Secondary Fiber Corrugated Paper Grammage Testing: Methods, Standards, and Applications

Grammage determination is the most fundamental quality control test for secondary fiber corrugated paper, measuring the basis weight (mass per unit area) of recycled corrugated medium in grams per square meter (g/m²). Accurate grammage testing ensures that corrugated paper produced from recovered fibers conforms to the specification limits established in international and national standards such as ISO 536 and GB/T 451.2, which directly governs board strength, packaging performance, and material cost. This article explains the technical principles, sampling requirements, test procedures, and practical applications of grammage determination for secondary fiber corrugated paper.

Technical Overview

Secondary fiber corrugated paper, also known as recycled corrugated medium or fluting, is manufactured predominantly from recovered paper stocks such as old corrugated containers (OCC), mixed waste paper, and sorted de-inked grades. Because the furnish composition and fiber quality of recycled raw materials vary significantly between batches, the grammage of the finished sheet must be tightly controlled to guarantee consistent performance in corrugated board production.

Grammage, also referred to as basis weight or substance, is defined as the mass per unit area of paper or board expressed in grams per square meter (g/m²). It is the single most fundamental property specified for corrugated medium: nominal grammage values for common grades typically range from approximately 80 g/m² to 200 g/m², and nearly all key strength indicators, including ring crush, flat crush (CMT), short-span compressive strength (SCT), and bursting strength, are evaluated relative to grammage. Determination is performed in accordance with ISO 536, GB/T 451.2, and ASTM D646, while the product itself is graded against product specifications such as GB/T 13023 for corrugated paper.

Compared with virgin-fiber papers, secondary fiber sheets exhibit more pronounced grammage variation because recycled furnish contains residual adhesive fragments, fillers, coatings, and short fibers that alter drainage behavior on the paper machine wire. Systematic grammage testing therefore underpins process control at the mill, product certification, and commercial settlement between paper mills, corrugated converters, and end users.

Testing Samples

Corrugated Medium Rolls and Sheets from Production

Representative samples are drawn from production rolls or cut sheets following the sampling procedures of ISO 186 or GB/T 450. Full-width strips taken across the machine direction allow evaluation of both the average grammage and the cross-direction grammage profile. Sampling positions must avoid damaged edges, splices, wrinkles, and areas visibly affected by moisture, and the samples must be protected from contamination and physical deformation during transport to the laboratory.

Component Plies Separated from Finished Corrugated Board

To verify the grammage of recycled medium incorporated into finished corrugated board, the component plies are separated by controlled water soaking or steaming until the adhesive bond weakens. Residual adhesive and liner fibers are carefully removed from the fluting surface, the separated ply is dried without stretching or damaging the sheet, and it is then reconditioned in the standard atmosphere before measurement. This approach is widely used for incoming inspection, dispute resolution, and reverse engineering of board constructions.

Laboratory Handsheets and Pilot Trial Samples

Handsheets prepared from recycled pulp and sheets from pilot machine trials are tested to evaluate furnish blends, optimize secondary fiber proportions, and predict the grammage behavior of full-scale production before committing raw materials.

Testing Items

Grammage determination for secondary fiber corrugated paper typically covers the following measurable items:

• Nominal grammage (g/m²): the average mass per unit area of conditioned specimens.
• Grammage deviation: the difference between the measured average and the nominal value, compared against the tolerance specified in the product standard or purchase contract.
• Cross-direction grammage profile: variation across the web width, used to detect forming, pressing, or drying non-uniformity.
• Machine-direction consistency: variation among specimens taken from different positions along the roll length.
• Conditioned-moisture basis: mass determined after equilibrium conditioning in the standard atmosphere, ensuring results are comparable between laboratories.
• Coefficient of variation: the statistical uniformity of grammage across the entire sample, reflecting overall process stability.

Testing Methods

Sampling and Conditioning

Samples are selected in accordance with ISO 186 or GB/T 450 so that they are statistically representative of the lot. Prior to testing, specimens are conditioned in the standard atmosphere specified by ISO 187 or GB/T 10739, namely 23 °C ± 1 °C and 50% ± 2% relative humidity, until equilibrium moisture content is reached. Conditioning is essential because recycled corrugated paper is highly hygroscopic, and unconditioned weighings can deviate by several percent depending on ambient humidity.

