Choosing the right natural fibre can have a big impact

Bio-based composite materials have been used since time immemorial – at various points in history, homes, infrastructure and tools have all been constructed from clays and stones combined with fibrous plant and animal products. Improvements in the processing and characterization of these natural materials have driven their more recent industrial use. In 2016, the global biocomposites market was estimated at $4.46 billion, with the materials used in the construction, aerospace, automotive, and packaging sectors, amongst others. And that market is forecast to grow to $10.89 billion by 2024.

However, such materials have limited utility in exterior applications because biocomposites tend to degrade when left exposed to the elements. A new study from researchers at Université du Quebec á Trois-Riviéres (UQTR) has explore this environmental degradation in more detail, particularly with short natural fibre-reinforced thermoplastic (SNFT) composites in mind.

Writing in the open-access journal Composites Part C [DOI: 10.1016/j.jcomc.2022.100296], they report on two polypropylene biocomposites; one made with flax fibres and the other with pine fibres. The proportion of fibre to matrix was kept consistent, at 30 wt% for each. The samples – PP30-F, and PP30-P, respectively – were put through an accelerated aging test which involved irradiating them with UV light for a total of 960 hours, at a temperature of 60°C. Changes in the colour, roughness, and surface morphology, as well as the flexural properties and drop weight impact behaviour of the samples were evaluated at five time points: after 0 h, 160 h, 320 h, 640 h, and 960 h.

The researchers found that while UV aging did not change the fundamental chemical makeup of the biocomposites, peaks in the infrared spectra of the samples grew with increased exposure time. This suggests that both samples experienced photo-oxidation. However, the increase was more significant in the pine composite than the flax. The authors write, “pine fibres undergo more photo-oxidation reactions under UV irradiation.” They attribute this to the lignin-rich nature of pine – on average, it contain ten times more lignin than flax.

Visually too, the samples changed over time; bleached by exposure to UV light. This bulk of this shift in colour happened within the first 480 h, and because it is related to the structure of its lignin content, there was some variation between the two composites. The authors write that “after 960 h of exposure, the color changes reached 40.95% and 31.31% for PP30-F and PP30-P, respectively.”

In terms of roughness, both biocomposites had initially exhibited smooth surfaces, free of defects. As they underwent aging, their surface roughness increased – the longer the samples were exposed to UV, the rougher they got, and after 960 h of exposure, micro cracks were also observed. However, PP30-P exhibited fewer cracks and less roughness than the flax composite. The authors again attribute this to lignin, which they say has an “antioxidant effect”.

Bending tests showed that aging had a negative impact on the mechanical properties of the biocomposites, with the most significant degradation happening in the first 480 hours. Again, the flax composite (PP30-F) degraded more severely than the pine composite. Drop weight impact tests led to similar outcomes – both were deformed by the test, but the PP30-F performed most poorly.

The authors conclude that “The design of biocomposite structures for outdoor applications should consider the influence of ultraviolet (UV) irradiation on the mechanical performances to more accurately determine their durability characteristics and prevent significant damage.”


Khaled Nasri, Lotfi Toubal, Éric Loranger, Demagna Koffi. “Influence of UV irradiation on mechanical properties and drop-weight impact performance of polypropylene biocomposites reinforced with short flax and pine fibers,” Composites Part C, Open Access 9 (2022) 100296. DOI: 10.1016/j.jcomc.2022.100296