Pressure-sensitive labels

He made the first labels that could be applied without moistening or glue. His first product was the Kum Kleen self-adhesive label offered in only one color: white (Figure 4.1). When customers asked to have their names on the labels, he added print, using a hand-cranked printing press. Self-adhesive labeling usage has expanded rapidly since those early days.

In the beginning, the face of the label consisted of plain, white paper. Today the label face substrate may be made of fabric, plastic, or metallic films and it might carry brightly colored designs and pictures.

The self-adhesive system offers a labeling flexibility that wet-glue cannot provide. The lower cost of change parts needed for the applicator is much lower that wet-glue systems, the set-up time for self-adhesive application is much simpler and operates much cleaner.

The modern combination 'platform press' used in the self-adhesive industry will produce high quality graphics and embellishments in one pass on the press, giving the facility to produce the printed, embellished and converted labels in a much reduced manufacturing window, compared to wet-glue labels.

The use of plastics for packaging, which has shown a faster growth rate than any other packaging material, has particularly helped boost pressure-sensitive labeling. This especially applies to the shorter-run and luxury end of the market. Typical sectors include toiletries and cosmetics, household products, motor oils and pharmaceutical goods.

Glass containers for foods, drinks and dairy products have historically been labeled with wet-glue labels, or are printed direct.

Nevertheless, certain designer products, such as drinks and beers now appear with pressure-sensitive labels. The usage of 'a no-label look’ filmic label is of particular note here (Figure 4.2).

METHOD OF MANUFACTURE

Pressure-sensitive labels, also known as self-adhesive labels, have been considerably refined over the years.

It is essentially a sandwich of substrates (Figure 4.3). The first part of the sandwich is the face material, which becomes the finished printed label.

The reverse of the face material is coated with a pressure sensitive adhesive and the face paper is then laminated onto the liner substrate which is coated with a silicone layer.

A self-adhesive label must have both the ability to 'hold' and the ability to 'release'. It is a contradictory requirement that it must stick to different types of surfaces under varying conditions, yet release easily from its backing paper.

This release coating permits easy removal of both the waste matrix and the label from the release liner, which becomes the carrier for the converted labels. Printing takes place on the face stock and is then die-cut to a shape and supplied in reels.

The face stock can be made from a wide range of substrates, with paper stock or filmic stock being the most widely used. These can be surface coated or uncoated depending on the properties required for the label surface.

Pressure-sensitive adhesives offer an extensive range of adhesive properties. The adhesive/labels are used in a wide range of conditions which demand very high performance levels. These conditions will include high and low temperatures, wet and dry environments and different product surfaces.

MANUFACTURING PRESSURE-SENSITIVE (PS) SUBSTRATES

The manufacture of PS substrates is primarily a coating and laminating process.

The adhesive coating and the laminating of the substrates is done on a coating machine as an in-line operation. A release coating is applied to the liner stock and then dried in-line. The face stock may also require a primer coating and this is also dried in-line. The adhesive coating can be applied directly to the reverse of the face-stock, or by transfer from the liner.

After the coating and drying operations has been completed the two separate substrates (the face-stock and liner) are laminated together to form the pressure sensitive substrate.

The method of PS manufacture is illustrated in Figure 4.4.

There are two methods of applying the adhesive coating to the reverse of the face-stock. The first and most widely used is the direct coating method in which the adhesive is coated directly onto the reverse of the face-stock.

The second is transfer coating when the adhesive is coated onto a release liner and then transferred to the reverse of the face-stock after the lamination process.

There are a number of different coating methods used for applying the adhesives.

The thickness of the adhesive is an important factor and must be established before the adhesive is applied.

The coating weight is important for two reasons:-

• It has to be even and consistent to provide the correct adhesion between the liner and the face stock and the label and the product container being labeled

• To apply the optimum/minimum adhesive coat to keep usage to a minimum

Typical coating weights would be in the range of 5-50 gsm for rubber based and aqueous based adhesives

The thickness of the adhesive coating is determined by the following factors:

• The type of face stock to be coated (absorbent – non-absorbent)

• The adhesive system to be used (solvent, emulsion, hot melt)

• The required thickness of the adhesive 0.8 - 5.0+ mils

These factors will determine if the adhesive should be a direct application to the face-stock or transfer coated to the liner.

When this information has been established it is easier to identify the most suitable method of applying the adhesive coating.

APPLYING THE ADHESIVE

There are several different methods of applying the adhesive. It is the type of adhesive that governs the method which is used for the adhesive application.

A typical coating system for aqueous and rubber adhesive coatings would be a Mayer Bar system or a metered system such as a gravure process application.

MAYER BAR COATING METHOD

The Mayer Bar method of application is commonly used to apply low-viscosity pressure-sensitive adhesives (PSAs).

