molecules Article Evaluation on the Photosensitivity of 2,20-Azobis(2,4-Dimethyl)Valeronitrile with UV Yi Yang 1 and Yun-Ting Tsai 2,* ID 1 Shaanxi Key Laboratory of Prevention and Control of Coal Fire, School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China; [email protected] 2 School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China * Correspondence: [email protected]; Tel.: +86-029-8266-5836 Received: 3 November 2017; Accepted: 12 December 2017; Published: 14 December 2017 Abstract: Azo compounds have high exothermic characteristics and low thermal stability, which have caused many serious thermal accidents around the world. In general, different locations (e.g., equatorial or polar regions) have different UV intensities. If the azo compound exists in an inappropriately stored or transported condition, the decrease in thermal stability may cause a thermal hazard or ageing. 2,20-Azobis(2,4-dimethyl)valeronitrile (ADVN) is investigated with respect to the thermal stability affected by UV exposure at 0, 6, 12, and 24 h. When ADVN is exposed to 24 h of 2 UV (100 mW/m and 254 nm), T0 is not only advanced, but the mass loss is also increased during the main decomposition stage. In addition, the apparent activation energy and integral procedural decomposition temperature (IPDT) of ADVN exposed to 24 h of UV is calculated by kinetic models. Therefore, the prevention mechanism, thermal characteristics, and kinetic parameters are established in our study. We should isolate UV contacting ADVN under any situations, avoiding ADVN being aged or leading to thermal runaway. This study provided significant information for a safer process under changing UV exposure times for ADVN. Furthermore, the research method may serve as an important benchmark for handling potentially hazardous chemicals, such as azo compounds described herein. Keywords: azo compound; free initiator; calorimetric and product analysis technology; kinetic parameters; environmental protection 1. Introduction In view of loss prevention, the appraisal of thermal characteristics for reactive chemicals has been a major concern of chemical industries. The main reason for many accidents is insufficient knowledge of reactants, intermediates, products, or various catalysts. In general, a small change of temperature or pressure may sporadically cause a serious runaway reaction for highly-energetic chemicals. The most hazardous information can be obtained from the literature, but if this information is scanty, the related safety parameters should be assessed by proper thermal analysis technology [1–3]. 2,20-Azobis(2,4-dimethyl)valeronitrile (ADVN) (the chemical structure is shown in Figure1), a common azo compound, is an excellent free radical supplier for chemical processes. When azo is involved in an exothermic reaction, it can amply provide free radicals and energy for synthesis of organic compounds or polymers, such as styrene, methyl acrylate, epoxy resin, or propylene, as an initiator, cross-linking, or curing agent. However, ADVN has thermal instability and sensitivity because of its –N=N– structure, which may incur violent thermal decomposition by external fire, other igniting sources, or irradiation [4–8]. In the past, many thermal accidents have occurred because the inherent safety of chemicals is not explicit and the accident process is unexpected, which may result in the stored temperature, cooling system, or pressure relief system to be wrongly designed Molecules 2017, 22, 2219; doi:10.3390/molecules22122219 www.mdpi.com/journal/molecules Molecules 2017, 22, 2219 2 of 9 Molecules 2017,, 22,, 22192219 2 of 9 safetyfor safety considerations. considerations. Wanasekara Wanasekara et al. et in al. 2011 in 2011 indicated indicated that thatthe increase the increase in the in temperature the temperature and exposureand exposure time time may may lead lead to surface to surface degradation degradation and and embrittlement embrittlement of of fibers fibers... WangWang Wang etet et al.al. al. inin in 2008 2008 mentioned that the UV light can cause the trans-cis isomerization mechanism of azo compounds (see Figure 22).). WangWang andand WangWang inin 2008 2008 alsoalso indicatedindicated thatthat thethe azoazo withwith aa transtrans isomerisomer hashas highhigh thermalthermal stability and low energy intensity [9–11]. [9–11]. Thus, Thus, it it is crucial that we understand the thermal hazard behaviors for azo which can be usedused efficientlyefficiently andand safely duringduring operating,operating,operating, transportation,transportation, and and storage [12]. [12]. In In the the past, most of the research of ADVN ADVN for for runaway runaway excursion excursion has has been been the the analysis analysis of pure