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Progress in Organic Coatings

journal homepage: www.elsevier.com/locate/porgcoat

Fluorinated photoinitiators: Synthesis and photochemical behaviors

S. Liang, Y.D. Yang, H.Y. Zhou, Y.Q. Li, J.X. Wang⁎

School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, China

A R T I C L E I N F O

Keywords:FluorinatedPhotopolymerizationPhotoinitiatorOxygen inhibitionMigration

A B S T R A C T

Two fluorinated photoinitiators, named as 2959-IPDI-F and 2959-IPDI-F-HDDA, were successfully synthesizedand characterized. The photochemical behaviors of 2959-IPDI-F and 2959-IPDI-F-HDDA were investigatedthrough the photolysis and polymerization of 1, 6- hexanediol diacrylate. The oxygen inhibition was obviouslydecreased due to the migration of fluorinated photoinitiator to the surface, which was proved through XPS, SEM-EDS. The higher concentration of photoinitiator at top layer accelerated the polymerization rate and reduced theoxygen sensitivity. A fluorine element enriched and folded surface could be developed only through simplephotopolymerization initiated by fluorinated photoinitiators.

1. Introduction

Due to the advantages of high speed curing, high efficiency, solventfree formulations, and low energy consumption, photopolymerizationhas garnered growing interest in both academia and industrial appli-cations such as in coatings, inks, adhesives, printing plates, dentalmaterials, etc. [1–8]. The formulation for UV curing contains UV cur-able oligomer or resin, diluent, photoinitiator and other functionaladditives. As the key component in photo-curing formulation, photo-initiators requires desirable qualities such as high absorptivity in thespectral region of lamp emissions, good solubility during formulation,high quantum yield, high photo reactivity, low odor, low toxicity, noyellowing with presence of migrating residues in the polymer, and goodstorage stability [9–12].

Generally, open-air curing is the simplest and cheapest method inmost industrial processing. The problem of oxygen inhibition iscommon in applications of free radical photopolymerization [13–16].The existent oxygen reacts with initiating radicals to form less reactiveperoxy radicals, which will reduce the polymerization rate or eventerminate the polymerization. To overcome this drawback, alternativeformulation with less sensitive or insensitive to oxygen are available, inwhich some component may capture oxygen to abate the oxygen in-hibition effect [17]. Oxygen inhibition could also be reduced by usinghigh intensity light or higher concentration of photoinitiator [18]. Al-though higher concentration of photoinitiator can reduce oxygen in-hibition, it also results in light shielding, volume shrinkage, especiallyhigher industrial cost. The sample thickness, initiation rate and oxygenconcentration would affect the photopolymerization kinetics [19]. Dueto the low surface energy and easy migration to the surface, the

fluorinated monomer and resin have been used in UV curing formulasto tune the photo polymerization behaviors and the surface properties[20–25]. The fluorinated photoinitiators have been reported [26], butonly few compounds can initiate the photopolymerization with lessoxygen inhibition. Nie et al. has modified the commercial photoinitiator2-hydroxy-2-methylpropiophenone (1173) and 2-hydroxy-1-(4-(2-hy-droxyethoxy) phenyl) −2-methylpropan-1-one (Irgacure 2959) withpentadecafluorooctanoyl chloride (PFOC) to prepare the fluorinatedphotoinitiator F-1173 [27] and F-2959 [28]. In order to find more ef-ficient photoinitiator, it is strongly necessary to explore more fluori-nated photoinitiator for UV curing application.

In our previous study, a novel polymerizable HMPP-type photo-initiator with carbamate was prepared by the reaction of NCO ofIsophorone diisocyanate with primary hydroxyl of Irgacure 2959 and 2-Hydroxyethyl methacrylate (HEMA) [29]. As part of our continuousinterest in developing unique photoinitiator, an amine modified Irga-cure 2959 was prepared by reaction of Irgacure 2959 with Isophoronediisocyanate (IPDI), and then following hydrolysis reaction. Twofluorinated photoinitiators were prepared through the aza-Michaeladdition of the amine modified Irgacure 2959 with perfluoroalkylethylacrylate (PFEA). The photochemical behaviors were evaluated by thephotopolymerization of 1, 6-Hexanediol diacrylate (HDDA). The mi-gration of initiators was proved by XPS and SEM analysis.