This coating system uses a wired rod to apply the adhesive and is best suited to adhesive types with good flow characteristics.

REVERSE - ROLL COATING

Reverse roll coating (also known as roll-to-roll coating) differs from other coating methods by having two reverse-running nips.

The metering roll and the applicator roll contra-rotate and have a fixed gap between the two rollers. The surface of the applicator roll carries an excess of adhesive coating and the thickness of the adhesive coat is established by the nip distance between the two rollers (Figure 4.5).

The meter roller (contra-rotating) wipes the surplus adhesive to leave a precise amount on the applicator roller. The applicator roll running in the opposite direction to the substrate, wipes the adhesive coating onto the substrate.

GRAVURE COATING

The gravure method of adhesive coating gives an accurate coat weight without any restrictions on the running speeds of the applicator. A low-viscosity adhesive requiring a low coating weight can therefore be applied at speed.

The coating weight (the amount of adhesive being applied to the substrate) is controlled by the screen cells engraved on the gravure cylinder. The depth and size of the cells can be engraved to a specified size. Smaller shallower cells deliver a lighter coating than a deeper larger cell formation, which delivers a heavier coating weight of adhesive.

Gravure applied coating give a very consistent and evenly coated result, suitable for coating clear filmic substrates used in the pressure sensitive industry.

Figure 4.6 shows the principle of the gravure process with the engraved gravure cylinder running in the tray which holds the viscose adhesive. The doctor blade wipes the cylinder leaving the adhesive in the cells.

The adhesive is then transferred onto the substrate which then enters the drying section.

KNIFE COATING

In the knife coating process the coating is applied to the substrate directly via a holding reservoir or an applicator roller (Figure 4.7). The knife which controls the thickness of the adhesive, can be a steel blade or alternatively an air knife. The excess adhesive is removed by the knife which is set to a predetermined height or air pressure.

This controls the thickness of the adhesive coating by wiping the surface of the adhesive, giving an evenly applied coating at the required weight.

HOT-MELT COATING

The hot-melt coating process is a system of applying an adhesive with a 100 percent solids content of wax and polymer resins. The adhesive is heated to a fluid state and then applied to the substrate by an applicator roll, the gravure process or extrusion process.

After the adhesive has been applied it is cooled using a chilled roller or chilled air system.

Temperature control is a key factor when using hot-melt adhesives. The temperature controls the adhesive viscosity and the thickness of the adhesive film. This in turn will affect the tack value and the speed of setting i.e. the thicker the adhesive coating the longer it will take to set.

HOT-MELT ADHESIVES

Hot-melt adhesives are 100 percent solids which are supplied in dry form and which melt at temperatures between 266 and 320 degrees F, (130 and 160 degrees C). After the coating process the adhesive coating dries at room temperature, leaving a tacky pressure-sensitive surface.

After the hot-melt coating has been applied, the backing liner is applied to the adhesive coated substrate, thereby laminating the substrate and backing liner together to form the self-adhesive material. The coated material is then passed over a chilled roller to complete the process.

Hot-melt adhesives are used for many adhesive applications, where their immediate speed of setting/bonding is ideal for fast production. It is often used for more difficult labeling applications where emulsion adhesives can be problematic.

With the hot-melt process higher adhesive coat weights improve adhesion to rough surfaces, but hot-melt self-adhesive formulations are generally unsuitable for labeling plasticized surfaces. The plasticizer will migrate into the adhesive and degrade it, so causing adhesive stringing and strike through of label papers, which may result in bond failure.

Hot-melt adhesives are popular because they are invisible on glass, PET and other plastic containers. They are also suitable for metal applications.

Their high tack characteristics make them suitable for difficult applications such as peelable, chill, permanent and deep-freeze label applications.

CONSIDERATIONS WHEN SELECTING PS ADHESIVES

When selecting an adhesive, the type of application and end use of the product to which the label is to be attached, will often determine the type of adhesive to be used.

Key considerations are:

• Is the label to be permanent or removable?

• Storage environment including the range of temperatures, moisture and their potential effect on the label

• The surface to which the label will be applied i.e. smooth/rough, hot, cold, frosted, wet, dry, paper, plastic or metal

• Will label be used for direct contact with food?

• Is there a medical application involving a sterilization process or skin contact?

• Is the label to be used for children’s toys?

TYPES OF ADHESIVE

There are a number of variables involved in the manufacture of pressure-sensitive adhesive products (PSA).

Although the process of coating and laminating is uncomplicated, not all PSAs can be coated using every type of coating system.

PSAs use differing adhesive technologies and each of these technologies have varying ranges of viscosity, solids content and coating weights, which may be required for specific applications.

