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Progress in Organic Coatings 45 (2002) 3342

The change of the properties of acrylic-based polyurethanevia addition of nano-silica

Shuxue Zhou a, Limin Wu a,, Jian Sun a,b, Weidian Shen ba Department of Materials Science, The Advanced Coating Research Center of China Educational Ministry,

Fudan University, Shanghai 200433, PR Chinab Department of Physics and Astronomy, Eastern Michigan University, Ypsilanti, MI 48197, USA

Accepted 24 May 2002

Abstract

Acrylic-based polyurethane coatings with nano-silica particles embedded were prepared. The compositions at the surfaces and at theinterfaces with substrates were investigated by X-ray photoelectron spectrometer (XPS). The mechanical and optical properties of thecoatings were studied intensively by using pendulum hardness tester, Nano-Indenter XP, Instron testing machine, dynamic mechanicalanalyzer (DMA), transmission electron micrograph (TEM), and UVVis spectrophotometer. The coatings with fumed silica and micro-silicaembedded were also investigated for comparison with the coatings containing nano-silica. The results showed that silica element neitherexisted at the surfaces nor existed at the interfaces of the coatings with nano-silica or micro-silica embedded, and the silicon atoms intend toreside inside the coatings. The macro-hardness, micro-hardness, abrasion resistance, and scratch resistance were apparently improved viaaddition of nano-silica. The tensile strength and Youngs modulus were also enhanced with the increasing content of nano-silica. However,the elongation at break decreased as nano-silica content increased. The UV absorbance in the wavelength of 290400 nm increased asnano-SiO2 content increased. In contrast, for the polyurethane coatings with fumed silica or micro-silica embedded, only hardness andabrasion resistance showed some increase. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Acrylic-based polyurethane; Nanocomposite coating; Property

1. Introduction

In the past decade, material scientists showed greatinterest in organicinorganic nanocomposites since theirapplication has dramatically improved material propertiesin engineering plastics, enhanced rubber, coatings, and ad-hesives [13]. The attractive improvement includes heatresistance, radiation resistance, mechanical and electricalproperties, which are usually resulted from the synergisticeffect between organic and inorganic components. Effectsof different nanoparticles on the properties of polymersvary a lot. To achieve the expected improvement by addingnanocomposites, understanding how these nanoparticlesinfluence the organic matrix is important.

Organicinorganic nanocomposites can be prepared by di-rectly blending with nanoparticles and organic compoundsor a solgel process with a metal alkoxide. The most com-monly used inorganic nanoparticles are SiO2, TiO2, ZnO,

Corresponding author.E-mail address: [email protected] (L. Wu).

CaCO3, etc., of them, nano-silica is the first nanoparticleproduced and has been studied in a lot of polymer systems.For example, Kaddami et al. [4] and Hajji et al. [5] hadcombined it with poly(HEMA), and Chang et al. [6] filled itinto poly(methyl methacrylate). Nano-silica could also im-prove scratch resistance of a coating and keep the coatingclear at the same time [7]. Petrovic et al. [8] found thatnanoparticle could enhance tensile strength and elongationof polyurethane elastomer.

In this project, nano-silica was embedded in theacrylic-based polyurethane, composition of the coatings atthe surface and at the interface, hardness, abrasion resis-tance, static and dynamic mechanical properties, scratchresistance, and optical properties of the coatings were in-tensively investigated by X-ray photoelectron spectrometer(XPS), pendulum hardness tester, Nano-Indenter XP, Instrontesting machine, dynamic mechanical analyzer (DMA),transmission electron micrograph (TEM), and UVVisspectrophotometer. For the sake of comparison, the effectsof fumed silica and micro-silica on polyurethane propertieswere also studied.

0300-9440/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0 3 0 0 -9440 (02 )00085 -1

34 S. Zhou et al. / Progress in Organic Coatings 45 (2002) 3342

2. Experimental

2.1. Materials

Nano-SiO2 (P640) with mean size of 20 nm and specificarea of 640 m2 g1 was purchased from Shanghai BONAWEILAI New Material Co., Ltd. of China. Micro-silica wasthe product of Huzhou HUANENG Fine Silica Powder Co.,Ltd. of China and fumed silica (Aerosil R972) came fromDegussa Company of Germany.

