Overview
ISO 15361:2000 specifies a standardized laboratory method for determining the zero-span tensile strength of pulp laboratory sheets - tested wet (never dried), rewetted, or dry (conditioned). The test measures the tensile force of a sheet at a clamp span effectively reduced to zero, giving insight into the intrinsic tensile strength of individual fibres and their retention through pulping and processing. The method applies to all fibre types, including recycled fibres, and is widely used in pulp and paper R&D, quality control and process optimization.
Key topics and technical requirements
- Scope and principle: Measures maximum force at rupture with clamps set to zero-span; result expressed as zero-span tensile strength and as zero-span tensile index (strength divided by grammage).
- Test pieces: Laboratory sheets formed from pulp slurry are required (oven-dry grammage target 60 g/m² ± 3 g/m²). Test pieces may be tested dry, rewetted (specific rewetting and dampness targets given), or never-dried.
- Apparatus essentials:
- Zero-span tensile tester with matched clamps (jaw width 15–25 mm, identical to ±0.01 mm) and clamping pressure adjustable 250–1 000 kPa.
- Force application rate: (25 ± 2) N/s per 10 mm jaw width; force measurement accuracy ±0.5% of range.
- Additional lab equipment: sheet former, disintegrator, devices to reduce fibre curl (Valley beater, PFI mill, or equivalent) and tools for cutting and wetting test pieces.
- Sample preparation: Dry/semi-dry pulps are disintegrated (ISO 5263); fibres may be mechanically treated to remove kinks before forming sheets. Rewetting procedures ensure saturation followed by controlled removal of excess water to a specified dry-matter range.
- Reporting and precision: Results must report conditioning, grammage basis (conditioned or oven-dry), and any mechanical pretreatment applied; precision and calibration procedures are defined in the standard.
Applications and users
- R&D teams evaluating new pulping, bleaching or refining processes to assess fibre strength retention.
- Quality control in pulp mills and papermaking operations monitoring raw fibre quality and effects of processing (including recycled fibres).
- Process engineers optimizing beating/refining to balance sheet strength versus fibre damage.
- Material scientists studying fibre mechanics and the impact of drying/rewetting on fibre integrity.
Related standards
Key referenced ISO documents that are commonly used alongside ISO 15361:
Using ISO 15361 helps ensure consistent, comparable zero-span tensile data for evaluating fibre strength and optimizing pulp and paper processes.
