Metal Packaging Materials PDF

Metal packaging Metallic elements: – Compact chemical structure (almost no diffusion phenomenon can take place in normal condition through a metal). – Good malleability (possible to obtain any solid shape). – High thermal conductivity. – High electric conductivity....

Metal packaging Metallic elements: – Compact chemical structure (almost no diffusion phenomenon can take place in normal condition through a metal). – Good malleability (possible to obtain any solid shape). – High thermal conductivity. – High electric conductivity. Metal alloys used as food packaging materials: – Aluminum and steels (iron alloys) mostly coated by tin, chromium oxides, and/or varnishes in special cases. – Stainless steel: food contact purposes (containers, vessels, kitchen utensils, pipelines). – Copper and cast iron: occasionally for food contact purposes (dairy boilers, grill tools). – Nickel: sandwich layer between steel and tin in low-tin coating steel (LTS). – Lead: soldering. – Copper: welding in side seam of a 3-piece can. Main metallic materials for food packaging: aluminum, stainless steel, tinplate and tin-free steel.  Metallic bonds are responsible for:  the strength  malleability (the ability to be hammered into various shapes)  ductility (the ability to be drawn out into thin wires) of metals  change material shape without cracking or breaking; e.g. aluminum sheet rolled into a very thin aluminum foil; aluminum disk hammered into soda cans. Although strong, metallic bonds are not bound to specific metal ions  forces from hammering and stretching can cause layers of metal ions to slide over each another  the movement attracts the free electrons to their new locations, while the sea of electrons is not broken and still holds the metal ions together.  Metallic bonds are responsible for high electrical and thermal conductivities of metals. – High electrical conductivity: due to the ability of the delocalized electrons to flow freely in the lattice of positive metal ions when a voltage is applied. – High thermal conductivity: due to the ability of metal ions to absorb heat energy by vibrating faster  collision with neighboring ions  transfer of heat energy.  Metal packaging Distinct advantages over glass: – Good heat transmission. – Not subject to thermal shocks  rapid heating and cooling are possible. – Lighter in weight. – Not subject to breaking. – Little or no interaction between the food and can only occurs provided the “correct” type of can is selected. – Resistant to physical damage. – Totally impervious to light and air. Main disadvantage: contents cannot be seen. Container designs  Raw materials Steel – A large family of iron alloys, with a low content of carbon (0.2-2%), widely used because they are hard, strong, durable and easy to shape. – Carbon: binding agent in iron alloy. The higher the carbon content of the steel, the higher its tensile strength, but also the higher its brittleness Commonly used: low-carbon steel, which is initially produced as blackplate  converted into tinplate or tin-free steel (TFS) for container and closure manufacture. – Cheap and strong materials, but the chemical inertness is never sufficient  the surface is commonly covered/layered to increase the chemical resistance and to avoid unwanted interactions with foods. Coating may be inorganic (other metals or metal oxides) or organic (varnishes and resins). Most important so far: tinplate.  STEEL Raw materials – Tinplate Coating blackplate (low carbon steel) with a thin layer of tin to provide good corrosion-resisting properties to steel and it is suitable for direct contact with many food products. – However, for most foods and drinks, it is still a necessary to apply an organic coating to the inside surface of the tinplate container to provide an inert barrier between the metal and the product packed  to prevent chemical reactions between the product and the container and to prevent taint or staining of the product by direct contact with the metal. The tin layer also provides good electrical current flow during welding process (i.e. fastening of two pieces of metal together by softening with heat and applying pressure). Being a very soft metal, the tin layer also acts as a solid lubricant during the wall ironing process of forming 2-piece thin wall cans.  