A Series of Novel Flame Retardants Produced with Nanosilica, Melamine, and Aluminum Diethylphosphinate to Improve the Flame Retardancy of Phenolic Resin.

To facilitate the flame retardancy of phenolic resin (PF), a series of novel flame retardants with nano-SiO2, melamine, and aluminum diethylphosphinate (ADP) were freshly prepared and tested. A thermogravimetric analysis, cone calorimeter, and scanning electron microscopy were employed to determine the thermal decomposition, flame retardancy, combustion properties, and structure of the carbon residue layer of PF. The pyrolysis kinetic parameters of modified PF were then computed, and the pyrolysis process was appraised. The results indicated that when 1.5 wt % of nano-SiO2, 3 wt % of melamine, and 15 wt % of ADP were added to PF, the limiting oxygen index value reached 39.6%, and UL-94 passed the V-0 level. A substantial synergistic effect was also observed. The thermogravimetric analysis revealed that the char residue at 800 °C reached 59.93 wt %. Furthermore, in the cone calorimeter test, the total thermal release and thermal release rate decreased to 30.7 MJ/m2 and 105.7 kW/m2, respectively.

Introduction

One of the earliest polymers adapted for use in industry is phenolic resin (PF). Research on this resin has been conducted for almost a century. PF is applied as an anticorrosion and flame-retardant material and as an adhesive due to its strong acid resistance, good thermal resistance, and immaculate mechanical properties.[1] However, the mass of methine groups present in the molecule of PF means that when an external force is applied the methine bridge is readily broken, rendering it brittle and substantially limiting its applications. Apart from its structural characteristics, the secondary methyl group is effortlessly oxidized and cleaved at high temperatures. This research focuses on the enhanced fireproof and flame retardant properties of PF. This is still a hot topic in the field of polymer materials.[2]

PF is primarily modified with boron (BN), silicon, nitrogen-based, and nanoscale-phosphorus-based flame retardants. Du et al. successfully synthesized an addition-curable hybrid phenolic resin containing silicon and boron. They proved the synergistic effect of boron- and silicon-modified PF through a series of FTIR, SEM/SEM-EDS, and other experiments.[3] Chemical cross-linking generated by the two-phase interface resulted in a sealed coating of glasslike ceramics on the interface layer, which improved corrosion resistance.[4,5] Mohammed et al. found that adding boron oxide to PF could promote PF carbonization and successfully found that its catalytic mechanism occurred in a multistage manner.[6] Li et al. employed borosilicate to modify PF. A new type of PF containing BN and silicon was prepared as part of this process. Its structure, molecular weight, curing property, gel property, and heat resistance were further characterized. Both BN and silicon were introduced to the PF to enhance thermal and oxidation resistance.[7] Xu and colleagues prepared phosphorus- and nitrogen-modified PF. Their experimental results illustrated that adding phosphorus and nitrogen to the cured resin system can produce a synergistic flame retardant effect, enhancing the flame-retardant properties of the fixed resin system and alleviating the thermal release rate. As the phosphorus and nitrogen contents increased, the flame retardant performance of the resin system increased after curing.[8]

In practice, nanomaterials are considered some of the most promising materials in the 21st century. The particle size, high surface energy, and elaborate surface coordination mean that a nano-SiO2 amorphous white powder (referred to as its agglomerate) can expeditiously form a bond with both itself and oxygen. This improves its intermolecular key force.[9,10] The three-dimensional mesh shape of nano-SiO2 substantially ameliorates the material’s thermal resistance.[11] It has been widely used in the modification of resin. For example, Chen et al. modified PF by coating nanosilica with graphene. A thermogravimetric (TG) experiment proved that the thermal stability of PF could be effectively improved and the carbon residue was increased by 6.54%, proving that coating nanosilica with graphene could promote the application of PF in the field of flame retardancy.[12] Noparve-Qarebagh et al. combined carbon nanotube aerogels with PF in an aerogel network to achieve a high carbon yield. Aerogel formation was confirmed using Raman spectroscopy, X-ray diffraction, and scanning electron microscopy (SEM).[13]

