Release Liner Base Paper: How to Specify It Before You Silicone It

A release liner base paper is uncoated paper made specifically to receive a silicone coating. You specify it on four axes: grammage, surface closure (air permeability and smoothness), mechanical strength for the coating line, and reel-to-reel consistency. Everything else on the datasheet exists to support one of those four.

Writing the actual specification takes longer than reading that answer, and most of the trouble starts because a release liner base is bought like a paper and used like a process input. The reel arrives with a grammage and a brightness on the label. What it decides, once it is on your coater, is how much silicone you burn, how many web breaks you take per shift, and whether the release force you qualified in March still holds in September.

What follows defines the category, names the incumbent grades honestly, and sets out what belongs on a spec sheet. It does not tell you which base to buy. That is a different question and it needs different evidence.

What is a release liner base paper?

A release liner base paper is an uncoated paper engineered to carry a release coating, almost always silicone, so that a pressure-sensitive adhesive separates from it cleanly and repeatably. It is sold as reel stock by grammage to coaters and converters. Its job is to make the coating work rather than to perform on its own.

The distinction matters because the base and the finished liner get discussed as though they were one product. They are not. A finished liner is a system of base plus silicone plus cure. The base contributes the surface the silicone sits on, the porosity that decides how much of that silicone vanishes into the sheet, and the mechanical envelope the coating line has to work inside. Change the base and you have changed the coating recipe, whether or not anyone meant to.

Commercially available coating bases cover a wide grammage span. Ahlstrom’s Silco range lists machine-glazed (MG) coating bases from 14 to 98 gsm and machine-finished (MF) coating bases from 41 to 163 gsm. Inside that span, standard glassine release base for labelstock has historically sat in a 50 to 80 g/m² band, roughly 35 to 75 µm thick, with silicone applied at about 1 to 2 g/m² full-surface coat weight. Those three numbers frame nearly every conversation about release liner base paper.

Definition, release liner base: the uncoated paper substrate a release coating is applied to. It is specified for what it does to the coating process, not for how it performs as a finished packaging material.

The definition stops holding at the edges. Filmic liners (PET, BOPP) are a separate category with different economics, and a paper base is not automatically right for high-speed die-cutting of very thin labels. If your application is failing on liner dimensional stability under tension rather than on release, a better paper may not be the answer at all.

What does the base actually have to do before the silicone goes on?

Four things, in this order of practical consequence: hold the silicone on the surface, run through the coater and cure oven without breaking or blocking, present the same surface on reel 40 as it did on reel 1, and stay chemically inert to the cure system. A base can pass every number on a datasheet and still fail on the fourth.

Silicone holdout is the commercial one. An open sheet absorbs silicone into the fibre network, so you raise coat weight to reach the same release, then pay for that silicone on every square metre for the life of the contract. A closed sheet keeps the silicone where it is useful. Supercalendered grades cost more per tonne and quite often less per square metre of finished liner, which is why the arithmetic that decides a base paper is always per square metre.

Runnability is the one that ends shifts early. Breaks at the unwind, in the coater nip, or in the oven draw come out of machine-direction tensile, tear resistance, moisture profile across the width, splice quality and reel geometry. Lighter paper has less of everything mechanical. That is the honest cost of lightweighting.

Cure chemistry is the one nobody specifies until it bites. Platinum-catalysed addition-cure silicone systems are poisoned by amines, certain sulphur compounds and some tin residues. A base carrying the wrong extractables can give you partial cure, silicone rub-off and adhesive contamination, and it will look like a silicone problem for several days before anyone questions the paper. If you are qualifying a new base, an extractables statement belongs in the first document exchange rather than the tenth.

Release itself is measured on the finished liner, never on the base. FINAT FTM 3 covers low-speed release force at 180° and 300 mm/min, and FINAT FTM 4 covers high-speed release force. An uncoated base cannot be given a release force value because it has no release coating yet. A supplier quoting one for an uncoated base is quoting something that does not exist.

Which properties belong on a release liner base paper specification?

Twelve properties carry almost all the decision weight, and each should appear with a target, a tolerance and a named test method. A specification that lists a value without the method it was measured by is not a specification. It is a preference, and two labs will disagree about it.

Two entries in that table are routinely missing from the Indian purchase specifications converters share with us during qualification: extractables and formation. Neither shows up on a standard mill test certificate, and both explain failures that otherwise get blamed on the coating. The testing guide for burst, tear and porosity goes deeper on the mechanical methods, and the paper converter glossary carries plain definitions of the terms used here.

Every value in a spec should also name its conditioning state. Paper properties are measured after conditioning to ISO 187, at 23 °C and 50% relative humidity. A tensile figure taken on unconditioned paper in a Gujarat July is not comparable to one from a European mill lab in February.

How do you specify air permeability without over-specifying it?

