Joining Systems for PVDF: What to Consider
Polyvinylidine fluoride is a popular choice for piping in high purity water and acid applications. Although it is widely used, proper recommendations for joining PVDF are often lacking. The most common joining systems are discussed in this article.
By David A. Seiler
Over the past 15 years, the need for fluid system purity in the semiconductor, pharmaceutical and biotechnology industries has led to a strong emphasis on material selection and specifications for piping products. Stainless steel is specified by type, and polymers are specified by grade name, additives, and manufacturer. Due to its extremely high purity, polyvinylidine fluoride (PVDF) has emerged as a popular choice for piping in high purity water and acid applications, where engineers are concerned about minimizing contamination to liquid wash streams.1-3 In fact, these three industries install about 10 million linear feet of PVDF piping and tubing each year in plants throughout the world.
Although PVDF is widely accepted by design engineers and contractors, projects that specify PVDF for high purity piping and tubing often lack recommendations on the proper method of joining the product. In this article, we will review the most common joining systems for PVDF.
System Availability
PVDF piping and tubing systems are available in a wide range of inner diameter sizes, wall thicknesses, and supported structures. Various forms of PVDF are available, depending on the desired stiffness.4
After deciding on the right size, thickness, etc., designers have to determine which joining method works best for their particular project. The method they select will impact system installation cost, pressure-rating, potential for disassembly and cleaning, purity, inner smoothness at welds, safety, and suitability for joining with other types of materials. Available piping and tubing sizes are listed in the table below.
Welded Systems
PVDF is easily welded and can be thermoformed to meet specific joining and component design requirements. Contact welding, radiant heat and RF induction are all proven ways of joining PVDF.
The first PVDF high purity systems were joined via socket fusion. This method involves placing the end of a pipe inside the end of a fitting, melting the outside of the pipe and the inside of the connector or fitting, and pressing the two ends together. When the heated sections of PVDF cool, the pipe and fitting become one continuous mass of polymer. Generally, the result is a strong joint with weld stress lines that are deflected, have a slight interference fit, and an inner bead. Socket fusion forms a joint quickly, with little cure time, and requires only simple tools and a heat source. Take care to avoid cold welds (in which the components are not fully molten) or misalignment of the pipe and fitting.
Butt fusion, which became popular during the 1980s, is another method of welding PVDF components. During butt fusion, the ends of two pipe components matched to size and wall thickness are heated until molten, then placed together with pressure. If done properly, the resulting joint should be at least 90 percent as strong as unwelded pipe. Stress lines are not deflected with butt fusion, and you can inspect the bead that forms at the joint. Like socket fusion, butt fusion forms a joint quickly with little cure time.
Although butt fusion can be done by hand in small tubing sections, special tools are typically used to apply predictable pressure to the pipe and fitting to hold them in place during heating, joining and cooling.
Beadless fusion is an advanced form of butt fusion designed to eliminate inner or outer weld beads on a pipe. It differs from butt fusion in that the bead is forced out of the weld by a heated, expandable bladder on the inside of the pipe. The pipe and fitting are both heated, and the molten area solidifies into a continuously molded system. When properly done, the beadless fusion weld line is almost undetectable, and the joint is as strong as a molded part. Beadless fusion requires special equipment with programmed timing and temperature controls.
Infrared is the newest welding method for joining PVDF. In socket, butt, and beadless fusion, the final joint consists of pure PVDF with no other materials present, i.e., solvents, gaskets, or embedded metals. But to complete the actual weld, the polymer must come in contact with metal surfaces to provide a heat sink for melting. Infrared joining uses radiant heat instead of contact welding, eliminating the possibility of contact with non-PVDF substrates during joining. The result is a strong joint with a minimum, uniform bead.
Infrared joining requires special equipment and a specific computer program, and the procedure will take longer than socket or butt fusion. Following the manufacturer`s guidelines should ensure a proper weld.
Mechanical Systems
Because of the ease with which PVDF can be thermoformed, machined, or injection-molded, the material is ideal for mechanical joining systems.
The oldest and simplest mechanical system for joining PVDF is threading. Molding or cutting threads in PVDF pipes and fittings allows joining of PVDF with itself, with metals, and dissimilar plastics. This joint has a bumpy surface, a lower pressure-rating than welded systems, also size and temperature limitations because of possible leakage from expansion and contraction. However, PTFE tape can be wrapped around the joint to prevent leaks. Of course, proper tightening of the components is critical to avoid harming threads and causing a leak.
