Oxidative stabilization of acrylic fibres

A new model for the structure of oriented acrylic fibres is presented. The polyacrylonitrile molecules (or the acrylic sequences in a co-polymer) are suggested to form two distinct regions within a fibril: amorphous (disordered) and partially ordered. In the partially ordered regions, the polymer molecules assume a contorted helical shape to form rods with a diameter averaging about 6.0 Å in which the nitrile units are oriented at various angles to the rod axis, but are spaced irregularly on or near the surface of the rod. The nitrile groups of adjacent rods can interpenetrate to form dipole pairs. The rods are ordered into a liquid crystal-type array, giving in some cases a lamellar-like texture oriented perpendicular to the fibril axis, with the ordered lamellae regions interspersed with amorphous regions. Evidence for the structure is obtained from transmission electron microscopy observations, a transient peak observed in small-angle X-ray scattering when fibres are thermally treated, as well as wide-angle X-ray diffraction patterns. The proposed model is consistent with the absence of a periodic repeat unit along the chain direction, with the h k 0 reflections seen in wide-angle X-ray and electron diffraction, with the spherulitic morphology reported in some studies, and with the platelike morphologies obtained under some conditions of precipitation from dilute solution.     

Oxidative stabilization of acrylic fibres

The mechanism of oxidative stabilization of acrylic fibres is characterized by two limiting cases which are determined by the fibre chemistry, the reaction conditions, and the diameter of the filament. These limiting cases correspond to diffusion-limited and reaction-limited kinetic processes. Although the chemistry of stabilization is too complex to specify, the various reactions are separated into two categories: those which occur prior to or concurrently with polymerization of the nitrile groups, called prefatory reactions; and those which occur subsequent to nitrile polymerization, called sequent reactions. Under conditions which allow the prefatory reactions to occur significantly before the sequent reactions, the diffusion of oxygen to reactive sites is limited by previously oxidized material; and the fibre shows a typical two-zone morphology. Under conditions where the prefatory and sequent reactions occur sequentially, the overall stabilization process is limited by the rate of the prefatory reactions; but a skin is established at the fibre surface which acts as an oxygen barrier. Data from a variety of sources, including oxygen analysis, microscopic examination, fibre residue after etching, tension developed in fibres held at constant length, and small-angle X-ray patterns, are cited as evidence for the two limiting cases.Based in part on a thesis submitted by SBW in partial fulfilment of the requirements for the Sc.D. degree in materials engineering, MIT, 1976;     

Oxidative stabilization of acrylic fibres

When acrylic fibres are heat treated for various times at 220 to 250° C, they form dark, insoluble structures of uncertain chemical character which are inert to many strong oxidizing and reducing agents. The heat-treated fibres are, however, rapidly decoloured by warm alkaline hypochlorite solutions. When fibres which have undergone short-time heat treatment are subjected to the hypochlorite, incubation periods are observed before decolouration is noted; and a swollen acrylic network remains after decolouration is complete. The acrylic network is primarily unreacted precursor units save for a small amount of hydrolyzed material. The decoloured reaction is zero order, indicating a reaction at the surface. The rate of the decolouration reaction also increases with increasing duration of the stabilization heat treatment. In fibres which have undergone partial diffusion-controlled stabilization, a dark mantle surrounds a lightly coloured core. The rate of decolouration is unaffected as the decolouration interface passes from the mantle to the core, indicating that the decolouration reaction is not influenced by the occurance of any sequent reactions. The existence of the acrylic residue indicates that the prefatory reactions are continuing in both mantle and core during the course of stabilization.¹³C-NMR spectra of the acrylic residue show the same triad methine peak areas as those obtained on the untreated fibre; hence the stereoregularity of the nitrile groups has no influence on the rate of nitrile polymerization. The mechanisms of nitrile initiation and of decolouration are discussed. The residue obtained by sulphuric acid etch is different from that obtained by hypochlorite treatment. These results suggest that during the early-to-intermediate stages of stabilization, the fibre consists of interpenetrating networks of original material, i.e., fibre which has undergone only the prefatory reactions and fibre which has undergone the sequent reactions.     

