Coating Base Paper Glossary for Converters: 24 Terms, Defined by the Test You Would Actually Run

A coating base paper glossary is only useful if every word in it points at a test. This one defines the 24 terms a coater or laminator uses to specify a base before silicone or barrier goes on, and names the TAPPI, ISO, ASTM or FINAT method that makes each term a number a supplier can be held to.

Most glossaries in this category are written by people selling reels. They define “silicone holdout” or “formation” in a sentence that sounds right and commits to nothing. The test is the commitment. A term without a method is an adjective, and adjectives are how a base gets approved on a sample and then fails on the fortieth reel.

The terms fall into four groups: how open the sheet is, how it behaves under silicone, whether it will run, and what the coating (not the paper) has to add. A quick-reference table with every method follows the definitions, and a short section after it lists the properties that have no agreed test at all, because that list matters as much as the rest.

What is coating base paper, and what does a glossary for it have to cover?

Coating base paper is uncoated paper made to receive a functional coating downstream: silicone, a barrier polymer, a biopolymer or an extrusion layer. A glossary for it has to cover the properties that decide how much coating the sheet absorbs, how evenly it takes it, and whether the reel runs at speed without breaking.

That is a different job from a packaging paper glossary. A carry-bag converter reads burst and tear because the bag has to carry a load. A coater reads air permeability and holdout because the base decides the coating bill. The same words appear in both, sometimes with different weight, and the paper converter glossary for bag makers covers the bag side of that vocabulary.

Coating base paper

Coating base paper is paper sold by grammage as reel stock to coaters and converters, and judged on what it does to the coating process rather than on how it looks. There is no single test for “coating base”. It is specified through the properties below, usually grammage, air permeability, smoothness, Cobb, tensile and moisture, each against a named method.

Commercial ranges are wide. Ahlstrom lists its Silco machine-glazed coating bases from 14 to 98 gsm and its machine-finished grades from 41 to 163 gsm. A grade name tells you very little. The numbers under it tell you almost everything.

Release liner base

Release liner base is the coating base paper that becomes a release liner once it is siliconised: the carrier behind a label, a tape or a hygiene adhesive. It is the largest single use of coating base paper, and the one where base properties show up most directly in cost, because silicone is the most expensive input on the line. The release liner base paper specification guide walks through how to write one.

GSM (grammage)

GSM, or grammage, is the mass of paper per square metre, measured to TAPPI T 410 or ISO 536. For a coater it is also the area you get per tonne you pay for, which is why it sits at the top of every certificate.

A worked example makes the point. One tonne of 70 g/m² base covers about 14,300 square metres. The same tonne at 50 g/m² covers 20,000 square metres, roughly 40% more area before anyone has discussed price. General-purpose glassine release base is usually sold somewhere in a 50-80 g/m² band, and delfort now sells a 35 gsm release base. The catch is that every other property on the certificate moves when grammage drops, and most of them move the wrong way.

Which terms describe how open or closed the sheet is?

Porosity, air permeability, supercalendering, formation, refining and the size press all describe one thing from different angles: how open the fibre network is at the surface a coating lands on. An open sheet drinks coating. A closed one holds it at the surface, where it does the work you paid for.

Porosity

Porosity is the proportion and structure of void space inside a paper sheet. Nobody measures it directly on a mill floor. It is inferred from air permeability, which is why the two words get used interchangeably on certificates and in sales calls, and why a supplier who says “low porosity” without a Gurley or Bendtsen figure has not said anything yet. The porosity and air permeability deep-dive covers why it drives silicone consumption.

Air permeability

Air permeability is how readily air passes through a sheet, and it is the working proxy for porosity. Two methods dominate. Gurley, to TAPPI T 460 or ISO 5636-5, reports the time in seconds for 100 mL of air to pass through 6.45 cm² of paper, so a higher number means a tighter sheet. Bendtsen, to ISO 5636-3, reports a flow in mL/min, so a higher number means a more open sheet.

That inversion causes real mistakes. A buyer who switches suppliers from a European mill to an American one can read “lower is better” onto a spec where the opposite is true. Put the method name in the specification line, every time, next to the number.

Air permeability is also the property most likely to vary across the width of a reel, not just between reels. Edges dry differently from the centre, and calender nips load unevenly, so a single test point from the middle of the sheet can hide an open edge that soaks up coating on every metre of the run. Ask for air permeability measured at several positions across the deckle, reported as a profile rather than an average. A base with a good mean value and a wide spread will cost more silicone than a base with a slightly worse mean and a flat profile, because the coater has to set coat weight for the most open strip on the web, not for the average one. That single change to a certificate request tells you more about a supplier’s process control than any brochure claim about consistency, and it costs the supplier nothing to report if the tests are already being run.

