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Synthesis and photopolymerization of novel UV-curable macro-photoinitiators
Progress in Organic Coatings 141 (2020) 105546
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Synthesis and photopolymerization of novel UV-curable macrophotoinitiators
T
Lin Denga, Liuyan Tangb, Jinqing Qua,* a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510641, People’s Republic of China School of Biotechnology and Health Sciences, Wuyi University, Jiangmen, Guangdong, 529020, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Photopolymerization Oligomer Kinetics Low migration
The design and development of macro-photoinitiators applicable to UV system has been attracted increasing attention for their excellent performance of low migration and high compatibility. Herein, four kinds of novel macro-photoinitiators (PI1-PI4) used in UV-curable coatings were synthesized by different isocyanates, 1-hydroxycyclohexyl phenyl ketone (Irgacure 184), 2-hydroxyethyl methacrylate (HEMA) and pentaerythritol triacrylate (PETA). The structures were analyzed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Moreover, the photopolymerization kinetics of the macro-photoinitiators were investigated by real-time FTIR. Results revealed that the macro-photoinitiators display higher curing efficiency, lower migration performance and better compatibility with formulation compositions compared to Irgacure 184. On the other hand, the macro-photoinitiators also served as self-initiating oligomers and had good thermal stability with 10 % weight loss temperatures (T10 %) above 299.7 °C. The UV-cured films consisting of oligomers and tripropylene glycol diacrylate (TPGDA) showed excellent physical properties and high curing efficiency and rapid curing rate.
1. Introduction
photoinitiators and the research of self-initiating oligomers [19–22]. Inorganic particles such as titanium dioxide as well as quantum-sized zinc oxide particles have been reported to increase the conversion rates and the physical properties in the UV-curing process of some acrylic monomers [23,24]. Apart from low toxicity, water-soluble photoinitiators also show excellent compatibility with aqueous formulations, even after evaporation of the water due to the exiting of hydrophilic residues [25]. For the macromolecular photoinitiators, they are known to have high compatibility, low migration and less volatile [26,27]. Hence, macromolecular photoinitiators are receiving more and more attentions in recent years. On the other hand, self-initiating oligomers containing the π-P-π conjugated groups or the electronic donor-acceptor groups effectively initiate the polymerization of monomers, such as thiol-ene group and thiol-vinyl group [28,29]. Most importantly, the self-initiating system can provide a good strategy for UV coating in high security applications at room temperature without any PI [30]. Besides, integrate the photoinitiator into the back bone of oligomers to synthesis polymeric photoinitiators, which can also regard as self-initiating oligomers, are of great importance due to their superiority in overcoming the drawbacks of conventional photoinitiator [31–34]. As a widely used Type I photoinitiators, Irgacure 184 can be cleaved into highly reactive free radicals with a maximum absorption bands at
As an environmental friendly green technology at present, UVcuring exhibits striking advantages of high efficiency, room temperature polymerization and almost no volatile organic compounds [1–6]. Therefore, it has attracted increasing interests in the development of coating, inks, microelectronics, dental repair, adhesives and biological materials fields [7–11]. During the UV-curing process, photoinitiators (PIs) are the key component because of their capacity to absorb energy radiation of appropriate light wavelength and generate active intermediate (free radical or cation) that initiate the prepolymer system into liner polymer or crosslinked network [12–15]. Despite the photoinitiators showed excellent performance and had been widely used in the industry, the problems of toxicity, oxygen inhibition, poor compatibility and migration of the residual photoinitiator had restricted their development in food and biomedical applications [16–18]. Hence with the updated of laws and regulations on energy conservation and environmental protection, developing new photoinitiator alternatives is emergent. Approaches that have been reported so far to improve or supersede the conventional PIs properties included the use of inorganic particles, new water-soluble photoinitiators, the synthesis of the macromolecular ⁎
Corresponding author. E-mail address: [email protected] (J. Qu).
https://doi.org/10.1016/j.porgcoat.2020.105546 Received 5 November 2019; Received in revised form 25 December 2019; Accepted 6 January 2020 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 141 (2020) 105546
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200 nm and 248 nm [35]. However, Irgacure 184 exhibits poor migration. To solve this problem, researches were carried out to combine the advantages of polymerization with the properties of commercial photoinitiators [36,37]. Monofunctional macromolecular photoinitiators based on Irgacure 184 were studied, which have the potential to initiate free radical polymerization under light-induction [38]. While this type of macro-photoinitiators have not widely used in industry. Herein, another type of macro-photoinitiators were proposed by grafting Irgacure 184 to isocyanate skeleton. The structures were characterized with FT-IR and 1H NMR spectra, and curing kinetics were performed by real-time FT-IR. The synthesized photoinitiators with similar or increased photoinitiating activity compared to the commercial precursors appear to be promising photoinitiators. Also, by comparing with Irgacue 184, the migration amounts of the macro-photoinitiators were all decreased.
when the NCO % reaches the theoretical value. After cooling to room temperature, the mixture was dissolved in petroleum ether, washed three times with distilled water, and dried by anhydrous sodium sulfate. Viscous liquid products were obtained after removing petroleum ether by rotary evaporation. 2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy FTIR was recorded with a Perkin Elmer spectrum 2000 spectrometer at the wave number from 400 cm−1 to 4000 cm−1 to identify the functional groups and chemical bonds changes. The liquid products were attached in KBr pallets and the solid was mixed directly with KBr powder. 2.3.2. 1H nuclear magnetic resonance 1 H NMR spectra were recorded using a Bruker AVANCE III400 (400 MHz) nuclear magnetic resonance spectrometer. About 20 mg samples were dissolved in dimethyl sulfoxide-d6 or CDCl3.
