Investigation on thermal characteristics and desensitization mechanism of improved step ladder-structured nitrocellulose

Cheng-kai Pu, Yu Luan, Ming-jun Yi, Zheng-gang Xiao

Key Laboratory of Special Energy Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, People's Republic of China

Keywords:Nitrocellulose Decomposition Desensitization Ladder structure Thermal characteristic Polyethylene glycol

ABSTRACT Nitrocellulose,or cellulose nitrate,has received considerable interest due to its various applications,such as propellants,coating agents and gas generators.However,its high mechanical sensitivity caused many accidents during its storage and usage in ammunition.In this work,two kinds of insensitive step ladderstructured nitrocellulose (LNC) with different nitrogen contents were synthesized.The products were characterized by FT-IR, Raman, XRD, SEM, elemental analysis, TGA, DSC, accelerating rate calorimeter analysis (ARC), and drop weight test to study their molecular structure, thermal characteristics and desensitization performance.Compared with raw nitrocellulose, LNC has a sharper exothermic peak in the DSC and ARC curves.The H50 values of the two kinds of LNC increased from 25.76 to 30.01 cm for low nitrogen content and from 18.02 to 21.84 cm for high nitrogen content, respectively.The results show that the ladder-structure of LNC which provides regular molecular arrangement and a soft buffer made with polyethylene glycol could affect the energy releasing process of LNC and reduce the sensitivity of LNC.Insensitive LNC provides an alternative to be used as a binder in insensitive propellants formulation.

1.Introduction

Propellants are the energy source of barrel armaments and one of the key factors determining the safety performance of armaments[1,2].The sensitivity of propellants makes them highly prone to burning or exploding when subjected to external stimuli,such as heat,impact and shock[3].Consequently,there are many accidents caused by the high sensitivities of propellants that had happened in the last few decades, and these catastrophic disasters are often accompanied by a large number of casualties and property losses[4], which are summarized in Table 1.Therefore, it is meaningful and significant to develop the propellants with enhanced insensitivity performance.

Nitrocellulose (NC) was firstly used to make single-base propellants in the 1880s, the French chemist Vieille used an etheralcohol solvent to mix NC with barium or potassium nitrates to produce propellants [9].Even now, NC is still an important constituent of gun propellants.Compared with other commonly used ingredients of gun propellants, NC has some disadvantages in its sensitivity performance.For example, the value of the impact sensitivity of NC is 3 Nm, while the impact sensitivity of hexogen(RDX),octogen(HMX)and nitroguanidine(NQ)are 7.5 Nm,7.4 Nm and >50 Nm respectively[10].And NC is more sensitive than high explosives, such as RDX, HMX, NQ, etc.It is easy to be accidently initiated by various external stimuli.In the development of insensitive propellants, reducing the sensitivity of NC is an attractive method to increase the safety performance of solid propellants.In the last few decades,many researches have been focusing on partly replacing NC with insensitive energetic binders to reduce the sensitivity performance of propellants, such as cellulose acetate(CA) [11], cellulose acetate butyrate (CAB) [12]and glycidyl azide polymer (GAP) [13].However, there are still some problems that need to be overcome,such as the poor mechanical properties which are caused by the migration of plasticizer [14]or processing techniques of these binders in propellant chemistry and technology[15].The poor mechanical properties of gun propellants will increase the breaking probability of propellant powder grains during burning process.Therefore, directly reducing the sensitivity of NC provides a new method to reduce the sensitivity of propellants.

In recent years,some studies have been conducted involving the sensitivities of NC.Meng et al.[16]coated the nitrocellulose surface with the silica gel, the impact sensitivities of NC/SiO2composites were reduced significantly in comparison with the raw NC.Meng et al.[17]also prepared nano-sized spherical NC composite crystal by pretreatment, degradation and nitration of microcrystalline cellulose.The nano-size spherical NC had a higher crystallinity and nitrogen content and less impact sensitivity in comparison with the raw NC.However,there are still two limitations that prevent these products from replacing the raw NC.One is the decrease of energy because the mass ratio of inert materials is higher than 30%.Another is the poor mechanical properties, similar to the insensitive energetic binders, the problem caused by the migration of plasticizer is still not solved.

