3.1. Characterization of Cyclotriphosphazene Derivatives
The synthetic schemes of the cyclotriphospazene-based derivatives are illustrated in
Figure 1 and
Figure 2. Results of EA confirmed full substitution of chlorine groups with cyclohexylamine functional groups and full substitution of amine and cyclohexylamine functional groups for HCACTP and DTCATP, respectively (
Table 2). No chlorine was detected in both samples. The determined concentrations of C, H, N and P were found to be in a good correlation with theoretical values as represented in
Table 2. Similarly, no significant differences were found between the experimental and theoretical values of amine number (see
Table 3), which corresponds to high purity of the synthetized cyclotriphosphazene derivatives.
The measured FTIR spectra of the synthetized derivatives are depicted in
Figure 3. The presence of the absorption bands around 900–1000 cm
−1, corresponding to the P=N stretching vibration of the phosphazene groups [
72], is clearly visible. Furthermore, the absorption bands at 3200 and 3400 cm
−1 corresponding to the absorption of N–H bonds [
72] and the bands at 2950 and 2850 cm
−1 corresponding to the presence of C–H bonds were detected [
72]. The FTIR spectrum of the second reagent, cyclohexylamine, was already described in literature [
73] and comprises the absorption bands at around 3360, 3340 and 1619 cm
−1 that are ascribed to the presence of N–H bonds [
73]. These bands appear as doublets in the spectrum reflecting the symmetric and asymmetric stretching of primary amines molecules. On the contrary, secondary amines (–N–H–) show only one singlet at around 1550 cm
−1 [
74]. Due to a successful conversion of reagents into the final HCACTP derivative, no bands of the N–H bond are visible in the recorded FTIR spectrum (see
Figure 3, spectrum 1). On the other hand, in the FTIR spectrum of DTCATP, the absorption band at around 1619 cm
−1 was detected due to the presence of 2 primary amines groups in the DTCTP structure that stayed preserved after the reaction with cyclohexylamine, as illustrated in the reaction scheme depicted in
Figure 2.
Stretching vibrations of P–N–C bonds were observed at wavenumbers in the range of 1115–1186 cm−1 for HCACTP and in the range 1100–1115 cm−1 for DTCATP. Deformation vibrations bands of CHx groups (x = 1–3) were detected in the region around 1400–1450 cm−1 in the case of HCACTP, and in the region of 1380–1450 cm−1 in the case of DTCATP. Stretching vibrations of ‘endocyclic’ P–N bonds were presented in the range of 1230–1289 cm−1 for DTCATP and at around 1230 cm−1 for HCACTP. The absence of P-Cl bonds was confirmed because no absorption bands around 600 cm−1 were detected.
NMR spectra of final derivates are shown in
Figure 4. HCACTP contains chemically equivalent phosphorus atoms in its structure, it corresponds to only one singlet with δ = 15.13 ppm in the spectrum (
Figure 4a). In the case of DTCATP derivative (
Figure 4b), a doublet at 14.38–14.39 ppm corresponding to two amine groups on cyclotriphosphazene unit and a triplet at 17.03–17.09 ppm corresponding to four cyclohexylamine groups were found with an intensity of 1:2. The
31P NMR spectra of HCCTP and DTCTP reagents were already reported [
74]. One singlet with δ = 20.12 ppm and two multiplets (doublet for –P–Cl
2 with chemical shift δ (D) = 18.3 ppm and triplet of –P–(NH
2)
2 with δ (T) = 9.03 ppm) were found for HCCTP and DTCTP reagents, respectively. Due to absence of these signals in measured NMR spectra of HCACTP and DTCATP, full substitution of chlorine groups in molecular skeletons was confirmed.
Molecular weights of HCACTP and DTCATP, determined using MS, were found to be in a good correlation with theoretical values. Molecular weights of HCACTP and DTCATP were detected to be 724.00 g/mol (theoretical value 723.94 g/mol) and 560.0 g/mol (theoretical value 559.65 g/mol), respectively.
