Bio-inspired writable multifunctional recycled paper with outer and inner uniform superhydrophobicity

Abstract

The papermaking industry always causes many disastrous problems to humans, such as energy consumption, environmental pollution and destruction to the ecosystem. However, paper is not only one kind of life's necessities but also a necessity of the office. But the service life of paper is reduced easily because of the invasion of water. Here we creatively prepared one kind of multifunctional inner and outer uniform-superhydrophobic paper by the secondary use of waste paper inspired by nature and traditional papermaking knowledge, which not only has a great water proof ability for various liquids, but also has wonderful self-cleaning, anti-fouling and oil absorption abilities. It is worth mentioning that superhydrophobic recycled paper still has good writability, suppleness, foldability and tailorability to meet our daily needs. Unique outer and inner uniform-superhydrophobicity make this paper able to tolerate any degree of abrasion. This groundbreaking work will not only avoid harm by the water invasion and expand the usable range of paper, but also it will ease the energy and environment crisis.

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Introduction

Paper, the most common disposable consumable for its superior properties, such as environmentally friendly, scalable, light weight, mechanically flexible and disposable, can be used for writing, printing and packing, whether in our daily life, office or industry. So, the papermaking industry holds an important position in global economics. However, the existing problems in the papermaking industry have become the focus of people's concern, along with the development of science and the progress of civilization. Enormous destruction to the ecosystem during the process of collecting raw materials, the massive energy consumption and serious atmospheric, soil and water pollution during the process of manufacturing are all problems faced by all humans.^1–4 Recycled paper, for the moment, must be the best answer to solve these problems.^5–7 The process of recycled paper involves mixing used paper with water to break it down. After being chopped up and heated, it is further broken down into strands of cellulose, which is calling pulp or slurry. Pulp can be made into new recycled paper through de-inking, bleaching and other cleaning methods. Compared with virgin pulp, generally, recycled paper can reduce energy consumption and cause less water pollution and less air pollution. Similarly, recycled paper decreases the demand for virgin pulp, namely, raw materials, as well trying to avoid destruction to the ecosystem.

Cellulose is one important ingredient of both paper and recycled paper which is a hydrophilic and hydroscopic material by nature. It is well known that water has long been the leading destroyer of paper. Once wet, the strength of paper soon diminishes significantly because celluloses are bonded together through the phenomenon of hydrogen (H–O–H) bonding, but the presence of water molecules weakens the intermolecular forces. Even drying again after water evaporation, paper appears wrinkled with the deformation of small amounts of soluble components so that paper loses its aesthetic property and practicality.^8,9 Thus, water-repellent paper has been one kind of much-anticipated functional product.

Recently, inspired by the micro- and nano-scale structure of the lotus in nature, a superhydrophobic surface which exhibit an extremely high water contact angle (WCA) (>150°) and low water sliding angle (SA) (<10°) has been a hot area of research in many areas.^10–15 Numerous fabrication methods have been developed for chemical and physical preparations of superhydrophobic surfaces in the literature, each providing varying degrees of control of the nano–micro roughness and wettability to various substrates which, without question, provide feasible ideas for water-repellent paper.^16–20 But it still will be a challenge to create water-repellent or superhydrophobic paper while keeping its intrinsic properties well. Actually, superhydrophobic paper has been reported by many groups before and these creative works have opened new doors to expand the range uses of traditional paper.^21–24 However, it is also undeniable that these perpetration methods always have questions about the weak strength of the paper and the complex process, and cannot be prepared on a broad scale. Furthermore, these second modifications will also inflict damage to the aesthetic property and practicality. But above all, present research just concentrates on the surface treatment of paper. If only the surface of paper is modified, the superhydrophobicity of the modified paper is easily lost due to wear and tear from friction during repeated use. Considering the above situation, there is an urgent need to design one kind of novel, wear-proof and uniform both within and outside superhydrophobic paper.

