Recent Advances in Metal Can Interior Coatings - ACS Publications

Chapter 17

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 16, 2016 | http://pubs.acs.org Publication Date: March 9, 1988 | doi: 10.1021/bk-1988-0365.ch017

Recent Advances in Metal Can Interior Coatings Raymond H . Good Holden Surface Coatings Limited, Bordesley, Green Road, Birmingham, B9 4TQ, England

Interior Can Coatings have been undergoing a period of considerable change over the last twenty years. Environmental Legislation, new can making technologies and the need to offer increased protection to less corrosion resistant substrates haveallinfluenced coating selection. Tough new air pollution regulations have forced the can producers to adopt compliant water based and high solids coating formulations. The move to eliminate lead from cans has resulted in fundamental changes in can fabrications. The advent of Draw-Redraw (DRD), Drawn and Wall Ironed (DWI) and Welded Technologies all require more highly sophisticated coatings. The coating supplier has responded rapidly to these requirements and continues to work on developing new organic film formers that will withstand the demands of future technologies. I n t e r i o r Can C o a t i n g Background Although metal food cans have been w i t h us f o r 150 years o r more, t h e canning r e v o l u t i o n o n l y r e a l l y s t a r t e d 70 - 80 y e a r s ago. I n t h e 19th century, cans were made by hand and a heavy c o a t i n g o f t i n p r o t e c t e d t h e f o o d s t u f f s from t h e i r o n s u b s t r a t e . W i t h the change i n l i f e s t y l e s f o l l o w i n g t h e V i c t o r i a n e r a , and t h e g r e a t e r need f o r convenience f o o d s , t h e " t i n c a n " became the necessary package f o r f r u i t , meats, f i s h and v e g e t a b l e s (J_). Many o f these foods b e i n g grown s p e c i f i c a l l y t o go i n t o cans. The improvements i n can making and t h e more widespread use o f cans p l a c e d an ever i n c r e a s i n g i n t e r e s t i n r e d u c i n g t h e package c o s t . One o f t h e a r e a s f o r c o s t s a v i n g s was t o reduce t h e t i n c o a t i n g weight and r e p l a c e i t w i t h o r g a n i c f i l m formers t h a t provide e x c e l l e n t c o r r o s i o n p r o t e c t i o n f o r t h e m e t a l l i c c o n t a i n e r and a b a r r i e r between t h e f o o d s t u f f and the m e t a l . Lacquering o r coating of t h e i n t e r i o r o f the can was f i r s t used t o p r e s e r v e t h e c o l o u r o f red f r u i t s and v e g e t a b l e products when packed i n uncoated t i n p l a t e

0097-6156/88/0365-0203$06.00/0 © 1988 American Chemical Society

Hotchkiss; Food and Packaging Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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FOOD AND PACKAGING INTERACTIONS

