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Low-density polyethylene (LDPE) is a type of thermoplastic, a synthetic polymer that softens to a liquid when heated and freezes when cooled. It is made from petroleum. LDPE has a wide variety of applications because of its toughness and low reactivity at room temperature.
As a primary component of plastic bags, food and drink containers, trays and computer equipment such as disk drives, low-density polyethylene is an important plastic. It is resilient, easy to weld and shape and flexible to the point of being almost unbreakable. This makes it a popular choice for parts that need to be flexible in order to function correctly.
The resilience of low-density polyethylene is because of its chemical structure. Like other polymers, LDPE consists of repeating units of carbon and hydrogen atoms that form bonded chains. LDPE exhibits branching on about 2 percent of its carbon atoms, meaning that in some places, a hydrogen atom is replaced by another carbon-hydrogen chain. This makes LDPE’s tensile strength and intermolecular forces weaker, resulting in lower density and greater flexibility.
Linear low-density polyethylene (LLDPE) is a variety of low-density polyethylene that is widely used in commercial and industrial applications. It is composed of shorter branching structures than LDPE, which gives it a lower viscosity and the ability to elongate when stretched. LLDPE is used in plastic wrap and plastic bags where a thinner, stretchier material than LDPE is required.
LDPE is widely used in laboratory equipment. Its flexibility and translucence make it useful for wash bottles and tubing, and its chemical resistance allows it to be used in conjunction with chemicals that might corrode other materials. For example, LDPE has good resistance to acids, bases, alcohols, aldehydes and vegetable oils.
Laboratory equipment manufacturers state that LDPE can be used in temperatures as high as 176 degrees Fahrenheit (about 80 degrees Celsius) and as low as minus-58 degrees Fahrenheit (about minus-50 Celsius). It is recommended that special care be observed in maintaining LDPE equipment, because the material can be weakened by oxidizing agents and might soften and swell over time.
LDPE was initially developed as a variation on high-density polyethylene (HDPE). HDPE exhibits less branching in its hydrocarbon chains and is therefore a harder material than LDPE. It is used in some of the same products as LDPE, such as plastic bags, but it also can be found in more rigid materials such as milk jugs and bottle caps.
The global market for LDPE and LLDPE has grown rapidly since its inception during the mid-20th century. Although polymer science has continued to develop new materials to meet the challenges of packaging and manufacturing, LDPE has remained a popular material because of its versatility and durability. LDPE can also be recycled, which gives the material staying power in an increasingly environmentally conscious society.