Hydrophobic and Flame-Retardant Foam Based on Cellulose

A study aimed to prepare cellulose-based hydrophobic, lightweight, and flame retardant foam composites. Cellulose was activated by phosphoric acid followed by blending with dolomite clay of different loading ratios. Gelatin/tannin as an adhesive system was used as a binder. A solution of the environmentally friendly silicone rubber (RTV) was applied onto foam samples via spray-coating to improve their water repealing performance, which was explored by investigating both of water contacting angle and wettability time of the coated foam samples. Flammability characteristics, thermal decomposition, surface morphology, and chemical structure of treated and untreated foams were investigated by flammability test, thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, and Fourier-transform infrared. The fire retardancy of foam composites was optimized at low and medium loading of dolomite. Also, the addition of RTV improved the hydrophobicity of composites maintained the fire retardancy of composites with medium loading.


Introduction
Cellulose and its derivatives have various and vital applications in a wide range of fields, such as textiles, paper industry, medicine, ink, and paints [1][2][3][4]. The development of fireretardant cellulosic materials has received attention due to conflagration that is causing massive losses for society. Fire retardant cellulosic materials have been manufactured based on blending and chemical surface modifications [3,5,6].
Recently, the manufacturing of foam materials has been a big target, especially from natural and renewable resources [7]. The foam materials exhibit unique properties such as low density, high porosity, high specific surface area, and high thermal insulation [8]. Therefore, cellulosic materials are promising for manufacturing available foams and aerogels to replace petroleum-based foam products. The preparation of cellulosic foam is dependent on the combination of cellulose substrate with inorganics materials, e.g., clays, oxides [9] or/and organics, e.g., carbon nanotubes, graphene oxide [10]. The varieties of such combinations can produce various hybrid foam composites having distinctive properties with specific functionalities such as high compressive strength [11], thermal insulation [7], magnetic and conducting properties [11]. The number of incorporated components and their origin play an essential role in the industrial production of such hybrid foams. Indeed, using as few as possible components from abundant natural resources would effectively simplify the processing conditions and give rise to sustainable material concepts [12].
Freeze-drying is a promising technique to obtain welloriented and highly porous aerogels. The unidirectional freezing of the aqueous suspension enables the ice crystals to grow along the temperature decreasing gradient resulting in a highly aligned porous structure by sublimation. In addition, the cooling rate can control the porosity of foam material since a higher cooling rate leads to smaller pores [13,14]. An essential disadvantage of cellulosic foam is low fire resistance due to (C, H, and O) in the cellulose structures that help to start combustion. Also, the available highly distributed hydroxyl groups can catch fire leading to its poor thermal stability [15]. The free hydroxyl groups along make hydrophilic properties for the cellulose chain. The association of water molecules with cellulose enhances the chemical degradation, decreasing the mechanical performance due to the collapse of the foam cell structure [16]. The efficiency of fire resistance cellulosic foam is affected by fibers' crystallinity and orientation. The high crystallinity of cellulose leads to high levels of levoglucosan during the pyrolysis, increases the flammability. While increasing the cellulosic fiber orientation could decrease the pyrolysis rate and reduce the flammability [17].
On the other hand, clay nanoparticles embedded in the cellulosic foam can contribute to the fire retardant properties of foam due to reducing heat transfer and catalyzing the char formation from cellulose due to the presence of metal ions in the chemical structure of clay. Recently, the flame retardant properties of cellulose-based freeze casting foam have been enhanced by TEMPO modification of cellulose substrate in blending with boric acid and sepiolite clay (a microfibrous magnesium silicate) [18]. The increasing efficiency of the flame retardant properties of the formed foam was attributed to the crosslinking among the main cellulosic macromolecules due to borate anions and clay. Consequently, the char oxidation process could be shifted to a higher temperature resulting in reducing in the formation of volatile combustible materials such as leuvoglucosane [19].
The development of water-repellent surfaces with a sliding angle < 10° and a contacting angle > 150° has been recently a vital research field for various applications, such as self-cleaning and antifouling purposes [20]. Both nanoand micro-hierarchical morphologies can introduce surface roughness as outstanding heuristic models to achieve a hydrophobic character. Nonetheless, the surface energybased materials are usually fluorine-containing agents, which are highly poisonous and expensive [20]. Several chemical and physical methods have been reported for the development of hydrophobic surfaces, such as electrospun and self-assembled nanofibers, sol-gel, lithography, and chemical and plasma etching. Nonetheless, those methods are typically time-consuming, costly, and require multiplestep complicated processing and instruments [21]. The spray-coating method has been applied as an easy and cheap method for developing functional surfaces at room temperature [22]. Room temperature vulcanized (RTV) silicone is a category of eco-friendly polymers that can cure at room temperature in the presence of a catalytic amount of dibutyltin dilaurate. RTV exhibits high resistance to chemicals, acids, and bases, aging, and high temperatures. It is characterized by excellent thermal stress and mechanical properties, low shrinking, low viscosity, and high hardness. These properties made it applicable in different fields, such as aviation and electronics [23,24].
The study aimed to prepare a fire-retarding composite from phosphorylated cellulose blended with dolomite clay in foam by using gelatin/ tannin as adhesive. Loading of silicon rubber to enhance the water repealing and fire retarding also was studied. Foam composites' characterization could be investigated using FTIR, SEM, EDX, TGA, hydrophobic measurements, and flammability properties.

