The complete guide to kraft paper

Everything you need to know about kraft paper: the process that makes it strong, the grade families that matter, how to spec it, and why the industry keeps confusing process with aesthetics.

Kraft paper is the backbone of industrial packaging. It covers everything from the liner on a corrugated shipping box to the bag holding 50 kg of Portland cement. When someone calls something "brown packaging paper," they almost certainly mean kraft. When a buyer says they need "strong wrap," they mean kraft. The word has become so broad it barely functions as a specification anymore — which is exactly the problem this guide exists to solve.

The name comes from the Swedish kraft, meaning "strength" or "force." That etymology is accurate. Kraft paper is genuinely stronger than papers made by competing pulping processes, and the reason is chemical rather than mechanical: the kraft process removes lignin selectively while leaving long cellulose fibres substantially intact. Those intact fibres are what deliver the tear resistance, burst strength, and tensile properties that define the grade family.

This guide covers the kraft process, the main grade families, the specs that actually matter, the mill geography, and the use-cases where kraft is the right answer and the GSM range to target.

The kraft process, in one page

Kraft paper starts as timber — overwhelmingly softwood, typically pine or spruce from cold-climate boreal forests. Softwood fibres are long: 2–4 mm versus 0.5–1.5 mm for hardwood. Fibre length is the single biggest predictor of tear strength in paper. Everything downstream from the choice of raw material is amplification, not compensation.

Wood chips to digester. Timber is chipped to roughly 25 mm × 25 mm × 5 mm, then screened for uniformity. The chips go into a batch or continuous digester with white liquor — a hot aqueous solution of sodium hydroxide (NaOH) and sodium sulphide (Na₂S). Temperature runs 155–175 °C; pressure 700–900 kPa; cook time 1–4 hours depending on target kappa number.

Lignin removal. Lignin is the glue binding wood cells together. In the digester, the sulphide and hydroxide ions attack ester and ether bonds in lignin, fragmenting it into smaller soluble molecules that dissolve into the spent liquor (black liquor). Roughly 45–50% of the dry wood weight dissolves out. The remaining pulp — the brown stock — is fibre with most of the lignin removed.

Why does this matter? Lignin left in the paper causes two problems: it weakens fibre-to-fibre bonding, and it yellows under UV (the reason newspapers yellow). Mechanical pulping (groundwood, TMP) retains most lignin. Sulphite pulping removes lignin but degrades fibre length in the process. The kraft process removes lignin efficiently without attacking cellulose — hence superior strength retention.

Bleaching. Unbleached kraft paper retains residual lignin and has the characteristic brown colour. Bleached grades go through a multi-stage sequence: modern mills use ECF (elemental chlorine-free, using ClO₂) or TCF (totally chlorine-free, using oxygen, ozone, and hydrogen peroxide). ECF is the dominant approach globally; TCF accounts for a small fraction of production, concentrated in markets with tight effluent regulations. Full bleaching raises brightness to 85–92% ISO while reducing residual lignin (kappa number) from ~25 to <1.

Refining. Pulp fibres enter refiners — rotating disc or conical machines — where mechanical shear causes fibrillation: the fibre surface is roughened and minor fibrils are raised. Fibrillation increases bonding surface area between fibres and is the primary way mills dial up paper strength. Over-refining collapses drainage and reduces bulk; the optimum is grade-specific.

Fourdrinier and drying. Diluted pulp (typically 0.3–0.5% consistency) is metered onto the moving wire of a Fourdrinier machine. Water drains through the wire; vacuum boxes and suction rolls remove more. The wet web passes through press nips (increasing consistency to ~40%) then over steam-heated drying cylinders (reducing to ~93% solids). For extensible sack grades, a creping or microcreping step follows to build elongation-at-break.

Why kraft beats sulphite and mechanical. Sulphite pulping uses sulfurous acid and bisulphite, which removes lignin effectively but also cleaves cellulose chains, shortening fibres and reducing tear strength by 20–40% relative to kraft at equivalent bleaching level. Mechanical processes retain lignin (high yield, 90–95% vs. kraft's 45–50%) but sacrifice strength and permanence. For demanding packaging applications, there is no practical alternative to kraft chemistry.

Grade families

Kraft paper is not one product. The term covers at least five commercially distinct grade families, each with its own fibre recipe, machine configuration, and specification regime.

Kraft liner (corrugated outer face)

Kraft liner is the premier grade for corrugated box facings — the outer and inner liner of the three-ply sandwich. It is unbleached (occasionally white-top coated), made from virgin long-fibre softwood pulp at 125–225 g/m². The key specifications are burst index (typically 4.0–5.5 kPa·m²/g for top liner) and compression strength (SCT or RCT).

The test liner grade above uses recycled OCC fibre, which is the competitive alternative to virgin kraft liner. Recycled fibre is shorter and weaker; testliner achieves burst indices of 2.5–3.5 kPa·m²/g at equivalent GSM — broadly 20–35% below virgin kraft liner. The choice between them is a cost-versus-performance decision: single-use consumer goods often tolerate testliner; export shipping or heavy industrial goods typically requires virgin kraft liner.

