Property, Production & Application
LDPE and HDPE are thermoplastic materials under the family of polyolefins, which are petrochemical-based plastics that include polypropylene (PP) and polyethylene (PE). Polyethylene is the most common plastic material in the world and is known for its simple structure, the simplest of all polymers commercially available.
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 is well known for being widely used in plastic bags as its low density makes it light and flexible, rendering 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, used instead of ABS material. It is also used to produce durable plastic parts such as HDPE pipes, toys and plastic chairs.
Although LDPE and HDPE are both thermoplastic ethylene polymers, they differ in several properties and uses. One stark contrast is that LDPE has more branching than HDPE. Branching takes place during polymerization, where polymer chains have secondary polymer chains attached to them by replacing an atom in the primary chain with a monomer group. This weakens the intermolecular forces in the polymer. That is why HDPE has a higher strength-to-density ratio than LDPE with increased tensile strength.
The varying uses of LDPE and HDPE are based on this fundamental structural difference, among many other properties.
Here, you will learn about:
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.
Table 1. Comparison of the material properties of LDPE and HDPE
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.
Table 2. Advantages and disadvantages of using LDPE vs HDPE
LDPE is made through either a stirred autoclave or a tubular reactor. Its general manufacture involves the compression of ethylene gas, polymerization using an initiator, and gas separation.
Most HDPE materials are produced by either slurry polymerization or gas-phase polymerization. The process starts with polymerization 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 mold. 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 into a pre-configured mold cavity, which also cools down the material. After solidifying, the plastic material is ejected out of the molding machine.
Similar to injection molding, 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 mold.
Above article source: https://matmatch.com/learn/material/ldpe-vs-hdpe
