Analytical Techniques for Assessing the Effects of Radiation on UHMWPE

Transcription

1 Analytical Techniques for Assessing the Effects of Radiation on UHMWPE Stephen Spiegelberg Cambridge Polymer Group, Inc. Ward St. Somerville, MA Outline Common analytical techniques used in the industry Pre-irradiation Post-irradiation Case studies

2 Analytical Test Techniques Prior to Irradiation Powder Consolidated Resin Post-Irradiation Puck, Rod, or Slab Machined Component Prior To Irradiation Electron microscopy SEM, TEM Molecular weight analysis GPC, dilute solution viscometry, light scattering Chemical analysis Net ash, trace element, FTIR, DSC 2

3 Post-Irradiation Mechanical Analysis Tensile, compression, fatigue, J-integral, small punch Crosslink density sol-gel, transient swelling Differential scanning calorimetry crystallinity, melting point FTIR TVI, VI, OI, IR-crystallinity Pre-Irradiation Analysis 3

4 Electron Microscopy TEM SEM -Chlorosulfonic acid -Uranyl acetate -Gold coating -ESEM Molecular Weight Analysis Polymers are a distribution of chain lengths (molecular weights) #chains Molecular Weight Moments of distribution n M n 2 M 2 n 3 M 3 M z = m i= m i= nm i i nm z i z i 4

5 Molecular Weight Analysis M n m m 2 nm i i nm i i i= i= M m w m = = n nm i i i i= i= M PDI = M w n Wt fraction Mn Mw Mn Mw Molecular weight Molecular weight Nomenclature in Radiation Chemistry Chain scission degradation Crosslinking improvement G value = # of events (yield) per ev or radiation energy ev =.62-7 kgy.g 5

6 Radiation Effects on Molecular Weight [ ] n = n, + s x / av M M G G D N [ ] w = w, + s/2 2 x / av M M G G D N G(X)=.4, G(S)=.5 G(X)=.5, G(S)=.4.6E+7.2E+6.6 molecular weight.4e+7.2e+7.e+7 8.E+6 6.E+6 4.E+6 2.E+6 Mn Mw PDI PDI molecular weight.e+6 8.E+5 6.E+5 4.E+5 2.E+5 Mn Mw PDI PDI.E dose [kgy].e dose [kgy] Guven, O., Crosslinking and Scission in Polymers Measuring G-Values [ ] n = n, + s x / av M M G G D N [ ] w = w, + s/2 2 x / av M M G G D N M n - [ G G ] s x M w - [ G /2 2G ] s x D, radiation dose level 6

7 How to Measure Molecular Weight Dilute Solution Viscometry Viscosity-averaged Molecular Weight Gel Permeation Chromatography Gives molecular weight distribution Dilute Solution Viscometry Measures size of polymer chain Empirically related to molecular weight for linear polymers ASTM D2857, F42 η rel = ηη tt [ η] = [(ln η rel)/ c] = c lnh rel /c [h] Ubbelohde Huggins, J. Am. Soc. (942) Concentration, c 7

8 Relating to Molecular Weight [ η ] = K' M a Log [h] Yields M v (-2 below M w ) K and a determined empirically from monodisperse samples of known molecular weight function solvent system used Log M Polyethylene in decahydronapthalene (35C) 4.37 M vnom, = 5.37 [] η Polymer Handbook, Brandrup, ed. Gel Permeation Chromatography M Calibration standards Mix solution and filter columns Detector h(t) h time Complete dissolution difficult Lose high molecular weight components time 8

9 Post-Irradiation Analysis Swelling Studies Used to characterize degree of crosslinking in polymer networks Competition between free energy of mixing and free energy of elasticity Crosslinks prevent complete dissolution Polymer chains at rest In solvent at temperature 9

10 Swelling Theory Flory, Principals of Polymer Chemistry (953) Assumes a tetrafunctional network free ends do not contribute to elastic (retractive) forces m [ lnυ χ υ ] F = kt n + n [ kt ] 2 F = 2 3 el νe/2 3αs 3 lnαs νd = ( ln( q ) + q + χq )/ Vq 2 /3 Measure swell ratio Vf H f q = = V H 3 Swelling Studies Per ASTM D2765 Gravimetric approach Solvent loss Handling of hazardous hot solvent weight [mg] sample removed from solvent Sample weight Swell Ratio Swell Ratio time [s]

