What High-Shear Mixing Window Prevents Sub-50 μm Microvoids in Epoxy Flooring?

Flooring compounds formulated with bisphenol A/F liquid epoxy resin having an epoxide equivalent weight of 180–190 g/eq and cycloaliphatic amine or amine-adduct hardeners are compounded on pin mixers at blade-tip speeds of 8–12 m/s. In this shear regime air is dispersed into microvoids with diameters below 50 μm because the letdown viscosity at 25 °C is normally 800–1500 mPa·s. Tech-3722 is introduced at 0.15–0.40 wt% of total batch during pigment and filler dispersion, before viscosity adjustment with reactive diluents or air-release additives. The polyether-modified silicone phase orients at the air–resin interface; the polyether segments reduce interfacial elasticity, allowing fine bubbles to coalesce into larger voids that rise to the surface within the 15–25 min pot-life window. In production trials with continuous mixing and post-cure film thickness of 2–3 mm, visual pinhole count per 1 m² falls below 5 when the compound reaches dynamic viscosity below 1200 mPa·s at 25 °C and is subjected to a subsequent 1–2 min vacuum deaeration at 50–100 mbar. Overdosing above 0.50 wt% produces crawling on polyurethane-primed concrete and reduces intercoat adhesion to less than 2.0 MPa when tested according to ISO 4624:2016 on damp substrates. Addition after letdown or after hardener is ineffective because localized high defoamer concentration stabilizes surface defects. The formulation should maintain pigment volume concentration between 10 and 15 % and volume solids above 90 % by ISO 3233-1:2019. Filler moisture above 0.5 wt% requires pre-drying of silica or calcium carbonate at 120 °C for 4 h when relative humidity exceeds 60 %, otherwise the required defoamer dosage shifts toward 0.35 wt% and batch-to-batch variance increases.

In filled epoxy potting compounds based on bisphenol A diglycidyl ether with an epoxide equivalent weight of 188 g/eq, anhydride hardeners, and silica or alumina filler loadings of 55–65 wt%, vacuum degassing at 5–15 mbar frequently fails to remove air trapped in narrow gaps between filler particles. The 70% active polyether-modified silicone defoamer is pre-blended into the resin component at 0.05–0.25 wt% of total compound prior to filler addition; this addition point matters because absorption onto filler surfaces above 0.30 wt% reduces dielectric strength after thermal cycling. Under vacuum, the additive lowers the surface tension differential between resin and filler and accelerates bubble coalescence at processing temperatures of 40–60 °C. Cured blocks 10 mm thick exhibit dielectric strength above 15 kV/mm when measured in accordance with ASTM D149-20, but this value decreases when residual microvoids exceed 0.5 vol% or when silicone migration creates a surface film. Published data for this specific filled configuration is limited; production observations indicate that at 0.20 wt% the cured surface remains non-oily and passes a tape adhesion test for encapsulation compounds. The additive is not suitable for anhydride systems cured below 80 °C because incompletely reacted polyether chains exude and reduce UL 94 V-0 performance in post-cure flame tests. Mixing should use a planetary vacuum mixer at 20–30 rpm for 15 min, not a high-shear disperser above 500 rpm, to prevent stable micro-emulsification that would otherwise delay air release. For thermal cycling from -40 °C to 125 °C at 10 K/min, keeping the dosage below 0.20 wt% avoids void growth at filler–resin interfaces and maintains volume resistivity above 10¹⁴ Ω·cm by ASTM D257-14.

When Solvent-Free Aromatic Polyurethane Primers Are Mixed With Calcium Carbonate Extenders

Mixed with calcium carbonate extenders, solvent-free aromatic polyurethane primers containing polymeric MDI with NCO content of 30–32 % and branched polyether polyols generate carbon dioxide during moisture curing. A portion of that carbon dioxide remains as pinholes in 80–120 μm dry film thickness when applied at 20 °C and 60 % relative humidity. The defoamer is added to the polyol side at 0.10–0.60 wt% of total formulation before filler dispersion and isocyanate mixing. In a two-component high-pressure airless spray unit with an output of 4–6 kg/min, the material reduces foam formation in the mixing chamber and prevents pinholes after 24 h at 23 °C and 50 % relative humidity. The primary control parameter is the ratio of polyether segment chain length to silicone backbone; insufficient polyether character causes intercoat adhesion failure, while excessive compatibility lowers antifoam efficiency at addition levels below 0.20 wt%. Adhesion to grit-blasted steel with surface preparation Sa 2½ according to ISO 8501-1:2007 should remain above 5 MPa according to ISO 4624:2016 when the defoamer is used at 0.30 wt%. At 0.80 wt% there is visible cratering over weld seams and solvent pop after forced drying at 60 °C. Viscosity of the mixed primer should be maintained between 350 and 700 mPa·s at 25 °C for airless sprayability; the defoamer has negligible influence on ISO 2431:2019 flow cup viscosity below 0.40 wt% but may reduce gloss at 60° specular angle when overmixed. Pot life measured by ISO 9514:2019 should remain at least 2 h at 23 °C; at 0.30 wt% the defoamer does not alter gel time by more than 5 min.

