CHEMILUMINESCENCE COMBINED WITH THERMAL ANALYSIS FOR STUDYING POLYMER OXIDATION
N.C. BILLINGHAMa, P.K. FEARONa, M.N. GRIGGa, S.W. BIGGERb
aDepartment of Chemistry, University of Sussex, Brighton, BN1 9QJ, UK
bVictoria University of Technology, P.O. Box 14428, Melbourne 8001, Australia
Because of the effectiveness of modern stabilisers, accelerated ageing tests must be used to evaluate their performance. Oven ageing testing is the most reliable but is time consuming. Differential Scanning Calorimetry (DSC) has been widely used for measuring oxidation induction times (OIT) but is usually (mis)used at temperatures above the melting point of the polymer.
Many polymers emit light (chemiluminescence, CL) during oxidation. CL offers an alternative method to monitor oxidation, its advantage being very high sensitivity. CL imaging of oxidative degradation has recently been made possible by the development of low noise, high sensitivity cameras, which allow multiple sampling, reducing experimental times and cost. Imaging also allows study of inhomogeneity.
In collaboration with Mettler Toledo, we have combined CL measurement with DSC in a single instrument. The combination has obvious advantages where oxidation occurs simultaneously with other thermal events, e.g. melting, since the CL is sensitive only to oxidation. CL in inert atmospheres can easily detect the presence of peroxides at concentrations well below those detectable by DSC.
Measurements on unstabilised samples of polypropylene show good correlation of CL with heat flow data, with kinetics consistent with the Russell mechanism for CL. Studies of the formation of carbonyl groups show that the relation of CL to C=O content is also consistent with the Russell mechanism.
For stabilised samples OITs measured by both CL and DSC are similar. It is easier to operate the CL at very high sensitivity and CL can easily detect OITs above 70 h. Increased oxygen pressures allow analysis by CL under conditions where DSC signals are noisy. There is good correlation between CL values measured by single-sample CL and imaging detection.
Multiple detection needs some care in order to avoid cross sample contamination but allows major economy of equipment time.
Some applications of the DSC/CL method will be discussed, including the assessment of stabiliser performance and the correlation of OIT and oven ageing.
RADIATION AGEING OF POLYMER MATERIALS
J.L. GARDETTE, A. RIVATON
Laboratoire de Photochimie Moléculaire et Macromoléculaire, UMR CNRS 6505, Université Blaise Pascal, 63177 Aubière cedex France
The modifications of structure and properties of polymeric materials resulting of the ageing initiated by ionising radiations have been a subject of interest as soon as nuclear programs began. Exposure of polymers to radiative environments causes indeed a deterioration of their properties. It has been approximately 50 years since researchers first began studying the effects of ionising radiations on polymers. The main radiation-induced changes consist in scissions and/or crosslinking of the macromolecular chains, formation of gaseous products such as hydrogen or low molecular weight hydrocarbons, accumulation of oxidation products, loss and/or formation of unsaturations.
Among the numerous studies devoted to the radiochemical ageing of polymers, many of them report on the mechanism of the -initiated degradation of polyolefins. This presentation concerns the behaviour of EPDM copolymers, which are composed of ethylene (PE), propylene (PP) and diene monomer (5-ethylidene 2-norbornene (ENB) in this study). These polymers find numerous applications and are often used as wire insulators or joints. If the mechanisms of photo- and thermo-oxidations of EPDM can be considered as well known, the radiation induced degradation of these polymers has only received little attention.
