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1. Introduction
The material surface treatment not only beautifies the appearance of the object but also protects the material body from the erosion of external factors [1]. Organic coating has been widely used due to its wide range of materials, simple process, strong practicability, and low cost. At present, the commonly used organic coatings mainly include alkyd resin, phenolic resin, chlorinated rubber, acrylic resin, epoxy resin, polyurethane resin, and so on [2]. However, these materials all come from the petrochemical industry, which was not only limited in source but also relatively harmful to the environment and does not conform to the concept of green chemistry [3]. Chinese lacquer, a typical candidate of sustainable resources with super-high durability, was a renewable raw material with excellent oxidation resistance and water barrier, good durability, chemical resistance, and mechanical properties. It was considered to be a promising natural coating for applications [4–6]. The main components of Chinese lacquer are urushiol, laccase, and resin, of which urushiol is the main film-forming substance in Chinese lacquer. Urushiol is a mixture of several derivatives of catechol with unsaturated side chains, consisting mainly of 15 carbons and possibly have zero to three double bonds, of which 60–70% are triene-side urushiol [4, 7]. In fact, Chinese lacquer has been used for coating object before more than 7000 years ago in China. However, aside from slow curing speed, there are still many issues to be resolved in practical application [8, 9]: (1) during the curing process of pure lacquer, there is a large amount of bubbles and shrinkage appeared at the surface [10]. Due to the defective sites on the surface, it is easy for erosion from the environment. This ultimately affects the overall protection of the coating and has a significant impact on the stability of the coating [11]; (2) pure lacquer coatings has the characteristics of brittleness and low impact resistance, which is not conducive to long-term use in high-impact environment [12]; and (3) pure lacquer coatings have the disadvantage of poor flame retardancy like most natural polymers. Since the limiting oxygen index of the lacquer film is only 20.1%, it is an extremely flammable material and the heat release rate is very high, which seriously restricts its application in special occasions.
Adding the small amount of flame retardants into Chinese lacquer coatings is the best strategy to improve the flame retardancy. Meanwhile, it is also important to ensure that the addition of flame retardants does not affect the performance of the lacquer films. At present, the commonly used flame retardants include the following categories [13]: (1) Halogen flame retardants. It is mainly bromine-containing compounds, but a large number of toxic substances are produced after the thermal decomposition of bromine-containing compounds. (2) Metal flame retardants. It mainly includes metal oxidation, and the compatibility between metal oxides and organic compounds restricts its application. (3) Phosphorous flame retardant. It mainly includes phosphate ester compounds, and the toxicity of organophosphorus compounds is also the main reason restricting its application. (4) Intumescent flame retardants. With C, N, and P as the core flame retardant, this kind of flame retardant has a synergistic flame retardant effect, which has attracted more and more attention [14]. Chen et al. studied and prepared a series of intumescent flame retardants and blended the synthesized flame retardants with polyurethane to prepare flame retardant TPU, which broadened its application range [15–18]. At present, there is very little research work on the flame retardancy of Chinese lacquer, especially the research on phospho-nitrogen co-flame retardancy that has not been reported. Therefore, the development of low-toxicity flame retardants with good compatibility and flame-retardant effects were the key to the flame retardant properties of Chinese lacquer [19].
Based on the above, in this paper, the surface silicon modified ammonium polyphosphate was used as an environmentally friendly phosphorus and nitrogen synergistic flame retardant, which was uniformly dispersed into lacquer by mechanical blending. After the addition of silicon-modified ammonium polyphosphate, it had the following characteristics: (1) The flame retardant can be evenly dispersed into the lacquer. The flame retardant has good compatibility with Chinese lacquer and does not affect the performance of the lacquer films. (2) The addition of silicon-modified ammonium polyphosphate does not affect the curing of Chinese lacquer. (3) Ammonium polyphosphate has the synergistic effects of phosphorus and nitrogen. The condensed-phase and vapor-phase flame retardant mechanisms were used to achieve the purpose of flame retardant, with high flame retardant efficiency and green environmental protection features [20].
2. Materials and Methods
2.1. Materials
Chinese lacquer was purchased from the Xi’an Research Institute of Lacquer. Silicon-modified ammonium polyphosphate (SAPP) was purchased from Hefei Combustion New Material Technology Co., Ltd. All reagents above were used as received without purification.
