What Is Kraft Paper? GSM, BF Grades, Virgin vs Recycled, and Its Uses in Packaging

Kraft paper is paper made from pulp produced by the kraft process, also called the sulphate process, in which wood chips are cooked in a hot alkaline solution of sodium hydroxide and sodium sulphide. The cooking dissolves the lignin and leaves long cellulose fibres nearly intact, and that fibre length is the whole point: kraft is the strongest of the common papers by tear, burst and tensile strength, which is why almost every corrugated box, paper sack and wrapping sheet in Indian packaging is made from it. The name is the German word for strength.

For a packaging buyer the material is defined by four things: the fibre source (virgin, semi-kraft or recycled), the grammage in GSM, the burst factor, and the finish. Get those four right on a specification and the paper will do what the box design assumes it will.

How kraft is made, and why that makes it strong

Wood chips, mostly softwood in Scandinavia and North America and a mix of hardwood, bamboo and bagasse in India, are cooked for two to four hours at 160–180 °C in white liquor. The lignin that glues the fibres together dissolves into the liquor; the fibres are washed, screened and, for brown kraft, sent straight to the paper machine without bleaching. The residual lignin gives kraft its brown colour and also contributes some stiffness. Bleached kraft goes through an oxygen or chlorine-dioxide bleaching stage that removes it, giving a white sheet with 5–10% lower strength but a printable surface.

Two process facts affect what you buy. First, the cooking chemicals are recovered and reused, which is what makes kraft the dominant chemical pulping route worldwide. Second, softwood fibres are 2.5–4 mm long against 0.7–1.5 mm for hardwood and recycled fibre, so virgin softwood kraft holds a burst factor of 30–40 where recycled kraft from the same machine settles at 14–22. Every recycling cycle shortens the fibres further; after four to six cycles they drop out as fines.

Virgin kraft, semi-kraft, recycled and test liner

Indian mills produce kraft along a spectrum of virgin-to-recycled content, and the names are used loosely. The table gives the working definitions:

The term "test liner" is European usage for a recycled liner that has been tested to meet a burst or ring-crush minimum; in Indian trade the same paper is simply called kraft at a stated BF. Ask for the BF and GSM rather than the trade name, and ask whether the mill is quoting the BF on a dry sheet at 50% RH, because a sheet tested at 65% RH will show 5–10% less.

GSM: what the grammage number tells you and what it does not

GSM is grams per square metre, the mass of one square metre of the sheet. It is measured under IS 1060 (Part 1) by cutting a 10 × 10 cm sample, weighing it in grams and multiplying by 100. Kraft is sold by weight, so GSM converts directly into how many square metres a tonne gives you: a tonne of 150 GSM liner is 6,667 m², a tonne of 120 GSM is 8,333 m².

GSM on its own says nothing about strength. A 150 GSM sheet of 14 BF recycled kraft bursts at about 2.1 kg/cm²; a 150 GSM sheet of 28 BF semi-virgin kraft bursts at 4.2 kg/cm², twice the figure at the same weight and roughly the same thickness. Buyers who specify only GSM get whichever fibre the mill has that week. The relationship is:

Bursting strength (kg/cm²) = BF × GSM ÷ 1000

and, turned around, BF = bursting strength (g/cm²) ÷ GSM. The bursting strength is what the Mullen tester measures; BF is what lets you compare a 120 GSM paper with a 180 GSM one on equal terms.

Common Indian kraft grades and what they burst at

The board a box is made from adds the burst of its liners, roughly, and the flutes contribute little to burst. A 5-ply board with 150/24 outer liners and a 120/18 middle liner will come out near 10 kg/cm², which is why "5-ply, 10 BS" is such a common Indian carton specification. Whether that is the right number to specify is a separate question; for stacked loads the edge crush test matters more, as covered on our corrugated box page.

Finish: MG, MF and the surface you get

Kraft is finished on the machine in one of two ways. Machine glazed (MG) kraft is dried against a single large polished Yankee cylinder, which gives one smooth, slightly glossy side and one rough side; it is the standard for wrapping paper, gummed tape base and paper bags, and prints well on the glazed face. Machine finished (MF) kraft passes through calender rolls and has a similar matt surface on both sides; liners and fluting are MF. A third finish, ribbed kraft, is embossed with fine lines and used for wrapping where the texture hides scuffing.

