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
  • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
• • 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
  • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
  • G01N3/10—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces generated by pneumatic or hydraulic pressure
  • G01N3/12—Pressure testing
• • 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
  • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
  • G01N3/18—Performing tests at high or low temperatures
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
  • F16L9/00—Rigid pipes
  • F16L9/12—Rigid pipes of plastics with or without reinforcement
• • 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
  • G01N2203/0012—Constant speed test
• • 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
• • 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
  • G01N2203/006—Crack, flaws, fracture or rupture
• • 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
  • G01N2203/0071—Creep
• • 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
  • G01N2203/0075—Strain-stress relations or elastic constants
• • 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
  • G01N2203/0202—Control of the test
  • G01N2203/0212—Theories, calculations
  • G01N2203/0218—Calculations based on experimental data
• • 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
  • G01N2203/022—Environment of the test
  • G01N2203/0222—Temperature

Definitions

• the present invention relates to a method for determining the time to brittle failure of a pipe made of a polypropylene composition.
• ISO 1 167 specifies the method of preparation of test pieces, the equipment used to pressurize test pieces, the testing procedure to be applied and the test report. The time to failure and the type of failure (brittle, ductile or mixed) at predetermined hoop stresses are determined by this test. Such methods require large volumes of material and long testing times of more than one year. These extremely long testing times are a serious concern in using these methods for determination of time to failure behavior of resins and manufactured pipes as well as in the development of new grades, in batch release testing and quality control.
• Resistance to slow crack growth of a pipe made of a high density polyethylene can be determined by the measurement of the strain hardening modulus according to IS018488.
• the slow crack growth behavior of HDPE can be regarded as a
• the disentanglement capability of HDPE will determine its resistance against slow crack growth.
• the strain hardening modulus of HDPE is a measure of the disentanglement capability of the tie molecules of this polymer and is an intrinsic property. Thus, the measurement of the strain hardening modulus of HDPE will give an insight on the slow crack growth behavior or HDPE.
• the present invention provides a method for determining the time to brittle failure of a pipe made of a polypropylene composition, comprising:
• step d) calculating true stress-true strain curve from the stress strain curve obtained by step c) and calculating the tensile strain hardening modulus G p from the true stress- strain curve, according to the method as described in ISO/DIS 18488,
• the inventors have found that the quotient G p Y and the time to brittle failure show a correlation.
• the determination of G p Y can be performed much easier than the measurement of the time to brittle failure.
• the time to brittle failure of a given pipe made of a polypropylene composition can be estimated by determining G p Y of the same material.
• the correlation can be obtained by measuring the time to brittle failure and G p Y for a sufficient number of pipes of different polypropylene composition. Once the correlation is obtained, it is no longer necessary to actually measure the time to brittle failure for new pipes.
• the measurement of G p Y can be used to estimate the time to brittle failure with a relatively high accuracy.
• time to brittle failure of a pipe made of polypropylene cannot be determined only by the tensile strain hardening modulus like with the Full Notched Creep Test (FNCT) of a pipe made of HDPE.
• FNCT Full Notched Creep Test
• the inventors have found that the determination of the time to brittle failure of a pipe made of polypropylene composition requires the measurement of the yield stress Y in addition to the tensile strain hardening modulus. step a)
• the method according to the invention comprises a) providing a specimen of the polypropylene composition having a predetermined shape.
• step a) involves compression molding a sheet from a polypropylene composition according to ISO 1873-2 and punching a specimen having a geometry of the test specimen described in ISO/DIS 18488 from the sheet.
• the geometry of the test specimen is described in section 6.1 of ISO/DIS 18488.
• the sheets After compression molding of the sheets, the sheets are stored for minimal 7 days at room temperature. After 7 days the sheets are annealed to remove any orientation or thermal history and maintain isotropic sheets. Annealing of the sheets are performed by conditioning the sheet for 1 h in an oven at a temperature of 95 to 120 °C, preferably 96 to 1 10 °C, more preferably 97 to 105 °C, more preferably 98 to 102 °C, most preferably around 100 °C and slowly cooling down to room temperature by switching off the closed temperature chamber. The period of conditioning is preferably 1 h, but may also be shorter or longer, e.g. 0.5 to 2 h. During this operation free movement of the sheets is allowed. After cooling down, specimens having the predetermined shape as described in ISO/DIS 18488 are punched from the sheet. step b)
• the method according to the invention comprises b) elongating the specimen at a constant traverse speed at a predetermined temperature.
• the constant traverse speed is 5-40 mm/min, more preferably 10-30 mm/min, more preferably 15-25 mm/min, more preferably about 20 mm/min.
