Process for producing high molecular weight polyethylene

Abstract

Processes for producing an (ultra) high molecular weight polyethylene (HMWPE) article include incorporating into the HMWPE resin a Hindered Amine Light Stabilizer (HALS) and cross-linking the (U)HMWPE during or after molding the (U)HMWPE resin.

Claims

1. A process for making a cross-linked article from (ultra) high molecular weight polyethylene ((U)HMWPE) comprising steps of: (i) forming a composition by incorporating at least one Hindered Amine Light Stabilizer (HALS) compound in a (U)HMWPE resin, wherein the (U)HMWPE is a substantially linear ethylene homopolymer or copolymer with a molecular weight distribution (Mw/Mn) of between 2 and 18; (ii) adding an initiator to the composition; and (iii) molding the composition to form an article having a cross-link density of 0.09 mol/dm.sup.3 or more.

2. The process according to claim 1 comprising the steps of: a) forming a composition by incorporating in the (U)HMWPE a HALS compound according to one of the following general formulas or combinations hereof: ##STR00003## wherein R.sub.1 up to and including R.sub.5 are herein independent substituents selected from the group consisting of hydrogen, ether groups, ester groups, amine groups, amide groups, alkyl groups, alkenyl groups, alkynyl groups, aralkyl groups, cycloalkyl groups and aryl groups, wherein the substituents may in turn contain functional groups selected from the group consisting of alcohol groups, ketone groups, anhydride groups, imine groups, siloxane groups, ether groups, carboxyl groups, aldehyde groups, ester groups, amide groups, imide groups, amine groups, nitrile groups, ether groups, urethane groups and any combination thereof; b) adding an initiator and optionally a coagent to the composition; c) molding or extruding the composition to form a cross-linked article or stock shape; d) optionally further cross-linking and/or sterilizing the article or stock shape via gamma radiation or electron beam radiation; e) optionally, if step c) results in a stock shape, machining the stock shape into an article, wherein step d) and step e) can be performed in either order.

3. The process according to claim 1, wherein the (U)HMWPE has an intrinsic viscosity of 8 dl/g or more.

4. The process according to claim 1, wherein the (U)HMWPE comprises one or more comonomers.

5. The process according to claim 1, wherein the HALS is present in an amount of between 0.01 and 2% by weight, based on the total weight of the (U)HMWPE.

6. The process according to claim 1, wherein the HALS has a molecular weight of 1000 g/mol or more.

7. The process according to claim 1, wherein the HALS compound is selected from the group consisting of N,N-[1,2-ethanediylbis [[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-iperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N,N-dibutyl-N,N-bis(1,2,2,6,6-pentamethyl-4-piperidinyl) or poly{[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-hexamethylene [(2,2,6,6-tetramethyl-4-piperidinyl) imino]]}.

8. The process according to claim 1, wherein the HALS is incorporated in the (U)HMWPE by mixing the HALS with (U)HMWPE resin or (U)HMWPE melt, by impregnating (U)HMWPE resin with a solution comprising the HALS, or by spraying a solution comprising the HALS on (U)HMWPE resin.

9. The process according to claim 1, wherein the initiator is a peroxide selected from the group consisting of tert-butyl cumyl peroxide, tert-butyl peroxybenzoate, di-tert-butyl peroxide, 3,3,5,7,7-pentamethyl-1,2,4-trioxepane,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, butyl 4,4-di(tert-butylperoxy)valerate, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, di(4-methylbenzoyl) peroxide, dibenzoyl peroxide, di(2,4-dichlorobenzoyl) peroxide, dicumyl peroxide, 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, 1-(2-tert.-butylperoxyisopropyl)-3-isopropenyl benzene, 2,4-diallyloxy-6-tert-butylperoxy-1,3,5-triazine, di(tert-butylperoxyisopropyl)benzene, diisopropylbenzene monohydroperoxide, cumyl hydroperoxide, and tert-butyl hydroperoxide.

10. The process according to claim 1, wherein the initiator is present in an amount of between 0.001 and 2.5% by weight based on the total weight of the (U)HMWPE.

11. The process according to claim 1, wherein the initiator is present in an amount of between 0.01 and 1% by weight based on the total weight of the (U)HMWPE.

12. The process according to claim 2, wherein the coagent is selected from the group consisting of divinylbenzene, diallylphthalate, triallylcyanurate, triallylisocyanuarate, triallyltrimellitate, meta-phenylene bismaelimide, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate, trimethylopropane timethacrylate, trimethylopropane, timethacrylate, pentaerytritol tetramethacrylate, zinc diacrylate, zinc dimethacrylate, and polybutadiene.

