LyondellBasell Icorene 3545 GRY 7401 is a rotomoulding-grade linear low-density polyethylene supplied as a pigmented powder; the GRY 7401 suffix identifies a factory-formulated grey colour package rather than a natural resin. The base polymer falls within the linear low-density polyethylene density band of 0.915 g/cm³ to 0.940 g/cm³ under ISO 1183-1:2019 and ASTM D792-20, although the exact released density of the grey configuration must be taken from the supplier’s batch certificate because pigment addition can shift density by several thousandths of a gram per cubic centimetre. The molecular architecture incorporates short-chain branches introduced by an alpha-olefin comonomer; these branches limit crystallinity and lamellar thickness while retaining sufficient linear backbone for strain hardening under slow biaxial deformation during rotational moulding. In carousel, shuttle, rock-and-roll and fixed-arm turret machines, the powder is rotated biaxially inside a closed mould and heated by forced-convection air until densification occurs. Typical rotomoulding powders are controlled by sieving, with inspection points commonly including a 500 µm screen for oversize particles and a 75 µm screen for fines, because coarse particles can contribute to pinholes and fine fractions can segregate. The numeric portion 3545 identifies the grade within the Icorene rotomoulding portfolio, while the colour suffix is supply-chain-specific. Compared with high-pressure low-density polyethylene, the LLDPE class generally shows a different balance of melt flow, tensile strength, puncture resistance, and environmental stress-cracking behaviour because it does not contain the same long-chain branching population. Compared with high-density polyethylene, Icorene 3545 GRY 7401 is expected to deliver lower flexural modulus, higher ductility at low temperature, and improved environmental stress-cracking resistance at equivalent melt index. Published data for this specific grey configuration is limited; the colour-compounded datasheet and certificate of analysis remain the controlling sources for process capability studies.

What Practical Limits Does the Peak Internal Air Temperature Impose on Wall Thickness Uniformity?

In rotational moulding of LLDPE, oven set-point alone is an incomplete control variable. The part reaches densification only after the internal air temperature inside the mould passes through the crystalline melting range; for LLDPE class materials, differential scanning calorimetry under ISO 11357-3:2018 typically places the main melting peak between 120 °C and 130 °C. Full densification and bubble removal generally requires a peak internal air temperature between 180 °C and 220 °C, though the exact window depends on wall thickness, mould gauge, and the absorptivity of the grey mould surface. Below the lower boundary, residual porosity remains at the inner surface and weld lines do not fully heal; above the upper boundary, the polymer may undergo thermo-oxidative chain scission, colour shift, and excessive melt fluidity that promotes localised wall thinning during cooling. The controlling measurement should be made with a Type J or Type K thermocouple inserted through the vent tube into the rotating mould cavity. Inferring polymer temperature from the mould surface alone introduces error because the internal air temperature can lag the external mould surface by 20 K to 40 K during heating. The presence of the grey pigment changes infrared absorption compared with natural Icorene 3545; therefore, cycle times should not be copied directly from natural-grade records. Cooling rate is equally important: forced-air cooling at 0.5 °C/s to 1.0 °C/s typically produces finer crystallites and lower warpage than still-air cooling at 0.05 °C/s to 0.2 °C/s, but excessive cooling can freeze residual stress and raise post-demoulding deflection. For large flat panels, peak internal air temperature repeatability within ±2 °C is used as an operational target to reduce thickness variation and warpage. Vent tubes with an inside diameter of at least 10 mm are normally required to prevent pressure imbalance during heating and cooling; clogged vents can produce blowholes and partial demoulding. Published data for this specific grey configuration is limited; mould thermocouple verification on the actual tool is therefore required before transferring the process from any other LLDPE grade.

Mechanical Response Characteristics Under Slow Biaxial Deformation and Low-Temperature Impact

Rotomoulded parts are not isotropic; test specimens should be taken from known low-thickness zones and moulded-in corners because tensile, flexural, and impact values vary with local thermal history. Tensile yield strength is measured under ASTM D638-14, typically at a crosshead speed of 50 mm/min; flexural modulus is determined under ISO 178:2019 or ASTM D790-17. LLDPE rotomoulding grades in this density range generally exhibit lower flexural modulus than HDPE but higher elongation at break and greater slow crack growth resistance. Charpy impact strength under ISO 179-1:2023 is highly sensitive to crystallinity, wall thickness, and notch preparation; a ductile-to-brittle transition may appear between −40 °C and −20 °C depending on cooling rate and pigment dispersion. Environmental stress-cracking resistance is evaluated by ASTM D1693-15e1; it is here that the short-chain-branched architecture separates LLDPE from HDPE and high-pressure LDPE in many chemical-storage applications. Cross-linked polyethylene grades may offer higher creep and heat resistance, but they cannot be reprocessed as readily; LLDPE grades provide repairability and a simpler recycling route. Published data for this specific grey configuration is limited, so comparative values should be generated on the colour-compounded material after the actual rotomoulding cycle, not on injection-moulded plaques.

