ASTM F2214-02
(Test Method)Standard Test Method forIn Situ Determination of Network Parameters of Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE)
Standard Test Method for<i>In Situ</i> 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 SI units are to be regarded as the 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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Designation:F2214–02
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 F 2214; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.2 molecular weight between crosslinks, M —the theo-
c
retical average molecular weight between crosslinks [g/mol].
1.1 This test method describes how the crosslink density,
3.1.3 swell ratio, q —the ratio of the volume of the sample
s
molecular weight between crosslinks, and number of repeat
in an equilibrium swollen state to its volume in the unswollen
units between crosslinks in ultra-high molecular weight poly-
state.
ethylene (UHMWPE) crosslinked by ionizing radiation or by
chemical means can be determined by measuring the swelling
4. Summary of Test Method
ratio of samples immersed in o-xylene. Examples of experi-
4.1 The height of a cubic specimen is measured, and the
mental techniques used to make these measurements are
specimen is placed in a dry chamber. A selected solvent is
discussed herein.
chosenaccordingtotheFlorynetworktheoryandisintroduced
1.2 The test method reported here measures the change in
into the chamber. The chamber is heated to the reference
height of a sample specimen while it is immersed in the
temperature. The sample height is monitored as a function of
solvent. Volumetric swell ratios assume that the sample is
time until steady state (equilibrium) is achieved. The swell
crosslinked isotropically, and that the change in dimension will
ratio is calculated from the final steady state (equilibrium)
be uniform in all directions. This technique avoids uncertainty
height and the initial height.
induced by solvent evaporation or temperature change.
1.3 SI units are to be regarded as the standard.
5. Significance and Use
1.4 This standard does not purport to address all of the
5.1 This test method is designed to produce data indicative
safety concerns, if any, associated with its use. It is the
of the degree of crosslinking in ultra high molecular weight
responsibility of the user of this standard to establish appro-
polyethylene that has been crosslinked chemically or by
priate safety and health practices and determine the applica-
ionizing radiation.
bility of regulatory limitations prior to use.
5.2 The results are sensitive to the test temperature, solvent,
and method used. For the comparison of data between institu-
2. Referenced Documents
tions, care must be taken to have the same test conditions and
2.1 ASTM Standards:
reagents.
D 2765 Test Methods for Determination of Gel Content and
2 5.3 The data can be used for dose uniformity analysis,
Swell Ratio of Crosslinked Ethylene Plastics
fundamental research, and quality assurance testing.
E 691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
6. Apparatus
6.1 The apparatus shall include any device that allows a
3. Terminology
non-invasive measurement of the change in one dimension of
3.1 Definitions of Terms Specific to This Standard:
the sample as it swells in the solvent. This measurement could
3.1.1 crosslink density, n —the theoretical average number
d
include, but is not limited to:
of crosslinks per unit volume [mol/dm ].
6.1.1 Mechanical measurements, such as linear variable
displacement transducers (LVDTs).
6.1.1.1 If a mechanical probe is used, it must be constructed
This test method is under the jurisdiction ofASTM Committee F04 on Medical
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
of a material that exhibits little thermal expansion, such as
F04.15 on Material Test Methods.
quartz or ceramic.
Current edition approved Nov. 10, 2002. Published January 2003.
6.1.2 Optical measurements, such as cameras or laser mi-
Annual Book of ASTM Standards, Vol 08.02
Annual Book of ASTM Standards, Vol 14.02 crometers.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F2214–02
FIG. 1 Marked Measurement Direction Before (a) and After (b) Swelling
6.1.2.1 Optical measurements should be insensitive to any fume hood, or vented with an elephant trunk should space
refractive index changes in the UHMWPE sample, given the considerations be an issue. Do not inhale the o-xylene vapors,
changing temperature of the system. as dizziness or a headache could result.
6.1.3 Inductive measurements, such as proximity sensors. 8.2 Irganox 1010, the antioxidant, is identified by the
Inductive measurements must be insensitive to temperature or manufacturer as an irritant and an inhalation hazard.
solvent composition.
