Enhanced flame retardant efficiency of in-situ polymerizing barium phenolic resin modified with carbon nanotubes

The experimental materials and main testing instruments used in this paper are shown in table 1.

The white solid phenol was heated in a water bath at 45 °C and maintained for a certain time until the solid phenol became colorless and transparent liquid. The electronic balance was used to weigh the quantitative barium hydroxide catalyst with the mass ratio of phenol to barium hydroxide 1: 0.04. As shown in figure 1(a), connect three flasks, mixer, thermometer and other experimental devices. A certain mass of melted liquid phenol and formaldehyde solution were weighed into a three-mouth flask. The molar ratios of formaldehyde and phenol were 1.3:1, respectively, for six comparative experiments. The temperature of the water bath was maintained at 60 °C–65 °C.

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After 30 min, the barium hydroxide catalyst was added into the three-mouth flask. The sample was heated for 4–5 h in water bath at 80 °C–85 °C. It was then heated at 90 °C–95 °C for about 60 min. Finally, PR after initial modification was obtained from being cooled down with cold water. When it drops to 20 °C the brown-red PR liquid was poured into the beaker, as shown in figure 1(b), and was then dried in the vacuum drying oven. The temperature was set at 30 °C–45 °C, and the pressure was maintained at about 0.084 MPa. After vacuum drying for about 30 min, take out the beaker. It was then put into the air drying oven to cure. The temperature of the drying oven was set at 60 °C, and it was heated to 80 °C after drying for about 1 h. After it was maintained at a certain temperature gradient for 10 h, it was taken out of the mold when cooled to room temperature. The final specimens were bagged and recorded.

LOI test is used to characterize the flame retardant properties of the composites. The diffusion ignition method is applied in the test. The flame of the igniter is contacted with the vertical surface through the top surface of the sample and reaches the 6 mm mark. The material is ignited for 30 s continuously and then removes igniter every 5 s. Check the burning condition of the specimen until the vertical surface can be stably ignited. When the burning part reaches the upright line, it is considered that the specimen is ignited.

TG test is to characterize the thermal stability of the material. The specimen is ground into powder about 3–5 mg. In the air atmosphere, the sample is increased from 50 °C to 800 °C at the heating rate of 10 °C min⁻¹ to study the relationship between the temperature and the char content.

The cone calorimeter is to characterize the flame resistance of the material. According to the international standard ISO 5660, the sample size is about 100 mm × 100 mm × 4 mm and the radiation heat flux experimental value is 50 kW m⁻² [24]. The room temperature was about 22 °C; the pressure was about 1 atm; relative humidity was 33%; air flow rate was 24 l s⁻¹.

FESEM is to characterize the microstructure of the material. Firstly, ETD-2000 ion sputtering instrument was used to spray gold for samples, and then charring of samples with different proportions was observed by FESEM. FESEM used in the test is ΣIGMA series thermal field emission scanning electron microscope produced by Carl Zeiss company of Germany.

LOI is a way to evaluate the relative flammability of polymer composites. From table 2, it can be seen that the LOI of PR is 32.9%, and the LOI are increased by 3.6% and 2.4% respectively after adding 0.2 wt% and 0.6 wt% MWCNTs-OH. The flame retardant effect is more obvious than the pure PR. When MWCNTs-OH is 1.0 wt% and 1.4 wt%, LOI of the samples are decreased by 5.5% and 9.1% respectively. With the increase of MWCNTs-OH, the LOI of PR decreases gradually. When PR is curing, its molecules are randomly arranged and the collision is random. The addition of MWCNTs-OH can induce the accumulation of PR molecules in the surroundings and increase the probability of intermolecular collision. When the proportion is appropriate, the combination of them is sufficient, and its flame retardant will increase. Excessive MWCNTs-OH will cause agglomeration, resulting in specimen voids and roughness, affecting the integrity of the carbonized carbon layer and weakening the role of blocking heat transfer. Finally, heat can be transferred to the carbon layer, causing more intense combustion.

