This paper mainly discusses the effect of coupling on the tensile properties of glass fiber (GF)/carbon fiber (CF) reinforced polypropylene (PP) hybrid composites which were made through a new injection molding process named direct fiber feeding injection (DFFIM) process. It is mainly divided into two parts which discusses the functional of coupling agent in the composites system, and the different contents of coupling agent (PA6 and MAPP) on the tensile properties of composites. DFFIM progress is a new method that by directly feeding of continuous carbon fiber into the barrel of injection molding machine to make the hybrid composites. The continuous CF roving strands are guided into the vent of devolatilizing unit of injection barrel and fed into the melt by the shearing motion of the screw during plasticization process. By using DFFIM process to make composites, the fiber attrition during extrusion compounding will be eliminated. It is a great improvement in reduction of material cost. And also the cost of reinforcing compounded pellet in the traditional composites market value chain could be lower. Polyamide 6 (PA6), Maleic anhydride-grafted polypropylene (MAPP) or both of them were mixed with pellets during the DFFIM process and PA6 and MAPP were used as coupling agent for CF/GF reinforced PP system. The CF and GF contents in each hybrid composites were tested to analysis the influence of fiber contains on the tensile properties of composites. Usually, better interfacial bonding between fiber and matrix in composites, better tensile properties of composites. So the effect of coupling agent (PA6 and MAPP) on the interfacial bonding between CF and PP in hybrid composites were firstly analyzed. And then the influence different contents of PA6 and MAPP on the tensile properties of GF/PP composites and GF/CF reinforced PP hybrid composites were investigated. It is found that the addition of PA6 did not improve the interfacial bonding but the addition of MAPP has shown a little improvement to the bonding between CF and PP. And when using PA6 and MAPP together as co-coupling agent, the tensile properties of composites has greatly increased. And, there is fiber aggregation in the core layer of the hybrid composites which made by DFFIM process, while there is no such phenomenon happened in the condition of normal injection molding process. It is the main reason that the tensile strength of hybrid composites without coupling agent is weaker than the GF/PP composites. And the tensile modulus of composites would be increased considerably. That is due to the addition of the carbon fiber which has high tensile modulus. In the condition of composites with 1wt.% PA6, the 1wt.% PA6 shows positives effect on tensile properties and while PA6 has negative role when the amount of PA6 has improved. Within a certain range, the larger amount of MAPP in the system of MAPP-PA6 composites, the better on the tensile properties of composites is. And MAPP has positive effect on the tensile properties of composites.

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