How to Test the Sun Resistance (Lightfastness) of Water-Based Inks

The sun resistance (lightfastness) of water-based inks is a critical factor determining the durability of printed products in outdoor or high-light environments. Depending on formulation and application, some prints can maintain color stability for over a year, while others fade within months.

To accurately evaluate performance, laboratory testing uses accelerated weathering methods to simulate long-term sunlight exposure under controlled conditions.

 

1. Key Standards for Lightfastness Testing

Several international standards are widely used to evaluate the light resistance of water-based inks:

• ISO 105-B02: Color fastness to artificial light (xenon arc lamp)
• ASTM G154: UV exposure of non-metallic materials
• AATCC TM16: Lightfastness testing for textiles
• ISO 4892 / EN ISO 4892: Artificial weathering using light sources

These standards define test conditions, exposure time, and evaluation methods to ensure consistent and comparable results.

2. Sample Preparation

Proper sample preparation is essential for reliable results:

• Print water-based ink on representative substrates (e.g., paper, BOPP, PET, textiles)
• Ensure uniform ink thickness and coverage
• Condition samples under standard temperature and humidity before testing

The substrate should reflect the actual application environment to ensure meaningful test outcomes.

3. Testing Equipment: Xenon Arc Weathering

The most widely used equipment is the xenon arc aging tester, which simulates the full solar spectrum:

• Ultraviolet light (UVA, UVB)
• Visible light
• Near-infrared radiation

This allows accurate reproduction of outdoor sunlight exposure, including heat and moisture effects when combined with humidity control.

4. Test Conditions and Parameters

Test conditions are set according to standards and application requirements:

Longer exposure times simulate extended outdoor usage conditions.

5. Evaluation Methods

5.1 Color Difference Measurement (ΔE)

Color change is measured using a spectrophotometer based on the CIELAB color space:

• ΔE < 1: No visible change
• ΔE 1–3: Slight fading
• ΔE > 5: Significant color change

5.2 Grey Scale Rating

Visual comparison using standard grey scales provides a grading system (1–5 or 1–8):

• Grade 5: Excellent (no fading)
• Grade 1: Poor (severe fading)

5.3 Additional Performance Checks

Besides color, other properties should be evaluated:

• Gloss retention
• Adhesion strength
• Surface hardness
• Cracking or chalking

6. Test Report and Data Interpretation

A complete test report should include:

• Testing standard and equipment used
• Detailed test conditions
• Exposure duration
• ΔE values and grading results
• Performance changes before and after exposure

Results can be expressed as:

• Lightfastness grade (1–5)
• Fading index
• Color difference (ΔE)

7. Correlation Between Accelerated and Natural Aging

Accelerated aging tests simulate long-term exposure in a shorter time. However, real-world conditions vary significantly:

• Geographic location (UV intensity)
• Seasonal changes
• Pollution and environmental factors

Therefore, laboratory results should be interpreted with caution and, if necessary, supplemented with outdoor exposure testing.

8. Practical Recommendations for Improving Lightfastness

• Use high lightfastness pigments (inorganic or weather-resistant organic pigments)
• Optimize resin systems for UV stability
• Add UV absorbers and light stabilizers
• Apply protective varnish or coatings

These strategies can significantly extend the outdoor durability of printed products.

Conclusion

Testing the sun resistance of water-based inks requires a combination of standardized methods, controlled laboratory conditions, and scientific evaluation techniques. By understanding test procedures and interpreting results correctly, manufacturers can optimize formulations and ensure long-term performance in real-world applications.