Dow: High performance oil dispersion adjuvant development

Date:06-12-2019

Oil dispersion (OD) formulation is an emerging formulation in Ag industry, which is claimed as environmental friendly, better pesticide efficacy than water-based formulation. Because the continuous phase is oil, like methyl oleate, methyl soyate, it shows high affinity with plant leaves, thus providing better spreading, wetting and penetration performance. However, the existing OD products have common issues of inadequate long-term storage stability, phase separation and caking in the bottom after several months storage. This hinders the booming development of OD formulation. Therefore, it is of high importance to explore high performance adjuvants of emulsifier, dispersant and thickener, exclusively used in OD formulation, promoting the industry technology progress.

Dow possesses rich product portfolios in emulsifiers and dispersants. How to develop the high performance products to address industrial challenges is our high priority these years. In this paper, high performance OD adjuvant development process had been described, followed by formulation preparation and performance evaluation. Currently, POWERBLOX^TM OD products is ready to deliver to the market for formulation trial and performance validation. 

Lego Unit Approach

It is well known that the OD formulation is a complex system, even for adjuvants, including wetting agents, emulsifiers, dispersants and thickeners. If all of the factors are considered together in the design of experiments, it will generate a huge amount of experiments and heavy workload. In order to accelerate product development progress, Lego unit approach was utilized to reduce design parameter as shown in Figure 1. At the initial stage, we investigated the performance between thickener and oil and the performance between emulsifier and oil, to select proper thickener and emulsifier. And then, the thickener and emulsifier in oil were fixed as the base system to evaluate the compatibility and stability of screened dispersants. Finally, the active was incorporated into above system to prepare OD full formulations and evaluate their storage stability. The technical hypothesis for this approach is:
-    Thickener plays a dominant role in oil phase thickening. The better thickening effect, the higher viscosity of oil phase, little related to active and other additives.
-    Emulsifier is decisive to emulsion quality and stability when OD formulation is diluted into water.   

Thickener Performance Evaluation
 
Methyl oleate is the largely used oil phase in OD formulation. In the following experiments, the methyl oleate was used as received from Wilmar. Various kinds of thickeners from different producers, more than 30, were collected to evaluate their thickening performance to methyl oleate, including organic, inorganic and organic-inorganic combination type. The comprehensive performance was tested, in term of oil compatibility, thickening effect, dispersing effect and cost effectiveness. It revealed that organobentonite was still the best choice in the market. The recommended dosage was 2.0-4.0 wt%. 

Emulsifier Performance Evaluation

 
In the application, OD formulation needs to be diluted into water firstly, prior to spraying. Therefore, emulsifiers play a critical role to get the high quality emulsion, thus affecting drug efficacy. Emulsifiers with various HLB values were selected to evaluate their effect on initial emulsification performance and emulsion stability over time. The emulsifier dosage was fixed at 10wt% in methyl oleate and the dilution was 200 times in water. When oil phase was added into water phase, turned the mixture upside down for 20 times for further emulsion stability observation over time. As shown in Figure 3, the performance of one series of emulsifiers with HLB value from 8.5-13.6 demonstrated that the higher the HLB value, the better the initial emulsification performance and the emulsion stability. However, high HLB value generally means high polarity, it may have compatibility issue with oil phase, which is of relatively low polarity.   
  
Span and Tween were taken as starting emulsifiers to mix in different ratios to get various HLB value emulsifier mixtures, which were used to study the compatibility with methyl oleate. As shown in Figure 4a, with the increase of HLB value, the compatibility between emulsifier mixture and oil phase became worse and worse, from transparency and homogeneous to turbid. In Figure 4b, the performance of the screened Dow’ products with even the HLB value at 12.0-13.0 showed excellent compatibility with oil phase with transparency and homogeneous appearance, which resulted from the specially designed structure. Besides the good compatibility, excellent emulsification performance is the final target. It was shown in Figure 4 c1, the mixture of oil and emulsifier with mediocre emulsification ability was added into water; as a result, the oil droplets couldn’t be emulsified spontaneously, but fell to the bottom still as oil droplets. When the mixture of Dow’s emulsifier and oil phase was dropped into water phase, the oil could be emulsified quickly with fine emulsion and excellent stability.

Dispersant Screening and OD formulation Development

 
Dispersant is used to disperse and stabilize pesticide particles, providing OD formulation long-term storage stability and ensuring sufficient shelf life. After thickener and emulsifiers were confirmed, dispersant was screened firstly by verifying its compatibility with oil and then by formulation development and optimization via altering the ratio of each components. As mentioned above, the components were complex in OD formulation with wide changing range, it was of high workload and time consuming. Dow high throughput platform could prepare a large amount of formulations in short time, facilitating to performance comparison and formulation optimization. Through scientific experiments design to get a lot of formula, the high throughput process was utilized to prepare target formulations as seen in Figure 5, more than 200 formulation within 1 week. Followed by, the excellent formulation could be screened out, on the basis of performance evaluation.   

 
Taking 2.5wt% penoxsulam OD formulation development as example, 12 formulation had been selected out of all of the designed experiments, due to superior thermal storage stability to benchmark (seen in Figure 6). After 2 weeks thermal storage, benchmark product (commercially available product and used as received, non fresh sample) had around 8% phase separation. For our developed samples, there was no phase separation by naked eyes, keeping homogeneous system. 

 
In order to differentiate the formulation perforamance difference caused by dipersant, accelerated stability testing method combining centrifugation and transmitted light scanning was used for performance evaluation to the formulations with good thermal storage stability. As seen in Figure 7 of each graph, the X axis stands for sample bottle height, the left is bottleneck and the right is bottom of bottle. And the Y axis indicates the tranparency of the sample in the bottle. The lower the transparency, the better the sample stability. Moreover, the more the high transparency, the better the sample stability. The results demonstrated that the number 12 sample showed best stability, thus providing an optimization adjuvant package to target formulation.
   
 
At the same time, to know more clearly the detail difference among each formulation, instability index was introduced to quantitatively measure formulation stability. Instability index is a specific value combining with Stokes’ Law and Lambert-Beer’ Law, demonstrating the relative value of the sum of transparency change to the maximum clarification during centrifugation, with the value range of 0-1 ^{[1, 2].} For the same active particles (same density) in close viscosity, the lower instability index, the better formulation stability. It was seen in Figure 8, the instability index of all of evaluated formulations were not so high, and the value of number 12 was the lowest, indicating the best stability.   

In a word, the thorough process had been built-up on adjuvant development, formulation preparation and performance evaluation. The explored high performance POWERBLOX^TM OD adjuvant is ready to go to the market for customer trial, addressing the incumbent issues effectively.

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[2] T. Detloff, T. Sobisch, D. Lerche, Instability Index, Dispersion Letters Technical, T4 (2013) 1-4, Update 2014