chemicals
By Chris Rozett
Recent findings in oil-free release aids
Conventional, oil-based release technology has served well to control sheet adhesion on the Yankee dryer, provide
lubrication for creping and cleaning blades, and remove excess coating from the dryer surface. However, oil-free
technology is now available that provides advantages over traditional release technology
T
here are two main directions in
developing oil-free release aids:
plasticization of the coating to
improve crepe density and control hard edge
buildup, and replacing oil-based release aids
with a cost effective, oil-free alternative. This
paper recounts recent findings in tailoring oilfree release aids for plasticizing and release
properties.
Let’s start with a brief description of how
traditional, oil-based release agents interact
with adhesives. Yankee coatings are generally
composed of two components, adhesives and
release agents. A number of tissue
manufacturers use a third component, a coating
modifier; but for our purposes, we’ll restrict
this discussion to two component systems.
Adhesives (responsible for holding the
sheet against the Yankee during creping) are
polymers - natural or synthetic - that crosslink on the Yankee surface. Left to themselves,
adhesives would hold the sheet so strongly
against the Yankee that it would be difficult,
if not impossible, to control creping without
causing sheet holes and breaks. This is where
release agents come in. Release agents
interfere with the integrity of the polymer
matrix on the Yankee surface. Oil-based
release agents create pockets of ‘dead space’
in the matrix by inserting droplets of oil in
between adhesive molecules. These droplets
help to prevent too much cross-linking of the
adhesives, which is good. However, the
droplets tend to weaken the matrix so much
that the coating tends to be stripped off the
dryer when the ratio of release to adhesive is
slightly out of balance.
Typically, adhesives and release agents
are applied to the Yankee surface by a low
pressure shower. The concentration of the
adhesive and release (taken together) is
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Figure 1: Oil-based and oil-free release concentration gradients
Figure 2: Release concentration gradients at the point of creping
typically less than 1%. Before the coating
matrix can form on the Yankee surface, the
carrier water must be evaporated. As the
carrier water is evaporated, the volume of
coating on the dryer decreases and adhesive
molecules come closer together and start to
associate with each other. Consequently, the
adhesive portion of the coating becomes more
compact as the coating dehydrates. The release
agent, on the other hand, is an emulsion of
oil in water. The oil does not compact during
coating dehydration. In fact, it expands on
heating, migrating away from the dryer
surface. Because the release oil expands while
the adhesive compacts, a concentration
gradient of release oil in the coating matrix
develops. More release oil is present in the
upper layer of the coating matrix; less is
present in the harder layer near the dryer
surface. Figure 1 shows a hypothetical
concentration gradient of an oil-based release
in the coating matrix between the dryer surface
and the sheet. A potential problem with having
a concentration gradient of release oil in the
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coating is that the coating remaining on the
dryer after creping can be quite hard. Under
the right conditions, it can become quite
difficult to control the build-up of hardened
coating on the dryer, especially just outside
the sheet edge. Edge build-up is a potentially
serious problem, often associated with sheet
breaks and/or heavy blade wear.
Oil-free release agents use a different
approach to modifying the coating on the
Yankee dryer. Instead of interfering with the
ability of adhesive molecules to bond by
inserting dead space in the coating matrix,
oil-free plasticizers associate with adhesive
molecules but do not cross-link with them
the way another adhesive molecule would.
Let’s first define the term ‘plasticizer’. A
plasticizer acts to make something more
pliable or flexible - more plastic. Since
plasticizers associate intimately with the
adhesive in the coating matrix, the net effect
is to make the entire matrix more flexible.
Creating a flexible coating matrix is the goal
of any soft coating program.
Looking again at Figure 1, we see that
there is essentially no concentration gradient
of oil-free release in the coating matrix. Both
the oil-free release and the adhesive portions
of the coating matrix compact during
dehydration. That means the oil-free release
has no driving force to concentrate in one
portion of the coating, leaving a uniform
amount of oil-free release throughout the Zdirection of the matrix. This lack of a
concentration gradient is especially important
when we consider what happens at the creping
and cleaning blades. Sheet softness is built
in the soft part of the coating matrix. It is here
that the creping blade deforms both the coating
and the sheet, which is the basis of creping.
But what happens to the coating that is left
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on the dryer after the creping and cleaning
blades have removed the softer part of the
coating? It is here that Yankee protection is
built. It is also here that an excessively hard
residue can be developed which interferes
with machine runnability and blade wear.
Figure 2 shows the hypothetical difference
between the concentrations of oil-based and
oil-free release agents in the harder portion
of the Yankee coating matrix- the fraction
that is not removed by the creping and cleaning
blades. In this example, the average
concentration of oil-based release in the cured
portion of the coating is 15%, while that of
the oil-free is 35%. The difference can lead
to a dramatic difference in pliability of the
coating left on the dryer.
