Auxetic polypropylene films

Auxetic materials are those exhibiting negative Poisson's ratio (ν) behavior. Polymeric auxetic extruded products in the form of cylinders and fibers have previously been reported. This article reports the successful production of auxetic polypropylene films (～0.15‐mm thick) using a melt extrusion process. Video extensometry and tensile testing techniques have been used to measure the in‐plane Poisson's ratios and Young's moduli of the auxetic film, both on an Instron tensile testing machine and a Deben microtensile testing machine. The film is elastically anisotropic with the Poisson's ratio and Young's modulus along the extrusion (x) direction being ν_xy = ?1.12 ± 0.06 and E_x = 0.34 ± 0.01GPa, respectively, while the Poisson's ratio and Young's modulus in the transverse (y) direction to the extrusion direction are ν_yx = ?0.77 ± 0.01 and E_y = 0.20 ± 0.01GPa, respectively. POLYM. ENG. SCI., 45:517–528, 2005. © 2005 Society of Plastics Engineers     

Novel variations in the microstructure of the auxetic microporous ultra‐high molecular weight polyethylene. Part 1: Processing and microstructure

Recent publications have detailed how auxetic (negative Poisson's ratio) polymers have been fabricated by a novel thermal processing route consisting of three welldefined stages—compaction, sintering and extrusion. In this paper, the compaction stage of the processing route is omitted and the effects of this on the structural integrity, microstructure and Poisson's ratios of the extrudates examined. The effects of varying the processing parameters of the sintering and extrusion stages are studied so that a set of conditions that produce a highly fibrillar auxetic material with sufficient structural integrity to allow mechanical properties to be evaluated can be defined. Poisson's ratios as low as ?4 have been obtained.     

Viscosity characterization and flow simulation and visualization of polytetrafluoroethylene paste extrusion using a green and biofriendly lubricant

Polytetrafluoroethylene (PTFE) and expanded PTFE (ePTFE) are ideal for various applications. Because PTFE does not flow, even when heated above its melting point, PTFE components are fabricated using a process called paste extrusion. This process entails blending PTFE powder particles with a lubricant to form PTFE paste, which is subsequently preformed, extruded, expanded (in the case of ePTFE), and sintered. In this study, ethanol was proposed as an alternative green lubricant for PTFE processing. Not only is ethanol benign and biofriendly, it provides excellent wettability and processing benefits. Using ethanol as a lubricant, the shear viscosity of PTFE paste and its flow behavior during paste extrusion were investigated. Frequency sweeps using a parallel-plate rheometer were performed on PTFE paste samples and various grits of sandpaper were used to reduce wall slip of PTFE paste. A viscosity model was generated and a multiphysics software was used to simulate PTFE paste extrusion. The simulated extrusion pressure was compared to experimental data of actual paste extrusion. Flow visualization experiments using colored PTFE layers were conducted to reveal the flow profile of the PTFE paste. The morphology of the expanded ePTFE tubes was examined using scanning electron microscopy and the effect of expansion ratio on ePTFE morphology was quantified.     

Novel fabrication route for auxetic polyethylene. Part 1. Processing and microstructure

One method for producing synthetic auxetic materials is starting with polymer powders and using a combination of compaction, sintering, and extrusion. This article presents a novel variation on this route, omitting the extrusion stage and generating the required microstructure by compaction followed by multiple sintering. The effects of single, double, and quadruple sintering on compacted cylinders are examined in terms of a detailed microstructural examination, study of density and dimensional variations, and measurement of Poisson's ratio. The best results were obtained by compaction followed by double sintering, resulting in a strain dependent Poisson's ratio as low as ν = ?0.32. This new technique has great potential for increasing the range of geometries that can be fabricated and is very akin to ceramic sintering techniques. POLYM. ENG. SCI., 45:568–578, 2005. © 2005 Society of Plastics Engineers     

Paste extrusion of polytetrafluoroethylene (PTFE): Surface tension and viscosity effects

The effects of the lubricant physical properties on the processing of polytetrafluoroethylene (PTFE) fine powder resins are studied. Lubricants having different surface tension and viscosity were used; the two properties changed independently. These effects were studied by using dies of various contraction angle and reduction ratio for resins having a variety of molecular architecture. It was found that the wettability (surface tension) of the lubricant strongly affects the pressure needed to extrude the PTFE pastes. The viscosity of the lubricant was also found to play a significant role in the process since a lubricant with a low viscosity causes the paste to be extruded at a lower pressure. These effects of the physical properties on the extrusion pressure influence significantly the mechanical properties of the final extrudates. The latter are functions of the degree of fibrillation, which is significantly influenced by the wettability and viscosity of lubricants. Finally, the effects of die geometry on extrusion pressure and mechanical properties of extrudates were also assessed in order to determine the geometrical characteristics and operation conditions for the optimization of the process.     

