Effect of the cellular structure on thermal conductivity of rigid closed-cell foam polymers during long-term aging

The thermal conductivity of rigid closed-cell polyurethane foams during long-term aging has been studied. The similarity between the kinetics of changes in the physical and mechanical characteristics of PU foams on progressive aging is established, which is attributed to the effect of matrix destruction. It is found that rigid foams have cell walls of various strength, whose impact on the kinetics of changes in the physical characteristics of the foams during long-term aging is ascertained. The results of predicting the thermal conductivity of PU foams by the method of temperature-time analogy and establishing the limits of its application are discussed. The research presented is of interest both in determining the foam durability and in replacing freons by alternative, ecologically less harmful blowing agents.     

Change in the optical properties of ski-3 rubber in deformation,fatigue, and thermal and radiation aging

Conclusion Optical methods can be used in studying the kinetics of mechanical fatigue, thermal, and radiation aging; some general characteristics and differences in the development of these processes in elastomers with higher extinction can be detected with them. The sensitivity of the measurements is increased by the rational selection of the light wave-length of preliminary homogeneous biaxial stretching. A decrease in the thickness attained by deformation by one order of magnitude did not convert the optical medium to the class of singly scattering media, but the changes in the optical properties in aging were more pronounced against the background of decreasing extinction.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 261–265, March–April, 1991.     

Deformability modelling of foam elongation

The relationships concerning the deformation of rigid cellular foams during elongation are reviewed under the conditions of a monotonous load increase. The cellular structure behavior is modelled by the operation of a certain piston structure. The proposed equation leads to a satisfactory approximation of the experimental stress-strain diagram for polystyrene and poly(vinyl chloride) foams.V. A. Kucherenko Central Scientific-Research Institute of Building Constructions, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 154–157, January–February, 1972.     

Correlation between the strength and thermal conductivity of glass-reinforced plastics

The effect of changes in the component ratio and porosity of glass-reinforced plastics on their strength and thermal conductivity is considered. A correlation is shown to exist between the bending strength and thermal conductivity of a glass laminate.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 749–751, July–August, 1969.     

Effect of extension on the structure and thermophysical properties of kapron (nylon 6)

The effect of extension on the structure and thermal conductivity of kapron has been investigated experimentally in the temperature range +20–130° C. The thermophysical characteristics were determined via a method based on the principle of the regular regime. The structure of the kapron was investigated by infrared spectroscopy and x-ray and microstructural analysis. The mechanical loads produced important structural changes that affected the thermophysical parameters of the kapron.Mekhanika Polimerov, Vol. 4, No. 1, pp. 180–182, 1968     

Effect of heat treatment on the static and fatigue strength of polycaproamide

The changes in strength and the polymorphous transformations which occur during the aging in air of polycaproamide heat-treated under various conditions were investigated. It was shown that the strength after prolonged aging in air with normal relative humidity depends on the structural changes occurring in the heat-treatment process. The structural changes lead to changes in fatigue strength and also in the mechanism of the breakdown of polycaproamide on aging.Kaunas Polytechnic Institute. Translated from Mekhanika Polimerov, No. 1, pp. 59–62, January–February, 1972.     

Effect of the chemical structure of the polymer matrix on the properties of foam polyurethanes at low temperatures

The dependence of physical and mechanical properties of oligoether-based foam polyurethanes on the molecular mass (M_c) of polymer chains between the nodes of the polymer network and on the content of rigid segments in the polymer is investigated at 293 and 98K. The values of M_c at which the foam plastics have the best mechanical properties at low temperatures are determined. The content of rigid segments in the polymer at which foam polyurethanes have the best combination of the linear thermal expansion coefficient and mechanical properties in tension at a temperature of 98K is found.Latvian State Institute of Wood Chemistry, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 4, pp. 517–526, July–August, 1999.     

Study of the thermophysical and antifriction properties of polyethylene-based compositions

The results of a study of the effect of the filler properties (thermal conductivity, density, particle shape and size, percentage content) on the thermal conductivity of the composite are presented; the factors responsible for the increase in the thermal conductivity of the polymer are indicated. Formulas are recommended for calculating the thermal conductivity of composites based on polyethlene and various fillers.Institute of Polymer Mechanics, Academy of Sciences of the LatvianSSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 259–266, March–April, 1973.     

