Introduction

The use of polymeric materials in a day-to-day life is increasing due to their remarkable combination of properties, varying range of molecular weight and ease of processing. However, polymers are also known for their relatively high flammability; most often accompanied by the evolution of corrosive or toxic gases and smoke during burning. Consequently, improving the fire-retardant behavior of polymers is a major challenge for extending their use to most high-performance industrial applications [1,2,3].

Epoxy resin is one of the most important thermoset engineering polymers with a wide range of industrial applications, such as packaging, adhesives, encapsulating materials for electronics, coatings, aerospace and marine systems because of their good chemical resistance, excellent thermal and electrical insulating properties, and high tensile strength [4,5,6,7,8,9]. However, the neat epoxy resin is highly flammable, which significantly restricts its applications. Therefore, the modification of epoxy in order to improve its flame retardancy is an important issue and needs to be addressed. Generally, there are several approaches to reduce the polymer flammability, i.e., (1) to use the inherently flame-retarded polymers (polyimide, polybenzoxazine, etc.,) [10], (2) to chemically modify the existing polymers (copolymerization of flame-retardant monomers into the polymer chains) [11]; and (3) to incorporate flame retardants into the hosting polymer matrix [12,13,14,15,16,17,18]. All these methods could be used for enhancing the flame retardancy of epoxy resin. Because inherently flame-retardant polymers can lead to high production costs, the modification of already existing systems is still valued by industries. As thermally stable materials, polybenzoxazines possess good flame-retardant behavior and are useful for electronic and aerospace applications as sealant and coating materials. Benzoxazine resins can be easily synthesized from phenol, formaldehyde, and amine through Mannich reaction by either solution or bulk method. The polymerization of resins proceeds via ring opening of oxazine rings by a thermal cure. On curing, benzoxazine resins develop a phenolic-like structure with inherent flame-retardant properties. Polybenzoxazines exhibit the characteristics that found in traditional phenolic such as heat resistance, flame retardance, and good electrical insulation behavior. Besides, they also exhibit lots of unique characteristics including low melt viscosity, dimensional stability, excellent mechanical properties, as well as low water absorption [19,20,21,22,23,24,25]. Even though having aromatic bisphenol and the aromatic diamine as raw materials, the resultant polybenzoxazines do not possess enough flame retardancy for electronic materials where a flame retardancy of the UL-94 grade is required. Hence, the incorporation of flame-retardant materials in polymers is still widely used. It is considered as a simple and cost-effective method for preparing flame-retarding polymers.

Phosphazenes, particularly polyphosphazenes, are a well-known class of versatile functional materials having an inorganic backbone structure. The compounds containing both phosphorus and nitrogen are expected to display enhanced flame retardancy when compared to that of similar compounds containing phosphorus alone. The reason is that the thermal decomposition of the phosphazene-based polymers is an endothermic process, and phosphate, metaphosphate, poly-phosphate generated in the thermal decomposition form a non-volatile protective film on the surface of the polymer to isolate it from the air; meanwhile, the inflammable gases released such as CO2, NH3, and N2 cut off the supply of oxygen and contribute to the synergistic flame-retardant behavior. The properties of organic polymers can be modified significantly to improve their fire resistance, ionic conductivity, biological compatibility, or other properties by the incorporation of a small amount of a specifically tailored phosphazene. This makes phosphazenes particularly good candidates for the development of fire-resistant materials for use in electric and electronic applications [26,27,28,29,30,31,32,33,34]. Only a few of the researcher worked with phosphazene fiber-reinforced polymer composites for flame resistant applications. Here, the cyclophosphazene nanofiber/sphere with the unique combination of phosphorus and nitrogen contributes to imparting the good flame-retardant behavior of the resulting polymer composites [31,32,33,34].

In the present work, an attempt has been made to develop non-flammable halogen-free hybrid nanocomposites based on polybenzoxazine/epoxy (PBZ/EP) matrix by reinforcing with PZT nanofiber, and their thermal, flame-retardant, and dielectric properties were studied and discussed. As the data results, 1.0% PZT-reinforced PBZ/EP/PZT nanocomposites possess V-0 classification (UL-94 test), indicating an excellent flame-retardant behavior.

Experimental

Materials

Hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfonyldiphenol (BPS) were purchased from Sigma-Aldrich, India. The epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) was supplied by Ciba-Geigy Ltd., India. Bisphenol-A, aniline, paraformaldehyde, tetrahydrofuran (THF), triethylamine (TEA), and methanol were received from SRL (India) Ltd. Bisphenol-A/aniline-based benzoxazine (Bz) was synthesized as per reported procedure [22].

