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 |
Density |
Low density 0.91-0.94 g/cm3 |
High density 0.95-0.97 g/cm3 |
Flexibility |
Low crystallinity (50-60%), and therefore more flexible |
High crystallinity (>90%), which makes it tougher and more rigid |
Heat resistance |
Drastic decrease in density when subjected to temperatures beyond 20°C |
Able to withstand heat with temperatures more than 100°C |
Melting point |
~115°C |
~135°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 |
While both materials have their respective benefits, they also carry with them a number of disadvantages. Here's a list of the pros and cons of each of LDPE and HDPE.
|
LDPE |
HDPE |
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 |
Wide variety of uses Low cost High tensile strength Able to resist low temperature Relatively strong and stiff Waterproof property Good electrical insulation |
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 |
May crack under stress Higher risk of mould shrinking Little to no UV resistance
|
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.
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.
Once produced, the thermoplastic material may be processed for consumer or industrial use by the following methods:
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.
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.
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.
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[2] A. Peacock, Handbook of Polyethylene: Structures, Properties, and Applications, NY: Marcel Dekker, Inc., 2000.
[3] NIIR Board of Consultants and Engineers, The Complete Technology Book on Plastic Films, HDPE and Thermoset Plastics, India: Asia Pacific Business Press, Inc., 2006.
[4] "Highlights of Low-Density Polyethylene", [Online] Available from: https://www.plasticsmakeitpossible.com/about-plastics/faqs/professor-plastics/professor-plastics-highlights-of-low-density-polyethylene/
[5] Yashoda, "Difference Between HDPE and LDPE", [Online] Available from: http://pediaa.com/difference-between-hdpe-and-ldpe/, 2016.
[6] J.P. Plog, "Processing Polyethylene", [Online] Available from: https://www.thermofisher.com/blog/materials/processing-polyethylene/, 2015.