In the traditional papermaking industry, fillers are the second most important component used to meet the needs of papermaking, such as decreasing the dependence on forestry resources and promoting drainage drying in the papermaking process with high benefits in terms of costs and energy saving. Importantly, fillers also can enhance and impact some essential properties of paper such as brightness, gloss, opacity, smoothness and printability. So, many types of fillers have been applied in the papermaking industry, including calcium silicate, precipitated calcium carbonate, calcium carbonate, titanium dioxide and so on.^25–27 In the same way, the wettability of paper also can be regulated by superwetting the filler, as far as we know, which has never been reported before.

For their amazing adhesion ability to various kinds of surface or substance, mussels have been attracting much attention in the research community;^28–31 this ability derives from L-3,4-dihydroxyphenylalanine (dopamine) with excellent biocompatibility and low cytotoxicity. Many studies found that the interfacial poly-dopamine (PDA) layers not only facilitate the dispersion of the filler, but also strengthen the stress transfer from the substrate to the filler, resulting in greatly improved mechanical properties.^32,33 Inspired by a wise mentor of humans – nature – we creatively combined the lotus effect and the mussel adhesion effect into a recycled papermaking process and obtained one kind of multifunctional uniform both inside and outside the superhydrophobic recycled paper with wonderful self-cleaning, anti-fouling and oil absorption abilities while still keep the paper's intrinsic properties.

Experimental

Materials

Waste paper obtained from our laboratory room was used as cellulose pulp fibers. 1,1,1,3,3,3-Hexamethyl disilazane (HMDS, 98%) was obtained from Shanghai KEFENG Chemical Reagent Co. Inc. 3-Hydroxytyramine hydrochloride (dopamine hydrochloride, 99.5%) was purchased from Sigma-Aldrich. All other chemicals were analytical-grade reagents and used as received. Millipore water (resistivity ∼ 18 MΩ cm) was used throughout this study.

Preparation of superhydrophobic silica gel powder (SSGP)

At first, 5 mL tetraethoxysilane (TEOS) was dissolved in 10 mL methanol, then a mixture of 7.5 mL NH₄OH (0.02 M) and 10 mL CH₃OH was added into this solution with stirring. 3.5 mL of HCl (0.1 M) was added dropwise to this mixture. Now, the pH of this system was close to 5. Then, 1.5 mL NH₄OH (25–28%) was slowly added dropwise to turn the pH into 8–9. After aging for 12 h, a white opaque silica gel was formed. The obtained silica gel, 50 mL of n-hexane and 7.5 mL of 1,1,1,3,3,3-hexamethyl disilazane (HMDS) were added into an autoclave together and kept at 80 °C for 10 h. After being filtered and washed with n-propanol twice, the obtained gel was dried at 60 °C under air for 6 h to remove the solvent and residual reactants. Then the obtained dry gel was ground to get the SSGP.

Preparation of superhydrophobic recycled paper (SRP) using a papermaking technique

Waste paper fragments (0.3 g) were immerged into deionized water (60 mL) under vigorous stirring conditions at 80 °C for 3 h. Then, an aqueous suspension of rough cellulose fiber pulp was obtained. 1 M NaOH was used to turn its pH into 9. Under room temperature, 0.01 g dopamine hydrochloride was added and kept stirring for 10 h. Here, one kind of brown-colored PDA-cellulose fiber pulp was formed. After being filtered and washed with deionized water and ethanol twice, respectively, the PCF pulp was re-dispersed into ethanol (30 mL) under gentle agitation at room temperature for 30 min. Different weight ratios of SSGP (0%, 10%, 20%, 30%, 40%, 50% and 60%, on the basis of the weight of waste paper fragments) was added into this system. After ultrasonication (50 Hz) for 15 min and kept stirring for 30 min, “wet pulp cake” was obtained after filtering. The “wet pulp cake” was dried at 60 °C under air for 3 h. Finally, the “dry pulp cake” was pressed into paper using a Tablet press under 5 MPa. Then the SRP was in its final formation.