As one of the new flame retardants, melamine is nitrogen-based and halogen-free. The mechanical and physical properties of products can be enhanced by adding melamine. The advantages of melamine are thus clear in comparison with other flame retardants. For instance, Ge et al. modified PF by adding melamine, which indicated that 4.5 wt % was the optimal melamine composition. Moreover, the oxygen index increased and the free formaldehyde content decreased.[14]

Another new flame retardant, aluminum diethylphosphinate (ADP), has been widely applied in recent years because it is halogen-free, has excellent flame retardancy and strong thermal stability, and is outstanding in hydrophobic smoke suppression. By adding bamboo-based porous carbon and ADP to epoxy resin (EP), Wang and Hao scrutinized the action and synergistic mechanism of bamboo-based porous carbon and ADP. UL-94 vertical combustion test, limiting oxygen index (LOI), and cone calorimeter studies demonstrated that a combination of 3 wt % of the phase change material (PCM) and 4.4 wt % of ADP can increase the LOI of EP composites from 24.6 to 42.6 wt %; moreover, the UL-94 test achieved the V-0 level and the peak thermal release rate was reduced to 60.7%. Bamboo-based porous carbon and ADP have a remarkable effect on flame-retardant EP.[15]

Although research on the pyrolysis mechanics of materials has matured, it has not yet reached saturation. Kayacan and Doǧan analyzed the thermal decomposition kinetics of low-density polyethylene and high-density polyethylene (HDPE). They employed TG tests to evaluate the apparent activation energy (Ea) and pre-exponential factor (A).[16] Ding et al. conducted a comparative study on the pyrolysis behavior and reaction mechanism of hardwood beech and softwood sequoia. The Flynn–Wall–Ozawa (FWO) model-free method estimated various Ea values and predicted reaction mechanisms using the Coats–Redfern model-fitting method.[17] Employing FWO, Friedman, and Kissinger–Akahira–Sunose (KAS) methods, Aboulkas et al. measured the thermal degradation behavior of polypropylene and polyethylene to obtain Ea values. They also determined the appropriate conversion of the process model using Criado and Coats–Redfern methods.[18] From these advances, this study adopted thermokinetic methods, such as FWO and KAS, to compute the Ea and A values of the chemical parameter reaction of the modified PF.[19,20]

To achieve this goal, a series of novel flame retardants with nano-SiO2, ADP, and melamine were mixed and added to PF. Applying TGA, flame retardant tests, and cone calorimeter tests, we determined that the synergistic effect of the three flame retardants caused the modified PF to exhibit impeccable flame retardancy thermokinetics and combustion properties. In addition, Fourier-transform infrared (FTIR) spectroscopy and SEM were applied to determine the flame-retardant mechanisms of the modified PF.

Experimental Section

Materials

Industrial products were chosen as the raw materials. PF was provided by Baiqian Chemical Co. (Qingdao, Shandong Province, China), and melamine (Analytical Reagent, 99%), aluminum diethylphosphinic acid (Analytical Reagent, 99.5%), and nano-SiO2 (15 nm) were obtained from Sinopharm Chemical Reagent Co. (Nanjing, Jiangsu Province, China). Anhydrous ethanol (Analytical Reagent, 99.7%) was acquired from Yonghua Chemical Tech Co. (Shenzhen, Guangdong Province, China).

Preparation of Modified Phenolic Resin

First, a mixture of PF and anhydrous ethanol was stirred and dissolved, after which melamine, ADP, and nano-SiO2 were added and dissolved by stirring for 1 h at 60 °C with a constant-temperature magnetic stirrer. An ultrasonic disperser was then utilized to oscillate the mixture for 30 min ultrasonically. The mixture was then left to stand for 30 min, after which the bubbles were removed. The defoamed PF hybrid solution was poured into a Teflon mold, and the mold was placed in a vacuum drying oven for temperature curing. After the spline temperature dropped to room temperature, the standard test spline of PF composite materials was removed from the mold. The three different types of flame retardants were divided into six groups by three factors and three levels through orthogonal decomposition. The factors in the orthogonal decomposition method were the reasons affecting the results. For example, ADP, melamine, and nano-SiO2 were three factors that affected the results. Next, each element was divided into different values, called levels. The different ratios of ADP, melamine, and nano-SiO2 are given in Table .

