Halogen-free, phosphorus decorated, bio-waste derived nanocomposite for highly ecient ame retardant for cotton fabric

We have prepared an ecient ame retardant composite using biowaste derived phosphorous groups decorated graphene supported nanomaterial. The eggshell was utilized as a source of calcium carbonate, which was converted to monocalcium phosphate (CP) by phosphoric acid treatment. As-prepared monocalcium phosphate was functionalized with graphene to prepare graphene functionalized monocalcium phosphate (GCP). The GCP-coated fabric didn't ignite during the ame test and sustained more than 600s on continuous exposure to ame without changing its initial length and shape. Whereas, graphene oxide (GO), and CP coated cotton fabric burnt out very easily within a short time. The synthesized GCP coated cotton fabric also conrmed ecient ame retardant property with a high limiting oxygen index (34.1) and char length of 2.5cm was generated from the VFT test. This facile method enables an easy process for mass production of cost-effective, bio-waste derived nanomaterial for a signicantly highly ecient candidate for different applications in sustainable chemistry, including ame-retardant application.


Introduction
Fire is the major revolutionary discovery of humans with a two-edged sword in that it is used in almost every aspect of daily life and industry, but it also causes signi cant damage. Every year approximately 0.2% of the global economy is lost due to re disasters without counting its reconstruction and other medical-related issues (Li et  The eggshell employed as a bio-ller was commonly utilized as an intumescent ame retardant coating because it includes a high quantity of calcium carbonate (CaCO 3 ), i.e., about 95% while 5% of other organic components. Intumescent re-retardant coating, which is largely composed of an intumescent re-retardant system, a synergist, a binder, and an auxiliary agent, is considered as one of the most effective materials for shielding substrates from re hazards (Yan et al., 2018). Researchers are interested in using polymer because of its simple compatibility with bio-lters, re resistance, and mechanical characteristics (Yew et al., 2018). The intumescent ame retardant has lately become a pandemic FR for polymers because of its low smoke, halogen-free, low toxicity, and excellent e ciency (Xueying et al., 2015;Zhang et al., 2018). Researchers have been drawn to developing novel intumescent ame retardant systems using bio-based ame retardants like starch (Olivares et  These ame retardants mainly consist of acid source, blowing agent, and binder. When these types of ame retardants come in contact with the ame, they produce black smoke, heat, and toxic gases (Chen et al., 2016). Also, such type of ame retardants has a limited scope of application, which means they cannot be applicable on cotton fabric. Graphene is forthcoming as a very attractive support material for ame retardation systems. It is a perfect t for the skeleton of ame retardant materials due to its large surface area, high oxygen-rich active functionalities, low toxicity potential and thermal stability. As a result, society continues to demand e cient materials that are simple to produce, environmentally benign, cheaper in cost, and gentle in nature. According to the literature, the modi ed bio llers-based ame retardant may be enhanced by the functionalization of phosphorous groups in the presence of graphene as a high surface area containing a suitable carbon-based skeleton.
As a result of the need for an environmentally friendly, easy-to-prepare ame retardant nanocomposite, we synthesized one from biowaste. In this, the biowaste (eggshell) was treated with phosphoric acid before being reacted with graphene oxide to prepare a nanocomposite for ame retardant applications.
Eggshell was converted to monocalcium phosphate by phosphoric acid treatment, which was functionalized with graphene to prepare graphene functionalized monocalcium phosphate (GCP). The prepared composite can sustain in the continuous ame for more than 10 min (600 s) as compared to the blank cotton fabric (5 s), coated fabric GO (15 s), and CP coated fabric (10 s). The coating of the GCP is a very simple and easy method that requires a concise time. Compared to the blank cotton fabric, GO coated cotton fabric, and CP coated fabric, the produced composite can maintain the ame for more than 10 minutes.

Preparation of graphene oxide (GO):
As mentioned in the preceding paper, the graphene oxide was synthesized using a modi ed Hummers technique (Dhumal et al., 2021).

