Ethanol-induced ammonium polyphosphate–silver gel paint: breaking the trade-off between conductivity, flame retardancy and adhesion in single-layer functional coatings

Abstract

Electrical fires pose significant threats to the lives and property safety of people. Although utilizing coatings to impart conductivity and flame retardancy to materials is convenient and reliable, traditional layer-by-layer preparation methods have the limitations of cost, convenience and scalability. Therefore, a single-layer coating that simultaneously imparts excellent conductivity and flame retardancy to materials presents broader application prospects. And good adhesion of the coating is another essential aspect. However, the trade-off between conductivity, flame retardancy, and adhesion creates huge challenges in the development of such coatings. Here, we report an ethanol-induced ammonium polyphosphate–silver (APP–Ag) gel paint to completely address the above issues. High molecular weight APP served as both a flame retardant and an adhesive, while the coordinating action of phosphate groups ensured the effective dispersion of nanosilver, and the nitrogen-containing carbon layer formed from triethanolamine and ascorbic acid at high temperature significantly enhanced the conductivity of the coating by connecting the silver nanoparticles. The coated materials could exhibit an electrical conductivity of over 200 S m⁻¹, with the limiting oxygen index (LOI) exceeding 60%. Meanwhile, the peak heat release rate (PHRR) and total heat release (THR) decreased by more than 30% compared to those of the untreated materials. Additionally, we utilized this gel paint to fabricate electric heating fabrics, motion sensors, and fire alarm devices. Finally, we have thoroughly explored the potential mechanisms of conductivity, flame retardancy, and adhesion of the gel coatings.

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New concepts

Herein, a novel ethanol-induced APP–Ag gel was reported to prepare a high conductivity and flame retardancy coating with excellent adhesivity. The coated materials could exhibit an electrical conductivity of over 200 S m⁻¹, with the limiting oxygen index (LOI) exceeding 60%. Meanwhile, the peak heat release rate (PHRR) and total heat release (THR) decreased by more than 30% compared to those of the untreated materials. Importantly, this gel paint can be used to fabricate electric heating fabrics, motion sensors, and fire alarm devices. This new design concept opens up a new research direction for the development of the next generation of functional conductivity and flame retardancy coatings.

1. Introduction

Natural biomass materials, such as wood and cotton fabrics, are widely used in furniture, clothing, and construction.¹ However, their susceptibility to ignition poses significant risks to human life and property. Therefore, it is essential to modify these materials to enhance their flame retardancy.^2,3 Furthermore, combining electrical conductivity with flame retardancy can greatly expand the application range of polymer substrates. The resulting dual-functional conductive flame-retardant device can be utilized in areas such as human motion monitoring,^4,5 electromagnetic shielding,^6,7 fire alarm systems,⁸ and static electricity prevention.^9,10 This combination can mitigate the risks of fires and equipment damage caused by the electrical heating effect and the inherent flammability of polymer substrates.^11,12 According to statistics, there were a total of 143 030 fire incidents in the United States from 2021 to 2022, of which 26 130 were caused by electricity, accounting for 18.3% of the total. The losses caused by these fires were substantial. Thus, the integration of electrical conductivity and flame retardancy holds significant importance.

Surface coating technology offers advantages such as economy, adaptability, reparability, and functionality.¹³ Utilizing coatings to impart conductivity and flame resistance to materials is undoubtedly a promising method. Many reports have utilized layer-by-layer (LBL) methods to sequentially coat flame-retardants and conductive layers onto materials.^14–17 However, the complexity, long reaction times, and stringent reaction conditions of the preparation methods significantly restrict their practical applications.^18–20

Single-layer coatings that simultaneously impart good conductivity and flame resistance to materials undoubtedly have a broader range of applications. Additionally, the coatings should also have good adhesion with materials to overcome the challenges of complex external environments. However, there are few relevant reports on such a coating. Aksam et al.²¹ developed a single-layer coating with conductive and flame-retardant properties using polyvinyl alcohol (PVA), folic acid (FA), and polyaniline (PANI) nanoparticles, which was applied to cotton fabric. However, the resulting product still has room for improvement in both conductivity and flame resistance. The challenges arise from the need for three components in single-layer conductive coatings: flame retardants, adhesives, and conductive fillers. To enhance the flame resistance of the coating, flame retardants must be added; however, the inherent flammability of adhesives can diminish the overall flame-retardant effectiveness. To improve the conductivity, conductive fillers needed to be added, but the inherent insulativity of both the flame retardants and adhesives increases the amount of conductive fillers significantly. This often leads to filler aggregation, which will affect the dispersion of metallic conductive fillers. To enhance the adhesion of the coating, adhesives needed to be added, but their intrinsic flammability and insulating properties can further compromise the flame-retardant and conductive performance of the coating.^22–25 The trade-off among conductivity, flame resistance, and adhesion makes it exceptionally difficult to develop strong adhesion coatings that can simultaneously provide materials with good flame resistance and conductivity.

