Polyethylene

How is PE Made?

Polyethylene is made by addition or radical polymerization of ethylene (olefin) monomers. (Chemical formula of Ethene – C₂H₄). Ziegler-Natta and Metallocene catalysts are used to carry out polymerization of polyethylene.

Common Types of Polyethylene (PE)

PE belongs to polyolefin family of polymers and is classified by its density and branching. The most common types of polyethylene are:

• • Branched Versions
    • Low-density polyethylene (LDPE)
    • Linear low-density polyethylene (LLDPE)

• • Linear Versions
    • High-density polyethylene (HDPE)
    • Ultra-high-molecular-weight polyethylene (UHMWPE)

• Cross-linked polyethylene (PEX or XLPE)

In addition PE is also available in other types such as: (not discussed in detail in this guide)

• Medium-density polyethylene (MDPE)
• Very-low-density polyethylene (VLDPE)
• High-molecular-weight polyethylene (HMWPE)
• Ultra-low-molecular-weight polyethylene (ULMWPE)
• Chlorinated polyethylene (CPE)

Some of the polyethylene suppliers include: Borealis, Celanese Corporation, Dow Chemicals, ExxonMobil Chemical, Lyondell Basell, NOVA Chemicals, SABIC. See all Polyethylene Suppliers

High Density Polyethylene (HDPE)

High Density Polyethylene (HDPE) is a cost-effective thermoplastic with linear structure and no or low degree of branching. It is manufactured at low temperature (70-300°C) and pressure (10-80 bar) & derived from either:

• Modifying natural gas (a methane, ethane, propane mix) or
• The catalytic cracking of crude oil into gasoline

HDPE is produced majorly using two techniques: Slurry Polymerization or Gas Phase Polymerization.

High density polyethylene is flexible, translucent/waxy, weather resistant, and displays toughness at very low temperatures.

Properties of High Density Polyethylene

1. HDPE Melting point: 120-140°C
2. Density of HDPE: 0.93 to 0.97 g/cm³
3. High Density Polyethylene Chemical resistance:
   • Excellent resistance to most solvents
   • Very good resistance to alcohols, dilute acids and alkalis
   • Moderate resistance to oils and greases
   • Poor resistance to hydrocarbons (aliphatic, aromatic, halogenated)
4. Continuous temperature: -50°C to +60°C, Relatively stiff material with useful temperature capabilities
5. Higher tensile strength compared to other forms of polyethylene
6. Low cost polymer with good processability
7. Good low temperature resistance
8. Excellent electrical insulating properties
9. Very low water absorption
10. FDA compliant

Disadvantages of HDPE

• Susceptible to stress cracking
• Lower stiffness than polypropylene
• High mold shrinkage
• Poor UV- and low heat resistance
• High-frequency welding and joining impossible

However, weathering resistance of HDPE can be improved by the addition of carbon black or UV absorbing additives. Carbon black also helps to reinforce the material.

Applications of High Density Polyethylene (HDPE)

Excellent combination of properties makes HDPE an ideal material in diverse applications across industries. It can be engineered according to the end use requirements.

Some of the major uses of high density polyethylene include:

1. • Packaging Applications – High Density Polyethylene is used in several packaging applications including crates, trays, bottles for milk and fruit juices, caps for food packaging, jerry cans, drums, industrial bulk containers etc. In such applications HDPE provides the end product a reasonable impact strength.

1. • Consumer Goods – Low cost and easy processability make HDPE a material of choice in several household/ consumer goods like garbage containers, housewares, ice boxes, toys etc.

• Fibers and Textiles – Thanks to its high tensile strength, HDPE is widely used in ropes, fishing and sport nets, nets for agricultural use, Industrial and decorative fabrics, etc.

                

Other Popular Applications of HDPE

Low Density Polyethylene (LDPE)

Low Density Polyethylene (LDPE) is a semi-rigid and translucent polymer. Compared to HDPE, it has a higher degree of short and long side-chain branching. It is produced at high pressure (1000-3000 bar; 80-300°C) via free radical polymerization process.

The LDPE is composed of 4,000-40,000 carbon atoms, with many short branches.

Two basic processes used for the production of low density polyethylene: stirred autoclave or tubular routes. The tubular reactor has been gaining preference over the autoclave route due to its higher ethylene conversion rates.

