http://ago.helion.pl ISSN 1733-4381, Vol. 6 (2007), p-71-78
Waxes – products of thermal degradation of waste plastics –
obtaining, capabilities, and application
Urbaniak W.1,2' Wasiak W.2, Fall J.3
1 Faculty of Technology and Chemical Engineering, Uniwersytet Technologiczno- Przyrodniczy, Seminaryjna 3, 85-326 Bydgoszcz urbaniakw@interia.pl
2 Faculty of Chemistry, A. Mickiewicz Uniwersity, Grunwaldzka 6, 60-780 Poznań wasiakw@amu.edu.pl
3 Instytut Technologii Nafty, Łukasiewicza 1, 31-429 Kraków, fallj@poczta.onet.pl
Streszczenie
Woski – produkty termicznej degradacji odpadów tworzyw sztucznych – otrzymywanie, właściwości oraz zastosowania
Termiczna degradacja poliolefinowych tworzyw sztucznych, takich jak różnego typu polietyleny czy polipropylen, prowadzi do otrzymania produktów gazowych, ciekłych, materiałów mających charakter wosków oraz skoksowanej pozostałości. Zazwyczaj proces prowadzi się w taki sposób aby uzyskać jak najwięcej produktów ciekłych, które wykorzystywane są jako komponenty paliw. Stosunkowo niewielkim zainteresowaniem cieszy się obecnie produkcja wosków, praktyczne zawsze powstających w pierwszym etapie procesu termicznej degradacji. Biorąc jednak pod uwagę tendencje wystepujące na rynku surowców chemicznych, należy oczekiwać wzrostu zainteresowania produktami tego typu. Jak wykazano w poniższym artykule, woski mogą być efektywnie wytwarzane w wyniku rozkładu termicznego odpadów tworzyw sztucznych zawierających różnego rodzaju poliolefiny, a otrzymane produkty praktycznie bardzo nieznacznie różnią się od wosków parafinowych czy syntetycznych wosków polietylenowych.
Abstract
Thermal degradation of thermoplastic polymers, such as polyethylene, polypropylene and polystyrene, leads to obtaining of several gaseous and liquid products, waxes and coke. Usually, the process is performed in a way to obtain as much liquid products as possible, which in turn are used as fuel components. Relatively little attention is currently paid to production of waxes, which are practically always produced at the first stage of thermal degradation process. However, taking into account tendencies in the chemical raw materials
market, increased interest in such products can be anticipated. As it was proved in this paper, waxes can be effectively produced as a result of thermal decomposition of waste plastics containing various polyolefins, and practically there is little difference between the obtained products and paraffin waxes or synthetic polyethylene waxes.
1. Introduction
One of the basic recycle methods of waste plastics is related to raw-material recycling. In case of waste polyolefin plastics, in particular polyethylene (HDPE and LDPE) and polypropylene, such recycling aims at processing waste plastics to the liquid components for the production of motor fuels (petrol and motor oil) and fuel oils [1-4]. Relatively little attention is currently paid to production of waxes, which are practically always produced at the first stage of thermal degradation process. The waxes obtained as a result of thermal decomposition of polyolefins, may be used directly as fuel in candles and lights, as a highly caloric additive and modifier for anti-moisture impregnation of fuel produced from waste materials (such as wood and biomass).
Usually, however, waxes developed as a result of thermal decomposition of polyolefins are further developed in the following processes into different gaseous and liquid products. Such situation is probably caused by potential sale problems. However, taking into consideration the observed increasing demand for waxes and their derivates (such as emulsions), it seems that the production of waxes from waste plastics will increase, all the more market value of such waxes is greater in comparison to liquid fuel components [5,6].
2. Wax types
Waxes cover a wide range of products, both of natural- and artificial origin. These are solid substances characterized by melting point above 40°C (usually, from 40°C to 130°C). At such temperature they melt without decomposition. Most of waxes show lattice composition. Waxes are characterized by quite good chemical resistance, limited solubility, and low viscosity at temperatures below the melting point. The weight of wax molecule ranges from few hundred to few thousands g/mol. The most popular hydrocarbon waxes of natural origin are the following: ceresin, microwaxes, Montana waxes, and Carnauba waxes, to mention also the most frequently used product – paraffin [7, 8].
Paraffin is a mixture of high molecular hydrocarbons, from C18 to C36 (boiling point from
350°C to 560°C), with a little addition of cyclic hydrocarbons [8]. Raw paraffin may be further developed, as a result of refining process, to macro and microcrystalline paraffin of molecular mass from 300 to 500. As a basic raw material for the paraffin synthesis, oil fractions of some types of crude oil, subjected to the distillation under lowered pressure, are used. Apart from machine oils, waxy crude is obtained as a result of distillation. Nowadays, due to decreased supply of paraffin waxes, caused by deepened processing of crude oils in the refineries, synthetic waxes gained importance, in particular synthetic hard paraffin and polyethylene waxes.
