Mark J. Kirwan Paper and Paperboard Packaging Technology
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PAPER-BASED FLEXIBLE PACKAGING
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can also be produced specifically for applications where high twist retention is
required, for example wrapping of sugar confectionery. Coloured RCF is available.
3.2 Packaging needs which are met by paper-based flexible packaging
It should be noted that this discussion is limited to paper-based flexible packaging
as used for food products in various forms together with, for example, confectionery,
pharmaceuticals, tobacco, horticulture, agrochemicals, DIY and electrical goods.
The needs of medical packaging will be discussed separately in Section 3.4.
3.2.1 Printing
Flexible packaging must carry information, including safety information where
appropriate, about the product and its use. In many instances, depending on the
product and how it is marketed, flexible packaging must provide visual impact at
the point of purchase.
The main processes used in printing paper for flexible packaging applications
are flexography and gravure. Several systems are available including water based,
solvent based, UV cured and digital printing. The print quality can be varied
depending on the surface and colour of the paper. In terms of colour, the choice is
usually white, i.e. based on chemical-bleached pulp, or brown, i.e. based on either
unbleached chemical pulp or dyed recovered fibre or mixtures of these two types
of pulp. The surface may be either machine finished (MF), machine glazed (MG),
supercalendered (SC), on-machine coated or cast coated. The whiter and smoother
coated papers will give the best print reproduction – sharper dots in colour illustrations,
fine text, good contrast and solid uniform colour.
3.2.2 Provision of a sealing system
A sealing system may be used to construct the pack, for example a sachet which is
sealed on two or three sides prior to filling, and to seal the package after filling.
Where the package has to provide product protection through the provision of
barrier properties, it is necessary for this sealing to be pinhole free.
The main method for providing sealing to paper so that it can be formed
and sealed is through the application of a sealable material. Examples are by
applying:
•
Polyvinylidene dichloride, PVdC, from an aqueous dispersion, also acrylics.
Another vinyl-based heat-seal coating is VMCH (Dow Chemical Co.) which
is applied from a solvent-based solution. Where a higher solids or lower
viscosity coating is required, Dow recommends VMCC. (Note: VMCH and
VMCC are carboxyl modified vinyl copolymers. They are functional terpoly-
mers of vinyl chloride, vinyl acetate and maleic acid [Dow, 2005].)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
•
Hot melt coating by roll application, or slot die, based on a wax blend
containing plastic polymers, such as ethylene vinyl acetate (EVA) and other
additives which improve initial tack and seal strength.
•
Extrusion coating with polyethylene (PE) and EVA modified PE. Other plastics
can also be heat sealed but they would not be chosen purely for their heat
sealability on grounds of cost. They may be chosen where they perform an
additional function such as that of an oil or grease barrier, for example an ionomer
such as Surlyn
®
(DuPont), and heat resistance, where polypropylene (PP),
polyethylene terephthalate (PET or PETE) or polyamides (PA) would be used.
•
Cold seal, based on either natural rubber or synthetic latex on a smooth paper
surface, such as glassine or bleached kraft, sealing is activated by pressure.
Cold sealing can be carried out at high speed and is particularly favoured for
the packaging of chocolate and other heat-sensitive products.
3.2.3 Provision of barrier properties
3.2.3.1 Introduction to barrier properties
Barrier properties are necessary in paper-based flexible packaging to protect the product.
It is always necessary to define the nature of the product protection, for example
texture, flavour, aroma, etc., required to achieve the required shelf life in the
expected environment(s) of storage, distribution and use. It is then necessary to
determine the type and amount, in terms of thickness and coating weight, of the
barrier material(s) required to provide the product protection required.
Several types of protection relating to moisture, texture, flavour and aroma,
etc., and the choice of barrier materials which are available to provide that
protection will be discussed in this section. It must be appreciated that the materials are
not necessarily interchangeable because the degree to which each provides a
particular barrier property varies, for example, aluminium foil compared with,
PVdC or PE.
Furthermore, the amount or degree of any particular form of protection will
vary with the amount of material applied in terms of thickness or coating weight.
