Stress softening in natural rubber vulcanizates. Part IV. Unfilled vulcanizates

Stress softening (Mullins effect) is observed in gum natural rubber vulcanizates. The magnitude of this softening is similar to that in carbon black-filled vulcanizates. The amount of stress softening is slightly greater in vulcanizates cured to produce predominantly polysulfide crosslinks than in those containing monosulfide or carbon–carbon crosslinks. The total recovery of stress softening in the vulcanizates containing monosulfide or carbon–carbon crosslinks suggests that the phenomenon is attributable to a quasi-irreversible rearrangement of molecular networks due to localized nonaffine deformation resulting from short chains reaching the limit of their extensibility. This non-affine deformation results in a displacement of the network junctions from their initial random state.     

Stress softening in natural rubber vulcanizates. Part III. Carbon black-filled vulcanizates

Stress softening (Mullins effect) occurs in rubber vulcanizates during the first and subsequent deformations. This paper shows the similarity of the degree of stress softening in both unfilled and carbon black-filled vulcanizates of natural rubber when stressed almost to break. This confirms the earlier investigations which were confined to moderate stresses. A simple interpretation of the tensile stress–strain results for filled rubber is that the strain in the rubber is increased by the presence of the filler, so that the ratio of the average strain in the rubber to the measured overall strain is given by a strain amplification factor. The usefulness of this concept is confirmed by showing the similarity of the stress-softened curves after normalizing the strains, provided the vulcanizates of both gum and filled vulcanizates were subjected to the same initial stress.     

Stress softening in natural rubber vulcanizates. Part V. The anomalous tensile behavior of natural rubber

Stress-strain cycling of natural rubber to high strains produces greater softening than amorphous rubbers, but only if the force on the sample is relaxed below a certain value during the cycle. This phenomenon, attributed to crystallization shows why nonrelaxing tests give a longer fatigue life than relaxing tests.     

Strain energy functions of rubber. I. Characterization of gum vulcanizates

A plot of principal stress difference versus principal extension ratios has been used as a graphic representation of general deformation. Two analytic forms of the strain energy function for isotropic, incompressible materials are suggested. These involve five or nine terms, the coefficients of which are found by regression to the general deformation plot. The resulting stress–strain equations are used to predict particular deformations, for example, simple extension, and are also evaluated in model engineering desing experiments. These experiments use interative techniques to predict the shapes and pressures of inflated diaphragms and tubes, and it is shown that the equations lead to accurate results even at relatively high extensions.     

Stress softening in elastomer blends

Compressive stress softening (Mullins effect) in black-filled vulcanizates of the blended system natural : nitrile : brominated butyl rubber has been investigated. The results are expressed in terms of parameters derived from successive stress-strain cycles. The ratio of the work done on loading in the first and second cycles is called the load work ration, R_w. A similar quantity, R_h, is defined for the loss or hysteresis work. Both load and loss work ratios show a compositional dependence similar to other properties of blends. The variation of R_w and R_h with composition agrees with the recent thesis of Mullins, Harwood, Payne and others that the Mullins effect is a property of the elastomer. Sudden variation of R_h with elongation has been interpreted in light of recent results of R. W. Smith as indicative of vacuole formation.     

Strain energy functions of rubber. II. The characterization of filled vulcanizates

In part I it was shown that if the strain energy function was expanded to a sufficient degree and the coefficients C_ij were found by regression to pure homogeneous strain data, then stress–strain equations could be derived to give accurate solutions to design problems even at relatively high extensions. The problems of applying this theory to filed vulcanizates are discussed and a way of obtaining pure homogeneous strain data for for such materials is suggested. Stress–strain equations fitted to the data are found to be general within the range of experimental strains and in some cases will extrapolate outside this range. The equations can be used in design applications where strains are greater than would be experienced in normal engineering practice.     

The dynamic mechanical properties of oil-extended,carbon black-reinforced,and peroxide-crosslinked EPDM rubber

The effects of aromatic, naphthenic, and paraffinic oils on the dynamic mechanical behavior of an EPDM rubber (Nordel 1560) have been investigated as have the influences of oil level, carbon black loading, and peroxide crosslinking.     

Molecular dynamics study on diamond nanowires mechanical properties: Strain rate, temperature and size dependent effects

Using molecular dynamics simulations, strain rate, temperature and size dependent mechanical properties of < 001 > orientation diamond nanowires are investigated. It is found that, for the same cross-sectional areas, strain rates have almost no effect on yield strength and Young's modulus, provided strain rates are within the range from 0.001 to 0.025 ps^− 1. Our calculated results have also indicated that, at the temperature ranging from 100 to 500 K, diamond nanowires' yield strength, Young's modulus, fracture strength and fracture strain are all decreasing with increasing temperature. Furthermore, at the temperature of 300 K, yield strength, Young's modulus, fracture strength and fracture strain increase dramatically with increasing cross sectional area. Finally, orientation dependent diamond nanowires mechanical properties are studied.     

