Dispersant and concrete additives
Dispersants and concrete additives (plasticizers) account for the widest use of lignosulfonates and sulfonated lignin. This is due to their appropriate molecular weight (10 000–50 000 g mol−1) and anionic charge density (0.1–0.9 meq g−1), stemming from the presence of functional groups. Studies have also noted low toxicity of lignosulfonates in marine life, which further increases potential for their safe use.
When ammonium is used as a base in sulfite pulping, the lignosulfonates produced usually have a higher molecular weight than that when either sodium or calcium are employed. This higher molecular weight was claimed to be related to the increased severity and rate of ammonium-based sulfite pulping, resulting in condensation reactions that would increase the lignosulfonate molecular weight.
Sodium lignosulfonates typically have a lower apparent viscosity than that of calcium lignosulfonates; this is attributed to sodium having a stronger electrokinetic repulsive force than that of calcium, which increases repulsion, and thus, reduces viscosity.
In one study, increasing the molecular weight of the lignosulfonates through oxidation and sulfomethylation enhanced their plasticizing abilities in concrete from 161 (unmodified) to 185 mm at a dosage of 0.3 wt % lignosulfonates.Another study stated that the fluidity of the oxidized and sulfomethylated product was comparable to that of commercial naphthalene sulfonate.
Increasing the molecular weight of kraft lignin by oxidation and sulfomethylation increased the adsorption of the modified kraft lignin on cement particles to 6 mg g−1 lignin dosage.This also increased the fluidity of the cement paste to 200 mm. In this case, the fluidity of unmodified kraft lignin was 70 mm and that of lignosulfonic acid was 190 mm at the same applied dosage.
Increased viscosity is indicative of increased molecular weight. Increasing the molecular weight of the kraft lignin through sulfomethylation with sodium sulfite and cross-linking with formaldehyde increased the viscosity of the product (3700 cP, compared with 1680 cP without cross-linking) to a level that allowed for effective use as a dye dispersant. The thermal stability of lignosulfonates was also an important factor for use as a dye dispersant.An increase in molecular weight from approximately 2000 to 14 000 g mol−1 by the hydyroxypropyl sulfonation of alkali lignin resulted in its improved heat stability, dispersibility of dye, and dye adsorption.
It was claimed that the hydrophobicity of lignosulfonates played an important role in their dispersing performance. In one study on calcium lignosulfonates, when lignosulfonates were fractionated based on molecular weight, in the higher molecular weight fraction with a lower charge density and decreased hydrophilicity were collected. As a result, increased hydrophobicity allowed for increased surface activity and decreased surface tension (surface tensions of 41.5 mN m−1 for the largest fraction and 66 mN m−1 for the smallest fraction).
The nitration of lignosulfonates also seemed to have an impact on the plasticizing capabilities of concrete admixtures. In one study, nitration to approximately 0.6 wt % allowed for less water to be added to concrete mixtures (45 L less water per m3 concrete), while the strength properties of the concrete were maintained at 25 mN. Increased nitrogen levels are also associated with corrosion prevention when applied as a concrete plasticizer.
The sulfur content of lignin-based products appears to significantly impact on their dispersion performances. In one report, reducing the sulfur content of lignosulfonates aided in increasing dispersion abilities by increasing hydrophobicity. This is because lower amounts of sulfonate groups result in less hindrance of hydrophobic adsorption between the hydrocarbon backbone of lignin and the material to which it adsorbs. Maintaining sulfur levels at 3.5 wt % still allows for adequate solubility, while zeta potential is also increased by enhanced adsorption.
An increase in sulfur content from 0.65 to 1.45 mmol g−1 from oxidation and sulfomethylation of lignosulfonates contributed to improving plasticizing ability, as well as increased fluidity from 161 to 185 mm. An increase in the degree of sulfonation through the same modification of kraft lignin to 2.04 meq g−1 increased the charge density to 4.6 meq g−1, which allowed for increased fluidity of cement paste compared with that of commercial lignosulfonates and lignosulfonic acid.Therefore, it may be concluded that the sulfur content of sulfonated lignin is essential for its solubility and dispersion, but, at a high sulfur content or high molecular weight, the dispersion of lignosulfonate may be hindered.
An increase in the molecular weight, phenolic hydroxyl content, and sulfonation as a result of modifying lignosulfonates with hydroxybenzyl alcohol allowed for better dye dispersion capabilities. The adsorption of lignosulfonates onto dye was increased due to increased molecular weight and hydroxyl content, whereas the sulfonate groups caused stabilization through strong repulsive charges.
Increasing both the molecular weight and degree of sulfonation of alkali lignin through oxidation and hydroxymethylation pretreatment before sulfonation decreased the surface tension (48 mN m−1) and increased zeta potential (30 mV), adsorption (8.5 mg g−1 cement), and dispersibility (216 mm) over that of commercial lignosulfonates (47.4 mN m−1, 26 mV, 5.9 mg g−1 cement, 210 mm, respectively).
By increasing the available functional groups through oxidation, the dispersing capabilities of lignosulfonates were increased dramatically. As mud dispersants, the oxidized lignosulfonates outperformed commercial products by at least 90 % based on gel point determination.For gypsum dispersion, dispersion efficiency was improved by 50 % following oxidation of lignosulfonate. Increasing the thermal stability of lignosulfonates as a dispersant for drilling mud through hydroxymethylation allowed for values similar to those of the commercial additive ferric chromium lignin sulfonate.