Structural changes and defect development in epoxy adhesive compounds during thermal aging

The structural characteristics of epoxy adhesive compounds and the development of surface defects during thermal aging have been investigated. Profilograms of the fracture surfaces of bonded joints have been obtained and certain causes of the increased dispersion of the bond strength characteristics explored.Mekhanika Polimerov, Vol. 4, No. 2, pp. 293–295, 1968     

Thermal conductivity of unidirectionally reinforced hybrid composites

Conclusion The thermal conductivity of organic-glass, organic-carbon, and carbon-glass plastic in dependence on the volumetric content of organic, glass, and carbon fibers was experimentally investigated. The solution for transverse thermal conductivity of unidirectional hybrid composite, obtained in [8] by generalizing the method of self-congruence to the case of a triphase model, is in satisfactory agreement with the experimental data.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 817–822, September–October, 1990.     

Effect of porosity and component ratio on the thermal conductivity of glass-reinforced plastics

The dependence of the thermal conductivity of glass-reinforced plastics on porosity and component ratio is investigated. The possibility of determining the porosity, specific weight, and resin content of glass-reinforced plastics from their thermal conductivity is confirmed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 487–491, May–June, 1969.     

Intensity of mechanical action and mechanical degradation of rigid polymers

The authors have investigated the relation between the degradation of rigid carbon-chain polystyrene, polyvinyl acetate, and polymethyl methacrylate polymers with the kinetics of mechanical degradation during vibromilling. They determine the critical value of the intensity necessary for the onset of the degradation process.Moscow Technological Institute of Light Industry. Translated from Mekhanika Polimerov, No. 2, pp. 362–364, March–April, 1974.     

Properties of rigid polyurethane foams filled with milled carbon fibers

The effect of milled carbon fibers of two types (differing in length) on the properties of rigid polyurethane foams in the density range from 50 to 90 kg/m³ is investigated. The coefficient of thermal expansion and properties of the foams in tension and compression as functions of fiber content in them are determined. It is found that the long fibers are more efficient in improving the properties of the foams in their rise direction. The elongation at break of the foams decreases significantly with increasing fiber content.     

Influence of physical aging on creep of rubber

The effect of physical aging on the creep response of chloropren rubber filled with carbon black was studied. The method of reduced variables and analytical method of determination of influence of physical aging on the creep compliance in short creep time test are proposed.Glówny Instytut Górnictwa, Pl. Gwarków 1, 40–166 Katowice, Poland. Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 112–118, January–February, 1997.     

Tensile behavior of plastic foams

The tensile stress-strain diagram of plastic foams is calculated on the basis of a previously proposed 14-faced cell model. An S-shaped inflection due to the flexural deformations of the ties is detected on the initial section of the diagram. It is shown that for closed-celled foams this inflection may degenerate. The results of testing PVC and polyurethane foams (volume content of polymer base about 3–6%) prove to be in satisfactory agreement with the calculations.Vladimir Scientific-Research Institute of Synthetic Resins. Translated from Mekhanika Polimerov, No. 4, pp. 670–675, July–August, 1971.     

Predicting the porosity of glass-reinforced plastics from their thermal conductivity

The effect of porosity, reinforcement ratio and specific weight on the thermal conductivity of a series of glass-reinforced plastics has been investigated. It is shown that, if the thickness, the weight characteristic (weight per square meter), and the number of layers of reinforcing fabric are known and the thermal conductivities of the components are constant, then the porosity of the glass-reinforced plastic can be predicted from its thermal conductivity. Data on the relation between the specific weight and the thermal conductivity of the glass-reinforced plastics investigated are presented.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 522–525, May–June 1971.     

Water absorption and swelling of glass/epoxy syntactic foams

The results of experimental and theoretical investigation of glass/epoxy syntactic foams at a long-term (10 years) exposure in water are presented. Specimens for experimental investigation were made from EDT-10 epoxy filled with MSO-779 hollow glass spheres. Seven types of specimens with filler volume fraction varying from 0 to 0.6 were tested. The curves of long-term water absorption and swelling were determined. It was expected that the addition of hollow glass spheres must substantially reduce the sorptional capacity of material because of the reduction of the volume content of the matrix and, therefore, because of reduction of the part of the volume where the molecules of water can diffuse. However, the water absorption process in syntactic foams was found to continue during the total term of the experiments without a tendency to stop. After ten years the limiting equilibrium state was not achieved and the concentration of water in composite specimens became higher than the equilibrium concentration of water in unfilled EDT-10 epoxy. In long-term tests the swelling strain was found to be a nonlinear function of water content. The degree of nonlinearity increased when the volume content of the filler was increased. The calculational prediction of absorption and swelling curves of syntactic foams with different volume contents of the filler was done using the experimentally determined characteristics of absorption of the matrix. The water transport process in the matrix was considered to obey Fick's law. It was found that the initial parts of the experimental curves (during the first year of exposure) show a good enough agreement with calculated data for engineering applications. Subsequently, the discrepancy between the experimental results and calculated data increased and reached such large values that the predicted results became unacceptable. The possible reasons of such discrepancy (breaking of adhesion bonds between the matrix and the filler with subsequent sorption of water at the glass sphere surface, and diffusional filling by water of the inner parts of the damaged glass spheres, etc.) are discussed.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995. University of Colima, Colima City, Colima, Mexico. Institute of Polymer Mechanics, Latvian Academy of Sciences, Riga, LV-1006, Latvia. Published in Mekhanika Kompozitnykh Materialov, No. 1, pp. 83–97, January–February, 1996.     

