Epoxy Resin Chemistry - ACS Publications - American Chemical Society

2 Photosensitized Epoxides as a Basis for Light-Curable Coatings WILLIAM R. WATT

Downloaded by MONASH UNIV on December 17, 2015 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch002

Princeton Research Center, American Can Company, Princeton, ΝJ 08540

Coatings which harden or "cure" by photoinduced polymeriza tion offer a way to reduce air pollution and to conserve energy. With such coatings, the change from liquid to solid is accom plished by an increase in molecular weight rather than by removal of solvent. Although there has been increasing interest in light-curable coatings over the past decade, most published investigations deal with coatings based on free-radical polymerization. Much less has been written about light-curable coatings based on ionic polymeri zation; and what has been written is of relatively recent origin. One of the earliest descriptions of this subject was a paper pre sented by the author in 1974 describing photosensitized epoxide coatings which cure by cationic polymerization on exposure to ultraviolet radiation (1). Since that time, several technical papers concerned with coatings based on photoinduced cationic polymerization have appeared (2-11). Photoinitiators for Cationic Polymerization of Epoxides The development of light-curable epoxide coatings depends on the availablility of a suitable photoinitiator. Several compounds are known (see Table I) which w i l l 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 on exposure to l i g h t , but not a l l o f these are s u i t a b l e f o r use i n commercial c o a t i n g o p e r a t i o n s . Some are c o s t l y , and t h e i r use i n coatings a p p l i c a t i o n s would be uneconomical. With o t h e r s , the rates o f p o l y m e r i z a t i o n are too slow to be o f i n t e r e s t i n high-speed c o a t i n g o p e r a t i o n s . These p h o t o i n i t i a t o r s have gener a l l y not been a v a i l a b l e as o f f - t h e - s h e l f items from chemical s u p p l i e r s . The n e c e s s i t y to s y n t h e s i z e them, together with the f a c t that t h e i r use as p h o t o i n i t i a t o r s i s p r o t e c t e d by p a t e n t s , has been a hindrance to t h e i r widespread use. From such informa t i o n as i s a v a i l a b l e , i t appears t h a t among the compounds l i s t e d in Table I , the onium s a l t s show the g r e a t e s t promise o f meeting 0-8412-0525-6/79/47-114-017$07.00/0 © 1979 American Chemical Society

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

18

EPOXY RESIN CHEMISTRY

Table I.

Photoinitiators for Cross-Linking of Epoxides


Type

Reference

1)

Unsaturated nitrosamines ( a c t i v a t e d by l i g h t and heat)

(54)

2)

Diazonium Fluoroborates

(14)

3)

Diazonium P e r c h l o r a t e s ; P e r f l u o r o c a r b o x y l a t e s

(15)

4)

F l u o r i n a t e d alkane s u l f o n i c a c i d s a l t s o f s i l v e r and t h a l l i u m ; or a metal s a l t o f a polyboron a c i d and a s i l v e r h a l i d e or aromatic h a l i d e .

(55)

5)

Cyclopentadienylmanganese t r i c a r b o n y l compounds

(56)

6)

B i s ( p e r f l u o r o a l k y l s u l f o n y l ) methane s i l v e r s a l t

(57)

7)

Diazonium s a l t s other than t e t r a f l u o r o b o r a t e s

(18)

8)

Aryliodonium s a l t s

(35) (36) (37)

9)

Aryliodonium s a l t s plus a c a t i o n i c dye

(40)

10) Diazonium d i f l u o r o p h o s p h a t e , phosphotungstate, phosphomolybdate, tungstogermanate, s i l i c o t u n g s t a t e and m o l y b d o s i l i c a t e

(58)

11) Group V i a Aromatic onium s a l t s

(47)

12) Group Va Aromatic onium s a l t s

(44)

13) T h i o p y r y l l i u m s a l t s

(9)


2.

Photosensitized Epoxides

WATT

19


the requirements o f commercial c o a t i n g s . On p h o t o l y s i s , they w i l l i n i t i a t e p o l y m e r i z a t i o n o f some epoxides, and they have the capac i t y to promote the r a p i d cure required i n high-speed c o a t i n g operations. Three types o f onium s a l t s most often mentioned i n the l i t e r ature as p h o t o i n i t i a t o r s f o r epoxide p o l y m e r i z a t i o n are diazonium s a l t s , aryliodonium s a l t s and onium s a l t s of the elements o f Groups Va and V i a . Representative examples o f each type are shown i n Table I I . Diazonium S a l t s . The f i r s t use o f a diazonium s a l t as an i n i t i a t o r f o r c a t i o n i c p o l y m e r i z a t i o n o f c y c l i c ethers was r e ported i n 1965 by Dreyfuss and Dreyfuss ( 1 2 ) , who announced the p o l y m e r i z a t i o n o f tetrahydrofuran i n i t i a t e d by thermal decompos i t i o n o f a "new c a t a l y s t " which they i d e n t i f i e d as benzenediazonium hexafluorophosphate. (In a l a t e r p u b l i c a t i o n (13) t h i s material was c o r r e c t l y i d e n t i f i e d as p-chlorobenzenediazomum hexaf1uorophosphate). In the same y e a r , a patent was issued to L i c a r i and Crepeau (14) f o r the photoinduced p o l y m e r i z a t i o n o f epoxide r e s i n s by diazonium t e t r a f l u o r o b o r a t e s f o r use i n the encapsulation o f e l e c t r o n i c components and the preparation o f c i r c u i t boards. In 1966, E. F i s c h e r was granted a U . S . patent (15) c l a i m i n g a method f o r p o l y m e r i z i n g c y c l i c ethers by means o f diazonium s a l t s o f p e r c h l o r i c a c i d and p e r f l u o r o c a r b o x y l i c a c i d s decomposed by heat and/or u l t r a v i o l e t r a d i a t i o n . The storage s t a b i l i t y o f mixtures o f these s a l t s w i t h c y c l i c ethers was poor, and the handling o f diazonium p e r c h l o r a t e s would r e q u i r e s p e c i a l c o n s i d e r a t i o n before i n t r o d u c t i o n i n t o l a r g e - s c a l e c o a t i n g o p e r a t i o n s . Dreyfuss and Dreyfuss (13) reported t h a t p-chlorobenzenediazonium hexafluorophosphate was not a very e f f e c t i v e i n i t i a t o r f o r p o l y m e r i z a t i o n o f epoxides, based on an observation t h a t thermal decomposition of the diazonium s a l t i n the presence of propylene oxide d i d not y i e l d a high molecular weight polymer. However, S. I . S c h l e s i n g e r found t h a t epoxides could be p o l y merized by a wide v a r i e t y o f diazonium s a l t s , i n c l u d i n g p - c h l o r o benzenediazonium hexafluorophosphate, on exposure to u l t r a v i o l e t r a d i a t i o n (16, 17). He d e l i n e a t e d d i f f e r e n c e s i n behavior between various s a l t s and i d e n t i f i e d those which are most e f f e c t i v e i n promoting r a p i d p o l y m e r i z a t i o n to high molecular weight. In part i c u l a r , he showed t h a t the hexafluorophosphates are much more e f f e c t i v e as p h o t o i n i t i a t o r s than the t e t r a f l u o r o b o r a t e s (18). More than one mechanism has been proposed to e x p l a i n the c a t a l y t i c a c t i v i t y o f diazonium s a l t s i n i n i t i a t i n g p o l y m e r i z a t i o n of c y c l i c e t h e r s . Dreyfuss and Dreyfuss (13) postulated t h a t i n i t i a t i o n i n v o l v e s hydrogen a b s t r a c t i o n from the c y c l i c e t h e r by a carbenium ion formed v i a decomposition o f the diazonium s a l t , followed by p o l y m e r i z a t i o n v i a t e r t i a r y oxonium ions a s s o c i a t e d with P F ' c o u n t e r i o n s . The p o l y m e r i z a t i o n r e a c t i o n s s t u d i e d by Dreyfuss and Dreyfuss were i n i t i a t e d by thermal decomposition o f 6