2. Experimental 2.1. Materials The following compounds were used in this study: toluene-2,4-diisocyanat (TDI), isophorone diisocyanate (IPDI) and hexamethylene diisocyanate trimer (HDI trimer, NCO % = 21.8–22.1 wt %) was supplied by Wanhua chemical Co., Ltd. Hydroxyethyl methacrylate (HEMA), pentaerythritol triacrylate (PETA), 1-hydroxycyclohexyl phenyl ketone (Irgacure 184) and dibutyltin dilaurate (DBTDL) were purchased from Aladdin Industrial Corporation (Shanghai, China). Acetone, hydrochloric acid was procured from Sinopharm Chemical Reagent Co., Ltd. Tripropylene glycol diacrylate (TPGDA) and trihydroxymethyl propane triacrylate (TMPTA) were obtained from Jiangsu Litian chemical co., Ltd (industrial grade) and used as benchmark monomer. All chemicals and solvents were used directly without further purification. The chemical structures of the compounds in this research are shown in Scheme 1.
2.3.3. UV–vis spectroscopy Ultraviolet spectra were measured by the U-3010 spectrophotometer (Hitachi, Tokyo, Japan) with detected wavelength from 190 nm to 600 nm. The samples were diluted with acetone to appropriate concentrations and the test temperature was kept at 25 °C. 2.4. Real-time FTIR spectroscopy experiments For kinetic measurements, FTIR spectrometer equipped with an ultraviolet light source was used to detect the photopolymerization. The total time was set to 300 s and the infrared spectra ranged from 4000 to 600 cm−1 was recorded. The mixture including different monomers and the synthesized photoinitiators were prepared and typically Irgacure 184 was added to the formulation for comparison. The mixture was spread on a polyvinyl chloride (PC) film homogeneously with about the same mass and then covered with another PC film to prevent oxygen inhibition during the test. The conversion of double band at intervals were monitored according to the C]C twisting band peak area at 810 cm−1 and taken the characteristic absorption peak of C]O at 1720cm−1 as the internal standard. The formula is shown below:
2.2. Synthesis of monomeric photoinitiators The synthesis process of PIs was shown in Scheme 2. In the first step, a mixture of HDI trimer and DBTDL were charged into a 250 ml fournecked round bottom flask equipped with a mechanical stirrer. The mixture was then stirred at 40 °C in a thermostat oil bath. Irgacure 184 was dissolved in acetone and then added into the flask slowly, afterwards the reaction temperature was maintained at 60 °C for 3−4 h until the NCO % reaches the theoretical value (NCO value in the system was measured according to HG/T 2409-1992 standard). Secondly, PETA was added into the flask using a dropping funnel within 1 h followed by setting the temperature to 70 °C. Finally, the reaction was terminated
Conversion (%) = [1 − (A810/A1720)t/ (A810/A1720)0]×100 %
(1)
Where (A810/A1720)0 being the ratio of the relative absorption peak area of the double bond to the carbanyl group before UV curing, (A810/ A1720)0 represents the ratio of the relative absorption area of the double bond to the carbanyl group peak after UV curing at time t.
Scheme 1. Chemical structures of monomers being used in this study. 2
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Scheme 2. The synthetic routes of PIs.
2.5. Migration studies
2.6.2. Properties of UV-cured films The pencil hardness of the films was monitored using the standard indentation method (GB/T6739-96). Viscosity of resins was measured using a rotational viscometer at room temperature. The water resistance of the film coatings was carried out using the GB/T1733-1993 method. Gloss at 60 °C of coatings was conducted according to the GB/ T9754-2007.Moreover, the gel contents were measured by submerging films in acetone for 30 min, then the films were dried in an oven at 60 °C until to constant weight and then calculated according to the equation below:
In order to compare the migration of the synthesized photoinitiators with Irgacure 184, the extraction value was applied to estimate the migration value according to the literatures [37,39,40]. The films including TPGDA monomer, 5 wt% of the synthesized novel photoinitiators and Irgacure 184 were prepared followed by fully cured under UV irradiation. After that those films were immersed in 25 mL of acetone for 12 h at room temperature. In addition, the migration in water medium was also carried out, which was much more concerned for industry application. The films were immersed in deionized water for five days at room temperature. After filtration, UV–vis absorption measurements were carried out to quantify the extracted amount of the photoinitiators. Finally, according to the Bear-Lambert law, the migration weight was calculated by the following Eq. (2): m = M ×c × V
solution=
(A × M×V
solution)/
(ε× b)
gel ratio %=m1/m0×100 %
(3)
where m0, m1 represent the films weight before submerged into acetone and after dried respectively. 3. Results and discussion
(2)
Where A is the absorption, M is molecular weight of photoinitiator, V is the total volume of solution, ε is the molar absorption coefficient of photoinitiator in solution and b is optical path length.
3.1. Synthesis and characterization of PIs
solution
As important variables, reaction time and temperature have significant influence on polymer synthesis process. Therefore, this paper first explores the influence of the reaction conditions on the synthesis of PIs depending on the NCO value. Fig. 1 displayed that the NCO value decreased with the increase of reaction temperature. Fig. 1 (a) showed that under low reaction temperature, the conversion rate between -NCO and −OH group was low, resulting in the inadequate reaction. However, while the reaction temperature is above 70 °C, the pre-reaction became severe, accompanied by a sharp rise in viscosity and side reactions. On the other hand, prolonging the reaction time can promote the transformation of NCO group, and after reaction for 3 h, the NCO value reached the theoretical one. Hence in the first step the reaction time and temperature were set to be 3 h and 60 °C respectively. Then HEMA or PETA was added as the end capping agent and introduced C]C bond to the final product. As shown in Fig. 1 (b), when under low reaction temperature at 50 °C, there was more residual NCO in the
2.6. Physical properties of oligomers and UV-cured films In this work, the UV-curable films were prepared by mixing the oligomers and monomer with the same content (5 wt %) of Irgacure 184. Following fully stirred, the mixture was casted on glass plate use an applicator with the thickness of the films about 50 μm. Subsequently, the films were cured under a mercury lamp (150–200 mJ/cm2). 2.6.1. Thermogravimetric analysis (TGA) The thermal stability of films was carried out with a 209 F1 thermogravimetric–differential thermal analyzer of German NETZSCH company, and the temperature range was set from 30 to 600 °C at a heating rate of 10 °C min−1 under nitrogen flow rate of 100 mL min−1. The sampling amount was 5−10 mg. 3
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Fig. 1. Effect of reaction time and temperature on the NCO value.