Both these causes of difference have commercial implications in that the weight or
thickness necessary to achieve the required performance may rule out the use of
a particular material where the cost is uncompetitive.
3.2.3.2 Barrier to moisture and moisture vapour
The effect of moisture on packaged products depends on the product. For some it
is necessary to maintain the moisture content at a high level to prevent the product
from drying out. For others the reverse may be the case as by absorbing moisture the
product may loose texture, (for example, crispness in a snack food) and metal products
containing ferrous iron can rust when exposed to high humidity and oxygen.
Every food product has an optimum moisture content with respect to its stability
as well as its texture and flavour. Non-food products, such as tobacco, also have an
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optimum moisture content range within which the quality is satisfactory. There
will, however, be a moisture content level at which the quality of the product
concerned would be deemed unsatisfactory. Assuming that the product is at the
correct moisture content when packed, it is the aim of the packaging to ensure that
an unsatisfactory moisture level is not reached within the intended shelf life in the
recommended storage conditions.
Food products affected by moisture will, however, gain or lose moisture until
equilibrium is established with the relative humidity (RH) of the atmosphere to
which they are exposed. In the case of a packed product, this environment will be
that existing within the sealed package.
It is therefore necessary to know the moisture content specified at the point of
manufacture/packaging and the equilibrium relative humidity (ERH) at that moisture
content. The next important consideration is the relative humidity of the environment
in which the packaged product will be stored, distributed and merchandised. In order
to simulate temperate conditions, testing is carried out on sample packages stored at
25°C and 75% RH where the product will gain moisture and 40% where it would
lose moisture. For tropical conditions, 38 °C and either 90 or 40% RH is used.
At 25°C/75% RH there will be a tendency for packaged products with a low
ERH to gain moisture from the external environment and for those with a higher ERH
to lose moisture to the external environment. It is the function of the packaging to
ensure that any movement of moisture either into, or from, the product still
ensures that the product does not achieve a moisture content, within the specified
shelf life, at which the product is judged unacceptable.
Many flexible packaged food products are required to have shelf lives of from
6 to 18 months depending on the product and where they are marketed and used.
This would be an inconvenient length of time to carry out a shelf-life test and
therefore laboratory studies have been developed to give guidance on shelf life
and barrier based on accelerated storage tests (Paine, 2002).
Dried foods, such as instant coffee and potato chips (crisps), have a typical
moisture content of around 3% and the ERH is 10–20%. These products require
a high, or good, barrier to water vapour – note this means a low water vapour
transmission rate (WVTR). Dried foods such as breakfast cereals with ERH 20–30%
are less stringent with respect to the water vapour barrier. For dried fruits and nuts
the ERH is 30–60% and for salt and sugar, 75 and 85% respectively. A cake has an
ERH of around 90% and it maybe thought that the barrier should prevent the loss
of moisture from the cake. However, an RH of 90% inside the package is an ideal
condition for mould growth. So rather than trying to prevent moisture loss it is
necessary to allow some loss but to slow down the rate to prevent the creation of
too high an RH within the package (FOPT, 1996).
Low water vapour permeability, i.e. a good or high barrier, may be provided in
several ways:
•
laminate/extrusion coating, paper/aluminium foil//PE
•
laminate/extrusion coating, metallised PET/paper/PE
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
•
dispersion coating, paper/PVdC
•
hot melt coating, paper/hot melt
•
wax coating, paper/wax blend (microcrystalline wax plus additives)
•
extrusion coating, PE/paper/PE or paper/PE
•
laminate/extrusion, SiO
x
coated PET/paper/PE.
A moderate or medium barrier to water vapour would be provided by PE extrusion
coated paper.
3.2.3.3 Barrier to gases such as oxygen, carbon dioxide and nitrogen
This barrier is necessary for the protection of products sensitive to flavour and
aroma loss or contamination. Products where atmospheric oxygen causes product
deterioration through oxidative rancidity, for example potato chips (crisps), may
be gas flushed with an inert gas such as nitrogen and carbon dioxide. Ground
coffee is sensitive to oxygen and may be vacuum packed.
