Polyoxyl (35) Castor Oil Veterinary Grade API for Tablets / Injections / Capsules / Powders / Granules / Premix / Solutions is a nonionic amphiphilic excipient produced by ethoxylation of castor oil to a nominal 35 mol ethylene oxide per mole of triglyceride. The material is identified by CAS 61791-12-6 and is typically controlled under the compendial identity for Polyoxyl 35 Castor Oil; no discrete model number is assigned because grade equivalence is defined by the oxyethylene chain length, acid value, hydroxyl value, residual ethylene oxide profile, and carbonyl distribution. The product is a pale yellow to amber viscous liquid at 25°C, with density near 1.05–1.06 g/cm³ and dynamic viscosity commonly 600–800 mPa·s. The pour point is approximately 21–26°C, so cold-weather transfer into manufacturing vessels requires heated storage at 30–40°C and jacketed transfer lines to limit back pressure. The veterinary-grade API designation indicates manufacture in a quality system aligned with 21 CFR Part 211 for finished pharmaceutical components, and the nonsterile raw material must be terminally sterilized or aseptically processed before use in injections. Typical release limits include acid value not more than 2.0 mg KOH/g, hydroxyl value 65–85 mg KOH/g, saponification value 60–75 mg KOH/g, iodine value 28–32 g I₂/100 g, and water content not more than 3.0%.

Because the product is synthesized by ethoxylation, residual ethylene oxide and 1,4-dioxane are controlled in the release specification. Residual ethylene oxide is typically limited to not more than 1 ppm, and 1,4-dioxane to not more than 10 ppm, using headspace gas chromatography according to Ph. Eur. 2.4.25 or USP <228>. The manufacturer should also supply a risk evaluation for nitrosating impurities if the excipient is used in premixes containing nitrite-preserved carriers, because polyether chains can generate trace nitrosamines under acidic storage. Bulk storage in stainless steel, glass-lined, or epoxy-phenolic lined containers is preferred; direct contact with unlined carbon steel may promote iron-catalyzed oxidation. The recommended storage temperature is 15–25°C, with excursions not exceeding 40°C for more than 72 h under validated transport conditions. Opened containers should be blanketed with nitrogen at 0.1–0.3 bar; headspace oxygen below 5% is recommended to keep peroxide value below 10 meq O₂/kg over a 24-month shelf life. In high-humidity environments above 60% RH, bulk containers must be sealed immediately after sampling because water absorption increases the risk of hydrolysis and microbial growth.

The product should not be confused with Polyoxyl 40 Castor Oil or Polyoxyl 40 Hydrogenated Castor Oil. The 35-mole ethoxylate has a lower nominal ethoxylate chain length than the 40-mole product, which modifies the cloud point and the ratio of polyethylene glycol monoesters to diesters; the practical HLB difference is typically within 1–2 HLB units. Hydrogenated versions reduce iodine value below 2 g I₂/100 g, improving oxidative stability but raising the melting range and often requiring pre-melting in cold-process formulations. The unsaturated 35-mole product is therefore preferred when a pourable liquid at 20–25°C is required without pre-melting, but it is less suitable than the hydrogenated version for oxygen-sensitive actives in long-term parenteral emulsions. The product is miscible with ethanol, isopropanol, dichloromethane, and toluene; it disperses in water. It is insoluble in mineral oil at 25°C but soluble in polar oils such as oleic acid and tributyrin. Co-solvent selection should follow VICH GL18(R2) or ICH Q3C residual solvent limits.

How Does the Oxyethylene Chain Distribution Affect Solubilization in Aqueous Veterinary Formulations?

In aqueous vehicles, the product forms micellar structures; the critical micelle concentration in purified water at 25°C is reported in the range of 0.02–0.04 wt%, although published data for specific target-species formulations is limited. The nominal 35-unit oxyethylene chain length produces a distribution of ethoxylated species that influences the solubilization of poorly water-soluble active pharmaceutical ingredients such as ivermectin, closantel, and fenbendazole in injectable and oral solutions. Because the castor oil backbone is unsaturated, the product remains liquid at room temperature and can be filtered through 0.22 μm sterilizing-grade membrane filters when diluted below 10 wt% in aqueous phase; at concentrations above 20 wt%, cold-vehicle filtration encounters high back pressure and membrane fouling. In continuous dilution using an in-line static mixer with 12 helical elements, the concentrated surfactant stream is pre-warmed to 35°C and injected at 0.5–1.0 kg/min into the aqueous phase to avoid high local viscosity and gel formation. Electrolytes reduce the cloud point; for a 1.0 wt% aqueous solution, the cloud point may fall below 80°C when chloride ion is present at 0.9 wt%, imposing an upper processing temperature limit in saline diluents and excluding steam sterilization of some saline-containing preparations.