Table 1

Samples of the Modified PF with Different Additives

sample code	ADP (wt %)	melamine (wt %)	SiO2 (wt %)
K0	0	0	0
K1	10.0	3.0	1.0
K2	10.0	4.0	1.25
K3	10.0	5.0	1.5
K4	12.5	3.0	1.25
K5	12.5	4.0	1.5
K6	12.5	5.0	1.0
K7	15.0	3.0	1.5
K8	15.0	4.0	1.0
K9	15.0	5.0	1.25

Testing and Characterization

The samples were characterized by infrared spectroscopy, thermogravimetry, LOI, vertical combustion, cone calorimeter, and scanning electron microscopy. First, measurements using an FTIR (Nicolet IS5 Spectrometer, Thermo Scientific, MA, USA) infrared spectrometer were recorded. Next, a SDTQ600 thermogravimetric analyzer (TA Company, DE, USA) was utilized to conduct TG experiments by heating the samples from 50 to 800 °C at a linear heating rate of 10 °C/min under a nitrogen flow of 20 mL/min. In the LOI test, 100 × 10 × 4 mm3 samples were tested using a JF-3 LOI instrument (China Jiangning Analytical Instrument Factory Co., Nanjing, Jiangsu China) by the ISO4589-2 standard. In the UL-94 vertical combustion tests, 125 × 13 × 3 mm3 samples were tested using a CFZ-2 vertical burner (CZF-2, China Jiangning Analytical Instrument Factory). Finally, in the cone calorimeter tests (CCTs), 100 × 100 × 3 mm3 sheets were prepared and tested under a 50 kW/m2 heat flux atmosphere using a cone calorimeter (Fire Testing Technology, West Sussex, UK) by the ISO5660 standard. Following the CCTs, the char layer of the samples was examined through SEM (a Zeiss EVO18 microscope was employed; Carl Zeiss AG, Oberkochen, Germany) to determine the morphological structure. The data of pure PF as the standard comparision sample is from our previous tests.[921]

Results and Discussion

Results of Fourier Transform Infrared Analysis

Pure PF and modified PF were characterized by FTIR, as presented in Figure . In comparison with the pure PF, it is concluded that the antisymmetric vibration of NH2 causes the characteristic absorption peak at 3471 cm–1 in K1 and K9. At the same time, the peak at 818 cm–1 corresponds to the characteristic peak of the para-substituted phenol of PF. Therefore, the characteristic peak at 818 cm–1 can also be found in K0 without melamine. However, the characteristic absorption peak at 818 cm–1 in the infrared images of K1 and K9 is caused by the deformation vibration absorption of triazine.[21] These are typical melamine characteristic peaks. A PO symmetrical stretching vibration was found at 1095 cm–1 and a P=C double bond appeared at 1153 cm–1. These are typical ADP characteristic peaks. The 1093 cm–1 absorption peak was associated with a Si–O–Si antisymmetric stretching vibration, whereas the broad peak at 3449 cm–1 was a structural water −OH antisymmetric stretching vibration. These are typical nano-SiO2 characteristic peaks. The results suggested that the ADP, melamine, and nano-SiO2 were successfully incorporated into the PF matrix.

Figure 1

FTIR spectra of the PF and modified PF.

