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Characterization of waterborne polyurethane for ecofriendly functional floor plate
Progress in Organic Coatings 67 (2010) 102–106
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Characterization of waterborne polyurethane for ecofriendly functional floor plate Eun-Hee Kim a,∗ , Woo-Ram Lee a , Sang-Wong Myoung a , Jeong-Pyo Kim a , Yeon-Gil Jung a,∗∗ , Yoon-Suk Nam b , Woon-Sub Kyoung b , Hyun Cho c a b c
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Eden Bio Wallpaper Co., Ltd., Anyang, Kyunggi 430-826, Republic of Korea Department of Nanosystem and Nanoprocess Engineering, Pusan National University, Miryang, Kyungnam 627-706, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 June 2009 Received in revised form 12 August 2009 Accepted 20 August 2009 Keywords: Waterborne polyurethane Nonmetallic mineral particle Ecofriendly floor tiles Mechanical property Crosslinking density
a b s t r a c t Three types of nonmetallic mineral particles (CaCO3 , TiO2 and loess) were incorporated into waterborne polyurethane acrylate (PUA) to improve the surface properties of ecofriendly floor tiles. Several properties of PUA containing nonmetallic mineral materials were measured by Fourier transform infrared spectroscopy (FT-IR), a dynamic mechanical thermal analyzer (DMTA), swelling tests, and contact angle measurement. Upon decreasing the molecular weight between crosslinks (variation of molecular weight of the polyol), the modulus and glass transition temperature (Tg ) of the PUA film increased because of the increase in crosslinking density. Resistance properties such as swelling and contact angle against water were enhanced with the addition of nonmetallic mineral particles because of the increase in the hydrophobic nature of the polymer matrix. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane (PU) is a most versatile polymer material, with a wide variety of physical and chemical properties, that can be prepared from many commercially available and relatively inexpensive polyols, isocyanates, and chain extenders [1–3]. These properties allow PU to be used in various industry parts, building materials, sports goods, medical equipment, adhesives, and coatings [4,5]. In particular, waterborne PU has emerged as a new chemical technology for ecofriendly materials that are nontoxic and nonflammable. While waterborne PU has drawbacks such as poor surface properties caused by low crosslinking density, it has many advantages including being tack-free before curing, which allows flooring and rolling operations, and high elasticity for high deformation coatings as well as having an environmentally friendly nature. Therefore, new curing methods such as the UV technique have been introduced, having various industrial applications because of its unique advantages, viz., energy savings, fast process, high production speed, high product durability, and excellent surface properties of scratch resistance and antiabrasion [6–8].
∗ Corresponding author. Tel.: +82 55 213 2742; fax: +82 55 262 6486. ∗∗ Corresponding author. Tel.: +82 55 213 3712; fax: +82 55 262 6486. E-mail addresses: [email protected] (E.H. Kim), [email protected] (Y.-G. Jung). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.10.019
Over the past few years, much attention has been given to improve the surface and mechanical properties of polyurethane acrylate (PUA) film by the incorporation of inorganic particles [9–11]. However, inorganic particles and polymer matrix have a low adhesion due to the incompatibility. Therefore, incorporation of these particles into the PUA matrix induces high interface adhesion between the PUA composite film as coating layer and the substrate comprised of inorganic materials. In this work, we added different types of nonmetallic mineral particles into a conventional PUA matrix to improve the surface properties for ecofriendly floor tiles and to produce an increase in adhesion between the PUA film and the substrate by increasing the surface area of the PUA film. The results were studied in terms of elasticity, contact angle between the PUA film and water, and swelling ratio of the PUA film against water. 2. Experimental 2.1. Material and synthesis of PUA–nonmetallic mineral composite film Urethane acrylate oligomer was prepared from poly (propylene glycol) (PPG, Mw = 400, 500, 600, Sigma–Aldrich Korea, Yongin, Korea) as a polyol, hexamethylene diisocyanate (HDI, Sigma–Aldrich Korea) as a isocyanate, dibutyltin dilaurate (DBT, Sigma–Aldrich Korea) as a catalyst, hydroxyethyl methacrylate (HEMA, Sigma–Aldrich Korea) as a modifier, dimethylol butanoic
E.H. Kim et al. / Progress in Organic Coatings 67 (2010) 102–106
103
Fig. 1. Particle size distribution of nonmetallic mineral materials: (a) loess, (b) TiO2 , and (c) CaCO3 .