Characterizations and techniques

FTIR spectra were recorded on a PerkinElmer 6X spectrometer. About 500 mg of optical-grade KBr was ground in a mortar with a pestle, and enough amount of solid sample was ground with KBr to make 1 wt% mixture for making KBr pellets. After the sample was loaded, a minimum of 16 scans were collected for each sample at a resolution of ± 4 cm−1. The DSC was performed on a NETZSCH DSC-200PC (TA instruments USA) with a heating rate of 10 °C/min, the measurements were taken from 30 to 300 °C under nitrogen atmosphere (20 mL/min). The instrument was calibrated with indium supplied by NETZSCH. About 10 mg of the samples was taken for each analysis. Thermogravimetric analysis was performed in TGA-Thermal Analyst NETZSCH STA 409PC (TA instruments USA) at a heating rate of 20 °C/min from 30 to 800 °C under a continuous flow of nitrogen (20 mL/min). The instrument was calibrated with the help of calcium oxalate and aluminum supplied by NETZSCH. About 10 mg of the samples was taken for each analysis. The dielectric constant and dielectric loss of the polybenzoxazine systems (length, width, and thickness of the sample specimen 10mmX10mmX3 mm, respectively) were determined with the help of an impedance analyzer (Solartron impedance/gain-phase analyzer 1260) using a platinum (Pt) electrode at 1 MHz at a temperature range of 30 °C to 200 °C. This experiment was repeated four times under the same conditions. The UL-94 vertical burning test was carried out for the PBZ/EP/PZT based on the testing method proposed by Underwriter Laboratory according to ASTM D-1356 standard. The SEM morphology of the residual chars was performed using a JEOL JSM Model 6360 microscope. The char residues of the specimen were coated with gold and were exposed to an accelerating voltage of 20 kV. A JEOL JEM-3010 analytical transmission electron microscope, operating at 300 kV with a measured point-to-point resolution of 0.23 nm, was used to characterize the phase morphology of the polymers.

Synthesis of the sulfone-containing phosphazene nanotubes (PZT)

The required stoichiometric quantity of HCCP and BPS as per the procedure reported in the literature [35] was charged to yield PZTs containing active hydroxyl groups: 8.73 g of triethylamine (0.0862 mol) was added to a solution of HCCP (5 g, 0.0143 mol) and BPS (14.4 g, 0.0575 mmol) in THF (100 mL). The reaction mixture was kept in an ultrasonic bath (100 W, 80 kHz) at 30 °C for 6 h. The product resulted was filtrated and then washed three times with THF and deionized water. The filtered solid was dried in a vacuum oven, and the sample PZT was obtained as white powder (Scheme 1).

Scheme 1
scheme 1

Synthesis of sulfone-containing phosphazene fiber (PZT)

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Development of PBZ/EP/PZT nanocomposites

PZT fiber of 0, 0.5, 1.0, and 1.5 wt% was reinforced with PBZ/EP (1:1 weight ratio) in THF solvent and stirred for 12 h at 30 °C. After 12 h, the resultant PBZ/EP/PZT hybrid nanocomposite samples were separately cast on a smooth glass substrate and thermally treated at 50 °C for 1 day, and at 80, 100, 120, 140, 160, 180, 200 for 1 h each and 220 °C for 2 h. The light brown-colored films formed were stripped from the glass substrates, and then they are utilized for further characterization (Scheme 2).

Scheme 2
scheme 2

Schematic representation of PBZ/EP/PZT nanocomposites

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Results and discussion

Halogenated flame-retardant materials are usually incorporated into the polymer matrix to improve its flame-retardant performance, whereas these materials release toxic gases and corrosive smoke during combustion; hence, it results in unwarranted environmental concerns. Accordingly, the development of non-halogenated flame-retardant system becomes an attractive and emergent subject [36]. As an alternative to halogenated flame retardants, phosphorus-based flame retardants are expected to play a major role, [8, 37, 38] because phosphorus can act in the condensed phase promoting char formation on the surface, which acts as a barrier to inhibit gaseous products from diffusing to the flame and to shield the polymer surface from heat and air. Nitrogen-containing compounds are another class of halogen-free flame retardants which can release non-flammable gases or decompose endothermically to cool the pyrolysis zone at the combustion surface. An incorporation of these phosphorus- and nitrogen-containing phosphazene fiber into PBZ/EP matrix by the chemical method, which contributes to improved flame retardancy. The introduction of cyclotriphosphazene moieties into the backbone is expected to enhance the thermal resistance, thermal stability, and most importantly, the fire resistance of the resulting PBZ/EP matrix.

