- February 28, 2013, Timothy J. Walker
Question: Can you help us understand how a roll will react to increases or decreases in storage temperature? How long does it take a roll to heat or cool to a new storage temperature?
Answer: First off, I don't recommend changes in storage temperature. If you can avoid changing the temperature of a winding or wound roll, please do so. Don't wind hot. Don't store your roll in a hot semi-trailer in the summer. Don't refrigerate your roll. Do ship your rolls in temperature controlled shipping containers or trucks.
That said, I'm sure many of you still will have 'reasons' to heat or cool your wound rolls.
How long will it take for a roll to change temperature? This question is similar to how long it takes a paper roll to change moisture content. It is all about diffusivity. You can imagine a wound roll is like a Thermos or cooler. When you put hot coffee in your Thermos, how long does it take before your coffee is cold? When you pack your cooler full of ice, how long does it take before the ice melts? It depends on the temperature differential and the amount of insulation.
In heating a wound roll, the roll starts at one temperature, say 70 deg F / 21 deg C, and you place the roll in a 140 deg F / 60 deg C oven. If the roll is unpackaged, the heat will immediately raise the temperature of the outside layer of the roll and begin to heat up the roll's side wall. If the core is open to the heat, the inside layer of the core will also heat up. (If the core is plugged, it prevents or delays heat from entering the roll through the core.) The heat will continue to penetrate the roll, heating up more outer layers, more distance in from each side wall, and begin to heat up layers near the core (with an open core). Small stack and narrow rolls will heat up quicker than large stack and wide rolls. Rolls that are highly conductive and have good contact between layers will heat up quicker than rolls of web with high R-values or insulating air layers.
I have not worked on modeling heat transfer in wound rolls, but I know Rheologic's TopWeb software models address both heat and moisture diffusion in rolls. I believe at least three of my web handling colleagues have worked on moisture or heat diffusion in rolls: Dave Roisum has studied moisture diffusion in paper rolls; Kevin Cole of Optimation has studied heating of coated and uncoated polymer film rolls; and Dilwyn Jones of Emral Ltd. has studied both temperature and viscoelastic effects in wound rolls.
The TopWeb software has a thermal (and moisture) diffusion model, though you need the diffusivity value for your material to use it, which is something most people don't know. TopWeb can model the temperature map in a roll over time, including options for insulated core, constant (and different) temperature core, or open core. In the case of the open core, the layers near the core are not the last to heat up. Instead, the last cool layers are about one-third up in the radial buildup, since the core allows heat to transfer through it.
The TopWeb model recommends for polyester film to use thermal diffusivity of 0.001 cm^2/s. This gives a heatup time of about 24 hrs. This number agrees with work I am aware of that involved inserting thermocouples into rolls of PET and then mapping the temperature over time during heated storage. These rolls took about 24 hrs to get up to temperature.
Your safest bet is to avoid temperature fluctuations if you can exercise some control.