135. Chemistry of Food Packaging (1974).pdf

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Chemistry of Food Packaging

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Chemistry of Food PackagingCharles M. Swalm,E d i t o r

A symposium sponsored bythe Div is ion of Ag r i cu l tu ra land F ood C hemistry at the166th M eet in g of theA mer ican Chemical Society,Chicago, Ill., A u g . 30, 1973.

ADVANC ES IN CHEMISTRY SERIES 135A M E R I C A N C H E M I C A L S O C I E T YWASHINGTON, D . C. 1974

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Library of CongressCIPDataChemistry of food packaging.

(Advances in chemistry series, 135)"A symposium sponsored by theDivision of Agriculturaland Food Chemistry at the 166th Meeting of the AmericanChemical Society, Chicago, Ill., Aug. 30, 1973."Includes bibliographical references.1. FoodPackagingCongresses.I. Swalm Charles M., 1918- ed. II. AmericanChemical Society. III. American Chemical Society. Division of Agricultural and FoodChemistry. IV. Series.

QD1.A355no. 135 [TP374] 540'.8s [664'.09] 74-17150ISBN0-8412-0205-2 ADCSAJ 135 1-109 (1974)

Copyright 1974AmericanChemical SocietyAll Rights ReservedPRINTED IN THE UNITED STATES OF AMERICA

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Advances in Chemistry SeriesRobert F. Gould, Editor

Advisory BoardKenneth B. BischoffBernard D. BlausteinEllis K. FieldsEdith M . FlanigenJesse C. H. HwaPhillip C. KearneyEgonMatijeviThomas J. MurphyRobert W. Parry

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F O R E W O R DA D V A N C E S I N C H E M I S T R Y S E R I E S was founded in 1949 by theA m e r i c a n Chemica l Soc ie ty as an outlet for symposia andcollections of data inspecial areas of topica l interest that coul dnot be accommodated in the Society's journals. I t provides amedium for symposia that would otherwise be fragmented,their papers di str ibu ted am ong severa l journa ls or not pu b l i shed at a l l . Papers are refereed cr i t i ca l l y accord ing to A C Sedi tor ia l standards andreceive the careful at tent ion and processing characterist ic of ACS pub l i cat ions . P apers pub l i sh edi n A D V A N C E S I N C H E M I S T R Y S E R I E S are original contributionsnot publ ished elsewhere in who le or major part and i n c l u dereports of research as w e l l as reviews since symposia mayembrace both types of presentat ion.

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P R E F A C EA s society develops and technology increases, researchers are modifying

o ld , accepted foods an d in tr oducin g new pr oducts in to the moder ndiet. T h e i r years of constant research have brought us from the colonialwa ys of "eat off th e vi n e" to th e present u se of pr eprocessed, mod i fi ed,an d synthetic foods.

F ood packag in g has s i m i l a r l y undergone rad i ca l changes. A s th epl ace of pr oducti on grows fa rt her fr om the ur ban centers w here mostof the food is consumed, the demands on food containers are greatlyincreased. Society used to be content to deal w i t h natu ra l food packag ing ,but now food must be shipped over long distances and stored for undetermined t ime periods and under often uncertain condit ions.

T h e technologies of glass, t i n , an d al u mi n um contai ners ar e bein gimproved, and the field of polymer containers is r a p i d l y expand ing . N owa food container may be r i g i d or flexible and may be made up of manycombinations of films, la yers, an d coatings. I t must be compa ti ble w i t hthe food cont ai ned, must protect th e product d u r i n g processing, shi pment,an d stora ge, an d mu st also satisfy ma r k etin g r equi r ement s for consu meracceptance.Camden, N . J . C H A RL E S M . S W A L MJuly 1974

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1Trends in the Design of Food ContainersR . E . B E E S E a nd R . J . L U D W I G S E NMateria l Sciences, R esear ch, and D evelopment, A mer ican C a n C o.,B arrington, I l l .

Des igns o f metal cans f or foods a r e co n t i n u o u s l y m od i f i edt o reduce cost a n d t o i m p r o v e c on t a i n er i n t eg r i t y a n d q u a l i t y . A d v a n ces i n co n t a i n er m a t er i a l s i n cl u d e st eel -m a k i n gpr ocesses f or t i n m i l l p r o d u c t s, co r r o si o n r esi s t a n t E T P (el ect r o l y t i c t i n p l a t e) f or m i l d l y a c i d foods, t i n fr ee steel t i nm i l l p r od u ct s, a n d n ew o r g a n i c co a t i n g s . Recent trendsi n co n t a i n er co n st r u ct i o n r esul t fr om a n t i p o l l u t i o n l e g i s l a t i o n , n ew ca n -m a k i n g technol ogy, a n d p u b l i c safety cons i d e r a t i o n s . Water base a n d U V cur ed o r g a n i c coatingsreduce p ol l u t i o n . D r a w i n g , d r a w a n d i r o n i n g , cementing,a n d w el d i n g p r o v i d e a l t er n a t e methods f or m a k i n g cans a n dp o t en t i a l l y u p g r a d e con t a i n er p er f o rm an ce. F u l l i n si d esolder f i l l et s i n soldered s a n i t a r y cans also i m p r o v e co n t a i n eri n t e g r i t y a n d thus co n t r i b u t e t o p u b l i c safety. T h e solderedsa n i t a r y c a n r em a i n s a n i m p or t a n t factor i n p r e ser v i n g foods.

A l t h ou g h soldered t i npl at e cans now domin ate the processed food i n -dustry ( J ) , changes a re bein g made. T he cann in g in dust r y an dconta in er man ufa ctur er s are r espondi ng to in creased social an d economi cpressures to change the t rad i t iona l methods. Some of th e most i mp or ta n tof these pressures are the efforts to protect the environment and theconcern over t he pu bl i c heal th aspects of ca nn ed foods. R enewed a t tent ion i s be ing gi ven to im pr oved conta i ner in tegr i ty an d safe can ni ngpracti ces . N ew can-ma k i ng mater ia l s an d manu factur in g techn iques a r econtr ibut ing to th e solu ti on of these pr oblems. R ecent chan ges i n container construction permit the use of lower gage stee l , lower t in coat ingweights w i t h improved corrosion resistance, beading of can bodies, andin creased use of in side organ ic coatin gs, w h i ch have al l hel ped to m i n i mize the cost of the t i n can wi th out r educi ng conta i ner qua l i t y orin tegr i ty .

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2 C H E M I S T R Y O F F O O D P A C K A G I N GT h e techn ica l aspects i nf l uenci ng these changes are r eviewed i n thi s

pa per. D i scu ssi on of these tr ends is l i m i t ed to the steel-based ma teri al s.T h e cur r ent dem a n d for easy-open ends for food containers has led toth e development of ma n y scored easy-open ends. T h i s is a subject i ni tsel f and is not inc luded in this discuss ion.Ma t er i a l s

T o appreciate the potenti al changes i n food cans, it is necessary todescri be bri ef ly the steel-based mat erial s used in moder n can ma nu fact u r i n g opera t ions. T h e t i n can is mad e from a specia l gra de of t h i n gage,low carbon, cold-rol led steel, which is general ly referred to as a t in m i l lproduc t. T he base steel is coated w i t h ei ther t i n , a ch r omiu m-chr omiu moxide system, or i t is just cl ean ed a n d oil ed. I t m ay a lso be coat ed w i t horganic coat ings.

Electrolytic Tinplate. M u ch of t he t i n m i l l pr oduct is ma de int oelect ro lyt i c t in p la te (E T P ) . A schemat ic of an E T P cross section i sg iven in F ig ur e 1. T h e steel st r ip is c leaned e lectro lyt i ca l l y i n an a l ka l i nebath to remove ro l l i ng l ub r i can t s and d i r t , p i ck l ed i n di l u te m in era l ac id ,usua l l y w i t h electr ic current appl ied to remove oxides, and plated w i t ht in . I t is th en passed t hr ough a mel t i ng tower to melt an d ref low the t i ncoat i ng to form the shi ny t i n surface an d the t i n - i ron al loy layer , chemi c a l l y tr eated to sta bil iz e th e sur face to pr event gr owt h of t i n oxide, an dl ubr i ca ted w i t h a th in layer of synthetic oi l .

T h e t i n coat i ng on E T P can be pur chased in 10 thickness ranges.Di f fe ren t i a l l y coated plate, such as #100/25 E T P , i s coa ted w i t h 60 X10"6 i n ch of t i n on the #10 0 side an d 15 X 10"6 i n ch of t i n on the others ide. T he use of di f ferent i a l ly coated E T P has ma r k edly reduced therequir ement for t i n metal (2 ) .

I n p l a i n t in pla te cans for ac i d foods, t i n provides cathodic pr otect ionto steel (3,4). T h e slow dissolu t ion of t i n prevents steel corrosion. M a n yinvest igators (5-11) have def ined this mechan ism i n detai l an d h aveshow n th at the t i n dissolu t i on rate is a fun ct i on of the cath odic act i vi tyof the base steel, the steel ar ea exposed th r ough the t i n an d the t i n -i ron al loy layers, an d the stannous ion concentr at ion. K a m m et a l . showedtha t control of the growth of the t i n - i r o n al loy layer provides a near lycontinuous t i n - i r o n alloy layer and improves the corrosion resistance ofheav i ly coated (over 45 X 10"6 i n . t i n ) E T P for m i l d l y a ci d foodpr oducts i n w h i ch t i n pr ovides cathodic pr otect ion to steel (12 ) . T h econtro l led t i n - i r o n alloy layer reduces the area of steel exposed to thep r od uct . E T P w i t h the cont r ol l ed al l oy is design ated type K , an d since1964, #75 type K E T P has been used to pr ovide the same pr otect ionas #100 E T P p rov ided p reviously (13 ) .

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1. B E E S E A N D L U D W I G S E N Des i g n of Food Co n t a i n e r s 3

F i g u r e 1 . Schem a t i c cross secti on of 5 5 2CR t i n p l a t e ( # 2 5t i n coating)T i n p l a t e can be pur chased i n a w i de ra nge of tempers a nd th i ck

nesses. C u r r e n t l y 17 different basis weights are avai lable commercial ly,fr om 5 5 #/ B B (pounds per base box or 217.78 ft 2 ) t o 1 3 5 #/ B B . T h es eweight s r ange i n n omi n al thi ckness fr om 6.1 to 14.9 mils .

