Basic Properties And Applications of Polyethylene (PE)

Basic Properties and Applications of Polyethylene (PE)

I. Basic Information

Name: Polyethylene

Abbreviation: PE

Chemical Formula: (C2H4)n

Melting Point: 85 to 136°C

Water Solubility: insoluble

Density: 0.91 to 0.96g/cm3

Appearance: Low molecular weight colorless liquid, high molecular weight colorless milky white waxy granules or powder

Flash Point: 270°C

II. History of Polyethylene (PE) Research

Polyethylene research began at the beginning of the 20th century with the accidental discovery of its early form by the German scientist Hans von Peckmann in 1899, and its re-discovery by the British company I.C.I in 1933, which attracted attention, and the production of low-density polyethylene by the British company I.G.I in the late 1930's, and the production of high-density polyethylene in the 1950's by the West Germans and the United States of America. Since then, research has gone deeper into the structure and properties, including macromolecule stretching chain and high-performance fibers, etc., and the field of application has been expanding.

III. How is PE made?

Polyethylene is made by the polymerization of ethylene (CH2=CH2), a process that involves linking ethylene molecules to form long chains of molecules with repeating -CH2- units through addition polymerization reactions. In industrial production, catalysts are often used to improve the efficiency of the polymerization, for example by using catalysts such as chromium trioxide (Cr2O3). In addition, polyethylene is prepared by polymerization reactions at different pressures and temperatures, such as high-pressure, medium-pressure and low-pressure polymerization methods, which affect the density and physical properties of polyethylene.

IV. Properties of PE

1. Physical properties

• PE has a low water vapor transmission rate, but a high vapor transmission rate for organic compounds. Water absorption is very small, about 0.03%.

2. Mechanical properties

• PE toughness and impact strength is good, hardness, modulus of elasticity and strength of commonly used plastics is low, and the relative molecular mass and its distribution, crystallinity and density and so on.

• High-density polyethylene (HDPE) crystallinity is high, strength is better; low-density polyethylene (LDPE) branched chain, low crystallinity, but impact strength and elongation at break is higher.

3. Thermal properties

• PE fire temperature is about 350℃, PE dust fire temperature is 450℃.

• Low temperature resistance is good, with the increase of relative molecular mass, the narrower the relative molecular mass distribution, the better the low temperature resistance.

4. Chemical properties

• PE is inert to water and chemical reagents, insoluble in general organic solvents at room temperature except for a few solvents.

• Fatty diameter, aromatic light, halogenated by the PE can be dissolved, the temperature exceeds 60 ℃, may be part of the solvent dissolved.

• Soluble in tetrahydronaphthalene and decahydronaphthalene, solubility and crystallinity, relative molecular mass.

• Resistant to dilute acid, alkali, salt solution at room temperature, not resistant to strong oxidizing acid.

• Different density PE oxidation resistance is different, LDPE oxidation resistance is worse than HDPE.

• The effect of gaseous chlorine and fluorine on PE increases with temperature.

• PE and other polymers are poorly compatible, difficult to bond and print, strong oxidizing agents and other treatments can improve adhesion and printability.

5. Electrical properties

• PE has good electrical insulation, which is related to its hydrophobicity and structural characteristics.

• The type and amount of additives are small, and the electrical insulation property is excellent.

• Relative dielectric constant and dielectric strength are related to relative molecular mass, environment and other factors, at room temperature in a specific frequency range and frequency-independent, suitable for high-voltage electrical insulation materials.

6. Environmental stress cracking resistance

• PE environmental stress cracking resistance is related to density, HDPE is more sensitive.

• According to the national standard GB 1842-80 test, the breakage rate of 50% of the time F50 (h) for the environmental stress cracking time, the results of the logarithmic probability of coordinate graphing method.

7. Hygienic

• PE is non-toxic to human body, can be used for food packaging materials, but need to pay attention to the toxicity of additives.

V. Classification and Properties of PE

Polyethylene is a thermoplastic that can be categorized by density into types such as low density polyethylene (LDPE), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE).

