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Hard-coating materials for poly(methyl methacrylate) from glycidoxypropyltrimethoxysilane-modified silatrane via a sol–gel process
Surface & Coatings Technology 200 (2006) 2784 – 2790 www.elsevier.com/locate/surfcoat
Hard-coating materials for poly(methyl methacrylate) from glycidoxypropyltrimethoxysilane-modified silatrane via a sol–gel process Walairat Tanglumlerta, Pattarapan Prasassarakichb,*, Pitt Supapholc,*, Sujitra Wongkasemjitc,* a
Petrochemistry and Polymer Science Graduate Program, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand c The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand Received 9 July 2004; accepted in revised form 10 November 2004 Available online 5 January 2005
Abstract Hard-coating suspensions for improving the scratch resistance of poly(methyl methacrylate) (PMMA) surfaces have been developed by a sol–gel processing method of silatrane in the presence of 3-glycidoxypropyl-trimethoxysilane (GPTS) using 1 M HCl as a catalyst. Poly(methyl methacrylate) substrates were dipped into the coating solutions and dried at 60 8C for half an hour before being cured at 120 8C for 2 h. The addition of GPTS to silatrane suspensions enabled the formation of dense and transparent thin-film with smooth surface on the substrates. The scratch resistance of the coated surface was found to increase with increasing GPTS to silatrane ratio. Both the curing time and curing temperature were found to affect the scratch resistance and adhesion properties of the coating layer. D 2004 Elsevier B.V. All rights reserved. Keywords: Silicate coating; Scratch resistance; Sol–gel process; Poly(methyl methacrylate); Silatrane
1. Introduction Nowadays, many types of plastics are used as optical materials due to their lightness in weight, ease of mass production, and inexpensiveness. Among these, poly (methyl methacrylate) (PMMA) and polycarbonate (PC) are well-known materials for making sheets or articles of excellent optical and mechanical properties. One of the major drawbacks limiting their uses is the poor scratch resistance. To overcome such a problem, many different kinds of hard-coating media, such as melamine-, acrylic-, and urethane-based chemicals, have been developed and used [1–3]. Organic–inorganic hybrid composites have been used as effective hard-coating materials, which show excellent advantages of both organic and inorganic precursors. These composites are based mainly on the use of metal alkoxide * Corresponding author. E-mail addresses: [email protected] (P. Prasassarakich)8 [email protected] (P. Supaphol)8 [email protected] (S. Wongkasemjit). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.018
and organosiloxane via sol–gel processing, which has been known to be one of the practical methods for preparing metal oxide thin-films as coating materials for glasses [4], metals [5], and organic polymers [6]. Thin-film coating of metal oxide based on sol–gel process offers an important advantage over other techniques in its relatively lowtemperature processing conditions. A number of published works have been focussing on improving the scratch resistance and mechanical strength of sol–gel-derived, low-refractive index silica thin-films. Lee and Jo [7] used condensed matters from mixtures of colloidal silica and methyltriethoxysilane (MTES) as hardcoating materials for PMMA. They found that the optimal amount of MTES for the hard-coating formula was 100 wt.% to the colloidal silica. The addition of tetramethylammonium formate, a curing catalyst, helped enhance the adhesion between the coating layer and the PMMA substrates as well as reduce the curing time. Li et al. [8,9] have developed many abrasion-resistant coatings based mainly on sol–gel reactions of aluminum, titanium, zirconium, or silicon alkoxide mixed with an organic compound
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containing two or more triethoxysilyl groups, from which the abrasion resistance of coated substrates was found to markedly improve. Piboonchaisit et al. [10] reported a synthetic route for a silicon alkoxide structure, called bsilatrane,Q which was found to have a remarkable stability in moist atmosphere. This advantage has made silatrane a good candidate for preparing silicon dioxide network via sol–gel transition, which was later reported in a subsequent work by Charoenpinijkarn et al. [11] as being successful. In the present contribution, silatrane was used as a precursor for preparing hard-coating suspensions specific for improving the scratch resistance of PMMA surfaces. The effects of ingredient compositions and curing conditions (i.e. curing temperature and curing time) on the adhesion between the coating layer and the PMMA substrates and the scratch resistance of the coated surface were thoroughly investigated.
2. Experimental details 2.1. Materials In silatrane synthesis, fumed silica (SiO2, 99.8%; SigmaAldrich, USA) and triethanolamine (TEA; Carlo Erba, Italy) were used as reactants; ethylene glycol (EG; J.T.Baker, USA) was used as both the solvent and a possible reactant, and acetonitrile [Labscan (Asia), Thailand] was used to cleanse the silatrane products and was distilled prior to use. In the preparation of the hard-coating suspensions, the synthesized silatrane and poly(vinyl alcohol) (PVA; SigmaAldrich, USA) or 3-methacryloxypropyl-trimethoxysilane (MPTS; Acros Organics, USA) or 3-glycidoxypropyltrimethoxysilane (GPTS; Acros Organics, USA) were used as reactants; HCl (BDH Laboratory Supplies, UK) was used as the acid catalyst; diethylenetriamine (DETA; Facai Group (2000), Thailand) or 3-aminopropyl-triethoxysilane (APS; Fluka, Switzerland) was used as a curing agent; and isopropanol (IPA; Labscan (Asia), Thailand) was used as a solvent. Lastly, PMMA substrates were prepared by cutting a 3-mm-thick sheet into specimens of 1.5 cm7 cm. 2.2. Preparation of silatrane Silatrane [tris(silatranyloxy-ethyl)amine or Si(TEA)2] was synthesized directly from inexpensive and abundant raw materials, SiO2 and TEA, via the oxide one-pot synthesis (OOPS) [10]. A mixture of 0.125 mol of TEA, 0.1 mol of SiO2, and 100 ml of EG was first heated at 200 8C under nitrogen atmosphere. The reaction was completed within 10 h, and the mixture was slowly cooled down to room temperature before distilling the mixture to get rid of the excess EG under vacuum (i.e. 8 mm Hg) at 110 8C for another 6 h. The white solid precipitate was washed three times with dried acetonitrile to remove unreacted TEA and