Fig. 2. The FT-IR spectra of raw materials and products.
system. The increase of temperature improved the conversation of NCO visibly. However, when the reaction temperature was more than 80 °C, the reaction system tend to be gelled. Besides, the NCO value decreased distinctly at the beginning of reaction. With the prolongation of time, there was only a small amount of NCO exists after reaction. Therefore, the reaction conditions of 70 °C and 4 h was selected for the subsequent end capping experiments.
Fig. 3. The 1H NMR spectra of PI1-PI4.
absorption peak at 3490 cm−1 corresponding to the eOH stretching vibration absorption peaks of Irgacure 184 and PETA disappeared. In addition, no absorption peak of –NCO at 2278 cm−1 was found, but a strong deformation vibration at 3320 cm−1 and a stretching vibration at 1536 cm−1 appeared, indicating the existence of eNH group. It also suggested that NCO had reacted with OH into NHCOO. Moreover, the other absorption peaks at 1720 cm−1 (C]O), 1450 cm−1 (]CH2) and
3.2. FTIR spectra analysis FTIR spectra of PETA, Irgacure 184, TDI, the first step product (1S) and PIs were shown in Fig. 2. In the FTIR curves of the PIs, a typical 4
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Fig. 4. UV–vis absorption spectra of Irgacure 184 and PIs in acetone (5 × 10−5 M) solution. Table 1 Absorption properties of Irgacure 184 and the synthesized PIs. Photoinitiators
λmax(nm)
εmax(M−1 cm−1)
Irgacure 184 PI1 PI2 PI3 PI4
243 244 244 245 255
15000 22200 22400 28900 29200
810 cm−1 (C]C) were found, indicating that the Irgacure 184 and acrylates have successfully incorporated into the polyurethane chains. 3.3. 1H NMR spectra The expected structure of PIs was further evidenced by 1HNMR as depicted in Fig. 3. For the spectrum of PI1 in Fig. 3, the peaks at δ = 9.65 ppm, δ = 8.92 ppm were considered to the proton of NH at the methyl para-position and the side position of the phenyl. As for the spectrum of PI3, the signals at 7.21 ppm can be assigned to the proton of NH without attached to phenyl. Moreover, the peaks at the range of 7.51–7.94 ppm are identified as the aromatic protons, indicating that Irgacure 184 has been grafted into the HDI trimer. The peaks at 1.23 ppm, 1.86–2.51 ppm and 3.35 ppm are attributed to the methylene, demonstrating that the existence of methylene group in the HDI trimer and HEMA. The feature at 5.88–6.02 ppm refers to the protons of eCH]CH2. In general, the target products are synthesized as the expected design according to FT-IR spectra and 1H NMR spectra. 3.4. UV–vis spectral characterization of the macro-photoinitiators The UV–vis spectra of the synthesized PI1-PI4 together with Irgacure 184 were measured and showed in Fig. 4. The typical values of maximum wavelength (λmax) and the molar absorption coefficients (ε) at λmax in acetone solution were listed in Table 1. Based on the results, PI1 to PI3 exhibit similar maximum absorption to Irgacure 184, while the absorption of PI4 is red shift (λmax = 255 nm) compared to Irgacure 184, which may be contributed to the multiple double bands attached to PI4. However, it should be noticed that all synthesized photoinitiators show higher molar absorption coefficient (εmax) than Irgacure 184.
Fig. 5. Photopolymerization efficiency of (a) different photoinitiators and (b) different concentration of PI3 using TPGDA as monomer (light intensity = 50 mw/cm2). (c) Comparison of different monomers (5 wt% PI3, light intensity = 50 mw/cm2). (d) Photopolymerization of TPGDA with different light intensity (5 wt% PI3).
3.5. Photopolymerization kinetic investigations of PIs Photopolymerization experiments of PIs that used in UV-curable were performed upon the real-time FT-IR in the absence of hydrogen 5
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Fig. 6. UV–vis absorption spectra of extraction solution from cured films.
Fig. 7. The mass fraction of the migrated Irgacure 184 in the PIs. Fig. 8. TG (a)and DTG (b) curves of the PIs.
donors. Fig. 5(a) presented the photopolymerization kinetic curves of TPGDA initiated by different PIs in comparison with Irgacure 184. As seen from Fig. 5(a), PI1, PI2 and PI3 exhibited almost the same conversion as the Irgacure 184, while PI1 and PI2 show higher initiation efficiency in the first forty seconds. However, due to the high viscosity and poor solubility in TPGDA, PI4 shows low initiation efficiency of C]C bond. Under the initiation of PI1-4, the final conversion rates of the double bond are all higher than 80 %. In addition, under the condition of 50 mW/cm2 light intensity, the experiments with different concentrations of PI3 using TPGDA as monomer were performed. According to Fig. 5(b), with the concentration of PI3 increased from 3 wt% to 7 wt%, the double conversion gradually increased and the time that reached the maximum conversion was reduced. Besides, it is clearly figured out that the final conversions in the condition of 7 wt% or 5 wt% PI3 were almost the same. High concentrations of photoinitiators can cause a dramatic increase in viscosity and lead to gel formation. On the other hand, optical shielding effect also weaken the photopolymerization activity. Therefore, 5 wt% was preferred as the amount of photoinitiator in the following UVcuring study.