HDPE has a density between 0.94 and 0.97 g/cm³ and is mainly produced at atmospheric or lower pressures, but can also be made at high pressures. It is highly crystalline and strong and is mainly used for injection and extrusion molding. Traditional production methods such as the Ziegler method, the Philips method and the standard oil method are effective but have low catalyst efficiency, which affects product properties.

The density of LDPE ranges from 0.91 to 0.935g/cm³, and its characteristics include short branched chain and long branched chain structures, which distinguish it from HDPE.The density and degree of branched chain of LDPE are closely related to its performance: the increase of density improves the tear strength and hardness, but the impact strength decreases. Its molecular weight distribution affects the processing performance and physical and mechanical properties, the wider the distribution, the better the processing fluidity, but the impact strength and stress cracking resistance may be reduced.LDPE is chemically stable, acid and alkali corrosion resistance, excellent electrical properties, but heat resistance and aging resistance is poor.

In recycling labeling, PE usually corresponds to recycling codes 2 and 4, which indicates that these plastics can be recycled. Different applications of polyethylene may have different crystal structures, which affect the performance characteristics of the final product.

Relative Molecular Mass Ranges of Different Polymerization Methods of Polyethylene

VI. Modification

Graft Modification

Graft polymerization, a way to polarize PE, adds functional polar monomers to PE without changing its backbone structure. This method keeps PE's original properties and adds new functions. Key approaches for grafting include:

• Solution Method: PE, monomers, and initiators are dissolved in solvents like toluene or xylene. The solvent's polarity and chain transfer constant greatly affect the reaction.

• Solid-Phase Method: PE powder reacts directly with monomers and initiators. Benefits include ambient temperature and pressure operation, maintenance of polymer properties, no solvent recovery, and simple, efficient post-processing.

• Melt Method: In the molten state, initiators thermally decompose to produce free radicals, which induce graft copolymerization, attaching monomers as side chains.

• Radiation Grafting Method: γ-rays or β-rays irradiate the material to generate free radicals, which then react with monomers for surface modification. Methods include co-irradiation, pre-irradiation, and peroxide-assisted irradiation.

Cross - linking Modification

Cross - linking modification of PE significantly boosts its physical properties, stress crack resistance, corrosion resistance, anti - creep properties, and weatherability, expanding its applications like PEX pipes. There are three main cross - linking methods:

• Radiation Cross - linking: High - energy rays (γ - rays, X - rays) create active particles in PE, inducing chemical reactions to form a cross - linked network.

• Chemical Cross - linking: Free radicals from peroxides or azo compounds react with unsaturated sites in PE molecules, forming active centers that cross - link via monomer bonding.

• Silane Cross - linking: Silanes with unsaturated vinyl groups and hydrolyzable alkoxy groups are grafted onto PE. Hydrolysis and condensation then form —Si—O—Si— cross - linkages.

Copolymerization (Blending ) Modification

1. Copolymerization of PE

Through coordination copolymerization (such as EPR, EPDM), free radical copolymerization (such as EVA) and ionic copolymerization (such as ethylene-(meth)acrylic acid copolymer), etc., to change the characteristics of the PE macromolecular chain or the introduction of reactive functional groups, to enhance the performance and as a reactive bulking agent.

2. Blending Modification of PE

• HDPE/LDPE Blending: Combines LDPE's flexibility with HDPE's strength. Adding LLDPE or VLDPE further improves performance.

• PE/CPE Blending: Adding chlorinated polyethylene (CPE) enhances flame retardancy, printability, and toughness. A compatibilizer is needed to improve compatibility

• PE/EVA Blending: Boosts flexibility, transparency, breathability, and printability, but slightly reduces mechanical strength.

• PE/Rubber Blending: Significantly improves HDPE's impact properties.

• PE/PA Blending: Enhances oxygen and hydrocarbon solvent barrier properties. Compatibility improvement is required.

Filler Modification

Filler modification involves adding inorganic or organic particles to thermoplastic resins to reduce costs or alter product properties, and it can be divided into general and functional filling based on objectives.

1. General Filling: Mainly impacts the mechanical properties of PE. Inorganic fillers like calcium carbonate and talcum powder can reduce costs, increase rigidity, heat resistance, and dimensional stability, but may affect mechanical and flow properties. Coupling agents or MPEW coating can enhance interfacial adhesion. Organic fillers such as straw and wood fiber are also commonly used.

