Chapter 9: Aluminium foil packaging – Packaging Technology

9

Aluminium foil packaging

J. Kerry,     University College Cork, Ireland

Abstract:

This chapter reviews the manufacture and use of aluminium foil as a packaging material. It discusses processing from refining and smelting to the production of foil, printing and embossing. The chapter also describes the use of aluminium foil as a laminate as well as aluminium metallised films.

Key words

aluminium foil

aluminium laminate

aluminium metallised film

9.11 Acknowledgements

I gratefully acknowledge the assistance of Mr Eddie Beatty and Mr Dave Waldron in the preparation of the photographic material presented in this chapter and to Dr John F. Kerry, Echo Ovens International Ltd, Raheen, Limerick, Co. Limerick for his assistance with the preparation of figure material for this chapter. All ownership of such material resides with The Food Packaging Group in University College Cork and with Echo Ovens International Ltd.

9.1 Introduction

Aluminium is a silvery white metal belonging to the boron group of chemical elements and has an atomic number of 13. It has a number of important characteristics:

• It has a third of the density of steel, making it a lightweight metal.

• In alloy form it is strong: whilst pure aluminium has a yield strength of 7–11 MPa, aluminium alloys have yield strengths ranging from 200 to 600 MPa.

• It is resistant to corrosion due to ‘passivation': the formation of a thin layer of aluminium oxide on the metal surface which prevents further oxidation.

• It is ductile and malleable at normal temperatures, making it easy to form.

• It is non-magnetic but is a good conductor of heat and electricity.

Aluminium is used widely in foil form for packaging at thicknesses of just over 6 to around 150 microns (a micron is one millionth of a metre). The key properties of aluminium as a packaging material are summarised in Table 9.1. One of the most important properties of aluminium as a packaging material is its inertness compared to most metals. As noted earlier, when exposed to air, aluminium forms a transparent oxide layer which prevents further oxidation. As well as being resistant to corrosion, aluminium is non-absorbent and thus an effective barrier against gases and liquid. Heavier gauges of foil (above 17 µm) provide a complete barrier to gases and liquids. Thinner gauges do allow some transmission: as an example, the typical water vapour transmission rate (WTVR) for foil of 9 µm thickness is 0.3 g/m2 per 24 hours (at a temperature of 38 °C and a relative humidity of 90% RH). Aluminium is also stable in cold conditions and can be used for frozen food. It is resistant to temperatures below 150 °C, making it suitable, for example, for the storage and heating of chilled foods.

Table 9.1

Key properties of aluminium as a packaging material

Property Advantages
Appearance Bright, reflective gloss makes for an attractive appearance
Stability When exposed to air, aluminium forms an oxide layer that prevents further oxidation. It is also inert and does not form toxic compounds when exposed to most chemicals, including most foods and cosmetics
Barrier properties Heavier gauges form a complete barrier to gases and water. Aluminium reflects light, making it a suitable material to protect light-sensitive products
Hygienic properties Aluminium’s smooth metallic surface is non-absorbent. It can be easily cleaned and sterilised
Formability Aluminium’s ductility makes it easy to form. It has excellent deadfold properties. Its friability (ability to crumple) makes it useful for blister packaging
Conductivity Aluminium conducts electricity and heat, making it useful for applications such as induction heat sealing of containers
Recyclability Aluminium can be recycled at relatively low cost (recycling requires about 5% of the energy required to refine aluminium)

Aluminium does not generate toxic residues or react with most chemicals, including the majority of foods and beverages. It produces a metal with a smooth surface that is easy to clean. Aluminium does not generate sparks, making it suitable for storage of flammable and volatile materials. Its conductivity makes it useful for such applications as electrostatic shielding and induction heat sealing for packaging closures (see Chapter 15). Its reflectivity means that it can be used to protect lightsensitive products.

As well as these functional properties for packaging, aluminium also has a smooth, bright and reflective appearance which gives it a decorative value. In foil form, the ductility of aluminium means that it can easily be moulded into a variety of container shapes as well as be processed into foil. It also means aluminium has excellent dead-fold properties, i.e. once folded it retains the shape of the fold. Another increasingly important advantage is aluminium’s recyclability. Aluminium and its alloys can be recycled at about 5% of the energy consumption required to refine the original ore. Aluminium can easily be separated from other metals for recycling as it is non-magnetic.