Now that we have a basic understanding
of how non-oil release agents work, it’s time
to decide whether to focus on plasticizing or
release.
P L A S T I C I Z I N G
R E L E A S E
V S
Strictly speaking, when dealing with oil-free
release agents, the question is not either/or
regarding plasticizing and release. It is rather
a question of degree. The reason is there is
an inverse relationship between plasticizing
and release properties. There are four key
factors in determining where a particular
release fits on the curve of plasticizing vs.
release: the type of base polymer used; the
molecular weight of the base polymer; the
type and degree of modification of the
polymer; and the emulsification package.
By working with these key properties, it is
possible to create oil-free release agents that
give strong release (much stronger than oil-
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based release agents on an actives basis) as
well as and some plasticization. It is also
possible to create highly plasticizing release
aids that have less aggressive release
properties. However, even highly plasticizing
release aids can be more efficient than oilbased release agents.
A D V A N T A G E S O F
P L A S T I C I Z I N G
R E L E A S E
There are two essential characteristics of
plasticizing release agents: they are soft
polymers that associate intimately with creping
adhesives and they compact along with
creping adhesives during coating dehydration.
These two aspects give the following benefits:
A softer, thicker, more even coating.
Because the plasticizer does not strip the
adhesive off the dryer, the coating matrix
actually gains in bulk. This is helpful in
dealing with moisture streaks that interfere
with proper coating buildup on the dryer.
More even creping. Unlike oil-based
release agents that form discrete droplets in
the coating matrix, the plasticizers associate
intimately with the creping adhesive on the
dryer. This leads to fewer pockets of coarser
crepe in the sheet. Virtually all plasticizing
release trials to date have shown a significant
improvement in MD and CD creping
uniformity.
Finer creping. Because plasticizing release
aids don’t strip the adhesive from the dryer,
the soft part of the Yankee coating (where
creping takes place) tends to be softer, which
usually gives an extra 10 crepe bars per cm
in crepe density.
Less hard edge buildup. Having a
relatively large fraction of release aid in the
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chemicals
CASE HISTORIES
hard part of the Yankee coating helps to keep
the coating left on the dryer flexible and easier
to manage.
C O N C L U S I O N
Certainly, oil-based release agents have served
tissue makers well for decades. However,
there are now alternative, oil-free release
technologies that provide real advantages over
conventional release agents. Plasticizing
releases allow the tissue maker to soften the
Yankee coating without stripping it; improving
creping, roll building, blade wear, and
controlling problematic Yankee coating
buildup. Oil-replacement releases offer
significant program savings over their oilbased counterparts. TW
Chris Rozett is Technical Marketing Manager,
Contaminant Control & Tissue, with Amazon
Papyrus Chemicals
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1: Removing edge build-up
A mill in the Middle East suffered from hard edge build-up, causing
it to change creping blades every 1-2 hours due to edge blade wear.
Figure 3 is a photograph of the tending side of the Yankee; the edge
build-up was several centimeters wide and could not be removed
under normal operating conditions. A hard creping adhesive was
chosen because of the machine’s operating speed (1750 m/minute)
and the desire to protect the Yankee. Figure 4 is a photograph of the Figure 3: Buildup of hardened
coating at the sheet edge
same area after introducing a plasticizing release; the edge build-up
was brought under control. As is typical with plasticizing release
applications, there was a significant reduction in creping adhesive
use. In this case, the reduction was 37.5%. There was also an increase
in creping density: the crepe bar count increased from 65 to 75 per
cm.
2: Controlling blade wear
A mill in Southeast Asia suffered from severe edge wear of the creping
blade and uneven roll building. Replacing the previous creping program Figure 4: Hardened buildup removed
with a hard, moisture-tolerant coating and a plasticizing release gave by plasticizing release
significant improvements in blade wear, roll building, and creping.
Figure 5 shows typical pre-trial blade wear of >5 mm at the edges.
Figure 6 shows even blade wear at the same blade life. In addition,
crepe density increased from 45 to 60 bars/cm. All this was achieved
with a reduction of 24% in creping adhesive use.
Figure 5: Heavy blade wear from edge
buildup
3: Oil replacement
A mill in Southeast Asia was interested in reducing its creping program
cost. Replacing the oil-based release aid with an oil-free plasticizer
allowed the mill to enjoy a release aid program cost reduction of
>20% while improving the evenness of the Yankee coating and
maintaining hand-feel, crepe density, roll building, and sheet Figure 6: Blade wear under control
specifications.
with plasticizing release
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