The boiling water shrinkage of high-density polyethylene (HDPE) tape yarn is a critical quality parameter in woven sack, flexible intermediate bulk container, and technical fabric production because it dictates dimensional stability during downstream hot-air heating, lamination, and coating processes. In laboratory practice, yarn specimens are conditioned for at least 24 h at 23 ± 2 °C and 50 ± 10 % RH in accordance with ISO 139:2005, and the initial length L₀ is recorded under a pretension of 0.5 cN/tex. The specimens are suspended without tension in boiling distilled water at 100 °C for 15 min as described in yarn shrinkage method ASTM D2259, then blotted, reconditioned, and measured. Boiling water shrinkage is calculated as S = 100 × (L₀ − L₁)/L₀, where L₁ is the post-exposure length. The draw ratio in tape yarn extrusion is defined as the linear speed ratio between the draw godet and the feed godet, DR = V₂/V₁. Commercial HDPE tape yarn lines typically operate at draw ratios between 5.0:1 and 9.0:1, with the exact value selected to balance tenacity, elongation at break, and boiling water shrinkage. Because boiling water at 100 °C is above the α-relaxation region of high-density polyethylene but below the melting endotherm of 128–138 °C, the test selectively releases the entropic strain of oriented amorphous tie chains while leaving the crystalline orientation largely intact except for local lamellar thickening and lamellar rearrangement. Consequently, the test is a sensitive indirect measure of the tie-chain population and the residual stress frozen into the tape during drawing and subsequent annealing.

What Occurs When Draw Ratio Approaches the Natural Draw Ratio of the HDPE Grade?

When the applied draw ratio approaches the natural draw ratio of a specific HDPE resin, the draw zone changes from a stable neck to a more distributed strain-hardening process. The natural draw ratio of commercial HDPE tape grades with melt flow index 0.3–1.2 g/10 min at 190 °C/2.16 kg under ISO 1133-1:2022 is typically observed in the range 4.5:1 to 7.0:1 depending on molecular weight distribution, short-chain branching content, and comonomer type. Below the natural draw ratio, the tape exhibits incomplete neck propagation, visible gauge bands, and periodic thickness variation along the running direction. Above the natural draw ratio, the tape enters the strain-hardening regime, where draw tension rises steeply and the crystalline lamellae fragment into smaller blocks connected by an increasing number of taut tie molecules. This structural transition is mechanically detectable as an upward deviation from the quasi-linear tension–draw ratio curve on a godet load cell. On production lines using single-screw extruders with L/D 30:1 and slot die gaps of 0.8–1.2 mm, the draw tension at 5.0:1 is commonly 0.15–0.25 N/tape, while at 8.0:1 the tension can reach 0.45–0.65 N/tape for a 1000 dtex tape. This tension increase enlarges the risk of surging on smooth draw godets, particularly when the first draw godet temperature is below 95 °C. To suppress draw resonance, production lines often preheat the tape over a heated godet or in a hot-air oven at 100–120 °C before the draw gap. The exact values recorded in production audits vary with quench bath temperature, line speed, and slot die output rate; published data for a specific configuration combining draw ratio and quench temperature is limited, and machine-specific trials are required.

Morphologically, the orientation of the orthorhombic HDPE unit cell along the tape axis intensifies with draw ratio, as measured by wide-angle X-ray diffraction. The Hermans orientation factor for the c-axis, f_c, increases from approximately 0.30–0.45 at 5.0:1 to 0.70–0.85 at 8.0:1, while the orientation of the a-axis and b-axis remains much lower. Differential scanning calorimetry according to ISO 11357-3:2018 shows that strain-induced crystallization raises the crystallinity from 60–68 % in undrawn or low-draw tapes to 74–82 % after high draw. The crystalline lamellae become thinner and more elongated, and the amorphous phase is transformed from a relatively isotropic random-coil state to a highly extended network of tie chains and bridging segments. Boiling water shrinkage correlates most strongly with the fraction of oriented amorphous material rather than with total crystallinity, because the oriented amorphous chains have a large entropic retraction force at 100 °C. This explains why shrinkage can continue to increase when crystallinity plateaus at draw ratios above 7.0:1. At the same time, the lamellar thickness distribution broadens, and some crystalline blocks undergo melting and recrystallization during the boiling water test, contributing to irreversible dimensional loss. The relative contribution of amorphous relaxation versus crystalline reorganization depends on the draw temperature: tapes drawn at high temperatures near 115–120 °C retain less amorphous orientation and therefore exhibit lower boiling water shrinkage at equivalent draw ratio than tapes drawn at 90–100 °C, because chain relaxation dissipates some of the entropic stress during drawing.

