Makmmol. Chem., Rapid Commun. 11, 159-167 (1990) 159

A new class of highly reactive acrylic monomers, 1

Light-induced polymerization

Christian Decker: Khalil Moussa

Laboratoire de Photochimie GtnCrale (CNRS), Ecole Nationale SupCrieure de Chimie, 3 rue Alfred Werner, 68200 Mulhouse, France

(Date of receipt: December 18, 1989)=)

Introduction

Ultraviolet-curable acrylic resins have found widespread applications in various industrial sectors, mainly to protect materials by fast drying coatings l v 2 ) and to pro- duce high-resolution relief patterns 3). Extensive studies have led to the development of very efficient photoinitiators 4* 5, and to a large number of functionalized oligo- m e r ~ ~ - ~ ) , which permits to obtain crosslinked polymers with tailor-made properties. The monomer used as reactive diluent plays a key role, since it affects both the speed of cure and the polymerization extent, as well as the physical characteristics of the final product lo-'*). The objective of the present work was to develop acrylic monomers that would be as reactive as triacrylates, but still allow to reach close to 100% conversion, while improving at the same time some of the polymer properties. We describe here the results obtained with some new monoacrylates that contain cyclic carbonate structures and compare their performance with that of the mono- di- and tri-acrylates commonly used in ultraviolet curable systems.

The following carbonate-acrylate monomers 2-0x0- 1,3-dioxolan-4-ylmethyl acrylate (l), 2-acryloyloxyethyl 2-0x0-I ,3-dioxolan-4-ylmethyl carbonate (2), and 2-0x0- 1,3- dioxolan-4-ylmethyl3-acryloyloxypropionate (3) were used in our kinetic investigation of the photoinitiated polymerization.

CH2=CH-C-0-CH,-CH-CH2 CH,=CH-C-O-CHz-CH2-O-C-O-CH,-CH-CH-CHz

0 II A, ,d 0 II 0 II A, ,A .. c C 1 II 2 II

0 0

CH,=CH-C-O-CH,-CH,-C-O-CH2-CH-CH2

0 II A d II 0 '-'

a) Revised manuscript of January 25, 1990.

0 1990, Hiithig & Wepf Verlag, Basel CCC 0025-1 16X/90/$03.00

160 Ch. Decker, K. Moussa

Experimental part

2-0x0-1,3-dioxolan-4-ylmethyl acrylate (l), 2-acryloyloxyethyl 2-0x0-l,3-dioxolan-4-ylmethyl carbonate (2), and 2-0x0- 1,3-dioxolan-4-ylmethyl3-acryloyloxypropionate (3) were synthesized by the Sociktt Nationale de Poudres et Explosifs (SNPE) according to the procedure described recently13* 14).

In order to compare the reactivity of these new compounds, we performed under identical con- ditions the photopolymerization of some acrylic monomers which are widely used in radiation- curable systems: monoucrylates: ethyldiethyleneglycol acrylatea) (EDGA from Norsolor) or an oxazolidone-monoacrylate b, (ActicryP trade name SNPE); diacrylates: hexanediol-diacrylate ') (HDDA from Norsolor) or tripropyleneglycol diacrylated) (TPGDA from Union chimique Belge); triucrylute: trimethylolpropane triacrylate e, (TMPTA from Norsolor).

Except otherwise stated, all the formulations contained a radical-type photoinitiator, Ga-di- methoxydeoxybenzoin (Irgacure 651 from Ciba-Geigy), which was added to the monomer system at concentrations up to 5 wt.-Vo.

Irradiation

For kinetic investigation of the photopolymerization, the formulation was applied as a uniform film of 25 pm thickness on a sodium chloride disk by means of a calibrated wire-wound applicator. Samples were exposed in the presence of air to the radiation of a 2 kW medium pressure mercury lamp, equipped with a semi-elliptical reflector, which has a power output comparable to that of the light sources used in industrial UV curing lines (80 W per linear centimeter). The UV irradiance at the sample position was measured by actinometry and found to be 1,5. i O - 6 einstein . s . cm -2 (= 500 mW cm -2 for a wavelength of 360 nm). A camera shutter was used to select precise exposure times in the range of 2 to 100 ms.

For the evaluation of properties, the formulation was coated onto a glass plate and passed under the UV lamp at a belt speed of 10 m/min. The number of passes needed to get a tack-free coating was determined, as well as the hardness and unsaturation content of the cured polymer.

