Scientists Introduce a New “Highly Flexible Waterproof and Fire-Resistant Inorganic Paper” but not made from trees.

Traditional Papers

Sometimes the word tradition might resemble as a safe norm. So, many people would like to do things in a traditional manner. But, behind every benefit, there is a loss. Coming to the part of trees, we made papers from plant cellulose fibers. These are easily destroyed by either liquid or fire. By doing this, the papermaking industry consumes a large number of natural trees and thus causes serious environmental problems including excessive deforestation and pollution. So, we called it the traditional process of papermaking. In consideration of the intrinsic flammability of organics and minimizing the effects on the environment and creatures, “biocompatible ultralong hydroxyapatite nanowires” are an ideal building material for the fire-resistant inorganic paper.

New Fire-resistant Inorganic Paper

The Highly Flexible and Non-flammable Inorganic Paper is made from ultralong hydroxyapatite nanowires. The new fire-resistant inorganic paper was discovered by the research group led by Prof. Dr. Ying-Jie Zhu from Shanghai Institute of Ceramics, Chinese Academy of Sciences (P. R. China).  The new kind of the fire-resistant paper can be used for printing and writing, and is promising for the permanent and safe storage of information, such as archives and important documents, avoiding being destroyed by fire. This invention provides a promising perspective for the fire-resistant inorganic paper as a possible substitute for the traditional cellulose paper, which has caused serious resource and environmental problems.

Hydroxyapatite is a natural mineral, and is the main inorganic component of human bone and teeth. The human bone contains about 70 wt. % hydroxyapatite; tooth enamel has more than 90 wt. % hydroxyapatite. Hydroxyapatite exhibits excellent biocompatibility, environmental friendliness, high-quality white color, high-temperature resistance, and excellent nonflammability. Therefore, it is an ideal raw material for building new fire-resistant inorganic paper.

However, a great challenge arises: hydroxyapatite materials usually have high brittleness and poor flexibility, making them difficult to make soft fire-resistant paper.

How to solve this problem? The Zhu’s group found that ultra long hydroxyapatite nanowires with lengths of more than 100 microns have good flexibility and can effectively solve the problem of high brittleness of hydroxyapatite materials.

However, the synthesis of ultra long hydroxyapatite nanowires is a big challenge. The Zhu’s group developed the calcium oleate precursor solvothermal method for the synthesis of ultra long hydroxyapatite nanowires with lengths up to several hundred microns (the maximum length is even close to 1000 microns). The as-prepared ultra long hydroxyapatite nanowires have diameters of about 10 nanometers, which is about 10,000 times smaller than the thickness of a human hair, and the nanowires can bend naturally with a high flexibility (Figure 1). However, this synthetic method uses an oleic acid/ethanol/water mixed reaction system. Then, the team has developed a new low-cost synthesis method using an environmentally friendly single water reaction system, and the aqueous suspension containing ultra long hydroxyapatite nanowires has a high whiteness (Figure 2), greatly reducing the amount of ethanol used, and does not require use of oleic acid.  This preparation method is environmentally friendly, and the chemical raw materials are common, readily available, and inexpensive, and no toxic and harmful substances are produced.

Figure 1. Scanning electron microscopy (SEM) image of ultra long hydroxyapatite nanowires synthesized by the calcium oleate precursor solvothermal method. The as-prepared ultra long hydroxyapatite nanowires can bend naturally, exhibiting a high flexibility.

Figure 2. An aqueous suspension containing ultra long hydroxyapatite nanowires prepared by the calcium oleate precursor hydrothermal method.

In addition, the scaled-up synthesis of ultra long hydroxyapatite nanowires is demonstrated with an autoclave with a volume of 10 liters and even a larger scale with a volume of 100 liters.

The Zhu’s group has successfully developed a new type of fire-resistant inorganic paper based on ultra long hydroxyapatite nanowires as the building material, which has high biocompatibility, is environmentally friendly, and artificially synthesized without consuming precious natural resources such as forests. The new fire-resistant paper is highly flexible, exhibits high-quality white color without bleaching, and excellent resistance to high temperature and fire (Figure 3). The new fire-resistant paper does not burn in fire even when being heated to red heat, and can withstand high temperatures of 1000 °C or higher; while the ordinary paper is burned to ash after exposure to fire in a few seconds. In addition, the new fire-resistant inorganic paper has superior performance in resistance to mould compared with the traditional paper.

