Improvement in Carbonization Efficiency of Cellulosic Fibres Using

Article Improvement in Carbonization Eﬃciency of Cellulosic Fibres Using Silylated Acetylene and Alkoxysilanes

Maria Mironova *, Igor Makarov , Lyudmila Golova, Markel Vinogradov, Georgy Shandryuk and Ivan Levin A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Moscow, Russia; [email protected] (I.M.); [email protected] (L.G.); [email protected] (M.V.); [email protected] (G.S.); [email protected] (I.L.) * Correspondence: [email protected]; Tel.: +7-495-647-5927

Received: 6 September 2019; Accepted: 25 September 2019; Published: 28 September 2019

Abstract: Comparative studies of the structure and thermal behavior of cellulose and composite precursors with additives of silyl-substituted acetylene and alkoxysilanes were carried out. It is shown that the introduction of silicon-containing additives into the cellulose matrix influenced the thermal behavior of the composite fibers and the carbon yield after carbonization. Comparison of the activation energies of the thermal decomposition reaction renders it possible to determine the type of additive and its concentration, which reduces the energy necessary for pyrolysis. It is shown that the C/O ratio in the additive and the presence of the Si–C bond affected the activation energy and the temperature of the beginning and the end of the pyrolysis reaction.

Keywords: Cellulose ﬁber; Lyocell; N-methylmorpholine-N-oxide; silicon-containing additives; thermal properties

1. Introduction By means of the solid phase dissolution of cellulose in N-methylmorpholine-N-oxide (NMMO), a direct solvent for cellulose, it is possible to obtain highly concentrated solutions and to spin the Lyocell-type fibers [1]. Recently, these fibers have attracted a lot of attention, not only as an alternative to rayon textile material, but also as precursors of carbon fibers [2]. The main problems of thermal transformation of cellulose fibers into carbon fibers are the increase of the carbon yield and formation of the necessary structures that provide good mechanical properties [3]. The theoretically possible value of the carbon yield in the carbonization process of cellulose is 44.4% [4]. In order to get as close as possible to this value, it is recommended to use active substances (the catalysts) that direct the process of pyrolysis towards the formation of carbon structures and prevent the byproducts (e.g., levoglucosanes) caused by the removal of carbon in gas form [5,6]. Catalysts of pyrolysis and flame retardants can be applied by impregnation of the precursor fibers with solutions of active substances followed by their drying [7,8]. The most widely used pyrolysis catalysts for cellulose are ammonium chloride and ammonium sulfate. Recently, silicon-containing catalysts have been proposed as an alternative to these active agents. Their use allows carbon fibers with a mechanical strength of more than 1 Gpa to be obtained. In [9–14], predominant data were given on the use of silicon-containing compounds for obtaining carbon fiber, in particular, in [10], the application of Tetraethoxysilane (TEOS) makes it possible to obtain carbon fibers with a diameter of 5.5 µm, a tensile strength of 930 Mpa, and an elastic modulus of 38 GPa, with a carbon yield of ~14.5%. The introduction of the pyrolysis catalysts and flame retardants into the polymer matrix can be considered as an alternative method of their application, which achieves a better distribution

Fibers 2019, 7, 84; doi:10.3390/fib7100084 www.mdpi.com/journal/fibers Fibers 2019, 7, x FOR PEER REVIEW 2 of 11 fibers with a diameter of 5.5 µm, a tensile strength of 930 Mpa, and an elastic modulus of 38 GPa, Fiberswith 2019a carbon, 7, 84 yield of ~14.5%. 2 of 12 The introduction of the pyrolysis catalysts and flame retardants into the polymer matrix can be considered as an alternative method of their application, which achieves a better distribution of the ofactive the substance active substance in the volume in the volumeof the fiber of the and fiber redu andces the reduces number the of number stages of processing stages of processing (Figure 1) (Figure[15,16].1 )[15,16].

Figure 1.1. BasicBasic stages stages of cellulose composite precursors andand carboncarbon ﬁbersfibers production.production. TEOS, Tetraethoxysilane; VTEOS, Vinyltriethoxysilane; BTMSA, Bistrimethylsilyl acetylene; NMMO, N-methylmorpholine-N-oxide.

