Compatibilization of highly sustainable polylactide/almond shell flour composites by reactive extrusion with maleinized linseed oil

Abstract

Highly sustainable composites were produced by melt compounding polylactide (PLA) with almond shell flour (ASF), a processed byproduct of the food industry, at a weight content of 30 wt.-%. However, due to the lack of miscibility between PLA and ASF, both being raw materials obtained from crops, resultant green composite presented poor ductility and low thermal stability. To overcome this limitation, maleinized linseed oil (MLO), a multi-functionalized plant-derived additive, was originally incorporated as a reactive compatibilizer during the extrusion process. Both chemical and physical characterizations showed that 1-5 parts per hundred resin (phr) of MLO successfully serve to function as a novel compatibilizer on the PLA/ASF composites, leading to highly sustainable materials with balanced mechanical, thermal, and thermomechanical properties. Achieved compatibilization was particularly related to a dual effect of plasticization in combination with grafting. The latter effect was ascribed to the formation of new carboxylic ester bonds through the reaction of the multiple maleic anhydride groups present in MLO with the hydroxyl terminal groups of both PLA chains and lignocellulose on the ASF surface. The fully bio-based and 2 biodegradable composites described herein give an efficient sustainable solution to upgrade agrofood wastes as well as contributing to reducing the cost of PLA-based materials.

Effect of different compatibilizers on environmentally friendly composites from poly(lactic acid) and diatomaceous earth

“…Concerning the use of ESAO a 1wt% was added, the manufacturer recommends an amount between 0.1–1 wt% since higher loadings lead to undesired effects such as gel formation which, in turn, leads to a decrease in mechanical properties. This behaviour has also been reported by Torres‐Giner et al Finally, regarding MLO, it is a relatively new compatibilizer and previous results suggest 5 wt% offers comparable results to the other compatibilization systems …”

“…The tensile strength decreases from 62.5 MPa for PLA‐DE without any coupling agent/compatibilizer to 40 MPa for PLA‐DE‐MLO biocomposite. This is a direct consequence of the plasticization effect that MLO can provide . Accordingly, the elongation at break increases to 13.8%, which is almost three times the PLA‐DE value.…”

“…On the one hand, MLO acts as a compatibilizer between the polymeric matrix and the silica DE filler. This compatibilization effect is directly related to the reaction/interaction between the maleic anhydride pendant groups in MLO and the hydroxyl groups contained in both PLA (end‐chains) and siloxane/silanol groups in DE as observed in other polyester composites . On the other hand, MLO provides a plasticizing effect that allows chain motion with a slight decrease in T g …”

Manufacturing and compatibilization of PLA/PBAT binary blends by cottonseed oil-based derivatives

“…Nevertheless, the efficiency of vegetable oil-based plasticizers is very poor as, in general, a very slight decrease on the glass transition temperature is achieved. Nevertheless, the highly reactive groups in modified vegetable oils (mainly epoxy, acrylate, maleic anhydride groups, among others) can react with hydroxyl groups present in the end chains of polyesters thus leading to additional reactions that could lead to chain extension, branching and crosslinking simultaneously as reported by Quiles-Carrillo and coworkers [41][42][43] in several works. With regard to Joncryl ® , typical amounts of 1 wt% are recommended by suppliers to avoid gel formation by crosslinking.…”

“…So that, it is expectable a more intense overlapping of chain extension, branching and crosslinking when using ECSO and MCSO than in the case of Joncryl ® (Figure 3c). A FTIR study carried out by Quiles-Carrillo et al [43] on poly(lactic acid) with lignocellulosic fillers demonstrated the reaction between maleinized linseed oil (MLO) and hydroxyl groups in PLA end chains and cellulose particles therefore giving clear evidences of reactive extrusion.…”