Reconstruction of the Microstructure of Cyanate Ester Resin by Using
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applied sciences Article Reconstruction of the Microstructure of Cyanate Ester Resin by Using Prepared Cyanate Ester Resin Nanoparticles and Analysis of the Curing Kinetics Using the Avrami Equation of Phase Change Hongtao Cao, Beijun Liu, Yiwen Ye, Yunfang Liu * and Peng Li * Research Institute of Carbon Fiber and Composite Materials, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (H.C.); [email protected] (B.L.); [email protected] (Y.Y.) * Correspondence: [email protected] (Y.L.); [email protected] (P.L.); Tel.: +86-010-644-34914 (Y.L.) Received: 4 May 2019; Accepted: 5 June 2019; Published: 10 June 2019 Featured Application: The addition of the BADCy resin nanoparticles can accelerate the formation of the microgels in the BADCy resin prepolymer. The kinetic paraments of systems with different BADCy resin nanoparticles contents were obtained using the Avrami equation and Arrhenius equation and explained the formation and growth of the microgels. Abstract: Bisphenol A dicyanate (BADCy) resin nanoparticles were synthesized by precipitation polymerization and used to modulate the microstructure of the BADCy resin matrix. A microscopic mechanism model was used to characterize the curing process of BADCy resin systems with different contents of the prepared nanoparticles. Due to the curing process of the thermosetting resin being analogous to the crystallization process of the polymer, the Avrami equation was used to analyze the microscopic mechanism of the curing process. The reactive functional groups, structure, and size of the prepared BADCy resin nanoparticles were characterized by FT-IR, SEM, and TEM, respectively. The kinetic parameters of different systems were then obtained using the Avrami equation, and they adequately explained the microscopic mechanism of the curing process. The results showed that the Avrami equation effectively described the formation and growth of gel particles during the curing process of the BADCy resins. The addition of nanoparticles can affect the curing behavior and curing rate. Since the reaction between the BADCy resin nanoparticles and the matrix is dominant, the formation process of the gel particles was neglected. This phenomenon can be understood as the added BADCy resin nanoparticles replacing the formation of gel particles. The reasons for accelerated curing were analyzed from the perspective of thermodynamics and kinetics. Besides this, the Arrhenius equation for non-isothermal conditions correctly accounted for the change in the cross-linked mechanism in the late-stage curing process. A comparison of the theoretical prediction with the experimental data shows that the Avrami theory of phase change can simulate the curing kinetics of different BADCy resin systems well and explain the effects of BADCy resin nanoparticles on the formation of the microstructure. Keywords: bisphenol a dicyanate resin; Avrami equation; Arrhenius equation; curing behavior 1. Introduction Cyanate ester (CE) resins are novel high-performance thermosetting resins similar to epoxy (EP) and bimaleimide (BMI) that have been developed in recent decades [1]. CE monomers contain two Appl. Sci. 2019, 9, 2365; doi:10.3390/app9112365 www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci.2019 2019,,9 9,, 2365 x 2 2of of 15 15 or more -OCN functional groups which can form a highly symmetrical triazine ring structure after or more -OCN functional groups which can form a highly symmetrical triazine ring structure after the curing reaction [2]. It is the chemical nature of the CE monomers and the unique structure of the the curing reaction [2]. It is the chemical nature of the CE monomers and the unique structure of the cured resin that give rise to a series of properties, such as excellent mechanical properties, a high glass curedtransition resin thattemperature, give rise tolow a series contractibility of properties, rate, such low as water excellent absorption, mechanical and properties, ultralow a dielectric high glass transitionconstant temperature,and dielectric low loss contractibility values [3–6]. rate, Compared low water with absorption, other thermosetting and ultralow resins, dielectric CEs constant have andexcellent dielectric overall loss performance. values [3–6]. Therefore, Compared CEs with are other a proven thermosetting replacement resins, for thermosets, CEs have excellent such as epoxy overall performance.or polyimide, Therefore, in applications CEs are in the a proven aerospace, replacement electronics, for thermosets,and communications such as epoxy industries or polyimide, [4,7–11]. in applicationsCE monomers in the aerospace, polymerize electronics, via a ring-forming and communications