When Boiling Water Shrinkage Exceeds 4 Percent in Woven Sack Coating Lines

In woven sack production, the cut tape fabric is often extrusion-coated with low-density polyethylene or polypropylene at melt temperatures between 280 °C and 320 °C. If the HDPE tape yarn has a boiling water shrinkage above 4 %, the fabric can contract during preheating or coating, causing wrinkles, loss of flatness, and inconsistent coating thickness across the width of the line. Process audits on coating laminators with 1.2–1.6 m working width indicate that shrinkage-induced width loss becomes severe when the tape fabric enters a drying or preheating zone at 90–120 °C under web tension below 50 N/m. The immediate corrective action is to reduce the draw ratio on the tape extrusion line from the high-tenacity setting of 8.0:1–9.0:1 to a moderate setting of 6.0:1–6.5:1. This adjustment lowers boiling water shrinkage by roughly 1.5–3.0 percentage points, but it also decreases tenacity from 5.5–6.5 cN/dtex to 4.0–4.8 cN/dtex. The reduced tenacity can increase end-break frequency on circular looms, especially in high-speed weaving with reed frequencies above 700 picks/min. A more effective approach is to maintain the high draw ratio and install an additional hot-air relaxation zone between the last draw godet and the winder. In this configuration, the drawn tape passes through a hot-air oven at 110–125 °C for a residence time of 2–5 s while the winder speed is set 3–5 % lower than the last draw godet speed. This controlled relaxation permits the oriented amorphous chains to contract without substantial loss of crystalline orientation, reducing boiling water shrinkage to 1.0–2.5 % while retaining tenacity above 5.0 cN/dtex. The window for this annealing step is narrow: if the oven temperature exceeds 130 °C, the tape surface can soften and stick to the idler rolls, whereas if the relaxation exceeds 6 %, tensile modulus and hot-air shrinkage under 80 °C can fall below the minimum values required by the end-use specification.

At draw ratios below 4.5:1, the tape yarn retains a large undrawn core and exhibits low orientation, high elongation at break above 25 %, and boiling water shrinkage below 0.8 %. The tape surface is typically smooth but may display a pronounced yield point and non-uniform stretching when loaded in weaving, which creates difficulties in maintaining consistent warp and weft tension. The tensile modulus is low, and the tape is prone to plastic deformation during winding and loom acceleration. In extrusion lines using a 0.8 mm slot die and water quench at 35 °C, the thickness of the quenched primary film is typically 100–180 μm, and after slitting to 2.5 mm width the low-draw tapes can have a titre of 1200–1600 dtex. Such tapes are rarely used in technical fabric because the boiling water shrinkage test shows almost no change, but the deflection temperature under load is low and the creep rate under sustained load is high. The use of low draw ratios is therefore limited to applications that require very low thermal shrinkage and accept substantially reduced stiffness.

Draw Ratio Selection Limits for HDPE Resins with MI 0.3–1.2 g/10 min

The balance between boiling water shrinkage and tenacity is strongly influenced by the melt flow index and molecular weight distribution of the HDPE resin. Resins with melt flow index 0.3–1.2 g/10 min and density 0.945–0.955 g/cm³ are common for tape yarn because they provide sufficient melt strength to survive the air gap between slot die and water quench at 10–30 mm. Tenacity and elongation data are typically measured according to ISO 2062. The table below summarizes representative production ranges for a single-screw extruder with L/D 30:1, die gap 1.0 mm, water quench temperature 38–42 °C, and annealing godet temperature 110 °C with 4 % relaxation.

These ranges are not universal material constants; they depend on the relaxation history, quench bath temperature, and the degree of fibrillation induced by the draw gap. For example, a HDPE resin with a broader molecular weight distribution may sustain 8.5:1 draw without excessive tape breakage but may also produce a higher boiling water shrinkage because the low-molecular-weight fraction relaxes less efficiently during drawing. Conversely, a resin with a narrower distribution and a higher comonomer content may exhibit lower shrinkage at 7.0:1 due to a higher amorphous phase mobility and more effective relaxation on the annealing godet. The annealing temperature must be at least 5–10 °C above the expected use temperature to stabilize dimensions, but should remain below the onset of rapid oxidation, which becomes significant in HDPE above 140 °C in air. Published data for the effect of draw ratio on boiling water shrinkage across all possible HDPE grades is limited, and extrusion trials with the specific resin batch and line configuration are necessary to establish a stable control window.

