Plastic is potentially flammable. Flammability is a trait tested in commercial plastic products by Underwriters Laboratories, the industry standard in plastic flammability ratings. Underwriters Laboratories testing assigns a horizontal or vertical burn rating to plastic products, certifying those that meet its standards.
To obtain UL certification, a plastic product must pass the horizontal or vertical burn level test based on its thickness. A plastic less than 3 millimeters thick is tested horizontally and must burn at a rate less than 76 millimeters per minute. Thicker plastic components are tested vertically, and burning must self-extinguish within 10 to 30 seconds to obtain certification.
Plastic is not just one substance. It’s a collective term that includes synthetic or semi-synthetic malleable organic material. It includes, but not limited to, polyester, polyethylene, polyvinyl chloride, polypropylene, polystyrene, polyamides, polycarbonate etc.
They are ordinary combustibles. UL 94 is a plastics flammability standard.
UL 94 Classifications relating to materials commonly used in manufacturing enclosures, structural parts and insulators found in consumer electronic products.
HB: slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm
V-2: burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.
V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.
V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.
5VB: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole.
5VA: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole.
There are 6 other UL 94 flame classifications. Three for low-density foam materials and three for very thin films.
So, different plastics will have different flammability ratings.