Results of Thermogravimetric Analysis

The thermal performance of both pure and modified PFs was then analyzed. Figure depicts the TG curve of the samples after heating from 50 to 800 °C at a linear heating rate of 10 °C/min under a nitrogen flow of 20 mL/min. The results of T5%, T10%, and Tdmax, respectively representing the 5 and 10 wt % temperature and maximum mass loss rate, are summarized in Table . In comparison with pure PF, the T5% values of PF composites with flame retardants were significantly reduced, which was believed to be due to an ethanol residue and water volatilization in PF composites. There were generally three TG stages in all of the curves, and the trend in the TG curves for the K1–K9 samples was similar to that for K0 of the pure PF. The temperature range of the first stage is between 50 and 350 °C, and the mass loss in this stage is mainly due to the evaporation of water and the escape of excess phenols, aldehydes, and small-molecule oligomers in PF. The second stage mainly occurs between 350 and 465 °C, mainly due to the thermal degradation of the main chain fracture of PF. The third stage is between 490 and 700 °C and is attributed to the thermal decomposition of the added flame retardants ADP, melamine, and nano-SiO2. The pure PF started to decay when the temperature reached 178 °C, the thermal decomposition rate was the fastest when the temperature reached 550 °C, and the carbon residue was 51 wt % at 800 °C. As presented in Figure and Table , the residual carbon contents of the modified PFs were 55.5%, 52.7%, 59.2%, 53.5%, 52.3%, 50.3%, 59.9%, 50.7%, and 53.5% at 800 °C, respectively. Following the addition of 1.0 wt % of nano-SiO2, 3.0 wt % of melamine, and 10.0 wt % of ADP into PF, the Tdmax value of K1 was 561 °C. The residual amount of carbon increased by 55.5% at 800 °C. In comparison with pure PF, char residues increased by 8.7%. Following the addition of 1.5 wt % of nano-SiO2, 5.0 wt % of melamine, and 10.0 wt % of ADP into PF, the Tdmax value of K3 was 561 °C, and the residual amount of carbon increased by 59.2% at 800 °C. In comparison with pure PF, char residues were augmented by 16%. Following the addition of 1.5 wt % of nano-SiO2, 3.0 wt % of melamine, and 15.0 wt % of ADP to PF, the residual carbon amount increased by 17.5% in comparison with pure PF, reaching a maximum of 59.9%. This demonstrated that nano-SiO2, ADP, and melamine played a synergistic role that enhanced the thermal stability of PF.

Figure 2

TG images of the modified PF samples under a nitrogen atmosphere.

Table 2

TG Data of the Pure PF and the PF Composites

sample code	T5% (°C)	T10% (°C)	Tdmax (°C)	char residue at 800 °C (wt %)
K0	178.0	313.0	550.0	51.0
K1	102.6	218.0	561.0	55.5
K2	155.3	287.0	560.0	52.7
K3	144.0	280.0	561.0	59.2
K4	122.8	247.5	560.0	53.5
K5	146.8	285.0	560.0	52.3
K6	157.1	287.5	558.0	50.3
K7	109.4	247.5	563.0	59.9
K8	153.0	292.8	559.0	50.7
K9	181.9	296.7	560.0	53.5

Flame-Retardant Properties of Modified Phenolic Resin

LOI and UL-94 vertical burning tests were conducted to assess flame-retardant properties of all the samples.[22] The test values are given in Table . As presented, the LOI of pure PF was 30.0%, but the dripping phenomenon was rarely observed in the vertical burning test due to its inherent flame retardancy. All the modified PFs were clearly at the V-0 level in the vertical burning tests. Following the addition of 12.5 wt % of ADP, 5.0 wt % of melamine, and 1.0 wt % of nano-SiO2, the LOI value reached a maximum of 39.6%, higher than that of pure PF.

Table 3

LOI and UL-94 Results of the PF and the Modified PF

sample code	LOI (vol %)	UL-94
K0	30.0	V-2
K1	37.6	V-0
K2	37.4	V-0
K3	38.5	V-0
K4	38.4	V-0
K5	38.3	V-0
K6	39.6	V-0
K7	38.8	V-0
K8	38.2	V-0
K9	38.3	V-0

As observed, the optimal value of the LOI and UL-94 tests was achieved by K6, which had incorporated the addition of 5.0 wt % of melamine, 12.5 wt % of ADP, and 1.0 wt % of nano-SiO2, and the flame retardancy of the modified PF K6 improved considerably. During the combustion process, gases from the decomposition of ADP were volatilized, and free radicals, such as PO, were generated and captured to inhibit combustion.[23,24] Furthermore, part of the residual formaldehyde was consumed in a reaction with melamine to form melamine–formaldehyde-containing nitrogen rings, which had a flame-retardant effect.[25] When the nano-SiO2 added to PF was sufficient, the nanoparticles became attached to the resin molecules. Because of their strong adsorption, they were attached to the surface of the readily oxidized methylene group, thereby promoting carbonization and reducing the possibility of PF being corrupted, as showcased in Scheme . Moreover, the mass of Si was greater than that of C, and therefore, Si–O bonds with higher bond energy entered into the modified phenol resin. Consequently, more power generated by the PF molecular chain was absorbed, which enhanced the flame retardancy of PF.

