THE BEHAVIOUR OF ABS AND PEI IN PRESENCE OF FIRE
The burning mechanism of polymeric materials
Combustion indicates a chemical reaction of esothermic oxidation-reduction (which means that it warms up), where a “combustibile” oxidises and a “oxidizing“ decreases (in the case of air, it’s oxigen). This reaction produces heat and electromagnetic radiations. It starts from reactants, the combustibile and the oxidizing, in our case respectively the polymer and oxygen, creating then products. When plastic materials are burnt, you mainly obtain water and carbon dioxide, but if the polymeric chain contains additional atoms other than carbon, hydrogen and oxigen, you can obtain subproducts too.
Polymeric materials usually burn in presence of a flame, but it’s important to stress that some polymers burn very easily (for example PE and PP) while others offer higher resistance (for example PA and most of all PEI as in our case, or PTFE). These differences can be quantified by the LOI (Limit Oxygen Index), an essential parameter able to highlight how a material can keep an active combustion once the reaction starts. LOI indicates the minimal quantity of oxigen required to substain a certain material combustion. Given that in the atmosphere the oxigen concentration is around 20%, the values of LOI exceeding 20 stress materials which use to individually turn off the combustion, while the lower values express that there is enough oxigen in the air to substain the combustion of that given material.
- ABS 19 LOI
- PEEK 35 LOI
- PEI 49 LOI
The experiment focuses on two samples, one in ABS and the other in PEI. Both have been put in contact with a flame generated by pressured butane gas.
- ABS burns creating a flame, a lot of smoke and final dripping.
- PEI carbonizes, gene rating a low quantity of non-toxic smoke (comparable to wood).
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