- Email: [email protected]
Ultraviolet applications for packaging
Radiat.
Phys.
Chem.
1977,
Yol.
9,
pp.
307-324.
Pergamon
Press.
Printed
in Great
Britain.
V . c - 3 .i
ULTRAVIOLET
APPLICATIONS
FOR PACKAGING
By M e s r o b i a n ~:~ Vice President Research & Engineering Continental Can Company, Inc. 1350 W. 76th S t r e e t Chicago, Illinois 60620 R.
B.
Over the past five years packaging manufacturers have compared "conventional" coating and printing systems with newly developed p r o c e s s e s w h i c h o p e r a t e at h i g h e r s p e e d s u n d e r m o r e e n v i r o n m e n t a l l y a c c e p t a b l e c o n d i t i o n s a n d , in s o m e c a s e s w i t h l e s s i n v e s t m e n t in c a p i t a l e q u i p m e n t . Water dispersed materials, dry powders, h o t m e l t s , h i g h s o l i d s c o m p o s i t i o n s h a v e b e e n e x a m i n e d in d e p t h . High velocity hot air as well as ultraviolet and electron beam curing have been investigated. M a n y of t h e a b o v e p r o c e s s e s h a v e f o u n d a p p r o p r i a t e a p p l i c a t i o n s in p a c k a g i n g m a n u f a c t u r e . Ultraviolet (UV) c u r i n g h a s e n c o u n t e r e d c o n s i d e r a b l e s u c c e s s in t h e a r e a o f d e c o r a t i v e p r i n t i n g and c o a t i n g . Development
of P h o t o c h e m i c a l l y
Reactive
Monomers
W h e n o u r w o r k w a s i n i t i a t e d in d e v e l o p i n g c o m m e r c i a l materials f o r UV c o n v e r t i b l e d e c o r a t i v e i n k s a n d c o a t i n g s , s e v e r a l i m p o r t a n t needs became evident. The conversion of these materials from l i q u i d c o m p o s i t i o n s t o d r y a n d f a i r l y t o u g h f i l m s u n d e r UV l i g h t h a d to b e a c h i e v e d in a s h o r t t i m e ( l e s s t h a n o n e s e c o n d of irradiation) b e c a u s e s p a c e in m o s t p l a n t s i s e x p e n s i v e a n d n o t c o n d u c i v e to i n s t a l l a t i o n of l o n g b a n k s o f UV l i g h t s . In a d d i t i o n , c o n v e r s i o n s h a d t o b e a c c o m p l i s h e d in o p e n a i r b e c a u s e i n e r t atmospheres were hard to control and added extra costs. It was observed that alkyl acrylates would convert from monomers to p o l y m e r s u n d e r UV l i g h t f a s t e r t h a n a n y o t h e r o l e f i n i c s y s t e m s which were studied. In t h a t r e s p e c t t h e y w e r e f o u n d to b e s u p e r i o r to alkyl methacrylates, vinyl esters, styrene and its derivatives, as well as unsaturated polyesters. H o w e v e r , it w a s a l s o o b s e r v e d t h a t t h e r e a r e c o n s i d e r a b l e d i f f e r e n c e s in t h e r a t e s of c o n v e r s i o n s between the various alkyl acrylates themselves. These differences went beyond the observation that the higher the acrylic f u n c t i o n a l i t y of a p a r t i c u l a r e s t e r t h e g r e a t e r i s t h e e n s u i n g c o n version rate. S u b s t i t u e n t g r o u p s l o c a t e d on t h e a l k y l p o r t i o n s of t h e a c r y l a t e e s t e r s i n f l u e n c e t h e r a t e s of t h e p h o t o p o l y m e r i z a t i o n of t h e a c r y l a t e e s t e r s . To illustrate this effect, a series -':-" In c o l l a b o r a t i o n
with:
J. F. Ackerman, E. T. Bailey, A. R a v v e a n d J . E . H a n l e 307
308
R.B. Mesrobian V.c-3.2
of beta substituted alkyl acrylates w e r e prepared and screened by irradiating t h e m under controlled conditions. Figure 1 shows h o w these m o n o m e r s c o m p a r e d in their rates of photopolymerization with one another w h e n a typical c o m m e r c i a l photoinitiator, diethox-yacetophenone, w a s e m p l o y e d and a m e r c u r y l a m p (100 watts/in., m e d i u m pressure) w a s used for irradiation: A s can be seen, the beta methox-y and beta hydrox-y substituted alkyl acrylates photopolymerized at a m u c h faster rate than for instance did the ethyl acrylate. T h e a b o v e i n f o r m a t i o n w a s u t i l i z e d in d e v e l o p i n g m o n o m e r s o l i g o m e r s f o r u s e in UV c u r a b l e c o a t i n g s a n d i n k s .
and
O n e q u i c k and s i m p l e r o u t e t o ~ - h y d r o x y a l k y l e s t e r s i s t h r o u g h r e a c t i o n of c a r b o x y l i c a c i d s w i t h e p o x i d e s . Because bisphenol A d i g l y c i d a l e t h e r is known to i m p a r t t o u g h n e s s and a d h e s i v e p r o p e r t i e s to c o a t i n g s , t h i s w a s o n e of t h e f i r s t m a t e r i a l s w h i c h were converted to diacrylate esters.
/0\
II2C - C
-CH2
- 0.
©
.
C[II. ~
©
.
0 -CH2
CII 3
0
..
0II
j
H2C--~ C - C - 0 - CIt 2 - ~ H
~ -CH2
+2H2C =C -C-OH
H
@ - Cll 2 - 0 -
>
,,
-0
CH 3 - C
I
CH 3
OH ~
0
- 0CH 2 - C - C H 2 - 0 - C - C}I : CH 2
J
H
UV i r r a d i a t i o n of t h i s ~ - h y d r o x y d i e s t e r d e m o n s t r a t e d t h a t it p h o t o p o l y m e r i z e s e v e n w i t h o u t a d d i t i o n of a p h o t o i n i t i a t o r . Moreo v e r , this p h o t o p o l y m e r i z a t i o n is a p p a r e n t l y not m u c h a f f e c t e d by a i r b e c a u s e the m a t e r i a l w a s c o n v e r t e d to a h a r d d r y f i l m without a n y s i g n s of an o i l y r e s i d u e o n t h e s u r f a c e , w h i c h is s o c o m m o n to m a n y m a t e r i a l s w h i c h a r e p h o t o p o l y m e r i z e d in o p e n a i r in t h i n films.
T h e presence of the beta hydroxy group is quite important to the reactivity of this molecule. Several derivatives w e r e prepared
U. V. applications for packaging
309 V.c-3.3
by reacting it with an isocyanate:
0 H 0
?-C-N
H2C =CH-C
=
.0@D
*0CH 2 =C-CH
/
0
2
~H 3
@
-C
tI
I
0C=N= - 0CH 2 -C
I
Q
- CH 2 - 0 = C = C H = C H 2
I
H
CH 3
0
an acetate:
0 0 2)
H 2 C = C H = C"
0
0 - C - CH = 0-
CH 3
0 - C - CH 3
C I t $ - C ' = C H 2 - 0' -
0c ,
I C - CH 2 = 0 - C - C H
= CH 2
0
CI{ 3
and an acrylate:
O
0
,i
o
c
3)
c
.c.2
©
©
O - C = CH = CH 2
I - c, - c , , ~ H
CH 3
N o n e of t h e s e m a t e r i a l s parent compound.
photopolymerized
-
~-
~.-
c~
/
as rapidly
0
as did the
This compound is now available from at least two sources in this c o u n t r y , D o w C h e m i c a l C o . a n d S h e l l C h e m i c a l C o . A l s o , it h a s since been described in scientific literature. 2, 3 Measurement
of C u r e
A r a p i d a n d u s e f u l w a y t o e s t i m a t e t h e c u r e of a n y c r o s s l i n k a b l e c o a t i n g , i n c l u d i n g UV c u r a b l e m a t e r i a l s , is to measure solvent resistance. In i t s s i m p l e s t f o r m t h i s i s d o n e w i t h a m e t h y l e t h y l ketone (MEK) rub test. For some work, a more precise method is desirable. I n v e s t i g a t i o n of v a r i o u s t e c h n i q u e s l e d t o t h e u s e of photocalorimetry. A Differential Scanning Calorimeter was converted into a Photocalorimeter f o r m e a s u r i n g t h e e x o t h e r m of p o l y m e r i z a t i o n w h i c h r e s u l t s f r o m e x p o s u r e of a s e n s i t i v e c o a t i n g o r i n k to UV r a d i a t i o n . The exotherm was used as a measure of t h e a m o u n t of p o l y m e r ization reaction that occurs. F i g . 2 s h o w s a s c h e m a t i c of t h e CCC Photocalorimeter.
