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Published on June 1, 1974 on http://pubs.acs.org doi: .comwww.k8449r.weebly.comChemistry of Food Packaging

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.fw001

Chemistry of Food PackagingC h a r l e s M. S w a l m ,Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.fw001EditorA symposium sponsored bythe Division of Agriculturaland Food Chemistry at the166th Meeting of theAmerican Chemical Society,Chicago, Ill., Aug. 30, LD.C.SERIESSOCIETY1974135

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.fw001Library of Congress CIP DataChemistry of food packaging.(Advances in chemistry series, 135)"A symposium sponsored by the Division of Agriculturaland Food Chemistry at the 166th Meeting of the AmericanChemical Society, Chicago, Ill., Aug. 30, 1973."Includes bibliographical references.1. Food—Packaging—Congresses.I. Swalm, Charles M., 1918ed. II. AmericanChemical Society. III. American Chemical Society. Di vision of Agricultural and Food Chemistry. IV. Series.QD1.A355 no. 135 [TP374] 540'.8s [664'.09] 74-17150ISBN 0-8412-0205-2ADCSAJ 135 1-109 (1974)Copyright 1974American Chemical SocietyAll Rights ReservedPRINTED IN THE UNITED STATES OF AMERICA

Advances in Chemistry SeriesPublished on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.fw001R o b e r t F . G o u l d , EditorAdvisory BoardKenneth B. BischoffBernard D. BlausteinEllis K. FieldsEdith M. FlanigenJesse C. H . HwaPhillip C. KearneyEgon MatijevićThomas J. MurphyRobert W. Parry

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.fw001FOREWORDA D V A N C E S I N C H E M I S T R Y SERIES wasfounded i n 1 9 4 9 bytheAmerican Chemical Society as an outlet for symposia andcollections of data i n special areas of topical interest that couldnot be accommodated i n the Society's journals. It provides amedium for symposia that would otherwise be fragmented,their papers distributed among several journals or not published at all. Papers are refereed critically according to A C Seditorial standards and receive the careful attention and processing characteristic of A C S publications. Papers publishedin A D V A N C E S I N C H E M I S T R Y SERIES are original contributionsnot published elsewhere i n whole or major part and includereports of research as well as reviews since symposia mayembrace both types of presentation.

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.pr001PREFACEA s society develops and technology increases, researchers are modifyingold, accepted foods and introducing new products into the moderndiet. Their years of constant research have brought us from the colonialways of "eat off the vine" to the present use of preprocessed, modified,and synthetic foods.Food packaging has similarly undergone radical changes. As theplace of production grows farther from the urban centers where mostof the food is consumed, the demands on food containers are greatlyincreased. Society used to be content to deal with natural food packaging,but now food must be shipped over long distances and stored for undetermined time periods and under often uncertain conditions.The technologies of glass, tin, and aluminum containers are beingimproved, and the field of polymer containers is rapidly expanding. N o wa food container may be rigid or flexible and may be made up of manycombinations of films, layers, and coatings. It must be compatible withthe food contained, must protect the product during processing, shipment,and storage, and must also satisfy marketing requirements for consumeracceptance.Camden, N . J.July 1974CHARLES M . SWALMix

1Trends in the Design of Food ContainersR. E . B E E S E and R. J . L U D W I G S E NPublished on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001Material Sciences, Research, and Development, American Can Co.,Barrington, Ill.Designs of metal cans for foods are continuously modifiedto reduce cost and to improve container integrity and quality. Advances in container materials include steel-makingprocesses for tin mill products, corrosion resistant ETP (electrolytic tinplate) for mildly acid foods, tin free steel tinmill products, and new organic coatings. Recent trendsin container construction result from antipollution legislation, new can-making technology, and public safety considerations. Water base and UV cured organic coatingsreduce pollution. Drawing, draw and ironing, cementing,and welding provide alternate methods for making cans andpotentially upgrade container performance.Full insidesolder fillets in soldered sanitary cans also improve containerintegrity and thus contribute to public safety. The solderedsanitary can remains an important factor in preserving foods.Although soldered tinplate cans now dominate the processed food i n dustry ( J ) , changes are being made. The canning industry andcontainer manufacturers are responding to increased social and economicpressures to change the traditional methods. Some of the most importantof these pressures are the efforts to protect the environment and theconcern over the public health aspects of canned foods. Renewed attention is being given to improved container integrity and safe canningpractices. N e w can-making materials and manufacturing techniques arecontributing to the solution of these problems. Recent changes i n container construction permit the use of lower gage steel, lower tin coatingweights with improved corrosion resistance, beading of can bodies, andincreased use of inside organic coatings, which have all helped to minimize the cost of the tin can without reducing container quality orintegrity.1

