Joanna Maszybrocka

Change of micromechanical properties of polyethylene induced by a tribological process in polymer/metal system

The influence of a tribological process in the polymer/metal system on changes in microhardness (H) and Young’s modulus (E) of the surface under friction and UHMWPE volume has been studied. Twelve samples of polyethylene characterized by a different degree of initial plastic deformation and different electron irradiation dose were analyzed. For four of the samples a decrease by 5÷10 times in polyethylene degradation has been found. This result is promising in that a prolonged durability can be expected in case of application for endoprostheses acetabular cups. It has been found also that a polyethylene material irradiated after initial prestraining only once shows enhanced micromechanical properties after friction. It has been found that this result was due to reorientation of lamellas on the friction surface taking place during deformation which accompanies the tribological process. It has been proved that the increase of H and E values in the layers situated at a depth of 1.5 mm÷7.5 mm is due to a decrease of the free volume centers in the UHMWPE polyethylene and to an increase of the degree of spatial arrangement of the structure (absorption in crystallinity band of FTIR spectrum).

The Influence of Plastic Deformation Performed before and after Electron Beam Irradiation on the Morphology and Structural Properties of UHMWPE

The paper presents changes in UHMWPE morphology and structure caused by irradiation with an electron beam and plastic deformation. The input material consisted of two grades of polyethylene, i.e. GUR 1050 and GUR 1020 (Poly Hi Solidur Medi TECH), used for producing the bearing components of endoprostheses applied in total joint alloplasty. Tests involved three groups of samples: the ones subjected to modification through radiation (N) only, those modified by plastic deformation before (ON) and after (NO) irradiation with an electron beam (N). The obtained variants were subjected to comprehensive morphological and structural tests, which made it possible to document changes in the lamellar structure (SEM) and orientation of the crystalline structure (SAXS) as well as to determine the content of the crystalline phase (DSC) and to define crosslinking density.

Changes in UHMWPE Morphology and Structure after Irradiation with an Electron Beam and Thermal Treatment

The paper presents changes in UHMWPE morphology and structure induced by exposure to a beam of electrons and thermal processing applied after the modification through irradiation. Two groups of samples were tested: those that underwent modification through irradiation only and those that underwent a sequential process which consisted of radiation modification and thermal processing. The obtained variants were subjected to comprehensive morphological and structural tests, which made it possible to: document changes in the lamellar structure (SEM); determine the content of crystalline phase (DSC); determine the degree and density of crosslinking; state the degree of polymer oxidation (FTIR); examine the influence of the irradiation dose on the intensity of free radicals occurrence (EPR) and to determine the effectiveness of their elimination using recrystallisation in an oxygen-free environment.

The Influence of Plastic Deformation Performed before and after Electron Beam Irradiation on Mechanical Properties and Wear of UHMWPE

The paper compares the modification methods of polymers GUR 1050 and GUR 1020 by means of plastic deformation before (ON) and after (NO) irradiation with an electron beam (N). Evaluation of the effectiveness of the modification methods applied was carried out by means of micromechanical examinations and tribological tests on T-01 and T-05 testers (polymer/Vitalium alloy). The results of micromechanical, sclerometric and tribological tests indicate unequivocally the effectiveness of the modifications applied, as compared to the polymer modification through irradiation with an electron beam only. A decrease in hardness and elasticity modulus was found for materials subjected to the ON and NO technology as compared to N, while maintaining the growing trend of these values along with the applied radiation dose. There was also noted a large increase of the wear resistance coefficient, Wβ, and an increase of the microploughing component’s share in the wear mechanism β, which had a direct influence on the reduction of the linear wear by 2.5–3 times and a nearly twofold decrease of the mass wear. The surface subjected to friction processes became smoother and free of lamella fragments which were visible for the base material not subjected to any modifications. This was documented with the observations performed on a scanning microscope and in the stereometric examination of the surface.

