Hard coatings on polycarbonate plate by sol-gel reactions of melamine derivative, PHEMA, and silicates.
During the past thirty years, there has been considerable research into substitutes for glass in cars, such as polycarbonate, because glass is heavy and quite fragile. The density of glass is more than double that of polycarbonate. Therefore, the weight of the glass in cars could be reduced by more than 30% if the glass in cars could be substituted with plastics such as polycarbonate. The advantage of such a weigh reduction would be an improvement in fuel efficiency, and the possibility of a smaller running motor in the door of cars so that very thin car door can be designed. In addition, a curved surface and coloring could be diversified, and more aerodynamic figure can be designed. Because polycarbonate plate is difficult to break, the driver and a passenger in cars cannot be propelled out from the car during an accident, and it can be helpful to prevent car thefts. Because polycarbonate is a thermoplastic, it can be recycled more easily than glass. Moreover, high technological functions such as UV protection, conductivity, as well as hydrophilic and hydrophobic properties can be provided more easily in a polycarbonate plate than in glass.
A polycarbonate plate has a > 90% transparency of light and a high impact strength. Therefore, it can be used instead of glass in many fields e.g., construction, decoration, and optical lenses. However, it cannot be used broadly due to its poor scratch resistance. For example it can be used in the lens of head lamps in cars but cannot be used for car windows. The reason for this is that the physical properties of the coating currently used on the head lamps of cars are unsuitable for use in car windows. Car windows must allow drivers and passengers to see everything. Hence, tougher scratch resistance is need. Therefore, there is a need for a coating system with excellent scratch resistance.
The sol-gel process is a method for synthesizing oxidized materials, in which the sol made by the hydrolysis in the liquid phase, which is then changed to a gel. Using the sol-gel process, a product can be produced a high degree of purity, and a uniformly mixed product can be produced when more than two reactants are used. In addition, the product can be synthesized at comparatively low temperatures using the sol-gel process. Therefore, many types of figures can be produced, for example thin membranes, fibers, and nano-size materials. In particular, a thin coating on plastics can be performed at a comparatively low temperature using the sol-gel process. Despite its advantages, there are some disadvantages. Generally, the reactants in the sol-gel process are expensive. In the reaction process, shrinkage and crack can easily occur. Therefore, it is difficult to make a large size product material. In addition, special techniques are needed to manage the reactants in the sol-gel process [1-14].
In this lab, a hard coating was deposited on a polycarbonate plate using the sol-gel process. When tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) were used in the coating process, the coating had the hardness of a 2H pencil at the optimum condition (15). When (3-glycidoxypropyl)trimethoxysilane (GPTMS), (3-aminopropyl)triethoxysilane (APTES) and diethylenetriamine were used in the coating process, the coating had the hardness of a 3H pencil (16).
In our study, the hard coatings on polycarbonate plate were deposited by the sol-gel process using poly(2-hydroxyethyl methacrylate) (PHEMA). TEOS, MTES, methylated poly(melamine-co-formaldehyde) (MPMF) and (3-isocyanatopropyl) triethoxysilane (IPTES). PHEMA was used to connect silicate based polymer and melamine based polymer. IPTES was reacted with PHEMA to functionalize PHEMA that the polymer could get triethylsilyl groups. MPMF was used as the reactive monomeric melamine derivative. The hydroxyl group in PHEMA can be partially reacted with an isocyanate group in IPTES to form a urethane group. The remaining hydroxyl group in PHEMA can be reacted with the methoxy group in MPMF through transetherification. In addition, the triethoxysillyl group in IPTES, which was reacted with PHEMA, can be reacted with TEOS and MTES. Therefore, a coating containing both a melamine and silicate structure can be prepared.
