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Synthesis and properties of PDMS modified waterborne polyurethane–acrylic hybrid emulsion by solvent-free method
Progress in Organic Coatings 63 (2008) 238–244
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Short communication
Synthesis and properties of PDMS modified waterborne polyurethane–acrylic hybrid emulsion by solvent-free method Chuyin Zhang, Xingyuan Zhang ∗ , Jiabing Dai, Chenyan Bai Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 25 February 2008 Received in revised form 24 May 2008 Accepted 31 May 2008 Keywords: Polyurethane–acrylic Hybrid emulsion Solvent-free Polydimethylsiloxane
a b s t r a c t A new type of polysiloxane modified polyurethane–acrylic hybrid emulsion was synthesized by solventfree method and the polysiloxane was introduced into the soft segment of polyurethane chains using dihydroxybutyl-terminated polydimethylsiloxane (PDMS). The formed film from the hybrid emulsion could provide obviously higher water-resistance property. The preparation technologies such as the content of carboxy group and acrylic monomer, the rate and the time of emulsification were discussed systematically. The chain structure and the particle size were confirmed by the analysis of Fourier transform infrared spectroscopy and transmission electron microscopy, respectively. The effect of PDMS content on the water resistance and the mechanical property were investigated by absorbed water ratio, water contact angle and dynamic mechanical measurement. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane materials are known to offer high performance with their toughness, abrasion resistance, mechanical flexibility and chemical resistance. Waterborne polyurethane has become one of the major research and development fields in recent years because of its environmental friendliness. During the synthesis of waterborne polyurethane, the water is used as dispersant instead of organic solvent, therefore the emission of volatile organic compounds can be largely reduced. However, in some cases, acetone or other organic solvent has to be added in the synthesis process to reduce the high viscosity of reactive system, and these organic solvents must be removed at last [1–10]. This method not only adds the cost of organic solvent and equipment investment, but also pollutes the atmosphere. In order to resolve the problem, solvent-free process was proposed and has attracted more and more attention [11–20]. Galgoci et al. [20] discussed the cost/performance advantages and disadvantages for the urethane–acrylic hybrid polymer dispersions (HPDs) and N-methylpyrrolidone (NMP)-free versions of HPDs, and found that due to the true hybrid nature similar to an interpenetrating network, NMP-free HPDs could perform favorably in their preparation and some good physical properties compared to analogous solvent-containing HPDs.
∗ Corresponding author. E-mail address: [email protected] (X. Zhang). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.05.011
In this work, we make modifications of the polyurethane–acrylic (PUA) hybrid emulsion by polysiloxane using the solvent-free method. We used dihydroxybutyl-terminated polydimethylsiloxane (PDMS) to incorporate the polysiloxane into the soft segment of polyurethane chains and obtained a new material of PDMS modified waterborne polyurethane–acrylic (Si-PUA). The preparation technology was discussed systematically and the effect of PDMS content on Si-PUAs was investigated.
2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI), Junsei Chemical Co., Ltd.; dimethylol propionic acid (DMPA) and 1,6-hexanediol (HDO) were the products of Aldrich Chemical Company; dihydroxybutylterminated polydimethylsiloxane (PDMS), COH = 60 mgKOH/g, Dow Chemical Company; polypropylene glycol (PPG), Mn = 2000, Daicel Chemical Industries, Ltd.; hydroquinone used as an inhibitor, Luoyang Chemical Reagent Company; butyl acrylate (BA), n-butyl methacrylate (BMA), triethylamine (TEA) used as neutralization agent, di-n-butyltin dilaurate (DBT) used as catalyst, ammonium persulfate (APS) used as initiator and sodium bicarbonate (NaHCO3 ) used as pH buffer solution were all purchased from Shanghai Chemical Reagent Co., Ltd. DMPA, PDMS, PPG and HDO were vacuum desiccated and IPDI was vacuum distilled before using. BA and BMA were used as received.
C. Zhang et al. / Progress in Organic Coatings 63 (2008) 238–244
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Scheme 1. Preparation process of Si-PUA hybrid emulsion.
2.2. Preparation of Si-PUAs The hybrid emulsion of Si-PUA was prepared according to the procedure shown in Scheme 1. IPDI, PPG and PDMS were first added into a dry vessel equipped with a reflux condenser, a mechanical stirrer and a thermometer. The prepolymerization of polyurethane was carried out at 90 ◦ C under N2 atmosphere until the NCO content reached a theoretical value A. Then DMPA as a chain extender was added into the system and reacted at 80 ◦ C until the NCO
content reached a theoretical value B. After it was diluted with suitable acrylic monomer, HDO and catalyst DBT were added to react for another 5 h at 70 ◦ C. During the process, suitable inhibitor was needed, and TEA as a neutralization agent was used to react with carboxy group in the side chain of prepolymer at 35 ◦ C. Finally high speed shearing was used to emulsify the solution after suitable deionized water was added into the reaction system. The PDMS modified polyurethane (Si-PU) aqueous dispersion containing acrylic monomer was obtained. Continued the next
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Table 1 Compositions for Si-PUAs Sample code
PDMSa (wt.%)
IPDI:PPG:PDMS (mol:mol)
DMPA (mol)
HDO (mol)
TEAb (mol)
BA/(BA + BMA) (wt.%)
Si-PUA0 Si-PUA1 Si-PUA2 Si-PUA3 Si-PUA4 Si-PUA5 Si-PUA6
0 1.5 3.5 6.4 8.1 9.8 12
1:0.24:0 1:0.23:0.01 1:0.21:0.02 1:0.19:0.05 1:0.18:0.06 1:0.17:0.07 1:0.16:0.08
0.32 0.32 0.32 0.32 0.32 0.32 0.32
0.39 0.39 0.39 0.39 0.39 0.39 0.39
0.35 0.35 0.35 0.35 0.35 0.35 0.35
51.2 51.2 51.2 51.2 51.2 51.2 51.2
a b
Relative to the total of IPDI + PPG + PDMS + DMPA + HDO. Mole ratio of TEA/DMPA is fixed at 1.1:1 to secure full neutralization.
