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RadTech Europe 2005 Conference & Exhibition Photoinitiating Systems for Hybrid Sol-Gel Coatings : Toward
Enhanced Materials for UV-Curing Applications
C. Croutxé- Barghorn, M. Feuillade, D.J. Lougnot, Department of Photochemistry, University of Haute-Alsace, France Abstract

Radiation curing in organic photopolymers has proved its value in terms of high technical performance
with both ecological and economic advantages. Understanding and control ing the photochemical
processes both in time and space represent the research in our Laboratory for more than twenty
years. Nowadays, organic-inorganic sol-gel materials have gained special interest since they combine
the characteristics of glass and polymer and improve the properties of the final material. New horizons
are thus opening for radiation curing of photopolymerizable hybrid organic/inorganic materials that are
today very attractive for different applications such as coating technology, optics, sensors, catalysis…
Although understanding of the photochemical step is of fundamental interest, little is known about the
photochemistry in hybrid sol-gel medium. In this paper, Real Time Fourier Transformed Infra-Red
spectroscopy was used to follow the photopolymerization of organic modified silicates upon UV-
irradiation. Various photoinitiating systems were tested for inducing radical polymerization of pendant
polymerizable moieties anchored on the partially condensed silicate network. Experiments performed
in both laminated and nonlaminated conditions al ow to shed some light on unexplored aspects of the
hybrid sol-gel photochemistry for coating applications. It provides new insights into
photopolymerization processes of interpenetrating organic-inorganic networks leading to advanced
materials.
1. Introduction

ORMOSILs (Organical y modified silicates) are of increasing interest as constituents of coating
materials for a wide spectrum of applications since they combine the hardness and thermal stability of
the inorganic component and the mechanical toughness and flexibility of the organic moiety [1, 2].
Material elaboration is achieved by the classical sol-gel route that forms an inorganic network at room
temperature by the hydrolysis and condensation of alkoxy compounds. Then, polymerizable acrylate,
methacrylate or epoxy functions grafted onto modified silicone alkoxides can react via a free-radical or
cationic mechanism initiated either by heating or UV-light. However, to avoid the loss of chemical
stability of organic molecules incorporated in the material for specific applications, a consolidation step
under mild conditions can be preferred. It corresponds to a photochemical process in which the
organic groups are polymerized under UV or visible irradiation.
The aim of this study is to understand the mechanisms of polymerization in hybrid sol-gel materials
and to improve the photosensibility of HSG by an appropriate choice of the photoinitiating system.
Hydrolysis and condensation reactions were precisely followed by liquid 29Si NMR spectroscopy,
which offers a description of the gel material before irradiation. Photopolymerization step was followed
by RT FTIR.
2. Experimental details

Methacryloxypropyltrimethoxysilane (MAPTMS, Aldrich) was used as a matrix precursor. Partial hydrolysis and condensation were carried on by directly adding 0.75 molar equivalent of acidified water (HCl 0.01 M). The mixture was stirred for one hour. Then, 0.75 molar equivalent of deionized water was introduced and the solution was stirred for 48 hours of maturation. Under this acidic catalyzed condition, hydrolysis is supposed to occur rapidly whereas the condensation reaction is slow [5]. Photoinitiator was added to the solution. After stirring for a few hours, the mixture was allowed to mature for 48 hours. Hydrolysis and condensation reactions release methanol and water in the medium. They were removed in a vacuum oven during 8 hours at 10 mbar and at room temperature. Partial elaboration of the silicate backbone was thus achieved (Figure 1). Then, free-radical polymerization of the methacrylate functions was proceeded by irradiating the sample under UV-light. Final y, the material consists of an inorganic-organic network. RadTech Europe 2005 Conference & Exhibition Figure 1: elaboration of hybrid sol-gel material Liquid state NMR measurements were performed on a Bruker MSL400 spectrometer. Liquid 29Si NMR experiments (79.48 MHz) were recorded with a 10.5 @s pulse width ( /4). All 29Si NMR spectra were recorded at room temperature. 29Si NMR spectra modeling was led with the WinFit program to estimate the species distribution. The photochemical process taking place under il umination was followed by Real Time FTIR. The irradiation source was a Hamamatsu Hg-Xe lamp (150W) coupled with a lightguide, transparent to near UV and visible light. Infrared spectra were recorded with a FTIR (Nexus 870, Nicolet). The photosensitive layer was coated onto BaF2 chips. The film thickness was adjusted with calibrated bar coaters or spacers to achieve suitable absorbance (typ. 100 µm films for an optical density below 1).
3. Discussion

