I will focus here on what it takes to be a good heat-sealing surface in a coextruded [or coated] film for use with a VFFS and HFFS packaging machines, leaving other sealing situations to a later date. In these forms of seal, two adjacent surfaces are forced together under heat and pressure for a fixed time and the inner most surfaces of the films are melted together to form the heat seal.
In general, an inner heat sealing layer for the formation of a flat or crimp seal has to have two main characteristics, the first is the seal initiation temperature (SIT), sometimes called the minimum seal temperature or MST. This is the temperature at which the sealing polymer is molten enough to flow and to melt bond to an adjacent layer in the seal. In addition, the heat sealing layer must melt below the melting point of the rest of the film, so that the sealing film is not distorted, shrunk or melted before the seal can form. So as you can imagine, the heat sealing surface polymers melting point is very important to the formation of a heat sealing film. Figure 1 shows a typical relationship between the heat sealing polymer melting point and the SIT for a three layer coex film. The seal range of any film is defined as the temperature range between the SIT and the upper temperature is determined by the thermal distortion of the total structure such that an unusable package is formed.
FIGURE 1. Crimp seal, Seal Initiation [threshold] Temperature as a function of EP copolymer melting point
But SIT is not enough to form robust packages at high speed. A second important attribute of the sealing resin is that it must hold together as the product is put into the formed bag, even while it is hot. This is called Hot Tack. Hot tack is measured by how far a seal will peal apart when subjected to a load as the seal jaws are released. Just like with heat sealing, where a seal strength vs. temperature curve can be produced, and Hot Tack curves as a function of temperature can also be produced. And for many applications it is the overlap of the seal strength and hot tack profiles which define the true seal range of the film as shown in Figure 2.
Hot tack is a much more important sealing characteristic for VFFS packaging than for HFFS. This is because in HFFS machined the film is gripped and pulled by a lower fin seal and the product is placed into the forming package as it is carried by the fin seal. So there is little to no stress on the end seals when they are formed, usually by a rotary cut off. But in VFFS, the lower seal is formed when the preceding bags top seal is formed and the bag cut and released from the remaining web above. Once the bag is cut free, the seal jaw opens and moves upward to grasp the forming bag, after it is filled and advances the film as it forms the top seal and cuts the filled bag free. During this sequence the product is dumped directly into the bag onto the still hot lower seal. This places a separating force onto the warm seal and it is the hot tack of the sealant that holds the bag together and prevents product from opening the lower seal. So for VFFS operations it is the Hot Tack which is perhaps the most critical heat sealing property. Hot tack appears to be related to the speed of recrystallization of the sealant resin.
In my next post, I will focus on how the polymerization technology can be used to produce polymers with the most desirable combination of minimum seal initiation temperature and hot tack in the heat-sealing surface.