Research Article www.acsami.org
Graphene-Borate as an Efficient Fire Retardant for Cellulosic Materials with Multiple and Synergetic Modes of Action Md J. Nine, Diana N. H. Tran, Tran Thanh Tung, Shervin Kabiri, and Dusan Losic* School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia S Supporting Information *
ABSTRACT: To address high fire risks of flamable cellulosic materials, that can trigger easy combustion, flame propagation, and release of toxic gases, we report a new fire-retardant approach using synergetic actions combining unique properties of reduced graphene oxide (rGO) and hydrated-sodium metaborates (SMB). The single-step treatment of cellulosic materials by a composite suspension of rGO/SMB was developed to create a barrier layer on sawdust surface providing highly effective fire retardant protection with multiple modes of action. These performances are designed considering synergy between properties of hydrated-SMB crystals working as chemical heat-sink to slow down the thermal degradation of the cellulosic particles and gas impermeable rGO layers that prevents access of oxygen and the release of toxic volatiles. The rGO outer layer also creates a thermal and physical barrier by donating carbon between the flame and unburnt wood particles. The fireretardant performance of developed graphene-borate composite and mechanism of fire protection are demonstrated by testing of different forms of cellulosic materials such as pine sawdust, particle-board, and fiber-based structures. Results revealed their outstanding self-extinguishing behavior with significant resistance to release of toxic and flammable volatiles suggesting rGO/ SMB to be suitable alternative to the conventional toxic halogenated flame-retardant materials. KEYWORDS: graphene, borate, fire-retardants, cellulosic materials flammable and susceptible to degradation under high temperature. Their protection from the fire is a significant challenge by using conventional fire-retardants to fulfill the demand of the projected market value of $12.81 billion USD by 2021.7 The most currently used fire-retardant materials for wood, polymers, and natural-fiber based products are based on brominated and chlorinated organic flame-retardants (BFRs and CFRs).8−10 Unfortunately, the use of these halogenated fire-retardants for last few decades have led to a concerning environmental contamination affecting wildlife and humans. The harmful consequences of using BFRs and CFRs eventually have led these products to be banned in 2010 following the “San Antonio Statement”.11 These BFRs and CFRs have been identified as mutagenic, carcinogenic, and extremely toxic, demanding for their urgent replacement with eco-friendly alternatives.11,12 To address these problems, several nonhalogen based fire-retardants have been developed using inorganic metal hydrates (aluminum hydroxide, magnesium hydroxide, zinc borate, and borax),13−18 expandable graphite,19,20 phosphorus,10,21,22 nitrogen,23,24 and silicon-based compounds,25,26 and other nanocomposites.7,27,28
1. INTRODUCTION The use of fire as primarily a source of energy was one of the key driving forces for the rise of human civilization from the Stone Age to today’s highly industrial stage.1 However, at the same time, fire presents one of the most significant hazards to people and environment causing enormous consequences of loss of lives, properties, and environmental destruction. The total costs and losses associated with fire hazards are enormous. In the U.S.A. alone the cost in 2011 was estimated to be USD $329 billion (2.1% of GDP),1 that, in Australia, is about AUD $12 billion (1.3% GDP).2 The origin of fire is well tied to the plants and trees which are cellulose-rich materials and an essential fuel causing their rapid oxidation in the exothermic chemical process of combustion in the presence of oxygen and heat.3 Wood and natural fibers are also the key structural materials used for both exterior and interior designs of buildings and constructions in different forms such as solid timber, plywood, boards, and so forth. There is a strong trend in the last few decades to replace compact natural wood with artificially engineered wood composites made of wood chips or fibers mixed with polymers and other ingredients.4−6 According to the Wood Based Panels International (WBPI), the worldwide consumption of particle boards and medium density fiber board (MDF) will reach 84.8 million m3 within 2017. The natural or engineered wood mainly contains cellulose, hemicellulose, lignin, waxes, and polymers which are highly © 2017 American Chemical Society
Received: January 13, 2017 Accepted: February 28, 2017 Published: February 28, 2017 10160
DOI: 10.1021/acsami.7b00572 ACS Appl. Mater. Interfaces 2017, 9, 10160−10168
Research Article
ACS Applied Materials & Interfaces
Scheme 1. Graphene-Borate Composites for Higher Efficacy of Fire Protection for Cellulosic Materials Based on Synergetic Effect of Several Modes of Fire-Retardant Action
composites.37 Dai et al. reported the preparation of graphene phosphonic acid (GPA) composite as a fire protective coating using mechanical ball-milling to modify graphene by introducing phosphonic acid for covalent attachment of phosphorus to the edges of graphene nanoplatelets.35 Furthermore, rGO containing hexachlorocyclotriphosphazene (phosphorus source), have been reported to outperform conventional fireretardants.36 Graphene was also introduced into nanocellulose to prepare a high performance thermal insulator that exhibited enhanced fire-retardancy particularly to reduce the flame propagation of the foam structure.38 In view of the outstanding properties of graphene and inorganic metal-hydrates demonstrated in previous works,7,37 herein we present a new concept to design an advanced fireretardant material for fire protection of cellulosic materials based on graphene-borate composites. Our approach is based on engineering of rGO and hydrated SMB composites with synergetic combination of fire retardant properties of graphene and SMB to provide multiple modes of fire protection, not possible to achieve with conventional fire retardants. The designed composite in the form of biomimetic nacre-like structures is created by in situ formation of sodium metaborate (NaBO2·xH2O or SMB) inside graphene layers. This type of composite is proposed to provide enhanced fire retardant properties based on a combination of several modes of action including: resisting access of oxygen which is key element for propagating fire, delay ignition and preventing the escape of toxic gases, failing to do these are main cause of life lost, providing water molecules in the stricture for self-extinguishing actions, and heat-sink performances, and providing a strong
