High-density polyethylene

Class of polyethylenes

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High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes.[1] With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber. HDPE is commonly recycled, and has the number "2" as its resin identification code.

In , the global HDPE market reached a volume of more than 30 million tons.[2]

Properties

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Thermophysical properties of high density polyethylene (HDPE)

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Density 961 kg/m3 Melting point 131.8 °C (269.24 °F) Temperature of crystallization 121.9 °C (251.42 °F) Latent heat of fusion 188.6 kJ/kg. Thermal conductivity 0.54 W/m.°C. at °C. Specific heat capacity to J/kg-K Specific heat (solid) 2.9 kJ/kg. °C. Crystallinity 61%

HDPE is known for its high strength-to-density ratio.[4] The density of HDPE ranges from 930 to 970 kg/m3.[5] Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength (38 MPa versus 21 MPa) than LDPE.[6] The difference in strength exceeds the difference in density, giving HDPE a higher specific strength.[7] It is also harder and more opaque and can withstand somewhat higher temperatures (120 °C/248 °F for short periods). High-density polyethylene, unlike polypropylene, cannot withstand normally required autoclaving conditions. The lack of branching is ensured by an appropriate choice of catalyst (e.g., Ziegler&#;Natta catalysts) and reaction conditions.

HDPE is resistant to many different solvents, and is exceptionally challenging to glue; joints are typically made by welding.

The physical properties of HDPE can vary depending on the molding process that is used to manufacture a specific sample; to some degree, a determining factor is the international standardized testing methods employed to identify these properties for a specific process. For example, in rotational molding, to identify the environmental stress crack resistance of a sample, the notched constant tensile load test (NCTL) is put to use.[8]

Owing to these desirable properties, pipes constructed out of HDPE are ideally applicable for drinking water[9] and waste water (storm and sewage).[10]

Applications

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• HDPE fibers can be spun into rope

• Disposable suits; nonwoven HDPE fabric

• Housewrap

• Plastic mailing envelopes

• Flexible HDPE pipes

• Corrugated HDPE pipe installation in storm drain project in Mexico

• The monobloc chair

• Bottle crates

• Toys and playground equipment

• Clear plastic bags (shown) are made of LDPE; blown-film shopping bags with handles are now made of HDPE

• HDPE is used to make sturdy bottles that resist oils. Transparent bottles are usually made of other plastics, such as polyethylene terephthalate

• Milk jug

• HDPE jerrycans resist softening and swelling from aromatic components of fuels

• Roller on HDPE construction entrance mat

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HDPE has a wide variety of applications; for applications that fall within the properties of other polymers, the choice to use HDPE is usually economic:

HDPE sheet which has been extrusion welded

HDPE is also used for cell liners in United States subtitle D sanitary landfills, wherein large sheets of HDPE are either extrusion welded or wedge welded to form a homogeneous chemical-resistant barrier, with the intention of preventing the pollution of soil and groundwater by the liquid constituents of solid waste.

HDPE is preferred by the pyrotechnics trade for mortars over steel or PVC tubes, being more durable and safer: HDPE tends to rip or tear in a malfunction instead of shattering and becoming shrapnel like the other materials.

Milk bottles, jugs, and other hollow goods manufactured through blow molding are the most important application area for HDPE, accounting for one-third of worldwide production, or more than 8 million tonnes.

Above all, China, where beverage bottles made from HDPE were first imported in , is a growing market for rigid HDPE packaging as a result of its improving standard of living. In India and other highly populated, emerging nations, infrastructure expansion includes the deployment of pipes and cable insulation made from HDPE.[2] The material has benefited from discussions about possible health and environmental problems caused by PVC and polycarbonate associated bisphenol A (BPA), as well as its advantages over glass, metal, and cardboard.

