Chapter 6
Polyethylene Degradation and Degradation Products
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Ann-Christine Albertsson and Sigbritt Karlsson Department of Polymer Technology, The Royal Institute of Technology, S-100 44, Stockholm, Sweden
The increasing use of different polymers in materials such as packaging etc. means increasing problems of disposing of garbage. Plastic waste is a considerable part of the garbage and demands are put on using degradable materials as well as increasing the possibility of recycling. Predicting the changes in long-term properties of new polymers is therefore even more important than before. Parameters which must be controlled are the rate of degradation and the evolution of low molecular weight degradation products. The degradation rate is important in case of using the plastic waste as landfills. It is somewhat contradictory to use degradable waste as landfill, it will be quite difficult using land for example to build on if the plastic waste is degradable. Various techniques are used for studying the degradation of polymers. Some of the more important ones are size-exclusion chromatography (SEC) for measuring of the molecular weight changes during degradation, infra-red spectroscopy (IR) for following changes in for example carbonyl-index and liquid scintillation countings (LS) following evolution of carbon-14-dioxide ( CO ), a measurement of degradation. LS is a very convenient and sensitive method which has been developed and described in a series of papers (1, 2, 3). Even small degradation rates can be monitored, below 0.1%, which is about 100 times better than traditional methods. 14
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Chromatography is a relevant technique when studying the evolution of low molecular weight degradation products. Using gas chromatograpgy (GC) and liquid chromatography (LC) it was possible to monitor the biodegradation of casein (biopolymer) used as additive in some cement products (4, 5, 6). O097-6156/90/0433-O060$06.00/0 © 1990 American Chemical Society
In Agricultural and Synthetic Polymers; Glass, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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6. ALBERTSSON & KARLSSON
Polyethylene Degradation & Byproducts
Arbin et al. (7) have studied the contamination of intravenous solutions from polyvinylchloride (PVC) bags using LC-diode-array detection. Examples of products identified in the solutions were different phthalates used as plasticizers. This is one example of the enormous importance of controlling what type of degradation products evolved during degradation of polymers. Phthalates are known to be carcinogenic and the levels of phthalates in for example blood-bags were often too high. Nowadays the manufacturers have managed to lower this level. It is equally important to monitor the degradation products of polymers used as packaging. Different additives are incorporated in otherwise inert polymers in order to make them degradable (in abiotic as well as biotic environments). Biodegradable additives Griffin (8, 9, 10) introduced the idea of increasing the biodegradability by adding a biodegradable additive to the polymer material. By mixing biodegradable biopolymers such as starch with PE and studying the degradation of LDPE film in compost he found that the autooxidation was enhanced. When a biodegradable additive is employed, microorganisms can easily utilize the additive. The porosity of the material is thereby increased and a mechanically weakened film is obtained. The surface area will be increased, and this film will be more susceptible than the original film to all degradation factors including biodégradation. Mixing 10% C32H56 with high density polyethylene (HDPE) powder gave a film containing an additive that will be degraded by fungi (11). Over a period of two years then degradation of this film was followed and compared with pure PE film containing C . Using the C U 2 LS method it was concluded that the liberation of CC>2 was slightly increased by the additive during the first years, but somewhat retarded thereafter. In combination with other degradation factors, the biodegradable additive presumably has a positive effect on the degradation factors. 1 4
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Molecular weight and degradation Paraffins may not only be additive in PE but can also be regarded as the low molecular counter part of synthetic polyolefins. Several groups have performed studies on the biodégradation of alkanes. Jen-Hao and Schwartz (12) were probably the first to claim that the number of bacteria that PE was able to support was dependent on the molecular weight of the polymer. Initial photooxidation and several other factors will diminish the molecular weight of polymers thereby releasing low molecular weight portions of polymeric chains which eventually can be biodegraded. The total environmental effect on polymers is therefore put together by
In Agricultural and Synthetic Polymers; Glass, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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several separate degradation factors working synergistically towards deterioration of polymeric materials outdoors (2).
