Synthesis and characterization of UV-curable dual hybrid oligomers based on epoxy acrylate containing pendant alkoxysilane groups

Progress in Organic Coatings 57 (2006) 50–55

Synthesis and characterization of UV-curable dual hybrid oligomers based on epoxy acrylate containing pendant alkoxysilane groups G¨ulay Bayramo˘glu, M. Vezir Kahraman, Nilhan Kayaman-Apohan, Atilla G¨ung¨or ∗ Marmara University, Faculty of Art & Science, Department of Chemistry, G¨oztepe, Kadikoy, 34722 Istanbul, Turkey Received 15 November 2005; accepted 2 June 2006

Abstract This work involves the synthesis of novel hybrid oligomers based on a UV-curable epoxy acrylate resin (EA). The EA resin was modified with various amount of 3-isocyanatopropyl trimethoxysilane (IPTMS) coupling agent. The modification percentage of the hybrid oligomer was varied from 0 to 50 wt.%. UV-curable, hard and transparent organic–inorganic hybrid coatings were prepared on Plexiglas substrates and their characterization was performed by the analyses of various properties such as hardness, gloss, tape adhesion test and stress–strain test. Results from the mechanical measurements show that the properties of hybrid coatings improve with the increase in modification ratio. The thermal behavior of coatings was also evaluated. It is observed that the thermal stability of epoxy acrylate coatings is enhanced with incorporation of siloxane. Gas chromatography/mass spectrometry analyses showed that the initial weight loss obtained in thermogravimetric analysis is due to the degradation products of the photoinitator and the acrylic acid moiety of acrylic monomers. © 2006 Elsevier B.V. All rights reserved. Keywords: UV-curable coatings; Organic–inorganic hybrids; Epoxy acrylate resin; Thermal stability; Stress–strain measurements

1. Introduction UV-curable coating applications have gained wide interests, due to their advantages such as lower energy consumption, less environmental pollution, lower process costs, high chemical stability and very rapid curing even at ambient temperatures [1–3]. UV-curable coatings are continually being developed by many leading suppliers in an effort to reduce any detrimental effects to the environment and to meet high standards required by industry [4]. Especially, in the field of UV curing industries, epoxy and epoxy acrylate derivatives have been widely used as coatings, structural adhesives and advanced composite matrices [5]. What distinguishes epoxy resins from the other polymers is their excellent chemical and solvent resistance, good thermal and adhesion properties, versatility in cross-linking. On the other hand, UVcurable acrylic urethane coatings are also used in a variety of applications due to their versatility, durability, appearance and superior weatherability compared to other resin systems [6]. UVcurable coatings are widely used for protective purposes. For this

∗

Corresponding author. Tel.: +90 216 3487759; fax: +90 216 3478783. E-mail addresses: atillag [email protected], [email protected] (A. G¨ung¨or). 0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.06.002

reason, often the requirements ask for enhanced hardness, and superior thermal stability. Moreover, during the two last decade’s higher demands on aesthetic qualities, in particular on gloss and color retention, have gradually increased. Because of this strong need for high performance coatings; extensive researches have been made to obtain materials that combine the desirable properties of organic polymers (elasticity, processability) and inorganic solids (hardness, chemical inertness and thermal resistance) [7]. Organic–inorganic hybrid materials offer the opportunity to combine both these properties [8]. These materials manifest some advantages such as low optical propagation loss, high chemical and mechanical stabilities as well as good compatibility with different surfaces to be coated [9,10]. Additionally, hybrid materials have low water absorption and water vapor permeability [11]. The most commonly employed preparation procedures for these materials are the use of sol–gel method. The formation of interpenetrating networks is one of the several approaches, which is used in design of hybrid systems. Difficulties of such an approach are potential incompatibilities between the moieties, leading to phase separation. In addition a major problem arises from the different stabilities of the materials [12]. However, the class of true hybrids in which mutual chemical bonds between organic and inorganic polymeric systems are formed can solve

