Surface and bulk properties of severely fluorinated carbon fibres

The development of ultra-inert composites using fluorinated carbon fibres as the reinforcement requires fluorinated carbon fibres with a durable surface composition. Here we report the effect of direct fluorination using an F₂/N₂ mixture at 653 K on the surface and bulk properties of two types of high strength carbon fibres. These were treated up to a surface fluorine content of ∼64 at.% and a bulk fluorine content of ∼15 mass%. A colour change was observed after fluorination caused by the changes in the graphitic band structure of the carbon fibres by the introduction of carbon sp³ hybridisation. The tensile strength and Young's modulus decrease after fluorination by up to 33 and 22%, respectively. XRD shows marginal changes in the interlayer distance but the crystallite size increases. Changes in the electrical conductivity of the fluorinated carbon fibres indicate that the modification is confined to the near surface volume. Predominantly covalent C-F bonds are formed as shown by X-ray photoelectron spectroscopy (XPS) and measured zeta (ζ)-potentials. Hence the fluorinated fibres are hydrophobic and have low surface tensions. This and the large increase in fibre surface area, as determined by nitrogen adsorption, is expected to facilitate interfacial interaction between fluorinated carbon fibres and fluoropolymers.     

用活性炭纤维同时脱除二氧化碳中COS与H_2S的研究

采用固定床动态吸附实验考察了用NaOH、KOH、Na2CO3、K2CO3和Fe3+、CO2+等对活性炭纤维(ACF)进行改性,并分别对其脱除氧硫化碳(COS)和硫化氢(H2S)的性能进行了测试。重点是以N2和CO2为稀释气,对改性离子的种类、含量、反应气氛和反应温度等的脱硫性能进行研究,还考察了在COS和H2S同时存在的情况下,ACF脱除COS和H2S的性能。结果表明,碱改性ACF比空白ACF脱除效果好,金属离子改性ACF与碱改性ACF相比可以有效地提高活性炭纤维的硫容量。其中,用CO2+改性ACF能同时提高脱除COS和H2S的效果。     

Structure and superstructure of carbon fibres

The structure of carbon fibres consists of stacks of carbon layers oriented parallel to the fibre axis. From X-ray wide-angle scattering studies (WAXS) one obtains various structural parameters characterizing the perfection of the stacking and the orientation of the layers. The latter is quantitatively related to the tensile modulus of the fibres. Small-angle scattering studies (SAXS) are used to determine size, shape and orientation of microvoids. For carbon fibres with high heat treatment temperatures (HTT), a correlation between microvoid content and elongation at break is observed which indicates an optimum value for the microvoid content. Electrochemical intercalation is used to obtain information on the accessibility of the carbon layer structure.     

Investigation of interfacial interaction between uncoated and coated carbon fibres and the magnesium alloy AZ91

Unidirectionally reinforced metal-matrix composites with a fibre volume content between 63 and 68% were processed by squeeze casting using T800 H carbon fibres and the magnesium alloy AZ91. The surface of the fibres was prepared by thermal desizing of the fibres or by deposition of a pyrolytic carbon (pyC) coating. Different interfacial conditions could be identified by transmission electron microscopy (TEM) and the single-fibre push-in test. TEM confirmed the formation of needle-like phases at the fibre surface or, for coated fibres, within the pyrolytic carbon coating. During loading by the Vickers type indenter an intense response was observed for composites of coated fibres and the magnesium alloy. This could by caused by stick-slip effects within the pyrolytic carbon coating.     

Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water: Synergistic effect of phenol and KOH on the decomposition efficiency

The combination of phenol and potassium hydroxide (KOH) was used to chemically recycle carbon fibre reinforced epoxy resin cured with 4,4′-diaminodiphenylmethane in subcritical water. This combination had a synergistic effect on decomposing this kind of epoxy resin. The main decomposition products from the epoxy resin were identified by means of GC-MS, and a possible free-radical reaction mechanism for the decomposition of epoxy resin is proposed. The recovered carbon fibres were characterized using single fibre tensile tests, scanning electron microscopy and X-ray photoelectron spectroscopy. Compared to virgin carbon fibres after sizing removal, the surface compositions of the recovered carbon fibres had little change and the tensile strength of the recovered carbon fibres was well retained.     

