Synthesis of UV-curable organic–inorganic hybrid urethane acrylates and properties of cured films

Thin Solid Films 514 (2006) 69 – 75 www.elsevier.com/locate/tsf

Synthesis of UV-curable organic–inorganic hybrid urethane acrylates and properties of cured films Jianwen Xu a , Wenmin Pang b , Wenfang Shi a,⁎ a

State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 8 June 2005; received in revised form 13 December 2005; accepted 14 February 2006 Available online 6 March 2006

Abstract An organic–inorganic hybrid urethane acrylate hydrolytic condensate (HUA-HC) was prepared through sol–gel method with an acid catalyst of low concentration from a hybrid urethane acrylate prepolymer (HUA), which is a disfunctional silane containing hydrolysable ethoxysilane groups (Si–OEt). The synthesis was monitored by Fourier-transformed infrared spectroscopy, 1H nuclear magnetic resonance (NMR), 13C NMR and 29Si NMR. The photopolymerization kinetics studied by photo differential scanning calorimetry showed that HUA-HC had a faster apparent photopolymerizaton rate and a higher conversion of double bonds than HUA under 72 °C due to the vicinity of double bonds in HUA-HC. However, HUA-HC showed a slower apparent photopolymerizaton rate and a lower conversion of double bonds than HUA at higher temperatures above 72 °C, which indirectly indicates that more extensive condensation between ethoxysilane (Si–OEt) and silanol (Si–OH) groups in HUA-HC than that in HUA. Both 100-μm-thick films of HUA-HC and HUA cured by ultraviolet (UV) light showed high pencil hardness (> 4 H) and excellent abrasion resistance (< 40 mg after 600 cycles). 2-mm-thick specimen of HUA-HC and HUA cured by UV light showed high tensile strength (> 30 MPa). The cured HUA-HC film showed better performance than the cured HUA film in the investigated aspects due to more inorganic Si–O–Si linkages in HUA-HC, which is also the reason that HUA-HC film had higher decomposition temperatures and more residue than HUA in thermal gravity analysis. © 2006 Elsevier B.V. All rights reserved. Keywords: Coatings; Silane; Organic–inorganic; Nuclear magnetic; Resonance; Hardness; Urethane acrylates; Thermo gravimetry

1. Introduction Thermoplastics have lower scratch and mar resistance compared with metals, ceramics, or thermosets. The physical impacts on polymers usually lead to a loss in their transparency, and thus to a reduction in the performance of materials. To avoid this deterioration, hard coatings have been proven to be an efficient and economical method to protect the underlying polymers. Different from conventional polymeric composite systems where an inorganic component is mixed with a polymeric component at micrometer level, organic–inorganic hybrid coatings, in which inorganic components and organic components interact at a nanoscale level, have attracted great interest in the past years due to the resulted synergic properties [1–4]. ⁎ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.032

Sol–gel technology was widely employed for preparing cross-linked inorganic glasslike matrixes composed of Si–O–Si linkages. The sol–gel process is, in fact, a rather complex process involving two basic reactions, first hydrolysis of alkoxysilane to give a silanol, followed by the condensation of a silanol with alkoxysilane or with another silanol to give siloxane linkages. Trialkoxysilane bearing an organic functional group is often used to yield organic–inorganic hybrid materials [5]. Recently, Crivello et al. [6] developed a kind of sol–gel technique involving the use of a weakly acidic ion-exchange resin as the catalyst for the controlled preparation of a series of fluid epoxy and 1-propenyl ether functional siloxane monomers and oligomers. These hybrid resins were shelf-stable for over 2 months, and no increase in the viscosity or gelling was observed on standing. They also showed high photopolymerization response and resulted in hard, transparent and insoluble thin films. Soppera et al. [7] studied the photopolymerization kinetics

