MARKET OVERVIEW: MULTILAYER FILMS, RAPRA, BRUSSELS 2005
Multilayer films 2005
Rapra International Conference
8th-9th November 2005, Brussels Belgium
To put some order into the many types of multilayers, a description is often used to define the type of multilayers. The various layers are defined by letters, to describe the symmetrical or non symmetrical type of construction. For instance five layers will commonly be defined as:
ABCBD, like PE-LLD/tie/PA/tie/PE-LD
Or another order of letters, showing asymmetrical structures.
The multilayer construction concept is applicable to flexible films, sealable into bags and pouches, to rigid sheet, thermoformable into trays, dishes and cups, and to blow molded containers.
Multilayer constructions of films and sheet aim at combining a number of properties, each particular to the various layers used, to arrive at a set of performances which could not be obtained by any of the composing materials used alone. Better barrier is insured because of the interface between possible defects of each single layer. Better mechanical properties can also be obtained.
Multilayer construction of films and sheet can be obtained by a number of processes:
Lamination or adhesive lamination
Lamination, adhesive lamination
Lamination is a process that makes multi layers by taking and putting together different layers of materials, with adhesives and possible association of partial extrusion and coating.
Lamination permits an almost unlimited range of multilayer structures, made of infinite combinations of different and incompatible materials, with almost complete freedom regarding thickness, as long as it is less than 400-500 µm.
Laminates will continue, because they often offer gas barrier better than that of coextrusion. Moreover, lamination is the only process that can be used with non plastics materials, aluminium foil and paper.
Coextrusion is a process of extruding a multi ply structure with several layers of resins in it, through one die simultaneously, so that all the different resins come together as a single integral structure.
This technique permits the use of melt heat to bond the various plastics, or using the center layer as an adhesive. Coextrusion is an economical competitor to conventional laminating processes, as it reduces material handling costs, raw material cost and machine time costs. Pin holing is also reduced with coextrusion, even when it uses one extruder and divides the melt into a two-layer structure. Other advantages are the elimination of delaminating and air entrapment.
The coextrusion of commodity plastics films, essentially non barrier films, is the largest in terms of volume. It includes, for instance:
Coextruded PE film made of PE-LLD and high molecular weight PE-HD.
Coextruded bi axially oriented PP film
Coextruded PE-HD/EVA, for carton box lining of dry grocery foods
Coextruded PP/PE, for unit packaging of peelable packages.
The coextrusion of commodity resins is thus a well known and widespread technique. The coextrusion machines used for commodity films are generally designed for not more than three or four layers. In most cases, the structure is made of two layers of the same, or closely same material, and of similar thickness. Or it may be made of one thick, recycled scrap layer, in the middle, sandwiched between two thinner layers of virgin material.
The coextrusion of specialty and barrier plastics films is more recent, about twenty years ago.
The more recent coextrusion structures, developed in the early 1980s, are generally made of five layers minimum. Most barrier materials require tie layers to prevent delaminating from olefinic outer layers, making a minimum of 5 layers a virtual necessity for large coextruded barrier film markets.
The advantages of barrier coextrusion are to obtain a multilayer material in one operation and to use only a thin layer of expensive barrier material coextruded with cheaper supports and dry adhesives. The technique offers broad potential range, but in order to keep the total film price within acceptable levels, it is necessary to produce large amounts of one given multilayer combination. This means that there is a relatively more limited choice of coextruded layer materials offered to the industrial customers than there may be when these materials are fabricated by lamination. The broader the application markets, the better accepted these limitations are.
The paradox is that the multiplication of layers permits an almost custom made adaptation of materials, but that the intrinsic higher productivity of the coextrusion equipment requires large volume output of few single types of multilayers. As time passes, since coextrusion began, this conflict has been reduced, in favor of more coextrusion, as the film suppliers are fewer, concentrated, and offering a more streamlined supply of multilayer combinations, adapted to many end-use markets.
This is definitely why coextrusion is more commonly used for commodity plastics, PE and PP, than for lower volume carefully designed barrier multilayer materials. And even this is not entirely true in the sense that multilayer materials often are obtained by two or more operations, like using a coextruded relatively commodity film of polyolefins, then further laminated, or coated, still expanding the range of possibilities.
The coextrusion process only permits surface printing. Most multilayer film producers want to protect the printing by sandwich printing or reverse printing. Sandwich printing achieves better and glossier appearance.
This is to show that there are many parameters leading the choice between coextrusion and lamination. The choice involves the materials, the width, the length, the size and speed of the runs.
There will always be a need for lamination and for coextrusion, and there will never be a complete replacement of lamination by coextrusion.
Extrusion lamination, to extrude a resin which then acts as a bond between two layers, is a variant from the extrusion coating process, when the resin extrusion is made on the surface of another material. In this latter process the supporting materials may be paper or aluminium foil, as well as plastics films.
Coating is more versatile than coextrusion. It can be combined with coextrusion and lamination in the broad range of possible multilayer film constructions.
In extrusion coating, an extruder forces melted plastics through a horizontal slot die onto a moving substrate or web of material. The rate of application controls the thickness of the continuous film deposited on the web. The melt stream, extruded in one layer or coextruded layers, can be used as a coating or as an adhesive to laminate or sandwich two or more materials together such as plastic film, foil or paper.
PVDC coating is one of the oldest gas barriers, having first been used on cellophane, for many years.
Coating is widely used on commodity films such as OPP. The coating process makes the basic OPP film, generally coex, transparent, with 3 layers, sealable on both sides, a more specialty film.
Metallizing originally was a replacement technique, to offer a substitute for aluminium foil. A very thin layer of aluminium deposited on plastics or paper appeared attractive as the price of aluminium increased. Metallization thus is just a coating, metal powder deposit, but the adhesion is more difficult to achieve than with inks.
Metallized films have a number of advantages over foil, such as overall cost, higher converting machine speeds, availability, flexibility and even brighter appearance. On the other hand, the main advantage of aluminium foil is the deadfold ability.
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