AN INTRODUCTION TO FOOD PACKAGING

Dr Gordon L. Robertson
University of Queensland &
Food•Packaging•Environment
Brisbane
Australia
CHAPTER 3: METAL PACKAGING

3.1 Learning Objectives

3.2 Introduction

3.3 Tinplate

3.3.1 Manufacture of Pig Iron

3.3.2 Steelmaking
3.3.3 Tinplating

3.4 Chromium-Coated (ECCS)

3.5 Aluminum Foils & Containers

3.6 Container-Making Processes

3.6.1 End Manufacture

3.6.2 Three-Piece Can Manufacture
3.6.2.1 Welded Side Seam
3.6.2.2 Soldered Side Seam
3.6.2.3 Double seaming
3.6.3 Two-Piece Can Manufacture
3.6.3.1 Drawn and Ironed (D&I)
3.6.3.2 Drawn and Redrawn (DRD)
3.6.4 Protective and Decorative Coatings
3.6.4.1 Protective Coatings
3.6.4.2 Decorative Coatings

3.7 Aluminum Foils and Containers

3.7.1 Aluminum Foils

3.7.2 Aluminum Tubes
3.7.3 Retort Pouch
3.7.4 Aluminum Containers

3.8 Corrosion of Metal Packaging

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 3.8.1 Tinplate Corrosion
3.8.2 Corrosiveness of Foods
3.8.3 External Corrosion of Cans

3.9 Exercise

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3.1 Learning Objectives properties to gases, moisture and
light, ability to withstand wide
This module has been written to extremes of temperature and ideal
provide a basic understanding of, and surfaces for decoration and
introduction to, metal packaging lacquering.
materials. On completion of this
module, the student will have a 3.3 Tinplate
general appreciation of the raw
materials and processes used to The term tinplate refers to low carbon
manufacture metal packaging mild steel sheet varying in thickness
materials, as well as the major from around 0.15-0.5 mm with a
categories of metal packaging coating of tin between 2.8-17 gsm
materials and their use in the (grams per square meter) (0.4-2.5 µm
packaging of food. thick) on each surface of the material.
The combination of tin and steel
3.2 Introduction produces a material which has good
strength combined with excellent
Four metals are commonly used for fabrication qualities such as ductility
the packaging of foods: steel, (the capability to undergo extensive
aluminum, tin and chromium. Tin and deformation without fracture) and
steel, and chromium and steel, are drawability (these attributes arise
used as composite materials in the from the grade of steel selected and
form of tinplate and electrolytically the processing conditions employed in
chromium-coated steel (ECCS), the its manufacture) as well as good
latter being somewhat unhelpfully solderability, weldability, non-toxicity,
referred to as tin-free steel (TFS). lubricity, lacquerability and a
Aluminum is used in the form of corrosion resistant surface of bright
purified alloys containing small and appearance (these latter properties
carefully controlled amounts of being due to the unique properties of
magnesium and manganese. Two tin). Furthermore, the tin coating
other metals are used during the adheres sufficiently well to the steel
soldering or welding of three-piece base that it will withstand any degree
tinplate and ECCS containers: lead and of deformation that the steel is able to
copper. However, since they are not withstand without flaking.
used for the fabrication of containers
in their own right, they will not be 3.3.1 Manufacture of Pig Iron
discussed further in this chapter.
The chemical composition of the base
Today, tinplate and aluminum have steel has a very significant effect on
become widely adopted for the the subsequent corrosion resistance
manufacture of containers and and mechanical properties of the
closures for foods and beverages, due tinplate. The iron ores used are
largely to several important qualities generally hematite (Fe2O3) with some
of these metals. These include their magnetite (Fe3O4). Commercial
mechanical strength and resistance to extraction of iron from its ores is
working, low toxicity, superior barrier carried out in blast furnaces, where a

Introduction to Food Packaging: Metal Packaging Chapter 3: Page 3 of 21

mixture of iron ores, solid fuel (coke) 15% sulfuric acid) near its boiling
and fluxes (limestone and dolomite) point. After pickling the strip is
are heated to around 1800°C. This recoiled and coated with an oil to
results in the reduction of most of the prevent rust formation and act as a
iron oxides to metallic iron (m.p. lubricant in subsequent operations.
1200°C). Today modern blast
furnaces are capable of producing The final stage of thickness reduction
molten iron of near constant (typically 90% from about 2 mm to 0.2
composition at high rates. mm) is carried out by cold rolling. The
effects of cold rolling are to increase
3.3.2 Steelmaking the strength and hardness of the steel,
but this is done at the expense of
The pig iron from a blast furnace ductility.
contains 3.5-5.0% carbon, 0.3-1.0%
silicon, and up to 2.5% manganese, 1% In the next stage (that of annealing),
phosphorous and 0.08% sulfur the steel is heated to temperatures of
depending on the ore. These 600-700°C, causing recrystallisation of
metalloids must be substantially the elongated ferrite grains into new
reduced in the steelmaking stage, and fine grains. This results in a marked
this is commonly accomplished using a increase in ductility and a
basic oxygen furnace. From the corresponding decrease in strength.
furnace the steel is cast into ingots
which are subsequently rolled into To reduce the possibility of severe
slabs about 250 mm thick or, more fluting, paneling or creasing, and to
commonly today, continuously cast impart the desired surface finish, the
into slab form. steel is given a final, very light cold
rolling (generally a reduction of 0.5-
2.0% in thickness) in a ‘Temper’ mill.
This imparts ‘springiness’ to the steel
but changes the temper or surface
hardness only slightly.

