Overview
ISO 4660:2020 - Rubber, raw natural - Colour index test defines standardized procedures to determine the colour of raw natural rubber against a recognized colour scale. The document specifies two methods: Method A - colour matching against standard coloured glasses (Lovibond comparator) and Method B - colour determination using a colour spectrophotometer. Method B (instrumental measurement) is the preferred method in case of dispute. The standard covers sample preparation, apparatus, calibration, test procedure and expression of results.
Key topics and technical requirements
- Two accepted methods:
- Method A: Visual matching to standard coloured glasses (Lovibond Comparator discs) under diffuse daylight (standard illuminant B) against a matt white background.
- Method B: Automatic measurement with a colour spectrophotometer using a standard illuminant (D65) and digital colour coordinates (e.g., CIE Lab*, XYZ, RGB).
- Sample preparation:
- Test portion: about 20 g ± 5 g from a homogenized piece.
- Mill passes: two passes, final sheet thickness 1.7 mm ± 0.1 mm; doubled sheet yields 3.4 mm ± 0.2 mm before cutting.
- Punch produces discs ≈13 mm diameter; moulded discs pressed at 150 °C ± 3 °C and ≥3.5 MPa for 5 min ± 0.2 min.
- Apparatus: laboratory mill (ISO 2393), mould (1.6 mm ± 0.05 mm cavities), punch, comparator, standard coloured glasses (Lovibond scale 1–16) and colour spectrophotometer.
- Calibration: Spectrophotometer must be calibrated per manufacturer’s instructions; standard glasses have CIE chromaticity coordinates (illuminant B).
- Expression of results: Report colour index to the nearest 0.5 unit for indices 1–5 and to the nearest 1 unit for values >5. If strong non-standard tints (yellow/green/grey) prevent matching, report inability with reason.
- Comparative data: Annex A discusses a study comparing methods and uses Delta E (ΔE) for colour difference evaluation (perception thresholds provided).
Applications
- Quality control and acceptance testing of raw natural rubber batches.
- Incoming raw material inspection for tyre, rubber goods and polymer compounding industries.
- Supplier/specification verification where consistent colour index is part of material grading.
- Laboratory standardization of colour reporting and digital archival of colour data.
Who should use this standard
- Rubber manufacturers, compounders and processors
- Materials testing and quality assurance laboratories
- Procurement and specification engineers specifying raw natural rubber grades
- Third‑party inspection and certification bodies
Related standards
- ISO 2393 - Rubber test mixes: preparation and mixing equipment (referenced for milling)
- ISO 2007 (referenced regarding punch design)
Keywords: ISO 4660:2020, raw natural rubber, colour index test, colour spectrophotometer, Lovibond comparator, colour determination, Delta E, CIE Lab*.
