coatings Article
Kinetic Research on the Curing Reaction of Hydroxyl-Terminated Polybutadiene Based Polyurethane Binder System via FT-IR Measurements Jiahu Guo 1,2 ID , Tao Chai 1, *, Yucun Liu 1, *, Jianlan Cui 1 , Hui Ma 1, *, Suming Jing 1 , Lunchao Zhong 1,3 , Shengdong Qin 1 , Guodong Wang 1,4 and Xiang Ren 2,5, * 1
2 3 4 5
*
Chemical Engineering and Environment College, North University of China, Taiyuan 030051, China;
[email protected] (J.G.);
[email protected] (J.C.);
[email protected] (S.J.);
[email protected] (L.Z.);
[email protected] (S.Q.);
[email protected] (G.W.) Department of ESH, Science and Technology University of Sichuan Staff, Chengdu 610101, China Graduate School at Shenzhen, Tsinghua University, Shenzhen 518000, China School of Ammunition Engineering, Army Engineering University, Shijiazhuang 050003, China College of Engineering, Sichuan Normal University, Chengdu 610101, China Correspondence:
[email protected] (T.C.);
[email protected] (Y.L.);
[email protected] (H.M.);
[email protected] (X.R.); Tel.: +86-288-481-8712 (T.C.)
Received: 5 April 2018; Accepted: 30 April 2018; Published: 4 May 2018
Abstract: Polyurethane binder systems based on hydroxyl-terminated polybutadiene (HTPB) possess several superior properties such as superior adhesion, high solid-loading capacity, outstanding mechanical performance, etc. They have been widely used in coatings and adhesives as well as in medical and military industries. The cure reaction between hydroxyl-terminated polybutadiene (HTPB) and diisocyanates plays a key role in the properties of final products as well as the adjustment of process parameters. FT-IR spectroscopy is applied to investigate the kinetics of the curing reaction of HTPB and isophorone diisocyanate (IPDI) in the presence of a low toxic and low viscosity catalyst, stannous isooctoate (TECH). The concentrations of the isocyanate groups (NCO) characterized by FT-IR during the cure reaction with respect to time were recorded at different temperatures and at constant stoichiometric ratio Rn[NCO]/n[OH] = 1.0. The kinetic parameters, i.e., activation energy (Ea ), pre-exponential factor (A), activation enthalpy (∆H) and activation entropy (∆S) were determined. In addition, the curing process and mechanism of the HTPB-IPDI reaction are discussed. Keywords: HTPB; binder; cure reaction; kinetics; mechanism
1. Introduction Hydroxyl-terminated polybutadiene (HTPB) is a form of liquid telechelic rubber with different molecular weight (approximately 1500–10,000 g·mol−1 ) and a high level of reactive functionality [1]. The liquid rubber possesses a unique combination of properties including a low glass transition temperature, low temperature flexibility, high solid-loading capacity, and excellent flow ability. They have been widely used in adhesives, coatings, sealants, medicine, as well as energetic materials [2–6]. As a result of the dimensional stability and structural integrity inherent from the cured polyurethane (PU) network, a HTPB-based binder system has found extensive applications in polymer-bonded explosives (PBXs), underwater weapons, composite solid propellants, and missile and launch vehicle technology [7–9]. The curing reaction of the polyurethane binder system which is based on HTPB and various diisocyanates (TDI Toluene diisocyanate, IPDI Isophorone diisocyanate, HDI Hexamethylene diisocyanate, etc.) to form a PU network (as shown in Figure 1), plays an important role in the manufacturing process as well as the properties of final products [9–13]. Coatings 2018, 8, 175; doi:10.3390/coatings8050175