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EG. The silatrane products were dried in a vacuum desiccator overnight prior to being characterized to confirm their successful synthesis. 2.3. Preparation of hard-coating solutions Prior to the successful preparation of hard-coating suspensions for PMMA, there were a number of fail trials, which are worth noting. In the first fail trial, suspensions of silatrane and PVA were prepared by mixing 1 g of silatrane with 10 ml of solutions of PVA in water with the concentration ranging from 4% to 6% w/v (based on the weight of PVA per volume of water). Potassium hydroxide (KOH)etched PMMA was then dip-coated in these suspensions, pre-cured at 60 8C for 30 min, and then cured at 120 8C for 2 h. Even though the coating layer was transparent, it was soluble in water. In the second fail trial, partially hydrolyzed silatrane suspension was prepared by dissolving an amount of silatrane in IPA, using 1 M HCl as the catalyst. Various amounts of MPTS were then added to the silatrane suspension and stirred for 24 h. The weight ratio between MPTS and silatrane was varied between 1 and 10 (i.e. 1, 3, 5, 7, and 10, respectively). KOH-etched PMMA was then dip-coated in these solutions, pre-cured at 60 8C for 30 min, and then cured at 120 8C for 2 h. The resulting coating layer appeared to be yellow in color and the as-prepared precuring suspensions tended to phase-separate rather easily. In the third fail attempt, GPTS was used to prepare the pre-curing suspensions of silatrane instead of MPTS. Before dip-coating PMMA in the suspensions, DETA in various amounts ranging from 1 to 5 wt.% (based on weight of DETA per total weight of the pre-cured suspensions) were added to the pre-curing suspensions. Coated PMMA specimens were pre-cured at 60 8C for 30 min and later cured at 120 8C for 2 h. It was found that the coating layer was improperly cured. In the fourth, successful attempt, APS was used as the curing agent instead of DETA. It was found that, among the various amount of APS added (i.e. from 1 to 5 wt.%), 3 wt.% was the best in giving the resulting curing or hardcoating suspensions in the conditions appropriate for dipcoating for a long enough time (i.e. long enough gelation period). The resulting coating layer was found to be transparent, but the adhesion between the coating layer and the PMMA substrates was not good, as the coating layer could be peeled off from the substrate surface rather easily. To improve the interfacial adhesion between the coating layer and the PMMA surface, plasma etching was utilized along with chemical etching in a KOH solution. Based on the aforementioned preliminary studies, the successful method for preparing the hard-coating suspensions can be capitulated as follows. First, partially hydrolyzed silatrane suspension was prepared by dissolving a known amount of silatrane in 1 M HCl solution and was later diluted by the addition of IPA. HCl was used as the
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catalyst. Various amounts of GPTS were then added to the silatrane suspension and stirred for 24 h. The weight ratio between GPTS and silatrane was varied between 1 and 10 (i.e. 1, 3, 5, 7, and 10, respectively). Prior to dip-coating, PMMA substrates were plasma-etched and then chemically etched in 10 wt.% KOH solution for 10 min before being washed by de-ionized water twice and later dried in an oven, and, immediately before dip-coating the as-prepared PMMA substrates, 3 wt.% APS was added to the pre-curing suspensions. Coated PMMA specimens were pre-cured at 60 8C for 30 min and later cured at 120 8C for 2 h. Chemical transformation of the as-prepared hard-coating suspensions was characterized by FT–IR, while the effects of the GPTS to silatrane ratio (i.e. 1, 3, 5, 7, and 10), curing temperature (i.e. 80, 100, and 120 8C), and curing time (i.e. 1, 2, and 3 h) on the interfacial adhesion between the coating layer and the surface of the substrates and the scratch resistance of the coating layer were characterized. 2.4. Characterization A Bruker Optics EQUINOX55 Fourier-transformed infrared spectrometer (FT–IR) was used to verify the chemical structure of the as-synthesized silatrane products, as well as to follow chemical transformation of the asprepared hard-coating suspensions. The measurement resolution of 2 cm 1 was used. A DuPont TGA 2950 thermogravimetric analyzer (TGA) was used to determine the ceramic yield of the as-synthesized silatrane products. The measurement was carried out under a heating rate of 10 8C min 1 from room temperature to 750 8C in a nitrogen atmosphere. A Fison Instruments VG Autospec 7070 E mass spectrometer under its positive fast atomic bombardment mode (FAB+-MS) was used to verify the chemical structure of the as-synthesized silatrane products. The mass range of the samples was set from m/e=20 to 1500. The adhesion between the coating layer and the PMMA substrates was assessed according to ASTM D 3359-02 standard test method [12], and the scratch resistance of the coating layer was assessed according to a steel wool test (i.e. L-11-12-05 Colts standard test method, Colts Laboratories, USA) [13].
