Study on UV Curable Waterborne Polyurethane Modified by Epoxy Resin Wenyuan Li 1, a, Youming Cao 1, b, Xinqi Zhou 1, c

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1 Applied Mechanics and Materials Vols (2013) pp (2013) Trans Tech Publications, Switzerland doi: / Study on UV urable Waterborne Polyurethane Modified by Epoxy Resin Wenyuan Li 1, a, Youming ao 1, b, Xinqi Zhou 1, c School of Materials and Energy, Gongdong University of Technology, Guangzhou Higher Education Mega enter, Guangzhou, , hina a leewenyuan@126.com, b youmingcao@sina.com, c zhouqi @163.com Keywords:UV curable; waterborne polyurethane; modification; epoxy resin; coating Abstract. In this paper, UV curable waterborne polyurethane emulsions were prepared by fractional step method using toluene diisocyanate (TDI), polyethylene glycol (PEG, MW=1000), α,α-dimethylol propionic acid (DMPA), 2-hydroxyethyl methacrylate (HEMA) as raw material, and UV-WPUs were modified with epoxy resin. The effect of different content of epoxy resin on the viscosity, the water resistance, and other properties was investigated. The synthesized polyurethane structure was confirmed by the Fourier transform infrared spectroscopy (FTIR). The FTIR analysis identified that the target product was expected epoxy modified waterborne UV polyurethane. And with H content increasing, the emulsion viscosity of the WPU increased and the water absorption of the films increased; with epoxy content increasing, the viscosity of the WPU emulsion increased and the water resistance of the films was improved. Introduction Since 1960s, countries around the world have formulated the corresponding environmental regulations, and the V (Volatile organic compounds) emissions of coatings have become stricter and stricter. Study of environment-friendly coatings, which have low V and low toxicity, has got more and more attention [1]. As one of the replacements of solvent-based coatings, UV curable waterborne polyurethane (UV-WPU) coatings have become the most favorable domestic and industrial coatings, because of their non-toxic, pollution-free, low energy consumption, easy operation and rapid curing rate [2]. ombining UV curable technology and waterborne coatings, UV-WPU coatings possess greater environmental and technological advantages, meeting the current green trend of environmental protection and representing the future of the coatings industry [3]. Waterborne polyurethane has such many advantage such as excellent adhesion, abrasion resistance and what mentioned above, but it also exists some shortages compared with solvent-based polyurethane, like low solid content, unideal heat resistance and water resistance and so on [4]. As reported, WPUs modified with acrylate, organosilicon or epoxy resin, have better mechanical properties and their solid content, adhesion and water resistance were improved. It is known that epoxy resin has high modulus, high strength, high adhesion, thermal stability, low shrinking rate, etc [5, 6].Generally, there are three ways to modify polyurethane with epoxy resin. The first one is mechanical blending; the second one is the graft copolymerization of polyurethane with epoxy resin; the third is that the epoxy open loop and react with -N of the prepolymer of WPU. In this paper, UV curable waterborne polyurethane emulsions were prepared by fractional step method using toluene diisocyanate (TDI), polyethylene glycol (PEG, M W =1000), α,α-dimethylol propionic acid (DMPA), 2-hydroxyethyl methacrylate (HEMA) as raw material, and UV-WPUs were modified with epoxy resin. As a result, epoxy modified WPU, which had high solid content, better water resistance, higher hardness, were gained. Experimental Material. Toluene diisocyanate (TDI, Tianjin Jinfeng o. Ltd.); Polyethylene glycol (PEG, Tianjin Baishi hemical o. Ltd.); α,α-dimethylol propionic acid (DMPA, Shanghai hunjin Reagent o. Ltd.); 2-hydroxyethyl methacrylate (HEMA, Shanghai hunjin Reagent o. Ltd.); N-methyl All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /11/12,04:01:18)

