Improvement of the surface properties of polycarbonate by organic–inorganic hybrid coatings

Improvement of the Surface Properties of Polycarbonateby Organic–Inorganic Hybrid Coatings

Paola Fabbri,1 Cristina Leonelli,1 Massimo Messori,1 Francesco Pilati,1 Maurizio Toselli,2

Paolo Veronesi,1 Sandrine Morlat-Therias,3 Agnes Rivaton,3 Jean Luc Gardette3

1Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita di Modena e Reggio Emilia,Via Vignolese 905/a, 41100 Modena, Italy2Dipartimento di Chimica Applicata e Scienza dei Materiali, Universita di Bologna,Via Risorgimento 2, 40100 Bologna, Italy3Laboratoire de Photochimie Moleculaire et Macromoleculaire, Unite Mixte De Recherches Centre National de laRecherche Scientifique 6505, Universite Blaise Pascal (Clermont-Ferrand), 63177 Aubiere Cedex, France

Received 21 April 2006; accepted 13 November 2007DOI 10.1002/app.27765Published online 23 January 2008 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Organic–inorganic hybrids (ceramers) wereprepared through the sol–gel process with opportunealkoxysilane-terminated polymer chains as the organicphase and tetraethoxysilane as the inorganic network pre-cursor. The consolidation process, used to reach a highdegree of crosslinking between the two phases, was car-ried out with either conventional oven heating or micro-wave irradiation. High conversion degrees were obtainedwith both treatments even when microwave postcuring

turned out to be much faster than conventional heating(5–10 s vs 40 min). Scratch-test and photooxidation investi-gations showed a significant improvement in scratch,yellowing, and photodegradation resistance for coated poly-carbonate. � 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108:1426–1436, 2008

Key words: coatings; curing of polymers; nanocomposites;photochemistry; transparency

INTRODUCTION

Polycarbonates (PCs) represent an extremely usefulclass of engineering thermoplastics known for theirtoughness and optical properties. The vast majorityof PCs are based on bisphenol A and have glass-transition temperatures in the range of 140–1558C;they are widely regarded for their optical clarity andexceptional impact resistance and ductility at orbelow room temperature. Other properties, such asthe modulus, dielectric strength, and tensile strength,are comparable to those of other amorphous thermo-plastics at similar temperatures below their respec-tive glass-transition temperatures. However, althoughmost amorphous polymers are stiff and brittle belowtheir glass-transition temperatures, PCs retain theirductility.1

The main limitations are due to their poor scratch,ultraviolet (UV), and solvent resistance. Any improve-ment of such properties would represent a veryimportant goal from an industrial point of view anda main scientific challenge. These enhancements canbe pursued by the physical incorporation of suitableadditives even if this approach presents several dis-advantages: the specific additive is often required in

high loadings to be effective, and this may result inadverse changes in the physical and mechanical prop-erties of the polymer. Additives can also be leachedfrom the polymer through normal service and aging,and this poses an environmental threat in addition tothe primary result of the reduction in its effect.

An alternative approach is represented by theapplication of functional protective coatings directlyto the manufactured items. Inorganic protective coat-ings produced by plasma techniques (chemical vapordeposition (CVD) and plasma enhanced chemicalvapor deposition (PE-CVD)) are known to showgood abrasion resistance in general,2 and completelyinorganic zinc oxide (ZnO) layers deposited onto PCby sputtering have been recently demonstrated to beeffective toward its photodegradation due to solarUV radiation.3

However, fully inorganic protective coatings havesome problems of poor adhesion due to the differ-ence in the thermal expansion coefficients betweenthe substrate and coating. The use of the afore-mentioned techniques is also limited by high costs,small item dimensions, and simple geometric formrequirements.

The coating of PC surfaces by organic–inorganichybrids prepared by the sol–gel process could be inmost cases an interesting alternative to gas-phase orvacuum-deposition techniques because of the high ver-satility associated with the application mild conditions.

Correspondence to: P. Fabbri. ([email protected]).

Journal of Applied Polymer Science, Vol. 108, 1426–1436 (2008)VVC 2008 Wiley Periodicals, Inc.

The classical sol–gel process4 consists of a two-step acid- or base-catalyzed reaction of metal alkox-ides [M(OR)x] with their partial and/or completehydrolysis followed by condensation, which leads tothe buildup of a three-dimensional inorganic net-work through the formation of M��O��M linkages.An efficient incorporation of organic chains into thisnetwork is also possible, particularly when the or-ganic molecules are characterized by the presence offunctional groups reactive in the aforementionedreactions. This approach leads to organic–inorganichybrid materials,5 also known as ceramers, in whichthe intimate mixing favored by the covalent bondingbetween organic and inorganic moieties can lead tothe formation of crosslinked structures composed ofnanosized organic and inorganic domains. Theresulting material is transparent and macroscopicallyuniform even if phase-separated.

These materials have already gained much interestbecause of the remarkable improvement in proper-ties such as the optical, mechanical, thermal, electri-cal, and magnetic properties in comparison withthose of pure organic polymers. Several applicationshave been developed for this kind of hybrid mate-rial,6–8 particularly in the field of protective coatingsof both organic and inorganic substrates.

