Synthesis of Ultradispersed Powders from Products of Polytetrafluoroethylene Pyrolisis †

Powdered materials occupy several specific segments of international production and markets, while in some cases they form the whole sector, in particular, powder metallurgy. In the case of polymers, powders are used to produce bulk half-finished and finished goods whose quality is to a great extent determined not only by chemical composition but also by the size and shape of powder particles. In view of this, the tendency to produce specific types of powders, initially of minimum particle size, is quite understandable. One of the directions of using powdered polymer materials is concerned with their application as components during the development of composite materials. In this case, their properties will depend significantly on the powder material used. All the above can also be applied to fluoropolymers. Modern industrial technologies provide polytetrafluoroethylene (PTFE) in a powder form which is then sold as a commodity or used to produce granules of bulk Fluoroplast half-finished goods for subsequent processing into finished ones. Most of the respective industrial technologies apply the reaction of polymerization of the gaseous monomer of tetrafluoroethylene (TFE) in aqueous medium under specific technological conditions with adding initiators . The powders thus obtained have particle sizes of 50-500 micrometers. Although when further processed in a jet-type mill, it is possible to attain particle sizes of 10-50 micrometers , the practical needs sometimes require even finer particles. The technology of synthesis in a gas phase has been widely used in the production of inorganic powders, in particular, metallic ones . For PTFE, this technology was not applied for a number of reasons. First, it was thought by some that the polymer heating results in chemical decomposition with the emission of a gaseous monomer C2F4 (more than 90%) that does not have the tendency for simple polymerization 1, . Second, since polymers can be characterized as “weak” objects tending to significantly change their properties under slight external impact, it appears complicated to find an optimal technological regime for the synthesis. Third, polymers are also characterized by complex mechanisms of thermal decomposition of initial polymer and formation of aerosols and particles in a gas phase. As a result, the processes of product formation from the gas phase and their structure are dif ficult for understanding and theoretical interpretation. It was shown by a number of authors’ studies that the process of PTFE production from pyrolysis Abstract


Introduction
Powdered materials occupy several specific segments of international production and markets, while in some cases they form the whole sector, in particular, powder metallurgy.In the case of polymers, powders are used to produce bulk half-finished and finished goods whose quality is to a great extent determined not only by chemical composition but also by the size and shape of powder particles.In view of this, the tendency to produce specific types of powders, initially of minimum par ticle size, is quite understandable.One of the directions of using powdered polymer materials is concerned with their application as components during the development of composite materials.In this case, their properties will depend significantly on the powder material used.All the above can also be applied to fluoropolymers.Modern industrial technologies provide polytetrafluoroethylene (PTFE) in a powder form which is then sold as a commodity or used to produce granules of bulk Fluoroplast half-finished goods for subsequent processing into finished ones.Most of the re-spective industrial technologies apply the reaction of polymerization of the gaseous monomer of tetrafluoroethylene (TFE) in aqueous medium under specific technological conditions with adding initiators 1) .The powders thus obtained have particle sizes of 50-500 micrometers.Although when further processed in a jet-type mill, it is possible to attain particle sizes of 10-50 micrometers 1) , the practical needs sometimes require even finer particles.The technology of synthesis in a gas phase has been widely used in the production of inorganic powders, in particular, metallic ones 2) .For PTFE, this technology was not applied for a number of reasons.First, it was thought by some that the polymer heating results in chemical decomposition with the emission of a gaseous monomer C2F4 (more than 90%) that does not have the tendency for simple polymerization 1,3) .Second, since polymers can be characterized as "weak" objects tending to significantly change their properties under slight external impact, it appears complicated to find an optimal technologi-products is principally possible and, moreover, is economically and technologically sound to be applied on an industrial scale [4][5][6][7][8] . Te achievements made include: patents on the method of powder production, the product itself, respective equipment for its production [4][5][6] , and FORUM product trademark 7) .A pilot-plant-scale production of ultradispersed powder has been established at the facilities of an academic institute.One of the main advantages of the method consists in the possibility of using PTFE waste as raw materials.Since there is no efficient technology of secondary processing of PTFE products at present, besides an economic problem, an ecological problem arises-the necessity of processing waste accumulated at industrial facilities.The present work discusses the processes of producing PTFE powder from the gas phase of the products of pyrolysis of bulk polymers.

