A Wide Array of Kraft Lignosulfonates for a Wide Array of Uses

 

It all starts from a renewable resource – the pine tree. The two most abundant bio-based polymers on earth, cellulose and lignin, are harvested from trees. Cellulose is separated from the lignin to make paper and packaging materials. Lignin and its most common derivative, lignosulfonate, find use in diverse applications including dispersants for crop protection formulations. 
 

Two dominant technologies exist to separate cellulose from lignin: the Sulfite process and the Kraft process. The Sulfite process isolates cellulose fibers by sulfonating the lignin using salts of sulfurous acid, which solubilize and extract the lignin. In this process, the sulfonation of the lignin is performed in the paper mill, whose main objective is to isolate cellulose and convert it into paper. Their focus is not on making specialty chemicals. Sulfite lignosulfonates are highly sulfonated, making them very hydrophilic and water soluble. The Kraft process, however, utilizes high alkaline conditions to solubilize lignin and separate it from cellulose. The un-sulfonated Kraft lignin is isolated by precipitation. Kraft lignin has a high purity and can be chemically modified to produce specialized lignosulfonates. 
 

Conversion of pure Kraft lignin to various types of lignosulfonates takes place in a chemical plant under controlled conditions. The chemical architecture, molecular weight and level of sulfonation occur under controlled, reproducible conditions generating consistent materials. Most Kraft lignosulfonates are lightly sulfonated, retaining their hydrophobic character and making them highly surface active. Sodium lignosulfonates have different CAS numbers, which differentiates them by type of sulfonation reaction. 
 

The mechanism by which lignosulfonates stabilize dispersions is different from surfactants or small molecule dispersants. Anionic dispersants, like naphthalene sulfonate, form a double layer and stabilize water insoluble particles by charge repulsion. Lignosulfonates are polymers with a smaller charge to mass ratio. They also stabilize by steric repulsion, making molecular weight an important factor in stabilization. Since lignosulfonates are anionic and “highly branched” they provide both steric and electrostatic stabilization. This makes them unique compared with other classes of dispersants. 

 

 

In figure 1A, small highly charged dispersants function with a repulsive force, generated from the presence of an electrical double layer at the solid-liquid interface. In figure 1B, Kraft lignin dispersants function by providing steric stabilization. Higher Mz+1 helps with steric stabilization.
 

Crop protection formulations are highly engineered and regulated to cover a wide range of formulations, from liquids to solids. Each distinct crop protection formulation requires a unique dispersion chemistry. For example, liquid crop protection formulations, such as suspension concentrates (SC), need a different dispersant chemistry than do solid formulations, such as water dispersible granules (WDG). To meet these challenging and diverse formulation requirements, a range of dispersant products is required, each with its own unique chemistry tailored to the needs of the crop protection formulation. 
 

In a suspension concentrate, a hydrophobic active ingredient is suspended in an aqueous liquid. SC formulations place a high demand on the dispersant. The most important performance factor influencing the dispersing agent is the extent to which it absorbs to the surface of the active ingredients in solution. In lignosulfonates, the degree of sulfonation is the most important chemical parameter that influences the strength of adsorption to the surface of the active ingredient. Degree of sulfonation is defined as the moles of sulfonate per kilogram of salt-free lignosulfonate. It represents the amount of sulfonate chemically bonded to the lignin. A lower degree of sulfonation makes the lignosulfonate less water soluble, giving it a higher affinity for a hydrophobic surface, such as the surface of an active ingredient in an SC. This is demonstrated in Figure 2, which shows that a lower degree of sulfonation yields a higher affinity for the surface of the active ingredient. 

In water dispersible granules (WDG) and wettable powders (WP), both disintegration rate and suspension of disintegrated product are important attributes for the crop protection product. Low degree of sulfonation products have strong suspension properties but are slower to disintegrate than higher degree of sulfonation products. Therefore, WDG and WP applications require a slightly higher degree of sulfonation that still provides surface adsorption (dispersion) while providing more balanced, faster disintegration rates. 
 

Suspension concentrates and water dispersible granules are the fastest growing formulations in the Latin American market. With team members based in Brazil and the United States, MWV Specialty Chemicals proudly serves this region. Our competencies include commercial excellence, technical support, and a deep understanding of chemistry.

About the authors:

Tom Sisson is a business manager at MWV Specialty Chemicals. Based out of Charleston, South Carolina, USA, he supports the agrichemicals market in the Americas. Tom can be reached at thomas.sisson@mwv.com.

Luciana Lion is a sales manager at MWV Specialty Chemicals. Based out of Campinas, São Paulo, Brazil, she supports the agrichemicals market in South America. She can be reached at luciana.lion@mwv.com