Fire-Retardant Nanocomposite Coatings Based on Nanoclay and POSS

Nanoclays and polyhedral oligomeric silsesquioxane (POSS) are found to be effective fire retardants that are widely used for fabricating fire-retardant nanocomposite coatings (FRNCs). FRNCs based on various nanoclays, such as montmorillonite (MMT), halloysite (HNT), vermiculite (VMT), and POSS, are discussed here. Classification of fire-retardant coatings; different types of FRNC fabrication techniques; fire-retardant mechanisms; various test methods; and the effect of nanofiller content on the fire retardancy of acrylates, epoxies, urethanes, and biopolymers are discussed in detail.

Nanotechnology in Textiles: Advances and Developments in Polymer Nanocomposites Edited by Mangala Joshi

Copyright © 2020 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-81-5 (Hardcover), 978-1-003-05581-5 (eBook) www.jennystanford.com

Fire tetrahedron

Figure 20.1 Fire tetrahedron.

Introduction

Fire can be described as the output of combustion reaction that takes place in the presence of fuel, oxygen, thermal energy (heat), and a chemical chain reaction. As represented in Fig. 20.1, generally these four elements are essential for a fire to occur and are usually called the "fire tetrahedron” or the "fire pyramid" [1]. Removing any of the essential elements will result in the fire extinguishing.

When a material burns, it undergoes a chemical change, with the formation of four important products of combustion: flammable gases, flame, heat, and smoke. Flammable gases constitute the gaseous parts of the combustion products. Flame is the visible luminous body of the burning gas, which is formed after a certain point, called the "ignition point," in the combustion reaction, consisting of carbon dioxide, water vapor, oxygen, and nitrogen [1]. Heat is the form of energy measured in terms of temperature to signify the intensity of the fire. Smoke produced in most of the fires consists of oxygen, nitrogen, and carbon dioxide.

The combustion process of polymers is similar to that happening in other materials. Generally, the combustion process of polymers

Combustion process of a polymer

Figure 20.2 Combustion process of a polymer.

includes three main steps: heating, decomposition, and ignition (Fig. 20.2). The polymer is heated by means of radiations or flames, and the thermoplastic material gets softened but the thermoset material, due to its cross-linked structure, won’t show any softening behavior. The peculiarity of a polymer is that even on further supply of thermal energy it won’t transform into a gaseous phase because of its high molecular weight; rather it will decompose. Decomposition is an endothermic process that involves the breaking of individual atomic bonds according to their unique bond dissociation energy. Usually, the decomposition process is a free-radical chain reaction that corresponds to the fourth element in the fire tetrahedron. Flammable gases formed as a result of decomposition mix with atmospheric oxygen and reach their ignition limits and are combusted by either flash ignition or selfignition. The flame produced combines with oxygen, which is an exothermic reaction, and it intensifies the decomposition of the polymer and increases the spread of the flame. Also ignition depends on factors like the amount of oxygen available, temperature, and the physical and chemical properties of the polymer.

Highly combustible materials, such as wood, plastics, and textiles, correspond to the fuel in the fire tetrahedron. Since we use these widely in our daily lives, these materials are responsible for most of the fire accidents leading to loss of life and destruction of property worth crores of rupees. According to national crime record bureau data, about 1.3 lakh Indians were killed in fire accidents during the period 2010-2014. Figure 20.3 shows the number of deaths and fire accidents versus the years they happened. Due to the increased awareness about fire safety, both numbers—that of fire accidents and that of deaths—have come down, though they are still way

Number of deaths and fire accidents versus the year

Figure 20.3 Number of deaths and fire accidents versus the year.

higher than the world average [2]. Of the number of people who died, two-thirds were female and one-third male. Figure 20.4 shows the impact of fires on the Indian economy, which is about 0.8% to 1% of the gross domestic product (GDP), which amounts to approximately 1 lakh crores per annum. The loss of GDP is the highest for India, followed by Italy, Germany, the United Kingdom, Japan, the United States of America, and Singapore.

