Sack Kraft Paper.

Sack kraft is the paper that carries the world's cement, flour, animal feed, fertiliser, and building chemicals — somewhere between four and five million tonnes of it every year WPI-g-000894. It is a long-fibre, unbleached softwood kraft, engineered for a single dominant job: to survive the shock loading of a high-speed bag-filling machine without bursting. A 50 kg cement bag is filled in roughly two seconds. The paper that makes that possible has to do three things other kraft grades do not. It has to stretch — four to eight percent before it breaks, so the sudden inrush of product decelerates against a cushion of elongation rather than a brittle wall. It has to breathe — a bag of fresh cement displaces 50 litres of air during filling, and if the paper doesn't let that air out in milliseconds, the bag ruptures. And it has to stay strong when wet, because cement warehouses are humid and industrial loading docks are not dry. Nothing else on the market combines those three properties at the cost that cement, flour, and fertiliser industries can absorb. That's what sack kraft is, and why — a century after the multi-wall paper sack replaced the jute bag — it has no serious competitor.

What it's used for

The cement industry is, by a large margin, the largest single consumer of sack kraft globally — roughly 40% of the world's tonnage goes into the familiar 50 kg cement bag printed with a mill brand and a batch code. China, India, and sub-Saharan Africa dominate incremental demand; Europe and North America have largely shifted to 1-tonne bulk bags (FIBCs) for large construction sites, but the 25–50 kg paper sack remains the default format for bagged cement reaching small contractors, rural distribution, and retail DIY channels. A single mid-sized cement plant in India or Vietnam will consume several thousand tonnes of sack kraft per year.

Flour, sugar, starches, and industrial food ingredients make up the second-largest slice — the classic two-to-four-ply multi-wall industrial bag with a pinch-bottom or valve closure. Animal feed and pet food represent a growing segment, particularly in the premium dry-pet-food category where paper sacks increasingly displace the plastic-woven alternative on sustainability grounds. Agrochemicals and fertilizer — urea, NPK compounds, ammonium sulphate — ship in sack kraft at enormous volumes, though polypropylene woven bags have taken share in tropical markets where monsoon humidity challenges paper.

Building chemicals — tile adhesive, grout, ready-mix mortar, gypsum products, self-levelling compounds — represent the highest-value end of the sack kraft market, because these products demand printed multi-colour bags with strong moisture resistance and often wet-strength treatment. Industrial powders covering everything from carbon black to lime to food-grade additives round out the picture. One category that sack kraft has not captured is consumer-facing retail bags: e-commerce growth and the shift from plastic shopping bags to paper alternatives has mostly benefited lighter kraft grades and MG sulphate paper, not sack kraft, which is over-engineered for applications that don't need shock-load survival. The sack kraft market remains, and will remain, dominantly industrial and B2B.

Origins and history

The history of sack kraft is the history of how industrial commodities came to be packaged. Before the 1920s, cement, flour, sugar, and fertiliser moved in cloth sacks — jute imported from Bengal, burlap, or heavy cotton. Cloth was strong but expensive, reusable but filthy after one fill with cement or lime, and impossible to fill at industrial speed.

The paper sack revolution has a specific inventor and a specific date. Guy O. Bates, working with his brother in Chicago, was granted US Patent 1,550,175 on 18 August 1925 for a multi-wall paper bag with a valve closure — four plies of heavy kraft paper glued together at the seams, with a small tubular opening at one corner that allowed the bag to be filled on a machine and then self-sealed under the pressure of the contents WPI-g-000894. Bates Valve Bag Co. built the first commercial production line in Chicago in 1926, and the economics were immediate: a paper valve sack cost perhaps a fifth of the equivalent jute bag, could be filled by a single-operator machine at hundreds of units per hour, and was disposable (no return freight, no washing, no reuse of a bag that had held lime). Portland Cement Company placed the first major order in 1927. By 1935 the multi-wall paper sack had displaced cloth for cement across the industrialised United States.

European development ran in parallel. German and Scandinavian mills — Waldheim in Germany, Billerud and Munksjö in Sweden — adapted the American multi-wall format to local filling standards and developed the heavy kraft pulp grades needed. Mondi and Klabin (Brazil) became major players in the post-war expansion.

The second transformative innovation came in the 1950s. Clupak Inc. in New York patented a mechanical creping process — credit goes to Sanford L. Cluett's US Patent 2,624,245 (1953) and the Clupak extensibility unit that followed. Before Clupak, kraft paper had maybe 2% machine-direction stretch; after Clupak, 4–8%. The creping process compacted the wet web against a rubber blanket, then relaxed it, producing a microscopic concertina pattern invisible to the eye but measurable as tensile energy absorption (TEA) — the area under the stress-strain curve, measured to ISO 1924-3. TEA is the physical quantity that predicts whether a sack will survive a drop from the filling spout or a forklift tumble. Pre-Clupak sacks burst at industrial filling speeds; post-Clupak, the modern 60-bags-per-minute cement filling line became possible. Every major sack kraft mill in the world operates a Clupak (or licensed-equivalent) extensibility unit today.

How it's made

Sack kraft begins as unbleached long-fibre softwood kraft pulp — the same chemistry first patented by Carl Dahl in 1884 WPI-g-000888, but with a critical difference. Where kraft liner is cooked to a lower kappa number (more delignified, slightly shorter fibres), sack kraft is given a low-yield, low-intensity cook designed to minimise fibre shortening. Fibre length is the single most important determinant of the finished paper's tear and TEA performance, so the pulper accepts a higher residual lignin content in exchange for preserving 2.5–4 mm softwood tracheids intact. Typical furnishes: US Southern pine (loblolly) for North American mills, Nordic spruce and Scots pine for Scandinavian mills, radiata pine for Brazil and Chile.

