Micromechanical Characteristics of Nanocomposites on the Basis of Polypropylene
The optimum combination of the filling agents with a polymeric matrix allows us to improve the physicomechanical, electrotechnical, adhesive and other characteristics of the nanocomposites [1, 2]. Due to their dielectric and optical properties, the ultradispersed silicon dioxide is considered to be one of the most promising filling agents. In particular, of a considerable interest is research of its influence on the structural and micromechanical properties of the polypropylene.
Preparation of samples
Components of nanocomposites can be prepared in different ways, for example, by chemical processing of the surface of the silicon dioxide or functionalization of the polyolefines by polar groups. The authors used the second way. The grafted copolymers with polar groups are effective compatibilizers (combiners) which improve the interphase adhesion. The mechanism of their action consists in a thermodynamic affinity and a good compatibility with a filled nonpolar polymer. Due to the active functional groups a compatibilizer can form chemical bonds with a filling agent, which makes possible to realize the strengthening effect of the nanoparticles in a composite [3–5].
Compatibilizing agent was a polypropylene (PPF), functionalized under a reactionary extrusion in a melt of the itaconic acid. Granulated PPF was crushed by Pulverisette 14, high-speed rotor mill, up to the thinnes less than 300 micrometers;
Base polymer was polypropylene (PP) Tatren IM 3570 from Slovnaft Co. (Slovenia);
Filing agent was silicon dioxide (Т-150).
Firstly, the filling agent of the silicon dioxide was combined with PPF powder by means of "the drunken barrel" gravitational mixer. Then as a result of mixing in a melt of PPF/Т-150 with the PP the base polymer nanocomposite granulated material was obtained on a laboratory twin-screw extruder (diameter of screws – 22 mm, L/D=25). Hot pressing samples were prepared from it for researches. The composition of the mixes for manufacture of the samples is presented in Table 1.
Microhardness of PP/PPF/Т-150 samples was determined by the method of Vickers on PМТ–3М microdurometer within the range of loads on the identor m=40–80 g and indentation duration of 10 s. The contact angle (CA) of wetting of the surfaces of the polymeric composites with distilled water was measured by the method of "a sedentary drop". Pictures of the profiles of the drops placed on the surface of the samples were made by means of МBS-10 horizontal microscope equipped with a digital video camera. Calculation of CA was done with the help of NanoImages software.
The pressed samples were also examined by the method of sclerometry with the help of a microsclerometer of in-house development . An indentor (a steel ball with diameter of 1.5 mm) implemented reciprocal motions on the surface of a sample, leaving a friction track. The load applied to the indentor was within the range of 0.5–1.5 N. The wear of the samples was estimated by the width of the friction track.
Evaluation of the supramolecular structure of the composite samples was done with the help of Micro–200T microscope with a digital camera by the method of optical polarizing microscopy in the transmitted light. Thin films of the composites formed at 220°С between two subject glasses were prepared separately. The speed of the samples’ cooling did not exceed 3°С/min. The temperature modes of heating and cooling were the same for all the samples.
Properties of the nanocomposite
The results of estimation of microhardness are presented in fig.1. If introduction of PPF into polypropylene resulted in an insignificant decrease of microhardness, addition to the composite of nanoparticles of Т-150 silicon dioxide beginning from 0.22 vol.% contributed to its increase. When the level of filling reached 0.88 vol.%, the microhardness of the composite increased by 20% in comparison with the initial polypropylene.
Figure 2 presents the results of the microsclerometric measurements. The growth of the content of the silicon dioxide in the samples was accompanied with narrowing of the width of their wear tracks. Better microhardness and wear resistance of the composites was due to the presence of nanoparticles, the hardness of which was considerably higher, than that of a polymer matrix, and also due to the changes of their supramolecular structure, which was examined with an optical polarizing microscopy.
Figure 3 presents micropictures of thin composite films of PP/PPF/Т-150 in transmitted polarized light. An average size of the spherulites of the initial polypropylene is 150 μm. Introduction of PPF resulted in their reduction up to 50 μm, and filling of the polypropylene with nanoparticles of the silicon dioxide was conducive to the further reduction of the sizes of the spherulites and wettability of their surfaces with water (Table 2). Pictures of the profiles of the drops of distilled water on the samples’ surfaces are presented in fig.4.
Thus, introduction into the polypropylene of nanoparticles of Т-150 silicon dioxide within the range from 0.22 vol.% up to 0.88 vol.%, leads to an increase of microhardness. With the growing content of nanoparticles, the wear resistance of the composite is improved. Changes in its microhardness and wear resistance to a great degree are due to the reduction of the sizes of the supramolecular structures – spherulites, which was demonstrated with the help of the polarizing optical microscopy. Besides, introduction of nanoparticles of the silicon dioxide into composition of PP contributed to a decrease of the wettability of its surface with water.
The work was done within the framework of the integration project of the basic researches of the Siberian Branch of RAS and NАS of Belarus. ■