2 Applied Mechanics and Materials Vols pyrrolidone (NMP, Tianjin Damao hemical reagent o., Ltd.); Triethylamine (TEA, Tianjin Fuyu fine chemical o. Ltd.); Epoxy resin (E44, Guangdong Nanhai Danzao Yongxiong hemical Plant); Acetone (Hengyang Kaixin hemical Reagent o. Ltd.); Irgacure 500(BASF ompany); Deionized water (Laboratory Preparation). Synthesis and modification of UV-WPU. Waterborne polyurethane was synthesized by reaction of isocyanate with active hydride. When isocyanate reacted with nucleophilic reagent containing active hydrogen, the oxygen atom of -N group accepted hydrogen and hydroxyl group was formed. But the hydroxyl group linking with unsaturated carbon atom was very unstable and it caused intramolecular rearrangement to generate urethane group. The reaction was as follow: R 1 N + H R 1 N H R 1 Synthesis route of waterborne polyurethane was shown in Figure 1. The specific synthesis procedures of UV-WPU were as follows: PEG and DMPA were vacuum-distilled at 100 in the vacuum drying oven for 12h to remove water. TDI was added first into the round-bottom, four-necked flask with mechanical stirrer, nitrogen inlet, pressure-equalized dropping funnel and thermometer. The whole reaction must be dried and carried out in the protection of nitrogen. When the temperature rose to 40-50, PEG was added into the system and reacted at for 2h. Prepolymer 1 was synthesized. When the system was cooled to 40-50, DMPA (dissolved with NMP) and epoxy resin were added into the system and reacted at for 2h. Prepolymer 2 was gained. When cooled to 40-50, HEMA and suitable inhibitor were added and reacted at for 2.5h. Prepolymer 3 was obtained. After cooling, TEA was added to react with carboxylic group for 30min. Finally high speed stirring rate was used to emulsify the solution for 15min after suitable deionized water was added into the reaction system. Then UV curable waterborne polyurethane emulsion was obtained. Preparation of WPU films. Photoinitiator (Irgacure 500) weighted 2wt.% of WPU emulsion was added into WPU emulsion and mixed. After stirred uniformly, the UV-WPU coating was smeared on a glass plate with a coating spreader. The coated film was dried at 60 for 20min and then cured by a laboratory UV curing machine (UVA201-20, Shanghai Pushen hemical Machinery o. Ltd.). The UV wavelength was 365nm, and the light intensity was 100W/m. The UV curing process was as follows: First, Photoinitiator in the system was decomposed under the UV irradiation and free active radicals generated. hv AR A + R Second, the free active radicals attacked the double bonds of prepolymer and chain propagation reaction occurred. R + R Third, the chain propagation reaction continued and the network structure was formed by crosslinking with each other. At last, the cured film was obtained.

3 844 Applied Mechanics and Mechanical Engineering III H R 1 H + N N PEG1000 TDI N R 1 N Prepolymer 1 H R 3 H H DMPA R 5 H Epoxy resin N R 1 R 3 R 5 Prepolymer 2 H H 3 H 2 R 4 H HEMA HN R 1 R 3 R 5 Prepolymer 3 H R 4 H 2 H 3 H 2 2 H 5 N 2 H 5 2 H 5 TEA HN R 1 R 3 R 5 Polymer 1 NEt 3 R 4 H 2 H 3 H 2 R 1 = = H(H 2 H 2 ) n R 3 = H 3 R 4 = H 2 H 2 H 3 H 2 R 5 = H 2 H H 2 H 2 H H 3 H 2 H 2 H H 2 H 2 H H 2 Fig 1 Synthesis Route of epoxy modified UV-WPU by fractional step method Measurement. Infrared spectrum of WPU sample was obtained on a Fourier transformed infrared spectrometer (FTIR, 6700, Nicolet Instrument orporation, USA). The viscosity of the WPU emulsion was measured by a rotational viscometer (NDJ-1B, Shanghai hangji Geological Instrument o. Ltd.). Water absorption was measured by immersing a film in deionized water at room temperature for 24h according to hina National Standard GB/T And the water absorption (w) was calculated by the following formula: w= ( m m0 ) m 100% 0 Where m 0 is the weight of dried film before soaking and m is the weight of the film that has absorbed water and wiped off its surface water. H 3

