Plastic Film

What is it?

Flexible plastic, or "film", is one of the most versatile of all the plastic mediums. It can be manufactured from a variety of resins, the more common including polyethylene, polypropylene and the polyvinyls. It can be laminated, both with other resins, and materials such as paper, card, or aluminium. The products manufactured from plastic films cover an enormous range and include wrapping for sweets, pallet wrap, shrink wrap on food, milk carton inner linings, grocery bags, freezer bags, blood plasma bags, supermarket bags, greenhouse cladding, horticultural ground film, raincoats, shower curtains, carpet backings, and protective packaging for export foods of all descriptions.
 

How is it made?

• Extrusion - hot, melted plastic resin is forced through a circular die, forming a thin tube. Air is introduced, expanding the size of the cooling tube which is then flattened as it passes through converging slats and rollers.
• Extrusion coating - resin is melted and forced through a long, slit-shaped die. The sheet of plastic falls evenly onto a continually moving roll of material, such as paper, cardboard, aluminium foil or fabric. The plastic cools and sets on the material.
• Coextrusion - this involves complex die design and multiple extruders, allowing two or more melt streams to be combined. Increasingly, up to five or seven layers are now being used.
• Calendering - this process follows the principle of squeezing laundry out in an old mangle. Hot plastic is poured and evenly squeezed between rollers.

Films and Hi-Tech Products

Plastic is often regarded as a cheap material for a technically undemanding task - the humble shopping bag being an example. But even this example exhibits some remarkable characteristics - there is no other material which could carry so much (5-10kg) with so little (5-1Og) - 1000 times its own weight.

Plastic films are typically only from a few hundredths to a few tenths of a millimetre thick. They can be extremely tough, so that puncturing by sharp objects, such as nails, is very difficult. They can, however, contain predetermined breaking points allowing for simple opening, as in crisps and peanut packaging. They can be self-adhesive as in cling film, or made airtight by welding, giving an air-tight pack as in freezer bags. They can be made transparent or opaque and they can have barrier properties to water vapour (for meat), oxygen (for oils and fats) and flavours and fragrances (herbs and spices). They can also be selectively permeable to one or more of these to allow certain packaged foods to breathe.

The range of properties required is often impossible to achieve with a single material, or "monofilm". In such cases a multilayer structure is used. Three layers are regularly used, with the middle layer providing the barrier properties while the outer two mainly determine properties such as strength, toughness, processibility, sealability or printability. Adhesion-promotion layers are often added which ensure the film structure remains stable and durable.

Film and Responsible Resource Use

Products manufactured from plastic film are often targeted with ill-informed and unfair comment about their impact on the environment and the resources consumed in their production.

An objective look at the subject, however, shows that film is far kinder to the environment than the available alternatives. If, for example, all polyethylene films (eg: bags, packaging film, shrink-on film, pallet wrap, etc) were replaced with other materials, the packaging weight necessary would more than quintuple, and the energy requirement would more than triple.

In multi-layer or composite films the difference is even greater. The weight would increase by as much as nine times and the energy by 3.6 times, since the only real alternatives here (air tight sealing of food and flavour retention) would be glass or metal.

Process changes and technical innovation within the manufacturing community are continually making advances in resource reduction. Radical reductions have occurred in the thickness of the film manufactured, thus conserving raw materials and energy. Plastic carry bags, for example, have progressively been reduced in thickness from 45 - 50µm to 15 -17µm (note: µm = micron = 1 millionth of a metre). Similarly the 1400g of shrink film required in the past for transport protection of 1.5 tonnes of palletised goods has now been reduced to 350g of stretch film

Plastic films are still nowhere near exhausting their development potential. Innovation and technical advancement in this area is ongoing and the future will see even more sophisticated applications for this adaptable, cost effective and resource efficient packaging medium.

Plastic film makes up over 60% of the plastics recycled in New Zealand. See our environment and recycling sections for more information on these important topics.