Synthetic hard paraffin is a wax obtained from carbon oxygen and hydrogen, as a result of Fischer-Tropsch synthesis. Such paraffin is composed of n-paraffin hydrocarbons of molecule mass ranging from 700 to 1000. This paraffin is a non-polar substance, with melting point by 25°C÷45°C higher than melting point of typical paraffin obtained from crude oil [7].
Polyethylene waxes are hydrocarbon compounds, obtained in controlled thermal decomposition process of polyolefins in the presence of neutral gas. Such waxes are a mixture of higher paraffin hydrocarbons and input substances, which were not decomposed during the thermal process, usually having lowered molecule mass than input substances. Polyethylene waxes may also be obtained as a result of controlled polymerization of ethylene to the products of molecule mass ranging form 1700 to 2500. Such waxes are characterized by more homogeneous composition, and thus they melt in quite limited temperature range [7].
3. Wax market in Poland
USA is the world’s largest manufacturer of crude oil waxes, from which, in turn, paraffin and microwaxes are developed. Annual production level is close to 1150 thousand of tones. Next places have been taken by: China (900 thousand of tones), France (200 thousand of tones), and Germany (190 thousand of tones). Total European production of waxes (both natural and artificial) is estimated at 750 thousand of tones, among these 15-20% is related to polyethylene waxes and the products of Fischer-Tropsch process [5]. In Poland, by 2003, the production of waxes reached 115 thousand of tones, and national market was estimated at 40 thousand of tones of paraffin, worth approximately 75 millions of Polish zloty. Nowadays, the Polish national market is estimated at 150 thousand of tones of paraffin-related products [6].
Sales of waxes and paraffin substances in Poland still shows seasonal properties, due to the fact that paraffin is mainly used for manufacturing of candles. However, continuous development and modernization of Polish economy will unquestionably results in a change in the structural usage of paraffin, waxes, and related products, towards high-quality products. Nowadays, in Poland, approximately 45% of paraffin, waxes and related substances are used for production of candles and lights. Further 25% is used in the chemical, rubber, and cosmetics industry. The remaining amount is related to production of matches, wood industry (chipboards), paper industry (wax paper, paraffin wrappings), and smelting industry (casting waxes, parting waxes, greasing of metal-sheet, etc.). Limited amount (5%) is related to different industries and technological processed not mentioned above [5, 6].
Polish paraffin market shows good prospects for development as the demand for paraffin-related products continuously increases. However, that’s a “shallow” market, in contrast to other oil-related substances, e.g., those used as fuels. There are only four refineries in southern Poland involved in production and modification of paraffin. On a small scale, many SMEs are being involved, mainly in the candle industry, in production of waxes from waste plastics.
4. Wax development by thermal decomposition of polyolefines
High molecular, string-like polyolefine plastics, during heating in anaerobic conditions, are being broken down to limited-molecule fragments, characterized by wax-like properties. As a basic raw material for this process, any type of polyethylene, polypropylene, and polybutene may be applied. However, polyethylene is the most frequently used substance. This substance, stable during heating up to 290°C in anaerobic conditions, is subject to thermal decomposition above this temperature, most effectively in 400°C. Under such conditions, practically only wax-related products of molecule mass similar to the waxes obtained as a result of high-pressure polymerization are developed. Gas and low-boiling-point products are barely present [7].
The following conventional technologies, successfully used for years in the refining industry, may be applied in production of waxes from polyolefines: thermal cracking (pyrolisis), catalytic cracking (FCC), and hydro-cracking. Thermal cracking is the most frequently used technology in case of waste plastics materials. There are, however, two disadvantages of this method. First, high temperatures must be applied, and second, several interim products must be further developed and refined. On the other hand, pyrolisis of waste plastics is technologically a not very sophisticated process.
Controlled thermal decomposition (350°C÷400°C) of different types of polyethylene in the presence of a neutral gas is one of the basic methods for obtaining polyethylene waxes. The products obtained in this process are characterized by complex composition. The liquid low molecular products they contain, as well as undecomposed polyethylene, broaden the temperature range of melting point of these kinds of waxes [7].
Polyethylene waxes, obtained in thermal decomposition, contain n-paraffin hydrocarbons and irregular iso-paraffin. In some products limited presence of oxygen, in the form of hydroxy groups, ester groups, and oxygen bonds may be observed. Final products, with molecular mass at a level of 1000, show different capabilities according to the process parameters and the type of de-polymerized polyethylene [9].
Detailed research on thermal decomposition of other polyolefines showed that the obtained products are not substantially different, according to the mass-molecule layout, from the polyethylene-related products [9]. This fact shows that the development of a mixture of polyolefine waste plastics, from which it is hard to distinguish homogeneous fractions of particular types of plastics, may lead to obtaining of a good-quality, homogeneous waxes of reproducible composition and contents.
5. Laboratory research of development of waxes from waste plastics obtained from selective collections of municipal wastes
To determine a possibility of production of waxes from the polyolefine waste plastics, some research of the thermal decomposition of such plastics materials towards obtaining mainly wax-related fractions was made. The research also aimed at determination of basic
requirements for the continuous technological process of production of wax from waste plastics.
