Photoinitiated Cross-Linking Polymerization of Monomer Blends

Chapter 8

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Photoinitiated Cross-Linking Polymerization of Monomer Blends Christian Decker Département de Photochimie Générale (UMR-CNRS Νº7525), Ecole Nationale Supérieure de Chimie de Mulhouse, Université de Haute-Alsace - 3, rue Werner, 68200 Mulhouse, France

Introduction The photoinitiated polymerization of multifunctional monomers is one of the most effective methods for producing rapidly tridimensional polymer networks and transform quasi-instantly a liquid resin into a solid and insoluble material (1-5). In most applications of this UV-curing technology a single type of monomers and telechelic oligomers is being used, mainly acrylates or epoxides which undergo polymerization by a radical or cationic-type mechanism, respectively. For some particular applications, the photopolymerization of monomer blends may provide some additional advantages, as it is an easy way to produce interpenetrating polymer networks (IPN) or crosslinked copolymers which may combine the properties of the two homopolymer networks (6-9). In the case of IPNs, photoinitiation is a unique method to generate separately each polymer network in a sequential timing (1) by a proper selection of the two photoinitiators and the radiation wavelength. A distinct feature of photoinitiation lies in the high polymerization rates which can be reached under intense illumination, together with the advantage of a solvent-free formulation curable at ambient temperature. The photocrosslinking of monomer blends is likely to find a number of industrial applications, in particular to achieve a fast drying of varnishes and printing inks, a quick setting of adhesives and composite materials and a selective insolubilization of photoresists in microlithography. The objective of this work was to study the light-induced crosslinking -polymerizationof various monomer blends, focusing our attention mainly on the kinetic aspect of these ultrafast reactions. In this respect, infrared spectroscopy proved to be a technique particularly well suited because it allows

92

© 2003 American Chemical Society

Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

93 one to monitor in real time the disappearance of each one of the monomers undergoing polymerization and to record directly the monomer conversion versus time curves in a timescale as short as 1 s. The important kinetic parameters can thus be determined, in particular the actual polymerization rate and the final cure extant, a quantity which governs the physico-chemical characteristics of the crosslinked polymer formed.


Experimental The following monomers and functionalized oligomers have been used in this study: • acrylate monomers, like hexanedioldiacrylate, HDDA, and a diacrylate derivative of bisphenol A, BPDA (Ebecryl 150), bothfromUCB Chemicals. • epoxy monomers, like a biscycloaliphatic diepoxide, BCDE (Araldite CY-179) from Ciba Specialty Chemicals, or epoxidized polyisoprene (EPI). • vinyl ether monomers, like the divinylether of triethyleneglycol (DVE-3) from ISP • vinyl fiinetionalized polymers, like polybutadiene in thermoplastic block copolymers (Kraton SBS)fromShell. The photoinitiators used are generating upon UV-exposure either free radicals (Darocur 1173, Lucirin TPO, Irgacure 819,fromCiba Specialty Chemicals and isopropyl thioxanthone, ITX) or protonic acid (a triarylsulfonium salt TAS, Cyracure 6990 from Union Carbide, or a PF diaryliodonium salt DAI from Ciba Specialty Chemicals). The liquid resin was applied onto a transparaît polypropylene film or onto a silicon wafer at a thickness ranging typically between 10 and 30 μιη. The curing was performed in the presence of air by UV-irradiation of the sample placed either in the compartment of an IR spectrophotometer (light intensity between 20 and 100 mW cm" ), or on the belt of a UV-line operated at a web speed between 5 and 60 m/min. The maximum light intensity received by the irradiated sample passing under the lamp was measured by radiometry (International Light ÏL-390) to be on the order of 500 mW em" . All the experiments have been carried at ambient temperature. The disappearance of each one of the functional groups of the monomer blend, was monitored continuously by real-time infrared (RTIR) spectroscopy, by selecting the proper wavenumber of die IR detection, at 795 cm" for the epoxy ring, at 1411 cm" for the acrylate double bond and at 1635 cm" for the vinyl ether and vinyl double bonds. Conversion versus time curves can be readily obtainedfromthe value of the IR absorbance, initially (A ) and after a given exposure (At), by plotting the ratio 1-A/Ao as a function of the exposure time. With the FTIR spectrophotometer used (Brucker IFS 66), up to 50 spectra can be recorded each second, thus allowing an accurate monitoring of fast 6

