Hamid Reza Pouretedal,Sajjad Damiri,Zynab Karami

Faculty of Applied Sciences,Malek Ashtar University of Technology,Iran

Keywords:HNS explosive Photodegradation Photostabilizer Kinetic rate constant

ABSTRACT The photostabilies of hexanitrostillbene(HNS)were studied at 1% wt.of photostabilizers.The photostabilizers of Chiguard BP-12(UV-531),ADK STAB LA-326(UV-326),ADK STAB LA-32(UV-P),Irganox 1010 and Everstab LS944 were used in this study.A photoreactor including a 30 W monochromatic UV lamp was used for investigation of photodegradations of solid HNS.The changes in surface area of HNS peak in HPLC chromatogram at time intervals of 2,4,6 and 8 h were used for calculation of kinetic rate constants of photodegradation process.The compound of UV-P photostabilizer was found to have the greatest effect on enhancing photostability for HNS among the investigated photostabilizers.The kinetic rate constants were obtained 6.2×10-2 and 4.1×10-2 h-1,respectively,for photodegradation process of HNS in absence and in presence of UV-P.

1.Introduction

2,2′,4,4′,6,6′-hexanitrostillbene(HNS,Fig.1)is an explosive with excellent properties such as thermal stability,impact and shock insensitivity.HNS has been used as an initiating explosive in the exploitation of oil,aviation,and spaceflight fields due to its structural stability and good detonation properties and there are an attractive for study of chemical,physical and practical properties and applicability of it[1,2].

Many energetic materials show the different of degradation pathways that depends on the initiation conditions,such as photo,thermal,shock and or spark[3].The study of photodegradation of energetic materials such as explosives is important for many scientific researches and defence industries.For better prediction of explosives lifetime and them storage conditions,the investigation of the photodecomposition mechanism,photolytic degradation kinetics,the degradation products and identification of intermediates are very important.Additionally,the obtained results from these studies are used to improvement of the performance of explosives in combustion and explosion[4,5].Breaking the bond of C-NO2,elimination of O atom,formation of OH,and isomerization of nitro-nitrite are proposed as an initial step in different mechanisms of photo-induced decomposition reaction for many explosives[6,7].

The ultraviolet(UV)and visible radiations may cause significant decomposition of many materials.Specifically,UV radiation can due to photooxidative degradation of molecular materials through the breaking of the bonds in them[8,9].Photooxidation of explosives is a major cause of irreversible changes.The degradation of explosives may be induced by UV radiation or catalytic process(or both)and can be accelerated at high temperature.The result of this photodegradation is production of radicals and reduction of molecular weight.These changes are due to change of chemical,physical and practical properties of materials and leading to reduction of lifetime and useless them.Exposure to UV radiation may cause the significant degradation of many materials such as dyes and pigments,weathering,plastics and polymers[10].

Photostabilizers are used for reduction of damage of UV radiation to materials such as explosives.Photostabilizers reduce photodamage and to ensure acceptable life times under outdoor exposure conditions.The photostabilization of materials may be resulted in many pathways.The proposed stabilizing mechanisms have been developed that depend on the behavior of stabilizer.These mechanisms are:(i)light screeners,(ii)UV absorbers,(iii)excited-state quenchers,(iv)peroxide decomposers,and(v)radical scavengers.However,among these mechanisms,the excited-state quenchers,peroxide decomposers,and radical scavengers are the most effective[11,12].

Fig.1.Molecular structure of HNS(2,2′,4,4′,6,6′-hexanitrostillbene).

Based on the obtained results,several efforts have been devoted to studying the photodecomposition of some energetic materials involving both cyclic and noncyclic compounds such as TATB,RDX,HMX,CL-20 and HNS[13-15].But,there is not any report about the photostabilization of energetic materials.

In the present work,the photostabilization of HNS was studied by addition of several UV stabilizers.Effect of each stabilizer in increasing of photostabilization of HNS was followed by the study of UV-Vis spectra and HPLC chromatogram of HNS before and after UV-irradiation.

