Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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702
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3.3.
I
Acrylic-Isocyanate (Acrylic-Urethane)
Acrylic-urethanes are formed exactly the same
as
their polyester counterparts. The acrylic
resins are linear instead
of
aromatic. However, they use hydroxy functionality and blocked
isocyanates to form the urethane bonds.
OH OH
HO-Acrylic
=
R
L
Blocked Isocyanate
0-I-0-T-0-I-R
‘0
\I
\R
Cross-link Intermediate
3.3.2
Acrylic-Diacid (Glycidyl-Acrylic)
Epoxy- (glycidyl-)functional acrylic resins can be compared to the hybrid systems dis-
cussed earlier. They are generally reacted with dicarboxylic acids or anhydrides. The most
common cross-linker is 1,12-duodecanoic acid (1,12-dodecane dioic acid).
oa
oa
I
I
I
I
I
I I
I
I
-R-C C-R-
CH2 CH2
0
0
CH2 CH2
0
0
2C
-
R
-
C02H
+
a
CO
0
CH2
I
I
I
0
I
-o-co-R-co-o-c
Glycidyl-Acrvlic
=
R
Diacid Cross-link Intermediate
4.0
FORMULATION
4.1
Resin
Systems
Formulation of thermoset powder coatings is much the same
as
that of liquid coatings
used for similar purposes. A coating may be chosen for functional or decorative purposes.
THERMOSET POWDER COATINGS
703
All
of the coating types used
in
powder coatings can offer decorative options. Nonetheless,
it is function that dictates system choice. The resin chemistry must be chosen to suit
service needs. Various pigments, fillers, and additive materials are then included to enhance
decorative or functional requirements.
There are two primary functions for any protective coating. They are chemical pro-
tection and exterior durability.
As
with many coating properties, these two tend to be in
opposition. The best systems for chemical protection are usually the poorest for exterior
durability.
Epoxy powder coating systems deliver the best chemical and corrosion resistance.
However, they have the least effective exterior durability. The double bonds in the aromatic
rings are easily broken by the ultraviolet light from the sun. Glossy finishes will go flat
with as little as six months exposure, with film degradation following soon thereafter.
Urethane systems are also very good for chemical resistance, and they have fairly
good exterior durability as well. Hybrids offer good chemical resistance, but the polyester
component makes them less effective. They are also poor with relation to exterior exposure
due to the epoxy portion of the cross-link network.
Acrylic and TGIC powder coatings provide the best exterior durability. Some sys-
tems can survive for up to twenty years exposure. They offer fair to good chemical resis-
tance. Urethane systems seem to be the best compromise between chemical and exposure
properties.
As
mentioned, they are very good in both areas.
4.2
Pigments and Fillers
Most pigments and fillers used
in
liquid coatings are suitable for use in powder coatings.
There are only a few special requirements for use. They must be sufficiently heat stable
so
they withstand the heat of extrusion and curing without degradation or color change.
Normally, the heat of extrusion is 125°C
or
less for a minute or two. The heat of cure is
usually 160-200°C for
10
to 20 minutes.
Secondly, they must be insoluble and nonreactive in the resin system. Blooming
and color shift are the most common results from pigments that are partially soluble or
reactive with the binder. Some epoxy system curing agents are especially susceptible
to
reaction with pigments.
4.3
Additives
4.3.1
Flow
and
Leveling
Flow and leveling agents are designed
to
minimize surface defects such as craters, pinholes,
and orange peel. The mechanism of their function alters the surface tension and/or rheology
of the coating. The likelihood
of
a smooth defect-free film is improved by reducing one
(or both) of these properties in a coating.
Most flow and leveling agents are liquids. Many are blended with inert inorganic
materials to offer them in a conveniently solid form. Chemically these are usually polyacry-
lates or polysiloxanes.
A
few new flow agents are available, however, in solid organic
form.
4.3.2
Debuhbling (Degassing)
The most common debubbling agent is benzoin (2-hydroxy- 1,2-diphenyl ethanone). It is
used to keep the surface of a curing film open long enough to allow for entrained air and
704
WAELDE
evolved gases to escape. Trapped air and gas bubbles
are
cause for premature failure of
coating films because they make the coating brittle. The one drawback
to
the use of
benzoin is its tendency to cause yellowing in lighter colors. A number of new advances
have entered the market in an attempt
to
match the efficiency
of
benzoin without the
challenge
of
yellowing.
