Overcome Stability Challenges in SC and SE Formulations with Sustainability in Mind

By
James Sawyer, Business Development Manager, Global Agriculture, Stepan Company
Laure Canal, Business Manager, Agricultural Solutions – Europe, Stepan Company

The agrochemical industry continues to see growth in aqueous-based formulations, including suspension concentrates (SC) and suspoemulsions (SE). SCs specifically have shown strong growth to become the most valuable formulation type globally after surpassing emulsifiable concentrates (EC) in 2014¹. Aligned with industry and grower objectives on sustainability, SCs offer many advantages including an improved safety profile, the elimination of costly solvents and the ability to formulate new active ingredients that often have limited solubility in solvent-based formulations.

When developing suspension-based formulations, there are two key technical challenges that must be overcome to achieve good product performance. First, the concentrated formulation must be stable over a wide range of temperatures under prolonged storage. Second, the formulation must dilute homogeneously while also demonstrating stability during the application window. Working against stability are the multiple physicochemical forces that must be overcome, even in formulations with only a single active ingredient (AI). As these systems tend to be thermodynamically unstable, particles will try to stabilize by reducing their surface area via agglomeration, Ostwald ripening or other mechanisms.

The right dispersants can help overcome these challenges, however, as microplastic and additional regulations come into force in the coming years, formulators will need to take further considerations when selecting the appropriate co-formulant for aqueous formulations.

Improving Dispersion with Paired Wetting Agents

Stepan Agricultural Solutions considered these factors and created a Polymeric Dispersants Kit that meets formulators’ performance requirements and the expectations of the European Chemicals Agency (ECHA) in regards to microplastics. The kit combines dispersants with various affinity groups and mechanisms of dispersion with paired wetting agents to help overcome these forces and greatly improve the quality of the dispersion.

Highlighted in Figure 1 are the primary mechanisms of electrostatic repulsion and steric hindrance that dispersants can provide to the formulation. The proper matching of the affinity of the dispersant to the active ingredient additionally plays a crucial role in performance.

Figure 1

Wetting agents are commonly paired with polymeric dispersants in aqueous dispersion formulations to help avoid agglomeration because they are easily adsorbed onto freshly generated, non-polar active surfaces during the milling process as illustrated in the image below. Wetting agents also can provide steric hindrance and help lower the interfacial surface tension between active particles and water, so are often additionally considered as co-dispersants. The structure of these molecules is based largely on ethylene oxide/propylene oxide (EO/PO) block copolymers and, in some instances, straight ethoxylates.

The potential impact of utilizing the most appropriate wetting agent is illustrated in Figure 2 for a metalaxyl 300 g/L formulation using Stepan’s STEP-FLOW^® 3000 polymeric dispersant and STEP-FLOW 4890-SC wetting agent. Metalaxyl was chosen for the test formulation because the active is notorious for causing crystal growth instability issues and is particularly problematic for many formulators.

Stepan’s testing results demonstrate the ability of STEP-FLOW 3000 and STEP-FLOW 4890-SC each to improve formulation stability under both freeze/thaw and extended elevated temperature stability. However, it is not until the dispersant and wetting agent are used in combination that the synergistic power is realized.

Figure 2

Comprehensive Toolbox for Aqueous Formulations

But it is not only metalaxyl that formulators would need to work with. There is a diversity of active ingredients available today, and there is no single universal product that is capable of solving all formulation challenges. Stepan Agricultural Solutions recognizes that in order to successfully address the challenges associated with SC and SE formulation development, formulators require a diverse toolbox of dispersants and wetting agents to provide the necessary stabilization. As such, Stepan has continued to expand our portfolio and now offer a broad range of polymeric dispersants and wetting agents that, when properly matched, can provide a synergistic enhancement of aqueous formulations. Highlighted in the tables below are our STEP-FLOW line of aqueous polymeric dispersants and the recommended selection of paired wetting agents that formulators can mix and match to find the optimum combination for their formulation. Stepan Agricultural Solutions also provides the products below in an all-in-one sample kit.

