Choose a cap liner from the formulation outward: the chemistry of the fill picks the material family, the neck finish tolerance and capping load pick the thickness, and the destination market picks the declarations. PE foam, EVA, pulpboard, foil laminate, silicone and nitrile each seal by a different mechanism, so each fails in a different and predictable way when the contents change. Sealing force and barrier are separate requirements, and a liner specification has to state both.
What to define before naming a material
A liner (also called a cap gasket) is the compressible layer between two rigid parts that are never truly round, flat or on nominal size. It only works when the closure shell can press it onto the sealing land, the flat ring at the top of the neck. That is why a liner proven on one bottle can leak on another of equal capacity from a different mould, and why a liner that holds water can fail on an essential oil blend with the same cap.
Five inputs describe the interface, and we ask for all of them before shortlisting anything:
- The sealing land. Its width, flatness and the radius where it turns into the bore. Narrow lands concentrate the load and suit a liner that flows into them; wide lands spread it and suit a liner that keeps its thickness when squeezed.
- The closure shell. A metal lug or continuous thread shell loads the liner through a defined roof. A plastic shell with a deep skirt and inward bead loads it differently, and some shells add a plug or spigot that seals inside the bore.
- The contents. "Oil", "alcohol" or "cream" is too short a description. What counts is the full formulation, the concentration of its aggressive fraction, water activity, pH, any surfactants or terpenes, fill temperature, and whether the pack is stored upright, on its side or inverted.
- The service conditions. Claimed shelf life, transport and warehouse temperatures, humidity, and how often the pack is opened and reclosed.
- The shell as a material. Shell and liner touch each other. A plasticiser, colourant or mould release can move from shell to liner, and a solvent in the liner can soften the shell.
Where the shell has a plug, the seal has two stages and both need evaluating. A plug that closes the bore can hide a marginal land seal, and the leak only shows once the plug relaxes.
Read together, these inputs produce three things: a material family, a thickness and compressibility band, and a list of evidence documents.
Six liner families compared
Most glass packs for food, drinks, pharmaceutical and cosmetic products use one of six families. Use the table from the fill outward: find the chemistry, cross to the family that withstands it, then note the documents to request and the testing a switch would bring. No row stands alone as a recommendation, because thickness, closure type and the filling line all shift the answer.
| Liner | How it seals | Chemical behaviour | Typical fills | Documents to ask for | Checks after a change |
|---|---|---|---|---|---|
| PE foam | Closed cells compress elastically; their recovery keeps the land sealed as the pack cools | Holds against water, dilute acids and aqueous solutions; weak against terpenes, aromatic solvents and long alcohol contact | Water, beverages, aqueous cosmetic bases, low-solvent household liquids | Food contact statement, migration report, safety data sheet, no-prohibited-substances declaration | Leak and vacuum decay, new torque window, weight loss in accelerated storage |
| EVA | Soft, compliant compression at a lower closing load; follows a wavy land | Moderate with oils and low-alcohol fills; limited with strong solvents; more compression set than foam | Cosmetic emulsions, low-alcohol lotions, personal care gels | Food or cosmetic contact statement, migration data, odour and taint assessment | Compression set, seal force after warm storage, repeated reclosure |
| Pulpboard | Mechanical compression only, with no meaningful elastic recovery | Lets moisture and oxygen through and wicks liquid; not for wet or oily contact | Dry powders, tablets, dry foods, dry botanicals | Dry food contact statement, virgin fibre declaration, no recycled content where required | None unless the fill becomes wet, in which case the family is replaced, not re-tested |
| Foil laminate | A compressible backing takes the load; the aluminium layer stops moisture, oxygen and aroma once sealed | Barrier is the main property; resistance at the cut edge is only that of the backing | Moisture- or oxygen-sensitive fills, aroma-critical beverages, induction-sealed packs | Food contact statement, migration report, barrier data for that foil construction, induction compatibility note | Barrier verification, headspace oxygen or moisture, induction seal peel and integrity |
| Silicone | Elastic deformation over a wide temperature band, very high recovery, low compression set | Broadly resistant to oils, alcohols, acids and water; some non-polar solvents still affect it | Essential oils, alcohol-based fills, aggressive cosmetic actives, packs reclosed many times | Food contact statement, migration data, extractables and leachables summary, volatile and odour assessment | Leak and vacuum decay, weight loss study, aroma transfer on the finished product |
| Nitrile (NBR) | High elastic recovery that keeps its closing force in warm conditions | Strong with oils, fats and hydrocarbons; poor with ketones, esters and strong solvents | Cosmetic oils, lipid-based serums, fatty technical fills | Food or cosmetic contact statement, migration data, odour and colour transfer assessment | Swelling and hardness, weight loss study, colour and odour transfer on the filled product |
A few points sit behind the rows. PE foam forgives modest land waviness, but a foam that is flawless on water may creep within weeks in a citrus oil. EVA lets a thinner liner seal an imperfect land, yet stacked packs kept under load through a warm season can lose sealing force for good, so it is a poor fit for long reclosure duty.
