Semicrystalline polyamide 6 monofilament for pelagic trawl twine and gillnet mesh is converted from melt-extruded water-quenched strands that possess low axial molecular orientation and an elongation at break commonly above 300%. Polyamide 6 granulate is dried to a moisture content below 0.06% in a desiccant dryer with a dew point no higher than −40 °C before being fed to a single-screw extruder with an L/D ratio of 30:1 and a compression ratio between 2.8:1 and 3.4:1. The relative viscosity of the feedstock typically ranges from 3.2 to 3.8 when measured according to ISO 307. Barrel temperatures in the feed, compression, and metering zones are held at 240 °C to 270 °C, while the die head is maintained at 260 °C to 280 °C. The melt is filtered through a screen pack with nominal retention of 25 µm to 40 µm before passing through a spinneret capillary with a diameter of 0.5 mm to 2.0 mm. Immediately below the die, the filament enters a water quench bath at 20 °C to 35 °C; the rapid cooling produces a predominantly γ-phase crystalline structure with spherulitic texture suppressed by the high cooling rate. In this as-spun condition, the linear tenacity measured according to ISO 1805 is generally below 0.25 N/tex, and the residual free volume allows subsequent plastic deformation to high extension. Drawing imposes a true strain of ε = ln(DR), where DR is the ratio of take-up speed to feed speed. Industrial two-stage drawing lines for PA6 fishing net monofilament operate at total draw ratios from 3.4:1 to 5.0:1, with the first stage in hot water or steam at 88 °C to 120 °C and the second stage in dry heat at 135 °C to 170 °C. The first-stage draw ratio is commonly 2.8:1 to 3.6:1, while the second-stage ratio ranges from 1.10:1 to 1.45:1. Between the draw stages, the oriented amorphous chains relax partially, and the subsequent heat-setting step at 140 °C to 180 °C under 2% to 6% relaxation stabilizes the molecular network and reduces boiling-water shrinkage. Draw ratio affects not only the average tensile strength but also the distribution of stress-transfer tie molecules, the skin-core orientation gradient, and the propensity for surface fibrillation; these secondary effects are often more important than linear tenacity in determining wet knot performance and mesh service life.

Does Wet-Knot Strength Retention Deteriorate Above a Total Draw Ratio of 4.2:1 in PA6 Gillnet Twine?

In drawn PA6 monofilament, the relationship between tensile strength and draw ratio is positive and nearly linear over the range 3.0:1 to 4.6:1, but the relationship between knot strength and draw ratio is non-linear. When tested at 23 ± 2 °C and 50 ± 5% relative humidity after conditioning for 24 h according to ISO 1805, a 0.45 mm three-strand gillnet twine may show an increase in linear tenacity from approximately 0.50 N/tex at 3.5:1 to 0.75 N/tex at 4.5:1 and 0.82 N/tex at 5.0:1. The elongation at break declines from 34% to 24% and then 19% over the same interval, while the work-to-rupture decreases because the stress-strain curve loses its post-yield drawing plateau and approaches a brittle failure envelope. The wet knot breaking force, determined by tying a single overhand knot in the monofilament and applying force at 300 mm/min, rises from 0.39 N/tex at 3.5:1 to a maximum of approximately 0.66 N/tex at 4.2:1, then falls to 0.55 N/tex at 5.0:1. The corresponding knot efficiency, defined as wet knot breaking force divided by wet linear breaking force and expressed as a percentage, increases from 76% at 3.5:1 to 86% to 90% at 4.0:1 to 4.3:1, and then falls below 72% at draw ratios above 4.8:1. This loss of knot efficiency is associated with a reduction in the ability of the oriented amorphous phase to redistribute transverse compressive stress; the heavily drawn fibrils act as stiff rods that fail by axial splitting rather than by shear yielding. Wide-angle X-ray scattering data show that the crystalline orientation factor increases monotonically with draw ratio, but the amorphous orientation factor saturates between 4.0:1 and 4.5:1, while the density of overstrained tie molecules increases. The overstrained tie molecules are load-bearing in a straight line but cannot withstand the bending and compression imposed by an overhand knot. Consequently, a specification based solely on linear tenacity would permit draw ratios that produce unacceptable wet knot performance in gillnet and purse-seine twine.

