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“Generational Advancements in Fluoropolymer Lined Piping Systems”Author / Presenter:Mr. David YanikProduct Development Engineering ManagerCRANE ChemPharma Flow Solutions, ResistoflexCopyright 2011 by Crane Co., ALL RIGHTS RESERVED
ABSTRACTPolytetrafluoroethylene, or PTFE, is a widely used plastic suitable in countless applications. Itsunique properties, from chemical inertness, low thermal and electrical conductivity, hightemperature resistance and mechanical strength, make it an ideal solution to many applicationchallenges. In particular, these properties are widely exploited in chemical process applications.When molded and installed in a metallic housing, the resulting composite system offers users atruly unique option in the industry, a piping system with the chemical inertness of PTFE, and thestrength of the steel housing which process engineers had come to depend on since the earlydays of the industrial revolution.Following World War II, a revolution took place within the chemical processing industry as newfluoropolymer-based piping designs were qualified and used with great success. At the sametime, competition increased among plastic-lined piping suppliers worldwide, and those supplierssought technical differentiation. In an industry where technical differences were sometimes quitesubtle, a great deal of anecdotal information has been used in the promotion process. In somecases, inferences were drawn from data which were not strictly logical or fact-based. The netresult has been devolution of the quality of the technical argument for individual product offerings.This paper is designed to inject facts back into discussions and thus empower end-users,specifiers and sellers, as to the salient points to know and inquire about when selecting plasticlined piping systems.Recent developments in PTFE processing have achieved significant improvements in permeationcontrol and understanding of this property. Collectively, the improvements have come to beconsidered a generational step forward for the plastic-lined piping industry, and thus are termed“Next Generation” plastic-lined pipe and fittings. Although specific methodologies are proprietary,a primary intent of this paper is to highlight performance differences between the Next Generationof plastic-lined pipe and fitting and all previous offerings in the industry.This paper seeks to provide practical insight on the phenomenon of permeation in plastic linedpiping. This is followed by technical overview of differentiating factors of three common PTFEpipe liner manufacturing techniques. Finally, the results of practical experimentation conductedby the authors and others will be presented and used to support the claims of this paper.Generational Advancements in Fluoropolymer Lined Piping Systems, Page 2 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
OVERVIEWPolytetrafluoroethylene, or PTFE, is a widely used plastic suitable for countless applications. Itsunique properties, from chemical inertness, low thermal and electrical conductivity, hightemperature resistance and mechanical strength, make it an ideal solution to many applicationchallenges. PTFE was invented by Roy J. Plunkett, researcher for El DuPont DeNemours, justbefore World War II, (reference U.S. patent number 2,230,654). At this time, the industry was justbeginning to understand the uses for this new material. One early important use of PTFE was asa liner for flexible hoses in aggressive chemical services; a natural evolution of this concept wasto use the new plastic as a liner for the more permanent installations in chemical plants – theprocess piping. This composite system offered users a truly unique option in the industry, apiping system with the chemical inertness of PTFE, and the strength of the steel housing whichprocess engineers had come to depend on since the early days of the industrial revolution.In parallel with these developments, innovation in the plastic industry itself was burgeoning.DuPont and other plastic manufacturers realized that many of the unique properties of PTFEwere directly attributable to the strength of the chemical bond of the carbon-fluorine chain in thebackbone of the PTFE molecule. This same chemical bond also introduced processingcomplexity as the material is so viscous at melt temperatures that standard manufacturingmethods for plastics, such as extrusion or injection molding, are simply not feasible for PTFE.Rather, special manufacturing techniques are required, which became a barrier to acceptance asfew plastics processors were willing to invest in the specialized equipment required to producePTFE. This proved to be the motivation for development of other fluorine containing plastics, allgrouped into the overarching category of fluoropolymers. Modification to the basic carbon-fluorinebackbone, primarily in the form of side chains, introduced new properties that enhancedprocessability and, in most cases, altered cost and performance.Within the chemical processing