CSPC LLDPE 2235C is a butene-1 copolymer linear low-density polyethylene produced by CNOOC and Shell Petrochemical Company Limited. The grade is supplied for blown-film extrusion where melt flow, density and molecular weight distribution control downgauging limits. Supplier technical documentation lists a nominal melt flow rate of 2.0 g/10 min at 190 °C under ISO 1133-1:2022 and a nominal density of 0.925 g/cm³ under ISO 1183-1:2019. The pelletized product is manufactured without a universal additive package; slip and antiblock loading should be confirmed on the certificate of analysis because film coefficient of friction and blocking behaviour depend on additive concentration.

The resin melt index and density are primary specification parameters because they control extruder back pressure, melt strength, film stiffness and sealing temperature. A melt flow rate of 2.0 g/10 min is high enough for stable bubble formation at moderate head pressure but low enough to retain melt strength for quench and film haul-off. A density of 0.925 g/cm³ yields a semi-crystalline morphology with a crystallisation temperature between 106 °C and 112 °C as measured by differential scanning calorimetry at 10 °C/min according to ISO 11357-3:2018. The exact crystallisation exotherm depends on cooling rate and additive nucleating effects. This thermal transition influences frost line crystallisation and the onset of film stiffness development.

The density of 0.925 g/cm³ places the resin in the medium-density range of linear low-density polyethylene. The crystalline fraction at this density produces a measurable haze level and limits clarity against metallocene grades. Because butene-1 is a shorter α-olefin than hexene-1 or octene-1, the tie-molecule population in the amorphous region is lower at constant density. This molecular architecture affects falling-weight impact, Elmendorf tear and puncture resistance in the final film. For a 25 μm monolayer film, machine-direction tensile strength at break typically falls in the 30–40 MPa range and transverse direction in the 25–35 MPa range under ISO 527-3:2018; elongation at break typically exceeds 800% in both directions. These ranges represent the butene-copolymer LLDPE class near 0.925 g/cm³, not grade-specific values. Published open-literature data for this specific grade are limited; the producer datasheet remains the primary source for film property values.

What separates a butene-copolymer Ziegler-Natta LLDPE from metallocene film grades?

The catalytic route controls short-chain branching distribution and molecular weight distribution. CSPC LLDPE 2235C is produced with Ziegler-Natta catalysis, which creates a broader molecular weight distribution than a typical metallocene-catalysed LLDPE. The broader distribution increases shear sensitivity and reduces apparent viscosity at high die shear stress. On a 45 mm grooved-feed blown-film extruder with L/D 30:1, the resin therefore reaches lower head pressure at equivalent output than a metallocene grade of the same nominal melt index. The trade-off is measured in film properties: metallocene grades of equivalent density generally provide higher dart impact, lower haze and higher hot-tack strength. Direct comparative values for 2235C against specific metallocene grades are limited to supplier technical data and application-specific pilot trials.

On production-scale lines, the rheological differences are observed most clearly in bubble behaviour. A Ziegler-Natta LLDPE bubble lacks the strain-hardening character of autoclave LDPE, so bubble stability depends on mechanical air-ring control, frost line position and line speed. Blow-up ratios above 3.0 are uncommon unless the resin is blended with LDPE. Operators typically maintain a blow-up ratio of 2.0–2.8 to balance machine-direction and transverse-direction shrinkage. Frost line height is set between 4 and 8 die diameters; a lower frost line decreases crystalline orientation and may improve dart impact, while a higher frost line increases film modulus at the expense of tear resistance.

Extrusion parameters and bubble stability limits in blown-film lines

A standard monolayer line configuration includes a barrier screw with L/D ≥ 24:1, a double-lip air ring and internal bubble cooling when throughput exceeds 150 kg/h. For single-screw extrusion in the 45–65 mm diameter range, a barrier screw with feed-section depth of 8.0–10.0 mm, metering-section depth of 2.5–3.5 mm and compression ratio of 3.0–3.5 is commonly used. The grooved feed zone is kept between 40 °C and 80 °C to prevent premature pellet melting and maintain feed conveying efficiency. Barrel setpoints from 180 °C to 200 °C in the feed zone and 200 °C to 220 °C in the metering zone are used to limit viscous heating. Melt temperature at the adapter should remain below 240 °C; extended operation above 220 °C increases the risk of oxidative gel formation. Die gap is normally set at 1.2–2.5 mm. If the die gap is reduced below 1.0 mm, high shear stress can produce sharkskin melt fracture at commercial output rates.

Because the resin does not contain the long-chain branching of LDPE, extensional viscosity is lower and the bubble is less resistant to sudden air-ring fluctuations. Stable operation at a blow-up ratio of 2.0–2.5 requires consistent air flow and a fixed frost line. When the frost line rises above 8 die diameters, the film enters the collapsing frame with higher crystallinity and reduced transverse-direction tear resistance. A low frost line below 4 die diameters may increase blocking tendency unless the film is adequately cooled and the additive package includes antiblock. Air-ring and internal bubble cooling conditions determine frost line height and gauge distribution. The use of internal bubble cooling reduces gauge spread and allows higher throughput; without it, the frost line is controlled by external air-ring velocity and bubble diameter. These relationships are standard for blown-film LLDPE and are observable on equipment with frost line position control.