2.2. Methods
The homogeneous dispersion of Chinese lacquer and silicone-modified ammonium polyphosphate was synthesized by the following method. Silicon-modified ammonium polyphosphate was added to the lacquer under rapid stirring (500 RPM), and then the suspension was stirred for another 2 hours to obtain a homogeneously dispersed system. Silica-modified ammonium polyphosphate was added with a mass fraction of 0%–30%, and the resulting homogeneous dispersions were marked as RLA, RLB, RLC, RLD, and RLE (see Table 1).
Table 1
The homogeneous dispersions of Chinese lacquer and SAPP.
| Sample | RLA | RLB | RLC | RLD | RLE |
| SAPP content (%) | 0 | 5 | 10 | 20 | 30 |
| Hardness | 3H | 3H | 3H | 3H | 3H |
| LOI (%) | 20.1 ± 0.3 | 20.5 ± 0.2 | 23.0 ± 0.1 | 26.5 ± 0.2 | 30.6 ± 0.3 |
Adding silicon-modified ammonium polyphosphate to natural raw lacquer and mix thoroughly, the quality fraction of SAPP is 0%, 5%, 10%, 15%, 20%, and 30%, respectively. Washing the slides and the iron slides and put them in the oven to dry for later use. Take an appropriate amount of evenly mixed sample on a clean slide or tinplate sheet, select a 75 μm film maker, evenly coat the sample on the slide, and prepare a composite paint film to naturally cure into a film for 7 days at room temperature.
2.3. Characterization
Composite films impact resistance performance test was conducted using the CQJ-II film impactor. The biggest shock distance where the film was not cracked was recorded as the films impact resistance values.
Composite films hardness was measured using a pencil hardness tester (QHQ-A, China). This feature is an important index for film mechanical properties. Pencil hardness was measured in order of B, HB, H, 2H, 3H, 4H, 5H, and 6H, indicating an increasing hardness level.
Fourier transform infrared (FTIR) spectra were recorded using a Thermo Nicolet iS5 spectrometer with KBr pellets. A total of 32 scans were conducted with a resolution of 4 cm−1.
Thermogravimetric analysis (TGA) were recorded using a Mettler TGA/DSC 3+ (Switzerland) under an atmosphere of nitrogen at a heating rate of 10°C/min from 30°C to 800°C.
Surface morphologies were investigated via field emission scanning electron microscopy (SEM) (Zeiss supra55) at a 5 kV acceleration voltage.
Microcalorimetry test was performed with a microcombustion calorimeter. Heated in a mixed flow atmosphere (80% nitrogen, 20% oxygen) at a heating rate of 1°C/s.
The limiting oxygen index (LOI) was measured according to ASTM D2863.
X-ray photoelectron spectroscopy (XPS) spectra of the char residue were recorded with an EscaLab 250Xi using Al excitation radiation (hy. 1253.6 eV).
3. Results and Discussion
3.1. Effect of SAPP on the Impact Resistance of Composite Films
Impact resistance expresses the elongation, static hardness, and adhesion to the substrate of the films under test. This index is important for the quality identification of the films. The impact resistance is the common manifestation of films adhesion, film strength, and ductility, but it is different from the deformation under static load, which is the rapid deformation caused by impact load. Hence, it is crucial to give high impact resistance to the films [21].
Generally, impact resistance is closely related to the mechanical loss of the films (internal consumption). Internal consumption denotes that the material will convert mechanical energy from impact energy into thermal energy through the polymer chain segment movement of internal friction. Therefore, the greater the internal dissipation, the greater the ability to absorb impact energy, and the mechanical loss factor can measure the size of the internal dissipation [22].
The impact resistance was significantly improved by adding SAPP increasingly (see Figure 1). The reason for this result is that with the increasing the SAPP contents, there is a stronger two-phase interaction between the SAPP and Chinese lacquer of the composite films. This interaction can absorb external impact energy, so the film has a stronger impact resistance.
