Thermoplastic PLA-LCP Composites: A Route toward Sustainable, Reprocessable, and Recyclable Reinforced Materials

Abstract

Reprocessing of reinforced composites is generally accompanied by loss of value and performance, as normally the reinforcing phase is damaged, or the matrix is lost in the process. In the search for more sustainable recyclable composite materials, we identify blends based on poly( l -lactide) (PLA) and thermotropic liquid crystalline polymers (LCP) as highly promising self-reinforced thermoplastic composites that can be recycled several times without loss in mechanical properties. For example, irrespective of the thermal history of the blend, injection molded bars of PLA containing 30 wt % LCP exhibit a tensile modulus of 6.4 GPa and tensile strength around 110 MPa, as long as the PLA matrix has a molecular weight of 170 kg mol –1 or higher. However, after several mechanical reprocessing steps, with the gradual decrease in the molecular weight of the PLA matrix, deterioration of the mechanical performance is observed. The origin of this behavior is found in the increasing LCP to PLA viscosity ratio: at a viscosity ratio below unity, the dispersed LCP droplets are effectively deformed into the desired fibrillar morphology during injection molding. However, deformation of LCP droplets becomes increasingly challenging when the viscosity ratio exceeds unity (i.e., when the PLA matrix viscosity decreases during consecutive reprocessing), eventually resulting in a nodular morphology, a poor molecular orientation of the LCP phase, and deterioration of the mechanical performance. This molecular weight dependency effectively places a limit on the maximum number of mechanical reprocessing steps before chemical upgrading of the PLA phase is required. Therefore, a feasible route to maintain or enhance the mechanical properties of the blend, independent of the number of reprocessing cycles, is proposed.

Importance of Viscosity Control for Recyclable Reinforced Thermoplastic Composites

“…To maintain the mechanical performance upon reprocessing, the use of a higher viscosity PLLA matrix (in combination with CE-1:1 ) or a different LCP ( CE-3:2 with the same PLLA matrix) will be beneficial, as this prevents infeasible viscosity ratios upon reprocessing. 16 …”

“… 14 , 15 For an LCP-PLA composite, we have demonstrated that the morphology and mechanical performance remain constant irrespective of the number of reprocessing cycles, given that the viscosity of the dispersed LCP phase remains lower than that of the PLA matrix (i.e., viscosity ratio < 1). 16 …”

Tailoring of advanced poly(lactic acid)‐based materials: A review

“…The final material is usually non‐porous. PLA can be processed by HME alone or with the addition of other polymers as well as different organic and inorganic substances 72–84 . When the molten PLA is extruded through a sheet die and quenched on polished chrome rollers that are cooled with circulating water, PLA films (usual thickness 0.076 mm) and sheets (usual thickness 0.25 mm) are produced 23 .…”

Design of PLA/ENR thermoplastic vulcanizates with balanced stiffness‐toughness based on rubber reinforcement and selective distribution of modified silica

“…Many studies have shown that the phase morphology of TPV has a considerable effect on PLA toughening. 12,36,38 Therefore, in order to remove the PLA phase, dichloromethane was used to etch the cryo-fractured surface of the PLA/ENR/SiO 2 /KH550 TPVs, and then SEM was employed to study the effect of KH550 on the morphology of ENR phase in the TPVs. According to our previous report, ENR owned a higher melt strength than PLA melts, and a certain volume percentage of ENR could sustain a good continuous during the dynamic vulcanization, which resulted in the formation of a special co-continuous phase structure.…”

“…Renewable resources have drawn increasing concerns for decades in consideration of environmental pollution and energy crises. [1][2][3][4] As a biodegradable polyester derived completely from biomass materials such as corns and potatoes, polylactic acid (PLA) exhibits excellent biocompatibility, [5][6][7] high mechanical strength, 8,9 and good recyclability, [10][11][12] which shows an enormous potential in alternative candidate for petroleum-based plastics. However, the inherent brittleness is one of the major barriers that hinder the widespread application of PLA.…”