The relationship between smoke emission and molecular structure of plastics

The smokiness of plastics during combustion is not inherent to the polymer. The composition and concentration of smoke produced by plastic combustion mainly depend on the combustion conditions, such as heat flow,oxidizer supply,material shape, presence or absence of flame, etc., and combustion environmental conditions (such as ambient temperature,volume of combustion space, ventilation, etc.). However, the smoke properties of plastic combustion are also related to the molecular structure of the polymer.In general, the smokiness of plastics during combustion has the following laws in relation to the molecular structure of polymers:

1,The higher the thermal stability, the less smoke.The reason why polymers with high thermal stability emit less smoke is that their carbon formation process occurs not in the gas phase but in the condensed phase.

2,Aliphatic polymers, especially aliphatic polymers with oxygen atoms in the main chain, have a very low smoke amount,and polymers containing benzene rings on the main chain have more smoke,while polymers containing polyene structures and polymers with benzene rings on the side chain have more smoke.Polymers containing polyene structures and polymers with benzene rings on the side chain  produce more smoke  because they are easy to graphitize through cyclization and polycondensation.

3,Halogen-containing(except fluorine-containing) polymer smoke is quite large,but not necessarily the higher the halogen content, the more smoke.

4,The higher the carbon/hydrogen ratio,the more smoke.The reason why the polymer with a high carbon/hydrogen ratio emits more smoke is that when burned, the C in the  polymer molecule is converted to CO2 and the H is converted to HO2 at the same reaction rate,so the polymer with a high carbon/hydrogen ratio will precipitate excess carbon.

5,Adding flame retardants to the polymer usually increases the amount of smoke. Adding flame retardants to polymers increases the amount of smoke. This is because flame retardants inhibit oxidation,reduce combustion heat release, resulting in incomplete combustion.

Plastic combustion not only releases a lot of heat, but also is often accompanied by a lot of smoke and toxic gases. The smoke produced by plastic combustion consists of two parts,the visible part and the invisible part.The visible part is the solid suspended matter,of which the black smoke is carbon black; White smoke is the tiny particles suspended in the air.The invisible part is gas, such as HCI, CO2, CO, HCN and methane,among which HCI, CO and HCN are toxic gases.There are different theories about the mechanism of black smoke produced by plastic combustion, the most  representative of which is the theory of carbon double bond polycondensation.The black smoke produced by plastic combustion is mainly formed by carbon particles suspended in the gas. According to the theory,the formation of these carbon particles is the result of further breaking and cyclization of chain segments containing common double bonds produced by thermal decomposition of plastics into aromatic or polycyclic polymers,and then condensation and graphitization.