Investigation of morphological, physical and mechanical properties of continuous hemp fiber reinforced polyurethane composites

Polyurethanes (PUR) are one of the most common polymers used in a wide range of industries, from the automotive to the aerospace industry, from white goods to the furniture industry. The global annual production amount of polyurethane ranks fourth after polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC). Polyurethanes have high CO2 emissions per kilogram because they are obtained from two separate industrial chemicals: polyols and diisocyanates. Therefore, reducing the PUR carbon footprint is important for the development of more environmentally friendly materials. Among natural fibers, hemp attracts attention due to its superior properties such as high strength, light weight, corrosion resistance, good sound insulation and moisture absorption, resistance to high temperatures, pilling and static electricity problems. In addition, for the industrialization of hemp, government institutions encourage the development of both high value-added and environmentally friendly materials with new strategies. In this thesis study, biocomposites were prepared by continuous hemp fiber reinforcement into a rigid polyurethane (RPU) matrix. To improve the interfacial interactions between the fiber and RPU in the composite, the fiber surfaces are coated with thermoplastic polyurethane (TPU). Additionally, multi-walled carbon nanotube (MWCNT) was added to the coating to increase its mechanical properties. Thus, in addition to unreinforced RPU, hemp, TPU-hemp and TPU/MWCNT-hemp fiber reinforced composites were prepared. The internal structures and polymer-fiber interactions of the composites were examined with scanning electron microscope (SEM) images. While void formation was not observed in the internal structure of unreinforced polyurethane from the SEM images, it was observed that large volume closed cell foam structure was formed in the hemp reinforced composites. It is thought that the formation of these cell structures is due to the remaining air molecules in the fiber expanding with the heat released during the curing of the RPU matrix and acting as a swelling agent. It was observed that the cell number and size decreased in the TPU-hemp reinforced composite. This result is due to the fact that the coated TPU layer has barrier properties and cannot prevent air molecules from passing into the matrix. It was determined that cell formation was further reduced in the TPU/MWCNT-hemp reinforced composite. The reason of this is interpreted as the fact that the free volume between the TPU chains in the coating decreases with the MWCNT filling, which further reduces the passage of air molecules through the TPU coating. While the RPU density was measured as 0.92 gcm3, the densities of hemp, TPU-hemp and TPU/MWCNT-hemp composites were determined as 0.58, 0.72 and 0.82 gcm3, respectively, depending on the cell formation rates. While the lowest value in the tensile tests was determined by RPU, these values increased with fiber reinforcement and the highest value was obtained in TPU/MWCNT-hemp reinforced composite. Yield strength values showed a similar change and were measured as 9.9 MPa for RPU and 13.9 MPa for TPU/MWCNT-hemp reinforced composite. While the tensile strength for RPU was determined as 12.8 MPa, the highest value was determined as 16.7 MPa in the TPU/MWCNT-hemp reinforced composite. While the maximum stress value from 3-point bending tests was determined as 25.3 MPa for RPU, this value was determined to be 27.4 MPa for TPU/MWCNT-hemp reinforced composite. From the Charpy impact results, it was determined that while the RPU toughness was 6.23 kJ/m2, the TPU/MWCNT-hemp reinforced composite toughness reached 10.2 kJ/m2. From these results, a biocomposite with a lower carbon footprint, lighter weight due to density reduction, and improved mechanical properties was developed by reinforcing RPU with TPU/MWCNT-coated continuous hemp fibers. It has been understood that by improving the fireproof properties of this developed composite, it can be used as a structural material in sectors such as aviation and automotive, construction, white goods and furniture.