1. Nanostructured Material
The nanohybrid could significantly enhance the polymer properties without a loss of clarity in the polymer. Although many different polymer-inorganic hybrids have been developed and studied recently, the water-based polymer encapsulated nanocomposite dispersion has not been successfully developed. It has been well known that water-based latexes have many advantages over other types of polymers, particularly when they are used in painting, coating, ink, toner, adhesives, and drug delivery. Obviously, if well-dispersed and polymer encapsulated inorganic nanohybrid latexes can be developed, new applications of nanohybrids can be achieved.
A novel polystyrene-encapsulated laponite composite system has been developed via a miniemulsion polymerization approach in our laboratory. The encapsulation mechanism and process parameters have been examined in detail. The nanoclay was encapsulated through a miniemulsion polymerization process. The stability of both miniemulsion and latex depends on initiation loci, premixing procedures, intensity and time of ultrasonification, and surfactants and co-stabilizer used. Hydrophobicity of the laponite clay played a vital role in both the encapsulation of the clay and the stability of the latex. It was found that the hydrophobicity not only favored the clay dispersion in the oil droplets but also aided the entry of the monomer into the clay’s intergalleries during polymerization. Through miniemulsion polymerization, hemisphere nanoparticles were also synthesized (Figure 1). These hemispheres could be used as a template for synthesis of asymmetrical nanostructured and multifunctional materials.
Polyaniline, as one of the most useful conducting polymers, has been attracting great research interests in recent years because of the low cost, ease of preparation, environmental stability, and reversible acid/base doping/dedoping characteristics. One-dimensional nanostructured polyanilines, such as fibers, tubes and wires have been synthesized primarily by a template method or self-assembly via conventional chemical oxidative polymerization using both single-phase and interfacial polymerization routes. Recently, we successfully synthesized polyaniline nanorods with extremely high crystallinity (>95%). These nanorods could self-assemble into nanorod microspheres, as shown in Figure 2.
Synthesizing high aspect ratio aragonite nanoparticles by mass and diffusion control in liquid emulsion membrane is also one our group research interests. The most interest in this area is to use mass and kinetic control approaches to synthesize nanoparticles. The most important contribution of this research is to provide an innovation method of assembling nanoparticles for chemical engineering, materials science and bioenginering. One possible application of nano-PCC is its high surface area, high reflect index and light scattering coefficient.
Nano-size inorganic oxide particles have many advantages over micrometer size particles. Precipitated calcium carbonate (PCC) is one of the most common fillers used in papermaking, paper coating, and polymer composites. Nano-sized PCC materials, including particles and wires, have been synthesized and used in many areas such as detergent additives of lubricating oil. As a long-term goal, our research focuses on both fundamental understanding of nanoparticle synthesis, such as PCC, and their applications in a real industry, such as filler in papermaking and coating.
The individual emulsion droplet in the LEM is designed as a microsize reactor, which is separated from other reactors. Nanoparticles with desired sizes can be synthesized by the total masses of reaction materials in the microsize reactors (mass control).
The layer-by-layer self-assembling technique has been used in synthesis of need shape TiO2 nano-hollow materials. This high specific surface material has great potential in high effective catalysts.
2. Fundamental Study of Papermaking Chemistry
Filler flocculation and retention study and their control at high turbulence conditions are one of our interested research areas. Papermakers are continuously searching for methods to decrease costs and improve product quality. One approach to achieve cost savings is increasing the filler content of the sheet, thereby replacing expensive fiber and improving paper drainage, paper dying, and optical properties of final paper products. However, high filler content usually reduce the paper strengths.
Aggregation of colloidal particles suspended in an aqueous media can be achieved by flocculation. Recently, nonimaging reflectance scanning laser microscopy (also known as focused beam reflectance measurement, FBRM) as a characterization technique has been used for studying the flocculation of wood fibers and fillers. The advantages of FBRM are this equipment can directly measure the flocculation of particles at real time and turbulence conditions.
Charge neutralization and charge control are very important in papermaking. However, the detail of charge neutralization reaction between two oppositely charged polyelectrolytes still remains unknown. Common methods used for studying charge neutralization reaction are colloid titration using a water-soluble dye as an indicator, and using a Mutek titrator to measure the end point of the reaction, but they cannot tell us if the end point of the titration can really represent a stoichiometric reaction in the system. It is still unclear that if all the anionic components in pulp slurry, such as anionic polymers vs small anionic organics, have been fully neutralized at end point. It is reasonable to believe that charge neutralization reaction and stoichiometric rule are molecular structure dependant. However, both colloidal titration using a dye as indicator or streaming current methods cannot tell the detail mechanism of charge neutralization.
Recent our study using a semipermutable membrane method provided a unique tool for studying charge neutralization in aqueous solution. In this study, an effective membrane and an UV spectrum meter (or a total organic analyzer) will be used to study the real charge neutralization reaction between two oppositely charged polyelectrolytes or particles. The method is not only be able for studying charge neutralization reaction, but also be able for polymer adsorption study. This work has been accepted to publish in Journal of Colloid and Interface Science. The future interest in this area is to expand our preliminary study to broad and complicate systems, and eventually to develop an online sensor for measuring cationic demand and charge neutralization reaction in pulp slurry.
