3.1. Properties of HCP
The effects of additives on the performance of the products can be concluded from their characterization and analysis.
As shown in
Figure 2a, the structure of the obtained HCPs was first characterized using an ATR-FTIR spectrometer. In the spectra, the peaks at 1635 cm
−1, 1608 cm
−1, and 1450 cm
−1 can be attributed to the characteristic stretching vibration of the polystyrene phenyl skeleton, while the peaks at 1700~2000 cm
−1 indicate that the benzene ring is mono-substituted, and the peaks at 2850 cm
−1 and 2920 cm
−1 are attributed to –CH
2 asymmetric stretching vibration and symmetric stretching vibration, respectively [
43]. With the addition of the flame-retardant additives AN and ME, the absorption bands near 3440 cm
−1 and 1450 cm
−1 were enhanced, corresponding to the N-H stretching vibration and C-N vibration, respectively. This indicated that AN and ME were successfully doped into the HCP backbone. The enhancement of the characteristic peaks at 3440 cm
−1 and 1450 cm
−1 of HCP-ME was smaller than that of the peaks of HCP-AN, implying that it was physically doped with HCP. Conversely, the characteristic peaks at 3440 cm
−1 and 1450 cm
−1 of HCP-TPP were greatly weakened, which indicates that TPP has successfully reacted.
Afterward, the XRD spectra of the obtained samples were characterized. As shown in
Figure 2b, it was found that the XRD diffraction profiles of the samples were similar due to the similar crystalline shape. The profiles of HCP-AN were weakened after 45°, whereas for HCP-TPP, new diffraction peaks were added near 21° and 32°. HCP and HCP-ME-1:25 showed similar XRD characteristic peaks; there was basically no change in the positions at which the diffraction peaks were located, and there were no obvious changes in the slit width of the diffraction peaks, which indicated that the addition of a small amount of ME would not change the crystal structure of the polystyrene composite flame-retardant material, and the change in the intensity of the diffraction peaks might be due to the slowing down of the nucleation rate caused by the addition of ME. It can be deduced that there are no chemical reactions in the preparation process with the addition of ME, but only physical and mechanical mixing.
Figure 3a,b show the TGA curves and derivative thermogravimetry (DTG) curves of HCP. The TGA curves of HCP show a clear one-step thermal decomposition process, which proceeds in three main stages. In the first stage, from 30 °C to 180 °C, the mass loss is mainly attributed to the volatilization of free and bound water and the decomposition of side chains in HCP [
44]. In the second stage, from 240 °C to 320 °C, the main chain of HCP breaks down and decomposes to produce non-flammable gases [
45]. The third stage is from 320 °C to 490 °C and is dominated by the continuous pyrolysis of HCP [
46].
Table 2 shows the TGA test data of the samples under a nitrogen atmosphere. The analysis shows that the maximum loss rate of the HCP sample is −10.87%/min at 445 °C, and the residual carbon content is 35.85 wt% at 600 °C, which is elevated by 35 wt% relative to pure PS, whose maximum loss rate is −26.54%/min at 413 °C. Compared with HCP, HCP-AN-1:25 has 36.96 wt% residual carbon, but its thermal decomposition temperature starts earlier, with a maximum loss of −12.11%/min at 442 °C, which is attributed to the influence of NH
3 groups in HCP. In addition, the thermal stability of HCP-TPP-1:25 is reduced, with a residual carbon content of 30.73 wt%, and its thermal decomposition starts earlier than that of HCP, with a maximum loss rate of −12.00%/min at 441 °C, whereas, for the sample physically doped with the ME flame retardant, the residual carbon content of HCP-ME-1:25 is 40.63 wt% at 600 °C, much higher than indicated in previous research on flame-retardant materials for PS [
40,
46,
47], and the maximum loss rate is −9.81%/min at 446 °C. The maximum loss rate is −9.81%/min, but the weight loss at 1% and 5% occurs at a much lower temperature, which may be because the sample contains more moisture and has ME distributed on its surface, which is pyrolyzed in advance. It is important to note that a low thermal decomposition temperature does not mean low flame retardancy. The lower the decomposition temperature of the sample, the earlier it decomposes to produce a non-flammable gas/film, resulting in a flame-retardant effect.
