Michal Cermak

Structural and mechanical behaviour of LLDPE/HNT nanocomposite films

The paper briefly describes structural and mechanical influences of Halloysite nanotubes (HNT) in different level of fulfilment (0, 1, 3, 7 wt%) in the LLDPE commonly used in the cable industry. The influence of HNT on the polymer has been observed and evaluated through the average crystallite size and the micro- deformation by X-Ray diffractometry and the imaging of SEM. Despite the certain inter-phase tension between the polymer and HNT, the influence on the mechanical and combustion behaviour was observed. Measurement showed a higher content of agglomerates in the sample with 7 wt% HNT fulfilment.

Dielectric analysis of halloysite nanotubes LLDPE nanocomposite compounds

The aim of this paper is to investigate the influence of HNT on the dielectric properties of LLDPE (linear low density polyethylene) nanocomposite compounds in the different level of fulfillment (0, 1, 3, 7 wt%). The utilization of these HNT/LLDPE nanocomposite compounds can be seen in the cable industry, where the dielectric properties are essential. Used experimental compounds have been prepared in the laboratory blades mixer Brabender W50 EHT and the plasticorder Brabender PLE 651. Immediately after mixing, the molten compounds were removed from the mixer, inserted into the spacer, and pressed into films of 0.25 mm thickness. Dielectric properties in the DC field (polarization current, reading of polarization index, volume resistivity) of the individual samples were measured and the temperature and frequency range by Broadband dielectric spectroscopy Alpha-A (Novocontrol) were investigated. Samples have been subjected to the dielectric strength test according to standards. As results showed, experiment...

In-situ monitoring of resin curing process based on interdigital impedance sensors

The online monitoring of resin curing process is described in the presented article. There exists several monitoring technique useful for resin curing monitoring and consequently for the curing technology improving, at the present. The advantage of the described monitoring system and monitoring method is in the using a unique sensors directly inserted into the tested resin or technology part which allows observing the resin characteristics as well as the local temperature directly at the sensor area. The data acquisition is processed by the in-house developed portable curing monitoring device and wirelessly transfered to the PC for further analysis. The study describes the experimental work of using the monitoring system and sensors and also the curing process of resins and composites using electrical impedance magnitude.

Investigation of filling level and processing technology influences on dielectric behavior of PE/HNT nanocomposites

This paper provides an overview of the dielectric behavior of composites consisting of polyethylene and halloysite nanotubes. The aim is to evaluate the changes in the dielectric properties depending on the different level of HNT filling and differences in processing technologies. The emphasis is placed on the evaluation of dielectric polarizations and their basic mathematical description. Furthermore, the attention is given to the behavior of electric conductivity. Composites were divided into two groups according to the used processing technologies and were prepared with three different filling levels. The first processing method included a simple kneading and compressing of the mixture and the second method used a small scale extrusion. The real part and the imaginary part of a complex relative permittivity were selected as key variables through the Broadband Dielectric Spectroscopy. Results obtained by measuring over a wide frequency and temperature range were supplemented by a mathematical model of dielectric polarizations according to Havriliak-Negami theory. Moreover, the most probable relaxation times of observed polarizations were calculated for each temperature and the fitting according to Arrhenius theory was performed.

Optimization of the curing process of epoxy resins using methods of structural analysis

Epoxy resins belong among materials frequently used for the production of impregnating composites especially for the construction of motors, generators, and transformers. During production, great emphasis must be focused on the proper curing process of these materials. Otherwise, it is not possible to guarantee the satisfactory properties of the final product. The paper is therefore focused on the analysis of the level of curing of epoxy resin by methods of structural analyses. The Fourier Transform Infrared Spectroscopy (FT-IR) and glass transition temperature evaluated from a reversible signal of heat flow measured by modulated differential scanning calorimetry (MDSC) were used. Curing process was performed in a laboratory oven. Partly and fully cured samples were analyzed during two independent experiments. These two experiments differ to each other especially by different times of sampling. The trend of the changes of infrared spectra measured on samples during the curing process corresponds well with the results of the glass transitions temperature measurements obtained by MDSC. Insufficient curing state is relatively well detectable by the presence of the spectral bands corresponding to the vibration of epoxy and anhydride groups (901, 1875, 1786 cm-1). Both methods are acceptable to detection of the curing level but with different sensitivity. While infrared spectroscopy is possible to use for prediction especially short times of curing, analysis of the glass transition is sensitive during the whole curing process.

