The large usage of petroleum-based conventional plastics has incurred a lot of environmental issues, such as white pollution due to their non-degradability and costly recyclability. Nowadays, new plastic materials, which are environmentally friendly and bio-degradable, are calling for more and more attention. Meanwhile, the substituting plastic material should be cheap, easy to synthesize and recyclable while possessing comparable mechanical properties to conventional plastics.
Natural materials made by mineralization or biominerals such as the shells of invertebrates have inspired scientists to design plenty of new artificial materials with remarkable properties, since living organisms exhibit perfect control over the composition, morphology and hierarchical structures of biominerals . Apparently, biominerals can meet most of the requirements for substituting conventional plastics such as the good mechanical performance and bio-degradability, but they are not shapeable and recyclable. Once formed, biominerals have definite shapes owing to the extrusion of the organic matrix and/or the irreversible crystallization of amorphous precursors. Nevertheless, during the mineralization of calcium carbonate in the presence of ppm amounts of acidic macromolecules, a transient “polymer-induced liquid precursor” (PILP) phase consisting of amorphous calcium carbonate (ACC) and polyacid is often observed, which can be formed into any shape . Therefore, if the liquid or amorphous phase can be long-term stabilized, a mineral-based plastic material may be obtained.
Production and Features of Hydrogel
Guided by the principles of biomineralization and green chemistry, in our recent paper published in Angewandte Chemie
, we reported a hydrogel composed of very small ACC nanoparticles and polyacrylic acid (PAA), as shown in figure 1. The synthetic procedure is very simple. In brief, we mixed two stock solutions, one with 0.1 M CaCl2
and 0.1 M PAA (Mw
~ 100, 000 g/mol) and the other with 0.1 M Na2
The high amount of PAA stabilizes the amorphous phase of CaCO3 perfectly, resulting in a dough-like soft hydrogel. The hydrogel is shapeable and self-healable like the cartoon character, “Barbapapa”, and more importantly, can solidify reversibly resulting in transparent objects with the structure resembling that of shrimp shell . With it, we successfully introduced “plasticity” into mineral-based hybrid materials, and thus we dubbed it “mineral plastic”.
The hydrogel exhibits a few interesting features. As shown in figure 2, the hydrogel is plastic, and can be formed into different shapes such as films, cylinders, and stars. The hydrogel is also stretchable. It may be stretched into very long fibers with plastic deformation without any elastic recovery. Additionally, the hydrogel can self-heal rapidly; within 5 s when linking two parts together. A SEM image of the freeze-dried hydrogel shows a porous structure, typical for hydrogels. Very small ACC nanoparticles (1.5-3 nm) can be identified in the TEM image of the dry gel, indicating that the hydrogel is actually a complex of ACC nanoparticles physically cross-linked by PAA chains via coulomb interactions between COO- and Ca2+. The solid content of the fully swollen ACC/PAA hydrogel was estimated to be 38-42 wt%. For the dried gel, the molar ratio of different components, [CaCO3]:[PAA]:[water], was calculated to be 39:74:44 by TGA. Such high amount of CaCO3 makes the hydrogel very tough (both the storage and loss moduli are larger than 103 Pa). Rheological measurements (not shown here) reveal that the hydrogel exhibits shear-thinning and thixotropic behavior that is typical for pseudoplastic fluids. All those features indicate that the hydrogel can be a very good candidate for plastic materials.
Furthermore, dried in air, the hydrogel can form free-standing, macroscopically continuous objects, and this process is totally reversible (fig. 3a,b). The dry objects can recover the original hydrogel state without any loss of mechanical performance by swelling in water for one day, and this drying-swelling process can be repeated many times. In other words, the final rigid material can be designed through the hydrogel and can be easily recycled by simply immersing in water and reshaping it! The dry objects are rigid with very smooth surface, and a nano-indentation measurement estimated its hardness and modulus to be 1.0 ± 0.07 GPa and 22.4 ± 1.1 GPa, respectively, which are apparently higher than its analogous biomaterial, shrimp shell (hardness, 0.172 MPa; modulus, 1.87 GPa) as well as conventional plastics (e.g., for PMMA, hardness, 0.187 MPa; modulus, 3.83 GPa).
Colored Mineral Plastic
Additionally, like in conventional plastics, colors can be easily introduced to the mineral plastic. 10,12-Pentacosadiynoic acid (PCDA) is a thermometric molecule which can spontaneously assemble in water to yield PCDA vesicles (transparent) that can be polymerized by UV irradiation (PDA vesicles, blue) . Upon heating, an irreversible color change from blue to red occurs as a result of the disruption of the planar conjugated structure. By introducing PCDA vesicles in the hydrogel, we found that both the hydrogel and the dried objects can reproduce all the color changes of PDA vesicles (fig. 3c). The easy processibility and functionality of mineral plastic are important for its industrial application.
From the above descriptions, we know that mineral plastics combine the advantages of biomaterials and conventional plastics. First, comparing with biominerals, the shape and structure of the hydrogel can be designed. Second, comparing with conventional plastics, mineral plastics are economic, environmentally friendly, biodegradable and easily recycled. For example, in case of a crack in the dry plastic material, simple addition of a drop of water will transform it to the gel state, which will self-heal the crack and subsequent drying yields the intact plastics again. Or addition of more water will allow complete reshaping of the gel to a new shape after use of the first plastic material. Simple addition of an acid like citric acid or acetic acid will decompose the carbonate and the whole plastic material will dissolve in case it shall be completely removed. Third, the preparation of mineral plastics is extremely simple and can just be performed by mixing in water in any vessel. The production process of the hydrogel can directly be adapted by the industry, especially since the source materials are industrially produced at low cost. Simple processing steps with the gel can allow for multiple applications of the dry plastics. For example, any shape can be casted in a mold, a foil can be extruded or by simple freeze drying, a porous plastic material can be obtained and its porosity utilized for various applications.
Although the road to practical applications of mineral plastics remains long, we can imagine how the material can be used in the future. Housings for electronic equipment or any other plastic application in the dry state could be realized with mineral plastics. Mineral plastics will also be useful for all kinds of applications, where a plastic part is spontaneously needed in a certain shape for example as (temporary) replacement of a broken part since mineral plastics can be easily synthesized by simple mixing of the components in water at room temperature. There may be also applications in lifestyle or decoration products since mineral plastics can be colored and shaped easily. Besides, the gel itself might find applications in cosmetics or could be loaded with agrochemicals, which would be easy to deliver in the dried granular state and only swell upon application in the field without environmental concerns.
Finally, we believe that there will be a family of materials for “mineral plastics” possessing similar properties. The next step of our work will be exploring more possibilities of “mineral plastics” in terms of suitable systems and thus range of chemical and physical properties. We would also like to explore the range of applications of this new plastic material in its different forms like casted parts, porous plastics, films etc. The use of biodegradable binder polymers such as polyaspartic acid could also be advantageous, since it might lead to the development of entirely environmentally friendly plastic-like materials.
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Prof. Dr. Helmut Cölfen
University of Konstanz, Germany
Dr. Shengtong Sun
School of Chemical Engineering
East China University of Science and Technology