In this study, organic–inorganic hybrid materialswere prepared from triethoxysilane-terminated poly-mer chains [i.e., poly(ethylene oxide) (PEO) or poly-caprolactone (PCL)] as the organic componentbecause of their good thermodynamic miscibilitywith PC9–12 and from tetraethoxysilane (TEOS) asthe inorganic network precursor; the hybrids weresubsequently obtained as self-consistent materials orcoated onto PC substrates.

Because one of the major points regarding theapplication of PC is related to its high sensitivity to-ward abrasion and an enhancement in this propertyshould be an important goal, the scratch resistanceof our coated samples was evaluated with respect tothe coating composition (in terms of the organic/inorganic ratio) and the type of postcuring treatmentapplied, namely, conventional oven heating ormicrowave irradiation. In fact, coupled to the sim-plicity of the coating system preparation and appli-cation, a fast curing step should also represent animportant factor for future industrial applications,and in this view, microwave-assisted curing is par-ticularly interesting and innovative.

In addition to the poor scratch resistance, furtherlimiting aspects for PC applications are related toyellowing and degradation due to the impact of so-lar light on the material. One can simply consider allthe problems related to the outdoor applications ofPC or to its use in the automotive industry becauseof the loss of transparency and brilliance due to thematerial’s photooxidation. The possibility of protect-

ing PC surfaces against photodegradation with anopportune transparent coating should then alwaysattract greater attention. For the aforementioned sig-nificant advantages presented by the sol–gelapproach, this method should be strongly suggestedfor the preparation of tailored hybrid coatings ensur-ing a barrier effect toward UV radiation.

We report the results of photooxidation studiesperformed on PC coated with hybrids based on thePCL organic phase. PCL was chosen for this part ofthe study rather than PEO because PEO is known tobe very sensitive to UV-light-induced oxidation.13 Inthis first investigation on the protective effect ofceramers, the attention has been mainly focused onthe barrier effect of the coatings. The oxidative deg-radation induced by exposure to UV light of PC sub-strates coated with PCL ceramers was followed byinfrared (IR) and ultraviolet–visible (UV–vis) spec-troscopy. The effects of the composition and thick-ness of the coatings are discussed. The screen effectthat these coatings can have when formulated withorganic or mineral UV absorbers will be reported ina forthcoming article.

EXPERIMENTAL

Materials

a,x-Hydroxy-terminated PEO (purchased from Fluka(Milan, Italy) and having a number-average molecu-lar weight of 600 g/mol), 3-isocyanatopropyltriethoxy-silane (ICPTES; Fluka), TEOS (Aldrich, Milan, Italy),hydrochloric acid at a 37% concentration (CarloErba, Milan, Italy), and ethanol (EtOH; Carlo Erba,Milan, Italy) were high-purity reagents and wereused without further purification.

a,x-Hydroxy-terminated PCL was prepared inbulk by the ring-opening polymerization of caprolac-tone (CL) monomer (purchased from Aldrich andused after distillation over calcium hydride) in thepresence of tin(II) 2-ethylhexanoate (SnOct2) as theinitiator and with ethylene glycol (EG; obtained fromAldrich and used as received) as the transfer agent.The reaction was typically carried out in a glass flaskunder magnetic stirring at 1208C for 24 h. The molarratio used, 20/1/0.02 CL/EG/SnOct2, led to a nomi-nal degree of polymerization of 20, which corre-sponded to a molecular weight of about 2300 g/mol.

The PC substrates used for photooxidation studieswere commercial, untreated, transparent films (Mak-rofol DE 6-2), 125 lm thick, from Bayer (Milan,Italy). For the scratch test, PC substrates were com-mercial, untreated, transparent slabs 3.0 mm thick.

Preparation of a,x-triethoxysilane-terminatedPEO and PCL

a,x-Triethoxysilane-terminated polymer chains (calledPEOSi and PCLSi in the following) were prepared

POLYCARBONATE ORGANIC–INORGANIC COATINGS 1427

Journal of Applied Polymer Science DOI 10.1002/app

by the bulk reaction of the corresponding a,x-hydroxy-terminated polymers with ICPTES (molarratio of 1/2). The reaction was usually carried out ina 50-mL glass flask equipped with a calcium chlo-ride trap and under magnetic stirring at 1208C for 3h, as already reported in a previous article.14

Preparation of the organic–inorganic hybrids

The same procedure was followed for the prepara-tion of PEO- and PCL-based hybrids.

Mixtures of TEOS and the a,x-triethoxysilane-ter-minated polymer (PEOSi or PCLSi) were dissolvedin EtOH at a concentration of about 10% (w/v), andthen water (for the hydrolysis reaction) and hydro-chloric acid (as the catalyst) were added in the fol-lowing molar ratios with respect to the ethoxidegroups of both the functionalized polymer andTEOS: EtO��/H2O/HCl 5 1/1/0.05.

A typical preparation for PEOSi/TEOS hybridswas as follows: PEOSi (1.20 g) and TEOS (0.80 g)were added to 20 mL of EtOH in a screw-threadglass vial and mixed until a homogeneous solutionwas obtained. Then, water and HCl were addedunder vigorous stirring at room temperature forabout 10 min. The closed vial was placed in an air-circulating oven at 708C for 30 min to allow partialprogression of the sol–gel reaction. The clear solutionwas then cast into a closed polytetrafluoroethylenedish, and the solvent was slowly evaporated at roomtemperature for about 1 week to get self-consistentsamples for further analysis. As an alternative, thesolution was deposited onto clean PC films by spincoating with a Laurell WS-400B-NPP-Lite instrument(North Wales, PA) operating at a spin rate of 2000rpm over 30 s (ACL 15) to obtain a deposited hybridfilm having a typical thickness of about 0.3–0.4 lm.