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is based on HTPB and various diisocyanates (TDI Toluene diisocyanate, IPDI Isophorone 2 of 9 diisocyanate, HDI Hexamethylene diisocyanate, etc.) to form a PU network (as shown in Figure 1), plays an important role in the manufacturing process as well as the properties of final products [9– The reaction and mechanism of the cure reaction the physicochemical properties 13]. kinetic The kinetic reaction and mechanism of the cure determine reaction determine the physicochemical and mechanical performance of the cured product [14]. Therefore, knowledge of the kinetics of properties and mechanical performance of the cured product [14]. Therefore, knowledge of the the polyurethane-curing reaction is of utmostisimportance for the design anddesign preparation of specific kinetics of the polyurethane-curing reaction of utmost importance for the and preparation products suitable properties of specificpossessing products possessing suitable [15–17]. properties [15–17]. In recent several publications havehave reported on theon research of the kinetics PU formation recentyears, years, several publications reported the research of the ofkinetics of PU through use of the viscosity waves, microcalorimetry, differentialdifferential scanning formationthe through use ofbuildup, viscosityultrasonic buildup, ultrasonic waves, microcalorimetry, calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and(FTIR), nuclear magnetic scanning calorimetry (DSC), Fourier-transform infrared spectroscopy and nuclearresonance magnetic (NMR) measurements [18–25]. Although there are several to study kinetics, FT-IR is resonance (NMR) measurements [18–25]. Although there areways several ways curing to study curing kinetics, known a very reliable method well suited for monitoring the curing process of the FT-IR isas known asrapid a veryand rapid and reliable method well suited for monitoring the curing process of PU formation. Such a test can reveal the extent of functional groups (such as the NCO group) the PU formation. Such a test can reveal the extent of functional groups (such as the NCO consumption and during the cure reaction [21–25]. In this work, FT-IR measurement is applied to study with stannous isooctoate (TECH) as aascatalyst. Through the study the the cure curereaction reactionofofHTPB HTPBand andIPDI IPDI with stannous isooctoate (TECH) a catalyst. Through study of the PU formation kinetics, an insight into the curing reaction may be obtained. In addition, the study of the PU formation kinetics, an insight into the curing reaction may be obtained. In the kineticthe parameters, i.e., the pre-exponential factor (A), rate constant (k),constant activation (Ea ), addition, kinetic parameters, i.e., the pre-exponential factor (A), rate (k),energy activation activation enthalpy (∆H), and activation (∆S)entropy for the (ΔS) cure for reaction arereaction calculated. energy (Ea), activation enthalpy (ΔH), andentropy activation the cure are calculated. Coatings 2018, 8, 175
Figure 1. Schematic diagram of polyurethane network formation from from HTPB HTPB and and IPDI. IPDI.
2. Materials and Methods 2.1. Materials Hydroxyl-terminated (HTPB, number-average molecular weight weight = 2700, hydroxyl Hydroxyl-terminatedpolybutadiene polybutadiene (HTPB, number-average molecular = 2700, value = 45.2 mg KOH/g) obtained Liming Research Chemical hydroxyl value = 45.2 mgwas KOH/g) was from obtained from LimingInstitute ResearchofInstitute of Industry Chemical(Luoyang, Industry China). Curing agent, isophorone diisocyanate (IPDI) was (IPDI) supplied Bayer (Leverkusen, Germany). (Luoyang, China). Curing agent, isophorone diisocyanate wasbysupplied by Bayer (Leverkusen, Catalyst, stannous isooctoate (TECH) was purchased from Sinopharm Chemical Reagent (Shanghai, Germany). Catalyst, stannous isooctoate (TECH) was purchased from Sinopharm Chemical Reagent China). Potassium (KBr) was delivered Merck (Darmstadt, Germany). Germany). (Shanghai, China). bromide Potassium bromide (KBr) wasbydelivered by Merck (Darmstadt, HTPB was dried under vacuum at 105 ◦°C C for 2 h then put under under aa nitrogen nitrogen atmosphere atmosphere before before use. Other materials were used as received. received.
