Arkema Rilsan Fine Powders T SILVER 7556 PA11 is a silver-pigmented polyamide 11 fine powder supplied for solvent-free thermoplastic coating of metal parts. The grade designation places it within the Rilsan T family of PA11 powders, in which the base resin is produced from 11-aminoundecanoic acid derived from renewable castor oil. The renewable carbon fraction of the base resin is measurable using ASTM D6866-22 and typically exceeds 90%. Unlike epoxy, polyester, or polyurethane powder coatings, this material does not cure by chemical cross-linking; it is heated above the crystalline melting region and forms a film by sintering, coalescence, and flow under finite melt viscosity. This distinction is operationally significant because a fused PA11 coating can be remelted, but it also displays a narrow thermal processing window over high-speed coating lines and no thermoset cure plateau.

The silver designation is not solely a colourant difference. T SILVER 7556 belongs to the fine powder product class, which is intended for electrostatic spray and fluidised-bed deposition where particle size distribution controls transfer efficiency, edge coverage, and film smoothness. The exact particle size distribution is batch-specific and must be read from the certificate of analysis; published product-grade values for this specific metallic grade are limited. In general industrial practice, Rilsan fine powder grades used for electrostatic application are controlled to a nominal top size below 250 µm, with fine fractions below 40 µm minimised to avoid powder cloud instability. The fine powder designation also separates the material from coarser PA11 grades that are used for different deposition techniques or thicker sintered linings.

Moisture uptake in PA11 is lower than PA6 and PA66. Equilibrium water absorption measured on injection-moulded specimens according to ISO 62 is normally 1.8–2.0% at 23 °C in water. PA12 fine powder can show saturation uptake near 1.3–1.5%, which is marginally lower, but PA11 retains a higher melting point and better resistance to stress cracking in the presence of polar hydrocarbons. For corrosion-protection coatings, the semicrystalline PA11 matrix limits chloride-ion and oxygen transport more effectively than lower-crystallinity polyamide grades when the film thickness is maintained above 150 µm. These bulk resin properties should not be read as direct coating performance values; adhesion, edge coverage, and pinhole density usually control the salt-spray result more than the polymer’s inherent water uptake.

Why does metallic silver pigmentation alter charging, reclaim, and film aesthetics in PA11 fine powder?

Metal-effect pigmentation in T SILVER 7556 modifies the powder’s bulk electrical behaviour and can reduce corona transfer efficiency if the powder is overcharged or if reclaim fractions are not controlled. Silver pigments, commonly platelet-shaped metallic flakes, create local field enhancement at the corona gun tip. Electrostatic spray lines running metallic PA11 typically set corona voltage between 50 and 70 kV and maintain a maximum powder output of 120–150 g/min to prevent free-ion accumulation on the metallic flakes. Reclaimed powder with high flake concentration can produce visual striping, gloss drift, and inconsistent film build; sieve classification at 125–150 µm and controlled virgin-to-reclaim ratios are used to limit non-uniform metallic pigment recovery. The difference from black or natural PA11 is therefore not only colorimetric but also rheological and electrostatic, because the metallic pigment modifies the perceived melt flow at the coating surface and can generate local hot spots during corona charging.

The following representative values apply to the PA11 base resin used in Rilsan fine powder grades; the T SILVER 7556 certificate of analysis and application-specific film tests supersede these values.

Film-level mechanical values are thickness-dependent and differ from resin values because the fused powder boundary layers and pigment orientation influence fracture propagation. Testing should be performed on coated coupons with the actual substrate and dry film thickness, using ISO 4624 for adhesion and ASTM D3363 or ISO 15184 for surface hardness when relevant. Published data for this specific T SILVER 7556 configuration is limited; coating qualification therefore requires a line-specific trial.

A direct comparison with alternative polyamide powder grades clarifies the positioning of T SILVER 7556. PA11 fine powder is specified for low-temperature impact service where PA6 may become brittle. The glass transition temperature of PA11 is near 40–50 °C, while PA6 has a glass transition near 50–60 °C and significantly higher water absorption at saturation. PA12 has a lower melting peak near 176–180 °C and lower density, but it is typically petrochemical-derived and may show lower hardness at elevated temperature. For decorative silver coatings, the PA11 base of T SILVER 7556 offers a balance of renewable carbon content, lower water uptake than PA6, and higher thermal stability than many polyolefin-based powder coatings.

When fluidised-bed coating replaces electrostatic spray for high film build on steel sections

For steel sections thicker than 3 mm, fluidised-bed dipping is selected when the required dry film thickness exceeds 250 µm or when edge protection is critical. The part is cleaned, blast-profiled to an average roughness Ra of 2.5–5.0 µm, and preheated to 300–350 °C. It is then immersed in a fluidised bed of T SILVER 7556 for 1–5 s, depending on thermal mass, removed, and post-heated at 200–240 °C for 2–5 min to complete melt flow. Electrostatic spray lines, in contrast, often operate with preheat temperatures of 180–230 °C and target a single-pass film build of 80–150 µm. The difference is not merely equipment preference; the fluidised-bed route yields higher residual thermal energy, which lowers melt viscosity and produces a denser film but increases the risk of sagging on vertical surfaces if the part is not rotated during the early post-heat phase.