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Degradation products nf PF, In connection with an ongoing project aiming at evaluating and developing new methods for studying the low molecular weight degradation of polymers a preliminary abiotic degradation ôf LDPE was performed. LDPE with a melt index (MI2) of 45 g/10 minutes and a thickness of 150 μπι was kept in hot water (95°C) in three different pH (0.5, 6 and 12.5). LDPE samples were withdrawn at regular intervals and studied in fourier transform infra-red spectroscopy (FTIR) and in differential scanning calorimetry (DSC) (a complete description of treatments, instrumental, etc. is described in a paper under preparation) (13). The water fraction was also withdrawn and extracted with diethylether. The ether extracts were analysed in GC showing increasing amount of degradation products as the degradation times were increased. Series of low molecular weight compounds were evolved at the same time as the crystallinity increased from about 32 % to 43 % during 40 weeks of degradation (13). Initially the amorphous part of the PE chain is degraded thereby giving an increase in the crystallinity. It is also possible that water diffuses into the polymer leading to a more ordered structure and a higher crystallinity. During the degradation the keto-carbonyl groups increased in amount as obtained by attenuated total reflectance (ATR)-FTIR. In the basic environment a growing peak corresponding to carboxylate anion is observed (13). After 15 days of degradation only low amounts of degradation products have evolved, the amount being lower than the detection limit of the GC system. After 17 weeks, however, 2-butanol, propionic acid, 1pentanol, butyric acid, valeric acid and caproic acid were detected in pH 6 water fraction. After another 20 weeks several alkanes could also be detected: η-octane, n-nonane, n-decane, n-dodecane, n-tridecane and n-tetradecane (13). Carlsson and Wiles have in an early work (14) discussed the ketonic oxidation products of PP films. The volatile products were analysed in GC with a flame ionization detector (FID) and a thermal conductivity detector (TCD) giving the major oxidation products carbon monoxide and acetone. Other products detected were water, formaldehyde, formic acid, propane, acetic acid and iso-propylalcohol. The monomelic yield from thermally degraded PE is very low, less than one percent. The mechanism is a random scission type giving many volatile degradation products with a very complex pattern. This
In Agricultural and Synthetic Polymers; Glass, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
6. ALBERTSSON & KARLSSON
Polyethylene Degradation & Byproducts
makes analyses and identification of low molecular weight degradation products of PE very difficult and demands skilful and careful emthod developments. Important areas for the study of PE and its degradation products are in particular the packaging industry. Demands are put that all packaging materials should be degradable, i.e. its life-time should be controlled and in this context it is important to show degradation products.
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Degradation mechanisms Photooxidation increases the low molecular weight material by breaking bonds and increasing the surface area through embrittlement. The hydrophicility is also increased. The formation of carbonyl groups, usually expressed as carbonyl index is one way of monitoring the photooxidation effect. Thermolytic degradation of PE starts with initiation: scission of primary bonds randomly or at chain ends. The depropagation occurs mainly through intermolecular or intramolecular hydrogen abstraction and the termination proceeds by combination or disproportionation. The accelerated degradation briefly described in this work is a combination of different degradation mechanisms. A photooxidation is unavoidable and by keeping a moderately high temperature (95°C) a thermolytic degradation can be accounted for. The influence of different pH will primarily affect the low molecular weight compounds giving rise to several organic acids besides the hydrocarbons to be expected from the thermolytic degradation mechanism. Conclusions A brief description of evolved low molecular weight degradation products of LDPE has been given. The experiment was conducted in order to evaluate the usefulness of GC for detecting degradation products of polymers. Further studies will be performed in order to improve the possibility of studying degradation products of degradable polymers and also discuss how different biodegradable additives will affect the degradation product pattern.
References 1. 2. 3. 4.
A-C. Albertsson J. Appl. Polym. Sci., 22, 3419 (1978). A-C. Albertsson: Advances in Stabilization and Degradation of Polymers, Volume 1, A. Patsis (ed.), Technomic Publishing Co., Lancaster, Pennsylvania, 1989. A-C. Albertsson and S. Karlsson: J . Appl. Polym. Sci., 35, 1289 (1988). S. Karlsson, Z.G. Banhidi and A-C. Albertsson: J . Chrom., 442, 267 (1988).
In Agricultural and Synthetic Polymers; Glass, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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5. 6. 7.
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8. 9. 10. 11. 12. 13.
AGRICULTURAL AND SYNTHETIC POLYMERS
S. Karlsson, Z.G. Banhidi and A-C. Albertsson: Materials and Structures, 22, 163 (1989). S. Karlsson and A-C. Albertsson: Materials and Structures, (in press) (1990). A. Arbin, S. Jacobsson, K. Männinen, A. Hagman and J. Östelius, Int. J. Pharmaceut., 28, 211 (1986). G.J.L. Griffin, British Pat. Appln. No. 55 195/73 (1973). G.J.L. Griffin, J. Polym. Sci., Poly. Symp., 57, 281 (1976). G.J.L. Griffin, Pure Appl. Chem., 52, 399 (1980). A-C. Albertsson and B. Rånby: in J.M. Sharpley and K.M. Kaplan (eds.) Proc. of the 3rd Int. Biodegrad. Symp., Appl. Science, London, 1970, p. 743. L. Jen-Hao and A. Schwartz, Kunststoffe, 51, 317 (1961). S. Karlsson and A-C. Albertsson, Manuscript under preparation.
RECEIVED
February 16, 1990
In Agricultural and Synthetic Polymers; Glass, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
