0262 1762/04 © 2004 Elsevier Ltd. All rights reserved WORLD PUMPS November 200436

Amajor article on thesignificance of thermo-plastic pumps written a few

years ago1, by Andy Bould of VantonPumps (UK) and Ken Comerford of itsparent company Vanton Pump &Equipment Corp, raised thisdisturbing question: “Considering thebasic engineering data that clearlyindicate their superior resistance tocorrosive and abrasive fluids, why thereluctance of editors to include themin major articles on materialselection?” The reasons they gaveincluded the lack of emphasis on thesubject by engineering schools, thebroad field experience with andapplication information available onthe various metals, plus thetemperature and strength limitationsof available plastics.

There was also the natural reluctanceof human beings to change from theirstandard practice, which rituallyindicated the use of the stainless andhigh alloy materials for corrosive

fluids, the existence of broadlycirculated specifications such as theASME ANSI process pump standardsthat excluded consideration ofplastics, and similar restrictions by thepharmaceutical industry for thehandling of high-purity water requiredin the production of medical products.But times have changed.

Responding to the development of abroad range of homogeneouschemically inert thermoplastics withhigher temperature and physicalproperty parameters, with provenabrasion resistance far greater than thestainless steel alloys, with no-rust, no-contamination guarantees and withthe availability of an extensive choiceof pump designs for everything fromdosing to metering to flow capacitiesof 1500 gpm, the nonmetallic pumpindustry has grown rapidly.

Recent studies by independentresearchers indicate that approx-imately 50% of those contacted have

used or are using nonmetallic pumpsfor the handling of aggressive andhigh-purity liquids. It is interesting tonote that ANSI process pumpstandards were changed in 1995 toallow polymer pumps and that thepharmaceutical industry standardshave more recently been revised tosuggest consideration of polymers forhandling high-purity water.

Selecting the rightmaterial is critical

Not all plastics are alike. Selectingthe right nonmetallic material for agiven application is just as difficult asselecting the right metal. All metalscorrode, plastics do not. But todetermine the most-effective metalfor a given application is relativelyeasy. Tabular data are readilyavailable in printed literature: databased on immersion tests in thespecific chemical and with time andtemperature controls. The results are

An alternative to metalThe acceptance of nonmetallic pumps in the industry has gathered pace overrecent years. In this article, George Black provides a brief guide to nonmetallicmaterials and gives an introduction to the top choices of materials available tothe pump industry today.

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ECTFE 6000 - 7000 psi

ETFE 6000 - 6500 psi

PVC 6000 - 7500 psi

UHMW PE 5600 psi

CTFE 4500 - 6000 psi

PP 4500 - 6000 psi

PFA 4000 - 4300 psi

FEP 2700 - 3100 psi

PTFE 2500 - 6000 psi

PE 1200 - 4550 psi

Table: Tensile Strength of Plastics ASTM 638

Figure 1. The inherent strength in a general mechanical form of the various polymers suitable for pump manufacture (1000 psi ≈ 70 bar; FEP = fluorinated ethylene propylene; PFA = perfluoroalkoxy).

66 psi 264 psi Melt Point

ECTFE 240°F 170°F 464°F

CTFE 258°F 167°F 424°F

ETFE 220°F 165°F 518°F

PVC 135°F 140°F <285°F

PTFE 250°F 132°F 620°F

FEP 158°F 124°F 554°F

PP 225°F 120°F 330°F

PFA 164°F 118°F 590°F

UHMW PE 155°F 110°F 265°F

LDPE ? 104°F 221°F

*Ultrapure Water, July 1987

PVDF 298°F 235°F 338°F

Figure 2. The strength of polymers under temperature and mechanical load – even thoughKynar has a lower melting point it outperforms higher melting point polymers whenplaced under temperature and mechanical stresses. Kynar is the registered trademark ofAtofina’s PVDF resins. (LDPE = low density PE.)

Table: Deflection Temperature of Plastics(ASTM D648)*

PVDF Homopolymer 7000 - 8000 psi

All images courtesy of Atofina Chemicals Inc. (NB: Atofina became Arkema in October 2004.)

www.worldpumps.com WORLD PUMPS November 200438

given in metal loss per period of time.Plastics are either inert in a givenmedium or they fail rapidly, but theyvary in terms of temperaturelimitations, weight, strength, specificresistance to chemicals and abrasionresistance.

Tables are helpful, but they are nosubstitute for field experience.Fortunately, much of this experienceis now available (Figures 1 to 6)thanks to the chemical firms whocreate and manufacture the variousresins, the pump manufacturers whohave been willing to share theirexperience in their productcatalogues and through the pages of trade magazines, and the many plant operating managers whohave been willing to providedocumented case history data onresults achieved with plastic pumpinstallations.

Highlighting the topchoices

Polyvinyl chloride (PVC)

This relatively low-cost thermoplasticmaterial is characterized by highphysical properties and good chemicalresistance to a broad range of acids,caustics and salt solutions. PVC is not

recommended for solvents such asketones, chlorinated hydrocarbonsand aromatics. Temperature is limitedto 60 °C (140 °F).

Chlorinated polyvinylchloride (CPVC)

This has similar properties to PVCbut is suitable for temperatures to 99 °C (210 °F) and higher pressure ratings at these elevatedtemperatures.