Aging of polymer composite materials exposed to the conditions in outer space

A comprehensive investigation is made of glass, carbon, organic fiber-reinforced plastics, and epoxy-based hybrid composite materials employed in Salyut-type spacecraft which remained in space for up to 1501 1501 days. In particular, the properties, aging mechanism, and strain-strength variations in these materials due to exposure to the conditions in outer space were studied. After a series of tests were performed in space the standard strain and strength parameters as well as the mass, density, and thickness changes in the composite materials were estimated. Electron-microscopic and dynamic-mechanical analyses were performed, and the thermal expansion was estimated for a wide range of temperatures. The principal, dominant process occurring due to the continuous presence in outer space was found to be post-curing of the resin materials, which in turn affected the mechanical characteristics of the composite materials. After 456–1501 days in space the room-temperature strength of the composite materials (except for organic plastics) did not decrease, while at high temperatures it even increased. The post-curing and restructuring of some composite materials lowered their dynamic shear moduli in the glassy state of the resin. Due to consolidation of the surface layer of hybrid composite materials irradiated and subjected to thermal cycles, failure during bending varied from transverse fracture to delamination. The negative effect of the post-curing process was expressed as higher internal tension in the hybrid composite materials with different linear thermal expansion coefficients. The magnitude of this effect depended on the amplitude of the thermal cycles. The unprotected surface of the composites bombarded by atomic oxygen, microparticles, and space garbage were subjected to pickling and microerosion, the maximum effect occurring at the initial stage of exposure. Desorption of moisture and low-molecular products during the first 100–200 days of thermal cycling in the vacuum of near-earth orbit must be considered when estimating the total mass loss of composite materials. Data from microscopic, dynamic-mechanical, and other types of analyses revealed that the outer-space factors improved the supermolecular order of the resin volume, while the subsurface layer structure of the composite materials had loosened. Microcracks formed in the plastic's surface during 1501 days in outer space did not, in general, affect the mechanical parameters of the composite materials. Most of the observed effects of exposure to conditions in outer space were less pronounced for plastics protected by aluminum foil or other plastic coatings. The data obtained can be used for designing external elements of spacecraft by selecting materials with specified and predictable properties for long-term service.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 4, pp. 457–467, July–August, 1993.In conclusion we thank I. G. Zhigun and R. P. Shlits for assisting in determining the mechanical properties of PCM as well as the crew of the Salyut-6 and Salyut-7 space stations for setting up, monitoring, and delivering samples to earth.     

Investigation of curing an epoxy resin by the polarization current method

Conclusions 1. An investigation of the curing kinetics of an epoxy resin by the polarization current method at constant temperature reveals, on the infralow frequency range f=0.001–1 Hz, the existence of a relaxation process which during polymerization changes shape and is shifted toward longer times.2. In all stages of polymerization the frequency dependences of the complex dielectric constant are satisfactorily described by the Havriliak-Negami dispersion equation.3. Whereas the dielectric characteristics on the audio- and radio-frequency range can be used to explore the initial stage of polymerization, the dielectric characteristics on the infralow frequency range can be used as a source of information on the state of the material in the final stages of cure (the end of the gelling period and the post-gelling phase). The characteristics that provide the most information are the spectrum parameters, which indicate three stages of polymerization.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 140–146, January–February, 1978.     

Prediction of the thermal conductivity of oriented fiberglass plastics

An analytical equation has been derived for calculating the thermal conductivity of fiberglass plastic pipes in the axial direction. The pipes, which represent items with the most general type of reinforcement, were manufactured by uninterrupted slanting and longitudinal-transverse winding. Equations for calculating the thermal conductivity of oriented fiberglass plastics with other types of reinforcement featuring various angles to the direction and plane of reinforcement are particular cases of the derived equation. The results of an experimental check for this equation for calculating the thermal conductivity of epoxy fiberglass plastics are presented.Translated from Mekhanika Kompozitnikh Materialov, Vol. 29, No. 3, pp. 381–386, May–June, 1993.