Key takeaways
- BOPP is biaxially stretched in both the machine and transverse directions to deliver high stiffness, gloss, and a good moisture-vapour barrier, while CPP is quenched on a chill roll for softness, deep-draw thermoformability, and a wide heat-seal window.
- Process drives resin choice: BOPP core layers run high-isotacticity homopolymer at roughly 2.5–4 g/10 min MFI (ASTM D1238 / ISO 1133, 230 °C/2.16 kg), whereas cast lines favour MFI in the 5–9 range for melt-curtain stability and gauge control.
- Sealability is a layer-engineering problem, not a base-resin property — both films rely on random copolymer or terpolymer skins (seal initiation roughly 115–135 °C) to close packages without distorting the stiff homopolymer core.
- BOPP wins on stiffness-to-weight, clarity, and barrier for labels, snack laminates, and overwraps; CPP wins on seal strength, hot-tack, and conformability for retort pouches, lamination plies, and textile/garment bags.
BOPP and CPP are both polypropylene films, and a buyer who treats them as interchangeable will get the spec wrong. They share a feedstock — propylene polymer, HS heading 3902, same melt-flow test bench — but diverge the moment the melt leaves the die. One is biaxially stretched; the other is quenched flat. That single processing fork decides clarity, stiffness, the moisture barrier, the heat-seal window, and ultimately which resin grade lands in your contract.
The naming is literal. BOPP is biaxially-oriented polypropylene: the primary sheet is stretched in both the machine direction (MD) and the transverse direction (TD), typically by ratios on the order of 4–5× in each axis, aligning the polymer chains into a stiff, optically clear, dimensionally stable web. CPP is cast polypropylene: the melt is extruded through a flat die onto a chilled roll and rapidly solidified with no orientation, leaving the chains relaxed and the film soft, conformable, and easy to seal. Same polymer, opposite molecular architecture.
Orientation is the whole story. When BOPP is stretched biaxially, the crystalline structure reorganises and chain alignment drives up the modulus — you get a film that is stiff, glossy, and far stronger per micron than its cast counterpart. That same orientation tightens the structure enough to cut water-vapour transmission, which is why BOPP is a workhorse moisture barrier for dry goods. The trade-off is that oriented chains are locked: the bulk film resists softening and flowing under heat, so its native seal performance is poor.
CPP takes the opposite path. The chill-roll quench freezes a low-orientation, lower-crystallinity structure, so the film stays soft and the chains remain mobile. Under a heat-seal jaw it softens and flows readily, delivering high seal strength and strong hot-tack — the ability to hold a seal while still warm, which matters on fast vertical form-fill-seal lines. CPP also draws deeply without tearing, making it the better thermoforming and lamination ply. What it gives up is stiffness, gloss, and barrier.
Orientation buys you stiffness, clarity, and barrier; the quench buys you seal strength and draw. You cannot maximise both in one monolayer — which is why most real packaging laminates one of each.
The table below sets the two films side by side on the attributes a packaging buyer actually specs against. Treat the figures as indicative ranges for standard grades — exact values shift with grade, gauge, and additive package.
| Attribute | BOPP (biaxially-oriented) | CPP (cast) |
|---|---|---|
| Process | Primary sheet then biaxial stretch (MD + TD) | Flat die onto chill roll, no orientation |
| Stiffness / modulus | High — rigid, crisp hand | Low — soft, drapeable |
| Clarity / gloss | Very high | Good to high (below BOPP) |
| Moisture (WVTR) barrier | Good | Moderate |
| Heat-seal strength | Low in bulk; needs copolymer skin | High |
| Hot-tack | Modest | Strong |
| Thermoformability / draw | Poor (oriented, low draw) | Excellent (deep draw) |
| Typical gauge range | 12–50 micron | 20–100+ micron |
| Typical uses | Labels, snack & confectionery laminates, overwrap, tape, metallised barrier | Sealant web, retort pouches, lamination ply, textile/garment & bakery bags |
This is where many buyers slip — ordering a generic "PP film grade" instead of one qualified for the line. BOPP core layers demand a high-isotacticity homopolymer with tight molecular-weight control, low xylene-solubles, and an MFI typically in the 2.5–4 g/10 min band (ASTM D1238 / ISO 1133 at 230 °C / 2.16 kg). That lower flow gives the melt strength needed to stretch the web 4–5× in both directions at high speed without splitting. A homopolymer optimised for fibre spinning or injection moulding will often not orient cleanly — confirm the grade is explicitly BOPP-qualified. For the mechanics of that number, see our note on reading melt flow index.