Properties of Low Density Polyethylene

1. LDPE Melting point: 105 to 115°C
2. Density of LDPE: 0.910–0.940 g/cm³
3. Chemical resistance of LDPE:
   • Good resistance to alcohols, dilute alkalis and acids
   • Limited resistance to aliphatic and aromatic hydrocarbons, mineral oils, oxidizing agents and halogenated hydrocarbons
4. Temperature resistance up to 80°C continuously and 95°C for shorter times.
5. Low cost polymer with good processability
6. High impact strength at low temperature, good weatherability
7. Excellent electrical insulating properties
8. Very low water absorption
9. FDA compliant
10. Transparent in thin film form

Disadvantages of LDPE

• Susceptible to stress cracking
• Low strength, stiffness and maximum service temperature. This limits its usage in applications requiring extreme temperatures.
• High gas permeability, particularly carbon dioxide
• Poor UV resistance
• Highly flammable
• High-frequency welding and joining impossible

Applications of Low Density Polyethylene (LDPE)

Low Density Polyethylene (LDPE) uses majorly revolve around manufacturing containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components, and various molded laboratory equipment. The most popular application of low density polyethylene is plastic bags.

1. Packaging – Thanks to its low cost and good flexibility, LDPE is used in packaging industry for pharmaceutical and squeeze bottles, caps and closures, tamper evident, liners, trash bags, films for food packaging (frozen, dry goods, etc.), laminations etc.
2. Pipes and Fittings – Low Density Polyethylene is used to manufacture water pipes and hoses for the pipes and fittings industry due to Its plasticity and low water absorption.

Other applications include consumer goods – housewares, flexible toys, agricultural films, wiring & cables – sub-conductor insulators, cable jacketing.

Linear Low Density Polyethylene (LLDPE)

LLDPE is produced by polymerization of ethylene (or ethane monomer) with 1-butene and smaller amounts of 1-hexene and 1-octene, using Ziegler-Natta or metallocene catalysts. It is structurally similar to LDPE.

The structure of LLDPE has a linear backbone with short, uniform branches (unlike longer branches of LDPE). These short branches are able slide against each other upon elongation without becoming entangled like LPDE.

In the present day scenario, Linear low density polyethylene (LLDPE) has been quite successful in replacing Low Density Polyethylene.

Properties of LLDPE

• Very flexible with high impact strength
• Translucent and natural milky color
• Excellent for mild and strong buffers, good chemical resistance
• Good water vapor and alcohol barrier properties
• Good stress crack and impact resistance

Applications of LLDPE: Suitable for a variety of film application such as general purpose film, stretch film, garment packaging, agricultural film, etc.

Benefits of Polyethylene Films

• PE films burn to carbon dioxide and water with no residue. There are no toxic fumes or gases and no cinders produced in this process
• PE films contain no plasticizers and no heavy metals. They are physiologically harmless
• No odor pollution or wastewater are produced in the manufacture of PE films

Ultrahigh-Molecular-Weight Polyethylene (UHMWPE)

Ultrahigh-molecular-weight polyethylene or UHMWPE has a molecular weight about 10 times higher (usually between 3.5 and 7.5 million amu) than High Density Polyethylene (HDPE) resins. It is synthesized using metallocene catalysts and ethane units resulting is structure where ethane units are bonded together resulting in UHMWPE structure typically having 100,000 to 250,000 monomer units per molecule.

• It has excellent mechanical properties such as high abrasion resistance, impact strength and low coefficient of friction.
• The material is almost totally inert, therefore it is used in the most corrosive or aggressive environments at moderate temperatures.
• Even at high temperatures, it is resistant to several solvents, except aromatic, halogenated hydrocarbons and strong oxidizing materials, such as nitric acid.
• These special properties allow the product to be used in several high performance applications.
• UHMWPE is suitable for high wear applications such as tubes, liners, silos, containers and other equipment.

Cross-linked Polyethylene (PEX or XLPE)

High-density crosslinked polyethylene, or XLPE, is a form of polyethylene with crosslinked structure specifically designed for critical applications.

Cross linked polyethylene is produced from polyethylene under high pressure with organic peroxides which creates a free radical. The free radical generates the crosslinking of the polymer which results in a resin that is specifically designed for critical applications like chemical storage pipework systems, hydronic radiant heating and cooling systems, and insulation for high voltage electrical cables.