Table 3 Thermogravimetric data of PIs. Runs
T10 % (°C)
T50 % (°C)
Tmax (°C)
Rmax (%/min)
Residue rate at 600 °C (%)
PI1 PI2 PI3 PI4
308.5 299.7 301.9 363.0
435.9 414.3 423.3 445.2
457.1 431.0 434.5 446.9
11.67 12.95 8.50 14.87
2.01 5.94 11.56 12.96
In order to evaluate the influence of different monomer on photoinitiation activities, HEMA, TPGDA and TMPTA were added in the study of the photopolymerization kinetics of PI3. As shown in Fig. 5(c), the photopolymerization reaction and final conversion of TPGDA is higher than HEMA and TMPTA. The result presumably attributed to the fact that the fewer number of double bonds in HEMA leading to lower photoinitiation efficiency. However, as a trifunctional monomer, TMPTA exhibited the lowest double band conversion. That is because
Table 2 Application properties with different photoinitiators. Films
Viscosity / mPa.s
Appearance
Pencil hardness
Gloss (60 °C)
Curing time/ s
Gel ratio / %
Water resistance
PI1 PI2 PI3 PI4 Irgacure 184
5000 6700 9000 14400 —
A B B B —
HB H HB H-2H B
92 97 98 95 96
45 30 30 15 60
98.58 96.71 97.20 90.42 91.63
No No No No No
A- Faint yellow transparent liquid; B- Colorless transparent liquid. 6
change change change change change
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TMPTA would cause a significant increase of viscosity and the reduction of mobility, resulting in less diffusivity and lower termination rate comparison with lower functional monomer [41]. Light intensity is a key factor in the photopolymerization process, hence different light intensity was carried out to explore the photopolymerization of UV-curing with PI3 as the photoinitiator. The results were showed in Fig. 5(d) and revealed that the final double band conversion gradually increased with the enhance of light intensity. When the light intensity was 10 mW/cm2, there was only 40 % double band conversion. The conversion boost to 76 % and 93 % with the light intensity increased to 30 mW/cm2 and 50 mW/cm2 respectively. This could be interpreted that higher light intensity promoted the cleavage of the photoinitiator, so it also increased the generation of free radicals and further accelerated the double band conversion.
The resins tended to decomposed fully when the temperature was more than 600 °C and PI4 showed the best performance with 12.96 % residue rate at 600 °C, which mainly because the oligomer that synthesized by HDI trimer and PETA had a higher molecular. 4. Conclusions In this article, the synthesis and properties of four UV-curable monomeric photoinitiators (PI1-PI4) based on isocyanate, Irgacure 184 were reported. The photopolymerization of HEMA, TPGDA and TMPTA, initiated by PIs, was studied by real-time FT-IR. The results point out that the products could successfully initiate acrylic monomer with excellent efficiency. Moreover, in contrast with Irgacure 184, low migration performance was obtained, among which the PI4 displays the lowest migration performance. Based on the results, it can be concluded that the designed photoinitiators have great potential to be used in UV curing systems.
3.6. Migration of the photoinitiators The migration stability of Irgacure 184 and the synthesized PIs was evaluated by measuring the residue photoinitiators of the cured films. As exhibited in Fig. 6, the absorption spectra of extracted acetone solution from cured films were determined by UV–vis spectroscopy. The results demonstrates that the extraction solution of resins cured by PI1−4 had weaker absorption than the resin cured with Irgacure 184 at 250 nm. In addition, Fig. 7 presented the mass fraction of the migrated Irgacure 184 moieties from PI1-4. The mass fractions were 19.6 %,18.2 %,14.3 %,10.9 % and 36.4 % for PI1, PI2, PI3, PI4 and Irgacure 184, respectively. Because the lower solubility in deionized water, the mass fraction of Irgacure 184 were less than the acetone solution. The mass fractions were 6.0 %, 5.6 %, 3.2 %, 2.1 % and 10.5 % for PI1, PI2, PI3, PI4 and Irgacure 184 in deionized water. Hence compared with Irgacure 184, the synthesized PIs proved to be significantly improved in migrant stability, which mainly attributed to the long alkyl chains. And high molecular weight endow the photoinitiators with good compatibility of acrylate monomers [42]. All these demonstrated that PIs could be served as efficient polymerizable one-component photoinitiators with toxicity in the cured materials.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. CRediT authorship contribution statement Lin Deng: Methodology, Formal analysis, Investigation, Writing original draft. Liuyan Tang: Writing - review & editing. Jinqing Qu: Resources, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
3.7. Physical properties of oligomers and coatings
This work was founded by the National Natural Science Foundation of China (No. 21878109), and the Science and Technology Program of Guangdong Province, China (No. 2015B090925006, 2016B090930005 and 2016B030302004).
As self-initiating oligomers, the physical properties of the films being cured by PIs including viscosity, appearance, pencil hardness, gloss, curing time and water resistance were summarized in Table 2. From Table 2, it can be seen that PI1, PI2 and PI3 exhibited lower viscosity than PI4, which mainly because the raw material of HDI trimer and PETA has high viscosity. In addition, the oligomers all showed excellent appearance except for PI1. PI1 showed a little bit yellow, thus resulting in a lower gloss at 60 °C. As for the result of hardness, the film being initiated by the synthesized oligomers in this work increased in contrast with the one being initiated by Irgacure 184. Especially, the films cured by the initiation of PIs completely in a short time (15−60 s). However, in terms of the properties of gel ratio, the PI4 oligomers had the worst performance, which in consistent with the photopolymerization kinetic results. Besides, the water resistance of the films were remarkable. The thermogravimetric analysis and difference thermal gravimeter were carried out from 30 °C to 600 °C and the curves were displayed in Fig. 8. The detail information of T10 % (the temperature of 10 % weight loss), T50 % (the temperature of 50 % weight loss), Tmax (temperature of maximum weight loss occurs), Rmax (maximum decomposition rate) and residue rate at 600 °C were listed in Table 3. These oligomers exhibited T10 % at 308.5 °C, 299.7 °C, 301.9 °C and 363.0 °C respectively, demonstrating that the oligomers had good heat resistance. Besides, according to the DTG curves, there were two stages in the decompose process. The first stage was due to the break of CeO of the -NHCO groups and the other one was ascribed to the further decompose to carbon volatiles in the temperature range from 345 °C to 484 °C [43].