Characterization

Field emission scanning electron microscope (FESEM) images were obtained on a JSM-6701F, both with Au-sputtered specimens. X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) measurements were made using the Al Kα line as the excitation source. TGA (thermogravimetric analysis) measurements were done with a NETZSCH STA 449C using a dynamic heating rate of 10 °C min⁻¹. Fourier transformer infrared spectra (FTIR) were recorded using a Thermo Scientific Nicolet iS10. The water contact angles were measured with a JC2000D with a 5 μL distilled water droplet at ambient temperature. The average WCA values were obtained by measuring the same sample at five different positions.

Results and discussion

Preparation and structural characterization

The fabrication process of SRP is schematically illustrated in Fig. 1. An oyster white aqueous suspension of rough cellulose fibers (CF) pulp was obtained from waste paper fragments under heating and magnetic stirring. On turning the pH to 9 and adding 0.01 g dopamine powders, the suspension became brown after 10 h aging because of the self-polymerization of dopamine. The cellulose fiber surface is richly endowed with hydroxyl groups and dopamine tended to react with the hydroxyl groups, resulting in dehydration and the formation of a charge-transfer complex. As a result, the cellulose fiber surface was clad by a PDA layer. The PDA layer endowed the micro-fibrillated cellulose with a strong bonding which was beneficial to enhance the mechanical strength of the cellulose fibers.³⁴ This is only the first effect of the PDA layer. As a biologically active binder, PDA is apt to make it possible to attach various cells or nanoparticles.³⁵ So, in the next step, the added SSGP can stick to the PDA-cellulose fiber surface easily, which increased the residue of SSGP in SRP. This is the second effect of the PDA layer. After the hydrophobic modified process, the obtained SSGP contained a lot of hydrophobic Si-(CH₃)₃ groups, which can be proved by the FTIR spectra of SSGP in Fig. S1 (see ESI†). Through filtration and dry treatments, the “dry pulp cake” of the SSGP/PDA/cellulose fibers was obtained finally. By tableting, the “dry pulp cake” was squashed into a round, flat paper with the diameter of about 4 cm, namely, SRP. By the way, the diameters of all the round SRP that are shown in all figures are 4 cm.

Fig. 1 Schematic illustration of the synthesis procedure of SRP from the waste paper via traditional papermaking processes.

The cellulose fiber paper (CFP) and PDA/cellulose fiber paper (PCFP) were also prepared in a similar way as control experiments. All these three kinds of papers (CFP, PCFP and SRP) are uniform in composition with smooth surface. The PCFP and SRP have weaker whiteness than CFP due to the brown PDA layer and the optical images of CFP, PCFP and SRP are shown in Fig. S2, see ESI.† In fear of damage to the aesthetics and writable of the PDA layer, the original amount of dopamine was controlled within a small range (0.01 g) and the polymerization time is relatively short. The FESEM images of CFP, PCFP and SRP are shown in Fig. 2. According to low resolution images of CFP, the diameters of the original fibers vary from about ten to dozens of micrometers, which are of a criss-crossing, formed porous structure. PCFP is so compact due to the DPA layer's adhesion action, which can enhance the interaction forces of the fibers. The compactness of SRP is less than that of PCFP because SSGP, an inorganic filler, has a negative impact on the fiber-to-fiber bonding with the opposite effect to the PDA layer.³⁶ Compared with the CFP, SRP is still more close-grained based upon the interaction of all of these factors. The high resolution FESEM images of CFP, PCFP and SRP can clearly display the surface topography of a single fiber. In Fig. 2d, the original fiber surface is smooth without a distinct wrinkle. The PDA/cellulose fiber surface shows a wavy integument with many grooves resulting from the PDA layer's cladding. The FTIR spectra of PCFP can also be proved from the band at 1400 cm⁻¹ that can be assigned to C=C resonance vibrations and the bands at 867 cm⁻¹ and 712 cm⁻¹ that can be assigned to –H vibrations in the aromatic rings of PDA (Fig. S3, see ESI†). The surface topography of the SSGP/PDA/cellulose fiber is different from that of the previous two. There are lots of flaky objects attached to the surface. In the higher resolution image, the flaky object was composed of SSGP of around 30 nm in size (Fig. S4, ESI†). XPS spectra can also give proof of this preparation process. The XPS spectra of CFP mainly include two peaks at 532 eV and 284.8 eV which are, respectively, labeled as O 1s and C 1s. Compared to CFP, the XPS spectra of PCFP has one weak peak at 398.4 eV assigned to N 1s, which indicated the existence of PDA. And, apparently, the XPS spectra of SRP show Si 2s and Si 2p at 155.1 eV and 105.8 eV that be consistent with the XPS spectra of SSGP, thus proving the existence of SSGP (the XPS spectrum are shown in Fig. S5, ESI†).