The reproducibility of boiling water shrinkage measurements is affected by sample pretension, immersion time, and the degree of agitation in the boiling water bath. If the samples are allowed to float freely, twisted tapes can entangle and reduce the apparent shrinkage. If the bath is not vigorously boiling, the temperature can stratify and produce lower shrinkage values. Laboratory audits using a water bath with 10 L capacity and a recirculating heater show that temperature variation of ±1.5 °C around the set point can change the measured shrinkage by 0.3–0.8 percentage points. The test should therefore be carried out in a covered bath with a minimum water volume of at least 50 mL per specimen and with no direct contact between specimens. After removal from the boiling water, the specimens should be blotted and reconditioned for 2 h under 23 ± 2 °C and 50 ± 10 % RH before final length measurement. The use of initial length measured after conditioning is critical because HDPE tape yarn can absorb only negligible moisture, but the dimensional state can be altered by winding tension and storage temperature.

The Draw Gap Is the Primary Control Surface for Shrinkage Reduction

On a single-screw extrusion line equipped with a 90 mm screw and L/D 30:1, the output rate is typically 180–250 kg/h for HDPE with a melt density of approximately 0.76 g/cm³ at 220 °C. The molten web exits a slot die of 1.0 mm gap and enters a water quench bath at 38–42 °C. The quenched primary film is slit to 2.0–3.0 mm widths and then drawn between two godet stands. The first godet stand operates at 15–30 m/min and heats the tapes to 95–110 °C; the second godet stand operates at 120–250 m/min depending on draw ratio. If the draw gap is too short, adiabatic heating can raise the tape temperature above 125 °C and reduce the effective draw stress, producing a tape with lower shrinkage but also lower modulus. If the draw gap is too long, necking becomes unstable and the tape can break at the draw point. The measured draw tension per tape at 8.0:1 is often 0.5 N or more, and the breakage frequency increases sharply when the draw ratio exceeds the maximum drawability of the resin batch. Batch-to-batch variation in melt flow index of ±0.05 g/10 min can shift the maximum draw ratio by 0.5:1 and change the boiling water shrinkage by 0.5–1.0 percentage points. Therefore, the draw ratio should not be set as a fixed dimensionless number but rather as a function of online draw tension at a given line speed and tape titre.

Relating Draw Work to Residual Shrinkage Force

Annealing after drawing reduces boiling water shrinkage by converting some of the oriented amorphous tie chains into more relaxed conformations while preserving the high crystalline orientation responsible for tenacity. In a tape drawn at 8.0:1, the initial boiling water shrinkage may be 5.5 %; after passing over a hot godet at 120 °C with 5 % relaxation, the shrinkage typically falls to 1.5–2.5 %. The mechanism involves two simultaneous processes: the retraction of taut tie chains that were stretched beyond their equilibrium length, and the melting and recrystallization of thin lamellar blocks that have low thermal stability. The extent of shrinkage reduction is governed by the annealing time, temperature, and the applied tension. If annealing is carried out at constant length rather than with relaxation, the entropic stress is retained and boiling water shrinkage remains high; if the tape is allowed to relax too much, the c-axis orientation falls and the tenacity is lost. The optimum relaxation is therefore a narrow process window that is often monitored by continuous measurement of online tension after the annealing zone. On a tape line running at 250 m/min, the residence time on a 2 m long hot-air annealing table is only 0.48 s, which is usually insufficient to reduce shrinkage below 3 % at high draw ratios. For this reason, production lines targeting very low shrinkage at high tenacity use tandem annealing zones or a heated godet set with a slight speed differential to increase the effective relaxation time. The temperature uniformity across the annealing zone must be within ±5 °C to avoid streaky shrinkage behavior across the tape width, which can lead to curling in woven fabric.

In tape yarn formulations, addition of white masterbatch containing 5–10 wt% titanium dioxide or calcium carbonate influences draw behavior and shrinkage. At filler loadings above 8 wt%, the draw tension increases and the maximum draw ratio before fibrillation decreases by approximately 0.5:1–1.0:1. The boiling water shrinkage can become more variable due to cavitation around filler particles. The use of processing aids or fluoropolymer-based polymer processing additives at 200–500 ppm can reduce die lip deposit but does not significantly change the draw ratio–shrinkage relationship. Food-contact HDPE grades used in packaging sacks are evaluated under FDA 21 CFR 177.1520 and EU Regulation 10/2011, and any annealing or surface-treatment step must not introduce substances that compromise these compliance requirements. The selection of a draw ratio for a given HDPE tape yarn therefore operates within a multi-constraint space bounded by tensile specification, loom performance, coatability, and regulatory compliance, with boiling water shrinkage serving as the most direct indicator of residual orientation that will respond to downstream thermal exposure.