Each of these items can affect the adhesive-coating. These variations will affect the adhesive properties and the choice of the right adhesive and method of application is very important. Figure 4.8 summarizes the main types of PSAs.

MAIN TYPES OF PRESSURE SENSITIVE ADHESIVES

The main adhesive formulations used for pressure sensitive adhesives are as follows;

• Acrylic

• Emulsion acrylics (water-based)

• Solution acrylics (organic solvent-based)

• Radiation curable (UV and EB) – 100 percent solids

• Rubber

• Solvent rubber

• Hot melt rubber (100 percent solids)

• Radiation curable (UV and EB-electron beam) - 100 percent solids

• Silicone adhesives

BASE COATING AND TOP COATING OF SUBSTRATES

There are other processes that are used to optimize the performance of materials. Base coating is used to create a base layer to assist the printing and embellishment processes by improving the ink key and ink lay, and by producing a smoother substrate surface, particularly when the foiling process is used.

The method of application for this type of coating is predominantly via the flexo process, although the screen and the gravure processes are also used. Roller coating too is also used as a base coating process.

The printing units are generally positioned immediately after the unwind unit of the press and can be used to apply an overall coat or alternatively a spot coating.

Top coating or over varnishing of the face substrate is widely used in the self-adhesive label market. The coating is applied as a protective layer after the printing process has taken place giving protection from abrasion and increasing the product resistance. It is also used to produce an alternative finish to the label and enhance the shelf display qualities of the product.

Top coating is also used to aid ink adhesion when secondary overprinting is required, for example date or batch coding data.

Top coated UV over varnishes are typically used to assist product detection via the label applicator or to improve filling line performance.

CORONA DISCHARGE

Corona treatment is another process often used to improve the basis for adhesion of applied printing inks, adhesives, lacquers, etc.

To obtain good adhesion it is necessary to increase the surface energy of the substrate. The surface of film or other material to be treated is bombarded with electrons to increase the surface wettability of the substrate (generally filmic).

Corona discharge is often used on polymer-based substrates that have low surface energy leading to poor adhesion of inks, glues and coatings.

MATERIAL SELECTION

Every label application must be carefully considered to ensure that the substrates selected can meet the expected performance criteria.

The following issues will impact on substrate selection;

• The ability of the substrate to withstand the printing process to be used

• The level of absorbency required i.e. the ability of a label substrate to retain liquids, moisture and inks

• The required rub resistance of the label surface

• The required material surface characteristics i.e. smoothness/roughness

• The chemically compatibility of the material to cater for inks, varnishes, solvents and the contents of the products the label may come into contact with

• Stability in different end-use environments and during application and handling

SUBSTRATES USED IN THE MANUFACTURE OF PRESSURE SENSITIVE LABELS

A wide range of paper and filmic materials are used in the manufacture of pressure-sensitive labels.

PAPER SELF-ADHESIVE SUBSTRATES

The main types of paper substrate used in the label market give excellent anti-fungal and wet strength features and fall into the following categories;

Coated Paper – Gloss and Semi-Gloss

• Matt coated

• Semi-gloss

• Gloss

• Cast coated

Uncoated Papers – matt finish

• Litho

• Vellum

• Textured paper - gives a 'laid' traditional appearance

• Metallics  - highly reflective surface – supplied in a range of colors and tints and holographic effects

FILMIC AND PLASTIC SELF-ADHESIVE SUBSTRATES

Whilst paper face materials continue to be the most common face-stock used in the label industry there has been an increasing rise in the use of filmics (Figure 4.9).

These substrates are available in a wide variety of constructions, such as co-extruded, blown and engineered films, all with a wide range of functional properties.

Some of the advantages and disadvantages of filmic substrates are highlighted below.

Advantages

• Can offer transparent features including a 'no-label' look when used on clear containers

• Durable and can offer conformable, 'squeezable' properties.

• Resistance to chemicals, grease, water, moisture etc

• Plastic labels can be compatible with the packaging material being used and this can aid recyclability

• High gloss finish without over laminating or varnishing

Disadvantages

• Can be more difficult to print and convert

• They are often more expensive than paper substrates

Filmic substrates fall into four main groups which form the majority of the film face-stock used today. These are PE, PP, PET, and PVC.

POLYETHYLENE – PE

Low density polyethylene has the highest share of the filmic facestock market and is widely used in the primary self-adhesive labeling market, particularly in health and beauty and the household chemical markets.

There are two types of PE film which are available; Blown PE and Cast PE films.

The key characteristic of polyethylene labels are;

• Excellent tear resistance

• Minimal shrinkage

• Good printability

• Low stiffness

• Good squeeze-ability

• Good resistance to moisture and chemicals

• Environmentally friendly

• Low transparency

• Can be prone to stretching

POLYPROPYLENE – PP

Polypropylene films offer cost-effective and high-yield face-stock and are steadily challenging other films for primary product labeling and variable information printing.

