Effect of temperature on initial stage of methyl methacrylate free radical bulk polymerization

Classic theory of free radical polymerization is based on two assumptions. The first is that the concentration of initiator is slightly changed and could be taken as a constant value. The second assumption is that the rate of initiation is equal to the rate of termination. Equation for the polymerization rate based on these assumptions can not successfully describe the initial stage of free radical polymerization. In order to solve this problem, three mathematical models were developed and used to verify of the mentioned assumptions. The models were fitted to experimental data and qualities of their fits were compared. Experimental data of isothermal bulk polymerization of methyl methacrylate were obtained by differential scanning calorimetry at 60, 70, 80 and 90°C with initiator concentration (AIBN) of 0.5 wt%. The best fit was shown by the model that assumes constant concentration of initiator during initial stage but takes into account that the rate of initiation is not equal to the rate of termination at the beginning

Zdravstvene karakteristike starijeg stanovništva Srbije i korišćenje zdravstvene zaštite

The analysis of both survey and routine statistical data has shown that seniors most frequently have cardiovascular and muscle bone health problems. During one year they make approximately 10 visits, one third more then 5 visits, 20% makes 5 visits and 30% 2-3 visits, 30% of this population are hospital attendants.Sa starenjem dolazi do značajnih promena u zdravstvenom stanju i zdravstvenim potrebama. U cilju sagledavanja zdravstvenih karakteristika i obima korišćenja zdravstvene zaštite starih lica u Srbiji, analizirani su podaci anketnog istraživanja (2000. godine) slučajno izabrane 2.301 stare osobe u Srbiji, kao i podaci rutinske zdravstvene statistike (1991-1999. godina). Rezultati su pokazali da u morbidite tu starih osoba dominiraju kardiovaskularne bolesti i bolesti mišićnokoštanog sistema, zatim povrede, bronhitis i dijabetes melitus. Oni koriste oko 10 usluga godišnje po jednoj osobi, odnosno 30% koristi po 2-3 usluge, oko 20% po 5 usluga, a veći broj usluga koristi trećina starih lica. Bolničku zaštitu koristi oko 30% njih. Oni odlaze lekaru najčešće zbog bolesti i povreda (44%), kontrole (28%) i propisivanja lekova (18%). Znači, stare osobe znatno češće obolevaju i koriste zdravstvenu zaštitu, nego osobe od 19 do 64 godine

The High-Energy Irradiation Ageing of Reinforced Elastomers Based on Rubber Blends

Elastomers are very often used in severe environments, for instance, in nuclear power plants, where they may be degraded by high-energy radiation and heat. The ageing behaviour of materials used in different equipments is very important. Elastomers based on chlorosulfonated polyethylene (CSM) are used for cable jacketing materials and have excellent radiation resistance needed in nuclear power stations. In the current work, the influence of gamma-irradiation dose (100, 200 and 400 kGy) on the ageing of reinforced blends based on CSM, styrene butadiene rubber (SBR) and natural rubber (NR) has been evaluated. The content of silica in CSM/SBR and CSM/NR rubber blends was varied. The curing behavior of compounds was estimated using the oscillating disk reometer. The irradiation of nano composites has been performed in air in the Co 60 radiation sterilization unit with the dose rate of 10 kGy h(-1). The thermal properties were studied by thermogravimetric test. The mechanical properties (hardness, modulus at 100% elongation, tensile strength and elongation at break) were determined before and after irradiation. A decrease in the elastic modulus and in the strain at break has been observed with increasing irradiation. At higher doses, the network chain scissions become the main degradation process, the cross-linked topology becomes irregular, and material contains more and more weak zones, which deteriorate the ultimate properties.4th International Conference on Radiation and Applications in Various Fields of Research (RAD), Proceedings, May 23-27, 2016, Nis, Serbi

Thermo-mechanical properties of rigid polyurethane foams with cement addition

The parameters of foam preparation, such as addition of fillers, have a significant effect on the properties of polyurethane foams. The goal of this work was to study the effect of the cement addition on the thermo-mechanical properties of rigid polyurethane foams. Isocyanate component was based on polymeric diphenylmethane diisocyanate, and polyol component was polyether type, with addition of castor oil as polyol. It was estimated that the addition of cement increased the values of foam compressive strength and permanent deformation. Compressive strength increased even more than 90% with 20 wt% of cement. Further increase of the cement loading deteriorates the mechanical properties of foamed material because it distorts cellular structures of obtained materials. Addition of cement increased the value of the glass transition temperature of polyurethane foam. The highest increase in the value of glass transition temperature, for 7.5ºC, was achieved by the addition of 20 wt% of cement