13. The process according to claim 2, wherein the co-agent is present in an amount of between 0.001 and 2.5% by weight based on the total weight of the (U)HMWPE.

14. The process according to claim 2, wherein the co-agent is present in an amount of between 0.1 and 1% by weight based on the total weight of the (U)HMWPE.

15. The process according to claim 1, wherein the step of molding or extruding comprises compression molding, ram-extrusion, direct compression molding, or hot isostatic pressing.

16. The process according to claim 2, wherein the step of further cross-linking and/or sterilizing is performed with an irradiation dose of from 10 to 40 kGy.

17. The process according to claim 2, further comprising annealing the cross-linked article or stock shape at a temperature below the melting temperature of (U)HMWPE.

18. The process according to claim 2, further comprising annealing the cross-linked article or stock shape at a temperature of between 60 and 140 C.

19. A crosslinked (U)HMWPE article obtained by the process according to claim 1.

20. The article according to claim 19, having a thickness of at least 2 mm.

21. A medical implant which comprises the article according to claim 19.

Description

EXAMPLES

(1) Materials:

(2) UHMWPE:

(3) The used UHMWPE had an Intrinsic Viscosity, measured according to ISO 1628-3, of 27 dl/g, which corresponds with a molecular weight of 7.3 million g/mol, as calculated using Margolies equation Mw=53700[I.V].sup.1.49

(4) The average particle size of the used UHMWPE resin according to ISO 13320 was 157 micron.

(5) Stabilizers:

(6) Vitamin E; (Alpha tocopherol from DSM Nutrional Products) Poly{[[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-hexamethylene [(2,2,6,6-tetramethyl-4-piperidinyl) imino]]}; (Chimassorb 944 from Ciba Specialty Chemicals) 1,3,5-Triazine-2,4,6-triamine, N,N-[1,2-ethanediylbis [[[4,6-bis[butyl(1,2,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]]-bis[N,N-dibutyl-N,N-bis(1,2,2,6,6-pentamethyl-4-piperidinyl); (Chimassorb 119 from Ciba Specialty Chemicals) 1,6-hexanediamine, N,N-bis(2,2,6,6-tetramethyl-4-piperidinyl)-, polymer with 2,4,6-trichloro-1,3,5-triazine, reaction products with 3-bromo-1-propene, n-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine, oxidised, hydrogenated; (Tinuvin NOR 371 from Ciba Specialty Chemicals)
Preparation of Solvent Blended Compounds

(7) The stabilizers were added to the UHMWPE by solvent blending. The stabilizers were first added to the polymer as a solution in Chloroform (about 100 ml/100 gr polymer); in a second step the chloroform was evaporated.

(8) Irradiation of the Samples

(9) Irradiation was performed by gamma irradiation (dose 25, 75 and 150 kGray (kGy)) on stock samples under nitrogen that were vacuum sealed into paper bags with an aluminum coating on the inside. To prepare the test samples for the Swell test a stock sample was prepared that was irradiated and was later machined into the smaller test samples.

(10) Preparation of the Samples

(11) Powder was compression molded into samples according to ISO 11542-2.

(12) Needed sample dimensions for analyses were machined from the molded stock samples.

(13) Ageing:

(14) The samples for the tensile test and for color determination were aged during two weeks in an air venting oven (Binder FDL115) at 110 C.

(15) Cross-Link Density Measurement

(16) The cross-link density was determined according to ASTM F2214-02, using samples with the dimensions 5 mm*5 mm*5 mm that were machined out of stock samples that were irradiated. These samples were subjected to swelling in o-xylene.

(17) Colour Determination

(18) Colour determination was done according to ISO 7724-1-2-3 (CIELAB, D65, 10, d8). The determination was done in reflection using a black background with a Minolta spectrophotometer. As samples 1 mm thick plaques were used that were machined out of the stock samples after irradiation and ageing.

(19) Tensile Tests

(20) The tensile tests (elongation at break and ultimate tensile strength) were performed according to ISO 527.

(21) Tensile bars (Type ISO 527-5B) were punched from 1 mm thick samples, that were machined out of stock samples after irradiation and ageing.

(22) Oxidation Index Determination

(23) The oxidation indices were determined from the Infrared Spectra measured in transmission on coupes of about 100 m, which were cut from cubes of 5*5*5 mm. The spectra were recorded on a Perkin Elmer Auto Image using 20 scans and a resolution of 4 cm.sup.1. The spectra were normalized as in ASTM F2102-06 to 1370 cm.sup.1 (1330-1370, base 1400 cm.sup.1). The oxidation index was defined as the peak height at 1717 cm.sup.1 using a baseline drawn from 1680-1765 cm.sup.1.