An inherent advantage of polyolefin films over other film types is their lower density range, facilitating removal and recovery in recycling operations with non-polyolefin containers and in their obvious compatibility with polyolefin containers.

Other key characteristic of polypropylene films labels are as follows;

• PP films may be stretched in one direction (OPP) or two directions (BOPP), or co-extruded usually in 3 layers

• High resistance to tearing and excellent dispensability

• Good die-cutting and printability

• Film flatness and resistant to moisture, abrasion, chemicals

• Good dimensional stability (OPP/BOPP) and excellent conformability

• High clarity and low cost

POLYESTER (PET)

PET films are preferred where durability, higher temperatures and enhanced chemical resistance are required. The major applications are in consumer durables and in the automotive sector.

The key characteristic of PET labels are;

• High durability and clarity and therefore Ideal for clear-on-clear/no-label look applications

• Good resistance to heat, stretch, tearing, chemicals, moisture

• Good dimensional stability and dispensability

• High cost

MANUFACTURING BLOWN AND CAST FILMS

Filmic substrates are manufactured by extruding liquid polymers. This process involves feeding polymer materials from a hopper into an extruder unit in pellet, powder or granule form. The material is heated and the liquefied polymer is injected into a die and by using air directed through the die a bubble, is formed at the desired thickness of film.

The manufacture of cast film differs from blown film manufacturing as the liquid polymer is passed through a flat die which produces flat film.

After leaving the flat die, the continuous sheet of liquid film is cooled which freezes the film, the edges of the film are trimmed and the film is wound into reels.

Cast films can be co-extruded in multi-layers (usually three) with each layer optimized for different characteristics (Figure 4.10).

RELEASE LINERS

The release liner (also known as the backing paper or carrier) forms the base of the self-adhesive substrate sandwich. The surface of the liner material is coated with silicone which allows the label face substrate to be easily and accurately dispensed on the labeling line.

Release liners are produced using both paper and filmic materials.

Super-calendared papers offer a robust and consistent surface which is an important pre-requisite for accurate and consistent die-cutting. They also offer good transparency which allows easy label edge detection during application. There are three main types of paper liners;

• Glassine

• Kraft - super calendared

• Paper with thin laminated PE film

FILMIC LINERS

Filmic liners offer a number of advantages over paper liners.

They are light and thin and because of their very smooth surface give an excellent wet out for the adhesive layer. This property enhances the adhesive transparency necessary for missing label identification.

The high tensile strength of filmic liners makes them ideal for high speed label application by removing the possibility of web breaks on the filling and label applicator lines.

There are two main types of filmic liners;

• Polyester

• Polypropylene

THE PRINTING PROCESSES USED IN THE MANUFACTURE OF SELF-ADHESIVE LABELS

Self-adhesive label substrates are printable by all the conventional printing processes, and increasingly by digital methods.

Many paper and filmic grades are also printable by dot-matrix, thermal, laser and inkjet printers for encoding, numbering and personalization.

Roll-label converters use narrow-web presses with integral die-cutting and waste matrix rewinders to print and convert self-adhesive materials.

Self-adhesive labels can be over-varnished, film laminated, hot-foil stamped and embossed.

The wide variety of decorative and surface embellishments techniques can be explored in the Label Academy book Label Embellishments and Special Applications.

SELF-ADHESIVE LABEL CONVERSION

In label production, converting covers any process performed to manufacture a complete label from a raw material or an unfinished material. The process of transforming rolls of self-adhesive material into labels on a release liner (carrier), presented in rolls or sheets so as to enable end users to apply them to products, packages or surfaces. This process includes slitting, die-cutting and matrix stripping operations.

DIE CUTTING

In the die-cutting process the face material is profile cut using a cutting tool to form the label shape. The liner or backing material is kept intact to protect the tacky adhesive and provide support to the cut-out labels, through to the moment of application.

The die-cutting process can be a flatbed or rotary system dependent on the design of the press. Fastest output is achieved when both printing and cutting operations are rotary, as this permits a continuous pass of the laminate through the press. With flat die-cutting the laminate has to either stop and start or reverse during its pass through the press, slowing the overall throughput.

Cutting tools come in a number of forms. Flat tools are normally made up from bent steel rule some 0.4 mm in width and 12 mm in height (Figure 4.11).

Rotary tools may be formed from a solid bar of steel around which the cutting profiles are etched or engraved, leaving them projecting from the surface.

Laser cutting systems are also used for the for manufacture of flexible die-cutting shims made of sheet-steel some 0.50 mm thick, these flexible cutting shims are mounted on a magnetic cylinder and give extremely accurate die-cutting and manufacturing consistency (Figure 4.12).

