The product designation Evonik VESTAMID L2141 BK 9.750 Nylon 12, Dry refers to an unreinforced polyamide 12 homopolymer supplied in black pellet form with a controlled dry-as-packaged moisture level. The grade belongs to the VESTAMID L series, in which PA12 is polymerised from laurolactam and produced without fibre reinforcement. The dry label indicates that the producer has reduced surface and internal moisture below 0.10 % by Karl Fischer titration, but this condition is stable only in sealed, undamaged packaging. The material is used in injection moulding, profile extrusion, and tubing extrusion where lower moisture absorption, chemical resistance, and low-temperature impact behaviour are required. Its density is near 1.01–1.02 g/cm³ according to ISO 1183-1, which places it below PA6 and PA66 grades at equivalent filler content. The product should not be confused with glass-filled or impact-modified VESTAMID L grades; the L2141 designation identifies a base resin without reinforcement, although the BK colour code introduces carbon-black pigment that changes optical, electrical surface resistivity, and processing behaviour relative to natural resin.

For incoming material control, the lot certificate should be matched to the purchase order for the BK 9.750 colour reference because visual colour alone is not sufficient to distinguish carbon-black masterbatch types. The receiving inspection should also verify melt volume-flow rate when the part geometry is sensitive to lot-to-lot viscosity drift. The most frequently referenced test standards for unfilled PA12 are ISO 1183-1 for density, ISO 11357-3 for melting peak, ISO 527-1/-2 for tensile properties, ISO 179-1/1eA for Charpy impact, ISO 62 for water absorption, and ISO 1133-1 for melt volume-flow rate. When the grade is stored in an unheated warehouse, the pallet should be allowed to reach room temperature before opening to prevent condensation onto cold pellet surfaces. If the packaging is cut and the material is not consumed in a single shift, the opened liner must be re-sealed under dry air or nitrogen; a conventional polyethylene bag tied by hand is not an adequate moisture barrier at relative humidity above 60 %.

Why does dry-state delivery matter for hydrolytic stability and moulded surface finish?

Water is the primary process-related risk in PA12 conversion. At melt temperatures above 220 °C, residual moisture hydrolyses the amide linkages, reducing molecular weight and increasing melt flow in an uncontrolled manner. The direct consequence is a loss of melt strength and notched impact resistance, while surface defects such as splay, silver streaks, and weld-line porosity appear on the moulded part. A moisture content of 0.10 % is a practical upper boundary for injection moulding of thin-walled parts; for extrusion of tubing with tight diameter tolerances, the preferred inlet moisture is below 0.06 %. The dry-as-delivered condition therefore shortens start-up time only if the packaging remains intact. Once opened, moisture regain follows the ambient dew point; at 23 °C and 50 % relative humidity, surface adsorption reaches a process-relevant level within hours, while at 80 % relative humidity the penalty appears in the first shift.

Pre-drying for regrind or material from opened containers is carried out in a desiccant dryer with a dew point of -30 °C or lower and an air inlet temperature of 80 °C. The residence time depends on starting moisture; pellets wetted by condensation require 4–12 h, while dry-state material exposed for one shift typically requires 2–4 h. Vacuum drying at 80 °C for 4 h is an alternative for small lots, but the vacuum level must remain below 100 mbar. Hot-air ovens are not equivalent because they cannot extract moisture from the hopper atmosphere at the necessary dew point. After drying, the pellets should be transferred through closed conveying lines or a desiccated hopper. The hopper capacity should not exceed 0.5–1.0 h of consumption; larger hoppers expose material to plant air and partially undo drying.

Under production conditions, the most commonly reported failure mode is not thermal degradation but moisture-induced silver streaking in the core of thick bosses. The defect appears after the first hour of processing from an opened bag, and operators may misdiagnose it as contamination. The corrective action is to return to a closed dryer with dew point below -30 °C and to reduce hopper residence time. Another field observation concerns black colour streaks when regrind is returned to the main feed at high ratios. For injection moulding, regrind addition should remain below 20–30 % unless the process is validated for higher reuse. The regrind fraction has a lower bulk density and can segregate in the feed throat; a crammer feeder or consistent granulate size improves melt stability.

The black-pigmented VESTAMID L2141 differs from PA6 and PA66 in moisture uptake, density, and impact retention. PA12 saturates at roughly 1.5–2.0 % water by ISO 62 immersion, while PA6 and PA66 reach 8–10 %. Consequently, the PA12 part retains a larger fraction of its dry tensile modulus in humid conditions and shows smaller dimensional change from water absorption. The material also has a lower density than PA66; the unfilled black grade is near 1.01–1.02 g/cm³, whereas unfilled PA66 is near 1.13–1.15 g/cm³. In comparison with glass-filled PA12 or PA66 grades, VESTAMID L2141 BK 9.750 has lower stiffness and lower heat-deflection temperature, so it is not a direct substitute when a glass-filled material has been validated for structural brackets under continuous load. However, the unfilled PA12 can provide higher elongation at break and better resistance to zinc chloride, automotive fuels, and road salt than many PA66 compounds.

