Bleached vs Unbleached Coating Base: What Actually Changes for the Coater

Bleaching changes four things a coater can measure: brightness, opacity, surface wettability and the residual chemistry the fibre carries into the coating station. It does not, on the published evidence, change silicone release or cure at equal coat weight. Most of the specification arguments about white versus natural base paper are arguments about appearance and print, dressed up as coating problems.

That is worth saying plainly because the two grades are usually sold on folklore. Unbleached is “stronger and more water-resistant”. Bleached is “cleaner and cures better”. Each of those claims has a mechanism behind it and a limit to it, and the limit is usually where the money is.

Bleached coating base is chemical pulp taken through oxidative stages after cooking to remove residual lignin and raise brightness before the sheet is formed. Unbleached coating base, also called natural or kraft-coloured, is the same chemical pulp with the lignin left in. Both are made to be coated, so both are judged on how they take a coating rather than on how they look, and that is the frame this piece keeps to.

What is the difference between a bleached and an unbleached coating base?

The difference is residual lignin. Unbleached kraft pulp leaves the digester with roughly 3 to 6 percent lignin still bound to the fibre, which is what makes it brown. Bleaching strips that lignin to near zero with chlorine dioxide, oxygen and peroxide stages, which is what makes the sheet white. Everything else that differs between the two grades follows from that one fact. The industry tracks lignin with the kappa number, measured under ISO 302 or TAPPI T 236. According to the United States Office of Technology Assessment’s 1989 review of bleached-pulp technology, bleachable-grade softwood kraft leaves the digester at kappa 20 to 40 and hardwood at 15 to 25, and kappa 35 corresponds to roughly 5 percent lignin. The same review puts the brightness of unbleached kraft at 10 to 20 percent GE and notes that kraft pulp can be bleached to 90 percent GE, or 91 to 92 for special grades, by taking lignin content to near zero.

Brightness itself is a defined measurement. ISO 2470-1 reports diffuse blue reflectance at 457 nanometres under illuminant C, the “ISO brightness” that European mills quote; ISO 2470-2 does the same under D65 outdoor daylight and gives a different number for the same sheet if it carries optical brighteners; TAPPI T 452 is the GE brightness method used in North America. A specification that says “brightness 85 minimum” without naming the method leaves enough room between the three scales for a reel to pass at the mill and fail at incoming inspection.

Bleaching leaves two things alone. It does not change the fibre source, so a bleached and an unbleached grade from the same mill share the same furnish, the same refining response and the same formation habits. And it does not, by itself, close the sheet. Porosity in a coating base is set by refining, sizing and calendering, which is why porosity and air permeability get their own treatment. What bleaching changes is the chemistry of the fibre surface, and that is where the coater’s questions start.

Does bleaching change how the base takes silicone?

Not measurably, on the one published head-to-head trial. In 2016, John Theisen of Expera Specialty Solutions reported to the Pressure Sensitive Tape Council that a 3.2 mil bleached SCK and a 3.2 mil natural SCK, made on the same paper machine, finished on the same supercalender and coated on the same five-roll coater with the same platinum solventless silicone at equal coat weight, gave equal release at 28 to 30 grams and equal cure, with silicone extractables below 1.0 percent for both.

The mechanism explains why. Silicone holdout depends on how closed and how dense the sheet surface is, because the silicone has to stay on top of the fibre network long enough to cure rather than wick into it. Neither of those is a colour property. A supercalender closes a bleached sheet and a natural sheet by the same mechanism, and at equal density the silicone sees the same surface. Theisen’s trial held everything but fibre colour constant, and the release and cure numbers did not move.

The same paper makes a second point that a buyer specifying white for cleanliness should read. Bleached liners dominate the tape market, and Theisen attributes that to a historical belief that they carried less foreign matter and debris. His applicator-roll sponge wipes from the two runs showed no difference, and he calls the cleanliness advantage a perception rather than a measured property. A modern unbleached grade from a mill with proper brownstock washing is not a dirtier sheet.

Colour does matter for silicone in one indirect way, through cure chemistry. Platinum-catalysed addition-cure silicone, which is the bulk of what runs on paper release lines, is poisoned by a defined list of contaminants. Dow’s guidance on platinum-catalysed silicone inhibitors names compounds containing nitrogen, sulfur, tin and phosphorus, plus arsenic, antimony, selenium and tellurium, and residual solvents such as chlorinated hydrocarbons, alcohols and esters. Dow lists primers and coatings that inhibit cure, not paper grades, and there is no published evidence that a properly washed unbleached kraft inhibits cure. But an unbleached sheet does carry more residual wood chemistry than a bleached one, and kraft cooking is a sulfur process, so the honest position is to test rather than assume.

