Mark J. Kirwan Paper and Paperboard Packaging Technology
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In operation, glue is applied onto the leading edge of the film via mechanically
driven rollers. The film is cut to length and wrapped around the bottle or can and
then glued on the trailing (overlap) edge to form a complete wrap. Glass, plastic
and metal containers – square, cylindrical or oval – can be handled. Label application
speeds up to 800 bottles a minute are achievable when using 2-applicator stations.
A new variant allows wrap-around labelling of returnable PET bottles with
very thin plastic film labels. A small amount of a casein-based adhesive is applied
to the bottle for label pickup. This cold glue crystallises after curing, which facilitates
subsequent label removal from the returned PET bottle. The lap overlap is sealed
with hot-melt adhesive. Label removal is carried out using a de-labelling machine,
which cuts the label vertically with a knife. The label can then be flushed from the
bottle using a high-pressure water jet.
The wrap-around reel-fed labelling system is primarily designed for the
wrap-around body labelling of straight-sided bottles or cans. It is possible to apply
a wrap-around label to a slightly contoured bottle or jar but it is not possible, using
standard equipment, to apply a neck label to a bottle with a tapered neck. However,
this can be achieved with a combination applicator line that can apply a wrap-around
body label and a separate wet-glue or pressure-sensitive neck label on the same line.
As far as cans are concerned, wrap-around film labelling provides a number of
advantages over the use of pre-printed cans, including reduced can inventory,
reduced lead times, greater flexibility for sales/marketing activities (special offers,
proof-of-purchase promotions, etc.).
With the addition of a second carousel in the labelling system, it is also possible
to positively shrink the label film to fit contoured cans or bottles. Label technology
for wrapping-around or rolling-on film from reels, followed by a light shrink, may
also be referred to as ROSO labelling.
4.2.7 Other labelling techniques
A fairly recent development is the use of a printed paperboard wrap-around label on
a tapered plastic pot for single portion desserts. The label is held in place by the plastic
rim of the pot, though the label has a glued side seam. It enables easy separation of
the plastic pot from the label for recycling purposes.
4.3 Label adhesives
The vast majority of labels require an adhesive to bond the label to the container,
or to some other surface. Being between the label material and the container or
product surface, the adhesive must be compatible with both. If all the variations of
label substrate (paper, paper-backed foils, metallised materials and plastics) and
container/surface type (metal cans, glass bottles, plastic bottles, wrapping films,
corrugated cases, etc.) are incorporated into the adhesive requirements, the
challenges for adhesive manufacturers are quite formidable.
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In addition, there are many different types of plastic containers (PE, PP, PET, etc.),
different metal container types – some with special coatings and varnishes – and
glass bottles that also have a variety of coatings to increase surface strength and
minimise breakage. There are also certain performance criteria that have to be
incorporated into adhesives, from cold-temperature application, wet-bottle application,
food contact, removability, immersion in water, repulpability, recyclability and more.
Consequently, adhesives are custom designed to meet all the required label types
(wet-glue, self-adhesive, gummed), the required surfaces to be bonded and the
necessary performance characteristics.
4.3.1 Adhesive types
To meet the demands of different label types, different surfaces to be bonded, and
different user-performance characteristics, labelling technology uses four main types
of adhesive – hot-melt, water-based, solvent-based and curable adhesive systems.
4.3.1.1 Hot-melt adhesives
These are thermoplastic materials with 100% solids that are heated to temperatures
above their melting point and applied to substrates in the molten state. Unlike water-
based or solvent-based adhesives, hot-melt adhesives do not require drying. They have
a high initial tack and set as quickly as they can cool down to their solidification
temperature, which makes them ideal for picking up labels in high-speed labelling
lines. They are popular because of their high setting speed and because they form
a virtually invisible line on glass, metal, PET and other plastic containers.
Key criteria in the use of hot-melt adhesives are temperature – which controls
viscosity – and adhesive film thickness, which affects speed of setting, tack and
open time. The thicker the adhesive applied, the longer it will take to set.
4.3.1.2 Water-based adhesives
They have had a dominant place in the label industry for many years and are used
in wet-glue, self-adhesive and gummed paper formulations. They are made up of
materials or compounds that can be dissolved or dispersed in water to become tacky
and form a bond and they dry by losing water through evaporation or by penetration
into the label substrate. At least one surface must be absorbent or porous to form a
strong bond.
