The Photoinitiated Cationic Polymerization of Epoxy Resins - ACS

1 The Photoinitiated Cationic Polymerization of Epoxy Resins J. V. CRIVELLO and J. H . W. L A M

Downloaded by VITERBO UNIV on January 6, 2015 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch001

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

The importance of epoxy resins in the fabrication of surface coatings is well recognized in the coatings and plastics industry. In fact, the chief use of epoxy resins, amounting to over 100 million pounds last year, was for coating applications (1). Traditionally, these coatings have been applied from solvents and cured by a baking process. Current efforts to reduce the amount of energy required to carry out coating processes as well as an increasing concern for the environment have provided the impetus to begin the search for new curing chemistry and coating application techniques which circumvent these problems. Ultraviolet curing has emerged as a rapidly growing method for the fabrication of essentially pollution-free coatings, having only a fraction of the energy requirements of traditional thermally cured materials. At the same time, the generally excellent properties of the coatings which are obtained using uv curing resins together with their high application and cure speeds have made it attractive for many industries to install uv-cure lines. While the bulk o f the work i n uv c u r i n g to date has i n v o l v e d the r a d i c a l p o l y m e r i z a t i o n o f v i n y l compounds, during the past f i v e years discovery of several new p h o t o i n i t i a t o r compounds now makes i t p o s s i b l e to e f f e c t the uv-cure o f epoxy r e s i n s by a c a t i o n i c process. With such systems, s o l v e n t l e s s l i q u i d epoxy r e s i n s can be cured c o n t i n u o u s l y a t very high l i n e speeds using approximately one tenth the energy which would be consumed by a comparable thermal process. The f i r s t o f these new p h o t o i n i t i ators are the a r y l d i a z o n i u m s a l t s ( I ) (2_>3.»4,5j. when these compounds are i r r a d i a t e d using uv l i g h t , a fluoroaromatic compound, n i t r o g e n and the strong Lewis a c i d , boron t r i f l u o r i d e , are produced (equation 1 ) . +

Ar-N EN B F " 4

—

>

Ar-F +

N

2

+

BF^

(1)

I I f the p h o t o l y s i s o f an a r y l d i a z o n i u m s a l t i s conducted i n the 0-8412-0525-6/79/47-114-001$05.00/0 © 1979 American Chemical Society

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

EPOXY RESIN CHEMISTRY

presence o f an epoxy r e s i n , boron t r i f l u o r i d e c a t a l y z e s the c a t i o n i c p o l y m e r i z a t i o n o f the r e s i n . Due to the production o f nitrogen as a byproduct o f the p h o t o l y s i s o f a r y l d i a z o n i u m s a l t s , the major uses o f c o a t i n g systems employing these p h o t o i n i t i a t o r s l i e s p r i m a r i l y i n t h i n f i l m a p p l i c a t i o n s such as c o n t a i n e r c o a t ings [6) and p h o t o r e s i s t s ( 7 J . R e c e n t l y , we have reported t h a t d i a r y l i o d o n i u m s a l t s ( I I ) are a second c l a s s o f h i g h l y e f f i c i e n t p h o t o i n i t i a t o r s f o r the Ar-I-Ar' X" = B F " , A s F 4

, PF

6

6

, SbCl

, etc.

6


II c a t i o n i c r i n g opening p o l y m e r i z a t i o n o f epoxy r e s i n s ( 8 , 9 J . P a r a l l e l work i n two other l a b o r a t o r i e s has l e d to the same conc l u s i o n (10, ]V). M e c h a n i s t i c s t u d i e s have shown t h a t on p h o t o l y s i s , d i a r y l i o d o n i u m s a l t s l i b e r a t e strong Br0nsted acids of the type, HX, which subsequently i n i t i a t e c a t i o n i c p o l y m e r i z a t i o n . In t h i s paper, we would l i k e to r e p o r t some recent work which has l e d to the development o f t r i a r y l s u l f o n i u m s a l t s ( I I I ) as a t h i r d c l a s s o f useful p h o t o i n i t i a t o r s f o r c a t i o n i c polymeriz a t i o n and i n p a r t i c u l a r , describe t h e i r a p p l i c a t i o n to the p o l y m e r i z a t i o n o f epoxides. ^r Ar-S