2

2

1

1

1

0



94 proceeding reactions. From the maximum slope of the conversion versus time curves recorded, the formulation reactivity, ratio of the rate of polymerization (Rp) to the monomer concentration [Mo], was evaluated for each monomer. The amount of unreacted functionalities in die UV-cured polymer was determined from the value of the final conversion readied at the end of the UV exposure. To evaluate the extent of the crosslinking reaction, the fraction of the polymer which has become insoluble in chloroform after a certain UV exposure was measured, as well as the swelling ratio defined as the amount of chloroform retained by the crosslinked polymer (Wsoivwu / Wdiy polymer)- The hardness of the UV-cured polymer was evaluated by monitoring the damping of the oscillations of a pendulum placed onto a glass plate coated with a 30 μιη thick film. Persoz values are expressed in seconds and are typically ranging from 30 s for soft elastomeric materials up to 400 s for very hard and glassy polymers.

UV-curing of acrylate / epoxide blends Multifunctional acrylate and epoxy monomers, as well as telechelic oligomers aid-capped with these functional groups, are widely used in today's UV-curable resins (4). By mixing these two types of monomers which polymerize by different mechanisms, interpenetrating polymer networks will be formed upon UV-irradiation in the presence of both radical and cationic-type photoinitiators. With the particular monomer blend selected, a 1/1 mixture by weight of Araldite CY-179 (BCDE) and Ebecryl 150 (BPDA), phase separation did not occur, neither before of after UV-curing. In the kinetic analysis of the polymerization of the epoxide by infrared spectroscopy, we encountered a problem due to the overlap of the epoxy ring IR band at 795 cm" by the acrylate double bond IR band centered at 810 cm' (Figure 1). It was yet possible to overcome this difficulty by monitoring at 1082 cm" the build-up of the ether bond which is formed upon the ringopening polymerization of the epoxide. In the neat epoxide, the intensity of the ether band was found to increase linearly with the monomer conversion, measured from the decrease of the epoxy band at 795 cm" . Since this relationship is expected to remain valid in the monomer mixture, we were able to evaluate accurately the epoxy conversion in the BCDE/BPDA blendfromthe increase of the IR absorbance at 1082 cm" . Figure 2 shows the polymerization profiles of the epoxy and acrylate monomers upon UV exposure of a 1/1 BCDE/BPDA blend containing both types of photoinitiators : [DAI] = 2 wt% and [Darocur 1173] = 2 wt%. As expected, die polymerization of the diacrylate proceeds fester and more extensively than the polymerization of the diepoxide because of a higher value of the propagation rate constant. It should be noted that a nearly complete 1

1

1

1

1


95


Absorbance

1140

1100

1060

1020

880 860 840 820 800 780

Wavenumber cm Fig. 1

-1

Variation of the infrared spectrum of an acrylate / epoxide monomer blend (BPDA/ BCDE) upon UV exposure. [DAI] = 2 wt%. I = 60mW.cm 2

Conversion (%) 100 75

/ I

I

/ / / / / /

50

•

* r

1/ 7 1

'EPOXIDE

25 \S 0

ι 10

20

UV exposure time (second) Fig.2

Photopoiymerization of a BPDA / BCDE blend in the presence of air. [Darocur 1173] = 2 wt% ; [DAI] : 2 wt%. Light intensity : 60 mW.cm" . Film thickness : 10 μτη. — neat BPDA. 2