G. Bayramo˘glu et al. / Progress in Organic Coatings 57 (2006) 50–55

this problem and presents uniform mixing at microscopic level with phase continuity [13]. Recently, combining silica groups into epoxy backbone for the formation of dual hybrid systems became an attractive way to significantly improve the electrical, thermal and flame retardance properties of the epoxy resins [14–19]. The thermal stability of the organic–inorganic hybrid network material based on a UV-curable epoxy acrylate resin can be improved with increasing silica content. The excellent thermal stability of silicon-containing epoxy compounds satisfies the requirements for application in advanced electronics [14,15]. UV-curable hybrid coatings represent a major class of hybrid materials. This kind of formulations consist of one or more than one photosensitive organic groups, usually unsaturated C C bonds, which can be polymerized under UV radiation. The organic oligomer can be functionalized with pending functional groups forming a second network [18]. Hydrolysis and condensation reactions of the inorganic part and photopolymerization of the organic moieties lead to a glass-like material at room temperature. In this view, the present study deals with preparation of novel bifunctional resin prepared by the reaction between UV-curable epoxy acrylate oligomer and 3-isocyanatopropyl trimethoxysilane (IPTMS). The hybrid networks are characterized by analysis of various properties such as hardness, gloss, tape adhesion test and stress–strain test. The thermal behavior of coatings is also evaluated. 2. Experimental

51

2.2. Characterization FT-IR spectrum was recorded on a Shimadzu 8303 FT-IR Spectrometer. To evaluate the coating properties of cross-linked films, the coating formulations were coated on Plexiglas® panels using a 30 ␮m applicator and cured in a bench type UV processor (EMA, 120 W/cm medium pressure mercury UV lamps). Contact angle measurements of hybrid films coated on Plexiglas panels were determined by using Kr¨uss GmbH DSA 10 Mk2 goniometer and ultra pure water. The coating properties were measured in accordance with the corresponding standard test methods as indicated. This includes gloss (ASTM D-523-80), pencil hardness (ASTM D3363), tape adhesion (ASTM D-3359), pendulum hardness (DIN 53157). MEK rub tests (ASTM D-5402) were performed to check the through cure of the coatings. Mechanical properties of the UV-cured free films were determined by standard tensile stress–strain tests to measure the modulus (E), ultimate tensile strength (δ) and elongation at break (ε). Standard tensile stress–strain experiments were performed at room temperature on a Materials Testing Machine Z010/TN2S, using a cross-head speed of 5 mm/min. Thermogravimetric analyses (TGA) of the UV-cured free films were performed using a Perkin-Elmer Thermogravimetric analyzer Pyris 1 TGA model. Samples were run from 30 to 750 ◦ C with a heating rate 10 ◦ C/min under air atmosphere. GC–MS analyses of hybrid films were obtained by Thermo Finnigam Trace GC Ultra and Thermo DSQ.

2.1. Materials 2.3. Synthesis of resin DGEBA epoxy resin (Epon 828, epoxy group content: 5260–5420 mmol/kg) kindly supplied by Izomas Shell-Turkey. Henkel, Turkey provided acrylic acid. Triphenylphosphine (TPP) was purchased from Fluka. 3-Isocyanatopropyl trimethoxysilane (IPTMS, 99%) was kindly supplied by Wacker. Dibutyl tin dilaurat (DBTDL) was provided by Merck. Hexanedioldiacrylate as reactive diluent (HDDA) was supplied by AGI Corporation. Photomer® -4006F and photoinitiator (Irgacure184) were provided by Cognis and Ciba Specialty Chemicals, respectively. All reagents were used as received. Plexiglas panels (75 mm × 150 mm × 0.82 mm) were used as substrates in all coating applications.

2.3.1. Preparation of epoxy acrylate oligomer (EA) Epon-828 (100 g) and triphenylphosphine (0.2 g) were charged into a three-necked 500 mL round-bottom flask, equipped with a nitrogen inlet and a dropping funnel. The mixture was stirred for 30 min and heated to 70 ◦ C under nitrogen. Acrylic acid (34.14 mL) was then added dropwise. After the addition was completely charged, the reaction mixture was kept at 80 ◦ C during 4 h. The product was clear viscous liquid. To reduce the viscosity of the neat resin 20 wt.% HDDA (27.6 mL) was added and stirred well. A representation of this reaction is shown in Scheme 1.

