Molten caprolactam at 69–71 °C is transferred into the continuous hydrolytic polymerisation line under a dry nitrogen pad to prevent moisture uptake and oxidative yellowing of the ring monomer. The feed is combined with 2.0–4.0 wt% demineralised water and 0.05–0.20 wt% acetic acid chain regulator, then passed through a vertical VK tube reactor with the upper heating zone maintained at 255–270 °C and the finishing zone at 240–250 °C. Residence time in the VK tube is 12–20 h, during which the water-driven ring opening of caprolactam yields ω-aminocaproic acid and subsequent polycondensation proceeds until equilibrium leaves 8–10 wt% caprolactam monomer and cyclic oligomers in the melt. The extruded strand is quenched in water at 15–25 °C, pelletised, and extracted in countercurrent hot water at 95–100 °C for 10–16 h to reduce extractables below 0.5 wt%. Extracted chips are dried under vacuum or with -35 °C dew-point air to a final moisture level below 0.06 wt%. Textile-grade chip is melt-spun through a filter pack with 20–40 µm filtration media and a 24–48 hole spinneret at 260–290 °C, with quench air at 18–22 °C and 60–75% relative humidity. Partially oriented yarn is wound at 4000–5000 m/min; draw texturing then uses a draw ratio of 1.25–1.45 and heat-setting at 160–190 °C to generate bulk and controlled interfilament cohesion. Fully drawn yarn is drawn at 2.8–3.5 and exhibits tenacity of 4.0–5.5 cN/dtex and elongation at break of 25–45% when tested to ISO 2062. Batch acceptance for apparel and sports textiles is anchored to ISO 307 for viscosity number, ISO 62 for water absorption, and OEKO-TEX Standard 100 Annex 4 for extractable residues; REACH SVHC content is verified below 0.1 wt%. Melt breaks and drip defects on the spin line become statistically relevant when chip moisture exceeds 0.08 wt%, because hydrolytic degradation in the extruder reduces die-pressure stability and lowers relative viscosity by more than 0.05 within 30 min of residence time.
What Processing Window Separates High-Tenacity Yarn Grade from Textile-Grade PA6?
In high-tenacity industrial yarn production, the hydrolytic route is operated at a higher viscosity target and tighter extractables control. The caprolactam feed water ratio is reduced to 1.5–2.5 wt%, chain regulator input is trimmed to 0.02–0.06 wt%, and finishing is extended until relative viscosity reaches 3.2–3.8 in 96% sulfuric acid, with total extractables held below 0.3 wt%. Spinning is performed through spinnerets with 48–120 holes at 270–290 °C, followed by multi-stage drawing on heated godets at 180–210 °C with a total draw ratio of 4.5–5.5. The resulting yarn shows tenacity of 8.0–9.5 cN/dtex, elongation at break of 15–20%, and hot-air shrinkage at 190 °C of 6–10% measured to ASTM D4974. Tire cord constructions are then twisted, woven, and dipped in a resorcinol-formaldehyde-latex bath with dry dip pickup controlled to 3–6 wt%; cord-to-rubber adhesion is tested to ASTM D2229. Industrial yarn acceptance is based on ASTM D885 tensile and fatigue data, and production-scale draw-twisters with 144-position creels require bobbin-to-bobbin tension variation below ±5% to avoid tyre cord modulus scatter. This segment does not tolerate residual caprolactam above 0.25 wt% because monomer migration during hot drawing creates deposits on godets and increases filament breaks.
