Douglas J. Gardner

Adhesion and Surface Issues in Cellulose and Nanocellulose

This paper provides a review of the scientific literature concerned with adhesion and surface properties of cellulose and nanocellulose. Cellulose is the most abundant chemical compound on earth and its natural affinity for self-adhesion has long been recognized. The ease of adhesion that occurs in cellulose has contributed to its use in paper and other fiber-based composite materials. Cellulose adhesion, which has received considerable attention over the past half century, occurs over a practical length scale ranging from the nanoscale to millimeters. Adhesion theories that have been examined in the bonding of cellulose fibers include: mechanical interlocking, adsorption or wetting theory, diffusion theory, and the theory of weak boundary layers. Cellulose fibers on the nanoscale are prepared in four different ways: (1) bacterial cellulose nanofibers, (2) cellulose nanofibers by electrospinning, (3) microfibrillated cellulose plant cell fibers and (4) nanorods or cellulose whiskers. Structure and properties of nanocellulose that are important include: morphology, crystalline structure, surface properties, chemical and physical properties, and properties in liquid suspension. Cellulosic nanofibers present a very high surface area which makes the adhesion properties the most important parameter to control for nanocomposite applications. In this paper, we will focus on discussion of the adhesion and surface characteristics of cellulose nanofibers that impact its properties and application in nanomaterials.

Drying cellulose nanofibrils: in search of a suitable method

Increasing research activity on cellulose nanofibril-based materials provides great opportunities for novel, scalable manufacturing approaches. Cellulose nanofibrils (CNFs) are typically processed as aqueous suspensions because of their hydrophilic nature. One of the major manufacturing challenges is to obtain dry CNFs while maintaining their nano-scale dimensions. Four methods were examined to dry cellulose nanocrystal and nanofibrillated cellulose suspensions: (1) oven drying, (2) freeze drying (FD), (3) supercritical drying (SCD), and (4) spray-drying (SD). The particle size and morphology of the CNFs were determined via dynamic light scattering, transmission electron microscopy, scanning electron microscopy, and morphological analysis. SCD preserved the nano-scale dimensions of the cellulose nanofibrils. FD formed ribbon-like structures of the CNFs with nano-scale thicknesses. Width and length were observed in tens to hundreds of microns. SD formed particles with a size distribution ranging from nanometer to several microns. Spray-drying is proposed as a technically suitable manufacturing process to dry CNF suspensions.

The Pyrolytic Behavior of Selected Lignin Preparations

Abstract A pyrolytic study of a variety of lignins prepared from hardwood and softwood species, and of selected commercial lignins, was performed using isothermal and dynamic thermogravimetric analysis and pyrolysis-gas chromatography. The lignins were evaluated according to weight loss, rate of weight loss, and products formed during the pyrolysis in an inert atmosphere. These data were then compared to nitrobenzene oxidation data. Infrared spectroscopy was used to determine structural changes of lignin during initial weight loss in the temperature range of 250° to 320°C. Structural changes appear to occur without any appreciable weight loss. Lignins thermal behavior during pyrolysis depends upon the degree of condensation present in the initial lignin. Thermogravimetric analysis is shown to be a simple and accurate method to determine the extent of lignin condensation. The yield of monomeric phenols is also dependent on the degree of lignin condensation.

Factors influencing contact angle measurements on wood particles by column wicking

The present work focuses on a capillary rise technique, referred to here as column wicking, for determining contact angles on wood particles. The liquid front rise versus time for different probe liquids has been measured for extracted and non-extracted spruce wood particles packed into glass columns. Wood is a porous, heterogeneous, and hygroscopic material. The sorption process of certain polar liquids in the wood substance, i.e. bulk sorption, is exothermic and causes swelling. This bulk sorption process and the resulting release of heat are observed as a distinct temperature increase within the columns during the wicking of water, formamide, and methanol. No temperature increase is observed for ethylene glycol, diiodomethane, and hexane. In some cases, the increase in temperature is observed in advance of the moving visible liquid front line. This may indicate that vapor is moving in advance of the liquid front, resulting in bulk sorption and the corresponding release of heat. An apparent non-linearit...

Preparation and characterization of transparent PMMA–cellulose-based nanocomposites

Nanocomposites of polymethylmethacrylate (PMMA) and cellulose were made by a solution casting method using acetone as the solvent. The nanofiber networks were prepared using three different types of cellulose nanofibers: (i) nanofibrillated cellulose (NFC), (ii) cellulose nanocrystals (CNC) and (iii) bacterial cellulose from nata de coca (NDC). The loading of cellulose nanofibrils in the PMMA varied between 0.25 and 0.5 wt%. The mechanical properties of the composites were evaluated using a dynamic mechanical thermal analyzer (DMTA). The flexural modulus of the nanocomposites reinforced with NDC at the 0.5 wt% loading level increased 23% compared to that of pure PMMA. The NFC composite also exhibited a slightly increased flexural strength around 60 MPa while PMMA had a flexural strength of 57 MPa. The addition of NDC increased the storage modulus (11%) compared to neat PMMA at room temperature while the storage modulus of PPMA/CNC nanocomposite containing 0.25 and 0.5 wt% cellulose increased about 46% and 260% to that of the pure PMMA at the glass transition temperature, respectively. Thermogravimetric analysis (TGA) indicated that there was no significant change in thermal stability of the composites. The UV-vis transmittance of the CNF nanocomposites decreased by 9% and 27% with the addition of 0.25 wt% CNC and NDC, respectively. This work is intended to spur research and development activity for application of CNF reinforced PMMA nanocomposites in applications such as: packaging, flexible screens, optically transparent films and light-weight transparent materials for ballistic protection.

