Aluminium Foil and Lidding Lacquers

Aluminium foil behaves differently from plastic film, and the coating has to be built for it. Foil is non-absorbent, so nothing soaks in: the solvent or the water has to leave through the coated surface, and the film forms and cures at that surface rather than partly in the substrate. Foil also has low thermal mass and high conductivity, so the web heats quickly in the oven and cure is fast, which is why foil coating lines run at high web speeds and short oven dwell. The consequence is that adhesion rests entirely on the lacquer chemistry and on the cleanliness of the foil surface. Foil supplied for lacquering is normally fully annealed and carries little residual rolling oil, but the level still varies between coils and across the width of a coil, and residual oil is the most common root cause of adhesion failure on foil. Confirm the residual oil level on the incoming foil before any coating is blamed.

The defining specification in this range is the seal. A seal is made on a filling line at a jaw temperature, a jaw pressure and a dwell time, and the lacquer has to reach its sealing initiation temperature within that dwell or the seal will not form. Seal strength on a web-to-web seal is measured by ASTM F88, and the peel of a finished lid from a filled container is measured on the pack itself using the supported technique of that method. The result is always specified as a window rather than a maximum: too weak and the pack leaks or opens in transit, too strong and the consumer cannot get into it. Easy-peel and controlled-peel grades are engineered to fail in a predictable way, either cohesively within the lacquer layer or cleanly at the interface, at a controlled force. The single most important selection question is the polymer of the container, because a lacquer formulated to seal to polypropylene will not seal to polystyrene, and a grade for rigid PVC will not serve a PET tray.

The range also covers work where the lacquer is not the seal. Protective baking lacquers carry the foil through forming and filling: retort grades have to survive sterilisation, typically 121 to 135 degrees Celsius under steam and pressure, without delamination, blistering or loss of seal, and deep-drawable grades have to stretch with the foil as it is formed into a tray or a blister cavity without cracking at the corners or the draw radius. Primers give a key on difficult, hard-tempered or oily foil and on foil composite, and overprint and spacer varnishes protect print and control winding on the reel. Pharmaceutical blister lidding is a regulated application, and the qualification sits with the finished pack rather than with the lacquer alone. Arctic issues a compliance dossier and declarations on request with the Technical Data Sheet.

Two boundaries are worth stating. Against Speciality Coatings for Packaging, Arcseal is a sealing coating applied inline by the flexible converter onto plastic film and board, while Arcfoil is a roll-coated, oven-cured lacquer on aluminium foil and foil laminate, including retort and pharmaceutical work. Against Metal Packaging Coatings, that range is for rigid metal, a can body or an end formed from sheet or coil, while Arcfoil is a foil web or a die-cut lid. If the substrate is a foil web that will be printed, formed, filled and sealed, Arcfoil is the range.

Heat-seal lacquers for lidding foil, selected by the polymer of the container they have to seal to: polyolefin, styrenic and rigid PVC, or polyester. Each grade is built around a sealing initiation temperature and a seal strength window that suit the filling line.

Grades that form a seal at a lower jaw temperature and a shorter dwell, for heat-sensitive fills, thin-wall containers that distort under a hot jaw, and fast lines where dwell is the limiting factor.

Peelable lidding systems engineered to fail at a controlled force, either cohesively within the lacquer or at the interface, so the lid opens cleanly without tearing the foil or lifting the container flange.

Baked protective lacquers for the outer or unsealed face of the foil, including retort-resistant grades for sterilised packs and deep-drawable grades that stretch with the foil during forming.

Thin-film primers that key to foil and foil composite and carry the topcoat, used where a hard temper, a laminate face or a variable residual oil level makes direct adhesion unreliable.

Varnishes applied over print on the foil web to protect the image through converting and to control the coefficient of friction and blocking behaviour on the reel and in the die-cutting station.

Heat-seal lacquers for hard-tempered lidding foil sealing to PVC, PVdC-coated PVC and polypropylene blister bases, formulated for a consistent seal at the high jaw temperatures and short dwell of a blister machine.

The figures below are the typical operating window for this range, not a measured result for any one batch. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

The figures in the table above are the typical operating window for the range, not a measured result for any one grade. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

On the coating line

Arcfoil lacquers are roll coated onto a foil web, by smooth roll, reverse roll or gravure depending on the coat weight, and cured in a hot air oven. The controlling variable is the peak metal temperature the web actually reaches, not the oven air setting. Foil has low thermal mass and heats quickly, but whether it gets to the air temperature depends on web speed, oven length and zone profile, so a change in speed changes the cure as much as a change in the setting. Measure the peak metal temperature with a travelling thermocouple when the line is qualified and after any speed change. Coat weight is set gravimetrically on the running web and should be checked across the width, because a light edge is where seal failures start. Under-cure leaves retained solvent, which shows up later as blocking on the reel, taint in the pack or a seal that drifts with storage. Over-cure embrittles the film and reduces the draw available on formable grades.

