Which Polymer for Which Application? A Process-Based Guide to Choosing PP and PE

Choosing the right raw material depends as much on which machine and which method you will process it with as on the end product's service conditions. Two grades from the same polymer family can give a flawless result in one process and an unusable one in another, simply because their melt flow rate (MFI) differs.

The general rule: a low MFI (high molecular weight) is preferred in processes where the melt has to carry its own weight — pipe, blow molding, blown film. A high MFI stands out in injection molding, where the melt has to fill the mold quickly, and in fiber production, where fine filaments are drawn.

Important note: the MFI of PP is measured at 230°C/2.16 kg and that of PE at 190°C/2.16 kg. The MFI values of a PP grade and a PE grade should therefore not be compared directly (see the 'What Is MFI' article). All values below are in g/10 min.

Blown film (general packaging, shrink, greenhouse film, heavy-duty sacks): LDPE is the most common choice. In our catalog, G03-5 with an MFI of 0.30 for shrink, greenhouse and heavy-duty sack film, and F2-12 with an MFI of 2.5 for food packaging and general-purpose film, are typical examples of this distinction.

Stretch film and thin films requiring high strength: LLDPE stands out in applications such as pallet stretch film because it offers higher tensile and puncture resistance than LDPE. Blending it with LDPE to improve processability is also common in blown film production.

Shopping bags and garbage bags: HDPE film grades are used for thin but tear-resistant film; YY-F00556 and YY-F00756 are grades made for these applications, with an MFI in the 0.05-0.07 range.

BOPP film: biaxially oriented PP film used for packaging and label films requires special homopolymer grades; FH250 (MFI 2.5) is intended for this use.

Products such as woven sacks, big bags, ropes and strapping use homopolymer PP drawn into tape (raffia) or monofilament. These grades have a low-to-medium MFI: MH180 (1.7), MH220 (2.5), MH318 (3.5) and MH418 (4.7) in our catalog are examples of this family.

HDPE can also be used in some monofilament and raffia applications that require higher flexural strength (e.g. YY-S0564).

Because the melt must flow easily when drawing fine fibers, MFI is higher. EH241 (24) and EH102 (11) are used for carpet (BCF), blanket and upholstery yarns; EH251 (24) for spunbond nonwovens for hygiene, agriculture and geotextiles; and EH341 (34) for high-speed lines.

In spunbond grades, a narrow molecular weight distribution (narrow MWD) is important for stable fiber spinning at high line speeds.

Both PP and HDPE are common in crates, buckets, caps, housewares and technical parts. PP stands out with higher heat resistance and the ability to form 'living hinges'; HDPE with better impact strength at low temperatures.

Examples from our catalog: EH082 (7.5) and EH161 (16) on the PP side; YY-I668 (5.5) and YY-I860(UV) (7.5) on the HDPE side for rigid parts such as crates and boxes. For caps of water and still-drink bottles, organoleptic HDPE grades that limit taste and odor transfer (YY-I860(O), YY-I457(O)) are preferred.

High-flow LDPE grades (e.g. I10-19T, I22-19T) are used in applications such as flexible lids, toys and masterbatch.

For parts used outdoors (pallets, crates, garden products, etc.), choosing a UV-stabilized grade slows down sunlight-related aging.

HDPE is the dominant material in hollow products such as detergent and shampoo bottles, food bottles, jerrycans and drums. MFI is low so that the molten tube (parison) does not sag before the mold closes: YY-S0452, YY-S0459 and YY-S0464 (0.36-0.40) for small and medium containers; YY-S00750 (0.076) and YY-S00356 (0.04), with a much lower MFI, for large containers up to 100-200 liters.

For details, see our article 'What Is Blow Molding?'.

Pressure pipes for drinking water and sewage: bimodal HDPE pipe grades (e.g. YY-B00552, MFI 0.05). Special PP types such as PP-R are used for hot and cold water plumbing.

Extrusion coating (paper, board, aluminum foil, woven sack fabric): high-flow grades are needed; H5-21T (4.7) and H7-20T (6.9) on the LDPE side and EC261 (26) and EC281 (28) on the PP side are made for this purpose.

Thermoforming (clear cups, boxes, trays): special homopolymer PP grades for clarity (e.g. MH220N). Wire and cable jacketing: general-purpose LDPE grades (e.g. H2-21T). Rotational molding (large hollow parts such as water tanks): generally LLDPE ground into powder.

Which machine and which method will you use? Will the product come into contact with food? Will it be used outdoors? Are low-temperature, impact or chemical resistance expected? Is clarity important? The answers quickly narrow down the list of candidate grades.

The MFI values in this guide are typical values taken from the manufacturer technical data sheets of the grades in our catalog. For the final choice, we recommend reviewing the current technical data sheet and running a sample trial when switching to a new grade. If you share your application with us, we can determine the right grade together.