Abstract
A three-component flame-retardant system was prepared based on in situ polymerization of ammonium polyphosphate (APP), diatomite (DE), and aluminium trihydroxide (ATH) to improve the flame retardance and smoke suppression properties of fibrous materials. Compared to the two-component system of APP-DE, the addition of APP-10% DE-4% ATH dosed at a filler load of 20% of the fibrous material reached a limited oxygen index of 27.5%, which was approximately 9.1% higher than the two-component system. The lower mass loss rate and higher residual mass at high temperatures resulted in excellent flame retardance. The synergistic effect on alleviating combustion and reducing heat release was shown by the 16.6%, 22.1%, and 12.5% decreases in the peak heat release rate, total heat release, and average effective heat combustion, respectively. Superior fire resistance was demonstrated by a higher fire performance index and a lower mass loss. A smoke suppression effect was shown by the peak smoke release rate and the total smoke release results that were 28.7% and 15.8% lower than the two-component system, respectively. Based on the porous structure of DE and generated aluminum oxide (Al2O3), the outstanding adsorption effect and flame-retardant effect was also demonstrated by the production rate of carbon monoxide (CO) and carbon dioxide (CO2).
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Ternary Flame Retardant System Based on the in-situ Polymerization of Ammonium Polyphosphate-Diatomite-Aluminium Trihydroxide
Jing Li,a,b,c,* Huifang Zhao,b Lizheng Sha,b Hui Zhang,a,* Mingqi Ding,c and Changlv Liao c
A three-component flame-retardant system was prepared based on in situ polymerization of ammonium polyphosphate (APP), diatomite (DE), and aluminium trihydroxide (ATH) to improve the flame retardance and smoke suppression properties of fibrous materials. Compared to the two-component system of APP-DE, the addition of APP-10% DE-4% ATH dosed at a filler load of 20% of the fibrous material reached a limited oxygen index of 27.5%, which was approximately 9.1% higher than the two-component system. The lower mass loss rate and higher residual mass at high temperatures resulted in excellent flame retardance. The synergistic effect on alleviating combustion and reducing heat release was shown by the 16.6%, 22.1%, and 12.5% decreases in the peak heat release rate, total heat release, and average effective heat combustion, respectively. Superior fire resistance was demonstrated by a higher fire performance index and a lower mass loss. A smoke suppression effect was shown by the peak smoke release rate and the total smoke release results that were 28.7% and 15.8% lower than the two-component system, respectively. Based on the porous structure of DE and generated aluminum oxide (Al2O3), the outstanding adsorption effect and flame-retardant effect was also demonstrated by the production rate of carbon monoxide (CO) and carbon dioxide (CO2).
Keywords: Ammonium polyphosphate; Aluminum hydroxide; Diatomite; Flame retardancy; Smoke suppression
Contact information: a: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, 210037, China; b: School of Environmental and Natural Resources, Zhejiang University of Science and Technology, Hangzhou 310023, China; c: Zhejiang Jingxing Paper Stock Co. Ltd., Jiaxing 314214, China; *Corresponding author: ljing1987@gmail.com; zhnjfu@163.com
INTRODUCTION
Flame retardant cellulosic materials provide various applications for armed forces, home furnishing and decorations, uniforms for firefighting personnel, auto filters, and more (Mohamed 2005). To meet the safety standards against the inhalation of smoke and toxic combustion gases like carbon monoxide (CO), the requirements for retardant properties are increasingly necessary. There are many approaches to improve flame retardancy, including the use of fillers or additives, surface treatments, and chemical or physical modification. However, the addition of fire-retardant chemicals still initially occurs in the mill (Visakh and Arao 2015).
Among flame retardants, phosphorus-based chemicals are still one of the most widely used additives. One of the most important phosphorus-containing flame retardants, ammonium polyphosphate (APP), has been favored due to its minimal environmental impact, low cost, and high retardant efficiency (Sha and Chen 2014, 2016). In order to achieve high fire performance levels, it is necessary to develop a flame-retardant system based on a combination of different flame-retardant agents. The concept of synergism is used to optimize flame-retardant formulations and enhance the performance of mixtures of two or more additives. Synergism can be achieved with a mixture of additives (Yen et al. 2012), and the synergism between APP and aluminium trihydroxide (ATH) has been shown in previous studies. Metal oxides or hydroxides are the least effective in terms of dosage but are the least expensive of all flame-retardants. They have a certain synergistic effect with phosphorus-containing flame retardants and are widely used as flame retardant synergists for the plastic, rubber, coating, and papermaking industries (Laachachi et al. 2006; Gu et al. 2007; Lin et al. 2011; Friederich et al. 2012; Zhou et al. 2013). Nonetheless, due to the complexity of chemical additives in cellulosic products, the synergism effect can be weakened due to the lower retention rate. Diatomite (DE) is a non-metallic mineral with a porous structure, strong adsorption capabilities, and a large specific surface area, which is why it is widely used as a filler in cellulosic materials, such as flame-retardant paper (Li et al. 2008; Sha and Chen 2014). Previous work has confirmed that APP-DE composite filler can adsorb some smoke and toxic gas (Sha and Chen 2014, 2016). Moreover, the main products of completely burnt cellulosic materials are carbon dioxide (CO2) and water vapor. However, when paper burns incompletely, it releases CO, which creates smoke, carbon black particles, and other pyrolysis products. The smoke released in the burning process of cellulosic materials can be controlled by reducing the generation and volatilization of the pyrolysis volatiles. Condensed phase char formation can effectively suppress the smoke and toxic gases (Lu and Wilkie 2010; Qian et al. 2014). The condensed phase smoke suppression is consistent with the flame-retardant mechanism, and the goal of flame retardance and smoke suppression can be achieved by the design of flame-retardant formulas. To reach a superior flame-retardant effect, better synergism among APP, DE, and ATH should be considered.
