ASTM F2214-02(2008)
(Test Method)Standard Test Method for In Situ Determination of Network Parameters of Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE)
Standard Test Method for <span class="bdit">In Situ</span> Determination of Network Parameters of Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE)
SIGNIFICANCE AND USE
This test method is designed to produce data indicative of the degree of crosslinking in ultra high molecular weight polyethylene that has been crosslinked chemically or by ionizing radiation.
The results are sensitive to the test temperature, solvent, and method used. For the comparison of data between institutions, care must be taken to have the same test conditions and reagents.
The data can be used for dose uniformity analysis, fundamental research, and quality assurance testing.
SCOPE
1.1 This test method describes how the crosslink density, molecular weight between crosslinks, and number of repeat units between crosslinks in ultra-high molecular weight polyethylene (UHMWPE) crosslinked by ionizing radiation or by chemical means can be determined by measuring the swelling ratio of samples immersed in o-xylene. Examples of experimental techniques used to make these measurements are discussed herein.
1.2 The test method reported here measures the change in height of a sample specimen while it is immersed in the solvent. Volumetric swell ratios assume that the sample is crosslinked isotropically, and that the change in dimension will be uniform in all directions. This technique avoids uncertainty induced by solvent evaporation or temperature change.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: F2214 − 02(Reapproved 2008)
Standard Test Method for
In Situ Determination of Network Parameters of Crosslinked
Ultra High Molecular Weight Polyethylene (UHMWPE)
This standard is issued under the fixed designation F2214; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
3.1 Definitions of Terms Specific to This Standard:
1.1 This test method describes how the crosslink density,
3.1.1 crosslink density, ν —the theoretical average number
molecular weight between crosslinks, and number of repeat
d
units between crosslinks in ultra-high molecular weight poly- of crosslinks per unit volume [mol/dm ].
ethylene (UHMWPE) crosslinked by ionizing radiation or by
3.1.2 molecular weight between crosslinks, M —the theo-
c
chemical means can be determined by measuring the swelling
retical average molecular weight between crosslinks [g/mol].
ratio of samples immersed in o-xylene. Examples of experi-
3.1.3 swell ratio, q —the ratio of the volume of the sample
mental techniques used to make these measurements are s
in an equilibrium swollen state to its volume in the unswollen
discussed herein.
state.
1.2 The test method reported here measures the change in
height of a sample specimen while it is immersed in the
4. Summary of Test Method
solvent. Volumetric swell ratios assume that the sample is
4.1 The height of a cubic specimen is measured, and the
crosslinked isotropically, and that the change in dimension will
specimen is placed in a dry chamber. A selected solvent is
be uniform in all directions. This technique avoids uncertainty
chosenaccordingtotheFlorynetworktheoryandisintroduced
induced by solvent evaporation or temperature change.
into the chamber. The chamber is heated to the reference
1.3 The values stated in SI units are to be regarded as
temperature. The sample height is monitored as a function of
standard. No other units of measurement are included in this
time until steady state (equilibrium) is achieved. The swell
standard.
ratio is calculated from the final steady state (equilibrium)
height and the initial height.
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
5. Significance and Use
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
5.1 This test method is designed to produce data indicative
bility of regulatory limitations prior to use.
of the degree of crosslinking in ultra high molecular weight
polyethylene that has been crosslinked chemically or by
2. Referenced Documents
ionizing radiation.
2.1 ASTM Standards:
5.2 The results are sensitive to the test temperature, solvent,
D2765 Test Methods for Determination of Gel Content and
and method used. For the comparison of data between
Swell Ratio of Crosslinked Ethylene Plastics
institutions,caremustbetakentohavethesametestconditions
E691 Practice for Conducting an Interlaboratory Study to
and reagents.
Determine the Precision of a Test Method
5.3 The data can be used for dose uniformity analysis,
fundamental research, and quality assurance testing.
This test method is under the jurisdiction ofASTM Committee F04 on Medical
6. Apparatus
and Surgical Materials and Devicesand is the direct responsibility of Subcommittee
F04.15 on Material Test Methods.
6.1 The apparatus shall include any device that allows a
Current edition approved May 1, 2008. Published June 2008. Originally
non-invasive measurement of the change in one dimension of
approved in 2002. Last previous edition approved in 2002 as F2214 – 02. DOI:
10.1520/F2214-02R08. the sample as it swells in the solvent. This measurement could
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
include, but is not limited to:
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
6.1.1 Mechanical measurements, such as linear variable
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. displacement transducers (LVDTs).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2214 − 02 (2008)
FIG. 1 Marked Measurement Direction Before (a) and After (b) Swelling
6.1.1.1 If a mechanical probe is used, it must be constructed 7.2 Anti-oxidant, 2,2'-methylene-bis (4-methyl-6-tertiary
of a material that exhibits little thermal expansion, such as butyl phenol).
quartz or ceramic.