industry, a revolutiontook place as new fluoropolymer-based piping designswere qualified and used with great success. At the sametime, competition increased among plastic-lined pipingsuppliers worldwide, and those suppliers soughttechnical differentiation. In an industry where technicaldifferences were sometimes quite subtle, a great deal ofanecdotal information was used in selling. In somecases, inferences were drawn from data which were notstrictly logical or fact-based. The net result has beendevolution of the quality of the technical argument forindividual product offerings. This paper is designed toinject facts back into the discussion and thus empowerend-users, specifiers and sellers, as to the salient pointsto know and inquire about when selecting plastic-linedpiping systems.“This paper is designed toinject facts back into thediscussion and thusempower end-users,specifiers and sellers, as tothe salient points to knowand inquire about whenselecting plastic-lined pipingsystems.”In particular, the phenomenon of permeation has been widely expounded upon, however, notalways factually. It is, at bottom, a simple three stage physical process involving the absorptionof a fluid into its container wall, diffusion through that wall, and escape, or evaporation on theexterior of that container wall. In the case of plastic-lined piping, that container is intimately andcompletely surrounded by the metallic housing, although infinitesimal interfacial gaps do exist.The housing may or may not be vented to atmosphere. The permeation phenomenon is of deepinterest to plastic-lined piping users, as the plastic wall, regardless of the exact type of plastic orpiped media, is known to permeate at rates much higher than alternate piping systems, such asthe solid metals. This is relevant to these users only in light of the subsequent potential for harmdone by the permeating chemical species.Generational Advancements in Fluoropolymer Lined Piping Systems, Page 3 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
The harm can take several forms:1. Fugitive emissions, or unintended escape of process media, which are potentiallydangerous and a target of increasingly strict regulation.2. Damage to the outer shell, reducing the sought after strength of the reinforcing housing,and thus also creating potentially unsafe conditions.3. Deformation of the liner - housing interface through the buildup of corrosion byproducts.This can degrade the liner’s vacuum resistance and, in extreme cases, seriously impingeflow of fluid through the pipe.Fugitive emissions generally occur throughout the length of time a lined piping component is inservice, beginning after a lag known as “break-through time” and increasing gradually until asteady state is reached; this will continue as long as the media and its motive force are present inthe component. The motive force derives from the media vapor pressure, itself temperaturedependent.The second and third items noted above also begin at the break-through time, but are generallyonly detectable via inspection or when a significant problem results, such as catastrophic shellfailure or when an unacceptable pressure drop is noted. These detections generally signal theend of the product’s useful life. All these effects have been perceived in actual applications andare also the subjects of claims of varying veracity made by producers of plastic-lined pipingproducts. The following section will provide information as to the factors that impact permeation.Key Factors Affecting PermeabilityFundamental understanding of the permeation phenomenon is not the aim of this presentdocument. Readers interested in an academic treatment are encouraged to study the appropriatereference materials noted in the appendices. This paper will provide a practical primer onpermeation, with a particular aim at plastic-lined piping installations. The results of practicalexperimentation conducted by the authors and others will be used to support the claims of thispaper. At a high level, most of the factors affecting permeation are relatively simple and intuitive;others require a brief exploration of some more subtle aspects of the physical process at work inpermeating systems.Vapor PressureOften, permeation is considered strictly in terms of outward effects and is equated with leaking.This however, is not entirely accurate. Where the distinction applies is important because, unlikeleakage, for liquids, the phenomenon is essentially independent of applied system pressure;instead vapor pressure in liquid piping systems is important. If there is an unhindered regionoutside the lining, vapor pressure is known to be directly related to permeation rate. If insteadthere is some hindrance in this outer region – e.g. an unvented pipe wall - as permeationproceeds, vapor will build up in the hindered region; the driving force for further permeation isthen the difference in vapor pressure on either side of the lining, and is thus reduced comparedwith the initial stages. Thus, when considering the severity of service and projecting rate ofpermeation, the value of importance when a liquid is present is the pressure of its vapor presentin systems which are less than hydraulically filled.In contrast, if the fluid is gaseous, vapor pressure is actual pressure; so that for a gas,permeation may be seen as a miniscule form of leakage. It should be noted that some aqueousfluid mixtures, when at high temperatures, may involve gas dissolved in water; in this case, thedriver for any water permeating will be its partial vapor pressure, whereas the gas will be drivenby its ‘partial actual pressure’.Generational Advancements in Fluoropolymer Lined Piping Systems, Page 4 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