R. B. M e s r o b i a n
310
V.c-3.4 I n f o r m a t i o n u s e f u l in t h e d e v e l o p m e n t o f c o a t i n g s a n d i n k s w a s obtained by this technique. For example, the effects of inhibitor c o n c e n t r a t i o n o r l e v e l of c a r b o n b l a c k p i g m e n t a t i o n w e r e a s s e s s e d w i t h r e s p e c t to c u r i n g c h a r a c t e r i s t i c s . Fig. 3 shows the primary output from the Photocalorimeter where f o u r e q u a l UV r a d i a t i o n p u l s e s r e s u l t e d in p l o t s of e x o t h e r m r a t e w h i c h , w h e n i n t e g r a t e d , p r o v i d e a m e a s u r e of t h e h e a t o f p o l y m e r ization. T h e i n f l u e n c e of " i n h i b i t o r " c o n c e n t r a t i o n o n UV p o l y m e r i z a t i o n rate was determined. A c o m b i n a t i o n o f h y d r o q u i n o n e (HQ) a n d P - m e t h o x y p h e n o l ( M E H Q ) w a s e v a l u a t e d in a UV c u r a b l e v e h i c l e at d i f f e r e n t l e v e l s . T h e r e s u l t s s h o w n in F i g . 4 i n d i c a t e t h a t w i t h equivalent exposure conditions (under nitrogen) doubling the " i n h i b i t o r " c o n c e n t r a t i o n r e d u c e d t h e d e g r e e of r e a c t i o n 18%. F u r t h e r , t h e s m a l l i n d u c t i o n p e r i o d ( p r o b a b l y d u e to r e s i d u a l d i s s o l v e d o x y g e n in t h e c o a t i n g ) a n d t h e r e d u c t i o n in s l o p e at higher concentration indicate that this combination of HQ and MEHQ is a "retarder" rather than an "inhibitor". A true inhibitor (such a s o x y g e n ) w o u l d s h o w a r e l a t i o n s h i p l i k e t h a t s h o w n in F i g . 5. T h e e f f e c t of c a r b o n b l a c k p i g m e n t a t i o n o n t h e p o l y m e r i z a t i o n rate of a UV c u r a b l e c o m p o s i t i o n w a s i n v e s t i g a t e d . Photocalorimetric d a t a w e r e o b t a i n e d f o r a n ink c o n t a i n i n g 8% c a r b o n b l a c k a p p l i e d at v a r y i n g f i l m t h i c k n e s s e s . F i g . 6 s h o w s ~hat a m o u n t o f p o l y m e r i z a t i o n r e a c h e d a c o n s t a n t l e v e l at a b o u t 2 m i c r o n s (a p r a c t i c a l i n k thickness). T h e UV r a d i a t i o n a p p a r e n t l y c o u l d n o t e f f e c t i v e l y penetrate through a thickness greater than this. In a n o t h e r s e r i e s of e x p e r i m e n t s t h e c a r b o n b l a c k c o n c e n t r a t i o n w a s v a r i e d at c o n s t a n t f i l m t h i c k n e s s (16 m i c r o n s ) . Fig. 7 shows the results. The film thickness here is equivalent to that of a c o a t i n g ( r a t h e r t h a n t h a t of an i n k ) . T h e e x o t h e r m i s r e d u c e d a s t h e p e r c e n t c a r b o n b l a c k i s i n c r e a s e d , a n d t h e r e i s no i n d i c a t i o n o f a n o p t i m u m l e v e l of b l a c k p i g m e n t . It c a n t h e r e f o r e b e c o n c l u d e d t h a t d e v e l o p m e n t of a UV c u r a b l e b l a c k c o a t i n g c a p a b l e of c o m p l e t e through-cure is very unlikely. Metal Containers
D e v e l o p m e n t s in UV c u r a b l e m e t a l d e c o r a t i n g s y s t e m s h a v e f o l l o w e d an e v o l u t i o n a r y p l a n . T h e e a r l y e x p l o r a t o r y w o r k o n m a t e r i a l s a n d p r o c e s s w a s c a r r i e d o u t in t h e l a t e 1 9 6 0 ' s . The f i r m d e c i s i o n to w o r k t o w a r d c o m m e r c i a l development took place e a r l y in 1970. A p i l o t l i n e ( F i g u r e 8) f o r c o a t i n g , p r i n t i n g , a n d
U. V. applications for packaging
311
V.c-3.5 varnishing with U V curable material w a s completed in June 1972. The original commercial UV " S e t P a s s " m e t a l p r i n t i n g l i n e h a s t h e c a p a c i t y t o u s e f o u r t e e n UV l a m p s (12 a n d 2 s p a r e s ) , 200 w a t t s / i n . m e d i u m p r e s s u r e (15 p s i g ) . T h e l a m p s h a v e q u a r t z e n v e l o p e s and a r e m e r c u r y a n d A r g o n f i l l e d . T h e y o p e r a t e at a c o n s t a n t c u r r e n t of 6.5 amperes. W o r k d u r i n g 1973 d e m o n s t r a t e d t h a t , at a l i n e s p e e d of 85 s h e e t s (300 ft. ) p e r m i n u t e , a l l c u r r e n t UV i n k s w o u l d cure with four lamps or less. Therefore, a m u c h s m a l l e r UV dryer was specified for all flat sheet printing installations after 1973. A l l UV s h e e t d r y e r s a r e e q u i p p e d w i t h e l l i p t i c a l r e f l e c t o r s f o r t h e l a m p s , p o s i t i o n e d s o t h a t t h e p l a t e s u r f a c e i s a t t h e f o c a l p o i n t . In l a t e r w o r k w i t h p r e f o r m e d c a n s , u s i n g f a s t e r c u r i n g c o a t i n g s , it w a s r e c o g n i z e d t h a t t h e s u r f a c e of t h e c a n d o e s n o t h a v e t o b e at o r n e a r t h e f o c a l p o i n t o f t h e r e f l e c t o r to e f f e c t a d e q u a t e c u r i n g . Figure 9 shows schematically a n u m b e r of M e t a l D e c o r a t i n g p r o c e s s e s w h i c h h a v e b e e n o r w i l l in t h e n e a r f u t u r e b e put i n t o commercial production. S y s t e m I UV " S e t P a s s " w a s p u t i n t o c o m m e r c i a l p r o d u c t i o n in J u l y 1972. T h e r e a r e p r e s e n t l y i I l i n e s o p e r a t i n g in 9 C o n t i n e n t a l Can Company plants. This system has been used primarily for 3-piece beer and soft drink cans. S y s t e m I I UV " S e t S t a c k " w a s put i n t o c o m m e r c i a l p r o d u c t i o n in J u n e 1974. T h e r e i s p r e s e n t l y o n e C o n t i n e n t a l C a n C o m p a n y l i n e o p e r a t i n g a n d a n o t h e r in t h e l a t t e r s t a g e s of c o n s t r u c t i o n . This system has been used primarily for aerosol cans. In d e v e l o p i n g s u i t a b l e UV c u r a b l e c o a t i n g s , i n k s a n d v a r n i s h e s f o r t h e a b o v e s y s t e m s , t h e n a t u r e of t h e v a r i o u s c o m m e r c i a l metal s u b s t r a t e s h a d t o b e c o n s i d e r e d ( F i g . 10). A l l of t h e s e c o n d i t i o n s w o u l d b e m o d i f i e d b e f o r e UV d e c o r a t i v e materials were applied to the other side if conventional coatings w e r e b a k e d on o n e s i d e of t h e p l a t e . T h e m e t h o d o f a p p l y i n g UV c u r a b l e m a t e r i a l s of t h e f i n i s h e d d e c o r a t i o n ( F i g . 11).
affects the quality
UV i n k s t e n d t o b e m o r e a d v e r s e l y a f f e c t e d t h a n t r a d i t i o n a l o l e o r e s i n o u s i n k s w h e n i n c o n t a c t w i t h f o u n t a i n s o l u t i o n s u s e d in t h e
312
R.
B. Mesrobian
V.c-3.6 lithographic process. Absorption of water tends to lower the v i s c o s i t y a n d c o n s i s t e n c y of t h e i n k a n d m a k e s t h e m p r o n e to "scumming" a t e r m u s e d t o d e s c r i b e t h e t e n d e n c y of t h e i n k t o t r a n s f e r c o l o r to t h e n o n - p r i n t e d a r e a s . Careful control of the c o m p o s i t i o n o f t h e f o u n t a i n s o l u t i o n i s n e c e s s a r y to o v e r c o m e t h i s p r o b l e m in p r o d u c t i o n . T h e c o m p o s i t i o n a n d h a r d n e s s of t h e c o a t i n g r o l l o r o f f s e t a l s o a f f e c t t h e a p p e a r a n c e of t h e c o a t i n g .
blanket
UV W h i t e C o a t i n g While development and commercialization of UV i n k s h a s b e e n r e l a t i v e l y r a p i d , p r o g r e s s w i t h UV w h i t e c o a t i n g s f o r c a n s h a s b e e n s l o w . W h e n w o r k w a s i n i t i a t e d , it w a s r e c o g n i z e d t h a t t h e r e w o u l d be greater problems with white coatings than with inks. Early efforts confirmed this prognosis and indicated three primary problem areas during the subsequent plant trials. 1) R h e o l o g y ( a p p l i c a t i o n c h a r a c t e r i s t i c s ) - ability to lay down a n d a c h i e v e a s m o o t h f i l m e q u i v a l e n t in a p p e a r a n c e t o t h e solvent base conventional white coatings using commercial c o a t i n g e q u i p m e n t at n o r m a l l i n e s p e e d s . 2) A d h e s i o n
f o l l o w i n g UV e x p o s u r e
3) D i s c o l o r a t i o n
- after the thermal
- picking at the printing press. bake.
