Coatings Technology Handbook Episode 3 Part 2 docx

Polyurethane Associative Thickeners for Waterborne Coatings

85

-7

PEUPU thickeners are used extensively in high gloss industrial or decorative applications. Here they
are typically used as single thickeners. The main criterion for selection is the precise flow required.
Industrial spray-applied paints need strongly shear-thinning behavior with relatively low high-shear
viscosity for good spray characteristics. Decorative paints, on the other hand, need more high-shear
viscosity to give adequate film build. For paints that are to be applied with both brush and spray, a
compromise is necessary.
The flow of a wood clear-coat is generally more difficult to control because of potential side effects
caused by the additives. Small changes in gloss or haze can be detrimental to the visual impression of
the surface. Special PEUPU grades have to be used for these applications. For spray-applied furniture
coatings, the thickener should be hydrophobic for greatest efficiency at the lowest possible loading. This
will minimize the risk of unwanted side effects. For parquet coatings, the thickener should give Newtonian
flow to optimize leveling and retain high-shear viscosity for film build.

85.7 Summary

PEUPU thickeners are now available in many forms, offering a variety of rheological characteristics.
Their thickening mechanism is purely associative, which leads to excellent flow and application properties.
The benefits of using these additives on the quality of the coating are tremendous. Properties such as
leveling, spatter resistance, film build, transparency, and gloss can all be improved relative to those found
with more traditional classes of thickeners. With careful selection, almost all flow problems can be solved.
Choice of the correct PEUPU depends on understanding not only the final flow requirements of the
coating, but also the potential interactions with the other raw ingredients. Optimization of the flow can
only be achieved if these interactions are well understood. Of greatest influence are the cosolvents used,

the surfactant or emulsion stabilizer package, and the latex particle size. For effective formulation, close
consultation with the additive supplier is recommended. This will ensure that the most appropriate
thickener is chosen and potential problems are avoided.

References

1. T. Annable, R. A. Brown, J. C. Padget, and Avd Elshout, “Improvements in the rheological control
of latex paints,” prepared for Advances in Coatings Technology, 2nd International Conference,
Katowice, 1996, Paper 17.
2. J. H. Bieleman, F. J. J. Riesthuis, and PMvd Velden, in D. R. Karsa, Ed.

Additives for Water-Based
Coatings

. Cambridge: Royal Society of Chemistry, 1990, pp. 156–180.
3. P. Bissinger, H. -R. Seelig, in

Waessrige Siliconharz-Beschichtungssyteme fuer Fassaden

. W. Schultze,
Ed. Renningen-Malmsheim: expert-Verlag, 1997, pp. 298–319.
4. P. A. Reynolds,

Prog. Org. Coat., 20

, 393–409 (1992).
5. H. N. Naé and R. H. Bank,

Rheology, 91

, 170–178 (1991).
6. M. T. Tetenbaum and B. C. Crowley, U.S. Patent 4,499,233, February 12, 1985.
7.

Rheology Handbook: A Practical Guide to Rheological Additives

. Hightstown, NJ: Elementis, Inc.,
2000, pp. 11–16.
8. R. D. Hester and D. R. Squire, Jr.,

J. Coat. Technol., 69

(864), 109–114 (1997).
9. U. Thies, in

Grundlagen zum Formulierung von Dispersions-Silikat-Systemen

. Ladenburg: BK Lad-
enburg, 1988, pp. 135–145.
10. A. Karunasena and J. E. Glass,

Prog. Org. Coat., 17

, 301–320 (1989).
11. J. E. Glass, D. J. Lundberg, M. Zeying, A. Karunasena, and R. G. Brown, “Viscoelasticity and high
shear rate viscosity in associative thickener formulations,” in

Proceedings of the Water-Borne and
Higher-Solids Symposium

,



New Orleans, 1990, pp. 102–120.
12. F. Sauer and P. Manshausen, “Neue rheologie-additive fuer dispersionsfarben und industrielack-
systeme,” in

VILF Seminar, Neue Additive, Fuellstoffe und Pigmente fuer neue Lacksyteme

,



Duessel-
dorf, 2000, pp. 62–73.

