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Characterization of UV-cured polyester acrylate films containing acrylate functional polydimethylsiloxane
European Polymer Journal 39 (2003) 2235–2241 www.elsevier.com/locate/europolj
Characterization of UV-cured polyester acrylate films containing acrylate functional polydimethylsiloxane H.K. Kim, H.T. Ju, J.W. Hong
*
Department of Polymer Science and Engineering, College of Engineering, Chosun University, 375 Seosuck-dong Dong-gu, Gwangju 501-759, South Korea Received 23 October 2002; received in revised form 25 April 2003; accepted 28 May 2003
Abstract Acrylate functional polydimethylsiloxane (AF-PDMS) was tested as a reactive additive in UV-curable coating formulations. Pencil hardness, solvent resistance, and gloss of the UV-cured films were measured to study the influence of AF-PDMS content on coating properties. Depth-profile analysis by FTIR-ATR and Raman spectroscopy was also performed to investigate the effect of AF-PDMS on the behavior of film formation during UV curing. The kinetics of photopolymerization were monitored by photo differential scanning calorimetry (photo-DSC). Our results show that AF-PDMS containing coating formulations are very sensitive to oxygen inhibition, so that an inert environment such as nitrogen purging is required to avoid coating defects. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Acrylate functional polydimethylsiloxane; UV curable; FTIR-ATR; Raman spectroscopy; Depth profile
1. Introduction The development of any coatings is generally a compromise between performance attributes (both in terms of the properties of the finished coating and the application of it) and cost. Neglecting the cost aspects for the moment, the first stages of the development of a coating concentrate on the polymeric binder. For a radiation-curable coating, this aspect is the same situation whether it is an epoxy, polyester, or urethane oligomer. Having chosen a suitable oligomer, the cure characteristics are considered. This is usually a compromise of photo-initiator level and type as well as functionality level. Lastly comes the additive package designed to cover the holes in the property portfolio or application method. While this may be a gross oversimplification of the development process of radiation-curable coatings, there can be little doubt that the additive package is
*
Corresponding author. E-mail address: [email protected] (J.W. Hong).
usually the last resort of a frustrated formulator and its development is often afforded ‘‘art’’ status [1]. The most prevalent additive used is silicone. The use of silicone additives has progressed from the early days of polydimethylsiloxane (PDMS), as it is commonly known [2]. In addition to changing the surface tension of the coating, silicone additives also influence other properties such as slip, leveling, and mar and scratch resistance. PDMS contains no chemical groups that could take part in the cross-linking reactions of any binder. Therefore, it may be removed (at least partly) from the coating surface relatively easily, e.g., by cleaning the surface with solvents. This means that the improvement to wetting, slip properties, and scratch resistance by PDMS is only a temporary effect that gradually reduces as the concentration of silicone in the surface decreases. Thus the ability to incorporate and fix silicone functionality into a film has much appeal. Radiation-curable silicones offer this possibility, and radiation-curable silicone oligomers and monomers are now widely used in numerous applications. Any method of fixing silicone requires that reactive groups are attached to the PDMS structure.
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00133-2
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H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
There are three approaches to preparing radiationcurable silicones: thiol/thiolene-type silicones, cationic epoxy silicones, and acrylate functional silicones. However, it is claimed that acrylate functional silicones offer the best comprise of overall properties, including better shelf life and producing fewer side effects [3–5]. Although many studies have investigated acrylate functional silicones [6–10], little attention has been given to depth-profile analysis of coating films containing AFPDMS. In the study reported in this paper, various methods were used to analyze the influence of the amount of AFPDMS on the surface properties and the morphology of UV-cured films. In addition, the distribution of AFPDMS in UV-cured films was observed at the film–air (FA) and film–substrate (FS) interfaces using FTIRATR and Raman spectroscopy.