acid (DMBA, Sigma–Aldrich Korea) as an ionic material, triethylamine (TEA, Fluka Korea, Yongin, Korea) as a neutralizer, and ˛,˛ -dimethoxy-˛-hydroxy acetophenone (Darocur 1173, Ciba Korea, Seoul, Korea) as a photoinitiator. Three types of nonmetallic mineral particles, namely loess (Eden Bio Wallpaper Co., Kyunggi, Korea), titanium dioxide (TiO2 , Wooshin Pigment Co., Ltd., Seoul, Korea), and calcium carbonate (CaCO3 , Sangdong Pila Co., Seoul, Korea), were incorporated into the PUA matrix. The reactor con-
sisted of four-necked separable flask of round-bottom with a mechanical stirrer, thermometer, condenser with a drying tube, and nitrogen gas inlet. First, HDI and DMBA were added to the dried flask. While stirring, the mixture was maintained at 80 ◦ C for 1 h to obtain NCOterminated oligomer, and then blended with nonmetallic mineral particles. DMBA is essential for ionic dispersion although it retards drying and reduces hydrolytic stability. It is important to minimize
Fig. 2. Synthetic scheme of PUA–nonmetallic mineral composite film.
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Fig. 3. IR analysis of HEMA-capped PUA: (a) before and (b) after NCO reaction.
the content of DMBA as much as possible while maintaining a stable dispersion. Ionic groups were located in soft segments, giving a good dispersion in water for the PUA oligomer. DMBA was first reacted with an excess amount of HDI, which was subsequently reacted with PPG and HDI. Then, the reacted mixture was cooled to 60 ◦ C, and HEMA was added to obtain HEMA-capped urethane oligomers. Oligomer capped by HEMA was cooled to 50 ◦ C, and the carboxylic acid groups were neutralized by TEA during the next 2 h. An aqueous dispersion of oligomer was obtained by adding water to the mixture using a tubing pump at a constant flow rate. Finally, Darocur 1173 of 4 wt.% and three types of nonmetallic mineral materials (loess, TiO2 and CaCO3 ) were fed to the reacted mixture under a high rotational speed wheel, and subsequently cast on a Teflon tile and dried before UV curing. The sizes of the nonmetallic mineral particles in this work are given in Fig. 1. All PUA films were cured by UV (1.5 mW/cm2 , 365 nm) for 3 min. The synthetic scheme for the PUA–nonmetallic mineral composite film is given in Fig. 2, and the basic experimental ranges to fabricate the PUA composite films are shown in Table 1.
3. Results and discussion 3.1. Synthesis and mechanical properties Chemical reaction of NCO-terminated PU oligomer and HEMA measured by FT-IR is shown in Fig. 3. The peak at about 2270 cm−1 corresponding to the stretch vibration of NCO groups completely disappears upon capping the NCO-terminated oligomer with HEMA. The resulting product is a stable urethane acrylate oligomer with a solid content of about 30%. The dynamic mechanical behavior of the PUA films of different molecular weights is shown in Fig. 4. Regardless of the molecular weight of polyol (Mw ), the glass transition temperature (Tg ) shows a single point, which indicates that the soft segment (polyol) and hard segment (isocyanate) of the PUA oligomer are phase-mixed at a segment level. If two segments are immiscible then, generally, two discrete glass transition peaks are obtained. The Tg and rubbery elastic modulus increase as Mw decreases, resulting from the reduction in molecular weight between crosslinks (Mc ). In this work, the PUA chains are chemically bonded to reactive sites of
2.2. Characterization Particle sizes of the nonmetallic mineral materials were measured by a particle size analyzer (Mastersizer/E, Malvern, England). Chemical reaction of NCO-terminated PU oligomer and HEMA were measured using Fourier transform infrared spectroscopy (FT-IR, Nicolet, Thermo Fisher Scientific, MA, USA). Mechanical properties of the PUA–nonmetallic composite films were measured using a dynamic mechanical thermal analyzer (DMTA, MKIV, Rheometrics Scientific, NJ, USA) with a tensile mode at a heating rate of 4 ◦ C/min and 10 Hz. Contact angle of the films with a deionized water was measured with a conventional contact angle goniometer (G-1, Erma, Tokyo, Japan) at room temperature. To measure the swelling ratio, samples were swollen in water at room temperature for 30 min and 12 h, and then water on the sample surface was removed using paper scraps. The surface morphology of the films was characterized by an atomic force microscope (AFM, Nanoscope III, Digital Instruments Inc., NY, USA). Table 1 Experimental ranges of PUA composite films prepared under various conditions. Nonmetallic mineral (wt.%)
Initiator (wt.%)
w/o Loess TiO2 CaCO3
1 5 1 5 1 5
4
Run number 1 2 3 4 5 6 7
Fig. 4. Storage modulus of PUA films with different molecular weights of PPG.