On the basis of a recent study involving the identification of the various products that are formed, a mechanism accounting for the radiochemical degradation of EPDM has been proposed, with a peculiar attention given to the role played by oxygen and by the ENB termonomer. Two main processes are involved in the radiooxidation of EPDM. The random -radiolysis of the polymer provides a constant source of macroalkyl radicals mainly formed on ethylene units. The secondary radicals so formed are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In particular, the decomposition of hydroperoxydes and the degradation of the ENB moieties, this latter being the most oxidisable site and the source of crosslinking, may result from hydrogen abstraction by radicalar species.4
OVERVIEW AND PARTICULARITIES IN DEGRADATION OF POLYOLEFINS DERIVED FROM FISCHER-TROPSCH SYNTHESIS
I. TINCUL
Sasol Technology, PO Box 1, Sasolburg, 1947, South Africa
Polyolefins are without doubt the largest volume family of commercially polymers and a vast literature is dedicated to this field in the last half of century. The most complex pool of monomers is available from the Fischer Tropsch process that is gaining momentum as a major market source. When compared with ethylene oligomerization processes the main advantage of the Fischer Tropsch process is the production of odd carbon number linear and branched olefins that showed new application possibilities in both the polyethylene and polypropylene families. We prepared a large number of new polyolefins with Fischer Tropsch derived alpha olefins that have particular microstructures and morphology according to the type of branch, length of branch and number of branches / 1000 carbons.
It is known that steric factors may play an important role in polyolefin degradation and that tertiary carbon is labile to oxidative attack. A decrease in crystallinity with the increase of comonomer content may favour auto-oxidation due to higher oxygen absorbtion. Very little literature on degradation of polyolefins prepared with Fisher Tropsch derived olefins is available. In this paper is presented and discussed some particularities on the degradation of polyolefins Fischer Tropsch derived.
The manifestation of oxidation weather by temperature or aging concerning polymer appearance as discoloration, loss of optical properties, loss of mechanical properties and surface cracks were observed. This behaviour was similar to known polyolefins. Oxidation induction time as measured by DSC varies with the number of branch and length of branch introduced in the polymer chain by a certain comonomer. The thermal stability was measured also by thermogravimetry and compared with the polymer structure and thermal behaviour. The degradation temperature increases with an increase of carbon number of the comonomer. Investigation for a series of ethylene co-polymers and ter-polymers with linear and branched alpha olefins showed an increase of pre-exponential factor as a function of carbon number in an Arrhenius type equation. A decomposition kinetic study showed that the activation and the rate constant is also related to the size and type of branch. Rheological investigation showed a similar pattern. It was concluded that while polyolefins obtained with Fischer Tropsch derived olefins showed a behaviour similar with known polyolefins, particularities in degradation does exist and are related to the number and type of branch.
ESSENTIAL CRITERIA FOR THE SUCCESSFUL RECOVERY OF PLASTICS
N. MAYNE
Association of Plastics Manufacturers in Europe, Avenue E. Van Nieuwenhuyse 4, Box 3, B – 1160 Brussels
E-mail address: neil.mayne apme.org
Of the many options for recovery of plastics at the end-of-life, mechanical recycling is in principle the preferred route. There is an increasing understanding of the mechanisms, which affect the degradation of polymers or physical property enhancement by increasing their compatibility with other polymers. While many laboratory scale experiments demonstrate the potential for high levels of recycling, in practice the levels achieved on an industrial scale with post-consumer plastics can be viewed as disappointingly low. In addition to economic considerations, this is because there are a limited number of applications allowing closed loop recycling and there is a mismatch between the supply of recyclate and demand from the market. New developments will increase the potential, but such recycling will nevertheless remain limited.
Feedstock recycling technologies offer versatility and many large scale processes have been developed and demonstrated. Logistics and supply commitments are key factors in determining viability. To date, fully commercial plants are only operational in Germany and Japan. A number of active developments are, however, currently underway in a number of European countries.
Energy recovery will become an increasingly important recovery option as it is also being stimulated by restrictions on the landfilling of waste. Even if the full potential of mechanical recycling were to be realized, the major part of total plastics waste would still be available for use as an alternative fuel. This means that with the expected growth of plastics, this represents an increasingly important source of energy, equivalent to millions of tonnes of imported oil each year.
The challenge for the successful recovery of plastics is to identify and use the optimum combination of options, underpinned by continuing innovation in process, product and recovery technologies in order to increase the resource efficiency of products throughout their whole life-cycle.