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2.2. Sample Preparation
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HTPB and IPDI were precisely weighed with a stoichiometric ratio Rn[NCO]/n[OH] = 1.0 then 2.2. Sample Preparation mixed for 2 min at high-speed stirring (2500 rpm) by a mechanical stirrer. At the beginning of the blending process, the corresponding ofwith catalyst TECH was ratio added to the system. Themixed materials HTPB and IPDI were preciselyamount weighed a stoichiometric Rn[NCO]/n[OH] = 1.0 then 2 min high-speed stirringand (2500the rpm) by a mechanical stirrer. At the beginning the blending were for mixed atatroom temperature concentration of the catalyst used in the of binder system was process, the corresponding of all catalyst TECH samples was added to the system. The materials were 0.1 wt %. Having thoroughlyamount blended reactants, were prepared for immediate kinetic mixed at room temperature and the concentration of the catalyst used in the binder system was 0.1 measurement. The polymerization reaction was performed not in anhydrous conditions and not under wt %. Having thoroughly blended all reactants, samples were prepared for immediate kinetic nitrogen atmosphere. measurement. The polymerization reaction was performed not in anhydrous conditions and not under nitrogen atmosphere. 2.3. Fourier-Transform Infrared (FT-IR) Spectroscopy Analysis
The curing reaction Infrared between HTPB and IPDI Analysis was investigated by measuring the changes of the 2.3. Fourier-Transform (FT-IR) Spectroscopy −1 in the FT-IR spectroscopy. The infrared spectra characteristic peaks of isocyanate groups (~2260 cm The curing reaction between HTPB and IPDI was) investigated by measuring the changes of the of thecharacteristic samples were analyzed by a Perkin Elmer 100 MA, USA). Measurements −1) in the FT-IR (Waltham, peaks of isocyanate groups (~2260 cmSpectrometer spectroscopy. The infrared spectra were of carried out by the transmittance in theElmer range100 of 4000–400 cm−1(Waltham, . Approximately 60 mg of the samples were analyzed bymode a Perkin Spectrometer MA, USA). −1. MPa Measurements carried by the mode in by theexerting range ofa pressure 4000–400 of cm25 crystalline KBr waswere pressed into out a wafer withtransmittance a diameter of 1.3 cm Approximately 60 mg of crystalline KBr was pressed into a wafer with a diameter of 1.3 cm by for 1 min. A tiny drop of the binder mixture was coated between two KBr windows right after the exerting a pressureThe of 25samples MPa formeasured 1 min. A tiny drop of the binder coated between twopreset sample was prepared. in the FT-IR test weremixture kept inwas a vacuum oven. The KBr windows right after the sample was prepared. The samples measured in the FT-IR test were kept temperature was in the range of 35–75 ◦ C with an accuracy of ±0.1 ◦ C. The oven was located close to in a vacuum oven. The preset temperature was in the range of 35–75 °C with an accuracy of ±0.1 °C. the FTIR Spectrometer to minimize thermal transients during the handling of the FT-IR wafer. The cure The oven was located close to the FTIR Spectrometer to minimize thermal transients during the reaction was monitored by testing the infrared spectra of the sample at appropriate time intervals with handling of the FT-IR wafer. The cure reaction was monitored by testing the infrared spectra of the respect to the temperature. sample at reaction appropriate time intervals with respect to the reaction temperature. 3. Results andand Discussion 3. Results Discussion 3.1. Determination of the Reaction Rate 3.1. Determination of the Reaction RateConstants Constants ◦ in the presence of The FT-IR spectra versus the time curingreaction reaction at 75 The FT-IR spectra versus the timeofofthe theHTPB-IPDI HTPB-IPDI curing at 75 °C inCthe presence of TECHTECH (0.1 wt is shown (0.1%) wtas %)catalyst as catalyst is shownininFigure Figure2. 2.
Figure 2. FT-IR spectra versus time HTPB-IPDI reaction reaction at TECH (0.1(0.1 wt %) Figure 2. FT-IR spectra versus time ofofHTPB-IPDI at75 75°C◦ Cwith with TECH wtas %)catalyst. as catalyst.
From the spectra, one can find that, as the reaction proceeded, the intensity of the characteristic
From the spectra, one−1 can find that, as the reaction proceeded, the intensity of the characteristic peaks around 2260 cm (NCO stretching) decreased and ultimately disappeared once the reaction peaks around 2260 cm−1 (NCO stretching) decreased and ultimately disappeared once the reaction was totally completed while the C=C stretching bands around 1640 cm−1 remained practically unaltered.