Si((OCH2CH2)3N)2H2, which was found to be very close to the theoretical value of ca. 18%. FAB+-MS showed the highest m/e at 669 of Si3((OCH2CH2)3N)4H+ and 100% intensity at 323 of Si((OCH2CH2)3N)2H+. All of the FT–IR, TGA, and FAB+-MS results confirmed the formation of silatrane. 3.2. Characterization of the coating suspensions The preparation of the coating suspensions started with partial hydrolysis of silatrane in a 1 M HCl solution, with HCl acting as the catalyst. IPA was then added to the suspension. During this step, the silatrane monomers underwent a polycondensation reaction to form oligomers. Prior to the addition of GPTS, the hydrolyzed silatrane suspension exhibited a pH level of 4 and became stable for a few days. Upon the addition of GPTS, the suspension became basic, with the pH level of around 8 to 9. The GPTS to silatrane ratio was varied between 1 and 10 (i.e. denoted in Table 1 as R1 to R10, with the control condition being R0). The aging step continued for another 24 h, after which time 3 wt.% APS was added as the curing agent. After the as-prepared PMMA substrates were dipped in the asprepared coating suspensions, they were pre-cured at 60 8C for 30 min and then cured at 120 8C for another 2 h. After curing, the coating layer appeared to be transparent and smooth. The sol to gel transition of the as-prepared coating suspensions was followed by FT–IR. Fig. 1 illustrates FT– IR spectra of the as-prepared coating suspension for a GPTS to silatrane ratio of 1 at various aging times ranging from 1 to 24 h. The obtained FT–IR spectra confirm that the starting chemicals transformed into Si–O–H. Clearly, the Si–OH stretching peaks observed at 3400 and 900 cm 1 corresponding to the hydrolysis of the alkoxy group of the starting chemicals increased with increasing the aging time. The Si–O–C group is recognized by the bands observed at 1170 to 1075 and 970 to 940 cm 1. As the aging time increased, the intensity of the band specific to Si–O–Si at 1200 to 1100 cm 1 [14–16] increased and leveled off at aging times longer than 24 h (results not shown), while that specific to Si–O–C bands decreased. This only means that condensation reaction already proceeded to some extent at 3
3. Results and discussion 3.1. Characterization of silatrane products FT–IR bands observed were 3000 to 3700 (w, intermolecular hydrogen bonding), 2860 to 2986 (s, rC–H), 1244 to 1275 (m, rC–N), 1170 to 1117 (bs, rSi–O), 1093 (s, rSi–O– C), 1073 (s, rC–O), 1049 (s, rSi–O), 1021 (s, rC–O), 785 and 729 (s, rSi–O–C), and 579 cm 1 (w, SipN). TGA showed a sharp mass loss, with the first derivative peak centering at 390 8C. The percentage of ceramic yield of the products was calculated to be ca. 18.5%, corresponding to
Table 1 Composition of the as-prepared coating suspensions and transparency and adhesion to PMMA substrates of the obtained coating layer Sample
Silatrane (g)
GPTS (g)
Transparency of coating layer
Adhesion quality rating
R0 R1 R3 R5 R7 R10
0 0.1 0.1 0.1 0.1 0.1
0.5 0.1 0.3 0.5 0.7 1.0
Transparent Transparent Transparent Transparent Transparent Transparent
0B 5B 5B 5B 5B 5B
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Fig. 1. FT–IR spectra of the as-prepared coating suspension for a GPTS to silatrane ratio of 1 (denoted as R1) at various aging times ranging from 1 to 24 h.
h, since hydrolysis and condensation reactions should start just after the addition of water. Fig. 2 exhibits the FT–IR spectra of the as-prepared coating suspensions for various GPTS to silatrane ratios ranging from 1 to 10 (denoted as R1 to R10) after aging for 24 h. Apparently, the coating suspension having the greatest amount of GPTS (i.e. R10) showed the most intensified Si– O–Si absorption band at 1200 to 1100 cm 1, while the coating suspension exhibiting the most intensified Si–OH peak at 3400 cm 1 was observed at the GPTS to silatrane ratio of 5 (i.e. R5). With further increase in the GPTS to silatrane ratio, the intensity of the Si–OH peak was found to gradually decrease. Since the initial water to GPTS ratio for the coating suspensions decreased with increasing GPTS to
silatrane ratio (from about 710 for R1 to 71 for R10), the observed increased intensity at 3400 cm 1 with increasing GPTS/silatrane ratio (of up to 5) should not be a contribution from the free water in the system, as most of the water molecules should participate in the hydrolysis of certain Si–O–R groups into Si–OH groups, some of which being subsequently transformed into Si–O–Si bonds. Silatrane-based coating suspensions containing GPTS having epoxy functionality can be cross-linked by the addition of an amine curing agent such as APS. When the amine curing agent was added, the suspensions became gellike, suggesting the formation of structural networking. For industrial utilization of the as-prepared coating suspensions, the effect of GPTS content (representing as the GPTS to
Fig. 2. FT–IR spectra of the as-prepared coating suspensions for various GPTS to silatrane ratios ranging from 1 to 10 (denoted as R1 to R10) after aging for 24 h.
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silatrane ratio R=1 to 10) on the gelation time in the presence of the amine curing agent is indispensable. In this investigation, a fixed amount of APS of 3 wt.% was used. The gelation time was found to decrease from ca. 200 h for the suspension of R=1 to 40 h for the suspension of R=10. In Chu et al. [14], triethylenetetramine (TETA) was use as the amine curing agent for the coating suspension system of colloidal silica and GPTS, from which they found that the gelation time decreased from ca. 300 h for the suspension of R=0.03 to ca. 5 h for the suspension of R=0.12 (the molar ratio between TETA and GPTS was 1:2). It should be noted that, without the presence of APS, the suspensions did not gel for all GPTS to silatrane ratios investigated. The results confirmed that the gelation of the silatrane/GPTS suspensions occurred because of the curing of the epoxyfunctionalized particles and silane species in the suspensions, and it is logical to postulate that the decrease in the gelation time with increasing GPTS to silatrane ratio was a result of the increase in the GPTS to APS ratio or the increase in the amount of GPTS (i.e. more reacting species) [3,17,18]. 3.3. Characterization of coated PMMA When only the solution of GPTS was used as the coating solution without the presence of silatrane, the coating film was transparent but adhered poorly on the PMMA substrates. As previously been discussed by Kim et al. [19], without the presence of a metal alkoxide, GPTS and APS formed siloxane bonds only in the dimeric or oligomeric form, leading to no adhesion between the coating film and the substrates. On the contrary, with the presence of the metal alkoxide in the coating suspensions, the siloxane bonds can be generated in a network manner. As a result, the addition of GPTS to the silatrane solution resulted in the formation of a transparent film with good binding property with the PMMA substrates. The optical property of the coating film from the coating suspension of different GPTS to silatrane ratios (i.e. R0 to R10) is summarized in Table 1. The cross-cut tape test (according to ASTM D 3359-02 standard test method [12]) was used to characterize the adhesion between the coating layer and the PMMA substrates. In this test method, a lattice pattern with either six or eleven cuts in each direction was made on a coated PMMA surface. A pressure-sensitive adhesive was then applied over the lattice and subsequently removed. Quality rating of the adhesion was determined from fractional area of the lattice in which a part or the whole coating layer was removed from the substrates. According to the standard, five quality ratings are assigned: 5B for 0% removal, 4B for less than 5% removal, 3B for 5 to 15% removal, 2B for 15 to 35% removal, 1B for 35 to 65% removal, and 0B for more than 65% removal, respectively [12]. According to the results shown in Table 1, only the coating layer prepared from the suspension containing only GPTS failed the