Figure 3. The new fire-resistant paper exhibits excellent resistance to high temperature and fire, it does not burn in fire even when being heated to red heat, and can withstand high temperatures of 1000 °C or higher.

Through a large number  of experiments, the team explored and optimized formulations for the ultra long hydroxyapatite nanowire suspension and inorganic adhesives, as well as the preparation technology for the fire-resistant paper, which greatly improved the mechanical strength and other properties of the fire-resistant inorganic paper. The fire-resistant paper can be used for writing and color printing, and is expected to be used for long-term safe storage of books and important documents such as archives and certificates. It also has many other uses and has good application prospects in many fields.  The manufacturing process of the fire-resistant inorganic paper is environmentally friendly, does not pollute the environment, and has good industrial application prospects.

The fire-resistant inorganic paper sheets with a A4 size (21 cm x 29.7 cm) have been successfully developed, which can be directly printed with a commercial colour printer, and the printing effect is good (Figure 4). The team has also successfully prepared the fire-resistant paper with a A3 size (42 cm × 29.7 cm).

Figure 4. Digital images of the as-prepared highly flexible and fire-resistant inorganic paper sheet with a A4 size. The fire-resistant inorganic paper can be directly printed with a commercial color printer.

New Waterproof Fire-resistant Inorganic Paper

Subsequently, by imitating the structure and effect of lotus leaf, a new kind of highly flexible, super hydrophobic (waterproof), and fire-resistant layered  inorganic paper has been successfully prepared by the same team using ultra long hydroxyapatite nanowires as a building block after the surface modification with sodium oleate (Figure 5). During the vacuum filtration, ultra long hydroxyapatite nanowires assemble into a porous hierarchical network with a layered structure and form the highly flexible, waterproof, and fire-resistant paper. The layered structure can significantly improve the resistance to the mechanical damage of the as-prepared waterproof and fire-resistant paper. The super hydrophobic property of the waterproof fire-resistant paper has a high stability and can be well preserved even in the high temperature environment. The as-prepared waterproof and fire-resistant paper shows excellent non-flammability, high thermal stability, excellent liquid repellency to water (Figure 6) and various commercial drinks. The waterproof fire-resistant paper has a good self-cleaning function, and dust and contaminants on the paper surface can be easily washed away by water, thereby achieving self-cleaning of the surface. If it is used for open-air billboards, it can automatically clean the dust by rain, avoiding the trouble of manual cleaning, and lowering cost.

Figure 5. A new kind of highly flexible, waterproof and fire-resistant layered inorganic paper prepared using ultra long hydroxyapatite nanowires after the surface modification with sodium oleate. Water (dyed with methyl blue) has a contact angle of more than 150oC on the surface of the waterproof fire-resistant paper, and water cannot be absorbed into the paper, but forms spherical water droplets rolling on the paper surface.

Figure 6. When the waterproof fire-resistant paper is immersed in the blue ink and the surface of the paper is not contaminated with water or blue color.

Moreover, the potential applications of the waterproof and fire-resistant inorganic paper have the efficient qualities as a highly effective adsorbent for oil/water separation, fire-shielding protector, and writing paper.

Various Types of New Fire-resistant Inorganic Paper and Their Promising Applications

Up to now we aware that the fire-resistant inorganic paper might replace the flammable traditional paper in the future.  Now find out the additional features. The hydroxyapatite nanowire paper with excellent fire-resistant and adiabatic properties is highly demanded in various high-temperature applications.

The Zhu’s group has also developed many other types of the fire-resistant paper based on ultra long hydroxyapatite nanowires for various applications, such as high-temperature-resistant label paper, fire-resistant calligraphy and painting paper, antibacterial fire-resistant paper, luminescent fire-resistant paper, multi-mode anti-counterfeit fire-resistant paper, electrically conductive waterproof fire-resistant paper, catalytic fire-resistant paper, waterproof magnetic fire-resistant paper, automatic fire alarm fire-resistant wallpaper, high-temperature battery separator, fire-resistant wrapping tape for optical (electric) cables, high-efficiency filter paper for organic pollutants/air PM2.5/anti-fog  mask/air purifiers, biomedical paper for bone defect repair and skin wound healing, and rapid test paper, etc.

So, a revolution may happen through this new discovery, which will minimize the environmental pollution and  safely protect paper-based  books and documents.

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