Rheology, as indicated by appearance, is the major problem area. It h a s b e e n p u r s u e d in t w o w a y s : (a) c h e m i c a l c h a n g e s in t h e c o a t i n g formulation, a n d (b) m e c h a n i c a l c h a n g e s on t h e c o a t i n g m a c h i n e . Optimization of the coating material composition is essentially complete. Current mechanical modifications in the coating application process provide significant p r o m i s e for a c o m m e r c i a l l y acceptable U V white coating on cans. PAPER
& PAPERBOARD
Folding Carton Folding carton printers m o s t often use sheet fed lithographic printing equipment to produce the decorated cartons. Their c u s t o m e r s use these cartons for packaging cosmetics, frozen food, drugs, cereals, and a m y r i a d of other c o m m e r c i a l products. In the process of completing the cartons it is necessary to die
U. V. applications for packaging V.c-3.7 cut, fold, crease and glue the decorated blanks. With conventional oxidizing inks a waiting period after printing is n e c e s s a r y to allow the ink to dry to a state which yields sufficient hardness and rub resistance for satisfactory p e r f o r m a n c e in the cutting and forming operations. In addition, it is often necessary to dust the freshly printed sheets with starch or w a x to prevent ink transfer f r o m sheet to sheet. B o t h t h e d e l a y in t h r o u g h p u t a n d t h e u n d e s i r a b l e d u s t i n g a r e l o n g s t a n d i n g d e f i c i e n c i e s in t h e f o l d i n g c a r t o n d e c o r a t i n g p r o c e s s . The u s e of u l t r a v i o l e t c u r a b l e i n k s o b v i a t e s b o t h o f t h e s e p r o b l e m s . T h e r e f o r e an i n c r e a s i n g n u m b e r of f o l d i n g c a r t o n p r i n t e r s a r e either using or contemplating the use of UV. T h e UV c u r i n g e q u i p m e n t u s e d in f o l d i n g c a r t o n p r i n t i n g i s s i m i l a r to t h a t u s e d f o r c u r i n g on m e t a l . The differences are dictated by the press configuration and space limitations rather than by any b a s i c d i f f e r e n c e in c u r e r e q u i r e m e n t s . E a r l y developments c o n c e n t r a t e d on l o c a t i n g t h e UV e q u i p m e n t in t h e d e l i v e r y p o r t i o n of t h e p r e s s o n l y , s i n c e m a n y l a m p s w e r e r e quired to achieve the desired degree of cure. Today one !amp b e t w e e n c o l o r s i s c o m m o n , w i t h n o t m o r e t h a n 4 l a m p s u s e d in the delivery section for final cure. A n o t h e r i m p o r t a n t c o n s i d e r a t i o n in c a r t o n d e c o r a t i o n w h e n u s i n g UV c u r a b l e m a t e r i a l s i s t h e a p p l i c a t i o n o f o v e r p r i n t v a r n i s h . On a m u l t i c o l o r p r e s s e q u i p p e d w i t h UV d r y i n g c a p a b i l i t y , it i s c o m m o n p r a c t i c e t o d r y t h e i n k s in t h e n e x t t o l a s t c o l o r s t a t i o n a n d a p p l y o v e r p r i n t v a r n i s h in t h e l a s t u n i t . I n t h i s w a y c o m p l e t e l y f i n i s h e d sheets having adequate gloss and excellent rub and chemical resistance are delivered from the press. Since no starch spray is r e q u i r e d , t h e s u r f a c e h a s a s m o o t h f e e l w h i c h h a s m o r e s a l e s a p p e a l . In m a n y c a s e s t h e UV c u r a b l e p r e s s - a p p l i e d varnish can r e p l a c e a c a t a l y z e d o v e r p r i n t c o a t i n g w h i c h r e q u i r e s b a k i n g in c i r c u l a t e d h o t a i r . T h e u s e o f UV v a r n i s h e l i m i n a t e s a i r p o l l u t i o n f r o m t h e s o l v e n t s in t h e c o a t i n g , a n d d o e s n o t r e q u i r e s c a r c e f u e l such as natural gas. In the printing of Christmas liquor cartons or cosmetic packages, as many as eight colors can be used and quite often the substrate is aluminum foil laminated to paperboard. In the lithographic printing of foil laminates, where n o r m a l inks dry poorly due to l a c k o f a b s o r p t i v i t y in t h e s u b s t r a t e , t h e u s e o f u l t r a v i o l e t c u r i n g is p a r t i c u l a r l y a d v a n t a g e o u s .
313
314
R.B. Mesrobian
V.c-3.8 S o m e printers find that the use of U V is desirable because of the higher productivity possible, particularly on m o d e r n high speed sheet fed presses. In C a n a d a a major printer has converted to U V exclusively for printing of cigarette packages. T h e sheets are printed with U V inks with in-line varnishing and embossing, a very efficient process. T h e penetration of ultraviolet ink drying into the folding carton business has been slow. T h e r e are, however, between 30 and 40 n e w presses and a like n u m b e r of retrofitted presses capable of running U V inks. Paper
Labels
Small narrow web presses are compact in nature and the simplicity and low space requirement of UV dryers are an excellent combination. The press can feed a web into a series of print stations using either lithographic or letterpress printing, and when coupled with a cutting device, can deliver a dry decorated label or box blank at acceptable speeds (in the order of 200 ft. /min.). Many presses of this type have been fitted with lamps between printing units and can use dry trapping if desired.
T h e binder which results f r o m the ultraviolet curing of unsaturated m o n o m e r s confers chemical resistance to the printed label. C h e m i c a l resistance is one of the prime requirements for labels, particularly for cosmetics, detergents, or other aggressive package contents. T h e need for overlacquer is obviated by the use of U V inks. Corrugated
Display
Board
In the decoration of corrugated board for advertising displays, the application of ink is by letterpress and a thick film of ink must be applied to make certain that all valleys receive ink. The result is that sheets printed with conventional inks are very susceptible to ink transfer in the pile. The use of UV drying solves this problem. However, there are technical difficulties in drying the thick ink films, especially black, because of the low efficiency in utilization of the UV energy. Plastic
Containers
Decorated polystyrene tubs have become a staple of the rapidly expanding quick-food business. Polystyrene, and especially foamed polystyrene, is a very heat sensitive substrate. Decoration of this surface is predominately accomplished on dry offset printing
U. V. a p p l i c a t i o n s
for packaging
V.c-3.9 equipment. In t h e o p e r a t i o n o f t h i s p r i n t i n g p r o c e s s t h e i n k i s e x p o s e d in t h i n f i l m s to t h e a t m o s p h e r e f o r l o n g p e r i o d s of t i m e on t h e p r e s s b e f o r e it i s t r a n s f e r r e d to the substrate. To keep the ink from drying prematurely, it i s n e c e s s a r y in t h e c a s e of s o l v e n t b e a r i n g i n k s , to u s e v e r y h i g h b o i l i n g s o I v e n t s . After the print is made, the conflicting situation is created where high boiling solvents must be removed from a heat sensitive substrate. Conventional oven design, therefore, c o n s i s t s of a l o n g d w e l l t i m e at r e l a t i v e l y l o w a i r t e m p e r a t u r e s . It i s n o t s u r p r i s i n g , therefore, t h a t u l t r a v i o l e t c u r i n g of i n k s h a s b e e n w i d e l y a d o p t e d f o r t h e p r i n t i n g of p l a s t i c c u p s a n d t u b s . The i n k s do n o t d r y o n t h e ink r o l l e r s a n d , a f t e r p r i n t i n g , r a p i d d r y i n g of the inks can be effected without distorting the plastic. C o m p o s i t e p l a s t i c o i l c a n s a r e b e i n g p r i n t e d w i t h UV c u r i n g b y several companies for similar reasons. In t h e s e c o n t a i n e r s t h e s t r u c t u r a l m a t e r i a l i s f r e q u e n t l y a d u a l - p l y s y s t e m of p o l y p r o p y l e n e and polyethylene. A d h e s i o n of UV i n k s to p o l y s t y r e n e h a s b e e n d e f i c i e n t in s o m e c a s e s c h i e f l y d u e to m o l d r e l e a s e a n d a n t i s t a t i c a g e n t s i n c o r p o r a t e d in t h e polystyrene compositions. T h e a g e n t s t e n d t o s p e w to t h e p l a s t i c surface and present a very low energy surface to which the ink has poor adhesion. On the other hand, adhesion to polyolefins has been uniformly good if treated with corona discharge immediately before printing. T h e f u t u r e of u l t r a v i o l e t c u r i n g s e e m s t o b e s e c u r e in plastic decoration. O n e m a j o r p r o d u c e r of p o l y e t h y l e n e s q u e e z e t u b e s , b y c h a n g i n g t o UV, i s r e c o v e r i n g a n e n t i r e f l o o r in a n installation where long ovens were required with conventional inks. F DA The FDA has not been involved to any appreciable extent up until n o w b e c a u s e m o s t a p p l i c a t i o n s f o r UV c u r i n g a r e o r i e n t e d t o u s e s that include no direct food contact with printing inks. Direct f o o d - c o n t a c t u s a g e o f UV c u r e d c o a t i n g s p r o b a b l y w i l l r e q u i r e e v a l u a t i o n of c h e m i c a l s a f e t y a n d w i l l b e s u b j e c t to t h e d e l a y s of the regulatory process. These coatings generally incorporate one or more components, such as photoinitiators, t h e u s a g e of w h i c h w a s n o t c o n t e m p l a t e d in c o n v e n t i o n a l h e a t - c u r e d c o a t i n g s . Consequently, such materials, if n o t c o v e r e d b y p r e v i o u s F D A regulations, will require a petition for acceptance. This proc e d u r e i n v o l v e s a c o n s i d e r a b l e a m o u n t of e x t r a c t a b i l i t y s t u d y a n d a v a r i a b l e a m o u n t of t o x i c o l o g y , d e p e n d i n g o n t h e s i m i l a r i t y of t h e n e w m a t e r i a l t o o l d e r w e l l - k n o w n m a t e r i a l s .
