Arkema Rilsan BESN BLACK TL is a black-pigmented polyamide 11 homopolymer extrusion compound formulated for rigid tubing manufacture. The PA11 chain, obtained from 11-aminoundecanoic acid, spaces amide linkages along a hydrocarbon segment of ten methylene units; this architecture reduces hydrogen-bonding density and equilibrium water uptake relative to PA6 and PA66. The grade is distinguished from plasticised PA11 cable coating compounds by the absence of external plasticiser addition, a deliberate formulation choice that preserves tube-wall modulus and resists creep in pressurised service. Density under ISO 1183-1 is reported in the range 1.03–1.05 g/cm³; dry melting temperature under ISO 11357-3 lies between 186 °C and 190 °C. The melt viscosity is controlled for annular die extrusion, not thin-wall injection filling, so published melt volume-flow rate values should be read only at 235 °C and 2.16 kg and compared with grade-specific lot certificates. Carbon black in the black TL formulation functions primarily as UV screening and surface marking contrast, but outdoor lifetime claims require component-level weathering data rather than raw-material pigment data alone. The semicrystalline morphology after extrusion typically shows crystallinity in the range 25–35 % by differential scanning calorimetry, with the exact fraction dependent on cooling rate, die geometry, and downstream annealing.

What separates PA11 rigid tubing from PA6 and PA12 in service?

In comparative terms, the property envelope of PA11 sits between PA6 and PA12 for moisture resistance and low-temperature impact. Under ISO 62, 24 h immersion at 23 °C gives water uptake of approximately 0.3 % for PA11, against 1.8–2.0 % for PA6; equilibrium moisture at 50 % RH is approximately 1.1 % for PA11 and 2.5–2.8 % for PA6. Lower water uptake limits the loss of yield stress after humid conditioning and reduces diameter growth in close-tolerance tubing. PA12 offers water uptake slightly below PA11 and generally higher notched impact at −40 °C, but PA11 retains a higher melting point and a higher modulus in the dry state in many commercial formulations. PA6 and PA66 provide higher dry tensile yield stress but require more drying before extrusion and show greater dimensional movement in service. The rigid tubing designation of BESN BLACK TL therefore targets applications needing a balance of hydrocarbon resistance, lower moisture shift than short-chain polyamides, and a stiffer tube wall than plasticised PA11 or PA12 flexible grades.

The values in the table are representative published ranges for unplasticised long-chain polyamide and PA6 materials; lot-specific batch data must be used for final design. The lower moisture uptake of PA11 relative to PA6 directly affects the retention of tensile yield stress after conditioning: PA11 may lose roughly 30–35 % of dry yield stress at 50 % RH, whereas PA6 loses a larger fraction due to greater equilibrium water absorption. The lower density of PA11 relative to PA6 also reduces weight per unit length of tubing at equal wall thickness. Against PA12, the dry melting point advantage of PA11 becomes relevant when short-term excursions exceed 110 °C, although continuous oxidative service above 90 °C requires specific heat-stabiliser verification.

Pre-extrusion Moisture Control and Barrel Profile Limits

Moisture management is the primary processing boundary for BESN BLACK TL. Pellets are pre-dried in desiccant dryers at 80–90 °C for 4–6 h with a supply air dew point below −40 °C; the residual moisture before extrusion is held below 0.10 % by weight. Higher residual moisture depresses viscosity through hydrolysis, creates splay and surface roughness on the tube, and can shift diameter control during vacuum sizing. On production-scale single-screw extruders with grooved feed sections and screw lengths of 24:1–30:1 L/D, the compression ratio is maintained between 2.5:1 and 3.0:1. Screen packs of 60/80/100 mesh are typical for removing unmelt and carbon black agglomerates, with replacement frequency determined by melt-pressure rise. Barrel temperatures from feed to metering run 220–260 °C; die temperatures are held at 240–250 °C. Melt temperature must remain below 270 °C, because excursions above this threshold lead to chain scission, black speck formation, and loss of impact strength. The cooling and sizing section uses water at 20–40 °C with vacuum calibration to control outside diameter and ovality. Batch-to-batch variation in carbon black masterbatch dispersion can shift apparent melt viscosity by 3–5 % at constant screw speed; therefore gravimetric feeding and inline melt-pressure monitoring are required on transfer lines. Surface melt fracture and sharkskin are controlled by die land length-to-gap ratios in the range 10:1–15:1 and by limiting shear rate at the die lip; residual moisture above 0.15 % produces bubbles in the tube wall and lower burst strength.

