Isotactic polypropylene (PP) composite drawn fibers were prepared using melt extrusion and high-temperature solid-state drawing at a draw ratio of 7. Five different fillers were used as reinforcement agents (microtalc, ultrafine talc, wollastonite, attapulgite and single-wall carbon nanotubes). In all the prepared samples, antioxidant was added, while all samples were prepared with and without using PP grafted with maleic anhydride as compatibilizer. Material characterization was performed by tensile tests, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy. Attapulgite composite fibers exhibited poor results in terms of tensile strength and thermal stability. The use of ultrafine talc particles yields better results, in terms of thermal stability and tensile strength, compared to microtalc. Better results were observed using needle-like fillers, such as wollastonite and single-wall carbon nanotubes, since, as was previously observed, high aspect ratio particles tend to align during the drawing process and, thus, contribute to a more symmetrical distribution of stresses. Competitive and synergistic effects were recognized to occur among the additives and fillers, such as the antioxidant effect being enhanced by the addition of the compatibilizer, while the antioxidant itself acts as a compatibilizing agent.

The thermal and mechanical properties of polypropylene-wollastonite composite drawn fibers were optimized via experiments selected with the Box-Behnken approach. The drawing ratio, the filler and the compatibilizer content were chosen as design variables, while the tensile strength, the melting enthalpy and the onset decomposition temperature were set as response variables. Drawn fibers with tensile strength up to 535 MPa were obtained. Results revealed that the drawing ratio is the most important factor for the enhancement of tensile strength, followed by the filler content. All the design variables slightly affected the melting temperature and the crystallinity of the matrix. Also, it was found that the addition of polypropylene grafted with maleic anhydride as compatibilizer has a multiple effect on the final properties, i.e., it induces the dispersion of both the antioxidant and the filler, tending to increase thermal stability and tensile strength, while, on the same time, deteriorates mechanical and thermal properties due to its lower molecular weight and thermal stability. Such behavior does not allow for simultaneous maximization of thermal stability and tensile strength. Optimization based on a compromise, i.e., targeting maximization of tensile strength and onset decomposition temperature higher than 300 °C, yields high desirability values and predictions in excellent agreement with verification experiments.

A large portion of the produced Polypropylene (PP) is used in the form of fibers. In this industrially oriented study, the development of composite PP drawn fibers was investigated. Two types of fillers were used (ultra-fine talc and single-wall carbon nanotubes). Optimization of the thermal and mechanical properties of the produced composite drawn fibers was performed, based on the Box-Behnken design of experiments method (surface response analysis). The effect of additives, other than the filler, but typical in industrial applications, such as an antioxidant and a common compatibilizer, was investigated. The drawing ratio, the filler, and the compatibilizer or the antioxidant content were selected as design variables, whereas the tensile strength and the onset decomposition temperature were set as response variables. Fibers with very high tensile strength (up to 806 MPa) were obtained. The results revealed that the maximization of both the tensile strength and the thermal stability was not feasible for composites with talc due to multiple interactions among the used additives (antioxidant, compatibilizer, and filler). Additionally, it was found that the addition of talc in the studied particle size improved the mechanical strength of fibers only if low drawing ratios were used. On the other hand, the optimization targeting maximization of both tensile strength and thermal stability was feasible in the case of SWCNT composite fibers. It was found that the addition of carbon nanotubes improved the tensile strength; however, such improvement was rather small compared with the tremendous increase of tensile strength due to drawing.