That test is cheap and should sit in every qualification protocol regardless of colour. Coat a trial reel at target coat weight, cure at line conditions, and run extractables and rub-off on the same samples used for release. An unbleached base that cures cleanly at equal extractables to your incumbent has answered the question; one that needs an extra pass through the oven has not, and the difference shows up as line speed. Our release liner base paper specification sets out where cure and anchorage sit in a full qualification.

What changes for a water-based or biopolymer barrier coating?

Wettability changes, and it changes in the direction most buyers do not expect. Lignin is hydrophobic, so an unbleached fibre surface repels water more than a bleached one, which means an aqueous dispersion coating wets and spreads less readily on a natural base at equal sizing. For silicone that is irrelevant. For a water-based barrier, a biopolymer or a heat-seal dispersion, it decides coat uniformity, pinholing and how hard the size press has to work. The evidence sits in fibre-level measurement. A 2022 paper in Scientific Reports on the wettability of pulp fibres reviews measurements of water contact angle on kraft fibres of different lignin content and confirms that contact angle rises with lignin content because of lignin’s hydrophobic effect; the same paper cites a contact angle of 52 degrees for Douglas fir unbleached kraft fibres and attributes the higher angles of high-lignin pulps to lignin and wood resin together. Bleaching removes both.

So the trade runs like this. An unbleached base has a head start on water holdout before any sizing goes on, which is why Cobb values under TAPPI T 441 or ISO 535 tend to come in lower at equal size-press addition, and why the grade has a long history in wrapping papers that see moisture. A bleached base wets more readily, which is exactly what an aqueous barrier coating wants at the applicator: even spreading, fewer skips, less dependence on wetting agents in the formulation. The same property that helps the natural sheet resist water hurts it as a substrate for a water-borne coating.

The practical consequence is that the specification for a water-based barrier base should carry a wettability figure, not a colour. Surface energy by contact angle to TAPPI T 458, or a Cobb 60 window, tells the coater what the coating will meet. If the end use needs natural fibre for a reason of its own, the coating formulator compensates with surfactant or the mill compensates with a surface size, and either choice costs money that a white base would not. That is a real limitation of the unbleached route, and it is the right reason to specify bleached where the barrier is aqueous.

Two cautions. First, extractives and lignin that stay in the sheet can migrate into a wet coating film during drying, and a dispersion barrier is thin. On an unbleached base, run the barrier test on the cured coating and not on the base alone; a grease or water barrier that passes on the sheet can fail on the coated structure. Second, none of this makes natural fibre the wrong answer for extrusion or hot-melt coating, where the coating is not aqueous and mechanical keying into a rougher, more open surface is what anchorage depends on. The glassine, SCK and CCK comparison shows how surface closure rather than colour sets the running order there.

How do brightness, opacity and show-through change the finished pack?

They change what the pack looks like and what a print process can do with it, and for most of the eight application families that a coating base serves, that is the decisive difference. A bleached base prints a wider colour gamut and reads cleaner under a clear barrier. An unbleached base is more opaque at equal grammage and looks like kraft on the shelf, which some brands want and others do not.

This site has already covered the print mechanism for bag converters, so the short version will do here: ink is semi-transparent, the substrate is part of every printed colour, and a substrate at 15 to 20 percent brightness cannot render a process-colour brand palette that a substrate at 85 percent can. The bleached versus unbleached comparison for carry bags sets out the Flexographic Technical Association’s print-contrast guidance and the cost gap; the substrate physics is the same for a coating base.

Opacity moves the other way. Lignin absorbs light, so at equal grammage an unbleached sheet hides more of what is behind it. Theisen’s descriptive property table for commercial liner grades shows the effect at the extremes: 77 percent opacity for a 60 lb SCK against 51 percent for a 37.2 lb glassine, with the glassine at higher density and higher gloss. Opacity is measured under ISO 2471, and the value belongs on a coating-base specification wherever the pack has to hide product or has print on one side that must not show through the other.

That translucency is a feature in one application family. Label liners are often run through gap sensors that read the difference in light transmission between liner and label, and a translucent bleached glassine gives the sensor a cleaner signal than an opaque natural sheet. Theisen puts it as the case where “smoothness and a clear adhesive look are desired”. A tape liner has no such requirement, which is why natural SCK is common in tapes and rare in pressure-sensitive label stock.

There is a second-order effect on the barrier layer itself. A clear biopolymer or silicone coating on a white base reads as white; the same coating on a natural base reads as brown, and any yellowing of the coating with heat or age is invisible on the natural sheet and visible on the white one. For a sachet or a confectionery wrap that carries a brand, that argues for bleached. For a dry-food liner or a release liner nobody sees, the natural sheet gives away nothing. Our sachet paper substrate requirements treat print as one of the six things the sachet substrate has to do before the barrier goes on.

Does the choice change food-contact compliance?