Specimen Preparation

Test pieces are cut using a precision die or template, with a minimum of ten specimens distributed systematically across the sample. Common specimen formats include a 100 cm² circle (112.8 mm diameter) or a rectangular 200 mm × 250 mm template equivalent to 1/20 m². The area of each specimen must be determined with an error not exceeding 0.5%, and specimens must be flat, free of folds, creases, water marks, and visible defects.

Weighing and Calculation

Each specimen is weighed on an analytical balance accurate to 0.001 g, such that the overall error of the mass determination does not exceed 0.5% of the result. The grammage is calculated using the following relationship:

Grammage (g/m²) = specimen mass (g) × 10,000 ÷ specimen area (cm²). For example, a 100 cm² specimen weighing 1.20 g corresponds to a grammage of 120 g/m².

Reporting and Evaluation

The test report includes the average grammage, the maximum and minimum individual values, the standard deviation, the coefficient of variation, the conditioning atmosphere, and the specimen dimensions. The measured average is compared with the nominal grammage and the permitted deviation, which is defined by the applicable product standard or by contractual agreement and commonly falls within approximately ±4% to ±5% for secondary fiber corrugated grades. Consistent negative deviation indicates underweight production and is frequently the root cause of downstream strength failures.

Applications

Grammage determination of secondary fiber corrugated paper supports quality assurance and commercial decision-making across the entire packaging supply chain:

• Incoming material verification at corrugated box plants, confirming that recycled medium delivered by suppliers matches the purchased nominal grammage.
• Mill process control, where cross-direction and machine-direction grammage profiles guide adjustments to stock flow, slice opening, pressing load, and drying strategy.
• Commercial settlement and weight-based invoicing, since the mass of paper traded between mills and converters is derived directly from grammage.
• Certification of compliance with product standards and recycled-content packaging specifications required by brand owners and regulatory bodies.
• Research and development activities, including furnish optimization, lightweighting studies, and evaluation of alternative recovered fiber sources.
• Failure analysis of packaging, where insufficient grammage of the recycled medium can be identified as the cause of reduced flat crush, edge crush, or stacking performance.

FAQ

What does grammage mean for secondary fiber corrugated paper?

Grammage is the mass per unit area of the paper, expressed in g/m². It defines how much fiber material is present in each square meter of the recycled corrugated medium and serves as the primary specification around which strength properties, pricing, and board construction are designed.

Which standards govern grammage testing of secondary fiber corrugated paper?

The core test methods are ISO 536, GB/T 451.2, and ASTM D646. Sampling is governed by ISO 186 and GB/T 450, conditioning by ISO 187 and GB/T 10739, and product grading of corrugated paper by GB/T 13023. Accredited third-party laboratories routinely perform these tests against the above standards.

How many specimens are required for a reliable result?

A minimum of ten conditioned specimens is recommended for a routine determination. Additional specimens should be tested when a cross-direction profile study or a statistical uniformity assessment is required, or when the material is suspected of being non-uniform.

What accuracy must the equipment provide?

The cutting die or template must deliver a specimen area with an error not exceeding 0.5%, and the balance must be accurate to 0.001 g so that the total uncertainty of the reported grammage remains well below 1% for standard specimen sizes.

Why is conditioning necessary before weighing?

Recovered-fiber corrugated paper absorbs or releases moisture rapidly with changes in ambient humidity. Conditioning at 23 °C and 50% relative humidity brings every specimen to a reproducible equilibrium moisture state, eliminating humidity-induced mass differences and ensuring that results from different laboratories are directly comparable.

Can grammage be tested on finished corrugated board?

Yes. The recycled medium ply is first separated from the linerboard facings by controlled soaking or steaming, cleaned of adhesive residue, dried, and reconditioned. Its grammage is then determined in the same manner as for a standalone sheet, allowing verification of the board construction against the declared specification.