Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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542
BOWER
3.3.
l
Pr1raflrl.s
Paraffins are extracted form oil. The amount and type
of
wax present depends on the
source of the
oil.
The paraffin is normally separated from the oil, distilled into several
fractions, and sold as commodity items by the oil refineries.
3.3.2 Microcrystalline Waxes
Microcrystalline waxes are normally the higher molecular weight fractions from the paraf-
fin extractions. They vary
in
hardness and melt properties depending
on
molecular weight
distribution and branching. The name “microcrystalline” evolved from the discovery that
these higher melting paraffins were not amorphous but consisted, to
a
high degree,
of
microcrystalline structures.
3.3.3
Montan
Waxes
Montan waxes are extracted from soft coal deposits located predominantly in Germany,
where the wax content in certain coal deposits is
10-
15%.
The coal is crushed and extracted
with solvents to yield a dark brown to black crude montan wax with a wax content of
50-60%.
3.4
Synthetic Waxes
3.4.
l
Polyetkylene urd Polypropplerw Waxes
Polyethylene waxes are made by polymerization
of
ethylene to form ethylene chains similar
to
polyethylene resin but much lower in molecular weight (2000-10,000). The waxes can
be oxidized by a variety
of
methods to produce waxes that are emulsifiable.
Polypropylene waxes are formed by controlled polymerization of propylene.
3.4.2 Modified Montan Wcrxes
Modified montan waxes are derived form coal. The crude montan waxes are subjected
to
a series of solvent extractions and oxidized with chromium trioxide (Cr03) generically
referred to as the Gersthofen process. The Cr03 selectively attacks tertiary carbon struc-
tures, leaving behind only the linear montanic esters and acids. After oxidation has been
completed, a light-colored product consisting mainly
of
montanic acid is formed. The
montanic acids can be reesterified with various alcohols and polyols and partially saponi-
fied to give a series
of
montan ester wax products having various properties.
3.4.3 Miscellaneous Synthetic Waxes
Fischer-Tropsch waxes are made by reacting carbon monoxide (from the coal gasification
process) with hydrogen gas to form a hydrocarbon wax.
Polyglycol waxes are high molecular weight chains of ethylene oxide that fornl a
wax-like product.
Oxidized hydrocarbons (paraffins. microcystalline. and polyethelyene waxes) are
formed by oxidation (generally air blown) of hydrocarbon waxes to form a large variety
of emulsifiable waxes.
4.0
COATING APPLICATIONS
Various waxes are used in different application areas for coatings to enhance certain
physical properties.
WAXES
543
4.1
Mechanism
Waxes generally function by one of two basic mechanisms.
The “ball bearing” mechanism is a dispersion of discrete wax particles throughout
the coating matrix. The particles that protrude above the film surface prevent the abrading
media from contacting the coating surface. Instead, the other surface glides over the pro-
truding wax particles, causing little damage
to
the surface.
The “migration” mechanism involves migration
of
the wax to the film surface,
where it aids in formation of a smooth glossy film. In some applications, the plastic nature
of the wax aids in filling voids formed
as
the solvent evaporates or the specific volume
changes during curing
of
the resinous components.
4.2
Property
Enhancement
The major properties affected by waxes are surface related.
4.2.
l
Gloss
A
high
gloss
is obtained from waxes predominantly by the migration mechanism. The
waxes tend to
fill
the microvoids formed as the coating dries to provide a smooth continu-
ous film. Often only small amounts of waxes are needed
(0.5-2%
of solids)
to
obtain
a
gloss
improvement. In fact, too much wax can lead to formation of wax solid on the
surface, resulting
in
a dull haze.
A
high
gloss
can be obtained by buffing the surface
(property
6
in Table
l),
but this is not practical in most situations. If the excess max were
to continue to migrate, the haze could return after several days
(or
weeks).
4.2.2
Mcrttirlg
A
mat (flat) surface is the opposite of high
gloss,
but both effects can be obtained using
waxes. To obtain a matting effect, the type
of
wax and method
of
incorporation must be
chosen
to
maximize the “ball bearing” mechanism. The wax particles that protrude above
the surface diffuse the light to reduce glare. Particle size is very critical in obtaining a
smooth, silky mat finish that does not feel rough or look dull or hazy. Dispersibility in
the paint or varnish vehicle is important to obtain a uniform surface.