96 polymerization of the acrylate double bond was achieved after a 20 s UVirradiation of the monomer mixture, while the conversion value was levelling off at 85% in the neat BPDA monomer (Figure 2). This behavior can be accounted for by considering that die less reactive epoxy co-monomer acts as a plasticizer during the early polymerization of the acrylate monomer : 80% conversion of the acrylate after 3 s, compared to only 15% conversion of the epoxide which is still liquid at that stage. The polymerization of the diepoxide is hardly affected by the previous buildup of die acrylate polymer network and reaches 50% conversion after a 20 s U V exposure. It should yet be noted that the remaining epoxy groups continue to react slowly upon storage of the sample in the dark because of the living character of cationic polymerization. The polymerization of the epoxide was found to be somewhat enhanced in the presence of air. This behavior was attributed to some catalytic effect of the atmosphere humidity (35%) which fevors chain transfer reactions involving the hydroxyl groups of water (10). As expected, an increase of the light intensity is speeding up the polymerization of both monomers. By working with an industrial type UV-line, a tack-free coating was already obtained after a single pass under the high intensity mercury lamp (500 mW cm") at a web speed of 10 m/min, which corresponds to an exposure time of 0.5 second. The cationic polymerization of the epoxide can be substantially accelerated by the addition of a photosensitize like isopropylthioxanthone. ITX absorbs more effectively the UV-radiation of the mercury lamp than DAI does, and it generates protonic acid by a redox reaction with the iodonium salt (3). Moreover, it produces free radicals by a photoindueed hydrogen transfer reaction with a Η-donor molecule. With the ITX+DAI combination of photoinitiators, the epoxy monomer was found to polymerize half as fast as the acrylate monomer (9). Similar polymerization profiles were recorded in the presence of air and in 0 diffusion-free conditions (laminate), thus showing the beneficial effect of the epoxide polymerization on the 0 inhibitory effect in the acrylate polymerization. The accelerated polymerization of the epoxide in the presence of a photosensitizer is generally attributed to an increase of the initiation rate due to a faster photolysis and decomposition of the diaryliodonium salt. This could be demonstrated by monitoring the disappearance of the DAI photoinitiator by infrared spectroscopy, upon UV-irradiation of the BCDE monomer. This PF iodonium salt was indeed found to exhibit a distinct IRtendcentered at 844 cm" , which disappears upon UV exposure. It is clearly apparent from die RTIR curves recorded for the DAI decay and the epoxy conversion (Figure 3), that the two processes are strongly correlated, in both the unsensitized and the ITX sensitized resins. Actually, a linear relationship was found to exist between the polymerization rate of the epoxide and the loss rate of the diarylidonium photoinitiator. 2

2

2

6

1



97 The crosslinking polymerization of the two monomers leads to a rapid insolubilization of the UV exposed resin, with formation of tight polymer networks, as shown by the low value of the swelling ratio (0.4). A tack-free coating was obtained after a 1 s exposure to intense UV light (0.5 s for the ITX sensitized sample). The polymer film continued to harden upon farther exposure to reach Persoz values on the order of350 s, nearly as high as mineral glass (400 s). hi spite of its great hardness, this aerylate/epoxide IPN was found to be moreflexiblethan the epoxy homopolymer, most probably because of the presence of the acrylate network Similar results have been obtained by UV-irradiation of a mixture of an epoxidized polyisoprene (EPI) and a diacrylate (HDDA) in the presence of both a radical photoinitiator (Lucirin TPO) and a cationic photoinitiator (Cyracure 6990). The ring-opening cationic polymerization of the epoxy groups proceeds nearly as fast as die polymerization of the acrylate double bonds (Figure 4), because of the plasticizing effect of the liquid monomer (20 wt%) introduced in EPI ; it increases markedly the molecular mobility and leads to a more complete cure than in the neat EPI. Here the two interpenetrating polymer networks are bound together by copolymerization of the acrylate double bonds of HDDA and the isoprene double bonds which are still present in the partly epoxidized polyisoprene (8). Owing to their performance regarding both processing and properties, UV-cured aerylate/epoxide polymers are expected to find their main applications as fest-drying protective coatings and for the manufacture of composite materials and optical components at ambient temperature.

