[ACS Symposium Series] Radiation Curing of Polymeric Materials Volume 417 || UV Cure of Epoxy-Silicone Monomers

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  • Chapter 28

    UV Cure of Epoxy-Silicone Monomers

    J. V. Crivello1 and J. L. Lee

    Corporate Research and Development Center, General Electric Company, Schenectady, NY 12301

    Epoxy-functional silicone monomers are a new class of versatile monomers which are particularly attractive in their application to UV cationic curing. These monomers can be readily prepared by the platinum catalyzed hydrosilylation of Si- containing compounds with epoxy compounds bearing vinyl groups. Novel epoxy monomers containing cyclic siloxane rings were prepared as well as multifunctional epoxy monomers with star and branched structures. Those monomers containing cyclohexylepoxy groups are characterized by their high rates of cationic photopolymerization. In addition, excellent cured film properties are obtained which make the new monomers attractive for potential applications in coatings.

    As a consequence of their high cure and application speeds, essentially pollution-free operation, very low energy requirements and generally excellent properties, coatings prepared by photopolymerization techniques (UV curing) have made a substantial impact on the wood coating, metal decorating and printing industries. Early developments in this field centered about the photoinduced free radical polymerization of di and multifunctional acrylates and unsaturated polyesters. Still today, these materials remain the workhorses of this industry. While the bulk of the current research effort continues to be directed toward photoinduced free radical polymerizations, it is well recognized that ionic photopolymerizations also hold considerable promise in many application areas. Photoiniduced cationic polymerizations are particularly attractive because of the wealth of different chemical Pand physical properties which can potentially be realized through the polymerization of a wide variety of different vinyl as well as heterocyclic monomers. Further, photoinitiated cationic polymerizations have the advantage that they are not inhibited by 1Current address: Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590

    0097-6156/90/0417-0398$06.00/0 1990 American Chemical Society

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  • 28. CRIVELLO & LEE UV Cure of Epoxy-SUicone Monomers 399

    oxygen and thus, may be carried out in air without the need for blanketing with an inert atmosphere to achieve rapid and complete polymerization (1).

    P h Q t Q i n i t i a t Q T s The origin of our interest in photoinitiated cationic polymerization began with the discovery that certain onium salts, namely, diaryliodonium (I) and triarylsulfonium () salts, could rapidly and efficiently photoinitiate the polymerization of virtually all types of cationically polymerizable monomers (2-4).

    Ar I

    A r - A r A r S + - A r

    x"

    II

    Where " = BF4", PF6", AsF6", SbF6"

    The photolysis of the above compounds results in the production of strong Br0nsted acids which initiate cationic polymerization by direct protonation of the appropriate monomers. Over the past few years, we have successfully prepared a wide variety of different onium salts and have modified their structures for the purposes of tailoring their spectral absorption characteristics, enhancing their photoefficiency and changing their solubility. The ability of these compounds to be photosensitized at wavelengths both within the UV and visible regions of the spectrum adds a further dimension to the potential utility of these photoinitiators (5). Due to the above mentioned factors as well as to their commercialization by several companies, onium salts I and are the most widely employed cationic photoinitiators in use today.

    The Synthesis of Di. Tri and Tetrafunctional Epoxv-Silicone Monomers

    As mentioned previously, the photoinitiated polymerization of almost any cationically polymerizable monomer can be carried out using onium salt photoinitiators I and II. However, among the most advantageous substrates for UV cationic polymerization are epoxide-containing monomers. The major reasons for this are as follows. Epoxide-based coatings are widely used in industry today and are noted for their outstanding chemical resistance and mechanical properties. Further, monomers containing the epoxide group are readily UV polymerized using onium salt photoinitiators (6). Accordingly, recent work in these laboratories has been directed to the preparation of new epoxy-containing monomers designed specifically for UV curing applications.