Methylated poly(melamine-co-formaldehyde) (average [M.sub.n], 511) (MPMF), TEOS, MTES. 2-hydroxyethyl methacrylate (HEMA), azobisisobutyronitrile (AIBN), 1-dodecanethiol, IPTES, poly(methyl methacrylate) (PMMA) (average [M.sub.w] 120,000), isopropanol (IPA), and anhydrous N,N-dimethylformamide (DMF) were purchased from Aldrich Chemical. The polycarbonate plate was obtained from Spolytec Co.
Infrared and UV spectroscopy was performed using a FTIR 680 (Jasco International) and PU 650 (Beckman) spectrophotometer, respectively. Scanning electron microscopy (SEM) was performed using both a Hitachi S-2500C and Hitachi S-5200V scanning electron microscope, Opticle microscopy was performed using an Olympus Mic-D microscope. Thermogravimetry analysis was performed using a Perkin Eylmer TRY 910.
The pencil hardness was detected using a Pencil hardness tester 221D and a Mitsubishi pencil was used in this pencil hardness tester. The adhesion test was carried out using 3 M Scotch tape. The abrasion resistance test was performed using a Taber abraser Model KPM-042 (CS-10 wheel).
Synthesis of Poly(2-hydroxyethyl methacrylate)
PHEMA, whose molecular weight was controlled using a chain transfer agent, was synthesized by free radical polymerization using AIBN as the initiator and 1-dodecanethiol as the chain transfer agent. HEMA (65.1 g. 0.500 mol) and ethanol (500 mL) were mixed in a 1 L round bottom flask. AIBN (1.30 g, 7.92 mmol) and 1-dodecanethiol (0.202 g, 1.00 mmol) were added to the resulting solution, which was then heated under reflux for 24 h. The polymer was precipitated by pouring the reaction solution into a mixture of benzene and hexane (1:1 v/v). The polymer was washed five times with the benzene/hexane mixture to remove the unreacted monomer. The synthesized PHEMA was dried for 10 h at 60[degrees]C under vacuum.
Synthesis of PHEMA Containing Triethoxysillyl Group (PHEMA-S)
Three types of PHEMA-S (PS40, PS60, PS80) were synthesized using PHEMA and IPTES. 40, 60, 80 indicate the mole percent of triethoxysillyl groups in hydroxyl groups of PHEMA. For example. PS60 was synthesized as follows: PHEMA (0.750 g, 5.76 mmol of OH group) and anhydrous DMF (6.0 mL) were mixed in a 50 mL round bottomed flask. IPTES (0.854 g, 3.46 mmol) was then added to this solution under a nitrogen atmosphere. The reaction was carried out for 15 h at 90[degrees]C. This solution was used to prepare the coating solution without further purification.
Preparation of the Coating Solution
Many formulations were used to prepare the coating solution. A typical example was as follows. Initially, a solution of silicates was prepared. TEOS (1.04 g, 5.00 mmol), MTES (0.892 g, 5.00 mmol), and 3.0 mL of 2-propanol were mixed in a 50 mL round bottomed flask. 2.0 mL of 1.00 mM HN[O.sub.3] was added to the solution. HN[O.sub.3] was used as the catalyst, and the water in a HN[O.sub.3] solution was used to hydrolyze the silicates. The reaction was continued for 48 h at 20[degrees]C.
MPMF (1.50 g) was added to the solution containing PHEMA-S which was already prepared, and the solution was stirred thoroughly. The solution of silicates was then added to this solution.
The polycarbonate plate was precoated with PMMA (average [M.sub.w] 120,000). PMMA (0.50 g) was dissolved in 30 mL of acetone and dropped on to a polycarbonate plate, which was hung down from a horizontal bar perpendicularly for 10 min at room temperature. This plate was dried in an oven for 15 min at 90[degrees]C.
The coatings were carried out by dropping the coating solution on to the polycarbonate plate that had been precoated with PMMA. The size of the polycarbonate plate was 8.0 X 6.0 cm and the amount of the coating solution dropped on to the polycarbonate plate was 1.5 mL. This coated polycarbonate plate was maintained for 10 min at room temperature. The curing was then carried out in three steps. First, curing was performed for 3 h at 80[degrees]C. The second curing was carried out for 12 h at 130[degrees]C Final curing was conducted at 130[degrees] C for 9 h under vacuum.