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of the sol of methacryloxypropyl trimethoxysilane (MAPTMS) through real-time Fourier-transformed infrared spectroscopy (FTIR), and demonstrated that the condensation state of the formed silicate network was of crucial importance for the photopolymerization kinetics, and a higher degree of condensation resulted in more efficient photopolymerization of acrylic double bonds. Up to now, UV-curable organic–inorganic hybrid coatings were usually prepared from MAPTMS, tetraethyl orthosilicate and commercial organic oligomers, etc. The cured hybrid coating showed excellent abrasion resistance for protecting the underlying soft substrates [2,3,8]. However, the processing complexity and instability of these mixtures limited their further application. Urethane acrylate oligomers have many industrial applications, ranging from adhesives, UV-curable coatings to bullet proof glass. The microphase separation in urethane segment governed by the length and type of soft segment is a key factor to achieve their excellent mechanical properties [9]. In this work, we synthesized a kind of UV-curable hybrid urethane acrylate prepolymer (HUA) containing Si–OEt, which is then hydrolyzed to form the hybrid urethane acrylate hydrolytic condensate (HUA-HC). Their photopolymerization kinetics under different temperatures was studied by photo differential scanning calorimetry (photo-DSC). The abrasion resistance, the tensile strength and the thermal properties of their UV-cured films were also investigated. 2. Experimental details 2.1. Materials 2-Hydroxyethyl acrylate (HEA) was obtained as a gift from Eternal Chemical Co. Taiwan, China, and was distilled under reduced pressure before use. 3-Aminopropyl triethoxysilane (KH550) was purchased from Nanjing Yudeheng Co., China. 2Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173) supplied by Ciba-Geigy, Switzerland. Toluene-2,4-diisocyanate (TDI) and other reagents employed in this work were analytic quality products from the First Reagent Co. Shanghai, China, and used without further purification. 2.2. Measurements The Fourier-transformed infrared (FTIR) spectra were recorded with a Nicolet MAGNA-IR 750 spectrometer. The nuclear magnetic resonance (NMR) spectrum was performed on an AVANCE AV 400 instrument (Brucker Co., Switzerland) using (CD3)2CO as a solvent and TMS as a reference. The 29Si NMR spectrum was recorded at 49.7MHz with a 30° pulse width, a 15s recycle delay and 4800 scans using an inverse gated sequence. The photopolymerization rate was monitored using a modified CDR-1 differential scanning calorimeter (DSC) (made by Shanghai Balance Instrument Co., China). About 5mg homogeneous formulation was weighed into the aluminum pan, and kept at the required temperature for 5min before the exposure to a UV spotcure system BHG-250 (Mejiro Precision Co. Japan). The incident light intensity at the sample pan was 0.9mW cm− 2.