Figure 2 shows the thermo gravimetric curve (TG curve) and the weight loss rate curve (DTG curve) of different samples. There are three stages of degradation in the process of heating. The first stage is from 50 °C–300 °C. In the initial stage, as the temperature increases, the physical bulk density changes, and the unreacted formaldehyde and other substances volatilize. At about 200 °C, the ether bond begins to undergo thermal degradation, and small molecular groups such as carboxyl groups and methyl groups are removed (about 300 °C), and its weight loss is relatively small. The second stage is the thermal decomposition of methylene and the dehydration condensation of hydrogen between phenolic hydroxyl and benzene ring (about 450 °C). Further crosslinking reaction of PR results in the formation of cyclic carbon with increasing temperature; methylene-bridged is converted to hydroperoxide, further triggering decomposition to produce alcohol and ketone compounds [25, 26]. In the third stage, PR occurred dehydrogenation and carbonization, and decomposed to CO, CO₂, CH₄, H₂ and other small molecules. When the temperature is higher than 570 °C, the weightlessness is basically stable. Comparing the TG curves of MWCNTs-OH/PR and PR, the PR curve varies obviously, while the MWCNTs-OH/PR curves of 0.6 wt% and 1.4 wt% are relatively flat and smooth. It is due to that MWCNTs-OH could promote the condensation and cyclization reaction between hydroxyl and PR to make MWCNTs-OH insert into the PR molecules by chemical bond and form a highly stable heterocyclic structure. Meanwhile, it improves the aromatic of resin to reduce the emission of H₂, CO, CH₄, CO₂ and other small molecules, thereby reduces the amount of decomposition of phenolic resin in high temperature, and effectively improves the carbon residue rate of PR. It will lead to the aggregation of MWCNTs-OH and result in reducing flame retardant effect with the increasing of the amount of MWCNTs-OH. However, when reaching a large number MWCNTs-OH, the flame retardant of PR will increase again because of the nature of MWCNTs-OH. According to the data of TG curve, it is found that the carbon residue rate of MWCNTs-OH/PR is increased at 800 °C, and it is increased by 6.1% when MWCNTs-OH is 0.2 wt%. Compared with PR, the addition of MWCNTs-OH was beneficial to reduce the thermal decomposition rate of PR and improve the carbon residue ratio. The temperature (T_20%) and time (t_20%) of the weight loss obtained from figure 2 are shown in table 2. The results show that the t_20% of MWCNTs-OH/PR is prolonged. When MWCNTs-OH is 1.4 wt%, the t_20% of modified resin is extended by 45 s compared with PR. It coincides with the analysis of TG. The peak of the DTG curve indicates the temperature at maximum weight loss rate. According to the curve, the temperature at maximum weight loss rate of the 0.2 wt% MWCNTs-OH curve is about 440 °C. The time to reach the temperature of samples of 0.6 wt%, 1.0 wt% and 1.4 wt% MWCNTs-OH are significantly delayed, in which 1.4 wt% MWCNTs-OH is delayed the most. The main reason is that MWCNTs-OH is beneficial to delay the thermal decomposition of PR and effectively decrease the temperature at maximum weight loss rate, reducing the decomposition of PR at high temperature, and thus improving the flame retardant performance of PR.

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The thermal stability of the composites was calculated by Coats-Redfern method. The thermal stability of the composites was analyzed from the mathematical point of view by the apparent activation energies. The Coats-Redfern method is a classical integral method for calculating the single heating rate in pyrolysis kinetics. And when the material consists of one or more irreversible reactions, the pyrolysis kinetic expression is divided into two types [27].

When n = 1,

$$\begin{eqnarray}&&\mathrm{ln}\left[\displaystyle \frac{1-{(1-\alpha )}^{1-{\rm{n}}}}{{T}^{2}(1-n)}\right]=\,\mathrm{ln}\left[\displaystyle \frac{AR}{\beta E}\left(1,-,\displaystyle \frac{2RT}{E}\right)\right]-\displaystyle \frac{E}{RT}\end{eqnarray} \tag{ 1 }$$

When n≠1,

$$\begin{eqnarray}&&\mathrm{ln}\left[\displaystyle \frac{1-{(1-\alpha )}^{1-{\rm{n}}}}{{T}^{2}(1-n)}\right]=\,\mathrm{ln}\left[\displaystyle \frac{AR}{\beta E}\left(1,-,\displaystyle \frac{2RT}{E}\right)\right]-\displaystyle \frac{E}{RT}\end{eqnarray} \tag{ 2 }$$

Where α is the decomposition rate, T is the temperature corresponding to the decomposition rate, A is the preexponential factor, E is activation energy, β is the heating rate, and R is the gas constant (8.314 J · mol⁻¹ · K⁻¹), n is the reaction order.

