There may also be legislative requirements that the applied label has to comply with, for example marine applications where up to three month immersion in sea water and chemical resistance are required.
These different requirements place significant demands on the label and, in particular, the label face material and adhesive used in the construction.
So how is a pressure-sensitive label constructed? The multiple layers of the laminate include a label face material of paper, film, synthetics or foil; a pressure-sensitive adhesive; and a silicone coated backing paper or film, called the release liner.
This structure can be seen in the following diagram (Figure 2.1).
Figure 2.1 Construction of pressure-sensitive laminate showing backing paper and silicone release coating
Some laminate constructions may also have an additional top coating to make them suitable for subsequent operations, such as HP Indigo or inkjet digital printing, or for thermal printing.
WHAT IS THE FUNCTION OF EACH LAYER IN THE PS SANDWICH?
The backing or release liner layer. The release liner protects the pressure-sensitive adhesive during handling, printing, converting, die-cutting and re-winding of the labels and right up to the point of dispensing and application. At this stage, the backing is peeled away from the adhesive immediately prior to its application.
The release liner must also stop the adhesive sticking to the backing or carrier material during the whole printing and converting process.
Depending on the particular requirements of the laminate, the release liner may be paper (super-calendered, coated, polymer coated, glassine, Kraft) or a plastic film (PET, OPP, LDPE or HDPE). Whatever the material used it must be smooth and consistent in caliper right across the coating machine web width. Some liner materials may need to be transparent, have different strength or stiffness requirements.
The silicone coating layer. Silicone coatings, which may be solventless, water-based, solvent-based or radiation curable, have long been the most widely used release agent for pressure-sensitive adhesive applications, dominated by the label market. The silicone is coated on the backing material to provide all the essential features of a typical self-adhesive label construction.
The specific adhesion between the silicone surface and the adhesive being used can be fine-tuned by the use of controlled release additives to the silicone coating to ensure the laminate is held together, yet provide appropriate release levels to suit labels that need to be applied automatically at great speed, or that can withstand the turns and flexing in the paper patch of a laser printer or copier without the labels dispensing inside the machine.
The silicone release system needs to allow the backing to be removed from the label, regardless of the tack level of the adhesive.
Silicon coating weights and coating performance have improved significantly in recent years and are discussed in more detail in the following chapter.
The pressure-sensitive adhesive layer. Pressure-sensitive adhesives are adhesives which in their dry state at room temperature are always tacky and will adhere or stick to a wide range of surfaces by simple contact under light pressure.
The simplest method of categorising pressure-sensitive adhesives is to class them according to their permanence or removability. Within these two categories there are a range of different adhesive types – natural rubber, synthetic rubber and acrylic polymers – which in turn may be solvent-based adhesives, hotmelts or dispersion (emulsion) adhesives.
The choice of adhesive will be dictated, for example, by the nature of the face material and the influences to which the label is exposed. There are also regulations and standards that must be observed.
A full explanation of pressure-sensitive adhesive technology can be found in chapter 4.
The label face material layer. Label face materials are those that finally appear on the product being labeled. The choice of material depends on many different performance, printing, converting, application, storage, handling, distribution and usage requirements. Selection of a label face material may depend on a great many factors, such as visual appearance and image, surface texture, degree of transparency, durability, long life, production flexibility, environmental considerations, volumes required, speed of application, the performance needs of the label (chemical or water resistance, sterilizing).
Because so many different factors and requirements may be involved in the choice of face material, there is invariably a wide range of different face materials to choose from. These include coated and uncoated papers, woven or laid papers, filmic substrates, non-paper synthetics, metalized papers and films and metalic foils.
Some materials may have additional top coatings added for specific performance requirements.
Paper is a fiber pulp-based material and can be printed with a variety of printing techniques. Spruce and birch wood from managed forest is often used as a basic raw material. Sustainably managed forests are certified by either PEFC or FSC.
The wood is separated from the bark and chaffed into chips. The wood chips are then pulverized in presses or in grinders with the addition of water. The particles are finally filtered and cleaned in several successive baths in order to achieve a homogenous fiber pulp. The pulps of today are generally a mixture of wood fibers and paper, to which a binding agent is added for the better formation of the final paper.
Modern paper production is undertaken on machines which can be over 100 meters long and up to 10 meters wide. The paper webs are produced at speeds of up to 1,800 m/min (Figure 2.3).
Figure 2.2 Paper mill and paper machine
Figure 2.3 Paper machine structure
When the pulp reaches the machine, it is dissolved in water, generally at a ratio of 5 percent pulp to 95 percent water. The dissolved pulp flows into a reservoir tank at the beginning of the paper machine which creates a uniform paper pulp to run through the paper machine. This is known as the ‘wet’ end of the process line.
