Film substrates are subjected to many different processes once they leave the manufacturing plant. In general, they must remain dimensionally stable until used, when subjected to various conversion steps or, as in the case of a shrink film, maintain its advertised shrinkage levels until heated. While a substrate’s dimensional stability can be impacted by several environmental changes, such as tension levels, relative humidity and temperature exposure, I will focus here on how various manufacturing methods impact the general thermal stability of a film substrate. I am using thermal stability as opposed to shrinkage because, in some instances, film dimensions will increase on heating.
While the apparent thermal stability of a substrate is dependent on the basic thermal properties of the polymer, such as its glass transition temperature (Tg) or melting point (Tm), for any specific polymer, it is ultimately controlled by the recoverable strain remaining in the substrate. The level of recoverable strain for a polymer is a measure of the level of molecular orientation “frozen” in the substrate. The temperature at which a film is “frozen” will be dependent on the substrate polymer’s Tg and Tm.
How frozen is frozen?
The level of this frozen molecular orientation will vary primarily based on the film-manufacturing method, its processing temperatures and the tension levels used to make the substrate. Because it is not possible to make a film substrate, from a melt-based film process, without pulling or drawing the melt from the die, all film substrates from polymer melts will have some initial level of molecular orientation and during the quenching process some of the molecular orientation will remain.
The primary manufacturing processes for films are melt-blown bubble lines, roll-cast films and solid-state stretched films based on tenter and double [and triple] bubble lines . Melt-blown films rely on a draw-down from the circular die and a transverse inflation using air pressure to blow the bubble. As the film is drawn and expanded, it thins and cools to a solid state. MD melt orientation may be lowered by increasing TD blow-up and balanced or unbalanced MD/TD orientation ratios can be set. At the point of freezing, often called the frost line, any residual strain remaining in the melt is frozen and becomes a recoverable strain (shrinkage) when the film is heated. Consequently, blown films generally have an MD and TD shrinkage.
1. Mount III, E.M., “Films, Manufacture,” Encyclopedia of Polymer Science and Technology, Kroschwitz, J.I. ed.; John Wiley & Sons, Inc., Third edition, (2002)