Bioplastics from Biomass - Acetylation of Xylans with Green Chemistry
There is social, environmental and increasing economic pressure on the industrial sector to substitute non-renewable resources with renewable ones as the increasing World population is exponentially depleting the fossil fuel supplies of the Earth. Each year about 260 million tons of plastics are produced from crude oil and most of it ends up as waste. Producing biodegradable plastics from renewable resources could be a contribution to a sustainable development. Hemicelluloses are the second most abundant biopolymers on Earth, with about 60 billion tons biosynthesized each year by plants. Xylans are the largest group of hemicelluloses and were designed by nature to primarily function as a matrix in plants. So far, they are unutilized valuable biopolymers with broad potential applications. Many xylans are acetylated in nature, and the degree and pattern of acetylation influences the material properties of the plant cell wall. Controlled chemical esterification is a powerful tool for tailoring the structure and material properties of hemicelluloses as a renewable raw material for bio-based plastic production.
First, chemical acetylation was carried out on corncob arabinoxylan (CCAX), and the films were then compared with the unmodified pure CCAX films. There was clear improvement of the thermal and water resistant properties of the xylan films after acetylation. In following work, the structure-property analysis showed a positive effect of arabinose side chains on the elongation at break, the thermal stability and glass transition temperature of the acetylated xylan.
To make hemicellulose acetylation more sustainable, green chemistry was applied. The first approach included the utilization of ionic liquids (IL) as reaction media. Rye AX and spruce AGX were both fully acetylated while maintaining a high degree of polymerization (DP) in a very fast reaction. Another green chemistry alternative is to use enzymes. In this thesis the surfaces of rye AX films were acylated with acetate and stearate using lipases and cutinases in a water-free environment. The advancing contact angle of the surfaces was increased, the stearated surface being most hydrophobic.
Finally, AcAX was combined with spruce nanofibrillated cellulose (NFC) to form films. The thermal properties, stiffness and stress at break of the composite were superior to the neat AcAX films. The uniqueness of this composite is the water resistance and high elongation at break, even at a 10 % NFC content.
This work is a contribution to the designing of novel xylan based materials and their feasible and environmentally friendly production. Acetylated arabinoxylans have a potential to replace many of the oil based plastics and become a future bioplastics.