At this stage the uncoated steel sheet

is referred to as black plate, so called
because some of the early production
was covered with black iron oxide. It is
the raw material for electrolytic
Blast furnace in steel mill
tinplate (ETP) and electrolytically
chromium-coated steel (ECCS).
The thick slabs are hot rolled down to
about 2 mm, and during this process
3.3.3 Tinplating
substantial layers (0.01 mm thick) of
oxides or scale are formed as a
The traditional method for tinplating
consequence of the steel being heated
involved dipping or passing the steel
to elevated temperatures for rolling.
through a bath of molten pure tin but
Next the scale is removed by a process
since the 1930s the process of
called ‘pickling’ which uses a dilute
depositing tin by electroplating has
aqueous solution of acid (typically 10-

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been used. The introduction of the manufacturers.
electroplating process enabled a
different thickness of tin to be applied The final structure of the completed
to the two surfaces of the steel. This coating is shown below and consists of
‘differential tinplate’ is of economic a tin/iron alloy layer (principally FeSn2)
benefit to the user since it enables the adjacent to the steel base, free tin, a
most cost-effective coating to be film of mixed oxides formed by the
selected to withstand the different passivation process, and an oil film.
conditions of the interior and exterior
of the container.

Plating is preceded by cleaning in a

pickling and degreasing unit, followed
by thorough washing to prepare the
surface. After the plating stage, the
coating is flow melted, passivated and
finally lightly oiled.

Flow melting consists of heating the

strip to a temperature above the
melting point of tin (typically 260-
270°C), followed by rapid quenching in
water. During this treatment a small Schematic structure (not to scale) of
quantity of the tin-iron compound tinplate showing main layers
FeSn2 is formed. The structure and
weight of this alloy layer plays an The majority of tin mill products are
important role in several forms of used by the container industry in the
corrosion behavior. manufacturing of cans, ends and
closures for the food and beverage
T steel strip is then given a passivation industry. Over 120 tinplate lines and
treatment to render its surface more 30 ECCS lines are operating in the
stable and resistant to the world today.
atmosphere. This generally involves an
electrolytic treatment in a sodium Tinplate sheets are described in terms
dichromate electrolyte that results in of a base box, a hangover from earlier
the formation of a film (usually <1 µm times when tinplate was sold in units
thick) consisting basically of chromium of 112 sheets, each 356 by 508 mm
and chromium oxides and tin oxides. (14 by 20 inches). Such a package was
known as a base box, and the area it
After passivation the plate is given a contained (20.2325 m2 or 31,360 in2)
light oiling to help preserve it from survives today as the unit area for the
attack and to assist the passage of selling of tinplate. To convert to
sheets through container-forming decimal thickness multiply the weight
machines without damaging the soft per base box by .00011. In the original
tin layer. Finally the strips are sheared system, 1 lb base box meant that 1 lb
into sheets or coiled, and then packed of tin was applied evenly to both sides
for shipment to the can of the plate, i.e. each side received 0.5

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lb (equivalent to 11.2 gsm) of tinplate.
This was given the designation in the
US of No. 100. Tinplate is now
commonly graded using the metric
unit SITA (Système International
Tinplate Area) which is based on 100
m 2.

Schematic structure (not to scale) of

ECCS showing main layers

The ECCS surface is more acceptable

Tinplate sheets ready for dispatch. for protective enamel (lacquer)
coatings or printing inks and varnishes
3.4 Chromium-Coated (ECCS) than tinplate, and the lack of a low
melting point (232°C) tin layer means
The production of electrolytically that higher stoving temperatures and
chromium/chromium oxide coated consequently shorter stoving times
low carbon steel sheet (to give ECCS can be used for the enameling of
its full name) is very similar to ECCS. Unlike flow brightened tinplate,
electrotinning, the only essential ECCS is a dull bluish color which
differences being that in the former necessitates modification of
case flow melting and chemical decoration processes to allow for its
passivation are not involved. The poor reflection.
initial development work was carried
out in Japan in the 1960s when tin was However, ECCS is less resistant to
on occasions in short supply, and the corrosion than tinplate as it has no
price extremely variable. sacrificial tin layer, and therefore must
be enameled on both sides. In
The process involves deposition in a addition, ECCS containers cannot be
dilute chromium plating electrolyte at soldered with traditional lead or tin
a temperature in the range 50-70°C. solders and therefore bonding of ECCS
As shown below, ECCS consists of a components must be by welding or
duplex coating of metallic chromium the use of organic adhesives. If
and chromium oxide. The ideal total welded, ECCS must be edge cleaned
coating weight is approximately 0.15 prior to welding to remove the
gsm which is much thinner than the chromium layer. This is a slow, costly
lowest grade of electrolytic tinplate and mechanically inefficient process.
which has a tin thickness of 5.6 gsm. ECCS ends are commonly used with
tinplate bodies.