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was totally completed while the C=C stretching bands around 1640 cm−1 remained practically A new characteristic absorption peak in the unique of range 1710–1760 cm−1 was to the −1 was attributed unaltered. A new characteristic absorption peak in therange unique of 1710–1760 cmattributed C=O stretching vibration, which confirmed the formation of the polyurethane group during the cure to the C=O stretching vibration, which confirmed the formation of the polyurethane group during reaction [25]. To ensure the that quantitative results were independent of the thickness of the sample the cure reaction [25]. Tothat ensure the quantitative results were independent of the thickness of the 1 between two KBr wafers, the change in the intensity of the NCO stretching band of IPDI at 2260 cm− sample between two KBr wafers, the change in the intensity of the NCO stretching band of IPDI at was monitored and divided bydivided the intensity the reference at 1640 cm−at1 for the C=C stretching −1 for 2260 cm−1 was monitored and by theofintensity of theband reference band 1640 cm the C=C in HTPB [21]. According to the Lambert-Beer law, a given peak area is directly proportional to the stretching in HTPB [21]. According to the Lambert-Beer law, a given peak area is directly proportional reagent concentration. Thus, the conversion of NCO can be derived from the FT-IR spectra recorded as to the reagent concentration. Thus, the conversion of NCO can be derived from the FT-IR spectra a functionasofa time, which can bewhich calculated the FT-IR data [25]: data as [25]: recorded function of time, can befrom calculated from theasFT-IR C2260 C C C2260 2260 2260 −− C C C16401640 1640 T=0 1640 C T= t T= t T=0 PNCO (1) PNCO== (1) C 2260 C2260 C1640 T=0 C1640 T=0 where P PNCO representsthe theconversion conversionof ofisocyanate isocyanategroups, groups,CC represents represents the the absorbance absorbance values values at at NCOrepresents where −1 /1640 cm−1 in the FT-IR spectra, and the subscripts of 0 and t denote reaction times. 2260 cm 2260 cm−1/1640 cm−1 in the FT-IR spectra, and the subscripts of 0 and t denote reaction times. The rate rate constant constant for for the the PU PU formation formation between between HTPB HTPB and and IPDI IPDI was was obtained obtained from from the the slope slope of of The the straight line by plotting the P/(1 − P) versus time according to the following equation [26]: the straight line by plotting the P/(1 − P) versus time according to the following equation [26]: PNCO PNCO == k tk t 1− 1 −PPNCO NCO
(2) (2)
where PNCO theconversion conversionofofNCO NCOgroup, group,kkisisthe therate rateconstant, constant,t tisisthe thecuring curingtime. time. NCOisisthe ◦ ◦ Five different different temperatures temperatures (from (from 35 35 °C C to to 75 °C) C) were were conducted conducted to to calculate calculate the the kinetic kinetic Five parameters of the curing reaction, as shown in Figure 3. 1.0 0.9 0.8 0.7
P / (1-P)
0.6
Curing Temperature o 35 C o 45 C o 55 C o 65 C o 75 C
0.5 0.4 0.3 0.2 0.1 0.0 0
300
600
900
1200
1500
1800
2100
2400
t (min)
Figure 3. The plot of P/(1 − P) versus time at various curing temperatures for the HTPB-IPDI Figure 3. The plot of P/(1 − P) versus time at various curing temperatures for the HTPB-IPDI binder system. binder system.
Clearly, the profile of the cure reaction of the HTPB-IPDI binder system showed a discontinuity Clearly, the profile of the cure reaction of the HTPB-IPDI binder system showed a discontinuity at each temperature. Two steps were featured, which included a faster initial step (i.e., Step 1 with at a each temperature. Two steps were featured, which included a faster initial step (i.e., Step 1 with a rate rate constant k1) and a slower consecutive step (i.e., Step 2 with a rate constant k2). The corresponding constant k1 ) and a slower consecutive data for both steps are listed in Table step 1. (i.e., Step 2 with a rate constant k2 ). The corresponding data for both steps are listed in Table 1.