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Synthesis and photopolymerization of novel UV-curable macrophotoinitiators
T
Lin Denga, Liuyan Tangb, Jinqing Qua,* a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510641, People’s Republic of China School of Biotechnology and Health Sciences, Wuyi University, Jiangmen, Guangdong, 529020, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Photopolymerization Oligomer Kinetics Low migration
The design and development of macro-photoinitiators applicable to UV system has been attracted increasing attention for their excellent performance of low migration and high compatibility. Herein, four kinds of novel macro-photoinitiators (PI1-PI4) used in UV-curable coatings were synthesized by different isocyanates, 1-hydroxycyclohexyl phenyl ketone (Irgacure 184), 2-hydroxyethyl methacrylate (HEMA) and pentaerythritol triacrylate (PETA). The structures were analyzed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Moreover, the photopolymerization kinetics of the macro-photoinitiators were investigated by real-time FTIR. Results revealed that the macro-photoinitiators display higher curing efficiency, lower migration performance and better compatibility with formulation compositions compared to Irgacure 184. On the other hand, the macro-photoinitiators also served as self-initiating oligomers and had good thermal stability with 10 % weight loss temperatures (T10 %) above 299.7 °C. The UV-cured films consisting of oligomers and tripropylene glycol diacrylate (TPGDA) showed excellent physical properties and high curing efficiency and rapid curing rate.
1. Introduction
photoinitiators and the research of self-initiating oligomers [19–22]. Inorganic particles such as titanium dioxide as well as quantum-sized zinc oxide particles have been reported to increase the conversion rates and the physical properties in the UV-curing process of some acrylic monomers [23,24]. Apart from low toxicity, water-soluble photoinitiators also show excellent compatibility with aqueous formulations, even after evaporation of the water due to the exiting of hydrophilic residues [25]. For the macromolecular photoinitiators, they are known to have high compatibility, low migration and less volatile [26,27]. Hence, macromolecular photoinitiators are receiving more and more attentions in recent years. On the other hand, self-initiating oligomers containing the π-P-π conjugated groups or the electronic donor-acceptor groups effectively initiate the polymerization of monomers, such as thiol-ene group and thiol-vinyl group [28,29]. Most importantly, the self-initiating system can provide a good strategy for UV coating in high security applications at room temperature without any PI [30]. Besides, integrate the photoinitiator into the back bone of oligomers to synthesis polymeric photoinitiators, which can also regard as self-initiating oligomers, are of great importance due to their superiority in overcoming the drawbacks of conventional photoinitiator [31–34]. As a widely used Type I photoinitiators, Irgacure 184 can be cleaved into highly reactive free radicals with a maximum absorption bands at
As an environmental friendly green technology at present, UVcuring exhibits striking advantages of high efficiency, room temperature polymerization and almost no volatile organic compounds [1–6]. Therefore, it has attracted increasing interests in the development of coating, inks, microelectronics, dental repair, adhesives and biological materials fields [7–11]. During the UV-curing process, photoinitiators (PIs) are the key component because of their capacity to absorb energy radiation of appropriate light wavelength and generate active intermediate (free radical or cation) that initiate the prepolymer system into liner polymer or crosslinked network [12–15]. Despite the photoinitiators showed excellent performance and had been widely used in the industry, the problems of toxicity, oxygen inhibition, poor compatibility and migration of the residual photoinitiator had restricted their development in food and biomedical applications [16–18]. Hence with the updated of laws and regulations on energy conservation and environmental protection, developing new photoinitiator alternatives is emergent. Approaches that have been reported so far to improve or supersede the conventional PIs properties included the use of inorganic particles, new water-soluble photoinitiators, the synthesis of the macromolecular ⁎
Corresponding author. E-mail address: [email protected] (J. Qu).
https://doi.org/10.1016/j.porgcoat.2020.105546 Received 5 November 2019; Received in revised form 25 December 2019; Accepted 6 January 2020 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
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200 nm and 248 nm [35]. However, Irgacure 184 exhibits poor migration. To solve this problem, researches were carried out to combine the advantages of polymerization with the properties of commercial photoinitiators [36,37]. Monofunctional macromolecular photoinitiators based on Irgacure 184 were studied, which have the potential to initiate free radical polymerization under light-induction [38]. While this type of macro-photoinitiators have not widely used in industry. Herein, another type of macro-photoinitiators were proposed by grafting Irgacure 184 to isocyanate skeleton. The structures were characterized with FT-IR and 1H NMR spectra, and curing kinetics were performed by real-time FT-IR. The synthesized photoinitiators with similar or increased photoinitiating activity compared to the commercial precursors appear to be promising photoinitiators. Also, by comparing with Irgacue 184, the migration amounts of the macro-photoinitiators were all decreased.
when the NCO % reaches the theoretical value. After cooling to room temperature, the mixture was dissolved in petroleum ether, washed three times with distilled water, and dried by anhydrous sodium sulfate. Viscous liquid products were obtained after removing petroleum ether by rotary evaporation. 2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy FTIR was recorded with a Perkin Elmer spectrum 2000 spectrometer at the wave number from 400 cm−1 to 4000 cm−1 to identify the functional groups and chemical bonds changes. The liquid products were attached in KBr pallets and the solid was mixed directly with KBr powder. 2.3.2. 1H nuclear magnetic resonance 1 H NMR spectra were recorded using a Bruker AVANCE III400 (400 MHz) nuclear magnetic resonance spectrometer. About 20 mg samples were dissolved in dimethyl sulfoxide-d6 or CDCl3.