Scheme 1.

52

G. Bayramo˘glu et al. / Progress in Organic Coatings 57 (2006) 50–55

Scheme 2.

2.3.2. Preparation of alkoxysilane modified epoxy acrylate oligomer (MEA) MEA was prepared through reacting EA and IPTMS with various feeding ratios. The preparation of MEA-30 is given as an example. 12.72 g EA (80:20 EA:HDDA, wt.%) was charged into a flame-dried three-necked, 500 mL round-bottom flask, equipped with a nitrogen inlet and a dropping funnel. DBTDL (0.1 wt.%) was added into the reaction flask as a catalyst and the system was heated to 40 ◦ C. Then, IPTMS (3.32 g, 0.016 mol, equivalent to 30 wt.% of theorical hydroxyl content of EA) was added dropwise to the well-stirred reaction mixture. Reaction mixture was kept at 40 ◦ C for 3 h. Disappearance of the characteristic –NCO peak at 2275 cm−1 in the FT-IR spectrum confirmed that the reaction was completed. 20 wt.% HDDA (3.2 mL) was added to product and stirred well. The preparation of modified epoxy acrylate (MEA) is shown in Scheme 2. 2.3.3. Preparation of hybrid coatings Modified epoxy acrylate containing UV-curable formulations were prepared by mixing (MEA) 70 wt.%, Photomer® 4006F (27 wt.%) and photoinitiator (3 wt.%) The composition of formulations is given in Table 1. Each formulation was prepared in a beaker with adequate stirring. In order to remove air bubbles formed during mixing, the beaker content heated to 40 ◦ C was kept under vacuum for 3 min. After homogenization, the prepared formulations were coated on to Plexiglas panels using a wire-gauged bar applicator obtaining a layer thickness of 30 ␮m. Before coating application, Plexiglas panels were cleaned using 99% pure methanol after removal of the protective temporary Table 1 Composition of hybrid systems Sample

MEA-0 MEA-10 MEA-20 MEA-30 MEA-50

IPTMS content (%)

Coating formulations (10 g) Resin (g)

Photomer® 4006F (g)

Photoinitiator (g)

0 10 20 30 50

7 7 7 7 7

2.7 2.7 2.7 2.7 2.7

0.3 0.3 0.3 0.3 0.3

foil. The applied wet coatings were hardened by three pass in a UV processor cabinet equipped with a medium pressure mercury lamp (120 W/cm) situated 15 cm above the speed controllable moving belt. The speed of the belt is kept at 2 m/min. After UV curing was successfully achieved, coated panels were immersed in distilled water at 40 ◦ C and remained for 2 h to facilitate the hydrolysis. Then the panels were annealed at 80 ◦ C for 6 h and then stored at room temperature for 1 day. Via annealing, postcuring of silanol groups were performed. Hybrid free films were prepared by pouring the light sensitive viscous liquid formulations on to a surface modified glass mold (10 mm × 50 mm × 1 mm). In order to prevent the inhibiting effect of oxygen, the resin in the mold was covered by a transparent 100 ␮m thick TeflonTM film before irradiation with a high pressure UV-lamp (OSRAM, 300 W). A quartz glass plate was placed over. The TeflonTM film was used to obtain a smooth surface. After 60 s irradiation under UV-lamp a 100 ␮m thick free hybrid films were obtained. 3. Results and discussion The aim of this work was to improve the physical and mechanical properties of UV-curable epoxy acrylate (EA) protective coatings. For this purpose, 3-isocyanatopropyl trimethoxysilane (IPTMS) was used to modify the EA resin. From 0 to 50 wt.% of hydroxyl groups of EA resin was capped reacting with IPTMS systematically to obtain alkoxysilane group via urethane linkages (Scheme 2). The conversions of the reactions were monitored by FT-IR measurement. As can be seen from Fig. 1, the disappearance of the characteristic absorption band at 2275 cm−1 assigned to the isocyanate group of IPTMS indicated the completion of the reaction. It also shows the characteristic N–H stretching band at 3335 cm−1 , carbonyl stretching band at 1726 cm−1 and Si–O–CH3 band at 1083 cm−1 . In Table 1, the compositions of the UV-Curable coating formulations based on a epoxy acrylate oligomer with pendant alkoxysilane are summarized. A total of five samples with different compositions were prepared and characterized. Each result reported in this paper is an average of four separate measurements. Various physical and mechanical characteristics of hybrid coatings before and after annealing is given in Table 2. The sol-

