Oil-modified PUDs: cross-linkable, VOC compliant, cost effective

Transcription

1 Oil-modified PUDs: cross-linkable, VOC compliant, cost effective Waterborne Polyurethane Dispersions (PUDs) tend to show undesirably low mechnical performance and chemical resistance, which is due to their merely physical drying mechanism. Oil-modified polyurethanes (OMPUs) offer a way to introduce additional oxidative crosslinking during film formation, which is adjustable through the choice of oils that are incorporated and their degree of unsaturation. Results based on OMPUs modified with linseed, soybean and castor oil - three very cost effective materials - show a considerable performance improvement. Sunil N. Peshane, Vilas D. Athawale Waterborne coatings are steadily gaining significance as environmentally protective alternative to solvent-based coating systems. They are more comparable in their handling and processing with conventional, allied coating systems. Ever increasing industrial development has brought with it volatile organic compounds (VOC). Therefore, in the context of a growing concern for ecology, it is the prerogative of the coatings polymer researchers to reduce drastically the solvent content of coating system by any possible means. In terms of volume, oil-modified polyurethanes (OMPUs) are among the most important coatings. Due to their excellent film formation, they provide superior properties compared to alternative technologies such as polyurethane dispersions (PUDs) and acrylic latex. This is because OMPUs dry by auto-oxidation, resulting in 3-D chemical crosslinking, while PUDs and latexes dry by coalescence of polymer particles which is a mere physical phenomenon. The present work aimed at developing VOC compliant OMPUs with competitive properties, while keeping their costs very low at the same time. The main steps involved in the synthesis were similar to the two step prepolymer technique used for waterborne polyurethane dispersions [1, 2], with the only difference that the oils were first converted to oil-esters by alcoholysis to incorporate hydroxyl functions, which act as soft segments. The resins were produced by reacting dimethylol propionic acid (DMPA), poly(propylene glycol) (PPG) and hexamethylene diisocyanate (HDI) with the oil-esters (usually monoglycerides). The main oils used were linseed, soybean and castor oil. The different oils were selected according to their different degrees of unsaturation [3-5] in the fatty acid chains, leading to different efficiencies of film formation and extents of crosslinking (Figure 3). This factor predominantly affects the performance of the cured films, helping to find structure property relationships. OMPUs were compared with a PUD based on the same isocyanate, which was studied earlier [6], and also with a hard acrylic latex. Mechanical Properties Hardness (Pencil /Shore A) The pencil hardness results indicated hardness range of H to 2H for all the coating films under study. No significant trends and conclusions could be deduced from the pencil hardness test - showing the ineffectiveness of this method. Further evaluations with the Shore A hardness tester confirmed that the linseed oil based OMPU-1 film showed the highest hardness value, while castor oil based OMPU-3 gave the lowest hardness among all oil-modified polyurethanes. This can obviously be understood in terms of the degree of crosslinking of the cured films. The linseed oil based film was highly cross-linked owing to the higher degree of unsaturation (linolenic fatty chain content 46 %) (Table 1), resulting in a dense 3-D polymer matrix and certainly improved properties of OMPU-1 compared to both OMPU-2 and OMPU-3. OMPU-3, which used castor oil, reflected poor performance among all the OMPUs because it did not have any significant linolenic unsaturation, and the low degree of linoleic (3.1%) and oleic (7.4%) unsaturation was not high enough to compensate the absence of linolenic chains. However, although OMPU-3 showed lower hardness, the value was much greater compared to PUD-5, where the crosslinking was almost negligible. The Shore A hardness of the hard acrylic latex was between that of OMPU-2 and OMPU-3 (Figure 5). These results clearly indicated the supremacy of OMPUs over uncrosslinked PUDs and to some extent over acrylic latex in terms of their mechanical performance. Flexibility All the coating binders easily passed the conical mandrel (1/8'') bend test. No cracking from the edges was found, indicating that all OMPUs, PUD-5 and acrylic latex showed control and balance of the physico-mechanical properties (Table 3). Impact Resistance The balance in mechanical properties was further confirmed by the impact resistance study, in which full-scale (160 in-lb) direct and reverse impact was tolerated by all OMPUs and PUD-5 (Table 3). The acrylic latex film could not sustain full-scale reverse impact, revealing its brittleness compared to OMPUs and PUD-5 (Figure 6) and the need of an external plasticizer in the acrylic to achieve the benchmarks set by the OMPUs. The excellent balance in properties of OMPUs is mainly attributed to the fact that oil-esters act as soft domains and contribute to the softness of the polypropylene glycol (Mn = 2000). At the same time, the glyceride chains are partially demobilised by the cross-links between adjacent chains, avoiding excessive softness. Drying Time The required curing time was estimated according to the Indian Standard IS 101. The drying times of PUD-5 and of the acrylic latex film were almost the same. -The different OMPUs, however, differed in full through-cure time, despite essentially equal dosages of dryers (Co and Mn metal salts) and identical curing conditions, implying a dependance of the drying time on the type of oil-ester used. The linseed oil-ester based OMPU-1 dried very fast compared to all other OMPUs under study, and was competitive to both PUD-5 and the acrylic latex, with a slight improvement over both of them (Table 3). The drying performance of the soybean oil-ester based OMPU-2, although not better than PUD-5 and acrylic latex, was not below average. Castor oil-ester based OMPU-3, however, gave undesirable long curing times. This behaviour is again explained by the degree of unsaturation in the OMPUs, determining the rate of oxidative curing. The highest degree of unsaturation leads to the fastest curing. During the auto-oxidation of OMPU films, the dangling fatty chains within the monoglyceride, forming a side chain of the main polyurethane backbone, cross-link via double bonds present onto it (Figure 4). The drying time trends of the coatings with respect to their set to

