Martin A. Hubbe

CELLULOSIC NANOCOMPOSITES: A REVIEW

Because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties, a great deal of attention has been paid recently to cellulosic nanofibrillar structures as components in nanocomposites. A first major challenge has been to find efficient ways to liberate cellulosic fibrils from different source materials, including wood, agricultural residues, or bacterial cellulose. A second major challenge has involved the lack of compatibility of cellulosic surfaces with a variety of plastic materials. The water-swellable nature of cellulose, especially in its non-crystalline regions, also can be a concern in various composite materials. This review of recent work shows that considerable progress has been achieved in addressing these issues and that there is potential to use cellulosic nano-components in a wide range of high-tech applications.

Theory of detachment of colloidal particles from flat surfaces exposed to flow

Abstract Theoretical models are presented for the detachment of colloidal particles from solid surfaces exposed to shear flow. The models are most relevant to cases of hard, spherical particles, which are small enough to display Brownian motion. It is concluded that the component of hydrodynamic force acting parallel to a sheared wall is usually much larger than the lifting force. Thus, in most cases, one can expect the downstream component of force to govern the critical or rate-determining step in the process of entrainment. Alternative limiting modes of incipient motion, e.g., rolling, sliding, and lifting, can be distinguished, based on the dependency of the shear stress required for detachment on the size of particles. Rate laws for detachment and the dependency of rates on the applied shear stress permit one to discriminate between processes limited by viscous flow, Brownian motion, and fluctuations in hydrodynamic forces. Finally, it is proposed that separate geometric models of sphere-wall interaction be employed in computing long-and short-range forces.

Dependency of polyelectrolyte complex stoichiometry on the order of addition. 1. Effect of salt concentration during streaming current titrations with strong poly-acid and poly-base

Titrations were carried out between solutions of a strong poly-acid (polyvinylsulfate, potassium salt) and a strong poly-base (poly-diallyldimethylammonium chloride (poly-DADMAC)) over a range of salt concentrations. Streaming current (SC) analysis of the titration endpoints appeared to show increasing deviations from 1:1 stoichiometry of complexation with increased salt. The results depended on the direction of the titration, such that a stoichiometric excess of the titrant (second additive) was required to achieve a SC reading of zero. These symmetrical results, depending on the order of addition, were obtained despite the fact that the plastic surfaces of the SC device had a slight negative charge and differing adsorption tendencies for the two kinds of polymer. A qualitative model of molecular events, based on non-equilibrium entrapment of non-complexed polymer segments was found to be inconsistent with results of tests carried out over a range of initial polymer concentration. Results were better described by a qualitative model involving formation of polyelectrolyte complexes (PECs) in solution, in which near-stoichiometric core complexes are stabilized by an excess of the second additive on their surface. Implications of the latter model were compared with the results of turbidimetric tests, aqueous contact angles on polymer-treated plastic surfaces, and microelectrophoresis of PECs. Results of this study have consequences for interpretation of polyelectrolyte titrations, as well as for industrial operations that involve the mixing of oppositely charged polyelectrolytes.

Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow 2. Mechanism of release

Abstract Experiments show how small particles are detached from a flat window exposed to turbulent shear flow. The onset of detachment is governed by the component of hydrodynamic force which pulls the particles in a downstream sense. An adhesive torque opposes the applied hydrodynamic torque. In the rate-determining step, a released particle rolls from its initial site of attachment. Resistance to rolling is proportional to the product of the net adhesive force and a characteristic length of the region of contact. The kinetics of release indicate that the process is governed by random events. Continued shearing at the same average shear stress results in continued entrainment of particles. The data are consistent with an idealized model of fluctuations in the local hydrodynamic force within the viscous sublayer of turbulent shear flow. Brownian motion does not properly account for the effect of shear stress on the rate of detachment.

Control of tacky deposits on paper machines - A review

SUMMARY: Wood-derived pitch and tacky materials of synthetic origin in recovered fiber streams often cause serious deposit problems on papermaking equipment. Ideally such materials would be completely removed in processes such as screening, cleaning, washing, or flotation de-inking. In practice, tacky materials that remain in the fiber furnish can build up within paper machine headboxes, forming fabrics, press sections, and dryer sections, reducing production efficiency. Product quality is likely to suffer, especially if deposited material ends up in the sheet. This review considers a variety of chemical additives that papermakers have used to combat deposit problems. The premise of this article is that knowledge of the chemistry and colloidal behavior of existing deposit-control agents can guide us in the selection, usage practices, and further development of strategies for the control of tacky deposits, especially in the case of pitch, adhesive-based stickies, and wax-like deposits.

Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow 1. Experimental system

Abstract This article describes an improved technique for studying the detachment of very small particles from solid walls exposed to turbulent shear flow. The technique is useful as an assay of the strength of adhesion between solids immersed in solution. It is also useful in determining the mechanism by which detachment takes place. The experiment is designed so that the particles rest on a window in the outer annular wall of a system of coaxial cylinders. This arrangement permits more rapid counting of particles remaining on precisely the same area throughout the experiment. Results are presented for the detachment of uniform colloidal hydrous oxide spheres from cellulose and glass substrates. Independently controlled variables included the applied shear stress, the size of the particles, the composition of the aqueous solution, and the time of shearing.

The Dispersion Science of Papermaking

Abstract Paper is one of the most important inventions in the history of civilization, and it is an essential commodity to all people in the world. The fact that we make ubiquitous use of a score of paper products makes it easy to underestimate its value and significance. This review is intended to put into perspective the dispersion science involved in papermaking and to describe how our understanding of key processes has evolve since its conception, approximately 2000 years ago, from art to science. Paper is formed from a slurry of fibers and much smaller particles that are often called “fines” and other chemical additives. Ahead of the paper forming process the slurry is subjected to a series of steps, including treatment with polyionic species and passage through unit operations that impose shear forces on the papermaking suspension. These steps alternately disperse the solids apart or re‐gather them back together. The overall process is optimized to achieve a highly uniform product, while at the same time achieving high efficiency in retaining fines in the sheet and allowing water to drain relatively quickly from the wet paper as it is being formed. As we approach the 1900‐year anniversary of the first detailed account of the papermaking process, it is the goal of this review to explore the scientific principles that underlie the art of papermaking, emphasizing the state of dispersion of the fibrous slurries during various stages of the manufacturing process. Some concepts that arise out of the experience of papermakers have potential applications in other fields.

Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow 4. Effect of polyelectrolytes

Abstract Pretreatment of cellulose and glass surfaces with cationic polyelectrolytes greatly increased the force needed to detach titanium hydrous oxide spheres. The force of adhesion was as much as 30 times greater than the highest values obtained in the absence of polymers. The hydrodynamic shear stress required for detachment increased with pretreatment level, molecular mass, and decreasing cationic charge of the polymer. The results are consistent with the presence of polymeric bridging between the solids.

On the Surface Interactions of Proteins with Lignin

Lignins are used often in formulations involving proteins but little is known about the surface interactions between these important biomacromolecules. In this work, we investigate the interactions at the solid-liquid interface of lignin with the two main proteins in soy, glycinin (11S) and β-conglycinin (7S). The extent of adsorption of 11S and 7S onto lignin films and the degree of hydration of the interfacial layers is quantified via Quartz crystal microgravimetry (QCM) and surface plasmon resonance (SPR). Solution ionic strength and protein denaturation (2-mercaptoethanol and urea) critically affect the adsorption process as protein molecules undergo conformational changes and their hydrophobic or hydrophilic amino acid residues interact with the surrounding medium. In general, the adsorption of the undenatured proteins onto lignin is more extensive compared to that of the denatured biomolecules and a large amount of water is coupled to the adsorbed molecules. The reduction in water contact angle after protein adsorption (by ~40° and 35° for undenatured 11S and 7S, respectively) is explained by strong nonspecific interactions between soy proteins and lignin.

Oil Spills Abatement: Factors Affecting Oil Uptake by Cellulosic Fibers

Wood-derived cellulosic fibers prepared in different ways were successfully employed to absorb simulated crude oil, demonstrating their possible use as absorbents in the case of oil spills. When dry fibers were used, the highest sorption capacity (six parts of oil per unit mass of fiber) was shown by bleached softwood kraft fibers, compared to hardwood bleached kraft and softwood chemithermomechanical pulp(CTMP) fibers. Increased refining of CTMP fibers decreased their oil uptake capacity. When the fibers were soaked in water before exposure to the oil, the ability of the unmodified kraft fibers to sorb oil was markedly reduced, whereas the wet CTMP fibers were generally more effective than the wet kraft fibers. Predeposition of lignin onto the surfaces of the bleached kraft fibers improved their ability to take up oil when wet. Superior ability to sorb oil in the wet state was achieved by pretreating the kraft fibers with a hydrophobic sizing agent, alkenylsuccinic anhydride (ASA). Contact angle tests on a model cellulose surface showed that some of the sorption results onto wetted fibers could be attributed to the more hydrophobic nature of the fibers after treatment with either lignin or ASA.