Flammability

Flammability

The most used standard for flammability of polymers is UL 94, of Underwriters Laboratories. A short description and comparison follows.

The classifications are HB, V1, V2 and V0. HB class materials pass a horizontal flame test of relatively modest requirements. V0 class materials have a very good flame resistance.

One should remember that a standard describes an ideal and well defined case. V1 and V2 classed materials will partly burn, without and with burning drops respectively. Test specimens, fixtures and burners are defined.

Classes 5VA and 5VB are for tests with stricter requirements.

UL also classifies films for vertical flame cases, those classes are VTM0, VTM1 and VTM2.

There are other national standards for flammability. DIN has such for construction, NES for cables etc.

An other standard, UL 746, describes flammability in connection with electric sources. Other such standards are for example IEC 60112, 60695 and 60950.

A very relevant measure of flammability is the Limiting Oxygen Index, LOI. This value corresponds to the minimum oxygen concentration in air, required for a material to burn. At LOI values below 32%, materials will burn, at values above 32 the flames will perish.

Flame retardants can be added to decrease flammability. Halogenated flame retardants are very effective, but environmentally not a good solution. Compositions based on phosphorus and silicone oils are examples of liquid systems, which are very easy to add. Metallic oxides or hydroxides are other (e.g. magnesiumoxide). Not seldom, attempts are made to combine systems to obtain synergetic effects. In excessive concentrations, flame retardants will deteriorate the mechanical properties of the polymer. Lately, there has been a significant interest in using nanocomposites as flame retardants.

The designer of flame retardant systems seeks to catch the radicals in the flame process, and at the same time create a shell of carbon or silica around the surface, to isolate and hinder oxygen transport to the flame, and also to conduct heat away from the burn site. That explains why some filled polymers gain flammability resistance by addition of mineral and graphite, as well as glass and carbon fibers.