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 3.6 Container-Making Processes
3.5 Aluminum
3.6.1 End Manufacture
Aluminum is the earth's most
abundant metallic constituent, The can end or lid is of complex design
comprising 8.8% of the earth's crust, developed for optimum deformation
with only the nonmetals O2 and silicon behavior since it is important that the
being more abundant. Alumina or ends are able to deform under internal
aluminum oxide (Al2O3) is the only and external pressure without
oxide formed by aluminum and is becoming permanently distorted. In
found in nature most commonly as effect they must act like diaphragms,
bauxite. The Hall-Héroult process is expanding during thermal processing
still the only method used for the and returning to a concave profile
commercial production of aluminum. when vacuum develops inside the can
Due to the chemical stability of its on cooling. The cross-section of a
oxides, the energy requirements for typical end design is shown below:
smelting are extremely high. This has
lead to the production of aluminum in
areas where cheap electrical power is
available.

Most commercial uses of aluminum

require special properties that the
pure metal cannot provide. Therefore,
alloying agents are added to impart
strength, improve formability
characteristics and influence corrosion The ends are stamped on power
characteristics. A wide range of presses from tinplate sheet which has
aluminum alloys is available been previously enameled. After
commercially for packaging stamping, the ends fall through the
applications, depending on the press into the curler to form the
container design and fabrication outside curl and diameter.
method being used. Commercially
pure aluminum is used for the A lining or sealing compound is then
manufacture of foil and extruded applied into the seaming panel; the
containers since it is the least sealant used is based on natural or
susceptible to work hardening. An synthetic rubber and is dispersed in
alloy containing 4-5% Mg and 0.35% water or solvent. Its constituents are
Mn produces a very rigid material subject to stringent food regulations.
suitable for manufacturing beverage The purpose of the sealant is to assist
can ends. the formation of an hermetic (air-
tight) seal by providing a gasket
Compared with tinplate and ECCS, between adjacent layers of metal.
aluminum is a lighter, weaker but
more ductile material that cannot be Several types of easy-opening devices
soldered. such as the key opening scored strip
found in solid meat or shallow fish

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cans have been available for many
years. However, an increased demand 3.6.2.1 Welded Side Seams
for convenience features has seen the
development of easy-open ends of Today in most countries the majority
two broad types: those which provide of three-piece tinplate cans used for
a pouring aperture for dispensing food have welded side seams.
liquid products, and those which give a Compared to soldered side seams (see
near full aperture opening for below), welding offers savings in
removing more solid products. material, since the overlap needed to
produce a weld uses less metal than
Most designs incorporate an easy- an interlocked soldered seam. As well,
open end consisting of a scored the side seam is stronger, it is easier to
portion in the end panel and a levering seam on the ends, and a greater
tab (formed separately) which is surface area is available for external
riveted onto a bubble-like structure decorating.
fabricated during pressing. Most but
not all of the entire aperture Prior to welding, sheets of steel are
circumference is scored, leaving enameled, and if necessary printed,
sufficient unscored portion to function with the area where the weld will be
as a hinge when the tab is pressed in. made left bare. The sheets are then
Close control of scoring conditions is slit into individual blanks, each blank
vital to ensure adequate resistance to being rolled into a cylinder with the
bursting without requiring an unduly two longitudinal edges overlapping.
high tearing load to open. Recently a The two edges are then welded
resealable end for carbonated together.
beverage cans has been
commercialized (see below).

Welded can line

The wire welded operation used today

for the high speed welding of tinplate
and ECCS containers utilizes a sine
wave alternating current (and in the
case of tinplate a continuous copper
3.6.2 Three-Piece Can Manufacture

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wire electrode) to produce a weld with plastic. However, since the advent of
an extremely low metal overlap (0.4 – high speed welding operations, the
0.8 mm). The use of copper wire as an use of chemically-bonded side seams
intermediate electrode is necessary to has declined.
remove the small amount of tin picked
up from the tinplate during the 3.6.2.2 Soldered Side Seams
welding process, which would
otherwise reduce welding efficiency. Today, with the exception of some
The tensile strength of a good weld is developing countries, very few food
equal to that of the base plate. To cans are produced with soldered side
prevent traces of iron being picked up seams, the concern of public health
by some types of beverages and acidic authorities being that lead from the
foods, repair side striping (enameling) tin/lead (2:98) solder would migrate
of the internal surface of the weld is into the food. Since the 1970s, most
required. countries insisted that only pure tin
solder be used on cans intended for
baby foods, this adding significantly to
the cost of such cans. The use of
tin/lead solder ceased when the US
FDA issued a final rule in July 1995
prohibiting its use in food containers,
and now the much more expensive
tin/silver (96:4) solder must be used.