step, APS aqueous solution was added drop by drop to the abovementioned system while NaHCO3 was used to maintain pH value. The reaction lasted for about 4 h at 80 ◦ C. Finally the emulsion was cooled down to the room temperature and PDMS modified waterborne polyurethane–acrylic (Si-PUA) hybrid emulsion was obtained. Changing the material ratio, a series of Si-PUAs with different PDMS content of 0, 1.5%, 3.5%, 6.4%, 8.6%, 9.8%, and 12% (named as Si-PUA0, Si-PUA1, Si-PUA2, Si-PUA3, Si-PUA4, Si-PUA5, and Si-PUA6, respectively), as shown in Table 1, were synthesized. In order to compare the properties of synthesized materials, some PUAs with different BA (or BMA) content were also synthesized and the compositions are shown in Table 2. The theoretical value of NCO is calculated based on the equation
Tensile strength test was carried out on a tensile tester (Model TY8000, Jiangdu Tianyuan Test Machine Co., Ltd., China) at room temperature with a speed of 50 mm min−1 . All measurements have an average of four runs. The dumbbell type specimen was 30 mm length at two ends, 0.2 mm thickness and 4 mm wide at the neck. The water-resistance property was determined as follows. A latex film was prepared by casting the hybrid emulsion on a leveled PTFE plate, curing at room temperature for 7 days and then vacuum drying at 80 ◦ C for 24 h. The weighed latex film (W0 ) was immersed into distilled water at room temperature for 24 h, followed by wiping off the surface water with a piece of filter paper to determined the weight W1 . The absorbed water ratio A of the film was calculated by the formula A=
(MNCO − MOH ) × 100% NCO% = MNCO
(W1 − W0 ) × 100% W0
MNCO is the mole number of NCO group in diisocyanate whereas MOH is the mole number of OH group. The real NCO content is measured by the standard di-n-butylamine titration method.
3. Results and discussion
2.3. Sample preparation and characterization
In the synthesis process of PUA hybrid emulsion by solvent-free method, it was found that the content of carboxy group and acrylic monomer, the rate and the time of emulsification had an important influence on the appearance of emulsion, the film formation and film properties.
The size and morphology of the emulsion particles were viewed on a Jeol JEM-100SX transmission electron microscope (TEM). The sample was prepared by depositing the emulsion onto a copper net after being stained by phosphor-wolframic acid. A thin latex film (thickness less than 20 m) for FTIR was directly fixed on a sample frame and measured ranging from 500 to 4000 cm−1 with a Nicolet Magna-IR 750 FTIR spectrometer. Dynamic mechanical analysis (DMA) measurement was carried out on a viscoelastometer (DMTA Mark IV, Rheometric Scientific Inc., USA), with temperature range from −80 to 140 ◦ C at 1 Hz. The heating rate was 5 ◦ C/min, and the dimension of the sample was 6 mm × 60 mm × 0.4 mm. The storage and the loss modulus were calculated by measuring the complex modulus and plotted versus temperature. Water contact angle on the cast film was measured at 25 ◦ C by the sessile-drop method using a contact angle goniometer (JC2000C1, Shanghai Zhongchen Digital Technical Equipment Ltd., China) and the reported results were the mean values of five times.
3.1. Investigation of preparation technology for PUA
3.1.1. Effect of carboxy group content In this experiment, we fixed the acrylic monomer content and the emulsification condition to investigate the effect of COOH content, the results were shown in Table 3. It was found that the hybrid emulsion could not be formed when the COOH content was lower than 1.7%. When the content was higher than 1.7%, the hybrid emulsion became more and more transparent but the characteristic of water resistance for the formed film became poor with increasing the COOH content. In the followed radical polymerization for BA with BMA, the polyurethane prepolymer acts as a macromolecular emulsifier, and the more the COOH content, the good the emulsification effect. Considering the characteristic of water resistance for the formed film, the optimum COOH content was between 1.9% and 2.1%.
Table 2 Compositions for PUAs Sample code
IPDI:PPG (mol:mol)
DMPA (mol)
HDO (mol)
TEAa (mol)
BA/(BA + BMA) (wt.%)
PUA1 PUA2 PUA3 PUA4 PUA5
1:0.24 1:0.24 1:0.24 1:0.24 1:0.24
0.32 0.32 0.32 0.32 0.32
0.39 0.39 0.39 0.39 0.39
0.35 0.35 0.35 0.35 0.35
14.6 28.0 51.2 81.7 100
a
Mole ratio of TEA:DMPA is fixed at 1.1:1 to secure full neutralization.
