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Campo, N.

Torrisi, L.

Visco, A.M.

EnIn the last years, an useful collaboration developed between the material engineers and the physicians of Messina University. The study of the intimate structure of a material, before and after its modification induced by an external ionizing radiation source (electrons, ions, gamma) requires the simultaneous presence of specialist in Chemistry, Physics and Engineering in order to define the best modification conditions and the consequent featuresof the new synthesized material. In particular the polyethylene, employed in different fields, such as microelectronics and biomedicine, was chosen as an important target to modify its properties through ions and electron beam irradiations. Ion beams, ith energy of the order of some hundred keV and doses of the order of 1014/cm2, are able to improve the polyethylene surface properties without change the pristine bulk. Instead, electron beams with energy of about 5 MeV and high dose, improve significantly bulk properties of the polymer. The effects of the ion and electron modifications were investigated with several physics characterization methods, as will be discussed in the following

Università del Salento: ESE - Salento University Publishing

 38   Abstract  In the last years, an useful collaboration de-veloped between the material engineers and the physicians of Messina University. The study of the intimate structure of a material, before and after its modification induced by an external ionizing radia-tion source (electrons, ions, gamma) requires the simultaneous presence of specialist in Chemistry, Physics and Engineering in order to define the best modification conditions and the consequent features of the new synthesized material. In particular the polyethylene, employed in different fields, such as microelectronics and biomedicine, was chosen as an important target to modify its properties through ions and electron beam irradiations. Ion beams, with energy of the order of some hundred keV and doses of the order of 1014/cm2, are able to improve the polyethylene surface properties without change the pristine bulk. Instead, electron beams with en-ergy of about 5 MeV and high dose, improve sig-nificantly bulk properties of the polymer. The ef-fects of the ion and electron modifications were investigated with several physics characterization methods, as will be discussed in the following.   INTRODUCTION   Ultra-high-molecular-weight-polyethylene (UHMWPE) is employed in the biomedical field due to its peculiar properties, such as the biocom-patibility, chemically inertia, electrical insulation, high mechanical resistance and lightness. Some fixed and mobile prosthesis are based on UHMWPE; the antifriction bearings of shoulder prostheses, the vertebral hip and knee (Fig.1) and discs are some examples [1].  In order to improve the already appreciable chemical and physical aspects, surface treatments by means of ion implantation and bulk treatments by means of the electrons treatment were investi-gated [2-4].  The ions implantation have been carried out N. Campo(1), A.M. Visco(1), L. Torrisi(2)  (1)University of Messina , Industrial Chemistry and Material Engineering Department, University of Messina, Italy  ninocampo@ingegneria.unime.it   avisco@unime.it (2)University of Messina, Physics Department,University of Messina, Italy lorenzo.torrisi@unime.it Polymer modifications due to absorption of different ioni-zating  radiations with 3 different ion types: N+, Ar+ and Xe+. It was demonstrated that the treatment with ions increases the UHMWPE wear resistance and decreases the permeability to the liquids [5-6].  The electrons treatment have been employed with high doses, of the order of the MGy, in oxygen presence. It induces an increase of the wear, the Young modulus and the tensile strength with the increasing of the electron dose.  The goals of the our research have been the follow-ing:  • to estimate the induced effects due to the Ar+, N+, Xe+ ions on the UHMWPE in terms of sur-face wear resistance;  •  to estimate the induced effect due to the elec-trons on the UHMWPE in terms of bulk com-pressive, traction and wear resistance.       EXPERIMENTAL SETUP  Material modification   Two modification methods were applied at the UHMWPE (GUR 1020, supplied by TICONA) in order to study and compare their effects:  1) Modification by means of ion implantation with N+, Ar+, Xe+ at 300 keV energy, with an im-plant current of 3 µA, in an ion implanter sta-Fig. 1.  An example of UHMWPE application to the intervertebral disk prostheses.  39   tion supplied by the University of Catania (Fig.2) at 400 keV. Ion implantation is per-formed in high vacuum conditions (10-7 mbar).  2) Modification by means of the electron use at 5 MeV energy and a current of 40 mA produced by a LINAC supplied by the Physics Depart-ment of Messina University (Fig. 3).  Electron irradiation is performed in air (1 atm).  The tables I and II show some data relatives to the type of ion and electron implantation, respec-tively. The effects of the ion implantation depend on the atomic number, energy, dose and dose-rate of the ion. The effects of the electron irradiation depend on the energy, dose and dose-rate of the electrons. Observing the data of the two tables, it’s evident that the two methods produce a great differ-ence in the deepness of penetration, which is of the order of hundred of nanometres for ions and centi-metres for electrons. The total stopping power of the electrons is, instead, very lower than that of the ions. The ion stopping power is nuclear, while the electron stop-ping power is electronic. The consequence is a lighter effect of the electron beam in the polymeric material with respect of that of the ion beam.  