Supercalendering

Supercalendering is passing paper through a stack of alternating hard and soft rolls under high pressure to compact and close the surface. It is how glassine gets its density and low air permeability. It is a process, not a property, so you do not specify “supercalendered”. You specify the air permeability and smoothness it is supposed to deliver, for example Bendtsen roughness to ISO 8791-2.

Formation

Formation is the evenness of fibre distribution across the sheet, the thing you see when you hold paper up to a light. Poor formation means thin spots, and a thin spot is where coating soaks in, where a web tears, and where a release value wanders. It matters more as grammage falls, because a light sheet has fewer fibre layers to average out the unevenness.

Refining

Refining is the mechanical treatment of pulp fibres before the paper machine, which fibrillates and flattens them so they bond more tightly. More refining generally means a denser, less permeable sheet with higher tensile strength, at the cost of energy and drainage speed on the machine. Mills track it through freeness, TAPPI T 227 (Canadian Standard Freeness) or ISO 5267-1 (Schopper-Riegler). A coater rarely specifies freeness, but it helps to know that most of the base’s closure is decided here, before any calender.

Size press

Size press is the station on the paper machine where a starch or other sizing solution is applied to the surface of the formed sheet. It seals the surface, controls water pick-up and shapes how a coating wets out. Its effect shows up in Cobb (below) and in contact angle to TAPPI T 458. If a coating behaves differently between two reels with the same grammage and air permeability, the size press is one of the first places to look.

Which release liner terms does a silicone coater specify against?

Silicone holdout, coat weight, release force and anchorage are the four terms a release-liner coater actually buys against, with RPS as the chemistry that can compensate for a more open base. Glassine, SCK and CCK are the three incumbent base grades any new base gets priced and benchmarked against.

Silicone holdout

Silicone holdout is the degree to which a base keeps applied silicone on its surface instead of letting it soak into the fibre network. High holdout means less silicone to reach a target release. There is no single holdout test. It is judged indirectly: the coat weight needed to reach a stable release on that base, plus rub-off and extractables after cure. That is why a base cannot be qualified on a lab sheet alone. The holdout number is really your coat weight on your line.

Coat weight

Coat weight is the mass of coating applied per unit area, reported in g/m². On a well-holding release base the silicone layer is very thin, roughly a micron, so small drifts in coat weight are a large share of silicone cost. Silicone coat weight is normally measured by X-ray fluorescence against calibration standards, on the coated web rather than on the base.

Release force

Release force is the force needed to separate a liner from the adhesive it carries, measured at a stated angle and speed. It is the performance figure a release liner is bought on. FINAT test method FTM 3 covers low-speed release at 180° and 300 mm/min, and FTM 4 covers high-speed release. Quote the method, the peel speed and the adhesive together, because a release figure without all three cannot be compared with anything.

Anchorage

Anchorage is how well the cured silicone bonds to the base paper. Poor anchorage shows up as silicone rub-off or transfer onto the adhesive, which contaminates the laminate and makes release drift. It is usually traced to cure conditions, surface contamination or a base whose surface chemistry works against the silicone system. The common check is still a finger rub-off test after cure, which says something about how young the formal methods here still are.

Reduced-penetration silicone (RPS)

Reduced-penetration silicone is a silicone system formulated to resist soaking into the fibre network, so a slightly more open base can still run at acceptable coat weight. It is the honest compensation route when a base is not as closed as glassine. It costs something, and it moves the question from “is the base closed enough” to “is the base plus RPS cheaper than glassine at equal release”.

Glassine

Glassine is a dense, translucent, supercalendered paper and the quality reference for release liner base. Its low air permeability comes from refining and supercalendering together. For uncoated glassine and greaseproof grades, TAPPI’s turpentine test for voids, T 454 is the named check on how closed the sheet is.

Supercalendered kraft (SCK)

Supercalendered kraft (SCK) is kraft paper densified by supercalendering and used as a lower-cost release base. It trades some surface closure and uniformity for price, so it typically needs more silicone to reach the same release. Naming is loose across markets: suppliers use “SCK” and “glassine” for grades that overlap, and differ mainly in surface treatment. Ask for the air permeability and the surface treatment rather than trusting the grade name.

Clay-coated kraft (CCK)

Clay-coated kraft (CCK) is kraft with a mineral coating applied to close the surface before siliconising. It reaches holdout through a coating layer rather than through densification, which changes its stiffness, weight and recyclability relative to glassine and SCK. John Theisen’s paper-based release liner review for the Pressure Sensitive Tape Council sets the commercial liner grades side by side, and the glassine, SCK and CCK decision matrix scores them on cost, release and recyclability.

Which terms predict whether a light base will run without breaks?