Fabrication of polyamide 12T micro-cellular foams with ultra-high compressive strength via in situ polytetrafluoroethylene fibrillation using supercritical carbon dioxide foaming

The development of micro-cellular foams with ultra-high compressive strength and high volume expansion ratio (VER) is a challenging task. Herein, polyamide 12T (PA12T) micro-cellular foams with ultra-high compressive strength were fabricated via in situ polytetrafluoroethylene (PTFE) fibrillation using supercritical CO₂ foaming technology and a chain extender. The resulting branched structure showed considerably improved viscoelasticity and foaming performance, thus improving the cell morphology of the PA12T foam and exhibiting high VER. The PTFE fibrillation network induced melt strength enhancement, crystallization nucleation, and cell nucleation. The branched PA12T foam with 1.5 wt% PTFE exhibited the smallest cell diameter (15 μm) and highest cell density (3 × 10⁹ cells/cm³). The compressive strength of the foam (0.50 MPa under 5% strain) was 70% higher than that of pure PA12T. This research offers an effective method for producing high-VER PA12T foams with adjustable micro-cellular structures and excellent mechanical properties.     

Paste Extrusion of Polytetrafluoroethylene (PTFE) Fine Powder Resins

The rheology of non‐melt processible polytetrafluoroethylene (PTFE) pastes has been studied using a capillary rheometer. It was found that fibrils of submicron dimensions are created during PTFE paste processing, which are responsible for the final strength of the extrudate. The mechanism of fibrillation is explained in terms of the unwinding of crystallites. To describe the effects of die design, a simple mathematical model has been developed. The model takes into account the elastic‐plastic (strain hardening) and viscous nature of the material in its non‐melt state. The model predictions are found to be consistent with experimental results obtained from macroscopic pressure drop measurements and flow visualization experiments.     

Time,temperature, and strain effects on viscoelastic Poisson's ratio of epoxy resins

Poisson's ratio of polymeric materials, although generally assumed as a constant, is known to display a viscoelastic dependence on time, temperature, and strain. This article investigates the phenomenology of this dependence on two crosslinked epoxy systems with different glass transition temperatures. Poisson's ratio measurements are performed by contact extensometers simultaneously measuring the axial and transverse deformations under two different tensile testing conditions: (i) constant deformation rate, in which the effects of strain, strain rate, and temperature are highlighted; (ii) stress relaxation (or constant deformation), where the dependence of Poisson's ratio on time is studied at various strain levels. The viscoelastic Poisson's ratio increases as strain, temperature, and time increases, with trends markedly depending on the materials glass transition. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

The effects of powder morphology on the processing of auxetic polypropylene (PP of negative poisson's ratio)

With the use of a three-stage thermal processing route, similar to that used previously for the production of ultra high molecular weight polyethylene possessing a negative Poisson's ratio (i.e. displaying auxetic behavior), an auxetic form of polyethylene has been fabricated. The polypropylene is processed by the compaction, sintering, and extrusion of a powder. The importance of powder morphology on the ability of polypropylene to achieve auxetic behavior has been examined, revealing that particle shape, size, and surface roughness are critical variables for successful processing. Negative Poisson's ratios of up to −0.22 at 1.6% strain have been obtained. The data have been successfully interpreted by use of a simple geometric model based on the polymer microstructure. These suggest that, by further optimizing the processing conditions, much larger negative Poisson's ratios should be achievable.     

Poisson's ratio and volume change accompanying deformation of randomly oriented electrospun nanofibrous membranes

ABSTRACT

The present work focuses on the determination of volume change accompanying deformation and Poisson's ratio for electrospun nanofibrous membranes. For this purpose, polyurethane (PU) is considered for the fabrication of electrospun nanofibrous membranes. Three different sample thicknesses are fabricated. Following this, surface morphology analysis and fibre orientation analysis are conducted to investigate the variation of properties between electrospun PU membranes of different thicknesses. Subsequently, PU specimens are subjected to uniaxial extension test where the changes in sample width and thickness are recorded as a function of applied strain. Volume changes are computed while further analysis on the relationship between transverse strains and axial strain provided the values of Poisson's ratio. For all three electrospun PU samples investigated, significant volume changes are observed while the in-plane Poisson's ratio is found to be around 0.55. However, the out-of-plane Poisson's ratio of electrospun PU membranes are not classical and remains undetermined.     