Aluminium does have a number of disadvantages. Pure aluminium loses significant strength at temperatures above 150 °C which means a protective coating is needed if it is to be exposed to further heat processing, e.g. cooking. Whilst useful for formability, the ductility of aluminium means that aluminium foil is easily torn or punctured. A particular problem is flex cracking, i.e. the tendency of foil to split when folded or stretched. At gauges below 17 µm, the foil can also suffer from ‘pinholes', minute holes caused by impurities in the metal or process variations. Pinholing allows water and gas to penetrate the metal. A protective coating or incorporation of the foil as part of a laminate (composite layer) may therefore need to be added to increase strength, prevent flex cracking and counteract pinholing. Coating and lamination are discussed in Section 9.6.

These weaknesses in aluminium foil can be an advantage in some packaging applications, for example in the production of blister packs for tablets, where the foil needs to break easily to allow access to the product. It is also important to be aware that aluminium foil is not resistant to all chemicals. Whilst the fats or mild organic acids generally found in food have little or no effect on aluminium, strong mineral acids will corrode bare foil. Mildly alkaline components (e.g. soaps and detergents) may also have a corrosive effect. Salt and other caustic agents will also corrode aluminium. In these conditions, a coating or lacquer is required to protect the metal.

9.2 Aluminium processing

Aluminium is the most abundant metal found in the Earth’s crust (approximately 8%) and is the third most abundant element found on Earth, after oxygen and silicon. Due to its reactive behaviour, aluminium is never found as a pure metal in nature but combined with hundreds of minerals. The chief source of commercially manufactured aluminium today is bauxite. Bauxite is a reddish-brown clay-like deposit containing iron, silicates and aluminium oxides, the latter comprising the largest constituents. At present, bauxite is so plentiful that only deposits containing a content of aluminium oxides greater than 45% are selected to manufacture aluminium. Bauxite derives its name from a small French town called Les Baux, where bauxite was first discovered in 1821. Today, the largest bauxite mines are located in North America, the West Indies, Australia and Northern Europe.

Since bauxite occurs naturally at the surface of the Earth’s crust, mining practices tend to be straightforward. Surface pits are opened using explosives to reveal bauxite beds. The bauxite ore is then excavated and loaded into trucks or rail cars for transportation to the converting or processing centre. In order to produce commercial-grade aluminium from bauxite, essentially two processes must be employed:

• the bauxite ore must be refined to remove impurities (this is called the refining step).

• aluminium metal is extracted from aluminium oxide using electrolysis (this is the smelting stage).

Following on from these initial steps two further processes are required:

• the aluminium is processed and rolled into foil

• special surface finishes are applied to enhance appearance, decorate, strengthen, protect or provide specialist functions, for example giving foil heat-sealing properties.

A flow process diagram for aluminium manufacture from bauxite is shown in Fig.9.1.

9.1 Illustration of the aluminium manufacturing process encompassing the chemical extraction process (A), electrolysis (B) and alloy casting (C) operations (1. Raw material (bauxite) is processed into pure aluminium oxide (alumina) prior to its conversion to aluminium via electrolysis. This primary step is achieved through the ‘Bayer Chemical Process’. Four tonnes of bauxite are usually required in order to generate two tonnes of finished alumina which ultimately produces approximately one tonne of aluminium at the primary smelter. 2. Bauxite feed hopper. 3. Mechanical crusher employed to reduce bauxite particle size and increase surface area for chemical extraction. 4. Input chemical (sodium hydroxide). 5. Input chemical (lime). 6. Aluminium oxide is effectively released from bauxite in the presence of caustic soda solution within the primary reactor (digestion) tank. 7. The aluminium hydroxide is then precipitated from the soda solution. 8. Spent solids/ tailings discard a red mud residue generated as a byproduct of the process. 9. Precipitation tank: aluminium hydroxide is precipitated from the soda solution. The soda solution is recovered and recycled within the process. 10. Drying system (air heater system). 11. Drying system (hot air blower system). 12. Drying system (cyclone fines recovery system): post calcination, the anhydrous end-product, aluminium oxide (Al2O3), is a fine grained free flowing, white powder. 13. Input chemical (aluminium fluoride – AlF3). 14. Input chemical (cryolite – Na2AlF3). 15. Fuel source (e.g. coke, petroleum and pitch). 16. Molten aluminium: the reduction of alumina into liquid aluminium is operated at around 950 °C in a fluorinated bath under high intensity electrical current. This electrolytic process (A) takes place in cells or ‘pots', where carbon cathodes form the bottom of the pot and act as the negative electrode. Positive electrodes (anodes) are held at the top of the pot and are consumed during the process when they react with the oxygen generated from the alumina. Two types of industrial anodes are currently in use. All potlines built since the early 1970s use the pre-bake anode technology where anodes manufactured from a mixture of petroleum coke and coal tar pitch (acting as a binder) are pre-baked in separate anode plants. In the Soderberg technology, the carbonaceous mixture is fed directly into the top part of the pot, where self-baking anodes are produced using the heat released by the electrolytic process. 17. At regular intervals, molten aluminium tapped from the pots is transported to the cast house crucible. 18. The aluminium is alloyed in holding furnaces by the addition of other metals (according to end user needs), cleaned of oxides and gases. 19. The liquid metal is then cast into ingots. These can take the form of extrusion billets, for extruded products, or rolling ingots, for rolled products, depending on the way it is to be further processed. Aluminium mould castings are produced by foundries which use this technique to manufacture shaped components.)

