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.

In this review, traditional and novel techniques for producing micro- and nano- fibers are discussed and various nanofillers, their modifications and polypropylene (PP) functionalization routes are presented. Their influence on PP properties is discussed and new PP composite fiber applications are presented. This review reveals interesting conclusions, such as that in terms of mechanical reinforcement, there is no nano-filler that can improve tensile strength to the extent that it is improved by drawing. However, in some cases, composite drawn fibers are characterized by higher tensile strength than drawn neat PP. With some notable exceptions, the PP nanocomposites lack of “dramatic” properties improvement is mainly due to the non-polar nature of the hydrocarbon chain, which does not favor strong intermolecular interactions with most popular (mainly inorganic) nano-fillers. However, other properties such as electric conductivity, water contact angle and others can be effectively altered using various nanofillers in PP matrices.

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.

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