Tinplate cans are easily soldered

because the tin solder alloy readily
fuses with the tin on the surface of the
steel. Enamel stripes are sometimes
applied to one or both sides of the
seam (‘side striping’) in an attempt to
A system of chemical bonding of side repair damage made to the previously
seams has been developed, mainly for applied enamel by the heat of the
dry or otherwise neutral products such solder. This is essential on beverage
as powders and oils. It utilizes a cans and those likely to contain highly
thermoplastic adhesive which is corrosive products.
applied to one edge of the pre-heated
body blank before it is rolled into a 3.6.2.3 Double Seaming
cylinder, giving complete protection of
the raw edges of the blank. A strong After the side seam has been formed,
bonded lap seam is produced that is the bodies are transferred to a flanger
able to withstand the high in-can for the final metal forming operation:
pressures generated by beers and necking and flanging for beverage
carbonated soft drinks during can cans, and beading and flanging for
warming or pasteurization. This food cans. The can rim is flanged
method can only be used with ECCS outwards to enable ends to be
cans since the melting point of tin is seamed on. The top of beverage cans
close to the fusion temperature of the is necked to reduce the overall

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diameter across the seamed end to resistance by acting as failure rings.
below that of the can body wall,
yielding savings in the cost of metal The end is then mechanically joined to
through the use of smaller diameter the cylinder by a double seaming
ends, allowing more effective packing operation as illustrated below. It
and stacking methods to be adopted, involves mechanically interlocking the
and preventing damage to the seams two flanges or hooks of the body
from rubbing against each other. cylinder and end and is carried out in
Simultaneous creation of the neck and two stages. In the so-called first
flange using a spin process is used. operation, the end curl is gradually
Double-, triple- and quadruple-necking rolled inwards radially so that its
is now quite common, the latter flange is well tucked up underneath
reducing the end diameter from 68 the body hook, the final contour being
mm to 54 mm for the common governed by the shape of the seaming
beverage can. roll.

First seaming operation

In the second operation, the seam is

tightened (closed up) by a shallower
seaming roll.

Necked beverage can

For food products where the can may

be subjected to external pressure
during retorting or remain under high
internal vacuum during storage, the
cylinder wall may be beaded or ribbed
for radial strength. There are many
bead designs and arrangements, all of
which are attempts to meet certain
performance criteria. Basically,
circumferential beading produces Second seaming operation
shorter can segments that are more
resistant to paneling (implosion), but
such beads reduce the axial load

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First and second operation

The final quality of the double seam is

defined by its length, thickness and
the extent of the overlap of the end
hook with the body hook. The degree
of interlocking between the body hook Finally the cans are tested for leakage
and cover hook is known as the using air pressure in large wheel-type
overlap. It is the amount of overlap testers; leaking cans are automatically
that gives the can an hermetic seal, rejected.
preventing microorganisms from
penetrating the can. The body hook
and cover hook are both formed by
the interlocking of the can body with
the can lid. The body hook was
originally part of the can body (flange).
The cover hook was initially from the
can lid. If either is excessively long or
short, problems with a short overlap
may develop.

The width of the seam is the vertical

distance from the bottom of the Seam projector used industrially to
double seam to its top. An excessive assess seam dimensions
width will also compromise the safety
of the overlap. Rigid standards are laid 3.6.3 Two-Piece Can Manufacture
down for an acceptable degree of
overlap and seam tightness. The main A major innovation in canmaking was
components of a double seam are the introduction of the seamless or
shown below: two-piece aluminum can in the 1950s
and tinplate can in the 1970s. For
many years canmakers have
manufactured in a single pressing
shallow drawn two-piece containers
such as the familiar oval fish can.
However, the technology to produce
deep drawn cans is a more recent

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innovation, although the basic concept the two-piece can has no side seam
dates back to the Kellver system for and only one double seam which is
producing cartridge cases developed more easily formed and controlled
in Switzerland during the Second because of the absence of a side seam
World War. lap juncture. The internal enamel does
not have to protect a soldered side
There are two main methods used seam or weld cut edge, and there are
commercially to make two-piece cans: material savings in solder and (in the
the drawn and ironed (D&I) process case of D&I cans) plate, the latter
which can be adapted to produce a being up to 35% lighter than a
can for pressure packs (including standard three-piece can. Since 1970,
carbonated beverages) and for food through the conversion of three- to
containers, and the drawn and two-piece cans and subsequent light-
redrawn (DRD) process which is a weighting, the weight of a tinplate soft
multistage operation and produces a drink can has been reduced by 40% to
can mainly suitable for food products,. 35 g, and that of the inherently lighter
Both processes depend on the two-piece aluminum can by 24% to18
property of the metal to ‘flow’ by g. Technology exists to continue this
rearrangement of the crystal structure trend, especially with tinplate cans.
under the influence of compound Finally, the absence of a side seam
stresses, without rupturing the permits all-round decoration of the
material. outside of the can, increasing the
effective printing area and leading to a
more aesthetically pleasing
appearance.