2. Experimental 2.1. Materials The following compounds were used in this study: toluene-2,4-diisocyanat (TDI), isophorone diisocyanate (IPDI) and hexamethylene diisocyanate trimer (HDI trimer, NCO % = 21.8–22.1 wt %) was supplied by Wanhua chemical Co., Ltd. Hydroxyethyl methacrylate (HEMA), pentaerythritol triacrylate (PETA), 1-hydroxycyclohexyl phenyl ketone (Irgacure 184) and dibutyltin dilaurate (DBTDL) were purchased from Aladdin Industrial Corporation (Shanghai, China). Acetone, hydrochloric acid was procured from Sinopharm Chemical Reagent Co., Ltd. Tripropylene glycol diacrylate (TPGDA) and trihydroxymethyl propane triacrylate (TMPTA) were obtained from Jiangsu Litian chemical co., Ltd (industrial grade) and used as benchmark monomer. All chemicals and solvents were used directly without further purification. The chemical structures of the compounds in this research are shown in Scheme 1.
2.3.3. UV–vis spectroscopy Ultraviolet spectra were measured by the U-3010 spectrophotometer (Hitachi, Tokyo, Japan) with detected wavelength from 190 nm to 600 nm. The samples were diluted with acetone to appropriate concentrations and the test temperature was kept at 25 °C. 2.4. Real-time FTIR spectroscopy experiments For kinetic measurements, FTIR spectrometer equipped with an ultraviolet light source was used to detect the photopolymerization. The total time was set to 300 s and the infrared spectra ranged from 4000 to 600 cm−1 was recorded. The mixture including different monomers and the synthesized photoinitiators were prepared and typically Irgacure 184 was added to the formulation for comparison. The mixture was spread on a polyvinyl chloride (PC) film homogeneously with about the same mass and then covered with another PC film to prevent oxygen inhibition during the test. The conversion of double band at intervals were monitored according to the C]C twisting band peak area at 810 cm−1 and taken the characteristic absorption peak of C]O at 1720cm−1 as the internal standard. The formula is shown below:
2.2. Synthesis of monomeric photoinitiators The synthesis process of PIs was shown in Scheme 2. In the first step, a mixture of HDI trimer and DBTDL were charged into a 250 ml fournecked round bottom flask equipped with a mechanical stirrer. The mixture was then stirred at 40 °C in a thermostat oil bath. Irgacure 184 was dissolved in acetone and then added into the flask slowly, afterwards the reaction temperature was maintained at 60 °C for 3−4 h until the NCO % reaches the theoretical value (NCO value in the system was measured according to HG/T 2409-1992 standard). Secondly, PETA was added into the flask using a dropping funnel within 1 h followed by setting the temperature to 70 °C. Finally, the reaction was terminated
Conversion (%) = [1 − (A810/A1720)t/ (A810/A1720)0]×100 %
(1)
Where (A810/A1720)0 being the ratio of the relative absorption peak area of the double bond to the carbanyl group before UV curing, (A810/ A1720)0 represents the ratio of the relative absorption area of the double bond to the carbanyl group peak after UV curing at time t.
Scheme 1. Chemical structures of monomers being used in this study. 2
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Scheme 2. The synthetic routes of PIs.
2.5. Migration studies
2.6.2. Properties of UV-cured films The pencil hardness of the films was monitored using the standard indentation method (GB/T6739-96). Viscosity of resins was measured using a rotational viscometer at room temperature. The water resistance of the film coatings was carried out using the GB/T1733-1993 method. Gloss at 60 °C of coatings was conducted according to the GB/ T9754-2007.Moreover, the gel contents were measured by submerging films in acetone for 30 min, then the films were dried in an oven at 60 °C until to constant weight and then calculated according to the equation below:
In order to compare the migration of the synthesized photoinitiators with Irgacure 184, the extraction value was applied to estimate the migration value according to the literatures [37,39,40]. The films including TPGDA monomer, 5 wt% of the synthesized novel photoinitiators and Irgacure 184 were prepared followed by fully cured under UV irradiation. After that those films were immersed in 25 mL of acetone for 12 h at room temperature. In addition, the migration in water medium was also carried out, which was much more concerned for industry application. The films were immersed in deionized water for five days at room temperature. After filtration, UV–vis absorption measurements were carried out to quantify the extracted amount of the photoinitiators. Finally, according to the Bear-Lambert law, the migration weight was calculated by the following Eq. (2): m = M ×c × V
solution=
(A × M×V
solution)/
(ε× b)
gel ratio %=m1/m0×100 %
(3)
where m0, m1 represent the films weight before submerged into acetone and after dried respectively. 3. Results and discussion
(2)
Where A is the absorption, M is molecular weight of photoinitiator, V is the total volume of solution, ε is the molar absorption coefficient of photoinitiator in solution and b is optical path length.
3.1. Synthesis and characterization of PIs
solution
As important variables, reaction time and temperature have significant influence on polymer synthesis process. Therefore, this paper first explores the influence of the reaction conditions on the synthesis of PIs depending on the NCO value. Fig. 1 displayed that the NCO value decreased with the increase of reaction temperature. Fig. 1 (a) showed that under low reaction temperature, the conversion rate between -NCO and −OH group was low, resulting in the inadequate reaction. However, while the reaction temperature is above 70 °C, the pre-reaction became severe, accompanied by a sharp rise in viscosity and side reactions. On the other hand, prolonging the reaction time can promote the transformation of NCO group, and after reaction for 3 h, the NCO value reached the theoretical one. Hence in the first step the reaction time and temperature were set to be 3 h and 60 °C respectively. Then HEMA or PETA was added as the end capping agent and introduced C]C bond to the final product. As shown in Fig. 1 (b), when under low reaction temperature at 50 °C, there was more residual NCO in the
2.6. Physical properties of oligomers and UV-cured films In this work, the UV-curable films were prepared by mixing the oligomers and monomer with the same content (5 wt %) of Irgacure 184. Following fully stirred, the mixture was casted on glass plate use an applicator with the thickness of the films about 50 μm. Subsequently, the films were cured under a mercury lamp (150–200 mJ/cm2). 2.6.1. Thermogravimetric analysis (TGA) The thermal stability of films was carried out with a 209 F1 thermogravimetric–differential thermal analyzer of German NETZSCH company, and the temperature range was set from 30 to 600 °C at a heating rate of 10 °C min−1 under nitrogen flow rate of 100 mL min−1. The sampling amount was 5−10 mg. 3
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Fig. 1. Effect of reaction time and temperature on the NCO value.
