Polyolefin is a general-purpose resin with excellent comprehensive performance and wide application. Because of its many excellent characteristics, its development is very rapid and its application is very common. Clay, as a source of abundant sources and low price in China, has also become one of the goals of scientific research. This article briefly summarizes the development of polyolefin / clay nanocomposites.
Polyolefin
Polyolefins are a class of polymer compounds obtained by the polymerization or oligomerization of olefins and certain cyclic olefins. Due to its simple processing, low production energy consumption, and abundant raw material sources, it has developed very rapidly, and its proportion in synthetic resins and plastics has increased year by year. By volume, polyolefin resin has surpassed steel and has become an indispensable material for humans. However, there are also shortcomings and shortcomings in its performance: for example, compared with engineering plastics, the tear strength is small and the hardness is small; the friction resistance, heat resistance and flame resistance are poor; the chemical resistance and environmental resistance drugs are poor [1]. In order to further improve the performance of the material, modifying it not only has high academic value, but also brings epoch-making significance to the upgrading and updating of traditional products [2]. Therefore, to solve various problems existing in existing polyolefin materials, research and development of new technologies for polyolefins with better performance, more advanced technology, lower cost, and no environmental pollution are important goals of the 21st century petrochemical industry. In today's academia, the engineering research of general-purpose plastics has become one of the research directions of polymer materials. The preferred method used in this field is the filling modification of polyolefin plastics. Adding fillers to polyolefins can improve the mechanical properties of the material and improve its processing properties, while also reducing costs [3].
2. Nanocomposites
The concept of nanocomposites was first proposed by Roy in 1984. It refers to a material in which the dispersed phase of the composite has at least one phase with a one-dimensional size reaching the nanometer level (1-100 nm). In recent years, the development of nanocomposites has been rapid. It is called "one of the most promising materials in the 21st century" and has received widespread attention from the scientific and technological circles, thus forming a wave of research on nanocomposites. The research of nanocomposites is more extensive and in-depth in the field of metals and ceramics, while the research of polymer nanocomposites started late, but it has developed rapidly in recent years, causing widespread concern in the field of polymer science [4].
3. Polymer nanocomposites
Polymer nanocomposites are new polymer composites in which the polymer is the matrix (continuous phase) and the inorganic particles are dispersed in the matrix at the nanometer scale (less than 100 nm). Compared with traditional composite materials, polymer nanocomposites have better mechanical and thermal properties than conventional polymer composites of the same composition due to the nano-effects brought by nanoparticles and the interface interaction between nanoparticles and the matrix. A new generation of high-performance, multi-functional composite materials provides the possibility [6].
According to the characteristics of each component of the composite material (layered silicate, organic cation and polymer matrix) and the preparation method of the composite material, three types of composite materials can be prepared: when the polymer cannot be inserted into the layered silicate Between the sheets, you get a phase-dispersed composite material, which is the traditional "micro-composite". When the polymer chain is inserted between the layered silicate sheets, a polymer / layered silicate alternates. The ordered multi-layer morphology results in "intercalated nanocomposite", and when the layered silicate sheet is completely uniformly dispersed in a continuous polymer matrix, the "peeled nanocomposite" is obtained. X-ray diffraction (XRD) and transmission electron microscopy (TEM) can distinguish between intercalated and exfoliated nanocomposites. Exfoliated nanocomposites are characterized by no XRD diffraction peaks, either due to the layer The spacing of the layered silicate layers is too large, or the layered silicate sheets are completely disordered. TEM can observe the morphology of the composite material, especially the completely disordered structure of the layered silicate sheets. In addition to the above-mentioned two types of nanocomposites with a clear structure, another type of intermediate structure is between intercalated and exfoliated nanocomposites, that is, intercalated structures and exfoliated structures exist at the same time. The XRD diffraction peak becomes wider, so it is necessary to determine the overall structure of the composite material in combination with TEM.
Various methods including "in-situ compounding" and the like can be used as the method for preparing the nanocomposite material. In recent years, the preparation of nanocomposites by in-situ compounding has become a relatively new subject in the field of materials science. The intercalation in-situ composite method is a typical in-situ composite method, which refers to the preparation method of inserting polymer monomers into the middle of the clay sheet to form a two-dimensional ordered nanocomposite during the polymerization process. Because the nano-scale clay dispersion sheet is formed during polymer polymerization, it is also called "in situ compound". Because the interface area between the dispersed phase and the matrix of the nanocomposite is large, the performance of the dispersed phase and the matrix can be fully combined, and the performance is greatly improved compared with the matrix material.
