Abstract
The dry pulp direct kneading method is an industrially viable, low-energy process for manufacturing cellulose nanofiber (CNF)-reinforced polymer composites, where the chemically modified pulps are nanofibrillated and uniformly dispersed in the polymer matrix during melt compounding. In the present study, cellulose fibers of various sizes ranging from surface-fibrillated pulps (20 μm in width) to fine CNFs (20 nm in width) were prepared from softwood bleached kraft pulps using a refiner and a high-pressure homogenizer. These cellulose fibers were modified with alkenyl succinic anhydride and dried. The dried fibers were used as a feed material for melt compounding in the dry pulp direct kneading method to fabricate CNF-reinforced high-density polyethylene (HDPE). When surface-fibrillated pulps were employed as a feed material, the pulps were nanofibrillated and dispersed uniformly in the HDPE matrix during melt compounding. The resulting composites had much better properties—i.e., much higher tensile modulus and strength values, and much lower coefficient of thermal expansion values—than the composites produced using pulps without pre-fibrillation. However, when CNFs were used as a feed material, they were shortened and agglomerated during melt compounding, and the properties of the composites consequently deteriorated. The study concludes that surface-fibrillated pulp, which can be produced cost-effectively using a refiner on an industrial scale, is more suitable as a feed material than CNFs for melt compounding in the dry pulp direct kneading method. This finding enables the elimination of a preliminary step in the preparation of CNFs from pulps, which is a time-consuming and energy-intensive process.Graphical abstract
Highlights
A cellulose nanofiber (CNF) is a fundamental element of plant cell wall, where cellulose molecules are extended and form semi-crystalline structures(Moon et al 2011)
When surface-fibrillated pulps were employed as a feed material, the pulps were nanofibrillated and dispersed uniformly in the high-density polyethylene (HDPE) matrix during the melt-compounding, and the composites had much better properties than the composites produced using the pulps without pre-fibrillation
The surface-fibrillated pulp, which can be produced cost-effectively using a refiner at an industry scale, was a more suitable form than the CNF as a feed material for melt-compounding in the dry-pulp direct kneading method
Summary
A cellulose nanofiber (CNF) is a fundamental element of plant cell wall, where cellulose molecules are extended and form semi-crystalline structures(Moon et al 2011). The extended and aligned cellulose molecules give a CNF excellent mechanical properties and coefficient of thermal expansion, which attracts industry to utilize CNFs to reinforce polymers. Many researchers have been trying to improve the low affinity by modifying CNF surface chemically (Hassan et al 2014; Sato et al 2016; Yano et al 2018), adding compatibilizers (Qiu et al 2005; Volk et al 2015; Sakakibara et al 2016; Suzuki et al 2016), and/or modifying mixing process of CNFs and polymers (Hietala et al 2013; Suzuki et al 2013; Oksman et al 2016)
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