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Film manufacturing
February 6, 2008
Films used as substrates are produced many different ways, and the properties will vary greatly for the same polymer depending on how it is made. The properties which vary the most are the mechanical, barrier and dimensional stability. For instance, in polypropylene the manufacture of film by extrusion can be done by simple casting or by orientation. Cast film is a relatively soft film with minimal tensile properties close to that of the base resin. The cast film is also very extensible because the polymer molecules are randomly arranged in the film. During the casting, the melt is drawn from the die and rapidly quenched on a chilled roll. During the draw down, the molecules might align in the melt but the relaxation rate will be high, and the residual orientation will be low. This is a lot like blown film in its properties, but the cooling rates will be lower generally giving a film which is hazier due to the lower cooling rate. Because the bubble is stretched in two directions the blown film will be more oriented than that of the cast film which is drawn in only one direction. Because the orientation is performed in the molten state before quenching, the film will have only moderate strength.
In contrast, film that is biaxially or uniaxially oriented after it is quenched will have greatly increased tensile properties in the direction in which it is stretched. The increase in tensile strength is due to the alignment of the molecules which cannot relax after the stretching is performed. In this type of process, the film is quenched as in the cast or blown process and then reheated to above the film's glass point but below the melting point. Therefore, the film is stretched in a relatively solid state. During this process the molecules will be elongated due to the stress applied to the reheated film. The stretching can be performed in several ways usually between moving rollers at different speeds or stretched by large diverging chains in large ovens called tenters. In this process the temperatures are too low for the molecules to relax significantly after the stretching. The alignment of the molecules gives rise to the increased strength because the strength of the polymer along the molecules length is due to the carbon-carbon bonding of most polymer chains. Perpendicular to the chains, the strength will be less, and uniaxially drawn films will be weak in that direction. During the stretching, the chains also are brought closer together and this increases the film's density, and this gives rise to an increase in the barrier of the film. However, this increased orientation frozen into the polymer can be released by heating the film, and unless the film is annealed, it will shrink when heated. Therefore, if it is not desired to have a heat-shrinkable film, the annealing process must be designed properly.
In coming posts, I will discuss the differences in film properties and manufacturing methods used to produce them.
Posted by Eldridge M. Mount on February 6, 2008 | Comments (0)