RPC Vol. 9, No. I-3--V
315
R. B. Mesrobian
316
V.c-3.10 OSHA W o r k e r safety does not present any unusual or insurmountable problems. Suitable shielding can eliminate radiation exposure and good engineering design can provide adequate ventilation to r e m o v e ozone and chemical vapors f r o m w o r k areas. M o s t of the c o m m o n l y used U V curable m o n o m e r s have no m o r e than slight to m o d e r a t e oral toxicity. Table I shows eye and skin irritations ratings for s o m e typical m o n o m e r s and coatings. In s o m e instances, protective clothing and rubber gloves m a y be necessary, but this is no different f r o m the handling of m a n y other chemicals, including s o m e solvents. L a b o r in the can industry has endorsed the adoption of U V lines since they eliminate s o m e of the noise and m o s t of the intense heat of gas fired ovens. T h e r e have been no O S H A objections to U V drying, and the process does not involve any n e w skills or c o m p l e x operating technique s.
3/18/76
(1) (2) (3) (4)
Data f r o m a paper submitted for publication in J. Appl. P olym. Sci. T a k y a m a , E., Japan Plastics, 5 (4), 6-19 (1971) Nishkobo, etal., J. Appl. Poly. Sci., 18, 3445 (1974) Burnett, G. M. and Melville, H. W . , Nature, gond. 156 (1945) 661; Proc. R o y Soc. A. 189 (1947) 456, 481 and 494
U. V. applications for packaging
317
IRRADIATION OF B--SUBSTITUTED ETHYL ACRYLATES UNDER NITROGEN FLUSH % Conversion vs. Time
% conversion to polymer
(figure 1 )
OH]
14
~CIHs
12
, Br
10
8
OCH=
iq~
6
4
CN
2 20
40
60 80 Time,Seconds
AlzacReflector MercuryLamp
100
1
120
=-
b.\\\\\\\\\\\\\\\~l
t~\\\\\\\\\\\\\\\'~l
\ IY'7-/-/77-/~
\
/
)
/
/
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FilterHolders ~.'=.W,n~,ow,~, S.,~,~,.,',o,. t
f.~.~.~
' ~ os~ c.,,
PHOTOCALORIMETER
(figure2)
318
R.B.
Exotherm Rate, dH at
Mesrobian
TYPICAL PHOTOCALORIMETRIC DATA (figure 3)
Heat of Reaction,
/
at
OH dt
at
.... f
I
l
\ u
tgL~ m m i ~
I ! mlm,=l
...JI
~tmm
J
I
Time
Exotherm, calories/gram
40
I
30
,
e.I
(figure 4)
•/
e/y
10
INFLUENCE OF INHIBITOR ON EXOTHERM
?/ /m• / / S•
• Standard Vehicle • Standard Vehicle plus 1200 ppm mehq, 200 ppm hq
ul S I / ./
0
II
,r
l
.10
I
.20
~ Reiative ~ UV Exposure "
I
.30 I t Im~
.40
• Standard Vehicle plus 2400 ppm mehq, 400 ppm hq
U. V. applications for packaging
319
PHOTOPOLYMERIZATION O F V I N Y L A C E T A T E A T 250C (figure 5)
Conversion %
4
0
1.86
3 . 8 6 5.80
Millimols/liter of p-Benzoquinone,
2
I
80
160 240 Time, Minutes
H E A T O F R E A C T I O N vs. F I L M T H I C K N E S S AT CONSTANT IRRADIATION
A H, calories x 10 -4
(figure 6)
30-
• 0 ~O
•
•
•
o ,o
•
8% Carbon Black in Photo Sensitive Vehicle
lO/ I
2
i 4
i 6
i 8
Film Thickness, Microns
I 10
I 12
I 14
320
R.B. Calories/ tl Gram
Mesrobian
HEAT OF REACTION VS.
5O
40
30
\
C A R B O N BLACK C O N C E N T R A T I O N IN PHOTO SENSITIVE VEHICLE AT 16 M I C R O N S T H I C K N E S S AND CONSTANT IRRADIATION (figure 7)
20
10
I
I
i
I
6
8
10
12
% Carbon Black
U V SECTION
SECOND PRINTING PRESS
UV SECTION
INFRARED DELIVERY SECTION CONVEYORS
TRAILING VARNISH UNIT
(figure 8)
DELIVERY SECTIONS
PRINT 2-UV INKS
I
I
I
PRINT 2-CONV. INKS
CONV. TRAILING
BASE COAT UV
UV SET
STACK
VARNISH LINE
2 COLOR AND
PRINT 2 CONV. INKS
CONV. TRAILING VARNISH
r~OATE R
D UV SET
PRINTER
am (Iz PIN O V E N
J
200
~
~,/ICKET OVEN P O L L U T I O N CONTROL DEVICE
BASE COAT
ROLLER PRINT SET PRINT UV STACK CONVEYOR 2 - UV 2 - UV T R A I L I N G INKS INKS VAR~JISH
TI"'--"
UV
STACK
+ C O N V E N T I O N A L I/S ENAMEL LINE
S Y S T E M 3 - UV D I R E C T P R I N T - 4 UV I N K S A N D V A R N I S H "SET STACK" LINE
S Y S T E M 4 - UV W H I T E
UV SET
FEET
I
150
/VICKET OVEN P O L L U T I O N C O N T R O L DEVICE
S Y S T E M 5 - UV W H I T E B A S E C O A T FOR T W O - P I E C E C A N S
"~OLLER 3ONVEYOR
::rOLLER PRINT .~ONVEYOR 2 - UV INKS
I
100
SYSTEM 1 - UV'~;ET PASS"LINE - 2 UV INKS
75
50
25
S Y S T E M 2 - UV SET STACK LINE
ROLLER CONVEYOR
)
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~TACK
J
260
--= L,3
I ,,,J
c,
t~
I"h 0
P}
I~0 O
,..4
322
R.B.
Mesrobian
Figure I0
COMMERCIAL A.
-
SURFACE CONDITION
Cathodic dichromate on tinplate Chromium - chromium oxide on steel Phosphate on aluminum
Lubricated i. 2.
C.
SUBSTRATES
Chemically treated i. 2. 3.
B.
METAL
Dioctyl sebacate (DOS) or acetyl tributyl citrate (ATBC) on tinplate Residual oil or butyl stearate on steel
Detergent washed
U. V. applications for packaging
Figure ii
METHOD OF APPLYING ULTRAVIOLET CURABLE MATERIALS q~i_n_%
[[i_5~i_n_% A.
Dry offset
A.
Direct roll
B.
Wet offset (lithography)
B.
Offset gravure
C.
Direct gravure
D.
Offset gravure
323
Severe Slight
Moderate
Extreme
Pentaerythritol triacrylate, PETA
H e x a n e d i o ] d i a c r y l a t e , HDDA
Trimethylol propane, triacrylate TMPTA
Neopentyl glycol d i a c r y l a t e , NPGDA
SKIN
Moderate
Moderate
Severe
Moderate
Moderate
Extreme
Mild
PRIMARY IRRITATION
** M o n o m e r components: H D D A , N P G D A
Extreme
Tetraethylene glycol, diacrylate TEGDA
I","M o n o m e r c o m p o n e n t s : P E T A and T E G D A
Severe
Coating B'**
IRRITATION
Extreme
EYE
RATINGS MATERIALS
Coating A*
MATERIAL
TOXICOLOGICAL U. V. CURABLE
TABLE I
and T M P T A ~
oI
0
f~
b~
Phys.
