Overview
ISO 3688:2022 - Pulps: Preparation of laboratory sheets for the measurement of optical properties defines standardized procedures to prepare laboratory sheets (test sheets or pads) from pulp suspensions prior to measuring optical properties such as brightness, whiteness and colour (including D65 conditions). The document specifies two preparation routes: forming pads in a Büchner funnel using a filter paper or wire screen, and making sheets in a standard sheet former (conventional or Rapid Köthen). It applies to most wood and non-wood pulps but excludes recycled pulps and pulps with very long fibres unless fibres are shortened.
Key topics and technical requirements
- Two preparation methods
- Funnel procedure: use a Büchner funnel (115–150 mm internal diameter), filter paper (free of fluorescent whitening agents, ~84 ± 4 g/m², filtration time ≈ 20 ± 4 s) or wire screen; vacuum drainage (recommended ≥ 60 kPa); careful removal and marking of sheet sides.
- Sheet former procedure: conforming to ISO 5269-1 / ISO 5269-2 (conventional or Rapid Köthen), controlled drying and pressing.
- Pulp pretreatment
- Disintegration per ISO 5263-1 / ISO 5263-2; prepare a stock suspension (typically 4 g/l).
- For slush pulp, determine stock concentration and withdraw appropriate volume.
- Chemical control
- pH adjustment to near-neutral 7.0 ± 0.3 (NaOH 0.1 mol/l; H2SO4 ~0.05 mol/l or 10% acetic acid available). Measurement at pH 5.0 ± 0.5 may be reported if required.
- Use distilled or equivalent water free from colouring and iron/copper ions.
- Optional retention aid for non-wood pulps (example: ~0.4% polyacrylamide noted).
- Equipment & materials
- Standard disintegrator, vacuum device, blotters (≈250 g/m²), pressing plates, hydraulic disk press, calibrated pH-meter (±0.1).
- Avoid iron/copper materials to prevent colour reversion.
- Sampling and sample size
- Sampling per ISO 7213 when assessing lot quality; prepare sufficient pulp for at least five sheets of ~225 ± 25 g/m² (more sheets may be needed to ensure ≥10 valid optical measurements).
- Exclusions
- Not applicable for recycled pulps (see ISO 21993), opacity measurements, or determination of scattering/absorption coefficients.
Applications and users
- Who uses it: pulp and paper laboratories, quality control teams, R&D departments, independent testing laboratories, and instrument manufacturers.
- Practical uses: producing repeatable laboratory sheets for optical testing (brightness, whiteness, colour), ensuring comparability of reflectance factor measurements across labs, supporting product development and quality assurance of pulp and paper products.
Related standards
Keywords: ISO 3688:2022, pulps, laboratory sheets, optical properties, Büchner funnel, Rapid Köthen, pulp preparation, brightness measurement, whiteness, paper testing.
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