2. Experimental

2.1. Materials

Isophorone diisocyanate (IPDI) was purchased from Sinopham

http://dx.doi.org/10.1016/j.porgcoat.2017.10.009Received 21 January 2017; Received in revised form 2 June 2017; Accepted 11 October 2017

⁎ Corresponding author.E-mail address: wangjiaxi@hebut.edu.cn (J.X. Wang).

Progress in Organic Coatings 114 (2018) 102–108

0300-9440/ © 2017 Published by Elsevier B.V.

MARK

Chemical Reagent Co. Ltd. Irgacure 2959 was supplied by Jiuri NewMaterials Tianjin, China. Dibutyltin dilaurate (DBTDL), acetone,ethanol and ethyl acetate (EA) were purchased from Tianjin ChemicalReagent Co. 1,6-Hexanediol diacrylate (HDDA) was obtained fromTianjin Tianjiao Chemical Co. Perfluoroalkylethyl acrylate (PFEA) wassupplied by Fuxin Hengtong Fluorine Chemical Co. Acetone was driedby anhydrous magnesium sulfate over night before use. All other sol-vents and starting materials were reagent grade and used as received.

2.2. Measurement

1H NMR and 13C NMR spectra were recorded on a Bruker (400 MHz)spectrometer using CDCl3 as the solvent. The properties of UV photo-lysis were studied through a Bruker Lambda 25 spectrometer and UV-curing box (Youwei General Machinery equipment Co.) in the range of200–500 nm. FT-IR spectra and real-time FT-IR spectra were recordedon a Bruker Tensor-27 spectrometer with a UV LED light source of365 nm (Uvata Precision Optoelectronics Co.). The photopolymeriza-tion was performed in a UV-curing box. X-ray photoelectron spectro-scopy (XPS) analysis was obtained on a Thermo Scientific Escalab250Xi. The etching experiments were carried out at Argon gas clustersat large mode with 2000 eV for 30s. The SEM morphology analysis wascarried out on a field emission scanning electron microscopy (FESEM,Nova NanoSEM 450). Energy dispersive spectrometer (EDS) was per-formed by EDAX Octane Plus.

2.3. Syntheses of 2959-IPDI-F and 2959-IPDI-F-HDDA

2.3.1. Synthesis of 2959-IPDI-F2.56 g (11.4 mmol) of Irgacure 2959 in 20 mL acetone solution was

added dropwisely to a mixture of 0.03 g of DBTDL and 2.53 g(11.4 mol) of IPDI in 10 mL acetone, with mechanical stirring at 40 °C,the solution was stirred for 5 h. The reaction mixture was cooled toroom temperature, and then 50 mL H2O was added dropwisely at room

temperature. The organic phase can be extracted by 50 mL EA. Afterremoving the EA, 4.50 g white solid named as 2959-IPDI-NH2 wasobtained (98% yield).

1H NMR (400 MHz, CDCl3,δ): 0.88 ∼ 1.23 (m, 13H, C-CH2-C, CH-CH2-C and CH3), 1.62 (s, 6H, CH3), 1.71 (m, 2H, CH-CH2-C), 2.93 (b,2H, CH2-NH), 3.76 (b, 1H, CH-NH2), 4.20 ∼ 4.45 (m, 4H, CH2-O), 6.95(d, 2H, Ar-H), 8.07 (d, 2H, Ar-H).

2.20 g (4 mmol) PFEA and 0.082 g LiOH were added into 1.62 g(4 mmol) 2959-IPDI-NH2 in 15 mL ethanol. The mixture was stirred for5.5 h at 30 °C, quantitative concentrated hydrochloric acid was addedto neutralize LiOH. The LiCl was removed by filtration. After removingthe ethanol, 3.85 g colorless solid named as 2959-IPDI-F was obtained(99% yield).

