PVDF Changes Improve High-Purity Water Piping Systems
During the preliminary years of the development of semiconductor technologies, manufacturers came to know that the performance of the computer chips depends on the quality of the water in the wafer rinse process. In the 1980s, it was often believed that a start-up manufacturing plant will generally have chip yields of 75% (1). Such low yields were found to be partly associated with construction materials in the process fluid handling system employed for cleaning silicon wafers.
The original systems were developed from steel pipes, and it was discovered that the high ionic extractables associated with metals do not give significant performance for the produced chips. This resulted in a paradigm shift toward plastic piping, which for a brief period of time was found to lower metal contamination. However, this eventually resulted in new forms of contamination such as total organic carbon (TOC) and remaining extractables associated with the stabilizers, processing aids and other additives that are common to commodity plastic piping. Another issue regarding commodity plastic piping was that these materials were not suitable for higher temperatures, which apparently enabled a better cleaning performance.
Early Movement to PVDF
Some of the first engineering pioneers in the design of high purity piping systems have been testing polyvinylidene fluoride or PVD, perhaps as early as 1980 (2-4). Based on the data being developed, it was obvious that the use of PVDF was a potential game changer in the field of semiconductor high-purity water processing. PVDF, unlike other polymers used for water at the time, was stable in nature and did not require the addition of processing aids, pigments and light stabilizers to be made into a smooth surface molded or extruded product that retained its original properties for a lifetime of use (5, 6).
Since 1965, the PVDF material has also been routinely used in challenging chemical applications such as hydrocarbons, strong acids and chlorinated solutions in service temperatures up to 145 °C. Cold or hot water did not pose any major problem for this specialty polymer that earlier was popular in niche applications, for example the nuclear reclamation industry, chlor-alkali, bromine, pulp and paper, and petrochemical industries. Filtration equipment, tower packing, pumps, valves and piping already existed commercially to service these other industries (7).
The original PVDF piping systems that were considered for the processing of high purity water were based on these available designs, which were favored for those applications where a complex chemical needs to be moved from one place to another and, more significantly, contained. These piping systems were connected by methods that now would be considered outdated by the present high purity industry standards. Butt fusion joining, socket fused solid piping and flanged plastic lined steel are systems that were quite common and used on some relatively large jobs. Examples of butt fusion and socket fusion systems are shown in Figures 1 and 2.
Figure 1. An example of butt-fused plastic piping.
Figure 2. View of plastic fittings designed for socket fusion and flange style joining.
Each joining method presented a potential of some kind of location that encouraged bacterial growth. Since almost all systems in the early stages were ambient water, the initial joining methods for PVDF pipe called for regular sterilization, which lowered the overall efficiency of the chip manufacturing process. At this moment in the 1980s, the discovery of a highly pure and strong material to process pure water was benefiting the industry. Based on the data being developed, it was indubitable from a starting point that PVDF was relatively low in water-related extractables as a polymer of construction, but still work was required on the mechanical design of the fluid handling systems to fulfill the requirements of this rapidly growing industry at the time (8, 9).
New Pipe Joining Methods
In the late 1980s, a new method to mechanically join piping that was initially designed for the drug, food and beverage industries markets came on the scene with PVDF as one of its key available materials of construction. “Sanitary” PVDF piping provided a means to join plastic systems in a better way than flanged systems. Compared to plastic-lined steel, the sanitary system was lighter and gave users an option to never throw away a pipe or fitting that had been poorly welded.
These systems became so popular that they are still used in small laboratories today, but eventually they were found to be suspect when the piping’s outside diameter (OD) was increased above 2.5 inches. One reason for this is that the plastic could be deformed in long runs of piping as a result of expansion and contraction with any substantial heat variation.