Ammonium polyphosphate (APP) is the most typical phosphorus-based flame retardant, composed of polyphosphoric acid and ammonia. During combustion, APP catalyzes the formation of a dense amorphous char layer on a substrate, creating an insulating barrier that hinders the contact between the substrate and the flame as well as oxygen, while also releasing ammonia gas to exert both condensed-phase and gas-phase flame retardant effects.^26–29 Furthermore, APP contains a large number of phosphate groups that exhibit strong coordination ability with metal ions.^30,31 Unexpectedly, we discovered that APP could form stable complexes with silver nitrate, triethanolamine and ascorbic acid solutions, forming gels upon the addition of ethanol. Our research into these unexpectedly obtained gels revealed their potential to effectively address the aforementioned issues. APP in the amorphous state could function both as a flame retardant and an adhesive, while the phosphate group's strong chelation with metal ions ensured good dispersion of nanosilver. The nitrogen-containing carbon layer formed by triethanolamine and ascorbic acid at high temperature could significantly enhance the conductivity of the coating. After prolonged storage in ethanol, the polymer network could solidify into a paintbrush that can be conveniently used for coating.

We applied this gel paint onto a cotton fabric and wood, and after a brief high-temperature treatment, we obtained a conductive and flame-retardant cotton fabric and wood with excellent comprehensive properties. The coating could endure 100 cycles of abrasion with 1200 mesh sandpaper. The coated wood and cotton fabric could attain an electrical conductivity greater than 200 S m⁻¹, with the limiting oxygen index (LOI) surpassing 60%. In addition, the peak heat release rate (PHRR) and total heat release (THR) were reduced by more than 30% compared to those of the untreated materials. We also utilized this gel paint to prepare a flame-retardant electric heating fabric, a motion sensor, and a fire alarm, which demonstrated good performance. Furthermore, we have thoroughly explored the potential mechanisms of conductivity, flame retardancy, and adhesion of the gel coatings. Our research has resolved the trade-off in the preparation process of single-layer conductive flame-retardant coatings, and the acquired paint may play a constructive role in fire prevention and protection for future buildings and electronic devices.

2. Experimental section

2.1. Materials

Cotton fabric (200 g m⁻², woven) and paulownia wood were purchased from a local store. Silver nitrate, triethanolamine, ascorbic acid, and ethanol were purchased from the Sinopharm Chemical Reagent Co., Ltd. Ammonium polyphosphate (APP) (n ≥ 1500) was obtained from Aladdin.

2.2. Preparation of APP–Ag gel paint

A triethanolamine solution of silver nitrate at a concentration of 25 mg mL⁻¹ (25 mL) was prepared, and then 0.5 g of APP was added. The suspension was sonicated to dissolve, and then 5 mL of ascorbic acid solution (0.2 g mL⁻¹) was mixed to reduce for 30 min. The resulting complex solution was transferred to a test tube, and subsequently, ethanol was instilled (solution : ethanol = 1 : 2). The test tube was shaken vigorously to immediately form gels. The resulting gels were stored in ethanol for a period to prepare a gel paintbrush.

2.3. Preparation of coated cotton fabric and wood

Before coating, the wood, cotton fabric, and gel paintbrush were pre-wetted with deionized water. Then, using the gel paintbrush, the coating was evenly applied to the substrate with the brushing method. Finally, the substrate was fully encapsulated by the coating. The film thickness was ensured consistent by controlling the same weight gain. The coating substrate was dried in an 80 °C oven, and then baked in the oven at 200 °C for 1 min.

The preparation process and mechanism of the APP–Ag gel paint, as shown in Fig. 1, began with the addition of triethanolamine to a silver nitrate solution. Due to the lone pair electrons present on the N and O atoms in triethanolamine, a complexation reaction occurred with silver ions, forming triethanolamine–silver ion (TEA–Ag⁺) complex. Subsequently, APP was introduced, where the negatively charged ammonium phosphate groups could chelate silver ions alongside triethanolamine, and the phosphate oxygen groups in APP interacted with the hydroxyl groups in triethanolamine to form hydrogen bonds. This led to the formation of a silver ion complex. Additionally, there are moderate-strength hydrogen bonds both intramolecularly and intermolecularly in APP, resulting in a certain degree of network crosslinking that transforms APP into a robust crystalline structure. The longer the molecular chain, the more significant the effect, which hinders solvent molecule penetration and causes a dramatic decrease in the water solubility of high polymerization APP. After the silver ion complex formation, the hydrogen bonds in the ammonium phosphate groups were disrupted, affecting the crystallization process of APP and transforming APP into an amorphous state, allowing water to permeate its interior and causing swelling and dissolution. After the addition of ascorbic acid, silver ions were reduced. When ethanol was added, the polarity of the solvent rapidly decreased, weakening the dipole moments of solvent molecules, and because the complex was a strongly polar molecule, the interaction forces between the solvent and solute molecules became less than those among solute molecules, leading to aggregation and precipitation of the silver complex, forming black linear structures. After shaking the test tube, these black linear structures immediately formed APP–Ag gels due to electrostatic and hydrogen bond attractions. After being stored in ethanol for an extended period, residual water in the APP–Ag gels was expelled, tightening the connections within the network and allowing the formation of a gel paintbrush that could be used for drawing when moistened. High-temperature baking could significantly enhance the conductivity of the material. As shown in Video S1 (ESI†), we used the unheated coated wood as a connector to connect the power source and a 12 V bulb. The flame was used in place of an oven for heating, visually demonstrating the process of resistance change. The detailed conductivity enhancement mechanism will be discussed in Section 3.4.

