Radiation Curing of Polymeric Materials - American Chemical Society

Chapter 31

UV-Radiation- and Laser-Induced Polymerization of Acrylic Monomers

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch031

C. Decker and K. Moussa Laboratoire de Photochimie Générale, Unité Associée au Centre National de Recherche Scientifique, Ecole Nationale Supérieure de Chimie, 3 rue Alfred Werner, 68200 Mulhouse Cedex, France

The kinetics of ultrafast polymerization of acrylic monomers exposed to UV radiation or laser beams has been investigated by IR spectroscopy. An 8 fold increase of the cure speed was observed by using diphenoxybenzophenone as photoinitiator instead of benzophenone. The reactivity of polyurethane-acrylate or epoxyacrylate systems was markedly improved by adding acrylic monomers that contain carbamate or oxazolidone groups and which impart both hardness and flexibility to the cured polymer. Time-resolved infrared spectroscopy was used to directly record the actual polymerization profile for reactions taking place within a fraction of a second upon UV or laser exposure. Comparison with other techniques of real-time analysis show the distinct advantages of this method for an accurate evaluation of the important kinetic parameters and of the dark polymerization which develops just after the irradiation. Radiation curing has now become a well accepted technology which, owing to its distinct advantages, has found widespread industrial applications, going from the surface protection of materials by fast-drying coatings to the patterning of electronic components by in solubilization of photoresists (1τ4). New polymer materials with tailor-made properties can thus be obtained instantly by simple exposure of a photosensitive resin to an intense source of UV radiation. This method of curing still suffers some inherent shortcomings which have somewhat restricted the fast growth that was expected to occur during the last decade. Recent efforts have been directed towards an improvement of the performance of UV-curable systems, trying to overcome some of the most severe limitations by acting at different levels : increasing the resin sensitivity, with the development of more efficient photoinitiators and more reactive monomers ; lowering the residual unsaturation content of the UV-cured material ; developing new reactive diluents, less toxic and less irritating ; reducing the strong inhibition effect of atmospheric oxygen ; improving the properties of radiation cured polymers ; increasing the speed of cure by using more powerful radiation sources, like lasers. 0097-6156/90/0417-0439$06.00/0 c 1990 American Chemical Society In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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In this paper we report some of the progress recently made in these various areas, concentrating on the light-induced polymerization of multiacrylic monomers which are today the most widely used UV-curable systems. We also describe here a new analytical method, based on IR spectroscopy, that permits the kinetics of photopolymerizations, which develop extensively in a fraction of a second, to be followed quantitatively and in real time.


EXPERIMENTAL SECTION Materials. The photopolymerizable resin was made of 3 main compounds : (i) a photoinitiator that generates free radicals upon exposure to UV radiation ; (ii) a prepolymer end-capped with acrylate groups ; (iii) a reactive acrylic diluent to lower the viscosity of the resin. For most experiments, the selected photoinitiator was α,α' dimethoxyphenylacetophenone, DMPA (Irgacure 651fromCiba-Geigy), because of its high initiation efficiency. The performance of a new photoinitiator, diphenoxybenzophenone (DPB), has been evaluated and compared to benzophenone (BZP) and Irgacure 651. DPB was synthetized by phosgenation at 90°C of diphenylether in the presence of A1C1 . A small amount of a tertiary amine, methyldiethanolamine (MDEA), had to be introduced in the formulation since, like for BZP, the radical production upon photolysis proceeds by hydrogen transfer from the donor molecule to the excited state of DPB. Two types of functionalized prepolymer were used, either an aliphatic polyurethane-diacrylate (Actilane 20 from SNPE), or a diacrylate derivative from the glycidylether of bis-phenol A (Actilane 72 from SNPE), incorrectly called epoxyacrylate. The reactive diluent consisted of mono, di or triacrylic monomers, namely, ethyldiethyleneglycol-acrylate (EDGAfromNorsolor), ethylhexylacrylate (EHA from Norsolor), hexanediol-diacrylate (HDDA from UCB), tripropyleneglycol- diacrylate (TPGDA from UCB) and trimethylolpropane-triacrylate (TMPTA from UCB). Two new monomers recently developed by SNPE were also employed as reactive diluent : a carbamate-monoacrylate (Acticryl CL 960) and an oxazolidone- monoacrylate (Acticryl CL 959). Typical resin formulations contained 2 to 5 % of photoinitiator and equal parts of the acrylic prepolymer and of the diluent. The resin was applied as a uniform layer of controlled thickness, between 12 and 36 μπι, on a KBr disk with a calibrated wirewound applicator. 3

