Linear low-density polyethylene (LLDPE) film resins designated FD21HS and FD21HN belong to a producer-specific nomenclature in which F denotes film-grade, 21 identifies a nominal melt flow rate near 2.1 g/10 min under ISO 1133-1:2022, H signals a higher alpha-olefin comonomer such as 1-hexene, and the terminal S or N differentiates the surface additive package. The phrase “LLDPE FD21HS and FD21HN: How Chinese Producers Name Film Resins” describes a practical converter-facing problem because these suffixes are not defined by ISO, ASTM, or GB standards. A technical data sheet and certificate of analysis from the producing entity remain the only authoritative source for exact batch properties. The base polymer is a copolymer of ethylene with an alpha-olefin, typically produced in a low-pressure gas-phase or solution process, with density below 0.925 g/cm³ by ISO 1183-1. Commercial film applications include blown film for industrial packaging, agricultural film, and sealant webs in multilayer laminates, where melt flow rate and density are selected to balance drawability, bubble stability, and mechanical strength.

What Do the Suffix Codes H, S, and N Signal in Chinese LLDPE Film-Grade Naming?

The alphanumeric structure FD21HS and FD21HN follows a common Chinese producer convention: the first letter F is assigned to film-grade resins; the second position D may be a product-series or reactor-train marker rather than a physical property; the numeral 21 is a nominal melt flow rate indicator; and the final suffix carries the subtlest information. In domestic Sinopec-affiliated product lines, H is used for hexene-modified LLDPE, while analogous butene grades may carry a different suffix. S is commonly interpreted as slip, meaning the pellet includes a migratory slip additive such as erucamide or oleamide, often with an anti-block material such as synthetic silica. N is commonly read as no-slip or neutral, meaning the surface-additive package is absent or limited to processing stabilizers. Because the suffix definitions are producer-specific and not governed by a public standard, a converter cannot rely on the letters alone. If the S designation corresponds to a slip package, FD21HS would be expected to develop a lower film-to-film coefficient of friction after additive migration, while FD21HN would retain higher surface friction and may be preferred for printing or laminating. The base melt flow rate by ASTM D1238 and density by ASTM D1505 or ISO 1183-1 may be identical between the two grades; the differentiation lies in surface performance. Published data for exact FD21HS/FD21HN additive compositions is limited outside producer documentation.

Melt Index, Density, and Comonomer Type as Grade-Definition Variables

For a film-grade hexene LLDPE of the FD21HS/FD21HN class, the nominal melt flow rate is determined by ISO 1133-1:2022 at 190°C with 2.16 kg. The nominal 2.1 g/10 min class balances extruder motor load, die lip build-up, and bubble stability: lower melt flow rate grades increase melt pressure and shear heating, while higher melt flow rate grades reduce melt strength and may compromise bubble stability. Density is measured by ISO 1183-1 or ASTM D1505; film-grade LLDPE commonly falls between 0.918 g/cm³ and 0.922 g/cm³. The comonomer type is more important than melt index in determining impact and tear resistance. 1-Hexene introduces C₄ branches along the polyethylene backbone, which increase the probability of tie-molecule formation between adjacent lamellae after crystallization; 1-butene introduces shorter C₂ branches and gives a different property balance. The following representative envelopes compare butene and hexene LLDPE film grades at a nominal melt flow rate of 2.0 g/10 min and density near 0.920 g/cm³. Values are compiled from published LLDPE film data and do not replace producer certificates for FD21HS or FD21HN.

Chinese producers often specify batch-to-batch variation of ±0.2 MFR units and ±0.002 g/cm³ density; converters should verify incoming lots against these limits before extrusion. The improvement in dart drop and tear strength for hexene grades is the main technical reason Chinese producers label H grades separately.

During continuous blown film extrusion of a hexene LLDPE grade such as FD21HS or FD21HN, the processing window is controlled by extruder screw geometry, die gap, blow-up ratio, and air-ring cooling. LLDPE has lower melt tension than low-density polyethylene and therefore requires a narrower die gap, typically 1.4 mm to 2.0 mm, to limit melt fracture and improve gauge uniformity. A blow-up ratio between 2.2:1 and 2.8:1 is common, with a dual-lip air ring operated at high air velocity to increase cooling rate and stabilize the bubble stalk. On a 75 mm grooved-feed single-screw extruder with a 30:1 L/D ratio, barrel zones from 170°C to 210°C and die temperatures from 190°C to 220°C are used for 2.1 g/10 min hexene LLDPE; die-head pressure can range from 250 bar to 380 bar depending on screen-pack condition and backpressure. A grooved-feed section improves solids conveying but increases motor amperage; if a slip/antiblock grade such as FD21HS contains harder additive agglomerates, the screen pack may load faster than with FD21HN. Extended runs of slip-modified grades can produce die-lip deposition after 4–6 h, requiring the line to stop for die cleaning; no-slip or neutral grades generally exhibit lower plate-out but may create higher film-to-film friction downstream. Bubble stability is adjusted through frost line height: a high frost line above 600 mm at high output can increase haze and gauge scatter, while a low frost line below 300 mm can trap moisture and generate microvoids in film stored at high relative humidity. To increase melt strength, converters blend LLDPE with 10–20 wt% LDPE, but this blend changes dart drop impact and tear balance. Published data for exact FD21HS/FD21HN rheology on a specific production line is limited; the safe procedure is to start from the producer’s recommended temperature profile and adjust air-ring pressure, die gap, and screw speed within the observed bubble-stability window.