G. Bayramo˘glu et al. / Progress in Organic Coatings 57 (2006) 50–55

53

Fig. 1. FT-IR spectrum of MEA-50.

Fig. 2. Influence of modification ratio on the water contact angles of hybrid coatings.

vent resistance of coatings was examined by performing MEK rubbing test. Hybrid coatings were unaffected after 500 double rubs. In all cases chemical resistance is excellent, exceeding 500 MEK double rubs while pencil hardness is greater than 4H, also indicative of highly cross-linked film. However, hybrid coatings do not show good adhesion on Plexiglas panels. When tape adhesion is tested, around 70% adhesion loss is observed. In order to investigate the surface property of hybrid materials water contact angle measurements were performed. Each contact angle value given in Fig. 2 represents an average of 8–10 readings. As can be seen in Fig. 2, the epoxy acrylate coating with 10% modification has a fairly polar surface and shows a water contact angle of 66.2 ± 6.4◦ . Upon 50% of modification with 3-isocyanatopropyl trimethoxysilane (IPTMS), the contact angle shows a shift to 72.5 ± 5.45◦ . This is an expected behavior assuming that pendant triethoxysilanes make the surface more hydrophobic. The contact angle value of uncoated Plexiglas panel was 63.8 ± 2.0◦ . The hydrophobicities of uncoated panels and of the coating are close to each other. Therefore, one can estimate that the surface polarity is not responsible for the poor adhesion. It is assumed that the polymerization of acrylate groups is responsible for the shrinkage of the coated film and this probably played a role in poor adhesion. The hardness of the coating is the most important factor affecting the abrasion and scratch resistance. Hard coatings give

better scratch resistance, whereas abrasion resistance is also affected by surface friction [20]. Chain flexibility and crosslinking degree of the network play a major role in the value of hardness [21]. The hybrid resin used to coat Plexiglas panels, has an increased hardness of about 5–9%, depending onto coating formulation and annealing time. The increase in the hardness can be attributed to alkoxysilane pendant group’s hydrolysis and post-curing via condensation by annealing. Coating’s gloss is a complex phenomenon resulting from the interaction between light and the surface of the coating [22]. In Table 2, the influence of post-curing on the gloss can be clearly seen. Unmodified epoxy acrylate (EA) coatings are not affected by annealing. Upon annealing at 80 ◦ C for 6 h, the gloss of MEA10, MEA-20 and MEA-30 coatings shifts to higher values. The increased gloss may be due to the post-curing of silanol groups, which resulted in more uniform film surface. On the contrary the gloss of MEA-50 is slightly decreasing with annealing period. This result shows that after a critical point, increasing the SiO2 content diminishes the gloss. Evaluated stress–strain data of hybrid coatings as Young’s modulus, ultimate tensile strength and elongation at break are listed in Table 3. As can be seen in Table 3, the ultimate tensile strength of pure EA coating is 29 MPa. The coating has a very low elongation value (0.8%). Test samples were broken without yield. This result reveals that the EA coating is hard and

Table 2 Physical and mechanical characterizations of hybrid coatings on Plexiglas panels Sample

MEA-0

Modification ratio (%)

0

MEA-10

10

MEA-20

20

MEA-30

30

MEA-50

50

Post-cure (h)

0 6 0 6 0 6 0 6 0 6

Pendulum hardness (k¨onig)

MEK rubbing test

99 99 102 105 101 104 103 105 102 103

>500 >500 >500 >500 >500 >500 >500 >500 >500 >500

Tape adhesion loss (%)