The demand for the reduction or prevention of the fire hazards posed by these materials is increasing day by day due to more stringent fire regulations. Approaches that increase ignition resistance, retard the combustion rate, and prevent the sustained

Impact of fires on the Indian economy

Figure 20.4 Impact of fires on the Indian economy.

burning of the material are used. Four such approaches are (i) addition of additives that are nonreactive in nature, (ii) addition of reactive fire-retardant (FR) additives that form chemical bonds with the polymer backbone during the polymerization or postprocessing step, (iii) addition of an inherent FR polymer, and (iv) surface modification of the polymer substrate. Nonreactive FR additives are incorporated into the polymer matrix through physical methods, such as blending [3-5]. When compared to reactive FR additives, nonreactive FR additives are required in higher dosages to provide the required fire retardancy to the polymer, tend to leach out with time, and provide a short-term FR effect. Even though reactive FR additives do not have such disadvantages, they are very expensive and difficult to prepare. These approaches affect significantly the physical and mechanical properties. The fourth approach, surface modification, retains the mechanical properties of the polymer and furnishes good surface finish to the polymer and is carried out by applying fire-retardant coatings (FRCs).

Fire-Retardant Coatings

FRCs are mainly used in the fields of construction, electrical and electronic devices, transportation, textiles, etc., since the materials in these fields can intensify the impact of a fire when a fire hazard has occurred [6, 7]. Traditional fireproofing coatings are cementitious coatings based on Portland cement, magnesium oxychloride cement, vermiculite, gypsum, and other minerals. FRCs appear like normal paints, are mainly available in a solvent form, and are either applied to the substrate by means of brushes and rollers or sprayed onto the substrate [8]. The majority of the research work focuses on textiles in which the FRCs are used as such. Normally, the fabrics are coated with a reactive fire retardant or back-coated with a polymer matrix to impart fire retardancy. The common polymer matrices used are polyacrylates, silicones, epoxy resins, polyurethane (PU), and poly(vinyl chloride) (PVC). PVC-coated fabrics find application mainly in architectural textiles. In the field of building and construction, FR additives are added to paints and then coated on the surface. Besides, FR-coated wood and steel

Table 20.1 Commercially available intumescent FRCs

FRC

Tradenames

Manufacturer

Latex based

T-102

Firetect, USA

Ammonium polyphosphate

Exolit420,422,

Clariant, Switzerland

(APP) based

423,462

substrates are also being used in the construction field. In the electrical and electronics fields, FR-coated cables and wires are used. The main function of an FRC is to inhibit the combustion reaction by forming a protective layer or coating over the substrate. So it is possible to shield the condensed and combustible phase of the polymer, leading to the cooling of the condensed phase. This will ensure that flammable gases are not produced by a decomposition reaction, oxygen availability is reduced, and heat transfer is hindered. Thereby, the fire will be extinguished.

The ideal FRC should have least flame spreadability, negligible release of toxic gases, good thermal stability, exterior durability, ease of application, good wear resistance, flexibility, cleanability, scrub resistance, stain resistance, tolerable solvents, hardness, gloss, good adhesion to the underlying substrate, and economic viability. Satisfying all the above criteria, there are many types of FRCs available in markets (Table 20.1). They are commonly classified into intumescent and nonintumescent coatings (Fig. 20.5) [9]. Intumescent coatings are those that have the ability to swell and form a 3D char coating that serves as an insulating barrier on the top of the substrate under the influence of a fire [10]. This layer decreases the heat and mass transfer between the condensed and vapor phases. Generally, the intumescent FRC contains one char source, an acid and gas source, and one binder resin in which all the components are suspended and coated on a substrate [10, 11]. A carbon source is the char former, an acid source is a dehydrating catalyst, and a gas source or blowing agent forms the 3D porous structure. The details of the acid source and the blowing agents used are tabulated in Table 20.2.

On the other hand, nonintumescent coatings are those that contain FR additives designed to inhibit/retard the spreading of flame and smoke by the substrate [12]. There are many types of nonintumescent coatings, like halogen based, phosphorous based,

Table 20.2 Ingredients of an intumescent FRNC

Carbon source

Acid source Blowing agent

Binder resin

Pentaerythritol, starch, phenolic, or urea resins

Salt of an inorganic, Melamine, nonvolatile acid, guanidine, and such as boric, chloroparaffins sulfuric, or phosphoric acid

Thermoplastics,

chlorinated

rubbers

Classification of FR coatings

Figure 20.5 Classification of FR coatings.

nitrogen based, phosphorous-nitrogen based, silicon based, and nanocomposite based, available in the market [12,13]. In the coming sections, we will focus on the nanoclay-and polyhedral oligomeric silsesquioxane (POSS)-based FRCs.

 
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