Steel – Tin free steels Tin is expensive  encourages development of steel plates protected with a different passivation film but leading to the same performance in can making and food protection. So far, the best choice has been the cold-rolled plate coated eletrolytically with chromium and chromium oxide (= ECCS, electrolytic chromium/chrome oxide coated steel). TFS has good functionality in heat resistance, lacquer coating adhesion and printing. However, it is less resistant than tinplate to corrosion in acidic environment, whereas it is more resistant at neutral or alkaline pH  TFS is always used in the forms coated with organic lacquer. Principal uses of ECCS: food can ends, crown caps, vacuum closures for glass preserve jars, and deep-drawn cans. Welding can not be performed on ECCS, since chromium hydroxyde has low electrical conductivity Side seam of TFS: cementing or adhesive bonding, or the chromium layer is removed if welding has to be applied.  Raw materials Steel – Polymer coated steels Plastic lacquer coating applied onto tinplate and TFS plate. Plastics: thermoplastic polymers. Benefits: excellent appearance, abrasion resistance, corrosion resistance, high moisture barrier. Difference polymers and thicknesses can be applied simultaneously. Example: TFS laminated with PET for manufacturing 2-piece and 3- piece cans https://www.youtube.com/watch?v=X1pB6O6AYMU&t=120s  Raw materials Steel – Stainless steel Iron alloys with better corrosion resistance than any other steels since they contain a large amount of chromium (minimum 11%)  spontaneous auto-passivation: chromium reacts with oxygen producing a molecular layer of chromium oxide  protecting the alloy from corrosion. Have equal composition both on the surface and in the mass  do not need any further protective coating. 3 basic groups of stainless steel: – Austenitic: contains 18% chromium and 8% nickel (widely known as 18-8); most important in food applications; nonmagnetic in annealed condition (annealing: hardening by heat treatment); can only be hardened by cold working. – Ferritic: small amounts of nickel and either 17% or 12% chromium plus other elements, e.g. aluminum or titanium; always magnetic; can be hardened only by cold working. – Martensitic: 12% chromium and no nickel; magnetic; can be hardened by heat treatment.  Raw materials Steel – Stainless steel Expensive  use mostly for processing plants, large storage containers, kitchen tools, and returnable container for beverages. Characteristics: high resilience, good thermal conductivity, ease of welding and casting ,hygienic (for standard cleaning processes: higher bacterial removing capacity and lower bacterial retention on the surface). – Difficulty of biofilm formation on the glossy surface of the material. – High resistance to mechanical, thermal and chemical applications  allowing stainless steel equipments to be cleaned with high temperature and concentrated detergents.  Raw materials ALUMINUM – Most abundant metal in earth’s crust (8.1%). – Light metal packaging. – Expensive material (high energy requirement for aluminum production). – Has a high negative value in the electrochemical series  high thermodynamic tendency to ionize  spontaneous reaction with oxygen  formation of a thin passivated film (1-5 nm)  slight protection to the corrosion phenomena. Practically, industries will do a chemical or electrochemical passivation (anodization) which increases the aluminum oxide thickness (50-200 nm) and makes it more regular  continued by a heat treatment that reduces greatly (but not eliminate completely) the porosity of the oxide film  a very thin ceramic layer onto a metallic surface. – The oxide layer on the surface of aluminum is not a complete protection since it is removable both at low (< 4) and high (> 8) pH values and is a porous coating permeable to many ions  the aluminum surface is usually protected further by a lacquer adhesion or a polymer coating.  Raw materials ALUMINUM – Thermal properties: At high temperatures (200-2500C), aluminum alloys tend to lose some strength. At negative temperatures: increased strength with maintained ductility (i.e. the malleability of something that can be drawn into threads, wires or hammered into thin sheets)  does not become fragile even at ultra freezing temperatures. At reduced pressure, it is possible to evaporate pure aluminum  condense onto a flexible surface (plastic film, paper sheet) as a very thin layer of just some hundreds of nm. – Advantages: High mechanical resistance Extraordinary malleability (the property of something that can be worked, hammered or shaped without breaking). – It is possible to reduce the thickness of aluminum foil to 3-6 µm) – It can be cast in any forms, rolled, roll-formed, stamped, drawn, spun, forged, and extruded into a variety of shapes High purity of aluminum alloys: easy and convenient recycling (uses only 5% of the energy used in primary production).  