In our previous work [18], a novel binder named step ladderstructured nitrocellulose was synthesized.It has a regular crystal structure, and its mechanical sensitivity was reduced greatly.However, there are still several shortages of the step ladderstructured nitrocellulose that need to be overcome, such as the large solvent consumption and the low energy level of products.

In this paper, several improvement methods were used to synthesize the improved step ladder-structured nitrocellulose(LNC)by isophorone diisocyanate (IPDI) reacting with free hydroxyl groups of NC, then reacting with polyethylene glycol (PEG).Each IPDI molecule has two functional isocyanate groups.The reactive activity of the secondary group(directly linked to the ring)of the IPDI is 10 times higher than that of the primary group(near the methyl)because of the steric hindrance of-CH3[19].The unequal reactivity prevents both isocyanate groups from joining the reaction at the same time[20].And,the temperature will be raised when the PEG is added into reaction system because most of the remaining isocyanate groups in the system are less reactive, needing more energy to react with hydroxyl groups of PEG.

In order to decrease the consumption of solvent, concentrated NC solution was used as the reaction system instead of dilute NC solution,and acetone was chosen as the solvent.Acetone is a nonpolar organic solvent and widely used as the solvent of NC [21].Although the solubility of NC is influenced by its nitrogen content,NC with a wide range of nitration levels has good acetone solubility in acetone[22,23].In order to increase the energy of LNC,two new approaches were introduced.One is the increase of the nitrogen content of the raw NC,which is the main chain of LNC.The nitrogen content of the raw NC was increased to 12.50% and 13.15%, and another is the decrease of the molecular weight of polyethylene glycol used in the preparation process.The LNC prepared by these improvement methods overcomes the shortages in solvent consumption and energy loss.Less solvent consumption will reduce the cost of mass production in the future.And the higher energy level ensures that LNC can be used as a high-energy binder to partly replace NC in LOVA gun propellants systems and insensitive high energy propellants [24].

Abbreviations

NC nitrocellulose

NC-LN nitrocellulose with low nitrogen content

NC-HN nitrocellulose with high nitrogen content

LNC the improved step ladder-structured nitrocellulose

LNC-LN the improved step ladder-structured nitrocellulose with low nitrogen content

LNC-HN the improved step ladder-structured nitrocellulose with high nitrogen content

FT-IR Fourier-transform infrared spectrometer

XRD X-ray diffractometer

SEM scanning electron microscope

TGA thermogravimetric analysis

HPTGA high pressure thermogravimetric analysis

DSC differential scanning calorimeter analysis

ARC accelerating rate calorimeter analysis

RDX hexogen

HMX octogen

NQ nitroguanidine

CA cellulose acetate

CAB cellulose acetate butyrate

GAP glycidyl azide polymer

IPDI isophorone diisocyanate

PEG polyethylene glycol

PEG-400 polyethylene glycol with a molecular weight of 400

DBTL dibutyltin dilaurate

H50critical drop height of 50%initiation probability

Table 1Major accidents related to propellants and nitrocellulose.

2.Experimental

2.1.Reagents and instruments

Nitrocellulose with a low nitrogen content of 12.50% (NC-LN)and nitrocellulose with a high nitrogen content of 13.37%(NC-HN)were provided by Luzhou North Chemical Industries Co.,Ltd,NC-LN and NC-HN were dried at 45°C for 10 days.Isophorone diisocyanate(IPDI) and dibutyltin dilaurate (DBTL) were provided by Macklin Co., Ltd.Polyethylene glycol with a molecular weight of 400 (PEG-400) and reagent grade acetone were provided by Sinopharm Chemical Reagent Co.Ltd.Distilled water was purified from tap water by Direct-Pure Genie deionized water machine.All chemical agents were used as received unless otherwise noted.