Hybrid materials based on PEO were character-ized by final organic/inorganic weight ratios of 7/3and 3/7, assuming the completion of the sol–gelreactions reported in Scheme 1. PEO-based sampleswere coded as follows: PEOSi/SiO2 7/3 indicates acoating with an organic/inorganic ratio of 7/3 (w/w) and a coating thickness of 0.3–0.4 lm, andPEOSi/SiO2 3/7 indicates a coating with an organic/inorganic ratio of 3/7 (w/w) and a coating thicknessof 0.3–0.4 lm.

Hybrid materials based on PCL were characterizedby final organic/inorganic weight ratios of 4/6 and8/2, assuming the completion of the aforementionedsol–gel reactions. The EtOH solutions were roll-coated onto clean PC films, and different coatingsthicknesses (0.6–0.75 and 1.3–1.5 lm) were obtained,depending on the solution concentration. Thickercoatings were obtained with more concentrated solu-tions (ca. 20% w/v). PCL-based samples were codedas follows: PCLSi/SiO2 4/6 L indicates a coatingwith an organic/inorganic ratio of 4/6 (w/w) and acoating thickness of 0.6–0.75 lm, PCLSi/SiO2 4/6 Hindicates a coating with an organic/inorganic ratioof 4/6 (w/w) and a coating thickness of 1.3–1.5 lm,PCLSi/SiO2 8/2 L indicates a coating with an or-ganic/inorganic ratio of 8/2 (w/w) and a coatingthickness of 0.6–0.75 lm, and PCLSi/SiO2 8/2 Hindicates a coating with an organic/inorganic ratioof 8/2 (w/w) and a coating thickness of 1.3–1.5 lm.

Curing treatments

PEO-based hybrids were subjected to two differentpostcuring treatments to allow the completion of thepolycondensation reaction, namely, conventionalheating in an air-circulating oven (WTB Binder, Tut-tlingen, Germany) at 808C for times ranging from 20to 60 min or microwave irradiation with an AlterTE10n single-mode applicator (Reggio Emilia, Italy)at 2.45 GHz and 350 W with irradiation times rang-ing from 3 to 15 s.

PCL based hybrids were postcured by conven-tional heating in an air-circulating oven at 1208C for24 h.

Characterization

Differential scanning calorimetry (DSC) was per-formed with a TA DSC2010 instrument (Vimodrone,Italy) to determine the degree of curing that wasreached through the curing treatments. Thermalscans were performed in the temperature range of0–2008C with a heating rate of 108C/min. The extentof the residual curing reaction was calculated duringthe first heating scan by the enthalpy value of theendothermic peak corresponding to the volatilizationof EtOH and water that formed during the progres-sion of the sol–gel reaction. At least three independ-ent DSC runs were carried out for each sample (typeof hybrid and type and time of postcuring treat-ment), and an average value was derived from eachseries of tests. A second heating scan was also per-formed to evaluate the presence of a residual sol–gelreaction or any other thermal transitions. Just beforethe measurements, all samples were accurately driedunder a dynamic vacuum at 408C for 1 h to elimi-nate any possible volatile product.

Scheme 1 Hydrolysis and condensation steps in the sol–gel process.

1428 FABBRI ET AL.


Scratch tests were carried out on a CSM Micro-Combi tester with a Rockwell C diamond scratch in-denter (tip radius (R) 5 0.2 mm) with the load pro-gressively increasing from 100 to 2000 mN at a loadrate of 494 mN/min for a scratch length of 1.0 mm.

Photooxidation was carried out by the exposure ofthe samples in a Sepap 12–24 accelerated artificialaging device. This medium-accelerated photoagingdevice has been described previously.14 It is equippedwith four medium-pressure mercury lamps andallows irradiation at wavelengths longer than 300 nmat 608C and in the presence of atmospheric oxygen.UV–vis spectra were recorded on a Shimadzu UV-2101 PC equipped with integrating spheres. The IRspectra were recorded on Nicolet Magna-IR 510(Waltham, MA) and 800 Fourier transform infraredspectrophotometers (resolution of 4 cm21, summa-tion of 32 scans). Samples were regularly analyzedduring irradiation.

RESULTS AND DISCUSSION

Microwave and oven-heating postcuringof the PEOSi/SiO2 hybrids

To evaluate the degree of conversion of the sol–gelprocess, dynamic DSC analysis was carried out onself-standing PEO-based hybrids before and after thetwo different types of postcuring treatments, namely,conventional heating and microwave irradiation. Theresidual degree of conversion was evaluated by themeasurement of the enthalpy corresponding to theendothermic peak attributable to the vaporization ofvolatile products during the first heating scan; thismeans evaluating the advancement of the crosslink-ing reaction in the hybrid by the monitoring of theformation of its volatile byproducts.