E.H. Kim et al. / Progress in Organic Coatings 67 (2010) 102–106
105
Fig. 5. Contact angle (a) and swelling ratio (b) of PUA–nonmetallic mineral composite films.
the PUA oligomer, and have a network structure of the oligomer with two vinyl groups, as evidenced by the existence of a rubbery plateau; the molecular weight corresponds to the molecular weight between Mc , which can be calculated based on the ideal rubber theory given by Eq. (1) [12,13]: o EN =3
RT Mc
(1)
o ), , R, and T are the plateau modulus, density, gas conwhere (EN stant, and absolute temperature, respectively. Using (1.1 g/cm3 ) o ) tabulated in Fig. 4, M was calculated. It and the plateau values (EN c is found that as Mw of the polyol increases from 400 to 600, the Mc increases by more than a factor of two, from about 547 to 913, causing an decrease in elastic modulus. From here on, the experiments were carried out on oligomer synthesized from PPG 400 with high polymer elasticity.
3.2. Contact angle and swelling ratio The contact angles against water on the surface of the PUA composite films are shown in Fig. 5(a) for each additional amount of nonmetallic mineral particles. Generally, incorporation of nonmetallic mineral particles into polymer chain leads to large changes in the surface property of water repellency, resulting from the low hydrophobicity of the particles [14–16]. As expected, when the nonmetallic mineral materials were added, contact angles were significantly increased and are dependent on the type and additional amount of mineral particle. In the case of loess and TiO2 , contact angles for the films having 5 wt.% (Runs 3 and 5) are lower than those for the films having 1 wt.% (Runs 2 and 4), resulting from aggregation between the nonmetallic mineral particles. However, CaCO3 (Runs 6 and 7) shows a different response because of the smaller particle size of CaCO3 , compared to the cases of loess and
Fig. 6. AFM topologies in fracture surfaces of PUA films with and without different types of 1 wt.% nonmetallic mineral materials: (a) w/o particle, (b) loess, (c) TiO2 , and (d) CaCO3 .
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TiO2 . The coalescence of particles is an important phenomenon in determining the properties of the polymer matrix and can be analyzed by dimensional analysis under assumptions that can explain a set of variables governing the phenomenon with Eq. (2). According to Buckingham’s Pi theorem, the number of dimensionless groups among q quantities that are given in u units may be (q − u). Assuming that droplet coalescence depends on interfacial tension (), viscosity (), particle diameter (d), and density (), q = 4 and u = 3 (mass, length, and time) and only one dimensionless group, called the coalescence number, is obtained [17]. d Coalescence number = 2
(2)
This coalescence number may be viewed as the ratio of coalescence force to the force of resistance. Coalescence may occur when the number exceeds a critical coalescence number. Viscosity appearing as a square term makes it a most important variable in Eq. (2). Viscosity is calculated by the Einstein equation [18]: = 1 + kE f + kE f2 o
(3)
where , o , kE , and f are the viscosities of the suspension and pure fluid, Einstein coefficient, and volume fraction, respectively. Particles of small size have high viscosity as a result of increasing the volume fraction. In this work, particle sizes of loess, TiO2 , and CaCO3 are 5.65, 3.73, and 1.60 m, respectively (see Fig. 1). CaCO3 has the highest viscosity because it has the smallest size. The film must have a small particle size and high viscosity to decrease coalescence. Therefore, the loess and TiO2 having larger sizes are more aggregated than CaCO3. Fig. 5(b) shows the swelling ratio of the PUA composite films prepared using various types of nonmetallic mineral particles. The swelling ratio was calculated by: Swelling ratio =
Wt − Wd × 100 Wd
(4)
