Regina C. R. Nunes

Performance of radar absorbing materials by waveguide measurements for X- and Ku-band frequencies

Abstract In this work, samples consisting of an elastomeric matrix containing different kinds of active materials in particulate form were prepared for reflectivity measurements. The materials individually used in weight contents of 80% in polychloroprene matrix were: carbonyl-iron (CI) and doped ferrite powders. The microwave reflectivity levels were determined from the magnetic and dielectric properties of the elastomeric composites obtained from scattering data, by fitting the samples in a waveguide, for measurements in the frequency range from 8 to 16 GHz. Better microwave absorption for X-band was obtained for CI while doped ferrite absorbed at higher frequencies (Ku-band).

Polymer–filler interactions and mechanical properties of a polyurethane elastomer

Films of a polyester-based polyurethane containing silica particles in a concentration range from 1 to 20 phr were prepared from THF solutions. Two types of silica were used resulting from two different thermal treatments of precipitated silica, yielding different surface hydroxyl concentrations. Some of their mechanical properties were measured and the reinforcement mechanism was discussed in terms of interactions between the hydroxyl groups and hard or soft segments of the polymer chains.

X-ray studies on compositions of polyurethane and silica

Abstract Compositions of polyurethane elastomer and precipitated silica were investigated by X-ray. The elastomer was characterized as a polyurethane based on 4,4-diphenylmethane diisocyanate (MDI), a low molecular weight polyester derived from adipic acid and 1,4-butanediol. The precipitated silica was submitted to thermal treatment in order to eliminate the surface hydroxyl groups. The mixtures were made into solutions using THF (tetrahydrofuran) as solvent. The silica concentration was 5 phr. The influence of two kinds of silica (thermally treated and non-thermally treated) on the crystallinity index and chain orientation was investigated. Based on the experimental results, propositions have been formulated to explain the different behaviors observed for the polyurethane–filler systems analyzed.

Flame retardancy in thermoplastic polyurethane elastomers (TPU) with mica and aluminum trihydrate (ATH)

Abstract Two types of aluminum trihydrate (ATH), with or without surface treatment, and mica were incorporated into TPU composites. The quantity of mica that should be used has been previously evaluated from mechanical properties. In the present work, different mixtures of TPU/ATH/mica were prepared and those compositions presenting fire retardancy were identified through the two principal tests in this area: vertical burning (UL94) and oxygen index (ASTM D2863). The influence of mica on fire resistance was also evaluated. The objective of this work was to obtain good flame retardancy in TPU–mica composites and an optimum cost-performance balance as a consequence of mica addition.

Aging effect on the reflectivity measurements of polychloroprene matrices containing carbon black and carbonyl-iron powder

Abstract The effect of aging, arising from salt spray exposure, on the microwave reflectivity was studied for polychloroprene (CR) matrices, containing different kinds of active material in particulate form. The materials used as microwave absorbers in different contents were carbon black (CB) and carbonyl-iron powder (CIP). The microwave reflectivities were determined for the frequency range of 8–16 GHz. The formation of a superficial layer of iron oxide on CIP:CR composites was responsible for a reduction and change of microwave absorption towards a higher frequency. The reflectivity curves of CB:CR composites did not display any significant difference before and after the exposure.

Mechanical properties of thermoplastic polyurethane elastomers with mica and aluminum trihydrate

Abstract In the present work two types of fillers were incorporated into thermoplastic polyurethane elastomers (TPU): mica and aluminum trihydrate (ATH). In the first stage, mixtures were prepared with TPU and 5, 10, 20, 25 and 30 phr of mica. The optimum amount of filler was evaluated for mechanical properties. In the second stage, ATH was used in mixtures with TPU and mica in the preparation of compositions with good flame retardant. All mixtures were obtained by extrusion and after compression molding they were evaluated through mechanical properties of tensile resistance, Shore A hardness and abrasion resistance. The objective of this work is to obtain good mechanical properties from TPU composites with mica–ATH as fillers and optimum cost-performance balance, in addition to flame retardant characteristics.

Characterization of Microwave-Devulcanized Composites of Ground SBR Scraps

The recycling of waste rubber has considerable significance in terms of environmental protection and energy conservation. Considering that most of the relevant literature is concerned with tire recycling, the objective of this study is to develop and characterize styrene-butadiene rubber composites containing only industrial rubber scraps devulcanized by microwave. The styrene-butadiene rubber extruded profile scraps were collected and ground under ambient conditions. The obtained powder styrene-butadiene rubber-r was physically, thermally, and chemically characterized. From the devulcanized styrene-butadiene rubber-r two composites were prepared, varying the exposure time of the powder in the microwave (3 and 4 min). These composites were compared to a control sample supplied by the industry from which the styrene-butadiene rubber extruded profile scraps were collected. Vulcanization parameters were determined by oscillatory disk rheometry. Vulcanized composites were characterized by crosslink density and physical-mechanical properties (Shore A hardness, tensile and tear strength, and compression set) before and after a postcure process. The mechanical properties of the compositions were ∼25% for tensile strength and 41% for tear strength compared to the control sample. The results for the crosslink density verified those for the mechanical properties of the composites.

Dielectric properties of flame resistant EPDM composites

Formulations of ethylene-propylene-diene rubber containing mixtures of aluminum hydroxide (ATH) and carbon black (HAF) as fillers were developed, aiming to achieve a synergism in properties, to fit national and international standards for application as insulation in electric wires and cables, yet keeping the fire resistance. After flammability determination, the developed compositions were analyzed for dielectric constant, dissipation factor, dielectric strength, and volumetric and surface resistivity. It was found that for flame resistance, a minimum amount of 160 phr of ATH must be present. However, this composition contains 15 phr of HAF, which is harmful to the electric properties. Only the composition with 170/7.5 (ATH/HAF) exhibited the desired performance.

Investigation of the degradation of thermoplastic polyurethane reinforced with short fibres

Abstract The thermal behaviour of short fibre reinforced composites was studied by DSC and TGA techniques. A thermoplastic polyurethane elastomer was used as the matrix and carbon fibre or aramid fibre as reinforcing fillers in each composite. The influence of short fibres on the thermal resistance of the matrix, glass transition temperature and kinetic parameters of the degradation reaction of the composites were studied. The thermal resistance of aramid fibre reinforced composites was greater than those of carbon fibre reinforced composites or the pure matrix polymer. In addition to this, analysis of the kinetics and the glass transition temperature suggest greater interaction between aramid fibre and elastomer matrix.

Mica as a filler for ABS/polycarbonate blends

Mica was incorporated in ABS/PC:65/35 blends prepared on a Haake Torque Rheometer System 9000 with Rheomix 600 mixer. To improve the polymer/filler interaction, a treatment using silane on mica surface was performed. Studies were conducted concerning mechanical properties (flexural and impact strength), thermal properties (DSC and Vicat softening temperature), rheology, and flame retardancy. The composition with 30 phr of mica gives the best properties and efficiency in reducing the rate of burning, then becoming an alternative material to be explored and possibly meet the new requirements sought by these market blends.