Combustion Basics

Rubber fabricators must have a working understanding of polymer flammability to maintain operational safety and efficiency.

There are three necessary components for combustion, as evident in the Fire Triangle model.  To list, the three parts include oxygen, fuel, and an ignition source. Significantly, you can prevent or extinguish a fire by eliminating one of the three elements.

Different plastics undergo the process of combustion in different ways.  However, a widely accepted general process includes a sequence of thermal and chemical changes, decomposition of a polymer, ignition of gases, combustion, and spread of a flame.  First, the source of ignition leads the plastic to rise in a temperature specific to that plastic’s chemical structure. Next, this plastic starts to break down as free radicals form.  Then, the gases surrounding the bulk material ignite and begin to combust. Once enough energy accumulates, burning of gas starts combustion at the plastic surface. Finally, the process results in flames and any combination of charring, smoke, and gases. 

Testing for Flammability

Polymer industry professionals use a range of tests to determine polymer flammability levels when planning for safety or efficiency.  These include:  

  • Limiting Oxygen Index (LOI)
    • This test determines the level of oxygen necessary to yield the combustion of a substance.  An LOI test involves the placement of a plastic sample into a glass column. The sample acts as a wick as a flame ignites it, then the substance burns down.  Once the previous step is complete, a researcher records the oxygen and nitrogen levels, which are unique to all plastic types.
  • Vertical Burning Test (94V)
    • For this test, one burns a candle-like sample plastic piece from the bottom up.  The substance then gets a classification in accordance with the duration of flame after placement.  
  • Horizontal Burning Test (94HB)
    • The Horizontal Burning test involves the placement of a flame at one end of a horizontal plastic sample. Next, the flame makes contact for 30 seconds or until it reaches a certain mark on the sample piece. A plastic material then gets a classification for the time required to reach the determined mark.       
Factors Affecting Rubber Tolerances
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