where Wt is the film weight at immersion time t and Wd is the weight of the dried film. As expected, the swelling ratio sharply decreases as the nonmetallic mineral particles are added to the PUA matrix. This is because it is difficult for water to penetrate into the PUA film as a result of the higher hydrophobic nature of the polymer matrix induced by the mineral particles. In general, contact angle and swelling ratio are surface and bulk properties, respectively. In this work, the swelling ratio is significantly varied as a function of the contact angle with addition of the nonmetallic mineral particles, indicating that mineral particles could be homogeneously dispersed in the entire polymer matrix, independent of nonmetallic mineral species. 3.3. Morphology Fig. 6 shows three-dimensional AFM images of PUA films with and without the nonmetallic mineral particles. The PUA composite
films containing the nonmetallic mineral particles show a rough surface morphology, while the pure PUA film has a much smoother appearance. This is because of an increase in the surface area of the PUA film by the nonmetallic mineral particles. Also, it is seen that the depth variation of the surface is increased as the size of the particles increases: CaCO3 < TiO2 < loess. 4. Conclusions Three types of nonmetallic mineral particles (CaCO3 , TiO2 and loess) were incorporated into the conventional waterborne PUA by free radical polymerization to fabricate the PUA composite films for ecofriendly floor tiles. The elasticity of the PUA films is enhanced upon decreasing the molecular weight of polyol (Mw ), being about 16.2 and 9.7 MPa as the molecular weight of the polyol increases from 400 to 600. Also, the glass transition temperature (Tg ) decreases as Mw increases. The reduction of molecular weight between crosslinks (Mc ) produces an increase in crosslinking density of the polymer matrix, which results in restricting the mobility and flexibility of the polymer chain. The contact angle against water is increased by the addition of the nonmetallic mineral particles because of the increase in hydrophobicity caused by the nonmetallic mineral particle. The Nonmetallic mineral particles make penetration of water into the PUA film more difficult, which makes it sufficient to apply the PUA composite film as a coating material for ecofriendly floor tiles. Acknowledgements The authors acknowledge financial support from the Korea Energy Management Corporation (KEMCO); the Korea Research Foundation Grant (KRF-2006-005-J02701). References [1] N.J. Iyer, T.P. Gnanarajan, G. Radhakrishnan, Macromol. Chem. Phys. 203 (2002) 712. [2] B.K. Kim, J.C. Lee, J. Polym. Sci., Polym. Chem. 34 (1996) 1095. [3] K.L. Noble, Prog. Org. Coat. 32 (1997) 131. [4] N.M.K. Lamba, K.A. Woodhouse, S.L. Cooper, Polyurethane in Biomedical Applications, CRC Press, New York, 1998. [5] C. Hapburn, Polyurethane Elastomers, Elsevier, Oxford, 1991. [6] C. Deker, Macromol. Rapid Commun. 23 (2002) 1067. [7] B.K. Kim, S.H. Paik, J. Polym. Sci., Polym. Chem. 37 (1999) 2703. [8] B.K. Kim, B.U. Ahn, M.H. Lee, S.K. Lee, Prog. Org. Coat. 55 (2006) 194. [9] L. Chu, M.W. Daniels, L.F. Francis, Chem. Mater. 9 (1997) 2577. [10] P.J. Dionne, R. Ozisik, C.R. Picu, Macromolecules 38 (2005) 9351. [11] F.D. Blum, E.N. Young, G. Smith, O.C. Sitton, Langmuir 22 (2006) 4741. [12] B. Yang, H. Xu, J. Wang, S. Gang, C. Li, J. Appl. Polym. Sci. 106 (2007) 320. [13] A.N. Gent, Engineering with Rubber, Hanser, New York, 1992. [14] B.S. Kim, S.H. Park, B.K. Kim, Colloid Polym. Sci. 284 (2006) 1067. [15] Y. Yuan, M.S. Shoichet, Macromolecules 33 (2000) 4926. [16] H. Lee, L.A. Archer, Macromolecules 34 (2001) 4572. [17] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, New York, 1960. [18] M. Angel, Polymer Science Dictionary, Chapman & Hall, New York, 1997.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Characterization of waterborne polyurethane for ecofriendly functional floor plate Eun-Hee Kim a,∗ , Woo-Ram Lee a , Sang-Wong Myoung a , Jeong-Pyo Kim a , Yeon-Gil Jung a,∗∗ , Yoon-Suk Nam b , Woon-Sub Kyoung b , Hyun Cho c a b c