The barrier is provided by either aluminium foil, EVOH, PVdC, aluminium
metallised PET, SiO
x
coated PET or wax. If aluminium foil is used, the heat sealing
is provided by hot melt or PE. Some end-users have discouraged the use of
aluminium foil on the grounds that the structure is not easily recyclable in
post-consumer waste management systems. EVOH is an excellent gas barrier but
the barrier deteriorates significantly in a high RH environment. It is therefore
sandwiched, when used in a paper-based flexible material, between layers of PE or
PP. An EVOH structure of this type is offered for liquid packaging.
3.2.3.4 Barrier to oil, grease and fat
This is required where the product has an oil, grease or fat content which must be
retained within the pack because loss of these ingredients would be reflected in
a lower quality product and because any permeation to the surface of the package
would be unsightly.
This barrier is provided by aluminium foil, PVdC or ionomer extrusion coating.
If aluminium foil is used, the heat seal needs to be a plastic coating with oil, grease
and fat resistance. Medium density PE, high density PE, PP and ionomer resin
such as Surlyn
®
all provide oil, grease and fat resistance, the degree of the barrier
property being given here in ascending order, with Surlyn
®
providing the best
barrier.
It may be sufficient to use an ionomer resin as a thin coating between PE
and aluminium foil. This will provide a good product barrier and also ensure
that the adhesion between the aluminium foil and the PE does not break down in the
presence of the product during storage. The use of ionomer resin in this way
also ensures that good PE adhesion is achieved without resorting to higher
temperatures at which there would be a danger of odour and product taint.
Greaseproof paper and glassine also have oil, fat and grease resistance, but will
also require a heat or cold seal coating for flexible packaging applications.
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3.2.3.5 Barrier to light
Some, particularly fat containing, products can deteriorate in light, especially
sunlight, containing a UV component, which can promote oxidative rancidity.
Therefore an opaque barrier is required which is only really guaranteed by including
aluminium foil in the construction.
3.3 Manufacture of paper-based flexible packaging
3.3.1 Printing and varnishing
Printing is normally the first process in the manufacture of flexible packaging.
Flexible packaging is printed reel to reel. The main processes used are flexography
and gravure. In addition, some printers use web offset litho. Digital printing can be
used, and normally it would be used on short runs. All the usual ink systems are
used, including metallic and fluorescent inks. UV cured systems are used to give a
high gloss, a rub and product resistant surface together with high heat resistance in
the, subsequent, heat sealing areas. Whereas printing is used for text, illustrations
and overall decoration, varnishing is used for surface finish, i.e. gloss, matt or satin,
protection of the print for rub and product resistance and for the control of surface
friction.
The print quality depends on the paper surface. To obtain the best print
results, a mineral pigment-coated paper should be used. Some applications of
printed paper do not require further conversion. Examples of paper products
supplied, printed on reels, to end-users for forming, filling and sealing include
flour/sugar bags (Fig. 3.1) and labels for use in injection moulding, for example
ice cream tub lids.
Patterned heat-seal and cold-seal coatings may be applied on the reverse
side of paper in register with print on the other (outside) surface using printing
techniques.
Figure 3.1 Sugar/flour bags. (Reproduced, with permission, from Robert Bosch GmbH.)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
3.3.2 Coating
Coating is the simplest method of adding other functions to paper. The active
functional material, for example VMCH, PVdC, PE, wax, etc., is either applied from
a solvent solution, water-based dispersion or as a solid, in the molten state.
3.3.2.1 Solvent-based coatings
Solvent-based coatings applied to paper by gravure mainly comprise varnishes
which impart heat resistance so that the surface does not pick under heat-sealing
bars. The varnish could be 100% solids UV cured and has high gloss, product and
heat resistance.
3.3.2.2 Water-based coatings
Water-based coatings for paper include PVdC with the coating applied by roll.
The coating weight and smoothness is controlled by air knife – a non-contact
technique (Fig. 3.2). Several applications may be applied and dried in one pass
through the coating machine, giving a total coating weight up to around 30g/m
2
.
3.3.2.3 Coatings applied as 100% solids, including wax and PE
Wax is the oldest paper-based functional coating. Originally paraffin wax was used.
