O. Delatycki

Characterizing the solid-state thermal oxidation of poly(ethylene oxide) powder

Abstract The oxidative degradation of powdered poly(ethylene oxide) (PEO) resin was studied by polarized optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), solution viscometry and Fourier-transform infra-red spectroscopy (FTi.r.). Powdered PEO readily oxidizes under mild ageing conditions (60°C) owing to its large surface area, its strained crystalline lattice and the weak carbon-oxygen bonds in its backbone. As a result, its physical properties deteriorate after an induction period of about 23 days and, in the extreme case, the free-flowing powder is transformed into a soft wax. With increasing oxidation, there is also a pronounced change in the morphology of PEO from a spherulitic to an axialitic structure. This transition is due to oxidatively induced changes in molecular weight and dispersity that affect the crystallization conditions. Examination of the PEO powder by SEM shows that it has an intricate, fibrillar, surface structure, which produces a large surface area available for oxidation. The emergence of multiple d.s.c. melting peaks after oxidation indicates that a number of low-melting, low-molecular-weight fractions are formed as a result of chain scission processes.

The morphology, chain structure and fracture behaviour of high-density polyethylene

The fracture behaviour of high-density polyethylene has only recently become the subject of comprehensive studies. Few of these studies have utilized a group of resins with systematic variation in molecular properties. In this work, a series of samples with controlled variation in chain structure have been prepared using commercial polymerization facilities. The fracture behaviour of these samples has been measured at both a constant rate of deflection and in static fatigue. Comprehensive statistical techniques were used to correlate these fracture results with the chain structure and morphology of the samples. Part I of this work presents the results for the work conducted at a constant rate of deflection. Both the fracture toughness and crack-growth rate were found to be most strongly dependent on the molecular weight of the resin. This is not an unexpected result. However, when variations in molecular weight are minimal, it was found that increasing the short branch content offers considerable scope for improving the fracture performance. Furthermore, longer short branches were found to be more effective at enhancing fracture behaviour. These results, which are of significant commercial importance, are interpreted in terms of existing models for the fracture process in polyethylene.

The effect of molecular structure and polymer morphology on the fracture resistance of high-density polyethylene

Abstract The fracture resistance of a range of polyethylenes was examined. The tearing modulus, T, was used as a measure of the resistance of the polymer to crack propagation. The values of the tearing modulus were calculated from the gradient of the J-integral versus crack extension lines, J(Δa). Strain rates in the range from 10−7 s−1 were tested. The strain rate dependency of crack initiation and crack propagation have been correlated with the composition and morphology of the polymers. The thickness of the lamellae, side chain concentration and degree of order in the crystalline regions have been shown to influence the resistance of the polymer to crack initiation, J c ( e ) . The tearing modulus appears to be influenced mainly by changes to the morphology that are due to short side chain branches.

Structural morphology and compaction of nascent high-density polyethylene produced by supported catalysts

The morphologies of three nascent high-density polyethylene (HDPE) powders, polymerized in the gas phase by different catalysts, were investigated using scanning electron microscopy (SEM). Silica-supported catalyst systems comprising TiCl4/MgCl2,bis(triphenylsilyl)chromate andbis(cyclopentadienyl)chromium were found to produce polymers with globular, nodular and worm-like microstructures, respectively. The topographies of the fluff particles are related to the compaction behaviour of the HDPE powders. Long, worm-like strands that protrude from the particles are capable of forming more extensive entanglements than the shorter, nodular structures. The entanglements are the main cause of agglomeration of the particles during their long-term bulk storage. Furthermore, the rate of thermal oxidation is influenced markedly by the polymer microstructure. The microstructure determines the surface area available for oxygen attack. High-resolution SEM combined with low-temperature plasma etching reveals that the worm-like structures consist of folded-chain lamellae that are coiled around a core of extended chains.

The effects of pigments on the photostability of polyethylene

The affect of carbon black and various colourizing pigments on the ultraviolet (UV) stability of high and low density polyethylene (HDPE and LDPE) was determined using a novel method for the analysis of oxygen uptake profiles. Samples were exposed to 0.27 Wm−2 (measured at 340 nm) UV irrdiation at 25.0±0.1° C in air at 1.0 atm. The usefulness of this method of assessment of UV stability is demonstrated. The method also enables the rapid collection of data that enable the comparison of the relative photostabilities of experimental and commercial formulations containing pigments and stabilizing additives. The results show that carbon black is an effective UV screening agent for HDPE when added at levels as low as 0.05% (wt/wt) and that increased photoprotection is achieved with increasing concentration of carbon black, up to 5% (wt/wt), above which there is no further significant increase in photostability. LDPE containing ultramarine blue pigment (Na7Al6Si6O24S3) exhibits relatively poor photostability, whereas ferric oxide (Fe2O3) and chrome orange (PbCrO4.PbO) pigments are better photostabilizers for this material. Cadmium sulphide (CdS) was found to photosensitize LDPE. A compound containing 0.10% (wt/wt) carbon black, 0.12% (wt/wt) titanium dioxide (TiO2) and 1.78% (wt/wt) phthalocyanine green (C33H2N8Cl14Cu) is an effective formulation for the stabilization of LDPE. Formulations of LDPE containing ultramarine blue-TiO2 or ferric oxidecarbon black combinations absorb heat on exposure and this may affect their photostability.

