Nano boron oxide and zinc oxide doped lignin containing cellulose nanocrystals improve the thermal, mechanical and flammability properties of high-density poly (ethylene)

Authors: BAJWA, DILPREET - Montana State University; Holt, Gregory; STARK, NICOLE - Forest Products Laboratory; BAJWA, SREEKALA - Montana State University; CHANDA, SAPTAPARNI - Montana State University; QUADIR, MOHIUDDIN - North Dakota State University

Submitted to: Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2023
Publication Date: 12/21/2023
Citation: Bajwa, D., Holt, G.A., Stark, N., Bajwa, S., Chanda, S., Quadir, M. 2023. Nano boron oxide and zinc oxide doped lignin containing cellulose nanocrystals improve the thermal, mechanical and flammability properties of high-density poly (ethylene). Polymers. 16(1). https://doi.org/10.3390/polym16010036.
DOI: https://doi.org/10.3390/polym16010036

Interpretive Summary: Functional items made from multiple materials are often referred to as composites and have been the focus of many research projects over the years. To produce functional composites, bio-based sustainable materials are often sought as raw materials. One of the challenges of using bio-based sustainable materials is their tendency to clump together making it difficult to manufacture the composites. To overcome some of the manufacturing issues, the addition of various additives have been researched. This study evaluated the use of lignin containing cellulosic nanofibers, known as LCNC, to help overcome clumping of materials and improve manufacturing. In addition to LCNC, eco-friendly hybrid systems of Zinc and boron-based additives were used to help improve performance characteristics and reduce the flammability of the composites. Results showed improvements in the mechanical properties, such as the elastic modulus, of 27% for the boron-based additive and 30% for the Zinc additive. However, only the Zinc additive helped improve the thermal stability of the composite. The boron-based additive had an adverse impact due to Oleic acid in the additive. The results showed that both mechanical and thermal properties of the LCNC composites can be improved further but better mixing and/or dispersion of the raw materials used to manufacture the composites is needed.

Technical Abstract: Functional polymer composites have captured the attention of researchers and industrial sectors. To produce functional composites, bio-based sustainable nanofillers like cellulose nanocrystal (CNC) and its derivatives are being exploited. The central challenge of using hydrophilic CNC as nanofiller for polymer composites is their tendency of self-agglomeration in the hydrophobic matrix. Therefore, lignin containing CNC (LCNC) can be used as nanofiller instead of pristine CNC to improve the interfacial interaction between the filler and matrix. Nanoscale fire retardant (FR) additives are added to improve the thermal properties of petroleum-based composites. Nano metal oxides reduce the flammability of polymers by modifying their degradation pathway. Zinc and boron-based compounds are well known for their superior flame retardancy, low toxicity and eco-friendly nature. In this research, safe, effective, and eco-friendly hybrid systems of nano ZnO/LCNC and nano B2O3/LCNC were prepared and incorporated in high density polyethylene (HDPE) matrix to improve their physio-mechanical and FR properties. Nano ZnO nanoparticles were prepared using zinc acetate and sodium hydroxide in aqueous dispersion and were coated onto LCNC particles in different concentrations. Nano B2O3 was produced by ultrasonication of B2O3 powder and was coated onto LCNC particles in different concentrations. These hybrid systems were incorporated in HDPE matrix separately to produce composites via melt blending extrusion process. The composite properties were evaluated using Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), tensile testing, horizontal burning test and microcalorimetry test. In this research, the mechanical properties of the composites were improved because of the reinforcement effect of LCNC and inorganic oxides. There is a 30% increase in elastic modulus observed in the case of ZnO coated composites, while the B2O3 coated composites showed a 27% increase in elastic modulus compared to neat HDPE. The presence of inorganic oxides in the composite system, especially in the case of ZnO, suppressed the adverse effect of the lower thermal stability of lignin and further degradation was prevented. The B2O3 coated composites showed a decline in fire retardant properties, due to the presence of Oleic acid in its structure. The above findings were evidenced by the TGA, horizontal burning test and microcalorimetry test. The SEM micrographs and EDX images demonstrated large agglomeration and irregular distribution of the inorganic oxides in the polymer matrix. Therefore, the overall mechanical and thermal properties of the LCNC incorporated composites can be further improved by improving filler dispersion in the HDPE matrix.