Silicone Leveling Agent RH-T1033

Product identification for Silicone Leveling Agent RH-T1033 defines the material as a polyether-modified polydimethylsiloxane liquid supplied at a representative active content greater than 99 %. The model designation RH-T1033 is applied to the solvent-free version intended for solventborne, high-solids, and radiation-curable systems; a separate waterborne grade is required for direct incorporation into aqueous dispersions. Quality control requires the following representative values, obtained on production batches at 23 °C and 50 % relative humidity.

These values are production-control targets rather than guaranteed specification limits. Published data for this specific configuration in water-reducible alkyds is limited; storage stability and pigment dispersion interactions must be confirmed on the target formula before scale-up. Production release includes batch-to-batch comparison of refractive index and viscosity using Shewhart control charts with upper and lower action limits. In an automotive basecoat line, a batch with viscosity at the upper control limit of 350 mPa·s did not cause transfer pump cavitation, but required a 5 % increase in stirrer torque. Viscosity is therefore not only a quality parameter but also a dosing pump sizing factor. Positive-displacement gear pumps with a turn-down ratio of 10:1 are recommended when dosing the neat liquid at 0.05 wt%.

What Limits Surface Tension Reduction in High-Solids Coatings?

In high-solids polyurethane clearcoats applied by rotary bell atomization, air entrainment and overspray-induced craters are often controlled only when the dynamic surface tension of the liquid film is reduced below that of the contaminants. Silicone Leveling Agent RH-T1033 migrates to the air–liquid interface during flash-off; migration is driven by the limited solubility of the siloxane segment in the matrix and is observed as a surface excess layer after 60 s of ambient flash at 23 °C and 50 % relative humidity. In a solventborne acrylic melamine clearcoat, a dosage of 0.1 wt% to 0.3 wt% based on total formulation reduced long-wave amplitude measured by wave-scan to below 5 units over a scan length of 10 cm. The threshold depends on solvent blend, substrate roughness, and film build; lower-solids systems may require the lower dosage because the additive accumulates more rapidly in the shrinking film. In a production-scale electrostatic disc line running at 45 m/min with a dry film thickness of 35 µm to 50 µm, cratering was eliminated when the additive was pre-diluted at 10 % in n-butyl acetate and dosed after the pigment dispersion stage. Leveling was evaluated according to ASTM D4062-11; specular gloss was measured with ASTM D523-14.

Surface tension reduction is dynamic; the additive must diffuse to the interface before viscosity build-up freezes the film. In high-solids systems with flash-off below 15 s before cure, a split addition of 50 % at let-down and 50 % after thixotrope addition is used to reduce migration distance. In a pilot line with a drawdown bar and a flash-off tunnel at 70 °C for 3 min, a single addition at 0.15 wt% gave equivalent leveling to split addition, but at 23 °C and 45 % relative humidity the split addition produced fewer edge defects.

Incorporation practice is governed by shear sensitivity and solubility. The additive should be pre-diluted before addition to the let-down tank; direct addition to high-viscosity mill base can create a localized silicone-rich phase that produces fisheyes. A pre-dilution of 10–20 % solids in xylene, n-butyl acetate, or Aromatic 100 is typical for solventborne systems. Let-down agitation with a stainless steel propeller at 5–12 m/s tip speed is sufficient; high-shear dispersion above 20 m/s is unnecessary and does not improve performance. In two-component polyurethanes, the additive is added to the polyol component before crosslinker addition. Batch stability at 40 °C for 28 days shows viscosity drift of less than 5 % when the additive is stored in sealed containers. Flow time measured by DIN 53211 must be re-established after dosing because the additive may alter the low-shear viscosity in high-thixotropy formulations.

At relative humidity above 60 %, moisture-cure one-component polyurethane systems may exhibit pinholes from carbon dioxide release; RH-T1033 does not suppress gas expulsion and is not a substitute for dehydration of solvents or pigment pastes. The additive should not be combined with amine-functional silanes in moisture-cure systems because amine-catalysed condensation can increase viscosity during storage. For baked alkyd systems, the additive is generally resistant to yellowing under standard schedules of 30 min at 130 °C, measured by ASTM D1925; beyond 150 °C for 60 min, discoloration should be evaluated on the actual resin because the organic modification can degrade and release lower-molecular-weight siloxanes.

Chemical stability in the presence of common sulfonic acid catalysts was evaluated at 40 °C for 7 days with 0.5 % p-toluenesulfonic acid in xylene; no exotherm or viscosity increase was observed. Aqueous alkali above pH 10 should be avoided because siloxane hydrolysis may occur during long-term storage.

Dosage Gradients and Film Defect Thresholds

In solventborne short-oil alkyd baking enamels, the effect of Silicone Leveling Agent RH-T1033 is nonlinear. At 0.05 wt%, surface tension reduction is insufficient to suppress cratering caused by 5 µm silica dust; at 0.15 wt%, crater count per 100 cm² drops from 12 to 2, while specular gloss at 20° rises from 78 to 84. At 0.50 wt%, intercoat adhesion and slip can become problematic in recoatable systems, even though distinctness of image remains stable. The boundary between defect suppression and recoatability loss is therefore formulation-dependent, with a typical upper limit of 0.5 wt% for solventborne bake enamels and 0.3 wt% for two-component clearcoats. Table 2 summarizes a representative gradient study using a forced-air laboratory oven at 130 °C for 30 min; published data for this specific configuration is limited and values should be confirmed on target line stock.