The Nature of Formation of Nanoaerosols in Gas eous Products of PTFE Pyrolysis
As was mentioned earlier, it had been an established opinion that upon heating, PTFE decomposes with a predominant emission of the monomer tetrafluoroethylene (TFE) that does not tend to polymerize in a gas phase without initiators or radiation ef fect.However, under specific conditions a mist forms in the gaseous products of PTFE pyrolysis while a white powder emerges on the reactor walls.
The mist observation reflects the light dispersion on the formed particles that is possible only in the case when the particles have sizes comparable to a quarter of the visible light range wavelength.Aerosols form as a result of the interaction of molecular radicals-the products of PTFE pyrolysis and monomer molecules.The conditions of the emerging radicals are related to the pyrolysis temperature conditions while the possibility of their interaction in a gaseous medium is determined by pressure and the reactor design.
Different aspects of PTFE thermal decomposition were studied in a number of works, for example, in 3,9,10) .The polymer is stable up to the temperature 300℃, however, at higher values it softens due to melting.At higher temperatures and prolonged exposure one can observe a marked destruction, in particular at a temperature of around 800℃, decomposition results in 97% output of monomer-С2F4 9) .Mass-spectrometry studies 8) have shown that in the temperature range 357-410℃, the main products in the Fluoroplast gas phase include C2F4, C3F6 and C3F5 with the ratio of evaporated components during the whole experiment 1:0.04:0.02,respectively.The pyrolysis product composition and the components ratio depend on the temperature conditions, but it is evident that multiple components in the gas phase are characteristic for any temperature conditions.The product's diversity in the gas phase is expressed also in IR-spectra presented in Fig. 1.
The study of aerosol formation in PTFE pyrolysis products was conducted in 11) .Measurements of the aerosol disperse composition were performed with the nanoaerosol diffusion spectrometer 12) .The dependence of the average particle size on technological conditions was established-these conditions include the time and temperature of initial product pyrolysis and the difference between the pyrolysis and nucle- ation temperatures (Fig. 2).Increase of the pyrolysis time results not only in the average aerosol particle size growth, but also in the radial distribution dispersion that is clearly seen from the figure.Since the the particles sizes of the observed aerosols are tens of nanometers, they can be called nanoaerosols.
The dependence of the aerosol particle sizes on the initial PTFE pyrolysis time has been stated in 11) .The 15-fold change of the particle median diameter was observed when varying the decomposition temperature in the range 400-580℃ while the nucleation temperature was constant (60℃).
The overall nucleation rate in all channels available in PTFE thermal decomposition products has the tendency to grow with increasing decomposition temperature at constant nucleation temperature.This appears to be natural, since with increasing decomposition temperature, one can observe a growth of the component's partial contents at invariable nucleation temperature which results in an increase of the nuclei formation intensity.
One can assume the following picture of nanoaerosol formation in the gas phase of PTFE pyrolysis products: The fluorocarbon medium of pyrolysis products contains a set of molecular radicals and molecules, and a variety of them stipulates the possibility of multi-channel nucleation process.Nucleation formations emerge due to density fluctuations of gaseous radicals, and this is accompanied by a polymerization process.Under these temperatures, destr uction of the monomer (C2F4) molecules is possible which results in the formation of polymerization-active elements, and this in turn leads to the emergence of an additional nucleation channel.Since the monomer share is high, this channel can be efficient and can improve the formation of nanoaerosols.
In the fluorocarbon molecules medium nanoaerosols would increase their particle sizes due to condensation of gas-medium molecular formations on them.The condensation process is just as possible on nanoaerosol surfaces as on reactor walls.
It is evident from the presented results that variation of the technical parameters (temperature, pyrolysis time, initial product particle shape and size, gaseous fluorocarbon product pressure, etc.) can contribute to controlling the nanoaerosol particle sizes and, therefore, the ultradispersed PTFE particle structure.

