Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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742
SATAS
4.1
Tapes
Tapes can be subdivided according to the backing material used: film tapes, fabric, paper,
foil, foam, etc. Special tape products are double-coated tapes, which have adhesive on
both sides
of
a
supporting sheet (film, paper, nonwoven fabric, foam), and transfer tapes,
which consist of an unsupported adhesive film. Double-coated tapes and transfer tapes
are protected by a silicone coated release liner. These tapes are heavily used for product
assembly applications and mounting and holding uses.
Tapes are also subdivided according to their application. Packaging tapes are the
largest single application. This field is dominated by OPP film tape, but other film, paper,
and glass fiber reinforced tapes are also used. Medical tapes and related products constitute
an important product area. Office tapes are dominated by matt acetate film tape, although
cellophane film tapes are still used. Electrical tapes include many products starting with
black vinyl electrician’s tape and ending with high-temperature fibedfilm tapes with sili-
cone adhesive. The automotive industry consumes many different tapes: electrical harness
wraps, paint masking tapes, product (including foam) assembly tapes, and tapes for many
other applications. The construction industry requires paint masking tapes, foam tapes for
gasketing, tapes for insulation
of
heating and air conditioning ducts, and many other uses.
The appliance industry uses wood grain and other surface decorating tapes, tapes for
attaching name plates, foam gasketing tapes, etc. Tapes are used in all industries for many
different applications. In most cases pressure sensitive adhesives are used for easier and
more convenient application.
4.2
Labels
Pressure sensitive adhesives are growing increasingly used for label. Although they are
more expensive than labels secured by waterborne
or
hot melt adhesives. because a release
liner is required, PSA label application equipment is less expensive, it is easier and cleaner
to
run, and the adhesive is quite suitable for plastic surfaces, such
as
polyethylene contain-
ers. PSA adhesives are used for impervious foil labels; they are easier
to
use for intricate
label design, and they are used for transparent film invisible labels.
The label industry consists of two separate groups. Large manufacturing companies
produce label stock: unprinted adhesive-coated large rolls or sheets. These are sold to
label manufacturers who print and die-cut the labels. While there are only few label
stock manufacturers, there is a large number of label printers often servicing a limited
geographical area.
4.3
Other Products
Pressure sensitive adhesives are used for many different products other than tapes and
labels. The medical area is a large one for a variety of dressings, transdermal drug delivery
systems, and other products and devices that use pressure sensitive adhesives to secure
these devices to the human body. Easily removable stickers (Post-its) have been developed
for office uses. There are many other office uses that employ PSAs. The list of other
products is difficult to exhaust, and this area is growing fastest and carries the highest
markup.
5.0
PROCESSING
Pressure sensitive adhesives are applied in thin layers on various substrates. Most
of
the
coating is carried out roll-to-roll,
i.e.,
the substrate is supplied as
a
wound
roll,
it is
PRESSURE SENSITIVE ADHESIVES AND ADHESIVE PRODUCTS
743
~ETERING
ROLL
t-t”-+
METERING
ROLL
APPLICATOR ROLL
Figure
5
Top
nip
fed
three-roll
reverse
roll
coater.
unwound, the adhesive is applied to the substrate’s surface, the adhesive is dried or other-
wise hardened, and then the substrate is rewound into a large roll. Further processing then
starts with the material in
a
large coated roll.
5.1
Coating
The adhesives are applied by specialized coating equipment. They must be liquid, or
sufficiently fluid, to form a thin layer
in
the coating head. Coating equipment is discussed
in several books?.’ Several techniques are used for
PSA
coating. The reverse roll coater
(see Fig.
5)
is the most versatile and frequently used machine for solvent-borne and aqueous
adhesives. Knife-over-roll coaters are used for heavier solvent-borne adhesive coaters.
The slot orifice coater is the main technique for the application of hot melt adhesives, and
this technique is also useful for the coating of aqueous emulsion adhesives. Calendering
is used for some specialized adhesive products. The compounded solid adhesive is
squeezed and formed into a thin sheet between heavy calender rolls. Calendering was the
main coating technique early
in
the development
of
PSA
products.
5.2
Drying
If the adhesive is applied as solution or emulsion, it requires drying before it can be wound
up. Forced air convection dryers dominate this application, with less frequently used
infrared heating. The web is carried through the oven supported
on
idlers, sometimes on
an apron, or sometimes supported by an air cushion in floatation ovens. More detailed
information
on
web drying can be found in several books.‘.’
No
drying is needed in hot
melt coating: cooling is sufficient to solidify the adhesive.
