A Primer on How to Put Substrates Together

To extrusion coat, extrusion laminate, or film laminate? If a composite construction is your goal, there is more than one way to get there.

Extrusion coating, extrusion laminating, and film laminating are three different manufacturing techniques a converter can use to make a composite construction. The physical properties and performance characteristics of something made by extrusion coating and laminating can be identical to that made by film laminating. Many of the major components of the final constructions are also the same.

Two Examples
To understand the differences among extrusion coating, extrusion laminating, and film laminating, consider examples of two constructions. One is a pouchstock, and the other is a construction that has use in the medical packaging industry. A typical pouchstock is a combination of paper/polyethylene/foil/PE. The material used for medical packaging consists of polyester/PE/foil/PE.

Some people will make the pouch material by using a laminating adhesive at the three interfaces. In some order, they will laminate the four substrates, probably using three separate laminating operations or perhaps doing the lamination on tandem laminators. Other manufacturers will combine the paper to foil using extrusion laminating. This portion of the construction will then undergo extrusion coating of the remaining PE layer or perhaps adhesive lamination of the original material to a PE film.

The assembly of the medical packaging material can occur in similar fashion. All four substrates may undergo combination using adhesive lamination. Alternately, the polyester-to-foil combination may use extrusion lamination. The final step in this construction would be an additional extrusion coating or adhesive lamination to introduce the other layer of PE.

An obvious pattern emerges from these examples. Options exist whenever a construction will contain PE. The PE can be present in one or more layers. Introduction of the PE can use a PE film in a laminating process or PE pellets in an extrusion process.

Extrusion Coating, Extrusion Laminating
The extrusion process as part of a converting operation that is making a composite material for a packaging application can perform either extrusion coating or extrusion laminating.

Extrusion coating is the process that lays a molten layer of extrudate onto a substrate. The substrate can be paper, foil, or even a plastic film that will withstand the temperature of the extruded molten polymer. The molten polymer is a very viscous liquid that actually flows on the substrate. During this process of flowing, the polymer wets the entire surface evenly. For porous substrates such as paper, it also enters the interstices of the uneven surface. Both phenomena are important contributors to adhesion.

Another factor that influences the resulting bond is the specific adhesion — how well the molten polymer conforms to or matches the chemical composition of the substrate.

Extrusion laminating in a converting operation is the combination of two substrates using a molten polymer. In this case, the extrudate enters the nip formed by two rolls. Two substrates also enter the nip by traveling over each roll. The extrudate is therefore the center part of a sandwich material. The same factors mentioned above — flow, substrate nonuniformity, and specific adhesion — are the factors that control the bonding of the three materials in the resulting sandwich composite.

Film Laminating—A Different Breed
The process of film laminating differs completely from the extrusion coating and extrusion laminating processes; it is the combination of a film to another substrate — film, paper, or foil—by using a laminating adhesive.

The adhesive is coated onto one substrate of the lamination, dried in an oven if it contains solvent or water, and then combined with the other substrate in a heated nip station using pressure. For finished products that contain more than two substrates, additional laminating steps are needed.

The bond values in a laminating operation depend on the specific characteristics of the laminating adhesive. It must have sufficient cohesive strength and the necessary adhesive strength to bond sufficiently to each of the substrates. Other variables such as coating weight, nip temperature, treatment level, etc., also will influence the final bond value.

In film laminating, PE becomes part of the composite through its introduction as a film. Since the PE film a laminator purchases was actually made by the film supplier from PE resin pellets, one might consider the film laminating process the introduction of an intermediate step in the total operation to make the final composite. The difference is that the converter or resin supplier is not making the film.

Where Do Primers Fit In?
The discussion above on extrusion coating and extrusion laminating did not mention a very important requirement of many extrusion operations — the use of a primer to provide specific adhesion. Consider again the construction mentioned above for use in the medical packaging industry. It was a polyester/PE/foil/PE composite. This is an oversimplification, because the actual construction will more likely be polyester/primer/PE/foil/primer/PE.

The primer between the polyester and PE is necessary so that the polyester will adhere properly to the PE. Likewise, the primer between the foil and PE is necessary to form a sufficient bond between those two substrates.

Application of a primer to a substrate before an extrusion coating or extrusion laminating operation requires some pieces of equipment that are necessary in a film laminating operation: a coating station and a drying station. In some instances, a laminating adhesive such as a polyurethane or polyester adhesive in solvent can find use as a primer. A general rule of thumb is to use the adhesive at about half the normally applied coating weight when using the material as an adhesive. Some materials such as PE imine or ethylene acrylic acid polymers are formulated specifically for use as a primer.

Which Technique Should I Use?
That is the obvious question at this point. Unfortunately, the answer is not simple.

First, the selection depends on the equipment available. If a converter has only an adhesive laminating line, that will be the method he will use to make a construction. Likewise, someone who has only an extruder obviously will opt to make all composite materials that way.

Those manufacturers that have equipment that will do adhesive laminating and an extruder will have to decide which technique they want to choose.

Extrusion operations require extended runs. On occasion, a converter may need to produce a small quantity of material that requires only a few hours to make. In this case, adhesive laminating using a film is the appropriate method to select. Economical use of an extrusion line requires lengthy runs. The time necessary to have all the variables reach equilibrium conditions is much longer than for an adhesive laminating process. Once all the line reaches equilibrium, it should run as long as possible without any changes.

Extrusion operations may offer a converter more opportunities for using a variety of polymers. Film users are under the restriction imposed by the range of films available from suppliers. Extruders can select from a large number of PE materials. They also can use copolymers or possibly even mixtures. Therefore, more options are available.

For a converter that is purchasing new equipment and can design a line to use either extrusion or film laminating, economics obviously plays a major role. Although capital equipment for extrusion operations might be more expensive than the simplest laminating equipment, some extensive laminating equipment with all the available options or auxiliary equipment might be more expensive than the most inexpensive extrusion line.

The number of processes that a converter is willing to perform also will play an important role in the selection of technique. Laminating many substrates might take more operations than extrusion. This is the case especially for coextrusion. Coextrusion is the combination of two or more molten polymers into a single extrudate. It offers many advantages, including the possibility of decreasing the amount of expensive polymer used, using recycled materials, minimizing thickness, etc.

One big advantage of coextrusion is that it can reduce the number of processes necessary to make a finished composite. With a sufficient number of openings in the die, a single extruder theoretically can make a composite comprising many different layers.

With so many factors to consider, none of the methods discussed here is the “one best way” to make any specific packaging composite. A good laminator can make a lamination of the same materials that an extruder would use with all the necessary properties. Likewise, a good extruder can make an extrusion coating or extrusion lamination with the same materials that a film laminator would use, with all the necessary properties.

So, careful analysis of many factors is required to choose the best method for you and your specific applications.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. He is the author of the monthly “PLC Probe” column. Contact him at e-mail: dbentley@unm.edu.

Subscribe to PFFC's EClips Newsletter