Cast nylon stock shapes are produced by anionic polymerisation of caprolactam in the absence of water. The monomer is melted at 80–90 °C under vacuum of 1–5 mbar and dried to below 0.03 wt% moisture; sodium caprolactamate at 0.2–0.6 mol% and N-acetylcaprolactam at 0.3–0.8 mol% are added as catalyst and activator. The reactive mixture is poured into a mould preheated to 140–160 °C and polymerises exothermically to a solid within 5–30 min. Because chain growth proceeds rapidly from the wall inward, thick sections require controlled mould temperature and post-cure annealing at 150–170 °C for 2–6 h to reduce internal stress and frozen orientation. The resulting cast PA6 G material exhibits 50–60% crystallinity measured by ISO 11357-3 differential scanning calorimetry, tensile stress at yield of 70–85 MPa to ISO 527-2, and flexural modulus of 2800–3200 MPa to ISO 178. Cut parts from cast PA6 are used in wear pads, sheaves, and gears where the high molecular weight improves creep resistance and fatigue life; machined tolerance is normally kept at IT9–IT11 because saturation water absorption of 6–8% to ISO 62 swells the part dimensionally. Anionic polymerisation is incompatible with moisture above 0.03 wt%, and the mould must not contain active-hydrogen impurities such as amines, alcohols, or residual water, which quench the lactam anion and stop chain propagation prematurely. Thick-wall castings above 80 mm section thickness require staged cooling or annealing interruption to prevent centre porosity, as the exotherm can exceed 200 °C locally despite the 140–160 °C mould temperature.
When 30 wt% Short-Glass Reinforcement Is Compounded into Injection-Grade PA6
When short glass fibre is compounded into caprolactam-derived PA6 at 30 wt%, the extrusion operation is typically performed on a co-rotating twin-screw extruder with an L/D ratio of 40:1 and side feeding of the glass roving downstream of the melting zone. The base resin is pre-dried in a desiccant dryer with a -35 °C dew-point air supply to below 0.10 wt% moisture, because melt moisture above 0.15 wt% produces visible surface splay and reduces notched impact strength by more than 20% in moulded bars tested to ISO 179-1/1eA. Barrel temperatures are profiled from 240 °C at the feed throat to 275 °C at the die, and screw speed is limited to 250–350 min⁻¹ to minimise fibre attrition below 0.25–0.40 mm number-average fibre length in the moulded part. Injection moulding uses a clamp force of 5–8 kN/cm² of projected area, a melt temperature of 260–280 °C, and a tool temperature of 80–90 °C to promote crystallinity and achieve an HDT/A value above 200 °C at 1.8 MPa to ISO 75-2. The anisotropic shrinkage between flow and transverse directions is 0.2–0.5% and 0.6–1.0% respectively to ISO 294-4; unwetted glass bundles or poor fibre coupling raise the transverse-to-flow shrinkage ratio above 3.0 and cause warpage in flat connector housings. Electrical and automotive parts must meet IEC 60093 surface resistivity, UL 94 flammability, RoHS 2011/65/EU, and REACH SVHC restrictions; polyamide mouldings in power-train applications are additionally screened to ISO 11403-3 for environmental stress cracking resistance against hot oil and calcium chloride road salt.
| Property | 0 wt% GL | 15 wt% GL | 30 wt% GL | 50 wt% GL | Test standard |
|---|
| Density (g/cm³) | 1.14 | 1.24 | 1.36 | 1.55 | ISO 1183-1 |
| Tensile stress at break (MPa) | 70 | 100 | 175 | 220 | ISO 527-2 |
| Tensile modulus (MPa) | 3000 | 5500 | 9000 | 16000 | ISO 527-2 |
| Charpy notched impact (kJ/m²) | 5 | 6 | 10 | 14 | ISO 179-1/1eA |
| HDT/A at 1.8 MPa (°C) | 65 | 195 | 205 | 215 | ISO 75-2 |
Representative values are derived from dry-as-moulded specimens conditioned to ISO 291 at 23 °C and 50% relative humidity; moisture conditioning lowers stiffness and raises impact toughness in proportion to absorbed water content.