Cone-and-plate rheometry at 25 °C indicates Newtonian behavior up to 1,000 s⁻¹; the additive contributes less than 2 % to low-shear viscosity at 0.3 wt% dosage. The lower use limit is also controlled by substrate contamination: on phosphate-treated steel with residual drawing oil, crater control may require 0.2 wt% or a combination with a wetting agent. Above 0.3 wt%, foam stabilization increases in air-spray application; the liquid should be evaluated with a foam test according to ASTM D3519 if air entrapment is expected.

When Silicone Leveling Agent RH-T1033 Replaces Polyacrylate Leveling Additives

Compared with medium-molecular-weight polyacrylate leveling agents, RH-T1033 provides lower equilibrium surface tension and faster migration. In a solventborne polyester/melamine clearcoat, replacement of a polyacrylate at 0.3 wt% with RH-T1033 at 0.15 wt% reduced surface tension from 28 mN/m to 22 mN/m and improved wave-scan long-wave values by 3 to 4 units. However, the silicone additive is less effective at maintaining intercoat adhesion after sanding; cross-cut adhesion on the subsequent layer was rated GT 1 to GT 2 under ISO 2409, compared with GT 0 for the polyacrylate control. The mechanism is attributed to the low surface energy siloxane layer that remains after curing; it reduces wetting by waterborne basecoats and is not fully removed by solvent wiping. Recoatability therefore requires abrasion, plasma treatment, or an adhesion-promoting wash. In high-gloss clearcoats where polishing or recoat adhesion is critical, a blend of RH-T1033 at 0.05 wt% to 0.10 wt% with a polyacrylate at 0.20 wt% is used to separate surface tension control from interlayer wetting.

Relative to fluorocarbon-modified leveling agents, RH-T1033 provides less wetting of oily fingerprints and lower cost-in-use, but it may produce more stable foam in air-spray application. Compared with low-molecular-weight polydimethylsiloxane oils, the polyether modification gives a controlled incompatibility that reduces surface slip and improves recoatability. The additive is not a standard silicone oil; its cloud point in water is not directly applicable because the product is not designed for direct aqueous incorporation. Published data for specific waterborne polyester dispersions is limited. Unlike silicone polyether copolymers with higher HLB, RH-T1033 is not designed for aqueous pigment wetting; surface tension reduction in water is muted. In solventborne systems, its critical micelle concentration is below the use range, so excess addition does not create a reservoir that enhances long-term slip. Compared with high-slip silicone additives based on high-molecular-weight dimethylsiloxane, the product is less slippery after cure; coefficient of friction tested by ISO 8295 typically increases by 0.03 to 0.05 relative to the unmodified control, not the 0.10 to 0.15 increase typical of high-slip products.

Use in UV-curable urethane acrylate clearcoats requires evaluation of oxygen inhibition at the surface. Because RH-T1033 migrates to the air interface, it can influence the cure response of the top 2 µm. At 0.1 wt%, pendulum damping after 5 passes at 10 m/min under a gallium-doped mercury lamp was comparable to the silicone-free control when measured by König pendulum damping in accordance with ISO 1522:2022. At 0.3 wt%, the surface tack was slightly extended; full cure required an additional 10 % incident energy. The additive should therefore be introduced after the oligomer and monomer blend, before photoinitiator addition, to avoid competition at the initiation stage. In electron-beam curable systems, no such limitation is expected because initiation is not oxygen-sensitive.

On a reverse roller coater with chrome-plated rolls, foam generation is minimized when RH-T1033 is pre-diluted in isophorone at 10 wt% before entering the recirculation line. Extended recirculation at 35 °C for 8 h did not cause gelling or filter blocking through a 50 µm bag filter; pressure drop increased by less than 0.2 bar. The diluted stream is preferably metered with a mass-flow-controlled dosing skid to maintain batch-to-batch reproducibility; manual addition into an open mixing tank is acceptable only if the addition point is below liquid level and agitation is continuous.

Storage in unopened containers at 5 °C to 30 °C is recommended. A single freeze-thaw cycle at −10 °C for 24 h followed by 24 h at 25 °C did not change viscosity beyond 10 %; nevertheless, repeated cycling should be avoided because polydimethylsiloxane segments may orient and create a hazy layer.

For regulatory compliance, the supplier documentation should be consulted for current REACH registration status and food-contact clearances. No FDA 21 CFR 175.300 clearance is automatically assumed for RH-T1033 in direct food-contact coatings. If the coating is specified under RoHS Recast 2011/65/EU, the final cured film must be screened for restricted substances because the formulated coating may contain other additives or pigments that carry the compliance obligation. The silicone leveling agent itself is not a reportable solvent under Directive 2004/42/EC; however, the pre-dilution solvent contributes to VOC and must be included in the coating VOC calculation according to ISO 11890-2.