Secondary Recycled Fiber Morphology Testing Experiment: Methods, Parameters and Standards

Secondary recycled fiber morphology testing is a laboratory experiment that quantifies the length, width, coarseness, curl, kink, and fines content of fibers recovered from waste paper and board. Because each recycling loop shortens fibers, generates fines, and reduces cell-wall swelling capacity, these morphological descriptors are direct predictors of pulp strength, drainage behavior, and sheet formation quality. This article explains the complete experimental workflow—from sample disintegration to automated optical analysis and microscopy—in line with ISO, GB, and related international test standards.

Technical Overview

Secondary recycled fibers, recovered from waste paper and board through repulping, screening, cleaning, and optional deinking or bleaching, are the dominant fiber source in modern papermaking. Unlike virgin fibers, recycled fibers have already passed through at least one forming, pressing, and drying cycle. This history causes hornification (irreversible bonding inside the cell wall), reduces water retention and swelling, and shortens fibers through mechanical action in pulpers, pumps, screens, and refiners. A fiber morphology testing experiment is designed to quantify these changes objectively by measuring the geometric characteristics of individual fibers suspended in dilute water.

The core output of the experiment is a statistically robust set of descriptors, including fiber length and its distribution, fiber width, coarseness, curl index, kink index, fines content, and the proportion of broken or folded fibers. These parameters directly govern bonding area, tensile and tear strength, drainage resistance on the wire, and formation uniformity of the finished sheet. The experiment is typically performed in mill quality-control laboratories or accredited third-party laboratories using standardized wet disintegration followed by automated optical image analysis, complemented by microscopy-based fiber furnish identification.

The resulting data support furnish design for containerboard, tissue, and printing grades, optimization of refining and screening operations, research on multi-loop recycling behavior, and the technical acceptance of purchased recovered paper or market recycled pulp.

Testing Samples

The experiment accepts wet or dried recycled pulp samples, machine chest stock, and re-pulped finished paper. Sample identity, recycling history, and any prior drying steps must be recorded, because drying before testing can exaggerate hornification artifacts and depress apparent bonding potential.

Old Corrugated Container (OCC) Pulp

OCC is the highest-volume recovered fiber grade and is sampled at points such as the pulper outlet, coarse-screen accept, and machine chest. Its furnish is predominantly unbleached softwood and hardwood kraft, so testing focuses on fiber shortening across the stock-preparation line and on the accumulation of fines in the short circulation.

Mixed Office Waste (MOW) and Deinked Pulp (DIP)

Bleached chemical fibers recovered from office waste and deinking lines carry fillers, toner residues, and stickies that interfere with bonding. Morphology testing verifies that deinking, flotation, and bleaching stages have not excessively cut fibers, and it is often paired with brightness and cleanliness measurements to evaluate the strength–optics trade-off.

Newsprint and Magazine Recovered Furnish

These grades contain high proportions of mechanical fibers (groundwood, thermomechanical, or chemithermomechanical pulp), which are shorter, stiffer, and fines-rich compared with chemical fibers. Testing documents the high fines fraction and elevated curl typical of these furnishes, which explains their fast drainage but limited tensile potential.

Laboratory Recycling-Loop and Reference Pulps

For research purposes, handsheets can be repeatedly formed, dried, and re-disintegrated in the laboratory to simulate several recycling loops. Parallel testing of a virgin reference pulp under identical conditions isolates the true morphological effect of recycling from process-related cutting.

Testing Items

The experiment reports both weighted statistics and full distributions so that changes in the fiber population, not only the average value, become visible.

• Fiber length: arithmetic mean, length-weighted mean, and weight-weighted mean length, together with the complete length-distribution histogram.
• Fiber width: mean width and width distribution, including thin-walled versus thick-walled fiber ratios where the analyzer provides wall-thickness indices.
• Coarseness: mass per unit length (mg/m), indicating how much fiber wall material is carried by a fiber of a given length.
• Curl index: contour length divided by endpoint distance minus one, describing straightness of the fiber population.
• Kink index and kink count: frequency and severity of localized angular bends along the fiber axis.
• Fines content: fraction of fibers or fragments below approximately 0.2 mm, or below a defined area threshold, reported as a percentage.
• Broken and folded fibers: percentage of the population showing fracture ends or permanent creases introduced by processing.
• Shives and fiber bundles: oversized, unseparated fiber aggregates that reduce sheet appearance and strength.
• Fiber furnish composition: proportions of softwood, hardwood, mechanical, and non-wood fibers identified by microscopy.

