Polyethylene (PE): Synthesis, Safety, Applications
Polyethylene (PE) is the world's most produced plastic, and its preparation process is a classic paradigm in the chemical industry.
This polymer material generated by the polymerization reaction of ethylene monomers has built a huge application system from micron-level films to 10,000-ton storage tanks with its process flexibility and performance adjustability.
1. Polymerization mechanism: chain revolution of ethylene molecules
The essence of PE synthesis is the chain polymerization reaction of ethylene (C₂H₄) monomers. According to different initiation methods, it is mainly divided into two categories:
(I) Free radical polymerization: disordered growth under high temperature and high pressure.
Reaction conditions: Under 100-300℃ and 100-300MPa, with peroxide (such as benzoyl peroxide) as initiator, ethylene molecules are formed into long chains through free radical addition.
Molecular characteristics: The chain structure contains a large number of branches (15-30 branches per 1000 carbon atoms), resulting in low crystallinity (50%-65%) and low density (0.910-0.925g/cm³), i.e. low-density polyethylene (LDPE).
Process limitations: High-pressure resistant equipment is required (reactor wall thickness reaches 100mm), and energy consumption accounts for 40% of the total energy consumption of PE production, but it is suitable for the production of high-transparency films (such as cling film, light transmittance>90%).
(II) Coordination polymerization: precise growth controlled by catalyst
Ziegler-Natta (Z-N) catalysis: TiCl₄-Al (C₂H₅)₃ is used as a catalyst to initiate ethylene polymerization at 60-90℃ and 1-10MPa. The product is a linear structure with a crystallinity of 80%-95% and a density of 0.941-0.965g/cm³, i.e. high-density polyethylene (HDPE).
Metallocene catalysis: Using the Cp₂ZrCl₂-AlMe₃ system, the molecular weight distribution can be precisely controlled (the polydispersity index is close to 1.0), and linear low-density polyethylene (LLDPE) is produced, whose branches are short and uniform (5-15 branches per 1000 carbon atoms), and the puncture resistance is 30% higher than that of LDPE.
Technological breakthrough: Unipol gas phase process of Univation Company in the United States realizes ethylene polymerization in fluidized bed reactor, with catalyst efficiency of 10⁴-10⁵g polymer/g catalyst, and energy consumption is reduced by 25% compared with traditional processes.
2. Synthesis process: the leap from laboratory to industrialization
The industrial production of PE is divided into three major routes according to product type:
(I) LDPE: the classic inheritance of high-pressure process
Using kettle or tubular reactor, the typical process includes:
Ethylene compression (multi-stage compressor boosted to 200MPa);
Initiator injection (concentration 0.01%-0.1%);
Polymerization reaction (residence time 30-60 seconds, conversion rate 20%-35%);
Product separation (removal of unreacted ethylene by flash evaporation);
Pelletization (melt extrusion pelletizing, particle size 2-4mm).
The LDPE produced by this process is used to make plastic bags (thickness 8-12μm) and agricultural films (tensile strength 15-20MPa), with an annual global output of about 15 million tons.
(II) HDPE: Efficiency Revolution of Low-Pressure Process
Slurry method and gas phase method are the mainstream processes:
Slurry method: Ethylene is polymerized in hexane solvent, and the product exists in the form of suspended particles, which are dried and granulated after centrifugal separation. BP's Innovene process uses a double-loop reactor with a single production line capacity of 300,000 tons/year, which is used to produce pipe-grade HDPE (such as PE100, hoop stress>10MPa).
Gas phase method: Ethylene contacts the catalyst in a gas-phase fluidized bed, and the heat of reaction is removed by circulating gas. ExxonMobil's ClearFlo process can produce HDPE with a wide molecular weight distribution for injection molding (such as trash cans, impact strength>25kJ/m²).
(III) LLDPE: Performance optimization of copolymerization process
It is produced by copolymerization of ethylene and α-olefins (such as 1-butene, 1-hexene), with a copolymer content of 3%-10%.
Dow Chemical's Insite technology uses solution polymerization to produce LLDPE with an octene content of up to 20%. Its tensile strength is 50% higher than that of LDPE and is used to make heavy packaging films (such as 25kg rice packaging bags).
3. Safety attributes: dialectics of solid non-toxicity and process risks
The safety of PE needs to be evaluated from the material form and production links respectively:
(I) Environmental friendliness of solid PE
Pure PE resin has stable chemical properties in the solid state, and no harmful substances such as heavy metals and phthalates have been detected. It is listed as a safe material for food contact by the US FDA (21 CFR 177.1520). Its biocompatibility allows it to be used in the medical field, such as hemodialysis tubing (inner wall smoothness Ra＜0.8μm, reducing the risk of thrombosis).
In the environment, the degradation cycle of PE is more than 500 years, but by adding photodegradants (such as benzophenone) or bio-based modifications (such as starch blending), the complete degradation time can be shortened to 5-10 years.
(II) Potential hazards in the production process
Ethylene gas risk: Ethylene is a flammable and explosive gas (explosion limit 2.7%-36%), and the production equipment must be equipped with explosion-proof walls (fire resistance limit > 4 hours) and emergency release systems.
Catalyst toxicity: Titanium tetrachloride in Z-N catalyst reacts violently with water to release hydrogen chloride gas (LC₅₀=200ppm), and chemical protective clothing must be worn during operation (penetration time > 60 minutes).
Thermal cracking products: PE may produce low molecular weight olefins such as acrolein (LD₅₀=46mg/kg) at high temperatures (>400℃), and a catalytic combustion device must be installed in the exhaust gas treatment system (removal efficiency > 99%).
4. Environmental Challenges and Sustainable Paths
The widespread use of PE has brought about dual impacts:
(I) White pollution dilemma
About 14 million tons of PE waste enters the natural environment every year, and PE microplastics account for more than 60% of the ocean. Solutions include:
Mechanical recycling: Germany uses near-infrared spectroscopy to sort PE and PP, with a purity of 98%, and recycled materials are used to produce outdoor flooring (bending strength>30MPa).
Chemical recycling: The closed-loop recycling technology developed by Loop Industries in the United States decomposes PE into monomers with an atomic utilization rate of 95%. A demonstration unit with an annual processing capacity of 10,000 tons has been built.
(II) Innovative breakthroughs in bio-based PE
Brazil's Braskem uses sugarcane ethanol to produce ethylene to produce "green PE", which has a carbon footprint 70% lower than traditional processes and is used to manufacture cosmetic packaging (such as Natura's shampoo bottles). The production cost of bio-based PE has dropped from $3,000/ton in 2010 to $1,800/ton in 2023, and its market share has increased year by year.
5. Future Outlook: Finding a Balance in Innovation
The development of polyethylene is a microcosm of the "efficiency and responsibility" game in the chemical industry. From the roughness of high-pressure processes to the precision of metallocene catalysis, from the dependence on fossil raw materials to the exploration of bio-based routes, the evolution of PE has always been accompanied by technological breakthroughs.
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