Paints, Coatings and Solvents Episode 7 potx

Table 8.1. Summary of pretreatment methods Method - Material Impurities Temporary Improved corrosion corrosion corrosion rolling shavings agents products layers after treat- tion with protection skin oils Fe Al Zn Lubricants, Scale, Dirt, Drawing Corrosion Old paint protection protec- ment coatings Mechanical methods Steam and high-pressure * - - - water blasting +++ + + 0 - __ - Jet blasting (e.g wet jets) + + + - ++ + 0 ++ + Manual abrasive methods (e.g., brushing, grinding) + + + - Wet chemical methods - + + 0 + + Solvent degreasing - - - - 0 0 0 - Steam +++ + Dipping +++ + - - - - - - Ultrasonic dipping +++ + Alkaline degreasing - - - - - + - Spraying +++ + Dipping +++ + Phosphoric acid +++ * + - - - - - - 0 Pickling * 0 * - + - Sulfuric acid + Hydrochloric acid + +- Caustic soda - * * * Phosphating Alkali phosphate +++ * - + + 0 + 0 + + 0 + 0 + + - * * * - - Zinc phosphate Chromating Adhesion promotors ++ ++ - - - - - (wash primer) +++ - The symbols have the following meanings: + + Extremely satisfactory. + Satisfactory. o Moderately satisfactory. - Unsatisfactory. - - Particularly critical. * Satisfactory only with special methods. Po !-J s 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 pickling 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 immersion 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 advantage 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; oxidizing 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, complex 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-coating 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 electrodeposition 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) compounds 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 hexavalent 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 fluorides, 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 properties 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 physical 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 Table 8.2. Pretreatment methods for plastics Pretreatment methods Use Physical and mechanical methods Abrasion Blowing-off Sanding Blasting Steam degreasing dry, or wet dust cloth oil- and water-free compressed air ionized compressed air Washing Spraying Dipping Chemical methods Oxidative Cross-linking solvent, aqueous surfactant solutions antistatically adjusted solutions conducting solutions Miscellaneous methods Tempering Storage, aging Application of a dielectric layer flame treatment corona discharge, plasma treatment oxidizing acids benzophenone solution with UV irradiation removal of contamination (cleaning) + reduction of electrostatic charge increase in adhesive strength, elimination of surface defects and foreign substances increase in adhesive strength, elimination or reduction of interfering constituents and adhering process auxiliaries, as well as foreign substances + reduction of electrostatic charge + increase in electrical conductivity increase in adhesive strength in special plastics, particularly polyolefins elimination or reduction of surface defects, constituents, or process auxiliaries reduction of electrostatic charge 8.2.3. Pretreatment of Wood [8.1], [8.2] Properly graded sanding with appropriate sandpapers is a prerequisite for a satisfactory wood surface. Industrially, sanding is performed on cylindrical abrasive-belt machines or automatic grinders, followed by brushing and suction to remove abrasive dust. Surface pretreatment includes the following steps: acetone) 1) Removal of resins, e.g., by hydrolysis (wood soaps or soda solution) or dissolution (e.g., alcohol, 2) Removal of adhesive residues 3) Rectification (filling. patching) of processing and growth defects in the wood 4) Structuring by brushing, burning, sandblasting. embossing, or leaching 5) Staining with dyes dissolved in water or solvents, or with pigment dispersions 8.3. Applicution Methods 203 8.3. Application Methods 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 protection), 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 methods that best comply with these requirements are electrodeposition coating, electrostatic atomization, and electrostatic powder spraying. 8.3.1. Spraying (Atomization) [8.3]-[8.3], [8.9] 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 Advantages Disadvantages Examples of areas of use Compressed air (pneumatic) atomization Universally employable Simple to use Uniform layer thicknesses Applicable to complicated workpiece geometries Small amounts can be applied Rapid change of colors Very good optical paint film Suitable for special-effect 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 (low losses) one application thickness paints as-supplied drums and containers Substrate with deep pores can be wetted Airmix atomization Combines advantages of pneumatic and hydraulic atomization Hot spraying High-viscosity paints can be Large film thicknesses Low sagging tendency Quicker drying of the paint applied film spraying experience required paint mists constitute a health hazard industry) ventilation required in enclosed spaces industrial coatings (furniture, compressed air supply required large-scale series coating applications (automobile repair and touch-uP finishes domestic appliances, etc.) unsatisfactory material utilization danger of film defects due to 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 coating of large objects (shipbuilding, steel con- ~&~tion work, machines, lorries3 etc.) paint pressure generator and compressed air supply required industrial coating trained workforce required industrial coating additional heaters necessary 8.3. Applicution Metliods 205 Table 8.3. Continued Advantages Disadvantages Examples of areas of use Two-pack coating Advantageous for temperature-sensitive workpieces Hardening at room temperature (energy saving) Better paint film quality High resistance to mechanical, chemical, and climatic influences Low content of organic solvents expensive and complicated equipment exact metering and mixing required (automatic monitoring) safety measures required with isocyanate-containing systems trained workforce industrial coating, coating of wood and plastics protection of buildings and structures corrosion protection large equipment and apparatus (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. 