The pulp is refined lightly — heavy refining develops bonding but also cuts fibres — and run on a Fourdrinier paper machine at relatively low consistency to preserve formation. The critical step comes just before the dryer section: the Clupak extensibility unit, or a licensed equivalent such as the Expanda system. The wet web, at around 60% solids, is pressed against a thick rubber blanket under heated steel rolls. As the rubber blanket decompresses on the far side of the nip, the paper compacts into microscopic crepe folds in the machine direction. This gives sack kraft its defining 4–8% stretch: when tension is applied, the microcrepe unfolds before the cellulose fibres take load.

Porosity is engineered with equal care. A sack kraft sheet with too-dense formation traps air during bag filling and ruptures on the first shock. Porosity is tuned by furnish (coarse vs. fine softwood fraction), refining energy, and wet-end chemistry, and measured as Gurley air permeance — typically 8–30 seconds for sack-grade paper.

Typical basis weight is 70–130 g/m² per ply; most industrial sacks are built from two to four plies glued or sewn together. Modern sack kraft machines are among the largest in the paper industry: widths of 7–10 metres, speeds of 1,200–1,500 m/min, annual capacities of 300,000–500,000 tonnes per machine. Reference machines include Mondi Štětí (Czech Republic), BillerudKorsnäs Gruvön (Sweden), Klabin Monte Alegre (Brazil), and Stora Enso Skutskär.

Specs that distinguish it

The numbers that matter when a converter is buying sack kraft for a high-speed filling line:

• Basis weight — 70–130 g/m² per ply; 90–110 g/m² is the dominant commodity range, measured to ISO 536.
• Tensile Energy Absorption (TEA) — 4.0–8.0% stretch at break, measured to ISO 1924-3. This is the defining spec. Below 4% and the sack will not survive industrial filling; above 8% gets harder to run cleanly on the bag-making machine.
• Burst strength (Mullen) — 480–720 kPa at 90 g/m², measured to ISO 2758. A proxy for combined tensile and elongation strength.
• Tear strength (Elmendorf) — 900–1,500 mN, measured to ISO 1974. Directly correlated with softwood fibre length preservation.
• Porosity (Gurley) — 8–30 seconds for 100 ml air. Too low and the sheet bursts from trapped air; too high and contaminants can migrate through. Measured to ISO 5636-5.
• Wet strength — retained strength after water immersion must exceed 35% of dry strength for grades destined for humid warehouse environments. Achieved with melamine-formaldehyde or polyamine-epichlorohydrin (PAE) wet-strength resins.

EUROSAC, the European trade body for multiwall paper sacks, publishes the standard test procedures and benchmark values used across the European sack industry, and their drop-test protocol (filled sack dropped from defined heights onto concrete) is the ultimate empirical check that the paper specs translate into survivable bags.

Variants and family

The sack kraft family splits along a small number of technical axes:

• Extensible sack kraft — Clupak-creped, 4–8% TEA, the dominant grade and the default spec for high-speed filling lines. Represents the large majority of global production WPI-g-000018.
• Non-extensible sack kraft — a straight-run kraft without Clupak treatment, roughly 2% stretch. Cheaper, but restricted to slower filling operations and lighter-duty products.
• Wet-strength sack kraft — with wet-strength resin added at the wet end. Essential for cement and lime sacks stored outdoors on pallets, and for any product with hygroscopic tendencies.
• Multiwall laminated sack kraft WPI-g-000434 — plastic-laminated or aluminium-laminated variants for waterproof or barrier applications. The aluminium-laminated version is used for moisture-sensitive chemicals, pharmaceutical intermediates, and powdered metals.
• Bleached sack kraft — a niche grade with bleached pulp, used for food-contact consumer-facing sacks (premium pet food, specialty flour) where brand aesthetics demand a white sheet.

Buying notes

Three technical red flags on a mill spec sheet worth catching:

TEA below 4%. Some mills market a low-cost "economy sack kraft" with TEA in the 2.5–3.5% range. On a low-speed filling line (say 15 bags/minute), you may survive. On any modern high-speed line running 40–60 bags/minute, you will see burst failures within the first hundred bags. The spec sheet number tells you whether the paper has been through a functioning Clupak stage or not.

Porosity under 8 seconds Gurley. A dense sheet might look high-quality to a buyer comparing burst numbers, but during filling, the trapped air has to escape somewhere. If it can't escape through the paper, it escapes by rupturing the bag. Good sack kraft is deliberately engineered porous. Test the Gurley on a sample lot before switching suppliers.

Inconsistent caliper. Multiwall bag converting lines glue multiple plies together and print on the outer layer; caliper variance of more than about 5% causes print register problems and machine jams on the tuber and bottomer. Ask for a caliper profile across the roll width, not just an average.

FSC/PEFC status and moisture. FSC Mix 70 is the realistic baseline for commodity sack kraft; FSC 100% available at premium from Nordic mills. Delivered moisture must be 6–8% — outside that window, printing and converting run into trouble.

Related reading

• Kraft paper — the complete guide — the parent hub for the kraft family.
• Kraft liner — the corrugated cousin that shares the kraft-pulp lineage but diverges on extensibility.
• Tensile strength — the underlying property that TEA measures as an energy integral.
• Burst strength (Mullen test) — the companion spec to TEA on every sack kraft datasheet.
• GSM — how basis weight is defined and measured.