Cyclical pressure on compression-moulded bioresorbable phosphate-glass fibre reinforced composites

Abstract

Event Abstract Back to Event Cyclical pressure on compression-moulded bioresorbable phosphate-glass fibre reinforced composites Fernando Barrera Betanzos1, Miquel Gimeno Fabra1, David Grant1 and Ifty Ahmed1 1 University of Nottingham, Materials, Mechanics and Structures, United Kingdom Introduction: The use of composites based on poly(lactic) acid ‘PLA’ and phosphate glass fibres ‘PGF’ to replace metallic devices used for bone fracture repair has been deemed as highly beneficial due to their excellent biocompatibility and biodegradability. However, difficulties in achieving thorough melt impregnation whilst limiting polymer degradation during manufacture has been one of the main issues precluding their implementation. Therefore, alternative manufacturing techniques, other than those involving temperature or time increases that improve fibre wet-out will inevitably pave the way for product adoption. Materials and Methods: PGF with the composition 45P2O5-16CaO-13Na2O-24MgO-2Fe2O3 mol.% were melt drawn to fabricate unidirectional composites of 4 volume fractions vf (0.15, 0.25, 0.35, 0.45) by alternately stacking layers of PGF and PLA films inside a purpose-built compression moulding tool. The mould was placed into a heated press at 180°C for 10min, followed by consolidation for additional 10min following two loading schemes: i) static pressure ‘SP’ (pressure held constant at 40 bar) and, ii) cyclical pressure ‘CP’ (pressure cycled for 1.5min at the end of the consolidation stage). Finally the system was cooled under constant pressure. Composite mechanical properties were evaluated via 3-point bending test. Effects of pressure profile on the crystallisation kinetics were assessed through Differential Scanning Calorimetry. PLA thermal degradation was evaluated via Advanced Polymer Chromatography. Results and Discussion: In all cases the flexural strengths of CP composites were approximately 30% higher than those in which SP was implemented (cf. Figure 1). This was explained by the CP ability to affect fibre network permeability through relaxation and reapplication of pressure, leading to improved impregnation. CP could also alter the melt viscosity and the capillary pressure. The CP composites mechanical properties were found to be the highest reported up to now for this composite system (ca. 480 MPa and 23 GPa for flexural strength and modulus respectively for the 0.45 vf) and even greater to cases were the fibre was treated with coupling agents [1][2][3]. The observance of a crystalline shoulder in the DSC traces (Figure 2) suggested transcrystallinity formation exclusively in the CP composites, probably as a result of the local chain alignment induced by the pulsating motion, and possibly accounting for enhanced stress transfer. The PLA molecular weight showed a maximum reduction of 20% in the case of the 0.45 vf composite due to the moisture absorbed by the hydrophilic glass surface. The extent of degradation did not deleteriously affect the PLA mechanical properties in the composites. Figure 2.- Influence of the pressure profile on composites flexural strength with progressively higher fibre content (error bars indicate ± standard deviation). The blue dashed line represents the theoretical composite strengths calculated using the rule of mixtures. Figure 4.- DSC traces of representative samples of (upper) Cycled Pressure and (down) Static-Hold Pressure composite series. Insert shows an enlarged view of the cold crystallisation peaks for both composite series. Conclusion: Implementation of cyclical pressure during the consolidation stage of compression moulding led to enhanced fibre impregnation with limited PLA thermal degradation due to its influence on the fibre preform permeability, melt viscosity and capillary pressure. Additionally, cyclical pressure appeared to promote the development of transcrystallinity which may further enhance mechanical properties upon annealing. Consejo Nacional de Ciencia y Tecnología (CONACYT); MeDe Innovation