Modern Container Coatings - American Chemical Society

6

Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0078.ch006

Photocurable Epoxide Coatings for Metal Containers DENNIS KESTER and W. A. TARWID American Can Company, 433 N. Northwest Hwy., Barrington, IL 60010

The curing of coatings by u l t r a v i o l e t radiation offers several significant advantages when compared to thermally cured solvent and waterborne materials. Radiation curing units consume less energy and require less floor space than gas-fired ovens. Furthermore, UV cured coatings contain little or no solvent and therefore s i g n i f i c a n t l y reduce p o l l u t i o n . U l t r a v i o l e t radiation curable epoxide coating formulations contain a blend of epoxide monomers and oligomers and a photosensitive diazonium s a l t . When exposed to u l t r a v i o l e t radiation, photodecomposition of the diazonium s a l t occurs releasing a Lewis acid as shown i n Figure 1(1, 2, 3). When thermally decomposed, this reaction i s known as the Schiemann reaction(4). Among the Lewis acids that may be generated by this reaction are boron t r i f l u o r i d e (BF ), pentafluorophosphate (PF ), antimony pentafluoride (SbF ), and antimony pentachloride (SbCl ). Once generated, the Lewis acid complexes with the oxirane oxygen causing opening of the epoxide ring and thereby generating cationic polymerization of the blend as shown i n Figure 2. 3

5

5

5

An advantage of this system over the free radical i n i t i a t e d systems i s that cationic polymerization i s not inhibited by oxygen. A disadvantage i s that the cationic polymerization is inhibited whenever the epoxide blends are exposed to materials containing substances which can act as Lewis bases, e.g., nitrogen containing molecules. By properly blending epoxide monomers and oligomers, UV cured coatings with a wide variety of properties can be prepared. Coatings possessing good film 0-8412-0446-2/78/47-078-038$05.00/0 © 1978 American Chemical Society Strand; Modern Container Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

KESTER AND TARwiD


Figure 1.

Photocurable

Epoxide

Coatings

Photodecomposition of diazonium salt

Initiation Θ

Figure 2.

Polymerization

Strand; Modern Container Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

40

MODERN CONTAINER

COATINGS


i n t e g r i t y , adhesion, f l e x i b i l i t y , hardness, chemical r e s i s t a n c e , and toughness a r e needed f o r end v a r n i s h e s where f l a t m e t a l p l a t e i s c o a t e d and then formed. A d d i t i o n a l p r o p e r t i e s such as g l o s s r e t e n t i o n and m o b i l i t y a r e needed f o r o v e r p r i n t v a r n i s h e s f o r con tainer labels. With the l a r g e number o f epoxide r e s i n s a v a i l a b l e , t h e s e p r o p e r t i e s may be a c h i e v e d by proper b l e n d i n g . Three epoxide monomers and one epoxide o l i g o m e r a r e shown i n F i g u r e 3. T a b l e I l i s t s a s e r i e s o f measure ments which were made on f o r m u l a t i o n s u s i n g t h e s e materials. These measurements were o b t a i n e d on prebaked t i n f r e e s t e e l p l a t e w i t h s e v e r a l d i f f e r e n t f o r m u l a t i o n s i n o r d e r t o i l l u s t r a t e the p r o p e r t y v e r s a t i l i t y o f UV c u r a b l e e p o x i d e c o a t i n g s . T a b l e I I i l l u s t r a t e s how d i f f e r e n t r a t i o s o f two epoxide monomers, i . e . , CY-179 and RD-2, a f f e c t f i l m properties. I t might be e x p e c t e d t h a t the h i g h e r the epoxide v a l u e , the f a s t e r the b l e n d would c u r e o r become t a c k f r e e . However, as shown i n T a b l e I I , the epoxide v a l u e s f o r f o r m u l a t i o n s A t h r o u g h Ε a r e the same and f o r m u l a t i o n Β c u r e s t h e f a s t e s t . I t i s not c o m p l e t e l y u n d e r s t o o d why f o r m u l a t i o n Β c u r e s the f a s t e s t , however, i t may be r e l a t e d t o the f a c t t h a t f o r m u l a t i o n Β has a h i g h e r c y c l o a l i p h a t i c epoxide v a l u e than f o r m u l a t i o n s C, D and E. Cycloaliphatic e p o x i d e s t e n d t o be more r e a c t i v e than g l y c i d y l epoxides. By t h i s argument, f o r m u l a t i o n A would be e x p e c t e d t o c u r e the f a s t e s t , i . e . , h i g h e s t c y c l o a l i p h a t i c epoxide v a l u e ; however, i t has the h i g h e s t viscosity. Cure r a t e tends t o d e c r e a s e a t h i g h v i s c o s i t y v a l u e s ( > 300 cps a t 2 5 ° C ) . I t can be seen t h a t t h e r e has t o be a f o r m u l a t i n g t r a d e - o f f between v i s c o s i t y and c y c l o a l i p h a t i c epoxide v a l u e s . The d a t a shown i n T a b l e I I i l l u s t r a t e t h a t a wide range o f s o f t e n i n g p o i n t s may be o b t a i n e d w i t h d i f fering resin ratios. I t can a l s o be seen t h a t t h e s o f t e n i n g p o i n t almost always i n c r e a s e s when the UV cured f i l m i s given a thermal treatment. This i s p r o b a b l y the r e s u l t o f f u r t h e r p o l y m e r i z a t i o n and the p o s s i b l e v o l a t i l i z a t i o n o f u n r e a c t e d low m o l e c u l a r weight m a t e r i a l s p r e s e n t i n the f i l m . The a d h e s i o n grades shown i n T a b l e I I i n d i c a t e no d e f i n i t e trend with cure r a t e , s o f t e n i n g p o i n t s , e t c . However, the t h e r m a l postbake always improves the a d h e s i o n p r o p e r t i e s o f the f i l m s . Again, t h i s i s