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85

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Coatings Technology Handbook, Third Edition

13. A. J. Whitton and R. E. Van Doren, “Formulating with rheological additives for latex paints,”
presented at the Advances in Coatings Technology 1st Conference, Singapore, 1991.
14. J. E. Glass,

J. Coat. Technol.,

50(640), 53–60 (1978).
15. J. E. Glass,

J. Coat. Technol.,

50(641), 56–71 (1978).
16. D. N. Smith and R. Klein, Optimising the Performance of Associative Thickeners in Waterborne
Coatings. AddCoat 2001, Orlando, 2001, Paper 13.
17. D. N. Smith, Rheological Additives — Making the Choice. 23rd FATIPEC Congress, Brussels, 1996,
pp. C151–182.
18. S. O. Williams, J. Neely, S. S. Kraus, and E. A. Johnson. Rheology Modifier and Dispersant Com-
patibility in Latex Paints. AddCoat 2001, Orlando, 2001, Paper 7.

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IV

-1

IV

Surface Coatings

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86

-1

86

Flexographic Inks

86.1 Introduction

86-

1
86.2 Process

86-

1
86.3 Substrate

86-

2
86.4 Vehicles

86-

2
86.5 Colorants

86-

3
86.6 Formulations

86-

3

86.1 Introduction

Flexography is a high speed printing process based on roll-to-roll mechanics where the inks are printed
on a multitude of different substrates. In some cases, the substrate is sheet fed. The inks are based on
colorants, resins, and solvents (organic or water), as well as various additives. The particular set of
ingredients is determined by the substrate, product to be packaged, and application and process specifics.
The last few years have seen major changes and improvements in the finished, printed materials;
therefore, the market share of this process in the packaging market has grown at a high rate. Some of
the changes and improvements have included the inking system, the number of colors available on the
presses, drying capacity of the equipment, press speeds, and changes in the inks.

86.2 Process

The materials printed using the flexographic process include much of the flexible packaging found in a
supermarket, corrugated containers, folding cartons, and many printed consumer products as well
(towels, tissues, diapers, cups, etc.). The upgrading of the process used to print these items has been
dramatic in the last few years. Traditionally, the process was a three-roll system. Today, the predominant
system uses enclosed doctor blades for the inking of the plates.
The three-roll system uses a rubber roll to pick up the ink in the fountain and transfer it to an anilox
roll (an engraved metal roll or ceramic roll). The ink fills the engraved cells and is then transferred to
plates (rubber or photopolymer). The image on the plate is raised from the surface background nonimage
area. A final transfer then occurs of the ink to the substrate to be printed. The printed material then gets

a final drying and is either rewound into a roll for future finishing or goes immediately to a lamination step.
The enclosed doctor blade system is the present inking system of choice. A doctoring blade (metal or
plastic) is positioned so that it forms a trough for the ink. The ink is fed into this area that is against the
anilox roll. The ink fills the engraved anilox cells, and excess ink is metered off by the blade. The metered
ink is then transferred to the plate and continues on as in the three-roll system.
In both systems, the amount of color to be transferred to the substrate is determined by the amount
of color in the ink, its viscosity, and the volume of cell engraving in the anilox as well as the number of
cells present. What the system changes have brought about is the following:

Sam Gilbert

Sun Chemical Corporation

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87

-1

87

Multicolor Coatings

87.1

87.2 Practice of the Art

87-

1

References

87-

2

87.1 Introduction

A standard paint is a single shade attained by a mixture of many finely ground pigments so that one
perceives a homogeneous shade. But natural products are seldom homogeneous in color. They are striped
and spotted with dark and light regions of varying size. To reproduce this appearance in a painted surface,
multicolor coatings are used that yield a multicolor appearance in one coat. Zola

1

discusses multicolor
coatings of a natural appearance, with striations from brush application and mottling from roller or
spray application. Multicolor paints are used as interior architectural coatings for public buildings and
have been used in automotive trunk interiors.

87.2 Practice of the Art

Little is published on the practice of formulation or making the multicolor coating. Zola patents show
multicolor formulations.

2–8

The major sources of information are the patents and publications

9–23

and
the commercial brochures describing the products used and specific formulation technology.

87.2.1 Continuous Phase

The continuous phase in a colloidal mixture is called the dispersion medium (e.g., water in milk, air
supporting the droplets of water in a fog). The continuous phase must be a fluid that has a sufficiently
low viscosity at high shear rates to allow easy application of the paint. If it is too stiff, the drag on the
brush will tire the painter, or the fluid will not be properly atomized in a spraying application. However,
at low shear rates, the viscosity must be sufficiently high to slow the settling out or the agglomeration
and coalescence of particles.
Colloidal additives are used to protect such dispersions. Protective colloids are added to increase storage
stability. Methyl cellulose, polyvinyl alcohol, and various nonionic surfactants are the most often used
protective additives. These materials absorb onto the surface of the suspended materials to form a jellylike
layer that inhibits coagulation. Another approach is to induce an electrical charge on the globule surface,
so that the globules will repel each other. Ionic surfactants, salts, or charged clay particles are commonly
added to induce surface charges.
The stabilization of suspended globules is absolutely necessary to prevent the coalescence and mixing
of different colors. This ability must extend during the coating drying as well: the individual spots must
have distinct boundaries with the next spot of a different color. This requires that the paint dry quickly.

