Performance Evaluation of Secondary Fiber Corrugating Medium: Test Items, Methods, and Standards

Secondary fiber corrugating medium — the fluted core paper produced mainly from recovered paper such as old corrugated containers (OCC) — is the structural backbone of corrugated packaging, and its performance directly determines box compression strength and runnability. A systematic performance evaluation covering grammage, moisture, tensile strength, ring crush, short-span compression, and Concora flat crush properties verifies conformity with specifications such as GB/T 13023 and the ISO 7263 series. This article explains the sample types, key testing items, standard test methods, and practical applications involved in evaluating recycled-fiber corrugating base paper.

Technical Overview

Corrugating medium (also known as corrugating base paper or fluting paper) is the corrugated middle layer bonded between linerboards to form the flute structure of corrugated board. Medium manufactured from secondary fiber relies predominantly on recovered paper streams such as old corrugated containers (OCC), old kraft bags, double-lined kraft clippings (DLK), and mixed waste paper, typically in grammages ranging from about 80 g/m² to 200 g/m².

Because each recycling loop shortens fibers, increases fines content, and raises ash and contaminant levels, 100% secondary-fiber furnishes deliver lower and more variable strength than virgin fiber. For this reason, performance evaluation is not a single measurement but a coordinated program of physical, mechanical, water-interaction, and compositional tests. In the Chinese market, product conformity is judged against GB/T 13023, which assigns grade classifications based on grammage-dependent minimum requirements for flat crush (CMT30), burst, tensile, and moisture, while international trade commonly references ISO methods and, for certain indices, ASTM and EN standards. Testing is typically performed in mill laboratories for process control and by accredited third-party laboratories for conformity assessment, certification, and dispute resolution.

Testing Samples

Production and Process Control Samples

Full-width strips or reel-end samples taken at scheduled intervals from the paper machine, representing each production batch, shift, or grade change. These samples support statistical process control of grammage, moisture, CMT/CCT, and strength indices before shipment.

Incoming and Outgoing Inspection Samples

Lots submitted by suppliers or presented to customers, sampled according to ISO 186 or GB/T 450 to obtain representative test pieces from reels or pallets of sheets for acceptance testing against contractual specifications.

Conformity Assessment and Market Surveillance Samples

Random samples drawn from finished goods or the open market for testing by accredited third-party laboratories, supporting product certification, regulatory supervision, and verification of recycled-content or grade claims.

Research and Benchmarking Samples

Laboratory handsheets and machine-trial papers from furnish optimization programs (for example, OCC fractionation, blending strategies, or strength additives), together with competitor benchmark samples, used to correlate furnish changes with end-use performance.

Testing Items

A comprehensive evaluation of secondary fiber corrugating medium typically includes the following indices:

• Grammage (basis weight) and caliper (thickness), which govern flute geometry and board weight
• Moisture content of the paper as delivered and after conditioning
• Tensile strength, elongation at break, and tensile energy absorption in machine and cross directions (MD/CD)
• Bursting strength
• Ring crush strength (RCT) in MD and CD
• Short-span compressive strength (SCT) in MD and CD
• Concora Medium Test flat crush after laboratory fluting (CMT30/CMT60) and the corrugated crush test (CCT30)
• Water absorptiveness (Cobb 60) and related surface sizing behavior relevant to adhesive bonding
• Air permeability, which influences corrugator speed and glue penetration
• Folding endurance (MIT) for evaluating flexibility and crack resistance during fluting
• Brightness and visual cleanliness for applications where the medium remains exposed
• Ash content on ignition, an indicator of filler and inorganic residue carried over from recycled furnish
• Fiber furnish composition, average fiber length, fines content, and contaminant counts (dirt, stickies, plastics)

Testing Methods

Sampling and Conditioning

Samples are taken per ISO 186 or GB/T 450, and test pieces are conditioned to equilibrium at 23 °C ± 1 °C and 50% ± 2% relative humidity in accordance with ISO 187 or GB/T 10739 before all physical testing. Grammage is determined per ISO 536 / GB/T 451.2 (or ASTM D646), caliper per ISO 534 / GB/T 451.3, and moisture content by oven drying per ISO 287 / GB/T 462.