In the tables 1 and 2 are shown data relatives to the ion and electron implantation, respectively.  Table I. Table II.  Wear test   The traditional system "pin on disc" was real-ized with the aim to measure the wear resistance of the pure and radiation modified UHMWPE samples [7]. It’s composed of a motor, a speed meter, a guide and a fix a tip made of steel AISI 316L. The vertical force of the tip can be increased adding a know weight in order to apply a know pressure. In our system, the pressure produced by the tip on the polymer surface was maintained at 8.50 MPa by using a spherical with a tip roughness of 50 nm. The wear measurements were performed with sam-ples immersed in a bath containing distilled water at room temperature (23°C).   Tensile and compressive tests   In order to estimate the change, of the prop-erty of the polymer bulk due to electron irradiation, tests of traction and compression of the electron modified UHMWPE samples were performed. Such investigations use an universal test machine (LLOYD INSTRUMENTS LR10K) which can be used at temperatures ranging between room tem-perature (23 °C) and 60 °C. For the tensile tests was Fig. 2. The ion Implanter.  Fig. 3.  The LINAC. Ion type Ion energy (keV) Total stopping power (keV/µm) Range (nm) N+ 300 390.4 926 Ar+ 300 656.0 460 Xe+ 300 1662 212 Energy e (MeV) Total stopping power (keV/100µm) Range (mm) 0.5 18.0 2.8 1 14.5 6.9 5 12.8 39.0  40  used a cross-head speed of 1 mm/min, according to ASTM 638 protocol. For each radiation dose, 10 specimens were analyzed. The tensile sample length, width and thickness were  60 mm, 10 mm and 1 mm, respectively. Compression tests were performed according to ASTM D 695M protocol. Samples shape was cylindrical with a high of 20 mm and a base diameter of 10 mm. The mechanical tests permitted to measure the elastic module of the polymer (E), the yielding stress (σy), the elongation at break (εr) and the ultimate strength (σr) of the pristine and irradiated samples.   FTIR and DSC analysis   FTIR (transformed of Fourier to the Infra Red) spectra have been record with a Perkin Elmer 2000 spectrometer equipped with a MCT detector. ATR spectroscopy was performed in polymeric films before and after the ion irradiation. Special attention was devoted to investigate in the wave number range 600 – 3000 cm-1 containing informa-tion about the main C-H and C-C chemical bond absorption peaks.  Differential Scanning Calorimeter (DSC) analyses of a pristine and irradiated polymer have been performed using the mod. TA DSC-2920 in-struments at a heating rate of 10 °C/min. The ana-lysed sample weights varied in the range 10 ÷11,5 mg.   CONCLUSION AND RESULTS  The macroscopic modifications caused by the two particle irradiation techniques (ion and electron beams) in the UHMWPE cause an optical effect clearly visible in the following photographs of Fig. 4 and Fig. 5. The photos of UHMWPE samples N+ ion implanted and electrons beam irradiated vs. doses show a progressive darkening with respect to the pure un-irradiated polyethylene. The electron beam do not completely darkens the polymer, al-though the employed high energy produces high absorbed dose. This result is in agreement with the data of the tables upper discussed and it is due to the electrons which reach high deep into the poly-mer dilute their energy density along their long track. On the contrary, ions have high stopping power and release their energy in the first superfi-cial layers depositing high energy-density. Further-more, in the case of the ion beam, the color inten-sity let to define an ionic dose of threshold beyond which the sample is totally blackened (~ 3x1015 N+/cm2). At this threshold dose, all the material is full modified. The wear resistance of the pure un-treated and ion implanted UHMWPE samples has been expressed like weight loss of material as a function of the tip covered travel-space (L). Fig.5 shows the wear behavior of the pure and ion implanted UHMWPE. All the ion implanted samples show a better wear resistance with respect to the pristine UHMWPE. The polymer implanted with lighter ion (N+) show a better wear resistance on long times with respect to the sample implanted with heavy ions (Ar+ and Xe+). The wear resistance of the pristine and elec-tron beam modified UHMWPE samples decreases with increasing the dose (Fig.6). Only the samples irradiated with low electron dose (0.1 MGy) show a better wear resistance than the pristine ones. This unexpected result could be due to the oxygen pres-ence during the irradiation and/or to a too high ab-sorbed dose. Mechanical wear test suggests that the ion effect upon the wear resistance is better than that of the electron beam. Particularly, light ions, as the  N+ ions, are preferred to the heavy ones for their effect on longer stress times.  The mechanical tensile and compressive tests have been performed at two different temperatures: 23 °C and 60°C. Measurements give: the tensile and the compressive modules Et and Ec; the tensile and the compressive yield strength, σty and σcy; the a b c d ea b c d Fig. 5.  HMWPE irradiated with electron beam at a dose of: (a) 0 MGy ; (b) 0.8 MGy; (c) 1.6 MGy ; (d) 3.2 MGy. Fig. 4.  UHMWPE implanted with a dose of: (a) 0.5x1015 N+/cm2 ; (b)1x1015 N+/cm2; (c) 3x1015 N+/cm2; (d) 6x1015 N+/cm2; (e)10x1015 N+/cm2 .  