Mark J. Kirwan Paper and Paperboard Packaging Technology
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PAPERBOARD-BASED LIQUID PACKAGING
387
There were alternatives launched in the US – some ten companies had launched
a paper-based package for milk, by the 1930s. In Germany, Jagenberg had
introduced Perga, which was also waxed. It had a circular base and a square top.
Perga developed quickly and by 1939, there were 26 factories in 8 countries
including Germany, England, Sweden, US, Canada and Australia. Jagenberg
founded PKL in 1958 and introduced a package known as Blocpac.
Figure 13.1 Pure-Pak
®
gable top carton. (Reproduced, with permission, from Elopak.)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
Meanwhile, from 1943, a different approach was in development in Sweden.
The company of Åkerlund and Rausing had been formed in 1929 to manufacture
consumer packaging. This company developed Satello, during World War II.
This was a wax-coated cylindrical container produced during the time when glass
and tinplate containers were in short supply. Satello was used to pack jams and
marmalade. The company then set out to develop a milk package which would be
formed from the reel by cutting and sealing a tube of moisture-proofed paperboard
at alternate right angles below the level of the liquid. This produced a tetrahedral-
shaped package and a patent was applied for in Sweden in the name of R. Rausing
in 1944 (Leander, 1996, p. 27).
It did not have an auspicious beginning. Ruben Rausing afterwards said that it was
‘a package (shape) which you only ever saw in geometry classes, to be manufactured
by a machine of whose appearance nobody had even the faintest idea, made
from a material which did not exist and intended for one of our most vulnerable
foodstuffs’. Of the person who invented the package, Rausing said that ‘this was
what you get for hiring people who have no idea how a package is made and what
it ought to look like’ (Leander, 1996, pp. 25–27) – clearly, a justification of lateral
thinking!
As with Pure-Pak
®
, the development of a forming, filling and sealing machine
together with that of a suitable packaging material took several years. The name
Tetra Pak was registered in 1950 and a subsidiary company of Åkerlund and Rausing,
AB Tetra Pak, formed in 1951. Tetra Pak became an independent company under
the ownership of Ruben Rausing in 1965.
Several plastic materials had been tried by Tetra Pak and a polystyrene blend
had been successful. The first packaging machine was installed in a dairy in Lund,
Sweden, in 1952. This produced a tetrahedral-shaped package. Today, it is known
as Tetra Classic (Fig. 13.2).
Polyethylene (PE) had been invented in England in 1933 by ICI and was given
the brand name ‘polythene’. Its use as a packaging material was not envisaged.
Subsequently, DuPont with a licence to manufacture PE encouraged the firm of
Frank W. Egan to develop an extrusion coating machine in 1954/55. Tetra Pak
began coating paperboards with PE in 1956 (Leander, 1996, p. 53).
Figure 13.2 Tetra Classic. (Reproduced, with permission, from Tetra Pak.)
PAPERBOARD-BASED LIQUID PACKAGING
389
A brick-shaped PE-coated paperboard pack was introduced by Zupack
(subsequently bought by Tetra Pak in 1982) in 1959 and, slightly earlier, PKL had
introduced Blocpac with a similar shape. As noted already, the Pure-Pak
®
gable
top from Elopac was being sold in Europe since the 1950s. These developments
prompted Tetra Pak to develop a brick-shaped pack and this was achieved by 1963.
The main advantage of a brick-shaped pack called Tetra Brik, with a square or rect-
angular cross section, is that it makes the best use of storage volume and is easier to
handle mechanically in the dairies and in distribution (Fig. 13.3).
In addition to developments in machinery, materials and pack shapes, it was also
recognised that it would be a major benefit to develop an aseptic paperboard-based
packaging system for milk. Aseptic packaging requires the separate sterilisation
for the package and the product, for them to be brought together in an aseptic
environment, i.e. one from which micro-organisms are excluded, and for the pack to
be hermetically sealed. Sterilising food, particularly liquid food, in this way achieves
its objective with a shorter exposure of the food to heat thereby minimising the loss
of nutritional content and flavour. This process would also result in an extended
product shelf life. It was demonstrated by Tetra Pak on tetrahedral-shaped packs in
1961 and on the Tetra Brik in 1969.
