BOPP / PET / PE / CPP 4 Common Films: How to Choose — 3-Dimensional Comparison of Transparency, Stiffness & Heat Seal Temperature + 6 Common Misuses

Last year, a customer was making sauce pouches and couldn't seal the bag opening. They raised the temperature from 130°C to 160°C, but the seal still wouldn't hold. After investigation, it turned out they were using BOPP film as the inner layer — BOPP's heat-sealing temperature requires above 130°C, and it must be paired with a PE or EVA co-extruded layer to seal. Single-layer BOPP cannot be heat-sealed.

When it comes to the "film" line, the reason customers often choose wrong isn't defects in the material itself, but a failure to understand the "boundaries" of each film type. Over the past year, we've seen cases of misuse across the 4 most commonly used films, which can be grouped into 6 categories — each one carries a real cost.

The Real Boundaries of 4 Films

First, a clear premise: there is no "best film" — only "the film suited to a specific scenario." The judgment is based on 3 dimensions: transparency, stiffness, and heat-seal temperature (corresponding to downstream processing).

BOPP (Biaxially Oriented Polypropylene)

Transparency ★★★★★ Stiffness ★★★★ Heat-seal temperature: Does not support independent heat-sealing

BOPP is an "outer layer material" and does not heat-seal directly. It needs to be laminated with a layer of PE or CPP to achieve sealing. BOPP has high tensile strength, making it suitable for food outer bags, gift bag outer layers, and label base substrates. Common mistake: customers use it as an inner layer for sealing bags, and the bag opening simply won't seal.

PET (Polyester Film)

Transparency ★★★★★ Stiffness ★★★★★ Heat-seal temperature: Does not support independent heat-sealing

PET has even better transparency than BOPP, with higher stiffness (a harder feel), making it suitable for gift box lamination, transparent windows, and high-end cosmetic outer layers. It also cannot be heat-sealed independently and requires lamination. Common mistake: a customer wanted to make a stand-up pouch and chose pure PET for the outer layer, but PET cannot seal to itself — the sealing layer must be PE or CPP.

PE (Polyethylene Film)

Transparency ★★★ Stiffness ★ Heat-seal temperature 120-160°C, sealable

PE is the "softest" film and offers the best heat-sealing performance. However, its transparency is lower than BOPP/PET, and its stiffness is low — it cannot provide a stiff outer-layer effect, so it is mainly used as an inner heat-seal layer. Common mistake: a customer wanted a transparent, stiff gift outer bag and chose PE for the outer layer; the resulting bag was soft and floppy, lacking the "gift feel."

CPP (Cast Polypropylene)

Transparency ★★★★ Stiffness ★★★ Heat-seal temperature 130-160°C, sealable

CPP has good transparency, can be heat-sealed, and is heat-resistant (commonly used in retort pouches) — it is the workhorse for "heat-resistant inner layers." Common mistake: a customer making frozen food packaging (-18°C) used CPP for the inner layer. CPP becomes brittle at low temperatures and cracks easily; frozen food inner layers should use PE or co-extruded film.

3-Dimension Real Comparison Table

Remember 3 key facts from this table:

1. BOPP/PET cannot be heat-sealed independently — a heat-seal layer must be laminated on
2. PE cannot serve as a high-stiffness outer layer — the floppy feel is a structural issue, not a processing issue
3. CPP is not resistant to low temperatures — use PE for frozen applications

6 Common Misuse Cases

Case 1: PE used as outer layer, insufficient stiffness

A customer was making sauce outer bags and wanted transparent material, so they chose single-layer PE bags. The resulting bags were soft and floppy, like shopping bags, lacking the crisp feel of food packaging. Switching to a BOPP/PE laminate — BOPP outer layer with PE inner layer — immediately improved stiffness.

Case 2: CPP used as inner layer, bag cracking in frozen conditions

For frozen dumpling packaging, the customer chose a PET/CPP laminate. After 3 months, customer complaints came in that the bag openings cracked after freezing. Switching to a PET/PE laminate solved the problem. CPP becomes brittle at -18°C, while PE stays flexible at low temperatures.

Case 3: PET used as inner layer in a retort pouch

A customer was making 121°C retort pouches (canned food grade) and chose a PET/PE laminate. PE's temperature resistance is 100°C; after 121°C retorting, the PE layer melted and the bags deformed and were scrapped. Switching to a PET/CPP laminate eliminated the problem at 121°C retort. CPP's temperature resistance is 140°C.

Case 4: BOPP outer layer without corona treatment

BOPP requires corona treatment before printing (surface tension ≥ 38 dyne); otherwise ink adhesion is poor. A customer purchased BOPP film on their own and didn't require the supplier to provide corona treatment — the printed ink rubbed right off. Remediation: add a corona treatment step, or switch to a film supplier that already provides corona-treated film.

Case 5: Recycled PE used, non-compliant for food contact

A customer, trying to cut costs, chose "recycled PE." Recycled PE comes from waste plastics and contains unknown additives — food contact material regulation GB 4806.7 does not allow recycled material to directly contact food. Switching to "virgin food-grade PE" raised the unit price by 30%, but achieved compliance.

Case 6: PE used for cosmetic outer bags, no premium feel

An emerging cosmetics brand was making sample outer bags and chose PE material bags. PE is soft and transparent, but lacks the premium "cosmetics" feel to the touch. Switching to a PET/PE laminate — PET for the outer layer, PE for the inner layer — provided both stiffness and sealability. Unit price increased 25%, and the customer accepted it — the brand premium from packaging upgrades covered the cost.

2 Judgment Rules of Thumb

Rule 1: Outer layer — look at transparency + stiffness; inner layer — look at heat-seal + temperature resistance

The outer layer handles "looking good + stiff" — choose between BOPP and PET. The inner layer handles "sealable + temperature resistant" — choose between PE (room temperature) and CPP (medium-high heat). Laminated together, you get combinations like BOPP/PE, PET/CPP, PET/PE, CPP/PE, etc.

Rule 2: Frozen — use PE; room temperature — use CPP; retort — use CPP

In frozen scenarios (below -18°C), the inner layer must be PE or co-extruded film, because CPP becomes brittle and cracks at low temperatures. In room-temperature scenarios, both CPP and PE work for the inner layer. In retort scenarios (above 100°C), the inner layer must be CPP or PET, because PE cannot handle high temperatures.

3 Procurement Reminders

Reminder 1: Check the material's outgoing inspection report

Film suppliers must provide an outgoing inspection report: thickness tolerance, transparency, heat-seal strength, surface tension, and corona value. Use suppliers with incomplete reports with caution.

Reminder 2: Food contact requires food-grade material

Food contact scenarios must use "food-grade" film, and the factory must hold an SC license or corresponding certification. Recycled material and industrial-grade PE are prohibited for food contact scenarios.

Reminder 3: Laminate pouches require overall compliance

Compliance for laminate pouches must be evaluated "as a whole," not on a single layer. GB 4806.7 requires the entire packaging bag to undergo migration testing — any single non-compliant layer will cause the entire bag to fail. We recommend customers submit the "entire finished bag" for testing, not just one layer of material.

Further reading:

• How is plastic packaging migration actually calculated: 4 common migration tests + which one food/cosmetics/toy companies should run
• Real data on barrier layers in composite materials: oxygen transmission rate comparison across 4 paper-plastic/aluminum-plastic/co-extruded film structures
• 3 real incidents involving corrugated carton moisture content: the full process from rain exposure to cardboard deformation
• GB 4806 food contact packaging testing in practice: 5 commonly failed items + 3 remediation approaches for non-conformance