Water-based adhesives are available in a variety of chemistries and composi-
tions, and are categorised as either natural or synthetic polymers, as follows:
Synthetic polymers Natural polymers
Polyvinyl acetate (PVA) Casein
Acrylics Dextrine/starch
Polychloroprene Natural rubber latex
Polyurethane dispersions
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Key points relating to some of these water-based adhesives are as below:
•
Dextrine/starch-based adhesives were one of the first ready-to-use label
adhesives. Made up from water-soluble natural polymers, they are used
either cold or warm, depending on application. Resin/dextrine adhesives are
used for the labelling of some types of coated glass and plastic containers.
•
Casein adhesives are some of the best-known adhesive types for wet-glue
labelling. Fast drying, they adhere well to cold, wet bottles, yet can be easily
removed in caustic cleaning solutions when used for labelling returnable bottles.
•
Water-based acrylic adhesives are usually used in self-adhesive applications
where, unlike organic-solvent-based adhesives, their flame-spread resistance is
an advantage. They are used in many UL applications. (Underwriters Laborato-
ries (UL) is an independent not-for-profit product testing and certification
organisation which helps manufacturers to achieve global product acceptance.)
•
Polychloroprene – used for some self-adhesive applications – was developed as
a substitute for natural rubber and offers a unique combination of adhesive
properties:
– outstanding toughness
– chemical resistance
– weathering resistance
– heat resistance
– oil and chemical resistance.
4.3.1.3 Solvent-based adhesives
These are noted for their fast bond strength development, good heat resistance,
adhesion to a wide range of substrates and tolerance to a wide variety of pro-
duction conditions – including low temperatures and high humidity. However,
organic-solvent-based pressure-sensitive formulations have been widely displaced
by water-based and hot-melt systems for economic as well as ecological reasons.
Solvent recovery and/or incineration is now essential to meet clean air legislation
requirements. This is expensive and can only be justified for large output
operations.
4.3.1.4 Curable adhesives
Those that use UV or electron-beam systems to ‘cure’ or set the adhesive are a
relatively new development and are used in some speciality tape applications and
have also found some (limited) application in the self-adhesive label sector.
Curable adhesives are cross-linked during the curing process.
4.3.2 Label adhesive performance
Label adhesives are a key element in the label application process and have to
perform as required on the labelling line and throughout all handling, shipping and
end-use stages.
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Important features of a label adhesive are as follows:
•
It must ‘wet’ the label surface, i.e. flow out onto the label substrate to which
it is applied. It must not reticulate.
•
It must have a good ‘initial tack’ as the label is applied to the container or
product surface.
•
The initial bond must be maintained, as applied, until the adhesive is fully
set, and there must be no ‘edge lifting’ or blistering.
•
It must meet any required end-usage performance criteria – food contact,
chemical or product resistance, water resistance, heat or cold storage, etc.
With the wide range and variety of end-use and performance requirements of
self-adhesive labels, there are, of necessity, also a wide range of special adhesive
performance needs which include:
•
Semi-permanent adhesives which remain removable and re-positionable for
some time after application before fully becoming permanent. Often used
for labelling high-value items.
•
Removable adhesives have low tack and peel values to avoid damage to labels
or goods when the labels are removed. The removal must be accomplished
without leaving any adhesive residue behind.
•
Filmic adhesive grades can be permanent, semi-permanent or removable
and are used specifically with synthetic film labelstocks.
•
Freezer-grade adhesives are permanent adhesives that stand up to the full range
of temperature extremes found in cold storage and freezer chest applications.
•
Repulpable adhesives are specifically designed so as not to interfere with the
paper repulping process used during the recycling of paper materials. A major
requirement of such adhesives is that they must not produce ‘stickies’ in the
pulp-filtration process.
•
Permanent adhesives need to bond quickly to the surface of the paper and
paperboard to which they are applied and also have an instant fibre-tearing
bond. Permanent adhesives are also used with fragile substrates to provide
tamper-evident label properties.
•
UV-curable adhesives have been developed to stand up to the toughest
challenges and provide outstanding heat, plasticiser and chemical resistance.
As far as wet-glue label adhesives are concerned, there are three main adhesive
groups:
1. Glass-container label adhesives are water-based casein, non-casein, synthetic or
resin-based adhesives which offer high performance, superior water resistance,
moisture and ice resistance throughout the label application, bottle line
handling and conveying, palletising, shipping and end-use stages. If labels
need to be removed after use then a non-water resistant adhesive is used.