Morphological Structure and Self-organization of Ultradispersed Powders
The particle shapes of the obtained powder were studied by the electronic microscopy method (Fig. 3).
The particle surface metallization distorted the particle true shape to a spherical one.Besides particles that we should call monoparticles, one can observe aggregates built from monoparticles and larger agglomerates formed from aggregates and molecules.
The respective terminology was taken from 2) .
The presence of the above-mentioned formations was confirmed by particle size measurements (Fig. 3) with the Sympatec HELOS-H1084, whose measurement principle is based on the analysis of powder particles subjected to laser irradiation scattering 15) .A dr y powder was introduced into the measurement chamber by the air flow at a controlled rate that enabled one to study the effect of air flow on the powder particles' aggregation.One could obser ve several peaks of the size distribution function (Fig. 3) corresponding to monoparticles, aggregate and agglomerate formations.Monoparticles have diameters more detail.As seen from the figure, the particles do not have ideal spherical shapes, and that is very clearly seen for the left-hand particle-it consists of interlocked blocks of sizes from tens to hundreds of nanometers.The phase AFM-images represent areas of different color/shade (dark and light) that corresponds to the presence of fluoropolymers of different structure.As was shown in our studies, monoparticles consisted of high-and low-molecular fractions.
The monopar ticle-forming blocks are of a highmolecular nature while the binding and coating mass is of a low-molecular one.Besides the block-type monoparticles, one can observe particles of a solid structure (Fig. 8, left-hand picture).The presence of monoparticles of two types-block and solid-was also confirmed by the method of transmission electronic microscopy 6) .One should mention that solid particles have the coating (possibly film-like) from a low-molecular polymer, their thickness is around 10 nm.
One can suggest the following scheme of nanoparticle formation: Molecular radicals serve as starting material to form dimers and larger oligomer formations, in other words, the macromolecule formation process is underway.
Macromolecules then form nanoparticles observed in a gaseous medium that transform into monoparticles in two ways: first, nanoparticles grow to the sizes of monoparticles, and second, nanoparticles coalesce to form block-type monoparticles.Coalescence can be accompanied by the formation of molecular (covalent) and supramolecular bonds which provides the block-type monoparticles' stability.The method of monoparticle formation depends on the respective thermodynamic conditions.
One can develop a multi-level hierarchical scheme of the powders' self-organization.The first level includes molecular radicals of angstrom size.The second level describes nanoblocks (nanoaerosols) with sizes of several dozens of nanometers.The third level is related to block-type solid monoparticles (100-700 nm).The fourth level includes monoparticle aggregates with weak bonds and, as a result, low mechanical stability (500-5000 nm).The fifth level is made up of agglomerates of monoparticles and aggregates (10000-30000 nm) with even weaker bonds between elements.

Peculiarities of Structure and Properties of Ultradispersed PTFE Powders
The X-ray diffraction studies conducted on PTFE samples of Fluoroplast-4 TM and FORUM TM 16) have shown the difference in the polymers' cr ystalline fraction structures.At room temperature, an ultradispersed sample has the crystal phase disordered along the axis of the hexagonal atomic packing of fluorocarbon chain molecules.One should mention that in industrial PTFE samples, this is observed only above +30℃.The high-temperature phase hardening takes place.Disordering is the result of CF2 groups rotating around the macromolecule axis.Spectroscopic studies per formed using the IRand 19 F NMR methods 17) have also shown the difference in chemical composition of the ultradispersed macromolecule sample .Additional lines are expressed in the spectra which can be explained by the presence of side trifluoromethyl (CF3) and end olefin (CF=CF2) groups, along with CF2-fragments, in fluororcarbon chains.The groups are characteristic for macromolecules of the low-molecular fraction of an ultradispersed fluoropolymer.
The difference in the ultradispersed PTFE structure results in the corresponding difference of properties as compared to fluoropolymer industrial samples, in particular, thermal samples.Derivatograph studies have revealed that the temperature of weight loss for an industrial sample of Fluoroplast-4 starts above 475℃ , while the whole temperature range is within one hundred centigrade 10,18) .The DTA curve shows an endothermic effect around 315℃that is attributed to sample melting.The exothermic peaks (525 and 575℃) emerge as a result of oxidation processes 10,18) .The thermal behavior of the FORUM TM -powder is markedly dif ferent: the weight loss temperature starts in the range 60-70℃; the polymer decomposition range is 60-550℃; one can also distinguish the areas of slow (60-290℃ ) and fast (290-550 ℃ ) thermal decomposition.Such a behavior is explained by the presence of phases of different thermal stability and different molecular weight.
Mass spectrometry data on the analysis of PTFE gaseous products strongly depend on the pyrolysis conditions.Pyrolysis of a sample of F-4 at temperatures above 400℃ produces just a few main components (C2F4, C3F6 and C3F5), while for FORUM TM -, more than 45 gaseous components were found at a pyrolysis temperature of 144℃ 18) .The main components on concentration include C3F5 (peak intensity 100), CF3 (97.4), while the concentration of the C2F4 monomer is not high and corresponds to the intensity of 30.One can also observe small concentrations of relatively large formations (for example, C18F35) with mass number 881.
Another difference in the FORUM® temperature behavior, as compared to industrial PTFE samples, is the absence of phase transitions at temperatures 19℃ и 30℃, while at the same time, the heat capacity temperature dependence shows a wide peak at a temperature of 21℃ 16) .Phase transitions are concerned with restructuring of the PTFE cr ystalline component, while the FORUM TM has a cr ystalline structure at room temperature corresponding to the PTFE high-temperature phase.The low-temperature anomaly is most probably related to restructuring in the amorphous phase of the ultradispersed product.

