Advanced plastics and new welding techniques are driving alternative materials into advanced life sciences facilities
By Gary Dennis
Thanks to the explosive growth of the semiconductor industry beginning in the 1980s, polymer technology for high-purity water and chemical processes has advanced significantly. Polymers, for the most part, have become the material of choice for the semi industry since the technology meets the increasing extreme non-contamination demands.
The advancements in polymer technology are now being incorporated into pharmaceutical and biotechnology processes with the advent of welding technology—engineered specifically for pharma/biotech applications—that permits bead-and-crevice-free fusion joints that reduce bioburden and subsequent plant downtime.
Polymer materials offer high purity, satisfy Food and Drug Administration (FDA) test requirements, lower extractables below current industry standards and, since the fluoropolymer's greatest leaching occurs during the first week after installation, no finishing steps (such as passivation) are needed after installation of the system. This characteristic has created a growing number PVDF and PP installations in Purified Water (PW) and PVDF for water-for-injection (WFI) systems.
For many years, stainless steel has been the material selected for pharmaceutical industry process systems. High mechanical strength and a low coefficient of thermal expansion allow stainless steel components to be sterilized or sanitized in a variety of ways while withstanding high temperatures.
Chemical passivation is frequently required to remove free ions from the surface and restore the oxide film that gives stainless steel its corrosion resistance.1 But orbital welding techniques used with stainless steel piping systems can cause rough surfaces that are often inspected by an outside contractor via boroscope (endoscope) technology.
Roughing in high-purity water, metallic ion contamination and post-installation maintenance of inner surface finishes are often tolerated in stainless steel systems based on long-term use and validation.
Properties and system performance of plastics vary—the biopharm engineer can match system need with price to optimize performance/cost requirements. Key piping system attributes to consider are hardness (particulation), tensile strength, smooth glass-like surface finish, purity characteristics and welding technology to meet the water service and cleaning methods performance criteria.
When selecting the proper polymer resin, the biopharm engineer optimizes performance by matching particular system requirements with cost efficiencies. Non-contamination, surface-finish smoothness (reducing the possibility for micro-organic deposits and growth), sanitation and sterilization adaptability, as well as welding and system capabilities, are among the chief criteria.2
A survey of polymer materials
The list of candidate plastics for pharmaceutical processing includes those currently used as standard piping materials. They include the vinyl resins polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC), the polyolefin polypropylene (PP), the fluoropolymers polyvinylidene fluoride (PVDF) and perfluoroalkoxy (PFA), both of which have inherent high purity and chemical resistance.
PVC has excellent fundamental properties as a piping material, and its low cost makes it commonly used across a variety of industries (Table A). CPVC has a higher use temperature than PVC compounds due to its higher chlorine content. Additives, such as stabilizers, lubricants and fillers required by the vinyl resin family, make them susceptible to de-plasticization in clean water.
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For this reason, PVC resins would best be considered for potable water and related applications. Table B shows typical polymer criteria for various water services.
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PP is an extremely lightweight, low-cost polymer. It is readily weldable, and acceptable for more advanced pipe fusion technologies useful to the pharmaceutical industry. Although PP contains processing additives, it's FDA approved and compatible with many industrial disinfectants, including hydrogen peroxide at 5 to 10 percent aqueous and chorine at 5 parts per million (ppm). Service temperature parameters limit the use of steam sterilization for PPs, and it's not suitable for ozone sterilization techniques.
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Natural PVDF resin has traditionally been the workhorse in high-purity applications. A pure fluoropolymer containing no additives and requiring no stabilizers for processing, PVDF is compliant with FDA and U.S. Pharmacopoeia (USP) Class VI regulations. PVDF also features high tensile strength and heat deflection properties, and provides the rigidity and thermal properties needed for high temperature system components. Tables C and D (above) show polymer strength and heat deflection temperature data.
The low-cost member of the fluoropolymer family, PVDF has broad resistance to the various acids, halogens (chlorine, bromine) and ultra-high-purity water employed in life science industry processes. PVDF is available in a complete line of system components—solid and lined pipe, pumps, valves, membranes, powder coatings and stock shapes. It's not recommended, however, for continuous exposure to strong alkalis or ketone solutions.
Perfluoroalkoxy (PFA) is a member of the fluoropolymer family of resins offering high-purity characteristics for biopharm processes. PFA is a fully fluorinated copolymer that is melt-processable and has broad chemical resistance to a host of chemical solutions.