The precision and care required for the die-cutting operation is very important. The cutting tool must be in good condition and be located in a die-cut unit that is accurately engineered and equipped with an anvil roller that runs true and is undamaged. The unit must also have the facility for fine adjustments to be made to the pressure system used to control the die tool and anvil roller pressures used for rotary die-cutting.

The depth and consistency of the cut is very important and Figure 4.13 shows the correct depth of cut when converting self-adhesive substrates.

WASTE STRIPPING

In order to achieve maximum economy and depending on the size of label and width of press, the labels may be printed more than one across the width of the press. Waste margins between and around each label may then be stripped away. This process is done immediately after the die-cutting operation and involves removing the waste matrix as a continuous skeleton, the waste is reeled up as the press runs (Figure 4.14).

METHOD OF SELF-ADHESIVE LABEL APPLICATION

The application of self-adhesive labels can be performed using a table-top, stand-alone, or in-line device/machine which is used to automatically apply pressure-sensitive labels from a roll of labels onto a container or product.

There are many different types of pressure-sensitive applicators, but they all have one thing in common: a means of peeling the release liner or backing away from the labels.

This is usually accomplished by unwinding a reel of die-cut labels and then pulling under tension around a stripper plate or beak. As the backing is taken around the sharp angle of the plate the front edge of the label is peeled away (Figure 4.15 and Figure 4.16).

Stripper plates may be found in a variety of configurations – fixed plate, floating plate, pneumatic, etc. (Figure 4.17).

Rigid stripper plates for example, can be used for very uniform products, while spring biased plates may be recommended for certain side labeling applications.

Floating stripper plates, either solenoid or pneumatically operated, may be used to place labels accurately into recessed areas on a pack at a required point. Powered stripper plates need to be accurately controlled by, say, electronic timers, so that the label is pre-dispensed or maintained in contact with the pack for the correct period.

Once the labels have been detached from the backing there are various ways of feeding them forward and pressing (applying) them on to the pack, container or product to be labeled in the correct position. An example of a self-adhesive label applicator can be seen in Figure 4.18.

The effectiveness of the labeling operation is determined by the uniform application of pressure to the label in order to ensure a positive contact of the adhesive with the surface of the product being labeled.

In their simplest form various rubber or foam rollers and/or drum application devices are used to fix the label to the product, perhaps also supplemented by brushes (Figure 4.19).

Other methods of uniformly applying pressure to the label to ensure that it sticks with a positive contact to the product or container include pressure pads, pneumatic plungers, air jets, knurled rings or powered rotating brushes. Alternatively, the label may be held in position on a vacuum box or drum and released onto the container when it is in the correct position. Once the label has been applied the backing paper is rewound on to a take-up spool.

Roller and/or drum application devices are one of the most common methods of affixing pressure-sensitive labels to products or containers. Ideal for flat or cylindrical packages, they may also be used to label concave and convex surfaces.

By employing two application heads it is possible to apply labels to the top and bottom of objects simultaneously, with only one pass through the machine. By setting the applicator heads horizontally it then becomes possible to apply labels to the sides of the objects.

With two units facing each other it is possible to label both sides of the object simultaneously. With some systems it is feasible to apply labels onto the corners of boxes, such as medicines or confectionary box sealing. A variation of conventional roller application used on some machines is the use of rollers made up of knurled aluminum rings, so allowing packs with uneven or flexible surfaces to be handled. 

Powered rotating brushes, sometimes profiled to the corner of the product, may also be used to press the label into difficult areas.

In some methods of label application a pad or plunger is used to fix the labels in position. These methods are often referred to as Tamp-on applications.

Here the dispensed labels are sucked onto a vacuum pad, perhaps on the end of a plunger, and then applied to the product when it is in the correct position by releasing the suction. The result is a highly accurate labeling system (Figure 4.20).

With such systems it is possible to select any surface, direction or angle, up or down, simultaneously to both sides, for label application, e.g. to place labels at an inclination of 45 degrees. By employing a longer plunger arm it is possible to apply labels into deep recesses, or even into the bottom of something like a cup or glass.

Air jet/air blow is another frequently used method of label application in which the labels are stripped as usual and then, after release, are again held on a low vacuum, but this time are blown onto the container by applying air pressure.

Using this method of label application it is possible to apply labels in positions that would be virtually impossible by other methods. Absolutely no damage will be done to the surface or product to be labeled.

A variation of air jet application is where the application head is built into a mechanical arm which can then be positioned to label by air jet, products such as television tubes.

LABEL AND PRODUCT CONTROL

To ensure that labels are applied accurately and consistently to products, the self-adhesive label applicator may be fitted with various label or product tracing/control devices.

