Quality and Durability of Different Polyethylene Pipe Coating Solutions T · 2016. 10. 24. · For...

36 www.paintsquare.comJPCL February 2008

he quality and durability of polyethylene (PE) pipe coating systems havebeen questioned by some sources recently, and the industry is strugglingto provide a definitive answer.Some high profile pipeline coating failures have been reported by

David Norman et al.1,10,11,12 These failures put pipelines at risk of corro-sion and rupture, which would damage the environment and reduce con-fidence in the ability of the pipe coating industry to provide durable and“fit for purpose” coating solutions. Photographs showing examples oftypical coating problems reported are shown in Figs. 1 and 2.While many problems and subsequent failures can be attributed to

By B. Turner, Borealis Business Unit Pipe, UK

Quality and Durabilityof Different Polyethylene

Pipe Coating Solutions

T

poor substrate preparation, incorrect coating application,and incorrect material selection, it is also evident that prod-ucts generally available to applicators can vary consider-ably in application characteristics and final coating proper-ties. This variation can also affect the quality of the appliedcoating system and the ultimate durability or service life ofthe pipeline coatings.This article looks at the properties of different three-layer

PE coatings (topcoat and adhesive) and discusses how thevariations among these different coatings might affect ulti-

JPCL February 2008 37www.paintsquare.com

mate performance and durability. This author intends toopen a discussion and will propose some potentially newideas on how different test methods, some from other pipeapplications, might be used or adapted to provide a betterestimation of durability and service life.The test schedule chosen for this study compared various

mechanical and physical properties that provide benefits todifferent members of the pipe coating value chain. The valuechain concept (the value each interested party seeks in aproduct) is one that this author strongly believes in and hasrecently used in developing products and business opportu-nities. Pipeline owners and specifiers, for example, may lookfor coating materials to have long-term durability and lowmaintenance requirements, whereas applicators may tend tofocus on ease of application and the ability of the system tomeet specification requirements. Although described as a“chain,” it may be more beneficial if we think of a matrix,with interactions and exchanges of value potential in a non-linear way, rather than a straight line. A typical pipe coatingvalue chain is shown in Fig. 3.

Materials and Pilot LineHigh density (HD) and medium density (MD) PE topcoatsand grafted PE adhesives generally available in the marketwere selected for testing, obtained in their original,unopened packaging, and taken from stock still in shelf life.References 1 and 2 in the tests are described as bimodalhigh-density compounds. The pipe coatings were applied onthe Borealis pilot line in Porvoo, Finland. No special condi-tions or application parameters were used, and all materi-

Holiday detection on coated pipelinePhoto courtesy of Borealis Business Unit Pipe, UK

Fig. 1: Problems associated with FBE coatingsPhotos courtesy of David Norman, Corrosion Control Ltd.


als were treated in a similar manner.The test schedule for this evaluation

is shown in Fig. 4.Other values reported in this article

are taken from technical data providedby the manufacturer.The pilot line (Fig. 5) will coat steel

pipe up to 168 mm external diameterand is capable of applying multi-layercoatings (up to five layers by side wrapand four layers by cross head extru-sion). The line was prepared to coat ina typical three-layer set-up, with epoxypowder applied by electrostatic spray,and the adhesives and topcoats appliedby side extrusion through a flat die.Pipe preheating was performed byinduction coil and was set at 185–195C (~365–383 F), depending on thematerial; topcoat extrudate tempera-ture was 220–230 C (~428–446 F).

Test ResultsOne immediate and obvious conclusionis that, for the chosen test conditions,the physical characteristics of thematerials vary considerably.