For tablet and dry premix manufacturing, the viscous liquid is normally adsorbed onto a porous carrier before addition to high-shear granulators. A loading of 20–30 wt% liquid product onto colloidal silicon dioxide yields a free-flowing adsorbate with a Carr index below 15 after sieving through 0.5 mm mesh. In a high-shear mixer operated with an impeller speed of 120 rpm and a chopper speed of 1,500 rpm, the adsorbate is added after the binder solution to limit overwetting; granulation endpoint is assessed by impeller torque rather than time alone. Torque values above 0.8 N·m/kg per unit of wet mass have been associated with subsequent tablet picking and lamination during compression. The product functions simultaneously as a wetting agent for poorly water-soluble actives and as a nonionic solubilizer that can increase dissolution rate in tablet matrices. However, the unsaturated castor oil backbone is susceptible to autoxidation when tablets are stored above 40°C or under high-oxygen-transmission packaging; stability protocols aligned with VICH GL45 should include peroxide value and related-substance monitoring by Ph. Eur. 2.5.5.

Batch-to-batch viscosity at 25°C can vary by ±10%; therefore powder premix loading is mass-metered rather than volume-metered. In a production-scale powder blender, the liquid product is sprayed onto a moving bed of microcrystalline cellulose or silica at nozzle air pressure 0.8–1.2 bar and bed temperature 25–30°C to prevent agglomeration. Near-infrared spectroscopy calibrated against the carbonyl band at 1,740 cm⁻¹ is used to confirm surfactant concentration in the final premix within ±0.5 wt% of target. When the product is introduced into a twin-screw granulator with L/D 25:1, barrel torque excursions above 20 N·m may indicate surfactant slip or insufficient powder wetting, requiring a reduction in liquid feed rate rather than an increase in screw speed alone.

Veterinary Injection-Grade Manufacturing and Endotoxin Control

Parenteral applications require bacterial endotoxin limits appropriate to the intended route and maximum dose. A typical release specification is ≤0.25 EU/mg for injectable-grade material, although the exact limit is calculated using USP <85> or Ph. Eur. 2.6.14 from the maximum dose per kilogram body weight. Biocompatibility assessment follows ISO 10993-1:2018 and USP <88> biological reactivity tests, with the test battery selected for the intended tissue contact route. During large-scale compounding, the product is filtered through a 0.2 μm filter cartridge after dilution to 10 wt% in water-for-injection at 35–40°C. Production-scale experience with 1,000 L jacketed stainless-steel vessels indicates that rapid addition faster than 0.5 wt% of batch mass per minute can generate foam in the vessel headspace; foam reduces downstream transfer accuracy and increases the surface area for oxidation. A nitrogen overlay at 0.2–0.5 bar positive pressure is recommended during hydration and tank transfer. Because nonionic surfactants can inactivate preservative systems, solutions containing benzalkonium chloride require preservative efficacy testing at the final excipient concentration according to USP <51> or Ph. Eur. 5.1.3. The product is not a sterile API; terminal sterilization or aseptic filtration remains mandatory for injectable dosage forms.

When Polyoxyl 35 Castor Oil Replaces Polysorbate 80 in an Aqueous Oral Solution

Replacement of polysorbate 80 with polyoxyl 35 castor oil requires re-evaluation of the free fatty acid load, pH-shift behavior, and preservative interaction because the castor oil-derived product contains ester-linked ricinoleic groups that can hydrolyze during terminal sterilization. In phosphate-buffered systems at pH 6.0, ester hydrolysis is minimized relative to unbuffered systems; preformulation screening according to VICH GL5 is required because the extent of hydrolysis depends on buffer species, ionic strength, and sterilizer load. Unlike polysorbate 80, the castor oil polyether has a slightly lower HLB and a higher proportion of triglyceride-derived hydrophobe, which can increase viscosity contribution in concentrated oral syrups and may reduce clarity in high-electrolyte diluents. The cloud point in 0.9% sodium chloride solution is typically lower than that of polysorbate 80, limiting steam sterilization above 60°C for saline-containing formulations. In drinking-water solutions, the product can lower surface tension but may promote foaming in recirculating pipelines; a defoaming agent may be required only if the recirculation flow rate exceeds 0.5 m/s.

When semisolid filling of hard and soft gelatin capsules is required, the product is blended with medium-chain triglycerides or propylene glycol monocaprylate at 10–30 wt% to reduce fill viscosity below 400 mPa·s at 35°C. In hard-capsule liquid-filling systems using a piston pump with 0.5 mm nozzle diameter, dissolved gas removal is critical; vacuum not greater than 50 mbar for 15 min prevents bubble formation and weight variation greater than ±3% relative standard deviation. For powders and granules, the product is rarely used as a dry powder; it is premixed onto silica or microcrystalline cellulose. When preparing feed premixes, addition of 2–5 wt% surfactant relative to the active premix can improve wettability of hydrophobic actives in drinking water, but hard water above 300 mg/L calcium carbonate equivalent may reduce performance through cloud point depression and micelle destabilization. Stock solutions for veterinary drinking water should be prepared as 5–15 wt% concentrates in softened water and diluted at point of use to avoid microbial growth; if unpreserved, hold time should not exceed 24 h at 20–25°C.