Purge transitions from HDPE or LDPE should use a medium-viscosity LDPE if the screw is not designed for LLDPE. The higher low-shear viscosity of LLDPE can increase compression-zone pressure and torque. On a 90 mm barrier screw, output is frequently limited by extruder torque rather than melting capacity. This limitation is an operational boundary observed on lines without high-torque gearboxes.

When downgauging below 20 μm, dart impact becomes the controlling criterion

At film thickness below 20 μm, the butene-1 comonomer content no longer provides sufficient tie-molecule density to sustain falling-weight impact unless the film is drawn under controlled low frost line conditions. Dart impact tested according to ASTM D1709A on a 25 μm monolayer film is the usual reference point. For butene-copolymer LLDPE with density near 0.925 g/cm³, the dart impact of a 25 μm film commonly falls in the 80–120 g range, but the producer datasheet for CSPC LLDPE 2235C should be used for the grade-specific value because die gap, blow-up ratio and additive loading shift the result.

When a converter draws the film to 15 μm, the addition of 10–15% LDPE or a hexene/octene LLDPE is a common measure to maintain bubble stability and impact performance. The LDPE blend increases extensional strain hardening but increases haze and may reduce machine-direction tear strength. A hexene/octene LLDPE blend increases tie-molecule density and dart impact but can alter heat-seal initiation and film stiffness. In either case, the operating window narrows; the frost line height should be lowered and the blow-up ratio reduced to 2.0–2.3 to preserve gauge uniformity.

Comonomer chemistry changes tear resistance and sealing response

The short-chain branching distribution from butene-1 differs from that obtained with hexene-1 or octene-1. At constant density, butene-copolymer LLDPE has a lower concentration of tie molecules in the amorphous regions, which reduces Elmendorf tear in both machine direction and transverse direction when tested under ASTM D1922-19. However, the lower melting onset from butene branches permits sealing at lower jaw temperatures than HDPE. The sealing window of butene-copolymer LLDPE is wider than that of HDPE but narrower than that of a high-alpha-olefin metallocene grade. Heat-seal strength is tested under ASTM F88/F88M-21; hot-tack is tested under ASTM F1921-20. On high-speed vertical form-fill-seal machines, jaw temperature is typically set between 115 °C and 135 °C for 50 μm film, but the exact setpoint must be determined by line trial because seal bar pressure, dwell time and contamination affect the result.

Relative to HDPE film grades, CSPC LLDPE 2235C has lower stiffness and lower film yield strength, but the lower melting onset allows lower heat-seal temperatures. Relative to autoclave LDPE, the resin has higher puncture and tear resistance at equal thickness but lower extensional strain hardening, which makes high-stalk bubbles more difficult. Relative to hexene/octene LLDPE, the butene-1 short-chain branching yields lower dart impact and reduced tear resistance at the same density; however, the butene-copolymer grade is often selected for lower resin cost and adequate processability on conventional LDPE film lines.

Primary applications include general-purpose mono-layer and coextruded films, heavy-duty sacks, agricultural film, overwrap and lamination film where the resin is not required to provide ultimate optical clarity. In heavy-duty sack formulations, 2235C is blended with LDPE at 10–20% to increase impact resistance and tear propagation resistance. In lamination film, the resin is used in the core layer to provide stiffness and melt strength; the skin layer is usually a lower-density metallocene or LDPE to improve heat-seal strength and optics.

Additive migration in LLDPE films follows Fickian diffusion with concentration-dependent diffusion coefficients; slip agents such as erucamide migrate to the surface over 24–72 h at ambient temperature. The migration rate is faster in low-crystallinity butene LLDPE than in HDPE, meaning surface coefficient of friction decreases more rapidly after winding. This is process-critical for printing and bag conversion. The resin should not be processed with high levels of acidic or amine-based additives unless the stabilization package is confirmed, because nucleating and acid-scavenging interactions can shift cooling behaviour and colour. Contamination with polypropylene in recycle streams above 5 wt% can create melt immiscibility, reducing dart impact and increasing haze.

Resin handling for 2235C follows standard polyethylene practice. The pellets have a bulk density in the range of 0.55–0.60 g/cm³. Drying is not normally required when moisture content remains below 500 ppm, but storage in environments above 60% RH may require pre-drying at 70–80 °C for 1–2 h to avoid surface defects caused by moisture vaporisation at the die exit.

For direct food-contact assessment, CSPC LLDPE 2235C falls under the olefin polymer provisions of FDA 21 CFR 177.1520 when used in accordance with the conditions of use and additive limitations specified in that section. European Union food-contact compliance is normally demonstrated through Regulation (EU) No 10/2011 and its migration limits; the overall migration limit of 10 mg/dm² applies to the final article unless a different value is set for the specific packaging application. REACH documentation should confirm that no substance of very high concern is present above 0.1 wt% in the article. RoHS Directive 2011/65/EU Annex II restrictions apply to electrical and electronic equipment, not to packaging; when the resin is used as packaging for EEE, the supplier certificate should confirm the absence of restricted phthalates and heavy metals above the tolerated maxima.