Phenolic and lignosulfonate-based matrices reinforced with untreated and lignosulfonate-treated sisal fibers

Abstract The present investigation addressed the use of treated and untreated sisal fibers (3 cm, 30 wt%, randomly distributed) as a reinforcement in bio-based composites. Sodium lignosulfonate (NaLS) was used to physically treat the sisal fibers and also as a macromonomer to prepare phenolic-type thermosets, namely, the matrices (LCs). The introduction of NaLS moieties on the surface of the fibers and in the chemical structure of the matrix aimed to increase the affinity of the fiber-matrix interface in addition to increasing the bio-sourced character of the final material. Sisal fibers were treated (and subsequently characterized) with aqueous 5 wt% NaLS solutions via heating (70 °C/1 h, SFT1), ultrasound irradiation (1 and 2 h, SFT2 and SFT3, respectively), and room temperature treatment at 24 h followed by ultrasound (1 h, SFT4). Briefly, considering only some properties of the composites, the following results can be highlighted: all the phenolic composites (PC, control samples) exhibited impact and flexural strengths considerably greater than the unreinforced phenolic thermoset (PT). The phenolic composite reinforced with untreated sisal fiber (PC-UF) and PC-SFT4 exhibited an impact strength of approximately 435 Jm −1 , and the others, PC-SFT1 and PC-SFT2, had a tendency to have a greater impact strength, with the exception of PC-SFT3 when compared with PC-UF. Based on the results exhibited by the PC-treated fibers, only LC-SFT1 and LC-SFT2 were prepared among the LC-treated fibers besides LC-UF. The set of PCs exhibited a lower impact strength than their LC composite counterparts. For example, the impact strength of LC-UF was approximately 1000 Jm −1 (compared to approximately 435 Jm −1 for PC-UF). This is a very good result for a thermoset matrix reinforced with natural fibers and clearly shows that the fiber-matrix interaction at the interface was favored when the thermoset was prepared from NaLS, and then, the load received during the impact was more efficiently transferred to the sisal fibers. SEM images showed good adhesion at the fiber-matrix interface of LC-treated fibers due to the similarity of the chemical structure of the matrix and treated sisal fiber. Concerning the assessment of the thermal conductivity (at 10, 25 and 50 °C), only the thermosets (phenolic, PT, lignosulfonate-based, and LT), the composites reinforced with untreated fibers (PC-UF and LC-UF), and PC-SFT2 (due to its high impact strength) were evaluated. The results showed no significant variation in the thermal conductivity as a function of temperature. There was no significant variation in the thermal conductivity of both thermosets (the thermal conductivity of PT and LT at 25 °C, for instance, were 0.30 and 0.28 Wm −1 K −1 , respectively) to their respective composites (at 25 °C, the thermal conductivity was 0.40 and 0.36 Wm −1 K − 1 for PC–UF and PC–SFT2, respectively, and 0.24 Wm −1 K −1 for LC–UF). A tendency towards lower conductivities (at 10, 25 and 50 °C) was observed for LC-UF when compared with PC-UF. Overall, these results meet the current expectations concerning the production of materials prepared from a high percentage of raw materials from renewable sources with good properties.