The FTIR spectra of neat PBZ/EP and PBZ/EP/PZT are shown in Fig. 1. The stretching bands at 1234, 1178, and 823 cm−1 correspond to the P=N and P–N characteristic absorption; the characteristic peaks appeared at 1358, 1290, and 1105 cm−1 are assigned to O=S=O; 1591 and 1496 cm−1 correspond to C=C absorption; the intense absorption appeared at 941 cm−1 corresponds to the P–O–(Ph) band [35]. From the figure, the disappearance of peaks at 937 cm−1 (for glycidyl groups present in DGEBA epoxy), 1520 cm−1 for trisubstituted benzene ring and 947 cm−1 for oxazine ring, and appearance of new peaks at 1505 cm−1 represents the tetra-substituted benzene ring [22], 2962 cm−1 and 2871 cm−1 for symmetric and asymmetric aliphatic CH2 groups and the broad peak at 3435 cm−1 corresponding to –OH becomes weak indicating that the DGEBA undergoes a complete curing reaction with BZ-a to form an inter-cross-linked network structure.

Fig. 1
figure 1

FTIR spectra for neat PBZ/EP and PBZ/EP/PZT nanocomposites

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Thermal resistance and stability of the nanocomposites

The thermal resistance is one of the most important properties of thermosets because it establishes the service environment for the epoxy-based functional materials. Usually, the glass transition temperature (Tg) represents the thermal resistance of the thermosets, which indicates the suitability of the material for specific applications. Therefore, it is very important for thermoset to achieve a high Tg using appropriate molecular structure. The Tg’s of PZT fiber-reinforced PBZ/EP thermosets were determined by DSC, and the data obtained are presented in Table 1. From the data, observed that all the thermosets show higher values of Tg when compared with that of the conventional epoxy [28, 37]. The values of Tg of neat PBZ/EP and PZT incorporated PBZ/EP are 171, 188, 195, and 208 °C, respectively (Fig. 2). The values of Tg are increased with increase in weight percentages of PZT nanofiber reinforced into PBZ/EP matrix. It is well known that glass transition is generally ascribed to the segmental motion of the polymeric networks, and the Tg is determined by the degree of freedom for the segmental motion, cross-linking and entanglement constraints, and the packing density of the segments. In this work, PZT fiber of varying weight percentages was incorporated into the backbone chain of the hybrid PBZ/EP matrix to improve the network structure in addition to the presence of sterically hindered aromatic substituents on the phosphazene ring to enhance the value of Tg. Thus, the values of Tg of cyclotriphosphazene-linked PBZ/EP are increased as a result of the retarded segmental motion. In addition, the abundant rigid aromatic rings and sulfone groups in the PZT fiber can further increase the rigidity of the resulting hybrid nanocomposites, which in turn contributes to the enhanced value of Tg.

Table 1 Thermal, flame resistant, and dielectric properties of neat PBZ/EP and PBZ/EP/PZT nanocomposites
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Fig. 2
figure 2

DSC thermogram for neat PBZ/EP and PBZ/EP/PZT nanocomposites

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The thermal stability of PZT fiber-incorporated PBZ/EP thermosets was investigated by TGA under a nitrogen atmosphere. The TGA thermogram of PBZ/EP/PZT and the data resulted are shown in Fig. 3, and the details of the thermal analysis are presented in Table 1. From the TGA traces of PBZ/EP/PZT, it was observed that there was a two-stage thermal decomposition occurred due to the nature of constituents while PBZ/EP shows single-stage degradation. The first stage of thermal degradation is caused by the hydrocarbon segments of the thermoset, and the second stage is attributed to the decomposition of the major backbones of polymeric networks due to their higher thermal stability. The TGA results also demonstrate that the initial degradation temperatures (Td) of all the thermosets corresponding to 5 wt% weight loss (Td5) and 10 wt% weight loss (Td10) are in the ranges of 270–328 °C and 308–362 °C, respectively. It was also noticed that the major decomposition occurs beyond 400 °C which indicate that the PZT fiber-reinforced PBZ/EP nanocomposites have good thermal stability. This may be due to the decomposition of the phosphorus and nitrogen groups in the PZT fiber that triggered the formation of the phosphorus- and nitrogen-rich char to hinder further decomposition of the PBZ/EP. Also, PZT fiber with many phenolic groups participated to increase the high cross-linking density of PBZ/EP. From the data presented in Table 1, it can be seen that the neat PBZ/EP shows the char yield of 21 wt%, whereas that of PZT fiber-reinforced PBZ/EP (0.5, 1.0, and 1.5 wt%) shows 37, 38, and 41%, respectively. An improvement of such higher char yields may be explained by the highly thermally stable phosphazene rings (phosphorus and nitrogen group in the PZT) in the PBZ/EP, which promotes the char formation during pyrolysis. The formation of phosphorus- and nitrogen-rich char in the decomposition of PBZ/EP/PZT not only increased the degradation temperature but also increased the high char yield. On the basis of these results, it can be concluded that the heat resistance of PZT incorporated PBZ/EP nanocomposites is superior to that of other known phosphorus-containing epoxy resins that have been reported [39].

