Progress in Organic Coatings Preparation and

Progress in Organic Coatings 65 (2009) 49–55

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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Preparation and characterization of phosphine oxide containing organosilica hybrid coatings by photopolymerization and sol–gel process Sevim Karatas¸, Zuhal Hos¸gör, Nilhan Kayaman-Apohan, Atilla Güngör ∗ Department of Chemistry, Marmara University, 34722 Göztepe/Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 17 May 2008 Received in revised form 16 September 2008 Accepted 21 September 2008 Keywords: Phosphorus Flame retardant UV-coating Sol–gel Photopolymerization

a b s t r a c t A series of UV-cured organic–inorganic hybrid coating materials containing up to 20 wt.% silica were prepared by sol–gel method from tetraethoxy silane (TEOS) which is used as the primary inorganic precursor, and diallylphenylphosphine oxide monomer (DAPPO), aliphatic urethane diacrylate resin (Ebecryl 210) are employed as the source of the organic components. In addition, methacryloxypropyltrimethoxy silane (MAPTMS) was used as both a secondary inorganic source and a silane-coupling agent to improve the compatibility of the organic and inorganic phases. The DAPPO content in all the coating formulations were from 0 to 20 wt.%. The physical and mechanical properties such as gel content, hardness, adhesion, gloss, contact angle as well as tensile strength were measured. These measurements revealed that all the properties of the hybrid coatings improved effectively, in case of adding the sol–gel precursor and DAPPO monomer content in the hybrid systems. The photo-calorimetric-DSC studies showed that the double bond conversion of the hybrid coatings was faster than the coating materials without silica. The thermal stabilities of the UV-cured hybrid materials were investigated by thermogravimetric analysis. The results showed that the addition of sol–gel precursor and DAPPO into the organic network also improves the thermal-oxidative stability of the hybrid coating materials. The surface morphology was also characterized by scanning electron microscopy (SEM). SEM studies indicated that inorganic particles were dispersed homogenously throughout the organic matrix. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In recent years, organic–inorganic hybrid materials have been extensively investigated as a promising advanced materials due to the fact that they combine both the advantages of organic polymers; ease of processing, good impact resistance, flexibility, etc. and inorganic materials; high mechanical strength, transparency, good chemical resistance and thermal stability, etc. These materials have been employed in a variety of applications such as optics, electronics, mechanics, sensors, membranes and specialty coatings [1–3]. The sol–gel process is the most commonly used method for the preparation of organic–inorganic hybrid materials at macro- or micro-scale, even at molecular level in mild conditions. It involves a series of hydrolysis and condensation reactions starting from a hydrolysable multi-functional alkoxysilane as precursor for the inorganic domain formation. Properties of the resulting hybrids heavily rely on the distribution of inorganic nanoparticles within the organic matrix. The use of suitable coupling agent provides

∗ Corresponding author. Tel.: +90 216 346 45 53/1379; fax: +90 216 347 87 83. E-mail address: atillag [email protected] (A. Güngör). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.09.022

bonding between the organic and the inorganic phases, which are linked by chemical covalent bonds, hydrogen bonds or physical interaction, therefore, well-dispersed nanostructured phases may result [4–7]. Curing with UV irradiation method has been widely employed in sol–gel processing of hybrid coatings bearing a photopolymerizable organic functionalities, for instance, vinyl, epoxy and acrylate groups. The main advantages of UV-curing technique are high-speed process, lower energy consumption as well as the curing process occurs at ambient temperature, lower process costs, high chemical stability and environmental friendliness by avoiding solvent exposure. In addition to above mentioned advantages, this technology can also meets the new requirements for traditional and advanced applications, since it can offer a broad range of final properties by modest changes within the formulation and the curing conditions [8–11]. Numerous UV-curable organic–inorganic hybrid materials based on silica prepared by sol–gel process have been reported in the literature. Zou et al. reported the preparation of UV-curable organic–inorganic hybrid materials based on hyperbranched polyester. The hybrid materials exhibited good thermal stability and morphology, mechanical properties. It was also found that the organic phase of the hyperbranched polyester-based hybrids

