Converting Magazine

Eldridge M. Mount

The impact of polymer density on substrates
April 3, 2008

When designing a modern coextruded film you are combining polymers to obtain a set of properties that you cannot obtain from a single polymer. That of course is because no single polymer has all of the desirable properties necessary for a modern film. Several polymers are fairly capable and supply a lot of properties, such as Polyvinylidene chloride (PVDC) which has fairly good mechanical properties and the rare combination of good moisture and oxygen barrier, a rare combination. However, it suffers from a very high density, about 1.6 gm/cc, one of the highest densities for a polymer. What is the problem with this? Well it means that the film yield (area/weight) is low, i.e. you get fewer square inches of film for a pound of polymer and while film is sold by the pound it is usually used by the square inch or coverage area!

This is the classic trade off in properties verses film yield. This is what won the war which raged between between cellophane and oriented polypropylene. In the early days of film substrates there were not enough various polymers to permit effective coextrusion and good polymers like PVDC were hard to extrude and had low yield. But the film substrates had limited properties, in particular barrier properties. This required that the film carry other polymers to add the desired properties. Usually these other polymers were added by coating. 

Cellophane was one of the first effective film substrates, it is mad by dissolving wood and then precipitating it into a film. Because it is cellulose it burns before it melts so is not heat sealable. Also, because it is made from wood it is very sensitive to moisture and has to be softened with plasticizer to make it flexible and coated to give heat sealing and moisture and oxygen barrier. So cellophane developed the infrastructure for producing coated films from limited substrates. But the density of cellophane is greater than 1 gm/cc so it sinks in water.

Along comes PET and it is an easy replacement for cellophane and needs all of the coating technology that cellophane and has similar yields due to its density of 1.3 gm/cc. So while it is functional it may have some competitive problems due to the young age of orientation when it was introduced and the overall cost to manufacture it. But once orientation was available, then other polymers were examined with orientation but many suffered from low melting points such as the polyethylene’s. Then in the 1960s polypropylene was becoming available (but no copolymers for coextrusion!) and was found to work well with orientation. Still the OPP needed to be coated for heat sealing and oxygen barrier but it came with a fairly good moisture barrier and unlike PET and cellophane was not affected by moisture. 

But even better it had a density of 0.91 gm/cc and a relatively high melting point of 165 C. This low density, one of the lowest for a polymer, gives OPP a huge yield advantage relative to cellophane and with volume growth it became progressively less expensive to produce. Initially OPP was hard to use because it is not as strong as cellophane or polyester, so the machines for using films had to be modified for lower tensions and better temperature control, to prevent shrinkage of the OPP film etc during sealing.

Even with these “limitations” the high yield and low cost of resins and manufacturing cost of OPP drove the machine modifications and once new machines were designed they were designed to easily handle the OPP films and its sensitivities to high tensions and high temperatures.  Once this happened it was just a matter of time before cellophane was replaced. In my next posting I will discuss the impact of replacing the OPP coatings with coextruded copolymers.

Posted by Eldridge M. Mount on April 3, 2008 | Comments (0)