LyondellBasell Icorene 3546 GRY 7735 is supplied as a linear low-density polyethylene (LLDPE) rotational moulding powder. The designation carries the LyondellBasell Icorene 3546 base resin and a pre-compounded gray colorant identified by shade code 7735, eliminating the need for dry-blended pigment masterbatch at the hopper. This configuration is intended for one-step manufacture of hollow parts such as storage tanks, industrial containers, cladding components, and structural panels in which colour uniformity and stabiliser dispersion are required after demoulding. The product is handled as a free-flowing powder rather than a pellet, which imposes specific particle-size, bulk-density, and dry-flow controls that are not relevant to injection-moulding grades of the same polymer family. Published data for this specific shade configuration are limited; therefore, mechanical and thermal values should be read from the current grade certificate and controlled against the base-resin method set: ISO 1133-1:2022, ISO 1183-1:2019, ISO 527-1:2019, ISO 178:2019, ISO 179-1:2010, and ISO 306:2022.

How does dry-flow behaviour govern moulded wall consistency?

In rotational moulding, the powder is not plasticated before mould filling. It is charged into a closed metal tool that is heated and rotated biaxially. Dry-flow time through a standardized funnel is therefore a direct predictor of cycle-start filling uniformity. For LLDPE rotomoulding powders of this density class, pourability measured by ISO 6186:1998 through a 25 mm orifice is typically below 45 s for a 500 g charge; longer values often indicate excessive fines, moisture uptake, or irregular particle shape. A powder with a dry-flow time above 60 s may still be processed, but the operator must compensate by increasing mould pre-rotation time or reducing the charge weight to avoid non-uniform wall thickness at the parting line.

Particle-size distribution is controlled by sieve analysis according to ISO 4610:2001 or ASTM D1921-18. For a general-purpose LLDPE rotomoulding powder, a typical specification retains not more than 5 g/100 g on a 500 µm sieve and not more than 20 g/100 g passing a 125 µm sieve. The coarse fraction delays sintering at the mould surface, while the fine fraction increases dust and raises the risk of agglomerates that persist as pinholes. For gray pre-compounded grades, the pigment dispersion step can shift the effective particle-size distribution; the product should be re-sieved after any pneumatic transfer because fines may segregate into the center of a silo as the powder settles.

Bulk density and tapped density are measured by ISO 60:1977. Rotational moulding powders with bulk densities in the 0.33–0.40 g/cm³ range provide sufficient mass per mould charge without inducing bridging in the feed system. A pre-compounded gray grade may exhibit a bulk-density shift of <0.01 g/cm³ relative to natural powder due to pigment loading, but this shift should be verified by an aluminium cylinder 1000 mL test before changing the charge mass on a production line. When converting from natural to pre-coloured powder, a twin-cone blender or low-shear ribbon blender set to 12–15 rpm is commonly used for pre-conditioning homogeneization; the final dry-flow check is then repeated to separate flow changes caused by particle-size shift from those caused by humidity.

When peak internal air temperature exceeds 220 °C during rotational moulding

Oven air temperature is not a sufficient control parameter. Production-scale machines equipped with in-mould thermocouples should be programmed around peak internal air temperature, not merely burner setpoint. For LLDPE rotational moulding powders, the recommended peak internal air temperature is usually 190–210 °C. If the measured internal air temperature exceeds 220 °C, the moulded part may develop inner-surface oxidation, which appears as yellowish streaks, embrittled weld lines, and a measurable reduction in tensile elongation. A ten-minute hold at 230 °C can reduce the failed part proportion in a batch by improving interparticulate diffusion, but the same dwell may shift the outer surface colour of gray components toward yellow if the antioxidant package has been partially consumed by the pigment compounding step.

The heating phase in a single-carriage shuttle rotational moulder with biaxial rotation ratio of 4:1 typically runs from 8 min to 16 min depending on wall thickness and oven setpoint. A mold surface temperature between 230 °C and 280 °C is required to transfer heat into the powder bed; however, the heat-transfer rate falls when the powder begins to sinter and forms an insulating skin. Venting is a critical production bottleneck: closed molds without PTFE-lined vent tubes retain moisture and degradation gases, leading to porosity at the inner surface. For a 500 kg-capacity rotational moulder, a vent opening of 6 mm diameter per 100 L of internal volume, checked for blockage after each cycle, reduces pinhole defects in thin-wall tanks to <2%. This is an operational boundary, not a property of the polymer alone.

Cooling rate controls crystallinity and warpage. Rapid quench with forced air at 8–10 m/s produces lower crystallinity and higher impact retention but can increase distortion in asymmetric parts; slow cooling in still air produces higher stiffness and higher shrinkage. Gray-pigmented LLDPE parts may cool at a different surface rate than natural parts because the pigment package modifies spectral emissivity in the infrared. The moulder should therefore not transfer cooling settings directly from a natural-powder tool to a gray-powder tool without verifying internal air-temperature decay curves and post-moulding shrinkage on a coordinate measuring machine.

