Valorization of waste biomass as functional fillers and additives for polymer-based materials

Wood-polymer composites (WPCs) are a particular group of composite materials consisting of one or more plant-based fillers and one or a mixture of polymers. Over the last decades, research on their development has aimed to address the main problems related to their processing: (i) variations in the quality of plant-based materials (PBMs); (ii) limited thermal stability of PBMs, narrowing the processing window for WPCs; and mostly (iii) limited compatibility between the often hydrophobic polymer matrix and hydrophilic fillers.

The first issue concerns the nature of PBMs and the structural differences associated with the cultivation environment, atmospheric conditions, or harvesting procedure. The structure and composition of biomass fluctuate within certain limits characteristic of particular materials. However, the most efficient applications often require narrowing this range, which implies the need to modify PBMs appropriately.

The insufficient thermal stability of PBMs noticeably limits the range of polymers that can be used as a matrix for composites. Numerous studies indicate 200 °C as the upper threshold for WPC processing, limiting or even excluding the use of engineering plastics such as polyamides (PA), poly(ethylene terephthalate) (PET), polyoxymethylene (POM), or acrylonitrile-butadiene-styrene (ABS). To avoid unfavorable effects of filler degradation during melt blending with thermoplastics, resulting in uncontrolled darkening of the material, porosity generation, and loss of the composites' impact resistance or tensile strength, fillers' pre-treatments are required, aimed at the removal of hemicellulose, lignin, and other components characterized by insufficient thermal stability.

Considering compatibility, its low level is a critical issue, as strong interfacial interactions are crucial for achieving satisfactory mechanical properties in composites. Enhancing the compatibility may be realized by providing possibilities for chemical bonding of modified fillers with functional groups present in the polymer backbone or by changing the character of the fillers' surface from hydrophilic to hydrophobic, simultaneously enhancing the filler-matrix mutual affinity. It often requires proper modification of the introduced fillers.

It can be seen that the most critical problems related to the processing of WPCs may require an adequate selection or modification of introduced PBMs. Beyond the most popular chemical treatments, thermal or thermomechanical modifications should be considered promising alternatives. Unlike chemical approaches, they avoid harmful solvents and are often far less complex. Thermal and thermomechanical modifications can also provide additional features to PMBs through thermally induced chemical changes associated with non-enzymatic browning reactions, as well as structural changes related, e.g., to the release of bound compounds, primarily phytochemicals acting as efficient antioxidants. Moreover, in a properly designed process, degradation products generated during treatment can be collected and used as feedstock in different sectors.

Given these aspects, the potential project aims to enhance the utilization of various types of waste-based biomass, focusing on underutilized materials, to add value and reduce the use of synthetic and virgin raw materials, such as antioxidants. Moreover, after improving the thermal stability of PBMs, they could be used as fillers for WPCs based on high-temperature-processed polymer matrices like PA, PET, POM, or ABS. Achieving both goals would be a significant step beyond the state of the art and would boost WPCs' sustainability.