Fig. 2 (a–c) Low resolution FESEM images of CFP, PCFP and SRP. (d–f) High resolution FESEM images of CFP, PCFP and SRP.

Wetting properties of SRP

The wetting properties of CFP, PCEP, and SRP were quantified by CA and SA using water as the test liquid. Owing to the inherent superhydrophilicity of cellulose and PDA, there is no doubt that both CFP and PCFP are superhydrophilic. When the water liquid contacts with the surface, it spreads out immediately and the paper gets soaking wet (Fig. S6, ESI†). However, SRP-obtained SSGP shows a great superhydrophobicity and a CA up to 158°, according to Fig. 3a. A water drop (5 μL) can stand on the paper surface in a near spherical shape and can maintain this over a long period of time. Furthermore, when the SRP is placed on a table with a certain angle, even smaller than 5°, the water droplet (about 12 μL) dripping off the microinjector can roll down easily and quickly, see Fig. 3b. When water droplet is placed in contact with a flat solid surface, it tends to reach a relatively low energy state and has a CA measured at the edge of each droplet which can described by Young's eqn (1):³⁷where γ_SV, γ_SL and γ_LV are the surface tension at the solid–vapor, solid–liquid, and liquid–vapor interfaces, respectively. For the rough solid surface situation, two classic well-established models developed by Wenzel and Cassie & Baxter should be considered. The Wenzel equation is valid for the homogeneous wetting regime described as follows:³⁸

cos θ_w = r cos θ (2)

here, r is the roughness factor.

Fig. 3 (a and b) The photographs of water droplets CA and SA of the SRP. (c) Optical images of SRP immersed in water with a mirror-like silvery white layer. (d) Optical images of a water column rebound phenomenon on the SRP surface. (e) Optical image of 1 M NaOH and 1 M HNO₃ droplets on the SRP surface and pH test papers wetted by 1 M NaOH (left, blue) and 1 M HNO₃ (right, red). (f) Optical images of all kinds of near-spherical common liquid droplets on the SRP surface.

Cassie & Baxter proposed the heterogeneous surfaces concept and they believed that the liquid drop will not penetrate into cavities on the rough surface in all cases and that air is trapped in these cavities to become air pockets. So, there is a new equation for this model:³⁹

cos θ_CB = f_SL(1 + cos θ) − 1 (3)

where f_SL is the liquid–solid interface fraction.