3.6.3.1 Drawn and Ironed (D&I)

The D&I (also known as DWI for drawn

and wall ironed) tinplate or aluminum
container is made from a circular disc
stamped from a sheet or coil of
uncoated plate and formed into a
shallow cup having effectively the
same side wall and base thicknesses as
the starting material, as shown below.
The forming process involves a flat
sheet being formed into a cup or
cylinder by the action of a punch
drawing it through a circular die, the
wall thickness of the cup being
Modern two-piece beverage cans uniform throughout. The plate is
covered with a thin film of water
Two-piece cans have technical, soluble synthetic lubricant prior to
economic and aesthetic advantages in forming.
comparison with soldered or welded
three-piece cans. In terms of integrity,

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 Tinplate is the best material for D&I
cans as the tin coating is soft and
ductile and imparts lubricity to the
steel while remaining bonded to it
throughout. Some aluminum is made
into D&I cans for food packaging but
these are mainly shallow drawn
containers. Most D&I aluminum cans
are used for beverage packaging (i.e.
The sequential stages in production of beer and soft drinks). ECCS plate is not
D&I cans. suitable for ironing as the chromium-
1. Disc cut from coil based coating is too hard.
2. Drawn into shallow cup
3. Redrawn into smaller diameter 3.6.3.2 Drawn and Redrawn (DRD)
cup
4,5 & 6. Wall thinning by ironing For many years canmakers have
7. Trimming to required height manufactured shallow drawn
containers; however, the novelty of
The cup is transferred to an ironing the DRD process is the use of multi-
press where it is held on a punch and stage drawing to produce a can with a
passed successively through a series of higher height-to-diameter ratio. This
ironing dies. As a consequence of the process is essentially identical to the
ironing process, the wall thickness is initial stages of the D&I technique
reduced (typically from 0.30 mm to except that the final height and
0.10 mm) and the body height diameter of the container is produced
correspondingly increased. by sequentially drawing cups to a
Concurrently the integral bottom end smaller diameter, i.e. causing metal to
is domed and profiled to provide flow from the base to the wall of the
added strength, the end retaining container rather than ironing the
essentially the original sheet container wall. As a consequence, the
thickness. Because the can wall may wall and base thickness as well as the
not iron to the same height all around surface area are identical to the
the circumference due to slight original blank, as opposed to the D&I
variation in material properties, cans can where the wall thickness is much
are ‘overdrawn’ and then trimmed to less than the base thickness. A typical
the correct height. DRD process is illustrated below.

The trimmed cans are chemically

cleaned to remove drawing lubricants
and prepare the surface for receiving
exterior and interior coatings. If the
cans are to be used for beverages,
they are then necked; D&I food cans
are commonly beaded for added The sequential stages in production of
strength against body collapse under D&I cans.
partial vacuum conditions. The cans 1. Body blank
are then flanged. 2. Drawn cup

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 3 & 4. Diameter decreases as cup 3.6.4.1 Protective Coatings
is redrawn
4. Finished trimmed can with Internally enameled (lacquered) metal
profiled base. containers are used when the product
and the plain (uncoated) container
Whereas in the D&I process the would interact to reduce the shelf life
internal diameter of the body remains or the quality of the product to an
constant throughout the ironing unacceptable level. Thus acidified
stages, the internal diameter of the beetroot, colored berry fruits, beer
DRD can is progressively reduced as and soft drinks are packed in
the height is increased during the enameled containers, i.e. containers in
various redrawing stages. Therefore, which organic coatings have been
the DRD cans do not offer the same applied to the inside (and sometimes
economies as D&I cans because in the the outside) surfaces.
former the metal cannot be selectively
distributed as it can during wall The primary function of interior can
ironing. Since the end is integral and is coatings is to prevent interaction
normally the thickest region, this between the can and its contents,
governs the material gauge and the although some enamels have special
result is often excessive side wall properties which allow products such
thickness. Typically 0.2 mm thickness as meat loaf to be easily removed
pre-lacquered tinplate and ECCS is from the cans, while others are used
used for the DRD process. DRD cans merely to improve the appearance of
are currently used in the packaging of the pack. Exterior can coatings may be
food rather than beverages since a used to provide protection against the
greater wall thickness is required to environment (e.g. when the cans will
withstand pressure reversals. The be marketed in particularly humid or
body is beaded, and ECCS is used more salt-laden climates), or as decoration
than tinplate since better enamel to give product identity as well as
adhesion is achieved with the former. protection. Generally some external
lacquering of tinplate and ECCS
3.6.4 Protective and Decorative containers is necessary for products
Coatings stored in hot humid atmospheres to
prevent external corrosion,
Container coatings provide a number particularly at the side seam region for
of important basic functions: three-piece cans.
 protect the metal from the
contents; For most containers the enamel is
 avoid contamination of the applied to the metal in the flat before
product by metal ions from the fabrication, typical film masses being
container; in the range 3-9 gsm (4-12 µm thick).
 facilitate manufacture; However, because of the considerable
 provide a basis for decoration amount of metal deformation with
and product identification; substantial disruption of the surface
 form a barrier to external which takes place in the D&I
corrosion or abrasion. operation, such containers must be
coated internally after fabrication.

