Allylic alcohols improve acrylic polyols Efforts to reduce the VOC content of two-component urethane coatings have focused primarily on lowering the molecular weight of the polyols. Unfortunately, the OH functionality of conventional polyols decreases linearly with molecular weight. With that, the crosslinking ability of the polyol and the performance of the coating also decrease. This inherent limitation has been overcome by developing a new class of acrylic polyols based on allylic alcohols. Daniel B. Pourreau, Wei Wang, Stephen H. Harris, Ronald S. Blackwell. Two-component (2K) urethane technology has undergone a gradual evolution over the past twenty years. Incremental changes in acrylic polyols and isocyanate crosslinkers have been, for the most part, driven by lower VOC content limits and increased performance requirements. Until recently, however, the basic chemistry of acrylic polyols had not changed. Efforts to reduce the VOC content of 2K urethane coatings focused primarily on lowering the molecular weight (Mn) of the polyols. Unfortunately, the OH functionality of conventional polyols decreases linearly with molecular weight. Hence, as Mn decreases, so does the crosslinking ability of the polyol and the performance of the coating. Consequently, high solids polyols did not perform as well as low-solids polyols. This inherent limitation has been overcome by developing a new class of acrylic polyols based on allylic alcohols. Allylic alcohols are efficient chain-transfer agents and do not readily homopolymerize. This has led to the commercialization of acrylic polyols with improved OH functionality and lower solvent demand compared to polyols made from hydroxy-functional acrylate or methacrylate monomers [1]. In addition, it has been discovered that blending of hard and soft resins instead of using a single resin offers unexpected benefits, including lower VOCs, improved appearance, longer pot life, shorter cure times, and improved weatherability [2]. Blending solid and liquid polyols also allows the coating formulator to minimize resin inventories while maximizing coating formulations and technologies. The following paragraphs describe the benefits of this new acrylic polyol technology and formulation approach. Improved high-solids acrylic polyols from allylic alcohols In conventional acrylic polyols, the acrylate ester and hydroxy-functional acrylate (HFAs) co-monomers have approximately the same reactivity. This means that the distribution of OH functionality in the resulting polymer is essentially random. Also, the amount of non-functional polymer increases linearly as the molecular weight of the polymer is lowered. This affects the coating's performance, especially its hardness, chemical and abrasion resistance. The solution to this problem is to replace the HFA with an allylic alcohol (AA). Allylic alcohols are much less reactive than acrylates and behave as chain-transfer agents [3]. This change in chemistry yields acrylic polyols with even distributions of OH groups in the polymer, high terminal OH content, and much less non-functional polymer than in HFA-based polyols. Analysis [4] of an AA-based polyol has shown it to contain only 13% polymer with less than two OH groups and no non-functional polymer, whereas a HFA-based resin contained approximately 28% polymer with less than two OH groups and 11% polymer with less than one OH group (Figure 1). Mono- and non-functional polymers act as plasticizers and extractable components in coatings, reducing their hardness, chemical and abrasion resistance. This is graphically illustrated in Figure 2. The resin management concept - few resins, many uses Unlike conventional solution polyols that have T g s in the 0 to 45 C range, polyols have been developed with T g values below - 40 C ("P60", "P90", and "P120") and above 50 C ("A90" and "A140") (Figure 3). A solution resin, "M100", has also been developed with an intermediate T g and OH number. This serves as a general-purpose solution resin. The low T g polyols are liquid and flow at ambient temperatures despite being solvent-free. This reduces shipping costs and handling hazards and offers the coating formulator a greater choice in solvents. This also allows the formulator to develop 100% solids coatings, inks or adhesives. In addition, the liquid resins are excellent pigment grinding vehicles and can be used as raw materials for moisture- and UV-curable urethanes resins and oligomers. Coatings formulated with blends of hard and soft polyols flow and level better, have longer pot life and open time, lower VOCs, and greater flexibility than coatings formulated with a single acrylic polyol. Hence, this