PEER-REVIEWED ARTICLE bioresources.com Jutila et al. (2023). “Flax in polymer composites,” BioResources 18(1), 899-925. 899 Effects of Pine Rosin on the Degradation of Mechanical Performance in Flax-Reinforced Polymeric Composites after Soil Burial at Low Temperatures Lauri Jutila, a Rama Layek, b Farzin Javanshour, a Lijo George, a Essi Sarlin, a and Mikko Kanerva a, * The effect of pine rosin (RO) was studied relative to the biodegradation of poly(lactic acid) (PLA), starch-based polymer (Mater-Bi), and PLA-flax composites. It was hypothesized that rosin can alter – either speed up or slow down – biodegradation in plastics depending on the specific species of polymer. The biodegradation was brought about by soil burial over 56 days. First, the effect of rosin was studied alone without any effects of soil burial. Second, the effects of soil burial were studied in terms of biodegradation. The results showed that RO increased the degree of crystallinity (+100%) and Young’s modulus (+14%) of PLA. For Mater-Bi, RO decreased the strength by 22% and led to brittleness (56% lower ultimate strain) of the specimens. After 56 days of soil burial, the presence of RO in PLA was found to speed up the degradation when compared to pure PLA (the decrease of strength was 9.3% and 6.6%, respectively). For Mater-Bi, the RO blending led to 2.1% slower biodegradation of strength during 56 days of soil burial. The effect of RO, in terms of affecting the biodegradation, was comparable in flax-reinforced and non-reinforced PLA. The strength of the fiber-matrix bonding remained equal for RO- impregnated fibers compared to the as-received flax fibers. DOI: 10.15376/biores.18.1.899-925 Keywords: Composite; Flax; Rosin; Soil burial; Interface Contact information: a: Unit of Materials Science and Environmental Engineering, Tampere University, P. O. Box 589, Tampere, FI-33720 Tampere, Finland; b: LUT University, Department of Separation Science, FI-15210 Lahti, Finland; *Corresponding author: mikko.kanerva@tuni.fi INTRODUCTION Structural components have requirements of stiffness, strength, and durability. The requirement of durability is controversial regarding the needs of developing degradable or compostable parts. Biological degradation and recycling of polymeric materials are the two necessary routes to make these materials sustainable. The barriers in the implementation of sustainable strategies are related to the lack of knowledge and time constraints within industrial material development (Veshagh et al. 2012). The recycling of fiber-reinforced composite materials often leads to heavy down-cycling (Yazdanbakhsh and Bank 2014; Oliveux et al. 2015). Moreover, biodegradable composites lack mechanical performance, such as strength and toughness, compared with the high-performance composites with rival matrix polymers and fibers of carbon, aramid, and glass (Kumar and Kumar 2012). Natural fiber composites are increasingly intended for novel applications (Hoffmann et al. 2021; Graupner et al. 2022). For the transmission line poles of data networks (Di Vito et al. 2020), large amounts of poles would be placed in rural areas. The protection covers, as