Hexene-based linear low-density polyethylene grades designated 6201 and 6201XR derive their practical behavior from 1-hexene comonomer incorporation. The butyl short-chain branches generated by 1-hexene disrupt lamellar crystallization more effectively than the ethyl branches of butene-1 at equivalent density. The result is a higher tie-molecule population, which shifts thick-wall failures from brittle slow crack growth to ductile yielding. The 6201 series is supplied in a melt flow rate class of 5.0 g/10 min at 190°C/2.16 kg per ISO 1133-1 and a density range of 0.935–0.938 g/cm³ per ISO 1183-1. The XR designation introduces an extended stabilizer system and a measurable increase in subzero notched impact strength while retaining the same melt-flow index class. Exact specification limits must be confirmed against the producer certificate; the values discussed below are representative of the density and melt-flow class rather than guaranteed lot-to-lot minima.

How Does Hexene Short-Chain Branching Control Slow Crack Growth?

In Ziegler-Natta-catalysed LLDPE, 1-hexene insertion creates butyl branches that deflect lamellar growth and increase the probability of tie molecules between adjacent crystallites. At a density of 0.935 g/cm³, differential scanning calorimetry per ISO 11357-3 typically records a crystalline fraction of 47–52% and a melting endotherm maximum of 122–125°C. The melting trace is broad, with a full width at half maximum of 15–20°C, indicating a heterogeneous short-chain branching distribution. This heterogeneity is functionally significant: it suppresses the sharp brittle transition that can occur in narrow-metallocene copolymers of the same density.

Environmental stress-crack resistance measured by ASTM D1693 in 100% Igepal CO-630 at 50°C exceeds 1,000 h F50 on 2 mm notched strips for both grades. The 6201XR grade gives a more repeatable F50 under high-stress bending, which is relevant for rotationally molded tanks with internal corners and threaded inserts. The brittle-to-ductile transition is depressed by at least 10°C relative to a butene-based LLDPE at equivalent density, although published data for this specific configuration is limited to producer technical bulletins.

Capillary rheometry at 190°C per ISO 11443 shows that a 5.0 g/10 min hexene LLDPE has a shear-thinning power-law index of 0.55–0.65 between 100 s⁻¹ and 1,000 s⁻¹. Zero-shear viscosity is approximately 2,000 Pa·s; at 1,000 s⁻¹, apparent viscosity falls below 250 Pa·s. On a 60 mm single-screw extruder with 30:1 L/D, melt temperature limits are 200–230°C for thick-wall profiles. Processing above 250°C initiates oxidation and increases gel formation in hexene-based LLDPE, producing visible specks in unpigmented parts. The screw should maintain 3.0–4.0 barrel zones with reverse-pitch mixing elements to avoid solid-bed breakup; excessive shear heating above 450 s⁻¹ in the adapter can lead to melt fracture in thick cross-sections.

In pulverized form, the particle-size distribution of a 5.0 g/10 min hexene LLDPE depends on cryogenic grinding conditions. Producers target a median particle size of 250–350 µm with 98 wt% passing through a 500 µm sieve. Oversize particles above 800 µm melt slowly and can create pinholes in tight mold radii. Dry-flow properties measured by a Hall flowmeter per ASTM D1895 are 30–40 s/100 g for pulverized 6201; the XR grade is slightly slower because of the higher molecular weight stabilizer package. This difference is not large enough to require changes in carousel speed, but it affects powder transfer through venturi loaders at feed rates above 120 kg/h.

Rotational Molding, Oven Residence, and Wall Thickness Limits

For rotational molding, the 6201 series processes at oven set temperatures of 280–320°C and a peak internal air temperature of 200–210°C. The practical wall thickness range is 3–12 mm. Thin sections below 3 mm may show incomplete densification because the heat transfer from biaxial rotation is insufficient to collapse all powder interstices. At wall thicknesses above 12 mm, the temperature gradient across the polymer melt produces three distinct layers: a fully consolidated inner surface, a bulk region with limited crystalline orientation, and an outer skin that may retain residual stresses. Controlled cooling at 15–25°C/min from 180°C to 80°C is required to minimize shrinkage. Linear mold shrinkage per ASTM D955 is typically 1.5–2.2% for natural resin, increasing by 0.2–0.4% when high loadings of inorganic pigment are present.

The XR grade shows less surface pitting after repeated oven cycles because the stabilizer package suppresses oxidative degradation in the powder bed. On a carousel machine with four arms, mold release agents containing fluoropolymer dispersions are used at 0.5–1.0 wt%; excess agent causes weld-line weakening at the mold split line. Peak internal air temperature should not exceed 210°C for pigment levels above 1.0 wt%; the combination of high pigment loading and high peak internal air temperature accelerates chain scission at the pigment-polymer interface and reduces ESCR below 500 h in filled systems.

Outdoor storage tanks and chemical containment parts use 6201XR because of weathering and chemical stress-cracking resistance. Ultraviolet exposure according to ISO 4892-2 cycle A shows 70–80% retention of tensile elongation after 1,500 h in natural grades with the producer’s hindered amine light stabilizer package. The exact UV-stabilizer concentration is supplier-controlled; part-specific weatherability is verified by ISO 4892-3 for through-thickness UV degradation of thin skins. Chemical contact with concentrated nitric acid above 20 wt% at 23°C is not recommended due to oxidative attack. Contact with aliphatic hydrocarbons can reduce ESCR; immersion in diesel fuel at 40°C requires derating of hoop stress by 40–50% relative to water service.

Food-contact compliance is available for 6201 under EU Regulation 10/2011 and FDA 21 CFR 177.1520. The XR variant contains higher molecular weight stabilizers; these stabilizers have molecular weights above 1,000 g/mol and therefore low migration potential, but specific overall migration limits must be confirmed per the producer’s certificate. The stabilizer package is not compatible with copper-based contact surfaces; copper stearate contamination from bronze bushings can catalyze thermo-oxidative chain scission during extended oven cycles.

When Low-Temperature Impact and ESCR Are Simultaneously Required

In impact-critical applications, the simultaneous requirement for low-temperature ductility and ESCR is met by the higher tie-molecule fraction in the XR grade. Charpy notched impact at -40°C per ISO 179-1eA is typically 7.5 kJ/m², while the base 6201 grade is 6.0 kJ/m². The difference is modest in energy units, but it corresponds to a shift from mixed-mode crack propagation to full ductile tearing in parts with a wall thickness of 6 mm. Drop-weight impact on flat plaques at -20°C per ISO 6603-2 gives a puncture energy of 12–15 J for 3 mm thickness, with the XR grade more consistently exceeding 14 J.

Operational boundaries apply. At melt temperatures above 240°C, the stabilizer package in 6201XR may generate volatile decomposition products that create voids in thick sections. The recommended heat stabilizer package is not compatible with copper-based contact surfaces; copper stearate contamination from bronze bushings can catalyze thermo-oxidative chain scission. Avoid blending with recycled polypropylene because the phase separation reduces notched impact by 30–50%. If regrind is used, the maximum regrind addition is 20 wt% for XR grades in pressure-rated parts. Weld-line strength in secondary injection overmolding decreases by 15–20% when the melt temperature is below 215°C; therefore, hot-runner manifolds are maintained at 230°C and the nozzle is not allowed to cool below 220°C during color changes.

Avoid combining 6201XR with carbon black masterbatches containing high-sulfur carriers, because sulfur-containing species reduce the oxidation induction time measured by ISO 11357-6 by more than 50%.