8. Safety Precautions
6.1.2 Optical measurements, such as cameras or laser mi-
crometers.
8.1 O-xylene is toxic and flammable, and should be handled
6.1.2.1 Optical measurements should be insensitive to any
only with heat and chemically protective laboratory gloves.
refractive index changes in the UHMWPE sample, given the
Theswellingapparatusshouldideallybeplacedinsideavented
changing temperature of the system.
fume hood, or vented with an elephant trunk should space
6.1.3 Inductive measurements, such as proximity sensors.
considerations be an issue. Do not inhale the o-xylene vapors,
Inductive measurements must be insensitive to temperature or
as dizziness or a headache could result.
solvent composition.
8.2 Irganox 1010, the antioxidant, is identified by the
6.2 The sensitivity of the measurement shall be 1 % of the
manufacturer as an irritant and an inhalation hazard.
initial height of the sample, H . An uncertainty analysis has
9. Test Specimens
demonstrated that this sensitivity will produce a relative error
in crosslink density less than 10 % for samples swollen to a
9.1 At least three specimens with a minimum sample height
fraction 50 % beyond their initial height. Thicker samples will
of 500 µm should be machined. The top and bottom surfaces
allow a less sensitive measurement.
should be parallel and smooth. The width and length (or
diameter,inthecaseofcylindricalsamples)shouldbelessthan
6.3 The solvent in the temperature chamber shall be able to
one third the size of the sample chamber (see 6.4). The
reach a temperature of at least 150°C, with an expanded
height-to-width aspect ratio should be at least 1:2 to minimize
uncertaintyof 61°C.Gradientsshallnotexceed0.2°C/cm(NB
buckling, with 1:1 preferred. The machining should be per-
o-xylene boils at 144°C).
formed so as to minimize thermal degradation of the samples.
6.4 The smallest chamber dimension shall be at least three
9.2 Orientation of Samples—Given that the swelling behav-
times the size of the largest initial sample dimension.
ior can depend on molecular alignment induced by processing
6.5 The volume of the chamber shall be at least ten times
conditions, the test specimens should be machined so that the
thatofthesample.Thechambershouldbesufficientlysealedas
relevant processing direction can be easily identified. The
to prevent gross solvent evaporation during the course of the
samples can then be oriented in the swelling apparatus relative
experiment (typically 2 h).
to the molding direction (that is, perpendicular to the extrusion
NOTE 1—The data acquisition software should collect both sample
of compression molding direction). The specimens can be
dimension and temperature at a rate of at least 0.1 Hz.
marked as shown in Fig. 1 to aid in sample alignment.
7. Reagents
10. Procedure
7.1 Ortho-Xylene (o-xylene), ACS grade, boiling point
10.1 Add approximately 0.5 to 1 % (mass fraction) of the
144°C.
antioxidant to the o-xylene to make a stock solution.
Trade name: Irganox 1010 has been found satisfactory for this purpose.
Available from Ciba-Geigy, 540 White Plains Rd., P.O. Box 2005, Tarrytown, NY
10591–9005.
F2214 − 02 (2008)
TABLE 1 Summary of Mean (±S , Absolute Interlaboratory
10.2 The initial sample height should be measured with a
R
Uncertainty) Swell Ratio (q), Crosslink Density (ν ), and
d
resolutionof1 %ofthesampleheightusingamicrometer.This
Molecular Weight Between Crosslinks (M ) for the Four Samples
c
value should be recorded. The measurement direction on the
Molecular Weight
sample can be indicated with a permanent marker.An example
Dose, Swell Ratio, Crosslink Density, Between
is shown in Fig. 1. kGy q ν [mol/dm ] Crosslinks,
d
M [g/mol]
c
10.3 The sample should be pre-wet with o-xylene, then
54.2 3.37 ± 0.26 0.133 ± 0.017 7,650 ± 1,010
71.5 3.12 ± 0.24 0.151 ± 0.021 6,720 ± 920
quickly placed in the dry chamber with the sample correctly
89.2 3.12 ± 0.24 0.152 ± 0.020 6,700 ± 890
oriented as marked in 10.2.
110.1 2.97 ± 0.31 0.170 ± 0.032 6,150 ± 1,190
10.4 The initial sample dimension, as determined with the
measurement system of the instrument described in 6.1, should
be recorded.
density, ν , as a function of the steady state swelling ratio, the
d
10.5 Start recording the sample dimension at a minimum
Flory interaction parameter, and φ , the molar volume of the
rate of 1 point every 10 s.
4,5
solvent.
10.6 Introduce the o-xylene stock solution into the chamber 21 21 22
1n 1 2 q 1q 1χ q
~ !
s s 1 s
ν 52 (2)
at a slow rate to prevent disturbing the sample. d 21/3 21
φ ~q 2 q /2!
1 s s
10.7 Raise the temperature of the solvent in the chamber to
12.2.1 The expression in Eq 2 assumes a three-dimensional
130 6 1°C.
network comp
...
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