The polar regions are characterized by extremely low temperatures and wide daily and annual temperature range. These distinctive climatic conditions cause many problems for vehicles and equipment, such as intensification of materials aging, growth of microcracks on equipment parts, decrease of strength, leaking at joints and seals, etc. There are many ways to solve these problems. One of them is use of frost resistant polymer nanocomposites (PNC). Recently, ultra-high-molecular-weight polyethylene (UHMWPE) has attracted attention of researchers as a perspective polymer for wide application fields [1-3]. Modification of UHMWPE using different types of filler can lead to significant enhancement of performance properties. PNC based on UHWMPE often exhibit enhanced mechanical and thermal properties, which can increase equipment durability . Such PNC are promising materials for replacing metals and glass in equipment intended for Arctic use [5-7].
Layered silicates are interesting for researchers as filler for PNC due to their unique structure. Layers of silicates can be exfoliated in polymer volume improving properties such as strength, gas permeability, wear resistance, etc. However, preparation of exfoliated nanocomposites still remains intractable problem because of low compatibility between nonpolar polymers and polar silicates. Synthetic inorganic nanoparticles (NP) are another type of fillers. NP usually form agglomerates, which hardens PNC preparation. Thereby, prevention of agglomeration and disintegration of agglomerates are the most important tasks in PNC preparation. In this paper, we review several methods of PNC preparation which can provide uniform distribution of filler particles in a polymer matrix and improvement of mechanical characteristics.
Powdered UHMWPE (Ticona GUR 4120, Germany) with an average molecular weight of 5×106 was used as a polymer matrix. Nanoparticles of Al2O3, AlN, SiO2, and Si3N4 (Emfutur Technologies, Spain) and Kaolinite, which is a layered silicate (Al4[Si4O10](OH)8), were used as a filler. Cetyltrimethylammonium bromide (CTAB) was used as a surfactant. Ethanol was used as a liquid media for ultrasonic treatment.
NP and the polymer matrix were mixed in a planetary mill (Pulverisette-5, Fritsch, Germany) for 2 min before hot pressing. Surface of kaolinite was modified by mechanical activation in a planetary mill (“Activator-2S”, Russia) for 2 min with the surfactant. All composites were prepared by hot pressing under 10 MPa for 20 min and 175-180 ∘C temperature. Filler content was varied between 0.1 – 5.0 wt.%.
PNC structure was characterized with a scanning electron microscope (SEM, JSM-7800F, JEOL, Japan) and by energy-dispersive X-ray spectroscopy (EDS, X-MAX 20, Oxford, United Kingdom) for chemical composition analysis. Mechanical tests were carried out on a universal testing machine Autograph AGS-J (Shimadzu, Japan) according state standard 11262-80.
2.1. Mechanical Activation of Layered Silicates with Surfactants
The mechanical properties of UHMWPE filled by mechanically activated kaolinite in amounts of 0.5–5.0 wt% have been evaluated . Strength increased by 15% and elongation at break increased by 20% in comparison with the initial UHMWPE. The best results were shown by composites with 0.5–1.0 wt% kaolinite. Thereafter, increase of filler content lead to the deterioration of the mechanical properties of composites.
Kaolinite modified with surfactant can also be used as a UHMWPE filler. However, surfactant solution cannot be applied in the case of kaolinite, because there are no exchangeable cations between silicate layers. There is another way to modify kaolinite surface using mechanical activation in a planetary mill with surfactant. Surfactant can be used as a surface modifier due to long hydrophobic hydrocarbon chain and hydrophilic positively charged end. Composites prepared using the surfactant revealed a 17% increase in strength and 33% increase in elongation at break (Table 1) . Enhanced mechanical properties can be explained by an increase in the adhesive interaction between kaolinite and UHMWPE in the interfacial layers [1, 3]. Also, surfactants can support the disintegration of the agglomerates of particles with high surface energy .
