1、Designation: F 2214 02Standard 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 oforiginal adoptio
2、n or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method describes how the crosslink density,molecular weight betw
3、een 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
4、 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 in all direction
5、s. This technique avoids uncertaintyinduced by solvent evaporation or temperature change.1.3 SI units are to be regarded as the standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to e
6、stablish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D 2765 Test Methods for Determination of Gel Content andSwell Ratio of Crosslinked Ethylene Plastics2E 691 Practice for Conducting an In
7、terlaboratory Study toDetermine the Precision of a Test Method33. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 crosslink density, ndthe theoretical average numberof crosslinks per unit volume mol/dm3.3.1.2 molecular weight between crosslinks, Mcthe theo-retical average molecul
8、ar 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 measured, and thespecimen is placed in a dry chamber. A selected solvent isc
9、hosen 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 swellratio is calculated from the final steady state (equilibrium)he
10、ight 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 byionizing radiation.5.2 The results are sensitive to the test temperature, s
11、olvent,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 research, and quality assurance testing.6. Apparatus6.1 The apparatus shall include any device t
12、hat 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 transducers (LVDTs).6.1.1.1 If a mechanical probe is used, it must be
13、constructedof a material that exhibits little thermal expansion, such asquartz or ceramic.6.1.2 Optical measurements, such as cameras or laser mi-crometers.1This test method is under the jurisdiction of ASTM Committee F04 on Medicaland Surgical Materials and Devices and is the direct responsibility
14、of SubcommitteeF04.15 on Material Test Methods.Current edition approved Nov. 10, 2002. Published January 2003.2Annual Book of ASTM Standards, Vol 08.023Annual Book of ASTM Standards, Vol 14.021Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United
15、States.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 orsolvent composition.6.2 The s
16、ensitivity 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. Thicker samples willall
17、ow 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 least threetimes the size
18、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 software should collect
19、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).48. Safety Precautions8.1 O-xylene is toxic and flammable, and should be handledonly with heat
20、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 Irganox 1010, the antioxidant,
21、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 case of cylindrical sampl
22、es) 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 of SamplesGiven that the
23、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 direction (that is, perpendi
24、cular 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 height should be measure
25、d 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 in the dry chamber with t
26、he sample correctlyoriented as marked in 10.2.10.4 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.4Trade name: Irganox 1010 has been found sa
27、tisfactory 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) SwellingF221402210.6 Introduce the o-xylene stock solution into the chamberat a slow rate to prevent disturbing the sample.1
28、0.7 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 an environmentally responsible mann
29、er, 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 Ratio11.1 The swell ratio, qs, is co
30、mputed 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 MolecularWeight Between Crosslinks12.1 Given
31、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 the polymer-solvent sys-tem.12.2 Fro
32、m 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 state swelling ratio, theFlory int
33、eraction parameter, and f1, the molar volume of thesolvent.5,6nd521n 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 rather than at the chain ends.Additio
34、nally, 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 The expression in Eq 2 can be red
35、uced 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(o-xylene) cm3/mol 136n21g/dm3 9201
36、3. 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 weight between crosslinks, and13.
37、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 ultra high molecularweigh polyethy
38、lene test samples tested by six laboratories.7For 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 HighMolecular Weight Polyethylene were machine
39、d into 5 mmcubes, packaged in nitrogen, and irradiated four gammadifferent irradiation doses (54.2, 71.5, 89.2, and 110.1 kGy).814.2 The data indicates that for the range of irradiationdoses currently used to produce highly crosslinked UHMWPEfor orthopedic applications, the swell ratio measurement i
40、sassociated with interlaboratory standard uncertainty of 8 to11 %.5Flory, P. J., Principles of Polymer Chemistry, Ithaca and London, CornellUniversity Press, 1953.6Flory, P. J., and Rehner, J., “Statistical mechanics of cross-linked polymernetworks. II. Swelling,” J. Chem. Phys., 11(11), 1943, pp. 5
41、21-526.7Spiegelberg, S., Kurtz, S., Muratoglu, O., Greer, K., Costa, L., Wallace, S., andCooper, C., Interlaboratory Reproducibility of Swell Ratio Measurements forCrosslinked Polyethylene, 48thAnnual Meeting of the Orthopedic Research Society,Dallas, TX, 2002.8Cubes of 5 mm per side are available f
42、rom 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 Summary of Mean (6SR, Absolute InterlaboratoryUncertainty) Swell Ratio (q), Crosslink Density (nd), andM
43、olecular 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 1,01071.5 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 0.17
44、0 6 0.032 6,150 6 1,190F221402314.3 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 Repeatability, rComparing two test results for
45、 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 same material, obtained by different opera
46、tors 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) probability of beingcorrect.14.4 There is no r
47、ecognized 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 followed in order to measure the swelling
48、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 thi
49、scontribution has been calculated to be 2.5 %.9X1.3 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 contact measure-ments, can cause erroneously low measured swelling ratios ifthe probe constrains the sample. This contribution will dependon the modulus of the sample, which in turn depends on thedegree of swelling.As an example, a sample with a modulus of100 MPa, when sub
