Mark J. Kirwan Paper and Paperboard Packaging Technology
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PAPER BAGS
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The paper carrier bag is used extensively as a ‘carry-out’ or ‘carry-home’ pack
for a wide range of ready meals and snacks for High Street fast-food outlets.
A smaller and specialised market exists for luxury gift carrier bags. These bags,
often handmade, use high quality art papers with board attachments and coloured
rope or cord handles. They are used as point-of-sale bags for luxury goods, such as
perfumes and jewellery. In addition, they are sold empty, mainly in boutiques and
greeting card shops for customers’ use as presentation carriers.
Finally, mention should be made of the ‘chuck bag’ or supermarket check-out
bag. These large capacity brown kraft SOS bags have fulfilled a demand in Europe
and America for transporting goods from a supermarket check-out to a car boot
(trunk). They are often re-used in the home as waste bags.
The papers used for this sector (apart from the chuck bags) are usually high
quality coated papers to achieve the best printing results, combined with sufficient
physical strength to protect the contents. Frequently, the contents are already
in sealed containers to protect against moisture or grease penetration. If not, a
wide range of coatings and protective layers can be added to the inside of the
carrier bag.
Figure 5.5 SOS carrier bag with handle.
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5.3 Types of paper used
Section 5.2 refers to the different papers used in paper bags according to the
application. This section examines these papers in more detail.
5.3.1 Kraft paper – the basic grades
‘Kraft’, the German word for strength, is the grade of paper most commonly used.
Previously, sulphite paper has been used for flat and satchel bags but it lacks
whiteness. Kraft paper is used in both unbleached form (brown kraft) and bleached
form (white kraft). Within these two main categories numerous qualities are avail-
able according to the mill process. Machine glazed (MG) kraft, supercalendered
(SC) and machine finished (MF) are all available according to the degree of
smoothness required. Striped or ribbed krafts are used and are traditional for
certain retail trades, particularly clothes and fashion shops. Coloured krafts can be
obtained but minimum tonnages mean that a solid ink coverage is often a more
practical proposition.
Other grades of paper used for special applications are as follows.
5.3.2 Grease resistant and greaseproof papers
Used widely for foodstuffs and pet foods where protection from greasy and oily
products is required.
5.3.3 Vacuum dust bag papers
Special grades of paper have been developed for paper bags used in vacuum cleaners.
The important factors are porosity and filtration to ensure the air flows freely
through the paper but not the dust.
5.3.4 Paper for medical use and sterilisation bags
A special government approved paper has been developed for this purpose. Poros-
ity, bacteriaproofness and wet-strength properties are all important factors, to
ensure that sterilisation bags withstand steam autoclaving and remain sterile.
5.3.5 Wet-strength kraft
This grade is often specified where exposure to damp or moisture is likely. It is an
essential requirement for hospital bags which are steam sterilised.
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5.3.6 Recycled kraft
Recycled kraft is used for carrier bags and other applications where a recycled
paper is acceptable and is often specified by customers. It is not suitable for med-
ical or food grades where virgin kraft must be used.
5.3.7 Coated papers
Kraft papers can be coated with china clay to provide an extra smooth finish and
achieve enhanced print standards. Extrusion coating of kraft papers with various
grades of plastic film is used to both protect and enhance appearance.
5.3.8 Laminations
Kraft papers are used in conjunction with all plastic films and/or with aluminium
foil to provide up to 4-ply laminations depending on the level of protection
required from the bag. Aluminium is used particularly where odour, flavour retention
and light protection are important factors.
5.3.9 Speciality papers
Many other grades have been used to produce bags with specialised requirements,
some on an experimental basis. These include:
•
Water soluble paper
•
Flame retardant paper
•
Wax impregnated paper
•
Bitumenised kraft
•
Creped kraft.
