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Nanoplastics refer to resin-based nanocomposites formed by metal, inorganic non-metal or polymer materials dispersed in a resin matrix in nanometer size. Layered silicates are mainly derived from natural de-fogging soil, so they are also called polymer/clay nanocomposites.
According to the different nanomaterials added, nanoplastics can be divided into: inorganic nanoplastics (including: filled nanoplastics, layered nanoplastics), organic nanoplastics and metal nanoplastics, which can be prepared by intercalation composite method, in-situ polymer method, gel-gel method, blending method and other methods.
Because of its high strength and heat resistance, high barrier and self-extinguishing properties, excellent processability and low price, nanoplastics are widely used in various high-performance industrial and agricultural pipes, automobile and mechanical parts, electronic and electrical components and other fields. At the same time, nanocomposites with excellent barrier properties have great potential in the packaging material market for food, especially beer packaging, meat and cheese products. Studies have shown that nanoplastics may cause growth deformities, Parkinson's disease lesions, and there is a risk of invading human cells and major organ tissues, which is harmful to human health.
Nanoplastics can be divided into the following categories according to the different nanomaterials added:
Inorganic nanoplastics refer to inorganic fillers dispersed in a polymer matrix in nanometer size.
Filled nanoplastics refer to inorganic nanopowders filled into a polymer matrix. The research on this type of nanoplastics started with nano-CaCO3 plastics. Currently, the nano-CaCO3 plastics that have been developed include:
Nano-CaCO3 added to high-density polyethylene (HDPE) materials. The test results show that when the CaCO3 content is 25%, its toughening effect is the best. After treatment with titanate coupling agent, the toughening effect of plastics is more obvious, and its maximum impact strength is 70% higher than that of pure HDPE. After the treated nano-CaCO3 system, even if the filler content is high, the nano-inorganic particle HDPE plastic still has good processing properties.
Nano-CaCO3 (particle size of 30nm) was added to polyvinyl chloride (PVC). The test results show that with the increase of the amount of nano-CaCO3, the tensile strength of the material increases, reaching the maximum value (58Mpa) at a content of 10%, which is 123% of pure PVC (47Mpa), while micron-level CaCO3 has no obvious strengthening effect. When the content is 10%, the impact strength of nano-plastic can reach 313% of pure PVC.
Nano-CaCO3 was added to polypropylene (PP), and the study found that the tensile strength showed a trend of first increasing and then decreasing with the increase of nano-level CaCO3 content. When its content was 4%, the tensile strength reached the maximum value. However, micron-level CaCO3 has no obvious strengthening effect on the tensile strength of the material. In addition, the toughening effect of nano-PP on the notched impact strength and unnotched impact strength of the material is very obvious, reaching the maximum value when the nano-level CaCO3 content is 4%.
Layered nanoplastics refer to inorganic nanomaterials as sheets, polymers inserted between nanosheets or sheets dispersed in polymer matrices. This type of material mainly refers to silicates, such as montmorillonite, kaolin, and diatomaceous earth. Montmorillonite is currently the most widely used, which is composed of lamellar crystals with a crystal thickness of about 1nm, a distance between lamellar layers of about 1nm, and a length of about 100nm. This type of nanoplastic has been studied: nylon 6 nanoplastics. The study found that when the amount of montmorillonite added was less than 10%, the strength of the material still increased significantly, and greatly exceeded the increase in traditional blended composite materials. Its heat deformation temperature increased from 65¡æ of nylon 6 to 152¡æ. Studies using wide-angle X-ray scattering (WAXD) and small-angle laser scattering (SALS) have shown that the addition of montmorillonite also plays a heterogeneous nucleation role, which increases the crystallization temperature of nylon 6 (PA6) and reduces the supercooling crystallinity. Moreover, the organic clay completely destroyed the spherulite structure of nylon 6, but the crystallinity of nylon 6 remained basically unchanged. When the spherulite size is controlled to be smaller than the wavelength of visible light, the intercalated material can have good air permeability and can be used for automobile engine parts. When used as a structural material, this nylon 6/montmorillonite nanoplastic has the characteristics of high specific strength and a weight reduction of 25%.
