Low density polyethylene (LDPE)
Low density polyethylene is a thermoplastic polymer of the olefinic family, consisting of multiple ethylene units. It is also often referred to by its acronym in Spanish (PEBD, or low density polyethylene) and in English (LDPE, or Low Density Polyethylene).
LDPE is a thermoplastic polymer. This means that at high temperatures it becomes flexible or malleable, melting when heated and hardening when cooled. Like all other thermoplastics, LDPE is perfectly recyclable and its density, compared to HDPE, is much lower due to its highly branched chain structure.
What are the properties of low density polyethylene?
Density of HDPE: 0.93 to 0.97 g/cm3
High Density Polyethylene Chemical Resistance:Excellent resistance to most solvents
Very good resistance to alcohols, dilute acids and alkalis
Moderate resistance to oils and greases
Poor resistance to hydrocarbons (aliphatic, aromatic, halogenated).
Continuous temperature: -50°C to +60°C, relatively rigid material with useful temperature capabilities
Coefficient of thermal expansion 100 – 220 x 10-6
Impact resistance without rupture Kj/m².
Tensile strength 0.20 – 0.40 N/mm².
Higher tensile strength compared to other forms of polyethylene.
Low cost polymer with good processability
Good low temperature resistance
Excellent electrical insulation properties
Very low water absorption
Resistance to chemicals:
The chemical resistance table is only a general guideline. Because there are so many factors that can affect the resistance of a specific product, it should be tested under its own conditions. It should be noted that the reactive combination of compounds from one or more classes may cause a synergistic or undesirable effect.
Dilute acid – Very high
Dilute alkalis – Very high
Oils and fats – Moderate
Aliphatic Hydrocarbons – Low
Aromatic Hydrocarbons – Low
Halogenated Hydrocarbons – Low
Alcohols – Very High
Disadvantages of HDPE:
Susceptible to stress cracking
Lower stiffness than polypropylene
High mould shrinkage
Poor UV and heat resistance
Inability to weld and join at high frequency
Low density polyethylene (LDPE) applications
The uses of low density polyethylene (LDPE) mainly revolve around the manufacture of containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components and various moulded laboratory equipment. The most popular application for LDPE is plastic bags.
Packaging – Due to its low cost and good flexibility, LDPE is used in the packaging industry for pharmaceutical and squeeze bottles, caps and closures, security seals, liners, rubbish bags, food packaging films (frozen, dry goods, etc.), laminates, etc.
Pipes and fittings – Low density polyethylene is used to manufacture water pipes and hoses for the pipe and fittings industry due to its plasticity and low water absorption.
Other applications are consumer goods – household goods, flexible toys, agricultural films, wiring – sub-conductor insulation, cable sheathing.
How is low density polyethylene obtained?
Polyethylene is made by chemically combining ethylene molecules. The chemical process under which this occurs is called polymerisation. In this process, the double bonds formed by the carbon atoms are broken and replaced by ethylene molecules that form long chains. In addition to ethylene, a catalyst is used as an auxiliary element for the chemical reaction.
Polymerisation takes place in a reactor. Inside the reactor, the temperature reaches 99°C. The reaction starts when the ethylene gas that is introduced into the reactor is put under pressure (100-300 PSI). This is when the gas and the catalyst are transformed into plastic.
However, there is a portion of the ethylene that does not polymerise. This portion is cooled and recycled in the reactor. On the other hand, the part that does polymerise is transformed into granules, also called pellets, which are extracted from the base of the reactor.
Difference between high and low density polyethylene?
Being composed mainly of the same polymerised ethylene molecules, LDPE and HDPE share many characteristics. For example, both materials have the following properties
Low material weight
tensile strength ranging from 0.20 to 0.40 N/mm2
High impact resistance
Chemical, water vapour and weather resistance
Low manufacturing and fabrication cost
When used in injection moulding operations, both materials also demonstrate the following
Melting temperatures of 180 ̊ to 280 ̊ C (355 ̊ to 535 ̊ F)
Fast injection speeds
No drying of the finished part required
The similarities in the above characteristics, among others, make LDPE and HDPE suitable for similar applications. Some of the industries that typically use both materials are:
Hydraulics and pneumatics
Piping and ducting
How is low density polyethylene recycled?
LDPE is processed by forming methods such as injection moulding and extrusion. As it melts rapidly with heat, it can be moulded repeatedly without altering its original properties too much, but they do. Therefore, it cannot be recycled more than 6 or 7 times, as it undergoes changes during the various reprocessing cycles.
Differences between LDPE and HDPE:
Although LDPE and HDPE share many characteristics, their internal compositions are fundamentally different, so there are also many differences. The polymer chains that make up both materials are branched in LDPE, whereas in HDPE the polymers have a more crystalline structure. This difference in the organisation of the polymers results in different characteristics in each material.
Differences in physical characteristics:
LDPE is softer and more flexible than HDPE. It also has a lower melting point (115°C) and is more transparent. Compared to HDPE, it is more likely to crack under stress.
HDPE is rigid and durable and offers higher chemical resistance. Its higher melting point (135°C) allows it to withstand higher temperatures than LDPE. Its more crystalline structure also results in higher material strength and opacity.
Differences in recyclability
Both LDPE and HDPE are recyclable; however, they must be recycled separately. LDPE is classified under recycling number 4, and HDPE under recycling number 2. Depending on the product, LDPE can also be more difficult to recycle, as it is softer and can get caught in recycling machinery. HDPE is easier to transport and pass through recycling equipment.
Differences in production methods
LDPE is produced by compressing ethylene monomer gas in an autoclave or tubular reactor to facilitate polymerisation, i.e. the joining of the monomers into polymer chains.
HDPE is created by heating the oil to very high temperatures. This process releases the monomers from the ethylene gas, which then combine to form polymer chains.