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.