PVDF HIGH PURITY PIPING MATERIAL FOR DE‐IONIZED WATERBENEFIT COST COMPARATION

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Key issues :Years ago, high purity water was used only in limited applications. Today, deionized (Dl) water hasbecome an essential ingredient in hundreds of applications including : Pharmaceutical, Biotechnology,Food Processing and Electronic Industries. Many industries are seeking ways to leave behind thecurrently‐used troublesome materials.Stainless steel imposed constant and increasing problems : rouging, pitting, corrosion, metallic‐poisoning, aggravated compliance issues, costly and environtmentally adverse cleaning protocols, andinadvertent fracture, plus costly biofilm issues.

Stainless steel has historically been adopted for containment of chemical processing because it isresistant to more chemicals than is iron or mild steel. It is an inorganic chemical combination of theessentially iron, chromium, and nickel. The chemicals resistance of stainless steel to certainchemicals can be improved, with a corresponding increase in cost. But even such chemical resistanceimprovement is not sufficient to overcome chemical attack or the corrosive attack of biofilmcomponents. Stainless steel corrode over time as the minor ingredients are lost and aselectrochemical potensials arise which promote the oxidation in even the “mildest” of chemicalconditions, i.e. hot steam, and the resulting rust (”rouging”) contaminates and compromises thequality of the products being produced in such equipment. Stainless steel can be furtherchemically treated to be made less reactive, i.e., passivated in a time consuming and expensivetreatment that must be performed regularly to ensure that the iron in this material doesn’toxidize_i.e, rust.

PVDF HIGH PURITY PIPING _ DEIONIZED (DI) WATER 

Steel Corrosion vs Temperaturesources :Materials Engineering for the Chemical Process Industries

Higher temperature increases chemical reactionrate. And since corrosion is an electrochemicalreaction, an increase in temperature will increasecorrosion rate. Generally, for every 30°F increasein temperature, corrosively doubles. In an opensystem, where the dissolved oxygen (DO) isallowed to escape at higher temperature,corrosion peaks at about 150°F and then drops offas the DO drops. In a closed system, where theDO is trapped, corrosivity continues to increasewith temperature.Hot water systems, if desired are compatiblewith thermoplastic systems and do notexperience accelerated corrosion as withstainless systems.

Passivation is costly, is only temporarily durable, and must be repeated if additional weldmentsare incorporated in to the system. Passivated or not, stainless steel is reactive to many harshchemical, particularly chloride and other halides, preventing their beneficial use inpharmaceutical and biotechnologis applications.Stainless steel are formed using various processing techniques including melting, extrusion,casting, forging, shaping, heat treating, welding, cutting, and rolling. Such processing result inmicroscopic irregularities in the structure. The surface irregularities of stainless steel_rangingfrom 180 grit to 400 grit_can be ameliorated, although with only temporary beneficial effect, todouble digit microinches by electropolishing. But electropolishing is also expensive, nonpermanent, and needs to be repeated often to maintain such a surface.

Temperature accelerates a chemical reaction rate

even so, electro‐smoothing only miniaturizes theheight of the asperities in the metalurgical surface,but does little to remove the nooks and cranniessurrounding the base of the asperities

Worse still, electropolishing can remove inclusionsin the metal creating pits, which in turn, can harbormicroorganisms and biofilm components toperfectly shelter them from even the most vigorouscleaning (Sources : Material of Construction forPharmaceutical and Biotechnology Processing ;Dupont Fluoroproducts . Fleming James R; et. all)

Surface physical chemistry of stainless steel is anothersignificant negative for its use in the Pharmaceuticaland Biotechnology industries_ it is wettable byaqueous solution, a characteristic which enhances notonly chemical corrosion, but also biofilm adhesionand biofilm resistance to detachment (Sources :www.solvaysolexis.com)

These table below, compares internal surface finishbetween PURAD PVDF, 316 EP (electropolished) and316 MP (mechanically polished). It can be seen thatcomparable PVDF system provide a smoother surfacefinish as compared to stainless steel.

source : www.asahi‐america.com

Plastics _PVDF Wide availability of plastics offering lower initial cost, less weight, complete resistance tocorrosion, good thermal insulation, elimination of the passivation process, extremely smoothinterior surface (measured in Ra = roughness average in µ). have increased their application,particularly in system such as purified water distribution loops and other critical processes.During the last five years, the technical requirements for laboratory water have tightened to matchthose of the semiconductor industry. The quality of recommended water system should be verylow conductivity, low total organic carbon (TOC), and low microbial content.FDA requirements for pharmaceutical piping and equipment stipulate that materials must havelow extractables (prohibit leaching of contaminant) , tolerance for sanitizing procedures andoperating temperatures, and an interior surface smooth enough to resist microbial colonization.Microbial colonization can induced corrosion in steel : certain microorganisms can also thrive in adeposit/crevice. This can lead to bacteria producing acid metabolites as part of their metabolism.Some bacteria produce slime which can attract deposits causing differential cell corrosion.

