LyondellBasell Icorene 3545 GRY 7469 is a grey-pigmented linear low density polyethylene (LLDPE) rotational moulding powder. The base resin is built around a linear molecular architecture with short-chain branching, which distinguishes it from high-pressure low density polyethylene and from high-density polyethylene. Published typical values for the base grade include a melt flow rate of 5.0 g/10 min at 190 °C/2.16 kg under ISO 1133-1 and a density of 0.934 g/cm³ under ISO 1183-1. These values position the product in the medium-density segment of the LLDPE rotomoulding range, where the material is specified for parts requiring a balance of creep resistance, stress crack resistance, and low-temperature impact strength.

The powder is intended for biaxial rotational moulding machines with cast aluminium or fabricated steel moulds. Particle morphology, dry-flow behaviour, and bulk density directly affect the uniformity of the melt-formed wall section. For this grade, the melt flow rate of 5.0 g/10 min supports coalescence of the powder bed during the heating stage without generating the severe sagging that can occur with high-flow HDPE grades in large flat panels. The use of the 7469 grey colourant produces a non-uniform heating response when compared with unpigmented natural material, and oven residence times should be adjusted by direct internal air temperature measurement rather than by visual melt appearance.

Position Within the Icorene Rotomoulding Portfolio

Within the Icorene rotomoulding line, Icorene 3545 GRY 7469 occupies a mid-stiffness position. Compared with a 0.924 g/cm³ LLDPE rotomoulding grade, the 0.934 g/cm³ density increases flexural modulus and tensile yield stress, while notch impact strength at sub-zero temperatures is generally reduced. Compared with a 0.940–0.950 g/cm³ HDPE rotomoulding material, the LLDPE grade shows lower flexural modulus but markedly better environmental stress crack resistance and low-temperature ductility. The principal structural difference is the presence of short-chain branches in LLDPE, which limits lamellar thickness and increases the probability of inter-lamellar tie molecules compared with HDPE at equivalent molecular weight. Where the comonomer is 1-hexene, the branch distribution tends to produce a more uniform tie-molecule network than butene-based LLDPE grades, contributing to higher stress crack resistance under ASTM D1693 conditions. The following table lists the typical property set for the base resin as reported in public datasheets; actual lot-to-lot variation and pigment dispersion must be verified against the certificate of analysis.

Because the material is a thermoplastic and not a cross-linked polyethylene, moulded parts can be ground and reprocessed. The amount of regrind added to virgin powder is limited in production by the progressive reduction in environmental stress crack resistance and impact strength; regrind levels above 20 wt% are generally avoided in stressed-panel applications unless physical testing confirms that the finished part meets the minimum specification. In comparison with cross-linked polyethylene, the material has lower creep resistance at elevated temperature but offers easier recycling and a wider processing tolerance during oven heating.

What Limits the Processing Window for Icorene 3545 GRY 7469 in Rotational Moulding?

Processing limits for this grade are controlled primarily by the peak internal air temperature and the cooling rate. On carousel machines with a 4:1 rotation ratio and forced-air oven heating, LLDPE rotomoulding powders are typically heated to an internal air temperature of 190–210 °C. Oven set-points between 280 °C and 310 °C are used, but the relationship between oven set-point and internal air temperature depends on mould mass, wall thickness, and the emissivity of the mould surface. If peak internal air temperature remains below 180 °C, the powder particles do not fully coalesce at the mould surface, and a porous inner skin is observed on the demoulded part. If peak internal air temperature exceeds 220 °C, oxidative chain scission accelerates, leading to a measurable increase in melt flow rate and a reduction in notched impact strength. For grey-pigmented grades, the 7469 colourant system can increase infrared absorption in certain oven wavelength bands. Field measurements on steel moulds with wall thicknesses of 3–6 mm indicate that grey parts can reach peak internal air temperature 5–8 % faster than natural powder under identical oven settings; the oven residence time must therefore be shortened accordingly when the colour change is made.

Cooling rate after densification is a second critical threshold. The mould should be cooled in forced air until the internal air temperature falls to approximately 80 °C before water-spray cooling is introduced. When water cooling is initiated above 95 °C, rapid quenching of the outside surface creates differential shrinkage through the wall; this can produce warpage, sink marks, and residual stress around inserts. Water-spray cooling from 120 °C to 70 °C at a rate greater than 10 K/min is associated with part distortion on flat-bottomed tanks and should be avoided unless the tooling is specifically designed for high-rate cooling. Demoulding below 60 °C is usually required for dimensional stability.

The powder should be kept dry. Although polyethylene is hydrophobic, fines and surface moisture can generate steam bubbles during the melt cycle. Production-scale experience shows that storage in non-condensing conditions with relative humidity below 60 % is adequate for most rotomoulding operations. At higher relative humidity or in cold warehouse conditions, moisture condenses on the powder surface and becomes trapped in the melt, creating internal pinholes and surface cratering. A pre-drying step of 70–80 °C for 1–2 h in a circulating-air dryer may be used before charging the mould.

