Process for producing high molecular weight polyethylene

Abstract

The invention relates to a process for producing an (ultra) high molecular weight polyethylene (HMWPE) article comprising: incorporating into the HMWPE resin a Hindered Amine Light Stabilizer (HALS) and cross-link the (U)HMWPE during or after molding the (U)HMWPE resin. In particular the invention relates to a process comprising the following steps: a) incorporating into (U)HMWPE resin a Hindered Amine Light Stabilizer (HALS) according to one of the following general formulas or combinations hereof: ##STR00001## wherein R.sub.1 up to and including R.sub.5 are herein independent substituents; for example containing hydrogen, ether, ester, amine, amide, alkyl, alkenyl, alkynyl, aralkyl, cycloalkyl and/or aryl groups, which substituents may in turn contain functional groups, for example alcohols, ketones, anhydrides, imines, siloxanes, ethers, carboxyl groups, aldehydes, esters, amides, imides, amines, nitriles, ethers, urethanes and any combination thereof; b) molding the (U)HMWPE resin comprising the HALS, resulting in an article; c) cross-linking and sterilizing the article via gamma radiation or electron beam radiation; d) optionally, if step b results in a stock shape, machining the stock shape into an article; wherein step c and step d can be performed in either order.

Claims

1. A process for producing an (ultra) high molecular weight polyethylene ((U)HMWPE) article comprising: (i) forming a composition by incorporating at least one Hindered Amine Light Stabilizer (HALS) compound into a (U)HMWPE resin having a molecular weight distribution (Mw/Mn) of between 2 and 18; and (ii) cross-linking the composition during or after molding by irradiating the composition with an irradiation dose of from 30 to 250 kGy, thereby forming an article having a cross-link density of 0.09 mol/dm.sup.3 or more.

2. The process according to claim 1, wherein the HALS compound is according to one of the following general formulas or combinations hereof: ##STR00004## wherein R.sub.1 up to and including R.sub.5 are herein independent substituents; for example containing hydrogen, ether, ester, amine, amide, alkyl, alkenyl, alkynyl, aralkyl, cycloalkyl and/or aryl groups, which substituents may in turn contain functional groups selected from the group consisting of alcohols, ketones, anhydrides, imines, siloxanes, ethers, carboxyl groups, aldehydes, esters, amides, imides, amines, nitriles, ethers, urethanes and any combination thereof.