The processes of refining and smelting require abundant electrical power and for this reason aluminium production is frequently located in areas where cheap electricity is readily available, e.g. northern Scotland and Scandinavia, where hydro-electric power is used. It is estimated that it takes 4 kg of bauxite to produce 2 kg of aluminium oxide, which, with the consumption of about 8 kW of electricity, produces 1 kg of pure aluminium. Due to the high costs associated with aluminium manufacture, metallurgists are continually investigating new approaches to the extraction of aluminium from bauxite in an attempt to reduce overall cost and environmental impact.

9.3 Refining

The refining step, which is also known as the Bayer process, is carried out in four stages:

• digestion

• clarification

• precipitation

• calcination.

In the digestion stage, ground bauxite is generally mixed with sodium hydroxide and pumped to heated and pressurised digestion tanks where the ore breaks down to soluble sodium aluminate and insoluble components which settle out in the tank. The clarification stage then follows. This entails passing the sodium aluminate through a series of filtered presses which are connected to tanks. The textile-based filters remove contaminants from the solution. This process can be repeated a number of times, depending on the level of contaminants present in the sodium aluminate solution. Finally this solution is then forwarded to cooling towers. From here, the aluminium oxide (alumina) is transferred to a large agitated tank or silo where the aluminium oxide fluid is seeded with hydrated aluminium crystals, thereby promoting the formation of aluminium particles. As the aluminium particles form, the hydrated and seeded aluminum crystals are attracted to each other and entrap the aluminium particles. This causes agglomeration to occur which causes clumping. As clumping develops, large agglomerates of aluminium hydrate form. As this stage in the process suggests, precipitation of aluminium hydrate occurs. The material is filtered and washed. The final stage in the refining process is calcination. This occurs when the aluminium hydrate is exposed to high temperatures in rotary kilns which drive off water and produce a white powdered material called aluminium oxide, or pure alumina, which resembles granulated sugar in appearance.

9.4 Smelting

The smelting step is employed to process the alumina. Its primary function is to separate alumina into metallic aluminium and oxygen by means of electrolysis, a procedure that was originally devised by Charles Hall and Paul-Louis-Toussaint Héroult in the late nineteenth century. The modified electrolytic method used today requires that the alumina is firstly dissolved in what is described as a smelting cell.

A smelting cell is made from steel, lined with carbon and filled with heated cryolite. Cryolite is an aluminium-based compound that has strong conductive properties. Once the smelting cell is filled with cryolite, an electric current is passed through the cryolite and this causes a surface crust to form on the alumina. This crust is not a permanent feature and is broken regularly through further alumina additions and via stirring. As the alumina decomposes during this electrolytic process, pure molten aluminium metal falls out of solution to the bottom of the smelting cell. The oxygen gas generated as part of this process combines with the carbon lining in the cell to produce carbon dioxide.

The purified molten aluminium is now collected from the bottom of the smelting cell by siphoning it off into crucibles which are then emptied into furnaces. At this point other elements may be added to the aluminium. The addition of other elements is dependent on the characteristics required from the aluminum alloy in question. Elements added include copper, zinc, magnesium, manganese and/or chromium. Aluminium alloys containing small amounts of these elements have excellent strength properties. As an example, Alloy 3003, which contains manganese, has greater stiffness as well as improved processing properties for cans or containers for pastries and pies. Alloys 1100, 1145 and 1235 are most commonly used for reroll stock for foil. Alloy 1200 is commonly used for packaging applications. Aluminium foils are made in several tempers (i.e. degrees of hardness), dependent on their application. As an example, blister packs require the foil to be in a half-hard temper so that the foil can be easily punctured for ease of access to the product.