STRUCTURE

REFERENCES

6

Θ P F

C) W A T T , W.R. US P A T E N T 3 , 7 9 4 , 5 7 6

B) F E I N B E R G , J . H. US P A T E N T 3 , 8 2 9 , 3 6 9

A) SCHLESINGER, S. 1., US PATENT 3 , 7 0 8 , 2 0 6

PM

HEXAFLUOROPHOSPHATE

p-METHOXYBENZENEDIAZONIUM

OCH3

ù

® N =N

Table II.

HEXAFLUOROPHOSPHATE TPS

DPI

B) CRIVELLO,J.V. a J.H.W. L A M , M A C R O M O L E C U L E S , 10,4*6, 1307-1315 ( 1977 )

J. RADIATION CURING J A N . 1978

5(1)

B) C R I V E L L O , J.V. E T A L ,

A) CRIVELLO,J.V. 8 J.H.W. L A M , A) KNAPCZYK,J.W.,W.E.MC EWEN, J A C S , 91 , 145 ( 1969) J. POLY. SCI., SYMPOSIUM * 5 6 , 3 8 3 - 3 9 5 ( 1976)

TRIPHENYLSULFONIUM

DIPHENYLIODONIUM

9 é

HEXAFLUOROPHOSPHATE

I® P F ®

Representative Onium Salt Photoinitiators


2.

WATT

21



the s a l t r a t h e r than p h o t o l y s i s . T h e i r mechanism would account f o r a "dark r e a c t i o n " , e x p l a i n i n g the l i m i t e d storage s t a b i l i t y o f some p h o t o s e n s i t i z e d epoxides i n the absence o f l i g h t . The Schieman r e a c t i o n (19) was c i t e d by S c h l e s i n g e r (16) and by L i c a r i and Crepeau (14) as the probable mechanism by which i n i t i a t o r s f o r c a t i o n i c p o l y m e r i z a t i o n are released by diazonium salts. Rutherford et a l (20) showed s i d e r e a c t i o n s to be m i n i mized when the diazonium s a l t i s a hexafluorophosphate. Although heat i s g e n e r a l l y used to promote decomposition o f diazonium s a l t s by the Schieman r e a c t i o n , the r e a c t i o n has been shown to occur a l s o under the i n f l u e n c e o f u l t r a v i o l e t r a d i a t i o n (21, 2 2 ) :

Phosphorous p e n t a f l u o r i d e was shown by M u e t t e r t i e s (23, 24) to i n i t i a t e p o l y m e r i z a t i o n o f c y c l i c ethers to h i g h - m o l e c u l a r weight polymers. The r e l e a s e o f a Lewis a c i d such as B F or P F i n the pre sence o f a m a t e r i a l capable o f undergoing c a t i o n i c p o l y m e r i z a t i o n would be expected to y i e l d polymeric products, and the photoinduced p o l y m e r i z a t i o n o f epoxides can be e x p l a i n e d according to the mechanism f o r c a t i o n i c p o l y m e r i z a t i o n proposed by Rose (25) and others (26, 27, 2 8 ) . 3

^ - Λ 4- P F

5

5

+H 0 2

PF ,0® P F

5

5

ΟΗΘ

O HΘ

5



22

EPOXY RESIN

CHEMISTRY

The length o f time r e q u i r e d f o r a c o a t i n g to go from the l i q u i d s t a t e i n which i t i s l a i d down on the s u b s t r a t e to a s o l i d nontacky s t a t e i s dependent upon the c o n c e n t r a t i o n o f photoinitiator. Coatings c o n t a i n i n g v a r i o u s amounts o f p-methoxybenzenediazonium hexafluorophosphate (PM) were a p p l i e d at a t h i c k ness o f 0 . 2 - 0 . 3 m i l to aluminum, and the time to become t a c k - f r e e f o l l o w i n g two seconds' exposure to a General E l e c t r i c UA-3 mercury arc (60 watts per inch) was determined. Coatings c o n t a i n i n g 1.0 to 1.2 parts o f PM per 100 parts o f epoxide cured r a p i d l y . Tackfree times o f l e s s than one second could not be measured a c c u r a t e l y , and there appeared l i t t l e advantage i n using l a r g e r amounts of p h o t o i n i t i a t o r . . At lower concentrations o f p h o t o i n i t i a t o r there was a d r a s t i c increase i n the time r e q u i r e d f o r the c o a t i n g to become nontacky, p a r t i c u l a r l y at concentrations below 0.6 parts of p h o t o i n i t i a t o r per 100 parts of epoxide. Table III.

Effect of Photoinitiator Concentration on Rate of Cure

Tack-Free l m e (seconds)

Amount o f P h o t o i n i t i a t o r ^ per 100 grams o f ECC* Grams Moles 0.2 0.4 0.6 1.0 1.2 1.4

0.0007 0.0014 0.0021 0.0035 0.0043 0.0050

(1)

p-Methoxybenzenediazonium

(2)

Time following mercury arc at 0.3 mil coating

*

ECC = 3,4-Epoxy hexane

( 2 )

60+ 60+ 4 3 1 1 hexafluorophosphate

two seconds' exposure to G.E. a distance of 4.5 inches for on aluminum to become tack cyclohexylmethyl-3

4-epoxy

3

(PM) UA-3 0.2free. cyclo-

carboxylate.

