Nanodiamond Powders with High Magnetic Characteristics
Of special interest are polyolefines because of their great volumes of production, high adaptability to manufacture and low costs. However, a nanomodification of those materials is complicated due to their low superficial energy and high viscosity of the melts, complicating a uniform or set distribution of nanoparticles in the mass of the matrix systems of composites and products from them. Therefore, in recent years researches have been going on in the functionalization of the polyolefines, and polypropylenes, in particular [5-7].
Another important problem is selection of nanomodifiers of the polymeric matrixes, the main characteristics of which are low cost and opportunity for improvement of the mechanical, physical and other characteristics . These requirements are met by dielectric nanosilicates and inexpensive versions of the electroconductive carbon nanomaterials (CNМ) , and also their complexes . Below are the results of research of a joint influence of a complex of nano-sized fillers of SiO2/CNМ on the optical transparency of the thermoplastic polymers and their adhesion to hard surfaces.
Preparation of samples
As a dielectric filler the nano-sized silicon dioxide of "tarkosil" brand (Т-150) was selected. It was obtained by evaporation of a pure quartz sand with a subsequent condensation of a high-temperature vapor in the form of nanoparticles (diameter of the primary particles was equal to 20 nm, specific surface by VET method was 130-150 m2/g) . The applied electroconducting filler was CNМ – a product of pyrolysis of the carbon-containing gases in a catalytic boiling layer, representing an interlaced multiwall carbon nanotubes with diameter of 10–20 nm (95%) and an impurity of a metal catalyst (up to 5%). The specific surface of CNМ measured by the method of VET was 113.5 m2/g . As a polymeric matrix for obtaining of composites a polypropylene was used, the melt of which was functionalized by itaconic acid during the process of a reactionary extrusion (FPP). FPP granulate was crushed into a dispersed powder (grinding fineness was not less than 300 microns) in Pulverisette 14 high-speed rotor mill with a continuous supply of a liquid nitrogen. Preparation of the composite mixes of FPP/CNМ/Т-150 was done by means of Pulverisette 0 vibrating micromill, with a striking-abrasive action of a steel milling sphere. The composition of the prepared composite mixes is presented in table1.
The samples for research of the optical characteristics were composite films with thickness of 120÷160 microns obtained by the method of hot pressing. The dependences of light transmission (T) and optical density (D) on the wavelength (λ) in the visible area were determined on Cary 100 spectrophotometer.
For research of the adhesive characteristics of the nanomodified FPP the modeling samples of glutinous compounds were prepared. The role of the glued substrates were played by sheets of galvanized steel. Overlapping gluing of the steel sheets by melt on the basis of FPP was done by hot pressing at a temperature near 180°С and specific pressure of 0.5-0.7 МPа. The maximum load necessary for a sliding failure of the glutinous compound was determined on tensile-testing machine at the speed of deformation of 10 mm/min.
The limiting wetting angle (LWA) with distilled water on the surfaces of the polymeric composites of FPP/CNМ/Т150 and PP/FPP/Т150 was measured by the method of a sedentary drop. Pictures of the profiles of the drops placed on the surface of the samples were obtained by means of МBS-10 horizontal microscope equipped with a digital video camera. Calculation of LWA was done by means of nanoImages program.
The supramolecular structure of all the samples of the composites was estimated by the method of the optical polarizing microscopy in a transmitted light by means of Micro–200T microscope equipped with a digital camera.
The morphology of the surface of destruction of the samples of glutinous connections was studied by means of NT-206 atomic-force microscope (АFМ) in a static mode of scanning by CSC 12/15 silicon cantilever. For processing and visualization of the experimental data the SurfaceExplorer (ОDО Microtestmachines) and NanoImages (NITsPR ITMO NAS of Belarus) software was used.
The spectral dependences of D(λ) and Т(λ) composite samples in the range of wavelengths of 330÷800 nanometers are presented in fig.1. As is known, introduction of CNМ in the structure of the transparent polymeric film materials leads to a decrease of their optical transmission in the visible area of the spectrum [12, 13]. As is visible on fig.1а, the FPP initial film is characterized by a much higher optical transmission, than a composite film containing 0.1 mass % of CNМ (curve 2а). A little bit better optical transmission is characteristic for the composite FPP films containing simultaneously 0.1 mass % of CNМ and 0.1÷2.0 mass % of Т-150 nanoparticles (curves 3а, 4а, 5а, 6а). The latter can be due to a more effective destruction of CNM units under the influence of Т-150 particles during the process of preparation of the powder composite mixtures in a vibrating mill. Besides, one should pay attention to the fact that introduction of CNМ and Т-150 in the composition of FPP films promotes lessening of the non-uniformity of the optical transmission.
During calculation of a light absorption coefficient (α, cm-1) the thickness of the composite films was taken into account:
where h – is thickness of a film in microns. The spectral dependences of the light absorption coefficient in the visible range are presented in fig.2. As is obvious, introduction in the composition of the composite films of nanoparticles of silicon dioxide Т-150 promotes an essential decrease of the light absorption coefficient in comparison with the films modified only by CNM.