Label tracing devices monitor the gap between the individual die-cut labels and interrupt the web-feed at the end of each label. This resets the system to receive the next start signal from the product tracing devices, which include micro-switches, photocells or spot color readers. Other controls on the web may be used to detect missing labels, the end of the reel or web breaks.

With many applicator systems the actual product or container triggers the application of the label through the use of product tracing/ control devices. This means that even with erratic or irregular product flow the system will automatically compensate and only dispense a label as required.

Many different scanning/sensing technologies are available on self-adhesive applicators, from photocells with reflector mirrors, transmitter/receivers, proximity switches, spot or color readers, or for certain applications, micro-switches. For semi-automatic applications, foot-switches may be used.

ADVANTAGES AND DISADVANTAGES OF SELF-ADHESIVE LABEL SYSTEMS

The advantages and disadvantages of self-adhesive labels as a decorative label technology are summarized here;

Advantages

• Clear self-adhesive filmic substrates can achieve a 'no label' look on clear containers

• Receptive to high quality graphics/embellishments (combination printing)

• Wide selection of substrates and adhesives

• Compatible with plastic containers for recycling

• Quick change-overs facilitate logistic flexibility and inventory control and therefore suited to short run, just in time applications

• Application efficiency – quick changeovers, accuracy

• Fast conversion and finishing time

• The diversity of applications is impressive. It includes underwater labeling, high and low-temperature labeling, also labels for sterilizing and autoclaving.

Disadvantages

• High cost and environmental issues related to the disposal of backing liner material

• High substrate cost

• Slower application than wet-glue on some jobs. For example the application rates may not be fast enough to deal with today's modern filling lines for soft drinks and beers, although this is becoming less of an issue.

LINERLESS LABELING

As already discussed conventional pressure-sensitive labels consist of a face material, an adhesive and a siliconized release liner backing that functions as a carrier for the label and protects the adhesive layer during handling, printing, finishing and right up to the point that the label is ready to be applied.

Applicators and labelers for conventional pressure-sensitive labels need to peel away the backing release liner for each label to be dispensed and successfully apply it to a container, product or pack. The liner then needs to be disposed of as waste.

Conventional pressure-sensitive laminated labels like this produce the highest waste levels of any packaging component, with over 50 percent lost during conversion and final end-use application.

Linerless pressure-sensitive labels however enable the release liner to be eliminated.

Indeed, the development of the linerless process was driven by the need to remove the backing liner used in pressure-sensitive labeling, in order to minimise wastage.

Linerless pressure-sensitive label technology is not new, it has been around for over 35 years. Today the improved techniques being used and developed have coincided with a greater awareness of the importance of sustainability and the need for retail chains and brand owners to prioritize the environmental issues facing the packaging industry. Millions of tons of used silicone-coated paper and filmic release liners are land filled each year and this is not good environmental practice.

Today, linerless labels are most commonly found in the form of pressure-sensitive labels for the blank label industry and as thermal labels used in print and apply weigh-price label dispensers. They are also popular in market sectors such as food and logistics, but the impact and growth in the other label markets has been slow.

That situation is now beginning to change as more countries start to tax liner waste as packaging material rather than industrial process waste and it is becoming more difficult and complicated to dispose of the liner. Some of the major packaging users have the ambitious objective of ensuring 100 percent of its packaging designs are reusable, recyclable or suitable for good environmental waste management.

Linerless technology ticks many of the sustainability packaging requirements and with improved coating technology and advanced applicator equipment, there is the potential to impact on several prime label sectors, particularly in food packaging and the labeling of glass bottles and jars.

Manufacturers of packaging converting equipment are now realising the full potential of the linerless decoration system.

HISTORICAL CONTEXT - MONOWEB

Monoweb introduced in the mid 1980’s by Waddington PLC was the first truly linerless pressure-sensitive label.

A silicone coating applied over the surface of the printed substrate allowed a non-contaminated release from the reel. The reel of printed substrate was positioned into the applicator and the profile shape of the label being applied was die-cut at the point of application and then removed from the web by direct application to the container.

There were some drawbacks with the Monoweb system. Any problems with the die-cutting operation immediately stopped the line and the quality and durability of the die tooling was critical. The system also introduced a new set of skills for the line operators, which required additional knowledge and operating skills, particularly with the die-cutting process, in order to extract the best performance out of the cutting tool.

The system of print to die-cut registration was achieved using a sprocket punched hole system using pins which located into the punched holes to ensure the correct registration between the printed image and the die-cutter. The punched holes located at the edges of the web required removal using slitting and waste extraction.

THE EVOLUTION OF THE LINERLESS LABEL

The development of the linerless labeling has evolved considerably in recent years.

Linerless labels use the same adhesive method as a pressure sensitive label, in that the label has an adhesive layer on the back of the label substrate. There are however some fundamental differences.