In 1970, Tetra Pak bought an Austrian company, Selfpack, which had also
introduced an aseptic system. PKL replaced Blocpac with Quadrobloc, and
offered an aseptic system. About the same time, this company changed its name
to Combibloc, and is SIG (Swiss Industrial Company) Combibloc, today. Later,
the aseptic Pure-Pak
®
was launched using aseptic processing licensed from
Liquipak.
In 1989, an expert panel on food safety and nutrition at the Institute of
Food Technology, Chicago, ranked the aseptic process and packaging as the
most significant of ten food science innovations of the previous 50 years
(Asepticpackaging, 1989).
Figure 13.3 Tetra Brik. (Reproduced, with permission, from Tetra Pak.)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
The leading companies in both aseptic and non-aseptic paper-based liquid
packaging today are Tetra Pak, Elopak (with Pure-Pak
®
cartons) and SIG
Combibloc. Despite the fact that Pure-Pak
®
was initially the leading paperboard-based
liquid package for many years (1915–1955), Tetra Pak is by far and away the
dominant company worldwide today with over 80% of the market. Speculation on
the reasons for the market developing in the way that it did is beyond the scope of
this book, but they are discussed in detail in INSEAD (2003).
The market expanded significantly from 1960 onwards. Initially, only milk and
cream were packed in paperboard-based packaging; then juices were launched and
from the 1980s, soups, sauces, cooking oil and, more recently, water. The range of
pack sizes is extensive – from Tetra Classics of 12–65ml through the popular range
of packs with square or rectangular cross sections from 200 ml to 2 l. Even larger
4 and 5 litre pack cartons are available in the Pure-Pak
®
range. Pack shapes have
expanded beyond the brick and gable-topped cartons to include multifaceted,
wedge, pouch and hexagonal designs.
The range of products and the shelf life requirements have extended the types
of packaging materials to include aluminium foil, high barrier and ionomer
plastics – the latter providing easier heat sealing, product resistance where required
and excellent adhesion to aluminium foil.
A major feature of pack development in recent years has been the attention paid
by the manufacturers to open, pour, reclose and tamper evidence. In 2001, a fully
retortable paperboard-based carton called Tetra Recart was launched to compete
with the processed food can. A major component of the success of these forms of
liquid packaging has derived from the attention paid to logistics – the science of
material flows. Various designs of brick-shaped transit packaging have been
developed to fit the pallet bases in common use. This in turn fixed the dimensions of
the individual package to minimise the volume required in storage and distribution.
Mini pallets and roll cages were developed to minimise handling and provide
suitable displays at the point of purchase.
In addition to the liquid packaging so far discussed, there are other ranges of
liquid packaging on the market. These include the bag-in-box pack with a corrugated
fibreboard outer and a high-barrier plastic inner, which is used for wine, in sizes of
2–3 l. The commercial use of the bag-in-box concept has been reported handling
30 gal semi-skimmed milk, for the retail chain Pret-A-Manger, where capacity in
storage and transportation savings were claimed (Paperboard, 2001). This type of
packaging has been extended to dry products, for example catering products in
large Pure-Pak
®
gable cartons, and also, to a limited extent, non-food liquid products.
A major feature of paper-based liquid packaging, in Europe and North America,
has been the attention paid to environmental issues in terms of minimising the
use of materials, energy savings in the packaging chain and to the recovery and
recycling of the paperboard, plastics and aluminium.
In 1996, the aseptic package received the Presidential, US, Award for Sustainable
Development – the first package to receive this environmental award (Environ,
1996; Asepticpackaging, 2000).