This brief period of discovery resulted in better ideas on how to further enhance the usual contact butt-fusion systems that were competing with the sanitary concept at the time. While sanitary systems continued to remain in use, it was only for smaller or highly specific applications for PVDF in laboratories. From the available range of options, two types of new joining methods evolved:
Beadless fusion was a novel welding method which when perfected makes a pipe appear to have no weld line on the internal surface. Although this form of welded joint consumes a lot of time to make, the welding apparatus suppliers have developed a machine that was highly consistent and largely free of human error if instructions are properly followed. As predicted, this type of welded joint can be mostly preferred in the light chemical and pharmaceutical water processing industries. A faster and easier joining process is required in large semiconductor plants. The smooth inside of the joints in a beadless weld system is shown in Figure 3.
Infrared (IR) welding is a welding method where the contractor has a well controlled machine that generates heat without making any contact with each area of a piping system that will be joined together. While this may seem a small problem, standard butt fusion has a wide window of operator skill that can create a weld of varying bead formation. Moreover, in contact welding, materials can adhere to the heater plate and cause unwanted material contamination and geometry in the weld area.
IR welding prevents many aspects of error that are associated with butt fusion and which would not be a major concern in many end applications for a high performance pipe. The inception of the IR welding method in the early 1990s together with years of development of joining systems has led to this method being the most popular option by the most prominent semiconductor manufacturing facilities.
Progression of PVDF in High-Purity Water
In the year 1980, about the time that PVDF was under development for use in high-purity water, the majority of PVDF piping systems were pigmented in green, blue, black, and red to differentiate a chemical plant from other polymers like polytetrafluoroethylene (PTFE), PVDC, and polypropylene. To the inexperienced plastic pipe user, most of these polymers appear to be the same and the use of color prevented a less advanced user base from inadvertently putting a material in 130 °C service and having a catastrophe occur. It was an easy process to no longer add pigment to the PVDF piping system and market a “natural” product to the new engineer team designing high-purity piping systems for the semiconductor industry.
At first, PVDF suppliers only offered homopolymers of vinylidene fluoride for piping systems and hence only a limited amount of grades could be used for fittings and pipes. In the 1980s, there was a great deal of research that led to PVDF grades which gave an effective surface finish and cycle time in molding, and the optimum in surface smoothness in extrusion.
Manufacturers developed a process in the 1990s that enabled the in-line deionized (DI) water washing of PVDF emulsion in the reacted latex form when the material is still at its smallest particle size prior to being processed into a pellet. This is later sold to the component manufacturers for making pumps, pipes, valves, tanks, instrumentation and filtration equipment.
This was believed to be a major achievement because combining this with the newer joining technologies developed by the pipe manufacturers benefited the end user, who now had access to an even more pure product to the parts per billion (ppb) extraction levels together with a piping system that reduced bacteria growth areas (10). To this date since the mid 1990s, the grades for the most favored high-purity PVDF piping systems had remained unchanged and continue to provide exceptional service performance.
As PVDF homopolymers went on to perform at the levels required in cold and hot high-purity water piping systems, engineers started exploring the other applications for PVDF-type chemistry in areas where water purity is ensured by using more expensive materials. PVDF copolymers of hexafluoropropylene and vinylidene fluoride (VF2) became popular in the area of flexible tubing (11).
PVDF homopolymers are good piping materials, as they are well known for their high strength across a wide temperature range (12). For tubing, the opposite is required and a break up of crystallinity of the polymer is created through the addition of comonomer to the polymer backbone, resulting in a material with greater flexibility.
Since the comonomer is fully fluorinated, and hence very stable, purity is not lost and also the amount of flexibility can be varied depending on the requirement of the user (13). To put it in simple terms, it is possible to generalize the flexural modulus of the PVDF homopolymer at about 2,070 mega pascal (MPa) (plus or minus, based on the molecular weight distribution) and there are commercially available PVDF copolymers in high purity versions down to 200 MPa.
Since both PVDF and PVDF copolymers act as good barriers to water, the lining thickness can be down to 1.5 millimeters (mm) (60 mils), and still provide enough permeation resistance to prevent the extractables that would otherwise be associated with the substrate. This special combination of PVDF is known to be the strongest commercially available fluoropolymer up to 140 °C (15) and is available as a highly flexible fluoropolymer within the same manufacturing process, and hence it is an exclusive polymer meant for engineering design.