The amount o f r a d i a t i o n r e q u i r e d to decompose the PM photoi n i t i a t o r i n a c o a t i n g 0 . 2 - 0 . 3 mil t h i c k on aluminum was d e t e r mined s p e c t r o p h o t o m e t r i c a l l y by changes i n the u l t r a v i o l e t absorbance f o l l o w i n g exposure to a mercury arc (29). By t h i s method i t was determined t h a t more than 90% o f the PM was photol y z e d during two seconds' exposure to the UA-3 arc at a d i s t a n c e of 4.5 inches (see Figure 1 ) . The r a d i a t i o n dose r e c e i v e d by the sample under these c o n d i t i o n s was 0.70 j o u l e / c m . In mixtures o f epoxides and diazonium s a l t s , there i s g e n e r a l l y a "dark r e a c t i o n " which takes place even i n the absence of u l t r a v i o l e t r a d i a t i o n . Mixtures stored i n the dark at average room temperatures w i l l g r a d u a l l y become more v i s c o u s and eventually solidify. The r a t e at which t h i s occurs i s i n f l u e n c e d by 2




WATT

Figure 1. Change in UV absorbance of a photosensitized coating on exposure to the mercury arc (Photosensitizer = PM)



24


several f a c t o r s , i n c l u d i n g storage temperature and sample s i z e . I t i s s t r o n g l y dependent upon the epoxides which are present. Some epoxides react very s h o r t l y a f t e r mixing w i t h a d i a zonium s a l t . Epoxides such as a l l y l g l y c i d y l e t h e r , v i n y l cyclohexene d i o x i d e and butanediol d i g l y c i d y l ether react v i g o r o u s l y when mixed with p-chlorobenzenediazonium hexafluorophosphate, e v o l v i n g heat and forming s o l i d products. Diglycidyl ethers o f bisphenol "A" (DGEBA) gel slowly over a p e r i o d o f several hours. Other epoxy r e s i n s show a very gradual increase i n v i s c o s i t y over a p e r i o d o f days or weeks (see Table I V ) . Another f a c t o r i n f l u e n c i n g storage s t a b i l i t y o f photosensit i z e d epoxides i s the s t r u c t u r e o f the aromatic diazonium s a l t . The nature and p o s i t i o n o f s u b s t i t u e n t s on the benzene r i n g o f the diazonium s a l t s i q n i f i c a n t l y i n f l u e n c e s r e a c t i v i t y toward epoxides (see Table V ) . Storage s t a b i l i t y can be f u r t h e r improved by a d d i t i o n o f " s t a b i l i z e r s " which i n h i b i t the dark r e a c t i o n without r e t a r d i n g r a t e o f cure. M a t e r i a l s f i t t i n g the d e s c r i p t i o n o f Lewis Bases or e l e c t r o n - d o n o r compounds are g e n e r a l l y e f f e c t i v e i n i n h i b i t i n g the dark r e a c t i o n . Many compounds o f t h i s t y p e , however, tend to quench the c a t i o n i c r e a c t i o n c o m p l e t e l y , so t h a t c u r i n g does not occur even on exposure to u l t r a v i o l e t r a d i a t i o n . There are some donor compounds which, w h i l e not completely preventing the dark r e a c t i o n , w i l l i n h i b i t i t and, at the same t i m e , not i n t e r f e r e with the c u r i n g r e a c t i o n . C y c l i c amides ( 3 0 ) , n i t r i l e s ( 3 1 ) , s u b s t i t u t e d ureas (32) and s u l f o x i d e s (33) have been shown to promote storage s t a b i l i t y o f p h o t o s e n s i t i z e d epoxides without preventing r a p i d cure when exposed to u l t r a v i o l e t radiation. Figure 2 i l l u s t r a t e s the e f f e c t o f an added s t a b i l i z e r (N-methylpyrrolidone) on storage s t a b i l i t y o f a mixture o f epoxy resins. S t a b i l i z a t i o n o f p h o t o s e n s i t i v e epoxides by e l e c t r o n donor compounds i s probably due to formation o f a complex with the Lewis a c i d which i s s l o w l y l i b e r a t e d by decomposition o f the diazonium s a l t on storage (23, 3 4 ) . Diary!iodonium S a l t s . The use o f d i a r y l i o d o n i u m s a l t s as p h o t o i n i t i a t o r s f o r epoxide p o l y m e r i z a t i o n was f i r s t described i n Belgian Patents issued i n 1975 and 1976 (35, 36, 37_). D i a r y l iodonium s a l t s had been used e a r l i e r i n photoinduced p o l y m e r i z a t i o n ( 3 8 ) , but they were used f o r f r e e - r a d i c a l p o l y m e r i z a t i o n s . Diaryliodonium s a l t s are not strong absorbers o f l i g h t at wavelengths above 300 nanometers ( 4 , 3 9 ) . They do not make e f f i c i e n t use of the output o f the high-pressure mercury arc and, as a r e s u l t , cure i n i t i a t e d by d i a r y l i o d o n i u m s a l t s i s apt to be slow and to r e q u i r e longer exposure than i s the case with d i a zonium s a l t s . Rates o f cure can be increased by adding to the d i a r y l i o d o n i u m s a l t a p h o t o s e n s i t i z e r which absorbs longer wavelength r a d i a t i o n and permits c u r i n g under sources o f v i s i b l e



0.75

Butanediol d i g l y c i d y l

8 to 30 cps i n 5 days

0.47 0.35

Bis(3,4-Epoxy-6-methyl methyl) adipate

alkyl glycidyl

928 to 1885 cps i n 5 days

0.56

Epoxy phenol novolak ( V i s . @52°C 1400-2000 cps)

Ci2-ii»

Gels i n 20 hours

0.57

DGEBA ( V i s . @25°C 4000-6000 cps)

ether

cyclohexyl

V i s c o s i t y goes from 470 to 2600 cps i n 5 days

0.74

3,4-Epoxy c y c l o h e x y l m e t h y l - 3 , 4 epoxy cyclohexane c a r b o x y l a t e

glycidyl

ether

0.87

ether

Allyl

reaction

Vigorous exothermal

1.39

V i n y l cyclohexene d i o x i d e

E P O X I D E

Result when mixed with p - e h l o v o b e n z e n e d i a z o n i u m * PFe

Reactivity of Epoxides Toward Diazonium Salt

Epoxy Value

Table IV.



Table V .

Photoinitiator

N PF


2

Benzenediazonium Hexafluorophosphates Influence of Substituents

Melting Point (°C)

Ultraviolet Absorption Maxima (nm)

Cure Time (seconds)

Potlife (hours)

1

2

6

N PF 2

Gelled within 24 hours

162-164

273

149-151

313

1

28

132

230 300 335

1

96

6

OCH3 N PF 2

6

OCH3

Gel led.within 24 hours

1

Time for ECC containing one pph photoinitiator to become tack-free following two seconds' exposure to GE UA-3 mercury arc at 4.5 inches. Sample temperature = 20°C.

1

Length of time in storage at 40°C for doubling of viscosity consisting of 55 parts DGEBA, 30 parts ECC, 15 parts Cu-ih ether and 1 part photoinitiator.

of alkyl


formulation glycidyl


2.

27


WATT

0

1

2

3 4 STORAGE TIME, DAYS

5

6

Figure 2. Effect of electron donor compound on storage stability of photosensitized epoxide mixture stored at 40°C (Photosensitizer = PM): (A) without Nmethylpyrrolidone; (B) 0.005% N-methylpyrrolidone added; (C) 0.02% N-methylpyrrolidone added; (D)0.06% N-methylpyrrolidone added


28


light

(40). The mechanism by which d i a r y l i o d o n i u m s a l t s i n i t i a t e p o l y m e r i z a t i o n o f epoxides has been e x p l a i n e d as i n v o l v i n g a c i d s produced from p h o t o l y s i s o f the d i a r y l i o d o n i u m s a l t (2^ 3^ 4 ) : Ar IPF 2

6

— ^ A r l + ArYH ( s o l v e n t )

+

Y- +

HPF

6

T h i s i s followed by 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 epoxide i n i t i ated by the proton from H P F . The d i a r y l i o d o n i u m metal h a l i d e complex s a l t s are not commercially a v a i l a b l e at t h i s w r i t i n g . Methods f o r t h e i r prep a r a t i o n are reviewed i n a paper by C r i v e l l o and Lam ( 4 ) .


