Polyethylene (PE) is one the most used plastics worldwide for a wide range of applications due to its good mechanical and chemical resistance, low density, cost efficiency, ease of processability, non-reactivity, low toxicity, good electric insulation, and good functionality.
In recent times, polymers have been used in a wide range of industrial applications, such as packaging, electronics, and construction . To better the performance of the aforementioned applications, polymers need to be more environmentally friendly, safe, and durable . The utilization of polymers is encouraged due to good functionality, acceptable durability, and cost-efficiency; however, high flammability, and poor thermal properties are noticeable limitations for their applicability . Most polymers are flammable and, by exposure to enough energy, they will leave a considerable amount of smoke and heat . It is reported that annually about 5000 people in Europe and 4000 people in the United States are killed by fires. Furthermore, the economic loss of these fire accidents is estimated to be about 0.3% of the GDP . Following the analysis conducted by the Geneva-based Aircraft Crashes Record Office (ACRO), it is possible to reduce the possibility of fire catastrophes and their subsequent fatalities by reducing the flammability of polymeric materials . Therefore, related to the use of polymers, there are environmental, economic, and health issues that are considered as the driving forces to motivate the conduction of research and different studies on the flammability of polymers .
Polyolefins are a group of the most popular polymers in various applications . One of the main polymers of this group is PE, with more than 100 million tons production per year, that is, 34% of the total plastic market, and is extensively used due to good mechanical and chemical resistance, low density, cost efficiency, ease of processability, non-reactivity, low toxicity, good electric insulation, and good functionality . However, like other polymers, the concern about PE is its flammability in applications requiring good flame retardancy. Although the emissions from PE have low toxicity, it has a low limiting oxygen index (LOI) and drips during burning, which causes rapid flame spread .
Typically, physical or chemical modifications, as well as applying FRs, are recommended to reduce the flammability of PE products . Flame retardancy of polymers increases by incorporation of FRs and is estimated to triplicate their survival time in case of fire . FRs are necessary as they enhance the flammability of polymeric materials by delaying the fire and its spread . The common commercial FRs used for PE systems are based on phosphorus, borate, inorganic hydroxides, silicon, and nitrogen and are introduced to polymer matrixes during processing . Each of the FR approaches uses a different mechanism and their application is attributed to the polymer matrix and the intended usage. For example, regarding the inorganic hydroxides, they will reduce the burning rate by releasing water to decrease the temperature since decomposition occurs at higher temperatures. Although gas-phase reactions are important in controlling flames in PE systems, quenching FRs based on halogens are not recommended due to environmental concerns . Moreover, intumescent flame retardants (IFRs) are of great importance and efficiency in restricting the fire by forming a swollen char and are applicable for polymeric materials . Considering the high production and applicability of PE, the issue of its flammability is of great importance and hence, there is an urgent need to develop strategies that minimize the flammability of PE.
PEs are one of the most potential materials of value-adding in the case of accurate formulation modifications . It is of great importance to know the grades of PE as their flame performance is correlated with their chemical structure (i.e., branching type) and FR formulations . The structure and properties of PE main grades, including density, crystallinity, LOI, thermal conductivity, melting temperature, and molecular weight are summarized in Table 1.
|PE Grades||Structure’s Description||Density (g/cm−3)||Crystallinity||LOI (%)||Thermal Conductivity (W/m·K)||Melt Temperature (°C)||Mw (g/mol)|
|Branched structure containing long and short chains||0.915–0.932||Lower degree of crystallinity||17–18||0.32–0.35||105–115||<50,000|
(Linear low-density PE)
|Branched structure containing short chains||0.910–0.930||Slightly higher than LDPE||17–18||0.35–0.45||120–130||<50,000|
|Linear structure||0.940–0.970||Higher degree of crystallinity||17–18||0.45–0.5||128–136||Up to 200,000|