Polymeric dispersants:

Wetting agents:

* Commercialization in progress

The continued development of our polymeric dispersant portfolio has led to the recent introduction of the styrene-based STEP-FLOW 5000 and STEP-FLOW 6000. STEP-FLOW 5000 is an anionically modified styrene acrylic polymer that is highly recommended for use in both SC and SE formulations. A representative formulation example is provided below wherein STEP-FLOW 5000 is demonstrated to pair well with our specialty surfactant blend STEP-FLOW 4894-SC to deliver another robust solution for a metalaxyl formulation.

Our newest offering, STEP-FLOW 6000, is the result of a multi-year research effort focused on developing novel molecular architecture in order to achieve enhanced performance, particularly for inhibiting crystal growth. As highlighted in Figure 3, the use of this patented technology is effective in inhibiting crystal growth for a large range of actives. A particularly striking example of the power of STEP-FLOW 6000 is demonstrated in the formulation stability demonstrated with metalaxyl.

Figure 3

A final product of note is our latest high performance polymeric (HPP) dispersant STEP-FLOW T6. Based on market demand, Stepan has developed this fluidized HPP dispersant that provides formulators with processing and handling advantages in addition to its key feature of enabling active ingredients to interact most strongly with affinity groups possessing matching chemistry. This enables strong association through mechanisms such as hydrogen bonding and π-π stacking.

Meeting Sustainability Needs

When considering the use of polymeric dispersants, a key question that formulators must be aware of is the potential for microplastic issues they may cause in the finished formulation. According to ECHA, around 42,000 tons of microplastics end up in the environment when products containing them are used. The concern raised by the European Food and Safety Authorities (EFSA) is that when microplastics are released in the environment they do not biodegrade and lead to irreversible pollution of our ecosystem and food chain.

As microplastic and additional regulations come into force in the coming years, formulators will face additional challenges to select the appropriate co-formulant for SC formulations. Current work in the proposed regulation, which is not expected to fundamentally change, defines a microplastic as a solid polymer contained in particles where at least 1 wt% of those particles have a size range below 5 mm or as a coating on solid particles where at least 1 wt% of those particles have a size range below 5mm. Derogation criteria will apply for water soluble polymers (>2 g/L), biodegradable polymers² and natural polymers without any chemical modification.

The evaluation of the polymeric dispersants highlighted in this article indicates that all products do not meet the definition of a microplastic or fulfill one of the derogation criteria. While this makes our STEP-FLOW range a great solution for formulation development, Stepan aims to go beyond the microplastics definition and propose biodegradable³ formulation solutions for SCs and SEs. Through our formulation expertise and innovative mindset, we are confident that we will forge a path to meet this challenge without compromising on performance.

As a sustainability-driven company we are committed to bringing solutions to the market that help growers and help reduce the accumulation of persistent chemicals in the environment. Our in-house formulations experts and global manufacturing network are dedicated to fostering growth throughout the agricultural industry. Stepan Agricultural Solutions is committed to providing innovative products that have outstanding performance and unique sustainability benefits that create a positive impact, extending from the farmers in the field to the growing population. We deliver a diverse product line including, but not limited to, adjuvants, dispersants, green solvents, emulsifiers and specialty blends. Our Stepan Agricultural Innovation Center in Winder, U.S., demonstrates our commitment to liquid and dry formulation design, strategic partnerships, education and sustainable agriculture. In addition, our global R&D teams continue to expand our technological capabilities for organic farming, biologicals and microplastics alternatives. Cultivating a tradition of excellence is a continuous journey and we rely on science to fuel our growth and inspire the future of agriculture.

1 REFERENCES: Featured Insight, Issue 266: Agrochemical Market by Formulation Type. his Markit. April 2021

2 OECD guidelines for biodegradation testing such as OECD 301 (B, C, D, F).

3 Achieve a minimum of 60% biodegradation after 60 days in OECD 301F.