Biopolymers in elastomers: lignins as biofiller for tyre compound

Lignocellulosic biomass is a natural complex composite of cellulose, hemicellulose, lignin, ashes and other soluble substances called extractives. The significant difficulties related to the separation of lignin-carbohydrates complexes are the major obstacle to overcome for lignocellulosic biomass utilization. In order to free the locked polysaccharides in cellulose, a number of lignocellulose pretreatment technologies is under intensive investigations, such as steam explosion, organosolv process, chemical treatment with acids or bases (ammonia, NaOH) and ionic liquid pretreatment. The relevance of lignocellulosic biorefinery relies not only on the recovery of carbohydrates, but also on the added value of lignin which is the second most abundant natural polymer, exceeded only by cellulose and hemicellulose. Lignin’s structure is determined by its botanical origin and the adopted isolation process. Depending on the plant source, lignins can be divided into three classes: hardwood (angiosperm), softwood (gymnosperm) and annual plant (graminaceous); on the other hand, according to the isolation process, lignins can be divided into two groups: lignin from sulfite process and sulfur free lignin. The latter is receiving increasing attentions because it offers a greater versatility than the former and it can be heat-processed avoiding the irritating odor-release commonly associated with commercial kraft lignin. In addition to cost advantages, annual renewability and huge availability are factors that could promote the use of sulfur-free lignin. Lignin’s structure contains a variety of chemical functional groups that affect its reactivity making it able to meet the needs of industry. It’s worth noting that lignin can be used for several industrial applications owing to its surface-active properties. It has also been applied as a filler in many elastomers (butadienestyrene- butadiene, isoprene-styrene-butadiene; styrenebutadiene) or in natural rubber. Moreover, lignin has shown a high antioxidant efficiency both as it is and in combination with commercial antioxidants. The main purpose of my doctorate has been testing sulfur free lignins (obtained from herbaceous plants as by-products of steam explosion and soda pulping processes) as fillers in rubber compounds in order to evaluate their reinforcement ability and their use as a partial replacement of carbon black. The objective is to realize lighter tyres characterized by a low rolling resistance and a reduced amount of material derived from no renewable sources. Lignin has some disadvantages that make its application as a rubber-reinforcing filler difficult, such as large particle size, strong polar surface and high tendency of its particles to link together by intermolecular hydrogen bonding arranging agglomerates. To improve the interaction between filler and elastomer, two strategies have been adopted: the chemical modification of lignin and the reduction of the size of its particles. Concerning the chemical modification, lignin can be functionalized by way of esterification, etherification, reaction with coupling agents (silane) and with hexamethylenetetramine (HMT), so that also its dispersion in the elastomer is improved. Instead, spray drying and the co-precipitation of latex with lignin have proved to be effective in reducing the particles’s size. All the products obtained have been characterized by IR, 31P NMR, GPC and microscope analysis and tested in rubber compounds as it is or as partial replacement of carbon black.