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Eden Bio Wallpaper Co., Ltd., Anyang, Kyunggi 430-826, Republic of Korea Department of Nanosystem and Nanoprocess Engineering, Pusan National University, Miryang, Kyungnam 627-706, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 June 2009 Received in revised form 12 August 2009 Accepted 20 August 2009 Keywords: Waterborne polyurethane Nonmetallic mineral particle Ecofriendly floor tiles Mechanical property Crosslinking density
a b s t r a c t Three types of nonmetallic mineral particles (CaCO3 , TiO2 and loess) were incorporated into waterborne polyurethane acrylate (PUA) to improve the surface properties of ecofriendly floor tiles. Several properties of PUA containing nonmetallic mineral materials were measured by Fourier transform infrared spectroscopy (FT-IR), a dynamic mechanical thermal analyzer (DMTA), swelling tests, and contact angle measurement. Upon decreasing the molecular weight between crosslinks (variation of molecular weight of the polyol), the modulus and glass transition temperature (Tg ) of the PUA film increased because of the increase in crosslinking density. Resistance properties such as swelling and contact angle against water were enhanced with the addition of nonmetallic mineral particles because of the increase in the hydrophobic nature of the polymer matrix. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane (PU) is a most versatile polymer material, with a wide variety of physical and chemical properties, that can be prepared from many commercially available and relatively inexpensive polyols, isocyanates, and chain extenders [1–3]. These properties allow PU to be used in various industry parts, building materials, sports goods, medical equipment, adhesives, and coatings [4,5]. In particular, waterborne PU has emerged as a new chemical technology for ecofriendly materials that are nontoxic and nonflammable. While waterborne PU has drawbacks such as poor surface properties caused by low crosslinking density, it has many advantages including being tack-free before curing, which allows flooring and rolling operations, and high elasticity for high deformation coatings as well as having an environmentally friendly nature. Therefore, new curing methods such as the UV technique have been introduced, having various industrial applications because of its unique advantages, viz., energy savings, fast process, high production speed, high product durability, and excellent surface properties of scratch resistance and antiabrasion [6–8].
∗ Corresponding author. Tel.: +82 55 213 2742; fax: +82 55 262 6486. ∗∗ Corresponding author. Tel.: +82 55 213 3712; fax: +82 55 262 6486. E-mail addresses: [email protected] (E.H. Kim), [email protected] (Y.-G. Jung). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.10.019
Over the past few years, much attention has been given to improve the surface and mechanical properties of polyurethane acrylate (PUA) film by the incorporation of inorganic particles [9–11]. However, inorganic particles and polymer matrix have a low adhesion due to the incompatibility. Therefore, incorporation of these particles into the PUA matrix induces high interface adhesion between the PUA composite film as coating layer and the substrate comprised of inorganic materials. In this work, we added different types of nonmetallic mineral particles into a conventional PUA matrix to improve the surface properties for ecofriendly floor tiles and to produce an increase in adhesion between the PUA film and the substrate by increasing the surface area of the PUA film. The results were studied in terms of elasticity, contact angle between the PUA film and water, and swelling ratio of the PUA film against water. 2. Experimental 2.1. Material and synthesis of PUA–nonmetallic mineral composite film Urethane acrylate oligomer was prepared from poly (propylene glycol) (PPG, Mw = 400, 500, 600, Sigma–Aldrich Korea, Yongin, Korea) as a polyol, hexamethylene diisocyanate (HDI, Sigma–Aldrich Korea) as a isocyanate, dibutyltin dilaurate (DBT, Sigma–Aldrich Korea) as a catalyst, hydroxyethyl methacrylate (HEMA, Sigma–Aldrich Korea) as a modifier, dimethylol butanoic
E.H. Kim et al. / Progress in Organic Coatings 67 (2010) 102–106
103
Fig. 1. Particle size distribution of nonmetallic mineral materials: (a) loess, (b) TiO2 , and (c) CaCO3 .
