A Study on Accelerated Ageing of Flax Fiber Reinforced Composites with Hygrothermal Cycling
Authors: Ivkov Sava 
Published: August 2022

In terms of environmental impact, flax fiber reinforced composites are more sustainable and green than their contemporaries like carbon fiber and glass fiber reinforced composites while having similar mechanical properties. Rising crude oil prices and improvements in extraction and the use of natural fibers are making materials like flax fibers increasingly attractive for the composites industry. However, the main problem of flax fibers remains the fact that they are hydrophilic in nature. This makes them less suitable for high-humidity environments because of their tendency for moisture sorption, causing damage to the surrounding matrix when used in composites. The resulting internal swelling and shrinking accelerates the rate at which the material ages leading to a decrease mechanical properties. The goal of this thesis is to investigate accelerated ageing on flax fiber composite samples through means of hygroscopic cycling. Testing is carried out on a lab scale to simulate and predict how the mechanical properties of the material change throughout the course of its lifetime (up to 18 years of in-service behavior). This is done by expanding on previous research which suggests that an initial decline in strength and stiffness over the first few years curves back to an increase in the later years. In other words, the mechanical properties are observed to not decline continuously due to aging and start increasing after long exposure. This knowledge is used to lower the aforementioned primary drop in mechanical properties and rethink the possible fields of application for flax fiber reinforced composites. Practically, three main pillars are distinguished in this research, all of which involve the production and ageing of flax fiber composite samples: Accelerated lab-scale hygroscopic cycling of samples, natural ageing of samples and lastly precycling of flax fabrics before composite production. The precycling is carried out to induce fiber hornification, a phenomenon which aims to: improve compatibility with the matrix, improve the strength of the fibers and increase their hydrophobicity as options to slow down the ageing of the composites which they reinforce. The primary matrices that are used for this study are the thermoplastics maleic anhydride polypropylene (MAPP) and polyoxymethylene (POM; an engineering thermoplastic). Epoxy is used within the hygroscopic cycling and natural ageing because of its importance as an industrial standard. POM is six times stiffer than MAPP and boasts higher strength as well. The main problem of the POM matrix, however, is its inferior fiber-matrix adhesion compared to MAPP resulting in a weaker interphase. Flexural testing on the regularly cycled samples is still ongoing because of the long nature of the ageing of the samples. Testing on the shorter cycles suggests a significant decrease in strength of all materials, with properties dropping by 20%-40% after four hygroscopic cycles, similar to the damage caused by aging naturally. After eight cycles the strength increases by up to 30% for flax/MAPP and up to 23% for flax/POM when comparing to four cycles. This points to a clear prevalence of restrengthening over the course of the composites in-service lifetime. Thus, ageing of flax fiber composites paired with decreasing mechanical properties is not a continuous process. Flax/epoxy does not show significant drops in strength after eight cycles and retains at least 96% of its initial strength. For the precycled samples, continuously increasing mechanical strength is measured with increasing precycle time. For MAPP the strength increases with up to 12% in longitudinal fiber direction with the transverse fiber direction (the fiber-matrix interphase) increasing up to 6%. For POM, the increase is more profound: increases up to 15% in longitudinal fiber direction and up to 43% increase of interphase strength. With all data for precycling exhibiting these upward trends it can be said that the precycling of flax fiber fabrics before composite production can be employed as a tool to improve in-service properties and expand on the fields of application for flax fiber composites.