Arkema Rilsan Fine Powders T SILVER 7537 PA11 is a silver-pigmented thermoplastic polyamide 11 powder coating grade supplied for fluidised-bed immersion and electrostatic spray application to ferrous and aluminium components. The resin phase is a castor-oil-derived PA11 homopolymer with a crystalline melting peak near 186 °C when tested by ISO 11357-3. The unfilled PA11 matrix has a density of approximately 1.03 g/cm³ to 1.05 g/cm³ per ISO 1183-1, while the silver grade includes platelet pigments that alter the powder bulk density and charge acceptance. Lot-specific values for particle size distribution, ash content, and moisture are recorded on the supplier certificate of analysis; typical Rilsan fine powder grades are supplied with a top cut in the region of 250 µm and a median particle size in the range of 90 µm to 120 µm, although grade-specific distributions for T SILVER 7537 should be confirmed before setting cyclone and sieve parameters.

Because the PA11 backbone contains a repeating undecanamide unit, the amide-group density is lower than that of PA6 or PA66, which reduces equilibrium water uptake and contributes to ductile response at sub-zero temperatures. This molecular characteristic is relevant when the silver-coated part is exposed to road salt spray or hot detergent solutions. The coating is used where a bright silver metallic finish must be combined with chemical resistance, abrasion resistance, and the low-temperature toughness of PA11. Production-scale uses include dishwasher baskets, automotive metal clips, seatbelt anchors, fluid-line brackets, and outdoor furniture. The powder is processed through standard PA11 coating lines, but the metallic flake package shifts recovery and reprocessing limits compared with non-pigmented Rilsan Fine Powders grades. Corona electrostatic spray booths tend to show more frequent electrode fouling and a narrower reclaim ratio window. The precise silver appearance depends on film thickness, substrate profile, and quench conditions; published data for this specific configuration is limited, so pre-production panels are recommended for colour match verification.

What Limits Fluidised-Bed Processing of Silver-Metallic PA11 Powders?

In fluidised-bed dip lines, the main process conflicts are bed stratification, moisture uptake, and the rheology of the metallic flake fraction during coalescence. With production tanks of 200 L to 500 L, continuous air flotation of a powder containing high-aspect-ratio silver platelets can generate a top-bed layer enriched in flake and a bottom layer enriched in spherical PA11 fines. This alters the silver intensity of the coating over the course of a shift. A typical control strategy is to maintain a fluidising air pressure between 0.1 bar and 0.3 bar, with the air dried to a pressure dew point no higher than −20 °C. Compressed air with a dew point above 0 °C produces moisture absorption at the particle surface, causing cohesion, reduced charge transfer, and pinhole defects at the part edge.

Metal parts are preheated to a peak metal temperature above the PA11 melting point. Oven profiles are typically set so that the component leaves the preheat zone at 250 °C to 270 °C, immersion time is 3 s to 8 s, and excess powder drains before entering a cure tunnel. With T SILVER 7537, extended fluidisation above 30 min should be avoided on small batch tanks because the metallic flakes can concentrate near the top of the bed and produce a visible mottled band on the first few centimetres of a dipped part. A density-gradient check of the bed at 100 mm intervals is therefore part of the production start-up procedure. Air velocity through the bed should remain between 2 m/min and 4 m/min linear velocity; lower values produce dead zones, while higher values elutriate fine particles into the extraction system and increase silver flake loss in cyclone recovery.

Predrying Thresholds, Surface Conditions, and Adhesion Test Protocol for T SILVER 7537

The pre-dry step is required when the powder has been exposed to ambient air above 60 % relative humidity or held in an opened container for more than 24 h. In such cases, drying in a fluid-bed hopper with desiccant air at 60 °C for 2 h to 3 h is recommended before spraying. The target moisture level in the recovered powder is below 0.3 % by Karl Fischer titration; above 0.5 %, the powder tends to form micro-agglomerates that pass through the spray gun as lumps and create localised thick film spots.

Steel substrates should be degreased and blasted to Sa 2½ per ISO 8501-1 with an anchor profile of 40 µm to 75 µm. Aluminium substrates are prepared by chromate-free conversion coating to limit filiform corrosion under the silver film. Electrostatic spray is performed with a gun voltage of 60 kV to 90 kV and transport air volume of 2 m³/h to 4 m³/h; film thickness is controlled by gun passes and part reciprocation rather than by prolonged spray time. For corrosion-critical parts, a minimum thickness of 200 µm is specified, with tighter limits of ±25 µm measured in line with ISO 19840.

Grade-specific values for T SILVER 7537 may differ from the unfilled PA11 matrix, particularly flexural modulus and impact properties, because the silver platelet pigment acts as a rigid filler. The values above are not a substitute for the Arkema certificate of analysis and should be used only for line-feasibility calculations. During the coalescence stage, the zero-shear viscosity of the PA11 melt follows an Arrhenius dependence on peak metal temperature; a temperature drop of 10 °C can slow bubble release and produce microporosity. In T SILVER 7537, the flakes further restrict bubble mobility, so the melt must remain above 220 °C long enough for air and moisture volatiles to escape. Thick steel sections with a heat capacity above 1 kJ/K often require an extended soak rather than a simple conveyor-speed increase.