2CHEMISTRY OF FOOD PACKAGINGThe technical aspects influencing these changes are reviewed i n thispaper. Discussion of these trends is limited to the steel-based materials.The current demand for easy-open ends for food containers has led tothe development of many scored easy-open ends. This is a subject i nitself and is not included i n this discussion.Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001MaterialsTo appreciate the potential changes i n food cans, it is necessary todescribe briefly the steel-based materials used i n modern can manufacturing operations. The tin can is made from a special grade of thin gage,low carbon, cold-rolled steel, which is generally referred to as a tin millproduct. The base steel is coated with either tin, a chromium-chromiumoxide system, or it is just cleaned and oiled. It may also be coated withorganic coatings.Electrolytic Tinplate. M u c h of the tin m i l l product is made intoelectrolytic tinplate ( E T P ) . A schematic of an E T P cross section isgiven i n Figure 1. The steel strip is cleaned electrolytically in an alkalinebath to remove rolling lubricants and dirt, pickled i n dilute mineral acid,usually with electric current applied to remove oxides, and plated withtin. It is then passed through a melting tower to melt and reflow the tincoating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, andlubricated with a thin layer of synthetic oil.The tin coating on E T P can be purchased in 10 thickness ranges.Differentially coated plate, such as # 1 0 0 / 2 5 E T P , is coated with 60 X10" inch of tin on the #100 side and 15 X 10" inch of tin on the otherside. The use of differentially coated E T P has markedly reduced therequirement for tin metal (2).In plain tinplate cans for acid foods, tin provides cathodic protectionto steel (3,4). The slow dissolution of tin prevents steel corrosion. M a n yinvestigators (5-11) have defined this mechanism in detail and haveshown that the tin dissolution rate is a function of the cathodic activityof the base steel, the steel area exposed through the tin and the t i n iron alloy layers, and the stannous ion concentration. K a m m et al. showedthat control of the growth of the tin-iron alloy layer provides a nearlycontinuous tin-iron alloy layer and improves the corrosion resistance ofheavily coated (over 45 X 10" in. tin) E T P for mildly acid foodproducts i n which tin provides cathodic protection to steel (12).Thecontrolled tin-iron alloy layer reduces the area of steel exposed to theproduct. E T P with the controlled alloy is designated type K , and since1964, # 7 5 type K E T P has been used to provide the same protectionas #100 E T P provided previously (13).666

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch0011.BEESE AND LUDWIGSENFigure 1.3Design of Food ContainersSchematic cross section of 55 2CR tinplatetin coating)(#25Tinplate can be purchased in a wide range of tempers and thicknesses. Currently 17 different basis weights are available commercially,from 5 5 # / B B (pounds per base box or 217.78 ft ) to 1 3 5 # / B B . Theseweights range i n nominal thickness from 6.1 to 14.9 mils.The final thickness of the steel for tinplate is achieved by twoprocesses. For conventionally reduced or single reduced plate, steel isannealed after cold reduction to restore ductility. The annealed coilis then temper rolled with only 1-2% reduction to make the final adjustment to its tensile strength, hardness, and surface finish. To reduce thecost of the lighter basis weight plates, double reduced, or 2CR, platewas introduced (14). A second 3 0 - 4 0 % cold reduction is given steelafter the anneal, which imparts a significant amount of cold work. Thisprovides 2 C R plate with generally greater hardness and tensile strength,a loss i n ductility, and an increase i n directionality. A t first, these factors2

