However, if we were to form a hot wire seal, all that is necessary is that the film itself melt and form a weld. But these hot wire seals were not generally hermetic and are good for bundling, packaging publications or nonperishable produces or in essence where the absolute film barrier is not as important for the shelf life of the product as the simple act of wrapping it. Much like the shrink wrapping of produce, which is used to contain problems to a single fruit, such as mold, and protect the other produce units. This single unit wrapping is used for preventing the bad apple from spoiling the barrel of apples. Of course these are general statements and there are always some exceptions where the fitness for use of the film is narrower than for a more universal application.
So the early paper, metal and cellophane films were not generally weldable to themselves at all and cans and jars prevailed for long term storage. But as new materials were found, generally polymeric materials they began to replace the natural combinations of wax and paper. This began as WWII was ending. LDPE, which was a strategic war materials during the war was now available to be used for civilian purposes. If you realize that polyethylene is just high molecular weight wax, then it is easy to understand the first large scale use of LDPE after the war, i.e. LDPE extrusion coated paper board which was used for the production of heat sealed boxes for milk. It is at this point that the production of synthetic polymers for packaging begins in earnest as new polymerization catalyst systems are discovered. Up to this point early synthetic polymers were made by condensation reactions which give us nylons, polyesters and urethanes to name a couple, but cannot produce polyolefins such as polyethylene. PVC was produced by free radical polymerization free radical polymerization and LDPE by a high temperature and pressure, free radical polymerization, but this polymerization method will not produce polypropylene or even high density polyethylene.
After the discovery of Ziegler Natta polymerization there was a general explosion of polyolefin polymers which greatly increased to palette from which to produce heat sealable polyolefin films. About this time there was an explosion in coextrusion technology which also facilitated the large scale production of coextruded polyolefin films. This technology evolution in polymerization technology has not been the case with condensation polymers so there are fewer polymers choices for making heat sealable coextruded films with PET or Nylon, both of which are generally limited to coating or laminating technologies to produce heat sealable structures.
In my next Post, I will focus on what it takes to be a good heat sealing surface in a coextruded [or coated] 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.