Metal Packaging Materials PDF

METAL PACKAGING MATERIALS FST 648 FOOD PACKAGING TECHNOLOGY OUTLINE Introduction Advantages & Disadvantages of metal container Steel manufacturing Can manufacturing Fabrications of can Corrosion of metal can OUTCOMES Able to describe...

METAL PACKAGING MATERIALS FST 648 FOOD PACKAGING TECHNOLOGY OUTLINE Introduction Advantages & Disadvantages of metal container Steel manufacturing Can manufacturing Fabrications of can Corrosion of metal can OUTCOMES Able to describe steel and 1 can manufacturing process Able to differentiate between 2 steel and can materials Able to explain the 3 fabrication process of can Able to discuss the corrosion 4 of metal can INTRODUCTION In 1804, French confectioner Nicholas Appert discovered method of “conserving all kinds of food substances in containers” He successfully produced preserved meat for French navy by partially cooking it, sealed in glass bottle with wired cork stopper & immersing the bottle in boiling water for several hours Awarded 12,000 francs ex-gratia payment on the condition that he published the details of his work In 1810 - French inventor, Phillipe de Girard convinced an Englishman, Peter Durand to patent the process by substituting the glass jars & bottles with tin cases Granted a patent from King George III for idea of preserving food in “vessels of glass, pottery, tin or other metals or fit materials”  METAL CONTAINER ADVANTAGES Flexible in processing – DISADVANTAGES can be adapted for different filling and sterilisation techniques Shipping of empty can take up a lot of space Excellent sealing properties - can be vacuum and hermetically seal, Canned products takes up more shelf- double seaming – very reliable and intact space during storage/ warehouse and retail Canned products have long shelf life Metal cans in storage must be protected High rate production – from moisture and/or humidity (if not in can manufacturing and canning operation lacquered or coated) Corrosion resistance surface of bright Steel (main component in tin plate cans) appearance (tin) is not easily recycled Coating adheres sufficiently well to the steel- Cans are only produced in certain sizes base – able to withstand any degree of and shapes deformation without flaking Considered old fashioned by the modern consumer MANUFACTURING OF STEEL STEEL-BASED METAL PACKAGING MATERIALS MATERIALS FOR IRON MAKING Iron Ores rocks that contains iron : hematite (Fe2O3) and magnetite (Fe3O4) with iron contents of 70% and 72% respectively. 2-6% silica, 1-3% alumina, phosphorus, sulfur etc. Coke (Fuel) Process involves carbonization of coal at high temperature (1100°C) for 18 hrs in O2 deficient atm Fluxes (limestone & dolomite) to collect impurities during iron and steel making causes chemical reaction in which unwanted elements are combine to form slag (by-product)  IRON MAKING Process of converting iron ore into molten iron, which is the primary ingredient for steel. BLAST FURNACE  PIG IRON Consist of: - Iron (≥ 90%) - Carbon (3.5 to 5%) - Others: manganese (~2.5%), sulfur (0.08%), phosphorus (1%) & silicon (0.3-1.0%) ­Steel stronger with carbon content: 0.5 to 1.5%. To eliminate most of carbon - create steel: Melt using BOF (Basic Oxygen Furnace) or EAF (Electric Arc Furnace) STEEL MAKING REFNING PROCESS Further refining process after producing molten iron or sponge iron to produce steel. Involve 2 process: Basic Oxygen Furnace (BOF) Electric Arc Furnace (AEF) BASIC OXYGEN FURNACE (BOF) BOF vessel is tilted – filled with pig iron and steel scraps. Vessel is then stood upright – lance is lowered down, blows 99% pure O2 causing Temp rise to ~1700°C – melts iron - lower Carbon content and helps remove unwanted elements. Fluxes are fed into vessel to form slag. BOF vessel is tilted – molten steel is poured into giant ladle. Slag is poured off.  ELECTRIC ARC FURNACE (AEF) ­ Used to lower cost and conserves raw materials (iron ore, coke and fluxes). ­ Used more extensively in steel making because it can handle large capacity and high production efficiency. 