Characterization of the inorganic part of the hybrid sol is of crucial importance to understand the mechanisms involved in the photopolymerization process. 29Si NMR is an appropriate technique to describe the condensation state of the silicate network. Chemical shift of Si is directly linked to the close environment of the atom of Si. In the case of trifunctional alkoxysilanes, the chemical shifts of 29Si range from 35 ppm to 75 ppm. Four groups corresponding to the different environments of the silicon atom are observed. The conventional Tn notation, where T represents a silicon atom and n the number of bridging oxygen atoms, was used to describe the 29Si spectra [3]. According to this notation, T0 represented monomeric groups, T1 end groups of chains, T2 middle groups in chains or cycles and T3 ful y branched sites. Final y, the letter % referred to linear species. Each condensation reaction induced a chemical shift of 8 to 9 ppm in the high field [4]. Within each group, different peaks could be distinguished, corresponding to Si atoms which differed according to their hydrolysis degree. The substitution of OH for OCH3 (hydrolysis) led to a chemical shift between 0.4 and 0.8 ppm. The liquid state 29Si NMR spectrum of a sol after the evaporation step is presented in figure 2. Figure 2: liquid state 29Si spectrum of a reference sol It can be seen that no T0 species (uncondensed precursor molecules) can be distinguish at -41 ppm. This result means that there is no more free precursor molecules in the formulation. They are al involved in oligomer species. Few T3 species are distinguishable. This result is not surprising since the precursor is not very reactive and acid conditions favour linear chains rather than crosslinked ones [5]. The sol-gel system mainly consists of linear chains. The ratio between T2 and T1 concentration can give information on the average linear SiOSi chains length (LCL) deduced by the following equation: RadTech Europe 2005 Conference & Exhibition It was found that LCL=3 in the synthesis conditions. Consequently, after an ageing time of 48 hours, the sol essentially consisted of oligomers with 3 Si atoms and thus, with 3 polymerizable moieties. The fundamental point emerging from this study is that the sol-gel material after 48 h. ageing is not far, from a structural viewpoint, from a multiacrylate (triacrylate) monomer. Consequently, photopolymerization of this precursor should be compared to the case of multimethacrylate systems. 3.2 Characteristics of photopolymerization in hybrid sol-gel materials The choice of the photoinitiator is of great importance to reach high efficiency of UV-curing. The commercial photoinitiators that have been optimized for “al -organic” monomer systems can be used for hybrid sol-gel. However, some specific properties of the sol-gel medium have to be taken into account for the choice of the photoinitiator [6, 7]: it must be soluble, stable and pH-compatible if different protonation states are possible. Indeed, the influence of the environment (polarity, temperature) can induce a loss of the photoinitiator efficiency. In this work, various commercial photoinitiators (Irgacure from Ciba Speciality Chemicals I 784, I 907, I 184, I 651, I 819) currently used for UV-curing of acrylate monomers were tested. Polymerization rates and final conversion ratios are summarized in Table 1. Under our experimental conditions, photopolymerization occurs within a few seconds for most photoinitiators used. Rp values are high enough to ensure fast photopolymerization and conversion ratio levels reach 90% for an efficient system. Table 1. Maximum polymerization rates (Rp,max/M0 s-1) and final conversion ratio (%max) obtained after 5 min. irradiation (laminated films of 100 @m, 2 wt. %, 210 mW/cm², Hg-Xe lamp emission spectrum). Although the final conversion ratio is of 90% in laminated systems, some difficulties are encountered for the curing of the hybrid sol-gel resin in contact of atmosphere: the final conversion is lower than in laminate and the polymerization at the surface of the film is not achievable even for long irradiation times at room temperature. This is a consequence of the oxygen inhibition reaction. Since acrylates are known to be more reactive than methacrylates, it was interesting to verify if the change of the polymerizable moiety could improve the photoreactivity of the sol-gel layer. For this purpose, acryloxypropyltrimethoxysilane (APTMS) was tested. Basical y, it is the same structure as MAPTMS. The only difference lies in the polymerizable function, ie. acrylate was substituted for methacrylate in APTMS. Both the polymerization rate and final conversion ratio are boosted up as compared to methacrylate monomer (Figure 3A). Another dramatic difference is the possibility to cure the surface of APTMS films whereas the film from MAPTMS can never become tack free. Reactivity of APTMS is indeed so high that radicals produced under UV-irradiation react much faster with monomers than with O2. Even at the surface of the film where the stationary concentration of O2 remains high, a total free- radical polymerization is possible. However, the use of APTMS remains limited for industrial applications because of its higher cost. An interesting way to overcome oxygen inhibition and improve the efficiency of the photopolymerization is to take advantage of the particularity of the hybrid sol-gel composition. Titanium or zirconium alkoxides are currently added to hybrid sol-gel materials in order to improve the optical and physical properties of the final material. In particular, complexion of the metal with carboxylic acid decreases the reactivity of the alkoxide, al owing the titanium to be incorporated as metal oxo-clusters with a nanometric size [8]. The influence of titanium oxo-clusters on photopolymerization kinetics is evident as shown in Figure 3. In addition, the films become tack free. RadTech Europe 2005 Conference & Exhibition Figure 3: A) Influence of the nature of the polymerizable group of the hybrid precursor on the
photopolymerization kinetic. B) Effect of the presence of titanium-oxo clusters (Ti:Si=1:10) on photopolymerization
of MAPTMS. Due to the high reactivity of titanium (IV) isopropoxide, this component was complexed with
isobutiric acid (AIB) in a molar ratio Ti:AIB = 1:2.
(3 wt. % I 651, laminated films of 100 @m, 210 mW/cm², Hg-Xe lamp emission spectrum)
Effect of titanium oxo-clusters is complex. First of all, it is concerned with their photocatalytic
properties. The presence of titanium alkoxides strongly increases the condensation state of the
medium that favours a spatial configuration of the silicates chains in which the pendant organic
moieties are arranged propitiously for polymerization. Secondly, it was demonstrated that they are
able to generate radicals under UV-irradiation [8]. The photoinduced mechanism involves mixed-
valence transient species (Ti3+-Ti4+) undergoing radicals. The improved photosensitivity of the resin
results, thus, from an addition of the photoactivity of both organic and inorganic species but also from
the sol configuration that accounts for extensive polymerization reactions resulting in higher Rp value
and conversion ratio [7]. In consequence, titanium-oxo clusters favor both the reticulation of the
organic and inorganic network. This piece of work demonstrates that the use of bicomponent
photoactive systems is an elegant route to improving the efficiency of the photopolymerization and
obtaining tack-free photosensitive films.
It has to be reported that I 819 is not stable in sol-gel formulations containing titanium alkoxides. Its
degradation is related to the oxidation of benzoyl phosphinoxides in acid phosphides and their
saponification by residual water or alcohols even at room temperature [9].
4. Conclusion

