LDPE is well known for being widely used on plastics bags, it's low density makes it lighter and flexible making it perfect for this kind of application. HDPE, on the other hand, is harder and offers higher strength and better resistance to heat, recently it has become very popular as a source material for 3d printing filaments, it is also used to produce durable plastic parts such as pipes, toys and plastic chairs.
Collectively under the polyethylene class of materials, LDPE (low-density polyethylene) and HDPE (high-density polyethylene) have shaped the landscape of the packaging and manufacturing industry. LDPE and HDPE are both thermoplastic ethylene polymers, although they differ in several properties and uses. One stark contrast is that LDPE has more branching than HDPE, thereby lending the latter stronger forces between molecules. This causes HDPE to have a higher strength-to-density ratio and increased tensile strength.
The varying uses of LDPE and HDPE are based on this fundamental structural difference, among many other properties.
Applications
LDPE
- Plastic bags
- Light packaging materials: Six-pack rings, waterproof carton lining, plastic wraps, snap-on lids
- Wash bottles
- Corrosion protection layer for work surfaces
- Computer hardware covers and packaging
HDPE
- Filament for 3D printers
- Strong packaging materials: bottle caps, plastic milk bottles, drums, bulk containers for industrial use
- Fibres for ropes, nets, and industrial fabrics
- Vehicle fuel tanks
- Boat parts
- Pipes and tubing
- Plastic chairs and tables
- Playground structures: slides, swing seats
- Consumer products: trash and recycling bins, ice cube containers, toys, ice chests
Properties of LDPE and HDPE
Although both materials come from the same ethylene monomer, the difference in chemical structure yields a wide variety of unique properties.
As its name suggests, low-density polyethylene (LDPE) has a lower density, strength, and temperature resistance. Meanwhile, high-density polyethylene (HDPE) is characterized by higher specific strength and heat resistance.
|
Property |
LDPE |
HDPE |
|
Chemical structure |
More branching |
Less branching, more linear |
|
Mechanical |
Low crystallinity, and therefore more flexible |
High crystallinity, which makes it tougher and more rigid |
|
Heat resistance |
Drastic decrease in density when subjected beyond room temperature |
Able to withstand more than 100 °C |
|
Chemical resistance |
Resistant to most alcohols, acids, and alkalis; low resistance to oxidising agents and selected hydrocarbons |
Superior resistance to solvents, alcohols, acids, and alkalis; low resistance to most hydrocarbons |
|
Strength |
Relatively increased impact strength in cold conditions |
High tensile and specific strength |
|
Transparency |
High, due to amorphous condition |
Low, due to increased level of crystallinity |
|
Maximum allowable stress at 20 °C |
6–17 MPa |
14–32 MPa |
Pros and cons of using LDPE and HDPE
While both materials have their respective benefits, they also carry with them a number of disadvantages.
LDPE
Advantages: Wide variety of uses, low cost, resistant to acids and bases, easy to process and mould, good electrical insulation, waterproof property, may be processed as transparent.
Disadvantages: More prone to crack stress, not useful in extremely high or low temperatures, high permeability to carbon dioxide and other gases, little to no UV resistance.
HDPE
Advantages: Wide variety of uses, high tensile strength, low cost, able to resist low temperature, waterproof property, relatively strong and stiff, good electrical insulation.
Disadvantages: May crack under stress, higher risk of mould shrinking, little to no UV resistance.
How LDPE and HDPE are produced and processed
LDPE production
LDPE is made through either a stirred autoclave or tubular reactor. Its general manufacture involves the compression of ethylene gas, polymerisation using an initiator, and gas separation.
HDPE production
Most HDPE materials are produced by either slurry polymerisation or gas phase polymerisation. The process starts with polymerisation from a solution of ethylene monomers, followed by separation and drying.
Processing
Once produced, the thermoplastic material may be processed for consumer or industrial use by the following methods:
Injection moulding: This rapid process converts LDPE and HDPE pellets or granules into custom shapes and sizes defined by the mould. The thermoplastic pellets are sent to a hot barrel, from which the materials are melted through a screw barrel and heater bands. The molten plastic is then injected to a pre-configured mould cavity, which also cools down the material. After solidifying, the plastic material is ejected out of the moulding machine.
Extrusion: Similar to injection moulding, this also makes use of heat to melt the plastic granules. The difference comes at the final section of the machine – in this case, the melted plastic goes through a pre-designed opening and is then cooled down to solidify.
Blow moulding: This type of processing is usually carried out to manufacture hollow-shaped plastic goods. Instead of injecting the molten plastic, the process uses compressed air to blow the material into the mould.
