Dow Chemical: Rain Fastness Testing Method Build-up and Adjuvant Exploration

Rainfall is a common and serious factor causing the loss of pesticides, especially in tropical areas that typically have intense precipitation. The development of rain fastness adjuvants is of great significance to avoid pesticide loss caused by rainfall and enhances its efficacy, thus reducing the pesticide usage amount.

The so-called rain fastness is the ability of the chemicals to resist removal or wash-off due to rainfall. Currently, popular mechanisms for rain fastness include “spreader” concept and “sticker” concept. “Spreader” is aimed at increasing the coverage of pesticides on the foliar surface, and then bringing pesticide quick uptake. “Sticker” could enhance the pesticide retention via a formed polymer film or net-like structure. Dow Chemical has a wide portfolio of technology in terms of wetter and polymer film former. It is a great opportunity to provide high quality products with superior performance to the market.
Herein, the sticker concept is used for pesticide retention enhancement in Figure 1. The incorporated polymer sticker with a specially designed structure and dosage will generate lots of domains on the plant leaf, which are formed by the polymer chains and pesticide particles. The polymer chain provides strong interaction with the plant leaf and pesticide particles, thus fixing the pesticide onto the plant surface strongly to resist rainfall wash-off. The properly added dosage ensures the pesticide particles are not coated by polymer film, maintaining the contact action for some pesticides, such as a fungicide.

In order to explore high performance adjuvants, Dow Industrial Solution Asia Pacific team managed to build up the rain fastness testing method in the lab, and then validate testing method reproducibility and repeatability, which could support quick adjuvant performance evaluation in a facile way. It is found that a lot of factors should be considered during testing procedure set-up, such as substance to mimic leaf surface, equal simulated rainfall to each sample etc; the testing parameters are confirmed after several rounds of trials and equipment debugging. Through the testing procedure, lots of “stickers” are screened on their rain fastness performance. POWERBLOXTM ADJ -35 and POWERBLOXTM ADJ-65 are two outstanding products, showing excellent performance. Taking 430g/L mancozeb SC as a starting material, lab-testing results were shown in Figure 2. The obvious difference can be seen before (Figure 2a) and after (Figure 2b) rainfall. The adjuvant introduction into the pesticide system helps dramatically for active ingredient retention on the substance.
Adjuvants rain fastness performance to various tested pesticide formulations are summarized in Table 1. The incorporating of adjuvant indeed endows pesticides with stronger rainfall resistance, to make them retained for a larger amount. Moreover, the highlighted feature of both products is that they are under U.S. EPA registration, having a good regulation profile.

The practical application effect is critical to reflect adjuvant performance. Third-party capability was utilized to conduct field-testing on weed control effect. Some 18% Glufosinate SL was used as a herbicide and eleusine indica was the weed target. After herbicide dilution was sprayed for 30 minutes, an equal amount of tap water to simulate rainfall was sprayed to each testing zone. Some 15 days later, the weed effect was shown in Figure 3. Compared to CK sample without herbicide, the other two samples exhibited excellent weed control effect. When it came to the comparison between no adjuvant sample and adjuvant added sample, less green tint could be found when POWERBLOXTM ADJ-35 was incorporated. It is demonstrated that the introduction of POWERBLOXTM ADJ-35 can offer excellent rainfall resistance for pesticides, thus providing higher efficacy.  

As it is known, field-testing results is statistic data. Actually, four testing zones were kept for each sample, and the weed effect was calculated by three typical plants for each zone. From the average result in Figure 4, it can be seen that the adding of POWERBLOXTM ADJ-35 will improve the efficacy 15-20% for each plant weed effect and aerial parts fresh weight and improve around 50% on underground parts fresh weight.
   
In a word, POWERBLOXTM ADJ series efficacy enhancers with excellent rain fastness and an eco-friendly profile are recommended for use in water-borne formulations, as in-can additive with dosages around 6wt%, and in solvent-borne formulations as tank mix additive with dosage around 20wt%.

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