Gloss 20◦

60◦

80 80 70 70 70 70 70 70 70 70

190 190 214 223 197 223 209 216 218 217

182 182 189 194 183 194 192 193 194 193

Pencil hardness >4H >4H >4H >4H >4H >4H >4H >4H >4H >4H

54

G. Bayramo˘glu et al. / Progress in Organic Coatings 57 (2006) 50–55

Table 3 Strain–stress analysis Sample

IPTMS (%)

Yield strength (Mpa)

Tensile strength (Mpa)

Elongation at break (%)

Modulus (Mpa)

MEA-0 MEA-10 MEA-20 MEA-30 MEA-50

0 10 20 30 50

– – – 38 39

29 38 42 36 33

0.8 0.9 0.8 2.0 2.3

3437 3739 4342 4243 4513

Fig. 3. GC–MS chromatograms of 10 wt.% modified hybrid coating at 125 ◦ C.

brittle. In the case of a hybrid coating with 20 wt.% of modification (MEA-20), an increase in tensile strength by a factor of 1.45 with respect to the pure EA coating was observed. Young’s modulus also shows an increase by a factor of 1.26. However, the variation in elongation at break is almost negligible. As the modification percentage of oligomer increases a yield appears. In addition the elongation at break and modulus increase moderately. These results demonstrate that the hybrid coatings with higher inorganic content behave as hard and strong. The isothermal GC–MS studies were performed at 125 ◦ C for all hybrid coatings to investigate qualitatively the volatile species at low temperature. No thermal degradation product can be found at 100 ◦ C and below. Fig. 3 displays the GC–MS spectrum

recorded for 10 wt.% of hybrid coating at 125 ◦ C (MEA-10). The GC–MS analyses showed that some unreacted photoinitator and reactive diluent are found as degradation products. The presence of peaks at 15.75 min (m/z: 99, cyclohexanol) derived from cleavage of the Irgacure 184, proved this occurrence. In addition, peak corresponding to fragments of acrylate esters (m/z: 55, at 16.40 min and m/z: 44, at 23.06 min) can also be detected. The thermal properties of the hybrid coatings were also characterized by TGA in air atmosphere. Fig. 4 shows the TGA thermograms of the hybrids and the results are collected in Table 4. All samples show a small weight loss around 125 ◦ C implying the release of volatile degradation products such as unreacted photoinitator and reactive diluent. One can see that all

Table 4 TGA analysis of hybrid networks Sample

Modification ratio (%)

10% weight loss temperature (◦ C)

50% weight loss temperature (◦ C)

Final weight loss temperature (◦ C)

Residue (%)

MEA-0 MEA-10 MEA-20 MEA-30 MEA-50

0 10 20 30 50

371 350 342 350 339

442 425 425 433 438

649 664 666 675 692

– 1.7 2.4 3.7 5.9

G. Bayramo˘glu et al. / Progress in Organic Coatings 57 (2006) 50–55

55

other properties such as gloss, tensile strength, and modulus are also improved. The adhesion of coating onto the Plexiglas substrate is poor. The thermal stability of hybrids is also higher than that of the pure epoxy acrylate resin. The enhancement of the mechanical and thermal properties was correlated with an increase in cross-linking density of the system. Acknowledgements The authors are pleased to acknowledge Wacker and Basf for their kind support for chemicals and Sabanci University for their kind support in chemical analysis. This work was supported by Marmara University, Commission of Scientific Research Project under grant FEN-043/030303, 2003. References

Fig. 4. Weight loss vs. temperature as a function of modification percentage. (×) MEA-0; (-·-) MEA-10; (- - -) MEA-20; (—) MEA-30; (· · ·) MEA-50.