KESTER AND TARwiD

Photocurable

Epoxide

î

CH OC(CH ) COCH 2

2

4

2

CH.

CY-178

(Ciba P r o d u c t s Co.)

CY-179


CH

RD-2

2

Coatings

χ»

CHCH 0(CH ) OCH CH 2

2

4

2

CH

2


Λ

CH

0

CH~

CHCHr

CH. A r a l d i t e 6004 (Ciba P r o d u c t s Co.) Figure 3.

Structures of material used in blends


42

MODERN" CONTAINER COATINGS

TABLE I, PARAMETERS AND PHYSICAL PROPERTY MEASUREMENTS


1. 2.

Tack Free Time - Number of seconds until tack free after exposure to two 170 watts/inch UV lamps at a belt speed of 110 feet/min. Viscosity - cps at 25°c.

3.

Epoxide Value - 100/epoxide equivalent weight

4.

Cycloaliphatic Epoxide Value - 100/cycloaliphatic epoxide eq. weight Softening Point - Determined by Perkin Elmer thermomechanical analyzer after UV exposure Softening Point After Thermal Postbake - Determined after UV exposure and a thermal postbake of 380o f 9 minutes.

5. 6.

Fe

7.

8.

9.

10.

11.

12. 13. 14.

o r

Adhesion - Coating is scribed with an "x" and tested with #610 Scotch Tape. Number represents relative amount of loss (0 = no loss and 10 = 100% loss). Adhesion After Thermal Postbake - Same test as No. 7 after UV cured coating has been postbaked at 380OF. for 9 min. Pasteurization Adhesion - Same test as No. 7 after UV cured coating is exposed to 160°F. water for 20 minutes. Pasteurization Adhesion After Postbake - Same test as No. 8 after coating is exposed to 160θρ. water for 20 minutes. Reverse Impact Resistance - 30 inch-pounds - The number represents the relative amount of metal exposed after impact (0 = no metal exposure and 10 = 100% metal exposure). Reverse Impact Resistance After Postbake - Same test as No. 11 after UV cured coating has been postbaked at 380°F. for 9 minutes. Wedge-Bend "30 inch-pounds - The number repre sents the relative amount of metal exposed after impact. Wedge-Bend After Postbake - Same test as No. 13 after UV cured coating has been postbaked at 380°F. for 9 minutes.



Properties^ 6 25 0.72 0.18 8 12 5 1 10 7 9 9 9 8

D 25 75

E x p l a n a t i o n s o f p r o p e r t y measurements g i v e n i n T a b l e I

2 47 0.72 0.36 44 56 3 1 10 7 9 5 7 5

C 50 50

2

1 100 0.71 0.53 74 89 8 0 10 2 3 3 5 6

Β 75 25

The p e r c e n t o f p h o t o i n i t i a t o r and s u r f a c t a n t s i s t h e same for a l l formulations.