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241
Table 3 Effect of carboxy group content Test sample
COOH content (wt.%)
Emulsion appearance
Appearance of formed film dipped into water for 24 h
1 2 3 4 5
1.7 1.9 2.1 2.3 2.5
White emulsion with seston Sub-transparent Sub-transparent Transparent Transparent
No film forming Film became a little white Film became a little white Film became white Film swelled
3.1.2. Effect of acrylic monomer content In the solvent-free method, acrylic monomers were used as dilution agent to reduce the viscosity of the polyurethane prepolymer. The more the acrylic content, the lower the viscosity of reaction system. It was found from Table 4 that when the acrylic content was higher than 40%, the acrylic monomer could not be wellencased by polyurethane emulsifiers, the emulsion appearance changed from transparent to sub-transparent and some concretionary appeared in the emulsion which resulted in the deposition and poor stability of the emulsion. Moreover the compatibility between the polyurethane prepolymer and polyacrylate was not well, and the last formed PUA film had poor performance. Combined the emulsion appearance and the film performance such as the absorbed water ratio, it is suitable to control the acrylic content at about 40% during the synthesis process of hybrid emulsion. 3.1.3. Effect of the emulsification rate and the emulsification time The rate and the time of emulsification had an unneglectable effect on the preparation process of the emulsion. We used different shearing rate and time during the emulsification to observe the effect of prepared emulsion, and results were shown in Table 5. Though the polyurethane prepolymer and the acrylic monomers could be well dispersed into water for the emulsification rate of 1200 or 2000 min−1 , more or less concretionary appeared for the test sample 1–3. Therefore in order to avoid the appearance of concretionary, emulsification rate of 2000 min−1 and emulsification time of 15 min should be used to get the emulsion during the radical polymerization process. 3.2. The structure of Si-PUA by FTIR Fig. 1 illustrates the spectra of polyurethane and PUA1 samples. The characteristic absorption of the C C bond at 1640 cm−1 disappears, indicating that the acrylic monomers have been
Fig. 1. FTIR spectra of polyurethane and PUA1 samples.
polymerized. The absorption at 1450 cm−1 [ı(C H)], 1728 cm−1 [(C O)] becomes larger, (C O C) of ester at 1150 cm−1 and out-of-plane bending vibration of –(CH2 )n – at 748 cm−1 confirm acrylate structure. As the peak at 1538 cm−1 belongs to N H group, we can see that the height of N H peak decreases after the radical polymerization, which indicates the interactions of N H groups with the acrylate component, most likely with the ester C O group. The structure of Si-PUAs is confirmed by FTIR as shown in Fig. 2. The absorption peaks at 3330 cm−1 [(NH)], 2855–2955 cm−1 [(CH2 ) and (CH3 )], 1728 cm−1 [(C O)], 1538 cm−1 [ı(NH)] and 1110 cm−1 [(C O C)] are the typical absorption peaks for the spectra of polyurethane. Compared with the sample of polyurethane without polysiloxane Si-PUA0, the peaks at 1033 cm−1 [(Si O)] and 803 cm−1 [CH3 –Si rocking] can be clearly detected in the spectrum of Si-PUA1, Si-PUA3 and Si-PUA5, indicating that PDMS groups have been successfully introduced into the polyurethane chains. In addition, the peak strength at 803 cm−1 enlarges as the PDMS content increases for the series of Si-PUA samples. 3.3. The morphology of emulsion particles by TEM The microstructure of emulsion particles was observed by TEM. In Fig. 3, the images of emulsion particles of Si-PUA3 and PUA3 are shown at 105 magnifications. The particle is spherical and the average particle size is about 45 nm. As the polyurethane prepolymer could act as macromolecular emulsifier for its hydrophilic carboxy group, the hydrophobic acrylic monomer could migrate into the polyurethane particle adequately before the radical polymerization to achieve the homogeneous particle size. It may be concluded that the outer component is more polyurethane while the inner component is more hydrophobic polyacrylate. It can be found from Fig. 3 that there is little influence to particle configuration when PDMS is introduced into PUA.
Fig. 2. FTIR spectra of Si-PUAs.
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Table 4 Effect of acrylic content Test sample
Acrylic content (%)
Emulsion appearance
Absorbed water ratio (%)
Concretionary
1 2 3 4 5
30 35 40 50 60
Transparent Transparent Transparent Sub-transparent Sub-transparent
13 11.5 8.9 8.5 9.0
No No No A little A little
Table 5 Effect of emulsification rate and time Test sample
Emulsification rate (min−1 )
Emulsification time (min)
Emulsion appearance
Concretionary
1 2 3 4
1200 1200 2000 2000
7 15 7 15
Milk white Sub-transparent Sub-transparent Sub-transparent
More A little A little No