11 Swelling Studies Per new ASTM standard In Situ measurements Transient and steady state results Round Robin in progress lase r mi crometer probe sample H/H Hf 6.5 Series. T time [s] 8 Temperature [C] FTIR Oxidation Index (7 cm - ) Trans vinyl groups (965 cm - ) Terminal vinyl groups (9 cm - ) ir crystallinity (33 cm - ) Good spatial resolution (2 µm)

12 FTIR absorbance [A.U.] Oxidation Trans-vinyl Terminal vinyl %crystallinity Hydroperoxides frequency [cm - ] FTIR µ-ftir 2 µm 3 µm 25 µm x-y stage wavelength [cm-] 2

13 Sterilized (nitrogen), bomb-aged 7 6 Oxidation Index [75/222] days 23 days 5 days days Factor of X position [mm] Electron Spin Resonance Detects residual free radicals in system A-C: 3- kgy Multiple peaks indicate free radicals resulting from peroxide disassociations 2 Nitrogen Stored 5 5 Intensity A A B C -5 C - -5 B Gauss 3

14 Mechanical Analysis J-Integral Testing: Crack Propagation J [kj/m 2 ] Troom 9.2 mm OFFSET LINE 8 y =.57x x x JQ 3 2 aq.5 mm EXCLUSION LINE crack length [mm]. ASTM J-83.2 in.8 in J (diameter).2 I in Specimens usually in plane stress conditions Extensive stress whitening (microcavitation) Not a good test for ductile materials Mechanical Analysis Uniaxial compression 33% modulus reduction with temp 25 to 37C 5% modulus reduction with radiation Very sensitive to compliance in grip, sample slippage, sample uniformity stress [Pa] 3.E+7 2.5E+7 2.E+7.5E+7.E+7 5.E+6.E+ E=.25 Gpa.5..5 strain [mm/mm] 4

15 Other Mechanical Techniques Tensile testing (ASTM D299, D638) Fatigue (ASTM E647) Small Punch (Kurtz et al. 999) Case Study : Effects of Nitrogen Packaging on Shelf- Storage Used micro-ftir to examine oxidation index of shelfstored gamma sterilized UHMWPE inserts sterilized in nitrogen packaging Silvio Schaffner, Sulzer Orthopedics 5

16 Procedure g-irradiated, kgy Co6 Shelf-age 5- yrs GUR 2 N 2 or air filled No white band formation observed in N 2 -packaged samples 2 mm Air-sterilized 2 oxidation index (A7/A9) Air years depth (mm) 6

17 Nitrogen sterilized 2 oxidation index (A7/A9) Air years Nitrogen years Nitrogen 5 years depth (mm) Case Study II: Effects of MWD on Crosslinking Used GPC to determine molecular weight distribution before crosslinking Swelling data to determine crosslink density after chemical and radiation crosslinking A. Edidin, Stryker Howmedica Osteonics S. Kurtz, Exponent 7

18 UHMWPE Ticona (TX) Material Density (g/cc) M v, g/mol IV (dl/g) , million 2 5/ million 28 Molecular weight distributions Weight fraction (%) E+3.E+4.E+5.E+6.E+7.E+8 molecular weight [g/mole] 8

19 Crosslink density dependence on Molecular Weight.25 crosslink density [mole/dm 3 ] kgy required for UHMWPE.E+4.E+5.E+6.E+7 molecular weight [g/mole] Mn: number-averaged Mw: weight-averaged D gel ( ) 7.5 = G X M w High M w Low M w Same dose level/ peroxide concentration Highly crosslinked Minimally crosslinked 9

20 Conclusions Suite of testing procedures Pre- and Post-irradiated UHMWPE Proper test design and data interpretation useful for manufacturing and R&D Ward St. Somerville, MA Go to for application notes on these subjects 2