Not in the rules, and possibly in the test results. Neither the Food Safety and Standards (Packaging) Regulations, 2018 nor Germany’s BfR Recommendation XXXVI requires a food-contact paper to be bleached or unbleached; both regulate the substances used, the migration limits and the test methods, and both are colour-blind. What the choice can change is what the migration test finds, because an unbleached sheet carries more wood-derived extractives into the extract.

The Indian frame is set out in the FSSAI Packaging Regulations, which reference Indian Standards for paper and board in Schedule I and require overall migration testing under the conditions the Regulations set out. Bleaching status does not appear as a criterion anywhere in that schedule. The German frame, BfR XXXVI, lists the permitted substances for paper and board and the limits on extractable metals, formaldehyde, primary aromatic amines and antimicrobial activity in the aqueous extract; it does not prescribe fibre colour either. The food-contact compliance guide for coated paper covers the regulations, the test conditions and the paperwork in full.

Where the grades diverge in practice is the extract. Resin acids, fatty acids and lignin fragments are more abundant in an unbleached sheet and can raise the overall migration figure or, more often, show up as taint in a sensory test. Both effects depend on the mill’s washing and on the coating that sits between paper and food, so they are a reason to test the coated structure rather than a reason to rule a grade out.

The bleaching chemistry itself is an environmental question, not a food-contact one. India’s discharge standard for large pulp and paper mills above 24,000 tonnes a year sets AOX, the adsorbable organic halogen that comes from chlorine-based bleaching, at 1 kilogram per tonne of paper produced, and the Central Pollution Control Board’s Charter 3.0 for the Ganga and Yamuna basin states, issued in June 2024, tightens the treated-effluent norm for integrated chemical-pulp mills to under 8 milligrams per litre. Elemental chlorine bleaching produces far more AOX than chlorine dioxide; a mill that bleaches with ECF or TCF sequences is meeting a discharge limit, not a food-safety one. A buyer who needs to state the bleaching route on a customer questionnaire should ask the mill for the sequence by name and for its consent-to-operate limits, because “chlorine-free” on a datasheet can mean either.

What actually changes: a side-by-side for the coating line

The table puts the properties above against the coating processes that care about them. It describes the two grades in general; it is not a specification for any particular product.

For a coater the reading is to choose by coating chemistry and end use first and let colour follow. Silicone lines can run either. Aqueous barriers favour bleached unless the formulation is built for a natural surface. Branded packs favour bleached for print. Unseen liners and moisture-exposed wraps are where natural fibre earns its place.

How do you write the choice into a base paper specification?

Write the properties, not the colour. A coating base specification that says “bleached” or “unbleached” and stops there has handed the mill a free hand on everything the coater actually meets at the applicator. Name the measurement, the method and the window for each property the coating depends on, and let the colour be the consequence.

1. Brightness, with its method named. ISO 2470-1 for ISO brightness, ISO 2470-2 for D65 brightness, or TAPPI T 452 for GE brightness. Give a minimum for a printed pack, a maximum for a natural-look pack, and say whether optical brighteners are permitted; some food-contact frameworks restrict them.
2. Opacity under ISO 2471 wherever the pack has reverse print, sees a label sensor or has to hide product. Natural fibre gives opacity cheaply; a bleached sheet may need filler to reach the same number at the same grammage.
3. Wettability, for aqueous coatings. Contact angle under TAPPI T 458 or a Cobb 60 window under TAPPI T 441. State which coating system the figure is for; a number that suits a dispersion barrier will be wrong for a silicone.
4. Cure and extractables on the trial reel, for silicone lines: release to FINAT FTM 10, silicone extractables by the method your silicone supplier specifies, rub-off by finger test. Run them on the same reel as the release test, and run them on both a bleached and a natural candidate if both are in play.
5. Migration on the coated structure, not the base. Overall migration under the FSSAI Packaging Regulations’ conditions, plus BfR XXXVI or 21 CFR 176.170 where the customer requires them. A sensory test alongside the migration test catches the taint that an unbleached sheet can carry.
6. The bleaching sequence by name, if the customer questionnaire asks: ECF, TCF or unbleached, with the mill’s AOX consent limit. “Chlorine-free” is not specific enough to answer a European questionnaire.
7. Reel-to-reel tolerance on each of the above, because the coater’s problem is rarely the average and almost always the spread. Ask for the certificate of analysis format before the trial, not after.

Where a paper coating base fits

Start with the limits. A paper base of either colour does not match a polymer film for gas barrier at equal thickness, and nothing about bleaching changes that; gas barrier on a paper structure comes from the coating. At the light end of the grammage range, porosity and moisture profile are harder to hold reel to reel, and that is true whether the fibre is white or brown. A mill offering both colours has not thereby solved either problem, and a coating technologist should treat any porosity or holdout figure quoted for a base that has not completed pilot production as an intention rather than a result.

