1、Designation: F2214 16Standard Test Method forIn Situ Determination of Network Parameters of CrosslinkedUltra High Molecular Weight Polyethylene (UHMWPE)1This standard is issued under the fixed designation F2214; the number immediately following the designation indicates the year oforiginal adoption
2、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,molecular weight between
3、 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 techniques used to make th
4、ese 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 in all directions.
5、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 concerns, if any, associate
6、d 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:2D2765 Test Methods for Determination of Gel Content andSwell R
7、atio of Crosslinked Ethylene PlasticsE691 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, dthe theoretical average numberof crosslinks per unit volume mol/dm3.3.1.2 mo
8、lecular 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 cubic specimen is measure
9、d, 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 (equilibrium) is achieved. The
10、 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 chemically or byionizi
11、ng radiation.5.2 The results are sensitive to the test temperature, solvent,and method used. For the comparison of data betweeninstitutions, care must be taken to have the same test conditionsand reagents.5.3 The data can be used for dose uniformity analysis,fundamental research, and quality assuran
12、ce 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 variabledisplacement tra
13、nsducers (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 Oct. 1, 2016. Published October 2016. Originallyapproved in 2002. L
14、ast previous edition approved in 2008 as F2214 02 (2008).DOI: 10.1520/F2214-16.2For 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 o
15、nthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16.1.1.1 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,
16、such as 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
17、orsolvent 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
18、height. 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 (NBo-xylene boils at 144C).6.4 The smallest chamber dimension shall b
19、e at least 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 3 to 8 h, depending on th
20、e crosslinkdensity).NOTE 1The data acquisition software should collect both sampledimension and temperature at a rate of at least 0.1 Hz.7. Reagents7.1 Ortho-Xylene (o-xylene), 98 %, boiling point 144C.7.2 Anti-oxidant, 2,2-methylene-bis (4-methyl-6-tertiarybutyl phenol).38. Safety Precautions8.1 O-
21、xylene is toxic and flammable, and should 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 dizzi
22、ness or a headache could result.8.2 Irganox 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.
23、 The width and length (ordiameter, in the 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 d
24、egradation of the samples.9.2 Orientation 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 swe
25、lling apparatus relativeto the molding direction (that is, perpendicular to the extrusionof compression molding direction). The specimens can bemarked as shown in Fig. 1 to aid in sample alignment.9.3 If the radiation dose can differ from the surface to thecenter of the sample, the location in the p
26、art where thespecimen is taken should be noted, as the swell ratio willdepend on the radiation dose.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
27、 (a) and After (b) SwellingF2214 16210. 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 height should be measured with aresolution of 1 % of the sample height using a micrometer. Thisvalue should be recorde
28、d. 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 in the dry chamber with the sample correctlyoriented as marked in 10.2.10.4 The initial sample dimension, as determined
29、 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 to prevent disturbing the sample.10.7 Raise the temperature of
30、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 an environmentally responsible manner, and cleanthe sample cell
31、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 Ratio11.1 The swell ratio, qs, is computed as indicated from theh
32、eight 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 MolecularWeight Between Crosslinks12.1 Given the steady state swell ratio, q
33、s, 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, 1, for the polymer-solvent sys-tem.12.2 From Florys network theory, which e
34、xplains 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, d, as a function of the steady state swelling ratio, theFlory interaction parameter, and 1, the mo
35、lar volume of thesolvent.4,5x52ln 1 2 qs21!1qs2111qs221qs21/32 0.5qs21!(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 rather than at the chain ends.Additionally, network entanglements may parti
36、ally 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 The expression in Eq 2 can be reduced to calculate themolecular weight
37、between crosslinks, Mc, where is thespecific volume of the polymer.Mc5 d!21(3)12.4 For polyethylene in o-xylene at 130C, the followingapproximate parameters shall be used:Parameter Value1(o-xylene-PE, 130C) 0.33 + 0.55/qs1(o-xylene) cm3/mol 13621fg/dm3g92013. Report13.1 Report the following informat
38、ion: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 weight between crosslinks, and13.1.6 The orientation of the specimens relati
39、ve 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 study4Flory, P. J., Principles of Polymer Chemistry, Ithaca and London, CornellUniversity Press, 1953.5Flory, P. J., and Re
40、hner, J., “Statistical mechanics of cross-linked polymernetworks. II. Swelling,” J. Chem. Phys., Vol 11, No. 11, 1943, pp. 521526.TABLE 1 Summary of Mean (SR, Absolute InterlaboratoryUncertainty) Swell Ratio (q), Crosslink Density (d), andMolecular Weight Between Crosslinks (Mc) for the Four Samples
41、Dose,kGySwell Ratio,qCrosslink Density,dmol/dm3Molecular WeightBetweenCrosslinks,Mcg/mol54.2 3.37 0.26 0.17 0.02 5,475 80471.5 3.12 0.24 0.20 0.03 4,741 72389.2 3.12 0.24 0.20 0.03 4,724 698110.1 2.95 0.32 0.23 0.05 4,235 998F2214 163conducted in 2001 involving four sets of ultra high molecularweigh
42、 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 Reference 8456, Ultra HighMolecular Weight Polyethylene were
43、 machined into 5 mmcubes, packaged in nitrogen, and irradiated four gammadifferent irradiation doses (54.2, 71.5, 89.2, and 110.1 kGy).714.2 The data indicates that for the range of irradiationdoses currently used to produce highly crosslinked UHMWPEfor orthopedic applications, the swell ratio measu
44、rement 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:14.3.1 Rep
45、eatability, 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 forthe
46、 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 % (0.95) pro
47、bability 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 procedureto be
48、 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 relat
49、ive 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 relative change over 130C isless than 0.05 %.X1.4 The weight of the sample probe, for contactmeasurements, can cause erroneously low measured swellingratios if the probe constrains the sample. This contribution willdepend on the modulus of the sample, which in turn