Key Features of XLPE

• High and low temperature
• Hydrolysis resistance
• High electrical and insulation properties
• High abrasion resistance
• Potable water approved
• High extrusion speed on standard lines
• Lower cost
• Mechanically tougher

Comparison Between Main Types of Polyethylene

	LDPE	LLDPE	HDPE
Polymer Full Name	Low Density Polyethylene	Linear Low Density Polyethylene	High Density Polyethylene
Structure	High Degree of short chain branching + long chain branching	High Degree of short chain branching	Linear (or Low degree of short chain branching)
Catalyst and process	Using radical polymerization using tubular method or auto clave method	Using Ziegler-Natta catalyst or metallocene catalyst	Ziegler-Natta catalyst in: – Single-stage polymerization – Multi-stage polymerization or a Cr or Phillips-type catalyst
Density	0.910-0.925 g/cm3	0.91-0.94 g/cm3	0.941-0.965 g/cm3
Crystallinity	Low crystalline and high amorphous (less than 50-60% crystalline)	Semi-crystalline, level between 35 to 60%	High crystalline and low amorphous (>90% crystalline)
Characteristics	Flexible and good transparency Good moisture barrier properties High impact strength at low temperature Excellent resistance to acids, bases and vegetable oils	As compared to LDPE, it has: higher tensile strength higher impact and puncture resistance	Excellent Chemical Resistance High tensile strength Excellent moisture barrier properties Hard to semi-flexible
Recycling Code
General Applications	Shrink wrap, films, squeezable bottles garbage bags, extrusion moldings, and laminates	High performance bags, cushioning films, tire separator films, industrial liners, elastic films, ice bags, bags for supplemental packaging and garbage bags	Molecular weight distribution is relatively narrow, has applications in injection moldings or flat yarns, and the latter type Molecular weight distribution is wide, is used to make film products, hollow plastic products and pipes

Difference Between PE, PU and PVC Tubing

PE, polyurethanes and PVC are widely used thermoplastics for agricultural pipes, tubing, hose pipes and for creating custom pipe solutions. Although no plastic tubing product can universally handle all applications, and there are certain differences that need to be considered per application area.

As compared to PU, polyethylene is less flexible but offers good moisture resistance. Polyurethane pipes are used where flexibility, kink resistance along with exceptional abrasion resistance is needed like cable jacketing, pneumatic controls, analytical instrumentation, etc. Whereas, polyethylene pipes exhibit high strength, good corrosion- and chemical resistance & hence suitable for use in municipal, industrial, marine, mining, landfill, duct and agricultural applications.

While, flexible PVC has several benefits like good chemical and corrosion resistance, excellent abrasion and wear resistance, rubber-like flexibility, visual contact with the flow (with clear styles), and outstanding flow characteristics. These properties enable PVC tubing use in general industrial, food and beverage, potable water lines, medical, chemicals, fuels, oils, and mechanical applications.

How to Process PE Plastic?

Various forms of Polyethylene can be used in processes like injection molding, blow molding, extrusion and various film creation processes such as calendaring or blown film extrusion.

• High density polyethylene can be easily processed by injection molding, extrusion (tubes, blow and cast films, cables, etc.), blow molding and rotomolding. Being and ideal material for injection molding process, it is majorly used for batch and continuous production.
• The most common processing technique used for Low Density Polyethylene is extrusion (tubes, blow and cast films, cables…). Low Density Polyethylene can be processed by injection molding or rotomolding also.

• UHMWPE is processed variously by compression molding, ram extrusion, gel spinning, and sintering. It conventional methods such as injection, blow or extrusion molding, because this material does not flow even at temperatures above its melting point.
• PE is not available for 3D Printing processes because it is more difficult to work with it. But now, recycled and green PE are gaining popularity for processing by 3D Printing. The sheer availability of PE is encouraging efforts to apply this material for additive manufacturing.

Polyethylene Recycling and Toxicity

Resin Identification Code for two main forms of polyethylene are:

LDPE and HDPE are non-biodegradable in nature and contribute significantly to the world’s plastic waste products. Both the forms of polyethylene are recyclable and used to produce bottles for non-food items, plastics for outdoor applications, compost bins, etc.

In solid form, Polyethylene is safe and non-toxic in nature but could be toxic if inhaled and/or absorbed as a vapor or liquid (i.e. during manufacturing processes).

PE (HDPE and XLPE) is widely used for water-related applications. Cross-linked polyethylene has become popular for potable water in recent years but PEX requires special fittings and is not recyclable. High-Density Polyethylene (HDPE) pipes are non-potable water applications. For potable water, HDPE can be used for both hot water and cold water service applications.