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A Summary of Flame Retardant Ratings and Testing Standards

October 28, 2025

A Summary of Flame Retardant Ratings and Testing Standards

I. Concept of Flame Retardant Ratings

Flame retardancy testing is a method used to evaluate a material's ability to resist flame spread. Common standards include UL 94, IEC 60695-11-10, and GB/T 5169.16. In the standard UL 94 - "Test for Flammability of Plastic Materials for Parts in Devices and Appliances," based on the severity of the test and the target application, flame retardant ratings are classified into 12 grades: HB, V-2, V-1, V-0, 5VA, 5VB, VTM-0, VTM-1, VTM-2, HBF, HF-1, and HF-2.

Commonly used flame retardant ratings generally range from V-0 to V-2, with V-0 representing the best flame retardancy performance among these.

1.1 Definitions of the Primary Ratings (HB, V-2, V-1, V-0)

  • HB: Horizontal Burning
    The HB rating indicates that the material burns slowly but does not self-extinguish. It is the lowest rating in UL 94 and is often used when the vertical burning tests (V-0, V-1, or V-2) are not applicable.

  • V-2: Vertical Burning - Level 2
    The V-2 rating indicates that after two 10-second flame applications in a vertical position, the burning time of the material does not exceed 30 seconds after flame removal. The burning droplets are allowed to ignite cotton placed 30 cm below the specimen, but the flame must not spread beyond the specified marker.

  • V-1: Vertical Burning - Level 1
    The V-1 rating indicates that after two 10-second flame applications in a vertical position, the burning time of the material does not exceed 30 seconds after flame removal. The flame must not spread beyond the specified marker, and burning droplets must not ignite cotton placed 30 cm below.

  • V-0: Vertical Burning - Level 0
    The V-0 rating indicates that after two 10-second flame applications in a vertical position, the burning time of the material does not exceed 10 seconds after flame removal. The flame must not spread beyond the specified marker, and burning droplets must not ignite cotton placed 30 cm below.

1.2 Introduction to Other Flame Retardant Ratings

  • 5VA & 5VB: These ratings belong to the vertical burning test classification using a 500W test flame (125mm flame height).

    • 5VA: After flame application under specified conditions, the material's after-flame time does not exceed 60 seconds after flame removal, the flame must not spread beyond the marker, and the burning time of dripping particles must not exceed 60 seconds.

    • 5VB: The criteria are similar to 5VA regarding after-flame time and flame spread, and the burning time of dripping particles must not exceed 60 seconds.

  • VTM-0, VTM-1, VTM-2: These are vertical burning test classifications for thin materials (thickness < 0.025 mm), using a 20mm flame height, applicable to plastic films.

    • VTM-0: After flame application, the after-flame time does not exceed 10 seconds.

    • VTM-1: After flame application, the after-flame time does not exceed 30 seconds.

    • VTM-2: After flame application, the after-flame time does not exceed 30 seconds (specific pass/fail criteria differentiate it from VTM-1).

  • HBF, HF-1, HF-2: These are horizontal burning test classifications for foamed materials, using a 38mm flame height.

    • HBF: The burning rate does not exceed 40 mm/min, and the flame should extinguish before the 125mm mark.

    • HF-1: After flame removal, the after-flame time does not exceed 5 seconds, and the flame must not spread beyond the marker.

    • HF-2: After flame removal, the after-flame time does not exceed 10 seconds, and the flame must not spread beyond the marker.

II. Purpose of Flame Retardancy Testing

The purposes of flame retardancy testing include:

2.1 Evaluating the Combustion Performance of Materials: It determines the burning rate, flame spread propensity, and the extent of fire development under fire conditions. This helps ascertain the material's safety, reliability, and suitability for fire protection applications.

2.2 Determining the Flame Retardant Capability of Materials: Testing identifies a material's ability to inhibit flame spread when exposed to an ignition source. This is crucial for preventing fire expansion and reducing losses caused by fires.

2.3 Guiding Material Selection and Use: By testing and comparing the flame retardant properties of different materials, informed choices can be made for material selection and application. This aids in selecting appropriate materials for construction, transportation, electronic devices, etc., to enhance fire safety.

2.4 Compliance with Regulations and Standards: Flame retardancy testing is often conducted according to national or industry regulations and standards. Testing ensures that materials meet the requirements, guaranteeing compliance and safety in specific applications.

In summary, the purpose of flame retardancy testing is to provide a critical basis for selecting suitable materials, improving fire safety, and ensuring compliance with regulations and standards by evaluating the combustion performance and flame retardant capabilities of materials.

III. Reference Standards

  • UL 94: "Test for Flammability of Plastic Materials for Parts in Devices and Appliances."

  • IEC 60695-11-10:2013: "Fire hazard testing - Part 11-10: Test flames - 50 W horizontal and vertical flame test methods."