Analysis

The extent of the polymerization process after a given exposure was evaluated quantitatively by IR spectroscopy by monitoring the decrease of the sharp peak centered at 812 cm-' (twisting vibration of the acrylic CH2=CH bond). The rate of polymerization (R,) was determined from the slope of the conversion versus time curves:

where [MI,, is the initial acrylate concentration and (Asl& the absorbance at 812 cm-' after irradiation for a given time t .

It should be emphasized that the R, value thus determined is not the actual polymerization rate, since it includes the dark polymerization, which continues to develop during the few second- time lapse between the end of the exposure and the measure of the IR absorbance at 812 cm-'. We have recently shown '3 by real-time infrared (RTIR) spectroscopy that, in those acrylic monomers exposed to intense UV radiation, the post-polymerization takes a substantial part in the overall process (up to 70%), even for experiments carried out in the presence of air.

a) Systematic IUPAC name: 2-(2-ethoxyethoxy)ethyl acrylate. b, 2-(2-0~0-3-0xazolidinyl)ethyl acrylate. ') Systematic IUPAC name: hexamethylene diacrylate. d, 1,4,7-Trimethy1-3,6-dioxaoctamethylene diacrylate. e, 2-Acryloylmethyl-2-ethyltrimethylene diacrylate.

A new class of highly reactive acrylic monomers, 1 161

The hardness of the cured film was evaluated by monitoring the damping time of the oscillations of a pendulum (Persoz hardness), which is directly related to the softness of the sample. For a 50 pm thick UV-cured film, coated onto a glass plate, Persoz values typically range from 50 s for elastomeric materials to over 300 s for hard and glassy polymers.

Results and discussion

Kinetics of light-induced polymerization

When acrylic monomers, like EDGA, HDDA or TMPTA (see Expl. port) were exposed as thin films to intense ultraviolet (UV) radiation in the presence of a photoinitiator, the polymerization developed within a few tenths of a second 12) . As the monomer functionality was increased, the rate of polymerization, R,, was found to raise substantially, while the maximum degree of conversion decreased from close to 100% for monoacrylates to only 50% for triacrylates. The oxazolidone- monoacrylate (ActicrylQ) was found to exhibit a distinct behaviour in so far as it polymerized as rapidly as a triacrylate and as extensively as a monoacrylate 1 2 ) .

An even more pronounced effect was observed with the three cyclic carbonate- monoacrylates 1-3 studied 16), as shown by the polymerization profiles of Fig. 1. For monomers 1 and 3, an ultraviolet exposure as short as 0,02 s proved to be sufficient to polymerize about half of the original amount of acrylic double bonds, compared to 0.09 s for ActicrylO, 0,2 s for HDDA and as much as 0.5 s for EDGA. With monomer 2, which contains a cyclic and a linear carbonate function, the polymerization proceeds even faster, reaching 90% conversion after only 5 milliseconds of irradiation in the presence of air. Such an outstanding performance makes this compound one of the most reactive monomers ever synthetized. The overall quantum yield of the polymerization (@,), which is defined as the amount of monomer polymerized per photon absorbed, can be directly determined from the conversion versus time curves. For monomer 2 the @,, value was found to be as high as 6000 mol/photon, thus reflecting how effectively the chain reaction develops in this monomer, even at very high initiation rates.

..g 100 I I I I I I I - Fig. 1. Polymerization profiles of new acrylic monomers 1-3 exposed to UV radiation in the presence of air. For comparison profile of ActicryP, see c

Exptl. part (- - -). (Light intensity: 500 mw cm -’; photoinitiator: 5 w t . 4 of Irgacure 65 1 (see Exptl. part); film thickness: 25 pm)

c-

Irradiation time in seconds

162 Ch. Decker, K. Moussa

The rate of polymerization of monomer 2 was found to drop by a factor of 10 when the acrylate was replaced by a methacrylate group. A tack-free coating could still be obtained after a short UV exposure of a 50 pm thick film in the presence of air. This monomer ranges amongst the most reactive methacrylates known and should be of great interest for applications where toxicity considerations prevent the use of acrylic monomers.

Cure extent

The maximum rate of polymerization of monomer 2 was found to reach a value as high as 750 mol * kg-’ * s-’, i. e. a 100-fold increase over a conventional monoacrylate like EDGA (Tab. 1). A single pass under the lamp at a speed of 10 m/min was sufficient to obtain a tack-free coating with monomer 2 while with HDDA, EDGA or ActicryP the films remained tacky even after more than 10 passes. One of the most remarkable features is that the polymerization of these cyclic carbonate-acrylates proceeds extensively, up to an almost complete exhaustion of the monomer, thus leading to a cured polymer that contains a very low amount (2 to 4 wt.-Yo) of residual unsaturation (Rb. 1).