Tensile strength, burst strength, Cobb value, web tension and dead fold are the runnability and converting terms. They matter more as grammage falls, because a lighter sheet has less strength in reserve for the same line speed, the same drying and the same nip.

Tensile strength

Tensile strength is the force per unit width a paper strip withstands before breaking, measured in machine and cross directions to TAPPI T 494 or ISO 1924-2 and reported in kN/m. On a coating line it is the number the web tension has to stay well below. Always ask for both directions. A sheet with good machine-direction tensile and weak cross-direction tensile will split along the web the first time an edge nicks.

Burst strength

Burst strength is the pressure a sheet withstands before rupturing, to TAPPI T 403 or ISO 2758, often expressed as burst factor (burst divided by grammage). It matters most where the converted pack takes a load. For food-grade paper the FSSAI packaging provisions set a burst factor of not less than 20, a tear factor of not less than 30 in any direction and grammage of at least 35 g/m² (FSSAI Compendium, Packaging Regulations). The burst factor and tear index testing guide explains how the calculation is done.

Cobb value

Cobb value is the mass of water a sheet absorbs from a standing pool in a fixed time, usually sixty seconds, reported in g/m² to TAPPI T 441 or ISO 535. For a coater it is an early read on how an aqueous coating will wet out and how far it will penetrate. A Cobb that wanders between reels usually means the size press is wandering, and the coating will follow it.

Web tension

Web tension is the pulling force held on a moving paper web between unwind and rewind, usually set in N/m of web width. It has no paper standard. It is a line setting, chosen as a safe fraction of the base’s tensile strength and adjusted for moisture, speed and drying. The lighter the base, the narrower the window between tension high enough to track and tension low enough not to break. A lighter base therefore needs a tighter moisture profile across the web, because moisture changes tensile faster than most operators expect.

Dead fold

Dead fold is a sheet’s ability to stay folded without springing back, which matters for twist wraps and folded packs. There is no TAPPI or ISO method for it on paper. The procedure most often borrowed comes from film patent literature, which measures the opening angle of a folded strip after a fixed load and dwell. The confectionery wrap substrate post covers how to write a working procedure.

Which barrier terms describe what the coating, not the base, has to deliver?

MVTR (or WVTR) and OTR describe moisture and oxygen transmission through the finished coated structure. They are not base paper properties. Uncoated paper is not a meaningful moisture or gas barrier on its own, so these numbers belong to the coating and the laminate. The base’s job is to let the coating form a continuous, pinhole-free film.

MVTR / WVTR

MVTR, or WVTR, is the mass of water vapour passing through a square metre of material per day under stated temperature and humidity. It is measured to ASTM F1249, an infrared sensor method, or gravimetrically to ASTM E96. The test conditions are part of the result. A figure at 23 °C and 50% relative humidity and a figure at 38 °C and 90% are not comparable, however similar they look.

OTR

OTR is the volume of oxygen passing through a square metre per day, measured to ASTM D3985 and reported in cc/m²/day. On paper structures it is set almost entirely by the barrier coating. What the base contributes is surface closure: a porous or rough surface needs more coating to reach a continuous film, and a pinhole in that film costs more OTR than any coat weight can buy back. Food-contact rules for that finished structure are covered in the coated paper food-contact compliance guide.

Which test method sits behind each term?

The table below is the quick reference: each term, what it tells a coater, the method to name on a specification, and how the result is reported. Where no method exists, the table says so rather than inventing one.

Two habits make this table useful rather than decorative. Write the method next to every number on a purchase specification. And ask a supplier to report every row from the same reel, because a certificate assembled from different reels tells you about the mill’s best day, not your delivery.

Which of these properties have no agreed test method?

Formation, silicone holdout, anchorage, web tension and dead fold have no single agreed paper test, and they are the properties converters complain about most. That is not a coincidence. They are where suppliers and buyers argue, because there is no number to settle the argument.

The practical answer is to write your own procedure and make it part of the specification. For holdout, that means a fixed coat-weight target on a named silicone system, with release measured to FINAT FTM 3 after a fixed ageing time. For formation, it means agreeing an instrument or a reference sample before the first reel ships. For anchorage, a rub-off check at a fixed time after cure. None of these is elegant. All of them beat an adjective.

A base paper specification that lists grammage, air permeability, Cobb and tensile against named methods covers the measurable half of what a coater needs. The other half, holdout, formation, anchorage and runnability at your line speed, only shows up in a trial on your own coater with your own silicone or barrier chemistry. No certificate replaces that trial, and a supplier who suggests otherwise is selling the sample rather than the reel. Budget the trial into the qualification plan from the start: a few reels, a named chemistry, fixed coat-weight targets, release to a stated FINAT method, and a written pass level agreed before the reels are unwound. That is how the words in this glossary turn into a decision. Keep the trial record with the specification, too. When a second supplier or a second mill comes along a year later, the pass levels you wrote down are what let you compare the new reels against the old ones without repeating the whole argument.

