Introduction

Paper has various applications and can be considered as energy source. Also, paper is used extensively in a variety of industries, including printing, writing, packing, construction, interior design, agriculture and daily necessities, because of its low cost, light weight and other benefits [1,2,3]. However, due to its inherent cellulose content that facilitates easier combustion, the potential fire hazards associated with paper have been recognized as an environmental concern, leading to a reduced demand for paper. Consequently, the incorporation of flame-retardant materials into paper makes it a safer and more convenient material to utilize [4]. As a result, various flame retardants, including nanofillers [5, 6], metal ions [7, 8] and halogen-based materials [9], have been employed to enhance its flame retardancy. Notably, integration flame retardancy feature to biomolecules based materials is required for expand their applications [10]. On the other hand, rice straw is utilized to fabricate paper in order to harness its abundance and is one of the important agricultural leftovers that merits attention owing to environmental contamination [11, 12]. Rice straw pulp production is beneficial process in different aspects such as easy extraction and high-quality fiber for certain kinds of paper; it may be used continually as a sufficient replacement in areas with limited forest wood resources [13]. Pulping is a crucial process that involves converting wood into a fibrous mass [14]. In Egypt, rice straws are collected promptly and managed through distinct disposal methods. However, paper extracted from rice straws is highly flammable and will ignite once exposed to fire source [15]. Therefore, fire retardancy feature must be maintained during the application by using an appropriate flame-retardant material [16,17,18]. There are two main approaches for incorporation of flame retardants system to paper; the paper is fabricated via mixing of inorganic mineral fibers and natural fibers, and asbestos, mineral wool, glass fiber and sepiolite fibers are examples of inorganic fibers. However, the second approach is performed via incorporation of flame retardant into the pulp or by coating on it [19]. In contrast, because of bentonite clays have excellent mechanical and thermal properties, researchers interested in developing a synthetic composite focusing especially on this type of aluminosilicate. Note to mention that it is abundant in nature, lightweight and eco-friendly, bentonite is a phyllosilicate clay that mostly consists of montmorillonite, which consists from tetrahedral silica sheets and an octahedral alumina sheet. Interfacial interactions with organic and inorganic materials are improved by this structure encouragement of hydrogen bonding [20, 21]. Bentonite has been previously combined with a number of soft phases, such as alginate [22], cellulose [23] and wood [24] to improve its physical and chemical properties. Moreover, bentonite has been utilized as flame-retardant fillers to afford flame retardancy and mechanical properties [25,26,27,28,29]. It is interesting to note that our group has been actively involved in the development of various flame-retardant nanocomposite materials for a variety of cellulosic and noncellulosic materials, including textiles [30,31,32]. In this study, paper nanocomposites were developed, resulting in new paper nanocomposites with improved flame-retardant and thermal stability characteristics. Rice straw pulp was exploited to prepare paper sheets, which were then filled with a layer of bentonite. Lead oxide nanoparticles were then prepared and then decorated on bentonite sheets and then coated on the surface the produced paper sheet. The physical, chemical and mechanical properties of the treated paper sheets under research were evaluated; also, the flame retardancy of the developed sheets and an untreated paper were fully studied.

Experimental

Materials

Rice straw was collected from local farm, Egypt. Bentonite was supplied from ICMI company for minerals, El Sadat city, Egypt. Lead nitrate and sodium hydroxide were purchased from Sigma-Aldrich. Distilled water was used in all experiments.

Rice straw pulping

The cellulose fiber was separated from the raw material as follows; 1100 g of rice straw was combined with 6 L of water and 75 g of NaOH (based on 7.5% mass/mass of raw material) in an autoclave digester, which was then heated to 160 °C for 2 h. When the time was up, the pressure was released, and the pulp was separated by filtering and repeatedly washed it in water until it became neutral.

Hand-sheet making and paper nanocomposites

In briefly, about 1.8 g of dried pulp prepared in Sect. 2.2 was homogenized with 5 to 7 L of tab water and air agitation. The attained suspension was separated through a screen under suction using a sheet former (S.C.A model-AB Lorentzen and Wettre), and then, it is pressed for 4 min using a hydraulic press. The wet sheet was then collected on blotting paper protected between two sheets. Afterward, dried as prepared sheets via using a rotary drum dryer for 4 h at 80 °C. The paper nanocomposite preparation was similar process with dispersion of several masses of bentonite (10–50 mass%).