4CHEMISTRYO F FOOD PACKAGINGPublished on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001made 2 C R plate more difficult to fabricate into containers. However,modification in manufacturing procedures has enabled this lower cost basesteel to be used.The chemistry of the base steel is carefully regulated to control boththe physical properties and corrosion resistance ( I S ) . Recent changesin steel manufacture have generally benefited tinplate performance. Basicoxygen processes, which permit steel to be made at a faster rate, tendto produce low carbon steel with lower levels of residual elements. Ingeneral this is believed to improve corrosion resistance. However, i n onecase, there is a reduction in the corrosion resistance of steel for cola typecarbonated beverages when the residual sulfur concentration is loweredfrom 0.035 to 0.018% (16, 17). In lemon-lime beverages, however, thelower sulfur levels improve corrosion performance. It is the copper/sulfur ratio which determines the corrosion resistance of steel for carbonated beverages.The new continuous casting processes, in contrast to ingot cast products, provide tin mill products which are exceptionally clean and formable.The deoxidizing processes required for continuous casting involve eitheraluminum or silicon killing, which adds aluminum or silicon to the steel.Experience with type D steels indicates that the added aluminum w i l lnot cause a corrosion problem. Laubscher and Weyandt (18) haveshown that the silicon found in silicon killed, continuous cast, heavilycoated E T P w i l l not adversely affect the corrosion performance of plaincans packed with mildly acid food products in which tin usually protectssteel. The data on enameled cans is not definitive. Additional publisheddata are required to determine whether or not silicon actually reducesthe performance of enameled cans made from enameled, heavily coated,silicon killed, continuous cast E T P .Tin Free Steel—Electrolytic Chromium-Coated. A less expensivesubstitute for tinplate, electrolytic chromium coated-steel, has been developed and is designated T F S - C T (tin free steel-chromium type) orT F S - C C O (tin free steel-chromium-chromium oxide) (19). This material can be used for many products where the cathodic protectionusually supplied by tin is not needed. A schematic cross section is shownin Figure 2. Electrolytic, chromium-coated steel is made by electrolytically depositing a thin layer of metallic chromium on the basic tinmill steel, which is in turn covered by a thin passive coherent layer ofchromium oxide.Organic coatings adhere to the electrolytic chromium-coated steelsurface exceptionally well. The surface is stable and does not discolorduring baking of enamels. It is resistant to staining from products containing high levels of sulfide, such as meat, fish, and some vegetables.

1.BEESE ANDLUDWIGSENDesign of Food Containers5Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001It resists pinpoint rust formation before enameling and filiform corrosionafter enameling.T F S - C T or T F S - C C O is a primary material for cemented andwelded beer and carbonated beverage containers (20-22) and can beused in sanitary food cans. It is currently used for ends on solderedsanitary food cans and is a candidate for drawn containers which donot require soldering.Figure 2.Schematic cross section of tin free steel (TFS-CT)Tin Free Steel—Can-Maker's Quality. C M Q (can-makers quality)steel is the basic tin mill product. C M Q can be either single or doublereduced steel. Rolling oils are removed, and the surface may or maynot be passivated. A schematic cross section of passivated C M Q is shownin Figure 3. Q A R (quality as rolled) 2 C R plate is the basic 2 C R tinmill product with the rolling oils on the surface. N o further treatmentis given. Figure 4 is a schematic cross section of Q A R plate.

6CHEMISTRYO F FOOD PACKAGINGPublished on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001C M Q is commonly referred to as black plate and has borderlinecorrosion resistance both before and after enameling. Handling andstorage before enameling must be carefully controlled to minimize pinpoint rust. Special organic coatings are required to control both internalcorrosion and external filiform corrosion. They are colored to cover thebrown appearance which forms during enamel baking. Undercuttingcorrosion resistance, as shown in Figure 5, is very poor. The chromatephosphate T F S is a heavily passivated C M Q which is currently not beingconsidered because of undercutting resistance and cost. Containers fordry products can be fabricated from C M Q .Figure 3. Schematic cross sectionof TFS-CMQ plateFigure 4. Schematic cross sectionof TFS-QAR plateQ A R is being evaluated for cemented or welded beverage cans.Special organic coatings and manufacturing techniques are required because of the high level of residual rolling oils. Reasonable success hasbeen achieved in making beer and beverage cans from Q A R plate.Organic Coatings. Organic coatings or lacquers protect the steel ortin from external or internal corrosion. The can interiors are coated toprevent undesirable reactions between the interior metal surface andthe product. These reactions involve: (1) corrosion of the tin coatingcaused by oxidants i n the product, (2) color or flavor loss by the productbecause of metal ion pickup, or (3) staining of the metal by sulfur-con-