2. Experimental 2.1. Materials Polyester acrylate (Ebecry 830: UCB chemicals) and trimethylol propane triacrylate (TMPTA: Miwon) were used as the oligomer and the triacrylate monomer, respectively. Tripropylene glycol diacrylate (TPGDA: Miwon) and 1,6-hexandial diacrylate (HDDA: BASF) were used as the diacrylate monomer. 2-Hydroxy-2methyl-1-phenyl-propane-1-one (DC 1173: Ciba-Geigy) was added as the photoinitiator. The chemical structure of the silicone acrylate (a; x-acryloxy organofunctional PDMS: Goldschmidt) used in the experiment is shown in Fig. 1. 2.2. Formulation, coating, and curing procedure The basic chemical compositions of the formulations are shown in Table 1. The liquid formulations were spread with a 15 lm bar coater on glasses and then cured in the presence of air, by one pass under a medium pressure mercury lamp (80 W/cm). The light intensity was measured to be 380 mW/cm2 at the sample position. 2.3. Measurements The film properties were measured 24 h after UV exposure. Pencil hardness (ASTM D 3363-74) and
O
CH3
H2C CHC OCH2 CH CH2 O(CH2)3 Si OH
CH3
Table 1 Composition of the tested formulations (%) Polyester acrylate TMPTA TPGDA HDDA DC 1173 AF-PDMS
UV1
UV2
UV3
UV4
40 25 20 5 4 –
40 25 20 5 4 1
40 25 20 5 4 3
40 25 20 5 4 5
methylethyl ketone (MEK) double rubs (ASTM D 4752) were measured according to the standard methods as indicated. In addition, the gloss (ASTM D 523) of the coatings was measured on Leneta test papers using a gloss meter from Sheen. The values reported here represent the averages from 10 measurements. Swelling was detected by differences in weight between the dried film and the swelled film. For accurate evaluation of the weight of the dried film, the original films were immersed in MEK for 1 day to remove impurities from the film surface and unreacted materials, and dried to a constant weight in a vacuum oven for 1 day. The weight of the film swelled with solvent was measured after quickly blotting the film between sheets of filter paper. The percentage swelling was calculated as follows: Swelling ð%Þ ¼ ðWs Wd Þ=Wd 100 where Ws and Wd are the weight of the swelled film and the dried film, respectively. The surface tension of the UV-cured films was estimated by Lewis acid–base theory using the contact angle data with water, formamide, and diiodomethane as wetting liquids. For each liquid, five drops were placed on the surface of the UV-cured film, and contact angle readings were taken from both left and right sides of the liquid–air–solid interface. Microscopic FTIR-ATR spectra were recorded using an IRls from Spectra Tech, which was purged continuously with purified air that was free of carbon dioxide and water (using an air dryer from New Techniques). To enhance the signal-to-noise ratio, each of the reference and sample spectra represent a collection of 128 scans recorded at 8 cm1 resolution. The spectrometer was equipped with a variable-angle ATR attachment. In an
CH3 O Si
n
CH3
O (CH2)3 O CH2 CH CH2 OC CH OH
Fig. 1. The chemical structure of silicone acrylate.
CH2
H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
effort to obtain molecular information from various depths, 45° parallelogram prisms of ZeSe and Ge were used as an internal reflection element, respectively. The depth of penetration (dp ) is evaluated according to the following equation [11]: dp ¼
k 2pðn21
sin h n22 Þ1=2 2
where k is the wavelength of the electromagnetic radiation and h is the angle of the incidence. n1 and n2 are the refractive indices of the reflection element and the sample, respectively. The dp values at 811 cm1 are approximately 0.8 lm (n1 ¼ 4) and 2.4 lm (n1 ¼ 2:4) from the FA and FS interfaces, respectively. FTIR-ATR spectra were analyzed using GRAMS/32 software from Galactic Industries Corporation. Raman spectra were taken of the free-film clearcoats using a Renishaw 1000 Raman microscope that utilized a 632.8 nm HeNe laser and a 50 objective. Laser power on the sample was controlled at 3 mW and focused to a spot size of 1 lm. Samples were put on the slide glass and all the spectra were calibrated to the silicon band at 521 cm1 . The spectra were recorded 5–10 scans depending on the samples and the spectral resolution was within 2 cm1 . The depth profiles of Raman spectra were obtained by moving the focus down into the sample in steps of 1 lm. Measurements were performed at 0, 1, 2 and 4 lm from the FA and FS interfaces of the sample. The morphology of the UV-cured films was studied using scanning electron microscopy (SEM; Hitachi S-4700). The photo-DSC experiments were conducted using a differential scanning calorimeter equipped with a photocalorimetric accessory (TA 5000/DSC 2920), with the samples being placed in uncovered aluminum pans. The initiation light source was a 200 W high-pressure mercury lamp, which produced a radiation intensity at the sample of 33 mW/cm2 over a wavelength range of 200– 440 nm. TA Instruments software was employed to obtain the results from the photo-DSC experiments.
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3. Results and discussion The effect of the inclusion of AF-PDMS on the surface properties of the UV-curable coating is summarized in Table 2. As the concentration of AF-PDMS in the formulation increases from 0% to 5%, pencil hardness increases to a maximum value for a concentration of 1% and then decreases at higher concentrations. It is apparent that solvent resistance and gloss show the same trend as pencil hardness. The first question is why there is a peak in the surface properties of coating film. It is expected that the addition of 1% AF-PDMS imparts a low coefficient of friction to the coating film and also improves the scratch resistance. Thus, an increase in the pencil hardness from 2H for sample UV1 (as defined in Table 1) to 3H for UV2 is not surprising. What is surprising is the sudden drop in pencil hardness as well as solvent resistance and gloss as the PDMS concentration is increased above 3%. It is expected that there are potential incompatibility problems with other components present in the coating formulation. In general, it is known that increasing the amount of PDMS above a certain level produces no improvements in the scratch resistance and slip; and using more PDMS only increases the possibility of negative side effects such as loss of intercoat adhesion as well as the above-mentioned incompatibility problem [12]. Since pencil hardness and solvent resistance are determined primarily by structural parameters such as chain flexibility and cross-link density, it is appropriate to consider the cross-link density of the samples. Because swelling ability is dependent on the texture of the film, we expected the swelling ratio to correlate with the cross-link density: if the film has higher cross-linking, it will show less swelling. This is reflected in the results of swelling ratios of various samples shown in the Fig. 2. Samples UV1 and UV4 exhibit the lowest and the highest swelling ratios, respectively, whereas no significant difference is evident between the swelling properties of UV2 and UV1. Henceforth we will not discuss UV4 because its surface properties and cross-link density are outside the specifications required in this paper.
