Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/0001—Type of application of the stress
  • G01N2203/0005—Repeated or cyclic
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/0014—Type of force applied
  • G01N2203/0016—Tensile or compressive
  • G01N2203/0017—Tensile
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/0058—Kind of property studied
  • G01N2203/0069—Fatigue, creep, strain-stress relations or elastic constants
  • G01N2203/0073—Fatigue
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/0058—Kind of property studied
  • G01N2203/0092—Visco-elasticity, solidification, curing, cross-linking degree, vulcanisation or strength properties of semi-solid materials
  • G01N2203/0094—Visco-elasticity
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/02—Details not specific for a particular testing method
  • G01N2203/026—Specifications of the specimen
  • G01N2203/0298—Manufacturing or preparing specimens

Definitions

• the present invention relates to a method for evaluating the fatigue strength of a polymeric composition.
• a polymeric composition selected by the evaluation method makes it possible to produce pipes or other articles capable of withstanding extremely severe operating conditions, such as those encountered in the offshore oil industry.
• the invention also relates to a pipe for conveying a petroleum fluid comprise this selected polymeric composition by means of the method of evaluation of the resistance to fatigue.
• This metal inner layer which gives shape to the pipe, is coated, generally by extrusion, with a polymer layer intended to provide sealing.
• a polymer layer intended to provide sealing.
• Other protective and / or reinforcing layers such as metal fiber webs and rubbers may also be arranged around the sealed polymer layer.
• the polymer is HDPE (high density polyethylene).
• polyamide is used; up to 90 ° C.
• Cross-linked polyethylene (PEX) can also be used when the pressure is not too great.
• PVDF polyvinylidene fluoride
• VDF vinylidene fluoride
• Polyamides and in particular polyamide 11 and fluorinated polymers and in particular polyvinylidene fluoride (PVDF), are known for their good thermal resistance, their chemical resistance, in particular to solvents, their resistance to weather and radiation (UV, etc. .), their impermeability to gases and liquids and their quality of electrical insulators. They are used in particular for the manufacture of pipes or pipes intended to convey hydrocarbons extracted from oil deposits located under the sea (offshore) or not (on-shore).
• PVDF polyvinylidene fluoride
• compositions Polymers are as easy as possible, which imposes a viscosity adapted to the transformation process (typically extrusion).
• the composition used does not have a viscosity that is too low (for example a melt index measured under hot conditions according to ASTM D-1238 (at 230 ° C. with 5 kg) of less than 15 g / 10 min. ).
• the selection of a polymeric composition is therefore crucial to withstand without damage the conditions of manufacture, handling, installation of offshore flexible pipes and also good use.
• the construction of the hoses is complex.
• the juxtaposition of the polymeric layers and the metallic elements leads to subjecting the used polymer to locally tri-axial stresses and deformations, in particular when the hoses are bent. This occurs repeatedly during the various handling operations and laying of hoses. This is also the case when used for dynamic applications such as flexible hoses connecting the seabed and the surface ("riser" in English terminology) and are subject to swell.
• the Applicant has developed a method for evaluating the fatigue strength of polymeric compositions which make it possible to select those which satisfy the criteria set out above and which have interesting technical characteristics, in particular a fatigue strength under tensile stresses. axial which results in a high number of cycles to failure (NCR), for example greater than 500.
• NCR cycles to failure
• the new criterion based on fatigue resistance makes it possible to better design compatible polymeric materials and compositions for use as a pressure sheath and intermediate sheath or outer sheath of offshore flexible pipes. This is particularly true in the case of VDF homopolymer or copolymer pressure sheaths or mixtures thereof.
• the evaluation method according to the invention uses notched axisymmetric specimens which are subjected to loading and unloading cycles, which constitute locally triaxial stresses simulating the stresses of the pressure sheaths of an off-shore flexible hose in service.
• WO2006 / 045753 discloses a fluorinated polymeric composition comprising a VDF homopolymer a fluorinated thermoplastic copolymer and a plasticizer, said composition having a ductile-brittle transition temperature of less than 5 ° C.
