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Journal of Sol-Gel Science and Technology 13, 735–738 (1998)c© 1998 Kluwer Academic Publishers. Manufactured in The Netherlands.

Sol-Gel Processing of Organic-Inorganic Nanocomposite Protective Coatings

YUNFA CHEN, LIANMING JIN AND YUSHENG XIEInstitute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China

Abstract. Organic-inorganic nanocomposite protective coatings are prepared on aluminum substrates by thespinning technique with the concept of incorporating homogeneously nanosized particles (of AlOOH, Al2O3,ZrO2, SiC) into molecular organic-inorganic hybrid matrices. The hybrid matrices are prepared from epoxysilaneand bisphenol A with imidazol as catalyst. The AlOOH particles are derived from aluminum isopropoxide andintroduced into the hybrid sols directly, and Al2O3, ZrO2, SiC particles are first surface-modified with SiOH fromhydrolyzed TEOS. The coatings are dense, smooth and flexible and inhibit corrosion.

Keywords: sol-gel, organic-inorganic nanocomposite, protective coating, nanosized particles

1. Introduction

Sol-gel processing of new organic-inorganic hybridmaterials has gained increased interest in the lastdecade. These materials are synthesized by chemi-cally incorporating organic polymers into inorganicnetworks, resulting in excellent and even uniqueproperties [1–3]. Due to avoidance of the disadvan-tages in processing of monoliths, such as the relativelyhigh cost of precursors, drying stresses and effective re-moval of volatiles, sol-gel coatings have already beenused in a wide variety of applications, in which organic-inorganic hybrid coatings constitute an important newfamily [4].

Particulate sols can be prepared using colloid chem-istry and polymeric sols by hydrolyzing alkoxides [4].Thus, incorporating particulate sols (nanosized parti-cles) into the molecular hybrid structure normally de-rived from modified silicon alkoxides will open thepossibility of achieving new multifunctional materials,especially those including non-oxide components [5].

In this work, organic-inorganic nanocomposite coat-ings are prepared by incorporating homogeneouslynanosized particles (of AlOOH, Al2O3, ZrO2, SiC) intomolecular organic-inorganic hybrid matrices. The hy-brid matrices are derived from epoxysilane and bis-phenol A. AlOOH particles were introduced into thehybrid sols directly, and Al2O3, ZrO2, SiC particles

were first surface-modified with SiOH from hy-drolyzed TEOS. The coatings thus obtained are dense,flexible, inhibit corrosion and are chemically durable.

2. Experimental

Polymeric Sol Preparation

The precursors were 3-glycidoxypropyltrimethoxy-silane (GLYMO) and bisphenol A, imidazol being usedas a catalyst. The preparation procedure is as follows.First, GLYMO (1 mol) was prehydrolyzed with 6 mo-lar equivalents of water (pH 5–6) at 40◦C for 1 hour,bisphenol A (0.1 mol) was then added. After stirringfor 10 min, imidazol was put into the mixture, increas-ing the temperature to 60◦C, and stirring continued foranother 30 min.

Particulate Sol Preparation

Boehmite (AlOOH) sol was prepared at 90◦C by hy-drolysis of Al-isopropoxide in excess water, and pep-tization with HNO3 (molar proportion of Al(OR)3/H2O/HNO3, 1/100/0.07), using the procedure reportedby Yoldas [6]. The boehmite size in the sol was deter-mined as<50 nm. The Al2O3, ZrO2 and SiC powderswere prepared in the laboratory using a coprecipitation

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736 Chen, Jin and Xie

or plasma method, with an average size of 40 nm, 70 nmand 80 nm, respectively. In order to improve the dis-tribution/bonding of the ceramic particles in the hybridmatrix, they were first surface-modified by hydrolyzedtetraethoxysilane (TEOS) using Sacks’s method [7],using∼70 wt% powders/30 wt% SiO2. The hydro-lysis was stopped after stirring 4–6 hours at 40◦C andmonitored by IR spectral analysis.

The two kinds of sol prepared were subsequentlymixed to produce nanocomposite (coating) sols. Ifboehmite, Al2O3 and ZrO2 were used as the particu-late phase, the mixed sols sometimes became turbid,but a little ethanol addition would cause the system tobecome transparent. In the case of SiC particles, thesol had the color of the SiC particles. The low viscos-ity nanocomposite sols could be stored for a long timeand remain suitable for coating. Commercial aluminumplate (1.5 mm in thickness) was used as the substrate.The spinning technique was used for coating with aspeed of 3000 rpm. The coatings were then dried atroom temperature and subsequently heated very slowly(1◦C/min) to 130◦C and held at this temperature for1 hour.

The characterization techniques used included vis-cosity measurement, thermal analysis (DTG-DTA),infrared spectroscopy (IR), surface profilometry, scan-ning electron microscopy (SEM) and corrosion inhibit-ing tests.

3. Results and Discussion

3.1. Viscosity Measurement

In order to obtain the most suitable time for carrying outthe coating process, the viscosity of the nanocompositesols was measured by a rotational viscometer at 60◦C.In all the cases with different nanosized particles, theviscosity variation with time initiated from the momentwhen the polymeric sol was mixed with the particulatesol was similar and can be divided into three stages,as shown in Fig. 1 for the Al2O3 system. The first in-crease may be due to further polymerization and theinteraction of the two kinds of sol. A constant viscos-ity of about 15 cP is subsequently obtained for almost90 min, which is suitable for coating. The gelationbegins in the third stage. However, some precipitateparticles were observed if the particle size was largerthan 50 nm.