The sol-gel technique allows to obtain hybrid materials over ful range of compositions, giving a way to tailor their mechanical or optical properties through judicious control of the chemistry: hydrolysis conditions, drying process, heating temperature, ratio and nature of the organic and inorganic structural units, presence of metal oxide, spatial y controlled photo-patterning due to the presence of photopolymerizable functions… Thus, hybrid sol-gel process can be an attractive low-cost alternative to conventional UV-curing of “all organic” photopolymers. Despite the strong interest in exploiting this material, only a few reports on the photopolymerization process occurring in hybrid sol-gel materials were realized. This study stressed out the characteristics of photopolymerization in hybrid sol-gel materials. It also pointed out the difficulties encountered for the curing of the hybrid liquid resin in contact to atmosphere. The use of bicomponent photoactive systems appears as an interesting route to increase the cohesion reached in the crosslinked film and overcome the inhibition effect of oxygen that could totally avoid UV-curing of thin films. The versatility of hybrid sol-gel materials combined to their unique physical properties and an improved reactivity resulting from the present study, should boost up the broadening of their application fields in the near future. RadTech Europe 2005 Conference & Exhibition 5. References

[1] G.
Chem. Mater., 13, 3422 (2001).
[2] H. Schmidt, J. Non-Cryst. Solids, 112, 419 (1989).
[3] G. Engelhardt, D. Michel, High Resolution Solid-State NMR of Silicates and Zeolites, Wiley & Sons, New York, 125 (1987).
[4] F. Brunet, J. Non-Cryst. Solids, 231, 58 (1998).
[5] C.J. Brinker, G.W. Scherrer, Sol-Gel Science, the Physics and Chemistry of Sol-Gel Processing, Ed. Academic Press, San-Diego, CA, (1990). [6] O. Soppera , C. Croutxé-Barghorn, J. Polym. Sci. Part A, Polm. Chem, 41, 716 (2003).
[7] O. Soppera , C. Croutxé-Barghorn, J. Polym. Sci. Part A, Polym. Chem, 41, 831, (2003).
[8] O. Soppera, C. Croutxé-Barghorn, D.J. Lougnot, New. J. Chem., 25, 1006, (2001).
[9] M. Jacobi, A. Henne, Polym. Paint Colour J., 175, 636, (1985).

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