cans. [2). The l a c q u e r i n g of cans was necessary t o prevent the b l a c k e n i n g of the can and i t s c o n t e n t s which o c c u r s w i t h c e r t a i n s u l p h u r p r o d u c t s c o n t a i n e d i n meat, f i s h and v e g e t a b l e p r o d u c t s . I t was Bohart (3_) who was a c t i v e i n developing sulphur r e s i s t a n t l a c q u e r s i n the 1920's. These l a c q u e r s c o n t a i n e d a suspension of z i n c o x i d e which r e a c t s w i t h s u l p h u r compounds formed d u r i n g p r o c e s s i n g to form w h i t e z i n c s u l p h i d e r a t h e r than the normal b l a c k t i n o r i r o n s u l p h i d e , (e.g. "corn b l a c k " ) . Most t i n p l a t e and a l l t i n f r e e s t e e l c o n t a i n e r s have t o be protected. The e x c e p t i o n s are t h r e e p i e c e t i n p l a t e cans w i t h heavy t i n weights. These are used p a r t i c u l a r l y where the t i n a i d s the f l a v o u r and appearance o f the p r o d u c t , p a r t i c u l a r l y l i g h t c o l o u r e d f r u i t s , f o r example p i n e a p p l e , g r a p e f r u i t and peaches (_4 ). P r o t e c t i o n of a food c o n t a i n e r has always r e q u i r e d the c a r e f u l s e l e c t i o n of l a c q u e r type depending on a number o f f a c t o r s : Lacquers must have s u i t a b l e 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 e i t h e r by roll coat, spray, c o i l , e t c . P h y s i c a l l y and c h e m i c a l l y , l a c q u e r s must have e x c e l l e n t adhesion t o the s u b s t r a t e , f l e x i b i l i t y d u r i n g forming and d u r a b i l i t y which p r o v i d e s a f i l m t h a t i s o d o u r l e s s and non toxic. Over the y e a r s , r e t e n t i o n of the f l a v o u r and c o l o u r of the product has become o f paramount importance. T h i s means the l a c q u e r must be s t a b l e over a wide temperature range b e i n g a b l e to r e s i s t the heat from the s i d e seam process and a l s o to the food steam processing conditions. More r e c e n t l y , c o a t i n g manufacturers have been r e s t r i c t e d i n the lacquer c h o i c e by components or a d d i t i v e s r e c o g n i s e d by food l e g i s l a t i o n , e . g . American Food and Drug A d m i n i s t r a t i o n . Chemistry

of I n t e r i o r Can C o a t i n g s

pre-1950

Oleoresinous Coatings. The o r i g i n a l can l a c q u e r s were based on oleoresinous products. T h i s group has the l o n g e s t h i s t o r y i n the i n d u s t r y and c o v e r s a l l those c o a t i n g m a t e r i a l s which are made by f u s i n g n a t u r a l gums and r o s i n s and b l e n d i n g them w i t h d r y i n g o i l s , such as l i n s e e d o r tung o i l . (_5) T h i s b a s i c combination e x p l a i n s the term " o l e o r e s i n o u s o r o i l based" l a c q u e r s . The mechanism o f d r y i n g i n v o l v e s o x i d a t i o n which i s a c c e l e r a t e d u s i n g d r i e r s and temperatures up t o 415°F f o r c u r e . Tung o i l , o f which a l p h a - e l e o s t e a r i c a c i d i s a major c o n s t i t u e n t , has been used e x t e n s i v e l y i n o l e o r e s i n o u s systems. Its c o n j u g a t e d s t r u c t u r e a s s i s t i n g i n the o x i d a t i v e cure ( F i g u r e 1 ) . Trans Trans C,H - CH = CH - CH = CH 4 9

Cis

n

CH = CH CH - CH - CH = CH - ÇH 4 9 J Τ 0 0

οο·

2

?

2

CH = CH

J,

C.H - CH - CH = CH - CH 4 9 j n

CH

(CH ) C0 H

CH = CH

CH

F i g u r e 1 - A l p h a - E l e o s t e a r i c A c i d (Tung O i l ) O x i d a t i v e Cure


17.

205

Advances in Metal Can Interior Coatings

GOOD


The reason o l e o r e s i n o u s systems a r e s t i l l used today i s t h e i r low c o s t and good a c i d r e s i s t a n c e (see Table 1 ) . However, because o f the open m i c e l l a r s t r u c t u r e o f o l e o r e s i n o u s m a t e r i a l s , they a r e prone t o c o r r o s i o n / s t a i n i n g problems w i t h s u l p h u r bearing products u n l e s s they a r e pigmented w i t h z i n c o x i d e p a s t e . I t was f o r t h i s reason t h a t a d e p a r t u r e was made from these n a t u r a l l y o c c u r i n g raw m a t e r i a l s to s y n t h e t i c p h e n o l i c r e s i n s d i s s o l v e d i n a blend o f s o l v e n t s . Phenolic Coatings P h e n o l i c r e s i n s a r e produced by the a c t i o n o f formaldehyde on phenol o r o t h e r s u b s t i t u t e d phenols i n the presence o f a b a s i c c a t a l y s t . These p h e n o l i c a l c o h o l s r e a c t immediately w i t h themselves, f o r m i n g e t h e r b r i d g e s o r methylene b r i d g e s t o produce extremely complex m o l e c u l e s which a r e t h r e e d i m e n s i o n a l and t h e r e f o r e provide the h i g h c h e m i c a l r e s i s t a n c e ( F i g u r e 2 ) . E t h e r i f i c a t i o n o f the p h e n o l i c s u s i n g a mono a l c o h o l improves s o l u b i l i t y i n aromatic solvents. OH

