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Publication numberUS20030139555 A1
Publication typeApplication
Application numberUS 10/243,636
Publication dateJul 24, 2003
Filing dateSep 12, 2002
Priority dateSep 13, 2001
Also published asDE60217053D1, DE60217053T2, EP1312636A1, EP1312636B1, US7204947, US20050070625
Publication number10243636, 243636, US 2003/0139555 A1, US 2003/139555 A1, US 20030139555 A1, US 20030139555A1, US 2003139555 A1, US 2003139555A1, US-A1-20030139555, US-A1-2003139555, US2003/0139555A1, US2003/139555A1, US20030139555 A1, US20030139555A1, US2003139555 A1, US2003139555A1
InventorsNeil Hubbard, Cherryl Cooper
Original AssigneeHubbard Neil Trevor, Cooper Cherryl Ann
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method of crosslinking polyolefins
US 20030139555 A1
Abstract
A method of forming an engineering component comprising the steps of:
subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.
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Claims(9)
1. A method of forming an engineering component comprising the steps of:
subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.
2. A method as claimed in claim 1 wherein the dosage rate is less than 3 kGy/hour.
3. A method as claimed in claim 2 wherein the dosage rate is less than 1.5 kGy/hour.
4. A method as claimed in a preceding claim wherein the polyolefin is an unblended homopolymer.
5. A method as claimed in claim 4 wherein the polyolefin is polyethylene.
6. A method as claimed in claim 5 wherein the polymer is ultra high molecular weight polyethylene.
7. A method as claimed in any preceding claim wherein the crystallinity of the crosslinked polyolefin is at least 40%.
8. An engineering component comprising polyolefin crosslinked in accordance with the method of any preceding claim.
9. A surgical implant or prostheses comprising a polyolefin crosslinked in accordance with the method of any of claims 1-7.
Description
  • [0001]
    This invention relates to the method of crosslinking polyolefins, particularly but not exclusively polyethylene using gamma radiation. Cross-linking is beneficial as it improves the wear resistance of polyethylene used in orthopaedic implants and other engineering applications, especially ultra-high molecular weight polyethylene (UHMWPE) used for these applications.
  • [0002]
    The crosslink density, that is the distance between bonds, is proportional to the radiation dose received. Wear resistance increases with higher crosslink density that occurs following use of higher doses of radiation. The detrimental counter effect of crosslinking is to reduce many mechanical and physical properties of the polymer. This reduction also occurs in proportion to the dose received. Higher doses cause greater reduction in physical properties. A reduction in strength may lead to a physical failure of the component. The maximum dose, and hence the maximum enhancement of wear that can be used in a particular circumstance is limited by this reduction in other physical properties. Commercial highly crosslinked UHMWPE has previously been treated with a specified total gamma radiation dose. The actual dose rate has not been considered important and has generally been left to the convenience of a vendor or contractor. Typically the crosslinking of polyethylene has been carried out at a dose rate of about 5 kilo Gray (kGy) per hour or higher (0.5 MRad/h).
  • [0003]
    Accordingly to the present invention a method of forming an engineering component comprises the steps of:
  • [0004]
    subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.
  • [0005]
    In a preferred method the dosage rate is less than 3 kGy/hour, more preferably less than 1.5 kGy/hour.
  • [0006]
    The dosage time is adjusted to provide a sufficient total dosage to cause efficient crosslinking and sterilisation. The total dosage may be selected by conventional means. A common total dose of 100 kGy may be used, although doses from 40 kGy to more than 102 kGy may be used for UHMWPE for orthopaedic prostheses and implants.
  • [0007]
    By reducing the dose rate to below 5 kGy/hour, preferably below 3 kGy/hour and most preferably below 1.5 kGy/hour, there is a significant improvement in the mechanical properties, particularly the crystallinity, impact strength and elongation at break. Conventional cross-linking by irradiation at higher dosage rates may decrease the crystallinity of UHMWPE from 50% to 35% for a total dose of 100 kGy. The use of the lower dose rate in accordance with this invention can maintain a level of crystallinity over 40%, leading to a consequent reduction in the loss of impact strength and elongation at break; The loss of these mechanical properties has been found to be less at lower dose rates for the same total dose level.