3.4. The DMA analysis Fig. 4 shows the DMA analysis results for Si-PUA3 and PUA3. It can be seen that PUA3 has higher storage modulus E in the region of low temperature. When it reaches the glass transition temperature, the value of E for Si-PUA3 and PUA3 is approximately the same. As E is a measure of material stiffness, the difference of storage modulus is related to the mechanical property of materials.[21] The incorporation of polysiloxane into the soft segment of PUA chains is expected to decrease the mechanical property due to the high flexibility of polysiloxane. Moreover, two samples have the same peaks at about 23 ◦ C, indicative of anneal at testing temperature and crystallization to enlarge the storage modulus. In Fig. 5, there are two distinct transitions, the similarity between the two curves indicating that the addition of PDMS does not influence the phase behavior of PUA. It is reported that the glass transitions of the components of polyurethane and polyacrylate blends are clear in both cases, [22] the broad peak at lower temperature of PUA shows that the compatibility between polyacrylate and polyurethane is good. In our system the acrylic monomer is allowed to diffuse into the polyurethane particles before polymerization, after which the polyacrylate is trapped inside the polyurethane. By the interaction between hydrogen bond and similar C O group polarity, polyacrylate mixes with polyurethane at a certain extent. 3.5. The water contact angle analysis Water contact angle on the surface of Si-PUAs increases with increasing the content of PDMS, as shown in Fig. 6. When PDMS content is below 4%, it influences contact angle greatly, the curve ascends sharply. The contact angle is nearly 97◦ when the PDMS
content increases to about 10%, but above that point the increase of PDMS content has low impact. The increase of contact angle could be attributed to the surface activity of the hydrophobic polysiloxane. When PDMS concentration is low, it migrates to the surface to shield the polymer chain with high surface energy, and substantially lowers the surface free energy. But when the PDMS content increases to a degree where there is enough polysiloxane at the surface, their low surface free energy is up to the limit point. 3.6. The water-resistance property Correlation between absorbed water ratio and content of PDMS for the sample of Si-PUAs is shown in Fig. 7. It is observed clearly that the absorbed water ratio decreases with the increase of the PDMS content. PDMS has an obvious influence on the absorbed water ratio. With PDMS content increases from 0% to12%, the absorbed water ratio decreases from 6.23% to 3.02%, the water absorbability administers obvious decrease with little PDMS content. It can be concluded that by introducing hydrophobic PDMS into soft segment of polyurethane chains, the water-resistance of PUA material can be enhanced easily. 3.7. The mechanical properties The tensile strength and the extensibility of PUA with different BA content is investigated. Fig. 8 shows that by keeping the mass of polyurethane unchanged while increasing the ratio of BA/(BA + BMA), the extensibility of film increases while tensile strength decreases. It can be seen that PUA1 has extensibility of 246% and PUA5 of 613% whereas their tensile strengths are 28.5 and 12.7 Mpa, respectively. As BA is a soft monomer compared
Fig. 3. TEM of Si-PUA3 (A) and PUA3 (B).
C. Zhang et al. / Progress in Organic Coatings 63 (2008) 238–244
Fig. 4. Correlation between log E and temperature for the samples of Si-PUA3 and PUA3.
Fig. 5. Correlation between tan ı and temperature for the samples of Si-PUA3 and PUA3.
Fig. 6. Water contact angle increases with increasing the content of PDMS for SiPUAs.
243
Fig. 7. Absorbed water ratio decreases with increasing the content of PDMS for Si-PUAs.
Fig. 8. Tensile strength decreases and extensibility increases with increasing the BA content for PUA samples.
Fig. 9. Tensile strength decreases and extensibility increases with increasing the PDMS content for Si-PUA samples.
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with BMA, the copolymer with more BMA component becomes hard for its less activity. From the testing result, it is concluded that polyurethane and polyacrylate chains are in interaction with each other at some degree because the change of ratios for different acrylic monomer has great impact on the mechanical property. As is known that the polysiloxane is very flexible and has excellent low temperature properties but poor tensile strength, the PUA films are likely to increase in extensibility and decrease in tensile strength when PDMS is added. Fig. 9 shows the mechanical properties at different content of PDMS for Si-PUAs. It can be seen that the tensile strength decreases but the extensibility increases with increasing the PDMS content. It is also found that both the extensibility and the tensile strength change sharply when the PDMS content is above 6.5%. This indicates little effect of PDMS content on the mechanical properties when PDMS content is below 6.5%. 4. Conclusions A series of Si-PUAs had been synthesized by solvent-free method using PDMS as a soft segment. In the method acrylic monomers acted as dilution agent and the polyurethane having carboxy groups acted as macromolecular emulsifier without using any other solvent and surfactant. The optimum preparation technology for the solvent-free method was achieved when the COOH content was between 1.9% and 2.1%, the acrylic content was 40% and the emulsification rate was 2000 min−1 for 15 min. The chain structure and the particle size were confirmed by FTIR and TEM analysis. The emulsion particle size was uniform and around 45 nm, which ensures better performance of the formed film. The obtained Si-PUA proved to possess higher contact angle and better water resistance when