Testing Methods

Sample Preparation and Wet Disintegration

Wet pulp samples are homogenized, and their consistency is determined gravimetrically. A defined oven-dry equivalent of pulp is soaked and then disintegrated in a standard laboratory disintegrator at low consistency, following the procedures of the ISO 5263 series, until the sheet or flake structure is fully separated without additional fiber cutting. The suspension is subsequently diluted to the measurement concentration required by the analyzer. Samples must never be oven-dried before disintegration, because irreversible hornification would bias the morphological results.

Automated Optical Image Analysis

The diluted suspension flows through a flat measurement cell of an automated fiber morphology analyzer, where a high-speed camera captures two-dimensional images of thousands to tens of thousands of individual fibers. Image-processing software traces the fiber spine and contour to compute true contour length, mean width, curl, kink angles, and fines classification, in accordance with ISO 16065-1 for fiber length and ISO 23713 for fiber width. Instrument performance is verified with certified reference fibers, and replicate measurements are averaged. Typical analyzer settings include:

• Measurement range of approximately 0.05 mm to 7 mm fiber length.
• Measurement of 5,000 to 20,000 fibers per sample, depending on the required statistical confidence.
• Fines defined as fragments shorter than about 0.2 mm or below the specified area limit.
• Curl and kink calculated from the traced spine geometry of each classified fiber.

Microscopic Fiber Furnish Analysis

A stained fiber suspension is prepared on glass slides and examined under an optical microscope, following the principles of ISO 9184 and the corresponding national method GB/T 4688. Herzberg-type staining differentiates chemical softwood, chemical hardwood, mechanical, and non-wood fibers by color reaction, and systematic field counting yields the furnish composition. This step is essential for recycled fibers, because furnish shifts between recycling lots change strength expectations even when average length remains constant.

Screen Fractionation and Data Verification

As a cross-check on optical results, the pulp can be classified by dynamic screen fractionation into length classes, and each fraction is weighed to obtain the mass distribution. Complementary drainage tests, such as Schopper-Riegler freeness per ISO 5267-1, correlate the measured fines level with expected wire drainage behavior, closing the loop between morphology data and process performance.

Applications

Fiber morphology data from secondary fibers are used across the recovered-paper value chain, from recovered-paper purchasing to board machine operation and academic research.

• Acceptance testing of purchased recovered paper and market deinked or packaging pulps.
• Optimization of pulper energy input, refining intensity, and screen slot configuration to limit fiber cutting.
• Furnish design for linerboard, corrugating medium, tissue, and printing papers with defined recycled content.
• Tracking of morphological degradation across multiple recycling loops in sustainability research.
• Technical evidence in supplier benchmarking, trade settlement, and development of enzymatic or chemical fiber-restoration treatments.
• Supporting documentation for recycled-content declarations and environmental product assessments.

FAQ

What is the difference between arithmetic, length-weighted, and weight-weighted average fiber length?

The arithmetic mean is dominated by the numerous short fibers and fines, the length-weighted mean gives intermediate emphasis, and the weight-weighted mean is proportional to fiber mass and therefore dominated by long fibers. Reporting all three values prevents fines-rich furnishes from being misjudged, because a pulp can show an acceptable weight-weighted length while containing excessive fines.

How much sample is required and how should it be preserved?

Typically 2 to 5 grams of oven-dry equivalent pulp per measurement are sufficient. Samples should be kept wet in sealed containers, refrigerated, and delivered to the laboratory within days; drying, freezing, or microbial degradation alters fiber properties and invalidates comparison.

Which fibers are counted as fines?

Fines are commonly defined as fiber fragments shorter than approximately 0.2 mm or below a specified projected-area threshold. In recycled furnishes, fines include both parenchyma and ray cells from the original wood and secondary fines generated by mechanical processing, and they strongly increase drainage resistance while diluting bonding strength.

How does repeated recycling change fiber morphology?

Successive recycling loops generally reduce average fiber length, increase fines content and curl, and—through hornification—reduce cell-wall swelling and bonding capability. The largest relative change usually occurs in the first drying and re-pulping cycle, after which the morphological decline flattens even though fiber potential remains permanently lower than that of virgin pulp.

What is the typical testing cycle and reporting format?

Once a wet sample is received, automated analysis and microscopy can normally be completed within three to seven working days. Reports include all weighted length statistics, distribution diagrams, width and coarseness data, curl and kink indices, fines and shive levels, furnish composition, representative fiber images, and the applied ISO or GB test methods.

Can the experiment distinguish hardwood from softwood recycled fibers?

Yes. Microscopic furnish analysis with differential staining, supported by width-distribution data from the optical analyzer, reliably separates softwood, hardwood, mechanical, and non-wood fiber populations, which is critical when evaluating unknown or blended recovered-fiber sources.