Table 2 Film properties and curing parameters of UV-cured coatings Sample Properties Pencil hardness (on glass) Solvent resistance (times) Gloss (20°) Surface tension (mN/m) Film–air Film–substrate Induction time (s) DH (J/g)
UV1
UV2
UV3
UV4
2H <200 69
3H >200 74
2H <200 70
F <200 58
38.1 43.2 0.66 370
37.4 44.3 0.77 357
40.3 45.3 0.97 321
44.1 46.0 1.05 302
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Fig. 2. Swelling ratio of four different UV-cured films as a function of time.
Another possible reason for the peak in surface properties is the behavior of film formation of the UVcured samples as a function of AF-PDMS concentration. Fig. 3 shows SEM images of the FA interface of various UV-cured films. The surface of UV2 is remarkably different from those of UV1 and UV3, which are wave-like and rough. These SEM data are consistent with the gloss data in Table 2. In principle, coating gloss is a complex phenomena resulting from the interaction between light and the surface of the coating, and is affected strongly by surface roughness [13]. Therefore, one would expect that the increased gloss of UV2 is due to its surface morphology being more uniform than that of the other UV-cured films (i.e., UV1 and UV3). At this point, the question arises why wrinkles form in the coating networks of UV1 and UV3. A depth-profile analysis would clearly illustrate the above phenomena, so surface tension measurement, FTIR-ATR, and Raman spectroscopy were used to build a depth profile of UV-cured films from the interfaces to the core. Let us begin with surface tension measurements of the various films. The surface tensions of the FA and FS interfaces as a function of AF-PDMS concentration are shown in Table 2. It is clear that the surface tensions at the FA interfaces are lower than at the FS interfaces, indicating that the former interface is more hydrophobic. The lowest surface tension occurs in sample UV2, so we predict that the highest concentration of AF-PDMS occurs at the FA interface of the UV2 film. In order to explain the above observations, the content of AF-PDMS and unreacted acrylic double bonds in the coating network was measured by FTIR-ATR, as shown in Fig. 4. The most pronounced changes in the ATR spectra induced by adding the AF-PDMS into the coating formulation occurred at 1022 cm1 which is characteristic of asymmetric siloxane (Si–O–Si) stretch-
Fig. 3. Scanning electron micrographs at the film–air interface of UV-cured films: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
ing vibrations. The intensity of the 1022 cm1 band increases as AF-PDMS concentration increases in the formulation. This peak can thus be used to calculate the concentration of siloxane segments in the UV-cured film. It should be also noted that the relative intensity of 811 cm1 increases with increasing amounts of AFPDMS. Since the infrared band at 811 cm1 is due to the C–H deformation mode of the acryl groups, it can be concluded that adding AF-PDMS into the formulation
H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
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1635
(a) 1599
(a)
(b)
1022
811
2000
Rel.Int
(c) 1800
1600
1400
1200
1000
(b)
800
Wavenumbers (cm ¯1) Fig. 4. FTIR-ATR spectra at the film–air interface of UVcured films: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
decreases the degree of cross-linking. We admit that there is no appropriate internal reference in the infrared spectra that can be used to measure quantitatively the concentration of AF-PDMS and unreacted acrylic double bond. Nevertheless, we tried to derive the subtraction spectrum between infrared spectra of the FA and FS surfaces. We applied an automatic spectral subtraction program, which utilizes an iterative leastsquares method to optimize the scaling factor, and results are shown in Fig. 5. The bands at 1020 cm1 and 811 cm1 are observed as positive bands. This means that the siloxane group of AF-PDMS and unreacted acrylic double bonds are enriched at the film–air interface. It is surprising that the degree of cross-linking is lower at the surface than at the bottom for a 15-lm film.
(a)
(b)
1022
(c) 2000
1800
1600
1400
1200
1000
811
800
Fig. 5. FTIR-ATR spectra of UV-cured films containing 1 wt.% of AF-PDMS: (a) film–air interface, (b) film–substrate interface, and (c) subtraction spectrum (a–b).
(c)
1660
1650
1640
1630
1620
1610
1600
1590
1580
wavenumbers (cm-1 )
Fig. 6. Raman spectra in the 1650–1585 cm1 range recorded at the film–air interface of UV-cured films: (a) without AFPDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AFPDMS.