OH

OH

OH

R = H, A l k y l Figure 2

P o s s i b l e Resole

Phenolic Structure

One o f the l i m i t i n g f a c t o r s o f p h e n o l i c c o a t i n g s i s t h e i r l i m i t e d flexibility, h i g h bake r e q u i r e m e n t s and f i l m weight latitude. However, they a r e s t i l l used on t h r e e p i e c e bodies where f l e x i b i l i t y i s not r e q u i r e d (see Table 1 ) . V i n y l Coatings V i n y l c o a t i n g m a t e r i a l s most r e l e v a n t t o can c o a t i n g s a r e based on copolymers o f v i n y l c h l o r i d e and v i n y l a c e t a t e o f low m o l e c u l a r weight d i s s o l v e d i n s t r o n g k e t o n i c s o l v e n t s . M i n o r m o d i f i c a t i o n o f the v i n y l c h a i n w i t h m a l e i c a c i d enhances t h e adhesion o f the c o a t i n g t o s t e e l and aluminium s u b s t r a t e s . The l o n g carbon-carbon c h a i n s i n v i n y l r e s i n s make them t h e r m o p l a s t i c i n nature. V i n y l r e s i n s can be blended w i t h a l k y d , amino and p h e n o l i c r e s i n s t o enhance t h e i r performance (_5 ). See F i g u r e 3 f o r a t y p i c a l v i n y l co-polymer s t r u c t u r e .

-(CH

- CH ) 0

- (CH - CH ) 0

I

I

CI

0C0CH

- (CH - CH) -

I 3

I

C0 H C0 H 2

2

F i g u r e 3 - T y p i c a l v i n y l co-polymer



206


The f l e x i b i l i t y o f these m a t e r i a l s a l l o w s them t o be used f o r caps and c l o s u r e s and drawn cans. The backbone o f a v i n y l r e s i n a l s o makes them s t r o n g and immune from a t t a c k by c h e m i c a l r e a g e n t s , such as a c i d s o r a l k a l i s . The development o f v i n y l p r o d u c t s has made i t p o s s i b l e t o develop s a t i s f a c t o r y l i n i n g s f o r cans t o c o n t a i n beer and s o f t d r i n k s whose f l a v o u r would be a f f e c t e d by the l e s s s u i t a b l e types of organic f i l m formers. Main d i s a d v a n t a g e s o f v i n y l c o a t i n g s are their sensitivity t o heat and a l s o t o steam p r o c e s s i n g o f food products. V i n y l c o a t i n g s were t h e r e f o r e r e s t r i c t e d t o hot f i l l and beer and beverage p r o d u c t s (Table 1 ) . Table I

F i l m Wt g/m 2

4-6 Dleores Dleores 7-9 f ZnO Phenolic 2 - 3 Vinyl

KEY; 1950

6-8

- E a r l y I n t e r i o r Three P i e c e Can

Int. Int. B o d i e s Ends

• • • •

Meat Fish

Vegetables Soups

0

_

_

0

•

0

t

• •

•

-

-

Coatings

Acid Fruits

Beer Beverages

t

-

_

_

•

-

-

t As Topcoat

· = V. S u i t a b l e , 0 = B o r d e r l i n e , - = Not s u i t a b l e . - 1986 Can C o a t i n g Developments