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S. Karatas¸ et al. / Progress in Organic Coatings 65 (2009) 49–55

showed much better compatibility with inorganic phase compared to linear polyester-based hybrids [12]. Chiang and Ma studied silicon and phosphorus-containing epoxy-based nanocomposites prepared by sol–gel method [13]. It was reported that the presence of silicon and phosphorus in the hybrid networks enhances flame resistance. Kim et al. studied the effects of acrylate functional polydimethylsiloxane (AF-PDMS) on the surface properties of the UV-curable polyester acrylate-based coating [14]. It was found that the pencil hardness, gloss, solvent resistance of the coating material increased with increasing AF-PDMS content in the UV-curable formulation. The photo-DSC experiments revealed that the photopolymerization of UV-curing materials containing AF-PDMS in air atmosphere is strongly inhibited by the oxygen compared to the UV-curing without AF-PDMS. Zhang et al. investigated the effects of the HCl concentration and silica content on morphology and mechanical properties of UV-curable acrylated polyurethane/silica hybrid materials [15]. It was found that the interfacial interaction between organic and inorganic phases was strengthened with the increase of HCl content. In addition, it was demonstrated that the hardness of the hybrid materials increased with increasing acid and silica content. Xu et al. synthesized organic–inorganic hybrid urethane methacrylate hydrolytic condensate (HUA-HC) from its prepolymer (HUA) by sol–gel method with low concentration of acid catalyst [16]. It was found that the thermal stability and abrasion resistance of the UV-cured HUA-HC films were higher than the cured HUA films due to the reinforcement by inorganic Si–O–Si linkages in the HUA-HC system. Polymers containing phosphine oxide moieties provide good flame resistance and excellent adhesion properties. It has been proven that the phenyl phosphine oxide moiety provides a strong interacting site for imparting miscibility with several systems [17,18]. In the present study, sol–gel material-derived UV-curable organic–inorganic hybrid coating compositions were prepared from an aliphatic urethane diacrylate resin (Ebecryl® 210) and the diallylphenylphosphine oxide (DAPPO) monomer synthesized by Grignard reaction as an organic source and silanealkoxide compounds (tetraethoxysilane (TEOS) and (3-methacryloxypropyl) trimethoxysilane (MAPTMS)) as the inorganic sources. The effects of DAPPO monomer and silica content on the physical, mechanical and thermal properties of the UV-cured hybrid coatings were systematically investigated. Moreover, the nanostructure of the hybrid materials was examined. 2. Experimental part 2.1. Materials Ebecryl® 210 (aromatic urethane diacrylate resin, average molecular weight Mw = 1500 g mol−1 ) was supplied by Akzo-Nobel. Dichlorophenylphosphine oxide, allyl bromide, vinyl trimethoxysilane (VTMS) and magnesium were purchased from Merck. N-vinyl-2-pyrrolidone was obtained from Aldrich. TEOS, purchased from Merck and MAPTMS donated from Wacker were used as inorganic precursor. p-Toluenesulfonic acid used as an acid catalyst was purchased from Fluka. Photomer® 6230 as reactive diluent was supplied by Cognis. Irgacure 184 (1-hydroxycyclohexyl phenyl ketone) as a photoinitiator was provided by Ciba Specialty Chemicals. BYK331 as a wetting agent was obtained from BYK. Tetrahydrofurane (THF) provided by Merck was dried over sodium wire and distilled before used. The other common solvents and chemicals were used as received. Plexiglass® panels (75 mm × 150 mm × 2 mm) provided by Birles¸ik Akrilik Sanayii (Turkey) were used as substrates in all coating applications.

