With the massive use of plastic packaging, it poses a serious threat to the environment and the sustainability of petroleum resources. The development of green, safe and biodegradable bio-based packaging materials is crucial. As a traditional packaging material based on lignocellulose, paper products have the advantages of good biodegradability, low cost and scalable production, and thus can be a powerful alternative to plastic packaging. However, paper products also have the inherent disadvantages of large paper surface pores and poor barrier properties. Therefore, further improvement of the barrier properties of paper products is a necessary way to expand the application areas of paper-based packaging materials. Lignin consists of three different phenylpropane units, coumaryl alcohol, coniferyl alcohol and mustard alcohol, which are called p-hydroxyphenyl (H), guaiacyl (G) and silylene (S) lignin precursor monomers, respectively, and are randomly bonded together by strong C-C and C-O bonds.
Application of lignin in barrier packaging paper
Lignin can be developed as a functional component of barrier packaging materials due to its unique properties such as hydrophobicity, flame retardancy, UV-blocking and antioxidant properties. Due to the conjugation of functional groups such as aryl, phenolic hydroxyl, ketone and carboxyl groups within the lignin molecule, lignin has better UV radiation resistance.
Oxygen barrier
Paper itself has large pores, which makes it difficult to block oxygen penetration. The reticulated dense structure of lignin molecules and the filling effect on the pore space of the fiber make it have better performance of inhibiting oxygen penetration.
Waterproof
As a structurally complex and inexpensive natural polymer, lignin consists of a number of different functional groups with a wide range of polarities throughout its structure, so that lignin has the potential to be either hydrophobic or hydrophilic, e.g. lignosulfonates are hydrophilic, whereas lignin sulfates are hydrophobic. The factors affecting the hydrophobicity of lignin-based barrier paper are mainly hydrogen bonding, micro- and nano-roughness of paper surface, in addition, the stability of hydroxyl groups in lignin affects the reactivity of lignin and solubility in water. Therefore, the modification of lignin through the modification of groups or the generation of new groups and the construction of micro- and nano-composite structures to improve the barrier properties of packaging paper have broad application prospects.
water vapor barrier
The hydrophobic properties of lignin attenuate the movement of water molecules on the surface of cellulose paper, thereby reducing the water vapor transmission rate (WVTR) of the paper. In addition, lignin also reduces the interaction between cellulose paper and water molecules, preventing the cellulose paper from wetting and swelling due to water absorption, and maintaining the material's water vapor barrier performance in high humidity environments.
Grease Barrier
Currently, most of the methods for constructing oleophobic interfaces are complex and expensive. Due to its unique physicochemical properties, lignin is effectively coated on the surface of paper to provide low surface energy on the surface of the paper, thus realizing the effect of oleophobicity.
flame retardant
The aromatic ring structure present in the molecular structure of lignin gives it a good charring ability, and the continuous and tight coke layer produced in the process of charring avoids the direct contact between the fire and the paper base, which is conducive to flame retardancy.
Lignin is abundant, inexpensive, green and biocompatible. Therefore, it is economically and technically feasible to apply lignin in the field of bio-based barrier packaging materials, especially in the field of food packaging where food quality and safety requirements are more stringent. At the same time, the development of lignin-based functional materials to achieve its industrial application in the barrier packaging paper, to reduce the consumption of fossil resources, to promote industrial upgrading of the pulp and paper industry, to achieve the dual-carbon goals are of great significance.
In addition, the development of lignin-based barrier packaging paper should also consider the following issues: (1) further improve the understanding of the molecular structure of lignin by using different technical analysis means; (2) establish a database to analyze the influence of lignin on the barrier properties of wrapping paper materials under different process conditions; (3) explore the preparation methods and applications of new green functionalized lignin materials applicable to the field of high-barrier packaging.
