By John Stitt
When a coating builds on the surface of a yankee, it develops
a hardness profile through its thickness. These profiles can vary
depending on the machine conditions, adhesive package and type
of release/modifier used. When making soft, light, dry crepe there
are two primary requirements, yankee protection and explosive
crepe for softness. Of course, blade life is important, too, and
is somewhat related to this topic, but it is really more of a
dependent factor.
In the figure, the typical hardness profile of a number of different
types of coating packages is shown. The left side of the figure
representing the area next to the yankee surface is ideally hard,
providing a base of hard coating that protects the yankee surface.
This area we call the ‘heel’ of the profile. Then,
moving to the right, toward the sheet contact, the coating becomes
softer and more pliable. This area can be called the ‘toe’ of
the graph.
The toe area of soft coating allows for intimate contact between
the coating and the sheet at the pressure roll. The sheet is never
truly flat because of formation marking from the fabric. (fine
fabrics for soft tissue!) The soft layer allows the coating to
flow into the structure and effectively reduce the column length
of the tissue contact area, thus resulting in explosive crepe
(microcrepe) instead of deflection crepe (macrocrepe). Intimate
contact between the sheet and the coating is essential for high
stretch, high softness creping.
This leads us to query how to maintain a significant toe in
the coating profile? Of course the nature of the coating package
is one factor. Polyamide coatings such as wet strength resin and
more engineered forms of the chemistry at the right temperature
and pH provide a very hard coating with a huge heel and little
toe. These are great for yankee protection but poor for softness.
The nature of the release/modifier can soften this toe somewhat
but it is not ideal and is very sensitive. Such a coating is great
for semi-wet crepe or away-from-home commodity tissue where softness
is not very important. On the other hand, the early very soft
coatings provided good softness when all conditions were perfect
but they were very sensitive and subject to being cut off completely
at blade changes because they essentially had little or no heel
on which the blade could ride.
What is needed is a coating that exhibits both a significant
heel and a significant toe. This, along with a variety of soft
coatings for varying yankee temperatures and dwell times, were
our prime objectives in developing the SELECT package approach.
We usually think of blade loading as PLI or kN/m, but in reality
the loading is more two-dimensional. Only for a short time is
the width of the wear pad a point of near-zero dimensions. As
the blade wears, the wear pad develops and the loading becomes
not just PLI but PSI of blade surface against the coating. With
constant air pressure and PLI as the wear pad increases, the load
is spread over more area and the PSI force against the coating
decreases.
In the second diagram one sees the tip of a doctor blade. On
this a wear pad is shown but of course, a new blade would not
have a wear pad. With a new or very sharp type of blade, the force
against the coating is maximum, the tip is sharp, and the blade
cuts deep into the coating.
If it is a very soft coating throughout the Z direction profile,
the blade with the sheet off the reel while the blade wears and
the coating rebuilds.
With a hard coating or a hybrid coating, you do not lose the
sheet but the toe is cut off and the blade rides on the hard surface.
You do not have the soft toe to produce maximum intimacy and explosive
microcrepe. With the hybrid coating you do not lose the sheet
but, because there is little toe, even with a moderately hard
coating, the second reel is usually softer than the first reel.
The coating rebuilds a toe.
We want the blade to ‘swim’ or
slice through the soft layer just under the sheet. If we go too
deep, there is no toe left
It is certainly true that all types of blades eventually wear.
But the hard blades stay sharp and penetrate deeply into the toe
of the coating for a long time. We want the blade to ‘swim’ or
slice through the soft layer just under the sheet. If we go too
deep, there is no toe left.
It is very common with ceramics and bi-metals, with soft tissue,
that the blade does not wear out but is removed because of operational
issues. Some have heard of blade suppliers who recommend that
a mill use the ceramic blade until the operation starts to falter,
remove it carefully, and then use it again with commodity type
tissue.
Of course, theoretically, one could say that there should be
a blade loading increase profile throughout the life of the blade
so the PSI of the edge on the coating would be constant (sort
of the opposite of winder relief) but, practically, this is not
possible. With a good coating package the toe will be long enough
to provide good intimacy throughout the blade life.
In summary, with sharp blades one needs to maintain the toe
of soft coating. Therefore we reduce blade loading to some degree.
Time and again, it has happened that sheet softness and stretch
go up when the loading on a ceramic or bimetal blade is reduced. TW
John Stitt is a tissue expert with Buckman Laboratories,
USA: Telephone 1-800-BUCKMAN/901-272-6641; email
jbstitt@buckman.com.
Idiosyncrasies of bimetal and ceramic blades