• the characterization parameters for the anticipation of the mechanical properties are the molecular weight expressed in relative viscosity measured in solution and in the molten state, and the ductile-brittle transition temperature of the polymer compositions measured in Charpy impact on notched specimens. Mechanical properties such as modulus E and plastic flow threshold elongations and breakage are also measured.
• the application WO2006 / 097678 describes multilayer flexible hoses which contains a layer of polyamide 12 (PA 12).
• PA 12 polyamide 12
• the only characterization parameter mentioned in this application for the anticipation of the mechanical properties is the molecular weight expressed in relative viscosity, measured in solution.
• the application EP 1342754 describes a composition for hoses used in the operation of oil and gas fields "off shore” comprising a polyamide, a plasticizer and elastomer NBR or H-NBR. Only the molecular masses are determined by steric exclusion chromatography.
• EP0608939 discloses polymeric compositions for the manufacture of pipes for the transport of hydrocarbons.
• the polymeric compositions are based on a PVDF homopolymer, a thermoplastic copolymer of VDF and a plasticizer. Mechanical properties such as plastic flow threshold elongations and fracture, and IZOD impact resistance are measured.
• the subject of the invention is a method for evaluating the fatigue strength of a polymeric composition comprising the following steps: i) providing a polymeric composition; ii) making a plurality of axisymmetric specimens scored from said composition; iii) subjecting said test pieces to a tensile fatigue test comprising several cycles of uni-axial loading and unloading of the specimen inducing thereto triaxial stresses simulating the conditions for stressing the pressure sheath of a pipe flexible, especially in an off-shore application, and iv) determine the number of cycles to failure for said polymeric composition.
• Each specimen used in the process according to the invention is axisymmetric to the longitudinal axis z, having a maximum diameter d and a curved notch of curvature radius R.
• Each specimen has in its notched part a minimum radius a, the ratio a Wherein R is from 0.05 to 10 and d is greater than 2a and preferably from 2 ⁇ + 0.5R to 2 ⁇ + 2R.
• the tensile fatigue test consists of an elongation along the longitudinal axis of the test piece, with a sinusoidal signal of frequency ranging from 0.05 Hz to 5 Hz, preferably from 0.5 Hz to 2 Hz, at a temperature ranging from 15 ° C to 23 ° C, preferably from -15 ° C to 5 ° C, preferably from -15 ° C to -5 ° C, the maximum elongation of the same fatigue cycle in traction is selected from 0.05i? at 11, preferably 0.075? at 0.4R.
• Compositions which have a number of average failure cycles (NCR) on several test specimens> 500, preferably> 1000, advantageously> 5000, even more preferably> 10000, are preferred.
• the minimum elongation of the same traction cycle is greater than or equal to 0 up to 0.25 R and preferably up to 0.08 R.
• the curvature radius R of the notch ranging from 0.5 mm to 10 mm, preferably from 3 mm to 5 mm, each test piece has in its notched part a minimum radius ranging from 0.5 mm to 5 mm, preferably 1.5mm to 2.5mm; and a maximum diameter d ranging from 2mm to 30mm, preferably from 6mm to 10mm.
• the maximum elongation along the longitudinal axis z ranges from 0.2 mm to 4 mm, preferably from 0.3 mm to 1.6 mm.
• the ratio between the minimum elongation and the maximum elongation of the cycle is chosen from 0 to 0.8 and preferably up to 0.5, advantageously up to 0.25.
• the polymeric composition evaluated by means of the process according to the invention comprises at least one semicrystalline thermoplastic polymer having a glass transition temperature (Tg) less than or equal to 130 ° C.
• the polymeric composition resulting from the determination process comprises a fluorinated polymer.
• the polymeric composition resulting from the determination method comprises a homopolymer or copolymer of VDF.
• the invention also relates to a flexible metal pipe comprising one or more metallic elements and at least one layer comprising the polymeric composition selected by means of the process according to the invention and optionally one or more layers of a polymer material different from that of the polymeric composition.