Silicon-containing compoundscompounds as as additives additives allow allow not not only only an increase an increase in the in carbon the carbon yield during yield heatduring treatment heat treatment of fibers, of but fibers, also but to obtain also to carbon-silicon obtain carbon-silicon carbide fiberscarbide [17 fibers], and [17], it is exactlyand it is this exactly class ofthis fibers class which of fibers is thewhich most is interestingthe most interesting for obtaining for obtaining heat-resistant heat-resistant materials [materials18]. [18]. Thermogravimetry Analysis (TGA) and Differential Differential Scanning Calorimetry (DSC) methods are widely used to study the propertiesproperties of carbon fiberfiber precursors and to represent the conditions for their production [19]. [19]. However, However, the the results results of of ex experimentsperiments cannot cannot always always be be interpreted interpreted definitely. definitely. To obtainobtain moremore accurateaccurate information, information, it it is is recommended recommended to to process process the the results results mathematically. mathematically. Thus, Thus, to determineto determine the ditheff erencesdifferences in thermal in thermal behavior, behavior process, process staging, staging, thermostability thermostability of synthetic of polymers,synthetic polysaccharides,polymers, polysaccharides, cellulose fibers, cellulose and sofibers, on, it isand reasonable so on, it to is evaluate reasonable the kineticto evaluate parameters the kinetic of the decompositionparameters of the using decomposition the Broido method using the [20 Broido–23]. method [20–23]. The Broido formula allows for estimation of the activation energy of the thermal decomposition of cellulose [[24]24] ln[ln(1/y)] = A Ea/(RT), (1) ln[ln(1/y)]=A-−Ea/(RT), (1) Ì where y = (Mt Me)/M0-Me,M0 is the mass of the initial sample, t is the sample mass at temperature where y = (Mt − Me)/M0-Me, M0 is the mass of the initial sample, Мt is the sample mass at temperature T in the TG experiment, Me is the final mass of the sample, Ea is the activation energy, T is the T in the TG experiment, Me is the final mass of the sample, Ea is the activation energy, T is the temperature, and R is the universal gas constant. temperature, and R is the universal gas constant. The purpose of this study was to compare the structure and thermal behavior of cellulose and The purpose of this study was to compare the structure and thermal behavior of cellulose and composite precursors containing silyl-substituted acetylene and alkoxysilanes additives, by the TGA composite precursors containing silyl-substituted acetylene and alkoxysilanes additives, by the TGA and DSC methods. and DSC methods. In this research, we report that the addition of silyl-substituted acetylene and alkoxysilanes into In this research, we report that the addition of silyl-substituted acetylene and alkoxysilanes into the cellulose matrix led to a decrease in the activation energy. Valuable information was obtained the cellulose matrix led to a decrease in the activation energy. Valuable information was obtained from the TGA; it is shown that 5% of the additive is sufficient to significantly increase the carbon yield from the TGA; it is shown that 5% of the additive is sufficient to significantly increase the carbon compared to cellulose during heat treatment of fibers to 1000 ◦C. yield compared to cellulose during heat treatment of fibers to 1000 °C. 2. Materials and Methods 2. Materials and Methods Concentrated cellulose dopes were prepared according to the method described in [25] from Concentrated cellulose dopes were prepared according to the method described in [25] from cellulose of the Baikal Cellulose and Paper Mill (Russia) (degree of polymerization was 600, H2O contentcellulose ~8%, of the and Baikalα-cellulose Cellulose content and notPaper less Mill than (Russia) 94%). As (degree a solvent, of N-methylmorpholine-N-oxidepolymerization was 600, H2O content ~8%, and α-cellulose content not less than 94%). As a solvent, N-methylmorpholine-N-oxide with a Tm = 120–160 ◦C (H2O < 10%) (Demochem, China) was used. The silicon-containing additives arewith shown a Tm = in 120 Table–1601 (Sigma-Aldrich,°C (H2O < 10%) (Demochem, USA). China) was used. The silicon-containing additives are shownFor inhibition in Table of 1 the(Sigma-Aldrich, of thermo-oxidative USA). degradation, 0.5% of propylgallate (Sigma-Aldrich, USA) was introduced to solution. FibersFibers 20192019,, ,77,, x,x 84 FORFOR PEERPEER REVIEWREVIEW 3 of 11 3 of 12

TableTable 1. 1.TheThe main main characteristics characteristics of organosilicon of organosilicon additives. additives.

Viscosity, η 3 Viscosity,3 Viscosity, , Ò Ò Density,Density, g/cm g3 /cm TheThe Compound Compound StructuralStructural Formula Тmm, °CC Тboil, °C, C η, mPa·s mPa s The Compound Structural Formula Тm,, °C°C◦ Тboilboil,, °C°C ◦ η,, mPa·smPa·s (T(T ==(T 2525 =°C)°C)25 ◦C) · (T = 25 °C) (T = 20 ◦C) (T(T == 2020 °C)°C)

TetraethoxysilaneTetraethoxysilane Tetraethoxysilane −82.5 170 170 0.933 0.933 0.75 0.75 (TEOS) (ID 24848042) −82.5 170 0.933 0.75 (TEOS)(TEOS) (ID(ID 24848042)24848042) −