Monomers butyl acrylate (BA, 96%), styrene (St, 97%),methyl methacrylate (MMA, 97%) and 2-hydroxyethylmethacrylate (HEMA, 98%) were purchased from Shang-hai Gaoqiao Petrochemical Company and used as supplied.Butyl acetate (98%) and 2-mercaptoethanol were purchasedfrom Shanghai Chemical Reagent Company, and act as thesolvent and chain transfer agent, respectively.

Catalyst dibutyltin dilaureate (98%) and cross-linkingagents: 1,6-hexamethylene diisocyanate homopolymer(HDI, Desmodur N 3300) were obtained from Bayer Com-pany and used as received. t-Butyl peroxy 2-ethyl hexanoate(TBPH) was supplied by Aztec Chemical Company (USA)and used as received. Coupling agent: methacryloylpropy-ltrimethoxysilane (KH570) was the product of NanjingSHUGUANG Chemical Factory of China.

2.2. Synthesis of acrylic polyol resins

A 500 ml round-bottom flask equipped with mechani-cal stirrer, thermometer with a temperature controller, N2inlet and a Graham condenser was charged with half ofthe total amount of butyl acetate to be used and heated to105 C. A solution of mixture of MMA, St, BA, HEMA,2-mercaptoethanol, TBPH and butyl acetate with/withoutnano-silica was added over some period of time under a slowstream of N2. During the process of addition of monomermixture, the temperature was maintained at 105 22 C.When addition was complete, another 10 wt.% TBPH of theinitial used TBPH weight was added, and refluxing was con-tinued for another 1 h. The resin solution is 70 wt.%.

2.3. Preparation of nanocomposite films

Two methods were adopted for preparation of nanocom-posite coatings. One was that modified or unmodifiednano-SiO2 particles was directly mixed with acrylic resinsat 60 C for about an hour under vigorous stirring, anotheris using in situ polymerization in which nano-silica wasfirstly dispersed in monomer mixture by ultrasonic irradi-ation for half an hour then polymerized according to themethod described in the preceding part. The nanocompositeacrylic resin without any further dilution was mixed withHDI based on 1/1 weight ratio of resin to HDI at roomtemperature. Just before application, dibutyltin dilaurate(0.05 wt.% of the total weight of the resin and polyiso-

cyanate on total solids) was mixed thoroughly into thecoating. Polyurethane coats with different thickness wereprepared by casting the above solution on Sn-coated fer-rous panels using a drawdown rod and dried at 120 C for30 min or on glass substrates dried at room temperature.

2.4. XPS analysis

A Microlab 310F (VG SCIENTIFIC, UK) multifunctionalXPS was used to determine the composition of typical ele-ments at the surface and interface of the films with Sn-coatedferrous panels. K radiation of Al was used as the excitationsource with a pass energy of 40 eV.

2.5. Macro-hardness

Macro-hardness was determined using pendulum hard-ness tester according to national standard of ChinaGB/T1730-93. The coating films were prepared on glassboards and dried at room temperature. The time swingingfrom 5 to 2 for the pendulum on the glass with and with-out films were named as t and t0, respectively, the ratio oft/t0 is regarded as macro-hardness.

2.6. Abrasion resistance

Abrasion resistance was determined on a round glassboard according to GB1768-79. 120# rubber abrasive wheelwas used. The film was first rubbed flat for about 100 cyclesthen recorded the initial weight. For every 200 cycles rub-bing, the abrasive wheel was renewed and the weight losswas recorded, which can be used to judge the abrasion re-sistance.

2.7. Static mechanical property

Tensile properties were acquired by an Instron modelDXLL 100020 000 testing machine (Shanghai, China). Thespecimens for tensile test were dumbbell (according to DieC of ASTM-D412) cut from the films that were prepared onglass substrates and had a thickness of about 7090m, andcarried out at a crosshead speed of 200 mm/min. A 20 mmbenchmark and the original cross-sectional area were uti-lized to calculate their tensile properties. The ultimate ten-sile strength and elongation were automatically calculatedby the computer connected to Instron. The average of atleast five measurements for each sample was reported, theexperimental error is 10%.