Extruder Output TestThe extruder used for the output testsis a Krauss Maffei 45/30D. The sameextruder was used for the “neck-in”tests (described below). It is easy tounderstand that higher output can real-ize faster application speeds, which canbe an important factor for the applica-tor if his coating line capacity is con-strained. On the other hand, higheroutput can also translate to lower rev-olutions, resulting in less power con-sumption at an equivalent output. Inthis test, the difference between the

best and worst output is about 10%(Fig. 6). In theory, it should be possibleto convert this to a 10% increase inapplication speed. In cases where theapplication equipment was furtheroptimized for temperature profile andscrew configuration in the extruder,and then for bimodal HDPE com-pounds, output improvements of up to15% have been reported to the author.

Neck-In or Draw Down TestThe neck-in (or draw down) testinvolves measuring the difference

Fig. 2 (Above and right): Problems associatedwith 3-layer PE coatings. Photos courtesy of

David Norman, Corrosion Control Ltd.

Fig. 3: Pipe Coating Value Chain Example

Other influences include


between the die width and the moltenfilm width at the pressure roller for dif-ferent rotational speeds of the pipe.Empirically, the faster the rotationalspeed, the higher the draw downshould be for the same extruder set-tings. Draw down or “neck-in” is anindication of the melt strength of themolten polyethylene film. This testshows how reliable and stable themolten plastic will be during process-

ing. It also shows if there are any gelsor unmelted pellets in the molten massthat may lead to film breakages. Morestable and reliable molten films haveobvious benefits for applicators.All compounds but Reference 1 suf-

fered film breaks, with Reference 2breaking at 15 m/min. rotationalspeed. Strangely, the neck-in forReference 4 actually improved until itbroke at 20 m/min. (Fig. 7). This indi-

cates some difference in the rheologicalproperties of Reference 4 that mayaffect application properties adversely.

VICAT TemperatureVICAT temperature broadly indicatesthe upper operating temperature limitsof the applied three-layer PE coating,because the higher the VICAT temper-ature, the higher the softening point ofthe compound. The topcoat should bematched with a suitable adhesive (andepoxy) to optimize the performance ofthe system.Figures 7 and 8 show the variation

in measured VICAT temperatures forthe tested PE adhesives and PE top-coats. Of the topcoats, References 1and 2 should offer the best perfor-mance, with at least a 10 degrees C(~30 degrees F) difference fromReference 3. Of the adhesives, Reference3 should offer worse high-temperatureperformance (Figs. 8 and 9).

Surface HardnessSimilarly, surface hardness measure-ment at various temperatures is anindication of the mechanical propertiesand upper operating temperature.Density also correlates closely to sur-face hardness. Here, the tests indicateReferences 3 and 4 of the topcoatshave the lowest densities and Shore Dhardness, and would provide the worstmechanical properties at higher tem-peratures. This correlates well to theVICAT temperature measurements(Fig. 10).

Peel Force TestsPeel force tests are used for qualitycontrol in the coating plant and also forprequalification purposes, and it is gen-erally recognized that the higher thepeel force, the better. The retention ofpeel force at higher temperatures willalso indicate suitability for higher oper-ating temperatures and possibly longerservice life. Adhesion is generallyaccepted as an indication of the quality

Fig. 6: Processing output at different speeds at Krauss-Maffei 45/30 D

Fig. 5: General view of the pilot line

30 60 90 120RPM

100

90

80

70

60

50

40

30

20

10

0

kg/h

Reference 1 Reference 2 Reference 3 Reference 4

10% extra output from ref. 1 vs.ref. 3 under these conditions

Top Coats

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of the whole coating system, and speci-fications are calling for higher valuesthan before. In this test we look at thedifferences in peel time force for the 3-layer PE system as applied onto pipescoated on the Borealis pilot line.The main governing factor in deter-

mining peel force is the chemicalnature of the adhesive. An adhesivewith grafted active sites will providemuch better performance than, forexample, a copolymer type, and thenature and concentration of the graft-ing should also affect the peel force.9

(Note: the selection of an epoxyprimer with a high enough glass tran-sition temperature [Tg] is alsorequired for higher operating temper-atures, especially over 110 C.) In ourtests, the adhesives—References 3and 4—clearly give much lower per-formance at 80 C (~176 F). Reference2, strangely, appeared to increase inpeel strength at 80 C (~176 F).Reference 1 and Reference 5 appearto offer the best overall performance(Fig. 11).

