The light fastness of fabric refers to its ability to resist fading or color change when exposed to light. It is an important characteristic to consider, especially for textiles used in outdoor or sunlit environments. The measurement of light fastness involves various testing methods. Here is a detailed explanation of one common method - the Xenon Arc test method, as per ISO 105-B02 standard. Please note that there are other methods available depending on specific industry requirements.
1. Test Equipment and Materials:
- Xenon Arc tester: A specialized instrument that produces an artificial light source simulating natural sunlight.
- Fabric samples to be tested
- Control fabric samples: lightfast fabrics used as a reference
- Lightfastness assessment panels: clear plastic panels used to hold the fabric samples during testing
- Color assessment equipment: spectrophotometer or colorimeter to measure color changes
2. Preparation:
- Cut fabric samples into identical sizes (usually 10x10 cm or 4x4 inches).
- Identify and label each sample to ensure accurate measurements and comparisons.
- If needed, wash and dry the fabric samples based on specific requirements or industry standards.
3. Test Procedure:
- Precondition fabrics: Expose fabric samples to the testing laboratory's standard conditions (temperature and humidity) for a minimum of 24 hours before testing.
- Mount fabrics: Secure fabric samples onto the lightfastness assessment panels for easy handling and exposure.
- Control samples: Include a few control fabric samples with known lightfastness properties to assess the effectiveness of the test.
- Test exposure: Place the panels with fabric samples in the Xenon Arc tester, ensuring even and consistent exposure across all samples.
- Exposure conditions: Set the exposure conditions, including exposure time, temperature, humidity, and any specific requirements based on the testing standard or industry specifications. Common exposure times range from a few hours to several hundred hours.
- Monitoring: During exposure, periodically monitor the samples using the color assessment equipment to measure any color changes.
- Test endpoint: Establish the end point of the test based on specific requirements, such as a certain level of color change or a predetermined exposure time.
4. Evaluation and Results:
- After the exposure period, remove the fabric samples from the lightfastness assessment panels.
- Measure color changes: Use a spectrophotometer or colorimeter to measure color changes in the fabric samples, comparing them to the initial color or the control samples.
- Analysis: Calculate and compare the Delta E (ΔE) values, which represent the degree of color difference between exposed and unexposed samples. Lower Delta E values indicate better lightfastness.
- Rating scales: Evaluate the fabric samples based on rating scales specified in relevant standards or industry guidelines. ISO 105-B02 standard, for example, provides a scale ranging from 1 to 8, with 1 being very low lightfastness and 8 being excellent lightfastness.
5. Reporting:
- Document and report the test results, including exposure conditions, evaluation criteria, and lightfastness ratings.
- Include photographs or color measurement records to support the evaluation.
- Any additional information such as specific fabric treatments or finishes should be noted in the report.
It is important to note that this is just one common method for measuring light fastness using a Xenon Arc tester. Other methods may involve different light sources, such as carbon arc lamps or fluorescent UV lamps. Additionally, specific industries may have their own testing standards or variations. It is advisable to consult relevant standards and guidelines specific to your industry for accurate and comprehensive light fastness testing.