2.Experimental

2.1.Reagents

The HNS III sample(purity;99.7%,melting point;316°C,particles size distribution;5-10μ)was prepared by the Iranian Chemical Company.The solvents of acetonitrile,ethanol and methanol were used as AR grade.Several photostabilizers including UV-531,UV-326,UV-P,Irganox 1010 and Everstab LS944 were purchased from Sigma-Aldrich Company with purity≥99%.The chemical and physical properties of photostabilizers are collected in Table 1.

Table 1The chemical and physical properties of photostabilizers.

UV-531,UV-326 and UV-P are the kinds of high efficient antiaging auxiliary with good performance.Ultraviolet absorbent can adsorb the UV ray in range of 270-380 nm.The features include non-toxicity,good compatibility,low transfer,non-inflammable,non-explosive,non-corrosive matter with good storage stability and easy to process are advantages of these materials.These UV photostabilizers can reduce the color and physical retarding loss.Dosage of them is 0.1-0.5 wt%.in a sample[16].Irganox 1010 is a sterically hindered phenolic antioxidant,a highly effective and nondiscoloring stabilizer for organic substrates such as plastics,synthetic fibers,elastomers,adhesives,waxes,oils and fats.It protects these substrates against thermo-oxidative degradation[17].Everstab LS944 is recommended for various polymers which require high light stabilization[16].

2.2.Procedure

The solid state HNS powders in absence and presence of photostabilizers(1%wt.)were irradiated using a 30 W monochromatic UV lamp(200-390 nm).They were stirred 3 times per hour to provide uniform UV exposure.The experiment was conducted at ambient temperature(nominally 30°C).The analysis of samples was performed in interval times of 2,4,6 and 8 h after irradiation process and the residual concentration of HNS was determined by high performance liquid chromatography(HPLC)methods.

The coating of HNS particles were made by sol-gel method.The particles of HNS were dispersed in ethanol and the dissolved stabilizer in EtOH was added while the sol was vigorously stirred by a mechanical stirrer at temperature of 50°C.After 5 min,the sol was cooled to room temperature and the obtained gel was filtered.The coated HNS particles were dried at temperature of 40°C.

Acetonitrile(AN)solvent with grade liquid chromatography(Merck)was used for preparation of samples of spectroscopy and chromatography tests.UV-visible spectra of HNS in acetonitrile(2 mg/L)were measured and registered using Perkin-Elmer-Lambd II UV-visible spectrophotometer in the 190-600 nm regions,in the presence of a solvent as a background.HPLC chromatograms of HNS in acetonitrile(100 mg/L)were recorded using Agilent 1200 with conditions:detector UV-254 nm,column C-18,mobile phase 60-40 of methanol-water and flow rate 1 mL/min.

3.Results and discussion

3.1.UV-Vis.spectra

Fig.2 shows the UV-Vis.spectra of HNS solution in AN solvent(2 mg/L)at times 0,2,4,6 and 8 h after irradiation process.

Fig.2.UV-Vis.spectra of HNS in AN solvent at times 0(unirritated sample),2,4,6 and 8 h after irradiation process.

The obtained spectra indicate that the maximum absorption peak is appeared at 225 nm that is attributed to theπ→π*transition region of the trans-stilbene,the base chromophore group of HNS.When the HNS is irradiated,two new absorption peaks occur:an intense absorption in the 290-320 nm range with a maximum at 300 nm,and a small band in the 410-440 nm range,centered at 425 nm[3-5].Also,the increasing the absorption intensity in wavelength 225 nm is seen after irradiation of sample.These changes in UV-Vis.spectra may be due to the photochemical reaction of the HNS samples.It is known that many stilbene compounds undergo a photodecomposition of trans-cis photoisomerisation reaction.However it should not affect the photodegradation of HNS which is attributed to the large intramolecular steric hindrance in HNS molecule[15,18,19].

The UV-Vis.spectra of photostabilizers of UV-P,UV-531 and UV-326 are shown in Fig.3.As seen,these compounds show a wide absorbance spectrum in ranges of 200-400 nm.Therefore,based on Figs.2 and 3,there is a strange overlapping between the absorption spectra of HNS and UV stabilizers in ranges 200-400 nm.Thus,it was not possible to follow the rate of photodegradation of HNS in presence of UV stabilizers by UV-Vis.spectrophotometry method.

