Final Report: Water-Based Coatings via Miniemulsion PolymerizationEPA Grant Number: R825326
Title: Water-Based Coatings via Miniemulsion Polymerization
Investigators: Schork, F. Joseph
Institution: Duke University
EPA Project Officer: Karn, Barbara
Project Period: October 1, 1996 through September 30, 1999 (Extended to December 31, 1999)
Project Amount: $274,984
RFA: Technology for a Sustainable Environment (1996) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Objective:The objective of this proposal was to develop a new class of water-based, low volatile organic compound (VOC) coatings by grafting various coatings resins into an acrylic polymer matrix using the miniemulsion polymerization technique. Emulsion polymerization currently is used to produce latex coatings. These are based on acrylic, styrenic, and vinyl monomers. They provide reasonably low VOC coatings, but do not have the hardness associated with oil-based paints. The hardness of an oil-based paint comes from the fact that the alkyd resins used in paints cure in air to develop a very hard, crosslinked coating. Conventional latex paints do not crosslink, and so do not have the hardness associated with alkyd-based coatings. Our technology involves dissolving the alkyd or other coating resin in an acrylic monomer mix, and then miniemulsion polymerizing. The resin is incorporated into the acrylic matrix, forming a graft copolymer. The product is a water-based coating, because the miniemulsion process by its nature involves an emulsion of resin, rather than a solution, in organic solvent. However, the presence of alkyd or other unsaturated resins allow curing (crosslinking) in air to form a coating of hardness comparable to that of oil-based coatings. It is projected that such coating products may actually be lower in VOC than conventional latex paints, because latex paints require a coalescing aid to enhance film formation. The coalescing aid is usually an organic solvent and contributes to VOC. It may be possible to replace the coalescing aid with a low molecular weight material (alkyd or other) that will crosslink in air and, thus, not contribute to VOC.
In keeping with the experimental plan in the proposal, we have investigated a number of potential resins for these hybrid coatings. We have focused on three: alkyds, polyesters, and oil-modified urethanes. The alkyd/acrylic graft copolymer delivered in a latex form would provide a water-based, air curable coating system applicable to architectural coatings. The polyester/acrylic graft copolymers would be useful in specialized industrial coatings where solvent-based polyesters currently are used. The oil-modified urethane/acrylic graft copolymer is targeted as a water-based replacement for solvent-based urethane varnishes and coatings. The current availability of these resins (alkyd, polyester, or urethane) in a water-based form is very limited. All three classes of hybrid coatings have been successfully made and show promising properties. We currently are negotiating the licensing of these products for commercialization.
Summary/Accomplishments (Outputs/Outcomes):Monomer Conversion. A great deal of effort was consumed in tackling a problem that was not apparent from our preliminary work. We have found that is it very difficult to get the conversion of monomer past 90 percent in the presence of the grafting resins. We have investigated many
different initiator systems, various temperature programs, and other standard procedures for increasing the monomer conversion in emulsion and miniemulsion polymerization. None has been fully successful, but we believe we now understand the root of this problem. At 90 percent monomer conversion, with a standard recipe, each polymerizing free radical is surrounded by 10 grafting sites for every residual monomer molecule. The result is that late in the polymerization, a great deal of grafting takes place. Because the free radicals on the graft sites are known to be less reactive than acrylic free radicals, the polymerization reaction rate slows near to zero. Independent research in the Netherlands has confirmed this conversion-limiting phenomenon. Various combinations of high initiator types and levels and high temperature reduce the problem. However, it may be necessary to steam-strip residual monomer from the latex in a commercial product. In this way, it will be possible to produce the desired zero-VOC coating.
Alkyds. We have selected alkyds (from McWhorter Corporation) to graft into acrylic miniemulsions. The acrylic monomer is a mix of 49 percent methyl methacrylate, 50 percent butyl acrylate, and 1 percent acrylic acid. This blend was chosen because this is a standard recipe for conventional acrylic latex paint resin. The kinetics of the reaction are quite reproducible, and the resulting latex is of the correct particle size and molecular weight. Monomer conversion ranges between 85 and 97 percent. The resulting latex is stable and coagulum free. Good films have been produced. The level of grafting is high, and the level of crosslinking is low, both of which are optimistic indicators for a commercial product.
