LLDPE Grade Codes Decoded: What the Digits in 218WJ, 118WJ and 7042 Tell You

Linear low-density polyethylene is a copolymer of ethylene and a short-chain alpha-olefin, typically butene, hexene or octene. The linear backbone carries controlled short-chain branches that depress crystallinity and density relative to high-density polyethylene. When a converter specifies LLDPE for blown film, cast film or sheet, the grade designation operates as a shorthand for a producer’s formulation and rheology. There is no single ISO or ASTM procedure that assigns the digits in 118WJ, 218WJ or 7042 a universal meaning. The digits are producer-specific and must be read against the technical data sheet and batch certificate of analysis.

For 118WJ and 218WJ, the numeric field can be decoded from published technical data. The first digit separates two melt mass-flow rate classes, while the two-digit segment 18 identifies the nominal density. For 7042, the sequence is a product code rather than a property cipher. This distinction matters in resin replacement and supplier qualification because two grades with identical density and melt mass-flow rate can still differ in molecular weight distribution, comonomer distribution, additive package and regulatory status. The grade code on a sack is a sales nomenclature rather than an engineering specification.

What Does the Numeric Prefix in 118WJ and 218WJ Actually Encode?

Published technical data sheets for these two LLDPE film grades show that the first digit in 118WJ and 218WJ is not arbitrary. It corresponds to the nominal melt mass-flow rate measured at 190°C/2.16 kg and reported as 1.0 g/10 min for 118WJ and 2.0 g/10 min for 218WJ. The two-digit field 18 corresponds to a nominal density of 0.918 g/cm³ at 23°C. The melt mass-flow rate is determined in accordance with ISO 1133-1 or ASTM D1238, using a 2.16 kg load. Density is determined in accordance with ISO 1183-1 or ASTM D1505.

The suffix letters WJ are not governed by a public standard. They identify a producer-specific film product family and additive package. A W or J character should not be assumed to have the same meaning in another supplier’s nomenclature. ISO 1043-1 defines symbols for plastics and feature identifiers, but it does not assign meaning to producer-specific commercial suffix letters. In some sales regions, the same numeric sequence may appear with different letter suffixes to indicate anti-block, slip or heat-stabilised variants.

In the Chinese domestic resin market, 7042 is widely supplied as a DFDA-7042 or equivalent butene-based LLDPE film resin. Published producer data sheets for this grade commonly list a nominal melt mass-flow rate of 2.0 g/10 min and a nominal density of 0.918 g/cm³. The alphanumeric prefix DFDA is not a numeric code; the digits 7, 0, 4 and 2 do not carry the density or melt-flow logic seen in 118WJ and 218WJ. A converter should not treat the first digit 7 as a density indicator or the final digit 2 as a melt-flow indicator. The designation is an internal product code whose technical meaning is fixed only by the manufacturer’s technical service documentation and the lot certificate of analysis. Published data for this specific configuration is limited outside the producer’s technical bulletins, particularly when a converter needs a complete additive disclosure for food-contact or medical packaging. Compliance under FDA 21 CFR 177.1520 or EU Regulation 10/2011 must be supported by a supplier statement; the grade code alone does not convey regulatory clearance.

Because LLDPE is a semicrystalline polymer, a density of 0.918 g/cm³ corresponds to a crystallinity level that is lower than high-density polyethylene but higher than very low-density polyethylene. The melting peak of butene LLDPE is typically in the range 120°C to 126°C, but the exact peak depends on comonomer content and thermal history. The Vicat softening point is commonly below 100°C. Differential scanning calorimetry is performed in accordance with ISO 11357-3 or ASTM D3418, and Vicat softening point is determined by ISO 306 or ASTM D1525. These thermal limits matter in sterile packaging or hot-fill applications. A converter should not use ambient density as the sole predictor of thermal resistance.

When 218WJ Replaces 118WJ on a Blown-Film Line

On a blown-film line with a 55 mm grooved-feed extruder, 30:1 L/D, 250 mm die, 1.8 mm die gap, and 2.5:1 blow-up ratio, a material change from 118WJ to 218WJ changes melt pump behaviour before it changes film density. Both grades are nominally 0.918 g/cm³, but 218WJ has a higher melt mass-flow rate and therefore a lower average molecular weight. The lower molecular weight reduces shear viscosity, die pressure and extruder torque. A line operator may observe lower motor load and may increase screw speed to maintain throughput; however, the reduction in melt strength can destabilise the bubble, especially if the frost line height exceeds 6 die diameters. LLDPE is shear-thinning; apparent viscosity falls as screw speed rises, but the lower molecular weight of 218WJ shifts the onset of sharkskin melt fracture and can alter the air ring cooling balance.