• the predetermined temperature is 95-105 °C, preferably 100 °C. step c)
• the method according to the invention comprises c) measuring the load sustained by the specimen during the elongation of step b). A stress-strain curve is obtained. In step c), the yield stress Y is also measured. The Yield stress is the tensile stress at the first point on the stress- strain curve where some further increase in strain occurs without any increase in stress.
• steps b) and c) are performed as follows:
• the sample shall remain between the clamps for a certain period, e.g. at least 1 minute, before the load is applied and measurement starts.
• test specimen A sufficient number (e.g. 5) of test specimen is subjected to the measurement and averages are measured to obtain the final results. Preferably, if straining of the test specimen takes place in the clamps, the test results are discarded. step d)
• the method according to the invention comprises d) calculating true stress-true strain curve from the stress strain curve obtained by step c) and calculating the tensile strain hardening modulus G p from the true stress-strain curve, according to the method as described in ISO/DIS 18488.
• the method of the calculations are described in ISO/DIS 18488, section 8 "Data treatment".
• the draw ratio, ⁇ is calculated from the length, /, and the gauge length, / 0 , as shown by formula 1 .
• ⁇ / is the increase in the specimen length between the gauge marks.
• the true stress, o irue is calculated according to formula 2, which is derived on the assumption of conservation of volume between the gauge marks:
• Neo-Hookean constitutive model (formula 3, see Annex A of ISO/DIN 18488) is used to fit and extrapolate the data from which ⁇ Gp> (MPa) for 8 ⁇ ⁇ ⁇ 12 is calculated.
• the method according to the invention comprises e) calculating a quotient of the tensile strain hardening modulus G p divided by the yield stress Y. step f)
• the method according to the invention comprises f) determining the time to brittle failure of the pipe based on the quotient G p Y. Preferably, the time to brittle failure of the pipe at 4.2 MPa and 95 °C is determined.
• step f) involves comparing the quotient G p /Y determined for the pipe to a reference formula which shows the relationship between the brittle time to failure and the quotient Gp Y for pipes made of polypropylene composition.
• the reference formula which shows the relationship between the brittle time to failure and the quotient Gp/Y for pipes made of polypropylene composition may be a calibration curve which is a best fit of plots taking time to brittle measurements performed on pipes on one axis and Gp/Y measurements performed on test specimens on another axis.
• the axis for the time to brittle may be on a logarithmic scale.
• each of the pipes was prepared according to ISO 1 167-2.
• the measurement of time to brittle failure was performed at 95 °C and at a hoop stress of 4.2 MPa measured according to IS03213.
• G p Y The measurement of G p Y was performed on test specimens made according to IS01873-2 and ISO/DIS18488 at a constant traverse speed of 20 mm/min and a temperature of 100 °C.
• the polypropylene composition of the pipe for which the time to brittle failure is determined may contain additives, in particular additives suitable for use in pipes.
• additives suitable for use in pipes include but are not limited to nucleating agents, stabilizers, anti-oxidants pigments and/or colorants, impact modifiers, flame retardants, acid scavengers, anti-microbials and the like.
• Such additives are well known in the art. The skilled person will choose the type and amount of additives such that they do not detrimentally influence the aimed properties of the composition.
• the polypropylene composition may consist of a propylene polymer or may consist of a propylene polymer and additives.
• the amount of the additive in the polypropylene composition is chosen from 0 to 5 wt% based on the total weight of the polypropylene composition (the polypropylene composition comprises 95 to 100 wt% of the propylene polymer), preferably the amount of additive is from 0.01 to 3 wt%, for example from 0.1 to 2 wt%, based on the total weight of the polypropylene composition.
• propylene polymer as used herein is meant propylene homopolymer, a random or a block copolymer of at least 70 wt% of propylene with up to 30 wt% of an a-olefin having 2 or 4 to 10 C-atoms.
• the examples of ⁇ -olefin in the random or block propylene copolymer include ethylene, 1 -butene, 1 -pentene, 4-methyl-1 -pentene, 1 -hexene, 1 -heptene or 1 -octene, preferably ethylene.
• the propylene copolymer may consist of at least 70 wt% of propylene and up to 30 wt% of a-olefin, preferably 1 -10 wt% of a-olefin, based on the total weight of the propylene copolymer.
• Polypropylene homopolymer and the copolymer of propylene with an ⁇ -olefin can be made by any known polymerization technique as well as with any known
• polymerization catalyst system Regarding the techniques, reference can be given to slurry, solution or gas phase polymerizations; regarding the catalyst system reference can be given to Ziegler-Natta, metallocene or single-site catalyst systems. All are, in themselves, known in the art.
• the propylene-based polymer used in the invention is a propylene homopolymer or a copolymer of propylene with ethylene, wherein the amount of ethylene is 1 -10 wt% based on the copolymer.

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• Physics & Mathematics (AREA)
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• General Health & Medical Sciences (AREA)
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• Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)

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• 2017
  • 2017-07-11 WO PCT/EP2017/067342 patent/WO2018011177A1/en unknown
  • 2017-07-11 EP EP17735595.5A patent/EP3485189A1/de not_active Withdrawn

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