Intelligent design of Yankee Coatings - Paper Technology International 2023 - Paper Technology International 2023 - Flipbook - Page 125

Intelligent design of Yankee Coatings - Paper Technology International 2023

PAPERTECHNOLOGYINTERNATIONAL Intelligent design of Yankee Coatings - Paper Technology International 2023
Yankee Coating Design Considerations
Before discussing design of Yankee coatings for specific
machine types, it is important to acknowledge other tissue and
towel grade factors that will impact coating performance and the
choice of components. Primarily these will include furnish, wet end
chemistry and end product performance targets like softness and
strength. Specifically, a number of these factors contribute to or
influence the natural coating components like hemicellulose content,
ash, and fines as well as wet end chemistries like strength aids and
debonders/softeners (reference 6). Operational considerations,
particularly, creping moisture will also play a major role in coating
design.
We will now discuss coating design considerations for the
major machine types or technologies of CWP, TAD and hybrid. A
primary consideration here is the sheet moisture prior to transfer
to the Yankee dryer. Figure 6 provides this moisture range for the
three machine types.
Figure 6: Sheet moisture at Yankee transfer by the three major
machine types.
These ranges are approximate but encompass most
machines within the machine type classifications. The driest transfer
moistures are encountered in TAD with values as low as 10% and
the wettest transfer moistures in CWP with values close to 90%.
Hybrid machines bring an intermediate moisture content to the
Yankee.
CWP – The vast majority of new CWP machines are
crescent formers and so we will restrict our discussion to these
machines. Crescent formers can run with all furnish types and
tissue manufacturers will utilize a wide range of creping moistures
(3-7% typical) dependent on softness and energy use targets. When
considering an appropriate adhesive for crescent formers, it must
be durable enough to withstand the wet transfer conditions (~8090% sheet moisture) and essentially flooded conditions within the
pressure roll nip (see reference 7). Another consideration is the film
softness at the creping blade which must be accommodating of the
creping moisture. Typical adhesive add-ons can be in the 1-5 mg/
m2 range. Both release and modifier chemistries may be employed.
Oil-based releases are more typical for away from home (AFH)
and higher creping moisture grades, whereas modifiers are more
common in at home (AH) higher softness grades creped at lower
moisture levels. Sometimes humectants can be employed to keep
the coating soft at low creping moisture conditions. Phosphate use is
common but will be dependent on water chemistry.
Figure 7: Cross directional profile on a CWP machine showing
blade wear (mils/hr), crepe structures per inch (CSI), and tensile
strength (g/in) for TULIPTM (in blue) vs. a conventional PAE (in
red). The light-blue shaded columns in the CD represent areas
of higher sheet moisture coming to the Yankee and hence areas
subjected to potential durability issues for the coating. On the
x-axis TS and DS represent the tending and drive sides of the
machine, respectively.
CWP Case Studies – Nalco Water’s TULIPTM adhesive
product line, based upon a modified vinyl polymer architecture,
offers a unique set of properties including high transfer and creping
adhesion, soft film characteristics and good moisture tolerance.
It has proven a good fit for CWP machines and references 8 and
9 may be consulted for further details. A particularly compelling
example of Tulip’s moisture tolerance is provided in Figure 7. Here
a cross directional profile of blade wear and sheet properties is
provided for Tulip (in blue) vs. a conventional PAE (in red). Note that
the light-blue shaded columns in the CD represent areas of higher
sheet moisture coming to the Yankee and hence areas subjected
to potential durability issues for the coating. In these high moisture
areas, the PAE adhesive has problems with higher blade wear,
lower crepe structures per inch, and higher sheet tensile. The Tulip
adhesive provides a relatively flat profile for these properties across
the CD showing its higher tolerance for moisture variation and thus a
wider operating window.
Table I: Result summary from a CWP Yankee coating trial
utilizing TULIPTM vs. a competitive PAE adhesive on a 16 gsm
tissue grade.
On another machine running at 1900 m/min (see Table I),
the Tulip adhesive was able to improve tissue stretch values from an
average of 15.3% to as high as 22.1% due to the improved creping
adhesion. This improved stretch capability was utilized to decrease
the crepe ratio while running at higher Yankee and reel speeds. In
turn, this resulted in an approximately 2.8% production increase.
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