Photosensitivity

Another parameter widely used to compare the reactivity of photoresists in micro- lithography is the sensitivity, S, which is defined as the energy required to reach a certain cure extent, usually 50% conversion. The lower the S value, the more sensitive the system will be. The S values listed in Rb. I range from 50 mJ ’ cm-’ for the least reactive monomer (EDGA) to a remarkably low 0,4 mJ - cm-’ for the most reactive one (2).

Photopolymerization of neat monomers

Considering the observed great reactivity of monoacrylates containing cyclic carbonate structures, it was tempting to see whether these monomers would polymerize under UV exposure even without any photoinitiator added. After 2 s of irradiation in the presence of air, 12% of the pure monomer 1 or 3 had polymerized and as much as 40% for monomer 2 (Fig. 2), despite its low absorbance in the near UV region: A365nm = 0,002 for a 100 pm thick film.

When the UV exposure was performed in a N,-saturated atmosphere, the polymerization of the neat monomer 2 developed much faster, 70% conversion being reached after 0,25 s (Fig. 2). The 0, inhibition effect can also be reduced by covering the monomer film with a transparent polyethylene film”). After a short induction period where the oxygen dissolved in the monomer is consumed, the polymerization of this laminate, exposed to UV radiation in the presence of air, develops almost as fast as in an inert atmosphere (Fig. 2). One of the great interests for these photoinitiator- free systems is that samples a few centimetres thick can thus be easily photopolymer- ized, while avoiding at the same time the drawbacks inherent to the use of photoinitia- tors (discoloration, toxicity, lower durability, higher cost, etc.).

9 2 0, 6 r?,

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164 Ch. Decker, K. Moussa

Fig. 2. Light-induced poly- merization of pure acrylic monomers 1, 2, and 3 exposed to ultraviolet radiation in the presence of air. Photopolymerization of monomer 2 in an N, atmosphere and as laminate NaC1/2/polyethylene (film thickness: 25 pm)

0 1 2 Irradiation time in seconds

Insolu bilisation

The polymers formed upon ultraviolet exposure of the three monomers considered are all found to be strictly insoluble in organic solvents, thus indicating that an extensive crosslinking process is taking place, even though these monomers contain only one acrylate function in their structural unit. Crosslinking appears to be most important for monomer 2 where, at a degree of conversion of 0,6, the insoluble polymer represents already 80% of the irradiated material, i.e., a similar figure than for diacrylate monomers (Bb. 1). Furthermore, the fully cured polymers show very little swelling in organic solvents (less than 50/0), which demonstrates that a very tight tridimensional network is formed upon irradiation. The nature of the chemical bonds that connect the polymer chains is now being investigated. Early IR studies revealed no opening of the carbonate cycle upon photopolymerization, which suggests that the linkage between polymer chains probably results from a transfer reaction involving a labile hydrogen of the monomeric unit.

Hardness and residual unsaturation

The hardness of the ultraviolet-cured polymers was found to depend strongly on the chemical structure of the monomeric unit, with the following trend (Tab. 1):

(cyclic + linear) carbonate < ester + cyclic carbonate < cyclic carbonate

2 3 1

The increased concentration of both acrylate and cyclic carbonate groups by going from monomers 2 to 3 and then to 1 and the resulting increase of crosslink density, may account for the enhanced hardness.

A new class of highly reactive acrylic monomers, 1 165

Fig. 3. Variation of the Persoz hardness (see Exptl. part) of ultraviolet-cured acrylic coatings upon further irradiation. (For TMPTA, HDDA and EDGA, see Exptl. part)

100 c I I I I

0.5 1.0 1.5 2.0 lrradiotion time in seconds

As shown by Fig. 3, the hardness of the tack-free coating continues to raise upon further exposure, up to a constant value which is reached after about 1 s of irradiation. This can be partly correlated with a further polymerization of the unreacted monomer, as shown by the concomitant drop of the amount of residual unsaturation which was determined precisely by IR spectroscopy (Fig. 4). In addition, to their hardness these polymer films also exhibit a good flexibility, probably because of the monofunctional- ity of the monomer, in marked contrast to the stiff and brittle di- and triacrylate polymers (Tab. 2).

