A problem of curl in metallized paper

We are facing a problem of curling in the metallized paper.

After top coating the metallized paper, the paper curls.

We have tried to give steam on the back side of the paper just before the rewind station.

At that time, it seems that the paper is not curling.

But after a couple of days, after slitting the paper if starts curling.

1.What is the best way of measuring the curl in a paper?

2.What is the sureshot way of producing curl free paper?

3.Does giving steam helps in reducing the curl in the paper?  If so, how much moisture should be added in the paper? Should we add some other material in the steam?

4.What can be the different causes of curling in the paper?

Please guide us on these issues.

Answer

A simple way of measuring curl is to cut a thin strip of material and let it rest on its edge on a flat surface and measure the radius of curvature.  This can mean cutting strips at an odd angle to get a sample that only has a curve and not also a twist in the strip. The curvature allows you to calculate the bending stress on the strip.

Curl is a measure of a difference in stress between two layers.  This is most commonly caused by the swelling and drying of one or more layers of material.

The first thing to do is to establish the sense of the stress.  If the metal surface is on the inside of the curve then the coating added on top of the metal would appear to have contracted.

If the curvature is such that the metal surface is on the outside of the curve then it is the paper that has contracted most causing the curvature.

If it is the coating on top of the metal that has contracted then this might be caused by the coating drying too rapidly so that the coating cures from the top surface first and then through the coating thickness. This solidifies the top surface as a skin and then the solvent has to be extracted from below this skin and as it is removed the lower levels want to shrink but as the top surface is fixed it puts stress into the coating.  If the drying is slower the solvent has time to migrate out from the depth of the coating with the solids settling down to the surface and this reduces the stress in the coating.

If the problem is a problem of shrinkage of the coating then adding water to the paper could be helping and is something that is worth checking out.

If you reduce the water does the problem get worse and start sooner?  If so then it might be that adding more water will swell the paper more before the coating is added so that as the coating shrinks the paper shrinks a similar amount.

If the curvature is the opposite direction with the metal surface outside the curvature it may be that the addition of moisture is the problem. As the water is dried off possibly more slowly than the coating is dried it contracts more than the coating and thus the curvature is the opposite direction. Again the test is to reduce the water addition and see what happens to the curvature.  What you are looking for is a balance to the shrinkage over the longer timescale.

Bear in mind that the drying rates for the paper and the coating may well be different, which is why you see no curl early on and curl later. What you want is no curl when the material is used later and so it may be that you will see curl when you test immediately after coating and drying but have to trust that the additional time and drying will improve the film to bring it too a neutral stress point with no curl.  So when you do your changes in moisture level you need to not only check the curl immediately but also plot any changes to the curl over time.

Where stress is concerned and curl has been a continuing problem at least one company solved the problem by coating both sides of the substrate with the same material so that the stress remained balanced. This was a drastic solution but in their case cost was not an issue and it did stop the problem. 

I hope these suggestion help.

Comments

[Murat ISIK]

We are interested in the KnowHow for metallizing paper. Do you know an expert for this ? Could I have the contact mail ?