Cast PP runs a higher-flowing resin — MFI commonly in the 5–9 g/10 min range — to lay a stable, even melt curtain onto the chill roll and hold gauge across the web. CPP grades are frequently random copolymers (ethylene incorporated into the chain) rather than straight homopolymer, because the copolymer lowers the seal-initiation temperature and softens the film. The homopolymer-versus-copolymer decision is foundational here; we cover it in depth in homopolymer vs copolymer PP.
Sealing, for both films, is a layer-engineering problem rather than a base-resin property. Stiff homopolymer cores will not seal at a useful temperature, so producers co-extrude a lower-melting skin — a random copolymer or a C2/C3/C4 terpolymer — with a seal-initiation temperature usually around 115–135 °C, well below the homopolymer core. That skin closes the package without distorting the structural layer. When you brief a converter, specify the seal-initiation temperature you need, not just "sealable".
- BOPP core: high-isotacticity homopolymer, MFI ~2.5–4 g/10 min, low xylene-solubles, BOPP-qualified.
- BOPP skin: random copolymer or terpolymer for seal/slip; metallisable or coatable variants for barrier.
- CPP base: random copolymer, MFI ~5–9 g/10 min for melt-curtain stability and gauge control.
- CPP retort: dedicated high-temperature grade rated for ~121–135 °C sterilisation, with the right additive package.
- Both: match additive systems (slip, antiblock, antistat) to end use and confirm food-contact status early.
Most polymer film ends up against food, so food-contact status is a gating spec, not an afterthought. For US-bound product, confirm the resin and its additives meet the relevant FDA 21 CFR provisions; for the EU, the food-contact framework regulation and the plastics measure apply. Get the documentation at the resin-selection stage — retrofitting compliance after a grade is locked is expensive. Our guide to food-contact polymers walks through what to demand from suppliers, and any incoming lot should be checked against its certificate of analysis.
The practical takeaway: do not choose between BOPP and CPP as if it were either/or — choose the structure. A printed BOPP outer web laminated to a CPP sealant ply is one of the most common flexible-packaging builds precisely because it captures the stiffness, gloss, and barrier of orientation on the outside and the seal strength and hot-tack of the cast film on the inside. Spec the resin to the process for each layer, lock the seal-initiation temperature and food-contact documentation up front, and you avoid the costly re-qualification loop. When you are ready to source qualified film-grade PP, the OmniaStrata polypropylene desk can match grade, MFI, and compliance to your line — or talk to us directly.
Frequently asked
Questions on the desk
What is the difference between BOPP and CPP film?
BOPP (biaxially-oriented polypropylene) is stretched in both the machine and transverse directions after extrusion, which aligns the polymer chains and produces a stiff, glossy, dimensionally stable film with a good moisture barrier. CPP (cast polypropylene) is extruded through a flat die onto a chilled roll with no orientation, giving a softer, more flexible film with superior heat-seal strength, hot-tack, and deep-draw thermoformability. The processing route — orientation versus rapid quench — is the root of every performance difference.
Which PP resin grade should I buy for a BOPP line?
BOPP core layers use a high-isotacticity homopolymer with MFI around 2.5–4 g/10 min (ASTM D1238 / ISO 1133, 230 °C/2.16 kg) and low xylene-solubles for clean, break-free stretching at high line speeds. Skin layers that need sealing use random copolymer or C2/C3/C4 terpolymer grades. Always confirm the grade is qualified for biaxial orientation — not every fibre or moulding homopolymer will stretch cleanly.
Why does CPP seal better than BOPP?
CPP is not oriented, so its polymer chains retain mobility and the film softens and flows readily under a sealing jaw, producing high seal strength and strong hot-tack. BOPP's biaxial orientation locks the chains in place, raising the effective melting threshold of the bulk film and narrowing its seal window — which is why BOPP almost always carries a lower-melting copolymer skin to achieve a usable seal. For demanding seals such as retort or liquid pouches, CPP is the default sealant ply.
Can BOPP and CPP be used in the same laminate?
Yes, and they frequently are. A common flexible-packaging structure laminates a printed BOPP outer web — for stiffness, gloss, and print fidelity — to a CPP sealant web that provides the heat-seal and hot-tack performance. The BOPP carries the graphics and structure; the CPP closes the pack. Pairing them captures the strengths of both processes in one specification.
Is CPP suitable for retort and hot-fill packaging?
Retort-grade CPP is one of the standard sealant films for pouches processed at roughly 121–135 °C, because cast PP retains seal integrity and softness at elevated temperatures better than oriented films. Specify a dedicated retort CPP grade with the right copolymer chemistry and additive package, and verify food-contact compliance — for US shipments under FDA 21 CFR — before committing volume.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.