The results of research of the adhesion and activity of the surfaces and their connection with the supramolecular structure of the considered polymeric nanocomposites are presented in table 2. Introduction in the composition of FPP 0.1 mass % of CNМ (sample 1а) leads to an essential decrease of the shear strength of a glued connection, which, apparently, is due to an intensive agglomeration of nanoparticles of CNМ. Introduction in composition of FPP/CNM of nanoparticles Т-150 (beginning from 0.044 vol. %) promoted an increase of the strength of the glued connections. The maximum increase of the shear strength of the glued connections is reached at the content in thermoplastic adhesive of 0.5 mass % of nanoparticles of Т-150 (sample 4а). With the further increase of the filler content the strength of the glued connections gradually decreases.
Introduction of nanoparticles (both SiO2, and CNМ) in the composition of the specified materials leads to an increase of the water repellency of a surface and, accordingly, reduction of its wettability. The greatest values of LWA (113°) were observed for the composites on the basis of FPP, containing simultaneously nanoparticles of CNМ and Т-150.
Fig.3 and 4 present micropictures of the composite films of FPP/CNM and FPP/CNМ/Т-150. From them it follows, that in FPP/CNM films there are large aggregates with average sizes of 35 microns. In FPP/CNМ/Т-150 film the sizes of aggregates are essentially less. Particles of CNМ, as a rule, are situated in the centre of the spherulites, i.e. they are embryos of a structure formation.
During an atomic-force research of the surface of the destructed glued connections the rupture plane was scanned with various resolutions and apertures of 17 µm2, 9 µm2 and 3 µm2. Such modes of panning allowed us to reveal interesting features of the morphology of the surface on the images and then to detail them.
On AFM images with a low resolution (fig.5) the areas of the glued connections with the integrity broken by a disruptive effort were revealed. The profile lines were drawn in such a way that they cross a glued surface in the zone with the preserved integrity, or one of its edges and a zone with the broken integrity. Fig.5 presents the corresponding profilograms.
For the surface of destruction of the glutinous connection of FPP (fig.5а) a difference of 0.7 microns was registered in the heights from the edge to the pit bottom. The width of the pit "in the zone of the undestroyed adhesive was 8 microns. For the surface of destruction of the glued connection of FPP with 0.1% of CNМ (fig.5b) the fixed difference of heights from the edge to the bottom of a pit was 1 micron. At that, this edge was obviously deformed by a compression effort and lost a thinner morphology of the surface. For the surface of destruction of the glued connection of FPP with 0.1% of CNМ and 0.5% of SiO2 (fig.5с) the recorded difference of the heights from the edge to the pit bottom was 0.5 microns and the further smooth rise was equal to 0.25 microns. The width of the pit "coming" into the zone of the undestroyed adhesive was 5 microns.
On AFM images with average and high resolutions (fig.6) a thin morphology appear of the surface of destruction of the glued connections. The destruction surface of the adhesives was basically covered with spherical formations with characteristic diameter of about 300 nm. Besides, on the AFM images of the surface of FPP (fig. 6a and 6b) and FPP with 0.1% of CNМ (fig.6с and 6d) it is possible to see small infringements of the integrity– circular deckle-edged pits with diameter from 200 up to 800 nm. In fig.6d also visible is rupture of the surface of the adhesive of a triangular form of a considerably smaller size – 50 nm.
On the AFM images of the surface of destruction of FPP with 0.1% of CNМ and 0.5% of SiO2 (fig.6e) a disrupture was registered between the two adhesive masses with a lateral size less than 3 micrometers. The image presented in fig.6f was obtained in the depth of the rupture. It is obvious that the adhesive substrate was not completely removed from the metal substrate. Interesting is the presence of an elongated nonspherical element with a correlation of the lateral diameters of 5:1.
From the analysis of the above-stated experimental data it is possible to draw a conclusion, that a change of the adhesive characteristics of the nanocomposites during introduction of Т-150 nanoparticles is caused by a decrease in aggregation of CNМ and a change of the supramolecular structure of the composite, ensuring an increase of the number of the microplatforms and, as result, of a total area of contact of FPP with a metal surface.
Results of modification
Thus, during research of the spectra of the optical transmission in the visible range an increase was discovered of the transparency of the composite films based on FPP and CNM, when small additives of Т-150 nanoparticles were introduced into their composition. Besides, another fact was revealed – a decrease of the wettability of the surface of such composite materials with distilled water. Filling of FPP with a set of Т-150/CNМ nanoparticles improves adhesion to the metal surfaces, which is manifested in 80-90% higher shear strength of the glued connections "galvanized steel – adhesive - galvanized steel". The changes in the considered characteristics of the composites were accompanied by a reduction of the sizes of their supramolecular structures (spherulites). An increase of the content of Т-150 nanoparicles by more than 0.5 mass % reduced the adhesive strength owing to the effect of screening of formation of new adhesive platforms.
The research was done within the framework of an integration project of fundamental research № 9 (2012-2014) of SB RAS and NAS of Belarus. ■