Analysing monomer performance Three types of reactive diluents were studied in detail regarding their performance in polyurethane acrylate based UV curing coating formulations. The results indicate that HDDA is superior to DPGDA and TPGDA in mechanical properties, whereas TPGDA exhibits better gloss. The effect of difunctional reactive diluents on the properties of UV-curable polyurethane acrylates Radiation-curable coatings offer a solution to the problems associated with the conventional coating techniques. Indeed there are many advantages of UV curing over the conventional processes. In this article, polyurethane acrylate as an oligomer has been looked at in combination with various reactive diluents and their effect the mechanical, optical, chemical and stain-resistance properties in a UV-cured coating film. An important feature of the activities of the coating industry is the provision of protective, decorative and economically viable coatings for the wood, paper, flooring and plastic industries. Conventional curing processes are based on solvent-based formulations, which release environmental pollutants in the form of volatile organic compounds (VOC) and other hazardous air pollutants (HAP) into the atmosphere [1,2]. Increasing awareness of the environment has emerged as the driving force in the search for alternative methods such as the radiation-curing process [3,5]. This technology uses a polymerisation technique which transforms a liquid resin in seconds into completely solid polymer that is insoluble in organic solvents and very resistant to heat as well as chemical and mechanical treatments. [6, 7] A UV-cured material consists of three components [8]: a) A resin, i.e. an oligomer or a prepolymer containing double-bond unsaturation or a cyclic structure capable of ring opening. b) Reactive diluents i.e. monomers with varying degrees of unsaturation. Monomers have dual function: they reduce the viscosity of the systems and also render cross-linking possible. c) A photoinitiator capable of absorbing UV radiation and generating reactive species which in turn can initiate polymerisation. Here we are considering the preparation and properties of UV curable polyurethane (PUR) acrylates. NCO terminated PUR prepolymers were synthesised from toluene diisocynate (TDI) and polyester polyol (made up of 1,6hexane diol, adipic acid and ethylene glycol) followed by tipping with 2-hydroxy ethyl methacrylate (HEMA). In addition, three types of reactive diluents: 1,6 hexane diol diacrylate (HDDA), dipropylene glycol diacrylate (DPGDA) and dipropylene glycol diacrylate (TPGDA) were used in different concentrations. A series of coating formulations having varying ratio of PUR acrylate and reactive diluents were prepared, and their mechanical, chemical, optical and stain resistance properties studied. Experimental Synthesis of the polymer The purity of the IPDI used was 98% and was used undiluted. This material as well as adipic acid (LR) grade, 1,6 hexane diol, ethylene glycol (LR) grade and HEMA, hydroquinone, the reactive diluents and photo-initiators were supplied by major manufacturers. The synthesis of the various polymers was carried out using the following methods: For urethane acetate, initially ethylene glycol, adipic acid and 1, 6 hexane diol were mixed in a three neck flask round bottom flask in the molar ratio of 0.8 : 1.7 : 2.0 respectively. The esterification reaction was catalyzed by adding 0.1% of dibutyl tin oxide. The reaction temperature was 140 C -150 C. An inert atmosphere was maintained by blanketing dry N2. The acid and hydroxyl values of the resin were checked at defined time intervals. (Figure 1) For urethane acrylate, a reaction flask filled with 2.2 moles of TDI under dry N2 gas at 50 C and blend of 1.2 moles of HEMA with hydroquinone (25 ppm) was added continuously over a period of half an hour at a maximum temperature of 60 C. The reaction mass was kept at 60 C for half an hour to assure the complete conversion of the hydroxyl group of HEMA. In the case of polyurethane acrylate, the temperature of the reaction flask containing urethane methacrylate precursor was maintained at 50 C. A mixture of hydroxyl-functional polyester and hydroquinone (25 ppm) was added over 30 minutes. An inert-gas atmosphere was maintained in the reaction flask throughout the reaction period with blanketing dry N2 gas. The completion of reaction was determined by monitoring the hydroxyl value of the resins at defined intervals. (Figure 2) An FTIR spectrum of PUR-acrylate resin was recorded using a NaCl cell on a FT-IR spectrophotometer. The