The LyondellBasell Icorene 3545 GRY 7407 grade is a rotational-moulding linear low density polyethylene supplied as a pretinted grey powder. The suffix GRY 7407 identifies the factory-dispersed grey pigmentation system rather than a separate post-mould coating; batch documentation therefore reports both the base-resin melt properties and the colour-specific compound. As a powder, the product is typically packed in 25 kg sacks or 1000 kg bulk containers, and it is designed for direct charging into moulds on biaxial rotational moulding machines. The particle-size distribution is controlled to a top cut near 500 µm, which corresponds to a 35 mesh sieve classification, while the median particle size is typically in the 250–300 µm range. This particle-size window is not a cosmetic attribute: oversized particles heat more slowly and can survive as visible pinholes, whereas an excessive fines fraction raises dusting and can impede powder flow during mould charging. Core polymer properties are determined by ISO 1133-1:2022 for melt-flow rate at 190 °C/2.16 kg and by ISO 1183-1:2019 for density.

Product naming in the Icorene range separates the base grade from the colour pack. The pigmented variant should not be specified by density alone because the colour compound can shift rheology within the manufacturer’s lot tolerance. Actual lot density may vary within a typical tolerance band of ±0.002 g/cm³, while melt-flow rate can vary within ±0.5 g/10 min at the standard load. Incoming inspection commonly confirms melt-flow rate and bulk density because these two measurements detect gross lot substitution or heat-history damage without requiring a full mechanical workshop.

Thermal, Mechanical and Chemical Resistance Baseline

Published data for the Icorene 3545 base resin are more extensive than data for the colour-specific compound; where pigment loading influences a value, the shift remains within normal lot reproducibility for most mechanical properties. A specification review should therefore treat the following table as a nominal baseline rather than a substitute for the batch certificate supplied with each production lot.

Density is the primary indicator of short-chain branching and crystallinity; a value near 0.935 g/cm³ places the material in the medium-density portion of the linear low density polyethylene family. The effect is practical: higher density raises modulus but reduces stress-crack resistance, while lower density improves ESCR but decreases top-load strength. The measured melt-flow rate near 4.0 g/10 min is in the medium-flow window for rotational moulding, high enough to promote particle coalescence at lower peak internal air temperature, yet low enough to retain melt integrity during the long unpressurised bake. If the flow rate is too high, the molten wall can pull away from sharp mould features under gravity; if too low, the powder may not fully densify without excessive oven time. Tensile yield strength and flexural modulus follow from the density and short-chain branching distribution; the reported values support load-bearing service at ambient temperature but do not eliminate creep under continuous load. The Charpy notched impact result at 23 °C is a single-point ranking value; low-temperature impact should be requested from the manufacturer if the part will be handled or struck at sub-zero conditions. Environmental stress crack resistance is the most important ranking for chemical containment because it separates materials that survive intermittent detergent or surfactant exposure from those that fail by brittle cracking after short service. The >1000 h result under ASTM D1693-15, condition A, is a comparative index, not a direct extrapolation to a specific chemical cargo. The condition A test involves a notched bent specimen immersed in 100% Igepal at 50 °C, with the reported value corresponding to 50% specimen failure.

Because rotational moulding does not apply melt pressure, the viscosity response of the polymer in the zero-shear region controls bubble removal and inner-surface finish. The melt-flow rate is a practical release test, but it does not capture the full sintering curve; a lot with acceptable MFR can still behave differently if its particle shape or stabiliser system shifts. Consequently, processors often qualify a new lot by producing a thin-wall cylinder and inspecting the inner surface under 20× magnification for pinholes and particle ghosts.

In shuttle-type rotational moulding machines fitted with peak internal air temperature sensors, the powder charges are processed with an oven air temperature between 280 °C and 315 °C. The densification sequence is governed by particle sintering at the mould wall, coalescence into a continuous melt, and subsequent removal of trapped air from the inner surface. The peak internal air temperature target for this grade is 190–205 °C; below 190 °C, the inner free surface retains residual particle boundaries that appear as pinholes or hairline porosity after demoulding. Above 205 °C, the unpressurised melt begins to undergo more rapid thermo-oxidative chain scission, generating odour, surface greasiness, and progressive darkening of the grey surface. The degradation chemistry is not linear: once the stabiliser is consumed, oxidation accelerates sharply, so a moulding that appears satisfactory at 210 °C can still fail subsequent ESCR or impact testing because of molecular-weight loss.