Fig. 1 Schematic diagram and the mechanism of the preparation of the APP–Ag gel paint.

2.4. Characterization

The microstructure of samples was investigated using a field emission scanning electron microscope (JSM-7500F). Chemical composition analysis of the samples was performed through attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR, Nicolet 6700) and X-ray photoelectron spectroscopy (XPS, AXIS SUPRA). The crystal structure of the samples was characterized by X-ray diffractometry (XRD) using a D8 Advance diffractometer. The adhesion strength of the samples was studied using a universal testing machine (Instron 5943), a sandpaper test and a drop test. The electrical resistance changes of the samples were studied using a static electricity meter (TH2690) and a desktop multimeter (DMM6500). The surface resistance of the samples was measured using the four-probe method (FT-340). The surface temperature of the sample and the corresponding thermal imaging images were documented utilizing a thermal camera (H21 pro, Hikmicro). The thermal stability of samples under nitrogen atmospheres was investigated using a simultaneous thermal analyzer (TG, STA449F3) with a heating rate of 10 °C min⁻¹. The limiting oxygen index (LOI) of the samples was determined using an oxygen index meter (JF-3A), with sample dimensions of 100 × 10 × 3 mm³ (wood) and 100 × 50 mm² (cotton fabric). Vertical flame tests (VFTs) were performed to evaluate the flame retardancy of samples, with sample dimensions of 100 × 30 × 3 mm³ (wood) and 100 × 40 mm² (cotton fabric), and the ignition time was 12 s, repeated twice, with a flame height of 3 cm. The combustion behavior of the samples was investigated using a cone calorimeter test following ISO 5660, under a heat flux of 50 kW m⁻². The sample dimensions were 100 × 100 × 1 mm³ (wood) and 100 × 100 mm² (cotton fabric). The water contact angles of the APP–Ag gels were measured using a droplet-based instrument (DSA100S SN 30009557) with a droplet volume of 5 μL, and the sample was evenly applied to the glass slide during the testing process. The thermogravimetric-infrared (TG-IR) test was used to analyze thermal decomposition gas products from 30 °C to 800 °C with a rate of 10 °C min⁻¹ under a nitrogen flow of 30 mL min⁻¹ (TGA8000-Spectrum3).

2.5. Durability and stability tests

The durability of coatings was investigated. Durability tests of coatings included resistance to ultrasonic and UV irradiation, and organic solvents. In the ultrasonic resistance test, the coated wood was soaked in ethanol solution and then sonicated continuously in an ultrasonic machine (40 kHz, 160 W) for 120 min, and the sheet resistance and LOI of coated wood were measured every 30 min. In the UV irradiation resistance test, the coated wood was exposed to a UV lamp for 24 h, and the sheet resistance and LOI of coated wood were measured every 6 hours. In the organic solvent resistance test, the coated wood was immersed in different organic solvents for 24 h. Subsequently, coated wood was cleaned with ethanol, following that placed in an 80 °C oven for 1 h, after which the sheet resistance and LOI of coated wood were measured.

3. Results and discussion

3.1. Flame retardant properties

Vertical burning and LOI experiments were conducted to evaluate the flame-retardant properties of the samples. The corresponding results are shown in Fig. 2a, b and Table 1. The weight gain of the coating on the samples was calculated using the following equation:

Weight gain (%) = 100% × (W₂ − W₁)/W₁ (1)

where W₁ and W₂ are the weights of the samples before and after treatment, respectively. As shown in Fig. S1 (ESI†), it is evident that once ignited, flames spread rapidly across the surface of the cotton fabric and wood, continuing to burn even after the flame was removed. Uncoated samples did not pass the UL-94 test and the LOI values were 18.3% and 22%, respectively. In contrast, the flame spread on the coated wood and cotton fabric was extremely slow, self-extinguishing after the flame was removed. The flame retardant rating of coated wood and cotton fabric reached the V-0 level. Furthermore, with an approximate 30% increase in weight, the LOI of the coated cotton fabric and wood reached 66% approximately.

Fig. 2 Photographs of the coated wood (a1–a6) and coated cotton fabric (b1–b6) during the vertical burning test. TG and DTG curves of the samples under N₂ atmospheres (c) and (d). HRR and THR curves of the samples (e) and (f).