OPTIMISAZION OF PROCESSING CONDITIONS IN THE RECYCLING OF POLYCONDENSATION POLYMERS
F.P. LA MANTIA, R. SCAFFARO
Dipartimento di Ingegneria Chimica dei Processi e dei Materiali
Università di Palermo, Viale delle Scienze, 90128, Palermo, Italy
Mechanical recycling of polymers is a very complex and difficult process because repetitive melt processing can lead to severe degradation of these materials with subsequent deterioration of all the properties. For condensation polymers, the degradation rate is much faster in the presence of water that induces depolymerization reactions, having also a synergistic effect with thermomechanical degradation and exalting the loss of properties. Indeed, a careful drying prior to processing can drastically reduce the degradation phenomena but increasing the complexity of the process and its cost.
The aim of this work is to study the effect of the reprocessing of some polycondensation polymers with and in absence of water. Moreover, the possibility to process wet polymer in the presence of proper stabilisers to avoid the drying step has been studied too.
The materials chosen for the study have been polyamide 6, polycarbonate and polyethyleneterephtalate.
The results indicate that for the three different systems considered, the wet processing causes, as expected, a strong reduction of the mechanical properties and of the molecular weight, as evidenced by rheological and thermal measurements.
If a degassing stage is added to the processing apparatus, a certain improvement of the properties is observed. This can be explained in terms of humidity removing with consequent reduction of the hydrolytic chain scission.
When nitrogen flow is added to the degassing, both humidity and oxygen are practically eliminated from the system, and a further improvement of the properties is observed. This can be explained with the further reduction of the humidity but it also suggests that degradation mechanism involves reactions.
The presence small amounts (500 ppm) of a particular antioxidant has a positive effect in all the cases. In particular, the adding of antioxidant can substitute the drying stage as it is able to protect the macromolecules from hydrolysis. This behaviour can be explained considering that this antioxidant is more prone to hydrolysis than the polymer, thus removing the water from the melt and decreasing the degradation rate. Such consideration has been proved using different antioxidants with different tendency to hydrolysis: using antioxidants with theoretical reduced hydrolysis tendency caused a parallel reduction of the protection with consequent loss of properties.
The positive effect of the presence of the antioxidant has been also verified for several reprocessing steps (up to five). The results indicate that the presence of the antioxidant still have positive effects also after several recycling operation.
It can be concluded that the mechanical recycling of condensation polymers can be effectuated with practically any loss of properties if an adequate removal of humidity from the system is performed. A valid alternative to the complex and expensive pre-drying operation can be the adding of antioxidants that undergo hydrolysis in vice of the polymers.
FEEDSTOCK RECYCLING OF PLASTICS IN JAPAN
A.OKUWAKI
Tohoku University, Graduate School of Environmental Studies,07 Aoba, Sendai, 980-8579
Various commercial plants for feedstock recycling of waste plastics have started up in Japan. In 1999, only Niigata liquefaction plant was operated. Now they are liquefaction at Sapporo, and Mikasa, Blast Furnace application by NKK, Coke Oven application by Nippon Steel Corp., Gasification by EBARA-UBE, and Glycolysis of PET bottles by Teijin. Separated PET bottles are now used as raw material for polyester fibers, 30,000t/y in capacity from April, 2002, and the two commercial plants for the Bottle to Bottle recycling are under construction for the first time in the world.
Clearly this is related to the practical implementation of the Containers and Packaging Recycling Law carried out in 1997 for PET bottles and in 2000 for the other plastics. This law affords allocating the cost for feedstock recycling of waste plastics separated only in homes. It has created attractive markets and thus, they say that the separated waste plastics are even in short supply in Japan. Recovery of PET bottles increases successfully since 1997, but that of plastics increases dramatically since 2000 when the law has carried out.
On the other hand, their cost for recycling decreases gradually. At present, their recycling fees are quite expensive compared to the cost for treatment of industrial waste plastics. However, it is expected that the cost will continue to drop. This trend suggests that the active competitions among various techniques proceed including the improvement of collection system. This situation creates big chances for the world recycling technologies in the Japanese market and refine necessarily both techniques and logistic systems.