2. Functional Filling: Aims to enhance performance in optical, electrical, magnetic, and combustion aspects.

• Biodegradable PE: Adding modified starch enables microbial degradability.

• Conductive PE: Compounding with conductive fillers like carbon black and metal powder yields conductive materials for anti-static, conductive, heating element control, and electromagnetic shielding applications.

• Flame-retardant PE: Incorporating halogen flame retardants, organic acids, ammonium phosphate, tribromobenzene, or flame-retardant inorganic fillers (e.g., Al(OH)₃, Mg(OH)₂) achieves flame retardancy.

Reinforcement Modification

Reinforcement modification is to enhance the performance of PE by adding reinforcing materials or special molding methods. Among them, self-reinforcement modification makes use of special molding process and mold design to make PE molecular chains oriented in parallel and form straight chain crystals to improve mechanical properties. Reinforcement materials such as glass fibers, synthetic fibers (e.g., polyacrylonitrile, polyamide, etc.) and whiskers (e.g., calcium carbonate, potassium titanate) can significantly improve the mechanical strength and heat resistance of PE and become engineering plastics. Adding interfacial reaction reagents and their grafts can enhance the interfacial bonding properties of composite materials.

Nanoparticle Modification

Nanomaterials refer to materials with particle size less than 100 nm, which can be combined with polymers to form multifunctional new materials due to their unique physicochemical properties. Nano-modified PE materials, including nano montmorillonite, nano zinc oxide, nano alumina and nano clay modified PE, have become the forefront of materials science research.

VII. Fields of application

1. Food packaging and agricultural mulching film

Application scenarios: food packaging bags, cling film, agricultural mulch film, greenhouse materials, etc.

Functions: Provide good sealing and protection performance to extend the shelf life of food; improve crop yield and water management efficiency.

2. Construction and Waterproof Membrane

Application Scenario: Waterproof membrane for construction, cement packaging, etc.

Function: Enhance the waterproof performance of construction, protect the building structure; ensure the storage safety of cement and other building materials.

3. Pipe and Transportation System

Scenarios: water supply, sewage treatment, natural gas transportation, agricultural irrigation pipes, etc.

Function: Excellent tensile strength, abrasion resistance and chemical stability to ensure the safety and efficiency of fluid transportation.

4. Wire and cable manufacturing

Application Scenario: Insulation layer and sheath material for low-voltage wires and cables.

Role: Provide excellent insulating properties and weather resistance to ensure the safety and stability of power transmission.

5. Packaging and Coating Materials

Application Scenario: Pharmaceutical packaging, cardboard coating, polyester film coating, etc.

Role: Improve the sealing and protection of packaging materials and enhance the performance of the substrate.

6. Biomedical Applications

Application scenarios: polymer films, nanocomposites, drug delivery systems and hydrogel materials, etc.

Role: Provide transparent and durable materials to support biomedical research and applications.

7. Industrial and mechanical products

Application Scenario: Injection molded products, plates, gears, etc.

Function: High strength and abrasion resistance, suitable for various industrial applications.

8. Automotive Parts Manufacturing

Scenarios: Fuel tanks, interior parts, etc.

Role: Provide good mechanical properties and chemical resistance to ensure the safety and reliability of automotive parts.

9. Adhesive and coating manufacturing

Scenarios: Hot-melt adhesives, emulsions, polymer-modified asphalt, paving and roofing coatings, etc.

Role: Provide strong adhesion and weatherability to support a variety of construction and engineering applications.

VIII. Conclusion

In recent years, polyethylene production processes have been improved to enhance efficiency and quality. Global demand for polyethylene continues to grow, with the Chinese market benefiting from economic growth and policy support. China will be the main driver of capacity growth in the future, but this may result in low prices. With the promotion of the “dual carbon” strategy, the industry focuses on environmental protection and sustainable development, and promotes the recycling of waste plastics. In the market segments, HDPE, LDPE, LLDPE, etc. continue to develop, metallocene polyethylene and modified polyethylene applications are promising. Overall, the polyethylene industry ushers in opportunities and challenges driven by technology, demand, production capacity and environmental protection.

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