Overview - ISO 249:2016 (Rubber, raw natural - Determination of dirt content)
ISO 249:2016 specifies a laboratory method for determining the dirt content of raw natural rubber (not including surface contamination). The standard defines sample preparation, solvents and peptizer options, dissolution and filtration procedures, sieve specifications, drying/weighing procedures, and reporting requirements. It is intended for routine quality control and acceptance testing of raw natural rubber.
Key topics and technical requirements
- Scope: Method applies to dirt contained within the rubber matrix; surface contamination is excluded.
- Sampling: Start from a homogenized laboratory sample per ISO 1795 and mill the test portion twice between cold rolls (nip 0.5 mm ± 0.1 mm, per ISO 2393).
- Test portion: Weigh 10 g to 20 g (use 20 g for low-dirt (“clean”) rubbers; smaller portions for heavily contaminated material). Carry out determinations in duplicate.
- Reagents / solvents: Mixed xylenes (139–141 °C), white spirit (155–198 °C), light petroleum (60–80 °C), toluene. All solvents must be dry and free from dirt; observe flammable-solvent safety precautions.
- Peptizers: Examples specified include xylyl mercaptan, 2‑mercaptobenzothiazole, di-(2-benzamidophenyl) disulfide, and tolyl mercaptan - quantities and solvent volumes for preparation are given.
- Dissolution / heating: Dissolve rubber with peptizer at 125–130 °C (avoid local overheating to prevent gel formation). Agitate occasionally.
- Filtration and sieve: Filter hot solution through a corrosion-resistant sieve with nominal openings 44–45 μm (325 mesh) mounted on a holder; retain dirt on the sieve.
- Cleaning / drying: Wash residue and sieve with specified solvents; dry at 100 °C (30 min if light petroleum used, 1 h if white spirit used). Cool in a desiccator and weigh to 0.1 mg.
- Calculations & precision: Dirt content is reported as percentage by mass (to 0.01%). Precision data and guidance are provided in Annexes A and B.
- Reporting: Report reference to ISO 249, sample ID, mean of two results, solvent/peptizer used, and any deviations or observations.
Applications and users
ISO 249 is used by:
- Rubber manufacturers and processors for incoming raw-material inspection and process control.
- Quality control and analytical laboratories performing routine dirt-content testing.
- Raw rubber suppliers and buyers for acceptance testing and contract verification.
- Regulatory bodies and standards compliance teams assessing material cleanliness.
Practical benefits include standardized, reproducible measurement of embedded contaminants, improved batch acceptance decisions, and clearer supplier communication.
Related standards
- ISO 1795 - Sampling and preparative procedures for raw natural and synthetic rubber.
- ISO 565 - Test sieves - nominal sizes (relevant for the 44–45 μm sieve).
- ISO 2393 - Laboratory mill roll settings (nip specification referenced).
Keywords: ISO 249, dirt content, raw natural rubber, peptizer, 325 mesh sieve, solvent dissolution, laboratory test, quality control.