Coated Aluminum Foil
As evidenced in the Coating Materials section at the start of this chapter, virtually any type of film-former can be formulated for use on foil, and many are in standard availability and daily use by the foil converting industry.
End users range from the completely aesthetic eye-catching decorative gift wrap to the most basic, functional and almost unnoticeable transparent coating on a foil package label, employed to protect the printed design from abrasion. And, as in coating everything, whether printing a book cover or painting a yacht, adhesion is the primary, but far from only requirement.
Modern coating materials, discussed early in this chapter, and the converter equipment and techniques to be discussed in this section, combine to satisfy every type of coated aluminum foil end-use requirement.
Aluminum foil is coated with basically the same equipment used for roll-to-roll web coating most other materials. The adhesive applicator stations on foil laminating machines are, of course, coating units. Also, specific foil coating facilities are today often incorporated in foil laminating machines.
Whether arranged in-line or separately, web coaters, like laminators, include unwinds, coating applicator stations of various types, spreader rolls, idler rolls, drying and/or chill-roll section or sections, and rewinds.
Functions of Foil Coatings
While bare aluminum foil is most attractive and durable for a host of applications and its use in that form is constantly increasing, the addition to the foil of any of a wide variety of coatings open still other fields, each with its growing list of profitable end uses.
One such field is packaging; another is advertising displays, brochures, and advertisements. Aluminum foil is often chosen in both of these fields for the striking colored metal effects obtained by the use of transparent, colored coatings, along or in conjunction with printed designs.
The greatest percentage of coated (and all) foil stock is used for packaging and is often laminated to paper or to film for increased strength and product protection.
The basic reasons for coating aluminum foil include the following:
- To produce a full range of attractive metallic colors (through the use of pigments and stable dyes in transparent coatings)
- To render the foil surface heat sealing
- To lubricate during processing (as to prevent galling-chafing of foil surface in rolls of foil for rigid container stock)
- To impart water and frost shedding characteristics to help avoid frosting over of frozen food package labels, etc.
- To increase the foilí»s scratch and scuff resistance
- To enhance water vapor barrier characteristics and gas resistance of light gauge foils
- To increase the foilí»s resistance to contamination by corrosive agents, products, or atmospheres.
- Improve the adhesion of other coatings or printing inks.
- To produce a specific surface, i.e., slip, non-slip or release.
- To increase the tear-tensile and Mullen (bursting) strengths
- To develop electrochemically certain special purpose films on the foil surface (such as dielectric anodized films on electrical foil)
- To impart high gloss and third dimensional depth to the decoration or printing on the foil, and to provide integral scuff resistance to protect the decoration
- To increase heat absorption
Table 6 gives a general indication, in idealized form, of some typical film formers employed to achieve the above foil coating objectives.
The purpose of a coating machine is to apply a uniformly controlled amount of film forming material to a web or sheet product. Whether the coating is a lacquer, protective coating, or adhesive, the basic processes can be the same.
Choice is determined by considerations of foil gauge, coating materials, or ultimate use requirements and economy. The common methods for coating and adhesive application are squeegee, direct roll, transfer, reverse roll coating and intaglio or engraved roll. These methods are illustrated in Figs. 11, 12, 13, and 14.
Lightweight low-viscosity coatings are usually applied by the squeegee, direct-roll transfer and intaglio (gravure) rolls. Reverse roll coaters are the preferred equipment for heavier coatings.
Drying Equipment For Solvent Removal
Equipment for solvent removal in coating, printing or laminating aluminum foil is usually identical in design to that used for other flexible sheets. The common type of drying is forced air convection. The heat source is gas, steam or electricity. Important factors to be considered in choosing equipment type and capacity are the impervious, nonsorptive surface of aluminum foil, its high thermal conductivity and high reflectivity for radiant heat.
In practically all of these operations, large amounts of volatile solvents are employed and ample ventilation facilities must be provided for their removal, since they are potential health and fire hazards. Since aluminum foil has a high reflectivity value (95 percent), radiant dryer heat should be applied to the paper or plastic side of a coated or printed lamination, where a substantial film or ink or coating must be dried. An infrared heat source also is effectively utilized in conjunction with counterflow air currents for solvent removal from coatings on foil or other materials.
In the case of both lacquered and laminated foils, cooling rolls, placed immediately ahead of the rewind unit, are also used to lower the temperature of the heated web and thus prevent blocking or sticking.
In coating and practically all other web converting operations, special attention must be given to the proper handling of the foil. Controlled-web tension is essential to smooth unwinding and rewinding of the coiled stock. It assists in maintaining the foil flat and free from wrinkles during coating, printing, laminating, and other web converting operations. Adequate power facilities and controls for braking and driving the machine are essential in providing a satisfactory tension.
A constant tension, regardless of coil diameter, is desirable at all times. However, it is essential for high speed operations; consequently, tension is maintained automatically and is compensated to adjust immediately to uniform acceleration or deceleration of the drive.
Applied tension, of course, should not exceed the yield strength of the web, and a safe tension is normally about one half of the ultimate tensile strength. In the case of annealed foil, this is three pounds per inch of width per mil of thickness. To avoid breaking or strain hardening, annealed foil should not be subjected to any greater tension than is necessary to promote and maintain a smooth web.
Support and Idler Rolls
Transport sections of coaters, laminators and similar web processing equipment should be so designed as to minimize all distances over which the foil is unsupported. This can be accomplished by compact design and/or by the use of properly spaced, well balanced idler rolls having good bearing systems and smooth surfaces.
Proper arrangement of guide rolls to alter the direction of travel of the foil web, or of rolls (idler) that provide compression contacts (e.g. nip rolls), assists in maintaining flatness and tension. Appropriate use of these design features can be particularly advantageous in preparing the foil web for contact with coating rolls.
A widely used device for supporting the foil and removing wrinkles and fullness in the metal is a spreader roll of one type or another. One of these is a herringbone roll, Fig. 15. This is a helically grooved roll, and generally is from four to twelve inches in diameter. Foil is threaded over the herringbone roll with a wrap of about 30 percent, depending on the gauge and condition of the foil, so that the metal spreads outward to prevent the formulation of wrinkles and creases. An adjustable curvature, flexible roll is another type of spreader roll which has a somewhat similar smoothing effect.
Several types of conveyors are used in drying ovens of finishing machines to support the foil web between the application unit and the rewind stand. Among these are slat type conveyors or rod type conveyors, as well as idler roll types, with the rolls placed 12 in. to 18 in. apart.