Polypropylene vs polyethylene in flexible packaging
Polypropylene vs polyethylene is the first decision in most flexible packaging structures. Both are polyolefins, built from the same two elements, and they look alike on a roll. One structural difference separates them, and almost every practical choice traces back to it.
Polypropylene carries a methyl group on every other carbon of the chain. Polyethylene carries nothing. That single substituent is why one survives a hot fill and the other survives a freezer. Neither does both.
This page compares the two polymers against each other. Each is documented on its own terms elsewhere. The polypropylene page covers tacticity, the copolymer families and where clarity comes from. The polyethylene page covers density, comonomer, melt flow index and surface energy.
One methyl group, two opposite temperature limits
A polyethylene chain is a plain run of CH2 units. Nothing sticks out of it, so the chains pack closely and slide past each other easily. A polypropylene chain has a methyl group hanging off every second carbon. That group is small, but it is in the way.
It has two consequences that pull in opposite directions. The chain is forced into a helix and packs into a more ordered crystal, which pushes the melting point up. The same group also restricts how freely the amorphous chains move. That pushes the temperature at which they stop moving up as well.
So polypropylene gains at the hot end and loses at the cold end, for the same reason. The published melting range for the homopolymer is 160 to 165 °C, against 105 to 115 °C for LDPE. At the cold end the transition for isotactic polypropylene is reported near the freezing point of water. That is why a PP pack can crack coming out of a freezer while a PE pack of the same thickness does not.
Polyethylene does not have one glass transition, and that is not a detail
Look up the glass transition of polypropylene and you get one number. Look up the glass transition of polyethylene and you get an argument that has been running for fifty years.
Gaur and Wunderlich, writing in Macromolecules in 1980, reproduce a histogram of fifty published reports on the glass transition of polyethylene. It does not have one peak. It has three, clustering roughly at 140 to 160 K, 185 to 200 K and 230 to 260 K. In everyday units that is about −133 to −113 °C, −88 to −73 °C and −43 to −13 °C.
The disagreement is not sloppiness. Polyethylene shows several distinct relaxations, and researchers differ on which one deserves the name. Dynamic mechanical work reports loss maxima near −20 °C and near −120 °C. These are assigned to amorphous material sitting in different places between the crystals. Others have argued for a value near −90 °C, and others again for two glass transitions rather than one.
What this means for a specification: do not ask what the glass transition of polyethylene is. Ask whether the film stays ductile at the temperature the pack will actually see. For polyethylene that answer holds well below freezing whichever relaxation you call the glass transition. For polypropylene it does not, and no amount of grade selection moves the transition far.
Polypropylene vs polyethylene: property comparison
The polypropylene vs polyethylene comparison is best read as ranges rather than single values, because both are families rather than single materials.
| Property | Polypropylene film | Polyethylene film | Which one wins |
| Density | 0.90 to 0.92 g/cm³ | 0.910 to 0.940 for LDPE, above 0.940 for HDPE | PP, more area per kilogram |
| Melting point | 160 to 165 °C, homopolymer | 105 to 115 °C for LDPE, higher for LLDPE and HDPE | PP |
| Behaviour below zero | Transition near 0 °C, embrittles | Ductile well below freezing | PE |
| Stiffness | High, particularly BOPP | Low to moderate | PP |
| Clarity and gloss | Excellent | Moderate | PP |
| Moisture barrier | Better at equal thickness | Weaker | PP |
| Oxygen barrier | Poor without a barrier layer | Poor without a barrier layer | Neither |
| Puncture and impact | Moderate | High | PE |
| Sealing window | Narrow in an all-PP structure | Wide | PE |
| Hot tack | Moderate | High with LLDPE and metallocene sealants | PE |
| Recycling code | 5 | 4 for LDPE, 2 for HDPE | Separate streams |
Why a kilogram buys more polypropylene film
Film is bought by weight and used by area. The bridge between the two is density, and it works against polyethylene.
At a given thickness, the area you get from one kilogram is inversely proportional to density. Taking the published ranges above, polypropylene at 0.90 against LDPE at 0.925 gives about 3 per cent more area per kilogram. Against HDPE at 0.96 the gap is closer to 7 per cent.
Worth saying plainly: this is the arithmetic of density alone, at equal thickness. It is not a claim about total cost. Resin price, film gauge and how thin each structure can actually be run all move the answer. They move it by more than three per cent.
Why the two cannot share one recyclable structure
Polypropylene and polyethylene are immiscible. Blended, they separate into phases rather than forming one material. The interface between those phases is where the blend fails. That is why they are collected and reprocessed as separate streams. A laminate that puts a BOPP web over a PE sealant is recyclable in neither.
The way out is to stay inside one family for the whole structure. Both routes exist and both work: all-PP structures and all-PE structures. What each route can claim under the PPWR design for recycling rules is set out in the sustainability section.
Which one a line actually needs
In most cases polypropylene vs polyethylene comes down to three questions, asked in this order.
Does the pack go below zero?
If it does, polyethylene. Polypropylene embrittles near the freezing point and seals crack under handling, which no grade choice fixes.
Does the pack see heat?
Hot filling, retort or a microwave means polypropylene. LDPE softens fifty degrees before polypropylene begins to.
Does the pack have to look sharp on shelf?
Polypropylene, and specifically BOPP. Clarity and gloss are where the ordered crystal pays back.
Is the product heavy, sharp or dusty?
Polyethylene. Puncture resistance and hot tack are what hold a fast vertical fill together.
Frequently asked questions
Is polypropylene stronger than polyethylene?
Not in the way the question usually means. Polypropylene is stiffer and holds its shape better, so it feels stronger in the hand. Polyethylene absorbs impact and resists puncture better. A BOPP pack survives being stacked; a PE pack survives being dropped.
Can a polypropylene film be sealed to a polyethylene film?
Not reliably. The two do not fuse, because they are immiscible and their melting points are around fifty degrees apart. A seal made between them holds by mechanical grip rather than by welding, and it fails under load. Structures that combine the families bond them with an adhesive or a tie layer instead of sealing them together.
Are both approved for food contact in the EU?
Both polymers appear on the authorised list of Regulation (EU) 10/2011, so both can be used in food contact plastics. The approval attaches to the finished material and to the additives in it, not to the polymer name. That is why every structure carries its own declaration of conformity rather than inheriting one.
Why does polypropylene crack in the freezer when polyethylene does not?
Below its glass transition the amorphous part of a polymer stops moving. A material that cannot move absorbs an impact by cracking. For isotactic polypropylene that transition is reported near the freezing point of water. A freezer puts the film on the wrong side of it. Polyethylene stays ductile far below that, whichever of its several relaxations is called the glass transition.
If polypropylene is stiffer and goes further per kilogram, why is polyethylene still the bigger volume?
Because more packs are flexible, sealed fast and handled cold than are stiff, clear and hot filled. Polyethylene also has the wider sealing window, which forgives variation on a line. Its collection stream in the EU is the more established one today.
The choice is made on the product, not on the polymer
Send the product, the temperature it sees and the line it runs on. We come back with a structure, the family it belongs to and what it can claim on recyclability.
Related resources
Polypropylene as a polymer
Tacticity, the glass transition that governs cold behaviour, the homopolymer and copolymer families, and where clarity comes from.
Polyethylene as a polymer
Density, comonomer, melt flow index, additives and surface energy, and how recycled PE differs from virgin.
All-PP structures
The recyclable polypropylene route: BOPP and CPP variants, barrier and sealing behaviour, and where the structure fits.
All-PE structures
The recyclable polyethylene route: LDPE, LLDPE and HDPE variants, the MDO-PE print web and forgiving sealing.