1 INTRODUCTION
With the increasing awareness of green environmental protection, the non-incineration utilization of biological waste is one of the subjects that has been widely studied in many countries. Coffee is one of the world’s top three drinks, with huge consumption every year(Franca and Oliveira, 2019). Spent coffee grounds (SCG), wastes from the coffee production process, are rich in fiber, oil, and protein (Ballesteros et al., 2014; Bomfim et al., 2022; McNutt and He, 2019) and have been developed to recycle utilization in clothing (Kalebek, 2021), organic fertilizer (Ragauskaitė and Šlinkšienė, 2022; Santhanarajan et al., 2021), medicine (Jamari et al., 2021; Nurman et al., 2021) and other fields. In addition, SCG is contained in natural pigments to dodge or reduce the use of artificial colors (Arya et al., 2022; Koh and Hong, 2019; Parra-Campos and Eduardo Ordonez-Santos, 2019). Biomass natural fiber has the advantages of low cost, abundant sources, and better biodegradability (Madyaratri et al., 2022).
It is worth noting that microplastic (Frias and Nash, 2019) produced by the slight degradation of conventional plastic polypropylene (PP) can directly harm human beings by polluting soil (Watteau et al., 2018; Zhang et al., 2018), fresh water (Rodrigues et al., 2018), and air (Zhang et al., 2020). Degradable plastics are conducive to solving these problems, such as starch-based plastics, polylactic acid (PLA), etc. But their application is always constrained due to the high development costs, certain performance limitations, and immature manufacturing technology.
A certain amount of biodegradable biomass waste is added to traditional plastics as fillers to prepare biomass-plastic composite materials (Bensalah et al., 2021; Qasim et al., 2020), which can effectively utilize the waste biomass resources, reduce the use of plastics and upgrade the degradability of the plastic. The characteristics of SCG, particularly the residual oil content, are vital for the performance of the subsequent SCG/PP composite plastics (Kalebek, 2021). The lower oil content in SCG is believed to obtain composites with a denser fiber structure and higher mechanical properties (Wu et al., 2016; Zarrinbakhsh et al., 2016). The extraction methods of oil from biomass include organic solvent extraction (Mueanmas et al., 2019; Unugul et al., 2020), ultrasonic-assisted extraction (Le et al., 2020), pressurized liquid extraction (de Almeida-Couto et al., 2022), and pyrolysis (Kumar et al., 2020). Hesham Moustafa et al. (Moustafa et al., 2017) heat-treated SCG at 270 °C in the nitrogen atmosphere. The obtained SCG was less oil and more hydrophobic. The resulting SCG/PBAT composite had better mechanical properties. Carmen Branca et al. (Branca et al., 2006, 2005) specified that the non-inert atmosphere is also beneficial to the extraction of bio-oil from cellulose and wood. Nonetheless, the effect of the atmosphere on SCG and SCG plastic composites has not been studied systematically.
In the present study, the effect of heat treatment parameters on the properties of SCG and SCG/PP composite plastics is investigated from the point view of the atmosphere. The results show that the SCG treated under air atmosphere is less oil and higher abrasive than under nitrogen atmosphere at the same temperature, which holds better affinity with PP by increasing the hydrophobicity at a relatively low cost and an eco-friendly process. The resulting SCG/PP composite is lower water absorption, elevated hydrophobicity, and mechanical properties, which have potential applications in the field of food packaging.