Development of innovative techniques for the manufacture of bioresorbable composites intended as fracture fixation devices

Barrera Betanzos, Fernando (2018) Development of innovative techniques for the manufacture of bioresorbable composites intended as fracture fixation devices. PhD thesis, University of Nottingham.

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In this project innovative manufacturing processes have been developed and characterised for the efficient and effective fabrication of PGF-PLA composites which have adequate properties for the fixation of fractured bones in flat and curved geometries.

Fully bioresorbable composite plates where produced by compression moulding of unidirectional phosphate glass fibre mats and PLA films alternatively stacked inside the mould cavity. The implementation of cyclic pressure during the compression moulding consolidation stage resulted in the production the strongest PGF-PLA composites hitherto reported. The strength of the composites consolidated via cyclic pressure was at least 30% higher with respect to the control samples fabricated using the conventional static pressure profile, reaching values of 480 MPa for plates reinforced with 0.45 fibre volume fraction. The increases in composite strength were attributed to the influence of pressure cycling on the fibre network permeability, melt viscosity and capillary pressure, leading to improved fibre impregnation with respect to statically applied pressure, obviating the need for elevated processing temperatures to reduce the melt viscosity. Pressure cycling seemed to promote the formation of transcrystalline layer around the fibres which could have also contributed to the superior properties of composites consolidated under cyclic pressure.

PGF-PLA composite rods with cylindrical cross sections were manufactured via uniaxial compression and plane strain forging of preconsolidated composite blanks obtained from compression moulded composite plates. Forging under uniaxial conditions reduced both the flexural strength and the elastic modulus of all the composite rods in comparison to the compression moulded composite plates by as much as 85% and 47%, respectively, for the 0.35vf samples consolidated under cyclic pressure, as a result of the fracture of the fibres consequent to the extensional flow.

Fibre fracture was prevented through confinement of the deformation to the x-y plane during plane strain forging. The mechanical properties of the compression moulded composite plates were preserved in the forged composite rods with 0.15 and 0.25 fibre volume fractions, but the high segregation of fibres in the 0.35vf and 0.45vf compression moulded composite plates fabricated with thick glass fibre mats led to the reduction of both the flexural strength and modulus of the forged composite rods with respect to the compression moulded composite plates by as much as 74% and 29%, respectively, for the 0.45vf consolidated under cyclic pressure, as a result of the nucleation of intra-ply cracks during plane strain forging. The deleterious effect of intra-ply cracks in the plane strain forged composite rods was significantly reduced by stacking a larger number of thinner phosphate glass fibre mats prior to compression moulding of the 0.35vf and 0.45vf composite plates.

The properties of the forged composite rods decreased in aqueous media. The initial loss of strength and modulus was attributed to the water-mediated lubrication of the fibre-matrix interface. Further decreases in mechanical properties were related to the fibre diameter reduction as a result of quick glass dissolution, which impaired the frictional stress transfer. Complete loss of the reinforcing effect of glass fibres was observed after 15 days of immersion in aqueous media.

Cylindrical PGF-PLA composite rods were also produced via a new and leaner manufacturing method consisting in compression moulding of thermoplastic hybrid preforms. Hybrid preforms were produced by depositing a sheath of PLA on phosphate glass fibre bundles continuously fed into a cross head extrusion die. Due to the characteristics of the technique, composites could be reinforced with both as-drawn and annealed phosphate glass fibres which were embrittled as a result of anisotropy relaxation and the formation of a surface tensile layer following heat treatment.

On account of the effect of annealing on the phosphate glass fibres mechanical properties, the flexural strength of as-drawn fibre reinforced compression moulded composite rods was higher with respect to annealed fibre reinforced ones, the former reaching maximum strength values of 371 MPa for the 0.45vf samples in comparison to 278 MPa for the latter.

The flexural moduli of the composites rods manufactured through consolidation of PLA-sheathed fibre bundles were the highest hitherto reported for bioresorbable composites reaching values in excess of 30 GPa for the samples with 0.45vf. The higher modulus values of this type of rods with respect to conventional laminated composites were associated with their unique reinforcement distribution achievable by compression moulding of PLA-sheathed fibre bundles.

Annealed fibre reinforced composites showed a better retention of mechanical properties in wet conditions with respect to as-drawn fibre reinforced ones, as a result of the slow degradation of annealed fibres, managing to present values even exceeding the human cortical bone range during the 30 day in vitro degradation study.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Ahmed, Ifty
Grant, David
Segal, Joel I.
Keywords: Bioresorbable, Composites, Phosphate Based Glasses, Fibres, Annealing, Degradation
Subjects: R Medicine > RD Surgery
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 49087
Depositing User: Barrera Betanzos, Fernando
Date Deposited: 13 Jul 2018 04:40
Last Modified: 24 Jul 2019 04:30

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