- 346 - 0.71 - 0.71 - 64 - 74 0 0 1 0 - 10 - 10 8 9 -

8

A 100 0

BLENDS

1

Tack F r e e Time V i s c o s i t y - CPS @ 25°C. Epoxide V a l u e C y c l o a l i p h a t i c Epoxide V a l u e Softening Point S o f t e n i n g P o i n t A f t e r Postbake Adhesion A d h e s i o n A f t e r Postbake P a s t e u r i z a t i o n Adhesion P a s t e u r i z a t i o n A d h e s i o n A f t e r Postbake Reverse Impact R e s i s t a n c e Reverse Impact R e s i s t a n c e A f t e r Postbake Wedge Bend Wedge Bend A f t e r Postbake

Formulation CY-179 RD-2

PROPERTIES OF CY-179 AND RD-2

TABLE I I .


MODERN CONTAINER

44

COATINGS


p r o b a b l y t h e r e s u l t o f f u r t h e r r e a c t i o n s i n t h e UV c u r e d f i l m s and t h e v o l a t i l i z a t i o n o f t h e u n r e a c t e d m a t e r i a l s from t h e f i l m . Even though p a s t e u r i z a t i o n a d v e r s e l y a f f e c t e d most o f t h e f o r m u l a t i o n s , p o s t b a k i n g t h e f i l m s a l s o improved a d h e s i o n a f t e r pasteurization. Wide v a r i a t i o n s were a l s o o b s e r v e d w i t h t h e r e v e r s e impact and wedge bend t e s t s . F o r these t e s t s , postb a k i n g t h e UV c u r e d f i l m u s u a l l y does n o t s i g n i f i c a n t l y improve f i l m p r o p e r t i e s . T a b l e I I I shows t h e e f f e c t o f d i f f e r e n t r a t i o s o f CY-179 and CY-17 8 on f i l m p r o p e r t i e s . F o r t h e s e two monomers, t h e e p o x i d e v a l u e s and t h e c y l o a l i p h a t i c epoxide values are i d e n t i c a l . I t can be seen t h a t t h e c u r e r a t e d e c r e a s e s as t h e v i s c o s i t y i n c r e a s e s and as t h e epoxide v a l u e s d e c r e a s e . The s o f t e n i n g p o i n t s o f these formulations again u s u a l l y i n c r e a s e d a f t e r postbake. The a d h e s i o n o f f o r m u l a t i o n s A, F G, H and I i s q u i t e good, b o t h b e f o r e and after pasteurization. T h i s i s p r o b a b l y because b o t h o f t h e s e m o l e c u l e s a r e q u i t e p o l a r . As shown i n T a b l e I I , o n l y f o r m u l a t i o n A had good a d h e s i v e properties. When s u c c e s s i v e amounts o f t h e r e l a t i v e l y n o n - p o l a r RD-2 was added t o CY-179, t h e a d h e s i v e p r o p e r t i e s o f t h e c o a t i n g s were a d v e r s e l y a f f e c t e d . f

T a b l e IV shows t h e e f f e c t o f d i f f e r e n t r a t i o s o f CY-179 and A r a l d i t e 6004 on f i l m p r o p e r t i e s . I t c a n be seen t h a t A r a l d i t e 6004 c u r e s v e r y s l o w l y because o f i t s low epoxide v a l u e and h i g h v i s c o s i t y . The t r e n d s i n d i c a t e t h a t i n c r e a s i n g amounts o f A r a l d i t e 6004 d e c r e a s e t h e a d h e s i v e p r o p e r t i e s o f t h e f i l m and g e n e r a l l y i n c r e a s e f l e x i b i l i t y and impact r e s i s t a n c e . A l t h o u g h o n l y f o u r epoxide m a t e r i a l s and t h i r t e e n f o r m u l a t i o n s were i n v e s t i g a t e d , t h i s study i l l u s t r a t e s the parameters and f o r m u l a t i n g t r a d e - o f f s t h a t must be c o n s i d e r e d when b l e n d i n g u l t r a v i o l e t c u r e d e p o x i d e coatings. As w i t h any r e l a t i v e l y new t e c h n o l o g y , t h e s e experiments i n d i c a t e t h a t much i s n o t understood. F o r example, i t i s n o t r e a l l y known how t h e s e monomers and o l i g o m e r s c o p o l y m e r i z e o r which m a t e r i a l s p r e f e r t o homopolymerize and which p r e f e r t o c o polymerize. These q u e s t i o n s w i l l u l t i m a t e l y have t o be answered i n o r d e r t o f u l l y u t i l i z e t h e s e systems. However, c o a t i n g s f o r m u l a t e d u s i n g t h i s c h e m i s t r y a r e now b e i n g used c o m m e r c i a l l y . As p o l l u t i o n r e g u l a t i o n s and energy c o s t s i n c r e a s e , t h e s e t y p e s o f



