What is LDPE?
Low-density polyethylene, also known as high-pressure polyethylene (LDPE), is the lightest variety of polyethylene resin. It is milky white, tasteless, odorless, non-toxic, and matte waxy particles. It has good softness, extensibility, electrical insulation, transparency, easy processing, and certain air permeability. It has good chemical stability and is resistant to alkali and general organic solvents.
Natural low-density polyethylene (LDPE) is very flexible and translucent in appearance. It is a soft, flexible, lightweight plastic material known for its low-temperature flexibility, toughness, and corrosion resistance.
Overview
Ethylene polymerizes into high molecular weight polyethylene by free radical mechanism under high temperature and pressure, and has the following characteristics:
(1) The ethylene polymerization process is a strongly exothermic process. The polymerization heat of ethylene is about 93.5 kJ/mol (or 3.3 kJ/g). The specific heat capacity of ethylene is 2.51~2.85 J/(g·℃) at 235 MPa and 150~300℃. If the reaction heat cannot be removed in time, the temperature will rise by 12~13℃ for every 1% ethylene polymerization. If the temperature is too high, it will also cause ethylene decomposition.
(2) Under high pressure, ethylene has been compressed to a gas-tight phase state with a density of 0.5 g/mL, which is similar to an incompressible liquid. At this time, the distance between ethylene molecules is significantly shortened, thereby increasing the probability of collision between free radicals and ethylene molecules, so polymerization reactions are likely to occur.
(3) At high temperatures, the free radical activity of the growing chain is large, and chain transfer reactions are likely to occur. The resulting polymer is a linear structure with many branches. Usually, there are 20 to 30 branches per 1,000 carbon chain atoms, the crystallinity is 45% to 65%, the density is 0.910 to 0.925 g/mL, it is light, flexible, and has good low temperature resistance and impact resistance.
What is LDPE plastic used for?
LDPE is used to make a variety of containers, dispensing bottles, squeeze bottles, tubing, plastic parts for computer components, molded laboratory equipment, and a variety of caps and closures. It is very strong and virtually unbreakable.
Its most common uses are in plastic bags for food shopping and dry cleaning bags to protect freshly washed clothes.
Commercially, it is primarily used as plastic film and packaging to protect many products, taking advantage of its tough, flexible, and transparent properties.
Product performance
The main characteristics of low-density polyethylene are as follows:
(1) The film is slightly milky white and transparent, and soft. The strength is lower than that of high-density polyethylene, but the impact strength is higher than that of high-density polyethylene.
(2) It is cold-resistant, low-temperature-resistant and high-temperature-resistant. Thicker films can withstand the sterilization process of immersion in 90°C hot water.
(3) It has good moisture-proof performance, stable chemical properties, and is insoluble in general solvents.
(4) It has high air permeability, so when used as packaging for easily oxidized foods, the storage period of its contents should not be too long.
(5) It has poor oil resistance and the product can be slowly dissolved. When packaging oily foods, the food will have a rancid smell after long-term storage.
(6) It will age under the action of ultraviolet rays and heat for a long time, affecting its physical properties and dielectric properties.
(7) The melting point is 110~115℃, and the processing temperature is 150~210℃. If it is in an inert gas, the temperature can reach 300℃ and remain stable. However, the melt is easily degraded when in contact with oxygen.
LDPE Recycling
While flexible low-density polyethylene (LDPE) bags and wrappers are not always collected through kerbside recycling programs, many plastic shopping bags can be reused. We are seeing many supermarkets start to offer recycling services for these soft plastics. For example, the Co-op is setting up collection points within its network to recycle plastic film lids from yogurt pots, plastic crisp bags, pasta bags, and chocolate and crisp wrappers.