T h e final thi ckn ess of the steel for t i n pl at e is ach ieved by tw oprocesses. F or conventi onal l y r educed or single r educed pl ate, steel isannealed after cold reduct ion to restore duct i l i ty . T he annea led co i li s then temper ro l led w i t h onl y 1-2% r educt i on to ma k e the final adjust ment to its tensile strength, hardness, and surface finish. T o r educe thecost of the l i ghter basis weigh t pla tes, doubl e r educed, or 2C R , pl atewas introduced (14 ) . A second 30 -4 0% cold redu ct i on is given steelafter the ann eal , w h i ch impa rts a s igni f icant amount of co ld w ork. T h isprovides 2C R p la te w i t h general ly greater hardness and tensi le strength,a loss i n duct i l i ty , an d an increase i n d i r ec t iona l i ty . A t first, these factors

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4 C H E M I S T R Y O F F O O D P A C K A G I N Gmade 2C R p l ate more di f f i cu lt to fabr i cate int o conta iners . H owever,modif icat ion i n ma nu fact ur in g procedur es has enabl ed this l ower cost basesteel to be used.

T h e chemistr y of the base steel is careful ly regulated to control boththe ph ysi cal propert ies an d corr osion resistance ( I S ). R ecent changesi n steel man ufa ctur e ha ve genera l ly benefited t in pl ate perform ance. B asi coxygen processes, which permit steel to be made at a faster rate, tendto produce low carbon steel w i t h l ower levels of resi du a l elements. I ngeneral this is beli eved to im pr ove cor rosion resistance. H owever , in onecase, there is a reduction in the corrosion resistance of steel for cola typecarbonated beverages wh en the r esidu al su l fur concentr at ion is l oweredfrom 0.035 to 0.018% ( 1 6 , 17 ) . I n l emon -l i m e beverages, however , th elow er sul fur levels im pr ove corr osion per for man ce. I t is the cop p e r /su l fur ratio which determines the corrosion resistance of steel for carbonated beverages.

T he new cont in uous cast ing processes, i n contr ast to ingot cast p r oducts, prov ide t i n m i l l products which are exceptional ly clean and formable.T he deoxid i z ing processes required for continuous casting involve eithera l u m i n u m or s i l i con k i l l i n g , w h i ch adds a l um in um or s i l i con to the steel .E xper i ence w i t h type D steels indicates that the added a l um in um w i l lnot cause a corr os ion problem. L au bscher an d W eyan dt ( 1 8 ) haveshown that the s i l i con found in s i l i con k i l l ed , continuous cast, heavi lycoa ted E T P w i l l not adversely affect the corrosion performance of p l a i ncans packed w i t h m i l d l y a c i d food products i n w h i ch t in usua l l y protectssteel. T h e dat a on ena meled cans is not defini ti ve. A d d i t i o n a l pub l i sheddata ar e r equi r ed to determi ne whether or not s i l icon actu al l y reducesthe performance of enameled cans made from enameled, heavily coated,s i l i con k i l l ed , con t i nuous cast E T P .

T in Free SteelElectrolytic Chromium-Coated. A less expensi vesubst itute for ti np l at e, electr olyt ic ch r omi u m coated-steel, has been developed and is designated T F S - C T ( t in free stee l-chromium type) orT F S - C C O ( t in f ree s teel - chr omium -chromiu m oxide) ( 1 9 ) . T h i s m a ter ia l can be used for many products where the cathodic protectionusua l ly su ppl i ed by t i n is not needed. A schemati c cross section is show ni n F ig u r e 2. E l ectro l yt i c , chr omium -coated steel i s made by electro-l y t i c a l l y deposit ing a th in layer of meta l l i c chromium on the bas ic t inm i l l steel, which is in t u rn covered by a th in passive coherent layer ofchromium oxide.

O r g a n i c coatings adhere to the electrolytic chromium-coated steelsurface exceptional ly we l l . T h e sur face is stable an d does not discolord u r i n g bak in g of enamels. I t is resistant to stain in g fr om products cont a i n i ng high levels of sulfide, such as meat, fish, a n d some vegetables.

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1. B E E S E A N D L U D W I G S E N Des i g n of Food Con t a i n e r s 5I t resists pinpoint rust formation before enamel ing and filiform corrosionafter enameling.

T F S - C T or T F S - C C O is a pr im ar y mat er ia l for cemented an dwelded beer and carbonated beverage containers (20-22) and can beused in sanitary food cans. I t is cur r entl y used for ends on solderedsanitary food cans and is a candi date for d r aw n contain ers w h i ch donot require soldering.

F i g u r e 2 . S c h em a t i c cross secti on o f t i n r ee steel ( T F S - C T )T in Free SteelC an-M aker's Quali ty. C M Q (ca n -m a k er s q u a l i t y )

steel is the basic tin m i l l product . C M Q can be either s ing le or doublereduced steel. R o l l i n g oils are removed, and the surface may or maynot be passivated. A schematic cross section of passivated C M Q is showni n F igu r e 3. Q A R (qua l i ty as ro l l ed) 2C R p la te i s the basic 2C R t inm i l l product w i t h the ro l l i ng oil s on the surface. N o fur th er treatmentis given. F i gu r e 4 is a schematic cross section of Q A R pl ate.

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6 C H E M I S T R Y O F F O O D P A C K A G I N GC M Q is comm onl y referr ed to as bl ack pl at e an d has borderl in e

corrosion resistance both before and af ter enamel ing . H an d l in g andstorage before enamel ing must be careful ly contro l led to minimize p i n poin t ru st. S pecia l organ ic coat in gs are r equi r ed to cont r ol both i n te rna lcorr osion an d externa l f i l i f orm corr osion. T h ey ar e color ed to cover thebr own appearance wh i ch forms dur in g enamel bak in g . U nder cut t i ngcor rosion r esistance, as shown i n F i gu r e 5, is very poor. T h e c h r o m a t e -phosphate T F S i s a heav i ly passivated C M Q wh ich i s cur r ent l y not be ingconsidered because of un dercu tt in g resistance an d cost. Containers fordr y products can be fabr icated f rom C M Q .

F i g u r e 3 . Schem a t i c cross secti on F i g u r e 4 . Schem a t i c cross secti onof T F S -C M Q plate of T F S - Q A R plate

Q A R is bein g evalu ated for cemented or wel ded beverage cans.Spec ia l organ ic coat ings a nd man ufa ctur in g techniques are r equi r ed because of the high level of residual ro l l i ng oils. R easonable success hasbeen achi eved in ma k in g beer a n d beverage cans f rom Q A R p l a te.

Organic Coatings. Organic coatings or lacquers protect the steel ort i n from external or i n te rna l corr osion. T h e can interi ors are coated topr event u ndesir abl e react ions between the in teri or meta l sur face an dthe pr oduct. T hese reactions i n vol ve: (1) cor r osion of the t i n coat in gcaused by oxida nts i n th e pr oduct , (2) color or flavor loss by the pr oductbecause of metal i on pi ck u p, or (3) stai ni ng of the meta l by sul fur -con-

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1. B E E S E A N D L U D W I G S E N Design of F ood Containers 7

t a i n i ng products . E x ter n a l i nks andcoatings are used to decorate thecan, tom i n i m i z e rus t ing , a n d to impr ove mob i l i t y .

H u n d r ed s of coatings area v a i l a b l e to thecan maker . C oat ings areselected for each use on the basis of w h i c h w i l l give adequate performance at lowest cost (23). R ecent advances i ncoating technology result

F i g u r e 5. U nd er c u t t i n g c or r o si o n r esi stance ofenameled platef r om curr ent app l ica t i on an d usage requi r ements . F or example, the newvended can for fo rmu la ted food products requires enamel performanceleve ls wh ich cou ld not be met by the oleoresinous coatings commonlyused. The organosol andw h it e sani tar y coati ngs were developed tomeet this need. T h e pigm ented w hi te coat ings not only provide a p leas ingaesthetic effect but also conceal underf i lm sta in ing produced by someproducts . I nadd i t i on to organosol white coatings, white coatings basedon epoxy-ester, acr yl ic, an d polyester resins ha ve been devel oped w h i chmeet sanitary food can requi rements . A l u m i n u m pigmented epoxy-phenolics and organosols have also been developed to concea l underf i l msta in ing.

T he mechanism which permits can coat ings to prevent meta l corrosion or s ta in ing has not been e lu cidat ed compl ete ly . C onta in er coat ingsare only 0.2 or 0.3 m i l s th i ck , w h i c h is 1/10 to 1/100 as t h i c k as convent i ona l coatings used to pr otect tan ks, pip es, or s id ing f rom atmosphericcorr osion. Some in vestigat ors beli eve org an ic coati ngs combat corr osionby phys i ca l barr ier , chemica l i nh ib i t i on , and /o r e le c t r i ca l effects (24).C a n coat ings permit water absorpt ion anddi f fus ion, t ransport of ionssuch as hydrogen and ch lor ide , and gas di f fus ion (25). T hese diffu sionmechanisms suggest that corrosion can occur th r ough cont in uous container coat ings. T hu s i t is reasonable to concl ude that w hen coat i ngfa i lures occur, i t is because of one or a l l of these mechanisms.T r e nd s

T h e r e are several areas to cons ider when discuss ing the fu tu re ofthe food can . A n t i - pol l u t i on leg i s la t ion , new can-making technologies ,and public safety aspects w i l l have a pronounced effect onfood conta inerdesign.

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8 C H E M I S T R Y O F F O O D P A C K A G I N GAnti-Pollution L egislation. A nt i -pol l u t i on l egi s l a t i on covers a broad

area of soc ia l responsib i l i ty . F or the can man ufactur er i t ranges f romthe requir ements of the C l ean A i r A ct of 1970 to reduce or el i mi na tecontaminants which po l lute the a ir to the ban-the-can type leg is lat ionas ena cted b y the state of O r egon. T h e la tter type of legisl at i on is d i rected m ai nl y at deterr in g roadside l i t ter of beer a n d beverage containers.T he C l ean A i r A c t has a si gn i fi can t effect upon the operations of i ndus t r i a lcoat i ng users. T h e requir ements to r educe or el i mi na te organ ic emissionsand noxious fumes from organic coat ing operat ions is of part icular concern . A s a resul t , the conta in er coat in g in du str y is act i vely t ry i ng todevel op al terna tiv es to solvent base coating systems.

T h e fol l owi ng are the most pr omin ent developments:1. W ater -base coat in gs for spr ay and rol l er coat app l i ca t i on2. E l ectr odeposit i on of water-base coat ings3. H eat cur ed h i gh sol i ds coat in gs4. R ad ia t i on cur ed h i gh sol ids coat ings5. E lectr ostat i c spra yed powder coat in gs6. H ot melt spray coat in gs

T hese techn ica l developments an d th eir merits ha ve recent ly been w el ldocumented by R . M . B r i c k (26 ) . T he potent ia l effect of these developments w i t h regard to the ordin ar y hot - f i l l ed or steam-ster i l i zed sanitar yfood container w i l l take some time to discover because the use of organiccoat ing materials on the inside of food containers is control led by theF ood and D r u g A dmin i s t ra t i on . T he i r gu idel ines res tr i c t t he compos i t ional structure of coat ing materials and l i m i t the amount of organicmater ia l which can be extracted from the coat ing by a food product (27 ) .I n general, container coat ings for steam-steri l ized food products mustwi ths tand the most stringent tests as described in these regulations.