2.2. Characterization The chemical structure of synthesized monomer was identified by FT-IR method. The FT-IR spectrum was recorded on a Schimadzu 8303 FT-IR spectrometer. In order to assess the coating performance of the hybrid coating compositions, each formulation was applied on Plexiglass® panels using a 30 ␮m applicator and cured in a bench type UV processor (EMA-Turkey, 120 W/cm medium pressure mercury UV-lamps). The coating properties were measured in accordance with the corresponding standard test methods as indicated; gloss (ASTM D-523-80), cross-cut (DIN 53151) and pendulum hardness (DIN 53157). The recorded values were the average of four measurements. The gel contents of the UV-cured hybrid films were determined by Soxhlet extraction method for 4 h employing acetone. To evaluate the thermal behaviour of the coatings, thermogravimetric analysis (TGA) was performed using a NETZSCH STA 409C model thermogravimetric analyzer. The samples were run from 20 to 800 ◦ C with a heating rate of 10 ◦ C/min under nitrogen atmosphere. Mechanical properties of the free films were determined by standard tensile stress–strain tests in order to measure the ultimate tensile strength (ı), the modulus (E) and elongation at break (ε). Stress–strain measurements were carried out at room temperature by using a universal test machine (Zwick Rolle, 500 N) with a crosshead speed of 3 mm/min. The measurements represent the average of at least five runs. Scanning electron microscopy (SEM) was performed on a JOEL JSM-5910 LV to investigate the morphologies of the hybrid coatings. The solid state Si-cross-polarization (CP)/magic-angle-spinning (MAS) NMR spectra were recorded using a Varian Unity Inova Spectrometer operated at 500 MHz frequency. Contact angle measurements were carried out with a Krüss FM41 instrument, equipped with a camera. Analyses were conducted at room temperature by means of the sessile drop technique. For each sample, at least three measurements were made and the average was taken. The measuring liquid was distilled water. The photo-DSC experiments were conducted using Pyris Diamond DSC equipped with UV spotcure system (EXFO Omni-CureTM 2000). A UV spotcure system was employed for irradiating the samples. Approximately 150 mg sample was placed in an aluminum pan, which was covered with a clear quartz disc. Filtered light (250–450 nm) with an intensity of 20 mW cm−2 was used. Heat flow vs. time curves was recorded in an isothermal mode under a nitrogen flow of 20 mL min−1 at 30 ◦ C. The heat liberated during the polymerization reaction was directly proportional to the number of C C double bonds (allylic and acrylic) in the system. By integrating the area under the exothermic peak, the conversion of the double bonds (C) could be calculated by: C=

Ht H0theor

(1)

where Ht is the heat evolved at time t, and H0theor is the theoretical heat for complete conversion. 2.3. Synthesis of diallylphenylphosphine oxide (DAPPO) DAPPO was synthesized by the reactions of allylmagnesium bromide and dichlorophenlyphosphine oxide according to the literature [19]. Firstly, allylmagnesium bromide was prepared from allylbromide and magnesium turnings via Grignard technique. Briefly, 20.85 g (0.858 mol) magnesium turnings and 250 mL of


dried THF were charged into a flame dried 500 mL three-neck round bottom flask equipped with a mechanical stirrer, a dropping funnel, a condenser and a nitrogen inlet. Then, the flask was cooled with an ice bath to 0–5 ◦ C and 24.5 mL (0.286 mol) of allyl bromide was slowly added into the flask. After the addition was completed, the reaction mixture was refluxed gently for 3 h. In the second step, 20 mL (0.143 mol) of dichlorophenylphosphine oxide was added dropwise into the allylmagnesium bromide solution and stirred at the room temperature under nitrogen over night. Finally, the crude product (DAPPO) was hydrolyzed with 10% sulfuric acid solution, washed with distilled water and then neutralized with 10% NaHCO3 solution and re-washed with distilled water several times, respectively. A light yellow product was obtained in a yield about 55%. IR (KBr pellet, cm−1 ): 3050 (aromatic –CH), 2958 (aliphatic –CH), 1639 (C C), 1439 (P–Ph), 1265 (P O). 1 H NMR (CDCl3 ): ı 7.8–7.3 ppm (O P–C6 H5 , 5H, multiple), 5.6 ppm (–CH2 –CHa CH2 –, 2H, multiple), 5.4–5.0 ppm (–CH2 –CH CHa Hb –, 4H, each of Ha and Hb doublet of doublets), 2.7 ppm (–CH2 –CH CH2 –, 4H, multiple). 31 P NMR: 42.0 ppm.

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Scheme 1. Synthesis of diallylphenylphosphine oxide monomer (DAPPO).

