Polypropylene homopolymer grade T30S is processed on cast-film tape lines into high-tenacity flat tapes for woven sacks. The grade is specified with a nominal melt mass-flow rate of 3.0 g/10 min when tested at 230°C under a piston load of 2.16 kg according to ISO 1133-1:2022, a tensile yield stress in the range 33–36 MPa when measured on type 1B specimens according to ISO 527-2:2012, a flexural modulus near 1,500 MPa according to ISO 178:2019, a density near 0.90 g/cm³ according to ISO 1183-1:2019, and a melting peak temperature near 165°C according to ISO 11357-3:2018. In a conventional tape line, the material is melt-extruded through a flat coat-hanger die with a die gap between 0.8 mm and 1.6 mm, quenched in a water bath maintained between 30°C and 45°C, and orientated in a hot-air oven or on heated godet rolls between 130°C and 150°C. The term high downstream draw refers to the speed ratio between the first godet stand that stabilises the quenched cast film and the second godet stand that draws the tape at elevated temperature. For homopolymer PP with a nominal MFR near 3.0 g/10 min, the stable solid-state draw ratio on conventional cast-tape equipment is usually limited to 6:1 to 7:1, with a practical upper boundary of approximately 8:1 before edge fibrillation, width loss, and break frequency increase sharply. Published data for the exact high downstream draw behaviour of T30S on a specific line configuration are limited; however, production data from water-quenched homopolymer PP tape lines indicate that the drawability limit is governed by quench-generated crystalline morphology, molecular weight distribution, orientation temperature, line speed, and die-edge thickness homogeneity. The primary stress mode is tensile orientation of a semi-crystalline film below the melting point; this is not melt-phase draw resonance, which occurs above the melting point and is governed by melt extensional viscosity, but solid-state orientation, where the load-extension behaviour is controlled by lamellar slip, amorphous-chain orientation, and fibril formation within the oriented spherulitic structure. High downstream draw reduces tape denier and increases machine-direction tensile strength, but the woven sack fabric also requires sufficient transverse tear resistance and dimensional stability during subsequent coating or lamination. A secondary effect is the increase in residual shrinkage and the narrowing of the processing window for acceptable tape width and elongation at break; both parameters influence weaving efficiency and final sack burst strength.

Why does T30S tape edge fibrillation become process-limiting above a 7:1 solid-state draw ratio?

The edge-limited drawability of T30S under high downstream draw arises from the non-uniform crystalline morphology that is frozen into the cast film during water quenching. The edges of the cast film cool faster than the centre and therefore contain a higher nucleation density and smaller spherulitic or smectic domains; the edge region is also slightly thinner because of die lip neck-in and edge bead removal. During solid-state drawing, the tensile stress concentrates in these thinner and structurally distinct edge zones. At draw ratios above 7:1, the stress transmitted to the edge zones approaches the cohesive strength of the inter-spherulitic boundaries, and the tie-chain network cannot redistribute the applied load over the full tape cross-section. Edge fibrillation initiates when fibrils begin to separate from the tape body along the machine direction, producing a ragged edge that reduces weaving performance and increases dusting. Tensile properties measured on oriented tapes according to ISO 527-3:2018 show a decline in elongation at break to below 15% as the draw ratio crosses 8:1, while tenacity at break increases only marginally because the additional orientation is converted into fibrillation rather than coherent molecular alignment. The fibrillation count per unit length is not defined in ISO 527-3:2018 and therefore must be measured by plant-specific optical inspection, but a practical control limit is often set at 0.5 fibrous defects per 1,000 m of tape. The residual width loss also becomes significant: a tape drawn at 7:1 can retain approximately 88–92% of the inbound cast-film width, while a tape drawn at 9:1 may retain only 72–80%, depending on die gap, quench temperature, and godet speed. This width loss changes the woven fabric cover factor, shifts the tape spacing required to maintain fabric dimensional stability, and can force downstream weavers to operate outside the normal reed and pick settings for a target bulk density. The defect mechanism is promoted by the relatively low entanglement density of a homopolymer with MFR near 3.0 g/10 min, as compared with random copolymers that contain comonomer defects and can exhibit a broader stable draw window. T30S therefore reaches a property cliff at high draw ratio: the narrow processing boundary separates acceptable tape tenacity from unacceptable fibrillation and break rate, and the boundary moves with small changes in quench bath temperature, oven temperature, die gap, or regrind content.

Values in the table represent process windows reported for water-quenched homopolymer PP tapes with MFR near 3.0 g/10 min, not guaranteed values for any single coil; published data for exact T30S under all listed draw ratios are limited, and shifts of 0.5–1.0 unit in the practical draw limit may occur with quench temperature, orientation oven set point, regenerated regrind content, and die gap.

