester resin. The combination of the two is intended to achieve a mix of the benefits from each of the films [6,12].

Acrylic latex emulsion coatings which are used in types of hybrid coatings are the formation of acrylic acid and acrylate polymerizations. Typical acrylic coatings are comprised of methacrylic acid and methyl methacrylate or butyl acrylate. Other forms of acrylic acids and acrylates can be used. The choice in ratios and monomer can be adjusted to achieve a target glass transition temperature. A higher glass transition temperature correlates to a more brittle structure at the operating temperature of the film. These polymers can be made in steps to achieve different blocks of polymer chain. Step and rate of addition can be used to impart surface active properties in cases where a series of A components are combined with a series of B components, where A is an acrylate and B is an acrylic acid. Ionic and hydrophobic portions of the block copolymer would change how the latex interacts with the emulsion. [13,14].

The synthesis of the acrylic emulsion begins typically with a pre-emulsion made from the emulsifier chosen and the acrylates and acrylic acids. The addition of an initiator such as sodium persulfate begins the reaction. The mop up of the reaction to complete the reaction is done using oxidation and reduction initiators such as tert-butyl hydroperoxide and sodium hydroxymethane sulfinate. The size of the micelle and the distribution of the sizes of micelles determine the stability of the emulsions as well as the viscosity and film forming properties of the emulsion [6,12,13,14].

Emulsions:
To better understand where some of the challenges arise from when using an emulsion system instead of a solvent based system, some information will be discussed about emulsions and surfactant types and choices. Emulsifiers can exist in different forms, non ionic, cationic, anionic, slightly anionic, and slightly cationic. Typical non ionic surfactants are created by the combination of ethoxylated long chain alcohols. A non ionic surfactant can be described using the HLB system (hydrophobe/lipophobe balance). The ratio between the hydrophobic portion and the lipophobic portions can be used to describe each non ionic surfactant with an HLB number. When creating emulsions it is ideal to test a range of HLB surfactants in order to find the most ideal emulsion. It is possible that two stable emulsions can be found across the range of HLB numbers, one on the lower end and one on the higher. One of the emulsions will be an inversion of other, or, one emulsion will be oil in water and the other will be water in oil. Typically water in oil emulsions have a higher viscosity than oil in water emulsions in room temperature (think mayonnaise vs. milk). The HLB system does not work for ionic surfactants and an HLB number can be assumed only by matching the performance of an ionic emulsifier to its non ionic counterpart. Non ionic emulsions are more sensitive to temperature extremes compared to ionic surfactants since they lack charge repulsion stabilization [15,16,17].

Slightly anionic and cationic surfactants are the combination of non ionic and cationic/anionic surfactants to achieve the charge stabilization of an ionic surfactant with the steric stabilization of a non ionic surfactant. Non ionic surfactants (VII) typically consist of an ethoxylated fatty alcohol. The fatty chain length or the moles of ethylene oxide added determine the HLB number [1,18]. Cationic surfactants (VIII) are typically amine based surfactants where in acidic conditions the hydrophobic tail of the amine has affinity for the hydrophobic portion of the additive and the cationic portion remains faced to the aqueous portion [19]. Anionic surfactants (IX) are comprised of a long chain alkyl with an acidic functional head, typically a carboxylic acid or sulphate. These types of surfactants operate in pH ranges above 7 where the salt of the acid can be formed [1].

Alkyd emulsions are emulsified in non ionic surfactants, anionic surfactants, or a combination of the both. The length of the oil chain is important when determining the HLB of the surfactant to use or even which anionic surfactant to use. The stability and performance of the emulsion can be determined through the droplet size and size distribution of the micelles formed. Ideally, small droplet sizes with a very narrow distribution of sizes are desired for an emulsion that will not flocculate or form agglomerations. It is observed that anionic surfactants provide smaller droplet sizes compared to non ionic surfactants at equal concentrations [1].

Modified emulsifiers can be chosen to have reactive properties with the alkyd polyester resin in the autoxidative curing process. Emulsifiers with diene systems or Diels-Alder adducts can aid in the curing time of the resin and impart stronger film properties [1,2]. With typical surfactants, as the film dries and water evaporates from the surface of the film, the surfactants migrate to the surface as well. The concentration of surfactant at the surface of the film is higher than anywhere else in the film. This can lead to poor water sensitivity. Preventing the migration of surfactant can be done using polymerizable surfactants. These surfactants react with the alkyd polyester during the curing process and allow for better cross linking [2].