9. Test Specimens
6.2 The sensitivity of the measurement shall be 1 % of the
initial height of the sample, H . An uncertainty analysis has 9.1 At least three specimens with a minimum sample height
demonstrated that this sensitivity will produce a relative error of 500 µm should be machined. The top and bottom surfaces
in crosslink density less than 10 % for samples swollen to a should be parallel and smooth. The width and length (or
fraction 50 % beyond their initial height. Thicker samples will diameter,inthecaseofcylindricalsamples)shouldbelessthan
allow a less sensitive measurement. one third the size of the sample chamber (see 6.4). The
6.3 The solvent in the temperature chamber shall be able to height-to-width aspect ratio should be at least 1:2 to minimize
reach a temperature of at least 150°C, with an expanded buckling, with 1:1 preferred. The machining should be per-
uncertainty of 61°C. Gradients shall not exceed 0.2°C/cm. formed so as to minimize thermal degradation of the samples.
(NB o-xylene boils at 144°C) 9.2 Orientation of Samples—Given that the swelling behav-
6.4 The smallest chamber dimension shall be at least three ior can depend on molecular alignment induced by processing
times the size of the largest initial sample dimension. conditions, the test specimens should be machined so that the
6.5 The volume of the chamber shall be at least ten times relevant processing direction can be easily identified. The
thatofthesample.Thechambershouldbesufficientlysealedas samples can then be oriented in the swelling apparatus relative
to prevent gross solvent evaporation during the course of the to the molding direction (that is, perpendicular to the extrusion
experiment (typically 2 h). of compression molding direction). The specimens can be
marked as shown in Fig. 1 to aid in sample alignment.
NOTE 1—The data acquisition software should collect both sample
dimension and temperature at a rate of at least 0.1 Hz.
10. Procedure
7. Reagents 10.1 Add approximately 0.5 to 1 % (mass fraction) of the
antioxidant to the o-xylene to make a stock solution.
7.1 Ortho-Xylene (o-xylene), ACS grade, boiling point
10.2 The initial sample height should be measured with a
144°C.
resolutionof1 %ofthesampleheightusingamicrometer.This
7.2 Anti-oxidant, 2,2’-methylene-bis (4-methyl-6-tertiary
value should be recorded. The measurement direction on the
butyl phenol).
sample can be indicated with a permanent marker.An example
is shown in Fig. 1.
8. Safety Precautions
10.3 The sample should be pre-wet with o-xylene, then
8.1 O-xylene is toxic and flammable, and should be handled
quickly placed in the dry chamber with the sample correctly
only with heat and chemically protective laboratory gloves.
oriented as marked in 10.2.
Theswellingapparatusshouldideallybeplacedinsideavented
10.4 The initial sample dimension, as determined with the
measurement system of the instrument described in 6.1, should
be recorded.
Trade name: Irganox 1010 has been found satisfactory for this purpose.
10.5 Start recording the sample dimension at a minimum
Available from Ciba-Geigy, 540 White Plains Rd., P.O. Box 2005, Tarrytown, NY
10591–9005. rate of 1 point every 10 s.
F2214–02
TABLE 1 Summary of Mean (6S , Absolute Interlaboratory
10.6 Introduce the o-xylene stock solution into the chamber
R
Uncertainty) Swell Ratio (q), Crosslink Density (n ), and
d
at a slow rate to prevent disturbing the sample.
Molecular Weight Between Crosslinks (M ) for the Four Samples
c
10.7 Raise the temperature of the solvent in the chamber to
Molecular Weight
130 6 1°C. Dose, Swell Ratio, Crosslink Density,
Between Crosslinks,
kGy q n [mol/dm ]
d
10.8 Continue to monitor the temperature and sample di-
M [g/mol]
c
mension until equilibrium is achieved (within 610 µm) over a
54.2 3.37 6 0.26 0.133 6 0.017 7,650 6 1,010
period of 15 min. 71.5 3.12 6 0.24 0.151 6 0.021 6,720 6 920
89.2 3.12 6 0.24 0.152 6 0.020 6,700 6 890
10.9 Decrease the temperature to below 50°C. Discard the
110.1 2.97 6 0.31 0.170 6 0.032 6,150 6 1,190
o-xylene in an environmentally responsible manner, and clean
the sample cell thoroughly.
10.10 Examinethesampleafterthetestiscomplete.Ifithas
12.3 The expr
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