For over 30 years, one of the most common materials specified for acid waste piping systems in North America has been polypropylene. Polypropylene is widely resistant to a wide range of acids and bases, having a capability to withstand a pH range of less than 2 to more than 13. However, while polypropylene is an economical choice for standing up to a wide range of chemicals, it is combustible and, as such, building codes often do not permit combustible pipe to be exposed in certain areas including return air plenums.

Polypropylene is a polyolefin thermoplastic that is polymerized from an oil derivative known as propylene. Propylene, which is an olefin, is like most derivatives of crude oil in that the basic molecule consists mostly of carbon and hydrogen molecules (CH3 - CH3 = CH2). Like most carbon-based products, polypropylene is combustible. It often is blended with additives to make it "flame-retardent" (also referred to as "fire-retardant"). However, it is not possible to retard combustion in a carbon-based product without increasing smoke generation to some extent due to the production of partially combusted hydrocarbon byproducts.

The standard form of polypropylene for indoor building use is the fire retardent type. While fire retardant polypropylene is less combustible than standard polypropylene, there are no formulations available for pipe that can meet a flame spread of less than 25 and a smoke density of less than 50, when tested according to ASTM E-84. It is this requirement of less than 25 flame spread and less than 50 smoke density that is required by most building codes for products installed in return air plenums. In some buildings, the entire areas between floors can sometimes be classified as return air areas, which means that a substantial portion of the building available to the acid waste piping installation is often in return air areas requiring this classification.

In the past, for acid waste installations where parts of the system had to pass through return air plenums, standard materials used included borosilicate glass or silicon-alloy iron (Duriron) piping. These materials, while being very chemically-resistant, have proved to be very expensive and difficult to install due to their brittle nature and the subsequent high level of breakage that is unavoidable.

In recent times, some manufacturers have promoted the application of a fire-retardant to polypropylene wrap as an alternative to borosilicate glass or silicon-alloy iron. However, some wraps have been tested by UL to result in a flame spread of less than 25 and smoke density of less than 50, according to the ASTM E-84 test. The final product assembly will not have been tested by UL according to this test, and as such cannot be considered to carry this rating. The application of this product is often difficult to install as required (with minimum 50% overlap) in many areas, and particularly when the piping is in a ceiling, behind walls or near other building structures that provide interference.

The PVDF Alternative

For several years now, there has been a viable to the other choices mentioned. This new answer is a specialized polymer known as polyvinylidene fluoride (usually abbreviated as PVDF), which is trademarked by Orion as "SuperBlue," using KynarR PVDF resin manufactured by Elf Atochem, N.A., and available in complete, plumbing-code approved drainage systems through 6-inch diameter. The SuperBlue PVDF product has been tested by UL according to ASTM E-84 and has demonstrated actual results of a flame spread of 5 and a smoke density of 35, according to the tests (Mfr.'s Ref. No. R15537, Control No. 1P91). This makes the product in many return air classified areas, whereas no other thermoplastic material was previously accepted. It is also price competitive when compared to glass and silicon-alloy iron, while also eliminating the further costs associated with breakage of these alternative products.

While PVDF does provide a new answer in terms of satisfying the E-84 smoke and flame spread requirements, it does not come at the expense of lessened chemical resistance. In fact, PVDF is even more chemically resistant to a wider range of chemicals and pH as compared to PP, and has better structural strength and temperature resistance. It is useful over a temperature range from -20 F up to 280 F.

PVDF is a member of the fluoropolymer family, which consists of polymers that contain fluorine as part of their make-up. Other fluoropolymers include the various forms of TeflonR (DuPont) such as PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene-propylene), and PFA (perfluoroalkoxy-ethylene), in addition to ECTFE (ethylene-chlorotrifluoroethylene copolymer; the most recognized brand of which is HalarR (Ausimont). Both PVDF and ECTFE are partially fluorinated fluoropolymers, whereas PTFE, FEP and PFA are fully fluorinated (all possible bonds to the carbon chain consist of carbon-fluorine).

