Rajesh K. Jain

Fiber-reinforced cellulosic thermoplastic composites

Steam-exploded fibers from Yellow poplar (Liriodendron tulipifera) wood were assessed in terms of their thermal stability characteristics, their impact on torque during melt processing of a thermoplastic cellulose ester (plasticized CAB) matrix, their fiber–matrix adhesion and dispersion in composites, and their mechanical properties under tension. Fibers included water-extracted steam-exploded fibers (WEF), alkali extracted fibers (AEF), acetylated fibers (AAEF), and a commercial milled oat fiber sample (COF) (i.e., untreated control). The results indicate that the thermal stability of steam-exploded fibers increases progressively as the fibers are extracted with water and alkali and following acetylation. The greatest improvement resulted from the removal of water-soluble hemicelluloses. The modification by acetylation contributed to improved interfacial wetting that was revealed by increased torque during melt processing. Whereas modulus increased by between 0 and 100% with the incorporation of 40% fibers by weight, tensile strength either declined by ⅓ to ½ or it increased by a maximum of 10%, depending on fiber type. AAEF composites produced the best mechanical properties. Fiber–aspect ratio was reduced to an average of 25–50 from ≫ 200 during compounding. The superior reinforcing characteristics of AAEF fibers were also reflected by SEM, which revealed better fiber–matrix adhesion and failure by fiber fibrillation rather than by fiber pullout.

Lignin derivatives. I: Alkanoates

The synthesis, isolation, chemical and molecular structure determination, and the thermal properties are described for several esters of aliphatic monocarboxylic acids with non-sulfonated lignins.

Thermoplastic Xylan Derivatives with Propylene Oxide

Polymeric xylan can be reacted with propylene oxide (PO) in aqueous alkali homogeneously. Since xylan is isolated from biomass in aqueous alkaline solution, an ‘in-line’ modification with PO as part of the isolation protocol, is most practical. Hydroxypropyl xylan (HPX) is a low molecular weight, branched, water-soluble polysaccharide with low intrinsic viscosity and thermoplasticity. Following peracetylation of HPX in formamide solution, water-insoluble acetoxypropyl xylan (APX) is formed that is also thermoplastic but no longer water soluble. The glass transition temperature (Tg) of APX varies in relation to degree of substitution with hydroxypropyl groups (DSPO), and this is found to decline from 160 to 70°C as DSPO rises from 0.2 to 2.0. At a temperature above the Tg of HPX a molecular reorganization is noted, and a faint transition due to melting (Tm) is observed at 205°C. HPX thermally degrades with a weight loss maximum at 317°C, or approximately 60°C below that of a corresponding cellulose derivative. HPX forms clear films when solvent cast from aqueous solution. Films are higher in ultimate tensile strength and lower in toughness than corresponding cellulose derivative films. The properties of HPX and APX derivatives qualify this material as a potential biodegradable and thermoplastic additive to melt-processed plastics. Blend characteristics with polystyrene reveal a shear-thinning effect in melt and a plasticization effect in solid state.

Isolation options for non-cellulosic heteropolysaccharides (HetPS)

The isolation of non-cellulosic heteropolysaccharides (HetPS) from barley husks (Hordeum spp.) and yellow poplar wood chips (Liriodendron tulipifera) was accomplished using mild steam explosion followed by extraction with water and ultrafiltration. The generally low yields, low purity, and low degree of polymerization (DP) improved when the HetPS were isolated following either alkali extraction of hammermilled or disk-refined biomass, or from holocellulose preparations generated by the conventional chlorite method or by organosolv delignification. Several purification methods were examined including precipitation using methanol; treatment with hydrogen peroxide (H2O2) or activated carbon (C) followed by precipitation with methanol; and H2O2-treatment followed by ultrafiltration. The isolation protocols were judged based on product yield, xylan content, and DP. The results indicate that, although steam explosion is effective in removing HetPS from the fiber source, virtually none remain in polymeric form. By contrast, alkali extraction succeeds in separating polymeric HetPS from the fiber source; and HetPS purity increases and polydispersity decreases with fiber prehydrolysis and delignification. Significant processing difficulties were attributed to the intimate association of HetPS with lignin which was effectively disrupted by acid-catalyzed pretreatment and treatment with H2O2. Ultrafiltration of H2O2-treated HetPS solutions represents the best procedure for isolating a xylan-rich polymer in high yield, with high DP and with high purity. Aqueous HetPS solutions can be spray- or freeze-dried into powderous products.

Multiphase materials with lignin. XV. Blends of cellulose acetate butyrate with lignin esters

Commercially available cellulose acetate butyrate (CAB, unplasticized) was blended in melt and solution with lignin esters having different ester substituents—acetate (LA), butyrate (LB), hexanoate (LH), and laurate (LL). All lignin esters formed phase-separated blends with CAB with domain size depending on processing conditions and the interaction between phases depending on blend components. CAB/LA and CAB/LB revealed the strongest interactions with domain sizes on the 15–30 nm scale as probed by dynamic mechanical thermal analysis and differential scanning calorimetry. The glass transitions (Tg) followed the Fox equation. Broader transitions corresponding to the Tgs of the two parent components were observed for CAB blends with LH and LL. Transmission electron micrographs revealed differences in the phase dimensions of the blends in accordance with chemical and processing (i.e., melt vs solvent) differences. Modest gains in modulus were observed for low contents (<20 wt %) of LA and LB.

Lignin Derivatives. II. Functional Ethers

Two hardwood (kraft and organosolv) lignins were reacted with propylene oxide, ethylene oxide, and chloroacetic acid. The primary lignin derivatives, hydroxypropyl kraft lignin, hydroxyethyl organosolv lignin, and carboxymethyl kraft lignin were converted into secondary derivatives by acetylation. The synthesis of the hydroxy and carboxy-functional ether derivatives involves the generation of oxyanions, the concentration of which determines the eventual degree of derivatization. Whereas the pH of the alkaline (aqueous) reaction medium rises during the reaction with the oxirane species, this declines during the reaction with chloroacetic acid. Careful pH control during the synthesis is needed for controlling the eventual extent of modification. Because of the volatility and reactivity of ethylene oxide, hydroxyethylations arc best performed in anhydrous isopropanol under pressure. Because of their surfactant characteristics, functional lignin ethers are difficult to isolate from aqueous solution. However, ether derivatives with as much as 80% lignin content have properties that are dramatically different from their parent polymers in terms of solubility, color, molecular weight distribution, and glass transition temperature. The latter is between 40 and 80°C lower than the corresponding underivatized lignin. The secondary acetate derivatives are easier to isolate than the primary ones.

Lignin Derivatives III. Sulfonate Esters

Several ionic, and a new class of non-ionic ligninsulfonate ester products were prepared using simple chemistry that may be adapted for industrial scale-up. This chemistry produces little known derivatives of ligninsulfonates by homogeneous phase reaction. Ionic esters are prepared in formamide; they have greater degrees of substitution than are typically achieved with heterogeneous phase chemistry. Non-ionic Ligninsulfonate esters are the product of the reaction of free sulfonic acids with oxiranc-containing compounds in a non-aqueous solvent (isopropanol). Non-ionic ligninsulfonate esters are water-insoluble, thermoplastic polymer derivatives with unique properties.