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Where it is essential to minimize pigmented with aluminum powder or
product-container interactions, e.g. other materials are also used. The
for canned beer and soft drinks where latter were described above as sulfur-
metal pick-up can affect flavor and resisting enamels, but they are also
clarity, the cans are given a post- used in premium quality packs where
fabrication repair lacquering. sulfur staining is not a problem, simply
to improve the appearance of the
Many types of internal enamel are inside can surface.
available for food containers. The
original can lacquers were based on 3.6.4.2 Decorative Coatings
oleoresinous products which include
all those coating materials which are Although the primary purpose in
made by fusing natural gums and decorating the external surface of a
rosins and blending them with drying metal container is to improve its
oils such as linseed or tung (Chinese appearance and assist its
wood oil). Although oleoresinous marketability, it also significantly
coatings are still used today (largely improves the container's external
because of their low applied cost), a corrosion resistance. Decoration of the
move has been made to synthetic external surface is similar in many
phenolic resins dissolved in a blend of respects to the process used to
solvents. protect the internal surface, the
constituents generally being dispersed
Sulfur resistant enamels are used to in volatile solvents, applied on roller
prevent staining of tinplate surfaces by coating machines (apart from the
sulfur compounds released from foods printed image) and baked in tunnel
such as meat, fish and vegetables ovens.
which have sulfur-containing amino Offset lithography has been used for
acids that breakdown during heat over a century for decorating sheet
processing and storage to release metal.
sulfides. These react with tin to form
black tin sulfide, or accumulate in the 3.7 Aluminum Foils and Containers
headspace and give out an unpleasant
odor. To overcome this problem, two 3.7.1 Aluminum Foil
approaches have been used. Enamels
are pigmented with zinc oxide or zinc Aluminum foil is a thin-rolled sheet of
carbonate which reacts with the sulfur alloyed aluminum varying in thickness
compounds to form white zinc sulfide from about 4-150 µm. Foil can be
(these are known as the sulfur- produced by two methods: either by
absorbing enamels), or the enamels passing heated aluminum sheet ingot
are pigmented with aluminum powder between rollers in a mill under
or white pigment to obscure any tin pressure and then rerolling on sheet
sulfide which might form (these are and plate mills until the desired gauge
known as sulfur-resisting enamels). is obtained, or continuously casting
and cold rolling. This latter method is
Special enamels having additives such much less energy intensive and has
as waxes to assist the release of the become the preferred process.
product from the can, or enamels

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Aluminum foil is available in a variety The aluminum tube is formed by the
of alloys. In the softest temper, cold impact extrusion of an aluminum
aluminum foil exhibits dead fold slug using a plunger. To relieve the
characteristics, i.e. when wrapped hardness, the tube is annealed in an
around an object it will assume the oven at 600 °C, after which the inside
profile of the object with no is enameled with a lacquer. Aluminum
springback. While this is frequently tubes are closed by folding after
advantageous, soft temper foil also application of a latex or heat sealable
wrinkles very easily which necessitates lacquer inside the fold area and heat
the use of great care during handling. applied; this ensures a hermetic seal.
Today the aluminum tube is relatively
Aluminum foil is essentially rare with most food tubes being made
impermeable to gases and water of plastic. Although early plastic tubes
vapor when it is thicker than 25.4 µm, contained aluminum foil as a barrier
but it is permeable at lower layer, it is now common to coextrude
thicknesses due to the presence of LDPE with EVOH to obtain a tube
minute pinholes. For example, 8.9 µm which provides an excellent barrier to
foil has a WVTR of up to 0.3 mL m-2 air and moisture. Plastic tubes are also
day-1 at 38°C and 100% RH. printed by a dry offset process.

Aluminum foil can be converted into a Plastic barrier laminate (PBL) tubes are
wide range of shapes and products used to pack foods, personal care lines
including semirigid containers with and pharmaceuticals. The plastic
formed foil lids, caps and cap liners, structure retains its good looks after
composite cans and canisters; handling. A five-layer structure
laminates containing plastic and includes an ethylene vinyl alcohol
sometimes paper or paperboard (EVOH) barrier layer to protect the
where it acts as a gas and light barrier; contents from oxygen and also
and foil lidding, the latter being sealed prevents oils or volatiles from leaching
using inductive sealing. Processes out of the pack.
involved may include converting,
forming, laminating, coloring, printing
and coating. It can also be embossed
to provide textured surfaces.