Biaxially Oriented PA6 Film as an Oxygen-Barrier Layer in Laminates
Biaxially oriented PA6 film begins with caprolactam-derived PA6 resin having relative viscosity of 3.0–3.5 and extractables below 0.4 wt%. The resin is dried to below 0.10 wt% moisture and extruded at 250–270 °C through a flat die onto a chill roll at 40–60 °C. Sequential or simultaneous biaxial stretching is carried out at 110–150 °C with machine-direction draw of 2.7–3.2 and transverse-direction draw of 2.8–3.4, followed by heat-setting at 190–210 °C. A 15 µm film exhibits oxygen transmission of 25–35 cm³/(m²·d·bar) at 23 °C and 0% relative humidity to ASTM F1249; at 85% relative humidity the oxygen transmission rate increases to 200–300 cm³/(m²·d·bar) because nylon barrier performance is strongly dependent on moisture. Tensile strength in the machine and transverse directions is 220–250 MPa and elongation at break is 80–100% to ISO 527-3. Laminates are produced with polyethylene or polypropylene sealant webs for vacuum pouches, deep-frozen food packaging, and retortable trays. Food-contact compliance is documented to FDA 21 CFR 177.1500, and EU 10/2011 overall migration is below 10 mg/dm² with caprolactam-specific migration limit of 15 mg/kg. Processing limitations include dimensional instability above 70 °C in high-moisture laminate structures and the need to condition rolls for 24–48 h at controlled humidity before slitting to prevent bag curl.
Extractable Content and Air-Jet Pressure Govern Carpet BCF Performance
In carpet yarn extrusion, caprolactam-derived PA6 chips with relative viscosity of 2.7–3.0 and extractables below 0.4 wt% are fed to single-screw extruders at 250–270 °C. The melt passes through 70–150 hole spinnerets with trilobal cross-section capillaries; air-jet bulking at 1.8–2.2 bar entangles the filaments to generate bulk and covering power. Draw ratio in the draw-texturing stage is 3.0–3.6, and hot-air crimp set temperature is 180–200 °C; final yarn count ranges from 1000–3000 dtex. Carpet bulked continuous filament must maintain tenacity of 2.5–3.5 cN/dtex and elongation at break of 35–55% to ISO 2062. After acid dyeing in continuous or batch equipment, quality checks include colour fastness to light to ISO 105-B02 and rub fastness to ISO 105-X12. Caprolactam extractables above 0.6 wt% cause dye uptake variability and visible streaking in continuous dye ranges; spin finish application must be limited to 0.3–0.7% by weight and applied uniformly to avoid burnt deposits on heat-setting rolls. Tile and broadloom producers require yarn denier variability below ±3% across the creel to maintain finished pile weight tolerances.
A different industrial route converts caprolactam to high-viscosity PA6 monofilament through single-hole die extrusion at 260–285 °C. The monofilament is quenched in a water bath at 20–40 °C, drawn in heated liquid or vapour zones at 90–180 °C with a total draw ratio of 4.0–5.5, and relaxed by 5–10% to control diameter and residual stress. Finished diameters range from 0.08–0.80 mm, with tensile tenacity of 5.5–7.0 cN/dtex and knot strength at 80–90% of straight tenacity measured to ISO 2062. Paper machine clothing and filter belt applications rely on this monofilament for dimensional stability and abrasion resistance. Wet conditioning reduces tensile strength by 10–20% relative to dry as-moulded values because polyamide 6 absorbs water and plasticises the amorphous phase; this must be included in working-load calculations. Acidic process conditions remain a known boundary for PA6 monofilament, and long-term exposure to mineral acids at elevated temperature leads to progressive hydrolysis, so published data for continuous service in hot acidic filtrate is limited.
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Caprolactam (IUPAC: azepan-2-one; CAS 105-60-2) is a seven-membered cyclic amide with the molecular formula C6H11NO and molar mass 113.16 g/mol. The compound is the exclusive industrial monomer for polyamide 6, obtained on a commercial scale by Beckmann rearrangement of cyclohexanone oxime in the presence of oleum or by vapour-phase catalytic rearrangement. Its crystallisation point for polymerisation-grade product is typically 68.8–69.2 °C, and the closed-cup flash point is approximately 125 °C. Molten caprolactam is handled at 75–90 °C to avoid solidification in jacketed 304L stainless steel lines. The product is supplied as hot liquid, crystallised flake, or pelletised chip; grade designations usually encode water content, permanganate absorption number, and UV absorbance, and these grades are selected according to whether the downstream application is textile filament, engineering resin, or film. The substance is registered under REACH and listed with EC number 203-313-2.