VinyltriethoxysilaneVinyltriethoxysilane (VTEOS)(VTEOS)(VTEOS) (ID (ID(ID <<00 160 160 0.910 0.910 0.7 0.7 24850510) 24850510)

Bistrimethylsilyl BistrimethylsilylBistrimethylsilyl acetyleneacetylene (BTMSA) (BTMSA) 2222 136 136 0.752 0.752 - - (ID(ID(ID 24851275) 24851275)24851275) (ID 24851275)

For inhibition of the of thermo-oxidative degradation, 0.5% of propylgallate (Sigma-Aldrich, As the main objects under consideration, the fibers obtained from the filled cellulose solutions USA) was introduced to solution. in theAs NMMO the main with objects a weight under ratioconsideration, of 95/5 and the 90 fibe/10rs (cellulose obtained/ organosiliconefrom the filled cellulose additive) solutions were studied inin(Table thethe NMMONMMO2). withwith aa weightweight ratioratio ofof 95/595/5 andand 90/1090/10 (cellulose/organosilicone(cellulose/organosilicone additive)additive) werewere studiedstudied (Table(Table 2).2). Table 2. Designation of the composite fibers. Table 2. Designation of the composite fibers. Ratio of Components Sample 0Ratio 95of/ 5Components 95/10 Sample Cellulose/TEOS0 L Ò-5 95/5 Ò-10 95/10 Cellulose/VTEOS L VT-5 VT-10 Cellulose/TEOS L Т-5-5 Т-10-10 Cellulose/TEOS Cellulose/BTMSAL L B-5Т-5 B-10 Т-10 Cellulose/VTEOS L VT-5 VT-10 Cellulose/BTMSA L B-5 B-10 The cellulose and composite precursors were spun on a capillary viscometer equipped with the fiberThe winding cellulose system and composite Rheoscope precursors 1000 (CEAST, were Italy) spun(d onon= aa 1 capillarycapillary mm, l/d viscometerviscometer= 40) by a dry-jetequippedequipped wet withwith method thethe with fiberfibera spinning windingwinding speed systemsystem of ~100 RheoscopeRheoscope m/min. 10001000 (CEAST,(CEAST, Italy)Italy) (d(d == 11 mm,mm, l/dl/d == 40)40) byby aa dry-jetdry-jet wetwet methodmethod with aThe spinning thermal speed behavior of ~100 ofm/min. the fibers was examined on a TGA/DSC combined thermal analysis instrument,The thermal Mettler behavior Toledo of (Switzerland).the fibers was examin The measurementsed on a TGA/DSC were combined carried out thermal in the analysis temperature instrument,instrument, MettlerMettler ToledoToledo (Switz(Switzerland). The measurements were carried out in the temperature3 instrument,range of 30–1000 Mettler◦C Toledo at a heating (Switz rateerland). of 10 The◦C/ min.measurements The flowrate were of carried the argon out was in the 10 cmtemperature/min. Alumina 3 rangecrucibles of 30–1000 of 70 µ L°C were at a used. heating rate of 10 °C/min. The flow rate of the argon was 10 cm3/min./min. Alumina crucibles of 70 µL were used. AluminaThe crucibles morphology of 70 ofµL the were fiber’s used. surfaces and cross-sections was studied by the scanning electron The morphology of the fiber’s surfaces and cross-sections was studied by the scanning electron microscopeThe morphology JSM U-3 of (JEOL, the fiber’s Japan). surfaces and cross-sections was studied by the scanning electron microscope JSM U-3 (JEOL, Japan). microscopeThe structure JSM U-3 of(JEOL, the fibersJapan). was studied by the X-ray diffraction method on a 12-kW generator The structure of the fibers was studied by the X-ray diffraction method on a 12-kW generator RigakuThe (Japan)structure with of the a rotating fibers was copper studied anode by the mode X-ray at roomdiffraction temperature. method on The a 12-kW CuKα generatorradiation with Rigaku (Japan) with a rotating copper anode mode at room temperature. The CuKα radiationradiation withwith aa a wavelength λ = 0.1542 nm was used. Equatorial diffractograms of the fibers were obtained from wavelength λ == 0.15420.1542 nmnm waswas used.used. EquatorialEquatorial diffractogdiffractograms of the fibers were obtained from parallel bundles of fibers (~100 pieces, average diameter of monofilament 12 µm). parallel bundles of fibers (~100 pieces, average diameter of monofilament 12 µm). Interplanar distances were calculated using the Bragg formula: InterplanarInterplanar distancesdistances werewere calculcalculated using the Bragg formula: d d= =λ/(2sin(/(2sin(λ/(2θ sin)))) (θ)) (2)(2) (2) where λ isis thethe wavelengthwavelength ofof thethe X-rayX-ray radiation,radiation, andand θ isis thethe diffractiondiffraction angle.angle. where λ is the wavelength of the X-ray radiation, and θ is the diffraction angle.