Because the particle size of nanoparticles is very small and the specific surface area is very large, about 100m2 / g, it has surface effect, volume effect, quantum size effect, and macroscopic quantum tunneling effect. Many excellent characteristics, such as corrosion and easy processing, make polymer nanocomposites exhibit many characteristics different from polymer composites. Nanoparticles not only significantly improve the strength, rigidity and toughness of polymers, but also because of their small size and good light transmittance, they can increase the density of plastics and improve the transparency, waterproofness, barrier properties and heat resistance of plastics. Sexual and anti-aging features. The polymer-based nanocomposite has the following characteristics: ⑴ lighter than traditional blends; ⑵ has excellent air tightness, can be repeatedly processed and used; ⑶ has good comprehensive properties (including mechanical properties, solvent resistance And thermal stability, etc.) [5].
4. Polyolefin / clay nanocomposites
In the study of polyolefin nanocomposites, the most researches using layered silicate as the dispersed phase are due to the in-depth study of the intercalation chemistry of layered silicate and the fact that layered silicate is easily available , The layered silicate used to prepare polyolefin / layered silicate nanocomposites belongs to the 2: 1 type layered silicate structure family, such as montmorillonite (MMT), hectorite, sepiolite, etc. . The crystal structure of MMT is shown in Figure 1.1. Their crystal lattice is a layered structure formed by an aluminum oxide (magnesium oxide) octahedral sandwiched between two silicon-oxygen tetrahedra by sharing oxygen atoms. Each structural unit has a thickness of about 1 nm, and its length and width range from It varies from 30nm to several micrometers, and the layers are combined by van der Waals force to form a van der Waals groove (also called layer gap). Since the trivalent aluminum in the aluminum-oxygen octahedral part of the 2: 1 type layered silicate unit cell is replaced by divalent magnesium isomorphism, the wafer is electronegative, so cations are adsorbed on the surface of the sheet to compensate for excess Negative charge to maintain electrical neutrality. The cations adsorbed in the layered silicate are mainly Na +, Mg2 +, Ca2 +, etc., and can be ion-exchanged. Due to the weak force between the layers, small molecules are easily inserted between the layers. The researchers used the advantages of clay as a nanocrystalline layer to enhance the polymer. However, there is a strong van der Waals force between the crystal layers of the clay. Under normal circumstances, the crystal layers are agglomerated and cannot reflect the nano-characteristics. Only the polymer is inserted between the layers, the spacing between the crystal layers is increased, and the clay crystal layer is evenly dispersed in the polymer, thereby preparing a nanocomposite material. However, the surface of the clay crystal layer is hydrophilic and cannot be directly intercalated with the molten polymer. Therefore, the clay must be organically modified.
Studies have shown that organic cations can also enter the layers through ion exchange. So that the hydrophilic montmorillonite has good compatibility with most polymers or monomers, this is the process of organic modification. In order to improve the compatibility of the layered silicate and polyolefin, certain organic cations (such as alkyl ammonium salts or alkyl phosphorus salts) can be used for ion exchange, and the organic layered silicate (abbreviated as Organic soil) The internal and external surfaces are changed from hydrophilic to hydrophobic, reducing the surface energy of silicate, so as to have better compatibility with organic polymers, and also increase the interlayer distance of silicate. Studies have shown that the increase in the interlayer distance depends on the ion exchange capacity of the organic soil itself and the length of the alkyl chain of the intercalator. There are three main structures of intercalation agent in organic soil: single-layer arrangement structure, double-layer arrangement structure and oblique arrangement structure. Vaia et al. Found different order degrees of intercalated chains by monitoring the asymmetric CH2 tensile and flexural vibration frequency changes. Through molecular dynamic simulation, the structural model of intercalated chains was given.
Filling polyolefin with clay has the following advantages:
â‘´ The content of clay is generally only 3% to 5%, but it can greatly improve the physical and mechanical properties of the material, and the filling amount of traditional reinforcing fillers such as SiO2 and carbon black reaches 20% to 60%.
⑵ Clay particles have an anisotropic sheet-like morphology and a highly consistent structure, thereby improving the barrier properties of solvents and other molecules of plastic products.
⑶ Under low stress conditions, it can improve the dimensional stability of plastic products.
â‘· Higher heat distortion temperature.
⑸ Nano-montmorillonite / thermoplastic polyolefin composite is easy to recycle, and its mechanical properties can be improved in the regeneration.
⑹ The surface of clay particles with colloidal properties is easy to chemically modify and can be successfully used in dyeing, printing and bonding.
⑺ Has antistatic and flame retardant properties.
â‘» The filler particles are small, and the surface of plastic products is more smooth [7 ~ 9].
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