For regulatory and process-release documentation, the film is tested against the standard framework used for PA11 powder-coated components. The silver pigment package may affect specific migration limits, and the final article declaration must include the pigment and stabiliser loading. Compliance to RoHS directive 2011/65/EU and the REACH substance-of-concern list is documented on the safety datasheet; food-contact status under FDA 21 CFR 175.300 or EU 10/2011 must be confirmed with Arkema technical service because the metallic flake may behave differently from the unpigmented PA11 film in simulant exposure. No release decision should be based on a single lot; full process capability requires film thickness, gloss, and impact testing over at least three production shifts.

When T SILVER 7537 Replaces PA12 in Abrasive Contact Zones

Compared with a PA12 fine powder of equivalent pigment loading, T SILVER 7537 shifts the melting point upward by approximately 10 °C, from the 176 °C region for PA12 to 186 °C for PA11. Preheat and cure ovens must therefore be run hot enough to fuse the film but not so hot that the metallic pigment flakes migrate to the air surface and lose orientation. In comparative technical literature, PA11 is generally reported to have a higher modulus and better resistance to hot hydrocarbon fluids than PA12, which supports use in underhood brackets and diesel fluid-line clamps. However, PA12 remains the lower-water-absorption option at saturation, with a typical value near 1.5 % compared with 1.8 % for PA11 per ISO 62. For applications involving continuous exposure to 80 °C water, the selection between T SILVER 7537 and a PA12 silver grade should be based on dimensional stability and adhesion after water immersion rather than on water uptake alone.

Against other PA11 fine powders within the Rilsan range, T SILVER 7537 differs primarily in the presence of the silver flake package. The platelet pigments reduce the translucency of the PA11 film but lower the permissible percentage of recovered powder in the feed. On corona-charging automated lines, a reclaim ratio above 30 % by weight is associated with colour drift and impact-resistance loss because recovered powder contains a higher concentration of fine particles and partially fragmented flakes. For tribo-charging lines, the same effect is usually observed above 40 % reclaim. Electrostatic charge acceptance also differs; line trials quantify the required gun current and powder-output settings against a non-pigmented control under the same ambient conditions.

When comparing T SILVER 7537 to short-chain aliphatic nylons such as PA6 or PA66 powder coatings, the processing and property differences are larger. PA6 and PA66 have higher melting peaks, often above 220 °C per ISO 11357-3, and higher water absorption at saturation, typically 8 % to 10 % per ISO 62. This makes PA6 and PA66 unsuitable where the silver film must maintain dimensional stability in humid outdoor exposure. PA11 also offers better low-temperature impact performance due to its amide linkage spacing. The use of T SILVER 7537 therefore targets applications that combine a decorative silver surface with the moisture dimensional stability and ductility that short-chain nylons cannot provide.

Post-cure Quench Rate Controls Haze and Gloss in Silver PA11 Films

After the powder has coalesced into a continuous silver film, the cooling path determines final optical appearance. A slow air-cooling profile, in which the part remains above 150 °C for more than 60 s, allows large PA11 spherulites to develop at the film-air interface; these scatter light and create a frosty or hazy appearance that obscures the metallic platelet reflection. Rapid cooling by water mist or forced air from 250 °C to below 100 °C within 30 s suppresses gross spherulite growth and maintains silver orientation. Production lines therefore position the quench station before the unload point and not at the end of the cure tunnel.

If the quench is too aggressive, thermal contraction of the PA11 matrix over a rigid metallic flake can generate microcracks at the pigment-matrix interface. The usual control is to limit the cooling rate to less than 5 °C/s during the initial 20 °C of sub-melt cooling and then accelerate to 10 °C/s once the film temperature falls below the Vicat softening range. Impact tests per ISO 6272-1 are performed on first-off parts before full batch release. A silver PA11 film of 250 µm thickness on 2 mm steel is generally expected to resist impact energies in the range of 2 J to 4 J without adhesion loss, but published data for this exact silver grade are limited; the value must be established on the production substrate and profile.

Surface defects associated with this grade are generally traced to substrate outgassing or to reclaimed powder fines. Castings and galvanised steel often require an additional preheat hold above 250 °C to burn off residual oils that would otherwise create pinholes in the silver film. On parts with sharp edges, the total film thickness at the edge should be checked separately because PA11 melt flow away from edges during cure can leave a thin region that fails ISO 9227 scribe creep testing before the rest of the component.

Storage and shelf-life controls for T SILVER 7537 follow the same protocol as other PA11 fine powders: sealed, moisture-barrier bags stored at 15 °C to 25 °C and below 60 % relative humidity. Opened packaging must be re-closed under dry conditions, and powder left in a spray booth hopper overnight should be purged with dry air before startup. Lot-to-lot variation in the silver flake concentration is controlled by the pigment supplier and verified by ash content and colour panel tests before the powder is released to the coating line.