1

+

X"

Ar" III Results and D i s c u s s i o n In our l a b o r a t o r y , we have found t h a t t r i a r y l s u l f o n i u m s a l t s ( I I I ) i n which the anions are o f the type BF " A s F " , P F " , SbF ~ e t c . , are e x c e l l e n t p h o t o i n i t i a t o r s f o r the p o l y m e r i z a t i o n of epoxy r e s i n s as w e l l as a v a r i e t y o f other monomers (12, 13). S i m i l a r r e s u l t s have a l s o been reported by another group o f investigators (14). T r i a r y l s u l f o n i u m s a l t s may be c o n v e n i e n t l y prepared v i a a number o f s y n t h e t i c r o u t e s . In equations 2 , 3 and 4 are shown three o f the most d i r e c t p r e p a r a t i v e methods. 6

6ArH

+

Ar ^S-* 0 Ar'

S C1 2

+

2

+

3C1

Ar'-MgBr

A 1 C 1 2

3

benzene r e f 1 ux

^

+

2(Ar) S Cl" 3

Ar ^ . Ar +

>

s

A r

-

x

"

+ +

6HC1

M

6

(2)

gBrOH (3)


1.

(Ar) S


2

+

(Ar') I 2

3

Photoinitiated Cationic Polymerization

CRIVELLO AND LAM

+

Cu

X"

1 1

(cat)

> Ar Ar»S

+

2

X" +

Ar'I

(4)

Although the r e a c t i o n shown i n equation 2 i s simple and d i r e c t , the products which are obtained are g e n e r a l l y impure ( 1 5 ) . The condensation o f d i a r y l s u l f o x i d e s w i t h Grignard reagents (equation 3 ) gives pure t r i a r y l s u l f o n i u m s a l t s ; however, the o v e r a l l y i e l d s are g e n e r a l l y o f the order o f 2 0 - 3 0 % ( 1 6 ) . In a very recent p u b l i c a t i o n from t h i s laboratory,we d e s c r i b e d a general s y n t h e s i s which affords these compounds i n high y i e l d s ( 1 7 _ ) . This new route i n v o l v e s the copper ( I I ) c a t a l y z e d a r y l a t i o n o f d i a r y l iodonium s a l t s with d i a r y l s u l f i d e s (equation 4 ) . Both o f the methods shown i n equations 2 and 3 give r i s e to t r i a r y l s u l f o n i u m h a l i d e s which are i n a c t i v e as p h o t o i n i t i a t o r s for c a t i o n i c p o l y m e r i z a t i o n . These s a l t s must, t h e r e f o r e , be converted to the corresponding s a l t s i n which the anion i s o f the type X" = BF4", ASF5", P F c " , e t c . Th is conversion may be accomplished using e i t h e r o f the two methods shown i n equation 5 . Ag o

_>

?

Ar S 3

+

OH" +

AgCl

(5)