Pencil Hardness Test
The pencil hardness was measured using a pencil hardness tester with the pencil at an angle of 45[degrees] under a 1 kg loading. The pencil used in this experiment was made by Mitsubishi, the lead of the pencil was ground perpendicularly to give the lead of the pencil an angle of 90[degrees] before measuring the pencil hardness each time.
Adhesion of the coating was measured using a crosscut test method. The coating was divided using the sharp knife and 3 m Scotch tape was then attached to the coating surface. This tape was pulled up by holding the end of the tape, and the amount remaining was counted.
Abrasion Resistance Test
The abrasion resistance was measured using a Taber abraser in which a CS-10 wheel was loaded. The absorbance of the coating was measured at 370 nm using a UV spectrophotometer after rotating the wheel 500 times under a 500 g load.
RESULTS AND DISCUSSION
Synthesis of PHEMA
PHEMA with a low molecular weight was synthesized by radical polymerization using 1-dodecanethiol as the chain transfer agent. The molecular weight of the synthesized PHEMA was determined by measuring the viscosity (17) and GPC. The viscosity average molecular weight of the synthesized PHEMA determined by the viscosity measurements was 9,600. The number average molecular weight and weight average molecular weight of the polymer determined by the GPC was 5,900 and 9,800, respectively. The molecular weight distribution of the polymer was 1.66.
Synthesis of PHEMA Containing Triethoxysillyl Group (PHEMA-S)
The three types of PHEMA-S synthesized in this study were PS40, PS60, and PS80, which contained 40, 60, and 80 mol percent of the triethoxysillyl group, respectively, against the hydroxyl groups of PHEMA. The extent of the reaction was monitored using an IR spectrophotometer by analyzing the reaction solution. The absorption for iso-cyanate groups by IR decreased after the reaction had started, which continued for 12 h. After that, the absorption for isocyanate group was almost dwindled away, and there was no change in the absorption for isocyanate groups. Therefore, the reaction was carried out for 15 h.
Preparation of the Coating Solution
The silicate solution was prepared by mixing TEOS and MTES. Nitric acid, as a catalyst, was then added to this solution with water, which can be reacted to hydrolyze TEOS and MTES. IPA was added as the solvent. This reaction was continued for 48 h at 20[degrees]C. In this reaction, both hydrolysis and condensation reactions proceeded. Scheme 1 shows all the reactions in the coating process.
MPMF was mixed with a PHEMA-S solution, which was already prepared, and the silicate solution was also added to the solution.
A precoating with PMMA was conducted before to coating process to improve the adhesion between the polycarbonate plate and the coating.
The main coatings were carried out by dropping the coating solution on to a polycarbonate plate precoated with PMMA. This coated polycarbonate plate was maintained for 10 min at room temperature to stabilize the coating. Curing was performed in three steps. First curing was carried out at 80[degrees]C for 3 h to evaporate the solvent and byproducts. The second curing step was conducted at 130[degrees]C for 12 h to evaporate the water and solvent. The final curing was performed at 130[degrees]C for 9 h under vacuum to evaporate all the volatile compounds in the coating including DMF. This stepwise curing helps prevent cracking during the curing process. Figure 1 show the entire coating process.
[FIGURE 1 OMITTED]
The Effect of Mole Ratio of TEOS and MTES
Using the polymers with three different compositions, the effect of mole ratio of TEOS and MTES on the properties of the coating was investigated. The results were showed in Table 1.
