Polyurethane Associative Thickeners for Waterborne Coatings 85-7PEUPU thickeners are used extensively in high gloss industrial or decorative applications.. Van Doren, “Formulating with r
Trang 2Polyurethane 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)
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)
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,
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Trang 385-8 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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Trang 4Surface Coatings
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Trang 586 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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Trang 687 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 Zola1 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 publications9–23 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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Introduction 87-1
and Continuous Phases
Trang 788 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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Trang 889 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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Premixture • Extrusion • Grinding • Sifting and Classifying •
Epoxy Systems • Polyester Systems • Acrylic Systems Resin Systems • Pigments and Fillers • Additives
Trang 9Thermoset 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
O CH2 CH CH2
O O
CH2 CH CH2
O
CH2 O Diglycidyl Ether of Novolac Resin
CH2 CH
CH2 O
O
CH2
n
R1 O CH2 CH2
O
CH HO R2 Epoxy Resin Phenolic Crosslink Intermediate
R2 O
R1 O CH2
OH CH +
N C NH C
NH
NH2
O
CH2 CH CH2
O
R
O
CH2 CH CH2 OH
R N
O CH2C CH3
OH R
N C N C
NH CH2 CH CH2 O
OH
R +
DICY Epoxy Resin Crosslink Intermediate
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Trang 10Thermoset 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
naturally at curing temperatures in the oven
N
H
O
CH3
CH3 N
C O
H3C CH2 N C
O
C H 3 N N
C O
C O
R
OH HO
OH
ε -Caprolactam Isophorone Diisocyanate Toluene Diisocyanate Triol
N O
N C O
N
O
CH3
CH3 N
C O
H3C CH 2 N C
O
CH3
CH3
N C O
CH3
CH2 N C O
CH3
CH3 N
C O
H3C CH 2 N
O
R O O
O
HO R1
"
OH
OH +
ε -Caprolactam Blocked IPDI HO-Polyester
C
CH3
CH3 N
C O
H3C CH2N C
O
CH3
CH3
N C O
CH3
CH2 N C O
CH3
CH3 N
C O Crosslink Intermediate
H3C CH2N
O
R O O O
HO R1
"
O
OH
O R1
"
OH OH
O R1" OH OH
N O H +
ε -Caprolactam