2.4. Preparation of the silane precursor The precursor sol was prepared by using TEOS (5.2 g, 0.025 mol) and MAPTMS (12.42 g, 0.05 mol) as the precursor alkoxides, ethanol (EtOH) (2.3 g, 0.05 mol) as solvent, distilled water (3.15 g, 0.175 mol) and p-toluensulfonic acid (0.062 g) as a catalyst for hydrolysis. Initially, TEOS, MAPTMS and EtOH were mixed into a 25 mL glass vial and then water which had been acidified by p-toluensulfonic acid was added dropwise into the vial under stirring. The whole mixture was allowed for partial hydrolysis approximately for 5 h at room temperature. The pH of the silane precursor was adjusted to be between 4 and 5. 2.5. Pretreatment substrate The Plexiglass® panels were cleaned using acetone after removal of the protective foil. In order to enhance the adhesion between the coating material and substrate, the Plexiglass® panels were treated with oxygen plasma designed by Ugur Electronic UD-600 Corona Generator under atmospheric pressure at a power 1.5 kW at room temperature before applying the coating materials. 2.6. Preparation of the hybrid coating formulations UV-curable formulation, which is named as control formulation (CF), was prepared by mixing the calculated amounts of aromatic urethane diacrylate resin (Ebecryl® 210), vinyl trimethoxysilane, Irgacure 184 as a photoinitiator, BYK 331 as a wetting additive, Photomer® 6230 and NVP as reactive diluents. The hybrid coating formulations were prepared mainly from the CF and 20 wt.% of the silane precursor. In addition, Ebecryl® 210 resin is replaced with DAPPO monomer with different percentages (5, 10, 15 and 20 wt.%) in order to investigate the influence on hybrid material property. The composition of all coating materials is listed in Table 1. Each composition was prepared in a 10 mL beaker and stirred until become clear and homogenous. Then, the formulations were applied onto corona-treated Plexiglass panels using a wire gauged bar applicator obtaining a layer thickness of 30 ␮m. The applied coatings were hardened by a UV processor houses a medium pressure mercury lamp [120 W/cm, max : 365 nm (320–390 nm), total lamp power = 3.24 kW] situated 15 cm above the moving belt after six pass (180 s). The speed of the processor is 2 m/min. The light dose is calculated as 720 mJ/cm2 .

In addition, the free films were prepared by pouring the prepared compositions into a TeflonTM wells (10 mm × 5 mm × 1 mm). Besides that, to prevent the inhibiting effect of oxygen, the mixture in the well was covered by transparent, 100 ␮m thick TeflonTM film before irradiation with a high pressure UV-lamp (OSRAM, 300 W). A quartz glass plate was also placed over the Teflon film in order to obtain a smooth surface and uniform thickness. After 180 s irradiation under UV-lamp, 100 ␮m thick hybrid-free films were obtained. The hybrid coated Plexiglass® and the free films were annealed at 65 ◦ C for 24 h and stored at room temperature. The hybrid coated Plexiglass® and the free films were annealed around at 65 ◦ C for 24 h in order to achieve high degree cross-linking via condensation of remaining silanol groups. 3. Results and discussion The main goal of this work was to investigate the effects of phosphorus and silica coated on the physical and mechanical properties of UV-curable protective hybrid coatings. For this purpose, the phosphine oxide containing monomer (DAPPO) was synthesized from allylmagnesium bromide and dichlorophenylphosphine oxide as shown in Scheme 1. The FT-IR and 1 H NMR spectrum of DAPPO were confirmed the formation of the expected structure. UV-curable organic–inorganic hybrid-based coatings on Plexiglass and free films were prepared by sol–gel method. The oligomer based on aromatic urethane diacrylate and diallylphenylphosphine oxide monomer were used as organic part, whereas inorganic part was composed of MAPTMS and TEOS. MAPTMS acts as coupling agent between the organic and inorganic phases, which is crucial for the formation of the nanocomposite. A set of samples existing of eight different compositions were prepared and characterized as can be seen in Table 1. Which first four samples in the set contain varying amounts of diallylphenylphosphine oxide monomer, in the range of 5–20 wt.%. Other four samples contain fixed amount silane precursor on the phc (per hundred compositions) basis. The detailed flow chart of the preparation procedure of UV-curable organic–inorganic hybrid coating was shown in Scheme 2. In order to evaluate cross-link density and related properties, soluble fractions extractable in UV-cured free films were removed with acetone. As seen in Table 2, insoluble content of the polymeric films were found to be between 90 and 95 wt.%. This result can provide additional information curing degree of the coatings.