2.8. Micro-hardness and scratch test

Micro-hardness and scratch test were carried out byNano-Indenter XP made by MTS, Inc. in USA. Thecoatings were deposited on Sn-coated ferrous panels. ABerkovich diamond tip (3-faced pyramid) was used in the

S. Zhou et al. / Progress in Organic Coatings 45 (2002) 3342 35

tests. Micro-hardness and scratch resistance were mea-sured under a normal load ranging from 200N to 50 mN.The micro-hardness was calculated by dividing the nor-mal force used in the indentation by the projected area ofindent at the surface. The scratch resistance was definedas the normal force applied during the scratching dividedby the cross-section of the trough after scratching. TheYoungs modulus was calculated from the slope of theload (P)penetration (h) curves at the turning point of theunloading. The details of the experiment methods and cal-culations of hardness and Youngs modulus measurementshave been described in the Ref. [9].

2.9. Dynamic mechanical property

Dynamic mechanical properties of polyurethane filmswere measured using DMA 242 (Netzsch, German) startingfrom 150 and 200 C with heating rate of 5 C/min at thefrequency of 10 Hz.

2.10. TEM observe

Transmission electron micrographs of the nanocompositefilms were obtained by a Hitachi H-600 apparatus (HitachiCorporation, Japan). Samples were prepared by ultramicro-tomy at room temperature, giving sections of nearly 100 nmin thickness. No further staining was used to improve con-trast.

2.11. UVVis spectra

An UVVis spectrophotometer (Hitachi UV-3000) wasused to measure the absorbance and transmittance spectraof the films in the range 200700 nm wavelength light.

3. Results and discussion

3.1. Composition of the surface and interface

The composition of the surface and interface ofpolyurethane films containing 5 wt.% nano-silica or micro-silica were determined by XPS. The results are listed inTable 1. There is no Si element detected at the surfaces and

Table 1The composition of the surface and interface of polyurethane coats con-taining nano- or micro-silica (at.%)Element Surface Interface

Nano-silica Micro-silica Nano-silica Micro-silica

C 69.1 70.9 56.3 46.1O 30.9 29.1 40.1 38.7N 0 0 3.6 15.2Si 0 0 0 0

interfaces of the films, no matter nano-silica or micro-silicaparticles are contained in the films, suggesting that silica arenot like organic silanizing compounds in which Si segmentsprefer to orient at the surface, silica like to immigrate intothe bulk. The interfaces contain N element while the sur-faces have no N element, indicating that urethane segmentsintend to orientate at interfaces while acrylic segmentslike to cover the surface since the former have higher freesurface energy than the latter [10]. The N content at the in-terface of the film containing nano-silica is lower than thatof micro-silica, this is possibly because nano-silica haveconsiderably greater specific area than micro-silica, and theOH groups on the surfaces of nano-silica can react withNCO groups from HDI, resulting in more NCO groupsabsorbed on the surfaces of nano-silica, which like to hideinto the bulk.

3.2. Hardness

3.2.1. Macro-hardnessA series of polyurethane films containing silica were pre-

pared and the pendulum hardness were determined as shownin Table 2. Sample nos. 17 were prepared by an acrylicresin A with HDI, sample nos. 822 were prepared fromanother acrylic resin B with HDI. Sample nos. 2 and 3were obtained by in situ polymerization using unmodifiednano-silica and modified nano-silica with silane couplingagent, respectively, other samples were obtained by directlymixing acrylic resin with silica then cured by HDI. The datain Table 2 display that addition of nano-silica can increasethe macro-hardness of the films no matter which preparationmethod was used (see sample nos. 15). The data from twoacrylic resins (see sample nos. 1, 57 and 1215), differ-ent thickness of film (see sample nos. 811 and 1215), anddifferent drying time, show that macro-hardness increaseswith the content of nano-silica. For sake of comparison, theeffects of fumed silica and micro-silica on macro-hardnesswere also investigated. Table 2 tells us that all silica stud-ied here can enhance the hardness of films (see sample nos.10, 17, 22), but excessive micro-silica led to decrease inmacro-hardness.

