Recycled corrugated base paper (recycled corrugating medium) is the fluted backbone of corrugated packaging, and its quality directly determines the stacking strength, burst resistance, and runnability of corrugated boxes. Professional quality testing evaluates grammage, CMT flat crush, ring crush, short-span compression, moisture, Cobb water absorptiveness, and contamination levels against ISO, GB, EN, ASTM, and JIS standards. This technical guide explains the sampling requirements, testing items, methods, and industrial applications of recycled corrugated base paper quality testing for paper mills, corrugators, and packaging buyers.
Technical Overview
Recycled corrugated base paper, commonly known as recycled corrugating medium, is manufactured almost entirely from recovered paper grades such as old corrugated containers (OCC), mixed waste paper, and cardboard cuttings. It forms the fluted middle layer of corrugated board, where it provides the vertical compression resistance that ultimately determines box stacking strength. Because the fiber source varies continuously, the properties of recycled medium fluctuate far more than those of virgin fiber grades, making systematic quality testing indispensable.
The dominant product specification in China is GB/T 13023, which classifies corrugated base paper into grades and sets minimum requirements for grammage, bursting strength, ring crush resistance (RCT), flat crush resistance after laboratory fluting (CMT), moisture content, and ash. Internationally, test results are harmonized through methods published by ISO, EN, ASTM, and JIS, allowing mills, converters, and buyers to compare data on a common basis. Accredited third-party laboratories routinely perform these evaluations for supplier qualification, incoming inspection, trade documentation, and dispute resolution.
From an engineering perspective, the key performance indicators of recycled medium—CMT, RCT, and short-span compression strength (SCT)—correlate strongly with the edgewise crush strength of finished corrugated board and, through established box-compression models, with the compressive performance of the shipping container. Chemical and cleanliness indicators such as ash content, stickies, pH, fluorescent whitening agents, and heavy metals govern printability, machine runnability, and regulatory compliance, particularly for food-contact and export packaging.
Testing Samples
Production Roll and Lot Samples
Sampling follows ISO 186 and the corresponding Chinese sampling standard GB/T 450. Full-width strips are taken from finished reels after discarding the outer damaged wraps, so that properties can be assessed across the entire machine profile (front side, center, and back side). For lot inspection, the number of reels or sheets sampled is determined statistically from the consignment size, and each sample is labeled with mill identification, production date, grammage, and machine direction to preserve traceability.
Conditioned Laboratory Specimens
All test pieces are cut with precision guillotines or strip cutters in both machine direction (MD) and cross direction (CD), then conditioned to equilibrium at (23 ± 1) °C and (50 ± 2) % relative humidity per ISO 187 and GB/T 10739 for a minimum of 24 hours before measurement. Typical specimen formats include 100 cm² squares for grammage, 12.7 mm wide strips for RCT, 15 mm wide strips for SCT, 12.7 mm × approximately 152 mm strips for CMT fluting, and circular discs for bursting strength.
Sample Handling for Moisture-Sensitive Tests
When as-received moisture content must be verified, samples are immediately sealed in moisture-proof bags at the sampling point and weighed on arrival at the laboratory before conditioning. This prevents moisture exchange with the atmosphere and provides a true picture of the reel condition as delivered to the corrugator.
Testing Items
Structural and Basis Properties
- Grammage (basis weight), typically 80–200 g/m² for commercial medium grades
- Caliper (thickness) and apparent density
- Moisture content, normally specified as 8% ± 2% at delivery
- Ash content, reflecting filler and inorganic residue carried over from recovered paper
Strength Properties
- Concora Medium Test (CMT30 and CMT60) — flat crush resistance after laboratory fluting
- Ring Crush Test (RCT) in both MD and CD
- Short-Span Compressive Test (SCT) in both directions
- Bursting strength and burst index
- Tensile strength, breaking length, and stretch at break
- Tearing resistance
Surface and Water-Interaction Properties
- Cobb 60 water absorptiveness, critical for glueability of the flute tips
- Brightness and color (L*, a*, b*) for visual consistency
- Surface roughness and absorbency where printing is applied to the medium
Contamination, Chemical, and Safety Items
- Stickies and plastics content of the recycled furnish (ISO 15360 series)
- pH of cold-extract and surface extract
- Fluorescent whitening agents, screened under UV 365 nm illumination
- Heavy metals (lead, cadmium, mercury, hexavalent chromium) by ICP-MS or ICP-OES
- Fiber furnish composition by optical microscopy
- Microbiological load and odor for food-contact applications
Testing Methods
All mechanical tests are performed on calibrated instruments in a temperature- and humidity-controlled laboratory. The table of core methods below maps each property to the applicable internationally recognized procedures.
