The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling. Each material is fed to the die from a separate extruder, but the orifices may be arranged so that each extruder supplies two or more plies of the same material. Coextrusion can be employed in film blowing, free film extrusion, and extrusion coating processes. The advantage of coextrusion is that each ply of the laminate imparts a desired characteristic property, such as stiffness, heat-sealability, impermeability or resistance to some environment, all of which properties would be impossible to attain with any single material.

“Coextrusion is another tool in the well-outfitted converter’s tool chest,” according to Bruce Foster, technical sales manager of Mica Corp. “It provides better quality monolayer extrusion coatings over a wider variety of widths and line speeds; the potential to save on the amount of premium resins by filling with lower cost materials; and the possibility to make multi-layer, multi-functional structures in a single pass.”

A group at Battenfeld Gloucester Engineering Co. feel that coextrusion offers targeted performance using specific polymers in specific layers, a reduction in the number of process steps, and lower cost, says spokesperson Bill Hellmuth, senior product manager — blown film systems.

Hellmuth states lower cost is possible because of the reduced amount of expensive polymer for equal performance, the use of inexpensive polymers without sacrificing performance, a reduction in setup and trim scrap, and the potential for use of a recycle layer.

Scott B. Marks, senior technical specialist at E.I. duPont de Nemours & Co., Packaging & Industrial Polymers, divides the coextrusion process into two main uses — non-barrier and barrier applications. “In the non-barrier applications, coextrusion allows a converter to maximize the properties he needs in a given structure such as sealing characteristics, coloration, opacity, and physical properties. For barrier application, coextrusion allows incorporation of a barrier layer to control transmission of oxygen, carbon dioxide, water vapor, etc.”

“Handling resins that could not run on their own” is a characteristic of coextrusion that David Timm, VP of Charter Films, considers an advantage.

“The disadvantage of coextrusion is that some differences in physical properties that make a combination desirable also make the combination incompatible,” says Gary D. Oliver, senior corporate scientist at Cloeren Inc. “Polymers for coextrusion should have similar melt viscosities to maintain laminar flow. Viscosity differences can be more or less tolerable depending on the location of the material in the composite structure and the thinness of the layer.”

Andy Christie, principal consultant of Optex Process Solutions, notes coextrusion has three disadvantages: It requires a more sophisticated extruder operator; it necessitates significant planning and forethought in system design; and it provides the potential to create expensive scrap quickly.

Richard W. Halle, senior staff engineer at ExxonMobil Chemical Co., observes the biggest disadvantage of coextrusion is “probably that the added complexity of the equipment means added maintenance cost.”

In an interesting comment regarding cost, Timm says, “Many people might consider the first disadvantage of coextrusion to be cost. I do not. Depending on the intricacies of die design, coextrusion dies can be difficult to purge and can be prone to buildup especially with barrier materials. Improvements in materials and die design can minimize such degradation problems.”

Application Advances
Expanded markets and uses are other focal points for capitalizing on the advantages of coextrusion and trying to reduce the influence of its disadvantages. Foster’s thinking in this area is simple: “Anywhere you see a rigid package today, you may see a flexible package next year.”

Perdikoulis offers a very futuristic remark: “Advanced control systems ultimately will have some simulation capability or ‘intelligence’ that will allow users to select the properties they want in a product, and the control system will specify the structure. Imagine selecting the shelf life requirements, strength, and heat seal characteristics and having the control system select the materials and adjust the layer ratios accordingly.”

“Applications and markets will continue to diversify into what we now call the ‘industrial segments,’” according to Marks. “The need for property enhancement and economics drove coextrusion into the packaging film industry. In the market for ‘industrial films and laminations,’ this need for coextrusion is now accelerating. Functionality needs vs. direct economic needs of an extrusion drive this today. Economic needs relate more to altering the assembly process, simplifying the assembly process, or both. Combining the technical advantages with product assembly changes may reduce total costs. In cases where it does not, the technical advantages are sufficient to propel coextrusion growth.”

Oliver notes the recent appearance of a hybrid container that is a multilayer coated paperboard. “We are seeing this round container show up more frequently in the United States and in more large-size liquid container boxes that have been prevalent in Europe for some time. I think another application that will continue to grow is the stand-up pouch.”

Looking at coextrusion growth globally, Christie notes, “Second and Third World markets will bypass many recent technologies and jump to leading-edge technology as they invest in new equipment and develop products for internal consumption and export.”

Hellmuth’s group reports “food and water packaging will top the list of products as the world strives to feed itself. Housing market pressure will push plastics to the forefront in an effort to conserve natural resources and provide an outlet for plastic package recycling. The group at Equistar Chemical thinks converters will find creative ways to use coextrusion to replace many laminated structures because of cost pressures and intensified competition: “This will involve a combination of resin engineering, equipment engineering, and innovation. The market also will see a significant jump in down gauging for certain applications that already seem over-engineered in total gauge. The ‘supply chain’ will drive creativity by accessing vantage points and knowledge bases that were previously not on the table for discussion. This will lead to new models for doing business and developing new products.”