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.[2, 13, 45, 78, 111, 112] 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 sulfomethy lated 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 hydyr-
oxypropyl 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.
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 con-
crete plasticizer.