Continuous Atmospheric Plasma Oxidation of Carbon Fibres: Influence on the Fibre Surface and Bulk Properties and Adhesion to Polyamide 12

Continuous atmospheric plasma oxidation (APO) was used to introduce oxygen functionalities to the surface of carbon fibres in an attempt to enhance interfacial adhesion between carbon fibres and polyamide-12 (PA-12). APO only affects the surface properties of the fibres while their bulk properties remained unchanged. Contact angle and ζ-potential measurements demonstrated that APO-treated fibres became significantly more hydrophilic due to the introduction of polar oxygen-containing groups on the fibre surface, which also resulted in an increase of surface energy on the carbon fibres. The interfacial shear strength of single carbon fibre/PA-12 model composites, determined by single fibre fragmentation tests, showed an increase from 40 to 83 MPa with up to 4 min of APO treatment time which confirms that the fibre/matrix interfacial adhesion was enhanced. This highlights that the incorporation of APO into composite manufacturing will allow tailoring of the fibre/matrix interface.     

NiSO_4改性对聚丙烯腈原丝及其碳纤维结构与性能的影响

碳纤维具有高比强度、高比模量、导电、耐热、自润滑等优异的综合性能,在纤维增强复合材料中得到了广泛的应用.可制备碳纤维的前驱体有人造丝、沥青、聚丙烯腈纤维、木质素、聚乙烯纤维、聚苯并噻唑(PBO)纤维等.但大多数高强碳纤维目前仍然是由聚丙烯腈纤维制备的,同时,许多工作都集中在更进一步提高碳纤维的机械性能.特别是在我国, 碳纤维质量与某些发达国家相比,还有较大的差距,急需解决的问题就是如何尽快研制出高力学性能的碳纤维.采用氨基硅氧烷、脂肪族羧酸[1]、CuCl[2]、KMnO 4[3]、CoCl2[4]等有机或无机化学试剂对聚丙烯腈原丝进行化学处理, 以改进最终碳纤维的结构与性能是一种有效的方法.国内在这方面的研究还很少.文献[1 ～4]中所采用的方法都是利用商业聚丙烯腈原丝在碳化前进行洗油、浸渍、洗涤烘干处理 ,增加了碳纤维制备的工序,同时,原丝损伤较大,在连续生产中难以适用.我们在原丝连续制备的同时采用NiSO4溶液浸渍处理聚丙烯腈纤维,本文主要研究了采用NiSO4浸渍改性后聚丙烯腈原丝及其碳纤维的结构与性能.研究表明,采用NiSO4在线浸渍改性聚丙烯腈原丝,生产工艺简单,且能有效地改进最终碳纤维的结构与性能.     

Recycling of carbon fibre reinforced polymeric waste for the production of activated carbon fibres

Composite waste composed of carbon fibres and polybenzoxazines resin has been pyrolysed in a fixed bed reactor at temperatures of 350, 400, 450, 500 and 700 °C. Solid residues of between 70 and 83.6 wt%, liquid yields 14 and 24.6 wt% and gas yields 0.7 and 3.8 wt% were obtained depending on pyrolysis temperature. The derived pyrolysis liquids contained aniline in high concentration together with oxygenated and nitrogenated aromatic compounds. The pyrolysis gases consisted mainly of CO₂, CO, CH₄, H₂ and other hydrocarbons. The carbon fibres used in the composite waste were separated from the char of the solid residue via oxidation of the char at two different temperatures and investigated for their mechanical strength properties. The carbon fibres recovered from the sample pyrolysed at 500 °C and oxidised at 500 °C exhibited mechanical properties which were 90% of that of the original virgin carbon fibres. Steam activation of the recovered carbon fibres was carried out at 850 °C at different times of activation. The effect of activation time on BET surface area, activated carbon fibres yield, porosity and the morphological structure of activated carbon fibres was evaluated. A maximum BET surface area of over 800 m² g⁻¹ was obtained for the activated carbon fibres produced at 850 °C for 5 h of activation. Nitrogen adsorption-desorption isotherms showed that the adsorption capacity increased as the activation time increased up to 5 h of activation and then after that decreased.     

Analytical characterization of coated carbon fibres

Summary Carbon fibres must be coated with a barrier layer in their application as reinforcement in metal matrix composites (MMC) to avoid reactions between the matrix and carbon. Materials frequently used are SiC, pyrolytical carbon or these materials in combination. The reactivity between the metallic matrix and the barrier layer depends on the stoichiometry and structure of the barrier material. XPS and AES are suitable methods for the investigation of the surface and of composition of the layer of the coated fibres. Additionally Raman-spectroscopy has been used to aid the distinction between the bonding states of carbon in the barrier layer and the fibre. The problems of the analysis of small, fibre-like materials are discussed.     