Pakka is developing flexC base, a coating base paper in the 40-60 GSM band, offered in bleached and unbleached form, intended as a substrate for barrier chemistries generally rather than for one coating system: silicone, biopolymer or plastic. The company has disclosed eight target applications for it, namely 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). The bleached and unbleached band is the reason this piece exists: the choice between the two is one a coater will have to make for each of those applications, and the decision runs on the properties above rather than on the colour.

The name needs care, because two products share part of it. flexC is Pakka’s finished compostable flexible-packaging laminate, sold to brand owners as a converted material with a published specification. flexC base is the uncoated coating base paper described here, sold to the coater or converter who applies their own barrier chemistry. They have different buyers, and a datasheet written for one does not describe the other.

We are not attaching performance figures to flexC base. No brightness, opacity, wettability, Cobb, cure or release figure is published for either colour, because the material is a development programme and pilot data does not exist yet. What a coater can usefully do now is decide which of the properties in the table their line depends on, write those into a trial protocol with the methods named, and hold any base, ours included, to that protocol before a reel is qualified.

Frequently asked questions

Is unbleached kraft paper better for silicone coating than bleached?

Not on the published evidence. The Expera head-to-head trial reported to the Pressure Sensitive Tape Council coated bleached and natural supercalendered kraft of the same caliper from the same machine with the same platinum silicone at equal coat weight and measured equal release, at 28 to 30 grams, and equal cure. Silicone holdout is set by surface closure and density, which bleaching does not change.

Why does a water-based barrier coating spread better on bleached paper?

Because lignin is hydrophobic. Bleaching removes it and leaves a fibre surface with a lower water contact angle, so an aqueous dispersion wets and levels more readily. An unbleached sheet resists water better before sizing, which helps a wrapping paper and hinders a water-borne coating on the same sheet.

Does bleached or unbleached paper make a difference to FSSAI food-contact approval?

The Food Safety and Standards (Packaging) Regulations, 2018 do not prescribe bleached or unbleached fibre; they regulate substances, migration limits and test methods. An unbleached sheet can carry more wood-derived extractives into an overall migration or sensory test, so the coated structure should be tested rather than assumed from the base.

What brightness should I specify for a coating base paper?

Specify it with the method. ISO 2470-1 gives ISO brightness under illuminant C, ISO 2470-2 uses D65 and reads higher on sheets with optical brighteners, and TAPPI T 452 gives GE brightness. Fully bleached kraft typically sits between 80 and 92 percent; unbleached kraft sits around 10 to 20 percent. Set a minimum for printed packs and say whether optical brighteners are allowed.

Is unbleached coating base paper more environmentally friendly?

At the mill, an unbleached grade avoids the bleach plant and the AOX discharge that chlorine-based bleaching produces, which Indian standards cap at 1 kilogram per tonne of paper for large mills. Downstream, both grades recycle and compost the same way; the coating on top of the base, not the fibre colour, decides the end-of-life route.

Last reviewed: September 2026.

Specifying a coating base in bleached or unbleached? Pakka’s flexC base programme covers the 40-60 GSM band in both forms, for coaters, laminators and release-liner producers building their own barrier structures. If you would like to discuss a trial, get in touch with a flexC base trial enquiry and tell us the coating chemistry, the end use and which of the properties above your line depends on. Technical brochures for our paper and packaging range are on the resources page, and general enquiries reach us through the same contact page.

Sources

• United States Congress, Office of Technology Assessment, Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp, Chapter 4: Pulp Bleaching Technology (1989), kappa number ranges, lignin content and brightness of unbleached and bleached kraft
• John Theisen, Expera Specialty Solutions, Paper Based Release Liners, Pressure Sensitive Tape Council technical paper (2016), bleached versus natural SCK release trial, opacity and density table
• Dow, FAQ: Platinum Catalyzed Silicone Inhibitors, contaminant classes that inhibit addition-cure silicone
• Sci Rep 12, 1560 (2022), Relationship between wettability of pulp fibers and tensile strength of paper during recycling, lignin content and water contact angle of kraft fibres
• Central Pollution Control Board, Charter for Water Recycling and Pollution Prevention in Pulp and Paper Industries, Charter 3.0 (June 2024), notified AOX standard of 1 kg per tonne for large mills and the 8 mg/l treated-effluent norm
• Food Safety and Standards Authority of India, Food Safety and Standards (Packaging) Regulations, 2018 (Compendium, Version II, 1 February 2022)
• German Federal Institute for Risk Assessment (BfR), Recommendation XXXVI, Paper and Board for Food Contact
• Pakka Limited, Investor Presentation Q1 FY2026-27, filed 18 August 2026 (BSE 516030 / NSE PAKKA)

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