Fig.3.UV-Vis.spectra of photostabilizers of UV-P,UV-531 and UV-326 in AN solvent.

3.2.HPLC analysis

HPLC chromatogram of HNS solution in AN solvent is shown in Fig.4.

Fig.4.HPLC chromatogram of HNS solution(5% wt.)in AN before irradiation.

The peak in retention time of 4.5 min is related to the trans-HNS,while,the cis-HNS shows a peak in retention time of 5.25 min[20].It is expected to reduce the peak area of HNS after irradiation and photodegradation of it.HPLC chromatogram of HNS after 2 and 8 h irradiation time is indicated in Fig.5.The new peaks in retention times of 1.5,3.3 and 3.5 min and reduction of HNS peak area may be related to the photodegradation of HNS and the new peaks can be related to the photodegradation products of HNS.However,peak area of HNS is used to follow as a signal in study of photodegradation of it.

The rate of photodegradation of HNS is determined by using peak area of HNS after irradiation at times of 2,4,6 and 8 h.The kinetic equation of pseudo-first order(Eq.(1))is used for calculation of kinetic rate constant of photodegradation reaction[21,22].

Fig.5.HPLC chromatogram of HNS solution(5% wt.)in AN solvent after 2 and 8 h irradiation.

Where,Coand Ct,k and t are,respectively,initial amounts of HNS before irradiation,amounts of HNS after irradiation at time of t,kinetic rate constant and time of irradiation.With respect to the relationship between concentration and peak area in HPLC chromatogram,the peak area of HNS before(So)and after(St)irradiation are used in Eq.(1).

The color of samples of HNS as solids and or in solution is changed from light yellow to orange brown when exposed to UV,sunlight and/or room light over a period of several hours(Fig.6).

E.G.Kayser reported that the exposed sample of HNS contained numerous organic compounds including 1,3,5-trinitrobenzene,4,6-dinitro[2,l]benzisoxazole, 2,4,6-trinitrobenazldehyde, 4,6-dinitroisoanthranil and 2,4,6-trinitrobenzoic acid[23].However,the aim of this work was the increasing of photostabilization of HNS by addition of photostabilizers.In Table 2,the kinetic rate constants of HNS photodegradation in absence and in presence of 1 wt% of photostabilizers are collected.Also,the curve of ln(So/St)versus time for samples of HNS and HNS/UV-P is shown in Fig.7.

Fig.6.Color of HNS samples before(A)and after(B)exposed to UV light.

Fig.7.Curves of ln(So/St)versus time for samples of HNS and HNS/UV-P.

Table 2The kinetic rate constants(k)of HNS photodegradation.

Performance of the UV stabilizers is slightly showing in different ways.As usual they take part in a particular section of the degradation chain reaction.For example,the interaction of UV stabilizers with the chain scission mechanism is due to preventing the propagation of the degradation mechanism and preventing a runaway reaction.Therefore,they are known as“free radical scavengers”or“hydroperoxide decomposers”[24].The optimum amount of these additives are typically at very low amounts such as 0.1-0.5% of a sample.UV stabilizers are also used as quencher because they are able to take over the energy absorbed by the chromophore such as conjugated double bonds in the molecule of a sample[25].The energy absorbed is disposed as heat and or fluorescent radiation with lower energy light.The most important class of UV absorbers is the hydroxybenzophenones typically stabilizer such as UV-P[26].

4.Conclusions

HNS explosive has been used in a lot of area and increasing of lifetime of it is very important for manufacturers and consumers.However,degradation of HNS by UV and or sunlight irradiation is a disadvantage and therefore,it is necessary the increasing of its photostabilization.Photostabilizers such as UV-P,UV-531 and UV-326(benzotriazole and benzophenon compounds),could be used for increasing the photostability of HNS.The obtained results showed that the photostability of HNS is increased more than 30%in presence of 1%wt.of UV-P reagent.HPLC technique was used to demonstrate increased stability of the HNS by following peak area changes.

Acknowledgement

We would like to thank the research committee of Malek-ashtar University of Technology(MUT)for supporting this work.