Typical DSC thermograms of PEOSi/SiO2 3/7before and after microwave irradiation and for dif-ferent times of irradiation are reported in Figure 1.Similar curves were recorded also for PEOSi/SiO2

7/3 and for the other samples postcured by conven-tional oven heating. The endothermic peak fromabout 30 to 1508C can be correlated to the vaporizingof volatile products (H2O and EtOH) that formedduring the first heating scan according to the afore-mentioned reaction scheme. This endothermic peakcan be considered an indirect indication of the resid-ual degree of cure remaining in the hybrid after thepartial progression of the sol–gel reaction betweenPEOSi and TEOS that occurred in solution at 708Cfor 30 min; as expected, the peak decreases withincreasing irradiation time. The second heating scanevidenced in all cases the absence of any peak, indi-cating that the sol–gel process was completed duringthe first heating scan and that no other reversibletransitions (crystallization and/or melting of PEOsegments) were present.

A quantitative evaluation of the conversion degreereached at different curing times can be calculatedby integration of that endothermic peak. The corre-sponding enthalpy values are reported in Table I.

In all cases, the enthalpy peak area was signifi-cantly lower than the corresponding value beforepostcuring, indicating that both microwave and ther-mal postcuring treatments were effective in promot-ing the conversion of ��Si��OH and ��Si��OEtgroups in ��Si��O��Si�� linkages.

The highest conversion degrees (proportional tothe inverse of the enthalpy peak area) were obtainedby oven heating for both hybrid compositions. Inthis respect, microwave irradiation could be consid-ered a less efficient postcuring treatment. On theother hand, microwave irradiation resulted in anenormously faster curing method if we compare thetime necessary to obtain similar values of the en-thalpy peak area. For example, enthalpy values of130–150 J/g were recorded after 10 s in the caseof microwave irradiation and after 2400 s in the caseof oven heating for PCLSi/SiO2 3/7 samples. Inother words, microwave irradiation represents avery fast curing method for organic–inorganichybrids and seems particularly interesting for indus-trial applications when the process time is a strictlyrestrictive parameter.

Furthermore, a different trend of the enthalpypeak area as a function of the treatment time wasnoted for the two postcuring methods. In particular,samples postcured by oven heating showed a mono-tonic decrease of the enthalpy with a correspondingincrease of the degree of crosslinking. Contrary tothis behavior, enthalpy values recorded for micro-wave-irradiated hybrids showed a minimum for in-termediate treatment times (5 s for PEOSi/SiO2 7/3

Figure 1 DSC traces of PEOSi/SiO2 3/7: (n) first scanbefore postcuring, (l) first scan after 5 s of microwaveirradiation, (*) first scan after 15 s of microwave irradia-tion, and (&) second scan after 15 s of microwave irradia-tion.



and 10 s for PEOSi/SiO2 3/7). To explain this unex-pected result, the occurrence of some degradationreaction, which in turn leads to the formation of vol-atile byproducts, could be supposed. In this case, thevolatilization of byproducts during the first heatingscan of DSC analysis can cause an overestimation ofthe enthalpy value attributed to the completion ofthe sol–gel process alone. Of course, this interpreta-tion needs to be supported by a specific analysis ofthe volatile products that formed during heating, butnumerical modeling and in-site instrumental mea-surement of the sample temperature during micro-wave irradiation can be of great interest for estimat-ing the occurrence of localized overheating duringthe cure. For this, both numerical modeling andinstrumental measurements were performed. Nu-merical modeling of the microwave heating in thesingle-mode applicator was performed with the com-mercial software Concerto 4.0 (Vector Fields, UK).The software allows the calculation of the specificabsorption rate (W/kg) in a lossy medium exposedto microwaves of a given frequency, thus providingan input to thermal calculations. In the case of 600W of incident microwave power at 2.45 GHz, equiv-alent to an effective power of 350 W because of im-pedance mismatching, for a 20 mm 3 20 mm poly-meric PC substrate coated with 300 nm of PEO,assuming a residual content of 0.3% EtOH after thepartial progression of the reaction, it was possible tocalculate the temperature rise in the coating, pro-vided that all the solvent was completely evapo-rated. For this simple model, assuming the PC sub-strate to be a nondissipative medium with respect tothe more lossy coating, only heat generation in thesample was considered, conduction, convection, and

irradiation being neglected. As far as instrumentalmeasurements are concerned, temperature evolutionduring the curing treatment was monitored withoutthe samples being contacted by means of a SitelIKST14-09 optical pyrometer (Milan, Italy) precali-brated on the sample emissivity, and the heat distri-bution was recorded with an Irisys 1011 thermalcamera, which was pointed at the sample during themicrowave exposure. The pyrometer was able tomeasure only the surface temperature, which wascooled by the surrounding environment, and it pro-vided an average temperature value on a relativelylarge area of the sample (spot diameter 5 10 mm).

A comparison of the calculated and measuredtemperature profiles is reported in Figure 2, andthey show good agreement. The small differencein the temperature distribution between the modeland the real samples can be ascribed to the variationof the substrate material permittivity with tempera-ture (i.e., the material tends to couple better withmicrowaves as the temperature increases), which hasbeen omitted in the simplified model. The resultsshow that there is a pronounced temperature mis-match between those regions of the sample that arelocated in an inner position inside the applicator andthe outer regions; this can be ascribed to the nonho-mogeneous electromagnetic field distribution. Thevalues of the temperature increase obtained by thesimulation are affected by a slight underestimationdue to the imperfect impedance matching in themodel between the microwave source and the load,which in the experimental tests was partially com-pensated by the use of a movable plunger. Furtheruncertainties in this calculated temperature valuescan be ascribed to the spectrum of the real micro-wave source (magnetron), which is centered around2.45 GHz but can have a 0.01-GHz broad band.Nevertheless, both the simulation and measurementsconfirm the existence of a localized overheating ofthe sample that, if prolonged in time, can lead to its

Figure 2 (Top) Calculated and (bottom) measuredtemperature profiles after 5, 10, and 15 s of microwaveirradiation.