Hydrated-sodium metaborate (SMB) is an inorganic metalhydrate that has barely been used as a flame-retardant despite possessing water molecules and boron in its structure, which could mitigate thermal degradation of wood and natural fibers.29 The hydrated-SMB being nontoxic, low cost, having a high thermal stability and ability with controlled release of water molecules is recognized to be a good candidate as a fireretardant material. SMB and other metal hydrates can also act as a chemical heat sink during combustion as they undergo endothermic decomposition that can cause ignition delay.16−18 In addition, the incorporation of other properties is also essential for better protection such as char formation, alteration of thermal properties, dilution of flammable gases, and inhibition of chemical reactions to resist combustion.7 To meet common fire-retardant requirements, graphene with the desirable properties of high thermal stability,30 gaseous and ionic impermeability,31,32 reduced moisture uptake,33 and nonflammable properties,34 has raised great interest, emerging as a new fire-retardant material.35,36 Graphene formulated as a 2D layered structure with a lamellae blocking effect could provide several unique features not only to prevent oxygen access, but also to delay the heat transfer between interfaces, resist the escape of pyrolysis products, and mix the oxygen. In addition, it could act as a carbon donor (charring agent) to create an insulating carbonaceous layer (char) between the burnt and unburnt structures during combustion.36 To further improve these properties graphene was combined with other common fire-retardant compounds (e.g., P-, Si-, or Ncontaining molecules graphene-nanocomposite) to formulate new and highly effective graphene-based fire-retardant 10161
DOI: 10.1021/acsami.7b00572 ACS Appl. Mater. Interfaces 2017, 9, 10160−10168
Research Article
ACS Applied Materials & Interfaces
Figure 1. Morphology and mechanism of rGO/SMB composite demonstrated for fire protection of cellulosic materials. (a) SEM image of the top surface and (b) the cross-section of rGO/SMB composite. (c) Structural model of graphene-borate composite with rGO layers and intercalated SMB nanocrystal. (d) The photos of pine sawdust before and after treatment with rGO/SMB solution showing changes of color with a model of coated wood particles pressed into wooden pellets resembling wooden particle board (photos on the right). were then covered by a one-sided closed glass cylinder to observe physical changes under constant heating. The instant reaction of samples placed on the hot plate was recorded for 30 min by a high definition video camera (Sony HDR-PJ260). To carry out the test of self-ignition properties, both the untreated and the treated sawdust (80 mg) were placed on a screen mesh wellset above a Bunsen burner (3 cm apart from the tip of burner) to be in contact with the flame. The flame height and gas flow of Bunsen burner was set by keeping a half quarterly opened air hole that is constant for both types of the samples and placed in the middle of the flame. The combustion phenomena (self-igniting, flame propagation) were recorded for further analysis by a high definition video camera (Sony HDR-PJ260). The pellets of a dimension of 120 × 13 × 3.5 mm3 were made from untreated and treated sawdust under a hydraulic pressure of 5 tons. The fire retardant behavior of these pellets were assessed by UL-94 standardized vertical burning tests.38,45 Five specimens of each type of sample were measured to ensure reproducibility of data and to grade their flammability. The time until the flame extinguished itself and the distance the burn propagated have been measured, and then the linear burning rate in mm per minute was determined. To examine rag-paper flammability with the application of rGO/ SMB material as coating, fiber-based paper was dip coated and cured under 50 °C for several times that increases the material loading up to 15 wt %. The samples with and without coating were placed under the candle burning test for flame retardant test. 2.4. Thermogravimetric Analysis. TGA (Thermogravimetric analysis) and DTG (Derivative Thermogravimetry) of treated and untreated sawdust were analyzed by a TA Instruments (Q-500, Tokyo, Japan) in air atmosphere. The temperature was raised from ambient temperature to 600 °C at a rate of 5 °C/min for the combustion in air environment. Thermogravimetric analysis coupled with Fourier transform infrared (TGA-FTIR) for the real-time analysis of multiple gas phase compounds released from the combustion samples were done by a PerkinElmer TG-IR EGA System connected to TL 8000 (TG-IR EGA, PerkinElmer Ltd., U.K.). The operation is accomplished in air atmosphere for an approximate sample mass of 16 mg at a rate of 6 °C/min.
intumescent and compact chair formation effect for creating thermal barrier with restraining structural collapse presented in Scheme 1. 39,40 To prove the proposed concept, and demonstrate these performances and multiple modes of action, wood in the form of sawdust, pressed sawdust pellets (resembling particle board), and fibrous rag paper were used as a model of cellulosic materials and tested by a series of flammability tests using standard procedures.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Graphene Oxide (GO). Graphite flakes (
