Foams based on low density polyethylene/hectorite nanocomposites: Thermal stability and thermo-mechanical properties

Foamed Phase Change Materials Based on Recycled Polyethylene/Paraffin Wax Blends

Foamed phase-change materials (FPCMs) were prepared using recycled linear low-density polyethylene (LLDPE) blended with 30 wt.% of paraffin wax (PW) and foamed by 1,1'-azobiscarbamide. The protection of pores' collapse during foaming process was insured through chemical cross-linking by organic peroxide prior foaming. This work represents one of very few attempts for a preparation of polymeric phase change foams without a use of micro-encapsulated phase change component leading to the enhancement of the real PCM component (PW) within a final product. The porous structure of fabricated foams was analyzed using micro-computed tomography, and direct observation, and reconstruction of the internal structure was investigated. The porosity of FPCMs was about 85-87 vol.% and resulting thermal conductivity 0.054-0.086 W/m·K. Differential Scanning Calorimetry was used to determine the specific enthalpies of melting (22.4-25.1 J/g) what is the latent heat of materials utilized during a heat absorption. A stability of samples during 10 heating/cooling cycles was demonstrated. The phase change changes were also investigated using the dynamic mechanical analysis from 0° to 65 °C during the 10 cycles, and the mechanical stability of the system and phase-change transition were clearly confirmed, as proved by DSC. Leaching test revealed a long-term release of PW (around 7% of its original content) from samples which were long term stored at temperatures over PW melting point. This is the usual problem concerning polymer/wax blends. The most common, industrially feasible solution is a lamination of products, for instance by aluminum foils. Finally, the measurement of the heat flow simulating the real conditions shows that samples containing PW decrease the energy passing through the sample from 68.56 to 34.88 kJ·m^-2. In this respect, FPCMs provide very effective double functionality, firstly common thermal insulators, and second, as the heat absorbers acting through melting of the PW and absorbing the excessive thermal energy during melting. This improves the heat protection of buildings and reduces temperature fluctuations within indoor spaces.

Effect of Chemical Blowing Agent on the PVC Cellular Coating Extrusion

Depending on the type and application, the coatings of power, electric, telecommunication cables as well as other types of conduits are made of various kinds of polymer plastics. However, most often, because of good mechanical properties and many other advantages, they are first of all made from polyvinyl chlorine (PVC). This paper contains characteristics of the developed cellular extrusion of cable coatings, as well as specification of the blowing agent (BA) used and selected research results of the obtained cellular extrusion product. In technological tests the coating extrusion technological line was used. The material was modified with a new blowing agent of exothermic distribution of process characteristics, which was introduced into the material in quantities from 0.2 to 0.6% wt. The amount of blowing agent used has a direct impact on the density and structure of the received result for the extrusion of modified polymers. The cellular structure of the cellular coatings was presented. The results of the study are thin-walled properties of single- and double-layer cellular outer coatings, forming an outer surface on a steel wire. The research on the structure of manufactured materials, density and the degree of porosity, water and oil absorptivity, mechanical strength is presented.

Porous Open-Сell UHMWPE: Experimental Study of Structure and Mechanical Properties

Ultra-high molecular weight polyethylene (UHMWPE) is a bioinert polymer that is widely used as bulk material in reconstructive surgery for structural replacements of bone and cartilage. Porous UHMWPE can be used for trabecular bone tissue replacement, and it can be used in living cell studies as bioinert 3D substrate permeable to physiological fluids. It is important to develop techniques to govern the morphology of open-cell porous UHMWPE structures (pore size, shape, and connectivity), since this allows control over proliferation and differentiation in living cell populations. We report experimental results on the mechanical behavior of porous open-cell UHMWPE obtained through sacrificial removal (desalination) of hot-molded UHMWPE-NaCl powder mixtures with pore sizes in the range 75 µm to 500 µm. The structures were characterized using SEM and mechanically tested under static compression and dynamic mechanical analysis (DMA), bending, and tensile tests. Apparent elastic modulus and complex modulus were in the range of 1.2 to 2.5 MPa showing a weak dependence on cell size. Densification under compression caused the apparent elastic modulus to increase to 130 MPa.

Nanoclay Intercalation During Foaming of Polymeric Nanocomposites Studied in-Situ by Synchrotron X-Ray Diffraction

The intercalation degree of nanoclays in polymeric foamed nanocomposites containing clays is a key parameter determining the final properties of the material, but how intercalation occurs is not fully understood. In this work, energy dispersive X-ray diffraction (ED-XRD) of synchrotron radiation was used as an in-situ technique to deepen into the intercalation process of polymer/nanoclay nanocomposites during foaming. Foamable nanocomposites were prepared by the melt blending route using low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) with surface treated nanoclays and azodicarbonamide (ADC) as the blowing agent. Foaming was induced by heating at atmospheric pressure. The time and temperature evolution of the interlamellar distance of the clay platelets in the expanding nanocomposites was followed. Upon foaming, interlamellar distances of the nanocomposites based on LDPE and PP increase by 18% and 16% compared to the bulk foamable nanocomposite. Therefore, the foaming process enhances the nanoclay intercalation degree in these systems. This effect is not strongly affected by the type of nanoclay used in LDPE, but by the type of polymer used. Besides, the addition of nanoclays to PP and PS has a catalytic effect on the decomposition of ADC, i.e., the decomposition temperature is reduced, and the amount of gas released increases. This effect was previously proved for LDPE.