Table 1 Vertical flame test and LOI data of different samples

Sample	Weight gain (%)	UL-94 rating	LOI (%)
Cotton fabric	0	NR	18.3
Coated cotton fabric	32.9	V-0	66
Wood	0	NR	22
Coated wood	36.2	V-0	64

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The thermal stability of various samples was investigated using TGA, as illustrated in Table 2 and Fig. 2c, d. T_10% denotes the temperature at which the sample experiences a 10% mass loss, while T_max and R_max, respectively, represent the temperature and rate at which the sample reaches its maximum degradation rate. All experiments were conducted under a N₂ atmosphere. The pyrolysis process of cotton fabric is primarily divided into two stages. The first stage occurred at 280–380 °C, where the depolymerization of cellulose predominantly takes place, with generation of a large amount of volatile products and aliphatic carbon. The second stage occurred after 380 °C, where the aliphatic carbon continued to undergo oxidative decomposition.³² The cotton fabric exhibited T_10%, T_max, and R_max values of 320 °C, 355 °C, and 1.49% °C⁻¹, respectively, with a residual char of 4.49 wt% at 800 °C. In contrast, the T_10%, T_max, and R_max values of the coated cotton fabric decreased significantly to 249 °C, 261 °C, and 0.68% °C⁻¹, respectively, accompanied by a char residue of 38.46 wt%. The pyrolysis of the wood is mainly divided into three stages. The first stage occurred at 30–200 °C, primarily due to the evaporation of moisture within the wood. The second stage occurred at 200–360 °C, which is the main pyrolysis phase of the wood. Lignin, cellulose and hemicellulose (main components of wood) rapidly degraded in this stage. The third stage occurred after 360 °C, where the wood undergone carbonization, forming char layers.³³ The wood exhibited T_10%, T_max, and R_max values of 250 °C, 346 °C, and 1.1% °C⁻¹, respectively, with a residual char of 18 wt% at 800 °C. In contrast, the T_10%, T_max, and R_max values of coated wood decreased to 248 °C, 312 °C, and 0.45% °C⁻¹, respectively, accompanied by a char residue of 31 wt%. These findings suggest that our coatings could facilitate the char layer formation of wood and cotton fabric, forming a protective carbon layer on the surface, thereby inhibiting further degradation and increasing the char residue. The above effects can be attributed to the addition of the flame retardant APP.^31,34

Table 2 TG and DTG data of different samples under a N₂ atmosphere

Samples	T 10% (°C)	T max (°C)	R max (% °C^−1)	Residue at 800 °C (wt%)
Cotton fabric	320	355	1.49	4.49
Coated cotton fabric	249	261	0.68	38.46
Wood	250	346	1.1	18
Coated wood	248	312	0.45	31

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CCT was performed to further assess the combustibility of the samples. The results of the CCT are shown in Fig. 2e, f and Table 3. For the wood, the peak heat release rate (PHRR) achieved was 182.7 kW m⁻², with a total heat release (THR) of 4.17 MJ m⁻². The PHRR and THR of the coated wood were reduced to 114.0 kW m⁻² and 2.7 MJ m⁻², respectively. For the cotton fabric, the PHRR was 111.5 kW m⁻², with a THR of 1.41 MJ m⁻². After coating, the PHRR and THR of the cotton fabric were reduced to 38.7 kW m⁻² and 0.7 MJ m⁻², respectively. The PHRR and THR of the coated wood were reduced by 37.6% and 35.3%, and those of the coated cotton fabric were reduced by 65.3% and 50.4%, compared with those of uncoated samples, respectively. Meanwhile, the fire growth rate (FIGRA, defined as the ratio of the PHRR to T_PHRR) of the coated wood and cotton fabric decreased by almost 42% and 48% compared with that of uncoated samples, respectively, indicating that the flashover time is delayed, leaving people more time to escape and sharply reducing the risk of fire.³⁵ As shown in Fig. S2 (ESI†), after cone calorimetry analysis, the coating enabled the residue of the cotton fabric and wood to maintain a relatively intact structure compared to uncoated substrates. The detailed flame-retardant mechanism will be discussed in Section 3.4.

Table 3 CCT results for various samples

Sample	PHRR (kW m^−2)	T PHRR (s)	THR (MJ m^−2)	FIGRA (kW m^−2 s^−1)
Cotton fabric	111.5	28.7	1.41	3.89
Coated cotton fabric	38.7	18.9	0.70	2.04
Wood	182.7	29.0	4.17	6.30
Coated wood	114.0	31.0	2.70	3.68

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3.2. Conductive properties

In addition to excellent flame-retardant properties, our functional coating also imparts superior electrical conductivity to the substrate. Using the four-probe method, we measured the sheet resistance of the coated wood and cotton fabric, and the sheet resistance values of coated wood and coated cotton fabric were 3 Ω sq⁻¹ and 10 Ω sq⁻¹, respectively. Despite some variations, both substrates demonstrated excellent conductivity, and the fabricated coated wood and cotton fabric were sufficiently capable of serving as conductive connectors, successfully making the power source illuminate a 12 V bulb, as shown in Fig. 3f. The coating film we prepared had a thickness of less than 0.5 mm, according to the formula:

k = 1/(R_s·w) (2)

where k represents the coating conductivity, R_s denotes the sheet resistance, and w is the coating thickness. Our coating exhibited a conductivity greater than 200 S m⁻¹ on the cotton fabric and greater than 667 S m⁻¹ on the wood. The flame-retardant and conductive properties of our coatings surpass those of the less-reported single-layer flame-retardant conductive coatings and some LBL conductive flame-retardant coatings. Furthermore, the performance of our coatings also has advantages compared to that of various flame-retardant coatings reported in recent years. Specific comparisons are shown in Table S1 (ESI†), further confirming that our gel paint addresses the trade-off issues encountered in the preparation of single-layer conductive flame-retardant coatings. Furthermore, excellent electrical conductivity could endow the coated cotton fabric with good electrothermal properties. As shown in Fig. S3 (ESI†), when a voltage of 1.5 V was applied to the coated cotton fabric, the surface temperature of the coated cotton fabric increased from 27.8 °C to 62.5 °C after 1 min.