1. Furnace fill with ~86% scrap steel & 14% iron 2. Electrodes lowered close to the surface of metal 3. Electrical current supplied through electrodes 4. Heat generated produce an arc of electricity (~lightning bolt) 5. Temperature raises to 1600°C - melt the metal 6. After ~80 mins, molten steel is transferred into ladle STEEL MAKING CASTING PROCESS STEEL MAKING CASTING PROCESS Steel is relatively weak due to uneven metal crystals. Causes metal crystal to re- crystallise into finer structure – gives steel greater toughness, shock resistance and tensile strength. Steel slab (250 mm thick) is rolled through series of rollers that reduces the slab progressively to 2mm and increase its length from 4 to 370m. STEEL MAKING CASTING PROCESS Process of producing thin, smooth, and high-strength steel sheets or strips STEEL MAKING CASTING PROCESS Heat process: steel is heated to specific temperature (600–700°C) then allowed to cool slowly. Cause recrystallisation of elongated ferrite grains into new fine grains. Soften steel - easy to bend, cut and shape, increase ductility but decrease strength. STEEL MAKING ELECTROPLATING PROCESS Electroplating was done using dilute chromium oxide (CrO3) and sulfuric acid as electrolytes at temperature 50 to 70°C Steel-based plate is degreased, cleaned by light pickling and thoroughly washed to prepare the surface The electroplating process produces: Electrolytic tinplate (ETP) Tin is coated on the surface of steel Electrolytic chromium-coated steel (ECCS)/ Tin free streel (TFS) Chromium is coated on steel  ELECTROLYTIC TINPLATE (ETP) Involves flow melting and passivation processes. Flow melting process - heating the tin-plated steel at 260 to 270°C followed by rapid quenching in water. Passivation process is done using electrolytic treatment in sodium dichromate electrolyte (stable and resistant to the atmosphere conditions) CROSS SECTION PROPERTIES OF ECCS COMPARED TO ETP Much thinner than the lowest grade of ETP plate Base layer of Cr on ECCS acts as corrosion barrier whereby the superimposed layer of Cr oxide prevents rusting ECCS surface is more acceptable for protective enamel coating or printing ink and varnishes than ETP, and organic coatings adhere exceptionally well on ECCS Easier to fabricate (simpler process than ETP) Has good chemical and thermal resistance and resistance to greater internal pressure Has poor reflection ECCS plate cannot be soldered by welding or using organic adhesives Cannot used for acid products MANUFACTURING OF CAN INTRODUCTION Al is the earth’s most abundant metallic constituents ~ 8.8% of earth crust In nature, Al found as Al.oxide (Al2O3), diaspore (Al2O3.H2O), gibbsite (Al2O3.3H2O) or bauxite (impure form of gibbsite). - 2 kg bauxite produce 0.5 kg of Al metal Other constituents (impurities) in bauxite - small quantity - eg. silicon, iron, titanium, copper & zinc Total impurities content in commercial grade Al should not exceed 1%. For food and chemical industries: 99.5% Al purity is usually used.  ALUMINIUM CAN ADVANTAGES Basic material (bauxite): available in abundance. DISADVANTAGES Lightweight: provide economic advantage. Use high electricity: energy intensive Excellent properties: corrosion resistance, non-toxic, production. odourless, does not impart metallic taste to Cannot be sealed easily by existing seaming products and provide good barrier (moisture, machines & cannot be soldered - has to be gasses etc.) weldedto produce 3-pc can. Recyclable: easily and economically recycled into Softer than tin plate – must be handled with new products. care during packing and transportation. Versatile in terms of performance, aesthetic appeal Newly fabricated Al cans cannot be supplied & design, has modern, clean & bright image – good to food manufacturers in flattened condition – consumers perspective. require more space, high transportation cost. Strong but workable/flexible: retains strength under Has greater tendency to bleach some of the extreme cold without becoming brittle, can be highly pigmented products impact-extruded/Drawn & Ironed process. Not suitable for high acid or alkali foods - Compatible with variety of substances & capable need special coatings. of accepting protective coatings and decorative finishes. Easy open-ends & apertures can be prepared from Al. Conducts heat and electricity and reflects light.  ALUMINIUM MANUFACTURING PROCESS Stage 1: Bayer process – refining bauxite ore to obtain Al.oxide Stage 2: Hall-Heroult process – reduction of Al.oxide to release pure Al ­Pure Al Very soft therefore alloying agents are used. Functions: to impart strength, improve formability and corrosion characteristics  METAL CAN FABRICATION 2-piece can 3-piece can 3 - PIECES CAN Made out of ETP or ECCS materials Cover lid (Manufacturer’s End) Bottom (Can End) Body 3 - PIECES CAN Cover Lid Can end must be able to deform under internal & external pressure without becoming permanently distorted. Act like a diaphragm - expand during thermal processing and returning to concave profile when vacuum developed inside can upon cooling. 