Abstract

The PhD project was details on the polyolefin nanocomposites compounding, processing and preparation. Two different types of polymer matrix with low melt flow rate for fiber forming polymers have been selected; high density polyethylene (HDPE) and isotactic polypropylene (PP). High density polyethylene was compounded with double layered hydrotalcite (LDH) while in case of polypropylene reinforcement by adding fumed silica and kaolinite was performed. In this way the influence of the nanofiller type on the thermo-mechanical properties of the prepared nanocomposites were studied. In recent years several research efforts have been focused on the preparation of polymer/layered inorganic nanocomposites because of the excellent properties in comparison to the neat polymer. The main reason of this interest lies certainly in the properties of the nanoclay, like high stiffness, and high aspect ratio, that induce enhancement of various polymer properties (thermal stability, mechanical properties, flame resistance and gas barrier) even with small amount of filler. Moreover, nanocomposites can be processed more easily than microcomposite. Recently literature evidences a lot of progress in the nanofilled bulk materials; on the other hand, there are relatively a few publications on fibers made of nanofilled polyolefins. For instance, PP fibers were produced with various types of nanofillers, e.g. layered silicates, carbon nanotubes and montmorillonite. In the case of HDPE, composite fibers containing calcium carbonate, carbon nanotubes, silica and layered silicates were reported. It is worth to mention that so far, no publication could be found on this work using the same nanofillers with the same matrix. This thesis is divided into six chapters; Introduction and Background, Experimental activities, after obtained Results with discussions are reported and finally Conclusions. In the Introduction and Background (Chapter I and II) general information about nanocomposites and characteristic of different nanofillers type were summarized. After that polymer processing method with particular attention on the melt extrusion and fiber spinning were described. Third Chapter is dedicated to the experimental part. Here, the used material characterization, nanocomposite preparation procedure and description of experimental techniques were reported. All nanocomposites were characterized by different experimental techniques. First nanofiller morphology by microscope (SEM and TEM) and X-ray diffraction technique was tested. Thermal stability was investigated by Thermal Gravimetric Analysis (TGA) and crystallization behavior by Differential Scanning Calorimetry (DSC). Finally mechanical properties were characterized by tensile test, Dynamical Mechanical Thermal Analysis (DMTA) and creep test. The Results and Discussion have been divided into two parts; first one was dedicated to the high density polyethylene layered double hydrotalcite nanocomposites (HDPE-LDH), while in the second polypropylene with fumed silica (PP-FS) and kaolinite (PP-K) nanocomposite were described. i. High density polyethylene hydrotalcite (HDPE-LDH) nanocomposites after different process of plates and fibers production will be compared in Chapter IV. At the beginning a polypropylene matrix, suitable for fiber production, was firstly melt compounded with organically modified hydrotalcite up to 5% by wt. Similar compositions with up to 3% wt. of LDH were performed by melt spinning. The incorporation of the clay into both bulk and fiber nanocomposite enhanced the thermal stability and induced heterogeneous nucleation of HDPE. Hydrotalcite positively affected the mechanical properties in term of higher Young’s modulus and tensile strength. After the preliminary characterization on bulk and as-spun material the fibers were hot drawn up to draw ratio (DR) 20. XRD analysis revealed intercalation with high degree of exfoliation for the composites with 1-2% wt. of LDH. For this compositions higher elastic modulus 9.0 GPa - 9.3 GPa (with respect to 8.0 GPa of the neat HDPE), and maintain tensile strength and deformation at break were observed. Moreover, the addition of low amount of LDH significantly improved the creep stability. ii. Nanocomposites of isotactic polypropylene fumed silica (PP-FS) were described in the Chapter V. Two types of hydrophobic fumed silica with different surface area (170m2•g-1 and 150m2•g-1) and surface treatment (treated respectively by dimethyldichlorosilane and octylsilane) up to 2% vol. were used. Similar as in case of HDPE-LDH nanocomposites plates production and characterization was a preliminary step to select the best compositions for the fiber preparation. After that, the work has been focused on the iPP-FS fiber production. Introduction of the nanofiller enhanced thermal stability and mechanical properties of the nanocomposite. Elastic modulus at draw ratio 10 increased from 5.3 GPa for neat iPP up to 7.5 – 8.6 GPa for compositions with 0.25 – 0.5% vol. Together with this improvement enhancement in strength at break and maintaining deformation at break were observed. Moreover, isothermal creep tests evidenced improvement in the creep stability due to the FS introduction, over the whole range of investigated draw ratios. iii. The last results of recent research dedicated to the polypropylene kaolinite (PP-K) nanocomposites are reported in Appendix 1. Nanocomposite fibers were successfully spun up to draw ratio (DR) 15 at very high nanofiller content up to 30% wt. The presence of kaolinite not only increased the thermal stability but also enhanced elastic modulus up to 5.6 GPa – 7.0 GPa for compositions with 1% up to 30% wt. of kaolinite, in comparison to 5.4 GPa for neat PP at draw ratio 10. Moreover, for the composition with 10% wt. of kaolinite better drawability with maximum modulus was obtained in comparison to neat PP. Finally the most important observation made on polyolefin nanocomposites fibers were summarized in the Chapter VI. It can be concluded that polyolefin fibers nanocomposites were successfully prepared by two different processing conditions: melt compounding and melt spinning followed by hot drawing. In case of plates the introduction of nanosilica remarkably improved the thermal stability and elastic modulus, with retention of the pristine tensile properties at break. Nanocomposites fibers showed a higher improvement of the elastic modulus with respect to the nanocomposites plates containing the same percentage of nanofiller. Moreover, the introduction of the nanofiller enhanced tensile dynamic mechanical properties especially for higher draw ratio. Similar behavior was also observed in case of creep compliance. Higher creep stability was observed for the drawn fibers with nanofiller in comparison to neat polymer. This behavior could be a consequence of the different orientation and morphology related to the crystallinity developed in the spinning. These results confirmed that polyolefin containing nanofiller could be easily spun into nanofilled fiber. TEM images revealed how the experienced improvements of the mechanical properties could be probably related to the orientation of nanofiller aggregates along the strain direction and to the consequent increase of the filler-matrix interfacial area.