Agricultural Silicone Spreading and Penetrating Agent RH-213

Agricultural Silicone Spreading and Penetrating Agent RH-213 is a nonionic, low-foam, polyether-modified trisiloxane liquid supplied at high active content for use as a tank-mix adjuvant in pesticide, plant growth regulator, and foliar nutrient applications. The siloxane backbone reduces surface tension at the air-water interface, while pendant polyether chains provide water dispersibility and modify adsorption kinetics at the leaf cuticle. This structural combination allows spreading across waxy, hairy, and difficult-to-wet foliage at concentrations below those required by conventional nonionic alcohol ethoxylates. RH-213 is not a standalone pesticide and does not replace sprayer calibration, nozzle selection, or carrier water pH adjustment. Product-specific toxicological and regulatory status must be verified on the current label and safety data sheet because peer-reviewed data for this exact designation are limited.

What Physical and Chemical Parameters Define RH-213?

At 25°C, the material is a clear to slightly hazy viscous liquid. Because RH-213 is sold as a concentrated surfactant, its active content is typically reported at ≥ 99% w/w; water or solvent content is minimal. Specification compliance is usually verified by lot-specific certificate of analysis rather than by a single international standard. The class-typical values shown below are drawn from agricultural polyether trisiloxane adjuvant technical data and should not replace the product certificate of analysis.

Surface tension is the primary control parameter. At 0.1% w/w in deionized water, the equilibrium surface tension falls to 20–23 mN/m under ASTM D1331-20; this is below the 28–34 mN/m range typical of linear alcohol ethoxylates at the same concentration. The low surface tension is necessary but not sufficient for leaf spreading; droplet impact energy, leaf roughness, and cuticle wax crystallinity also govern the final wetted area. Because surface activity is concentration-dependent, field dilutions should be prepared gravimetrically or with calibrated injection systems. Published data for this specific configuration is limited at concentrations above 0.2% v/v, where run-off losses may overcome any additional spreading benefit.

In spray-tank practice, RH-213 is introduced at 0.025%–0.1% v/v of finished spray volume. The lower end of this range is generally reserved for low-volume ground application and easily wetted foliage; the upper end is considered only where strong cuticle penetration is required or where spray water hardness exceeds 200 ppm CaCO₃. No product-specific rate-response curve for RH-213 is available in the peer-reviewed literature, so field validation under the specific nozzle and pressure configuration is required. The addition order follows standard compatibility hierarchy: fill the tank to 50–70% of final volume with clean water, add water-soluble products and buffering agents first, add RH-213 under continuous agitation, then add emulsifiable concentrates or suspension concentrates, and finally top up to volume. High-shear recirculation should be avoided because the siloxane surfactant can generate stable foam if entrapped air contacts the spray pump intake. A diaphragm pump with bypass agitation at 1.5–2.5 bar is sufficient to maintain a homogeneous mix without excessive foaming.

Because RH-213 reduces the surface tension of the spray fluid, droplet size spectra from conventional flat-fan and air-induction nozzles can shift. Operational experience with related trisiloxane adjuvants indicates that air-induction nozzles at 3.0–4.0 bar retain coarse-droplet characteristics, whereas hollow-cone nozzles may produce a larger proportion of driftable fines. Therefore, nozzle selection should be repeated with the adjuvant present and not extrapolated from water-only calibration. This requirement is especially critical in aerial application, where spray volume may be below 50 L/ha and the surface tension reduction increases the fraction of small droplets.

Spreading Dynamics on Leaf Surfaces

On hydrophobic leaf cuticles, the contact angle of pure water often exceeds 90°. Polyether trisiloxane adjuvants lower the contact angle to below 40° within seconds because the siloxane backbone adsorbs rapidly at the liquid–air and liquid–solid interfaces. This rapid interfacial adsorption is measured by dynamic surface tension rather than equilibrium surface tension alone. According to ASTM D7334-08 advancing contact angle measurements on paraffin film, the spreading behavior of the polyether trisiloxane class is stronger than that of alcohol ethoxylates at equivalent active concentration. Wetting diameter in film tests for related trisiloxane products is commonly 30–50 mm at 0.1% w/w, although RH-213-specific values are not published. The practical consequence is that spray droplets flatten and cover leaf area, but excessive spreading can reduce retention and cause coalescence-induced run-off. Thus, the optimum rate is not the highest rate that produces visible wetting; it is the lowest rate that achieves target coverage without run-off.

Spreading is also affected by leaf microtopography and trichome density. On smooth, waxy leaves, the surfactant-induced contact angle reduction dominates. On hairy leaves, droplet penetration into the trichome canopy can become the rate-limiting step. In such cases, the penetration-enhancing property of RH-213 may be less pronounced than on smooth cuticles, and a spreader-sticker or crop oil concentrate may be required to achieve adequate retention. This distinction is important because silicone adjuvants are often recommended for difficult-to-wet crops, but their performance is not uniform across all morphological surfaces. A single field rate cannot be assigned without crop-specific and leaf-stage-specific validation.