Fig. 3 Pressure sensors for human motion detection: elbow flexion (a), wrist flexion (b), and finger flexion (c). Sensors for continuous fire alarms: repetitive electrical resistance transition performance (d) and snapshot (e). Coated cotton fabric (f1) and coated wood (f2) as conductive connectors to illuminate a light bulb.

We also used this coating to fabricate motion sensors and fire alarm devices, and the preparation process is illustrated in Fig. S4a and b (ESI†). For the creation of a motion sensor, the two layers of the coated cotton fabric (50 × 20 mm²) were stacked, and conductive copper tape was used to connect the ends of the top and bottom layers to wires. Finally, the coated cotton fabrics were bonded together with polyimide tape (7413D, 3M) to obtain the sensor. For the creation of a fire alarm device, the coated wood was subjected to continuous flame treatment. Copper tape was then applied to both ends of the board, and the device was connected to a homemade fire alarm circuit (the thyristor model was TYN1225).

The flexible sensor was affixed to the elbow, wrist, and finger joints of the test subject. During bending movements, the sensors underwent varying degrees of deformation, resulting in distinctly different resistance signals, as illustrated in Fig. 3a–c. The sensor could be used to detect human motion. The reason is that the sensor has pressure-induced reversible changes in its conductive pathways under external pressure.⁵ When pressure is applied to the sensor, the air gaps between the fibers decrease, increasing the contact points between adjacent fibers. This results in more conductive pathways, leading to a reduction in resistance.

The fire alarm device is demonstrated in Video S2 (ESI†) and Fig. 3d, e, which could quickly and repeatedly trigger alarms when exposed to fire. The ability of fire alarms mainly depends on the flame retardancy of our coating. The detailed fire alarm mechanism will be discussed together with the flame-retardant mechanism in Section 3.4.

3.3. Coatability, adhesivity and durability

Our gels could form a paintbrush after being stored in ethanol for an extended period, exhibiting excellent coatability. As shown in Videos S3, S4 (ESI†) and Fig. 4, after wetting the substrate, the paintbrush could easily create a film on the cotton fabric and wood. In addition, the microscopic morphology of the APP–Ag gels has been presented, as shown in Fig. S5 (ESI†). The above demonstration can prove that our coating method offers significant advantages over traditional techniques such as LBL methods, solution immersion and spraying. First, as mentioned in the Introduction, achieving both good flame retardancy and electrical conductivity in a substrate generally requires a composite strategy, which typically involves multiple layers of functional coatings. These coatings are usually applied through immersion or spraying.^36,37 In contrast, our APP–Ag gels only require the application of a single layer through brushing, followed by simple thermal treatment, to impart good electrical conductivity and flame retardancy to the substrate. The approach is more convenient. Moreover, compared to flame retardants like phytic acid and conductive fillers such as MXenes, carbon nanotubes or graphene, the low cost of APP and Ag is undoubtedly appealing.³⁸ Second, due to the limited liquid retention of the substrate in a single immersion step, most immersion methods require repeated soaking and drying steps to achieve a satisfactory weight gain, which inevitably increases time costs.^39,40 In contrast, our APP–Ag gel paintbrush could make the substrate achieve a reasonable weight gain only in one-step, significantly reducing time costs. Additionally, the high solid content in the APP–Ag gels improves the utilization rate of the material. Third, immersion methods require suitable containers to ensure that the substrate is fully immersed in the solution. Finding an appropriate container for substrates with irregular shapes is difficult. Our APP–Ag gels can directly apply the coating, eliminating this limitation. Lastly, the APP–Ag gel coating is environmentally friendly, because the raw materials are harmless. In addition, the adhesion of the coating was characterized using a drop test, a sandpaper test and a single-lap shear test. The drop test was performed using the coated wood (overlap length around 20 mm), which was suspended in the air at a height of 300 mm above the desktop and freely impacted numerous times, as shown in Fig. 5a and Video S5 (ESI†). After multiple impacts, the two woods were still firmly adhered together. In the sandpaper test, only one side of the wood was treated with APP–Ag gel paint. The sample loaded with a 100 g weight was then placed on a 1200 mesh sandpaper. In one cycle, the sample underwent a slow movement of 150 mm, after 100 cycles, the sample was connected to the circuit. As shown in Fig. 5b and Video S6 (ESI†), after 100 cycles of abrasion, the brightness of the bulb and the current flowing through the circuit showed almost no change. The single-lap shear test was carried out using a universal testing machine. In detail, woods were cleaned with ethanol and water, and then each specimen was cut into small pieces with 50 mm in length, 20 mm in width and 3 mm in thickness. Finally, these samples were stuck using our gels (overlap length around 20 mm). The stretching speed was 10 mm min⁻¹, and the test illustration is shown in Fig. S6 (ESI†). The bonding efficiency of the APP–Ag gel paint was measured by determining the adhesive strength and adhesion energy of the system. Adhesive strength is the force the composite needs to break per unit length or area. Adhesion energy is the total amount of work essential for each unit length or area during the test.⁴¹ Unfortunately, due to the weak tensile strength of the wood used, the boards broke before the adhesive joint could delaminate (Fig. S7, ESI†), indicating that the actual bonding efficiency should be greater than what was obtained in the experiment. As shown in Fig. 5c, the adhesive strength of our gel paint coating exceeded 230 kPa, with a adhesion energy greater than 875 J m⁻². Even so, our APP–Ag gel paint exhibited a higher adhesive strength with the substrate than the commercial adhesives and other hydrogel coatings reported in the literature.^42–45 The above test indicates that our coating has good adhesion to the substrate. The detailed adhesion mechanism will be discussed in Section 3.4.

