STUDY ON MECHANISM OF FLAX TOUGHENING PLA

摘要

PLA has been one of the most promising candidates of petroleum- based plastic substitute material, not only because of its biodegradability, biocompatibility, no toxicity, high stiffness, clarity, gloss, and UV stability, but also because of its renewability. However, the brittleness of PLA is critical defect for its wide application. An effective way to improve toughness is introducing fibers into matrix. A lot of researches were published to discuss about all kinds of fibers filling PLA, however, the mechanism of fibers toughening PLA was not revealed yet. In this paper, PLA/Flax bio-composite was compounded by melt extrusion. Impact strength, elongation at break and stress-strain curves were obtained by mechanical properties test. SEM was used to observed structures of flax fiber and impact fracture morphology of bio-composite. Furthermore, emergence and development of bio-composite crazes were recorded by polarizing microscope. The mechanical properties test results showed that the addition of flax obviously improved toughness of bio-composites. Elongation at break and impact strength were improved 327% and 21.2% compared to pure PLA, respectively. Stress-strain curves of bio-composite appeared yield point and cold- draw stage, which did not happen for pure PLA. For PLA/Flax bio-composite, the flax fibers were torn into microfibre or pulled out when they subjected shock impact, fracture surfaces have characteristic of dimples. Polarizing microscope photos illustrated crazes stop developing when they met flax ahead across the road. At last, toughening mechanism was derived by experimentphenomena and results. Flax fibers scattered randomly around matrix, two special orientations were selected to analyze, perpendicular or parallel. At first, , when orientation direction of flax fibers were perpendicular to stress, stress on the flax fibers exceeded binding force between fiber bundles, flax bundles were torn into microfibers. The impact energy was absorbed, which made impact strength increase. Secondly, when orientation direction of flax fiber was parallel to stress, crazes appeared in stress concentrating area of PLA matrix, and were perpendicular to stress and orientation direction of fiber. Crazes moved forward until came across crabwise fibers. The new crazes continued generating and expanding, and stopped ahead of flax fibers. When stress was stronger than molecular link chemical binding force, flax fibers broke and pulled out from matrix, and fracture surfaces traversed fiber bundles. Flax fiber hindered crazes developing and slowed negative growth, which was favorable to consume impact energy and improve toughness. Toughening mechanism of fibers not perpendicular or parallel to stress has mixed characteristics of them. This research has a significance of revelation for the development of fiber toughening composites.