The pencil hardness was measured by a pencil hardness tester QMY (Tianjin Material Testers Co., China) according to the State Standard Testing Method (GB/T6739-1996 [10], equivalent of American Society for Testing and Materials (ASTM) D3363). The abrasion resistance was measured with a QMX abrasion apparatus (Tianjin Material Testers Co., China) using a wheel under 500g load in accordance with the State Standard Testing Method (GB 4893.8-1985 [11], equivalent of ASTM 4060). The stress–strain curves were measured with a MTS 809 Axial/Torsional Servohydraulic System (MTS systems Co. USA) at room temperature. The dumb-bell shaped specimens were prepared in aluminum molds with the size according to ASTM D412 [12] and irradiated under 1kW UV lamp (80 W cm− 1, Lantian Co. China) for 5 min. The crosshead speed was 60mm min− 1. A digital extensometer (Model 632.26E-20, MTS Systems Co.) was used to directly measure the strain within the specimen gage length for accurate measurement of the strain. Thermal gravimetric analysis (TGA) was carried out on a Shimadzu TG-50 instrument using a heating rate of 10 °C min− 1 in air atmosphere. 2.3. Synthesis 2.3.1. Hybrid diol HEA (58.06g, 0.50mol) was introduced into a 250-ml threeneck flask equipped with a mechanical stirrer, a drop funnel and a nitrogen inlet. KH550 (55.34g, 0.25mol) was then dropped slowly into the flask below 30°C. After stirred vigorously for 20h, a viscous transparent fluid was obtained, named hybrid diol (HD). 1 H NMR ((CD3)2CO) (ppm): 0.55 (2H, Si–CH2), 1.18 (9H, CH3–CH2–O), 1.53 (2H, Si–CH2–CH2), 2.42 (6H, N–CH2), 2.74 (2H, CH2–COO), 3.66 (4H, CH2–CH2–OH), 3.77 (6H, O–CH2–CH3), 4.09 (4H, COO–CH2). 13 C NMR ((CD3)2CO) (ppm): 8.29 (Si–CH2), 18.62 (CH3– CH2–O), 21.00 (Si–CH2–CH2–), 33.11 (CH2–COO), 49.98 (N–CH2–CH2–COO), 57.12 (N–CH2–CH2–CH2–Si ), 58.71 (CH3–CH2–O), 60.47 (CH2–OH), 66.47 (COO–CH2), 173.15 (–COO–CH2). 2.3.2. Hybrid urethane acrylate TDI (34.83g, 0.20mol) was introduced into a 250-ml three-neck flask equipped with a mechanical stirrer, a drop funnel and a nitrogen inlet. HD (45.36g, 0.10mol) was then dropped into the flask at 50°C. The system was raised to 70°C after finishing HD addition, and was stirred for 6h. Finally, HEA (23.23g, 0.20mol) was dropped into the flask with the above reactants added after cooling to 50°C, and then stirred for 3h at 70°C, yielding a slightly yellow viscous liquid, named hybrid urethane acrylate (HUA). 2.3.3. Hydrolysis and condensation of HUA HUA (10.34 g) was dissolved in equal amounts of isopropyl alcohol containing 0.1 M HCOOH (0.18 g) at 70 °C, and refluxed for 8h. Then the volatiles were removed from the system under reduced pressure, yielding a transparent viscous liquid, named hybrid urethane acrylate hydrolytic condensate (HUA-HC).

J. Xu et al. / Thin Solid Films 514 (2006) 69–75

71

HOCH2CH2OOCCH2CH2

NH2CH2CH2CH2Si(OCH2CH3)3

f e+g h

i

5

4

3

N

NCO

CH2CH2CH2Si

70 oC

2 3 H NMR (ppm)

1

0

Fig. 2. 1H NMR spectrum of HD.

CH H2 OC OCH2CH3 O

3

NCO + HD

TMS

c

d

1

CH 2C H

2OCN

a

j

CH2CH2COOCH2CH2OH

Hybrid Diol (HD) OCN

c

b

CH2CH2CH2Si

2 HOCH2CH2COOCH=CH2

e d

a

The overall synthesis process was schemed in Fig. 1. A kind of diol containing alkoxysilane groups (HD) was synthesized by Michael addition of KH550 and HEA, which is a quantitative N

j

b

Intensity (arb. units)

3.1. Synthesis and characterization of HUA

CH2CH2CH2Si

3. Results

i

h

CH2CH2COOCH2CH2OH

N

H3 H 2C OC OCH2CH3 OC H2 C H3

An appropriate amount of HUA or HUA-HC was blended with 4wt.% Darocur 1173 as a photoinitiator at 60°C for about 30 min until the solution became homogeneous. The transparent viscous formulations were cast onto glass plates by an applicator with a 100-μm gap. After being exposed to a medium pressure Hg lamp (1 kW, 80 W cm− 1, made by Lantian Co., China) for 5 min to ensure complete curing, the films were used for hardness and abrasion resistance measurements. The specimens used for stress–strain measurements were prepared in an aluminium mold with the size of 50 × 10 × 2mm according to ASTM D412 [12] and the two sides were irradiated using the same Hg lamp for 5 min. The crumbs used in TGA measurements were cut from the 2-mm-thick samples.