TABLE 1. The effect of the mole ratio of TEOS and MTES on the property of the coating. Polymer Mole ratio of Coating Ex. no. composition TEOS:MTES Hardness Adhesion surface PS40-1 PS40 4:0 - - [CHI] PS40-2 3:1 2H 96 [DELTA] PS40-3 2:2 2H 100 [OMICRON] PS40-4 1:3 1H 100 [OMICRON] PS40-5 0:4 1H 100 [OMICRON] PS60-1 PS60 4:0 - - [CHI] PS60-2 3:1 3H 94 [DELTA] PS60-3 2:2 3H 100 [OMICRON] PS60-4 1:3 2H 100 [OMICRON] PS60-5 0:4 1H 100 [OMICRON] PS80-1 PS80 4:0 - - [CHI] PS80-2 PS80-3 3:1 2H 92 [DELTA] PS80-4 PS80-5 2:2 2H 100 [OMICRON] 1:3 1H 100 [OMICRON] 0:4 1H 100 [OMICRON] TEOS + MTES, 0.010 mol; PHEMA, 0.750 g; MPMF, 1.50 g; [OMICRON], very good; [DELTA] not bad; [CHI], bad.
The coating was not available when PS40 was selected and TEOS was used only in silicates (PS40-1). After completing the coating process, the coating had many cracks, which made it difficult to measure the hardness and adhesion.
In the case of using PS40-2, the pencil hardness was 2H and the adhesion was not perfect, the coating surface was not entirely clean. In the case of PS40-3, the adhesion was perfect and the surface of the coating was quite clean. The pencil hardness was 2H. In the case of PS40-4 and PS40-5, the adhesion was perfect and the surface of the coating was quite clean but the pencil hardness of both was 1H. which is lower than that of PS40-3. PS40-3 was the best result in the case of using PS-40. TEOS has four ethoxy functional groups but MTES has three. Therefore, the crosslinking density of the coating increased with increasing amount of TEOS. The results showed that increasing the crosslinking density was helpful in increasing the hardness of the coating but an excessive amount of crosslinking was harmful to the coating resulting in the formation of cracks. All the coatings were transparent.
PS60 showed similar results to PS40 but the pencil hardness of the coating was higher. The pencil hardness of the PS60-3 coating was 3H. PS80 showed similar results to PS40. Overall. PS60-3 showed the best result.
The Effect of Amount of Silicates
The effect of the amount of silicates on the properties of the coating was examined. The mole ratio of TEOS/ MTES was 1:1, and the number of moles of silicates means the total number of moles of TEOS and MTES.
Table 2 shows that the pencil hardness of the PS60-7, PS60-8, and PS80-8 coatings was 3H. This means that mole ratio of silicates and MPMF play an important role in the hardness of the coatings. This means that the balance between the silicate and melamine structures is important.
TABLE 2. The effect of the amount of silicates on the property of the coating. Polymer Total Mole Coating Ex. no. composition TEOS: MTES Hardness Adhesion surface PS40-6 PS40 0.0050 1H 100 [OMICRON] PS40-7 0.010 2H 100 [OMICRON] PS40-8 0.020 2H 100 [OMICRON] PS40-9 0.040 2H 100 [OMICRON] PS60-6 PS60 0.0050 2H 100 [OMICRON] PS60-7 0.010 3H 100 [OMICRON] PS60-8 0.020 3H 100 [OMICRON] PS60-9 0.040 2H 100 [OMICRON] PS80-6 PS80 0.0050 1H 100 [OMICRON] PS80-7 0.010 2H 100 [OMICRON] PS80-8 0.020 3H 100 [OMICRON] PS80-9 0.040 2H 100 [OMICRON] Mole ratio of TEOS: MTES, 1:1: PHEMA, 0.750 g; MPMF, 1.50 g; [OMICRON], very good; [DELTA] not bad; [CHI], bad.
The Effect of Amount of MPMF
The effect of the amount of MPMF on the properties of coating was examined. The mole ratio of TEOS/MTES was 1:1, and the total number of moles of silicates was 0.010 mol.