Melt-phase rheology remains relatively stable across the normal extrusion temperature range, but the transition from cast film to oriented tape introduces process constraints that are not predicted by the nominal MFR alone. On a 90 mm single-screw extruder with 30:1 L/D and a 1,000 mm coat-hanger die, a melt temperature of 245–255°C produces a stable cast film at output rates between 350 kg/h and 450 kg/h, with a quenched film thickness before drawing of approximately 90–130 µm depending on die gap and first godet speed. The high downstream draw then reduces final tape thickness to a typical range of 25–40 µm, with denier values commonly between 250 denier and 500 denier (28–56 tex). Melt pump discharge pressure is typically held below 15 MPa, while the screen pack and filter system must keep pressure drop below 5 MPa to avoid melt-temperature gradients and locally degraded gel particles. If the draw ratio is increased while melt output is not synchronised, a mismatch of 2–3% between first-godets speed and second-godets speed produces thickness variation and edge instabilities that become visible as transverse thickness bands on the oriented tape. These bands act as stress risers in the weaving process and can generate weft breakage. The first godets speed is generally set from 15 m/min to 25 m/min on tape lines; high downstream draw is achieved by increasing the second-godets speed rather than reducing the first-godets speed, because a low first-godets speed produces a thicker cast film that must be drawn more aggressively and often increases the occurrence of internal cavitation and whitening in the oriented tape. Post-extrusion ageing also plays a role: the smectic fraction formed during quenching can transform to the alpha-crystalline form over 24 h at ambient temperature, so tensile data measured immediately after extrusion may overstate the tape elongation retained in warehouse-aged material. Production trials therefore require conditioning and retesting of tapes after at least 24 h at 23 ± 2°C and 50 ± 10% relative humidity before accepting the high-draw process window.

Quench water temperature, smectic fraction, and orientation drawability interplay

Water-quench temperature and orientation oven residence time control the solid-state draw window more strongly than any other single process variable. A quench water temperature between 30°C and 40°C generates a high fraction of smectic or microcrystalline material that can be transformed into a fibrillar morphology with a comparatively low drawing force; this range is preferred for high-downstream-draw operation with T30S. When the quench water is held below 25°C, the surface layers of the cast film solidify so rapidly that a highly oriented and brittle skin is produced, and the practical draw ratio limit can fall to 6:1. When the quench water is raised above 50°C, the quench rate decreases, spherulite size increases, and the force required to draw the film rises; the same orientation oven set point may then produce a lower maximum draw ratio and a higher coefficient of variation in tape tenacity. The orientation oven set point should be kept between 135°C and 150°C for homopolymer PP tape. At temperatures below 120°C, insufficient chain mobility produces brittle fracture at draw ratios above 7:1. At temperatures above 150°C, the tape can soften locally, stick to heated godets or oven guides, and exhibit width and thickness distortions. Residence time in the orientation zone is typically 0.4–1.2 s, depending on oven length and line speed; shorter residence times require higher oven temperatures to achieve the same draw ratio, while longer residence times may produce thermal relaxation and reduce the orientation gained. The drawing force measured at the second godets is an indirect indicator of drawability: a force that rises non-linearly when the draw ratio increases from 6:1 to 8:1 indicates that the material is approaching the fibrillation limit, and a plateau or decrease in force at higher draw ratio can signal incipient fibrillation rather than stable drawing. In high-draw operation, the second godet stand must be run in speed-control rather than torque-control to maintain an exact speed ratio; otherwise draw-ratio drift can occur as tape tension changes during roll acceleration. Godet contact wrap angle should be sufficient to prevent slippage at the higher orientation tension, typically at least 210° of wrap on the second godet roll.

When a 90 mm 30:1 single-screw tape line raises the second-godets speed beyond 145 m/min

On a production line with a fixed first-godets speed of 18 m/min, a draw ratio of 8:1 requires a second-godets speed of 144 m/min. Raising the draw ratio to 9:1 or 10:1 requires second-godets speeds of 162 m/min or 180 m/min, respectively, and at these speeds the orientation tension rises sharply. The tension increase is not linear with draw ratio because the tape undergoes strain hardening at moderate draw ratios and then incipient fibrillation at high ratios. Process data from water-quenched homopolymer PP tapes indicate that edge fibrillation and break frequency increase steeply above 8:1, with the break frequency often rising from 0.1 breaks per 1,000 m at 7:1 to between 1.0 and 2.0 breaks per 1,000 m at 9:1. The second-godets drive must be sized to handle the higher torque, and the godet shell must provide enough wrap length to prevent slippage; tape tension per unit width values above 0.4 N/mm are generally interpreted as an upper control limit for stable operation on conventional smooth godets. Raising the oven temperature to compensate for higher draw ratios can extend the limit by 0.5–1.0 draw ratio unit but simultaneously increases hot-air shrinkage and produces a softer tape surface. If line speed is reduced instead, productivity falls and the melt pump output must be reduced to maintain cast-film thickness. Incompatibilities and operational boundaries are specific: regrind from edge trim and rejected tapes, when incorporated at more than 20%, shifts the molecular weight distribution and reduces the maximum stable draw ratio by 0.5–1.0 unit; formulations containing nucleating agents or slip agents can alter quench crystallinity and require revalidation of the entire orientation window. High-downstream-draw tapes should be annealed at 120–130°C for 0.5–1.5 s after orientation to reduce residual shrinkage to ≤ 4% when tested at 130°C for 15 min using an internal method aligned with ISO 11501:2022. Without this annealing step, woven sacks made from high-draw tapes may develop fabric distortion during subsequent lamination or coating at 110–150°C, and the seam strength can degrade because the tapes have lost part of their cold-draw reserve. The practical maximum draw ratio must therefore be established not only by tape tenacity but also by the acceptable width, shrinkage, fibrillation count, and break rate under the weaving conditions specified for the final sack.