In this post, I will focus on what it takes to be a good heat sealing surface resin in a coextruded film and 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. My Sept 5, 2016 Post defines what a usable seal range is based on MST for packaging speed and hot tack, which is very important for vertical form fill and seal packaging.
For OPP films, there has been a great deal of sealant resin development. It was discovered, that if simple copolymers were used as skins, such as ethylene Propylene (EPcopolymer) or butene propylene (PBcopolymer) copolymers, we find that in general the EPcopolymers gives inherently good hot tack but is difficult to obtain lower MST values as the higher ethylene content copolymers were difficult to manufacture. In comparison, the BPcopolymers give low MST values but have little hot tack. This I believe is due to the inherent differences in crystallization of the two different copolymers. It is well known that polyethylene is highly crystalline and has a very rapid crystallization. In comparison, polybutylene has a very complicated two stage crystallization. These differences appear to carry over into the copolymers, though I must admit that I have never experimentally verified this myself, i.e. it is an informed intuition [fancy words for guess?].
As more terpolymers were being commercially produced it was possible to begin to see optimum combinations of comonomer which were useful in improving the sealing performance of coextruded OPP films. After a good bit of experimentation Figures 1 and 2 give a summary of the overall results.
In Figure 1 we see that as the butene concentration in the terpolymer increases at constant ethylene composition that the MST decreases. Increasing the ethylene concentration at fixed butene composition give improved hot tack. This is better shown in Figure 2, where Hot Tack is plotted with increasing butene concentration and is found to decrease with increasing butene concentration. Here again the variation in hot tack with increasing ethylene concentration is also seen.
From Figures 1 and 2 it is easy to understand that for any given film application there will likely be an optimum copolymer or terpolymer composition. For vertical applications depending on the package size and product characteristics, the hot tack could be increased by increasing the ethylene comonomer content at a fixed butene content. Likewise for a horizontal application, a simple PBcopolymer might be adequate with a very low MST or perhaps with a low ethylene content to prevent end seal opening as product is inserted into the forming package.
The challenge of course is to characterize the available resins in terms of comonomer composition and performance in your film design. Composition information can be obtained from resin suppliers or by laboratory measurements and film performance from packaging machine by direct testing.