Evonik VESTOSINT® 2178 white Polyamide 12 is a fine-particle thermoplastic coating powder based on polyamide 12 resin. The material is supplied as a white powder with a controlled particle-size distribution centred near 60 µm D50 and a crystalline melting endotherm between 176 °C and 180 °C when measured by differential scanning calorimetry under ISO 11357-3. The powder is intended for electrostatic spray deposition, fluidised-bed dip coating, and rotational lining on metallic substrates. After thermal fusion, the coating forms a semi-crystalline layer with the low equilibrium moisture uptake of polyamide 12, high impact toughness, and resistance to aliphatic hydrocarbons and neutral salt solutions. Unlike solvent-borne nylon lacquers, the powder contains no organic solvents and is therefore applied without volatile organic compound emissions from the coating itself.

Why Does Particle-Size Distribution Govern Electrostatic Transfer Efficiency on Thin-Wall Stampings?

Electrostatic spray lines using corona guns typically operate at 60 kV to 80 kV with a charging current of 20 µA to 50 µA. Transfer efficiency declines when the D50 drops below 30 µm because fines remain suspended in turbulent booth air instead of depositing on the grounded part. VESTOSINT 2178 white is classified to a nominal D50 of approximately 60 µm and a D90 below 100 µm, which supports single-pass film builds of 80 µm to 150 µm without excessive orange peel. Retention on a 125 µm sieve is held low, and production lots are screened before packing to remove agglomerates. On thin-wall stampings with low thermal mass, a powder finer than the specified window can produce charge saturation at the film surface; the result is back-ionisation, cratering, and poor pattern coverage before the part enters the fusing oven.

In fluidised-bed dip coating, particle size controls bed expansion and weight pick-up. Compressed air for fluidisation is dried to a dew point below -40 °C, and air flow is adjusted until bed expansion reaches 20 % to 40 %. Steel parts preheated to 260 °C to 400 °C are immersed for 3 s to 8 s; the higher preheat range is used for thin sections that lose heat rapidly, while heavier sections require lower preheat to avoid interface degradation. When the powder bed contains excessive fines from reclaimed overspray, fluidisation becomes uneven and the coating thickness on vertical surfaces varies by more than 30 %.

Corona Charging, Back-Ionisation, and Fluidised-Bed Immersion Time on Zinc-Phosphated Cold-Rolled Steel

On zinc-phosphated cold-rolled steel, the phosphate layer can reduce surface resistance to 10³ Ω to 10⁶ Ω per square. The conductive phosphate layer accelerates charge dissipation, so first-pass film build may be lower on phosphated steel than on lightly blasted bare steel. An applied voltage of 80 kV at a gun-to-target distance of 150 mm can deposit approximately 120 µm on clean bare steel but only 70 µm to 90 µm on heavily phosphated surfaces with ambient moisture. Drying the phosphate layer at 120 °C for 20 min before spraying stabilises surface resistance and reduces star-shaped craters caused by back-ionisation above 100 kV.

For fluidised-bed application on zinc-phosphated steel, immersion times are normally 3 s to 8 s with part preheat from 300 °C to 350 °C. After the substrate heat is exhausted, additional powder may adhere but will not sinter. Incomplete coalescence can be corrected in a post-heat oven at 190 °C for 5 min only when the powder layer has not absorbed moisture. Sharp edges and weld fillets should be masked or supported because cure stresses concentrate where coating thickness exceeds 200 µm.

When White Colour Retention and Hydrolytic Resistance Are Required in Coated Dishwasher Racks

The low equilibrium moisture uptake of polyamide 12 underpins wet-heat performance. Under ISO 62, polyamide 12 absorbs 0.6 % to 0.8 % moisture at 23 °C and 50 % relative humidity, compared with 2.5 % to 3.0 % for polyamide 6. In 80 °C water immersion, PA12 retains more dry tensile strength than PA6 or PA66 because fewer amide groups per chain length are available for hydrogen bonding with water. This characteristic supports use on dishwasher racks, medical trolley wire goods, and food-processing trays exposed to neutral-to-alkaline cleaning agents.