acid (DMBA, Sigma–Aldrich Korea) as an ionic material, triethylamine (TEA, Fluka Korea, Yongin, Korea) as a neutralizer, and ˛,˛ -dimethoxy-˛-hydroxy acetophenone (Darocur 1173, Ciba Korea, Seoul, Korea) as a photoinitiator. Three types of nonmetallic mineral particles, namely loess (Eden Bio Wallpaper Co., Kyunggi, Korea), titanium dioxide (TiO2 , Wooshin Pigment Co., Ltd., Seoul, Korea), and calcium carbonate (CaCO3 , Sangdong Pila Co., Seoul, Korea), were incorporated into the PUA matrix. The reactor con-
sisted of four-necked separable flask of round-bottom with a mechanical stirrer, thermometer, condenser with a drying tube, and nitrogen gas inlet. First, HDI and DMBA were added to the dried flask. While stirring, the mixture was maintained at 80 ◦ C for 1 h to obtain NCOterminated oligomer, and then blended with nonmetallic mineral particles. DMBA is essential for ionic dispersion although it retards drying and reduces hydrolytic stability. It is important to minimize
Fig. 2. Synthetic scheme of PUA–nonmetallic mineral composite film.
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Fig. 3. IR analysis of HEMA-capped PUA: (a) before and (b) after NCO reaction.
the content of DMBA as much as possible while maintaining a stable dispersion. Ionic groups were located in soft segments, giving a good dispersion in water for the PUA oligomer. DMBA was first reacted with an excess amount of HDI, which was subsequently reacted with PPG and HDI. Then, the reacted mixture was cooled to 60 ◦ C, and HEMA was added to obtain HEMA-capped urethane oligomers. Oligomer capped by HEMA was cooled to 50 ◦ C, and the carboxylic acid groups were neutralized by TEA during the next 2 h. An aqueous dispersion of oligomer was obtained by adding water to the mixture using a tubing pump at a constant flow rate. Finally, Darocur 1173 of 4 wt.% and three types of nonmetallic mineral materials (loess, TiO2 and CaCO3 ) were fed to the reacted mixture under a high rotational speed wheel, and subsequently cast on a Teflon tile and dried before UV curing. The sizes of the nonmetallic mineral particles in this work are given in Fig. 1. All PUA films were cured by UV (1.5 mW/cm2 , 365 nm) for 3 min. The synthetic scheme for the PUA–nonmetallic mineral composite film is given in Fig. 2, and the basic experimental ranges to fabricate the PUA composite films are shown in Table 1.
3. Results and discussion 3.1. Synthesis and mechanical properties Chemical reaction of NCO-terminated PU oligomer and HEMA measured by FT-IR is shown in Fig. 3. The peak at about 2270 cm−1 corresponding to the stretch vibration of NCO groups completely disappears upon capping the NCO-terminated oligomer with HEMA. The resulting product is a stable urethane acrylate oligomer with a solid content of about 30%. The dynamic mechanical behavior of the PUA films of different molecular weights is shown in Fig. 4. Regardless of the molecular weight of polyol (Mw ), the glass transition temperature (Tg ) shows a single point, which indicates that the soft segment (polyol) and hard segment (isocyanate) of the PUA oligomer are phase-mixed at a segment level. If two segments are immiscible then, generally, two discrete glass transition peaks are obtained. The Tg and rubbery elastic modulus increase as Mw decreases, resulting from the reduction in molecular weight between crosslinks (Mc ). In this work, the PUA chains are chemically bonded to reactive sites of
2.2. Characterization Particle sizes of the nonmetallic mineral materials were measured by a particle size analyzer (Mastersizer/E, Malvern, England). Chemical reaction of NCO-terminated PU oligomer and HEMA were measured using Fourier transform infrared spectroscopy (FT-IR, Nicolet, Thermo Fisher Scientific, MA, USA). Mechanical properties of the PUA–nonmetallic composite films were measured using a dynamic mechanical thermal analyzer (DMTA, MKIV, Rheometrics Scientific, NJ, USA) with a tensile mode at a heating rate of 4 ◦ C/min and 10 Hz. Contact angle of the films with a deionized water was measured with a conventional contact angle goniometer (G-1, Erma, Tokyo, Japan) at room temperature. To measure the swelling ratio, samples were swollen in water at room temperature for 30 min and 12 h, and then water on the sample surface was removed using paper scraps. The surface morphology of the films was characterized by an atomic force microscope (AFM, Nanoscope III, Digital Instruments Inc., NY, USA). Table 1 Experimental ranges of PUA composite films prepared under various conditions. Nonmetallic mineral (wt.%)
Initiator (wt.%)
w/o Loess TiO2 CaCO3
1 5 1 5 1 5
4
Run number 1 2 3 4 5 6 7
Fig. 4. Storage modulus of PUA films with different molecular weights of PPG.
