Plastic Combustion Process And Factors Affecting Plastic Combustion

Plastic Combustion Process And Factors Affecting Plastic Combustion

Plastics possess excellent properties and their application scope is expanding increasingly. They are particularly used in large quantities in industries such as construction, transportation, furniture, electronics and electrical appliances, and daily necessities. Plastics have facilitated and beautified people's lives and environments, achieving remarkable economic and social benefits, and have gradually replaced traditional materials.

However, the vast majority of commonly used plastics are combustible or flammable in air. When they burn, they produce a large amount of smoke and toxic gases, which can cause people to die from poisoning and suffocation, and also affect fire-fighting and rescue operations.

Therefore, the flame-retardant modification technology of plastics has attracted high attention from countries around the world. Before carrying out flame-retardant modification, it is necessary for us to understand the combustion process of plastics and the factors that affect the combustion of plastics.

I. The Combustion Process of Plastics

The combustion process of plastics is a very intense and complex thermal oxidation reaction, characterized by dense smoke or intense flames. The general process of plastic combustion can be divided into five basic stages.

Stage One: Heating and Temperature Rise. The reason why plastics undergo combustion is primarily due to the increase in temperature under the influence of an external heat source. Thermoplastic plastics will exhibit melting at this stage.

Stage Two: Thermal Decomposition. When the temperature of the plastic rises to the decomposition temperature due to heating, thermal decomposition will occur, producing combustible gases and other thermal decomposition products, which mainly include:

1. Combustible gases, such as methane, ethane, propane, formaldehyde, acetone, carbon   monoxide, etc.

2. Non-combustible gases, such as carbon dioxide, nitrogen, etc.

3. Liquid products, namely melted degraded polymers and prepolymers.

4. Solid products, such as carbonaceous materials, etc.

5. Smoke, which refers to solid particles (such as carbon) suspended in the air.

Stage Three: Ignition. When the concentration of combustible gases produced by the thermal decomposition of plastics reaches the ignition point, and they come into contact with oxygen and are heated to the ignition temperature, the combustion of plastics can be triggered and will spread from a local area to the entire material.

Stage Four: Sustained Combustion. When the net heat of combustion of the plastic, that is, the difference between the heat of combustion of the plastic and the heat required to heat adjacent materials to the combustion state is equal to or greater than 0, the plastic will continue to burn on its own.

Stage Five: Combustion Cessation. When the combustion of plastics progresses to the point where the net heat of combustion becomes negative, the ignited plastic will cease to burn after the ignition source is removed.

II. The Combustion Reaction of Plastics

The combustion of the thermal decomposition products of plastics proceeds via a free radical chain reaction mechanism, which is similar to the thermal oxidative degradation of plastics and includes the following four steps:

1.Chain Initiation

2. Chain Propagation

3. Chain Branching

4. Chain Termination

III. Factors Affecting the Combustion of Plastics

There are numerous factors that affect the combustion of plastics, including the intrinsic combustion characteristics of polymers and the extrinsic combustion environmental conditions.

When the combustion environmental conditions are the same, the main determinants are the combustion characteristics of the polymers, which include chemical composition and structure, specific heat capacity, thermal conductivity, decomposition temperature, heat of combustion, flash point and ignition point, flame temperature, limiting oxygen index, and combustion rate, etc.

• Chemical Composition and Structure

The combustion of polymers is essentially the burning of flammable gases produced by the thermal decomposition of polymers when they are heated. Therefore, polymers with different chemical compositions and structures have different combustion characteristics due to the varying content of flammable gases in their thermal decomposition products.

Obviously, polymers with a higher content of flammable gases in their thermal decomposition products are more likely to burn.

• Specific Heat Capacity

Specific heat capacity refers to the amount of heat required to raise (or lower) the temperature of 1 gram of a substance by 1°C. Under the same conditions, polymer materials with a higher specific heat capacity require more heat during the heating phase of the combustion process, thus making them more difficult to ignite. This concept is further explained in the context of polymer combustion, where materials with a higher specific heat capacity absorb more heat when their temperature is raised, which can make the ignition process more challenging due to the greater amount of energy needed.

• Thermal Conductivity

Thermal conductivity, also known as heat conductivity or thermal coefficient, is a physical quantity that represents the ability of a material to conduct heat. Under the same conditions, polymer materials with a higher thermal conductivity will heat up more slowly during the heating phase of the combustion process, and thus are more difficult to ignite. This is because materials with higher thermal conductivity can dissipate heat more effectively, requiring more energy to reach the ignition point.

• Decomposition Temperature

Since polymer combustion is essentially the combustion of flammable gases produced by the thermal decomposition of the polymer when it is heated, polymers with lower thermal decomposition temperatures are generally easier to burn.

• Heat of Combustion

The heat of combustion is an important factor in maintaining combustion and delaying ignition. The combustion of most polymers is an exothermic reaction.

• Flash Point and Autoignition Point

Flash point is the lowest temperature at which an open flame can ignite, while autoignition point is the lowest temperature at which spontaneous combustion can occur without an open flame.

• Flame Temperature

As with the heat of combustion, flame temperature is an important factor in maintaining combustion and delaying ignition. Most polymers have flame temperatures much higher than matches and cigarettes, up to about 2000°C.

• Limiting Oxygen Index

The minimum volume fraction of oxygen in the gas mixture that will sustain the combustion of a polymer is called the limiting oxygen index (LOI) of the polymer, or oxygen index (OI) for short.

The Oxygen Index is an important indicator of whether or not a polymer material will burn. Since the volume fraction of oxygen in air is 20.9%, polymers with an oxygen index less than 21% can generally be ignited in air.

• Rate Of Burning

The speed of combustion affects the development and spread of a fire. Various polymeric materials do not burn at the same rate and, therefore, have a fast or slow rate of propagation during combustion.

In an actual fire, the burning speed of a material is affected by the disturbance and diffusion of external gases, conduction of heat, convection and radiation.

IV. Conclusion

With a deep understanding of the combustion process of plastics and the factors affecting the combustion of plastics, YINSU Flame Retardant Company has developed a series of environmentally friendly flame retardants for different plastics as science and technology continue to advance and people's awareness of environmental protection continues to grow. These flame retardants not only improve the fire resistance of plastics and help fire safety, but also are environmentally friendly and can meet the requirements of fire safety and sustainable development of different industries. YINSU will continue to pay attention to the latest research results and application trends of plastics materials, and contribute to the promotion of green and sustainable development of the plastics industry.