White colour retention is influenced by the titanium dioxide package and the stabiliser system. UV-A exposure at 340 nm in cyclic condensation testing can reveal yellowing if the fusion oven exceeds 210 °C for more than 10 min or if the powder is reclaimed through more than 3 heat cycles. Salt-spray exposure under ISO 9227 for 1,000 h is a common qualification criterion for this type of coating on zinc-phosphated steel; continuous films of 120 µm to 150 µm thickness are generally required to prevent red rust at the scribe. Published data for this specific grade and scribe configuration is limited and must be verified with end-use parts.

Processing Boundaries on Galvanised Substrates: Outgassing, Quench Cracking, and Adhesion

Hot-dip galvanised steel releases gas from zinc-iron intermetallic layers when heated above 200 °C. Fluidised-bed dip coating with VESTOSINT 2178 at preheat temperatures above 300 °C can therefore cause pinholing unless the zinc layer is passivated and pre-baked to remove trapped moisture. Electro-galvanised sheet with zinc thickness below 10 µm produces less outgassing, but rapid cooling after fusion can reduce adhesion. Cooling rates faster than 20 °C/min freeze amorphous regions before complete spherulitic crystallisation, and the resulting residual stress can produce micro-cracks at the zinc-polymer interface.

Adhesion testing under ISO 4624 is performed on blasted steel and galvanised steel. A continuous PA12 coating on blasted steel commonly requires pull-off adhesion of at least 8 MPa, but on galvanised substrates failure may occur within the zinc layer at lower values without indicating a coating defect. Moisture control for the powder is a separate boundary: storage above 60 % relative humidity for more than 4 h warrants pre-drying, because moisture levels above 0.1 % can reduce fluidisation and create micro-voids during fusing.

Surface preparation has a stronger influence on performance than powder grade selection. Cold-rolled steel should be degreased and blasted to Sa 2½ under ISO 8501-1, with an anchor profile of 50 µm to 75 µm for heavy corrosion protection. Zinc-phosphated surfaces may be used for electrostatic spray, but the conversion coating must be free of white corrosion products and dried before powder deposition. Aluminium substrates require chromium-free conversion coating or abrasive blasting; direct application to untreated aluminium can produce adhesion failure after thermal cycling because the oxide layer is mechanically weak.

As supplied, the powder is packaged in moisture-barrier liners inside fibre drums. Bulk density determined by ISO 8130-3 is normally between 0.45 g/cm³ and 0.55 g/cm³. Cyclone recovery systems on electrostatic lines should limit reclaimed overspray to 30 % of virgin powder; reclaim fractions with D50 below 20 µm should be kept below 10 % of the fluidised-bed charge. Without this control, bed density increases and transfer efficiency drops, producing visible film thickness variation on deep-recessed areas.

The fused film typically develops a degree of crystallinity between 35 % and 45 % after controlled cooling from the melt. The crystalline phase contributes solvent resistance and scratch hardness, while the amorphous phase contributes impact toughness. The glass transition of polyamide 12 is approximately 40 °C to 50 °C; below this temperature the amorphous domains are rigid. Quenching from 180 °C below the glass transition suppresses crystallisation and produces a softer, more transparent layer; this is undesirable for a white coating because hiding power and chemical resistance are reduced. Thermal degradation of polyamide 12 becomes significant above 250 °C; local hot spots in the fusing oven above 220 °C can generate gel defects and discoloration that are masked initially by the white pigment but later appear as brittle regions.

