Low Density Polyethylene (LDPE)
Learn about the properties, applications, advantages, disadvantages, and toxicity of this material.
In this article, you will learn what low-density polyethylene is, how it is made, its properties, advantages, disadvantages, it's uses, and health safety information.
What is Low Density Polyethylene (LDPE)?
Low Density Polyethylene (LDPE) is a flexible, odorless, transparent, 100% recyclable thermoplastic polymer popular in products like grocery/garbage bags, juice containers, and cling wrap. Its flexibility, toughness, and corrosion resistance combined with its low-cost, high-efficiency production process make it an appealing choice for engineering applications, fueling demand for the production of millions of tons of LDPE each year.
LDPE is identified on plastic products as a number “4” within an arrow triangle. LDPE is a flexible, odorless, transparent, 100% recyclable thermoplastic polymer. It is popularly used in products like grocery/garbage bags, juice containers, and cling wrap.
Polyethylene is a thermoplastic polymer derived from petroleum composed of long chains of ethylene molecules known as monomers, which can be represented by the form (C2H4)n.
Properties of the resulting material will vary depending on the manufacturing methods used. In the family of polyethylene materials, the unique properties of LDPE are traced back to its highly branched, tree-branch-like bonding structure. This branching gives LDPE its signature flexibility and ductility due to its lower crystallinity, making it quite different from more linear varieties of polyethylene like high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). This branching effect is visualized in the image below:
Branch like bonding structure for LDPE.
How is Low Density Polyethylene (LDPE) made?
Low Density Polyethylene is made through a process known as free radical polymerization. This process requires heat (420-570 degrees K) and a lot of pressure (1000-3000 atm), so it is commonly conducted in an autoclave or tubular reactor in large batches. The first step is to gather the raw materials from petroleum by breaking down the petroleum-based hydrocarbons into smaller molecules and then separating the ethylene gas from the resulting mixture. This ethylene gas is then pumped into the reactor, where it bakes alongside an initiator like oxygen or organic peroxide to initiate polymerization. A slurry of polyethylene forms and gets extruded into granules, with any unreacted gas left in the reactor being used for the next batch. The image below shows the molecular structure of polyethylene and ethylene.
Polyethylene (polythene, PE, polyethene) and ethylene (ethene) molecule.
Image Credit: Bacsica/Shutterstock.com
Once extruded, the LDPE granules can be packaged or processed further to create products through a variety of fabrication processes, such as:
- Injection Molding
- Blow Molding
- Thermoforming
- Vacuum Casting
- Film Making
- Extrusion
These methods are used to produce a range of low density polyethylene products, which are explored in greater detail in this article. For more information on alternative production methods for other types of polyethylene, see our article on High Density Polyethylene (HDPE).
What Are the Properties of Low Density Polyethylene (LDPE)?
Table 1 below illustrates the material properties of low density polyethylene:
| Property | English | Metric |
|---|---|---|
Property Density | English 0.0331-0.0347 (lb/in3) | Metric 0.915-0.960 (g/cm3) |
Property Tensile Strength, Yield | English 1100-2030 (psi) | Metric 7.60-14.0 (MPa) |
Property Tensile Strength, Ultimate | English 1300-7830 (psi) | Metric 8.96-54.0 (MPa) |
Property Modulus of Elasticity | English 22.0-42.1 (ksi) | Metric 0.152-0.290 (GPa) |
Property Elongation at Break | English 226-650 (%) | Metric 226-650 (%) |
Property Hardness, Shore D | English 42.0-56.0 | Metric 42.0-56.0 |
Property Melting Point | English 216-235 (oF) | Metric 102-113 (oC) |
Property Processing Temperature | English 293-644 (oF) | Metric 145-340 (oC) |
Property Moisture Vapor Transmission (@37.8oC/100oF) | English 1.04 (cc-mil/in2-24hr-atm) | Metric 0.410 (cc-mm/m2-24hr-atm) |
Property Water Vapor Transmission | English 0.0167-1.55 (g/100in2/day) | Metric 0.260-24.0 (g/m2/day) |
The most notable properties of LDPE are its toughness, its high flexibility at low temperatures, and its corrosion resistance. It has good strength relative to its density, which explains why it has widespread use in the production of small, lightweight carrying containers like plastic bags.