The modified molten aluminium is then poured into chilled casting moulds where it cools and sets to form large aluminium blocks or slabs called ‘rolling ingots’ which typically range in weight between 10 and 25 tons. These ‘rolling ingots’ are reduced to ‘ reroll stock’ or sheets approximately 3–6 mm thick. All reroll stock is supplied to aluminium foil manufacturers in 0 temper (i.e., in its softest form) so that it can be worked easily.

9.5 Production of aluminium foil

Aluminium foil is typically less than 150 µm in thickness. Foils are available in gauges as low as 6.3 µm. Heavier foil gauges (> 17 µm) provide an absolute barrier to gases and liquids. A typical water vapour transmission rate (WVTR) for 9 µm foil is 0.3 g/m2 per 24 hours at 38 °C and 90% RH. As thickness is reduced, foil becomes more vulnerable to tearing or pinholing. Aluminium foil is produced by two basic processes:

• the traditional method of rolling aluminium slabs, ingots or thick plates into a narrow gauge aluminium web stock using heavy rolling mills (Fig. 9.2),

9.2 The conventional rolling-mill method of producing reroll aluminium stock and ultimately, aluminium foil.

• by continuous casting or hot-strip casting (similar to an extrusion process) which takes place immediately after the aluminium has left the furnace (Fig. 9.3).

9.3 Continuous casting or ‘hot-strip’ casting to produce aluminium reroll stock and ultimately, aluminium foil.

Because it has been established for longer, the rolling-mill method of producing reroll stock is still widely used. When being rolled to foil, ingot-rolled stock must be re-annealed (reheated) between mill passes to overcome work hardening and restore workability. The most economical means of manufacturing reroll stock, however, is via continuous casting. A typical continuous-casting production line runs directly from the furnace to a winding reel. The system continuously feeds, casts, chills and coils the reroll stock. Since it is heated during production, continuous-cast reroll stock does not need to be re-annealed when being made into foil.

After the foil stock has been manufactured, it must be further processed on a rolling mill. The work rolls have finely ground and polished surfaces to ensure a flat, even foil with a bright finish. The work rolls are paired with heavier backup rolls which exert very high pressure on the work rolls to ensure stability. This pressure ensures a uniform gauge (thickness) across the resulting aluminium foil sheet (known as a web). Each time the foil stock passes through the rolling mill, it is squeezed, its thickness is reduced and its length increases, but its width remains the same. This means the required width for the final foil product must be set at the beginning of the process.

The rolling process can be viewed as a form of extrusion. Foil gauges under 25 µm are often passed through the work rolls in two webs at a time. This is done so that the two webs of foil support each other and are less likely to tear when rolled. During the rolling process, the aluminium stock requires annealing to maintain workability. The addition of lubricants to the aluminium surfaces also maintains the workability of the material. A final annealing stage removes the lubricating oil, thus making the foil receptive to printing inks and adhesives.

9.6 Foil finishes, coatings and lacquers

Rolling produces two natural finishes on foil: bright and matte. The foil surfaces in contact with the work rolls are polished to a bright and shiny finish. When a single web is run, both sides are bright. If thinner foils are rolled together, the foil-to-foil face of each web develops a satin-like matte finish. Other finishes can be produced with special patterns on the work rolls or, more commonly, by using separate or in-line mechanical finishing machines (Table 9.2).

Table 9.2

Standard aluminium foil finishes and treatments used in packaging

Type of finish Type of finish
Bright both sides Uniform bright specular finish, both sides
Extra-bright both sides Uniform extra-bright specular finish, both sides
Matte one side Diffuse reflecting finish, one side
Matte both sides Diffuse reflecting finish, both sides
Embossed Pattern impressed by engraved roll or plate
Annealed Completely softened by thermal treatment
Chemically cleaned Chemically washed to remove lubricants
Hard Foil fully work-hardened by rolling
Intermediate temper Foil temper between annealed and hard

In most packaging applications, aluminium foil is combined with other materials such as coatings, inks, papers, paperboards and plastic films. A very useful characteristic of aluminium foil is that it has the capacity to readily accept many different types of coating materials such as inks (for printing), varnishes and lacquers (for embossing), adhesives and polymers (for heat sealing, etc.). The selection of foil alloys, gauges and tempers needs to take into account the type of coating or lacquer required.