PAPERBOARD-BASED LIQUID PACKAGING
391
13.2 Packaging materials
13.2.1 Paperboard
Paperboard provides strength, structure, a hygienic appearance, and a good printing
surface for liquid packaging cartons. The basic construction is multi-ply paperboard
made from virgin fibres to ensure a high standard of odour and taint neutrality.
The outer layer is always made from bleached, i.e. white, chemical pulp. (Note:
This is also referred to as bleached sulphate pulp as the chemical separation of the
cellulose fibres from wood is carried out by the sulphate process.) This outer layer
may be white pigment coated to give the best print reproduction. The other layers
may comprise either bleached or unbleached, i.e. brown, chemical pulp.
It is also possible to incorporate chemically treated thermomechanical pulp
(CTMP) which has a lighter shade than ordinary thermomechanical or refiner
mechanical pulp but is not as white as bleached chemical pulp. Where it is used, it
is sandwiched between layers of bleached chemical pulp in the paperboard. CTMP
provides more bulk and hence more stiffness than chemical pulp of the same
grammage, or basis weight, and is lower in cost. The thickness and hence grammage
or basis weight used depends on the size of carton and whether CTMP is used in
the construction.
The main difference between liquid packaging board and board used for folding
cartons is in the type of internal sizing applied at the stock preparation stage. Even
though the pack design ensures that neither the edges nor the flat surfaces of the
paperboard are exposed to the liquid product, because dairies have high humidity
and wet environments, it is necessary to ensure that raw edges of board which are
exposed to that environment do not readily absorb water or product.
Liquid packaging paperboard has developed into a mature product with
capital-intensive large-scale production from relatively few producers (SPCI, 2002).
13.2.2 Barriers and heat sealing layers
The basic construction requires compatible heat sealing polymers on both the face,
top, or print side, and the reverse side. This is usually provided by extrusion coatings
of low density polyethylene (LDPE). In some designs, as we will see later, PE is
sealed outside surface to inside surface and hence the PE on the two surfaces must
be heat-seal compatible; and in other designs, the sealing is inside to inside. Most
designs also require outside to outside sealing. Figure 13.4 shows a 2-side PE
lined paperboard.
This construction will provide liquid tightness and humidity protection. It is
mainly used for fresh products requiring a relatively short shelf life in chilled
distribution.
The thicknesses of PE used depend on the size of carton. The outer layer
may be either 14 or 26 g/m
2
and the inner layer 26, 41 or 56 g/m
2
, the latter two
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
being applied in two or three consecutive applications to achieve the total PE
thickness required.
Polyethylene, as with most plastics, should not be thought of as a single material
in the way, for instance, that we think of a specific inorganic chemical compound.
There is a large family of PEs of different densities and molecular structures, which
are controlled by the conditions under which the ethylene was polymerised – the con-
ditions referred to concern pressure, temperature and the type of catalyst used. The
recent introduction of metallocene (cyclopentadiene) catalysts has had a major impact
on the properties of PE and other plastics. Hence this is an active area of development
for the manufacturers of PE. The LLDPE (linear low density polyethylene) is superior
to LDPE in most properties such as tensile strength, impact and puncture resistance.
It is also possible to blend these and other compatible polymers to enhance or
vary the properties. In this connection, ethylene vinyl acetate (EVA) should be
mentioned. As an extrusion coating, EVA improves heat sealability. In films, such
as in the laminates used in the bag-in-box applications, EVA improves strength
and flexibility as well as heat sealability.
For higher barriers in liquid packaging, it is necessary to use other materials in
addition to PE. Ethylene vinyl alcohol (EVOH) and polyamide (PA) are good
oxygen barriers and they also provide flavour protection and oil/fat resistance.
EVOH and PA are not particularly good barriers to moisture vapour, the barrier for
which is provided by the PE. These structures are used for medium and long-term
shelf-life products in ambient and chilled distribution and provide an alternative to
the use of structures which include aluminium foil (Fig. 13.5).