Influence of different functionalization methods of multi-walled carbon nanotubes on the properties of poly(L-lactide) based nanocomposites

This paper presents influence of the type of carbon nanotube functionalization on properties of poly(L-lactide) (PLLA) based nanocomposite materials. For this purpose surface modifications of multi-walled carbon nanotubes (MWCNTs) were performed by chemical and irradiation techniques, while thermo gravimetric analysis, UV-Visible and Fourier-transform infrared (FT-IR) spectroscopies confirmed successful covalent functionalization. Series of PLLA bionanocom-posites with different contents of functionalized MWCNTs (0.7; 1.6; 2.1 wt%), were synthesized via ring-opening solution polymerisation of L-lactide. FT-IR analysis confirmed that grafting of L-lactide, under controlled condition, is possible to perform starting from the surface of functionalized MWCNTs. From differential scanning calorimetry results it was concluded that even low contents of chemically and irradiation functionalized MWCNTs had a significant effect on thermal properties of the prepared nanocomposites, raising the values of melting and glass transition temperatures. Thermogravimetric analysis (TGA) has shown that the degradation onset temperature for composites with chemically functionalized MWCNTs, was much higher than that for the neat poly(L-lactide) sample and composites with irradiation functionalized MWCNTs. Morphology studies by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that poly(L-lactide) covered surfaces and separated functionalized MWCNTs. Good dispersion of carbon nanotubes in polymer matrix enabled conductivity of synthesized materials, as determined by conductivity tests

Isoconversional kinetic analysis of the alkyd/melamine resins curing

The curing reaction for the mixtures of alkyd resins based on ricinoleic acid, phthalic anhydride and three polyols (glycerin, trimethylolpropane or ethoxylated pentaerythritol) with two different commercial melamine resins was investigated by differential scanning calorimetry (DSC). The curing kinetics analysis was performed using the isoconversional methods (Ozawa-Flynn-Wall, Kissinger-Akahira-Sunose and Friedman). Isoconversional methods were carried out with three heating rates (5, 10 and 20°C/min) in a scanning temperature range from 40 to 250°C. It was found that the curing activation energy of resin mixtures is influenced by alkyd and melamine resin type due to the catalytic effect of hydroxyl group on the reactions. The dependence of apparent curing degree on time, which was obtained by mathematical transformations of dynamic DSC data using Ozawa-Flynn-Wall method, describes well the isothermal DSC experiments

Low power laser efficacy in the therapy of inflamed gingive in diabetics with parodontopathy

Background/Aim. There is clear evidence on direct relationship between periodontal disease and diabetes mellitus. Many investigations point out greater prevalence and severity of periodontal disease among diabetic patients. During last decade, low level laser therapy has been used in periodontal therapy. It has biostimulative effect, accelerates wound healing, minimizes pain and swelling, and there is almost no contraindication for its usage. The aim of the paper was to investigate the efficiency of low level laser therapy as adjuvant tool in reduction of gingival inflammation in diabetic patients. Methods. The study incuded 150 participants divided into three groups: group I (50 participants with diabetes mellitus type 1 and periodontal disease), group II (50 participants with diabetes mellitus type 2 and periodontal disease), group III (nondiabetic participants with periodontal disease). Gingival health evaluation was done using gingival index Löe-Silness. Soft and hard deposits were removed, periodontal pockets cleaned and GaAlAs low level laser therapy (5 mW) applied five consecutive days. In each patient, low level laser therapy was not applied on the left side of the jaw in order to compare the effects of the applied therapy. After the first, third and fifth therapy and one month after the last visit gingival index was evaluated. Before the first and after the fifth therapy exfoliative cytology of gingiva was done and nuclei areal was analyzed morphometrically. Results. After all investigated periods, gingival index and nuclei areal were significantly decreased comparing to values before the therapy, at both jaw sides (p < 0.001). After the 1st, 3rd and 5th therapy, the t-test showed a significantly decreased gingival index at the lased side of jaw comparing to non-lased side. Conclusion. Low level laser therapy is efficient in gingival inflammation elimination and can be proposed as an adjuvant tool in basic periodontal therapy of diabetic patients

Modification of epoxy resins with thermoplastic segmented polycarbonate-based polyurethanes