In
Figure 4a–d, SEM images show the microscopic morphology of the samples. There are particles on the surface of the material, and there is a crosslinking phenomenon. In
Figure 4a–c, it is observed that the surface of the sample is relatively rough and has a porous structure, and there are smaller particles of material distributed on the surface. The particles are the ferric chloride catalyst, and they are distributed on the surface of the material after the material is milled. This may help to improve its flame retardancy. Comparing
Figure 4a–c, it can be observed that the microscopic morphology of HCP-ME is quite different from the other three groups, with larger particulate matter distributed on its surface in
Figure 4d, which is the reason that the ME molecules cannot participate in its reaction but are added to the material in the form of physical mixing, and thus, there are ME particles distributed on the surface of the material as well as in its interior. When the material burns, the ME molecules distributed on its surface will decompose and absorb heat, which improves the flame retardancy of the material.
3.2. Flame-Retardant Performance Test
To evaluate the material’s flame-retardant properties, we conducted a loss-on-ignition test, which represents the amount of gaseous products released by the physical evaporation or chemical decomposition of raw materials due to heat [
48,
49].
Table 3 shows the loss-on-ignition data of the HCP flame-retardant materials, from which it can be seen that with the increase in AN addition, the loss on ignition of the materials decreases, and the loss on ignition was less than that of the control group without additives. From the loss-on-ignition test curve (
Figure 5a), it was found that the mass of the samples changed a lot in the first 5 min, which is the main decomposition process. The mass curve for HCP-AN-1:10 stabilized at 14 min, HCP-AN-1:15 at 12 min, HCP-AN-1:20 at 16 min, and HCP-AN-1:25 at 22 min, indicating that the change in AN has a significant effect on its loss on ignition. This is because AN is an alkaline material and FeCl
3 is an acidic catalyst; when too much of the flame-retardant additive AN is added, a mutual reaction between AN and FeCl
3 occurs, which affects the structure of the product, so its loss on ignition increases and the curve stabilization time is prolonged. However, from the results of the loss on ignition, it can be seen that the loss on ignition of HCP-AN is smaller than that of HCP, which has a positive effect on the flame-retardant properties of the material. With the increase in TPP addition, the loss on ignition of the material increased. This may be attributed to the fact that TPP is a triphenyl structure containing multiple reaction sites, and the more TPP that is added, the less homogeneous the reaction is, leading to a decrease in the stability of HCP-TPP and an increase in the rate of loss on ignition. From the loss-on-ignition curve (
Figure 5b), it was found that HCP-TPP has similar properties, and the main decomposition process occurs in the first 6 min; its final loss on ignition is much smaller than that of HCP-AN, which is mainly because HCP samples doped with different flame-retardant additives had significantly different combustion products under aerobic conditions. Therefore, it is not appropriate to compare samples with different additives through loss on ignition, which should only be used to compare the same kinds of additives. The loss on ignition of HCP-ME did not show regular changes: HCP-ME-1:10 and HCP-ME-1:25 had a smaller loss on ignition, and HCP-ME-1:15 and HCP-ME-1:20 had a larger loss on ignition, the reason being the uneven distribution of ME molecules in the interior of HCP. However, it was found that the addition of ME reduced the burnout rate of HCP, which is because the addition of ME makes HCP-ME thermally decompose to produce ammonia and other gases, and ammonia decomposition is a kind of heat-absorbing reaction, as the resulting gas will take away heat in the form of thermal convection, which slows down the whole thermal decomposition process and reduces the rate of product loss on burning (
Figure 5c).
As shown in
Figure 6, the PHCP sample (60 mm × 20 mm × 20 mm) was clamped to the iron frame table of the combustion test bench and burned with an alcohol lamp for 1 h. After that, the remaining length of the combusted sample was measured, and during this process, infrared thermal imaging cameras were used to observe the temperature distribution of the material during the combustion process. As can be seen from
Table 4, the addition of the flame retardants AN, TPP, and ME has a positive effect on the flame retardancy of the materials. With the increase in the addition of the flame-retardant additive AN, the residual burning length of the PHCP-AN sample showed a gradual increase of 43 mm, 45 mm, 45 mm, and 47 mm, indicating that its flame-retardant effect increases with the increase in the molar fraction of flame-retardant additives. This is because of the presence of ammonium ions, so HCP-AN is thermally decomposed to produce ammonia, nitric oxide, and other non-combustible gases. Among them, ammonia will further decompose with oxygen and absorb heat. This process not only reduces the concentration of oxygen but also takes away the heat, so its flame-retardant properties are enhanced with the increase in the dosage of AN additives.