3. Results and Discussion Cellulose and composite fibers were obtained by a dry-jet wet method. The SEM images of spun fibers with different contents of silicon-containing additives are illustrated in Figure2. Fibers 2019, 7, x FOR PEER REVIEW 4 of 11 Fibers 2019, 7, x FOR PEER REVIEW 4 of 11

3. Results and Discussion 3. Results and Discussion Cellulose and composite fibers were obtained by a dry-jet wet method. The SEM images of spun Fibers Cellulose2019, 7, 84 and composite fibers were obtained by a dry-jet wet method. The SEM images of 4spun of 12 fibers with different contents of silicon-containing additives are illustrated in Figure 2. fibers with different contents of silicon-containing additives are illustrated in Figure 2.

(a) (b) (a) (b)

(c) (d) (c) (d) Figure 2. Microphotographs of surface and cross-section of cellulose (a), composite precursors 95% Figure 2. Microphotographs of surface and cross-section of cellulose ( (a),a), composite precursors 95% cellulose + 5% BTMSA (b), 90% cellulose + 10% VTEOS (c), 95% cellulose + 5% BTMSA (d). cellulose + 5%5% BTMSA BTMSA ( (b),b), 90% cellulose + 10%10% VTEOS VTEOS ( (c),c), 95% 95% cellulose cellulose ++ 5%5% BTMSA BTMSA ( (d).d). As shown, the introduction of silicon-containing compounds into the cellulose matrix did not As shown,shown, thethe introductionintroduction of of silicon-containing silicon-containing compounds compounds into into the the cellulose cellulose matrix matrix did notdid leadnot lead to a significant change in the morphology of the fibers. The surface and cross section of both tolead a significantto a significant change change in the morphologyin the morphology of the fibers. of the Thefibers. surface The andsurface cross and section cross of section both cellulosic of both cellulosic and composite fibers did not contain obvious defects. The cross-section was circular. The andcellulosic composite and composite fibers did fibers not contain did not obvious contain defects. obvious Thedefects. cross-section The cross-section was circular. was circular. The average The average diameter of the obtained fibers was 15 to 17 microns. diameteraverage diameter of the obtained of the obtained fibers was fibers 15 to was 17 microns.15 to 17 microns. The structure of precursor fibers was one of the factors determining their properties, including The structure of precursor fibersfibers was one of thethe factorsfactors determining their properties, including mechanical and thermal behavior [26]. That is why before comparing the thermal properties of mechanical andand thermal thermal behavior behavior [26 [26].]. That That is why is why before before comparing comparing the thermal the thermal properties properties of cellulose of cellulose and composite fibers, their structure was studied with X-ray diffraction (Figure 3). andcellulose composite and composite fibers, their fibers, structure their structure was studied was with studied X-ray with diff ractionX-ray diffraction (Figure3). (Figure 3).

(a) (b) (a) (b) Figure 3. Cont. Fibers 2019, 7, x FOR PEER REVIEW 5 of 11

FibersFibers2019 2019, 7,, 84 7, x FOR PEER REVIEW 5 of5 of 12 11

(c)

Figure 3. Equatorial diffractograms of cellulose (1) (andc) composite fibers: (a) T-5 (2) and T-10 (3); (b) VT-5 (2) and VT-10 (3); (c) B-5 (2) and B-10 (3). FigureFigure 3. 3.Equatorial Equatorial di diffractogramsffractograms of of cellulose cellulose (1)(1) andand compositecomposite fibers:fibers: ( (aa)) T-5 T-5 (2) (2) and and T-10 T-10 (3); (3); (b) (b)VT-5 VT-5 (2) (2) and and VT-10 VT-10 (3); (3); ( (cc)) B-5 B-5 (2) (2) and and B-10 B-10 (3). (3). For all the samples studied, the main peaks are observed in regions 2θ~12.3° and 2θ~20.6° with interplanarForFor all all thedistances the samples samples of studied,3.61 studied, and thethe2.19 mainmain Å, respectively. peaks are are observed observed The positions in in regions regions of 2theseθ 2~12.3°θ~12.3 peaks and◦ and refer2θ~20.6° 2θ ~20.6to the with◦ withpolymorphicinterplanar interplanar formdistances distances cellulose of of3.61 3.61II and[27]. and 2.19The 2.19 Å, Å,intensities respectively. respectively. of the The peaks positionspositions decrease ofof thesethese with peaks peaks an referincrease refer to to the inthe polymorphicconcentrationpolymorphic form of form cellulosethe additive,cellulose II [27 ].whichII The [27]. intensities indicatesThe intensities of a the partial peaks of decreasedisorderingthe peaks with decreasein an the increase basal with in planes concentrationan increase of the in ofcelluloseconcentration the additive, structure. which of the indicates additive, a partialwhich disorderingindicates a in partial the basal disordering planes of thein the cellulose basal structure.planes of the celluloseUnlikeUnlike structure. cellulose fibers,fibers, in which the carbon yieldyield tends to bebe thethe minimumminimum valuevalue withwith linearlinear heatingheatingUnlike up toto cellulose 10001000 ◦°C,C, fibers, forfor almostalmost in which allall compositecomposite the carbon precursorspr yiecursorseld tends the to increaseincreasebe the minimum ofof thethe carboncarbon value yieldyield with waswaslinear observed,observed,heating whichup to 1000 can be°C, seenseen for fromalmostfrom thethe all thermogravimetricthermogravim composite pretricecursors curves the inincrease Figure 4 of4 obtained obtainedthe carbon atat various yieldvarious was temperaturetemperatureobserved, intervals.whichintervals. can be seen from the thermogravimetric curves in Figure 4 obtained at various temperature intervals.