Internal bubble cooling systems and high-velocity air rings can compensate for reduced bubble stability, but the converter should log die pressure, melt temperature and frost line height when switching between 118WJ and 218WJ. A lower die pressure may mask poor melt homogeneity if the screen pack is partially plugged or if regrind levels change. Typical melt temperatures should be maintained between 180°C and 220°C at the die. Temperatures above 240°C accelerate oxidative degradation, particularly if the resin contains unsaturated slip additives or if regrind is added. If bubble instability persists, production lines commonly blend 218WJ with 10–20 wt% high-pressure LDPE to increase melt strength. This blend changes haze, tear balance and seal initiation behaviour and must be tested in accordance with the film property standards listed below.

Capillary rheometry data for 118WJ and 218WJ show that at 190°C and a shear rate of 100 s⁻¹, the steady-shear viscosity of 218WJ is lower than that of 118WJ. The exact ratio depends on molecular weight distribution and should not be estimated from melt mass-flow rate alone. A converter can obtain the viscosity curve using ISO 11443. For LLDPE, the melt flow ratio MFR21.6/MFR2.16 is a rough proxy for molecular weight distribution and shear sensitivity. The high-load melt flow rate should be measured at 190°C/21.6 kg if shear sensitivity is critical to the extrusion setup.

The relationship between melt mass-flow rate and impact toughness is not linear. In butene LLDPE, increasing melt flow rate from 1.0 g/10 min to 2.0 g/10 min reduces the number of high-molecular-weight chains and tie-chain concentration. The loss of tie molecules reduces slow crack resistance and dart impact more than it reduces tensile yield strength. A converter should not specify 218WJ for heavy-duty sacks or thick agricultural film unless the lower impact performance has been validated for the intended end-use.

Film Property Signatures and Downstream Converting Limits

The density equivalence of 118WJ, 218WJ and 7042 does not make them interchangeable in film converting. The molecular weight difference between 118WJ and 218WJ affects orientation, tie-chain density and impact resistance. At equivalent film gauge, a higher melt flow rate butene LLDPE film generally exhibits lower dart impact and lower Elmendorf tear resistance in the machine direction than a lower melt flow rate grade. These differences must be quantified on the actual film line because die gap, blow-up ratio, frost line height and extrusion rate shift the balance of machine-direction and transverse-direction orientation.

Heat-seal initiation temperature is not directly encoded in the grade designation. It depends on short-chain branching distribution, additive bloom and film thermal history. A converter should generate heat-seal curves in accordance with ASTM F1921 or ASTM F88 rather than infer seal behaviour from density alone. Similarly, a 7042 grade with the same nominal melt flow rate and density as 218WJ may have a different comonomer distribution due to catalyst and reactor conditions, which alters hot tack and seal strength.

At 25 µm gauge, a butene LLDPE film with 0.918 g/cm³ density commonly shows a yield strength in the machine direction below 15 MPa and elongation at break above 500%. These values shift with extrusion temperature and draw ratio. Elmendorf tear resistance in butene LLDPE is typically low in the machine direction and high in the transverse direction; the ratio can exceed 1:4 depending on blow-up ratio. The higher melt flow 218WJ may shift this ratio further because lower melt strength reduces machine-direction orientation.

Slip Migration and Antiblock Loading Are Not Encoded in the Suffix Letters

Slip additives such as erucamide and oleamide migrate to the film surface after extrusion. The coefficient of friction of a slip-modified LLDPE film can decrease from its initial value to a plateau after 24 h to 72 h at 23°C. Migration rate depends on additive solubility in the amorphous phase, storage temperature and film crystallinity. Higher storage temperatures accelerate migration but can also increase blocking if the film is wound tightly. Antiblock additives such as synthetic silica or talc reduce film-to-film blocking and increase haze. The balance between slip and antiblock is part of the producer’s additive package; it is not revealed by the numeric grade code alone.

LLDPE is not hygroscopic and normally does not require pre-drying. If film regrind or stored pellets are exposed to condensation at high relative humidity, surface moisture can enter the extruder and create porosity or gauge variation. Storage at high ambient humidity above 80% RH should be avoided unless the resin is in moisture-protected packaging. Incompatible additives such as some amine-based colour concentrates or oxidising agents should not be combined with LLDPE film formulations without a compound stability study because they can decompose antioxidants or react with unsaturated slip additives. Processing temperatures above 240°C can consume phenolic and phosphite stabilisers more rapidly and cause yellowing or gel formation. Multiple extrusion passes or high regrind levels can deplete the antioxidant package and increase gel formation, film appearance defects and extractable levels.

For incoming resin acceptance, the lot certificate of analysis should be treated as the governing document. It reports the actual melt mass-flow rate, density, additive loading and, where applicable, food-contact status. The grade code is a commercial shorthand; it does not replace batch-specific analytical data. Because 218WJ and 118WJ differ in melt rheology, they are not interchangeable in film production without line adjustment. A 7042 grade may share nominal melt flow rate and density with 218WJ but may not share molecular weight distribution, catalyst residue, additive package or regulatory status. The converter should avoid uncontrolled blends of 218WJ and 118WJ in the same hopper unless the blend ratio is fixed and validated by melt flow, film property and seal performance testing.