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Coextrusion: opportunities and challenges

Modified on Saturday, 31 January 2015 03:38 AM by mpieler Categorized as Consultants Corner
Sheet and film fabricators should care fully examine coextrusion technology for their exact application, considering, both the opportunities and challenges it presents. Coextrusion may give a vastly superior product for a given application with lower costs than any other method, but on the other hand, if approached improperly, it will give nothing but headaches. Obviously, coextrusion can succeed in a big way. Witness the current publicity and excitement regarding plastic barrier containers or packages which are replacing metal cans and glass jars in some applications and creating completely new package designs in others. But behind those successful containers is development work which started over 20 years ago. Certainly not all coextrusion developments have taken such a long time. For example, the simple two-layer, GPPS/HIPS, structures used for dairy containers took less than a year to become fully commercial.

The opportunities of coextrusion are better physical properties for a given application which range from: barrier properties to the diffusion of oxygen, superior heat resistance, better thermoformability, low taste and odor effects, needed toughness improvement, etc. Reduced multilayered sheet or film costs are always a goal of coextrusion. This may be achieved by judicious substitution of a lower priced polymer for a more expensive polymer. In some cases, major cost reduction and process simplification are achieved by the elimination of a more expensive manufacturing process, such as, multi pass laminating or coating processes. The challenges to fully achieving the named benefits are frequently formidable and always require careful development work, unless the coextrusion is a virtual duplication of a proven system with regard to exact polymer selections, equipment design, and coextrusion operating conditions.

In coextrusion some unique problems may be encountered which do not exist in monolayer extrusion. The major unique problems are: layer non-uniformity, melt interfacial instability, choosing an optimum adhesive, and the scrap recycle of dissimilar polymers. Solutions to these problems are dependent on proper equipment design and polymer selection for the particular structure and application. Today more development time is probably connected with selecting the best polymer for an application through pilot coextrusion runs. The viscoelastic and chemical properties of the different polymers in coextruded structures have major effects on the named coextrusion problems. Demonstration coextrusion runs are normally required to be certain that the polymers will function together satisfactorily. For example, the relative viscoelastic proper ties, which cannot be defined adequately by laboratory measurements, have profound effects on the layer distribution and on the interfacial instability problem (a type of melt fracture, but experienced at a melt interface between two polymers).

Fortunately, modifications to the equipment design (especially, feedblock and die) can overcome some of the viscoelastic mismatches. This is an area where the careful original design of the feedblock can facilitate problem solving by allowing easy modification of polymer flow channels if initial trial coextrusion runs reveal, for example, a layer non-uniformity problem requiring a design correction. The relative chemical properties of polymers (not actually a definable term, but frequently connected to relative polarities of the adjacent polymer layers) characterize the adhesion between layers and the possibility of scrap recycle with retention of adequate physical properties. When the polymers of choice do not adhere, an adhesive layer will be required. The adhesive choice can be crucial to the success of the coextrusion, for it can give the needed bonding and in some cases can act as a compatibilizer for scrap recycle. The adhesive layer also requires adequate viscoelastic matching to the other resins for best performance.

The challenges of coextrusion are best resolved by a very systematic approach to selecting and proving: the exact design of the structure, the specific polymers for each layer, and the integrated equipment system. It is important to resolve early in the considerations whether the desired structure is a developed structure (commercially proven) or one which is going to require development time and expense. This decision can have a major effect on whether or not a given company should undertake the project. Despite the challenges, coextrusion is growing in major ways after successful development work shows the way. Once the total system is satisfactorily developed, the coextruded structure should be consistently producible with about the same control care required for monolayer extrusion.

- Charles R. Finch

See also:
  • Coextruded multilayer distribution
  • Coextruded - why?
  • Coextruded sheets
  • Correcting flow instabilities in coextrusion
  • Interfacial instabilities during coextrusion of LDPEs
  • Project management
  • Sheet/film coextrusion grows
  • "Wave" pattern instability in multilayer coextrusion

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