FigureFigure 4.4.Thermogravimetry Thermogravimetry Analysis Analysis (TGA) (TGA) curves curves of cellulose of cellulose and compositeand composite fibers: fibers: complete complete curves andcurves parts and of parts the curvesof the incurves the temperature in the temperature ranges 30–260ranges ◦30–260C and °C 260–315 and 260–315◦C. Atmosphere: °C. Atmosphere: argon Figure3 4. Thermogravimetry Analysis (TGA) curves of cellulose and composite fibers: complete 10argon cm 10/min. cm3 Linear/min. Linear heating heating at a rate at ofa rate 10 ◦ Cof/ min.10 °C/min. curves and parts of the curves in the temperature ranges 30–260 °C and 260–315 °C. Atmosphere: argon 10 cm3/min. Linear heating at a rate of 10 °C/min. Fibers 2019, 7, x FOR PEER REVIEW 6 of 11 Fibers 2019, 7, 84 6 of 12 When the samples were heated up to 200 °C, the fibers lost 8%–9% of their mass, which was stipulated with the adsorbed water removal. The highest rate of weight loss for cellulose fibers was When the samples were heated up to 200 ◦C, the fibers lost 8–9% of their mass, which was stipulatedobserved withup to the 100 adsorbed °C. At further water heating, removal. VT-10 The precursors highest rate lose of weightwater the loss most for actively cellulose and fibers this process continues up to 160 °C. The process of removing water from T-10 fibers proceeds at the was observed up to 100 ◦C. At further heating, VT-10 precursors lose water the most actively and lowest rate and ends at around 200 °C. In the next stage of sample heating (in the temperature range this process continues up to 160 ◦C. The process of removing water from T-10 fibers proceeds at 200–340 °C), two competing reactions occured–dehydration and depolymerization [28]. These the lowest rate and ends at around 200 ◦C. In the next stage of sample heating (in the temperature reactions correspond to the extremes on the differential curves of the mass loss rate (DTG) (Figure 5). range 200–340 ◦C), two competing reactions occured–dehydration and depolymerization [28]. These reactions correspond to the extremes on the differential curves of the mass loss rate (DTG) (Figure5).

Figure 5. Mass loss rate (DTG) curves for cellulose and composite fibers. Figure 5. Mass loss rate (DTG) curves for cellulose and composite fibers. Although the course of the DTG curves is the same, the positions of the temperatures at the beginningAlthough and the the end course of the of decomposition the DTG curves reaction, is the same, as well the as theposition maximums of the of temperatures the mass loss at rate, the beginning and the end of the decomposition reaction, as well as the maximum of the mass loss rate, differ. The temperatures corresponding to the extremum points and DTGmax values for all the testing samplesdiffer. The are giventemperatures in Table corresponding3. to the extremum points and DTGmax values for all the testing samples are given in Table 3. Table 3. Parameters of the thermodestruction rate of cellulose and composite fibers: the temperatures ofTable minimums 3. Parameters and the DTG of values.the thermodestruction rate of cellulose and composite fibers: the temperatures of minimums and the DTG values. Sample Tmax, ◦C DTGmax, %/min Sample Tmax, °С DTGmax, %/min L 331 1.74 − T-5L 332331 −1.74 2.02 − T-10T-5 335332 −2.02 2.49 − VÒ-5 337 2.35 T-10 335 −2.49− VÒ-10 344 2.38 VТ-5 337 −2.35− B-5 328 2.03 − B-10VТ-10 332344 −2.38 2.14 B-5 328 −2.03− B-10 332 −2.14 The temperature range of pyrolysis, determined from the first derivative of the TGA curves, was used for estimation of the activation energy of thermal decomposition. According to [29], these values The temperature range of pyrolysis, determined from the first derivative of the TGA curves, allow us not only to estimate the energy necessary for the reaction, but also the possible nature of was used for estimation of the activation energy of thermal decomposition. According to [29], these the product obtained. For more accurate determination of the pyrolysis starting temperature, TGA values allow us not only to estimate the energy necessary for the reaction, but also the possible derivatives of a higher order could be used. The corresponding temperature dependences of the weight nature of the product obtained. For more accurate determination of the pyrolysis starting loss rate is presented in Figure6, and calculated activation energy, as well as intrinsic points of the temperature, TGA derivatives of a higher order could be used. The corresponding temperature decomposition process, is shown in Table4. dependences of the weight loss rate is presented in Figure 6, and calculated activation energy, as well as intrinsic points of the decomposition process, is shown in Table 4. Fibers 2019, 7, x FOR PEER REVIEW 7 of 11 Fibers 2019, 7, 84 7 of 12