3.2.2. Micro-hardnessMicro-hardness of the coatings with different concentra-

tion of nano-silica under different normal load is shown inFig. 1. The data points used in the plot is the average valueof three individual measurements performed at differentspots on the surface. The large fluctuation in the plot maybe attributable to the small size of measurement, m2,roughness of the surface, and inhomogeneity of the coat-ing. However, the tendency is clear that the micro-hardnessincreases with the increasing concentration of nano-silica,which is consistent with macro-hardness measurement.Fig. 2 shows the change of the coatings with different typesof silica embedded in their micro-hardness. Overall, it canbe seen that the order of micro-hardness from large to


Table 2Macro-hardness of polyurethane films containing silica prepared at different conditions

Sample no. Componenta resin solution/silica/HDI Drying time (day) Thickness (m) Macro-hardness Type of silica1 100/0/100 1 79 0.57 Nano-silica2b 99/1/100 1 74 0.603b 99/1/100c 1 78 0.624 99/1/100c 1 71 0.615 99/1/100 1 74 0.616 95/5/100 1 70 0.757 90/10/100 1 73 0.77

8 100/0/100 19 51 0.53 Nano-silica9 99/1/100 19 58 0.59

10 95/5/100 19 47 0.6211 90/10/100 19 50 0.69

12 100/0/100 5 88 0.28 Nano-silica13 99/1/100 5 75 0.4014 95/5/100 5 81 0.5415 90/10/100 5 78 0.75

16 99/1/100 7 54 0.49 Micro-silica17 95/5/100 7 52 0.5818 90/10/100 7 53 0.49

19 99/1/100 7 68 0.45 Micro-silica20 95/5/100 7 62 0.5821 90/10/100 7 76 0.46

22 95/5/100 7 55 0.63 Fumed silicaa Sample nos. 17 were obtained from acrylic resin A with HDI and sample nos. 822 from acrylic resin B with different monomer composition

from A, resin solid content for A and B are 70 wt.%.b Nano-silica is combined with acrylic resin by in situ polymerization method.c Nano-silica is treated with KH570 coupling agent.

small is nano-silica, fumed silica, and micro-silica. Fig. 3further shows the effect of concentration of micro-silicaon the micro-hardness of the coatings. In contrast to thenano-silica case, the micro-hardness of the coatings withdifferent concentration of micro-silica are about the sameunder the normal force less than 10 mN. Therefore, theaddition of nano-silica to coatings can more efficiently en-hance the hardness since they have considerably greaterspecific surface area than micro-silica.

Fig. 1. The change in micro-hardness of the films with different nano-silicacontent with peak load.

3.3. Abrasion resistance

The weight loss of the polyurethane films with differentnano-silica content at different abrasion cycle is shown inFig. 4. The weight loss gradually decreases as nano-silicacontent increases, indicating that nano-silica can improve theabrasion resistance of the coating film. Figs. 5 and 6 manifestthe effect of the types of silica and micro-silica content onthe weight loss of film, respectively. It was seen from Fig. 5

Fig. 2. The change in micro-hardness of the films with different types ofsilica and peak load.


Fig. 3. The change in micro-hardness of the films with differentmicro-silica content and peak load.

Fig. 4. The weight loss of the polyurethane films containing differentnano-silica content.

Fig. 5. The weight loss of the polyurethane films containing differenttypes of silica.

Fig. 6. The weight loss of the polyurethane films containing differentmicro-silica content.

that the abrasion resistance of the films containing differenttypes of silica are nearly the same except for the fumedsilica. Fig. 6 shows that the weight loss does not change ifonly a small amount of micro-silica (e.g. 1 wt.%) is added,but the abrasion resistance increases as micro-silica contentincreases.

3.4. Static mechanical property

The static mechanical properties of polyurethane filmscontaining different nano-silica or micro-silica contents de-termined on Instron testing machine, the effect of silica con-tent on strength and elongation are plotted in Fig. 7. The filmstrength considerably increases and elongation at break ob-viously decreases with increasing nano-silica content. Thisis likely due to the reaction of isocyanate with hydroxylgroups on the surfaces of nano-silica, which leads to a highercross-linking degree of the film. On the other hand, the smallspecific area of micro-silica particles cannot change the ten-sile property of the film greatly.

