A lignocellulosic filler reinforced polymer is a composite material made through the combination of vegetal fibres or particles (also called "fillers") and a matrix of organic polymers. Composites are designed to improve the mechanical properties and lower the impact on the environment. Fillers perform the role of bridges for stress transfer inside the material, permitting them to sustain higher loads. At the same time, adding a filler inside a material leads to the reduction of the percentage of polymer used in the synthesis of said material. By lowering plastic use, composites allow for a reduction of both their carbon footprint and the cost related to their fabrication.
Lignocellulosic fillers
Lignocellulosic fillers are organic materials coming from plant sources, like leaves, stalks, or food shells, peels and seeds. The majority of them are sourced from agri-food residues or wastes and find a new end-of-life in the formulation of biodegradable biocomposites. They are primarily composed of three organic polymers, namely lignin, cellulose and hemicellulose. The plant cells composing the fillers present an outer thin primary wall which is made of all these three organic polymers plus pectin. Internally a secondary wall exists, whose composition accounts of around 45% cellulose, 20% hemicellulose and 25% of lignin. Lignin is responsible for tying together the other polymers and strongly binding them together. Food waste production around the world has been almost stable in the last years, with just a barely increasing trend. The major sources of waste come from maize and rice products, cassava, potatoes, sugar cane, fruits and vegetables. Despite being considered as wastes, these residues showed great potential in strengthening polymers, demonstrating the possibility to grant a new purpose to products that otherwise would be discarded.
Morphology Filler morphology could change according to the desired final use. In most applications, fillers are used as one of the following conditions:
as particles as fibers as they are without further modifications The sizing of the filler is decided upon use in accordance with the polymeric matrix they are coupled with. To achieve better adherence and stability of the final composite, in many instances fillers are reduced to dimensions in the order of micrometers (μm). Typical fibre fillers have measures around 300 μm to 500 μm, showing an aspect ratio (L/D) around 3. Particles, instead, found applications in smaller dimensions around 100 μm. Smaller filler diameters, such as 1 μm, were incorporated more easily inside the matrices, but the decrease in particle size could pose the risk of particles becoming airborne. Small particulate powders present a fire hazard and the high temperature required for the processing of polymers presents the risk of igniting the powder and ultimately exploding.
Thermal degradation The filler as well as the polymeric matrix are processed at high temperature to permit their blend and further molding. This increase in temperatures is related to the thermal degradation that can take effect to both the filler and polymer. Thermal degradation happens when a material is subjected to a high enough temperature that a chemical decomposition takes place. Since filler and polymers have different degradation temperatures, with the former being lower, processes are usually set to perform under polymer's melt temperatures. Generally, for lignocellulosic fillers the degradation region starts at 150 °C and ends at 180 °C. From 200 °C to 300 °C they fall into the char production region and in the 300-360 °C in the pyrolysis front formation region. As such, usually, fillers are subjected to thermal degradation during the composite synthesis. Filler degradation endangers the structural integrity of the final material and could also even lower the mechanical properties instead of increasing them as desired.
Treatment methods
Lignocellulosic materials are hydrophilic in nature, whereas the employed polymers are hydrophobic and non-polar. This discrepancy is responsible for the difficulty in merging polymers and fillers in one composite material. For this reason, fillers undergo one or more pretreatment methods to remove impurities which remained on the external surface after washing and to expose the hydroxyl (OH) groups present on the filler's backbone. Increasing the number of hydroxyl groups affects the ability to form more interfacial bonds and permits a better adhesion between fillers and polymers. A large number of pretreatments exist, among them the largest categories are physical processing, chemical treatment, physicochemical processing and biological treatments.
Physical processing Physical processing is used for both decreasing filler size and to change structural and surface properties, affecting the mechanical bonding between fillers and polymers. An important consideration is that physical methods do not change the chemical composition of the material but they only influence the surface properties. Methods belonging to this category are extrusion, milling, pyrolysis, ozonolysis, ultrasounds, microwaves, corona and plasma treatments.
Chemical treatment Chemical treatments can be used to change the surface structure of the filler as well as to modify its molecular structure. In doing this, different chemical compounds could be used, among which the most used are alkali and silane treatments, ionic liquids, organic solvents, dilute acids and oxidative lignification. Sometimes, some of these chemical treatments are used in combination to further modify the filler structure. The methods see the soaking of the lignocellulosic filler in the selected treatment solution for a specific amount of time, then they are usually washed and dried. The procedure could be repeated in more reprises according to the specific effect and desired outcome. Chemical treatments are generally considered the quickest and most effective among the treatment methods.
Physicochemical processing Physicochemical processing is a combination of physical and chemical methods, used to reduce the filler size and implement molecular modifications at the same time. These treatments show effectiveness in the cases of delignification, depolymerization, reduction of crystallinity etc. The effects of different physical processing coupled with all the possible chemical treatments vary according to the compatibility with the filler used.
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