2.2.Preparation of LNC

First,36 g raw NC was added into 2.4 L acetone solvent in a 3 L flask slowly and stirred at 260 rpm for 24 h until the raw NC was fully dissolved.6.72 g IPDI and 6.40 g DBTL were added to the NC acetone solution within the flask dropwise.The mixture was heated at a temperature of 50°C under the condition of stirring at 90 rpm,and the reaction was maintained at the temperature for 2 h.Second,3 g PEG-400 was added to the reacting flask and heated at 75°C.The reaction continued maintaining at 75°C for 2 h.Finally, the reaction solution was poured into 4.5 L distilled water, and the products were separated with vacuum filtration.The products were dried for 48 h at 45°C.Approximately 42 g LNC was obtained.Fig.1 shows the preparation process of LNC.

The NC-LN and NC-HN were both modified by IPDI and PEG-400,thus there were two kinds of reaction products with different nitrogen contents,i.e.,improved step ladder-structured NC with low nitrogen content(LNC-LN)and improved step ladder-structured NC with high nitrogen content (LNC-HN).

2.3.Characterization

The molecular structure of raw NC and LNC was characterized by Fourier-transform infrared spectrometer (FT-IR, Nicolet iS10) and Raman spectrometer (INVIA, UK Renishaw Company).The FT-IR spectra were collected in the transmission mode with a resolution of 4 cm-1.The scans range of FT-IR was 550-4000 cm-1and scans times were 32.The scans range of Raman was 500-4000 cm-1,raster type was 1200 1/mm and laser wavelength was 785 nm.

The crystal structure of raw NC and LNC was characterized by Xray diffractometer (XRD, D8 Advance, Germany Bruker Company)with Ni-filtered Cu-Kα radiation(λ=0.1542 nm)generated at 40 kV and 40 mA.

The surface profile of raw NC and LNC was recorded by scanning electron microscope (SEM, Quanta FEG-250).Samples were prepared by dispersing dry powder on double-sided conductive adhesive tape, and they were coated with gold by arc discharge method.The experimental voltage of the SEM was 15 kV.

The thermal characteristics of raw NC and LNC were characterized by thermogravimetric analysis (TGA, 851E TGA/SDTA, Swiss Maitre Toledo Company),high pressure thermogravimetric analysis(THEMYS HP, France Setaram Company), differential scanning calorimeter analysis (DSC, DSC3+, Swiss Maitre Toledo Company)and accelerating rate calorimeter analysis (ARC, ARC ES, UK THT Company).

Fig.1. Preparation process of LNC.

The TGA scan was carried out under a N2atmosphere from 50 to 600°C.The heating rate of TGA was 10°C/min.The sample mass of TGA was 0.5 mg.

The temperature range of the high pressure thermogravimetric scan was 50-380°C.The characterization process was carried out at a heating rate of 10°C/min under a N2atmosphere.And the pressure was 10 bar and 30 bar, respectively.The sample mass of high pressure thermogravimetric was 1.5 mg.

The DSC scan was carried out under a N2atmosphere from 50 to 350°C.The heating rate of DSC was 10°C/min.The sample mass of DSC was 0.5 mg.

The temperature range of the ARC scan was 80-220°C,and the temperature step was 5°C.The maximum safety pressure was 200.0 bar,the maximum pressure rate was 100000 bar/min and the maximum exotherm rate was 1000°C/min-1.The sample mass of ARC was 100 mg.

The heat of explosion of raw NC and LNC was characterized by oxygen bomb calorimeter(ZDHW-6W).The sample mass was 1.0 g.

The nitrogen content of raw NC and LNC was characterized by elemental analysis(Vario EL cube, Germany Eiementar Company).

The sensitivity of raw NC and LNC was tested by the drop hammer apparatus.The test was carried out according to GJB/J 3052-97 standard method 601.2 [25,26], which is equivalent to MIL-STD of American.The testing apparatus uses a 5 kg steel drop weight with a 35 mg sample resting between two steel anvils.

3.Results and discussion

3.1.Molecular structure of LNC

The molecular structure of LNC is shown in Fig.2.The NC chains are linked by IPDI and PEG.IPDI is used as a linkage to graft PEG onto NC chains.IPDI is an aliphatic ring with one primary and one secondary isocyanate group, which has unequal reactivity at different temperatures [18,27].PEG is used to form a buffer between the two NC chains, which plays a role of a soft cushioning layer when LNC faces external stimuli.PEG,a linear polymer with a regular structure,good performance of bonding and flexibility,has been widely used as a thermoplastic binder in the propellants[28].

