Heat Sealable BOPP Film: One Side or Two? And What Temperature Actually Seals?

The Seal That Holds Everything Together

Walk through any snack food packaging line and you will see the same critical moment repeated thousands of times per hour: two heated jaws close on a film tube, press for a fraction of a second, and open to release a perfectly sealed pouch. The seal must hold through distribution, resist grease and moisture, and open cleanly when the consumer pulls the two sides apart. It sounds simple. It is not. And at the center of it all is a thin layer of copolymer polypropylene that melts at the right temperature, bonds under the right pressure, and solidifies at the right speed.

Heat sealable BOPP film is not a single product — it is a family of films defined by which side seals, at what temperature, and with what strength. Understanding the differences is the difference between a packaging line that runs at full speed and one that stops every twenty minutes for seal failures.

The Temperature Gap That Makes Sealing Possible

Standard BOPP film is made from homopolymer polypropylene with a melting point around 160–165°C. If you tried to heat-seal this film to itself, you would need to bring the entire film thickness above 160°C. By the time the interface was hot enough to bond, the rest of the film would have shrunk, distorted, or melted through. The film would fail long before the seal formed.

The solution is a co-extruded sealant layer made from a copolymer of propylene and ethylene. Adding a small percentage of ethylene (typically 3–7%) disrupts the regularity of the polypropylene crystal structure, lowering the melting point to 125–140°C depending on the ethylene content. This 30°C gap between the sealant melt temperature and the core melt temperature creates a processing window: the sealant layer melts and flows under heat and pressure, forming a bond, while the core layer stays solid and maintains the film dimensions.

This is why the seal temperature specification on a datasheet is not just a number — it is the entire operating philosophy of the film. Set your sealing jaws too low, and the sealant layer never reaches its melt point: you get a weak seal or no seal at all. Set them too high, and you intrude into the core layer’s softening range: the film shrinks, wrinkles, or the seal bar sticks and tears the film on opening.

Single-Side vs. Double-Side Sealable: Matching the Film to the Job

Single-side heat sealable BOPP has the sealant layer on one surface only, with the reverse side being a standard non-sealable surface — often corona-treated for printing or lamination. This is the most common configuration for horizontal form-fill-seal (HFFS) overwrap applications: cigarette cartons, CD and DVD wraps, box overwrap for tea and confectionery. In these applications, you only need to seal film-to-film on the inside surface. The outside stays non-sealable to prevent sticking during transport and display.

Double-side heat sealable BOPP carries the sealant layer on both surfaces. This is essential for vertical form-fill-seal (VFFS) pillow pouches used for snacks, pasta, rice, and frozen foods. The back seal of the pouch requires film-inside to film-outside bonding — both surfaces must be sealable. The end seals also bond the inner surfaces of the pouch front and back, which again requires sealant on the inside. Double-side sealable film is the standard workhorse of the snack packaging industry.

There is also a third, less common variant: heat sealable on one side with a cold-seal release coating on the other. This is used for heat-sensitive products like chocolate bars, where the cold seal adhesive does the bonding at room temperature and the heat sealable side acts as the release surface during unwinding.

Reading the Heat Seal Curve: Strength, Temperature, and Dwell Time

Every heat sealable BOPP film comes with a heat seal strength curve — a graph showing seal strength (in N/15mm) versus sealing jaw temperature at a fixed dwell time and pressure. The curve has a characteristic shape: a steep rise as the sealant layer begins to melt and flow, a broad plateau where seal strength is stable, and then a decline as the film begins to shrink and distort at excessive temperatures.

The plateau is your operating window. A typical copolymer sealant layer begins sealing around 115–120°C, reaches full strength at 125–135°C, and maintains that strength up to about 150–155°C before thermal shrinkage starts to degrade the bond. The width of this window — roughly 20–30°C — determines how forgiving the film is on a production line. Wider windows tolerate temperature variation from jaw heating inconsistencies, line speed fluctuations, and ambient temperature changes. Narrower windows demand tighter process control.

Dwell time is the silent variable. Typical sealing dwell times range from 0.3 to 1.0 seconds on high-speed snack lines. Halving the dwell time does not halve the seal strength — the relationship is non-linear. Below a critical minimum dwell time, the heat does not penetrate the sealant layer to the interface, and the seal fails regardless of temperature. Above that threshold, additional dwell time adds marginal strength. In our experience working with converters on seal optimization, the single most common discovery is that their dwell time was 30–50% longer than needed — which means they were running their entire line slower than necessary. Finding the minimum effective dwell time for your film and machine is one of the highest-ROI optimization exercises in a packaging operation.