HOCH2CH2OOCCH2CH2

g

f

2.4. Film preparation and UV curing

reaction and was confirmed by 13C NMR, 1H NMR (Fig. 2) and FTIR (Fig. 3). The peaks at 1637 cm− 1 and 810 cm− 1 for acrylic double bonds (CfC) and the peak at 3371cm− 1 for –NH2 group in the FTIR spectrum of HD disappeared, while the peak at 956cm− 1 for hydrolysable Si–OCH2CH3 group still exist. The stability of the synthesized diol was greatly improved compared to that of KH550, which is easy to gel after standing for several minutes when exposed to atmosphere due to the self-catalysis effect of the basic –NH2. The gelling time of HD was found to -1

810 cm

3

CH H2 OC OCH2CH3 OC H 2C H

HUA-HC

70 oC

N

CH2=CHOOCCH2CH2OOCNH

NHCOOCH2CH2COOCH=CH2

CH2CH2CH2Si 3

3 CH H2 OC OCH2CH3 OC H 2C H

Hybrid Urethane Acrylate (HUA)

Fig. 1. Scheme for the synthesis of hybrid urethane acrylate (HUA).

Transmittance (arb. units)

3

+2 HEA

-1

HUA

1410 cm

-1

956 cm

-1

1410 cm

HD

956 cm

-1

-1

956 cm 4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1)

Fig. 3. FTIR spectrum of HD, HUA and HUA-HC.

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prolong even for over several hours at room temperature in atmosphere. HUA was synthesized according to the traditional two-step approach. Firstly, a disfunctional polyol (HD) reacts with 200% (mol/mol) excess isocyanate groups to hydroxy groups to give an isocyanate group (–NCO) terminated intermediate, which is then end-capped with HEA to give an acrylate functional urethane. The synthesis of HUA was conducted without the addition of external catalysts, and finished in a short time due to the catalytic effect of tertiary amino of HD. The FTIR measurement and a di-n-butylamine back titration method were used to monitor the reaction process. The-NCO absorbance (2270 cm− 1) doesn't exist in the spectrum of final product (Fig. 3), while the characteristic bands of acrylate appear at 1410cm− 1 and 810 cm− 1. On the other hand, the existence of a single peak at − 44.79ppm in 29Si NMR spectrum indicates that Si–OEt groups are not affected by the synthesis reactions (Fig. 4). 3.2. Sol–gel process of HUA The rate and extent of sol–gel reactions are strongly dependent on the strength of acidic or basic catalysts used. Strong acids and bases usually give fast hydrolysis and condensation of alkoxysilanes and high conversion to polymers. When the sol–gel condensation of alkoxysilanes is carried to high conversion, cross-linked matrixes were usually obtained. Furthermore, many chemical bonds, such as CO–O, CO–NH, are unstable under strong acid or base and thus side-reactions happen. At the same time, it is also quite difficult to control a fast sol–gel process to stop at immediate stages and reproducibly make soluble fluid oligomers with low viscosity. Indeed, even when such materials are obtained, they exhibit poor pot-life and normally gel on standing due to further condensation. For these reasons, less active catalysts are desired. In this work, a low-concentration solution of formic acid (0.1 M HCOOH) as the catalyst and insufficient H2O for all Si–OEt to condensation (H2O/Si–OEt = 1) were employed. The proposed structure of the final product HUA-

HUA 0.1 M HCOOH OC2H5 Si

f

OC2H5

f

Si

Si

O

OC2H5

O O

Si

Si

f

C2H5

O

O

n

OC2H5

f O

m

f

f : (CH2=CHCOOCH2CH2OCONHC6H3(CH3)NHCOOCH2CH2COOCH2CH2)2N(CH2)3

Fig. 5. Scheme for hydrolysis and condensation of HUA and the proposed structure of the condensate HUA-HC.