Due to the simple preparation method, wide source of materials and many types of applicable polymers of layered nanoplastics (i.e. silicate nanoplastics), this type of nanoplastics has attracted much attention and is extremely active in research.
Organic nanoplastics are formed by adding nano-organic materials to a polymer matrix, and most of these organic substances are liquid crystals. For example, adding 10% of thermotropic liquid crystal polymer to polyimide (PI) can increase the elastic modulus from 1.7Gpa to 6.9Gpa and the tensile strength from 125Mpa to 470Mpa. 5% poly(p-phenylene terephthalamide) (PPTA) is blended with nylon 6 (PA6) liquid to obtain a molecular composite material in which PPTA microfibers (diameter 15~30nm, length about 600nm) are dispersed in the PA6 matrix. A small amount of PPTA microfibers are also used as a reinforcing agent to form a molecular composite material with thermosetting polyimide (PI), PVC, ABS, ionomers, etc. with the help of solvents.
Metal nanoplastics refer to the addition of nanometal powders to polymers. For example: adding nano Cu to polyoxymethylene (POM) to improve the plastic's resistance, or adding other nanometal powders to polypropylene (PP) to improve the plastic's conductivity and mechanical strength.
Intercalation composite technology can realize the composite of organic matrix and inorganic dispersed phase at the nanoscale. The resulting nanoplastics can perfectly combine the rigidity, dimensional stability and thermal stability of inorganics with the toughness, processability and dielectric properties of polymers. The amount of montmorillonite in nanoplastics is relatively small, generally below 10wt.%, usually only 3-5wt.%, but its rigidity, strength, heat resistance and other properties are comparable to those of conventional glass fiber or mineral-filled reinforced composite materials (filling amount of about 30wt.% or even higher). Therefore, nanoplastics have a low specific gravity, high specific strength and specific modulus without losing their impact strength, which can effectively reduce the weight of products and facilitate transportation. At the same time, since the size of nanoparticles is smaller than the wavelength of visible light, nanoplastics have high gloss and good transparency.
Due to the good combination of the polymer matrix and the montmorillonite sheet, by controlling the planar orientation of the nano-silicate sheet, nano plastic products show good gas (including water vapor) barrier properties. For example, the oxygen permeability of nylon 6 nanocomposite (containing 2wt.% montmorillonite) is reduced by half compared with pure nylon 6, and the water vapor permeability is reduced by more than one-third. Some nano plastics also have flame retardant and self-extinguishing properties. In addition, due to the addition of nano-sized inorganic particles and the close combination between the filler and the matrix, the size stability of nano plastics is much better than that of ordinary plastics.
Nano plastics have high melt strength, fast crystallization speed, and low melt viscosity, so they have excellent processing properties for injection molding, extrusion and blow molding. In particular, extrusion-grade and injection-molding-grade nano ultra-high molecular weight polyethylene solves the problem of ultra-high molecular weight polyethylene processing. The improvement of processing properties is of great significance to the practical application of nano plastics.
The preparation methods of nanoplastics are summarized as intercalation composite method, in-situ polymer method, gel-gel method, blending method, etc.
