Understanding UL 94 Flammability Ratings

When selecting a plastic material for an electrical application you may be required to have a product with a UL 94 flammability rating. The Underwriters Laboratory (UL) has published their UL 94 standard entitled, Test for Flammability of Plastic Materials for Parts in Devices and Appliances. This standard was developed to describe the test requirements for flammability of plastic materials used in devices and appliances and is intended to serve as a preliminary indication of their acceptability with respect to flammability for a particular application.

The UL 94 flammability testing uses standard test specimens and is used to determine the flammability properties of plastic materials. Tests conducted on the plastic material under the specified conditions are intended to provide information for comparing the relative burning characteristics of different materials or assessing any change in burning characteristics prior to or during actual use. It should be noted that UL 94 test specimens represent small-scale testing and do not cover flammability of plastic materials when used as materials for building and construction applications.

Test specimens can be injection molded, compression molded or cut from sheet. Standard test specimens are 125mm long by 13mm wide and are provided in the minimum thickness and 3mm thickness. Specimens can be supplied for approval in natural, black or colors as well as in a range of densities, melt flows or reinforcement levels.

Standard thickness for UL 94 test specimens is 1.5mm and/or 3mm thick for bars and 3mm thick for plaques. However, the test can be conducted at any thickness up to 13 mm. It is important to consider what thickness your application requires and ensure that you select a material that meets your flammability requirements at your application thickness.

Prior to testing, specimens are conditioned at 23°C and 50% relative humidity for a minimum of 48 hours. A second set of specimens is conditioned in an air-circulating oven for 168 hours at 70°C.

Basic Testing Procedures

• Horizontal Burn (HB) – A 125mm x 13mm injection molded specimen is placed horizontally in a clamp and the bar is exposed to a calibrated flame at the unclamped end for a set time. Burn rate timing and distance on the test bar is started once flame is removed.
• Vertical Burn (VO, V1 and V2) – A 125mm x 13mm injection molded bar is placed vertically and clamped at the top. At the bottom of the bar a calibrated flame is applied for a specified time. Dry surgical cotton is placed 300mm below the test specimen to catch drips. Timing begins after flame is removed.
• Surface Burn (5VB & 5VA) – Material must first pass the Vertical Burn testing before surface testing is conducted. A 150mm x 150mm plaque is placed horizontally with a calibrated flame at a 20-degree angle from below the plaque. Timing begins after the flame is removed.

Testing conditions and requirements for the Horizontal and Vertical flammability ratings are shown in Table 1. Materials that meet the specific requirements are given a final rating of HB, VO, V1, V2, 5VB or 5VA. Flammability ratings and other properties can be found for plastic materials on their UL Yellow Card.

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