The concentration of unreacted acrylic double bonds as a function of depth beneath the film surface was observed using Raman spectroscopy. Fig. 6 shows the 1650–1585 cm1 region of the Raman spectra collected at the FA interface of the UV-cured films. Unlike infrared spectra, Raman spectra have an isolated internal reference peak at 1599 cm1 due to aromatic C@C stretching in polyester oligomer next to the 1635 cm1 band due to acryl C@C stretching. We assume that this band is independent from the cross-linking reaction. It is clearly seen from the figure that the degree of surface cross-linking decreases as the concentration of AFPDMS increases in the formulation. From these data we are able to perform a depth-profile analysis of the various films using Raman spectroscopy. Fig. 7 shows the relative intensity of the 1635 and 1599 cm1 bands as a function of the depth of penetration of Raman light. Again, the degree of cure at the FA interface is lower than that of FS interface for all samples, which is due to the effect of oxygen inhibition that is notorious in freeradical photopolymerization. However, this effect is more important for PDMS derivatives, because in many of these oxygen is both freely soluble and exhibits a high diffusion coefficient [14,15]. It is interesting to compare
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H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
600
UV 1 UV 2 UV 3
4
a
Heat Flow (mW)
Relative intensity (1635/1599)
5
3
2
b
450
c 300
150 1 0
2
4
6
8
10
12
14
16
Depth of penetration (µm) Fig. 7. Plot of Raman intensity ratio (1635/1599) vs. depth of penetration (d, UV1: without AF-PDMS; j, UV2: with 1 wt.% AF-PDMS; m, UV3: with 3 wt.% AF-PDMS).
the slope of the relative intensity vs. depth of UV2 with those of UV1 and UV3: the slopes for UV1 and UV3 films are notably steeper than that for UV2 film. In light of these results and the SEM data of Fig. 3, it is evident that the abrupt change of degree of cure with film thickness results in the wave-like roughness of the UV1 and UV3 films. It should also be noted that the boundary area of oxygen inhibition can be measured by extrapolating the relative intensity–depth curves to meet. As the concentration of AF-PDMS in the formulation increases, the boundary area of oxygen inhibition is increased from surface to core. It is due to the AF-PDMS in the coating network enhancing the transportation of oxygen from air to film core, resulting in oxygen inhibition in the free-radical photopolymerization. Our data demonstrate that UV curing with AF-PDMS is very sensitive to the oxygen inhibition, indicating that an inert environment such as nitrogen purging is required. In order to clarify the oxygen inhibition effect of UV curing with AF-PDMS on the curing behavior, we investigate the kinetics of photopolymerization with and without AF-PDMS using photo-DSC in an air atmosphere. Fig. 8 shows the photo-DSC exotherm curves for photopolymerization of UV1, UV2, and UV3. From the peak symmetry, induction time (IT), and the time to peak maximum (PM), one can obtain information such as the optimum ratio of monomer to oligomer, the photoinitiator efficiency, and the curing rate [16]. Specially, the oxygen inhibition can be compared by the induction time, corresponding to 1% of the conversion. The results presented in Table 2 show that IT increases and DH decreases with increasing concentration of AFPDMS, indicating that both the initial cure rate and the conversion decrease. These photo-DSC results clearly demonstrate that the photopolymerization of UV curing
0 1.4
1.5
1.6
1.7
1.8
1.9
2.0
Time (min) Fig. 8. Photo-DSC exotherm curves for photopolymerization: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
with AF-PDMS in an air atmosphere is strongly inhibited by the oxygen compared to UV curing without AFPDMS.
4. Conclusion The effect of the amount of AF-PDMS included in UV-curable coatings on the properties of the cured film has been investigated. We have demonstrated that less than 1 wt.% of AF-PDMS should be added into a UVcurable polyester acrylate-based coating. If the AFPDMS content is greater than this, surface and bulk properties decline dramatically. The depth-profile analysis shows that the degree of cure at the FA interface is higher than that at the film–substrate interface, which is due to oxygen inhibition in films with a thickness of less than 15 lm. The microscopic roughness of UV-cured film––which we have shown in our SEM analyses–– arises from the abrupt change of cure degree with film thickness. Raman depth-profile analysis proved to be a powerful technique for investigating the film formation behavior of UV-cured films. Photo-DSC is also well suited to studying the oxygen inhibition effect of UV curing system with AF-PDMS.
Acknowledgement This study was supported by research funds from Chosun University, 2002.
H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
References [1] Tonge JS, Blizzard JD, Washer TR. In: Proceedings of Rad Tech, North America, 1994. p. 581. [2] Paul S. Silicones resins in surface coating. New York: Wiley; 1985. p. 200. [3] Kawakami Y, Murthy RAN, Yamashita Y. Macromol Chem 1984:185. [4] Mazurek M, Kinning DJ, Kinishita T. J Appl Polym Sci 2001;80:160. [5] Allen NS, Johnson MS, Oldering PKT, Salim S. In: Oldering PKT, editor. Chemistry and technology of UV and EB formulation for coatings, inks and paints, vol. II. SITA Technology; 1991. p. 140. [6] Davidson RS, Ellis R, Tudor S, Wilkinson SA. Polymer 1992;35:3031. [7] M€ uller U, Screhmel B, Neuenfeld J. Makromol Chem Rapid Commun 1989;10:539.