substances. However, the photo effect of ADVN has has rarely beenbeen explored.explored. Since Since ADVN ADVN has has extreme photosensitivity with UV, thethe decompositiondecompositionion mechanismmechanism maymay be changed changed with with the the exposureexposure timetime and and mass mass [13]. [[13].13]. Often,Often, UVUV UV irradiationirradiation irradiation existsexists exists ubiquitously,ubiquitously, ubiquitously, and and even even its its intensityintensity maymay bebe differentdifferent between locationlocation latitudes.latitudes. Simultaneously,Simultaneously, if if the the optical opticaltical isolation isolationisolation of ofof UV UVUV for forfor the thethe package packagepackage or oror container containercontainer is isnotis notnot complete, complete,complete, the the photothe photophoto effect effecteffect may decreasemaymay decreasedecrease the thermal thethe thermalthermal stability stabilitystability of ADVN, ofof resultingADVN,ADVN, resultingresulting in the increased inin thethe increasedriskincreased of a thermal riskrisk ofof a runawaya thermalthermal reaction runawayrunaway [14 reactionreaction]. [14].[14]. Figure 1. ChemicalChemical structure of ADVN. Figure 2. LightLight effect effect for the trans-cis isomerization mechanism of ADVN. The calorimetrycalorimetry andand kinetickinetic modelsmodels are are used used to to analyze analyze the the thermal thermal stability stability parameters, parameters, such such as asapparent apparent onset onset temperature temperature (T (0T),00),), peak peakpeak temperature temperaturetemperature (T ((Tp),pp),), final finalfinal temperature temperaturetemperature ( ((TTfff),),), apparentapparent activation activation energy ( Eaa),),), frequencyfrequency frequency factorfactor (ln(ln k00),),), andand and reactionreaction reaction orderorder order (( (nn),),), toto establishestablish thethe hazardhazard characteristicscharacteristics and and toto explore explore thethe decompositiondecomposition products products by by employingemployemployinging differential differential scanningscanning calorimetrycalorimetry (DSC) (DSC) and and thermogravimetrythermogravimetry (TG) (TG) forfor determinationdeterminatiodetermination of UV effectseffects onon ADVNADVNADVN [[15,16].[15,16].15,16]. 2. Results Results and and Discussion 2.1. TG Results Figure3 3 showsshows thethe massmass lossloss versusversus temperaturetemperature diagramdiagram fromfrom TGTG testingtesting forfor ADVNADVN andand ADVNADVN ◦ with different exposure durations of 6, 12 andand 2424 hh ofof UVUV irradiationirradiation atat aa heatingheating raterate 2020 °C/min.C/min. There areare two two decomposition decomposition stages stages for ADVN for ADVN and ADVN andand exposedADVNADVN toexposedexposed UV, and to theto UV,UV, main andand decomposition thethe mainmain decompositionstage is the second stage stage. is the We second found stage. that whenWe foun ADVNd that was when exposed ADVN to was 6 and exposed 12 h of to UV, 6 and the profiles12 h of of the TG curve are not obviously different between each other, and for both T is 78 ◦C. However, UV, the profiles of the TG curve are not obviously different between each other, and0 for both T00 isis 7878 °C.°C. although the T of the first stage of ADVN is close to ADVN exposed to 24 h of UV, the T of the However, although0 the T00 ofof thethe firstfirst stagestage ofof ADVNADVN isis closeclose toto ADVNADVN exposedexposed toto 2424 hh ofof UV,UV,0 thethe T00 ◦ ◦ ofsecond the second decomposition decomposition stage is stage curtailed is curtailed from 132 fromC to 132 127 °CC to and 127 the °C massand the loss mass is increased loss is increased from 70% to 80%. According to the literature [17], T and mass loss can be used to determine the thermal stability fromfrom 70%70% toto 80%.80%. AccordingAccording toto thethe literatureliterature0 [17],[17], T00 andand massmass lossloss cancan bebe usedused toto determinedetermine thethe 2 thermalforthermal chemicals. stabilitystability Therefore, forfor chemicals.chemicals. if ADVN Therefore,Therefore, is exposed ifif ADVN toADVN a UV isis intensity exposedexposed of toto 100 aa UVUV mW/m intensityintensityfor over ofof 100100 24 mW/m h,mW/m the onset22 forfor over 24 h, the onset temperature and temperature of the maximum mass loss can decrease, causing abnormal aging or lessening the thermal stability during preparation, storage, transportation, and operating conditions. Molecules 2017, 22, 2219 3 of 9 temperature and temperature of the maximum mass loss can decrease, causing abnormal aging or Molecules 2017, 22, 2219 3 of 9 lesseningMolecules 2017 the, 22 thermal, 2219 stability during preparation, storage, transportation, and operating conditions.3 of 9 ADVN (Without