Pulpboard is dimensionally stable and the traditional answer for dry, low-risk goods. Put on a wet product, it gives a pack that looks closed, holds torque and still fails a leak test after a humidity cycle. Foil laminate adds no sealing force of its own; it governs what can cross a seal that already exists, which makes it the family for oxidation, aroma loss or moisture pickup, and for closures that weld a membrane to the land by induction.
Silicone keeps its properties longer than the organic families, does not support microbial growth the way a foam can, and releases cleanly from tacky products. It is, however, relatively permeable to water vapour and to some aroma compounds. Nitrile is picked for fatty and oily fills, including some agrochemical packs, but its odour and colour need review before cosmetic use: rubber odour in a cream is noticed by the customer before the laboratory.
Pairings that work and pairings that fail
Certain combinations come up often enough that their failure pattern can be named before a sample is ordered.
| Pairing | What happens | What to do instead |
|---|---|---|
| Virgin pulpboard on a liquid cosmetic | The board wicks product into the cap, softens and loses load; the pack leaks on the second or third opening. A line leak test on production day shows nothing because wicking takes time. | Foam, EVA or silicone, with a barrier layer if the product also oxidises |
| PE foam on a citrus terpene fill | Terpenes swell the foam and the recovery force goes; product seeps at the thread after a few weeks, with no visible defect at capping. | Silicone, or a foil laminate on a chemically resistant backing; expect a new torque window because the liner is stiffer |
| Foil laminate under a hand-applied lug cap on a wavy land | Seals in the middle of the tolerance band and leaks at both ends, since the laminate creates no sealing force. | Change the closing method, not the liner; this is a fit and torque matter |
| Silicone on a moisture-sensitive powder | The seal is liquid-tight, but moisture passes through the liner itself. The complaint is caking or poor flow, not leakage. | A construction that provides barrier as well as seal |
The combinations that hold are just as predictable: silicone on an essential oil blend, nitrile on a cosmetic oil, foam on an aqueous lotion, foil laminate on an aromatic beverage, pulpboard on a dry botanical. In each, mechanism, chemistry and reclosure duty line up. How much margin they have still depends on thickness and on the land, which the material name alone cannot tell you.
Chemical resistance depends on concentration, temperature and time
Resistance is not a fixed property of a material. It belongs to a material at a given concentration and temperature for a given duration. Four kinds of aggressive fraction account for most problems in glass-packed products.
Oils and fats
These swell foam and some EVA grades and pull out low molecular weight fractions. The liner thickens, softens and stops pushing back, while substances from it migrate into the product. Nitrile and silicone are the usual candidates. The deciding test is weight and hardness change after immersion in the real fill, or an agreed simulant, at storage temperature for a defined period, with a migration test added for regulated food or cosmetic categories.
Acids
Acids work by hydrolysis and by extracting fillers and plasticisers, and pH and temperature strongly govern how fast. Most families cope with dilute aqueous acid at ambient temperature; none of them can be assumed safe with concentrated or hot acid. Failure tends to start at the crevice between liner and land, where the acid concentrates. An acidic fill that is also carbonated or hot filled puts a pressure differential on the liner, which is a fit question checked by pressure decay, not immersion.
Alcohol
Concentration catches buyers out. A product sold as alcohol free may still hold several percent ethanol, and a toner may hold more than fifty percent. Ethanol draws polar and semi-polar components out of foams and some EVA grades, causing odour and taste defects. Silicone and barrier constructions are normally examined for high-alcohol fills. Ask for migration data at the achievable concentration and run an organoleptic check on the finished product, since a liner can pass migration and still add a note to a fragrance.