Strength and Compression Testing

Tensile properties (strength, strain, and tensile energy absorption) are measured on constant-rate-of-elongation testers per ISO 1924-2 / GB/T 12914 or ASTM D828. Bursting strength is determined per ISO 2758 / GB/T 454 (or ASTM D774). Compressive performance — the defining characteristic of corrugating medium — is evaluated by ring crush per ISO 12192 / GB/T 2679.3, short-span compression per ISO 9895 / GB/T 2679.10, Concora flat crush after laboratory fluting per ISO 7263-1, and the corrugated crush test per ISO 7263-2. Folding endurance follows ISO 5626 / GB/T 457.

Water Interaction and Structure-Related Tests

Water absorptiveness by the Cobb method (Cobb 60) is measured per ISO 535 / GB/T 1540, while Bendtsen air permeance is determined per ISO 5636-3 / GB/T 458. These results predict behavior at the corrugator glue station, where rapid but controlled adhesive penetration is essential for strong bonds at high production speeds.

Optical, Compositional, and Furnish Analysis

Brightness is measured per ISO 2470-1 / GB/T 7974; ash content on ignition per ISO 2144 / GB/T 742; fiber furnish composition by staining and microscopic analysis per ISO 9184 / GB/T 4688; dirt in paper per ISO 15755; and stickies and plastics in recycled pulps per ISO 15360. Freeness of the recycled pulp may additionally be characterized per ISO 5267-1/-2 to support furnish optimization.

Applications

Performance evaluation data for secondary fiber corrugating medium support decisions across the entire packaging value chain:

• Mill quality control and grade certification: confirming that production meets GB/T 13023 grade requirements and internal release limits
• Incoming inspection at corrugating plants: verifying supplier consistency and calculating safety margins for flute take-up and bonding
• Board and package design: using SCT and CMT/CCT values to model edge crush (ECT) and box compression strength for transport packaging
• Trade and contract settlement: providing objective, standardized evidence when lots are disputed between buyers and sellers
• Regulatory conformity and sustainability claims: substantiating recycled-content declarations and market surveillance compliance through accredited third-party laboratories
• Research and development: quantifying the effect of furnish changes, refining strategies, and chemical programs on the strength of recycled fiber

FAQ

Why is the Concora Medium Test (CMT) considered the most representative indicator for corrugating medium?

CMT measures the flat crush resistance of the medium after it has been fluted under standardized laboratory conditions, so it directly simulates the paper's primary function in service. It integrates fiber quality, formation, and bonding potential into a single value, which is why GB/T 13023 uses CMT30 as the principal strength criterion for grade classification.

How does a 100% secondary fiber furnish influence test results?

Recycling shortens and hornifies fibers, reducing inter-fiber bonding. Compared with virgin medium, recycled grades generally show lower RCT, SCT, tensile, and burst values, higher ash, and greater lot-to-lot variability. Mills compensate through OCC sorting and cleaning, long-fiber fractionation, gentle refining, strength additives, and strategic blending with a proportion of virgin or high-quality recovered kraft fiber.

What is the difference between RCT, SCT, CMT, and CCT?

RCT measures the edgewise compressive resistance of a flat specimen supported in a ring; SCT measures compressive strength over a very short (0.7 mm) free span and reflects intrinsic fiber network strength; CMT measures flat crush resistance after laboratory fluting; and CCT measures the edgewise compression of a laboratory-fluted specimen held vertically. All four correlate with the stacking performance of finished corrugated boxes, and test programs often run them together to build a complete compressive profile.

Why is conditioning so important when testing recycled paper?

Paper strength is strongly humidity-dependent, and secondary fiber medium with higher fines and ash content can be particularly sensitive. All physical tests must be performed on specimens equilibrated at 23 °C and 50% RH per ISO 187 / GB/T 10739; otherwise results may shift by a double-digit percentage and cannot be compared between laboratories.

Can 100% recycled corrugating medium meet high-grade specifications?

Yes. With high-quality OCC furnish, efficient screening and cleaning to remove stickies and ash, fractionation of long fiber, and appropriate chemical programs, modern mills routinely produce 100% secondary fiber medium that satisfies the highest CMT and RCT grades of GB/T 13023, and the evaluation program described here provides the objective evidence to prove it.