Outdoor service life of gray LLDPE parts is dominated by the interaction between the hindered-amine light stabilizer package, the gray pigment, and the base resin. Accelerated weathering by ISO 4892-2 cycle 1 and ASTM D4329-13 is used to benchmark colour retention and surface microcracking. For non-structural appearance components, a Delta E value below 3.0 after 2000 h of xenon-arc exposure is a common acceptance threshold; for structural parts, tensile impact retention by ISO 8256:2004 should be measured at 0 h, 1000 h, and 2000 h to verify that surface oxidation has not propagated into a loss of load-bearing capacity. Gray shades based on organic pigments may exhibit less surface-temperature rise than carbon-black-filled grades, but they do not provide the same UV-shielding efficiency as a high-load carbon-black concentrate. The selection of this pre-compounded gray grade over a dry-blended carbon-black masterbatch therefore depends on whether the part is appearance- or structural-critical.

Comparative positioning against natural and black LLDPE rotomoulding powders

The principal difference between Icorene 3546 GRY 7735 and an unpigmented LLDPE rotational moulding powder of the same base resin is the elimination of post-moulding colouring and the associated handling of dry pigment masterbatch. A natural powder gives the highest translucency and is often preferred for tanks where internal fluid level visibility is required. A black powder, particularly with 2.0–2.5 wt% carbon black, provides the strongest UV-screening performance and is used for long-service outdoor parts. A gray pre-compounded powder occupies an intermediate position: it commonly provides better colour uniformity than dry-blended masterbatch because the pigment is dispersed during compounding, but its stabiliser demand is higher than a natural grade because the gray shade reflects less ultraviolet light than white but absorbs less than black.

Melt-flow and density differences must be checked when substituting one pre-compounded shade for another. Rotational moulding grades are generally supplied in the 0.935–0.945 g/cm³ density range and 3–6 g/10 min melt flow rate at 190 °C and 2.16 kg load. Higher melt flow promotes faster sintering but lowers melt strength, which can increase sag on vertical walls in thick sections; lower melt flow improves moulded stiffness but may require a 5–8 °C higher peak internal air temperature to reach full surface replication. When changing from a natural grade to a pre-compounded gray grade, the pigment carrier can act as a low-viscosity diluent, so the effective melt flow may deviate from the base-resin certificate. The processor should measure the as-received powder by ISO 1133-1:2022 after compaction moulding, not assume equivalence from the base-resin datasheet.

Thermal and mechanical comparison is made by the same method set. For LLDPE rotomoulding powders, tensile yield stress by ISO 527-1 is typically 14–20 MPa; elongation at break is above 400%; flexural modulus by ISO 178 is in the 500–800 MPa range; and Charpy notched impact by ISO 179-1 at -40 °C is often above 6 kJ/m². These values are class-level and not a replacement for the lot-specific Certificate of Analysis. Pre-compounded pigments can reduce elongation at break by 5–15% relative to natural resin because pigment particles act as local stress concentrators. The result is that a gray part may still meet a 400% elongation criterion but may not retain the same low-temperature impact as the natural equivalent; therefore, the part designer should derate by one service temperature class if no grade-specific impact data are available.

Chemical contact service for gray LLDPE tanks requires verification with the specific stored fluid because environmental stress cracking resistance is influenced by molecular weight, comonomer type, and part processing history. ESCR testing by ASTM D1693-15 in 10% Igepal CO-630 at 50 °C is a severity benchmark, but it does not replace immersion tests in the actual chemical. For a linear low-density polyethylene rotomoulding grade, stress-cracking failures are typically observed as longitudinal cracks at the tank bottom radius, where the combination of wall thickness reduction, residual stress from cooling, and external load is highest. The processor should machine test bars from the sidewall and bottom of a moulded tank rather than from compression-moulded plaques to capture real production variables. If the stored fluid is aggressive, the gray pigment package should be examined for catalytic interaction with the antioxidant system; certain transition-metal pigments can reduce the oxidative induction time measured by ISO 11357-6 under 190 °C oxygen. This limitation requires supplier confirmation before the product is used for chlorinated water, strong oxidizing agents, or diesel-additive blends.

Regulatory constraints do not transfer automatically from base-resin approval.

Polyethylene intended for food-contact use is regulated on the basis of the base polymer and the additives, not on the finished color alone. FDA 21 CFR §177.1520 permits olefin polymers intended for food-contact use, provided the extraction fraction does not exceed the prescribed limits under the specified food-simulant and temperature conditions. The gray pigment package of Icorene 3546 GRY 7735 must be assessed separately because the colorants are not automatically covered by the base-resin clearance. For potable water or food-contact service, the finished fabricator should obtain a compliance letter from LyondellBasell or the final compounder stating that the specific shade 7735 meets the applicable positive list in Regulation (EU) No 10/2011. Without that confirmation, the product should be limited to non-food, non-potable industrial service. The same restriction applies to medical packaging unless USP <661.1> extractables testing is completed on the moulded article.