John Scheirs

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.

A review of oxygen uptake techniques for measuring polyolefin oxidation

Abstract The application of oxygen uptake techniques to the determination of polyolefin stability is extensively reviewed and includes the consideration of both manometric apparatuses and devices which incorporate pressure transducers. The sensitivity of oxygen uptake instruments, types of heating facilities, molecular sieves and adsorbents, the shape of the oxygen uptake profile and the technique of oxygen uptake as related to other techniques are also discussed. A comprehensive table listing oxygen uptake apparatuses in chronological order is included in the review.

An investigation of the kinetics of cellulose degradation under non-isothermal conditions

Abstract The degree of polymerization (DP) of a polymer system subjected to non-isothermal depolymerization is examined kinetically with a preliminary application to the study of cellulose degradation. An equation for the time dependence of the DP observed under non-isothermal degradation using a linear temperature ramp is derived from the relevant isothermal kinetic equations. An iterative computerbased method of fitting non-isothermal degree of polymerization (NIDP) data is also presented. The activation energy and the Arrhenius A-factor associated with the depolymerization process can be varied independently to achieve an optimized fit to experimental NIDP data and may be used to calculate the predicted lifetime of the polymer in service. The potential of NIDP data analysis to quickly deliver these kinetic parameters is identified and the confidence ranges in the predicted lifetimes derived from NIDP data analyses appear to be similar to those derived from isothermal analyses of DP data. Experimental NIDP data support the notion that the prolonged degradation of cellulose does not produce oligomers of DP much less than ca. 200.

Study of the mechanism of thermal degradation of cellulosic paper insulation in electrical transformer oil

Cellulosic winding paper (made by the Kraft process) was heated dynamically in naphthenic oil in sealed glass ampoules to 200°C. Samples of paper were analysed at selected temperatures for degree of polymerization while the oil medium was analysed for furanic compounds, dissolved gases and water content. An abrupt decrease occurs in the degree of polymerization of the winding paper at ∼100°C. This temperature coincided with the onset of formation of carbon oxides, methane, ethane, water and furanic compounds. It was shown that the paper, and not the oil, is the predominant source of CO, CO 2 and water in both inert and oxidizing atmosphere. A number of furanic degradation products of the paper were detected in the oil. 2-Furfuraldehyde was the furanic compound that was detected in the highest concentration in oil with progressive paper ageing. The results support the model that, at least below 200°C, the above degradation products directly originate from the decomposition of unstable chain ends formed by chain scission.

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.

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.

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.

REVIEW OF VOLATILE ORGANIC COMPOUNDS DERIVED FROM POLYETHYLENE

The formation of volatile organic compounds (VOCs) in polyethylene (PE) is a topic of concern to industries involved in the packaging of items such as foodstuffs and pharmaceuticals that are sensitive to organoleptic contamination. This article reviews the available literature on VOCs that originate from PE during its manufacture, processing, storage, and service life. The package–product interactions that may occur between PE and packaged foodstuffs are also considered together with the wide range of methods for the analysis of VOCs. The following analytical methods are discussed: (i) sensory evaluation, (ii) chromatographic techniques and their associated sampling techniques, including the “hot-jar” method and dynamic headspace sampling, (iii) gas chromatography–olfactory sensing, and (iv) artificial olfaction or “electronic nose” technology.

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.

Effect of chromium on the oxidative pyrolysis of gas-phase high-density polyethylene as determined by dynamic thermogravimetry

Abstract The oxidative degradation of HDPE samples containing various chromium compounds was studied by chemiluminescence and thermogravimetry. The importance of the valence state and solubility of chromium in relation to its pro-oxidant behaviour is reported. Chemiluminescence experiments confirm that the effectiveness of chromium compounds in promoting thermal oxidation is dependent on their solubility in the polymer. In order to study the effect of chromium catalyst residues on the stability of HDPE, a chromocene/silica polymerization catalyst was deactivated by exposing it to air and was then added to the polymer. Although the deactivated catalyst is quite insoluble in the polymer, it exhibits strong pro-oxidant behaviour at 125°C due to the high effective surface area of its silica substrate. Thermogravimetry shows that the onset of thermal oxidation of the virgin HDPE occurs at about 280°C whereas rapid oxidative pyrolysis commences at 360°C. The deactivated catalyst increases the temperature of onset of pyrolysis by up to 70°C. The increase in the degradative resistance of HDPE containing the deactivated catalyst is attributed to the ability of chromium ions to complex macroalkylperoxyl radicals, thereby terminating the oxidative chain process. This termination occurs most efficiently under conditions where the polymer hydroperoxide concentration is very low. Oven ageing of HDPE in air leads to oxygenated groups which are eliminated in the early stages of thermogravimetric analysis to produce conjugated sequences in the polymer backbone. The presence of conjugation in the polymer suppresses the rate of volatilization during the analysis and thus retards the rate of oxidative pyrolysis.