In the T SILVER 7556 grade, the metallic pigment introduces an additional process variable during high-temperature fluidised-bed deposition. Excessive preheat above 350 °C or extended post-heat can oxidise the silver pigment surface, producing a dull grey or non-uniform metallic appearance even when adhesion remains acceptable. Oven dwell should therefore be controlled by part heat capacity rather than by a fixed time, and thermocouple verification on the thickest section is required to prevent local overheating. This constraint is less severe in non-metallic PA11 grades because colour drift is less visible, but the metallic silver finish exposes any melt-flow inconsistency at the surface.

Fluidisation behaviour is evaluated using ASTM D1895 bulk density and tap density. PA11 fine powder bulk density is typically 0.45–0.55 g/cm³. In fluidised-bed hoppers, the powder bed should be aerated with dry compressed air at a dew point below -40 °C to avoid moisture pickup. The metallic silver grade may require lower air pressure than black powder because the metallic flakes increase bed density and reduce expansion; operators typically start at 0.8–1.2 bar and adjust to a bed height increase of 10–20%. Published data for this specific grade is limited, so the fluidised-bed settings should be established on the coating line with the actual hopper geometry and powder batch.

Dishwasher basket coating lines represent one production-scale use of PA11 fine powder. On stainless steel wire frameworks, Rilsan T SILVER 7556 is applied at 200–300 µm to combine detergent resistance, impact toughness, and noise abatement. The coating must survive repeated thermal cycles from 20 °C to 75 °C wash liquor containing alkaline detergents and rinse aid. Long-term exposure data from production lines indicate that film thickness below 150 µm on wire intersections is the dominant failure point, because local abrasion between adjacent wires initiates cracking at the fused particle boundaries. The base resin’s compliance with FDA 21 CFR 177.1500 and EU Regulation 10/2011 supports use in repeated food-contact service, but the final coated article must be evaluated for overall migration under the intended food simulant and time-temperature conditions. No claim of global food-contact approval should be taken from the resin designation alone.

Salt-spray performance of PA11 coatings on grit-blasted steel with zinc phosphate pre-treatment is typically assessed using ISO 9227 neutral salt spray. With a uniform dry film thickness of 250–350 µm and complete edge coverage, coated steel panels can exceed 1,000 h without red rust. Thinner sections, sharp edges, and pinholes from trapped moisture are the main causes of early failure; the polymer film itself has low water transmission but cannot compensate for a discontinuous coating. For aluminium substrates, chromate conversion or thin-film silane pre-treatments are used instead of zinc phosphate to prevent galvanic acceleration at exposed edges.

Surface preparation, adhesion loss mechanisms, and edge coverage at 200 µm nominal thickness

Adhesion of T SILVER 7556 to steel is dominated by mechanical interlocking rather than polar bond formation. A minimum angular blast profile of 2.5–5.0 µm Ra is specified for load-bearing corrosion service, with sharp-edged steel media typically producing higher pull-off values than round shot. Adhesion can be measured using ISO 4624 pull-off on flat coupons or ASTM D3359 cross-cut on thin sections. Pull-off failure mode is more important than the absolute value in production troubleshooting; cohesive failure within the coating indicates adequate substrate preparation and possible thermal degradation, while adhesive failure at the substrate interface indicates insufficient profile depth or preheat inconsistency. On sharp machined edges, the melt flow can pull back during post-heat, leaving a thin coating zone below 100 µm; this edge recession is a known limitation of thermoplastic powder coating and must be addressed by geometry changes or increased local preheat.

Powder storage and handling affect the silver appearance and process stability. Moisture uptake by the powder during storage at relative humidity above 60% can create micro-voids during melt coalescence, visible as pinholes in the final film. Pre-drying at 80 °C for 4 h is a standard corrective step for hygroscopic polyamide powders, but the drying system must avoid mechanical shear that could disorient the metallic pigment. Storage below 25 °C and 60% RH is recommended to maintain the powder’s electrostatic charging characteristics and metallic pigment distribution.

For automotive fluid-handling clips and furniture hardware, the metallic silver surface is exposed to UV and abrasion. The PA11 base resin is not inherently UV-stable; exterior silver coatings may exhibit gloss reduction and chalking under accelerated weathering, and qualification should include ISO 16474-3 or equivalent cyclic weathering with the specified final coating thickness. The metallic pigment contributes to early gloss retention but can exhibit surface chalking at the film boundary. Corrosion protection can remain intact after cosmetic chalking because the underlying PA11 film is not subject to the same degradation mechanism, provided the full coating thickness is maintained.