Where a coating is needed, gravure coating is used for most low-viscosity materials. Heavier coatings require some form of roll coater. Coatings generally can be classified as protective or decorative. Protective functions for coatings include:

• making the foil more heat-resistant

• increasing tensile strength

• increasing resistance to potentially corrosive agents

• enhancing the barrier properties of low gauge foil

• increasing resistance to scratching or scuffing

• increasing the UV resistance of a printed foil.

Vinyl heat-seal coatings are widely used with aluminium foil. It is important that the coating is compatible with the product. It is also important to note that a normal heat-seal coating does not add significantly to a foil’s bursting strength. Coatings are only likely to add to foil strength above thicknesses of 25 µm. Table 9.3 lists the chemical resistance of some coating materials. Table 9.4 lists the general properties of the various coatings applied to foil.

Table 9.3

Chemical resistance of coating materials

Source: Aluminium Association, Aluminium Standards and Data.

Table 9.4

General properties and characteristics of typical aluminium foil coating compounds

The quality rating score equates to: 1 = best performance, 2 = satisfactory performance or frequently used and 3 = unsatisfactory performance.

Decorative and other functions include giving additional gloss and depth to a decorated or printed foil, or improving the adhesive quality of the foil for other coatings or printing inks. The use of transparent lacquers and varnishes gives foil a particularly bright metallic sheen. Coloured lacquers help to impart colour; for example, yellow gives aluminium a gold appearance.

9.7 Printing and embossing

If foil is to be printed, it is important to be aware that its glossy surface may make small print difficult to read. Reverse type is best avoided, unless it is large. It is sometimes necessary to print a matt white background on which black type will be more visible. Printers usually print foil on the same presses used for paper or other material. If a foil is to be printed, it is given a primer or wash coat to ensure a clean surface and to provide a foundation for the ink. Shellacs and vinyls are common primers for gravure and flexographic printing. If a thicker coating is required, e.g. for lithographic printing, vinyl copolymer or nitrocellulose may be used. A second film coat may then be applied on top of the printing to protect it from scuffing as well as to reduce surface friction.

Aluminium foil is particularly suited to embossing. This gives both a three-dimensional quality to a design and increases the number of reflective surfaces able to reflect light to create a more eye-catching effect. It also increases stiffness (see below) and allows cut pieces of foil to be easily separated, e.g. stacks of pre-cut lids. Thinner gauge foil in web (sheet) form is passed between a steel roll containing the engraving pattern and a soft matrix roll (usually paper). Heavier-gauge, coated or laminated foil is embossed with two engraved steel rolls, one with the positive image, the other the negative image. Since it requires further processing with rollers, embossing tends to improve foil stiffness and dead-fold properties.

9.8 Using aluminium foil as a laminate

Lamination involves combining sheets of different materials into a single layer, using a mixture of adhesives, pressure and sometimes temperature to bond the materials together (see also Chapter 14). Aluminium foil is laminated on web-fed rotary equipment which sometimes includes a coating unit to add further protection. Adhesives are selected on the basis of their suitability for the materials to be joined as well as such issues as any toxicity or contamination risk they might present, potential odour or colour issues, moisture and heat resistance. The main types of adhesives used are:

• heat-sealable adhesives

• water-soluble emulsions and dispersions

• thermoplastics in lacquer-type solvents

• hot melts (natural or synthetic waxes)

• extruded-film adhesives.

Four methods are used to laminate aluminium foil:

• wet bonding

• dry bonding

• extrusion bonding

• hot-melt bonding.

As the name suggests, wet bonding involves combining the various layers before the adhesive is dry. A water-or solvent-based adhesive is used and is normally applied to the foil. Further layers are then applied on top and the laminate passed through a combining or nip roll at varying drying temperatures, depending on requirements (Fig. 9.4). Materials laminated to the foil base need to be porous so that the liquid medium of the adhesive can be taken up and the adhesive properly dried, thus paper is an ideal material for wet bonding. The smoother, denser and less porous the paper, the less adhesive is required. A low paper moisture content is important to ensure good adhesion and prevent problems such as staining. It is important to combine and glue the materials quickly to ensure good adhesion and prevent materials slipping as they are fed through the machine. The use of a rapidly setting adhesive also helps to minimise the risk of air pockets or blisters in the laminate, particularly if the laminate is stressed or flexed in subsequent processing, e.g. coating operations. High drying temperatures improve bonding and water resistance, but they can over-dry and damage papers or boards.