6

Sulfonium S a l t s . Other onium s a l t s beside the diazonium and halonium s a l t s w i l l , on exposure to l i g h t , i n i t i a t e polymerization of epoxides and other monomers capable of undergoing c a t i o n i c p o l y m e r i z a t i o n . The use o f onium s a l t s o f the elements of Groups Va and V i a was f i r s t described i n 1975 ( 4 1 , 42) and 1976 (43). More r e c e n t l y , U . S . patents have been granted c o v e r i n g the use o f onium s a l t s o f the elements o f Groups Va (44, 45J and V i a (46, 47) w i t h epoxides and with other monomers which are known t o polymerize c a t i o n i c a l l y . There seems to be some confusion concerning the n e c e s s i t y to add p h o t o s e n s i t i z e r s to increase s p e c t r a l absorbance i n order to o b t a i n r a p i d cure o f epoxides c o n t a i n i n g t r i a r y l s u l f o n i u m s a l t s . The aromatic sulfonium complex s a l t s are claimed to be photosens i t i v e o n l y to the s h o r t e r wavelength r a d i a t i o n (_5, 3 9 ) , and they have been reported to be s l i g h t l y l e s s p h o t o r e a c t i v e than d i a r y l iodonium s a l t s ( 6 ) . The l i m i t e d s p e c t r a l response has been c i t e d as a s e r i o u s inherent l i m i t a t i o n w i t h respect to t h e i r use as p h o t o i n i t i a t o r s i n p h o t o s e n s i t i v e compositions ( 4 8 ) . I t was shown t h a t a c o a t i n g made up o f 4 parts t r i phenylsulfonium hexafluorophosphate (TPS) i n 100 parts o f an epoxy r e s i n (DER 331) f a i l e d to cure i n 5 minutes' exposure to a G . E . H3T7 mercury a r c ; but f o l l o w i n g a d d i t i o n o f .02 parts o f 2 - e t h y l - 9 , 1 0 - d i m e t h o x y anthracene, c u r i n g occurred i n f i f t e e n seconds. On the other hand, i t has been reported t h a t t r i a r y l s u l f o r r i u m s a l t s are very r e a c t i v e p h o t o i n i t i a t o r s for the r i n g opening of epoxy monomers (_7). I t was a l s o reported (49) t h a t a c o a t i n g composed o f 4% by weight o f t r i p h e n y l s u l f o n i u m hexafluorophosphate i n 3 , 4 - ë 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 cyclohexane c a r b o x y l a t e cured i n 20 seconds' exposure. I t has been p o s t u l a t e d (6) t h a t p h o t o l y s i s o f Group Va and Via onium s a l t s proceeds as shown below, and that the a c i d HPF then i n i t i a t e s r i n g opening and p o l y m e r i z a t i o n o f the epoxide. This i s s i m i l a r to the mechanism proposed by Dreyfuss and Dreyfuss (13). The n o n n u c l e o p h i l i c counterion PF ~does not react w i t h the growing c a t i o n i c species 6

6


2.

WATT


AR SPF 3

6

^

29

• A r S + ArH + 2

Y- +

HPF

6


The a r y l s u l f o n i u m metal h a l i d e complex s a l t s are not r e a d i l y a v a i l a b l e . T r i p h e n y l s u l f o n i u m hexafluorophosphate can be prepared from diphenyliodonium hexafluorophosphate by heating w i t h phenyl s u l f i d e , as described by Knapczyk and McEwen ( 5 9 ) , or by r e a c t i o n o f phenyl magnesium bromide w i t h d i p h e n y l s u l f o x i d e followed by r e a c t i o n w i t h hexafluorophosphoric a c i d according to the procedure o f W i l d i , T a y l o r and P o t r a t z (60). Comparison o f P h o t o i n i t i a t o r s Two p r o p e r t i e s of fundamental importance to the successful f u n c t i o n i n g o f a p h o t o i n i t i a t o r f o r l i g h t - c u r a b l e coatings are cure r a t e and storage s t a b i l i t y o f the p h o t o s e n s i t i z e d formulations. In high-speed c o a t i n g o p e r a t i o n s , the i n t e r v a l between a p p l i c a t i o n o f the c o a t i n g and r o l l i n g up or s t a c k i n g o f the coated s u b s t r a t e i s often no more than a few seconds. In t h i s b r i e f i n t e r v a l the c o a t i n g must be converted from a f r e e - f l o w i n g l i q u i d to a nontacky s o l i d . While capable o f r a p i d c u r e , the c o a t i n g must be s u f f i c i e n t l y s t a b l e t h a t i t does not undergo a p p r e c i a b l e change i n the period between a d d i t i o n o f the p h o t o i n i t i a t o r and a p p l i c a t i o n to the object being coated. T h i s i n t e r v a l may be a few hours ( o n - s i t e mixing) or several months, as would be the case w i t h coatings prepared at a c e n t r a l l o c a t i o n and stored f o r extended periods p r i o r to d i s t r i b u t i o n and use. Comparison o f Rates of Cure. The r a t e a t which coatings cure i s g e n e r a l l y based on personal judgment o f the time r e q u i r e d to become nontacky to the touch. The t e s t i s simple and w i d e l y used, but i t i s h i g h l y s u b j e c t i v e . We have developed an instrument which e l i m i n a t e s much of the s u b j e c t i v i t y o f the touch t e s t . T h i s instrument, which i s i l l u s t r a t e d i n Figure 3, c o n s i s t s o f an o s c i l l a t i n g stage which carries a coated metal coupon i n t o an enclosure c o n t a i n i n g a shuttered mercury arc ( G . E . UA-3, 60 watts per inch) i n a p a r a b o l i c r e f l e c tor. When the o s c i l l a t i n g stage enters the e n c l o s u r e , a s h u t t e r opens, exposing the coated coupon to the u l t r a v i o l e t r a d i a t i o n from the mercury a r c . At the end o f a measured i n t e r v a l , the s h u t t e r c l o s e s , and the o s c i l l a t i n g stage r e t r a c t s . The s h u t t e r operates on a t i m e r , p e r m i t t i n g v a r i a t i o n s i n exposure time from one second to f i f t e e n minutes.



30


Figure 3. Device for measuring tack-free time: (1) probe (absorbent cotton); (2) timer controlling interval between exposure and activation of probe; (3) oscillating stage; (4) exposure timer; (5) constant temperature bath and circulator; (6) air pressure regulator controlling pressure on probe; (7) General Electric UA-3 mercury arc, reflector, and shutter (inside enclosure)

210

235

260

285

310

335

360

WAVELENGTH^ NANOMETERS ) Figure 4.