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[8] M€ uller U, Timpe HJ, Neuenfeld J. Eur Polym J 1991;27: 621. [9] M€ uller U. JMS Pure Appl Chem 1996;33A:34. [10] Miller HC. In: Proceedings of Rad Tech, North America, New Orleans, 1988. p. 173–82. [11] Harrick N. Internal reflection spectroscopy. New York: Interscience; 1967. p. 30. [12] Bieleman J, Scholz W, Heilen W, Ferner U, L€ uers G. In: Bieleman J, editor. Additives for coatings. Wiley-VCH Verlag; 2000. p. 143. [13] Wicks ZW, Jones FN, Pappas SP. Organic coatings: science and technology, vol. 1. New York: John Wiley; 1994. [14] Ohyanagi M, Nishide H, Snenaga K, Tsuchida E. Polym Bull 1990;23:637. [15] M€ uller U, Jockusch S, Timpe HJ. J Polym Sci Polym Chem 1992;30:2758. [16] Hong JW, Lee HW. J Korean Ind Eng Chem 1994;5:860.
Characterization of UV-cured polyester acrylate films containing acrylate functional polydimethylsiloxane H.K. Kim, H.T. Ju, J.W. Hong
*
Department of Polymer Science and Engineering, College of Engineering, Chosun University, 375 Seosuck-dong Dong-gu, Gwangju 501-759, South Korea Received 23 October 2002; received in revised form 25 April 2003; accepted 28 May 2003
Abstract Acrylate functional polydimethylsiloxane (AF-PDMS) was tested as a reactive additive in UV-curable coating formulations. Pencil hardness, solvent resistance, and gloss of the UV-cured films were measured to study the influence of AF-PDMS content on coating properties. Depth-profile analysis by FTIR-ATR and Raman spectroscopy was also performed to investigate the effect of AF-PDMS on the behavior of film formation during UV curing. The kinetics of photopolymerization were monitored by photo differential scanning calorimetry (photo-DSC). Our results show that AF-PDMS containing coating formulations are very sensitive to oxygen inhibition, so that an inert environment such as nitrogen purging is required to avoid coating defects. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Acrylate functional polydimethylsiloxane; UV curable; FTIR-ATR; Raman spectroscopy; Depth profile
1. Introduction The development of any coatings is generally a compromise between performance attributes (both in terms of the properties of the finished coating and the application of it) and cost. Neglecting the cost aspects for the moment, the first stages of the development of a coating concentrate on the polymeric binder. For a radiation-curable coating, this aspect is the same situation whether it is an epoxy, polyester, or urethane oligomer. Having chosen a suitable oligomer, the cure characteristics are considered. This is usually a compromise of photo-initiator level and type as well as functionality level. Lastly comes the additive package designed to cover the holes in the property portfolio or application method. While this may be a gross oversimplification of the development process of radiation-curable coatings, there can be little doubt that the additive package is
*
Corresponding author. E-mail address: [email protected] (J.W. Hong).
usually the last resort of a frustrated formulator and its development is often afforded ‘‘art’’ status [1]. The most prevalent additive used is silicone. The use of silicone additives has progressed from the early days of polydimethylsiloxane (PDMS), as it is commonly known [2]. In addition to changing the surface tension of the coating, silicone additives also influence other properties such as slip, leveling, and mar and scratch resistance. PDMS contains no chemical groups that could take part in the cross-linking reactions of any binder. Therefore, it may be removed (at least partly) from the coating surface relatively easily, e.g., by cleaning the surface with solvents. This means that the improvement to wetting, slip properties, and scratch resistance by PDMS is only a temporary effect that gradually reduces as the concentration of silicone in the surface decreases. Thus the ability to incorporate and fix silicone functionality into a film has much appeal. Radiation-curable silicones offer this possibility, and radiation-curable silicone oligomers and monomers are now widely used in numerous applications. Any method of fixing silicone requires that reactive groups are attached to the PDMS structure.
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00133-2
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H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
There are three approaches to preparing radiationcurable silicones: thiol/thiolene-type silicones, cationic epoxy silicones, and acrylate functional silicones. However, it is claimed that acrylate functional silicones offer the best comprise of overall properties, including better shelf life and producing fewer side effects [3–5]. Although many studies have investigated acrylate functional silicones [6–10], little attention has been given to depth-profile analysis of coating films containing AFPDMS. In the study reported in this paper, various methods were used to analyze the influence of the amount of AFPDMS on the surface properties and the morphology of UV-cured films. In addition, the distribution of AFPDMS in UV-cured films was observed at the film–air (FA) and film–substrate (FS) interfaces using FTIRATR and Raman spectroscopy.
