1、Designation: F 2214 02 (Reapproved 2008)Standard Test Method forIn Situ Determination of Network Parameters of CrosslinkedUltra High Molecular Weight Polyethylene (UHMWPE)1This standard is issued under the fixed designation F 2214; the number immediately following the designation indicates the year
2、oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method describes how the crosslink density,mole
3、cular weight between crosslinks, and number of repeatunits between crosslinks in ultra-high molecular weight poly-ethylene (UHMWPE) crosslinked by ionizing radiation or bychemical means can be determined by measuring the swellingratio of samples immersed in o-xylene. Examples of experi-mental techni
4、ques used to make these measurements arediscussed herein.1.2 The test method reported here measures the change inheight of a sample specimen while it is immersed in thesolvent. Volumetric swell ratios assume that the sample iscrosslinked isotropically, and that the change in dimension willbe uniform
5、 in all directions. This technique avoids uncertaintyinduced by solvent evaporation or temperature change.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This standard does not purport to address all of thesafety concern
6、s, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 2765 Test Methods for Determination of G
7、el Content andSwell Ratio of Crosslinked Ethylene PlasticsE 691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 crosslink density, ndthe theoretical average numberof crosslinks per unit
8、volume mol/dm3.3.1.2 molecular weight between crosslinks, Mcthe theo-retical average molecular weight between crosslinks g/mol.3.1.3 swell ratio, qsthe ratio of the volume of the samplein an equilibrium swollen state to its volume in the unswollenstate.4. Summary of Test Method4.1 The height of a cu
9、bic specimen is measured, and thespecimen is placed in a dry chamber. A selected solvent ischosen according to the Flory network theory and is introducedinto the chamber. The chamber is heated to the referencetemperature. The sample height is monitored as a function oftime until steady state (equili
10、brium) is achieved. The swellratio is calculated from the final steady state (equilibrium)height and the initial height.5. Significance and Use5.1 This test method is designed to produce data indicativeof the degree of crosslinking in ultra high molecular weightpolyethylene that has been crosslinked
11、 chemically or byionizing radiation.5.2 The results are sensitive to the test temperature, solvent,and method used. For the comparison of data between institu-tions, care must be taken to have the same test conditions andreagents.5.3 The data can be used for dose uniformity analysis,fundamental rese
12、arch, and quality assurance testing.6. Apparatus6.1 The apparatus shall include any device that allows anon-invasive measurement of the change in one dimension ofthe sample as it swells in the solvent. This measurement couldinclude, but is not limited to:6.1.1 Mechanical measurements, such as linear
13、 variabledisplacement transducers (LVDTs).1This test method is under the jurisdiction of ASTM Committee F04 on Medicaland Surgical Materials and Devices and is the direct responsibility of SubcommitteeF04.15 on Material Test Methods.Current edition approved May 1, 2008. Published June 2008. Original
14、lyapproved in 2002. Last previous edition approved in 2002 as F 2214 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe AS
15、TM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.6.1.1.1 If a mechanical probe is used, it must be constructedof a material that exhibits little thermal expansion, such asquartz or ceramic.6.1.2 Optical measurements, such a
16、s cameras or laser mi-crometers.6.1.2.1 Optical measurements should be insensitive to anyrefractive index changes in the UHMWPE sample, given thechanging temperature of the system.6.1.3 Inductive measurements, such as proximity sensors.Inductive measurements must be insensitive to temperature orsolv
17、ent composition.6.2 The sensitivity of the measurement shall be 1 % of theinitial height of the sample, H0. An uncertainty analysis hasdemonstrated that this sensitivity will produce a relative errorin crosslink density less than 10 % for samples swollen to afraction 50 % beyond their initial height
18、. Thicker samples willallow a less sensitive measurement.6.3 The solvent in the temperature chamber shall be able toreach a temperature of at least 150C, with an expandeduncertainty of 61C. Gradients shall not exceed 0.2C/cm.(NB o-xylene boils at 144C)6.4 The smallest chamber dimension shall be at l
