LyondellBasell Icorene 3560 G GRY 7275 is a linear low-density polyethylene powder supplied for dry-flow rotational molding. The suffix GRY 7275 identifies a pre-compounded gray pigment package rather than an unpigmented natural grade. The material is typically characterized by a density of 0.935 g/cm³ (ISO 1183-1) and a melt flow rate of 5.0 g/10 min at 190°C/2.16 kg (ISO 1133-1). Tensile modulus measured on compression-molded plaques is approximately 600 MPa (ISO 527-2/1B), with yield stress near 17 MPa and elongation at break greater than 600%. These values are screening data; rotomolded parts typically display lower stiffness and impact than compression-molded specimens because cooling-rate-dependent crystallinity and microvoid populations differ. The powder is intended for rotational molding, but lot-specific certificates of analysis should be consulted for exact pigment loading, stabilization package, and particle size distribution.

What Limits Sintering and Bubble Removal in Thin-Wall Parts?

During the heating phase, the powder tumbling inside the mold passes through tackifying and densification stages. Sintering is not complete when the first melt layer forms; trapped air must diffuse outward, and polymer particles must coalesce under capillary pressure. A nominal differential scanning calorimetry melting peak of 126–130°C (ISO 11357-3) means that a peak internal air temperature of 195–205°C is commonly required to remove subsurface bubbles. On a shuttle-type rotational molding machine with a 2.0 mm nominal wall thickness, oven air at 295°C typically brings the part to this peak internal air temperature within 18–22 min. A 6 mm wall may require 28–34 min. If the peak internal air temperature remains below 190°C, residual porosity and pinhole defects occur. Above 210°C, the gray surface may show oxidative gloss loss and a measurable drop in notched low-temperature impact. Cooling then governs morphology. Forced-air cooling at approximately 15–25 K/min through the crystallization range reduces warpage but can retain amorphous material. Slower still-air cooling raises crystallinity and stiffness but increases shrinkage and can introduce internal stress. Although the base LLDPE is not hygroscopic to the same extent as polyamide, condensation on powder stored in uncontrolled warehouses can introduce steam during heating. Pre-drying at 70°C for 2 h is therefore used when relative humidity exposure exceeds 60%.

Powder dry-flow is a separate critical input. The bulk density of the powder is commonly 0.35–0.40 g/cm³ (ISO 60), and the particle size distribution should be checked by sieve analysis. Fractions below 150 µm promote flow into narrow ribs but increase dusting and can reduce bulk density. Fractions above 500 µm slow sintering and leave visible orange-peel melt defects. The grade is typically handled in moisture-resistant bulk bags with a storage limit of 12 months when kept below 40°C. Mold release agents based on zinc stearate or silicone are applied to the cavity; excessive release agent can reduce paint adhesion and weld-line strength.

When the Gray Pigment Package Alters Thermal Absorption and Cycle Time

The GRY 7275 colorant is not inert in the radiative and convective heating environment of a rotational molding oven. Gray and black powders absorb radiant energy more efficiently than natural translucent powders. In air-circulating ovens with infrared contribution, a dark gray rotomolding powder can reach a given peak internal air temperature 5–8 K higher than an unpigmented LLDPE under the same oven setpoint and dwell. Transfer from a natural grade to the gray 7275 variant therefore requires compensation: reducing oven air temperature by 10–15°C or shortening dwell by 2–4 min is common until thermocouple data confirm the target. Failure to adjust can lead to oxidation on exterior surfaces, increased yellowness index, and embrittlement after outdoor weathering. Process monitoring should track peak internal air temperature, oven air temperature, and cooling time. The melt flow rate of a degraded lot increases relative to the virgin powder; a rise above the specification range indicates thermo-oxidative chain scission. For outdoor parts, the gray package contributes opacity but not necessarily weatherability; UV stabilization must be confirmed separately using accelerated weathering data such as ASTM D2565 or ISO 4892-2.

In outdoor equipment and chemical containment applications, the material’s performance profile is governed by impact toughness, environmental stress crack resistance, and chemical permeation. Rotationally molded housings, agricultural tanks, and playground components are produced from LLDPE of this density when low-temperature ductility is more important than maximum stiffness. The ESCR of a 2 mm notched plaque tested under ASTM D1693-B in 10% Igepal at 50°C typically exceeds 500 h for the unpigmented base resin; a pigmented version may be lower if pigment agglomerates act as crack-initiation sites. Chemical resistance is consistent with LLDPE: acceptable resistance to dilute aqueous acids, alkalis, and many polar solvents, but limited resistance to aromatic hydrocarbons, chlorinated solvents, and strong oxidizing acids. Hydrocarbon permeation is higher than HDPE. For fuel or solvent applications, fluorination, sulfonation, or a multilayer barrier is normally required. The grade is not recommended for continuous exposure to diesel, gasoline, or ketones without application-specific permeation and stress-crack testing.

Low-Temperature Impact and Environmental Stress Crack Resistance

Resistance to low-temperature impact is a primary differentiator for this LLDPE relative to HDPE rotational molding powders. The notched Charpy impact at -40°C (ISO 179-1/1eA) on compression-molded samples is typically 10–14 kJ/m²; rotomolded parts may fall to 7–10 kJ/m² depending on cooling rate and part wall thickness. Brittleness temperature by ASTM D746 is usually below -70°C, but this single-point value should not replace part-level drop testing. Instrumented drop-weight tests on rotomolded boxes at -40°C with impact velocity around 4.4 m/s are recommended for packaging applications. ESCR data are sensitive to processing: fast cooling reduces spherulite size and can improve ESCR, while slow cooling increases crystallinity and may reduce stress-crack resistance. The gray 7275 pigment package can lower ESCR if pigment dispersion is poor. Scanning electron microscopy of fracture surfaces from notched creep tests is used to identify agglomerates. The grade is not formulated for sustained stress above 4–6 MPa in aggressive wetting agents because slow crack growth can occur even below yield stress.

A Shift in Stiffness, Permeability, and Creep Versus HDPE and MDPE

Against a standard HDPE rotational molding powder of density 0.945–0.955 g/cm³, Icorene 3560 G GRY 7275 has lower tensile and flexural modulus, higher elongation, better low-temperature impact, and better environmental stress crack resistance. In exchange, it has a lower Vicat softening point, higher gas and hydrocarbon permeability, and lower creep modulus. Against an MDPE of approximately 0.940 g/cm³, the material trades a small reduction in stiffness for improved low-temperature ductility. Compared with higher-MFR LLDPE rotomolding grades of 7–8 g/10 min, the nominal 5.0 g/10 min flow may require slightly higher peak internal air temperature or longer heating to fill fine ribs but supports better impact retention. The following table provides screening comparisons based on compression-molded specimens; these ranges are not a specification.

Because regulatory compliance for a colored LLDPE variant is not automatically transferred from the base resin, the GRY 7275 grade must be evaluated against the final part’s end-use requirements. FDA 21 CFR 177.1520 covers olefin polymers for food contact subject to extractives limitations and end-use restrictions, but the pigment and stabilizer package must be cleared for the intended food type and use temperature. In the European Union, EU 10/2011 requires overall migration testing and specific migration limits for pigments and additives. For potable water contact, approvals such as NSF/ANSI 61 or WRAS are not conferred by the polymer alone; the molded article and any post-treatment must be tested. The presence of the gray 7275 package may also affect recyclability sorting because carbon black-containing grades are often not detectable by near-infrared sorting equipment. The grade is not recommended for use above 60°C under continuous hydrostatic stress without creep-rupture evaluation.