Since the thermal decomposition of PR and MWCNTs-OH/PR samples varies greatly from 10% to 80%, the weight loss rate of 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% were selected according to the DTG curve. The first four components are in the low temperature phase and the last four components are in the high temperature phase. Select the two levels of reaction stages n = 1 and n = 2, and use DTG data for efficient calculation mapping. As a result of the thermal decomposition of PR is a relatively complex process, two temperature intervals were chosen in this study, namely low temperature stage (1st Stage) and high temperature stage (2nd Stage). Figure 3 shows the curves obtained by fitting the pyrolysis data by the Coats-Redfern method, where n = 1 and n = 2, respectively. When n = 1, $\mathrm{ln}\left[-\,\mathrm{ln}\left(1-\alpha \right)/{T}^{2}\right]$ and $1/T$ are plotted to produce a straight line with a slope of $-E/R.$ When n = 2, the straight line can be got in the same way.

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The apparent activation energy of different specimens can be calculated as shown in table 2. The value of α is based on TG and DTG curve data. The polymer activation energy represents the minimum energy required for a chemical reaction of a polymer, and it indicates how easy the polymer pyrolysis process is. According to table 2, when n = 1, the activation energy can be increased by 1.628 kJ mol⁻¹ and 3.530 kJ mol⁻¹ respectively when MWCNTs-OH is 0.2 wt% and 0.6 wt% at low temperature stage. And the activation energy is increased by 11.052 kJ mol⁻¹ with MWCNTs-OH of 0.2 wt% at high temperature stage. When n = 2, the activation energy is increased by 1.739 kJ mol⁻¹ and 4.001 kJ mol⁻¹ when MWCNTs-OH is 0.2 wt% and 0.6 wt% at low temperature stage. The activation energy is increased by 4.584 kJ mol⁻¹ when MWCNTs is 0.2 wt% at high temperature stage. It shows that when MWCNTs-OH is 0.2 wt%, the activation energy of both MWCNTs-OH is improved in the low temperature stage and high temperature stage, which is beneficial to enhance the thermal stability of PR. When MWCNTs-OH is 0.6 wt%, the activation energy increases most at the low temperature stage, but decreases at the high temperature stage.

Figure 4 shows the heat release rate (HRR), total heat release (THR), mass loss rate (MLR) and effective heat combustion (EHC) curves of PR and MWCNTs-OH/PR. Heat release rate (HRR) indicates the fire intensity. According to figure 4(a), HRR curve is single peak type. When the sample is burning, the carbon layer formed prevents the heat transfer to reduce the heat transfer rate and slows down the decomposition rate of the material. It leads to the inhibition of the heat release rate to flame retardant effect. It can be seen from table 2 that when MWCNTs-OH is 0.2 wt%, 0.6 wt% and 1 wt%, the ignition time is delayed by 20 s, 6 s and 11 s respectively. And the peak of HRR (pHRR) is reduced by 21.4%, 13% and 12% respectively compared with pure PR. However, when the addition is 1.4 wt%, the ignition time is shortened by 106 s and pHRR is risen by 11%.

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Total heat release (THR) is the total amount of heat released from the material per unit area. The greater the THR, the greater the danger of fire. As shown in figure 4(b), when MWCNTs-OH is 0.2 wt%, 0.6 wt% and 1 wt%, THR is decreased by 0.5%, 6.7%, 1.3% respectively compared with pure PR. When MWCNTs-OH is 1.4 wt%, THR is increased by 13.4%. Mass loss rate (MLR) is the rate of change in mass loss of material during combustion. It reflects the pyrolysis speed and behavior of materials at a certain fire intensity. Figure 4(c) shows that when the MWCNTs-OH is 0.2 wt%, 0.6 wt%, 1.0 wt%, the time to the peak of MLR (pMLR) is delayed for 62 s, 9 s and 34 s respectively. The average of MLR in the first 120 s is 0.068 g s⁻¹, 0.062 g s⁻¹, 0.068 g s⁻¹, 0.066 g s⁻¹ and 0.081 g s⁻¹ respectively. Effective heat of combustion (EHC) indicates the heat generated by the combustion of the combustible component in the volatiles formed by the thermal decomposition of the material during combustion. It reflects the degree of gas combustion in the gas phase flames, the higher the ratio, the higher the risk of fire. From figure 4(d), it is found that time to peak of EHC (PEHC) of the samples with 0.2 wt%, 0.6 wt% and 1.0 wt% MWCNTs-OH are delayed to 110 s, 24 s and 72 s compared to PR. When MWCNTs-OH is 0.2 wt%, the average of EHC is reduced by 4.0%.

