Influence of thiol and ene functionalities on thiol–ene networks: Photopolymerization, physical, mechanical, and optical properties

Polymer Testing 32 (2013) 608–616

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Material properties

Influence of thiol and ene functionalities on thiol–ene networks: Photopolymerization, physical, mechanical, and optical properties Jian Zhou, Qiu-yu Zhang*, Shao-jie Chen, He-peng Zhang, Ai-jie Ma, Ming-liang Ma, Qing Liu, Jiao-jun Tan Key Laboratory of Applied Physics and Chemistry in Space, Ministry of Education, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, 127 Youyi West Road, Xi’an 710072, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 November 2012 Accepted 30 January 2013

Photopolymerization behaviour of thiol–ene networks consisting of di-, tri-, tetra- and hexa-functional thiol and acrylate monomers was evaluated. For understanding the effect of monomer functionality on polymer properties, a comprehensive investigation of the fundamental physical, mechanical and optical properties of thiol–ene networks was conducted. The results indicate that monomer functionality can considerably influence the physical properties, thermal behaviour and optical performance of such networks. As the monomer functionality improved, the number of functional group conversions, shear strength, Tg, storage modulus and reflective index increased. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Photopolymerization Thiol–ene Functionality Polymer properties

1. Introduction The use of photocurable polymers has demonstrably been very effective for producing tailor-made polymers with desirable physical and mechanical properties. In particular, thiol–ene photopolymerization has attracted great attention since the generally accepted polymerization mechanism proposed by Kharasch in 1938 [1]. The mechanism (Scheme 1 [2–7]) indicates that thiol–ene network polymers are formed via a radical addition reaction, which is known to produce an efficient step-growth mechanism. Compared with traditional acrylate photopolymerization, thiol–ene curing affords unmatched levels of insensitivity towards oxygen inhibition [8], highly uniform crosslinking [9] and narrow glass transition regions [10]. In addition, many types of enes and thiols can be incorporated into thiol–ene networks [11]. Reactivities of the three basic * Corresponding author. Tel./fax: þ86 2988431653. E-mail addresses: [email protected], dancyzj1983@yahoo. com.cn (Q.-y. Zhang). 0142-9418/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2013.01.013

types of thiolsdmercaptopropionate, thioglycolate and alkyl thioldwith various types of enes such as norbornene, vinyl ether, allyl ether, allyl triazine, allyl isocyanurate, alkene, acrylate, methacrylate and styrene have been reported [2,3,12,13]. There have been sporadic reports on the kinetics of photocured thiol–ene networks and their selected physical properties [14]. Jacquelyn [15] et al. evaluated triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetramercapto propionate (PETMP) systems as novel dental restorative materials; these systems displayed shrinkage stress of only 0.4 MPa or approximately only 14% of the maximum shrinkage stress of the traditional bis-glycidyl methacrylate/triethylene glycol dimethacrylate (Bis-GMA/TEGDMA) system. Li [16] et al. discussed the effect of thiol and ene monomer structures on thiol–ene network properties; specifically, they compared the differences in the performance of secondary and primary thiols. Todd [17] et al. investigated the effect of alkene structures on the mechanism and kinetics of thiol– alkene photopolymerization reactions using real-time infrared spectroscopy. They concluded that terminal enes

J. Zhou et al. / Polymer Testing 32 (2013) 608–616

h

609

I*

Step 1

I* + RSH H C CH2

RS

Step 2

R'

H C

SR + RSH

R'

CH2

RS

SR

I Initiation

RS + R' Propagation R'

H C

CH2

2 RS 2 R'

H C

CH2

SR

Termination

R' R'

RS

+ R'

H C

CH2

SR

R'

H C C H H C

CH2

SR SR + RS

CH2

Step 3 Step 4 Step 5

CH2

SR

CH2

SR

CH2

SR

Step 6

Step 7

SR Scheme 1. General thiol–ene photopolymerization process.

react very rapidly with thiol and follow the basic thiol–ene two-step reaction mechanism. Internal cis enes are characterized by an insertion–isomerization–elimination reaction series, which results in trans-ene formation. However, most previous studies on thiol–enes were focused on low functionality (di-, tri-, or tetra-functional) monomers. There has not been a detailed evaluation on higher-functionality monomers. To obtain a more comprehensive understanding of the relationship between monomer functionality and polymer properties, we studied thiol and ene monomers with di-, tri-, tetra-, and hexa-functionality in detail in this work. An investigation of higher-functionality monomer, especially the hexa-functional ene and thiol monomers, was firstly implemented. The photopolymerization behaviour, physical, mechanical and optical properties of different thiol–ene polymers for different functional monomers were characterized and compared, which provide more theory guidance for UV curing material formulation design.

