A new intumescent insulation emergency material for thermal protection of storage tanks –potassium polyacrylate & organic modified hectorite & intumescent flame retardant

Highlights

• A new type of intumescent insulation emergency material (PPHI) is proposed for thermal protection of storage tanks.

• The optimum content of intumescent flame retardant in the PPHI material was determined.

• A series of thermal insulation properties of PPHI material were studied.

• The possible insulation mechanism of PPHI is proposed.

Abstract

Thermal protection of adjacent equipment such as chemical storage tanks is very important when a fire accident occurs. In this paper, a new intumescent insulation emergency material, potassium polyacrylate & organically modified hectorite & intumescent flame retardant (PPHI), was prepared successfully. The material was characterized by Fourier Transform Infra-Red (FTIR) and X-ray photoelectron spectroscopy(XPS). Thermal insulation performance and thermal stability of the material were studied by cone calorimeter, fire resistance test, scanning electron microscope and energy dispersive spectroscopy (SEM/EDS), and thermogravimetric analysis (TGA), respectively. The results showed that thermal insulation performance of PPHI was the best when the addition content of intumescent flame retardant (IFR) reached 0.54 wt%. The morphology and structures by SEM gave positive evidence that char layers formed from the PPHI composites were improved greatly due to the addition of intumescent flame retardant. Finally, a possible thermal insulation mechanism of PPHI was presented.

Introduction

In recent years, with the continuous expansion of the scale of the chemical industry, a large number of large-scale hazardous chemical storage tanks have been put into use. When a fire occurs in one of the storage tanks, the thermal radiation it brings will even ignite the adjacent storage tanks, worsening the consequences of the accident (Ovidi et al., 2021; Shi et al., 2019). Therefore, the prevention of chemical tank fire needs to be strengthened. Nowadays, there are mainly four traditional ways to protect adjacent tanks, which are safe distances between tanks, fire dike, sprinklers, and fire-retardant coatings on tank walls(Ramsden and Abusaieda., 2017). Meng et al. used PHAST and FDS software to evaluate the consequences of representative accident scenarios for oil tanks of different sizes. The results show that, when the safety distance between storage tanks is designed according to current standards, the safety performance decreases as the capacity increases (Meng et al., 2012). In addition to the proper safety distance, the most commonly method is using cooling water. In general, it is possible to reduce the distance between tanks if they are protected by a cooling water system. In the early days, Droste used water spray systems and thermal insulation protection measures for liquefied petroleum tanks to conduct a full-enclosure fire experiment. The results show that neither water mist nor thermal insulation will cause the explosion of the tanks within 90 min under the action of fire. The protection of the thermal insulation layer is more effective than the water spray systems at the beginning of the fire(B. Droste and Schoen, 1988; W.Schoen and Droste, 1988). Shirvill studied the effectiveness of the water spray system to protect the tank under conditions of jet fire impingement from nearby releases of liquid propane and butane in 13 tons liquefied petroleum gas tank. The study found that, in the state of jet fire, it is difficult for the water spray device to completely cover the surface of the liquefied petroleum gas storage tank(Shirvill, 2004). The experiment results from Roberts show that when the actual spray intensity reaches 10 L/(min·m²), the water spray system can effectively protect the storage tank(Roberts, 2004).

However, the effectiveness of the cooling water system depends on many factors, including regular overhaul and maintenance schedule, service life, and so on. Dry spots will probably appear on the surface of the tank when the cooling water system fails or does not absorb radiant heat enough to provide adequate protection. When part of the tank is exposed to the high radiant heat of the adjacent pool fire, this situation will lead to the uneven temperature inside the tank and the deformation and failure of the tank. Due to the decline of the mechanical properties of the tank, the strength of the tank wall will be weakened or even collapse(Rana et al., 2010). Excessive high temperatures can cause deformation and prevent it from recovering to its original state. Carbon steel starts to lose its strength at about 400 °C. For example, when a liquid storage tank is exposed to thermal radiation, the temperature and pressure of the liquid in the tank increase. When the rising pressure value exceeds the set pressure value, pressure relief valve will be opened automatically, and flammable steam is released into the atmosphere, which is likely to lead to the escalation of the accident.