Figure 6. Dependence ln(ln(1/y)) VS 1000/T for determining the activation energy of the thermal decomposition.Figure 6. Dependence ln(ln(1/y)) VS 1000/T for determining the activation energy of the thermal decomposition. Table 4. DTG data for cellulose and composite ﬁbers. Table 4. DTG data for cellulose and composite fibers. Sample L (1) T-5 (2) T-10 (3) B-5 (6) B-10 (7) VT-5 (4) VT-10 (5) SlopeSample 26.14 L (1) 19.47 T-5 (2)25.08 T-10 (3) 19.4 B-5 (6) 20.6 B-10 (7)19.6 VT-5 (4)17.1 VT-10 (5) − − − − − − − TonsetSlope, ◦C 281−26.14282 −19.47 292−25.08 279−19.4 279−20.6 282−19.6 287 −17.1 TrangeTonset, ◦C, oC 281–306 281 282–307 282 292–317 292 279–304279 279–304 279 282–307 282 287–312 287 Ea, kJ/mol 234 162 209 162 172 164 143 Trange, oC 281–306 282–307 292–317 279–304 279–304 282–307 287–312 Weight loss, % 5.6 4.7 7 5.3 3.4 3.5 4.8 CarbonЕa,yield kJ/mol at 234 162 209 162 172 164 143 3.5 14.4 12.3 14.8 13.1 12.6 5.7 1000Weight◦C, loss, % % 5.6 4.7 7 5.3 3.4 3.5 4.8 Carbon yield at 1000°C, % 3.5 14.4 12.3 14.8 13.1 12.6 5.7

Thus, the lowest point of the pyrolysis starting (279 C) was for the precursors B-5 and B-10, Thus, the lowest point of the pyrolysis starting (279 °C)◦ was for the precursors B-5 and B-10, and and the highest was for the sample T-10 (292 C). All other formulations were located inside this the highest was for the sample T-10 (292 ◦°C). All other formulations were located inside this temperature interval. This means that depending on the nature of the additive, the thermal stability temperature interval. This means that depending on the nature of the additive, the thermal stability of the samples was slightly varied. Reducing the structural order in the system did not lead to a of the samples was slightly varied. Reducing the structural order in the system did not lead to a decrease in thermal stability. Hence, it can be assumed that the chemical interaction between the decrease in thermal stability. Hence, it can be assumed that the chemical interaction between the silicon-containing additive and the cellulose matrix, and the amount of the additive, plays a more silicon-containing additive and the cellulose matrix, and the amount of the additive, plays a more important role in the decomposition process than the crystalline structure. It is known that in the important role in the decomposition process than the crystalline structure. It is known that in the presence of water, organosilicon compounds are extremely unstable compounds, prone to hydrolysis presence of water, organosilicon compounds are extremely unstable compounds, prone to reactions, and subsequent condensation and polycondensation with the formation of oligomers and hydrolysis reactions, and subsequent condensation and polycondensation with the formation of polymers with siloxane bonds of a linear or cyclic structure [30,31]. In the studied cellulose solutions in oligomers and polymers with siloxane bonds of a linear or cyclic structure [30,31]. In the studied NMMO, water was present in an amount of ~10% to 13%. The introduction of tetraethoxysilane (TEOS) cellulose solutions in NMMO, water was present in an amount of ~10% to 13%. The introduction of to cellulose solutions in NMMO led to the formation of a discrete phase of polysiloxanes. The resulting tetraethoxysilane (TEOS) to cellulose solutions in NMMO led to the formation of a discrete phase of polysiloxane particles were uniformly distributed in the cellulose matrix. The vinyltriethoxysilane polysiloxanes. The resulting polysiloxane particles were uniformly distributed in the cellulose (VTEOS) molecule contains double bonds that can open and integrate into cellulose chains during matrix. The vinyltriethoxysilane (VTEOS) molecule contains double bonds that can open and thermolysis, ensuring the formation of Si–C bonds (as evidenced by IR spectroscopy). Bistrimethylsilyl integrate into cellulose chains during thermolysis, ensuring the formation of Si–C bonds (as acetylene (BTMSA) is chemically inert and does not chemically interact with the matrix when forming evidenced by IR spectroscopy). Bistrimethylsilyl acetylene (BTMSA) is chemically inert and does not the fiber. chemically interact with the matrix when forming the fiber. Based on the Broido method, the values of activation energy (Ea) of the thermal decomposition Based on the Broido method, the values of activation energy (Ea) of the thermal decomposition for the spun fibers were calculated. The activation energy for Lyocell fibers was 234 kJ/mol, which for the spun fibers were calculated. The activation energy for Lyocell fibers was 234 kJ/mol, which is is closer to the values for the cellulose samples discussed in [32–34]. The activation energy of the closer to the values for the cellulose samples discussed in [32–34]. The activation energy of the thermal decomposition of T-5, VT-5, and B-5 practically coincided. Further increases in the content of thermal decomposition of T-5, VT-5, and B-5 practically coincided. Further increases in the content Fibers 2019, 7, x FOR PEER REVIEW 8 of 11

