Coated paper: how to avoid cracking at the fold: what is the best way to handle fold cracking? The first step is to discover whre the problem lies.

Cracking at the fold is an aesthetic defect that takes place when coated paper is folded. The fold can be in a magazine, a book, a bag, or even a box. This defect can be merely cosmetic, or may lead to a complete paper failure at the location of the fold. Cosmetic cracking can be characterized by a small disruption of the coating layer at the location of the fold, making the fold unappealing, especially if the fold is in a heavily inked area.

Identifying the cause of fold cracking is the first step. The problem could be with any of these areas:

* the base paper

* the coating layer

* the pigments

* the additives

* the binders or co-binders used in the process

* the moisture level in the final printed paper

" the printing process.

The amount of coating and the number of coating layers could also be a factor; hut if the paper itself is failing before the coating is applied, it is useless to focus on the coating.

COATING LAYERS AND MOISTURE

For this discussion, we will focus primarily on the influence of the coating layers (coating ingredients) and the effects of moisture, if investigation identifies the coating layer as the cause of the defect, the choice of latex and its physical properties will definitely help minimize a fold-cracking problem. A number of commercially available products can help reduce this type of cracking-at-fold issue.

Another way to reduce fold cracking is to increase moisture in the coated paper. Howeve1; increasing the moisture level in web off= set grades can lead to other coating defects, such as blister. Thus, papermakers must optimize the formulation to balance different needs. The ultimate goal is to design a coating/paper system that is flexible enough to be folded without disrupting the coating layer. The moisture of the coated system plays the most important role in this problem. A printer may reduce or eliminate this fold issue by applying a bead of water at the location of the fold.

In the end, mills can solve many fold crack issues by increasing the moisture level and using the appropriate latex in the coating system; but first and foremost they must determine the cause of the problem The ability to test and diagnose the problem is the key to solving any fold cracking problem.

TAPPI's PLACE (Polymers. Laminations, Adhesives, Coatings, and Extrusions) Division is made up of hundreds of technical professionals working in flexible packaging, laminations, adhesives, coatings, and extrusions. The PLACE Division offers excellent opportunities for you to learn from industry experts, as well as your peers who experience and resolve the same challenges you face in your job. For more information, visit TAPPI's website at www.tappi.org and click on "The PLACE" beneath the "Journals" heading.

Jamel Attal is product development leader for Dow Emulsion Polymers' Paper Coating Technical Service and Development group. Contact him by phone at [phone], or by email at [e-mail]