Converting Magazine

Eldridge M. Mount

Great metallizing substrates, Part 3
February 1, 2008

Continuing from the last two discussions, I want to talk about what controls the barrier properties of a metallizing substrate. In Figure 1, we see a mapping of barrier properties for a variety of metallized films[i].

WVTR: ASTM test method F1249-90 at 37.8 °C (100 °F) and 90% relative humidity (RH). 

OTR: ASTM test method D 3985-81 at 23 °C (73.4 °F) and 0% RH.

Figure 1: Plot of Log (WVTR) vs. Log (OTR) for various barrier films

In the research leading up to this chart, it was discovered that the barrier properties of the metallized films were controlled by the surface on which the metal was deposited and not on the bulk polymer of the substrate.  This was especially true for the oxygen barrier.  This is seen from the examples of the Al/amPA/OPP and AL/APET/opp which are coextrusions of amorphous nylon (amPA) and amorphous polyester (APET) onto a propylene core and then stretched to produce a coextruded OPP.  These films have the approximate barrier profile of the MET PET film as opposed to the 70 MET which is metallized OPP.   Based on these discoveries EVOH was coextruded onto the OPP base and the MET-UHB sample was produced[ii] which taught that EVOH had the best metallized barrier profile of all the possibilities and could overlap the properties of aluminum foil. 

These results tell us that it is the surface chemistry of the metallizing surface that controls the final barrier properties of the film.  Nylon, polyester and EVOH are all polar polymers and the high surface free energy seems to improve the quality of the deposited aluminum layer giving an improved gas barrier as compared to the use of a polyolefin surface such as ethylene propylene copolymer (EP copolymer) or homopolymer propylene (PP) typical of the 70 MET and other metallized OPP films. In general, the use of a EP copolymer or PP skin does not impact the moisture barrier, but the use of EVOH clearly improves the moisture barrier as well as the moisture barrier relative to the other metallized surfaces.

Why is this?  Examining the surface chemistry of EVOH we find that it is composed of an ethylene polymer with hydroxyl (-OH) groups located along the chain.  If you recall our earlier discussions about treatment, where oxygen is added to the surface of a polymer film such as polyethylene, EVOH is just equivalent to a well treated polyethylene.   Therefore, if we could “properly treat” a HDPE surface we might obtain EVOH like metallized barrier properties at a lower cost than using EVOH.  This makes use of the idea that the control of the metallizing surface chemistry is what is necessary to control the barrier properties. 
 
So how best to control the metallizing substrates surface chemistry? Well, coextrusion of the best surface is the most direct way and insures the best control of the surface as with the coextrusion of the EVOH shows, but in our ongoing discussions of treatment we have seen that the control of film surface oxidation is also related to the type of treatment process we employ. If we want to replicate EVOH on a surface without actually using EVOH then we need to start with a polymer like EVOH and then place the –OH groups onto it. This is what was done[iii] , a HDPE layer was coextruded onto a PP core, oriented to produce the OPP film which was then flame treated to optimize the production of –OH groups on the film surface. The result of this is seen in Figure 1 as the MET-HB film.  This modified surface gave properties similar to the EVOH in that the moisture barrier was improved relative to the other films and the oxygen barrier was improved relative to the 70 MET metallized OPP.

What this teaches is that a film with an hydroxilated polyethylene surface (EVOH) is the best metallizing surface for the control of metallized film oxygen and moisture barrier.  Metallized EVOH is the best byt a flame treated HDPE surface is great.  Research in the production of high barrier metallized films has been focused by these discoveries to methods of creating –OH rich surfaces on metallizing films. 

Posted by Eldridge M. Mount on February 1, 2008 | Comments (0)