In conclusion, the effect on the thermal stability and flame retardancy of the material is the most obvious when the MWCNTs-OH is 0.2 wt%. When adding the appropriate proportion of MWCNTs-OH, it can better prevent the heat transfer, slow down the HRR and thermal degradation rate, and effectively reduce THC and EHC under heat radiation. The proliferation of the inhibition plays a role in enhancing the flame retardant and thermal stability of the sample. And the following thermogravimetric analysis shows that when MWCNTs-OH is 1.4 wt%, the thermal stability is relatively better because MWCNTs-OH/PR shows more MWCNTs-OH characteristics after PR is grinded into powder. However, the whole resin composite is analyzed in cone calorimeter experiment, which is closer to reality application. And the results indicate that excess MWCNTs-OH will affect the dispersion of MWCNTs-OH in PR and destroy the structural properties of PR, thus PR thermal stability and flame retardancy will be reduced.

When the material burns, the amount of toxic gas is an important parameter to measure the fire safety of materials. PR produces CO when insufficient combustion occurs. Most of the people who died in the fire were suffocated by inhaling excessive toxic gas, so the use of PR in the combustion process must take full account of the toxicity of the material. The production of CO (COP) can be seen from table 2. The time to reach the peak of COP of the composites with 0.2 wt%, 0.6 wt% and 1.0 wt% MWCNTs-OH are all delayed compared to PR which is 225 s. The modified PR with the addition amount of 0.2 wt% is obviously changed, and the peak time is delayed to 338 s; the peak value is decreased by 25.9%. At the same time, the average yield of CO (COY) is decreased by 21.4%. It displays that MWCNTs-OH have a certain effect on inhibiting the production of toxic gas CO and reducing the release rate of CO. The peak and average production capacity of the 1.4% MWCNTs-OH added increase more than pure PR's. The rate of COY during combustion is faster that is easier to cause personal security threats and it is not conducive to evacuation of the people after the fire.

Figure 5 shows combustion residues of different samples. There are different degrees of cracks in the residue of the sample, in which PR cracks are the most, and the sample is divided into irregular small pieces. The samples with 0.2 wt% and 0.6 wt% MWCNTs-OH have relatively few cracks and good residue integrity. It can be seen that there are circular residues of different sizes on the surface of the residue. This is due to the generation of carbon dioxide, water vapor and a small amount of carbon monoxide during combustion. When the sample is carbonized to form a carbon layer, it cannot be released in time to form a cavity. The presence of the cavity can reduce the heat transfer and reduce the burning rate of the material, and also reflects the relatively complete carbonization of the sample from the side. The residues of the samples with 1.0 wt%, and 1.4 wt% MWCNTs-OH have a good degree of fragmentation compared to the PR. It indicates that the addition of MWCNTs-OH is beneficial to the integrity of PR as a whole.

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In order to confirm the effect of MWCNTs-OH on carbonization of PR, the residue after cone calorimeter is observed under thermal field emission scanning electron microscope (FESEM). Because PR is an insulator, the sample must be sprayed with metal film before it is observed. It can prevent the accumulation of charge, thus increasing the stereo sense of the image and reducing the thermal damage.

Pyrolysis of resin produces a carbonaceous residue entitled 'char' [28]. Figures 6(a) and (b) indicates the surface of pure PR is relatively symmetrical; white carbonized char is relatively intact that can play a crucial role in ablation behavior because it reduces the pyrolysis rate by emitting considerable incident radiation into the gas phase [29, 30]. The heavier color parts are not completely carbonized which represents that the effect of blocking heat transfer is not obvious. Figures 6(c) and (d) show MWCNTs-OH/PR carbonized image after adding 0.2 wt% MWCNTs-OH. On the surface there are many uneven size of the raised particles wrapped in a thick layer of carbon. This carbon layer is PR wrapped outside of MWCNTs-OH that formed by high temperature carbonization. The carbon layer is tightly connected. Thus, the sample with 0.2 wt% MWCNTs-OH plays a significant role in the carbonization during the combustion of PR. Also it shows that MWCNTs-OH has a good combination with PR which is helpful to improve the flame retardancy. It will generate conjugation of two π bonds with adding MWCNTs-OH in PR. It is helpful to promote the carbonization of carbon layer to improve the integrity and tightness of the composites, thus improving the ability of the sample to block heat transfer and fire reaction compared with PR. Figures 6(e) and (f) show MWCNTs-OH/PR carbonized image after adding 1.4 wt% MWCNTs-OH. The carbon layer is obviously uneven, and MWCNTs-OH appears obvious agglomeration that damages the compactness and integrity of the carbon layer, reduces the blocking heat transfer effect.

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