Substrate Secrets

 I received a question a few days ago relating to observed differences in the sealing of an OPP film.  In particular the observation reported was that untreated surface was sealing better to itself (UT/UT) than the treated surface to itself (T/T).  The outcome was that the UT/UT orientation would run quite a bit faster on the packaging machine and the question was why.

 In my mind there are two likely answers.  While I don’t know the exact or approximate structure of the film in question I would assume it is an A/B/C film with differential skin resins where the A surface is treated.  I will first address the problem assuming this and then answer as if the film is an A/BA film with the same skin polymers.

 The most obvious answer to me is that the treated A surface is a higher melting copolymer than the untreated C polymer.  This is the general product design for most heat sealable OPP films.  Typically the heat sealing C layer is a Ethylene, butylene propylene terpolymer (EBP terpolymer) with a melting point somewhere around 120C or lower while the treated A skin would be a 3.5% ethylene 96.5% propylene or EP copolymer melting around 135C.  this melting point difference will translate into a noticeable sealing difference and a difference in packaging machine speed.  Typically the A layer is chosen for its converting performance and could be EP copolymer, a higher melting EBP terpolymer, HDPE, MDPE or homopolymer PP.  Generally I would expect it to be the EP copolymer if it was a commodity film.

 However, if the film is an A/B/A film with one side treated I would still expect the treated surface to seal at a higher temperature.  In my experience treating a copolymer or terpolymer will increase the seal initiation temperature up to 10 degrees.  While no one really knows exactly why this is there are several possibilities.   The best description I have read was by Briggs where he found lower molecular weight polymer components on the surfaces after treating.  The low molecular weight materials will create a weak boundary layer on the film surface which must be melted at a higher temperature to get the polymer surfaces in intimate contact to form the seal.  An alternative would be that light surface cross linking of the surface occurred which interferes with sealing but this would require and excellent treatment process and I am skeptical that this would be a leading cause of the SIT increase.  I would tend to think the weak boundary layer formed from higher treater intensities would predominate.