Kraftliner vs Testliner: Choosing Corrugated Liner Grades

The short answer: kraftliner is made from virgin softwood pulp and wins decisively on burst, tear and performance under moisture. Testliner is made from recovered fibre and, at the right grammage, can compete on the property most shipping cases are actually graded on - edge crush. Which one belongs in your spec depends less on fibre origin than on two things: whether your box is graded on burst or on ECT, and how much moisture the pack will see between your plant and the customer.

A corrugated board is three papers, specified separately

Before the grades make sense, be clear about what is being graded. Every corrugated board is at least three distinct papers glued together:

• Outer liner - the printed face. It takes the ink, carries bending stress and sees the abrasion.
• Fluting medium - the corrugated middle. It carries compression when the case is stacked and absorbs energy when it is dropped.
• Inner liner - the face against the product.

Kraftliner and testliner are liner grades. The medium has its own vocabulary: semichemical fluting (NSSC) is the virgin-fibre medium, and recycled fluting - Wellenstoff in European usage - is the recovered-fibre equivalent. A specification that names a liner grade and leaves the medium unstated is two-thirds written, and the third that is missing is the one carrying the stack load.

What kraftliner is

Kraftliner is made predominantly from virgin softwood pulp produced by the sulphate (kraft) process. The fibres are long and largely undamaged, which is the whole point: long fibres form a sheet with high strength per unit mass and, more importantly, high resistance to the modes of failure that involve a sheet tearing or rupturing rather than buckling.

It comes brown by default. The common variants you will be quoted are unbleached brown kraftliner, white top kraftliner where a bleached top ply gives a print surface without a separate coating, and mottled white where the coverage is deliberately partial. Grammages run wide - roughly 115 to 440 g/m2 across the catalogue - but the everyday shipping range for liners sits between about 125 and 200 g/m2.

What testliner is

Testliner is made from recovered fibre. It is normally multi-ply: the better-sorted, longer recovered fibre goes on the outer ply where appearance and strength are visible, and lower-grade stock sits underneath. That construction is why a good testliner can look and print far better than its average fibre quality would suggest.

Grades are numbered by descending fibre quality - T1, T2, T3, and in some catalogues T4 - with T1 typically carrying a meaningful fraction of virgin or high-grade fibre in the top ply. Schrenz is the lowest grade, essentially unsorted recovered fibre, and belongs in the medium or in light-duty inner liners rather than on a face that has to print. White-top testliner exists and is the usual answer when a recycled-content requirement and a print requirement arrive together.

The properties that actually differ

Fibre origin does not move every property equally. It moves the ones that depend on fibre length and bonding far more than the ones that depend on grammage and sheet density.

Property Test method Favours Burst (Mullen) ISO 2759 Kraftliner, strongly Tear ISO 1974 Kraftliner Ring crush RCT ISO 12192 Grammage-driven; gap narrows Short-span SCT ISO 9895 Grammage-driven; gap narrows Medium flat crush CMT, ISO 7263 Grammage and process driven Humid performance ISO 187 climate Kraftliner Print surface - Either; white top on both

Read that table as a guide to where to look, not as a substitute for data. The only defensible comparison is two mill certificates for the two candidate papers, at the grammages you are actually considering, tested by the same method. Ask for RCT or SCT in both machine and cross direction, not a single headline number.

Burst-graded or ECT-graded - that is the real decision

This is where most liner arguments are actually settled, and most people do not realise the decision has already been made for them.

Burst measures the pressure required to rupture the board. Rupture resistance depends heavily on fibre length and inter-fibre bonding, which is precisely where virgin kraft fibre is strong. A burst-graded specification therefore quietly mandates virgin fibre, whether or not anyone intended that.

ECT measures the compressive load a board edge carries before it buckles, and it is the input that predicts stacking strength. Compression resistance is driven mainly by the compressive strength of the components - RCT and SCT - and by sheet density and grammage. Recovered fibre competes here in a way it cannot compete on burst.

So the practical sequence is: settle the grading basis first, then choose the fibre. If you inherited a burst spec written decades ago for a product that is now stacked on pallets rather than thrown into a rail car, the fibre question is downstream of a spec question you should reopen.

Worked example: what a liner swap costs in board weight

The arithmetic that matters is board grammage, because that is what you buy and what you ship. Board grammage is the two liners plus the medium multiplied by a take-up factor - the medium is longer than the board because it is corrugated.