adhesion test, in which the test result exhibited 0B quality rating, while the coating layer prepared from all of the silatrane/GPTS suspensions investigated received 5B quality rating. Based on the results obtained, the presence of silatrane unquestionably helped improve the adhesion between the coating layer and the PMMA substrates. It is because the amine curing agent, APS, was able to link to the PMMA substrates by reacting with the ester groups through its amine group [8]. The GPTS coupling agent itself was able to form strong bonds with partially hydrolyzed silatrane and the PMMA substrates, linking the different materials together [19–22]. The steel wool abrasion test (according to L-11-12-05 Colts standard test method [13]) was used to characterize the scratch resistance of the coated PMMA surface. In this test method, a steel wool of grade #000 or #0000 was used. The steel wool was rubbed across the substrate surface continuously for 10 cycles under an application of a 1 kg load. In this particular test, the rubbing was carried out automatically. The quality rating of the test was determined by the number of scratched lines present on the surface. According to the standard, six quality ratings are assigned: A for no evidence of scratched lines, B for 1 to 3 scratched lines, C for 4 to 20 scratched lines, D for 21 to 40 scratched lines, E for 41 to 100 scratched lines, and F for more than 100 scratched lines being observed, respectively [13]. Table 2 summarizes steel wool abrasion test results for pure PMMA substrate and PMMA substrates coated in GPTS and silatrane/GPTS suspensions. Apparently, pure PMMA substrate failed the abrasion test, with the observed scratched lines being more than 100. For the substrate dipcoated in the suspension containing only GPTS, its scratch resistance improved only by a little, exhibiting E quality rating. For the coating suspension having a GPTS to silatrane ratio of 1, the scratch resistance of the resulting coated surface improved appreciably, with the number of scratched lines being only within 1 to 3. For the coating suspensions containing GPTS to silatrane ratios ranging from 3 to 10, no scratched lines were observed on the coated surface. Fig. 3 shows the optical micrographs for pure PMMA substrates both before and after the abrasion test and for PMMA substrates coated with the coating suspensions Table 2 Scratch resistance of the uncoated PMMA, PMMA surface dip-coated in GPTS suspension, and PMMA surface dip-coated in silatrane/GPTS suspensions Sample
Abrasion quality rating
Corresponding number of scratched lines
Uncoated PMMA R0 R1 R3 R5 R7 R10
F E B A A A A
More than 100 41 to 100 1 to 3 0 0 0 0
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Fig. 3. Transmission optical micrographs exhibiting the surface of (a) uncoated PMMA surface before the steel wool abrasion test, (b) uncoated PMMA surface after the abrasion test, (c) PMMA surface coated in suspension having a GPTS to silatrane ratio of 1 after the abrasion test, and (d) PMMA surface coated in suspension having a GPTS to silatrane ratio of 10 after the abrasion test. The marker bar is for 100 Am.
having the GPTS to silatrane ratios of 1 and 10. Obviously, the hybrid coating layer consisting of silatrane, GPTS, and APS greatly improved the scratch resistance of the PMMA substrates. In order to illustrate the effect of curing temperature and time on the adhesion and the scratch resistance of the coating layer, PMMA substrates were dip-coated in the coating suspension having the GPTS to silatrane ratio of 1. Table 3 summarizes the adhesion and the abrasion test results of R1-coated PMMA substrates which were cured at 80, 100, or 120 8C for 1, 2, or 3 h. For the samples cured for 1 h, all of the test results were unacceptable, regardless of the curing temperature. For the samples cured for 2 h, only
the samples cured at 120 8C showed acceptable adhesion and scratch resistance test results. For the samples cured for 3 h, the samples cured at 100 and 120 8C exhibited acceptable adhesion test results, but only the samples cured at 120 8C showed acceptable scratch resistance test result. The extent of curing strongly affects the scratch resistance of the coating layer. The curing process is based on the homo-condensation of silanol groups and the heterocondensation between the silanol and alkoxysilane groups. The higher concentration of the silanol groups or the higher extent of hydrolysis of alkoxysilane groups in the system can certainly help promote the extent of curing and scratch resistance of the final coating layer [23,24].
Table 3 Adhesion and abrasion test results of PMMA surface dip-coated in a suspension containing a GPTS to silatrane ratio of 1 after curing at 80, 100, or 120 8C for 1, 2, or 3 h Curing temperature (8C)
80 100 120
Curing time (h) 1
2
3
Adhesion quality rating
Abrasion quality rating
Adhesion quality rating
Abrasion quality rating
Adhesion quality rating
Abrasion quality rating
0B 0B 0B
F F D
0B 0B 5B
E C B
0B 5B 5B
E C B
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4. Conclusions
References
Hard-coating suspensions for improving the scratch resistance of poly(methyl methacrylate) (PMMA) surface have been successfully developed by a sol–gel processing of silatrane in the presence of 3-glycidoxypropyl-trimethoxysilane (GPTS) using 1 M HCl as a catalyst. Prior to dip-coating, 3-aminopropyl-triethoxysilane (APS), an amine curing agent was added into the suspensions. Poly(methyl methacrylate) substrates were dipped into the coating solutions and dried at 60 8C for half an hour before being cured at 120 8C for 2 h. The addition of GPTS to the silatrane suspensions enabled the formation of dense and transparent thin-film with smooth surface on the substrates. The coating layer obtained from all of the silatrane/GPTS suspensions was found to adhere to the PMMA substrates particularly well. The scratch resistance of the coated PMMA surface was found to significantly improve from that of the pure PMMA surface. When the GPTS to silatrane ratio in the suspensions was increased from 3 to 10, the scratch resistance of the coated PMMA surface was impeccable. Lastly, it was also demonstrated that both of the curing time and curing temperature affected a great deal the scratch resistance and adhesion properties of the coating layer.
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Acknowledgments The authors wish to thanks partial supports from Commission on Higher Education, Ministry of Education; Ratchadapisek Somphot Endowment Fund, Chulalongkorn University; Petroleum and Petrochemical Technology Consortium; and Petroleum and Petrochemical College, Chulalongkorn University.