Chemical immersion testing under ISO 2812-1 is used to evaluate contact with diesel fuel, hydraulic oil, grease, and salt solutions. The PA12 coating is resistant to aliphatic hydrocarbons but should not be used in continuous contact with strong mineral acids, phenols, or hot polar solvents such as methanol and glycol-based brake fluids, because these agents plasticise the amorphous phase and reduce abrasion resistance. Taber abrasion testing under ASTM D4060-19 with CS-10 wheels is commonly applied at 1,000 cycles; a fully fused 200 µm PA12 film can show mass loss below 20 mg depending on substrate preparation and cure. Poorly fused layers have lower Shore D hardness and cannot be corrected by post-heating if the powder has already absorbed moisture.

Triboelectric spray guns charge the powder by friction rather than corona discharge. VESTOSINT 2178 white can be applied tribostatically with lower free-ion density, which reduces back-ionisation on deep recesses and Faraday-cage areas. However, tribo guns require fluidising air pressure to be adjusted between 1.5 bar and 2.5 bar and may require a powder hose length above 3 m to develop sufficient charge. In high humidity booths above 60 % RH, tribo charging efficiency drops because moisture increases surface conductivity of the powder grains.

Polyamide 12 is electrically insulating. Volume resistivity of a fused white powder layer can be expected above 10¹³ Ω·cm when measured under IEC 62631-3-1. This limits use in electrostatic discharge protected environments unless an antistatic topcoat or conductive primer is applied. The white titanium dioxide pigmentation does not provide meaningful charge dissipation; therefore the coating should not be regarded as static-dissipative.

Values for PA11 and PA6 columns are generic resin-class reference data, not grade-specific certification values for other VESTOSINT products.

Compared with PA11, PA12 in this grade exhibits a lower melting point by approximately 10 °C and lower density. The lower melting point permits faster fusion on heat-sensitive zinc-coated steel and reduces the preheat temperature required for fluidised-bed dip coating. Compared with PA6, PA12 offers lower equilibrium moisture uptake and better dimensional stability in humid environments, while PA6 provides higher surface hardness and higher tensile modulus for applications where stiffness dominates. Compared with lower-melting copolyamide coating powders that fuse at 110 °C to 130 °C, VESTOSINT 2178 requires a higher fusing temperature but provides better chemical resistance and lower creep under sustained load.

Within the VESTOSINT polyamide 12 range, the white grade is differentiated from natural unpigmented grades by the titanium dioxide pigment package and higher hiding power. Processors switching from a natural PA12 powder may need to reduce gun feed air by 0.2 bar to 0.4 bar because pigmented powders can charge differently under corona conditions. Black and coloured grades may contain alternative pigment systems that influence melt viscosity and surface smoothness; therefore the same oven settings cannot be transferred between colours without first verifying film build and gloss.

Production-scale batch experience indicates that classifier wear can shift D50 from 58 µm to 64 µm over a campaign. This shift may appear minor but can alter fluidised-bed expansion and electrostatic transfer on high-speed lines. Manufacturers therefore record the D10, D50, and D90 values of each incoming lot and adjust gun voltage or bed air flow before production. If the D90 rises above 100 µm, films thicker than 150 µm can exhibit surface texture that cannot be eliminated by lowering oven temperature alone.

Bulk powder exposed to humid air can be pre-dried at 80 °C for 4 h to 6 h in a recirculating air dryer with tray depth below 50 mm. Drying at higher temperatures can cause the powder to agglomerate because the surface of individual grains begins to sinter near the melting onset. Fluidised-bed hoppers should be purged with dried air and not loaded with cold powder directly from an unheated warehouse in winter, because condensation on the powder surfaces increases charge dissipation and reduces film build.

Regulatory documentation for the base polyamide 12 commonly references EU RoHS Directive 2011/65/EU for lead, cadmium, mercury, hexavalent chromium, PBB, and PBDE restrictions, and REACH Regulation (EC) No 1907/2006 for monomer registration and substances of very high concern. Food-contact suitability must be confirmed on the finished article under 21 CFR 177.1500; the powder contains auxiliary stabilisers and pigments that are not automatically covered by the resin clearance. No statement in this material constitutes a food-contact certification without end-use migration testing under the applicable national regulation.