At a receiving terminal, polymerisation-grade material is specified to limit the impurities that affect photostability, colour, and chain regularity. The permeation of trace water into bulk storage is a dominant parameter because water acts as an initiator in the subsequent ring-opening equilibrium and shifts molecular weight distribution. Consequently, silos and tanks are blanketed with dry nitrogen having a pressure dew point below −40 °C. Heating coils maintain product above its melting range but below 100 °C; prolonged exposure above 100 °C promotes oligomerisation and yellowing. Contact with carbon steel is excluded because iron pickup catalyses colour bodies; transfer is performed with DIN 11850-compliant stainless steel piping and magnetically coupled positive-displacement pumps.
What Does Polymerisation-Grade Caprolactam Demand in Bulk Receiving and Storage?
Receiving specifications for the low-permanganate polymerisation grade are shown in Table 1. The values are derived from standardised methods rather than supplier-specific wet chemistry; they are used to release incoming tankers and isotanks before discharge. The tight control of volatile bases and permanganate absorption number is necessary because unsaturated carbonyl impurities and basic nitrogen compounds affect both downstream colour and molecular weight stability in continuous polymerisation.
Table 1 — Polymerisation-grade caprolactam specification profile
| Parameter | Typical limit | Test method |
|---|
| Crystallisation point | 68.8–69.2 °C | ISO 7060 |
| Water content | ≤0.05 wt% | ISO 760 |
| Permanganate absorption number | ≤5 mg/kg | ISO 8660:2002 |
| Colour of 50% aqueous solution | ≤5 Hazen units | ISO 8112 |
| Volatile bases | ≤0.5 mg/kg | ISO 8661 |
| Acidity or alkalinity | ≤0.05 mmol/kg | ISO 8111 |
Discharge from a road tanker is normally conducted at 80–85 °C using nitrogen pressure or a heated screw pump. The molten monomer is filtered through a 10 µm stainless steel element to remove particulate matter before entering the day tank. For flaked product, a drum flaker with internally chilled water at 15–25 °C produces flake thickness between 1 mm and 3 mm. Pneumatic conveying of flakes uses dry air with a pressure dew point below −40 °C to prevent condensation in dense-phase lines. Storage tanks are designed for recirculation through a self-cleaning strainer; dead-legs are minimised because localised solidification in stagnant molten caprolactam can create hard blockages that require line tracing and reheating.
Hydrolytic polymerisation of caprolactam in a VK tube reactor proceeds through ring-opening, step-growth, and equilibration with approximately 10 wt% residual monomer at thermodynamic equilibrium. The reactor is a vertical tubular vessel with a length-to-diameter ratio between 18:1 and 22:1, fitted with internal static mixing elements. Temperature zones are maintained between 240 °C and 270 °C; residence time is ordinarily 10 h to 20 h depending on target relative viscosity. Water, aminocaproic acid, or acetic acid is metered as a chain regulator. The number-average molecular weight of the polymerised melt is controlled by the ratio of chain regulator to monomer; for fibre-grade polyamide 6, a relative viscosity of 2.4–2.6 is typical, while engineering resin grades are polymerised to 2.8–3.4. Relative viscosity is measured on a 1 wt% solution in 96% sulfuric acid according to ISO 307.
After discharge, the polymer strand is quenched in circulating water, pelletised, and unreacted monomer is extracted with hot water at 95–100 °C to reduce residual caprolactam below 0.5 wt% for engineering resin grades. Residual caprolactam after extraction is measured by gas chromatography with flame-ionisation detection. Vacuum tumble drying under 100 mbar and 90–100 °C lowers moisture to below 0.02 wt% before melt processing; if the moisture content exceeds this threshold, hydrolytic degradation in the extruder can cause a loss of relative viscosity greater than 10% after a single heat history. Extraction water containing unreacted caprolactam is concentrated in a multi-effect evaporator and recycled to the reactor; the monomer recovery loop is operated at 80–90 °C and 0.2–0.4 bar absolute to suppress oligomer formation.
