Intelligent design of Yankee Coatings - Paper Technology International 2023 - Paper Technology International 2023 - Flipbook - Page 124

Intelligent design of Yankee Coatings - Paper Technology International 2023

PAPERTECHNOLOGYINTERNATIONAL Intelligent design of Yankee Coatings - Paper Technology International 2023
Figure 3: Yankee Creping Performance – Some Key Factors.
Yankee Coating Requirements and Definitions
Given the importance of creping, the chemical component
of creping or the Yankee coating is also critical to the overall
performance of the tissue machine both from an operational
efficiency and finished product quality perspective. The Yankee
coating may be described as a thin film which is an agglomerate
of (a) materials sprayed directly onto the Yankee surface and (b)
“natural” coating components deposited from the furnish and wet
end of the tissue machine as depicted in Figure 4 (reference 3).
To support efficient machine operation and good quality (i.e., good
creping) of the tissue, the Yankee coating must perform three critical
functions.
1. Protect the Yankee surface against mechanical and
chemically induced defects.
2. Provide adequate adhesion to transfer the tissue sheet
to the Yankee dryer and then to crepe it effectively to
develop the required properties of softness, bulk, and
absorbency.
3. Have the correct film softness to allow the tip of the
doctor blade to reside within the coating layer and
provide good creping and machine operation, yet still
be durable enough to protect the Yankee surface.
A practical way to understand the characteristics and
behavior of Yankee dryer coatings is to use a model known as
“Coating Space” (Figure 5), (reference 4). This model corresponds
to the functions listed above where the protection function is
now described as durability. It is easily understood by tissue
technologists and mill operations personnel.
Figure 4: Depiction of Yankee Coating showing various
components comprising the film (reference 3).
Before discussing Yankee coating design considerations
and to provide a common understanding of the synthetic coating
components listed in Figure 3 and depicted in Figure 4, it is helpful
to offer the following Yankee coating component definitions.
•
Adhesive – The adhesive functions to form the main
cohesive protective film or coating on the Yankee
while providing the necessary adhesion for transfer of
the sheet to the Yankee and then to crepe the sheet
effectively. More than one adhesive can be used, and
these materials are typically film forming polymers.
•
Release – Release products have traditionally been
oil-based with added surfactants for emulsification and
application through the spray boom. These products
provide release from the Yankee and blade, while
controlling excessive coating film build-up.
•
Modifier – Modifiers are typically surfactant-based
and more compatible with adhesives than release oils.
They can soften the coating film while increasing its
uniformity in terms of appearance and thickness (see
reference 5). Modifiers are more commonly used on
advanced machines where they provide the necessary
release at the blade as well.
•
Humectant – Humectants can be used to help retain
moisture in the coating to keep it flexible and soft while
remaining adhesive.
Figure 5: Depiction of
•
Phosphate – Phosphates are used as inorganic
the Coating Space Model
protective agents for the Yankee surface. They can
concept showing the
also help anchor the coating thus providing uniformity.
three axes defining film
Monoammonium phosphate (MAP) is a typical
properties (reference 4).
example.
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