PET film haze

Is there any relation between Pet Film Haze with orientation of film or crystallinity or amorphousity?  Can we correlate HAZE with anyone or with all?

Answers

The answer is yes things such as crystallinity and haze are linked. The draw ratio of can affect the amount of crystallinity and in general the greater the draw ratio the clearer the film (lower haze) but this can then be worsened by the thermal setting process. The thermal setting allows the film to relax and helps reduce the problem of shrinkage but the longer the time or higher the temperature the more the film will relax and some clarity will be lost. (Bear in mind these may be quite small differences). If the draw is not equal in both orientations the crystalline regions within the film can be oriented and so there can be optical differences with orientation. This is most easily measured by plotting the refractive index with orientation. 

Similarly the polymerisation process, which affects things like the molecular weight can also have an effect on tensile performance which then affects the draw and so can affect crystallinity and haze.

Crystalline regions within the polymer are denser and harder and the amorphous material flows around these crystallites. It can be noticeable that cast film may be very clear but after the initial forward draw the film has more haze but after the sideways draw the haze reduces again.  This may be more noticeable with filled film. This is thought to be from crystallites or filler begin slower to re-orientate and so protruding through the surface and roughening the surface and increasing the haze. With the sideways draw this helps flatten then back into the film and so the haze reduces but possibly not to the low level of the cast film.   

I hope this helps.

Additional answer from Dilwyn Jones (AIMCAL Instructor in web handling)

Most thin gauge PET film (including that for metallizing) has a small amount (<0.5%) of inorganic filler, such as calcium carbonate, silica, china clay and glass bead, added to improve the handling behaviour both during manufacture and subsequent processing.  Without it, reels would telescope if wound at realistic speeds, and block during storage.  The composition, particle size and volume fraction of the filler are the main factors influencing haze.  Haze has contributions from both the bulk and the surface, as the filler particles near the surface increase the surface roughness.  There may also be particles that are so large they give individual optical effects, such as Newton's Rings, or visible marks in the metal layer.  Finally, voids form around some particles during the stretching steps of film manufacture, increasing the haze.

Stretching ratios, temperatures during crystallisation, and PET molecule factors such as glycol and IV have a smaller effect.

Without inorganic filler, there is still catalyst residue and internal contamination to scatter light and cause haze.  This is a mechanism in clear, thicker film.

A lot of the detailed knowledge on this is of course proprietary to the major manufacturers.  However, there may be examples in the patent literature especially, and the open scientific literature also.