Hydrolytic stability is the main operational boundary for RH-213. In aqueous solution, polyether-modified trisiloxanes can undergo siloxane bond cleavage, with rates increasing below pH 5.0 and above pH 9.0. At ambient temperature and neutral pH, the hydrolysis half-life of non-end-capped trisiloxane adjuvants is on the order of several hours to days; end-capped variants such as RH-213 are designed to extend this window, but specific kinetic data for this designation are limited. Tank mixtures prepared outside the pH range 5.0–9.0 should be sprayed within 2 h to avoid loss of surface activity. Alkaline sources such as potassium carbonate, ammonium hydroxide, or high-concentration phosphate buffers can accelerate degradation more than acidic sources at equivalent pH.

Hard water above 500 ppm CaCO₃ may reduce clarity and interfacial performance. Calcium and magnesium ions interact with the polyether chains and can induce clouding or salting-out in concentrated fertilizer mixtures. If the carrier water is extremely hard, a water-conditioning agent should be added before RH-213. The user should perform a jar test with the actual tank mixture at ambient temperature and observe for phase separation, gel formation, or floating oil layers. Any persistent turbidity or settled solids indicates incompatibility and requires a different addition order or water treatment.

Foliar application under slow-drying conditions on sensitive flowers and buds presents a phytotoxicity threshold. Field observations from related trisiloxane adjuvants have shown that concentrations above 0.2% v/v increase the risk of leaf margin necrosis and flower petal spotting when drying time exceeds 30 min. The mechanism is not fully established but is likely associated with surfactant penetration into stomatal chambers and cell membrane disruption. For RH-213, published data for this specific configuration is limited; however, the same threshold should be assumed until local small-plot testing demonstrates otherwise. Crops with thin cuticles, such as certain berry species, greenhouse ornamentals, and young transplants, are the most sensitive.

When Tank Mixtures Contain High-Electrolyte Fertilizers

Addition of RH-213 to high-electrolyte tank solutions requires compatibility testing. Ammonium sulfate at 10–20% w/v is commonly used to improve herbicide uptake; however, the high ionic strength can reduce the cloud point of nonionic surfactants and destabilize polyether-modified trisiloxanes. If the tank mixture becomes cloudy or separates into oily droplets, the adjuvant is no longer uniformly distributed and spray deposition will be uneven. The optimum sequence is to add water-conditioning agents first, then the high-electrolyte fertilizer, then RH-213 under agitation, and finally the pesticide. In some cases, dilution with water before addition reduces the local concentration shock.

Boron-containing foliar nutrients present a separate incompatibility risk. Sodium borate and boric acid solutions can crosslink or dehydrate polyether chains in silicone adjuvants, leading to stringy precipitates and blocked nozzles. Manganese sulfate solutions above 5% w/v may also reduce solubility through metal-polyether complexation. If such combinations cannot be avoided, the tank mixture should be agitated for 10 min and allowed to settle for 10 min; any settling or viscosity increase indicates incompatibility. A 150-μm nozzle strainer should be inspected after spraying high-electrolyte mixtures because precipitated silicone complexes can lodge in strainer gaps and reduce flow rate.

Use with oil-based formulations is not categorically prohibited, but emulsification may be modified. Crop oil concentrates contain aromatic or paraffinic solvents that compete for the interface; the silicone surfactant can transiently destabilize the oil-in-water emulsion if added before the oil concentrate. The best practice is to add the oil concentrate before RH-213 or to use a compatibility aid. Spray mix pH should be retested after all components are added because alkaline herbicides and acidifying adjuvants can push the final pH outside the 5.0–9.0 hydrolytic stability window.

Compared with conventional nonionic wetting agents, RH-213 achieves a lower surface tension at lower concentrations. Alcohol ethoxylates typically require 0.1–0.25% v/v to achieve surface tensions of 28–34 mN/m, whereas the polyether trisiloxane class reaches 20–23 mN/m at 0.05–0.1% v/v. This concentration advantage reduces adjuvant loading, but it also narrows the margin for error: overdosing above 0.2% v/v can increase run-off and phytotoxicity risk more sharply than with alcohol ethoxylates. Compared with crop oil concentrates, RH-213 contains no petroleum solvent and is less likely to cause visible phytotoxicity on sensitive foliage at labeled rates. However, it does not provide the same film-forming or UV-screening functions that oil-based adjuvants can offer. Compared with unmodified trisiloxane adjuvants, RH-213 is formulated with lower foam tendency, but the hydrolytic degradation pathway is still present.

The regulatory status of RH-213 must be confirmed for the intended jurisdiction. In the European Union, REACH registration and classification should be verified before import or formulation use. In the United States, related polyether-modified trisiloxanes may be listed as inert ingredients under 40 CFR 180.910, but the specific RH-213 designation requires label confirmation. No conclusion regarding suitability for a specific crop can be drawn from generic class data; small-plot compatibility and efficacy testing under local growing conditions remains the definitive method. The user is responsible for verifying that the product label, safety data sheet, and regional pesticide adjuvant regulations permit the intended application.