Thermal Study of Anhydrides Cured Tetrafunctional Cardo Epoxy Resin

Tetrafunctional cardo epoxy resin (EBCF) was cured by using 10 wt% maleic anhydride (MA), pyromellitic dianhydride (PMDA), phthalic anhydride (PA), tetrahydrophthalic anhydride (THPA), tetrabromophthalic anhydride (TBPA), and tetrachlorophthalic anhydride (TCPA) as hardeners at 120C for 40–105 min (gel time) and then postcured 1 h at 130C. Gel time is found to depend on the structure of the anhydrides used. Cured samples were found insoluble in common solvents. Cured and uncured EBCF were characterized by FTIR, DSC, and TGA techniques. Cured and uncured resins followed multistep degradation reactions. Kinetic parameters, namely, order of degradation, energy of activation, frequency factor, and entropy change, were determined according to the Anderson-Freeman method and interpreted in light of the nature of hardeners used for curing purpose. The resins followed integral or fractional order degradation kinetics. Complex degradation reactions are due to different types of linkages in cured resins. Both nature and structure of resin and hardeners affected the curing behavior and the resultant thermal properties of the cured resins.


Introduction
Epoxy resins are well known for their outstanding processing behavior and physicochemical properties such as mechanical stiffness and toughness; chemical, moisture, and corrosion resistance.They are most widely used in advanced technologies, aerospace, electronics, communication, adhesives, primers, coatings, and semiconductor encapsulation industries.They are also used for storage and management of nuclear waste as matrices for advanced fiber-reinforced composites [1][2][3][4][5][6].The basic properties of epoxy resins can be modified by blending different resins, by selection of curing agents, and by the use of modifiers and fillers.
Multifunctional epoxy resins are well known for their high glass transition temperatures, high decomposition temperatures, long term high temperature performance, and good wet strength performance [7][8][9][10][11].These materials suffer two important limitations because of their intrinsic brittle nature and considerable moisture absorption tendency from environment, which adversely affect most physicomechanical properties of the fabricated articles.Both these drawbacks increase by enhancing the crosslink density of the network.
To the best of our knowledge, no work has been reported on anhydrides cured multifunctional epoxy resin containing cyclohexyl as a cardo (Latin meaning a loop) group, which encouraged us to investigate the present work.In this paper, we have reported curing study of cardo group containing tetrafunctional epoxy resin (Scheme 1) by using 10 wt% of various anhydrides.The cured resins are characterized by IR, DSC, and TGA techniques.Our future objective is to prepare and characterize fiber reinforced composites of this resin and hardeners used.

Experimental
2.1.Materials.Solvents and chemicals used were of laboratory grade and purified prior to their use [12].Tetrafunctional cardo epoxy resin (EE 800) was synthesized according to our recent work [13].Maleic anhydride (MA)

Measurements.
Fourier transform infrared FTIR spectra (KBr pellets) of EBCF and anhydrides cured EBCF samples were scanned on a Shimadzu FTIR-8400 spectrometer over the frequency range from 4000 to 400 cm −1 .Differential scanning calorimetric (DSC) and thermogravimetric analyses (TGA) were carried out on a Shimadzu DSC60 and Pyris-I Perkin Elmer TGA at 10 ∘ C min −1 heating rate in nitrogen atmosphere.weight loss over those temperatures in its TG curve (Figure 6).TG curves of cured and uncured EBCF are presented in Figures 5 and 6, from which it is clear that decomposition reactions are very complex and followed multistep degradation reactions.Initial decomposition temperature ( 0 ), decomposition range, temperature of maximum weight loss ( max ), % weight loss involved in each step, and % residue remained at 700 ∘ C are reported in Table 2. Decomposition of epoxy resin begins with dehydration of secondary hydroxyl groups followed by homolytic cleavage of the formed allylic bond [14,15].Repetition of bond cleavage of the epoxy network leads to the evaporation of the low molecular weight fragments, whereas polymerization of unsaturated fragments results from dehydration and subsequent aromatization contributing into charring.Crosslinking density depends on the chemical structure of the resin, curing agent, and their functionalities, also on the curing mechanism and conditions selected.Thermal stability order is EBCF-THPA > EBCF-MA > EBCF-PMDA = EBCF-TBPA > EBCF-PA > EBCF.The use of MA and THPA as hardeners for EBCF resulted in improvements in good thermal stability, while other hardeners improved it to some extent.Associated kinetic parameters such as energy of activation (  ), frequency factor (A), order of reaction (),

Results and Discussion
where / is the weight loss with time,  is the active weight of the substance,  is the heating rate, R is the gas constant, h is the Planck's constant, T is the temperature, and  is the Boltzmann constant.The least square kinetic parameters ,   , and  are reported in (Table 3) along with regression coefficients ( 2 ).The entropy change (Δ * ) was determined at corresponding  max and also included in Table 3, from which it is clear that both cured and uncured resins followed either fractional or integral order decomposition kinetics.Ether and ester linkages are thermally weak points in the polymer chain, and hence, selective degradation occurs from those points on heating.The degradation may result in the formation of free radicals, which may further undergo recombination and degrade at high temperatures.The degradation process is a complex process, which involves a variety of reactions such as crosslinking, branching, recombination, and rearrangement.EBCF decomposed completely into low molecular mass substances, while EBCF-MA (37%), EBCF-PMDA (34%), EBCF-PA (26%), EBCF-THPA (21%), EBCF-TBPA (30%), and EBCF-TCPA (53%) showed a considerable amount of residue at 700 ∘ C confirming the formation of highly thermally stable crosslinked product, which may further degrade at elevated temperatures.Large and negative magnitudes of Δ * indicated that transition state is more in orderly state than individual resin molecules and vice versa [13,[17][18][19][20][21][22].Thus, both nature and structure of the hardeners affected the thermal behavior of the cured resins.

Table 1 :
Gel time of EBCF cured by various anhydrides at 120 ∘ C.

Table 2 :
DSC and TGA data of anhydrides cured and uncured EBCF.

Table 3 :
Kinetic parameters of anhydrides cured and uncured EBCF.