Paints, Coatings and Solvents Episode 7 potx

Nonelectrostatic atomization methods Compressed air pneumatic atomization Rapid change of colors Very good optical paint film quality paints Airless hydraulic atomization Very hig

198 8 Paint Application

and is a good base for the subsequent coating Previously, sulfuric acid was preferred for pickling because of its low vapor pressure; acid losses are therefore slight and environmental pollution is low Nowadays the tendency is to use hydrochloric acid because it allows better cleaning of the metal surface (even slightly alloyed steels) Evaporation of hydrochloric acid from the pickling baths is limited by using self- contained or completely enclosed units Organic inhibitors are generally added to pickling acids to limit the pickling action to the oxidic impurities and to minimize the dissolution of the base material Addition of surfactants has a limited degreasing effect

Pickling in aqueous caustic soda solution is widely used for cleaning aluminum workpieces on account of their amphoteric behavior However, after alkaline pick- ling the article generally has to be treated with acid to remove loosely adhering layers

of pickling sludge and to brighten the surface

8.2.1.2 Degreasing

Processes using aqueous solutions or organic solvents have become extremely important for removing organic impurities In special cases, salt melts and treat- ments at elevated temperature in the gas phase are also employed

Organic Solvents Chlorinated hydrocarbons are widely used They remove oils

and greases extremely effectively (generally in the vapor phase) Cleaning in immer- sion baths can be significantly improved by using ultrasound Increasingly stringent environmental protection legislation will, however, greatly restrict the future use of chlorohydrocarbon solvents

Aqueous Media Degreasing may be performed with alkaline, neutral, or acidic aqueous media Approximately neutral (mild alkaline) degreasing agents with fairly high concentrations of wetting agents and surfactants are mainly used An advan- tage of these agents over alkaline or acidic media is a simpler and more economical effluent treatment The degreasing agents hydrolyze animal and plant oils and grease Non-hydrolyzable components (mineral oils and grease) are dissolved and dispersed by adding colloidal emulsifiers and wetting agents These baths are oper- ated at 60-80 "C and a pH of 8-9 The concentration of cleansing agents is between

1 g/L (spraying methods, pressure 0.15 - 0.25 MPa) and 50 g/L (dipping methods) Both stationary and flow-through baths are used

8.2.1.3 Formation of Conversion Layers [8.1]-[8.5]

Conversion layers generally consist of inorganic compounds formed on the metal surface They are used to increase the corrosion resistance and to improve the paint adhesion of the metal surface

Industrially, phosphate layers are the most important and phosphating is used to treat steel, aluminum, and zinc Chromating produces layers containing trivalent or hexavalent chromium compounds and is mainly used with aluminum and zinc

8.2 Preireutnieni of Subsiruie Surjkes 199 Special oxide layers and inorganic-organic coatings are used for special purposes in strip treatment

The surface weight of conversion layers is 0.05-5 g/mZ, With higher surface weights the flexibility of the layers decreases, which has an adverse effect on the flexural adhesion of the organic coating

Phosphating Processes The most important phosphating processes are alkali,

zinc, and zinc-calcium phosphating In alkali phosphating the layer-forming cation originates from the substrate, in zinc phosphating processes it originates from the phosphating solution

Alkali phosphating (iron phosphating) is mainly used when corrosion protection does not have to satisfy stringent requirements The solutions (pH 4-6) consist of acid alkali phosphates, free phosphoric acid, and small amounts of additives; oxidiz- ing agents (e.g., chlorates, chromates, or nitrites), condensed phosphates (e.g., py- rophosphate or tripolyphosphate), and special activators (e.g., fluorides or molyb- dates) The first reaction is the pickling reaction which produces Fez+ ions from the substrate (steel) These ions react with phosphate ions from the solution to form sparingly soluble iron phosphate that precipitates and adheres strongly to the metal surface Zinc phosphate layers are formed in an analogous reaction sequence on zinc surfaces Aluminum is usually treated with fluoride-containing solutions; thin, com- plex coatings are formed that contain aluminum, phosphate, and fluoride The baths are adjusted to a concentration of 2- 15 g/L Contact with the surface may take place via spraying, flooding, or dipping The bath temperature is normally 40-70 "C, but can be lowered to 25-35 "C with special bath compositions Treatment times are