Solid-State Sublimation and Moisture Pick-Up in Crystalline Storage Silos
Crystalline flake and chip forms are not static solids; caprolactam exerts a measurable vapour pressure at ambient temperature and will sublimate onto cool surfaces in bulk storage. Silos are therefore operated with low-velocity ventilation and a dry nitrogen sweep. Moisture ingress above 0.05 wt% changes the melt crystallisation point and broadens the melting range, leading to bridging in hoppers and feed-rate variation at the extruder throat. This is particularly critical for lost-in-weight gravimetric dosing systems in compounding lines where feed consistency must remain within ±0.5% of setpoint. A single-dump hopper is not recommended; the flaked product is best discharged through mass-flow silos with polished cone angles above 70° and a discharge diameter greater than 600 mm to prevent rat-holing.
The crystalline form also exhibits limited storage stability in humid coastal environments. Bagged flake exposed to relative humidity above 60% for extended periods can crust on the outer layer and alter the angle of repose during hopper drawdown. For this reason, opened bags are typically consumed within one shift and not returned to the warehouse. Silos with external condensate traps are used for long-term flake storage; the traps are inspected weekly because sublimed caprolactam deposits can reduce vent capacity and create overpressure in the headspace.
When Nylon 6 Replaces Nylon 66 in Glass-Filled Injection Moulding
The primary competitive product landscape for caprolactam is the two-monomer nylon 66 system based on adipic acid and hexamethylene diamine. Table 2 compares the resulting polymer properties relevant to injection moulding. Nylon 6 has a lower melt temperature, which permits barrel settings 10–30 °C lower and reduces energy consumption, but it has comparable or slightly higher equilibrium water absorption under ISO 62. Glass-filled nylon 6 compounds require mould temperatures between 70 °C and 90 °C for adequate crystallinity; nylon 66 typically requires 80–100 °C. The narrower processing window for nylon 6 in semicrystalline thin-wall parts results from its lower crystallisation temperature and fast skin formation in cold runners; screw back-pressure is usually limited to 0.3–0.5 MPa to avoid excessive shear heating.
Table 2 — Comparative polymer data for caprolactam and competing monomer systems
| Monomer system | Resulting polyamide | Melting peak | Water absorption | Injection barrel profile | Density |
|---|
| Caprolactam | PA6 | 215–225 °C | 2.5–3.0 wt% | 240–270 °C | 1.12–1.14 g/cm³ |
| Adipic acid + hexamethylene diamine | PA66 | 255–265 °C | 2.2–2.8 wt% | 270–300 °C | 1.13–1.15 g/cm³ |
| Laurolactam | PA12 | 170–180 °C | 0.7–1.5 wt% | 190–240 °C | 1.01–1.03 g/cm³ |
For monomer replacement at the chemical level, caprolactam differs from laurolactam not only by ring size but also by moisture sensitivity and crystallisation rate. PA12 produced from laurolactam has lower water absorption and is preferred for dimensionally stable fuel-contact components under SAE J2260; caprolactam-based PA6 requires nucleating agents and glass-fibre reinforcement to approach the same dimensional stability. For substitution of nylon 66 with nylon 6 in structural brackets, the lower heat deflection temperature of unfilled PA6 under ISO 75-1/-2 at 1.8 MPa is often 20–30 °C lower, which must be compensated by higher glass loading or mineral filling. Because caprolactam is a single-ring monomer, the polymerisation plant does not require equimolar salt balancing as in nylon 66; this removes the pH control step associated with hexamethylene diamine/adipic acid salt preparation but introduces an extraction and monomer recovery loop as an additional cost centre. Published data for this specific configuration is limited; plant-scale validation under ISO 1133-1:2022 melt flow rate and ISO 527-2 tensile testing is therefore required before substitution.
