CPP film: cast polypropylene and what the chill roll decides
CPP film is polypropylene extruded through a flat die and quenched on a cooled roll. It is usually described as polypropylene without orientation. True, and not very useful. The more useful description is that CPP is polypropylene frozen at a chosen rate.
That rate is set by the temperature of the chill roll, and it does more than cool the film. It decides which solid form the polymer takes.
This page covers how cast polypropylene is made and what that does to it. The structure it goes into, where it sits as the sealant against BOPP in an all-PP laminate, is covered separately. The polymer before any processing is on polypropylene.
Cast extrusion is a cooling process, not a shaping one
The melt leaves a slit die, crosses a short air gap, and lands on a water-cooled roll. Nothing stretches it in the transverse direction. It is drawn slightly in the machine direction as it crosses the gap, and then it is frozen.
Two things happen in those few centimetres. The short draw in air starts orienting the melt. Published work shows this affects how crystals nucleate and grow once the film reaches the roll. Then the roll pulls heat out of one side faster than the other. The two faces end up with different structures.
That asymmetry is real and measurable. Work on cast PP sheet reports a skin layer whose thickness depends on the roll temperature. The colder the roll, the thicker the skin and the smaller the spherulites underneath. A CPP film is not the same material on both faces. That is part of why the roll side and the air side seal and print differently.
The chill roll temperature decides which solid form you get
Polypropylene does not have one solid state. Cooled slowly it forms the monoclinic alpha crystal. Cooled fast enough it does not have time, and freezes into a mesophase, an intermediate order that is neither properly crystalline nor amorphous.
Which one you get is set on the line. A study varying only the chill roll temperature found the transition mapped out cleanly.
| Chill roll temperature | What forms in the film | What that gives you |
| Below 40 °C | Mesophase only | Softest, clearest, lowest stiffness |
| Around 60 °C | Mesophase and spherulitic crystal together | The middle ground, and the least predictable |
| Around 80 °C | Fully monoclinic alpha crystal | Stiffest, hazier, most dimensionally stable |
The same body of work finds the chill roll to be the dominant processing parameter. Output rate and line speed change crystallinity only a little. They change transparency more, mainly through surface roughness. Die temperature matters least of the three.
Why CPP film is clear, and why it is not the reason you would guess
The usual explanation for cast film clarity is low crystallinity. That is close but wrong in a way that matters when you are chasing haze on a specification.
Research on cast iPP found haze correlating with the mesophase content rather than with alpha crystallinity. Two films can hold nearly the same total crystallinity and still differ in haze. What scatters light is the size and shape of the domains, and the roughness of the surface. Not how much crystal is present.
Practical version: if a CPP film is hazier than the last lot at the same specification, the first question is not the resin. It is whether the chill roll ran warmer, or whether the surface finish changed. Both move haze without moving the datasheet.
Cast against blown, on the same polymer
Blown film is extruded through a ring die and cooled by air while a bubble stretches it in both directions at once. Same polymer, different history, different film.
| Cast | Blown | |
| Cooling | Contact with a chilled roll, fast and one-sided | Air, slower and from outside |
| Crystal form reported | Alpha only | Alpha plus beta modification |
| Stiffness | Lower, and tunable across a wide range | Higher, around 50% stiffer than the softest cast film |
| Range available | Wide, the roll gives a large tuning window | Narrower, one study found crystallinity moving only between 62 and 65% |
| Clarity | Better | Lower at comparable crystallinity |
| Gauge uniformity | Better | Harder to hold |
The line worth keeping is the last one about range. Cast polypropylene is not simply the softer process. It is the process with the wider window, because the roll temperature is a dial and the bubble is not. That is why sealant grades are cast: the seal behaviour has to be tuned, not accepted.
Which side of the laminate CPP belongs on
Short answer, the inside. Orientation raises the temperature at which a film distorts. That is what you want in a print web and the opposite of what you want in a sealant. BOPP is stretched and CPP is not, so BOPP goes outside and CPP faces the product.
How that pairing behaves as a structure, what it can claim on food contact and which recycling stream it enters, is on monomaterial PP. The equivalent pairing in the polyethylene family, where an oriented PE web replaces the BOPP, is on BOPE film.
Frequently asked questions
Is CPP film the same as BOPP?
No, although both are polypropylene. BOPP is stretched in both directions after extrusion. CPP film is not stretched at all beyond a slight draw in the air gap. The orientation is what separates them. BOPP is stiffer, clearer and more heat resistant. CPP is softer and seals at a lower temperature. In an all-PP laminate they are usually used together, one outside and one inside.
Can CPP film be retorted?
Retort grades exist, and they are a specific family rather than standard CPP. Sterilisation puts the film through steam at temperatures well above what a general purpose sealant tolerates. The grade, the structure and the adhesive all have to be selected for it together. It is specified as a duty, not chosen from a thickness list.
Why does the same CPP specification seal differently between lots?
Most often for two reasons. The film is not identical on both faces, and the chill roll temperature moves the solid form of the polymer. A film quenched cold holds more mesophase and seals earlier. The same nominal grade run on a warmer roll holds more alpha crystal and needs more heat. The datasheet may not show it.
Can CPP be metallised?
Yes, and it is done, although polyester and BOPP are the more common substrates. Metallising CPP keeps the structure inside the polypropylene family. That is the point when the target is a recyclable all-PP pack. How the deposited layer behaves and why pinholes decide the barrier is on metallised film.
Is cast film always thicker than blown?
No. Both cover overlapping thickness ranges and the choice is not made on gauge. It is made on what the film has to do. Cast for sealing, clarity and gauge uniformity. Blown for stiffness and toughness at a given thickness.
Specifying a CPP sealant
Send the product, the filling line and the seal you need to hold. We come back with a grade, a structure and the documentation that goes with it.
Related resources
Monomaterial PP structures
Where CPP sits as the sealant against BOPP: the all-PP route, barrier and sealing behaviour, food contact and recyclability.
Polypropylene as a polymer
Tacticity, the glass transition that governs cold behaviour, the homopolymer and copolymer families, and where clarity comes from.
BOPE film, the PE equivalent
The same logic in the polyethylene family: an oriented web outside, a softer grade inside, and the whole pack in one family.
Polypropylene vs polyethylene
Which family the structure should be in before the film type is chosen: temperature limits, sealing and recycling streams.