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

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-Η 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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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).

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 (Π) salts, could rapidly and efficiently photoinitiate the polymerization of virtually all types of cationically polymerizable monomers (2-4).

Ar I

A r — Γ - A r A r — S + - A r

x" χ

ι II

Where Χ" = 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 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. 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 ΠΙ.

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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 Epoxy—Silicone 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-Η 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-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 1,3,5,7-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. 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 epoxysilicone monomer, XIII, shown was prepared.

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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 epoxysilicone 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 Epoxy—Silicone 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

ÇH 3 ÇH3

+ H—Si—Ο Si—Η I 1 CH3 CH3

Oay/H£04

ÇH3 » y n 3 ν y n 3

H—Si O-i-S Ο J—Si—Η

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 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

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"Epoxy equivalent weight Dow

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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 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 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 {(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 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 change from very 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 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 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 quality films with 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

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RADIATION CURING OF POLYMERIC MATERIALS

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.

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Φ . c

' Π 3

I I I I 0 1 2 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 % (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate as photoinitiator.

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CH3

CH3

CH

3

-I

1 1

1 1

1 1

I I

I I

' ι

ι ι

ι

01

23

01

23

01

23

01

23

Irrad

. Tim

e (m

in.)

Figu

re 3

. C

ompa

riso

n of

the

diff

eren

tial

scan

ning

ph

otoc

alor

imet

er

UV

cur

e re

spon

se

of

epox

y-te

rmin

ated

si

licon

e ol

igom

ers

with

di

ffere

nt c

hain

len

gths

. T

he

olig

omer

s w

ere

cure

d us

ing

0.5

mol

e %

(4-

octy

loxy

phen

yl)

phen

ylio

doni

um

hexa

fluor

oant

imon

ate

as

phot

oini

tiato

r.

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Table III

Film Properties of UV Cured 1 Epoxy-Silicone Monomers

Gel Fract ion* P. Hardness Solv. Resis t* Compound T g * 5 sec. hv 5 sec. hv 5 sec. hv

5 sec. hv (1 sec. hv) (1 sec. hv) (l sec. hv)

>750

(>750)

>750

(260-300)

>750

(>750)

1 6 mi l f i lms 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 e C / m i n . + E x t r a c t e d wi th acetone. * Methyl ethyl ketone double rubs.

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nu

100 CH,

90

CH,

80

70 \ 60

Weig

r 50 \

40 Heating rate = 10eC/min \

30 -

20 -\ ^ Air

10 -

0 1 1 -±._ 1 L_

V N 2

» 1 1 1 0 100 200 300 400 500 600 700 800 900

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 hexafluoro-antimonate as photoinitiator.

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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

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