HC is given in Fig. 5. The FTIR spectra (Fig. 3) indicate that the peaks for organic functional groups in HUA are mostly maintained after the sol–gel process except that the peak for Si–OEt at 956cm− 1 decreases greatly. Due to the difference in the hydrolysis and condensation rates between three ethoxy groups in each silane unit, the complete condensed unit (T3) and partly condensed unit (T2, T1, T0) (T is the species with three hydrolysable silane monomer, x in Tx represents the number of siloxane bound on the Si-atom) both exist in HUAHC, which is evidenced by the 29Si NMR spectrum (Fig. 6). The inorganic condensation degree calculated by the integration of individual signal (condensed degree (%) = T1 + 2T2 + 3T3/3(T0 + T1 + T2 + T3) [13]) was about 30%. The hydrolysis and condensation of HUA at a higher concentration of HCOOH (1.0 M), stronger acid (0.1 M HCl) or more H2O (H2O/Si–OEt > 1) would result in more condensed resins, which are rather unstable on standing, inevitably leading to gel. Whereas HUA-HC prepared under low concentration acid catalyst and low H2O/Si–OEt have showed relative stability, and no obvious viscosity change was observed on standing for 1 month. 3.3. Photopolymerization kinetics The Photo-DSC measurement assumes that in a photocuring process the measured heat flow is proportional to the conversion

Intensity (arb. units)

Intensity (arb. units)

T1

0

-20

-40

-60

29

Si NMR (ppm)

Fig. 4. 29Si NMR spectra of HUA.

-80

-20

T2

T3

T0

-30

-40

-50

-60

29

Si NMR (ppm)

Fig. 6. 29Si NMR spectra of HUA-HC.

-70

-80

J. Xu et al. / Thin Solid Films 514 (2006) 69–75

where dα/dt is the conversion rate or the polymerization rate, ΔHtotal is the theoretical total exothermic heat per milligram at 100% conversion, (dH/dt)T is the measured heat flow per milligram at a constant temperature (T). For calculating the polymerization rate and ΔHtotal, the value for the heat flow of polymerization ΔH0 = 86 Jmmol− 1 (for per acrylic double bond) was taken. Figs. 7 and 8 show the apparent photopolymerization rate and conversion of double bonds at different temperatures for HUA and HUA-HC, respectively. Both HUA and HUA-HC show high photopolymerization response and reach the highest rate in 10s after starting irradiation. The apparent photopolymerization rate and unsaturation conversion of HUA both increase with raising temperature, which is consistent with traditional acrylate oligomers. This is mainly due to the decrease in viscosity with increasing temperature and thus increasing the collision probability of CfC bonds at higher temperatures. However, the apparent photopolymerization rate and the unsaturation conversion of HUA-HC increase with raising temperature at first, but then decrease when temperature

Photopolymerization rate (s-1*100)

  da 1 dH ¼ dt DHtotal dt T

4.0 49 °C

3.5

57 °C

3.0

72 °C 82 °C

2.5

92 °C

2.0 1.5 1.0 0.5 0.0 0

10

20

30

40 50 60 70 Irradiation time (s)

80

90

100

30

40 50 60 70 Irradiation time (s)

80

90

100

80 49 °C

70

57 °C 72 °C

60 Conversion (%)

rate at a constant temperature (T). The rate of change in the conversion can therefore be defined as follows:

73

82 °C

50

92 °C

40 30 20 10 0 0

10

20

Fig. 8. Photopolymerization rate and unsaturation conversion of HUA-HC at different temperatures. Photopolymerization rate (s-1*100)

5.0 4.5

49 °C

4.0

57 °C 72 °C

3.5

82 °C

3.0

92 °C

2.5 2.0 1.5 1.0 0.5 0.0 0

10

20

30

40 50 60 70 Irradiation time (s)

80

90

100

30

40 50 60 70 Irradiation time (s)

80

90

100

80 49 °C

70

57 °C 72 °C

Conversion (%)

60

82 °C 92 °C

50 40 30 20 10 0 0

10

20

Fig. 7. Photopolymerization rate and unsaturation conversion of HUA at different temperatures.

