These functions include:
A pre-coat, or primer makes the substrate surface receptive to ink
Value-adding decoration effect – for example spot matte or gloss and various kinds of tactile effects
Adding slip and static characteristics which meet the handling requirements of different application systems such as for in-mold labeling (IML) and shrink sleeves
Providing regulatory compliance – for example meeting food migration requirements or (in the case of screen) Braille specifications
Providing mechanical and chemical resistance to the ink, including scratch, scuff resistance, outdoor protection, anti-curling properties and barrier properties
Anti-counterfeit and promotional properties, for example scratch off to reveal codes.
Coatings can be supplied in a range of chemistries, including UV, solvent and water-based, and can be applied using gravure, flexographic, screen and, more recently, inkjet processes. Coatings are typically applied in-line on a narrow web label press, a hybrid digital/flexo press or a sheet-fed press (for wet-glue or IML). On a standalone digital press, coating is applied as an off-line process or with additional inkjet heads integrated into the print engine.
The choice of print process used to apply coatings will depend on a number of factors, including: the thickness of coat required; the size/abrasiveness of the ink particles within the coating film; and process considerations such as press speed/investment factors. The thickness of a coating film typically varies between 2-2.5mm compared to a typical ink thickness of 1mm (see Figure 6.2).
Figure 6.2 Relative thickness of coating and ink layer
The screen process, either flatbed, semi- or full rotary, allows the heaviest laydown of even the largest and most abrasive particles such as those found in high security coatings, but at the cost of speed and higher capital investment.
Flexographic coating is a cost-effective process which will run at the highest press speeds, but reduced viscosity limits the range and build height of coatings. As well as standard print stations on flexo presses, flexo coating units can appear as modules in both in-line and sheet-fed offset presses, as well as letterpress machines.
Gravure is ideal for long runs of low-viscosity coatings such as cold seal and laquers.
We should also draw attention to specialist coating machines which allow converters to build their own special laminates or linerless labels, such as those produced by ETI and Maan Engineering.
Water-based coatings are formulated from styrene acrylate resin (see Figure 6.3). Acrylate dispersions form the backbone of the coating itself, providing mechanical properties including runability and abrasion resistance.
Waxes are used for scuff resistance; silicone provides the wetting agent and ammonia is used to dilute the resin and give stability to the coating. The styrene acrylate provides gloss to the resin. Water is used to adjust the viscosity and as a solvent. The manufacturing process is very similar to inks, only that coatings have a higher viscosity.
Figure 6.3 Formulation of water-based coatings
The water-based coating is dried by passing through an infra-red tunnel with large volumes of air circulating. The water evaporates leaving polymers on top of the ink. Although water-based coatings are normally dry by the time they leave the press, 12-24 hours is required for the coating to become fully activated (or fully functional).
In the case of conventional offset inks, during this time the ink underneath will still be wet, as it dries more slowly than the coating (see Figure 6.4).
Figure 6.4 Drying of water-based coatings
UV-based coatings use epoxy acrylate as an adhesive agent (see Figure 6.5). Other ingredients include reactive diluent monomers, oxygen blockers, photo-initiators, a reaction activator, and additives to confer different performance characteristics, as well as matting agents for matt coatings. When the coating layer is exposed to UV radiation, it will polymerize and be instantly dry (see Figure 6.6).
Figure 6.5 Formulation of UV-based coatings
Figure 6.6 Relative thickness of coating and ink layer
SPECIFYING A COATING
While the main function of a coating is to protect the printed image, there are additional functions, as noted earlier, including regulatory compliance, reducing environmental impact, visual and tactile effect, running/productivity characteristics, and special properties such as security features.
When deciding on the coating for a particular application, trade-offs need to be considered. For example, a high degree of visual impact can make it harder to comply with all regulatory requirements. Similarly, with a highly tactile coating, it will be more difficult to meet environmental reduction targets on that product – for example where coatings need to biodegrade along with the substrate. So, a balance is required when creating a coating strategy depending on the requirements of a particular product.
For applications where, regulatory compliance is the most important factor, there are a range of factors to consider. These include EU regulations on plastics in food contact (Regulation (EU) No 10/2011), where the raw materials used in coatings are taken into account in the overall pack assessment, while in the US coatings might form part of the FDA approval process for a plastic container.
Similarly, coatings will need to be correctly specified where labels which may come into contact with skin, such as stickers for toys or temporary tattoos for children. In Europe these come under Safety of Toys certification (DIN EN 71 part three), which regulates migration of different raw materials.
In the industrial sphere, coatings for labels used on chemical drums will need to be compliant to BS5609 (Labelling Hazardous Goods for Marine Transportation), while ROHS (Restriction Of Hazardous Substances) certification is required for electronics goods. REACH (see article on Impact of legislation on ink manufacture and usage) presents the same key compliance issues for coatings as it does for inks for any coatings imported into or used in Europe.
Manufacturers are only allowed to use a list of positively tested raw materials in their product formulations.
USE OF COATINGS
The mechanical or inkjet application of a coating on a machine system involves a complex set of variables which help ensure high levels of efficiency and productivity. Factors include the type of printing process, press speed and how fast the coatings and inks need to be cured or dried.
Water-based coatings in particular require a high level of expertise from press operators, with requirements for special anilox rollers and maintenance of tight tolerances for dryer temperatures, as well as the ability to assess post-cure activation times.
UV coatings involve fewer variables, but still require care, particularly around press speed to ensure a full cure. Very high reactivity UV coatings are available for press speeds of 150-200 m/min, but in practice most presses run at much lower speeds.
There are further issues to consider around the development of LED-UV coatings. As with LED-UV inks, these coatings require photo-initiators tuned to specific narrow wavelengths. Printers need to be aware that LED-UV coatings have a different reactivity compared to arc UV coatings, and this will affect curing speed and runability.
In-line processing needs to be taken into account when specifying pressure-sensitive label coatings. These processes may include cold or hot foil stamping, die-cutting and embossing.
Converters will also need to know what kind of labeling machines will be used at the packaging plant. Different types of label will require different properties on the label applicator line.
In-mold labels in particular require very specific coating properties. The coating will have to avoid any static build-up during printing and converting, but must have the ability to accept static when placing the label inside the mould.
Shrink sleeves require specific coating properties. The coatings will need to conform to the shrinkage ratio of the label without cracking, and also provide the correct degree of slip coefficient.
For direct thermal labels, the coatings need to be resistance to heat.
In some cases, the tactile impact of a coating is of key importance. A range of special tactile effects can be created, including smoothness, roughness and softness.
Luxury goods manufacturers are now looking to create a ‘natural’ impression using coatings to make films look like paper. Another key trend in this market is the no-label look, which requires very clear coatings. ‘Soft touch’ coatings which give a velvety feel to labels are also growing.
A wide range of special visual effects can be delivered by a coating, including spot matt and gloss coatings, metallic coatings, pearlescent coatings, ‘drip off’ effects for bathroom products, three dimensional effects and cast and cure, and simulating embossing effects with coatings – including Braille.
Coatings can also be specified from a sustainability viewpoint, for example where fully compostable labels are required, or the label structure needs to be ‘climate neutral’. The use of standard water-based products can also be viewed as a sustainable solution.
Primer coatings are required to assist ink keying to a substrate surface and to assist accurate color management. Primers allow the presentation of consistent surface properties irrespective of variations in substrate porosity and surface tension. Primer coatings are required for some digital processes, including for all HP Indigo ElectroInk systems, and for printing onto films with water-based inks.