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105
Fig. 5. Contact angle (a) and swelling ratio (b) of PUA–nonmetallic mineral composite films.
the PUA oligomer, and have a network structure of the oligomer with two vinyl groups, as evidenced by the existence of a rubbery plateau; the molecular weight corresponds to the molecular weight between Mc , which can be calculated based on the ideal rubber theory given by Eq. (1) [12,13]: o EN =3
RT Mc
(1)
o ), , R, and T are the plateau modulus, density, gas conwhere (EN stant, and absolute temperature, respectively. Using (1.1 g/cm3 ) o ) tabulated in Fig. 4, M was calculated. It and the plateau values (EN c is found that as Mw of the polyol increases from 400 to 600, the Mc increases by more than a factor of two, from about 547 to 913, causing an decrease in elastic modulus. From here on, the experiments were carried out on oligomer synthesized from PPG 400 with high polymer elasticity.
3.2. Contact angle and swelling ratio The contact angles against water on the surface of the PUA composite films are shown in Fig. 5(a) for each additional amount of nonmetallic mineral particles. Generally, incorporation of nonmetallic mineral particles into polymer chain leads to large changes in the surface property of water repellency, resulting from the low hydrophobicity of the particles [14–16]. As expected, when the nonmetallic mineral materials were added, contact angles were significantly increased and are dependent on the type and additional amount of mineral particle. In the case of loess and TiO2 , contact angles for the films having 5 wt.% (Runs 3 and 5) are lower than those for the films having 1 wt.% (Runs 2 and 4), resulting from aggregation between the nonmetallic mineral particles. However, CaCO3 (Runs 6 and 7) shows a different response because of the smaller particle size of CaCO3 , compared to the cases of loess and
Fig. 6. AFM topologies in fracture surfaces of PUA films with and without different types of 1 wt.% nonmetallic mineral materials: (a) w/o particle, (b) loess, (c) TiO2 , and (d) CaCO3 .
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TiO2 . The coalescence of particles is an important phenomenon in determining the properties of the polymer matrix and can be analyzed by dimensional analysis under assumptions that can explain a set of variables governing the phenomenon with Eq. (2). According to Buckingham’s Pi theorem, the number of dimensionless groups among q quantities that are given in u units may be (q − u). Assuming that droplet coalescence depends on interfacial tension (), viscosity (), particle diameter (d), and density (), q = 4 and u = 3 (mass, length, and time) and only one dimensionless group, called the coalescence number, is obtained [17]. d Coalescence number = 2
(2)
This coalescence number may be viewed as the ratio of coalescence force to the force of resistance. Coalescence may occur when the number exceeds a critical coalescence number. Viscosity appearing as a square term makes it a most important variable in Eq. (2). Viscosity is calculated by the Einstein equation [18]: = 1 + kE f + kE f2 o
(3)
where , o , kE , and f are the viscosities of the suspension and pure fluid, Einstein coefficient, and volume fraction, respectively. Particles of small size have high viscosity as a result of increasing the volume fraction. In this work, particle sizes of loess, TiO2 , and CaCO3 are 5.65, 3.73, and 1.60 m, respectively (see Fig. 1). CaCO3 has the highest viscosity because it has the smallest size. The film must have a small particle size and high viscosity to decrease coalescence. Therefore, the loess and TiO2 having larger sizes are more aggregated than CaCO3. Fig. 5(b) shows the swelling ratio of the PUA composite films prepared using various types of nonmetallic mineral particles. The swelling ratio was calculated by: Swelling ratio =