4.3.3
UV
Inhibitors
Various ultraviolet light inhibitors are available
to
aid coating resistance to degradation
by the sun’s rays. The most common are hindered amines, phosphites, sulfates, and pheno-
lics. Most will have some positive effect on any coating’s UV resistance. However, each
system will require testing to determine the best combination of inhibitors. Some systems,
like epoxies and hybrids, will not develop any substantial UV resistance due to their
aromatic nature.
4.3.4
Catulysts
Catalysts or accelerators are used to reduce the reaction time or curing temperature of the
resin and cross-linker. They allow for faster production time by shortening the gel or set
time of the thermosetting coating. Energy can be conserved because
full
cure may be
attained at lower oven temperatures. The most common catalysts are thiazoles (used
in
polyesters), phosphines and ammonium halides (used in epoxies), and thiocarbamates
(used
in
urethanes).
5.0
END
USES
The following table details some of the applications currently using the chemistries dis-
cussed in this chapter.
Type Typical Applications
EPOXY Shelving, transformer cases, primers, bathroom fixtures, refrigerator
racks, sweepers, sewing machines, power tools, room air conditioners,
office furniture, instrument cases, garden tools, kitchen furniture, fire
extinguishers, toys, refrigerator liners, dryer drums, microwave ovens,
mixers and blenders, fertilizer spreaders, screening,
oil
filters, automo-
bile springs, hospital equipment, bus seat frames, business machines,
glass bottles
Hybrids Tool boxes, farm equipment, electrical control boxes, hot water heat-
ers, hot water radiators, primerkurfacers, grain storage bins, trans-
former covers.
0
1.1 filters and air cleaners, air conditioner housings,
fire extinguishers, toys, screening wire, power tools, shelving, office
furniture
Urethane Fluorescent light fixtures, steel and aluminum wheels, patio furniture,
playground equipment, fence fittings, chrome wheels and trim, garden
tractors, range side panels and broiler, ornamental iron, air conditioner
cabinets, restaurant furniture supports, transformer cases
TGIC Irrigation pipe and fixtures, outdoor furniture, air conditioning units,
steel and aluminum, wheels, wire fencing, fence poles and fittings,
farm equipment, aluminum extrusions, transformers
THERMOSET
POWDER
COATINGS
705
Acrylic Range side panels, refrigerator cabinets and doors, washing machine
parts, dishwasher exterior, aluminum extrusions, microwave ovens,
garden tractors, automotive trim coating
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
IO.
11.
12.
13.
14.
15.
Patent GB
#643,691; 9/27/50;
P. G. Clements (to Schori Metallizing Process, Ltd.).
Patent
DE
A933.019; 9/15/55;
E. Gemmer (to Knapsack-Griesheim AG).
Patent
US
R.844.489; 7/22/58;
E. Gemmer (to Knapsack-Griesheim AG).
Patent GB
#915,575.
E.
P. Miller, Electrostatic finishing methods, paper presented
at
the annual meeting of the
National Paint. Varnish, and Lacquer Assn., Colorado Spgs., CO,
9/12/63.
D.
A. Bates,
The
Science
of
Powder
Cocctings,
Vol.
1.
Selective Industrial Training Associates,
Ltd.,
p.
4.
London,
1990.
Ibid.
p.
4.
Ibid.
p.
4.
Ibid.
p.
4.
Statistics from Powder Coatings Institute, Alexandria, VA.
Rohm and
Haas
Brochure 82F2, Primid XL-552-A Novel Crosslinker for Powder Coatings,
1990.
Josef
H.
Jilek,
Powder Coatings
(monograph), Fed. of Soc. for Ctg. Tech., Blue Bell, PA,
1991.
Douglas
S.
Richart, Powder coatings-A review of the technology,
Amer. Point
Crg.
J.,
4/
22/9
1.
Pulverizing Machinery
(product brochure), Mikropul Corp., Summit, NJ.
R. Campbell and R. Kumar, Organic pigments for powder coatings,
Amer. Paint
Ctg.
J.,
41
2219
I.