1H NMR (400 MHz, CDCl3,δ): 0.88–1.23 (m, 13H, C-CH2-C, CH-CH2-C and CH3), 1.62 (s, 6H, CH3), 1.71 (m, 2H, CH-CH2-C), 2.40 (m, 2H,CH2-CF2), 2.58 (m, 2H, CH2-CH2-NH), 2.93 (b, 2H, C-CH2-NH), 3.51 (m,2H, CH2-COO), 3.76 (b, 1H, CH-NH2), 4.01 (m, 2H, CH2-O-CO),4.20–4.45 (m, 4H, CH2-O), 6.95 (d, 2H, Ar-H), 8.07 (d, 2H, Ar-H).

2.3.2. Synthesis of 2959-IPDI-F-HDDAAccording to procedure described as in 2.3.1, under promotion of

0.041 g LiOH, 1.94 g (2 mmol) 2959-IPDI-F reacted with 0.45 g(2 mmol) HDDA at 40 °C for 5 h to give 2.37 g light yellow solid namedas 2959-IPDI-F-HDDA (99% yield).

1H NMR (400 MHz, CDCl3,δ): 0.88–1.31 (m, 13H, C-CH2-C, CH-CH2-C and CH3), 1.41 (b, 4H, CH2-(CH2)2-CH2), 1.53–1.76 (m, 13H, CH3,OCH2-CH2, CH-CH2-C), 2.40 (m, 2H, CH2-CF2), 2.58 (m, 4H, CH2-CH2-NH), 2.93 (b, 2H, C-CH2-NH), 3.51 (m, 4H, CH2-COO), 3.67 (m, 5H, CH-N, OC-O-CH2), 3.96–4.30 (m, 6H, OC-O-CH2, CH2-O), 5.82 (d, H,CH= CH2), 6.13 (m, H, CH = CH2), 6.40 (d, H, CH = CH2), 6.95 (d,2H, Ar-H), 8.07 (d, 2H, Ar-H).

Scheme 1. Synthesis pathway of 2959-IPDI-F and2959-IPDI-F-HDDA.

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2.4. Photopolymerization kinetics

The real-time FT-IR was used to monitor the photopolymerizationkinetics. The monomer solution (HDDA) containing 1 wt.% of photo-initiator (relative to photoactive group, Irgacure 2959 + PTFA, 2959-IPDI-F and 2959-IPDI-F-HDDA) was painted on a KBr slice, and thenexposed to UV irradiation. FT-IR spectroscopies were taken at intervals.The conversion of double bond was calculated by measuring the peakintegrating of 810 cm−1.

3. Results and discussion

3.1. Syntheses and characterization of 2959-IPDI-F and 2959-IPDI-F-HDDA

Due to the activity of primary hydroxyl to NCO is much higher thanthat of tertiary hydroxyl, the Irgacure 2929 reacted with IPDI at 40 °Cproducing semi-adduct urethane 2959-IPDI [30], along with cycle ur-ethane dimer 2959-IPDI-cycle as byproduct as shown in Scheme 1 [31].2959-IPDI was further hydrolyzed to give 2959-IPDI-NH2. Under pro-motion of LiOH, 2959-IPDI-NH2 reacted with PFEA through the aza-Michael addition reaction at 30 °C generating 2959-IPDI-F, and furtherreacted with HDDA at 40 °C to give 2959-IPDI-F-HDDA. For 1H NMR,characteristic peaks for each proton had been marked in Fig. 1 a, 2, 3and the ratios of signal integration were in agreement with their ex-pected structure. The 13C NMR spectrum of 2959-IPDI-NH2 showedcharacteristic peaks as marked in Fig. 1b. The signals near 155.43 ppmwere assigned to ester-urethane group, which confirmed the formationof 2959-IPDI-NH2 [32]. Peaks a–e might be assigned to correspondingcarbon of byproduct 2959-IPDI-Cycle (see supporting information, Fig.S1). The 2959-IPDI-F and 2959-IPDI-F-HDDA were also characterized