Introductory behavior of rubber concrete

This article introduces a new type of concrete, the so‐called rubber concrete, and thereupon presents a way of modification of waste rubber to construction articles. The conventional cement concrete is made by mixing cement with sand and pebbles, but the rubber concrete proposed here virtually excludes cement completely. The manufacturing process of rubber concrete can be divided into two methods, which are designated for dry and wet processes, but this article focused just on the dry process. The physical properties of rubber composite increased with the silane treatment of added aggregates, but the volume of the aggregate might not be a critical factor affecting the compressive strength in the range of the aggregate contents used in this study, that is, the interfacial adhesion between the matrix rubber and the aggregates was a key factor to improve the mechanical properties of rubber concrete. The compressive strength of rubber concrete was about 89 MPa and the Poisson's ratio, which is the ratio of compressive‐to‐tensile strength, was 5.5%. From the viewpoint of the compressive strength and the Poisson's ratio, rubber concrete had better properties than those of conventional cement concrete. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 35–40, 1999     

Reactive polytetrafluoroethylene/polyamide compounds. I. Characterization of the compound morphology with respect to the functionality of the polytetrafluoroethylene component by microscopic and differential scanning calorimetry studies

Compounds of electron‐beam‐irradiated polytetrafluoroethylene (PTFE) and polyamide (PA) were produced by reactive extrusion. During extrusion, both a breakdown process of the PTFE agglomerates and a chemical reaction between PTFE and PA took place. The morphology of the compounds was characterized with differential scanning calorimetry using fractionated crystallization, with atomic force microscopy and scanning electron microscopy, and with dynamic light scattering. The particle size of the dispersed PTFE phase decreased as the irradiation dose increased. A simple theoretical model of the breakdown process of PTFE agglomerates was made for the discussion of the development of the observed degree of dispersion. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1308–1316, 2005     

The dependence of the elastic properties of silica/alumina materials on the conditions used for firing

The dependence of the elastic properties of a range of powder compact samples has been measured as a function of firing variables. It was found that both Young's modulus and Poisson's ratio are particularly sensitive to the peak temperature and the time for which the peak temperature is maintained, over a range of these variables for which density is not significantly affected. The material investigated is used industrially for the manufacture of wall tiles. Firing trials conducted in an industrially operated tunnel kiln have indicated that sufficient variation in firing conditions exists, in the cross-section of the tunnel kiln, to cause significant variation in the values of Young's modulus and Poisson's ratio of bodies fired in different positions in the kiln. Microstructural examination of bodies produced to have very similar densities but vastly different values of Young's modulus and Poisson's ratio has indicated that the dependence of Young's modulus and Poisson's ratio on firing conditions can be explained by the extent of sintering within the ceramic matrix.     

Influence of fibrillated poly(tetrafluoroethylene) on microcellular foaming of polyamide 6 in the presence of supercritical CO2

In this article, PA6/poly(tetrafluoroethylene) (PTFE) composites were prepared by internal mixer with high rotor speed. The existence of PTFE nano-fibrillation network structure was observed by scanning electron microscopy (SEM) analysis. The effect of PTFE on crystallization and rheological behavior of PA6 was evaluated. The result showed that the PTFE fibrils improved the crystallization properties of PA6 and do not change the crystal structure. The PTFE effectively enhanced the melt strength of PA6 by fibrillation. The PA6/PTFE composites were then foamed assisted by supercritical CO₂. The PTFE was used as cell nucleating agent, crystal nucleating agent and melt strength enhancement agent in the foaming process. Finally, the microcellular PA6 foams were successfully obtained with the cell density higher than 10⁹ cells/cm³, the cell size of ca. 14 μm and the volume expansion ratio of 16.     

