Janis Zicans

Epoxy composites filled with high surface area-carbon fillers: Optimization of electromagnetic shielding, electrical, mechanical, and thermal properties

A comprehensive analysis of electrical, electromagnetic (EM), mechanical, and thermal properties of epoxy resin composites filled with 0.25–2.0 wt. % of carbon additives characterized by high surface area, both nano-sized, like carbon nanotubes (CNTs) and carbon black (CBH), and micro-sized exfoliated graphite (EG), was performed. We found that the physical properties of both CNTs- and CBH-based epoxy resin composites increased all together with filler content and even more clearly for CBH than for CNTs. In the case of EG-based composites, good correlation between properties and filler amount was observed for concentrations below 1.5 wt. %. We conclude that CBH and, to a lower extent, EG could replace expensive CNTs for producing effective EM materials in microwave and low-frequency ranges, which are, in addition, mechanically and thermally stable.

A DSC study of the effect of ascorbic acid on bound water content and distribution in chitosan-enriched bread rolls during storage

Changes of tightly and loosely bound water relative content in bread were studied using differential scanning calorimetry method. Combination of chitosan with ascorbic acid changes water redistribution between starch and gluten and in such a way can be related to bread quality and sensory properties. The decrease of the water vaporization temperatures, melting temperatures and enthalpies in fresh bread containing chitosan were detected when ascorbic acid was added in combination with chitosan. The decrease of melting peak temperature has been attributed to the increase of interaction of loosely bound water and the decrease of vaporization peak temperature has been attributed to the decrease of interaction of tightly bound water with bread components as a result of ascorbic acid addition. Freezable water amount and total water amount in crumb decrease more rapidly during first stage of staling and more slowly at the second stage of staling in the bread nutritionally fortified with chitosan.

Creep of Poly(Vinyl Chloride)/ Chlorinated Polyethylene Blends

The results of experimental investigations of the creep behavior of blends of poly(vinyl chloride) (PVC) with chlorinated polyethylene (CPE) are presented. Eight types of specimens with the PVC/CPE weight ratios of 100/0, 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100 are examined. The creep tests were continued for 1000 h. It is discussed how the blend composition affects the elastic and inelastic behavior of the material. The elastic compliance of the blend can be determined from the properties of its components by using the Kerner and Budiansky equations for heterogeneous systems with a phase structure of statistic-dispersion type. The creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep with coefficients depending on the blend composition.

Thermomechanical and adhesive properties of radiation-modified polymer composites for thermosetting products

Gamma-irradiated blends of polyethylene (PE) with ethylene-propylene-diene copolymer (EPDM) and a thermotropic liquid-crystalline polymer (LCP) are investigated at absorbed radiation doses not exceeding 150 kGy (10 kGy=1 Mrad). The temperature dependences of elastic moduli, tension diagrams at a temperature above the melting point of the crystalline phase of PE, and long-term strain recovery curves for oriented test specimens are presented. The kinetics of thermal relaxation and shrinkage stresses in previously oriented composite specimens upon their heating and cooling is investigated. Data on the influence of LCP additions on the adhesive interaction of the compositions with steel are obtained. The peculiarities of thermomechanical and adhesion properties of these composites are discussed taking into account the morphologic and calorimetric data obtained.

A DSC study of the effect of bread making methods on bound water content and redistribution in chitosan enriched bread

The effect of bread making methods on bound water migration from crumb to crust and moisture redistribution during bread storage at room temperature was studied. Comparative analysis of water behavior in bread crusts and crumbs was performed using differential scanning calorimetry method. Water vaporization enthalpies and temperatures of water vaporization peaks were determined and compared for bread produced by a single stage, straight dough method and bread produced by a two stage, sponge-and-dough method. The effect of chitosan on the crust and the crumb properties was analyzed for the breads produced by both methods.

Thermal and elastic properties of poly(vinyl chloride) (PVC) + chlorinated polyethylene (CPE) blends

Abstract. Eight types of PVC + CPE blends with different weight ratios (PVC/CPE = 100/0, 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100) are tested. Data on the heat conductivity, thermal diffusivity, and heat capacity of the blends investigated are reported. Primary attention is paid to the structural approach for effective elastic constants: bulk, shear, and Youngs modulae. The blends are considered as random mixtures of two isotropic constituents. It is found that the elastic moduli may be well represented by the Kerner and Budiansky equations.