Scheme 1

Thermal Decomposition Model of Phenolic Resin Composites

Cone Calorimeter Test (CCT) of Modified Phenolic Resin

The CCT is one of the most effective approaches to delve into flame-retardant properties under actual fire conditions.[26] The results of the peak heat release rate (PHRR), the time corresponding to PHRR (TpHRR), the total heat release (THR), the smoke production rate (SPR), the ignition time (TTI), and the amount of carbon residue from CCT are summarized in Table . The curves of HRR, THR, and the mass losses of PF and modified PF are given in Figure .

Table 4

CCT Data of the PF and the Modified PF TPHRR

sample code	PHRR (kW/m2) ±30	TPHRR (s) ±3	THR (MJ/m2) ±1	PSPR (m2/s)	TTI (s) ±3	residue (wt %)
K0	304.4	86.0	78.6	5.55	22	32.2
K1	171.8	100.0	58.2	4.02	23	34.7
K3	120.4	196.0	32.9	2.56	28	36.0
K7	105.7	126.0	30.7	3.58	27	39.1

Figure 3

(a) HRR, (b) THR, and (c) mass loss of PF and modified PF.

Following ignition, a sharp increase occurred in the HRR of pure PF. The peak value of HRR was 304.4 kW/m2 at 86.0 s. THR reached 78.6 MJ/m2, and the carbon residue was 32.2%. When 10.0 wt % of ADP, 3.0 wt % of melamine, and 1.0 wt % of nano-SiO2 were added to PF, the HRR and THR of this K1 group decreased to 171.8 kW/m2 and 58.2 MJ/m2, respectively, and the residual carbon was 34.7%.

In sharp contrast to the pure PF, the HRR and THR of K1 decreased by 43.5% and 26.0%, respectively. Furthermore, the residual carbon increased by 7.8%. When 10.0 wt % of ADP, 5.0 wt % of melamine, and 1.5 wt % of nano-SiO2 were introduced into PF, the HRR and THR of the K3 group were reduced to 120.4 kW/m2 and 32.9 M/m2, respectively. According to the test data, the maximum heat release rates of K1, K3, and K7 decreased and the time to reach the maximum heat release rate increased. Significantly, the time for K3 to reach the maximum heat release rate was prolonged more obviously, which indicates that K3 accelerates the decomposition of noncombustible gas after the temperature increases and forms a gas-phase flame-retardant environment in advance.

In even sharper contrast to the pure PF, the HRR and THR of K3 decreased by 60.4% and 58.1%, respectively. Following the addition of 15.0 wt % of ADP, 3.0 wt % of melamine, and 1.5 wt % of nano-SiO2, the HRR and THR of the K7 group decreased to 105.7 kW/m2 and 30.7 MJ/m2, respectively, a decrease of 65.2% and 60.9%, which differed substantially from the values for pure PF. The residual carbon content of the K7 group was increased by 21.4%. In contrast, the pure PF was increased by 39.1 wt %. At the same time, the PSPR of pure PF decreased to 5.55 m2/s after flame retardant was added. The modified PF K3 was reduced by 53.9% to 2.56 m2/s. The TTI of modified PF with flame retardant was improved in comparison with pure PF. Thus, the three flame retardants prominently decreased the HRR, THR, and PSPR of the PF and increased the residual carbon ratio of PF. Consequently, the thermal stability of PF was effectively enhanced.

Char Residue Analysis

The flame-retardant effect of the modified PF was analyzed from the residual char using a CCT. Digital photographs of the PF and selected PFs before and after the CCT are illustrated in Figures and 5, respectively. According to the carbon residue content after cone combustion, the pure PF carbon residue content was 32.2 wt % at 800 °C. With the addition of flame retardants, the carbon residue content of the modified PF composite also increased. Among them, the modified PF K1 carbon residue increased to 34.7 wt %, the modified PF K3 carbon residue increased to 36.0 wt %, and the modified PF K7 carbon residue was increased to 39.1 wt %. The results showed that the three flame retardants promoted the formation of PF carbon residue.[27,28]

Figure 4

Digital photographs of samples taken before CCT of (a) K0, (b) K1, (c) K3, and (d) K7.

