Organic/Inorganic Composite Materials for CoatingApplications

JENG-I CHEN,1 RATCHAROJ CHAREONSAK,1 VUTHIPONG PUENGPIPAT,1 SUTIYAO MARTURUNKAKUL2

1 Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

2 Saha Sethapan (1978) Co., Ltd., Bangkok 10100, Thailand

Received 11 June 1998; accepted 25 February 1999

ABSTRACT: A new organic/inorganic coating material based on the modification of aconventional melamine/polyol system has been developed. Polyhydroxyethylmethacry-late functionalized with alkoxysilane group was mixed with hexamethoxymethylmel-amine. Upon heating under an acid catalyzed condition, both sol–gel reaction andmelamine/polyol reactions occurred simultaneously, leading to highly crosslinked hy-brid composites. The synthesis and characterization of the hybrid materials are re-ported. The organic/inorganic material was also coated and cured on polycarbonatesubstrates. The coated/cured samples exhibited excellent optical property. Surfacescratch and abrasion resistance of the samples was found better than those of pristinepolycarbonate substrate. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1341–1346, 1999

Key words: melamine; coating; hybrid material; organic/inorganic composite

INTRODUCTION

The use of plastic materials has become popularin a number of applications, such as lenses, pan-els, and lamp covers, etc., due to their lightweight, good mechanical and impact strength, aswell as easy processability. In order to enhancethe overall performance of the plastic parts forsuch applications, surface-hardening coatings areapplied on the plastic substrates. These coatingmaterials are usually based on organic com-pounds such as melamine, polyurethane, andacrylic-based polymers.1–5 The hardening effect isa result of chemical reactions that convert mole-cules of the coating layer into a highly crosslinkednetwork structure.

Melamine compounds are widely used as pro-tective coatings due to their transparency, good

adhesion, as well as heat and chemical resis-tance.4,5 Hexamethoxymethylmelamine (HMMM)is a fully alkylated melamine. Since active hy-droxyl groups are no longer present, it has alonger shelf life than other types of melamines.HMMM has been used together with acrylic co-polymers containing hydroxyl group on the sidechain for coating applications.6 Such a combina-tion is often referred as a melamine/polyol sys-tem. The crosslinking reaction of this system isbased on the transetherification between mel-amine and the pendant hydroxyl group of thecopolymer under an acid catalyzed condition. Thereaction scheme is shown below. The reaction ki-netics and mechanism of such a reaction havebeen studied extensively.7–9 Upon baking at anelevated temperature, a highly crosslinked poly-mer network is formed.

Correspondence to: J.-I Chen.Contract grant sponsor: Rachadapesaksompoch Founda-

tion at Chulalongkorn University.Journal of Applied Polymer Science, Vol. 74, 1341–1346 (1999)© 1999 John Wiley & Sons, Inc. CCC 0021-8995/99/061341-06

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Inorganic materials can also be used for sur-face coating on plastic substrates. This is usuallyachieved by a sol–gel reaction since the tradi-tional melting process for glasses at high temper-ature is not suitable when organic polymer sub-strates are used. A typical sol–gel reactionscheme based on alkoxysilane functionality isshown below.10 Through the hydrolysis and con-densation reactions, vitrification can occur lead-ing to glassy inorganic materials.

There is an increasing interest in synthesizinghybrids of organic and inorganic materials.11–23

Various approaches have been adopted to prepareorganic/inorganic composite materials. Recentstudies on organic/inorganic composite materialshave been reviewed by Wilkes and co-workers.13

Structure–property relationship and phase sepa-ration behavior of these hybrid materials havealso been investigated.13,14 The in situ polymer-ization of tetraethoxysilane in several organicpolymers was studied. The properties of thesematerials have also been reported.15–18 Low mo-lecular weight organic polymers were functional-ized with alkoxysilane groups. Through sol–gelreaction, organically modified sol–gel glasseswere prepared.19–21 Nanocomposites were pre-pared by aging of a solution that allows radicalpolymerization of monomers and sol–gel reactionto occur concurrently.22 Schmidt et al. reportedorganic/inorganic hybrid coatings for metal andglass surfaces. Nanoscaled composite coating ma-terials with very high scratch resistance havebeen developed.23 Other related works, such asnonshrinking organic/inorganic composites,24 andoptical materials,25,26 have also been reported.Most of these works were based on the sol–gelreaction of alkoxysilane functionalities in organicpolymer matrices.