continued to increase as shown in Fig. 8. This phenomenon seems to be strange if that the polymerization of double bonds was considered to be the only reaction under UV light. The condensation of Si–OH or Si–OEt should have taken place at higher temperature. At lower temperature (lower than 72 °C), HUA-HC has a faster apparent photopolymerzation rate and a higher conversion of double bonds than HUA. For example, the fastest rate and the final conversion of HUA-HC at 49°C are 0.0265 s− 1 and 68% (Fig. 8), while they are 0.0142 s− 1 and 60% for HUA, respectively (Fig. 7). Since the conversion of double bonds of HUA-HC and HUA are both not high at lower temperature, the vicinity of double bonds in HUA-HC because of the preformed Si–O–Si linking obviously has a positive effect on their polymerization when exposed to UV light. At higher temperature, HUA-HC shows a slower apparent photopolymerization rate and a lower conversion of double bonds. For example, 0.0246 s− 1 and 57% for HUA-HC, while 0.0460 s − 1 and 81% for HUA are obtained at 92 °C, respectively. It should be noted that Photo-DSC measurement is only valid for materials exhibiting a single reaction without other enthalpic events, such as the evaporation of solvents or volatile components, enthalpy relaxation, or significant changes in heat capacity with the conversion. Therefore, he unique photopolymerization kinetic of HUA-HC indicated indirectly that there are other reactions happening besides the polymerization of double bonds. In HUA-HC, there are more

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J. Xu et al. / Thin Solid Films 514 (2006) 69–75 60 HUA HUA-HC

55

HUA HUA-HC

50 45

30 Stress (MPa)

Weight Loss (mg)

40

20

40 35 30 25 20

10

15 10

0

5 0

100

200

300

400

500

0

600

0

2

4

6

Wear (cycles)

8

10

12

14

16

18

20

Stain (%)

Fig. 9. Abrasion resistance of UV cured HUA and HUA-HC 100-μm-thick films at room temperature.

Fig. 10. Stress–strain curves for UV cured HUA and HUA-HC 2-mm-thick films.

reactive Si–OH groups in addition to the remained Si–OEt groups. The condensation between Si–OH and Si–OH or Si– OEt, and the evaporation of small molecules (H2O or EtOH) coming from the condensation would have negative influence on the heat flow, which is negligible at lower temperatures, and become notable at high temperatures. The condensation of Si–OEt groups in HUA may also happen together with the polymerization of double bonds, whereas the condensation extent of HUA is lower than that in HUA-HC. Soppera [7] and Medda [14] also observed more distinct condensation occurred under UV irradiation in the condensated MAPTMS than unhydrolyzed MAPTMS. Therefore, it could be concluded from Photo-DSC results that more condensation took place in HUA-HC than HUA when exposed to UV irradiation, especially at high temperatures.

to higher crosslinking density of its cured film. As a result, HUA-HC film has higher abrasion resistance than HUA film. HUA and HUA-HC films both showed high transparency, and as well high pencil hardness (Table 1) due to the presence of acrylate structure in the molecules. HUA-HC film showed slight higher pencil hardness than HUA film because of the above mentioned more inorganic Si–O–Si linkages in HUA-HC. Fig. 10 shows the stress–strain curves of HUA and HUAHC. Both HUA and HUA-HC have relative high tensile strength of 32.8 and 44.9 MPa, and elongation-at-break of 14.1% and 9.6%, respectively. The slight higher tensile strength and lower elongation of HUA-HC indicate that it has denser network structure than HUA. 3.5. Thermal properties

3.4. Abrasion resistance and mechanical properties

Table 1 Viscosity, pencil hardness and thermal properties of HUA and HUA-HC Sample

M aw

Viscosity Viscosity Hardnessb T1d c at r.t at 70°C (°C) (cps) (cps)

HUA 1034 >10,000 HUA-HC / >10,000 a

6500 7000

5H 6H

T2d d (°C)

Residue at 700°C (%)

220.6 416.8 8.19 226.2 436.0 14.7

Theoretical molecular weight. b Taken until the pencil could plough the films. c Decomposition temperature at 5% weight loss at the first stage degradation. d Starting decomposition temperature at the second stage degradation.

