Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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152
LIPPERT
narrow production systems, steel rolls have been successful.
In
most cases, however, steel
rolls are not as forgiving as an elastomer roll and therefore have not yet been accepted
for production. We expect this to change as the technology matures.
Elastomer rolls have improved over the past several years to allow the precise roll-
to-lip conformation absolutely required for proximity coating. When specifying an elasto-
mer roll, the following items must be carefully considered:
runout tolerance
release characteristics
heat transfer
hardness
hardness uniformity
roll deformation (bow)
resistance
to
attack by the coating or cleaning agent
We see the use of rolls covered with urethane or Viton being most evident with diameters
of
around 300 mm and hardness
of
up to 90 durometer, Shore A. Runout tolerances of
0.0005
in. and better are being achieved.
The elastomer roll will be deformed by
a
given width web, and rolls nut be provided
to
match web width changes. Spare rolls are also necessary. as damage often occurs.
12.0
AUTOMATIC CONTROL
12.1
Die
Control
All comnlercially acceptable automatic die adjustment systems available today use flexible
lip and heated bolt arrangements (see Fig.
12).
Minor disagreements exist
on
details, but
total overall perforlnance does not seem to be greatly affected. The greatest confusion
surrounds the effect
of
pure mechanical response time and how it relates to process analysis
and its relation to response. All systems to date simply read the variation from target and
make a corrective response. Polymer lot-to-lot differences. temperature changes, in-plant
drafts, and many other factors that affect gage have made it impossible
to
anticipate flow
changes. Anticipation-based programs can be used
in
product changes. however,
if
known
effects will happen over a relatively fixed period or on start-up.
During a production run. if a variance is seen. make sure that it is
not
a short-term
effect, gone
in
the time
it
would take
to
make a change. We also may want to determine
the variance trend. Only after careful analysis of the problem can we make a die adjustment.
The time from discovery until an adjustment takes effect varies from line
to
line. however
start-up to
55%
control, assuming the total system has reached some stability, will be
10-20 minutes, and
t_
3%
control in 15-30 minutes.
Total and substrate weight separation. the first steps in the process control cycle.
are achieved by a layout similar to that shown in Figure
13.
Existing technology is available
and has been proved in production for nearly a decade.
The Autotlex die features thernlal expansion and contraction
of
the lip adjusting
bolts to make finer adjustments
in
lip opening than are possible with strictly mechanical
means.
Each lip adjusting bolt is fitted with
a
block containing
a
heater and air cooling
path. Approximate gage uniformity is established
in
the conventional manner before the
Autoflex system is engaged. Thickness variations are converted to lip opening correction
SLOT DIE
COATING
FOR LOW
VISCOSITY
FLUIDS
153
Figure
12
Automatic transverse coat weight control.
0
0
D
\
f
D
b
pre-coat
measurement
measurement
1r
1,
coated
web
Figure
13
Schcmatic drawing
of
total and substratc weight scparation in production control cycle.
154 LIPPERT
by increasing or decreasing power to the individual lip bolt control blocks
to
trim variations
to a minimum.
The key
to
the transverse thickness control is a microprocessor-based controller that
On ambient operation dies, care must be taken to isolate the heat from the Autoflex
is tied into the conventional computer control system.
bolts from the die body.
12.2 Die-To-Roll Position Adjustment System
The ability to repeat the original roll-to-die setup position is critical during start-ups and
normal web splice coating interruption.
During operation, minor changes in die position may have to be made to accommo-
date roll expansion, changes in adhesive viscosity and smoothability, and substrate thick-
ness variance. An automatic positioning device is available that will allow continuous
adjustment, if necessary, of the web to the lip face. This is accomplished through a device
similar
to
the Autoflex die bolt adjustment system. A heating and cooling device is installed
in the manual adjustment system
(U.S.
Patent 3,940,221) for die-to-roll gap setup. Heating
or cooling of this device will expand or contract the steel block and increase or decrease
the die-to-roll gap.
A
usable method for monitoring smoothing must be employed and
interfaced with the Autoflex computer.
13.0 DECKLING
Deckling may be necessary to reduce die width or
to
make stripe coatings. Be careful
when attempting this, to make sure that shim materials are soft. We recommend Teflon/
filled, Teflodaluminum, foilhluminum shim stock, or soft brass. For room temperature
applications, an adhesivelfoadadhesive mounting tape works very well.
In many applications, a rake-type device may be used for stripe coating (Fig. 14A).
This device does not clamp into lip and can be changed very quickly.