Polyethylene (PE)

This ultrahigh molecular weight(UHMW) material is impermeable towater and resistant to organicsolvents acids and alkalis. It is verysimilar to polypropylene and retainsgood physical properties totemperatures of 93 °C (200 °F).

Polypropylene (PP)

The lightest of the industrial-gradeplastics, PP offers an excellentstrength-to-weight ratio. It is suitablefor temperatures up to 85 °C (185 °F).In addition to resisting acids andalkalis, PP is highly resistant toorganic solvents. It is notrecommended for use with strong

oxidizing acid, chlorinated hydro-carbons or aromatics.

Polyvinylidene fluoride(PVDF)

PVDF is a strong, tough and abrasion-resistant fluoropolymer that resistsdistortion and retains its strength attemperatures to 149 °C (300 °F).PVDF is inert to most solvents, acidsand alkalis, as well as wet and drychlorine, bromine and otherhalogens. Its high hardness and lowcoefficient of friction make it ideal forabrasion-resistant applications. PVDFis recommended for use withultrapure water and reagent-gradechemicals and other applicationswhere freedom from contamination isimportant. Atofina Chemicals is oneof the major manufacturers of PVDFresins, producing a range of gradesunder the Kynar® trade name.

Ethylenechlorotrifluoroethylene(ECTFE)

This material resists a broad range ofacids including oxidizing types, thealkalis and most other corrosive andabrasive fluids. It is suitable for usewith ultrapure water and similar fluidswhere contamination could be a

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Figure 3. Deflection temperature as for Figure 2, but with temperatures in °C.

mg/1000 cyclesNYLON 6-10 5UHMW PE 5PVDF 5 - 10PVC (Rigid) 12 -20PP 15 - 20CPVC 20CTFE 13PS 40 - 20304 SS 50ABS 60 - 80PTFE 500 - 1000

Figure 4. Abrasion resistance (Taberabrasion tester) of various polymerscompared to Nylon 6 (commonly thoughtof as the most abrasion-resistant). (PS =polystyrene; 304 SS = common grade ofstainless steel; and ABS = acrylonitrilebutadiene–styrene.)

Table: Abrasion Resistance(Abrasion Ring CS - 10, Load 1 Kg)*

*Ultrapure Water, July 1987

WORLD PUMPS November 2004 www.worldpumps.com 39

problem. ECTFE offers high tensile strength and impactresistance. It is recommended fortemperatures to 149 °C (300 °F).Related materials are CTFE(chlorinated trifluoroethylene) andPCTFE (polychlorinated trifluoro-ethylene).

Polytetrafluoroethyl-ene (PTFE)

Most commonly referred to as Teflon,this material retains its mechanicalproperties at temperatures to 260 °C(500 °F). It offers excellent chemical,impact and abrasion resistance, but

compared with other fluoropolymersits tensile strength and creepresistance is low. Related materialETFE (ethylene-tetrafluoroethylene)has higher tensile strength but lower melt and decompositiontemperatures.

Fibreglass/glass-reinforced polyester(FRP/GRP)

These thermosetting materials arepolyesters reinforced with glass orglass fibres to provide additionalstrength. They are closer to metals instrength, but not as chemically

resistant. They are composites ratherthan homogeneous polymers; thusthere is potential for the formation ofcapillary passageways that can resultin wicking. This limits theirapplication if the same pump is to beused for different chemicals andreduces their ability to withstandabrasive materials. Maximum servicetemperature is 110 °C (230 °F). ■

Reference

(1) A. Bould and K. Comerford, ‘Thesignificance of thermoplastic pumpsfor pumping corrosive chemicals’,World Pumps, No. 413, pp. 50–54,(February 2001)

CONTACTGeorge BlackGeorge Black CommunicationsConsultants6218 Benhurst RoadBaltimoreMD 21209, USA.Tel: +1-410-764-9379Fax: +1-410-764-6949E-mail: coop641@aol.com

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Figure 6. Comparisonof the gas permeabilityof various fluoro-polymers.

Vanton Index

The most complete collection ofarticles of this nature can be foundin the Vanton Technical Libraryindex, which lists and identifiesmore than 160 of these articles onthermoplastic pumps and theirapplications. This index can beviewed and obtained on request viathe internet at www.vanton.com.

Figure 5. The melting points of the various polymers in comparison to their degradationtemperature and the width of the processing window each polymer offers. Kynar offersthe operators the largest error operating window in the fluoropolymer family.

• Data based on 100 µm film thickness at 23 °C• Method: ASTM D1434 for gases• Water vapour according to DIN 53122• PVDF film is made by Westlake Plastics from Kynar PVDF

Gas Permeability of Fluoropolymers PTFE PFA FEP ETFE CTFE ECTFE PVF

Water Vapour g/m2.d.bar 5 8 1 2 1 2 7

Air cm3/m2.d.bar 2000 1150 600 175 — 40 50

Oxygen cm3/m2.d.bar 1500 — 2900 350 60 100 12

Nitrogen cm3/m2.d.bar 500 — 1200 120 10 40 1

Helium cm3/m2.d.bar 3500 17000 18000 3700 — 3500 300

Carbon Dioxide cm3/m2.d.bar 15000 7000 4700 1300 150 400 60

Table: Gas Permeability of Fluoropolymers*

*Data published in 1980 Kunststoffe paper entitled 'Fluorocarbon Films - Present Situation and Outlook'.

PVDF

2

7

20

30

600

100