ISO TR 10837:1991 download.Determination of the thermal stability of polyethylene ( PE ) for use in gas pipes and fitting.
0.1 General
Polyethylene pipes and fittings are designed to be joined together by means o( fusion techniques which can Involve temperatures In excess of 200 °C. To prevent the polymers from which the pipes and fittings are made from degrading during fusion, It Is necessary to Incorporate stabilizers which Impart thermal stabirily to Llie polymers
The measurement of the thermal stability of a polymer can be determined by either measurement of the oxidation Induction temperature of the polymer, or measurement of the oxidation induction time at a set lemperature. These determinations are generally carried out in oxygen or air.
The task group (see foreword) agreed that (he measurement of the oxidation induction time should be the preferred method because it offered greater sensitivity. A standard method of test for the measurement of thermal stability by means of oxidation induction tIme has been prepared.
0.2 Basic technical problem
In studying the polyethylenes used In (lie production of gas pipes and fittings arid already shown to meet the performance requirements of national gas pipe and fitting specifications, the following major problem was identified which accounted (or the task group’s failure to reach agreement on proposals to be forwarded to the sub-committee.
When measuring the thermal stability of a polymer by determining the oxidation induction time at eIther 200 °C or 210 C, two disttnctly different stabilIty levels were identified due to the use of different stabilizer systems.
it was therefore not possible to agree a specification level which could be met by acceptable polymers, yet ensure in all cases sufficient thermal stability to permit the assembly of pipes end fittings by fusion.
The problem raced by the task group is illustrated by figure0.i.
This Technical Report specifies a method of measurIng the oxidative thermal stability of polyethylene pipe and filling m!nrial in oxygen at typfr’al pro. cesaln9 and welding temperatures. It may be used to measure the stability of either raw materials or imnished products, and may be taken as an Indication of polymer or anti-oxidant performance.
The recommended test temperatures of 20(3 °C and 210 C are suitable for adequately stabilized pipe and fitting materials.
The thermal stability measured by this method is dependent on test specimen mass and size.
2 NormatIve reference
The following standard contains provisions which, through reference in this text, constitute provisions of this Technical Report. At the time of publication. the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this Technical Report are encouraged to Investigate the possibility of applying the most recent edition of the standard indicated below. Members of IEC and ISO maintain regIsters of currently valid International Standards.
ISO 293:1986, Plastics — Compression movldinq test speclmns 01 Thermoplastic materials
3 PrincIple
The test measures the lime during which the antioxidant present in a test specimen inhibits oxidation whilst the specimen Is held at 200 °C (or 210 °C) In a stream of oxygen.
The progress of the oxidation is morutored by measuring the dilntererlce In temperature between the specimen compartment and reference compartment of a thermal analyser and recording this again5t time. The therrnI stability Is tlieii deilved from this record.
4 Apparatus and materials
4.1 Differential thermal analyser or differential ecannlng calorimeter, capable of
a) recording the difference in temperature or In energy flow between specimen nd reference compartments against time;
b) maintaining the test temperature wIthin ±0,5 °C for the duration or the test:
c) exposing the specimen to a flow of oxygen equal to 50 crn3lmln + 10 04.
d) programming the specimen lefnperalures over the range 150 DC to 250 DC. at a rate of I °C/min or lees;
a) programming the specimen temperature to rise from ambient temperature to the test temperature at a rate of 20 °C j I °C;
f) continuously recording the specimen temperature with a resolution of 0,1 °C (ii this facUlty is not available, then 4.2 appiles).
4.2 Temperature-measuring apparatus, capable of continously monitoring the specimen temperature with a resolution of 0,1 DC.
A high-Impedance digital voltmeter with a resolution of I iiV has been found suitable when connected to the specimen thermocouple and the associated cold junction, or cold-junction compensator, of the thermal analyser.
4.3 Analytical balance, capable of weighing the
15 mg ± 0,5 mg test specimen to an accuracy of
0,1 mg.
4.4 Oxygen and high-purity nitrogen supplies, able to be switched to give alternate flow. The changeover shall be made sufficiently close to the differential thermal analyser or differential scanning calorimeter cell so that the atmosphere is completely changed within I mm of switchover.
No purification train is deemed necessary In eIther cnmprcsed-gas supply.
4.5 Gas-flow-measuring devices.
Rotameters are suitable, but their calibration shall be checked against a positive-displacement device. 46 High-purity iiietal standards,
Indium: Melting poInt 156,8 °C ± 0,5 DC; Tin: Melting point 231.9 °C ± 0,5 °C.
5 PreparatIon of test specimens
5.1 Test specimens from pipe and fittings
The position from whch the test specimen is taken shall be as defined in the relevant product standard.
a) Saw a 2 cm to 3 cm ring from the ampie pipe or fitting.
b) Cut a 2 cm segment from the ring.
c) HoldIng the segment in a vice, cut a through-wail cylinder sample with a diameter just less than the Inner diameter of the sample paris of the thermal anaiyser.
d) Using a scalpel, cut a test specimen from the through-wall cylinder of appropriate thickness to give a specimen weight of 15 mg ± 0,5 mg.
The test specimen is designed to allow measurement of the thermal stability at points throughout the wall thickness.
e) 1-tandle the specimen with care and do not expose It to direct sunlight.
NOTE 1 A hole-borer, driven by an electric drill. is a ultabIe devTce for taking a through-wall core directly from a pipe or fitting. Care should be taken not to overheat the eample.
5.2 Test specimens from raw materials in the form of moulded sheet
a) Proporo a comproselon-mouldod plaquo in accordance with ISO 293. LimIt heating to 2 mm at 150 °C ± ‘C.
b) Cut a cylindrical sample with a diameter just less than the Inner diameter of the specimen.
c) Using a scalpel, cut a specimen from the cylinder to give a specimen weight of 15 mg ± 0,5 mg.
6 Procedure
6.1 CaHbrat Ion
Establish Rfl oxygen flow of 50 cm3/mln 10 % over the specimen and reference compartments of the apparatus at a temperature 10 °C below the mailing point of Indium or tin.
Heal a 2 mg specimen of indium or tin in a sealed aluminium pan, using an empty aluminium pan as reference, at a rate not exceeding I °C/mirt until the melting endotherm has been recorded. lithe apparalus does not automatically do so, mark the mdicated temperature on the chart at Intervals in the region of the endotherm so thai the meiting point can be determined to a precision of ±0,1 °C. Determine the melting points of both indium and tin In this way.
The melting point of the metal is taken ac the temperature given by the intercept of the extended baseline and the tangent to the first slope of the endoiherm (see figure 1).
Adjust the apparatus so that the indicated melting points of Indium and tin lie within the ranges 156,0 °C ± 0,5 °C and 231,9 C ± 0,5 °C, respevtively.