Using the above s y n t h e s i s , a wide v a r i e t y o f t r i a r y l s u l f o n ium s a l t p h o t o i n i t i a t o r s can be prepared. In Table 1 are shown some r e p r e s e n t a t i v e t r i a r y l s u l f o n i u m s a l t p h o t o i n i t i a t o r s which were prepared during the course of t h i s r e s e a r c h . A l l the compounds i n t h i s t a b l e are w e l l c h a r a c t e r i z e d c r y s t a l l i n e compounds w i t h w e l l defined m e l t i n g p o i n t s , elemental a n a l y s e s , u l t r a v i o l e t , p r o t o n , and C nmr s p e c t r a (1_7). The a b i l i t y of t r i a r y l s u l f o n i u m s a l t s bearing n o n - n u c l e o p h i l i e counterions to serve as p h o t o i n i t i a t o r s i s completely general and i n c l u d e s a l l the s y m m e t r i c a l , unsymmetrical, s u b s t i t u t e d and u n s u b s t i t u t e d as w e l l as p o l y n u c l e a r and h e t e r o c y c l i c s a l t s shown i n Table 1 . In c o n t r a s t to r a d i c a l p h o t o i n i t i a t o r s which are a l s o gene r a l l y thermally unstable, t r i a r y l s u l f o n i u m s a l t s display a surp r i s i n g l e v e l o f thermal s t a b i l i t y . Figure 1 shows the thermog r a v i m e t r i c a n a l y s i s curves for t r i p h e n y l s u l f o n i u r n h e x a f l u o r o arsenate performed i n a i r and n i t r o g e n at a heating r a t e o f 10°C/minute. In both c a s e s , thermal decomposition i s not observed below 3 5 0 ° C . This high l e v e l o f thermal s t a b i l i t y i s r e f l e c t e d i n the extremely long s h e l f l i f e s o f epoxy r e s i n s s e n s i t i z e d with t r i a r y l s u l f o n i u m s a l t p h o t o i n i t i a t o r s . Figure 2 shows the p l o t s o f the increase i n s o l u t i o n v i s c o s i t y v e r s u s time i n months f o r various concentrations of ( C s ^ J q S ASF5" i n the h i g h l y r e a c t i v e b i s e p o x i d e , 4 - v i n y l c y c l o h e x e n e d i o x i d e . Only a s l i g h t increase i n the o v e r a l l s o l u t i o n v i s c o s i t y was noted 1

3

+


In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 162-165

111-112

BF/

AsF "

6

-

133-136

AsFg"

6

195-197

AsF "

P F

191-193

BF/ 4

6

Mp (°C)

Anion

64.28

5.35

7.06

g

50.10

4.01

3.95

7.23

6.67

§

fnd.

50.00 307(24,000)

s

5

*

g

7.91

8.16

6.00

5.90

7.09

6.90

7.06

w

5.41

9.14 9.33

calc.

64.32

3.98

3.88

5.03

4.96

3.41

3.31

4.31

4.28

Analysis

287(36,800)

fnd.

46.70

46.49

57.05

56.90

47.78

47.63

62.00

61.71

Elemental

Salts

275(42,100)

calc.

243(24,700) 278(4,900)

calc. fnd.

225(21,740)

fnd-

280(10,100)

249(19,700)

calc.

fnd.

237(20,400)

calc.

227(21,000)

fnd.

298(10,000)

calc.

(e)

227(21,000)

a x

298(10,000)

Xm

Preparation of Triarvlsulfonium

Table 1


ι.

ο

oo ir>


in

5


CRivELLO AND L A M

LO

·—

oo cr> cô oo

*t

«3-

ο

to

CVI

CVJ

ο

CO

i— "O «Ο C

O ο Ν

in CNJ CO KO CVI

ο Ο Ο

^ Ο Ο

Ο ο νο

Ο ο 'ί

'—. ο Ο

CM CVI CD

* Γ*» KO

00

CVI

CVI

CO

CVI

CVI

ο Ο

ο ο νο

cô ex»

00 ·—

κο «Λ

• • - ο


6


100 A

φ (0

OU


§3

οΗ

—ι 100

Figure 1.

1 200

ι 300

1 400

1 500

1 600

ι 700

Thermogravimetric analysis of (C H ) S AsF ~ in nitrogen and air at a programmed heating rate of 10°C/min 6

3

4

5

5

S

+

6

6

TIME ( M O N T H S )

Figure 2.

Solution stability study with various concentrations of (C H ) S AsF in 4-vinylcyclohexene dioxide at55°C


6

5

s

+

6

1.