    Silicon-containing epoxides with hydrolytically stable carbon-silicon bonds were first prepared by Pleuddeman by the addition of hydrogen functional silanes to epoxy compounds containing double bonds (7,8). We have employed this reaction extensively to prepare several different difunctional epoxy monomers as shown in Table I. An example of this reaction is given in equation 1 for the preparation of difunctional monomer .

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  • 400 RADIATION CURING OF POLYMERIC MATERIALS

    Table I

    Characteristics of Silicon-Containing Epoxy Monomers

    Compound " J * " " EEW* Compound " J * * EEW*

    *Epoxy equivalent weight

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  • 28. CRIVELLO & LEE UVCure of EpoxySilicone Monomers 401

    ^ l catalyst CH3 CH3

    III

    eq.l

    The reactions proceed cleanly and quantitatively to give the desired epoxy functional silicones.

    An interesting branched tetrafunctional epoxy-silicone monomer, VIII, can be readily prepared as shown in the following equation by the platinium catalyzed condensation of the tetrafunctional SI- compound, tetrakis(dimethylsiloxy)silane, with 3-vinyl-7-bicyclo[4.1.0]heptane.

    VIII eq. 2

    In an analogous fashion, starting with methyltris(dimethylsiloxy)silane, the corresponding afunctional epoxy monomer, IX, was prepared in quantitative yield. Similarly, a wide variety of complex resins containing Si- groups and quaternary silicon are available within the silicones industry and can be appfied to this chemistry.

    The Preparation of Novel Cyclic Epoxy-Functional Siloxanes

    The prospect of preparing compounds containing both epoxide rings and siloxane rings appeared to present some interesting possibilities for the synthesis of novel monomers with unusual properties. Starting with the commercially available 1,3,5,7-tetramethylcyclotetrasiloxane, it was possible to carry out a fourfold hydrosilylation reaction with various vinyl containing epoxides provided that the reaction was carried out under nitrogen and rigorously dry conditions. Equation 3 shows an example of this reaction.

    CH 3

    CI

    I I -CH3 ..pf. s.

    eq. 3

    Tetrafunctional cyclic epoxy-silicone monomer, X, was obtained as a mixture of stereo and regio isomers.

    Using the synthetic route depicted in equation 4, the trifunctional cyclic epoxysilicone monomer, XIII, shown was prepared.

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  • 402 RADIATION CURING OF POLYMERIC MATERIALS

    XIII

    Poly(dimethylsiloxane) and poly(methylhydrogensiloxane) can be equilibrated in the presence of strong acids, such as trifluoromethanesulfonic acid to give cyclic componds. This is depicted in equation 5.

    Depending on the ratios of the two polymers used, one can produce equilibrium mixtures in which there are present as the major cyclic components six, eight and ten membered rings having one to five hydrogens attached per ring. These mixtures may be fractionated to give specific desired cyclic compound. However, in the usual case, an isomeric mixture of compounds of any given ring size will be obtained. For example, the above method was used for the synthesis of a cyclic difunctional epoxy-silicone monomer having an eight membered siloxane ring. This monomer actually consists of the two regio isomers shown below together with a number of related stereoisomers.

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  • 28. CRIVELLO & LEE UV Cure of EpoxySilicone Monomers 403

    C H 3 C H 3

    C H * O _ X S H - C H 3 C H \ O S i CH3

    I^CHs

    Shown in Table II are the structures of four novel cyclic epoxy-silicones which were prepared during the course of this work.

    The Preparation of a.w-Epoxv-Functional PolvCdimethvlsiloxane Oligomers

    A series of well characterized a,w-hydrogen difunctional polydimethylsiloxane oligomers were prepared as shown in equation 6 by the cationic ring opening polymerization of 2,2,4,4,6,6,8,8-octamethylcyclotetrasiloxane ( D 4 ) in the presence of tetramethyldisiloxane as a chain stopper (10).