From the 1950s, the main wax component has been microcrystalline wax to which
polymers such as PE and EVA have been added by blending to improve the barrier
in folded areas and also the hot tack in heat sealing. Wax is applied as either ‘dry
waxing’ or ‘wet waxing’. In the former, the wax is applied and the weight controlled
by roller application. It is then passed over heated rolls so that the wax is driven
into and saturates the paper. Alternatively, after application the wax can be rapidly
chilled, usually in a water bath, thereby creating a high gloss and keeping most of
the wax on the surface.
Polyethylene (PE) is supplied as pellets which are melted by a combination
of high pressure, friction and externally applied heat. This is done by forcing the
Breast
roll
Applicator roll
Coating pan
Air jet
Air
knife
Figure 3.2 Principles of air knife coating. (Reproduced, with permission, from The Paper Industry
Technical Association, PITA.)
PAPER-BASED FLEXIBLE PACKAGING
93
pellets along the barrel of an extruder using a polymer-specific screw under con-
trolled conditions that ensure a homogeneous melt prior to extrusion (Fig. 3.3).
The molten plastic is then forced through a narrow slot or die onto the paper, or
the aluminium foil surface of a previously laminated paper/aluminium foil laminate.
As it comes into contact with the paper or foil it is brought in contact with a large
diameter chill roll with either a gloss or sand blasted matte finish which determines
the surface finish of the PE (Fig. 3.4). To render the surface suitable for print and/or
adhesion with an adhesive, it is treated with a corona discharge. A coating weight
of 20g/m
2
would be typical though higher and lower coating weights are possible.
Usually the coating is confined to one side but if two side coating is required then
in order to avoid blocking in the reel only one surface may have a gloss finish.
Extruders having two extruding dies can coat both sides of a sheet in one pass
through the machine (Fig. 3.9).
The simplest PE application is Kraft/PE as an overwrapping material – e.g. as
a transit overwrap for 10 × 20 cigarette cartons. In this case, labels would be
applied over the envelope end seals.
Plastic granules
External electrical
heating elements
Motor
Molten plastic
Die
Figure 3.3 Plastic extruder.
Paper/paperboard
Extruder
Molten plastic
Chill roll
Slitting and reeling
Figure 3.4 Extrusion coating of paper or paperboard.
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There is a wide range in the choice of PE. Hot tack in heat sealing is improved
by blending EVA with LDPE. If a medium density PE is used, it improves the
puncture resistance. This is important for providing better puncture resistance
when packing granules with sharp edges. Surlyn
®
ionomer improves adhesion to
aluminium foil and provides resistance to essential oils and other oil and fat-type
products. EVOH can be sandwiched between outside and inside coating of
paper-coated PE to significantly improve the barrier to oxygen, which causes rancidity
in fat-containing products, and other gases.
3.3.2.4 Metallisation
Metallisation is a process whereby aluminium is vapourised in vacuum and deposited
to form a thin film on the surface of a substrate (Fig. 3.5). This process has been
applied to paper but it is more usually applied to film. There are three problems
with paper. First, the time required to create the vacuum in the material extends
the time required at a reel change and reduces the moisture content so that the
paper requires rehumidification after metallising. Second, being thicker than film
the length of material processed per reel is lower and hence the area coated in a
given time is lower than with plastic film, and third, the surface of paper is not as
smooth as film and the barrier improvement is not sufficiently significant.
These features can be overcome at a cost. The paper surface can be clay coated
and prelacquered to make the surface smoother and improve the metallised
appearance. The moisture changes can be avoided by using a transfer metallising
process whereby the metallising layer is first transferred to a reel of PP. The
metallised layer on the PP is transferred to the paper with the help of an adhesive.
In this process, the plastic film (PP) can be reused.
The advantage of metallised paper is that it can replace aluminium foil in some
applications. The barrier is, however, poor compared to foil unlike the situation
Rewind
Wire
feed
Vapourised
aluminium
Shutter
Unwind
Vacuum
Chilled
drum
Boat
Figure 3.5 Metallising process. (Reproduced, with permission, from The Institute of Packaging.)