Intercalation composite method is the main method for preparing nanoplastics at present. Intercalation composite method is to intercalate polymer monomers or polymers into the lamellar structure of silicate (such as montmorillonite, mica, vermiculite, etc.) sheets, thereby destroying the lamellar structure of silicate, peeling it into basic units with a thickness of 1nm, a length and width of 100~1000nm, and making it evenly dispersed in the polymer matrix, so as to realize the composite of polymer and lamellar silicate at the nanoscale. There are generally two ways of intercalation composite method: ¢Ù Intercalation polymerization method, that is, first disperse the polymer monomer, insert it into the layered silicate sheet, and then polymerize it in situ, and use the large amount of heat released during polymerization to overcome the Coulomb force between the silicate sheets and peel them off, so that the nano-sized silicate sheets are compounded with the polymer matrix in the form of chemical bonds. ¢Ú Polymer intercalation method, that is, mix the polymer melt or solution with the layered silicate, and use mechanical chemistry and thermodynamics to peel the layered silicate into nano-sized sheets and evenly disperse them in the polymer matrix.
The in-situ polymerization method is to use in-situ filling to evenly disperse the nanoparticles in the monomer, and then perform polymerization reaction, which not only realizes the uniform dispersion of the filled particles, but also ensures the nano characteristics of the particles. In addition, in the in-situ filling process, the polymer matrix is only polymerized once, and no thermal processing is required, which avoids the resulting degradation, thereby ensuring the stability of various properties of the matrix.
This method has been used since the late 1980s. It is to dissolve precursors such as silane metal oxides in water or organic solvents, and the solute is hydrolyzed to generate nano-scale particles and form a sol, which is then evaporated and dried to form a gel. The characteristic of this method is that it can be carried out under mild reaction conditions, the two phases are evenly dispersed, and can even reach the level of "molecular composite". The biggest problem is that during the drying process of the gel, due to the volatilization of solvents, small molecules and water, shrinkage stress will be generated inside the material, which may cause the material to crack. Despite this, the sol-gel method is still one of the most widely used and more complete methods.
The blending method is to directly disperse and mix various inorganic nanoparticles (including fiber tubes) with polymers. The characteristics of this type of method are that the process is relatively simple and easy to industrialize. Its disadvantage is that it is difficult to evenly disperse the nanoparticles at the original nanoscale, which brings new problems to the stability of the product. For this purpose, the following different processes have also been developed, such as solution blending, emulsion blending and melt blending.
In nano-plastic composite materials, the size of the added filler dispersed phase is less than 100nm in at least one dimension. Due to the nano-size effect, surface effect and strong interface bonding of the dispersed phase, nano-plastics have microstructures and properties that matrix plastics and micron-level plastic composites do not have, such as high strength, antistatic properties, radiation protection and antibacterial properties. It is a new high-performance material with broad application prospects.
For example, compared with general micro-composite materials, nano-plastics containing a small amount of montmorillonite show excellent comprehensive properties, so they are lighter than conventional filled composite materials. Good performance combination, simple processing technology and low price make nano-plastics widely used in various high-performance industrial and agricultural pipes, automobile and mechanical parts, electronic and electrical components and other fields. At the same time, nano-composite materials with excellent barrier properties have great potential in the packaging material market for food, especially beer packaging, meat and cheese products.
Researchers at the Leiden Institute for Biological Research in the Netherlands used chicken embryos as a model to study the possible extreme effects of polystyrene nanoparticles and found that nanoplastics can cause malformations. The researchers used high concentrations of polystyrene particles, which are not usually present in organisms. But it shows the effects of nanoplastics on very young embryos under extreme conditions. The researchers observed malformations in the nervous system, heart, eyes and other parts of the face.
Researchers at Duke University in the United States said in the journal Science Advances that nanoplastics interact with a specific protein naturally present in the brain, producing changes associated with Parkinson's disease and certain types of dementia.
A new study published in the Proceedings of the National Academy of Sciences on January 8, 2014 pointed out that researchers at Columbia University used a new technology to determine, calculate and analyze the chemical structure of nanoparticles in bottled water. The study found that bottled water contained 10 to 100 times more plastic particles than previously thought. The study used a new technology that can detect nanoplastics in bottled water. The study found that each liter of bottled water contained an average of 240,000 plastic particles, 90% of which were nanoplastics. Nanoplastics can invade human cells and major organ tissues, and are very harmful to human health.
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