3.7.2 Aluminum Tubes

The collapsible aluminum tube is a

unique food package which allows the
user to apply the product directly and
in precise amounts when required.
Typical applications include
condiments such as mustards,
mayonnaises and sauces, as well as
dessert sauces, cheese spreads and 3.7.3 Retort Pouch
pâté.
The retort pouch is a flexible package,

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hermetically sealed on three or four It is for all of the above reasons that
sides and made from one or more retort pouches have found wide
layers of plastic and/or foil, each layer acceptance by military forces, the US
having a specific functionality. The military term for this type of package
choice of barrier layers, sealant layers being ‘Meal, Ready-To-Eat’ or MRE;
and food contact layers depends on "Meals Rejected by Everyone" is a
the processing conditions, product popular nickname for MREs. NASA
application and desired shelf life. began using retort pouch food for
Typical processing conditions involve space missions in the 1970s and the
temperatures of 121°C for times of up US Army began delivering large
to 30 minutes (60 minutes for the quantities of MREs to the troops in
large (3.5 kg) catering packs). One of 1981.
the attractions of the retort pouch
compared to the metal can is the thin
profile of the package (12-33 mm for
200-1000 g pouches), enabling
retorting times to be reduced by up to
60%, final quality to be improved, as
well as rapid reheating prior to
consumption. Other advantages
include the ease of carrying, reheating
and serving, as well as weight and
space saving. Finally disposal of the
used pouch is much simpler than for
the metal can as it can be easily
flattened.

A typical three layer pouch structure

would be an outer layer of 12 µm PET
(polyethylene terephthalate) for
strength and toughness; a middle
layer of 7-9 µm aluminum foil as a
moisture, light and gas barrier; and an
inner layer of 70-100 µm CPP (cast
polypropylene) for heat sealability,
strength and compatibility with all
foods. An additional inner layer of 15-
25 µm PA (polyamide or nylon) is used
when a longer shelf life is required.

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Traditionally a three-sided seal pouch Metals are chemically reactive and can
was used for MREs and other be readily oxidized by O2 and other
commercial products, but recently a agents to form largely useless
multilayer four-side-seal retort pouch corrosion products. This vulnerability
has been developed. Stand-up pouch to oxidation accounts for the fact that
designs having a gusseted bottom with few exceptions (copper, silver
have also been commercialized. and gold), metals do not occur
naturally in the metallic state but are
Transparent retort pouches can be found combined with O2 or sulfur in
produced by replacing the aluminum their ores. A considerable amount of
foil layer with certain plastics that may energy is required to extract metals
also have an inorganic coating, thus from their ores, and the reverse
allowing the pouch to be reheated in a process (which releases energy) is
microwave oven. strongly favored as the metal reverts
back to its natural state. As a very
The shelf life of foods packaged in broad generalization it can be said
retort pouches is very dependent on that the more difficult it has been to
storage temperature. If stored at 16°C, win the metal from its natural form,
they will last for about 130 months; at the greater will be its tendency to
27°C 76 months; at 38°C 22 months, return to that form by corroding, but
and at 50°C, only a month. Because of the rate of return will of course
this, military MREs are stored in depend on the environment.
climate controlled warehouses where
they can be kept for up to ten years 3.8.1 Tinplate Corrosion
before being used.
The tinplate surface consists of a large
3.8 Corrosion of Metal Packaging area of tin and tiny areas of exposed
tin-iron alloy (FeSn2) and steel as a
Metals are important materials for the result of pores and scratches in the tin
packaging of foods, combining coating. The tin-iron alloy layer acts as
properties of strength, toughness, a chemically inert barrier to attack on
ductility and impermeability. the steel base. In the case of tinplate
However, the chemical structure exposed to an aerated aqueous
which gives them their valuable environment, all the anodic corrosion
practical properties is also responsible is concentrated on the minute areas of
for their main weakness: their steel and the iron dissolves, i.e. rusts.
susceptibility to corrosion. Corrosion is In extreme cases perforation of the
the term used to describe the sheet may occur. This is the process
chemical reaction between a metal which occurs on the external surface
and its environment to form of tinplate containers.
compounds; it is a universal process
affecting all metals to a greater or However, inside a tinplate can, the tin
lesser extent. Because the reaction may be either the anode or the
takes place at the metal surface, the cathode depending on the nature of
rate of attack can be reduced and the food. In a dilute aerated acid
controlled by modifying the conditions medium the iron dissolves, liberating
at the surface. H2. In deaerated acidic food, iron is the