UV-curing of acrylate/vinyl ether blends Vinyl ethers (VE) do not undergo homopolymerization when they are exposed to UV radiation in the presence of a radical-type photoinitiator because of the high electronic density of the VE double bond which prevents its attack by V E radicals. These monomers can however undergo a radical-type copolymerization when they are associated to an acrylate co-monomer (11). Unlike V E radicals, acrylate radicals react with both vinyl ether and acrylate double bonds, which leads to the formation of a copolymer network containing isolated vinyl ether units. Vinyl ether monomers are very effective in reducing the viscosity of telechelic acrylate oligomers, requiring typically half the amount of the homologous acrylate reactive diluents. An additional advantage of vinyl ether monomers lies in their lower odor and irritancy than acrylate monomers. By increasing the molecular mobility, vinyl ethers provide bothfesterand more complete curing of multifunctional acrylate monomers. However, since acrylate radicals are twice as reactive toward the acrylate double bond as toward the


98 Remaining DAI (%)

Epoxy conversion (%) 60

100

+ ITX

75 40 *****


50

*****··»

photoinitiator

20

25 I

I

10 20 UV exposure time (second)

I 30

Fig.3 Decay of the iodonium photoinitiator and polymerization of the epoxy monomer upon UV-irradiation of BCDE with and without photosensitizes [ITX] = 0.5 wt%; [DAI] = 2 wt%; ligjht intensity = 60 mW.cm* . 2

Conversion (%)

Exposure time (second) Fig.4

Influence of a diacrylate monomer on the photocrosslinking of an epoxidized polyisoprene. [HDDA] = 20 wt%; [Lucirin TPO] = 1 wt%; [Cyracure 6990] = 2 wt%. Light intensity = 600 mW.cm" 2



99 vinyl ether double bond, the UV-cured copolymer contains a relatively large amount of unreacted V E double bonds. The latter can not react further because there are no acrylate double bonds available anymore (the unreacted acrylate double bonds are trapped in die polymer network). In order to achieve a more complete polymerization of the monomer blend, it is necessary to introduce a second photoinitiator (TAS or DAI) to promote the cationic polymerization of the vinyl ether monomer. However, because arylonium salts undergo a fast photolysis, there was still an appreciable amount (~ 20%) of residual V E unsaturation in the UV-cured polymer. We have been able to overcome this difficulty by performing the UV-irradiation in a sequential timing. Thefirstexposure withfilteredlight (λ > 350 nm) induces only the photocleavage of the radical-type photoinitiator (Lucirin TPO), which initiates die radical copolymerization of the two monomers. The second exposure to unfiltered light induces the decomposition of the onium salt to generate the protonic acid which initiates the cationic polymerization of the unreacted V E double bonds. Figure 5 shows the polymerization profiles recorded by RTIR spectroscopy for a polyurethane-acrylate / DVE-3 mixture which was fully cured by this two step irradiation process. The properties of the polymer material obtained, depend mainly on the chemical structure of the acrylate oligomer selected, since the amount of V E monomer used as reactive diluent can be lowered typically below 20 wt%.

UV-curing of vinyl ether / epoxide Vinyl ethers are known to polymerize rapidly by a cationic mechanism in the presence of photogenerated protonic acid (12). When they are associated to epoxides, they can speed up the polymerization of these less reactive monomers and lead to a more complete cure. The product formed upon UVexposure of a VE/epoxide blend can be either a crosslinked copolymer or two interpenetrating polymer networks, depending whether these monomers undergo crosspropagation or not. Because of complex interactions between vinyl ethers and epoxides, copolymerization was not considered to take place in appreciable amount upon UV-exposure of a mixture of these monomers (13). However, when we exposed to UV-radiation an equimolar blend of BCDE and DVE-3, two monomers widely used in UV-radiation curing, in the presence of a triarylsulfonium salt, the polymerization profiles of the vinyl ether and the epoxide were found to be very similar (Figure 6), which argues in favor of a copolymerization process. An alternating copolymer network could be formed by a crosspropagation mechanism, the V E carbocation reacting preferentially with the epoxy ring, while the oxonium ion would react preferentially with the V E double bond :


100

Conversion (%) 100 /

acrylate

75


50

_

/

vinyl e t h e r ^ ^ ^ .

