Titanium/polymer sheets designed for biomedical application; Feasibility study
Solid State and Surface Chemistry, Material Synthesis
Synthesis and Properties of Functional Materials
Primary Shaping and Reshaping Technology, Additive Manufacturing
Final Report Abstract
Surface-confined poly methyl methacrylate (PMMA) chains were successfully grown on large titanium (Ti) substrates (up to DIN A6 size) through surface-initiated atom transfer radical polymerization (SI-ATRP) coupled with a “grafting from” method using phosphonic acids as coupling agents to ensure the metal-polymer covalent bonding. They were used as adhesive layers for the fabrication of biocompatible resin-free sandwich materials produced by hot-pressing. In particular, tethered PMMA chains were used to replace the epoxy resin - usually employed in sandwich materials to stick the polymer core onto the metal skin sheets - and ensure the bonding through the formation of entanglements caused by the interpenetration of the tethered PMMA chains into those of the PMMA core. In this way, to define the optimum Ti/PMMA/Ti sandwiches hot-pressing parameters (temperature, time and pressure) delivering the highest Ti/PMMA adhesion, optimal factorial experimental plan defined by Design of Experiments method was applied. At the found optimum conditions (180°, 90 min and 2 bar), a high bonding strength (pull-off strength higher than 20 MPa and a shear resistance of 10 MPa) and excellent forming properties were produced. The mechanical properties determined by tensile testing showed that the failure of the sandwiches mainly depends on the failure strain of the Ti skin sheets; whereas, the brittle failure nature of the PMMA did not remarkably influence the failure strain of the sandwich. Additionally, the rule of mixtures was verified with the obtained stiffness- and strength-related properties (E, YS and UTS). Applying this rule, two sandwich combinations (symmetric and asymmetric) possessing mechanical properties - especially Young’s modulus - close to that of the cortical bone were proposed. The Ti/PMMA/Ti forming behavior was evaluated performing three-point bending and deep-drawing tests. In particular, despite the low ductility of the PMMA core, delamination-free bending operation up to 180° were achieved without cracking of sandwiches, made of thicker Ti skin sheets (0.4 mm), for both smaller and larger bending punches (3 mm and 6 mm in diameter). However, for smaller bending punch (diameter of 3 mm), the bending angle was limited to 80° if thinner Ti skin sheets (0.2 mm) were used. Further, Ti/PMMA/Ti were completely deep-drawn without visual cracking of the titanium skin sheets, thanks to the outstanding forming behavior of this latter. The same behavior was found for the Erichsen test, where the brittle nature of PMMA has no negative influence on the forming limits or even the Ti/PMMA interface quality (no delamination took place). For the future activities, further experimental set-up has to be prepared to upgrade the possible size of the treated Ti sheets as well as to improve the homogeneity of the grafted layers. Regarding the formability of the Ti/PMMA/Ti sandwiches, the polymer used clearly represents the bottleneck for the system. Thus, their shaping capabilities can be improved carrying out warm/hot forming (around the melting temperature of PMMA) and/or using or even developing a more biocompatible ductile polymer.
Publications
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“A novel alkali-activation of titanium substrates to grow thick and covalently bound PMMA layers” ACS applied materials & interfaces, 2018
M. Reggente, P. Masson, C. Dollinger, H. Palkowski, S. Zafeiratos, L. Jacomine, D. Passeri, M. Rossi, N.E. Vrana, G. Pourroy, A. Carrado
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“Resinfree three-layered Ti/PMMA/Ti sandwich materials: adhesion and formability study”, Composite Structures 218 (2019) 107–119
M. Reggente, M. Harhash, S. Kriegel, P. Masson, J. Faerber, G. Pourroy, A. Carradò, H. Palkowski