If excessive force is applied when the deckle is clamped into the lip, the lip will
distort, causing lip wear or uneven coating at this point (Fig. 14B).
Deckling or stripe coating cannot be used in a system featuring the rotating rod lip
design.
13.1 Air Entrapment Behind Deckling
When the die ends are deckled in,
it
is common for air
to
become trapped in this area.
This air is slowly released, causing voids in the coating at the edges. A purge port with
shutoff valve should be installed at the die ends to eliminate this problem.
14.0
DIE CLEANUP
In most cases, it is important
to
change coatings or coating formulation frequently, There-
fore it is necessary to be able either to purge out the system or to clean the die completely.
This holds true for the complete system, including pump filters and piping.
SLOT
DIE
COATING
FOR
LOW VISCOSITY
FLUIDS
155
Figure
14
Deckling:
(A)
rake
style and
(B)
shim
style.
Purging is the easiest and most common method; but extreme care is necessary
to
stream-line all flow areas
to
eliminate dead areas.
The opening and cleaning of
a
die
can be an easy
30
minute experience
or
a
3-4
day nightmare. Slot die designs differ greatly; some are simple two-piece designs, while
others have complex assemblies.
It
is common
to
have a dual pumping and piping system, allowing quick changeover
from one coating
to
the next, cleanup then taking place after the line is up and running.
This Page Intentionally Left Blank
17
Extrusion Coating with Acid
Copolymers and lonomers
1
.O
PRODUCT CONSIDERATIONS
Acid copolymers and ionomers are high performance resins that offer adhesion, heat seal,
and barrier properties markedly superior to those of conventional polyethylenes. Figure
1
shows the structure
of
acrylic and methacrylic acid copolymers, the two types
of
acid
copolymer commercially available in the United States at this time. The presence of the
methyl side group in the methacrylic acid copolymers results in some subtle differences
between the two resin types, but they can be regarded as equivalent after allowing for the
difference in molecular weight of the comonomers. For example,
a
10
wt.
%
acrylic acid
copolymer is equivalent to a
12
wt.
%
methacrylic acid copolymer in carboxyl group
content.
Ionomers (Fig.
2)
are derived from acid copolymers by partial neutralization of the
carboxyl group with either sodium or zinc ions. Since the neutralization reaction results
in a substantial increase in melt viscosity, it is commonly referred
to
as
ionic cross-linking.
In both acid copolymers and ionomers, the melt and solid state properties are
strongly influenced by intramolecular hydrogen bonding, as illustrated in Figure
3.
The
forces involved in hydrogen bonding are almost
10
times stronger than the intramolecular
forces in the nonpolar polyethylenes.
The ionomers are distinguished by the combination of hydrogen bonding and in-
terchain ionic forces perhaps an order of magnitude stronger than the hydrogen bonds.
As
a consequence, the ionomers have a wide spectrum of melt and solid state properties.
including better hot tack and grease resistance than acid copolymers of equivalent acid
content.
With the acid copolymers, melt index and acid content are the major variables
available to the resin producer. Since melt index is
a
measure of melt viscosity, it is related
157
158
BREBNER
-
[,,,-,,g
1
-
COOH
Figure 1
Structure
of
ethylene acrylic (top) and methacrylic acid (bottom) copolymers.
c=o
I
0-
MM++
c=o
I
0
I
H
Figure 2
Ionomer structure.
CH3
Figure
3
Intrarnolccular hydrogen bonding.
EXTRUSION COATING WITH ACID COPOLYMERS AND IONOMERS
159
primarily to the processing characteristics of the resin. The resins now used for extrusion
coating applications fall in the
5-15
melt index range to accommodate a broad field of
processing needs.
Resins with acid contents of
3-15
%
are currently on the market. The effects of
increasing acid content are:
better foil adhesion
better hot tack
lower seal initiation temperature
better oil and grease resistance
better abrasion resistance
higher gas and moisture permeability
The most important effect is the improved adhesion to unprimed aluminum foil, since
most commercial applications
of
acid copolymers involved foil adhesion.
The ionomers are characterized by additional variables, which can be manipulated
to
obtain a broader spectrum of properties in the final product. These variables include
the following:
melt index (molecular weight) of base resin
percentage
of
acid in base resin
degree of ionic cross-linking (percent acid neutralized)
type of ion used for neutralization (Na
or
Zn)
In particular, the use of the sodium and zinc ions produces families
of
resins with distinctly
different properties. The zinc ionomers are used
for
the majority of extrusion coating
applications because
of
their excellent adhesion to unprimed
foil.