Nanocomposites in many systems have shown higher thermal stability than pure organic polymer due to the strong interaction between organic and inorganic components. Fig. 11 shows the TGA curves of UV cured HUA and HUA-HC films. It can be seen that the condensed inorganic linkages in HUA110 100 90 80 Weight Loss (%)

The abrasion resistance with the wear cycles of HUA and HUA-HC films is depicted in Fig. 9. Both of them showed excellent anti-abrasion ability (< 40mg after 600 cycles). The cured HUA-HC film shows higher anti-abrasion ability than HUA film, mainly due to the reinforcement by inorganic Si– O–Si linkages in HUA-HC film, which resulted from the hydrolytic condensation during sol–gel process and more condensation accompanying with polymerization of double bonds when exposed to UV light. Compared to HUA, HUAHC is a urethane oligomer of higher functionalities, leading

HUA HUA-HC

70 60 50 40 30 20 10 0 100

200

300 400 500 Temperature (°C)

600

700

Fig. 11. TGA curves of UV cured HUA and HUA-HC crumbs cut from 2-mmthick films.

J. Xu et al. / Thin Solid Films 514 (2006) 69–75

HC have obvious effect in improving the thermal stability of coatings. Both HUA and HUA-HC films show a two-stage decomposition process. The starting degradation temperatures at the two stages for HUA-HC film are 226.2 °C and 436.0 °C (Table 1), which are 5.6°C and 29.2 °C higher than those of HUA film (220.6 °C and 406.8 °C, respectively). More inorganic Si–O–Si linkages in HUA-HC greatly inhibited the heat diffusion, and thus postponed the decomposition of organic parts. The delaying of starting degradation temperatures in the second stage (29.2 °C) is much larger than that in the first stage (5.4 °C), indicating that the protection of organic part by inorganic networks is more distinct for the organic part which is more close to the inorganic networks. 4. Conclusion A novel organic–inorganic hybrid resin (HUA-HC) has been prepared by sol–gel method under a low concentration of acid catalyst. Both prepolymer (HUA) and hydrolytic condensate (HUA-HC) showed high photopolymerization response, long storage stability and processable viscosity. The UV cured films showed excellent transparency, antiabrasion ability, tensile behaviors and thermal properties. The condensate HUA-HC has a faster photopolymerzation rate and a higher conversion of double bonds than HUA at lower temperatures. The Si–OEt and Si–OH groups in HUA-HC under UV irradiation at high temperatures have more tendency to condense than those in HUA. Due to more

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inorganic Si–O–Si linkages, the UV cured HUA-HC film shows higher pencil hardness, tensile strength, and thermal stability than HUA. Acknowledgment The financial supports of China NKBRSF project (2001CB409600) and National Natural Science Foundation of China (50233030) are gratefully acknowledged. References [1] M. Avella, M.E. Errico, E. Martuscelli, Nano Lett. 1/4 (2001) 213. [2] K.H. Haas, S. Amberg-Schwab, K. Rose, Thin Solid Films 351/1-2 (1999) 198. [3] M.E.L. Wouters, D.P. Wolfs, M.C. van der Linde, J.H.P. Hovens, A.H.A. Tinnemans, Prog. Org. Coat. 51/4 (2004) 312. [4] J.Y. Wen, G.L. Wilkes, Chem. Mater. 8/8 (1996) 1667. [5] U. Schubert, N. Husing, A. Lorenz, Chem. Mater. 7/11 (1995) 2010. [6] J.V. Crivello, K.Y. Song, R. Choshal, Chem. Mater. 13/5 (2001) 1932. [7] O. Soppera, C. Croutxe-Barghorn, J. Polym. Sci. Polym. Chem. 41/5 (2003) 716. [8] C.H. Li, K. Jordens, G.L. Wilkes, Wear 242/1-2 (2000) 152. [9] S. Velankar, J. Pazos, S.L. Cooper, J. Appl. Polym. Sci. 62/9 (1996) 1361. [10] S.A.o. China, State Standards of China GB/T 6739-1996 (1996). [11] S.A.o. China, State Standards of China GB 4893.8-1985 (1985). [12] G. Xu, Y.B. Zhao, W.F. Shi, J. Polym. Sci., Part B, Polym. Phys. 43/22 (2005) 3159. [13] S. Sepeur, N. Kunze, B. Werner, H. Schmidt, Thin Solid Films 351/1-2 (1999) 216. [14] S.K. Medda, D. Kundu, G.T. De, J. Non-Cryst. Solids 318/1-2 (2003) 149.