PDMS content was higher than 6.5%. The mechanical property has great change when PDMS content is above 6.5%. Acknowledgements Financial support from the National Natural Science Foundation of China (No.20073041) and Anhui Provincial Education Department (No. 2004kj362zd) are acknowledged. References [1] C.K. Kim, B.K. Kim, H.M. Jeong, Colloid Polym. Sci. 269 (1991) 895. [2] C.H. Yang, S.M. Lin, T.C. Wen, M.S. Wu, J.S. Chang, Polym. Eng. Sci. 35 (1995) 722. [3] C.H. Yang, H.J. Yang, T.C. Wen, Polymer 40 (1999) 871. [4] T.T. Cheng, T.C. Wen, J. Chin. Inst. Chem. Eng. 29 (1998) 327. [5] K. Lewandowski, L.R. Krepski, D.E. Mickus, J. Polym. Sci. Polym. Chem. 40 (2002) 3037. [6] C. Zhou, X.Y. Zhang, J.B. Dai, H.P. Zhang, Chin. J. Chem. Phys. 18 (2005) 448. [7] C.Y. Bai, X.Y. Zhang, J.B. Dai, W.H. Li, Prog. Org. Coat. 55 (2006) 291. [8] C.Y. Bai, X.Y. Zhang, J.B. Dai, C.Y. Zhang, Prog. Org. Coat. 59 (2007) 331. [9] C.Y. Bai, X.Y. Zhang, J.B. Dai, Prog. Org. Coat. 60 (2007) 63. [10] J.B. Dai, X.Y. Zhang, J. Chao, C.Y. Bai, J. Coat. Technol. Res. 4 (2007) 283. [11] L. Peter, V.D. Kennetha, U.S. patent 4,644,030A (1987). [12] D. Richard, G.A. Bruce, V. R.Bheema, U.S. patent 5,173,526 (1992). [13] H.E. Richard, U.S. patent 5,508,340A (1996). [14] K. Eberhard, W. Dirk, G.U. Fritz, U.S. patent 5,672,229 (1997). [15] J.J. James, W.R. Wayne, U.S. patent 5,959,027A (1999). [16] S. W. Kenneth, T.L. Rick, L. Pault, W.R. Wayne, U.S. patent 6,087,440A (2000). [17] B. Hans, K. Wolfgang, H.Guenter, U.S. patent 2,003,083,428 (2003). [18] B. Vogtbirnbrich, Prog. Org. Coat. 29 (1996) 31. [19] Y. Nomura, A. Sato, S. Sato, H. Mori, T. Endo, J. Polym. Sci. Polym. Chem. 45 (2007) 2689. [20] E.C. Galgoci, C.R. Hegedus, F.H. Walker, D.J. Tempel, F.R. Pepe, K.A. Yoxheimer, A.S. Boyce, JCT. CoatingsTech. 2 (2005) 28. [21] D.M. Crawford, J.A. Escarsega, Thermochim. Acta. 357 (2000) 161. [22] C.R. Hegedus, K.A. Kloiber, J. Coat. Technol. 68 (1996) 39.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Short communication
Synthesis and properties of PDMS modified waterborne polyurethane–acrylic hybrid emulsion by solvent-free method Chuyin Zhang, Xingyuan Zhang ∗ , Jiabing Dai, Chenyan Bai Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 25 February 2008 Received in revised form 24 May 2008 Accepted 31 May 2008 Keywords: Polyurethane–acrylic Hybrid emulsion Solvent-free Polydimethylsiloxane
a b s t r a c t A new type of polysiloxane modified polyurethane–acrylic hybrid emulsion was synthesized by solventfree method and the polysiloxane was introduced into the soft segment of polyurethane chains using dihydroxybutyl-terminated polydimethylsiloxane (PDMS). The formed film from the hybrid emulsion could provide obviously higher water-resistance property. The preparation technologies such as the content of carboxy group and acrylic monomer, the rate and the time of emulsification were discussed systematically. The chain structure and the particle size were confirmed by the analysis of Fourier transform infrared spectroscopy and transmission electron microscopy, respectively. The effect of PDMS content on the water resistance and the mechanical property were investigated by absorbed water ratio, water contact angle and dynamic mechanical measurement. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane materials are known to offer high performance with their toughness, abrasion resistance, mechanical flexibility and chemical resistance. Waterborne polyurethane has become one of the major research and development fields in recent years because of its environmental friendliness. During the synthesis of waterborne polyurethane, the water is used as dispersant instead of organic solvent, therefore the emission of volatile organic compounds can be largely reduced. However, in some cases, acetone or other organic solvent has to be added in the synthesis process to reduce the high viscosity of reactive system, and these organic solvents must be removed at last [1–10]. This method not only adds the cost of organic solvent and equipment investment, but also pollutes the atmosphere. In order to resolve the problem, solvent-free process was proposed and has attracted more and more attention [11–20]. Galgoci et al. [20] discussed the cost/performance advantages and disadvantages for the urethane–acrylic hybrid polymer dispersions (HPDs) and N-methylpyrrolidone (NMP)-free versions of HPDs, and found that due to the true hybrid nature similar to an interpenetrating network, NMP-free HPDs could perform favorably in their preparation and some good physical properties compared to analogous solvent-containing HPDs.
∗ Corresponding author. E-mail address: [email protected] (X. Zhang). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.05.011
In this work, we make modifications of the polyurethane–acrylic (PUA) hybrid emulsion by polysiloxane using the solvent-free method. We used dihydroxybutyl-terminated polydimethylsiloxane (PDMS) to incorporate the polysiloxane into the soft segment of polyurethane chains and obtained a new material of PDMS modified waterborne polyurethane–acrylic (Si-PUA). The preparation technology was discussed systematically and the effect of PDMS content on Si-PUAs was investigated.
2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI), Junsei Chemical Co., Ltd.; dimethylol propionic acid (DMPA) and 1,6-hexanediol (HDO) were the products of Aldrich Chemical Company; dihydroxybutylterminated polydimethylsiloxane (PDMS), COH = 60 mgKOH/g, Dow Chemical Company; polypropylene glycol (PPG), Mn = 2000, Daicel Chemical Industries, Ltd.; hydroquinone used as an inhibitor, Luoyang Chemical Reagent Company; butyl acrylate (BA), n-butyl methacrylate (BMA), triethylamine (TEA) used as neutralization agent, di-n-butyltin dilaurate (DBT) used as catalyst, ammonium persulfate (APS) used as initiator and sodium bicarbonate (NaHCO3 ) used as pH buffer solution were all purchased from Shanghai Chemical Reagent Co., Ltd. DMPA, PDMS, PPG and HDO were vacuum desiccated and IPDI was vacuum distilled before using. BA and BMA were used as received.
C. Zhang et al. / Progress in Organic Coatings 63 (2008) 238–244
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Scheme 1. Preparation process of Si-PUA hybrid emulsion.