Three major f a c t o r s i n f l u e n c i n g t h e d i r e c t i o n o f t h e major can makers in the last 30 y e a r s have been Economics, Environment and Engineering. The d r i v e towards g r e a t e r economics f o r c e d t h e can makers i n t o reducing raw m a t e r i a l c o s t s . The use o f t h i n n e r gauge s t e e l w i t h r e d u c t i o n i n t i n c o a t i n g weight have p l a c e d more emphasis on more s o p h i s t i c a t e d o r g a n i c f i l m formers. The e n v i r o n m e n t a l movement i n the U.S.A. i n the '60's r e s u l t e d i n tough new a i r p o l l u t i o n r e g u l a t i o n s b e i n g demanded and l e g i s l a t i o n passed by the government. T h i s l e d t o can makers having t o reduce t h e i r s o l v e n t e m i s s i o n s from t h e i r c o a t i n g l i n e s . Can makers had two choices, either i n s t a l l i n c i n e r a t o r s o r use c o a t i n g s t h a t c o m p l i e d w i t h t h e new laws (_6 ). The Food and Drug A d m i n i s t r a t i o n had the r e s p o n s i b i l i t y t o r e g u l a t e the components t h a t can be used i n c o n t a c t with foodstuffs. The engineers o f t h e major can companies worked f e v e r i s h l y i n promoting new t e c h n o l o g i e s t h a t o f f e r e d both improved l i n e speeds and containers t h a t o f f e r e d q u a l i t y improvements t o the v e g e t a b l e , f r u i t and beverage packers. These t e c h n o l o g i e s were based on e l e c t r i c a l r e s i s t a n c e w e l d i n g o f t i n p l a t e c y l i n d e r s f o r 3 p i e c e food cans, Draw Redraw (DRD) f o r T i n Free S t e e l (TFS) food cans and the Drawn and Wall Ironed (DWI) p r o c e s s f o r aluminium o r t i n p l a t e beverage containers.


17. GOOD


207

I t was i n e v i t a b l e t h a t f o r the new can making t e c h n o l o g i e s t o succeed, o t h e r o r g a n i c f i l m formers had t o be developed. The most i m p o r t a n t developments were t h e s o l v e n t based epoxy p h e n o l i c , t h e PVC organosol dispersion and t h e water borne epoxy-g-acrylic t e c h n o l o g i e s . Each b a s i c technology has i t s s t r e n g t h s and weaknesses but when m o d i f i e d can produce o r g a n i c f i l m s h a v i n g superior properties and a b l e t o meet t h e c h a l l e n g e s o f t h e new can technologies.


Welded Food Cans The t r a d i t i o n a l method o f m a n u f a c t u r i n g 3 p i e c e cans f o r food and beverage products has been t h e l e a d / t i n s o l d e r p r o c e s s . In recent y e a r s , l e g i s l a t i o n has f o r c e d can makers t o adopt a l t e r n a t i v e methods o f s i d e seaming t o a v o i d p o s s i b l e l e a d c o n t a m i n a t i o n o f food products. The new p r e f e r r e d 3 p i e c e technology being that o f e l e c t r i c a l r e s i s t a n c e welding of t i n p l a t e bodies. The i n t e r i o r body l a c q u e r s used on welded cans a r e a n a t u r a l e x t e n s i o n o f food l a c q u e r s used on t h e s t r a i g h t w a l l e d 3 p i e c e s o l d e r e d cans. However, h i g h l y r e f i n e d epoxy p h e n o l i c technology has been developed t o s a t i s f y the new demanding areas o f the welded c a n . The use o f l e s s c o r r o s i o n r e s i s t a n t t i n p l a t e and t h i n n e r gauge m a t e r i a l w i t h post beading o f the can w a l l t o i n c r e a s e s t r e n g t h has v i r t u a l l y e l i m i n a t e d t h e use o f the b a s i c p h e n o l i c c o a t i n g used p r e v i o u s l y on the s o l d e r e d c a n . Epoxy P h e n o l i c

