High-Density Polyethylene, commonly known as HDPE, is one of the most widely used and sought-after thermoplastic materials in modern industry. The material was developed in 1951 by J. Paul Hogan and Robert L. Banks at Phillips Petroleum laboratories and was commercially launched in 1954 under the name Marlex. Since then, it has become the preferred choice for a wide range of applications due to its unique combination of properties: excellent flexibility, high chemical resistance, outstanding impact strength, and exceptional performance at low temperatures.
In the chemical industry, water and wastewater treatment facilities, food plants, and agricultural and energy industries, polyethylene is used to manufacture storage tanks, piping systems, containment basins, and a wide variety of process equipment. This article provides a professional overview of the material's properties, advantages, and disadvantages, and a comparison with other thermoplastic materials to assist in selecting the right material for each application.
What is Polyethylene? Physical and Chemical Properties
Polyethylene is a semi-crystalline thermoplastic polymer produced through the polymerization of ethylene gas. Its linear molecular structure, with minimal branching, provides relatively high density and excellent mechanical properties. HDPE belongs to the polyolefin family, along with polypropylene (PP), sharing many basic properties but offering its own unique advantages.
In terms of temperature resistance, polyethylene performs well in a range between- 50 and 80 degrees Celsius for continuous use, with the ability to withstand temperatures up to 120 degrees Celsius for short periods. The material's melting point ranges between 130 and 137 degrees Celsius. The outstanding advantage of HDPE over other thermoplastic materials is its excellent resistance to freezing temperatures, making it an ideal choice for applications in cold environments or systems exposed to significant temperature fluctuations.
The chemical resistance of HDPE is among the highest of thermoplastic materials. The material is resistant to most inorganic and organic acids, bases, salts, and aqueous solutions at various concentrations. It is suitable for storage and transport of sulfuric acid, hydrochloric acid, basic solutions such as sodium hydroxide, and a wide range of industrial chemicals. A significant advantage of HDPE over PP is its better resistance to oxidizing agents. However, HDPE should not be used in contact with certain organic solvents such as benzene and toluene, or with oils and pesticides at high concentrations and elevated temperatures.
The material's density ranges between 0.94 and 0.97 grams per cubic centimeter, slightly higher than polypropylene but still lighter than water. The tensile strength of HDPE ranges between 21 and 37 MPa, and it exhibits excellent flexibility and impact strength, making it particularly resistant to shocks and mechanical impacts.
Polyethylene Grades in Industry
In the piping and tank industry, two main grades of polyethylene are common: PE80 and PE100. The number indicates the Minimum Required Strength (MRS) in megapascals. PE100 offers higher strength and allows the use of thinner walls for the same pressure rating, making it more economical and efficient for high-pressure piping systems. Selecting the appropriate grade depends on project requirements, expected working pressures, and required safety standards.
HDPE Polyethylene Technical Specifications
The following data summarizes the physical, mechanical, and chemical properties of high-density polyethylene in standard commercial grades. It should be noted that these values may vary depending on the material grade, additives, and manufacturing conditions. When designing a specific system, it is recommended to verify the data with the material manufacturer and perform compatibility tests for actual working conditions.
Physical and Mechanical Properties
| Property | Value | Units | Notes |
|---|---|---|---|
| Density | 0.94-0.97 | g/cm³ | Relatively high density for polyolefins |
| Melting Point | 130-137 | °C | Lower than PP (160-170°C) |
| Continuous Service Temperature | -50 to 80 | °C | Excellent cold resistance |
| Maximum Short-Term Temperature | Up to 120 | °C | Limited time exposure |
| Tensile Strength | 21-37 | MPa | Depends on grade and additives |
| Elastic Modulus | 0.8-1.2 | GPa | More flexible than PP |
| Elongation at Break | 100-1000 | % | Very high flexibility |
| Water Absorption | <0.01 | % | Excellent water impermeability |
| Hardness | 60-70 | Shore D | Hard surface |
| Impact Resistance (Izod) | 20-210 | J/m | High impact strength |
Chemical Resistance
| Chemical Group | Examples | Resistance Level | Limitations |
|---|---|---|---|
| Inorganic Acids | Sulfuric acid, phosphoric acid, hydrochloric acid | ⭐⭐⭐⭐⭐ Excellent | Including diluted nitric acid |
| Bases | Sodium hydroxide, potassium hydroxide, ammonia | ⭐⭐⭐⭐⭐ Excellent | All concentrations |
| Salts and Aqueous Solutions | Chlorides, sulfates, nitrates | ⭐⭐⭐⭐⭐ Excellent | All concentrations |
| Alcohols | Methanol, ethanol, isopropanol | ⭐⭐⭐⭐⭐ Excellent | All concentrations |
| Oxidizing Agents | Hydrogen peroxide, diluted chlorine solutions | ⭐⭐⭐⭐ Good | Better than PP for oxidizers |
| Oils and Fats | Mineral oils, animal fats | ⭐⭐⭐ Moderate | At low temperatures only |
| Aromatic Solvents | Benzene, toluene, xylene | ⭐⭐ Limited | Swelling at high temperatures |
| Chlorinated Solvents | Chloroform, dichloromethane | ⭐ Not suitable | Swelling and damage |
| Fuels | Gasoline, diesel, kerosene | ⭐⭐ Limited | Short-term exposure only |
Comparison Between Polyethylene and Other Thermoplastic Materials
Selecting the appropriate material for an industrial project requires understanding the differences between the various available materials. Below is a comparison between polyethylene and other common materials:
Polyethylene vs. Polypropylene (PP): Both materials belong to the polyolefin family and share similar properties, but significant differences exist. HDPE is more flexible and more resistant at sub-zero temperatures, while PP is more rigid and stronger and performs better at high temperatures (up to 100 degrees Celsius for continuous use and 120 degrees Celsius for short periods, compared to 80 and 120 degrees for HDPE). The melting point of PP is higher (160-170 degrees Celsius compared to 130-137 degrees for HDPE). In terms of chemical resistance, HDPE is more resistant to oxidizing agents, but PP is more resistant to certain organic solvents. For projects requiring operation at freezing temperatures, maximum flexibility, or resistance to oxidizing agents, HDPE is the preferred choice.