Silicas are often used as matting agents, but their high density can lead to problems
with settling
out.
Combinations of waxes and silica can lead to optimum effects. Using
agents with differences
in
refractive indexes can further enhance matting effects without
forming a haze.
4.2.3
Slip
Waxes can reduce the coefficient of friction by aiding in the formation of a smooth surface,
as discussed
in
Section
4.2.1,
or by reducing the number of contact points by gliding over
the wax particles (ball bearing mechanism) discussed in Section
4.1.
Soft oily-type agents (soft, microcrystalline waxes or silicones) can also be used to
form a slippery lubricating film on the surface. Mar resistance and blocking generally are
very poor with this approach.
4.2.4
Abr-clsior~
Resistance
The ability of waxes to improve abrasion resistance is generally related to their ability to
increase slip.
544
BOWER
4.2.5 Antiblocking
Blocking can be reduced by incorporating hard wax particles (ball bearing mechanism)
to reduce surface contact. A compromise in gloss must often be accepted. Some hard
waxes can reduce blocking by absorbing oils and reducing the migration
to
the surface
of
other soft tacky components present in the coating formulation.
4.2.6
AntisettlinglAntisoggirlg
Agents
When dissolved and cooled in solvent-based systems, many waxes form a thixotropic
dispersion. The formation
of
a gel, which will liquify when stirred, will reduce the tendency
of denser materials (pigments, fillers, etc.) to settle out.
4.3
Incorporation
Methods
How a wax is incorporated into a coating formulation can affect the end performance.
Particle size and uniformity of dispersion are critical factors. The best method depends
on
the type
of
wax and the desired end effect. Four basic methods are used.
4.3.
I
Wax
Cornpounds
Solvent-dispersed pastes are formed by heating the wax in a suitable solvent (ideally the
same as the vehicle used in the coating formulation) and quickly cooling to room tempera-
ture under high shear conditions. The rapid cooling and high shear rates are required
to
prevent the formation
of
large seed crystals, which would appear as blemishes in the
coating when applied.
Minor variations in the production of wax compounds can cause changes in the
particle size distribution. Because
of
the sensitivity related to production conditions of
wax compounds, most users rely on wax compound producers instead of preparing such
waxes in-house.
4.3.2 Micronized
Powders
Air milling and classifying the wax powder gives the optimum control
of
particle size.
Micronized powders are based mainly on very hard, high melting waxes such
as
polyethyl-
ene waxes, amides, and polytetrafluoroethylene (PTFE). According
to
the wax definition,
PTFE is not a wax, but because it is used in the same area associated with waxes, it
is
often included in discussions on wax applications. Attempts
to
prepare these waxes
in
a
compound form is difficult. Their crystallinity invariably leads to formation of larger
particles
if
the procedure is not properly done.
The dry powder reduces the handling of flammable and hazardous solvents often
associated with waxy compounds. Both solvent-and water-dispersible grades are available.
On the negative side, improper handling can lead to dusty conditions and explosion hazards.
Micronized powders are also more expensive on a per-pound basis (however. greater
efficiency can lead to lower use levels).
4.3.3 Milling
The wax can be obtained as a coarse powder and milled
in
the same manner as pigments,
or
along with the pigments. This approach provides the end user with wax at the lowest
cost. However, the time involved and the cost of the equipment rarely justify the savings
in
raw materials. Those who currently use this approach do
so
because they already have
the equipment, which would be idle otherwise.
WAXES
545
4.3.4
Etnulsification
This approach
is
limited to water-based systems. The emulsion can be prepared in-house,
provided the equipment is available, or it can be supplied by outside producers. The
wax must be an emulsifiable type.
Most
commonly used for emulsification are oxidized
polyethylene waxes and the refined montan waxes.
REFERENCES
1.
A.
H.
Warth,
Chemistry and
Technology
qf
Waxes,
2nd ed., New York, Reinhold, 1956.
2.
H.
Bennett,
Industrial Waxes:
Vol.
l,
Natural and Syntlletic Waxes;
Vol.