Coatings

Epoxy p h e n o l i c c o a t i n g s a r e made e i t h e r by s t r a i g h t b l e n d i n g o f a s o l i d epoxy r e s i n w i t h a p h e n o l i c r e s i n o r a r e t h e p r o d u c t s o f t h e precondensation o f a m i x t u r e o f two r e s i n s i n a p p r o p r i a t e s o l v e n t s . During t h e c u r i n g and b a k i n g o f t h e coated f i l m , d i v e r s e c h e m i c a l r e a c t i o n s take p l a c e which form a t h r e e d i m e n s i o n a l s t r u c t u r e , combining t h e good adhesion p r o p e r t i e s o f t h e epoxy r e s i n and t h e high chemical r e s i s t a n c e p r o p e r t i e s o f t h e p h e n o l i c r e s i n . The chemical s t r u c t u r e o f t h e epoxy p h e n o l i c r e s i n can be r e p r e s e n t e d as shown i n F i g u r e 4. P r e c o n d e n s a t i o n o f t h e epoxy and p h e n o l i c r e s i n s l e a d s t o b e t t e r a p p l i c a t i o n p r o p e r t i e s o f the c o a t i n g (J).

Figure A

-

Epoxy P h e n o l i c

Chemistry

The balanced p r o p e r t i e s o f epoxy p h e n o l i c c o a t i n g s have made them almost u n i v e r s a l i n t h e i r a p p l i c a t i o n on food cans w i t h the e x c e p t i o n



208


of deep m u l t i s t a g e DRD cans. Other f i l m f o r m e r s , based on p o l y e s t e r t e c h n o l o g y , have made some i n - r o a d s i n t o food cans but their hydrolytic instability has made them o n l y a p p l i c a b l e f o r the l e s s c o r r o s i v e products. S i d e Seam P r o t e c t i o n ; There i s an exposed c u t edge on the i n s i d e o f the welded can where most o f the t i n , d u r i n g the s i d e seaming p r o c e s s , has been l o s t . I t i s n e c e s s a r y t o p r o t e c t t h i s area w i t h a l a c q u e r f i l m w i t h h i g h i n t e g r i t y . S i d e seam p r o t e c t i v e c o a t i n g s have a v e r y i m p o r t a n t r o l e t o p l a y . Without them the f o o d s t u f f would be t a i n t e d w i t h h i g h d i s s o l v e d i r o n and, i n extreme c a s e s , p e r f o r a t i o n s would o c c u r . For a g g r e s s i v e p r o d u c t s , the s i d e seam a r e a i s t y p i c a l l y p r o t e c t e d u s i n g an e l e c t r o s t a t i c a l l y a p p l i e d t h e r m o p l a s t i c p o l y e s t e r powder ( 8 ) . F u s i o n o f the powder r e s u l t s i n 100% p r o t e c t i o n o f the weld a r e a . For l e s s a g g r e s s i v e p r o d u c t s s o l u t i o n s t r i p e s , based on epoxy o r v i n y l o r g a n o s o l t e c h n o l o g i e s , can be applied using a conventional spray. Welded can s t o c k w i l l c o n t i n u e t o r e l y on sheet c o a t i n g because c u r r e n t r e s i s t a n c e w e l d i n g r e q u i r e s a narrow uncoated s t r i p t o be left a c r o s s the w i d t h o f the coated web. An o r g a n i c f i l m would i n t e r f e r e w i t h the w e l d i n g p r o c e s s . Laser welding could u t i l i s e p l a t e t h a t has r e c e i v e d 100% c o v e r i n g of a c o a t i n g m a t e r i a l . Draw Redraw Technology The s i n g l e s h a l l o w drawn cans f o r f i s h , meat and p e t f o o d p r o d u c t s nave e x i s t e d f o r the l a s t 30 y e a r s . In the 1970's s e v e r a l major can companies d e c i d e d to i n v e s t i n the m u l t i - d r a w n s t e e l c o n t a i n e r f o r a whole range o f processed f o o d s . T r i p l e drawn f o o d cans a r e now a commercial r e a l i t y . They are l e a d f r e e . T h e i r i n t e g r a l end and l a c k o f s i d e seam has d r a m a t i c a l l y reduced the i n c i d e n c e o f i r o n p i c k up and m i c r o - l e a k a g e . A l l of these problems have f o r many y e a r s been a d i s a d v a n t a g e o f the 3 p i e c e s o l d e r e d food can. In the DRD p r o c e s s , cups are punched from p r e - c o a t e d t i n f r e e s t e e l , redrawn i n two s t a g e s , the bottom p r o f i l e i s i n s e r t e d , cans are trimmed and f l a n g e d , beaded and 100% l e a k t e s t e d b e f o r e b e i n g p a l l e t i s e d and shipped to the packer. The p r e f e r r e d base s t e e l , t i n f r e e s t e e l known as TFS-CT p l a t e (CT d e s i g n a t e s chromium t y p e ) , c o n s i s t s of a v e r y t h i n l a y e r o f chromium metal covered w i t h a chromium o x i d e f i l m t o promote c o a t i n g adhesion and t o prevent u n d e r c u t t i n g and f i l i f o r m c o r r o s i o n . The DRD p r o c e s s p l a c e s extreme s t r e t c h i n g and compression f o r c e s on the upper s i d e w a l l o f the can. Add to t h i s p a r t i a l t h i n n i n g o f the s i d e w a l l by the i r o n i n g p r o c e s s and i t can be a p p r e c i a t e d t h a t c o a t i n g s f o r t r i p l e drawn food cans need v e r y h i g h performance r e q u i r e m e n t s , as o u t l i n e d i n Table I I . {9) Table I I - C o a t i n g requirements f o r t r i p l e drawn food cans 0 0 0 0