5- 10 s (spraying of strip material) and 1-3 min (spraying or dipping of individual

parts) Iron phosphating includes both thin-coating (0.2-0.4 g/m2) and thick-coat-

ing methods (0.6- 1 O g/m2) The color of the layers is blue-green, and in some cases

also reddish iridescent The surfaces become matter and grayer with increasing coating weight

Zinc phosphating is primarily used for the surface treatment of steel and zinc as well as composites of these metals with aluminum Aqueous phosphoric acid soh- tions (pH 2.0-3.6) containing dissolved acidic zinc phosphate, Zn(H,PO,), , are used

The phosphate layers are gray in color (weight 1.2-6.0 g/m2) and consist

of Zn,(PO,), 4 H,O (hopeite), Zn,Fe(PO,), 4 H,O (phosphophyllite), and Zn,Ca(PO,), 4 H,O (scholzite) Layer formation is complete when the metal is completely covered with a phosphate layer, and the pickling action initiating layer formation has stopped The treatment baths contain 0.4-5 g/L ofzinc and 6-25 g/L

of phosphate, calculated as P,O,

The phosphating baths are usually used in automatic or semiautomatic dipping, spraying, or flooding plants at 45-70 "C; low-temperature processes operating at

25-35 "C also exist Treatment times are 60- 120 s (spraying process) and 3-5 min

(dipping process) The iron phosphate sludge must be removed periodically or con- tinuously from the bath

Low-zinc processes were developed with the introduction of cathodic electrodepo- sition coating In the normal zinc processes flat, sheetlike crystallites (mainly hopeite) are formed which may project from the surface In the low-zinc process the

200 8 Paint Application

layers mainly consist of phosphophyllite They have a parallel orientation relative to the metal substrate and are more finely crystalline and compact than the hopeite layers Very thin layers with a higher iron content are produced with nitrite-free low-zinc processes

Chromating In chromating, metal surfaces (mainly aluminum and magnesium)

are brought into contact with aqueous acid solutions of chromium(V1) compounds and additives that activate and accelerate pickling The pickling reaction converts acidic Cr(V1) into basic Cr( 111); cations of the treated metal simultaneously accumu- late in the liquid film on the metal surface leading to precipitation of a gel-like layer containing chromium(III), chromium(VI), cations of the treated metal, and other components The final conversion layer is formed after aging and drying Treatment can be carried out by spraying or dipping (6-120 s) at 25-60 "C Effluent waste must be treated to remove Cr(V1); the most common method being reduction with sulfite to form Cr(II1) followed by precipitation of chromium(II1) hydroxide with milk of lime

For the yellow chromuting of aluminum, solutions containing chromium(V1) com- pounds as well as simple or complex fluorides and activators are used to accelerate layer formation The pH value is 1.5-2.5 at total bath concentrations of 5-20 g/L The conversion layers consist of oxides or hydrated oxides of trivalent and hexava- lent chromium and aluminum The color of the layer may range from colorless through yellowish iridescent to yellowish brown, corresponding to an increase in the surface weight from 0.1 to 3 g/m2

The essential constituents of the aqueous solutions for green chromuting are chromic acid, fluorides, and phosphates The pH value of the baths is slightly less than in the case of yellow chromating, and the bath concentration is normally 20-60 g/L The conversion layers consist largely of chromium(II1) phosphate and aluminum(II1) phosphate, with small amounts of fluorides and hydrated oxides The surface weight is 0.1 -4.5 g/m2, and the color ranges from iridescent green to deep green

Aqueous, chromium-free acidic solutions have also been developed for aluminum materials that may contain complex fluorides of titanium and zirconium, phosphate, and special organic compounds These solutions are applied by spraying or dipping (up to 60 "C) and produce thin, almost colorless conversion layers with a surface weight < 0.1 g/mZ

Aqueous chromic acid solutions containing chlorides, simple and complex fluo-

rides, sulfates and formates as activators, are used for chrornuting zinc The total bath

concentration is 5-30 g/L, the pH value 1.2-3.0 The bath temperature is in the range 25 - 50 "C, the process times are 5 - 120 s, the achievable surface weight is 0.1 -

3 g/m2 The layer color changes with increasing surface weight from iridescent through yellow to brown or olive green

Rinsing A passivating rinse is necessary to exploit the quality-improving proper-

ties of conversion layers The most important rinsing agents are dilute aqueous solutions of chromic acid, optionally with additional amounts of chromium(II1) Equally good or only slightly worse results can be obtained with solutions free from chromic acid and containing polyvalent metal cations (e.g., chromium(II1) or also