Ltd. Pentaerythritol tetraacrylate (PETA) and dipentaerythritol hexaacrylate (DPHA) were kindly supplied by Guangzhou Deco Composite Materials Technology Co., Ltd. (see Fig. 1 for chemical structures). 2.2. Sample preparation The monomers (or olimers) and initiators were weighed and mixed in clean, dry vials. Each vial was wrapped with aluminium foil to block light. To remove any air bubbles, we placed the vials in a vacuum oven at room temperature for several hours. It was necessary to maintain darkness throughout the process. To prepare the test specimens for dynamic mechanical analysis (DMA), a series of thiol-acrylate materials, ranging from flexible to rigid polymers, were molded into rectangular test pieces (50  5  2 mm) after photopolymerization among the four acrylate monomers and the four thiol monomers. For full curing, all the thiol–ene samples were treated at 50  C for 12 h.

2. Experimental

2.3. Characterization

2.1. Materials

The photopolymerization process of thiol–ene samples was monitored using a Bruker Tensor 27 real-time Fourier transform infrared spectroscope (RT-FTIR) operating at 5 scans/s [18–21]. RT-FTIR was effective for monitoring the photopolymerization process because the decrease in absorption in the peak area was directly proportional to the number of polymerized functional groups. Therefore, the degree of conversion of the functional groups could be calculated by measuring the peak area at different points of reaction time with OPUS6.5 software, and substituting the measured values in the following equation [22].

Pentaerythritol tetra(3-mercaptopropionate) (PETMP) and a free-radical photoinitiator, 1-hydroxycyclohexyl phenyl ketone (Iragcure 184), were purchased from TCI Chemical Reagent Co., Ltd. Trimethylol propane tris(3mercaptopropionate) (TMMP) was purchased from Acros Organics Chemical Reagent Co., Ltd. Dipentaerythritol hexakis(3-mercaptopropionate) (DPMP) was purchased from Guangzhou Sgsmt Special Materials Technology Co., Ltd. Tetraethylene glycol bis(3-mercaptopropionate) (EGMP) was purchased from Guangzhou Baimo Biological Technology Co., Ltd. Tripropylene glycol diacrylate (TPGDA) and trimethylol propane triacrylate (TMPTA) were purchased from Shanghai Polynaisse Resources Chemicals Co.,

Ct ¼ ðA0  At Þ=A0  100%

(1)

Ctdconversion at t timeA0dpeak area of function group before irradiationAtdpeak area of function group at t time

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Fig. 1. Chemical structures of monomers used in this study.

Thiol–ene samples were spread directly on the KBr tablets and irradiated at room temperature under continuous UV light, which was generated by a UV spot source (Lightningcure L8868, Hamamatsu, Japan) and transmitted through an optical fibre. The UV light intensity on the sample surfaces, detected using a UV light

radiometer (UV-INT 150, Germany), was about 18 mW/cm2 based on keeping 1 cm irradiation distance and 100% output light intensity. The peaks for thiol and acrylate were 2571 cm1 and 1624 cm1, respectively. A schematic representation of the RT-FTIR apparatus used here is shown in Fig. 2.

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611

Fig. 2. Schematic of real-time FTIR apparatus.