Synthesis of lead oxide nanoparticles

The lead oxide nanoparticles (PbO-NPs) were prepared based on previous reports [33, 34]. A sonication with structure director additive of 10 g for 30 min has been performed on solution of 0.02 mg lead nitrate, followed by dropwise addition of enough sodium hydroxide solution. Afterward, nanostructured lead hydroxide has been produced. Subsequently, the mixture was acoustically drained for 30 min, and precipitate lead hydroxide was eliminated using three cycles of filtration using distilled water and ethanol. After that, the precipitate was sonicated and filtered for 30 min in ethanol. The final precipitate was obtained after 3 h dehydration at 320 C, as indicated. Additionally, to prevent agglomeration, the attained nanoparticles were sonicated in ethanol for 30 min. Finally, the nanoparticles were filtered and dried at 110 C and denoted as PbO-NPs.

Preparation of coating solutions

A solution of 25 mL of (2% w/v) alginate with 0.5 g of lead oxide nanoparticles (PbO-NPs) was prepared and diluted to various proportions with (2% w/v) alginate 25%, 50%, 75% and 100% then applied on the surface of paper using a 120 micron-coating applicator (a type of film applicator combining 4 gap sizes in one unit (30, 60, 90 and 120 μm)). The coated papers were dried in an oven at 50 °C for 3 h.

Determination of retention of fillers

To determine the retention of fillers in the sheets, the prepared hand sheets were ignited in a muffle at 800 C for 4 h. The following equation was used to calculate retention: Filler retention value (%) = ( Mretained / Madded) × 100 where Mretained and Madded are the amounts (g) of retained and added fillers (Bentonite), respectively. Mretained was calculated using the ash contents determined on retention hand sheets.

Characterization

The structure of developed paper nanocomposites was elucidated using FT-IR spectroscopy using Bruker Tensor 37 in the 400 to 4000 cm−1 wavenumber regions. Also, the samples crystallinity was analyzed using X-ray powder diffraction (XRD, D2 PHASER, BRUKER). The diffraction angle 2 was modified from 0 to 100. The structure was determined using monochromatic CuKα radiation (l = 1.54 Å). Moreover, the surface morphology images of sample were obtained using scanning electron microscope using SEM (JEOL-JSM-IT200). The tensile strength was performed using a universal testing machine (LR10K; Lloyd Instruments, Fareham, UK) equipped with a 100-N load cell and a constant crosshead speed of 2.5 cm min−1 in accordance with the TAPPI (T494- 06) standard procedure. The gauge length is fixed at 10 cm, and 15 mm wide strips are utilized for the analysis. Thermogravimetric analysis of samples was carried out using Thermal Analysis LINSEIS STA PT1000 (TGA) (about 10 mg), was tested at 10 ◦C min−1 under N2 across the whole temperature range (750 °C). The flammability properties were evaluated using limiting oxygen index (LOI), and LOI was measured according to ISO-4589 [35].

Result and discussion

Fabrication of paper sheet nanocomposites

The paper sheets were fabricated via green route from rice straw pulp and bentonite sheets (B) and the bentonite sheets mass ratio was altered from 10–50 mass% and then PbO-NPs were coated on the paper composites yielding new paper nanocomposites as presented in Fig. 1. Therefore, the retention of the filling sheets was calculated and compared as depicted in Fig. 2. The results show that superior retention value was attained at 10 filling paper and 40% filling paper, respectively.

Fig. 1
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Schematic diagram representing the green synthesis of paper nanocomposites

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Fig. 2
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Retention percentage of bentonite in paper sheet

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Structural and morphological characterization of developed paper nanocomposites

The structure of the developed paper nanocomposites based on incorporated B and coated PbO-NPs layer were elucidated using XRD and FT-IR spectroscopy. Figure 3 presents the XRD patterns of bentonite, PbO, blank paper, paper filled with 40% of B and paper filled with 50 B and coated with PbO-NPs. Figure 3A shows peaks at around 20° and 30° (2θ) corresponding to bentonite structure; however, Fig. 3B shows XRD pattern with sharp peaks at approximately 30°, 37° and 40° (2θ) which exhibits the characteristic structure of PbO-NPs. Moreover, Fig. 3C displays the diffraction patterns of rice straw cellulose peaks which appeared as sharp peak at about 21.3°. However, the amorphous peaks noticed at 18.25° are ascribed to amorphous intensity [36]. For paper filled with 40% B, there are main peaks perceived at (2θ) = 20.7°, 26.5°, 36.3°, 54.7° (Fig. 3A) [36,37,38]. However, it appears in lower value and the shorter peak due to blend to cellulose peaks. Interestingly, the peaks appear in higher intensity and the sharper peak is ascribed to the presence of PbO-NPs in 50% B-PbO-NPs (Fig. 3E).