When forming an alkyd emulsion, the type of fatty acid used determines the shear rate and amount of heat needed to form the emulsion. Short oil alkyds require higher temperatures to form stable emulsions with small droplet sizes compared to long oil alkyds. Emulsification procedure usually involves the addition of the emulsifier to the liquid resin followed by the addition of water under heat and shear forces. Homogenizers are also used preparing emulsions with small droplet sizes [1].

Corrosion:
It is important to keep in mind that the emulsion model of the alkyd polyester can be conceptualized as spheres. When the film is dry there are spaces between the agglomerated spheres through which moisture can permeate. This creates new challenges when using alkyd emulsion coatings to protect ferrous metal substrates. Typical approaches are very mechanical in nature and aim at filling the spaces between the spheres with organic or metallic additives in order to reduce permeability. Solvent borne alkyd polyester paints perform well at corrosion inhibition since they form strong continuous films, properties enhanced by further degrees of cross linking and branching. Additional inorganic inhibitors are added to prevent the formation of rust products or to act as a barrier to water in the film. Utilizing acid polyols as backbones can increase the degree of branching. In the case of triethylolpropane and diethylolpropanoic acid, high levels of branching can be formed due to each site adding at least two more sites where an ester can be formed. In an AB copolymer of triethylolpropane and diethylolpropanoic acid, each dimer has four sites at which an ester can be formed with more diethylolpropanoic acid molecules or long chain fatty acids. Different choices can be made depending on the degree of branching desired. Addition of acrylic modified resins can also add branching through the addition of alcohol functional acrylates and acrylic acid. The functionality added via backbone polyester choice or acrylate modification can determine the degree of film impermeability [12,13,20,21,22,23].

Summary:
Now that alkyd resins are increasing in demand, thanks to the vast choice of renewable feedstock sources, and with the changing landscape of environmental laws, significant work has been done to reduce the volatile organic compound content in alkyd polyester systems. This has been done through the use of emulsifying the alkyd polyester resin in water to replace the need for volatile solvents such as toluene or mineral spirits. Moving to an emulsion system has created new unique challenges that did not exist in solvent based alkyd polyester formulations. Of the impacted properties, the ones of highest concern were film strength, film permeability, and curing time. The addition of emulsion stability as a factor also introduces area for improvement. The partitioning of water soluble and oil soluble components change the drying characteristics when using different types of metal drier solutions. Concentrating the drier in the oil phase improves the cure time as compared to concentrating it in the aqueous phase. Emulsifier choice also changes the characteristics of film strength, film permeability, and cure time. Reactive, polymerizable surfactants improve film strength and reduce permeability due to better agglomeration of the micelles during evaporation of solvent and oxidative cure [1,2]. Addition of acrylic modifiers can increase the rate of curing though the formation of new cross linking or Diels-Alder adducts. Replacement catalysts to cobalt driers also aid in the curing process and strength of the film [3,6]. Choosing the monomer choices in the alkyd polyester resin during synthesis can also enhance the film properties. Choosing monomers with more sites to branch and cross link will form stronger films with higher gloss and reduced permeability [23]. Emulsion stability is achieved by using a combination of non ionic and anionic surfactants in order to control the droplet size and distribution. This is also to achieve a balance between steric stabilization and charge stabilization of the emulsion to prevent agglomeration of the micelles during the storage of the emulsion [1,2].