Robert D. Athey, Jr.

Athey Technologies

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Continuous Phase • Dispersed Phase • Combining Dispersed
Introduction 87-1
and Continuous Phases

88

-1

88

Paintings Conservation

Varnish

88.1 Introduction

88-

1
88.2 Effect of Varnishes

88-

1
88.3 Types of Varnishes

88-

2
References

88-

3

88.1 Introduction

To understand the properties of a varnish that satisfy the requisites of paintings conservation, it is
necessary to introduce the rudiments of conservation and the artist’s original reasons for varnishing. It
is also important to mention that no varnish currently exists that satisfies all the needs of a paintings
conservator, though great progress has recently been made.

1–3

88.2 Effect of Varnishes

Picture varnishes used for the restoration of paintings are intended to create the same optical effect on
the painting that was achieved by the artist but subsequently lost as a result of chemical and physical
changes of the original varnish layer. An artist, for instance, relies on varnish to create the perception of
the third dimension and the saturation of colors as well as to achieve some degree of glossiness in the
process. The deleterious ramifications of a visibly aged varnish are not confined to the obvious yellowing
of the colors; in addition, the lightening of dark passages and the darkening of lighter areas may result
in a reduction in the range of color one visualizes. How these changes affect the painting depend on the
style and subject matter of the painting as well as the materials the artist used. It is, however, safe to say
that in any circumstance, these changes divert the purpose of the picture.
It is most often changes in the varnish layer that can obscure or confuse an artist’s original intentions.
Unfortunately, badly aged varnish is not the only factor that can upset the balance of a painting: some
pigments are fugitive and thus fade; binding media can yellow; and furthermore, the various components
of the painting as a whole respond differently to climatic changes in the environment, causing more
brittle areas to crack under stress. Fortunately, the sensitive, well-trained conservator, through scholarly

and scientific assistance, best understands how these inevitable acts of time affect the balance of a painting
and how the work was originally intended to read. With the greatest caution, using fully reversible
materials and techniques, conservators can restore much of this balance, depending on the extent of
change that has overcome the painting.
The effect of varnish on a painting cannot be ignored; in fact, there have been some movements in
art history that preferred the matte appearance of an unvarnished picture, most notably the Impressionists
and Cubists.

4–6

(The responsible conservator will honor the artists’s choice and refrain from ever var-
nishing such a picture.) On the other hand, an artist who chooses to varnish a painting has most certainly

Christopher W.
McGlinchey

The Metropolitan Museum of Art

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89

-1

89

Thermoset Powder

Coatings

89.1 Introduction

89-

1

Discovers Powder Coatings

89.2 Processing and Equipment

89-

3

Application Equipment

89.3 Chemistry

89-

6

89.4 Formulation

89-

11

89.5 End Uses

89-

12
References

89-

12

89.1 Introduction

In recent years, awareness of environmental conservation and pollution prevention has risen steadily.
Governmental regulation and true concern for the environment have motivated chemists to modify all
types of coatings to reduce environmental impact. The concept of environmental “friendliness” has
dramatically changed the way that coatings are formulated.
Powder coatings are arguably the most environmentally “friendly” coatings. They do not contain
solvents to be released as hazardous air pollutants (HAPs). Powder coatings release very low amounts of
volatile organic compounds (VOCs) during the baking cycle. They produce virtually no waste material.
And, they contain very few hazardous chemicals. (Note: The few hazardous chemicals that have found
their way into powder coatings are decreasing as they are replaced with safer materials.)

89.1.1 Powder Coatings Defined

Powder coatings can be described as “ground up dry paint.” They may also be referred to as pulverized
plastics. They have properties in common with both materials. The polymeric resins that are used in
producing powder coatings are similar in nature to those used in both paints and plastics. All three
materials are composed of combinations of resin, pigment, filler, and various additive materials. They
may be thermoplastic or thermosetting.
The primary difference between the three types of compounds is the molecular weight range of the

polymers used as binders. Plastics use the highest molecular weight resins, paint the next highest, and
powder coatings the lowest. Paint, of course, also contains various solvents to dissolve or dilute the coating
for easy application.

