Figure 8.1 - Example of a film laminated label
The lamination film can vary in thickness and clarity depending on the type of film used. This can be acetate, polyester or polypropylene based, all of which are supplied in reel or sheet format with either a matt or gloss finish.
Laminate film thickness is measured in microns. For instance 150 micron will be thicker than film at 75 microns.
Polypropylene or polyester laminates are suitable for application onto a printed web using an adhesive, combined with heat and pressure. The film requires careful handling and good control of the laminate tension during application is most important, ensuring that the laminate is applied smoothly and crease free onto the printed web.
Laminates are also supplied with a finish suitable for overprinting or as a non-printable surface. With products such as multi-layer labels, coupon and booklet labels a film laminate may be used to enhance, protect, hinge, and provide a reseal strip for the construction.
There are several types of lamination used in the packaging industry typically applied via a lamination unit (see Fig 8.2).
Figure 8.2 - Typical lamination unit
Solventless lamination: The adhesives used are solventless, which dry by chemical reaction and therefore do not requiring a drying system. This method is used widely in flexible packaging.
Wax lamination: The adhesive is a wax or hot melt which is applied in a liquid state to one of the substrates prior to the substrates being brought together.
This method is used widely in the food packaging industry
Dry lamination: Is the process whereby an adhesive is applied to the laminate film, which remains slightly tacky before it is applied to the printed web. This process is used widely for self-adhesive label lamination (see Figure 8.3).
Figure 8.3 - Dry Lamination - Shows the layout of a dry lamination unit with the reel of pre-coated laminate positioned above the base web which is to be over-laminated
Wet lamination: With wet lamination the adhesive is in a liquid state when the laminate and base substrates are brought together. This process is used in both the flexible packaging and self-adhesive labeling industry.
This article focuses on the two types of lamination most widely used in the self-adhesive label industry i.e. dry lamination and wet lamination.
Dry lamination tends to be used for short run applications. The type of lamination film used for dry lamination is heavier and thicker than the film used for wet lamination and because it is pre-coated it is more expensive.
A health and safety issue associated with dry lamination is the amount of noise generated as the ‘tacky’ adhesive separates when unwinding from the reel.
Within the self-adhesive industry the adhesives used for dry lamination are generally water-based acrylics. Although some solvent-based adhesive is still used it is reducing due to the environmental issues created.
One of the interesting developments in laminate adhesives is the use of 100 percent solids adhesive. These are reactive, cured adhesives and their use eliminates the need for drying systems and the energy costs are significantly reduced.
With dry lamination there are two method of bonding the laminate film to the substrate being laminated.
Hot lamination uses a heated nip roller to activate a pre coated ‘heat seal’ adhesive on the laminate film. This process bonds the laminate film to the printed substrate.
In some instances the adhesive can be applied on the press as a hot melt application.
To apply the hot melt adhesive it is necessary to heat the hot melt to the correct temperature and then apply the liquid adhesive through a die coater unit.
The two substrates then enter the nip roller assembly to apply the necessary pressure. The pressure of the nip rollers causes the adhesive to flow evenly, creating a consistent bonding between the two substrates.
Cold lamination uses a pressure sensitive adhesive which is pre-coated to the laminating film and remains in a ‘tacky’ state. The nip rollers are not heated, but do apply pressure to the laminating film and the substrate, bonding the laminate film to the printed base substrate.
This method of lamination avoids damaging the printed image.
With wet lamination a liquid adhesive is applied to the film laminate prior to the merging of the base web and the over-laminate.
In the process of wet lamination the adhesive is applied to either the reverse side of the film laminate or alternatively to the face of the printed substrate to be laminated. The liquid adhesive is applied by a roller coating system, usually the flexographic process. The two substrates, one of which has been coated with the adhesive, are then nipped to form the bond (see Fig 8.4)
Figure 8.4 - Wet Lamination Process
Types of wet adhesives
The types of adhesive used for wet lamination applications in the self-adhesive label manufacturing are;
The adhesive can be air dried using a heated drying unit to remove any solvent or moisture. If UV adhesives are being used, the web is passed through a UV drying system.
Water-based adhesives are considered better for bonding because of the good initial grab and fast drying characteristics, particularly if being applied to porous substrates.
Acrylic-based adhesives are a low cost liquid adhesive which provide excellent bonding properties and are suitable for UV curing. These UV cured laminating adhesives are solventless, low viscosity liquid adhesives, which can be applied through normal printing methods and then UV cured.