1.BEESE A N D LUDWIGSENDesign of Food Containers7Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch001taining products. External inks and coatings are used to decorate thecan, to minimize rusting, and to improve mobility.Hundreds of coatings are available to the can maker. Coatings areselected for each use on the basis of which w i l l give adequate performance at lowest cost (23). Recent advances i n coating technology resultFigure5.Undercutting corrosion resistance ofenameled platefrom current application and usage requirements. F o r example, the newvended can for formulated food products requires enamel performancelevels which could not be met by the oleoresinous coatings commonlyused. The organosol and white sanitary coatings were developed tomeet this need. The pigmented white coatings not only provide a pleasingaesthetic effect but also conceal underfilm staining produced by someproducts. In addition to organosol white coatings, white coatings basedon epoxy-ester, acrylic, and polyester resins have been developed whichmeet sanitary food can requirements. Aluminum pigmented epoxyphenolics and organosols have also been developed to conceal underfilmstaining.The mechanism which permits can coatings to prevent metal corrosion or staining has not been elucidated completely. Container coatingsare only 0.2 or 0.3 mils thick, which is 1/10 to 1/100 as thick as conventional coatings used to protect tanks, pipes, or siding from atmosphericcorrosion. Some investigators believe organic coatings combat corrosionby physical barrier, chemical inhibition, and/or electrical effects (24).Can coatings permit water absorption and diffusion, transport of ionssuch as hydrogen and chloride, and gas diffusion (25). These diffusionmechanisms suggest that corrosion can occur through continuous container coatings. Thus it is reasonable to conclude that when coatingfailures occur, it is because of one or all of these mechanisms.TrendsThere are several areas to consider when discussing the future ofthe food can. Anti-pollution legislation, new can-making technologies,and public safety aspects w i l l have a pronounced effect on food containerdesign.

Published on June 1, 1974 on http://pubs.acs.org doi: 10.1021/ba-1974-0135.ch0018CHEMISTRYO F FOOD PACKAGINGAnti-Pollution Legislation. Anti-pollution legislation covers a broadarea of social responsibility. F o r the can manufacturer it ranges fromthe requirements of the Clean A i r Act of 1970 to reduce or eliminatecontaminants which pollute the air to the ban-the-can type legislationas enacted by the state of Oregon. The latter type of legislation is d i rected mainly at deterring roadside litter of beer and beverage containers.The Clean A i r Act has a significant effect upon the operations of industrialcoating users. The requirements to reduce or eliminate organic emissionsand noxious fumes from organic coating operations is of particular concern. As a result, the container coating industry is actively trying todevelop alternatives to solvent base coating systems.The following are the most prominent developments:1. Water-base coatings for spray and roller coat application2. Electrodeposition of water-base coatings3. Heat cured high solids coatings4. Radiation cured high solids coatings5. Electrostatic sprayed powder coatings6. Hot melt spray coatingsThese technical developments and their merits have recently been welldocumented by R. M . Brick (26). The potential effect of these developments with regard to the ordinary hot-filled or steam-sterilized sanitaryfood container w i l l take some time to discover because the use of organiccoating materials on the inside of food containers is controlled by theFood and D r u g Administration. Their guidelines restrict the compositional structure of coating materials and limit the amount of organicmaterial which can be extracted from the coating by a food product (27).In general, container coatings for steam-sterilized food products mustwithstand the most stringent tests as described i n these regulations.Since most of the alternative coating systems above utilize polymermaterials or adjuvants which are not acceptable food contact materials,a change i n coating systems for the inside of food cans w i l l probablybe slow because of the testing required. Thus, the can maker w i l l initiallyuse incineration or adsorption of solvent emissions to comply with thepollution regulations. However, high solids, heat-cured sanitary, andC-enamels are now being evaluated to eliminate the need for pollutioncontrol. They have essentially the same chemical composition

Chemistry of food packaging. (Advances in chemistry series, 135) "A symposium sponsored by the Division of Agricultural and Food Chemistry at the 166th Meeting of the American Chemical Society, Chicago, Ill., Aug. 30, 1973." Includes bibliographical references. 1. Food—Packaging—Congresses. I. Swalm, Charles M., 1918- ed. II. American

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