Unlocking Potential: Functional Versatility of Biodegradable Polyether-Modified Trisiloxanes

Biodegradable polyether trisiloxane Technology

In today's marketplace, every commercially available molecule can be assigned a CAS number, including polyether trisiloxanes. Chapter three of the MSDS specifies the CAS number for the polyether trisiloxane molecule. However, it is important to recognize that this number does not provide a precise characterization of the molecule and its properties. The CAS number is quite general and does not account for variations in ethylene oxide (EO) and propylene oxide (PO) ratios or different molecular weights of polyethers. Moreover, it does not indicate whether a polyether-trisiloxane contains high-purity heptamethyltrisiloxane; lower purities may result in the presence of tetra- and even penta-siloxanes. Additionally, polyether trisiloxanes are complex formulations that often include excess polyether and various stabilizing agents.

Generic structures of polyether trisiloxanes

The products BREAK-THRU^® S 301 and BREAK-THRU^® S 233 share the same CAS number (CAS 134180-76-0), yet their effectiveness as tank mix additives varies significantly. This article will explore the differences in their physico-chemical properties.

Figure 1 illustrates a simplified generic structure of a polyether trisiloxane associated with CAS number 134180-76-0. Using conventional surfactant symbols, the hydrophobic tail is represented by the red line, which corresponds to the trisiloxane backbone, while the hydrophilic head group is depicted as the blue ball, representing the polyether.

Figure 1: Generic structure of Polyether Trisiloxanes having the CAS number of 134180-76-0

Both polyether trisiloxanes are biodegradable due to OECD 301 F

BREAK-THRU^® S 301 and BREAK-THRU^® S 233 are part of Evonik's patented Polyether Trisiloxane Technology. Both products are biodegradable according to OECD 301 F, which has more stringent criteria than OECD 301 B. BREAK-THRU^® S 301 demonstrates 66% biodegradability, while BREAK-THRU^® S 233 achieves an impressive 78% biodegradability within 28 days. The passing requirement for biodegradability is over 60% within this timeframe, making these results exceptional in the market.

Polyether trisiloxanes are extremely surface active

Both BREAK-THRU^® S 233 and BREAK-THRU^® S 301 lower the surface tension of water to approximately 22 mN/m at a treatment rate of just 300 ppm.

In contrast to BREAK-THRU^® S 301, which acts as a typical superspreader, BREAK-THRU^® S 233 does not exhibit a superspreading effect; instead, it functions as a super penetrant. This paper will explore the underlying reasons for this difference in performance.

Figure 2 illustrates the superspreading effect of BREAK-THRU^® S 301. Generally, superspreading is defined as the spreading of a 50 µl water droplet to cover an area greater than 35 cm² on a hydrophobic substrate. This definition is commonly referenced in various patents. The typical substrate used for testing is BOPP film, which has a surface energy of approximately 30 mN/m, simulating the hydrophobic surfaces found on most leaves. Achieving this superspreading effect is not possible with conventional chemistry.

Figure 2: Superspreading effect of BREAK-THRU^® S 301 in water

In this test, BREAK-THRU^® S 233 would only achieve a spreading area of 5 cm², despite its ability to lower the surface tension of water similarly to BREAK-THRU^® S 301.

The advantages of a non-superspreading polyether trisiloxane BREAK-THRU^® S 233 for systemic actives like herbicides

The significant reduction of surface tension (down to 22 mN/m) in tank mix spray droplets caused by BREAK-THRU^® S 233 results in excellent adhesion and retention of the droplets on all leaf surfaces. However, the subsequent performance on the leaf differs markedly from that of conventional trisiloxanes. Instead of spreading over a large area, the active ingredient (the plant protection product) becomes concentrated in a small area on the leaf. This higher concentration gradient facilitates significantly increased penetration into the leaf, particularly for some systemic active ingredients such as herbicides and fertilizers, enhanced by the ultra-low surface tension of the water. Consequently, BREAK-THRU^® S 233 functions as a super penetrant, making it particularly effective for systemic applications.

