Melt-processing and properties of thermoplastic composites based on ethylene-acrylic acid copolymer reinforced with wood nanocellulose

Doctoral thesis, 2020

The societal efforts to move towards a more sustainable future has resulted in the search for bio-based, biodegradable and renewable replacements among which cellulose is also being explored as a possible option. The main source of cellulose are plants and trees, with a global annual synthesis of 100 billion tons, and hence abundantly available in nature. Cellulosic materials are also biodegradable, renewable, inexpensive and have been used as structural material for centuries while still covering a large part of the industries like paper, forest products, textiles, etc. Sweden has always used cellulose as raw materials in various applications and with the stricter regulations from EU, the industries are opting for more sustainable materials/solutions. This has worked in the favour of wood-based materials which can be used as an inexpensive reinforcing material in thermoplastic composites.

A composite material contains two (matrix and reinforcement) or more (additives) components that are combined to obtain a material with properties different than the individual components. Wood is nature’s composite where lignin is the matrix and cellulose are the reinforcement. The reinforcement provides structural integrity and improves the properties, which is exactly what cellulose does in trees and plants.  In wood-polymer composites, the cellulose has been used as reinforcement, for decades, to strengthen the polymer matrix. However, an interest has been rekindled due to advancements in cellulose production technology which helps in commercially obtaining nano-sized cellulose reinforcements. Here, the nanocellulose is expected to improve the reinforcing capabilities more than the larger fibers, due to their excellent mechanical properties, resulting in composites with high strength and stiffness.

Despite the favourable properties of nanocellulose, it has a major drawback when used as reinforcement in thermoplastics, due to its relatively hydrophilic nature when compared to the usually hydrophobic polymer matrix, which drastically affect the properties. In addition, the main method of producing nanocellulose composites has been through laboratory scale methods and to make the production of nanocellulose composites commercially interesting  on an industrial scale, the feasibility with conventional melt processing technique should be considered. The results from this work helps us improve the understanding of melt processing of cellulose nanocomposites and highlight the importance of process details. It also highlights the possibility of upscaling the production while analysing its impact on nanocellulose modification and the different types of melt processing techniques. In this work, depending on the type of processing method and the hierarchical structure of the cellulose reinforcements used, the composites exhibited an increase in stiffness of up to 21 times than that of the polymer matrix.