Irradiation. Samples were exposed for a short time to the radiation of a 2 kW medium pressure mercury lamp with a power output of 80 W per linear centimeter. The emitted light was focused by means of a semi-elliptical reflector on the sample, at which position the fluence rate was measured to be 500 mW c m or 1.5 χ 10 eintein s^cnr . A camera shutter was used to select a precise exposure time in the range of 2 to 100 ms. A less intense UV source (Philips HPK 125 W) was used for the kinetic investigation of the photopolymerization by real-time infra-red (RTIR) spectroscopy, with a fluence rate of 4 mW cm- . The laser-curing experiments were performed with a continuous wave argon ion laser (Spectra Physics, Model 2000) tuned to its emission line at 363.8 nm. The radiant power at the sample position was measured to be in the range 30 to 200 mW cm , depending on the selected laser output. Some photopolymerization experiments were carried out with a pulsed nitrogen laser (SOPRA, model 804 C) which emits at a wavelength of 337.1 nm. The instantaneous radiant power was calculated to be 500 kW cm- , based on the energy of a pulse (5 m J) and its duration (8 ns). The laser was operated in a multiple-pulse mode at a repetition rate between 2.5 and 40 Hz. An electronic shutter was used to select the desired number of pulses. 2

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In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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The fraction of incident light absorbed by the sample was determined by differential actinometry or from the absorbance of the coating at the wavelength of the relevant laser emission. Irradiations were carried out a room temperature, either in the presence of air or in a nitrogen-saturated reactor equipped with polyethylene windows which are transparent to both UV radiation and the IR analyzing beam. Analysis. The extent of the polymerization process was evaluated quantitatively by IR spectroscopy (Perkin Elmer spectrophotometer, model 781) by monitoring the decrease of the sharp peak centered at 812 cm* (twisting vibration of the acrylic C H = CH bond) which occurs after a given UV exposure. This analytical method has proved very valuable for measuring the polymerization rate of reactions which develop in the millisecond time scale £51, but it suffers the major disadvantage of requiring tedious point by point measurements. A new method, RTIR spectroscopy, has recently been developed to study in real time the kinetics of ultra-fast photopolymerizations and has already been described (6). It was applied here to follow quantitatively the laser-induced curing of highly reactive acrylic photoresists. The method consists of exposing the sample simultaneously to the UV beam which induces the polymerization and to the analyzing IR beam, and monitoring continuously the resulting drop in the IR absorbance of the reactive double bond. Conversion versus time curves can thus be directly recorded for polymerizations developing extensively in 0.5 second or more. Faster polymerizations were followed by using a transient memory recorder ; the limiting factor is then the response of the IR detector, usually 30 ms. The hardness of the cured film was evaluated by monitoring the damping of the oscillations of a pendulum (Persoz hardness) which is directly related to the softness of the sample. For a 35 μηι thick UV-cured film, coated onto a glass plate, Persoz values typically range from 50 s for elastomeric materials to 300 s for hard and glassy polymers. The film flexibility was evaluated by bending UV-cured coatings 180° around mandrels of decreasing diameters and measuring the diameter at which cracks first appear ; for a 3 mm diameter, the film undergoes a 30 % elongation. Highly flexible coatings were subjected to the most severe zero-T-bend test where the coated substrate was bent onto itself, with no spacer between the two halves. Samples passing this test without film cracking or crazing were marked 0 in the flexibility scale. 1