UV absorbance spectra of onium salt photoinitiators



2.

WATT


31

When the s h u t t e r c l o s e s and the stage r e t r a c t s , a second timer i s a c t i v a t e d which c o n t r o l s the i n t e r v a l between exposure and a d m i n i s t r a t i o n o f the tack t e s t . This i n t e r v a l can a l s o be v a r i e d from one second to 15 raiautes i n increments o f one second. The tack t e s t i s administered by a small a i r - d r i v e n p i s t o n , to one end o f which i s attached a probe c o n s i s t i n g o f a b a l l of absorbent c o t t o n . Pressure on the probe i s c o n t r o l l e d by a i r pressure d r i v i n g the p i s t o n . A tacky c o a t i n g i s r e a d i l y recognized by the tendency o f cotton 1 i n t e r s to adhere to i t . The number of seconds f o l l o w i n g exposure f o r a c o a t i n g to become hard enough t h a t i t does not p u l l c o t t o n 1 i n t e r s from the probe i s defined as the " t a c k - f r e e t i m e " . An advantage o f measuring independently both the exposure time and the i n t e r v a l between exposure and the disappearance o f tack i s that i t allows a separation o f the e f f e c t s o f u l t r a v i o l e t r a d i a t i o n and the e f f e c t s of heat. The photoinduced c u r i n g of epoxides i n v o l v e s two s t e p s : 1. 2.

Photodecomposition o f the i n i t i a t o r P o l y m e r i z a t i o n o f the epoxide

U l t r a v i o l e t r a d i a t i o n i s r e q u i r e d only f o r the f i r s t s t e p . The second s t e p , w h i l e independent o f UV r a d i a t i o n , i s influenced by heat. Continuous exposure to a mercury arc to determine t a c k free time can i n f l u e n c e the course o f the p o l y m e r i z a t i o n because of an increase i n temperature of c o a t i n g and s u b s t r a t e caused by the intense heat o f the mercury a r c . To avoid t h i s c o m p l i c a t i o n and to maintain c o n t r o l o f the temperature o f the c o a t i n g during c u r e , exposure to the mercury arc was g e n e r a l l y l i m i t e d to two seconds. This proved adequate to g i v e a t a c k - f r e e c o n d i t i o n w i t h i n one second f o l l o w i n g exposure f o r the most r e a c t i v e epoxides. The PM and TPS p h o t o i n i t i a t o r s g e n e r a l l y gave s h o r t e r t a c k free times than DPI; and at the lower c o n c e n t r a t i o n l e v e l , PM was somewhat more e f f e c t i v e than TPS. T h i s can be a t t r i b u t e d to the more e f f i c i e n t use o f the mercury arc r a d i a t i o n by the PM p h o t o i n i t i a t o r , which has an a b s o r p t i o n peak a t 313 nanometers (Figure 4 ) , corresponding to a peak i n the emission spectrum o f the mercury a r c . Absorption maxima f o r DPI and TPS are at the lower end o f the spectrum, f a r removed from the peak output o f the high pressure mercury a r c . An e x p l a n a t i o n o f the d i f f e r e n c e s i n cure r a t e between DPI and TPS i s l e s s o b v i o u s , as the absorption spectra o f these two compounds are s i m i l a r . Depending on the method of p r e p a r a t i o n , however, the TPS p h o t o i n i t i a t o r frequently shows some absorbance i n the s p e c t r a l region between 290 and 340 nm, o v e r l a p p i n g the band at 310 i n the mercury lamp emission spectrum. This may be the r e s u l t o f a f o r t u i t o u s contaminant not completely removed i n s y n t h e s i s and p u r i f i c a t i o n o f the TPS p h o t o i n i t i a t o r .


32


Differences i n c a t a l y t i c a c t i v i t y o f the three p h o t o i n i t i a t o r s are most obvious when the coatings are exposed t o a source of predominantly v i s i b l e l i g h t , such as s u n l i g h t , or to a source of shortwave r a d i a t i o n , such as a g e r m i c i d a l lamp. Table VI shows the time r e q u i r e d f o r 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane c a r b o x y l a t e (ECC) p h o t o s e n s i t i z e d w i t h the three p h o t o i n i t i a t o r s to become t a c k - f r e e when exposed to v a r i o u s sources o f l i g h t .


Table VI.

Comparison of Photoinitiators

E f f e c t o f R a d i a t i o n Source on Rate of Cure o f 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate P h o t o s e n s i t i z e d w i t h PM, DPI or TPS

R a d i a t i o n Source, Exposure Time UA-3 (60W/in), 2 seconds II

^

II

II

S u n l i g h t (continuous

exposure)

not

cure

in

30

minutes

2

60

5

1

5

1

25-30

NC*

NC*

35

15

75

Germicidal lamp (continuous exposure) *Did

TACK-FREE TIME, seconds TPS PM DPI

continuous

exposure

I t can be seen t h a t under the i n f l u e n c e o f s u n l i g h t , PM w i l l i n i t i a t e p o l y m e r i z a t i o n r e a d i l y , w h i l e n e i t h e r DPI nor TPS produces a cure. Under the germicidal lamp, which emits 254 nm r a d i a t i o n almost e x c l u s i v e l y , the s i t u a t i o n i s r e v e r s e d , w i t h DPI and TPS which are strong absorbers o f the s h o r t e r wavelength r a d i a t i o n , being somewhat more e f f e c t i v e than PM. Storage S t a b i l i t y o f P h o t o s e n s i t i z e d Epoxides. Storage s t a b i l i t y o f p h o t o s e n s i t i z e d formulations i s dependent upon many f a c t o r s , such as temperature, sample s i z e , s t r u c t u r e and concent r a t i o n o f the p h o t o i n i t i a t o r , e x c l u s i o n o f l i g h t and the presence o f i m p u r i t i e s . Tests were run under p a r a l l e l c o n d i t i o n s using a mixture o f epoxides known to give a short p o t l i f e w i t h the PM p h o t o i n i t i a t o r to compare storage s t a b i l i t y o f the same formulation p h o t o s e n s i t i z e d w i t h DPI or TPS. Storage s t a b i l i t y was determined by p e r i o d i c measurement o f v i s c o s i t y . Results are summarized i n Table V I I .


2.


