Coatings Technology Handbook Episode 1 Part 7 pptx

Because the coating material flows under pressure into and through this mesh or matrix before being deposited onto a substrate, the resulting coating has a thickness far greater than tha

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22-4 Coatings Technology Handbook, Third Edition

End uses include velour backcoatings, automotive backcoatings, mattress ticking, fixation of pile on pile fabrics, flame retardant and cigarette-proof coatings, imitation suede, and antislip coatings

22.4 Advantages

1 Because substrate, screen, and counterpressure rollers have the same speed, coating is done without tension and friction Thus, virtually all substrates can be processed on this system, including knitted fabrics, velours, nonwovens, and shift-sensitive materials, such as skiwear, mattress ticking, and Lycra fabrics

2 The user has penetration control: penetration into the substrate can be completely avoided or, if desired, controlled

3 Thanks to the low system content, the coating method is clean, and fast changes are possible

4 Coatings are exactly reproducible As parameters, squeegee pressure, squeegee setting, mesh num-ber, and viscosity can be measured and read off, and any given coating can easily be repeated

5 Chemical savings (up to 20% of the coating weight) are realized in two ways: (a) through accurately controllable application and because the screen follows the web structure exactly (thus, the textile character is maintained, and an excess of paste, such as occurs with knife coating, is avoided); and (b) through great accuracy, in left/right and longitudinal directions, of the application amount

6 Application is both tensionless and frictionless

7 By means of the closed system, the user has total process control

8 The knife coating option can be attached above the whisper blade roller, mentioned earlier This knife coating (see Figure 22.3) can be used as a knife-on-air system for paste or unstable foam coatings and in the knife-over-roll coater made for foam applications In both cases, the apparatus is fitted with a paste or foam distribution system over the full working width In this way, it is possible to apply colored coatings with a totally even appearance

FIGURE 22.3 Adaptation of screen coater for knife coating.

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DK4036_book.fm Page 4 Monday, April 25, 2005 12:18 PM

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23 Screen Printing

23.1 Introduction 23-1 23.2 Geometry of the Printing Screen 23-2

The Rotary Screen

23.3 The Stencil 23-3 23.4 Dynamics of the Squeegee 23-3 23.5 Coating Transfer 23-4 23.6 Converting the Applied Coating 23-4 23.7 Conclusion 23-4 References 23-4

23.1 Introduction

The screen printing process is markedly different from most imaging processes generally associated with the graphic arts First, the printing plate is actually porous, formed by a woven mesh of synthetic fabric threads or metal wire (or in at least one case, by a nonwoven, electroformed metal matrix), which is then combined with a selective masking material, commonly called a stencil Because the coating material flows under pressure into and through this mesh or matrix before being deposited onto a substrate, the resulting coating has a thickness far greater than that of a material printed onto the substrate by offset lithography, gravure, flexography, xerography, or ink-jet printing

For this and other reasons, the screen printing process has many practical applications in industrial manufacturing areas in which other printing media have few or none

to a rigid framework of aluminum or steel In most applications, this framework forms a rectangular plane However, variations are possible, including the cylindrical screen, which is affixed and sealed at both ends In the case of mesh, whether of synthetic polyester monofilaments or stainless steel wire, tension is applied simultaneously in opposing directions to obtain a semirigid planar surface This stretched printing screen then performs three distinct functions: (a) meters the fluid coating (or ink) that flows through it under pressure, (b) provides a surface for shearing the viscous columns of coating material that form during transfer to the substrate, and (c) provide support for the imaging elements (the stencil)

Ink or coating transfer is initiated by the imposition of pressure on the screen by means of a flexible plastic blade, the squeegee Because of the flexibility of the blade material and its physical profile, a hydraulic action is caused by force exerted in two directions The blade presses into the screen, and its inherent flexibility enables it to be put into direct contact with the substrate, thus effecting ink transfer The blade also sweeps in a horizontal direction, thus applying the ink or coating

as it moves, and causing the columns of material to shear as the printing screen rebounds after the squeegee has passed

Timothy B McSweeney

Screen Printing Association International

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24 Flexography

24.1 Introduction 24-1

24.2 Flexo Press Systems 24-2

24.3 The Most Important Flexo Deck System 24-5

24.4 Printing Forms or Plates 24-7

24.5 Print Substrates and Printing Inks 24-9

Throughout the printing industry, flexography, or flexo, has established its reputation as a quality printing process bearing comparison with letterpress, gravure, and offset, which have been used industrially for many years Today, the whole packaging sector and other areas of the printing industry would be unthinkable without this highly economical quality printing process This is attributable primarily to the high flexibility flexo offers, its qualification for a wide range of materials, the large and variable range of print repeat lengths, the different press widths available, and the quite extraordinarily high production speeds Other advantages include the highly diversified flexo press specifications and the possibility of using flexo in line with other printing techniques and processing operations

