Tribological and Nanomechanical Behavior of Liquid Wood

Abstract

During the last decades, there has been an increased interest in the use of lignin-based composites following the ideas of developing green materials for fossil-based raw materials substitution. The biopolymer Arboform is a mixture of lignin, plant fibers, and additives, which is nowadays successfully used in many applications. As a thermoplastic, it can be molded and is therefore also called “liquid wood.” In this paper, we report a study comparing the nanomechanical and tribological properties of Arboform (AR), and Aramid-reinforced Arboform (AR-AF) composite biopolymers. The samples were produced in an industrial-scale injection molding machine. Nanoindentation experiments have revealed that, in both series of biopolymer samples, an increase in temperature or a change in the injection direction from 0 deg to 90 deg produces an increase in hardness. On the other hand, Young's modulus is slightly affected by the increase in temperature, and not affected by the injection angle. Tribological characterization has shown that all samples, except the AR-AF injected at 175 °C, present noticeable wear and have a similar friction coefficients μ ∼ 0.44–0.49 at Hertzian contact pressures p0 between 90 and 130 MPa. Interestingly, the reinforced polymer produced at 175 °C shows no wear and low friction of μ ∼ 0.19 at p0 = 90 MPa. Our results show that the reinforced Arboform biopolymers are a good candidate to replace other polymers in many mechanical and tribological applications.

Dynamic Mechanical Analysis and Thermal Expansion of Lignin-Based Biopolymers

“…The incorporation of aramid fibers, in a mass percentage of 15, increases not only the thermal resistance of the Arboform ® LV3 Nature biopolymer but also its mechanical characteristics, confirmed by the obtained results of the research groups in [ 12 , 37 , 38 , 39 , 40 , 41 , 42 ].…”

“…Additionally, the maximum temperature at which most of the polymeric bonds are lost by thermal degradation, over 83%, increases by up to 9 °C when reinforcing Arboform ® LV3 Nature with aramid fibers; There is no improvement on the residual mass found at the end of the analysis because the thermal interval in which the aramid fibers lose a high mass weight, over 97%, is 480–520 °C [ 33 , 36 ]. Thus, at higher temperatures, it is impossible for this reinforcement to further improve the thermal characteristics of a polymer; The incorporation of aramid fibers, in a mass percentage of 15, increases not only the thermal resistance of the Arboform ® LV3 Nature biopolymer but also its mechanical characteristics, confirmed by the obtained results of the research groups in [ 12 , 37 , 38 , 39 , 40 , 41 , 42 ]. …”

Tribological and nanomechanical properties of a lignin-based biopolymer

“…However, we notice that all measurements using various methodologies have values in a similar range. If we compare the results from Table 2 with our previous results of the pure Arboform polymer (18), with hardness H IT in the range 239-350 MPa and Young's modulus E IT in the range 5.4-5.9 GPa, we conclude that the higher content of lignin in the biopolymer Arboblend V2 Nature (>90%) is probably a slight detrimental factor on its mechanical properties, in comparison to Arboform, which contains only 30% lignin. Interestingly, we will see in the next sections that the biopolymer Arboblend V2 Nature performs tribologically much better than Arboform and the Aramid-reinforced Arboform polymers.…”

“…It is well known that the morphology of the polymers in the moldings is influenced by the process variables like mold and melting temperature, shear forces, applied pressure, and elongational flow (24). We have recently studied the morphological properties of Arboform biopolymers that were made with the same molds of Figure 1 and no textured lines were observed or high waviness W t values were measured (18). The skin texture in our Arboblend V2 Nature samples is probably due to temperature differences between the bulk and the surface, and the flow field in the molding process.…”

“…In a previous publication, we have compared the nanomechanical and tribological properties of another "liquid-wood" polymer named Arboform, and reinforced with aromatic polyamide (Aramid) fibers (18), which have a nonrenewable fossil origin. Arboform has only 30% of lignin, being the rest comprised of wood pulp fibers and other natural additives.…”