samples exhibited a 10% weight loss at around 350 ◦ C followed by a rapid loss over the temperature range 390–440 ◦ C. It was clearly observed that thermal stability of hybrid networks was improved compared with pure epoxy acrylate network. Thermogravimetric analyses showed that (Table 4) with increasing the silica content, final weight loss temperature shifted to higher values. The delay in degradation caused by the siloxane moiety may be attributed to the steadiness of inorganic structure, which may stabilize the epoxy acrylate resin. The weight loss appearing between 650 and 700 ◦ C is probably due to the further oxidation of siloxanes. In addition, hybrid networks gave higher char yield than pure epoxy acrylate network. The char yield is 6% at 750 ◦ C for 50 wt.% modified hybrid coating. The presence of residual char can be used as a preliminary indicator of flame resistance. 4. Conclusion In this paper a series of UV-curable organic–inorganic hybrid coatings were prepared based on a bifunctional oligomer bearing diacrylate and trimethoxysilane functionalities. Incorporation of trimethoxysilane functionalities into the organic coating improves the hardness. Upon increasing inorganic content, the

[1] T.Y. Inan, E. Ekinci, A. Kuyulu, A. G¨ung¨or, Polym. Bull. 47 (2002) 437–444. [2] S.P. Pappas, UV Curing: Science and Technology, Technology Marketing Corporation, Stanford, USA, 1978. [3] N.A. Kayaman, R. Demirci, M. C¸akır, A. G¨ung¨or, Radiat. Phys. Chem. 73 (2000) 254–262. [4] UV-Curable Coatings—Generic Testing and Quality Assurance Protocol, Prepared by National Defense Center for Environmental Excellence (NDCEE), Operated by Concurrent Technologies Corporation March 24, 1998. [5] J.W. Kim, K.D. Suh, Colloid Polym. Sci. 276 (4) (1998) 342–348. [6] L.W. Arndt, L.J. Junker, S.P. Patel, D.B. Pourreau, W. Wang, Proceedings of the 80th Annual Meeting of Federation of Societies for Coatings Technology, October 30 to November 1, 2002, 2343-VI-1202. [7] P. Judenstein, C. Sanchez, J. Mater. Chem. 6 (1996) 511. [8] M. Mager, L. Schmalstieg, M. Mechtel, H. Kraus, Macromol. Mater. Eng. 286 (11) (2001) 682–684. [9] X. Zhang, H. Lu, A.M. Soutar, Singapore Institute of Manufacturing Technology (SIMTech) Technical Reports, 2003 (STR/03/072/ST). [10] D.S. Yu, J.C. Han, L. Ba, Am. Ceram. Soc. Bull. 81 (9) (2002) 38–39. [11] K.H. Haas, K. Rose, Rev. Adv. Mater. Sci. 5 (2003) 47–52. [12] G. Kickelbick, Prog. Polym. Sci. 28 (2003) 83–114. [13] O. Soppera, C. Croutxe-Barghorn, C. Carre, D. Blanc, Appl. Surf. Sci. 186 (2002) 91–99. [14] C.S. Wu, Y.L. Liu, Y.S. Chiu, Polymer 43 (2002) 4277–4288. [15] W.J. Wang, L.H. Perng, G.H. Hisue, F.C. Chang, Polymer 41 (2000) 6113. [16] G.H. Hisue, W.J. Wang, F.C. Chang, J. Appl. Polym. Sci. 70 (1877). [17] W.-K. Chin, M.D. Shau, W.C. Tsai, J. Polym. Sci. Part A: Polym. Chem. 33 (1995) 373–379. [18] C.L. Chiang, C.C.M. Ma, F.-Y. Wang, H.-C. Kuan, Eur. Polym. J. 39 (2003) 825–830. [19] C.L. Chiang, C.C.M. Ma, Eur. Polym. J. 38 (2002) 2219. [20] S. Karatas¸, Z. Hos¸g¨or, Y. Mencelo˘glu, A.N. Kayaman, A. G¨ung¨or, J. Appl. Polym. Sci. (2006), in press. [21] M.V. Kahraman, M. Ku˘gu, Y. Mencelo˘glu, A.N. Kayaman, A. G¨ung¨or, J. Non-Cryst. Solids 352 (2006) 2143–2151. [22] M.D. Soucek, Prog. Org. Coat. 36 (1999) 89.