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Coextrusion: Changing the Face of Tubing

Medical Device & Diagnostic Industry Magazine MDDI Article Index Originally Published MDDI September 2005 Cover Story: Extrusion

Chris O'Connor

In the early 1950s, manufacturers began using extruded tubing in packaging applications for liquid medicines and cosmetics. By the 1970s and 1980s, extrusion technology had advanced to include numerous polymers, multilumen options, and bump tubing. In addition, coextrusion technology was developed, enabling multiple materials to be combined into a unified structure.

Coextrusion is obviously a versatile tool. There are literally thousands of plastic materials that are suitable for extrusion. Considering the possible combinations and permutations of coextrusions, there are numerous possibilities that can be explored. The technology enables device manufacturers to create films, sheets, profiles, and tubing with specific aesthetic and performance features. Coextrusion processes can form stripes and layers on tubes to reconcile seemingly contradictory requirements, such as good flexibility and good pressure and chemical resistance. In effect, coextrusion technology may be the best solution to many tubing application problems. And, with the constant development of new material formulations, new opportunities for coextrusion combinations are constantly presenting themselves.

Coextrusion

Extrusion is the process of melting and pumping a plastic through a heated die and then cooling the plastic into a specific shape. Coextrusion is the same process, except that multiple extruders pump more than one material into the product. Manufacturers can extrude the material onto the product as an additional overall layer or coating. Alternately, it can be placed in a defined area, such as in a stripe or a sequentially alternating material pattern.

The coextrusion process serves many purposes in today's medical device market. Most notably, combining multiple materials enables manufacturers to modify a product's properties. Changes that are possible include improved chemical resistance, good mechanical attributes, better visual and aesthetic appearance, and reduced cost. This article refers specifically to tubing products, but the basic ideas and technologies are applicable to many other extrusion products as well.

Plastic tubing is an integral part of many medical devices. It is diverse enough to be used as a 0.030-in.-diameter wire guide or as a 2-in.-diameter packaging sleeve. Helping this diversity is the fact that many different types of plastics can be extruded into tubing. Vinyl, polyethylene, and polypropylene are very commonly used tubing materials. Other more-specialized materials, such as nylon, acrylic, styrene, polycarbonate, and fluoropolymers, can also be used.

Depending on the materials used, tubing can have very different physical performance characteristics. For example, PVC can be formulated to be soft or hard, clear or opaque, and is typically durable enough for most medical applications. Polyethylene tubing is stronger and has additional chemical resistance, but it is translucent. Polycarbonate tubing is very clear and resistant to high temperatures, but it is very rigid. But some devices require tubing that exhibits seemingly contradictory characteristics, such as tubing that has good chemical resistance and is very flexible. If a tube made from one material lacks certain physical characteristics, coextrusion can correct the problem by giving the tube additional features.

Multilayer tubing, produced using coextrusion, can offer physical performance enhancements not available with monolayer tubing. Coextruded tubing is often made with one or more thin layers, or skins, on the inside or outside of the tube. The skins can be 0.002–0.005 in. thick. The thin skins are combined with a thicker (0.005–0.100-in.), more-functional inner or intermediate layer. The skins can either protect the functional layer of material from damage, or they can protect the tube contents or user from contact with the contents, or both.

Materials to Consider

There are many combination possibilities for coextrusions because there are many types of materials and layer thicknesses that can be created. Sometimes, combinations are made with materials of the same chemical family, such as vinyls, cellulosics, or polyethylenes. The combinations can be hard or soft, depending on the amount of plasticizer or the density in the formulations. Multilayer structures combining these related materials easily bond to each other because they have essentially the same makeup. Table I lists a few examples of materials that can be extruded together, as well as the medical applications for which each combination can be used.

Sometimes the chosen materials are not from the same family, such as vinyl and polyethylene. These materials cannot bond with each other because of differences in their molecular structures. Using coextrusion, additional layers can be added to bond the dissimilar layers. These additional layers are known as adhesive or tie layers. Some manufacturers have been able to coextrude structures combining as many as seven layers.

However, it is important to note that some materials cannot be coextruded. These include materials that have significantly different melting points and viscosity curves. For example, a material like polycarbonate, which processes around 500ºF, should not be coextruded with PVC, which processes at 300ºF. In that case, the PVC would degrade when it came into contact with the hot polycarbonate. Other combinations of materials, such as PVC and acetal, are explosively reactive and so should never be coextruded together.

Applications

Stripes. One of the oldest examples of coextrusion is the coextruded stripe (see Figure 1). A striped drinking straw is one simple example of a coextruded tube. If, for instance, a white straw is extruded from a polypropylene material, a colored polypropylene could be coextruded in a small area to create a striped tube. Various colors of stripes can be coextruded for a more aesthetically appealing product.

In medical applications, coextruded stripes are also often used as identifiers. For example, a swab with a blue stripe might be identified with a specific company brand. Also consider a suction or irrigation device connected to a pump that uses two very long tubes. If one tube is striped blue and the other striped gray, they can easily be identified from one end to the other. This can be important because such tubes can be 30 ft long and are often routed to remote connections. Also, using a stripe rather than simply coloring the entire tube maintains visibility of the tubes' contents.

Other striping options for coextrusion include radiopaque materials, such as barium sulfate. Radiopaque materials can be seen in x-ray scans. A stripe of such material can be coextruded into a tube so physicians can identify the tube's position when it has been inserted into a patient's body.

