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Flame retardance of plastics - UL 94

Standard 94 of the Underwriters Laboratories (UL94) is accepted world-wide as the predominant standard for classifying the flame retardance of plastics. The procedure according to UL94 is to test the ability of the material to extinguish itself after flaming. The classification is according to the speed of burning and the extinguishing time, droplet formation, and the duration of afterglow time. Depending on the wall thickness, several classifications are possible for each material. The basis for the appropriate specification of a material should correspond to the thickness of the moulding’s main wall. Information on the UL 94 classification is only comparable and meaningful if it specifies the relevant wall thickness.

The flammability category always refers to the raw material, tested using ideal test pieces. In the case of manufactured parts, deviations caused by different material thicknesses and the effects of processing are unavoidable.

Index: Flame retardance of plastics
SymbolIndexBrief descriptionDefinition
 


HB

The test specimen is held horizontally. For wall thicknesses up to 3 mm, the rate of combustion must be 76 mm/min, and less than 38 mm/min for wall thicknesses of 3 mm.Often misunderstood:
Non-flame-retardant qualities (or materials which are not intended for flame-retardant applications) do not automatically meet horizontal burning criteria. Although the least rigorous, UL 94 HB is a category of flammability and can only be achieved by means of testing.
 


V-2

Test specimen vertical, flame height 20 mm; self-extinguishing up to 30 s after the flame has been removed; no burning droplets; afterglow = max. 60 s.Cotton wool underneath the test piece may ignite.

 
 


V-1

Test specimen vertical, flame height 20 mm; self-extinguishing up to 10 s after the flame has been removed; no burning
droplets; afterglow = max. 60 s.
Cotton wool underneath the test piece may not ignite.

 
 


V-0

Test specimen vertical, flame height 20 mm; self-extinguishing up to 10 s after the flame has been removed; no burning
droplets; afterglow = max. 30 s.
Cotton wool underneath the test piece may not ignite.
 
 


V-5

Burning test to determine flammability classes UL 94 5VB and UL 94 5VA. Plastics which meet at least classification V-0 can be tested – in this case, the flame height is 125 mm.The process exposes a vertically-mounted specimen 5 times for a period of 5 seconds with pauses of 5 seconds.

In addition to the test criteria as per UL 94 V, this flammability test also takes into  consideration the formation of holes in panels.
 UL 94 5VAUL 94 5VB
Afterflame time / afterglow time of the test specimens after the 5th flame application [sec]< 60< 60
Burning dropletsnono
Hole formation (in panels)noyes

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