8.2 Preireuttnetit of' Siihstrare Surfaces 201 organic components) The concentration of active substances in the rinsing baths is 100-250 mg/L Rinsing is performed at 20-50 "C, treatment time ranges from a few seconds to about a minute The surfaces should be sprayed with demineralized water

as a last rinse to prevent crystallization of water-soluble salt residues

8.2.2 Pretreatment of Plastics [8.1]-[8.3], [8.6]-[8.8]

Pretreatment of plastic surfaces is necessary for the following reasons:

1) To increase adhesion strength

2) To reduce the concentration of interfering constituents and mold release agents

3) To eliminate surface defects (e.g., bubbles)

4) To remove interfering foreign substances

5) To increase electrical conductivity

Many pretreatment techniques are used in practice (Table 8.2) The normal phys- ical method used to improve the adhesive strength of the coating to the substrate is

to slightly roughen the surface by solvent treatment, abrasion, or blasting Some plastics (e.g., polyolefins) require special pretreatment methods; processes that mod- ify the surface molecular layers of the plastic to increase their polarity have proved suitable (e.g., flaming, immersion in an oxidizing acid, immersion in a benzophenone solution with UV irradiation, corona treatment, plasma treatment)

Corona discharge is performed in a high-frequency alternating field (14-40 kHz)

at 10-20 kV between two electrodes The plastic surface is oxidized in a very short period (milliseconds) Plasma treatment is carried out under a moderate vacuum down to ca 10 Pa The advantage of this technique is the better penetration depth and the fact that it is also possible to treat shaped parts more easily The plasma can also burn in gases (e.g., argon), whereby special effects (e.g., plasma polymerization) can be achieved

Adhering processing additives (lubricants, release agents) can be removed by cleaning with solvents or aqueous surfactants The solvent stability and solvent and water absorption of the plastic material should be taken into account In order to reduce migration of constituents (shaping agents, plasticizers, dyes, organic pig- ments, stabilizers) during and after coating, preliminary tempering is often recom- mended (at the same time surface defects can also be detected)

Surface defects resulting from production (e.g., pores, bubbles, flow seams, and projecting fibers) are rectified by surface appearance enhancement Deeper-lying defects are filled in and smoothed with putties

Elimination of foreign substances (e.g., dirt particles and fibers) is very difficult due to the electrostatic charge of the plastics material Alternative methods are wiping with a lint-free cloth wetted with water or a water-alcohol mixture or blowing with ionized compressed air Plastics can also be made permanently anti- static by applying a dielectric coating

202 8 Puinr Applicurion

Physical and mechanical methods

oil- and water-free compressed air ionized compressed air

irradiation

removal of contamination (cleaning)

increase in adhesive strength, elimi- nation of surface defects and for- eign substances increase in adhesive

of interfering constituents and adhering process auxiliaries, as well

as foreign substances

increase in adhesive strength in spe- cial plastics, particularly polyolefins

elimination or reduction of surface

auxiliaries reduction of electrostatic charge

Properly graded sanding with appropriate sandpapers is a prerequisite for a satis- factory wood surface Industrially, sanding is performed on cylindrical abrasive-belt machines or automatic grinders, followed by brushing and suction to remove abra- sive dust

Surface pretreatment includes the following steps:

acetone)

2 ) Removal of adhesive residues

5 ) Staining with dyes dissolved in water or solvents, or with pigment dispersions

8.3 Applicution Methods 203

Many techniques have been developed for the industrial application of coatings The individual industrial coating methods can, however, only be employed in limited areas if design and production sequence are not matched to the requirements of the coating technique Adoption of more environmentally friendly coating methods is therefore often less a problem of investment, than a problem of the application limits

of the relevant processes Analysis of the criteria for choosing a coating method is

a complex task Of particular importance are the workpiece (design, material), the coating material, the number of workpieces and batch sizes, range of workpieces, requirements demanded of the coating (e.g., decorative appearance, corrosion pro- tection), economic factors, legal provisions, and available facilities, premises, and equipment Economic factors generally have top priority for choosing an application method on a commercial basis Coating systems and processes are usually preferred that best satisfy the demands and requirements for thin coatings, high degree of material utilization, low energy costs, and good automation Modern coating meth- ods that best comply with these requirements are electrodeposition coating, electro- static atomization, and electrostatic powder spraying