With the increase in the flame-retardant additive TPP, the residual burning length of the PHCP-TPP sample showed a gradual increase of 44 mm, 46 mm, 46 mm, and 47 mm, and the overall residual length was better than that of the PHCP-AN sample. As a kind of organic phosphorus flame retardant, the flame-retardant mechanism of TPP is as follows: HCP-TPP will decompose to generate metaphosphoric acid during the combustion process, and the metaphosphoric acid will polymerize to generate polymethyl metaphosphoric acid. In this process, the covering layer generated by phosphoric acid plays the role of a coating, but because the generated polymethylphosphoric acid is a strong acid with strong dehydrating properties, it will also make the polymer dehydrated and carbonized, change the combustion process of the polymer, and form a layer of carbon film on the surface of the polymer in order to insulate the air.
However, the residual burning lengths of HCP-ME did not change regularly with the increase in flame-retardant additives, and the overall residual burning lengths were 42 mm, 47 mm, 42 mm, and 48 mm with the increase in flame-retardant additives. The flame-retardant mechanism is similar to that of HCP-AN, but ME is doped into HCP in a physical form, which is different from the traditional means of physical doping, and this method leads to the uneven distribution of ME inside HCP, resulting in large changes in its flame-retardant effect. Therefore, in the future, the physical doping of flame retardants should focus on the doping method to ensure the uniform distribution of flame retardants.
It can be seen that, compared with the physical addition of the ME flame retardant, the chemical addition of the AN and TPP flame retardants is more stable in improving the flame-retardant performance of the material.
Table 5 shows the infrared thermography of the sample combustion process. During the early stage of combustion (0~2 min), there will be a brief (<30 s) combustion phenomenon, and then the flame will extinguish on its own. From
Table 4, it was found that during the entire heating process of the alcohol lamp, only the front part of the material is in a high-temperature state, while the back part is at room temperature, indicating that the material has an excellent insulation effect. However, during the flame combustion process, although the front part of the material does not self-ignite, a slow oxidation process occurs, causing the phenolic resin prepolymer of the material to lose its binding, become a powder, and fall off. In addition, it was found that the front end of the material can still maintain a high-temperature state after prolonged combustion. Only when the material burns to a sufficiently short state will the temperature at its front end return to normal, as shown in S11 and S13. Unfortunately, because the experimental process is susceptible to the influence of the external environment, we cannot compare and analyze the thermal insulation effect of the material from infrared thermography but only analyze the approximate pattern. Therefore, subsequent thermal conductivity testing of PHCP samples was carried out.
Figure 5d shows the thermal conductivity of the materials. In addition, Tukey’s test was performed on the obtained data and labeled a–d to indicate that there is a significant difference between the thermal conductivity of the samples, while the same labeled letters represent non-significant differences between the data. It was found that the samples all have low thermal conductivity, which indicates their excellent thermal insulation effect. For PHCP-AN, the thermal conductivity of PHCP increased with a small amount of AN, which may be attributed to the fact that too little AN has little effect on the pore structure of PHCP, and some impurities, such as FeCl
3, remain in the pores of PHCP. With the increase in AN content, the thermal conductivity decreases and then increases. This is because when a small amount of AN, a flame-retardant additive, is added, the presence of AN increases the porosity of the product and raises its air sites, which increases its thermal resistance and enhances the thermal insulation effect of the samples. However, since AN is alkaline and FeCl
3 is an acidic catalyst, when too much of the flame-retardant additive AN is added, AN and FeCl
3 will react with each other, thus reducing the porosity of the product and weakening the heat insulation effect. For PHCP-TPP, a small amount of TPP addition does not have a significant effect on its thermal conductivity, while when too much TPP is added, the thermal conductivity of PHCP-TPP first increases and then decreases, which is mainly related to the homogeneity of the TPP reaction; when the amount of TPP is increased, the homogeneity of the reaction deteriorates, which leads to a change in the structure of the product [
43].
The thermal conductivity of PHCP-ME did not change regularly with the increase in the amount of ME added. Its thermal conductivity is mainly related to the distribution of ME; because PHCP is a porous material, when ME molecules are distributed inside the pores of the material, the pore diameter becomes smaller, the thermal resistance of materials is reduced, and the coefficient of thermal conductivity increases, which is especially obvious in S10, because S10 has more ME added. This also explains why S10 has the longest residual burning length in the combustion test, because ME is distributed inside the aperture of the material, which reduces the contact area of oxygen, on the one hand, and on the other hand, when the material burns, the flame retardant is distributed more widely and uniformly, which can better prevent combustion.