Browsing by Author "Dishovsky N."
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Item A Flexible Miniature Antenna for Body-Worn Devices: Design and Transmission Performance(2023-03-01) Al-Sehemi A.; Al-Ghamdi A.; Dishovsky N.; Atanasov N.; Atanasova G.The last few years have seen a rapid increase in body-worn devices because these devices cover a broad spectrum of potential uses. Moreover, body-worn devices still require improvements in their flexibility, size, and weight that necessitate the development of flexible and miniature antennas. In this paper, we present a new flexible miniature antenna for body-worn devices. To ensure flexibility and comfort when the antenna is in contact with the human body, a substrate from natural rubber filled with TiO2 is developed. The miniaturization is achieved using the quadratic Koch curve. The antenna design, optimization, and characterization are performed on a human body model. The performance of the antenna is analyzed in two scenarios: (1) in- to on-body, and (2) on- to off-body wireless communications. The results show that the antenna realized the maximum telemetry range of more than 80 mm for in-body communications and more than 2 m for off-body communications. Moreover, the highest 10 g specific absorption rate value was 0.62 W/kg. These results, in addition to the antenna’s compact dimensions (12 mm × 26 mm × 2.5 mm) and the low manufacturing price, make the proposed antenna an ideal candidate for health telemetry applications.Item A flexible planar antenna on multilayer rubber composite for wearable devices(2017-01-01) Al-Sehemi A.; Al-Ghamdi A.; Dishovsky N.; Atanasova G.; Atanasov N.This paper presents the design of a flexible antenna using planar dipole with a reflector to achieve optimal radiation efficiency and low specific absorption rate (SAR) when the antenna is placed directly over the skin of body model. The antenna is designed for the 2.45GHz frequency band. The parametric analysis of the proposed antenna is carried out. The proposed antenna achieves stable onbody performance: |S11| varies from −16.05 dB (on skin) at 2.47 GHz resonant frequency to −16.40 dB (in free space) at 2.44 GHz resonant frequency. It was found that the maximum 1 g average SAR value is only 0.23W/kg for an input power of 100mW when the antenna is placed directly over the skin of a three-layer body model, and radiation efficiency is 20.5%. The measured results are presented to demonstrate the validity of the proposed antenna.Item Antimycotic activity of sumac extract in composites based on epoxidized natural rubber for application in footwear soles production(2020-01-01) Dishovsky N.; Marinkova D.; Raykova R.; Vladimirova L.Biofilms are attached to different kinds of surfaces or associated with interfaces. The microbial communities are often composed of multiple species, which interact between each other. Attachment is complex process, regulated by diverse characteristics of the growing media, substrates and cell surface. Biofilms have different effects, which can affect changes in surface properties of polymer carriers. Accumulated biomass may provoke negative effects by direct attack, which leads to destruction of polymer matrixes. An option to solve the problem is introduction of bioactive ingredient into the composite. The aim of our study was to find and propose bioactive ingredients, derived from renewable sources, particularly such exhibiting of antimycotic influence, which can be included to elastomeric composite materials. These materials can be used for obtaining of winter footwear soles with adhesion, increasing to various types of icy surfaces. The influence of introduced antimycotic additive, extracted from Cotinus coggygria plant, was investigated. Antimycotic activity of obtained plant extracts against two eukaryotic strains was investigated by diffusion method in agar broth. The formation of biofilms from Candida lypolitica and Candida albicans strains onto natural rubber vulcanizates, kinetics and growth of biofilms strains were also observed.Item Biogenic amorphous silica as filler for elastomers(2018-06-01) Dishovsky N.; Malinova P.; Uzunov I.Natural products from agricultural wastes are finding importance in the polymer industry due to their many advantages such as being lightweight, low cost and environmentally friendly. In the present study the potential of the two types of rice husk ash (RHA) prepared under different conditions as fillers in natural rubber-based elastomer composites was investigated. The fillers were prepared by rice husks incineration and characterized by means of X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET) specific