II.
Compounded Waxes
3.
R. Sayers,
War:
An
Introduction.
London, European Wax Federation and Gentry Books Limited,
4.
“Wax technology,” extracted from
Proc. Clletn. Spec.
Manuf
Assoc. 61st Mid-year,
pp.
und
Technology,
New York, Chemical Publishing Co., 1975.
1983.
86-1
11,
1975.
This Page Intentionally Left Blank
63
Carboxymethylcellulose
Richey
M.
Davis
Hercules Incorporated, Wilmit~gton, Delaware
1
.O
INTRODUCTION
Carboxymethylcellulose
(CMC)
is a charged, water-soluble polysaccharide derived from
cellulose found in plants. It is used extensively in aqueous paper coatings for controlling
coating viscosity. Aqueous paper coatings are colloidal suspensions consisting primarily
of pigment, such as kaolin clay, and binders such as starch or latex. The solid concentrations
typically range from
55
to
70%.
The properties of a coating are determined by colloidal
forces acting between particles and by the properties of the suspending aqueous phase.
CMC
affects the colloidal forces by adsorbing on clay pigment and by increasing the
viscosity
of
the aqueous phase. These effects will be discussed after a brief description
of the coating process.
2.0
COATING APPLICATION
For the best performance in a paper blade coating machine, a coating should have a shear
thinning viscosity that decreases as the shear rate increases. Typical viscosities range from
about
3000
mPa.s at shear rates below
1000
sec-' when the coating is mixed and pumped
to less than
100
mPa.s at shear rates above
1,000,000
sec-' when the coating is applied
to the paper. The linear velocity of paper
in
a
coating machine can vary from less than
450
dmin to greater than
1400
dmin.
Once the fluid coating has been applied
to
the moving paper web, the paper passes
through a section of the machine in which the excess coating is removed, often by a doctor
blade. As the coating passes under the doctor blade, the paper adsorbs water from the
coating. The paper then goes to a dryer section, where most
of
the water is removed
to
This work was sponsored by Aqualon Company
547
548
DAVIS
form
a
solid coating layer on the paper.
If
the coating's water phase viscosity is too low
or
if
the coating particles form large aggregates with a porous structure, too much water
is lost prematurely from the coating under the blade. Solid aggregates form under the
blade and scratch the coated paper. Excessive water
loss
also
raises the coating solids
until
the coating becomes shear thickening (the viscosity increases with the shear rate).
This causes undesirable streaking on the paper. Shear thickening occurs as
a
result of
increased collisions between the colloidal particles
in
the coating. These collisions form
aggregates, which raise the viscosity. For good coating performance,
it
is
important to
viscosity the water phase and to suppress the tendency of the solid particles to aggregate
upon collision. The viscosity of the water phase increases as the concentration of CMC
increases.' Particle aggregation is a direct consequence of the colloidal forces acting be-
tween the particles. These forces and the role of CMC
in
affecting them are discussed
next.
3.0
COATING FORMULATION
The colloidal chemistry of paper coatings is governed by the attractive and repulsive forces
between the clay, binder, and polymer. Kaolin clay comes
in
the form of thin platelets
D
D
D
D
PARTS
CMC/100
PARTS
CLAY
Figure
1
Adsorption isotherm for kaolin clay and
CMC
at
pH
=
7
and
25°C.
Adapted from
Ref.
4
with permission.
CARBOXYMETHYLCELLULOSE
549
that have negative charges on the edges and faces at pH values exceeding
7.'
Paper coatings
are used typically at
a
pH
of
8-9.
Clay particles are attracted to each other by van der
Waals forces, which are particularly strong when platelets approach to within
l
nm
of
each other.3 Most clays are treated with an anionic dispersant such
as
a
low molecular
weight polyacrylate, which adsorbs on the clay surface. This increases the net negative
charge on the platelets and causes them to repel their neighbors, thus reducing the tendency
for particle aggregation.