Liner Wall ThicknessLiner wall thickness is inversely related to permeation rate; essentially, a doubling of wallthickness will, all other things being equal, halve the permeation rate. To fully understand thisfact, one must first appreciate the concept of a concentration gradient that is present inpermeating systems with unrestricted escape on the exterior surface. On the wetted side of apermeating system, the wall is fully saturated when at steady state. Just beneath the skin, thediffusion activity ensures that the wall is less than fully saturated. When steady-state conditionshave been reached, the concentration continues to reduce linearly until, on the exterior, anymolecules of permeant are removed by evaporation and the concentration is near zero. It isintuitive to think of the wall as eventually achieving full saturation but, in practice, this virtuallynever happens in vented PTFE lined piping systems. A good analogy for the situation withunrestricted exterior escape is the temperature gradient through an insulated building wall; theeffect of increasing insulation thickness is to thermal transfer exactly as wall thickness is topermeation rate.In a pipe situation, escape is not unrestricted. This means that a vapor pressure of the permeantbegins to build up in non-touching regions between lining and pipe wall, and the ‘line’ of permeantmolecules still within the lining waiting to escape at the exterior become increasingly held up.Thus the concentration slowly increases within the lining and the drive for permeation reducingaccordingly. Eventually, if the non-touching regions are not joined and/or not vented toatmosphere, theoretically a fully saturated situation could arise. It is likely in practice that someescape always happens, but nevertheless, in time a concentration increase will happen within thelining.Permeating Molecule SizePermeating molecule size seems to be an obvious factor in permeation rate; the larger themolecule of the permeating species, the lower the permeation rate. For liquids, permeationoccurs because most are soluble to a lesser or greater extent in the amorphous regions ofplastics. On a molecular level, what this means is that after the adsorption stage the liquidmigrates by diffusion through the plastic by jumping into intermolecular voids that exist in allpolymers, formed transiently by kinetic-energy-driven motion. Therefore, it is important toconsider not just the molecule size of the interior liquid, but also the molecule size of thepermeant. In addition, some permeant molecules are chemically attracted to the plastic, e.g., byhydrogen bonding, which can cause swelling in some instances. Genuine chemical attack is alsopossible, but unlikely to be met in practice providing well-established plastics have been selectedfor the service. The bottom line is that knowing the molecule size of the process media may ormay not be an indicator as to the ultimate permeation rate.Solubility CoefficientFluids are soluble in polymeric materials. The amount (mass) of liquid which can be absorbed bya given volume of polymer is given by the solubility coefficient multiplied by vapor pressure(atmospheres or bars), and the volume of gas likewise is given by the solubility coefficientmultiplied by actual pressure; the units for solubility coefficient are different in the two cases toallow for this subtly-different definition. In simple terms, it is a measure of the affinity a permeantmolecule has for the plastic container wall material. Increasing affinity or wettability correlatesdirectly to permeation rate.CrystallinityOn a molecular level, most plastics exhibit randomly located areas of ordered crystalline structurescattered throughout amorphous arrangements of molecules. These crystalline arrangements areeffectively small, void-free zones which, among other effects, inhibit permeant progress. Thepermeating molecules are thus forced to migrate along an alternate route. This has the net effectof slowing the overall permeation rate. There are two reasons for this effect. The commonlyreferred to point is that individual molecules are prevailed upon to adopt a more tortuous route.Equally or perhaps more significant is the traffic jam effect. As mentioned above, permeation is aphysical process and permeants migrate through a container wall by making a series of jumpsGenerational Advancements in Fluoropolymer Lined Piping Systems, Page 5 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
