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European Coatings Journal
03/2003
88
Oil-modified PUDs: cross-linkable, VOCcompliant, cost effective
Waterborne Polyurethane Dispersions (PUDs) tend to showundesirably low mechnical performance and chemicalresistance, which is due to their merely physical dryingmechanism. Oil-modified polyurethanes (OMPUs) offer away to introduce additional oxidative crosslinking during filmformation, which is adjustable through the choice of oils thatare incorporated and their degree of unsaturation. Resultsbased on OMPUs modified with linseed, soybean and castoroil - three very cost effective materials - show a considerableperformance improvement.Sunil N. Peshane, Vilas D. AthawaleWaterborne coatings are steadily gaining significance asenvironmentally protective alternative to solvent-basedcoating systems. They are more comparable in theirhandling and processing with conventional, allied coatingsystems. Ever increasing industrial development hasbrought with it volatile organic compounds (VOC).Therefore, in the context of a growing concern for ecology, itis the prerogative of the coatings polymer researchers toreduce drastically the solvent content of coating system byany possible means.In terms of volume, oil-modified polyurethanes (OMPUs) areamong the most important coatings. Due to their excellentfilm formation, they provide superior properties compared toalternative technologies such as polyurethane dispersions(PUDs) and acrylic latex. This is because OMPUs dry byauto-oxidation, resulting in 3-D chemical crosslinking, whilePUDs and latexes dry by coalescence of polymer particleswhich is a mere physical phenomenon. The present workaimed at developing VOC compliant OMPUs withcompetitive properties, while keeping their costs very low atthe same time.The main steps involved in the synthesis were similar to thetwo step prepolymer technique used for waterbornepolyurethane dispersions [1, 2], with the only difference thatthe oils were first converted to oil-esters by alcoholysis toincorporate hydroxyl functions, which act as soft segments.The resins were produced by reacting dimethylol propionicacid (DMPA), poly(propylene glycol) (PPG) andhexamethylene diisocyanate (HDI) with the oil-esters(usually monoglycerides). The main oils used were linseed,soybean and castor oil. The different oils were selectedaccording to their different degrees of unsaturation [3-5] inthe fatty acid chains, leading to different efficiencies of filmformation and extents of crosslinking (Figure 3). This factorpredominantly affects the performance of the cured films,helping to find structure property relationships. OMPUs werecompared with a PUD based on the same isocyanate, whichwas studied earlier [6], and also with a hard acrylic latex.
Mechanical Properties
Hardness (Pencil /Shore A)The pencil hardness results indicated hardness range of Hto 2H for all the coating films under study. No significanttrends and conclusions could be deduced from the pencilhardness test - showing the ineffectiveness of this method.Further evaluations with the Shore A hardness testerconfirmed that the linseed oil based OMPU-1 film showedthe highest hardness value, while castor oil based OMPU-3gave the lowest hardness among all oil-modifiedpolyurethanes. This can obviously be understood in terms ofthe degree of crosslinking of the cured films. The linseed oil
based film was highly cross-linked owing to the higherdegree of unsaturation (linolenic fatty chain content 46 %)(Table 1), resulting in a dense 3-D polymer matrix andcertainly improved properties of OMPU-1 compared to bothOMPU-2 and OMPU-3. OMPU-3, which used castor oil,reflected poor performance among all the OMPUs becauseit did not have any significant linolenic unsaturation, and thelow degree of linoleic (3.1%) and oleic (7.4%) unsaturationwas not high enough to compensate the absence of linolenicchains. However, although OMPU-3 showed lowerhardness, the value was much greater compared to PUD-5,where the crosslinking was almost negligible. The Shore Ahardness of the hard acrylic latex was between that ofOMPU-2 and OMPU-3 (Figure 5). These results clearlyindicated the supremacy of OMPUs over uncrosslinkedPUDs and to some extent over acrylic latex in terms of theirmechanical performance.
FlexibilityAll 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 showedcontrol and balance of the physico-mechanical properties(Table 3).
Impact ResistanceThe balance in mechanical properties was further confirmedby the impact resistance study, in which full-scale (160 in-lb)direct and reverse impact was tolerated by all OMPUs andPUD-5 (Table 3). The acrylic latex film could not sustainfull-scale reverse impact, revealing its brittleness comparedto OMPUs and PUD-5 (Figure 6) and the need of anexternal plasticizer in the acrylic to achieve the benchmarksset by the OMPUs. The excellent balance in properties ofOMPUs is mainly attributed to the fact that oil-esters act assoft domains and contribute to the softness of thepolypropylene glycol (Mn = 2000). At the same time, theglyceride chains are partially demobilised by the cross-linksbetween adjacent chains, avoiding excessive softness.