Enhancement of Carbon Nanofibers Dispersion on Epoxy Resin Foams Leading to Remarkable Electrical Conductivity Improvement

A simple procedure to evaluate and improve the dispersion of carbon nanofibers is presented and validated, relating the observed dispersion of the nanofibers in an intermediate stage of the production process to the final macroscopic properties of the foams. Epoxy/carbon nanofiber foams with optimal nanofiber dispersion are produced taking advantage of this procedure using inexpensive industrial vapor grown carbon nanofibers. Furthermore, the main characteristics and the electrical resistivity of the foams with improved nanofibers dispersion have been determined and related to the presence of the carbon nanofibers, greatly enhancing the electrical conductivity of the epoxy/carbon nanofiber foams.

Low Density Non-crosslinked Closed/Open Cell Polypropylene Foams with High Mechanical Properties

Low density polypropylene based foams with different cellular structures have been produced by the improved compression molding route using a high melt strength polypropylene as polymer matrix. In addition, different types of nanoparticles have been introduced in the formulation (multi-wall carbon nanotubes, organomodified nanoclays and natural nanoclays) to modify the structure and properties. The results have showed a clear correlation between the open cell content of the foams and the mechanical properties in compression. In the unfilled polypropylene high specific mechanical properties are only achievable with low values of open cell content. In comparison, for an equal value of the interconnectivity between cells, the samples containing nanoclays present much higher specific properties. This result is attributed to the reinforcement of these nanoparticles in the solid matrix, due to an improved exfoliation during the foaming process and the presence of a bimodal cellular structure. The produced foams have interesting properties with stiffness similar to those of commercial polymer foams used for the core of sandwich panels.

Effectiveness of Blowing Agents in the Cellular Injection Molding Process

The study was undertaken to investigate the effect of the type and content of blowing agents in the polymeric materials being processed on the structure and selected physical and mechanical properties of the obtained injection molded parts. In the study of the cellular injection molding process, three standard and generally available thermoplastics were used. In the experiments, the blowing agent content (0–2.0% by mass, i.e. 0, 0.4, 0.8 and 2.0% (wt)) fed into the polymer being processed was adopted as a variable factor. In the present study, the blowing agents with the endothermic decomposition characteristics and exothermic decomposition characteristics, coming in a granulated form with a diameter ranging from 1.2 to 3 mm were used.

Based on the results of investigating porosity, porous structure image analysis as well as microscopic examination of the structure was shown. It has been found, that the favorable content of the blowing agent in the polymeric material should be of up to 0.8% wt.. With such content of the blowing agent in the polymeric material, favorable strength properties are retained in porous parts, the pore distribution is uniform and the pores have similar sizes.

Properties and Physical Structure of Cellular PVC Coatings

The production of cellular coatings made of polymeric materials by the extrusion process differs from solid coating extrusion in that the polymer produced by the process based on the application of blowing agents has a diphase structure (gas-polymer) with small and evenly distributed gas bubbles. The research described in the paper was conducted using blowing agents with endothermic and exothermic decomposition behaviours, dosed as pellets in the range from 0.4% to 0.8% by weight relative to the weight of the polymer being processed. The extrusion process for modified PVC was conducted using the technological line for the production of electrical cable coatings.

The content of the blowing agents applied in the tests in the amount of up to 0.8 wt% was selected in such a way as to produce cables with a solid coating and center coating. The appearance of the produced cable coatings, their thickness and dimensions comply with the current norms.

In the investigated range of the blowing agent content, the temperature of the extrudate leaving the head decreased by approximately 7%, which can probably be attributed to the endothermic blowing agent decomposition behaviour in the extruder head.

The mechanical properties of cellular products, including their hardness, are affected by the macromolecular shape, orientation and bond. A lower number of cross-linkages in the cellular polymeric material means decreased strength properties of such product.

The results of the cellular coating resistance to water and oil absorption have demonstrated considerably increased absorption of the coatings produced, depending on the residence time in the aggressive environment and type of blowing agent applied.

Analyzing the results of density, porosity and optical examination of the physical structure of thin-walled cellular coatings, it has been determined that the favourable content of the blowing agent in the polymer should amount to 0.6-0.8 wt%, which leads to density decrease by approximately 40%.

Improving the Structure and Physical Properties of LDPE Foams using Silica Nanoparticles as an Additive

Low density polyethylene/silica nanocomposites have been produced by melt blending and foamed in a high pressure autoclave by gas (CO2) dissolution. Different amounts of nanosilica, (from 1 wt% to 9 wt%) were used in order to analyze the influence of silica content on thermal and mechanical properties of both foamed and un-foamed composites.

It has been proved that the presence of silica nanoparticles modifies the structure and properties of both foamed and un-foamed materials. An increase in the crystallinity of the polymeric matrix as well as a decrease in cell size (for low contents of silica) have been observed. In addition it was found a significant increment of melt strength, thermal stability and mechanical properties for both solids and foams. From the obtained results, it has been concluded that silica particles play a multifunctional role in this system, and in addition to this, it has been demonstrated that the use of nanoparticles can produce synergetic effects, increasing foam mechanical properties in a greater extend than that of the solids.