by 13C NMR spectra (see supporting information, Fig. S2, Fig. S3). TheFT-IR spectra of three compounds are shown in Fig. 4. The absorptionpeaks locating at 1148 and 1201 cm−1 could be attributed to stretchingvibrations of CeF. The wagging vibrations of CeF bonds could be de-tected at 659 cm−1 [33]. The CeN stretching vibration of 2959-IPDI-NH2 at 1170 cm−1 moved to 1148 cm−1 (overlapped with CeF stretch

Fig. 1. 1H NMR and 13C NMR spectra of 2959-IPDI-NH2.

Fig. 2. 1H NMR spectrum of 2959-IPDI-F.

Fig. 3. 1H NMR spectrum of 2959-IPDI-F-HDDA.

Fig. 4. FT-IR spectra of three compounds.

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vibration signal), confirming the aza-Michael addition of primaryamine to acrylate.

3.2. Photolysis and photopolymerization kinetic

The UV–vis spectral changes of 2959-IPDI-F and 2959-IPDI-F-HDDAin ethanol upon photolysis are shown in Figs. 5 and 6. Before the UVirradiation, the UV–vis spectra of 2959-IPDI-F and 2959-IPDI-F-HDDAare similar. As the photolysis proceeded, the photoinitiator decomposedand the intensity of the absorption peak around 273 nm decreased untilit almost completely disappeared, while a new peak emerged around256 nm. Those photolysis behaviors are similar to fluorinated photo-initiator reported by Nie group [28].

The photoinitiating ability was evaluated by the photopolymeriza-tion of HDDA. The HDDA solution containing photoinitiator Irgacure2959, 2959-IPDI-F and 2959-IPDI-F-HDDA was painted on a KBr sliceand then irradiated immediately. The Irgacure 2959 + PFEA andIrgacure 2959 were used as the comparison. The kinetics of photo-polymerization profiles are shown in Fig. 7(a). As the comparison formigration of photoinitiator, the KBr slice painted by the HDDA solutionwith photoinitiator was kept for 30 min, and then irradiated, the ki-netics of photopolymerization profiles are shown in Fig. 7(b). It is clearthat the final double bond conversions (DC) of HDDA initiated by 2959-IPDI-F and 2959-IPDI-F-HDDA are much higher than that by Irgacure2959 with or without PFEA. The photoinitiating ability is in the order of2959-IPDI-F-HDDA>2959-IPDI-F > Irgacure 2959. The fluorinatedphotoinitiator may migrate to surface, which may increase the initiatorconcentration, and result in higher polymerization rate on top layer.When the top layer cured, the oxygen was hard to diffuse inside thepaint to inhibit the polymerization. Compared to Irgacure 2959 withoutPFEA, Irgacure 2959 with PFEA had a slightly higher DC, which couldbe attributed to the migration of PFEA. However, if the solution waskept for 30 min before irradiation, most of fluorinated initiator mi-grated to top layer, and the lower photoinitiator concentration in thebulk caused lower polymerization rate at the beginning of the stage.Based in Fig. 7(b), the photopolymerization rate of HDDA initiated byIrgacure 2959 with PFEA was faster than that by 2959-IPDI-F or 2959-IPDI-F-HDDA in first two minutes, and then the photopolymerizationrate of HDDA initiated by 2959-IPDI-F or 2959-IPDI-F-HDDA was fasterthan that by Irgacure 2959 with PFEA. This result could be explainedthat the migration of PFEA was faster than 2959-IPDI-F or 2959-IPDI-F-HDDA, and the migration of PFEA might reduce the oxygen con-centration, resulting less oxygen inhibition effect. The different layerhas different polymerization rate due to gradient distribution of in-itiator and oxygen inhibition. It was worthy to notice that the top layerof HDDA solution of Irgacure 2959 with or without PFEA was still stickafter irradiation for 35 min, while the HDDA solution of 2959-IPDI-F or2959-IPDI-F-HDDA cured to produce a nice film after irradiation atsame condition.