25 /

/

Is 0

λ > 250 nm

λ > 350 nm ι

ι

1

2

3

ι

—ι

i

4

5

6

Exposure time (second) Fig.5 Sequential polymerization of a polyurethane-acrylate / divinyl ether mixture exposed successively to filtered and unfiltered UV radiation Ebecryl 284 / DVE-3 =2/1 molar ; [Lucirin TPO] = 1 wt% ; [Cyracure 6990] = 2 wt% ; Light intensity = 50 mW.cm" . 2

Conversion (%) 100

Exposure time (second) Fig.6 Photopolymerization of a vinyl ether / epoxide blend. [DVE-3] / [BCDE] = 1 molar. [Cyracure UVI-6990] = 4 wt%. Light intensity = 20 mW.cm" . 2


101

—CH -CH-OR 2

+ —CH-CH

—CH —CH-σ I 2

—CH2-CH-OCI + CH2=CH-OR — » — C H - Ç H - O - Ç H - Ç H - C H - C H - 0 R 6r ^chOR Τ Τ


2

2

An interesting observation was made by repeating this photopolymerization experiment in die presence of an excess of vinyl ether ([VE] / [Epoxy] = 2 molar). Here again similar polymerization curves were recorded for the two monomers until half of the epoxide had polymerized, as shown in Figure 7. This means that relatively small amounts of epoxides are sufficient to suppress the cationic homopolymerization of the vinyl ether, which is known to proceed very efficiently in the neat monomer. It is only once the epoxy ring concentration has been cut by more than half (a 3 fold excess of VE) that the amount of V E polymerized becomes superior to the amount of epoxide polymerized. But, surprisingly, the homopolymerization of the vinyl ether, quantified by the difference between the two curves of Figure 7, is still proceeding much slower under those circumstances than in the neat DVE-3. These kinetic results strongly suggest that the epoxy ring of BCDE and the vinyl ether double bond of DVE-3 are undergoing alternating copolymerization. Experimental evidence in favor of such a mechanism has been recently obtained by Kostanski et al. (14) who observed a single T for the UV-cured blend. They also showed that a monofunctional V E monomer is totally incorporated into the polymer network, necessarily by copolymerization with the diepoxide. g

UV-curing of acrylate and vinyl monomer blends Photoinitiated crosslinking polymerization may also proceed by a radical mechanism in the case of acrylate and vinyl double bonds, like for example upon UV-irradiation of a polybutadiene plasticized with a diacrylate monomer (15). It proved to be an effective way to speed up the curing reaction and increase the crosslink density in a triblock styrene-butadiene-styrene (SBS) thermoplastic elastomer containing a large amount of vinyl double bonds (59%) produced by 1-2 polymerization of butadiene. This boosting effect is due to both the plasticizing effect of the liquid acrylate monomer which increases the molecular mobility in the elastomeric phase, and to a greater reactivity of vinyl radicals towards the acrylate double bond (copolymerization) than toward the vinyl double bond (homopolymerization). Copolymerization occurs mainly in die very early stage of the UV exposure, about 70% of the acrylate double bonds being consumed within the first 0.2 s. Upon further exposure, vinyl double bonds disappear mainly by homopolymerization, pretty much like in neat SBS. The additional crosslinks are causing a substantial increase of the



102 polymer hardness but, because of the elastomeric character of the polybutadiene chains, the UV-cured film still maintains a highflexibility(zero-T-bend), thus making this UV-cured polymer well suited for protective coating applications, specially on flexible substrates. A similar behavior was observed in another type of thermoplastic elastomer, acrylonitrile/butadioie/aoylonitrile (ABA), where the polybutadiene chain contains mainly 2-butene double bonds (97%) formed by the 1-4 polymerization (16). The addition of a triacrylate monomer ([TMPTA] = 20 wt%) was found to accelerate considerably the insolubilization process, as shown in Figure 8. It also leads to the formation of a more tightly crosslinked copolymer, as shown by the drastic decrease of the swelling ratio : a drop from a value of 10 for the neat ABA polymer UV-irradiated for 1 s to a value of 0.8 for the sample containing TMPTA. A similar but less pronounced effect was found by using a diacrylate ([HDDA] = 20 wt%) as reactive plasticizer. It should be noted that the addition of these acrylate monomers (20 wt%) had no major effect on the hardness of the UV-cured rubber which remained soft and flexible, even after extensive UV-exposure (Persoz hardness value between 50 and 70 s). The elastomeric character, in particular the impact resistance and the adhesives properties (tackiness), was thus retained in the photocrosslinked ABA copolymer, thus making it well suited for adhesives and safety glass applications.