Pleochroism in thermally-treated acrylic fibres

Acrylic fibres, coloured by inert thermal-treatments up to 573K are highly pleochroic. The preferred absorption is parallel to the fibre axis, implying that the chromophore is a molecular sequence intrinsically part of the oriented polymer chain. The biabsorption, k-k, shows a direct correlation with amount of nitrile reacted in the polymer. Although the development of chromophore is directly related to polymerization of nitriles, the polymerized nitrile sequences themselves may not necessarily be the specific chromophore.Compensated biabsorption, corrected essentially for chromophore concentration, is substantially constant throughout the thermal-treatment, showing that the preferred orientation of both inert chromophore and polymer chain is maintained. An exception arises when light oxidation in air follows inert thermal-treatment, indicating that the oxidation chromophore is oriented perpendicular to the inert chromophore and polymer chain.     

Reinforcement of acrylic polymers with radiopaque cellulose fibres

The incorporation of BaSO₄ containing cotton threads into acrylic matrices was studied, as a means of reinforcing the polymer and producing radiopacity at the same time. Coating the threads with a thin poly(vinyl acetate) film containing BaSO₄ was defined as the most suitable method, providing strength and wettability to the fibres. A percentage of about 10 p.h.r. fibres can be characterized as satisfactory for both objectives, i.e. radiopacity and reinforcement.     

Effective stress intensities in stress corrosion cracking

The phenomena of branching and blunting of stress corrosion cracks are reviewed and their effects demonstrated for a martensitic steel. The stress intensity that a crack can sustain is proportional to the square root of its tip radius, so that blunt cracks require a higher apparent stress intensity. A simple procedure is outlined for converting apparent stress intensities to effective stress intensities, so eliminating anomalous effects due to crack branching and blunting.

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Résumé On passe en revue les phénomènes de ramification et d'arrondissement de l'extrémité des fissures de corrosion sous tension, et on examine leurs effets dans le cas d'un acier martensitique.L'intensité de contrainte qu'une fissure peut supporter est proportionnelle à la racine carrée de son rayon d'entaille, en sorte qu'à des fissures émoussées correspondent une intensité apparente de contrainte plus élevée. On propose une procédure simple permettant de convertir les intensités apparentes de contraintes ou intensité effective, ce qui permet d'éliminer les effets parasites associés à la ramification et à l'arrondissement des fissures.

     

Chemical durability studies on basalt fibres

Chemical durability studies carried out on basalt fibres indicated that this fibre has excellent resistance to alkaline attack, but it has poor resistance to acids. Basalt compares very well with Cem-Fil with respect to alkaline durability. Surface study by scanning electron microscopy and strength properties were used to ascertain the durability of basalt and Cem-Fil.     

Dental application of various kinds of fibres in heat curing acrylic resin

The effect of the type of fibres on bending behaviour and impact energy in reinforced acrylic resin was examined. Reinforcing materials such as inorganic glass fibre and cloth and organic polyester and Kevlar fibres were coated with a silane coupling treatment, whereas Co-Cr wire was directly used for the reinforcement. After silane coupling treatment each fibre was added to the heat curing base resin, except polyester fibre. It is concluded that the inclusion of glass linear fibre provides an effective improvement on plain acrylic base resin. An additional way to reinforce the resin matrix was given by the combined use of glass and Kevlar fibres.     

Environmental stress cracking of polycarbonate catheter connectors

A catheter connector allows medical staff to supply a range of fluids to ill patients using a single intravenous line. In the case of premature babies, such a line provides nutrition in the form of a synthetic milk (TPN) and antibiotics to prevent infection. The connectors are made from several alternative polymers, but polycarbonate is used widely. However, the material is susceptible to environmental stress cracking (ESC) from a variety of fluids. The case study shows how the first versions of one design were poorly moulded and cracked during service, allowing bacteria to enter the line and so infect the baby. The baby contracted meningitis, but lived although suffering brain damage. Evidence was produced which showed that it was a widespread problem in the mid-1990s in UK hospitals. The investigation suggested that poor design and manufacture were the root cause of the problem.     