A conventional pressure-sensitive label has a face substrate positioned onto a siliconized liner ensuring that the adhesive and face stock do not stick to each other in the roll. The liner also acts as the carrier for the printed and profile cut labels which are dispensed onto the product direct from the liner.

In the linerless process the face stock is printed as a single layer and the label surface is coated with a silicone layer and the adhesive coating applied to the back of the label stock.

Alternatively a separate liner is coated with the adhesive and laminated onto the printed label stock allowing the adhesive to transfer from the liner to the back of the label stock.

The liner and face stock are then de-laminated, the liner is re-wound for reuse leaving a single layer of printed face stock, which is then rewound into a reel.  Figure 4.21 compares the structure of a linerless label with that of conventional pressure-sensitive laminate.

The silicone coating on the face on the printed label, prevents the adhesive from adhering to the face of the label (Figure 4.22).

The removal of the need for the backing liner offers significant environmental and cost benefits.

Release liners used in conventional self-adhesive systems generally are not recycled and the bulk of them find their way into landfill.

By removing the need for a release liner the amount of waste material is significantly reduced and therefore the costs related to waste disposal, storage, transportation and substrate usage are drastically cut.

The removal of the release liner also increases the number of labels per roll resulting in fewer roll changes for press operators and also a reduction in downtime on the filling and label application lines. The increased number labels per reel also reduces the amount of inventory storage space required for the printed label stock.

PRINTING AND CONVERTING LINERLESS LABELS

A common misconception with the linerless system is that special printing equipment is needed to print them. This is not the case and labels are printed using conventional printing machinery.

A typical linerless press configuration will comprise of a single unwind unit, flexographic printing units and a rewind unit. As there is no liner, printing on a single side or both sides of the label substrate can be carried out.

When printing is completed the reels are ready for the second stage which is the application of the adhesive and the silicone coating which also helps to protect against UV and moisture exposure.

Next is the die-cutting process, waste removal and finally the checking/slitting operations.

Individual coating units are used for the hot-melt adhesive and the silicone coating process.

A base liner substrate is coated with the adhesive, at the same time the UV silicone coating is applied to the printed side of the face-stock.

The adhesive coated liner is then laminated to the reverse side of the printed face stock allowing the adhesive coating to be transferred to it. The laminated web then proceeds into the die-cutting unit, the waste matrix is removed and the liner and face stock are separated.

The liner is rewound ready for re-use and the finished linerless printed reels are slit and wound into smaller reels ready for application on the filling or labeling lines.

This method of linerless label manufacturing makes the labels suitable for the following end-use applications.

• Print and apply labeling which require variable data etc. as used in product distribution and logistics tracking

• Primary product decoration of a product or container which does not require a profile shape i.e. best suited for applications requiring square or rectangular cut labels.

APPLYING LINERLESS LABELS

It is at the application stage where additional equipment is required. A special cutting unit is required which separates the label ready for application.

To apply the label a mechanical or laser cutting system cuts/slits a single label from the adhesive coated linerless reel.

After the cutting process has been completed the label is applied by adhering the leading edge of the label to the container and then wiping the remainder of the label onto the container.

For difficult and uneven surface applications, the single label can be blown onto the container surface using suction to hold the label and air jet to apply the label.

Figure 4.23 shows a linerless applicator positioned on the left side of the filling line cutting and applying pre-printed linerless labels. 

One of the big differences between linerless and pressure-sensitive labels is that linerless labels cannot be profile die-cut during the printing operations.

As explained previously the printing and embellishing processes are carried out using only the single layer of face paper and not the sandwich of substrates used in the conventional pressure sensitive label system.

Because the labels are printed in a continuous reel with no liner to support the individual profile cut label, this means that there are limits on the profile shape of the linerless label.

This limits the label shape to a rectangular or square cut profile. Developments however are taking place to overcome this issue of profile shapes produced in a linerless format.

Because of the limits on the shapes which can be used, linerless systems often use clear film to create the effect of a profile shaped label. The design of the printed image is used to create the effect of a profile shaped label albeit that the label may be square or rectangular in shape.

ADVANTAGES AND DISADVANTAGES OF LINERLESS SYSTEMS

A summary of the advantages and disadvantages of linerless systems is summarized below;

Linerless Advantages

• No release liner required

• Printing, adhesive application, die-cutting and rewinding of finished reels takes place in one pass

• Eliminates the problem of waste removal and re-cycling of the liner material

• Minimal waste matrix required making a material cost saving on face substrate

• More labels per roll and less roll changes during application

• Lower reel weights and transportation costs

Linerless Disadvantages

• Investment is required for the linerless application equipment

• Some materials are unsuitable for linerless application

• The printer has to apply silicones and adhesives (hotmelt)

• The printer will require suitable coating units

• Filler/labeler will require suitable applicator to apply the linerless labels.