Phosphoric Acid Treatment
To defibrillation, bleached bagasse pulp (50 g dried weight) was soaked in distilled water at a liquor ratio of 1: 10 with continuous stirring for one hour. Next, the produced suspension pulp was de-watering by G2 sintered glass until obtaining pulp with consistency 1:4 (included with 200 ml distilled water). Then, the defibrillated produced pulp was treated with phosphoric acid (50% wt/wt). The addition of concentrated ortho-phosphoric acid (85% wt/wt) to the defibrillated pulp was based on the amount of water associated with the pulp to produce finally 50 g pulp immersed in 50% phosphoric acid with liquor ratio 1: 7. The suspended pulp in phosphoric acid was agitation at 50 °C for six hours followed by washing with distilled water giving activated cellulose [25].

Preparation of Cellulose-Based Foam
A typical experiment for foam preparation was performed as follows: 4 g dried weight phosphorylated pulp was suspended in 100 ml distilled water via stirring in a glass beaker using mechanical stirring. First, 0.4 g of gelatin was dissolved in 10 ml distilled water, and then 0.2 g tannin was mixed with gelatin solution via stirring till complete dissolution. Next, the mixture of gelatin/tannin was added to the pulp suspension with continuous stirring, and sodium bicarbonate was added. Finally, different amounts of dolomite clay (0.0, 0.5, 1.5, 2, and 3 g) were added to the suspended pulp after blending with gelatin/tannin mixture to obtain composite suspension constituents and coded as; Cellulose, F1, F2, F3, and F4, respectively. Then, the suspension was poured into a Petri dish and frozen in a deep-freezer at − 80 °C, followed by the freeze-drying process (ALPHA 1-2/LD PLUS, Martin Christ, Germany) to afford a foam.

Preparation of Superhydrophobic Foams
Solution of RTV in petroleum ether (15 g/l) was stirred for 60 min and then subjected to ultrasonic at 35 kHz for 60 min to guarantee the formation of a homogeneous solution. The foam samples were then spray-coated using the above-prepared solution and then air-dried for 15 min to afford samples FS1, FS2, FS3, and FS4 depending on the total content of dolomite clay.