Similarly, a stripe can be coextruded into a tube to indicate the position of a physical feature, such as a formed curl. A urology stent has a curled end that maintains the device's position during use. A guidewire holds the curl straight during insertion, and when the wire is extracted, the tube curls back to its formed shape. The curl must be in the correct position when this happens. A radiopaque stripe on the tube will indicate the orientation of the bend and is visible on the exposed portion of the stent. This ensures the device is correctly oriented.

Layers. Coextruding layers of different materials can produce tubing with different inside and outside properties. Figure 2 shows a thin layer of high-durometer thermoplastic rubber coextruded inside of a low-durometer thermoplastic rubber. This combination offers an inside surface that is slightly hard and through which it is easy to insert a cable. The basic physical property of the tube, however, is still soft and flexible.

Or, imagine a packaging tube that contains both liquid and a brittle device, such as a glass ampule. The package has been validated by several costly tests, and the tubing material has been approved for contact with the contained liquid prior to use. Unfortunately, in use, the brittle device occasionally punctures the package and causes spills. A coextruded package design could combine the original tube material with a different, more-puncture-resistant exterior material. The original material would form the inside surface of the package to maintain the already-established contact approvals. However, the additional material would encapsulate the outer surface of the package to avoid punctures and spills.

Another option involves increasing the strength of a product while maintaining its flexibility. Consider a supply-line tube that has traditionally been made of flexible vinyl. A new application is introduced that increases the supply-pressure requirement beyond the strength of the flexible tube. More-rigid materials cannot be used because the supply line feeds a hand-operated device, and the rigid tube creates too much resistance to movement. A coextruded tube that adds a layer of the more-rigid material to the flexible vinyl could increase the pressure rating of the supply line and maintain the device's flexibility.

Additional layers can also be added to extrusions that already contain additional features, such as multilumen structures. The inner surface of the multilumen urethane tube shown in Figure 3 creates less friction than an equivalent unlined component does. Figure 4 shows a three-layer extruded tube. The original device was a styrene tube that had a cotton end solvent-welded to it. However, the manufacturer needed to make the device steam sterilizable. The original styrene material would warp and distort when exposed to the high-temperature steam. Polypropylene could withstand the steam environment without warping, but the solvent bond could not adhere the fiber tip to the propylene. An outer layer of styrene was coextruded onto the propylene, which yielded both the temperature resistance and the solvent weld feature. Finally, a third adhesive layer was needed to bond the propylene to the styrene.

Permeation Resistance. Coextrusion can also be used to add a layer of material to provide a so-called barrier that protects the tube's contents. Many commonly used materials, like polyethylene and styrene, lack such barrier performance. Without a barrier, vapors or gases can permeate through the wall thickness of the tube. This permeation can result in two different problems. Either the contents of the tube will slowly evaporate out (egress) of the tube, or outside elements, such as oxygen, can enter (ingress) the tube. Ingress and egress can be problematic in many applications. The addition of a barrier layer will retard this permeability.

For example, a low-density polyethylene tube was designed to contain an adhesive product. The tube worked well, but the required shelf life of the product was so long that the adhesive would begin to lose functionality. A certain nylon that possessed the permeation barrier was used to keep oxygen from ingressing to the package contents. A three-layer tube was coextruded to provide the barrier for the contents while maintaining the contact-sealing surfaces of the polyethylene. The shelf life of the product was extended, and the functional integrity of the package was not compromised.

Economics

Coextrusion technology has gained considerable ground during the past 10 years. Tooling and control techniques specifically designed for coextrusion are now commercially available. Die sets can be purchased to produce almost any layer configuration imaginable. Measurement systems that include ultrasonic layer-measuring heads can be incorporated into feedback control loops that will automatically adjust diameter and layer thicknesses.

The technology does have a price, however. The up-front costs of coextrusion include additional extruders, additional floor space for the extruders, and higher tooling and adaptor costs. This can double the investment costs of the equipment upstream of the cooling section for the extrusion line. Also important to consider are ongoing costs, including commingled scrap and increased levels of process complexity. Also, because of the increased complexity of this technology, there is often a need for an experienced, senior line operator. It may be a good idea to consider hiring an experienced contract tubing manufacturer to perform complicated coextrusion processes (see sidebar on this page).

Often, a more-expensive material can be replaced with a less-expensive material using coextrusion. For example, a fluoropolymer-lined polyethylene tube should be less expensive than a plain fluoropolymer tube. This substitution does not always result in a less-expensive product, however. The additional process costs, such as longer startups and higher scrap rates, may offset the material cost savings.

Conclusion

Extruded tubing comes in many sizes, colors, and materials. Coextrusion can modify the physical features of a tube to add visual appeal, surface lubricity, or physical strength. There are many possibilities of material combinations that can be realized using coextrusion. Application solutions are now becoming more focused, such as by putting critical materials in locations where they will be most beneficial. Adding stripes and layers of highly specialized resin formulations in specific locations of a tube can yield a desired benefit. Also, rising raw-material costs often justify coextrusion to displace expensive formulations with less-expensive commodity materials in noncritical areas.

Newly formulated materials can be coextruded with traditional tube materials. Specific surface conditioning is now possible with new resin formulations. Tubes with physical properties that mimic standard commodity products can be coextruded with skins of new formulations to offer low coefficients of friction, static dissipation, and antithrombogenic properties in a tube that otherwise appears to be indistinguishable from other standard tubing.

New devices and new materials are being designed every day. Coextrusion can be used to combine these expanding technologies into future products.

Chris O'Connor is senior product development engineer at Teel Plastics Inc. (Baraboo, WI).

Copyright ©2005 Medical Device & Diagnostic Industry

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