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Multilayer films 2005

Rapra International Conference
8th-9th November 2005, Brussels Belgium



Plastics are a miracle and plastics films are an even greater miracle.

Plastics have been a success story, for their unique features. Plastics truly are “miracle materials”, with the almost infinite combinations of various molecules creating these multiple polymers, in the true heart of matter.

Why the success of plastics in packaging, and everywhere?

Plastics have many properties, often equivalent, or inferior, to those of traditional packaging materials, but the combination of these average properties generally gives a unique synergy, their very value for many applications. One of the most striking features of plastics is to be, at the same time, flexible glass and transparent metal.

Plastics are light, generally lighter than other packaging materials. Even when they are heavier, than wood, paper or board, their higher mechanical strength allows using less plastic. Plastics are generally less bulky than the materials they replaced, film versus paper and board, plastic bottles versus glass bottles.

The miracle of plastics films in packaging is like an "invisible bubble”, thin and light. The most extraordinary package probably is the 300 grams of stretch film, putting together a pallet of hundreds of kilos and costing less than 2 dollars. Plastics are the ideal ecological package.

Film definitions

There often are long, and pointless, discussions about the various definitions contained in the terms of “films” versus “sheet”, and the not completely consistent dividing lines between flexible, semi rigid, or semi flexible, and rigid.

In common use, films are planar forms of plastics that may be thick enough to be self-supporting, but thin enough to be flexed, folded or creased without cracking. The dividing line between film and sheet is sometimes given as:

2 to 200 µm, flexible
200 to 400 µm, semi rigid, or semi flexible
Over 400 µm, rigid.

However, in practice, a number of flexible materials are just in-between, like PA, nylon, and PVC. It is increasingly admitted that most thermoformed containers, at least the shallow trays, are in the flexible category.

The thickness criterion defines films as of 250 µm, or 0.25 mm, or 10 mils (of an inch), or less, down to 2-4 µm. Sometimes the film definition goes up to 300 µm, or 0.30 mm, or 12 mils.

Multilayer films, definitions

There are innumerable constructions for multilayers, many that seem practically custom designed for their specific use. Plastics can be most easily associated with traditional materials, boosting the overall performances to fit almost all packaging needs. Multilayers may be all plastics films, or include paper, board, aluminium foil.

The requirements for the selection of multi-layers are unlimited, aspect, machinability, speed, strength, functions, barrier, protection. The key idea is that the price/performance ratio is better achieved with the synergy of the layers than with a single material.

Simply put, the basic, most frequent structure of multilayer films is made of three basic types of components:

An outer layer, to provide protection against abrasion and scratches during processing and packaging operations. It must be printable, direct or reverse, and resistant to temperatures required to melt the sealant.

The middle layer provides the barrier to gas permeation. It may be EVOH or PVDC and others. In the case of EVOH used with polyolefins, there must be tie-layer between the polyolefins and EVOH.

The inner layer provides a hermetic seal by melting at selected temperatures. The most used sealants are PE, EVA and ionomers.

Barriers, definitions

With the exception of pinhole free aluminium foil, all flexible packs have some level of permeability to oxygen, carbon dioxide and water vapor. Achieving a good moisture barrier is relatively easy for plastics. Barrier to gases is more difficult. It is rarely economic to use solid single film as barrier, although there are a number of more or less polymer barriers which provide suitable levels of performance for particular situations.

The barrier concept really applies to plastics whose performances approach total barrier but never achieve it. The barrier materials whose price is at least two or three times higher than that of traditional materials or of commodity plastics, can be used, for economical reasons, only sparingly, as a thin layer, in association with cheaper plastics, in multi-layer materials.

There are a number of barrier materials / processes designed to achieve the best balance of cost/performance for the applications as selected. Each packaging solution is the result of a fine tuning between the cost, the protection and the consumer acceptability.

The very first barrier materials were aluminium foil, various coated papers and cellophane.

The main polymer-based barrier materials are:
Oxide coated films, SiOx, AlOx
Liquid crystal polymers, LCP
Nano films, latest entrants in barriers
Plasma techniques, so far more for blown containers
And semi barriers such as oriented PA 6 OPA.



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