surface area, Hg-porosimetry and N2-adsorption. The evaluation involved determining the vulcanization characteristics of the compounds and their physical and mechanical characteristics, resistance to thermal aging as well as the dynamic properties of the vulcanizates containing RHA. It has been found that the ash from rice husk has good potential as filler in elastomers, especially as a substitute for synthetic commercially available silica. RHA improves the grip on ice and snow better than standard silica and may be used in formulations for shoe soles used for winter sports, extreme hiking footwear, ski boot soles, winter tire protectors, etc. Its quality depends on the conditions under which rice husk is incinerated. It is recommended to run the process at a temperature not higher than 800 °C. In particular, cases in which the vulcanizates contain biogenic silica have very interesting characteristics like improved modulus at 300% of elongation, lower residual elongation, higher mechanical loss angle tangent at 0 °C and lower at 60 °C, and are superior to those of the vulcanizates containing commercially available filler Ultrasil® 7000 GR.Item Composites based on acrylonitrile butadiene rubber designed for sealants contacting potable water(2019-01-01) Nikolova S.; Mihaylov M.; Dishovsky N.The present work reports on the preparation and investigation of the properties of composites based on acrylonitrile butadiene rubber (NBR) aimed for production of sealants in water supply networks carrying hot and cold potable water. The studies involve 12 NBR-based composites. A conventional, semi-efficient and efficient sulphurvulcanization system is used for four of the composites. A vulcanizing agent, dicumyl peroxide, was used in the other eight composites. The results obtained show that the type of the vulcanization system used has a significant impact both on the vulcanization characteristics of the investigated rubber compounds and on the mechanical properties of their vulcanizates. The comparison of the results obtained and the requirements of standard EN 681-1:1996 referring to the mechanical properties of the elastomer composites used for production of sealants for pipes carrying hot and cold potable water reveal that composition P-10 is the most suitable for the purpose. The microbiological characteristics of the water in contact with the composites indicate that no bacterial growth occurs in the samples tested as a result of this contact. The latter does not lead to a change of the organoleptic parameters as well. Higher values of heavy metals, nitrates or nitrites are not observed.Item COMPOSITES OF WASTE RUBBER VULCANIZATES AND MODIFIED ADHESIVE SOLUTIONS TO CREATE CONTINUOUS COATINGS(2021-01-01) Dishovsky N.; Delchev N.; Ivanova N.; Malinova P.; Lachev A.Composites of waste rubber vulcanizates and polyurethane-based adhesive solutions have been developed, containing a waste sunflower oil as a modifying additive, intended for use as seamless floorings. The floorings are made in two versions: in laboratory conditions and in real conditions. The floorings of both variants are characterized by a number of important performance parameters: mechanical properties - in terms of tension and pressure, glass transition temperature, shock absorption according to head injury criterion (HIC) and criterion for maximum value of dynamic impact of free fall acceleration (G), potential elastic energy and energy density. It was found that the most important factors for the formation of the characteristics of the composites are the size, the specific surface of the rubber crumbs, respectively, the chemical nature of the adhesive used, the amount of the adhesive solution and the amount of the modifying additive. Based on the HIC and the G criteria, it is concluded that the created seamless coating has a safe fall height of 1 meter and can be used to cover playgrounds and sports facilities.Item Development And Verification Of Mechanical Characteristics Of A Composite Material Made Of A Thermoplastic Matrix And Short Glass Fibers(2021-01-01) Isametova M.; Abilezova G.; Dishovsky N.; Velev P.The paper presents the results of computer modeling and prediction of the mechanical properties of composite materials with a polycarbonate matrix filled with short glass inclusions. At the micro-level, the influence of the volume of inclusions on the mechanical properties of the designed composite based on polycarbonate matrix is studied in the DIGIMAT (France) program. It was found that with a ratio of the sizes of inclusions in the range of 468÷60, the particles have a needle shape, and the material