-s .c I Tack-free

7 6 - c C C U

C 0 ._ p 1 - 3 0

3

- L " -

- : 2 - 0 a, E -

Scrotch-free

2.0 0' 0 0.5 1.0 1,s

Irradiation time in seconds

Variation of the residual unsaturation content of ultraviolet-cured acrylic polymers from Fig. 4. 1-3 upon further irradiation

166 Ch. Decker, K. Moussa

Copolymerization

One of the shortcomings of the very reactive monomer 2 consists in its relatively high viscosity, which may straiten its range of applications. A fluid but still highly reactive formulation can be obtained by simply combining monomers 1 and 2, which are perfectly miscible. The light-induced copolymerization of a 1 / I (w/w) mixture of these monomers was found to proceed very rapidly, at a rate of 400 mol kg -l * s- l , which corresponds to a photosensitivity of 1 mJ cm-'. At the same time, the hardness of the cured polymer was substantially improved, as expected from the previously observed behaviour of monomer 1 (Tab. 2).

Tab. 2. Performance analysis of acrylic monomers. (Photoinitiator: Irgacure 651 = 5 wt.-%, cf. Exptl. part; light intensity: 500 mW * cm -*; film thickness: 25 pm)

Monomer a)

Maximum rate of polymerization in mol .kg- ' . s - '

Maximum degree of conversion in Vo

Maximum hard- ness in s

Monoacry- Diacrylate Triacrylate Cyclic carbonate-monoacrylates late HDDA TMPTA 1 2 1 + 2 EDGA

5 25 100 220 750 400

100 80 50 98 96 91

50 250 340 360 200 300

Flexibility good poor poor good good good

Surface aspect I tacky tacky glossy glossy glossy glossy

a) For abbreviations EDGA, HDDA, TMPTA, see Exptl. part.

Conclusion

The present study demonstrates for the first time the outstanding reactivity of acrylic monomers containing a cyclic carbonate structure, when they are exposed in condensed phase to ultraviolet radiation. Photopolymerization even occurs without any added photoinitiator, specially if the monomeric unit contains both a linear and a cyclic carbonate in close vicinity. These new monomers offer the distinct advantage of combining high reactivity and extensive cure to finally yield hard but still flexible polymer materials after a very short exposure to ultraviolet radiation.

A similar study has been carried out by taking the same monomers as reactive diluents in ultraviolet-curable systems, based on polyurethane-acrylates or epoxy- acrylates, which are most employed in radiation-curing applications. This work, which will be reported in a forthcoming paper, fully confirms the outstanding reactivity of these new acrylic monomers, as well as the excellent properties of the cured polymer.

A new class of highly reactive acrylic monomers, 1 167

The authors wish to thank SociPtP Nationale des Poudres et Explosifs (SNPE) for the grant of a graduate student fellowship and for the supply of monomer samples.

’) C. G. Roffey, Photopolymerization of Surface Coatings, Wiley Interscience Publ., Chichester

*) C. Decker, J. Coat. Technol. 59 (751), 97 (1987) 3, L. F. Thompson, C. G. Willson, J. M. J. Frechet, editors, Materials for Microlithography -

4, H. J. Hageman, Prog. Org. Coat. 13, 123 (1985) ’) G. F. Vesley, J. Radiat. Curing 13, 4 (1986) 6, A. Ledwith, in: “Developments in Polymerization 3’: edited by R. N. Harward, Appl. Sci.

’) F. S. Stone, R. Liberman, J. Radiat. Curing 14, 10 (1987)

’) J. J. Wildi, Proc. Radiation Technologies Conference, Florence 1989, p. 491 lo) C. Decker, T. Bendaikha, Eur. Polym. J. 20, 753 (1984) ’ I ) F. Chevalier, S. Chevalier, C. Decker, K. Moussa, SOC. Manuj Eng., Techn. Paper FC 87-9

1982

Radiation Sensitive Polymers, ACS Symp. Series 266 (1 984)

Publ., Barking 1981, p. 55

H. C. Miller, Proc. Radiation Technologies Conference, Florence 1989, p. 429

(1987) C. Decker, K . Moussa, Polym. Muter. Sci. Eng. 60, 547 (1989)

1 3 ) Fr. Patent 89.08651 (1989), J. C. Brosse, S. Chevalier, D. Couvret, C. Decker, K. Moussa 14) J. C. Brosse, S. Chevalier, D. Couvret, J. P. Senet, to be published Is) C. Decker, K . Moussa, Makromol. Chem. 189, 2381 (1988)

K . Moussa, Ph. D. Thesis, Mulhouse, November 16, 1988 C. Decker, K . Moussa, Macromolecules 22, 4455 (1989)