3. New Polymer Development
Developing novel polymer for paper industry was one of my research focuses. Cationic nanoparticle retention aid and nonionic retention aid are two types of retention systems that have been studied in past. In the past decades, wet-end operations have been dramatically changed mainly because of the shift from acid to alkaline papermaking, greater use of high-yield and recycled pulp, and faster machine speeds. Because of these changes, many new retention systems, such as dual retention, microparticle retention, and micropolymer retention, have been brought into the paper industry. Although these new systems may greatly improve first-pass retention and paper formation for fine paper grades, they are less effective for high-yield pulps containing large amounts of dissolved and colloidal anionic substances. Therefore, development of new retention systems that can achieve high first-pass retention and reduce the total cationic demand simultaneously is desired by the paper industry.
The cationic polymeric microparticle was proved to be an effective flocculate for negatively charged PCC suspension in our early study. Potentially, a cationic polymeric microparticle have advantages over a water-soluble polyelectrolyte when it is used as a retention aid or co-retention agent because: (1) there is no conformation change when the cationic nanoparticle adsorbs onto a negatively charged substrate, which results in a more effective patch formation; (2) the bridge length can be exactly controlled by particle size; (3) the total cationic demand of the pulp furnish in papermaking can be reduced; (4) the cationic microparticles do not penetrate into the pores of the fiber surface due to their inflexible structure; and (5) there is no accumulation of microparticles in a closed white water system because of the strong bonding force between cationic nanoparticles and negatively charged substances. Our previous research indicated that cationic nanoparticle system is a potential retention system that functions differently as traditional retention aids. However, the mechanism of this unique system has not been understood and further study is needed.
Biodegradable polymer and materials from wood fibers are also interested in our group. The extensive research on development of degradable plastic suitable for packaging application started in the early 1970s. Cellulose can be hydrolyzed with the help of cellulases in landfills. The functional group, hydroxyl of cellulose, can graft polyolefin side chains onto cellulose backbone with different degree of substitution (DS) if the reaction condition is controlled properly. The cellulose backbone can be hydrolyzed if the DS is low enough to be less obstructive for the penetration of cellulases. After the cellulose backbone is hydrolyzed, the physical properties of plastic film will lose dramatically and the short grafted polyolefin chains are easy to undergo further oxidizing or free radical degradation reactions to fulfill the entire polymer matrix environmental degradation. Since the reactions happen primarily at the interface of polymer film, adding faster degradable filler into polymer matrix to increase the spots, channels, and total surface areas for enzymes penetration into plastic film is necessary. Wood fiber flour is biodegradable but it is hydrophilic and amorphous parts can absorb water to swell. The surface of wood fiber powder has to be modified to hydrophobicity to be compatible with the hydrophobic polymer matrix. The properties of polymer composites, such as degradation speed, hydrophobicity, physical characteristic, etc., can be adjusted by different methods of cellulose and wood fiber modification, ratio of modified wood fiber flour versus polymer matrix, etc.
4. Filler Modification and New Filler Development for High Filler Containing Paper
Increasing filler content in pulp products has been interested for a long time by paper industry. The paper industry utilizes fillers either to improve process economics or to provide desired functional or end-use properties of paper products. It is estimated that $2.50/ton is saved for each one percent increase of filler in paper. Most filled papers are produced for the printing and writing grades, although recently tissue filler applications have been studied. Sheet properties normally improved by fillers include opacity, brightness, gloss, smoothness, porosity, and printability. In fact, some of these paper qualities cannot be achieved without the use of fillers.
However, there are disadvantages associated with higher filler loadings beyond a certain level, which include reduced paper strength, increased size demand, abrasion, and dusting. Researchers have been conducted to increase the filler loading without incurring those disadvantages. Approaches that have been sought include different pulp and filler combinations, preflocculation of filler, incorporation of starch in the sheet, and retention of filler in regions where it is unlikely to interfere with inter-fiber bonding (such as lumen loading). Therefore, filler modification and new filler is one of the solutions for reduced strength properties when filler content is increased.
Filler modification using novel polymer coating technique and new filler synthesis are active research area in our group. “Bulky” filler and filler aggregates will also been considered. Novel PCC fillers with different aspect ratios (up to 100) and structures have been synthesized in my research group. Assemble of fillers to give a high open structured using modified nanosilicate and low density bulky filler aggregates has many potential for different paper grades. Our past research also provided the fundamental information needed to engineer the next generation of fibrous additives for composite pulp product platforms. Few PCC samples with different structures synthesized in our group are shown below. Many of these fillers have never been reported in the literature before. A very large filler route (150 mm) was also synthesized by our group. We have not found any research group has been able to synthesize this large needle-like filler. The new filler synthesis and fundamental study will not only benefit paper industry, but also be very interesting projects for material science and engineering, chemical engineering, biomineral and plastic industry.