Introduction to Food Packaging: Metal Packaging Chapter 3: Page 18 of 21

anode initially, but later reversal of proceed at a rapid rate, resulting in H2
polarity occurs and the tin becomes swelling or perforation of the can.
the anode, thus protecting the steel; Thus it is easily possible to actually
tin has been described in this situation reduce the shelf life of a canned
as a sacrificial anode. This reversal product by using an enameled can.
occurs because certain constituents of
foods can combine chemically with 3.8.2 Corrosiveness of Foods
Sn2+ ions to form soluble tin
complexes. Food products and beverages are
extremely complex chemical systems
As discussed earlier in this chapter, covering a wide range of pH and
food cans with enamel (lacquer) buffering properties, as well as a
coatings are used to protect against variable content of corrosion
excessive dissolution of tin, sulfide inhibitors or accelerators. Factors
staining, local etching and change in which influence the corrosiveness of
color of pigmented products such as food products and beverages can be
berry fruits. However, the use of divided into two groups: intensity and
enamels will not guarantee the type of corrosive attack inherent in
prevention of corrosion and in some the food itself, and corrosiveness due
cases may actually accelerate it. to the processing and storage
Therefore, careful consideration must conditions. All these factors are
be given before selecting an enamel interrelated and may combine in a
system for a particular canned food. synergistic manner to accelerate
corrosion.
The general pattern of corrosion in
enameled cans is very different from The most important corrosion
that in plain cans, and is generally accelerators in foods include O2,
more complex. It depends not only on anthocyanins, nitrates and sulfur
the quality of the base steel plate, the compounds. From a corrosiveness
tin-iron alloy layer and the tin coating, point of view, it is convenient to divide
but also on the passivation layers and foods into five classes: those that are
the nature of the enamel coating. The highly corrosive such as apple and
only exposure of metal in an enameled grape juices, berries, cherries, prunes,
can is at pores and scratches in the pickles and sauerkraut; those that are
enamel coating and at cracks along moderately corrosive such as apples,
the side seam. Some of these peaches, pears, citrus fruits and
discontinuities in the enamel coating tomato juice; those that are mildly
may coincide with pores in the tin corrosive such as peas, corn, meat and
coating, thus resulting in exposure of fish; and strong detinners such as
the steel. Even if defects in the enamel green beans, spinach, asparagus and
film expose only the tin coating, the tomato products. Beverages are
availability of all the corrosion conveniently considered as a fifth
promoters in the can for attack on the class.
limited areas of tin ensures that steel
is soon exposed at them. Because 3.8.3 External Corrosion of Cans
these areas of exposed steel are
almost unprotected, corrosion may Although tinplate is very durable in a

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dry atmosphere, it rusts readily in the free chlorine content of around 5-10
presence of moisture, rusting ppm (parts per million).
occurring more readily the thinner the
tin coating. The presence in the When cans are water cooled, they
atmosphere of sulfur dioxide or oxides should be around 40°C when they are
of nitrogen accelerates the rate of removed from the retort. If they are
corrosion since they dissolve to form warmer than this, there is the risk of
acids. Chlorides (present in locations thermophilic spoilage, chemical
close to the sea) can also cause a rapid degradation and significant internal
increase in the rate of corrosion. Rusty corrosion; if they are cooler than this,
cans will not be purchased by insufficient heat will remain to
consumers even though the contents evaporate any water adhering to the
may be perfectly good. exterior of the can. Cans should be
stacked in such a way so as to enable
self-drying to occur prior to labeling
and packing.

If corrosion is to be prevented during

storage, then the atmosphere
surrounding the can must be free of
corrosive vapors or chemicals, and not
promote condensation of moisture. As
well, packaging materials in contact
with the cans (generally paperboard
Severely rusted tinplate cans cartons closed with adhesives) should
be as free as possible from soluble
The mechanism of external corrosion chlorides, sulfates or other salts which
is complex. Rust forms as a result of may promote condensation of
corrosion of the iron. Three stages of moisture and corrosion. Cartons
exposure to the risk of external rusting usually have a water content of 10-
may be differentiated: thermal 12%, and when the air temperature
processing; cooling of cans, and rises, moisture evaporates from the
storage. cartons in the warmer outer zone and
condenses in the cooler center of the
Cans are typically cooled after thermal stack.
processing (retorting) by passing water
through the retort or placing the cans The shrink wrapping of cans, while
in cooling canals, but in some protecting them from promoters of
canneries hot cans are removed from corrosion found in the atmosphere,
the retort and left to cool in the air. If can cause problems of condensation.
cans are cooled with water, it is a If the air inside the shrink wrap
regulatory requirement in virtually contains a considerable quantity of
every country that such water contain water vapor and the package is later
a measurable amount of bactericide subjected to a drop in temperature,
(typically chlorine) when it exits the condensation will occur.
retort. This usually means in the case
of chlorine that incoming water has a Rusting can also be caused by

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unsuitable conditions of transport and 3.9 Exercise
storage where cycling of the humidity
(described as ‘sweating’) occurs. This (a) Explain why an aluminum beverage
is especially so when cans are can would be unsuitable for packaging
transported from temperate to a meat product that required thermal
tropical areas, or temperate to processing.
temperate areas via the tropics (for
example, from Australia to North (b) Obtain three different local canned
America). Attempts to prevent foods and classify the foods in terms
condensation of moisture by free of their corrosiveness.
movement of air have usually been
unsuccessful because the center of a (c) List the procedures you would take
stack of cartons filled with cans takes a to minimize external corrosion of
long time to respond to the external tinplate cans both during distribution
temperature change and may remain and storage.
below the dew point for long periods.

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