In practice, the maximum allowable wall thickness for single-shot moulding is limited by the thermal conductivity of the polymer. Wall thicknesses above approximately 12 mm require sufficiently long oven residence for the inner surface to reach coalescence temperature. However, the outer surface then remains near oven temperature for an extended period, increasing oxidative degradation. For parts exceeding 12 mm, moulding trials with thermocouple-instrumented moulds are required to confirm that the inner surface reaches 190 °C while the outer surface does not exceed 220 °C.

Under service loading from internal liquid sloshing, stress crack resistance often determines tank life. The base resin has an F50 value above 1000 h under ASTM D1693 when tested in 100 % Igepal CO-630. This value indicates resistance to detergent-induced stress cracking, but it does not guarantee resistance to aromatic hydrocarbons, organic acids, or biodiesel blends. For diesel fuel tanks, full-scale testing under ASTM D1998 is required because laboratory ESCR tests do not replicate the combined effects of internal pressure, weld lines, and chemical swelling. The low-temperature impact behaviour of LLDPE is superior to that of HDPE at equivalent wall thickness; parts moulded from this grade can be specified for outdoor use at temperatures down to -20 °C provided the design eliminates sharp internal corners and abrupt section changes. The grey pigment package contributes to ultraviolet screening, but the grade should not be considered a replacement for long-term outdoor weathering validation under ISO 4892-2 or ASTM G154.

For rotationally moulded playground equipment, the impact requirements of EN 1176-1 may apply. Compliance depends on part design, wall thickness, and moulding quality, not on the resin alone. Sharp corners, knit lines, and pigment agglomerates are the primary initiation sites for brittle impact failure at low temperature. In production audits, through-wall pinholes are most commonly observed at the bottom corners of deep ribs and at the junction between the mould parting line and the inner surface. These failures are associated with insufficient powder flow into the corner during early rotation and can be reduced by increasing the initial rotation speed or by adding a draft angle greater than 3° to the rib sidewall.

The Grey Pigment Package Alters Thermal Absorption

The 7469 grey colourant changes the optical absorption behaviour of the powder bed during rotomoulding. Unlike natural polyethylene, which transmits a significant portion of near-infrared radiation, a pigmented grey material absorbs a larger fraction of the incident oven energy at the surface. This can accelerate the surface melting stage but can also create a solid skin before the interior powder has fully melted. On a production line, the variable is controlled by adjusting total oven time rather than by increasing oven temperature, because overheating the outer surface leads to oxidative degradation. Thermocouple-instrumented trials are the accepted method for setting the oven cycle; trial data from a steel mould with a 4 mm wall section indicate that the grey material may require an oven-time reduction of 5 % compared with natural powder to reach the same peak internal air temperature. Published data for this specific configuration is limited, so moulders should generate their own time-versus-temperature curves for each tool.

When Ambient Humidity Exceeds 60 % During Powder Storage

When the powder is stored in an unheated warehouse at relative humidity above 60 %, condensation on the cold surface of bulk powder containers can introduce sufficient moisture to generate steam during the rotomoulding cycle. The resulting internal voids are typically spherical and occur at the inner wall surface, where the last material to melt is located. These voids are distinct from chemical blowing voids and are not eliminated by extending the oven cycle. The corrective action is pre-drying at 70–80 °C for 1–2 h in a circulating-air dryer. Longer drying at temperatures above 85 °C should be avoided because partial sintering of the powder surfaces can reduce dry-flow characteristics and create feeding irregularities in automated powder-loading systems.

For water-based applications, chemical resistance under ASTM D543 should be evaluated at the intended service temperature. The grade is not recommended for continuous contact with strong oxidising acids, concentrated oxygenated solvents, or aromatic hydrocarbons at temperatures above 40 °C unless full-scale immersion testing is performed. It is also not cross-linked, so creep resistance under continuous load above approximately 60 °C is lower than that of cross-linked polyethylene or HDPE. The upper continuous service temperature is typically limited to 60 °C for stressed parts and 90 °C for unstressed, intermittently heated parts; service at higher temperatures must be validated through heat-aging tests under ISO 188.

For rotationally moulded agricultural tanks and sprayer bodies, the governing difference between this grade and a low-density polyethylene homopolymer is the retention of ductility at stress concentrations. A branched LDPE will generally yield at lower elongation and exhibit shorter crack initiation times under ASTM D1693 conditions, whereas the linear grade maintains a more ordered crystalline network. At the same density, the LLDPE structure also provides better resistance to slow crack growth in notched sections. Moulding trials on production tooling with formed-in threaded inserts have shown that the combination of insert geometry and cooling-rate control has a greater effect on crack resistance than the resin choice alone; inserts with a sharp thread root should be avoided or prototyped with thermocouple-instrumented moulds before release of the part.