Table 3 shows that the pencil hardness of the PS60-11 coating was 3H. The surface of the PS60-13, PS80-12, and PS80-13 coatings was quite rough. This means that mole ratio of silicates and MPMF plays an important role in the hardness of the coatings, and an excessive amount of melamine in the coating is harmful to the coating properties using the PHEMA-S with small amount of hydroxyl group.
TABLE 3. The effect of the amount of MPMF on the property of the coating. Polymer MI'MF Coating Ex. no. composition (g) Hardness Adhesion surface PS40-10 PS40 1.0 1H 100 [OMICRON] PS40-11 1.5 2H 100 [OMICRON] PS40-12 2.0 2H 100 [OMICRON] PS40-I3 3.0 1H 100 [OMICRON] PS60-10 PS60 l.0 2H 100 [OMICRON] PS60-11 1.5 3H 100 [OMICRON] PS60-12 2.0 2H 100 [OMICRON] PS60-13 3.0 1H 100 [DELTA] PS80-10 PS80 1.0 1H 100 [OMICRON] PS80-11 1.5 2H 100 [OMICRON] PS80-I2 2.0 2H 100 [DELTA] PS80-13 3.0 1H 96 [DELTA] Mole ratio of TEOS: MTES, 1:1; Total moles of silicates, 0.010 mol; [OMICRON], very good; [DELTA], not bad; [CHI], bad.
The Physical Properties of the Coatings
The IR spectrum of the PS60-3 coating revealed a Si--O--Si stretching vibration at 1080-1100 c[m.sup.-1], the OH stretch at 3100-3600 c[m.sup.-1] and a melamine group at 1568, 1495, and 815 c[m.sup.-1].
Figure 2 shows the TGA result of PS60-3 coating. The weight of the sample began to decrease at 300[degrees]C. At ~350[degrees]C the weight decreased abruptly. The amount of sample remaining at 700[degrees]C was ~40%. In this experiment, the coating contained both inorganic and organic compounds. The material remaining at 700[degrees]C contained inorganic silicates. However, the melamine part formed a char (18), (19).
[FIGURE 2 OMITTED]
The thickness and roughness of the coating was determined by cross section SEM. Figure 3 shows an SEM image of the PS60-3 coating. The mean thickness of the precoating and main coating was 5.10 [+ or -] 0.15 [micro]m, and 18.5[+ or -] 0.35 [micro]m, respectively. The surface of all of the coatings was quite flat in the SEM image.
[FIGURE 3 OMITTED]
The abrasion resistance of the coating was measured using a Taber abraser loaded with the CS-10 wheel. After rotating the wheel 500 times under a 500g load, the absorbance of the coating was measured at 370 nm using a UV spectrophotometer. PS40-3, PS60-3, and PS80-3 were selected for the abrasion resistance test. The results are shown in Fig. 4.
[FIGURE 4 OMITTED]
The results showed that the absorbance of an uncoated original polycarbonate plate increased from 0.135 to 0.800 after the abrasion resistance test, but the absorbance of the coated polycarbonate plate did not change significantly. The absorbance of the PS40-3, PS60-3, and PS80-3 coated plate increased from 0.137 to 0.168, 0.136 to 0.152, and 0.138 to 0.169, respectively. Among the coated samples, the PS60-3 coating shows the lowest increase in absorbance. The pencil hardness of PS60-3 was 3H, which is better than the other coatings. These results showed that the hardness improved the abrasion resistance.
In our study, hard coatings were deposited on a polycarbonate plate using a sol-gel process with PHEMA, TEOS, MTES, MPMF, and IPTES. The hydroxyl groups in PHEMA partially reacted with the isocyanate group in IPTES to form a urethane group. The remaining hydroxyl groups in PHEMA reacted with the methoxy group in MPMF through transetherification. Coatings that contained both the melamine and silicate structures were prepared. The conditions and formulation to obtain the optimal physical properties were determined. The balance between the silicate structure and melamine structure is important for improving the hardness of the coating. Smooth coatings with a pencil hardness of 3H were formed and showed excellent abrasion resistance and transparency.