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Table 1 The composition of UV-curable coating materials. Samples

CF CF-P:5 CF-P:10 CF-P:15 CF-P:20 CF-P:5/Si:20 CF-P:10/Si:20 CF-P:15/Si:20 CF-P:20/Si:20

Base formulation (BF) (g) Ebecryl 210 (wt.%)

Ph6230 (20%)

BYK331 (g)

IRQ 184 (3%)

NVP (10%)

TMVS (2%)

65 60 55 50 45 60 55 50 45

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

DAPPO (wt.%)

Sol–gel content (20 wt.%) (g)

– 5 10 15 20 5 10 15 20

– – – – – 2.0 2.0 2.0 2.0

Scheme 2. Flow diagram showing procedure for preparing the UV-cured hybrid coatings.

The results of some performance tests of the coating materials such as gel content, gloss, adhesion and impact strength are also collected in Table 2. Each result reported in this paper is an average of four separate measurements performed. Hardness measurements are a quick, reliable means of quantifying the mechanical properties and performance of coatings. The hardness of coating is the most significant factor affecting the abrasion and scratch resistance. Smoothness of the coating influences the measurements and also the type of substrate, adhesion to the

substrate and heterogeneity within the coating can influence the hardness measurements. Chain flexibility and cross-linking degree of the network play a major role in the determination of hardness [20]. As shown in Table 2, addition of DAPPO into the control formulation was moderately affected the pendulum hardness. As the content of DAPPO increased from 5 to 20 wt.%, the hardness also increases. This indicates that the addition of the DAPPO made enhancive influence on the hardness of the coatings due to the increasing cross-linking density of the network system. Moreover,

Table 2 The physical properties and curing parameters of the UV-cured coating materials. Sample

CF CF-P:5 CF-P:10 CF-P:15 CF-P:20 CF-P:5/Si:20 CF-P:10/Si:20 CF-P:15/Si:20 CF-P:20/Si:20

Gel content (wt.%)

95 95 94 94 90 93 90 88 88

Gloss 20◦

60◦

136 138 140 141 141 140 144 145 141

140 142 145 142 144 146 147 147 145

Cross-cut adhesion

Pendulum hardness

Contact angle (◦ )

H (J/g)

1 1 1 0 0 0 0 0 0

23 24 27 31 43 55 51 60 76

80 78 64 67 64 82 81 81 78

195 236 216 216 210 243 264 206 202


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Fig. 2. Young’s modulus of the UV-cured coating materials.

Fig. 1. Solid state 29 Si NMR spectrum of the hybrid material with 20 wt.% DAPPO monomer content.

it was determined that the incorporation of the 20% (phc) silane precursors (TEOS and MAPTMS) greatly improved the hardness of the coating materials. This can be explained by the formation of inorganic silica network as a result of silanol condensation [21]. Coating’s gloss is a complex phenomenon resulting from the interaction between light and the surface of the coating. Specifically, the gloss is affected strongly by surface roughness in the clear coating system [22]. As shown in Table 2, the changes are negligible at both angles (20◦ and 60◦ ) for all the type of samples in comparison with control samples. It is difficult to attribute what cause this phenomenon probably more extensive test must be carried on in order to obtain solid conclusion. In order to acquire a qualitative impression of the adhesion between the coating material and Plexiglass® substrate, the crosscut adhesion test was applied according to the DIN 53151 standard. Adhesion can be classified from zero, which represents a good adhesion, to five which represents a poor adhesion [9]. As can be seen in Table 2, all coatings on Plexiglass® panels exhibited good adhesion. The cross-cut test showed that DAPPO monomer acts as a determining factor in adhesion property. This can be ascribed to the phosphine oxide moiety, which may possess good miscibility to improve the interaction between resins in the coating system [17,18]. In addition, it was thought that another reason behind good adhesion was probably corona discharge treatment of Plexiglass® panels before applying coatings [23]. Contact angles are very sensitive to the surface composition changes. The reported data in Table 2 are the average ones, taken from left and right sides of the three droplets. The contact angles of distilled water were measured immediately after the drop was settled on the hybrid coated Plexiglass surface. As can be seen from Table 2, water contact angles, measured on polymer films ranged from 64◦ to 81◦ . It is interesting to note that, addition of DAPPO monomer into coating formulation, decreases the water contact angle due to the highly polar phosphine oxide group. Besides that, incorporation of silane precursor into the coating formulation increased the water contact angles. This can be attributed to the formation of nanostructured silica inorganic network. Similar observation reported previously for poly(phosphine oxide ether ketone)s [24]. To assess the condensation reactions of the organo alkoxy silanes, a 29 Si-solid state NMR spectroscopy was employed. As can be seen in Fig. 1, four different kinds of signals were observed at −58, −64, −90 and−104 ppm for the hybrid material containing 20 wt.% DAPPO monomer. The peak at −58 ppm is assigned to T2 species [R–Si (OSi)2 –(OH)], the −64 ppm peak to T3 species [R–(OSi)3 ], the −90 ppm peak to Q2 species [Si(OSi)2 –(OH)2 ], and