Based on the previous theoretical research, one can draw the conclusion that a smaller solid–liquid contact area and a lower solid surface energy are conducive to a higher water contact angle. Barthlott and Neinhuis⁴⁰ held that a nano- and micro-scale hierarchical structure benefited an extreme repellence against liquid droplets, inspired by the lotus leaf, which shows fine-branched nanostructures on top of microsized papillae. Therefrom, researchers have committed to create such a special hierarchical roughness structure to achieve wonderful superhydrophobic surfaces.⁴¹ Besides, fluorine substances always have attracted much attention for a low surface energy in this research field. However, two nearly inevitable shortcomings are high toxicity and high price. In this work, we chose HMDS with the aid of rich hydrophobic –CH₃ to avoid fluorine-substances successfully.⁴² So, this low toxicity SRP can satisfy the need of real life without worry. From the structural aspect, micron-scale PDA-cellulose and nano-scale SSGP can combine into a hierarchical structure of our SRP, see Fig. 2f. So, that is the reason why this SRP have excellent superhydrophobicity. In order to manifest its waterproof ability more intuitively, on dipping of the SRP into the water using tweezers, and then removing it out seconds later, there is no residual water and the SRP is as dry as before (see Movie S1, ESI†). More interestingly, when the SRP is immerged in water, a layer of silvery white mirror can be found. This is because there are abundant air pockets between water and the paper surface (Fig. 3c) which also proves that water on the paper surface exists in the Cassie & Baxter state. Therefore, the experiment results agree well with the theoretical analysis. Using an injection syringe to eject a water column into the SRP surface, the impacting water column bounces out without spread (Fig. 3d). In addition, the SRP has a strong acid and alkaline resistance as well. Let drops of HNO₃ (1 M) and NaOH (1 M) fall onto the SRP surface. We can observe both drops keep a perfect spherical shape on the paper that can be sustained over time (Fig. 3e). All in all, the extreme goal for our artificial SRP is to serve the people's everyday life. So, some common liquids in our daily life are studied too. Coffee, milk, green tea, black tea, methylene blue and water drops are both placed on SRP surface and they all showed near-spherical shapes with high CA (Fig. 3f). This indicates that the SRP can meet people's basic needs and can handle various situations.

Different from traditional fillers in the papermaking industry, SSGP played a key role in regulating the wettability of recycled paper in this study but to improve the optical properties and smoothness. The impact of the weight ratio of SSGP (here we use the term W(SSGP) on the basis of the weight of waste paper fragments) was revealed by measured CA and SA values. When W(SSGP) = 0%, namely, PCFP was tested first and the CA was 0° for the superhydrophilicity of cellulose and PDA as mentioned before. Only on increasing W(SSGP) to 10% from 0%, could the CA also significantly increase to 139.3 ± 2.3°. However, the water drop (∼12 μL) cannot roll down at an inclination of ∼10°. Furthermore, when we continued to improve the W(SSGP) to 20%, the hydrophobicity of recycled paper also improved (CA achieved 143.7 ± 1.2°). This time, the water drop can roll down with an inclination of ∼10° but it cannot roll down with an inclination of ∼8 degrees. When W(SSGP) = 30%, the CA finally surpassed and reached 157.8 ± 0.7°. Strictly speaking, this time the recycled paper is still not superhydrophobic because the water drop cannot roll down with an inclination of ∼8 and ∼5°. In other words, the SA value is bigger than 5°. Only when W(SSGP) = 40%, did we eventually obtain SRP. The CA of SRP is 158.9 ± 1.1° and SA is lower than 5°. If the SRP is tilted only a little, the water drop can roll down easily and rapidly. As the W(SSGP) continues to improve, the CAs are 160.9 ± 0.8° and 158.3 ± 1.7°, respectively when W(SSGP) are 50% and 60%. Similarly, the SAs of these two situations are both lower than 5°. In general, W(SSGP) = 40% is the boundary between high-hydrophobicity and superhydrophobicity (the images of CAs and SAs from 10% to 60% are shown in Fig. S7†). It is interesting to note that in our control experiment, when the SSGP was added into original the cellulose fiber pulp without PDA and then made into paper under the same conditions and methods, the CA was only 151 ± 3° and the water drop (∼12 μL) could not roll down with an inclination of ∼8°, even when the W(SSGP) = 40% (see Fig. S8†). This result reveals that PDA has an important influence on the final results. So, TGA was used to characterize the thermal degradation of three kinds of papers (CFP, SSGP/CFP and SRP) and the weight loss curves in N₂ are shown in Fig. S9.† The degradation of all of them occurs at around 300 °C, which was the result of the depolymerization of cellulose into volatile products (levoglucosan). But the residue weight percentages were significantly different at the end of degradation (800 °C). The weight percentage of residue of CFP was about 10%, which means a stable char residue of cellulose.^43,44 As a control experiment, the weight percentages of residue of SSGP/CFP (W(SSGP) = 40%) increased to 25%. Obviously, difficult-to-decompose inorganic filler (SSGP) still existed. In contrast, the weight percentage of a residue of SRP was as high as 35% with 40 percent in the PDA situation. The adhesion effect of PDA can reduce the loss of SSGP and increase the SSGP content in SRP, which led to a better superhydrophobicity of SRP as well.

