2002 Coating of Methyltriethoxysilane—Modified Colloidal Silica on Polymer substrates for abrasion

Journal of Sol-Gel Science and Technology24,175–180,2002c 2002Kluwer Academic Publishers.Manufactured in The Netherlands. Coating of Methyltriethoxysilane—Modified Colloidal Silica on PolymerSubstrates for Abrasion ResistanceMAN SUNG LEEDepartment of Chemical Industry,Dong-eui Institute of Technology,Pusan614-715,KoreaNAM JU JODepartment of Polymer Science and Engineering,Pusan National University,Pusan609-735,KoreaReceived January29,2001;Accepted November27,2001anic-inorganic hybrid composites were prepared by the sol-gel method for the hard coating agent of transparent plastic,and their abrasion resistance,optical and surface characteristics were evaluated.Methyltriethoxy-silane(MTES)and colloidal silica were used as starting materials.The addition of MTES to colloidal silica enabled the formation of dense thinfilms with very smooth surface on the substrates.The thinfilms were strongly adhered to the substrates without primer treatment.The abrasion resistance increased with the increase in the ratio of MTES to the colloidal silica.Optimal amount of MTES for the hard coating agent was100wt%to the colloidal silica.The addition of curing catalyst,tetramethylammonium formate was found to be very effective to enhance the adhesion strength of coating agent to the substrates and reduced curing time.Keywords:abrasion resistance,sol-gel method,hard coating agent,methyltriethoxysilane,colloidal silica1.IntroductionRecently transparent polymer materials have been utilized as windows in aircraft,buildings,and opti-cal lens.However,while transparent polymer materials have excellent optical transparency and many benefi-cial mechanical properties,they show poor abrasion re-sistance and weatherability.To solve these problems, many kinds of hard coating agents such as melamin, acrylic and urethane resin based hard coating agents were developed[1–4].However they were not satisfied with abrasion-resistance and weatherability. Organic-inorganic hybrid composites have created very effective hard coating agents possessing advan-tages of both organic and inorganic materials.These are based on the use of metal alkoxide and organosiloxane by the sol-gel method.The sol-gel method is a novel procedure among solution reactions,which is based on the preparation of macromolecular network through the typical hydrolysis of alkoxide groups and the conden-sation reaction in following Eq.(1):R -Si-(OR)3+3H2O→R -Si-(OH)3+3ROH(hydrolysis)R -Si-(OH)3+R -Si(OH)3→R (OH)2-Si-O-Si-(OH)2R +H2O(condensation)(1) where R and R are the alkyl group and short alkyl hydrocarbon,respectively.Hydrolysis of the metal-lic alkoxide groups provides unstable and highly ac-tive metallic hydroxides under acid or base condition in alcohol solution.And metallic hydroxides become macromolecules subsequently due to the condensation reaction of themselves.Moreover,the degree of poly-merization can be controlled according to the reac-tion condition such as concentration,temperature,and176Lee and Jocatalyst [5].Then,sol-gel method is one of the effective methods for the coating of colloidal silica at relatively low temperature on various organic polymer substrates such as polymethylmetacrylate (PMMA),polycarbon-ate (PC),polyethyleneterephthalate (PET),Nylon,and so on [6–8].With the development of synthesis technology for silicone compounds,E.I.Dupont made silicone based hard coating agents by using hydrolysis of tetraalkoxysilane and vinyl acetate at the first time in 1943[9].Owenes-Illinosis applied condensed mate-rials of methyltrialkoxysilane and phenyltrialkoxysi-lane to coatings [9].Dow-Corning established the basic technology of hard coating according to the con-densed materials prepared by cohydrolysis of colloidal silica and methyltrialkoxysilane in 1974.However,the traditional organic-inorganic hybrid hard coat-ing agent has shrinkage problem during curing pro-cess,and the abrasion resistance was not so good.And the traditional coating agent is very poor econ-omy,because the primer must be used to enhance the interfacial strength between substrate and coating