The effect of molecular structure of the prepared cyclotriphosphazene derivatives on their thermal stability was evaluated by TGA under air atmosphere.
Figure 5 depicts the TG thermograms for both cyclotriphosphazene derivatives. The results are summarized in
Table 4 and
Table 5. Both derivatives were exhibited a three-step thermal degradation. Based on the knowledge of dissociation energies of particular bonds [
75] present in HCACTP and DTCATP, it can be assumed that during the thermo-oxidative decomposition, the cyclohexylamine groups were cyclically decomposed into cyclohexane and ammonia together with a subsequent decomposition of the cyclotriphosphazene unit (
Figure 6 and
Figure 7). Comparing the results of molecular weight loss with the molecular weight of the considered decomposition products (17.03 and 84.16 g/mol for ammonia and cyclohexane, respectively) and with the molecular weight of the particular functional groups (16.03 and 98.17 g/mol for amine and cyclohexylamine group, respectively), it can be assumed that the thermo-oxidative decomposition of HCACTP proceeded as follows. In the first step, cyclohexane was apparently released from one cyclohexylamine group followed by oxygen attachment to the nitrogen residue of the cyclohexylamine group. After that, the elimination of four cyclohexylamine groups probably proceeded in the same manner in the second step. Finally, the remaining cyclohexylamine group and cyclotriphosphazene skeleton were decomposed in the third step. In the case of DTCATP thermo-oxidative degradation, the decomposition process seemed to proceed as follows. In the first step, one amine group was removed producing ammonia. In the second step, the additional amine group was eliminated and three cyclohexylamine groups were probably decomposed resulting in cyclohexane release and oxidation of nitrogen residuals. Finally, the remaining one cyclohexylamine group and the phosphazene base were decomposed in the third step. When comparing the thermo-oxidative stability of both synthesized cyclotriphosphazene derivatives (see
Table 5), it was found that DTCATP was more stable than HCACTP, which correlates well with dissociation energies of the particular bonds forming the molecular structures of the derivatives.
3.2. Monitoring of Curing Process
In general, the curing of epoxy resin by amine curing agents involves two major addition reactions; the active hydrogen in primary amine reacts with an epoxy group to form secondary amine and the secondary amine reacts with another epoxy group to cure [
76]. Epoxy resins with two terminal epoxide groups can be cured with agents such as diamines, anhydrides or isocyanates [
77]. The amine curing agent must have more than three active hydrogen atoms and two amino groups in a molecule so that the cured epoxy resin becomes cross-linked polymer. The same principles are also valid for the curing using HCACTP and DTCATP derivatives comprising primary and secondary amine groups in their structures. Reference epoxy materials cured with EDA and DPTA followed the curing procedures already described elsewhere [
78,
79].
The curing process of DGEBA with HCACTP and DTCATP was investigated using DSC tracings (
Figure 8). The results revealed that DTCATP was a more reactive curing agent than HCACTP. The significant curing reaction (demonstrated by heat evolution) of DTCATP was detected at temperatures above 50 °C. However, the curing reaction of HCACTP derivative was measured at the temperatures above 80 °C. These experimental findings are in a good agreement with the theoretical assumption based on the fact that the primary amine groups are more reactive than the secondary groups (DTCATP contains two primary amine groups and four secondary amine groups in the molecule, whereas HCACTP contains six secondary amine groups). To ensure a sufficient rate of the DGEBA curing reaction, the optimal precuring temperature for further experiments was set to 70 and 100 °C for DTCATP and HCACTP, respectively.
Furthermore, the conversion of DGEBA precuring process with HCACTP and DTCATP derivatives, at the respective optimal temperature was determined based on the extent of curing heat released during DSC measurements. The results are listed in
Figure 9. It visible that for both phosphazene-based curing agents a sufficiently high (around 80%) conversion of the curing reaction was achieved after 4 h of curing time at the selected temperature. Thus, the time interval of 4 h was chosen as the optimal duration of the precuring process for both types of the cyclotriphosphazene derivatives.