3 - PIECES CAN Can End Design for optimum deformation behaviour Depend on: - plate thickness - contour of the expansion rings - countersink depth Lining/Sealing/Gasket Compound: -consist of water or solvent dispersion of rubber placed in the curl of the can end (food grade compound). - to assist the formation of hermetic seal by filling the voids left after mechanical formation of the double seam. Functions: - To fill the void at the end of body hook (prime sealing area). - To fill the end hook wrinkles. - To prevent any seam areas having solely metal to metal contact. 3 - PIECES CAN Body The body part is produced by: Welded Side seam Soldered Side seam Welded Side seam Mostly used for food Material saving: overlap needed to produce weld uses less metal than an interlocked seam (soldered side seam) Stronger seam Easier to seam on the ends Greater surface area available for external decorating Requires enamel repair (internal surface): to prevent iron traces picked up by some types of beverages and acidic foods. Soldered Side seam Very few food cans have soldered side seams – due to health concern on lead migration into food from tin/lead (2:98) solder. 1970’s most countries insisted on using only pure tin solder for baby foods – increase cans cost significantly. USFDA (1995) prohibit use of tin/lead solder in food containers – replace by tin/silver (96:4) solder. DOUBLE SEAM Involves mechanically interlocking flanges/hooks of the body cylinder (can body) to the can end. Final quality of double seam defined by its length, thickness, extent of overlap of the end hook with body hook etc. Rigid standards - acceptable degree of overlap and seam tightness. DOUBLE SEAM STANDARD *Double seam components must be within the specified limit. DOUBLE SEAM OPERATION End curl gradually rolled inwards Seam is tightened (closed up) radially. by a shallower seaming roll. Flange well tucked up underneath body hook. form final contour according to profile of the seaming roll. DOUBLE SEAM DEFECTS 2 - PIECES CAN Constructed of ETP, ECCS or Al Seamless Easily formed and control Cover lid (Manufacturer’s End) Body + Bottom 2 - PIECES CAN Method Produce Drawn & Ironed (D&I) Drawn & Redrawn (DRD) 2 - PIECES CAN Method Produce PROTECTIVE COATINGS Internal Coating External Coating To prevent interactions between the To provide protection against can and its content (primary function) environment – prevent external corrosion To allow products to be easily removed from the can To improve appearance of the can – as decoration to give product identity Type of internal coating to be applied depend on: To prevent mechanical damage due - product to be canned to abrasion - expected shelf life Examples: acrylate oligomer, organic Examples: vinyl, acrylic, phenolic, pigment, acrylate monomer, wax epoxyphenolic REQUIREMENTS FOR INTERNAL COATING act as an inert barrier separation between container and its content not imparting any flavour to its content provide required chemical resistance must resist physical deformation during container fabrication enamel used must be flexible, spread evenly, completely cover and adhere to metal surface. METAL CORROSION CORROSION ?? Destruction of metal due to chemical or electrochemical reaction between metal and its environment. Metals are chemically reactive, readily oxidised by O2 and other agents to form corrosion compounds (rust). For food cans, corrosion is due to : a) reaction of contents with the container (internal) b) reaction of metal container with the environment (external). CORROSIVENESS OF FOODS  PROCESSING AND FOOD COMPOSITIONS STORAGE CONDITION CORROSIVENESS Acidity FACTOR pH Sulfur Oxygen compounds Thermal processing Nitrates Storage temperature Phosphates Plant pigments Synthetic colourings Copper  FOOD COMPOSITIONS ACIDITY No direct proportionality between product acidity and degree of corrosion Fruit juices that contained organic acids are more corrosive than pure solutions of organic acids itself Suggesting that fruit juices contain unidentified depolarisers that enhance corrosive action of organic acids No direct proportionality between pH and degree of corrosion of tinplate FOOD COMPOSITIONS SULFUR Sources of sulfur/sulfur compounds in canned foods: a) Spray residues from agricultural