#747
Author: Tim Walker,
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Subject: There are two factors that can cause BOPP roll to become tighter after winding
There are two factors that can cause BOPP roll to become tighter after winding (especially in thin films, as you mentioned).
1. Polypropylene polymer orientation residual stress relaxation
The PP biaxial orientation process strains and moves the PP polymer molecule chains. Quenching / cooling the polymer locks the molecules into their new positions / orientations. However, even at room temperature, there is some molecule movement towards a more ordered or denser structure.
I don't know if it is appropriate with BOPP, but with PET orientation, I think they use the term "free volume". In the first 24 hours after the orientation process, there can be some shrinkage due to loss of free volume.
This effect is seen strongly in the rolls wound at the end of the orientation process. Also, if the rolls are moved quickly to converting (slitting / rewinding), the "densifying" or shrinkage will happen in the slit rolls.
In either case, shrinkage causes the rolls to get tighter as each layer squeezes down on the core and the internal roll layers.
To learn if this is a big factor, delay the converting of the maker rolls by 24 hours and see if the slit rolls end up with the same tightness. I don't know enough about BOPP making to know what orientation process variable will increase or decrease this phenomenon.
2. Viscoelastic recovery
BOPP has some viscoelastic properties. A VE web will have a two-fold response to web tension. The first component is elastic and immediate (strain proportional to stress). The second component is viscous and time dependent (more strain over stress-time).
Vinyl electrical tape shows this behavior in its extreme. If you elongate a strip of electrical tape by hanging a weight on it (to create tension) for 10 seconds, then remove the weight, the sampel length recover over time. The longer you hold the sample under tension, the longer the recovery time will be.
In winding BOPP, this can tighten slit rolls. The tension before the winding roll is elongating the BOPP, partially with a viscous component. When the web goes into the winding roll, it will try to push down on the inner layers to releave its elongation. Over time, the BOPP will try to relax more (strain recovery or length shrinkage) causing the roll to get tighter.
Since the recovery is proportional to stress and stress-time, lowering the incoming tension and shortening the time it is stressed with reduce this roll tightening effect.
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