Composites were studied by XRD to find the explanation of mechanical properties enhancement at small amounts of kaolinite in the composites. The peak corresponding to the interlayer space of kaolinite was not detected in diffraction pattern of PNC with surfactant (Fig. 1). Absence of peak indicates that kaolinite exfoliates in the UHMWPE volume . Exfoliated silicate layers provide significant enhancement of the performance characteristics of PNC because of increase effect of silicate on the PNC structure formation process.
2.2. Mechanical Activation of Nanoparticles with Polymer
Determination of tensile strength and elongation at break allowed to estimate effect of mechanical activation on composite properties. As shown in Figure 2, composites with 0.5 – 1 wt.% of nanofiller possessed the best mechanical properties. At higher nanofiller content mechanical properties gradually decrease.
Use of mechanical activation led to 35 – 40% increase in elongation at break. Grinding balls of planetary mill deforms UHMWPE particles during mechanical activation and transform them into flat flakes with dimensions 50 – 200 microns . This process results in increase of mechanical properties because of increase in specific surface area of polymer particles.
Analysis of PNC mechanical properties show that an optimal NP concentration exists . Increase of filler content beyond optimal content leads to considerable decrease in mechanical properties. It is caused by the changes of adhesive interaction "UHMWPE-nanofillers". In this case, PNC's supramolecular structure consists of defective areas and deformed spherulites.
The correlation between the PNC's mechanical behavior and their microstructures is fundamentally important in determining the deformation mechanism. As shown in [11, 12], polymer deformation process consists of two major stages. The polymer's interlamellar amorphous phase is exposed to deformation at first, and this process has elastic character. After that, non-elastic deformation occurs in the spherulites.
The samples prepared in a paddle mixer are shown in Fig. 3b-d and the samples produced by joint mechanical activation in a planetary mill are shown in Fig. 3e-g. As a result, it can be concluded that the joint mechanical activation leads to more uniform distribution of the nanofiller in the polymer matrix than for composites made in a paddle mixer. Distribution uniformity is the explanation of the enhancement of mechanical properties.
2.3. Ultrasound Processing of Components during Mixing in Liquid Media
The scheme of mixing is presented in Fig. 4. Developed technology consists of three stages: 1) NP agglomerates disintegration under ultrasonic vibration impact; 2) formation of UHMWPE and NP suspension; 3) evaporation of liquid media under the ultrasonic vibrations to condense NP on the UHMWPE particles surface.
UHMWPE particles surface was investigated by SEM to estimate effect of ultrasonic treatment. As seen in Fig. 5, ultrasonic treatment provides uniform distribution of NP on UHMWPE particles surface. Distribution of filler in PNC is a critical factor for mechanical properties. Mechanical tests were carried out to clarify effect of ultrasonic treatment (Fig. 6). Significant increase of strength and elongation at break was found for all composites, but with increase of filler content mechanical properties deteriorate. Such changes of properties depend on filler content as usual for nanocomposites [3, 10]. This is due to the high surface energy of NP which increases agglomeration tendency with increase of filler content.
In this article, we reviewed several technologies of PNC preparation. New PNC with improved mechanical properties were obtained using these technologies. Shared feature of them is the reaching of filler uniform distribution in a polymer matrix.
Polymer nanocomposites with exfoliated structure were obtained. These nanocomposites with exfoliated silicates were prepared by treating layered silicates with surfactants during mechanical activation, and then mixing with initial UHMWPE. This type of nanocomposites exhibits improved mechanical characteristics in comparison with the pure polymer and the composites prepared without mechanical activation with surfactant.
The most perfect supramolecular and crystal structure in PNC was formed using the joint mechanical activation of composite components. Also, the presence of specific defective areas and imperfect spherulites in the supramolecular structure of the PNC reduced mechanical properties.
The technology of combining nanoparticles and initial polymer under the influence of ultrasound in a liquid media leads to the most uniform distribution of nanofiller on UHMWPE surface without any significant changes in the structure of the resulting nanocomposite.