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Table 1. Rate constants for the polymerization of HTPB-IPDI system at different isothermal curing temperatures in the presence of TECH (0.1 wt %) as catalyst. T (◦ C)
k1 ·103 (min−1 )
R-Square
k2 ·103 (min−1 )
R-Square
35 45 55 65 75
0.1503 0.2871 0.4850 0.7177 1.2300
0.9764 0.9960 0.9890 0.9668 0.9482
0.0519 0.1356 0.2338 0.3578 0.6618
0.9948 0.9791 0.9761 0.9948 0.9962
k1(T) /k1(35) k2(T) /k2(35) 1.00 1.91 3.23 4.77 8.18
1.00 2.61 4.50 6.89 12.74
k1 /k2 2.89 2.12 2.07 2.01 1.86
T = Curing Temperature; k1 , k2 = rate constant for step 1 and step 2 from FT-IR measurement.
The differences in the rate constants in different steps indicate that the polymerization between HTPB and IPDI obeyed second-order kinetics. Notably, both constants (k1 , k2 ) increase with the increase of curing temperature from 35 ◦ C to 75 ◦ C. On the other hand, the ratio of k1 /k2 showed a decreasing trend as the temperature increased, from 2.89 (35 ◦ C) decreased to 1.86 (75 ◦ C). This result is similar to Lucio’s research [14], in the presence of (ferrocenylbutyl) dimethylsilane, the ratio of k1 /k2 for HTPB-IPDI system decreased from 2.39 (50 ◦ C) to 1.60 (80 ◦ C). From the ratio of k1(T) /k1(35) and k2(T) /k2(35) , the catalytic effect in Step 2 was pronounced than that of Step 1 with the an increase in curing temperature. For example, the value of k1(75) /k1(35) was 8.18, whereas k2(75) /k2(35) was 12.74. The results indicate that k2 was more sensitive to the curing reaction temperature in the presence of catalyst TECH. In other words, when the curing temperature increased, k2 increased higher than k1 . This behavior may have been attributable to the catalytic activity of the organometallic compound (such as TECH) on the different NCO groups (such as primary NCO and secondary NCO) which was not identical. The addition of this type of catalyst may have expanded the disparity of the reactivity of the non-equivalence NCO groups in the asymmetric diisocyanates (such as IPDI and TDI) and, consequently, the rate constants of the two steps in the cure reaction were different. In the previous literature, similar behavior was also observed by Burel, whose research involved an analogous system in the presence of dibutyltin dilaurate (DBTDL) as catalyst [23]. 3.2. Kinetic Studies by FT-IR Measurements The thermodynamic parameters, such as pre-exponential factor (A), activation energy (Ea ), activation entropy (∆S), and activation enthalpy (∆H), were determined from the Arrhenius equation and Eyring equation: Ea + ln A (3) ln k = − RT ln
k ∆H ∆S R = − + + ln T RT R Nh
(4)
where k is the reaction constant, T is the curing temperature (K), R is the universal gas constant (R = 8.314 J·mol−1 ·K−1 ), N is Avogadro constant (N = 6.022 × 1023 mol−1 ) and, h is Planck constant (h = 6.626 × 10−34 J·s). From the Arrhenius plot (Figure 4a), the activation energy (Ea ) and pre-exponential factor (A) are computed with a first-order liner fitting. A similar evaluation of the kinetic data in the Eyring plot (Figure 4b) was adopted to obtain the value of activation entropy (∆S) and activation enthalpy (∆H). The values of A, Ea , ∆S, and ∆H are computed and listed in Table 2. Table 2. Thermodynamic parameters for the reaction of HTPB-IPDI system. Ea1
Ea2
(kJ·mol−1 ) 45.80
54.23
A1
A2
(L·mol−1 ·min−1 ) 8854
92,745
∆H1
∆H2
(kJ·mol−1 ) 43.01
53.51
∆S1
∆S2
(J·mol−1 ·K−1 )
−178.44
−158.91
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-6.5
-12.5
k1 for stage 1
k1 for stage 1
k2 for stage 2
-7.0
k2 for stage 2
-13.0
-7.5
-13.5
ln k
ln (k/T)
-8.0 -8.5
-14.0
-14.5
-9.0 -15.0
-9.5 -15.5
-10.0 2.85
2.90
2.95
3.00
3.05
3.10 -1
1000/T (K )
(a) Arrhenius plot
3.15
3.20
3.25
3.30
2.85
2.90
2.95
3.00
3.05
3.10
3.15
3.20
3.25
3.30
-1
1000/T (K )
(b) Eyring plot
Figure4.4.Arrhenius Arrheniusand and Eyring Eyring plots for and IPDI. Figure for the thecuring curingreaction reactionbetween betweenHTPB HTPB and IPDI.