At draw ratios between 3.0:1 and 4.0:1, differential scanning calorimetry at 10 K/min reveals a progressive reduction in the intensity of the low-temperature melting shoulder near 210 °C, which corresponds to the γ crystalline phase, and a sharpening of the dominant α-phase melting peak near 220 °C. The crystallinity computed from the heat of fusion, using a reference value of 230 J/g for fully crystalline polyamide 6, increases from 22% to 25% in as-spun monofilament to 42% to 48% after drawing to 4.2:1. Wide-angle X-ray scattering shows that the α(200) reflection intensifies at the expense of the γ(001) reflection as the total draw ratio increases, indicating a strain-induced γ-to-α transformation. Small-angle X-ray scattering long period values decrease from approximately 12 nm in as-spun material to 8 nm to 9 nm at 4.5:1, while the crystalline size along the fiber axis increases from 6 nm to 9 nm. This structural refinement produces a higher density and a lower equilibrium moisture regain; from the viewpoint of netting use, the reduction in moisture regain at high draw ratio can be beneficial because it reduces the plasticizing effect of seawater on the amorphous phase. However, the same structural constraint reduces the energy-absorbing capacity of the amorphous phase under cyclic bending and wave-induced oscillation. The oriented amorphous fraction increases up to a total draw ratio near 4.0:1 and then becomes overstrained; this fraction is the primary contributor to wet fatigue resistance and knot-induced energy absorption. Therefore, the structural optimum for combined tensile stiffness and knot durability is not the point of maximum crystallinity but the point where the oriented amorphous fraction is maximized without fibrillation.

Thermo-Mechanical Process Boundaries in Multi-Stage Drawing Plants

Commercial draw lines for PA6 fishing net monofilament typically arrange the first-stage draw in a wet bath at 88 °C to 98 °C and the second-stage draw in a hot-air or steam cabinet at 135 °C to 160 °C. The wet first stage lowers the drawing stress because water acts as a plasticizer; at 95 °C, the drawing stress of a 1.00 mm as-spun strand rises from approximately 18 MPa at draw ratio 2.5:1 to 55 MPa at 4.0:1 and 75 MPa at 4.5:1. The stress transmission between godet sets requires high-friction roller surfaces and precise temperature control; if the first godet set slips, the actual draw ratio can oscillate by ±0.15, producing a diameter variation of ±0.03 mm and a wet knot strength coefficient of variation above 6%. Staggered drawing with a first-stage ratio of 3.2:1 and a second-stage ratio of 1.25:1 produces a more uniform radial orientation profile than a single-stage draw at 4.0:1, because the intermediate relaxation allows amorphous tie chains to redistribute before the final crystalline locking step. The strain-rate sensitivity of PA6 at 100 °C is approximately 0.09 per decade of strain rate; a line-speed increase from 100 m/min to 300 m/min therefore requires either a higher first-stage bath temperature of 5 °C to 8 °C or a reduction in first-stage draw ratio of approximately 0.2 to avoid filament breaks. These constraints become more severe for thin monofilaments below 0.30 mm diameter, where the surface-to-volume ratio is high and the filament loses heat rapidly upon exiting the heating zone. The second-stage draw temperature must also remain below 170 °C for unstabilized PA6 unless a heat-stabilizer package with a copper-halide system is included, because prolonged exposure above this temperature initiates thermo-oxidative chain scission and surface yellowing. A two-stage draw line with 7-roll godet sets, a 2.5 m hot-air channel, and closed-loop diameter gauging provides sufficient control to hold the total draw ratio within ±0.05 of target, which is necessary for knot-strength consistency.