WATT

33

Table VII. Comparison of PM, DPI, and TPS: Potlife of an Unstabilized Formulation @ 4 0 ° C

Photoinitiator PM DPI TPS

Time f o r V i s c o s i t y to Double < 12 hours 13 days No change a f t e r 6 months


C h a r a c t e r i s t i c s o f the Curing Process Dreyfuss and Dreyfuss (13) showed t h a t the c a t i o n i c polymeri z a t i o n o f c y c l i c ethers has the c h a r a c t e r i s t i c s of a " l i v i n g " p o l y m e r i z a t i o n , i n t h a t there appears to be a l a c k o f t e r m i n a t i o n except through r e a c t i o n o f the c a t i o n i c growing chain end with i m p u r i t i e s ; and t h a t e v e n t u a l l y a steady s t a t e i s a t t a i n e d where the l i v i n g polymer i s i n e q u i l i b r i u m w i t h i t s monomer. In the p h o t o i n i t i a t e d c a t i o n i c p o l y m e r i z a t i o n o f epoxides, UV energy i s r e q u i r e d o n l y to cause decomposition of the photos e n s i t i v e s a l t r e l e a s i n g the m a t e r i a l s which i n i t i a t e p o l y m e r i z a t i o n . Once t h i s has been done, the propagation o f p o l y m e r i z a t i o n continues without f u r t h e r input o f UV energy. Crosslinking continues long a f t e r exposure t o u l t r a v i o l e t r a d i a t i o n has been stopped. The p r a c t i c a l consequence o f t h i s i s t h a t the coated s u b s t r a t e can be r o l l e d up or stacked as soon as the surface becomes nontacky. Curing w i l l continue i n s t o r a g e . Evidence o f t h i s c o n t i n u i n g r e a c t i o n can be obtained spectrop h o t o m e t r i c a l l y . The i n f r a r e d spectrum o f a p h o t o s e n s i t i z e d c o a t i n g on an aluminum s u b s t r a t e showed an a b s o r p t i o n band a t 910 c m " , as shown i n Figure 5. Immediately a f t e r c u r i n g t o a t a c k - f r e e s u r f a c e , the i n f r a r e d absorbance at 910 cm- had d i m i n i s h e d . A f t e r standing 20 hours longer i n the absence of l i g h t , i t had a l l but disappeared, i n d i c a t i n g disappearance o f n e a r l y a l l unreacted epoxide groups. Evidence o f a c o n t i n u i n g r e a c t i o n was a l s o obtained by S. I . S c h l e s i n g e r (17) who noted an increase i n the number o f steps on a photographic step t a b l e t p r o p o r t i o n a l to the delay between exposure and development. Changes i n the s o l v e n t and abrasion r e s i s t a n c e o f cured coatings w i t h age f o l l o w i n g exposure a l s o gave evidence o f a " l i v i n g " p o l y m e r i z a t i o n . Resistance o f coatings to methyl e t h y l ketone immediately a f t e r exposure i s g e n e r a l l y poor, even though the c o a t i n g s are nontacky. These same coatings an hour or more l a t e r e x h i b i t e x c e l l e n t r e s i s t a n c e to MEK. 1

1



34


Before Exposure

Figure 5.

After Exposure

After standing 20 Hrs. in dark

IR spectrum showing disappearance of epoxide band at 910 cm' following exposure of photosensitized coating to mercury arc


1


2.

WATT


35

E f f e c t o f Epoxide S t r u c t u r e on Rate of Cure. The r a t e at which p h o t o s e n s i t i z e d epoxides are converted from the l i q u i d to the s o l i d s t a t e i s i n f l u e n c e d by the epoxide s t r u c t u r e . Some p h o t o s e n s i t i z e d epoxides remain l i q u i d even a f t e r prolonged exposure to the output o f a mercury a r c , w h i l e others s o l i d i f y i n l e s s than one second. Rates a t which various epoxides become t a c k - f r e e are shown i n Table V I I I . Epoxidized o l e f i n s show the highest r a t e s o f c u r e . In the formulation o f c o a t i n g s , i t i s g e n e r a l l y necessary to use a mixt u r e o f epoxy r e s i n s i n order to o b t a i n the rheology needed f o r a p p l i c a t i o n or to o b t a i n the p a r t i c u l a r p r o p e r t i e s r e q u i r e d by the end use o f the object being coated. Coatings to be cured at ambient c o n d i t i o n s g e n e r a l l y r e q u i r e some a l i p h a t i c epoxide f o r r a p i d cure ( 5 0 ) . The data i n Table IX i l l u s t r a t e s the e f f e c t o f i n c l u d i n g a c y c l o a l i p h a t i c epoxide i n c o a t i n g formulations which are to be cured under ambient c o n d i t i o n s . Epoxides which cure r e l a t i v e l y s l o w l y or not a t a l l at o r d i n a r y temperatures can be used as components o f mixtures cont a i n i ng 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECC), i n which case the r a t e o f cure i s dependent upon the amount o f ECC i n the m i x t u r e . Mixtures o f epoxides which are slow to cure at o r d i n a r y temperatures w i l l cure more r a p i d l y at e l e v a t e d temperatures ( 3 ) . The e f f e c t s o f temperature on r a t e o f cure o f mixtures o f a c y c l o a l i p h a t i c epoxide and a DGEBA r e s i n are shown i n Table X. E f f e c t o f Humidity on Rate o f Cure C a t i o n i c p o l y m e r i z a t i o n i s terminated by the presence of contaminants, i n c l u d i n g water. In experiments to note the combined e f f e c t s o f temperature and humidity on t a c k - f r e e t i m e , temperature o f the s u b s t r a t e was c o n t r o l l e d by means o f the o s c i l l a t i n g s t a g e , as described above, and humidity was c o n t r o l l e d by conducting the experiments i n an environmental chamber. Res u l t s are shown i n Table X I . At r e l a t i v e h u m i d i t i e s o f 65% or l e s s the e f f e c t on t a c k free time ranged from n e g l i g i b l e to s l i g h t w i t h epoxides c o n t a i n ing the PM or TPS p h o t o i n i t i a t o r s . With DPI, r e s u l t s were s t r o n g l y dependent upon the epoxide. The 3:1 mixture o f c y c l o a l i p h a t i c diepoxide and DGEBA r e s i n showed no change i n t a c k - f r e e time when humidity was r a i s e d from 45% t o 65%; but the t a c k - f r e e time of the 3:1 mixture o f c y c l o a l i p h a t i c d i e p o x i d e and butaned i o l d i g l y c i d y l ether increased s i g n i f i c a n t l y . High humidity (85%) caused a d r a s t i c increase i n t a c k - f r e e time when the s u b s t r a t e was maintained at 35°C or l e s s . When the s u b s t r a t e temperature was r a i s e d to 45°C, the e f f e c t of high humidity on t a c k - f r e e time was overcome and r a p i d cure was observed. Under commercial UV c u r i n g c o n d i t i o n s ( i . e . , m u l t i p l e 200 watt per inch lamps and no c o n t r o l o f s u b s t r a t e temperature), coatings would o r d i n a r i l y reach or exceed 45°C on exposure to the mercury arcs and t h i s would tend to obscure the e f f e c t s o f high humidity.



^

Q

A

r

C H

3

2

)

4

(EEWI89Î

0

C H ^

- % ^ Q .

( EEWI75 Î

0 ^ C H

30

60

35

* * T I M E TO BECOME TACK-FREE FOLLOWING TWO SECONDS EXPOSURE TO GENERAL ELECTRIC U A - 3 ( 6 0 WATTS /INCH ) MERCURY ARC AT 4.5 INCHES. TEMPERATURE OF S U B S T R A T E 3 0 ° C .