As seen in Table 2, the activation energy of Ea1 (45.80 kJ·mol−1) was lower than Ea2 (54.23 kJ·mol−1), As seen in Table 2, the activation energy of Ea1 (45.80 kJ·mol−1 ) was lower than Ea2 (54.23 kJ·mol−1 ), relating to its faster urethane reaction rate k1 which was much higher than k2. The pre-exponential relating to its faster urethane reaction rate k1 which was much higher than k2 . The pre-exponential factor factor (also called frequency factor) for Step 1 was approximately A1 = 8854, and A2 = 92,745 for the (also factor) Step 1 the waskinetic approximately A1 = 8854, and caused A2 = 92,745 for reaction the Step 2. Stepcalled 2. Onefrequency explanation may for have been control mechanism which the cure One explanation mayHTPB have in been kinetic which caused the cure reaction between between IPDI and the the initial step control (Step 1);mechanism concurrently in the subsequent step (Step 2), the IPDI and HTPB in the initial step (Step 1); concurrently in the subsequent step (Step 2), the reaction was reaction was largely governed by a diffusion process [24]. largelyAs governed by a diffusion process [24]. −1 seen in Table 2, the activation enthalpy was found to be 43.01 kJ·mol (ΔH1) and 51.51 −1 (∆H ) and As −1seen Table 2, theentropy activation enthalpy wasabout found be 43.01 kJ−1·mol kJ·mol (ΔH2).inThe activation for the Step 1 was ΔS1to = −178.44 J·mol ·K−1, and ΔS 12 = − 1 −1 ·K−1 , 51.51 kJ·mol (∆H activation entropy for the Step 1and wasentropy about values ∆S1 = − 178.44 −158.91 J·mol−1 ·K−12 ). for The the Step 2. Clearly, activation enthalpy are usefulJ·mol to reveal and ·mol−1 ·K−1 fortheory, the Step 2. Clearly, activation enthalpy and entropy valuesofare the∆S mechanism. In Jtransition-state activation entropy is usually considered as the degree 2 = −158.91 disorder in the system Large negative values of theentropy activation entropyconsidered indicate that useful to reveal thereaction mechanism. In [27]. transition-state theory, activation is usually as the energy be partitioned into system a lesser [27]. state Large at the negative transition-state, illustrates entropy an associative degree of must disorder in the reaction values which of the activation indicate mechanism [28].beInpartitioned comparisoninto to the activation values of the which two steps, a larger that energy must a lesser state atentropy the transition-state, illustrates annegative associative value of ΔS 1 was found, which was related to the easier formation of the transition state. mechanism [28]. In comparison to the activation entropy values of the two steps, a larger negative value of ∆S1 was found, which was related to the easier formation of the transition state. 3.3. Analysis of Curing Mechanism of HTPB-IPDI Binder System 3.3. Analysis of Curing Mechanism Binder System which occurred during the curing To understand the causes of of HTPB-IPDI the two-step phenomenon reaction between HTPB and IPDI, aspects need to be analyzed. Firstly,during one must consider the To understand the causes of thetwo two-step phenomenon which occurred the curing reaction chemical structures of the HTPB and IPDI used in the experiment. Generally, a HTPB microstructure between HTPB and IPDI, two aspects need to be analyzed. Firstly, one must consider the