Low Temperature Elongation TestsThis laboratory test, for the topcoatcompounds, uses compression-moldedplates made from virgin pellets. Tensileelongation is measured at -45 C (-49 F)on 2-mm-thick test specimens (“dogbones”), with a pulling speed of 50mm/min.This test, taken from the Russian

GOST and Gazprom specifications,6 isparticularly useful in determining thelow-temperature performance of plas-tics during mechanical handling, suchas field bending or even moving pipesin or to the stock yard or right-of-way,and is important for cold climates suchas in parts of Russia, China, andCanada (Fig. 12).Reference 1 clearly shows much bet-

ter performance than any of the othercompounds tested.It should be noted that the method

of obtaining samples for this test can


Fig. 9: High Temperature Properties (VICAT Softening Temperature)

Fig. 7: Neck-in at different processing speeds

9 15 20 25Speed m/min

70

60

50

40

30

20

10

0

124

122

120

118

116

114

112

110

108

%Neck-In

°C



Top Coats

PE Top Coats

Fig. 8: High-Temperature Properties (VICAT)

Reference 3clearly worstHT performance

VICAT softening temperature A59, 10N

lead to variation in the values. TheGOST and Gazprom specifications callfor test samples to be taken from actu-al coated pipes, which will lead to vari-ations in the film thickness and conti-nuity, thus causing variation in elonga-tion and early breakages.

DiscussionAlthough all of the products tested aredesigned to meet the requirements ofvarious national, international, andcompany standards and specifications,there is a wide variation in the testedproperties of these commercially avail-able and often used topcoat and adhe-sive compounds. This raises the ques-tion, “How might that variation trans-late to actual field performance anddurability or lifetime?” Some sugges-tions for further tests that could beused to simulate or predict lifetime aregiven below in blue italics.The draft ISO 21809-1 for polyolefin

coated steel pipes will probably con-tain a short-term (24-hour) disbond-ment after a hot water soak test. Thisis intended for in-plant quality control.Robin John4 proposes a 60- or 100-dayhot water immersion test as an indica-tor of long-term durability. A thermalcycling test is included in the approvaltesting to Russian GOST andGrazprom specifications for 3-layer PEpipe coatings.6

Might a long-term hot water soakor thermal cycling test be suitable asa general screening test for durabili-ty? If so, for how long, and would itbe possible to extrapolate a lifetimeto failure?

When pipes are coated with polyeth-ylene by side wrap melt extrusion,stresses are frozen into the polymerand are said to aid adhesion. However,could these stresses also play a part indisbondment? It has been noticed thatlifting or detachment at pipe endssometimes occurs.1 This lifting might


Fig. 11: Peel Strength

Fig. 12: Low-temperature properties (Elongation at break: -15°C [-9 F], 50 mm/min, 2 mm pressed plate)

500

300

100Reference 1 Reference 1 Reference 1 Reference 1

Top Coats

This property isespecially important forpipelines to be laid incold climates, eg.,Russia, Canada

Fig. 10: Material Surface Hardness (Shore D and density)

62

61

60

59

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965

960

955

950

945

940

935

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d

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sity

ofco

mpound(kg/M3)


Top Coats

Peel Strength in +23 and +80 °C (~+73 and ~+176 F)

Grafted adhesives

Ref. 4 and ref. 5 arepowder versions

References 3and 4 clearlyworse HTperformance

%Elongation

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be a function of frozen-in stress and/ordifferential expansion and contractionbetween the plastic and steel.Perhaps a test to determine the

frozen-in stresses might be useful inevaluating lifetime. If so, how mightthe test be done?

