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), qualitative and quantitative functional group distribution (FTIR and 31 P-NMR). Then the lignins were incorporated in natural rubber by two different approaches, namely co-precipitation 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.
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.
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