Fig. 3
figure 3

XRD spectra of A bentonite, B PbO-NPs, C blank paper sheets, D paper filled with 40% of B E paper filled with 50% B and coated with PbO-NPs

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Figure 4 presents the FT-IR spectra of the characteristic absorption peaks of bentonite, PbO-NPs, blank paper, paper filled with 40% of B and paper filled with 50 B and coated with PbO-NPs. Thus, Fig. 4A-B exhibits the characteristic absorption peaks of bentonite PbO-NPs. However, Fig. 4C reflects the characteristic absorption bands of the cellulose chain structure. Therefore, the absorption peak noticed at 1060 cm−1 is correspond to the asymmetric bridge stretching of C–O–C groups in cellulose and peak positioned at 2900 cm−1 indicate the presence of (–CH) aliphatic groups stretching. However, the broad band appeared between 3100 and 3500 cm−1 corresponding to O–H vibration. However, after filling the paper with bentonite sheets, the corresponding bentonite peaks are also observed (Fig. 4d). Therefore, the absorption peaks observed at 794 and 779 cm−1 correspond to the -CH2 rocking modes, and absorption peak located at 3413 cm−1 ascribed to the vibrations of structural OH but overlapped by sharp intensity peak with OH peak observed in cellulose. Interestingly, the absorption peak noticed in the range of 950–1100 cm−1 is attributed to the stretching vibration of the Si–O group (Fig. 4E) [37]. Moreover, the absorption peak observed at 469 cm−1 revealed the presence of the Pb–O stretching vibration mode and the sharp peak noticed at 669 cm−1 represents the asymmetric bending vibration of Pb–O–Pb bond [38]. This result elucidates the successful fabrication of paper nanocomposite.

Fig. 4
figure 4

FT-IR spectra of A bentonite B PbO-NPs, C blank paper sheets, D paper filled with 40% of B and E paper filled with 50% B and coated with PbO-NPs

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The structure of paper sheets was further investigated using microscopic tools. Thus, the SEM images of blank paper, paper filled with 40% of B and paper filled with 50 B and coated with PbO-NPs are shown in Fig. 5. For blank paper, smooth surface morphology was observed for cellulosic fibers (Fig. 5A). However, after incorporation of 40% of bentonite sheets rough surface was noticed for the paper composites fibers (Fig. 5B). However, after coating the paper composite with PbO-NPs dense roughness surface was noticed for the cellulose fibers as visualized in Fig. 5C. The SEM images show that the bentonite sheets are dispersed into cellulosic matrix and PbO-NPs were distributed on the paper surface, confirming the existence of bentonite and PbO-NPs.

Fig. 5
figure 5

SEM of A blank paper, B paper filled with 40% of B and C paper filled with 50%B and coated with PbO-NPs

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Mechanical and thermal stability properties

Different mechanical properties of the developed paper nanocomposites were evaluated in terms of bulk density, maximum load, breaking length, tensile strengths, tensile index and Young's modulus, and data are tabulated in Table 1 and presented in Fig. 6. For paper bulk densities, Fig. 6a exhibits the relationship between paper bulk density and B mass loadings. Paper sheets with high bulk densities are generally opaque, thick, light and airy as expected the bulk density of the bentonite-filled paper sheets increased; the paper with 30% bentonite sheets has the highest bulk density. However, the paper nanocomposite composed from paper-bentonite nanosheets performed better mechanically than unmodified paper sheets. It was found that filling and coating paper led to an increase in measured of breaking length and maximum load (Fig. 6b) which reach its maximum in 40% bentonite + PbO-NPs paper. On the other hand, the tensile strength and tensile index (Fig. 6c) also calculated of the paper in order to investigate the effects of the bentonite and PbO-NPs on the paper, the tensile strength and index were improved by filling and coating paper but decrease as increasing the bentonite percentage. Young's modulus (Fig. 6d) shows higher value in 20% bentonite + PbO-NPs paper. The results indicated that the filled paper with 40% bentonite sheets revealed higher mechanical features compared with other papers. Thus, modifying the rice straw paper with bentonite and coated with PbO-NPs enhanced the mechanical proprieties of paper sheets.

Table 1 Mechanical properties of paper sheets
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Fig. 6
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a Bulk density, b maximum load, c tensile strength, d young’s modulus of paper sheets modified by bentonite + PbO-NPs

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On the other hand, the thermal stability of the developed paper nanocomposite was evaluated using TGA. The TGA curve demonstrates the samples have basically two mass loss steps. The first one is the dehydration of adsorbed moisture is a consequence of the first stage volatilization step, while the second one is main mass loss and was results of thermal decomposition. For blank paper, the first mass loss step was noticed in the temperature range of 30–80 °C and was attributed to evaporation of weakly tapped and bound moisture (Fig. 7B). However, the significant thermal degradation began around 250–300 °C due to the thermal breakdown of cellulose which is the typical of cellulosic materials [37, 39, 40]. However, for paper sample contained 40% of bentonite sheet it lost approximately 30% of it is initial mass and taking place in the temperature range of 30–100 °C (Fig. 7A). For the second step, the sample begins decomposition after 500 °C as presented in Fig. 7A. Thus, the thermal properties of rice straw paper are improved by filling it with bentonite, reducing mass loss as well as decreasing volatile products.