2. Experimental 2.1. Materials Polyester acrylate (Ebecry 830: UCB chemicals) and trimethylol propane triacrylate (TMPTA: Miwon) were used as the oligomer and the triacrylate monomer, respectively. Tripropylene glycol diacrylate (TPGDA: Miwon) and 1,6-hexandial diacrylate (HDDA: BASF) were used as the diacrylate monomer. 2-Hydroxy-2methyl-1-phenyl-propane-1-one (DC 1173: Ciba-Geigy) was added as the photoinitiator. The chemical structure of the silicone acrylate (a; x-acryloxy organofunctional PDMS: Goldschmidt) used in the experiment is shown in Fig. 1. 2.2. Formulation, coating, and curing procedure The basic chemical compositions of the formulations are shown in Table 1. The liquid formulations were spread with a 15 lm bar coater on glasses and then cured in the presence of air, by one pass under a medium pressure mercury lamp (80 W/cm). The light intensity was measured to be 380 mW/cm2 at the sample position. 2.3. Measurements The film properties were measured 24 h after UV exposure. Pencil hardness (ASTM D 3363-74) and
O
CH3
H2C CHC OCH2 CH CH2 O(CH2)3 Si OH
CH3
Table 1 Composition of the tested formulations (%) Polyester acrylate TMPTA TPGDA HDDA DC 1173 AF-PDMS
UV1
UV2
UV3
UV4
40 25 20 5 4 –
40 25 20 5 4 1
40 25 20 5 4 3
40 25 20 5 4 5
methylethyl ketone (MEK) double rubs (ASTM D 4752) were measured according to the standard methods as indicated. In addition, the gloss (ASTM D 523) of the coatings was measured on Leneta test papers using a gloss meter from Sheen. The values reported here represent the averages from 10 measurements. Swelling was detected by differences in weight between the dried film and the swelled film. For accurate evaluation of the weight of the dried film, the original films were immersed in MEK for 1 day to remove impurities from the film surface and unreacted materials, and dried to a constant weight in a vacuum oven for 1 day. The weight of the film swelled with solvent was measured after quickly blotting the film between sheets of filter paper. The percentage swelling was calculated as follows: Swelling ð%Þ ¼ ðWs Wd Þ=Wd 100 where Ws and Wd are the weight of the swelled film and the dried film, respectively. The surface tension of the UV-cured films was estimated by Lewis acid–base theory using the contact angle data with water, formamide, and diiodomethane as wetting liquids. For each liquid, five drops were placed on the surface of the UV-cured film, and contact angle readings were taken from both left and right sides of the liquid–air–solid interface. Microscopic FTIR-ATR spectra were recorded using an IRls from Spectra Tech, which was purged continuously with purified air that was free of carbon dioxide and water (using an air dryer from New Techniques). To enhance the signal-to-noise ratio, each of the reference and sample spectra represent a collection of 128 scans recorded at 8 cm1 resolution. The spectrometer was equipped with a variable-angle ATR attachment. In an
CH3 O Si
n
CH3
O (CH2)3 O CH2 CH CH2 OC CH OH
Fig. 1. The chemical structure of silicone acrylate.
CH2
H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
effort to obtain molecular information from various depths, 45° parallelogram prisms of ZeSe and Ge were used as an internal reflection element, respectively. The depth of penetration (dp ) is evaluated according to the following equation [11]: dp ¼
k 2pðn21
sin h n22 Þ1=2 2
where k is the wavelength of the electromagnetic radiation and h is the angle of the incidence. n1 and n2 are the refractive indices of the reflection element and the sample, respectively. The dp values at 811 cm1 are approximately 0.8 lm (n1 ¼ 4) and 2.4 lm (n1 ¼ 2:4) from the FA and FS interfaces, respectively. FTIR-ATR spectra were analyzed using GRAMS/32 software from Galactic Industries Corporation. Raman spectra were taken of the free-film clearcoats using a Renishaw 1000 Raman microscope that utilized a 632.8 nm HeNe laser and a 50 objective. Laser power on the sample was controlled at 3 mW and focused to a spot size of 1 lm. Samples were put on the slide glass and all the spectra were calibrated to the silicon band at 521 cm1 . The spectra were recorded 5–10 scans depending on the samples and the spectral resolution was within 2 cm1 . The depth profiles of Raman spectra were obtained by moving the focus down into the sample in steps of 1 lm. Measurements were performed at 0, 1, 2 and 4 lm from the FA and FS interfaces of the sample. The morphology of the UV-cured films was studied using scanning electron microscopy (SEM; Hitachi S-4700). The photo-DSC experiments were conducted using a differential scanning calorimeter equipped with a photocalorimetric accessory (TA 5000/DSC 2920), with the samples being placed in uncovered aluminum pans. The initiation light source was a 200 W high-pressure mercury lamp, which produced a radiation intensity at the sample of 33 mW/cm2 over a wavelength range of 200– 440 nm. TA Instruments software was employed to obtain the results from the photo-DSC experiments.