• Another object of the invention relates to the use of notched axisymmetric specimens to simulate the triaxial stresses of a polymeric material, notably forming a hose pressure sheath for off-shore use. , in which each test piece is subjected to a test of tensile fatigue comprising several cycles of loading and unloading the specimen.
• the polymeric composition according to the invention comprising at least one semi-crystalline thermoplastic polymer having a glass transition temperature (Tg) of less than or equal to 130 ° C., preferably less than or equal to 110 ° C.
• Tg glass transition temperature
• the glass transition temperature can be measured by differential scanning calorimetry (DSC).
• DSC differential scanning calorimetry
• the polymeric composition of the invention may also contain additives. As an additive it is possible to choose plasticizers, impact modifiers and mixtures thereof.
• the semicrystalline thermoplastic polymer having a glass transition temperature (Tg) of less than or equal to 130 ° C. may be chosen, without limitation, from: polyolefins such as polyethylene and polypropylene;
• thermoplastic polyurethanes TPU
• polyethylene terephthalate or butylene TPU
• silicone polymers TPU
• the fluorinated polymers comprising at least 50 mol% and preferably consisting of monomers of formula (I):
• VDF content must be greater than 80% by weight, or even better than 90% by weight, to ensure sufficient mechanical strength when hot (that is to say a good creep strength at 130 ° C.).
• the comonomer may be a fluorinated monomer chosen, for example, from vinyl fluoride; trifluoroethylene (VF3); chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro (alkyl vinyl) ethers such as perfluoro (methyl vinyl) ether (PMVE), perfluoro (ethyl vinyl) ether (PEVE) and perfluoro (propyl vinyl) ether (PPVE); perfluoro (1,3-dioxole); perfluoro (2,2-dimethyl-1,3-dioxole) (PDD).
• VF3 trifluoroethylene
• CTFE chlorotrifluoroethylene
• TFE tetrafluoroethylene
• HFP hexafluoropropylene
• perfluoro (alkyl vinyl) ethers such as perfluoro (
• the optional comonomer is chosen from chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), trifluoroethylene (VF3) and tetrafluoroethylene (TFE).
• CTFE chlorotrifluoroethylene
• HFP hexafluoropropylene
• VF3 trifluoroethylene
• TFE tetrafluoroethylene
• the comonomer may also be an olefin such as ethylene or propylene.
• the preferred comonomer is HFP.
• the plasticizer With regard to the plasticizer according to the invention, this is generally described in "Encyclopaedia of Polymer Science and Engineering", Wiley and Sons (1989), pages 568-569 and 588-593.
• the plasticizer must be compatible with the homopolymer or copolymer of vinylidene fluoride.
• the plasticizer that can be used in the invention can be dibutyl sebacate (DBS of formula C 4 H 9 -COO- (CH 2 ) S-COO-C 4 H 9 ), dioctyl phthalate (DOP ) or NBSA (Nn-butyl-butylsulfonamide).
• DBS dibutyl sebacate
• DOP dioctyl phthalate
• NBSA Nn-butyl-butylsulfonamide
• High-performance plasticisers that can also be used in the invention are polymeric polyesters such as those derived from adipic, azelaic or sebacic acids and diols, and mixtures thereof, provided however that their number-average molecular weight is at least about 1500, preferably at least 1800, and not more than about 5000, preferably less than 2500 g / mol. Polyesters of too high molecular weight indeed lead to polymeric compositions of lesser impact strength.
• An adipic acid polyester of average molecular weight of 2050 g / mol sold by CIBA under the brand RHEOPLEX 904 can also be used.
• a high performance plasticizer for the present invention is DBS which is easily incorporated with PVDF.
• a shock modifier of the core-shell type comprises at least one inner layer of a soft polymer and a bark based on an acrylic polymer (that is, the outer layer also refers to acrylic bark).
• Acrylic polymer means polymers that contain methacrylic and / or acrylic monomers.
• the impact modifier is in the form of particles whose average diameter generally is at most 1 micron, preferably between 50 and 400 nm.
• the polymeric composition used according to the present invention can be manufactured directly by synthesis: a polymerization.