  • [0008]
    The polyolefin is preferably a polyalphaolefin, preferably selected from polyethylene, polypropylene and copolymers and blends thereof. Use of ultra high molecular weight polyethylene with molecular weight>1106 g/mol preferably>3106 g/mol is especially preferred.
  • [0009]
    The invention is further described by means of example but not in any limitative sense.
  • EXAMPLE 1
  • [0010]
    Ultrahigh molecular weight polyethylene (UHMWPE) was crosslinked by gamma radiation from a cobalt 60 source at four different dose rates to the same total dose (100 kGy). The dose was assessed by dosimeters in accordance with BS EN 552. The exercise was repeated to provide 3 sets of test materials.
  • [0011]
    The degree of crosslinking (cross-link density) was measured using a SRT 1 (Swell Ratio Tester) supplied by Cambridge Polymer Group of Sommerville Mass., USA to the draft ASTM standard D27651 in accordance with the procedure used for the round robin tests. The four samples within each set were the same (no significant difference) thus demonstrating the same crosslink density and no effect of dose rate.
  • [0012]
    Mechanical and physical properties were measured in accordance with the standards stated in Table 1 and the results analysed using Student's t test for matched pairs to demonstrate significance.
  • [0013]
    The dose rate was demonstrated to have a significant1 effect on the Impact Strength, Elongation at break and Crystallinity. Lower dose rates provided materials with significantly better properties than those produced at high dose rates.
    TABLE 1
    Cystal- Impact Elongation
    Dose Rate Swell Crosslink linity Strength %
    kGy/hr Ratio Density % kJ/m2 ASTM F
    Method Calculation ASTM D27652 D.S.C3 Izod4 648
    DR1 1.0 3.1 0.15 42.3 63 239
    DR2 1.8 3.0 0.16 38.3 62 235
    DR3 6.1 30 0.165 36.7 58 234
    DR4 7.3 3.1 0.155 34.4 59 224
  • EXAMPLE 2
  • [0014]
    Orthopaedic grade, ram extruded GUR 1050 rods of 65 mm diameter were manufactured for gamma irradiation in air to a dose level of 100 kGy. Mapping of the irradiation plant was carried out using dosimetry to determine the placement of rods to achieve the specified nominal dose rates. Each set of four rods were irradiated at different nominal dose rates of 1, 2, 6, and 7.5 kGy per hour.
    TABLE 2
    Actual Dose Level
    Nominal Dose Rate KGy Range
    Dose rate kGy/hr kGy/hr EN 552 kGy
    1.0 1.0 99.9-101.4 1.5
    2.0 1.8 97.7-100.8 3.1
    6.0 6.1 95.8-102.7 6.9
    7.5 7.3 96.3-104.8 8.5
  • [0015]
    The rods were melt annealed in an air atmosphere at 150 C. with a slow cool down rate to ambient temperature. The rods were machined into test specimens with a minimum sample size of six for each dose rate. Tensile Strength, Yield Strength and Elongation at Break were determined in accordance with ISO 527 using Type 5 specimens. Impact Strength testing conformed to ASTM F648-00 Annex A1 using double notch Izod specimens. Crosslink density and swell ratio was determined using the SRT-1 (Cambridge Polymer Group) to the draft ASTM standard 3.2 Mar. 1, 2001. Samples of 150 μm were tested on a Nicolet FTIR with microscope to determine the Transvinyl Index (TVI) (Muratoglu, O.K.et al., 47th ORS 2001 p. 1013) and a Netsch Differential Scanning Calorimeter to determine the crystallinity. Gamma irradiation using the above mentioned dose rates and testing was carried out on three independently crosslinked sample sets. Statistical analysis was performed using Graphpad software and a p-value <0.05 was used to establish significance. A comparison was made to rods irradiated during a production run of two hundred rods of the same diameter and dose level, but using a dose rate of 0.4 kGy per hour.