Studies on Morphology, Rheological, Mechanical, and Thermorelaxation Properties of Chemically and Radiation Modified Polyethylene/Ethylene Propylene-Diene Copolymer Blends

Experimental studies of the blends consisting of chemically and radiation modified polyethylene and ethylene -propylene -diene copolymers have been carried out. Measurements of crystallinity, microhardness, strength-deformation, viscoelastic, adhesive and thermorelaxation properties, as well as electron microscope observations, have shown that the blends chemically vulcanized by a special elastomer phase crosslinking system has a typical biphasic structure within the entire composition range with characteristics specific for rubber. Subsequently, radiation vulcanized blends where crosslinking takes place in both disperse phases, formation of chemical bonds between these phases and fibrillation of molecular structures is observed. Consequently, it can be stated that radiation treatment shows the best properties of the blends in comparison with chemical vulcanization. Therefore, materials formed by such a systcm can be successfully used, e.g., as elastic and adhesive active thermosetting materials if the polymer is previously oriented.

The Use of Thermal Analysis in Assessing the Effect of Bound Water Content and Substrate Rigidity on Prevention of Platelet Adhesion

Implant surface-induced initiation of blood coagulation leading to thrombus formation is a major problem. It is an important challenge in biomedical research to enhance thromboresistance of polymer coatings of blood-contacting implants and devices by the reduction of platelet adhesion to their surfaces. Currently, the molecular mechanisms responsible for interfacial processes related to regulation of protein adsorption and platelet adhesion in antithrombotic coatings are not yet clearly understood. We studied the role of water binding energy in chitosan molecules using differential scanning calorimetry and thermogravimetric analysis and the role of chitosan coating rigidity using dynamic mechanical thermal analysis in the prevention of platelet adhesion. It was found that the presence of loosely bound water in chitosan coatings increases platelet adhesion. The high molecular weight (HMW) chitosan coating that contains only tightly bound water prevents platelet adhesion. The high molecular weight (HMW) chitosan coating is more rigid and has higher platelet adhesion compared with more soft LMW chitosan coatings. The degree of hydration of thromboresistant coatings is significant parameter that must be taken into account in the design of blood-contacting surfaces.

Properties of compositions based on post-consumer rigid polyurethane foams and low-density thermoplastic resins

Several composition systems based on post-consumer rigid polyurethane (PUR) foam and low-density thermoplastic resin - polyvinylchloride (PVC), polypropylene (PP) and thermoplastic polyurethane (TPU) - have been investigated to determine their optimal processing conditions. Detailed research work on the mechanical properties of the compositions has been carried out to expand application possibilities of the tested binary composition systems. Also the effect of the thermoplastic matrix on the molecular structure of the rigid PUR foams has been studied.

Features of thermomechanical properties of radiation-modified blends of high-density polyethylene with liquid-crystalline copolyester

The present paper surveys the investigation results of gamma-irradiated blends of high-density polyethylene (PE) with thermotropic liquid-crystalline polymer (LCP). The LCP used was a liquid crystalline copolyester of 40% poly(ethyleneterephthalate) with 60% p-(hydroxybenzoic acid). The LCP content in the blends was 0,5, and 10 wt.%. The constituents were blended with the use of a single-screw extruder. The samples were prepared by compression molding and irradiated by a Co60 γ-radiation source in an inert atmosphere (argon) to relatively low absorbed doses (up to 200 kGy; 1 Mrad=10 kGy). Morphological, thermal, and mechanical properties in a wide temperature range were investigated for the irradiated and nonirradiated samples. The effects of gamma-irradiation and LCP addition on the thermomechanical behavior of PE are discussed. It was found that the LCP addition affected significantly the stress-strain behavior of PE at temperatures above the melting point. The features of thermorelaxation properties of the PE/LCP blends previously irradiated and oriented, particularly thermorelaxation and residual shrinkage stresses at isometric heating and cooling, were also established. The results obtained testify that the LCP addition makes it possible to improve considerably the thermosetting properties of irradiated PE.