3.3. Characterization of Epoxy Materials
DGEBA-based epoxy resin cured with HCACTP, DTCATP and with two conventional aliphatic amine curing agents (EDA and DPTA) as a reference samples were evaluated and compared from the point of view of thermal stability and combustion behavior properties. A detail description of two-step curing process of DGEBA with the curing agents is reported in
Table 1. The postcuring process was same for all investigated samples—3 h at 120 °C.
The curing efficiency of the prepared cyclotriphosphazene derivatives was evaluated on the basis of glass transition temperature (
Tg) and the change of crosslinking density (see
Table 6,
Figure 10). According to the literature, DGEBA-based thermosets cured with linear amines usually exhibit
Tg values close to 110 °C [
80]. The resulting DGEBA-based epoxy materials cured with HCACTP, DTCATP possessed a significantly reduced
Tg. In accordance with the expectations, the curing efficiency of a respective curing agent HCACTP and DTCATP were influenced by its molecular bulkiness, which governs the mobility, and by the number of primary and secondary amine groups per molecule. Therefore, the control amine curing agents (EDA and DPTA) provided more densely crosslinked epoxy materials with elevated
Tg in contrast to both phosphazene curing agents, where EDA was shown to be the most effective while HCACTP the least effective. When comparing both phosphazene curing agents, DTCATP was found to be a more effective curing agent, probably due to the presence of primary amine groups providing a higher reactivity and crosslinking density. The properties of uncured DGEBA-based thermosets may be found elsewhere [
60].
FTIR spectra of samples after curing reaction are shown in
Figure 11. The intensity of the absorption band at 910 cm
−1 (corresponding to the stretching vibration of C–O bonds present in glycidyl groups [
81]) was found to be negligible for all samples. Thus, it can be stated that epoxy groups in DGEBA were fully consumed during reaction with amine-based curing agents.
Selected samples after curing reaction were observed under SEM to verify the surface compactness (
Figure 12). A compact surface was observed in the case of densely crosslinked samples cured using EDA, DPTA and DTCATP, while the HCACTP-cured epoxy resin exhibited a structure with many micropores as visible in
Figure 12a,b. The reason of high roughness of HCACTP samples could be ascribed to the presence of six cyclohexyl groups bounded to the nitrogen atoms in the skeleton of this compound. Such complex structure with high steric hindrance might probably affect the proper preparation of the cured epoxy resin.
In
Figure 13, TG thermograms for the prepared epoxy materials are depicted. The results of analysis are summarized in
Table 7. TGA profiles revealed that the DGEBA thermosets cured with HCACTP and DTCATP exhibited a lower thermo-oxidative stability in the comparison with the DPTA-cured epoxy resin. At the same time, the DGEBA thermosets cured with HCACTP and DTCATP were found to be more stable than the EDA-cured sample. The estimated limiting oxygen index (LOI) was calculated for all testing samples from the char yield results at 900 °C, in which weight of the residue practically remains constant. The lower the LOI value reflects the easier ignition of materials. As reported in
Table 7, the samples with cyclotriphosphazenes derivatives have LOI numbers always higher than commercially used aliphatic amines. The highest LOI was calculated for sample cured with HCACTP derivative. The LOI number was found to be higher for 6.4% or 4.0% in the comparison with EDA or DTPA, respectively.
The combustion behavior and the flame resistance of the prepared epoxy materials were characterized with the following parameters: time to ignition (TTI), peak heat release rate (pHRR), total heat release (THR), maximum average rate of heat emission (MARHE), total smoke production (TSP) and mass loss rate (MLR). TTI is often used to determine the influence of a flame retardant on the ignitability of a material. As summarized in
Table 8, TTI values of the epoxy materials cured with HCACTP and DTCATP, were significantly increased in the comparison with other samples. It has to be noted that the DTCATP-cured epoxy resin achieved two times higher TTI than both amine-cured thermosets. Such behavior suggests a considerable flame resistance effect of both phosphazene-based curing agents in epoxy materials.