Differences in phase behavior is responsible for the special performance of BREAK-THRU^® S 233

Both polyether trisiloxanes, BREAK-THRU^® S 233 and BREAK-THRU^® S 301, are similar in their ability to reduce the surface tension of water; however, only BREAK-THRU^® S 301 functions as a superspreader. Therefore, another factor is crucial for achieving superspreading.

Figure 3 presents Young's equation, which can be used to calculate the contact angle of a water droplet on a surface. The surface tension of the liquid (σl) is crucial, as is the interfacial tension between the liquid and the solid surface (σl/s). Here, the performance of the two polyether trisiloxanes, BREAK-THRU^® S 233 and BREAK-THRU^® S 301, differs. An "educated guess" suggests that for a substance to qualify as a superspreader, the interfacial tension (σl/s) must be less than 8 mN/m.

Figure 3: contact angle of a water droplet on a surface. Besides surface tension also the interfacial tension is important.

BREAK-THRU^® S 233 is clearly unable to achieve a sufficient reduction of interfacial tension between the solid and liquid phases to below 8 mN/m, which is necessary for superspreading. The reason for this lies in the different phase behavior of BREAK-THRU^® S 233 in water compared to that of superspreaders. This will be further explained in the next chapter.

Differences of phase behavior in water 

It is well known and commonly taught in schools and universities that surfactants can form micelles. However, micelles are not the only structures that surfactants can create. BREAK-THRU^® S 301 does not form micelles; instead, it forms bilayers. The presence of these bilayer aggregates (Lα, vesicles, L3) can be demonstrated using polarized light. These bilayer aggregates are birefringent, meaning that the lamellae have an anisotropic shape that alters the polarization plane of polarized light. Figure 4: illustrates the bilayer of BREAK-THRU^® S 301 in water.

Figure 4: Aggregates of BREAK-THRU^® S 301 in water 

(J. Phys.chem. B 2003, 107, 5382-5390; Dynamic Light Scattering and Viscosity Studies on the Association Behavior of silicone Surfactants in Aqueous Solutions)

In contrast, BREAK-THRU^® S 233 cannot rotate polarized light because it forms classic spherical micelles in water, which do not alter the polarization plane of polarized light.

Consequently, the phase behavior of both trisiloxanes is entirely different. While both aggregates achieve the same reduction in surface tension of water, the reduction in interfacial tension varies. Referring back to Young's equation, Figure 5 illustrates this effect.

Figure 5: Differences in interfacial tension between BREAK-THRU^® S 301 and BREAK-THRU^® S 233. BREAK-THRU^® S 233, which forms spherical micelles, does not achieve complete coverage of the interface. In contrast, BREAK-THRU^® S 301 offers the most effective reduction of interfacial tension.

The micelle-forming surfactant BREAK-THRU^® S 233 cannot cover the interface between the solid and liquid phases as densely as the bilayer-forming surfactant BREAK-THRU^® S 301. As a result, BREAK-THRU^® S 233 is unable to achieve an interfacial tension (σl/s) of less than 8 mN/m. With a σl/s value greater than 8 mN/m, the thermodynamic requirement for a positive spreading coefficient (see Equation 1) is not met. This positive spreading coefficient is essential for superspreading on hydrophobic surfaces, such as leaves.

Equation 1: Mathematical calculation of the spreading coefficient.

It is important to note that even if we significantly increase the amount of BREAK-THRU^® S 233, it will not further affect the interfacial tension.

The Reason for the spherical micelle formation of BREAK-THRU^® S 233 in water

A valuable concept for characterizing micelle geometry is the critical packing parameter (CPP), as shown in Figure 6. The CPP plays a crucial role in determining the performance of BREAK-THRU^® S 233 as a non-superspreading trisiloxane. BREAK-THRU^® S 233 forms micelles in water (CPP < 1/3) but does not form bilayers. This distinctive behavior of trisiloxanes is patented by Evonik.

Figure 6: Critical Packing Parameter CPP of surfactants in water. CPP is responsible for the aggregation of amphiphilic molecules in water.

