Polyethylene and polypropylene are the two highest-volume polymers in world trade, and the two most often confused at enquiry stage. They are not substitutes for one another, and treating them as such produces material that arrives correct on paper and fails on the line.
The material difference
Polyethylene is more flexible, performs better at low temperature and has good chemical resistance. It divides by density:
- HDPE — linear and dense. Rigid, strong, chemically resistant. Containers, caps, pressure pipe, industrial mouldings.
- LDPE — heavily branched. Soft, clear, easy to process. Flexible film, liners, coatings.
- LLDPE — linear low density. Flexible with higher tensile and tear strength. Stretch film, agricultural film, packaging.
Polypropylene is stiffer, tolerates higher temperatures and has better clarity in its homopolymer form. It divides by composition:
- Homopolymer — propylene alone. Stiff, clear, heat resistant. Rigid packaging, fibre, filament, general moulding.
- Copolymer — ethylene incorporated. Less stiff, markedly better impact resistance especially in cold. Containers, closures, automotive parts.
Where the decision is actually made
Three questions decide it, and none of them are commercial.
What temperature will the part see? PP tolerates heat better; parts that will be hot-filled, autoclaved or used near heat lean toward PP. PE performs better in cold, and parts used at low temperature lean toward PE.
Does it need to bend or hold shape? PE flexes. PP holds. A part that must resist deformation under load generally wants PP; a part that must fold, stretch or absorb impact generally wants PE.
What is the process? Film extrusion, blow moulding, injection moulding and fibre spinning each have their own requirements, and the same polymer family is not equally suited to all of them.
Melt flow index: the variable most often left out
Once the family is chosen, melt flow index decides whether the material will run.
MFI describes how readily the polymer flows when molten. Low MFI means higher viscosity — better for film and pipe extrusion, where melt strength matters. High MFI means easier flow — better for injection moulding, particularly thin-walled parts.
A grade with the right density and the wrong MFI will either fail to fill the mould or fail to hold the melt. The material is not defective; it is simply the wrong grade for that machine. This is the single most common cause of a rejected polymer delivery, and it is entirely avoidable at enquiry stage.
What to state in a polymer enquiry
Five things, in this order:
- Polymer and family — PE (HDPE / LDPE / LLDPE) or PP (homopolymer / copolymer).
- Melt flow index, or the process and part it is for if the MFI is not yet fixed.
- Application and any performance requirement — UV stability, food contact, antistatic.
- Packaging format and volume per shipment, and whether the requirement repeats.
- Discharge port and preferred Incoterm.
An enquiry stating “PP, 500 tonnes” cannot be qualified. An enquiry stating “PP copolymer, MFI 12, injection moulding for cold-chain containers, 25 kg bags, monthly” can be worked on the same day.