TABLE IDSC Analysis: Enthalpy of the PEOSi/SiO2 Samples at

Different Curing Times and Treatments

Hybrid

Type ofpostcuringtreatment

Treatmenttime (s)

Enthalpypeak

area (J/g)

PEOSi/SiO2 7/3 None — 109.6Microwave 3 80.8

5 61.910 65.915 91.0

Oven (808C) 1200 53.72400 23.43600 18.9


5 200.210 137.915 224.6

Oven (808C) 1200 303.42400 147.83600 69.0

1430 FABBRI ET AL.


partial degradation, thus explaining what wasobserved in the DSC measurements.

Scratch resistance of PC coated with the PEOSi/SiO2 hybrids

To evaluate the enhancement in the scratch resist-ance of PC slabs coated with PEOSi/SiO2 hybrids,samples were prepared as follows: hybrid solutionsin EtOH were deposited by spin coating onto 3.0-mm-thick PC slabs, and the systems were subjectedto the previously discussed postcuring treatments.Scratch tests were carried out by a scratch beingformed by a progressively increasing load (from 100to 2000 mN) for a final scratch length of 1 mm. Atypical output of the scratch test is reported in Fig-ure 3, in which the trend of the penetration depth(PD) is shown as a function of the scratch length ofuncoated PC and PC coated with PEOSi/SiO2 7/3before and after the thermal postcuring treatment(oven heating) lasting for 20, 40, or 60 min. Similartrends were also seen for the other samples. Asexpected, all the samples showed a progressiveincrease of PD due to the increasing applied load.

The slope of the PD curves was systematicallyhigher in the case of uncoated PC or PC coated withhybrids that were not subjected to any postcuringtreatment with respect to coated samples subjectedto a postcuring treatment (either microwave irradia-tion or oven heating). The results of scratch tests forall samples are reported in Table II, in which thefinal PD values (i.e., at a scratch length of 1.0 mm)of uncoated PC and PC coated with differenthybrids and subjected to different postcuring treat-ments (type and time) are reported.

According to the lower slope of the previously dis-cussed PD–distance curves, the final PD values werelower when hybrids coated onto PC were postcuredby oven heating or by microwave irradiation.

These data indicate that organic–inorganic hybridsare effective coatings for the improvement of thescratch resistance of polymeric substrates such as PC.

To obtain the desired protective effect, a postcur-ing step is necessary to improve the degree of thesol–gel reaction, thus providing a high crosslinkingdensity and hence a harder surface. Both oven heat-ing and microwave irradiation were effective post-

Figure 3 Scratch test: PD as a function of the scratch length of uncoated PC and PC coated with PEOSi/SiO2 7/3 beforeand after the postcuring treatment (oven heating).

TABLE IIScratch Test: Final PD (Scratch Length 5 1 mm) ofUncoated PC and PC Coated with Different Hybrids

Subjected to Different Postcuring Treatments(Type and Time)

Coating

Postcuring treatmentFinal

PD (nm)Type Time (s)

None — — 30.5PEOSi/SiO2 7/3 None — 31.1

Microwave 3 25.55 24.9

10 24.915 24.9

Oven (808C) 1200 27.92400 28.23600 28.5


5 23.310 23.815 23.3

Oven (808C) 1200 23.52400 23.23600 23.6



curing methods, and the scratch resistance (in termsof the final PD) was almost independent of the typeand duration of the treatment. On the other hand,the hybrid composition (organic–inorganic final ra-tio) also seems to have an effect on the scratch resist-ance; in fact, substrates coated with postcuredhybrids having a higher inorganic content (PEOSi/SiO2 3/7) showed an average value of the final PD(independently of the time of treatment) of 23.4 nmfor both oven heating and microwave irradiation,whereas postcured samples based on PEOSi/SiO2 7/3 hybrids showed average values of 25.0 (microwaveirradiation) and 28.2 nm (oven heating). For thesesamples, a harder surface was obtained throughthe microwave-irradiation curing treatment, andthis result is a further factor supporting the effective-ness of this method versus the traditional thermalone.

These considerations were further supported bythe optical micrographs of the scratches present onthe surface after the test. Some examples arereported in Figure 4, in which the higher scratch re-sistance related to coatings with the higher inorganiccontent is quite evident. From Figure 4(b), it isclearly evident that no detachment of the silica-richcoating from the substrate occurred in the regionaround the scratch profile, and this also means thatthe coating has good adhesion to the PC substrateand that the sol–gel approach leads to a hard, resist-ant, and effective surface coating.