Density and Porosity
The density of cellulose-based aerogels/foams is between 10 and 10.5 kg/m 3 . Weight and volume can measure the density of aerogels by dividing the weight by the volume. The porosity can be evaluated from the density of the aerogels (ρ * ) by utilizing the following equation: where the ratio ρ * /ρ c is the relative density, the density of cellulose (ρ c ) is assumed to be 1460 kg/m 3 [26].
FT-IR spectra were recorded in the range of 400-4000 cm −1 on (Shimadzu 8400S) FT-IR Spectrophotometer. The XRD patterns were investigated on a Diano X-ray diffractometer using CoKα radiation source energized at 45 kV and a Philips X-ray diffractometer (PW 1930 generator, PW 1820 goniometer) with CuK radiation source (λ = 0.15418 nm), at a diffraction angle range of 2θ from 10 to 70° in reflection mode. The thermal stability was carried out using a TGA Perkin-Elmer (STA6000), with a heating rate (10 ºC/min). The temperature ranged from room temperature up to 900 ºC under air atmosphere. The chemical composition of dolomite was also determined using sequential advanced AXIOS wavelength-dispersive X-ray fluorescence (WDXRF). The dolomite particle size was analyzed using high-resolution Transmission Electron Microscope (TEM) with a model type (JEOL-JEM-2100), and the images were taken at an acceleration voltage of 120 kV. The surface morphology of cellulose and foam with different concentrations of dolomite were analyzed using electron microscope FEI IN SPECTS Company, Philips, Holland, environmental scanning without coating. The microscope was attached to a dispersive energy spectrometer (EDX). The images were obtained using an accelerating voltage of 10-15 kV. EDX analysis was carried out to be supporting information confirming the presence of clay in the prepared composites.
Hydrophobic measurements, both of slide and water contacting angles, as well as wettability time [27] of the spray-coated foam samples were determined under ASTM(D-7334) standard test using DataphysicsOCA15EC (Germany). The flame test of the spray-coated foam samples was carried out under the standard BS5438(1989) test [28].

Preparation of Foams
Phosphoric acid was used to make decrystalization of cellulosic pulp to be more swollen, and the treatment of pulp was performed by phosphoric acid includes two main processes [25]. Firstly, an esterification reaction between hydroxyl groups of cellulose and phosphoric acid to form cellulose phosphate (H 3 PO 4 + cellulose → cell-O-PO 3 H 2 ). Secondary, a competition of hydrogen-bond formation between hydroxyl groups of cellulose chains and hydrogen-bond formation between one hydroxyl group of a cellulose chain and a water molecule or with hydrogen ions.
Meanwhile, another by-reaction, acid hydrolysis of β-glucosidic bonds of cellulose, will take place. However, such acid hydrolysis could be overcome by decreasing the dissolution temperature. Therefore, the freeze-drying approach has been adopted to prepare cellulose-based composite foam by blending pulp with dolomite clay micro-particles at different loading ratios. Upon freeze-drying, robust anisotropic aerogel was produced that may resist combustion and exhibited a low thermal conductivity. During the freezedrying process, microfibrillated cellulose aligns laterally and form layer structures of varying lengths due to the diffusion forces or inter-fibrillar hydrogen bonding. In addition, partial decomposition of sodium bicarbonate added during the preparation of foams resulted in the formation of carbon dioxide that enhanced pores formation in the pulp matrix.
Different techniques can be used to detect the elemental content of the material. One of these is the wavelengthdispersive X-ray fluorescence (WDXRF), which generally affords a detection limit of elements with a total content higher than 10 mg/kg. The elemental composition of dolomite was carried out using WDXRF, which indicated that the main constitutes are CaO and MgO with 27.85 and 15.94%, respectively (Fig. 1). The moisture and volatile material, water, and carbon dioxide from carbonates present in the sample represent 39.39% (Table 1). The average particle size of dolomite is ranges from 7 to 31 nm (Fig. 1).