S in ce most of the al tern at iv e coati ng systems above ut i l i ze polymermater ia ls or adjuvants which are not acceptable food contact materials,a chan ge i n coati ng systems for t he in sid e of food cans w i l l probab lybe slow because of the test in g r equi r ed. T hu s, the can mak er w i l l i n i t i a l l yuse in cin era ti on or adsorpt ion of solvent emissions to compl y w i t h thepo l lu t ion regulat ions. H owever , h i gh sol i ds, heat -cur ed sanita ry, an dC -enam els are now bein g eval ua ted to el im in at e the need for pol l u t i oncont rol. T h ey have essenti al l y the same chemi cal composit i on a nd p r operti es as cur r ent coat in g mat eria ls but do not requi r e contr ol of th e l owam ount of or gan ic solvents rel eased to the atm osphere d u r i n g oven bak i n g.

A t present, the ma jor effort to devel op an ti -pol l u ti on coat in g systemsis devoted to outside l it hogr aph ic decorat i ng m ateria ls for beverage a n dnon-food contai ners. W at er-based, c lear varn ishes an d wh it e coat in gsare in creasin gly ava i l abl e for specif ic end uses. Si gnif i cant advances ar e

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1. B E E S E A N D L U D W I G S E N Des i g n of Food Co n t a i n e r s 9bein g made i n us in g u l t ra v io let ra d ia t ion as a nonpo l l ut i ng cur i ng methodfor contai ner decora t ion. T h e ink s an d coat ings used i n the U V cur in gprocess are essenti al l y 100% sol i ds mat eria ls w h i ch are genera l l y composed of a cr yl i c or l i near un satu ra ted polyester monomers combin ed w i t ha suitable photosensit izer.

New C an-M ak in g Technologies. T h e second area affect ing conta in ert rends is the new can -mak in g technology. Severa l ma nu factu r i ng techniques are being considered which would compete w i t h the conventionalsoldered t in pla te sanitar y contai ner . T hese in c lu de dr aw i ng , dr aw n an di ron ing , an d cement in g and we ld i ng . T he commerc ia l success of the deepd r a w n a l u m i n u m food can i s we l l kn own. D r aw n and d raw n and i ronedprocesses for steel-based mat erial s are bein g eval ua ted. T h e techn ologyfor replacing the soldered side seam w i t h either cement ing or weldingtechni ques has been developed.

A dvan ces i n mater ia l s an d new contai ner constr uct ion techniquesare usu al ly evalu ated w i t h one- or two-year test packs. T h e t i me r equi r edto pr ove perform an ce of new ma teri al s or cont ai ner constr ucti ons slowsdevelopment pr ogra ms. H owever , severa l l abora tory techn iques areava i l ab le w h i ch pr ovide a reasonable est imate of conta in er perform an ce.

F i g u r e 6 . D r a w n cans f or oodW h i l e each container manufacturer has developed proprietary tests, mostare based on electrochemical techniques. C orr os ion i n enameled E T Por T F S cans can be eval ua ted u sin g one of the avai la ble procedur es ( 2 8 ,29 , 30 ) . C orr osion perform an ce of p l a i n t i n pl at e cans can be estim atedus ing the P rogress ive A T C T est developed by K a m m (6,7). T hese testsshou ld speed the developm ent of n ew cont ai ner s.

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10 C H E M I S T R Y OF F O O D P A C K A G I N GD R A W N C A N S . D r a w n cans, shown i n theF i g u r e 6, are pu nch ed fr om

a flat sheet of coated stock which has been lubr icated w i t h a so l id l u b r i cant such as wax (31). D r a w i n g is probably the simplest method ofconta iner manufacture , requ i r ing the fewest number of operations fromsheet or coil stock to finished can. The presses used u sua l l y ha ve m u l t i p l e dies , permitt ing the manufac tu re of two or more cans w i t h eachpress stroke. For sha l low drawing th i s might be done in a single operation. For deeper dr aw in g mul t i -s tage draw s are requ i red . The finaloperation is performed on a header, which beads and necks- in the bottomen d to form the stacking feature. The process is su i tab le for T F S , E T P ,a n d a l u m i n u m . The absence of a side seam andbottom endseam e l im i nates theproblems associated w i t h these areas and adds aesthetic appeal.H ow e ver , the process places severe requirements on the metal surfacea n d orga ni c coati ngs. Some enam el coati ngs w i l l meet the demands ofthis process, but care must be taken to preserve the i n tegr i t y of theenamel coat in g wh i ch u l t i ma te ly determi nes conta iner performan ce.

F i g u r e 7. D r a w n a n d r oned cans or oodD R A W N A N D I R O N E D S T E E L C A N S . Drawi ng and ironing grew out of

the dr aw in g process. F ig ur e 7 depicts D & I cans for foods. Thed r a w na n d i roned t inp la te can is usua l ly produced by d r a w i n g a cup f rom acoi l of lu br i ca ted matte t inp l a te .

T h e need for matte t inp la te is an import ant factor in the commercia lsuccess of the D & I steel-based beverage container (32). D r a w n c u p s

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1. B E E S E A N D L U D W I G S E N Des i gn o f Food Co n t a i n e r s 11are fed i nt o an ir oni ng press w h i ch thin s or i rons the s ide w al l s , ther ebyinc reas ing the heigh t of the can . T h e side w a l l of the cur rent t in pla teD & I can for beer is r edu ced fr om 0.013 to 0.0045 i nch by the i ron ingprocess. A fter i r oni ng the can is tr im med, an d lubr ican ts are r emovedby wash in g. W ash ed, dr y cans are decor ated on the outside, bak ed, an dthen spra y coated on the in sid e an d bak ed. T h e finished ca n is neck eda n d flanged. B ead i n g strengthens the th in i r oned side wa l l s . T hi s processi s commerci a l ly appl icabl e to both t in pla te an d a l um in um an d is presently used for beer an d car bonat ed bever age cans (33).

T her e are severa l disadvantages to dr aw n and ir oned cans for foods.T he D & I can is used onl y w i t h pr oducts sui ta ble for ena meled cans sincether e is not enough t i n on the int eri or sur face of a p l a i n D & I can toprovide cathodic protection to the large area of steel exposed throughthe ironed t in surface.

F i g u r e 8 . Cemented a n dwelded bever age cans

C E M E N T E D A N D W E L D E D C A N S . B eer an d car bonat ed bever age cans,made by the now f am i l i a r cement ing ( 2 2 ) and we ld ing ( 2 0 ) processes,are shown in F ig ur e 8. T hese processes could also be used for sanitaryprocessed food cans. E na mel ed T F S mater ia l s are used for these cans.Corros ion performance of the enameled, cemented, and welded cans iss imi lar to tha t of enam eled sol dered cans for p r oducts w h i ch do n otrequi re the cath odic protection usu al l y sup pl ied by the t i n coati ng.

T h e cemented l ap seam used i n these cans is a san dw i ch of p l at e,orga ni c coati ngs, a n d cement. T h e body is for med on a modi fied body-maker, and the process is based on the control of heat input and removal.W el ded cans are a lso made on a modi f ied bodymak er. C oated T F S bodyb lanks w i t h bare edges are fed into the bodymaker where the marginsare cleaned so that a un ifor m electr ica l resistance w i l l be presented tothe electric current provided to weld the side seam.

T h e ser vice l i f e perfor ma nce of the cemented or w el ded enam eledT F S container for many food products should be s imi lar to the perform-

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12 C H E M I S T R Y O F FO O D P A C K A G I N Gance pr ovided by an enameled #25 E T P soldered conta in er. T her e is noin crease i n the col d wor k of the base steel nor is there any damage tothe enamel coatings by the container forming processes. L ow t i n solderis qui te in ert, an d thu s the r emoval of the solder f r om the conta in er byeither the cementing or welding operations should not affect the containerperformance.

Public H ealth Aspects. T h e t h i rd factor affecting container trendsin volves pu bl i c heal th . I t is no coincidence th at the new conta in er-m a k i n g processes which have been described also have a strong potent ia l for up gr ad in g can in tegri ty for processed food products . T hetwo-piece can (dr a w n or dr aw n an d ir oned) has only one end to bedou bl e seam ed an d has no sol dered side seam. S in ce the open end of th ecan body is a smooth, continuous flange, there is less chance for falseseams an d r econt am in at ion du r i n g processing. S in ce there is no sideseam, there is no solder, an d hence, no chan ce for l ead mi gr at ion in tothe food. W el ded or cemented can bodies also provide the same potent ia l for e l im ina t i ng concern over l ead mi gr at ion but wi th out the advantageof end double seaming noted for two-piece cans.

I mp r oved can int egri ty can also be provi ded for soldered t i npl atecans. T h e recent tr end is to f low solder a l l the w ay th r ough the t h i rdfo ld of the side seam to provide a fu l l fillet of l ow ti n solder on the in sideof the can (21). T hi s I S F ( i nsi de solder f il let) sani tar y can has a hi gherlevel of can int egrit y. T h e solder fillet gives addi t iona l creep and b lowupstrength and is more easi ly coated w i t h organ ic side seam stri pes. T h eel ectr ochemical potenti al of l ow t i n solder is such tha t any t i n or steelexposed to the product w i l l give cathodic protection to the low t in (98%l ead ) solder exposed at th e sid e seam ( 3 4 ) . T he enam eled sani tar y canmay also be soldered w i t h pur e t i n solder. T hi s specia l constru ction isk n o w n as the h igh t in fillet ( H T F ) ca n . T i n i s a v ai l a b l e to provi d ecat hodi c pr otecti on to steel exposed th r ough the br eak s in the ena melcoat ing because the cans are soldered w i t h pu r e ti n solder so tha t a1/32 i n . w i de fillet of t i n is exposed al ong th e sid e seam (35, 36 ) . T h i scan i s par t i cu l ar l y su i ted for asparagus an d some tomato products.

Conc lus ion

I n spite of a l l these new conta in er inn ovati ons, there are ma ny s i tuat ions wh ere t in pl ate is r equi r ed because of its corrosion resistance andab i l i t y to m ain ta i n pr oduct qua l i t y. T hu s, i t is believed that the solderedt inp la te sani tar y can w i l l remain an important factor in preserv ing foodsfor many years to come.