with such inclusions has a higher stress limit and elastic modulus than with a shape coefficient less than 50. The components of the fiber orientation tensor were also determined, at which the values of computer modeling are in good agreement with experimental data. The influence of the size of the finite element grid on the characteristics of the composite at the macro level was studied, and recommendations were given for choosing the size of the face of the finite element. The adequacy of computer models was confirmed by the results of field tests. The paper presents the results of testing flat samples made by injection molding technology. Mechanical tests were carried out for three variants of samples made of composite material based on a polycarbonate matrix with 10 %, 20 % and 30 % inclusions. The discrepancy between the experimental and computer results for samples with 10 %, 20 % content of short chopped fibers is explained by the influence of technological factors on the properties of the material at the macro-level. The conducted research allowed us to develop a computer modeling technique used at the stage of development of polymer composites based on thermoplastic matrices with short glass inclusionsItem Dielectric and microwave properties of siloxane rubber/carbon black nanocomposites and their correlation(2011-01-01) Al-Hartomy O.A.; Al-Solamy F.; Al-Ghamdi A.; Dishovsky N.; Iliev V.; El-Tantawy F.In this paper, the dielectric and microwave properties of carbon black/siloxane rubber-based nanocomposites have been investigated in the frequency range from 1 GHz till 12GHz according to the content of carbon black and the frequency. It has been established that the increasing frequency and filler content lead to an increase in the relative permittivity and tangent of dielectric loss angle. At higher filler content, the effects become more pronounced, especially those upon dielectric loss. It has been also established that there are two well-distinguished areas in all dependences of microwave properties on frequency and filler content increasing. The first is between 1 and 8 GHz wherein the reflection and attenuation of microwaves do not change considerably with frequency and filler content alternation while shielding effectiveness worsens. The second area is between 8 and 12 GHz wherein the reflection and attenuation of microwaves increase drastically with the increasing frequency and filler content. Shielding effectiveness improves, too. It has been established that in all cases the degree of correlation between dielectric and microwave properties evaluated on the basis of the coefficients of correlation calculation is perfect. Copyright © 2011 Omar A. Al-Hartomy et al.Item Effect of activated carbons on the dielectric and microwave properties of natural rubber based composites(2017-10-01) Al-Sehemi A.; Al-Ghamdi A.; Dishovsky N.; Nickolov R.; Atanasov N.; Manoilova L.The effect of six different types of activated carbons has been studied in view of their applicability as functional fillers upon the dielectric and microwave properties of natural rubber based composites. It is found that the textural characteristics of the studied active carbons exert influence both on the real part of dielectric permittivity and dielectric loss angle tangent, as well as on the microwave properties of the studied composites. The composite containing activated carbon on the basis of wooden material possesses the highest value of the total shielding effectiveness in comparison with the composites, containing another activated carbons. It has the lowest values of specific surface area, area and volume of micropores and the highest values of external surface area, volume of mesopores and average pore diameter. With gradually increasing the values of the indices in the first group and gradually decreasing the values of the indices in the second group, the value of total shielding effectiveness decreases. It is obvious that these indices have influence on the rubber matrix-filler particle interactions, such as multicontact chain adsorption to the surface of the filler. The spatial inhomogeneities formed give rise to polarization phenomena and to a frequency dependence of the dielectric properties.Item Effect of the modified solid product from waste tyres pyrolysis on the properties of styrene-butadiene rubber based composites(2014-01-01) Delchev N.; Malinova P.; Mihaylov M.; Dishovsky N.Being resistant to moisture, oxygen, ozone, solar radiation, microbiological degradation, etc., waste car tyres and their deposition present a great environmental problem. Pyrolysis is an environment-friendly process