Kraft sack (multi-wall bags)

Kraft sack paper is the material for open-mouth and valve bags: cement, flour, fertiliser, animal feed, chemicals. The spec regime is different from liner: burst strength matters, but extensibility — the percentage elongation before rupture — is critical. Bags absorb dynamic impact loads when filled and dropped; a sack that cannot stretch will burst at seams.

Standard kraft sack runs 70–90 g/m² per ply; multi-wall bags use 2–5 plies. Virgin extensible sack (semi-extensible: elongation 2–4%, extensible: 4–8%+) is the workhorse for cement bags globally. The most demanding applications — wet-strength sacks for export shipment in humid climates — combine extensibility with wet tensile additives.

Bag kraft (lighter industrial wrap and bags)

Below sack kraft in grammage, bag kraft paper covers consumer bags, shopping bags, and industrial wrapping at 40–120 g/m².

The grade distinction from sack kraft is primarily extensibility specification — bag kraft for shopping bags does not require the elongation-at-break that cement sacks need. Virgin fibre is still predominant for strength and cleanliness; recycled grades exist but are restricted to applications where contamination control is less critical.

Unbleached kraft (industrial wrapping)

Raw unbleached kraft is the cheapest structural wrap: palletising paper, void fill, ream wrapping, interleaving. GSM runs 80–160 g/m². Brightness and printability are irrelevant; burst and tear are what matter. The brown colour is the visual signature of residual lignin.

White-top kraft (premium corrugated and coated applications)

White-top kraft is a hybrid: unbleached kraft base with a bleached or coated top layer. It delivers the structural strength of kraft liner (virgin long-fibre base) with a printable white surface suitable for retail-shelf corrugated display boxes. The top coating is typically a bleached chemical pulp furnish or a clay coating. GSM typically 120–200 g/m².

The comparison above illustrates the structural contrast between testliner (recycled liner) and fluting (corrugating medium): both industrial packaging grades, both OCC fibre, but with distinct structural roles in the corrugated sandwich.

Specs that matter

GSM

The most quoted number, and genuinely important — but always read it in context of the grade family. A 125 g/m² kraft liner and a 125 g/m² kraft sack are very different products. See GSM.

| Grade family | Typical GSM range | |---|---| | Extensible sack | 40–90 g/m² per ply | | Bag kraft / light wrap | 40–120 g/m² | | Kraft liner (inner) | 100–150 g/m² | | Kraft liner (top / outer) | 125–225 g/m² | | White-top kraft | 120–200 g/m² | | Heavy industrial liner | 200–450 g/m² |

Burst index

Burst index = burst pressure (kPa) / grammage (g/m²). For liner grades, this is the primary purchase specification in most trading relationships. Virgin kraft liner: 3.5–5.5 kPa·m²/g. Testliner: 2.0–3.5 kPa·m²/g. Sack kraft: 4.0–6.5 kPa·m²/g. See burst strength.

Tensile strength and elongation

Tensile strength (kN/m in machine direction and cross direction) matters for all grades. For sack kraft specifically, elongation at break is often the binding specification: standard sack kraft ≥2%; extensible grades ≥4%; super-extensible ≥6%.

Cobb value (water absorption)

Cobb 60 (water absorbed in 60 seconds, g/m²) indicates surface sizing quality. For industrial wrapping and corrugated liner, Cobb values below 25–30 g/m² are typical. Wet-strength sack grades specify lower Cobb values and add wet tensile retention.

Brightness

Unbleached kraft runs 25–45% ISO brightness — the brown kraft aesthetic. Semi-bleached: 55–70%. Fully bleached: 80–92%. White-top grades specify the top layer brightness separately (typically ≥80% ISO). For food packaging applications, brightness signals the use of virgin fibre; buyers in food markets often specify minimum brightness as an indirect proxy for cleanliness.

Caliper

Caliper (thickness in µm or mm) at a stated GSM tells you the bulk: bulk = caliper / GSM. Higher bulk = more stiffness per unit weight. See caliper.

Mills and origins

Kraft paper production is geographically constrained by its raw material requirement: long-fibre softwood. That means cold climates where Pinus sylvestris, Pinus radiata, Picea abies, and Pinus taeda grow at commercial scale.

Scandinavia is the historic heartland. Stora Enso (Finland/Sweden), Billerud (Sweden), and Mondi's Scandinavian operations produce premium kraft liner and sack grades with FSC-certified chain of custody. Nordic mills were early adopters of ECF bleaching and closed-loop effluent recovery. Labour costs are high; quality and certification depth are also high.

Brazil has grown rapidly through Klabin and Suzano, exploiting planted Pinus and Eucalyptus (for hardwood market pulp alongside kraft). Brazilian kraft sack and liner are now significant exports to Europe and Asia. Fibre quality is competitive; logistics to Asian markets are a genuine cost advantage over Nordic mills.