Production

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Industrial production of HDPE from ethylene happens through either Ziegler-Natta polymerization or the Phillips slurry process. The Ziegler-Natta method uses a combination of catalysts, including titanium tetrachloride, in contact with gaseous ethylene to precipitate high-density polyethylene.[17] In a similar way, the Phillips slurry process uses silica-based catalysts in contact with a fast-moving hydrocarbon and polyethylene slurry to precipitate high density polyethylene.[18]

Processing will determine the properties of the HDPE. The method used to synthesize the HDPE is crucial because the micro structure of the HDPE will vary. The Phillips Slurry process results in HDPE with less branching and more precise molecular weights than the Ziegler process, but the Ziegler process provides greater flexibility in the type of polyethylene produced.[18]

The molecular weight of HDPE refers to the length of the polyethylene chains, and helps determine properties such as flexibility, yield strength, and melt temperature. After the precipitate is formed, the temperature, pressure, and cooling time during processing will dictate the degree of crystallinity, with a higher degree of crystallinity resulting in greater rigidity and chemical resistance.[19] Depending on the application, the method and processing steps can be adjusted for an ideal result.

Once the HDPE has been synthesized, it is ready to be used in commercial products. Industrial production methods for HDPE products include injection molding for complex shapes such as toys. Extrusion molding is used for constant-profile products such as pipes and films. Blow molding is intended for hollow products, specifically bottles and plastic bags. Rotational molding is used for large, seamless parts such as chemical drums and kayaks.[19] The method used during processing depends on the product requirements, with each having benefits for a given application.

See also

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References

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High-density polyethylene or HDPE is a commonly used petroleum thermoplastic and the most used of the three polyethylenes for a wide range of applications. If you look at this plastic under a microscope, you would see that it has a linear structure with few branches lending to its optimal strength/density ratio. As a result of its molecular makeup, this polymer shines brightest in applications where moisture resistance and cost-effectiveness are needed.

HDPE was created in the s and used in high-frequency radar cables during World War II. It was introduced to the market commercially soon after. While its higher density versions yield a more rigid result, HDPE can vary in flexibility. Low-density grades of the thermoplastic are less stiff and the high-density grades have equally high crystallinity.

The Advantages and Disadvantages of HDPE

Even with its popularity, there are some advantages and disadvantages you should consider before using HDPE for your application:

Advantages

• Cost-effective
• Can withstand temperatures from -148 to 176 degrees Fahrenheit
• Non-leaching
• UV-resistant
• Dishwasher safe
• Resistant to most chemical solvents
• Stiff material

Disadvantages

• Poor weathering resistance
• Flammable
• Sensitive to stress cracking
• Difficult to bond

4 Common Uses for HDPE

1. Plastic bottles: Most people are informally introduced to the durable plastic through water bottles. HDPE is great for blow-molding applications, particularly food and beverage containers because it won&#;t leach into the bottles&#; contents. In addition, HDPE is recyclable which makes it a great choice for these highly disposable products.
2. Toys: HDPE is UV resistant which helps keep the toys resistant to the ray&#;s damaging and discoloring effects.
3. Chemical containers: With its chemical resistant properties, HDPE is great for laundry, shampoo, conditioner, household cleaning products, motor oil, antifreeze and recycling bins. The strength of these bottles is increased when they&#;ve been colored or pigmented.
4. Pipe systems: HDPE pipe grade sheet has a higher molecular weight compared to the standard HDPE used in the above examples. This strengthening with its UV-resistance makes it ideal for piping and outdoor applications. Pipe grade sheet has the ability to withstand -220 to 180 degrees Fahrenheit. It&#;s durable in most chemical interactions which make it useful in a lot of industrial applications.

AC Plastics has HDPE industrial sheets including pipe grade, smooth sheet, and matte sheet to suit a wide range of applications. No matter how large or small your order is, we can fulfill your order. Order today, or call us to speak to one of our specialists.

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The Advantages of HDPE Include

• Affordable
• High-Quality
• Operate and Harsh Temperatures
• Non-Leaching
• UV Resistant
• Resistant to most Chemicals
• Stiff Material
• Amazing Durability
• Highly Versatile

The Disadvantages of HDPE Include

• Poor Weathering
• Highly Flammable
• Sensitive to Stress Cracking
• Not Biodegradable
• Can't Be Composted
• Not Resistant to Oxidizing Acids
• Not Resistant to Chlorinated Hydrocarbons
• High Thermal Expansion
• Poor Temperature Capability

The Top Uses for HDPE Include

• Shampoo Bottles
• Toys
• Chemical Containers
• Pipe Systems
• Milk Jugs
• Recycling Bins
• Grocery Bags
• Cereal Box Liners
• Flower Pots