Fig. 4 Illustration of paintbrush painting on the cotton fabric (a) and wood (b). A snapshot of the APP–Ag gel paintbrush (c).

Fig. 5 Photographs of the coated wood during the drop test (a1)–(a4) and the sandpaper test (b1) and (b2). The single-lap shear test results of the coated wood (c).

The durability of the coating is essential for daily use. The ultrasonic and UV irradiation and organic solvent resistance of the coated wood was tested. The test results are shown in Fig. 6a–c. Prolonged ultrasonic treatment, UV irradiation, and organic solvent immersion did not affect the sheet resistance and LOI of our coated wood, which indicates that our coating has excellent durability.

Fig. 6 Durability tests of the coating: ultrasonic resistance test results of the coated wood (a), UV irradiation resistance test results of the coated wood (b) and organic solvent resistance test results of the coated wood (c).

3.4. Conductivity, flame-retardancy, fire alarm and adhesion mechanisms

The properties of the APP–Ag gels are highly related to the inner chemical composition. The chemical composition of the APP–Ag gels before and after heating was investigated using FT-IR, XRD and XPS, to reveal the underlying mechanisms behind the adhesion and conductivity enhancement. Fig. 7a shows the FT-IR spectra of the APP, APP–Ag gels and APP–Ag gels after heating. By comparing the FT-IR spectra of APP and APP–Ag gels, a new peak around 1140 cm⁻¹ appeared in the spectra of APP–Ag gels, which is attributed to the bending vibration of tertiary amine.⁴⁶ The observation confirmed that the APP–Ag gels contain triethanolamine. Additionally, some infrared absorption peaks showed a slight shift due to coordination interactions.⁴⁷ Comparing the FT-IR spectra of the APP–Ag gels before and after heating, we can observe that the peak at 1225 cm⁻¹ disappeared, while a broad peak appeared around 2294 cm⁻¹. The peak at 1225 cm⁻¹ is attributed to the stretching vibration of P=O,⁴⁸ whose disappearance suggests that P=O groups might act as binding sites for silver after heating.⁴⁹ The broad peak at 2294 cm⁻¹ is attributed to the bending vibration of P–OH,⁵⁰ originating from polyphosphoric acid formed during the heating of APP.

Fig. 7 FT-IR (a), XRD (b) and XPS (c) spectra of different samples. XPS high-resolution C 1s spectra of APP–Ag gels (d) and APP–Ag gels after heating (e).

XRD analysis further reveals the structure and composition of the samples. As shown in Fig. 7b, comparing the XRD patterns of APP–Ag gels and APP, we can discover that the peaks corresponding to APP completely disappeared for the APP–Ag gels, leaving only peaks corresponding to the silver crystal planes (111), (200), (220) and (311),⁵¹ indicating that the silver plating solution disrupts the crystallinity of APP. Comparing the XRD spectra of the APP–Ag gels before and after heating, a weak broad peak appeared between 20° and 30° after heating, which corresponds to the (002) crystal plane of amorphous carbon,⁵² indicating that a certain degree of carbonization occurred during the heating process.

XPS is a powerful tool for analyzing the elemental composition and chemical state of material surfaces, and was used to analyze the APP–Ag gels before and after heating. The spectra, as shown in Fig. 7c, reveal the presence of C, O, N, P, and Ag elements in the APP–Ag gels. After heating, the Ag peak weakened, possibly due to the limited depth of XPS detection,²⁷ as the thickness of APP–Ag gels increased during the heating process, which encapsulated the silver nanoparticles. As shown in Fig. 7d, the high-resolution C 1s spectra of the APP–Ag gels before heating were deconvoluted into four peaks at 284.60 eV, 286.20 eV, 287.66 eV and 289.05 eV, corresponding to C–C/C=C, C–O/C–N, C=O and O–C=O.^53,54 The presence of C=O and O–C=O indicates the residual presence of ascorbic acid in the APP–Ag gels. As shown in Fig. 7e, the C 1s spectra of APP–Ag gels after heating were also deconvoluted into four peaks at 284.60 eV, 286.30 eV, 287.78 eV and 288.92 eV, corresponding to C–C/C=C, C–O/C–N, C=O and O–C=O. The comparison of the C 1s spectra of APP–Ag gels before and after heating shows that the relative peak intensity at 284.60 eV (C–C/C=C) increased, suggesting that a certain degree of carbonization occurred during the heating process,^53,55 which is consistent with the XRD analysis. Additionally, the XPS analysis shows that the N element was present in the surface carbon layer.