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I. B E E S E A N D L U D W I G S E N Design of Food Containers 13Literature Cited1. Coonen, . H . , Mason, S. I., "RecentAdvances in Rigid Metal Containers,"Proc. Int. Congr. FoodSci. Technol., 3rd, SOS/70, Washington, D.C.A u g . 9-14, 1970, pp. 589-594.2. McKirahan, R. D., Connell, J. C., Hotchner, S. J., FoodTechnol. (1959)13, 228-232.3. Kohman, E. F., Sanborn, . H., Ind. Eng. Chem. (1928) 20, 76-79.4. Lueck, R. H., Blair, H. T., Trans. Amer. Electrochem. Soc. (1928) 54,257-292.5. Kamm, G . G . , Willey, A. R., Corrosion (1961) 17, 77t-84t.6. Kamm, G. G., Willey, A. R., "The Electrochemistry of Tinplate Corrosionand Techniques for Evaluating Resistance to Corrosion by Acid Foods,"Proc. Int. Congr. Metal. Corrosion, 1st, London, April, 1961,pp. 493-503.7. Kamm, G. G., Willey, A. R., Beese, R.E., Krickl, J. L., Corrosion (1961)17, 84t-92t.8. Koehler, E. L., J. Electrochem. Soc. (1956) 103, 486-491.9. Koehler, E. L., Canonico, C. M.,Corrosion (1957) 13, 227t-237t.10. Vaurio, V. W., Corrosion (1950) 6, 260-267.11. Willey, A. R., Krickl, J . L . , Hartwell, R. R., Corrosion (1956) 12, 433t-440t.12. Kamm, G. G ., Willey, A. R., Beese, R. E . , Mater. Prot. (Dec. 1964) 3 (12),70-73.13. Hotchner, S. J., Poole, C. J., "Recent Results on the Problems of Can Corrosion," Proc.Int. Congr. Canned Foods, 5th, Vienna, October 3-6,1967, pp. 151-171.14. Brighton, K. W., Riester, D. W., Braun, O. G., Nat. Canners Ass. Informa-tion Letter No. 1909, 61-64 (January 31, 1963).

15. Book ASTMStand. (1973) Part 3, A 623-672.16. Beese, R. E., Krickl, J. L., Bemis, L. E., Proc. Soc. Soft Drink Technol.(1968), 57-80.17. Mittelman, M. D., Collins, J. R., Lawson, J. ., "Corrosion Resistant T in -plate for Carbonated Beverage Containers," Tech. Meet. Amer. IronSteel Inst.,San Francisco, Calif., Nov. 18, 1965.18. Laubscher, A. N., Weyandt, G. N., J . FoodSci. (1970) 35, 823-827.19. Kamm, G. G ., Willey, A. R., Linde, N. J ., J. Electrochem. Soc. (1969) 116,1299-1305.20. Chiappe, W. T., Mod. Packag. (Mar., 1970) 43, (3), 82-84.21. Eike, E. F., Coonen, . H., "The Changing Food Container," Int. Congr.Canned Foods, 6th, Paris, November 14-17, 1972.22. Kidder, D. R., Kamm, G. G., Kopetz, . ., Amer. Soc.Brew. Chem., Proc.(1967) 138-144.23. McKirahan, R. D., Ludwigsen, R. J., Mater. Prot. (Dec. 1968) 7, (12),29-32.24. Hartley, R. ., J . Mater. (1972) 7, 361-379.25. Koehler, E. L., "Corrosion Under Organic Coatings," U. R. Evans Intern.Conf. Localized Corrosion, Williamsburg, V a . , Dec. 6-11, 1971, Paper 51.26. Brick, R. M . , "New Can Coating Systems to Meet 1975 Air Quality Regulations,"Protective Coatings Division of the Chemical Institute of Canada, Toronto andMontreal, Canada, March21-22, 1973.27. Ives, M . , J. Amer. Diet. Ass. (1957) 33, (4), 347-351.28. Bird, D. W., Jones, B. R., Warner, L. M . , Bull. Inst. Nat. Amelior. (1972)23, 128-153.29. Reznik, D., Mannheim, H. C., Mod. Packag. (Aug. 1966) 39, (12), 127-130.30. Walpole, J. F., Bull. Inst. Nat. Amelior. (1972) 23, 22-29.

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14 CHEMISTRY OF FOOD PACKAGING31. Johns, D. H ., "Manufacturing Methods for Metal Cans," Proceedings of aSeminar onMetal Cans for Food PackagingMaterials, Methods, Selection Criteria and New Developments, Univ. of California, Los Angelesand San Francisco, Calif., November 6-7, 1968.32. Bolt, R. R., Wobbe, D. E., U.S. Patent 3,360,157 (1967).33. Kaercher, R. W., Mod. Packag. (Oct., 1972) 45, (10), 66-70.34. Ass. Food Drug Offic. U.S, Quart. Bull. (1960) 24, 193-195.35. Hotchner, S. J., Kamm, G. G.,Food Technol. (1967) 21, 901-906.36. Kamm, G. G., U.S. Patent 3,268,344 (1966).RECEIVED October 1, 1973.

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2Glass Containers as Protective Packagingfor FoodsJ . M . S H A R FGlass Container M anufacturers Institute, Inc. , W ashington, D .C . 20006

T h e U . S . Food a n d D r u g A d m i n i st r a t i on regulates fooda d d i t i v e s der i ved f r om pac ka g i n g m a t er i a l s , especi a l l yheavy me t a l s , monomer s, p l a s t i c i z er s , s t a b i l i z er s , a n t i o x i d an t s , co l o r a n t s , o r other components. Soda - l i m e- si l i cacon t a i n e r glasses a r e inert a n d q u a l i f i ed b y t h e U .S .P h a r m a c op o ei a a s h a v i n g l ow aqueous ex t r a ct i on a n d l i g h tt r a n sm i s s i o n . T h i s glass s a n i m p erm ea b l e b a r r i er t o l i q u i d ,vapor, o r gas t r a n s f e r . Aqueous foods a r e gene r a l l y a c i d i c ,ex t r a ct i n g m i n u t e am ou n t s of t h e a l k a l i n e oxides, soda, a n dca l c i a from a co n t a i n er . S im u l t a n eo u sl y , a n adher ent h y d r a t ed si l i c a f i l m i s formed w h i ch l i m i t s t h e depth of r eact i o n . Rep r e sen t a t i v e aqueous d a t a show m i n o r elementspr esent only i n p p b . O n th e basis of inertness a n d b a r r i e rc ha r a c t e r i s t i c s , glass c o n t a i n e r s a r e found supe r i o r f o rhermetic p a c k a g i n g o f foods.

' T 'h e pur pose of food pa cka gin g is c lear ly s tated by the F ood P r otect ionA C om m i t t ee ( F P C ) of t h e N a t i on a l A cademy of Sc iences ( I ) . T he

food package is to protect the contents d u r i n g storageboth before saleand i n the homefr om contam inat ion by d i r t and other foreign mater ia l ;infestat ion by insects, rodents, and microorganisms; and loss or gain ofmoisture, odors, or f lavors. F r equen tl y deter iora ti on is contr ol l ed byprevent ing contact w i t h air, contaminating gases, or l i ght . B ecause thepackages are closely associated w i t h food, they must contribute l i t t le , i fany, acceptable , h arm less , in c iden ta l addi t ives w h i ch or ig in ate i n thepackag ing an d are tr an sfer r ed to the food mecha ni cal l y or by sol ut ion,extract ion, or decomposi t ion. T hese often un an t i c ipa ted addi t i ves havebeen lon g recognized an d are c losely r egul ated by th e U .S . F ood a nd

15

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16 C H E M I S T R Y O F F O O D P A C K A G I N GD r u g A dmi n i s t r at i on ( F D A ) wh i ch i s respons ib le for the safety of thefood supply .

E a r l y discussions w i t h the F D A l ead i ng to the present regu lat ionswere concerned w i t h the speci fic a ppl i cat ions of the basic F ood an d D r u gL a w of 1938. I nt erpr etati ons wer e der i ved fr om l ong experience w i t hi nherent ly ha rm less conta in ers of gla ss, w ood, a n d some metals . I n theF D A C ode of F eder a l R egu lat i ons ( C F R ) , a speci f i c sec tion dea ls w i t hfood addi t iv es fr om contai ners (2). T est procedur es a re giv en w h i chspecify extra ctant , t i me, an d tempera tu r e of exposur e. T hese routi neshave l i t t l e or no extraction effect on glass containers because of theirinertness. H owever , there appears to be gr owi n g restr i ct ion i n the in ter pretat ion of this section.

T h e un der l y in g cause for the rest r i c t ion i n the C F R has been theincreasing divers i ty an d compl exity of pa cka gin g mater ia l s , ma ny ofw h i c h contain substances quite foreign to food pr oduct s. Some of thesesubstances have been questioned as to long term physiological effect onthe consum er since they are often i n cid ent al , un an ti c ipa ted addi t i ves tothe contained foods du r i n g storage. Q uestions ar ise as to th e P b i n soldersused to join side-seams; m etal l i c organ ic complexes of S n, Zn , C d fav oredas polym er sta bi l izer s; as w el l as a var iety of orga ni c molecul es in tendedas pl ast i c izers, an t ioxidents, colora nts, an d relat ed agents. T h e plast icsthemselves ma y release un reacted monomers. T hi s is caused by di fferin gsolvent actions of the various foods and beverages.

T he cont inu ing concern i n t he U .S . and E ur ope w i t h i n c i den t a l a dd i t ives fr om pa cka gi ng has been cl ear l y stated by G ol ber g wh o has pa r t i c ipa ted extensively i n the discussions of the F P C (3). H e also r eview edthe so-cal l ed F r aw l ey proposal w h i ch presented the concept that any substance used as a fu nct ion al component i n food packaging (other thanpesticides an d heav y meta l s) at a l evel of 0.2% or less coul d not atta inan unsafe level in the food. H owever , G o lber g has found th at th is concept is not acceptabl e a nd that i n the pr evai l i n g circum stances, few conclusions w i l l be reached regarding the acceptabi l ity of these incidentaladdit i ves by the auth or i t ies. A dd i n g to this compl icat ion is the pr oblemof determining and measur ing the quant i t ies of the compounds migrat ingin to foods f rom any s ingle packag i ng mater i a l .

T h e inertness of contemporar y soda-l i me-si l ica glass is so great th atin vest igators r ar ely give i t second thought. T herefore, it is im porta ntto un dersta nd i n deta i l the composit ion of th e glass as w el l as its l oworder of extraction by aqueous foods. A s a r i g i d container, glass doesnot require plast ic izers . I t is composed exclusively of stable earthenoxides and does not employ stabi l izers or ant i oxida nts. H eav y metals arenot a component , a n d since i t is for med i n one cont in uous conta in erstructure , solders containing lead are not used.

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2. S H A R F Glass Cont ai ners 17M ost packaged foods or beverages (as w e l l as drugs and cosmetics)

are neutra l or aci di c. T herefore, the extract i on of the component earthernoxides from glass containers by neut ra l andac id i c aqueous solut ions isconsid ered below . S in ce glass is quite abras ion res istant and is i m p e r v i ous to fats or oi ls , theact ion of dr y or fa t ty foods is not discussed. Thet ransparency of glass is anadvantage in i dent i f y ing the contents, but i frestr ict ion of act ive wavelengths i n the near u l t ravio let though thev is ib le range is desirable, amber glass may be ut i l i zed . The r i g i d glasscontainer is also used tore t a in vacuum or posit ive pressure w i t h hermet icfunct ioning closures.

