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.

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.

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Polypropylene (PP) is one of the most commercially used thermoplastics, while a significant amount of PP is used in the form of fibers. In this study, the effects of modification of the filler on the thermal and mechanical properties of composite polypropylene/wollastonite drawn fibers were investigated. In this direction, the surface modification of wollastonite with various organic acids, such as myristic, maleic, malonic glutaric, pimelic, and suberic acid, and the use of two solvents were studied. The surface-modified wollastonite particles were used to produce composite polypropylene drawn fibers. The modification efficiency was found to be slightly better when a non-polar solvent (carbon tetrachloride) was used instead of a polar one (ethanol). FTIR experiments showed that myristic, maleic, malonic, and pimelic acid can strongly interact with wollastonite’s surface. However, the mechanical strength of the composite fibers was not increased compared to that of the neat PP fibers, suggesting inadequate interactions between PP and wollastonite particles. Furthermore, it was observed that the drawing process increased around 10% the crystallinity of all samples. Wollastonite modified with malonic acid acted as a nucleating agent for β-crystals. The onset decomposition temperature increased by 5–10 °C for all samples containing 2% wollastonite, either modified or not. The suggested modifications of wollastonite might be more suitable for less hydrophobic polymers.

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Polypropylene (PP), like all polymers, is susceptible to various forms of aging. Drawn fibers exhibit increased mechanical properties; however, the drawing results in non-equilibrium (decreased entropy) structures, due to the orientation of the polymer chains. Consequently, the drawn fibers are susceptible to an additional form of physical aging. In this work, the effect of common industrial additives on the mechanical strength of virgin and thermally aged PP fibers was studied. Thermogravimetry and tensile strength tests were used to characterize the drawn fibers, before and after physical thermal aging. PP drawn at 120 °C and at a drawing ratio of 7 exhibited a tensile strength of 549 MPa, while the incorporation of an antioxidant and a compatibilizer lowered the tensile strength down to 449 MPA. This reduction was related to the constraint of chain alignment due to the low molecular weight and poor dispersion of the additives. Depending on the aging temperature, shrinking occurred to different extents in pure PP fibers, accompanied by a 6–7% reduction in tensile strength. The fibers with incorporated additives exhibited higher rate and degree of shrinking. Briefly, the incorporation of such additives in drawn PP resulted in the deterioration of the fibers’ mechanical tensile properties. Since such additives have an indisputable value for non-drawn samples and their usage is necessary for various reasons also in drawn samples, e.g., for their protection from chemical aging/decomposition, additives specific for drawn samples should be developed.