Production of PTFE Low-temperature Fractions
A wide temperature range of the thermal destruction and sublimation of phases with different molecular weights of the product FORUM® makes it technologically possible to separate fractions by a repeated thermal treatment.Indeed, by heating the product at the low-molecular fraction destruction temperature, one can obtain the residue which would contain the high-molecular fraction exclusively.On the other hand, pyrolysis of the FORUM® product is accompanied with the emergence of large amounts of different fluorocarbon fragments capable of forming different structural and morphological forms of polymers.
We have analysed the FORUM® product at temperatures 70, 140 and 300℃, and it was stated that the samples of fluoropolymer groups differed significantly in morphological structure 19) .The first-group samples comprise films of micron-size area and nanosize thickness (Fig. 10).In the second-group samples, one can observe multilayer tubes of lengths from 10 up to 300 micrometers and diameters from 2 to 20 μ obtained through flat fragments rolling and other formations.As regards the third-group samples, they are represented by calibrated balls of around 1 μ in size (Fig. 5).
The investigations of FORUM® pyrolysis products by the mass-spectrometric method showed the availability of a wide range of molecular and radical components, for example, CF3, C2F4, C2F6, C3F5 C4F9 and others 16) .The ratio of these components depends on the pyrolysis temperature of the initial powder.It is possible that each component forms particles of a powder with various morphological structures.The difference in the particle's morphology must therefore be related to the fact that gaseous products were obtained at different temperatures of FORUM® pyrolysis.
The difference is also revealed by the fact that while the X-ray dif fractograms of the powder obtained at high temperatures is identical to that of industrially produced PTFE, quite a different picture characterizes the low-temperature fraction (Fig. 6) 20) : cr ystal peaks are absent at high values of Bragg angles, two clearly expressed diffuse halos and a set of sharp reflections in the small angles area are observed.Such a picture can be attributed to the presence of layered formations.
A significant difference is also characteristic for the sample's thermal parameters (Fig. 7).The weight loss of the low-temperature fraction starts at 50℃ and finishes at around 150℃, it proceeds in a single phase, and its rate is different at the initial and final stages, which is reflected by asymmetry of the minimum on the DTG-curve.It is possibly related to increasing decomposition intensity after melting.The DTA-curve shows an anomaly with extremum at 83℃ that is attributed to the polymer melting.The weight loss of the high-temperature fraction occurs in the temperature range 120-300℃, where one can separate two stages with a boundary region in the area above 200℃.The same area is characterized by an anomaly of the DTA-curve that could be connected to the polymer melting.
The dif ference in temperature behavior can be related with that of the molecular weights of the macromolecules: low-temperature-fraction molecules correspond to lower weight values.The NMR-and IR-spectroscopy studies have confirmed the latter Fig. 5. Micrographs of FORUM TM powder pyrolysis products at different temperatures (from left to right: 70, 150 and 300℃) 19) .Marks correspond to 30, 10, and 3 micrometers, respectively.
fact-the low-temperature-fraction macromolecules contain end olefin and center trifluoromethyl groups 21) .A marked intensity of respective signals reflects their significant quantity and small sizes of molecular chains.In high-temperature fractions, these signals are absent.
The advantage of the low-molecular fraction is its solubility in supercritical CO2 that allows using the solution technology to apply thin fluoropolymer coatings of thickness 2-4 nm 22) .Such coatings enable one to make the surface hydrophobic without disrupting its micro-and nano-profile, thus fulfilling the superhydrophobicity conditions and making the surface self-cleaning.We managed to develop surfaces with the water drop wetting angle of 160°2 3) .Use of the solution technology in supercritical CO2 also enabled us to encapsulate hydrocarbon paraffins into the fluoropolymer shell with formation of colloidosomes of a size 300 micrometers and the coating thickness up to 10 micrometers [23][24][25][26][27][28][29][30][31][32][33][34][35] .

