Synthesis and characterisation of polyamide 6 blends made by reactive extrusion

Abstract

Continuous anionic polymerisation of catalysed E -caprolactam has been undertaken on an intermeshing co-rotating twin-screw extruder for the preparation of polyamide 6. An optimised extruder screw and barrel configuration was used during reactive extrusion process to permit necessary feeding, mixing, devolatilising and pumping requirements. Various screw speeds and barrel temperature profiles were also employed to influence the thermal and shear history of the reaction mixture. Differential scanning calorimetry and wide angle X-ray diffraction techniques were used to characterise the thermal properties and crystalline order of the polymerisation product. It was observed that the extent and form of the crystalline structure can be greatly influenced by the material composition, method of preparation and annealing procedures during and after the polymerisation stage. Molecular weight, molecular weight distribution and residual monomer content of the reactive polymerised PA6 samples as determined by solution viscometry, gel permeation chromatography and gas chromatography analysis revealed that these properties could be influenced by polymerisation conditions in the extruder, yielding values of Mw up to 100 kg.mol-1 and polydispersity index as high as 6 with monomer residue contents of 4-6%. Selected mechanical properties of the PA6 samples as measured using standard test procedures exhibited superior impact and elongational properties under tensile deformation but showed a slightly reduced tensile strength compared to commercial polymer. A reactive polymerised blend of PA6 with 10 wt% EPR copolymer has been successfully synthesised in a similar manner using the twin-screw extruder at a screw speed of 150 rpm. This elastomer modified blend material exhibits superior tensile and impact strength compared to the pure PA6. Enhancement of the mechanical properties is postulated to be attributed to the formation of a PA6-gPE graft copolymer which can possibly functioned as an emulsifying agent to lower the interfacial tension between the two phases thereby compatibilising the blend. Experimental evidence from FTIR and phase solubility studies have supported the formation of this graft copolymer. Characterisation studies by DSC, DMA and W AXD suggested structural modification of the EPR copolymer has occurred as a result of chain scission during the polymerisation/ blending process leading to the formation of a crystalline PP phase and an amorphous PE component. Macroradicals formed from this mechano-degradation reaction by chain scission of the reacting polymers are postulated to be responsible for the formation of this graft copolymer. Blends of reactive polymerised PA6 and a commercial grade isotactic PP homopolymer of compositions 10%, 30%, 50% and 70% PP were also prepared with and without the presence of another commercial grade functionalised PP compatibiliser. Results obtained from structural, morphological and mechanical studies obtained for the non-functionalised blend of 10% PP showed apparent "miscibility" of the phase components which is an unexpected observation since PA6 and PP are well known to be incompatible. It is postulated that a branched PP and a PA6-g-PP graft copolymer has been formed, as a result of mechano-degradation reaction during the extrusion/blending process. Reaction blends with higher PP compositions however, sbowed incompatibility behaviour with coarse blend morphology and poorer mechanical properties. Phase inversion phenomena for this reaction blend is observed at PA6 composition of 30% with evidence of interpenetrating polymer network structure in the blend mophology. The functionalised reaction PA6/PP blends with 5% compatibiliser showed phase separation of the polymer components in all compositions as evidenced from DSC, W AXD, DMA and SEM studies. No improvement of mechanical properties was observed as compared to the blends without compatibiliser. A PA6-g-PP-g-MA graft copolymer has postulated to have formed between the PA6 and the compatibiliser but this compatibisation effect is not efficient enough to enhance the mechanical behaviour of the blends.