Fig. 3
figure 3

TGA thermogram for a neat, b 0.5 c 1.0 and d 1.5 wt% of PBZ/EP/PZT nanocomposites (char yield of the a–d is 21, 37, 38 and 41%, respectively)

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Flammability characteristics of the nanocomposites

The UL-94 burning experiment is most important to assess the burning behavior of materials and is a widely used method for determining the upward burning characteristics of PBZ/EP/PZT nanocomposites. The UL-94 burning test results of the PBZ/EP/PZT nanocomposites are presented in Table 1. As expected, 0.5 wt% PBZ/EP/PZT attained the V-1 rating and that of 1.0 and 1.5 wt% PBZ/EP/PZT nanocomposites are V-0 classification, whereas the neat PBZ/EP matrix is burning in a UL-94 test. It is surprisingly noticed that the PZT fiber-incorporated nanocomposites did not exhibit an aggressive combustion during the tests, and there were no flaming drips which were observed as well. One of the fascinating characteristics of combustion is that most of the testing bars just combusted slightly with a small blaze and quenched within 5 s when the flame agitator was removed during the vertical burning test, indicating a self-extinguishable behavior. At the end of burning experiments, the surfaces of these nanocomposites were covered with an expanded char network, indicating that the thermosets formed an effective char which was able to prevent the heat transfer and flame spread during combustion. These results provide an evidence for non-flammability of the PZT fiber-reinforced PBZ/EP nanocomposites.

The limiting oxygen index (LOI) is considered as another important method used to quantify the flame-retardant behavior of organic polymer matrices. LOI represents the lowest environmental oxygen required for sustaining a flame. The oxygen volume content of the ambient atmosphere is about 21%. Therefore, a material exhibiting an LOI value above 21% might show flame-retardant properties [40]. The oxidized layer serves as a barrier against heat and oxygen diffusion, which contributes to the flame-retardant behavior to polymers in the high-temperature region and also gives high char yields. It has also been reported that a high char yield leads to high flame retardancy.

In the present study, the flame-retardant behavior of PBZ/EP and the varying weight percentages of PZT fiber-reinforced hybrid nanocomposites were analyzed. The values obtained are presented in Table 1; the values are found to be in the range of 26–35. Furthermore, the behavior of flame retardancy increased as the concentration of PZT fiber was increased. Burning behavior was correlated with the cross-link density of the matrix; i.e., with increasing cross-link density, the flammability resistance increased due to the formation of higher char. Thus, it can be concluded that the developed PBZ/EP/PZT can be used as better flame-retardant materials, where a high-performance application is required.

Analysis of residual char of the nanocomposites

It was considered that the residual chars formed during combustion can give some important information regarding the inherent flammability characteristics of a polymeric material, and they also reflect the fire-resistant mechanisms to some extent. Furthermore, the physical structure of the charring layer also plays a very important role in the performance of flame retardancy. Therefore, the morphology of the residual chars obtained from UL-94 burning tests was investigated by SEM. Figure 4 illustrates the SEM images of these residual chars. As shown by Fig. 4b–d, the outline of these residual chars exhibited some irregular-shaped bulk and the inside of the char shows a multiporous structure according to the magnified micrographs of each sample, indicating a typical morphology after the intumescent char formation. Additionally, all the residual chars observed have displayed a very gassy surface with a few pores breaking through. These charred layers formed during the combustion provide a rigid and compact texture in nature, and there are lots of integrated honeycomb pores inside. Such a structural form favors the temperature grads in the char layer and protects the matrix inside. Therefore, it is concluded that the thermo-oxidative reaction of cyclotriphosphazene moieties enhances the char formation during combustion, results in a protective char layer formed on the surface of PBZ/EP, which serves as a barrier against heat and oxygen diffusion, and consequently, the flame retardancy of the nanocomposites was improved significantly. Moreover, the multiporous structure of residual char also depends on the other factors such as the condition of curing, gases released during combustion, the composition, and structural feature of PBZ/EP/PZT nanocomposites.