for their recycling. This paper presents data referring to the study of the effect of the solid product from waste tyres pyrolysis on the properties of styrene-butadiene rubber (SBR) based composites. It is found that the further modification of the pyrolysis carbon black decreases two times the ash content and six times the zinc oxide concentration in the ash but the curing characteristics of the investigated SBR based compounds remain unaffected. The values of the modulus 100, the tensile strength and Shore A hardness of vulcanizates containing modified pyrolysis carbon black increase, while the elongation at break and the residual elongation decrease slightly. The effects observed are related to the decreased ash content of the pyrolysis carbon black following the modification as well as to the increased silicon oxide concentration in the remaining ash.Item Flexible polymer/fabric fractal monopole antenna for wideband applications(2021-01-01) Al-Sehemi A.; Al-Ghamdi A.; Dishovsky N.; Atanasova G.; Atanasov N.A novel flexible polymer/fabric fractal monopole antenna with a wideband performance is presented. A thin sheet of highly conductive fabric and a natural rubber-based composite have been used for conductive and non-conductive parts of the antenna, which allow keeping the antenna as flexible and thin as possible. The proposed antenna has been simulated, prototyped and tested. Results show that the antenna has a simulated impedance bandwidth of 3.8 GHz (2.2–6.0 GHz) and a measured impedance bandwidth of 3.7 GHz (2.3–6.0 GHz) to cover the most commonly used standards in wireless communication systems. The radiation efficiency of the antenna reaches over 93% throughout the operating frequency band with satisfactory radiation patterns and gain.Item Friction behavior produced in the course of a contact enabled between composite materials and eco-friendly soles prototypes made of elastomeric material with regard to ice-covered surface(2019-03-01) Kandeva M.; Dishovsky N.The subject matter of the present research constitutes a study of the technique of friction between samples and prototypes of soles made of composite materials comprising natural rubber characterized with ingredients originating from renewable sources produced in the course of their contact with an ice surface. Methods and devices were developed in order to investigate the static and dynamic friction force and the change with regard to frictional force in the event of contact with regard to three varieties of forms of ice: melting ice (ice with a layer of water on it, temperature: 0 to 1 °C), wet ice (temperature: - 4° -5° C) and dry ice (temperature: - 10° - 12° C). Coefficients of static and dynamic friction, along with change of the friction coefficient were obtained for 14 types of elastomers and ingredients from renewable sources - natural rubber, silicon dioxide extracted from the process of burning rice husks and microcrystalline cellulose, and using rapeseed oil as a technological additive. It has been ascertained and concluded that all tested materials have demonstrated highest friction coefficient in the event of contact produced with dry ice being the maximum coefficient of friction equal to 0.61. The most unfavorable and dangerous hypothesis for exposure of pedestrians to the effect of on an ice- covered surface are the cases of contact with wet ice and melting ice. There is no correlation of the coefficient of friction for the different materials under conditions of dry ice, wet and melting ice. The composites with the highest coefficient of friction under circumstances of wet and melting ice have been used to produce prototypes of soles, having identical pattern, and experimental results were obtained for the coefficients of friction and the friction coefficient change /rise/ in the event of contact produced with regard to dry, wet and melting ice. By means of implementation of the 3D printing technology, patterns of footwear soles with identical dimensions were made of the same material but with different patterns and results were obtained for the impact of the pattern on the static and dynamic coefficient of friction produced in the course of a contact with an ice surface in its three relevant states - dry, wet and melting ice. The implementation of this study has been funded by the Operational Program 2014BG16RFOP002 - Innovation and Competitiveness, Procedure: BG16R-FOP002-1.002 - Support for the development of innovations by start-ups, Grant Agreement BG16R-FOP002-1.002-0028-C01 Eco-footwear soles on elastomeric basis with improved ice and snow grip.Item Gas chromatography-mass spectrometry for characterization