Kraft Liner.

Kraft liner is the warm brown paperboard you see on the outside of every large shipping carton that has ever crossed an ocean. It is the workhorse face layer of corrugated containerboard — a virgin-fibre, unbleached grade made from long-fibre softwood pulp using the kraft (sulphate) pulping process. In the middle of the family, at around 140 g/m², sits the weight that the global export-packaging industry has treated as the default outer liner for the better part of sixty years. That's the version this article is about, situated inside the canonical Kraft Liner family WPI-g-000888.

Three things distinguish kraft liner from the look-alike grades that compete with it. First: the fibre is virgin, not recycled, which preserves length and therefore strength. Second: it is unbleached, so the residual lignin in the pulp gives the sheet its characteristic warm brown tone — that colour is a by-product of the chemistry, not a dye. Third: it is specified by burst strength, not by appearance. When a buyer asks for "kraft liner 140" they are asking for a specific minimum burst index, a specific fibre provenance, and a specific compatibility with recycled inner liners and fluting. The colour is a diagnostic; the numbers are the contract.

What it's used for

The overwhelming majority of kraft liner in the world becomes the outer facing of a corrugated shipping box. In a standard double-wall construction, the outside liner is virgin kraft at 140 g/m²; the inside liner is usually testliner WPI-g-000863 at a similar weight but lower cost, made from recycled old-corrugated-carton fibre. Between the two liners runs a fluting medium WPI-g-000864 — a lightweight, corrugated sheet that provides vertical compression resistance.

That sandwich is the physical object behind nearly every e-commerce parcel you have ever received, every wine case that survives a pallet drop, every banana box shipped refrigerated from Ecuador to Rotterdam. Fast-moving consumer goods run the same construction with lighter liners. Export-grade shipping boxes — the category that has to tolerate humidity, rough handling, and month-long voyages — typically step up to heavier kraft liner on both faces (170 or 200 g/m²) and a stronger fluting.

Outside corrugated, 140 GSM kraft liner shows up as heavy-duty wrapping — reams of it roll off a mill straight into rolls used for pallet wrapping, industrial interleaving, and protective layer paper in steel coil or automotive parts shipments. It is the base sheet for many printed fast-food takeout bags (though retail shopping bags usually run heavier). It appears as the outer layer of multi-wall kraft sacks for cement, flour, and pet food, though those sacks are typically built around a stronger and lighter extensible kraft WPI-g-000018 and use the liner-weight sheet as a reinforcing outer ply rather than a primary structural layer.

The split between "140 kraft liner" and the heavier grades matters economically. At 125 g/m² you lose the burst margin that a box needs to survive stacked on a pallet six-high. At 170 g/m² you add about 20% to the cost of fibre for every square metre shipped. 140 is the number where the burst budget meets the material budget, and the industry has voted with its purchase orders for decades.

Origins and history

The kraft process was invented in 1879 by Carl Ferdinand Dahl, a German chemist working at Ulrich-Holtmann paper mill in Danzig — now Gdańsk, Poland. Dahl's innovation was the substitution of sodium sulphide (Na₂S) for sodium sulphite in the pulping liquor. The substitution looks incidental; it is not. Sulphide chemistry is far more selective: it cleaves lignin without attacking cellulose, which means the resulting pulp retains nearly the full length of the original wood fibres. The soda process of the day (NaOH alone, patented by Burgess and Watt in 1854) degraded fibres badly. Tilghman's sulphite process (1866–67), which dominated late-19th-century papermaking, was gentler but produced a weaker sheet because the acidic conditions attacked the hemicellulose binding matrix. Dahl's sulphate process threaded the needle.

He filed German patent DRP 25,629 on 1 October 1884, titled Verfahren zur Bereitung von Zellstoff — "Process for the preparation of cellulose." The name "kraft" came later, adopted from the Swedish and German word for "strength" or "force" by the Scandinavian mills that commercialised the process aggressively through the 1890s and 1900s. By 1910 the Munksjö and Billerud mills in Sweden were exporting kraft sack paper at industrial scale; by 1920 the process had displaced sulphite for nearly all strength-critical grades, and the sulphite mills were converting.

Sweden became the spiritual and industrial capital of kraft paper for two reasons that still hold. The cold boreal climate grows slow, straight softwoods — mostly Norway spruce and Scots pine — whose tracheid fibres are long (2–4 mm) and uniformly thick. And the winter ice lock on the Baltic gave Swedish mills predictable log supply through vertically-integrated forest holdings. The same industrial logic is why North American kraft is concentrated in the US Southeast (loblolly pine) and Canada (black spruce, jack pine), and why tropical kraft based on eucalyptus never quite competes on tear strength despite its dramatic cost advantage in the pulp itself.

The shift to 140 GSM as the industry-standard outer-liner weight happened in the mid-twentieth century, driven by corrugated export pallet economics: 140 was the lightest grade that could consistently deliver enough burst to survive the edge-crush loads of a six-high pallet stack at Atlantic humidity. The FEFCO (European Federation of Corrugated Board Manufacturers) grade code system, first published in 1963 and formalised through the 1970s, gave the weight a permanent identifier and standardised how it was ordered.

How it's made

Modern kraft liner is made by a process that has changed more in emissions control than in chemistry since Dahl. Softwood logs are debarked, chipped to nominal 25 × 25 × 5 mm, and screened. The chips go into a continuous digester — typically a Kamyr-style vertical reactor — with white liquor, an aqueous solution of NaOH and Na₂S at roughly 100 g/L total active alkali. Temperature runs 165–175 °C, pressure 800 kPa, cook time 2–5 hours depending on target kappa number (the residual lignin index). Roughly half the dry weight of the wood dissolves into the black liquor during cooking; the remaining pulp is the brown stock — long cellulose fibres with most of the lignin removed but enough residual to keep the characteristic brown colour.

