CUK (Coated Unbleached Kraft).

CUK — Coated Unbleached Kraft — is the brown-core folding-carton paperboard. Take a finished CUK carton, slice through it with a sharp knife, and the tell is immediate: the core is brown all the way through, top ply to bottom, because the pulp is unbleached kraft and the natural lignin colour has never been removed WPI-g-000893. What the consumer sees is the coated top ply — a clay-coated printable surface, typically at ISO brightness 72–80%, that carries lithographic print, foil stamping, and embossed graphics as cleanly as any bleached board. What the consumer does not see is the long-fibre southern-pine kraft that gives the carton its wet-strength and tear resistance. Three folding-carton families compete for shelf space: SBS (Solid Bleached Sulfate, white core throughout, premium tier) WPI-g-000891, FBB (Folding Boxboard, with a grey mechanical-pulp core visible at cut edges) WPI-g-000892, and CUK. Where wet-strength, tear resistance, and durability matter more than whiteness — where the package will meet condensation, humidity, or rough handling — CUK is the structural answer.

What it's used for

The defining application is beverage multipack carriers. The classic 6-pack, 8-pack, and 12-pack carriers for beer and soda — the paperboard handle-and-keel constructions that wrap cans for Budweiser, Coca-Cola, Heineken, Miller, PepsiCo, and essentially every major North American beverage brand — are almost universally CUK. The reason is physical: a cold can condenses atmospheric moisture, the condensate runs down onto the carton, and the carton has to hold can weight without tearing at the handle die-cut. Unbleached kraft fibres, long and lignin-rich, retain tensile strength when wetted far better than bleached fibres. Graphic Packaging International alone produces several million tonnes per year of CUK for this single application category.

Frozen-food outer cartons are the second major category. Microwave-ready cartons, frozen-dinner sleeves, frozen-pizza boxes, and ice-novelty multipacks from brands like Kraft Heinz, Tyson, Nestlé, and Conagra are commonly CUK — the freezer-to-ambient temperature cycling creates condensation that bleached boards tolerate poorly. The coated top ply accepts four-colour litho and matches SBS visually at retail shelf distance; the unbleached core handles the moisture load.

Heavy-duty consumer-electronics cartons — the outer folding cartons around power tools, small appliances, and big-box hardware items — favour CUK for its tear resistance and the economic advantage of unbleached pulp. Pet-food multipack boxes and hardware-store outer packaging round out the category. Industry estimates put CUK and natural-kraft folding boards at roughly 40% of the US folding-carton market by tonnage. European share is far smaller — under 10% — because FBB from Nordic and Central European mills has dominated the European carton landscape for half a century, and the regulatory climate increasingly disfavours unbleached boards for food-contact applications.

Origins and history

CUK emerged in the 1960s and 1970s in North American mills as a direct outgrowth of existing kraft linerboard production WPI-g-000888. The logic was industrial: Southern-pine mills in Georgia, Alabama, South Carolina, Louisiana, and Virginia were already producing virgin unbleached kraft pulp at enormous scale for linerboard and multi-wall sack kraft. Building a coated board grade from the same furnish required only the addition of multi-ply forming headboxes and a clay-coating station — incremental capital on top of an installed pulp base. Mead Paper (headquartered in Dayton, Ohio, and operating southern-pine mills including Stevenson, Alabama), Westvaco (West Virginia Pulp and Paper, whose Covington, Virginia, and Charleston, South Carolina, mills were flagship producers), Georgia-Pacific, and International Paper's Riegelwood, North Carolina, facility led the commercialisation.

The specific commercial driver was the rise of the paperboard six-pack beer carrier through the 1950s and 1960s. Before then, beer was retailed in wooden cases, metal-wire handle constructions, or heavy-waxed corrugated. The paperboard carrier — lighter, recyclable, and able to carry four-colour brand graphics — required a substrate that combined kraft's wet-strength with a litho-grade print surface. Clay-coating the top ply of a multi-ply brown-kraft board turned out to be the elegant solution. Mead's "Container Industry" division and Westvaco's packaging mills drove what became the US standard, and by the late 1970s the board was referred to interchangeably as CUK, CNK (Coated Natural Kraft), or simply "coated kraft folding board" in US industry literature.