Some of the key properties of low density polyethylene include:
- More branching in molecular structure
- Low temperature flexibility
- Toughness
- Corrosion/Chemical Resistance
What Are the Applications of Low Density Polyethylene (LDPE)?
Low density polyethylene has properties that are attractive for applications in food-safe bagging, waterproof packaging, and other packaging materials. Thanks to its low density and high strength, very thin sheets of LDPE can be used to hold relatively high loads (think of grocery bags or soda-can six-pack rings). This combined with its low cost and recyclability makes it an excellent choice for a range of high production volume applications, some of which are listed below:
The applications of low density polyethylene include:
- Juice Containers
- Cling Wrap
- Bags
- Trays
- Six-Pack Rings for Soda Cans
- Tubing
- Prosthetics
- Wash Bottles
- Ice Cream Lids
- Extrusion Molding
- Laminates
The pie chart below illustrates the broad uses of low density polyethylene:
Uses of low density polyethylene.
Image Credit: Poly(ethene) (Polyethylene)
What Are the Advantages of Using Low Density Polyethylene (LDPE)?
The advantages of Low Density Polyethylene include:
- Flexibility, low crystallinity
- Moisture resistance, low permeability
- Impact Resistance
- Chemical Resistance
- Cost-effective, economical, and widely recyclable
What Are the Disadvantages of Using Low Density Polyethylene (LDPE)?
The disadvantages of low density polyethylene are:
- Susceptibility to stress cracking
- Low strength, stiffness, and maximum service temperature
- High gas permeability, particularly with carbon dioxide
- Poor UV resistance
- Highly Flammable
What Are the Health Effects of Low Density Polyethylene (LDPE)?
Low Density Polyethylene is one of the safest plastics in widespread use today, mainly because it is nontoxic, and is not carcinogenic. Even when ingested (in small quantities), polyethylene does not cause any hazardous health effects. This means that Low Density Polyethylene is considered safe and non hazardous for human skin contact, inhalation, and ingestion.
However, it should be noted that during the production of LDPE, the material may come into contact with hazardous chemicals which may blend with the plastic as it is processed. As a result, chemicals may leach into food or the environment during use, compromising the sterility of the end product. This primarily applies to recycled low density polyethylene, with ‘virgin’ low density polyethylene products being considered food-grade by the FDA.
Is Low Density Polyethylene (LDPE) Toxic?
Low Density Polyethylene is non-toxic. It is considered to be safe enough to be in contact with food as seen from its widespread use in sandwich bags, bread bags, and juice container lamination.
It should be noted, however, that recycled polyethylene does not have the same guarantee of sterility and purity as ‘virgin’ polyethylene, since environmental contaminants it is exposed to during its lifecycle can become embedded in the recycled product. In those cases, despite polyethylene being nontoxic, the potentially hazardous foreign chemicals may leach into the environment or into food. For this reason, the FDA has only given a food-grade rating to ‘virgin’ low density polyethylene plastics. Plastic leaching increases with acidity, duration of contact, and heat, so it is important to be conscious of what and where you are storing your polyethylene.
What Are the Differences Between Low Density Polyethylene (LDPE) and High Density Polyethylene (Hdpe)?
Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE) differ in the more linear arrangement of molecules within the HDPEs polymer chain, giving it greater stiffness, strength, higher crystallinity, and lower permeability than LDPE.
The lower crystallinity comes as a consequence of reducing the frequency of branching within the polymer chain, allowing HDPE molecules to arrange closer together. Lower permeability is another upside to having closely packed molecules, giving HDPE greater chemical resistance. HDPE also benefits from improved thermal resistance, allowing it to operate up to temperatures of 100oC. These properties make HDPE a great candidate for some more heavy-duty applications where higher chemical resistance and stiffness are required.
Since their thermo-chemical-mechanical properties vary so much, the applications of HDPE and LDPE are different. When selecting the right plastic for an application, it is important to consider the operating conditions of the product, ensuring that it is optimal to meet the chemical-mechanical demands. For more information on the properties and applications of High Density Polyethylene, see our related article on HDPE.
Summary
This article summarized the properties, uses, applications, advantages, and toxicity of low density polyethylene (LDPE).
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