Drying TimeThe required curing time was estimated according to theIndian Standard IS 101. The drying times of PUD-5 and ofthe acrylic latex film were almost the same. -The differentOMPUs, however, differed in full through-cure time, despiteessentially equal dosages of dryers (Co and Mn metal salts)and identical curing conditions, implying a dependance ofthe drying time on the type of oil-ester used. The linseedoil-ester based OMPU-1 dried very fast compared to allother OMPUs under study, and was competitive to bothPUD-5 and the acrylic latex, with a slight improvement overboth of them (Table 3). The drying performance of thesoybean oil-ester based OMPU-2, although not better thanPUD-5 and acrylic latex, was not below average. Castoroil-ester based OMPU-3, however, gave undesirable longcuring times. This behaviour is again explained by thedegree of unsaturation in the OMPUs, determining the rateof oxidative curing. The highest degree of unsaturation leadsto the fastest curing. During the auto-oxidation of OMPUfilms, 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). Thedrying time trends of the coatings with respect to their set to
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touch and hard drying response are summarized in Figure 7.
Chemical / Solvent ResistanceThe resistance to chemicals of the OMPUs was quiteacceptable in comparison to PUD-5 and the acrylic latex,except for the alkali resistance. This, however, was theweak point of almost all candidates (Table 3). The pooralkali resistance of the OMPUs is obvious because of theglyceride backbones, in which ester groups are easilyattacked and hydrolyzed by alkali. PUD-5 was falling short inalkali resistance for the same reason - here, the estergroups are found in the polyester polyol.OMPUs also showed a better solvent resistance, evaluatedby the double rub test, except for methyl ethyl ketone(MEK), where the coating could withstand only 90 MEKdouble rubs without a considerable damage. The higherperformance of OMPUs over PUD-5 and acrylic latex isonce again attributed to the cross-linked polymer matrix ofOMPUs. The poor performance of PUD-5 demonstrates thatthe usually high chemical and solvent resistance ofpolyurethanes only applies to significantly cross-linkedresins - PUD-5 is a linear (slightly cross-linked)thermoplastic polymer.
ConclusionsThe majority of commercial waterborne PU dispersions arepredominantly linear thermoplastic polyurethanes. Thecoatings formulated with them show relatively poormechanical performance and chemical and solventresistance, compared to cross-linked varieties. Oil-modifiedpolyurethanes (OMPUs) can considerably improve theseperformances, due to their higher inter- and possiblyintra-chain crosslinking, effected by the unsaturated reactivesites in the oil modification. Curing of the PU dispersionschemically modified with oil-esters occurs by coalescence ofthe particles and subsequent oxidative crosslinking.Compared to mere coalescence, which occurs inconventional PUDs, this can enhance the performancedrastically.The selection of proper oil backbone, however, is veryimportant because the degree of unsaturation plays a keyrole, providing different extents of crosslinking and thusperformance. Comparing the OMPUs used in this study,linseed oil based oil-esters can be considered an idealcandidate, and soybean oil the second best choice.Generally, waterborne oil-modified polyurethane dispersionscan achieve an improvement of physico-mechanicalperformance, chemical and solvent resistance, and dryingrates (if proper selection of oil is made) - withVOC-compliant coating materials at pocket friendly prices.
References[1] D. Dieterich, Prog. Org. Coatings, 9 (1981), 281-340.[2] Szycher, M., Szycher's Handbook of Polyurethanes,CRC press, Washington, D. C., 1999.[3] Painter, E. P., and Nesbitt, L. L., Ind. Eng. Chem., Anal.Ed., 15, 123-128 (1943).[4] Hilditch, T. P., and Jasperson, H., J. Soc. Chem. Ind., 58,187-189 (1939).[5] Kaufmann, H. P., and Bornhardt, H., Fette u. Seifen, 46,444-446 (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, FourthEdition, Edited by D. Swern, John Wiley and Sons, Inc.,New York, (1979).[9] Goodman, S., Handbook of Thermoset Plastics, Noyes
Publications, New Jersey, 1986, 252-254.
Results at a glanceVOC compliant, waterborne oil-modified polyurethanes(OMPU) were synthesized by two step prepolymer method,based on oil-esters obtained from linseed, soybean andcastor oil backbones, polypropylene glycol (PPG, Mn = 2000and dimethylol propionic acid (DMPA), and hexamethylenediisocyanate (HDI) as an isocyanate precursor. One-pack,air-drying coatings with oxidative crosslinking - acceleratedby the inclusion of Co and Mn metal salts - were obtained,that are cost effective, environment friendly and showexcellent resistance against water, chemicals and solvents(except resistance to alkali), better drying times, and a blendof acceptable mechanical properties, compared withwaterborne PUD and acrylic latex coatings - further addingvalue for money and offering a good compromise to reduceVOC.