2. Rigid plastic-container label adhesives are designed to provide good wet
tack and adhesion when labelling at high speeds.
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3. Metal-container label adhesives are hot-melt adhesives developed specially
for the labelling of cans and offer hot pickup for roll-through labelling equip-
ment and good hot-melt adhesive performance in rotary labelling equipment.
4.4 Factors in the selection of labels
Each label technology or label type has its own advantages and disadvantages in
the provision of label solutions for different applications. Factors to be considered
when selecting a label type include:
•
cost of label material (non-adhesive or pre-adhesive, opaque or clear, metal-
lised, etc.)
•
cost of printing and converting (number of colours, in-line options, sheet or
web printed)
•
visual appearance and image (paper, film, design capabilities, aesthetic appeal,
premium quality, tactile ‘feel’, no-label look)
•
durability (resistance to scuffing, scratching or image deterioration)
•
production flexibility (ease and speed of label line changeover)
•
environmental considerations (recyclability, returnability, waste disposal)
•
volumes required (short, medium or long runs)
•
speed of application (high-speeds required?)
•
capital investment of label line equipment (wet-glue high; self-adhesive lower)
•
running cost/hourly rate of label line (operator and machine)
•
down-time (set-up, clean-up, removal of misapplied labels)
•
performance needs of labels (chemical or water resistance, labels for autoclav-
ing or sterilising, wet-strength, high or low temperature usage or application)
•
information needs on label (reverse-side printed, leaflet or booklet label)
•
security features (tamper-evidence, anti-theft, brand protection, hologram).
Rather than looking at any one or more criteria in isolation when selecting a label
type, it is more usual today to look at the chosen solution or possible solutions in
terms of total applied label cost. This is the end-of-line cost of labelling a bottle, jar,
can or other type of pack. It comprises both the cost of the application technology,
including investment, to achieve the desired application speed and the cost of the
materials used. The materials used mainly comprise the label which must meet the
needs of appearance and performance at every stage from application to end-use.
The label cost will depend on the cost of the plain label material together with the
costs of printing and conversion. In many cases, it will be found that there is more
than one labelling solution for any one particular application.
4.5 Nature and function of labels
Many packages and containers carry more than one label. Some unit containers
have two, three or more labels, each with its own specific brand or informational
purpose while transit and distribution packaging may carry more functional types
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of labels for addressing, tracking and tracing or security purposes. Some of the key
roles and functions of labels are set out below.
4.5.1 Primary labels
These are designed to carry the product’s brand name or image identification that
will attract attention on the retail shelves and appeal to prospective buyers. The
labels usually promote a specific logo or brand name, incorporate special brand or
house colours and may carry a picture, drawing or representation (which generally
may not hide, obscure or interrupt other matter appearing on the label). For some
applications, the primary face label may be reverse side printed with secondary
information. Used with clear liquids and clear bottles (glass or plastics), the informa-
tion is read from the back of the container, through the container and contents, thus
eliminating a separate secondary label.
4.5.2 Secondary labels
Secondary labels are usually smaller and are used to carry information such as
a list of ingredients, health or safety requirements, nutritional details, instructions
for use, EAN (European Article Numbering) Code, warnings, manufacturer or
supplier name and address or registered office of the manufacturer or packer,
prices, volume, weight or quantity and possibly promotional or special offer deals.
If appropriate, the label may also contain any special storage conditions or conditions
of use, and particulars of the place or origin.
Information contained on primary and secondary labels is normally required to
be clear, legible and indelible, readily discernible and easy to understand, and is likely
to have to conform to one or more of the regulations, standards or legislations that
relate to labels and to labelling.
With one-piece wrap-around can and bottle labels, the primary and secondary
elements are all incorporated into one label, rather than being two separate labels.
4.5.3 Logistics labels
While primary and secondary labels are found on unit packs – the individual
bottles, cans or packs – there is a need for labels on transit packages that are used
to distribute goods and track and trace their movement in the supply chain to
ensure they reach the correct delivery address. These logistics labels are frequently
printed with computer-generated graphics, variable text and unique numerical or
bar code symbols using thermal, laser or inkjet imprintable labels.