Difficulties with bubble retention in hot-cast TDI-prepolymer polyurethane elastomers are observed when the prepolymer NCO content is 3.2–6.5 % and MOCA is used as chain extender with a processing temperature of 80 °C. When the polyether-modified silicone defoamer is pre-dispersed at 0.03–0.20 wt% in the prepolymer phase prior to degassing at 80 °C and 3–10 mbar, time to achieve a bubble-free mixture is reduced from 30–40 min to 10–15 min. The addition should not exceed 0.25 wt% because residual silicone at the interface between casting and steel core reduces tensile strength below 25 MPa and tear strength below 35 N/mm according to DIN 53504:2017 and ISO 34-1:2022. For a 85–90 Shore A elastomer cured 24 h at 100 °C, compression set measured by ISO 815-1:2019 at 70 °C for 22 h can be affected by more than 2 percentage points when unreacted defoamer migrates to the friction surface. In production casting of conveyor rollers with wall thickness of 20 mm, circular voids larger than 0.2 mm are eliminated at 0.10 wt% without observable surface exudation. The material must be mixed under low shear at 80 °C and not added to the MOCA phase because the amine reacts with free NCO to generate additional bubbles. Effective degassing is achieved in a double-arm mixer at 30 rpm under vacuum with a surface area-to-volume ratio above 0.2 cm⁻¹; shorter mixing cycles below 8 min leave residual microfoam in the centre of thick sections.

Because clear amine-cured epoxy casting resins are processed in deep sections of 25–50 mm, entrapped air manifests as suspended microbubbles that reduce optical clarity after cure. The 70% active polyether-modified silicone defoamer is dispersed in the resin component at 0.05–0.20 wt% of total batch before tinting and before slow amine hardener addition. At a mixed viscosity of 400–800 mPa·s at 25 °C, bubbles can rise to the surface during a gel time of 8–12 h, but surface foam may be reincorporated during a second pour if the defoamer is depleted. Incorporation at 0.10 wt% followed by vacuum degassing at 10–20 mbar for 15 min yields a transparent casting with haze below 2% measured by ASTM D1003-21. Above 0.25 wt%, microvoids can become stabilized as cell-like defects because the silicone concentration exceeds the bulk compatibility limit. For decorative tabletop production at 22–25 °C, the defoamer does not eliminate amine blush caused by relative humidity above 70%; a moisture-controlled booth is still required. Published data for this specific decorative casting configuration is limited.

Unsaturated Polyester Gelcoat Pinhole Control and Thixotropy Adjustment

Gelcoat formulations based on ortho- or isophthalic unsaturated polyester dissolved in styrene at 35–42 % monomer content and thickened with fumed silica to a thixotropic index of 3.5–5.5 require both air release and rapid surface leveling because gelcoat cure with MEKP proceeds at ambient temperature while air bubbles are still rising. The gelcoat is applied at 0.4–0.6 mm wet film thickness by airless or conventional spray. Pinholes of 50–150 μm form when MEKP-initiated cure at 1.5–2.0 phr progresses before air bubbles escape from the wet film at 20–25 °C. The 70% active defoamer is introduced at 0.20–0.50 wt% of the base formulation before rheology modifier addition. When incorporated before fumed silica, the polyether-modified silicone rapidly wets the silica surface and prevents stable bubble nucleation during high-speed dispersion at 1500–2000 rpm. In sprayed laminates exposed to water immersion at 40 °C for 30 days, a dosage of 0.30 wt% shows no blisters or delamination, while a control develops osmotic blisters within 14 days. Operator observation on production spray booths indicates that 0.50 wt% can produce surface cratering over sharp mold radii because the local surface tension drops below 25 mN/m. Barcol hardness according to ASTM D2583-13a should remain above 35 after 2 h post-cure at 40 °C. The product should not be used above 0.60 wt% in gelcoat because over-defoaming creates a closed-cell surface layer that prevents styrene evaporation and causes under-cured surface under humid conditions. When fumed silica is added after the defoamer, the thixotropic gel network develops within 20 min; at 0.30 wt% the gelcoat Brookfield viscosity at 10 rpm does not vary by more than 5% relative to the control.

Reducing Entrapped Air During High-Filler-Loading Bulk Moulding Compound Mixing

Typically, bulk moulding compound (BMC) based on unsaturated polyester resin, low-profile additive, styrene, calcium carbonate filler at 60–70 wt%, zinc stearate release agent, and 10–20 wt% chopped glass fiber is mixed in a sigma-blade kneader at 30–50 rpm. Air entrapment during wetting of glass fibre and filler is the main source of voids in compression-moulded components. The polyether-modified silicone defoamer is added to the liquid resin phase at 0.10–0.40 wt% before filler addition and before glass fiber incorporation. In BMC formulations with paste viscosity of 20,000–40,000 mPa·s at 25 °C, the silicone-polyether copolymer accelerates air release without destabilizing the low-profile thermoplastic phase. The product should be added before the low-profile additive or with the resin to avoid localized surface tension gradients that produce glass fiber wet-out defects. At 0.10 wt% the product is preferred for parts requiring post-paint adhesion; at 0.40 wt% moulded surfaces may show paint pops after electrocoat because silicone concentrates at the surface during cure at 140–160 °C. Flexural strength measured by ISO 14125:2014 on 4-mm compression-moulded plaques remains within 10% of the control at 0.20 wt%, while void content determined by image analysis decreases from 1.8% to below 0.6%. The additive is not a replacement for tool vacuum or proper shear thinning; it will not remove large entrapped air pockets formed during material charge placement.