Polyethylene or PE is the largest volume commodity thermoplastic and cheapest packaging film. It is used for a wide variety of applications. It is easy to process and can be blended with a variety of materials, like EVA, other polyolefins, fillers, pigments to alter its basic properties. PE is soft, very flexible, clear, and has good tear and moisture resistance but only fair gas barrier properties. The two most important grades are low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). These two resins are often thought to be the same because they have very similar properties. For example, both have good to fair clarity, excellent chemical resistance, good moisture barrier but only fair or poor gas barrier properties. Both types can be heat-sealed and are strong, tough and highly flexible.
LLDPE was originally considered mainly a cost reducer because of its lower cost to manufacture, which is true for the most part. However, there are some differences in some properties that make them preferable for different uses. For example, LDPE is often chosen for its higher clarity, ease of processing and higher gloss whereas LLDPE is selected for its higher tensile and impact strength (i.e. its higher toughness) and better heat sealability.
Some major disadvantages and limitations of polyethylene films are high thermal expansion, poor gas barrier and weathering properties, low temperature resistance, and difficult to bond. For this reason, low density polyethylene resins are sometimes blended with other resins to improve certain properties or, vice versa, they are added to other film resins to improve their film properties, like toughness and flexibility.
Another noteworthy PE resin is ultra low density polyethylene or ULDPE. It is a soft and very flexible thermoplastic. Compared with LDPE and LLDPE, ULDPE is softer and offers greater low temperature flexibility and flex-crack resistance. It is ideal for applications where better optics and high tear resistance are required. However, unmodified ULDPE has poor UV resistance and very low gas barrier properties, except for water vapor.
Metallocene1 polyethylene or mPE is a unique polyethylene, which is made by utilizing a metallocene catalyst. This technology results in resins with very pure, and consistent properties. mPE films exhibit fast hot tack, which allows for rapid sealing. Many grades have excellent puncture resistance and permeability to oxygen and good tensile strength at break, i.e. they are much stronger than conventional polyethylene resins.
Metallocene catalyzed polymerization results in very pure, and consistent resins with specific properties. A wide variety of metallocene polyethylenes can be produced with very different properties ranging from
very soft with low melting point to high melting point with good heat resistance, that is, metallocene catalysts enable the resin manufacturers to tailor PE with specific film properties to meet precise performance requirements.
Many derivatives of early metal metallocenes are active catalysts for olefin polymerization As well. Unlike traditional and still widely used heterogeneous Ziegler-Natta catalysts, metallocenes are homogeneous.
For many applications, mPE is a suitable alternative to PVC and EVA. For example, some manufacturers claim that mPE has better hot tack than EVA, and is, therefore, an excellent alternative to Surlyn films, whereas other grades of mPE are suitable alternatives to PVC for certain applications.
Both LDPE and LLDPE films are used for similar applications, including stretch- and shrink-wraps, bags, liners, and packaging films. Both find uses for countless produce and applications including carryout bags, candy wrap, food packaging, trash and can liners, grocery bags, bubble packaging, overwrap and envelope films, industrial liners, shipping sacks, mattress bags, and agricultural and construction film.
ULDPE is often used for very specific film applications, such as stretch wrap and food packaging. Important applications for ULDPE resins and resin blends include heavy-duty sacks, consumer bags, turf bags, mulch films, silage wrap, and packaging for cheese, meat, coffee, and detergents. They are also widely used for injection-molded products such as heating and water pipes.
Due to the low odor, high chemical resistance and inertness, many PE grades are suitable for packaging applications under FDA regulations.
1Metallocenes are catalyst which consist of minute particles of positively charged metal ions sandwiched between two cyclopentadienyl anions. They are also known as single-site catalysts because they have only one active site per catalyst particle, which are all identical.