A sustainable approach to scalable production of a graphene based flame retardant using waste fish deoxyribonucleic acid

Highlights

• Graphene in presence of waste fish DNA was exfoliated and functionalized.

• Highly DNA grafted graphene (GnP^D) contained ∼27% P, N, and O contents.

• GnP^D acted as a strong flame retardant against polymer matrices.

• A compact layer and a porous layer were observed after polymers burning.

Abstract

Graphene based flame retardants have gained increasing attention among researchers due to some interesting properties such as the tortuous path effect. Nonetheless, their complex, unsustainable, eco-unfriendly, and costly production hinder their adoption in various fields. Here, we used waste deoxyribonucleic acid, generated by the fishing industry, as a sustainable source of phosphorus and nitrogen to functionalize graphene nanomaterials. Our scalable and one-step approach, which employs waste-derived deoxyribonucleic acid as a green modifier in the ball milling process, is capable of producing the deoxyribonucleic acid-functionalized graphene nanoplatelets from graphite with high production yield, high oxygen, nitrogen and phosphorus contents, and high water dispersion concentration. Such a synthesized approach led to the exfoliated nanoplatelets, mostly consisting of layers with a thickness <9 nm and a lateral size of 300–600 nm, with the Brunauer–Emmett–Teller specific surface area of ∼180 m²/g. The remarkable effect of the deoxyribonucleic acid-functionalized graphene as an efficient and sustainable flame retardant on fire extinguishing of a wide range of polymer matrices including epoxy resin, polyvinyl alcohol, and polystyrene nanocomposites was successfully evidenced by achieving V-0 rating in UL-94 vertical burning tests. A multilayer char residue consisting of a compact layer and a porous layer was found to be the dominated mechanism in the fire extinguishing. The combination of deoxyribonucleic acid and graphene can result in manufacturing value-added green flame retardants from indirect reuse of fish waste, which can be suitable for high performance polymer nanocomposites including construction, automotive and aerospace. It is envisaged that the loop from fish waste to green flame retardants may come to be closed soon, which can be the main goal of the circular economy in cutting-edge applications.

Introduction

Over the previous decades, flame retardant community have been shifting their attention towards sustainability, especially in term of environmental impacts (Landry, 2012). It has been shown that flame retardants, such as brominated flame retardants or synthesized intumescent systems based on phosphorous, have some devastating effects on the environment in terms of persistency (de Wit, 2002), bio-accumulation and environmental toxicity (Grand and Wilkie, 2000). Such toxic flame retardants can have long term impact on the environment since they can often leach out of the polymer-based products (Ravichandran et al., 2011). Based on this fact, some regulations regarding banning brominated flame-retardants in the USA and Europe have been introduced. Similarly, phosphorous based flame retardants are also under inspection because of their environmental and health safety (EHS) issues (Zammarano et al., 2011). It appears that there would be an urgent need for sustainable, low leaching, and non-toxic flame retardants with low EHS.

From long time ago, fish waste have been used for common everyday applications in different low-investment possibilities such as agriculture fertilizer (López-Mosquera et al., 2011), animal feed (Arruda et al., 2007), and biodiesel/biogas (Arvanitoyannis and Kassaveti, 2008) and the like. Although such mentioned functionalities can be efficient for green production, cleaner environment, and less energy consumption in these areas above, big investors in different cutting-edge technologies such as automotive, aerospace and construction will not be absorbed to such fish waste unless a considerable potential of fish waste is found for their applications. It is assumed that the manipulation of fish waste to make it valued-added product will help the reusing of fish waste more. Such expected trend will become a research hotspot in the circular economy chain since the global concerns including landfill space (Pan et al., 2015), resource efficiency and raw materials (de la Caba et al., 2019) are of importance.

Among different by-products of the fish industry, fish sperm is one of the most valuable renewable materials. Fish sperm is a renewable, sustainable, and inexpensive source of deoxyribonucleic acid (DNA) (Alongi et al., 2013). It has been reported that DNA can act as an intrinsically flame retardant on cotton fabrics and enhance fire retardancy of the system. In contrast to the conventional fire retardant materials, which are commonly phosphorous or halogen-based hazardous compounds, DNA is a green and natural flame suppressant and retardant which can potentially be replaced with traditional fire-retardant materials. As a result, the use of green alternatives can reduce the concerns related to the environmental impact including climate change, ecosystem impacts, human health effects, and resource depletion (Wang et al., 2019).

The general structure of DNA consists of sodium phosphate backbone groups, deoxyribose unites, and nucleobases (adenine, guanine, cytosine, and thymine) having hydrogen bondings together. The sodium phosphate groups can be decomposed into phosphoric acid and water, leaving behind a char during its combustion. Nucleobases, which also known as nitrogenous bases, are nitrogen-containing biological compounds that can act like a blowing agent releasing non-combustible gases because of burning. Thanks to the phosphate-deoxyribose backbone and amino acids in DNA structure, it can be considered a strong anionic polyelectrolyte (Wang et al., 2001), as well as a high source of phosphorus and nitrogen compounds (Zabihi et al., 2016). Nonetheless, from a practical point of view, although it seems that DNA has the potential to be considered a green flame retardant, it still has some hurdles to jump. Due to inherent weak nature of DNA (Chauhan, 2018), It seems that the incorporation of DNA itself in polymers cannot meet evolving market place demands in term of mechanical performance, which is needed for fabrication of high performance polymer composites/nanocomposites. The DNA can only act as a modifier for increasing the flame retardancy of reinforcing materials in composites/nanocomposites.

