UV Cure of Epoxy-Silicone Monomers - ACS Symposium Series (ACS

Chapter 28

UV Cure of Epoxy-Silicone Monomers 1

Downloaded by UNIV OF TEXAS EL PASO on August 19, 2014 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch028

J. V.Crivello 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-Η containing compounds with epoxy compounds bearing vinyl groups. Novel epoxy monomers containing cyclic siloxaneringswere 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 photoinducedfreeradical 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 1

Current address: Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 0097-6156/90/0417-0398$06.00/0 ©1990 American Chemical Society In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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).

PhQtQinitiatQTs The origin of our interest in photoinitiated cationic polymerization began with the discovery that certain onium salts, namely, diaryliodonium (I) and triarylsulfonium (Π) salts, could rapidly and efficiently photoinitiate the polymerization of virtually all types of cationically polymerizable monomers (2-4). Ar


I +

Ar—Γ-Ar

Ar—S -Ar

x"

χ

ι

II

Where Χ" = BF ", PF ", AsF ", SbF " 4

6

6

6

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 Π 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. Epoxidebased 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 ΠΙ.

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


400

RADIATION CURING OF POLYMERIC MATERIALS

Table I Characteristics of Silicon-Containing Epoxy Monomers Compound

*Epoxy equivalent

" J * " " EEW*

Compound

"J**

weight


EEW*

28. CRIVELLO & LEE

401

UVCure of Epoxy—Silicone Monomers

^

l

catalyst

CH

CH

3

3

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-Η compound, tetrakis(dimethylsiloxy)silane, with 3-vinyl-7-bicyclo[4.1.0]heptane.

eq. 2

VIII

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-Η 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 1,3,5,7tetramethylcyclotetrasiloxane, it was possible to carry out a fourfold hydrosilylation reaction with various vinyl containing epoxides provided that the reaction was carried out under nitrogen andrigorouslydry conditions. Equation 3 shows an example of this reaction. CH

3

CI

I

I -CH

3

..pf.

Ο

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 epoxy silicone monomer, XIII, shown was prepared.



402


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 memberedringshaving one to five hydrogens attached perring.These mixtures may befractionatedto 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 epoxysilicone monomer having an eight membered siloxanering.This monomer actually consists of the two regio isomers shown below together with a number of related stereoisomers.


28. CRIVELLO & LEE

UV Cure of Epoxy—Silicone Monomers

CH

C H 3

X

CH*

403

O_ SH-CH

C

H

3

\

3

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). ÇH +

3

H—Si—Ο I

CH

3

CH3

ÇH

ÇH3

3

Si—Η

Oay/H£0

4

H—Si

»

CH

y n

33

O-i-S

ν

CH y n

33

Ο J—Si—Η

1

CH

3

CH

3

\ CH

3

/ n CH

3

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 η = 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




404

"Epoxy equivalent weight



28. CRIVELLO & LEE


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 ΙΠ. The bisglycidyl ether IV is the least reactive, while monomer VI and monomer V u 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 ΠΙ. 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 ΠΙ 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 {(4octyloxyphenyl)phenyliodonium SbF6"}(i.e. 0.25 moles photoinitiator/100 mol monomer) in the above two monomers was spread as 1 milfilms,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 ΠΙ (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 changefromvery hard and brittle in the case where η = 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 smoothfilmswhich 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 250°C in air and 350°C in nitrogen. Conclusions Epoxy-containing silicone monomers are a novel class of monomers which are very attractive as substrates for photopolymerizable coatings, inks, adhesives as well as other applications. Among the advantages which may be cited for these new monomers possess are: 1) they are easily prepared by simple, straightforward techniques 2) show outstandingly high cure rates and 3) give high qualityfilmswith excellent physical and chemical properties. Moreover, these monomers are freely miscible with other epoxy monomers and when added in modest amounts, substantially increase the rates of cationic photopolymerization which those epoxy monomers undergo. Such monomers also may be thermally cured using a wide variety of




I

ι

ι

ι

0

1

2

Irrad. Time (min.) Figure 1. Differential scanning photocalorimeter UV cure response for various difunctional epoxy-silicone monomers using 0.5 mole % (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate as photoinitiator.