When a Converter Replaces FD21HN with FD21HS in High-Slip Blown Film

Replacing FD21HN with FD21HS changes the migratory surface additive package rather than the base resin architecture. In a high-slip film, erucamide or oleamide diffuses from the bulk to the film surface over 24–72 h at ambient temperature; the film-to-film coefficient of friction determined by ASTM D1894 can decrease from a no-slip kinetic value above 0.50 to below 0.20 after full migration. On horizontal form-fill-seal packaging machines running at 60 bags/min, film-to-metal friction above 0.30 can cause drag, misfeed, and seal misregistration; therefore slip grades are specified for high-speed packaging. However, the migrated amide layer reduces surface energy and can interfere with corona treatment and lamination. If a FD21HS film is corona treated to 38–42 mN/m by ASTM D2578 and laminated within 24 h, the initial bond may be acceptable; after further migration the bond strength may decrease because the amide layer forms a weak boundary layer. For print-grade or solventless lamination substrates, FD21HN is often used to avoid post-treatment wetting decay. The base resin density and melting range are not altered by the S/N additive switch, but slip agents can increase seal initiation temperature if they bloom onto the seal surface before sealing. The exact comparison must be verified with producer certificates because producer nomenclature is not standardized and N may denote neutral or no-slip with different stabilizer packages.

Dart Drop Impact and Elmendorf Tear Reflect Comonomer Architecture

For LLDPE film, rapid-impact failure is governed by tie-chain concentration, lamellar thickness, and orientation. Hexene comonomer introduces C₄ branches that reduce crystallinity and increase the probability that a single polymer molecule participates in multiple lamellae; this architecture improves impact and tear resistance relative to butene LLDPE of equivalent density and melt flow rate. At 25 µm film thickness, published ranges for hexene LLDPE show dart drop impact F50 values by ASTM D1709 above 120 g and machine-direction Elmendorf tear by ASTM D1922 between 2.5 N and 4.5 N, while butene grades commonly fall between 85 g and 130 g for dart drop and 1.5 N to 2.5 N for machine-direction tear. These values are representative, not batch specifications for FD21HS or FD21HN. Tear resistance is anisotropic in blown film: machine-direction tear is generally lower than transverse-direction tear because of machine-direction orientation. High-stalk extrusion at low blow-up ratio increases machine-direction orientation and may reduce MD tear, whereas higher blow-up ratio improves transverse-direction orientation and may increase MD tear but reduces bubble stability. Additive packages such as slip and anti-block do not materially alter these mechanical properties at typical addition levels below 3000 ppm, although micron-scale agglomerates above 10 µm can initiate micro-tears and reduce dart drop impact. At a density above 0.925 g/cm³, dart drop and tear resistance decline because higher crystallinity reduces tie-chain concentration; below 0.915 g/cm³, film blocking and bubble stickiness increase.

Regulatory assessments for FD21HS and FD21HN in food-contact applications require examination of both the olefin polymer and the migratory surface additives. In the United States, olefin polymers may meet FDA 21 CFR 177.1520 when density and extractable fraction limits are satisfied; slip and anti-block additives must be cleared separately under the appropriate food-contact regulations. In the European Union, plastic food contact materials are governed by Regulation (EU) No 10/2011; overall migration must not exceed 10 mg/dm², and erucamide is subject to a specific migration limit that must be verified using simulants and time-temperature conditions appropriate for the intended use. China applies GB 9685-2016 as the positive list for additives used in food contact materials, with specific migration limits for slip agents and anti-block substances. REACH Regulation (EC) No 1907/2006 imposes registration duties on additives in the EU market; the polymer itself is exempt from registration but not from authorization if a relevant SVHC concern arises. The following compliance matrix summarizes the instruments that apply to a Chinese LLDPE film grade of the FD21HS/FD21HN class.

Because the final S or N additive formulation may not be fully disclosed on the grade name, the converter should request the certificate of analysis, additive declaration, and migration test report. Pre-drying is typically not required for LLDPE at relative humidity below 60%; if resin has been stored in high-humidity conditions, surface moisture can be removed by drying at 80°C for 2 h. The presence of a slip additive does not remove the need for food-contact compliance testing; it adds a specific-migration verification step that must be completed under the intended film thickness and end-use temperature.