2.2. Preparation of Si-PUAs The hybrid emulsion of Si-PUA was prepared according to the procedure shown in Scheme 1. IPDI, PPG and PDMS were first added into a dry vessel equipped with a reflux condenser, a mechanical stirrer and a thermometer. The prepolymerization of polyurethane was carried out at 90 ◦ C under N2 atmosphere until the NCO content reached a theoretical value A. Then DMPA as a chain extender was added into the system and reacted at 80 ◦ C until the NCO
content reached a theoretical value B. After it was diluted with suitable acrylic monomer, HDO and catalyst DBT were added to react for another 5 h at 70 ◦ C. During the process, suitable inhibitor was needed, and TEA as a neutralization agent was used to react with carboxy group in the side chain of prepolymer at 35 ◦ C. Finally high speed shearing was used to emulsify the solution after suitable deionized water was added into the reaction system. The PDMS modified polyurethane (Si-PU) aqueous dispersion containing acrylic monomer was obtained. Continued the next
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C. Zhang et al. / Progress in Organic Coatings 63 (2008) 238–244
Table 1 Compositions for Si-PUAs Sample code
PDMSa (wt.%)
IPDI:PPG:PDMS (mol:mol)
DMPA (mol)
HDO (mol)
TEAb (mol)
BA/(BA + BMA) (wt.%)
Si-PUA0 Si-PUA1 Si-PUA2 Si-PUA3 Si-PUA4 Si-PUA5 Si-PUA6
0 1.5 3.5 6.4 8.1 9.8 12
1:0.24:0 1:0.23:0.01 1:0.21:0.02 1:0.19:0.05 1:0.18:0.06 1:0.17:0.07 1:0.16:0.08
0.32 0.32 0.32 0.32 0.32 0.32 0.32
0.39 0.39 0.39 0.39 0.39 0.39 0.39
0.35 0.35 0.35 0.35 0.35 0.35 0.35
51.2 51.2 51.2 51.2 51.2 51.2 51.2
a b
Relative to the total of IPDI + PPG + PDMS + DMPA + HDO. Mole ratio of TEA/DMPA is fixed at 1.1:1 to secure full neutralization.
step, APS aqueous solution was added drop by drop to the abovementioned system while NaHCO3 was used to maintain pH value. The reaction lasted for about 4 h at 80 ◦ C. Finally the emulsion was cooled down to the room temperature and PDMS modified waterborne polyurethane–acrylic (Si-PUA) hybrid emulsion was obtained. Changing the material ratio, a series of Si-PUAs with different PDMS content of 0, 1.5%, 3.5%, 6.4%, 8.6%, 9.8%, and 12% (named as Si-PUA0, Si-PUA1, Si-PUA2, Si-PUA3, Si-PUA4, Si-PUA5, and Si-PUA6, respectively), as shown in Table 1, were synthesized. In order to compare the properties of synthesized materials, some PUAs with different BA (or BMA) content were also synthesized and the compositions are shown in Table 2. The theoretical value of NCO is calculated based on the equation
Tensile strength test was carried out on a tensile tester (Model TY8000, Jiangdu Tianyuan Test Machine Co., Ltd., China) at room temperature with a speed of 50 mm min−1 . All measurements have an average of four runs. The dumbbell type specimen was 30 mm length at two ends, 0.2 mm thickness and 4 mm wide at the neck. The water-resistance property was determined as follows. A latex film was prepared by casting the hybrid emulsion on a leveled PTFE plate, curing at room temperature for 7 days and then vacuum drying at 80 ◦ C for 24 h. The weighed latex film (W0 ) was immersed into distilled water at room temperature for 24 h, followed by wiping off the surface water with a piece of filter paper to determined the weight W1 . The absorbed water ratio A of the film was calculated by the formula A=
(MNCO − MOH ) × 100% NCO% = MNCO
(W1 − W0 ) × 100% W0
MNCO is the mole number of NCO group in diisocyanate whereas MOH is the mole number of OH group. The real NCO content is measured by the standard di-n-butylamine titration method.
3. Results and discussion
2.3. Sample preparation and characterization
In the synthesis process of PUA hybrid emulsion by solvent-free method, it was found that the content of carboxy group and acrylic monomer, the rate and the time of emulsification had an important influence on the appearance of emulsion, the film formation and film properties.
The size and morphology of the emulsion particles were viewed on a Jeol JEM-100SX transmission electron microscope (TEM). The sample was prepared by depositing the emulsion onto a copper net after being stained by phosphor-wolframic acid. A thin latex film (thickness less than 20 m) for FTIR was directly fixed on a sample frame and measured ranging from 500 to 4000 cm−1 with a Nicolet Magna-IR 750 FTIR spectrometer. Dynamic mechanical analysis (DMA) measurement was carried out on a viscoelastometer (DMTA Mark IV, Rheometric Scientific Inc., USA), with temperature range from −80 to 140 ◦ C at 1 Hz. The heating rate was 5 ◦ C/min, and the dimension of the sample was 6 mm × 60 mm × 0.4 mm. The storage and the loss modulus were calculated by measuring the complex modulus and plotted versus temperature. Water contact angle on the cast film was measured at 25 ◦ C by the sessile-drop method using a contact angle goniometer (JC2000C1, Shanghai Zhongchen Digital Technical Equipment Ltd., China) and the reported results were the mean values of five times.