Raw materials ALUMINUM – Drawbacks: High price. Difficulties of welding. – Cannot be welded by can-making systems and can only be used for seamless (two-piece) containers. Poor chemical resistance. (prone to oxidize & corrode in its pure state) – Much appreciated for food packaging due to its flexible, rigid, and semi rigid applications. Pure aluminum: foils, deep drawn containers, impact-extruded cans or tubes. 2 most common alloying metals: manganese and magnesium – Manganese: increases the corrosion resistance slightly. – Magnesium: increases the strength, but reduces corrosion resistance against acid and alkali.  Raw materials Aluminum foil – The sheet of aluminum alloy that was reduced-rolled to thickness of 4-150 µm. Done by 1) rolling through heated rolls OR 2) cold rolling after continuous casting (less expensive)  annealing – Excellent barrier properties to water, gas and aroma. Absolute barrier if the thickness is above 25 µm. – Lightweight. – Resistant to most foods, except those with high acid and/or salt. – It can withstand high and cold temperature condition. – It has dead-fold property  need cautions handling to prevent pinhole formation. – Typical usages: cooking foil, laminated film structure, semi-rigid trays. For flexible packaging materials: the foil is laminated onto plastic film or paper to provide barrier against light, oxygen and moisture  used in retortable pouch bottle closure liner, composite can. For rigid containers: thick foil is formed into dishes or cups  used for packaging convenience foods, cakes, frozen foods, chilled foods in catering, institutional cooking, take-away hot meals.  Raw materials Recycled packaging metal – Both aluminum and steel-based packaging materials are able to be re-melted. – No loss of quality during the re-melting process  may be reused in unlimited number of times.  Can-making process Three-piece welded cans. – Consists of body blank and 2 ends. – Only constructed from steel (aluminum is not suitable for welding).  Can-making process Two-piece single drawn and multiple drawn (DRD) cans – Materials: pre-coated, laminated and printed tinplate or TFS.  Can-making process Two-piece drawn and wall ironed (DWI) cans. – Materials: uncoated tinplate (aluminum does not have sufficient strength to withstand the heat process cycle).  End-making process Plain food can ends and shells for food/drink easy-open ends.  End-making process Conversion of end shells into easy-open ends.  Coatings, film laminates, inks Organic materials are used to provide barrier or decorative coatings to metal containers and closures (liquid-applied coatings, inks, film laminates, or hot-extruded polymer). – 3-piece cans, 2-piece drawn containers and can ends: the metal is coated and printed while it is flat, in coil, or sheet form, prior to the can or end forming operations. – 2-piece drawn and wall-ironed containers: all coating and decoration is carried out after the can body has been formed. – Coating of metal coil or sheet is done by roller-coating. – Metal printing may be made onto flat sheet or circular cans, as appropriate.  Processing of food and drinks in metal packages Can reception at the packer.  Processing of food and drinks in metal packages Filling and exhausting – Filling should be carried out accurately and consistently. – No damage to the can. – Avoid external contamination. – Reasonable headspace  affects final vacuum level (residual oxygen affects internal corrosion and product quality) and internal pressure during heat processing and cooling. To achieve vacuum condition, cans may be: A. Hot-filled with product and closed with or without direct steam injection into the headspace during double seaming (steam flow closure). B. Filled with product (ambient or hot-fill) and then exhausted (with or without end clinched on) prior to double seaming (with or without steam flow close). » Exhausting removes entrapped air from the product, raise initial temperature of the product prior to heat processing (thereby shortening process time/cooking effect) and ensure good vacuum level is achieved. C. Closed in a vacuum chamber.  Processing of food and drinks in metal packages Seaming – Most important: double seam – Formed in two operations from the curl of the can end and the flange of the body. – CCP for heat-processed foods. – Seam parameters (in UK): Seam thickness Seam height (length) Overlap Free Space Body hook butting % wrinkle or tightness rating  not measured from the seam section but by visual inspection of the removed cover hook.  