Paper intrinsic properties of SRP

Can this well-designed modified functional water-proof paper keep its paper intrinsic quality such as writability, suppleness, foldability and tailorability, very much deserves our care. All this work will be wasted research if the intrinsic quality of the paper is lost during the modified process to obtain superhydrophobicity. So, it is a great challenge to have both.

It is impossible to write with a water-based ink pen on this SRP due to the great water-proof ability. The surface energy of water is about 72 mN m⁻¹, but oil is much low (20–30 mN m⁻¹).⁴⁵ Then, the superhydrophobic surface is superoleophilic in general and this SRP is no exception. On this understanding, an oil-based ink pen is undoubtedly a potential alternative and it is truly useful. According to Fig. 4a, using a blue oil-based ink marker pen, we write the word “writable” and draw a tree on the paper surface. Obviously, the smooth handwriting has not caused damage to the paper. It is worthwhile to indicate that water drops can still stand nearby-spherical even on the written place. This SRP has a very good suppleness as well and can be folded in half by tweezers, shown in Fig. 4b. Furthermore, the SRP can be cut into rectangle and then folded into a simple paper boat which can, different from the ordinary one, float on the water surface for quite a long time without wetting. Combining the above characteristics, we designed an artificial lotus colored by a green dye which also can float on water just like live one in nature (Fig. 4c and d). This work must open up new prospects for the application of paper crafts. The durability and instability of SRP are worth caring for as well. By clipping the middle area, the SRP can hang weights (50 g) easily without breakage and deformation (shown in Fig. 4e). High-temperature experiments were also conducted. In order to explain more objectively and intuitively, taking commercial tissues as a comparison, we can see that after heating at 100 °C for one half hour, there was no significant change of our SRP. However, commercial tissue folded slightly because of the loss of contained trace amounts of water, shown in Fig. 4f. These above two points illustrate that this SRP must be sufficient to meet the basic daily needs. In this lab-based fundamental research, the beating, deinking, bleaching and refining treatments of the traditional papermaking industry have been omitted for the mature development of these aspects and they are not our research focus. Likewise, we have avoided the use of industrial adhesives in this process. So, the mechanical property of SRP has certainly been a possible concern here. PCFP (SRP without PDA) shows very weak mechanical properties. No matter whether it is folded or stretched, both make it broken and we can't write on it. Considering accessibility and affordability factors, dopamine was chosen to solve this issue here. A PDA layer that covers the fiber surface enhances the interaction force between fiber–fiber and fiber-filler. This organic–inorganic composite construction not only helps maintain good superhydrophobicity, as mentioned before, but also contributes greatly to mechanical properties. Obviously, the mechanical property of SRP is associated with the initial content of dopamine increase. But the initial content of dopamine has to be controlled within a small value. The reason is that brown PDA has a negative impact on the brightness and appearance of SRP (see Fig. S2†). After comprehensive consideration, a small amount of dopamine is the best choice.

Fig. 4 (a) Optical images of the SRP read “writable” and a tree by a blue oil-based ink marker pen. (b) Optical images of the curved SRP. (c) SRP boat floats on the water surface without wetting. (d) Artificial lotus leaf. (e) Hanging experiment of 50 g weights. (f) Optical images of SRP and commercial tissue before (top) and after (bottom) the high-temperature experiment (100 °C).