Finally, the developments and improvements achieved in the field of press engineering, flexo printing plates, and flexo inks have recently contributed quite decisively to the position the process holds today

24.1.1 Historical Development of the Aniline and Flexographic

Printing Process

Today’s flexo process is far more than 100 years old According to historians, extremely primitive aniline work was produced in the United States as far back as 1860

The original name of this letterpress process, “aniline printing,” is traceable to the aniline dyes used

in the mid-19th century that were diluted in alcohol and had been used in printing for many years This rubber printing process — until 1970, rubber printing plates were used exclusively — was initially employed for the printing of wrapping papers The first aniline printing apparatuses are said to have been used in England and Germany beginning in 1890 From about 1910, some European machine manufacturers started to supply aniline printers in combination with paper bag machines to permit printed paper bags to be produced in a single pass From the early 1920s until approximately 1940, aniline

Richard Neumann

Windmöller & Hölscher

Historical Development of the Aniline and Flexographic End Printers • Stack Presses • In-Line Systems • Central Printing Process • The Flexo Process

Impression Machines Three-Roller System (Fountain Roller Color Deck) •

Rubber and Photopolymer Plate Making • Printing Plate Two-Roller or Fountainless Printing Deck

Mounting and Proofing Print Substrates • Flexo Inks

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24-10 Coatings Technology Handbook, Third Edition

consist of colorants (dyes or pigments), binding agents (natural resins, artificial resins, or plastics), and

a solvent or solvent blend Whereas flexo inks used to be based on basic (soluble) dyes, pigment inks are used primarily today because of more exacting demands on the ink’s fastness To obtain the desired properties such as brilliance, adhesion, and qualification for laminating, the correct binding agents and additives must be selected

Apart from the aforementioned properties, flexo inks are required to generate a quality end product Fast and perfect drying of inks on the substrate during printing is another aspect of paramount impor-tance, and in this respect, the solvent of the solvent mixture used is the decisive factor

The drying system involves evaporation of solvents after the ink has been applied to the web This drying process is substantially accelerated within the printing press using hot air, which is blown onto the web, and appropriate exhaust arrangements

The most important solvents are hydrocarbons, alcohols, glycols, esters, and ketones Recently, water-soluble pigment inks, once used exclusively in the printing of multiwall paper sacks, gift wrap, corrugated board, and wallpaper, have been playing an increasingly important role and also have been adopted in the fields of newspapers and plastic films The obvious reason for the growing trend to use water-soluble inks in package printing and for plastic films is the new set of laws calling for reduction of solvent emissions into the environment

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25 Ink-Jet Printing

25.1 Introduction 25-1 25.2 Continuous Jet Printing 25-1 25.3 Impulse Jet (Drop-On-Demand) Printing 25-2 25.4 Ink-Jet Inks 25-3 Bibliography 25-4

Ink-jet printing refers to any system in which droplets of ink are ejected onto a printing surface to form characters, codes, or other graphic patterns The ink-jet concept dates from the 1860s, when Lord Kelvin developed the first practical jet for pattern generation Early commercialization was in the oscillographic recorder area in the 1950s Since the 1960s, ink-jet developments have focused on computer output, with major contributions made by such scientists as Hellmuth Hertz in Europe (Lund Institute, Sweden) and Richard Sweet (Stanford University) and Steven Zoltan (Brush Instruments) in the United States Current commercial products range from printers for direct coding of packages, to high-speed–low-resolution direct mail printers (from Diconix), to graphic arts quality color plotters (from Iris Graphics),

to graphic arts quality color plotters (from Hewlett Packard) To address this range of applications, several variants of the technology have been developed Each approach involves trade-offs among cost, speed, reliability, and print quality, determined by interactions between hardware and supplies

Ink-jet printing functions include the following:

• Creation of an ink stream or droplets under pressure

• Ejection of ink from a nozzle orifice

• Control of drop size and uniformity

• Control of which drops reach the paper

• Placement of drops on the recording surface Control of these processes depends on several design variables, such as nozzle size, firing rates, drop deflection methods, and ink viscosity Changing any variable typically requires adjustments to other system variables, making R&D advances slow and expensive