handful of resins can be used to formulate a variety of low-voc and solvent-free coatings. The following paragraphs illustrate the versatility of this resin blending concept and the superior performance, appearance, and compliance of coatings formulated with allylic-monomer based acrylic polyols [5]. The physical properties [6] of AA-based acrylic polyols are listed in Table 1. Molecular weights and polydispersities are low, resulting in low solution viscosities. Solution viscosities increase with OH number due to inter-chain hydrogen bonding and with resin T g. Solution and melt viscosities of AA-based polyols are significantly lower than those of HFA-based polyols of similar T g and functionality. For example, a 70% solids MAK solution of A140 was half as viscous as a commercial resin with a T g of 30 C, and 10 times less viscous than a resin of similar T g and functionality. This means that AA-based acrylic polyols deliver significantly more hardness at a fraction of the VOC penalty of conventional acrylic polyols. High functionality of polyol for better chemical-resistance Industrial maintenance coatings [7] can be divided into light-, medium- and heavy-duty categories based on their performance requirements. Light-duty uses are mostly decorative and do not require the high chemical, corrosion, or abrasion resistance of topcoats for floor, chemical containment or marine uses. For heavy-duty, non-immersion use, it is advantageous to use a more highly functional acrylic polyol blend and an excess of isocyanate crosslinker ("over-indexing") to improve the chemical resistance of the coating. Over-indexing also improves coating hardness and abrasion resistance. Examples of all three types of coatings are given (Table 2). The pot life is longer than with conventional coatings and
cure speed is considerably faster, allowing for faster recoats. Impact resistance is also improved while other properties such as abrasion and salt-spray resistance are comparable to commercial topcoats. The first two formulations contain blends of hard and soft AA-based polyols whereas the third contains only "M100" [8]. Increasing the polyol blend hardness has several beneficial effects such as increased solvent resistance as well as gloss retention under accelerated weathering conditions at the expense of slightly higher formulation viscosity and dry times. The VOC content was reduced below 250 g/l by decreasing the amount of hard polyol in the blend and increasing the solids content of the formulations. This increased the formulation viscosities above 850 mpa.sec, which requires brush, roller, or airless spray application. European Commission set VOC contents limits for several coating systems Today's clearcoats for automotive refinishing must meet stringent VOC, appearance, durability, and productivity requirements. The European Commission has issued a Draft Directive that proposes to set VOC content limits for several coating categories, including decorative paints, varnishes and vehicle refinishing products. The proposed limit for refinish topcoats is 420 g VOC/l. With conventional acrylic polyols, this limit is difficult to achieve. Achieving excellent appearance, pot life and productivity in a compliant system is particularly difficult with conventional acrylic urethane systems. This is because productivity (fast dry times) is often achieved by using hard acrylic polyols and fast solvents to achieve a "lacquer dry" effect before the crosslinking reaction has occurred. This can result in a number of surface defects including "orange peel" and the "pinholing" (Figure 4). Weatherability testing The weatherability of the AA-based clearcoat was tested against three commercial benchmarks from leading suppliers of refinish coatings. The commercial clearcoats were prepared and applied according to the manufacturers' recommendations over a white commercial basecoat. The AA-based clearcoat was inhibited with 1.5% each of a hindered amine light stabilizer (HALS) and a benzotriazole UV screener [9]. Commercial clearcoat A is a low VOC "super-productive" spot and panel clearcoat and commercial clearcoats B and C are 500g VOC/l super-productive clearcoats from leading suppliers. The clearcoat based on AA-polyols outlasted the commercial low-voc product and one of the 500g VOC/l ultra-productive clears (Figure 5). Its 60 gloss retention and yellowing resistance was comparable to the best high VOC commercial clearcoat. Clear and pigmented formulations for wood have also been developed, including gloss and matt urethane and acid-catalyzed conversion varnishes, as well as dual cure formulations with both UV- and chemical-cure