Linear low-density polyethylene (LLDPE) is a copolymer of ethylene with an alpha-olefin comonomer, commonly 1-butene, 1-hexene, or 1-octene. The alphanumeric grade code printed on a supplier’s certificate of analysis often reduces three manufacturing variables—nominal density, melt index, and comonomer identity—into a single string. Reading an LLDPE grade code: density, melt index and comonomer in one string is not a globally standardized procedure under a single ISO or ASTM rule. The typical commercial encoding places the decimal increment of density after 0.9 in the first two or three digits, places an abbreviated melt index in the next numeric block, and uses a suffix letter or digit for the comonomer or catalyst package. A code field containing 18 and 10 therefore may decode to 0.918 g/cm³ and 1.0 g/10 min when confirmed against the supplier datasheet and the appropriate test methods. The three fields are not independent: comonomer concentration depresses crystallinity and density, while the melt index is controlled through reactor conditions and often changes with the same catalyst and comonomer environment. Substituting one grade for another solely on the basis of density without reading the comonomer and melt index positions causes process deviations in film extrusion, injection molding, and rotational molding.

The commercial code logic is best treated as a vendor-specific shorthand rather than a specification. Some suppliers use the first digit as a product family, the next two digits as the density increment, and the final digit as a melt index in tenths of a gram per 10 minutes. Others use a two-letter suffix in which B, H, or O denotes 1-butene, 1-hexene, or 1-octene, but the same letter may instead indicate an additive package or catalyst generation. Because no single data format dominates globally, the reader must verify the decoded values against the actual density measured by ASTM D1505-18 or ISO 1183-1:2019 and the melt index measured by ASTM D1238-20 or ISO 1133-1:2022. The code string is useful for rapid warehouse identification, but it does not replace the certificate of analysis for lot release.

What Are the Three Co-Dependent Variables Embedded in the Code?

The density block in an LLDPE grade code is the first variable because it correlates with crystallinity, stiffness, and barrier. The melt index block is the second variable because it defines the flow path in the melt state. The comonomer field is the third variable because it explains how a given density was achieved. A low density of 0.918 g/cm³ can be produced with a high concentration of 1-butene, a moderate concentration of 1-hexene, or a lower concentration of 1-octene, and the resulting tie-molecule population and mechanical toughness are not equivalent even when density and melt index are identical. The grade code therefore is not a quality ranking; it is a co-dependent data string that must be read as a set.

Industrial laboratories confirm these fields using separate methods. Density is determined after conditioning to 23 °C ± 2 °C and 50% ± 10% relative humidity according to ASTM D618-21. Melt index is measured in an extrusion plastometer with a 2.0955 mm diameter die and 8.000 mm length under a piston load of 2.16 kg at 190 °C. Comonomer identification is performed by Fourier transform infrared spectroscopy according to ASTM D5576 or by carbon-13 nuclear magnetic resonance according to ASTM D5017. These three measurements constitute the reference values that the grade code abbreviates. If the code and the measured certificate do not agree, the measured values govern for specification purposes.

Density determination for code compliance uses a gradient column per ASTM D1505-18 or the immersion method per ISO 1183-1:2019. LLDPE densities typically fall between 0.915 g/cm³ and 0.940 g/cm³, with metallocene grades extending downward to approximately 0.904 g/cm³. The two-digit density code normally reads after the decimal: 20 indicates 0.920 g/cm³, 25 indicates 0.925 g/cm³, and 30 indicates 0.930 g/cm³. However, producers occasionally use a three-digit field such as 918 to avoid ambiguity. Density in LLDPE is lowered by short-chain branching from the comonomer; each branch interrupts chain folding and reduces crystalline fraction. As a result, density cannot be interpreted without the comonomer field: an octene-based grade at 0.918 g/cm³ generally requires a lower molar comonomer content than a butene-based grade at the same density.

In production, the density block also sets the expected stiffness and tear balance. A film grade at 0.918 g/cm³ with 1.0 g/10 min melt index is typically used for stretch film or food packaging, whereas a film grade at 0.925 g/cm³ with 0.5 g/10 min may be selected for heavy-duty sacks. These applications are differentiated under ASTM D882-18 tensile properties and ASTM D1709-15A dart drop. The density value in the grade code should never be used alone to infer stiffness, because molecular weight distribution and comonomer distribution also influence the secant modulus. Published data for this specific structure-property relationship is available in supplier technical bulletins and must be confirmed on the target converting line.

When Melt Index Digits Are Read at 190 °C and 2.16 kg

Melt index digits in LLDPE grade codes refer to the melt mass-flow rate measured under ASTM D1238-20 Procedure A or ISO 1133-1:2022 Method A at 190 °C with a 2.16 kg load. A code field of 1.0 or 10 may denote 1.0 g/10 min; a field of 0.5 or 05 may denote 0.5 g/10 min. Since LLDPE is non-Newtonian, the single-point melt index is an inverse viscosity indicator rather than a complete rheological profile. The same melt index can be achieved with different molecular weight distributions, which is why two grades of 1.0 g/10 min may behave differently in blown film bubble stability or injection mold filling. The grade code provides only the single-point value, not the extrusion curve.

On a blown film line, a melt index of 0.5 g/10 min to 2.0 g/10 min is typical for LLDPE film resins, while injection molding grades often range from 20 g/10 min to 100 g/10 min, and rotational molding grades commonly fall between 5 g/10 min and 8 g/10 min. The melt index field therefore acts as a first-pass process selector. A high-MI grade reduces melt pressure and cycle time in injection molding but may lower dart impact and tear strength in film. A low-MI grade improves melt strength and bubble stability but raises extruder backpressure. The reading of the melt index field is reliable only if the laboratory uses the same die and load conditions; a melt flow rate under 5.0 kg or 21.6 kg loads is not interchangeable with the 2.16 kg melt index and must not be substituted when decoding the grade string.

