One of those packaging parts that are seldom noticed unless it doesn't work properly is the packaging film. Typically, a leak or seal failure occurs because the film, container and sealing conditions don't get along. Whether you are a manufacturer or a consumer of food packaging, knowing how seal strength develops and how to select the right packaging film for the product is the difference between a good package and a grievance that costs you a lot of money. You will find this guide easy to follow, as it decomposes the practical variables that are relevant.
What Actually Happens During Heat Sealing
The process of heat sealing is not just melting two surfaces. Heat sealing is a controlled process in which the sealant layer of the film is heated above its softening point but below its melting or degradation temperature, pressed against a suitable surface, and cooled under pressure. The outcome is a molecular entanglement at the interface.
The three parameters that govern this process are temperature, pressure and dwell time. All three interact with one another. Sometimes a lower temperature can be offset by an increase in dwell time but only to a certain extent. If the sealant is underheated, the polymer chains will not interdiffuse, and the seal will be weak or will not peel correctly. Too much heat, however, can lead to too much flow of sealant; can thin the sealant at the seal edge; can cause degradation of sealant, brittle seals and leakers.
The sealant layer itself is usually a lower-melting polymer (such as polyethylene, ethylene-vinyl acetate, or an ionomer) co-extruded or laminated onto a structural layer that provides strength, stiffness and barrier properties. The structural layer doesn't seal; it supports it. It is a bad idea to mix them up because it will not work well.
Seal Strength Is Not One Number
Seal strength is often specified as an average value in grams per 25 mm or newtons per 15 mm, but different applications require different failure modes. A ready meal should have a peelable lid which will separate easily without delamination or tearing. A pouch for carrying liquids should be able to withstand burst pressure. A fresh produce tray requires a seal that will stay in place in the stack, but will open without tearing out the fibers.
In practice there are three measurements. First, the peel strength is the force needed to pull the seal apart at a given angle. The second is burst strength, the internal pressure that the seal can resist. Third, hot tack—the seal strength at the hot state—is important in vertical form-fill-seal operations where the seal is put to the test before it cools. Hot tack can be an issue on a high-speed packaging line, even if a film has a good final peel strength.
Matching Film to Container Substrate
Most seal failures stem from compatibility considerations. An effective sealant for polyethylene trays would not work on polyethylene terephthalate, polypropylene or paperboard. The rule is simple: like seals to like, or the sealant should be specially designed to bond to the substrate.
Typically, a sealant layer of polyethylene provides good, uniform sealing for polyethylene trays. A sealant that melts at a lower temperature than the container is required to prevent deformation of containers made of polypropylene. Ionomer-based sealants are used often for PET and amorphous PET trays, due to their good bondability and peelability. Paperboard and molded fiber are more difficult to seal. The film must bond to a porous, fibrous surface, and the coating or dispersion on the board is crucial to whether a seal will form.
Other compatibility factors include contamination. The presence of grease, moisture, crumbs or powder in the seal area will weaken the seal. Films with thicker sealant layers and higher melt flow can tolerate some contamination; however, no film seal is reliable through a layer of oil. That's why food service applications frequently have a wipe or purge step prior to sealing.
Thickness, Structure, and Barrier Requirements
The other important factor is the structure of the film, which affects seal strength. A thin sealant layer can seal quickly but may be squeezed out under high pressure. A thicker sealant layer offers a larger amount of material to be diffused with the other materials and offers higher contamination tolerance but needs more heat to activate. Coextruded films that have a dedicated sealant layer, a tie layer and a barrier layer provide the greatest control but require narrow sealing windows.
The situation is complicated by requirements of barriers. When a film must exclude oxygen or moisture, the barrier layer is typically separated from the sealant so that it is not damaged during sealing. Films with aluminum foil or metallized layers can seal well, but the seal must be strong enough to prevent delamination between the barrier and sealant layers. Seals on these structures are not always the failure point; rather, the failure point is usually found at the bond between the layers.
Testing and Troubleshooting in Practice
A practical seal strength program begins by sealing a series of samples at various temperatures, using a constant pressure and dwell time, and measuring the peel strength. The curve typically rises to a plateau and then falls. The target operating window may not be the highest point of the plateau, it is actually the center of the plateau, as this allows tolerance for normal variation.
If the seal doesn't work, it gives you a clue as to where it has failed. Low strength, clean peel is indicative of underheating or a dirty seal area. If a seal is broken when it tears the substrate, the seal is over-sealed or using an incompatible sealant. A seal that separates between film layers indicates a coextrusion or adhesive defect. A seal that leaks when cold may be due to an inadequate pressure or very short cooling time.
Xiamen Xiefa Packing Co., Ltd. collaborates with clients in retail, food service, industrial, and hospitality industries to customize heat sealing film with various kinds of trays, cups, bags, and containers. The company has over 50 product design patents, independent mold-making workshop and over 2,000 self-owned SKUs to help with custom printing, structure, material and accessory development, design and even free samples for evaluation. Seal strength is a system property, so the best results are obtained by testing film and container together in line.
Conclusion
Heat sealing film is not a commodity to be chosen by price only. Seal strength is a result of the coupling of temperature, pressure, dwell time, sealant chemistry, substrate, and contamination. When you know what's at play, packaging buyers and users can select films that seal reliably, open reliably, and serve as a barrier through the distribution chain. The best approach is to consider the film and container as one packaging system, using real measurements to determine the sealing window and following the mode of failure when issues do develop.