Foil surface and adhesion

Adhesion on foil is decided before the lacquer arrives. Foil supplied for lacquering is normally fully annealed, but residual rolling oil from the mill pass still varies between coils and across the width of a coil, and it remains the most common root cause of adhesion failure. Confirm the residual oil level on incoming foil, keep the coils covered and dry, and treat a change of foil supplier as a change of specification that needs a trial. Where the oil level, the temper or a laminate face makes direct adhesion unreliable, prime with Arcfoil PR 110 and coat over it. Test adhesion on the cured web, not on a hand drawdown. On foil laminate and on heavier gauges a cross-cut to ISO 2409 can be made without cutting through the metal; on thin lidding foil the blade cuts or deforms the foil, so the result is a foil failure rather than an adhesion result, and a tape pull, a bend over a mandrel and a solvent rub are the usable checks. Agree the method with Arctic before it is written into a specification.

The seal on the filling line

A seal is made by three variables together: jaw temperature, jaw pressure and dwell time. The lacquer has to reach its sealing initiation temperature at the interface within the dwell available, and on a fast line the dwell is short enough that jaw temperature has to rise to compensate. Heat has to travel through the lid, so lid construction, foil gauge and any laminate layer all change the result. Seal strength on a web-to-web seal is then measured by ASTM F88, and the peel of the finished lid from the container is measured on the filled pack using the supported technique of that method, since a lid peeled from a rigid flange is not the flexible-to-flexible geometry that a T-peel method describes. Judge the result against a window agreed for the pack, not against a maximum. Run the window deliberately: seal a matrix of temperature against dwell at the working pressure, measure the peel force at each point, and set the line in the middle of the plateau rather than at its edge. For peel grades, record the failure mode as well as the force, because a peel that changes from cohesive to interfacial has changed even when the number has not.

Forming, retort and printing

Deep-drawable grades are qualified on the draw itself: form the actual cavity at the actual draw ratio and inspect the radius and the corners for microcracking, since a crack that passes visual inspection can still fail a dye penetration or a corrosion resistance check on the coated face. Retort grades are qualified on the full cycle, at the customer’s own temperature, hold time and overpressure, with the pack examined for blistering, whitening, delamination and flange seal loss after cooling. Where the foil is printed, the overprint or spacer varnish is set by the winding and die-cutting behaviour of the reel, so specify the coefficient of friction target, measured to ASTM D1894, and the storage conditions with the enquiry. Any grade that will be printed by a digital process, whether liquid electrophotographic, dry toner, UV inkjet or aqueous inkjet, needs a press trial on the customer’s own press before specification.

Handling

Stir before use and check the supply viscosity at the working temperature. Reduce only with the thinner named on the Technical Data Sheet, since the solvent blend sets the flow and the release of solvent in the oven. Keep containers closed, store within the temperature range on the label, and do not return line returns to the drum without a viscosity and solids check. Safety Data Sheets are issued with every delivery and apply to storage, handling and disposal.

A foil lacquer is specified against the pack, not against the foil alone. Arctic needs the following before a grade is recommended, and a line trial at the customer’s own filling or blister machine is part of every specification.

The pack

• What polymer is the container or blister base: polypropylene, polyethylene, polystyrene, rigid PVC, PVdC-coated PVC, PCTFE-laminated PVC, PET, APET, CPET or a composite? This is the first question and it decides the subfamily.
• Is the seal permanent, easy-peel or controlled-peel, and what opening force is acceptable to the end user?
• What seal strength window is required, measured by ASTM F88 on the web-to-web seal and on the filled pack, and what failure mode is acceptable?
• What is the product being filled, and is it hot filled, cold filled, aqueous, oily, acidic or alcoholic?
• What shelf life is claimed, and at what storage temperature?

The foil and the web

• What foil gauge and temper, and is it plain foil, printed foil, or a foil laminate with paper, PET, polyamide or board?
• What is the residual rolling oil level on the incoming foil, and does it vary by supplier or coil?
• Which face carries the lacquer, and what is on the other face?
• Is the lacquer to be printed over, and by which process?

The coating line

• Application method and coating head: smooth roll, reverse roll or gravure, and the anilox or cylinder specification if gravure.
• Target dry coat weight and the tolerance you can hold across the width.
• Web speed, oven length, oven zone temperatures and the peak metal temperature the web actually reaches, measured rather than assumed.
• Winding conditions and storage: how warm is the reel wound, and how long is it held before converting?

The filling or blister line

• Jaw temperature range, jaw pressure and dwell time available, and the number of lanes and their variation.
• Machine type: rotary cup filler, tray sealer, thermoform-fill-seal, or rotary or platen blister machine.
• Cycle rate, and whether dwell or temperature is the limiting factor.

Forming, sterilisation and regulatory

• Is the foil formed, and at what draw depth and draw ratio?
• Is the pack sterilised, and at what temperature, hold time and overpressure?
• Is the pack reheated by the consumer, in what type of oven and at what temperature, and is the lid removed first?
• Which regulations or customer specifications does the finished pack have to satisfy, and in which markets? Arctic issues a compliance dossier and declarations on request with the Technical Data Sheet so that you can complete the pack assessment; the qualification of the finished pack sits with the pack owner.
• For pharmaceutical lidding, what push-through and seal integrity criteria apply, and what documentation does your quality system require at change control?

The Technical Data Sheet, Safety Data Sheet and regulatory declaration for any grade in this range are issued by our laboratory against the substrate and process you are running. Tell us the job and we will specify against it.