Ninety percent of solvent-based lamination adhesives for film lamination are currently applied using gravure printing technology. Application systems using flexo printing technology have very interesting benefits for production, especially in the difficult area of laminating very thin aluminium foil.
Basic concepts of film lamination
A film laminate is a multi-layer substrate, the individual layers of which (made of plastic, aluminium or paper) are bonded together with adhesive either inline or in a separate process step. The lamination of different foils and films enables us to fabricate flexible foodstuff packaging materials to custom specifications.
These include acting as a barrier against light and gases such as nitrogen, carbon dioxide and steam; good visual aesthetics (the interplay of adhesive and printed colour); resistance (surface adhesion and sealing seam), sealability (secure closure) and peelability (the openability of the packaging).
Many options for making laminated film are available. These include lamination (LF = solvent free and LH = solvent based); multi-layer extrusion (adhesion promoters); LH primer (such as PU adhesive); water-based primer (EAA dispersions); molten primer (EAA); and maleic acid.
Adhesive application systems
The fabrication of an optimal film lamination depends largely on the uniformity of application of the adhesive, which affects not only the optical uniformity of the adhesive, but also its adhesion. In lamination, one must differentiate between two basic application techniques:
1. Solvent based lamination
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Polished roller application (dip roller/application roller)
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Anilox roller application (gravure cylinder/doctor blade)
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Flexo printing (Flexo-4-roll)
2. Solvent free lamination
Lamination techniques
If the adhesive is dissolved in an organic solvent, the latter must be “removed” in a drying tunnel before the lamination takes place. This means that when the webs are bonded in the lamination step, the adhesive layer is already “dry”. This process is called dry lamination for this reason.
If lamination is carried out immediately after adhesive application, without an intermediate drying stage, one speaks of wet lamination. The adhesives are both physically drying and chemically crosslinking systems. The bond strength is based on physical processes – i.e. the drying of the adhesive once the solvent has evaporated.
When absorbent materials such as paper, cardboard, felt or textile substrates are laminated, dispersions are the technology of choice.
The gravure coating system
The gravure process is currently most commonly used for applying solvent based adhesives on laminating machinery. This consists of a gravure cylinder and rubber coated impression roller, and is a simple configuration which works in a stable and reliable way in practice. Cylinders (chrome anilox rollers) with different cell volumes are used to define the quantity of adhesive applied to the substrate.
The electromechanical gravure process is used (in special cases, even laser engraving is used) to engrave micron-size cells in the copper surface of the cylinder, which are then galvanically coated with chrome. In the coating station, these cylinders
rotate in a tank filled with laminating adhesive, thus filling the cells which, once any excess has been scraped off with the doctor blade, transfer the adhesive to the substrate.
In order to guarantee uniform coating, no air bubbles must remain in the cells. To prevent this from occurring, the floor of the tank is often fitted with a bar to drive out any bubbles. In modern laminating machines, the gravure adhesive coating process can be very fast (max. 600 m/min). Solvent based adhesive is processed at room temperature with a viscosity of 15 to 20 seconds (4 mm cup).
The solid content of this adhesive is generally 20 to 35%, and the maximum dry application weight can be as much as 4 g/m2.
The Flexo-4-roll system
A technical alternative for uniform application of solvent based adhesive is the Flexo-4-roll system, which has certain advantages over the gravure process. As the name indicates, this flexo coating station compromises of four rollers: the rubber coated squeeze roller (A); the laser engraved ceramic anilox roller (B); the rubber coated application sleeve (85 Shore A); and the chrome coated impression sleeve.
With the exception of the squeeze roller, which is driven by motor of the anilox roller, each roller has its own drive. The anilox roller’s drive can be precisely calibrated to apply dry adhesive weights of 2 to 4.5 g/m2. Depending on the lamination application in question, the Flexo-4-roll system can handle adhesives with a wide viscosity range from 18 to 35 seconds (4 mm cup). The maximum coating speed is 450 m/min.