reaction under industries the [conditions4,7–11]. of heating or catalysts.CE monomers Simmon first polymerize proposed via the a ring-forming self-polymeri reactionzation mechanism under the conditions of CE resins of heatingin the absence or catalysts. of a Simmoncatalyst first[12]. proposed The curing the reaction self-polymerization of the CE resins mechanism was initiated of CEresins and catalyzed in the absence by impurities of a catalyst in the [12 ]. Themonomer curing or reaction moisture of in the the CE environment. resins was initiated However, and this catalyzed process proceeds by impurities with difficulty in the monomer when the or moisturepurity of inthe the monomer environment. is high [13]. However, Active hydrogen this process containing proceeds compounds, with difficulty such as when phenols, the purityamines, of theand monomer imidazole, is in high concert [13]. with Active metal hydrogen ions can containing catalyze the compounds, curing of CE such resins as phenols,[14]. This amines, reaction and is imidazole,depicted in in concertFigure 1. with A metalrange ionsof such can catalyzemetal ions the curinghas been of CE reported, resins [ 14including]. This reaction chromium is depicted [15], inmanganese Figure1. A [16], range iron of such[17], metalcobalt ions[18], hastin been[19], reported,zinc [20], includingand copper chromium [21]. However, [ 15], manganese the metal ion [16 ], ironcompounds [17], cobalt have [18 poor], tin solubility [19], zinc in [ 20the], CE and resins copper and [ 21promote]. However, the hydrolysis the metal reaction ion compounds of the triazine have poorring solubilityas well; as in a theresult, CE these resins metal andpromote salts are therare hydrolysisly used as reactiona catalyst. of Alternative the triazine catalytic ring as well;systems as a result,have always these metal been salts an important are rarely useddirection as a catalyst.of research. Alternative At present, catalytic room systems temperature have always ionic liquids been an important(RTILs) and direction organically of research. modified At layered present, silicate room have temperature been proved ionic to liquidsaccelerate (RTILs) CE resin and curing organically [22– modified24]. layered silicate have been proved to accelerate CE resin curing [22–24]. FigureFigure 1.1. Self-polymerizationSelf-polymerization mechanism of cyanate ester ester in in the the presence presence of of catalysts. catalysts. PhysicalPhysical and and structural structural changes changes a ffaffectect the the polymerization polymerization kinetics kinetics and and the the structure. structure. Therefore, Therefore, the identificationthe identification of relevant of relevant kinetic models kinetic can models provide can insight provide into theinsight curing into process the of curing thermosetting process resin of systems.thermosetting In the curingresin systems. process, In identifying the curing each process, step of identifying the chemical each reaction step of is the very chemical difficult. reaction Especially, is invery the difficult. late curing Especially, stage, the in the diff usionlate curing factor stage, plays the a controllingdiffusion factor role. plays Thus, a controlling the phenomenological role. Thus, model,the phenomenological rather than the mechanismmodel, rather model, than isthe used me [chanism25]. The model, Kamal kineticis used model[25]. The considers Kamal bothkinetic the autocatalyticmodel considers behavior both andthe theautocatalytic n-stage reaction, behavior and and it canthe ben-stage used toreaction, describe and the it curing can be process used to of thermosettingdescribe the curing resins process precisely of [ 26thermosetting]. However, theseresins models precisely are [26]. limited However, to describing these models the curing are limited reaction processto describing and do the not curing give specificreactionphysical process and meaning do not to give the specific parameters. physical Previous meaning studies to the demonstrated parameters. thatPrevious the Avrami studies equation demonstrated considerably that the illustrated Avrami equa thetion isothermal considerably and non-isothermal illustrated the isothermal curing behavior and ofnon-isothermal thermosetting curing resins [behavior25,27–29 ].of thermosetting resins [25,27–29]. TheThe essence essence ofof thermosettingthermosetting resinresin curing is the cross-linked chemical chemical reaction reaction of of linear linear polymer polymer chains.chains. Meanwhile, Meanwhile, this this is is accompanied accompanied by by microphase microphase separation, separation, the the shift shift of theof the glass glass transition, transition, and otherand processes,other processes, eventually eventually forming forming a three-dimensional a three-dimensional