Comparative study of the structural, morphological and oxidative characteristics of high-density polyethylene and poly(ethylene oxide)

A comparative study was made of the thermo-oxidative stabilities of high-density polyethylene (HDPE) and poly(ethylene oxide) (PEO) aged in air at 90 and 60°. The PEO was a commercially available grade and two types of HDPE were produced using organo-chromium catalysts supported on a porous silica substrate. Examination of the silica by scanning electron microscopy (SEM) showed it to consist of spherically shaped particles with rough, irregular particulates adhering to their surfaces. Fractured silica particles reveal a system of voids which influences the ultimate mechanical strength of the silica and hence determines the stage at which the silica particles shatter during the polymerization process. The particle size distribution of each silica support was determined by laser light scattering. It was found that the silica support which had the higher effective surface area to weight ratio increased the reactivity and productivity of the catalyst system, thus affecting the morphological characteristics of the nascent polymer particles. The SEM examination of nascent PEO showed ductile, drawn cobweb structures. Since HDPE catalysed using a bis(triphenylsilyl)chromate shows a similar cobweb morphology and is known to have an induction period proceeding steady-state polymerization, it can be inferred that PEO polymerizes after an induction period. The rate of polymer oxidation was assessed by carbonyl index measurements obtained by Fourier transform i.r. spectroscopy. The rate of oxidation correlates with the specific surface area of the polymer, which is determined by the nascent morphology. Polarized optical microscopy showed that isothermally crystallized films of the oxidized polymers display an axialitic morphology. After oxidation, it appears that the calcium oxide residue [ca 0.8% (w/w)] in the commercial grade of PEO can act as an efficient nucleating agent for axialitic growth, when the surfaces of the residue particles are wetted by oligomeric oxidation products.

Re-examination of the Crosslinking Process in Styrene-Unsaturated Polyester Systems

Abstract The network structure of a range of styrene-unsaturated polyester networks was investigated by degradative and spectroscopic studies, sol-gel analysis, and dynamic mechanical measurements. The results show that in addition to styrene-fumarate copolymerization, a fumarate-fumarate crossiinking reaction occurs when the styrene concentration is low. The extent of this reaction was found to be dependent on the proximity of the fumarate units to one another, and on the curing conditions employed. Although the sol-gel studies were qualitatively consistent with these observations, a quantitative analysis was not successful due to deviations of the polyester networks from the model.

Efficiency of processing stabilizers using a micro-oxygen uptake technique

Abstract The thermooxidative stability of various low density polyethylene (LDPE) film formulations was investigated using the technique of micro oxygen uptake measurement following multiple extrusions. The results show that the micro oxygen uptake technique is more sensitive than conventional test methods. High molecular weight hindered phenolic stabilizers are more effective in reducing gel formation during polymer film production than are lower molecular weight species such as butylated hydroxytoluene (BHT). The antioxidant 2,2′-ethylidene-bis-(4,6-di-tert-butylphenol) is an effective stabilizer, but it forms a highly coloured complex with transition metal impurities. The hindered phenol/organic phosphite system, comprising a combination of 0.008% (wt/wt) octadecyl-3-(3,5-di-tert-butyl)-4-hydroxy-phenol propionate and 0.032% (wt/wt) tris-(2,4-di-tert-butyl)phenyl phosphite, is effective in suppressing the formation of coloured products but does not provide adequate thermal stability.

Effect of selenium on the thermal oxidation and oxidative pyrolysis of high-density polyethylene

Abstract The performance of selenium as an antioxidant for high-density polyethylene (HDPE) was investigated by isothermal chemiluminescence (CL) and dynamic thermogravimetric analysis. It was found that 100 ppm of selenium powder (100-mesh) decreases thermo-oxidative degradation and displaces the onset of thermal pyrolysis by about 60°C.

Double torsion fracture testing of high-density polyethylene

The fracture of polyethylene has been studied extensively using conventional testing geometries such as three-point bending (TPB) and single-edge notch tension (SENT). These geometries are of limited utility for studying crack growth, because the crack speed is constantly changing and the crack front is in the centre of the specimen. Double torsion (DT) is a fracture geometry that suffers neither of these disadvantages, yet has only received limited attention in the literature. Its use has been limited to highly brittle materials such as glass, ceramics, thermosetting plastics and PMMA. In contrast to these materials, high-density polyethylene (HDPE) is an inherently ductile polymer. Before the advantages of DT can be exploited for testing HDPE, it is first necessary to demonstrate the validity of DT fracture measurements performed on such a ductile material. In this paper it is shown that at moderate rates of loading and at temperatures below 0‡C, valid double torsion fracture results can be obtained for an ethylene 1-butene copolymer. A novel technique for specimen preparation and a simple method for accurately monitoring crack growth are also described.