The FT-IR spectra of NC-LN, LNC-LN, NC-HN and LNC-HN are shown in Fig.3.As seen from the FT-IR spectra of NC-LN,the strong band at 1635 cm-1,1271 cm-1, 820 cm-1, 743 cm-1and 660 cm-1correspond to the characteristic absorption band of nitric ester.The band at 1056 cm-1refers to C-O stretching vibration among the rings.The band at 996 cm-1is assigned to C-O stretching vibration of rings.The weak band at 2917 cm-1is assigned to C-H stretching vibration[29].

Fig.2. Molecular structure of LNC.

Fig.3. FT-IR spectra of NC-LN, LNC-LN, NC-HN, LNC-HN, PEG-400 and IPDI.

Compared with the FT-IR spectra of NC-LN, the FT-IR spectra of LNC-LN has new absorption bands at 2955 cm-1and 2852 cm-1,referring to the C-H stretching vibration of IPDI rings and PEG-400,respectively.The band at 1565 cm-1is assigned to the stretching vibration of carbonyl groups.The result indicates that NC-LN was modified by IPDI and PEG-400 successfully.

As seen from the FT-IR spectra of NC-HN,the band at 2970 cm-1and 2922 cm-1correspond to the asymmetric stretching vibration of methylene and C-H stretching vibration, respectively.

Compared with the FT-IR spectra of NC-HN,the FT-IR spectra of LNC-HN has new absorption bands at 2964 cm-1and 2852 cm-1,referring to the C-H stretching vibration of IPDI rings and PEG-400,respectively.The band at 1562 cm-1corresponds to the stretching vibration of carbonyl groups.The bond at 2243 cm-1in the FT-IR spectra of IPDI is the characteristic absorption peak of -NCO.So this peak disappears in the FT-IR spectra of LNC-LN and LNC-HN after the modification.The result indicates that NC-HN was also modified by IPDI and PEG-400 successfully.

The Raman spectra of NC-LN, LNC-LN, NC-HN and LNC-HN are shown in Fig.4.As seen from the Raman spectra of NC-LN,the band at 570 cm-1is due to the C-O-C stretching vibration.The absorption band at 619 cm-1is attributed to twisting vibration of C-C.The weak band at 688 cm-1is assigned to the twisting vibration of O-NO2.The band at 860 cm-1and 1672 cm-1are due to-NO2deformation vibration and asymmetric stretching vibration,respectively.Besides, the band at 1299 cm-1and 1380 cm-1are attributed to the symmetrical stretching vibration of -NO2[30].The band at 1167 cm-1is assigned to C-O asymmetric stretching vibration.

As seen in the Raman spectra of LNC-LN,these absorption bands still exist and no new absorption band appears.It can be concluded that the grafting reaction of IPDI and PEG does not affect the O-NO2of raw NC,which is related to the energy level of raw NC.

The Raman spectra of NC-HN and LNC-HN are similar to the Raman spectra of NC-LN and LNC-LN.The O-NO2of NC is not influenced by the modification of the raw NC chains.

3.2.Crystal structure of LNC

Fig.4. Raman spectra of NC-LN, LNC-LN, NC-HN and LNC-HN.

The XRD spectra of NC-LN,LNC-LN,NC-HN,LNC-HN and PEG are shown in Fig.5.NC-LN has two diffraction peaks.One is at around 2θ= 12.54°, commonly assigned to the 100 and 1plans and considered as a superposing double peak[29,31].And another is at around 2θ=20.00°,commonly assigned to 200 plan and considered as the less ordered or amorphous region of the cellulose chains[32].After being modified by IPDI and PEG-400,the XRD spectra of LNC-LN shows two diffraction peaks at 2θ= 7.02°and 2θ= 20.86°.The left shift of the peak and the decrease of peak area, which assigned to the 100 and 1plans, indicate that LNC-LN has a different crystal structure.