(1.) S.K. Medda, D. Kundu, and G. De, J. Non-Cryst. Solids., 318, 149 (2003).
(2.) H.S. Yang, O.H. Kwon, J.D. Lee, J.S. Rho, and Y.H. Kim, J. Korean. Ind. Eng. Chem., 7, 823 (1996).
(3.) J. Chen, P. Chareonsak, V. Puengpipat, and S. Marturunkakul, J. Appl. Polym. Sci., 74, 1341 (1999).
(4.) Y.T. Lee, D.S. Jeong, and H.J. Jeong, Poiymer(Korea), 19, 753 (1995).
(5.) H.K. Kim, J.G. Kim, J.A. Yu, and J.W. Hong, J. Korean. Ind. Eng. Chem., 12, 287 (2001).
(6.) D. Rats, V. Hajek, and L. Martinu, Thin Solid Films, 340, 33 (1999).
(7.) S.E. Yoon, H.G. Woo, and D.P. Kim, Polymer (Korea), 24, 389 (2000).
(8.) T.H. Lee, E.S. Kang, and B.S. Bae, J. Sol-Gel. Sci. Techonol., 27, 23 (2003).
(9.) Y.J. Eo, D.J. Kim, B.S. Bae, K.C. Song, T.Y. Lee, and S.W. Song, J. Sol-Gel. Sci. Techonol., 13, 409 (1998).
(10.) D.G. Park, Polym. Sci. Technol., 8, 268 (1997).
(11.) D.K. Hwang, J.H. Moon, Y.G. Shul, K.T. Jung, D.H. Kim, and D.W. Lee, J. Sol-Gel. Sci, Techonol., 26, 783 (2003).
(12.) M.S Lee and N.J. Jo, J. Sol-Gel. Sci. Techonol., 24, 175 (2002).
(13.) T.P. Chou and G. Cao, J. Sol-Gel. Sci. Techonol., 27, 31 (2003).
(14.) J.D Mackenzie and E.P. Bescher, J. Sol-Gel. Sci. Techonol., 19, 23 (2000).
(15.) Y.J. Ji, H.Y. Kim, Y.S. Yoon, S.W. Lee, and J.S. Shin, J. Adhesion Interface., 6. 10 (2005).
(16.) J.Y. Kim, Y.J. Shin, Y.R. Shin, Y.J. Ji, Y.S. Yoon, and J.S. Shin, J. Korean. Ind. Eng. Chem., 17, 170 (2006).
(17.) R.J. Fort and T.M. Polyzoidis, Eur, Polym. J., 12, 685 (1976).
(18.) S.V. Levchik, A.I. Balabanovich, G.F. Levchik, and L. Costa, Fire, Mater., 21, 75 (1997).
(19.) B.K. Kandola, A.R. Horrocks, and S. Horrocks, Fire. Mater., 25, 153 (2001).
Correspondence to: Jae Sup Shin; e-mail: Jsshin@chungbuk.ac.kr Contract grant sponsor: Chungbuk National University.
Published online in Wiley InterScience (www.interscience.wiley.com). (c) 2008 Society of Plastics Engineers
Young Jae Shin, (1) Mee Hye Oh, (2) Yeo Seong Yoon, (2) Jae Sup Shin (3)
(1) Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843
(2) Korea Automotive Technology Institute, Chonan, Chungnam 330-912, Korea
(3) Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||poly(2-hydroxyethyl methacrylate)|
|Author:||Shin, Young Jae; Oh, Mee Hye; Yoon, Yeo Seong; Shin, ae Sup|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Jul 1, 2008|
|Previous Article:||Macroindentation of polymers.|
|Next Article:||Cocontinuous Cellulose acetate/polyurethane composite nanofiber fabricated through electrospinning.|