Self-cleaning and anti-fouling properties of SRP

Packaging, the technology of enclosing or protecting products for distribution, storage, sale, and use, are some of the important uses of the paper industry. Paper packaging can be described as a coordinated system of preparing goods for transport, warehousing, logistics, sale, and end use. The greatest risks to paper packaging must be abrasion and soaking during transport or the warehousing process. Researchers have long been trying to discover some good methods but there seemed to be no answer to this question. Fortunately, a superhydrophobic surface, particularly this SRP with great self-cleaning and anti-fouling properties^46,47 is a perfect solution.

A water droplet can roll off instead of sliding on the superhydrophobic surface for the high CA and low SA. For a favorable decease of the surface energy, the powders adhered on the surface are easy attached to liquid so that the dirt on the superhydrophobic surface can always be taken away by the water droplet. This is the reason why the lotus leaves keep a clean appearance in an unclean natural environment, called the “self-cleaning” ability. It is important inherent this feature of a superhydrophobic surface and it has been researched a lot. The significance of the “self-cleaning” ability of the SRP is self-evident. So, a self-cleaning experiment has been conducted as well. A bit of black graphite powder was spread on the SRP evenly with an inclination of about 30 degrees. These powders did not slip off at beginning. Using an injection syringe, water droplets were dripped continuously and slowly from 1 cm above the paper. The water drop could roll down and take the touched powders away. After few drops of water, on the area where the water passed by almost no graphite powder remained (Fig. 5a–c). Generally speaking, the SRP can always keep it clean even in an outside environment for a long time with rain scouring but this situation also inevitably leads to new questions. The rainfall has a certain amount of dust and pollutants in it and can this SRP withstand this? Namely, whether this SRP has anti-fouling ability, which was investigated using muddy water. Similarly, the muddy water was poured out using a beaker and it flowed down in strands from the paper surface. The paper still keeps clean consistently without any residue or pollution. Furthermore, with the addition of muddy water in a petri dish, the SRP can float on the liquid surface but wetted as shown in Fig. 5d–f. So, this SRP, no doubt, will bring dramatic changes into the paper packaging industry with good dust and water proof ability.

Fig. 5 (a–c) Time sequence of self-cleaning experiment process on SRP with a 30° incline and black graphite powders as the contaminants were taken away by water droplets. (d–f) Time sequence of an anti-fouling experiment process on SRP with the impact muddy water.

Oil adsorption ability of SRP

The frequently occurring oil leaking accidents in the ocean and industrial oily wastewater continue to threaten ecological systems.^48,49 A more everyday example is that we always need to handle the oil–water mixture situation in the kitchen. It a is huge trouble to separate a small amount of oil from useful water.^50,51 Taking advantage of the pore structure and superhydrophobicity/superoleophilicity, the SRP is believed to be an ideal candidate for the adsorption of oil from water, especially wastewater. To investigate the oil/water separation performance of the SRP, hexane was chosen as the absorbate. Fig. 6a shows that when the SRP was in contact with the surface of hexane-water mixtures, the hexane (stained with Sudan red IV) was adsorbed rapidly, and the SRP turned red gradually. When the paper reaches saturation, the hexane on the water surface significantly reduced but with no loss of water. The whole process is fast, within a few minutes. A heavy oil experiment was tried as well using 1,2-dichloroethane (stained with Sudan red IV). In contrast, 1,2-dichloroethane sinks to the bottom of a water beaker. Even when the entire SRP was immersed in the water, 1,2-dichloroethane was also adsorbed rapidly and without water loss, shown in Fig. 6b. The results indicate that SRP is a promising adsorbent for the cleanup and removal of oil with different densities. One interesting thing to note: after drying, the used SRP retains its original morphology and size, except it is colored by red, and still has great water proof ability with the high CA of the above two cases (see the insets of Fig. 6). That is to say, the SRP can be recycled.

Fig. 6 Snapshots of the oil adsorption experiments of (a) hexadecane (dyed with Sudan red IV for a clear observation) from the water surface by SRP and (b) 1,2-dichloroethane (dyed with Sudan red IV for a clear observation) from the bottom of a water beaker by SRP. The insets: the photographs of the water droplet CA of a dry oil-wetted SRP by hexadecane (in a) or 1,2-dichloroethane (in b) and both CA values are larger than 150°.