Two nomenclature points are worth fixing. First: European industry uses SUS (Solid Unbleached Sulphate) for what US industry calls CUK — the same grade, different abbreviation, reflecting the European preference for naming grades by pulp family. Second: CNK (Coated Natural Kraft) appears frequently in North American spec sheets and is used effectively as a synonym for CUK, though some mills reserve "CNK" for a slightly lighter basis-weight range (200–260 g/m²) and "CUK" for the full folding-carton range up to 350 g/m².

Today the US producers are Graphic Packaging International (the volume leader, with mills in Macon, Georgia, and Augusta, Georgia), WestRock (the merged successor to Mead and MeadWestvaco), and Packaging Corporation of America (PCA). European production is concentrated at Billerud (Sweden, the Gruvön and Skärblacka mills), and historically at Mondi Frantschach (Austria) and BillerudKorsnäs before its rename. Brazilian and Chilean kraft producers have entered the space at the lower-cost end using eucalyptus-softwood blends.

How it's made

The furnish is virgin unbleached kraft pulp, produced by the same kraft (sulphate) process described for kraft linerboard — sodium sulphide and sodium hydroxide cooking at 165–175 °C, dissolving lignin while preserving the cellulose fibre length. For CUK the wood is predominantly southern yellow pine (loblolly, slash, longleaf, shortleaf), which delivers tracheid fibres 3–5 mm long with thick cell walls — the structural basis for high tear and wet-strength performance. Middle plies on some commercial CUK grades incorporate secondary fibre (high-quality OCC or mill broke) to reduce cost without compromising the outer-ply strength.

The key chemical point is what the mill does not do: there is no bleaching stage. No oxygen delignification, no chlorine dioxide sequence, no alkaline peroxide. The pulp is washed, screened, refined, and sent directly to the board machine. The brown tone is unoxidised residual lignin — 3–6% by dry weight depending on the kappa target — and it is a structural asset, not a defect. Bleaching degrades cellulose hemicelluloses and shortens average fibre length by 8–15%, which is why bleached kraft (used in SBS) produces a measurably weaker sheet per unit basis weight than unbleached kraft at the same furnish.

The board machine is a multi-ply Fourdrinier, typically running three to five plies through separate headboxes. A representative construction: a top ply of well-refined unbleached softwood kraft (0.5–0.8% consistency, targeted for coating holdout), one or two middle plies of a kraft-plus-secondary-fibre blend, and a bottom ply of unbleached kraft for reverse-side strength. Machine speeds run 400–700 m/min — slower than linerboard because multi-ply forming is more delicate and the coating downstream sets a throughput ceiling.

Coating is applied on-machine or at an offline coater at 15–20 g/m² per side on the top ply (single-coat C1S is the dominant construction; double-coated top for premium print work is available at 20–30 g/m² total). The coating formulation is kaolin clay plus ground calcium carbonate in a styrene-butadiene or styrene-acrylate latex binder — similar chemistry to SBS coatings, with the key difference that CUK coatings are often loaded with more TiO₂ (titanium dioxide) to mask the dark substrate and lift surface brightness. The bottom ply remains brown and uncoated. See /glossary/brightness-vs-whiteness for why coating brightness on CUK is measured differently than on bleached boards.

Specs that distinguish it

The numbers that separate CUK from SBS and FBB at the point of purchase:

• Basis weight — 200–350 g/m². Beverage carriers run 230–280; frozen-food outer cartons 260–310; heavy industrial outer cartons 310–350.
• ISO brightness (coated top) — 72–80% (ISO 2470-1). The coating brightens the substrate dramatically — naked kraft brown base-sheet brightness is roughly 20–35%, so the coating is doing 40–50 ISO points of lift.
• ISO brightness (reverse) — 20–35%. The uncoated brown reverse is the visible signature of CUK at cut edges and inside assembled cartons.
• Caliper — 0.25–0.50 mm depending on basis weight (ISO 534).
• Bending stiffness (Taber) — comparable to SBS per unit weight at equivalent basis weight, typically 14–36 mN·m across the 230–320 g/m² range (TAPPI T-543). CUK does not sacrifice stiffness for its strength advantage.
• Tear strength (Elmendorf) — typically 2–3× SBS at equivalent basis weight (TAPPI T-414). This is the headline structural advantage.
• Wet-strength — significantly higher than SBS at equivalent basis weight; specific mill figures vary but a 10-minute soak retention of 30–40% of dry tensile is typical for CUK versus 15–25% for bleached SBS.
• Smoothness (PPS 10) — 1.0–2.0 µm on the coated top; suitable for offset litho and most high-quality graphic work.