In this work, a novel synergistic retardant system was created based on the in-situ polymerization of APP and ATH in the DE possessing porous structure and providing a reaction site for efficient retardancy potential. The optimum formula and mechanism were both explored.
EXPERIMENTAL
Materials
The softwood pulp and hardwood pulp were obtained from ARAUCO (Santiago, Chile) and CENIBRA (Belo Oriente, Brazil), respectively. Analytically pure phosphoric acid (85%) and urea were supplied by Shanghai Lingfeng Chemical Reagent Co. (Shanghai, China), and chemically pure DE was supplied by Chinasun Specialty Products Co. (Changshu, China). Aluminum hydroxide (Al(OH)3, 99.6%, 5000 sieve mesh) was provided by Xinxiang Jinsheng New Materials Co. (Xinxiang, China). Cationic polyacrylamide (CPAM) was provided by Nalco (Shanghai, China), and silica sol was supplied by Suzhou Tianma Specialty Chemicals Co. (Suzhou, China).
Preparation of the APP- DE-ATH Composite Fillers
A certain amount of phosphoric acid was poured into a three-necked flask and heated to 70 °C in an oil bath and a certain amount of urea was added into the agitated flask. The molar ratio of phosphoric acid to urea was controlled at 1:1.8. The reaction is shown in Eq. 1,
(1)
The mixture was heated at a rate of 2 °C/min to 3 °C/min. A DE amount equivalent to 10% mass of the generated APP and ATH equivalent to 1%, 2%, 3%, 4%, and 5% mass of the generated APP was added into the flask when the temperature reached 130 °C. The mixture was stirred rapidly, and the temperature was kept at 130 °C for 15 min. The product was poured onto a small tray and placed in an oven to solidify at 210 °C for 2 h. Then, the solid product was ground to powder and screened to obtain the APP-10% DE-ATH composite fillers with different compositions, which were designated as APP-10% DE-1% ATH, APP-10% DE-2% ATH, APP-10% DE-3% ATH, APP-10% DE-4% ATH, and APP-10% DE-5% ATH. The APP-10% DE composite filler was prepared in the absence of ATH.
Preparation of the Flame Retardant Paper
Flame retardant paper with a basis weight of 100 g/m2 was prepared with 25 wt% of softwood pulp and 75 wt% of hardwood pulp as the main fibrous raw materials and 0.2 wt% of cationic polyacrylamide and 0.3 wt% of silica sol were the dual retention aids. The prepared composite fillers at different dosages were used as the flame retardants.
Characterization
The thermogravimetric analysis (TGA) of the composite fillers was performed on a STA 449 F3 TGA thermal analyzer (NETZSCH, Selb, Germany) at a heating rate of 10 °C/min under a nitrogen atmosphere and a flow rate of 50 mL/min. A 10 mg sample was used for the thermal analysis.
The limited oxygen index (LOI) test of the paper samples were performed with a JF-3 digital display limiting oxygen index tester (Glomro, Shanghai, China) according to the ASTM standard D2863-13 (2013). The test samples were 100 mm × 10 mm.
The cone calorimeter tests (CCTs) of the paper sheet were carried out with an FTT 2000 cone calorimeter (Fire Testing Technology, East Grinstead, UK) in accordance with the ASTM standard E1354-16a (2016). The specimens (100 mm × 100 mm × 0.20 mm) were measured horizontally with a standard optional retainer frame and grid at a heat flux of 30 kW/m2. The distance between the cone heater and the specimen was 25 mm.
The scanning electronic microscopy (SEM) analysis was performed on the charred residues after the CCTs by using a TM3000 scanning electron microscope (Hitachi, Tokyo, Japan). The resolution of the SEM was 30 nm and its accelerating voltage was 5 kV.
The elemental analysis of the charred residues after the CCTs with and without filler was also determined using an Energy-Dispersive Spectrometer (EDS, JED-2300, JEOL, Tokyo, Japan) on Field-Emission Scanning Electron Microscope (FE-SEM, JSM-7001F, JEOL, Tokyo, Japan).
Tensile strength and bursting strength were tested according to TAPPI standard methods and calculated based on basis weight.
RESULTS AND DISCUSSION
LOI Values
Figure 1 shows LOI values of paper filled with different flame retardant recipes at different filler loadings. As can be seen in Fig. 1, the LOI values of the paper increased with the increased filler loading. The APP-10% DE-4% ATH sample yielded the best LOI value and therefore the best flame-retardance. Although a higher retardant loading resulted in better flame retardance, the LOI value even at filler loading of 20% had achieved 27.5%, which could meet the requirement for fire resistance. Moreover, the addition of ATH at 4% showed the best results and was therefore chosen as the optimum formula.