19、east threetimes the size of the largest initial sample dimension.6.5 The volume of the chamber shall be at least ten timesthat of the sample. The chamber should be sufficiently sealed asto prevent gross solvent evaporation during the course of theexperiment (typically 2 h).NOTE 1The data acquisition
20、 software should collect both sampledimension and temperature at a rate of at least 0.1 Hz.7. Reagents7.1 Ortho-Xylene (o-xylene), ACS grade, boiling point144C.7.2 Anti-oxidant, 2,2-methylene-bis (4-methyl-6-tertiarybutyl phenol).38. Safety Precautions8.1 O-xylene is toxic and flammable, and should
21、be handledonly with heat and chemically protective laboratory gloves.The swelling apparatus should ideally be placed inside a ventedfume hood, or vented with an elephant trunk should spaceconsiderations be an issue. Do not inhale the o-xylene vapors,as dizziness or a headache could result.8.2 Irgano
22、x 1010, the antioxidant, is identified by themanufacturer as an irritant and an inhalation hazard.9. Test Specimens9.1 At least three specimens with a minimum sample heightof 500 m should be machined. The top and bottom surfacesshould be parallel and smooth. The width and length (ordiameter, in the
23、case of cylindrical samples) should be less thanone third the size of the sample chamber (see 6.4). Theheight-to-width aspect ratio should be at least 1:2 to minimizebuckling, with 1:1 preferred. The machining should be per-formed so as to minimize thermal degradation of the samples.9.2 Orientation
24、of SamplesGiven that the swelling behav-ior can depend on molecular alignment induced by processingconditions, the test specimens should be machined so that therelevant processing direction can be easily identified. Thesamples can then be oriented in the swelling apparatus relativeto the molding dir
25、ection (that is, perpendicular to the extrusionof compression molding direction). The specimens can bemarked as shown in Fig. 1 to aid in sample alignment.10. Procedure10.1 Add approximately 0.5 to 1 % (mass fraction) of theantioxidant to the o-xylene to make a stock solution.10.2 The initial sample
26、 height should be measured with aresolution of 1 % of the sample height using a micrometer. Thisvalue should be recorded. The measurement direction on thesample can be indicated with a permanent marker. An exampleis shown in Fig. 1.10.3 The sample should be pre-wet with o-xylene, thenquickly placed
27、in the dry chamber with the sample correctlyoriented as marked in 10.2.3Trade name: Irganox 1010 has been found satisfactory for this purpose.Available from Ciba-Geigy, 540 White Plains Rd., P.O. Box 2005, Tarrytown, NY105919005.FIG. 1 Marked Measurement Direction Before (a) and After (b) SwellingF
28、2214 02 (2008)210.4 The initial sample dimension, as determined with themeasurement system of the instrument described in 6.1, shouldbe recorded.10.5 Start recording the sample dimension at a minimumrate of 1 point every 10 s.10.6 Introduce the o-xylene stock solution into the chamberat a slow rate
29、to prevent disturbing the sample.10.7 Raise the temperature of the solvent in the chamber to130 6 1C.10.8 Continue to monitor the temperature and sample di-mension until equilibrium is achieved (within 610 m) over aperiod of 15 min.10.9 Decrease the temperature to below 50C. Discard theo-xylene in a
30、n environmentally responsible manner, and cleanthe sample cell thoroughly.10.10 Examine the sample after the test is complete. If it hasshown signs of cracking, or is yellowed, thermal degradation islikely to have occurred. This data will be suspect and should bediscarded.11. Calculation of Swell Ra
31、tio11.1 The swell ratio, qs, is computed as indicated from theheight measurement:qs5 Vf/ V0! 5 Hf/ H0!3(1)where:Vf= final volume,V0= initial volume,Hf= final height, andH0= initial height.NOTE 2This calculation assumes that the sample is isotropic.12. Calculation of Crosslink Density and MolecularWe
32、ight Between Crosslinks12.1 Given the steady state swell ratio, qs, of a polymerimmersed in a specific solvent at a particular temperature, thecrosslink density, molecular weight between crosslinks, andnumber of crosslinks/chain can be computed if one knows theFlory interaction parameter, x1, for th
33、e polymer-solvent sys-tem.12.2 From Florys network theory, which explains the swellratio of a polymer-solvent system as a competition betweenelastic forces and forces derived from the free energy ofmixing, the following expression is derived for the crosslinkdensity, nd, as a function of the steady
34、state swelling ratio, theFlory interaction parameter, and f1, the molar volume of thesolvent.4,5nd521n 1 2 qs21! 1 qs211x1qs22f1qs21/32 qs21/2!(2)12.2.1 The expression in Eq 2 assumes a three-dimensionalnetwork composed primarily of “H-bonding,” or the formationof crosslinks along the main chain rat
