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Concept of Flame Retardant Rating

Flame retardant rating testing is a method used to evaluate a material’s ability to resist flame spread. Common standards include UL94, IEC 60695-11-10, and GB/T 5169.16. In the standard UL94, Test for Flammability of Plastic Materials for Parts in Devices and Appliances, flame retardant ratings are classified into 12 levels based on the test’s stringency and application: HB, V-2, V-1, V-0, 5VA, 5VB, VTM-0, VTM-1, VTM-2, HBF, HF1, and HF2.

Generally, the commonly used flame retardant ratings range from V-0 to V-2, with V-0 indicating the best flame retardant performance.

1.1 Definitions of the Four Flame Retardant Ratings

HB (Horizontal Burning):
The HB rating indicates that the material burns slowly but does not self-extinguish. It is the lowest level in UL94 and is typically used when vertical testing methods (V-0, V-1, or V-2) are not applicable.

V-2 (Vertical Burning – Level 2):
The V-2 rating means that the material undergoes two 10-second vertical flame tests. After the flame is removed, the material’s burning time does not exceed 30 seconds, and it may ignite cotton placed 30 cm below. However, the flame must not spread above the marked line.

V-1 (Vertical Burning – Level 1):
The V-1 rating means that the material undergoes two 10-second vertical flame tests. After the flame is removed, the material’s burning time does not exceed 30 seconds, and the flame must not spread above the marked line or ignite cotton placed 30 cm below.

V-0 (Vertical Burning – Level 0):
The V-0 rating means that the material undergoes two 10-second vertical flame tests. After the flame is removed, the material’s burning time does not exceed 10 seconds, and the flame must not spread above the marked line or ignite cotton placed 30 cm below.

1.2 Introduction to Other Flame Retardant Ratings

5VA and 5VB belong to the vertical burning test classification using a 500W test flame (125mm flame height).

5VA (Vertical Burning – 5VA Level):
The 5VA rating is a classification in the UL94 standard. It indicates that after the flame is removed, the material’s burning time does not exceed 60 seconds, the flame must not spread above the marked line, and any dripping flames must not exceed 60 seconds.

5VB (Vertical Burning – 5VB Level):
The 5VB rating is similar to 5VA, with the same criteria for burning time and flame spread.

VTM-0, VTM-1, VTM-2 are classifications for thin materials (thickness < 0.025mm) in vertical burning tests (20mm flame height), applicable to plastic films.

VTM-0 (Vertical Tray Burning – Level 0):
The VTM-0 rating means that after the flame is removed, the material’s burning time does not exceed 10 seconds, and the flame must not spread above the marked line.

VTM-1 (Vertical Tray Burning – Level 1):
The VTM-1 rating means that after the flame is removed, the material’s burning time does not exceed 30 seconds, and the flame must not spread above the marked line.

VTM-2 (Vertical Tray Burning – Level 2):
The VTM-2 rating has the same criteria as VTM-1.

HBF, HF1, HF2 are classifications for horizontal burning tests on foamed materials (38mm flame height).

HBF (Horizontal Burning Foamed Material):
The HBF rating means that the foamed material’s burning speed does not exceed 40 mm/min, and the flame must extinguish before reaching the 125mm marked line.

HF-1 (Horizontal Burning – Level 1):
The HF-1 rating means that after the flame is removed, the material’s burning time does not exceed 5 seconds, and the flame must not spread above the marked line.

HF-2 (Horizontal Burning – Level 2):
The HF-2 rating means that after the flame is removed, the material’s burning time does not exceed 10 seconds, and the flame must not spread above the marked line.

Purpose of Flame Retardant Rating Testing

The objectives of flame retardant rating testing include:

2.1 Evaluating Material Combustion Performance

Determining a material’s burning speed, flame spread, and fire propagation under fire conditions helps assess its safety, reliability, and suitability for fire-resistant applications.

2.2 Determining Flame Retardant Capability

Testing identifies a material’s ability to suppress flame spread when exposed to a fire source, which is crucial for preventing fire escalation and minimizing damage.

2.3 Guiding Material Selection and Use

By comparing different materials’ flame retardant properties, testing aids in selecting appropriate materials for construction, transportation, electronics, and other fields to enhance fire safety.

2.4 Compliance with Regulations and Standards

Flame retardant testing is often conducted according to national or industry regulations. It ensures materials meet safety and compliance requirements for specific applications.

In summary, flame retardant rating testing provides critical data for material selection, fire safety improvement, and regulatory compliance by evaluating combustion behavior and flame resistance.

Reference Standards

UL94: 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*

Test Methods for HB, V-2, V-1, and V-0

4.1 Horizontal Burning (HB)

4.1.1 Sample Requirements

Form: Sheets (cut, cast, extruded, etc.) with smooth edges, clean surfaces, and uniform density.
Dimensions: 125±5mm (length) × 13±0.5mm (width). Minimum and 3mm thickness samples required unless thickness exceeds 3mm. Maximum thickness ≤13mm, width ≤13.5mm, corner radius ≤1.3mm.
Variants: Representative samples for different colors/densities.
Quantity: Minimum 2 sets, 3 samples per set.

4.1.2 Test Procedure

Marking: 25±1mm and 100±1mm lines.
Clamping: Hold near the 100mm end, horizontal lengthwise, 45°±2° widthwise, with a wire mesh 100±1mm below.
Flame: Methane flow 105ml/min, back pressure 10mm water column, flame height 20±1mm.
Ignition: Apply flame at 45° for 30±1s or until burning reaches 25mm.
Timing: Record time and burned length (L) from 25mm to 100mm.
Calculation: Burning speed (V) = 60L/t (mm/min).

4.1.3 Test Records

Whether flame reaches 25±1mm or 100±1mm.
Burned length (L) and time (t) between 25mm and 100mm.
If flame passes 100mm, record time from 25mm to 100mm.
Calculated burning speed.

4.1.4 HB Rating Criteria

For 3–13mm thickness: Burning speed ≤40mm/min over 75mm span.
For <3mm thickness: Burning speed ≤75mm/min over 75mm span.
Flame must stop before 100mm.

4.2 Vertical Burning (V-2, V-1, V-0)

4.2.1 Sample Requirements

Form: Sheets with smooth edges, clean surfaces, and uniform density.
Dimensions: 125±5mm × 13.0±0.5mm. Provide min/max thickness samples; if results differ, intermediate samples (≤3.2mm span) are needed.
Variants: Representative samples for different colors/densities.
Quantity: Minimum 2 sets, 5 samples per set.

4.2.2 Sample Conditioning

Standard: 23±2°C, 50±5% RH for 48h; test within 30min after removal.
Oven: 70±1°C for ≥168h, then cool in desiccator for ≥4h; test within 30min.

4.2.3 Test Procedure

Clamping: Hold top 6mm, vertical orientation, bottom 300±10mm above cotton (0.08g, 50×50mm, ≤6mm thick).
Flame: Methane flow 105ml/min, back pressure 10mm water column, flame height 20±1mm.
Ignition: Apply flame at sample bottom edge (10±1mm distance) for 10±0.5s. Adjust if sample deforms.
Timing: Record afterflame (t1) after first ignition, reapply flame for 10±0.5s, then record afterflame (t2) and afterglow (t3).
Notes: If dripping occurs, tilt burner 45°. Ignore samples if flame extinguishes due to gas emission.