3.1. Investigation of preparation technology for PUA
3.1.1. Effect of carboxy group content In this experiment, we fixed the acrylic monomer content and the emulsification condition to investigate the effect of COOH content, the results were shown in Table 3. It was found that the hybrid emulsion could not be formed when the COOH content was lower than 1.7%. When the content was higher than 1.7%, the hybrid emulsion became more and more transparent but the characteristic of water resistance for the formed film became poor with increasing the COOH content. In the followed radical polymerization for BA with BMA, the polyurethane prepolymer acts as a macromolecular emulsifier, and the more the COOH content, the good the emulsification effect. Considering the characteristic of water resistance for the formed film, the optimum COOH content was between 1.9% and 2.1%.
Table 2 Compositions for PUAs Sample code
IPDI:PPG (mol:mol)
DMPA (mol)
HDO (mol)
TEAa (mol)
BA/(BA + BMA) (wt.%)
PUA1 PUA2 PUA3 PUA4 PUA5
1:0.24 1:0.24 1:0.24 1:0.24 1:0.24
0.32 0.32 0.32 0.32 0.32
0.39 0.39 0.39 0.39 0.39
0.35 0.35 0.35 0.35 0.35
14.6 28.0 51.2 81.7 100
a
Mole ratio of TEA:DMPA is fixed at 1.1:1 to secure full neutralization.
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Table 3 Effect of carboxy group content Test sample
COOH content (wt.%)
Emulsion appearance
Appearance of formed film dipped into water for 24 h
1 2 3 4 5
1.7 1.9 2.1 2.3 2.5
White emulsion with seston Sub-transparent Sub-transparent Transparent Transparent
No film forming Film became a little white Film became a little white Film became white Film swelled
3.1.2. Effect of acrylic monomer content In the solvent-free method, acrylic monomers were used as dilution agent to reduce the viscosity of the polyurethane prepolymer. The more the acrylic content, the lower the viscosity of reaction system. It was found from Table 4 that when the acrylic content was higher than 40%, the acrylic monomer could not be wellencased by polyurethane emulsifiers, the emulsion appearance changed from transparent to sub-transparent and some concretionary appeared in the emulsion which resulted in the deposition and poor stability of the emulsion. Moreover the compatibility between the polyurethane prepolymer and polyacrylate was not well, and the last formed PUA film had poor performance. Combined the emulsion appearance and the film performance such as the absorbed water ratio, it is suitable to control the acrylic content at about 40% during the synthesis process of hybrid emulsion. 3.1.3. Effect of the emulsification rate and the emulsification time The rate and the time of emulsification had an unneglectable effect on the preparation process of the emulsion. We used different shearing rate and time during the emulsification to observe the effect of prepared emulsion, and results were shown in Table 5. Though the polyurethane prepolymer and the acrylic monomers could be well dispersed into water for the emulsification rate of 1200 or 2000 min−1 , more or less concretionary appeared for the test sample 1–3. Therefore in order to avoid the appearance of concretionary, emulsification rate of 2000 min−1 and emulsification time of 15 min should be used to get the emulsion during the radical polymerization process. 3.2. The structure of Si-PUA by FTIR Fig. 1 illustrates the spectra of polyurethane and PUA1 samples. The characteristic absorption of the C C bond at 1640 cm−1 disappears, indicating that the acrylic monomers have been
Fig. 1. FTIR spectra of polyurethane and PUA1 samples.
polymerized. The absorption at 1450 cm−1 [ı(C H)], 1728 cm−1 [(C O)] becomes larger, (C O C) of ester at 1150 cm−1 and out-of-plane bending vibration of –(CH2 )n – at 748 cm−1 confirm acrylate structure. As the peak at 1538 cm−1 belongs to N H group, we can see that the height of N H peak decreases after the radical polymerization, which indicates the interactions of N H groups with the acrylate component, most likely with the ester C O group. The structure of Si-PUAs is confirmed by FTIR as shown in Fig. 2. The absorption peaks at 3330 cm−1 [(NH)], 2855–2955 cm−1 [(CH2 ) and (CH3 )], 1728 cm−1 [(C O)], 1538 cm−1 [ı(NH)] and 1110 cm−1 [(C O C)] are the typical absorption peaks for the spectra of polyurethane. Compared with the sample of polyurethane without polysiloxane Si-PUA0, the peaks at 1033 cm−1 [(Si O)] and 803 cm−1 [CH3 –Si rocking] can be clearly detected in the spectrum of Si-PUA1, Si-PUA3 and Si-PUA5, indicating that PDMS groups have been successfully introduced into the polyurethane chains. In addition, the peak strength at 803 cm−1 enlarges as the PDMS content increases for the series of Si-PUA samples. 3.3. The morphology of emulsion particles by TEM The microstructure of emulsion particles was observed by TEM. In Fig. 3, the images of emulsion particles of Si-PUA3 and PUA3 are shown at 105 magnifications. The particle is spherical and the average particle size is about 45 nm. As the polyurethane prepolymer could act as macromolecular emulsifier for its hydrophilic carboxy group, the hydrophobic acrylic monomer could migrate into the polyurethane particle adequately before the radical polymerization to achieve the homogeneous particle size. It may be concluded that the outer component is more polyurethane while the inner component is more hydrophobic polyacrylate. It can be found from Fig. 3 that there is little influence to particle configuration when PDMS is introduced into PUA.
Fig. 2. FTIR spectra of Si-PUAs.