The preparation of monocalcium phosphate (CP)
The waste eggshell was collected as a source of CaCO 3 from the local canteen and washed several times with water. The washed shells were dried at room temperature rst, then in an oven at 80°C for 6 hrs. To make powder, the dried shells were crushed in a grinder. This eggshell powder was then mixed in a 1:2 ratio with the phosphoric acid solution, as shown in equation 1. Subsequently, the solution was stirred for an hour and water was added to this solution. After that, the solution was ltered, and the white solid was collected. This white mass was then dried in an oven at 100°C for an overnight period. Results And Discussion X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy was used to characterize the as-prepared materials (SEM).
The surface morphology of the as-prepared materials was studied by scanning electron microscopy (SEM). In the case of GO, it has a thin, wrinkled sheet-like structure (Some et al., 2012). The morphological changes have been traced by performing SEM of GCP coated material before and after the re test and SEM pictures are displayed in Figure 1. The shape of the GCP in these images makes it appear like the CPs are evenly dispersed on the graphene surface. The CP particles are clustered together in a form that resembles a bundle (Figure 1a). The morphology of the CP alters following the ame test as it transforms into granular form, which is interesting. It might be because when the CP is exposed to heat, it forms a stable crystal structure in the form of granules or spheres, as shown in Figure 1b (Welzel et al., 2004).
Furthermore, as shown in Figure 2a, the cotton fabric was being uniformly coated by the composite throughout the surface, and its elemental mapping contains carbon, phosphorous, oxygen, and calcium ( Figure 2c). In addition, the cotton fabric retained its morphology and structure (Figure 2b) as well as its elemental mapping, depicting an equal distribution of the carbon, phosphorous, oxygen, and calcium elements, as shown in Figure 2d. The EDX mapping of the GCP composite was described on the Table  S1.
XPS analysis was used to study the surface electronic state and atomic composition of the composite.
The surface electrical state of the composite and its atomic composition was investigated using XPS analysis.
The GCP has a high oxygen content i.e., the C/O ratio of 0.95 as shown in Table S1. In the spectra survey of GCP (Figure 3a), the C1s, O1s, P2p, P2s, and Ca2p peaks were seen. At 531.2 and 533.2 eV, respectively, the O1s spectra (Figure 3b) comprise the C=O/P=O and C-O/P-O bonding. The GCP's C1s spectra (Figure 3c) reveals a strong peak for C-C bonding at 284.6 eV, a peak for C-O and C-P at 285 eV, and a peak for C-O and C-P at 285 eV. The existence of P2p and P2s at 133.2 and 134.6 eV, respectively, is con rmed in the GCP spectra, as shown in Figure 3d  The TGA was performed in an air environment with a ow rate of 20°C to assess the thermal stability of the GCP composite. Because of its thermal stability, there is no notable change in weight in the TGA spectra of CP and calcium carbonate (Figure 4a). Both curves have excellent thermal stability, with weight loss ranging from 15-20% in each case. The rst loss was noticed in the GCP curve at 100°C owing to the loss of intercalated water molecules. After that, at 240°C, a second weight loss was observed due to the burning of the carbon skeleton (Ylmaz et al., 2004). The GCP has lost about 41% of its total weight, demonstrating the composite's thermal stability. The crystallite structure of the GCP composite material has shown by the XRD pattern (Figure 4b). The usual graphene peak at 26.2° and the peak at 32.2° suggest the presence of CP crystals (Kamalanathan et al., 2014). Figure S1 depicts the graphene oxide XRD pattern, which exhibits a characteristic peak at 10°.
The FT-IR spectra of GO revealed the following peaks as shown in Figure 5. In the spectra of the CP, the peaks found at 569, 950, and 1092 cm −1 attributed to the vibrations of phosphate group PO4 −3 (Yelteen et al., 2016). Also, the hydroxy group (-OH) is represented by the steep peak at 3467 cm −1 and the peak at 650 cm −1 . Peaks for the phosphate group (PO4 −3 ) at 599, 977, and 1024 cm −1 in the GCP spectra demonstrate the incorporation of monocalcium phosphate into the graphene core. At 1440 cm −1 , the peak for C-H stretching was also seen. From the peak at 1627 cm −1 , the C=C stretching vibration bonding was con rmed.

