Better bond quality between sheet and Yankee dryer coating creates softer tissue

Pulp & Paper, Sep 2002 by Stitt, John

ISSUE FOCUS:

TISSUE

A review of adhesives and creping mechanics shows that the crucial factor in producing quality tissue is the "intimacy" of the bond between the sheet and Yankee dryer coating

IN THE PRODUCTION OF TISSUE, optimal control of the Yankee dryer coating is the most significant parameter in maintaining product quality and high production efficiency. For years, there has been tremendous focus on achieving the highest bond strength between the sheet and the Yankee dryer coating. Elaborate equipment has been developed to test this strength with various Yankee dryer coating formulations and even at varying moisture contents, trying to duplicate in the laboratory what happens on the Yankee.

A combination of theory and operational experience ultimately led to an understanding of the interaction of the adhesive, the release, and the sheet, assuming several other significant factors, such as moisture content and drying scheme, are held constant. While strength of the dryer adhesion is significant, it may not be the most important factor in achieving optimal operating conditions. Could it be too much emphasis has been placed on strength as the critical control factor?

It is almost impossible to model creping technology, and there is no creping model that accurately predicts what will happen on any Yankee when a change is made in the system.Trying to independently define the result of each input factor in the creping equation is a daunting task. The reality is that each factor is highly interrelated with the others on the machine. Only when a certain set of factors is designated as fixed will adjustment of a single factor result in a predictable outcome.

Data show that the most significant factor in achieving optimal sheet properties in most tissue grades, given a sufficient amount of adhesive, is the intimacy factor. This factor is the quality of the bond between the sheet and the dryer coating. The purpose of this article is to bring creping technology a few steps closer to quantifiable science instead of an art form.

TISSUE MAKING OBJECTIVES. The objective of premium quality tissue manufacturing is to produce softer, higher-quality facial and toilet tissue. In technical terms this means:

* Better crepe structure;

* More microcrepe;

* Better drape, less stiffness/limpness;

* Higher ratio of stretch to % crepe;

* Less abrasive surface;

* More surface depth;

* Higher bulk for a given basis weight.

Softness is a highly desirable property, and, to control softness, we must define, in technical terms, factors that do influence the consumer's perception of softness. Some of the most significant of these factors are listed previously. We may not be able to do a tight mapping of each independent factor to softness, but we can say that directional movement results in a softer or harsher tissue.

How do we get the desired properties? We get them by controlling the input parameters of the process. These outcomes require both technical knowledge and process knowledge. To better understand how these factors influence creping, a brief review of the physics of creping is helpful.

CREPING MECHANICS. Creping imparts energy into the sheet structure, causing wrinkling, as well as exploding the physical structure of the sheet-breaking the fiber bonds. This energy, especially the exploding of the sheet in the Z direction, gives stretch and softness. You should consider ideal creping not only as wrinkling the sheet, but also as mechanical debonding.

So, what does give good explosion of the sheet in the Z direction? Consider a model from civil engineering, based on the Euler Column-Buckling Equation, as detailed in Figure 1. Mechanical energy "pops" the sheet loose from the Yankee when the sheet hits the creping blade. The release of the sheet from the coating occurs in the coating under the sheet and, unfortunately, sometimes before the sheet contacts the creping blade.

The uncreped sheet hitting the creping blade deforms by either "column failure" or "plastic failure," depending on the quality of support (adhesion) of the sheet to the Yankee surface. A column fails by bending mode, shear-slip-plane mode, or bulging/explosion mode, depending on the ratio of the height of the column to the width. Bending or buckling failure occurs at one spot in the length of a tall thin column and the remainder of the column remains intact.

For soft tissue, a bulging/explosion failure mode is preferred, which produces a Z-direction explosion, mechanically breaking the inter-fiber bonds and decreasing strength while increasing bulk. With poor adhesion, or inconsistent irregular bonding, the failure mode is bending. If the support points of the column are reduced enough, then the failure mode becomes plastic failure, resulting in bulging/explosion.

We get this type of high-energy creping by increasing the strength and the uniformity of the adhesion of the uncreped sheet to the Yankee dryer. Yet, in most cases, increasing the bond strength also decreases bond uniformity. As bond strength increases, the coating hardens and contact, or bond intimacy, usually decreases.