Environmental stress cracking of PVC and PVC-CPE

The fracture toughness of Polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23°C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 °C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 °C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.     

Environmental stress cracking of PVC and PVC-CPE

The craze initiation of PVC and PVC-CPE in a number of vapour and liquid environments was studied. The observed decrease in the craze-initiation stress could be ascribed to the absorption of components by the PVC and the PVC-CPE matrix. The magnitude of the absorption depends on the activity of the components and on their interaction with the PVC matrix. The activity of the components was defined according to thermodynamics. The interaction of the components can be obtained from measurements of the angle of contact, though it is preferred to quantify this interaction by absorption experiments. A simple model is presented to predict the long-term craze-initiation stress of PVC in environments.     

Hydrogen-assisted stress cracking of high-strength wheel bolts

Early failures occurred with two sizes of wheel bolts used for attaching front wheels and dual rear wheels to heavy truck hubs. Failure resulted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly susceptible to hydrogen-assisted stress cracking. The microstructure resulted from an alloy steel that was carburized, heat treated, and then zinc plated. This combination of material and processing produced a high-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a hard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the threshold for hydrogen-assisted stress cracking.     

Environmental stress cracking of PVC and PVC-CPE

The failure of unnotched PVC and PVC-CPE was studied in a number of vapour and liquid environments. The failure mechanisms observed are related to craze initiation and logarithmic craze growth. A model to explain the origin of the logarithmic craze growth is presented. The failure mechanism in air and in benzene and toluene vapours is ductile, whereas it is brittle in n-hexane, n-decane and ethanol vapours, n-octane/benzene mixtures and natural gas condensate. Equations to describe the time to failure for both failure mechanisms are derived. The ductile failure is ascribed to plasticization and/or plastic deformation around the craze. Brittle failure is thought to arise from crazes which reach a critical size.     

Matrix cracking with frictional bridging fibres in continuous fibre ceramic composites

Interfacial debonding and matrix cracking due to residual axial stresses have been analysed for unidirectional fibre-reinforced ceramic composites. The analytical solutions for the crack-opening displacement, the axial displacement of the composite due to interfacial debonding, and the critical residual axial stress for matrix cracking have been obtained. The solutions were then compared with those for tensile loading in the fibre direction. Three issues related to Part I, i.e. the effective fracture toughness of the composite, the critical loading stress for matrix cracking in the presence of residual stresses, and the debonded fibre length due to loading, were also addressed in the present study.     

Matrix cracking with frictional bridging fibres in continuous fibre ceramic composites

Matrix cracking bridged by intact fibres, which debond from the matrix and then slip against the matrix in friction, has been analysed for unidirectional fibre-reinforced ceramic composites under tensile loading parallel to the fibre axis. The effect of bonding at the fibre-matrix interface, Poisson's effect of the fibre, and residual stresses were included in the analysis. Both the crack-opening displacement and the displacement of the composite due to interfacial debonding have been analytically related to the fibre bridging stress. The critical stress for matrix cracking was also analysed. The existing solutions can be recovered by considering a special case in the present generalized solution.     

The effect of flame-retardants on the properties of acrylic and modacrylic fibres

In this study, flame-retardant additives such as tributyl phosphate, tri(dibromopropyl) phosphate, aluminium hydroxide powder and aluminium hydroxide gel are used for acrylic fibre (acrylonitrile and vinyl acetate copolymer), and calcium phosphate and antimony oxide are introduced into modacrylic fibre (acrylonitrile and vinylidene chloride copolymer). Investigations are carried out on the effect of these flame-retardants on the thermal properties, mechanical properties, morphology and flammability of acrylic and modacrylic fibres. The results show that the effect of additives on the rate of coagulation during spinning can be understood from the changee of morphology of the fibres. Tri(dibromopropyl) phosphate and antimony oxide are good flame-retardant and good synergistic flame retardant, respectively.