Relationship Between the Shape of Giant Reed-Based Fillers and Thermal Properties of Polyethylene Composites: Structural Related Thermal Expansion and Diffusivity Studies

Abstract

This paper describes the effect of two different fillers derived from giant reed (Arundo donax L.), namely fibers and shredded aerial parts of the plant, on the thermal properties of polyethylene-based composites, as the analysis of dimensional stability of lignocellulose-based composites, and its relationship with their thermal diffusivity, has not yet been assessed in the literature. It has been found that the introduction of such materials resulted in a significant reduction of the coefficient of thermal expansion, particularly more important in the case of fibers, due to their higher aspect ratio; in particular, this coefficient is reduced to less than half for fibrous composites (from 1.6·10− 4 K− 1 to 6.1·10− 5 K− 1 or 3.5·10− 5 K− 1 for 20 and 40% loadings). This parameter also influences the thermal diffusivity of the final parts; the diffusivity of composites increases with the ratio of lignocellulosic filler used, particularly when using fibers, due to a better orientation of these fibers than the shredded material, which does not exhibit a fibrous shape. Composites with 20% share of the filler exhibited a thermal diffusivity increased by about 15% compared to neat polyethylene, while 40% loadings resulted in a 25% and 60% increase for ground and fibrous materials, respectively. These results provide additional features to lignocellulose-composites characterization, providing properties not usually mentioned in the literature to expand the knowledge about such composite materials beyond mechanical properties, providing a broader range of properties to offer a wider application area of such composites. Statement of NoveltyArundo donax L. is of great interest to biorefineries due to its fast growth and resistance to adverse environmental conditions. Most research on this plant species focuses on obtaining energy products or valuable chemicals, while very few are related to composite production, particularly on thermoplastic materials. The work found in the literature so far does not provide insights into the relationships between the types of filler (and their aspect ratio) and their thermal properties. Therefore, this work expands the knowledge on the thermal behavior of lignocellulose-polymer composites, supplementing the research, usually focused only on mechanical properties, in their characterization by correlative analysis of thermal influenced dimensional change with structure and thermal diffusivity. Determining the coefficient of thermal expansion (CTE) is a relevant parameter to assess the possibilities of using a material at high or low temperatures and evaluate the dimensional stability of a product during its service lifetime. On the other hand, thermal diffusivity brings together the capacity of a material to store thermal energy and distribute it throughout the material; that is, it relates heat capacity and thermal conductivity, which are also essential in using materials in market applications. Therefore, the work not only provides the results of thermal diffusivity and CTE of thermoplastic-reed composites but also correlates both parameters as a way to widen the range of application of plant-based composites in areas where dimensional stability (i.e., low thermal expansion) is required. Graphical Abstract