28. CRIVELLO & LEE



' Π

3

Φ

.c

I

I

0

1

I 2

I

3

Irrad. Time (min.)

Figure 2. Differential scanning photocalorimeter curves for difunctional epoxy-silicone monomer III compared with tetrafunctional monomer X cured with 0.5 mole % (4octyloxyphenyl)phenyliodonium hexafluoroantimonate as photoinitiator.


407


1

1

1

1

0

1

1

2

1

I

I

3

I

CH3

3 0 1 2 Irrad. Time (min.)

CH

I

3

'

ι

0

ι

1

Figure 3. Comparison of the differential scanning photocalorimeter UV cure response of epoxy-terminated silicone oligomers with different chain lengths. The oligomers were cured using 0.5 mole % (4-octyloxyphenyl) phenyliodonium hexafluoroantimonate as photoinitiator.

0 1 2 3

-I

CH3


2

ι

ι

3

28.

CRIVELLO & LEE

409


Table III 1


Film Properties of UV Cured Epoxy-Silicone Monomers

Compound

T * 5 sec. hv g

Gel F r a c t i o n * 5 sec. hv (1 sec. hv)

P. Hardness 5 sec. hv (1 sec. hv)

Solv. R e s i s t * 5 sec. hv (l sec. hv)

>750 (>750)

>750 (260-300)

>750 (>750)

1

6 m i l f i l m s cured w i t h a GE H3T7 medium pressure mercury arc lamp using 0.25 mole % (4-octyloxyphenyl)phenyliodonium Sbl^" * Measured at 2 0 C / m i n . E x t r a c t e d w i t h acetone. * Methyl ethyl ketone double rubs. e

+


RADIATION CURING O F POLYMERIC MATERIALS

nu 100

CH,

90 80

\

70 60 Weigr


410

\

50 40

e

Heating rate = 10 C/min

30

-

20

-

10

-

\

\

^

Air

V

0 1 0

1 100

-±._ 200

1 300

L_

400

500

N » 600

1 700

2

1 800

1 900

Temperature (°C)

Figure 4. Thermogravimetric analysis curves in N2 and air for monomer III polymerized by 5 seconds irradiation using 0.5 mole % (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate as photoinitiator.


28. CRIVELLO & LEE


411

conventional epoxy curing agents. Ongoing studies are currently under way to explore these latter aspects of epoxy-functional silicone monomers. Literature Cited


1.

Sitek, F. Soc. Mfg. Eng. Radcure Europe '87 Conference Technical Paper FC 87-274 1987. 2. Crivello, J.V.; Lam, J.H.W. J. Polym. Sci., Polym. Symn. No. 56 1976, 383. 3. Crivello, J.V.; Lam, J.H.W.; Volante, C.N. J. Rad. Curing 1977, 4(3), 2. 4. Crivello, J.V.; Lam, J.H.W. J. Polym. Sci., Polym. Chem. Ed. 1979, 17(4), 2. 5. Crivello, J.V. Adv. in Polym. Sci. 1984, 62, 1. 6. Crivello, J.V.; Lam, J.H.W. In "ACS Symp. Ser. 114;" Bauer, R.S., Ed.; Am. Chem. Soc.: Washington, 1978, 1 7. Pleuddemann, E.P. Chem. Eng. Data, 1960, 5(1), 59. 8. Pleuddemann, E.P.; Fanger, G. J. Am. Chem. Soc. 1959, 81, 2632. 9. McGrath, J.E.; Yilgor, I. Adv. in Polym. Sci. 1988, 86,1. 10. Crivello, J.V.; Conlon, D.A.: Lee, J.L. J. Polym.Sci.,Part A 1986, 24, 1197. 11. Moore, J.E. In "UV Curing Science and Technology;" Pappas, S.P., Ed.; Technology Marketing Corp.: Stamford, 1978, 1. RECEIVED September 13, 1989


UV Cure of Epoxy-Silicone Monomers - ACS Symposium Series (ACS

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