Morphology Studies
Porosity is an essential feature of aerogel structure since it determines the adequate air volume in the matrix. As shown in Fig. 2a, the cellulosic pulp has an organized fibrils layered structure. Fibrils are highly interconnected, leading to a dense aerogel structure with macro-pores at the surface of the foam. Figure 2b-d, illustrated SEM for composite foams comprised of phosphate cellulose pulp blended with different ratios of dolomite clay. Generally, the outer surface of these composites exhibited a smooth 2D sheet-like structure with few macro-pores. EDX has confirmed the loading concentrations of dolomite upon the outer surface of fibrils. It showed increasing the concentrations of Mg and Ca representing the main constituents of dolomite. On the other hand, the concentration of P was approximately the same for the composites sample as in Fig. 2b-d. At the high concentration of dolomite clay, the aerogel structure collapsed and formed a microstructure composed of smaller pores (Fig. 2d).
The presence of a high concentration of dolomite particles near cellulose fibrils may have hindered the interfibrillar attraction, causing the fibril layer structure disruption. The adhesive system components, tannin, and gelatin, added to a blended composite of pulp and dolomite, have an essential role in case of such high loading of clay. The adhesive system enabled the connectivity of the fibril layer structure to be still preserved, maintaining the physical integrity of aerogel. The dolomite particles have occupied the matrix pores in case of the highest dolomite concentration. In contrast, the partial occupation of matrix pores has been achieved in the case of low dolomite concentration. This agrees with the lowest porosity and highest density values of produced foam at the highest concentration of dolomite. In contrast, the lowest density and highest porosity could be obtained in the lowest concentration of dolomite (Fig. 3). Figure 4 illustrated SEM for blended composites of cellulosic pulp and dolomite after treatment with silicone rubber for acquiring the composites' water-repelling properties. The accumulation of silicon rubber has appeared as a co-centered layered aggregation on the surface of the composite matrix. In the case of composite with low dolomite concentrations, silicon rubber has light distribution on the composite matrix, although high Si concentration manifested from EDX data. On the other hand, the dense distribution of silicon rubber has been obtained in the case of composite with the highest dolomite concentration, despite the lowest concentration of silicon rubber obtained from EDX data. These abnormal results can be interpreted because of dolomite and silicon rubber incompatibility due to their zeta potentials. Silicone rubber has negative zeta potential, while dolomite can acquire positive zeta potentials as a result of alkaline pH due to the presence of  Fig. 4c, leaving most of the internal pore surface empty. These results agreed with results as in Fig. 3, where the highest porosity and lowest density of foam composite with silicone rubber were obtained in the case of the highest dolomite concentration. EDX data has confirmed that the lowest silicon rubber concentration for foam composite with silicone rubber foams has been obtained in the case of the highest dolomite concentration. This proved that silicone rubber has not diffused through the pores but only coats the external surface of fibrils.   1054, 896 cm −1 assigned for OH, C-H, C-C, C-O-C, and β-linkage, respectively [32]. The peaks at 1230 and 928 cm −1 were referred to P=O and P-OH, respectively [33]. The band at 1710 cm −1 [33] present in the spectra of foams can be attributed to C=O stretching mode and is probably due to the oxidation carried out during phosphorylation of cellulosic fibers [34]. Dolomite displays characteristic FT-IR absorption at 3020, 2626, and 730 cm −1 [35], shifted, as shown in Fig. 5, toward 2850, 2450, and 700 cm −1 , respectively, physical interactions with cellulosic fibers. As dolomite concentration was increased in the foam sample, the peaks indicated to P's presence were reduced due to the shielding effect of dolomite. Figure 5 also illustrates the characteristic bands of silicone rubber impeded in the foam matrix consisting of activated cellulose with dolomite. Generally, the absorption peak at 2960 cm −1 is assigned to the stretching vibration of CH 3 [36]. The absorption at 1410 cm −1 is referred to  [37]. The absorption peak at 1250 and 860 cm −1 are correspond to the bending and rocking vibrations of CH 3 . The absorption peak at 1000 cm −1 is assigned to the stretching vibration of Si-O-Si on the backbone of silicone rubber [38,39]. The absorption peak at 780 cm −1 is the stretching vibration of Si-C [40]. The peaks indicated to cellulose are significantly diminished due to the predominate peaks referred to as silicone rubber that diffused through the foam cavities or coated the external surface of cellulose. All of these suggested incorporations of a cellulosic matrix with dolomite clay and silicone rubber. The peak at 1240 cm −1 attributed to gelatin.

