Closure Liner Compounds

A closure liner is a gasket formed inside a cap, which seals against the glass or metal finish of the container when the cap is applied. It is not a coating. It is a compound deposited into the closure shell, either flowed in and gelled or dried, or moulded in the shell, and then compressed against the container finish by the capping head. The seal is mechanical: the compound deforms to the profile of the finish, is required to hold pressure or vacuum for the life of the pack, and releases in a controlled way when the consumer opens it.

The live issue in this category is the compound chemistry itself. Traditional liners for metal vacuum closures are PVC plastisol containing plasticisers, and plasticiser migration into fatty foods is under regulatory and customer pressure. PVC-free thermoplastic elastomer compounds are the replacement route, and packers are specifying them more often, particularly for fatty and oily fillings. We state the position plainly. Plastisol still performs and is still specified by many packers. The thermoplastic elastomer route removes the phthalate plasticiser question, although these compounds are not free of all softening additives, since styrenic block copolymer grades normally carry a polymer-compatible extender oil, and they behave differently on the lining line and in the capping head. Every conversion has to be re-qualified rather than assumed. Arcliner is built to serve both routes so the change can be made on the customer’s own timetable.

Closure Liner Compounds sit apart from Metal Packaging Coatings, which covers closure interior and exterior lacquers, and apart from the proposed Arcseam range of can end sealing compounds. Arcseam is a liquid water-based compound flowed into the curl of a can end and dried, where it is later formed into a double seam. Arcliner is a solid compound formed in a closure shell by moulding or flowing and then compressed onto a glass, metal or plastic finish by a capping head. Different plant, different customer, different unit of sale.

Qualification in this category is not a laboratory exercise. The compound is proven on the customer’s capping line and then by torque retention, vacuum retention, pasteurisation or retort survival and organoleptic panel testing over months of real storage. Sensory and taint performance is decisive for beer, water, carbonated soft drinks and mild-flavoured foods, and a compound that passes every mechanical test can still fail a taste panel. We do not claim food-contact approval for any grade. The declaration and the compliance dossier are issued on request with the Technical Data Sheet, so that the packer can complete its own assessment against the regulation that applies in its market.

Shallow liners for crimped steel crowns on carbonated beverages, where the seal must hold internal pressure and survive tunnel pasteurisation without taint.

Ring gaskets for lug caps and full-panel gaskets for press-on twist-off caps on glass jars, where vacuum retention, clean release and torque retention over shelf life decide the pack.

Liners for aluminium ROPP shells, where the thread is formed on the bottle during capping, so the compound must seal under the top load of the pressure block while the skirt is rolled.

Compression seals moulded or flowed into polyolefin closures at temperatures the shell will take, including valve and venting compounds for closures that must relieve pressure in a controlled way.

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 are the typical operating window for the range, and confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

Arcliner is applied in two steps that usually happen in two different factories. First the compound is placed in the closure shell: flowable plastisol grades are dosed and spun into the shell and then gelled in an oven, water-based flowable grades are dosed and dried, and thermoplastic elastomer grades are dosed as a hot melt and compression moulded in the shell against a cooled forming tool. This step is run by the closure maker on rotary lining machinery, and it fixes deposit weight, deposit distribution and the profile of the sealing surface. Second the lined closure is applied to the container by the packer, and only at that point does the compound become a seal.

The oven or tool condition follows the shell as much as the compound. A steel or tinplate shell will carry plastisol gelation at oven air temperatures of 180 to 210 C. A PP or HDPE shell will not, so compounds for plastic closures are water-based and dried at 60 to 90 C, or are moulded in the shell from the melt against a cooled tool. Under-gelled plastisol and under-consolidated elastomer both look correct in the shell and both leak later, so the oven profile and the tool temperature are process controls, not settings to be adjusted on the line.