Compos i t ion of S o d a - l i m e-S i l i c a Conta iner GlassT he major i ngred ient of this glass is selected sand, S i 0 2 , w h i c h is

fluxed a n d melted in large tonnage at commerc ia l ly a t ta inab le temperatures by theadd i t i on of soda ash, N a 2 C 0 3 , w h i c h u po n f i r i ng becomesN a 2 0 . T h i s two-component glass is clear but has no resistance to thehydra t i ng act ion of wa ter, an d the convent i ona l water-glass so lut ion w oul deasi ly fo rm. The stabi l i ty against wa ter results f rom ad di ng l i mestone,f requent ly do lomit i c , which in themelt furnishes the divalent elementsas oxides C a O andM g O . Theres istance and mechanica l forming properties are improved by a lesser amount of A 1 2 0 3 . Theresult ing g lass isnot a t rue chemical compound, but 'more a l a t t i ce or mice l l e of oxygenatoms i n w h i ch there is a stat is t i ca l l y ra ndom d is t r ibut ion of theposit iveions. I n these compositions, the 0 2 content of glass approaches 50% byweight .

T h e theoret ica l mechan ism of theact ion of water on such glass hasbeen f u l l y considered by D o u g l a s andE l -S h a my (4). Themost aggressive solut ion is doub le -d is t i l l ed water at neut ra l i t y . Theeffect of d i l u t eacidic solut ions is much less, the main ac t ion be ing the extract ion ofa l k a l i (N a+) ions which are replaced by hydrogen ions. The result is asurface zone w h e r e the glass is depleted of sodium . A l th ough t races ofextracted sil icates mayappear in the so lut ion, th is res ident dealkal izedlayer becomes a bar r ie r to further ionic di f fus ion, reducing theextract ionto a very low t erm in a l r a te. Theaqueous phase of themajor i ty of foodproducts is ac id ic .

T h e US. Pharmacopoe ia (USP) StandardsT h e U S P st i pu l a tes a test andl im i t s for a l ka l i extr act ion fr om con

tainer soda-lime glass (5). Spec ia l ly prepared doub le -d is t i l led water isused toextract the glass for onehour at 121 C in a steam autoclave on astr ict cyc le program . A n a l iquot of the extract is then back t i t rated w i t h

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18 C H E M I S T R Y O F F O O D P A C K A G I N G5 0 N H 2 S 0 4 us in g meth yl r ed indi cator. Soda- l i me-si l i ca g lass qual i f iesas T y p e I I I w i t h a l i m i t of 8.5 m l of th e ba ck t i t ra t ing acid per 50 ml ofextract. T h e cur rent r outin e empl oys pow dered glass (+50, 20 sieve)because i t is consider ed mor e representa ti ve of th e char acter of t he glasst han the in ner surface of a pa r t i cul ar bott l e. H owever , in the past thetest has applied to bottles, and the pass level was less than 30 mg/1.total extractables expressed as sulfates for the typical smal l bott le. F u r ther , th e exposure t o accel era ted condi ti ons of 121 C for one h ou r i nan au tocl ave is consi der ed to r epresent mor e th an one year of stor age atr oom tempera tur e an d probably a lmost a typi ca l two-year per iod.

T h e accelera ted test for soda-l im e glass conta in er s usi n g the steamautoclave has been reviewed f u l l y by B acon and B u r c h , whose findingsform the basis for cur r ent procedur es (6, 7). T h e r elat ionsh ip of thevar ious compositi ons of a l l si l ica te glasses has been compa r ed i n themore recent study by B acon (8 , 9, J O ) . E mph as is is p l aced on the depress ing effect of various ions in water which extensively s low the react ion. D i s t i l l e d water is considered the most aggressive except for a l k a l i n esolu tions w h i ch tend to remove the di ffusion contr ol l in g, adherent,hydrated s i l i ca la yer pr oducin g mi nu te flakes. T hi s effect is occasional lyseen i n stored r eagent bottl es of a mm oni a or causti c.

A n a l y s i s o f t h e Aqueous E x t r a c tT h e cha r acter ist ic aqueous extra ct fr om soda-l im e glass has been

identif ied in extensive ana ly t i ca l studies by Poole ( 1 1 ) . T hese areshown i n T ab l e I . A n empi r i ca l ra t io of ext ract pp m d iv i ded by component percenta ge gives an ap pr oxim at ion of the dif fusi on rates th r oug hthe hydrated in te r fe r ing s i l i ca l ayer. T h e occur r ence of the l ayer isind icated by the proport iona l ly low rat io for S i 0 2 i n the extract . A m ongthe maj or consti tuents, N a appears the most active (except for the K 2 0present in a smal l amount, probably derived from feldspar the source ofA 1 2 0 3 ) . T hi s pa r t i cul ar glass has a cont empora r y ra nge for these firsttwo major components , but the C a O and M g O leve l der ived from dolo-m i t i c limestone has been selected to show the trend of decreasing ionmobi l i t y . T he very low ra t io for A 1 2 0 3 indicates low mobi l i ty and theapparent block ing act ion in the s i l i ca film. T hi s expla ins the lower rateof extra ction foun d i n soda- l im e glasses w h i ch cont ai n a lum ina .

I r o n is associated wi th s i l i ca sand, usual ly as a l ight surface stainon the gr ai ns. A m ber glass devel ops i on i c color centers or compl exes ofF e - S - C added to the batch as i ron sul f ide and powdered anthraci te .A l t h ou g h the F e content be four or five times tha t show n i n th e exam pl ei n T a bl e I , it appear s to be bou n d i n the compl ex so that no greaterextract ion occurs w i t h the S and C. T i t a n i u m is associated w i t h sand as

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2. S H A R F Glass Cont ai ners 19i lme n i te or ru t i l e , a n d theother elements appear to be present at background levels associated w i t h the glass batch materials . C u l l e t is selected,broken soda- l ime glass which originates in thep l a n t or is purchased onthe outside. I t is used for approx imate ly 20% of the composit ion andappears to have l i t t l e effect on theproport ion of extractabl es. T her e hasbeen no observable " m u l t i p l i e r " effect on any of these res idua l elementswhen recycled cu l let is used instead of earthen batch materials a lone.

Table I. Relationship of Flint Glass C omposition and Aqueous E xtr ac t0G l a ss Com p o si t i o n , % Aqueous E x t r a c t * , ppm R a t i o , ppm / %

S i 0 2 71.6 20.0 0.28N a 2 0 13.5 7.0 0.52C aO 10.3 5.0 0.48M gO 2.50 1.0 0.40A 1 2 0 3 1.25 0.07 0.06B a 0 2 0.35 0.003 0.08K 2 0 0.23 0.2 0.87F e 3 0 4 0.04 0.02 0.50T o t a l s 99.77 33.293

T h e above accelerated extraction procedure employed special double distilled waterexposed to the soda-lime glass surfaces for 2 hours in a steam autoclave at 121C .T h i s schedule is considered to represent 3-4 years of room temperature shelf life of atypical container filled with distilled water. If thecontainer and water were aproductsterilized at 121C for an hour and then stored, this schedule would represent a shelflife of at least 2 years. A ci di c food p roducts would extract one-quarter toone-half thi samount because of ion interference.

6 Analysis by atomic absorpti on showed other residu al elements present in minuteamounts in theextract: T i 0 2 , 7.0 ppb; A s 0 2 0 3 , 1.0 ppb; P bO , 0.3pp b; and both C o 3 0 4and C d O less than 0.1 ppb.

Ex te rna l Su r f ace Pr o tec t i ve Coa t ings

Protect ive coatings are frequently used onglass containers to increasetheir performance and f low through h i g h speed filling l ines. T h e y areapp l i ed on ly to theexterior surfaces either by cold vapor fogging or hotsurface reaction at one or both ends of the annea l i ng lehr . The co ldsurface coatings may be minute , i nv is ib le amounts of l ubr i c i ous foodgrade stearates, oleates, or polyethylene. The hot endcoatings developby exposing containers which arest i l l hot f rom forming machines to d r y -air d i l u te d vapors of t i n ch lo r i de or t i t an ium ch lor ide . A noxide film ofeither metal forms on theexterior glass surface. The amount of film iscontro l led to prevent i r idesence which can occur when the coat ing is80 microns th ick . T hese oxide sur face coats are somewhat harder t hanthe glass, form a t i gh t l y adherent bond w i t h theco ld endcoatings, andmechan i ca l l y protect as we l l as l ubr i ca te the contact surfaces. B ei n grestr icted to external surfaces, they areremote from the contents.

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20 C H E M I S T R Y O F F O O D P A C K A G I N GCl osu r es

Closures for glass containers are commonly a one-piece meta l shellw i t h thr eads or lu gs for att achm ent to the glass cont ai ner openi ng (sincehomogeneous glass stoppers are of l im i ted use). T h e exposed m etal sur faces are coated w i t h impermeable continuous f i lms of f u l l y cured var n ishes or lacquer. T h e typ i ca l , r esi l ient seali ng gasket u sed for f u l l yher meti c closures is a sta bi l ized elastomer, pr esentin g a m i n i m u m sur face to the cont ai ned pr oduct. A l th oug h the surface coatings often ares im i l a r to those used i n th e in ter ior s of met al conta in ers, th e exposedclosure area is a smal l fr act i on of tha t for an a l l -meta l container so thatextractables, i f any, are minor by comparison.

L i g h t T r a n s m i s s i o nL i gh t tr an smi ssion char acteri stics of soda-l im e glass depend on th e

absence or development of i oni c color centers i nv olv i ng a fr act i onalpercentage of i ron an d i ts compl exing. T h e clear or flint glass is lowi n i ron an d other meta l l ic elements as T abl e I in dicates. T hu s i t ist ransparent which is advantageous in food conta iners . T he typ ica ltransmission curve for flint glass shows v i r t u a l l y zer o tra nsm ission of th enear ul tr avi olet at wavelengt h of 290 m/x, r ap id l y ascending to appr oxi mate ly 90% at about 40 m/*, an d conti nu in g in to the in fr ar ed. Somefood an d dr ug products m ay show specif ic, char acterist i c wa velengthabsorption bands in the 290-450 m/x range, and energy absorbed at thesebands may i n i t i a te some changes i n the pr oduct. H ence i n r ecognit ion,th e U S P X V I I I an d ear l i er edit i ons have st i pul ated for a " l i gh t resistantcont ai ner" the ma xim um percentage of l ig ht tr ansmi ssion of a cl osuresealed glass container is "not to exceed 10% i n any wa velength 290 to450 m/z." T hi s qual i f i cat i on is r eadi l y atta i ned by amber g lass . A s impl ea l ternat ive is placing a paper label of adequate area on a flint glassconta iner to act as a l igh t bar r ier .

