Oxygen and Radical Photopolymerization in Films - ACS Symposium

Chapter 14

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch014

Oxygen and Radical Photopolymerization in Films Vadim V . Krongauz , Chander P. Chawla , and Juliette Dupre 1,*

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DSM Desotech Inc., 1122 St. Charles Street, Elgin, IL 60120 Ecole Nationale Supérieure de Chimie de Mulhouse, Université de Haute -Alsace-3, rue Werner, 68200 Mulhouse, France

Introduction

Radical photopolymerization kinetics is sensitive to the presence of oxygen that reacts with radicals forming peroxides that do not promote chain growth [1]· R + O -> ROO •

•

2

In solvent-free coatings consisting of oligomer, initiator and additives dissolved in a polymerizable monomer (reactive diluent), the presence of oxygen leads to decrease in reaction rate and a spatial anisotropy of polymer yield [2-6].

Figure 1. A diagram of anisotropic photopolymerization of a coating

© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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166 Photopolymerizing resins are rarely degassed prior to exposure to light, and therefore contain, up to 10 M dissolved oxygen [1]. Special efforts are required to achieve high speed and uniformity of cure in oxygen presence. One-sided illumination results in photopolymerization anisotropy. Intensity of excitation light decreases with the distance from the illuminated surface exponentially in accordance with Lambert-Beer's law. In optically thin, 1-10μm thick films light may be reflected from substrate, adding to the complexity. Assuming bi-radical chain termination mechanism, the square root of the total average photopolymerization rate dependence on the distance from the illuminated surface, L, can be described by the equation (1)[1,7]: -3


1/2

av

1/2

2

1/2

= (4I /L εc) [1 - exp( - εcL/2)] (1) av

0

where I is the incident light intensity at the surface, c is the concentration of the 0

absorbing species, and ε is the total extinction coefficient (Fig. 2).

Figure 2. Computed photopolymerization rate dependence on resin layer thickness; polymerization rate is assumed proportional to a square root of light -8

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intensity [1, 7]; I = 2.5x10 Einsteins, extinction coefficient, ε = 10 cm 0

-l

(mol cm) were used.

The anisotropy of photopolymerization infilmsis amplified by the uptake of oxygen from surrounding atmosphere and the diffusion of dissolved oxygen and monomer towards illuminated surface. Diffusion of reagents to illuminated



167 surface is stipulated by their higher consumption in the regions where radical production is higher, that is, l-ΙΟμηι from the illuminated surface [2-6]. Various techniques are used to reduce the detrimental effects of oxygen on radical photopolymerization infilms.These include the use of a protective cover or inert gas flow over the resin film surface, preventing oxygen influx during photopolymerization. Amines may be added, since they react with oxygen, yielding radicals capable of continuing the chain polymerization [8]. We revisited oxygen inhibition of radical photopolymerization in solventfree acrylate compositions. We compared effectiveness of resin degassing by different inert gases and considered the effects of resin viscosity. We also detected cure rate increase in the presence of small amounts of aromatic thiols. This increase could be attributed to oxygen binding and radicals rejuvenation by thiols [9, 10].

Experimental

Hexanediol diacrylate (HDODA), and isobornyl acrylate (IBOA) reactive diluents, and urethane acrylate oligomers CN963J75, CN963B80 were used (Sartomer). 2,2-Dimethoxy-2-phenylacetophenone (Irgacure® 651) or 1Hydroxycyclohexy phenyl ketone (Irgacure® 184) photoinitiators (Ciba), were used at 0%, 1% or 3% by weight. 2-Benzoxazolethiol (BOT) and 2benzothiazolethiol (BTT) (Aldrich) were used at levels 0.01 to 5%. A Perkin-Elmer DSC-7, with 100W high pressure Hg lamp was used at 30°C. Light was attenuated by a neutral densityfilter(OD=0.1) to yield a light intensity of 62.9 mJ/cm without afilter,and 7.1mJ/cm with afilter.Flow rate of He or N (Air Products) was 24.0 ml/min[15]. Fourier transformed infrared spectroscopy (FTIR) was used to monitor polymerization at the acrylate double bond absorption maximum at 810cm" (Nicolet FTIR). For FTIR the resin was placed on a NaCl crystal (oxygen-rich environment) or between two NaCl crystals (oxygen-free environment). The resin side open to air was exposed to UV light (22 mJ/cm ), while the IR absorption was recorded. Dependence of shear modulus of resin on time of UV-exposure was monitored using a Stress Tech (Reologica) instrument. 2