Except for PVDF, high temperature capable fluoropolymers of flexibility range, varying performance and molecular design can never be welded to each other (16). Shown in Figure 4 is a high-purity tank constructed using fabric backed PVDF copolymer sheets that were welded and bonded to an existing metallic shell.
End Users of PVDF in Pure Water
As explained in several paragraphs of this article and validated in numerous other published articles, PVDF is extensively used in the semiconductor industry (17). Right from the development of semiconductor-based technologies through to the growing need for better processes in the potable water and biopharmaceutical industries, many specifications at top engineering organizations favor PVDF over other commodity polymer options or metals in the design of all types of pure water systems (18, 19).
Standards adopted by ASME-BPE (20) in the biopharma industry clearly help designers on factors that need to be considered when using a PVDF piping system. Since they relate to potable water, many PVDF grades are listed to NSF 61 — the testing and listing procedure defined by the National Sanitation Foundation. Users can visit the NSF.org website to get the specific grades of PVDF and PVDF copolymer that meet this listing.
The reason for selecting the PVDF in these industries as opposed to the low extractable concerns in the semiconductor industry is that in addition to being a pure material that conforms to regulatory compliance, PVDF can also be steam cleaned (21) and withstands cleaning agents in a higher performing manner compared to stainless steel. It also provides a light weight and easily installed option. This is combined with the fact that some forms of high-purity PVDF conform to the ASTM E84 (25/50) standards needed in building codes for fire safe products (22). Shown in Figure 5 are some examples of ASTM E84 code compliant piping deployed in an institutional building.
Since 2003, the use of PVDF in biotech applications has more than tripled around the world. Moreover, PVDF is welded to polyethylene in residential housing construction in Europe which has made the potable water industry experience double- and triple-digit growth over the last three years. It is estimated that compared to solid PE or PEX structures where chloride disinfectants are used, composite PVDF/PE structures can last 5 to 10 times longer.
New Applications for PVDF
In the last 10 years, PVDF has been increasingly used in filtration products. In fact, flat sheet PVDF membranes had existed for years and are extensively used in the semiconductor and biotech industries (23). The purification of water on a global basis has been made possible, thanks to the newer versions of PVDF and PVDF copolymers that lend themselves to the production of hollow-fiber membranes.
Aside from this development, woven and non-woven fabrics can be produced by extruding very fine fibers of PVDF and these fibers can be integrated in filtration devices for ultimate performance in water applications where high temperature, disinfectant resistance and overall general long life are required from a material. While this use of PVDF has now become commercial, there is still more scope for development and perhaps it is the most exciting and novel technology platform being explored at this time.
The speed at which the fabrics are made from PVDF will hopefully fill the gap that has existed for a long time where designers of filtration component were had to use dissimilar materials in the construction of the end products. Furthermore, it was very common to see PVDF filters with polyolefin supports that lacked the chemical resistance or temperature of PVDF, or PVDF filters that were supported by more expensive materials than if the whole structure was made with the more affordable PVDF polymer (24-26).
Composite disposable systems are another application for biotech fluids. Pure, flexible PVDF copolymers can be fixed in multilayer structures with other flexible substrates such as polyether block amide, thermoplastic urethane (TPU) and flexible polyesters. Such multilayer structures can be any kind of structure that is extruded, and this novel technology is generally seen in films or tubes that are made into bags (27). Shown in Figure 6 is an example of the use of PVDF multilayer films as a disposable system.
It was in the 1980s that PVDF emerged into the high-purity water scene. The original products, and techniques of joining the products, have all emerged in ways that enhance the overall quality of high purity water. Similar to many technology roadmaps, PVDF components came into the semiconductor industry with designs that were suitable at that time to support other industries that were less technologically advanced.
These polymers and designs have been further implemented for specific application in the semiconductor industry, which has resulted in product developments promoting adjacent growth to novel types of high purity water industries such as potable water, pharmaceutical and biotech. With ongoing developments, designers and end users across many apparently unrelated industries will most probably reap the benefits in terms of system availability, performance and cost from the PVDF components suppliers.
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