Summa r y

T h e contempora ry soda- l i me-si l ica glass is form ed by fu sin g selectedsand, soda ash, and limestone, w i t h lesser alt ern ate ma ter ia l such asfe ldspar . W h en such a glass is gi ven an accelerat ed test by extra ctionw i t h double dis t i l l ed wa ter for 2 hour s i n a steam au toclave at 121 C ,a t i gh t l y adhered h ydrat ed s i l i ca bar r ier forms on the glass int erfa cew h i c h ma r k edl y reduces the diffu sion of positi ve ions. I n descend in gorder of appear an ce in the aqueous extract were N a 2 Q , C a O , M g O , a ndA 1 2 0 3 . T h e test is consi dered to be th e equi va l ent of 3 to 4 years of sh elf

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2. SHARF Glass Containers 21life for distilled water in the glass container. The total extract of otherlesser oxides, was 33 ppm. A n acidic solution would reduce this level toone-quarter to one-half this amount. Analysis by atomic absorptionshowed other residual elements present in minute amounts in the extract:TiOo, 7.0 ppb; As203, 1.0 ppb; PbO, 0.3 ppb; and both Co304 and CdOless than 0.1 ppb. Theglass can be made amber color, effectively reducing transmission in the 290-450m/xrange to less than 10%. As a packaging material glasshas superior performance characteristics compared withmetallic, plastic, composite, or other barrier structures particularly forprocessed foods requiring hermetic containment.Literature Cited1. Food Protection Committee, "The use of chemicals in food production,processing, storage, and distribution,"Natl. Acad. Sci.-Natl. Res. Council, Washington, D.C., 1973.2. U.S.Food & Drug Administration,Code of Federal Regulations. Title 21Food and Drugs, Part 121Food Additives Subpart F., par. 121.2500,"Food additives resulting from contact with containers or equipment,and food additives otherwise affecting food," 1972.3. Golberg, L., "Trace chemical contaminants in food; potential for harm,"Fd. Cosmet.Toxco(1971) 9, 65.4. Douglas, R. W ., El-Shamy, T. M. M ., "Reaction of glasses withaqueoussolutions,"J.Am. Ceram. Soc. (1967) 50, 1.5. U.S. Pharmacopoeia XVIII, "Light transmission; Chemical resistance-glasscontainers," Easton, Pa.,p. 923, 1970.6. Bacon, F. R., Burch, O. G., "Effect of time and temperature onacceleratedchemical durability tests made on commercial glass bottles," J. Am.Ceram. Soc. (1940) 23, 1.7. Ibid. (1941) 24, 29.8. Bacon, F. R., "The chemical durability of silicate glass," The Glass Ind.(1968) 49, No. 8, 438.9. Ibid. No. 9, 494.10. Ibid. No. 20, 554.11. Poole, J. P., (1973) Brockway Glass Co., Brockway, Pa., privatecommunication.

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3Tinplate Containers for PackagingIrradiation-Sterilized FoodsJ . J . K I L L O R A N a nd E . W I E R B I C K IU . S . A rmy N ati ck L abora tories, N at ick , M ass. 01760

G . P . P R A T T , K . R . R E N T M E E S T E R , E . W . H I T C H L E R , andW . A . F O U R I E RA merican Can Co. , B arrington, I ll . 60013

T he r el i a b i l i t y o f t h e comm er c i a l l y a v a i l a b l e t i n p l a t e cont a i n er w a s d et erm i n ed f or p a c k a g i n g i r r a d i a t i o n - p r o cessedfoods. E i g h t enamel s, three en d - sea l i n g comp ou n d s, t w ot i n p l a t es, a n d t h e side-seam solder were i r r a d i a t e d w i t h3.0-4.0 M r a d a n d 6 -7 .5 M r a d a t 5 , -3 0 , a n d -90. T h eepoxy p h e n o l i c enamel w a s t h e pr efer r ed enamel. Therewere m i n i m a l extr acti ves fr om t h i s enamel i n t h e pr esenceof three f o o d - s i m u l a t i n g sol vents. T h e pr efer r ed end - s ea l i n gcompound w a s t h e blend of cur ed a n d u n c u r ed i s o b u t y l en e-i sopr ene copolymer . Component t est i n g o f t h e t i n p l a t e a n dsolder showed that t h e g amma r a d i a t i o n , even a t 90,d i d n o t t r a n sf or m t h e beta t i n , or si l v er y f orm , t o t h e a l p h at i n , or powder y f orm . I n a sm a l l - sca l e p r o d u ct i o n test, t h et i n p l a t e co n t a i n er w a s completely r el i a b l e f or p a c k a g i n gi r r a d i a t i o n - s t e r i l i z e d beef a n d h am .

A t first, the program which invest igated the packaging of i r r ad i a t i on -processed foods, concentrated on the most advanced type of con

ta iner , the t i np l at e can . I t h a d perfor med successful l y for a centur y as acontainer for thermoprocessed foods. H owever, as a conta iner for thei r radiat ion-processed foods, i ts ph ys ica l , chemi ca l , an d protect ive char acteristics had to be evaluated, i n c l u d i n g the effects of rad ia t ion onenamels and end-sealing compounds. T h i s container was satisfactory forpa c k a g i n g foods that were i r rad ia t ion s ter i l i zed whi le unfrozen (1 , 2 ) .

W i t h the advent of i r r ad i a t i on processing of frozen foods to m ain ta i nacceptable qua l i ty i n beef, h a m , pork , an d chi ck en, questions wer e posed

22

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3. K I L L O R A N E T A L . T i n p l a t e C o n t a i n er s 23as to wh ether the meta l can w oul d perform sat is factor i ly . W h a t is theeffect of r adi at ion a n d/or l ow temperat ure on the inter i or enamels an dend-seal ing compounds? Does the combin at i on of r adi at i on an d the l owtemper atu re promote the conversi on of t i n fr om th e beta, or s i l very for m,to the al ph a, or pow der for m, r enderi ng the t in coati ng i neffective i nprotect ing the base steel of the tinplate?

T h i s pa per describes the w or k that wa s per for med to answer thesequestions, inter a l i a , i n c l u d i n g

(1) A screeni ng study for eval ua ti ng an d selecting components ofthe t in plat e conta in er tin pl ate , enam el, end-sea l ing compound, an d s ide-seam so lder wh ich were i r r ad i a ted at designated doses and temperatures(2) A n extra ctive stu dy of one can ena mel i n the pr esence of food-s im u l a t i ng solvents to determi ne h ow gam ma r ad ia t i on fr om a cobalt-60

source altered the nature and amount of extractives of this enamel.(3) A p erfor ma nce test of cans of i r rad ia t ion-s ter i l i zed meat products (3).E x p e r i m e n t a l

Irradiation Conditions. T he gamma (cobalt -60) rad i a t i on f a c i l i t yand the source ca l i bra t ion are descr ibed by H ol m and J a r ret t (4 ) .I r r a d i a t i o n doses were 3-4 M r a d and 6-7.5 M r a d at 9 X 10 2 rads persecond for the screeni ng stu dy. I r r ad i at i on temper atu r es wer e 5, 30,a n d 90C . T he gamma source was ca l i bra ted w i t h the ferrous su l fate-c u p r i c sulfate dosimeter.

Enamels. E i g h t comm ercia l enamels , l i s ted i n T abl e I , were appl iedby ro l l coatin g to pan els (25 X 76 cm ) of 43 k g, T yp e M R - T U , N o . 25electro lyt ic ti np l at e. T hese panels wer e cut in to stri ps (10 X 25 cm )an d tested for flexibility before and after i r rad ia t ion w i t h the G enera lE l ect r i c im pa ct appar atus by the reverse im pa ct method. T h e im pactor

T able I. E namels C oated on T in pl ate0D r y WeightE n am e l (mg/ cm 2)

1. P oly bu ta di ene 3902. Polybutadiene w i t h zin c oxide pi gment 4953. E poxy phenol ic w i t h a l um in um p igment 4164. E poxy-w ax an d buta diene-styr ene copolym er w i t ha l u m i n u m pi gm ent 442, 785. E poxy-wax w i t h a l um in um p igment 4426. O l eoresinous 5987. P h enol i c 1568. Oleoresinous w i t h zinc oxide and epoxy w i t ha l u m i n u m pi gm ent 598; 234

43 kg (95 lb ), T y pe M R - T U , N o. 25 (5.6 g/m 2)

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24 C H E M I S T R Y O F F O O D P A C K A G I N Gwas dropped onto the strips fr om a heigh t of 76 cm. Str ips wer e gr adedfor percent elongation in the range of 0 .5 -60%. E na mel adhes ion tothe t i npl ate was deter mi ned by scr ibi ng an d ta pi ng the ena meled testspecimens w i t h cellophane tape.

End-Sealing Compounds. T h e comm er cia l end-sea l ing compoundsused in this study were:

C ompoun d A A b lend of cured an d un cur ed i sobuty l ene-i soprenecopolymerC ompoun d B A b lend of polych loroprene and butad iene-styrenecopolymerC ompoun d C A b lend of polych loroprene and un cured i sobuty l ene-isoprene copolymerSt r ips of tinplate (2.5 cm wide) coated on one side w i t h the test

enam els w ere di pp ed in solu tions of th e end-seal in g compounds w i t h aF i s che r -Payne di p coater. T h e viscositi es of the solu ti on wer e adju stedso tha t th e dr i ed compou n d wa s 7.5 X 10"2 cm thi ck. T ests were performed for cohesion, adhesion, and brittleness of the end-sealing compounds coated on the var ious enam els. C ohesi on is defined her e as ameasure of the combin ed elonga ti on-elast ici ty pr oper ty of the end -sea l ing compou nd . A dh esion is a measure of th e adh esive str engthbetw een the end-seal in g compoun d an d the enamel. B oth cohesion an dadhes ion w ere determi ned by ma nu al l y p i ck i ng at the end-sea l i ng compound w i t h a dissecting needle. A nu mer ica l gr ad in g scale betw een 1an d 10 was used to indicate the effect of i r rad ia t ion and/or temperature.T h e effect of temperature (5, 30, and 90C) on the el ast ici ty of theun i r r ad i a te d test specimens was determined by bending the specimens90 over a 6.4 m m glass r od. A n a rb i t r a ry nu mer ica l scale was used asthe basis of analysis. R i g i d i t y chan ges of the end-seal in g compoun dscaused by the i r rad ia t ion t reatment were determi ned by tors ional br a i danalysis. I n thi s test a glass br ai d coated w i t h an end-seal in g comp oun dis suspended vert ica l l y, an d a weigh t is hu ng at the l ower end to for ma tors ional pendul um . T h e per i od of r otat iona l osci l l a t i on of the pendu l u m measures cha nges i n th e r i g i d i t y of the end-seal in g compou n d coatedon the braid (5).