3.5. Youngs modulus

Fig. 8 shows the Youngs modulus of the polyurethanecoatings with different concentration of nano-silica, whichwas measured by Nano-indenter XP under different nor-mal load. Again, the large fluctuation may be due to thesmall size of measurement, m2, roughness of the sur-face, and inhomogeneity of the coating, as mentioned above.The tendency that the modulus increases with the increasingnano-silica concentration is clear. Fig. 9 is the Youngs mod-ulus of the coatings containing different types of silica un-der different normal load. The Youngs modulus of coatingwith nano-silica and the Youngs modulus of coating withfumed silica are about the same, while the Youngs modulusof coating with micro-silica is distinguishably smaller thanthem. Again, this is probably because some reaction of iso-cyanate with hydroxyl groups on the surfaces of nano-silica


Fig. 7. Tensile properties of polyurethane films with different silica content.

Fig. 8. The Youngs modulus of polyurethane films containing differentnano-silica content.

Fig. 9. The Youngs modulus of polyurethane films containing differenttypes of silica.

Fig. 10. The Youngs modulus of polyurethane films containing differentmicro-silica content.

had happened, resulting in a higher cross-linking degreefor nano-silica embedded film than for micro-silica con-tained film. Another reason is there should be greater inter-action strength between nano-silica and organic matrix thanmicro-silica and organic matrix since the former has greaterspecific surface area than the latter. Fumed silica has to someextent similar particle characteristic to nano-silica. As con-centration of micro-silica increases to 10 wt.%, the Youngsmodulus of the coating showed an apparent increase, espe-cially under the high normal loads, as indicated in Fig. 10.

3.6. Dynamic mechanical property

Storage modulus and loss tan as functions of tempera-ture for the polyurethane films without or with 5 wt.% sil-ica are presented in Figs. 11 and 12, respectively. Bothnano-silica and micro-silica can enhance storage modulusof the polyurethane film, but nano-silica seems to be more


Fig. 11. The storage modulus of polyurethane films with or without silicadetermined by DMA.

efficient for increasing modulus than micro-silica since theformer has larger surface interaction with polyurethane ma-trix than the latter. Fig. 12 shows that glass transition tem-perature (Tg) and dissipation peak decrease after applicationof silica, and the larger the particle size of silica is, the lowerthe Tg is. This is probably because Tg is related to free vol-ume in composites, addition of silica can increase the freevolume in composites, and the composite with larger parti-cles has higher free volume fraction, which results in lowerTg. The same phenomenon in nano-silica/PMMA systemwas also found by Ou et al. [11].

3.7. Scratch resistance

The scratch resistance of polyurethane coatings with andwithout nano-silica particles embedded under different nor-mal load was measured by Nano-indenter XP and plotted inFig. 13. The addition of small amount of nano-silica (e.g.1 wt.%) to the coating can dramatically improve its scratchresistance. Increasing the concentration of nano-silica canimprove the scratch resistance furthermore, as indicated in

Fig. 12. The dissipation peak of polyurethane films with or without silicaby DMA.

Fig. 13. Comparison of scratch resistance of acrylic-based polyurethanefilm before and after nano-silica filled.

Fig. 14. The scratch resistance of coatings containing differ-ent types of silica vs. the normal load and the scratch resis-tance of coatings with different concentration of micro-silicavs. the normal load are shown in Figs. 15 and 16, respec-tively. The polyurethane film containing nano-silica showsthe best scratch resistance, while the coating containingmicro-silica performs the worst, as shown in Fig. 15. Thecoating with a 10 wt.% micro-silica shows a superior scratchresistance over the coatings with a 1 wt.% and a 5 wt.%micro-silica under low normal loads. As the normal load in-creases beyond 1 mN, different concentration makes no dif-ference in the scratch resistance, as indicated in Fig. 16.

3.8. Optical property

3.8.1. AppearanceThe appearance of acrylic resin is waterwhite. However,

after nano-silica was added, there are some changes inappearance, as indicated in Table 3. The resin with lownano-silica concentration is still clear but becomes opaque

Fig. 14. Variation of the scratch resistance of coating films from resin Awith nano-silica concentration.


Fig. 15. The scratch resistance of coating films from resin B containingdifferent kinds of silica.