X-Ray Diffraction
XRD of dolomite shows characteristic peaks at 2θ = 31.5° and 41.5° that referred to CaO and MgO respectively and broadband in the region around 10° [41]. Figure 6 illustrates XRD for composite samples comprised of activated cellulose with various concentrations of dolomite. At low dolomite concentration, the peaks at 2θ = 31° and 41° are small, and only the broadband around 10° has appeared obviously. The peak that appeared around 2θ = 22° is the crystalline portion of the cellulose chain. Low intensities peaks characterized for dolomite may be due to the interaction of dolomite cations with cellulose chains and diffusing dolomite distribution inside pores and the cavities of the foam matrix. Increasing dolomite concentration, the peaks at 2θ = 31° and 41° are distinguished.
XRD of pure silicone rubber has a main characteristic peak, which is weak due to a poor crystalline nature in silicone rubber [42]. This feature is manifested obviously for cellulose/dolomite/Silicone rubber composite with low dolomite concentration (Fig. 6). As dolomite concentration increased in the foam matrix, weak dolomite peaks could be distinguished at 2θ = 31° and 41°. These weak intensities could be attributed to the scattering and shielding effects caused by silicone rubber that is distributed through the matrix. In general, the leak of composite peaks may be attributed to the thick layer of silicone rubber that interpretation or coats the cellulosic matrix or low content of dolomite composite.

Thermogravimetric Analysis (TGA)
TGA was used to investigate the thermal degradation behaviors, thermal stability, and residue formation of cellulose, activated cellulose, RTV, and foams with different ratios of dolomite before and after treatment with silicone rubber (Fig. 7). The pyrolysis is defined by two competitive pathways, the depolymerization of glycosyl units to volatile levoglucosan or dehydration followed by decomposition of the same units to aromatic char, which is the final residue at the end of the TGA test [43]. Cellulosic pulp denotes decomposition temperature at 5% weight loss less than that of activated cellulose. The final residue of cellulose pulp is about 8%, while activated cellulose denotes about 20% residue at 500 °C. Therefore, activated cellulose has higher thermal stability than cellulose pulp.
The addition of dolomite to activated cellulose caused multiple changes in its thermal characteristics. As shown in Fig. 7, the major decomposition peaks of low dolomite contents have been shifted to lower temperature 350 °C, while high dolomite content has been shifted to 220 °C. Generally, dolomite acts as a catalyst to possible decomposition pathways that take place for foam matrices. At low dolomite content, the dehydration pathway was predominant, and hence the formation of aromatic char was the major end product. On the other hand, at high dolomite content, the catalytic action influenced the dehydration pathway and depolymerization, forming volatile levoglucosan end product. Therefore, the residues at low dolomite content F2 and F3 (about 50% at 1000 °C) were higher than the residues resulting from high dolomite content matrix (about 20% at 1000 °C). Since the char formation can prevent the matrix from combustion, the increase of char residue indicates the flammability resistance. So, a significant increase of the thermal stability of low dolomite (F2 and F3), content has been achieved compared with high dolomite content (F5). Figure 7 shows TGA of foam composites consisting of activated cellulose, RTV, dolomite, and silicone rubber with different dolomite contents. As shown in Fig. 7, composite FS2 has the lowest thermal stability due to the majority of synthetic polymer (silicone rubber) blended with pulp in the present low dolomite content. Increasing the dolomite content can enhance thermal stability. The composite FS3 denotes 55% residue, while composite FS5 denotes 30% residue at 1000 °C. As the char yield of composites without silicone rubber differs significantly from that of silicone rubber, the polymer /clay system appears to inhibit the thermal degradation of the foam matrix. The mechanism of the polymer/ clay system may be explained as follows: firstly, dolomite can catalyze thermal degradation of foam matrix through dehydration reactions that result in carbonaceous materials. Secondly, these carbonaceous materials can fill the voids of polymer silicon rubber, which can shield and inhibit further degradation of these carbonaceous materials into volatile materials. Thus, silicone rubber preserves most of the char produced from the catalytic pyrolysis of foam matrix, and consequently, the residue product is higher than that of foam without polymer. In light of this visualization, it is possible to explain why FS3 composite produced more residue than that resulted from FS5. Despite the high dolomite content, FS5 composite has silicone content lower than FS3 composite, as mentioned before from EDX. Thus, the amount of polymer in FS5 composite is not able to catch all the produced char. Some of this char can be further converted into volatile substances resulting in loss of residue char yield.