Processing of food and drinks in metal packages Heat processing – Sterilization: typically 115 – 1350C. – Pasteurization: typically 90 – 1050C. – Development of internal pressure inside the container during heat processing may cause distortion of the container or opening of easy-open scores on can ends  most dangerous: maximum pressure differential between the process medium and the can interior, e.g. during the change from heating to cooling. Failure to control the air pressure can cause significant damage to cans, e.g. peaking (distention) or panelling (collapse).  Processing of food and drinks in metal packages Post-process: can cooling, drying and labeling – Cooling water: risk of microbial contamination, salt residues- cause rusting. – Cans are not supposed to be handled whilst still wet and hot  risk of micro-suction of pathogenic microorganism, e.g. C. botulinum spores into the can (through the seam). – Drying: important to minimize microbial recontamination (for sterilized food container after cooling) and prevent external container corrosion during storage (for both food and beverage cans). – Use the correct label paper quality and recommended adhesive for labeling.  Processing of food and drinks in metal packages Container handling – Any visual defect should be regarded as significant  affects consumer’s decision. – Impacts during handling after heat processing, esp. on seam areas, may allow microbiological recontamination of can contents. Guide-rails on conveyor system to avoid contact with sensitive seam areas. Necked-in designs for cans to reduce seam-to-seam impacts.  Processing of food and drinks in metal packages Storage and distribution – Air temperature and air humidity are important. Improper combination can lead to condensation which can further lead to corrosion. – Salt residues can lead to rusting and possible perforation of double seams. – Freezing of filled cans should be avoided to prevent deformation of cans due to ice expansion. – Absence of water  to minimize external corrosion and ingress of bacteria through the can seams. – Minimum mechanical damage. – Removal of damaged containers. – Care handling.  Shelf life of canned foods Canning: hermetic sealing of foods inside a metallic container followed by the sterilization or pasteurization of the food by heat treatment  Preservatives are basically not necessary. Possible chemical reactions that continue to take place inside the can: breakdown of color, flavor, etc., and also possible interaction of food with the container. Shelf life: – Minimum durability: the period of time under normal storage conditions during which a product will remain fully marketable and will retain any specific qualities (beyond this point, the food may still be satisfactory for consumption). – Technical shelf life: the period of time under normal storage conditions after which the product will not be fit to eat. 3 main factors that affect the shelf life of canned foods: – Sensory quality, including color, flavor (plus taints) and texture. – Nutritional stability. – Interactions with the container. Electrochemical corrosion of iron. Corrosion often begins at a location (1) where the metal is under stress (at a bend or weld) or is isolated from the air (where two pieces of metal are joined or under a loosely-adhering paint film.) The metal ions dissolve in the moisture film and the electrons migrate to another location (2) where they are taken up by a depolarizer. Oxygen is the most common depolarizer; the resulting hydroxide ions react with the Fe2+ to form the mixture of hydrous iron oxides known as rust.  Shelf life of canned foods Interaction between the can & its contents: – Most common: CORROSION Etching, pitting corrosion, staining of the surface – Use of internal lacquers (only tinplate has some resistance to corrosion to acids in food without lacquer). Even tinplate must be lacquered when it is used to pack aggressive products, e.g. tomato puree, or where there is danger of pitting corrosion or surface staining, e.g. in meat products. The role of tin: – It protects the steel base-plate Without tin, the exposed iron would be attacked by the product and it could cause discoloration and off-flavors in the product and swelling of the cans; the iron could even be perforated and the can would lose the integrity. – It provides a chemically reducing environment  any oxygen left in the can at the time of sealing will rapidly consumed by the dissolution of tin  Minimizes product oxidation and prevents color loss and flavor loss. Shelf life of canned foods  Shelf life of canned foods The dissolution of tin from the can surface – Tin in canned food is derived from the tin coating which dissolved into the product during storage  factors affecting the dissolution rate: Time and temperature Exposure of the tinplate Tin-coating weight Type and composition of the product, e.g. acidity. Presence of certain ions, e.g. nitrates.  