Effect of charcoal content on properties of charcoal/polypropylene composites

Abstract: As a green and environmental material, wood plastic composite is widely used in indoor and outdoor decoration, transportation, automobile and other fields for its low cost and good strength. However, there are a large number of hydrophilic polar groups on the surface of natural fiber materials, and the thermoplastic is non-polar, which creates a large number of voids in the material, making the mechanical properties of the material worse. Charcoal is produced from pyrolysis and carbonization of wood at high temperature. The polar groups on the wood surface are damaged by high temperature and the polarity is weakened, which make it easier to be compatible with thermoplastics. In this paper, charcoal/polypropylene (PP) composites were prepared by twin-screw extruder using charcoal and PP as the main raw materials. The properties of the composites were characterized by means of X-ray diffractometer (XRD), differential scanning calorimeter (DSC), electronic universal testing machine, dynamic thermal mechanical analyzer (DMA) and field emission scanning electron microscope (SEM). The experimental results showed that: 1) the composites had obvious peaks at 14°/17°/18°and 27° which corresponded to the crystal plane (110) (040) (130) (002), showing α -crystalline form, respectively, and this was consistent with pure PP. The peak strength of each sample decreased with the increase of carbon content, this indicated that the increase of charcoal content had little effect on the microcrystalline structure of the composite, but it would decrease the diffraction peak strength of the composite. 2) after the addition of charcoal, the melting temperature of the composite did not changed, but the crystallization temperature of the composite increased and the strength of the crystallization peak gradually decreased. This indicated that the addition of charcoal was beneficial to exothermic crystallization of composites and improved the thermal stability of composites. 3) when the charcoal content was 20%, the tensile strength of the charcoal/PP composite was low, which was only 12 MPa, when the content of charcoal was 60%, the tensile strength of the composite was about 25.47 MPa, which was the maximum value, as the increase of charcoal content to 80%, the tensile strength of the composite decreased to 7 MPa significantly. On the one hand, the lower amount of PP could not bind charcoal particles together effectively, forming a large number of agglomerations in PP, and the bonding interface of the composite was severely damaged, the stress concentration of the composites was easy to occur under the action of external forces. On the other hand, PP chain contains -CH3, and the three-dimensional resistance of the polymer chain is greater than PE, which is not conducive to the internal rotation of the polymer chain, increasing the rigidity of the chain and reducing the flexibility of the chain, This is why the tensile strength of composites is low. This indicated that the composites with better tensile strength could be obtained by adding proper amount of charcoal. 3) when the temperature was low, the storage modulus of the composite was larger, but with the increase of the temperature, the storage modulus of the composite decreased continuously. with the increase of charcoal content, the storage modulus of charcoal/PP composites increased and the loss factor of composites decreased. This indicated that the increase of temperature would have an adverse effect on the storage modulus of composites, but the increase of charcoal content could help to improve the rigidity of composites. 4) the SEM images showed that the microstructure of charcoal was porous structure, so the microstructure of charcoal/PP composites was different from that of wood-plastic composites. PP filled the gullies and pores of the charcoal, and effectively acted as the binder between the charcoal particles inside the composites, which formed a compact interlocking structure.