New Acrylic Polyols for Low-VOC Coatings
Since acrylic polyols have higher molecular weights and solution viscosity than isocyanate crosslinkers, efforts to reduce the VOC content of urethane coatings have primarily focused on lowering the solvent demand of the acrylic polyols. Usually, lowering the molecular weight (Mn) or hardness (Tg) of the polyols reduces solvent demand but also has a negative impact on coating performance and ease of use. The main reason for this drop in performance is that the functionality of the polyol decreases linearly with molecular weight. Hence, as Mn goes down, the crosslinking ability of the polyol and the performance of the coating decrease.
Over the past several years, we have been working to overcome this inherent limitation by developing a new class of acrylic polyols based on allyl alcohols. Lyondell is a leading producer of allyl alcohol, which it produces by isomerization of propylene oxide. This has led to the development of liquid and solids polyols with improved functionality and lower solvent demand compared to conventional polyols made from HFAs.1
In addition, we discovered that using a blend of hard and soft resins instead of a single resin had some unexpected benefits, including lower VOCs, improved appearance, longer pot life, and shorter cure times.2 Blending solid and liquid polyols allows formulators to minimize resin inventories while maximizing coating formulations and technologies. Following is a description of this new acrylic polyol technology and its benefits.
The Resin Management Concept(tm)DSM Coating Resins recently introduced a blending concept for polyester coatings.3 Solid polyesters suitable for powder coatings were blended with low-Tg polyesters to give solution polyester blends suitable for can and coil applications. The low-Tg polyesters, however, were not liquid at 100% solids and were supplied in solution.
Similarly, we have developed solid and liquid acrylic polyols based on allylic alcohols that can be blended and crosslinked with polyisocyanates or melamines. The resulting coatings typically have lower VOCs, and improved appearance and performance compared to coatings formulated with a single HFA-based polyol. However, since the low-Tg polyols are liquid, they can also be used to prepare 100%-solids moisture- and UV-curable resins. The solid polyols can be formulated into storage-stable powders that melt and flow at lower temperatures than commercially available systems. We have successfully developed a handful of acrylic polyols that can be used in solventborne, powder, and 100%-solids coating systems using UV, moisture, isocyanate, and melamine crosslinking technologies (see Figure 1).
Formulators also benefit from lower inventory requirements, lower raw material and development costs, and shorter time to market. New resins no longer need to be developed for each new coating application. Formulators can take the hard and liquid polyols and blend them to the desired coating specification. Since they will already be familiar with the properties of each resin and their performance in other blends, formulators can quickly arrive at the right formulation and be confident that the coating will perform according to expectations.
This occurs to some extent today. Most formulators add resin modifiers to their formulations when the properties of the base resin fall short of requirements. Examples include the use of polyester polyol flexibilizers in acrylic melamine baking enamels for flexible substrates and the use of high-solids acrylic polyols to reduce the VOC content of medium-solids resins. However, the resins they blend may come from different suppliers and may be incompatible. Overcoming these problems often extends time required to formulate and test new coating formulations and their eventual commercialization.
ACRYFLOW PolyolsConventional acrylic polyols are produced by free-radical polymerization of acrylate esters and HFAs to provide crosslinking sites on the polymer. Since all acrylates have approximately the same reactivity, the distribution of hydroxyl functionality in the polymer is essentially random. Consequently, as the molecular weight of the polymer decreases, the probability that it will contain a crosslinking site decreases.
This is unfortunate for two reasons. First, low molecular weight is desirable because it reduces the solution viscosity of the resin and, therefore, the VOC content of the coating. As acrylic polyol producers attempt to reduce the solvent demand of their resins, the amount of non-functional polymer in their polyols increases. Also, the random distribution of crosslinking sites in the polymer makes for an irregular crosslinked network. This reduced and uneven crosslink density affects the coating's performance, especially hardness and chemical and abrasion resistance.
A better approach is to use allyl alcohols (see Figure 2) as the hydroxy-functional monomers. Allyl alcohols are much less reactive than acrylate monomers and act as chain-transfer agents.5 This presents both a challenge and an opportunity. The problem of slow reactivity had already been overcome in our styrene-allyl alcohol (SAA) resin process.3 SAA resins have high terminal hydroxyl content and do not lose their functionality at low molecular weights.7,8 We have now further developed this technology to produce low-molecular-weight acrylic polyols with improved hydroxyl content and distribution.1
The use of allyl alcohols requires significant process changes but yields improved acrylic polyols. Improvements include a more even distribution of functionality in the polymer, high terminal hydroxyl content and fewer non-functional polymers chains. This is due to the fact that allyl alcohol-based polyols better maintain their functionality as molecular weight decreases and the alcohols act as chain-transfer agents.
In conventional high-solids polyols, producers often increase the amount of hydroxy-functional acrylate in the polymer to 140-175 gram KOH/g range, to compensate for the rapid loss in functionality. This, however, increases the isocyanate demand, traditionally the most expensive component of the coating. So, in effect, conventional acrylic polyols trade low VOCs for either performance or cost.