layer.In this study,nano-sized colloidal silica was mixed with MTES,which was hydrolyzed and condensed with the silanol groups on the surface of silica.Coat-ing films were made from these solutions on PMMA substrates.After curing,the abrasion resistance was measured and compared with an uncoated substrate.To improve the adhesion of the coating solutions with substrates,curing catalyst was introduced to the coating solution without the primer.In this report,the effects of MTES contents on abrasion resistance were exten-sively examined.2.Experimental2.1.MaterialsColloidal silica (LUDOX RLS,solid contents 30wt%,water dispersion solution)and MTES were purchased from Aldrich and used as received.PMMA sheets(GREENGLAS R,Bayer-Sewon Ltd.)with a thick-ness of 2mm were cleaned with isopropanol before use.Tetramethyl ammonium formate as a curing cat-alyst was purchased from Aldrich and used as re-ceived.Various compositions of the coating solutions were prepared and represented as RX,where X shows the wt%of MTES to colloidal silica in the startingsolution.Figure 1.The procedure for preparing the hybrid thin film.2.2.SynthesisThe procedure for preparing the hybrid thin films is schematically shown in Fig. 1.Colloidal silica and MTES were used as starting materials.Proper amounts of MTES and colloidal silica were poured into 4-necked flask and stirred slowly.0.2wt%of acetic acid was added to control the acidity of the solution and to increase hydrolysis reaction rate.The solution was controlled at pH 4.7and stirred for 24hours at 30◦C for enough hydrolysis reaction.The acid catalyst was added slowly into the alkoxide solution with stirring.After being stirred for about 24hours,partially condensed and hydrolyzed solution was obtained.To eliminate excess water and ethanol,additional ethanol was added for azeotropic elimination into the solution.It was stirred and heated at 40◦C under reduced pres-sure.To minimize additional condensation reaction,the coating solutions were kept at below 0◦C in the refrigerator.To make coating agents,isobuthylalcohol (IBA)and ethyleneglycolmonoethylether (EE)were added as diluent solvents.Also,0.2wt%of curing catalyst,tetramethylammonium formate,was added.Finally,the coating solution with 40wt%of solid content was obtained.The coating was carried out on the substrates in flowing or dipping-withdrawing manner at the atmo-spheric temperature.The obtained coating films were heat-treated at 85◦C for one hour.2.3.CharacterizationFT-IR spectra were obtained using a Nicolet Impact 400D FT-IR Spectrometer.Adhesion was measured byCoating of MTES —Modi fied Colloidal Silica 177tape test using ASTM D3359.A lattice pattern with eleven cuts in each direction is made in the film to the substrate,pressure-sensitive tape is applied over the lattice and then removed,and adhesion is evaluated by comparison with descriptions.Abrasion resistance was measured with a Teledyne Taber type abrader with CS-10F abrasion wheel under 500g loading.Surface roughness was measured with a Digital Instrument,NanoScope RE atomic force microscope (AFM)by tapping mode.Light transmittance was measured with a Toyoseiki Direct Reading Hazemeter.Refractive index was also measured with a spectroscopic Ellipsometer (V ASE,J.A.Woollam,USA).3.Results and Discussion 3.1.FT-IR SpectroscopyFigure 2shows the time dependence of infrared spec-tra for the coating solution,R100.The Si CH 3group in the MTES is easily recognized by a strong and sharp band at about 1260cm −1together with one or more strong bands in the range of 865–750cm −1.The Si O C 2H 5group is recognized by a small band at about 1170–1160cm −1together with small bands in the range 1100,1075(s),and 970–940cm −1.Siloxane shows one or more very strong infrared bands in the region 1130–1100cm −1.Disiloxane and small-ring siloxane show a single Si O Si band.As the silox-ane chain become longer or branched,the Si OSiFigure 2.Time dependence of FT-IR spectra of the coating solutions prepared from R100.absorption become broader and more complex,show-ing two or more overlapping band.The H-bonded Si OH group is recognized by a broad band at about 3400–3200cm −1with single broad