Journal of Photopolymer Science and Technology

Volume 29, Number 1 (2016) 133 －137 Ⓒ 2016SPST

Photopolymerization Kinetics of Different Chain Sizes of Bi-functional Acrylic using Real Time FT-IR

Kentaro Taki1*, Takehiro Taguchi 2, Ryota Hayashi 1, Hiroshi Ito 2

1Chemical and Materials Engineering Course, School of Natural Systems, College of Science and Engineering, Kanazawa University, Kakumacho, Ishikawa 920-1192, Japan. 2Department of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata, 992-8510, Japan

In this study, the photopolymerization kinetics of bifunctional acrylic monomers having different chain lengths, such as 1,4-bis(acryloyloxy)but ane, 1,6-bis(acryloyloxy)hexane, and 1,10-bis(acryloyloxy)decane, wa s investigated by real-time Fourier transform infrared (FTIR) spectroscopy, using Irgacure 184 ® (1 wt%) as the photoinitiator. Dark analysis was employe d for measuring the kinetic constants for propagation and termination. Plots of kinetic constants for propagation against double-bond conversion showed a plateau, su ggesting that the reaction rate is controlled at low conversion, and with increas ing conversion, the reaction rate decreases as the diffusion rate of the co ntrols propagation. At low conversion, as compared to the reaction for a monomer having a long chain length, the propagation reaction for a monomer with a short chain lengt h switched to a diffusion-rate controlled propagation reaction. The results suggested that short chain length monomers form a dense cross-linking network, which hinders the diffusion of the monomer, and the kinetic constants decrease at low conversion. The results obtained from the plot of versus conversion indicat ed that at a maximum kinetic chain length of up to 10 6, the reaction switches to the diffusion-rate controlled propagation of each monomer. Keywords: free polymeriza tion, photopolymerization, kinetics, FTIR

1. Introduction propagation reaction. However, if the rate of UV or photopolymerization diffusion of the monomer to the radical technology has been employed in a wide range becomes slow with increase of conversion, as of industries, such as electronics, cosmetics, the cross-linking network hinders the construction, and printing. For designing a diffusion of monomers, the propagation suitable formulation as well as UV curing reaction rate is controlled by the diffusion rate process conditions, it is imperative to of monomer [1]. understand the kinetics of UV curing. The conversion at which the The UV-cured coating process involves reaction-rate-controlled switches to the bulk -free cross-linking polymerization. diffusion-rate-controlled corresponds to the The polymerization consists of photo onset of the formation of cross-linking -initiation, propagation, and termination networks. It is imperative to investigate the reactions. After the phot o-initiation occurred, onset conversion for understanding the the propagation occurs when the monomer complex network structure formed by the approaches the primary or macro-radical. The photopolymerization of multi-functional propagation reaction proceeds ideally at low monomers. conversion that is reaction-rate controlled Numerical simulations of

Received April 22, 2016 Accepted May 24, 2016 133 J. Photopolym. Sci. Technol., Vol. 29, No. 1, 2016

photopolymerization are a powerful technique radicals of the initiator as well as the primary for analyzing UV curing, which has been radical and macro-radicals. Moreover, if the investigated by several researchers [2-9]. UV is stopped, the photoinitiator is not Numerical simulations can be categorized dissociated, which in turn does not result in into two approaches based on monomer the production of initiator radicals. Hence, the diffusion. One of the approaches explicitly monomer is consumed by either the primary includes the diffusion of the monomer in the or macro-radicals. Assuming that the primary reaction-diffusion equation of the monomer radical is a part of macro-radical, their kinetic [2]. The diffusion coefficient varies with constants could be the same, determined as double-bond conversion. However, it is the kinetic constant for propagation kp. The difficult to experimentally determine so-called dark polymerization has been diffusion-related parameters, such as mutual employed for determining kp and kt, which has diffusion coefficient and attenuation factor. been thoroughly reviewed by Andrzejewska The other approach includes the variation of [13]. If bimolecular ter mination is considered the kinetic constants for propagation and for termination, two parameters, A and B, are termination with conversion [3,4,9]. The determined by fitting to the conversion of parameters in the models utilized in the experimental data as follows:

aforementioned studies are determined by A  kkp/ t (1) fitting the measured data of kinetic constants.  Bk [M ] (2) The parameter fitting of the model t n0  significantly depends on i nitial values, and it 1 xtA   ln Aln Bt  te 1 (3) is difficult to avoid local minimum 1 xtA e convergence. k p    Moreover, the measured kinetic constants [M]0A0 1x IR [PI] p (4) k tend to change by the inhibition of and t  formation of cross-linking networks [9]. kp[M]0 1 xt A ( e ) [M n0 ]   (5) Although studies about network structure k [M ]2 by measurements of the kinetic constants were t n0 where kp and kt represent the kinetic constants reported so far [9-13], it is still challenging to for propagation and binary termination, investigate the kinetic of photopolymerization  respectively, [M ] represents the molar for elucidating the relationship between the n0 concentration of the macro-radical under UV photopolymerization kinetics and light shut-off conditions, xA represents the cross-linking network structures. double-bond conversion, t represents the time, te In this study, real-time Fourier transform represents the time under UV light shut-off infrared (FTIR) spectroscopy was employed conditions, [M]0 represents the initial molar for measuring the photopolymerization concentration of the double bond, η represents the kinetics of bifunctional monomers having quantum yield, I0 represents the UV intensity at different chain lengths, such as 1,4-bis 365 nm, ϵ represents the molar absorption (acryloyloxy)butane, 1,6-bis(acryloyloxy) coefficient, [PI] represents the photoinitiator hexane, 1,10-bis(acryloyloxy)decane, using concentration, Rp represents the polymerization Irgacure 184® (1 wt%) as the photoinitiator. rate under UV light shut-off conditions, and ν Dark polymerization analysis was employed represents the kinetic chain length. for measuring the kinetic constants for

propagation and termination. 3. Experimental

3.1. Materials 2. Theory Bifunctional monomers, such as Under UV irradiation, the dissociation of 1,4-bis(acryloyloxy)butane (butane, B2935), photoinitiator results in the continuous 1,6-bis(acryloyloxy)hexane (hexane, B2936), formation of its radicals, which readily react 1,10-bis(acryloyloxy)decane (decane, B2937) with the double bond, affording a primary were purchased from Tokyo Chemical radical; this primary radical then reacts with a Industry (Japan). The monomer chain length monomer and forms macro-radicals. As a increased in the order of butane < hexane < result, the monomer is consumed by the decane. Each monomer contained 25 ppm of

134 J. Photopolym. Sci. Technol., Vol. 29, No. 1, 2016 inhibitor MEHQ. The photoinitiator Irgacure elsewhere [9]. 184®was purchased from BASF (Germany). Table 1 summarizes the parameters used All other regents were used without further for calculating the kinetic constants. purification. The weight ratio of the monomer to photoinitiator was 99:1, and they were Table 1. Parameters for calculating kinetic mixed for 1 h and stored in a refrigerator at constants. 5 °C. Parameter Value −5 −2 UV intensity, I0 6.1 × 10 Em 3.2. Real-time FTIR measurement s−1 (20 mW cm−2) Two pieces of a 5 × 5 × 0.5t mm3 KBr plate Absorption coefficient 0.966 (JASCO, Japan) were used. A shim ring with a Photoinitiator 53.85 mol m−3 thickness of 10 μm, inner diameter ϕ of 3 mm, and concentration outer diameter ϕ 5 mm was used. A sample plate Temperature 320.15 K was prepared as follows: First, a shim ring was Molar concentration of 4.12 mol L−1 placed on a KBr plate, and a drop of a monomer the double bond solution was dropped on the center of the shim ring. Second, another KBr plate was placed over the 4. Results and Discussion shim ring. The shim ring maintained the solution 4.1. Conversion under UV light shut-off conditions thickness constant for every measurement. Fig. 1 shows the effect of irradiation time on the The sample plate was placed on a stage, which double-bond conversion under UV light shut-off was maintained at 47 °C, of the optical bench of a conditions. Conversion increased with increasing real-time FTIR (VERTEX 70, Bruker-Optics, irradiation time. In particular, conversion Germany) system. The beam from the beam drastically increased until 2 s. At the same splitter of the FTIR was transmitted to the normal irradiation time, conversion followed the order of direction of the sample plate. Simultaneously, the decane > hexane > butane. Difference in UV light was irradiated to the sample plate from an conversion was predominant for irradiation time incident angle of 45°. The UV light was greater than 1 s. Long chain length monomers transmitted from a high-pressure mercury lamp exhibited high conversion. (OmnicureTM S2000, EXFO Co., Canada) through a liquid light guide. The UV intensity was measured by a photometer (UT-150, USHIO, ] 1.0 ‐ [ Japan). The UV intensity was adjusted to 20 mW

−2 off cm by changing the position of the liquid light ‐ 0.8 guide and the iris of the lamp. The UV intensity shutt was adjusted to a desired value within ±0.1 mW 0.6 light −2 butane cm for increasing reproducibility. A photodiode 0.4