Where do bag converters and coaters use the same word differently?

GSM, porosity, Cobb and burst appear in both a bag converter’s vocabulary and a coater’s, but they carry different weight. For a bag converter, porosity is mostly about print and glue. For a coater it is the coating bill. Reading one glossary into the other job causes specification mistakes.

Take porosity. A carry-bag maker wants enough openness for water-based glue to key into the sheet, and rarely writes an air permeability limit at all. A silicone coater wants the surface as closed as the budget allows, and writes the air permeability limit first. Burst runs the other way. It is central to a bag, which has to hold a load, and secondary on a release liner, which is peeled and discarded.

Colour is another case. Whether a base is bleached or unbleached changes brightness, print and extractives, which matter differently for a label liner and a grocery bag. The bleached vs unbleached coating base comparison covers what changes for the coater, and the sachet paper requirements post shows how the same terms reorder when the application changes.

The rule that keeps both sides honest is simple. Define the term by its test method and state which job the number is for. “Air permeability 400 s Gurley, TAPPI T 460, release base” means one thing. “Low porosity” means whatever the person reading it wants it to mean.

Where a paper coating base fits

Everything in this glossary applies to any base paper a coater might qualify, including a new one. That is worth saying before naming our own, because a glossary is where suppliers most often slip a performance claim in by definition.

Start with the limits. Pakka’s flexC base is a development programme, not a product with a published data sheet. No trial results have been published, so there is no air permeability, Cobb, tensile or release figure a coater could hold it to today, and nothing on this page should be read as one. Its band also sits below the grammage most glassine release base is sold at. The runnability and holdout terms above are therefore exactly the ones it would have to prove on your line, with your chemistry, before it earns a place on a specification.

What the company has disclosed is this. flexC base is a coating base paper in a 40-60 GSM band, bleached and unbleached, intended as a substrate for silicone, biopolymer and plastic barrier chemistries. The eight named target applications are confectionery wraps, snack pouches, sachets, medical packaging, seed packs, release liners, dry food liners and tea pouches (Pakka Limited, Investor Presentation Q1 FY2026-27, filed 18 August 2026, BSE 516030 / NSE PAKKA).

flexC base is not the same product as flexC. flexC is Pakka’s finished compostable flexible-packaging laminate, sold to brand owners with its own published specifications. flexC base is the uncoated paper a coater or laminator would put its own chemistry on, and none of flexC’s figures carry over to it.

For a coater, the useful conversation is not about brochures. It is a trial written against your current base specification, using the terms and methods in the table above, with your own pass levels. That is also the fairest way to judge any new base, ours included. Grease-resistant liners are one application where paper already competes well, and the dry food liner post sets out where it does and does not.

Frequently asked questions

What is the difference between porosity and air permeability in paper?

Porosity is the void structure inside a sheet. Air permeability is how fast air passes through it, measured by Gurley (TAPPI T 460, ISO 5636-5) or Bendtsen (ISO 5636-3). Mills and buyers use air permeability as the measurable stand-in for porosity, because porosity itself is not practical to measure on a production reel.

Should a coating base be specified by Gurley or Bendtsen?

Either works, as long as the method is named next to the number. Gurley reports seconds, so higher means tighter. Bendtsen reports flow, so higher means more open. Use the method your silicone supplier and your incoming lab already run, and never convert one to the other with a rule of thumb.

What does silicone holdout mean on a release liner specification?

It means how much of the applied silicone stays at the surface rather than soaking into the paper. There is no direct holdout test. In practice it is the silicone coat weight needed to reach a stable release on that base, checked with rub-off after cure and release force to FINAT FTM 3.

What is the difference between glassine, SCK and CCK?

All three are release liner bases. Glassine is refined and supercalendered to a dense, translucent sheet. SCK is supercalendered kraft, cheaper and usually more open. CCK closes the surface with a clay coating instead of densification. Grade names overlap between suppliers, so compare them on air permeability and surface treatment.

Which tests should a coater ask for on a base paper certificate?

At minimum: grammage (TAPPI T 410), air permeability with the method named, Cobb (TAPPI T 441), tensile in both directions (TAPPI T 494) and moisture, all from the same reel. Holdout, anchorage and runnability need a trial on your own line, because no certificate measures them.

Last reviewed: September 2026

Talk to us about a trial

If a lighter or alternative coating base is on your qualification list, the practical first step is a flexC base trial enquiry. Send your current base specification and the methods you test to, and the conversation starts from your numbers rather than ours. Pakka’s product brochures are on the resources page, and general questions can go through the contact page.

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