Waterborne Oil-Modified Urethanes
Solventborne-oil-modified urethane resins are used to formulate coatings for wood and metal. These single-pack systems find great acceptance in the contractor and DIY market because of their ease of use and superior performance. However, they are high in VOCs, typically around 3.7 pounds per gallon.
Alternative technologies such as polyurethane dispersions and acrylic latexes or combinations of the two have been in the market place for years. These products meet the market criteria of a one-pack, easy-to-use, low-VOC system, but at a performance cost compared to their solventborne predecessors. These “lacquer-dry” technologies form thermoplastic films by the evaporation of water and co-solvent. In general, the acrylic lacquer-dry systems lack abrasion resistance and all the alternative technologies lack the chemical resistance of the more robust crosslinked solventborne systems.
In response to market demand, waterborne oil-modified urethane resins were developed. The WB OMUs cure by way of air oxidation and bridge the performance gap between the older non-VOC-compliant solventborne systems and alternative waterborne technologies, such as polyurethane dispersions and acrylic latexes. Market response to the first-generation products was very positive. Benefits of the product include low VOC content, ease of use and cleanup, quick re-coat time, and superior chemical and solvent resistance. Formulated coatings based on the first-generation WB OMU are typically supplied at 1.6 lbs. per gallon VOC.
More recently, a second-generation WB OMU has been introduced. This product maintains the performance of the first generation but exhibited considerably less applied and in-can yellowing.
ExperimentalWaterborne oil-modified urethanes were synthesized and formulated using a standard urethane paint formulation suitable for wood surfaces. The resin this study evaluates, known as Spensol® F-97, is a second-generation WB OMU. This article discusses the synthesis process, along with the physical properties of the neat resin. The waterborne OMU was then compared against the following commercial systems.
- Sample A - Conventional solventborne OMU varnish, Spenkel® F-77-M-60
- Sample B - First-generation waterborne OMU varnish, Spensol® F-96
- Sample C - Second-generation waterborne OMU varnish, Spensol® F-97
- Sample D - A commercial acrylic varnish (contains a small portion of PUD)
- Sample E - A polyurethane dispersion/acrylic blend varnish (50/50 blend)
- Sample F - A straight polyurethane dispersion varnish
The effect of blending an acrylic emulsion into the second-generation waterborne OMU was also studied.
SynthesisSelf-crosslinkable WB OMUs are synthesized by the typical prepolymer process.1,2 The reaction scheme is shown in Figure 1. The major difference from conventional waterborne urethane synthesis is the use of an oil-ester as the soft segment in the OMUs. The reaction involves the use of an OH-functional oil-ester, dimethyl-olpropionic acid and diisocyanate to form an isocyanate-terminated prepolymer.
The prepolymer is neutralized using tertiary amine and dispersed into water. An amine chain extender is then used to extend the prepolymer. NMP cosolvent is used in the prepolymer preparation and the solids content is controlled at 32% to 33%.
Crosslinking of Waterborne OMUKey to the performance of both solventborne and waterborne oil-modified urethanes is their ability to oxidatively crosslink. The waterborne OMUs cure by a two-step process. The first step is the evaporation of water and cosolvent from the coating to form a hard thermoplastic film. This is followed by oxidative crosslinking of the polymer through the unsaturation in the oil to form a three-dimensional (3-D) structure. It takes approximately 4 to 7 days before the films reach ultimate properties. The crosslinking and forming of an insoluble 3-D network is the primary reason for their superior chemical and solvent resistance compared to thermoplastic polyurethane dispersions (PUDs) and acrylic emulsions. The crosslinking reaction is typically accelerated by the inclusion of manganese and cobalt metal salts. The schematic of the crosslinking reaction is shown in Figure 2.
The most significant difference between the first- and second-generation WB OMU is the selection of the oil ester. By the proper selection of the oil, the second-generation system exhibits considerable less yellowing than either the first-generation WB OMU or its solventborne predecessor. The need for a less yellowing product is in response to a market need for the urethane to effectively compete against the clear acrylics and PUDs. This applies for both the “in can” color and the applied color.
Typical properties of the second-generation WB OMU are presented in Table 1, and the OMU was formulated according to the procedure shown in Table 2. Testing followed ASTM methodology. Formulations for the first-generation WB OMU and SB OMU appear in Addendum 1, which will appear in the online version of this article, at www.pcimag.com.
Results and DiscussionPreviously, Petschke and Ingle3,4 examined the performance of the first-generation WB OMU (Spensol® F-96) against a commercial solventborne OMU in varnish formulations. Spensol F-96 is the immediate predecessor to the second-generation Spensol F-97, the subject of this article.
Results show that the first-generation waterborne OMU has a significantly shorter dry time than its solventborne predecessor (see Table 3). Dry time is particularly important for contractors as an indicator of the amount of time required before second and subsequent coats can be applied.
Seven-day hardness, as measured by Taber abrasion, shows that the waterborne OMU reaches a higher level of hardness after 7 days compared to the solventborne OMU. Hardness is a crucial measurement in the commercial market, as it gives an indication of “return to full service” for the floor or wood surface. The results in Table 4 show better Tukon hardness and approximately a 27% less weight loss during Tabor abrasion.
Petschke and Ingle further evaluated chemical resistance according to ASTM D-1308. The data in Table 6 shows the chemical resistance of the first-generation WB OMU to be nearly identical to its solventborne predecessor. The conventional OMU had an aggregate score of 90 with an average of 4.1, compared with an aggregate score of 93 and average of 4.4 for the waterborne OMU.