Conclusion The photoinitiated polymerization of blends of multifunctional monomers is an effective method to produce polymer networks with well designed properties. Depending on the polymerization mechanism and on the kind of photoinitiator selected (radical or cationic-type), a variety of network architectures can be generated by combination of different types of monomers. By acting on the proportions of the two monomers one can finely adjust the properties of the UV-cured polymer and make it well suited for the considered application. Under intense illumination, the crosslinking polymerization of the various monomer blends occurs within a fraction of a second to generate insoluble polymer material. Such ultrafast reaction is best followed by real-time infrared spectroscopy, a technique that records directly conversion versus time curves for each one of the two types of monomers. Their intrinsic reactivity can thus be determined accurately, as well as the amount of unreacted monomer in the UV-cured polymer. Because of its process facility (short UV-irradiation of a solvent-free formulation at ambient temperature) and the broad range of product performance, the UV-curing of monomer blends is expected to attract attention


103 Polymer formed (mol/kg)


6

0

5

10

15 20 Time (sec)

Fig.7 Photoinitiated cationic polymerization of an epoxy monomer in (BCDE) in the presence of an excess of divinyl ether. [DVE-3]/[BCDE]=2 molar. [Cyracure 6990] = 4 wt%. Light intensity = 20 mW.cm" . 2

Gelfraction(%) 100

-A

\

Swelling ratio 20

A

15

10

0

0.5

1

1.5

2

Exposure time (second) Fig.8 Influence of a triacrylate monomer on the photoinduced insolubilization of an aerylonitrile - butadiene thermoplastic elastomer. [TMPTA] = 20 wt% ; [Irgacure 819] = 3 wt%. Light intensity = 600 mW.cm* . — neat ABA rubber. 2


104 in an ever-growing number of industrial sectors, in particular to achieve a fast drying of protective coatings and adhesives, and a rapid manufacturing of composite materials, microcircuits and printing plates.


Acknowledgements The authors wishes to thank his co-workers at the Polymer Photochemistry Laboratory, Drs. Danielle Decker, Trieu Nguyen Thi Viet and Khalid Zahouily. He also acknowledges the financial support from Ciba Specialty Chemicals (Basle-Switzerland) and the Centre National de la Recherche Scientifique (France). References 1. 2.

Decker C., Progr.Polym.Sci. 1996, 21, 593 Roffey C., Photogeneration

of Reactive Species for

UV-Curing,

Wiley,

New York 1997 3. 4.

Crivello J.V., J.Polym.Sci., Polym.Chem. 1999,37, 4241 Davidson R.S., Exploring the Science Technology and Applications

5.

and EB Curing, SITA Technology London 1999 Andrzejewska E., Progr.Polym.Sci. 2001, 26, 605

of UV

6.

Timpe H.J., Strehmel B., Angew. Makromol.

7. 8.

Moussa Κ., Decker C., J.Polym.Sci., Polym.Chem., 1993, 31, 2633 Decker C., Le Xuan H., Nguyen Thi Viet T., J.Polym.Sci., Polym.Chem.Ed.

1996,

Chem. 1990, 178, 131

34, 1771

9.

Decker C., Nguyen Thi Viet T., Decker D., Weber-Koehl E., Polymer 2001, 42, 5531 10. DeckerC.,Nguyen Thi Viet T., Pham Thi H., Polym. Intern,. 2001, 50, 986 11. Decker C., Decker D., J.Macromol.Sci. 1997, A34, 605

12.

Lapin S.C., in Radiation

Curing Science and Technology,

Pappas S.P. Ed.,

Plenum Press, New York, 1992 p.241 13. Rajaraman S.K., Moweers, Crivello J.V., J.Polym.Sci.Polym.Chem.Ed. 1999, 37, 4007 14. Kim Y.M., Kostanski L . K . , Mac Gregor J.F., Polymer (in press) 15. Decker C., Nguyen Thi Viet T., Macromol.Chem.Phys. 1999, 200, 358 16. Decker C., Nguyen Thi Viet T., J. Appl.Polym.Sci., 2001, 82, 2204


Photoinitiated Cross-Linking Polymerization of Monomer Blends

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