Chem.
1977,
Yol.
9,
pp.
307-324.
Pergamon
Press.
Printed
in Great
Britain.
V . c - 3 .i
ULTRAVIOLET
APPLICATIONS
FOR PACKAGING
By M e s r o b i a n ~:~ Vice President Research & Engineering Continental Can Company, Inc. 1350 W. 76th S t r e e t Chicago, Illinois 60620 R.
B.
Over the past five years packaging manufacturers have compared "conventional" coating and printing systems with newly developed p r o c e s s e s w h i c h o p e r a t e at h i g h e r s p e e d s u n d e r m o r e e n v i r o n m e n t a l l y a c c e p t a b l e c o n d i t i o n s a n d , in s o m e c a s e s w i t h l e s s i n v e s t m e n t in c a p i t a l e q u i p m e n t . Water dispersed materials, dry powders, h o t m e l t s , h i g h s o l i d s c o m p o s i t i o n s h a v e b e e n e x a m i n e d in d e p t h . High velocity hot air as well as ultraviolet and electron beam curing have been investigated. M a n y of t h e a b o v e p r o c e s s e s h a v e f o u n d a p p r o p r i a t e a p p l i c a t i o n s in p a c k a g i n g m a n u f a c t u r e . Ultraviolet (UV) c u r i n g h a s e n c o u n t e r e d c o n s i d e r a b l e s u c c e s s in t h e a r e a o f d e c o r a t i v e p r i n t i n g and c o a t i n g . Development
of P h o t o c h e m i c a l l y
Reactive
Monomers
W h e n o u r w o r k w a s i n i t i a t e d in d e v e l o p i n g c o m m e r c i a l materials f o r UV c o n v e r t i b l e d e c o r a t i v e i n k s a n d c o a t i n g s , s e v e r a l i m p o r t a n t needs became evident. The conversion of these materials from l i q u i d c o m p o s i t i o n s t o d r y a n d f a i r l y t o u g h f i l m s u n d e r UV l i g h t h a d to b e a c h i e v e d in a s h o r t t i m e ( l e s s t h a n o n e s e c o n d of irradiation) b e c a u s e s p a c e in m o s t p l a n t s i s e x p e n s i v e a n d n o t c o n d u c i v e to i n s t a l l a t i o n of l o n g b a n k s o f UV l i g h t s . In a d d i t i o n , c o n v e r s i o n s h a d t o b e a c c o m p l i s h e d in o p e n a i r b e c a u s e i n e r t atmospheres were hard to control and added extra costs. It was observed that alkyl acrylates would convert from monomers to p o l y m e r s u n d e r UV l i g h t f a s t e r t h a n a n y o t h e r o l e f i n i c s y s t e m s which were studied. In t h a t r e s p e c t t h e y w e r e f o u n d to b e s u p e r i o r to alkyl methacrylates, vinyl esters, styrene and its derivatives, as well as unsaturated polyesters. H o w e v e r , it w a s a l s o o b s e r v e d t h a t t h e r e a r e c o n s i d e r a b l e d i f f e r e n c e s in t h e r a t e s of c o n v e r s i o n s between the various alkyl acrylates themselves. These differences went beyond the observation that the higher the acrylic f u n c t i o n a l i t y of a p a r t i c u l a r e s t e r t h e g r e a t e r i s t h e e n s u i n g c o n version rate. S u b s t i t u e n t g r o u p s l o c a t e d on t h e a l k y l p o r t i o n s of t h e a c r y l a t e e s t e r s i n f l u e n c e t h e r a t e s of t h e p h o t o p o l y m e r i z a t i o n of t h e a c r y l a t e e s t e r s . To illustrate this effect, a series -':-" In c o l l a b o r a t i o n
with:
J. F. Ackerman, E. T. Bailey, A. R a v v e a n d J . E . H a n l e 307
308
R.B. Mesrobian V.c-3.2
of beta substituted alkyl acrylates w e r e prepared and screened by irradiating t h e m under controlled conditions. Figure 1 shows h o w these m o n o m e r s c o m p a r e d in their rates of photopolymerization with one another w h e n a typical c o m m e r c i a l photoinitiator, diethox-yacetophenone, w a s e m p l o y e d and a m e r c u r y l a m p (100 watts/in., m e d i u m pressure) w a s used for irradiation: A s can be seen, the beta methox-y and beta hydrox-y substituted alkyl acrylates photopolymerized at a m u c h faster rate than for instance did the ethyl acrylate. T h e a b o v e i n f o r m a t i o n w a s u t i l i z e d in d e v e l o p i n g m o n o m e r s o l i g o m e r s f o r u s e in UV c u r a b l e c o a t i n g s a n d i n k s .
and
O n e q u i c k and s i m p l e r o u t e t o ~ - h y d r o x y a l k y l e s t e r s i s t h r o u g h r e a c t i o n of c a r b o x y l i c a c i d s w i t h e p o x i d e s . Because bisphenol A d i g l y c i d a l e t h e r is known to i m p a r t t o u g h n e s s and a d h e s i v e p r o p e r t i e s to c o a t i n g s , t h i s w a s o n e of t h e f i r s t m a t e r i a l s w h i c h were converted to diacrylate esters.
/0\
II2C - C
-CH2
- 0.
©
.
C[II. ~
©
.
0 -CH2
CII 3
0
..
0II
j
H2C--~ C - C - 0 - CIt 2 - ~ H
~ -CH2
+2H2C =C -C-OH
H
@ - Cll 2 - 0 -
>
,,
-0
CH 3 - C
I
CH 3
OH ~
0
- 0CH 2 - C - C H 2 - 0 - C - C}I : CH 2
J
H
UV i r r a d i a t i o n of t h i s ~ - h y d r o x y d i e s t e r d e m o n s t r a t e d t h a t it p h o t o p o l y m e r i z e s e v e n w i t h o u t a d d i t i o n of a p h o t o i n i t i a t o r . Moreo v e r , this p h o t o p o l y m e r i z a t i o n is a p p a r e n t l y not m u c h a f f e c t e d by a i r b e c a u s e the m a t e r i a l w a s c o n v e r t e d to a h a r d d r y f i l m without a n y s i g n s of an o i l y r e s i d u e o n t h e s u r f a c e , w h i c h is s o c o m m o n to m a n y m a t e r i a l s w h i c h a r e p h o t o p o l y m e r i z e d in o p e n a i r in t h i n films.
T h e presence of the beta hydroxy group is quite important to the reactivity of this molecule. Several derivatives w e r e prepared
U. V. applications for packaging
309 V.c-3.3
by reacting it with an isocyanate:
0 H 0
?-C-N
H2C =CH-C
=
.0@D
*0CH 2 =C-CH
/
0
2
~H 3
@
-C
tI
I
0C=N= - 0CH 2 -C
I
Q
- CH 2 - 0 = C = C H = C H 2
I
H
CH 3
0
an acetate:
0 0 2)
H 2 C = C H = C"
0
0 - C - CH = 0-
CH 3
0 - C - CH 3
C I t $ - C ' = C H 2 - 0' -
0c ,
I C - CH 2 = 0 - C - C H
= CH 2
0
CI{ 3
and an acrylate:
O
0
,i
o
c
3)
c
.c.2
©
©
O - C = CH = CH 2
I - c, - c , , ~ H
CH 3
N o n e of t h e s e m a t e r i a l s parent compound.
photopolymerized
-
~-
~.-
c~
/
as rapidly
0
as did the
This compound is now available from at least two sources in this c o u n t r y , D o w C h e m i c a l C o . a n d S h e l l C h e m i c a l C o . A l s o , it h a s since been described in scientific literature. 2, 3 Measurement
of C u r e
A r a p i d a n d u s e f u l w a y t o e s t i m a t e t h e c u r e of a n y c r o s s l i n k a b l e c o a t i n g , i n c l u d i n g UV c u r a b l e m a t e r i a l s , is to measure solvent resistance. In i t s s i m p l e s t f o r m t h i s i s d o n e w i t h a m e t h y l e t h y l ketone (MEK) rub test. For some work, a more precise method is desirable. I n v e s t i g a t i o n of v a r i o u s t e c h n i q u e s l e d t o t h e u s e of photocalorimetry. A Differential Scanning Calorimeter was converted into a Photocalorimeter f o r m e a s u r i n g t h e e x o t h e r m of p o l y m e r i z a t i o n w h i c h r e s u l t s f r o m e x p o s u r e of a s e n s i t i v e c o a t i n g o r i n k to UV r a d i a t i o n . The exotherm was used as a measure of t h e a m o u n t of p o l y m e r ization reaction that occurs. F i g . 2 s h o w s a s c h e m a t i c of t h e CCC Photocalorimeter.