The XRD spectra of NC-HN only have a broad diffraction peak at 2θ = 21.84°.The difference between NC-LN and NC-HN may be caused by a decrease of the -OH groups, which can form many intramolecular and intermolecular hydrogen bonds.And the decrease of the -OH groups is responsible for the decrease of ordered crystalline arrangements [33].At the same time, the XRD spectra of LNC-HN has a peak at 2θ = 7.30°, and reveals a broad diffraction peak at 2θ = 20.74°.The XRD spectra of LNC-HN are similar to that of LNC-LN.

The XRD spectra of PEG has two sharp peaks at 2θ=19.34°and 2θ=23.46°.Because the mass proportion of PEG-400 in the LNC is only 6.56% and the position of the diffraction peaks is close to the broad diffraction peak of LNC,the diffraction peaks related to PEG-400 are not obvious in the XRD spectra of LNC.

The XRD results indicate that LNC has a different crystal structure.The difference between the spectra shows that the molecular structure of raw NC has changed after modification, which will affect its thermal and sensitivity properties.

3.3.Surface profile of LNC

The SEM image of raw NC and LNC was shown in Fig.6.The raw NC presents as fibrous.And the diameters of raw NC are about 15-25 μm.The SEM micrographs show that the morphology of LNC is changed, probably because it has gone through the process of precipitation from the acetone solution.In addition, the surface of the LNC-LN and LNC-HN is rough.The tiny rough nubs on the surface of LNC may be the crystalline lumps of PEG-400 [34,35].

Fig.5. XRD spectra of NC-LN, LNC-LN, NC-HN, LNC-HN and PEG.

3.4.Thermal characteristics of LNC

3.4.1.Thermogravimetric analysis of LNC

Fig.7 shows the thermogravimetric analysis of raw NC and LNC.As seen from the thermogravimetric curve of NC-LN, the thermal decomposition of NC-LN starts at about 150°C [36].And the thermal mass loss process of NC-LN only has one stage.Its mass loss ratio is about 93.7%, because there is not enough oxygen to converse carbon into gas under a N2atmosphere.

In contrast, the thermal mass loss process of LNC-LN can be divided into two phases.In the first phase, the sample has a mass loss process in the temperature range from 150 to 230°C, corresponding to the thermal decomposition of the NC chains.And the second phase is from 250 to 350°C corresponding to the decomposition of the connection chains.The mass loss ratio of LNC-LN is about 87.0%.

The thermogravimetric curve of NC-HN is similar to that of NCLN.Because of its high nitrogen content,its mass loss ratio is about 96.7%.

As can be seen from the thermogravimetric curve of LNC-HN,the peak of LNC-HN in the second phase is less obvious than that of LNC-LN.NC-HN has less reactive -OH on the NC chains to be modified by IPDI and PEG-400.Therefore, the peak of decomposition of the connection chains is weak.The mass loss ratio of LNCHN is about 89.2%.

To study the pressure influence on the decomposition process of raw NC and LNC,the thermogravimetric analysis was performed in 10 bar(Fig.8(a))and 30 bar(Fig.8(b))pressure,respectively.As can be seen in Fig.8(a), the mass loss process of NC-HN starts from about 180°C, and the mass loss process of LNC-HN starts from about 150°C.In the 10 bar pressure,the mass loss rate of the main mass loss process is almost the same.Furthermore, the second decomposition stage of LNC-HN, which is found in the common thermogravimetric analysis curve of LNC-HN and corresponding to the decomposition of the connection chains,does not appear in the curve.This may be because the high pressure environment accelerates the decomposition process of LNC-HN,then the non-obvious second thermal mass loss stage is covered by the first thermal mass loss stage.

When the pressure rise to 30 bar, the baseline of the curve becomes more uneven.This is because the mass of testing energy materials samples is small (1.5 mg) and the airflow disturbance of the shielding gas is more obvious under high pressure.Fig.8(b)shows that the increase in pressure causes an increase in the decomposition temperature of LNC-HN.The starting temperature of mass loss process of NC-HN increases from 180 to 215°C,and the starting temperature of mass loss process of LNC-HN increases from 150 to 225°C.