5.3.10 Weights of paper
Grams per square metre (g/m
2
) is used for measuring the weight of paper. The
range of weights used depends on the style of bag and machinery used. In general,
lighter substances can be used to make flat and satchel bags and range from 40 to
100g/m
2
. The range for SOS bags is between 60 and 110g/m
2
– the minimum
weight is higher due to the manufacturing process of SOS bags. The total
grammage of paper bags can, of course, be increased by making 2-ply bags. The
grammage of some coated and processed papers needs to be increased to match
the equivalent strength of a kraft paper. (Note: lbs/3000sq ft = 1.627 g/m
2
.)
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5.4 Principles of manufacture
5.4.1 Glue-seal bags
5.4.1.1 Flat and satchel bags
The bags are formed from a reel of the required material which is usually about
20 mm wider than the total width of the bag, i.e. face + gusset × 2. The 20 mm of
extra material becomes the overlap to form the seam along the bag.
The bag is shaped over a forming plate matching the size required. Guide wheels
tuck in extra paper to form a gusset if a satchel bag is required. Paper from the reel
is cut to the required length by a knife or a blade mounted on a rotating cylinder.
Glue is applied to the web at two separate stations to seal the bottom seam and the
side seam.
5.4.1.2 Self-opening satchel bags (SOS bags)
As can be seen from the illustrations in Section 5.2, the method of construction
for SOS bags is more complex. The free-standing bag is formed by a series
of folds, slits and creases. Heavier paper is needed to withstand this process and
machine speeds tend to be slower than for flat and satchel bags. The principles
relating to reel width and glue application are the same as for flat and satchel
bags.
5.4.2 Heat-seal bags
As the construction depends on a heat seal, the reel or reels of paper must include
a thermoplastic material to form the inside of the bag. This can be in the form of
an extrusion coating or a separate web of heat-sealable film. Virtually all heat-seal
bags are of SOS shape. The bottoms are formed by a series of folds, cuts and
creases. Heat is applied to both the base of the bag and the sidewall by heated
machine parts and a hot-air flow.
5.4.3 Printing on bag-making machines
Depending on the nature of the design required and the number of colours, the
printing process is often included in the bag-making machinery. Paper from the reel
is first fed through the printing unit – up to four colours – before passing through to
the bag-making process. Quick drying inks are essential to avoid reduction in
machine speeds.
5.4.4 Additional processes on bag-making machines
As paper from the reel passes through the bag-making machine, other processes
and attachments can be included.
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5.4.4.1 Punching
If windows are required in the bag, the paper web can have a punched cut-out.
Unless the bag is totally lined with film, a narrower reel of film to suit the width of
window is used. Bags can have a large number of very small holes punched, for
example ventilation holes for bulbs.
5.4.4.2 Paper handles
Narrow reels of folded or twisted paper and reinforcing paper to secure the
handles are fed on to the main web of paper to produce carrier bags.
5.4.4.3 Lacquers and adhesives
A strip of heat sealable polyvinyl acetate (PVA) coating can be applied to the top of
the inside of the bag. Hot-melt adhesive can be sprayed on to the outside of the bag.
These provide customers with the means to heat-seal bags when filled.
5.4.4.4 Metal strips
Metal strips can be attached to the bag when formed, to provide a means of
reclosure.
5.4.4.5 Reinforcement strips
A supplementary reel can be used to provide a reinforcing patch on the base of the bag.
5.4.5 Additional operations after bag making
Additional processes include the following:
•
Wicketting of bread bags to be used on automatic machines in bakeries. This is
a process whereby the bags are collated in any quantity on a U-shaped metal
wicket wire utilising holes pre-punched through the lip of the bag. The purpose
is to enable packing on automatic filling lines where the bags are mechanically
selected and air inflated to enable the product to be inserted. This packing
method is most suited to factory bakers of bread and farm-produce packers.
•
‘Stringing’ of flat and satchel bags for use in retail outlets. This consists of
threading a string through one corner of a stack of bags, which can then hang
on a hook.
•
Fitting of ‘tin-ties’ around the top of the bag for reclosure.