Fig. 2. The FT-IR spectra of raw materials and products.
system. The increase of temperature improved the conversation of NCO visibly. However, when the reaction temperature was more than 80 °C, the reaction system tend to be gelled. Besides, the NCO value decreased distinctly at the beginning of reaction. With the prolongation of time, there was only a small amount of NCO exists after reaction. Therefore, the reaction conditions of 70 °C and 4 h was selected for the subsequent end capping experiments.
Fig. 3. The 1H NMR spectra of PI1-PI4.
absorption peak at 3490 cm−1 corresponding to the eOH stretching vibration absorption peaks of Irgacure 184 and PETA disappeared. In addition, no absorption peak of –NCO at 2278 cm−1 was found, but a strong deformation vibration at 3320 cm−1 and a stretching vibration at 1536 cm−1 appeared, indicating the existence of eNH group. It also suggested that NCO had reacted with OH into NHCOO. Moreover, the other absorption peaks at 1720 cm−1 (C]O), 1450 cm−1 (]CH2) and
3.2. FTIR spectra analysis FTIR spectra of PETA, Irgacure 184, TDI, the first step product (1S) and PIs were shown in Fig. 2. In the FTIR curves of the PIs, a typical 4
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Fig. 4. UV–vis absorption spectra of Irgacure 184 and PIs in acetone (5 × 10−5 M) solution. Table 1 Absorption properties of Irgacure 184 and the synthesized PIs. Photoinitiators
λmax(nm)
εmax(M−1 cm−1)
Irgacure 184 PI1 PI2 PI3 PI4
243 244 244 245 255
15000 22200 22400 28900 29200
810 cm−1 (C]C) were found, indicating that the Irgacure 184 and acrylates have successfully incorporated into the polyurethane chains. 3.3. 1H NMR spectra The expected structure of PIs was further evidenced by 1HNMR as depicted in Fig. 3. For the spectrum of PI1 in Fig. 3, the peaks at δ = 9.65 ppm, δ = 8.92 ppm were considered to the proton of NH at the methyl para-position and the side position of the phenyl. As for the spectrum of PI3, the signals at 7.21 ppm can be assigned to the proton of NH without attached to phenyl. Moreover, the peaks at the range of 7.51–7.94 ppm are identified as the aromatic protons, indicating that Irgacure 184 has been grafted into the HDI trimer. The peaks at 1.23 ppm, 1.86–2.51 ppm and 3.35 ppm are attributed to the methylene, demonstrating that the existence of methylene group in the HDI trimer and HEMA. The feature at 5.88–6.02 ppm refers to the protons of eCH]CH2. In general, the target products are synthesized as the expected design according to FT-IR spectra and 1H NMR spectra. 3.4. UV–vis spectral characterization of the macro-photoinitiators The UV–vis spectra of the synthesized PI1-PI4 together with Irgacure 184 were measured and showed in Fig. 4. The typical values of maximum wavelength (λmax) and the molar absorption coefficients (ε) at λmax in acetone solution were listed in Table 1. Based on the results, PI1 to PI3 exhibit similar maximum absorption to Irgacure 184, while the absorption of PI4 is red shift (λmax = 255 nm) compared to Irgacure 184, which may be contributed to the multiple double bands attached to PI4. However, it should be noticed that all synthesized photoinitiators show higher molar absorption coefficient (εmax) than Irgacure 184.
Fig. 5. Photopolymerization efficiency of (a) different photoinitiators and (b) different concentration of PI3 using TPGDA as monomer (light intensity = 50 mw/cm2). (c) Comparison of different monomers (5 wt% PI3, light intensity = 50 mw/cm2). (d) Photopolymerization of TPGDA with different light intensity (5 wt% PI3).
3.5. Photopolymerization kinetic investigations of PIs Photopolymerization experiments of PIs that used in UV-curable were performed upon the real-time FT-IR in the absence of hydrogen 5
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Fig. 6. UV–vis absorption spectra of extraction solution from cured films.
Fig. 7. The mass fraction of the migrated Irgacure 184 in the PIs. Fig. 8. TG (a)and DTG (b) curves of the PIs.
donors. Fig. 5(a) presented the photopolymerization kinetic curves of TPGDA initiated by different PIs in comparison with Irgacure 184. As seen from Fig. 5(a), PI1, PI2 and PI3 exhibited almost the same conversion as the Irgacure 184, while PI1 and PI2 show higher initiation efficiency in the first forty seconds. However, due to the high viscosity and poor solubility in TPGDA, PI4 shows low initiation efficiency of C]C bond. Under the initiation of PI1-4, the final conversion rates of the double bond are all higher than 80 %. In addition, under the condition of 50 mW/cm2 light intensity, the experiments with different concentrations of PI3 using TPGDA as monomer were performed. According to Fig. 5(b), with the concentration of PI3 increased from 3 wt% to 7 wt%, the double conversion gradually increased and the time that reached the maximum conversion was reduced. Besides, it is clearly figured out that the final conversions in the condition of 7 wt% or 5 wt% PI3 were almost the same. High concentrations of photoinitiators can cause a dramatic increase in viscosity and lead to gel formation. On the other hand, optical shielding effect also weaken the photopolymerization activity. Therefore, 5 wt% was preferred as the amount of photoinitiator in the following UVcuring study.