Performance of BREAK-THRU^® S 301 and BREAK-THRU^® S 233 for plant protection products

Both polyether trisiloxanes are soluble in water and various oils, making them suitable for use in in-can formulations, particularly in EC, OD, and SE types. Due to the limited long-term hydrolytic stability of trisiloxanes, SC formulations should maintain a pH value between six and eight. The typical treatment rate for these formulations ranges from 1-3% v/v. This low treatment rate often results in only about 50 ppm of trisiloxane in the tank (depending on the formulation's treatment rate). As previously mentioned, this concentration is sufficient to significantly enhance the performance of the plant protection product in terms of adhesion and retention of spray droplets. Table 1 illustrates the impact of wetting with BREAK-THRU^® S 301 at 50 ppm on various leaves.

Table 1: Impact of BREAK-THRU^® S 301 on contact angle of water on different leaves.

The performance of BREAK-THRU^® S 301 as a tank mix adjuvant and superspreader has been well established over the years, and the product is used globally.

Below we present here a laboratory and field trial with BREAK-THRU^® S 233 as tank mix adjuvant.

Field Efficacy with BREAK-THRU^® S 233 

This study aimed to compare the efficacy of glyphosate with and without the use of adjuvants. The glyphosate formulation utilized was Roundup at a rate of 560 g ae/ha. The herbicide was applied both alone and in combination with a commercial nonoxynol-based adjuvant, as well as BREAK-THRU ^® S 233. Simulated rainfall was introduced 60 minutes after the herbicide application. Results showed that BREAK-THRU^® S 233 significantly enhanced the control of Bermuda grass (Cynodon dactylon) 30 days post-application, both without and with simulated rain, compared to glyphosate alone and glyphosate with the nonoxynol-based adjuvant (see Figure 7). Notably, this improvement in efficacy with BREAK-THRU^® S 233 was achieved at a rate of 200 ml/ha, while the nonoxynol-based adjuvant was applied at 500 ml/ha. This study clearly demonstrates that BREAK-THRU^® S 233 enhances the penetration of glyphosate, regardless of rain, in comparison to a commercial adjuvant.

Figure 7: Control of Bermuda grass (Cynadon dactylon) 30 days after herbicide application.  Rain was simulated 60 minutes after application. 

In a subsequent study, radiolabeled (14C) glyphosate was applied to the weed Capsella bursa-pastoris using a conventional trisiloxane and BREAK-THRU S 233, both at 0.1% v/v. BREAK-THRU^® S 233 significantly improved the uptake of glyphosate 10 minutes and 24 hours after application compared to a traditional trisiloxane with superspreading properties (Figure 8).

Figure 8: Uptake of radiolabelled glyphosate with a conventional trisiloxane compared to BREAK-THRU^® S 233 after 10 minutes and 24 hours.  

In summary, the biodegradable polyether trisiloxane technology exemplified by BREAK-THRU^® S 301 and BREAK-THRU^® S 233 demonstrates significant advancements in the formulation of effective tank mix additives. While both products share the same CAS number and exhibit comparable abilities to reduce surface tension, they differ markedly in their performance characteristics due to their distinct phase behaviors. BREAK-THRU^® S 301 acts as a superspreader, effectively covering hydrophobic surfaces and enhancing the distribution of active ingredients, while BREAK-THRU^® S 233 functions as a super penetrant, concentrating active ingredients for improved absorption, particularly in systemic applications.

The critical packing parameter (CPP) plays a vital role in determining the aggregation behavior of these trisiloxanes, with BREAK-THRU^® S 233 forming spherical micelles and BREAK-THRU^® S 301 forming bilayers. This fundamental difference impacts their ability to reduce interfacial tension, a key factor for achieving optimal spreading on plant surfaces. Furthermore, both products are suitable for various formulations, contributing to the overall efficacy of plant protection products while adhering to environmental standards through their biodegradability.

As the agricultural industry continues to seek effective and environmentally friendly solutions, the unique properties of BREAK-THRU^® S 301 and BREAK-THRU^® S 233 position them as valuable tools for enhancing the performance of plant protection products, ensuring better adhesion, retention, and penetration of active ingredients.

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