3.4.2.DSC analysis of LNC

As shown in Fig.9,the onset temperature and end temperature of the exothermic peak of NC-LN are 190°C and 241°C,respectively.And the onset temperature and end temperature of the exothermic peak of LNC-LN are 182°C and 201°C,respectively[37].The range of exothermic peak of LNC-LN is narrower than that of NC-LN.The DSC curves of NC-HN and LNC-HN are similar to the DSC curves of NC-LN and LNC-LN.The onset temperature and end temperature of the exothermic peak of NC-HN are 192°C and 232°C,respectively.And the onset temperature and end temperature of the exothermic peak of LNC-HN are 181°C and 205°C, respectively.The result of DSC shows that although the modification of IPDI and PEG-400 on raw NC will decrease the energy of LNC because of the decrease of nitrogen content the introduction of linking chains will shorten the exothermic time in the decomposition process and lead to the concentration of energy release.

After comprehensively considering the results of TGA and DSC,no matter how much nitrogen content LNC has, the mass loss process of LNC starts at about 150°C and ends at about 300°C,the exothermic process starts at about 180°C and ends at about 205°C.The energy release of LNC mainly occurs in the initial period of the thermal decomposition process.Then the thermal decomposition process of LNC is still going on in the following period, but it releases less energy.The energy release of LNC is uneven during the mass loss process due to the cooperative effects of-NO2group and PEG linking chains.By contrast, the mass loss process of raw NC starts at about 150°C and ends at about 240°C, the exothermic process starts at about 180°C and ends at about 240°C.Once the mass loss process of raw NC ends,the energy release of raw NC will end.The energy release process of raw NC is essentially consistent with the mass loss process of raw NC.

To eliminate the thermal history,mechanical history and crystal structure influence of the decomposition process [38], recrystallization DSC was performed.The samples were heated from 30 to 100°C at a heating rate of 10°C/min and held at 100°C for 5 min.Then, the samples were cooled at a cooling rate of 10°C/min to -30°C and held at -30°C for 5 min.Finally, the samples were heated from-30 to 350°C at a heating rate of 10°C/min.The DSC curves are shown in Fig.10,the heating and cooling process seems to have almost no effect on the thermal decomposition process of NC-HN and LNC-HN.The DSC curve of LNC-LN still has a sharp exothermic peak.The result shows that the elimination of the thermal history will not influence the decomposition process of LNC.

Fig.6. SEM micrographs of (a) raw NC, (b), (c) LNC-LN and (d), (e) LNC-HN.

Fig.7. Thermogravimetric analysis of NC-LN, LNC-LN, NC-HN and LNC-HN.

3.4.3.ARC analysis of LNC

The ARC was also used to research the thermal characteristics of relatively large mass raw NC and LNC.The sample mass in the ARC test was 100 mg.Fig.11(a) and Fig.11(b) both show that the massively exothermic process of LNC comes later than raw NC.

As can be seen from Fig.11(a),the exothermic process of NC-LN and LNC-LN starts at 150°C[39].The exothermic process of NC-LN ends at 173.7°C and 10.6 bar, the exothermic process of LNC-LN ends at172.3°C and 9.0 bar.This may be due to the introduction of non-energetic components (IPDI and PEG-400).IPDI and PEG-400 as the linking chains decrease the energy of LNC.Furthermore,the exothermic of LNC-LN takes a longer time than NC-LN in the adiabatic environment of ARC,which indicates that LNC-LN has a better high temperature resistance in the adiabatic environment.

The temperature and pressure curves of NC-HN and LNC-HN in the ARC experiment are shown in Fig.11(b).The exothermic process of LNC-HN also has a lower final temperature and pressure.Like the result of DSC, the sharp exothermic peak is also found in the ARC temperature curve of LNC-HN.In an adiabatic and constant volume environment, LNC-HN, which has a higher energy level than LNCLN, seems to rapidly thermally decompose after the temperature reaches 160°C.Before the temperature reaches 160°C,like LNC-LN,LNC-HN also needs a longer time to increase the temperature.

3.4.4.Decomposition process mechanism of LNC

Based on the above thermal analysis results,LNC seems to have a different decomposition process compared with the raw NC.We attempt to propose a mechanism to describe this decomposition process of LNC.

Fig.8. HPTGA of NC-HN and LNC-HN.

Fig.9. DSC curves of NC-LN, LNC-LN, NC-HN and LNC-HN.

