De Focatiis, Davide S.A.

Grant, David M.

Parsons, Andrew J.

Tomczynska, Magdelena M.

Ward, Michael

Nottingham ePrints

The influence of process conditions on the structure and properties of melt processed poly (lactic acid) nanohydroxyapatite composites  Michael Ward1, Magdalena M. Tomczynska, David M. Grant, Andrew J. Parsons and Davide S.A. De Focatiis  1Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD. m.ward@nottingham.ac.uk   Introduction  Poly (lactic acid) (PLA) is a biodegradable and bioresorbable thermoplastic polymer with good optical clarity, radio-translucency and biocompatibility. These properties make it a desirable material for medical implant application. At present, the use of PLA implants has been restricted to non-structural functions such as mandibular fracture plates due to its limited mechanical performance [1]. An area where PLA implants have been successfully employed is in bioresorbable interference screws. PLA screws for the fixation of bone-tendon and soft tissue grafts during ACL  and PCL reconstruction procedures are proving favourable [2]. The work presented here is part of an effort to incorporate novel nano-sized hydroxyapatite parti-cles (nHA) into bioresorbable PLA for healthcare applications via melt-processing, in order to broaden the application range of such bioresorbable materials. In this study the focus is on the effects of atmospheric process conditions on the post-processed molecular weight and me-chanical properties of these novel nanocomposites.  Experimental   The medical grade PLA employed in this study was RESOMER® LR706S (Evonik Industries) 70:30 poly (L-lactide-co-D,L-lactide) (PLDLLA). The nHA is produced through a novel continuous flow reactor, and can be synthesised to produce large batches of specific morphologies, with the potential to modify the surfaces of the HA with many different surfac-tants [3]. Composites have been compounded using a Haake MiniLab recirculating extruder at 210 °C for 15 minutes in different atmospheres.  Extrudates were compression moulded at 170 °C for 15 minutes to produce specimens for mechanical testing. Mechanical tests were carried out using an in-house custom miniature 3-point bending rig with specimen sizes of 7 x 2 x 0.5 mm. The molecular weights of the compression-moulded virgin polymer and extrudates were analysed using size-exclusion chromatography (SEC) in THF, calibrated with polystyrene standards.  Results and discussion   It is well known that melt processing of PLA is very sensitive to the presence of water. Fig. 1b reports measurements of the peak molar mass Mp as a function of process conditions. Relative to the pre-processed conditions, compounding under a blanket of bottled nitrogen gas causes a significant decline in the molar mass. A drop in mechanical properties is also ob-served as a result of compounding, as shown in Fig 1a for flexural strength. The introduction of just 2.5wt% nHA platelets provided a small improvement in the flexural strength. The ad-dition of nHA platelets results in both an improvement in the flexural strength and to the post-processed molar mass, possibly due to the nHA’s interaction with the residual water. In an attempt to reduce the water content, bottled nitrogen was passed through a cold trap to remove residual moisture. This mostly led to an improved flexural strength and molar mass after processing. With the nHA particles, the flexural strength of the unprocessed poly-mer was recovered, but the compounding still caused a significant reduction in molar mass.  The most significant improvement in post-processed molar mass was observed when the nHA was separately dried at a high temperature. This is believed to remove any remaining water from the PLA during processing, significantly reducing the effect of the depolymerisa-tion ordinarily taking place at the compounding temperatures. Although the mechanical prop-erties of the nanocomposites are not significantly affected by the moisture content, the re-duced drop in molar mass is likely to play a significant role once these materials experience hydrolytic degradation in service.   Conclusions  This work has investigated the effect of processing conditions on the molar mass and flexural strength of PLA and PLA nHA nanocomposites. Although compounding causes a drop in molar mass and flexural strength, much of this drop can be offset by compounding under dry nitrogen and by pre-drying the nanoparticles.   References  1. An J, Jia P, Zhang Y, Gong X, Han X, and He Y, Application of biodegradable plates for treating pediatric mandibular fractures. J Craniomaxillofac Surg, 2015. 43(4): p. 515-520. 2. McGuire DA, Barber FA, Elrod BF, and Paulos LE, Bioabsorbable interference screws for graft fixation in anterior cruciate ligament reconstruction. Arthroscopy, 1999. 15(5): p. 463-473. 3. Gimeno-Fabra M, Hild F, Dunne PW, Walton K, Grant DM, Irvine DJ, and Lester EH, Continuous synthesis of dispersant-coated hydroxyapatite plates. CrystEngComm, 2015. 17(32): p. 6175-6182.  Fig. 1 Properties of PLA and 2.5% nHA-PLA composites after melt processing in different conditions. (a) Flexural strength (b) Peak average molecular weight  (a) (b) 

The influence of process conditions on the structure and properties of melt processed poly (lactic acid) nanohydroxyapatite composites

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