In conclusion, the chemical composition of the APP–Ag gels consists of APP, silver, triethanolamine and ascorbic acid. Based on the above observations and analyses, a possible mechanism for the enhanced conductivity of the APP–Ag gels is proposed: the carbon-containing substances triethanolamine and ascorbic acid can undergo a certain degree of carbonization during heating with the help of APP, resulting in the formation of amorphous carbon layers. Triethanolamine introduces nitrogen-doping during the formation of the carbon layer, which can significantly enhance the conductivity.⁵⁶ These nitrogen-doped amorphous carbon layers crosslink the silver nanoparticles, forming a conductive network, which greatly increases the conductivity of the coating.

The possible mechanism for the enhanced adhesion of the APP–Ag gels is as follows: first, the APP–Ag gels exhibit excellent hydrophilicity to water (Fig. S8, ESI†). After the wood and cotton fabric are wetted with water, the APP–Ag gels can demonstrate excellent interfacial wettability. Second, APP with high molecular weight contains a large number of polymer chain segments, which provide the cohesion essential for adhesives. Third, ascorbic acid and APP generate acidic substances during high-temperature heating,⁵⁷ which can undergo an acid–base reaction with the triethanolamine molecules. The reaction increases inter-molecular interaction sites, resulting in stronger intermolecular forces, further enhancing the cohesion.⁵⁸ In general, the cohesion of the adhesive is related to the mechanical and viscoelastic properties of the adhesive itself, while the interfacial adhesion strength is mainly determined by the compatibility between the adhesive and the substrate surface.^59,60 The high cohesion and excellent interfacial wettability significantly enhance the adhesion strength of the APP–Ag gels.

Conducting an in-depth exploration of the flame-retardant and fire alarm mechanisms of APP–Ag gels coatings is necessary. SEM, XPS and XRD were used to examine both coated and charred coated wood to reveal the flame-retardant and fire alarm mechanisms of the condensed phase. The morphology of the wood is shown in Fig. S9 (ESI†), displaying a clear fiber structure. The morphology of the coated wood is shown in Fig. 8a, where the surface was covered by the APP–Ag gels and became uneven. Moreover, a large number of silver nanoparticles could be observed dispersed within the coating. The morphology of charred coated wood is shown in Fig. 8b, where a dense expanded char layer could be observed, with silver nanoparticles embedded within the char layer. XPS was used to analyze the chemical composition of the coated wood before and after charring, as shown in Fig. 8c. Both the coated and charred coated woods contain C, O, N, P and Ag elements, with the relative intensity of the N 1s peak decreasing after charring. The possible reason is that some N elements were converted into NH₃, which could play a role in gas-phase flame retardancy.⁶¹Fig. 8d1 shows the high-resolution C 1s spectrum of the coated wood, which was deconvoluted into four peaks at 284.60 eV, 286.26 eV, 287.8 eV and 288.85 eV, corresponding to C–C/C=C, C–O/C–N, C=O and O–C=O, respectively. As shown in Fig. 8d2, the high-resolution C 1s spectrum of the charred coated wood was also deconvoluted into four peaks at similar positions, with the relative intensity of the C–C/C=C peak increasing, indicating that the degree of carbonization of the coated wood had increased after the charring. The C–N bond suggests the formation of a nitrogen-containing char layer. Fig. 8e1 shows the high-resolution P 2p spectrum of the coated wood, which was deconvoluted into two peaks at 134.6 eV and 133.82 eV, corresponding to P=O and P–O–P/P–O–C,^62,63 respectively. Fig. 8e2 shows the high-resolution P 2p spectrum of the charred coated wood, where the relative intensity of the P–O–P/P–O–C peak increased, and a new peak appeared at 133.2 eV, corresponding to P–N.⁶⁴ The analysis results indicate that P, N and Ag were involved in the carbonization process, forming a P–N–Ag cross-linked carbon layer, and the carbon layer was the mixture of APP–Ag gels carbon and wood carbon. Fig. 8f shows the XRD spectrum of the charred coated wood, where a weak broad peak appeared between 20° and 30°, corresponding to amorphous carbon, and silver peaks were also observed, confirming that silver was incorporated into the char layer, consistent with the SEM and XPS results.

Fig. 8 SEM images of coated wood (a) and charred coated wood (b). XPS (c) spectra of coated wood and charred coated wood. High-resolution C 1s XPS spectra of the coated wood (d1) and charred coated wood (d2), high-resolution P 2p XPS spectra of the coated wood (e1) and charred coated wood (e2). XRD (f) spectra of the charred coated wood.

