Thermal degradation of polyethylene PE is a process in which the polymer's long chains break down due to the application of heat. This results in a decrease in molecular weight, leading to changes in the material's physical and mechanical properties. The mechanism behind the thermal degradation of polyethylene can be explained through a series of steps:1. Initiation: When polyethylene is exposed to high temperatures, the carbon-carbon C-C and carbon-hydrogen C-H bonds in the polymer chains become susceptible to homolytic cleavage. This process generates free radicals, which are highly reactive species with unpaired electrons.2. Propagation: The free radicals formed during initiation can react with other PE chains, leading to chain scission or cross-linking. Chain scission results in the formation of shorter polymer chains, while cross-linking leads to the formation of a three-dimensional network structure. Both of these processes contribute to the degradation of the polymer's properties.3. Termination: The free radicals can also react with each other or with other molecules present in the system, such as oxygen or impurities, to form stable products. This step terminates the radical reactions and limits the extent of degradation.To improve the thermal stability of polyethylene, several strategies can be employed:1. Stabilizers: Adding stabilizers, such as antioxidants and heat stabilizers, can help to prevent or slow down the thermal degradation process. Antioxidants, like hindered phenols or phosphites, can react with the free radicals generated during degradation, thus preventing further chain scission or cross-linking. Heat stabilizers, such as organometallic compounds, can help to absorb the heat and protect the polymer from thermal stress.2. Copolymers: Incorporating other monomers into the polyethylene structure can also improve its thermal stability. For example, ethylene-vinyl acetate EVA copolymers exhibit better thermal stability compared to pure polyethylene due to the presence of vinyl acetate groups, which can act as radical scavengers.3. Cross-linking: Controlled cross-linking of polyethylene can improve its thermal stability by creating a more thermally stable three-dimensional network structure. This can be achieved through methods such as irradiation, peroxide cross-linking, or silane cross-linking.4. Processing conditions: Controlling the processing conditions, such as temperature, pressure, and residence time, during the manufacturing of polyethylene can help to minimize the extent of thermal degradation.5. Blend with other polymers: Blending polyethylene with other thermally stable polymers, such as polypropylene or polyamide, can improve the overall thermal stability of the blend. This is due to the synergistic effect of the different components in the blend, which can help to reduce the rate of degradation.