Converting Magazine

Eldridge M. Mount

Reply to Juans questions on metallized laminations from Dimensional Stability Considerations
March 17, 2008

Juan asked the following series of questions:

 What is the recommended dimensional stability of a metallized film for an extrusion lamination process?

Eldridge replies:  Well I’m not sure that there is a particular specification on film shrinkage for metallization and this is made clear from the general ability to metallize biaxially oriented and cast and blown films .  I remember how surprised I was with the level of success we had with metallizing some shrink film.  One of the things it taught me was that in regards to the thermal impact to the film, the metallization process is somewhat self protecting.  This is I believe in large measure to the IR reflectivity of the aluminum deposit, so as the film is metallized the heat of the boats is reflected and then the primary heat load would seem to be the heat of the aluminum condensation.   This, I believe, is also why both supported and unsupported metallizers work

In general, the “shrinkage” or recoverable strain which is important is that which occurs on the film when it is wound in the metallizer, most of this is due to the tension which is applied to the film in the winding, which is why you can ruin an oriented PET film with excellent dimensional stability but can also successfully metallize cast or blown polyethylene films.   What you need to do is measure the temperature of the film exiting the metallization chamber and then insure that the film you are making or using will withstand that temperature without appreciable shrinkage or that the film is not stretched at these temperatures by the tensions in winding and transporting the film.   

Regarding my previous question Juan asks;

I would like to know what level of shrinkage (dim stab) of the met film provides a good level of adhesion of the metallized surface to the LDPE and also reduces the risk for cracking of the metallized layer.

Eldridge replies:  Film shrinkage is not in my opinion related to the adhesion of an extrusion lamination layer to the metallized surface of the film.  In general the adhesion is controlled by the surface oxidation of the laminating layer.  In the case of a LDPE extrusion lamination, this is related to how hot the melt temperature, the polyweight applied, the length of the gap and the line speed.  The LDPE layer has to be oxidized to adhere well to the aluminum layer and has to have sufficient time to wet the aluminum surface (this is related to the poly weight and the melt temperature).  I will address the adhesion of LDPE to the aluminum in extrusion lamination in more detail in a future posting.

Cracking or “crazing” of the aluminum is related to the melting point of the surface under the metal layer as well as the applied tension during the lamination step.  I.e. you can craze an adhesive lamination simply by over tensioning the film and this is the key mechanism in an extrusion lamination as well.  However, in extrusion laminations the thermal load on the film will impact the relative strength of the film (how easy it is to stretch at a given tension level) and perhaps the melting of the layer under the metal layer on the film.  In OPP technology, the metallizing skin layer is often times a copolymer PP or a polyethylene surface which melts around 135 C.  In these cases high temperatures combined with high poly weights can cause crazing.  To avoid this, the film manufacturers will thin the skin layer (us patent 5,194,318 col. 2 ll. 48 to 57) which minimizes the tendency to craze.  Homopolymer PP is a good crazing resistant surface but a poor adhesion surface.

Are the dimensional stability of a film and the adhesion level between LDPE and the film related in some way?

Eldridge replies:  Not in my opinion, the adhesion is controlled by the surface polymer and its method and level of treatment.  See the series of posts on “Great metallizing substrates”

What is the dimensional satiability or shrinkage of a film based on LDPE or BOPP?

Eldridge replies:  OPP films are made and heat stabilized in a hot air oven and will have shrinkage of approximately -4.5% MD and -2.5% TD at 135 ^oC.  At this temperature a LDPE film will be molten as the melting point of LDPE is approximately 110 ^oC.  As the LDPE film is heated to the below the melting point, it may shrink from between 5% to 50% in both the MD and TD.  So you have to look at the method which was used to make the LDPE film. 

Blown films will have much higher levels of melt induced orientation than a cast film and will therefore show a much larger shrinkage on heating than a cast film.  LDPE Shrink bundling films are usually made from blown films.  You have to measure the shrinkage as a function of temperature and plot it to have a good idea of the films behavior.  In a future post I will discuss heat shrinkable films in more detail

Posted by Eldridge M. Mount on March 17, 2008 | Comments (0)