The effects of composition on ionomer properties are shown in Table
1.
In general,
the ionomers based on high acid copolymers and with a high degree of neutralization have
the best heat seal and product resistance characteristics. However, these advantages are
balanced by a need for relatively high free carboxyl content for optimum foil adhesion.
Table
1
Ionomer Properties Versus Composition
~ ~~ ~~~ ~ ~
Higher free carboxyl:
greater adhesion to polar substrates
(foil, nylon)
Higher percentage of acid Higher ion linking
Na
Zn
Greater stiffness
Better oil resistance
Higher
gloss
Higher tensile and
impact strength
Lower haze
Lower melt and
softening points
Greater stiffness Better oil resistance Less water sensitive
Better oil resistance Higher
gloss
Better adhesion
Higher
gloss
Lower haze Less “neck-in”
Higher tensile and Higher draw
Higher toughness Higher toughness
Higher hot tack
(foil, nylon)
impact strength
160
BREBNER
In practice, most extrusion coating resins used for coextrusion with nylon have a relatively
high degree
of
neutralization. There are limited uses of sodium ionomers in extrusion
coating applications that call for maximum grease resistance, but these applications are
on either paper or primed foil.
2.0
END-USE CONSIDERATIONS
2.1
Foil Adhesion
One
of
the primary reasons for the selection of an acid copolymer or ionomer for use in
extrusion coating is adhesion to unprimed aluminum foil. Extensive studies in Du Pont
laboratories have found that aclylic and methacrylic acid copolymers of equivalent acid
content have equal adhesion to foil. At a particular acid content,
a
zinc ionomer will have
15-20% lower foil adhesion. Resins with high acid content
(10-
13%)
have
20-25%
better
adhesion than resins with low acid content
(3-5%).
The acid copolymers and ionomers have additional utility because of the enduring
nature of their adhesion to foil. High levels of adhesion of polyethylene to foil can be
achieved with primers or high melt temperatures in extrusion, but the bonding is often
deteriorated by extended exposure
to
aggressive environments. With acid copolymers
and ionomers, most hard-to-hold products can be packaged without sacrificing long-term
durability of the coating adhesion. Acid copolymers and ionomers of equal acid content
are essentially equivalent in product resistance.
2.2 Heat Seal Characteristics
The heat seal properties of acid copolymers and ionomers constitute the second major
area of superiority over low density polyethylene (LDPE). Seal initiation temperatures are
lower and strengths higher than low density polyethylene. Figure 4 is a generalized plot
of seal strength versus interface temperature for acid copolymers, ionomers, and LDPE.
The acid copolymers and ionomers have similar seal initiation characteristics but the acid
copolymers have a higher ultimate seal strength. Both have much higher seal strengths
than LDPE.
Hot tack strength is the ability of a heat seal to remain together when a force is
applied while it is still in the molten state. This is
a
critical property in vertical form-fill-
seal applications, in which the product is loaded immediately after the seal is made. It is
also
critical
in
any high speed packaging operation in which the package is exposed to
some form of abuse before the seal has cooled.
One method of measuring hot tack strength
is
the Du Pont spring test. A series
of
springs of different thicknesses and widths at the narrowest point provides varying levels
of spring tension (Fig.
5).
Figure
6
shows how the spring is located inside the sample.
The sample is heat sealed for
3
sec. at a pressure of 40 psi. The three-second dwell time
allows an accurate measurement of the interfacial seal temperature by using a very fine
thermocouple hooked up
to
a rapid response recorder.
When the heat seal bars are released, the springs apply an instantaneous force to
the seal. The separation of the
1
in. wide seal measured to the nearest tenth of an inch.
The results are plotted as the force required to obtain 20% seal separation
as
a function
of temperature.
Figure
7
shows the hot tack characteristics
of
an ionomer and acid copolymers
of
equivalent acid content in comparison with low density polyethylene. LDPE, a completely
EXTRUSION COATING WITH ACID COPOLYMERS AND IONOMERS
2000
5
I5O0
W-
E
E
1000
U
B
S
W
v)
-l
a
W
500
i
/’
””
)
220
240
260
SEAL INTERFACE TEMPERATURE,
‘F
i
D
Figure
4
Heat seal strength versus bar temperature
(1
mil coatings on
30
lb.
kraft
paper).
h
8‘
1
T
3”
l
161
Figure
5
Spring
for
Du Pont hot tack tester.