Stress relaxation in carbon-black-filled rubber vulcanizates at moderate strains

Stress-relaxation measurements have been made on rubber vulcanizates containing carbon black. Evidence has been produced for the existence of a distinct relaxation process which occurs within the first 60 s of relaxation at room temperature. This process becomes less important as the strain is increased and disappears when the temperature is increased or the rubber is swollen. It is suggested that this relaxation is the result of breakdown of structure formed from the carbon-black particles.     

AC conductivity and positive temperature coefficient effect in microcellular EPDM vulcanizates

Dielectric relaxation spectra of conductive carbon black reinforced microcellular EPDM vulcanizates were used to study their relaxation behavior in the frequency range of 0.01–10⁵ Hz over a wide range of temperature from 30 to 120°C. The effect of variation in filler loading and blowing agent loading (density) on dielectric characteristics such as impedance, dielectric constant, and conductivity has been studied. The experimental results show that the relative dielectric permittivity of the composites depends strongly on the extent of carbon black and blowing agent concentrations. The frequency dependence of AC conductivity has been investigated by using Percolation theory. The permittivity and conductivity of the microcellular composites have been analyzed based on scaling theory at increasing temperatures. The applicability of Lichtenecker‐Rother's “rule of mixture” to describe the complex permittivity of the composite has also been investigated. Irrespective of the blowing agent loading and temperature, the percolation threshold as studied by DC conductivity was found to be at 40 phr loading of the filler. Scanning electron microphotographs showed agglomeration of the filler above this concentration and formation of a continuous network structure. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

The tensile properties of strain‐crystallizing vulcanizates. II. Stress relaxation and hysteresis

Stress relaxation on cessation of extension and hysteresis were examined in conventional and peroxide vulcanizates over a range of crosslink densities at room temperature and at 90°C. The rate of relaxation decreases with an increase in temperature and is attributed to slower nucleation of strain‐induced crystallites. The decrease in the volume fraction of extendable material as a result of strain‐induced crystallites has only a small effect on the rate at which the slope of the stress–strain curve rises. Crystallites that form on cooling, melt on heating, leaving an unaltered network, but when strain‐induced crystallites are melted by heating, they do not reform on cooling. The relaxed network now extends further before failure than a network in which extension was not interrupted by a heating–cooling cycle. This supports proposals in the preceding article that strain‐induced crystals increase tensile properties by altering the network deformation pattern and not by the deflection of propagating flaws, whose mechanism would require the heated sample to fail earlier due to its lower crystalline content. Hysteresis increases sharply at strains at which crystallization in the network becomes possible. The hysteresis ratio reaches a plateau value at higher strains. Relaxation affects both the extension and retraction stress–strain curves and it is proposed that hysteresis is determined by differences in the crystallinity during extension and retraction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2423–2430, 2006     

炭黑与白炭黑增强聚烯烃弹性体硫化胶的比较

研究了炭黑、白炭黑对聚烯烃弹性体(POE)硫化胶的增强作用,考察了交联剂过氧化二异丙苯(DCP)和炭黑及白炭黑的用量对POE硫化胶物理机械性能的影响,并用扫描电子显微镜对其微观结构进行了观察.结果表明,用DCP交联POE,POE的主要物理机械性能比未交联前有所下降;用炭黑、白炭黑增强POE硫化胶,其物理机械性能显著提高,说明增强效果明显,炭黑、白炭黑最佳用量分别为60份和40份.     

高温碳化温度对碳纤维性能的影响

对经过相同预氧化、低温碳化和不同高温碳化处理的碳纤维样品进行了拉伸强度、拉伸模量、密度的分析,找出了2种原丝在经过碳化工艺处理后拉伸强度、拉伸模量、密度与高温碳化温度的相关性,并对其影响机理进行了研究。研究结果表明:在一定的温度范围内碳纤维拉伸强度随着高温碳化温度的提高而增加,在高温碳化温度达到一定值时,碳纤维拉伸强度将下降;碳纤维拉伸模量均随着高温碳化温度的提高而增加;碳纤维密度均随着高温碳化温度的提高而降低。     

High temperature vulcanization of elastomers: 2. Network structures in conventional sulphenamide-sulphur natural rubber vulcanizates

