In recent years, with the increasing consumption of fossil resources and increasingly severe environmental pollution problems, countries have successively introduced "plastic restriction orders" and "plastic ban orders", and the development and use of biodegradable polymer materials has become an inevitable trend. By introducing nano cellulose with excellent structure and good physical and chemical properties, the strength, thermal stability and barrier properties of degradable polymer materials can be improved or enhanced, so as to design and develop degradable nanocomposites with excellent properties. This article mainly introduces two kinds of nanocellulose and their preparation methods, expounds the reinforcement mechanism of nanocellulose and the construction method of composite materials, and focuses on the strengthening of cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF). Research progress in polylactic acid (PLA), thermoplastic starch (TPS) and polycaprolactone (PCL).

1. CNC and CNF enhancement mechanism

Nanocellulose mainly enhances the related properties of composite materials by forming hydrogen bonds, percolating networks, chain entanglement and promoting crystallization. The CNC with high crystallinity can act as a nucleating agent when it is uniformly dispersed in the polymer matrix, providing nucleation sites to promote crystallization, thereby improving the relevant properties of the composite material. The slender and filamentous CNF can be entangled with polymer chains to form a stable three-dimensional network structure, which significantly improves the impact resistance and toughness of the composite material. In some hydrophobic or low-polar polymers, nanocellulose is prone to agglomeration due to intermolecular hydrogen bonding, thereby reducing the reinforcement effect. Therefore, surface modification or modification of nanocellulose is needed to improve its dispersion and compatibility in the matrix.

2. Composite material construction method

Nano-cellulose reinforced biodegradable polymer composite materials are mainly divided into liquid medium processing method and dry melting compound method, which can be further subdivided into solution casting method, electrostatic spinning, layer-by-layer assembly method and melt compounding. Among them, the two most commonly used are the solution casting method and the melting compound method. The solvent casting method is mainly suitable for laboratories or small-scale research, and is not suitable for expanding production. Melt compounding method is usually used in the processing of thermoplastic polymers, suitable for large-scale industrial production, and the prepared composite material has higher strength than the solution casting method.

Figure 1 Different preparation methods of nanocellulose-reinforced polymer nanocomposites

3. Nano cellulose reinforced biodegradable polymer composites

The key to preparing high-performance biodegradable nanocomposites is to transfer or synergize the excellent properties of nanocellulose into the surrounding polymer matrix. This requires nano cellulose to be fully and uniformly dispersed in the polymer matrix.

3.1 Nano cellulose/PLA composite materials

PLA is a thermoplastic aliphatic polyester with tensile strength, elastic modulus and transparency comparable to traditional petroleum-based polymers. However, PLA is relatively brittle, has poor thermal stability, and has low gas barrier properties, which restrict its application. Nanocellulose limits its loading in PLA due to its inherent hydrophilicity, and its dispersibility and compatibility are poor. Modification of nano cellulose through surface adsorption, chemical modification and polymer graft copolymerization can improve the dispersibility of nano cellulose and achieve a significant enhancement effect.

Table 1 Modification of nano cellulose and its effect on the properties of PLA composites




3.2 Nano cellulose/thermoplastic starch composite material

Thermoplastic starch (TPS) is a kind of thermoplastic polymer material formed by starch granules ruptured and plasticized through chemical modification or plasticizer as raw material. Its production process is simple, low-cost, and biodegradable, but its mechanical properties are poor and it has strong water absorption. Nanocellulose and TPS are both hydrophilic substances with good affinity. The strong hydrogen bond between them makes the interface compatible, and limits the water swelling of TPS, reduces the moisture absorption performance, and significantly improves its mechanical properties. performance.


Table 2 The influence of nano cellulose on the properties of starch-based composites




3.3 Nanocellulose/PCL composite materials

PCL has good biocompatibility and extremely high ductility, but its tensile strength and elastic modulus are very low. The interface compatibility between the hydrophilic nanocellulose and the hydrophobic PCL matrix is poor, and the direct blending of nanocellulose and PCL does not have a good effect. However, by modifying the nanocellulose, the mechanical properties and thermal stability of the composite material can be significantly enhanced.

Table 3 The influence of nano cellulose on the properties of PCL-based composites



4. Summary and Outlook

With the continuous research and development of nano cellulose reinforced biodegradable polymers, renewable and biodegradable nano cellulose as a reinforcing medium can make the biodegradable polymer catch up with or even exceed the traditional in terms of mechanical properties, barrier properties, etc. Petroleum-based polymer. This kind of composite material with excellent properties will be widely used in many fields.