UV
irradiation is also used
as
means
of
solidifying reactive adhesive coatings by the initiation of
a
polymerization reac-
tion in the reactive oligomer and monomer mixture.
5.3
Slitting and Die-Cutting
Large rolls produced on the drying equipment are slit
to
narrow tapes
or
to wide material
in case
of
label stock to be printed and die-cut. Several slitting techniques are used: shear,
744
SATAS
score, and razor blade. See Ref.
3
for more detailed information
on
the slitting of
PSA
products. The label stock
is
die cut by cutting through the label material and leaving the
backup release liner uncut. Rotary die cutters are usually employed. The matrix between
the labels
is
peeled off and the labels are then dispensed as needed.
REFERENCES
1.
C. W.
Bemmels,
Pressure-sensitive tapes and labels. In
Hwzdhook
ofAr1he.siw.s
(I.
Skeist, ed.).
2.
W.
H. Shecut and H. H. Day,
U.S.
Patent 3,965
(1845).
3.
D. Satas, ed.,
Handbook
ofPressure
Semifive
Adhesive
Technology.
3d
ed.
Warwick, RI: Satas
4.
D. Satas, ed.,
Web
Processing
md
Corwerrirlg
Techlology
and
Equipment.
New York: Van
5.
E.
D.
Cohen and
E.
B.
Cutoff, eds.,
Modem
Corrtin<q
crnd
Dryiqq
Tetlrnology.
New York: VCH
New York: Van Nostrand Reinhold, 1977.
and Associates, 1999.
Nostrand Reinhold, 1984.
Publishers. 1992.
85
Self-seal
Adhesives
1
.O
INTRODUCTION
Cohesives, often referred to
as
“self-seal adhesives,” are coatings which
in
a dry state
exhibit the unique ability to selectively stick to themselves when brought together under
pressure.
1.1
Adhesion and Cohesion
Natural rubber is the ingredient common to all self-seal adhesives. It is this raw material,
generally utilized in the latex form, which confers or facilitates the ability
to
cohere.
Natural latex consists
of
long chain, high molecular weight molecules, the ends of
which are thought
to
contain an extra double bond protected with
a
weak hydrogen-bonded
protein molecule. It is believed that when such molecules are positioned
in
intimate contact
and mechanical
or
thermal energy is introduced, the following reaction takes place. First,
the weakly hydrogen-bonded protein molecule breaks from the long chain and second,
stored elastic energy from within the molecule is released, causing molecular vibration.
The subsequent entanglement of the chains and joining
at
the now-available bonding sites
produces
a
molecular inseparable condition referred
to
as a
cohesive seal (Figs.
1-3).
In application, the greater the number of bonding sites utilized
to
impart adhesion,
the smaller the number remaining
to
facilitate subsequent adhesion.
Laboratory evaluation on the functionality of self-seal formulations confirms the
significance of achieving an appropriate adhesivekohesive balance relative
to
a
specific
substrate.
Through product selection or formula modification, it is possible to enhance or
accentuate the adhesive or the cohesive properties, but
not,
however, without inversely
diminishing the other set.
A
product exhibiting the appropriate adhesivekohesive balance
after
a
consideration of the seal functionality requirements mandated by the application
745
746
TlMM
Figure
1
Long chain natural latex molecules.
and other pertinent performance parameters. In addition. product selection can be made
only after the influence of the specific substrate on the adhesive/cohesive balance has
been acknowledged.
Coating weight also affects seal functionality. Approximately
3.5
g/m’ is required
to ensure proper film formation, generating optimum adhesive functionality;
5.0
g/m’ is
generally suggested as the coating weight offering the most reliable performance. Conse-
quently, the adhesive/cohesive balance is typically evaluated at this coating weight,
whereas 7.0 g/m’ represents perhaps the highest commercially viable coating weight for
one-pass applications on high speed converting equipment. This coating weight is em-
ployed only in applications that require enhancement of the cohesive functionality over
that which is available through formulation.
Drying or, more significantly, insufficient drying, can influence the performance
of
the product as well. Excessive retained moisture is the equivalent of a plasticizer addition,
minimizing adhesion while accentuating cohesion. Additionally, retained moisture can
create
a
roll blocking problem. Polyamide-based overprint lacquers typically employed
for release can themselves become softened in the presence
of
excessive moisture.
Cohesives are dried at temperatures ranging from
50
to
90°C;
however, drying is
best accomplished at a web temperature of 60-70°C and through the use
of
a
scrubbing
zoned drying, or otherwise minimizing the amount of moisture-laden recirculated air.