The shear strength of the thiol–ene components was measured in accordance with ISO 6238: 2001 [23] using a CMT7204 computer-controlled electronic universal testing machine. Each sample was measured thrice, and the average of these three measurements was considered. The viscoelastic properties of these components were characterized using a dynamic mechanical analyser (DMA/SDTA 861e, Mettler Toledo, Switzerland) (cantilever mode). The samples were analyzed under the following conditions: frequency of 1 Hz and heating rate of 5  C/min between 20  C and 100  C in nitrogen atmosphere. Calorimetric analysis was performed in nitrogen atmosphere using a differential scanning calorimeter (DSC2910, TA Instruments) at a heating rate of 10  C/min between 20  C and 100  C. The shore

D hardness of the samples was determined according to ISO 868: 2003 [24], using a Shore hardness tester (LX-D, Xi’an Minsks Detection Equipment Co., Ltd.). Refractive index of the samples was measured using a digital Abbe refractometer (WAY-1S, Shanghai Physical Optical Instrument Factory). 3. Results and discussion 3.1. Photopolymerization analysis In order to study the influence of the formulation on the performance, four ene monomers and four thiol monomers were selected as thiol–ene candidate compounds. First,

Fig. 3. RT-FTIR spectra of TPGDA with four thiol monomers: (1) TPGDA-EGMP; (2) TPGDA-TMMP; (3) TPGDA-PETMP; (4) TPGDA-DPMP.

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Fig. 4. Effect of thiol monomer functionality on function group conversion with irradiation time: (1) thiol conversion and (2) alkene conversion.

formulations of TPGDA with stoichimetric EGMP, TMMP, PETMP, and DPMP were investigated. A few representative spectra are shown in Fig. 3, i.e., those obtained at 0, 10, 30, 60, and 120 s. It is evident from Fig. 3 that the areas of the thiol peak at 2571 cm1 and the alkene peak at 1624 cm1 decreased gradually during photopolymerization. For the four thiol monomers with different functionality, as shown in Fig. 4, thiol conversion increased with thiol monomer functionality. In the cases of TMPTA, PETA and DPHA, similar trends could be found albeit with small differences. Generally, for a given ene monomer, both the thiol and the alkene conversion increased with thiol monomer functionality. To understand the outcome when a given thiol monomer is polymerized with different ene monomers, we selected PETMP as the given thiol monomer and stoichiometric TPGDA, TMPTA, PETA, and DPHA as the ene monomers. The conversion data is presented in Fig. 5. From Fig. 5 it is evident that after irradiation for 120 s, the extents of PETMP-PETA and PETMP-DPHA functional group conversions are higher than that of the PETMPTMPTA one, which is in turn higher than that of the PETMP-TPGDA functional group conversion. The extent of PETMP-DPHA conversion was only marginally higher than PETMP-PETA conversion in spite of the difference in

functionality; this could be ascribed to the steric effect of multifunctional monomers. Therefore, the thiol–ene photopolymerization reaction can possibly be enhanced by improving monomer functionality, regardless of the thiol or the ene monomers used in the reaction. 3.2. Shear strength The shear strength of the different thiol–ene samples was measured according to ISO 6238: 2001. Each sample was measured thrice, the average of the three measured values and the relative standard deviation (RSD) were calculated. The results are listed in Table 1. It can be seen from the data for samples (1), (2), (3), and (4) that the shear strength of TPGDA-DPMP is higher than that of TPGDA-PETMP, which, in turn, is higher than those of TPGDA-TMMP and TPGDA-EGMP. When the TPGDA was replaced with TMPTA, PETA, and DPHA, the shear strength trends were analogous as expected, i.e. shear strength increased with thiol monomer functionality. Similarly, it is evident from the data corresponding to samples (1), (5), (9), and (13) that the shear strength of DPHA-EGMP is higher than that of PETA-EGMP, which, in turn, is higher than those of TMPTA-EGMP and TMPTA-EGMP. When the EGMP group was replaced with TMMP, PETMP, and DPMP, similar

Fig. 5. Effect of ene monomer functionality on function group conversion with irradiation time: (1) thiol conversion and (2) alkene conversion.

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Table 1 Shearing strength of different thiol–ene samples. Sample

Experiment 1

Experiment 2

Experiment 3

Mean value (MPa)

RSD

(1) TPGDA-EGMP (2) TPGDA-TMMP (3) PGDA-PETMP (4) TPGDA-DPMP (5) TMPTA-EGMP (6) TMPTA-TMMP (7) TMPTA-PETMP (8) TMPTA-DPMP (9) PETA-EGMP (10) PETA-TMMP (11) PETA-PETMP (12) PETA-DPMP (13) DPHA-EGMP (14) DPHA-TMMP (15) DPHA-PETMP (16) DPHA-DPMP