4 Applied Mechanics and Materials Vols Pencil hardness of WPU film was measured by a portable pencil scratch test instrument (QHQ-A, Tianjin Yonglida Material Testing Machine o. Ltd.) according to hina National Standard GB/T Adhesion force of WPU film was measured by a row lattices instrument of film (QFH, Tianjin Yonglida Material Testing Machine o. Ltd.) according to hina National Standard GB (89). Results and discussions In order to study the effect of -H content and epoxy content on properties of WPU, such as viscosity, water resistance, hardness and adhesion, a series of non-modified WPUs with -H content changing from 4% to 8% were synthesized. Meantime, when other synthesis condition is the same, epoxy modified WPUs with different epoxy resin weight percentage changed from 1% to 5% were prepared. Research results and discussions are as follows. The structure characterization of UV-WPU. The FTIR spectra of pure epoxy resin and WPU modified with epoxy were shown in Fig 2. A cm -1 B Wavenumber(cm -1 ) A: Epoxy resin, B: WPU modified with epoxy Fig 2 The FTIR spectra of the epoxy resin and WPU modified with epoxy A is the pure epoxy resin, and B is the WPU modified with epoxy resin. In the curve A, 3487cm -1 (-H)is one of the characteristic absorption peaks of epoxy resin, and 1296cm -1 is the absorption peak of the quaternary carbon atom in the epoxy resin. Besides, peaks at 829cm -1 and 916cm -1 represent epoxy group. In the curve B, these typical absorption peaks at 3286cm -1 (N-H), 2875cm -1 (H 2 and H 3 ), 1716cm -1 (=), 1539cm -1 (N-H), and 1107cm -1 (--), evidenced the existence of urethane in synthesized WPU polymer. Additionally, the other typical absorption peaks at 1668cm -1 (=), 1454cm -1 (=H 2 ) and 833cm -1 (=H) indicated that = bonds of HEMA had been incorporated in the polyurethane chains. It was needed to note that the absorption peak at 2270cm -1 (-N) did not appear, and it proved the -N group has been completely reacted. ompared with A, the absence of the absorption peaks at 3487cm -1, 829cm -1 and 916cm -1 implied that the -H group and the epoxy groups of epoxy resin had already been reacted with -N group. And the existence of the peak of the quaternary carbon atom at 1300cm -1 proved that epoxy group had been grafted in polyurethane chain. So the product was the expected target epoxy modified WPU, namely, the WPU modified with epoxy resin was successfully synthesized.

5 846 Applied Mechanics and Mechanical Engineering III Viscosity characteristics of WPU emulsion. For the WPU emulsion, the system viscosity is a very significant, because it influences on the processability and other properties of coatings. Fig 3 shows the relationship between the viscosity of WPU emulsion and different -H content (a) and epoxy content (b) Viscosity(Pa.s) Viscosity(Pa.s) H(%) Epoxy content(%) (a) Fig 3 Relationship between viscosity and -H content (a) or epoxy content (b) It can be seen from Fig.3.a that the viscosity of WPU increased with the raising -H content. Because as -H content raised, the content of carboxylate anions in the polyurethane chains increased, and intermolecular interaction increased accordingly. The movement of polyurethane chains became difficult. As a result, the viscosity of WPU emulsion increased. Besides, the relationship between viscosity and epoxy content is shown in Fig.3.b. The viscosity of emulsion increased slowly when the epoxy content was less than 2%; and the viscosity of emulsion rapidly increased, when the epoxy content was more than 2%. The intermolecular interaction force of polyurethane chains increased obviously with increasing epoxy content, because more rigid benzene rings which were introduced into polyurethane chains, which caused the increasing steric hindrance effects. As a result, movement of polyurethane molecule chains was restricted, leading to the increasing viscosity. n the other hand, the crosslinking structure formed by reaction between -H group of epoxy resin and -N of WPU prepolymer was another importance factor that brought into increasing viscosity. When epoxy resin content is over 4%, the emulsion viscosity could be too high, and excessive viscosity may result in the emergence of gels. Water resistance of WPU films. In general, the poor water resistance of waterborne polyurethane limits its application, so it is necessary to improve the water resistance of WPU. (b) 90 Water absorption(%) H(%) Water absorption(%) Epoxy content(%) (a) Fig 4 Relationship between water absorption and -H content (a) or epoxy content (b) (b) The water absorption of non-modified WPU films with different -H content and epoxy modified WPU films with different epoxy content are shown in. Fig 4.a and Fig 4.b, respectively.