chemicals example: thio and dithiocarbamic acid fungicides derivatives b) Residues from sulfur-containing preservatives example: sodium metabisulfite c) Components of sulfur-containing compounds example: proteins: sulfur-containing amino acids FOOD COMPOSITIONS SULFUR Two types of sulfide staining: iron sulfide and tin sulfide. Do not constitute a health hazard or lead to failure of can – consumer rejection based on aesthetic grounds. Iron sulfide stain Tin sulfide stain Black colour. Blue-black or brown colour Mainly occur in headspace region during Widespread throughout can or immediately after heat processing Occurs during/immediately after heat Not formed at pH < 6 processing Less likely to formed at content contact Prevented by using sulfur-resistant portion of the can enamels added with Zn or Al compounds Overcome by using enameled cans Not suitable for acid products due to formation of Zn or Al salts which could be harmful to health FOOD COMPOSITIONS NITRATES Found in fruits and vegetables grown in heavily fertilised soils & in water polluted by fertiliser Responsible for serious economic and toxicological problems in some canned foods, notably tomato products FOOD COMPOSITIONS PHOSPHATES Naturally present in meat or added as polyphosphates to processed meat products to maintain moisture during processing, improve flavour and taste of meat Caused increased discolouration due to formation of iron phosphate and sulfide formation. FOOD COMPOSITIONS PLANT PIGMENTS Anthocyanins - among most important corrosion accelerator Able to form complexes with cations (iron & tin salts) Tannins – discolouration of canned cranberries due to formation of complex between tannins and tin salts FOOD COMPOSITIONS SYNTHETIC COLORING Presence of azo dyes together with residual O2 in filled soft drink cans – capable of acting as corrosion accelerators and potentially active corrosive agents Caused tin and iron dissolution leading to perforation of can body and flavour defects FOOD COMPOSITIONS COPPER Presence of dissolved copper in acidic products packed in metal containers – accelerate corrosion Source: - Food contact equipment using copper-bearing metal - Certain fungicides containing copper PROCESSING & STORAGE CONDITION OXYGEN Acts as depolariser, accelerate corrosion by reacting with H2 through cathodic reaction Source: Dissolved O2 in food products (fruit & vegetable cells) and residual O2 in headspace Remove O2 as much as possible - part of good canning practice. Using hot filling, vacuum filling, exhausting, closure under vacuum, and also control of headspace volume. PROCESSING & STORAGE CONDITION THERMAL PROCESSING Very small quantity of metal found dissolved during heat sterilisation process However, degradation products formed during thermal processing can involved in corrosion (eg. Nonezymic browning intermediates & sugar derivatives). PROCESSING & STORAGE CONDITION STORAGE TEMPERATURE Generally known: chemical reaction rate increases 2X for each 10°C increase in Temperature To minimize undesirable reactions such as nonenzymic browning in canned foods – preferably storage Temperature kept as low as practicable CORROSION OF ENAMELED CAN Enamel coating used to protect against excessive dissolution of tin, sulfide staining, local etching and change in colour of pigmented products. Will not guarantee prevention of corrosion – in some cases, may accelerate corrosion – important to select enamel based on properties of product. Performance of enameled food cans affected by thickness of enamel coating. Corrosion of enameled depends on: Quality of base steel plate Tin-iron alloy layer and tin coating Passivation layer Nature of enamel coating CORROSION OF ECCS CAN Consist of Cr/Cr oxide layer: - provide rust protection - outstanding enamel coating adhesion - Good resistance to underfilm sulfide staining ECCS alone cannot be used in food packaging - lacks resistance to corrosion All ECCS must be enameled CORROSION OF ALUMINIUM CAN Al. forms protective oxide film when exposed to air and water – completely passive in pH range 4 to 9 Undergo severe corrosion if in contact with copper, iron or other more positive metals in presence of electrolyte (eg. fruit juice) Products containing brine: should not be packed in Al can – rapid corrosion within 24 hrs Cleaning solutions used in food processing plant (pH 13) – must not come into contact with Al packaging materials or thoroughly rinsed with water immediately after cleaning.