Aluminium foil is a well-established high-barrier material, which provides
a barrier to light, oxygen and moisture vapour. It provides excellent protection to
flavours and has oil/fat resistance. It may be laminated to either side of the
paperboard. When laminated to the outside (top side) of the paperboard, it
provides the carton with a metallic finish. In the majority of the cases, aluminium
is used solely for barrier properties and an additional layer of PE is applied as
Polyethylene
Paperboard
Polyethylene
Figure 13.4 2-side polyethylene-lined paperboard. (Reproduced, with permission, from Elopak.)
PAPERBOARD-BASED LIQUID PACKAGING
393
indicated in Figure 13.6. These laminates are used for aseptic and hot filled
products, requiring a long shelf life in ambient distribution, and fresh juices in
chilled distribution.
Ionomer (Surlyn
™
) is a moisture vapour barrier like PE and, additionally, has
oil/fat resistance. It has very good hot-tack and heat-sealing properties and is used
as a tie layer (process aid) on aluminium foil on which PE can be extruded. Ionomer
can be extruded onto aluminium foil at lower temperature than PE and thereby
avoid potential odour from PE extruded at the temperatures necessary to achieve
good adhesion to aluminium foil.
Polyethylene
Polyethylene
Paperboard
EVOH barrier
Tie layer
Figure 13.5 2-Side polyethylene-lined paperboard incorporating ethylene vinyl alcohol (EVOH).
(Reproduced, with permission, from Elopak.)
Polyethylene
Paperboard
Tie layer
Tie layer
Aluminium barrier
Polyethylene
Figure 13.6 2-Side polyethylene lined paperboard incorporating aluminium foil. (Reproduced, with
permission, from Elopak.)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
An alternative tie layer for PE/PA would be one of the Bynel
®
range of
coextrudable adhesion promoters from DuPont (Tampere, 2000; DuPont website).
Polyester (PET or PETE) film vacuum metallised with a thin layer of aluminium
can be a good barrier to oxygen and moisture vapour. It can be laminated to
paperboard. Other films such as polypropylene (PP) and polyamide can also be
metallised. Another high-barrier film construction has a plasma-coated layer of
silicon oxide (SiO
x
) on polyester film which also provides a completely non-metallic
barrier which can be extrusion-laminated to paperboard.
There are many possible combinations of extrusion coating and laminating.
For example, PP, with resistance to essential oils, is extrusion-coated onto paper-
board for cartons used to pack orange juice. The PP is overcoated with a blend
containing PP plus 20% LDPE and 4% tackifier to improve the heat sealing of the
inner layer (TAPPI, 2001). PP also has higher heat resistance than PE.
In practice, the product, the shelf life required and commercial considerations
will all have a bearing on what is used in any specific liquid packaging application.
Another possible approach to barrier coating which may find a place in future
developments is by way of size-press application on the paperboard machine of
polymers in water-based dispersions. Both Surlyn
™
and Nucrel
®
are thermoplastics
from DuPont with oil/grease resistance and low seal-initiation temperature properties,
which are available as dispersion coatings. Lower costs are favoured by the
treatment which takes place on the paperboard machine (SPCI, 2002; Tampere, 2000;
DuPont website).
Another reported development involves the production and use of polymer
nanocomposites in which nanoscale particles of surface-modified clay are dispersed
in the polymer. Enhanced barrier properties are claimed, making the material
suitable for the packaging of oxygen-sensitive beverages such as fruit juices
(Krook & Hedenqvist, 2001).
A relatively new material in production from Ecolean (www.ecolean.com)
comprises calcium carbonate mixed with PE plastics (Packaging, 2002).