The sealing mechanism differs by closure type, and the line settings follow from it. A crown is crimped: the skirt is formed down over the glass crown finish and the shallow liner is compressed against the top sealing surface in a single stroke, so deposit weight and crimp dimensions govern whether carbonation is held. A lug twist-off cap is applied over steam, so that condensing steam displaces the headspace air and the vacuum forms as the pack cools, and the gasket has to flow into the finish while hot and then hold as the glass and the metal contract at different rates. A press-on twist-off cap has no formed thread: the thread is impressed into the compound by the glass finish as the cap is pressed on, so the liner has to be a full-panel deposit with enough compound on the skirt to take that thread, where a lug cap can be served by a ring gasket. A roll-on pilfer-proof aluminium shell has no thread either until the capping head forms one, and the seal is made by the vertical load of the pressure block on the top panel while the thread and pilfer rollers form the skirt, so the liner must hold that seal through the spring-back of the aluminium. A plastic closure seals purely by compression under applied torque, which is the least controlled load of the four, particularly when the cap is reclosed by hand.

What governs a good result on the line is consistent in every case. Deposit weight and its distribution around the shell are the first variable to check when a seal fails. The condition of the shell lacquer and the cleanliness of the shell decide adhesion of the liner to the closure. Glass finish quality, chipping and out-of-round on the container are frequently the real cause of a leak attributed to the compound. Application torque, head pressure, crimp or roll-on settings and headspace steam must be recorded, because a change on the capper will show up as a torque or vacuum drift weeks later. Where the filled pack is tunnel pasteurised, the liner carries the full cycle, and cooling after the tunnel and storage temperature in the warehouse then determine how torque and vacuum behave over shelf life. Where the beverage is flash pasteurised in line before filling, the liner does not see that heat at all and the thermal demand on the compound is much lower, so the two routes are not interchangeable when a grade is selected.

Qualification runs on the customer’s own line. A trial fill is capped at production speed, then pasteurised or retorted to the customer’s cycle, then held and measured for removal torque, vacuum retention and leak rate at intervals over several months, with an organoleptic panel on the packed product at each interval. We expect to support that programme rather than to shorten it. The declaration and compliance dossier for the grade are issued on request with the Technical Data Sheet.

To specify an Arcliner grade correctly we need the closure, the container and the process, not only the product name. The questions below are the ones a customer must answer before we quote.

• Which closure type: crown, lug twist-off, press-on twist-off, roll-on pilfer-proof aluminium, or plastic closure, and at what diameter?
• Is the shell steel, tinplate, aluminium, PP or HDPE, and which interior lacquer or resin is on the sealing face?
• What is the container finish: glass, aluminium, PET or HDPE, and can you supply drawings or sample containers from current production?
• Does the pack hold pressure, hold a vacuum, or neither, and what internal pressure or residual vacuum is specified?
• What is the product being packed, and specifically is it fatty, oily, acidic, alcoholic or high in essential oils?
• Is a PVC-free compound required, or is plastisol acceptable for this pack, and is the change driven by a customer specification or by a regulation you must satisfy?
• Is the filled pack tunnel pasteurised, and at what temperature, dwell or pasteurisation unit target, or is the product flash pasteurised in line before filling? If the pack is retorted, at what temperature, time and overpressure?
• Is the lining step run in-house by flow and gel, by flow and dry, or on in-shell moulding equipment, and what oven profile, melt temperature and tool temperature do you run, at what line speed?
• What deposit weight is currently specified, and what is the tolerance you hold in production?
• What application and removal torque targets apply, and what removal torque range is accepted at end of shelf life?
• What shelf life must the pack hold, and at what storage and transport temperatures?
• What sensory standard applies to the packed product, and who runs the panel?
• Which regulation and which market must the declaration address, and is a compliance dossier required at the quotation stage?
• What is the annual volume, and in what form do you buy: drum, pail or bulk?

With those answers we can propose a grade and a trial plan. Final selection is confirmed by a capping line trial on your own closures and containers, followed by torque, vacuum, heat process and organoleptic testing over the intended shelf life.

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.