Hard-coating materials for poly(methyl methacrylate) from glycidoxypropyltrimethoxysilane-modified silatrane via a sol–gel process Walairat Tanglumlerta, Pattarapan Prasassarakichb,*, Pitt Supapholc,*, Sujitra Wongkasemjitc,* a
Petrochemistry and Polymer Science Graduate Program, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand c The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand Received 9 July 2004; accepted in revised form 10 November 2004 Available online 5 January 2005
Abstract Hard-coating suspensions for improving the scratch resistance of poly(methyl methacrylate) (PMMA) surfaces have been developed by a sol–gel processing method of silatrane in the presence of 3-glycidoxypropyl-trimethoxysilane (GPTS) using 1 M HCl as a catalyst. Poly(methyl methacrylate) substrates were dipped into the coating solutions and dried at 60 8C for half an hour before being cured at 120 8C for 2 h. The addition of GPTS to silatrane suspensions enabled the formation of dense and transparent thin-film with smooth surface on the substrates. The scratch resistance of the coated surface was found to increase with increasing GPTS to silatrane ratio. Both the curing time and curing temperature were found to affect the scratch resistance and adhesion properties of the coating layer. D 2004 Elsevier B.V. All rights reserved. Keywords: Silicate coating; Scratch resistance; Sol–gel process; Poly(methyl methacrylate); Silatrane
1. Introduction Nowadays, many types of plastics are used as optical materials due to their lightness in weight, ease of mass production, and inexpensiveness. Among these, poly (methyl methacrylate) (PMMA) and polycarbonate (PC) are well-known materials for making sheets or articles of excellent optical and mechanical properties. One of the major drawbacks limiting their uses is the poor scratch resistance. To overcome such a problem, many different kinds of hard-coating media, such as melamine-, acrylic-, and urethane-based chemicals, have been developed and used [1–3]. Organic–inorganic hybrid composites have been used as effective hard-coating materials, which show excellent advantages of both organic and inorganic precursors. These composites are based mainly on the use of metal alkoxide * Corresponding author. E-mail addresses: [email protected] (P. Prasassarakich)8 [email protected] (P. Supaphol)8 [email protected] (S. Wongkasemjit). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.018
and organosiloxane via sol–gel processing, which has been known to be one of the practical methods for preparing metal oxide thin-films as coating materials for glasses [4], metals [5], and organic polymers [6]. Thin-film coating of metal oxide based on sol–gel process offers an important advantage over other techniques in its relatively lowtemperature processing conditions. A number of published works have been focussing on improving the scratch resistance and mechanical strength of sol–gel-derived, low-refractive index silica thin-films. Lee and Jo [7] used condensed matters from mixtures of colloidal silica and methyltriethoxysilane (MTES) as hardcoating materials for PMMA. They found that the optimal amount of MTES for the hard-coating formula was 100 wt.% to the colloidal silica. The addition of tetramethylammonium formate, a curing catalyst, helped enhance the adhesion between the coating layer and the PMMA substrates as well as reduce the curing time. Li et al. [8,9] have developed many abrasion-resistant coatings based mainly on sol–gel reactions of aluminum, titanium, zirconium, or silicon alkoxide mixed with an organic compound
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containing two or more triethoxysilyl groups, from which the abrasion resistance of coated substrates was found to markedly improve. Piboonchaisit et al. [10] reported a synthetic route for a silicon alkoxide structure, called bsilatrane,Q which was found to have a remarkable stability in moist atmosphere. This advantage has made silatrane a good candidate for preparing silicon dioxide network via sol–gel transition, which was later reported in a subsequent work by Charoenpinijkarn et al. [11] as being successful. In the present contribution, silatrane was used as a precursor for preparing hard-coating suspensions specific for improving the scratch resistance of PMMA surfaces. The effects of ingredient compositions and curing conditions (i.e. curing temperature and curing time) on the adhesion between the coating layer and the PMMA substrates and the scratch resistance of the coated surface were thoroughly investigated.
2. Experimental details 2.1. Materials In silatrane synthesis, fumed silica (SiO2, 99.8%; SigmaAldrich, USA) and triethanolamine (TEA; Carlo Erba, Italy) were used as reactants; ethylene glycol (EG; J.T.Baker, USA) was used as both the solvent and a possible reactant, and acetonitrile [Labscan (Asia), Thailand] was used to cleanse the silatrane products and was distilled prior to use. In the preparation of the hard-coating suspensions, the synthesized silatrane and poly(vinyl alcohol) (PVA; SigmaAldrich, USA) or 3-methacryloxypropyl-trimethoxysilane (MPTS; Acros Organics, USA) or 3-glycidoxypropyltrimethoxysilane (GPTS; Acros Organics, USA) were used as reactants; HCl (BDH Laboratory Supplies, UK) was used as the acid catalyst; diethylenetriamine (DETA; Facai Group (2000), Thailand) or 3-aminopropyl-triethoxysilane (APS; Fluka, Switzerland) was used as a curing agent; and isopropanol (IPA; Labscan (Asia), Thailand) was used as a solvent. Lastly, PMMA substrates were prepared by cutting a 3-mm-thick sheet into specimens of 1.5 cm7 cm. 2.2. Preparation of silatrane Silatrane [tris(silatranyloxy-ethyl)amine or Si(TEA)2] was synthesized directly from inexpensive and abundant raw materials, SiO2 and TEA, via the oxide one-pot synthesis (OOPS) [10]. A mixture of 0.125 mol of TEA, 0.1 mol of SiO2, and 100 ml of EG was first heated at 200 8C under nitrogen atmosphere. The reaction was completed within 10 h, and the mixture was slowly cooled down to room temperature before distilling the mixture to get rid of the excess EG under vacuum (i.e. 8 mm Hg) at 110 8C for another 6 h. The white solid precipitate was washed three times with dried acetonitrile to remove unreacted TEA and