1.9004 2.2228 2.8419 2.7766 2.5218 3.7466 5.6016 6.4728 3.0559 5.2316 6.8624 7.5213 3.5994 5.5961 7.4326 9.4073

1.7623 2.1092 2.8113 2.9249 2.6713 4.2645 5.2206 6.0546 2.9832 4.9137 7.1373 7.0379 3.4261 5.4319 7.6776 8.1687

2.0778 2.4541 3.2859 3.3675 2.4463 4.6422 4.8433 6.4293 3.6721 5.4510 6.6057 7.7476 3.2078 5.6289 7.7468 8.6287

1.91 2.26 2.98 3.02 2.55 4.22 5.22 6.32 3.24 5.20 6.87 7.43 3.41 5.55 7.62 8.73

0.16 0.18 0.32 0.31 0.11 0.45 0.38 0.23 0.38 0.27 0.27 0.36 0.28 0.11 0.17 0.63

trends were observed. Therefore, we may conclude that the shear strength improved with monomer functionality. 3.3. Dynamic mechanical analysis The storage moduli of the four thiol–ene polymers, which were composed of a hexa-functional DPHA with EGMP, TMMP, PETMP, and DPMP, were analyzed (See Fig. 6(a)). It was evident that the storage modulus improved with thiol monomer functionality, with the ene monomer remaining unchanged. The storage modulus improved from 1200 MPa for the DPHA-EGMP system to nearly 3900 MPa for the DPHA-DPMP system. In addition, we analyzed the glass transition temperature (Tg) of these four thiol–ene polymers. The results, presented in Fig. 7(a), show that for the same ene monomer, the Tg improved with thiol monomer functionality. The Tg improved from 1  C for the DPHA-EGMP system to around 42  C for the DPHA-DPMP system. These trends of the storage modulus and the Tg can possibly be ascribed to the fact that as monomer functionality increased, crosslink density increased. To illustrate these trends in detail, the trends for the storage moduli and Tg versus temperature in another group of thiol–ene components were considered, namely, four acrylate monomers (TPGDA, TMPTA, PETA, and DPHA)

with a tetra-functional thiol monomer (PETMP). The relationship between storage modulus and temperature for different thiol-acrylate compounds is shown in Fig. 6(b). It is evident that as the ene monomer functionality increased, the storage modulus increased from 1600 MPa for the TPGDA-PETMP system to 3500 MPa for the DPHAPETMP system. In addition, it can be seen from the Tg versus temperature curve, shown in Fig. 7(b), that the Tg increased from 6  C for the TPGDA-PETMP system to 29  C for the DPHA-PETMP system with acrylate monomer functionality. From the above discussions pertaining to the two groups, we may conclude that the storage modulus and Tg of thiol–ene materials can be enhanced by adding thiol or ene monomer functionalities. The crosslink density of the samples increased with monomer functionality; therefore, the material properties were improved. 3.4. DSC analysis The compositions of a hexa-functional DPHA with EGMP, TMMP, PETMP, and DPMP were selected as the DSC test group. The curves are shown in Fig. 8. From Fig. 8, we found that the Tg of the four constituents showed a regular change, i.e. the Tg value increased from

Fig. 6. Storage modulus versus temperature of different constituents.

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Fig. 7. Tan d versus temperature of different constituents.

nearly 0  C for the DPHA-EGMP system to about 30  C for the DPHA-DPMP system with thiol monomer functionality. Obviously, the trend obtained by the DSC method was in accord with that of the DMA method, albeit with small differences between the Tg values. It is evident that the Tg increased with monomer functionality, which is closely related to the crosslinking density. 3.5. Hardness measurements The Shore D hardness experiments results of different thiol–ene polymers are shown in Fig. 9. It is evident from Fig. 9 that the Shore D hardness increased with the

monomer functionality. The double-hexa-functional system, i.e. the DPHA-DPMP group, exhibited the highest hardness, reaching 86. In contrast, the double-di-functional system (TPGDA-EGMP) showed a value of less than 10, expressing a flexible elastomer. 3.6. Reflective index characterization The refractive indices of all thiol–ene samples were measured at 20  C and the results are listed in Table 2. The relationship among the refractive index and the other properties of the samples is expressed as follows using the Lorentz–Lorenz equation [25].

