ASTM F2214-23

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
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Designation: F2214 − 16 F2214 − 23
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
1.1 This test method describes how the crosslink density, molecular weight between crosslinks, and number of repeat units
between crosslinks in ultra-high 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 healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D2765E177 Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene PlasticsPractice for Use
of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 crosslink density, ν —the theoretical average number of crosslinks per unit volume [mol/dm ].
d
This test method is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.15 on Material Test Methods.
Current edition approved Oct. 1, 2016Sept. 1, 2023. Published October 2016October 2023. Originally approved in 2002. Last previous edition approved in 20082016 as
F2214 – 02 (2008).F2214 – 16. DOI: 10.1520/F2214-16.10.1520/F2214-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2214 − 23
3.1.2 molecular weight between crosslinks, M —the theoretical average molecular weight between crosslinks [g/mol].
c
3.1.3 swell ratio, q —the ratio of the volume of the sample in an equilibrium swollen state to its volume in the unswollen state.
s
4. Summary of Test Method
4.1 The height of a cubic specimen is measured, and the specimen is placed in a dry chamber. A selected solvent is chosen
according to the Flory network theory and is introduced into the chamber. The chamber is heated to the reference temperature. The
sample height is monitored as a function of time until steady state (equilibrium) is achieved. The swell ratio is calculated from the
final steady state (equilibrium) height and the initial height.
5. Significance and Use
5.1 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.
5.2 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.
5.3 The data can be used for dose uniformity analysis, fundamental research, and quality assurance testing.
6. Apparatus
6.1 The apparatus shall include any device that allows a non-invasive measurement of the change in one dimension of the sample
as it swells in the solvent. This measurement could include, but is not limited to:
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 of a material that exhibits little thermal expansion, such as quartz
or ceramic.
6.1.2 Optical measurements, such as cameras or laser micrometers.
6.1.2.1 Optical measurements should be insensitive to any refractive index changes in the UHMWPE sample, given the changing
temperature of the system.
6.1.3 Inductive measurements, such as proximity sensors. Inductive measurements must be insensitive to temperature or solvent
composition.
6.2 The sensitivity of the measurement shall be 1 % of the initial height of the sample, H . An uncertainty analysis has
demonstrated that this sensitivity will produce a relative error in crosslink density less than 10 % for samples swollen to a fraction
50 % beyond their initial height. Thicker samples will allow a less sensitive measurement.
6.3 The solvent in the temperature chamber shall be able to reach a temperature of at least 150°C,150 °C, with an expanded
uncertainty of 61°C.61 °C. Gradients shall not exceed 0.2°C/cm0.2 °C ⁄cm (NB o-xylene boils at 144°C).144 °C).
6.4 The smallest chamber dimension shall be at least three times the size of the largest initial sample dimension.
6.5 The volume of the chamber shall be at least ten times that of the sample. The chamber should be sufficiently sealed as to
prevent gross solvent evaporation during the course of the experiment (typically 3 to 8 h, depending on the crosslink density).
NOTE 1—The data acquisition software should collect both sample dimension and temperature at a rate of at least 0.1 Hz.
7. Reagents
7.1 Ortho-Xylene (o-xylene), ≥98 %, boiling point 144°C.144 °C.
F2214 − 23
FIG. 1 Marked Measurement Direction Before (a) and After (b) Swelling
7.2 Anti-oxidant, 2,2'-methylene-bis (4-methyl-6-tertiary butyl phenol).
8. Safety Precautions
8.1 O-xylene is toxic and flammable, and should be handled only with heat and chemically protective laboratory gloves. The
swelling apparatus should ideally be placed inside a vented fume hood, or vented with an elephant trunk should space
considerations be an issue. Do not inhale the o-xylene vapors, as dizziness or a headache could result.
8.2 Irganox 1010, the antioxidant, is identified by the manufacturer as an irritant and an inhalation hazard.
9. Test Specimens
9.1 At least three specimens with a minimum sample height of 500 μm should be machined. The top and bottom surfaces should
be parallel and smooth. The width and length (or diameter, in the case of cylindrical samples) should be less than one third the
size of the sample chamber (see 6.4). The height-to-width aspect ratio should be at least 1:2 to minimize buckling, with 1:1
preferred. The machining should be performed so as to minimize thermal degradation of the samples.
9.2 Orientation of Samples—Given that the swelling behavior can depend on molecular alignment induced by processing
conditions, the test specimens should be machined so that the relevant processing direction can be easily identified. The samples
can then be oriented in the swelling apparatus relative to the molding direction (that is, perpendicular to the extrusion of
compression molding direction). The specimens can be marked as shown in Fig. 1 to aid in sample alignment.