60+

60+

60+ 60+ 40 60+

30

60+

60+

60+ 60+ 60+

60+

40

10 60+

60

60+

10

60+

60+

4

1

1

7

1

1

17

2

1

60+

15

3

4

1

1

6

1

1

TACK -FREE TIME, SECONDS** TPS DPI PM 0.005*| 0.01 * 0.005* | 0.01 * 0.005*1 0.01*

Tack-Free Time of Various Epoxides Photosensitized with PM, DPI, or TPS

* CONCENTRATION IN MOLES OF PHOTOINITIATOR PER GRAM EQUIVALENT WEIGHT OF EPOXIDE

0

o

EPOXIDES

Table VIII.


2.


WATT

37

Table IX. Correlation of Viscosity, Epoxy Value, and Cure Time of Epoxide Coating Showing Dependence of Rapid Cure on Cycloaliphatic Epoxide

EPOXIDE COATING COMPOSITION Parts by Resin 2


Resin 1

Weight Resin

Resin

3

4

20

50

—

20

80

--

20

— — — -10

---

Vis

@ 25°C

Epoxy

Cure

(ops)

Value

( seeoi

10

0.54

NC

13

0.67

NC

--

80

100

0.64

4

80

—

20

31

0.69

NC

--

80

20

841

0.67

2

50

~

50

110

0.64

NC

—

50

50

2253

0.62

5

10

--

80

622

0.53

NC

1504

0.53

9

10

—

10

80

10

40

—

50

94

0.60

NC

10

~

40

50

518

0.59

5

10

70

—

20

28

0.66

NC

10

—

70

20

314

0.63

3

5

80

—

15

24

0.67

NC

Each

composition

l

No.

2

seconds

continuous

coating *NC = No cure *0.2-0.

Ζ mil

contained

exposure

to become after

1 pph PM

60 seconds

coating

on

alkyl

to

UA-3 Eg Arc

@ 4.5

inches

tack-free exposure

aluminum

Resin

1 = Cxz-m

glycidyl

ether

Resin

2 = Butanediol

diglycidyl

Resin

3 =

3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane

ether

carboxylate Resin

4 = DGEBA

(vis.ê25°C

= 4000-6000

cps;

EEW =

172-178)


for



Table X.

Rate of Cure of DGEBA Resin

R e l a t i o n s h i p between amount of c y c l o a l i p h a t i c epoxide added and cure r a t e at v a r i o u s temperatures ECC ADDED (pph)

Tack-free Time (seconds) @30° @40° @50°

@23°

None 2 5 10 15 25 35

60+ 60+ 60+ 60+ 60+ 25 4

0.2-0.3

mil

4 sees, Photoinitiator:

coatings at

4.5

60+ 60+ 60+ 55 45 3 2

20 17 17 13 4 1 1

on aluminum

@60°

5 4 2 1 1 1 1

exposed

to

1 1 1 1 1 1 1

G.E.

UA-3

inches.

PM (1.3

parts

per

100

parts

epoxide)


for


25°C=550CPS

2

1

0

5

5

™

IT

™ °o°o?

0.01

°°°

0.01

0

5

TPS

b

0

0

°oT

K

0

«"

,

TPS

0

0.01

5

14 6 2 1

18 4 1

1

20

1

1

1

1

1

1

2

1

10

30 2

1 1

1 1

EPOXIDE.

1

1

2

9

1

1

1

1

1

4

1 1

1

5

13

60+

60+

4

5

1

1

35

1

1

3

15

35

1

1

1

1

2

60+

1

1 60+

1

1

e

e

1

1

1

3

15

1

1

1

1

1

60+

1

1

2 1

60+

1

60+ 60+

1

1 60+

1 60+

10

45

60+

60+

7

13

1

2

60+

60+

1

2

60+

60+

60+

60+

60+

60+

60+

60+

60+

60+

60+

60+

60+

1

1

1

1

1

1

1

2

5

25

1

1

1

1

1

60+

1

1

T I M E FOLLOWING E X P O S U R E TO UV FOR COATING T O B E C O M E T A C K - F R E E ( C O T T O N B A L L T E S T ).

e

2

TACK-FREE TIME, SECONDS 45%RH 65% RH 85%RH 25° 35 4 5 ° 2 5 35° 45° 25° 35 4 5 °

M O L E S PHOTOINITIATOR P E R GRAM E Q . WT. O F

VIS. A T 2 5 ° C = 8 6 5 C P S

E P O X Y EQ.WT.= 149

+1 PART

3 PARTS

VIS. A T 2 5 ° C = 8 0 C P S

EPOXY EQ. WT.= 138

^O-tCHglcO-^

4-1 PART

3 PARTS

VIS. AT

DPI

0

0.01

0

0

M

P

CONC.'

ΡM

TYPE

PHOTOINITIATOR

XI· Comparison of Photoinitiators (PM, DPI, TPS)—Influence of Temperature and Humidity on Tack-Free Time

EPOXY EQ.WT.= 140

EPOXIDE

Table


40


M a i n t a i n i n g the s u b s t r a t e at lower temperatures i n a humid atmosphere undoubtedly causes condensation o f moisture on the s u b s t r a t e and c o a t i n g . Excess water w i l l complex w i t h the a c i d i c i n i t i a t o r promoting p r e f e r e n t i a l l y the h y d r o l y s i s o f epoxy groups. The net e f f e c t i s to consume epoxy groups which do not c o n t r i b u t e to c r o s s l i n k i n g (53).


V o l a t i l i t y o f P h o t o s e n s i t i z e d Epoxy Coatings A p r i n c i p a l m o t i v a t i n g f a c t o r i n the development o f l i g h t curable coatings i s the reduction i n v a p o r i z a t i o n o f m a t e r i a l s which would c o n t r i b u t e to a i r p o l l u t i o n . I t i s therefore d e s i r able to minimize the amount o f low-molecular-weight m a t e r i a l i n a c o a t i n g f o r m u l a t i o n . However, v i s c o s i t y l i m i t a t i o n s imposed by the method o f a p p l y i n g the c o a t i n g to the s u b s t r a t e r e q u i r e the i n c l u s i o n o f some low-molecular-weight m a t e r i a l . L o w - v i s c o s i t y epoxide monomers are a v a i l a b l e which have high b o i l i n g points and low vapor pressures. However, the nature o f the c u r i n g process i n v o l v e s c o n d i t i o n s conducive to e v a p o r a t i o n . The c o a t i n g i s a p p l i e d i n the form o f a t h i n f i l m w i t h a very large surface. I t i s subjected to r a d i a t i o n from a h i g h - i n t e n s i t y mercury arc which i n c l u d e s s i g n i f i c a n t i n f r a r e d r a d i a t i o n . F u r t h e r , the coated surface i s g e n e r a l l y moving at a high v e l o c i t y , c r e a t i n g a strong flow o f a i r over the s u r f a c e . The v o l a t i l i t y o f l i g h t - c u r a b l e epoxy formulations was determined by observing changes i n weight at three stages: - A f t e r c o a t i n g but before exposure - A f t e r exposure - A f t e r baking a t 110°C f o r 2 hours a t 1 mm pressure Coatings were a p p l i e d a t a thickness o f approximately 0 . 3 m i l to weighed aluminum sheets. The t o t a l weight o f c o a t i n g was between 50 and 60 m i l l i g r a m s . Sheets coated w i t h uncured formul a t i o n s were allowed to stand on the platform o f an a n a l y t i c a l balance f o r ten minutes at ambient c o n d i t i o n s . T h i s i s c o n s i d e r ably longer than the normal i n t e r v a l between c o a t i n g and exposure i n a standard c o a t i n g o p e r a t i o n , but i t was d e s i r e d to provide exaggerated t e s t c o n d i t i o n s . Coated p l a t e s were then exposed to the mercury arc u n t i l the c o a t i n g was t a c k - f r e e . A f t e r being returned to e q u i l i b r i u m w i t h room temperature, the coated p l a t e s were weighed a g a i n . The aluminum p l a t e s w i t h cured coatings were then placed i n a vacuum oven and baked as described above. The method i s s e n s i t i v e to changes o f 0.1 m i l l i g r a m or 0.2% o f the o r i g i n a l c o a t i n g weight. I t was found t h a t the wet coatings could stand f o r more than ten minutes w i t h no measurable change i n weight, i n d i c a t i n g no s i g n i f i c a n t evaporation would be l i k e l y t o occur i n the b r i e f i n t e r v a l between a p p l i c a t i o n o f the c o a t i n g and i r r a d i a t i o n .