In conventional spraying, atomization is the result of external mechanical forces, i.e., the exchange of momentum between two free jets (air and paint) Atomization may be classified as compressed air atomization (air 0.02-0.7 MPa, paint 0.02- 0.3 MPa), airless atomization (paint 8-40 MPa), air-assisted airless atomization, also termed airmix process (air 0.02-0.25 MPa paint 2-8 MPa), and special tech- nologies (Table 8.3)

In compressed air (pneumatic) atomizurion, compressed air flows through an annu- lar gap in the head of the spray gun that is formed between a bore in the air cap and the concentric paint nozzle Further air jets from air-cap bores regulate the jet shape and assist atomization The expanding compressed air leaves the paint nozzle at high velocity A low-pressure area is formed in the nozzle aperture which exerts a suction effect and assists outflow of the paint The difference between the velocities of the compressed air and the exiting paint atomizes the paint into particles that are conveyed as spherical droplets in the free jet In the high-pressure process (0.2- 0.7 MPa) the exiting air jets can atomize the paint material extremely finely The size

of the liquid droplets varies from ca 10 pm to 100 pm (depends on the liquid viscosity, amount of delivered paint, and air pressure) In the low-pressure process (0.02-0.2 MPa) atomization is correspondingly coarser (20- 300 pm) Depending

on the viscosity and throughput, the paint can be fed to the nozzle via a suction cup,

a pressure cup, a flow cup, or pressure tank

204 8 Puiiit Applicution

Table 8.3 Nonelectrostatic atomization methods

Compressed air (pneumatic) atomization

Rapid change of colors

Very good optical paint film

quality

paints

Airless (hydraulic) atomization

Very high operating speed

Low spray mist formation

Large film thicknesses in

Uniform surface and film

Suitable for high-viscosity

Direct application from

High-viscosity paints can be

Large film thicknesses

Low sagging tendency

Quicker drying of the paint

ventilation required in enclosed

compressed air supply required

large-scale series coating applications (automobile repair and touch-uP finishes domestic appliances, etc.)

unsatisfactory material utilization

spray mist

expensive apparatus equipment parameters must be matched to the coating material limited number of spray jets due to overlapping

amount can be regulated during application

nozzle subject to high degree of wear

danger of sagging with sensitive paints

(shipbuilding, steel con-

lorries3 etc.)

paint pressure generator and

additional heaters necessary

Hardening at room tem-

perature (energy saving)

Better paint film quality

High resistance to

mechanical, chemical,

and climatic influences

Low content of organic

industrial coating, coating

structures corrosion protection large equipment and ap- paratus (machines, aircraft, ships, commercial vehicles, etc.)

In airless (hydraulic) atonzization the paint is forced through a slit nozzle of hard metal under high pressure (8-40 MPa) On account of the high degree of turbulence, the paint stream disintegrates immediately after leaving the fluid tip A similar atomization process occurs in spray cans where the paint pressure is produced by the propellant gas

The combined airmix process operates at a lower paint pressure (2-8 MPa) Additional low-pressure air jets (0.02-0.25 MPa) from the air-cap bores impinge on the spray jet to mix and homogenize it In addition to the atomizer and a compressed air generator (airless pump), the airmix unit therefore also requires compressed air for postatomization Advantages over the airless method are the less sharply defined spray jet and the smaller droplet size Compared with compressed air atomization,

a low-mist coating is possible

Hot spraying can be combined with all of the spraying methods described above

and is used for large film thicknesses or highly viscous, high-solids paints (lower solvent consumption) The paint is heated to 50-80 "C in a heat exchanger Imme- diately after atomization the heat content of paint droplets is transferred to the air and the workpiece The droplets therefore cool and their viscosity rapidly increases; the risk of sagging at larger layer thicknesses is thus reduced

In two-pack paints both the binder and the hardener have to be mixed before application In paints with a short pot life the paint must be metered and mixed in the atomization equipment immediately prior to use The two reactive components are normally mixed in static mixers after metering

In purely electrostatic spraying, the paint is atomized solely by electrostatic forces

In electrostatically assisted spraying, atomization takes place by the methods de- scribed in Section 8.3.1, with simultaneous or subsequent electrical charging

206 8 Puitir Applicutioti

T r a n s f e r o f Drops

due t o electric field e f f e c t

In all electrostatic coating methods an electric field is applied between the atom- ization equipment and the workpiece (Fig 8.1) The advantages and disadvantages

of this technology are listed in Table 8.4 The paint is electrically charged by a concentrated electric field at a high-voltage electrode In “lead charging” the nonat- omized paint is charged by direct contact with the electrode In “ionization charg- ing” mechanically produced paint droplets are charged by attachment of ions from the air; the electrode serves as a corona tip that generates these ions