Fig. 4
figure 4

SEM morphology of the residual chars of a neat, b 0.5 c 1.0 and d 1.5 wt% of PBZ/EP/PZT nanocomposites

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Dielectric constant of the nanocomposites at varying temperatures

The variation of values of dielectric constant with temperature at 1 MHz is shown in Fig. 5. The values of dielectric constant are decreased with increasing the temperature. The temperature coefficient of dielectric constant values with regard to the percentage weight concentration of PZT fibers is 0.006, 0.008, 0.0078, and 0.0078 for 0, 0.5, 1.0, and 1.5 wt%, respectively. From the values, it can be ascertained that the composite samples exhibit stable dielectric behavior with regard to variation in temperature. The dielectric properties of a polymer are determined by the free volume, charge distribution and also by the statistical thermal motion of its polar group. In the case of PBZ/EP/PZT nanocomposites, the values of dielectric constant are 4.4, 4.0, and 3.4 with an increasing PZT fiber loading level of 0.5, 1.0, and 1.5 wt%, respectively, for 1 MHZ at 30 °C, when the temperature was raised to 200 °C the values of dielectric constant are reduced to 2.9, 2.7, and 2.4, respectively. The reduction in the value of dielectric constant may be explained due to an enhanced free volume and also to the certain extent due to the influencing effect of PZT fiber.

Fig. 5
figure 5

Dielectric constant with various temperatures for neat PBZ/EP and PBZ/EP/PZT nanocomposites

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The value of dielectric losses of neat PBZ/EP is marginal at 30–140 °C, and above 140 °C, the value of dielectric losses is significant and this supports the concept of free volume theory according to which the molecular mobility near Tg depends mainly on free volume. In the glassy state below Tg, the free volume will be frozen in and remain fixed. As the temperature increases, the glassy state will expand due to the increase in free volume in the molecular system, which results from the changing irrational amplitude of bound distances. This leads to the maximum dielectric loss at and above Tg for neat PBZ/EP (Fig. 6). In the case of PZT fiber-reinforced PBZ/EP nanocomposites, the dielectric loss was reduced significantly with increasing temperature and the reverse trend in the case of dielectric loss was observed above 170 °C, due to an increase in Tg’s and is interpreted on the basis of the nanoreinforcement of PZT fibers, which could restrict the motions of macromolecular chains. In the present study, it was also noticed that the enhancement in the value of Tg is not only ascribed to the nanoreinforcement of PZT fiber but also attributed to the additional cross-linking of the nanocomposites resulting from the inter-component reaction between PZT and PBZ/EP.

Fig. 6
figure 6

Dielectric loss with various temperatures for neat PBZ/EP and PBZ/EP/PZT nanocomposites

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Morphology

Figure 7a–c shows the TEM images of 0.5, 1.0, and 1.5 wt% of PBZ/EP/PZT nanocomposites, respectively. From the figure, it can be ascertained that the PZT nanofibers are flexible and are about 100 nm in diameter and the PZT fibers are homogeneously dispersed within the PBZ/EP matrix. From the TEM images, it is concluded that the PZT fiber was completely and uniformly dispersed within the PBZ/EP matrix. This is due to the strong covalent interactions resulted between the PZT fiber and the PBZ/EP matrix. The nanometer level dispersion of PZT fiber in the PBZ/EP matrix also influences the thermal and electrical properties of the resulting PBZ/EP/PZT nanocomposites.

Fig. 7
figure 7

TEM images of a 0.5, b 1.0, and c 1.5 wt% of PBZ/EP/PZT nanocomposites

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Conclusion

Flame-retardant PBZ/EP/PZT hybrid nanocomposites have been successfully developed with varying weight percentages (0.0, 0.5, 1.0 and 1.5%) of PZT fiber reinforced with PBZ/EP matrix through thermal curing. Data resulted from DSC and TGA analysis indicate that the PBZ/EP/PZT possess the higher values of Tg as well as char yield and higher thermal stability than that of neat PBZ/EP. Furthermore, the PBZ/EP/PZT nanocomposites possess V-0 classification from the UL-94 test, indicating an excellent flame-retardant behavior. This is attributed to the structural feature of PBZ/EP covalently bonded with PZT fiber. The presence of such a unique combination of phosphorus and nitrogen in the PZT contributes to a good flame-retardant behavior. The compatibility between PZT fiber and PBZ/EP was confirmed by TEM analysis. In addition, the nanocomposites based on the PZT fiber exhibited lower values of dielectric constant and dielectric loss over a wide range of temperatures indicating its suitability for effective and thermally stable flame-retardant dielectric materials for microelectronic applications.