Influence of Halloysite nanotubes in low-density polyethylene on electric polarization

This paper deals with the influence of nanofiller on the behaviour of polymer dielectric in an electrical alternating field. As the basic polymer is selected a linear low-density polyethylene and as a nanofiller a perspective material based on aluminosilicates known as the Halloysite nanotubes is used. A broadband dielectric spectroscopy is applied as a diagnostic method. This method allows us to measure the real and imaginary part of the complex relative permittivity. The effect of different filling of polymer material with nanotubes to the ongoing polarization process is evaluated on the basis of the trend of the permittivity. Besides the influence of the simple presence of nanofiller, the effect of variable temperature is also taken into consideration. The measured data is evaluated as the frequency and temperature dependence with an emphasis on the interconnection of frequency and temperature effects. Subsequently, the data are processed as Cole-Cole diagrams (circular and linearized). These diagrams offer a slightly different view on the solved issue. The paper concludes with a calculation of the relaxation times of the polarization, which were registered in the selected frequency and temperature range. The relaxation times calculation is based on the theory of Cole-Cole diagrams. Linearized Cole-Cole diagram has been designed mainly for the calculation of relaxation times.

Polarization of LLDPE/HNT and HDPE/HNT blends in AC electric field

Halloysite nanotubes (HNT) represent a type of aluminosilicate clay of natural origin with nanotubular structure. HNT have been started to use in many industrial fields, including the application in the electrical engineering. This paper is focused on the determination of dielectric properties of pure halloysite nanotubes, as well as, a polyethylene (LLDPE and HDPE) based HNT nanocomposite (blends). Individual samples in each series had contained different amount of HNT filler. The impact of the HNT content and the impact of the type of polyethylene on the dielectric properties were observed and evaluated. Measurements were carried out under the AC field via a broadband dielectric spectroscopy. Dielectric properties were evaluated through real and imaginary parts of the complex relative permittivity. Results are displayed as surface visualizations of frequency and temperature dependence. The relaxation times of selected relaxation processes were analyzed according to the theory of relaxation polarization.

Fourier transform infrared spectroscopy as a tool for identification of the curing process of epoxy resin

Paper deals with analysis and description of the curing process of the samples made of the epoxy resin using method of Fourier transform infrared spectroscopy (FT-IR) in technique of Attenuated Total Reflectance (ATR). Infrared spectra were measured on samples cured in two steps with isothermals at 93 °C and 150 °C when samples in delivered state and cured for 20, 42, 52, 60, 70, 90 and 110 minutes were analysed. Infrared spectra were recorded on Nicolet 380 spectrometer (Nicolet Instrument) using Attenuated Total Reflectance technique (ATR) with ZnSe crystal in the range of wavenumbers of 4000 - 600 cm-1. Spectral bands of anhydride and epoxy group were used for the curing identification. The glass transition temperature was measured as a comparative parameter for the curing level investigation. On the basis of results, good applicability of the method of Fourier transform infrared spectroscopy for identification of un-cured samples is evident.

A comparative study of dielectric, mechanical and structural properties of fire-protective insulation based on XLPE

Paper deals with comparing of dielectric, mechanical and structural properties of primary cable insulation based on cross-linked low density polyethylene (XLPE) with different level of cross-linking. Testing material was divided into four groups according to time of exposition to low pressure steam, which initiates cross-linking reaction. The measurement of the degree of cross-linking by chemical extraction in xylene solution at 140°C according to ASTM D2795-11 followed. Dielectric properties (polarization index, volume resistivity) of the individual samples were then measured according to standards. Dielectric properties were compared with results of tensile strength test at various temperatures (50°C, 55°C, 60°C) measured by dynamic mechanical analysis. The degree of cross-linking represented by the gel content positively influences the tensile strength, the volume resistivity and the change of structural behaviour of the material by differential scanning calorimetry was also observed. But it negatively affects the polarization indexes. Changing measured parameters could be explained by the change of the reaction dynamics in accordance with the time of exposition which initiate cross-linking reactions.