Calibration of the extrusion plastometer follows ASTM D1238-20 die dimensions of 2.0955 mm ± 0.005 mm diameter and 8.000 mm ± 0.025 mm length; the melt temperature is controlled at 190 °C ± 0.2 °C. Some high-flow LLDPE grades require a shorter piston travel interval of 6.35 mm rather than 25.4 mm to maintain timing accuracy. The code abbreviation does not capture this procedural detail, so the certificate of analysis must be consulted for replicate variability. Because a single-point melt index cannot capture the shear-thinning behavior of LLDPE, processors sometimes read the grade code with an additional supplier code for high-load melt index or melt flow ratio. The ratio of high-load melt index at 21.6 kg to 2.16 kg melt index is used as an indirect indicator of molecular weight distribution. A grade string that includes only the 2.16 kg value leaves this information absent. Narrow molecular weight distribution metallocene LLDPE grades often display lower melt strength at the same melt index than broad-distribution Ziegler-Natta grades. This difference cannot be inferred from the density and melt index digits alone; the comonomer and catalyst field must be read in conjunction with the supplier’s rheology curve.

Comonomer identification in an LLDPE grade string is less uniformly encoded than density or melt index. Producers may use the letter B for 1-butene, H for 1-hexene, or O for 1-octene, or they may use a digit that refers to an internal catalyst system. The comonomer field has direct consequences for mechanical performance. At equivalent density and melt index, hexene and octene grades tend to produce a higher dart impact resistance and lower puncture propagation than butene grades because the longer branch length increases the probability of tie-molecule formation between adjacent lamellae. These differences are quantified under ASTM D1709-15A dart drop, ASTM D1922 Elmendorf tear, and ASTM D882-18 tensile elongation. A grade code that omits the comonomer identity should be treated as incomplete for applications requiring low-temperature toughness.

Because the comonomer position in the code is not standardized, a letter that means 1-butene for one producer may denote an additive masterbatch or catalyst type for another. The reader must verify the field against the supplier’s technical data sheet and the lot certificate. Misreading the comonomer field can lead to blending a butene-based LLDPE into an octene-based formulation, altering the dart impact and tear balance even though the density block remains 0.918 g/cm³. Structural verification by ASTM D5017 13C NMR remains the most direct method for resolving comonomer identity when the code is ambiguous. The exact density depression per unit comonomer is material-specific and cannot be read from the grade code alone.

Production-scale blown film lines read the LLDPE code string as an initial setup guide rather than a process replacement. For a 1-hexene grade with 0.918 g/cm³ density and 1.0 g/10 min melt index, a typical monolayer line with a 30:1 L/D grooved-feed extruder and a die gap of 1.5 mm to 2.0 mm may be started with a melt temperature of 190 °C to 205 °C. Deviations beyond ±5 °C from the supplier profile often alter frost line height and bubble stability because the low melt strength of LLDPE is sensitive to temperature and cooling rate. The density block informs the blow-up ratio selection: higher density grades at 0.930 g/cm³ can tolerate a lower blow-up ratio, while lower density grades at 0.918 g/cm³ require higher blow-up ratio and internal bubble cooling to maintain gauge uniformity. These relationships are not visible in the grade string alone; the code only directs the operator to the correct technical data sheet.

LLDPE grade selection on a blown film line is also constrained by melt fracture onset. At comparable melt index, an octene-based LLDPE with longer branches may exhibit a different shear-thinning exponent than a butene-based LLDPE, changing the critical shear rate at the die lip. A 2.0 mm die gap and a die land length of 10 mm to 20 mm are commonly used to reduce shear stress and delay sharkskin. The grade code does not state melt fracture parameters; however, the density and MI positions identify the datasheet that provides the critical shear rate and recommended die temperature. Published data for the exact melt fracture onset is specific to each supplier’s additive package and molecular architecture.

Injection molding of high-flow LLDPE grades at 20 g/10 min to 100 g/10 min uses the density and melt index fields to set barrel temperatures and clamp force. A thin-wall lid grade at 0.925 g/cm³ and 50 g/10 min may require a mold temperature of 10 °C to 25 °C and an injection velocity high enough to fill before the flow channel freezes. Injection molds for LLDPE thin-wall packaging often require vent depths of 0.02 mm to 0.05 mm and polished or textured tool surfaces. The density block in the grade code guides the packing pressure profile: a lower density grade requires a lower packing pressure but may exhibit higher shrinkage after demolding. Shrinkage of LLDPE measured by ASTM D955 or ISO 294-4 typically ranges from 1.0% to 2.5% depending on density, mold temperature, and part thickness. A processor reading only the melt index may underestimate the effect of density on cycle time and dimensional stability.

Rotational molding grades at 5 g/10 min to 8 g/10 min and 0.932 g/cm³ to 0.940 g/cm³ are pulverized to 35 mesh and processed at peak internal air temperature near 200 °C to 220 °C. The comonomer field is critical in this operation because slow cooling rates in rotational molding increase the crystalline fraction and intensify the effect of comonomer type on impact and warpage. Published data for the precise machine settings in these specific configurations is limited; the converter must validate the decoded code fields against the actual mold-filling and impact performance on the target equipment.