Hydrophobic Characterization
Surface roughness was created on the surface of the foam samples using spray-coating technology of RTV in petroleum ether under ambient conditions. We were able to produce foam samples spray-coated with RTV to improve water contacting angles in the range of 97.8°-162.4°. On the other hand, the uncoated foam sample displayed very poor hydrophobic properties. There have been diverse techniques previously described in the literature to accomplish the creation of a superhydrophobic surface using costly materials, time-consuming processes, and complex procedures and instruments [44]. Therefore, the current method can be described as a practical, simple, and cheap Furthermore, the present simple technique can be applied for the large-scale production of hydrophobic commodities. The contacting angle of the uncoated sample was monitored at 0°. The spray-coated foam samples illustrated higher water contacting and slide angles, as depicted in Table 2 and Fig. 8. Once the concentration of dolomite clay increased, the contact angle was from 97.8° (FS1) to 162.4° (FS4). However, the contact angle decreased from 162.4° (FS4) to 154.4° (FS5). The wettability time also increased from 20 (FS1) to 45 min (FS5). This could be attributed to the enhanced roughness and decreased surface energy by increasing dolomite clay and RTV [45][46][47][48][49][50].
However, the significantly higher concentrations of dolomite clay combined with RTV filled the pores, which negatively resulted in decreasing the surface roughness to decrease the water contacting angle [51]. In addition, the slide angle was determined to assess the hydrophobic efficiency of the spray-coated foam samples. The changes in the wetting performance were attributed to decreasing the slide angle with the increase of dolomite clay concentration. The slide angle reduced from 13° (FS1) to 8° (FS4) with increasing the engagement of dolomite clay and then raised from 8° (FS4) to 9° (FS5). Therefore, the slide angle measurements matched that of the water contact angles.

Flam Test
The flame retardancy of a material does not depend on the material's melting temperature itself, but on the transformations that occur inside the material, as well as on the compounds present after the exposure of a retardant material to high temperatures [52].
The fire retardancy of the prepared foams was tested by determining the char length and wider char length as given in Table 3. Untreated cellulose pulp could not pass the flammability examination and was entirely burnt. Thus, it was indicating its poor flame retardancy.
Loading of low and medium concentrations of dolomite improves the fire retardancy detected by the char lengths of these foams. Otherwise, the highest concentration of dolomite reduced the fire retardancy due to high dolomite clay increasing the rate of heat transfer to the cellulosic matrix. Consequently, that sample's char length rises, indicating the reduction of the fire retardancy property of this sample. This corresponded with the results of Gillani et al. They suggested that the dolomite clay enhanced the fire performance and formed dense and continuous char when incorporated as a filler in conventional intumescent fire resistive systems [38]. Moreover, the coating of foams by silicone  rubber increased their fire retardancies on comparison with uncoated foams. This can be attributed to the formation of a protective layer that can prevent heat transfer from the heat source and prevent oxygen flow to the flammable material and prevent the supply of pyrolysis gases to the material surface also. These results were agreed with that of TGA of foams.

Conclusions
The flame retardancy of foam composites has been agreed with the thermal stability that TGA carries out. The fire retardancy of composites depends on the decomposition reaction's inhibition, which makes transformations inside the cellulosic matrix. The addition of dolomite clay improved the fire retardancy of cellulosic foam at moderate concentrations, while the high concentration reduced the fire-retardant property due to the high heat transfer. Cellulose-dolomite foams sample was coated with silicone rubber to improve the water-repelling and fire-retardant properties. So, the simple production of hydrophobic foam was developed via spray-coating under ambient conditions utilizing a solution of RTV demonstrating water contact angles in the range of 97.8°-162.4°, while the sliding angles were in the range of 13°-8°.