1.2
Seal Performance
Eminently important in evaluating, testing, and anticipating seal performance is an under-
standing
of
the significance of the mode
of
failure. Simply stated, observe and record
Figure
2
The effect of energy in seal formation: molecular excitation; severing of the “protein”
molecule.
SELF-SEAL ADHESIVES
747
Figure
3
Cohesivc seal
consisting
of
both chemical bonds.
whether the seal strength obtained represents an adhesive, cohesive,
or
substrate failure.
In these respective cases, the self-seal is not adhering to the substrate, the self-seal is
not adequately sticking to itself, or delamination
or
fiber tearlfilm destruct, is occurring.
Some further descriptive commentary on the mode of failure
also
is appropriate:
Webby:
adhesive failure (en masse release from one
or
the other side).
Leggy:
adhesive failure (tenacious, elongating, rubber-like bands).
Stringy:
cohesive failure (short or fine elastic strands, splitting seal).
100%
cohesive
failure
(visually appears undisturbed, as before sealing).
Thus, significant
in
evaluating seal performance is the recording of the procedures
utilized in testing. Test methods used should relate as closely as possible
to
the actual
conditions employed in commercial seal formation.
Self-seals are pressure-respondent. The greater the pressure, the more immediate
the cohesion response. However, the introduction of mechanical energy can help
to
form
a bond and generate a suitable response in a mature seal.
A
seal is considered
to
be mature
after
1
hour, but some minimal increase
in
strength may continue to develop for
up
to 24
hours before a
fully
stabilized condition is achieved. Typically associated with an increase
in seal strength is a change
in
the mode of failure.
For
instance, a
100%
cohesive failure
at
300
g/25 mm could yield a substrate destruct seal at
800
g125 mm with increased
pressure
or
aging.
No
such similar change or improvement would be realized from a seal
exhibiting an adhesive failure. It is only the developing cohesive response that is subject
to this improvement.
As
already mentioned, it is an accepted practice to utilize polyamide-based overprint
lacquers to inhibit roll blocking. Careful consideration must, in fact, be given
to
selecting
an ink-overprint system that is specific to the substrate and suitable for use
in
conjunction
with self-seals. This
not
only
assures proper release and eliminates ink pick, but also
minimizes the potential for offset contamination. Offset contamination is represented by
an accumulation
of
migrating slip
or
release modifiers from the printed surface to the
surface of the self-seal, diminishing or inhibiting the subsequent cohesive response.
2.0
APPLICATION TECHNIQUES
In relaying techniques and procedures developed
to
facilitate cohesive applications via
gravure, it is appropriate to define some
of
the inherent product limitations and physical
characteristics that affect their use.
2.1
Handling
Cohesives are perishable. They have
a
suggested shelflife, and proper rotation of inventory
is required.
748
TlMM
Cohesives must
be
protected from freezing; ideally they are stored at 10-30°C in
tightly closed containers.
Although as a general rule cohesives are not subject to settling, they do, in fact,
stratify, as a result of their low viscosity and variable specific gravity, and must therefore
be well stirred before use.
Cohesives are susceptible to contamination. Transfer of material should be made
only to clean, water-rinsed containers.
Particulate matter and/or solvents, including alcohol, will reduce mechanical stabil-
ity, and cohesive formulations should not be adulterated. Any such modifications can
interfere with product performance.
It is advisable to discard used cohesive rather than returning it to a storage container;
this eliminates the possibility
of
cross-contaminating fresh material.
Cleanup is best accomplished with cool water. Ammoniated water
or
alkali detergent
may also be used. Completely dried material may be stripped or peeled from equipment.
The use of solvent is not recommended.
Cohesives
are
preserved with ammonia, which is highly corrosive to copper and/or
copper alloys. Additionally, the incorporation of copper ions into a cohesive will cause
accelerated degradation. Therefore, any such contact must be avoided.
As
copper is typically employed as the base for gravure cylinders, it is imperative
that a hard, impervious chrome plating be utilized over
100%
of the cylinder. Sometimes
a nickel flash is employed immediately under the chrome for added protection.
Cohesives stabilized for machinability may be shear sensitive, and excessive shear
must be avoided.
Cohesives should not be pumped by means of gear pumps routinely used for inks.
A diaphram pump can be utilized. However, a systaltic pump employing alkali-resistant
flexible tubing is preferred.
Doctor Blade
-
150-200pm
Backer Blade
-
350
-
500pm
"Gravure
Figure
4
Schematic
of
a rigid blade assembly.