Figure 5

Digital photographs of samples taken after CCT of (a) K0; (b) K1, (c) K3, and (d) K7.

The residual char after the CCT was analyzed using SEM, and the images are shown in Figure . As depicted in Figure a, porous holes and wide cracks are evident on the surface of the carbon layer of the pure PF. This caused the flammable, volatile gas to volatilize swiftly through the pristine PF surface during the combustion process. When ADP, melamine, and nano-SiO2 were added, the carbon layer of the modified PF became more continuous, dense, and compact. However, the rigid intermolecular structure of PF was disturbed when the melamine–formaldehyde was generated, and an irregular cross-linked interpenetrating network structure was formed. Thus, the decomposition of PF was hindered.[29,30] Furthermore, the surface energy of SiO2 was relatively low, and it migrated to the surface of the material during combustion to strengthen the carbon layer. In contrast, diethylhypophosphorous acid was formed by the degradation of ADP, and SiO2 formed a silicon-containing phosphate to attenuate the volatilization of the phosphorus-containing compound.

Figure 6

Electron micrographs of carbon layers after CCT of (a) K0, (b) K1, (c) K3, and (d) K7.

Pyrolysis Kinetics Analysis of the Modified Phenolic Resin System

To conduct a thermal analysis of the materials, three factors of solution kinetics are generally required: Ea of the reaction, pre-exponential factor A, and reaction mechanism f(α) (mechanical function). According to the recommendations of the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry,[31,32] the pyrolysis process can be expressed by eq , illustrated aswhere α is the conversion degree during pyrolysis, k(T) is the reaction rate constant and can be illuminated by Arrhenius law, and f(α) is a function of the reaction mechanism. Accordingly, α and k(T) can be calculated from eqs and 3, respectivelywhere W0, W, and W∞ represent the sample mass at the initial time, time t, and the end, respectively, A is the pre-exponential factor, Ea is the apparent activation energy of the reaction, R is the universal gas constant, and T is the reaction absolute temperature. From eqs and 3, eq can be expressed as follows:

Under linear heating conditions, the expression of the heating rate β is illustrated by eq :

From eqs , and 5, eq can be expressed as follows:

The function of the FWO method is as follows:[33]

If the values of pyrolysis conversion α are the same, then G(α) is a constant value, and ln β has a linear relationship with 1/T. The experimental pyrolysis data obtained the T value at the same conversion degree at different heating rates. Origin software was used to construct the ln β versus 1/T linear regression function graph. In conjunction with determination of the value of A, the Ea value at the conversion range was calculated directly from the slope.

The function of the KAS method is expressed as follows:[34]

Similarly to the FWO method, ln (β/T2) and 1/T exhibit a linear relationship. The pyrolysis data obtained the T value at the same conversion degree from different heating rates. FWO and KAS methods are derived from approximate ways to solve the dynamic parameters. Therefore, there is a specific deviation between the values obtained by the two methods.

Calculation and Analysis of Ea and A

The experimental material was a sample of group K7. Figure demonstrates the TG and DTG curves of the modified PF K7 at heating rates of 10, 20, 30, and 40 K/min. The pyrolysis curves exhibit better consistency. As is evident from the pyrolysis curve at a heating rate of 10 K/min, the pyrolysis of modified PF K7 was divided into three stages. A temperature range from 50 to 350 °C was defined as the first thermal mass loss stage. The mass loss was primarily created by the decomposition of a small number of additives due to residual moisture evaporation. The temperature range from 350 to 465 °C was defined as the second thermal mass loss stage, primarily caused by the PF primary chain fracture.[35,36] The temperature range from 490 to 700 °C was defined as the third thermal mass loss stage, which denoted secondary pyrolysis of the product of modified PF K7.[37]Figure indicates that the primary thermal decomposition of PF took place in the second and third stages. Therefore, the focus of this section is on presenting an analysis of the second and third stages. According to the TG/DTG curve, the main pyrolysis stage of the modified PF K7 occurred from 15.0% to 35.0%. Therefore, the Ea and A of the modified PF K7 in this conversion range was calculated using FWO and KAS methods.

Figure 7

(a) TG and (b) DTG curves of modified PF at different heating rates.