Fig. 8. Heat flow versus temperature of different constituents: (1) DPHA-EGMP; (2) DPHA-TMMP; (3) DPHA-PETMP; (4) DPHA-DPMP.

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the highest refractive indices, whereas the TMMP-TPGDA based networks have the lowest refractive index. Furthermore, we selected the TMPTA-PETMP system as the optimization group to understand the relationship between refractive index and the sulphur content. Stoichiometric TMPTA-PETMP includes around 45 wt% TMPTA and 55 wt% PETMP. A series of TMPTA-PETMP components with different weight ratios, i.e. 30/70, 40/60, 45/55, 50/50, 60/40, and 70/30, were prepared. The refractive indices of these components are shown in Fig. 10. From Fig. 10, it is evident that for a given system, the reflective index improved as the sulphur content increased, which is consistent with the findings presented in [16]. 4. Conclusions

Fig. 9. Hardness of thiol–ene crosslinked networks.

Table 2 Refractive index of thiol–ene samples. Constituent TPGDA

TMPTA

PETA

DPHA

TMMP PETMP DPMP TMMP PETMP DPMP TMMP PETMP DPMP TMMP PETMP DPMP

Refractive index

Sulfur content (wt%)

1.4857 1.4895 1.4909 1.5007 1.5062 1.5078 1.5054 1.5116 1.5138 1.5092 1.5140 1.5145

10.86 11.28 10.94 13.27 13.91 13.40 13.90 14.61 14.04 13.39 14.05 13.53

rNav a n2  1 ¼ n2 þ 2 3M0 ε0

(2)

ndthe refractive indexrdpolymer densityNavdAvogadro constantadaverage polarizabilityM0dpolymer molecular weightε0dvacuum permittivity It is evident from Table 2 that at 20  C, the double-hexafunctional networks prepared using DPHA and DPMA have

Fig. 10. Relationship between refractive index and sulfur content.

In this work, we investigated the effect of the functionality of different thiol and acrylate monomers on thiol–ene polymer properties. Both the thiol and acrylate groups have four functionalities including di-, tri-, tetra-, and hexa-. With a series of tests on more than a dozen samples, we concluded that the polymer crosslinking density increased with the increasing monomer functionality. As a result, the consequent series of performances on thiol–ene compounds were also enhanced. The shear strength improved from 1.91 MPa to 8.73 MPa, the Shore D hardness increased from less than 10 to more than 80, the Tg elevated from about 0  C to more than 40  C, and the reflective index changed from 1.4857 to 1.5145. These investigations suggesting the very attractive performances for UV adhesive that the thiol–ene compounds with higher-functionality would have the better adhesion to glass, appropriate mechanical strength and larger reflective index. Therefore, the research has a great significance on formula design of UV adhesive and other related products. References [1] M.S. Kharasch, F.R. Mayo, Chem. Ind. (London) 57 (1938) 752. [2] C.R. Morgan, F. Magnotta, A.D. Ketley, Thiol/ene photocurable polymers, J. Polym. Sci.: Polym. Chem. Ed. 15 (3) (1977) 627–645. [3] N.B. Cramer, C.N. Bowman, Kinetics of thiol-ene and thiol-acrylate photopolymerizations with real-time Fourier transform infrared, J. Polym. Sci., Part A: Polym. Chem. 39 (19) (2001) 3311–3319. [4] T.Y. Lee, T.M. Roper, E.S. Jonsson, C.A. Guymon, C.E. Hoyle, Thiol-ene photopolymerization kinetics of vinyl acrylate/multifunctional thiol mixtures, Macromolecules 37 (2004) 3606–3613. [5] S.K. Reddy, N.B. Cramer, C.N. Bowman, Thiol-vinyl mechanisms. 1. termination and propagation kinetics in thiol-ene photopolymerizations, Macromolecules 39 (2006) 3673–3680. [6] B.D. Fairbanks, T.F. Scott, C.J. Kloxin, K.S. Anseth, C.N. Bowman, Thiol-yne photopolymerizations: novel mechanism, kinetics, and step-growth formation of highly cross-linked networks, Macromolecules 42 (2009) 211–217. [7] T.Y. Lee, Z. Smith, S.K. Reddy, N.B. Cramer, C.N. Bowman, Thiol-allyl ether-methacrylate ternary systems: polymerization mechanism, Macromolecules 40 (2007) 1466–1472. [8] A.K. O’Brien, N.B. Cramer, C.N. Bowman, Oxygen inhibition in thiolacrylate photopolymerizations, J. Polym. Sci., Part A: Polym. Chem. 44 (6) (2006) 2007–2014. [9] N.B. Cramer, S.K. Reddy, A.K. O’Brien, C.N. Bowman, Thiol-ene photopolymerization mechanism and rate limiting step changes for various vinyl functional group chemistries, Macromolecules 36 (21) (2003) 7964–7969. [10] M. Kade, D. Burke, C.J. Hawker, The power of thiol-ene chemistry, J. Polym. Sci., Part A: Polym. Chem. 48 (2010) 743–750. [11] C.E. Hoyle, T.Y. Lee, T. Roper, Thiol-enes: chemistry of the past with promise for the future, J. Polym. Sci., Part A: Polym. Chem. 42 (21) (2004) 5301–5338.