A
polymeric dispersant
also
imparts stability through steric stabili-
zation, in which adsorbed polymer chains on one clay platelet repel polymer chains on
neighboring platelets
as
a
result
of
osmotic pressure.3
Negatively charged CMC adsorbs onto kaolin clay and acts
as
a
stabilizer. Figure
1
shows the adsorption isotherm
for
CMC with molecular weight
of
1
10
kg/mol onto
clay.' The amount of adsorbed polymer per mass of clay increases
as
the added polymer
concentration increases. This polymer adsorption increases the amount of negative charge
and hence the zeta potential on the clay, as shown in Figure
2.
The zeta potential, which
is the electrostatic potential near the surface
of
a
clay aggregate, ranged from
20
to
30
mV,
which is typical for suspensions that are moderately stabilized by electrostatic repulsion.
>
E
i
6
F
Z
W
l-
0
a
l-
6
W
N
36
34
-
32
-
30-
2a-
26
-
24
-
22
-
m
20
I
l
I
I
1
I
0
1
2
3
4
5
6
U
m
U
PARTS
CMC/lOO
PARTS
CLAY
U
Figure
2
Zeta potential of kaolin clay with adsorbed
CMC
at
pH
=
7
and
25°C
Adaptcd from
Ref.
4
with permission.
550
DAVIS
?
1000
10,o
0
0
100,000
SHEAR RATE, 1/SEC
O
00
GC,
0
0
C
C
0
0
mm
SHEAR RATE, 1/SEC
Figure
3
High shear viscosity for kaolin clay and CMC;
W
clay volume fraction
=
0.39.
Distilled
water
control:
0.10
part
CMC/I00 parts clay;
0
0.21
part CMC/100
parts
clay;
0
0.31
part
CMC/100 parts
clay.
(Adapted
from
Ref.
4
with
permission.)
The effect of CMC on coating viscosity at high shear rates is illustrated by the first
curve
in
Figure
3,
where
a
concentration kaolin clay suspension without CMC begins
to
shear thicken at
a
shear rate of
18,000
sec-
l.'
Adding
0.10
part of CMC per
100
parts
clay increases the viscosity and
also
slightly reduces the shear thickening tendency. As
the CMC concentration increases
to
0.21
part, the suspension becomes shear thinning.
Increasing the CMC level
to
0.31
part makes the suspension even more shear thinning. The
adsorbed CMC increases the degree of electrostatic and steric stabilization, thus inhibiting
collision-induced flocculation.
There are no quantitative models that predict accurately the onset
of
shear thickening
viscosity in paper coatings at very high shear rates. These are, however, numerous theories
for the viscosity
of
concentrated suspensions at low shear rates. These were reviewed by
Metzner.s A useful, general model was developed by Krieger." The suspension viscosity
-q
is correlated with the water phase viscosity
-qs
and the solid volume fraction
as
where
@
is the ratio
of
the solids volume and the total volume;
a',,,
and
B
are dimensionless
parameters determined by fitting viscosity data to Equation
1.
CARBOXYMETHYLCELLULOSE
551
An analysis
of
viscosity data using Equation
1
suggests that paper coatings, like
other concentrated suspensions, flow in ordered layers along streamlines rather like cards
in
a deck sliding over each other.' At low shear rates, this ordered structure results
in
a
low viscosity. Above a critical shear rate, this ordered structure breaks down and the
particles
in
the suspension collide far more frequently, resulting in flow-induced agglomer-
ation and the resulting shear thickening viscosity are suppressed by treating the coating
with
CMC.
REFERENCES
1.
Aqualon Cellulose Gum (Sodium Carboxymethylcellulose), (product bulletin), Aqualon Com-
2. A. E. James, and D.
J.
A.
William,
Adv.
Colloid Inteufcrce
Sci., /7:219 (1982).
3.
H.
Van Olphen,
An
Introduction
to
Clay
Colloid Chernistry.
2nd ed. New York: Wiley-lntersci-
4.
R.
M. Davis,
TAPPI
J.
70(5),
99 (May 1987).
5.
A.
B.
Metzner,
J.
Rhrol..
29
739 (1985).
6.
J.
W. Goodwin, in
Sciencv
and
Technology
of
Polymer Colloids.
G. W. Poehlein, Ed. 1982. p.
pany, Wilmington, DE 19850-54
17.
ence, 1977, p. 48.
552.