Lawrence R. Waelde

Troy Corporation

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Powder Coatings Defined • The First Powder Coatings •
Premixture • Extrusion • Grinding • Sifting and Classifying •
Thermoset Beginnings • The Beginning of Growth • The World
Epoxy Systems • Polyester Systems • Acrylic Systems
Resin Systems • Pigments and Fillers • Additives

Thermoset Powder Coatings

89

-7

The epoxy groups terminating each molecule react with acidic or basic curing agents. The three most
common are the phenols, dicyandiamides (DICY), and carboxylic acids, including carboxy-terminated
polyesters. Various acids, anhydrides, amines, and imidazoles are also used as cross-linkers with epoxy resins.

89.3.1.1 Epoxy-Phenols (Phenolic)

The curing of epoxy resins with phenols results in the opening of the epoxide ring and the formation of
a hydroxyl group, at either the primary or secondary position. The hydroxyl group is available for reaction

in the cross-linking of the resin. The aromatic ring attaches to the unreacted carbon of the epoxide.

89.3.1.2 Epoxy-Dicyandiamide (DICY)

DICY cured epoxy coatings react in a similar manner to that of the previous type, where nitrogen-bearing
groups replace the aromatic ring. All four functional groups will react with the epoxide, acting as a
primary or secondary amine.

89.3.1.3 Epoxy–Polyester (Hybrid)

Epoxy resins react with carboxy-functional polyesters in the same way as carboxylic acids. The hydroxyl portion
of the acid group reacts with the epoxide. The rest of the reaction follows as we have seen in the previous
examples. Because both reactants are considered primary resins, the system is referred to as a “hybrid.”
O
CH
2
CH
CH
2
O
O
CH
2
CH CH
2
O
CH
2
O
Diglycidyl Ether of Novolac Resin

CH
2
CH
CH
2
O
O
CH
2
n
R1
O
CH
2
CH
2
O
CH
R2
HO
Epoxy Resin Phenolic Crosslink Intermediate
R2
O
R1 O
CH
2
OH
CH
+
N

C
N
H
C
NH
NH
2
O
CH
2
CH
CH
2
O
R
O
CH
2
CH
CH
2
OH
R
N
O
CH
3
C
CH
2

OH
R
NC
N
C
NH
O
CH
2
CH
CH
2
OH
R
+
DICY Epoxy Resin Crosslink Intermediate

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Thermoset Powder Coatings

89

-9

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89.3.2.2 Polyester–Isocyanate (Polyurethane)
Polyester resins used to produce urethanes have hydroxyl functionality. They react at the carbon–nitrogen

bonds in an isocyanate. The two isocyanates most used in powder coatings are isophorone diisocyanate
(IPDI) and toluene diisocyanate (TDI). However, TDI melts at room temperature, and IPDI is a liquid.
This would cause any powder coating in which they were used to have poor package stability. The coating
would sinter (cake) into a solid very quickly. Furthermore, they would react with the polyester and gel
during the heat of extrusion, again making them unusable.
Consequently, isocyanates are reacted with triol materials to give them higher melting points and
better package stability when compounded in powder coatings. Then they are blocked, most commonly
with (epsilon)
ε-caprolactam, to prevent them from reacting with the polyester until they unblock
naturally at curing temperatures in the oven.
N
H
O
CH
3
CH
3
N
C
O
H
3
C
CH
2
N C
O
CH
3
N

N
C
O
C
O
R
OH
HO
OH
ε-Caprolactam Isophorone Diisocyanate Toluene Diisocyanate Triol
N
O
NC
O
N
O
CH
3
CH
3
N
C
O
H
3
C
CH
2
N
C

O
CH
3
CH
3
N
C
O
CH
3
CH
2
N
C
O
CH
3
CH
3
N
C
O
H
3
C
CH
2
N
O
R

O
O
O
HO R1
"
OH
OH
+
ε-Caprolactam Blocked IPDI
HO-Polyester
C
CH
3
CH
3
N
C
O
H
3
C
CH
2
N
C
O
CH
3
CH
3

N C
O
CH
3
CH
2
N
C
O
CH
3
CH
3
N
C
O
Crosslink Intermediate
H
3
C
CH
2
N
O
R
O
O
O
HO
R1