References:
1. G. Ostberg, M. Hulden, B. Bergenstahl, K. Holmberg. “Alkyd Emulsions.” Progress in Organic Coatings 24 (1994): 281-297. DOI 2. K. Holmberg. “Polymerizable Surfactants.” Progress in Organic Coatings 20 (1992): 325-337. DOI 3. R.P. Klassen, R.P.C. van der Leeuw. “Fast drying cobalt-free high solids alkyd paints.” Progress in Organic Coatings 55 (2006): 149-153. DOI 4. D. Iseri-Caglar, E. Basturk, B. Oktay, M. Vezir Kahraman. “Preparation and evaluation of linseed oil based alkyd paints.” Progress in Organic Coatings 77 (2014): 81-86 DOI 5. N. Thanamongkollit, M. Soucek. “Synthesis and properties of acrylate functionalized alkyds via Diels-Alder reaction.” Progress in Organic Coatings 73 (2012): 382-391 DOI 6. T. Nabuurs, R. Baijards, A. German. “Alkyd-acrylic hybrid systems for use as binders in waterborne paints.” Progress in Organic Coatings 27 (1996): 163-172 DOI 7. Z. Oyman, W. Ming, R. van der Linde. “Oxidation of model compound emulsions for alkyd paints under the influence of cobalt drier.” Progress in Organic Coatings 48 (2003): 80-91 DOI 8. C. Uzoh, O. Onukwuli, R. Odera, S. Ofochebe. “Optimization of polyesterification process for production of palm oil modified alkyd resin using response surface methodology.” Journal of Environmental Chemical Engineering 1 (2013): 777-785 DOI 9. P. Harris, R. Hughton, P. Taylor. “Role of ligand in cobalt(II)-catalysed decomposition of tert-butyl hydroperoxide. Evidence for the participation of bridged dicobalt complexes.” Polyhedron 16 (1997): 2651-2658 DOI 10. J. Mallegol, J. Lemaire, J. Gardette. “Drier influence on curing of linseed oil.” Progress in Organic Coatings 39 (2000): 107-113 DOI 11. S. Tanase, E. Bouwman, J. Reedijk. “Role of additives in cobalt-mediated oxidative crosslinking of alkyd resins.” Applied Catalysis A: General 259 (2004): 101-107 DOI 12. n. Heiskanen, S. Jamsa, L. Paajanen, S. Koskimies. “Synthesis and performance of alkyd-acrylic hybrid binders.” Progress in Organic Coatings 67 (2010): 329-338 DOI 13. E. Morozova. “Synthesis of Polymerization Film Formation Materials by Method of Emulsion or Direct Radical Polymerization of Acrylic Monomer.” Protection of Metals and Physical Chemistry of Surfaces 46 (2010): 239-254 DOI 14. K. Kang, C. Kan, Y. Du, D. Liu. “Synthesis and properties of soap-free poly(methyl methacrylate-ethyl acrylate-methacrylic acid) latex particles prepared by seeded emulsion polymerization. “ European Polymer Journal 41 (2005): 439-445 DOI 15. H. Kunieda, K. Shinoda. “Evaluation of the Hydrophile-Lipophile Balance (HLB) of Nonionic Surfactants (I).” Journal of Colloid and Interface Science 107 (1985): 107-121 DOI 16. H. Kunieda, N. Ishikawa. “Evaluation of the Hydrophile-Lipophile Balance (HLB) of Nonionic Surfactants (II).” Journal of Colloid and Interface Science 107 (1985): 122-128 DOI 17. H. Davis. “Factors determining emulsion type: hydrophile-lipophile balance and beyond.” Colloids and Surfaces A: Physicochemical and Engineering Aspects 91 (1994): 9-24 DOI 18. K. Suzuki, Y. Wakatuki, S. Shirasaki, K. Fujita, S. Kato, M. Nomura. “Effect of mixing ratio of anionic and nonionic emulsifiers on the kinetic behavior of methyl methacrylate emulsion polymerization.” Polymer 46 (2005): 5890-5895 DOI 19. X. Yan, W. Xu, R. Sao, L. Tang, Y. Ji. “Synthesis of polymerizable quaternary ammonium emulsifier and properties of its fiber crosslining emulsion.” Colloids and Surfaces A: Physicochemical and Engineering Aspects 443 (2014): 60-65 DOI 20. J. Alam, U. Riaz. S. Ashraf, S. Ahmad. “Corrosion-protective performance of nano polyaniline/ferrite dispersed alkyd coatings.” J. Coat. Technol. 5 (2008): 123-128 DOI 21. J. Alam, U. Riaz, S. Ahmad. “High performance corrosion resistant polyanline/alkyd ecofriendly coatings.” Current Applied Physics 9 (2009): 80-86 DOI 22. W. Araujo. I.C.P. Margarit, O. Mattos, F. Fragata, P. de Lima-Neto. “Corrosion aspects of alkyd paints modified with linseed and soy oils.” Electrochimica Acta 55 (2010): 6204-6211 DOI 23. K. Manczyk, P. Szewczyk. “Highly branched high solids alkyd resins.” Progress in Organic Coatings 44 (2002): 99-109 DOI