What Is Kraft Cardstock? Specs, GSM, & B2B Uses

What Is Kraft Cardstock? An Engineer’s Technical Breakdown

what is kraft cardstock – Biodegradable Barrier Coating and Fiber Pulp Molding (TadaPack Engineering Guide)

Kraft cardstock is an unbleached paperboard produced through the sulfate (kraft) pulping process, where wood chips are cooked in a sodium hydroxide and sodium sulfide solution. Unlike bleached grades, the lignin is not fully removed, giving the board its characteristic tan-to-brown color and superior fiber-to-fiber bonding. For B2B buyers, the defining property is strength-to-weight: kraft board routinely outperforms comparable-grammage bleached board in tensile, tear, and burst performance. It is the substrate of choice for rigid mailers, folding cartons, retail hang tags, and as linerboard in corrugated constructions.

This article belongs to our pillar on Packaging Materials & Processes, which covers the full substrate decision tree.

Manufacturing: Why the Sulfate Process Wins

The kraft (German/Swedish: Kraft, meaning “strength”) process dissolves lignin while preserving cellulose fiber length—typically 2–3 mm for softwood furnish versus under 1 mm in mechanical pulps. Longer fibers translate directly into higher tear resistance (Elmendorf) and fold endurance, critical for score-and-fold carton lines running above 300 units/min.

Unbleached grades retain 90–95% natural lignin. Bleached kraft (used in food-contact cartons) sacrifices roughly 10–15% strength to achieve whiteness above 85% ISO brightness. Most 2026 procurement specifications therefore split the difference with high-yield or semi-bleached grades for premium graphics, keeping unbleached for structural layers.

Kraft Cardstock Specifications That Matter

When writing a spec sheet, engineers evaluate four core metrics, all governed by TAPPI Standard Test Methods for Paper & Board:

• Grammage (GSM / basis weight): 200–450 GSM for cardstock; above 450 GSM boards are typically classified as paperboard or boxboard.
• Caliper: 0.25–0.60 mm (10–24 points), measured per TAPPI T 411.
• Bursting strength (Mullen): 350–900 kPa depending on furnish; measured per TAPPI T 810.
• Short-span compression (SCT): 2.0–4.5 kN/m — the best predictor of box crush performance.

When kraft cardstock is converted into linerboard for corrugated constructions, the same fiber metrics drive corrugated flutes selection and resulting ECT ratings (e.g., 32 ECT C-flute = ~4.8 mm flute height).

Virgin vs. Recycled Kraft: 2026 Procurement Reality

Virgin kraft delivers 15–20% higher burst and tensile values than recycled grades because recycled fibers shorten and lose bonding capacity with each loop (typically 5–7 usable cycles). However, Extended Producer Responsibility (EPR) fee modulation and the EU Packaging and Packaging Waste Regulation (PPWR) now economically favor recycled-content and mono-material board structures. Many 2026 state-level EPR programs in the US (Oregon, Colorado, California starting 2027 fee collection) apply eco-modulated fees that penalize plastic lamination—a clear tailwind for uncoated or aqueous-coated kraft board.

The tension between recycled-content mandates and performance requirements is analyzed in depth in our guide on Sustainable Packaging Growth Challenges. Where a rigid plastic insert must be replaced, consider hybrid approaches reviewed in our recycled post-consumer polypropylene sourcing guide.

Barrier Coatings and Convertibility

Uncoated kraft cardstock is inherently grease-permeable, limiting foodservice use. 2026 specification trends favor:

• Aqueous dispersion coatings: repulpable, kerbside-recyclable, ~8–12 GSM add-on.
• PFAS-free grease barriers: required under state bans (effective in 10+ US states by 2026).
• PLA or bio-wax coatings: compostable but industrial-compost-only.

Convertibility metrics: kraft cardstock scores and folds cleanly at 0.30 mm caliper with a standard creasing matrix; above 0.45 mm, die-cut crease depth must increase ~15% to avoid fiber cracking on the print side. Digital toner and offset inks adhere well to unbleached surfaces; expect ~5% ink coverage increase versus white top grades for equivalent opacity.

Cost Benchmarks

Indicative 2026 FOB pricing (Asia/US mills, per short ton): virgin kraft linerboard $780–$920; recycled kraft board $640–$760; food-grade aqueous-coated kraft $1,050–$1,250. Convert per-unit: a 350 GSM A4 rigid mailer runs $0.11–$0.16 at 50,000-unit volumes.

Bottom Line

Kraft cardstock is not a commodity choice—it is a performance substrate whose fiber physics (long cellulose, retained lignin) deliver structural strength that bleached and recycled boards cannot match at equal grammage. Specify GSM, caliper, Mullen, and SCT with TAPPI method references, and validate EPR fee exposure before locking furnish percentages.