the −104 ppm peak to Q3 species [Si(OSi)3 (OH)]. These signals give an indication of hydrolysis and condensation of MAPTMS and TEOS in order to form Si–O–Si bonds of the inorganic backbone. As can be seen in the figure the fraction of Q4 peak centered at 111 ppm is relatively low and this indicates incomplete condensation of inorganic network. This result is expected since the water/alkoxy silane ratio was chosen as r = 3 to obtain partial hydrolysis to overcome the problems of gelation of the inorganic part before UV-curing process. The ultimate tensile strength (), Young’s modulus (E) and elongation at break (ε) values of 10 wt.% DAPPO and 10% DAPPO–20 wt.% silane precursor containing films were compared with the control formulation. As can be seen in Fig. 2, the tensile strength of control film (CF) is 44 MPa. The coating has low elongation value (4.6%), and it was broken without yield. From this result, CF can be classified as hard and brittle material. This is due to the higher cross-linking density of the coating sample. The incorporation of DAPPO and silane precursor into the coating formulation decreased the elongation at break. Inorganic silica network act as a stress concentrators and affects the ultimate fail or point during extension. The trend in elongation is similar to that observed for tensile strength. However, Young’s modulus had different trend from that recorded for tensile strength and gradually increases by the addition of DAPPO and silane precursor. Good adhesion between organic and inorganic phases probably results in a higher Young’s modulus [9]. The thermal stability of polymeric materials are very important when they are employed for a flame retardant system, which mainly concerns the release of decomposition products and the formation of char. Fig. 3 shows the TGA curves of the UV-cured hybrid materials with varying DAPPO content and in the presence of 20 wt.% silane precursor. During the thermo-oxidative degradation in air, all the samples present a single step degradation. The evaluated data

Fig. 3. Thermal gravimetric analysis of the coating materials.

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Table 3 Thermal properties for the UV-cured coating materials. Samples

CF CF-P:10 CF-P:20 CF-P:10/Si:20 CF-P:20/Si:20 a b

Weight loss (5%) (◦ C)

280 250 240 180 160

Tmax (◦ C)

380 380 375 370 365

Char residuals (wt.%) Experimentala

Theoreticalb SiO2

6.33 7.43 9.95 14.07 15.00

– – – 12.00 14.00

Experimental results from TGA at 800 ◦ C. Theoretical values based on assumption that only inorganic moieties exist.

from these thermograms were listed in Table 3. One can see that CF begins to degrade after 100 ◦ C followed by a rapid loss around 380 ◦ C. However, addition of DAPPO into the coating formulations decreases the thermal stability at low temperatures. It is known that relatively low thermal stability of the thermosetted phosphoruscontaining polymeric coating comes from the phosphorus group degrading at relatively low temperatures [25]. The small weight losses at 100 ◦ C (5 wt.%) can be emerged from the evaporation of physically absorbed water, residual solvent and volatile organic compounds (unrelated photoinitiator, decomposed acrylic groups and remaining non-polymerized reactive diluents) [26,27]. This observation is characteristic of intumescent char formation. In contrast at higher temperatures they shows higher thermal stability

Fig. 5. (a) The conversion of the double bonds of the UV-curable coating materials and (b) the conversion of the double bonds of the UV-curable hybrid coating materials.