7


CRivELLO AND L A M

a f t e r one year storage o f these s o l u t i o n s i n the dark at 55°C. Although t r i a r y l s u l f o n i u m s a l t s are h i g h l y t h e r m a l l y s t a b l e , they undergo r a p i d p h o t o l y s i s when i r r a d i a t e d at wavelengths from 200-300nm. Under these c o n d i t i o n s , e f f i c i e n t (Φ3]3,360 0.170.19) homolytic rupture o f one o f the carbon s u l f u r ' b o n d s r e s u l t s . The f o l l o w i n g mechanism has been proposed f o r t h i s p h o t o l y s i s (12) =

Ar S

+

X"

[Ar S

+

3

Ar S"!"

[Ar S

χ"]*

>

Ar st

YH

>

Ar S -H

+

>

Ar S

H

2Ar-

>

Ar-Ar

+

>

ArH +

+


2

^

+

Ar S -H 2

Ar-

+

>

3

YH

χ"]*

3

+

2

Ar-

+

2

+

2

a +

X~

Y-

b

c

+

d e

Y·

f (6)

I n t e r a c t i o n o f the r a d i c a l - c a t i o n , A r S - , w i t h the s o l v e n t or monomer, YH, i n step c r e s u l t s i n the r e l e a s e o f a proton i n step d and the r e s u l t a n t formation o f the strong a c i d , HX. Br0nsted a c i d s such as H B F 4 , HAsFs, and HSbF are w e l l known very r e a c t i v e i n i t i a t o r s t o r the r i n g opening p o l y m e r i z a t i o n o f epoxides (18). Although i t i s improbable t h a t acids such as those mentioned above e x i s t i n the anhydrous s t a t e , when they are gen erated by p h o t o l y s i s i n s o l u t i o n , i t i s l i k e l y t h a t immediate p r o t o n a t i o n o f the monomer or the s o l v e n t occurs to produce species such as HM X " . Subsequent a d d i t i o n o f monomer molecules r e s u l t s i n propagation and chain growth. The o v e r a l l process i s shown i n equation 7. 2

6

+

AroS

+

X"

0

M nM

+

!™

>

+

HX

HM X"

+

0

Ar-

+

Y-

+ HX

2

YH +

Ar S

+

>

HM X"

>

H(M) M n

+

X"

(7)

T r i a r y l s u l f o n i u m s a l t p h o t o i n i t i a t o r s are capable o f p o l y m e r i z i n g almost a l l known types of mono and p o l y f u n c t i o n a l epox ide c o n t a i n i n g monomers. The r e a c t i v i t y o f these systems depends on a number o f f a c t o r s . The r e a c t i v i t y and number o f epoxide groups present play a major r o l e i n determining the o v e r a l l r a t e and extent o f p h o t o p o l y m e r i z a t i o n . Monomers most a c t i v e using d i a r y l i o d o n i u m s a l t p h o t o i n i t i a t o r s are those possessing no f u n c t i o n a l i t y other than the epoxide groups, i . e . , e p o x i d i z e d o l e f i n s (19). I n t r o d u c t i o n o f f u n c t i o n a l groups such as e t h e r s ,