PFA has a relatively low heat deflection temperature compared to other fluoropolymers commonly used in piping applications.3 As such, extra reinforcement systems designed to accommodate its softness must be incorporated. PFA is commonly used as a flexible tubing and, to date, has been limited to pipes up to two inches in diameter.
Welding technology considered
Over the years, one key area of improvement has been welding technology. Initially, welding of high-purity piping components meant butt fusion. Butt-fused pipe and fittings were married with a hot plate and required considerable craftsmanship in timing the heat. Handling the weld and butt-fused joints created the potential for contamination in high-purity applications.
The next generation of high-purity welds became possible with the introduction of infrared (IR) fusion. This technology allowed for dependable repeatability driven by the consistency of computerized welding equipment.
In this new technology, welding parameters could be programmed and documented with software, eliminating human error. Ultra purity is maintained in this technique since the pipe ends are welded without touching a heated surface; just as importantly, welding beads remain small in relation to inside diameter (ID) and wall thickness.
The latest generation of welding techniques was designed specifically for the biopharm community. Commonly termed bead-and-crevice-free (BCF) welding, or smooth inner bore (SIB) by high-purity pipe suppliers, this innovation goes one step beyond IR welds by creating nearly indistinguishable welds on the inner pipe surface.
This technique eliminates one large concern—bacteria build-up. The weld area can be easily inspected in translucent PP and PVDF joints with appropriate external lighting.
As with IR welding, joints remain consistently computer controlled and the fusion operation is documented. Table E shows joining methods for polymers and Table F shows a comparison of fusion technologies for metallic components and PVDF.
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Surface finish considerations
The primary resins used in high-purity pipe designs—PVDF and PP—offer smooth surface finishes due to their ease of processability. PVDF consistently exhibits roughness average (RA) measurements near 7 compared to 180 grit stainless steel measurements in the 20 to 25 range (per micro inch).
PP measurements are comparable to those of stainless steel. The smooth surface finish combined with the upgraded welding technology give polymer pipe some significant advantages over current steel technology. Polymer's less frequent sanitization and cleaning cycles translate into fewer interruptions in plant operations and lower overall production costs.
Life science processes employ a variety of cleaning and sterilization methods, including steam, ozone, hydrogen peroxide and chlorine at various concentrations and temperatures.
Polymer performance varies depending on the sanitation method (see Table G for polymer performance criteria). Ozone in particular seems to be gaining in popularity. With this in mind, the processing engineer must be sure to specify the appropriate material according to sanitization needs.
PVDF is considered the polymer of choice for steam and ozone. From a strict chemical compatibility standpoint, PVDF, PP, CPVC and PVC are all suitable in hydrogen peroxide at 5 to 10 percent aqueous, and chlorine at 5 ppm, depending on the overall system performance being sought by the engineer.
The stringent demands of a pharmaceutical high-purity water system regularly require 80º C periodic sanitation or steam sterilization temperatures of 121º C (WFI—water for injection). A study performed on a PVDF pipe system under pharmaceutical and biotechnology conditions showed that sterilization with saturated steam at temperature up to 127º C had no demonstrated effect on the mechanical properties of PVDF piping. 4
In addition, the sterilized PVDF and the referenced pipe demonstrated an average roughness of Ra 0.05 microns (µm), which indicates no changes to the inner surface.4
GARY DENNIS is marketing manager for ATOFINA Chemicals' technical polymers. He can be contacted at: [email protected]
References
- Shnayder, L. “Pharmaceutical Purified Water Storage and Distribution Systems—An Engineering Perspective,” Pharmaceutical Engineering, pp. 66-72 (November, December 2001).
- Wulf, B. “Pristine Processing—Designing Sanitary Systems,” Chemical Engineering, pp. 46-49 (November 1996).
- Hanselka, R; Williams, R; Bukay, M. “Materials of Construction for Water Systems—Part 1: Physical and Chemical Properties of Plastics,” Ultrapure Water pp. 46-50 (July-August 1987).
- Gruen, Harold; Burkhart, Marty; O'Brien, Greg “Steam Sterilization of PVDF Piping Systems in PW and WFI for the Pharmaceutical and Biotechnology Applications,” Ultrapure Water, pp. 31-38 (October 2001).