Study on Thermal Properties of Coffee Waste-Based Composites

Abstract

Coffee is the most consumed daily beverage by millions of people worldwide and therefore its production is constantly growing. In 2021, coffee production reached 7.7 million tons, which is almost double what it was 30 years ago (Bijla et al., 2022:e14190). In coffee-producing countries, more than 50% of the fruit mass consists of residue from the coffee fruit, and used coffee powder is the waste residue obtained during the brewing process (Campos-Vega et al., 2015:24). Different ways of evaluating SCG are being explored. For example, it is popular to use SCGs as a soil fertilizer (mineral addition) to promote plant growth. Due to their high nutritional value, SCGs have also been additionally tested in animal feed, or, phenolic extracts isolated from SCGs can be used as antioxidants. Carbohydrates present in SCGs are valuable carbon sources for the biotechnological production of polyhydroxyalkanoates, carotenoids, and bioethanol (Kovalcik et al., 2018:104). Apart from these, coffee waste can also be used as a filler in polymer matrices. Many different parameters such as filler type and ratio, the chemical structure of the matrix, modification processes, water and humidity, and UV rays can affect the composite properties. It is known that natural fiber composites exposed to direct sunlight are exposed to radiation and thus the covalent bonds in the organic polymer are broken, causing changes such as discoloration, weight loss, surface roughness, deterioration of mechanical properties, and brittleness (Mahzan et al., 2017:e012021). In this study, biocomposite materials have been developed by using untreated and treated with various methods (alkali (NaOH), microwave (MW), ultrasound (US), and alkali followed by microwave (NaOH+MW)) coffee wastes (CW) in the epoxy matrix. The filling ratio in composites was selected as 10-30-50 wt%. Composites were exposed to UV rays for 360 h. The changes in the chemical structure due to the effect of UV light were illuminated by Fourier-transform infrared spectroscopy (FTIR). The effect of UV radiation on the thermal properties of composites has been investigated using thermogravimetric analysis (TGA). As a result, it was determined that the thermal properties slightly increased as a result of the formation of free radicals from the groups in the epoxy structure and increasing cross-link density under the influence of UV (Korkmaz and Gültekin, 2022:e110004).