  • GB/T 5169.16-2017: "Fire hazard testing for electric and electronic products - Part 16: Test flames - 50W horizontal and vertical flame test methods." (National standard of China, technically aligned with IEC 60695-11-10).

IV. Test Methods for HB, V-2, V-1, V-0

4.1 Horizontal Burning: HB

4.1.1 Sample Requirements

  • Form: Sheets, produced by cutting, casting, extrusion, etc., with smooth edges, clean surfaces, and uniform density.

  • Dimensions: Length × Width = 125 ± 5 mm × 13.0 ± 0.5 mm. Provide samples at the minimum thickness and at 3 mm thickness. If the min thickness >3mm or max thickness <3mm, 3mm samples may not be needed. Max thickness ≤13mm, max width ≤13.5mm, corner radius ≤1.3mm.

  • Series: For different colors, densities, etc., provide representative samples.

  • Quantity: Minimum 2 sets, 3 samples per set.

4.1.2 Test Procedure

  • Marking: Mark lines at 25 ± 1 mm and 100 ± 1 mm.

  • Clamping: Clamp the end near the 100mm mark, keeping the long axis horizontal and the wide axis at a 45°±2° angle. Place a wire mesh 100 ± 1 mm below the sample.

  • Burner: Methane, flow rate 105 ml/min, back pressure 10 mm water column.

  • Flame: Flame height 20 ± 1 mm.

  • Application: Apply the flame at 45° to the lower edge of the sample, immersing it 6 mm, for 30 ± 1 seconds or until the flame reaches the 25mm mark, then remove.

  • Timing: Start timing when the flame front reaches the 25 ± 1 mm mark. Record the time taken for combustion to cease and the length burned.

  • Calculation: V = 60L / t, where V is burning rate (mm/min), L is burned length (mm), t is burning time (s).

4.1.3 Test Records

  • Whether the flame reaches the 25 ± 1 mm or 100 ± 1 mm mark.

  • If burning stops between 25mm and 100mm, record burned length (L) and time (t).

  • If the flame passes the 100mm mark, record the time from 25mm to 100mm.

  • Calculated burning rate.

4.1.4 HB Rating Criteria

  • For 3-13mm thick samples over a 75mm span: Burning rate ≤ 40 mm/min.

  • For <3mm thick samples over a 75mm span: Burning rate ≤ 75 mm/min.

  • Burning stops before the 100mm mark.
    Samples meeting these criteria are rated HB.

4.2 Vertical Burning: V-2, V-1, V-0

4.2.1 Sample Requirements

  • Form: Sheets, produced by cutting, casting, extrusion, etc., with smooth edges, clean surfaces, and uniform density.

  • Dimensions: Length × Width = 125 ± 5 mm × 13.0 ± 0.5 mm. Provide samples at the minimum and maximum thickness. If results differ, intermediate thickness samples may be needed. Max thickness ≤13mm, corner radius ≤1.3mm.

  • Series: For variations, provide representative samples.

  • Quantity: Minimum 2 sets, 5 samples per set.

4.2.2 Sample Conditioning

  • Standard Conditioning: 48 hours at 23 ± 2°C and 50 ± 5% RH. Test within 30 minutes after removal.

  • Oven Conditioning: At least 168 hours at 70 ± 1°C, then cool in a desiccator at room temperature for at least 4 hours. Test within 30 minutes after removal.

4.2.3 Test Procedure

  • Clamping: Clamp the top 6mm of the sample, oriented vertically downward. The sample's lower end should be 300 ± 10 mm above a layer of pure cotton (0.08g, 50x50mm, max thickness 6mm).

  • Burner: Methane, flow rate 105 ml/min, back pressure 10 mm water column.

  • Flame: Flame height 20 ± 1 mm.

  • Application: Place the flame center at the midpoint of the lower edge (10 ± 1 mm from burner tip to sample). Apply for 10 ± 0.5 seconds. Adjust burner if sample deforms. If dripping occurs, tilt burner to 45°. After 10 ± 0.5s, withdraw burner at 300 mm/min to at least 150mm away. Immediately record afterflame time t1. After flames cease, re-apply flame for 10 ± 0.5s. Withdraw and record second afterflame time t2 and afterglow time t3.

  • Note: Differentiate afterflame/afterglow. If flames are extinguished by material gases, re-ignite burner immediately and continue application until total 10 ± 0.5s is reached.

4.2.4 Test Records

  • First afterflame time (t1).

  • Second afterflame time (t2).

  • Second afterglow time (t3).

  • Whether the specimen burns completely.

  • Whether dripping particles ignite the cotton.

4.2.5 V-2, V-1, V-0 Rating Criteria
After testing, the flame retardant rating is determined based on the following criteria (summary of pass/fail limits for total afterflame time, individual afterflame times, afterglow time, cotton ignition, and specimen consumption).