3.3. Surface migration

3.3.1. XPSIn order to get more evidence about the migration of fluorinated

photoinitiator, the XPS measurement and etching experiments wereinvestigated. As shown in Fig. 8, the peaks at 284.80 eV, 532.08 eV and688.35 eV represented of carbon, oxygen and fluorine, respectively.Fig. 9 showed the high resolution C1s XPS and etching experiments offilm, which obtained by HDDA photopolymerization initiated by 2959-IPDI-F. Four peaks shown in Fig. 9a, situated at 284.70 eV (A),286.20 eV (B), 288.65 eV (C) and 291.55 eV (D), were assigned to the

Fig. 5. UV–vis spectra of 2959-IPDI-F (ethanol, 10−4 mol/L) upon photolysis.

Fig. 6. UV–vis spectra of 2959-IPDI-F-HDDA (ethanol, 10−4 mol/L) upon photolysis.

Fig. 7. Photopolymerization kinetics of HDDA. (Radiation intensity of the UV light was 356 mW/cm2; all sample were open to air).

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CeC of the hydrocarbons, CeO, C]O and CeF, respectively. Becausethe fluorinated photoinitiator is the only source of fluorine, the dis-tribution of photoinitiator in the film could be tracked by signal relativeto fluorine. The content of peak D decreased with the increase ofetching times (Fig. 9b), which verified the migration of fluorinated

photoinitiator. Fig. 10 showed the high resolution F1s XPS of etchingexperiments of film. As the etching proceeded, the XPS of F1s shifts alittle to lower energy and peak becomes a little broad. F/C atom ratio atthe surface of cured HDDA films initiated by 2959-IPDI-F and 2959-IPDI-F-HDDA is 0.153, 0.139, respectively. The content of fluorine atom

Fig. 8. XP spectra of films of HDDA initiated by 2959-IPDI-F (a); 2959-IPDI-F-HDDA (b).

Fig. 9. High-resolution C1s of cured HDDA film initiated by 2959-IPDI-F (a); etching experiments (b).

Fig. 10. Etch experiments of (a) cured HDDA film initiated by 2959-IPDI-F and (b) cured HDDA film initiated by 2959-IPDI-F-HDDA. (Ar gas cluster ion, power: 2000 eV, each etchingtime: 30 s).

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declined gradually with etching times.

3.3.2. SEM and EDSScanning electron microscope (SEM) was used to observe the sur-

face morphology and Energy dispersive X-ray spectroscopy (EDS) ana-lyses was used to locate the element distribution. The surface mor-phology and fluorine distribution are shown in Fig. 11. There were amass of folds on the surface of both two samples. Based on the XPS andetching experiments analysis, this textile bumps was resulted from thefluorinated photoinitiators migration to the top layer forming gradientdistribution, leading to different polymerization behaviors [28]. Thereis a complex balance between the high polymerization speeds owing tohigh initiator concentration on the surface, and larger oxygen inhibitionowing to high oxygen concentration and diffusion rate at interface offilm and air. The unique polymerization behavior makes the surfacemorphology uneven. The migration of fluorinated initiator enriches thefluorine element non uniform distribution.

4. Conclusions

Two fluorinated photoinitiators with good anti-oxygen inhibitionwere synthesized through the aza-Michael addition of fluorinated ac-rylate with amine modified commercial Irgacure 2959. The fluorinatedphotoinitiators can migrate to top layer and the liquid-air interface inthe gradient distribution. High concentration photoinitiator decreasesthe oxygen inhibition of the photopolymerization. Fluorine elementenrichment on the surface makes the film with a lower surface energy.A unique folded surface could be easily developed through photo-polymerization initiated fluorinated photoinitiator.

Acknowledgement

This research was conducted without specific grants from anyagency.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at http://dx.doi.org/10.1016/j.porgcoat.2017.10.009.

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