By Rod Anderson, Managing Director of Aflex Hose Ltd. (Original article published at www.pharmaceuticalonline.com)

PTFE (polytetrafluoroethylene) was discovered in 1938 and became commercially available in 1950. PTFE-lined flexible hose products were first introduced in 1955, mainly for military applications in the United States. It was soon realized, however, that two of the primary properties of this remarkable plastic made it particularly ideal for use in pharmaceutical and chemical plants as a lining material for components containing process fluids - the excellent chemical resistance and ease of cleaning are quite simply in a different league than those of the other materials that previously had been used.

Many pipes, vessels, pumps, valves, and other process fluid components have consequently been lined with PTFE and other fluoroplastics ever since, and various designs of PTFE-lined hoses have been developed to provide a flexible conduit for connecting between components.

Basically, PTFE-lined hose constructions include an inner PTFE liner tube with outer reinforcement and protection layers, including stainless steel wire or textile braids, steel helix wires, and rubber layers and covers in various combinations.

The hose constructions are designed to provide a hose that is able to flex sufficiently in the application without excessive stress being applied and without the risk of damage, kinking, or interference with the function of the hose.

This is where a problem arose, because hoses made up with smoothbore PTFE liner tubes in bore sizes above 3/4 inch (20 mm) were found to be hard to flex and prone to kinking.

This problem was partially solved by making the PTFE, FEP, or PFA liner thin-wall, then bonding it externally to an EPDM or white silicone rubber hose carcass. This was done in order to prevent the possibility of the liner tube kinking inside the hose construction. Such products are widely available today, but have significant disadvantages. First, the adhesive layer outside the thin wall liner is a chemically reactive, non-FDA approved material, which has been known to leak into the hose bore and contaminate process fluids in certain types of applications. Second, when the hose is flexed, the liner at the inside of the bend ripples severely, creating entrapment zones. Third, the hose is stiff, and hose bore sizes above 1-inch bore require excessive force to flex the hose, making it difficult to install on site.

Another widely available hose designed to overcome the problem includes a PTFE tube liner, which is helically convoluted, or 'concertina-ed'. This provides a hose liner design with excellent flexibility and kink resistance, but introduces the significant drawback that the internal convolutions are entrapment zones for the fluids passing through, and turbulent flow is induced by the internal convolutions, which severely reduces fluid flow rates.

In spite of the disadvantages, these PTFE, FEP, and other derivative fluoroplastic-lined hose products are available and in use in pharma, biotech, and chemical plants worldwide with varying degrees of customer satisfaction.

During the last few years, an alternative design of PTFE liner has become available for use in hose constructions that combines a substantially smooth bore (slight ripples) with an excellent level of flexibility. While the bore of the PTFE liner is effectively smooth, the outside surface is heavily convoluted to provide 'hoop strength', so the liner is self-strengthened and does not need to be bonded with an adhesive to the outside braid or rubber layers.

When flexed, the bore does not exhibit severe rippling. The ripples already present become a little deeper, but not severe.

The hose liner is externally reinforced with a stainless steel wire braid and an optional EPDM or silicone rubber cover, without any adhesive layers, to complete the hose construction.

Such hoses can be made into assemblies that are available with 'integral PTFE-lined' end fittings attached, in which the thick-wall PTFE liner is passed through the bore of the end fittings then flared out to form the sealing face. This protects the end fitting material from contact with the process fluid and ensures clean, restriction-free flow of the process fluid through the end fitting.

Finally, pharmaceutical, biotech, and chemical plant engineers are able to obtain what they have always wanted - a genuinely flexible, genuinely smoothbore, fully PTFE-lined hose product, eliminating all the disadvantages of previous designs.

These products are available under the trade names Bioflex, Pharmaline, and Pharmalex, and are available from Aflex Hose Ltd (UK), Aflex Hose USA LLC, or any of the Aflex Hose distributors worldwide.

Rod Anderson was one of the founders of Aflex Hose in 1973 and has been managing director of Aflex Hose Ltd for the past 15 years His primary achievement has been to develop worldwide market leadership in the PTFE-lined hose products that Aflex offers to pharmaceutical, biotech, and chemical plants for process fluids transfer applications.