Superior composite materials with better tensile fracture resistance and can be produced by choosing the correct sizing agent. Thermoplastic composites reinforced with carbon fiber (CF) are finding more and more applications in the aerospace, engineering, marine and automobile industries, due to their mechanical and electrical properties.1, 2 Polyamide (PA 6,6) is a good candidate for the polymeric matrix of these CF-reinforced thermoplastic composites due to its thermal stability, low dielectric constant, low cost and high mechanical strength.2, 3 CFs are used as reinforcing materials for polymeric matrices because of their remarkable mechanical properties and low density when compared with other reinforcing fibers.4 The performance of CF-reinforced composites is affected by the properties of both the CFs and the polymeric matrix. Strong interaction between CFs and the matrix improves the performance of CF-reinforced composites due to the better load transfer from the fibers to the matrix.1, 4 But generally, the interaction between CF and thermoplastic matrices is weak and some modification method such as sizing or coating must be applied to the CF surface to overcome this weakness.5–7 Sizing is an efficient and simple method and sizing materials are also known as matrix compatibilising agents. When using sizing, a thin layer of polymer adheres to the fiber surface and protects the fiber from damage during processing and handling.5Figure 1.Scanning electron microscope micrographs of tensile fracture surfaces of thermoplastic/carbon fiber (CF) composites. (a) Unsized ×1000, (b) polyamide (PA) ×1000, (c) polyurethane (PU) ×1000. Here we investigated the effect of sizing agent type on the performance of CF-reinforced polyamide 6,6 (PA 6,6) composites. Chopped unsized CF and sized CFs (0.6cm) were used as reinforcing materials. Selected sizing agents were polyurethane (PU), polyamide (PA), polyimide (PI), phenoxy (PHE) and epoxy/phenoxy (EPO PHE). We prepared PA 6,6 composites which contained %30wt carbon fiber in a laboratory-scale micro-compounder. All the compounds were subsequently molded with a laboratory-scale injection molding machine. We characterized the properties of the prepared composites with tensile and izod impact tests and a scanning electron microscope.Mechanical properties of the prepared composites and PA 6,6 are shown in Table 1. The tensile strength and modulus of the composites increased with the addition of CF to the PA 6,6 matrix. And the tensile strength and modulus of sized-CF-reinforced composites were higher than that of unsized-CF-reinforced PA 6,6 composites. We believe this is because of the increased interaction between the CF and the PA 6,6 matrix after the sizing process. In our study, while PA-sized-CF-reinforced composites gave the maximum tensile strength value, PU-sized-CF-reinforced composites gave the highest modulus value.The strain-at-break value of neat polymeric matrix decreases with the addition of the fiber due to the decreased polymer chain mobility.8 In our study, the highest strain-at-break value was obtained for the PA-sized-CF-reinforced composite. We also observed that strain-at-break values of the CF-reinforced PA 6,6 composites where sizing was used were higher than those where it wasn’t. We attribute this to the better interaction between PA 6,6 matrix and sized CFs.Table 1.Mechanical properties of prepared CF-reinforced composites and PA 6,6 matrix. PU: polyurethane, EPO PHE: epoxy/phenoxy, PI: polyimide, PHE: phenoxy.TensileStrain atImpactStrengthBreakStrengthModulus(MPa)(%)(kJ/m2)(MPa)PA 6.669.172.780.12030Unsized166.316.354.78870PU199.816.656.713600EPO PHE176.615.355.112900PI178.916.855.311000PHE199.516.956.212200PA207.817.961.312500It is known that impact strength value of neat polymer decreases with the addition of fiber, making it more brittle.9, 10 Though we observed that the impact strengths of composites in our study were lower than that of neat PA 6,6, sized-CF reinforcement enhanced the impact strength values as compared to the unsized materials. In addition, PU-sizing gave the highest impact energy values among the other sized-CF-reinforced composites.We evaluated tensile fracture surfaces of all the composites with a scanning electron microscope. The micrographs show that PU- and PA-sized carbon fibers have better interaction with the PA 6,6 matrix (see Figure 1) than other sizing agents.Our results indicate that the selection of proper sizing materials for the thermoplastic matrix used has a critical effect.11 The right sizing selection can enhance the properties of CF-reinforced polymeric composites. PA- and PU-sized CFs can be used for PA 6,6 matrix composites. Our future research in this area will evaluate the various sizing agents for CF in composites that use thermoplastic matrices other than PA 6,6.