At a total draw ratio of 4.4:1, the wet abrasion resistance of PA6 monofilament in a rotating drum test using silica sand and synthetic seawater at 20 °C is generally superior to that of the same polymer drawn at 3.5:1; typical mass loss is lower by 18% to 25% because the drawn surface contains fewer microvoids and the densified amorphous phase resists moisture-induced plasticization. When the draw ratio is raised to 5.0:1, surface fibrillation becomes visible under optical microscopy at 200× after 10,000 wet flex cycles, and subsequent abrasion removes the fibrils, increasing mass loss by 30% to 40% relative to the 4.4:1 condition. Loop tenacity follows a similar trend; the maximum loop strength occurs near 4.2:1, and a sharp decrease is observed above 4.8:1 because the fibrillar structure cannot tolerate the small bending radius at the loop. In cold-water fisheries where seawater temperatures range from 5 °C to 10 °C, the energy required to fracture the ligament between knots decreases substantially when the draw ratio exceeds 4.6:1, raising the probability of brittle mesh failure during hauling. The operational implication is that a total draw ratio window of 3.8:1 to 4.4:1 is preferred for wet-knot-critical gillnet products, while high-tenacity seine twines may tolerate 4.6:1 to 4.8:1 only when the net is heat-set under tension and the knot geometry distributes load away from tight bends. Above this range, the gains in linear tenacity are offset by a more fragile surface and a reduced ability to absorb local bending energy during net deployment and retrieval.

When Total Draw Ratio Exceeds 4.8:1, Surface Fibrillation and Splitting Govern Service Life

Published data for high-draw PA6 monofilament in wet-knot applications is limited, but production-scale observations consistently show that increasing the total draw ratio from 4.3:1 to 4.9:1 raises average linear breaking strength by only 3% to 5% while reducing wet knot efficiency from approximately 88% to 71%. The increase in tensile strength is accompanied by a broadening of the wet knot breaking force distribution; the coefficient of variation may rise from 4% at 4.2:1 to 9% at 5.0:1 because individual filaments in the same production batch experience slightly different local draw ratios due to diameter tolerance and transverse temperature gradients. At draw ratios above 4.8:1, the skin layer of the filament develops periodic craze-like features perpendicular to the drawing axis. These features are not always detected by online diameter gauges but appear as a loss of surface gloss or a whitening of the filament. The craze boundaries then act as initiation sites for filament splitting during knot tying and for crack propagation under cyclic wave loading. Process line data from two-stage drawing of 0.60 mm PA6 monofilament show that pushing the total draw ratio from 4.3:1 to 4.9:1 can increase filament breaks at the second drawing stage from less than one break per hour per threadline to several breaks per hour per threadline, while the online diameter coefficient of variation remains below 2%. This asymmetrical response—large loss of knot durability for a small gain in linear tenacity—explains why netting twine specifications often require a minimum elongation at break of 20% and a minimum wet knot efficiency of 75% rather than a maximum linear tenacity. Draw ratios above 4.8:1 are therefore reserved for products that are subsequently annealed at 170 °C for 20 s under 10% relaxation and used in low-knot applications such as leader material or longitudinal ropes, where tensile stiffness is more important than wet knot strength.

The ranges in this table are representative of industrial two-stage drawing of PA6 monofilament and should not be used as acceptance limits for a specific netting product. Single-lot acceptance values depend on filament diameter, stabilizer package, and destination regulatory requirements.

Demersal trawl mesh panels assembled from PA6 monofilament drawn at 4.2:1 exhibit a balanced combination of mesh opening stability and knot retention under cyclic loading because the oriented amorphous fraction retains sufficient molecular mobility to accommodate localized stress concentrations at the knot shoulders. When the same panel is constructed from monofilament drawn at 4.8:1, the initial mesh stiffness is higher, but the knot eyes open more rapidly under repeated loading because the overstrained fibrils split at the compression zone of the knot and the remaining load-bearing area decreases. In flume tests of netting panels at water velocities of 0.8 m/s to 1.5 m/s, the projected mesh opening changes by less than 2% for the 4.2:1 material after 200,000 load cycles, whereas the 4.8:1 material can show a change of 5% to 7% over the same test period. This difference arises not from the difference in linear tenacity but from the reduced capacity of the high-draw fibrils to redistribute local stress near the knot. Boiling-water shrinkage also decreases with draw ratio and subsequent relaxation; values below 4% are typical for monofilament heat-set at 170 °C after drawing, which is desirable for maintaining mesh dimensional stability during storage and deployment. However, the same relaxation step cannot repair surface fibrillation that has already occurred. Therefore, the selection of draw ratio for PA6 fishing net monofilament is a compromise among tensile stiffness, knot durability, abrasion resistance, and process stability, and the optimum for many wet-knot netting products lies between 4.0:1 and 4.4:1.