The polymers obtained were characterized in terms of molecular properties via GPC, 13C-NMR and DSC analysis, and extraction. The polymers produced in this case have a quite broad molecular weight distribution. NMR analysis has determined that the sites of grafting on the alkyd are double bonds nearest to acid groups. There are two possible forms of polymers, copolyacrylates-alkyd and polyacrylates. In these materials, the fraction of polyacrylate is small. This result supports the proposed mechanism of droplet nucleation followed by graft copolymerization.
Polyester. Hybrid miniemulsion polymerization has been performed with a three-component acrylic system (same as that for alkyds) of methyl methacrylate, butyl acrylate, and acrylic acid in the presence of a Bayer? Roskydal TPLS2190 unsaturated polyester resin. Novel latexes were obtained in which the polyester resin was grafted to the acrylic polymer, forming a water-based crosslinkable coating. Both emulsions and latexes were shelf stable for over 6 months, shear stable, and resistant to at least one freeze/thaw cycle. Resin to monomer ratios were studied as high as 1:l (wt-wt) and total emulsion solids as high as 45 percent. Monomer droplet and latex particle sizes were similar, suggesting the preponderance of droplet nucleation. A high level of crosslinking (>70 percent) during polymerization was observed in this particular hybrid system in contrast to those involving alkyd or polyurethane resins (<5 percent). Films, both homogeneous and hard, were achieved with exceptional adhesion.
The malleability of the dried latex changes with the amount of polyester resin. Reactions have been performed from a range of 10 percent resin (with respect to total miniemulsion weight) to 18 percent resin. Monomer conversion ranged between 85 and 92 percent. One goal achieved was increasing the total solids of the reaction medium to 45 percent while maintaining colloidal stability and product properties. Even at 45 percent total solids, the stability of the latex remained after thermal shock (freezer-thaw cycle), high shear, and shelf-life tests. The pH of the polymerized latex, regardless of percentage of resin solids, has been found to be near 1, due to the potassium persulfate initiator. Post addition of an ammonium hydroxide solution is used to raise the pH to its desired level.
A significant advancement in the research has been the elimination of the conventional hydrophobe from the reaction. In conventional miniemulsion polymerization, a hydrophobe such as hexadecane is always used to minimize the effects of Ostwald ripening. This phenomenon occurs in the time between sonication and the actual initiation of the reaction. For miniemulsion polymerization, the monomer droplet size must be under 500 nm in diameter, so the hydrophobe was used in our early reactions to maintain this requirement. However, hexadecane does not otherwise participate in the reaction and thus comes out of the coating eventually as a VOC. We have proven that you can omit this extra hydrophobe from the reaction and still avoid Ostwald ripening. The largely hydrophobic resins we use in the reaction serve this same purpose and, by doing so, we have eliminated the VOCs from the product. The particle size, as measured by dynamic light scattering, was measured to be 170 nm right after sonication and was unchanged at the time of reaction initiation, nearly 30 minutes later.
Transmission electron microscopy (TEM) has been used to determine particle morphology of the final latex. Electron microscopy showed the hybrid particle morphology to have internal domains of polyester resin in an acrylic matrix.
Unsaturated polyesters are commonly crosslinked by graft copolymerization with styrenic or acrylic monomers. Therefore, it is not surprising that high levels of crosslinking were observed. The fact that good films could be produced in spite of the high level of crosslinking is most likely because the reactivity ratios for acrylic/polyester systems are such that very long crosslinks of polyacrylate are formed between the polyester chains. This makes for a very loose crosslink structure that is capable of film formation. It is anticipated that if the same products were produced, replacing the acrylic monomer with styrene, a very tight gel consisting of short styrene crosslinks between the polyester chains, would result in a product that was incapable of producing good films.
Oil-Modified Polyurethane. An oil-modified polyurethane (OMPU) (McWhorter Corporation) has been selected as the initial candidate for evaluation in the new process. This is an unsaturated product (through the oil) that will allow for grafting, and air curing on drying, to a hard, crosslinked coating. The comonomer mix used for the alkyds work has been selected. Polymerizations have been carried out at ratios of 100, 60, and 30 parts OMPU per 100 parts of acrylic monomer. The resulting latexes at 33 percent total solids exhibit good latex stability. Particle sizes were in the range of 100-300 nm. Monomer conversions were between 90 and 95 percent. Reaction time is about 3-4 hours.
