Converting Magazine

Eldridge M. Mount

Orientation in Films-Part 1
June 22, 2008

I thought I'd begin with a discussion on film orientation. Orientation is the alignment of polymer chains in a film in particular directions in the film. In general there are three primary directions we consider, two in the plane of the film and the third through the film. For our purposes we will define them as the machine direction (MD) or X, which is the direction that the films moves through the machine form start to finish. Next there is the transverse direction (TD) or Y which is the direction perpendicular to the machine direction, and finally there is the thickness direction (TK) or Z direction which is perpendicular to the MD and TD directions. The MD and TD directions are sometimes called the in plane directions. These are general definitions of the principle directions of the film and are for reference purposes.

Orientation is due to the average direction which the polymer molecule shows. There are two potential parts to the film orientation, the crystalline and the amorphous phase orientation. In amorphous polymers such as atactic polystyrene or PMMA, there are no crystals and therefore only amorphous phase orientation. In simicrystalline polymers such as PP, PE and PET, there are both crystals and amorphous chains. The amorphous chains are best imagined as cooked spaghetti with no apparent order while the crystals will resemble the rigid uncooked spaghetti which can be held as a tight bundle. Try to imaging short bunches of uncooked spaghetti  mixed into a plate of cooked spaghetti (with the ends of the bunched spaghetti being the loose cooked spaghetti) and you get an idea of what a semicrystalline polymer is like at the level of the molecules.

What causes molecular orientation is a force which acts on the polymer molecule to pull it in a direction and then being frozen in place as by quenching from a molten state. This is generally the case in film manufacturing where the polymer molecule is stretched by pulling or inflation as a melt followed by rapid quenching as for cast and blown films and with solid state orientation is in Double Bubble or Tentered films.     

A truly unoriented film is isotropic, or uniform in all direction in the film. These are extremely rare and generally not easily made and are prized in applications which need uniform optical properties. The can be recognized by the uniformity of their optical properties, for example the refractive index, in the X, Y and Z directions. Films with uniform orientation in the X and Y planes of the film are orthotropic and unless the TK properties are measured might appear isotropic because the in plane properties (the X and Y directions) are the exactly the same. Films with variable orientation in all three directions are called anisotropic films where none of the properties in any direction are the same. If you think about it we only need one direction to be different to have an anisotropic film so the orthotropic film is a special case of anisotropic orientation.

Well what does all this special language mean to film people? Well the physical, barrier and optical properties will be different in each direction due to the molecular alignment of the polymer chains. Too much orientation in one direction will weaken the film in the other direction, such as tear strength and elongation to break. Higher in plane orientation, MD, TD or MD-TD will lower the TK (Z) orientation and this improves the moisture and gas barrier of a film which is why oriented films have better barrier properties. In general when we orient a film we orient both the crystals and the amorphous parts of the polymer. The crystalline orientation plays some part in changing the film properties but the amorphous orientation is more important in the overall strength and barrier properties of the film which we are most concerned about. Naturally it is more difficult to measure directly than crystalline orientation.

Relative levels of orientation can be measured many different ways both sophisticated (expensive) and easy (cheap) such as x-ray diffraction (expensive) and tensile properties (cheap). Optical properties are also related to orientation and can be expensive and cheap depending on the amount of sample preparation. Z direction orientation is always hard to measure directly because the films are thin so most orientation is measure in the MD and TD directions. The simplest ways to measure film orientation is by measuring the shrinkage and the tensile properties of the film. These measurements will give relative levels of orientation and can be correlated with the absolute measures of orientation. 

In general if a film (of the same material) shrinks more than another film it is oriented more, or annealed less. To tell the difference we would need the mechanical properties and here if the mechanical properties are better (better being higher) then the film is oriented more.

In my next posting I will discuss relative levels of orientation as in Blown and tenter films and how to interpret the various orientation measures we can make easily.

Posted by Eldridge M. Mount on June 22, 2008 | Comments (2)