    TABLE 3
    Radiation Rate in kGy/Hour
    Property 0.4 1.0 1.8 6.1 1.8
    Impact 64 63 62 59 59
    Strength
    kJ/m2
    Yield 20.4 19.6 19.5 19.4 19.3
    Strengh
    MPa
    Tensile 44.2 43.2 41.3 41.1 42.8
    Strength
    MPa
    Elongation at 246 239 236 234 224
    break %
    Swell Ratio 3.04 3.14 3.00 2.97 3.06
    Cross-link 0.16 0.15 0.16 0.16 0.16
    Density
    Mole/dm3
    Crystallinity 43.0 42.3 38.3 36.7 33.9
    %
  • [0016]
    Izod impact strength, elongation at break, yield strength and crystallinity showed a significant decrease with increasing dose rate (p<0.05). The square of the correlation coefficient (R2) for yield strength versus crystallinity was 0.9985. The square of the correlation coefficient (R2) for Izod impact strength versus dose rate was 0.9998. The level of crystallinity reduced by 20% with increasing dose rate whilst swell ratio and cross-link density showed no statistical significance between dose rate.
Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US5879400 *Feb 13, 1996Mar 9, 1999Massachusetts Institute Of TechnologyMelt-irradiated ultra high molecular weight polyethylene prosthetic devices
US6017975 *Aug 15, 1997Jan 25, 2000Saum; Kenneth AshleyProcess for medical implant of cross-linked ultrahigh molecular weight polyethylene having improved balance of wear properties and oxidation resistance
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US7344672Oct 13, 2004Mar 18, 2008Biomet Manufacturing Corp.Solid state deformation processing of crosslinked high molecular weight polymeric materials
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US8398913Mar 19, 2013Biomet Manufacturing Corp.Solid state deformation processing of crosslinked high molecular weight polymeric materials
US8641959Jul 24, 2008Feb 4, 2014Biomet Manufacturing, LlcAntioxidant doping of crosslinked polymers to form non-eluting bearing components
US9017416Jun 21, 2010Apr 28, 2015Derek J. McMinnMethod of forming a polymer component
US9017590Mar 5, 2013Apr 28, 2015Biomet Manufacturing, LlcSolid state deformation processing of crosslinked high molecular weight polymeric materials
US9283079Feb 25, 2013Mar 15, 2016Derek James Wallace McMinnCup with crosslinked polymer layer cable ties
US9421104Feb 3, 2014Aug 23, 2016Biomet Manufacturing, LlcAntioxidant doping of crosslinked polymers to form non-eluting bearing components
US20060079595 *Oct 13, 2004Apr 13, 2006Schroeder David WSolid state deformation processing of crosslinked high molecular weight polymeric materials
US20060079596 *Oct 13, 2004Apr 13, 2006Schroeder David WCrosslinked polymeric material with enhanced strength and process for manufacturing
US20090030524 *Jul 24, 2008Jan 29, 2009Biomet Manufacturing Corp.Antioxidant doping of crosslinked polymers to form non-eluting bearing components
US20090082546 *Dec 4, 2008Mar 26, 2009Biomet Manufacturing Corp.Crosslinked polymeric material with enhanced strength and process for manufacturing
US20090212343 *Apr 2, 2009Aug 27, 2009Actel CorporationNon-volatile two-transistor programmable logic cell and array layout
US20100298945 *Aug 3, 2010Nov 25, 2010Biomet Manufacturing Corp.Crosslinked polymeric material with enhanced strength and process for manufacturing
US20100314800 *Aug 3, 2010Dec 16, 2010Biomet Manufacturing CorporationSolid state deformation processing of crosslinked high molecular weight polymeric materials
US20110153025 *Jun 21, 2010Jun 23, 2011Mcminn Derek JMethod of Forming a Polymer Component
Classifications
U.S. Classification526/348
International ClassificationA61L27/16, B29C35/08
Cooperative ClassificationB29K2023/0683, B29C2035/085, B29C35/0805, A61L27/16, B29K2995/0087, B29L2031/7532
European ClassificationA61L27/16, B29C35/08B
Legal Events
DateCodeEventDescription
Dec 24, 2002ASAssignment
Owner name: PERPLAS LIMITED, UNITED KINGDOM
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:HUBBARD, NEIL TREVOR;COOPER, CHERRYL ANN;REEL/FRAME:013604/0029;SIGNING DATES FROM 20021121 TO 20021126