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EG. The silatrane products were dried in a vacuum desiccator overnight prior to being characterized to confirm their successful synthesis. 2.3. Preparation of hard-coating solutions Prior to the successful preparation of hard-coating suspensions for PMMA, there were a number of fail trials, which are worth noting. In the first fail trial, suspensions of silatrane and PVA were prepared by mixing 1 g of silatrane with 10 ml of solutions of PVA in water with the concentration ranging from 4% to 6% w/v (based on the weight of PVA per volume of water). Potassium hydroxide (KOH)etched PMMA was then dip-coated in these suspensions, pre-cured at 60 8C for 30 min, and then cured at 120 8C for 2 h. Even though the coating layer was transparent, it was soluble in water. In the second fail trial, partially hydrolyzed silatrane suspension was prepared by dissolving an amount of silatrane in IPA, using 1 M HCl as the catalyst. Various amounts of MPTS were then added to the silatrane suspension and stirred for 24 h. The weight ratio between MPTS and silatrane was varied between 1 and 10 (i.e. 1, 3, 5, 7, and 10, respectively). KOH-etched PMMA was then dip-coated in these solutions, pre-cured at 60 8C for 30 min, and then cured at 120 8C for 2 h. The resulting coating layer appeared to be yellow in color and the as-prepared precuring suspensions tended to phase-separate rather easily. In the third fail attempt, GPTS was used to prepare the pre-curing suspensions of silatrane instead of MPTS. Before dip-coating PMMA in the suspensions, DETA in various amounts ranging from 1 to 5 wt.% (based on weight of DETA per total weight of the pre-cured suspensions) were added to the pre-curing suspensions. Coated PMMA specimens were pre-cured at 60 8C for 30 min and later cured at 120 8C for 2 h. It was found that the coating layer was improperly cured. In the fourth, successful attempt, APS was used as the curing agent instead of DETA. It was found that, among the various amount of APS added (i.e. from 1 to 5 wt.%), 3 wt.% was the best in giving the resulting curing or hardcoating suspensions in the conditions appropriate for dipcoating for a long enough time (i.e. long enough gelation period). The resulting coating layer was found to be transparent, but the adhesion between the coating layer and the PMMA substrates was not good, as the coating layer could be peeled off from the substrate surface rather easily. To improve the interfacial adhesion between the coating layer and the PMMA surface, plasma etching was utilized along with chemical etching in a KOH solution. Based on the aforementioned preliminary studies, the successful method for preparing the hard-coating suspensions can be capitulated as follows. First, partially hydrolyzed silatrane suspension was prepared by dissolving a known amount of silatrane in 1 M HCl solution and was later diluted by the addition of IPA. HCl was used as the
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catalyst. Various amounts of GPTS were then added to the silatrane suspension and stirred for 24 h. The weight ratio between GPTS and silatrane was varied between 1 and 10 (i.e. 1, 3, 5, 7, and 10, respectively). Prior to dip-coating, PMMA substrates were plasma-etched and then chemically etched in 10 wt.% KOH solution for 10 min before being washed by de-ionized water twice and later dried in an oven, and, immediately before dip-coating the as-prepared PMMA substrates, 3 wt.% APS was added to the pre-curing suspensions. Coated PMMA specimens were pre-cured at 60 8C for 30 min and later cured at 120 8C for 2 h. Chemical transformation of the as-prepared hard-coating suspensions was characterized by FT–IR, while the effects of the GPTS to silatrane ratio (i.e. 1, 3, 5, 7, and 10), curing temperature (i.e. 80, 100, and 120 8C), and curing time (i.e. 1, 2, and 3 h) on the interfacial adhesion between the coating layer and the surface of the substrates and the scratch resistance of the coating layer were characterized. 2.4. Characterization A Bruker Optics EQUINOX55 Fourier-transformed infrared spectrometer (FT–IR) was used to verify the chemical structure of the as-synthesized silatrane products, as well as to follow chemical transformation of the asprepared hard-coating suspensions. The measurement resolution of 2 cm 1 was used. A DuPont TGA 2950 thermogravimetric analyzer (TGA) was used to determine the ceramic yield of the as-synthesized silatrane products. The measurement was carried out under a heating rate of 10 8C min 1 from room temperature to 750 8C in a nitrogen atmosphere. A Fison Instruments VG Autospec 7070 E mass spectrometer under its positive fast atomic bombardment mode (FAB+-MS) was used to verify the chemical structure of the as-synthesized silatrane products. The mass range of the samples was set from m/e=20 to 1500. The adhesion between the coating layer and the PMMA substrates was assessed according to ASTM D 3359-02 standard test method [12], and the scratch resistance of the coating layer was assessed according to a steel wool test (i.e. L-11-12-05 Colts standard test method, Colts Laboratories, USA) [13].
