How to Test the Flame Retardant Rating of Flame-Retardant Silicone?
Testing the flame-retardant rating of silicone must follow national standards (GB), international standards (such as UL, ISO), or industry regulations. Below are the common testing methods and procedures:
1. Main Flame Retardancy Testing Standards
(1) Vertical Burning Test (UL94)
Applicable to: Evaluation of the flame-retardant performance of plastics, rubber, silicone, and other materials.
Test Method:
The silicone sample (typically 125mm × 13mm × thickness) is fixed in a vertical position.
A standard flame (methane or butane) is applied to the lower end of the sample for 10 seconds, and the burning behavior is observed.
Rating Determination:
| Rating | Burning Time | Dripping Ignites Cotton? |
|---|---|---|
| V-0 | ≤10 sec | No |
| V-1 | ≤30 sec | No |
| V-2 | ≤30 sec | Yes |
| Fail | >30 sec | - |
(2) Limited Oxygen Index (LOI) Test (GB/T 2406 / ISO 4589)
Principle: Determines the minimum oxygen concentration in an oxygen-nitrogen mixture required to sustain combustion.
Procedure:
Place the silicone sample in a transparent combustion tube and introduce different O₂/N₂ gas mixtures.
Ignite the sample and determine the lowest oxygen concentration that sustains combustion for 3 minutes or burns a length of 50mm.
Results:
LOI ≥ 28%: Generally considered flame-retardant (ordinary silicone has an LOI of about 20%-24%).
(3) Horizontal Burning Test (GB/T 2408 / ISO 1210)
Applicable to: Evaluating the burning rate of materials in a horizontal position.
Rating Determination:
Burning speed ≤ 40mm/min (FH-1 level) or ≤ 75mm/min (FH-2 level).
(4) Glow Wire Test (IEC 60695-2-10)
Applicable to: Evaluating the flame retardancy of silicone used in electrical and electronic applications.
Test Method:
A 950°C heated wire is applied to the sample for 30 seconds, and any ignition or sustained burning is observed.
2. Key Testing Steps
Sample Preparation:
Cut the sample to standard dimensions (e.g., strip-shaped for UL94, rod-shaped for LOI).
Ensure samples are free of bubbles and impurities, and meet the required thickness (typically 1-3mm).
Pre-Treatment:
Store the samples at 23±2°C and 50±5% humidity for 48 hours to avoid environmental interference.
Testing Environment:
Conduct tests in a wind-free, temperature- and humidity-controlled laboratory to prevent external airflow from affecting the flame.
Data Recording:
Record burning time, dripping behavior, flame spread length, etc.
3. Common Flame-Retardant Silicone Ratings
Ordinary silicone: Generally non-flame-retardant (LOI ~20%-24%).
Flame-retardant silicone:
UL94 V-0: Highest flame retardancy level (commonly used in the electronics industry).
UL94 HB: Passes only the horizontal burning test (lower flame retardancy).
LOI ≥ 30%: High flame retardancy (e.g., silicone with flame retardant additives).
4. Factors Affecting Flame Retardancy
Filler Types: Inorganic flame retardants such as aluminum hydroxide and magnesium hydroxide can enhance flame retardancy.
Silicone Base Material: Phenyl silicone is more heat-resistant than methyl silicone.
Thickness: Thicker samples are more likely to pass the test (e.g., UL94 has lower requirements for 3mm samples).