Brown stock is washed in a counter-current drum sequence to recover black liquor for chemical regeneration (the kraft process's famous closed-loop chemistry: black liquor is burned in a recovery boiler, generating process steam and electricity, and the sodium salts are recausticised back to white liquor). For kraft liner, no bleaching stage is needed — the brown tone is a feature, not a defect. The washed pulp is refined in low-consistency disk refiners to develop fibre bonding, then pumped to the wet end of the paper machine.

A kraft liner paper machine is typically a Fourdrinier, often with a secondary top-ply former to enable a two-layer construction: a stronger virgin top layer for burst resistance and a cheaper fibre blend in the base. The stock hits the wire at 0.4–0.8% consistency, drains by gravity and vacuum through a 30–60 metre forming section, presses at the wet press section (the press nip is where most density development happens), dries through a 40–60-cylinder dryer section, sizes with a surface sizing station (typically starch-based for kraft liner; rosin size in the wet end for moisture resistance), calenders to target thickness, and winds to parent rolls. A modern machine produces kraft liner at 800–1,300 m/min — the width can exceed 8 metres.

The essential insight about the kraft process, the one every paper buyer should hold onto: the strength comes from fibre length, which comes from the chemistry. Sulphide selectively attacks lignin without degrading cellulose. Sulphite attacks both. Recycled fibre — even from high-quality kraft feedstock — has been through the process of dispersion in water, drying, and redispersion at least once, and every cycle shortens fibres by 10–15%. That is why a virgin kraft liner at 140 GSM will consistently out-burst a recycled testliner at the same weight, and why the two grades command different prices.

Specs that distinguish it

The numbers that matter when you're actually buying a roll of kraft liner 140:

• Basis weight — 140 g/m² nominal, measured to TAPPI T-220 or ISO 536. Mill tolerance is typically ±3 g/m²; audit lots tighter.
• Burst index — ≥ 6.5 kPa·m²/g for premium virgin kraft liner (ISO 2758 Mullen test). Entry-grade runs 5.5–6.0; specialty export grades reach 7.5. This is the headline spec.
• Tensile index (MD) — 60–80 N·m/g, measured to TAPPI T-494 or ISO 1924-2.
• Ring crush index — 10–14 N·m/g, a proxy for ECT performance once converted to board. Increasingly specified directly.
• Cobb value — 25–35 g/m² on surface-sized grades (ISO 535) — i.e. how much water the surface absorbs in 60 s. Lower = better moisture resistance.
• Brightness (ISO 2470) — 27–32%. Anything brighter than 35% suggests partial bleaching, which usually means the mill is trying to pass a semi-bleached grade off as unbleached.

Compared to the testliner alternative at a similar weight, the trade-offs stack up clearly:

Kraft liner is the premium answer. Testliner is the economical answer. The gap is roughly 15–20% on price and 20–30% on burst index. For high-value or export goods the math favours kraft; for domestic FMCG in dry conditions the math usually favours testliner.

Variants and family

The kraft liner family is wide. Within the same sheet structure and chemistry, the meaningful variants are:

• Kraft liner 125 g/m² — light-duty outer liner for domestic shipping; used where export survival isn't a requirement and where the printed graphics demand a smooth, lighter surface.
• Kraft liner 140 g/m² — the canonical mid-tier default covered by this article; the weight the corrugated industry treats as the standard face liner.
• Kraft liner 170–200 g/m² — heavy-duty export and industrial grades; chosen when burst or ring-crush margin dominates the spec.
• White-top kraft liner — a two-ply construction with a bleached white top layer for high-quality print and a brown kraft base for strength. A different economic tier; typically priced 25–40% above plain kraft liner.
• Extensible sack kraft WPI-g-000018 — not a liner but a close cousin; virgin kraft engineered for stretch and impact absorption in multi-wall paper sacks. Much lighter (70–100 GSM) and specified differently (TEA, stretch-at-break).

Buying notes

Three red flags worth catching on a mill spec sheet:

Recycled content masquerading as virgin. A sheet labelled "kraft liner" but sourcing from a mill that runs primarily OCC (old corrugated containers) feedstock is testliner regardless of what the label says. The tell is usually the burst index — below 6.0 kPa·m²/g at 140 g/m² is almost always recycled fibre. Ask for a fibre-composition analysis. Virgin softwood kraft runs 80-95% softwood tracheids on fibre microscopy.

Brightness above 35%. On an unbleached kraft liner, this almost always indicates partial oxygen delignification or brightening with alkaline peroxide at the brown stock washer. It isn't necessarily a quality problem — oxygen delignification is legitimate chemistry — but it signals either a mis-classified grade or a mill trying to reposition a semi-bleached product. Confirm the ISO 2470 figure on a physical sample, not the promotional literature.

Certification mismatch. For EU buyers under EUDR, FSC Mix 70 (minimum 70% FSC-certified fibre) is the realistic baseline for commodity kraft liner. FSC 100% is available at a 5–10% price premium from Swedish and Finnish mills. PEFC is accepted in some EU supply chains but not treated as equivalent to FSC by many downstream brand owners. Check your brand requirements before you buy on price alone.

Related reading

• Kraft paper — the complete guide — the parent hub for the kraft family of grades.
• Burst strength (Mullen test) — the primary spec for kraft liner.
• Fluting — the corrugated medium between the two liners.
• Edge crush test (ECT) — the board-level metric your liner rolls up into.
• GSM — how basis weight is defined and measured.