Si((OCH2CH2)3N)2H2, which was found to be very close to the theoretical value of ca. 18%. FAB+-MS showed the highest m/e at 669 of Si3((OCH2CH2)3N)4H+ and 100% intensity at 323 of Si((OCH2CH2)3N)2H+. All of the FT–IR, TGA, and FAB+-MS results confirmed the formation of silatrane. 3.2. Characterization of the coating suspensions The preparation of the coating suspensions started with partial hydrolysis of silatrane in a 1 M HCl solution, with HCl acting as the catalyst. IPA was then added to the suspension. During this step, the silatrane monomers underwent a polycondensation reaction to form oligomers. Prior to the addition of GPTS, the hydrolyzed silatrane suspension exhibited a pH level of 4 and became stable for a few days. Upon the addition of GPTS, the suspension became basic, with the pH level of around 8 to 9. The GPTS to silatrane ratio was varied between 1 and 10 (i.e. denoted in Table 1 as R1 to R10, with the control condition being R0). The aging step continued for another 24 h, after which time 3 wt.% APS was added as the curing agent. After the as-prepared PMMA substrates were dipped in the asprepared coating suspensions, they were pre-cured at 60 8C for 30 min and then cured at 120 8C for another 2 h. After curing, the coating layer appeared to be transparent and smooth. The sol to gel transition of the as-prepared coating suspensions was followed by FT–IR. Fig. 1 illustrates FT– IR spectra of the as-prepared coating suspension for a GPTS to silatrane ratio of 1 at various aging times ranging from 1 to 24 h. The obtained FT–IR spectra confirm that the starting chemicals transformed into Si–O–H. Clearly, the Si–OH stretching peaks observed at 3400 and 900 cm 1 corresponding to the hydrolysis of the alkoxy group of the starting chemicals increased with increasing the aging time. The Si–O–C group is recognized by the bands observed at 1170 to 1075 and 970 to 940 cm 1. As the aging time increased, the intensity of the band specific to Si–O–Si at 1200 to 1100 cm 1 [14–16] increased and leveled off at aging times longer than 24 h (results not shown), while that specific to Si–O–C bands decreased. This only means that condensation reaction already proceeded to some extent at 3
3. Results and discussion 3.1. Characterization of silatrane products FT–IR bands observed were 3000 to 3700 (w, intermolecular hydrogen bonding), 2860 to 2986 (s, rC–H), 1244 to 1275 (m, rC–N), 1170 to 1117 (bs, rSi–O), 1093 (s, rSi–O– C), 1073 (s, rC–O), 1049 (s, rSi–O), 1021 (s, rC–O), 785 and 729 (s, rSi–O–C), and 579 cm 1 (w, SipN). TGA showed a sharp mass loss, with the first derivative peak centering at 390 8C. The percentage of ceramic yield of the products was calculated to be ca. 18.5%, corresponding to
Table 1 Composition of the as-prepared coating suspensions and transparency and adhesion to PMMA substrates of the obtained coating layer Sample
Silatrane (g)
GPTS (g)
Transparency of coating layer
Adhesion quality rating
R0 R1 R3 R5 R7 R10
0 0.1 0.1 0.1 0.1 0.1
0.5 0.1 0.3 0.5 0.7 1.0
Transparent Transparent Transparent Transparent Transparent Transparent
0B 5B 5B 5B 5B 5B
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Fig. 1. FT–IR spectra of the as-prepared coating suspension for a GPTS to silatrane ratio of 1 (denoted as R1) at various aging times ranging from 1 to 24 h.
h, since hydrolysis and condensation reactions should start just after the addition of water. Fig. 2 exhibits the FT–IR spectra of the as-prepared coating suspensions for various GPTS to silatrane ratios ranging from 1 to 10 (denoted as R1 to R10) after aging for 24 h. Apparently, the coating suspension having the greatest amount of GPTS (i.e. R10) showed the most intensified Si– O–Si absorption band at 1200 to 1100 cm 1, while the coating suspension exhibiting the most intensified Si–OH peak at 3400 cm 1 was observed at the GPTS to silatrane ratio of 5 (i.e. R5). With further increase in the GPTS to silatrane ratio, the intensity of the Si–OH peak was found to gradually decrease. Since the initial water to GPTS ratio for the coating suspensions decreased with increasing GPTS to
silatrane ratio (from about 710 for R1 to 71 for R10), the observed increased intensity at 3400 cm 1 with increasing GPTS/silatrane ratio (of up to 5) should not be a contribution from the free water in the system, as most of the water molecules should participate in the hydrolysis of certain Si–O–R groups into Si–OH groups, some of which being subsequently transformed into Si–O–Si bonds. Silatrane-based coating suspensions containing GPTS having epoxy functionality can be cross-linked by the addition of an amine curing agent such as APS. When the amine curing agent was added, the suspensions became gellike, suggesting the formation of structural networking. For industrial utilization of the as-prepared coating suspensions, the effect of GPTS content (representing as the GPTS to
Fig. 2. FT–IR spectra of the as-prepared coating suspensions for various GPTS to silatrane ratios ranging from 1 to 10 (denoted as R1 to R10) after aging for 24 h.
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silatrane ratio R=1 to 10) on the gelation time in the presence of the amine curing agent is indispensable. In this investigation, a fixed amount of APS of 3 wt.% was used. The gelation time was found to decrease from ca. 200 h for the suspension of R=1 to 40 h for the suspension of R=10. In Chu et al. [14], triethylenetetramine (TETA) was use as the amine curing agent for the coating suspension system of colloidal silica and GPTS, from which they found that the gelation time decreased from ca. 300 h for the suspension of R=0.03 to ca. 5 h for the suspension of R=0.12 (the molar ratio between TETA and GPTS was 1:2). It should be noted that, without the presence of APS, the suspensions did not gel for all GPTS to silatrane ratios investigated. The results confirmed that the gelation of the silatrane/GPTS suspensions occurred because of the curing of the epoxyfunctionalized particles and silane species in the suspensions, and it is logical to postulate that the decrease in the gelation time with increasing GPTS to silatrane ratio was a result of the increase in the GPTS to APS ratio or the increase in the amount of GPTS (i.e. more reacting species) [3,17,18]. 3.3. Characterization of coated PMMA When only the solution of GPTS was used as the coating solution without the presence of silatrane, the coating film was transparent but adhered poorly on the PMMA substrates. As previously been discussed by Kim et al. [19], without the presence of a metal alkoxide, GPTS and APS formed siloxane bonds only in the dimeric or oligomeric form, leading to no adhesion between the coating film and the substrates. On the contrary, with the presence of the metal alkoxide in the coating suspensions, the siloxane bonds can be generated in a network manner. As a result, the addition of GPTS to the silatrane solution resulted in the formation of a transparent film with good binding property with the PMMA substrates. The optical property of the coating film from the coating suspension of different GPTS to silatrane ratios (i.e. R0 to R10) is summarized in Table 1. The cross-cut tape test (according to ASTM D 3359-02 standard test method [12]) was used to characterize the adhesion between the coating layer and the PMMA substrates. In this test method, a lattice pattern with either six or eleven cuts in each direction was made on a coated PMMA surface. A pressure-sensitive adhesive was then applied over the lattice and subsequently removed. Quality rating of the adhesion was determined from fractional area of the lattice in which a part or the whole coating layer was removed from the substrates. According to the standard, five quality ratings are assigned: 5B for 0% removal, 4B for less than 5% removal, 3B for 5 to 15% removal, 2B for 15 to 35% removal, 1B for 35 to 65% removal, and 0B for more than 65% removal, respectively [12]. According to the results shown in Table 1, only the coating layer prepared from the suspension containing only GPTS failed the