Polyethylene vs. PVC: Polyvinyl chloride (PVC) is a more rigid material than HDPE and has good fire resistance. However, HDPE is more flexible, more resistant to mechanical impacts, and performs better at low temperatures. PVC is more common in sewage and cold water systems in construction, while HDPE is preferred for pressure systems, underground piping, and industrial applications requiring flexibility.
Polyethylene vs. PVDF: Polyvinylidene fluoride (PVDF) offers even higher chemical resistance than HDPE, especially to oxidizing agents and halogens, and operates in a wider temperature range. However, its cost is significantly higher. PVDF is intended for extreme applications where HDPE does not provide an adequate solution, such as working with hydrofluoric acid or concentrated chlorine.
Industrial Applications of Polyethylene
The versatility of polyethylene is reflected in the wide range of its industrial applications. The material is used for manufacturing process equipment, storage and conveyance systems, and environmental safety solutions across various industrial sectors.
Polyethylene tanks are used for storing chemicals, water, and wastewater in chemical plants, water treatment facilities, and the food industry. The tanks are manufactured in volumes ranging from hundreds of liters to tens of cubic meters, with options for custom adaptation of shape, connections, and accessories. The main advantage of HDPE tanks is the combination of flexibility, impact resistance, and strength at low temperatures.
Polyethylene piping provides a proven solution for conveying water, wastewater, and chemicals in industry and infrastructure. The piping is manufactured in various diameters and connection types, adapted for operation at varying pressures. A significant advantage of HDPE piping is the ability to heat-weld, providing a homogeneous and strong connection, along with flexibility that allows easy installation even in complex routes.
Polyethylene containment basins are designed to contain leaks and chemical effluents in accordance with environmental regulatory requirements. The basins are manufactured in various sizes and adapted to the type of chemicals stored, with containment capacity of 110% of the largest tank volume or 25% of total tank volume in the area, according to regulations. HDPE is particularly suitable for containment basins in areas with temperature fluctuations or cold environments.
Polyethylene water storage tanks are especially common in the water industry, agriculture, and firefighting facilities. The material is approved for contact with drinking water and offers years of durability without special maintenance.
Plast Hen specializes in the design, development, and manufacture of polyethylene tanks and systems with full customization. With over thirty years of experience and the capability to produce large storage tanks without size limitations, the company provides solutions for the chemical, pharmaceutical, semiconductor, food, water, and agricultural industries. Each tank is designed and manufactured with strict adherence to international quality standards, including DVS 2205 for thermoplastic welding, and is accompanied by engineering support through to on-site installation.
Polyethylene Welding and Quality Standards
One of the significant advantages of polyethylene is the ability to heat-weld it without the need for adhesives. Thermoplastic welding produces a homogeneous joint with strength similar to the material itself, unlike glued or mechanical connections that may constitute a weak point.
Common welding methods for polyethylene include hot plate welding, extrusion welding for piping and large tank connections, electrofusion welding for field piping connections, and hot air welding for repairs and finishing work. The choice of welding method depends on material thickness, part size, and strength requirements.
The German DVS 2205 standard is the accepted industry standard for welding tanks and process equipment from thermoplastic materials. The standard defines requirements for design, manufacturing, and quality testing, ensuring that the final product meets safety and performance requirements. Additionally, HDPE piping for water infrastructure complies with international standard ISO 4427, which defines requirements for polyethylene piping systems for water supply and sewage and drainage systems.
Considerations in Selecting Polyethylene for a Project
When selecting polyethylene for an industrial project, several factors must be considered. First, chemical compatibility with materials that will come into contact with the equipment must be examined, taking into account concentrations and operating temperatures. Second, the expected temperature range must be considered. HDPE excels at low temperatures but is more limited at high temperatures compared to PP. Third, flexibility and impact resistance requirements must be examined, areas where HDPE particularly excels.
Regarding costs, the final price of tanks, piping, or other polyethylene equipment is determined by several factors: volume and dimensions, wall thickness, shape complexity, type of connections and accessories, and special requirements such as UV protection for outdoor use. Therefore, each project requires individual assessment and customized engineering design.