35、her than at the chain ends.Additionally, network entanglements may partially contributeto the refractive forces. Thus the calculated crosslink densitywill account for these contributions as well. The expression inEq 2 has been shown to be valid for swelling ratios up to q =10, or Mc10 000 g/mol.12.3
36、 The expression in Eq 2 can be reduced to calculate themolecular weight between crosslinks, Mc, where nis thespecific volume of the polymer.Mc5 nnd!21(3)12.4 For polyethylene in o-xylene at 130C, the followingapproximate parameters shall be used:Parameter Valuex1(o-xylene-PE, 130C) 0.33 + 0.55/qsf1(
37、o-xylene) cm3/mol 136n21g/dm3 92013. Report13.1 Report the following information:13.1.1 Complete identification of the sample,13.1.2 Solvent and temperature used,13.1.3 Initial heights of the samples,13.1.4 Final heights of the samples,13.1.5 Calculated swell ratio, crosslink density, and molecu-lar
38、 weight between crosslinks, and13.1.6 The orientation of the specimens relative to theprinciple processing direction (that is, ram extrusion direction,or compression molding direction).14. Precision and Bias14.1 Tables 1 and 2 are based on a round robin studyconducted in 2001 involving four sets of
39、ultra high molecularweigh polyethylene test samples tested by six laboratories.6For all sets of samples, all the specimens were prepared at thesame time by the same laboratory. Each laboratory tested threespecimens from each set of samples.14.1.1 Samples from NIST Referencet 8456, Ultra HighMolecula
40、r Weight Polyethylene were machined into 5 mmcubes, packaged in nitrogen, and irradiated four gammadifferent irradiation doses (54.2, 71.5, 89.2, and 110.1 kGy).74Flory, P. J., Principles of Polymer Chemistry, Ithaca and London, CornellUniversity Press, 1953.5Flory, P. J., and Rehner, J., “Statistic
41、al mechanics of cross-linked polymernetworks. II. Swelling,” J. Chem. Phys., Vol 11, No. 11, 1943, pp. 521526.6Spiegelberg, S., Kurtz, S., Muratoglu, O., Greer, K., Costa, L., Wallace, S., andCooper, C., Interlaboratory Reproducibility of Swell Ratio Measurements forCrosslinked Polyethylene, 48thAnn
42、ual Meeting of the Orthopedic Research Society,Dallas, TX, 2002.7Cubes of 5 mm per side are available from NIST as Referencet 8457Orthopaedic Grade Polyethylene Cubes. Available from National Institute ofStandards and Technology (NIST), 100 Bureau Dr., Stop 3460, Gaithersburg, MD20899-3460.TABLE 1 S
43、ummary of Mean (6SR, Absolute InterlaboratoryUncertainty) Swell Ratio (q), Crosslink Density (nd), andMolecular Weight Between Crosslinks (Mc) for the Four SamplesDose,kGySwell Ratio,qCrosslink Density,ndmol/dm3Molecular WeightBetween Crosslinks,Mcg/mol54.2 3.37 6 0.26 0.133 6 0.017 7,650 6 1,01071.
44、5 3.12 6 0.24 0.151 6 0.021 6,720 6 92089.2 3.12 6 0.24 0.152 6 0.020 6,700 6 890110.1 2.97 6 0.31 0.170 6 0.032 6,150 6 1,190F 2214 02 (2008)314.2 The data indicates that for the range of irradiationdoses currently used to produce highly crosslinked UHMWPEfor orthopedic applications, the swell rati
45、o measurement isassociated with interlaboratory standard uncertainty of 8 to11 %.14.3 Concept of r and RIf Srand SR, the absoluteintralaboratory and interlaboratory uncertainties, have beencalculated from a large enough body of data, and for test resultsthat are averages from testing 3 specimens:14.
46、3.1 Repeatability, rComparing two test results for thesame material, obtained by the same operator using the sameequipment on the same day, the two test results should bejudged not equivalent if they differ by more than the r value forthat material.14.3.2 Reproducibility, RComparing two test results
47、 forthe same material, obtained by different operators using differ-ent equipment on different days, the two test results should bejudged not equivalent if they differ by more than the R valuefor that material.14.3.3 Any judgment in accordance with 14.3.1 and 14.3.2would have an approximate 95 % (0.
48、95) probability of beingcorrect.14.4 There is no recognized standard by which to estimatebias of this test method.15. Keywords15.1 crosslink density; molecular weight; network; swell-ing; UHMWPEAPPENDIXES(Nonmandatory Information)X1. RATIONALEX1.1 This test method is intended to describe the procedu
49、reto be followed in order to measure the swelling behavior ofcrosslinked ultra high molecular weight polyethylene.X1.2 There are contributions to the measured swellingbehavior introduced when the polyethylene melts, as deter-mined by the change in density as the sample goes from a solidto a melt. The relative or fractional change in height from thiscontribution has been calculated to be 2.5 %.8X1.3 There are contributions from thermal expansion of thesample, which can be calculated from the thermal coefficient ofexpansion of UHMWPE. The re
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