Study on the Viscoelastic Poisson‘s Ratio of Solid Propellants Using Digital Image Correlation Method

Digital image correlation methods were used for further studies of the viscoelastic Poisson's ratio of solid propellants. The Poisson's ratio and the Young's relaxation modulus of solid propellants were separately determined in a single stress relaxation test. In addition, the effects of temperature, longitudinal strain, preload and storage time on the Poisson's ratio of solid propellants were discussed. The Poisson's ratio master curve and the Young's relaxation modulus master curve were constructed based on the time‐temperature equivalence principle. The obtained results showed that the Poisson's ratio of solid propellants is a monotone non‐decreasing function of time, the instantaneous Poisson's ratio increased from 0.3899 to 0.4858 and the time of the equilibrium Poisson's ratio occurred late when the temperature was varied from −30 °C to 70 °C. The Poisson's ratio increased with temperature and longitudinal strain, decreased with preload and storage time, while the amplitude Poisson's ratio increased with preload, decreases with longitudinal strain and storage time. The time of the equilibrium Poisson's ratio occurred in advance with the increase of longitudinal strain, preload and storage time.     

Properties of polytetrafluoroethylene (PTFE) paste extrudates

In this work, we have studied the effects of extrusion die design, resin molecular structure, and lubricant concentration on the properties of PTFE paste extrudates by performing macroscopic extrusion pressure measurements, Raman spectroscopy, differential scanning calorimetry and mechanical testing on the extrudates. Five resins of different molecular structures were tested. We have found that a balance between fibril quantity and quality (in terms of fibril orientation and continuousness) is necessary to ensure acceptable products, as illustrated through the effects of the operating variables on the extrudate tensile strength. The number of fibrils formed during extrusion can be increased by extruding the paste through a die of larger reduction ratio or by decreasing the lubricant content in the paste, thereby increasing the extrusion pressure. However, excessive pressure will cause fibril breakage. By using a die of larger entrance angle, the extent of fibrillation is also increased, although the quality of the fibrils is somewhat compromised. Increasing the die aspect (L/D) ratio does not increase the extent of fibrillation. However, it increases the degree of fibril orientation and ensures smoother extrudate. Finally, we have found that extrudates obtained using a paste of higher molecular weight are mechanically superior.     

Effect of irradiation modification above melting point on the wear resistance of polytetrafluoroethylene

The cross-linked polytetrafluoroethylene (PTFE) and PTFE/carbon fiber (CF) composites were synthesized through electron beam irradiation in the molten state of PTFE at a controlled temperature of 340 ± 3°C under an inert gas atmosphere for this study. The wear resistance of raw (raw-PTFE), irradiated modified PTFE (RM-PTFE), and CF-reinforced PTFE composites were evaluated using a friction and wear testing machine. The testing was conducted under varying ambient temperatures and dynamic loads. After irradiation, the samples were sectioned into specific sizes for subsequent testing purposes. Under the test conditions of 4.64 MPa positive pressure, 800 rpm speed, and a duration of 300 s at 20°C, the wear amount of PTFE after irradiation modification is significantly reduced from 1.4103 mm to only 0.0233 mm, representing a remarkable reduction by a factor of 60. Similarly, under the test conditions of 4.64 MPa positive pressure, 200 rpm speed, and a duration of 300 s at 20°C, the friction coefficient of PTFE after irradiation modification is substantially decreased from an initial value of 0.13 to just 0.03. The observed improvement can be attributed to the transformation of PTFE's crystalline form into spherulite, accompanied by a significant enhancement in the degree of cross-linking within its molecular chain. The PTFE was supplemented with 10% CF prior to irradiation. Under the test conditions of a positive pressure of 4.64 MPa, rotation speed of 800 rpm, and a duration time of 300 s at 20°C, the wear amount of the composite material measured only 0.0007 mm, representing a reduction by a factor of 2000 compared to that observed for pure PTFE. This improvement can be attributed to the CF filler's high wear resistance properties and the composite's enhanced thermal conductivity.     

PTFE聚合装置防火防爆安全设计

简述PTFE聚合生产工艺流程，介绍了主要危险物质的性质及装置火灾危险性分类，提出PTFE聚合装置设计时应考虑的防火防爆安全技术。     

On the elastic constants of amorphous carbon nitride

Elastic and thermomechanical properties of amorphous carbon nitrite thin films as a function of nitrogen concentration are reported. The films were prepared by ion beam assisted deposition with nitrogen concentrations ranging from 0 to 33 at.%. By using a combination of the thermally induced bending technique and nano-indentation measurements it was possible to calculate independent values for the Young's modulus, the Poisson's ratio, as well as the thermal expansion coefficient of the films. The hardness and elastic recovery are discussed in terms of the Young's modulus and the Poisson's ratio.