SELF-SEAL
ADHESIVES
749
Oscillating Blade
c
Scavenger Bars
(or)
overall etching cyilnder ends
Figure
5
Schematic
of
a
gravure cylinder
with
scavenger
bars,
100%
etching
of
a
cylinder end,
and
oscillating doctor blade.
2.2
Procedures
Experience has shown that shear generated by blade doctoring can
be
minimized through
maintaining as steep an angle as possible (i.e., 90" tangent to the cylinder). It is suggested
that a well-dressed, rigid blade assembly be utilized; typically blue steel
150-200
pm
doctor,
350-500
pm backer, recessed
1.5-3.5
mm (Fig.
4).
To
further reduce shear generated at the doctor blade in pattern applications, scaven-
ger bars on overall etching of cylinder ends should be considered. Oscillating the blade
through the widest parameters and at its fastest speed will also aid in the elimination
of
particles formed in the land areas. Also suggested is the under cutting of the impression
roll to eliminate inadvertent backside coating, should shear-generated particles cause the
ends of the blade to lift (Figs. 5,6).
Ammonia, used as a volatile preservative, functions as the secondary stabilizer as
well. During intermittent or low usage runs, aeration may cause a reduction in the ammonia
level sufficient
to
create machining difficulties. In such situations, nondetergent 26" Be
(29.4%) ammonium hydroxide should be added to a pH of
9.5-10.5.
Cohesives are quick drying, facilitating their utilization on high speed converting
equipment. However, unlike solvent-borne inks, once dry they will not redisperse in them-
selves. Therefore to minimize drying in the application station, it is suggested that
all
Impression
Roll
(undercut)
(substrate)
h
mi
Deep Pan
(vs.
Shailowllnner Pan
Splash
Guard
Pan Cover
(minimize drylng)
7
(not impeding clrculatlon)\u/
Figure
6
Schematic
of
a
gravure station.
750 TlMM
other “make-ready” be accomplished before cohesive is added to the pan. Once added.
the gravure cylinder must be kept rotating, ideally flooding the cylinder when not actually
coating. Before
it
is dropped or reseated, the blade should be wiped with a rag moistened
with water, remove any dried material or other contaminants.
The pan ideally should be deep, thereby increasing volume, and covered
if
possible,
with the flow not restricted by inappropriate positioning of the splash guard.
Additionally, thought should be given to minimizing hot drafts from the driers onto
the application station.
Rewind tension should be carefully controlled to avoid blocking from too tight
a
roll. Cohesives exhibit antitelescoping frictions, and low rewind tensions are indeed func-
tional in this respect. Cohesive-coated wrap typically produces noise through the rewind
and slitting operations because of the inherent tendency to cling.
Converted rolls should be cradled or stored
on
end, rather than
on
their sides. This
is to reduce internlittent blocking, which could be caused by the weight
of
the roll.
Sol-Gel
Coatings
1.0
INTRODUCTION AND BASIC PROCESSING
The sol-gel process has been suggested as an alternative to conventional methods such as
sputtering, chemical vapor deposition (CVD), and plasma spray for applying thin ceramic
coatings. Sol-gel thin films are in a few notable cases technically sound alternatives to
these methods as well as con~n~ercially viable alternatives.
The technology of sol-gel thin films has been around for more than
20
years. The
process is surprisingly simple. A solution containing the desired alkoxide is prepared with
a
solvent and water. It is applied to a substrate by spinning, dipping. or draining. A tacky
film forms that
is
typically
1
km thick, uniform over large areas, and adherent to the
substrate. The sol-gel process is able
to
apply a coating simultaneously
to
the inside and
outside of complex shapes. The film dries quickly
to
a microporous coating of oxide.
The chemistry of the sol-gel process can be simplified to:
Condensation-Polymerization: 2M(OR),
IOH"H,O
+
(OR),
-
lM-O"(OR),
~
I
Hydrolysis: M(OR),
+
H@-"(OR),
IOH
+
ROH
where M is a metal species (Si, AI, Ti, Zr, etc.) and R is an organic group (methyl, ethyl
isopropyl, etc.). These reactions generate an oxide skeleton
in
the solution. Upon contact
with the substrate and exposure to atmosphere, the solution gels, that is, becomes rigid.
Remaining solvent can be evaporated readily. These coatings can be heat treated to densify
them. Densification means that the microporosity closes and eventually is eliminated. The
limitation to this step is the temperature to which the substrate and the film can be subjected.
Hermetic films can be achieved in some but not all applications.
Among sol-gel processed materials, thin films are the true leader, and some exam-
ples are given for abrasion-resistant, corrosion-resistant. optical, and electrical thin films.
751