of liquid products from pyrolysis of municipal waste and spent tyres(2015-12-01) Pavlova A.; Stratiev D.; Mitkova M.; Stanulov K.; Dishovsky N.; Georgiev K.Liquid products produced from two different types of waste pyrolysismunicipal wastes and spent tyre wastes are investigated using gas chromatography- mass spectrometry. This method has been applied for detailed identification of composition of the samples. The components were characterized in terms of their Kovts retention indices on a PONA capillary column. The obtained analytical data were successfully used for the characterization of the samples. More than three hundred compounds were detected. The liquid products were complex mixtures, composed mainly of C4-C12 compounds. The examination of the selected m/z values very clearly indicates the existence of the different groups of compounds. With a lot of olefins content (31.9%), followed by aromatics (20.0%), paraffins (17.3%), and naphthenes (7.5%), it is described as the liquid product from pyrolysis of municipal solid wastes. The aromatic compounds in liquid product from pyrolysis of spent tyre wastes have the highest concentration (33.5%), and they are followed by naphthenes (28.6%), olefins (19.2%), and paraffins compounds (7.0%). The present study has shown that the pyrolysis of municipal waste and spent tyres can be used as a means for reduction of environmental pollution and production of liquid product which could be used as a fuel source.Item Influence of bis(triethoxysilylpropyl) tetrasulfide amount on the properties of silica-filled epoxidized natural rubber-based composites(2016-07-01) Al-Hartomy O.A.; Al-Ghamdi A.A.; Al Said S.A.F.; Dishovsky N.; Mihaylov M.The aim of the present article is to investigate the influence of the amount of bis(triethoxysilylpropyl) tetrasulfide on the curing characteristics and mechanical and dynamic properties of rubber composites based on epoxidized natural rubber (Epoxyprene 50) filled with 70 phr silica. The obtained results showed that although the interaction between the epoxy groups of epoxidized natural rubber and the silanol groups of silica through hydrogen bonds improves the dispersion of filler in the rubber matrix, the presence of silane coupling agents is necessary to obtain rubber compounds and vulcanizates with good vulcanization characteristics and mechanical and dynamic properties.Item Influence of bis(triethoxysilylpropyl) tetrasulfide amount on the properties of silica-filled epoxidized natural rubber-based composites(2016-07-01) Al-Hartomy O.A.; Al-Ghamdi A.A.; Al Said S.A.F.; Dishovsky N.; Mihaylov M.The aim of the present article is to investigate the influence of the amount of bis(triethoxysilylpropyl) tetrasulfide on the curing characteristics and mechanical and dynamic properties of rubber composites based on epoxidized natural rubber (Epoxyprene 50) filled with 70 phr silica. The obtained results showed that although the interaction between the epoxy groups of epoxidized natural rubber and the silanol groups of silica through hydrogen bonds improves the dispersion of filler in the rubber matrix, the presence of silane coupling agents is necessary to obtain rubber compounds and vulcanizates with good vulcanization characteristics and mechanical and dynamic properties.Item Influence of carbon black structure and specific surface area on the mechanical and dielectric properties of filled rubber composites(2011-01-01) Al-Hartomy O.A.; Al-Solamy F.; Al-Ghamdi A.; Dishovsky N.; Ivanov M.; Mihaylov M.; El-Tantawy F.Natural rubber based composites have been prepared using various amounts of two fillers: conventional Corax N220 carbon black or electrically conductive carbon black Printex XE-2B which has a very high specific surface area. The composites have been studied by dynamic mechanical thermal analysis, dielectric thermal analysis and SEM. It has been established that all vulcanizates investigated are in the glass state in the -80°C to -40°C interval. The storage modulus increases with the increasing filler content in the -40°C to +80°C interval when the vulcanizates are in the highly elastic state. DETA shows that the increase in filler content leads to an increase in the dielectric permittivity (ε′). ε′ also increases with temperature increasing. Higher frequency causes a decrease of ε′ values which becomes more pronounced with the increasing filler content. Obviously, when the content of Printex XE-2B carbon black in the vulcanizates is higher than 7.5 phr, the percolation threshold is reached and the ε′ values increase up to 102-104. The ε′ values for the vulcanizates comprising 20 and 50 phr Corax N220 