• the polymeric composition according to the invention comprises a semi-crystalline thermoplastic polymer having a glass transition temperature (Tg) of less than or equal to 130 ° C.
• Tg glass transition temperature
• the composition used according to the present invention can also be manufactured by mixing with the molten state, different constituents in any mixing device, and preferably an extruder. The polymeric composition is most often recovered in the form of granules.
• the polymeric composition used according to the invention may be a homopolymer of VDF which contains a plasticizer.
• the weight ratio of plasticizer to VDF homopolymer is in the range of 10 to 15%, preferably 10 to 12%.
• Another polymeric composition according to the invention may be a homopolymer of VDF which contains a plasticizer and a shock modifier.
• the mass ratio of plasticizer relative to the total mass of the polymeric composition is chosen from 1% to 5%, preferably from 2% to 4%.
• the mass ratio of the impact modifier relative to the total mass of the polymeric composition is chosen to be greater than or equal to 2% up to 10%, preferably 6% to 9%.
• the polymeric composition according to the invention may also contain a homopolymer or a copolymer of VDF.
• FIG. 1 shows a sectional view of a flexible metal pipe comprising a layer of the polymeric composition (1) covering a metal carcass (3), all reinforced by an armor (2).
• Fig. 2 represents the axisymmetric specimen shape for the tensile fatigue test making it possible to impose a triaxial stress field on the stressed material: longitudinal axis z, comprising a curved notch of curvature radius R, each specimen having a minimum diameter a and a maximum diameter d.
• longitudinal axis z comprising a curved notch of curvature radius R, each specimen having a minimum diameter a and a maximum diameter d.
• Fig. 3 schematically shows a servohydraulic dynamometer with a specimen.
• Test specimens for determining the fatigue strength according to the invention are prepared from the polymer composition manufactured.
• the specimens may be prepared by injection of the manufactured polymeric composition.
• Test specimens may also be prepared by extrusion, for example extrusions of the strips or tubes followed by machining of the specimens. In particular, the specimens are cut in the circular thickness of a tube of the polymeric composition.
• Each specimen is axisymmetric to the longitudinal axis z and has a curved notch of curvature radius R, a minimum radius a and a maximum diameter d. The specimen is defined by these three values a, d and R.
• the relationship between the minimum radius a and the radius of curvature a / R is 0.05 to 10, preferably 0.2 to 1 and more preferably 0.4 to 0.6.
• the maximum diameter d_ is greater than 2 times the minimum radius a and preferably ranging from 2 ⁇ + 0.5R to 2 ⁇ + 2R.
• the curvature radius R is 0.5 mm to 10 mm, preferably 3 mm to 5 mm and typically 4 mm; each test piece has a minimum radius a varying from 0.5 mm to 5 mm, preferably 1.5 mm to 2.5 mm and typically 2 mm; and the Maximum diameter d is chosen from 2mm to 30mm, preferably 6mm to 10mm and typically 7mm.
• the fatigue test according to the invention consists in biasing the test pieces to a tensile fatigue test consisting of an elongation along the longitudinal axis z, with a sinusoidal signal of frequency ranging from 0.05 Hz to 5 Hz, preferably from 0 , 5Hz to 2Hz and typically IHz, at a temperature ranging from -15 ° C to 23 ° C, preferably from -15 ° C to
• the maximum elongation of the same tensile cycle is from 0.05 R to IR and preferably from 0.075 R to 0.4 R, expressed in relative dimension from the radius of curvature of the notch R.
• the minimum elongation of the same cycle traction is greater than or equal to
• the maximum elongation is chosen from 0.2 mm to 4 mm, preferably from 0.3 mm to
• the minimum elongation of the fatigue test is determined by the ratio between the minimum elongation and the maximum elongation of the cycle. This ratio varies from greater than or equal to 0 up to 0.8 and preferably up to 0.5, and advantageously up to 0.25 and is typically 0.21.
• the result of the fatigue test is the average number of cycles to failure (NCR) of the set of test pieces.
• NCR cycles to failure
• composition which has an average number of cycles to failure is retained on several test specimens> 500, preferably> 1000, advantageously> 5000, and even more preferred> 10000.