Tinplate and Solder. T h e tests wer e car r ied out to deter mi ne theeffect of low temperature i r r ad i a t i on on the meta l l ur gica l propert i es ofthe t i npa l te, solder, an d sol der ed la p joints. T w o types of t i npl ate wereused: 43 k g (95 l b ), T yp e M R - T U an d 43 k g (95 l b) , T yp e M R - T 2 ,both coated w i t h N o. 25 electr olyt i c t i npl ate. T h e test specimens were20 X 20 cm panels.T he solder ( 2% t i n -9 8% lead) w as mol ded in to test specimens w i t ha 1.27 cm dia meter r educed section. T h e soldered la p joint specimenswere prepar ed fr om the t i npl at e an d solder. A solder flux was ap pl i ed

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3. K I L L O R A N E T A L . Tinplate Containers 25to each joint, the clamped specimens were dipped into molten solder,and the jo int was obta ined thr ough cap i l l a r y act ion . The tensi le specimens w i t h a 1.27 cm wide reduced section were prepared from thesoldered lap joints. T ensi l e tests were car r ied out w i t h the I n s t ronuniversa l test ing machine at a cross-head speed of 0.5 cm / m i n u t e .I mp act tests w ere cond uct ed on a P l as-T ech uni versa l tester at a cross-head speed of 12.7 X 103 cm / m i n u t e . The corrosion resistance of thet i n coatings was determined from the i r on-solut i on va lues ( ISV) a n d thea l loy- t in -coup le ( A T C ) m et hod (6). Themicros t ructure of thet i np la tewas analyzed w i t h the scanning electron microscope (SE M ) and thetransmission elect ron mi croscope ( T E M ) (7). W i t h S E M , the t i n - i r o na l loy was examined di rect ly on theplate surface after stripping the t i n .T E M requi red examinat ion of a carbon rep l i ca . The cast solder wasexamined w i t h T E M .

E xtractives from E namel. The nature andconcentration of extract ives f rom thei r r a d i a t e d epoxy phenolic enamel (coated on t in fo i l ) weredetermined by chemica l andmi croana l y t i ca l techn iques and comparedw i t h the extractives from the same, but un i r ra d ia t ed , enamel in contactw i t h thesolvent under s imi lar storage conditions (1). The food-s imulati n g solvents were demineral ized dist i l led water , 3% acetic acid, andn-heptane. B oth thewater andacetic acid extractives were treated w i t hchloroform to produce a chloroform -soluble fra ct ion conta in in g most ofthe organic components and a chloroform- insoluble fract ion conta in ingmost of the inorganic components. The aqueous solvents were storedafter i r rad ia t ion for six weeks at 38 C wh i l e the n-heptane was storedfor four hours at 21 C . The i r rad ia t ion dose was 4.7-7.1 M r a d at 21-40 C and at 30 10 C . The i r ra d ia t i on- induced changes in theenamel and ident i f i cat ion of extractives were determined by i n f ra redspectroscopy using a B eck m a n I R 10 grating spectrophotometer.

Resu l t s a n d Discussion

Enamels. The flexibility grades for theeight enamels (T ab l e I ) thatwere i r rad ia ted w i t h 3-4 M r a d and 6-7.5 M r a d at 5, 30, and -90Care shown in T abl e I I . T hese data ind i ca te that theepoxy-based enamelsshowed the best i n i t i a l flexibility at 90 C andmainta ined the i r flexib i l i t y after i r rad ia t ion . Thepreferred enamels were the epoxy phenol i cw i t h al um in um pigment, epoxy-wax and butadienestyrene copolymerw i t h a luminum p i gment , andepoxy-wax w i t h a lu minu m p i gment . Tin-p la te adhesion before andafter i r rad ia t ion was satisfactory for the eightenamels.

End-Sealing Compounds. The qua l i t at ive cohesion testin g of thethree end-sealing compounds w i t h the dissectin g needl e showed th at

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26 C H E M I S T R Y O F F O O D P A C K A G I N GT able II . F lexibility of I rradiated C an Enamels

E l o n g a t i on a t E l o n g a t i on a t E l o n g a t i on a t5a -30a -90a

Co n J i r 7 . 5 Con 4 7 . 5 Con 4 7 .5E n am e l trol M r ad M r ad trol M r ad M r ad trol M r ad M r ad1 40 40 40 20 40 40 10 5 52 20 20 20 20 20 20 5 5 53 60 60 60 60 60 60 20 10 104 60 40 60 60 60 60 20 10 105 20 40 40 60 60 60 20 10 106 40 40 40 20 20 20 5 5 57 20 20 20 20 20 20 10 5 58 60 40 60 20 20 20 5 5 5

a E xpr essed as percent elongation.C omp oun d A was affected most by the i r rad ia t ion , Compound B least , andC ompoun d C in termedia te. T he effect of the ir r ad i at ion on cohesionincreased w i t h i nc reas ing i r r ad ia t i on dose an d temperatu re. T he isobutyl ene- isoprene copolymer i n C ompou n d A a nd C ompou n d C degradesd u r i n g i r r ad ia t i on (9), becomi ng softer a fter i r rad ia t ion . Since the seamof a can is formed before i r rad ia t ion , some softening of the compoundi n th e seam i s not detr i men ta l to the in tegr i ty of the seam.

T he three i r r adi at ed end-seal i ng compounds h ad good adhesion toa l l the enamels except E n a m el 5, the epoxy-wax enam el w i t h a l u m i n u mTable II I. E ffect of I rradiation on C rack ing andAdhesion of E nd-Sealing C ompounds0

Compo u n d s *B C

E n am el C r a ck i n g Ad h esi o n C r a ck i n g Ad h esi o n C r a ck i n g A d h esi o n1 3 1 7 6 5 72 4 2 7 7 6 63 4 3 8 6 4 44 5 4 6 8 4 55 4 4 7 7 5 66 4 3 7 7 6 67 5 4 7 7 6 78 5 4 7 8 6 5

a Ir radiated at 6 to 7.5 M r ad at -9 0 C .6 Grading Scale:

R a t i n g035710

C r a c k i n gnoneslightmoderatesevereshattered

Adhesionno adhesion lossslight adhesion lossmoderate adhesion lcsevere adhesion loss100% adhesion loss

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3. K I L L O R A N E T A L . T i n p l a t e C o n t a i n e r s 27pi gmen t. W h en bent 90 over a 6.2 mm glass r od at 90 C , the orderof cr ack in g resistance an d adhesion to enamels for th e u n ir r ad i at ed an di r r ad i a ted end-seal ing compounds wa s: C omp oun d A > C ompou n d C >C om po u n d B , as s u m m a r i z ed i n T a b l e I I I .

T abl e I V shows the data on r i g i d i t y changes of the end-sealing compoun ds a t tw o dose l evels . R i g i d i ty was determ ined by to rs iona l b r a i danalysis (5). T hese data in dicate that the b l end of cur ed an d un cur edisobuty l ene-isoprene copolym er was softened most by the ir r ad i at i ontreatment, the bl end of pol ychl oroprene an d but ad ien e-styren e copol ymer softened the least, and the blend of polychloroprene and the uncuredisobutyleneisoprene copolym er was interm ediate. I ncr easin g the i r r a diat ion dose from 3-4 M r a d to 6-7.5 M r a d decreased the r i g i d i t y of thethree end-seal i ng compounds. T he i r ra di at ion temperatu re di d not s igni f i cant ly inf luence r i g i d i t y .T able I V . R igidi ty of E nd-Sealing Compounds After the I rr adiation

I r r a d i a t i o n Co n d i t i o n s Rel a t i v e R i g i d i t y aDose (M r a d ) T e m p e r a t u r e () A B C

3-4 5 0.81 0.85 0.786-7.5 0.52 0.78 0.64Z -A - 3 0 0.63 0.80 0.756-7.5 0.46 0.76 0.643-4 - 9 0 0.71 0.92 0.776-7.5 0.65 0.76 0.71

* R elative rigidi ty = P G 2 / P 2 where P Q is peri od of pendu lum of the con trol, an dP is period of pendu lum after ir ra di ation. A value of 1 indi cates no change; less than1 indicates softening.T able V . R elative R igidity of U nir radiated E nd-Sealing Compounds

Re l a t i v e R i g i d i t y aT em p er a t u r e (C) A B C

5 1.57 2.15 1.87- 3 0 5.12 26.6 11.4- 9 0 138.0 57.2 32.5 R elative ri gidity = P G 2 / P 2 where P Q is period of pendu lum at 27C , and P is periodat test temperature.

T he re lat i ve r i g id i t ies of u ni rr adi ated compounds at var i ous temperatures w i t h 27 C as standar d are shown i n T abl e V . T h e lower thetemperature, the higher the relat ive r i g id i ty . T h e patterns of the r i g i d i t ychanges w i t h decreasing temperature were different w i t h each compound.C ompou n d A showed less change tha n B an d C down to 30 C , butmore change at 90C . C omp oun d C showed more change th an C om poun d A at 30C , but less at 90C . C omp oun d B showed most

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28 C H E M I S T R Y O F F O O D P A C K A G I N Gchange at 5 and 30C an d r an ked between A an d C at 9 0 C . T hesere la t ive r i g i d i t i e s show the r i g i d i t y change of the i n d i v i d u a l compound,since each br a i d is compa r ed onl y w i t h i tself at different temperatures.

T h e r i g i d i t y changes caused by i r rad ia t ion would not preclude theuse of the th r ee end-seali ng compound s. Si nce the i r rad ia t ion takes placeafter the seam is form ed i n a t in pl at e conta in er , the end-seal in g compou n d is di str ibu ted i n the seam a nd a softeni ng of the compoun d shoul dnot affect seal in g performan ce. H owever , i n selecting an end-seal in gcompound for a container for i r rad ia t ion-s ter i l i zed foods, the overa l l dataon adhesion, cohesion, r i g i d i t y indicate that the Compounds A and Bw o u l d be preferr ed. C ompou nd A ha d the best low temperature characteristics, an d C omp oun d B was the least affected by i r rad ia t ion .

Tinplate and Solder. M et a l l u r g i ca l studies were performed to determ i n e the effect of i r r ad i a t i on at low temperature on the corrosion resistance of t i npl at e an d on the mecha ni cal propert ies an d mi crostr uctu r e oft i np l a te an d side-seam solder of the ti n pl at e cont ai ner. T h e ar ea of ma jorinterest was the effect of low-temperature i r rad ia t ion on the possible conversion of th e t i n fr om the beta form to the al ph a form . I n the case ofpure t in , the t rans i t ion occurs at 18 C . I t was fear ed that low-temper atu rei r r ad i a t i on w ou l d create dis l ocati ons i n the cr ystal l at t i ce of t i n an d enhan ce the conversion of t i n fr om the s i l very form to a powder y for mrender ing the t i n coati ng in effective i n pr otectin g the base steel . T i n usedfor i ndus t r i a l consu mpt ion contains tra ce amounts of solu ble impu r it ies ofl ead an d an ti mony to r etar d thi s conversion for severa l year s.