In contrast, ACRYFLOW polyols have fewer non-functional impurities, even in resins with lower OH numbers. For example, an HEMA-based commercial resin with an OH number of 144 was found to contain approximately 25% polymer with less than 2 OH groups and 13% polymer with less than one OH group (see Figure 3). An AP1.0-based polyol, on the other hand, was found to contain only 13% polymer with less than 2 OH groups and zero non-functional material despite having a lower OH number of 134.
The RMc at WorkTo demonstrate this concept and the improved performance of the allyl alcohol-based resins, we compared blends of hard and soft ACRYFLOW resins (see Table 1) with HFA-based acrylic polyols in 2K urethane, pigmented industrial maintenance coatings. We also developed additional resins blends that were tested in 2K clearcoats over white basecoats.
The low-end maintenance market requires low cost and low VOCs, so most commercial acrylic polyols sold for this application are high-solids resins with low hydroxyl functionality. The trade-off is often poor solvent and chemical resistance due to the reduced crosslink density of the coating. Higher end uses, where chemical resistance is required, use higher functionality polyols.
Table 2 lists the compositions and properties of four white basecoats suitable for the maintenance, general metal, or transportation markets. The coatings were sprayable at less than 340 g VOC/l (per U.S. EPA definition) without exempt solvents. Coating formulations with less than 2.1 lbs/gal (250 g/l) may also be developed using exempt solvents or if different application techniques are used, such as roller or brush. The viscosity of the pigmented coatings was dependent on the resin blend Tg and OH number. Overall, coating viscosities and VOCs were lower than those of the commercial controls and pot lives were, in some cases, twice as long.
Pot lives, as measured by a doubling of the initial coating viscosity, were all over 4 hours. With the addition of small amounts of acetic or other organic acids, pot life could be enhanced to the point where formulations with 6 hour surface cure times were still liquid after 24 hours. Longer pot life extends the application window and reduces paint waste and, therefore, cost.
Through cure was much faster than commercial controls but surface cure, also known as "open time," was longer. This is a significant advantage when coating large surfaces as overlapping coats can blend into each other with no visible line. On the other hand, rapid through cure allows for greater productivity as parts can be handled sooner without marring and floors can be walked on sooner after application.
The most significant improvement achieved with the ACRYFLOW blends was in chemical resistance. These coatings withstood 200 MEK rubs with almost no scratches while the coatings based on commercial polyols completely broke down between 100 and 200 MEK rubs. This superior chemical resistance was maintained even when the isocyanate index was reduced to 0.8 to match the crosslink density of the commercial systems (see Figure 5). Good chemical and abrasion resistance increase the time required between recoats, thereby reducing costs.
Eight urethane clearcoat formulations based on four ACRYLFLOW resins were also exposed to accelerated weathering with no appreciable loss of gloss or yellowness increase. All the resin blends easily passed SSPC level 3 weatherability specifications. These require less than 30% gloss loss and less than 2% increase in yellowness index after 2,000 hours' QUVA exposure or 48 months of Florida exposure
SummarySolvent-free, liquid acrylic polyols were prepared using allyl monopropoxylate (AP1.0) and acrylate monomers and solid acrylic polyols with low solution viscosity were prepared from allyl alcohol, methacrylate monomers, and styrene. Blends of these hard and soft polyols were crosslinked with polyisocyanates to give coatings with improved properties compared to commercial controls. Specifically, the resin blends yielded coatings with lower VOCs, longer pot lives and open-time, but faster through cure. In addition, the fully cured coatings had superior solvent and abrasion resistance, good hardness and flexibility, and excellent weatherability compared to commercial controls. Costs also are reduced by way of a combination of lower raw material costs, improved pot life and productivity, and greater coating durability.
Blending hard and soft polyols based on allyl alcohols allows the formulator to "dial-in" the desired properties and gives coatings suitable for a wide range of applications and coating technologies. Using a total of five acrylic polyols and this blending concept, we have developed starting formulations for industrial maintenance, automotive refinish, and OEM baking enamels for metal and plastics. We are currently developing UV- and moisture-curable compositions based on the liquid acrylic polyols and expect the solid resins to also find use in powder coatings. We believe that these new acrylic polyols and formulating approach will help formulators meet the ever-increasing performance, compliance, and economy requirements of their coatings.
For more information or the suggested formulations for polyols, contact Lyondell Chemical Co. via e-mail to firstname.lastname@example.org; visit www.lyondell.com; or Circle Number 130.
References1 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 A. Heijenk, Proceedings of the Twenty Eighth International Waterborne, High-Solids, & Powder Coatings Symposium", February 21-23, 2001, R. F. Storey & S. F. Thames, Editors, p. 217.
4 B. Ranby, Applied Polymer Symposium, 1979, 26, 327-344
5 V.P. Zubov, et. al., J. Macromol. Sci.-Chem., 1979, A13(1), 111-131
6 U.S. Pat. Nos. 5,444,141, 5,512,642, 5,886,114.
7 Guo, S. H. In Specialty Monomers and Polymers; Havelka, K. O.; McCormick, C. L., Eds.; ACS Symposium Series 755; American Chemical Society: Washington, DC, 2000, pp 147-158.
8 A commercial polymer was fractionated using supercritical CO2 extraction and each fraction analyzed by Gel Permeation Chromatography (GPC), wet chemical analysis, and high-resolution NMR spectroscopy.