Tamami et al.27 have developed an abrasionresistant transparent coating material based onmelamine functionalized with the trialkoxysilanegroup. After processing and heating of this low

molecular weight compound, a crosslinked organ-ic/inorganic hybrid coating layer (1–3 mm) wasformed through a sol–gel reaction. Although themelamine moiety is not involved in crosslinkingreaction in this work, it provides good adhesion tothe substrate as well as rigidity of the film. Theabrasion resistance of the coated polycarbonate(PC) samples was superior to that of an uncoatedone. This work demonstrated that highlycrosslinked materials based on sol–gel reaction ofthe alkoxysilane group exhibited optical trans-parency and good abrasion resistance.

In this work, the preparation of novel organic/inorganic composite materials based on HMMMand functionalized acrylic polymers is presented.The chemical structure of the hybrid material isshown in Figure 1.

The functionalized acrylic polymer containsboth hydroxyl and alkoxysilane pendant groups.The hydroxyl group reacted with HMMM throughtransetherification forming an organic polymernetwork. Meanwhile, the reaction among alkox-ysilane groups formed an inorganic networkthrough inter- and intramolecular sol–gel reac-tions. Both reactions were catalyzed by a strongorganic acid. As a result, a new class of organic/inorganic composite materials was prepared forthe first time. The synthesis, processing, andcharacterization of these hybrid composites ascoating materials are reported.

EXPERIMENTAL

Hydroxy ethyl methacrylate (HEMA, .95%) wasobtained from Siam Chemical Industry and wasused as received. Synthesis of poly[hydroxy ethylmethacrylate] (PHEMA) was carried out by freeradical polymerization using benzoyl peroxide(Merck) as an initiator and laurylmercaptan(Fluka) as a chain transfer agent. HEMA (73.0 g)

Figure 1 Chemical structure of the organic/inorganichybrid material prepared from HMMM and a polymercontaining both hydroxyl and alkoxysilane pendantgroups.

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and ethanol (663.3 g) were mixed in a round bot-tom flask. To this solution, benzoyl peroxide (2.06g) and laurylmercaptan (0.22 g) were added. Thesolution was heated to reflux and the reaction wasallowed to continue for 24 h. The polymer wasprecipitated out by pouring the reaction solutioninto a mixture of benzene and hexane (1:1 v/v).28

The polymer was washed several times with abenzene/hexane mixture in order to remove unre-acted monomer and impurities. The polymer,PHEMA, was then dried at 60°C under vacuum.

A solution made of 1.37 g of PHEMA and 11.28g of dried dimethylformamide (DMF, reagentgrade from Carlo Erba) was prepared in a roundbottom flask purging with dry nitrogen. To thissolution, 2.60g of 3-isocyanotopropyltriethoxysi-lane (IPSE, Fluka) was added in order to func-tionalize PHEMA. The reaction was carried out at90°C and the extent of reaction was monitoredusing IR spectroscopy by analyzing the reactionsolutions at various reaction time. The startingmolar ratio of isocyanate in IPSE to hydroxylgroup in PHEMA was 1 to 1. Polymers with var-ious degrees of functionalization were obtained bycontrolling the reaction time. The reaction solu-tion containing functionalized PHEMA as well asunreacted IPSE was used subsequently withoutfurther purification.

A coating solution was prepared by mixing thepolymer solution with HMMM (Monsanto, Res-imenet 747) and an acid catalyst. The polymersolution (1.01 g) and HMMM (0.039 g) were mixedthoroughly prior to the addition of the catalyst,p-toluenesulfonic acid (Fluka). The acid addedinto the coating mixture was in solution form(2%). When acid was added directly in solid form,the local concentration was too high, leading toinstant partial gelation of the solution. The acidadded was 0.3 wt % of the total solid contents inthe coating mixture. The resulting homogeneoussolution was applied on PC substrates using afour-sided applicator (Sheen Instruments) to forma coating layer. The side with a 90 mm gap of theapplicator was chosen for the coating process. Thecoated PC samples were dried at 60°C under vac-uum. Final curing of samples was carried out at135°C for 12 h.

