Microscopic studies on the polymers decomposition in a closed volume at elevated temperatures in the presence of bulk NiCr alloy
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In the present work, self-disintegration of the bulk metallic Ni–Cr alloy in the presence of different polymer compounds under the conditions of closed volume and elevated temperatures is studied. It was found that in the cases of polymers containing in addition to carbon only hydrogen atoms (polyethylene) or only halogen atoms (polytetrafluoroethylene), self-disintegration does not proceed noticeably even at temperatures as high as 800 °C. The predominating form of the solid product is non-catalytic carbon shaped as microspheres or their sintered agglomerates. On the contrary, for decomposition of the polymer containing hydrogen and halogen atoms (polyvinylchloride), self-dispersion of the bulk alloy is significantly facilitated. This process is accompanied with formation of catalytically derived forms of structured carbon within all the studied temperature range (500–800 °C). The most common form of the carbon deposits is nanofibers. At the same time, some unusual carbon structures were observed, among which the solid-phase carbon–nickel microcrystals of a regular cubic shape with pyramidal cavities on each facet are of special interest. The studied process can be considered as an alternative approach for utilization and recycling of the plastic wastes.
KeywordsPolymers decomposition Closed volume NiCr alloy RAPET Nanostructured carbon materials
Nowadays carbon nanofibers (CNF) attract a great attention due to their enhanced thermal, electric and mechanical properties [1, 2, 3]. Most of commercial technologies to obtain the CNFs imply chemical vapor deposition in various alternatives including the catalytic one. As it is well known, the catalytic properties in the catalytic chemical vapor deposition (CCVD) are performed by metals of iron group (Fe, Co, Ni) and their alloys supported on the surface of the carrier with developed surface area. On the other hand, recently we have demonstrated the possibility to use bulk metallic nickel or its alloys in a bulk state as a catalyst for the synthesis of CNFs from halogenated hydrocarbons [4, 5, 6]. During the interaction of such aggressive organic medium with bulk metal items the latter undergo self-disintegration with formation of nickel particles with uniform size of 200–250 nm. The particles formed exhibit excellent activity in CNFs growth. In order to obtain possible solid-phase intermediates, we have performed experiments on detail studies of the mentioned interaction at elevated temperatures in closed volume (autoclave conditions), when the gaseous products withdrawal is practically absent [7, 8]. A similar approach is well known in the literature to be successfully applied for the synthesis of various carbon and other inorganic materials [9, 10, 11]. The described process was named as reactions under autogenic pressure at elevated temperature (RAPET). As it was found in our recent studies, the interaction of metallic nickel with halogen-substituted hydrocarbons (for example, hexafluorobenzene) in closed volume leads to appearance of solid phase structures, hypothetically—microcrystals of nickel halogenides. These compounds are metastable and undergo rapid destruction at high temperatures. This process is accompanied by formation of carbon nanofibers doped with a small amount of halogen, which is very important for their further application due to enhanced surface properties (for example, better hydrophily). Thereby, the use of halogen-containing compounds as a source for CNFs production can be also considered as a perspective approach for processing of different organochlorine wastes into valuable carbonaceous product.
Besides the processing of organochlorine wastes, utilization of spent polymeric items (solid domestic wastes, plastics, etc.) is of great importance, since the world production of such items grows year by year. As a rule, industrial synthetic polymers are quite stable chemical compounds. Most of them (for instance, polyethylene) are resistant towards action of sun light and air oxygen, and do not undergo self-destruction for a long period of time. Among polymeric compounds, the halogen-containing ones (like polyvinylchloride, fluoroplastic) are the most dangerous. Their combustion in air leads to formation of variety of gaseous toxic compounds. One of the perspective approaches of polymers utilization known in the literature is their processing into various carbon materials (for example, carbon nanotubes) [12, 13, 14, 15]. As a rule, the catalyst is represented by transition metal or by salt of such metal. In some cases, the catalyst can be added into reactor in a gas phase [for example, Fe(CO)5] .
In the present research, an approach for processing of polymeric wastes (polyethylene, polyvinylchloride, Teflon) in the presence of bulk NiCr alloy self-disintegrated under the action of aggressive reaction medium with formation of uniform nickel particles catalyzing the growth of CNFs is suggested. The possibility to perform such a process in closed volume at elevated temperature was studied. The attractive advantages of this approach are that it goes via one stage and looks very simple in terms of technical realization.
Commercial Ni–Cr alloy (nichrome; 80 wt% Ni, 20 wt% Cr) in the shape of wire of 0.1 mm in diameter was used as a bulk metal item. The polymer samples to be decomposed were polyethylene of low pressure (household plastic bag), polyvinylchloride (insulating braid), and polytetrafluorethylene (Teflon tape).
