7 Types of Plastics You Should Know (Part 2)

In the last article, we have introduced PET, HDPE, and PVC. Today, we will focus on the type LDPE and the comparisons between HDPE and LDPE.

Low-density polyethylene, usually abbreviated to LDPE, is the lightest type of all polyethylene resins. It is tasteless, odorless, and non-toxic with a non-transparent milky white appearance. And the resin identification code of LDPE is the number “4”.

LDPE was produced in 1933 by Imperial Chemical Industries (ICI) using a high-pressure process via free radical polymerization. Its manufacture still employs the same method today. And LDPE has the following characteristics:

·Excellent low-temperature flexibility.

·Excellent moisture barrier property.

·Chemical stability. LDPE can withstand temperatures of 80°C continuously and 90°C for a short time. And it is not reactive at room temperature, except to strong oxidizers. LDPE has excellent resistance to acids, alcohols, bases, and esters, yet poor resistance to oil and fat.

·Excellent impact resistance.

·Low density. LDPE breaks down more easily than other plastics.

·Exposure to consistent sunlight will lead to chemical and physical aging of plastics.

One of the most common uses of LDPE is in plastic bags which are provided by grocery and other retailers. Besides, it is also widely used for manufacturing various containers, dispensing bottles, wash bottles, squeezable bottles, tubing, food wrappings, plastic parts for computer components, and various molded laboratory equipment.

Comparisons between HDPE and LDPE

Basic information

Similarities

Both HDPE and LDPE belong to thermoplastics under the family of polyethylene resins, one of the most widely used plastic materials in the world. As a result, they share many characteristics.

First of all, both of them can be seen everywhere in our daily life, from shopping bags to children’s toys to all types of containers. And some of the industries that commonly use both materials include automotive, electrical, hydraulics and pneumatics, packaging as well as pipe and piping. 

Secondly, they cost little. It is said that the average global price for HDPE will fall to $750 per metric ton in 2022, compared with the price of $936.1 in 2019. And LDPE is much cheaper, which ranges from $211.11 to $404.76. What’s more, HDPE and LDPE are able to be recycled, helping control environmental pollution by reducing the need for landfills.

Thirdly, they have similar mechanical properties as well as chemical resistance. On the one hand, HDPE and LDPE are lightweight, waterproof, with little resistance to UV, and can be used as good electrical insulation. On the other hand, both of these two materials are resistant to chemicals, acids, water vapor, and weathering, which means they are stable most of the time.

Differences

It is the structure of their molecules that causes their distinct properties. The polymer chains that make up both materials are branched in LDPE, whereas, in HDPE, the polymers have a more crystalline structure, which means that HDPE’s polymer chains line up more regularly, giving it a higher density than LDPE. And the forces of attraction between polymer molecules in HDPE are also stronger.

·Appearance. HDPE and LDPE look and feel totally different. HDPE is wax-like, lusterless, and opaque, while LDPE has a clear or translucent appearance. As for the tensile strength, HDPE is semi-rigid and tough, while LDPE is flexible.

·Producing process. Both the method of slurry polymerization and gas-phase polymerization can be applied to make HDPE. By heating petroleum to very high temperatures (we also call this process “cracking”), ethylene gas will be created. During its production, the gas molecules will attach to form polymers, which then produce polyethylene. The polyethylene we get will have a sludgy appearance, but after it’s put through a series of molds, it forms into granules. Once the molding process is complete, we’ll have a strong polymer material - that is HDPE. As for LDPE, there are also two basic methods available for the production of LDPE, namely stirred autoclave and tubular reactor. We take the tubular reactor as an example. The reaction is carried out in a tubular reactor. Compress monomer ethylene gas in the tubular reactor to speed up polymerization – the process in which relatively small molecules (monomers) combine chemically to produce a very large chainlike or network molecule (polymer). And high-pressure polymerization allows further copolymerization of ethylene with polar co-monomers.