PC sheets (UV concentrated grade KU1-1241)with a thickness of 2 mm were obtained fromEastern Polymer Group. The sheets were cut into12 37 cm pieces. The surface of PC substrateswas cleaned by ethanol and dried before the ap-plication of coating. Visible light transmissionspectra were measured using a spectrophotome-

ter (Macbetht, Model Color-eye 7000). The abra-sion resistance was performed by means of anabrasion scrub tester (Sheen Instruments). Twobrushes weighing 0.45 kg each were mounted ontwo holders. The brushes reciprocated on the test-ing samples to cause wearing of surfaces. The testwas carried out under dry condition without theuse of detergent or any other liquids. The scratchresistance of samples was measured using ascratch test apparatus (Sheen Instruments). Forthe measurement, a scratch test steel needleloaded with a weight was moved on a testingsample surface. Measurements were carried outby gradually adding weights until a line on thespecimen surface caused by the scratch test nee-dle was observed. The minimum weight causingan observable scratching line was then recorded.Infrared spectra were obtained using an Fouriertransform IR (Nicolet Impact 400D) spectropho-tometer.

RESULTS AND DISCUSSION

The functionalization of PHEMA was monitoredby means of IR spectroscopy. Figure 2 shows theIR spectra of the polymer solution right after add-ing IPSE and 8 h after the addition. A decrease inisocyanate peak at 2270 cm21 and a decrease inhydroxyl peak at 3465 cm21 indicated that thereaction of isocyanate with hydroxyl group hasoccurred forming urethane linkages. The ure-thane peak signal was not identified since it over-lapped with carbonyl peaks from both PHEMA(1727 cm21) and DMF (1679 cm21). The changesin the absorbance of the isocyanate peak werealso used to calculate the degree of functionaliza-tion. When the reaction proceeded for 3, 6, and 8 hat 90°C, the percentages of hydroxyl group re-acted (degree of functionalization of PHEMA)were calculated to be 10, 40, and 65 mole %,respectively. These three polymers are namedand coded with degree of functionalization as Co-poly-10, -40, and -65.

In a commercial melamine/polyol coating sys-tem, typical hydroxyl content in the polymer isapproximately 30–40%. In our system, a similaror even higher percentage of hydroxyl group isdesired as the steric hindrance effect from thebulky alkoxysilane group may obstruct the reac-tion between hydroxyl group and melamine. As aresult from steric hindrance, some hydroxylgroups will not be able to react and will remain inthe sample. Therefore, investigation was carried

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out only on polymers with hydroxyl contents of 35% or higher. It was also found that gelation ofreaction solution occurred when the reaction timewas longer than 15 h. This is a result of thecrosslinking process from the sol–gel reaction ofthe alkoxysilane group. IR spectrum of the gelledproduct showed a significant increase and broad-ening of the hydroxyl group at around 3400 cm21.It suggested that the ethoxysilane (Si—O—Et)linkages were hydrolyzed to Si—OH bonds. Asmall amount of alkoxysilane linkages may un-dergo hydrolysis and even condensation whenheated for a prolonged period. Cross-polymer sol–gel reactions can cause gelation of solution easily,especially when the degree of functionalization ofPHEMA is high.

The coating solutions were prepared by thor-oughly mixing the polymer solution with HMMM.The solid content in the coating solution is ap-proximately 28%. No reaction was observed asboth the reaction between the hydroxyl group andHMMM as well as the sol–gel reaction are veryslow at room temperature when an acid catalystis not present. The addition of acid catalyst leadsto partial gelation of coating solution in approxi-mately 10 min. The application of the coating

solution on substrates was carried out before ge-lation occurred. It was also found that the gellingtime of Copoly-65 was the shortest. As mentionedearlier, the inter- and intramolecular sol–gel re-action is responsible for one type of crosslinkingreaction while the reaction between hydroxylgroup and HMMM contributes to the other type ofcrosslinking reaction. Copoly-65, which possessedless pendent hydroxyl group than Copoly-10 andCopoly-40, gelled faster, indicating that the ini-tial networking process mainly arose from thesol–gel reaction. However, both types of reactionsare expected to proceed simultaneously.

The drying process at 60°C under vacuum re-moved most of the solvent and volatile contents.The final curing temperature was set at 135°Cbecause the glass transition temperature (Tg) ofPC is approximately 150°C. Curing at higher tem-peratures may have provided a higher extent ofcrosslinking reaction from the sol–gel process,but it led to the distortion of samples and, in somecases, samples became opaque probably due tocrystallization of PC.

