High barrier properties of nanocellulose: mechanism, modification and application

Nanocellulose , as an emerging bio-based nanomaterial, has shown great application potential in the field of barrier materials due to its unique structure and excellent properties. Based on the structural characteristics of nanocellulose, this paper deeply explores the mechanism of its high barrier performance, analyzes the strategies to improve its barrier performance through chemical modification and physical recombination, and summarizes its application prospects in food packaging, pharmaceutical packaging and electronic device packaging. Finally, the future research direction and development challenges of nanocellulose high-barrier materials are prospected.

1. Introduction

Barrier materials have important applications in packaging, electronics, medicine and other fields. Their main function is to block the penetration of gases (such as oxygen, carbon dioxide), water vapor, oil and other substances. Traditional barrier materials mostly rely on petroleum-based polymers, but their non-degradability and environmental problems have prompted researchers to turn to the development of green and sustainable alternative materials. As a renewable nanomaterial derived from natural cellulose, nanocellulose has become an ideal candidate for high barrier materials due to its high specific surface area, high crystallinity, excellent mechanical properties and adjustable surface chemical characteristics.

2. Structural characteristics of nanocellulose

Nanocellulose mainly includes cellulose nanofibers (CNF), cellulose nanocrystals (CNC) and bacterial cellulose (BC), and its structural characteristics have a decisive influence on its barrier properties:

The diameter of nanoscale size and high specific surface area
nanocellulose is usually between 1-100 nanometers, with a length of up to several microns, and has an extremely high specific surface area (up to hundreds of m²/g). This nanoscale size allows it to form a dense network structure that effectively extends the diffusion path of gas and water vapor.

The crystallinity of high crystallinity
nanocellulose is as high as 50%-90%. The crystallization area is composed of highly ordered cellulose molecular chains. The molecules are closely bound by hydrogen bonds to form a dense barrier structure, which can effectively block the penetration of small molecule substances.

Surface Chemical Characteristics
Nanocellulose surface is rich in hydroxyl groups (-OH), making it hydrophilic and chemically modifyable. Through surface modification, its surface energy, hydrophobicity and interface interaction can be regulated, thereby optimizing its barrier properties.

3. Mechanism of high barrier properties of nanocellulose

The high barrier properties of nanocellulose are mainly based on its unique structure and physical and chemical characteristics. The specific mechanism includes the following aspects:

3.1 Gas barrier properties

The high barrier properties of nanocellulose on gases such as oxygen and carbon dioxide are mainly attributed to its dense network structure and high crystallinity. The diffusion path of gas molecules in nanocellulose membranes is hindered by nanofibers, and the diffusion coefficient is significantly reduced. In addition, the highly crystallized regions of nanocellulose are closely arranged and have strong intermolecular forces, which further limits the permeability of gas molecules.

3.2 Water vapor barrier properties

Nanocellulose itself is hydrophilic and has relatively weak barrier properties to water vapor. However, its water vapor transmittance (WVTR) can be significantly reduced by surface chemical modification (such as acetylation, silanization) or in combination with other hydrophobic materials. For example, compounding nanocellulose with polylactic acid (PLA) can form a dense interface structure that effectively blocks the penetration of water vapor.

3.3 Oil barrier properties

The dense network structure and high mechanical strength of nanocellulose make it have excellent barrier properties to greases. Its nano-scale fibers can effectively prevent the penetration of oil and fat molecules while maintaining the flexibility and mechanical properties of the material.

4. Modification strategies to improve nanocellulose barrier properties

In order to further improve the barrier properties of nanocellulose, researchers have developed a variety of modification strategies, mainly including chemical modification and physical composite methods.

4.1 Chemical modification

Chemical modification is to regulate its performance by changing the chemical structure of the nanocellulose surface. Common chemical modification methods include:

Acetylation: The hydrophilicity of nanocellulose is reduced by introducing acetyl groups and its barrier properties to water vapor are improved.

Silenization: Use silane coupling agent to introduce hydrophobic groups on the surface of nanocellulose to enhance their compatibility with hydrophobic matrix materials.

Oxidation modification: Carboxylic groups are introduced on the surface of nanocellulose through TEMPO oxidation, enhancing its dispersion and interface interaction.

4.2 Physical Compound

Physical composite is to combine nanocellulose with other materials (such as polymers, nanoparticles) to improve barrier performance using synergistic effects. Common composite systems include:

Nanocellulose/polymer composite materials: Combine nanocellulose with polymers such as polylactic acid (PLA), polyethylene (PE) to form a dense interface structure, which significantly improves the barrier properties of gas and water vapor.

Nanocellulose/nanoparticle composite material: Combine nanocellulose with nanoparticles such as montmorillonium and graphene, and use the sheet structure of the nanoparticles to further extend the diffusion path of gas molecules.

4.3 Multi-layer structure design

By designing a multi-layer composite film, combining nanocellulose with other barrier materials (such as aluminum foil, polyvinylidene chloride), the advantages of each layer can be fully utilized to achieve efficient barriers. For example, coating a layer of hydrophobic polymer on the surface of the nanocellulose membrane can significantly reduce the water vapor transmittance.

5. Application of nanocellulose high barrier materials

5.1 Food packaging

The high gas barrier properties of nanocellulose can effectively extend the shelf life of food and reduce the use of preservatives. For example, packaging for meat, fruits and vegetables and dairy products can block oxygen and water vapor and keep food fresh.

5.2 Medical packaging

In the pharmaceutical field, nanocellulose membranes can be used in drug packaging to prevent the drug from getting damp or oxidizing, and ensure the stability and effectiveness of the drug.

5.3 Electronic device packaging

The high barrier properties of nanocellulose make it an ideal choice for flexible electronic device packaging materials, which can effectively prevent moisture and oxygen from eroding the device.

5.4 Environmentally friendly materials

Nanocellulose is derived from renewable resources and is biodegradable. It can be used to replace traditional petroleum-based plastics and reduce environmental pollution.

6. Challenges and future prospects

Although nanocellulose has great potential in the field of barrier materials, it still faces some challenges:

Cost issue: The preparation cost of nanocellulose is high and a more economical production process is needed.

Large-scale production: How to achieve efficient and large-scale production of nanocellulose still needs further research.

Performance optimization: The barrier properties and mechanical properties of nanocellulose are further improved through modification and composite technologies.

In the future, with the continuous development of nanotechnology, materials science and green chemistry, nanocellulose high-barrier materials are expected to be commercially applied in more fields, providing new solutions for sustainable development.

7. Conclusion

Nanocellulose is becoming a hot topic in the field of barrier materials with its unique structure and excellent barrier properties. Through chemical modification and physical composite, its barrier performance can be further improved to meet the needs of food packaging, pharmaceutical packaging, electronic device packaging and other fields. In the future, with the advancement of technology and the reduction of costs, nanocellulose high-barrier materials will play an important role in green environmental protection and sustainable development.