19 545 0.53 0.53 47 47 0 0 1 1 2 2 3 1

25 75

E x p l a n a t i o n s o f p r o p e r t y measurements g i v e n i n T a b l e I .

2

the same

22 465 0.59 0.59 20 35 0 0 1 0 1 9 2 8 surfactants is

12 392 0.65 0.65 32 52 0 0 0 0 4 9 7 9

50

1

The p e r c e n t o f p h o t o i n i t i a t o r and for a i l formulations,

8 346 0.71 0.71 64 74 0 0 1 0 10 10 8 9

2 Properties Tack F r e e Time V i s c o s i t y - CPS @ 25°C. Epoxide V a l u e C y c l o a l i p h a t i c Epoxide Value Softening Point S o f t e n i n g P o i n t A f t e r Postbake Adhesion A d h e s i o n A f t e r Postbake P a s t e u r i z a t i o n Adhesion P a s t e u r i z a t i o n A d h e s i o n A f t e r Postbake Reverse Impact R e s i s t a n c e Reverse Impact R e s i s t a n c e A f t e r Postbake Wedge Bend Wedge Bend A f t e r Postbake

F 75 25

CY-178 BLENDS

1

A 100 0

Formulation CY-179 CY-178

PROPERTIES OF CY-179 AND

TABLE I I I .


19 685 0.47 0.47 28 29 0 0 0 0 1 1 1 1

100

S*

G

ο

n

S,

H

Ο

3 S

Ci

ο ο


9 3000 0.62 0.36 57 65 1 0 2 1 9 3 8 6

12 1187 0.67 0.53 56 62 0 0 1 0 10 8 10 8 8 346 0.71 0.71 64 74 0 0 1 0 10 10 8 9

Tack F r e e Time V i s c o s i t y - CPS @ 25°C. Epoxide V a l u e C y c l o a l i p a t i c Epoxide V a l u e Softening Point S o f t e n i n g P o i n t A f t e r Postbake Adhesion A d h e s i o n A f t e r Postbake P a s t e u r i z a t i o n Adhesion P a s t e u r i z a t i o n A d h e s i o n A f t e r Postbake Reverse Impact R e s i s t a n c e Reverse Impact R e s i s t a n c e A f t e r Postbake Wedge Bend Wedge Bend A f t e r Postbake

E x p l a n a t i o n s o f p r o p e r t y measurements g i v e n i n T a b l e I,

2

same

The p e r c e n t o f p h o t o i n i t i a t o r and s u r f a c t a n t s i s the for a l l formulations.

11 5500 0.58 0.18 55 72 5 1 10 0 4 3 7 6

75

L

1

Properties^

Κ 50 50

J "75 25

A TÏÏÏÏ 0

6004 BLENDS'

Formulation CY-179 6004

PROPERTIES OF CY-179 AND

TABLE IV.


120 6000 0.5 3 0.00 52 75 0 0 10 0 1 2 3 4

M 0 100

4^

Ο

H

ο >

w w ο

> S

H

8

S ο α w

os

6. KESTER AND TARwiD

Photocurable Epoxide Coatings

materials w i l l necessarily occupy a more prominent position among container coatings.


References: (1)

S. I. Schlesinger, U.S. Patent 3,708,296

(2)

J . J. Licari and P. C. Crepeau, U.S. Patent 3,205,157

(3)

E. Fischer, U.S. Patent 3,236,784

(4)

For review, see Roe, Org. Reactions 5, 193-228 (1949)

RECEIVED MAY 22, 1978.

American Chemical Society Library 1155 16th St. N. W. Wuhtiftoi, L 6. 20038


Modern Container Coatings - American Chemical Society

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