This Page Intentionally Left Blank
80
Peelable Medical Coatings
Donald
A.
Reinke
Olilrer
Products
Cornparlv.
Grmd
Rapids,
Michigun
1
.O
INTRODUCTION
Peelable coatings used on packaging materials for disposable medical devices first became
practical in the late
1960s
with the introduction
of
a spunbonded polyethylene paperlike
material (Tyvek) by
E.
I. DuPont Company. Previously, disposable or single-use medical
devices were packaged in tear-open packages. Medical device manufacturers were only
beginning to be concerned about the particulates generated when such packages were torn
open. The industry was not regulated at this time, and the concern for product liability
was not to come for several years.
A
major change took place in the way medical devices were packaged when DuPont
began marketing Tyvek as a bacterial barrier material for packaging. Tyvek is a unique
material that meets all the critical requirements for medical device packaging. It is a good
bacterial filter, that is water resistant, as well as puncture and tear resistant. Being made
from polyethylene, it is stable during both gas and radiation sterilization. It is also nontoxic,
hence could meet the same
U.S.
Pharmacopoeia requirements listed for medical devices.
With the availability
of
an acceptable packaging material, peelable coatings could
be developed
to
seal this material to plastic films
to
form pouches or to thermoformed
plastic trays as a lid.
2.0
COLD-SEAL COATINGS
Peelable coatings for medical devices generally fall into two groups. First, cold-seal or
cohesive coatings, made from latex, are sealed by pressure alone and will seal to themselves
only. This type of coating has been used for self-sealing envelopes, for candy wrappers,
and in a variety of industrial packages. Peelability with cohesive coatings is achieved by
coating a release layer between the adhesive and the substrate. This allows the adhesive
to peel away from the substrate without leaving a residue or tearing fibers.
707
708
REINKE
3.0
HEAT-SEAL
COATINGS
The second type
of
coating, heat-seal adhesives. are more complex in their requirements
than the cold-seal adhesives. The latter (and) generally are applied only in the seal area,
seal only to themselves. The heat-seal adhesives are normally coated over the entire surface
of the web and have
to
seal to a wide range
of
plastics. Heat and pressure
are
required
to
form a seal.
Many different types of heat-seal coating are used for medical packaging, but
all
must meet the same requirements. The most important requirement is that they be stable
after sealing,
so
that a sterile package is assured for several years. Disposable devices are
sterilized after packaging and must remain sterile until used. They must also have a porosity
of under
200
seconds Gurley,
to
prevent the buildup of excessive internal pressure in the
package during a normal gas sterilization cycle. The coating must be nontoxic and must
meet the same requirements listed in the
US.
Pharmacopoeia for the plastics used in the
device being packaged. Other desired attributes are good hot tack, clean color, enough
strength to pass shipping tests, and good transfer of the adhesive onto the seal surface
when the package is peeled open.
The coatings used on heat-sealable Tyvek and papers come in many forms. They
can be solvent-type lacquers applied in
a
grid pattern to maintain porosity, water-based
dispersions dried very carefully
so
that they do not melt, or hot melt adhesives applied
in small dots.
The lacquer-type adhesives are applied by the gravure process at a rate
of
3-6
pounds per 3000 square feet. The adhesive is applied in lines that cross at an angle, leaving
an uncoated area between them resulting in good porosity. Also the lines break up the
peel forces
to
give better peelability.
The water-based dispersions are based
on
ethylene-vinyl acetate copolymer disper-
sions blended with waxes and tackifiers
to
form a blend
of
particles. This blend is coated on
the web by air knife coating or by gravure. The coating must be dried at
a
low temperature
to
keep the coating from melting and forming a film. The porosity is achieved by having
large particles
in
the blend and by mud cracking of the coating as it dries. The dried
coating weight is
5-8
pounds per 3000 square feet.
Hot melt coatings are a blend
of
an ethylene-vinyl acetate copolymer, terpene resins,
and wax. These are melted together and applied by a gravure roller engraved with
50-70
cells per inch. The small dots of adhesive that result from this have a small uncoated area
between them that makes the coating porous. The coating is applied at a rate of
9-12
pounds per
3000
square feet.