As expected, scratch resistance tends to improvewith the inorganic phase content increasing. How-ever, the differences between coatings with markedlydifferent organic–inorganic contents were not so high,and this behavior could be attributed to a preferen-tial surface segregation of silica domains independ-ent of the bulk composition, a phenomenon alreadyobserved, reported, and discussed for this kind ofmaterial.14

Photooxidation of PC coated with thePCLSi/SiO2 hybrids

Reactions that result from the exposure of PC to UVlight under conditions representative of solar lightcan be described by a dual photochemistry,15,16

which involves direct phototransformation andinduced photooxidation. These reactions provoke adramatic yellowing of the polymer and a degradationof the mechanical and physical properties of the ma-terial, thus representing one of the outmost limitingaspects for PC applications. Direct absorption of radi-ation below 330 nm provokes indeed two successivephoto-Fries rearrangements, as shown in Scheme 2.

UV radiation above 330 nm is likely to inducegem-dimethyl side-chain photooxidation and phenylring oxidation. Both processes produce various oxi-dized species with chain scissions and crosslinking.

Figure 4 Optical micrographs of scratches performed onPC coated with (a) PEOSi/SiO2 7/3 and (b) PEOSi/SiO2

3/7 hybrids postcured by microwave irradiation (irradia-tion time 5 5 s).

Scheme 2 Direct phototransformation of PC.

1432 FABBRI ET AL.


These photochemical reactions can be evidencedby the modifications of the UV and IR spectra of theexposed samples. Yellowing can be easily quantifiedby the measurement of the increase of absorbance at400 nm (Fig. 5), which mainly reveals the contribu-tion of the photoinduced oxidation of PC, whereasthe variations at 325 nm can be used to monitor thedirect phototransformation that generates the photo-Fries products L1 and L2.

The degrading effects induced by the exposure ofpolymers to outdoors conditions can be limited bystabilizers. Stabilizers include several types of addi-tives: UV absorbers used to protect PC from the UVradiation by competitive absorption, quenchers usedto deactivate the excited states, antioxidants withchain-breaking antioxidants, hydroperoxide decom-posing antioxidants, and hindered amine stabil-izers.17 The various processes that provoke the oxi-dation of polymeric materials can be generally effi-ciently limited by the addition of combinations ofthese various stabilizers to oxidizing polymers. Com-plementary antioxidants and protection of stabilizers(e.g., UV absorbers and phenolic antioxidants) areresponsible for excellent results.

As recalled previously, the photoyellowing of PCinduced by UV light results from both photoinducedoxidation and a photo-Fries rearrangement. Inhibi-tion of the photoyellowing can be indeed obtainedby a reduction of the absorption of the UV radiationwith UV absorbers and/or by the limitation of thepropagation of the oxidation with antioxidants.However, total absorption by a UV absorber of theradiation impinging the material cannot be achievedas far as the surface of the polymer is concerned.This is especially true in the case of those polymersthat strongly absorb in the UV range, such as PC.This direct result of the Beer–Lambert law, whichshows that the layers at the surface of the samplecannot be protected by the UV absorber, is sup-

ported by many experimental results.18,19 For thisreason, an efficient stabilization of PC against photo-yellowing cannot be achieved so simply. To suppressthe surface yellowing, an elegant solution is to pre-vent the radiation from reaching the surface of thepolymer. A coating containing a UV absorber capa-ble of absorbing the incident radiation is generallydeposited onto the substrate to absorb the UV radia-tion before it impacts on the PC substrate. This coat-ing also acts as a physical barrier to oxygen, whichdecreases the rate of oxidation of the polymeric sub-strate by limiting the oxygen flux at the surface ofthe polymer.

Two different approaches can achieve this goal.Aliphatic polymeric coatings are used with UVabsorbers. According to the absorber concentrationand the thickness of the coating, complete photopro-tection of the substrate can be obtained. However,because the coating is an organic polymer, the life-time of the multilayer part systems depends on thephotostability of this coating under irradiation,which necessitates complementary stabilization ofthe coating against photooxidation.

Another approach consists of depositing a ceramiccoating such as ZnO on the surface of PC. ZnOstrongly absorbs in the UV range and constitutes aphysical barrier to oxygen. As a result of these pro-perties, ZnO coatings decrease the rate of oxidationand the rate of photoyellowing of PC, the efficiencydepending on the thickness and the structural prop-erties.20 Surface properties of the ZnO/polymer as-sembly can be improved by a hard upper layer suchas Al2O3, which confers to the polymer antiscratchproperties and also acts as an excellent barrier foroxygen.21

Ceramers represent an interesting alternativeapproach because besides their valuable mechanicalproperties, these materials can act as protectivelayers in terms of a screen effect when mixed withUV absorbers and also as a barrier to oxygen.

In this study, we investigated the effect of PCLSi/SiO2 hybrids with various compositions on the oxi-dation and yellowing of PC induced by exposure toUV light in the presence of air. PCL was chosenrather than PEO because PEO is known to be verysensitive to UV-light-induced oxidation.13

Accelerated artificial aging was performed onuncoated PC films and PC coated with differentPCL-based hybrids under irradiation at long wave-lengths (>300 nm) at 608C in the presence of atmos-pheric oxygen. The effects of both the hybrid compo-sition and coating thickness were taken into account.The oxidative degradation of the samples inducedby UV light was followed by IR and UV–vis spec-troscopy. The kinetics of oxidation of PC were com-pared to those of coated PC through the plotting ofthe variation of the absorbance of some specific

Figure 5 UV–vis spectra of uncoated PC film duringphotooxidation.



wave numbers or wavelengths against the irradia-tion time:

� The variation of the absorbance at 3490 and 1687cm21 measured on the IR spectra.