616

J. Zhou et al. / Polymer Testing 32 (2013) 608–616

[12] A.F. Jacobine, in: J.D. Fouassier, J.F. Rabek (Eds.), Radiation Curing in Polymer Science and Technology III: Polymerization Mechanisms, Elsevier, London, 1993, pp. 219–268. [13] C.E. Hoyle, C.N. Bowman, Thiol-ene click chemistry, Angew. Chem., Int. Ed. 49 (2010) 1540–1573. [14] H. Lu, J.A. Carioscia, J.W. Stansbury, C.N. Bowman, Investigations of step-growth thiol-ene polymerizations for novel dental restoratives, Dental Mater. 21 (12) (2005) 1129–1136. [15] J.A. Carioscia, H. Lu, J.W. Stanbury, C.N. Bowman, Thiol-ene oligomers as dental restorative materials, Dental Mater. 21 (12) (2005) 1137–1143. [16] Q. Li, H. Zhou, C.E. Hoyle, The effect of thiol and ene structures on thiol-ene networks: photopolymerization, physical, mechanical and optical properties, Polymer 50 (2009) 2237–2245. [17] T.M. Roper, C.A. Guymon, E.S. Jonson, C.E. Hoyle, Influence of the alkene structure on the mechanism and kinetics of thiol-alkene photopolymerizations with real time infrared spectroscopy, J. Polym. Sci., Part A: Polym. Chem. 42 (2004) 6283–6298. [18] T. Clark, L. Kwisnek, C.E. Hoyle, S. Nazarenko, Photopolymerization of thiol-ene systems based on oligomeric thiols, J. Polym. Sci., Part A: Polym. Chem. 47 (1) (2009) 14–24.

[19] H. Zhou, Q. Li, T.Y. Lee, A. Guymon, E.S. Jonsson, C.E. Hoyle, Photopolymerization of acid containing monomers: real-time monitoring of polymerization rates, Macromolecules 39 (24) (2009) 8269–8273. [20] H. Wei, Q. Li, M. Ojelade, S. Madbouly, J.U. Otaigbe, C.E. Hoyle, Thiol-ene free-radical and vinyl ether cationic hybrid photopolymerization, Macromolecules 40 (24) (2007) 8788–8793. [21] G.V. Salmoria, P. Klauss, A.T.N. Pires, J. Roeder, V. Soldi, Investigations on cure kinetics and thermal degradation of stereolithography RenshapeÔ 5260 photosensitive resin, Polym. Test. 27 (6) (2008) 698–704. [22] C. Decker, T.N.T. Viet, H.P. Thi, Photoinitiated cationic polymerization of epoxides, Polym. Int. 50 (9) (2001) 986–997. [23] ISO 6238: 2001, Adhesive-Wood to Wood Adhesive BondsDetermination of Strength by Compressive Loading. International Standard, 2001. [24] ISO 868: 2003, Plastics and Ebonite – Determination of Indentation Hardness by Means of a Durometer (Shore Hardness). International Standard, 2003. [25] N.J. Mills, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, J.I. Kroschwitz (Eds.), Encyclopedia of Polymer Science and Engineering, second ed., vol. 10, John Wiley and Sons, New York, 1987, pp. 493–540.