13.3 Printing and converting
13.3.1 Reel-to-reel converting for reel-fed form, fill, seal packaging
The paperboard is printed reel to reel by flexo or gravure. The printed reels are
then either PE extrusion coated on both sides or, if an additional barrier layer is
required, the reels are PE extrusion laminated to aluminium foil, or PE extrusion
coated on both sides. With the heavier PE extrusion coatings on the inside of the
package, a second PE application is required. As noted above, an alternative barrier
layer may be applied – sandwiched between the paperboard and the outer coating
of PE. The reels are then creased and slit to size and sent to the packer where the
reels are formed, filled and sealed. An exception to this is Tetra Classic which is
not creased during conversion.
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395
13.3.2 Reel-to-sheet converting for supplying printed carton blanks for packing
There are options possible for producing flat carton blanks from extrusion coated
and laminated paperboard. These materials are printed and cut and creased in-line.
The printing process would most likely be gravure or flexo. In order to achieve
good print adhesion to the PE, it is necessary to treat the surface of the PE after
extrusion coating with an electric corona discharge, or direct gas flame, which oxi-
dises the surface of the PE, making the surface molecules more reactive. Where
printing is applied to the outside surface of the PE, the inks must have good wet
and dry rub resistance.
Polyethylene and barrier coatings can also be cut into sheets before printing,
usually by offset litho. The printed sheets are then cut and creased to produce indi-
vidual carton blanks. The print must also be product resistant.
The design includes a narrow fifth panel on the side of the blank about 15mm
wide. The edge of this panel is then ‘skived’. In this process, most of the
paperboard from a 4-mm wide strip on the edge is removed. This leaves a narrow
strip, 4mm wide, of the inner layer of material, i.e. PE+ aluminium foil + residual
fibre, which is then folded over through 180°, so that it is then in contact with PE on
the remaining 10 mm wide panel. As this is completed, the carton panels are folded
over, and the 10 mm wide side seam of the carton is formed using hot air or gas
flame. Skiving enables efficient folding and a side seam which is restricted to two
thicknesses of the paperboard. This construction seals the inner surface of the
narrow sealing panel to the inner surface of the joining panel and ensures that when
the carton is filled, the liquid does not have any contact with a raw or exposed edge of
paperboard, as shown in Figure 13.7. The side seam sealed carton blanks are
then ready for dispatch to the packer.
13.4 Carton designs
13.4.1 Gable top
The original Pure-Pak
®
(from Elopak) carton was a gable top as shown in Figure 13.1;
this is also available with a screw cap. Gable-top cartons have been available from
other suppliers for many years. Tetra Rex, as shown in Figure 13.8, is a gable top
Figure 13.7 Heat sealed side seam. (Reproduced, with permission, from Tetra Pak.)
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carton produced by Tetra Pak. These cartons are produced from carton blanks.
Other gable tops are Bowpack (Elopak from 2003) and Biopack (Field).
13.4.2 Pyramid shape
This is the shape of the first liquid pack launched by Tetra Pak. It is known today
as Tetra Classic and is shown in Figure 13.2. The board is supplied uncreased in
reel form. The pack is formed over a shoulder and around a vertical tube where the
side seam is heat sealed. Prior to sealing, a thin ribbon of PE is fed and heat sealed
across the raw, or exposed edge, of the longitudinal seal. This ensures that the liquid
is not in contact with the raw edge of the paperboard. The product is fed into the
tube where it enters the folded side seam–sealed tube. The horizontal seal is made
intermittently through the product. As succeeding heat seals are made at right
angles to each other, the resulting pack is shaped like a pyramid.
13.4.3 Brick shape
The Tetra Brik is made from reels and the Combibloc (SIG Combibloc) is made
from carton blanks. In the case of Tetra Brik, the side seam is also covered with a
strip of PE film to prevent the product from contacting the raw edge of the overlap-
ping seam. This style results in ears formed when the horizontal heat seals are made.
They are folded flat against the pack and lightly sealed. Tetra Brik Aseptic 200ml is
Tetra Pak’s first resealable portion pack. It has a square profile and features a screw
cap (Corrugated, 2002). Another brick shape is Quadrobloc.
Figure 13.8 Tetra Rex gable top. (Reproduced, with permission, from Tetra Pak.)