When Grey Pigment Dispersion Alters Powder Dry-Flow and Wall Thickness Distribution

Pigment incorporation in rotomoulding powder is not limited to surface colour. The GRY 7401 package introduces a dispersed phase that changes powder dry-flow angle, bulk density, tribocharging, and thermal absorption. Dry-flow and bulk density are measured by ASTM D1895-17; a shift of several degrees in angle of repose or a drop of 2%–4% in bulk density after high-shear blending indicates that feeder settings, load-cell calibration, and fill-shoe stroke may need adjustment. In automated filling systems, batch-to-batch variation in pigment concentration should be controlled within ±0.2 wt% to keep wall thickness distribution stable. Segregation can occur if the pigment particle size is significantly finer than the base powder; fines may concentrate in corners, producing darker bands and localised stress concentration. Static charge can be monitored with a charge plate monitor, but any antistatic additive must be confirmed compatible with the grey colour package and the final application. Because grey pigments can act as nucleating agents, crystallisation temperature may increase by a few kelvin and crystallite size may decrease slightly; the resulting part can show a small increase in flexural modulus and a possible reduction in impact if the crystallinity rise is not compensated by proper cooling. Optical microscopy at 100× magnification can detect pigment agglomerates, and dispersion should be verified on a fused panel rather than on the dry powder alone. Processors should not assume that a natural Icorene 3545 cycle is transferable to the GRY 7401 configuration without re-qualification of peak internal air temperature, demoulding time, and shrinkage on the exact grey material.

Olefin polymers intended for repeat-use food-contact articles may be examined under 21 CFR 177.1520, but the final grey formulation must have all colourants and adjuvants individually authorised for the intended contact conditions; the converter must obtain the supplier’s food-contact statement for GRY 7401. In the European Union, Regulation (EC) No 1935/2004 and, where applicable, Regulation (EU) No 10/2011 require verification of overall migration and any specific migration limits for the pigment package and stabiliser system. REACH duties under Regulation (EC) No 1907/2006 include communication of substances of very high concern in articles above the 0.1 wt% threshold under Article 33. RoHS Directive 2011/65/EU restricts lead, cadmium, mercury, hexavalent chromium, PBBs, and PBDEs at the homogeneous material level; typical grey pigment compositions based on carbon black and titanium dioxide do not introduce these elements, but incoming batch verification is required because recycled filler or pigment streams can carry trace metals.

Repeated Exposure Above the Crystalline Melting Range Does Not Leave the Antioxidant Package Unchanged

Rotational moulding scrap is commonly ground and blended with virgin powder for non-structural or buried components. For Icorene 3545 GRY 7401, each pass above 200 °C in an air atmosphere consumes part of the phenolic/phosphite antioxidant package; chain scission may increase the melt flow index measured at 190 °C with 2.16 kg under ISO 1133-1:2022, while crosslinking may reduce it. An oxidative induction time measured by ISO 11357-6:2018 or ASTM D3895-19 provides a more direct indication of residual stabiliser than melt flow index alone. Processors using a shuttle moulding machine and forced-air cooling should monitor the melt flow index before and after each grinding pass; a shift greater than 15% from the virgin grey powder is regarded as an operational boundary for thin-wall parts, because excessive chain scission leads to pinholes, surface roughness, and reduced low-temperature impact. The grey pigment provides surface UV protection but does not prevent thermal oxidation at the inner air-melt interface. Re-stabilisation of the recycled fraction may be required before blending at 20 wt% to 30 wt% with virgin powder. Published data for this specific grey configuration is limited; therefore, the boundary should be validated on the actual part geometry and oven cycle rather than adopted from natural or high-density grades.

Outdoor exposure of grey LLDPE parts introduces a separate set of oxidative and photo-degradation pathways. Grey pigment packages containing carbon black or mixed-metal oxide pigments screen ultraviolet radiation, but ultraviolet stabilisers may still be required for long-term colour and impact retention. Accelerated weathering under ISO 4892-2:2013 or ASTM G155-21 can rank formulations, yet xenon-arc cycles with water spray do not exactly replicate field service. In chemical storage, the environmental stress-cracking resistance of LLDPE does not confer universal chemical resistance; chlorinated hydrocarbons, certain aromatic solvents, and strong oxidising acids can swell or degrade the polymer. For each chemical fluid, immersion testing under the expected stress level and temperature should follow ASTM D543-21, and polyethylene upright storage tanks may be evaluated under ASTM D1998-21. Published data for this specific grey configuration is limited; compatibility testing on the actual colour-compounded material is required when the service fluid is not a simple aqueous system.

Demoulding temperature is a critical boundary that is sometimes overlooked because the part may appear dimensionally stable while still inside the chilled mould. The heat deflection temperature of LLDPE is low relative to HDPE; under ISO 75-2:2013 at 0.45 MPa, values for LLDPE class materials may fall below 50 °C. Removing the part from the mould at a surface temperature above this value, or allowing uneven airflow across the part, can create permanent warpage, sink marks at thick bosses, and localised stress whitening. Mould release agents, vent-tube placement, and internal air pressure during cooling all affect demoulding force. For Icorene 3545 GRY 7401, demould time should be determined by monitoring the part surface temperature, not by a fixed timer taken from a natural or high-density grade. Premature demoulding is one of the main causes of dimensional rejection in large flat panels and storage tank sidewalls. Published data for this specific grey configuration is limited; a practical boundary is to delay demoulding until the hottest mould surface is below the heat deflection temperature of the compounded material under the same load.