THE CATCHPOINT SYSTEM – A CASE STUDY

A number of companies offer proprietary linerless technology and applicator systems for both primary and secondary product decoration.

These companies include Catchpoint, Ravenwood Packaging, ETI Converting and Catchpoint licensee ILTI srl.

One of the systems which has been developed and patented is the Catchpoint system, this technology has been designed specifically for the application of linerless labels.

The system uses the same linerless manufacturing process explained earlier, but incorporates calibrated micro perforations (0.2 or 0.3 mm ties) positioned between each label. The perforations secure the labels within the web/reel but allow the label to be individually separated from the printed reel and accurately applied to the container at high speed, these perforations are known as Catchpoints™.

Catchpoint have worked closely with a number of material suppliers, printers and manufacturers of labeling applicators and are one of the first companies to develop a practical linerless label application system with none of the efficiency and safety issues experienced with the earlier Monoweb system.

Labels are shaped by a combination of die-cutting and micro perforations which are incorporated into the die-cutting tool and give an accurately repeated label shape. There are however limits on the shape of the label that can achieved using this method of cutting but shaped cutting of the top and bottom of the label can be easily carried out whilst still maintaining the perforated Catchpoints which are located in the non-profile cut areas.

Because the labels are linked together by the Catchpoints, spacing between the labels is not necessary and therefore eliminates the waste matrix which is normally associated with conventional self-adhesive labeling. Figure 4.24 below compares a conventional pressure-sensitive label layout to space saving layout required by the Catchpoint system.

CATCHPOINT LABEL APPLICATION

The Catchpoint application system is a simple way of applying labels to the container. The labels are held together in the web by micro perforations and at the point of application the labels are separated by momentarily pausing the web over a stationary separator edge (or in the ILTI version a moving separator tool) so that the label being applied is separated at the Catchpoint. This separates the label from the stationary web thereby allowing the single label to be accurately applied.

Figure 4.25 illustrates the Catchpoint label release sequence.

The peel plate or beak used for conventional pressure-sensitive labeling are replaced by a label separation system, labels can be separated and applied at high speed.

LINERLESS FROM A LAMINATE

Filmic substrates are a rapidly growing part of the pressure sensitive label market, Catchpoint has worked with laminate suppliers in the development of thin filmic labels which can be used in the linerless system.

The filmic face substrate is printed conventionally, but after printing the clear film base liner is de-laminated from the back of the printed face substrate and transferred onto the top of the face substrate. This leaves the adhesive layer on the back of the face substrate thereby creating a linerless label constructed with two layers of very thin filmic substrates and leaving the original release coating, which is on the liner to provide the release coating needed for a linerless format. The major advantage of this system is that it allows the use of filmic substrates with a much reduced face film calliper (down to 20-30 micron).

Linerless labels do not required the same stiffness characteristics that are required by conventional pressure-sensitive labeling. The Catchpoint application allows the label being applied to be attached to the container whilst the printed reel simultaneously gives stability and control of the thin label at the point of application.

CATCHPOINT LEANLEANTM

Catchpoint has extended the use of the very thin films to both enhance sustainability and deliver label shapes which cannot be provided in a linerless format. Catchpoint LeanLeanTM is a conventional PS label delivered from a filmic liner.

The new system overcomes the stiffness limits required by the conventional application by the use of limited catchpoints between the label images as shown in Figure 4.26. The minimal ties release the follower label to a defined pre-dispense and this label is then controlled by the next arriving container to repeat the sequence. 

This system uses a standard pressure sensitive filmic substrate construction which is printed and embellished using conventional methods, the die-cutting process which includes the catchpoints is the next process during which time the label is profile cut and the reduced waste matrix for the shape only is removed.

SUMMARY OF CATCHPOINT ADVANTAGES AND DISADVANTAGES

Catchpoint - Advantages

• No liner or carrier is required in a linerless format

• A good range of application options across the whole speed range

• The ability to offer very thin lower cost face materials as a conventional label with reduced matrix waste

• Applicator change unit suitable for use on existing application lines

• A label application system using micro-perforations which gives accurately applied label placing

• System uses existing conventional die-cutting systems

• A range of profile shapes which meet many existing label shape specifications on both paper and clear films

• Fewer web breaks and a claimed 2-3 percent improved line efficiency.

Catchpoint - Disadvantages

• The printer cannot apply silicone and adhesive as efficiently as the major laminators

• Linerless application systems can be more complex than self-adhesive systems

• Some very special materials cannot be used

• New investment is required to convert linerless labels and to install special application systems

• The cost of linerless labels may be higher