In purely electrostutic sprajiing the paint is atomized solely by electric field forces The paint flows as a thin film over a high-voltage sharp edge, where it is subjected

to high field forces The paint film breaks up into threads and then into charged droplets that follow the electric force lines to the workpiece Only paints with a

cm can be applied in this way The best known designs are the electrostatic spray gap

(AEG method), the electrostatic spray cone (diameter 70- 250 mm, max rotational

speed 1500 min- I ) , and the electrostatic spray disk (diameter 400-700 mm, Max rotational speed 3000 min- ’) Purely electrostatic spraying methods are only used in special cases on account of their limitations (workpiece geometries, type and throughput of the paint)

Electrostut icullj, ass is fed at o m i x t ion methods are more versatile than purely e lec- trostatic methods because atomization takes place mechanically The electric field serves only to charge the paint material and to transport the charged droplets to the workpiece The following systems are used:

to 5 x

Table 8.4 Electrostatic atomization methods

Purely electrostatic spraying methods

housings and flat parts in the

materials high consistency of paint data low flexibility

Low wear of structural parts

Electrostatically assisted rotation atomization

Practically all paints can be unsuitable for parts with

excessive edge coating may occur

industrial coating

Electrostatically assisted compressed air, airless, and airmix atomization

8.3.3 Dipping [8.1]-[8.3], [8.10]

Dip coating is one of the simplest and oldest coating methods In addition to dipping in solvent- or waterborne paints, electrodeposition has become important for large-scale series production (Table 8.5)

208 8 Puini Applirrriion

Table 8.5 Dipping methods

Conventional dipping

paint analysis control edge coating often unsatis- danger of sagging, dripping, foam and bubble formation

factory paint splashing

Electrodeposition coating

Complete and uniform

paint film, also in

Fully automatic operation

High parts throughput

Environmentally friendly

(waterborne paints)

complex equipment

priming in the automobile industry

articles complex bath monitoring

highly trained workforce high material costs

Conventional Dipping In conventional dipping the workpieces are immersed in the

paint and then removed (Fig 8.2) The liquid paint adheres to the surface and is then dried or stoved Care should be taken to ensure that the workpieces do not float during dipping and that air bubbles do not become trapped The speed at which the workpiece is removed from the bath must be selected so that excess paint adhering

to the surface can run off The draining and evaporation time must be sufficiently long to ensure satisfactory evaporation of the solvents (if necessary a hot air zone should be included for waterborne paints)

Electrodeposition Electrodeposition paints are suspensions of binders and pig-

ments in fully demineralized water with low concentrations (ca 3 %) of organic solvents (see Section 3.8) Electrodeposition coating may be either anodic or cathod-

ic

In anodic electrodeposition the workpiece acts as the anode This method is only

used to a small extent Disadvantages compared with cathodic electrodeposition are its poorer handling and corrosion protection Advantages include the lower paint price and lower expenditure on plant technology

8.3 Applicutiori Methods 209

-,

a ) Edge suction; b) Overflow; c) Circula- tion system with pump, filter piping and

roller-type covers); g) Raising and lower- ing system; h) Workpiece

In cathodic electrodeposition the workpiece acts as the cathode; this method is

more important than anodic electrodeposition The binders consist largely of non- water-soluble epoxy resins, and to a lesser extent of acrylic resins (one-layer coat- ings) These resins are converted into a water-soluble (i.e., ionized) state by neutral- ization with organic acids ( e g , acetic acid):

Electrodeposition equipment and technology is expensive and is therefore practi-

cable only for large-scale series coating (Fig 8.3) In addition to the dipping tank

and a storage tank, circulation systems for the paint and auxiliary materials, rinsing systems (ultrafiltrate, fully demineralized water), a regulated d.c supply (200-

400 V), and the workpiece transporting system (curent supply) also have to be installed Since electrodeposition systems require temperature control, production tanks are equipped with heaters and chillers

210 8 Point Applicurion

Figure 8.3 Cathodic electrodeposition coating unit

a) Dipping tank with overflow; b) Recycle circulation; c) Paint filter circulation; d) Paint cooling