R. B. M e s r o b i a n
310
V.c-3.4 I n f o r m a t i o n u s e f u l in t h e d e v e l o p m e n t o f c o a t i n g s a n d i n k s w a s obtained by this technique. For example, the effects of inhibitor c o n c e n t r a t i o n o r l e v e l of c a r b o n b l a c k p i g m e n t a t i o n w e r e a s s e s s e d w i t h r e s p e c t to c u r i n g c h a r a c t e r i s t i c s . Fig. 3 shows the primary output from the Photocalorimeter where f o u r e q u a l UV r a d i a t i o n p u l s e s r e s u l t e d in p l o t s of e x o t h e r m r a t e w h i c h , w h e n i n t e g r a t e d , p r o v i d e a m e a s u r e of t h e h e a t o f p o l y m e r ization. T h e i n f l u e n c e of " i n h i b i t o r " c o n c e n t r a t i o n o n UV p o l y m e r i z a t i o n rate was determined. A c o m b i n a t i o n o f h y d r o q u i n o n e (HQ) a n d P - m e t h o x y p h e n o l ( M E H Q ) w a s e v a l u a t e d in a UV c u r a b l e v e h i c l e at d i f f e r e n t l e v e l s . T h e r e s u l t s s h o w n in F i g . 4 i n d i c a t e t h a t w i t h equivalent exposure conditions (under nitrogen) doubling the " i n h i b i t o r " c o n c e n t r a t i o n r e d u c e d t h e d e g r e e of r e a c t i o n 18%. F u r t h e r , t h e s m a l l i n d u c t i o n p e r i o d ( p r o b a b l y d u e to r e s i d u a l d i s s o l v e d o x y g e n in t h e c o a t i n g ) a n d t h e r e d u c t i o n in s l o p e at higher concentration indicate that this combination of HQ and MEHQ is a "retarder" rather than an "inhibitor". A true inhibitor (such a s o x y g e n ) w o u l d s h o w a r e l a t i o n s h i p l i k e t h a t s h o w n in F i g . 5. T h e e f f e c t of c a r b o n b l a c k p i g m e n t a t i o n o n t h e p o l y m e r i z a t i o n rate of a UV c u r a b l e c o m p o s i t i o n w a s i n v e s t i g a t e d . Photocalorimetric d a t a w e r e o b t a i n e d f o r a n ink c o n t a i n i n g 8% c a r b o n b l a c k a p p l i e d at v a r y i n g f i l m t h i c k n e s s e s . F i g . 6 s h o w s ~hat a m o u n t o f p o l y m e r i z a t i o n r e a c h e d a c o n s t a n t l e v e l at a b o u t 2 m i c r o n s (a p r a c t i c a l i n k thickness). T h e UV r a d i a t i o n a p p a r e n t l y c o u l d n o t e f f e c t i v e l y penetrate through a thickness greater than this. In a n o t h e r s e r i e s of e x p e r i m e n t s t h e c a r b o n b l a c k c o n c e n t r a t i o n w a s v a r i e d at c o n s t a n t f i l m t h i c k n e s s (16 m i c r o n s ) . Fig. 7 shows the results. The film thickness here is equivalent to that of a c o a t i n g ( r a t h e r t h a n t h a t of an i n k ) . T h e e x o t h e r m i s r e d u c e d a s t h e p e r c e n t c a r b o n b l a c k i s i n c r e a s e d , a n d t h e r e i s no i n d i c a t i o n o f a n o p t i m u m l e v e l of b l a c k p i g m e n t . It c a n t h e r e f o r e b e c o n c l u d e d t h a t d e v e l o p m e n t of a UV c u r a b l e b l a c k c o a t i n g c a p a b l e of c o m p l e t e through-cure is very unlikely. Metal Containers
D e v e l o p m e n t s in UV c u r a b l e m e t a l d e c o r a t i n g s y s t e m s h a v e f o l l o w e d an e v o l u t i o n a r y p l a n . T h e e a r l y e x p l o r a t o r y w o r k o n m a t e r i a l s a n d p r o c e s s w a s c a r r i e d o u t in t h e l a t e 1 9 6 0 ' s . The f i r m d e c i s i o n to w o r k t o w a r d c o m m e r c i a l development took place e a r l y in 1970. A p i l o t l i n e ( F i g u r e 8) f o r c o a t i n g , p r i n t i n g , a n d
U. V. applications for packaging
311
V.c-3.5 varnishing with U V curable material w a s completed in June 1972. The original commercial UV " S e t P a s s " m e t a l p r i n t i n g l i n e h a s t h e c a p a c i t y t o u s e f o u r t e e n UV l a m p s (12 a n d 2 s p a r e s ) , 200 w a t t s / i n . m e d i u m p r e s s u r e (15 p s i g ) . T h e l a m p s h a v e q u a r t z e n v e l o p e s and a r e m e r c u r y a n d A r g o n f i l l e d . T h e y o p e r a t e at a c o n s t a n t c u r r e n t of 6.5 amperes. W o r k d u r i n g 1973 d e m o n s t r a t e d t h a t , at a l i n e s p e e d of 85 s h e e t s (300 ft. ) p e r m i n u t e , a l l c u r r e n t UV i n k s w o u l d cure with four lamps or less. Therefore, a m u c h s m a l l e r UV dryer was specified for all flat sheet printing installations after 1973. A l l UV s h e e t d r y e r s a r e e q u i p p e d w i t h e l l i p t i c a l r e f l e c t o r s f o r t h e l a m p s , p o s i t i o n e d s o t h a t t h e p l a t e s u r f a c e i s a t t h e f o c a l p o i n t . In l a t e r w o r k w i t h p r e f o r m e d c a n s , u s i n g f a s t e r c u r i n g c o a t i n g s , it w a s r e c o g n i z e d t h a t t h e s u r f a c e of t h e c a n d o e s n o t h a v e t o b e at o r n e a r t h e f o c a l p o i n t o f t h e r e f l e c t o r to e f f e c t a d e q u a t e c u r i n g . Figure 9 shows schematically a n u m b e r of M e t a l D e c o r a t i n g p r o c e s s e s w h i c h h a v e b e e n o r w i l l in t h e n e a r f u t u r e b e put i n t o commercial production. S y s t e m I UV " S e t P a s s " w a s p u t i n t o c o m m e r c i a l p r o d u c t i o n in J u l y 1972. T h e r e a r e p r e s e n t l y i I l i n e s o p e r a t i n g in 9 C o n t i n e n t a l Can Company plants. This system has been used primarily for 3-piece beer and soft drink cans. S y s t e m I I UV " S e t S t a c k " w a s put i n t o c o m m e r c i a l p r o d u c t i o n in J u n e 1974. T h e r e i s p r e s e n t l y o n e C o n t i n e n t a l C a n C o m p a n y l i n e o p e r a t i n g a n d a n o t h e r in t h e l a t t e r s t a g e s of c o n s t r u c t i o n . This system has been used primarily for aerosol cans. In d e v e l o p i n g s u i t a b l e UV c u r a b l e c o a t i n g s , i n k s a n d v a r n i s h e s f o r t h e a b o v e s y s t e m s , t h e n a t u r e of t h e v a r i o u s c o m m e r c i a l metal s u b s t r a t e s h a d t o b e c o n s i d e r e d ( F i g . 10). A l l of t h e s e c o n d i t i o n s w o u l d b e m o d i f i e d b e f o r e UV d e c o r a t i v e materials were applied to the other side if conventional coatings w e r e b a k e d on o n e s i d e of t h e p l a t e . T h e m e t h o d o f a p p l y i n g UV c u r a b l e m a t e r i a l s of t h e f i n i s h e d d e c o r a t i o n ( F i g . 11).
affects the quality
UV i n k s t e n d t o b e m o r e a d v e r s e l y a f f e c t e d t h a n t r a d i t i o n a l o l e o r e s i n o u s i n k s w h e n i n c o n t a c t w i t h f o u n t a i n s o l u t i o n s u s e d in t h e
312
R.
B. Mesrobian
V.c-3.6 lithographic process. Absorption of water tends to lower the v i s c o s i t y a n d c o n s i s t e n c y of t h e i n k a n d m a k e s t h e m p r o n e to "scumming" a t e r m u s e d t o d e s c r i b e t h e t e n d e n c y of t h e i n k t o t r a n s f e r c o l o r to t h e n o n - p r i n t e d a r e a s . Careful control of the c o m p o s i t i o n o f t h e f o u n t a i n s o l u t i o n i s n e c e s s a r y to o v e r c o m e t h i s p r o b l e m in p r o d u c t i o n . T h e c o m p o s i t i o n a n d h a r d n e s s of t h e c o a t i n g r o l l o r o f f s e t a l s o a f f e c t t h e a p p e a r a n c e of t h e c o a t i n g .
blanket
UV W h i t e C o a t i n g While development and commercialization of UV i n k s h a s b e e n r e l a t i v e l y r a p i d , p r o g r e s s w i t h UV w h i t e c o a t i n g s f o r c a n s h a s b e e n s l o w . W h e n w o r k w a s i n i t i a t e d , it w a s r e c o g n i z e d t h a t t h e r e w o u l d be greater problems with white coatings than with inks. Early efforts confirmed this prognosis and indicated three primary problem areas during the subsequent plant trials. 1) R h e o l o g y ( a p p l i c a t i o n c h a r a c t e r i s t i c s ) - ability to lay down a n d a c h i e v e a s m o o t h f i l m e q u i v a l e n t in a p p e a r a n c e t o t h e solvent base conventional white coatings using commercial c o a t i n g e q u i p m e n t at n o r m a l l i n e s p e e d s . 2) A d h e s i o n
f o l l o w i n g UV e x p o s u r e
3) D i s c o l o r a t i o n
- after the thermal
- picking at the printing press. bake.