9.3 If the radiation dose can differ from the surface to the center of the sample, the location in the part where the specimen is taken
should be noted, as the swell ratio will depend on the radiation dose.
10. Procedure
10.1 Add approximately 0.5 to 1 % (mass fraction) of the antioxidant to the o-xylene to make a stock solution.
10.2 The initial sample height should be measured with a resolution of 1 % of the sample height using a micrometer. This value
should be recorded. The measurement direction on the sample can be indicated with a permanent marker. An example is shown
in Fig. 1.
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
10591–9005.10591-9005.
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10.3 The sample should be pre-wet with o-xylene, then quickly placed in the dry chamber with the sample correctly oriented as
marked in 10.2.
10.4 The initial sample dimension, as determined with the measurement system of the instrument described in 6.1, should be
recorded.
10.5 Start recording the sample dimension at a minimum rate of 1one point every 10 s.
10.6 Introduce the o-xylene stock solution into the chamber at a slow rate to prevent disturbing the sample.
10.7 Raise the temperature of the solvent in the chamber to 130 6 1°C.1 °C.
10.8 Continue to monitor the temperature and sample dimension until equilibrium is achieved (within 610 μm) over a period of
15 min.
10.9 Decrease the temperature to below 50°C.50 °C. Discard the o-xylene in an environmentally responsible manner, and clean
the sample cell thoroughly.
10.10 Examine the sample after the test is complete. If it has shown signs of cracking, or is yellowed, thermal degradation is likely
to have occurred. This data will be suspect and should be discarded.
11. Calculation of Swell Ratio
11.1 The swell ratio, q , is computed as indicated from the height measurement:
s
q 5 ~V /V !5 ~H /H ! (1)
s f 0 f 0
where:
V = final volume,
f
V = initial volume,
H = final height, and
f
H = initial height.
NOTE 2—This calculation assumes that the sample is isotropic.
12. Calculation of Crosslink Density and Molecular Weight Between Crosslinks
12.1 Given the steady state swell ratio, q , of a polymer immersed in a specific solvent at a particular temperature, the crosslink
s
density, molecular weight between crosslinks, and number of crosslinks/chain can be computed if one knows the Flory interaction
parameter, χ , for the polymer-solvent system.
12.2 From Flory’s network theory, which explains the swell ratio of a polymer-solvent system as a competition between elastic
forces and forces derived from the free energy of mixing, the following expression is derived for the crosslink density, ν , as a
d
4,5
function of the steady state swelling ratio, the Flory interaction parameter, and φ , the molar volume of the solvent.
21 21 22
ln ~12 q !1q 1χ q
s s 1 s
ν 52 (2)
x 21/3 21
φ q 2 0.5q
~ !
1 s s
12.2.1 The expression in Eq 2 assumes a three-dimensional network composed primarily of “H-bonding,” or the formation of
crosslinks along the main chain rather than at the chain ends. Additionally, network entanglements may partially contribute to the
refractive forces. Thus the calculated crosslink density will account for these contributions as well. The expression in Eq 2 has been
shown to be valid for swelling ratios up to q = 10, or M >10 000 g/mol.
c
Flory, P. J., Principles of Polymer Chemistry, Ithaca and London, Cornell University Press, 1953.
Flory, P. J., and Rehner, J., “Statistical mechanics of cross-linked polymer networks. II. Swelling,” J. Chem. Phys., Vol 11, No. 11, 1943, pp. 521–526.
F2214 − 23
TABLE 1 Summary of Mean (±S , Absolute Interlaboratory Uncertainty) Swell Ratio (q), Crosslink Density (ν ), and Molecular Weight
R d
Between Crosslinks (M ) for the Four Samples
c
Molecular Weight
Dose, Swell Ratio, Crosslink Density, Between
kGy q ν [mol/dm ] Crosslinks,
d
M [g/mol]
c
0.17 ±
54.2 3.37 ± 0.26 5,475 ± 804
0.02
71.5 3.12 ± 0.24 0.20 ± 0.03 4,741 ± 723
89.2 3.12 ± 0.24 0.20 ± 0.03 4,724 ± 698
110.1 2.95 ± 0.32 0.23 ± 0.05 4,235 ± 998
TABLE 1 Swell Ratio (unitless)
Number of Repeatability Reproducibility Repeatability Reproducibility
A
Average
Material Laboratories Standard Deviation Standard Deviation Limit Limit
x¯
n S S r R
r R
A-GUR 1050, gamma (nitrogen) 6 2.950 0.183 0.332 0.512 0.930
crosslinked to 11
...