mechanisms. These formulations are suitable for parquet flooring, cabinetry, furniture and millwork [10]. Hard and soft polyols dial-in desired properties Improved acrylic polyols have been developed by using allylic alcohols instead of HFAs as the OH-functional monomers. Coatings formulated with these new polyols have lower VOCs, longer pot-lives and open-time, but faster through cure than those based on conventional acrylic polyols. In addition, the fully cured coatings have superior solvent and abrasion resistance, good hardness and flexibility, and excellent weatherability compared to commercial controls. Blending hard and soft polyols allows the formulator to "dial-in" the desired properties and provides coatings suitable for a wide range of applications and technologies. Using a total of six acrylic polyols and this blending concept, formulations have been developed for industrial maintenance, automotive refinish, OEMs, wood, metal and plastics. UV- and moisture-curable compositions based on these acrylic polyols have also been developed. This new acrylic technology and blending approach should help formulators meet the ever-increasing performance, compliance, and productivity requirements of their customers. For more information, please e-mail acryflow@lyondell.com; visit www.lyondell.com; or phone 610-359-6837. References [1] U.S. Pat. Nos. 6,294,607; 5,646,213; 5,571,884; 5,534,598; 5,525,693; 5,480,943; 5,475,073. [2] U.S. Pat. No. 6,294,607. [3] B. Ranby, Applied Polymer Symposium, 26, (1979), p. 327. V.P. Zubov, et. al., J. Macromol. Sci.-Chem., A13, (1979) 1, p. 111. [4] The polymers were separated into molecular weight fractions using supercritical CO2 extraction. Each fraction was then analyzed for OH content and molecular weight. [5] www.acryflow.com [6] http://www.lyondell.com/html/products/ techlit/2380-v10-0404.pdf [7] http://www.lyondell.com/html/products/ techlit/2497-v5-0404.pdf [8] http://www.lyondell.com/html/products/ techlit/2245-v12-0404.pdf [9] http://www.lyondell.com/html/products/ techlit/2573-v4-0404.pdf [10] Please check www.acryflow.com for a new brochure on wood coatings. Results at a glance - Allylic alcohols are much less reactive than acrylates so they yield acrylic polyols with even distributions of OH groups in the polymer, high terminal OH content, and much less non-functional polymer than in HFA-based polyols. - The low Tg polyols are liquid and flow at ambient temperatures despite being solvent-free. - Coatings formulated with blends of hard and soft polyols flow and level better, have longer pot life and open time, lower VOCs, and greater flexibility than coatings formulated with a single acrylic polyol. - Allylic alcohol-based acrylic polyols deliver significantly more hardness at a fraction of the VOC penalty of conventional acrylic polyols. - Clearcoats based on these new polyols outlast many commercial products and give comparable 60 gloss retention and yellowing resistance. The authors: -> Dr. Daniel Pourreau has been with Lyondell Chemical since 1991. He has held positions as senior scientist, technical service manager, market development manager, R&D manager, and business development manager. He holds a Ph.D. in chemistry from Penn State University and has co-authored 20 US patents and numerous articles. -> Dr. Stephen Harris received his BS in chemistry from Muhlenberg College in 1975 and his Ph.D. in Physical Organic Chemistry from the University of Rochester in 1979. He has worked in process and product research and technical service at Lyondell Chemical since 1980.
-> Dr. Wei Wang received a B.S. Degree in 1985 and a Ph.D. Degree from Peking University in 1991. Dr. Wei Wang is currently a senior scientist for Lyondell Chemical Company in the Performance Chemicals department. -> Ron Blackwell has 23 years experience formulating urethane adhesives, elastomers, sealants, and coatings. He currently works in the Analytical Department of Lyondell Chemical Company.
Figure 1: AA-based acrylic polyols better maintain their functionality as molecular weight decreases compared to HFA-based polyols.
Figure 2: Photograph of 2K urethane topcoats based on M100 and a conventional acrylic polyol after 200 methylene chloride rubs.
Figure 3: The "Resin Management Concept": solid and liquid acrylic polyols can be blended for numerous solution coating formulations or used alone for 100% solids systems.
Figure 4: Surface of an AA-polyol based urethane clearcoat vs. a commercial refinish clearcoat.
Figure 5: Gloss retention of a new urethane clearcoat vs. three conventional clearcoats under automotive UVB accelerated weathering.
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