Flute Nominal take-up factor A 1.54 C 1.43 B 1.36 E 1.27 F 1.23

Take-up varies slightly with the corrugator roll profile, so confirm the factor with your supplier before you cost anything. Now take a single-wall C-flute case:

Case A - kraftliner both faces Outer 150 g/m2 kraftliner Medium 120 g/m2 semichemical Inner 150 g/m2 kraftliner Board 150 + (120 x 1.43) + 150 = 471.6 g/m2 Case B - testliner both faces, grammage raised Outer 175 g/m2 testliner T2 Medium 120 g/m2 recycled fluting Inner 175 g/m2 testliner T2 Board 175 + (120 x 1.43) + 175 = 521.6 g/m2 Delta +50 g/m2 of board

The 25 g/m2 uplift per liner in Case B is a placeholder, not a rule. Replace it with the actual uplift needed to match RCT or SCT from the two mill certificates - it may be smaller, and for some grade pairings at some grammages it may be zero.

Then convert to something you can act on. A case with a blank area of 1.6 m2 weighs 471.6 x 1.6 = 754.6 g in Case A and 521.6 x 1.6 = 834.6 g in Case B, so the swap adds about 80 g of fibre per box. Whether that matters depends on your lane: on a volume-rated shipment it is invisible, on a weight-rated shipment it is not, and on a pallet that is already at a weight limit it can cost you units per layer. Run the number before you assume either direction.

Humidity is where the gap widens

All paper loses strength as it takes up moisture, and corrugated board loses it fast. The certified numbers on any mill datasheet are measured after conditioning at 23 C and 50 percent RH per ISO 187. That climate is a laboratory reference so that results are comparable between mills. It is not your warehouse, your container, or a monsoon-season cross-dock.

Recovered fibre has been through pulping and drying repeatedly. The fibres shorten and stiffen - hornification - so the sheet leans more heavily on refining and on added starch for its strength, and it recovers less completely after a humid cycle. The consequence is that the kraftliner-to-testliner gap you measure at 50 percent RH is not the gap you get at 85 percent RH, and creep under sustained load in a humid store widens it further.

If the pack sees ocean freight, tropical storage, or condensation coming out of cold chain, either specify virgin fibre in the load-carrying faces or specify a moisture-resistant treatment, and derate the board for the climate you actually ship in rather than the one it was tested in.

When not to specify kraftliner

• The spec is ECT-graded and a testliner at the corrected grammage meets it with margin.
• You have a recycled-content requirement from a customer scorecard or a regulatory scheme. Virgin fibre is the wrong tool for that goal.
• The box lives indoors in conditioned space and never sees a humid cycle.
• The real failure mode is geometry - an oversized box, a missing fitment, poor stack alignment. No liner grade fixes a product rattling in a void.
• Regional availability. A grade that is routine in one market can be a special order with a lead time in another, and a spec written to a fibre name rather than a performance number strands you there.

When not to specify testliner

• The spec is burst-graded and cannot be renegotiated. You will chase it with grammage and lose.
• Export, ocean freight, or sustained high-humidity storage where creep governs.
• Direct contact with unpackaged fatty or dry food, where migration from recovered fibre is a documented concern. Use a functional barrier or virgin fibre, and get the food-contact declaration in writing.
• Heavy loads where the board is already at the top of its grade and there is no grammage headroom left to give.

How to write the specification line

Specify performance and test method first, fibre second, and only name a fibre where a property genuinely demands it. That leaves your supplier free to source regionally without renegotiating the whole spec.

Board C flute, single wall Outer liner 150 g/m2 white top testliner, T2 Medium 120 g/m2 recycled fluting Inner liner 150 g/m2 testliner, T2 Performance ECT >= 6.0 kN/m per ISO 3037 Conditioning ISO 187, 23 C / 50 percent RH Moisture Board moisture at delivery, stated range Substitution Any construction meeting the ECT minimum is acceptable on written approval

The ECT figure is yours to set. Derive it from the box compression the stack actually requires and the safety factor your storage climate and stack duration justify - not from a catalogue default.

What to ask the mill before you commit

• Certified grammage and its tolerance band, not the nominal figure alone.
• RCT or SCT in both machine and cross direction, at the exact grammage quoted.
• Burst, if any downstream customer specification still references it.
• CMT for the medium. The medium carries the stack, and it is the component most often left unquoted.
• Ply construction and top-ply furnish for testliner. T2 from two mills is not necessarily the same paper.
• Recycled content as a stated percentage, if you intend to claim it anywhere.
• Food-contact declaration where relevant.
• Confirmation that every figure was measured after ISO 187 conditioning.

Choose the grading basis, get the certificates, do the grammage arithmetic, and derate for the climate. The fibre argument resolves itself once those four things are on the table.