UV hexane

(GaAsP, G5842, Hamamatsu Photonics, Japan) at decane was used to check if the UV light was on or off. 0.2 Typically, conversion is measured as 0.0 follows: (1) UV intensity is adjusted using a Conversion UV meter. (2) The sample plate is placed on a 0.1 1 10 100 temperature-controlled stage. (3) Real-time Irradiation Time [s] FTIR measurement is carried out under UV Fig. 1. Effect of irradiation time on conversion under exposure. (4) UV light is switched off when UV light shut-off conditions. Temperature was 47 °C. the exposure time passes the pre-determined UV intensity was 20 mW cm−2. butane: time. Real-time FTIR measurement was 1,4-bis(acryloyloxy)butane, hexane: continued for 60 s for monitoring dark 1,6-bis(acryloyloxy)hexane, decane: polymerization. The absorbance peak height 1,10-bis(acryloyloxy)decane. at 812 cm−1 was utilized for calculating conversion, and the wavenumber resolution Fig. 2 plots the kinetic constants for propagation −1 was 8 cm . Thirty-three spectra were kp versus conversion. The plots of kp vs. conversion recorded every 1 s. The details of the exhibited a plateau at low conversion, and the instrument and method have been reported kinetic constants decreased with increasing

135 J. Photopolym. Sci. Technol., Vol. 29, No. 1, 2016 ]

conversion. The kinetic constant kp of butane 1 ‐ s

1E+11 1 decreased at the lowest conversion of ~0.2. The ‐ 1E+10 mol kinetic constant kp of hexane decreased at a [L

1E+09

conversion of 0.5, and that of decane was 0.6. The kt

kinetic constant of short chain length monomers 1E+08 decreased at low conversion. Short monomers are 1E+07 hypothesized to form dense networks, thereby 1E+06 termination, switching to diffusion-rate-controlled 1E+05 of

polymerization at low conversion. 1E+04 butane 1E+03 hexane decane constant 1E+02 ] 1

‐ 00. 0.20.40.6 80. 1.0 s

1 1E+11 ‐

Kinetic Conversion [‐] 1E+10 mol

[L 1E+09 kp 1E+08 Fig. 3. Kinetic constants for termination kt versus conversion. The symbols used are the same as those 1E+07 shown in Fig. 1. 1E+06 propagation,

1E+05 of

1E+04 butane 1E+08 1E+03 hexane decane 1E+07 butane constant 1E+02 hexane ]

00. 0.20.40.6 80. 1.0 ‐ decane [ 1E+06

Kinetic Conversion [‐] length 1E+05

Fig. 2. Kinetic constant for propagation kp versus chain conversion. The symbols used are the same as those 1E+04

shown in Fig. 1. 1E+03 Kinetic

Fig. 3 shows the kinetic constant for termination 1E+02 0.0 0.2 0.4 0.6 0.8 1.0 kt versus conversion. The plots of kt decreased with Conversion [‐] increasing conversion. A plateau was not observed for the three monomers indicating that the Fig. 4. Kinetic chain length versus conversion. The termination reaction was controlled by the symbols used are the same as those shown in Fig. 1. diffusion rate of radicals. As bimolecular termination occurred between two radicals, the 5. Conclusion majority of the radicals were trapped in the In this study, the photopolymerization kinetics network. Hence, the kinetic constant for of bifunctional monomers, such as termination observed inherently includes the effect 1,4-bis(acryloyloxy)butane, 1,6-bis(acryloyloxy)- of the diffusion of radicals. hexane, 1,10-bis(acryloyloxy)decane, with Fig. 4 plots kinetic chain length, representing the different chain lengths was investigated by average number of monomer units consumed for real-time FTIR. Dark polymerization analysis was each radical initiator that begins the polymerization employed for measuring the kinetic constants for of a chain, of each monomer versus conversion. propagation and termination. Kinetic chain length varied with conversion. By the comparison of conversion among the Interestingly, kinetic chain length exhibited a peak three monomers at the same irradiation time, the at around 1 × 106 at the conversion at which longest monomer, 1,10-bis(acryloyloxy)decane, diffusion-rate-controlled polymerization started. exhibited the highest conversion. The kinetic The kinetic chain grows in reaction-controlled constant for propagation exhibited a plateau, polymerization. On the other hand, in indicating reaction-controlled polymerization. diffusion-rate-controlled polymerization, the Then, the kinetic constant for propagation kinetic chain length does not propagate because of decreased with conversion, indicating the limited diffusivity of monomers. Hence, the diffusion-rate-controlled polymerization. The kinetic chain length attains a peak value. kinetic constant for propagation for the short chain

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