adhesion test, in which the test result exhibited 0B quality rating, while the coating layer prepared from all of the silatrane/GPTS suspensions investigated received 5B quality rating. Based on the results obtained, the presence of silatrane unquestionably helped improve the adhesion between the coating layer and the PMMA substrates. It is because the amine curing agent, APS, was able to link to the PMMA substrates by reacting with the ester groups through its amine group [8]. The GPTS coupling agent itself was able to form strong bonds with partially hydrolyzed silatrane and the PMMA substrates, linking the different materials together [19–22]. The steel wool abrasion test (according to L-11-12-05 Colts standard test method [13]) was used to characterize the scratch resistance of the coated PMMA surface. In this test method, a steel wool of grade #000 or #0000 was used. The steel wool was rubbed across the substrate surface continuously for 10 cycles under an application of a 1 kg load. In this particular test, the rubbing was carried out automatically. The quality rating of the test was determined by the number of scratched lines present on the surface. According to the standard, six quality ratings are assigned: A for no evidence of scratched lines, B for 1 to 3 scratched lines, C for 4 to 20 scratched lines, D for 21 to 40 scratched lines, E for 41 to 100 scratched lines, and F for more than 100 scratched lines being observed, respectively [13]. Table 2 summarizes steel wool abrasion test results for pure PMMA substrate and PMMA substrates coated in GPTS and silatrane/GPTS suspensions. Apparently, pure PMMA substrate failed the abrasion test, with the observed scratched lines being more than 100. For the substrate dipcoated in the suspension containing only GPTS, its scratch resistance improved only by a little, exhibiting E quality rating. For the coating suspension having a GPTS to silatrane ratio of 1, the scratch resistance of the resulting coated surface improved appreciably, with the number of scratched lines being only within 1 to 3. For the coating suspensions containing GPTS to silatrane ratios ranging from 3 to 10, no scratched lines were observed on the coated surface. Fig. 3 shows the optical micrographs for pure PMMA substrates both before and after the abrasion test and for PMMA substrates coated with the coating suspensions Table 2 Scratch resistance of the uncoated PMMA, PMMA surface dip-coated in GPTS suspension, and PMMA surface dip-coated in silatrane/GPTS suspensions Sample
Abrasion quality rating
Corresponding number of scratched lines
Uncoated PMMA R0 R1 R3 R5 R7 R10
F E B A A A A
More than 100 41 to 100 1 to 3 0 0 0 0
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Fig. 3. Transmission optical micrographs exhibiting the surface of (a) uncoated PMMA surface before the steel wool abrasion test, (b) uncoated PMMA surface after the abrasion test, (c) PMMA surface coated in suspension having a GPTS to silatrane ratio of 1 after the abrasion test, and (d) PMMA surface coated in suspension having a GPTS to silatrane ratio of 10 after the abrasion test. The marker bar is for 100 Am.
having the GPTS to silatrane ratios of 1 and 10. Obviously, the hybrid coating layer consisting of silatrane, GPTS, and APS greatly improved the scratch resistance of the PMMA substrates. In order to illustrate the effect of curing temperature and time on the adhesion and the scratch resistance of the coating layer, PMMA substrates were dip-coated in the coating suspension having the GPTS to silatrane ratio of 1. Table 3 summarizes the adhesion and the abrasion test results of R1-coated PMMA substrates which were cured at 80, 100, or 120 8C for 1, 2, or 3 h. For the samples cured for 1 h, all of the test results were unacceptable, regardless of the curing temperature. For the samples cured for 2 h, only
the samples cured at 120 8C showed acceptable adhesion and scratch resistance test results. For the samples cured for 3 h, the samples cured at 100 and 120 8C exhibited acceptable adhesion test results, but only the samples cured at 120 8C showed acceptable scratch resistance test result. The extent of curing strongly affects the scratch resistance of the coating layer. The curing process is based on the homo-condensation of silanol groups and the heterocondensation between the silanol and alkoxysilane groups. The higher concentration of the silanol groups or the higher extent of hydrolysis of alkoxysilane groups in the system can certainly help promote the extent of curing and scratch resistance of the final coating layer [23,24].
Table 3 Adhesion and abrasion test results of PMMA surface dip-coated in a suspension containing a GPTS to silatrane ratio of 1 after curing at 80, 100, or 120 8C for 1, 2, or 3 h Curing temperature (8C)
80 100 120
Curing time (h) 1
2
3
Adhesion quality rating
Abrasion quality rating
Adhesion quality rating
Abrasion quality rating
Adhesion quality rating
Abrasion quality rating
0B 0B 0B
F F D
0B 0B 5B
E C B
0B 5B 5B
E C B
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4. Conclusions
References
Hard-coating suspensions for improving the scratch resistance of poly(methyl methacrylate) (PMMA) surface have been successfully developed by a sol–gel processing of silatrane in the presence of 3-glycidoxypropyl-trimethoxysilane (GPTS) using 1 M HCl as a catalyst. Prior to dip-coating, 3-aminopropyl-triethoxysilane (APS), an amine curing agent was added into the suspensions. Poly(methyl methacrylate) substrates were dipped into the coating solutions and dried at 60 8C for half an hour before being cured at 120 8C for 2 h. The addition of GPTS to the silatrane suspensions enabled the formation of dense and transparent thin-film with smooth surface on the substrates. The coating layer obtained from all of the silatrane/GPTS suspensions was found to adhere to the PMMA substrates particularly well. The scratch resistance of the coated PMMA surface was found to significantly improve from that of the pure PMMA surface. When the GPTS to silatrane ratio in the suspensions was increased from 3 to 10, the scratch resistance of the coated PMMA surface was impeccable. Lastly, it was also demonstrated that both of the curing time and curing temperature affected a great deal the scratch resistance and adhesion properties of the coating layer.
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Acknowledgments The authors wish to thanks partial supports from Commission on Higher Education, Ministry of Education; Ratchadapisek Somphot Endowment Fund, Chulalongkorn University; Petroleum and Petrochemical Technology Consortium; and Petroleum and Petrochemical College, Chulalongkorn University.