Cooling after the bake is as critical as heating. Forced-air cooling is normally used from the oven exit until the mould temperature falls below 100 °C. Water-mist or spray cooling may then be introduced, followed by a full water quench only after the part has cooled sufficiently to maintain dimensional control. Early water contact can lock in residual stress at thick-to-thin transitions, producing warpage in long flat panels or dimensional drift after installation. A common field issue on multi-arm machines is uneven airflow across the mould, which creates side-to-side temperature differences before demoulding; this is not a polymer deficiency but a fixture-level variable that must be mapped when changing moulds. Rotational ratio is typically maintained between 4:1 and 5:1, and the exact setting is matched to cavity geometry; for deep cylindrical tanks, the minor-axis speed determines the wall thickness distribution along the sidewall.

Typical part configurations for this grade include industrial chemical storage tanks with wall thicknesses from 4 mm to 12 mm, agricultural hoppers, outdoor electrical enclosures, and grey marine buoyancy components. The powder form allows uniform wall distribution in complex double-wall or multi-cavity tools where pelleted thermoplastics would require extensive grinding. For each application, the wall thickness and mould surface area must be matched to the oven heat uptake and the cooling capacity of the machine; large flat panels with high surface-area-to-volume ratios cool faster than thick bosses, and tooling inserts should be designed to avoid heat-sink effects that produce localised under-sintering.

What Process Control Limits Apply When the Grey Pigment Package Is Present?

Pigment systems used in rotational moulding powders can alter the rheology and thermal response of the powder. The grey tint in Icorene 3545 GRY 7407 is precompounded, so there is no free pigment dust at the charger; however, the pigment phase can nucleate crystallisation and slightly increase the low-temperature modulus of the part. The practical effect is usually limited to a shift of 2–4 °C in the observed peak internal air temperature needed to achieve a smooth inner surface compared with natural unpigmented powder, but published data for this specific colour code are limited. The tinted material may also absorb infrared radiation differently; a dark grey moulding in a steel mould can run hotter at the surface than a white natural part in the same oven. On production lines with infrared oven panels, operators should monitor mould-surface temperatures with contact or optical sensors rather than relying solely on oven air temperature. If the surface temperature exceeds 315 °C for periods longer than a few minutes, the stabiliser depletion rate increases and the grey surface may shift toward a bronze hue before mechanical failure is detected.

Post-processing additive blending with the precompounded powder should be avoided unless the additive system has been validated for dry-blend uniformity. Powder tumble blending of liquid antistats, silicones, or amine-based antistatic agents can create uneven surface gloss or interfere with grey pigment dispersion. If internal release is required, the release system should be evaluated in a plaque trial because some release chemistries can exude to the surface and alter the appearance of the grey finish. On production-scale shuttle machines, lot-to-lot variation in grey intensity is usually controlled tightly, but retaining samples from each lot for colour-matching inspection under D65 illumination remains prudent because multiple pigment lots can affect appearance even when mechanical properties remain within specification.

Oxidative induction time testing, often performed under oxygen at 200 °C by a method such as ISO 11357-6, provides a comparative measure of remaining stabiliser after processing. For rotational moulding grades, a post-bake oxidative induction time below 10 min may indicate stabiliser exhaustion and an increased risk of early field cracking. Because the grey pigment can interfere with simple colour-based degradation markers, oxidative induction time and carbonyl index measurements are preferred for release testing of precoloured compounds.