molecular weight of the resin was determined by GPC using polystyrene as a standard and THF was used as an eluent. The hydroxyl and acid values of the resin were obtained by titration. Preparation of the coatings Coating samples were prepared by using resin, hydroxyl cyclo hexyl phenyl ketone. (HPCK) (25% by weight) and reactive diluents in four different molar ratios: 50:50, 60:40, 70:30, 80:20 (The resin was a monomer) designated as sample code A, B, C, D. The viscosity of the coating samples were determined using a Brookfield viscometer and the refractive indices of the coating samples were measured using a Mettler refractometer. (Toledo RE 40D). (Table 1) Application and curing of the films The UV-cured, PUR-acrylate films were prepared by applying the various coating formulations on mild-steel panels using a bar applicator. For the evaluation of various mechanical and chemical resistance properties, the dry film thickness of the coating films was kept as far as possible at 12 microns. The panels were then exposed to UV lamp chamber having following radiation details: UV dryer type GT Ultra Cure 250 Curing width: 250mm Bulb type: UV (medium pressure) GT 250 pure
Transparent: 120 watt/cm Silica-quartz Air cooling: 100mm Operating-voltage: 440 volts 3 phase a.c. Speed: 50 cycles Power consumption: 120 w/cm for UV Testing procedures Panels of the various coating formulations were cured in a UV chamber and were tested for different mechanical and chemical resistance properties using the following test methods: gloss at 60 (ASTM: D-523-99), scratch hardness (ASTM: D-5178), pencil hardness (ASTM: D-3363-00), chemical resistance (EN: 438-2:1991) and stain-resistance test (ISO-4211). The adhesion properties were measured by using a crosscut adhesion tester. The tester consists of die made up of 9 parallel blades, 1/16 inches apart and 1 mm long. The die is pressed into the panel in two directions right angles to each other. A strip of self-adhesive tape was applied to the pattern, left in contact for 10 seconds and stripped rapidly by pulling the tape back on itself at an angle of approx. 120. The test was rated good if 5% of squares were removed. The mar resistance of the coating films was determined using an automatic tester with a hardened-steel, hemispherical point of 1 mm diameter as a scratching needle. Pencil hardness was measured wih a pencil hardness tester, and gloss was determined using a triglossometer. Flexibility measurements were carried out on a ¼ inch mandrel bend tester. To measure stain resistance the ISO-4211 test was used. Panels were coated with test materials and cured in a UV chamber. The coated panels were kept at room temperature for 24 hrs. Droplets of tea, coffee and hot water were pipetted onto the films and covered with glass cups for 24 hours to prevent evaporation. The films were carefully cleaned and the films surface was analysed. To determine the alkali resistance, coating films were exposed to a 0.1N NaOH solution. Infrared confirms absence of residual NCO In the IR spectrum of the PUR-acrylate resins (Figure 3), a weak band 1180 cm-1 is attributed to C-N stretching. The band at 1651 cm-1 confirms the formation of a urethane group. The band at 1534 cm-1 is attributed to N-H deformation. The strong band at 1735 shows the presence of an ester-carbonyl group of the acrylate present in PUR-acrylate resin. Further a strong band at 2963 cm-1 is believed to be due to C-H stretching and a strong band at 3345 cm-1 is thought to arise from OH stretching, due to the residual hydroxyl group present in the resin. Moreover, the absence of any band at 2270 cm-1 in the spectrum of PUR-acrylate resin confirms that no unreacted NCO group is present. From the GPC results, the molecular weight distribution of the PUR acrylate resin was determined as follows: Mn355Mw1853MP1735Mz4340Polydispersity 5.23This clearly shows that the synthesised resin is a low molecular weight resin. Properties of the coating films Table 2 summarises the test results. Good mar resistance is one of the important mechanical properties of a protective coating. The damage caused by scratches on a cured surface may be minor causing for example a change in the gloss properties of a coating, or be so severe that it causes deformation and finally induces cracking. From Table 2 it becomes clear that the mar resistance decreases with the type of reactive diluents in the order HDDA > DPGDA > TPGDA. This can be attributed to the structures of DPGDA and TPGDA. They contain flexible ether linkages, leading to lower load values required to deform and