(24) Results:

(25) TABLE-US-00001 TABLE 1 Color determination Radiation dose Example Stabilizer 0 kGy 25 kGy 75 kGy 150 kGy A 0 0.55 1.35 2.88 B 0.15 wt % 7.71 12.16 12.07 13.07 Vitamin E 1 0.05 wt % 0.14 0.47 1.39 3.00 Chimassorb 944 2 0.15 wt % 0.13 0.30 1.31 2.92 Chimassorb 944 3 0.05 wt % 0.21 0.41 1.31 2.84 Chimassorb 119 4 0.15 wt % 0.23 0.2 1.23 2.91 Chimassorb 119 5 0.05 wt % 0.36 0.46 1.48 2.93 Tinuvin NOR 371 6 0.15 wt % 0.94 1.44 2.29 3.41 Tinuvin NOR 371

(26) In table 1 the difference between the color of the b*-value of the different samples and the color of the reference sample (not stabilized, not irradiated sample) is given.

(27) From these results it was clear that the Vitamin E containing samples were more yellow than the HALS containing samples.

(28) TABLE-US-00002 TABLE 2 Cross-link density Cross-link density (Mol/dm.sup.3) for samples that were irradiated with different doses. Radiation dose Example Stabilizer 25 kGy 75 kGy 150 kGy C 0.148 0.223 0.234 D 0.15 wt % Vitamin E 0.087 0.160 0.215 7 0.05 wt % Chimassorb 944 0.145 0.258 0.335 8 0.15 wt % Chimassorb 944 0.157 0.233 0.381 9 0.05 wt % Chimassorb 119 0.114 0.213 0.196 10 0.15 wt % Chimassorb 119 0.131 0.187 0.248 11 0.05 wt % Tinuvin NOR 371 0.151 0.209 0.279 12 0.15 wt % Tinuvin NOR 371 0.120 0.162 0.323

(29) From the results in Table 2 it is clear that for the HALS stabilized samples a lower radiation dose is needed to get a cross-link density that is comparable with the Vitamin E containing sample.

(30) TABLE-US-00003 TABLE 3 Tensile strength Tensile strength (N/mm.sup.2) of samples that were irradiated with different doses, after ageing for two weeks at 110 C. Radiation dose 0 25 75 150 Example Stabilizer kGy kGy kGy kGy E 11.6 10.8 13.6 17.4 F 0.15 wt % Vitamin E 55 51 48.7 41.6 13 0.05 wt % Chimassorb 944 56.9 50.3 45.9 42.4 14 0.05 wt % Chimassorb 119 55.4 49.6 45.3 41.8 15 0.05 wt % Tinuvin NOR 371 56.2 48.5 45.1 40.6

(31) From these results it was clear that after ageing samples comprising 0.05 wt % HALS had a tensile strength that was comparable with a tensile strength for a sample comprising 0.15 wt % Vitamin E.

(32) TABLE-US-00004 TABLE 4 Oxidation index Oxidation index of samples that were irradiated with different doses after ageing for two weeks at 110 C. Radiation dose 0 25 75 150 Example Stabilizer kGy kGy kGy kGy G 9.74 8.96 10.5 8.8 H 0.15 wt % Vitamin E 0.012 0.042 0.095 0.215 16 0.05 wt % Chimassorb 944 0.112 0.019 0.184 0.239 17 0.05 wt % Chimassorb 119 0.024 0.139 0.276 0.210 18 0.05 wt % Tinuvin NOR 371 0.108 0.212 0.224 0.293

(33) From these results it was clear that 0.05 wt % HALS could prevent an increase of the oxidation index. The amount needed from the HALS was lower than the 0.15 wt % Vitamin E that was needed to obtain the same result.

(34) TABLE-US-00005 TABLE 5 Change in cross-link density Change in cross-link density (in mol/dm.sup.3) of samples that were irradiated with different doses after ageing for two weeks at 110 C. Radiation dose Example Stabilizer 25 kGy 75 kGy I 0.1 0.2 J 0.15 wt % Vitamin E 0.0 0.0 19 0.05 wt % Chimassorb 944 0.0 0.0 20 0.05 wt % Chimassorb 119 0.0 0.0 21 0.05 wt % Tinuvin NOR 371 0.0 0.0

(35) From these results it was clear that the HALS, as well as Vitamin E, were effective in preventing a decrease in cross-link density due to ageing.