� The evolution of the absorbance at 325 and 400nm measured on the UV–vis spectra.

Measurements were performed for all the samplesevery 20 h over 160 h of exposure, and spectra werecompared for the same sample at different times andalso for different samples; examples are shown inFigure 5 (for uncoated PC film) and Figure 6 (for PCcoated with PCLSi/SiO2 4/6 L, 600–750-nm thickness).A protective effect can be immediately deducedfrom the data plotted in Figures 7 and 8, in whichthe absorbance at 325 and 400 nm is plotted againstthe irradiation time for all samples and for uncoatedPC; the increase in the absorbance at 400 nm (relatedto yellowing) is lower for PC samples coated with

ceramers than for the uncoated PC. Moreover, Figure8 shows that the ceramers applied to samplesPCLSi/SiO2 8/2 L and PCLSi/SiO2 8/2 H have aslightly higher protective effect than PCLSi/SiO2 4/6L and PCLSi/SiO2 4/6 H. On the other hand, no sig-nificant effect of the coatings thickness was ob-served. A comparison of the increase of the absorb-ance at 325 nm on the one hand and at 400 nm onthe other hand shows that the coating used hereoffers protection as a barrier to oxygen. As expected,no important decrease of the rate of formation of L1and L2 (characterized by the increase of absorbanceat 325 nm) can be observed. The effect of the coatingon the oxygen diffusion is confirmed by the IR anal-ysis. The kinetics of oxidation of PC were comparedto those of protected PC films through the plottingof the variations of the absorbance at 3490 and 1687cm21 measured on the IR spectra after subtraction ofthe initial IR spectrum as a function of the irradia-tion time; as examples, Figures 9 and 10 are shownfor sample PCLSi/SiO2 4/6 L in the two different

Figure 6 UV–vis spectra of PC film coated with PCLSi/SiO2 4/6 L during photooxidation.

Figure 7 Evolution of the absorption band at 325 nm forPC films that were uncoated and coated with ceramers:(A) PCLSi/SiO2 4/6 L, (B) PCLSi/SiO2 4/6 H, (C) PCLSi/SiO2 8/2 L, (D) PCLSi/SiO2 8/2 H, and (PC) uncoatedfilm.

Figure 8 Evolution of the absorption band at 400 nm forPC films that were uncoated and coated with ceramers:(A) PCLSi/SiO2 4/6 L, (B) PCLSi/SiO2 4/6 H, (C) PCLSi/SiO2 8/2 L, (D) PCLSi/SiO2 8/2 H, and (PC) uncoatedfilm.

Figure 9 IR spectra of the PC film coated with PCLSi/SiO2 4/6 L during photooxidation in the hydroxyl domainof 3800–3100 cm21.

1434 FABBRI ET AL.


spectral regions. The IR spectroscopy results weresimilar for all samples coated with different hybrids.However, the comparisons of the kinetic rates of oxi-dation for the different samples reported in Figures11 and 12 show that hybrid-coated PC samples wereless degraded than uncoated PC, even though itappeared difficult to establish which kind of coatingwas the most protective. However, a long irradiationtime (>100 h) permitted discrimination of the vari-ous samples, and it seems that (see Fig. 12) samplesPCLSi/SiO2 8/2 L and PCLSi/SiO2 8/2 H mighthave given better results, confirming what alreadywas observed by UV–vis measurements.

These preliminary investigations give a firstapproach to the role played by hybrid coatingsbased on PCL against photooxidation when depos-ited onto PC films. The experimental results revealeda limited but significant improvement with respectto uncoated PC, which was due to the fact that theyconstituted a barrier only to O2 and not to the UVradiation that impinged on the PC substrate. It is

necessary to investigate in more detail the effect ofthe coating composition to maximize the protectionefficiency. The possibility of applying this kind ofprotective coating even to objects of complex shapescould draw widespread attention for several indus-trial applications.

CONCLUSIONS

PEO/silica and PCL/silica hybrids, to be appliedonto PC substrates, were easily prepared by a sol–gel process. To increase the degree of reaction andthus to crosslink the thermoset system, differentpostcuring treatments were carried out by conven-tional oven heating and microwave irradiation. Theextent of the postcuring reaction was investigated byDSC analysis, which showed that both postcuringtreatments were efficient enough to almost completethe sol–gel reaction. However, microwave irradiationled to crosslinked coatings with a process timeof some seconds (5–10 s), whereas the same levelof conversion degree was reached in about 40 minin the case of oven heating. Moreover, microwaveheating selectivity, because of the mismatch ofdielectric properties between the coating and thecandidate substrates to be used, offers interestingpossibilities of scale-up of the process to many low-loss dielectrics.

Scratch tests carried out on PC substrates coatedwith PEO/silica hybrids evidenced a significantincrease of scratch resistance with respect touncoated PC. Postcuring treatment (both microwaveirradiation and oven heating) was absolutely neces-sary to achieve a sufficiently high conversion degreeof the sol–gel reaction (crosslinking degree) and thusthe desired antiscratch effect of the coating.

Figure 10 IR spectra of the PC film coated with PCLSi/SiO2 4/6 L during photooxidation in the carbonyl domainof 1900–1500 cm21.