As alternatives to the solventborne OMUs, the first-generation waterborne OMUs have been very successful in meeting the demands of the marketplace. Their performance is equal to, or better than, the solventborne products in a much more environmentally compliant system. Samples A and B in this study are at 444 gm/L and 200 gm/L, respectively, representing greater than a 50% reduction in VOC.
Comparison with Alternative Waterborne TechnologiesNot only is the waterborne OMU intended to respond to the need for a compliant version of the solventborne product, it is also intended to compete against clear wood coatings such as acrylics, PUDs and their blends. Therefore, it is important to establish their base line performance against these alternative systems. The authors compare the second-generation waterborne OMUs against the first-generation WB OMU and alternative waterborne acrylic and polyurethane dispersions. The second-generation was chosen because of its improved yellowing resistance, making it more comparable to the clear wood systems.
Again, key properties investigated are dry time, yellowing, hardness, and stain and chemical resistance. Figure 3 shows the dry time results of the systems tested. The second-generation WB OMU maintains the significant dry time improvement of the first-generation WB OMU and is comparable to the other competitive technologies.
The second-generation WB OMU exhibits much less yellowing than the solventborne or first-generation products. The data in Table 5 shows the level of yellowing for the WB OMU approaching the level of the PUD. By their very nature, oil-modified urethanes yellow to a certain extent; in fact the development of color is considered a desirable trait as it adds warmth and richness to the wood surface. However, the in-can color is a concern that the second-generation WB OMU successfully addressed. Yellowness was not measured for samples D and E, but they are expected to be similar to sample F, the PUD.
Stain, chemical and solvent resistance were measured for all varnishes and includes previous data generated on the solventborne OMU. All data was collected after 7 days drying at constant temperature and humidity per ASTM D-1308. The ability of the solvent- and waterborne OMUs to crosslink significantly improves their chemical resistance. This is not seen in the thermoplastic acrylic and polyurethane dispersion. Chemical- and solvent-resistance data is presented in Table 6.
Evaluation of the summation and averages show the first- and second-generation WB OMUs are better than the conventional solventborne OMU and significantly better than the competitive thermoplastic products. Overall, the waterborne OMUs are better in almost all respects except in 10% sodium hydroxide solution exposure. This result is typical of most oil-modified products including alkyds and is the only area where the performance of the alternative systems was consistently better.
As reported earlier there is an increase in hardness and abrasion resistance for the first- and second-generation OMUs compared to the SB OMU. In comparison to the acrylic and acrylic/urethane blends the WB OMU exhibit superior Sward and pencil hardness and better abrasion resistance. The PUD had better abrasion and Sward than the WB OMU but at a loss of mar resistance as shown in Table 7.
Blending Study with Acrylic ResinAcrylic resins are often added to PUD and waterborne OMUs to reduce the formulation cost of the finished coating or varnish. As with any blend there is a trade off between performance and cost. To evaluate this effect, an acrylic was blended into the second-generation WB OMU formulation at levels between 0% and 30%.
For this test, a commercial acrylic resin was used to determine the effect on abrasion resistance and other properties. Figure 4 shows the effect of adding acrylic resin to the WB OMU.
The figures show that the Taber abrasion resistance decreases slightly with the addition of 10% or 20% acrylic resin, and significantly decreases at the 30% level. In addition, there is a definite drop off in mar resistance at both the 20% and 30% acrylic content. Subjective measurements of mar resistance are shown above each bar in Figure 4.
For practical formulating, the addition of 10% to a maximum of 20% acrylic resin is best for reducing the cost of the formulated coating without a severe loss of performance. This is particularly important for wood floor coatings where abrasion and mar resistance are crucial to the long-term durability of the floor. It is less crucial in vertical surface applications, where the coating or varnish is not subjected to the same level of abrasion.
ConclusionThe authors have presented data on the synthesis and performance characteristics of waterborne oil-modified urethane resins. The synthesis of the waterborne OMU is similar to typical waterborne urethane polymerization techniques but with a fatty acid oil added as a soft segment modifier and a crosslinking site. Curing of waterborne oil-modified urethanes is a two-step process. The first step is the evaporation of the water and cosolvent from the varnish to form a thermoplastic film, followed by crosslinking across the double bonds in the oil segment.
When comparing the WB OMU against its solventborne predecessor, better physical properties were found, such as hardness and abrasion resistance and near equivalent chemical resistance.
Comparison against a number of commercially available acrylic and polyurethane dispersions show the waterborne OMU to have superior physical and chemical properties across the board, except for abrasion resistance where the PUD was better.
Finally, the effect of adding 10% to 30% acrylic resin to reduce the cost of the formulated paint was studied. It was determined that including up to 20% acrylic does not significantly affect the abrasion resistance. However, levels in excess of 30% lead to significant drops in performance and are not recommended. As with any blending work, formulators must carefully weigh the cost/performance tradeoffs.
In summary, waterborne oil-modified urethane resins offer the best alternative to solventborne oil-modified urethanes in terms of performance, cost and environmental friendliness. They also offer an attractive alternative to acrylic resins and polyurethane dispersions in applications that demand a good combination of ease of use, hardness, abrasion and chemical resistance, and environmental compliance.
AcknowledgmentsThe authors wish to thank Glenn Petschke, Mike Ingle and Alicia Taylor, Mike Ellison, and the staff of Reichhold Inc. laboratories for their assistance in this article.
This article is based on a paper presented at the Western Coatings Societies’ 24th Biennial Symposium and Show February 15-17, 1999, in Reno, NV.
For more information on resins, contact Reichhold Inc., 2400 Ellis Road, Durham, NC. 27703.