Research Renders Results for our Customers

The manufacture of coated paper-based products such as magazines, books and cartons entails a folding operation. Representing nearly 40 percent of quality complaints from web-offset printers, “fold cracking” is one of their most frequent problems and is both an aesthetic and functional issue. Because Dow is dedicated to addressing customer concerns and developing innovative technologies that respond to their needs, fold crack resistance has been a topic of further research for us.

Dow frequently cooperates with leading external research institutes to generate a fundamental understanding of important technical problems and then uses this understanding to develop breakthrough solutions for the paper and coating industry. The cooperation with Abo Akademi University in Finland has been especially close and fruitful.

“Having a long-term, strategic cooperation between our laboratory and Dow makes it possible to start smaller and larger projects fairly quickly,” says Prof. Dr. Martti Toivakka, with the Center for Functional Materials Laboratory of Paper Coating and Converting at Abo Akademi University.“We understand each party’s needs and motivations, and Dow knows our strengths in research.”

The Dow Emulsion Polymers team led by Prof. Dr. Pekka Salminen, Global Application Technology Leader, in cooperation with research experts at the university recently conducted a two-part study on optimizing the balance of stiffness and fold cracking for coated paper. For Dow, this research enables us to respond to an important customer concern. For the university, the study provides insight on how a practical problem is linked to a solids mechanics issue that has scientific value in general.

Fold-crack resistance is a key coated paper quality that influences the functionality and appearance of the final product. Folding of coated paper can leave a visible crack line along the printed fold when the coating fractures under the duress of the folding operation. In addition, the fracture line in the coating reduces the strength of the coated paper and increases the possibility of the magazine or coated article falling apart in the folds. This loss of strength and stiffness upon folding has become even more critical with the reduction in fiber content through the use of more coating layers. It is also common knowledge in our industry that fold cracking becomes more severe when the moisture content is reduced. This is why the problem is especially acute in web offset application in which the paper is oven dried at high temperature during printing.

In the joint study, we conducted numerical modeling and simulations to calculate bending stiffness, predict the onset of failure, and based on this prediction, calculate the residual load carrying capacity of coated paper. In part one of this study, the modeling work suggested that the best possibility to optimize the balance between fold cracking and bending stiffness was seen for triple coated paper.

In part two of the study, pilot coater tests on both double and triple coated paper were implemented to confirm some of the results from the modeling studies. The key variables tested were strength of coating layer, strength distribution between coating layers, stiffness distribution between coating layers and thickness distribution between coating layers. Four different S/B latexes having different Tg (glass transition temperature) and modulus were used as the binders.

The best balance between stiffness and fold crack resistance was achieved with the triple coating tests. These simulations showed that a thin, stiff bottom layer can be allowed to crack with a thick, middle layer acting as a buffer absorbing the crack. This enables the use of a relatively stiff binder in the outer layer that contributes to the stiffness. This combination of layers allows one to achieve a higher level of stiffness than any of the double layer coating combinations while retaining good fold crack resistance. Ultimately, these studies proved to be a “triple threat” to fold cracking and a positive development for Dow customers.

“I think we increased our understanding of the mechanisms causing the fold cracking problem, and provided new insight that can lead to improvements,” explains Toivakka. “As is often the case, the results also pointed to some new issues that might be important, including the role of the base paper/coating interface, and the design of the folding machinery itself.”

“Working with external research groups like Abo Akademi allows for an amazing partnership and opportunity to continually provide our customers with the best products possible,” says Salminen. “As a leader in science and technology, Dow is always focused on developing innovative and cutting-edge solutions to improve the processes and products that concern the paper industry today and into the future.”