The mixture at this stage is called slurry and is transferred to an endless rotating polyester screen where the water content of the fiber pulp is reduced from 95 percent to 80 percent. Any fibers which escape this screening process are captured and recycled. By the end of the screen, the paper is strong enough to be transferred from the wire to the wet press section of the paper machine.
The wet paper is then fed through a succession of pressure rollers which are equipped with absorbent felts. At the end of this section, the paper has lost some of its weight and has a water content of just 60 percent.
The final phase, during which the paper attains its final water content of five percent, is the drying section. This consists of a sequence of steam-heated drying cylinders which are arranged on top of each other. Their temperature reaches up to 120 degC as a result of which the moisture evaporates. The temperature gradually decreases from one cylinder to the next. After the drying section the basic paper has been created.
Depending on the final specification of the paper, extended processes like coating or calendering can follow in-line or off-line.
The calendering process makes the paper surface smooth and glossy. To better understand calendaring, it can be compared to ironing textiles, where the textile is pressed on by a warm iron. The same happens with paper. The paper is fed through a pile of heated chrome cylinders and the paper comes out smoothed, or ‘ironed’ at the end.
Instead of making the paper appear glossy, it can also be made matt by replacing certain chrome rollers with ceramic rollers at this stage of the process. The pressure applied to the paper will affect the paper thickness.
Films are generally made from granulate. Granulates are small plastic particles which will melt at a certain temperature and during the film manufacturing process the granulate becomes liquid.
PE manufacturing. Polyethylene (PE) is a type of polymer that is thermoplastic, meaning that it can be melted to a liquid and re-molded as it returns to a solid state. The technology makes use of PE granulates as a basic raw material. There are two main processes to manufacture polyethylene films : casting and blowing technology.
Casting PE is manufactured by melting the PE granulate, pressing it through an extruder and guiding the film in a horizontal direction via rollers to the winding unit.
Blown PE is a process where PE granulates are melted and pushed out of the extruder in a vertical direction in the form of a bubble. At the end of the bubble the film is collected into double folded film webs and transported to slitter and winder units.
To make the films receptive to ink, corona treaters are installed at the end of the film production lines. The surface tension of the film should be 38 dyne or higher for most printing technologies, and because surface tension reduces over time, corona re-treatment is often required on the printing press. Alternatively the film can be top coated, which has the same effect of increasing ink anchorage.
PP manufacturing. The manufacturing of PP is very similar to the production of cast PE (Figure 2.4). The PP granulate is melted, pressed through an extruder and the film is stretched and cooled down by rollers in the horizontal direction to the winding unit.
PET manufacturing. Manufacturing PET is in its basics very similar to cast PP production (Figure 2.5).
Figure 2.4 PP manufacturing machine
Figure 2.5 PET manufacturing process
SELF-ADHESIVE LAMINATE PRODUCTION
The manufacture of self-adhesive materials today is predominately undertaken on wide-web high-speed coating and laminating lines incorporating sophisticated process monitoring and control systems to ensure that quality, coat weights, tension, rewind, slitting, etc, are all within demanding production and performance tolerances (Figure 2.6).
Figure 2.6 A high-speed self-adhesive coating and laminating line. This picture shows the UPM Raflatac coating line in Wroclaw, Poland
The most sophisticated of these lines takes a backing material, coats with a silicone release coating, dries or cures the coating, coats with adhesive, dries or cures this then applies the required face material, before rewinding and slitting to the required web widths for the converter (see next chapter for more details on coating technology).
With die-cutting at the label converting plant being so critical for high-speed applicator lines, all materials and coating are processed within tight tolerances.
Holograms can be divided into two groups:
Paper-based or filmic substrates with a web-wide holographic image, repeating in the machine direction. These are for general use, mostly for decorative labels.
Single paper-based or filmic holograms with an individual brand logo and selected security levels for high end applications like security labeling.
Holograms covering the web and repeating in the machine direction are produced in a rotary process. Depending on the diameter of the original transfer roller, there will be frequent image interruptions in the cross direction. These join-lines are called ‘shim’-lines, and are part of the hologram production process. Shim line specifications should be considered when holograms are involved.
ALUMINIUM FACE LABELS
Durable face labels (with serial numbers, UL approval references, etc) can be manufactured from pure aluminium in combination with a strong adhering solvent-based or UV acrylic based adhesives. For high quality printing, the aluminium needs to have a top coating.
Die-cutting of aluminium requires a suitable die-cutting tool. A typical example of a solid aluminium label is that found on whiskey bottles, often combined with embossing.
Figure 2.7 Hologram production