Multilayer Films
(Flexible Packaging Materials)

Properties

The majority of today's packaging films are multilayer structures ranging from three to 12 layers. They are also referred to as co-extruded films because they are made by a multilayer coextrusion process. The combination of several layers of different materials improves the mechanical and physical properties of the film including puncture, tear and heat resistance as well as moisture and oxygen barrier properties.

Multilayer films find many applications in the high-volume packaging industry including food and medical packaging. The combination of several polymer layers significantly increases shelf‐life by controlling the transmission rate of oxygen, carbon dioxide and moisture as well as the concentration of oxygen inside the package which is key in preserving the freshness of fresh produce for longer period of time.

The most common polymers utilized in the flexible packaging industry are polyethylene (PE), polypropylene (PP), ethylene-vinyl alcohol (EVOH), polyamide (Nylon, PA), ionomers (EAA, EMAA), and ethylene vinyl acetate (EVA). Among these, polyethylene is the largest and cheapest packaging film. It is easy to process and is often combined with gas/aroma barriers such as PA and EVOH. The most important grade is linear low density polyethylene (LLDPE). It is a high-clarity film widely used for food packaging and blow molding of bottles. It is sometimes combined with high density polyethylene (HDPE) which is stiffer, harder, and has higher tensile and bursting strength but lower impact and tear strength than LLDPE. The combination of LLDPE and HDPE provides superior properties, and allows for thinner films. For this reason, the majority of today's packaging films include more than one olefin layer. For example, cereal bags consist of several layers of HDPE, LDPE and LLDPE combined with an aroma barrier, whereas films that require higher mechanical strength and/or improved heat resistance (microwavable and hot-filled food packaging) often include a layer of polypropylene (PP). Besides superior heat resistance, PP provides the basic strength of the packaging and contributes to the moisture barrier.

Some important limitations of polyethylene films are poor gas barrier properties, low temperature resistance, and difficult to bond. To improve these properties, olefins are often combined with polar polymers such as PA, EVOH or Saran (PVDC). However, coextrusion of these mateirals requires tie resins¹ because olefins do not adhere well to these polymers. A typical packaging film for fresh produce consists of four to seven layers. Two examples are LLDPE-Tie-EVOH-Tie-LLDPE and LLDPE-HDPE-Tie-EVOH-Tie-HDPE-LLDPE² among many others. Most of these multilayer films consist of at least 50 percent olefins (see table below).

Another important class of packaging materials are ethylene copolymers such as ethylene-co-vinyl acetate (EVA), ethylene-co-methacrylic acid (EMAA), and ethylene-co-acrylic acid (EAA). Copolymerization with polar monomers increases adhesion, toughness and clarity but lowers crystallinity and melting point which, in turn, negatively affects strength. These copolymers function mainly as a heat-sealable food contact layer or as a tie layer for co-extruded olefins and polar monomers such as PA and PET.

For more demanding packaging applications, plastic films are either laminated to an aluminum foil or combined with a metalized film. The aluminum layer greatly reduces the water and oxygen transmission rate and also provides a metallic and glossy appearance. Aluminum is the most effective vapor and aroma barrier. However, in recent years, the amount of aluminum foil used in packaging has decreased in order to reduce recycling problems. Metalized films are also a popular choice for confectionaries.

¹Tie layers are functional polymers coextruded between two chemically different polymers to improve the adhesive strength and to prevent delamination.

²The combination of HDPE with LLDPE offers a better balance of stiffness and toughness and allows for thinner films, and thus, reduces material cost.