In this work, epoxy hybrid materials were synthesized by addition of thermoplastic segmented aliphatic polyurethanes with good elastic properties. The modified epoxy samples were obtained by curing of previously homogenized mixture of prepared polyurethane melts, epoxy resin and crosslinking agent Jeffamine D-2000. The influence of different weight content of polyurethanes (5, 10 and 15 wt. % compared to pure epoxy resin) as well the influence of different hard segments of elastomers (20, 25 and 30 wt. %) on the curing of modified epoxy systems was studied. The curing was followed by differential scanning calorimetry (DSC), in dynamic regime from 30 to 300°C, at three heating rates (5, 10 and 20°C/min). With the increase of hard segments content of polyurethanes added in higher concentration (10 and 15 wt. %) into epoxy matrix, the temperature of maximum ratio of curing was shifted to lower values (from 205 to 179°C). Obtained DSC data were analyzed using two integral methods (Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose) and one differential kinetic model (Friedman). The significant differences were observed in the second part of the epoxy curing (for the reaction degrees higher than 60 %), where the values of activation energies remarkably increase. The addition of polyurethane elastomers retarded the curing process due to decreased mobility of reactant molecules caused by higher viscosity of reaction mixture. By detailed analysis of determined kinetic parameters, it is concluded that the influence of slow diffusion is more pronounced in the presence of thermoplastic polycarbonate-based polyurethanes, which confirmed their effect on the mechanism of epoxy curing. The highest tensile strength and hardness showed the DGEBA modified with the polyurethane with highest hard segment content. Increasing the hard segment content of polyurethane and its concentration in matrix, the tensile strength of modified epoxy was increased. The elongation at break of modified epoxy samples was significantly improved by addition of polycarbonate-based polyurethanes with low hard segment content, due to higher content of flexible soft segment chains. [Projekat Ministarstva nauke Republike Srbije, br. III 45022), i Pokrajinski Sekretarijat za nauku i tehnološki razvoj (projekat 114-451-2396/2011-01). Autor iz Praga duguje zahvalnost „the Grant Agency of the Czech Republic“ (Czech Science Foundation, project No. P108/10/0195).

Incidence and treatment outcome of oral lichen planus in Southeast Serbia in a 10-year period (1997-2007)

Background/Aim. Lichen planus is a chronic, immunologic, mucocutaneous disease with a wide range of clinical manifestations. The aim of this retrospective study was to evaluate the most common forms of oral lichen planus (OLP) and its symptoms and to describe treatment responses in patients during 10-year period. Methods. The study was conduced on 163 OLP patients who came in the Department of Oral medicine and Periodontology between 1997 and September 2007. Each case was classified into one of four clinical subtypes: reticular, atrophic, erosive-ulcerative, bullous. Results. There was no significant difference in patients age. Women were found to be significantly more likely to have OLP (p < 0.001). Corticosteroids were effective in reducing symptoms, erythema and healing ulcers. Improvement was shown over a long term in 61.35% patients. Over the long term 38.65% patients maintained the same type of OLP or it became a more severe type. Two patients (1.22%) developed oral carcinoma during the follow-up period. Conclusion. The response of patients with erosive OLP to a short course of systemic corticosteroids often was quite remarkable. However, symptoms and signs tended to recur after this treatment. Periodic examinations, patient education, medical treatment, monitoring of side-effects as well as follow-up biopsies are necessary for management of OLP patients

The influence of montmorillonite content on the kinetics of curing of epoxy nanocomposites

In this work, the attention was paid at the investigation of montmorillonite dispersion in epoxy/amine systems due to improved final properties of the nanocomposites. The influence of different montmorillonite content on the kinetics of curing of epoxy/Jeffamine D-230 systems was followed by differential scanning calorimetry (DSC). The curing of epoxy nanocomposites was performed using dynamic regime at three different heating rates: 5, 10 and 20°C/min. Three isoconversional methods were applied: two integral (Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose methods) and one differential (Friedman method). The presence of montmorillonite (MMT) causes the beginning of curing at lower temperatures. The shape of the DSC curves has been changed by the addition of MMT, supporting the hypothesis of a change in the reaction mechanism. For hybrids with 3 and 5 wt.% of MMT, the Eα dependence is very similar to those found for the reference system (epoxy/Jeffamine D-230) for the curing degree less than 60%. The hybrid with 10 wt.% of MMT has lower energy activation in regard to the referent system without montmorillonite. Greater differences are observed in the second part of the reaction, where it is known that the curing process is more controlled by diffusion (α>0.60). The Ea value increases at the end of the reaction (α→1), which was observed for all systems, and is more pronounced in the presence of montmorillonite. [Projekat Ministarstva nauke Republike Srbije, br. III45022