Stepwise pyrolysis—gas chromatography of viscose fibres

Thermogravimetry and pyrolysis—gas chromatography are well known methods for investigating the thermal stability and structure of polymeric materials. In this paper, the possibilities of applying the little known stepwise pyrolysis—gas chromatography for evaluating the thermal stability of modified viscose fibres are discussed. The temperature dependence of the evolution of water, carbon monoxide and carbon dioxide on thermal degradation of flameproved viscose fibres has been investigated and the data are compared with the thermogravimetric curves.Stepwise pyrolysis—gas chromatography provides reproducible quantitative data on the decomposition of polymers and combines some of the advantages of thermogravimetry and pyrolysis—gas chromatography.     

Pattern formation and non-linear phenomena in stretched discotic liquid crystal fibres

This paper presents a non-linear numerical and bifurcation analysis of pattern formation phenomena in a discotic nematic liquid crystal confined to annular cylindrical cavities and subjected to extensional deformations. The results are of direct relevance to understanding the industrial melt spinning of mesophase carbon fibres, using discotic nematic liquid crystals precursor materials. Three types of orientation patterns are identified in this study: spatially constant (radial), monotonic (pinwheel), and oscillatory (zigzag). Numerical and closed form analytical results predicting continuous transformations between the radial, pinwheel, zigzag radial orientation modes are presented. The bifurcation analysis provides a direct characterization of the parametric dependence and the transitions between these three basic patterns, and provides a complete understanding of the multistability phenomena that is present in the oscillatory orientation patterns. In general it is found that small fibres of nearly elastically isotropic discotic nematic liquid crystals tend to adopt the classical ideal radial texture, while larger fibres with anisotropic elastic moduli tend to yield the zigzag texture. Fixed arbitrary surface orientation of intermediate size and anisotropy tend to adopt the pinwheel texture. The theoretical results are able to explain the main features and mechanisms that lead to the commonly observed cross-section textures of industrially spun mesophase carbon fibres.     

Effect of thermal transformation and stability on the flammability of PAN precursors-based carbon fibres

Journal of Thermal Analysis and Calorimetry - During the development of the carbon fibres and oxidized fibres, the aim was to get a material with extraordinary mechanical properties. Nowadays, it...     

Comparison of protein immobilisation methods onto oxidised and native carbon nanofibres for optimum biosensor development

The properties of native and oxidised graphene layered carbon nanofibres are compared, and their utilisation in enzyme biosensor systems using different immobilisation methods are evaluated. The efficient oxidation of carbon nanofibres with concentrated H₂SO₄/HNO₃ is confirmed by Raman spectroscopy while the introduction of carboxylic acid groups on the surface of the fibres by titration studies. The oxidised fibres show enhanced oxidation efficiency to hydrogen peroxide, while at the same time they exhibit a more efficient and selective interaction with enzymes. The analytical characteristics of biosensor systems based on the adsorption or covalent immobilisation of the enzyme glucose oxidase on carbon nanofibres are compared. The study reveals that carbon nanofibres are excellent substrates for enzyme immobilisation allowing the development of highly stable biosensor systems. Figure Immobilization of proteins on carbon nanofibres     

A novel enzymatic bioelectrode system combining a redox hydrogel with a carbon NanoWeb

A novel bioelectrode system has been prepared in which an enzyme and a conducting polymer hydrogel are combined in a nanostructured scaffold. The latter consists of fibres of carbon NanoWeb, grown by chemical vapour deposition onto reticulated vitreous carbon (RVC). The catalytic currents produced by this new bioelectrode system are significantly larger than those obtained using conventional electrodes.     

Characterisation of carbon fibres grown from carbonaceous gases by measurements of their density and oxidation resistance

In the present work it has been demonstrated, by measurements of their density and air oxidation resistance, that different sorts of carbon fibres grown from a gaseous stage, vapour grown carbon fibres (VGCF), present a duplex structure. By means of these two experimental techniques, structural differences between both constituent phases of this kind of fibres have been determined. It has been proved that the catalytic phase or nucleus presents higher level of crystalline perfection than the pyrolytic or cortical phase. This is corroborated by the fact that thin VGCF show better mechanical properties than thick fibres.In this way, it has been possible to determine the mean nucleus diameter of a batch of VGCF, and to established a mathematical expression, of the rule of mixtures type, for their density.     

Thermal degradation analysis and XRD characterisation of fibre-forming synthetic polypropylene containing nanoclay

Flammability of synthetic fibres is significantly worse than that of bulk polymers because of the high surface area to volume ratio and the low tolerance to high filler loadings in the fibre production process. Introducing nanocomposite structures has the potential to enhance the char formation at relatively low loadings of nanoparticulate fillers and hence can reduce the flammability of synthetic polymers and fibres.This paper reports thermal degradation analysis results in conjunction with TG analysis under different atmospheres and further studies of X-ray diffraction characterisation of fibre-forming polypropylene containing selected dispersed nanoclays.The concentrations of hydrocarbons, carbon monoxide and carbon dioxide released during the TG analysis have been monitored and analysed by using a combined electrochemical infrared analyser. The intensity changes of the crystallinity peaks and nanoclay peaks in the polymer and composites are discussed.     