For a steel structure chemical storage tank, the intumescent flame retardant coating is often used as one of the thermal protection methods to prevent fire (Lucherini and Maluk, 2019). In recent years, environment-friendly intumescent flame retardants have gradually attracted people's interest. Polylactide (PLA) composites containing a combination of oxidized corn pith fiber and a bio-based flame retardant have been prepared via an in-situ modification method(Yang et al., 2021). Intumescent coatings formulated with coconut fiber, wood waste, and peach stone biomasses were tested. Different formulations with variable concentrations of biomasses were carried out(de Souza et al., 2016). Epoxy resin coatings were prepared with vegetable compounds (ginger powder and coffee husk) to act as a char source in the intumescent system. These vegetable compounds showed potential application as a char source and decreased the temperature rise rate of the substrate (de Sá et al., 2017). Costes uses a melt blending method to combine lignin and phytic acid to develop a bio-based flame retardant system in PLA. Accordingly, its mechanics, thermal and fire performance were studied. Studies have shown that combining these two additives can provide an interesting way to limit the negative effects of each additive and improve thermal and flame retardant properties of the material(Costes et al., 2017).

Hectorite is a natural layered magnesium-lithium silicate, and it is a trioctahedral clay mineral. Traditionally, hectorite can be used as adsorbent, catalytic and rheological additives. More recently, hectorite is often used in the fields of advanced analytic and optical, diagnostic, medical materials and tissue engineering(Zhang et al., 2019). Hectorite can also be used to prepare heat-resistant or flame retardant materials. For example, a kind of nanocomposites was prepared by melt blending poly(lactic acid) with 5 and 7 wt% of an organically modified montmorillonite or an organically modified magnesium sodium fluoro-hectorite or unmodified sepiolite. The results show that these nanocomposites exhibit good clay dispersion into the polymer matrix and have significant thermal and thermos-mechanical performance improvements(Fukushima et al., 2012). Awad et al. prepared a kind of nanocomposites of poly(vinyl chloride) with hectorite- and bentonite-based organically-modified clays. The measurement results indicate that the addition of appropriately modified bentonite or hectorite nano-clay not only reduces the total amount of the material's smoke evolution but also increases the evolution time of smoke. At the same time, a decrease in the peak value of heat release rate can also be seen(Awad et al., 2009).

Tea saponin is a kind of green and natural plant extract, it is cheap and renewable, and it is often used as a foaming agent. Its structure contains many hydrophilic groups such as –COOH and –OH, which makes it more soluble in water (Qian et al., 2015). Hectorite can effectively improve the thermal stability of the material, which has many good properties such as hydrophilicity, thixotropy, and so on (Fukushima et al., 2012; Zhang et al., 2019). The combination of organic bentonite and tea saponin can significantly improve the fluidity, thixotropy, and storage stability of the material(Qian et al., 2014).

When the cooling water and protective coating fail, in order to prevent the expansion of the accident, an emergency heat insulation strategy is urgently needed. For this reason, we have successfully prepared sprayable emergency thermal insulation composite material PPH (potassium polyacrylate & hectorite)(Zhang et al., 2021). When encountering an emergency, PPH material is combined with fire-fighting water and sprayed on the surface of the storage tank to isolate the heat conduction between adjacent storage tanks.

In this work, the PPH material has been further upgraded. Firstly, silane coupling agent KH-570 was used to organically modify hectorite (Kotal and Bhowmick, 2015). Then, tea saponin, ammonium polyphosphate (APP) and pentaerythritol (PER) trinity intumescent flame retardant was prepared and added to the materials. Accordingly, a new type of emergency thermal insulation composite material PPHI was synthesized. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), cone calorimeter (cone calorimeter), thermogravimetric analyzer (TGA), and scanning electron microscope/energy spectrometer (SEM/EDS) were used to study the effect of flame retardant content on composite influence of material thermal performance.