Chapter 1: Introduction

Nowadays, polymeric materials are proliferating in nearly all industrial sectors because of their multiple merits such as light weight, balanced mechanical properties, and great resistance to chemical and corrosive environments. However, the chemical composition and aliphatic long chains make it very easy for organic polymeric materials to catch fire, which then leads to rapid flame spread accompanied by the generation of a great number of toxic substances when they are exposed to high temperatures and heat sources. Correspondingly, there has been a huge loss of life and property worldwide each year. According to the International Association of Fire and Rescue Services (CTIF) report, between 2012 and 2016, about 4 million fires were reported worldwide, which caused about 31 000 deaths and approximately 62 000 injuries.¹  Therefore, it has been necessary to develop flame retardants (FRs) for creating fire-retardant polymeric materials to mitigate their fire impacts.

Although halogenated FRs are widely used in a variety of polymeric materials for a long time due to their low cost and high efficiency, it has been known for over 20 years that some of these compounds such as tetrachlorobisphenol A (TCBPA) and polybrominated diphenyl ethers (BDEs) can leach into the environment, with particularly high concentrations recorded in fish and marine mammals, which can affect the nervous and cardiovascular systems of these animals.^2,3  Capozzi et al. found thirty-five types of PBDEs and eight other FRs in air samples (vapor plus particles) collected at six sites near the North American Great Lakes between 2005 and 2019 and indicated that the levels of FRs were significantly high in urban areas.⁴  Wang et al. investigated the concentrations and distributions of PBDEs and other brominated FRs (NBFRs) in the dissolved phase of surface seawater near a coastal mariculture area of the Bohai Sea in China, and the results showed that the total concentrations of PBDE and NBFRs were in the range of 15.4–65.5 and 2.12–13.6 ng L⁻¹.⁵  Since the Stockholm Convention on 22 May 2001 conducted in Stockholm, many countries including the United States, Europe, and China have already taken actions to control/restrict the production and application of these halogenated FRs.

As an alternative, organophosphate esters (OPEs) are predicted to contribute over 20% of flame retardants in the near future owing to their relatively high effectiveness.⁶  However, OPEs are reported to have high toxicity, persistency, and biomagnification potential in the ecosystems and human bodies.⁷  Their bioaccumulation has been reported to exist in various biota, surface water and sediments, air, fish, and even in human samples.^8,9 

The long-lasting environmental and health concerns associated with the above-discussed fire retardants have driven the development of “green” flame retardants for polymeric materials. “Green” flame retardants, here, not only include the fire retardants themselves but also include their synthetic process.

Generally, green fire retardants include nontoxic inorganic oxides (e.g., Mg(OH)₂ and Al(OH)₃); naturally derived inorganic fire retardants (e.g., montmorillonite and halloysite); and naturally-derived organic fire retardants (e.g., lignin, phytic acid, DNA, and dopamine) as well as their derivatives. In this book, the emphasis will be on naturally derived fire retardants from sustainable resources such as plant, animal, mineral, or other lower-carbon feedstocks, which are environmentally benign and do not introduce toxic compounds into our ecosystem. Still, major challenges with these sustainable feedstocks include how to use them to synthesise fire retardants that show comparable efficiency to existing petroleum-based counterparts in polymers and their massive production. A typical example is lignin, a by-product in the agricultural, paper, and pulp industries, and it is always deemed as a carbon source due to its high thermal stability and char-forming capability. However, polymer composites modified with lignin alone often fail to reach the required V-0 rating during UL-94 tests because of its limited efficiency.¹⁰  To boost its efficiency, chemical modifications are usually required, typically through chemically/physically introducing phosphorus (P) and/or nitrogen (N) into its structures.

As mentioned above, green synthesis of FRs is an equally important topic for the development of green fire retardants. Traditionally, fire retardant synthesis involves the use of massive organic solvents, which can cause volatile organic compound (VOC) issues and increased complexity of post-processing. Besides, longer synthesis periods and higher treatment temperatures/pressures also lead to higher energy consumption. Hence, it has been desirable to develop fire retardants via “Green Synthesis”—that is, using facile methods and a few reaction steps using water or other less toxic liquids as a solvent or even without using any solvents. In particular, recent years have witnessed the use of water as the media for the synthesis of fire retardants based on electrostatic interactions or hydrogen bonding.

In brief, this book mainly focuses on (i) the use of green fire retardants in polymeric materials and (ii) the green synthesis of eco-friendly fire retardants for polymeric materials, with the aim of mitigating the fire risk of existing and new polymeric materials while minimising their environmental and health impacts. This book aims to summarise the state-of-the-art advances of green fire retardants for creating fire-retardant polymeric materials and to provide a useful reference resource for scientists, engineers, postgraduate students, and policy-maker scientists who are interested in or working in the fire retardancy field. The ultimate goal of this book is to promote the development of green fire retardants that safeguard life and property while minimising their negative impacts on our ecosystem.