Processing window, screw recovery, and hot-runner thermal limits

The melt temperature for injection moulding is normally maintained between 210 °C and 250 °C. Barrel set points are not intended to be read as melt temperature; a needle pyrometer or injection purge should be used to verify actual melt. A reverse or flat profile is used: the rear zone is kept high enough for stable screw recovery, while the front and nozzle zones are lowered to limit thermal stress. For a general-purpose screw with L/D 20–25, the screw speed is set between 80–150 min⁻¹ for medium shot sizes, and back pressure is held at 20–80 bar hydraulic. Higher back pressure improves homogenisation of the black pigment but increases shear heating and melt residence time. The melt temperature must not exceed 260 °C for more than 10 min in the barrel, and the hot-runner manifold should be balanced to avoid dead zones where black PA12 can stagnate and degrade to a brown or brittle residue.

The mould temperature controls surface appearance, shrinkage, and post-mould dimensional change. For unfilled PA12, a tool temperature of 40–80 °C is common; higher temperatures reduce moulded-in stress and improve weld-line strength but increase cycle time. Lower temperatures around 40 °C may be used for fast cycle cosmetic parts, but the part should not be ejected too early because PA12 has a sharp crystallisation transition. The holding pressure profile is normally set to fill 95–98 % of the cavity by volume during injection and then pack at 600–900 bar hydraulic depending on flow length. The gate freeze time should be established by part weight stabilisation; a minimum of 0.2–0.5 s per millimetre of wall thickness is a practical starting point for unfilled PA12, but actual values depend on tool steel and cooling layout.

Extrusion of black PA12 tubing or profile requires a different thermal profile. The melt temperature should remain in a similar range but with a lower front zone to avoid excessive die swell. A single-flight barrier screw with a Maddock mixing section is generally suitable for black masterbatch distribution; the compression ratio is typically 2.5:1–3.5:1. The melt filter is specified at 100–200 µm for tubing applications. Downstream calibration must be selected for PA12 shrinkage; vacuum tank water temperature between 20 °C and 40 °C is used for dimension setting, but thicker walls may require water at 40–60 °C to avoid internal voids. These parameters are equipment-dependent and should be derived from a starting condition, not used as production limits without validation.

When the black-pigmented grade replaces natural PA12 in existing tools

Switching from natural VESTAMID L2141 to the BK 9.750 colour variant on an existing production line is not a drop-in substitution. The carbon-black pigment changes the cooling behaviour of the melt because black pigmentation can increase infrared absorption and slightly alter nucleation. In injection moulding, the black grade may require a lower front-zone temperature or a faster injection speed to prevent jetting and flow-line formation. The moulded part may also display higher gloss than natural PA12, and surface defects are more visible because of the black colour. Dimensional differences can appear because pigmentation modifies the crystallisation onset; the holding-pressure time may need to be extended by 0.1–0.5 s on thick sections to keep part weight stable. In extrusion, the black grade may produce a different die swell and melt fracture threshold, so the die land length may need adjustment if the line previously produced natural tubing. Published data for this specific colour-compounded configuration is limited; therefore only production trials with the exact lot are definitive.

Mould shrinkage for unfilled PA12 varies with wall thickness and processing conditions; typical values range from 0.7 % to 1.2 % in flow direction and 0.8 % to 1.3 % transverse when measured after 48 h at 23 °C and 50 % relative humidity. The heat-deflection temperature under 0.45 MPa is approximately 130 °C for unfilled PA12; under 1.8 MPa it is below 60 °C. Therefore structural use above 80 °C under load requires explicit verification. The grade exhibits inherent resistance to aliphatic hydrocarbons, hot oils, and salt solutions, but it is not resistant to strong acids, oxidising agents, or polar solvents such as phenol and cresol. Exposure to zinc chloride solutions, including road spray in winter, is generally tolerated by PA12 better than by PA6, but stress-cracking resistance must be tested on the finished part under the specific stress state. The black grade should not be processed with excessive regrind of degraded material because the carbon-black pigment masks early yellowing, making thermal degradation harder to detect by colour check alone.

Application areas documented for unfilled PA12 of this class include automotive fuel pipes, vapour return lines, air brake tubing, cable sheathing, flexible conduits, and injected connectors or clips. The material is often chosen where PA6 or PA66 would be too stiff or too moisture-sensitive, and where a polyolefin would lack hydrocarbon resistance. The low-temperature impact retention of PA12 is a critical factor in pneumatic systems used in cold environments; the material can remain ductile below -30 °C in the dry state, but the exact value depends on notching radius and processing conditions. The black pigmentation provides UV weathering resistance for outdoor cable and tubing applications, but it does not replace a UV stabiliser package for continuous sunlight exposure beyond the supplier’s stated weathering data. Air-brake tubing converted from PA12 is commonly tested to ISO 7628-1 or SAE J844, but conformance is a line-specific certification and not implied by the raw material designation alone.

Regulatory documentation should be obtained for the exact colour compound, not for the natural base resin only. The unfilled PA12 base polymer may be listed in FDA 21 CFR 177.1500 or EU Regulation (EU) No 10/2011 for specific food-contact conditions, but carbon-black pigmentation and processing aids must be evaluated separately. The supplier’s REACH registration under Regulation (EC) No 1907/2006 should be confirmed for the BK 9.750 grade, and the product should be checked against RoHS Directive 2011/65/EU as amended for restricted substances in electrical and electronic equipment. For automotive fuel-contact parts, the moulder or extruder must verify the finished component against the relevant SAE or OEM specification, because raw material certifications do not cover weld-line integrity or extraction behaviour after processing.