HPHP- A Component for Enviornmentally Advanced Coatings m 2 fd- Don Morris Texas Eastman Division, P.O. Box 7444 Longview, Texas 75607 The coating s industry is undergoing a dramatic shift from solvent based paints to technologies that are more environmentally acceptable. Much of this change is due to changing regulations on emissions and waste disposal. Technologies such as water-borne, high-solids, and powder coatings help the coatings industry meet the regulations and can offer increased performance over solvent based coatings. These technologies are often called Compliant Coatings because they meet or exceed EPA guidlines for coatings emissions. Markets for powder coatings are growing because of the high quality of the resulting coatings and the increased emphasis on reducing solvent emissions from commercial paint lines. Powder coatings markets have grown 10-15 percent per year since 1981 and are now used to coat metal products such as patio furniture, bicycles, lawn mower housings, many automobile components, and kitchen and laundry appliances. Most powder coatings systems are composed of a polyester resin and a crosslinking agent which react chemically to form the final coating. This crosslinking occurs at elevated temperatures after the powder has melted and flowed out into a smooth film. Compared to solvent borne paint systems, powder coatings offer reductions in the emissions of organic solvents to the environment. Typically 25 to 50% of the weight of a solvent borne paint is solvent which evaporates once the paint is applied to the substrate.
Powder coatings contain no solvent and produce less than 0.1 pounds volatile organic compounds per gallon (VOC) during application. This is considerably below the EPA control VOC of 2.8 pounds per gallon for coating metal surf aces. Powder coating resin manufactures must now rely on a fairly limited number of diols and diacids to improve the performance of polyester resins. Neopentyl glycol (NPG Glycol*) is an important diol in the preparation of many types resins because its chemical structure provides the weatherability and chemical resistance needed to make high performance coatings. NPG is of particular value in polyester powder coating resins because, in addition to its excellent performance, it results in resins with the desired glass transition temperature (Tg) and melt viscosity. The powder coating resin must have the hardness to be ground into a powder (friability) but must melt in a fairly narrow ) temperature range to provide controlled flow as the polyester is crosslinked. The Tg of the resin must be high enough that the resin does not agglomerate during storage, that is it must remain free flowing so it can be applied easily. The melt viscosity controls how the polymer flows as it is melted. The surface appearance of the coatings is greatly affected by the melt viscosity and Tg of the polyester resin. Smoothness, as measured by gloss and distinctness of image, is closely related to flow of the polymer as it is cured. Powder coatings can be used in additional applications if the smoothness is enhanced. A large potential market in automotive coatings could be served if the appearance of powder coatings can be made to match solvent based automotive coatings.
. A glass transiton temperature of between 50-8OoC is desired for the resin in order that the powder will be stable during storage and will flow correctly when melted. The Tg and melt viscosity of a powder resin is dependent on the monomers used to make the polyester resin. The resin is the largest component of the powder and provides the mechanical strength and overall performance for the resin. The resins are made by condensation of mixtures of glycols, polyols, diacids and/or polyacids to produce ester linkages by the elimination of water. Most polyester powder coating resins use neopentyl glycol as the major glycol component in combination with terephthalic acid as the major diacid to provide a tough weatherable coating. Improvements in the performance and appearance of the finished powder coating can be attained by using modifying monomers in the resin formulation during condensation step. The modifing monomer is used to change the Tg and melt viscosity of the polyester resin in order to enhance desired physical properties such as smoothness, UV stability, hydrolysis resistance, impact strength and flexibility. Larry Johnson and coworkers of Eastman Chemicals summarized the effects of several new polyester intermediates on resin properties in a paper given at the Eighteenth Water-borne, Higher-solids, and Powder Coatings Symposium in New Orleans in February, 1991. 3 Hydroxypivalyl hydroxypivalate (HPHP) is a monomer closely related to neopentyl glycol that is very useful in modifying the properties of polyester powder coating resins. This glycol is essentially two neopentyl glycol molecules joined through an ester linkage. The major change observed when substituting HPHP for
neopentyl glycol is a lowering of the Tg and melt viscosity as seen in tables 1-3. The resin manufacture substitutes HPHP for a portion of the neopentyl glycol to control the Tg and melt viscosity of the powder resin. Adjusting the Tg and melt viscosity allows the storage stability (resistance to agglomeration) to be balanced with the flow out of the resin as it cures. The appearance of coatings containing HPHP may be enhanced because of improved gloss and fewer surface defects and the performance of the coating may be improved becaused of greater flexibility and resistance to hydrolysis. Other glycols can be used to lower the Tg and melt viscosity of polyester resins, however, HPHP results in smoother more attractive and more durable coatings while maintaining storage stability of the powder. Substituting HPHP for neopentyl glycol increases the aliphatic character of the polymer because of the increased molecular weight of HPHP as compared to NPG. This increase in molecular weight means that less terephthalic acid, for example, is used in each pound of resin. Decreasing the amount of aromatic character of a resin can increase the UV stability of the resin. The decreasing aromatic charcacter can also result in coatings with increased flexibility because rigid terephthalic acid-npg links are replaced with more flexible HPHP links. The structures of both HPHP and NPG contain hydroxyl groups that are protected by two methyl groups on the next carbon atom. The ester linkage in HPHP is surrounded by four methyl groups. These protected hydroxyl groups and the protected ester group enhance the chemical stability of polyesters made with typical diacids such as terephtalic acid. The hydrolytic stability of the resulting polyester is increased dramatically versus
n 3 3 unprotected ester linkages. This makes coatings made with the resins more detergent and acid rain resistant. Acid catalyzed cleavage of the ester bonds in the polyester result in loss of polymer strength and eventual failure of the coating without the protected ester linkages. Eastman Chemicals Company is a manufacturer of a number of diols and diacids that are used in resin synthesis. Neopentyl glycol and HPHP are produced at the Texas Eastman Division in Longview, Texas. Neopentyl glycol has been manufactured since 1962. Eastman was the first manufacturer in the world of this key coatings intermediate. HPHP production at Texas Eastman began in 1991. HPHP is synthesized in a multipurpose manufacturing plant that was designed to provide for very low emissions. By-products from HPHP manufacture are processed in the neopentyl glycol plant where they are recovered as neopentyl glycol and isobutanol. A single high boiling stream from the HPHP plant is incinerated. The chemistry of HPHP manufacture is similar to neopentyl glycol manufacture in that both products are made by the aldol condensation of isobutyraldehyde and formaldehyde. Neopentyl glycol is made by hydrogenating this aldol product and HPHP is made by condensing two molecules of the aldol product in a Tischenko reaction that produces the ester linkage. Eastman Chemical Company is committed to providing the coatings industry with intermediates that can be utlilized to improve coatings performance and environmental acceptability. NPG is a trademark of Eastman Kodak Company
Table 1 The Effects of Substituting HPHP for NPGGlycol in Hydroxyl-Functional Powder Coating Resins IC1 Visc. 0 To 'C Mn (71700~1. E NG - 54 2426 30 NG/HPHP 4/1 49 2771 22 HPHP - 26 2885 4 Table 2 The Effects of Substituting HPHP for NPG-Glycol in Carboxyl-Functional Powder Coating Resins IC1 Visc. GLycols Ratio Ta. OC Mn (20OOC). E NG - 59 1979 12 NG/HPHP 3/2 50 1948 11 NG/HPHP 2/3 43 2005 9 HPHP - 36 1907 7 Table 3 The Effects of Substituting HPHP for NPG-Glycol in Carboxyl-Functional Powder Coating Resins IC1 Visc. CHDM/NG 4/1 68 1762 >loo CHDM/HPHP 4/1 56 1599 71