It is the presence of this carbon-fluorine (C:F) bond that gives these polymers their excellent chemical resistance, since the C:F bond is the strongest possible bond that exists compared to all other typical polymeric bonds (such as carbon-hydrogen, carbon-oxygen or carbon-chlorine). Most chemicals do not have the wherewithal to disrupt this bond, which would then lead to attack of the polymer. It is for this reason that the fully fluorinated fluoropolymers (PTFE, FEP and PFA) are notorious for displaying unparalleled chemical resistance compared to virtually all other materials. While PVDF and ECTFE are only partially fluorinated, they have enough of these strong C:F bonds to exhibit chemical resistance nearly that of the fully fluorinated types. In addition, PVDF and ECTFE have the added features that compared to the fully fluorinated types, they possess better mechanical strength, creep resistance, and are less permeable (the various forms of TeflonR, while chemically resistant, allow many chemicals to permeate through their walls).

The presence of the C:F also has a great deal to do with allowing the polymer to resistant combustion, since it takes a great deal more energy to disrupt this exceptionally strong bond. In normal atmospheres, PVDF has a limiting oxygen index of 44%, meaning that it takes an oxygen content of 44% for the product to continue to combust once the source of a flame is removed. In many tests, it is hard to even get the PVDF material to be able to combust according to the normal test methods, since it is so very fire resistant.

High Purity Water Systems

PVDF, like its fluoropolymer cousins, is also a product which can be manufactured in an extremely pure form, without the use of additives typically required of other polymers to provide mechanical strength and aid in processing. As a result, the material is good for high purity water systems where water of extreme purity is being conveyed (e.g. in 18 megaohm high purity water for semiconductor manufacturing, and in water for injection use in pharmaceutical manufacturing).

Compared to all other fluoropolymers, PVDF is the least expensive, and given its comparable chemical resistance and superb mechanical properties, is an excellent choice for rigid piping systems. Piping is manufactured in sizes from 1-1/2 inches through 12 inches and is available in molded fittings through 6-inch diameters.

PVDF is a heat fusible thermoplastic, and is typically joined by either socket fusion (ASTM D 2657, Practice 1) or butt fusion methods (ASTM D 2657, Practice 2). Unlike products such as PVC, CPVC or fiberglass, PVDF can not be solvent cemented or joined by adhesive bonding, mainly owing to the fact that its superb chemical resistance to most chemicals eliminates the possibility that common adhesives can soften or attack the surface of the tough polymer.

The SuperBlue product line also offers a mechanical means of joining, which for applications where installation is in areas that are restricted offers a reliable and easy method for joining the product. In addition, this mechanical joining offers the added feature that the PVDF product line can be transitioned to polypropylene via this form of joint (using a PVDF flexible joint liner), thereby offering the possibility of an economical installation. Therefore, polypropylene can be used outside of return air plenum rated areas, whereas PVDF can be used inside of all return air plenum rated areas. Since PVDF is more expensive than Fire-Retardant PP, such a combination makes for a very economical overall installation. This combination of materials is referred to by Orion Fittings as its "Triple-Play" combination, and is illustrated in the accompanying figure. Orion PVDF has been used extensively throughout the U.S. in schools, hospitals, and other research institutional buildings having a building occupancy, and thus, being subject to building fire codes.

In addition to having UL listing for flame spread of less than 25 and smoke density of less than 50, according to ASTM E-84 (Mfr.'s Reg. No. R15537, Control No. 1P91), SuperBlue is also approved by major plumbing codes. This includes listing according to the Uniform Plumbing Code (IAPMO Certificate of Listing, File No. 3738) and the International Plumbing Code (BOCA Certificate of Compliance, Research Report 98-38). The users of the product line can therefore have a system which meets both the letter and intent of the fire code, while also maintaining plumbing code approval, without having to resort to the expense and difficulty of installing a borosilicate glass system or high silicon-alloy iron system. PVDF piping is also available as dual containment piping systems, either with a PVDF/PVDF combination, or by combining PVDF inside of polypropylene for a more economical choice. PVDF is also available in a full array of other associated parts, such as tanks, valves, instruments, pump parts and others, to offer complete piping systems installations.