9.4 Wet-bonding foil lamination processes.

Where materials are not porous, dry bonding is used instead of wet bonding. Dry-bond adhesives use both natural and synthetic sealing agents and can be either water or organic solvent based (see Chapter 16). Synthetic agents include vinyls, epoxies, polyesters and urethanes. The adhesive is applied to the foil and allowed to dry. The layers are then aligned and passed through a heated combiner roll which reactivates the adhesive to create the bond. This method is well suited to non-porous materials such as polyester films, which add strength and flexibility when combined with aluminium foil.

Extrusion bonding involves extrusion of one or two molten plastic films which are then combined with the aluminium foil. As the aluminium foil web approaches the combiner roll, an extruder die deposits a layer of hot extrudate across the width of the web. The laminate then passes through the chilled nip of the combiner roll, cooling the plastic layer which solidifies. No drying is required. When only the two layers are involved, the process is known as extrusion coating.

The hot-melt process is used for high-speed lamination since there is no need for a drying stage. Hot melts include polymer resins, waxes and resin-wax combinations. The hot-melt can be melted at a lower temperature than extruded coatings, applied to the web and chill-set in the nip of the combiner roll. The plastic nature of the hot melt improves heat-sealability and the dead-fold characteristics of the foil.

Foil laminates intended for barrier applications should be evaluated for their barrier properties using the finished pack after all of the machining is complete, and preferably after a real or simulated shipping cycle. It is not uncommon for a prospective barrier laminate to have no measurable permeability when flat (i.e. at the point of manufacture) but to have significant permeability when formed, folded and creased into a final pack format.

9.9 Aluminium metallised films

Vacuum metallising or metallisation involves depositing a metal layer onto a substrate (e.g. paper or plastic film) in vacuum conditions. Aluminium is the only metal used for vacuum metallising in packaging applications. Initially used for decoration, metallisation is now widely used in flexible packaging since it improves gas and moisture barrier properties, heat resistance, light reflectance and electrical conductivity. Crisp packets, for example, are typically made of metallised polypropylene which provides effective protection for the required shelf life of this type of product. Metallised films are often a component in a laminate. Table 9.5 lists examples of laminates that use metallised films. Since aluminium is effective in converting microwave energy to heat, metallising is also used in ‘susceptor’ packaging of microwavable foods where the metallised component serves to create a local microwave energy ‘hot spot’. Given the high temperatures involved, the aluminium needs to be protected by a heat-resistant layer, for example of polyester.

Table 9.5

Typical examples of laminates using metallised films

Product Type of laminate
Coffee 12 μm metallised BON/50 μm LDPE
Savoury snacks 18 μm OPP/adhesive/18 μm metallised OPP
Condiments/spices 12 μm metallised PET/38 μm MDPE
Bag-in-box wine 50 μm ionomer/12 mm metallised PET/75 μm EVA
Biscuits OPP/18 mm metallised OPP
Medical products paper/adhesive/18 μm metallised OPP/ionomer
Cold meats metallised PET/PE

Batch processing is the most widely used approach to metallising. It involves a horizontal tubular chamber, up to 2 m in diameter and 3 m long (Fig.9.5). A series of rollers carry the substrate through the chamber from which the air has been evacuated by vacuum pumps. Pure aluminium wire is fed into containers ('boats') which are electrically heated to vapourise the aluminium. (The vacuum reduces the vapourisation temperature of the aluminium.) As the vapour rises, it condenses on the underside of the substrate as it passes over a chilled drum. The thickness of the metal deposited is controlled by a combination of web speed, wire feed rate and boat temperature. A batch can handle reels of up to 20,000 m long in just under an hour. The thickness of the aluminium layer is as low as three-millionths of a centimetre. Electrical resistance and optical density are the two main methods for testing deposition and layer thickness. Metallised patterns can be achieved by metallising a pre-printed film, or using caustic solutions to selectively remove the metallic layer to create patterns or windows in the metallised film.