Fibers 2019, 7, 84 8 of 12 of these additives in the system led to a multidirectional change in the activation energy. The value of Ea increased for silyl-substituted additives and decreased for BTMSA. The maximum value of 209 kJ/molthese was additives achieved in the for system T-10 ledsamples. to a multidirectional It should be noted change that in thethisactivation value of E energy.a is lower The than value that of E a obtainedincreased for for cellulose silyl-substituted fibers. additives and decreased for BTMSA. The maximum value of 209 kJ/mol wasAlong achieved with for the T-10 highest samples. activation It should energy be noted for T-10 that thisfibers, value the of highestEa is lower mass than loss that rate obtained was also for recordedcellulose (Table fibers. 4). Lyocell fibers, on the contrary, were characterized by the lowest rate of mass loss. As a result,Along in withthe temperature the highest range activation for which energy the foractivation T-10 fibers, energy the was highest calculated, mass lossthe largest rate was mass also lossrecorded of 7% was (Table observed4). Lyocell for fibers,the T-10 on sample, the contrary, while were the BT-5 characterized and B-10 by fibers the lowestlost about rate 3.5% of mass of the loss. initialAs a mass result, only in the (Table temperature 4). At T range = 1000 for °C, which the thecarbon activation yield of energy the Lyocell was calculated, fibers tended the largest towards mass minimumloss of 7% values. was observed The composite for the T-10 precursor sample, T-5 while samp theles BT-5 had and a B-10carbon fibers yield lost of about 14.4% 3.5% at ofthe the same initial temperature,mass only (Table which4). is At the T =maximum1000 ◦C, thevalue carbon among yield all of the the samples Lyocell fibersstudied. tended The towardsminimum minimum value (5.7%)values. was The observed composite for VT-10 precursor fibers. T-5 It samples is importan hadt ato carbon note that yield despite of 14.4% the atchemical the same nature temperature, of the additiveswhich is introduced the maximum into value the cellulose among all matrix, the samples the carbon studied. yield The initially minimum increased value (5.7%)with the was addition observed offor 5% VT-10silicon-containing fibers. It is important compounds, to note while that a further despite increase the chemical in the natureadditive of to the 10% additives led to a introduced decrease ininto its value. the cellulose matrix, the carbon yield initially increased with the addition of 5% silicon-containing compounds,The DSC curves while aare further shown increase in Figure in the7. additive to 10% led to a decrease in its value. The DSC curves are shown in Figure7.

(a) (b)

(c)

FigureFigure 7. 7.DSCDSC curves curves for for cellulose cellulose (1 (1)) and and composite composite fibers: ﬁbers: ( (aa)) T-5 T-5 (2), T-10 (3);(3); ((bb)) VT-5VT-5 (2), (2), VT-10 VT-10 (3); 3 (3);(c )(c B-5) B-5 (2) (2) and and B-10 B-10 (3). (3). Atmosphere: Atmosphere: argon argon 10 10 cm cm/3min./min. Linear Linear heating heating at at a a rate rate of of 10 10◦ C°C/min./min.

TableTable 55 summarizes summarizes the the most most important important quantitative quantitative indicators indicators of of DSC DSC data. data. Two temperature ranges can be distinguished for all samples (Table5). The first one (30–178 ◦C) characterizes an endothermic effect, connected to the removal of water. The second one is the thermal decomposition of cellulose (the temperature range of 286–376 ◦C), accompanied by endo- and exo-effects. The described ranges are well correlated with the TGA data. Fibers 2019, 7, 84 9 of 12

Table 5. The main parameters of ﬁbers obtained by DSC studies at a heating rate of 10 K/min in an inert atmosphere.