During the research, traditional extruder, capable of warming the contents up to 400°C, was applied. The contents were processed during 3÷4 minutes, and the obtained product was, after cooling and grinding down, subjected to this process for several times. Raw material composed of a mixture of waste plastics, pulled out from a stream of municipal waste. The plastics were cleaned in a water bath, and further the PET, PCV and polystyrene related plastics were removed. As a result, polyethylene (LDPE and HDPE) and, however in a limited amount, polypropylene made up for the main fraction of the plastics mixture. The products obtained as a result of thermal decomposition were extracted with toluene to isolate wax-related fraction, which in turn was subjected to simulated fractional distillation (SIMDIS). The method is described in details in [10]. The obtained results were compared with those obtained for waxes originating from crude oil.
In Table 1 some results of extraction of waxes (by the use of toluene in Soxhlet apparatus) from the polymers subjected to thermal decomposition were shown. The results show that, apart from temperature, the time of reaction is the most important factor. Time equal to 15 minutes is too small to obtain full conversion of polymer to the low molecular fraction. By extrapolation of these results, a conclusion may be drawn that, under conditions mentioned above, the reaction time should be at least two times longer. This conclusion is confirmed in the literature [7, 9], where authors reported an optimum reaction time equal to 30 minutes at the temperature of 400°C÷420°C. The reaction time at a lowered temperature must be substantially extended.
Table 1. Amount of extracted wax-related fractions, depending on number of cycles in the extruder
No Number of cycles * Amount of extracted wax-related fraction [%]
1 1x 15,4 2 2x 19,5 3 3x 28,2 4 4x 30,1 5 X** 63,2 *
(a)
(b)
(c)
0 10 20 30 40 50 0 1.0e5 2.0e5 3.0e5 S ig . 1 in C :\H P C H E M \.. .\0 0 1 F 0 1 0 1 .D Time (min.) 0 10 20 30 40 50 0 1.0e5 2.0e5 3.0e5 S ig . 1 in C :\H P C H E M \.. .\0 0 1 F 0 1 0 1 .D Time (min.) **6 hours at the temperature 300°C÷350°C
Fig. 1 shows sample results of SIMDIS analysis of the products of thermal decomposition of municipal waste polyolefines (sample 5 in Tab. 1) - (a), distillation products of pyrolisis of waste polyolefines (b) and commercial paraffin obtained from crude oil (c).
Fig. 1. Sample results of SIMDIS analysis
The analysis of SIMDIS results shows that for all studied cases products of the same type were present, mainly hydrocarbons containing from 18 to 30 carbon atoms in a molecule (Fig. 1). In the case of (a) they are products obtained as a result of thermal decomposition of municipal waste polyolefines (sample 5 in Tab. 1). The profile of SIMDIS spectrum indicates a very low content of low molecular products of thermal degradation and suggests that some part of polymer was not subject to degradation. The profile of products obtained from municipal waste polyolefines as a result of complete pyrolisis and distillation at a temperature of 350- 450oC was shown in fig 1b. A considerable share of fractions boiling at lower temperatures and symmetric distribution of spectra indicates more advanced fragmentation of plastics.
Moreover, both the layout and the type of main developed products are similar to the products appearing in the paraffin obtained from crude oil (Fig. 1c).
Partial degradation of polyolefines observed in case of sample 5 (Tab. 1) was also confirmed by the results of DSC (differential scanning calorimetry) – Fig. 2 – (curve 2), where peaks of not reacted polyolefines (for comparison, curves 1 and 4 show DSC chart for LDPE – 1 and HDPE – 2) and products of melting temperatures slightly higher than
melting temperatures of commercial paraffin obtained from crude oil (curve 3) can be clearly observed.
Fig. 2 Sample results of DSC analysis
6. Summary and final conclusions
Thermal decomposition of waste polyolefin plastics towards production of waxes is one of the most promising techniques. The obtained products show good quality and similar properties to waxes obtained from crude oil, and the process itself is quite efficient. The process was aimed at thermal processing of a mixture of plastic materials at a temperature of 400°C during 30 minutes. According to our research, the process may be continuous provided that a few reactors of extruder type are applied in a cascade mode. As raw material practically any kind of polyolefine type municipal waste plastics may be used, in particular polyethylene (HDPE and LDPE) and polypropylene.
References
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[6] www.tworzywa.com.pl/aktualnosci/aktualnosci.asp?id=1714
[7] Ullmans Encyklopaedie der technischen Chemie, vol. 24, pp. 1-49, Verlag Chemie, Weinheim, 1983
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[10] W.Wasiak, W.Urbaniak, M.Chojnacka, J.Fall „Analiza składu frakcji paliwowych z destylacji olejów przepracowanych i odpadowych tworzyw sztucznych”, in J.W.Wandrasz, K.Pikoń (eds.) „Paliwa z odpadów”, vol. IV, pp. 71-77, HELION Press, Gliwice 2003 r.