chemical comprises approximately 60%used trans-1,4-structure, 20% cis-1,4-structure andmicrostructure 20% vinyl-1,2-structure structures ofof the HTPB and IPDI in the experiment. Generally, a HTPB comprises [1]. In theory, the reactivity of OH groups in HTPB should be different. However, thanks to the of approximately 60% trans-1,4-structure, 20% cis-1,4-structure and 20% vinyl-1,2-structure [1]. In theory, relatively large molecular weight, the reactivity of all OH groups in HTPB were largely similar and the reactivity of OH groups in HTPB should be different. However, thanks to the relatively large were not likely to cause significant differences in reaction rates. The other reactant of the curing molecular weight, the reactivity of all OH groups in HTPB were largely similar and were not likely system was IPDI, which possesses two asymmetric NCO groups, as shown in Figure 5a. The to cause significant differences in reaction rates. The other reactant of the curing system was IPDI, commercial IPDI usually contains a ratio of 25~30% trans-isomer and 70~75% cis-isomer, thus, there which possesses two asymmetric NCO groups, as shown in Figure 5a. The commercial IPDI usually are two non-equivalent NCO groups were in four unequal positions [29], as shown in Figure 5b. contains of only 25~30% and 70~75% thus, there tworate non-equivalent Asaaratio result, the trans-isomer structure of IPDI, the eightcis-isomer, different products, andare eight constants NCO groups were in four unequal positions [29], as shown in Figure 5b. needed be considered. It is obviously that the PU formation of IPDI and HTPB was too complex to As a ifresult, only the structure oftoIPDI, the eight different products, and eight according rate constants analyze no assumptions were made simplify the reaction system [14]. Fortunately, to needed be considered. It is obviously that the PU formation of IPDI and HTPB was too complex to the study of Rochery, the main position of secondary NCO group was in the horizontal direction, analyze if nothe assumptions were madewas to simplify the reaction [14]. according therefore, major isomer of IPDI cis-horizontal-NCO sec, system as shown in Fortunately, Figure 5b with ellipses to the[29]. study Rochery, the main position ofinsecondary group was the horizontal direction, Theof relative reactivity of NCO groups IPDI couldNCO be simplified andin attention was focused on therefore, the major isomerbetween of IPDI was cis-horizontal-NCO , as shown in Figure 5b with ellipses [29]. the difference in activity primary NCO and secondary NCO. As a result of the larger steric sec effect of the cyclohexane ring groups and a-substituted methyl, the reactivity primarywas NCO was much The relative reactivity of NCO in IPDI could be simplified andof attention focused on the lower than secondary NCO. primary NCO and secondary NCO. As a result of the larger steric effect difference in activity between of the cyclohexane ring and a-substituted methyl, the reactivity of primary NCO was much lower than secondary NCO.
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Figure5.5.Structure Structureof of(a) (a) IPDI IPDI and and (b) Figure (b) four four isomers isomersofofIPDI. IPDI. Figure 5. Structure of (a) IPDI and (b) four isomers of IPDI.