Among different types of reinforcing materials, graphene, a monolayer to few-layer of carbon atoms assembled in a hexagonal lattice, has gained extensive attractions due to its exceptional properties including mechanical, electrical, thermal, and optical properties (Ahmadi et al., 2019a). Graphene is known as a prominent candidate for a vast range of applications such as high-performance structural composites, anti-corrosion coatings and paints, efficient and precise sensors, efficient electronics, flexible displays, solar panels, drug delivery, and the like (Ye and Tour, 2019). Since graphene discovery, massive advancements have been made to develop new methodologies for the production of functionalized graphene (Zabihi et al., 2017), and multifunctional graphene nanomaterials which resulted in emergence of a wide range of graphene-based materials family (Kuila et al., 2012). In recent years, graphene has gained enormous attractions to be used as a promising platform in the synthesis of high-performance graphene-based flame retardants (Hu et al., 2016). Owing to layered structures and high thermal stability, incorporation of graphene in polymer matrices provide a so-called “tortuous path” effect, hindering the diffusion path of pyrolysis products (Wang et al., 2017). Such barrier effect of graphene induces the formation of a compact residue when burning polymer matrices. Traditional flame retardant fillers generally include inorganic flame retardants such as aluminium and magnesium hydroxide, and organic flame retardants such as halogen, nitrogen, and phosphorus-rich compounds (Cheng et al., 2016). Inorganic flame retardants are restricted due to poor efficiency, needing high loading, and poor compatibility with polymer matrices (Zia-ul-Mustafa et al., 2017). Application of organic flame retardants is also limited due to releasing highly toxic and harmful gases, low thermal stability, and softening effects on polymer matrices.

General mechanisms involved in suppressing combustion using flame retardants are endothermic degradation, thermal shielding effect, releasing dilutant gas phase, and gas-phase radical quenching (Cheng et al., 2019). In the recent decade, inspired by these mechanisms, many attempts have been made to synthesise graphene oxide (GO) functionalized by species having such features. Due to owing various functional groups e. g., hydroxyl, carboxyl, and epoxide groups, GO can provide a facile accessible platform to functionalize graphene with flame retardant agents. Focusing on phosphorous based flame retardant, different strategies have been performed to functionalize graphene oxide by phosphaphenanthrene (Ran et al., 2019), phenoxycycloposphazene (Chen et al., 2018), phosphorous trichloride (Żelechowska et al., 2017), phosphorus oxychloride (Yu et al., 2015), phytic acid (Fang et al., 2019), polyphosphoric acid (Khose et al., 2018), and phosphorous based deep eutectic solvent (Pethsangave et al., 2017). All of these approaches are often costly and not eco-friendly due to the following reasons: (1) synthesis of GO from graphite by Hummer, Brodie and Staudenmaier’s methods (Tsagkalias et al., 2018) requires significant amounts of concentrated acids and strong oxidants, generating liquid and gaseous wastes, and (2) its functionalization approaches usually involve several sequence reaction routs requiring toxic organic solvents and harsh conditions.

As an alternative approach, synthesizing functionalized graphene nanomaterials using mechanical exfoliation through ball milling process has received great attention due to more environmentally friendly, i.e., no need of organic solvent, (Piras et al., 2019), low cost, and scalable process (Zhang et al., 2017). Due to high collision energy during ball milling, low aspect graphene nanoplatelets (GnP) ratio e. g., small lateral dimension are usually expected by this approach, limiting production of high-quality graphene (Zhang et al., 2017). In mechanical exfoliation using ball milling approach, the role of strength and concentration of various surfactants and additives in increasing exfoliation degree, avoiding graphite plates from extensive breaking, and simultaneous functionalization during ball milling of graphite have been well studied (León et al., 2014). The direct mechanical exfoliation of GnP from graphite by ball milling is an efficient approach for the low cost and scalable production of graphene nanomaterials.

The ball milling of graphite with functional additives serves two main goals: (1) breaking the strong interlayer interactions between graphite layers using high shear forces, which results in exfoliating graphite into thin graphene stacks, and (2) covalently/non-covalently attaching significant amounts of functional additives by interaction of additives with active sites produced on the edge or surface of exfoliated nanoplatelets during ball milling, to obtain multifunctional GnP. The presence of excessive surfactant or additive can hinder exfoliated GnP from restacking during milling. In the context of graphene-based flame retardants, various graphene nanosheets were synthesized through ball milling of graphite in presence of additives such as aluminium particles (Jeon et al., 2017) and red phosphorus powders (Kim et al., 2014), aiming to simultaneously synthesise and edge-functionalized graphene with aluminium hydroxide and phosphoric acid. Although the graphene functionalized with aluminium hydroxide and phosphoric acid can release water during combustion and yield char residues which hinder fire propagation, the resulting functionalized graphene using these methods often fall short of high lateral size and low thickness.

Based on the discussion mentioned above and considering the potential of DNA, it seems that DNA, as an efficient green surfactant in the ball milling process of graphite, alongside its flame retardancy effect during combustion, making it a sustainable and green candidate for production of graphene based flame retardant materials. Here, this work presents a novel fire retardant materials processing towards the use of green and sustainable materials with minimum waste generation to reduce the fire hazard in polymeric materials. For this purpose, graphene was functionalized with fish DNA through the ball milling process, synergistically contributing to produce a highly rigid and efficient flame retardant. It is assumed that DNA can act as not only flame retardants but also exfoliation modifier, leading to lessen the breakage of produced graphene during the ball milling process. The produced GnPs were incorporated into a wide range of polymers matrices including epoxy resin (EP), polyvinyl alcohol (PVA), and polystyrene (PS) to evaluate the effect of this novel graphene-based flame retardant on the fire safety performance of nanocomposites. It is hypothesized that such a green flame retardant can compete with other rivals including conventional flame retardants in terms of flame retardancy efficiency and compatibility with the polymeric matrices.