Changes of Free Radical Content in Exposed to Electrons GUR1020 and GUR1050 Polyethylene Types Intended for Endoprosthesis Acetabular Cups

The paper presents the results of the EPR examination of free radical content in GUR1020 and GUR1050 polyethylenes which were irradiated with an electron beam. Changes in the amount of radicals in the period of 12 months in the UHMWPE cross-linked with a dose of 26-156 kGy were analysed. Spectra registered 90 days after irradiation show a hyperfine structure. An image consisting of 7 characteristic lines is the most similar pattern that can be used to analyse this spectrum. Subsequent measurements (255 and 372 days after irradiation) produce images consisting of one resonance line. Alkyl and allyl radicals generated during the irradiation are the source of the EPR signal. The influence of atmospheric oxygen causes further changes of the radicals mentioned above, which leads to the formation of peroxyl radicals. The content of radicals in the polymer and its changes over time were calculated. The amount of radicals increases as the absorbed dose grows, but decreases over time. Comparative measurements of the material subjected to thermal processing at the temperature of 423 K have shown that in this case, no resonance lines are present, which testifies to the effective elimination of free radicals by means of UHMWPE recrystallisation.

Changes in the Molecular Structure of UHMW Polyethylene under the Influence of Thermal and Radiation Modification, as Well as Time

The paper presents the results of an examination of UHMW polyethylene type GUR1050 using the EPR, FTIR and SEM methods. The relationship between the number of radicals in the polymer and the absorbed dose of ionizing radiation has been shown. The radicals, being highly active macroparticles, disappear in time, so that after about a year their amount is reduced by approximately one order of magnitude. The disappearance of the radicals over time results in a change in EPR spectra, an increase in the degree of oxidation, and cross-linking. The most probable mechanisms are the formation of alkyl and allyl radicals and their gradual transitions, which lead to the absorption of atmospheric oxygen, and the recombination of radicals, which leads to the formation of cross linkages. Thermal treatment of the samples is an effective method of eliminating the radicals. Tribological tests and their analysis by means of SEM observation showed that the irradiated samples proved to have higher resistance to abrasion than the material that was not exposed to radiation.

The Influence of e-Beam Irradiation on the Morphology and Functional Properties of UHMWPE

The paper presents an analysis of the effectiveness of radiation modification of GUR 1050 and GUR 1020 polymers intended for endoprosthesis bearing components. It has been demonstrated through extraction and expansion that the crosslinking density increases as the applied dose of electron irradiation and the molecular weight of polyethylene grow. The recorded changes were accompanied by an increase in the degree of crystallinity, which was more significant in the case of the more heavy GUR 1050 (compared to GUR 1020). The morphological changes are supported by the results of micromechanical, sclerometric and tribological examinations. Tribological examinations were documented with SEM images of the surface of specimens subjected to oscillatory friction (roll-block tester). Series of crosswise valleys and projections, arranged along the friction direction, were found on the friction surface. The smoothening of the surface and a lack of lamella fragments on the base specimens which were not subjected to e-beam irradiation are observed along with a reduction of wear due to the increasing dose of irradiation.

The Influence of Plastic Deformation on Structure of Highly Crosslinked UHMWPE Intended for Endoprosthesis Cups

In this paper, an attempt has been made to determine the changes in the structure of UHMWPE under the influence of deformation, which can be the cause of the decrease of functional properties. It has been found that the degree of cross-linking slightly changes due to operational loads (deformation ef=0.15-0.6). The deformation leads, however, to the systematic lowering of the degree of crystallinity and an insignificant decrease of lamella width. Tests of wide- and small-angle X-ray scattering (WAXS, SAXS) prove that this is the result of operational deformation causing a rather weak texture of crystals and the fragmentation of some lamellae. Anisotropy of the lamellar structure also takes place, which changes 2D SAXS images into ellipses with a rapidly growing axial ratio with ef value.