T a b l e V I summ ar izes the results of tension tests on t in pl at e i r r a diated at 6-7.5 M r a d . R ad i a t i on ha d no apparent effect on the tensi le

T able V I . E ffect of R adiation on Tensile Properties of T inplateDose

(M r a d )(Contro l )6 to 7.56 to 7.56 to 7.5

T e m p e r a t u r e( )

5- 3 0- 9 0

Y i e l dS t r e n g t h 0(M P a )472482473478

T e n s i l e a tF a i l u r e0,(M P a )472487473479

E l o n g a t i o n * ( 5% )T e n s i l e Impact8.78.09.310.2

5.33.87.26.0a T ransverse to rollin g dir ection.b T ensi le, 8.4 X 10"2 m /s; impact 2.1 m /s.

proper ties of the t in pl at e s ince the mi nor var iat ions i n tensi le valuesreflect experi menta l err or. I mpa ct duc t i l i t y da ta , i n pa r t i cu l a r , was s ig ni f icant because i t in dica ted that no embri t t l ement occur red i n the t i n -p la te as a resul t of the low-temperat ur e i r rad ia t ion at 30 and 90 C .M et a l l o gr a ph i c examination showed that the base steel was not affectedby the low-temperature i r rad ia t ion . F i g u r e 1 shows the typ i ca l mi cro-

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3. K I L L O R A N E T A L . T i n p l a t e C o n t a i n er s 29

F i g u r e 1 . M i c r ost r u ct u r e o f t h e M R -T U steel ( 5 0 0 X )structures of spheroidal carbides and f ine ferri t ic gra in size before a n dafter th e i r r ad i a t i on for 43 k g (95 l b) , T ype M R - T U steelplate.

T a b l e VI I shows the data on the effect of the low-temperature i r r a diat ion on the tensi l e propert ies of cast 98-2 solder (98% l ea d -2 % t i n ).T hese data in dicat e that the r adi at i on ha d no effect on the tensi le properties of the comm erci al solder w h i ch is used for the s ide seam of t in pl at econta iners . M eta l l ogra ph ic examin at ion conf i r med the absence of changei n the mi cr ostr uct ur e of the solder after i r rad ia t ion . ( F i g u r e 2 ) .

T h e peel strengt h of sol dered l ap join t specimens w as also notaffected by i r r ad i a t i on . F or example, the i n i t i a l peel strength of a lapjo int , fabricated from the 43 kg (95 lb) M R - T U , N o. 25 t inpl ate , an d the

T able V I I . E ffect of R adiation on Tensile Properties of SolderY i e l d T e n s i l e a tF a i l u r e (M P a )Dose S t r e n g t h(M P a )M r ad )

(Contro l )6 to 7.56 to 7.56 to 7.55- 3 0- 9 0

9.58.48.28.318.417.917.918.3

54414553a Tensile, 0.2% offset; elongation, in 5.1 cm .

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30 C H E M I S T R Y O F F O O D P A C K A G I N G

F i g u r e 2 . M i c r ost r u ct u r e of t h e cast solder ( 1 5 0 X )98 -2 solder , was 1.58 X 10 4 N / m an d 1.55 X 10 4 N / m a ft er i r rad ia t ionat 6-7.5 M r a d at - 9 0 C .

C ha nges in corr osion r esistance of the el ectr olyt i c t i n coatings weredeter mi ned by th e ir on-sol uti on-val ue test and the al l oy-t in -coup l e test(6 ) . C orr osion resistance data for th e 43 k g (95 l b ), T yp e M R - T U , N o.25 t in pla te are presented i n T abl e V I I I . T hese data show tha t there isno s igni f icant di fference i n the corr osion r esistance of the un ir r ad ia ted

Table V I I I . E ffect of R adiation on C orrosion Resistance of T in plateR a d i a t i o n C o n d i t i o n s A l l o y - T i n C o u p l e I r o nDose T e m p e r a t u r e (/ A / cm 2) S o l u t i o n( M r a d ) () T o p B o t t om ( j t g Fe )

(Contro l ) 0.30 0.24 113 to 4 5 0.28 0.21 123 to 4 - 3 0 0.24 0.19 103 to 4 - 9 0 0.28 0.20 146 to 7.5 5 0.28 0.23 116 to 7.5 - 3 0 0.24 0.20 146 to 7.5 - 9 0 0.21 0.19 11

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3. K I L L O R A N E T A L . T i n p l a t e C on t a i n e r s 31a n d i r r adi at ed t in pla te . T he scanni ng electr on a nd tran smission electr onmi croscopic exami na tion of the i r o n - t i n al l oy sh owed no eviden ce ofchange as a result of i r rad ia t ion . T h e a l loy shown i n the ph otomicr ographs of F ig ur es 3 an d 4 is ty pi ca l of th e conti nu ous str uctu r e foun d onsteelplate w i t h superior corrosion resistance and indicates that the i r o n -t i n al loy crysta ls pr ovide r emar k abl y complete coverage of the base steel.

F i g u r e 3 . SE M ph ot om i c r og r a p h s of i r o n -t i n a l l o y of d e t i n n e d t i n p l a t e(20,000 X )

T h e meta l l ur gica l experiments showed that the beta-a lpha tr ans i t ionof th e t i n coati ng d i d not occur at i r r ad i a t i on doses of 3-5 M r a d a n d6-7.5 M r a d at 5, 30, and 90C and that the tensi le propert ies, impactduct i l i ty , peel str ength of solder ed l ap joint s, an d micr ostru ctur e of commerc ia l ti np l at e a n d solder wer e not affected by th e i r r ad i a t i on cond i tions that are used i n the ster i l ization of meat products.

E xtractives from C an E namels. E a r l i er work reported by P ra t t (1 )showed that in a comparison between i r r ad i a t i on processing and th erma lpr ocessin g, no sign ifi cant differences w ere foun d i n the amoun t of extractives obtain ed fr om three commer cia l can ena mels epoxy ph enoli c,polybutadiene, and oleoresinousin the presence of three aqueous

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32 C H E M I S T R Y O F F O O D P A C K A G I N G

F i g u r e 4 . T E M p h o t om i cr o gr a p h s o f i r o n - t i n a l l o y of detinned t i n p l a t e(20,000 X )

so lventssimulat ing neut ra l , ac id , and fa t ty foods. T h e i r r ad i a t i on -processed enam els yi el ded sma l l er amount s of extractives th an the th erm o-pr ocessed enamels. T hese enam els h ad been ir r ad i a ted w i t h 6 M r a d at25 C i n the presence of the food-si mu l at i ng solvents.

F o l l o w i n g the same pr ocedur es descr ibed i n the above-ment ionedstudy, ad di t i ona l extra ctive data wer e obta in ed for the epoxy phenol i cenam el tha t was i r r adi ated at 4.7-7.1 M r a d at 25 and 30C i n thepresence of dist i l led wa ter, 3% acetic aci d, an d n-heptane. T h e changesi n the amount of extractives resul t i ng fr om th e i r rad ia t ion treatment ares h ow n i n T a b l e I X . I n t he case of the water and acetic acid extractives,there wa s no chan ge i n either the chl oroform -solu bl e fr actions or thechloroform - insoluble fract ions. I n the case of the n-heptane extractives,the amount of extractives decreased when the i r rad ia t ion temperaturewas r educed from +25 to 30 C . I nfr ar ed spectra of the chl oroform -soluble residues from the water and acetic acid extractives of the uni r r ad i a ted an d i r r adi at ed enam el were id ent i ca l to the ch loroform -solubleresidues fr om the solvent blan ks. I n other word s, the epoxy phenol i c

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3. K I L L O R A N E T A L . T i n p l a t e C o n t a i n e r s 33enamel had no chloroform-soluble residue from the water and acet ic acidextract ives tha t coul d be attr ibu ted to the enamel. T h e n-heptan e-solu bleresidue of the ir r ad ia ted enamel was i dent i cal to th e residu e foun d i nthe un ir r adi ated enam el , being a l ow molecul ar w eight res idue of theparent enamel.

Production Test. I n a smal l -sca le pr oduct i on test, t inpl ate containersw i t h two commer cia l l y ava i l abl e enamels an d two end-seal i ng compoun ds, wh i ch wer e selected fr om the results of thi s stu dy, p erfor medsat is factor i ly when packed w i t h beef and ham . B eef was i r ra d ia ted w i t h4.5-5.6 M r a d at 5, -3 0 , and - 9 0 C ; ham was i r r ad ia ted w i t h 3-4 M r a dan d 6-7.5 M r a d at 30 C . F or this pr oduct ion test , beef an d ha m werepacka ged i n r oun d t inpl ate containers an d ha m in P u l l m a n t inplate containers, fr ozen a n d r efri gera ted pr oducts wer e sh i pped 1,200 mi l es byt ruck , and were gamma i r ra d ia ted at var ious doses and temperatures.I r r ad i a te d products were sh ip ped 1,200 m il es i n a non-r efr igera ted tr uckand stored at selected tempera tur es a nd hu mi di t i es. T h e in tegr it y of thecans was evaluated after storage for 10 days, 3 months, and 6 months.

T able I X. C hange in Amount of E xtractives After I rr adiationwith 4.7-7.1 MradC h a n g e n E x t r a c t i v es, mg/ cm 2

Sol vent 25 -30W a t er

C H C 1 3 soluble 0.00 0.00C H C I 3 i nsolu bl e 0.00 0.00A cet ic acidC H C I 3 solu ble 0.00 0.00C H C I 3 i nsolu bl e 0.00 0.00n - H ep t a n e 0.002 0.001

T h e pr oduct i on test showed th at the epoxy phenolic enamel was thepr efer r ed enam el for coat in g t in pl ate contai ners used in pack agi ngi r rad ia t ion-s ter i l i zed ham and beef. T h e preferr ed end-seal i ng compoun dfor the same appl icat ion was the b len d of cu r ed and un cur ed i sobutyl ene-isoprene copolymer.C o n c l u s i o n s

T h e eval ua t ion of the components of the t i npl ate conta in er showedthat the preferred enamel for i r r ad i a t i on processing was the epoxyph enoli c; the pr eferr ed end-seal in g compoun d was the bl end of cur edand u ncu r ed isobuty len e-isopr ene copolymer. C omponent test in g of t i n -plate and solder for possible changes i n mechan ica l propert i es, mi cro-structure, and corrosion resistance indicated that the radiat ion caused

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34 CHEMISTRY OF FOOD PACKAGINGno tin rot, .e.,conversion of tin from the beta, or silvery fo

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