PAPER-BASED FLEXIBLE PACKAGING
95
when plastic film is metallised. Due to the on-costs required to metallise paper the
price differential compared with aluminium foil is not significant. Metallised
polyester film (PET or PETE) has been laminated to paper and paperboard as an
aluminium foil replacement. A large usage of metallised paper is for the bundling
of cigarettes prior to packing in a carton or soft pack.
3.3.2.5 Hot melt coatings
Hot melt coatings are also wax-type blends but they have higher viscosities than
the simpler waxes discussed under Section 3.2.3. They are either applied by roll
coating or by extrusion, though the extruder would be a simpler design than the
type used to extrusion coat PE.
Hot melt-coated paper is used as an in-mould wrap-around label on thermofor-
med plastic tubs for yoghurt.
A typical specification is:
•
overlacquer for gloss and print protection
•
gravure or flexo print
•
bleached kraft paper (95–100 g/m
2
)
•
hot melt coating.
The labels are fed from the reel into the pot moulds just before the sheet of plastic,
for example polystyrene, also fed from reels, is thermoformed. The hot melt coating
seals to the plastic. It also seals where it overlaps the print. The coefficient of friction
is an important property of the overprint lacquer to ensure efficient runnability on
the machine.
One of the main applications for hot-melt-coated paper is in the wrapping of
soft cheese. The packaging in this application is designed to participate in the
maturing of the cheese.
A typical specification is:
•
20 µ OPP film reverse side printed by flexo or gravure
•
adhesive laminated – sometimes in stripes – to 25–40 g/m
2
bleached kraft
paper
•
hot melt coating.
The OPP is microperforated after printing to allow cheese respiration and a
controlled water vapour transmission rate. The hot melt coating which is used in
the direct wrappage of food provides:
•
controlled oxygen permeability (which influences the development of the
cheese bacteria and, therefore, the taste)
•
good folding around the product
•
heat sealing for a better pack closure
•
glossy finish to the product.
This specification can be modified by metallising the OPP film over the print.
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3.3.2.6 Cold seal coating for pack closure/sealing
For specific applications in flexible packaging, an alternative to heat sealing is
coldseal. This technology requires only mechanical pressure to bond two cold seal
coated surfaces together, without any heat being required. Sealing at room
temperature, excellent seal integrity and a wider variety of substrates are the main
advantages of cold sealing compared with heat sealing. When a packing machine
using heat seal stops, heat from the sealing jaws can result in significant product
damage and high rejection rates, especially with a product involving chocolate.
Coldseal is not particularly sensitive to the sealing dwell time and allows a higher
tolerance to variations in line speed. The main applications for cold seal are on
horizontal form, fill, seal (HFFS) bar lines and overwrapping, although some
vertical form, fill, seal (VFFS) operations exist as well.
Coldseal technology is based on a coating technique. A water-based emulsion
is printed onto a web substrate by means of the gravure process. The key component
of the emulsion is natural rubber latex which provides cohesive features. The coatings
stick preferentially to each other when two coated surfaces are brought together.
Other ingredients are water, ammonia, surfactants, anti-oxidants, anti-foam
agents, biocides and an acrylic component. The latter acts as an adhesive, bonding
the coldseal to the substrate.
In the wet state, coldseal has a shelf life limited to six months. It must be stored
away from frost and heat. Below 0°C, an irreversible loss of sealing ability
occurs. After printing, a shelf life, before use, of at least six months is guaranteed.
Excessive ageing allows physical, chemical and biological degradation to occur and
leads to loss of seal strength and the development of repulsive odours. After sealing,
coldseal packaging keeps its seal integrity in a freezer, for example ice cream sticks.
Typical constructions are:
•
release lacquer
•
printing inks
•
substrate, for example glassine
•
coldseal applied in a pattern
and
•
release film
•
printing ink
•
laminating adhesive
•
substrate
•
coldseal applied in a pattern.
Release lacquer or film is required to allow for easy unwinding (low cling) and
prevention of blocking of the material in the reel. Release lacquer consists of
polyamide resin. Release film is a low surface tension plain OPP. Glassine, which
may be coloured, for example chocolate brown, would be a suitable paper substrate.
(Typical plastic film substrates would be white pearlised polypropylene, transparent
and metallised polypropylene as well as metallised polyester.)