Logistics, bar code and variable information printed labels are predominantly
printed in one colour (black) only, often during the transit packaging, palletising or
warehouse/distribution chain stages; and applied to cartons, trays, boxes, cases and
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pallet loads prior to shipment. Print and apply label equipment is commonly used for
these applications, with the variable information entered through a keyboard or keypad.
More recently, major developments have been taking place in what are called
smart, intelligent or chip labels which are finding applications in the logistics chain
for identification, traceability, track-and-trace and smart logistics applications.
Many of these new label solutions are based on smart label RFID (Radio Frequency
Identification) technology where the label/tag costs less than one dollar. Most
smart labels are in the form of very thin labels or laminates to get the price down
and to make them suitable for targeted applications, which are, in the main, new
ones where the label must usually be disposable.
Smart (RFID) labels in the logistics chain are used to:
•
implement a step change into the supply-chain operation
•
provide full visibility of the stock in the supply chain
•
automate the processes associated with the management of stock.
The full implementation of RFID technology has been slow because: the (still)
relatively high cost of the labels/tags, the need to develop standards, there is no
one company that can manage large-scale implementation, and the economies of
scale are not yet available.
4.5.4 Special application or purpose labels
In recent years, an increasing need has arisen for additional product or pack labels
that are used to provide unique brand protection, and the prevention of retail theft,
tamper-evident or promotional solutions.
The range and variety of ‘security’ labels and solutions available to brand owners
and end-users now includes:
•
security papers and films for labels with special fibres, coatings, planchettes,
threads, watermarks, etc.
•
special security inks for labels from which the print cannot be erased or which
may have to be fluorescent, indelible, infrared, luminescent, magnetic, photo-
chromic, water-fugitive, thermo-chromic or optically variable
•
security-label design systems incorporating guilloches, fractals, special rasters,
microtext, curved distortion to scrambled indicia, anti-colour photocopy features
from special coated papers, void materials, reactive imaging, etc.
•
optically variable devices (OVDs) ranging through chromograms, colourgrams,
micrograms, destruct foils, hologram materials, micro-embossed films, etc.
Note: Guilloches are printed security lines, the layout of the intersections and
geometry is unique. Copying is inhibited by the layout arrangement of thin lines,
rainbow print and the exact colour calibration. Indicia are distinct marks, signs or
symbols – they are especially relevant to corporate or brand identity, and they are
also used for postal-address identification.
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The choice of security device(s) to be incorporated into brand protection,
anti-counterfeiting or authentication labels will depend on level of security, cost,
process, application method and effectiveness.
Such special labels may require purpose-built presses or converting lines incor-
porating silicone or adhesive pattern printing and folding units as well as special
units to combine the label with the product, by either application or insertion.
Leaflet and book labels with multiple pages are used in the labelling of chemicals
or agro-chemicals, for some DIY products and on computer software packs.
Variations may also be found in promotional applications. Tactile warning labels
for packaging of products for the blind are another special type of label.
Form and label combinations – where one or more self-adhesive labels are found
on a form – are used in distribution applications. One pass through a computer printer
provides both the paperwork and labels that can be peeled off for pack/distribution
labelling, order picking or job/production identification purposes.
Other specific label types include labels used to produce a collar or neck label.
These may be pre-formed, slipped on and then locked, rather than glued in place. Tie
on labels, tags or swing tickets may be used in the packaging/labelling of luxury items.
4.5.5 Functional labels
On-occasion labels may also be used during a packaging, processing or end-usage
operation, for example to indicate successful sterilisation or autoclaving by colour
change or provide evidence of significant adverse temperature change. Others may
be used to provide a tamper-evident or security seal (Fig. 4.3), or can be peeled off
and re-placed to provide a re-closure device.
Labelling/packing lines may incorporate systems for applying a carry-home
strip or label which can be applied to one or two bottles of lemonade or mineral water.
4.5.6 Recent developments
Over the past few years, ‘no-label’ look film labels, with matt or gloss surfaces, have
become popular and part of the lexicon of labelling. Marketeers see these labels in
Figure 4.3 Types of tamper evident security labels. Source: Labels & Labelling Consultancy.
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image-enhancing terms to encourage people to buy expensive products in glass or
plastic containers (hair-care products, cosmetics, near-beer drinks). This type of
labelling competes with direct decoration because it achieves similar graphic effects.