tear the coating. In the adhesion tests shown in Table 2, all coating samples showed good cross-hatch performance. Pencil hardness tests revealed that HDDA coating films showed the maximum hardness owing to the hydrocarbon backbone in the structure. In the gloss measurements at an angle of 60, the coating films formulated with TPGDA had a higher gloss than those with difunctional monomers. This is because of its trifunctionality, which forms a highly crosslinked structure, which in turn effects an increased angular reflectance of light (Table 2). Films of all the coatings compositions were flexible enough to pass the mandrel test.figure 4 and 5 show the stain resistance and chemical resistance results, respectively. All the coating films exhibit good resistance to distilled water. Samples with a high percentage of HDDA showed better resistance. This is due to the excellent chemical-resistance properties of the hydrocarbon chain present in the structure.dpgda and TPGDA films were attacked by an acid solution This was due to the presence of flexible ether linkages in the monomers. On the other hand, HDDA films showed excellent acid resistance. HDDA films also showed better alkali resistance due to the hydrocarbon backbone present in the monomer, which gives maximum cross-linking (Figure 5).? References: 1. C. Decker, Prog. Polym. Sci., vol.21 (1996), p.593 2. C. Devar, Handbook of Polymer Science and Technology, Cheremisinoff, N.P. (Ed.), vol.3, Marcel Decker, New York, NY, 1989, p.541 3. D. Patel, N.R. Kondekar, Paint India, June (2002), 35 4. J.P.Fouassier, Photoinitiator, Photopolymerisation and Photocuring, Hanser, Munich, 1995 5. J.T.Kanajppu, Paint and Coating Industry, October (2000), 164 6. P.K.T.Oldring, Chemistry and Technology of UV and EB formulation for Coatings, Inks and Paints, vol.1-4, Wiley SITA Technology, London, 1991 7. S.P.Pappas, Radiation Curing Science and Technology, Plennum Press, New York, NY, 1992 8. Swaraj Paul, Surface coatings Science and Technology, John Wiley and Sons, New York, p.717. - UV radiation was effectively used to cure polyurethane acrylate resin in the presence of suitable amounts of difunctional reactive diluents. - UV-cured wood coatings made from these materials exhibit excellent stain resistance properties. - Coating films based on HDDA out-performed those with DPGDA and TPGDA in mechanical properties. - Coating films of TPGDA exhibit better gloss than those of HDDA and DPGDA. THE AUTHORS? Abhishek Srivastava is a research scholar at the department of oil and paint technology of the Harcourt Butler Technology Institute at Kanpur in India. He obtained his B.Sc. degree in chemistry in 1999 and an M.Sc. in analytical chemistry in 2001 from Agra College, Agra (Dr. Bhimrao Ambedkar University, Agra).? Dr. Devendra Agarwal is a professor in paint technology and runs the department of oil and paint technology of Harcourt Butler Technology Institute, where he had obtained an M.Tech, Ph.D. (Tech.) degree. With 25 years teaching research experience, he has a number of international publication
to his credit and has organised and attended several national and international conferences on the subject of coatings and allied products.? Sukhen Mistry is currently a research scholar at the department of oil and paint technology of Harcourt Butler Technology Institute. He obtained his B.Sc. degree in chemistry in 2000 and an MSc degree in Organic Chemistry in 2003.? Dr. Jagbir Singh is deputy general manager-outsourcing and R&D at M/s Jubilant Organosys Limited, Noida in India. He obtained his BSc (Hons) in 1981 in chemistry and an M.Sc in Organic Chemistry in 1983 from Meerut University, Meerutin India, an M.Tech in polymer science in 1985 and a Ph.D in polymers from the Indian Institute of Technology in Delhi. Dr. Singh has seven publication to his credit and attended several international conferences and workshops. * Corresponding Author. Contact: Abishek Srivastavac/o Dr. Devendra AgarwalHarcourt Butler Technological InstituteDepartment of Oil and Paint TechnologyNawabganj, Kanpur-208 002Uttar PradeshIndiaTel. +91 9449958132, Fax +91 512 2533812abhishekhbti27@gmail.com
Figure 4: Stain resistance of coating films. 1 = highly marked, 2 = highly marked, but no modification of the surface appearance, 3 = slightly marked, 4 = very slightly marked, 5 = no marks and no modification of the surface appearance
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Figure 1: Acid and OH value of polyester polyol
Figure 2: Hydroxyl value of PUR-acrylate resin
Figure 3: Infrared spectrum of the PUR acrylate resin