Set a floor rather than a point value, and set it against the silicone system you actually run. Air permeability is a proxy for porosity, and porosity is what decides silicone consumption. The tightest sheet available is still not automatically the cheapest liner once base price enters the arithmetic.

The mechanism is simple enough. Silicone applied to an open sheet migrates into the void structure before it cures. That silicone is spent and does nothing for release, so coat weight climbs to compensate. Close the surface and less of it is lost. Surface closure comes from refining, furnish, size-press chemistry and calendering, with supercalendering being the route that gives glassine its characteristic density.

For the working range: release-base patent literature describes uncoated and unsized bases at around 300 seconds Gurley or higher, extending past 1000 seconds for tighter grades, with holdout improved by low porosity and low density together rather than by either alone. Those are category figures from the technical literature, not values to copy into a purchase order. They tell you the order of magnitude the conversation happens at.

The honest limitation belongs here rather than in a footnote. Porosity control at low grammage is the hardest thing to do in a coating base, and it is not a solved problem, at Pakka or generically in the industry. Lighter sheets have less fibre to close, and closing them by calendering alone costs strength, which comes back as web breaks. Anyone claiming their lightweight base has porosity and runnability both fully solved is selling rather than specifying.

There is a second route worth knowing. Reduced-penetration silicones (RPS) exist precisely because base porosity is hard to control, and they are formulated to sit up on a more open sheet. If your base is naturally more open, an RPS system may cost less overall than paying for a supercalendered base you do not otherwise need. Trading chemistry cost against substrate cost is a legitimate engineering decision, not a compromise.

Glassine, SCK and CCK: what the incumbents actually are

Three paper grades dominate release liner base globally, and knowing what each one is prevents most specification confusion. Glassine is a dense, translucent supercalendered sheet. SCK is supercalendered kraft. CCK is clay-coated kraft, where a mineral coating rather than calendering alone provides the closed surface.

The naming is a genuine trap. SCK is largely the North American convention and glassine the European one, and a European glassine is typically a supercalendered kraft base carrying a topcoat. Two suppliers can quote what sounds like two different grades and mean roughly the same paper, or use one word for two different constructions. Ask for the construction, not the trade name.

• Glassine has the highest surface closure and the best silicone holdout of the three. It is the most expensive per tonne, translucent, and the reference grade for label liners.
• SCK gets its closure from calendering rather than coating. It is the commodity workhorse a new base gets priced against.
• CCK gets its closure from a mineral layer. It is generally the cheapest route to a usable surface, with recyclability and coating-adhesion considerations of its own.

Ahlstrom, delfort and the other European mills publish product data on all three, which is why a technical search on these terms returns European datasheets and Indian trading listings and almost nothing in between. No Indian producer publishes converter-grade technical education on this category. That gap is why this post exists, and it seems better to say so than to pretend the incumbents are unknown.

Which of the three suits a given job is a scoring question covering release consistency, silicone consumption, cost, recyclability and runnability. It deserves a comparison built for the purpose rather than a paragraph here.

How do you write a tolerance rather than a target?

Write every property as a target plus a two-sided tolerance plus a sampling rule, because a base paper that meets its target on average and drifts within the reel will still fail on your line. The tolerance is the specification. The target is only the middle of it.

What matters is variation, not level. A coater sets tension, coat weight and cure to a surface. If that surface moves between reels, the settings that were right on Monday are wrong on Thursday, and the operator compensates by adding silicone, which is the most expensive way a specification failure can express itself. The complaint you hear from converters is almost never “the paper is bad” and almost always “the paper is not the same as last time”.

Sampling has a standard. ISO 186 sets out how to sample paper and board to determine average quality, and a specification that names it is meaningfully harder to argue about than one that does not. Pair it with a stated frequency, per reel or per lot or per shipment, and with which properties are certificate-reported against which ones you will test on receipt.

A workable sequence for writing one from scratch:

1. Write down the coating system first: silicone chemistry, target coat weight, line speed, cure temperature. The base spec derives from these, not the other way round.
2. Set grammage and its tolerance from your yield and tension requirements.
3. Set an air permeability floor consistent with your silicone system, and say which method and instrument it is measured on.
4. Set smoothness on the coated side, and say which side that is. Two-sidedness is real on MG grades.
5. Set MD and CD tensile and tear from your worst-case tension and splice conditions rather than your average ones.
6. State moisture content and the conditioning standard.
7. Require an extractables and cure-compatibility statement, and run a cure trial before volume qualification.
8. Name the sampling standard, the reporting frequency, and the properties you will verify on receipt.
9. Specify reel geometry, core, splice count and splice marking.
10. Agree in writing what counts as a rejectable deviation, before the first commercial reel ships.