Figure 11 Variation of the absorbance at 3490 cm21

[D(DO)3490] during photooxidation for PC films that wereuncoated and coated with ceramers: (A) PCLSi/SiO2 4/6L, (B) PCLSi/SiO2 4/6 H, (C) PCLSi/SiO2 8/2 L, (D)PCLSi/SiO2 8/2 H, and (PC) uncoated film.

Figure 12 Variation of the absorbance at 1687 cm21

[D(DO)1687] during photooxidation for PC films that wereuncoated and coated with ceramers: (A) PCLSi/SiO2 4/6L, (B) PCLSi/SiO2 4/6 H, (C) PCLSi/SiO2 8/2 L, (D)PCLSi/SiO2 8/2 H, and (PC) uncoated film.



Photooxidation tests carried out on uncoated PCthin films and the same films coated with PCL/silicahybrids demonstrated a protective effect of thehybrid coatings against photooxidation. This effectcan be easily improved by the variation of the compo-sition and formulation of the hybrids. This will be theobject of further studies undertaken by two researchgroups that collaborate in this area. The main objec-tive of the upcoming experiments is to focus on thescreen effect of the coatings to limit the UV radiationat the surface of the polymer. A screen effect will beobtained by the development of proper formulationswith organic and mineral UV absorbers.

As a general conclusion, from an industrial pointof view, great advantages can be obtained by theapplication of PCL- or PEO-based hybrid coatingsonto PC substrates: although transparency andclarity are maintained, enhanced resistance can beobtained toward different environmental factors,such as oxidative UV irradiation, scratching, and ab-rasion. As a second point, the possibility of perform-ing fast curing processes through microwave irradia-tion represents an attractive possibility for scalingup this application.

The authors thank Tania Zanasi for her support of the ex-perimental work.

References

1. Brunelle, D. J. In Technical Information Series; Center, G. E. R.D., Ed.; General Electric: New York, 2002; p 32.

2. (a) Vanlandeghem, A.; Greger, R.; Palmers, J. WO Pat. 01/89721 (2001); (b) Yanagihara, K.; Kimura, M.; Shinkai, M. Jpn.Pat. 61200134 (1986).

3. Moustaghfir, A.; Tomasella, E.; Rivaton, A.; Mailhot, B.; Jacquet,M.; Gardette, J. L.; Cellier, J. Surf Coat Technol 2004, 180–181, 642.

4. Brinker, C.; Scherer, G. Sol–Gel Science: The Physics andChemistry of Sol–Gel Processing; Academic: Boston, 1990.

5. Kickelbick, G. Prog Polym Sci 2002, 28, 83.6. Haas, K. H.; Wolter, H. Curr Opin Solid State Mater Sci 1999,

4, 571.7. Messori, M.; Toselli, M.; Pilati, F.; Fabbri, E.; Fabbri, P.; Busoli,

S. Surf Coat Int Part B: Coat Trans 2003, 86, 181.8. Messori, M.; Toselli, M.; Pilati, F.; Fabbri, E.; Fabbri, P.; Busoli,

S.; Pasquali, L.; Nannarone, S. Polymer 2003, 44, 4463.9. Cruz, C. A.; Paul, D. R.; Barlow, J. W. J Appl Polym Sci 1979,

23, 589.10. Jonza, J. M.; Porter, R. S. Macromolecules 1986, 19, 1946.11. Chaoxu, L.; Qingshan, K.; Qingrong, F.; Yanzhi, X. Mater Lett

2005, 59, 773.12. Balsamo, V.; Calzadilla, N.; Mora, G. A.; Muller, J. J Polym Sci

Part B: Polym Phys 2001, 39, 771.13. Morlat, S.; Gardette, J. L. Polymer 2001, 42, 6071.14. Messori, M.; Toselli, M.; Pilati, F.; Fabbri, E.; Fabbri, P.; Pas-

quali, L.; Nannarone, S. Polymer 2004, 45, 805.15. Scheirs, J.; Gardette, J. L. Polym Degrad Stab 1997, 56, 339.16. Rivaton, A.; Mailhot, B.; Soulestin, J.; Varghese, H.; Gardette, J.

L. Polym Degrad Stab 2002, 75, 17.17. Pospisil, J.; Nespurek, S. In Handbook of Polymer Degrada-

tion, 2nd ed.; Halim Hamid, S., Ed.; Marcel Dekker: NewYork, Chapter 6, p 191.

18. Mailhot, B.; Rivaton, A.; Gardette, J. L.; Moustaghfir, A.; Toma-sella, E.; Jacquet, M.; Ma, X.-G.; Komvopoulos, K. e-Polymer[online] 68, 2005.

19. Claude, B.; Gonon, L.; Verney, V.; Gardette, J. L. Polym Test2001, 20, 771.

20. Moustaghfir, A.; Rivaton, A.; Tomasella, E.; Mailhot, B.; Cellier,J.; Jacquet, M.; Gardette, J. L. J Appl Polym Sci 2005, 95, 380.

21. Moustaghfir, A.; Tomasella, E.; Jacquet, M.; Rivaton, A.; Mail-hot, B.; Gardette, J. L.; Beche, E. Thin Solid films 2006, 515, 662.

1436 FABBRI ET AL.


Improvement of the surface properties of polycarbonate by organic–inorganic hybrid coatings

Documents

Transcript of Improvement of the surface properties of polycarbonate by organic–inorganic hybrid coatings