"
O
OH
O
R1
"
OH
OH
O
R1
"
OH
OH
N
O
H
+
ε-Caprolactam

90

-1

90

Peelable Medical

Coatings

90.1 Introduction

90-

1
90.2 Cold-Seal Coatings

90-

2
90.3 Heat-Seal Coatings

90-

2

90.1 Introduction

Prepackaged sterile medical devices and supplies became necessary in the late 1960s with the growth in
prepaid health insurance programs. Insurers required that health care providers itemize the cost of all
the supplies used during a procedure. This led to the rapid growth in the development of disposable or
single-use devices. These were packaged in paper/plastic pouches, trays, or containers and then sterilized.
At the time of use, the package was torn open to expose the sterile device. When the package was torn,
the device was showered with particulates and bacteria that caused a great deal of concern. At that time,
the medical device industry was not regulated by the U.S. Food and Drug Administration (FDA).
In 1968, the E. I. DuPont Company introduced a polyethylene, paperlike material (Tyvek

®

) that had
most of the properties needed for packaging medical devices. Tyvek is a unique material that meets almost

all of the critical requirements for medical packaging. It is a good bacterial filter — very porous, water-
resistant, and puncture- and tear-resistant. Being made from polyethylene, it is stable during both ethylene
oxide gas and radiation sterilization. It does not stand up well during steam sterilization, so only a few
adhesives have been developed for this purpose.
With an acceptable packaging material available, peelable coatings were developed to seal Tyvek to
plastic films or thermoformed trays to form pouches and trays that could be peeled open at the point
of use without compromising the sterility of the device.
There are five basic types of adhesives used to seal Tyvek to plastic surfaces to form sterile packages.
These are cold seal, lacquer-based heat seal, water-based heat seal, hot-melt-based heat seal and low-
density polyethylene. All of these adhesives are very difficult to formulate and apply so that they meet
all of the requirements of the medical device manufacturers.
Some of the requirements these adhesives must meet are as follows:
•Must peel cleanly without generating particles
•Have a peel strength over 1 lb per inch of width and less than 3 lb per inch of width regardless
of the peel angle
•Be very stable both before and after sterilization (shelf life requirements can be as long as 10 years)
•Meet the U.S. Pharmacopoeia requirements for medical device plastics

Donald A. Reinke

Oliver Products Company (Retired)

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Peelable Medical Coatings

90

-3

integrity has been maintained. A 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 that 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 a parting layer between the adhesive and the
substrate. Release coating on the substrate surfaces are also used when paper is used instead of Tyvek.
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 percentage 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, causing a shift to paper and away from Tyvek. Also, environmental
concerns are pressuring medical device companies to use some method other than ethylene oxide gas
sterilization. The requirement for a peelable package, however, remains strong.
The medical device industry is now under the control of the FDA, which requires complete validation
of processes and materials. Formulating and validating a new adhesive will generally take from 1 to 3
years. For this reason, packagers of medical devices are very reluctant to change suppliers. They do not
have the engineering staff they had in the earlier years when the industry first started. To change an
adhesive and develop the documentation required by the FDA is very expensive. Most companies have
alternate suppliers they can use if they have problems.
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 through 1987.

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91

-1

91

Conductive Coatings

91.1 Introduction

91-

1
91.2

91.3 Commercially Available Conductive Coatings

91-

5
91.4 Applications

91-

6

91.5 New Developments

91-

7
References

91-

8

91.1 Introduction

In 1986, sales in the coatings industry exceeded $10 billion, and production approached a billion gallons.

1

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 Com-
munications 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 1, 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 coatings 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 pro-
cesses for metallizing plastics are electroless plating, metal spraying, sputtering, and vacuum metallizing.
The first commercial plating of plastics was recorded in 1905.

2

Metallizing of plastics occurred during
World War II, and large-scale production started in the early 1960s. All these processes are now multi-
million-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.

3

There are various reasons for metallizing plastics. In the automotive industry, metallized plastic
combines the consumer appeal of metal with light weight. Electroless copper metallization is an indis-
pensable 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 shielding on many plastics. The plastics that account for most of the sub-
strates metallized are acrylonitrile-butadiene-styrene (ABS), polypropylene, polyphenylene oxide,
epoxies, phenolics, polyimides, and polyesters. The commercial process for metallization of plastics merits
separate discussion and is not further considered in this chapter.
Polymers (coatings) with conductivities greater than 1(

Ω

cm)

–1

are defined as conductive polymers
(also metallically conducting plastics, synmetals).

4

Unfortunately, the literature is not clear-cut, and often,

materials that are semiconductors with conductivities less than 1(

Ω

cm)

–1

are also called “conducting.”

Raimond Liepins

Los Alamos National Laboratory

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Metallic • Filled Polymeric • Polymeric • Organometallic
Shielding from Electromagnetic Interference • The Stealth •
Types of Conductive Coatings
91-2
Miscellaneous