[figure(s) omitted; refer to PDF]
3.2. Effect of SAPP on of Composite Films Hardness
As the SAPP content increased, there was no change in the hardness of the composite films (see Table 1). This means that SAPP is evenly dispersed in the composite films and there is no agglomeration formed which affects the network structure of Chinese lacquer. This proves that the addition of SAPPD has no effect on the curing of the composite films. Therefore, there is no change in the hardness of the composite films [23].
3.3. FTIR Spectrum Analysis
The addition of SAPP has no significant effect on the infrared spectrum, which means that the addition of SAPP has no effect on the curing of Chinese lacquer (see Figure 2). The absorption peaks at 1073 cm−1 belong to the symmetric telescopic vibration of the P-O bond and the telescopic vibration of the Si-O-Si bond of SAPP. This proves that the SAPP is uniformly dispersed in the composite films. The disappearance of the crab-leg-like peaks at 1621 and 1595 cm−1 indicates complete curing of Chinese lacquer [4, 9]. This also indirectly proves that SAPP has no effect on the lacquer curing.
[figure(s) omitted; refer to PDF]
3.4. Morphology of the Composite Films
The microscopic morphology of different proportions of APP and lacquer was observed through SEM. The surface of the films containing only pure lacquer is rougher. Particles and porous structures are present on the film surface, and delamination can be clearly seen (see Figure 3). In contrast, after the addition of SAPP, the surface of the films has a trend towards smoothness, but there are still a few pores. The SEM image shows that the film with 30% SAPP content is the smoothest, with appropriate gloss and few pores. Other films with different content of SAPP also performed better than the pure Chinese lacquer films, with moderate number of pores and good smoothness.
[figure(s) omitted; refer to PDF]
The pores in the film after the curing of the pure lacquer film are the distribution of the dispersion phase of the lacquer. The cross-section of the film containing SAPP contains granular material. As the SAPP content increased, the amount of granular material increased, so these particles should be the result of the addition of modified SAPP. The films containing 5%, 10%, and 20% of SAPP show similar cross-sections and textures in the SEM images. The topography and texture of the films with 30% SAPP content are more uneven, which showed a variety of textures in the SEM diagram (see Figure 4). Overall, it can be concluded that the addition of SAPP makes the structure of the composite films denser.
[figure(s) omitted; refer to PDF]
3.5. Flame-Retardant Properties and Thermal Stability of Composite Films
The Intumescent flame retardant system goes through an expansion process during combustion and forms the carbon layer. Therefore, it is necessary to study the differences between the different carbon residues after combustion [23, 24]. Figure 5 shows SEM photographs of the different composite film residues collected after combustion tests. It can be observed that the pure lacquer has almost no condensed-phase carbon layer on the surface after combustion. After the addition of 5% SAPP, the surface appears to form a cracked shell and a slight condensed-phase carbon layer, and the condensed phase is very discontinuous. After further increasing the SAPP addition, a continuous, dense, and smooth condensed-phase carbon layer (in Figure 5 RLD) appeared on the postcombustion surface of the composite films [23, 25]. However, further addition of SAPP resulted in a large number of holes on the condensed-phase surface, mainly due to the release of vapor-phase material released from the decomposition of the surface flame retardant due to the excessive amount of intumescent flame retardant. In general, the lack of a condensed-phase carbon layer cannot resist the erosion of high temperature and thus cannot protect the substrate from burning. Instead, the composite material forms a continuous, dense, and highly swollen carbon layer during combustion. It slows down the transfer of heat and mass between the gas phase and condensed phase, further protecting the matrix material in the combustion chamber (in Figure 5 RLE), resulting in a system with low-peak heat release rate (PHR), total heat release (THR) values, and good flame retardant properties [26–28].
[figure(s) omitted; refer to PDF]
Flame retardant performance is usually measured by the limiting oxygen index (LOI). A high limiting oxygen index means that a higher level of oxygen is required for the combustion of the material. LOI increases drastically from 20.1 to 30.6 when SAPP content is increased from 0 to 30 wt % (see Table 1). The results indicate that SAPP shows good flame-retardant effect to Chinese lacquer films.
