Polymer composites for sustainable 3D printing materials

Abstract

Biodegradable and bio-based polymers have raised great attention since sustainable development policies tend to become more and more important with the growing concern for the environment and the decreasing reserve of fossil fuel [1]. The increasing demand for environmentally friendly materials attracted the attention on biopolymers reinforced with cellulose, that is a virtually inexhaustible source of raw material [2] and on new manufacturing ways such as additive manufacturing (AM) [3]. The most diffused AM technology for polymers is Fused Deposition Modelling (FDM), a technique where a filament of thermoplastic polymer is extruded through a nozzle and deposited layer by layer to form the final object with the support of computer aided design. The aim of this work is the development of different kind of thermoplastic biodegradable composites based on commercially available polymers reinforced with cellulose and to study their applicability in fused deposition modeling (FDM). The final goal is the production of plastic filaments suitable to feed a commercially available FDM 3D-printing machine. Starting from microcrystalline cellulose (MCC), two different types of nanocellulose: crystalline nanocellulose (CNC) and nanofibrillated cellulose (NFC) were produced and studied to be applied as natural reinforcing fillers for selected types of biopolymers. Cellulose nanocrystals in water solution were prepared from micro-cellulose through a sulfuric acid hydrolysis while the fibrillated nanocellulose was obtained with high energy ultrasonication. The commercial grade polymer matrices selected in this research were: i. polyvinyl alcohol (PVA), a water-soluble biodegradable material; ii. poly(lactic acid) (PLA), a biodegradable polymer that comes from the fermentation of agricultural waste; iii. poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) that belongs to the family of polyhydroxyalkanoates (PHA) and it is entirely synthesized by microorganism as an intracellular storage product under particular growth conditions. Composite materials containing various amounts of cellulose fillers produced by solution or melt mixing were grinded and extruded through a single screw extruder to obtain filaments. With the aid of a desktop 3D printer, dumbbell specimens were fabricated, and their mechanical properties determined. Several characterization techniques were used in order to assess the effect of micro- and nanocellulose on the physical and thermo-mechanical behavior of these thermoplastic composites. According to SEM analysis, CNC particles appear homogeneously dispersed in PVA without noticeable aggregates. Thermal degradation of PVA was shifted towards higher temperatures with the increase of filler content, enhancing the thermal stability of the composites as compared with neat PVA. An enhancement in the storage modulus with the amount of CNC was observed in both filament and 3D printed specimens. In particular, an increase of about three times in the storage modulus at room temperature was reached in 3D samples with a CNC concentration of 10wt%. An improvement of the dimensional stability was observed with a reduction of the creep compliance with the filler content. Quasi-static tensile tests evidenced an increase of the stiffness and the strength of PVA due to the CNC introduction. A comparison between the reinforcing effect of nanocellulose and microcellulose in 3D printed samples highlighted the higher efficiency of CNC over MCC in reducing the rubber-like behavior of polyvinyl alcohol. Maleic anhydride (MAH) was employed to improve the interaction between hydrophilic microcrystalline cellulose and the PLA matrix. Infrared spectroscopy confirmed the grafting of maleic anhydride on the PLA backbone during melt mixing and SEM analysis revealed that microcellulose was well dispersed in PLA and maleic anhydride was able to enhance the interface between the two components. Thermal degradation of PLA was not affected by the presence of MAH. On the other hand, glass transition temperature, crystallization temperature and melting temperature were lowered by the increasing amount of MAH. Glass transition temperature at 10wt% of MAH decreased from 70°C to 48°C. Tensile tests highlighted that microcellulose in low concentration was able to improve the stiffness and the stress at break of 3D printed specimens. The maximum in term of stiffness and strength is reached for composite at 1wt% of MCC and at 5 wt% with the presence of MAH. NFC was dispersed in PLA by solution mixing and nanocomposites were printed and characterized. The creep compliance curves of the 3D printed samples were well fitted by a power law model and resulted that NFC was able to reduce the time-dependent linear response under constant load conditions, improving the geometrical stability. Static tensile test on plates obtained by solution casting displayed an increase in stiffness of the filament samples with increasing amount of nanocellulose. The same effect was not observed on 3D printed samples where a poor adhesion between subsequent layers was evidenced from SEM analysis upon the introduction of NCF. Lauryl functionalized nanocellulose was incorporated in PLA with solution mixing technique but the limited quantity of materials did not permit to go further with the production of filaments. Scanning electron microscopy indicated that up to a filler content of 6.5 wt. %, LNC was well dispersed. Nanocomposites with 3 and 5 wt. % of LNC showed the highest strain at break and a large amount of plastic deformation due to a strong interfacial adhesion between the PLA and filler particles while for higher LNC fractions the presence of aggregates weakened the nanocomposite. A decrease in stiffness was measured upon the introduction of LNC related to the low stiffness of the short aliphatic chains attached to the surface of the cellulose and so the formation of a soft phase between filler and the matrix as highlighted also by gas permeability tests. Finally, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) was successfully extruded and 3D printed. PHBH and NCF were mixed in solution and extruded in form of filaments used to feed a 3D printing machine. The reinforcing effect of the nanocellulose in terms of stress at break and of elongation at break showed a maximum at a content of 0.5 wt%. An increase in stiffness for filament with increasing amount of nanocellulose was measured but also in this case it was not observed in 3D printed samples. Anyway, the presence of NCF did not affect the thermal behavior of the materials.