Good commercial a p p l i c a t i o n F l e x i b i l i t y and adhesion Process r e s i s t a n c e Chemical r e s i s t a n c e

0 0 0 0

H i d i n g power Colour choice Flavour free FDA a p p r o v a l


17. GOOD


209


Extremely high f l e x i b i l i t y and adhesion i s needed t o r e s i s t f r a c t u r e d u r i n g the drawing p r o c e s s . The s e l e c t i o n o f s u i t a b l e c o a t i n g s f o r t h i s c o n t a i n e r proved to be a l e n g t h y process r e q u i r i n g e x t e n s i v e t e s t pack e v a l u a t i o n s . The i n t e r i o r and e x t e r i o r c o a t i n g o f c h o i c e f o r t r i p l e drawn and partially i r o n e d cans i s the PVC v i n y l o r g a n o s o l . E x t e n s i v e s t u d i e s have shown t h a t c e r t a i n f l e x i b l e p o l y e s t e r s , f l e x i b l e epoxy p h e n o l i c s and h i g h m o l e c u l a r weight phenoxy c o a t i n g s may have the f l e x i b i l i t y and adhesion t o w i t h s t a n d a t r i p l e draw b u t n o t t h e p r o p e r t i e s needed f o r the e x t r a p a r t i a l w a l l i r o n i n g p r o c e s s . PVC Organosol C o a t i n g s : The most commonly used i n t e r i o r c o a t i n g on DRD cans i n the U.S.A. i s a b u f f c o l o u r e d v i n y l o r g a n o s o l . A basic PVC o r g a n o s o l f o r m u l a t i o n w i l l i n c o r p o r a t e a h i g h m o l e c u l a r weight PVC o r g a n o s o l d i s p e r s i o n r e s i n which i s t h e r m o p l a s t i c i n n a t u r e and extremely f l e x i b l e . I n o r d e r t o enhance t h e f i l m s product r e s i s t a n c e and a d h e s i o n , s o l u b l e t h e r m o s e t t i n g r e s i n s , i n c l u d i n g epoxy, p h e n o l i c and p o l y e s t e r s a r e added. P l a s t i c i s e r s a r e added t o a i d the f i l m formation. The h y b r i d system i s d e p i c t e d i n Figure 5. The c o l d wet film has the d i s p e r s e d h i g h m o l e c u l a r weight PVC p a r t i c l e s suspended in a blend o f a r o m a t i c and k e t o n i c s o l v e n t s . On h e a t i n g t h e f i l m , t h e r e i s a l o s s o f d i l u e n t o r non s o l v e n t which e n r i c h e s t h e p a r t i c l e s w i t h the s t r o n g e r k e t o n i c s o l v e n t s . The c o n t i n u i n g a c t i o n o f heat and s o l v e n t s causes t h e PVC r e s i n t o s w e l l . The p l a s t i c i s e r present a s s i s t i n t h e f o r m a t i o n o f a c o n t i n u o u s f i l m as t h e t r u e solvent i s slowly l o s t . During the l a t t e r stages o f d r y i n g , the s o l u b l e t h e r m o s e t t i n g r e s i n s w i l l cure t o g i v e the optimum p r o p e r t i e s needed f o r the DRD c o n t a i n e r .