The onset degradation temperature (T-5%) of the composite film decreases with the addition of SAPP, which may be due to the prior decomposition of the flame retardant. The onset degradation temperature (T-5%) decreases with increasing SAPP content and then increases while SAPP content exceeds a certain value (in Figure 6(a)). This is because flame retardants are easy to be pyrolyzed at low temperatures to protect the matrix materials. The flame retardants decompose quickly at low temperatures and the formation of a condensed-phase carbon layer on the material surface to prevent further pyrolysis, thus causing the maximum weight loss temperature backward [29–31]. The amount of residual carbon at 800°C also shows a significant increase with the addition of flame retardants. The DTG curve indicates a significant reduction in the rate of thermal decomposition with the addition of SAPP (in Figure 6(b)).
[figure(s) omitted; refer to PDF]
The heat release rate (HRR) is a representative parameter to characterize the thermal properties. The evaluation of the HRR allows the analysis of the thermal properties of the material and thus the evaluation of the flame retardancy of the composite films [32, 33]. Generally, heat release rate (HRR), especially maximum heat release rate (PHRR), plays an important role in the process of fire diffusion. PHRR refers to the rate of heat release per unit gram sample, measured in W/g. The higher the PHRR, the faster the thermal cracking rate caused by the heat feedback to the surface of the polymer material, thus producing more volatile combustible substances and accelerating the flame propagation [34, 35]. If the maximum heat release rate drops, making it difficult for the surrounding material to reach its ignition point, this reduces the likelihood of the flame spreading.
The HRR value of composite film with SAPP was significantly different from that of pure film (see Table 2). The peak value of RLA (PHRR) is 194.3 W/g, the corresponding value of RLB is 143.5 W/g, RLC is 117.9 W/g, RLD is 99.6 W/g, and RLE is 98.5 W/g. Obviously, the peak heat release rate can be substantially reduced by adding SAPP; for example, the peak value of raw paint can be reduced by 50% (as compared to RLA) with adding 20% of SAPP.
Table 2
HRR values of composite films.
| Sample | RLA | RLB | RLC | RLD | RLE |
| HR capacity (J·g−1·K−1) | 197 | 142 | 118 | 100 | 99 |
| Peak HRR (w/g) | 194.3 | 143.5 | 117.9 | 99.6 | 98.5 |
| Total HR (kJ/g) | 14.1 | 11.2 | 9.3 | 6.5 | 5.9 |
| Temperature (°C) | 457 | 470.9 | 469.3 | 479.7 | 473 |
THR is the amount of heat released per unit area of material from the beginning to the end, in kJ/g. The larger THR is, the greater the heat released by the burning of polymer materials, which means that polymer materials are more dangerous in fire. The combination of HRR and THR can better evaluate the combustibility of materials. The total heat release of pure raw film was 14.1 kJ/g, while the total heat release of SAPP composite films was 11.2 kJ/g, 9.3 kJ/g, 6.5 kJ/g, and 5.9 kJ/g, respectively (see Table 2). In combination with the relationship between their heat release rates, it can be found that the introduction of SAPP can significantly reduce the heat release amount and the heat release rate and improve the flame retardant effect of the material.
HR capacity is the heat released per unit temperature increase per unit mass of material, which is expressed in units of J·g−1·K−1. Polymer materials with a high HR capacity release more heat per unit of temperature increase in the combustion process. So the material is more dangerous when it burns. Compared with that of pure lacquer film, the HR capacity of SAPP composite films have decreased by 50% (see Table 2).
The maximum cracking temperature is the highest temperature the material can withstand. The higher the temperature, the less likely the material is to pyrolyze and therefore has better flame retardant properties. The thermal cracking temperature to 22.7°C by adding SAPP increasingly (see Table 2). It indicates that SAPP has good flame-retardant property.
A comprehensive analysis of HR capacity, peak HRR, total HR, and maximum cracking temperature shows that SAPP can significantly improve the flame retardancy when added to raw lacquer.
The chemical components of the residual char for composite film (heated under N2 atmosphere at 800°C for 10 min) were investigated by XPS (see Figure 7). As shown in the figure, there are two bands from C1s spectra: the peak at around 284.7 eV is attributed to C-H and C-C in aliphatic and aromatic species, and the peak at around 286.0 eV is assigned to C-O (ether and/or hydroxyl group). O1s spectrum has only single peaks at around 531.8 eV. It is assigned to the C-OH groups of lacquer [36, 37].