The finish decides two things for a converter. Flexo printing on a corrugated box goes onto the outer liner, and a smoother, higher-density liner takes finer text and solid colours. Adhesion of the starch used to glue the flutes to the liners is better on a rougher, more porous surface, so the inside face of a liner should not be sized too hard. Kraft with a surface smoothness under 200 ml/min Bendtsen prints well; above 400 ml/min it will look coarse.

Wet strength, Cobb, and monsoon

Kraft absorbs water, and wet kraft is weak: a fully saturated sheet keeps only 3–8% of its dry tensile strength, and a corrugated box at 90% RH keeps about half its compression strength. Three specifications control this:

• Cobb value (IS 1060 Part 1, Cobb 60): grams of water one square metre absorbs in 60 seconds. Plain liner runs 60–100 g/m²; sized liner for export or cold-chain runs 25–35 g/m². Specify Cobb 30 or lower for any carton that will be stuffed at Mundra or Kandla in monsoon.
• Wet-strength grade: a polyamide-epichlorohydrin resin added at the mill lets the sheet keep 20–35% of its dry strength when wet. Used for produce boxes, ice-packed seafood cartons and sack kraft for cement. It costs 8–15% more than plain kraft and slows repulping.
• PE or wax coating: turns the sheet into a barrier. Coated kraft is effectively not recyclable in the Indian OCC stream, so use it only where the exposure demands it.

Moisture content of the paper as delivered is also part of the specification: 6–8% at the mill, and a reel that has sat uncovered in a Rajkot godown through August will be at 10–12%, corrugate badly and wash-board on the machine.

Uses in packaging, and the grade for each

• Corrugated liner and medium: the largest use by far. Liners 100–250 GSM at 16–30 BF; medium 100–140 GSM. See the kraft paper glossary entry for the board make-ups we run in 3, 5 and 7 ply.
• Wrapping and interleaving: 40–80 GSM MG kraft around bundles of brass fittings, between sheet metal, over furniture. Thin 1 mm EPE sheet replaces it where surface abrasion or moisture is the concern; kraft wins where the buyer wants an all-paper pack.
• Void fill and cushioning paper: 50–80 GSM recycled kraft crumpled or fan-folded from a dispenser. Cheap, recyclable, poor at repeated impact.
• Multi-wall sacks: 2–4 plies of 70–90 GSM sack kraft, sometimes with a PE liner, for cement, chemicals, sugar, pulses.
• Gummed paper tape and honeycomb core: 60–80 GSM kraft base for water-activated tape; 100–140 GSM for the hexagonal core of honeycomb board.
• Angle boards and edge protectors: laminated 150–250 GSM plies pressed into an L-section that carries strap load on a pallet.

How to specify kraft paper on a purchase order

A kraft specification that a mill or converter cannot misread contains, in this order:

1. Grade name and fibre: virgin kraftliner, semi-kraft, recycled kraft or fluting medium, with minimum virgin content if it matters.
2. GSM with tolerance: 150 GSM ± 4%, tested to IS 1060.
3. BF or bursting strength: 22 BF minimum, or 3.3 kg/cm² minimum, at 50% RH and 27 °C.
4. Ring crush or CMT for medium and for liners going into a stacking-critical box; burst is not enough for compression.
5. Cobb 60: 30 g/m² maximum on the outer face for export.
6. Moisture: 7% ± 1% at delivery.
7. Finish and colour: MF natural brown, MG for wrapping, bleached or white-top if the box is printed in solid colours.
8. Reel width and core: matched to the corrugator or the converting machine; 76 mm core is standard.

On corrugated box orders the same information is written per layer, for example "150/24 – 120/18 – 120/18 – 120/18 – 150/24" for a 5-ply board, and the box is tested as a whole to IS 2771. Most disputes over box performance in our experience come from a spec that stated GSM and left BF to the mill.

Where Balaji International fits

We buy kraft against exactly this kind of specification and convert it into 5-ply and 7-ply cartons, partitions and die-cut sleeves at our Rajkot plant, with the layer make-up written on the drawing rather than left to whatever paper is in stock. If you have an existing box that is failing in stacking or in monsoon, send us the box and tell us the route; we will pull the liners apart, test them, and quote a board with the GSM, BF and Cobb that the load actually needs. For the broader picture of how liner grade turns into box strength, our corrugated box manufacturing page covers ply, flute and testing together.