In compressed-air brake and hydraulic control tubing, the grade is evaluated on finished assemblies rather than raw material. Air brake tube specifications such as SAE J844 impose burst-pressure retention after dry and humid ageing, low-temperature flexure at −40 °C, and resistance to compressor oil. Hydraulic control tubes are tested for volume change and tensile change after immersion in mineral oil or ester-based fluids under ISO 1817. Hydrocarbon resistance in PA11 is superior to PA6 for aliphatic fuels and mineral oils, but aromatic fuels and aggressive phosphate ester fluids can extract low-molecular-weight amide oligomers over sustained exposure. Published data for this specific configuration is limited, and component certification must be performed on the final tube dimensions and end-fitting geometry; raw-material fluid-immersion data alone do not establish burst lifetime of an assembled circuit.

When dimensional stability after moisture uptake dictates tube tolerances rather than dry as-moulded values

Conditioning from the dry state to 50 % RH equilibrium can increase tube diameter by up to 0.15 % in PA11; this shift is lower than PA6 but slightly higher than PA12. The dry tensile yield stress of 42–48 MPa decreases to 28–35 MPa after conditioning, while elongation at break rises from 200 % to above 300 %. Dry flexural modulus of approximately 1000 MPa falls to 700–900 MPa after moisture uptake. Dynamic mechanical analysis places the dry glass transition of PA11 near 45 °C; moisture plasticization moves the loss modulus peak below 0 °C, explaining the improvement in notched impact after humid conditioning. Post-extrusion annealing at 100 °C for 4 h under nitrogen reduces crystallinity gradients and free shrinkage caused by later exposure to 80 °C air. Tube metrology should therefore be performed after a fixed conditioning or annealing step; as-moulded dimensional inspection alone under-represents the service state and can cause tolerance rejection after shipment to humid environments.

Thermal oxidative stability is bounded by carbon black antioxidant synergy and wall thickness

Thermal oxidative stability in PA11 is a function of antioxidant package depletion kinetics, carbon black loading, and oxygen diffusion through the tube wall. Differential scanning calorimetry oxidative induction time testing under ISO 11357-6 is used as a batch-release indicator, but the value is not a direct substitute for long-term heat ageing at 80–100 °C. The black TL grade contains carbon black for UV screening, yet carbon black can also adsorb portions of the stabiliser package, shifting oxidative induction time if dispersion is uneven. Production-scale failures have been observed when carbon black masterbatch was poorly distributed in the feed throat, creating black streaks and localised brittleness in the tube wall; the resulting failure mode is low Charpy notched impact and cracking at fitting barbs. Wall thickness controls the rate of oxygen ingress and antioxidant depletion at service temperature; thicker walls provide a barrier effect only if the inner bore is not exposed to hot oxidising gases such as high-temperature compressed air.

Material control parameters for the grade are summarised below. These values are used for incoming resin acceptance and not as final component performance limits.

Operational boundaries for Rilsan BESN BLACK TL derive from oxidative and hydrolytic degradation kinetics. Continuous air service above 90 °C requires heat-stabilised validation, and the black carbon black loading does not prevent oxidative chain scission at elevated temperature. Concentrated mineral acids, formic acid, phenols, and strong oxidising agents are incompatible; exposure can cause surface etching and rapid loss of tensile strength. Stress cracking has been observed when PA11 tube is clamped against metal surfaces contaminated with zinc chloride-based flux residues in humid conditions, because zinc chloride attacks amide linkages; such contact should be prevented. The maximum allowable working pressure is not a raw-material constant; it is derived from long-term hydrostatic strength data according to ISO 9080, factored by service temperature, fitting loss, and safety margin. Published data for this specific configuration is limited, so designers must generate pressure-regression curves on the production tube dimensions and end fittings rather than extrapolating from short-term burst tests.