Sanitary joining has been used on PVDF systems for years. Similar to the process used for joining stainless steel components, sanitary joining involves molding a connector groove within a fitting, post-forming a tube or another fitting, then joining the two by placing one into the other, using a gasket as a seal. Placing a nut over the top of the joint will secure the fitted grooves. Rather than a bead, the resulting joint has a small crevice. Temperature limitations based on sizing are available from the manufacturer.
To form the connector groove (or trumpet), sanitary joining requires a contact heater. It is important to choose the appropriate gasket material, properly line up the joint, and secure it. Depending on temperature and pressure, sanitary joining is generally viable for joints up to 2.5 inches in diameter.
Riontite is a new mechanical joining system for PVDF, in which two faced-off components are forced together. The system incorporates fittings with male threads, a face seal for the end of the pipe, and a PVDF snap lock ring. A nut is included with a metal cutting ring which, when tightened on one end of the fitting, seals and locks the connection. Riontite is limited to joints 2 inches in diameter and has a crevice rather than a bead. All contact surfaces are PVDF, and joints can be made quickly and without heat.
In flanged systems for PVDF, one component is bolted to another with a gasket in between. This allows joining of dissimilar materials but is limited in pressure ratings, and the fluid comes into contact with the gasket. Avoid overtightening the bolts. And keep in mind that the gasket must be compatible with the liquid the system will handle.
PVDF-lined steel piping systems have been available since the 1960s. There are several ways to secure the extruded liner inside the metal pipe. Whatever way you choose to secure the liner, several inches of PVDF will extend out of the metal, and the polymer will be thermoformed against the metal flange. When the components are bolted together, the PVDF compresses and seals.
This system`s main advantage is in combining the purity and chemical resistance of PVDF with the structural support of steel. A crevice forms at the joint, and liquid should not be allowed to contact the metal and extract metallic ions back into the stream. Temperature and pressure will not distort these systems.
PVDF-lined steel can be prefabricated or the lining can be applied with a heating tool and die. In recent years, minimizing the number of fittings and joints in new plastic-lined piping systems has reduced labor costs and eliminated potential emission areas. Like plastic-lined steel, flanged systems can be wrapped with fiberglass. But for long-term reliability, the fiberglass and the PVDF must be securely bonded.
Flexible Hose and Tubing
Flexible PVDF is also obtainable for tubing applications. This material has a flexural modulus one-third that of pipe-grade PVDF and is available in coils. PVDF tubing can be joined by standard compression fittings.
Some manufacturers are introducing composite hoses consisting of polyethylene or polyurethane on the outside and PVDF on the inside. Other types of construction–for example, braided hose–are used in specialty applications such as high-pressure fluid transfer lines
Summary
The type of joining method used for PVDF depends on several factors, including design criteria, performance and cost. Some of the systems presented here may be short-term solutions for quick startup, repair, or pilot operations. Other methods are designed for long-term reliability and optimum performance. A large system may require several different joining methods.
Whatever the requirements, design engineers should evaluate all options before settling on any particular method. They should examine the advantages and pitfalls of each joining method with a reputable supplier before determining final specifications. n
Acknowledgments
The author thanks the following individuals and companies for their input: Cap Hyde, Acutech Plastics; Jack Hunt, Chemtrol; Axel Riester, George Fischer Signet Inc.; Mike Dennehy, Orion Fittings; Rob Marsiglia, Sanitech, Inc.; and Nancy Lindley, Dow Chemical USA.
References
1. Blume, R., “Preparing Ultrapure Water,” Chemical Engineering Progress, pp. 55-57 (Dec.1987).
2. Governal, Robert A., “Ultrapure Water: A Battle Every Step of the Way,” Semiconductor International, pp. 176-180 (July 1994).
3. Henley, Mike, “Semiconductor Industry Expansion Should Continue,” Ultrapure Water, pp. 15-20 (May/June 1995).
4. Barber, L., Seiler, D.A., “Handling of High Purity Fluids with New, Flexible PVDF,” Ultrapure Water, pp. 37-38 (May/June 1989).
David A. Seiler is manager of sales and marketing, Technical Polymers for Elf Atochem North America, Inc. (Philadelphia, PA). Seiler has been with Elf Atochem and its predecessor companies since 1983. He has more than a dozen years` experience in the development of PVDF piping and tubing applications for the chemical, pharmaceutical, semiconductor, and other industries.
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PVDF is thermoformed for joining in a mechanical system.
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One of the newest welding methods uses infrared for joining PVDF. Infrared uses radiant heat instead of contact welding. Shown is an infrared welding machine.
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An example of an installed PVDF piping system.
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The individual mechanical PVDF components are available in a wide variety of diameter sizes and thicknesses.