from void to void when they are near enough and large enough to accept the permeant. When acrystallite is encountered, not only must the permeating molecule adopt an alternate route, but itis statistically more likely that nearby voids are already occupied and thus unavailable to acceptnew passengers.This particular factor of crystallinity is relevant because, as it happens, it is one of the more costeffective and practical tools available to lined piping designers that helps to achieve significantgains in permeation rates. Additionally, processors of fluoropolymers have collectively developedinternal standards and controls to improve the crystalline nature of their products. Recentdevelopments in the market have achieved significant improvements in, and understanding ofthis, property. Collectively, the improvements have come to be considered a generational stepforward for the plastic-lined piping industry, and thus are termed “Next Generation” plastic-linedpipe and fittings. Although specific methodologies are proprietary, a primary intent of this paper isto highlight performance differences between the Next Generation of plastic-lined pipe and fittingand all previous offerings in the industry.TemperatureAs mentioned previously, the inter-molecular voids present in all plastics are the vehicle throughwhich permeants travel; their molecules jump from void to void as they become available. Thevoids move (and thus become available) as a result of the kinetic energy of the plastic molecules.The temperature of a system is, of course, a direct measurement of the molecular kinetic energy.Thus, physics dictates that higher temperature means faster moving voids which, in turn, providesincreasing statistical probability that an available void will present itself to a permeating moleculemore frequently in a hot system than the same system at a cooler temperature. Also, the plastic’sinter-molecular voids become larger through the increased kinetic energy at elevatedtemperature; again enhancing the probability of a diffusing molecule to enter it.From the statistics, it happens that absolute system temperature in degrees Kelvin and apermeation term are directly proportional; this term is the logarithm of the permeation coefficient,which in turn is the permeation rate applying to a unit cube at 1 bar pressure.In the next section, this paper provides a sharper focus on PTFE-lined pipe and fittings anddiscusses different manufacturing techniques and resin characteristics.PTFE Processing, Materials, and MethodsAs mentioned above, PTFE cannot be made into useable shapes using traditional meltprocessing methods. Rather, it is processed with unique, and generally very expensive,equipment. For the general shapes required for production of PTFE-lined pipe and fittings,manufacturers have several alternatives; these will be described in the coming subsections. Inorder to select the option or options that will be used, it should be evident than an investingmanufacturer must use a calculus that includes total cost of the investment, size range andconfigurations to be produced, and the likely payback of the various options. On the paybackside, an estimate is used of the likely market size, and how much of that market is addressablegiven the advantages and disadvantages of the options chosen. For maximum flexibility, amanufacturer would be capable of producing PTFE-lined piping using all of these methods. Belowis a brief overview of the available methods for processing PTFE into the shapes used for plasticlined pipe and fittings.Paste ExtrusionIn this method, finely cut PTFE powder is used. PTFE powders of this texture are highly sensitiveto mechanical damage and at normal room temperatures are easily “bruised”. The reasons forthis sensitivity are well known: thus, it is common to ship, store, and mix or blend the powderunder refrigerated conditions.Generational Advancements in Fluoropolymer Lined Piping Systems, Page 6 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
Step one in the paste extrusion process is to mix a carefully measured recipe of the powder witha liquid, which serves as a lubricant. Naphtha or similar aromatic petroleum distillates arecommon. Some manufacturers add fillers at this step in order to impart certain desirableproperties. Subsequently, the mix is sealed in containers, and allowed to age at roomtemperature, typically for a period of 24 hours or more, in order to improve homogeneity. Steptwo involves the compression of the aged mix in a vessel in order to form a billet. This stepdrives out air and further homogenizes the mix, as wetter, (higher pressure) areas donatelubricant to the dryer areas. In step three, the billet is placed in an extrusion chamber (at roomtemperature) and compressed with substantial hydraulic force such that it flows between a dieand core pin disposed at one end of the chamber. The resulting