The coated/cured samples showed very goodoptical quality. Figure 3 shows the appearance ofa pristine PC and a coated/cured sample. Al-

Figure 2 IR spectra of the polymer solution, Top: right after adding IPSE. Bottom: 8 hafter the addition.

1344 CHEN ET AL.

though both melamine/polyol and sol–gel reac-tions generate volatile contents such as alcoholand water, all samples exhibited excellent opticalclarity before and after curing. The results alsoindicated that there was no significant phase sep-aration in these organic/inorganic composite ma-terials. Inorganic components are distributedevenly in the coating layer. However, it is possiblethat minor phase separation can occur in the ma-terial. When the size of scattering sites due to thepresence of various domains is small relative tothe wavelength of visible light, severe light scat-tering will not happen. Therefore, the samplescan still exhibit good optical clarity. IR spectrumof the cured sample on a KBr window showedbroad peaks at 1200–1000 cm21, suggesting theformation of a sol–gel network. A differentialscanning calorimetry scan on the coating materialshowed no observable Tg due to a highlycrosslinked structure in the organic/inorganiccomposites.

The coating materials were tested for boththeir scratch and abrasion resistance. Figure 4shows the results from the scratch resistancemeasurement. The minimum weight required tomake a visible scratching line on the sample sur-faces is indicated. The performance of samplesfrom this measurement is related to materialproperties, such as hardness, adhesion, lubricity,resilience, etc. For uncoated PC, a scratching line

was first observed when the load was 300 g.Coated samples showed improved scratch resis-tance compared to that of pristine PC. It was alsofound that the scratch resistance was enhancedas the degree of functionalization increasedwithin the range studied. It is worthwhile to men-tion that Copoly-10 with 90% of hydroxyl groupsintact allows more reactions with HMMM due toless steric hindrance. However, the resultsshowed that Copoly-65 exhibited better scratchresistance than Copoly-10 and Copoly-35. WhenCopoly-65 was used, sol–gel reaction from thealkoxysilane moiety imparted more cross-polymerreaction in the organic/inorganic hybrid material.Consequently, the scratch resistance of Copoly-65is the best among the samples investigated. Peel-ing tests were also carried out for coated/curedsamples by using a pressure sensitive tape. Allcoated/cured samples showed good adhesion tothe substrate and no fragment was peeled offfrom the substrate by the tape.

Both coated samples and pristine PC were sub-ject to the abrasion test. As the abrasion cyclesincreased, the quality of the surfaces becameworse for all samples. However, coated samplesshowed better abrasion resistance than the pris-tine PC sample judged by the naked eye. In addi-tion, the transmittance of samples in the visibleregion of samples was measured. Figure 5 showstransmittance of pristine PC and coated/curedsamples (both Copoly-65 and Copoly-40 after 500cycles). The coated samples have better abrasionresistance and less scratches on the surface thanthe pristine PC after testing. As a consequence,higher transmittance of light was measured forthe coated samples due to their less scattering

Figure 4 Scratch resistance results for various sam-ples. The minimum weight load required to make ascratch line on a sample surface is indicated.

Figure 3 A comparison of the appearance of a pris-tine PC and a coated/cured sample (Copoly-65 basedcoating).

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surfaces. Copoly-65 also performed better thanCopoly-40 and Copoly-10 in the abrasion resis-tance test.

CONCLUSIONS

A melamine/Copoly system was developed as anorganic/inorganic hybrid coating material. Thecoated/cured samples exhibited excellent opticalquality, indicating the uniform distribution of theinorganic composition. Both surface scratch andabrasion resistance of PC substrates was en-hanced by the organic/inorganic coating. Theseproperties were also affected by the degree offunctionalization in the polymer.

The authors would like to acknowledge the financialsupport from the Rachadapesaksompoch Foundationand the Office of Academic Affairs at ChulalongkornUniversity. We also thank Professor R. J. Jeng at theNational Chung Hsing University in Taiwan for hishelp and fruitful discussion. Materials provided by SCIand EPG are appreciated.

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Figure 5 Transmittance of various samples after theabrasion test (500 cycles).

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