Glass-clear labels and adhesives are also used in pharmaceutical labelling
where glass vials have to be visually checked for contamination of the contents
before use. Any particle or bubble in the adhesive could be mistaken for contam-
ination of the contents and could lead to unnecessary rejection of the vial.
Furthermore, clear-on-clear labels eliminate pre-printing of containers for stock,
offering bottle or brand owners logistics flexibility in terms of total inventories
and sizes of containers. Coloured containers can also be used, with the clear label
allowing the colour to show through, rather than having to match the container
colour with printing ink.
Another more recent labelling development is the use of OVDs and diffractive
optically variable image devices (DOVIDs), generically called holograms, which have
gained considerable prominence in the fight against counterfeiting. They cannot
effectively be reproduced by colour copiers or scanners, or by conventional photo-
graphic and printing processes. They are versatile enough today to be bonded to labels,
tags and textiles, providing complex 3D structures which provide anti-counterfeit,
tamper-evident, plain, numbered or other imaging solutions.
4.6 Label printing and production
The printing and production of labels is undertaken on a wide variety of presses –
from sheet-fed to web-fed – and using almost every available printing process.
The type of press or process used is determined by:
•
the specific label or printing requirement
•
the availability and quantity of labels
•
the nature and quality of the printing
•
the number of colours
•
whether subsequent converting operations (die-cutting, embossing, metallic
foiling, laminating, waste stripping, etc.) are carried out in-line on the press
or are separate stand-alone operations
•
how the labels are to be shipped to the packaging or bottling line (in slit reels,
cut to size or punched to shape in stacks).
Apart from the mechanical-printing processes used to produce pre-printed labels –
rotary and semi-rotary letterpress, flexo and UV-flexo, screen process, offset litho,
gravure, hot or cold foiling – there are a range of on-demand, VIP solutions used
by the label industry. These include thermal printing, laser and ink-jet printing and,
more recently, complete digital colour printing presses.
The mechanical-label printing processes can be categorised by the way in which the
area of the printing plate or cylinder that carries the printing ink (the image carrier)
is defined when compared to the non-printing area. Printing plates and cylinders
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that print from a raised, inked surface are known as relief printing processes.
These include letterpress and flexographic printing.
Printing processes that carry the printing ink in a recessed pattern of etched or
engraved cells are known as intaglio printing. These include gravure and rotogravure.
A printing process that prints from a flat, chemically defined, printing plate is
known as planographic printing. The most commonly used planographic printing
process used for labels is offset litho.
Each of these mechanical printing processes has undergone developments in
recent years which have led to better quality, colour and speeds. As such there is no
‘best process’. Each of them is capable of high quality printing at economic speeds,
and each can impart a quality or characteristic to the overall design and image of
the finished label. These characteristics include such things as evenness or brightness
of colours, thickness of the ink film, fineness of reproduction, quality of halftones,
the texture and feel of the label, etc. Some can also impart a range of special effects
such as impregnated inks, which can be used with perfumes, fruit flavours or other
aromas, rub-off or scratch-off inks, colour or temperature change inks. Raised tactile
images, for example braille characters, can be printed to provide product information,
including the warning of hazardous contents, for the blind and partially sighted.
An important market need which none of these mechanical printing processes
can handle in a commercially viable way is that of printing variable information.
Examples of such information include bar codes, batch and date codes, sequential
numbers, price and/or weight information and print runs of short length, i.e. where
the time to print is short compared with the time to set up the press. To meet these
requirements a number of methods of electronically printing variable images or
short runs have been developed. Some can be run in line with fixed-image
mechanical presses, some are stand-alone machines and some are meant for adding
variable information off-line.
Most recently, digital printing technology – in which the image is defined or
created by computer – has begun to find a place in the label printing industry for
full-colour and spot-colour short-run printing of labels. By the end of 2002, there
were some 200 digital presses in label plants worldwide.
A brief guide to each of the key mechanical, variable information and digital
label printing processes and techniques is set out below.
4.6.1 Letterpress printing
Letterpress printing was the earliest method used to print labels back in the eighteenth
and nineteenth centuries, and is still a key technology in the printing of quality prime
and secondary self-adhesive labels today.
Modern letterpress printing (Fig. 4.4) uses photosensitive polymer plates onto
which an image (ink-carrying area) is produced by photographic or direct-imaging
plate-making techniques. After exposure, the plate can be treated in a special
wash-out solution (chemical or water-based) to develop the image and non-image