Ink-jet printers fall into two basic categories: continuous jet (synchronous) and impulse jet (drop-on-demand) Most early development took place in the continuous jet arena, but recent emphasis has shifted

to the less complex (and therefore less costly) drop-on-demand approaches

25.2 Continuous Jet Printing

Continuous ink-jet systems operate by forcing pressurized ink in a cylinder through nozzles in a contin-uous stream Nozzle diameters range from 3 to 0.5 mil; the smallest nozzles can require up to 600 psi of pressure to eject the ink The ink stream is unstable, breaking into individual droplets either naturally

Naomi Luft Cameron

Datek Information Services

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26 Electrodeposition

of Polymers

26.1 Introduction 26-1 26.2 Advantages 26-1 26.3 History 26-2 26.4 Process 26-2 26.5 Equipment 26-3

26.6 Laboratory 5 References 26-5 Bibliography 26-6

The electrodeposition of polymers is an extension of painting techniques into the field of plating and, like plating, is a dip coating process The art of metal plating utilizes the fact that metal ions, usually

Ni2+ or Cu2+, can be discharged on the cathode to give well-adhering deposits of metallic nickel, copper,

ions into ammonia-type, water-insoluble polymers known as cathodic deposition Alternatively, a large

be mentioned that “R” symbolizes any of the widely used polymers (acrylics, epoxies, alkyds, etc.) The electrodeposition process is defined as the utilization of “synthetic, water dispersed, electrodepos-itable macro-ions.”1

26.2 Advantages

Metal ions, typically 1/2 Ni2+, show an electrical equivalent weight 1/2 Ni2+ equal to approximately 29.5

g, while the polymeric ions typically used for electrodeposition exhibit a gram equivalent weight (GEW)

George E F Brewer*

George E F Brewer Coating Consultants

Throwing Power • Maintaining a Steady State • Rupture Voltage Conveyors • Metal Preparation • Tank Enclosures • Dip Tanks •

Water • Bake or Cure Rectifiers • Counterelectrodes • Agitation • Temperature Control • Ultrafilter • Paint Filters • Paint Makeup • Deionized

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27 Electroless Plating

27.1 Introduction 27-1 27.2 Plating Systems 27-2 27.3 Electroless Plating Solutions 27-3

27.4 Practical Applications 27-4 27.5

27.6 Stability of Plating Solutions 27-7 27.7 Electroless Plating 27-7

27.8 Properties of Chemically Deposited Metal Coatings 27-10 References 27-11

In electroless plating, metallic coatings are formed as a result of a chemical reaction between the reducing agent present in the solution and metal ions The metallic phase that appears in such reactions may be obtained either in the bulk of the solution or as a precipitate in the form of a ﬁlm on a solid surface Localization of the chemical process on a particular surface requires that the surface must serve as a catalyst If the catalyst is a reduction product (metal) itself, autocatalysis is ensured, and in this case, it

is possible to deposit a coating, in principle, of unlimited thickness Such autocatalytic reactions constitute the essence of practical processes of electroless plating For this reason, these plating processes are sometimes called autocatalytic

Electroless plating may include metal plating techniques in which the metal is obtained as a result of the decomposition reaction of a particular compound; for example, aluminum coatings are deposited during decomposition of complex aluminum hydrides in organic solvents However, such methods are rare, and their practical signiﬁcance is not great

In a wider sense, electroless plating also includes other metal deposition processes from solutions in which an external electrical current is not used, such as immersion, and contact plating methods in which another more negative (active) metal is used as a reducing agent However, such methods have a limited application; they are not suitable for metallization of dielectric materials, and the reactions taking place are not catalytic Therefore, they usually are not classiﬁed as electroless plating

Electroless plating now is widely used in modifying the surface of various materials, such as noncon-ductors, semiconnoncon-ductors, and metals Among the methods of applying metallic coatings, it is exceeded

in volume only by electroplating techniques, and it is almost equal to vacuum metallization

Electroless plating methods have some advantages over similar electrochemical methods These are as follows:

A Vakelis

Lithuanian Academy of Sciences

Deposition Rate • Solution Life • Reducing Agent Efﬁciency Copper Deposition • Nickel Plating • Cobalt, Iron, and Tin Factor • Solution Sensitivity to Activation

Plating • Deposition of Precious Metals • Deposition of Metal

Mechanisms of Autocatalytic Metal Ion Reduction 27-5

Alloys

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