For plastic pressure pipes, it is widelyaccepted that pressure testing at differ-ent temperatures can be extrapolatedusing regression curves to predict ser-vice lives of 50 years or more. Notchand slow crack growth is also widelyused in plastic pipes to predict servicelife and rank for duability.7,8

Environmental Stress CrackResistance (ESCR) is also used to indi-cate durability of plastics. Some PEresins for pipe coating have publicizedESCR at 1,000 hours, while others are5,000 hours.What could this difference in ESCR

mean in terms of lifetime or durability?

Could the tests above or similar testsbe used for pipe coating qualification,and where should the threshold be set?Marcel Roche et al.2,4 have called formore cooperation and information shar-ing about pipeline coatings, and I canonly echo that request. Additionally, across-industry research program todevelop an accelerated durability testor suite of tests would be the bestapproach to the uncertainty that cur-rently exists. Borealis would certainlybe willing to be an active participant insuch research.

ConclusionThree-layer polyethylene coatings forsteel pipelines that transport oil and gasare the coatings of choice in many areasof the world. Global energy supply iscoming more and more in focus asevery nation vies for security of supply.Correctly specified pipeline coatingsadd to the security of the pipeline, andensuring the quality and durability ofthe coating is one important factor. This

article has shown that the properties ofdifferent polyethylene coatings varyconsiderably and suggests how thisvariation in properties might affect per-formance. Tests to predict durability orservice life have yet to be considered,however, and the author hopes thatthis article will aid in the discussion ofthis subject in the future.

References1, David Norman, “Are We Protecting

Our Assets?” BHR Group PipelineProtection Conference, 2003.

2. M. Roche, D. Melot, G. Paugam,“Recent Experience with PipelineCoating Failures,” BHR GroupPipeline Protection Conference2005.



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3. V. Sauvant-Moynot, S. Duval, J.Kittel, X. Lefebvre, “Contributionto a Better FBE Election for 3-Layer Polyolefin Coatings,” BHRGroup Pipeline ProtectionConference 2005.

4. R. John, E. Alaerts, “The Importanceof the Hot Water Immersion Testfor Evaluating the Long Term

Performance of Buried PipelineCoatings,” BHR Group PipelineProtection Conference 2005.

5. Denis Grimshaw, “Ask the Expert:What Can Cause Disbondment inThree-Layer Pipe CoatingSystems?” CIS Oil and GasMagazine, 2006

6. Russian GOST R51164 and

Gazprom specifications for 3-layerPE coatings on steel pipe.

7. Mats Backman, Christer Lind, “NewGeneration Bimodal PE80 andPE100 Polymer Design BenefitsPipe Manufacture and End Use,”Plastics Pipe XI Conference,Munich, September 2001.

8. Gunnar Bergström, Stefan Nilsson,Kristian Thörnblom, Carl-GustafEk, Harald Herbst, AndersStenström, “Durability Testing for100-Year Lifetime for Buried Non-Pressure Plastic Pipes,” PlasticPipes XIII Conference, Washington,2006.

9. D. Nozahic and L. Leiden, “LatestDevelopments in Three-ComponentPolyethylene Coating Systems forGas Transmission Pipelines,”NACE EXPO 3, 2000.

10. K.K. Tandon, G. V. Swamy, G. Saha,“Performance of Three-LayerPolyethylene Coating on a CrossCountry Pipeline – A Case Study,”14th International Conference onPipeline Protection, BHR Group,Barcelona, Spain, October 29-31,2001.

11. M. Roche, “Field Experience andLaboratory Testing of PipelineCoatings–An Update,” CanberraJoint Technical Meeting, March 16,2007.

12. G. Portesan, J. Taves, G. Guidetti,“Cases of Massive Disbondmentwith Three-Layer PE PipelineCoatings, Cathotic Protection, andAssociated Coatings,” CEFRACOR,EFC Event nr254, Aixd-en-Provence, France, June 6-7, 2002.


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