Effect of the material structure and nanoadditive type on the thermal properties of nanocomposite alginate fibres

Thermal properties of sodium alginate and calcium alginate fibres containing nanoadditives were analysed. Thermal decomposition of fibres based on sodium alginate, both pure and containing nanoadditive, produces sodium carbonate (for fibres without nanoadditive) or, for modified fibres, a mixture of sodium carbonate with the corresponding nanoadditive, which does not undergo any changes within the range of measured temperatures. The chief gaseous products accompanying the decomposition are carbon (IV) oxide and water. The shape of the DTG curves and values of the *T ₅₀ coefficient indicate an improvement in the thermal properties of the fibres when ceramic nanoadditives (SiO₂ and HAp) are introduced, as compared with pure sodium alginate fibres. When ceramic nanoadditives (MMT, SiO₂, bioglass) are added to the material of calcium alginate fibres the nature of the thermal processes is not affected. However, similarly as for sodium alginate fibres modified with nanoadditive, here also it was note the effect on the value of the *T ₅₀ coefficient. The greatest thermal stability, expressed by the value of *T ₅₀, was observed when SiO₂, HAp and bioglass were used as nanoadditives. Considering that the porosity, sorption properties and cross section were similar for all fibres, it can be suggested that in all cases the structure of the fibres has a lesser effect than the type of nanoadditive on the thermal properties of the investigated fibres. The effect of the chemical structure of the material was reflected in the higher decomposition temperature of calcium alginate nanocomposite fibres compared with sodium alginate fibres. This also applies to fibres without any nanoadditive.     

Computational study of the texture formation in mesophase pitch-based carbon fibres

This paper studies the thermal relaxation phenomena after melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch using mathematical modelling and computer simulation. The Ericksen and Landau-de Gennes continuum theories are used to investigate the structure development and texture formation across mesophase pitch-based carbon fibres. The two-dimensional model captures five types of transverse patterns, which match the commonly observed textures for mesophase pitch-based carbon fibres. They are: random, zig-zagged radial, radial, quasi-onion and onion. These textures represent the various combinations possible from the interplay between structure (i.e. texture) development and cooling during the fibre spinning process. During the thermal relaxation after the cessation of extensional flow the discotic nematic molecules store elastic free energy decays. The distorted nematic molecular profiles reorient to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation result in different transverse textures. The rate at which the fibres are cooled is the main factor in controlling the structure development. A slow cooling rate would permit the nematic discotic molecules to reorient to a well-developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.     

Matrix properties and composite failure

The influence of matrix extensibility on the properties of a composite was studied using two glassy polymers of almost identical chemical structure but differing crosslink densities. The lower crosslink density gave a 73 % increase in tensile elongation at break and a 56% increase in specific fracture energy. Unidirectional laminates of glass, carbon, and Kevlar^® fibres were prepared with these two polymers and tested for shear strength, transverse tension, and dynamic fatigue.The shear strengths of the polymers were found to be almost independent of crosslink densities (about 100 MPa). The interlaminar shear strengths of the carbon fibre laminates corresponded to those of the matrix polymers (Kevlar^® fibre laminates failed at 60 %). In accordance with Griffith's equation the more extensible polymer and its laminates performed better in tensile tests transverse to the fibres due to improved fracture energy. Failure criteria based on strain magnification were useful in the case of glass fibre laminates, but proved inadequate for laminates based on anisotropic fibres such as carbon and Kevlar^®.The dynamic fatigue strengths of the two matrix polymers were unaffected by the difference in crosslink densities. Almost the same fatigue strengths were obtained for the matrix polymers as for the laminates (carbon, glass) transverse to the fibres. A lack of processability of the polymer with high functionality was identified as a source of deteriorating effects.     

Analysis of the microporous structure of the low-cost activated carbon fibres obtained from flax and jute cloth

The paper presents the results of research devoted to obtaining the low-cost activated carbon fibres from waste flax and jute cloth by carbonisation in inert atmosphere and activation with air as well as to the analysis of the microporous structure of materials obtained on the basis of the nitrogen, argon and benzene adsorption isotherms, using among others, the unique LBET method with implemented of the new models of the multilayer adsorption on the heterogeneous surfaces of the carbonaceous adsorbents. As part of the research conducted, imaging of the surface of the low-cost activated carbon fibres obtained has also been done using scanning electron microscopy.