The heat release rate (HRR) parameter is one of the most important parameter to quantify the size of fire and effective flame resistance of systems normally exhibits a lower HRR values. When comparing the HRR of the tested epoxy materials (see in
Figure 14), it is evident that the application of both phosphazene-based derivatives significantly decreased values of HRR in the comparison with samples with amine curing agents. Similarly, MARHE and THR parameters for the epoxy resin cured with cyclotriphosphazene derivatives were found to be lower in the comparison with those of the amine-cured epoxy materials. These results may be attributed to the decomposition of phosphazene, which resulted in the formation of phosphorus-rich char layer imparting an increased heat insulation and thermal stability. The formed char protected barrier probably inhibited heat and oxygen transfer into the interior of the epoxy material and suppressed the transfer of flammable volatiles into flame zone during the combustion process. Thus, the quantity of released heat and the intensity of combustion pyrolysis reactions were decreased.
The results revealed that the DTCATP-cured epoxy resin exhibited the highest TSP value, whereas the HCACTP-cured epoxy resin was shown to produce the lowest quantity of smoke. It can be supposed that the smoke production is partly associated with nitrogen concentration in the cured epoxy material (see
Table 9), which is responsible for ammonia release. Moreover, this phenomenon may be contributed to strength, compactness and porosity of the created char layer which affects the transfer of heat and the quantity of released smoke. Hence, a continuous and porous char layer was probably formed during combustion of the DTCATP-cured epoxy resin, while burning of the HCACTP-cured DGEBA resulted in a continuous char shield. Moreover, the results of MLR also confirmed the flame-retarding behavior of both phosphazene-based curing agents. In comparison with the amine-cured epoxy materials, the cyclophosphazene-cured thermosets exhibited a significantly delayed mass loss rate. Based on the results discussed above, it can be concluded that both cyclotriphosphazene derivatives imparted a pronounced flame retarding effect into the DGEBA-based epoxy materials, where the incorporation of HCACTP as a curing agent provided a higher flame retardancy than the curing with DTCATP.
In comparison with the literature [
32,
56,
83,
84,
85,
86], pHRR of bisphenol A bis(diphenyl phosphate) oligomer (phosphazene derivate) [
83] was reported to be worse in the comparison with the cyclotriphosphazene-based curing agents. Another work dealt with the usage of a modified epoxy resin with phosphoric acid [
84]. pHRR values were detected to be 650 kW/m
2 and 590 kW/m
2 (much higher values than for HCACTP and DTCATP as reported in
Table 8). The application of carbon nanotubes combined with hexaphenoxycyclotriphosphazene [
85] resulted also into the worse pHRR values. The best pHRR result (501 kW/m
2) was obtained for the system named EP1 (containing 2.0 wt % of P, 14.92 wt %) of hexaphenoxycyclotriphosphazene and 1 wt % of carbon nanotubes. Tested compound containing phosphaphenanthrene and phosphazene groups [
56] showed the best pHRR value of 383 kW/m
2. Phosphazene-based flame retardant with active amine groups of polyphosphazene resulted in the product with pHRR value of 474.78 kW/m
2 and TSP value of 19.44 m
2 [
32]. pHRR value of 426 kW/m
2 was reported in the case of usage of monomer with six functional epoxy groups combining by eugenol with hexachlorocyclotriphosphazene [
86].
Two novel flame retardants have been successfully prepared. Compared to the traditional curing agents such as DPTA and EDA, DCCATP and HCACTP-cured epoxy materials showed better flammability parameters including TSP. Thus, the usage of DCCATP or HCACTP may help to preserve the environment by decreasing the amount of released harmful pollutants into atmosphere in case of a fire accident. The prepared materials based on the amino-functionalized cyclotriphosphazene derivatives could find application as e.g., protective coatings, high-performance plastics and adhesives.