Mechanical property enhancement of flax fibers via supercritical fluid treatment

Abstract

The desire for lightweight, carbon-negative materials has been increasing in recent years, particularly as the transportation sector reduces its global carbon footprint. Natural fibers, such as flax fiber and their composites, offer a compelling combination of properties including low density, high specific strength, and carbon negativity. However, because of the low modulus and high variability in performance, natural fibers can't compete with glass fibers as structural reinforcements in polymer composites. In this study, flax technical fibers were treated in supercritical CO₂ (scCO₂), and the effects of this treatment on the morphology and properties of flax fibers are reported. Treatment in scCO₂ successfully resulted in higher fiber modulus and strength by 33% and 40%, respectively. Fiber porosity was reduced by 50% and morphological changes to the fibers were observed. Specifically, fiber lumen collapsed during treatment and micro/mesoporosity was reduced by 27%. Treated flax fibers were used to create 30 vol% unidirectional flax-epoxy composites. ScCO₂ treatment raised composite modulus and strength by 33% and 25%, respectively. Because of the dependence between technical fiber size and mechanical properties, the relationship between fiber modulus and fiber size were created and applied to the rule-of-mixtures. This relationship were found to be viable representations of the fiber performance within each composite. Overall, the treatment developed in this study has the potential to significantly improve natural fiber properties, enabling their consideration for use in lightweight, semi-structural composites.

Figure 1

Flax fiber cellular morphology shown via fiber fixation and microtoming. (a) Untreated fiber labeled with fiber widths D1 and D2, (b) scCO₂ treated fiber, (c) untreated fiber lumen, labeled with lumen widths d1 and d2, (d) scCO₂ treated fiber lumen. Parts of this figure reproduced with permission from the Proceedings of the American Society for Composites: Thirty-seventh Technical Conference, 2022 Lancaster, PA: DEStech Publications, Inc..

Figure 2

Flax fiber D1 and D2 measurements before and after treatment in scCO₂. In general, treatment reduces the smaller width (D2) to a greater extent than the larger width (D1).

Figure 3

Single fiber tensile properties of untreated flax fibers before and after correction for lumen porosity as well as properties of scCO₂ treated fibers.

Figure 4

Cumulative pore volume of untreated vs scCO₂ treated fiber.

Figure 5

Change in pore volume and surface area distributions due to treatment in scCO₂.

Figure 6

FTIR spectra for untreated and scCO₂ treated fibers, showing no significant changes to fiber composition upon treatment.

Figure 7

Fracture surfaces of (a) 30% untreated flax composite. (b) 30% scCO₂ treated flax composite.

Figure 8

Relationship between fiber modulus and cross-sectional area for untreated and scCO₂ treated fibers.

Figure 9

Design of pressure vessel used to incorporate nanomaterials within samples using supercritical CO₂.

Figure 10

Inside of pressure vessel during supercritical fluid treatment. Fibers were placed on top of fiberglass mesh and a magnetic stir-bar was used to agitate the fluid for the duration of treatment.

Figure 11

(a) Schematic of paper mount used for single fiber tensile testing of flax fibers. The gauge length was 10 mm for all samples tested. (b) Schematic of width measurement locations on technical fiber and area measurement via approximation of fiber cross-section as ellipse.

Figure 12

Silicone mold containing 8 rectangular cavities for composite creation. Dimensions for final composite specimens shown.

Figure 13

Vacuum setup used during composite curing. Samples were cured under vacuum for 8 h at 80 °C.

Figure 14

(a) Fixture used to apply tabs to composite specimens, (b) composites ready for tensile testing. (c) Composites after tensile testing, showing failure within the specimen gauge length.

Figure 15

Image processing of composite cross-sections to enable fiber size distribution measurements. (a) Original image of scCO₂ treated flax-epoxy composite after polishing. (b) Enhanced contrast of fibers within epoxy matrix. (c) Fibers detected using ImageJ Fiji software; sizes quantified.

Figure 16

The relationship between cross-sectional area of untreated fibers and fiber modulus. Points designated with a triangle were gathered from the literature. All other data points were generated via single fiber tensile testing in this work, as summarized in Table 3.