    CH3

    H 3 H3

    + HSi Si I 1 CH3 CH3

    Oay/H04 H3 y n 3 y n 3

    HSi O-i-S JSi

    CH3 \ CH3 /n CH3

    CH3 CH3

    eq. 6

    The platinum catalyzed condensation of the a,w-hydrogen difunctional polydimethylsiloxane oligomers with 3-vinyl-7-oxabicyclo[4.1.0]heptane proceeds smoothly and quantitatively. Under the above conditions, a,w-epoxy-functional polydimethylsiloxanes with = 17, 41, 59 and 111 were prepared as colorless and odorless mobile oils.

    DSC Characterization of Epoxv-Siloxane Monomers

    To obtain qualitative and quantitative data concerning the reactivity of epoxy-siloxane monomers we employed differential scanning photocalorimetry (3,11). This is a general method for obtaining both qualitative and quantitative information on photopolymerizations. Specifically, the height of the exothermic peak gives a qualitative indication of the reactivity of the monomer, while the time from the opening of the shutter to the maximum of the exothermic peak wich relates to the time required to reach the maximum polymerization rate, gives a quantitative measure of the

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  • 404 RADIATION CURING OF POLYMERIC MATERIALS

    "Epoxy equivalent weight Dow

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  • 28. CRIVELLO & LEE UV Cure of EpoxySilicone Monomers 405

    reactivity. Thirdly, the area under the exothermic peak gives a direct measure of the overall enthalpy of the polymerization and hence the conversion.

    Figure 1 shows a composite of the differential photocalorimetry curves of several of the difunctional silicon-containing epoxy monomers given in Table I. Clearly, the most reactive of these monomers is . The bisglycidyl ether IV is the least reactive, while monomer VI and monomer Vu which is not shown in the figure are intermediate in their reactivity. This order of reactivity is similar to that which we have noted in an earlier publication for carbon based monomers, (3) i.e., those monomers containing cycloaliphatic moieties are more reactive than monomers containing glycidyl ether-type functional groups. Monomer V, also containing cycloaliphatic epoxy groups, is comparable in its reactivity to monomer .

    A comparison between difunctional monomer III and cyclic tetrafunctional monomer X is given in Figure 2. While both monomers are very reactive, some differences in their photocalorimetry curves can be discerned. The polymerization of is slightly faster than that of X and is essentially complete within 3 minutes. The exceptional high reactivity of III and X were further confirmed by determining their tack-free times. When a 0.25 mole percent photoinitiator {(4-octyloxyphenyl)phenyliodonium SbF6"}(i.e. 0.25 moles photoinitiator/100 mol monomer) in the above two monomers was spread as 1 mil films, tack-free times of 500 ft/min were obtained using a single 300W medium pressure mercury arc lamp.

    The differential photocalorimetric curves of four epoxy end-group functional poly(dimethylsiloxane) oligomers are given in Figure 3. It is interesting to note that, compared to monomer (n =0), the longer chain compounds show a similar profile of their reactivities in cationic polymerization which are independent of their chain length. As one progresses from n = 0 t o n = l l l i n this series, the crosslinked polymers change from very hard and brittle in the case where = 0, to soft and flexible (n = 17-59) and finally to elastomeric (n =111).

    Film Properties of Photopolvmerized Epoxy-Silicone Monomers Some preliminary properties of photocured films of several of the epoxy-silicone monomers described in this paper are shown in Table III. Excellent properties are obtained for these materials even at short irradiation doses. Most noteworthy are the very high glass transition temperatures which were obtained for the crosslinked polymers after an irradiation time of 5 seconds. High gel contents are noted in all cases for these materials after a 1 second irradiation. The hardness of the cured resins appears to be dependent on the degree of functionality (epoxy equivalent weight) of the respective epoxy-silicone monomer, with the highest hardness obtained for the cyclic tetrafunctional epoxy monomer X. In general, the new monomers exhibit excellent solvent resistance as measured by the number of methyl ethyl ketone double rubs. When cured, the monomers give clear, glossy smooth films which show a surprising degree of flexibility. Lastly, Figure 4 shows the thermogravimetric analysis curves in nitrogen and air for the photocrosslinked polymer derived from monomer III. This polymer is stable to 250C in air and 35...

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