Other wet paint coating methods are summarized in Table 8.6 Application with

a brush or roller is now only used to a great extent in the handicrafts sector, do-it-

yourself sector, or on building sites

Flat workpieces (e.g., paper sheets, films, wooden boards, and panels) can be coated economically and quickly by rolling, pouring, or knife coating These meth- ods have become important because they are easily automated, have a high material

yield, and are environmentally friendly With roller coating the paint material is

transferred from rotating rubber rollers to one or both surfaces of the workpiece (Fig 8.4) In forward roller coating (layer thickness I ca 12 pm) the workpiece and paint application roller run in the same direction With reverse roller coating (layer thickness 3- 100 pm) they run in opposite directions

The pouring method is commonly used in the wood and timber trade Here the paint is pumped into a pouring head that has a paint outflow slit with adjustable lips (Fig 8.5) Paint that does not come into contact with the workpiece is returned to the tank via a collecting channel Since the paint is constantly circulated and is heated by the circulation pump, it must be cooled and must contain high-boiling solvents

Knife coating is used in the paper and textile industry for coating continuous material Knife coating can also be used to print and coat flat, two-dimensional

5.3 Applicarion Merhods 31 1

Table 8.6 Miscellaneous wet coating methods

Brushing

Simple equipment

High paint yield

No specially trained work-

Fast and easy to master

High paint yield

Uniform film thicknesses

Wiping

Fast application

Uniform film thickness

High paint yield

highly labor-intensive (high wage costs)

nonuniform film thicknesses danger of brush marks

only suitable for smooth surfaces

worse wetting of the substrate labor-intensive

Rolling, printing, strip (coil) coating

unsuitable for workpieces with complex shapes

High degree of automation

High paint yield

specially prepared materials only for flat strip, or panel- type parts

nonuniform film thicknesses danger of paint slurry formation

Centrifugation, drum application

High paint yield

no special surface quality points, etc.)

steel superstructures lattice constructions handicrafts do-it-yourself

steel superstructures handicrafts do-it-yourself

exterior coating of pipes application of bitumen to pipelines

wood coating

strip and panel coating (sheet metal, wood, films, paper, paperboard)

chipboard, paper, and card- board coating

wood panel coating

large, bulky articles (radia- tors, frames for commercial vehicles, etc.)

small mass-produced articles (hooks, eyelets, screws)

c) Transporting belt; d) Article being coated;

Paint filter; j) Paint line; k ) Valve for quanti- tative adjustment; 1) Pump

g) Paint film; h) Pressure release value; i)

4&d - ,

I

f

@

Figure 8.4 Roller coating

a) Metering roller; b) Coating material; c) Paint roller; d) Workpiece

workpieces The coating is applied to the substrate which then passes under a doctor knife The coating material is pressed onto the workpiece with the doctor knife (Fig 8.6) The knife also removes excess coating material and smooths the surface

Highly pigmented, pasty coatings and viscous putties and fillers are applied by troweling The materials are applied with a pair of counterrotating rollers Excess material is smoothed and “pressed” into the workpiece surface This method can only be used for flat, striplike, or panel-shaped workpieces (e.g., wooden panels) With flow coating gentle streams of paint are pumped over the workpieces via nozzles Excess paint flows into a collecting trough and can be recirculated (Fig 8.7) Small items (e.g., hooks, eyelets, clasps, and buckles) can be coated by centrifuga- tion or drum coating The workpieces must not adhere to one another, nor must they become entangled In centrifugation the workpieces are placed in a wire basket, and then immersed in a paint bath and centrifuged at a rotational speed of about

500min-’ With drum coating paint is fed into a rotating drum containing the

workpieces The paint feed can be achieved by adding special, low-viscosity paint or

by spraying with spray guns

Coil coating denotes the continuous coating of cold-rolled steel strip (including galvanized strip) or aluminum with organic polymers In automated plants the metal strip is first cleaned and chemically pretreated The strip is then roller-coated on one

or both sides, with one or more coats of liquid, thermosetting or thermoplastic coating materials The paint coat is dried in an oven after each application Through- put rates are of the order of 1-3 mjs Constant production conditions using appro- priate control devices ensure high quality

I I-

&[

Air blade

Figure 8.6 Knife coating

Rubber cloth blade

@-

Roller blade

sheets)

Figure 8.7 Flow-coating method

a ) Paint nozzle; b) Drip pan; c) Filter; d ) Paint adjustment valve; e) Pump; f ) Paint tank