Fig. 7
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TGA curves of A 40% bentonite and B blank paper

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Flame retardancy of properties

The flammability properties were investigated for some selected group of samples. Thus, flammability properties of paper sheets (Paper; P) and their developed composites, and coated with sodium alginate alone (PC), and filled with 10, 40 and 50% bentonite (B) (PB-10, 40 and 50). This is in addition to PB-10, PB-40 and PB-50 samples coated with sodium alginate and PbO-NPs nanoparticles (PBC-PbO-NP10 (40) (50)) were evaluated using the limiting oxygen index (LOI) based on ISO standard ISO-4589–2, and the results are tabulated in Table 2. The untreated paper was burned completely once ignited using flame source providing LOI value of 20%, and this worse flame retardancy is ascribed to their organic nature of cellulose. Interestingly, once bentonite layers were incorporated in the paper fabrication, the flame retardancy trend was noticed (Table 2). This flame retardancy attitude was further improved once mass loading of B increased achieving LOI value of 21 in PB-50. These flame retardancy properties were ascribed to the nature and chemical composition of B as aluminosilicate layers (contain SiO2 Al2O3) which trigger cellulose chains for formation of protective char layer rather than thermal decomposition [17,18,19, 41, 42]. Interestingly, upon PbO-NPs were incorporated in coating layer on surface of paper, a significant flame retardancy effect was attained (Table 2). This superior flame retardancy was corroborated from higher LOI values achieving 24% in PBC-PbO-NP50 compared to 20 and 21% for blank paper and PB-50 (PbO-NPs free) samples, respectively. This outstanding flame retardancy was attributed to the synergistic flame retardancy action occurred between B layer and PbO-NPs which trigger paper chains for creating high strength protective char layer and in turn isolating flaming zone from decomposition one [33, 41, 42]. This could be due to PbO-NPs catalyze B layers for protective barrier formation and then strengthen their structure, then successfully retard heat and mass transfer [33, 41, 42].

Table 2 Flammability properties of untreated and treated paper sheets
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The flame retardancy mechanism was further investigated using microscopic investigation of attained char layer from untreated and flame-retardant paper. Thus, Fig. 8a presents the SEM image of untreated paper char after LOI test which displays a broken char layer contains porous structure which facilitates the escape of combustible gases via pores feeding flaming zone. This is further elucidated at high-magnification SEM images (Fig. 8b-c). On contrast, Fig. 8b presents dense and compact char barrier free from pores which obtained after burning of PBC-PbO-NP50. Interestingly, high-magnification images visualize the formation of protective layer on the fiber surface own convenient strength which affords efficient isolation of flaming zone from decomposition one (Fig. 8 e–f) [17, 41, 42]. In conclusion, the developed char barrier efficiently retards the mass and heat transfer and in turn reflects good fire safety to treated paper.

Fig. 8
figure 8

SEM image of formed char after LOI test of a P, b-c P at high magnification, d PBC-PbO-NP50 and e–f PBC-PbO-NP50 at high magnification

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Conclusions

Rice straw paper was produced in this work containing bentonite and coated with PbO-NPs. The FT-IR findings confirmed the presence of bentonite and PbO-NPs, which was shown to be associated with an interface between a cellulose band and a bentonite band. The presence of bentonite and PbO-NPs was also detected on the XRD peaks. The SEM results proved the deposition of bentonite into fiber matrix and PbO-NPs on the paper surface. The results also show that the best mechanical properties observed in paper sheets prepared from rice straw filled with 40% bentonite and coated with PbO-NPs have been enhanced, when compared to a modified paper, according to improved tensile index. In view of the fact that paper with a 50% fill and coated with PbO-NPs provides increased fire safety for bentonite sheets, high flame retardancy has been attained by new covered and coated papers recording 24% of LOI value. This superior flame retardancy action was stemmed from bentonite as aluminosilicate triggers cellulose chains for formation of char layer and synergistic flame retardancy action occurred between bentonite layers and PbO-NPs which trigger paper chains for creating protective char layer. Thus, affords strong char barrier effectively prevents the emission of toxic gases.