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3. Results and discussion The effect of the inclusion of AF-PDMS on the surface properties of the UV-curable coating is summarized in Table 2. As the concentration of AF-PDMS in the formulation increases from 0% to 5%, pencil hardness increases to a maximum value for a concentration of 1% and then decreases at higher concentrations. It is apparent that solvent resistance and gloss show the same trend as pencil hardness. The first question is why there is a peak in the surface properties of coating film. It is expected that the addition of 1% AF-PDMS imparts a low coefficient of friction to the coating film and also improves the scratch resistance. Thus, an increase in the pencil hardness from 2H for sample UV1 (as defined in Table 1) to 3H for UV2 is not surprising. What is surprising is the sudden drop in pencil hardness as well as solvent resistance and gloss as the PDMS concentration is increased above 3%. It is expected that there are potential incompatibility problems with other components present in the coating formulation. In general, it is known that increasing the amount of PDMS above a certain level produces no improvements in the scratch resistance and slip; and using more PDMS only increases the possibility of negative side effects such as loss of intercoat adhesion as well as the above-mentioned incompatibility problem [12]. Since pencil hardness and solvent resistance are determined primarily by structural parameters such as chain flexibility and cross-link density, it is appropriate to consider the cross-link density of the samples. Because swelling ability is dependent on the texture of the film, we expected the swelling ratio to correlate with the cross-link density: if the film has higher cross-linking, it will show less swelling. This is reflected in the results of swelling ratios of various samples shown in the Fig. 2. Samples UV1 and UV4 exhibit the lowest and the highest swelling ratios, respectively, whereas no significant difference is evident between the swelling properties of UV2 and UV1. Henceforth we will not discuss UV4 because its surface properties and cross-link density are outside the specifications required in this paper.
Table 2 Film properties and curing parameters of UV-cured coatings Sample Properties Pencil hardness (on glass) Solvent resistance (times) Gloss (20°) Surface tension (mN/m) Film–air Film–substrate Induction time (s) DH (J/g)
UV1
UV2
UV3
UV4
2H <200 69
3H >200 74
2H <200 70
F <200 58
38.1 43.2 0.66 370
37.4 44.3 0.77 357
40.3 45.3 0.97 321
44.1 46.0 1.05 302
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Fig. 2. Swelling ratio of four different UV-cured films as a function of time.
Another possible reason for the peak in surface properties is the behavior of film formation of the UVcured samples as a function of AF-PDMS concentration. Fig. 3 shows SEM images of the FA interface of various UV-cured films. The surface of UV2 is remarkably different from those of UV1 and UV3, which are wave-like and rough. These SEM data are consistent with the gloss data in Table 2. In principle, coating gloss is a complex phenomena resulting from the interaction between light and the surface of the coating, and is affected strongly by surface roughness [13]. Therefore, one would expect that the increased gloss of UV2 is due to its surface morphology being more uniform than that of the other UV-cured films (i.e., UV1 and UV3). At this point, the question arises why wrinkles form in the coating networks of UV1 and UV3. A depth-profile analysis would clearly illustrate the above phenomena, so surface tension measurement, FTIR-ATR, and Raman spectroscopy were used to build a depth profile of UV-cured films from the interfaces to the core. Let us begin with surface tension measurements of the various films. The surface tensions of the FA and FS interfaces as a function of AF-PDMS concentration are shown in Table 2. It is clear that the surface tensions at the FA interfaces are lower than at the FS interfaces, indicating that the former interface is more hydrophobic. The lowest surface tension occurs in sample UV2, so we predict that the highest concentration of AF-PDMS occurs at the FA interface of the UV2 film. In order to explain the above observations, the content of AF-PDMS and unreacted acrylic double bonds in the coating network was measured by FTIR-ATR, as shown in Fig. 4. The most pronounced changes in the ATR spectra induced by adding the AF-PDMS into the coating formulation occurred at 1022 cm1 which is characteristic of asymmetric siloxane (Si–O–Si) stretch-
Fig. 3. Scanning electron micrographs at the film–air interface of UV-cured films: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
ing vibrations. The intensity of the 1022 cm1 band increases as AF-PDMS concentration increases in the formulation. This peak can thus be used to calculate the concentration of siloxane segments in the UV-cured film. It should be also noted that the relative intensity of 811 cm1 increases with increasing amounts of AFPDMS. Since the infrared band at 811 cm1 is due to the C–H deformation mode of the acryl groups, it can be concluded that adding AF-PDMS into the formulation
H.K. Kim et al. / European Polymer Journal 39 (2003) 2235–2241
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1635
(a) 1599
(a)
(b)
1022
811
2000
Rel.Int
(c) 1800
1600
1400
1200
1000
(b)
800
Wavenumbers (cm ¯1) Fig. 4. FTIR-ATR spectra at the film–air interface of UVcured films: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
decreases the degree of cross-linking. We admit that there is no appropriate internal reference in the infrared spectra that can be used to measure quantitatively the concentration of AF-PDMS and unreacted acrylic double bond. Nevertheless, we tried to derive the subtraction spectrum between infrared spectra of the FA and FS surfaces. We applied an automatic spectral subtraction program, which utilizes an iterative leastsquares method to optimize the scaling factor, and results are shown in Fig. 5. The bands at 1020 cm1 and 811 cm1 are observed as positive bands. This means that the siloxane group of AF-PDMS and unreacted acrylic double bonds are enriched at the film–air interface. It is surprising that the degree of cross-linking is lower at the surface than at the bottom for a 15-lm film.