band,in the range 950–810cm −1.Also,free Si OH group is recognized at 3690cm −1[10].Analysis of reaction medium by FT-IR indicated that firstly,hydrolysis reaction occurred and conden-sation reaction was observed at about 3hr spectrum.The peak intensities of Si O(H)stretching mode at about 900cm −1and the (Si)O H stretching mode at 3400cm −1corresponding to the hydrolysis of ethoxy group in MTES were increased with an in-crease in reaction time.Asymmetric stretching modes of Si O C 2H 5at 1160,970,1075and 1100cm −1were disappeared in 3hr spectrum.It has been known that the partial condensation was occurred by the Si O Si asymmetric stretching mode at 1130–1100cm −1,the Si O Si symmetric stretching vibration at 1120cm −1,the symmetric Si O Si stretching mode at 870cm −1.Finally,partially condensed silane sol with free silanol groups and H-bonded silanol groups was obtained dur-ing 24hr reaction time.3.2.The Formation of Coating FilmsTable 1shows the state of the coating films on PMMA sheets in the RX system.When colloidal silica alone was used,the thin film formed on PMMA sheet was easily cracked and the appearance was looks like white powder.The addition of MTES to the colloidal silica178Lee and JoTable 1.The state of the coating films on PMMA sheet in the R x system (compositions are represented as Rx,where x shows the wt%of MTES to colloidal silica in the starting solution).Colloidal MTES Formation of Composition silica (wt%)(wt%)coating film R0100–None R1010010None R2010020Haze R3010030Transparent R5010050Transparent R7010070Transparent R100100100Transparentyielded crack free and transparent films.Transparent thin films without crack and haze were obtained in the composition range from R30to R100.Measured thick-ness of coating film after curing was 8micrometer in average.It should be noted that the films with good transparency and low haze could be obtained when proper amounts of MTES were used.3.3.Adhesion and the Effects of Curing CatalystFigure 3represents the reaction of tetramethyl ammo-nium formate with silanol in the coating solution and formation of ester linkages.Formed ester groups have chemical attraction with other ester groups on the PMMA substrates.Thus curing catalyst was linked ef-fectively between PMMA substrate and coated film and reduced curing time.Thin coating films were not peeled off from the tape peel off test after cross cutting.Also,curing catalyst showed good adhesion for polycarbon-ate substrate without primer treatment.Table 2shows the results of adhesion test of coating film on PMMA sheet coated with R100.Maximum curing temperature on PMMA substrate was 90◦C.PMMA substrates were thermal-transformed at above 90◦C.When curing catalyst was not used,coating filmsFigure 3.Reaction mechanism of tetramethylammoniumformate as curing catalyst with silanol group.Table 2.The result of adhesion test of coating film on PMMA sheet coated with R100on curing temperature and time.Curing time (hr)Amount of curing catalysts Curing tempera-ture (◦C)0.51235None80–0/1000/1000/1000/10085–0/1000/1000/1000/10090–0/1000/1000/1000/1000.2wt%8080/100100/10085100/100were easily peeled off from the substrate having no concerned with curing time and temperature.There-fore,optimum curing time was 30min at 85◦C with 0.2wt%of curing catalyst to the coating solu-tions.When the amounts of curing catalyst were over 0.2wt%,curing time was reduced.But rough film sur-face was obtained.3.4.Light TransmittanceFigure 4shows total light transmittance in the range of visible light.In the case of coated PMMA sheet with R100,light transmittance (about 92%)increased over 2%in all visible light range (400–780nm)compared with uncoated PMMA sheet (about 90%).In general,the change of refractive index (RI)in fluences on the light transmittance.RI of uncoated PMMA sheet with 2mm thickness is 1.40and PMMA sheet coated with R100was 1.38.Higher light transmittance of coating film was probably due to lower RI because coating films exhibited lower RI than that of the PMMA sheet.ItisFigure 4.Light transmittance of PMMA sheets uncoated and coated with R100.Coating of MTES —Modi fied Colloidal Silica179Figure 5.AFM photographs of PMMA