Fig. 4. (a) Photo-DSC exotherm curves for the coating materials based on DAPPO monomer and (b) photo-DSC exotherm curves for the hybrid coating materials containing DAPPO monomer.

and this phenomenon plays an important role in improving fire resistance of the coating. In addition while the char yield of pure organic coating 6.33 wt.% at 800 ◦ C, this value reached to 15% for 20 wt.% DAPPO. Moreover, the formulation of the inorganic Si–O–Si linkages in the hybrid systems inhibited the heat diffusion. Therefore, the decomposition temperature of organic part is lifted up towards higher values [16]. The hybrid coatings have much more residue than organic coatings. The char yields of CF-P:10/Si:20 and CF-P:20/Si:20 are 14.0 and 15.0, respectively. This is owing to the presence of both phosphine oxide and SiO2 structures in the network systems. As shown in Table 3 the experimental residuals are higher than the theoretical estimated silica values in view of the trapping of the polymer moieties in inorganic networks that confirm the strong interaction between phases [28]. The photopolymerization kinetics was investigated using photoDSC in order to clarify the photo-curing process of the various coating systems listed in Table 1. Photo-DSC experiments are capable of providing kinetics data in which the measured heat flow can be converted directly to the ultimate percentage conversion for a given amount of formulation, with the data obtained reflecting the overall curing reaction of the sample. Fig. 4a and b shows the photo-DSC exotherm curves for photopolymerization of the coating materials containing DAPPO monomer and their hybrids with 20 wt.% sol–gel precursor. Table 2 also shows the heat liberated during photopolymerization and it was varied between 195 and 264 J/g and the greatest values obtained for CF-P:10/Si:20. Enthalpy of the polymerization increases with increasing DAPPO monomer content. Fig. 5a and b shows that the double bond conversion of all the coating materials. It can be seen that the conversion also increases


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4. Conclusions A series of UV-cured organic–inorganic hybrid coatings with 20 wt.% silica content were prepared by sol–gel method. The diallylphenylphosphine oxide monomer was successfully synthesized and added into the coating system to improve the flame retardance properties. The DAPPO content in all the coating systems was ranged from 0 to 20 wt.%. The physical and mechanical properties such as gel content, hardness, adhesion, gloss, contact were examined. The results revealed that all the properties of the hybrid coatings improved effectively by gradual increase in silane precursor and DAPPO monomer content. Good adhesion between organic–inorganic phases increased the modulus. Very high char yield (18%) demonstrates that DAPPO influences the flame retardant properties of the hybrid coatings strongly. Morphological investigation showed that silica particles varied between 30 and 40 nm were homogenously distributed through the hybrid coating. Acknowledgements This work was supported by TUBITAK TBAG Project No. 106T083. The authors would like to thank Burçin Yıldiz for 29 Si NMR measurements. References

Fig. 6. (a) and (b) SEM micrographs of the hybrid material with 10 wt.% DAPPO content at various magnifications.

with increasing DAPPO (Fig. 5a). In addition, it is observed that the conversion is higher when the silane precursor was added into the resin containing DAPPO (Fig. 5b). These results can be explained by the fact that increasing the chain propagation mobility of double bonds due to the reduction of viscosity[29]. The compatibility of the organic polymer and silica network greatly affects thermal, mechanical and optical properties. The morphology of the composite films was investigated by SEM from fractured surface. Fig. 6a and b shows the SEM images of the fracture surface of the hybrid coating with 10 wt.% DAPPO monomer at various magnifications. According to these figures, the inorganic moieties were uniformly dispersed throughout the polymer matrix. As can be seen the from Fig. 6b, the silica particles have different patterns such as spherical, trigonal, hexagonal and rod. Except rod shape silica, the sizes of all particles were ranged between 28 and 40 nm. The lengths of rod shape silica varied between 100 and 120 nm as well. The formation of different shaped particles can be attributed to the fact that partial condensation was achieved during sol–gel reaction. Consequently, silica networks are restrained at the molecular level in the hybrid system. The nanocomposite coating material exhibited good miscibility between organic and inorganic phases.

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