On the basis of the five α degrees, the five corresponding fitting curves were obtained by fitting with Origin software, corresponding to five α degrees of 15.0%, 20.0%, 25.0%, 30.0%, and 35.0%. Figures and 9 illustrate the fitted images for the FWO and KAS methods, respectively. Tables and 6 present the Ea values calculated using the FWO and KAS methods, respectively, at different heating rates.

Figure 8

FWO method curves of the thermal process of modified PF from 65 to 85 wt %.

Figure 9

KAS method curves of the thermal process of modified PF from 65 to 85 wt %.

Table 5

Values of the Pyrolytic Kinetic Parameters Calculated by the FWO Method

	conversion degree (%)
	15.0	20.0	25.0	30.0	35.0
k = −0.4567Ea/R	–5.772	–8.154	–11.96	–7.836	–6.145
Ea (kJ mol–1)	105.08	148.45	217.74	142.65	111.87
R2	0.96081	0.90924	0.96642	0.92942	0.92738

Table 6

Values of the Pyrolytic Kinetic Parameters Calculated by the KAS Method

	conversion degree (%)
	15.0	20.0	25.0	30.0	35.0
k = −Ea/R	–11.86	–17.20	–25.87	–16.27	–12.20
Ea (kJ mol–1)	98.61	143.01	215.10	135.28	101.44
ln A	9.11	14.46	23.86	10.67	4.38
R2	0.94706	0.90105	0.96196	0.91661	0.90991

The fitting results indicate that the Ea calculated using both methods exhibited solid and consistent values. The Ea range of the modified PF K7 composite material increased from 105.08 to 217.74 kJ/mol and from 98.61 to 215.1 kJ/mol, respectively. The Ea calculated using the FWO method was slightly larger than that derived from the KAS method, although the trend was consistent. An analysis was performed using the fitting result of the KAS method as an example, where the Ea at a conversion degree of 15.0% was 98.61 kJ/mol. As the reaction proceeded, Ea increased in line with the conversion degree and reached its maximum value of 215.1 kJ/mol when the conversion degree increased to 25.0%. Thus, the smaller the Ea, the stronger the reactivity. As the α degree increased, some substances, such as water and volatile matters, precipitated in succession, exacerbating the difficulty of continuing the reaction and raising the Ea of the material.[38] Moreover, Ea began to decrease when the pyrolysis of the modified PF K7 proceeded to the α degree at 30.0%. This might have been because the secondary pyrolysis process of the product in the third stage affected the main pyrolysis stage of the modified PF K7 and therefore reduced part of Ea.

Conclusions

Different ratios of nano-SiO2, melamine, and ADP were used and synthesized with pure PF. The FTIR results indicate that these flame retardants can be successfully added to the PF. The conclusions drawn from this study are as follows.

The relative optimized ratios of the three flame retardants were determined through TG analysis, LOI, the UL-94 test, and the CCT. Following the addition of 12.5 wt % of ADP, 5.0 wt % of melamine, and 1.0 wt % of nano-SiO2, the LOI value reached a maximum of 39.6%, 32% higher than that of pure PF. According to the CCT results, with the addition of 15.0 wt % of ADP, 3.0 wt % of melamine, and 1.5 wt % of nano-SiO2, the HRR, THR, and carbon residue of PF were optimal. In comparison with the values for pure PF, the HRR and THR of modified PF K7 decreased by 65.2% and 60.9%, respectively.

The experimental results thus revealed that the performance of PF was optimal when 15.0 wt % of ADP, 3.0 wt % of melamine, and 1.5 wt % of nano-SiO2 were added. As presented for the K7 group, this was the most suitable ratio. The FWO method was employed to calculate the group’s Ea range, which increased from 105.08 to 217.74 kJ/mol. In contrast, when the KAS method was used, the Ea of the modified PF K7 ranged from 98.61 to 215.1 kJ/mol. The SEM results indicate that the carbon layer of the phenolic composite material became more continuous, dense, and compact. Furthermore, the large cracks and holes almost vanished due to the reaction between melamine and formaldehyde.

Nano-SiO2, ADP, and melamine can exert a considerable synergistic effect in the PF, substantially enhancing its thermal stability and flame retardancy.