A natural rubber (NR) gum mix with a conventional N-cyclohexyl-2-benzothiazylsulphenamide (CBS) accelerated sulphur system (0.5:2.5 CBS/S) was vulcanized at temperatures from 140°C to 200°C. The influence of cure temperature on (a) the chemical crosslink density, (b) the distribution of crosslink types, (c) the extent of sulphidic main-chain modifications, and (d) the zinc sulphide formation was investigated. Results show that elevated cure temperatures produce a network with a lower crosslink density, in particular a lower polysulphidic crosslink density. The formation of intramolecular sulphidic groups and zinc sulphide increases with increasing temperatures. The possibility of chain scission during vulcanization was examined by a quantitative analysis of the sol-gel data. Less than 1 site of scission per 100 crosslinked isoprene units was established in the temperature range of 140–200°C. The network results can be satisfactorily correlated with the physical properties of a tyre tread mix of NR as reported in Part 1. Mechanistic interpretations are made to account for the network results.     

Preparation and properties of carbon black filled EPDM/POE thermaplastic vulcanizates

TPVs filled with different amounts (0–50 phr) of carbon black were prepared via melt mixing by dynamic vulcanization in Haake plasticorder at 150°C and 40rpm and then the properties of them were studied. Torque-time curves showed that the curing degree reached a biggest value at 10 phr and then decreased with the increase of filling content while the curing rate was always rising. Mechanical properties such as tensile strength, tear strength, modulus as well as hardness increased with the increment of carbon black content while the tension set at break was reduced dramatically. Two phase morphology was observed by SEM photographs and the effect of carbon black on curing extent was testified. To illuminate the effect of carbon black, curometer curves and carbon black dispersion pictures were also analyzed. Rubber processing analyzer (RPA) experiments proved that there was a progressive nonlinear behavior, which was more and more clearly expressed with the increment of carbon black content and could be explained via the Payne effect. But the TPVs containing highest carbon black exhibited the fastest drop of G′ with increasing strain amplitude as obtained from the value of G₀′ − G_∞′. The order of tanδ at different carbon black content was tanδ (10 phr) < tanδ (0 phr) < tanδ (30 phr) < tanδ (50 phr) at lower strain amplitude. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Hydrodynamic and temperature effects on the flow-induced local corrosion rate in pipelines

When a flow enters a pipeline or goes over an obstacle, a new boundary layer is developed and produces a local corrosion rate that is greater than the average corrosion rate. Equations are developed to calculate this local flow-induced corrosion rate. The calculated local corrosion rate varies from a fraction of mm/year to 1.27 mm/year (50 mils/year), depending on the temperature, velocity, and location of the flow. The calculations also show that the maximum local corrosion rate occurs at a short distance from the edge of the pipe entrance. The wavy appearance of erosion-corrosion failure on metal surfaces can be explained by the boundary layer theory and the convective mass transfer of ferrous ions.     

甲基丙烯酸锌与炭黑共同补强天然橡胶的研究

在开放式炼胶机上,分别以直接添加和原位生成2种方法向天然橡胶(NR)基体中添加甲基丙烯酸锌(ZDMA),制备了2种类型的ZDMA与炭黑共同补强的NR硫化胶,X射线衍射(XRD)分析证实,成功制备了ZDMA。扫描电镜(SEM)研究表明,原位生成ZDMA硫化胶的断面比直接添加ZD-MA硫化胶的粗糙很多,表现出更强的界面结合力。力学性能研究表明,用ZDMA部分替代炭黑,可以有效提高硫化胶的力学性能。在相同添加量下,原位生成ZDMA硫化胶的力学性能优于直接填充ZD-MA硫化胶的。     

Modern Aspects of Stress and Strain

This is the fourth contribution to an ongoing series of articles entitled ENDPIECE. The idea behind this series is to have a place where the Editor-in-Chief of the journal SILICON can reflect on some aspects of silicon (Si) science and on its applications. Hopefully, the collection of articles will build and the series will serve to show just how broad the field of silicon-based materials and technologies is. Indeed, this broadness is being reflected in the growing content of the journal SILICON. In silicon-based materials, we often use the terms stress and strain in the area of mechanical properties. Seven articles from SILICON have been briefly reviewed herein in order to illustrate the importance of the mechanical testing of silicon-based materials and of the experimental definition of stress and strain. The terms stress and strain have a number of wider definitions and these will be explored. Throughout our daily lives in the modern world we often encounter the word stress and we are all-too-familiar with the negative outcomes of prolonged time periods of being under stress. In this ENDPIECE I will share some of my thoughts and ideas on stress and strain in a broad context. It is worth noting that there are many books available which have stress as (or in) the title. I have used selected topics presented in the popular literature to illustrate various aspects of stress and strain.