Flame retardant test of the GCP coated cotton fabric
An ethanol lamp was utilized to conduct the ame retardant test.
A dipping procedure was used to coat the GCP sample on the fabric. The GO and CP coated cotton fabrics were treated in the same way. The GO coated cotton fabric and blank cotton fabric were tested for the ame experiment in the air atmosphere, as shown in Figure (6 a-c) and Video V1. The GO coated fabric burnt out within 15 s, whereas the blank cotton fabric burnt out within 5 s. The CP coated burnt out in 10 s, exhibited the same phenomena in comparison to other control samples (Figure 6d-f and Video V2). In the case of GCP, the coated fabric did not catch re for more than 10 minutes (600s) on continuous ame and retained its original shape and size, as illustrated in Figure ( The plausible mechanism of the GCP composite for ame retardancy is shown in Figure S2. As a result, GCP coated fabric was the most promising ame retardant material compared to GO and CP coated cotton fabric. The chemistry of ame retardancy is determined by the recurrence of chemical structural elements and their interaction during combustion. Because of the strong bonding interaction, the energy required to break the sp2 hybridized carbon atom is higher. As a result, any compounds containing the sp2 carbon's strong backbone, are more ame resistant (Walters et al., 2002;Lyon et al., 2009).
To evaluation of the ame retardant property of the GCP composite, the vertical ammability test (VFT) was performed. The test was done as per the ASTMD6413-09 standard method for the ame-resistant test of cotton fabric. The ame height was 38 mm and the sample dimensions were 30 x 7.6 cm 2 . The bottom cut edge of the fabric was subjected to a controlled ame for 12 s, followed by ashing over the surface, burning with ame time, burning with afterglow after ame stop, and char length measured as shown in Figure 6. Table S2 shows the results of the vertical ammability test, which were outstanding, as well as detailed ammability data. The blank fabric lit quickly and burned up completely in less than s (Figure 7b), with ashing across the surface afterwards. The GCP coated fabric did not display ash across the surface and did not burn, although it did have a 2s afterglow, resulting in a total burning time of 2s (Teli et al., 2017) and measured char formation is 2.5 cm (Figure 7c). Table S3 summarizes the data and LOI explanations for the composite. Also, table S4 contains a comparative study of the char length of similar materials.
The mechanical property of the ame retardant composite plays important role. Tensile strength of the GCP composite was discussed in the S3. Whereas, the washing fastness of calcium carbonate, CP, and GCP coated cloth/binder before and after washing was analyzed and weight percent loss calculated and mentioned in Figure S5.

Conclusions
We have synthesized halogen-free, phosphorus functional groups decorated, sustainable, bio-waste derived nanocomposite for e cient ame retardant for cotton fabric. Leftover eggshells were treated with phosphoric acid to prepare monocalcium phosphate, which was functionalized with graphene to prepare GCP. The as-prepared GCP composite coated cotton fabric can prevent a continuous ame for more than 600 s, whereas the CP and GO coated cotton fabrics burn out in 10 and 15 s, respectively, with smoke formation. The major advantage of the GCP coated fabric is to protect its initial shape and size after being exposed to a continuous ame for 10 minutes. The both LOI and vertical ammability test con rmed the GCP's ame retardant properties. The GCP coated fabric has a high LOI value of 34.1, indicating good ame retardant properties. The phosphorus decorated bio ller functionalized graphene nanocomposite was synthesized by using a simple and green approach, which has high e ciency as ame retardancy for cotton fabric. The present study indicates the environmentally benign, sustainable e cient ame retardant nanocomposite deserves exploration for different fabrics.