Production and Application of Ultradispersed powders
The technological set-up of the installation is presented in Fig. 8.The technological process of producing highly-dispersed PTFE FORUM® proceeds as follows: The bulk PTFE chips (1) are loaded into bunker (2) and fed by a worm feeder (3) to a pre-heating zone (5) and further into a reactor (6).Due to the water cooler (4), the PTFE melt hardens as a moving plug, thus preventing the intake of air into the reactor and the escape of PTFE destruction products into the room.The temperature and melt level are controlled by two gages (8, 9).The centrifugal fan (7) creates gas-dynamic conditions when a cooled monomer is fed to the PTFE melt surface, prevents further polymer decomposition into low-molecular fragments, facilitates its fast condensation into microparticles and carries the powder thus formed to cyclones (10) which separate the solid product from the gas phase.The monomer is then returned to the reactor.The monomer is continuously cooled by water coolers (12, 13).The excess of monomer and other gaseous pyrolysis products is removed through one of the working holes (4, 15, 16,  17, 18, 19) for further disposal 25) .
Fast destruction product cooling and conducting the process at temperatures above 520°enable one to completely exclude the formation of highly toxic perfluoroisobuthylene and to reduce down to minimum values the formation of toxic hexafluoropropylene.The installation design (raw materials fed from below through a liquid gate of PTFE melt) excludes the input of air into reactor and, therefore, eliminates the danger of toxic carbon fluoroxide formation.The safety interlocking systems automatically shut down the installation at critical operation conditions.The suggested method is not energy-consuming and enables one to use any PTFE waste, including composites, with a high degree of productivity that substantially reduces the product cost.Small particle sizes (0.1-1 micrometers), low molecular weight and the presence of active sites on the surface ensure high PTFE adhesion to metals, glass and other solid materials, thus enabling one to apply thin polymer layers.
The fields of practical application of the highly dispersed PTFE are extensive: protective coatings for metallic parts destined for use in aggressive and sterile media, anti-scale and anti-friction coatings, neutral fillers for cosmetics and medical preparations, dr y lubricants and fire-prevention components, additives for oils to reduce the vibroactivity of mechanisms, etc.
In order to extend the fields of application of the sub-micron PTFE FORUM® powder, the bulk PTFE thermal destruction was performed in a reduction atmosphere.In this case, low molecular weight and the presence of hydrogen in the PTFE polymer chain intensify the PTFE molecules' lyophilic behavior and reactivity, which enables one to use the obtained powder as an initial cathode material for lithium chemical batteries (CB).
The application of cathode materials on the basis of modified PTFE in lithium chemical batteries can result in a 35% increase of CB energy capacity as compared to graphite monofluoride and a 4-fold decrease of its cost, since the technology of producing graphite monofluoride is based on using gaseous fluorine and is associated with strict requirements relating to the production safety.
In order to increase the adhesive ability of the PTFE particles to metal and to stabilize PTFE suspensions in the case of the FORUM® powder application in oil mixtures for anti-wear additives to oils and lubricants, the PTFE thermal destruction was performed in the presence of oxygen 6) .

Conclusion
The possibility of obtaining nano-and microsize fluoropolymer particles from the gas phase of PTFE pyrolysis products has been stated.The process of nanoaerosol formation and its dependence on the temperature and pyrolysis time was studied and it was established that it is possible to control the fluoropolymer nanoaerosol sizes by changing the technological conditions.The powder monopar ticles' structural features were revealed: two structural forms-solid and blocktype-are generated from nanoaerosols.A multistage hierarchical scheme of organization of the powder obtained by using the gas-phase method was suggested.The presence of two types of polymers corresponding to low-molecular and high-molecular fractions was stated.Suggestions on the formation mechanisms of fluoropolymer particles in gas-phase products from PTFE pyrolysis were presented.
The possibility of separating low-and high-molecular fractions of fluoropolymers by secondary thermal processing of the FORUM®was shown, and the structure and properties of fractions obtained at different pyrolysis temperatures were investigated.
The possibility of industrial-scale production of ultradispersed powder was demonstrated, and the fields of its practical application were analysed.The powder production not only resolves commercial tasks but also provides solutions for the ecological processing of PTFE waste into commercial products.

Fig. 2 .Fig. 3 .Fig. 4 .
Fig. 2. The radial distribution of nanoaerosol particles as a function of the technological conditions and pyrolysis time.Distribution with the average diameter value 21 corresponds to the thermolysis time of 70 minutes; distribution with the value of 44 nm to the time of 365 minutes.The thermolysis temperature was 496℃, the nucleation temperature 60℃ 11) .