- Grammage: ISO 536 / GB/T 451.2 / ASTM D646 — conditioned specimens of known area are weighed on a 0.001 g analytical balance and expressed in g/m²
- Moisture content: ISO 287 / GB/T 462 — oven drying at (105 ± 2) °C to constant mass; moisture loss is calculated as a percentage of the original mass
- Bursting strength: ISO 2758 (paper) or ISO 2759 (board) / GB/T 454 / ASTM D774 — a hydraulic or pneumatic diaphragm applies increasing pressure to a clamped disc until rupture
- CMT: ISO 7263 / GB/T 2679.6 — a conditioned strip is fluted between the rollers of a laboratory fluter, placed in a compression tester with a guide, and crushed flat between parallel platens 30 seconds (CMT30) or 60 seconds (CMT60) after fluting; result reported in newtons
- RCT: ISO 12192 / GB/T 2679.8 — a 12.7 mm wide strip is supported in an annular ring holder and loaded edgewise until failure; the peak force in newtons is reported for MD and CD
- SCT: ISO 9895 — a 15 mm wide strip is compressed over a very short 0.7 mm span, isolating the pure compressive failure of the fiber network in N/mm
- Cobb 60: ISO 535 / GB/T 1540 — one side of a 100 cm² specimen is exposed to water for 60 seconds; the mass gain in g/m² measures water absorptiveness
- Ash content: ISO 2144 / GB/T 463 — combustion of specimens at (900 ± 25) °C with the residue expressed as a percentage of the oven-dry mass
- pH: ISO 6588-1 / GB/T 1545 — cold-extraction of specimen suspensions followed by electrometric measurement
- Stickies and plastics: ISO 15360-1 and ISO 15360-2 — image-analysis based quantification of tacky and polymeric contaminants in recycled pulp
- Fiber composition: ISO 9184 / GB/T 4688 — fiber staining and optical microscopy to identify hardwood, softwood, and non-wood fibers
- Fluorescent whitening agents: UV 365 nm screening, with food-contact conformity assessed per EN 648 (no positive fluorescence permitted)
- Heavy metals: acid microwave digestion followed by ICP-MS or ICP-OES, with the combined Pb + Cd + Hg + Cr(VI) limit of 100 mg/kg applied in line with European packaging legislation
For certification purposes, results are compared with the grade tables of GB/T 13023 or with contractual specifications agreed between supplier and customer. Retained specimens and raw data are archived so that any disputed result can be re-examined in an accredited third-party laboratory if required.
Applications
Quality testing of recycled corrugated base paper supports decision-making at every stage of the packaging value chain.
- Mill process control: continuous verification of CMT, RCT, and ash against grade targets during stock preparation and on the paper machine
- Grade certification: demonstrating conformity with GB/T 13023 or customer specifications for commercial release of each production lot
- Incoming inspection at corrugating plants: confirming grammage, moisture, and crush resistance before reels are committed to the corrugator, reducing warp, crush failures, and bonding defects
- Box design input: SCT and RCT data feed edgewise crush and box compression calculations used to design single-wall, double-wall, and heavy-duty containers
- Export and food-contact compliance: chemical and hygienic testing supports declarations for packaging intended for direct or indirect food contact and for shipments into regulated markets
- Trade settlement and dispute resolution: independent third-party test reports provide an objective basis for claims, price adjustments, and arbitration between trading partners
- Research and development: benchmarking recycled furnish recipes, refining strategies, and chemical additives to raise strength while maintaining high recovered-fiber content
FAQ
What is the difference between CMT and RCT?
CMT measures the flat crush resistance of the medium after it has been fluted in a laboratory fluter, so it directly simulates the performance of the flute structure in the board. RCT measures the edgewise compressive strength of an unfluted strip supported in a ring holder and reflects the intrinsic compression resistance of the paper. Both correlate with box stacking strength, and specifications normally quote both values.
What do CMT30 and CMT60 mean?
The numbers indicate the elapsed time between fluting and compression: 30 seconds (CMT30) or 60 seconds (CMT60). Because the fluted structure relaxes over time, CMT60 values are lower than CMT30 values for the same paper. Reports must always state which variant was used.
Why is moisture content so critical?
The compression strength of recycled medium falls sharply as moisture rises above the normal 6–10% window, while overly dry paper becomes brittle and generates dust on the corrugator. Moisture testing on delivery, combined with proper reel storage, protects both runnability and final box strength.
Can recycled corrugated base paper be used for food packaging?
Yes, provided the furnish is controlled and the finished paper passes the applicable safety assessments, including fluorescent whitening agent screening per EN 648, heavy metal limits, microbiological and odor evaluation, and any migration requirements of the target market. Functional barriers such as liners or coatings are often applied for direct food contact.
Who should perform the testing?
Paper mills and corrugators operate in-house laboratories for routine control, while accredited third-party laboratories are engaged for grade certification, supplier qualification, export documentation, and independent verification in commercial disputes. Using internationally standardized methods (ISO, GB, EN, ASTM, JIS) ensures that results are comparable across all parties.