(a)
(b)
1022
(c) 2000
1800
1600
1400
1200
1000
811
800
Fig. 5. FTIR-ATR spectra of UV-cured films containing 1 wt.% of AF-PDMS: (a) film–air interface, (b) film–substrate interface, and (c) subtraction spectrum (a–b).
(c)
1660
1650
1640
1630
1620
1610
1600
1590
1580
wavenumbers (cm-1 )
Fig. 6. Raman spectra in the 1650–1585 cm1 range recorded at the film–air interface of UV-cured films: (a) without AFPDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AFPDMS.
The concentration of unreacted acrylic double bonds as a function of depth beneath the film surface was observed using Raman spectroscopy. Fig. 6 shows the 1650–1585 cm1 region of the Raman spectra collected at the FA interface of the UV-cured films. Unlike infrared spectra, Raman spectra have an isolated internal reference peak at 1599 cm1 due to aromatic C@C stretching in polyester oligomer next to the 1635 cm1 band due to acryl C@C stretching. We assume that this band is independent from the cross-linking reaction. It is clearly seen from the figure that the degree of surface cross-linking decreases as the concentration of AFPDMS increases in the formulation. From these data we are able to perform a depth-profile analysis of the various films using Raman spectroscopy. Fig. 7 shows the relative intensity of the 1635 and 1599 cm1 bands as a function of the depth of penetration of Raman light. Again, the degree of cure at the FA interface is lower than that of FS interface for all samples, which is due to the effect of oxygen inhibition that is notorious in freeradical photopolymerization. However, this effect is more important for PDMS derivatives, because in many of these oxygen is both freely soluble and exhibits a high diffusion coefficient [14,15]. It is interesting to compare
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600
UV 1 UV 2 UV 3
4
a
Heat Flow (mW)
Relative intensity (1635/1599)
5
3
2
b
450
c 300
150 1 0
2
4
6
8
10
12
14
16
Depth of penetration (µm) Fig. 7. Plot of Raman intensity ratio (1635/1599) vs. depth of penetration (d, UV1: without AF-PDMS; j, UV2: with 1 wt.% AF-PDMS; m, UV3: with 3 wt.% AF-PDMS).
the slope of the relative intensity vs. depth of UV2 with those of UV1 and UV3: the slopes for UV1 and UV3 films are notably steeper than that for UV2 film. In light of these results and the SEM data of Fig. 3, it is evident that the abrupt change of degree of cure with film thickness results in the wave-like roughness of the UV1 and UV3 films. It should also be noted that the boundary area of oxygen inhibition can be measured by extrapolating the relative intensity–depth curves to meet. As the concentration of AF-PDMS in the formulation increases, the boundary area of oxygen inhibition is increased from surface to core. It is due to the AF-PDMS in the coating network enhancing the transportation of oxygen from air to film core, resulting in oxygen inhibition in the free-radical photopolymerization. Our data demonstrate that UV curing with AF-PDMS is very sensitive to the oxygen inhibition, indicating that an inert environment such as nitrogen purging is required. In order to clarify the oxygen inhibition effect of UV curing with AF-PDMS on the curing behavior, we investigate the kinetics of photopolymerization with and without AF-PDMS using photo-DSC in an air atmosphere. Fig. 8 shows the photo-DSC exotherm curves for photopolymerization of UV1, UV2, and UV3. From the peak symmetry, induction time (IT), and the time to peak maximum (PM), one can obtain information such as the optimum ratio of monomer to oligomer, the photoinitiator efficiency, and the curing rate [16]. Specially, the oxygen inhibition can be compared by the induction time, corresponding to 1% of the conversion. The results presented in Table 2 show that IT increases and DH decreases with increasing concentration of AFPDMS, indicating that both the initial cure rate and the conversion decrease. These photo-DSC results clearly demonstrate that the photopolymerization of UV curing
0 1.4
1.5
1.6
1.7
1.8
1.9
2.0
Time (min) Fig. 8. Photo-DSC exotherm curves for photopolymerization: (a) without AF-PDMS, (b) with 1 wt.% AF-PDMS, and (c) with 3 wt.% AF-PDMS.
with AF-PDMS in an air atmosphere is strongly inhibited by the oxygen compared to UV curing without AFPDMS.
4. Conclusion The effect of the amount of AF-PDMS included in UV-curable coatings on the properties of the cured film has been investigated. We have demonstrated that less than 1 wt.% of AF-PDMS should be added into a UVcurable polyester acrylate-based coating. If the AFPDMS content is greater than this, surface and bulk properties decline dramatically. The depth-profile analysis shows that the degree of cure at the FA interface is higher than that at the film–substrate interface, which is due to oxygen inhibition in films with a thickness of less than 15 lm. The microscopic roughness of UV-cured film––which we have shown in our SEM analyses–– arises from the abrupt change of cure degree with film thickness. Raman depth-profile analysis proved to be a powerful technique for investigating the film formation behavior of UV-cured films. Photo-DSC is also well suited to studying the oxygen inhibition effect of UV curing system with AF-PDMS.
Acknowledgement This study was supported by research funds from Chosun University, 2002.
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