8


a c e t a l s or e s t e r s was found to a p p r e c i a b l y attenuate the r e a c t i v i t y o f these monomers toward strong p r o t o n i c a c i d c a t a l y z e d p o l y m e r i z a t i o n . Analogous r e s u l t s were observed using t r i a r y l s u l f o n ium s a l t p h o t o i n i t i a t o r s . While the photochemistry o f t r i a r y l s u l f o n i u m s a l t s i s domi nated by the s t r u c t u r e o f the t r i a r y l s u l f o n i u m c a t i o n which i s the l i g h t absorbing s p e c i e s , the p o l y m e r i z a t i o n chemistry i s domi nated by the nature o f the a n i o n . The reason f o r t h i s derives from the f a c t t h a t the type of anion present determines what p r o t o n i c a c i d i s produced on p h o t o l y s i s . As we have already shown, t h i s a c i d i s the species r e s p o n s i b l e f o r the p o l y m e r i z a t i o n o f the epoxide monomer. In Figure 3 i s shown p l o t s of the percent conversion to polymer versus time f o r the uv induced photopolym e r i z a t i o n of cyclohexene oxide using 0.02 moles o f t r i p h e n y l s u l fonium s a l t s having d i f f e r e n t a n i o n s . Since i t has been shown t h a t the quantum e f f i c i e n c i e s f o r p h o t o l y s i s o f t r i p h e n y l s u l f o n ium s a l t s are independent o f t h e i r anions ( 1 2 ) , the d i f f e r e n c e s observed i n the curves shown i n Figure 3 are due to the r e l a t i v e r e a c t i v i t y o f the a c i d s i n p o l y m e r i z a t i o n . The curve f o r the SbF5 s a l t was omitted from t h i s f i g u r e s i n c e p o l y m e r i z a t i o n occurred at too f a s t a r a t e to be a c c u r a t e l y measured. Triaryl sulfonium s a l t s may thus be ranked on the b a s i s o f t h e i r r e a c t i v i t y i n the f o l l o w i n g order w i t h respect to t h e i r a n i o n s : SbF5~> AsF5~>PF6~>BF4~. S i m i l a r l y , Figure 4 compares the r e a c t i v i t y o f these same s a l t s i n the photopolymerization of another monomer, styrene o x i d e . Even on a weight b a s i s , the order o f r e a c t i v i t y i s the same as noted before although the molecular weight of_the t r i a r y l s u l f o n i u m s a l t decreases i n the order SbFg">AsF ">PFg">BF^". The above model s t u d i e s w i t h monofunctional epoxy compounds give c o n s i d e r a b l e i n s i g h t i n t o the r e a c t i v i t y o f various t r i a r y l sulfonium s a l t p h o t o i n i t i a t o r s . However, i n terms o f c o a t i n g s , one i s p r i m a r i l y i n t e r e s t e d i n the r e a c t i v i t y o f these photoi n i t i a t o r s i n d i - and m u l t i f u n c t i o n a l epoxy systems. Determina t i o n o f the cure r a t e s r e q u i r e d to produce t a c k - f r e e coatings i s a g e n e r a l l y accepted i n d u s t r y standard f o r the r e a c t i v i t y o f uv cured c o a t i n g s . In Table 2 are given the cure r a t e s i n f t / m i η which were determined f o r the c y c l o a l i p h a t i c b i s e p o x i d e , 3,4e p o x y c y c l o h e x y l m e t h y l - 3 ' , 4 ' - e p o x y c y c l o h e x a n e c a r b o x y l a t e (ERL 4221) using 1 mole % o f t r i phenylsulfoniurn BF ~ P F " , A s F / and SbF ' s a l t s . 4 6 b A more q u a n t i t a t i v e measure o f the degree o f cure as w e l l as the cure r a t e i n uv cure systems can be obtained w i t h the a i d of a s p e c i a l l y designed d i f f e r e n t i a l scanning c a l o r i m e t e r . This device was developed at General E l e c t r i c (20) and described by the present authors i n previous p u b l i c a t i o n s ( 1 9 , 2 1 ) . Figure 5 shows the type of data which can be obtained using t h i s d e v i c e . When the s h u t t e r i s opened a l l o w i n g uv l i g h t to s t r i k e the sam p l e , immediate exothermic r e a c t i o n ensues which i s recorded as a peak by the s t r i p c h a r t r e c o r d e r . The time i n t e r v a l from when the s h u t t e r i s opened to the peak o f the exothermic curve i s one 6

C



CRTVELLO AND L A M


80

Ε 0

ι ι 1 2

ι 5 UV CURE(MIN)