2.

WATT


41


S u r p r i s i n g l y , f o l l o w i n g exposure to the mercury a r c , the coatings do not l o s e weight, but a c t u a l l y increase i n weight. This gain i n weight on exposure to the mercury arc has been ob served c o n s i s t e n t l y over several hundred samples. I t i s a very r e a l e f f e c t and can be r a t i o n a l i z e d on the basis o f t e r m i n a t i o n of the p o l y m e r i z a t i o n by water, probably absorbed from the humid i t y i n the atmosphere. F i n a l l y , the cured coatings were baked two hours at 110°C under vacuum ( l e s s than 1 mm Hg), to remove any r e s i d u a l v o l a t i l e s i n the cured c o a t i n g . The r e s u l t s shown i n Table XII i n d i c a t e weight l o s s e s ranging from 0.28 to n e a r l y 7%, depending on compo s i t i o n o f the c o a t i n g . P r e p a r a t i o n of ρ-Methoxybenzenediazonium Hexafluorophosphate S i x t y m i l l i l i t e r s (0.30 mole) of 5M h y d r o c h l o r i c a c i d was added to 12.3 grams (0.10 mole) o f ρ - a n i s i d i n e , and the mixture was s t i r r e d w i t h heating u n t i l a l l s o l i d s d i s s o l v e d . The s o l u t i o n was d i l u t e d with 100 m l . o f d i s t i l l e d water and cooled on an i c e - s a l t bath. A t about 0°C, c r y s t a l s began to form. S e p a r a t e l y , a s o l u t i o n o f 6.9 grams (0.10 mole) of sodium n i t r i t e i n 50 m l . o f d i s t i l l e d water was prepared and cooled t o 50C. The n i t r i t e s o l u t i o n was added s l o w l y to the s t i r r e d a c i d s o l u t i o n , dropping the n i t r i t e d i r e c t l y i n t o the a c i d , not along the w a l l s o f the c o n t a i n e r . Rate o f a d d i t i o n was regulated so t h a t the temperature o f the r e a c t i o n mixture d i d not exceed 8°C. S t i r r i n g at 0°C was continued f o r 10-15 minutes, then 15 m l . o f 65% aqueous hexafluorophosphoric a c i d was s l o w l y added, keeping the temperature of the r e a c t i o n mixture below 8°C. ( C a u t i o n : HPF a t t a c k s g l a s s . A l l p l a s t i c containers and labware should be used f o r handling and t r a n s f e r r i n g t h i s a c i d . ) S t i r r i n g and c o o l i n g were continued f o r 10-15 minutes a f t e r a l l the HPF had been added. The p r e c i p i t a t e d product was recovered by f i l t r a t i o n , washed w i t h c o l d water u n t i l free o f c h l o r i n e (AgN0 t e s t ) and a i r d r i e d on the f i l t e r . The crude product was r e c r y s t a l l i z e d from methanol. Nineteen grams (68% y i e l d ) o f a white c r y s t a l l i n e product (m.p. 150-151°C) was o b t a i n e d . 6

6

3


42


Table XII.

1

Changes in Weight of Light Curable Epoxide Coatings During Various Stages of Processing

PERCENT CHANGE IN WEIGHT Coating Formula

On 10 mins


A Β C D

2

3

hours

&

23°C

2

+2.24 +2.33 +2.72 +1.37

Nil Nil Nil Nil

Coating 0.2-0. 200 watts per

2

On exposure to mercury arc

standing

δ mil thick inch

at 110°C under

on

vacuum

During bake

3

-0.28 -6.88 -2.40 -1.21

aluminum (less

than

1 mm Eg)

Conclusions The c r o s s l i n k i n g o f epoxides i n i t i a t e d by a c i d i c products o f p h o t o l y s i s i s a promising concept f o r the development o f nonp o l l u t i n g , energy-conserving c o a t i n g s . I t i s p o s s i b l e t o formu l a t e compositions from r e a d i l y a v a i l a b l e epoxy r e s i n s and mono mers which can be a p p l i e d by conventional c o a t i n g machinery and which cure r a p i d l y enough f o r high speed c o a t i n g o p e r a t i o n s . Onium s a l t s have many o f the c h a r a c t e r i s t i c s r e q u i r e d o f photosensitive i n i t i a t o r s f o r light-curable coatings. Aryldiazonium, aryliodonium and a r y l s u l f o n i u m s a l t s a l l i n i t i a t e r a p i d c u r e . Storage l i f e o f p h o t o s e n s i t i z e d formulations ranges from a few hours t o more than s i x months, depending upon s t r u c t u r e of the p h o t o i n i t i a t o r , composition o f t h e c o a t i n g and storage conditions. The c a t i o n i c process i s not i n h i b i t e d by oxygen. Curing can be i n i t i a t e d i n a i r under n a t u r a l s u n l i g h t o r l o w - i n t e n s i t y mercury arcs whose useful l i f e t i m e i s several times t h a t o f the h i g h - i n t e n s i t y arcs used i n f r e e - r a d i c a l c u r i n g processes. T y p i c a l o f c a t i o n i c p o l y m e r i z a t i o n , the r a t e o f cure i s i n f l u e n c e d by temperature and the presence o f moisture. By a j u d i c i o u s choice o f epoxides, i t i s p o s s i b l e t o formulate coatings which cure r a p i d l y under ambient c o n d i t i o n s . The combination o f arylonium s a l t s w i t h s e l e c t e d epoxy r e s i n s provides the e s s e n t i a l s f o r a v i a b l e system o f l i g h t curable coatings and should i n v i t e f u r t h e r i n v e s t i g a t i o n .


2. WATT


43


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RECEIVED


Epoxy Resin Chemistry - ACS Publications - American Chemical Society

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