carbon black are measurable with those for the vulcanizates comprising 5 and 10 phr Printex XE-2B carbon black respectively. The results obtained could be explained by the difference in the structure and specific surface area of the two types of carbon black - Printex XE-2B and Corax N220. Copyright © 2011 Omar A. Al-Hartomy et al.Item Influence of carbon black structure and specific surface area on the mechanical and dielectric properties of filled rubber composites(2011-01-01) Al-Hartomy O.A.; Al-Solamy F.; Al-Ghamdi A.; Dishovsky N.; Ivanov M.; Mihaylov M.; El-Tantawy F.Natural rubber based composites have been prepared using various amounts of two fillers: conventional Corax N220 carbon black or electrically conductive carbon black Printex XE-2B which has a very high specific surface area. The composites have been studied by dynamic mechanical thermal analysis, dielectric thermal analysis and SEM. It has been established that all vulcanizates investigated are in the glass state in the -80°C to -40°C interval. The storage modulus increases with the increasing filler content in the -40°C to +80°C interval when the vulcanizates are in the highly elastic state. DETA shows that the increase in filler content leads to an increase in the dielectric permittivity (ε′). ε′ also increases with temperature increasing. Higher frequency causes a decrease of ε′ values which becomes more pronounced with the increasing filler content. Obviously, when the content of Printex XE-2B carbon black in the vulcanizates is higher than 7.5 phr, the percolation threshold is reached and the ε′ values increase up to 102-104. The ε′ values for the vulcanizates comprising 20 and 50 phr Corax N220 carbon black are measurable with those for the vulcanizates comprising 5 and 10 phr Printex XE-2B carbon black respectively. The results obtained could be explained by the difference in the structure and specific surface area of the two types of carbon black - Printex XE-2B and Corax N220. Copyright © 2011 Omar A. Al-Hartomy et al.Item Investigation of long time stored propellants(2014-01-01) Nedkova M.; Shishkov P.; Serafimova E.; Dishovsky N.An investigation of double based propellants and pyroxiline, produced before the year 2000 was carried out. The process of ageing during long time storage of single and double-based propellants and pyroxilin in non heated military stores and in heated building at room temperature was studied by elemental analysis (EA). The velocities of burning were measured by DSC and the decomposition energy of the propellants was calculated. Obtaining of a shift of the bands, con-nected by hydrogen bonds formation between the obtained-OH groups at the place of-ONO2 groups after ageing was confirmed by FTIR spectroscopy.Item Microwave properties of natural rubber based composites containing carbon black-magnetite hybrid fillers(2018-04-25) Al-Ghamdi A.; Al-Hartomy O.; Al-Solamy F.; Dishovsky N.; Malinova P.; Shtarkova R.The paper presents the synthesis and characterization of a carbon black-magnetite hybrid filler. The complex study on the structure of the filler has shown the magnetite phase to be distributed both over the surface (inter-aggregately) and inside (intra-aggregately) the carbon black particles, thus forming a true hybrid material. The results from the investigations on the mechanical and microwave properties of natural rubber-based composites filled with the new hybrid filler have been also reported. They have been compared to those of a composite comprising the physical mixture of carbon black and magnetite (at the same ratio as in the hybrid filler). The determined microwave characteristics of the composite comprising the hybrid filler obtained reveal the possibility for its use in manufacturing elastomer-based microwave absorbers.Item MIXED PEROXIDE/SULFUR VULCANIZATION OF ETHYLENE-PROPYLENE TERPOLYMER BASED COMPOSITES. CURING CHARACTERISTICS, CURING KINETICS AND MECHANICAL PROPERTIES(2022-01-01) Nikolova S.; Mihaylov M.; Dishovsky N.Ethylene propylene diene monomer (EPDM) based composites filled with carbon black were studied to evaluate the effect that a mixed peroxide/sulfur vulcanization system in the absence or presence of the ethylene glycol dimethacrylate (EGDMA) co-agent has on the vulcanization characteristics, the kinetic parameters of the vulcanization process, as well as on the mechanical properties before and after accelerated heat aging. The results obtained show that, implementation of a mixed peroxide/sulfur vulcanization system in the presence of EGDMA as a co-agent is a promising opportunity to expand the fields of application of organic peroxides in rubber industry.