[figure(s) omitted; refer to PDF]
The composite films (sample RLD containing 20% SAPP) XPS test results are shown in Figure 8. There are still two peaks of C1s spectra at 284.7 eV and 286.0 eV. However, compared to the two peaks of the C1s spectra, the proportion of peaks containing SAPP at 286.0 eV became larger than that of pure lacquer films. This is mainly due to the dehydrating effect on the lacquer from the flame-retardant SAPP [14]. For the O1s spectra of composite films, there is a peak that appears as 531.0 eV, which belongs to the P=O group. N1s spectrum has two peaks at around 399.1 and 400.5 eV, the peak at 399.1 eV can be assigned the nitrogen in the NH3 groups of SAPP [38]. The peak at 400.5 eV corresponds to the formation of some oxidized nitrogen compounds. The single peak between 134.0 and 135.0 eV in the P2p spectrum can be assigned to the pyrophosphate and/or polyphosphate [20, 39–41].
[figure(s) omitted; refer to PDF]
4. Conclusions
In summary, we first have designed and prepared a method for incorporating silicone-modified ammonium polyphosphate into Chinese lacquer to improve the flame retardancy of organic coating. The silicone-modified ammonium polyphosphate and lacquer composites showed a 52% increase in limiting oxygen index and 186% improvement in Impact resistance with 30% SAPP content. Scanning electron microscopy results showed a smoother and flatter surface and a denser internal structure with the addition of SAPP, which was responsible for maintaining the high impact resistance and hardness of the composite films. Thermogravimetry and microcalorimeter analyzed the thermal performance of the composite films. The results showed that HR capacity, peak HRR, and total HR decreased significantly after adding SAPP, and maximum cracking temperature and residual carbon amount increased. Surface morphology and elemental analysis of the pyrolysis products showed that a dense and smooth condensed-phase carbon layer was produced on the pyrolysis products after the addition of SAPP, and elemental analysis showed that the aromatic carbon ratio increased after the addition of SAPP, while a large amount of NOx and pyrophosphate and/or polyphosphate appeared. This illustrates that SAPP achieving flame retardancy via the efficiency of carbonization, oxygen barrier, and the thermal interruption effect.
Acknowledgments
This work was financially supported by the Mindu Scholars Special-term Professor of Minjiang University (MJY18011), Natural Science Foundation of Fujian Province (2023H0051, 2023J011394, 2023C0028, 2022J011120, 2021J011019, and 2021J011021), and Science and Technology Project of Fuzhou (2022-P-004, 2021-SG-270).
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Abstract
The intumescent flame retardant has a high efficiency of carbonization, oxygen barrier, and thermal interruption effect, which is considered to be a promising green and environmentally friendly flame retardant. Herein, we demonstrated a new approach to improve the flame retardant performance of Chinese lacquer films by adding silicon modified ammonium polyphosphate (SAPP), leading to superior flame retardant. SAPP drastically raised the limiting oxygen index (LOI) from 20.1% to 30.6%, and at the same time, the impact resistance was increased from 7 cm to 20 cm without changing the hardness of the lacquer films. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy were used to investigate the chemical components and morphology of the residual char of Chinese lacquer and SAPP composite films. The thermal degradation process of films was detected and analyzed by thermogravimetry and microcalorimetry, and the possible flame retardant mechanism was discussed. This simple approach opens up a new way to design for improved flame retardancy of organic film polymers.
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Details
; Xia, Jianrong 2
1 College of Environmental and Resource Sciences College of Carbon Neutral Modern Industry, Fujian Normal University, Fuzhou 350108, China; Fujian Engineering and Research Center of New Chinese Lacquer Materials, College of Materials and Chemical Engineering, Minjiang University, Fuzhou 350108, China
2 Fujian Engineering and Research Center of New Chinese Lacquer Materials, College of Materials and Chemical Engineering, Minjiang University, Fuzhou 350108, China
3 Fujian Jiaofa High Tech Co., Ltd., Fuzhou 350108, China
4 College of Environmental and Resource Sciences College of Carbon Neutral Modern Industry, Fujian Normal University, Fuzhou 350108, China