## 1 Introduction  
Petrochemical-based polymers are known to negatively affect the environment [1], leading to increasing public awareness towards the promotion of \"green\" products. Hence, environmental rules are continuously legalated by governments [2], while authorities encourage environmental investments in the polymers sector. Among those eco-friendly solutions suggested for the promotion of \"green\" materials, attention has shifted to polymer composites based on lignocellulosic fibers' use as reinforcing agents [3, 4, 5, 6, 7], which may attribute significant economic benefits, with the ability to play a major role in the automotive and packaging industry, transportations, construction, etc. [8, 9, 10, 11, 12, 13]. It is well known that the preference for green composites over neat polymers arises from their environmental sustainability, improved performance, cost-effectiveness, and potential for weight reduction [14; 15; 16; 17; 18]. Though, a great deal of technical challenges need to be solved, such as fiber-matrix adhesion, the raise of mechanical performance, poor flammability resistance, and moisture-uptake control [15; 16; 17; 18]. Thereafter, there is a demand for the enhancement of the overall performance of natural polymer composites and therefore of the quality of the lignocellulosic fibers used, aiming toward the intensification of their applicability and adaptability in more applications [16]. Natural fibers can lead to composites with low densities and high specific strengths [19]. Furthermore, they possess a great variety of advantages over synthetic fibers, such as that they are low-cost, neutral to CO\({}_{2}\) emissions, biodegradable, have acceptable properties in terms of insulating properties and mechanical performance, are easy to handle and process, and come without health risks. Moreover, they are environmentally friendly, with a low density and high toughness, contributing to waste reduction, increasing energy efficiency, and promoting sustainability [16; 20; 21]. Though, a series of disadvantages characterize lignocellulosic fibers, such as low strength, high moisture content, limited processing temperatures, low durability, low fire resistance, quality and price variability, and, foremost, incompatibility with hydrophobic polymer matrices [12; 16; 17; 18]. A great variety of lignocellulosic fibers is available for further use in composite polymers. The quality of natural fibers, and therefore the properties they provide to polymer composites, depend on different factors, reported in detail by [PERSON] et al. [21]. In brief, the main factors that affect the fibers' quality when referred to polymer composite manufacture are the species of the plant, the crop location, the local climate, the fibers' ripeness during harvesting, the fiber-extraction methods, the storage conditions, and the age of the fiber [22; 23]. Furthermore, the quality of the fibers is greatly affected by the harvesting process, since cell-wall thickness, adherence with the surrounding structure, and the coarseness of fibers vary accordingly to the fiber-production stage in which reaping has been performed. Moreover, [PERSON] et al. [24] have reported that is quite difficult to define a lignocellulosic fiber's quality. Nonetheless, although the abovementioned factors affecting fibers' quality are difficult to examine, the composition of natural fibers can be investigated, leading to the estimation of a series of respective polymer composites' properties, such as thermal and mechanical properties [16; 25]. Lignocellulosic fibers are mainly composed of three components: cellulose, hemicellulose, and lignin [26]. Cellulose is a non-branched macromolecule which contains chains of different lengths of one-four linked \(\beta\)-d-anhydroglucopyranoside units [27]. On the contrary, lignin is a branched structure, serving as the matrix material in which cellulose is embedded, along with hemicellulose [28]. Each component provides to the system totally different properties.