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Results and Discussion

The results of Tryson and Schultz [7] confirmed by us [10] showed the dependence of acrylates photo-polymerization enthalpy on sample size (Fig. 3). The data are only in qualitative agreement with equation (1) (Fig. 2) [7]. We


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attempted to explain this [10]. No oxygen reactions were considered in eq. (1) derivation, yet residual oxygen is present in the resins even after nitrogen purge. Unidirectional illumination leads to higher oxygen consumption at the surface depleting oxygen in the depth of an optically thick film. Polymerization in the absence of oxygen is less exothermic as is evident from the experimental data (Fig. 4) and from a simple Hess's cycle estimates presented below [10] Sample Size Dependence of Total Heat of Photopolymerization 3000 ι —-

0

10

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30

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50

Weight (mg) Sample Size Dependence of Total Heat of Photopolymerization 900

ι

0

—

10

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30

40

50

60

Weight (mg)

Figure 3

Sample weight dependence of total heat ofpolymerization


169 In the presence of oxygen RCH/+0

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- » (ROO*)* + 29kcal

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R C H O O [RC H-O-O H* ]* RCHOOHCR + 37 kcal 2 RCHO* -> RCO + R C O H + 80 kcal Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch014

RCHO* + R,* - » RCHOR, + 45 kcal In the absence of oxygen, the chain propagation and termination reactions, such as listed below, are noticeably less exothermic. R* + R* -> R-R + 25 to 80 kcal chain termination R* + H C=C-R, -> •R-CH^C-R, + 2 to 20 kcal chain propagation R* + HR, -> R H + * R, + 2 to 20 kcal chain transfer 2

Thus, dissolved oxygen raises the specific total enthalpy of photopolymerization even higher in larger samples, than is predicted by the equation (1) (Fig. 3, 2). Removal of Oxygen Dissolved in Formulation with 97% HDODA 500 ι

1

s, 4oo S

no N2

:

g 300 ce

3 55 200 ο 100 Η

ο

5

ο Time (min) Figure 4.

Integral heat emission dependence on nitrogen purge time

Nitrogen purging does not remove oxygen from acrylated compositions efficiently [10]. Kinetics of heat evolution during photopolymerization indicated


170 that oxygen removal by helium is more effective than by nitrogen at short times of degassing. If degassing is prolonged, nitrogen is more effective (Fig. 5). Helium is less polar and less soluble in resins, but it diffuses faster than nitrogen. It may be concluded that inert gas diffusivity is dominant factor when the resin degassing time is short. To achieve higher photopolymerization rate at short light exposure, He should be used to purge oxygen from the resin. Comparison of Efficiency of Oxygen Removal by Helium and Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch014

Nitrogen: 59% H D O D A 50

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Time (min)

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Time (min) Figure 5.

Efficiency of oxygen removal from 200 cps resin by He, and Ν2


171 Photopolymerization and oxygen inhibition kinetics are diffusion controlled [1]. Thus, the differences in oxygen effects on the radical photopolymerization in resins of different viscosity were observed. Kinetics of acrylate photoconversion in the resinfilmsopened to air and protected from oxygen by two NaCl substrates vary in resins with different viscosities (Fig. 6).


FTIR-Detected Photopolymerization Kinetics: Resin Viscosity=2100 cps 100

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Oxygen and Radical Photopolymerization in Films - ACS Symposium

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