On the other hand, it was important to interpret all the reactions including simultaneous and On the other hand,which it was important to interpret all the reactions includingphenomenon simultaneous and competitive occurred during two steps. From the experimental and On the reactions other hand, it was important to the interpret all the reactions including simultaneous and competitive reactions which occurred during the two steps. From the experimental phenomenon and the simplified assumptions HTPB and IPDI, proposed the reaction mechanism of HTPB-IPDI competitive reactions whichof occurred during thewe two steps. From the experimental phenomenon and thebinder simplified assumptions HTPB6.and IPDI, we proposed the reaction mechanism HTPB-IPDI system, as shown inofFigure There were two main steps the curing process of ofofHTPB-IPDI HTPB-IPDI the simplified assumptions of HTPB and IPDI, we proposed the in reaction mechanism binder system, as shown in Figure 6. There were two main steps in the curing process of HTPB-IPDI binder system, system. as In shown Step 1, in two moles6.of secondary NCO groups reactedprocess with two OH groups binder Figure There were two main stepsininIPDI the curing of HTPB-IPDI binder system. In Step 1, two moles of secondary NCO groups in IPDI reacted with two OHlarge, groups in one mole HTPB. At the beginning of the curing reaction, the rate constant k 1 was relatively binder system. In Step 1, two moles of secondary NCO groups in IPDI reacted with two OH groups in one mole HTPB. AtAtthe beginning of reaction, the rate rate constant relatively large, mainly caused by kinetic mechanism. In Step 2, the primary NCO groups inwas IPDI reactedlarge, with in one mole HTPB. thecontrol beginning ofthe thecuring curing reaction, the constant k1k1was relatively mainly caused by kinetic control mechanism. In Step 2, the primary NCO groups in IPDI reacted with OH groups in HTPB and formed a three-dimensional network structure polyurethane, given the rate mainly caused by kinetic control mechanism. In Step 2, the primary NCO groups in IPDI reacted with constant k 2 in this step is relatively small. OHOH groups in in HTPB and network structure structurepolyurethane, polyurethane,given given rate groups HTPB andformed formedaathree-dimensional three-dimensional network thethe rate constant k in this step is relatively small. 2 constant k2 in this step is relatively small. O
O
HTPB
IPDI NCO
HTPB
IPDI
NCO
+ NCO
+ NCO
HO HO
OH
k1
nOH
k1
HN
O C
O
HN
C
O
O
O C
NH
nO
C
NH
n
NCO
NCO NCO
n
NCO O
O
HN HN
O C
O O
C
O
O C
NH
nO
C
NH
n
NCO
+
HO
+
HO
NCO
OH
k2
nOH
k2
n
NCO O
NCO O
HN
O C
O
HN
C
OO NH NH
4. Conclusions
C O O C O
O
O C
NH
nO
C
NH
O OH
n
OH
NH
C O
O
n OH
NH
C
O
n OH n
n Figure 6. Curing process of HTPB-IPDI binder system.
Figure6.6.Curing Curingprocess process of of HTPB-IPDI HTPB-IPDI binder Figure bindersystem. system.
4. Conclusions In this work, FT-IR was applied to monitor the cure reaction of HTPB and IPDI with TECH used 4. Conclusions as a catalyst. The presence ofapplied two steps in the polyurethane formation wasand clearly theused rate In this work, FT-IR was to monitor the cure reaction of HTPB IPDIobserved, with TECH In this work, FT-IR was applied to monitor the cureinreaction of HTPB and IPDI Since with TECH used as a constant was different in the first and second steps all curing temperatures. experimental as a catalyst. The presence of two steps in the polyurethane formation was clearly observed, the rate catalyst. presence two steps in and the polyurethane formation was clearly observed, rate constant resultsThe showed twoofbroken lines, accordingly, all the results are followed thisthe trend. Other constant was different in the first second steps in other all curing temperatures. Since experimental wasresults different in thetwo firstbroken and second in all curing temperatures. experimental showed showed lines,steps accordingly, all the other resultsSince are followed this results trend. Other
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two broken lines, accordingly, all the other results are followed this trend. Other kinetic parameters, such as activation energy (Ea ), activation entropy (∆S), and activation enthalpy (∆H), were determined from the Arrhenius formula and Eyring equation. All parameters had two values, with each standing for different steps of the curing reaction. This phenomenon was largely attributable to the two asymmetric isocyanate groups in IPDI, which was expected to lead to two different polymerization rates. Author Contributions: T.C. and Y.L. conceived and designed the experiments; J.G., H.M., J.C., S.J., L.Z. and S.Q. performed the experiments; H.M. analyzed the data; G.W. and X.R. contributed reagents/materials/analysis tools; J.G. and H.M. wrote the paper. Funding: This research was funded by NSAF grant number U1330131 and Sichuan Safety Supervision Bureau Safety Production Technology Project grant number aj20170518104846. Acknowledgments: This work was supported by NSAF (Grant No. U1330131) and Sichuan Safety Supervision Bureau Safety Production Technology Project (Grant No. aj20170518104846). Conflicts of Interest: The authors declare no conflict of interest.
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