Òmin Òmin Òmax Òmin Òrange, Endothermic Òrange, Endothermic Òrange, Exothermic Òrange, Endothermic Sample Endotherma, 1 Endotherma, 1 Exotherma, 1 Endotherma, 1 ◦C Effect, J g− ◦C Effect, J g− ◦C Effect, J g− ◦C Effect, J g− ◦C · ◦C · ◦C · ◦C · L 30–172 77 162 288–345 326 119 345–372 350 25 − − − Ò-5 30–178 112 155 295–319 311 22 319–355 347 56 − − Ò-10 30–177 111 157 277–341 317 66 342–351 348 8 351–376 354 11 − − − VT-5 30–174 106 178 286–333 318 74 333–352 348 18 352–370 354 8 − − − VT-10 30–177 99 224 289–326 323 95 326–345 341 62 345–376 351 14 − − − B-5 30–164 101 139 281–312 305 10 312–357 337 92 − − B-10 30–177 112 166 289–325 312 10 339–355 347 19 − − Fibers 2019, 7, 84 10 of 12

The adsorbed water is the most quickly removed from the cellulose fibers, a minimum of the endotherm is observed at 77 ◦C. The introduction of additives into the cellulose matrix leads to a change in the kinetics of the water removal and the shift of the endothermal minimum to higher temperatures. The dehydration and depolymerization reactions are accompanied by an endo-effect, which, depending on the type of additive and its concentration, starts at temperatures of 277–295 ◦C. The endo-effect decreases dramatically for T-5, B-5, and B-10 fibers, while for silyl-substituted additives it increases with increasing additive concentration, and for composite fibers of series B it remains unchanged. At higher temperatures, processes of gas formation (CO, CO2, etc.) [35] and their interaction with the transformed sample under study are observed, which are expressed as endo- and exo- peaks on the DSC curves. With an increase in the VTEOS content up to 10%, the thermal behavior of the system changes. On the DSC curve (Figure7, curve 3), there are also three peaks, but the maximums of the peaks are shifted. Thus, the position of the endopeak corresponding to dehydration processes shifts to 323 ◦C, while the position of the maximum of the exopeak shifts toward lower temperatures to 341 ◦C. The character of the curve also changes. The increased content of the additive leads to a significant acceleration of the processes of structural rearrangements, as evidenced by the heat of the exo-effect that has increased almost 4-fold. The results obtained, at first glance, are somewhat different from the TGA data, according to which an increase in the VTEOS content in cellulose to 10% leads to a decrease in the carbon yield. Apparently, the processes occurring in the heat treatment process are more complex. It can be assumed that high heat release in the bulk of the sample initiates the flow of deeper destruction processes leading to the mass loss.

4. Conclusions Thus, the introduction of silicon-containing additives into the cellulose matrix leads to a decrease in structure ordering and affects the thermal behavior of the resulting composite precursors. For all composite fibers, the activation energy of pyrolysis is less than that for cellulose fibers. At the same time, the reaction itself starts first for samples B-5 and B-10, and later on for T-10 fibers, i.e., the first type of additive acts as a pyrolysis catalyst, and the second one as a flame retardant. The type of observed peaks on the DSC curves for composite fibers cardinally changes from the endopeaks inherent for cellulose fibers by the combination of endo- and exo- peaks. This may indicate the occurrence of a number of complex chemical processes leading to an increase in the mass of the carbon yield at temperatures up to 1000 ◦C. Regardless of the type of silicon-containing agent, the maximum values of the carbon yield are achieved with the introduction of 5% of the composite additive into the cellulose. For further improvement of the values of fibers’ carbon yield, it is necessary to optimize the temperature profiles and residence times. Additional impregnation of the discussed composite precursors with ammonium chloride, diammonium phosphate, or other silicon-containing compounds also contributes to an increase in carbon yield values and is the subject of further research. The resulting carbon fibers from composite precursors based on cellulose in the future can be used to create composite materials with unique engineering properties such as high thermal stability and mechanical values.

Author Contributions: M.M. and I.M. provided the idea for this study, proposed the experiments and wrote the paper; I.M. analyzed the data; L.G. analyzed the data and reviewed the paper; M.V. produced the composite samples; G.S. performed thermal analysis and discussed the data; I.L. investigated the structure of the fibers; M.M. and I.M. edited the final paper. Funding: This research was funded by RFBR according to the research project No. 16-33-60218 mol_a_dk. Conflicts of Interest: The authors declare no conflict of interest. Fibers 2019, 7, 84 11 of 12

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