Wt − Wd × 100 Wd
(4)
where Wt is the film weight at immersion time t and Wd is the weight of the dried film. As expected, the swelling ratio sharply decreases as the nonmetallic mineral particles are added to the PUA matrix. This is because it is difficult for water to penetrate into the PUA film as a result of the higher hydrophobic nature of the polymer matrix induced by the mineral particles. In general, contact angle and swelling ratio are surface and bulk properties, respectively. In this work, the swelling ratio is significantly varied as a function of the contact angle with addition of the nonmetallic mineral particles, indicating that mineral particles could be homogeneously dispersed in the entire polymer matrix, independent of nonmetallic mineral species. 3.3. Morphology Fig. 6 shows three-dimensional AFM images of PUA films with and without the nonmetallic mineral particles. The PUA composite
films containing the nonmetallic mineral particles show a rough surface morphology, while the pure PUA film has a much smoother appearance. This is because of an increase in the surface area of the PUA film by the nonmetallic mineral particles. Also, it is seen that the depth variation of the surface is increased as the size of the particles increases: CaCO3 < TiO2 < loess. 4. Conclusions Three types of nonmetallic mineral particles (CaCO3 , TiO2 and loess) were incorporated into the conventional waterborne PUA by free radical polymerization to fabricate the PUA composite films for ecofriendly floor tiles. The elasticity of the PUA films is enhanced upon decreasing the molecular weight of polyol (Mw ), being about 16.2 and 9.7 MPa as the molecular weight of the polyol increases from 400 to 600. Also, the glass transition temperature (Tg ) decreases as Mw increases. The reduction of molecular weight between crosslinks (Mc ) produces an increase in crosslinking density of the polymer matrix, which results in restricting the mobility and flexibility of the polymer chain. The contact angle against water is increased by the addition of the nonmetallic mineral particles because of the increase in hydrophobicity caused by the nonmetallic mineral particle. The Nonmetallic mineral particles make penetration of water into the PUA film more difficult, which makes it sufficient to apply the PUA composite film as a coating material for ecofriendly floor tiles. Acknowledgements The authors acknowledge financial support from the Korea Energy Management Corporation (KEMCO); the Korea Research Foundation Grant (KRF-2006-005-J02701). References [1] N.J. Iyer, T.P. Gnanarajan, G. Radhakrishnan, Macromol. Chem. Phys. 203 (2002) 712. [2] B.K. Kim, J.C. Lee, J. Polym. Sci., Polym. Chem. 34 (1996) 1095. [3] K.L. Noble, Prog. Org. Coat. 32 (1997) 131. [4] N.M.K. Lamba, K.A. Woodhouse, S.L. Cooper, Polyurethane in Biomedical Applications, CRC Press, New York, 1998. [5] C. Hapburn, Polyurethane Elastomers, Elsevier, Oxford, 1991. [6] C. Deker, Macromol. Rapid Commun. 23 (2002) 1067. [7] B.K. Kim, S.H. Paik, J. Polym. Sci., Polym. Chem. 37 (1999) 2703. [8] B.K. Kim, B.U. Ahn, M.H. Lee, S.K. Lee, Prog. Org. Coat. 55 (2006) 194. [9] L. Chu, M.W. Daniels, L.F. Francis, Chem. Mater. 9 (1997) 2577. [10] P.J. Dionne, R. Ozisik, C.R. Picu, Macromolecules 38 (2005) 9351. [11] F.D. Blum, E.N. Young, G. Smith, O.C. Sitton, Langmuir 22 (2006) 4741. [12] B. Yang, H. Xu, J. Wang, S. Gang, C. Li, J. Appl. Polym. Sci. 106 (2007) 320. [13] A.N. Gent, Engineering with Rubber, Hanser, New York, 1992. [14] B.S. Kim, S.H. Park, B.K. Kim, Colloid Polym. Sci. 284 (2006) 1067. [15] Y. Yuan, M.S. Shoichet, Macromolecules 33 (2000) 4926. [16] H. Lee, L.A. Archer, Macromolecules 34 (2001) 4572. [17] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, Wiley, New York, 1960. [18] M. Angel, Polymer Science Dictionary, Chapman & Hall, New York, 1997.