Synthesis of bacterial cellulose using hot water extracted wood sugars

Bacterial cellulose (BC), a type of nanopolymer produced by Acetobacter xylinum is a nanostructured material with unique properties and wide applicability. However, a standard medium used for the cultivation of BC, the Hestrin-Schramm medium, is expensive and prevents wide scale extension of BC applications. In this research, a relatively low-cost culture media was successfully developed from wood hot water extracts for the Acetobacter xylinus 23769 strain. Hot water extract (HWE) is a residual material originating from pulp mills and lignocellulosic biorefineries and consists of mainly monomeric sugars, organic acids and organics. The effects of different pH (5, 6, 7 and 8) and temperatures (26, 28 and 30°C) were also examined in this research. There were no significant differences in the crystallinity and the recorded Iα fraction of cellulose produced between Hestrin-Schramm and the HWE medium. The maximum production of 0.15g/l of BC was obtained at a pH of 8 and temperature of 28°C. Glucose and xylose in the HWE were the main nutrient sources utilized in all BC cultivations based on high-pressure liquid chromatography (HPLC) results. HWE was shown to be a suitable carbon source for BC production, and a process was established for BC production from lignocellulosic feedstocks without using any modification of the HWE. HWE is an abundant and relatively inexpensive forest by-product. Using HWE for BC production could reduce burdens on the environment and also, achieve the goal of large scale BC production at low cost without using added culture nutrients.

Production and Characterization of Cellulose Nanofibers from Wood Pulp

Cellulose nanofibers with a size range of 5–100 nm have the potential to be a low cost renewable material that has application in a range of products. However, current chemical methods to produce crystalline nanofibers suffer from low yields and high chemical costs, while mechanical methods require high energy costs. Methods to lower the energy costs of the mechanical methods have not been well documented in the literature. A bleached softwood kraft pulp was processed using a mechanical dispersion mill and a homogenizer to produce cellulose nanofibers. Two different commercial enzymes were used to pretreat the wood fibers before the mechanical treatments. The resulting nanofibers were characterized by light microscopy, atomic force microscopy, and inverse gas chromatography. Results indicate that the dispersion mill does not affect the overall pulp fiber fibrillation, but does help prepare the sample for the homogenizer. Most fibrillation occurs after three passes through the homogenizer. The enzyme pretreatment has little effect on the size of the fibers, but does allow for higher solids to pass through the homogenizer without clogging. The dispersion component of surface energy of the resulting nanofibrils is impacted by the type of enzyme used. The measurement of acid–base properties proved to be challenging using current IGC experimental protocols.

Surface treatments of wood–plastic composites (WPCs) to improve adhesion

In an effort to improve the adhesive bonding between wood–plastic composites (WPCs) formulated with polypropylene and a commercial epoxy adhesive, surface treatments were performed to chemically and/or physically modify the surface of WPCs. The treatments were performed on extruded WPC that had been planed and consisted of chromic acid treatment, flame treatment, water treatment, flame then water treatment and water then flame treatment. The strength of the adhesive bonds of the treated samples was tested following ASTM D 905 and the maximum shear stress was calculated for each treatment. The chromic acid and flame treatments increased their respective average shear strengths by 97% and 67% compared to an untreated control group. The increase in bond strength due to these two treatments is believed to be a result of their oxidative mechanisms. The water treatment, which consisted of covering the planed surface of a WPC with water for 10 min, resulted in an increase in shear strength of 31% relative to the control. Characterization of the water-treated WPC surface with profilometry and scanning electron microscopy indicated that the likely mechanism for the increase in bond strength was the absorption of water and subsequent swelling of the wood present in the WPC, creating greater surface area for bonding. The combination of flame and water treatments showed increased shear strength relative to the individual treatments alone, indicating that the two processes might act synergistically to facilitate the formation of stronger adhesive bonds.

Influence of drying method on the surface energy of cellulose nanofibrils determined by inverse gas chromatography

Research and development of the renewable nanomaterial cellulose nanofibrils (CNFs) has received considerable attention. The effect of drying on the surface energy of CNFs was investigated. Samples of nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNC) were each subjected to four separate drying methods: air-drying, freeze-drying, spray-drying, and supercritical-drying. The surface morphology of the dried CNFs was examined using a scanning electron microscope. The surface energy of the dried CNFs was determined using inverse gas chromatography at infinite dilution and column temperatures: 30, 40, 50, 55, and 60 °C. Surface energy measurements of supercritical-dried NFCs were performed also at column temperatures: 70, 75, and 80 °C. Different drying methods produced CNFs with different morphologies which in turn significantly influenced their surface energy. Supercritical-drying resulted in NFCs having a dispersion component of surface energy of 98.3±5.8 mJ/m(2) at 30 °C. The dispersion component of surface energy of freeze-dried NFCs (44.3±0.4 mJ/m(2) at 30 °C) and CNCs (46.5±0.9 mJ/m(2) at 30 °C) were the lowest among all the CNFs. The pre-freezing treatment during the freeze-drying process is hypothesized to have a major impact on the dispersion component of surface energy of the CNFs. The acid and base parameters of all the dried CNFs were amphoteric (acidic and basic) although predominantly basic in nature.

Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase

The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (δ) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz–van der Waals work of adhesion (W a LW) and the acid–base interaction parameter (I a−b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid–base interaction (I a−b) or by reducing the chemical incompatibility (Δδ) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.