sheets uncoated and coated with R70or R100.also considered that the very smooth surface of coating agents composed of MTES and colloidal silica reduced light scattering on the film surface.3.5.Surface Roughness and Abrasion ResistanceIt is obvious that surface roughness affects abrasion resistance,light scattering and adhesion of coating film.Figure 5shows AFM images of the coating film prepared from R70,R100compalable with un-coated PMMA sheet.The value of surface roughness of the PMMA sheets,uncoated and coated with R70or R100was summarized in Table 3.The surface rough-ness substantially decreased with increasing MTESTable 3.Surface roughness of PMMA sheets uncoated and coated with R70or R100.Roughness Uncoated (nm)PMMA R70R100R max 40.24523.44314.773R mean6.2630.8340.421content.(Figure 5and Table 3)Smooth surface with unevenness of less than 1nm was obtained in coated PMMA sheets with R70and R100,while rough surface larger than a few nanometer was obtained in uncoated PMMA sheets.The abrasion resistances of PMMA sheets uncoated and coated with R100or AS4000that is one of the cur-rent top commercial abrasion resistant silicone resin solution was compared in Fig.6.Taber type abrader with CS-10F abrasion wheel under 500g loading was used.The haze of scratched surface in PMMA sheet coated with R100was considerably decreased in com-parison with uncoated PMMA sheet.At higher num-ber of cycles R100had more effective abrasion resis-tance compared with AS4000.The addition of MTES to the inorganic particle effectively improved the coat-ing toughness by formation of siloxane chemical bonds between MTES and inorganic particle.It is also con-sidered that the enhancement of abrasion resistance is caused by that silane treatment lowers the surface energy of the colloidal silica,which reduces capil-lary stresses during drying.On the other hands,coat-ing solution with excess MTES had worse abrasion180Lee andJoFigure 6.Haze (%)as a function of number of wear cycles for uncoated and coated PMMA sheets.CS-10F abrasion wheel was used under 500g loading.resistance than R100.Therefore,it could be concluded that the organic-inorganic hybrid coating film consist-ing of MTES and colloidal silica formed high dense siloxane network between particles,resulting in good abrasion resistance.4.ConclusionCoatings were prepared from suspensions containing colloidal silica and various amounts of MTES.Light transmittance of coated films increased over 2%com-pared with uncoated PMMA sheet.And it was origi-nated from the decrease of refractive index and surface roughness.Surface roughness substantially decreased by the increase of MTES contents.Abrasion resistance of PMMA sheet coated with silica coatings increasedwith the addition of MTES and the optimum amount of MTES was 100wt%to colloidal silica.The MTES adsorbed on the silica surface and formed a strong siloxane bonding between particles by the condensa-tion reaction.The curing catalyst,tetramethyl ammo-nium formate,was very effective material to enhance the adhesion strength of coating agent to the substrates.These hard coating agents could have ef ficient adhe-sion and durability under sunlight,which leads to good productivity and economic ef ficiency.AcknowledgmentThis work was supported by grant No.2000-2-30800-001-2from the Basic Research Program of the Korea Science &Engineering Foundation.References1.J.D.Blizzard and L.J.Cottington,U.S.Patent 5,403,535,1995.2.S.H.Schroeter and D.R.Olson,U.S.Patent 4,210,699,1980.3.M.Ogawa and Y .Hara,U.S.Patent 4,764,576,1988.4.R.E.Martison et al.,U.S.Patent 5,403,535,1995.5.C.J.Brinker and G.W.Scherer,Sol-Gel Science:The Physics and Chemistry of Sol-Gel Processing (Academic Press,New York,1990).6.R.Nass,E.Arpac,W.Glaubbit,and H.Schmit,J.Non-Cryst.Solids 121,370(1990).7.B.Wang and G.L.Wilkes,J.Macromol.Sci.Pure Appl.Chem.A 31,249(1994).8.K.Tadanaga,K.Iwashita,and T.Minami,J.Sol-Gel Sci.and Tech.6,107(1996).9.W.A.Finzel and H.L.Vincent,Silicones in Coatings ,New Federation Series on Coating Technology,(Federation of Societies for Coatings Technology,Blue Bell,PA,1996).10.B.Arkles,Silanes,Silicones,and Metal-Organics ,2nd ed.,(Gelest,PA,1998).。