Bio–polyamides (PA10.10 and PA6.10) montmorillonite composites: properties and Essential Work of Fracture (EWF)

The plastics industry is increasingly oriented towards the use of polymers from natural origin (biobased) replacing plastics derived from petrol chemistry (fossil-based). Recently, some aliphatic polyamides were revalued by industries for packaging and for long-term applications but still low investigated. Moreover, nanoclays have been proven to improve mechanical and barrier performance of fossil based PAs. On the other hand, these superior properties of PA nanocomposites usually have as a “side effect” the increase of the brittleness by suppressing plastic deformation at the crack tip, which greatly limits the application of these materials. In this study two kinds of bio-polyamides were investigated: PA10.10 and PA6.10. As filler a natural montmorillonite modified with methyl hydrogenated tallow bis-2-hydroxyethyl quaternary ammonium (Cloisite®30B-Cl30B) was used. PAs were melt blended with clay using a co-rotating twin screw extruder LEISTRITZ ZSE 18/40 D. Neat polymers were also processed to have the same thermal history. Pellets obtained by the first extrusion were dried before the film extrusion. A single screw extruder (Eurotech Extrusion Machinery S.r.l.) (L=80cm D=25mm) equipped with a flat die was used. Samples for mechanical characterization were die cut and tested with a crosshead speed of 1 mm/min using a Zwick 10 universal testing machine. Specimens for EWF were prepared according to the ESIS protocol and conditioned at 33% of R.H. and room temperature. Before testing, a fresh razor blade was carefully tapped into ligament of specimen to introduce two aligned sharp pre-cracks. The free ligament length (l) was left in the range from 5 to 22 mm. The specific work of fracture (wf) was obtained by dividing the integrated area of the load–elongation curve with the initial ligament area. The essential work of fracture (we) and the plastic term (β*wp) are easily obtained from the best linear regression of a set of values represented in a graph plotting wf versus l (Figure 1a). An estimation of the shape and dimensions of the necked zone surrounding the fractured ligament was done in order to derive the wp value (Figure 1b). Clay did not significantly affect the work required for the onset of crack propagation (we) in PA6.10 while decrease in PA10.10. The plastic work (β*wp) of both PA was increased with clay. The individual nanoparticles act as stress concentration points, which promoted cavitations and thus induced relatively large local deformation. By this way, the mechanical work content necessary to generate the plastic flow is increased, leading to a higher wp value.