Experimental
MaterialsDimethylol propionic acid (99 %) (DMPA) was purchasedfrom Aldrich, USA. Triethyl amine andN-methyl-2-pyrrolidone (s. d. fine-chem, India) were driedover 4A° molecular sieves for 7 days. Hexamethylenediisocyanate (HDI) was procured from Merck, Germany.Glycerol and the "Fascat 4100" catalyst were purchasedfrom s. d. fine-chem, India). The samples of refined linseedoil, soybean oil and castor oil were obtained from Jayant OilMill, Mumbai, polypropylene glycol (PPG) of molecularweight Mn = 2000 from E. Merck (India) Ltd. All chemicalswere used without any further purification.
Synthesis of Hydroxyl Terminated Oil-estersLinseed and soybean oils were reacted separately withglycerol at around 190°C using a "Fascat 4100" catalyst(0.05 % w/w based on oil) in a three-necked round bottomflask equipped with a mechanical stirrer and watercondenser for 30-35 minutes. These reactions aretechnically termed alcoholysis (Figure 1). The oil-ester(monoglyceride) formation was confirmed by 1:3 methanoltolerance, a clear and homogeneous solution indicatedconversion from oil to the desired product. The productsobtained were discharged into glass stoppered bottles andplaced in vacuum desiccator, before they were used ashydroxyl terminated oil-ester soft segments in furtherreactions. Castor oil was used as such since it alreadycontains a hydroxyl function, contained in a fatty acid chain(ricinoleic acid, 12-hydroxy octadecanoic acid) [7, 8].
Synthesis of OMPUsA resin kettle equipped with thermometer, mechanicalstirrer, nitrogen gas inlet and reflux condenser was chargedwith the preformulated amount of oil-ester, PPG (Mn =2000), DMPA and HDI. The reactions were carried out at80-90 °C for 3-4h on an oil bath using dibutyl tin dilaurate(DBTDL) as a catalyst (Figure 2). When the desiredpercentage of free NCO as determined by dibutylamineback titration [9] was achieved, the prepolymers wereneutralized with triethyl amine (TEA). The polyurethaneionomers thus formed were subsequently dispersed in waterand chain extended with ethylene diamine at 55 °C to obtain35% w/w solid content resins. N-methyl-2-pyrrolidone (NMP)was employed as a processing aid and co-solvent. Thegeneral characteristics of all OMPUs synthesized in this wayare discussed in Table 2, also in comparison with PUD-5and an acrylic latex.
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AnalysisBoth mechanical and performance properties of the coatingswere tested. Hardness was measured with a Shore AHardness Tester and also with the conventional pencilhardness test. Flexibility was tested by the conical mandrel(1/8") method. The coating adhesion was checked by a drytape adhesion test. Impact resistance was measured by aFalling Block Impact Tester (Komal Scientific, India). Theparticle size of the dispersions was determined using aParticle Size Analyzer model "SALD 1100", Shimadzu,Japan. All properties were evaluated according to ASTMmethodologies unless otherwise specified.
Preparation of Test SpecimenFor physico-mechanical and other performance testsunpigmented films were prepared by applying the resins onpretreated (degreased, derusted and zinc phosphated) mildsteel panels at 30 mm film thickness. Coatings were allowedto air dry at room temperature in fully ventilated atmosphereand were subjected to testing only after 7 days to ensure fullthrough-cure.
LIFELINES-> Dr. Vilas D. Athawale is Senior Professor in theDepartment of Chemistry of Mumbai University in India. Hehas participated widely in programs for the development ofscientific and technological education. The author´s specialresearch interest includes the study on blends, grafting,liquid crystallin polymers, coatings, interpenetrating polymernetworks and chemoenzymatic synthesis.-> Sunil N. Peshane is a research scholar at the Departmentof Chemistry, University of Mumbai working under theguidance of Professor Vilas D. Athawale.
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Figure 1: Synthesis of Oil-esters.
Figure 2: Synthesis of waterborne Oil-modified polyurethanes.
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Figure 3: Structure of Different Fatty Acid Chains Present in Oils.
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Figure 4: Oxidative Crosslinking of Oil-modified Polyurethanes.
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Figure 5: Trends in Scratch Hardness (Shore A).
Figure 6: Trends in Impact Resistance (Direct / Reverse).
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Figure 7: Trends in Drying Times.
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