Under fault or overload conditions, component systems or individual plastic parts may reach temperatures which causes them to deform, melt, or possibly ignite. Consideration must also be given to whether adjoining materials could aggravate these conditions or be ignited by a failing plastic material.
To limit failure consequences, compliance agencies have established performance criteria that can be used during material selection and part design.
The burning behavior of plastics is not just a material characteristic. Part design and nominal wall thickness are key factors in flammability certification.
Underwriters Laboratories (UL) is an independent organization that provides product safety testing and certification. Their UL94 testing procedures and rating system for thermoplastics flammability are the generally accepted standard throughout most of the world.
Different UL94 ratings are given based on variations of the test method and results:
- Horizontal Burn (HB Rating)
A specimen less than 3 mm thick is tested in a horizontal orientation. It must burn at a rate of less than 76 mm/min. - Vertical Burn (V-0 to V-2 Rating)
Specimen thickness is a element of this classification which is conducted in a vertical orientation. Burning must self extinguish within 10-30 seconds. The effect of dripped particles can also be monitored.
Material test results must be certified by UL in order to obtain a “yellow card” or Recognized Component status and bear their “UR” mark. Use of Recognized Component materials can be advantageous for obtaining a UL Listing for completed devices.
RTP Company has over 450 plastic compounds already Recognized by UL with a variety of flammability ratings. RTP Company also participates in UL’s Client Test Data Program which allows testing conducted by RTP Company to be accepted by UL, this procedure can expedite UL certification of new materials.
Glow wire and needle flame test methods are popular in Europe for ascertaining the flammability of plastics.
These tests simulate thermal stresses to which materials may be subjected in a failure situation, such as the glowing elements of an overloaded resistor, to assess fire hazard.
Glow wire testing can be conducted using two methods:
- Glow Wire Flammability Index
Flame or glowing must self extinguish within 30 seconds after removal of the glow wire. - Glow Wire Ignition Temperature
No flame or glowing for more than 5 seconds while the glow wire is applied.
- Automotive — FMVSS 302
- Aerospace — FAR 25.853
- Limiting Oxygen Index — ASTM D 2863 or ISO 4589
- Flame spread — ASTM E 162
- Smoke density — ASTM E 662
- Heat release — ASTM E 1354