The monomer emulsions prepared for hybrid miniemulsion polymerization showed excellent shelf-life stability (>5 months), and the polymerization was run free of coagulation. Solvent extraction indicated that the grafting efficiency of polyacrylics was greater than 29 percent for all the samples produced. 13C solution NMR spectrum showed that a substantial fraction of the original carbon double bonds (>61 percent) in oil-modified polyurethane remained after polymerization for film curing.
Film properties were good. ASTM 03359-78, "Standard Test Methods for Measuring Adhesion by Tape Test" gives a rating of "5" for the products produced ("5" is the best classification of this test). ASTM 03363-74, "Standard Test Methods for Film Hardness by Pencil Test" gives a "B" rating. This is quite acceptable for a general purpose coating. 13C solution NMR was used to determine that 61 to 72 percent of the carbon-carbon double bonds were left in the product after polymerization. These are important because they provide the cure mechanism. Solvent extraction indicates that the degree of crosslinking is less that 5 percent, while the degree of grafting is 37 to 60 percent. These parameters are important because excessive crosslinking will inhibit film formation during the coating operation, and too little grafting will result in domains of polyester and domains of polyactylate, rather than a homogeneous copolymer.
Commercialization. Three patents have been filed for the production of alkyd, polyester, and urethane coatings, respectively, by the hybrid miniemulsion process: (1) Water-borne alkyd coatings by miniemulsion polymerization, U.S. Patent Application Serial Number 06/696,361; (2) Water-borne oil-modified polyurethane coatings by miniemulsion polymerization, U.S. Patent Application Serial Number 09/312,329; and (3) Water-borne polyester coatings by miniemulsion polymerization, U.S. Patent Application Serial Number 09/312,327. We currently are negotiating the licensing of this technology for commercialization.
Future Work. There are three significant outstanding issues?two practical and one scientific?in this project. Two problems remain with all three of the products (alkyd, polyester, and urethane) made to date. First, the monomer conversion is at about 95 percent, whereas a commercial product should be produced at 99.9 percent. One can attack this problem with varied polymerization temperature policies and mixed initiator systems, or by steam-stripping to remove residual monomer. Second, the hardness of the products to date range up to "2H." Commercial coatings manufacturers will want a hardness of at least "2H," and preferably "4H." The choice of resin, the composition of the acrylic monomer mix, and the effect of the polymerization conditions on film hardness and glass transition temperature need to be further explored.
The solution of the two practical problems is tied up in the third outstanding issue in this work: the determination of the molecular architecture of the hybrid polymer, and the effect of the molecular architecture on coatings properties. The reactivity of conjugated versus unconjugated double bonds during the polymerization process and, subsequently, during curing needs to be explored. We believe the propensity for grafting of the various types of double bonds present in the resin is controlling both the final monomer conversion and the hardness of the film produced.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
|Other project views:||All 17 publications||6 publications in selected types||All 6 journal articles|
||Dong H, Gooch JW, Schork FJ. Water-borne oil-modified polyurethane coatings via hybrid miniemulsion polymerization. Journal of Applied Polymer Science 2000;76(1):105-114.||
||Samer CJ, Schork FJ. Miniemulsion copolymerization in batch and continuous reactors. Industrial and Engineering Chemistry Research 1999;38(5):1792-1800.||
||Samer CJ, Schork FJ. The role of high shear in continuous miniemulsion polymerization. Industrial and Engineering Chemistry Research 1999;38(5):1801-1807.||
||Schork FJ, Poehlein GW, Wang S, Reimers J, Rodrigues J, Samer C. Miniemulsion polymerization. Colloids and Surfaces (A) 1999;153(1-3):39-45.||
||Tsavalas JG, Gooch JW, Schork FJ. Water-based crosslinkable coatings via miniemulsion polymerization of acrylic monomers in the presence of unsaturated polyester resin. Journal of Applied Polymer Science 2000;75(7):916-927.||
||Wu XQ, Schork FJ, Gooch JW. Hybrid miniemulsion polymerization of acrylic/alkyd systems and characterization of the resulting polymers. Journal of Polymer Science (Part A) 1999;37(22):4159-4168.||