6 Applied Mechanics and Materials Vols As shown in Fig 4.a, it is found that water absorption increased from 35.7% to 80.9% rapidly as -H content increasing from 4% to 8%. Because -H group was reacted with TEA to neutralize and form a salt, that provides WPU with hydrophilicity. With -H content increasing, the content of carboxylate anions increased and the hydrophilic abilities of WPU films were enhanced accordingly. onsequently, water absorption of WPU films increased. In other words, water resistance of WPU films decreased. As shown in Fig 4.b, the water absorption of non-modified WPU film was 80.9%. As epoxy content changing from 1% to 5%, the water absorption decreased from 43.5% to 8.25% and the downtrend slowed down. That implied that modification with epoxy can improve water resistance of WPU films. It can be ascribed to the crosslinking structure. Epoxy has many hydroxyl groups that can react with -N group of WPU prepolymer, and then branch points were introduced into polyurethane chains. Therefore, crosslinking structure was formed so that it was difficult for water molecules to go into WPU chains to swell. So the water absorption of WPU films decreased and their water resistance was improved. onsidering emulsion viscosity and water absorption of the films, -H content at 4% is suitable for non-modified waterborne polyurethane. And as for epoxy modified waterborne polyurethane, epoxy content should be at 2%~ 3% and the WPU on this condition has better performance. Hardness and adhesion of WPU films. Table 1 shows the relationship between hardness, adhesion and epoxy content. As shown in Table 1, with the increasing epoxy content, the transparence of WPU emulsion turned from transparent to translucent, even opaque and the hardness and adhesion of WPU films presented rising trend on a whole. As mentioned above, crosslinking structure was formed by epoxy resin modification, which improved the crosslinking density. Greater crosslinking density was benefit to the hardness and adhesion. Taking appearance, viscosity, water resistance, hardness and adhesion of WPU emulsion and the curing films into consideration, the WPU with -H content 4% and epoxy content 2%~ 3% had better performance. onclusions Table 1 Relationship between epoxy content and properties of WPU films Epoxy content(%) Appearance Hardness Adhesion 0 Transparent 3H Grade 3 1 Transparent 4H Grade 3 2 Transparent 4H Grade 2 3 Transparent 4H Grade 2 4 Translucent 4H Grade 1 5 paque 5H Grade 1 In this paper, UV curable waterborne polyurethane were prepared using toluene diisocyanate (TDI), polyethylene glycol (PEG,MW=1000), α,α-dimethylol propionic acid (DMPA), 2-hydroxyethyl methacrylate (HEMA) as raw material, and UV-WPUs were modified with epoxy resin. The product obtained had fine transparence, good stability and low water absorption. And the WPU with -H content 4% and epoxy content 2% had better performance. The major conclusions are as follows: (1) FTIR analysis shows that the product is expected UV-WPU modified with epoxy resin. (2) With -H content increasing, the viscosity of WPU emulsion and the water absorption of the WPU films both increase. The appropriate -H content is 4%. (3) With epoxy resin content increasing, the viscosity of WPU emulsion increase. But the water absorption of the WPU films decrease rapidly. The appropriate epoxy content is 2%~ 3%.

7 848 Applied Mechanics and Mechanical Engineering III Acknowledgment Financial support of Guangdong Province 211 Subject onstruction (No ) is acknowledged. References [1] T. Zhang, W.J. Wu, X.J. Wang, Y.P. Mu. Effect of average functionality on properties of UV-curable waterborne polyurethane-acrylate. Progress in rganic oatings, 68 (2010) [2] H.D. Hwang, H.J. Kim. Enhanced thermal and surface properties of waterborne UV-curable polycarbonate-based polyurethane (meth)acrylate dispersion by incorporation of polydimethylsiloxane. Reactive and functional polymer, 71 (2011) [3] H.J. Dong, Y.K. Eun, S.K. Young. High solid and high performance UV cured waterborne polyurethanes. olloids and Surfaces A: Physicochemical and Engineering Aspects. 370 (2010) [4] Y.B. hen. Y.Z. Xing, et al. A new UV curable waterborne polyurethane: Effect of = content on the film properties. Progress in rganic oatings, 55 (2006) [5] F. Wang, J.Q. Hu, W.P. Tu. Study on microstructure of UV-curable polyurethane acrylate films. Progress in rganic oatings, 62 (2008) [6] Y.W. Bao, L. Tian, J.H. Gong. Structure and Properties of UV-urable Waterborne Polyurethane/Acrylate omposite Resin. Advanced Materials Research. 177 (2010):