Studies conducted by G. Husted of Micro Techno Research, Inc have compared the bioadhesionproperties of high performance thermoplastics (PVDF and ECTFE) with that of high grades of EPand MP 316L stainless steel. The samples were examined and had their biocolonization levelsdirectly counted. The results of which are shown on these table below. From these test, it isevident that PVDF and ECTFE are more resistant to bioadhesion than high grades of stainless steels.

While the surface profile is an important factorin terms of reducing bioadhesion, the leachoutfrom a pipe system is more important to asystem overall purity level. While the pipingcannot positively influence water quality it canhave a dramatic negative effect on waterquality. It is well known; stainless steel leachhigher rates of metal ions than thermoplasticssystems.

Table 2. Bio‐adhesion comparison between SS MP 316 L, SS EP 316L, PVDF and ECTFE materials

Microbial Fouling/ biofouling is special casebecause the foulant, that is themicroorganisms, can grow at the expense ofbiodegradable substances from the waterphase, turning them into metabolic productsand biomass. Therefore, microorganisms areparticles which can multiply. They produceextracellular polymeric substances, which keepthem together and glue them to the surface(bioadhesion) and also add to the fouling.Biofouling is not only a problem in technicalenvironments but also equally in health andmedical context. Contamination of drinkingwater frequently originates from biofilmdevelopment .

Stainless steel 316L systems have been shownto be more susceptible than PVDF to excessivebiofilm formation. High performance thermo‐plastics do not contribute to the nutrition (ioncontamination) levels of water systems.

Table 3. provides a comparison of detectable ionsand trace metals in high purity water systemswith materials of construction of PVDF and 316LSS. Water samples were drawn from similarpoints and analyzed for contamination levels. Asshould be expected, metal ion contaminationlevels in the PVDF systems are dramatically lowerthan those of the stainless steel system.

The higher rates of contamination of stainlesssteel system contribute to elevated conductivitylevels, high TOC levels and increased rates ofmicrobial activity by providing bacteria anutrition rich environment.

The iron content of austenitic stainless steelsincreases during the orbital welding process. Thisis turn disturbs the chromium/ iron balance atthe product surface. Systems become moresusceptible to corrosion when this balance isdisturbed. Passivation is required to restore thecorrosion resistance properties of austeniticstainless steels.

Table 3. Actual purity comparison of four sample points in system with piping material in HP PVDF and 316L SS. Measurement type is ion Chromatography

Dynamic Leachout Comparison of HP PVDF vs 316L SS

System Application and Material Usage

The de‐ionization of water removes mineral ions from water to limit its ability to transferelectricity. De‐ionized (DI) water is used in :Electrically sensitive applications such as the production of semiconductors, capacitors, andhigh purity acid and chemical compound; Pharmaceutical, Biotechnology, and Food Processing.PVDF pipe has the special ability to maintain the purity and DI quality of water. Other plasticpiping materials can experience a shearing action from the water flowing against the inside of thepipe. This can lead to high concentration of particulates in the piping system. The ultra‐smoothsurface of PVDF pipe reduces this effect substantially, making it essential for many high‐techapplications that require contamination‐free piping.PVDF‐lined steel piping systems are also available, combining the purity and chemical resistanceof PVDF with the structural support of steel. These systems are very resistant to distortion frompressure and temperature.

Benefit of Material to Systems Application_Total Cost Comparation

1. PVDF is easier to install and less expensive than metals suitable for high purity systems.

2. PVDF does not rust or rouge over time and doesn’t need passivation, It maintains its purity over the entire length of the installation.

3. PVDF is tolerance for sanitizing procedures and high operating temperatures

4.      Less expensive for environtmentally adverse cleaning protocols, doesn’t costly biofilmissues.

Source : ‐G.C. Shah, Fina Oil and Chemical Co

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