Failure analysis of rotationally moulded parts made from this grade commonly identifies four causes: incomplete densification at low peak internal air temperature, oxidative embrittlement at high oven temperature or long cycle time, contaminated mould-release chemistry, and moisture condensation on cold powder. Incomplete densification is characterized by pinholes or an irregular inner surface; the corrective action is not to increase oven air temperature alone but to verify that the peak internal air temperature reaches at least 190 °C throughout the cavity. Oxidative embrittlement is often detected only after impact testing at the weld line or after an outdoor exposure cycle; the ESCR value can drop by an order of magnitude before colour change becomes obvious. Mould-release contamination appears as non-uniform gloss or delamination at the outer surface, and it is usually traced to silicone or amine-based sprays that were not evaluated with the grey compound. Moisture-induced surface defects are typically elongated pinholes near the coldest mould regions, and they are prevented by storing powder in sealed containers below 60 % RH and allowing the material to reach at least 15 °C before charging.

Unlike high-pressure LDPE rotational moulding powders, the linear low density polyethylene backbone of Icorene 3545 GRY 7407 gives higher environmental stress crack resistance and tensile yield without a proportional increase in stiffness. In terms of ESCR ranking, an LDPE part of similar melt index may show failure before 100 h under the same Igepal condition, whereas the LLDPE value cited above exceeds 1000 h. This difference is one reason detergent and chemical tanks are increasingly moulded from LLDPE rather than LDPE. Compared with HDPE rotational moulding grades, this product has lower flexural modulus and lower heat-deflection performance but significantly better low-temperature crack resistance. HDPE may be selected for heavy structural bases or high-temperature liners, while the Icorene grade is appropriate for walls and covers where ESCR and impact toughness govern field life. Metallocene LLDPE rotational moulding grades can provide a narrower melting range, higher dart impact, and improved organoleptic properties, but they often demand tighter oven control because of their sharper sintering response and higher price. The Icorene platform is therefore specified for conventional rotomoulding lines that require a wider processing window and established long-term weathering data.

When Replacing Natural-Powder LLDPE in UV-Exposed Rotational Mouldings

Replacing a natural or white powder with the grey variant eliminates dry colour blending but changes solar load and inspection behaviour. In outdoor service, a grey moulding can reach a higher surface temperature than an equivalent white part under direct sunlight, and this temperature difference is not trivial for creep-sensitive components. A flat roof panel exposed to solar radiation may experience a surface temperature increase of 20–30 °C over ambient air temperature, and the grey tint raises the equilibrium surface temperature relative to white by an amount that depends on solar reflectance and air circulation. Structural designers should not substitute the short-term flexural modulus directly into long-term deflection calculations without applying the same creep modulus and temperature derating factors used for other pigmented polyethylene parts. The weathering stabiliser package in the pretinted powder provides resistance to UV-induced chalking and colour fade, but it does not confer universal outdoor performance; exposure in high-UV desert or coastal environments may still produce surface oxidation, loss of gloss, or gloss non-uniformity. Inspection protocols for grey parts should include surface gloss, chalking, or carbonyl index measurements because colour change alone is less visible in grey than in natural unpigmented polyethylene.

Outdoor enclosures made from this grade should have their outdoor performance validated under the intended exposure site, not solely under accelerated weathering. Accelerated weathering can rank formulations but does not reproduce the thermal cycling, humidity, and biological fouling present in service. Published data for the GRY 7407 tint across all exposure regimes is limited; customers holding multi-year outdoor requirements should request ultraviolet stabilization data for the specific colour compound from the compounder and generate an initial natural-weathering reference set before committing to high-volume production.

Specifying this material for potable water contact requires a written compliance statement for the finished rotational moulded article, because pigment dispersants and the GRY 7407 carrier system can modify extraction behaviour even when the natural base resin carries food-contact listings. The product is not intended for direct immersion in strong oxidizing acids, aromatic hydrocarbons, or fuels at elevated temperature; compatibility must be tested with the actual service fluid under the correct temperature cycle and surface stress. Published data for this specific configuration at continuous service above 60 °C under mechanical load is limited, and long-term creep design should use validated case-specific data rather than short-term tensile values. The powder should not be used in injection moulding or extrusion plastication lines; its particle morphology, sintering-oriented molecular design, and stabiliser package make it unsuitable for high-shear processing. The grade should be stored in dry conditions below 60 % RH and brought to at least 15 °C before processing if it has been kept in unheated storage, because cold powder can condense surface moisture and generate pinholes in the sintered wall.