Investigation of the crystallization and mechanical properties of wood fiber/polypropylene composites nucleated by a self-assembly β-nucleating agent

Isotactic polypropylene (PP) is a widely used and versatile commodity polymer with several polymorphs (α, β, γ). β-form isotactic PP attracted more interest due to its improved mechanical properties in comparison with the more common α-form. Addition of β-nucleating agents is an easy approach to induce β-form in isotactic PP. Few studies paid attention to the use of β-PP as the matrix of natural fiber/PP composites. In this study, isotactic PP was nucleated by a β-nucleating agent N, N’-dicyclohexylterephthalamide (TMB-5) and then melt compounded with poplar wood fiber (WF) to prepare WF/PP/TMB composites. The influence of adding different contents (0.05, 0.1, 0.15, 0.3 and 0.5%) of TMB-5 on the crystalline morphology, melting and crystallization behavior, mechanical properties and heating deflection temperature (HDT) of WF/PP composites was investigated. It was found that TMB-5 accelerated the crystal nucleation and induced β-crystal growth due to TMB-5 provided the nucleation site. The crystals size of WF/PP/TMB composite decreased and crystals boundaries became blurred with the increasing contents of TMB-5. The TMB-5 contents also affected the self-assembly morphology of TMB-5 which further influenced the morphology of crystals. The relative content of β-crystal ( $${\mathrm{K}}_{\upbeta }$$ ) reached maximum value of 65.3% when the content of TMB-5 was 0.3%. But WF decreased $${\mathrm{K}}_{\upbeta }$$ of PP/TMB under same content of TMB-5 which demonstrated the interference of WF with the generation of β-crystal. For mechanical properties, impact strength of WF/PP/TMB composite reached maximum value of 9.6 kJ·m−2 when adding 0.05% TMB-5. The TMB-5 contents had an insignificant effect on the flexural strength and modulus of WF/PP/TMB composite. Compressive strength of WF/PP/0.3%TMB composite reached maximum value of 64.4 MPa, which increased by 25.5% compared to WF/PP composite. Moreover, TMB-5 could improve the HDT of WF/PP composite as well. These results revealed that the addition of β-nucleating agent TMB-5 is an effective and practical method to improve crystallization rate and mechanical properties of WF/isotactic PP composites. WF/PP/TMB composite can be used in construction decoration, catering, automobile applications.