Fig.10. Recrystallization DSC curves of NC-HN and LNC-HN.

Fig.11. ARC temperature and pressure curve of NC-LN, LNC-LN, NC-HN and LNC-HN.

As shown in Fig.12,the decomposition of raw NC can be divided into two stages [2,36].According to the result of TGA (Fig.7) and high pressure TGA (Fig.8), the sample starts to lose its mass at about 150°C.Moreover,the ARC result(Fig.11)also shows that the starting temperature of the exothermic process is 150°C.Therefore,the decomposition of raw NC might start from about 150°C.The first stage of the decomposition of raw NC is the denitration reaction of O-NO2on the NC chains and the forming of NO2and the corresponding alkoxy (-CH2O).And compared with carbon 3, the preferred path of NC decomposition is the carbon 2 (Fig.13) [40].From the DSC result (Fig.9) and ARC result, the heat and gas generated from the first stage are little.The corresponding exothermic changes are not obvious in the DSC curves,the pressure increases slowly when the temperature rises from 150 to 160°C in the ARC curve(see Fig.14).

Fig.12. Schematic diagram of the decomposition of raw NC.

Fig.13. Schematic diagram of the chemical structure of raw NC.

However,the starting temperature of the second stage is hard to be defined.Under the heating rate of 10°C/min(TGA,high pressure TGA and DSC), the second stage seems to start at 180°C.But, according to the ARC result,the second stage seems to start at 160°C in the adiabatic environment.The second stage is the decomposition reaction of the raw NC chains under the catalysis of NO2gas[8].In this stage,numerous gas and heat are generated.The TG and DSC curves both have obvious changes in this process, and the ARC pressure curves also have a sharp upward trend.

Compared with the raw NC, the decomposition process of LNC has three stages.The first stage of the thermal decomposition of LNC is found slower than that of NC in the ARC results.NC can be written as [C6H7O2(OH)3-r(ONO2)r]n, and the nitrogen content of the two kinds of raw NC is 12.50%and 13.37%.After calculation,the values ofrof the two kinds of raw NC are 2.42 and 2.71 respectively.Therefore,there is only at most one free hydroxyl on the individual chain segments of raw NC which can be grafted by PEG-400.The linking chains on carbon 2 or carbon 3 will protect the O-NO2on the adjacent carbon from the catalysis of NO2gas because of the steric hindrance.This reduces the reaction rate of the first stage.

Fig.14. Schematic diagram of the decomposition of LNC.

On the contrary,the second stage of the thermal decomposition of LNC is found more rapid than that of raw NC both in the DSC and ARC results.This is also caused by the existence of the linking chain.The linking chains with a high molecular weight reduce the stability of the molecular chain after the denitration reaction of the first stage.In turn, this unstable structure accelerates the decomposition of the molecular chain in the second stage.

As shown in the TGA results, the third stage of LNC decomposition starts at about 260°C, which refers to the decomposition reaction of the linking chains.The linking chain segment is a hypoxia type alkyl group, which is not classified as energy materials.Therefore,as the DCS result,the decomposition process of the third stage under the N2atmosphere releases almost no energy.

3.5.Heat of explosion and sensitivity of NC and LNC

The results of the nitrogen content of raw NC and LNC are recorded by elemental analysis.The mass of IPDI occupied the main part of the mass of the linking chains of LNC, and the nitrogen content of IPDI is 12.60%.As the result, the nitrogen content has a slight drop after the raw NC was modified by IPDI and PEG-400.

The results of the heat of explosion of raw NC and LNC are based on the oxygen bomb method.The heat of explosion value for LNCLN is 2978 J/g,and the heat of explosion value for LNC-HN is 3315 J/g.Compared with NC-LN (3740 J/g) and NC-HN (4127 J/g) [41], it was reduced by 21.4% and 19.7%, respectively.After the corresponding calculation, the mass of the linking chain which is non-energetic components accounts for 21.26%of the total mass of LNC.It seems that the energy reduction of LNC is roughly equal to the mass ratio of the added non-energy components.The results show an obvious correlation between the energy loss and the proportion of the mass of non-energetic components.