The optimum amount of coating will vary for each type of adhesive. The controlling
factor is the melt viscosity of the adhesive at the sealing temperature. If the viscosity is
low, a portion
of
the adhesive will be pushed into the substrate, reducing the amount of
adhesive available to absorb stresses and to transfer during peeling. Good adhesion to a
smooth surface with coated paper or Tyvek can be achieved with
5-
12
pounds of adhesive
per 3000 square feet. An accepted rule of thumb in the coating industry is that
15
pounds
per
3000
square feet is equal to
0.001
inch of thickness.
While there are no regulations in this country requiring nontearing packages, the
medical device industry places a high priority on this feature. Another desirable feature
of the coating, which goes along with peelability, is the ability
of
the coating
to
transfer
to the plastic surface
to
which it is sealed. The transfer pattern must be unbroken when the
package is opened. This is interpreted to mean that the seal integrity has been maintained. A
PEELABLE MEDICAL COATINGS
709
clear track through the seal area will cause the package to be rejected because there
is
a
path for bacteria to enter the package.
Formulating a coating that is nontoxic and strong, with good hot tack and good
adhesion to a variety of plastic surfaces, and is able to peel with a controlled peel strength,
is a difficult task.
Several methods have been used to achieve peelability. The use of primer coats
between the adhesive and the substrate is the one most often chosen; this creates as a
parting layer between the adhesive and the substrate. Release coatings on the substrate
surfaces are also used where paper instead
of
Tyvek is used.
Most people in the industry feel that the best method is
to
have an adhesive with a
controlled cohesive strength. When the package is peeled open, the adhesive will split
with a controlled force that is below the delamination strength
of
the substrate. By using
this method, the surface fibers of
the
substrate are not raised
or
broken.
Cohesive failure of the adhesive can be achieved by formulating an adhesive that
has two phases, one a strong adhesive to hold the package together and the other a weak
friable material that breaks up the structure of the first phase. By varying the percentages
of
the two phases, the cohesive strength of the coating can be controlled within narrow
limits.
The nature
of
peelable medical packaging materials is currently undergoing a change.
There is a drive to reduce the cost of medical devices, which is causing a shift to paper
and away from Tyvek. Also environmental concerns are pressuring medical device compa-
nies to use some method other than ethylene oxide gas sterilization. The requirement for
a peelable package however still remains strong.
Further information on medical device packaging can be found in the proceedings
of
the Technical Association of the Pulp and Paper Industry’s Coatings and Laminations
conferences during the years from 1980-1987.
This Page Intentionally Left Blank
81
Conductive Coatings
1
.O
INTRODUCTION
In 1986 sales
in
the coatings industry exceeded $10 billion, and production approached
a
billion gallons.' The breakdown
of
sales was
$4.1
billion for architectural coatings,
$3.5
billion for industrial coatings, and
$2.4
billion for specialty coatings. Conductive
coatings-a minuscule part of these trade sales-have been used both as industrial coatings
and
as
specialty coatings. Regulations of the Federal Communications Commission (FCC),
in Docket
No.
20780, which regulates electromagnetic emissions from computing devices,
have provided a strong impetus for the commercial development of conductive polymeric
materials (including coatings and paints). Since October
I,
1983,
it
has been necessary
for any computing device that generated signals or pulses in excess of
10
kHz to comply
with the emission standards set forth in the docket. Although conductive polymeric coat-
ings have made inroads
in
areas where metallic coatings previously were used, progress
has been slow.
A product related to conductive coatings is metallized plastic. The most important
commercial processes for metallizing plastics are electroless plating, metal spraying, sput-
tering, and vacuum metallizing. The first commercial plating of plastics was recorded
in
1905.' Metallizing of plastics occurred during World War
11,
and large-scale production
started in the early 1960s. All these processes are now multimillion-dollar industries. Large
quantities of plastics are metallized each year, with automotive items making up more
than 60% of the market on a plated area basis.'
There are various reasons for metallizing plastics. In the automotive industry, metal-
lized plastic combines the consumer appeal of metal with light weight. Electroless copper
metallization is an indispensable part of the modern electronics industry. Printed circuit
boards use electroless copper to coat nonconductive plastic surfaces to define the circuit
patterns. Zinc arc and flame-spray techniques provide electromagnetic interference shield-
71
1