ô°-ôgQ%"o-

//////////

////////// C o l d Wet F i l m

Loss o f non solvent Δ

S w e l l i n g o f PVC p a r t i c l e s absorption of true solvent "gelling".

> //////////

F u s i o n o f PVC Loss o f t r u e s o l v e n t Solvents

Diluents Dispersants

eg Xylene - No s o l v e n t a c t i o n eg Ketones - S o l v e n t a c t i o n

Figure 5 - D i s p e r s i o n f u s i o n process



210


P i g m e n t a t i o n w i t h t i t a n i u m d i o x i d e i s commonly used t o produce a w h i t e o r b u f f c o l o u r e d i n t e r n a l c o a t i n g . I n t e r n a l l u b r i c a n t s based on h i g h m e l t i n g waxes f a c i l i t a t e the drawing p r o c e s s (J_0). DRD cans are now used e x t e n s i v e l y i n the USA and UK f o r meat, v e g e t a b l e s and pet f o o d . With the improvements g a i n e d by u s i n g the 2 p i e c e DRD and welded can technologies, the 3 p i e c e s o l d e r e d food can i s q u i c k l y d i s a p p e a r i n g from the s t o r e s h e l v e s . By the year 1990 the cans w i t h s o l d e r e d s i d e seams w i l l be a c o l l e c t o r s i t e m . Beer Beverage Can

