Stepan: Novel Polymeric Dispersants to Control Crystal Growth in Suspension Concentrate Formulations

Over the past decade, the agrochemical industry has seen a rise in suspension concentrate (SC) formulations on the market. In 2014, SC formulations surpassed emulsifiable concentrates (EC) as the most prevalent formulation type defined by market share (Phillips McDougall, 2018). Beyond the removal of costly solvents that may be flammable or irritating, this trend is driven by the pipeline of new active ingredients (AIs) that are generally becoming more complex, with compounds having increased molecular weight, more diverse functionality and chiral centers (Yuan, 2018). These molecules can be more challenging to formulate into solvent-based formulations as their poor solubility limits AI loading. SC formulations provide an excellent alternative to achieve high loading of these complex AIs, while also achieving improved safety and cost savings related to the elimination of solvents.

A key challenge for SC formulations is long-term stability of the dispersion, as AI particles must remain suspended in both the concentrate and dilution stages of the product. Dispersant selection is vital when developing these formulations as it strongly influences stability. One of the most common forms of instability in SC formulations is particle size growth, which can occur through a variety of mechanisms such as Ostwald ripening or agglomeration of the suspended AI particles (Tadros, 2017). The severity of this behavior, referred to as crystal growth, is a function of the dispersant performance. If not prevented, crystal growth will lead to numerous issues like decreased efficacy due to uneven spray application and clogged sprayers.

Dispersion is the process through which agglomerates of solid particles become separated, and a new interface forms between each of the smaller particles and the surrounding liquid. This process is facilitated by the application of external force (milling) and the use of ambiphilic additives such as dispersants. A key factor that governs the efficiency of this process and the stability of the resulting dispersion is the surface chemistry of the solid that is to be dispersed. The hydrophobic nature of modern AIs introduces a complication into this process, due to their poor interactions with conventional dispersants.

New High Performance Polymeric Dispersants

High performance polymeric (HPP) dispersants are one type of technology that have been developed to overcome this type of challenge. They are larger than surfactants and can have a variety of different molecular architectures (Figure 1). The key feature for this type of dispersant relies on the fact that AIs interact best with affinity groups that have matching chemistry, which enables strong association through available mechanisms such as hydrogen bonding and π-π stacking.

 

Stepan has recently developed a new class of nonionic, HPP dispersants. Originally designed for and applied to the preparation of pigment dispersions for coatings applications, this chemistry platform has found significant utility in the preparation of SCs of hydrophobic AIs. Formulations prepared using this novel, patent-pending technology exhibit superior particle dispersion and demonstrate crystal growth inhibition. A generalized structure of our new HPP dispersants is shown in Figure 2. These dispersants are composed of three discreet components:
1. a linker, which can vary in length, functionality, flexibility, and number of appendages;
2. affinity domains, which are designed to interact with the surface of AI particles, and can vary by the type, number and arrangement of anchoring groups; and
3. stabilizing segments, which can differ by length, hydrophobicity and functionality.

The modular nature of our chemistry platform affords significant flexibility in dispersant design.

Optimizing Structure Design

In order to optimize structure design for SC formulations, more than 60 derivatives of HPP dispersants were evaluated with six different AI’s. The STEPSPERSE® kit comprising three HPP dispersant molecules emerged as the best candidates for all the AIs screened. These dispersants vary by number of appendages, with the kit containing 1-tail, 2-tail and 3-tail architectures.   

This article will focus on the performance of the STEPSPERSE kit with Metribuzin and Metalaxyl, two of the most challenging AIs. Both AIs are partially soluble in water (1.1 and 8.4 g/L respectively), which introduces varying severities of crystal growth. SC formulations were made containing 2% dispersant (Table 1). Mill bases were prepared for each AI and were subsequently separated into samples for the dispersant to be shear mixed in. The experimental dispersants are 100% active and were solubilized in glycol prior to addition. It was important for the samples to be prepared immediately and particle size measured after processing to ensure no crystal growth occurred in lead time.

The SC formulations were evaluated for long-term stability at elevated temperature. Metribuzin was held at 54 ºC for four weeks and Metalaxyl, which has a lower melting point of 65 ºC, was held at 40 ºC for eight weeks. In addition, all samples were run through five cycles of freeze-thaw to ensure shelf-life stability. Stability was assessed by suspensibility (ASTM method E1673), particle size measurements and polarized microscopy. In this work, particle size is discussed in terms of d90, which indicates the diameter at which 90% of the total particle volume has a smaller particle size and 10% has a larger size.

Figure 3 summarizes the particle size growth that was observed with these HPP dispersants in comparison to a commercial graft copolymer, a common industry solution for crystal growth inhibition. The commercial control had poor performance with over 3,000% growth in d90 measurements for both Metribuzin and Metalaxyl, whereas the three HPP dispersants exhibited significantly reduced particle growth for both AIs across the stability periods. The 3-tail molecule is the top performing dispersant for Metribuzin, with an increase in d90 from 10 to 25 μm (120%) after four weeks at 54 ºC.The 1-tail molecule is the top performing dispersant for Metalaxyl, with an increase in d90 from 7 to 33 μm (350%) after eight weeks at 40 ºC.

 

Figure 3. Plot of the percent increase in d90 particle size measurements after stability at elevated temperature for commercial graft copolymer control and three optimized HPP dispersants.

Polarized microscopy supports that crystal distribution remains much smaller and more controlled with Stepan’s new polymeric dispersants. Figure 4 compares 200x magnification images of the commercial graft copolymer control versus the top STEPSPERSE candidate for each active.

                                                                                                                                                                              

The three HPP dispersants discussed in this article clearly show a significant decrease in crystal growth of challenging SC formulations. However, there is still room for improvement to further enhance control of crystal growth with these HPP dispersants in problematic formulations. For example, dispersants milled with the AI will likely have better performance as there will be increased surface contact and better opportunities for particle adhesion. In addition, the formulations assessed have not been optimized. The formulations were developed to stress the performance of the dispersants at a 2% use rate. As a result, stability should be improved with the optimization of dispersant and co-formulant concentrations.

Stepan’s new STEPSPERSE kit of HPP dispersants exhibit superior particle dispersion and demonstrable crystal growth inhibition with problematic AIs. In addition, our HPP dispersant platform has developed into a toolbox of more than 150 molecule designs, and Stepan can leverage the modular nature of our chemistry, to identify the best fit for a challenging SC formulation.

References:

1. PHILLIPS MCDOUGALL. (February, 2018). AgriFutura. AgriService, 220:1-6.

2. TADROS, T. F. (2017) Suspension Concentrates: Preparation, Stability and Industrial Application, p. 103-151.

3. VAN DEN HAAK, H. J. W. Kirk-Othmer Encyclopedia of Chemical Technology: Dispersants, 8:672-697.

4. YUAN,G. Agropages. (May, 2018). Formulation & Adjuvant Technology, p 10.

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This article was initially published in AgroPages '2020 Formulation & Adjuvant Technology' magazine. Download it to read more articles.