Common Seal Failures and What They Tell You

A seal that peels open easily with a clean separation at the interface is an under-temperature seal. The sealant layer never fully melted and interdiffused. Solution: raise jaw temperature in small increments or increase dwell time.

A seal that tears the film rather than peeling open is an over-temperature seal or a dwell-time issue. The bond is stronger than the film itself — which sounds desirable but is actually a problem because the consumer cannot open the package cleanly. Solution: reduce temperature slightly or switch to an easy-peel sealant formulation.

A seal that holds initially but fails hours later, especially in the presence of oily products, is a contamination issue. Grease and oil from the product migrate to the seal area during filling, contaminating the sealant surface before the jaws close. The solution is often a sealant layer specifically formulated for hot-fill or oily product resistance, or a design change that prevents product contact with the seal area. Our technical team spends more time helping customers diagnose seal failures than any other single issue — which tells you how universal these problems are across the packaging industry.

Frequently Asked Questions

• What temperature does heat sealable BOPP film seal at?

The copolymer sealant layer typically begins sealing at 115–120°C, reaches full strength at 125–135°C. The core homopolymer PP layer melts at 160–165°C, providing a 30°C processing window.

• What is the difference between single-side and double-side sealable BOPP?

Single-side has sealant on one surface only, used for HFFS overwrap where only inside-to-inside sealing is needed. Double-side has sealant on both surfaces, required for VFFS pouch making where inside-to-outside back seals and inside-to-inside end seals are both needed.

• Can you print on the sealable side of BOPP film?

Generally not recommended. The sealant layer has a lower surface energy and different surface chemistry than the corona-treated print side. Ink adhesion on the sealant surface is poor. Print on the treated, non-sealable side.

• What seal strength can I expect from BOPP heat seal film?

For standard copolymer sealant BOPP, typical seal strength is 2.5–4.5 N/15mm at the optimal sealing temperature. Easy-peel formulations are designed for 1.5–3.0 N/15mm. Higher strengths are achievable with specialty sealant resins.

• Does BOPP heat seal film work with all packaging machines?

Mostly yes, but seal parameters (temperature, dwell time, pressure) must be tuned to the specific machine. Horizontal and vertical FFS machines have different heat transfer characteristics. Always run a seal optimization trial on the target machine.

中文译文

热封型BOPP薄膜：单面封还是双面封？什么温度才能封住？

标准BOPP由均聚聚丙烯制成，熔点约160–165°C。热封此膜需整层加热到160°C以上，等界面够热时膜的其他部分早已收缩变形甚至熔穿。解决方案是共挤热封层：丙烯-乙烯共聚物，少量乙烯（3–7%）打乱PP晶体结构规整性，熔点降到125–140°C。热封层和芯层30°C温差形成加工窗口：热封层融化流动粘结，芯层保持固态维持尺寸。

单面热封型BOPP仅一面有热封层，反面经电晕处理用于印刷或复合。这是HFFS外包装最常见配置：香烟条盒、CD/DVD外包装、茶叶糖果盒外包装。双面热封型BOPP两面都有热封层，是VFFS枕式袋必需品 — 背封需要膜内侧对膜外侧粘结，两端封也需要内侧对内侧。第三种较少见：一面热封一面冷封离型涂层，用于温度敏感产品如巧克力棒。

每卷热封型BOPP附热封强度曲线：封口强度(N/15mm) vs 封口夹温度。曲线特征：热封层融化流动时陡升，强度稳定平台期，温度过高膜收缩变形导致下降。平台期就是操作窗口 — 约20–30°C宽，决定膜在产线上的宽容度。保压时间是沉默变量：高速线0.3–1.0秒。根据我们协助加工商做封口优化的经验，最常见的发现是他们的保压时间比需要的长了30–50% — 意味着整条线跑得比应有的速度慢。找到最低有效保压时间是包装作业中投资回报率最高的优化之一。

封口轻易剥离且界面处干净分离 = 温度不足。封口撕裂薄膜而非剥离 = 温度过高。封口最初牢固但数小时后失效（尤其含油产品）= 污染问题。我们的技术团队花在帮助客户诊断封口故障上的时间超过任何其他单项问题 — 这说明这些问题的普遍性有多高。