Developments

I t was back i n 1935 t h a t the Krueger Brewing Co. i n the U.S. f i r s t produced a 3 p i e c e s o l d e r e d t i n p l a t e beer can. Since that date, t e c h n o l o g i e s have improved tremendously. Aluminium impact e x t r u d e d 2 p i e c e cans were t r i e d i n the l a t e 1950's. Cemented and welded TFS cans were p o p u l a r i n the l a t e '60's and e a r l y '70's. However, from the e a r l y days o f 1959, developments a t the major p a c k a g i n g , aluminium and s t e e l companies ensured the 2 p i e c e Drawn and W a l l I r o n e d (DWI) c o n t a i n e r has a dominant p o s i t i o n today i n N o r t h America and Europe. Because o f r e c y c l a b i l i t y and d e p o s i t l e g i s l a t i o n , aluminium dominates the 70 b i l l i o n d r i n k s can market i n the U.S. In Europe, DWI was i n t r o d u c e d r a p i d l y i n the 1970's by the major can companies. T i n p l a t e i s used on over 60% o f the DWI l i n e s i n Europe. The process: U n l i k e the DRD p r o c e s s , the DWI m a n u f a c t u r i n g p r o c e s s b e g i n s w i t h a c o i l o f uncoated p l a t e t h a t i s unwound, l u b r i c a t e d and fed i n t o a p r e s s t o form s h a l l o w cups. Cups are then f e d i n t o an i r o n i n g p r e s s . The i r o n e r redraws and i r o n s the can w a l l s . During the drawing, i r o n i n g and t r i m m i n g o p e r a t i o n s the s h e l l i s covered w i t h a f i l m o f l u b r i c a n t and c o o l a n t . T h i s i s removed i n the washer where the cans are conveyed, u p s i d e down, through a s e r i e s o f c l e a n i n g and c h e m i c a l t r e a t i n g zones w i t h a f i n a l d e i o n i s e d water wash. These t r e a t m e n t s are n e c e s s a r y t o improve c o a t i n g a d h e s i o n . F o l l o w i n g the e x t e r i o r d e c o r a t i o n w i t h base c o a t i n g s , i n k s and v a r n i s h e s , the a p p l i c a t i o n o f the a l l i m p o r t a n t i n t e r i o r c o a t i n g i s made by s p r a y . The cans are then baked under s t r i c t l y c o n t r o l l e d c o n d i t i o n s . The f i n a l s t a g e o f metal f o r m i n g c o n s i s t s o f d i e n e c k i n g (or s p i n n e c k i n g ) f o l l o w e d by f l a n g i n g , t e s t i n g and p a l l e t i s i n g . Spray C o a t i n g C h e m i s t r y T r a d i t i o n a l l y , c o a t i n g s f o r beer and beverage cans have been based on a few s o l v e n t borne f o r m u l a t i o n s (Table I I I ) . I n i t i a l l y , a two c o a t s o l v e n t based system was used on t i n p l a t e 2 o r 3 p i e c e cans. The basecoat was an epoxy p h e n o l i c m a t e r i a l g i v i n g e x c e l l e n t a d h e s i o n and a l l o w i n g an impermeable v i n y l t o p c o a t t o be a p p l i e d . T h i s system i s s t i l l a r g u a b l y the b e s t p r o t e c t i v e system around f o r beer/beverage c o n t a i n e r s . The e p o x y / v i n y l system has been superseded i n Europe by 1 o r 2 c o a t s o f an epoxy amino c l e a r l a c q u e r system which meets the r e q u i r e m e n t s o f the demanding European market f o r t i n p l a t e DWI cans. In the U.S., w i t h the m a j o r i t y o f DWI can l i n e s b e i n g aluminium, the l o w e r c o s t water based i n t e r i o r l a c q u e r s now dominate t h i s market.


17. GOOD


211


T a b l e I I I - Spray C o a t i n g S e l e c t i o n f o r Beer Beverage Cans 1.

S o l v e n t based. Base c o a t - Epoxy p h e n o l i c Top c o a t - Solution vinyl.

2.

S o l v e n t based Epoxy amino, one o r two c o a t s .

3.

Water based E p o x y - g - a c r y l i c copolymer, one o r two c o a t s .

The tremendous growth i n the beer and beverage can market i n the U.S. and Europe h i g h l i g h t e d the r e a l need t o reduce p o l l u t i o n from organic solvents. In the U.S., the maximum p e r m i t t e d v o l a t i l e o r g a n i c c o n t e n t o f s p r a y i s 4.2 l b s / g a l l o n c a l c u l a t e d w i t h water excluded. I n approximate terms, a t y p i c a l water based spray l a c q u e r c o n t a i n s 20% o f f i l m f o r m i n g r e s i n s , 65% water and the r e m a i n i n g 15% i s organic co-solvents. To meet the r e q u i r e m e n t s o f the EPA, many r e s e a r c h departments w i t h i n the major c o a t i n g s u p p l i e r s , spent c o n s i d e r a b l e time and e f f o r t i n p u r s u i n g the need f o r a water based i n t e r i o r spray m a t e r i a l . In T a b l e IV the major r e q u i r e m e n t s f o r a s u c c e s s f u l water borne i n t e r i o r c o a t i n g f o r aluminium o r t i n p l a t e DWI cans are o u t l i n e d .

Table IV - Requirements f o r Water Borne Can C o a t i n g s 0 0 0 0 0 0 0

Low a p p l i e d c o s t Meet E.P.A. R e g u l a t i o n s (

Recent Advances in Metal Can Interior Coatings - ACS Publications

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