Title : Biopolymer filled rubber compounds with applied low molecular weight plasticizers
Abstract:
The depletion of resources, global warming, and negative environmental impacts have increased awareness of the bio-economy and green technologies. Lignocellulosic raw materials are promising alternatives to petroleum-based products because they are abundant, sustainable, and contribute positively to reducing global greenhouse gas emissions. Lignin is the second most abundant biopolymer worldwide, offering significant potential for various applications. Each year, about 50 to 70 million tons of technical lignin are produced, but only 1 to 2% of this amount is used to create value-added products. The remainder is either landfilled or burned for energy generation or chemical recovery. Lignin has a highly branched, amorphous aromatic structure with a variety of functional groups such as carboxyl, carbonyl, hydroxyl, and methoxy groups. These groups give lignin desirable properties, including antimicrobial activity, antioxidant effects, UV absorption, adhesive qualities, and hydrophobicity.
Its high availability, eco-friendliness, biodegradability, and ability to reinforce make it an ideal candidate as a filler or component in rubber compounds to create innovative green composite materials. Through the augmentation of polar groups, electrostatic attraction, and hydrogen bonding, it can establish a connected network with certain polar polymers. However, due to the formation of strong intramolecular interactions between the macromolecular chains, its compatibility and adhesion with non-polar polymers tend to be poor. Therefore, the incorporation of lignin in its original, unmodified form into polymer compounds usually results in the deterioration in the physical–mechanical and utility properties of the final materials.
Different modification methods have been employed to enhance the dispersion of lignin within rubber matrices and to improve the compatibility between the two components. The results have demonstrated very promising prospects and pointed to the high application potential of lignin into rubber compounds. However, many of these modification processes are time-intensive and involve additional costs. In some cases, opting for a straightforward approach to increase the uniformity and compatibility of rubber blend components is more practical, especially for products manufactured on an industrial scale.
In this work, calcium lignosulfonate as a biopolymer component was incorporated into acrylonitrile-butadiene rubber in the amount of 50 phr (parts per hundred rubber). To improve the adhesion and homogeneity between the rubber and the biopolymer, three low-molecular-weight plasticizers were used, namely 1,4-butanediol, ethylene glycol, and glycerol. Those plasticizers are highly available and cost-effective and were incorporated into rubber compounds in the amount ranging from 5 to 30 phr. The results revealed that plasticizers softened both the rubber matrix as well as the biopolymer filler. The higher the polarity of the plasticizer, the higher the plasticizing effect on lignosulfonate. The plasticizing effect increased in the following order: 1,4-butanediol < ethylene glycol < glycerol. Softened lignosulfonate formed small soft filler like domains well distributed within the rubber matrix. Good compatibility and adhesion between the rubber and the biopolymer on their interface was observed leading to the enhancement in tensile characteristics of composites plasticized with ethylene glycol and glycerol.
