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The aim of this work is to elucidate the relationship between lignin main features and its behavior in natural rubber compounds, in particular focusing on thermal stability and mechanical properties. Five lignins obtained from different sources and through different extraction processes were characterized in terms of purity, sulfur content, molecular weight distribution (GPC), and qualitative and quantitative functional group distribution (FTIR and 31P NMR). Then, the lignins were incorporated in natural rubber by two different approaches, namely coprecipitation and dry-mixing. Thermal stability and mechanical properties of lignin/natural rubber blends were investigated in both masterbatches and vulcanized compounds. The oxidation induction time (OIT) was used to determine the thermal stabilization of the lignin/NR masterbatchs, while tensile stress-strain properties of the compounds were evaluated after vulcanization. It was found that differences in the chemical and morphological characteristics of lignin influence its antioxidant and reinforcement capability. The addition of lignin to vulcanized compounds demonstrated the possibility to improve mechanical properties hypothetically through a tandem mechanism of protection and reinforcement.

A biorefinery process was set up to simultaneously produce cellulose nanocrystals (CNCs) and an inorganic-organic material composed by silica and lignin starting from rice husk, a largely available agricultural side product. The opportunity to synthesize this new material, combining lignin and silica at the microscopic level, ensued naturally as the two substances were extracted simultaneously during the process. The main parameters influencing the yield, the composition, and the properties of the lignin-silica material (LSM) were optimized. The overall yield of the process was 35% (12% CNCs and 23% LSM). The sub-micrometric irregular particles constituting the LSM were approximately equally composed by lignin and silica. Larger batches were produced to assess the possibility to use the LSM as a reinforcing biofiller in elastomeric composites based on natural rubber. The lignin-silica material reinforced natural rubber (Ultimate elongation +17%, ultimate strength +198%). The overall effect was rationalized in terms of individual contributions of the two constituents: silica was supposed to contribute largely to the reinforcement, whereas lignin seemed to protect natural rubber from degradation, conveniently enhancing the elongation at break and lowering the total density of the biofiller.

In this work, the possibility to conveniently exploit lignin as a functional additive for natural rubber was pursued following two strategies. The first was based on the fractionation of lignin: extraction with organic solvents is suitable to produce lignin fractions with better defined structural features, molecular weight distributions, and physicochemical properties. The second approach was based on the chemical modification of lignin in the attempt to overcome its poor affinity with the rubber: esterification with anhydrides was selected to modify relatively large samples of lignin at laboratory scale. The effectiveness of different modifications of lignin as a drop-in replacement for carbon black was evaluated analyzing the tensile mechanical properties of model elastomeric compounds. In addition, the behavior of the modified lignins was rationalized through Hansen solubility parameters predicted with the group-contribution method.

Il lavoro di tesi si colloca nel più ampio progetto del Corimav intitolato materiali elastomerici da fonti rinnovabili. In particolar modo è incentrato sulla valorizzazione di substrati lignocellulosici a basso costo tramite l’ottenimento di prodotti ad elevato valore aggiunto che possono essere utilizzati in sostituzione degli ingredienti tradizionali all’interno di mescole elastomeriche per pneumatici o per l’introduzione di nuove proprietà. In una prima fase del lavoro mi sono occupato della messa a punto di un processo di biorefinery che ci ha consentito di frazionare i substrati di partenza in tre componenti potenzialmente interessanti: lignina, nanocellulosa e silice. Il processo risulta interessante per la possibilità di recuperare simultaneamente tutti e tre i prodotti, con un elevato grado di purezza e una resa discreta. In un secondo memento una modifica del processo di biorefinery ci ha permesso di preparare un materiale ibrido lignina-cellulosa potenzialmente estremamente interessante in quanto combina le caratteristiche della silice (filler di punta nei pneumatici di ultima generazione) e della lignina (rinnovabile, leggera, proprietà antiossidanti). Dopo uno screening iniziale delle proprietà in mescola dei prodotti ottenuti dal frazionamento dei substrati lignocellulosici mi sono concentrato sull’utilizzo della lignina, cercando di migliorare le proprietà meccaniche dei compositi con la gomma naturale. Il lavoro è proseguito su tre fronti: identificazione delle caratteristiche strutturali della lignina che migliorano le proprietà termiche e meccaniche dei composti elastomerici, miglioramento delle proprietà finali mediante modifica della struttura della lignina con diversi trattamenti (frazionamento, processi termici e modifiche chimiche) e ottimizzazione delle caratteristiche del biofiller lignina-silice per aumentarne la capacità di rinforzo. The industrial PhD project was linked to the broader Corimav-Pirelli project named elastomeric materials from renewable resources. The work dealt with the valorization of low value lignocellulosic substrates, focusing on the production of new materials for elastomeric compounds. The target was to produce renewable materials that could be used as a replacement for traditional materials in tyres manufacturing. At first I`ve focused my efforts on setting up a biorefinery process that allowed to fractionate the raw lignocellulosic substrates into three potentially interesting products: lignin, cellulose nanocristals and silica. With the process, it was possible to recover simultaneously all the main fractions with high purity and reasonable yield. Later, through a modification of the biorefinery process a fourth material was prepared, a biofiller composed by lignin and silica. This material is potentially extremely interesting since it combines the desirable characteristics of both silica (state of the art filler in high performance tyres) and lignin (renewable, lightweight, with antioxidant properties). At first all the products generated through the process form the lignocellulosic substrates were tested in rubber model compounds. After the first screening of the properties in composites with natural rubber, the effort focused on lignin based materials, studying the influence of their properties on the thermal and mechanical properties of their composites with natural rubber. The work followed three main topics: identification of relationships between lignin molecular structure and the performances in the elastomeric compounds, improvement of the characteristics by means of different thechniques (fractionation, thermal processes and chemical modifications) and optimization of the lignin-silica biofiller in order to improve its reinforcing capability.