Steps 7 and 10 get skipped most often and cost the most when they are. The base paper procurement checklist covers the commercial half of this process for Indian buyers.

What changes when the base goes lighter than 60 GSM?

Yield rises, mechanical margin falls, and every tension setting on the line has to be established again. Lightweighting a release liner is real and commercially proven, but it is a re-qualification rather than a drop-in substitution. The paper behaves differently at the unwind, in the nip and at the rewind.

The commercial case is strong and public. delfort’s Tersil UltraLight release base runs at 35 gsm, which the company describes as 40% lighter than standard alternatives, yielding roughly 28,500 m² of labelling surface per tonne and cutting coater changeovers by around 38% because more metres fit on a reel of the same diameter. Against a 50 to 80 g/m² incumbent band, that is a step change in metres per rupee of freight and per hour of line time.

The engineering cost is just as real. Less fibre means lower tensile and tear, a narrower tension window, more sensitivity to nicks and splices, and less thermal mass through the cure oven. Moisture profile matters more because there is less sheet to buffer it. If your line runs older tension control, or your splices are inconsistent, a lighter base will find that out inside one shift.

The reasonable approach is to treat a grammage reduction as a project with a trial protocol, a waste allowance and a defined go or no-go, the same discipline you would apply to a new silicone chemistry. That turns a gamble into a measurement.

Where a paper coating base fits

Pakka manufactures coating base paper, so read this section knowing that. The limitations come first, because they are what a converter needs before anything else. A paper base is not a drop-in where dimensional stability under high-speed die-cutting is the binding constraint. Porosity control at low grammage is the hardest property in this category and it is not solved. Any base paper new to your line is a qualification project with trial waste attached, whichever mill it came from.

With that said, flexC base is Pakka’s coating base paper development programme: a 40-60 GSM base, bleached and unbleached, intended as a substrate for barrier chemistries generally rather than for one coating type. It is disclosed as a development programme in Pakka Limited, Investor Presentation Q1 FY2026-27, filed 18 August 2026 (BSE 516030 / NSE PAKKA), along with the target applications, of which release liners is one.

flexC base is not flexC. flexC is Pakka’s finished compostable flexible-packaging laminate, sold to brand owners with published specifications. flexC base is the uncoated coating base paper this post describes, sold to coaters and converters. Different products, different buyers, and no specification transfers from one to the other.

Because flexC base is in development, this post carries no performance figures for it. No porosity, no holdout, no coat weight, no release data. Numbers that have not been generated and verified do not belong on a listed company’s website, and a converter who has been handed an optimistic datasheet before will recognise why that line is drawn. What is available now is a technical conversation about your coating system and your specification, which is the useful part at this stage anyway.

Frequently asked questions

What is release liner base paper made of?

Chemical pulp, refined and formed into a dense, closed sheet. Glassine and SCK grades reach that closure through supercalendering; CCK grades use a clay coating. Furnish, refining level and size-press chemistry determine porosity and strength, which is why two 60 g/m² bases from different mills can behave completely differently on the same coater.

How much silicone does a release liner need?

Full-surface silicone coat weight for release liners typically runs about 1 to 2 g/m², with the exact figure set by the base’s holdout, the silicone chemistry and the release level required. A more open base pushes that number up and a supercalendered base pulls it down. Silicone is the dominant variable cost in a liner, so the base decision is largely a silicone-consumption decision.

What is the difference between glassine and SCK paper?

Mostly regional naming plus construction. SCK, supercalendered kraft, is the common North American term. Glassine is the European term and usually means a supercalendered base carrying a topcoat that closes the surface further. Ask any supplier for the construction and the measured surface properties instead of relying on the grade name.

Can you test silicone holdout on an uncoated base paper?

Not directly. Holdout is a behaviour of the base-plus-silicone system. Before coating it is inferred from air permeability, density and smoothness, then confirmed by a coating trial at your own coat weight and line speed. A holdout figure quoted for an uncoated base is a prediction rather than a measurement.

What should be on a release liner base paper spec sheet?

Grammage, thickness, air permeability, smoothness on the coated side, density, moisture, MD and CD tensile, tear, Cobb, and an extractables and cure-compatibility statement, each with a two-sided tolerance, a named test method and the conditioning standard. Add the sampling standard, the reporting frequency and reel geometry, then agree in writing what counts as a rejectable deviation.

Last reviewed: September 2026

If you are writing or revising a release liner base specification and want a second technical opinion on it, or you want to discuss a coating base against your own silicone system and line conditions, talk to Pakka’s technical team about a flexC base trial enquiry. Say “flexC base trial enquiry” in your message and it reaches the right people directly. Related substrate guides for converters, including sachet paper substrate requirements, food-contact compliance for paper packaging in India and barrier and MVTR benchmarks for paper pouches, sit alongside the printed technical material on the Pakka resources page.

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