9.5 Vacuum metallising is a form of physical vapour deposition, a process of combining metal with a non-metallic substrate through evaporation. The most common metal used in vacuum metallisation is aluminium for a variety of reasons including cost, thermodynamic and reflective properties. The evaporation takes place by feeding aluminium onto heated sources or boats, which operate at approximately 1,500 °C (2,700 °F). The atmosphere in the vacuum metallising chamber is evacuated to a vacuum level suitable for the evaporation of the aluminium wire. Upon contact with the substrate being processed, the aluminium vapour condenses and creates a uniform layer of vacuum deposited aluminium.

Metallised papers need to be high-quality virgin stock usually with a clay coating to provide a smooth surface and high reflection in the finished metallised surface. Papers also need to be lacquer coated using gravure coaters to seal the paper and ensure a good, even metal adhesion. The lacquer is dried using radiant heating, UV, electron beam or infrared curing. Developments in paper quality and lacquer technology have significantly expanded the use of metallised papers in areas such as labels and cartons for cosmetics. Since the vacuum and temperature conditions reduce the moisture content of paper substrates to below 5%, remoistening is an essential step in the manufacture of metallised paper to avoid curling. Although most plastic films can be metallised, oriented polypropylene (OPP), biaxially oriented polypropylene (BOPP), polyethylene terephthalate (PET) and biaxially oriented nylon (BON) are the most commonly metallised packaging films. Unlike paper, plastic films do not need to be sealed to be metallised.

9.10 Conclusion

Because of the numerous beneficial characteristics of the metal, the way it can be converted to foil, used in laminates or to metallise films, aluminium use is widespread throughout the packaging industry, particularly with respect to fast-moving consumer products. The numerous packaging formats and the products that it is associated with are presented in Table 9.6. Examples of container types are shown in Fig. 9.6.

Table 9.6

End uses for aluminium foil as packaging material

Packaging format Products
Rigid smooth wall containers Meat joints for roasting, large portioned ready-meals and convenience-style food products (Fig. 9.6)
Semi-rigid wrinkle wall containers Take-away meals, savory pies, bakery products, frozen and chilled ready-meals (Fig. 9.6)
Closure systems Milk, beverages, instant coffee, dried powders, health foods and pharmaceutical products, cosmetic creams
Labels Used widely from foodstuff to beverage packaging
Composite cans Powdered drinks, snack foods, juice-based beverages, instant biscuit/ cookie dough, chilled and frozen foods
Flexible packaging Dairy products: milk, cheese, butter and ice cream
Beverages: wines, juices, soft drinks, liquors, beer; non-beverage products: soaps, shampoos, conditioners, detergents, cosmetics
Dessicated and powdered products: coffee, tea, cocoa powder, custard, fruit, concentrates, vegetables, dehydrated powders, soups, herbs and spices, yeast and other powdered extracts, salt, sugar, pharmaceuticals, tobacco products
Cereal and baking foodstuffs: cake mixes, cereals, frosting mixes, pasta-based products, rice-based products, biscuits, crackers, snack foods, breads
Confectionery: chocolate, hard and soft sweets, all products containing volatiles contributing to flavours such as mint, orange, coffee, aniseed, clove, etc.
Muscle-based foods: meat, poultry, fish, game, casseroles, stews, soups and broths, general muscle-based retorted products, chilled and frozen products, pet foods

9.6 Examples of a smooth wall heavy gauge deep drawn aluminium container (above) and a wrinkle wall narrow gauge shallow drawn aluminium container (below).

9.12 Sources of further information and advice

Most general textbooks on aluminium discuss the metallurgical properties of the metal and its applications as a sheet or extruded material in construction, aerospace and automotive applications, and are thus of limited interest in the context of this textbook.

The following organisations provide useful information specific to aluminium foil in packaging:

• Aluminium Foil Container Manufacturers Association: http://www.afcma.org

• Aluminium Federation: http://www.alfed.org.uk/downloads/documents/ D53ZC9P4LP_15_aluminium_packaging.pdf

• European Aluminium Association: http://www.alueurope.eu/wp-content/ uploads/2011/10/Aluminium-in-packaging-brochure.pdf

• European Aluminium Foil Association: http://www.alufoil.org

• Australian Aluminium Council: http://aluminium.org.au/packaging

• The Packaging Federation: http://www.packagingfedn.co.uk

• Aluminium in Packaging: http://packaging.world-aluminium.org