The commercial comparison: CUK usually runs 10–15% below SBS on price at equivalent basis weight, and offers structural tear and wet-strength that SBS cannot match. It loses on white-on-white aesthetics — the brown core is visible at every cut and scored edge — and on regulatory headroom for food-contact in Europe.

Variants and family

The CUK family splits along coating and top-layer lines:

• C1S CUK (Coated One Side) — the dominant commercial construction. Coated print top, uncoated brown reverse. Standard for beverage carriers, frozen-food outer cartons, pet-food boxes, and hardware multipack boxes. 80–85% of CUK tonnage.
• C2S CUK (Coated Two Sides) — coated both faces, used where the interior of the assembled carton must also be printable or requires a controlled surface finish. Premium pricing; niche category.
• Uncoated natural kraft folding board — no coating at all, used where the rustic brown aesthetic is a deliberate brand choice (craft beer, artisan foods, premium direct-to-consumer subscription boxes).
• White-top CUK — the "CUK with a secret." A bleached white pulp top ply (30–60 g/m²) laminated over the unbleached kraft body, giving a brighter and whiter print surface — ISO brightness 82–86% — while retaining the brown core and its structural properties. Popular for premium beverage carriers and for brands migrating from SBS seeking cost relief without visible change to retail shelf appearance. The edge gives it away: cut the carton and the core is still brown.

Buying notes

Three red flags worth catching on a mill spec sheet or first-container inspection:

Inconsistent coating brightness. Unbleached base-sheet brightness is inherently more variable than bleached stock — batch-to-batch kappa-number variation of ±2 points translates to visible differences in the underlying substrate that the coating has to mask. Insist on a contractual minimum coated-top ISO 2470 brightness (typically 74% as the audit floor for lithographic print work) and pull samples from every fifth shipped pallet.

UV-driven yellowing. Residual lignin in the unbleached core absorbs UV and oxidises over time, yellowing further. For retail packaging with months of shelf life under fluorescent or LED lighting the effect is usually negligible; for long-lifecycle applications (archival boxes, display packaging with open-ended shelf life) it is a real failure mode. A light-exposure accelerated-ageing test to ISO 105-B02 is worth running on any CUK intended for long-term display.

Recycled-content claims. "SUR-certified" CUK (Sustainable Unbleached Responsible, an industry voluntary label) typically carries 0% post-consumer recycled content — the branded labels most consumers read as "recycled" refer to virgin-sourced certification, not recycled fibre. If genuine recycled content matters to your brand claim, insist on a chain-of-custody statement with specific fibre-source percentages, and remember that adding recycled content to CUK reduces the tear and wet-strength advantage that justified CUK selection in the first place.

One more context note for European buyers. The EU regulatory environment is tightening around unbleached board for direct food-contact — recent BfR Recommendation XXXVI updates and EFSA opinions on mineral-oil-hydrocarbon (MOH) migration have been less favourable to unbleached grades than to bleached. For primary food-contact cartons in the EU, FBB or SBS remains the safer regulatory choice. For secondary packaging — outer cartons, multipack carriers — CUK remains fully acceptable.

Related reading

• Brightness vs whiteness — why ISO brightness on a coated unbleached board reads differently from the same number on a bleached board.
• Kraft paper — the complete guide — the parent hub for the kraft family, covering the chemistry shared across CUK, kraft liner, and extensible sack kraft WPI-g-000018.
• SBS — Solid Bleached Sulfate — the premium white-core alternative; the primary cross-shop grade for CUK buyers.
• FBB — Folding Boxboard — the European mid-tier competitor with mechanical-pulp middle plies.
• Kraft Liner — CUK's industrial cousin; same furnish, different construction and end use.

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