(54a) Efficient Conversion of Pulp and Paper Sludge to Cellulose Nanomaterials and Their Application As Building Blocks for Constructing Conductive Nanopapers and Hydrogels

Recently, cellulose nanomaterials (CNM) include cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) produced from cellulosic materials (e.g. wood, agricultural and industrial waste, etc.) have received increasing attention due to their unique properties such as high specific surface area, high elastic modulus, high thermal stability, as well as renewability and biodegradability. These unique properties make CNM as promising building blocks for the manufacture of many sustainable nanomaterials.

In this work, we firstly tried to prepare CNFs from pulp and paper sludge (PPS) by a sustainable process of formic acid (FA) hydrolysis pretreatment combined with microfluidization. It was found that the mild FA hydrolysis (at 95 ºC for 3-6 h) pretreatment could hydrolyze most of hemicellulose, swell and break down the cellulose fibers, and the cellulosic solid residue (CSR) with a high yield (over 75%) could be obtained. The resultant CSR could be further converted to CNFs with relatively low-intensity microfluidization (only two passes). Then, we tried to introduce a small amount of FeCl₃ into the FA hydrolysis process as catalyst to enhance the hydrolysis efficiency. Results showed that CNCs could be produced with a comparable yield of 20-30 wt.% with traditional sulfuric acid hydrolysis, and the CSR could be further fibrillated to CNFs with low-intensity fibrillation. In addition, FA could be readily recovered and reused with the recovery rate higher than 90%. Intriguingly, both the resultant CNCs and CNFs showed good dispersibility in DMSO, DMF and DMAC respectively because of the introduction of ester groups on the surface of the products during FA hydrolysis.

Furthermore, cellulose nanopapers (CNP) were prepared from the above obtained CNFs dispersion in DMAC by a vacuum filtration approach. A thin layer of silver nanowires could be further introduced on top of the CNP surface to make conductive CNP. Results showed that the resultant conductive CNP had high transparency (over 80% at 550 nm), high thermal stability (onset thermal degradation temperature was over 320 ºC), low resistance (45.57 Ω/sq) and excellent water-resistance (could remain the shape in water over 100 days), which showed great promise for the development of flexible and multi-functional electronics (e.g. touch screen).

In addition, both of the CNCs and CNFs were coated with a thin layer of polypyrrole (PPy) via in situ polymerization. The PPy-coated CNCs and CNFs as conductive nanofillers were further incorporated into polyvinyl alcohol (PVA)-borax hydrogels respectively. It was found that the composite hydrogels showed high viscoelasticity and mechanical strength, as well as good conductivity, which had potential applications in multifunctional smart soft materials.