Based on the above analysis, the possible fire alarm mechanism for charred coated wood is as follows: the APP–Ag gel coating imparts good flame retardancy to the wood, and after the charring process, the coated wood can form a dense, silver-doped amorphous carbon layer with high initial electrical resistance. The carbon layer helps the wood to preserve the original structure, and the amorphous carbon layer has a negative temperature coefficient of resistance, meaning that the resistance of the carbon layer decreases as the temperature increases. The nanoparticles of silver doped within the layer can also serve as conductive pathways for charge carriers. When the device is exposed to flame, the temperature of the charred coated wood increases, enhancing the kinetic energy of charge carriers, enabling them to overcome potential barriers, thus reducing resistance. When the flame is removed from the device, the temperature of the charred coated wood decreases, and the charge carriers can not overcome the potential barriers, leading to an increase in resistance. In summary, the repeated temperature variations caused by flame exposure and removal alter the conductivity of the charred coated wood, enabling our product to function as a fire alarm sensor.^65–68

The gaseous phase pyrolysis products of wood and coated wood were detected using TG-IR. The FT-IR spectra at the temperatures corresponding to the peak values of the DTG curves for different samples are shown in Fig. 9a. Several typical pyrolysis volatiles were detected, including H₂O (3588 cm⁻¹), hydrocarbons (2964 cm⁻¹), CO₂ (2360 cm⁻¹), CO (2188 cm⁻¹), carbonyls (1731 cm⁻¹), aromatic compounds (1510 cm⁻¹) and ethers (1085 cm⁻¹).⁶⁹Fig. 9b–g illustrate the variation in the intensity of typical pyrolysis gases with temperature, revealing that the coating significantly reduced the production of pyrolysis-related gases from wood, thereby reducing heat and smoke release. Additionally, new peaks at 1180 cm⁻¹ and 1000–950 cm⁻¹ appeared in the coated wood, corresponding to P=O and NH₃.^70–72 P=O could transform into PO˙, which could capture H˙ and OH˙ within the flame, thereby terminating the chain reactions that sustained combustion effectively suppressing the flame.⁷⁰ In this process, the generation of H₂O was slightly inhibited.⁷³ NH₃ could dilute the concentration of flammable gases and oxygen, thereby limiting the combustion reaction.

Fig. 9 FT-IR spectra (a) of the volatiles of wood and coated wood at specific temperatures. The absorption peak intensities of H₂O (b), hydrocarbons (c), CO₂ (d), CO (e), carbonyls (f) and ethers (g) versus temperature.

Based on the above analysis, the possible flame retardant mechanism of the APP–Ag gel coating is as follows: when the substrate is exposed to flame, APP decomposes into phosphoric acid and polyphosphoric acid, and releases NH₃. In the gas phase, NH₃ and non-flammable gases such as H₂O dilute flammable gases and O₂ carry away some heat.³¹ Meanwhile, the polyphosphoric acid formed contains a large number of P=O groups, which can generate PO˙ and capture high-energy H˙ and HO˙, thereby suppressing the flame in the combustion zone.⁷⁰ In the condensed phase, APP and Ag can promote dehydration of ascorbic acid, triethanolamine and the substrate, forming a carbon layer.⁷³ The incorporation of triethanolamine plays an important role in promoting the formation of carbon–nitrogen–phosphorus heterocycles.³² The combination of nitrogen-containing components and phosphoric acid can lead to flame quenching and promote the expansion of the substrate into carbon. The complex char layer provides a barrier effect to inhibit the release of volatile flammable gases such as hydrocarbons, ethers and carbonyls. As a result, the combustion process becomes difficult to sustain, and the substrate can self-extinguish.

The mechanisms for conductivity, adhesion and flame retardancy enhancement have been summarized, as shown in Fig. 10.

Fig. 10 Mechanisms for conductivity (a), adhesion (b) and flame retardancy (c) enhancement.

4. Conclusion

In conclusion, we reported a unique functional gel paint. This paint was easy to apply, effectively resolving the trade-off between conductivity, flame retardancy, and adhesion found in single-layer functional coatings and coated materials exhibited excellent comprehensive properties. The coating could withstand 100 cycles of abrasion with a 1200 mesh sandpaper. The LOI of the coated cotton fabric reached 66%, with the PHRR and THR reduced by 65.3% and 50.4%, respectively, and an electrical conductivity greater than 200 S m⁻¹ can be achieved. The LOI of the coated wood reached 64%, with the PHRR and THR decreased by 37.6% and 35.3%, respectively, and an electrical conductivity greater than 667 S m⁻¹ can be achieved. The combined performance indicators for flame retardancy and conductivity were superior to those of less reported single-layer conductive flame-retardant coatings and some LBL conductive flame-retardant coatings. We successfully applied this functional paint to electric heating fabrics and sensors, which demonstrated good performance. In addition, we have thoroughly explored the mechanisms of conductivity, flame retardancy, and adhesion of the gel coatings. Our gel paint can be applied for bonding and coating on flammable substrates, suitable for fire retardancy, electric heating fabrics, fire alarms, and motion sensing, and may play a constructive role in fire prevention and protection for future buildings and electronic devices.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21774098).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh01684j

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