Table 2Elemental analysis, oxygen bomb test and impact sensitivity test results of NC-LN,LNC-LN,NC-HN and LNC-HN.

The impact sensitivities of raw NC and LNC are compared in Table 1.Critical drop height of 50% initiation probability (H50) is used to express the impact sensitivities and the 50%initiation point is determined by means of the Bruceton up-and-down method[42].The higher theH50value is,the lower the impact sensitivity is.TheH50values for LNC-LN and LNC-HN are 19.84 cm and 30.01 cm,respectively.As shown in Table 2, it has become obvious that the sensitivities of LNC are successfully reduced.

3.6.Desensitization Mechanism of LNC

According to the hot-spot ignition theory for propellants[43,44], the ignition of propellants is thermal in origin with mechanical or electrical energy converted into heat at the local hot spots.Then the formation of hot spots promotes the local temperature rise to the ignition temperature of propellants.Finally, it causes the propellants to burn or even explode.Based on the hotspot theory, the desensitization mechanism of LNC can be summarized as the combined effects of the regular structure of LNC and the buffer of the soft PEG segments.

The ladder structure in LNC provides regular molecular arrangement, it has better chemical stability and mechanical properties [45,46].The regular structure of LNC can reduce the defects of LNC crystal which can decrease the probability of hotspots generated by the slide of crystal[47].Thus the sensitivities of LNC will be reduced.

Moreover, as the commonly used chain extenders in the propellants formulation, PEG can improve the mechanical properties of the propellants [48].The introduction of PEG reduces the glass transition temperature of NC-based propellants [49,50], and the propellants containing PEG are softer [28].As can be seen from Fig.2, PEG-400 form a buffer between NC chains.When the external forces were acted to LNC,the buffer plays a lubrication role and leads to a reduction in the probability of formation of hot spots[51].

3.7.Comparative summary with previously reported work

In the previous initial study of step ladder-structured nitrocellulose [18], the consumption of tetrahydrofuran solvent for each gram of NC was 200 mL, and its heat of decomposition was decreased from 1471 to 646 J/g, the decline ratio is 56.1%.In contrast, the improved LNC only requires 66.7 mL acetone solvent for each gram of NC, and its heat of explosion was decreased not exceeding about 20%.These values indicate that the improved preparation method of LNC just requires less solvent and prepared LNC has a higher energy level than the reported initial step ladderstructured nitrocellulose.Certainly, further improvements need to be carefully studied since the energy loss needs a further cut.

4.Conclusions

In this paper, an efficient preparation approach for LNC by introduction of IPDI and PEG-400 between NC chains was developed.Concentrated NC solution with high nitrogen content was used as a reaction system,at the same time,the molecular weight of PEG was decreased to a certain value.Compared with the initial preparation method,this new route just requires less consumption of solvent.However, its products have a higher energy level.The heat of explosion value of LNC-LN is 2978 J/g, and the heat of explosion value of LNC-HN is 3315 J/g.The thermal decomposition process of LNC was studied.Compared with the raw NC, the decomposition process of LNC has three stages.The first stage of the thermal decomposition of LNC is the denitration reaction of secondary O-NO2on the NC chains, the second stage is the decomposition reaction of the NC chains, and the third stage is the decomposition reaction of the non-energetic linking chains.In addition, the release of gas and heat is mainly generated in the second stage.The decomposition process LNC seems to have a longer mass loss process and a more rapid exothermic process.In other words, the energy release of LNC is more concentrated.The combined effects of the regular ladder structure of LNC and the buffer of the soft PEG segments between NC chains cause the impact sensitivity(H50)of LNC less than that of the raw NC.The H50value LNC-LN increased from 25.76 to 30.01 cm compared with NCLN, and The H50value LNC-HN increased from 18.02 to 21.84 cm compared with NC-HN.The new insensitive LNC is potential to be used in gun propellants by partly replacing the raw NC.LNC provides an alternative as a binder in LOVA gun propellants and insensitive high energy propellants formulation [52,53].For the potential applications of LNC in gun propellant chemistry, its stability,compatibility,degradation products after forced use and the influence of LNC on the burning and interior ballistic performance of propellants needs to be further studied in the future.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China (No.22075146).