2 touch and hard drying response are summarized in Figure 7. Chemical / Solvent Resistance The resistance to chemicals of the OMPUs was quite acceptable in comparison to PUD-5 and the acrylic latex, except for the alkali resistance. This, however, was the weak point of almost all candidates (Table 3). The poor alkali resistance of the OMPUs is obvious because of the glyceride backbones, in which ester groups are easily attacked and hydrolyzed by alkali. PUD-5 was falling short in alkali resistance for the same reason - here, the ester groups are found in the polyester polyol. OMPUs also showed a better solvent resistance, evaluated by the double rub test, except for methyl ethyl ketone (MEK), where the coating could withstand only 90 MEK double rubs without a considerable damage. The higher performance of OMPUs over PUD-5 and acrylic latex is once again attributed to the cross-linked polymer matrix of OMPUs. The poor performance of PUD-5 demonstrates that the usually high chemical and solvent resistance of polyurethanes only applies to significantly cross-linked resins - PUD-5 is a linear (slightly cross-linked) thermoplastic polymer. Conclusions The majority of commercial waterborne PU dispersions are predominantly linear thermoplastic polyurethanes. The coatings formulated with them show relatively poor mechanical performance and chemical and solvent resistance, compared to cross-linked varieties. Oil-modified polyurethanes (OMPUs) can considerably improve these performances, due to their higher inter- and possibly intra-chain crosslinking, effected by the unsaturated reactive sites in the oil modification. Curing of the PU dispersions chemically modified with oil-esters occurs by coalescence of the particles and subsequent oxidative crosslinking. Compared to mere coalescence, which occurs in conventional PUDs, this can enhance the performance drastically. The selection of proper oil backbone, however, is very important because the degree of unsaturation plays a key role, providing different extents of crosslinking and thus performance. Comparing the OMPUs used in this study, linseed oil based oil-esters can be considered an ideal candidate, and soybean oil the second best choice. Generally, waterborne oil-modified polyurethane dispersions can achieve an improvement of physico-mechanical performance, chemical and solvent resistance, and drying rates (if proper selection of oil is made) - with VOC-compliant coating materials at pocket friendly prices. References [1] D. Dieterich, Prog. Org. Coatings, 9 (1981), [2] Szycher, M., Szycher's Handbook of Polyurethanes, CRC press, Washington, D. C., [3] Painter, E. P., and Nesbitt, L. L., Ind. Eng. Chem., Anal. Ed., 15, (1943). [4] Hilditch, T. P., and Jasperson, H., J. Soc. Chem. Ind., 58, (1939). [5] Kaufmann, H. P., and Bornhardt, H., Fette u. Seifen, 46, (1939). [6] Athawale, V. D., and Peshane, S. N., Eur. Coat. J. 1-2 (2003), p. 45. [7] Achaya, K.T., J. Amer. Oil. Chem. Soc., 48, 11, 758, (1971). [8] Bailey's Industrial Oil and Fat Products, Vol. 1, Fourth Edition, Edited by D. Swern, John Wiley and Sons, Inc., New York, (1979). [9] Goodman, S., Handbook of Thermoset Plastics, Noyes Publications, New Jersey, 1986, Results at a glance VOC compliant, waterborne oil-modified polyurethanes (OMPU) were synthesized by two step prepolymer method, based on oil-esters obtained from linseed, soybean and castor oil backbones, polypropylene glycol (PPG, Mn = 2000 and dimethylol propionic acid (DMPA), and hexamethylene diisocyanate (HDI) as an isocyanate precursor. One-pack, air-drying coatings with oxidative crosslinking - accelerated by the inclusion of Co and Mn metal salts - were obtained, that are cost effective, environment friendly and show excellent resistance against water, chemicals and solvents (except resistance to alkali), better drying times, and a blend of acceptable mechanical properties, compared with waterborne PUD and acrylic latex coatings - further adding value for money and offering a good compromise to reduce VOC. Experimental Materials Dimethylol propionic acid (99 %) (DMPA) was purchased from Aldrich, USA. Triethyl amine and N-methyl-2-pyrrolidone (s. d. fine-chem, India) were dried over 4A molecular sieves for 7 days. Hexamethylene diisocyanate (HDI) was procured from Merck, Germany. Glycerol and the "Fascat 4100" catalyst were purchased from s. d. fine-chem, India). The samples of refined linseed oil, soybean oil and castor oil were obtained from Jayant Oil Mill, Mumbai, polypropylene glycol (PPG) of molecular weight Mn = 2000 from E. Merck (India) Ltd. All chemicals were used without any further purification. Synthesis of Hydroxyl Terminated Oil-esters Linseed and soybean oils were reacted separately with glycerol at around 190 C using a "Fascat 4100" catalyst (0.05 % w/w based on oil) in a three-necked round bottom flask equipped with a mechanical stirrer and water condenser for minutes. These reactions are technically termed alcoholysis (Figure 1). The oil-ester (monoglyceride) formation was confirmed by 1:3 methanol tolerance, a clear and homogeneous solution indicated conversion from oil to the desired product. The products obtained were discharged into glass stoppered bottles and placed in vacuum desiccator, before they were used as hydroxyl terminated oil-ester soft segments in further reactions. Castor oil was used as such since it already contains a hydroxyl function, contained in a fatty acid chain (ricinoleic acid, 12-hydroxy octadecanoic acid) [7, 8]. Synthesis of OMPUs A resin kettle equipped with thermometer, mechanical stirrer, nitrogen gas inlet and reflux condenser was charged with the preformulated amount of oil-ester, PPG (Mn = 2000), DMPA and HDI. The reactions were carried out at C for 3-4h on an oil bath using dibutyl tin dilaurate (DBTDL) as a catalyst (Figure 2). When the desired percentage of free NCO as determined by dibutylamine back titration [9] was achieved, the prepolymers were neutralized with triethyl amine (TEA). The polyurethane ionomers thus formed were subsequently dispersed in water and chain extended with ethylene diamine at 55 C to obtain 35% w/w solid content resins. N-methyl-2-pyrrolidone (NMP) was employed as a processing aid and co-solvent. The general characteristics of all OMPUs synthesized in this way are discussed in Table 2, also in comparison with PUD-5 and an acrylic latex.