Polymer modifications due to absorption of different ionizating  radiations  41   ultimate tensile strength, σr; the elongation at break, εr (table III). The measurements were performed in order to give the average value on 8 independent tests. The Et and Ec modulus were plotted as a func-tion of the absorbed dose and of the two tempera-tures in the Fig 7. Results indicate that the material stiffness increases with the irradiation dose with respect to the pristine UHMWPE stiffness. This dependence, for the tensile and the compressive modulus, is stronger at lower temperature (23 °C) with respect to the higher temperature (60°C).  These results suggest that the radiation dam-age increases the mechanical properties in terms of elastic modulus, for compressive and tensile stresses. Unfortunately, the material becomes more fragile, as demonstrated by the decreasing of the rupture charge to elongation and compressive stress (Table 3). In order to improve the mechanical prop-erties of the polymer without increase excessively its fragility, it’s very important to irradiate the ma-terial with an optimal electron dose. The dose of 0.8 MGy could be considered the value at which the polymeric properties are well improved maintaining low the fragility. The ion implanted UHMWPE samples are mechanically improved only on the polymer surface, as demonstrated by the micro-hardness measurements [7]. FTIR spectra of the electron implanted UHMWPE samples show the trans-vinyl bond for-mation at ~ 960 cm-1 at high electron beam doses (a), the C=C bond at ~ 1462 cm-1 due to a cross linking formation process (b) and the characteristic peaks in the range 1700- 1750 cm-1 due to oxidation process due to the electron beam irradiation in air presence (Fig.8) [8]. These peaks are well evi-denced at high electron beam doses (400 kGy) while are little evidenced at lower electron beam doses (100 kGy) and they are completely absent in the pristine samples The FTIR analysis shows a a) b) Fig. 6. Wear measurements in water of ion implated specimens (a) and electron beam irradiated ones (b). Fig. 7. Tensile (a) and compressive (b) mechanical parameters of elec-tron beam modificated UHMWPE. Table III a) b) N. Campo, A.M. Visco, L. Torrisi  42  difference between the two kind of modifications due to ions and electrons. High ion doses modify the UHMWPE samples producing cross links on the surface chains and darkening the surface colour. The electron beam modification is less strong than that observed for ion implantation and its use in air produces vinylic double bonds and oxidized species not completely cross linked. Electron irradiated surface appears not completely dark but it is modi-fied in deep, up to about 2-3 centimeters. DSC measurements confirms that UHMWPE modifications occur with both the irradiations and that a different result is induced by the two kind of charge particle. The melting temperature of the pure polymer decreases after the ion implantation (Fig.9a) and increases after the electron beam irra-diation (Fig. 9b), changing also the polymer crys-tallinity. Ion implantation and electron beam irra-diation are two physical techniques capable to mod-ify the polyethylene material in a different way since they modify the surface and the bulk proper-ties of UHMWPE, respectively.  The visible effect is a progressive darkening of the initially white polymer which becames brown after the electron beam exposure and com-pletely black only after the high ions doses implan-tation.  The ion implantation process modifies the UHMWPE surface in terms of an increasing of its wear resistance. The weight loss generally de-creases with increasing the ion dose used to implant the polymer. Furthermore, the UHMWPE im-planted with lighter ions shows a better wear resis-tance than the heavier ones over longer wear simu-lation times.  The ion implantation increases the wear re-sistance better than the electron beam irradiation. The not good wear resistance of the electrons modi-fied samples can be due probably to the oxygen presence during the irradiation and to the too low deposited electron density. The electron beam in-creases the bulk mechanical properties of the UHMWPE in terms of stiffness and ultimate strength.  Further studies will be carried out on UHMWPE samples modified with electrons in inert atmosphere and in vacuum by using at low doses with the final aim to increase the bulk wear resis-tance properties of this interesting polymeric mate-rial.   REFERENCES  [1] S.C. Scholes, A. Unsworth and A.A. Gold-smith, Phys. Med. Biol. 45 (2000) 3721-3725. [2] J. Chen, F. Zhu, H. Pan, J. Cao, D. Zhu, H.  Fig. 8.  FTIR of electron implanted UHMWPE. Fig. 9. DSC analysis of ion implanted (a) and electron beam modifi-cated (b) UHMWPE.  a) b) Polymer modifications due to absorption of different ionizating  radiations  43   Xu, Q. Cai, J. Shen and Z. H., Nucl. Instr. and Meth. in Phys. Res. B, 169 (2000) 26-30. [3] W. Shi, X.Y. Li and H. Dong, Wear 250 (2001) 544-552.  [4] T. Venkatesan, L. Calcagno, B.S. Elman and G. Foti, P. Mazzoldi and G. W. Arnold Eds., Chapter 8, Elsevier, Amsterdam, 1987 . [5] L. Torrisi , A.M. Visco and A. Valenza, Rad. Eff. and Def. in Solids 158 (2003) 621-633. [6] L. Torrisi , C. Gentile, A.M. Visco, N. Cam-po, Rad. Eff. and Def. in Solids 158 (2003), 731-741. [7] A. Valenza , A.M. Visco, L. Torrisi , N. Campo, Polymer 45, 1707-1715, ( 2004)  [8] R.M. Silverstain, G.C. Bassler and T.C. Morril, 3rd ed., John Wiley and Sons Eds.,New York, 1995. 8. N. Campo, A.M. Visco, L. Torrisi 

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