The Home Appliances Recycling Law came into full force in April 2001 is to be revised within a few years. In such process, they say, not only the recycling of their plastics cases, but also that of printed circuit boards are introduced to elevate the recycling degree.
In addition, the Law for Recycling of End-of-Life Vehicles has passed through the Parliament, July 2002. They say, an average cost per car will be 20,000 Yen, namely 151Euro. Clearly this law should proceed the recycling of automotive shredder residue.
These environmental policy of Japanese Government are sure to promote more the recycling of waste plastics in Japan.
THERMAL DEGRADATION OF POLYMERS: BASIC PRINCIPLES AND UNSOLVED PROBLEMS
H. BOCKHORN
Institute of Chemical Engineering, University of Karlsruhe, Engesserstraße 20, D-76131 Karlsruhe, Germany
Thermal degradation of polymers is the initial step in smoldering of plastics, fires where plastics are involved in, or incineration of plastics containing wastes. Also, different recycling processes of plastics are based on thermal degradation. From this, thermal degradation of polymers has attracted ample attention in the scientific discussion.
According to the different molecular structure of polymers their thermal degradation follows different mechanisms. These different classes of mechanisms, e.g. thermal elimination mechanisms, radicalic chain mechanisms, are discussed and illustrated with appropriate examples. The experimental techniques for investigating mechanism of thermal decomposition of plastics will be discussed and their limitations with respect to deriving detailed mechanisms and corresponding rate coefficients will be illustrated.
The focus of the discussion lies on the development of detailed chemical mechanisms for the thermal degradation of different classes of polymers, the evaluation of rate data from different (and combined) experimental techniques and the possibilities of prediction of thermal degradation rates and evolution rates of gaseous products. In this context unsolved problems and some necessary developments will be identified.
NANOCOMPOSITES: STABILITY ISSUES & CHARACTERISATION
M. EDGE & N.S. ALLEN
Centre for Materials Science Research, Department of Chemistry and Materials, Faculty of Science and Engineering, The Manchester Metropolitan University, Chester Street Manchester Ml 5GD, U.K., (E-mail: M.Edge mmu.ac.uk)
Exceptional improvements in the properties of polymers have been reported upon the incorporation of nanoparticles. A key attraction of the technology is that these benefits can be achieved at lower filler loadings compared with conventional technologies. Nanoparticle dispersions also offer the option to utilise secondary functionality as delivery vehicles for benefit molecules: for example the controlled release of polymer additives. However, exploitation of these effects in commercial products; has so far been severely restricted for a number of reasons: e.g. in production-scale melt processing there are problems of reproducibly generating and dispersing nanoparticles. Surface treatments, essential for compatibility/dispersion of the nanoparticles, not only lead to morphological stability problems (nanoparticles tend to reform agglomerates), but to polymer degradation. By judiciously engineering the polymer-host interactions, nanocomposites may be produced with a broad range of properties. However our understanding of any interfacial interactions is limited by currently available methods of characterisation. The majority of techniques are unable to yield information on polymer behaviour in constrained environments, simply giving information on morphology or macroscopic mechanical properties. One exception to this is the development of microwave methods sensitive to static and dynamic structural variations at the molecular level. High frequency microwave dielectric loss, by network analysis, is capable of distinguishing relaxations of polymer chains (1- and (3-). On a local scale nanocomposite polymer chains exhibit higher flexibility along their backbone, along with a marked supression of cooperative dynamics typically associated with the glass transition: on a global scale relaxation of polymer chains either tethered to or in close proximity to the host surface are dramatically altered. Microwave measurements also have value where the nanoparticles are conductive or photoconductive. Measurements of conductivity induced by stationary excitation are able to monitor the fate of excess charge-carriers and their role in the interfacial transfer processes leading to degradation. This presentation aims to compare the physico-chemical properties of conventional composite with nanocomposite materials (encompassing silica, carbon-based and titania nanoparticles) in the context of interfacial interactions and associated polymer stability.