Table 3 Thermogravimetric data of PIs. Runs
T10 % (°C)
T50 % (°C)
Tmax (°C)
Rmax (%/min)
Residue rate at 600 °C (%)
PI1 PI2 PI3 PI4
308.5 299.7 301.9 363.0
435.9 414.3 423.3 445.2
457.1 431.0 434.5 446.9
11.67 12.95 8.50 14.87
2.01 5.94 11.56 12.96
In order to evaluate the influence of different monomer on photoinitiation activities, HEMA, TPGDA and TMPTA were added in the study of the photopolymerization kinetics of PI3. As shown in Fig. 5(c), the photopolymerization reaction and final conversion of TPGDA is higher than HEMA and TMPTA. The result presumably attributed to the fact that the fewer number of double bonds in HEMA leading to lower photoinitiation efficiency. However, as a trifunctional monomer, TMPTA exhibited the lowest double band conversion. That is because
Table 2 Application properties with different photoinitiators. Films
Viscosity / mPa.s
Appearance
Pencil hardness
Gloss (60 °C)
Curing time/ s
Gel ratio / %
Water resistance
PI1 PI2 PI3 PI4 Irgacure 184
5000 6700 9000 14400 —
A B B B —
HB H HB H-2H B
92 97 98 95 96
45 30 30 15 60
98.58 96.71 97.20 90.42 91.63
No No No No No
A- Faint yellow transparent liquid; B- Colorless transparent liquid. 6
change change change change change
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TMPTA would cause a significant increase of viscosity and the reduction of mobility, resulting in less diffusivity and lower termination rate comparison with lower functional monomer [41]. Light intensity is a key factor in the photopolymerization process, hence different light intensity was carried out to explore the photopolymerization of UV-curing with PI3 as the photoinitiator. The results were showed in Fig. 5(d) and revealed that the final double band conversion gradually increased with the enhance of light intensity. When the light intensity was 10 mW/cm2, there was only 40 % double band conversion. The conversion boost to 76 % and 93 % with the light intensity increased to 30 mW/cm2 and 50 mW/cm2 respectively. This could be interpreted that higher light intensity promoted the cleavage of the photoinitiator, so it also increased the generation of free radicals and further accelerated the double band conversion.
The resins tended to decomposed fully when the temperature was more than 600 °C and PI4 showed the best performance with 12.96 % residue rate at 600 °C, which mainly because the oligomer that synthesized by HDI trimer and PETA had a higher molecular. 4. Conclusions In this article, the synthesis and properties of four UV-curable monomeric photoinitiators (PI1-PI4) based on isocyanate, Irgacure 184 were reported. The photopolymerization of HEMA, TPGDA and TMPTA, initiated by PIs, was studied by real-time FT-IR. The results point out that the products could successfully initiate acrylic monomer with excellent efficiency. Moreover, in contrast with Irgacure 184, low migration performance was obtained, among which the PI4 displays the lowest migration performance. Based on the results, it can be concluded that the designed photoinitiators have great potential to be used in UV curing systems.
3.6. Migration of the photoinitiators The migration stability of Irgacure 184 and the synthesized PIs was evaluated by measuring the residue photoinitiators of the cured films. As exhibited in Fig. 6, the absorption spectra of extracted acetone solution from cured films were determined by UV–vis spectroscopy. The results demonstrates that the extraction solution of resins cured by PI1−4 had weaker absorption than the resin cured with Irgacure 184 at 250 nm. In addition, Fig. 7 presented the mass fraction of the migrated Irgacure 184 moieties from PI1-4. The mass fractions were 19.6 %,18.2 %,14.3 %,10.9 % and 36.4 % for PI1, PI2, PI3, PI4 and Irgacure 184, respectively. Because the lower solubility in deionized water, the mass fraction of Irgacure 184 were less than the acetone solution. The mass fractions were 6.0 %, 5.6 %, 3.2 %, 2.1 % and 10.5 % for PI1, PI2, PI3, PI4 and Irgacure 184 in deionized water. Hence compared with Irgacure 184, the synthesized PIs proved to be significantly improved in migrant stability, which mainly attributed to the long alkyl chains. And high molecular weight endow the photoinitiators with good compatibility of acrylate monomers [42]. All these demonstrated that PIs could be served as efficient polymerizable one-component photoinitiators with toxicity in the cured materials.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. CRediT authorship contribution statement Lin Deng: Methodology, Formal analysis, Investigation, Writing original draft. Liuyan Tang: Writing - review & editing. Jinqing Qu: Resources, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
3.7. Physical properties of oligomers and coatings
This work was founded by the National Natural Science Foundation of China (No. 21878109), and the Science and Technology Program of Guangdong Province, China (No. 2015B090925006, 2016B090930005 and 2016B030302004).
As self-initiating oligomers, the physical properties of the films being cured by PIs including viscosity, appearance, pencil hardness, gloss, curing time and water resistance were summarized in Table 2. From Table 2, it can be seen that PI1, PI2 and PI3 exhibited lower viscosity than PI4, which mainly because the raw material of HDI trimer and PETA has high viscosity. In addition, the oligomers all showed excellent appearance except for PI1. PI1 showed a little bit yellow, thus resulting in a lower gloss at 60 °C. As for the result of hardness, the film being initiated by the synthesized oligomers in this work increased in contrast with the one being initiated by Irgacure 184. Especially, the films cured by the initiation of PIs completely in a short time (15−60 s). However, in terms of the properties of gel ratio, the PI4 oligomers had the worst performance, which in consistent with the photopolymerization kinetic results. Besides, the water resistance of the films were remarkable. The thermogravimetric analysis and difference thermal gravimeter were carried out from 30 °C to 600 °C and the curves were displayed in Fig. 8. The detail information of T10 % (the temperature of 10 % weight loss), T50 % (the temperature of 50 % weight loss), Tmax (temperature of maximum weight loss occurs), Rmax (maximum decomposition rate) and residue rate at 600 °C were listed in Table 3. These oligomers exhibited T10 % at 308.5 °C, 299.7 °C, 301.9 °C and 363.0 °C respectively, demonstrating that the oligomers had good heat resistance. Besides, according to the DTG curves, there were two stages in the decompose process. The first stage was due to the break of CeO of the -NHCO groups and the other one was ascribed to the further decompose to carbon volatiles in the temperature range from 345 °C to 484 °C [43].
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