5.5 Performance testing
5.5.1 Paper
Standard tests exist in the paper trade for measuring the strength and properties of
paper. These include burst, tear, tensile, stretch, smoothness, stiffness, thickness,
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porosity and substance. More specific tests are carried out on special grades, for
example wet-strength and grease-resistant papers.
5.5.2 Paper bags
When the reels of paper have been converted into paper bags, performance tests
can be conducted both by the bag maker and the bag user to ensure conformity
with agreed standards. Many of these tests will be of a practical nature.
5.5.2.1 Hospital bags
These can be tested in a steam steriliser to ensure that they withstand the steam
sterilisation cycle and remain sterile.
5.5.2.2 Dust bags
Performance tests are conducted in the vacuum cleaner. These tests are designed
to ensure the maximum level of dust retention and maintain sufficient porosity to
prevent strain on the motor.
5.5.2.3 Paper bags for food use
Food manufacturers are, of course, concerned that paper bags will preserve their
product and provide the required shelf life. The moisture vapour transmission rate
of the bag (MVTR) is an important factor in heat sealed bags. In addition, accelerated
shelf-life tests can be conducted on the bags.
5.5.2.4 Physical strength
Drop tests have been devised to measure the resistance of a paper bag to
bursting. There are weight tests designed to gauge the strength of handles on
carrier bags.
5.6 Printing methods and inks
5.6.1 Printing methods
5.6.1.1 Flexographic printing, off-line
Due to advances in flexographic printing in the last 15 years, ‘off-line’ or pre-
printing by the flexo process has become the norm for multi-colour illustrations
with the additional use of ultraviolet (UV) lacquers. The use of the common
impression (CI) cylinder for half-tone printing has resulted in ever-finer screens
being printed on suitably smooth paper surfaces. It requires a trained eye to detect
the difference between half-tone flexo and photogravure print.
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5.6.1.2 Flexographic printing, in-line
The ability to print the paper reel on the bag machine from rubber plates was
viewed as a major development in developing paper bags. (See the chronicle of
events in Section 5.7.1) This in-line printing is used extensively for printing bags
but is generally restricted to four colours. It is confined to line and type matter and
coarser screens where close register is not essential.
5.6.1.3 Photogravure
This is an intaglio process as distinct from the relief printing of flexography.
Gravure printing is synonymous with excellence in print quality and definition of
very fine print. It has the drawback of high origination costs, which means it is
only viable for long print runs. Its use has become more limited in recent years due
to the advances in the quality of flexographic print.
5.6.1.4 Silkscreen
This stencil printing process is slow and expensive but achieves impressive
results. Silkscreen may be used for printing or over-printing on paper bags after
manufacture usually for the packaging of luxury items.
5.6.2 Inks
The nature and viscosity of printing inks depend, of course, on the process being
used. In addition, printing inks which are used on paper bags for food and hospital
products must conform to strict trade standards.
Special ‘indicator’ inks are used for paper bags for ethylene oxide (EtO) gas
and steam sterilisation to show that the packs have passed through the sterilisation
process.
5.7 Conclusion
5.7.1 Development of the paper bag industry
The introduction to this chapter referred to the age of the paper bag industry; listed
below is a chronicle of some major events in the development of the industry. It is
not comprehensive but serves to demonstrate the direction in which the industry
was evolving.
1630 The first recorded reference to grocery bags.
1858 Bibby & Baron of Bury installs the first machine to make paper bags.
1870 Margaret Knight founded the Eastern Paper Bag Company (USA). Just
before that, she had been an employee in a paper bag factory when she
invented a new machine part to make square bottoms for paper bags.
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1872 Luther Crowell also patented a machine that manufactured paper bags.
1872 Robinsons of Bristol acquires a New York patent for making ‘satchel’
shape bags, i.e. bags folded with a side gusset.
1902 Hele Paper Mills provided the idea of converting printed reels of paper
into bags. This cheaper process was soon seen as a powerful advert-
ising medium leading to rapid expansion of the bag industry.
1902 A Bristol factory has 17 paper bag machines but 400 workers are still
engaged in making bags by hand.