extrudate has the appearanceand texture of a very viscous paste, from which the general name of the process is derived. Thefourth and final step is a thermal sintering process, which first drives off the lubricant and thenmelts the extrudate, enabling molecular coalescence and crystal growth to occur. This sinteringprocess occurs either “in-line” with the extrusion process or in a subsequent batch process. Inboth cases, the temperature profile experienced by the extrudate during heat up and cool downare critical for imparting desired properties.The general properties of paste extruded PTFE products result from the molecular orientationwhich occurs during the extrusion process. In this process, fibrils form and orient themselvesparallel to the extrusion axis. This, in turn, results in a tendency to anisotropic mechanicalproperties, some of which may be undone during the coalescence that occurs during sintering.Regardless, it is considered axiomatic that paste extruded material has a degree of flexibility thatis absent in materials produced using other methods. It is typical that paste extruded PTFE has avery smooth surface finish imparted by the die and core pin. Conventional wisdom holds that thesmall particle used to form the original mix also results in smaller voids and hence lowerpermeation rates compared to methods that utilize coarser granular powders; this is, however,false. (See further discussion of this topic in Appendix C).Ram ExtrusionIn the ram extrusion process, a relatively coarse, granular PTFE powder is used. It is sometimespre-sintered to minimize the additional heat energy required to achieve coalescence and crystalgrowth. This, in turn, enables a relatively fast “in-line” sintering process. The process is generallycontinuous; the free-flowing resin is metered into a hopper positioned at the entrance to a die andcore pin, around and within which, electrical heating elements are disposed. An appropriatelysized “ram” is forced through the resin in the hopper, packing a small charge of the resin into theannulus between die and core pin. The ram then withdraws, the loose resin in the hopper falls bygravity back into the space vacated by the ram and then the process repeats. Each stroke of theram advances the charge (and all previous charges) through the heated section of the die/corepin assembly, and in this way the sintering process is accomplished. As the extrudate exits thedie, it encounters air, is relatively quickly “quenched,” and freezes into the shape imparted by thedie and core pin. This quick quench has historically resulted in rather low crystallinity of theproduct and given the product a poor reputation for permeation. Post extrusion thermaltreatments have been developed to address this shortcoming and it is now common for ramextruded PTFE products to exhibit some of the lowest permeation rates available.The properties of ram extruded PTFE products are randomly oriented molecular chains whichresults in isotropic mechanical properties; generally this is apparent to the casual observer as astiff smooth plastic with good dimensional consistency.Isostatic MoldingIsostatically molded PTFE is accomplished by making molds of the desired shape. In contrast tothe previously described techniques, complex interior and exterior geometries are possible. Aflexible bladder is disposed between the exterior and interior shape forming mold. On one side orthe other of the bladder, a vacuum is drawn, causing the bladder to be forced against that moldelement by atmospheric pressure. The resulting annulus between bladder and mold wall is filledagain with a coarse free-flowing granular PTFE powder. The evacuated side of the bladder isGenerational Advancements in Fluoropolymer Lined Piping Systems, Page 7 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
then pressurized, causing the bladder to squeeze the resin against the other tooling member.This caused the powder to pack into a near approximation of the desired shape. However, thearticle has very low strength at this point and must be carefully handled to avoid breakage. Themolded article is typically placed into a tray, or supporting cradle, and sintered in a batch process.The properties of isostatically molded PTFE articles are a high degree of stiffness caused by therandom molecular orientation. Very accurate dimensional properties are achievable throughcareful tool design and filling technique. A characteristic surface roughness on the bladder side ofthe molded article is one common characteristic.The above outlined processes require a heavy investment and thus appropriately equippedmanufacturers are uniquely positioned to make impartial judgments of the merits and deficienciesof each of the PTFE processing techniques described above. The table below summarizes these.Generational Advancements in Fluoropolymer Lined Piping Systems, Page 8 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