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A NEW PHOTOINITIATOR : DBPHENOXYBENZOPHENONE The efficiency of various substituted benzophenones for initiating the photopolymerization of multifunctional monomers in condensed phase was evaluated by IR spectroscopy from conversion versus exposure time kinetic curves. The best results were obtained with diphenoxybenzophenone (DPB) associated to an hydrogen donor, like methyldiethanolamine, for a one to one mixture of hexanediol-diacrylate (HDDA) and of a polyurethane-diacrylate (Actilane 20 from SNPE). Figure 1 shows some typical polymerization profiles obtained for a 30 μπι thick film exposed, in the presence of air, to the UV radiation of a medium pressure mercury lamp at a light intensity of 1.5 χ ΙΟ einstein s" cnr . At a photoinitiator concentration of 2 %, the reaction was found to develop 8 times faster with DPB than with benzophenone (BZP) and even more rapidly than with 2,2-dimethoxyphenylacetophenone (Irgacure 651 from Ciba Geigy), one of the most efficient photoinitiators. An exposure time as short as 0.05 s proved to be sufficient to make polymerize more than 75 % of the acrylate functions. The higher initiator efficiency of DPB can be attributed to either a larger absorbance in the UV region or/and to a higher production of initiating radicals. In order to differentiate these two -6

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effects, the fraction (f) of the incident radiation absorbed by the sample was determined by actinometry ; it was found to be twice larger in DPB than in BZP formulations because of a substantial bathochromic shift of the DPB absorption spectrum toward longer wavelengths. From the observed 8 fold increase in the reactivity of DPB over BZP, it can be inferred that the remarkable performance of this novel photoinitiator not only results from a better capability for absorbing the radiation emitted by the UV source, but alsofroma higher intrinsic initiation efficiency. The importance of this second factor can be assessed by evaluating the quantum yield of the polymerization (Φρ) i.e. the number of acrylate functions polymerized per photon absorbed : Φ = 110 mol photon for the BZP based formulation, and Φ = 530 mol photon- for the DPB one. For a given monomer (M), the variation of Φ are p 0.5 directly related to the variation in the initiation quantum yield Φ. : Φ = —— [Μ].Φ. . -1

ρ

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The five fold increase of Φ observed when BZP was replaced by DPB must therefore result from a much more effective production of initiating radicals with this novel photoinitiator. The reasons for such a large effect of the phenoxy substituents on the photolysis mechanism of benzophenone are now being investigated by laser spectroscopy. An additional advantage of this new photoinitiator is to provide a better through-cure of the coating and to reduce the amount of acrylate functions which have not polymerized. A 36 μπι tack-free coating photocured with DPB was found to contain only 12 % residual unsaturation, as compared to 25 % with benzophenone, thus improving its long-term stability. As a consequence of the enhanced polymerization at the coating-substrate interface, the adhesion of the polymer film onto the support was markedly increased ; this parameter is of prime importance with respect to the practical applications of curable systems as protective coatings and adhesives. Another interest of this new photoinitiator is to improve some of the mechanical properties of the cured polymer such as the hardness and scratch resistance. Polyurethane-acrylate coatings, UV cured with the DPB (2 %) + MDEA (5 %) photoinitiator system, appeared to be substantially harder (Persoz hardness = 200 s) than BZP + MDEA based coatings (120 s), as expected from the enhanced through-cure provided by DPB. Lowering the amount of tertiary amine, which acts both as plasticizer and chain transfer agent, was shown to further increase the coating hardness, but at the expense of the cure speed. Extremely hard and glassy polymers were obtained by using a bisphenol A-epoxy-acrylate as prepolymer instead of a polyurethane-acrylate, the Persoz hardness reaching then values beyond 300 s (Figure 2). It should finally be emphasized that the use of DPB is not restricted to acrylic compounds since it proved to be also a very efficient photoinitiator for the polymerization of vinyl monomers, like N-vinylpyrrolidone (NVP). In addition, DPB appeared to be particularly well-suited to photo-cure systems that need hydrogen abstraction type photoinitiators, like the thiol-polyenes resins (7), since it is then to be compared to the poor-performing benzophenone. ρ

NEW REACTIVE ACRYLIC MONOMERS The actual trends for the development of new diluents usually follow a number of important guide-lines. Besides decreasing the formulation viscosity, all tend to : lower the volatility and the toxicity ; increase the reactivity, especially for soft coatings ; decrease the amount of unreacted functional groups ; improve the final product properties.


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Conversion

Polymerization ofAcrylic Monomers

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(%)

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Diphenoxy - benzophenone

0 Exposure

Radiation Curing of Polymeric Materials - American Chemical Society

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