In coating by the spinning method, only a smallquantity of sol is normally needed, which leads the

Figure 1. Variation of viscosity with time in sol containing Al2O3

particles (60◦C).

hot sol to cool down quickly. The viscosity measuredat 60◦C was therefore compared with that at room tem-perature but only a minor difference was detected. It isworth noting that the performance of coatings derivedfrom the sol kept at room temperature is even betterthan those from the sol at 60◦C. This will make thecoating procedure more useful practically.

3.2. Thermoanalysis

Thermal analysis was completed in an air atmosphereusing a heating rate of 2◦C/min. The samples wereground xerogels obtained by drying the sols at 40◦Cfor 10 hours.

Figure 2 illustrates the TG-DTA curves recordedon the sample containing boehmite. The endothermic

Figure 2. TG-DTA curves on the xerogels (boehmite, air atmos-phere, 2◦C/min).

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Sol-Gel Processing 737

Figure 3. IR spectra of the surface modified particles by hydrolyzedTEOS.

peak accompanied by a weight loss is attributed to theremoval of residual water and solvent. An exotherm atabout 150◦C appears to be the result of decomposition.If the coating was heated to 150◦C, it became yellow.From the IR spectra, it is found that the SiOH in thecoating disappeared at 150◦C. So the curing tempera-ture in this system is normally fixed at 130◦C.

3.3. Infrared (IR) Spectroscopy [8]

Figure 3 shows the IR spectra for the surface-modifiedZrO2 and SiC particles using hydrolyzed TEOS. Forcomparison, IR spectra of the SiO2 gel obtained fromTEOS under the same reaction conditions is also in-cluded. Figure 3 indicates that the ceramic particleswere coated with a layer of SiO groups, which is use-ful to assist bonding of the particles with the hybridmatrix.

The epoxy group in GLYMO is capable of form-ing either diol units by hydrolytic ring opening orpolyethene oxide chains by polyaddition. To sepa-rate the two process with formation of the inorganicnetwork and organic crosslinking, the introduction ofbisphenol A and 1-methylimidazol (MI) into the sys-tem is normally necessary. MI acts as a catalyst for thecondensation of silanol groups and also an initiator forthe epoxide polymerization above 60◦C [3]. However,It was found that the MI addition resulted in a light yel-low color. The replacement of MI with imidazol in thepresent work avoided the color change until a slightlyhigher temperature.

Comparing the IR spectra of GLYMO before andafter prehydrolysis, the epoxy ring (∼1200 cm−1,930 cm−1 and 850 cm−1) is clearly opened. After

Figure 4. IR spectra of nanocomposite sol and gel (boehmite sys-tem).

hydrolysis of Si OR, the peak of SiO (Si OH)is near 860 cm−1, −CH3 in the form of Si OCH3

(∼2817 cm−1) has almost disappeared. Figure 4 showsthe IR spectra for the nanocomposite sol and thecured coating. Bisphenol A is covalently connectedwith the chain and the CO C bond is detected near1050 cm−1, also the broad peak of SiO Si is clearlyobserved. From Fig. 5, the differences for IR spectraof the gels cured at 90◦C and 150◦C are related to thebands belonging to the SiOH group. Above 150◦C,the Si OH disappeared as observed on the DTA curves.

3.4. Coating Thickness and SEM Observation

The coating thickness was measured using a surfaceprofiler (Sloan Tech, Dektak-3ST). The thickness ofcoatings after a single spinning was 1–3µm. If thicker

Figure 5. IF spectra of the cured gels at 90◦C and 150◦C (ZrO2

system).

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738 Chen, Jin and Xie

Figure 6. SEM micrograph of cured nanocomposite coating(Al2O3 system).

coatings were needed, repeated spinning could be used.In the present case, spinning for 2–3 times was neces-sary to obtain dense, smooth and flexible coatings witha thickness of about 7µm.

The surface characteristics of the coatings were ob-served by SEM. Figure 6 shows the cured coating withAl2O3 particles, but it is representative of all the cases.The particles were observed to be homogeneously dis-tributed in the coatings. The particle size on the SEMmicrographs was slightly larger than that in the raw ma-terials. This may be due to the particles being coatedwith Si O or organic groups. If AlOOH was used, thisphenomenon was less obvious.

3.5. Coating Performance

Coated aluminum samples were treated in 1 N HCl,1 N NaOH and 1 N KCl solutions at room temperaturefor one month, but no corrosion took place. However,for treatment in concentrated HCl (37 wt%), corrosionwas detectable after only 70 min.

If boehmite is added as a nanosized particle,Kassemann and Schmidt assumed that the interface isstabilized by the nanocomposite matrix through theheterocondensation of silanols with surface AlOHgroups. In the present case, a similar explanation canbe applied due to the very similar corrosion behaviorof the various nanocomposite coatings.

If the coated aluminum plate was subjected to bend-ing for several times, the coating did not peel off,

indicating that it was sufficiently flexible. In this sys-tem, the introduction of bisphenol A and boehmite solshould make the coating scratch resistant and the or-ganic groups should improve the flexibility, althoughtests have not been completed.

4. Summary and Conclusions

Organic-inorganic nanocomposite protective coatingswere prepared on aluminum substrates by a spin-ning technique. In the coatings nanosized oxideparticles (AlOOH, Al2O3, ZrO2, SiC) were incorpo-rated homogeneously into a molecular hybrid matrix.The hybrid matrix was derived from epoxysilaneand bisphenol A with imidazol as catalyst, whichavoids the light yellow color found in the case of1-methylimidazol. The AlOOH particles preparedfrom Al isoproxide were introduced directly into thehybrid sols, and the other particles were first surfacemodified with Si OH groups. The coatings are dense,smooth and flexible and inhibit corrosion.

Acknowledgment

The authors are grateful for financial support by theChinese Academy of Sciences (CAS) and Ministry ofHuman Affairs.

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