Solvents
Ketones, esters, terpenes and aromatic hydrocarbons leave the fewest options. Hardly any elastomeric liner survives long contact with a strong solvent, so the practical route is often a different closure system, such as a barrier membrane that keeps the solvent off the compressible element. Within the liner families, silicone is the usual first candidate and nitrile the usual exclusion, confirmed against the specific solvent and not the class name.
One condition covers all four. Resistance must be shown at the highest concentration and temperature the pack will meet, across the whole shelf life, on a liner of production thickness. A shortened test at ambient temperature on a laboratory thickness is a screening result and should be labelled that way in the technical file.
Thickness, compression and neck finish tolerance
Thickness is where the liner meets the glass, and it explains why two bottles sharing a finish code behave differently. The liner must be compressed enough to close the leak path and absorb land waviness, yet not so far that it yields, takes a permanent set or splits at the land edge.
Picture the stack at full seat. The shell roof is at a fixed height. The land height moves within the finish tolerance from mould to mould and during a run. The liner has its own thickness tolerance and thins further as it is capped. The case that decides whether a pack leaks is the thinnest liner on the lowest land under the lightest load the capper delivers. Average combinations never leak, so they prove nothing.
Liner thickness and land flatness are therefore a matched pair. Looser finish tolerances need either a thicker compressible liner or a tighter capping window; a very flat land at a closely held height allows a thinner liner and less closing load. A common mistake is to tighten the glass drawing to save weight and leave the liner untouched, which can move the pack from a comfortable compression band to a marginal one. Any change on one side of the interface should be reviewed against the other, starting from the neck finish drawing, which fixes the land, bore and thread family.
Land width has the same effect through contact pressure. At equal torque a narrow land presses harder and favours a stiffer liner; a wide land presses more gently and favours a softer one. If the neck is out of square, or the land is not perpendicular to the axis, the seal holds on one side and seeps on the other. No liner corrects that, so measure the finish before blaming the gasket.
Food-contact and cosmetic documents to request
The evidence set follows the market and the product category, and it comes partly from the liner supplier and partly from the closure supplier. Asking for it is normal. A supplier unable to produce a coherent set is a risk in its own right. How the liner's papers fit into the wider file is explained in our guide to the food contact declaration pack for a regulated fill.
- United States, food contact. A statement of compliance with the applicable provisions of FDA 21 CFR for the specific material, migration or extraction data where the regulation calls for it, and the conditions of use under which the statement is valid.
- European Union, food contact. EU 10/2011 for plastic materials and EU 1935/2004 as the framework regulation, with a written declaration of compliance naming substance restrictions, migration limits, conditions of use and supporting test data. Some specifications also cite national schemes such as the German LFGB recommendations; the supplier should explain which scheme covers the grade actually offered, not send a generic certificate for another grade.
- European Union, cosmetics. These fall under the cosmetic framework, not the food one. The liner must not introduce substances that undermine the safety assessment of the finished product. Buyers typically ask for a material composition statement, a migration or extractables evaluation, and a note on odour and colour transfer. For products with an IFRA-related fragrance declaration, consider that a liner absorbing fragrance notes can change what the consumer smells on first opening.
Four more documents are worth having in almost every case: a material safety data sheet; a declaration that prohibited substances such as certain phthalates or heavy metal colourants are absent; an organoleptic or taint assessment for flavoured or fragrant fills; and the maximum contact temperature and duration the declaration covers. That last one is missed most often, and a hot fill or pasteurisation step can push the pack outside the declared conditions.
This list describes what a liner dossier normally contains. It does not claim that any particular plant, grade or shipment holds these approvals. The documents must be issued for the material actually supplied, by the party legally responsible for it.
How much re-testing a liner change needs
A liner is a critical component, and the work a change triggers should match how far the sealing mechanism has moved. Treating every change as trivial causes field failures; treating every change as a development project stalls supply.
| Level | What changed | Work required |
|---|---|---|
| Same material | Only the supplier or lot; family, thickness and construction are identical | Incoming inspection against the existing drawing (thickness, diameter, compressibility or hardness), a short leak and torque check on filled packs, and a documented record |
| Same family | Thickness, density or barrier construction | New torque window, leak and vacuum decay across the tolerance band instead of at nominal, accelerated storage covering the claimed shelf life |
| New family | For example foam to silicone, or pulpboard to laminate | Fresh compatibility assessment on the full formulation, migration or extractables evaluation, new torque window, fit test at glass and closure extremes, accelerated and where warranted real-time storage, transit evaluation if the closure system changed, reissued declarations |
The middle level cannot be skipped. A thicker liner in an unchanged material raises the closing load, and too much load can crack a neck or make the pack impossible to open even when the seal is excellent. A new family is development work because mechanism, chemistry and barrier all move at once, and the material named in the compliance statement is no longer the one inside the cap.