3 Analysis Both mechanical and performance properties of the coatings were tested. Hardness was measured with a Shore A Hardness Tester and also with the conventional pencil hardness test. Flexibility was tested by the conical mandrel (1/8") method. The coating adhesion was checked by a dry tape adhesion test. Impact resistance was measured by a Falling Block Impact Tester (Komal Scientific, India). The particle size of the dispersions was determined using a Particle Size Analyzer model "SALD 1100", Shimadzu, Japan. All properties were evaluated according to ASTM methodologies unless otherwise specified. Preparation of Test Specimen For physico-mechanical and other performance tests unpigmented films were prepared by applying the resins on pretreated (degreased, derusted and zinc phosphated) mild steel panels at 30 mm film thickness. Coatings were allowed to air dry at room temperature in fully ventilated atmosphere and were subjected to testing only after 7 days to ensure full through-cure. LIFELINES -> Dr. Vilas D. Athawale is Senior Professor in the Department of Chemistry of Mumbai University in India. He has participated widely in programs for the development of scientific and technological education. The author s special research interest includes the study on blends, grafting, liquid crystallin polymers, coatings, interpenetrating polymer networks and chemoenzymatic synthesis. -> Sunil N. Peshane is a research scholar at the Department of Chemistry, University of Mumbai working under the guidance of Professor Vilas D. Athawale.

4 Figure 1: Synthesis of Oil-esters. Figure 2: Synthesis of waterborne Oil-modified polyurethanes.

5 Figure 3: Structure of Different Fatty Acid Chains Present in Oils.

6 Figure 4: Oxidative Crosslinking of Oil-modified Polyurethanes.

7 Figure 5: Trends in Scratch Hardness (Shore A). Figure 6: Trends in Impact Resistance (Direct / Reverse).

8 Figure 7: Trends in Drying Times.

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