Synthesis of effective NR-based toughening agent for poly(lactic acid)

The difficulty in producing packages from poly(lactic acid) (PLA) stemmed from its brittleness and poor processability. In this work, a toughening agent was prepared from natural rubber (NR) grafted with PLA (PLA-g-NR) through melt grafting technique by using a constant weight ratio of NR to PLA of 2:1 and di-tert-butyl peroxide (DTBP) as an initiator. The first effect studied was the peroxide content in PLA-g-NR. The FTIR and 1H-NMR results confirmed that PLA was successfully grafted onto NR. PLA-g-NR exhibited higher rheological properties than the unreacted PLA-NR blend indicating the presence of grafted structures and also possessed higher resistance to thermal degradation. The blending of PLA-g-NR with PLA at both NR contents of 10 and 15 wt% revealed that the optimum DTBP content of 0.4 phr provided an improvement in impact strength with smaller NR particle sizes and better dispersion of NR. For the second effect, the sequence of material addition for preparing PLA-g-NR (DTBP-0.4 phr) was investigated. It was found that the premixing of PLA-g-NR provided a similar rheological behavior and thermal degradation to the case of sequential mixing. However, the premixed PLA-g-NR led to a further increase in impact strength of the blends to 153.2 J/m at only 10 wt% NR, which corresponded to smaller NR particle sizes. The third effect studied was the incorporation of two types of chain extender (CE) including polycarbodiimide (PCDI) and Joncryl ADR-4368 (Jon). The rheological results found that 0.5 wt% Jon provided an increase in the rheological properties and inhibited thermal degradation better than PCDI indicating the effective chain extension of PLA. Furthermore, the presence of Jon in PLA-g-NR led to higher impact strength for PLA at both NR contents, while the addition of PCDI provided higher impact strength at only 15 wt% NR. The higher impact strength also related to a decrease in NR particle sizes. The fourth effect employed the compatibilizer (CP) to improve the phase compatibility between PLA and NR. The FTIR and 1H-NMR spectra revealed that the addition of CP resulted in better grafting of NR with PLA. Moreover, CP provided a significant increase in rheological properties and a strong influence on preventing thermal degradation of PLA-g-NR resulting from the increased grafted structures of PLA-g-NR. The torque values of PLA/PLA-g-NR blends with the addition of CP also increased during melt blending, indicating the improved interfacial compatibility of the blends. The method of adding CP also strongly affected the impact strength, where the addition of CP during the compounding of PLA/PLA-g-NR led to the highest impact strength of 352.6 J/m at 10 wt% NR and 589.7 J/m at 15 wt% NR. This was supported by the smallest NR particle sizes and good dispersion of NR in PLA matrix. Finally, the overall effects of NR content, CE and CP was integrated. The use of CP together with PCDI in the blend at 15 wt% NR caused a significant increase in impact strength with the most consistent impact strength value of 656.6 J/m. This indicated the improved compatibility, which was also reflected by the best dispersion and distribution of NR particles in PLA.