## 1 Introduction  
During the last years, there has been a significant increase in social concern for the environmental problem generated by petrochemical polymeric materials [1; 2; 3]. For this reason, one of the main objectives of the scientific community is the research and development of new highly environmentally friendly materials which could be suitable to replace petroleum-based polymers to reduce their carbon footprint [4]. Many of these researches focus on the field of polymer composites reinforced with lignocellulosic particles giving rise to the so-called wood plastic composites (WPC). A series of advantages make the use of lignocellulosic reinforcements very attractive for their use as reinforcement,such as their low cost, low density, non-abrasive properties, non-toxic, biodegradable, and their environmentally friendly nature [5; 6]. Besides, the lignocellulosic particles usually provide an aesthetic wood-like surface finish, which makes them very interesting for use in sectors where aesthetics is an essential factor, such as the furniture or automotive sectors. Furthermore, WPCs have several advantages over wood, such as low maintenance, high dimensional stability, and high resistance to biological attack [7; 8]. There are many research works that have focused on the effect of different types of lignocellulosic particles on the properties of both thermoplastic and thermoset matrices. For example, interesting works have been developed with rice husks [9; 10; 11], peanut shells [7; 12; 13], almond shells [14; 15; 16], hazelnut shells [17; 18], date palm seeds [19], lemon peel [20], _Posidonia oceanica_[21; 22], olive stones [23], among others. Most of these lignocellulosic particles come from agricultural by-products or wastes that are currently used for animal feeding and just left on controlled landfills. Therefore, the use of these wastes, widely available as a source of fillers for composite materials, can be a new economic opportunity for the agricultural sector and thereby contributing to generate sustainable circular economies [24; 25; 26]. Flux (_Linum usitatissimum L._) is a worldwide cultivated plant, mainly for obtaining fibers and oil-rich seeds [27]. It is estimated that the world production of flaxseed, also known as linseed, was approximately 3.2 million tons during 2018 [28]. Traditionally flaxseeds have been used as an oil source, due to its high triglyceride content (between 30 and 41 wt%), for use in paints and coatings, linoleum, inks, varnishes, cosmetics, soap production, among others [29; 30]. However, over the past few years, the flaxseed oil has also gained popularity as a nutritional supplement due to its high content of \(\alpha\)-linolenic acid (ALA), an omega-3 fatty acid beneficial in preventing cardiovascular disease or hypertension [31]. In addition, many studies have shown that flaxseed oil has a positive effect on diseases such as hyperlipidemia, colon and breast cancer, or atherosclerosis [32]. The main by-product generated during oil extraction is flaxseed cake, which is the solid mass left after the seeds are pressed during the oil extraction process. For this reason, flaxseed cake is widely available and, a cost-effective and environmentally friendly material to be used in composites [33]. The flaxseed cake is rich in cyanogenic glycosides, which may be degraded to toxic hydrogen cyanide (HCN) upon ingestion and may represent a risk to human health if used in food applications without prior detoxification treatment [31]. For this reason, currently, a part of this by-product is spray-dried to obtain flour (flaxseed flour), which is used as a low-value by-product for obtaining livestock feed or fertilizers [34; 35]. In other cases, this waste is used for composting or simply incinerated [36]. Therefore, the flaxseed flour can potentially be a candidate for use in composites.