Rheology, as indicated by appearance, is the major problem area. It h a s b e e n p u r s u e d in t w o w a y s : (a) c h e m i c a l c h a n g e s in t h e c o a t i n g formulation, a n d (b) m e c h a n i c a l c h a n g e s on t h e c o a t i n g m a c h i n e . Optimization of the coating material composition is essentially complete. Current mechanical modifications in the coating application process provide significant p r o m i s e for a c o m m e r c i a l l y acceptable U V white coating on cans. PAPER
& PAPERBOARD
Folding Carton Folding carton printers m o s t often use sheet fed lithographic printing equipment to produce the decorated cartons. Their c u s t o m e r s use these cartons for packaging cosmetics, frozen food, drugs, cereals, and a m y r i a d of other c o m m e r c i a l products. In the process of completing the cartons it is necessary to die
U. V. applications for packaging V.c-3.7 cut, fold, crease and glue the decorated blanks. With conventional oxidizing inks a waiting period after printing is n e c e s s a r y to allow the ink to dry to a state which yields sufficient hardness and rub resistance for satisfactory p e r f o r m a n c e in the cutting and forming operations. In addition, it is often necessary to dust the freshly printed sheets with starch or w a x to prevent ink transfer f r o m sheet to sheet. B o t h t h e d e l a y in t h r o u g h p u t a n d t h e u n d e s i r a b l e d u s t i n g a r e l o n g s t a n d i n g d e f i c i e n c i e s in t h e f o l d i n g c a r t o n d e c o r a t i n g p r o c e s s . The u s e of u l t r a v i o l e t c u r a b l e i n k s o b v i a t e s b o t h o f t h e s e p r o b l e m s . T h e r e f o r e an i n c r e a s i n g n u m b e r of f o l d i n g c a r t o n p r i n t e r s a r e either using or contemplating the use of UV. T h e UV c u r i n g e q u i p m e n t u s e d in f o l d i n g c a r t o n p r i n t i n g i s s i m i l a r to t h a t u s e d f o r c u r i n g on m e t a l . The differences are dictated by the press configuration and space limitations rather than by any b a s i c d i f f e r e n c e in c u r e r e q u i r e m e n t s . E a r l y developments c o n c e n t r a t e d on l o c a t i n g t h e UV e q u i p m e n t in t h e d e l i v e r y p o r t i o n of t h e p r e s s o n l y , s i n c e m a n y l a m p s w e r e r e quired to achieve the desired degree of cure. Today one !amp b e t w e e n c o l o r s i s c o m m o n , w i t h n o t m o r e t h a n 4 l a m p s u s e d in the delivery section for final cure. A n o t h e r i m p o r t a n t c o n s i d e r a t i o n in c a r t o n d e c o r a t i o n w h e n u s i n g UV c u r a b l e m a t e r i a l s i s t h e a p p l i c a t i o n o f o v e r p r i n t v a r n i s h . On a m u l t i c o l o r p r e s s e q u i p p e d w i t h UV d r y i n g c a p a b i l i t y , it i s c o m m o n p r a c t i c e t o d r y t h e i n k s in t h e n e x t t o l a s t c o l o r s t a t i o n a n d a p p l y o v e r p r i n t v a r n i s h in t h e l a s t u n i t . I n t h i s w a y c o m p l e t e l y f i n i s h e d sheets having adequate gloss and excellent rub and chemical resistance are delivered from the press. Since no starch spray is r e q u i r e d , t h e s u r f a c e h a s a s m o o t h f e e l w h i c h h a s m o r e s a l e s a p p e a l . In m a n y c a s e s t h e UV c u r a b l e p r e s s - a p p l i e d varnish can r e p l a c e a c a t a l y z e d o v e r p r i n t c o a t i n g w h i c h r e q u i r e s b a k i n g in c i r c u l a t e d h o t a i r . T h e u s e o f UV v a r n i s h e l i m i n a t e s a i r p o l l u t i o n f r o m t h e s o l v e n t s in t h e c o a t i n g , a n d d o e s n o t r e q u i r e s c a r c e f u e l such as natural gas. In the printing of Christmas liquor cartons or cosmetic packages, as many as eight colors can be used and quite often the substrate is aluminum foil laminated to paperboard. In the lithographic printing of foil laminates, where n o r m a l inks dry poorly due to l a c k o f a b s o r p t i v i t y in t h e s u b s t r a t e , t h e u s e o f u l t r a v i o l e t c u r i n g is p a r t i c u l a r l y a d v a n t a g e o u s .
313
314
R.B. Mesrobian
V.c-3.8 S o m e printers find that the use of U V is desirable because of the higher productivity possible, particularly on m o d e r n high speed sheet fed presses. In C a n a d a a major printer has converted to U V exclusively for printing of cigarette packages. T h e sheets are printed with U V inks with in-line varnishing and embossing, a very efficient process. T h e penetration of ultraviolet ink drying into the folding carton business has been slow. T h e r e are, however, between 30 and 40 n e w presses and a like n u m b e r of retrofitted presses capable of running U V inks. Paper
Labels
Small narrow web presses are compact in nature and the simplicity and low space requirement of UV dryers are an excellent combination. The press can feed a web into a series of print stations using either lithographic or letterpress printing, and when coupled with a cutting device, can deliver a dry decorated label or box blank at acceptable speeds (in the order of 200 ft. /min.). Many presses of this type have been fitted with lamps between printing units and can use dry trapping if desired.
T h e binder which results f r o m the ultraviolet curing of unsaturated m o n o m e r s confers chemical resistance to the printed label. C h e m i c a l resistance is one of the prime requirements for labels, particularly for cosmetics, detergents, or other aggressive package contents. T h e need for overlacquer is obviated by the use of U V inks. Corrugated
Display
Board
In the decoration of corrugated board for advertising displays, the application of ink is by letterpress and a thick film of ink must be applied to make certain that all valleys receive ink. The result is that sheets printed with conventional inks are very susceptible to ink transfer in the pile. The use of UV drying solves this problem. However, there are technical difficulties in drying the thick ink films, especially black, because of the low efficiency in utilization of the UV energy. Plastic
Containers
Decorated polystyrene tubs have become a staple of the rapidly expanding quick-food business. Polystyrene, and especially foamed polystyrene, is a very heat sensitive substrate. Decoration of this surface is predominately accomplished on dry offset printing
U. V. a p p l i c a t i o n s
for packaging
V.c-3.9 equipment. In t h e o p e r a t i o n o f t h i s p r i n t i n g p r o c e s s t h e i n k i s e x p o s e d in t h i n f i l m s to t h e a t m o s p h e r e f o r l o n g p e r i o d s of t i m e on t h e p r e s s b e f o r e it i s t r a n s f e r r e d to the substrate. To keep the ink from drying prematurely, it i s n e c e s s a r y in t h e c a s e of s o l v e n t b e a r i n g i n k s , to u s e v e r y h i g h b o i l i n g s o I v e n t s . After the print is made, the conflicting situation is created where high boiling solvents must be removed from a heat sensitive substrate. Conventional oven design, therefore, c o n s i s t s of a l o n g d w e l l t i m e at r e l a t i v e l y l o w a i r t e m p e r a t u r e s . It i s n o t s u r p r i s i n g , therefore, t h a t u l t r a v i o l e t c u r i n g of i n k s h a s b e e n w i d e l y a d o p t e d f o r t h e p r i n t i n g of p l a s t i c c u p s a n d t u b s . The i n k s do n o t d r y o n t h e ink r o l l e r s a n d , a f t e r p r i n t i n g , r a p i d d r y i n g of the inks can be effected without distorting the plastic. C o m p o s i t e p l a s t i c o i l c a n s a r e b e i n g p r i n t e d w i t h UV c u r i n g b y several companies for similar reasons. In t h e s e c o n t a i n e r s t h e s t r u c t u r a l m a t e r i a l i s f r e q u e n t l y a d u a l - p l y s y s t e m of p o l y p r o p y l e n e and polyethylene. A d h e s i o n of UV i n k s to p o l y s t y r e n e h a s b e e n d e f i c i e n t in s o m e c a s e s c h i e f l y d u e to m o l d r e l e a s e a n d a n t i s t a t i c a g e n t s i n c o r p o r a t e d in t h e polystyrene compositions. T h e a g e n t s t e n d t o s p e w to t h e p l a s t i c surface and present a very low energy surface to which the ink has poor adhesion. On the other hand, adhesion to polyolefins has been uniformly good if treated with corona discharge immediately before printing. T h e f u t u r e of u l t r a v i o l e t c u r i n g s e e m s t o b e s e c u r e in plastic decoration. O n e m a j o r p r o d u c e r of p o l y e t h y l e n e s q u e e z e t u b e s , b y c h a n g i n g t o UV, i s r e c o v e r i n g a n e n t i r e f l o o r in a n installation where long ovens were required with conventional inks. F DA The FDA has not been involved to any appreciable extent up until n o w b e c a u s e m o s t a p p l i c a t i o n s f o r UV c u r i n g a r e o r i e n t e d t o u s e s that include no direct food contact with printing inks. Direct f o o d - c o n t a c t u s a g e o f UV c u r e d c o a t i n g s p r o b a b l y w i l l r e q u i r e e v a l u a t i o n of c h e m i c a l s a f e t y a n d w i l l b e s u b j e c t to t h e d e l a y s of the regulatory process. These coatings generally incorporate one or more components, such as photoinitiators, t h e u s a g e of w h i c h w a s n o t c o n t e m p l a t e d in c o n v e n t i o n a l h e a t - c u r e d c o a t i n g s . Consequently, such materials, if n o t c o v e r e d b y p r e v i o u s F D A regulations, will require a petition for acceptance. This proc e d u r e i n v o l v e s a c o n s i d e r a b l e a m o u n t of e x t r a c t a b i l i t y s t u d y a n d a v a r i a b l e a m o u n t of t o x i c o l o g y , d e p e n d i n g o n t h e s i m i l a r i t y of t h e n e w m a t e r i a l t o o l d e r w e l l - k n o w n m a t e r i a l s .
