PLA/flax fiber bio-composites: effect of polyphenol-based surface treatment on interfacial adhesion and durability

Fiber treatments to enhance interfacial adhesion in natural fiber-reinforced composites generally induce the use of many chemicals. To address this issue, the present work aimed to develop innovative and ecological fiber treatments to produce 100%-green composites, by investigating interfacial adhesion between modified flax fibers and polylactic acid (PLA) using lignin and tannin, two types of eco-friendly polyphenolic compounds derived from waste or paper industry byproducts. The surface modification of flax fibers was performed using a dip-coating method and solutions of condensed tannin or Kraft lignin at different concentrations ranging from 1% to 7% (wt/V). The results showed that the use of coating solutions composed of 1% (wt/V) tannin or lignin significantly improved the interfacial adhesion of composites. Tannin showed better compatibility with PLA and flax fibers with a 17% and 29% increase in the ultimate flexural strength and interlaminar shear strength of the composites, respectively. These results were explained by tannin higher concentration of hydroxyl functions as compared to lignin and thus, good reactivity with cellulose and PLA. Coating solutions rich in lignin or tannin have reduced the mechanical properties of composites due to the thicker coating layers and the migration of lignin or tannin molecules into the PLA bulk.

Automated summary

The study aimed to develop ecological fiber treatments for improving interfacial adhesion in natural fiber-reinforced composites. Results indicated that tannin showed better compatibility with PLA and flax fibers, significantly enhancing the mechanical properties of composites.