• NCR average number of cycles to failure
• the polymeric composition selected by the determination method according to the invention can be used for the manufacture of pipes or conduits intended to convey a fluid under pressure and / or corrosive.
• a flexible metal pipe may comprise one or more metallic elements and at least one layer comprising the polymeric composition resulting from the manufacturing method according to the invention and optionally one or more layers of a polymer material, different from that of the polymeric composition. .
• ® KYNAR ® 400HDCM800 PVDF bimodal homopolymer marketed by ARKEMA.
• This product contains DURASTRENGTH ® D200: heart-bark shock modifier marketed by ARKEMA having a soft inner layer of T g
• EXL ® 2650 in place of the Durastrength, which is a shock modifier marketed by ROHM & HAAS having a soft inner layer of T g ⁇ - 60 ° C.
• ® KYNAR ® 50HDP900 PVDF bimodal homopolymer marketed by ARKEMA. containing DBS.
• KYNAR FLEX ® 3120-50 copolymer of 90% by weight VDF and 10% by weight HFP
• Composition A 89% KYNAR 50 and 11% DBS
• Composition B 89.5% KYNAR 400 and 7.5% EXL 2650 and 3% DBS
• composition C 100% KYNAR FLEX 3120-50 Composition D: 92% KYNAR 400 and 5% EXL 2650 and 3% DBS Composition E: 95% KYNAR 400 and 2% D200 and 3% DBS
• Charpy shock measurements are carried out following a protocol derived from the test of the ISO 179 IeA standard. This protocol has been adapted to be more severe than that of the standard in that the notch is made using a razor blade and therefore has a bottom radius of notch smaller than the value of 0 , 25 mm recommended in the standard.
• the thickness of the bars used is also greater than that of the bars recommended in the standard (6 or 7 mm typically against 4 mm). On 10 bars, we proceed by dichotomy in steps of 5 ° C to supervise the TDF. This corresponds to 50% brittle fracture.
• the impact velocity referenced is that recommended by ISO 179 IeA.
• This test consists in determining, for a given sample of polymer composition, the number of cycles to break (NCR), that is to say the number of cycles at the end of which the rupture of the sample occurs.
• NCR the number of cycles to break
• the test is performed at a temperature of -10 0 C over axisymmetric specimens of 4 mm radius notch of curvature (R4) and minimum radius of 2mm, with a servohydraulic load cell, for example of MTS 810.
• the distance between jaws is 10 mm.
• the test piece is subjected to a maximum elongation of 1.4 mm and a ratio between the minimum elongation and the maximum elongation of 0.21, which leads to a minimum elongation of 0.3 mm; with a sinusoidal signal having a frequency of 1 Hz.
• the result of the test is the average of the results obtained on 10 test pieces.
• the log mean found for 10 specimens corresponds to NCR (average number of cycles to failure).
• Example: the polymeric composition C has an NCR of 10000 on 10 test pieces tested. This means that there are on average 10,000 cycles before the rupture of the test pieces of the polymeric composition. Hot creep
• the resistance to hot creep is evaluated by performing a temperature pulling test according to ISO 527 on new test pieces of the polymeric composition, with a temperature conditioning of these test pieces of 20 min before the test.
• the threshold stress of these test pieces is measured at 130 ° C. for polymeric compositions based on homopolymers or copolymers of VDF. This stress corresponds to the maximum nominal tensile stress supported by the specimens before traction. The greater this stress, the better the creep resistance of the polymer.
• compositions A to E according to the Charpy test differs from that made on the basis of the fatigue test according to the invention.
• the design of a new polymeric composition will be different depending on whether the Charpy test or the fatigue test according to the invention is chosen as a criterion.
• composition C has a higher TDF
• the latter has a fatigue RCR (measured according to the process of the invention) lower than that of the composition C.
• the fatigue test according to the invention makes it possible to demonstrate the interest of the composition C with respect to the composition E, to withstand the actual fatigue conditions existing in the offshore hoses.

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• Physics & Mathematics (AREA)
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