A side job was given to our pilot line operator to, in his spare time, try and produce a biaxial-HDPE film. After struggling with the cold, chill-roll adhesion problem, he decided to go warmer as we often cast the OPP films relatively warm, say 45° C. Seeing some improvement, he kept at it and finally around 93° C the HDPE melt adhered to the chill roll, giving good edges to the sheet, and good biaxially oriented film could be produced on a “standard OPP line with an ordinary “chill” roll . We all felt this was quite a breakthrough. Then, doing some background reading in Sweeting Volume I , I came across this advice:
“The air layer [behind the film from poor adhesion] causes important film-quality problems also. Ripples and puckers are formed when the film does not adhere to the drum [cast roll] and differential cooling and lateral shrinkage of the film occur. Excessive neck-in and uneven neck-in (i.e.: scalloping) can also occur.
“These quality problems are eliminated if the film adheres satisfactory to the drum. The most effective step which can be taken to give satisfactory adhesion is to heat the drum (42).” Emphasis added (J. L. Hecht, “The Technology of Melt Casting”, Chapter 8, O. J. Sweeting, Ed., The Science and Technology of Polymer Films, Vol. I, Interscience Publishers, New York, (1968), p.407; Reference 42 [D. Grant, Plastics Progress 1961P. Morgan Ed. McMillian, New York, 1962, pp. 151-178].
Controlling surface crystallinity
It pays to read the old stuff. I believe what is at play here is the control of the surface crystallinity by quenching. The OPET film with a high Tg will easily quench an amorphous sheet, which is readily adhered to a chilled chrome roll. Polypropylene with a Tg of perhaps -40° C generally yields a sheet of perhaps 50% crystallinity while HDPE with a Tg of perhaps -90° C or lower typically gives films of 85% crystallinity. It is my thesis that the high casting temperature for the HDPE produces a more amorphous surface, which will wet the chrome roll surface. I may not be right, but the evidence suggests it as a plausible answer. Others with better analytical equipment might be able to test and perhaps confirm my thesis on surface crystallinity and sheet-casting success, but that’s my story, and I’m sticking to it.
On the other extreme is the production of thin cast films. If film adhesion to the chill roll is too great, it is not possible to strip the film from the roll surface. So for thin cast film the best approach is to “float” the film on a thin layer of air between the melt curtain and cast roll. This can be done with textured, cast-roll surfaces or perhaps cold, smooth-roll surfaces to minimize melt adhesion, especially for polyethylene. This approach to casting can give uniform quenching of the thin film while maintaining a high-quenching rate to control film morphology and film properties.
In regards to film morphology effects from chill-roll melt casting, It is known from the production of oriented tapes, that due to higher retained melt orientation, chill-roll casting gives lower levels of ultimate stretch ratios, and hence tenacity or tape strength than the lower quench rate of blown films with slower quenching allows .
2. US Patent 6689857, Larter et. al, “High Density Polyethylene Film With High Biaxial Orientation”, Feb. 10, 2004
3. J. L. Hecht, “The Technology of Melt Casting”, Chapter 8, O. J. Sweeting, Ed., The Science and Technology of Polymer Films, Vol. I, Interscience Publishers, New York, (1968), p.407
4. H. A. Krassig ET. al, Fiber Technology from Film to Fiber, Marcel Dekker, Inc., New York, (1984), pp. 104-107, Vol. 4, International Fiber Science and Technology Series, M. Lewin, series Ed.