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R. B. Mesrobian
316
V.c-3.10 OSHA W o r k e r safety does not present any unusual or insurmountable problems. Suitable shielding can eliminate radiation exposure and good engineering design can provide adequate ventilation to r e m o v e ozone and chemical vapors f r o m w o r k areas. M o s t of the c o m m o n l y used U V curable m o n o m e r s have no m o r e than slight to m o d e r a t e oral toxicity. Table I shows eye and skin irritations ratings for s o m e typical m o n o m e r s and coatings. In s o m e instances, protective clothing and rubber gloves m a y be necessary, but this is no different f r o m the handling of m a n y other chemicals, including s o m e solvents. L a b o r in the can industry has endorsed the adoption of U V lines since they eliminate s o m e of the noise and m o s t of the intense heat of gas fired ovens. T h e r e have been no O S H A objections to U V drying, and the process does not involve any n e w skills or c o m p l e x operating technique s.
3/18/76
(1) (2) (3) (4)
Data f r o m a paper submitted for publication in J. Appl. P olym. Sci. T a k y a m a , E., Japan Plastics, 5 (4), 6-19 (1971) Nishkobo, etal., J. Appl. Poly. Sci., 18, 3445 (1974) Burnett, G. M. and Melville, H. W . , Nature, gond. 156 (1945) 661; Proc. R o y Soc. A. 189 (1947) 456, 481 and 494
U. V. applications for packaging
317
IRRADIATION OF B--SUBSTITUTED ETHYL ACRYLATES UNDER NITROGEN FLUSH % Conversion vs. Time
% conversion to polymer
(figure 1 )
OH]
14
~CIHs
12
, Br
10
8
OCH=
iq~
6
4
CN
2 20
40
60 80 Time,Seconds
AlzacReflector MercuryLamp
100
1
120
=-
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f.~.~.~
' ~ os~ c.,,
PHOTOCALORIMETER
(figure2)
318
R.B.
Exotherm Rate, dH at
Mesrobian
TYPICAL PHOTOCALORIMETRIC DATA (figure 3)
Heat of Reaction,
/
at
OH dt
at
.... f
I
l
\ u
tgL~ m m i ~
I ! mlm,=l
...JI
~tmm
J
I
Time
Exotherm, calories/gram
40
I
30
,
e.I
(figure 4)
•/
e/y
10
INFLUENCE OF INHIBITOR ON EXOTHERM
?/ /m• / / S•
• Standard Vehicle • Standard Vehicle plus 1200 ppm mehq, 200 ppm hq
ul S I / ./
0
II
,r
l
.10
I
.20
~ Reiative ~ UV Exposure "
I
.30 I t Im~
.40
• Standard Vehicle plus 2400 ppm mehq, 400 ppm hq
U. V. applications for packaging
319
PHOTOPOLYMERIZATION O F V I N Y L A C E T A T E A T 250C (figure 5)
Conversion %
4
0
1.86
3 . 8 6 5.80
Millimols/liter of p-Benzoquinone,
2
I
80
160 240 Time, Minutes
H E A T O F R E A C T I O N vs. F I L M T H I C K N E S S AT CONSTANT IRRADIATION
A H, calories x 10 -4
(figure 6)
30-
• 0 ~O
•
•
•
o ,o
•
8% Carbon Black in Photo Sensitive Vehicle
lO/ I
2
i 4
i 6
i 8
Film Thickness, Microns
I 10
I 12
I 14
320
R.B. Calories/ tl Gram
Mesrobian
HEAT OF REACTION VS.
5O
40
30
\
C A R B O N BLACK C O N C E N T R A T I O N IN PHOTO SENSITIVE VEHICLE AT 16 M I C R O N S T H I C K N E S S AND CONSTANT IRRADIATION (figure 7)
20
10
I
I
i
I
6
8
10
12
% Carbon Black
U V SECTION
SECOND PRINTING PRESS
UV SECTION
INFRARED DELIVERY SECTION CONVEYORS
TRAILING VARNISH UNIT
(figure 8)
DELIVERY SECTIONS
PRINT 2-UV INKS
I
I
I
PRINT 2-CONV. INKS
CONV. TRAILING
BASE COAT UV
UV SET
STACK
VARNISH LINE
2 COLOR AND
PRINT 2 CONV. INKS
CONV. TRAILING VARNISH
r~OATE R
D UV SET
PRINTER
am (Iz PIN O V E N
J
200
~
~,/ICKET OVEN P O L L U T I O N CONTROL DEVICE
BASE COAT
ROLLER PRINT SET PRINT UV STACK CONVEYOR 2 - UV 2 - UV T R A I L I N G INKS INKS VAR~JISH
TI"'--"
UV
STACK
+ C O N V E N T I O N A L I/S ENAMEL LINE
S Y S T E M 3 - UV D I R E C T P R I N T - 4 UV I N K S A N D V A R N I S H "SET STACK" LINE
S Y S T E M 4 - UV W H I T E
UV SET
FEET
I
150
/VICKET OVEN P O L L U T I O N C O N T R O L DEVICE
S Y S T E M 5 - UV W H I T E B A S E C O A T FOR T W O - P I E C E C A N S
"~OLLER 3ONVEYOR
::rOLLER PRINT .~ONVEYOR 2 - UV INKS
I
100
SYSTEM 1 - UV'~;ET PASS"LINE - 2 UV INKS
75
50
25
S Y S T E M 2 - UV SET STACK LINE
ROLLER CONVEYOR
)
:IG. 9
~TACK
J
260
--= L,3
I ,,,J
c,
t~
I"h 0
P}
I~0 O
,..4
322
R.B.
Mesrobian
Figure I0
COMMERCIAL A.
-
SURFACE CONDITION
Cathodic dichromate on tinplate Chromium - chromium oxide on steel Phosphate on aluminum
Lubricated i. 2.
C.
SUBSTRATES
Chemically treated i. 2. 3.
B.
METAL
Dioctyl sebacate (DOS) or acetyl tributyl citrate (ATBC) on tinplate Residual oil or butyl stearate on steel
Detergent washed
U. V. applications for packaging
Figure ii
METHOD OF APPLYING ULTRAVIOLET CURABLE MATERIALS q~i_n_%
[[i_5~i_n_% A.
Dry offset
A.
Direct roll
B.
Wet offset (lithography)
B.
Offset gravure
C.
Direct gravure
D.
Offset gravure
323
Severe Slight
Moderate
Extreme
Pentaerythritol triacrylate, PETA
H e x a n e d i o ] d i a c r y l a t e , HDDA
Trimethylol propane, triacrylate TMPTA
Neopentyl glycol d i a c r y l a t e , NPGDA
SKIN
Moderate
Moderate
Severe
Moderate
Moderate
Extreme
Mild
PRIMARY IRRITATION
** M o n o m e r components: H D D A , N P G D A
Extreme
Tetraethylene glycol, diacrylate TEGDA
I","M o n o m e r c o m p o n e n t s : P E T A and T E G D A
Severe
Coating B'**
IRRITATION
Extreme
EYE
RATINGS MATERIALS
Coating A*
MATERIAL
TOXICOLOGICAL U. V. CURABLE
TABLE I
and T M P T A ~
oI
0
f~
b~