8.3.2. Electrostatic Atomization [8.3]-[8.3], [8.9] In purely electrostatic spraying, the paint is atomized solely by electrostatic forces. In electrostatically assisted spraying, atomization takes place by the methods described in Section 8.3.1, with simultaneous or subsequent electrical charging. 206 8. Puitir Applicutioti Transfer of Drops due to electric field effect Figure 8.1. Fundamentals of the electrostatic coating process In all electrostatic coating methods an electric field is applied between the atomization 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 charging” 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 moderate viscosity and an electrical conductivity in the range 5 x S/ 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 omixt 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 [...]... human eye [9.26] I S 0 77 24 describes methods for the instrumental determination of the color coordinates and color differences The standard is based on the CIE 1 976 (L*, a*, h*) color space I S 0 77 24 specifications are satisfied by many color measuring devices: tristimulus colorimeters, spectrophotometers, and abridged spectrophotometers Spectrophotometric data are now preferred and have replaced tristimulus... used for testing e.g., fast and slow movement, small or large load, high and low temperature, constant or intermittent contact The falling sand (abrasive) method, abrasive blast method, rotating disk method, rotating wheel method (Taber abraser, I S 0 77 84), and Gardner wet abrasion method (used with emulsion paints in DIN 5 377 8 T 2 ) are commonly used The stone impact resistance is an important property... In 1939 HUNTER JUDD and found that specular gloss measured at 60" in a reflectometer with specified light source and receptor apertures provided a useful classification of paint finishes according to glossy appearance [9.15] The instrument was standardized as ASTM D 523, improved, and subsequently standardized as I S 0 281 3 The 60" reflectometer is now used worldwide as the standard instrument for... in heating Paints, Coatings and Solvents Second, CompletelyRevised Edition Dieter Stoye, Werner Freitag copyright 0WILEY-VCH Verlae CirnhH I Y Y X 9 Properties and Testing Anyone wishing to test the quality of a paint or coating quickly realizes that only a few properties can be accurately scientifically defined In many cases there is a good correlation between defined physical properties and the behavior... flexibility of coatings after different curing processes The method can be used to determine the optimum conditions for curing with heat [9.36], [9. 37] Abrasion Resistance Abrasion (wear) resistance is a basic factor in the durability of a coating, and is determined by the interaction of the abrasive medium and the coating Many types of abrasive media and treatment are used for testing e.g., fast and slow... been developed for paints and coatings that are intended to simulate in-use conditions These testing methods are often similar but their results are not fully comparable Standard manuals provide a good overview of available test methods [9.1]-[9.5] In this chapter attention is focused on methods that are widely known and internationally standardized, or whose international standardization is in progress... drive and sensor are separate The motor drives the outer cylinder; the inner, stationary cylinder is connected to the sensor Viscometers with cylinder and cone/plate geometries can also be employed The cylinder viscometers are easier to use and provide more reproducible results Cone and plate systems can be used to investigate the hardening behavior of paints The system can easily be cleaned and only... uses of cone and plate systems are limited for several reasons and they cannot be used with dispersions [9.6], [9 .7] Falling ball viscometers and capillary viscometers are not generally used for testing paints Flow cups are, however, special capillary viscometers with a short capillary in which the force of gravity acts on the paint Other Rheological Properties Paint brushability, sagging, and leveling... pyknometers and glass Gay- Lussac or Hubbard pyknometers are suitable The Hubbard pyknometer is especially useful for highly viscous paints hnrner.red Boifr Merhod (DIN 53 2 17 T3) The density is calculated from the buoyancy of spherical bodies immersed in the paint This method is used for low- and medium-viscosity paints and is particularly suitable for production control Vibration Method(D1N 532 17 T5) A... Conditioning The physical and mechanical properties of paints and coatings generally depend on the environmental conditions used during testing; the most important variables are temperature and humidity The degree to which each of these variables needs to be controlled is determined by the effect the variable has on the property being measured Thus for the measurement of viscosity and density the temperature . 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. 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. 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 methods