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Table 4 Effect of acrylic content Test sample
Acrylic content (%)
Emulsion appearance
Absorbed water ratio (%)
Concretionary
1 2 3 4 5
30 35 40 50 60
Transparent Transparent Transparent Sub-transparent Sub-transparent
13 11.5 8.9 8.5 9.0
No No No A little A little
Table 5 Effect of emulsification rate and time Test sample
Emulsification rate (min−1 )
Emulsification time (min)
Emulsion appearance
Concretionary
1 2 3 4
1200 1200 2000 2000
7 15 7 15
Milk white Sub-transparent Sub-transparent Sub-transparent
More A little A little No
3.4. The DMA analysis Fig. 4 shows the DMA analysis results for Si-PUA3 and PUA3. It can be seen that PUA3 has higher storage modulus E in the region of low temperature. When it reaches the glass transition temperature, the value of E for Si-PUA3 and PUA3 is approximately the same. As E is a measure of material stiffness, the difference of storage modulus is related to the mechanical property of materials.[21] The incorporation of polysiloxane into the soft segment of PUA chains is expected to decrease the mechanical property due to the high flexibility of polysiloxane. Moreover, two samples have the same peaks at about 23 ◦ C, indicative of anneal at testing temperature and crystallization to enlarge the storage modulus. In Fig. 5, there are two distinct transitions, the similarity between the two curves indicating that the addition of PDMS does not influence the phase behavior of PUA. It is reported that the glass transitions of the components of polyurethane and polyacrylate blends are clear in both cases, [22] the broad peak at lower temperature of PUA shows that the compatibility between polyacrylate and polyurethane is good. In our system the acrylic monomer is allowed to diffuse into the polyurethane particles before polymerization, after which the polyacrylate is trapped inside the polyurethane. By the interaction between hydrogen bond and similar C O group polarity, polyacrylate mixes with polyurethane at a certain extent. 3.5. The water contact angle analysis Water contact angle on the surface of Si-PUAs increases with increasing the content of PDMS, as shown in Fig. 6. When PDMS content is below 4%, it influences contact angle greatly, the curve ascends sharply. The contact angle is nearly 97◦ when the PDMS
content increases to about 10%, but above that point the increase of PDMS content has low impact. The increase of contact angle could be attributed to the surface activity of the hydrophobic polysiloxane. When PDMS concentration is low, it migrates to the surface to shield the polymer chain with high surface energy, and substantially lowers the surface free energy. But when the PDMS content increases to a degree where there is enough polysiloxane at the surface, their low surface free energy is up to the limit point. 3.6. The water-resistance property Correlation between absorbed water ratio and content of PDMS for the sample of Si-PUAs is shown in Fig. 7. It is observed clearly that the absorbed water ratio decreases with the increase of the PDMS content. PDMS has an obvious influence on the absorbed water ratio. With PDMS content increases from 0% to12%, the absorbed water ratio decreases from 6.23% to 3.02%, the water absorbability administers obvious decrease with little PDMS content. It can be concluded that by introducing hydrophobic PDMS into soft segment of polyurethane chains, the water-resistance of PUA material can be enhanced easily. 3.7. The mechanical properties The tensile strength and the extensibility of PUA with different BA content is investigated. Fig. 8 shows that by keeping the mass of polyurethane unchanged while increasing the ratio of BA/(BA + BMA), the extensibility of film increases while tensile strength decreases. It can be seen that PUA1 has extensibility of 246% and PUA5 of 613% whereas their tensile strengths are 28.5 and 12.7 Mpa, respectively. As BA is a soft monomer compared
Fig. 3. TEM of Si-PUA3 (A) and PUA3 (B).
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Fig. 4. Correlation between log E and temperature for the samples of Si-PUA3 and PUA3.
Fig. 5. Correlation between tan ı and temperature for the samples of Si-PUA3 and PUA3.
Fig. 6. Water contact angle increases with increasing the content of PDMS for SiPUAs.
243
Fig. 7. Absorbed water ratio decreases with increasing the content of PDMS for Si-PUAs.
Fig. 8. Tensile strength decreases and extensibility increases with increasing the BA content for PUA samples.
Fig. 9. Tensile strength decreases and extensibility increases with increasing the PDMS content for Si-PUA samples.
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with BMA, the copolymer with more BMA component becomes hard for its less activity. From the testing result, it is concluded that polyurethane and polyacrylate chains are in interaction with each other at some degree because the change of ratios for different acrylic monomer has great impact on the mechanical property. As is known that the polysiloxane is very flexible and has excellent low temperature properties but poor tensile strength, the PUA films are likely to increase in extensibility and decrease in tensile strength when PDMS is added. Fig. 9 shows the mechanical properties at different content of PDMS for Si-PUAs. It can be seen that the tensile strength decreases but the extensibility increases with increasing the PDMS content. It is also found that both the extensibility and the tensile strength change sharply when the PDMS content is above 6.5%. This indicates little effect of PDMS content on the mechanical properties when PDMS content is below 6.5%. 4. Conclusions A series of Si-PUAs had been synthesized by solvent-free method using PDMS as a soft segment. In the method acrylic monomers acted as dilution agent and the polyurethane having carboxy groups acted as macromolecular emulsifier without using any other solvent and surfactant. The optimum preparation technology for the solvent-free method was achieved when the COOH content was between 1.9% and 2.1%, the acrylic content was 40% and the emulsification rate was 2000 min−1 for 15 min. The chain structure and the particle size were confirmed by FTIR and TEM analysis. The emulsion particle size was uniform and around 45 nm, which ensures better performance of the formed film. The obtained Si-PUA proved to possess higher contact angle and better water resistance when
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