Experiments on polymer decomposition in a close volume were performed using quartz ampoules (d = 4–5 mm, V ~ 0.2 ml) as a reactor [7, 8]. A piece of nichrome wire of about 0.2–0.3 mg was placed in an ampoule together with 2–3 mg of polymer (polyethylene, Teflon or polyvinylchloride). The ampoule with reaction mixture was sealed and brought to thermal treatment at certain temperature for 2 h. The accuracy of temperature measurements was ± 2 °C.
After the reaction procedure, the ampoule was carefully opened for microscopic investigation. The obtained carbon material was studied by scanning electron microscopy (SEM) on a JSM-6460 (Jeol, Japan) electron microscope with a resolution of 4 nm in the range of magnifications from 5 × to 300,000 ×. The local energy-dispersion X-ray (EDX) microanalysis was carried out using EDAX spectrometer with energy resolution 127 eV.
HRTEM images were obtained using a JEM-2010 electron microscope (Jeol, Japan) with a lattice-fringe resolution of 0.14 nm at accelerating voltage of 200 kV. The samples for HRTEM were prepared on a perforated carbon film mounted on a copper grid.
3 Results and discussion
As it was reported earlier, self-disintegration of bulk metallic nickel and its alloys can be significantly facilitated when the reaction mixture contains both hydrogen and halogen sources [7, 8]. In the case of organic compounds containing hydrogen or halogen only (for example, hexamethylbenzene or hexafluorobenzene) used as a carbon source, this process is rather complicated. Polyethylene, the most popular domestic polymer material and, therefore, the most common plastic waste, contains only carbon and hydrogen atoms. Thereby, taking into account the results of previous studies, it was expected that its catalytic decomposition accompanied with self-disintegration of the bulk NiCr alloy in closed volume at elevated temperature will be hindered. In order to examine this expectation, the ampules with reagents (piece of NiCr wire and polyethylene) were consecutively calcined at temperature from 400 to 800 °C with a step of 100 °C. For the resulted contents we have tried to apply ferromagnetic resonance (FMR) method as described in [5, 6, 7, 8]. This method was shown to be an efficient tool for investigation of nickel-containing samples. The most convenient case of its application is the study of nichrome, since this alloy does not have FMR signal in the initial state but appearance of particles of metallic nickel even in a small amount is well detectable. In the present study, it was found that FMR signal indicating the beginning of NiCr self-disintegration did not appear even for samples treated at 700 °C. Note that for the samples heated at higher temperature, the applicability of FMR method is restricted due to strong microwave absorption by carbon products resulting from thermal decomposition of the organic precursor. Moreover, at reaction temperatures of 500 °C and above the appearance of an intensive singlet g = 2.003 typical for various carbon materials becomes noticeable [7, 8].
Thereby, in the case of completely halogenated polymer (when all hydrogen atoms are substituted with halogen), self-disintegration of bulk nickel alloy is also complicated. Carbon nanofibers formed via catalytic route are present in a minor amount, while non-catalytic dense carbon deposits are found to be the predominating product.
In the present work, catalytic and non-catalytic decomposition of widely used plastic substrates (polyethylene, polytetrafluoroethylene, and polyvinylchloride) was studied under the RAPET conditions (closed volume and elevated temperatures). Self-disintegration of the bulk Ni-based metallic alloy (Ni–Cr wire) was shown to take place with significant intensity only when polyvinylchloride was used as a substrate. This plastic contains simultaneously hydrogen and halogen atoms required to initiate metal dusting of nichrome. Experiments with polyethylene and polytetrafluoroethylene do not lead to noticeable exposure of the self-disintegration process, even at temperatures as high as 800 °C. Dense carbon deposits was the predominant form of the carbon product resulting from decomposition of these hydrogen-only and halogen-only containing substrates similar to ones described in . At the same time, it should be noted that small amounts of catalytically derived nanofibrous carbon were observed while decomposing the polytetrafluoroethylene at 800 °C.
Due to these reactions, the surface of the bulk metal is loosened, thus facilitating self-dispersion of the alloy with the formation of nickel particles catalyzing the growth of carbon nanofibers.
Besides the mentioned above, formation of unusual solid-phase microcrystals of regular cubic shape with pyramidal cavities on each crystal facet was observed during the catalytic decomposition of polyvinylchloride. While these cubes consist of carbon (70–85 at.%) and nickel (15–30 at.%), nichrome self-dispersion is believed to take place under the studied conditions (RAPET at 800 °C). The carbon–nickel microcubes obtained are of great attractiveness, and thus will be studied in detail in the nearest future.
In order to conclude, the results of the present research are quite perspective for processing of plastic wastes. The proposed approach is attractive due to its simplicity, single-staging, absence of the catalyst preparation procedures, and, finally, formation of valuable carbon product with unique structure.
This study was funded by the Russian Foundation for Basic Research (Grant Number 16-33-60034).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 5.Bauman YI, Kenzhin RM, Volodin AM, Mishakov IV, Vedyagin AA (2012) Formation of growth centers of carbon nanofibres during self-dispersion of Ni-containing alloys: studies by means of ferromagnetic resonance. Chem Sustain Dev 20:143–155Google Scholar