Fig. 16. Dependence of the scratch resistance of coating films from resinB on micro-silica concentration.

when the concentration increases. Theoretically, the size ofnanoparticle is small than the wavelength of visible light,no scattering and reflecting occurs in the visible light range,so the nanocomposite should be transparent. But nanopar-ticles tend to aggregate due to its high surface energy. The

Table 3Appearance of acrylic resins and polyurethane films

No. Silica Appearance

Types Concentration(wt.%)

Acrylic resinsolution

Films

1 No silica Transparent Transparent

2 Nano-silica 1 Transparent Transparent3 5 Slight cloudy Semi-transparent4 10 Slight cloudy Semi-transparent

5 Micro-silica 1 Cloudy Transparent6 5 White Transparent7 10 White Transparent

8 Fumed silica 5 Transparent Transparent

Fig. 17. TEM micrograph of nano-SiO2/acrylic-based polyurethane film.

nano-silica particles used here are hydrophilic and have alot of OH groups at the surface. Thus, acrylic polyol hasbetter miscibility with nano-silica, but the solvent, butylacetate, is not miscible with nano-silica, causing acrylicresin solution and the corresponding film opaque. Fig. 17shows the TEM photograph of nano-silica dispersed in thepolyurethane film. It was seen that part of the nanoparticleshave reached nanometer scale, although some aggregatesare observed. When nano-silica content increases, the con-tent of aggregates larger than nanometer size also increases.For the fumed silica, both the acrylic resin solution and itsfilm are transparent. The appearance of the acrylic resinsolution containing micro-silica is completely opaque dueto its larger particle size, but the corresponding film be-come clear, which is possibly because the refractive indexof micro-silica is close to that of the polyurethane matrix.

3.8.2. UVVis spectraThe UVVis absorbance and transmittance spectra of the

films containing nano-silica are seen in Figs. 18 and 19, re-spectively. Fig. 18 indicates that the absorbance in the range290400 nm wavelength increases as nano-silica content in-creases. Because of absorbance and reflection, the transmit-tance of light reduces especially in the UV range. The re-duction of transmittance in the visible light range indicatesthat the film become opaque, which is consistent with thechange in appearance of the film observed. Comparison oftransmittance of the film containing nano-silica with othersilica is shown in Fig. 20. There is very small absorbance forfumed silica and no absorbance at all for micro-silica, even


Fig. 18. Absorbance spectra of polyurethane film containing differentnano-silica content.

Fig. 19. Transmittance spectra of polyurethane film containing differentnano-silica content.

Fig. 20. Transmittance spectra of polyurethane film containing differenttypes of silica.

Fig. 21. Absorbance spectra of polyurethane film containing differentmicro-silica content.

Fig. 22. Transmittance spectra of polyurethane film containing differentmicro-silica content.

at higher micro-silica concentration, just as seen in Figs. 21and 22. This means that nano-silica can more efficiently en-hance exterior durability of coatings or polymer films thanother silica.

4. Conclusions

Effect of nano-silica on the surface and interface, mechan-ical and optical properties of acrylic-based polyurethanefilms was investigated. Silicon element was not detectedat surface and interface of polymer film. Addition ofnano-silica can improve the hardness, abrasion resistance,scratch resistance, tensile strength, modulus and weather-ability of the polymer film, while fumed silica or micro-silicacan only increase the hardness and abrasion resistance.Moreover, dependences of the properties of the films onsilica content were completely different from various typesof silica.


Acknowledgements

We would thank Shanghai Nano-Special Foundation, KeyProject of China Educational Ministry, Doctoral Foundationof China Educational Ministry, Shanghai Shuguang Foun-dation and National Science Foundation of China for thefinancial support for this research.

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The change of the properties of acrylic-based polyurethane via addition of nano-silicaIntroductionExperimentalMaterialsSynthesis of acrylic polyol resinsPreparation of nanocomposite filmsXPS analysisMacro-hardnessAbrasion resistanceStatic mechanical propertyMicro-hardness and scratch testDynamic mechanical propertyTEM observeUV-Vis spectra

Results and discussionComposition of the surface and interfaceHardnessMacro-hardnessMicro-hardness

Abrasion resistanceStatic mechanical propertyYoung's modulusDynamic mechanical propertyScratch resistanceOptical propertyAppearanceUV-Vis spectra

ConclusionsAcknowledgementsReferences

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