At every level, validate with the production liner, glass, closure and capping head, at fill temperature, using the real product or an agreed simulant. Development-lot samples on a bench torque gauge answer a different question from the one the filling line asks. The procedure and acceptance criteria are those of the cap fit test that crosses glass against closure tolerances, and because a change in thickness or stiffness shifts the capper's numbers, the application and removal torque windows and their sampling plan need revisiting too.
Keeping the specification stable through reorders and shortages
Three habits decide whether a good liner choice survives a production schedule.
Specify with a drawing. An order for a "foam liner" leaves thickness, density, diameter, backing and barrier layer open, and any of them can drift between lots unnoticed. A toleranced drawing with a material designation and a named declaration keeps the seal inside its qualified band and gives incoming inspection something to measure.
Treat substitution as a controlled change. When stock runs out, the temptation is to fit whatever the closure supplier has on the shelf. Test the substitute against four questions:
- Does it seal by the same mechanism?
- Does it withstand the same chemistry for the same contact time?
- Does it give comparable sealing force at the same closing load?
- Does it carry the same declaration for the same market?
Four passes make it a same-family change, which still needs a confirmation run. One failure makes it a new family and calls for the full programme.
Record the qualified combination at the line. When the closure supplier, glass supplier or formulation changes, the liner is only presumed to still work. A record of the combination and its supporting evidence shows quickly whether a new change touches the seal, the load or the chemistry, and it puts the answer to a customer audit in one place.
On products that run for years, liner makers reformulate from time to time for regulatory or raw material reasons. With a qualified combination on file, a buyer can ask for advance notice of such changes and re-check only the affected level, instead of learning about it through a complaint.
Frequently asked questions
Where do I start when picking a liner for a new product?
With the formulation, not the bottle. Describe the aggressive fraction, water content, alcohol concentration, pH and fill temperature, then the shelf life, storage conditions, reclosure duty and destination market. That usually narrows the field to one or two families. Next, set thickness and compressibility against the finish tolerance and the capping equipment, and confirm the required declarations exist for the exact grade. A family picked before the chemistry is characterised tends to be changed later.
Can silicone replace a foam liner without new testing?
No. It is a change of family: the mechanism, the compression required, the force produced at a given torque and the barrier behaviour all differ, and so does the material named in the declaration. Swapping on the line without the full programme is among the most common origins of leak complaints that surface months after shipment.
Is a thicker liner always a better seal?
No. Extra thickness absorbs more land waviness and finish variation, which is why it is common on wider-tolerance glass. It also needs more load for equal compression and can push torque past the point where the neck cracks or the cap is hard to remove. Over-compression leaves a permanent set in some materials and weakens the seal in storage. The right thickness keeps compression within the working range across every tolerance of glass, liner lot and capping head.
Which liner suits essential oils?
Silicone and chemically resistant barrier constructions are the usual candidates, because terpenes and other aromatic fractions swell and soften foam and many EVA grades. Confirm against the actual blend and concentration: a citrus oil and a resinous oil do not attack materials equally. Oxidation- or aroma-sensitive fills may need a barrier layer on top of the resistant sealing element, and frequently reopened packs make recovery over many cycles part of the requirement.
What should come with a liner sample?
The compliance and safety documents listed above, plus the figures that turn a sample into a specification: thickness and diameter tolerances, the compressibility or hardness range, and the maximum contact temperature and duration the declaration covers.
Why would a pack pass leak testing and seep two months later?
Late failures are driven by time or temperature and cannot be seen on filling day. Swelling from an aggressive fill, compression set in a warm warehouse, wicking through pulpboard and slow relaxation of the closing load all build over weeks. Add an accelerated ageing or storage stage to the test plan, with liner, glass and closure at their tolerance extremes and a representative fill and fill temperature.
Will a different liner change my capper's torque setting?
Usually. Torque has to compress the liner enough to close the leak path without exceeding what the glass and closure can take. A thicker, softer or stiffer liner alters the torque needed to get there and how much of it remains as removal torque after storage. Anything beyond a like-for-like supplier swap should be followed by a new torque window, written into the capping specification and the incoming inspection plan instead of left as a note on a sample report.