I 10

Figure 4. Photopolymerization of styrene oxide using 10% (C H ) S* X' salts 6


5

S

10


Table 2 UV Cure o f ERL 4221 Using Various T r i p h e n y l s u l f o n i u m S a l t s Photoinitiator (C H ) S

+

(C H ) S

+

(C H ) S

+

6

5

6

5

6


3

C

H

}

195

PF "

228

AsF "

248

6

3

( 6 5 3

BF " 4

3

5

Cure Rate

S +

6

S B F

6~

2

8

(ft/miη)*

0

*1.0 mil f i l m s cured using three 200W/in. Hanovia Hg arc lamps a l i g n e d p a r a l l e l to d i r e c t i o n o f t r a v e l o f the conveyor. measure o f the r e a c t i v i t y o f the p o l y m e r i z a t i o n system under study: the s h o r t e r the time i n t e r v a l between these two p o i n t s , the more r e a c t i v e the system. In a d d i t i o n , the heat o f r e a c t i o n , ΔΗ, can be c a l c u l a t e d from the area under the curve. The ΔΗ v a l ues are a q u a n t i t a t i v e measure o f the extent o f p o l y m e r i z a t i o n . Comparing the various curves i n Figure 5 leads to the c o n c l u s i o n t h a t the c h a r a c t e r o f the anion present i n the p h o t o i n i t i a t o r has a marked e f f e c t on both the r a t e and extent o f p o l y m e r i z a t i o n . Other important r a t e determining f a c t o r s i n the p h o t o i n i t i ated p o l y m e r i z a t i o n of epoxies are the c o n c e n t r a t i o n of the p h o t o i n i t i a t o r , the wavelength o f the i r r a d i a t i n g l i g h t and i t s i n t e n s i t y and the temperature. The i n f l u e n c e o f these f a c t o r s can be c o n v e n i e n t l y determined using the modified d i f f e r e n t i a l scanning c a l o r i m e t e r (DSC). Figure 6 shows a p l o t o f the e f f e c t of c o n c e n t r a t i o n o f (C5H5)oS A s F " on the r a t e o f cure o f ERL 4221. The optimum cure rates are obtained at concentrations o f 2-3% o f t h i s p h o t o i n i t i a t o r . Further increase i n the photo i n i t i a t o r l e v e l does not produce a corresponding increase i n the cure r a t e , p o s s i b l y due to l i g h t screening e f f e c t s by the t r i a r y l sulfonium s a l t i t s e l f or i t s p h o t o l y s i s p r o d u c t s . A DSC study o f the e f f e c t o f l i g h t i n t e n s i t y on the cure r a t e of ERL 4221 c o n t a i n i n g 2% by weight ( C H ) 3 S As Fβ" i s d i s played i n Figure 7. In t h i s f i g u r e , I i s the i n c i d e n t l i g h t i n t e n s i t y d e l i v e r e d by a GE H3T7 medium pressure mercury arc lamp b a l l a s t e d at 200 W a t t s / i n c h of arc length and p o s i t i o n e d at 20 cm from the samples. As the curve shows, the cure times approach l i m i t i n g values at both high and low l i g h t l e v e l s . At high l i g h t i n t e n s i t i e s , the system i s l i m i t e d by the absorption c h a r a c t e r i s t i c s o f p h o t o i n i t i a t o r , w h i l e a t very low i n t e n s i t i e s , there appears to be some type o f t h r e s h o l d or i n h i b i t i o n e f f e c t . +

6

6

5

0



CRivELLO AND L A M


4r

t SHUTTER OPEN Figure 5.

IRRAD. TIME (MIN.)

Photopolymerization of ERL 4221 using 1.5 mol % (C H )sS* X" salts: 6

(—; 4>ss&F- ( s

+

6 ;

; 4> s A5F - (—) +

3

6 ;

The Photoinitiated Cationic Polymerization of Epoxy Resins - ACS

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