Overview
ISO 18908:2000 specifies a standardized laboratory method for measuring the folding endurance (fatigue resistance) of photographic film. Based on the MIT folding‑endurance tester (described in Annex B), this analytical test quantifies how many repeated back‑and‑forth folds a film sample will withstand before breaking. It applies to raw or processed film, with or without a gelatin backing, and is intended for controlled laboratory evaluation - not to simulate real‑world use.
Key topics and technical requirements
- Test principle: Repeated longitudinal folding (backwards and forwards) of a vertical sample until failure; results reported as number of double folds.
- Apparatus (MIT tester specifics):
- Oscillation: 135° ± 5° to each side of vertical.
- Frequency: 175 ± 25 double folds per minute.
- Folding surface radius: 0.38 mm ± 0.03 mm.
- Machine counter records double folds to break.
- Sampling & specimen size:
- Minimum set: at least 10 samples per test.
- Standard specimen: 120 mm long × 15–16 mm wide.
- Cut samples in ~23 °C and 50% RH; wear rubber gloves to avoid moisture transfer.
- Tension and conditioning:
- Applied tension during test: 9.81 N.
- Conditioning and test standard temperature: 23 °C.
- Recommended RH: 15% for emulsion/backing sensitivity, 50% for support evaluation.
- Conditioning: equilibrate in a controlled test chamber (air velocity ≥150 mm/s) until moisture equilibrium (typically up to 4 h; not to exceed 24 h).
- Environmental control tolerances: temperature ±1 °C; RH ±1% below 30% and ±2% above 30%.
- Interpretation guidance:
- For films ≈0.15 mm thick, ~20+ double folds indicates acceptable flexibility; ≤3 indicates very brittle film.
- Differences under 3 double folds are generally not significant.
- Note: polyester (PET) base films often exceed MIT test limits (impracticable).
- Alternative equipment: Allowed if correlated to the MIT tester described in the standard.
Applications and who uses it
ISO 18908 is used by:
- Photographic film manufacturers for quality control of film flexibility and fatigue resistance.
- Materials testing laboratories and QA/QC teams validating production batches.
- Conservation and archiving professionals assessing durability of photographic materials.
- Test equipment manufacturers to design or validate folding‑endurance testers.
- Research labs studying effects of processing, temperature and humidity on film brittleness.
Practical uses include comparing raw vs. processed film flexibility, checking directional effects (machine vs. transverse direction), and evaluating the influence of emulsion or backing layers on fold fatigue.
Related standards
- ISO 18907 - wedge brittleness test (detects emulsion cracks).
- ISO 5626 - other folding‑endurance test methods.
- ISO 18900–18999 series - numbering block for imaging materials standards.
Keywords: ISO 18908, folding endurance, MIT folding‑endurance tester, photographic film testing, imaging materials, film flexibility, conditioning, relative humidity, test chamber, photographic film standards.
