Plastic is indispensable and essential to modern civilized society because it has excellent properties and low price compared to other materials (Andrady et al. 2009). Thanks to these advantages, a wide range of applications, such as packaging materials, digital devices, and medical equipment, are made from plastics. However, the use of plastic has led to various environmental problem. During its manufacture, plastic emits greenhouse gases that causes global warming and leaks endocrine disruptors when incised or buried. In addition, plastics flowed into the sea are threatening the ecosystem because it has been poorly biodegraded in nature (Lechner et al. 2014; Zarfl and Matthies. 2010). Human health is threatened by such plastic problems. Therefore, more and more consumers are interested in eco-friendly products, and countries are imposing harsh regulations on the use of plastic (Li and Zhao. 2017).
For this reason, considerable interest has been generated for developing and using eco-friendly biocomposite products. Biocomposite is the combination of natural fibers and polymer matrix (Ramamoorthy et al. 2015). Generally, various types of cellulose fibers such as wood fiber, kenaf, cotton, flax, hemp, jute and sisal have been used as reinforcement or fillers for biocomposite because of their biodegradable, inexpensive and renewable advantages (Alemdar and Sain. 2008). The mechanical properties of biocomposite are influenced by filler and matrix properties. The high strength and stiffness characteristics of pulp fibers could improve the strength of the biocomposite, which has been confirmed in several studies (Xie et al. 2010; Thakur and Thakur. 2014; Trache et al. 2016).
Among various types of cellulose-based fibers, wood fibers are manufactured in large quantities in pulping and papermaking processes. Thus, stable supply is possible, and the price is lower than that of non-wood or mineral fibers. Numerous studies that use wood pulp as reinforcement for biocomposites have been reported. For example, Peltola et al (2014) investigated the use of bleached soft wood kraft pulp to produce PP and polylactic acid (PLA) composite and found that the tensile strength and modulus of biocomposite were improved significantly because of well-dispersed fibers in the polymer matrix. Besides, the addition of wood pulp reinforcement in the polymer matrix increases the degradation temperature and crystalline degree (López et al. 2012).
Although wood pulp is a suitable candidate for filler of biocomposite, there are challenges to overcome. Wood pulp has some drawbacks such as fluffy nature, low bulk density, high moisture absorption and incompatibility with most polymer matrix, etc. Notably, fluffy nature not only makes it difficult to feed raw materials into the extruder but also increase the transportation costs (Mahdavi et al. 2010). The fluffy nature of the pulp could be solved by manufacturing cellulose microfiber. Diverse manufacturing process to obtain cellulose microfiber exist, such as dry milling, chemical pretreatment and mixing of these technics. Acid hydrolysis is commonly used to produce cellulose microfiber like microcrystalline cellulose (MCC) on the industrial scale. However, the production process is not environmentally friendly because of the high concentration of sulfuric acid used in the manufacturing process (Araki et al. 1998). The use of high concentration sulfuric acid consumes a large amount of water for neutralization and generates high chemical oxygen demand (COD) of wastewater. Diverse chemical treatment methods had been studied for environmentally friendly production of cellulose microfiber. However, the effect of cellulose microfiber characteristics prepared by different chemical treatments on the performance of biocomposite has not been studied in detail.
In this study, the influence of chemical treatments on cellulose microfiber characteristics, such as morphology, chemical structure, crystallinity and so on, were investigated. Sulfuric acid hydrolysis, glyoxal crosslinking and acetylation were conducted to produce chemically treated cellulose microfiber. Polypropylene (PP) was selected as the matrix because it has easy processability, low price and high thermal stability. The biocomposites were produced by mixing cellulose microfiber, PP and compatibilizer at a ratio of 30%, 67% and 3%, respectively. In addition, the effect of microfiber types on physical, thermal and mechanical properties of biocomposites was also examined.