1930 A Bristol factory, opened in 1912, now produces 25 million bags a week.
1932 The opening of the Welton Packaging factory to make paper bags and
carrier bags.
During the 1930s, a European patent was registered for printing paper from
rubber stereos. The ultimate aim of printing the reel of paper on the bag-making
machine had now been achieved. The process was called Aniline or Densatone,
but is now generally known as flexographic printing.
5.7.2 The future
The paper bag industry has shown that it is adaptable and responsive to challenges.
Paper, the raw material, is a natural and renewable resource. Paper bags have an
assured future in the forefront of all the available packaging materials. Having
regard for the current packaging legislation trends in Europe, paper bags are well
placed for a period of sustained growth.
Reference
Wmich, 2004, http://www.wmich.edu/ppse/flexo/ Department of Paper Engineering, Chemical
Engineering & Imaging, Western Michigan University, 26 February.
6 Composite cans
Catherine Romaine
6.1 Introduction
By utilizing the best combination of materials, composite-can construction ensures
optimum presentation and mechanical strength as well as hermetic protection.
This choice of materials and production techniques for can components – body,
top and bottom – offers packaging solutions that are flexible and cost-effective for
mass-consumption and luxury products, as well as for numerous industrial appli-
cations. While dry-food packaging is the most common application for today’s
composite cans, more and more retailers are seeing its value as a customizable
option for vendable and non-food products.
When you look at the issues facing food packaging, which is the largest packaging
market segment, safety and cost are at the top as shown in Table 6.1. When
considering the composite can as a packaging option, it proves to be one of the most
versatile packages in the marketplace, primarily because of its ability to safely
deliver a product at a relatively low cost. In addition, it also satisfies basic
marketing/brand functions. In general, it provides some general performance
characteristics, such as containing the product and allowing it to be readily and
easily dispensed. It must also deliver adequate shelf life for the type of food it
contains. Finally, the can must fit the product and retail venue, and meet the
needs of the brand.
Table 6.1 Top 10 issues impacting food packaging
Note: Readers were asked to rate the impact of the following packaging
issues on their businesses in the next two years. (Based on an average rating
on the 5-point scale where 5 is most important and 1 is least important.)
Source: Food Engineering’s 2002 Packaging Trends Survey.
Ranking Packaging issue Rating
1 Product safety 4.07
2 Cost of materials 3.99
3 Faster packaging line speeds 3.75
4 Improved packaging line automation 3.74
5 Consumer convenience 3.73
6 Product shelf life 3.56
7 Increased flexibility/changeover 3.40
8 New packaging materials 3.32
9 More customized packaging 3.19
10 New labelling and coding technology 3.18
Paper and Paperboard Packaging Technology
Edited by Mark J. Kirwan
Copyright © 2005 by Blackwell Publishing Ltd
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
6.2 Composite can (container)
6.2.1 Definition
A composite can is a convolute-wound, spiral-wound, linear-draw or single-wrap
rigid body with one or both end closures permanently affixed. While paper is the
primary component of the canister body, the total construction of the package
involves several layers of material, often including aluminum foil and plastic,
Figure 6.1.
6.2.2 Manufacturing methods
There are several processes which can be used to manufacture composite cans.
6.2.2.1 Convolute winding
The convolute method of manufacture involves winding multiple layers of a
single-ply of material around a rotating mandrel to form a round (or non-round) can
body. The body material is coming in at a right angle to the mandrel, with all of
the paper going in the same direction (Fig. 6.2a). A cutting operation sizes
individual cans to customer specifications.
6.2.2.2 Spiral winding
The plies of a spiral can are wound around a stationary mandrel in a helical pattern,
bonded with an adhesive in high volume, continuous production, Figure 6.3.
Individual sizes are then cut to customer specifications. The spiral can is divided
into five distinct parts (Fig. 6.2b):
•
an inner liner
•
paperboard body, separate plies (typically two-ply)
•
label
•
top closure
•
bottom closure.
Label
Body ply
Liner
Figure 6.1 Composite-can construction. Source: Sonoco.