Russia (Segezha Group, Archangelsk Pulp and Paper) historically supplied a significant share of European sack kraft. Sanctions and supply disruptions since 2022 have reshuffled the market; buyers who relied on Russian supply have largely shifted to Nordic, Brazilian, or North American sources.

North America (International Paper, Packaging Corporation of America, WestRock) dominates the North American corrugated liner market. Pinus taeda (loblolly pine) in the US Southeast provides the long-fibre base. North American kraftliner often runs at slightly different specification conventions (using US basis weight rather than GSM) which creates confusion in cross-border trading.

Use-cases and the choice tree

Corrugated liner (125–200 g/m²)

The largest single end-use for kraft paper globally. A standard single-wall corrugated box (B-flute) uses two liner sheets and one corrugating medium. The outer liner typically runs 125–175 g/m² virgin kraft liner; the inner liner may be testliner at 100–150 g/m² to reduce cost. Applications where the box will be printed (retail display), exposed to humidity, or carrying high static loads — these are the cases where virgin kraft liner on both faces is worth the premium.

WPI-g-000863 (testliner) vs. virgin kraft liner is the choice tree node: if BCT requirements can be met with recycled fibre, testliner is cheaper. Run the McKee formula with your actual liner and flute specs before committing.

Shipping sacks (70–90 g/m² per ply, 2–5 plies)

Cement, fertiliser, flour, animal feed. The specification is always extensible sack kraft; the number of plies depends on the product weight and bag drop height. For 25–50 kg fills, a 3–4 ply bag of 70–80 g/m² extensible kraft is standard. For aggressive handling (conveyor discharge, pneumatic filling), specify elongation ≥4%.

Food bags and fast-food packaging (40–90 g/m²)

Consumer-facing brown paper bags — fast food, bakery, produce — run 40–80 g/m² virgin kraft. The specification here is driven partly by mechanical performance (the bag holds a coffee and croissant, not 50 kg of cement) and partly by food safety: virgin fibre, mineral oil migration compliance, and printability. Bright white or natural brown — both are kraft, the distinction is bleaching level.

Fruit wrapping and interleaving (40–80 g/m²)

A niche that demands virgin fibre and neutral pH: citrus fruit wrapping paper, avocado interleaving, stone fruit packaging. The paper contacts food, so OCC recycled grades are disqualified. Basis: 40–60 g/m² virgin unbleached kraft. Smooth surface finish to avoid mechanical damage to fruit skin.

Industrial wrapping (80–160 g/m²)

Steel coil wrapping, machinery packaging, ream wrapping. Unbleached, uncalendered, single-layer. Burst and tear matter; printability and brightness do not.

Sustainability and EUDR

"The industry's conflation of 'kraft' (the process) with 'kraft' (the visual tone — the brown colour) is the single most common source of spec-sheet confusion."

Kraft paper's sustainability profile is more complex than the brown-equals-natural narrative implies. Key dimensions:

Fibre certification. Virgin kraft, by definition, uses primary forest fibre. That means chain-of-custody certification matters. FSC 100% certifies that all fibre comes from FSC-certified forests. FSC Mix 70% allows up to 30% of fibre from controlled (non-certified-certified) sources. FSC Mix Credit operates via a mass-balance calculation across a mill's production. PEFC operates similarly and dominates in Scandinavian supply chains.

For buyers sourcing from European mills, FSC or PEFC certification is table stakes — you will not find uncertified Northern European kraft at major paper traders. For Brazilian and North American supply, certification coverage is high but patchy; ask for the specific CoC certificate code, not just the logo.

EUDR (EU Deforestation Regulation). From late 2025 (current enforcement date under revision), operators placing paper and board on the EU market must conduct due-diligence to demonstrate the products are not associated with deforestation or forest degradation after December 31, 2020. For kraft paper, this means tracing the wood fibre supply to the plot of origin — country, region, coordinates. Mills with robust FSC/PEFC systems are better positioned but not automatically compliant; the EUDR requires a specific due-diligence statement and documentation trail.

The practical implication: if you are importing kraft paper into the EU post-enforcement date, request your supplier's EUDR due-diligence documentation before confirming the order, not after.

Carbon. Black liquor — the spent cooking liquor — is burned in recovery boilers to regenerate cooking chemicals and generate energy. Modern kraft mills are typically energy self-sufficient and export surplus electricity. Biogenic CO₂ from black liquor combustion is the dominant emission, not fossil CO₂. This gives virgin kraft a lower scope 1–2 fossil carbon footprint than many competing materials, though the land-use accounting remains contested.

Glossary cross-references

This guide covers the structural mechanics of kraft paper but points outward for the measurement standards:

• GSM — the universal weight measurement
• Brightness vs whiteness — why brightness ISO ≠ whiteness
• Burst strength — the Mullen test and burst index
• Caliper — thickness and its relation to bulk
• Opacity — relevant for white-top and bleached grades
• Fluting — the corrugating medium that pairs with kraft liner
• Flute profiles — A/B/C/E/F/N flute dimensions
• CMT — concora medium test for fluting grades
• RCT — ring crush test for liner and medium
• ECT — edge crush test for the assembled corrugated board