3. The process according to claim 1, wherein the HALS compound is selected from the group consisting of 2,2,6,6-tetramethyl-4-piperidone; 2,2,6,6-tetramethyl-4-piperidinol; bis-(1,2,2,6,6-pentamethylpiperidyl)-(3,5-di-tert-butyl-4-hydroxybenzyl) butylmalonate; di-(2,2,6,6-tetramethyl-4-piperidyl) sebacate; oligomer of N-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-piperidinol and succinic acid; bis-(2,2,6,6-tetramethyl-4-piperidinyl) succinate; bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate; bis-(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate; N,N-bis-(2,2,6,6-tetramethyl-4-piperidyl) hexane-1,6-diamine; N-butyl-2,2,6,6-tetramethyl-4-piperidinamine; 2,2-[(2,2,6,6-tetramethylpiperidinyl)imino]-bis-[ethanol]; poly((6-morpholine-S-triazine-2,4-diyl)(2,2,6,6-tetramethyl-4-piperidinyl)-iminohexamethylene-(2,2,6,6-tetramethyl-4-piperidinyl)-imino); 5-(2,2,6,6-tetramethyl-4-piperidinyl)-2-cycloundecyloxazole); 8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro(4,5)decane-2,4-dione; polymethylpropyl-3-oxy[4(2,2,6,6-tetramethyl)piperidinyl)siloxane; copolymer of -methylstyrene-N-(2,2,6,6-tetra-methyl-4-piperidinyl)maleimide and N-stearylmaleimide; 1,2,3,4-butanetetracarboxylic acid, polymer with beta,beta,beta,beta-tetramethyl-2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol, 1,2,2,6,6-pentamethyl-4-piperidinyl ester; 2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol,beta,beta,beta,beta-tetramethyl-, polymer with 1,2,3,4-butanetetracarboxylic acid, 2,2,6,6-tetramethyl-4-piperidinyl ester; D-glucitol, 1,3:2,4-bis-O-(2,2,6,6-tetramethyl-4-piperidinylidene)-; oligomer of 7-oxa-3,20-diazadispiro[5.1.11.2]heneicosan-21-one, 2,2,4,4-tetramethyl-20-(oxiranylmethyl); propanedioic acid, [(4-methoxyphenyl)methylene]-,bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ester; formamide, N,N-1,6-hexanediylbis[N-(2,2,6,6-tetramethyl-4-piperidinyl; 1,3,5-triazine-2,4,6-triamine, 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); 1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic acid, bis (2,2,6,6-tetramethyl-4-piperidinyl) ester; 1,5-dioxaspiro (5,5) undecane 3,3-dicarboxylic acid, bis (1,2,2,6,6-penta-methyl-4-peridinyl) ester; N-2,2,6,6-tetramethyl-4-piperidinyl-N-amino-oxamide; 4-acryloyloxy-1,2,2,6,6-pentamethyl-4-piperidine; 1,3-benzenedicarboxamide, N,N-bis(2,2,6,6-tetramethyl-4-piperidinyl); 3-dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)-pyrrolidin-2,5-dione; 1,3-Propanediamine, N,N-1,2-ethanediylbis-,polymer with 2,4,6-trichloro-1,3,5-triazine, reaction products with N-butyl-2,2,6,6-tetramethyl-4-piperidinamine; 1,1-(1,2-ethane-di-yl)-bis-(3,3,5,5-tetra-methyl-piperazinone); 1,1,1-(1,3,5-triazine-2,4,6-triyltris ((cyclohexylimino)-2,1-ethanediyl)tris(3,3,5,5-tetramethylpiperazinone); 1,1,1-(1,3,5-triazine-2,4,6-triyltris((cyclohexylimino)-2,1-ethanediyl)tris(3,3,4,5,5-tetramethylpiperazinone); 1,2,3,4-Butanetetracarboxylic acid, tetrakis(2,2,6,6-tetramethyl-4-piperidinyl) ester; 1,2,3,4-Butane-tetra-carboxyllc acid, 1,2,3-tris-(1,2,2,6,6-penta-methyl-4-piperidyl)-4-tridecylester; mixture of esters of 2,2,6,6-tetra-methyl-4-pipiridinol and several fatty acids; Propanedioic acid, [(4-methoxyphenyl)methylene]-,bis(2,2,6,6-tetramethyl-4-piperidinyl) ester; 3-Dodecyl-1-(2,2,6,6-tetramethyl-4-piperidyl)-pyrrolidin-2,5-dione; 3-Dodecyl-1-(1,2,2,6,6-pentamethyl-4-piperidyl)-pyrrolidin-2,5-dione; 1,2,3,4-Butanetetracarboxylic acid, tetrakis(1,2,2,6,6-pentamethyl-4-piperidinyl)ester; 1,2,3,4-Butane-tetra-carboxyllc acid, 1,2,3-tris-(2,2,6,6-tetra-methyl-4-piperidyl)-4-tridecylester; mixture of: 2,2,4,4 tetramethyl-21-oxo-7-oxa-3.20-diazadispiro[5.1.11.2]-heneicosane-20-propionic acid dodecylester and 2,2,4,4 tetramethyl-21-oxo-7-oxa-3.20-diazadispiro[5.1.11.2]-heneicosane-20-propionicacid tetradecylester; 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]]}; 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); Poly[(6-morpholino-s-triazine-2,4-diyl)[1,2,2,6,6-penta-methyl-4-piperidyl)imino]-hexamethylene[(1,2,2,6,6 penta-methyl-4-piperidyl)imino]]1,6-Hexanediamine, N,N-bis(1,2,2,6,6-pentamethyl-4-pipiridinyl)-, Polymers with morpholine-2,4,6-trichloro-1,3,5-triazine; Poly-methoxypopyl-3-oxy[4(1,2,2,6,6-pentamethyl)-piperidinyl]-siloxane; 1,6-Hexanediamine, N,N-bis(2,2,6,6-tetramethyl-4piperidinyl)-polymer with 2,4,6-trichloro-1,3,5-triazine, reaction products with N-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine; reaction products of N,N-ethane-1,2-diylbis (1,3-propanediamine), cyclohexane, peroxidized 4-butylamino-2,2,6,6-tetramethylpiperidine and 2,4,6-trichloro-1,3,5-triazine; and 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, oxidized, hydrogenated.

4. 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]]}.

5. The process according to claim 1, wherein the HALS is 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]]}.

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

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

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

9. The process according to claim 1, wherein the HALS is present in an amount between 0.01 and 2% by weight, based on the total weight of the composition.

10. The process according to claim 1, wherein the HALS is present in an amount between 0.02 and 1% by weight, based on the total weight of the composition.

11. The process according to claim 1, wherein the irradiation dose is between 40 and 130 kGy.

12. An artificial medical implant comprising an article formed by the process according to claim 1.

13. An artificial medical implant comprising an article formed by the process according to claim 2.

14. An artificial medical implant comprising an article formed by the process according to claim 3.

15. An artificial medical implant comprising (U)HMWPE and at least one Hindered Amine Light Stabilizer (HALS) 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]]}, and wherein the artificial medical implant has a cross-link density of 0.09 mol/dm.sup.3 or more.

16. The artificial medical implant according to claim 15, wherein the artificial medical implant has been cross-linked by irradiation with gamma rays at a dose between 30 and 250 kGy.

17. The artificial medical implant according to claim 15, wherein the artificial medical implant has been cross-linked by irradiation with gamma rays at a dose between 40 and 130 kGy.

18. An artificial medical implant comprising (U)HMWPE and 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]]}, and having a cross-link density of 0.09 mol/dm.sup.3 or more.

19. The artificial medical implant of claim 18, wherein the artificial medical implant has been cross-linked by irradiation with gamma rays at a dose between 40 and 130 kGy.

20. The artificial medical implant according to claim 12, which is used for hip arthroplasty, knee replacement, shoulder replacement or spinal applications.

Description

EXAMPLES

Materials

(1) UHMWPE:

(2) 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

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

(4) Stabilizers:

(5) 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

(6) 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.

(7) Irradiation of the Samples

(8) 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.

(9) Preparation of the Samples

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

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

(12) Ageing:

(13) 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.

(14) Cross-Link Density Measurement

(15) 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.

(16) Colour Determination

(17) 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.

(18) Tensile Tests

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

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

(21) Oxidation Index Determination

(22) 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.

(23) Results:

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

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

(26) 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

(27) 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.

(28) 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

(29) 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.

(30) 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

(31) 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.

(32) 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

(33) 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.