Shelf life of canned foods Tin toxicity – High concentration of tin in food irritates the GI tract and may cause stomach upsets in some individuals, with symptoms e.g. nausea, vomiting, diarrhea, abdominal cramps, abdominal bloating, fever and headache  short-term symptoms with recovery expected soon after exposure. – Legal limit: 200 mg kg-1 of tin in food products (EU) – Increased risk of effects: > 250 mg kg-1 – Accelerating factors: heat, oxygen, nitrate, some chemical preservatives and dyes, and certain particularly aggressive food types (e.g. celery, rhubarb). – Effective method: high vacuum level  Shelf life of canned foods Iron – Dissolution of iron in tinplate and tin-free steel containers. – High levels of iron in food will make it unpalatable  flavor changes and/or color changes  end of shelf life. – High iron corrosion usually only occurs toward the end of tin corrosion. Lead – Used to be major problem with older, soldered cans. Aluminum – Aluminum dissolution is generally low in food products since all aluminum cans have very good lacquer system. – In low amount can affect sensitive products, e.g. beer (causing cloudiness or haze). Lacquers – Increasing use of lacquers since it effectively limits dissolution of tin into the product. Shelf life of canned foods  Internal corrosion Caused by mechanical damage on the cans or a manufacturing fault or an unusually aggressive reaction between the can and its content. – E.g.: poor handling  denting  cracking of internal lacquer  corrosion. Causes: – Insufficient lacquers or ineffective lacquer. – Formation of embossed codes on can ends  cracking of the lacquer  corrosion  Stress corrosion cracking The acceleration of corrosion in certain environment when metals are externally stressed or contain internal tensile stresses due to cold working. Can occur in many metals. Very difficult to predict where attack will occur. Sometimes seen in steel cans in the beaded area of the body, where cracks occur in the metal.  Environmental stress-cracking corrosion of aluminum alloy beverage can ends Aluminum alloy, e.g. in easy-open end cans for drinks can react with moisture in the air  environmental stress cracking corrosion. Greatly accelerated by the presence of contaminants e.g. residual salts esp. chlorides and other halides. – Risky areas: at the pull-tab and stay-on-tab easy-open ends. Care handling after can filling  easy-open can ends are thoroughly washed with clean water and dried before put in store. – During storage, humidity condition needs to be controlled by provision of adequate ventilation.  Sulfur staining (Sulfide) Characteristics: blue-black or brown marks on the inside of the cans. Caused by the sulfur compounds from the protein in the product (e.g. in peas, sweet corn, fish or meat) reacting with the can metal. – Cosmetic problem  not harmful, does not normally lead to further corrosion BUT often results in consumer complaints. – Most obvious in the headspace (with the presence of residual oxygen). Sulfur from protein product + residual oxygen (headspace) + iron (from steel exposure) = complex of iron sulphides, oxides & hydroxides. Suitable lacquers: grey in color, contain zinc or aluminum  will react with the sulfur compounds producing white metal sulfides  harmless and not readily visible  but NOT suitable for acid products (acid may attack the coating to produce zinc or aluminum salts  harmful to health) and for solid meat or fish product (the product may stick to the lacquer during heat sterilization).  External corrosion Rusting, often occurs at specific points, e.g. the end seams or the score lines on easy-open ends. Requires the presence of metal, oxygen and moisture. Possible causes: – Condensation due to temperature fluctuations, humidity changes, draughts, poor stacking. – Label with high chloride or sulfate. – Incomplete drying. – Hygroscopic deposits. – Low external tin coating weight. – Physical damage. – Rusty retorts. – Leaking product from neighboring cans.