RECENT ADVANCEMENTSIn making a thorough study of the current knowledge base of PTFE processing, and applying theresults of numerous experiments, a qualified research and development team has been able tosignificantly advance the state of the art. Specifically, the team has developed two newmethodologies and innovatively applied the statistical tool known as Design of Experiments,(DoE) and then used the results of these efforts to design a truly optimized PTFE-lined pipingsystem. These methodologies have been developed in these areas, and will be expanded on inthe following sections. A method which allows PTFE processors to discriminate among the many resinformulations on the market, in such a way that an assessment can be made ofthe potential of individual formulations to be processed for optimal properties.Rigorous conduct of carefully designed experiments to determine sinteringoven profiles which will consistently deliver optimal properties.A method of assessing the effect of each step in the lined piping manufacturingprocess to determine which have a significant impact on crystallinity or thedesired property of interest.Resin Formulation SelectionThere are a plethora of PTFE lined piping system options for use in producing the liners.Historically, the selection process has been made using a variety of decision factors includingcost, resin manufacturer claims, processability, and perhaps most importantly, equipmentavailability. During the research for this paper, the team had at their disposal, equipment suitablefor all three of the above described processing methods. Thus, a resin formulation selection fromvirtually the entire scope of product offerings was possible. This raised the difficult question ofhow to discriminate among them in a scientifically sound way. Ultimately, a set of analyticalprocesses which provided a factual, numerical selection process was developed. The exactmethodology is considered proprietary, but a hallmark of the process is that individual resins aresubjected to a series of mini-sintering cycles and important performance indicators are assessedafter each cycle. The process is continued until it becomes apparent that the resin has been overprocessed and the results are compared graphically to similar results for many other resinfamilies. In this way, the researchers were able to assess the potential of many resins and makean appropriate selection.Sintering Oven Profile DeterminationThe exact temperature profile to which PTFE products are exposed during sintering is the keyfactor affecting most physical properties, including crystallinity. Thus, it is vital that a profile bedeveloped that produces the desired properties. Most PTFE resin manufacturers can providesome guidance on the various parameters, such as the temperature ramp rate. However, eachoven has unique characteristics and peculiarities. It is the responsibility of the process designerto work with the available equipment to determine the profile. Most commonly, this has beendone by trial and error. Inevitably, this results in a marginally suitable thermal cycle, but almostcertainly one which is not optimized for the desired properties. However, by adopting a rigorousscientific approach to the problem, statistical techniques can be employed to derive an ovenprofile which provides the optimal properties sought by the designer.Lined Pipe Assembly Method AnalysisSix Sigma methodology provides a tool to consider inputs and outputs at each process step, andthen to experimentally determine the impact of varying inputs on the outputs. This is afundamental teaching of the Six Sigma “DMAIC” process, but one which is seldom applied withdiscipline and resources appropriately. The designing organization has reviewed each step in thePTFE lined pipe assembly process and used the findings to modify the process to produceoptimal outputs. In particular, traditional methods of achieving a liner-to-housing interference fitGenerational Advancements in Fluoropolymer Lined Piping Systems, Page 9 of 19Copyright 2011 by Crane Co., ALL RIGHTS RESERVED
have been scrutinized. Most PTFE manufacturers utilize a varianton the “Method of Lining and Jacketing Tubular Members with“Most PTFEPrestressed PTFE” developed and patented by Resistoflex in 1962manufacturers utilize(ref US Patent number 3,050,786). This method has made areliable and robust assembly, but has now been shown to includea variant on thea detriment to the overall permeation resistance of the assembly.process developedSpecifically, the heating and stretching of the PTFE which enablesand patented bythe liner to be installed in a housing smaller than the moldedResistoflex in 1962shape of the PTFE, does mechanical harm to the crystalline matrix(ref US Patentof the plastic. Through careful study and experimentation, t
interest to plastic-lined piping users, as the plastic wall, regardless of the exact type of plastic or piped media, is known to permeate at rates much higher than alternate piping systems, such as the solid metals. his is relevantT to these users only in light of thesubsequent potential for harm done by the permeating chemical species.