Out- and in-side uniform superhydrophobicity of SRP

As noted earlier, superhydrophobic paper has been prepared by many groups before. Wang et al. prepared one kind of superhydrophobic filter paper via a modified commercially available filter paper by a mixture of hydrophobic silica nanoparticles and polystyrene solution.²¹ Ogihara's group sprayed superhydrophobic and transparent alcohol suspensions of SiO₂ nanoparticles coatings to get superhydrophobic paper which can maintain its superhydrophobicity after being touched by a bare finger.²² Using a straightforward surface modification with poly(hydroxybutyrate), Sousa and coworkers also fabricated superhydrophobic paper to design essential nonplanar lab apparatus.²³ These outstanding works have inspired subsequent minds and point the way forward. However, it is well known that paper is one kind of daily consumable with a low wear resistance, and this is the reason why common surface modified superhydrophobic paper has huge limitations of utilization, as mentioned above.

Fantastically, different from previous research, the SRP has unique outer and inner uniform superhydrophobicity. Taking advantage of traditional papermaking industry knowledge here, the SSGP exists in each part of the SRP no matter the surface (outside or inside). So, even if the surface part is worn away by friction, the water-proof ability still exists. A sand paper abrasion test under a 50 g of load to the SRP was adopted as well. The SRP sample (1 cm × 1 cm) was adhered to a slide with double sided adhesive tape and then the sample was propelled for 8 cm on the sandpaper (800 Cw). One cycle experiment process of the sandpaper abrasion is shown in Fig. S10, see ESI.† After 5 cycles, the SRP clearly shows an edge with high roughness. But this did not impact the wettability of paper and water droplet can still “stand” on it. The CA value can reach ∼150° but fluctuates wildly (±4.3°) because of the uneven surface (Fig. 7a). The situation is also available over 10 cycles. But for 15 cycles, the instability of CA will drop because the sample is very thin now and it will turn flat. But what is astonishing is the CA can be up to 154.4 ± 1.2° after 20 cycles with almost no sample remaining on the double side adhesive tape, which can be seen with the naked eye clearly (Fig. 7a, the insets). This illustrates that the SRP can keep a great water proof ability even under an excessive wear situation. In order to clarify this property more intuitively, the SRP was torn apart from the middle to expose the internal part to air, as shown in Fig. 7b. Both the surface and the inner part can make a water droplet stand in a near-spherical shape, which also proves the unique outer and inner uniform superhydrophobicity of the SRP.

Fig. 7 (a) Broken line graph of CAs on the SRP in the 20 cycles of abrasion test. The insets: optical images of fifth cycle (left) and twentieth cycle of SRP (right). (b) Optical image of tearing SRP with water droplets (stained with methylene blue) on the surface and within.

Conclusions

Using SSGP as filler, we fabricated one kind of multifunctional uniform, both within and on the surface, superhydrophobic recycled paper which has wonderful self-cleaning, anti-fouling and oil absorption abilities, inspired by lotus effect and mussel from nature. Especially, this superhydrophobic recycled paper can keep its intrinsic properties intact such as writability, suppleness, foldability and tailorability. In brief, this work not only avoids harm by water invasion but also expands the usable range of paper. Simultaneously, taking advantage of traditional papermaking knowledge, this superhydrophobic recycled paper can ease the energy and environment crisis as well. The research to further improve the intensity, brightness and flame-retardancy and to expand other potential applications of the superhydrophobic recycled paper are currently under investigation in our laboratory, and this work must draw great inspiration to people who are interested in this field.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (No. 51522510), the “Funds for Distinguished Young Scientists” of Hubei Province (2012FFA002), the Co-joint Project of the Chinese Academy of Sciences and the “Top Hundred Talents” Program of the Chinese Academy of Sciences and the National 973 Project (2013CB632300) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04259g

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