Powder coating
Powder coatings are organic , mostly thermoset coating powders with a solids content of 100%. In contrast to all other coating technologies, coating with powder coatings does not require any solvents . Processing methods such as extrusion and grinding are used to produce powder coatings .
Today's powder coating technology has existed since the 1960s, when pure epoxy systems were first used which, due to the raw material situation at that time, were still very slow in response time, which significantly restricted the areas of application. There are now a number of suitable raw materials. This enables the variable setting of the optical and mechanical properties.
The main areas of application for powder coatings are general metal coating (35%), household appliances (21%), facade coatings (20%), furniture painting (13%) and automotive painting (8%).
Economical meaning
Around 1.1 million tons of powder coating were produced worldwide in 2006. In Europe, powder coatings account for around 10% of the overall coatings market. The majority, a little over a third each, is produced in Europe and Asia. The remaining third is split half between North America and the rest of the world. The region that grew the most in 2007 with around 10% is Eastern Europe , whereby Turkey recorded the greatest growth with an annual growth of 12%. The largest production location for powder coatings, on the other hand , remains Italy , which has an annual production volume of around 100,000 tons of powder coatings.
In Germany, around 71,000 tons of powder coating were produced in 2008 (+ 0.8% compared to 2007), which makes up around 3% of the total production volume of paints and coatings. The production value of powder coatings manufactured in Germany is around 260 million euros (−12.4% compared to 2007).
history
Development of technology
The first attempts with powdery coating compounds, which can produce a film when melted, were made in the 1940s. It was a thermoplastic plastic powder. These had almost no resemblance to today's powder coatings. In 1953 Erwin Gemmer, from the then Knapsack-Griesheim AG , which was later taken over by Hoechst , developed the fluidized bed sintering process with which coaters were given the opportunity to paint industrial quantities for the first time. Then as now, the process produces layers thicker than 200 µm, i.e. layers three to four times as thick as is usual with electrostatically applied powder coatings.
In contrast to the purely thermoplastic binders ( polyamide , polyvinyl chloride ) used up to now, a cross-linking epoxy resin was brought onto the market in the early 1960s , which is the cornerstone for later development. Another breakthrough followed in 1968 with the development of hybrid powder coatings (epoxy and polyester as binders). This resulted in shorter curing times and more constant quality. This is crucial for use in large systems. At the beginning of the 1970s, TGIC was also found, a highly weather-resistant hardener for polyester systems. At the same time, the first acrylate and polyurethane systems emerged, which initially did not catch on.
In 1995, with the powder filler , powder coating was used for the first time as an automotive coating. The powder slurry (a suspension of powder coating in water) has been used as a clear coating on the A-class since 1996 .
Application technology
In the field of application technology, a milestone was set in the mid-1960s with electrostatic spray guns ( corona application ). For the first time it was possible to fluidize the powder coating and apply it electrostatically. The initially low transfer efficiency were greatly improved by the introduction of the recovery, a hitherto completely unknown in the paint region procedure so that the process also economically interesting was. The second possibility of charging, the charging of the powder particles via friction ( tribo application ), was shown for the first time in 1972.
Industrial use
The basis for the spread of powder coating technology was a regulation, the Technical Instructions for Keeping the Air Clean ( TA Luft for short ), which was passed by the German Federal Government in 1974 and essentially aimed at restricting the use of solvents. In addition to powder coatings, innovations at that time were mainly water-based coatings, high-solid coatings (so-called high-solid coatings) and radiation-curing coatings. According to the VdL , the powder coatings, which appeared industrially from 1965, but until then mostly as experimental products from a few paint manufacturers, experienced a strong upswing. In 1966 the first powder coating plant in Germany was put into operation. In the first statistically recorded year of production, 1974, only 3,369 tons of powder coating were produced. This was subsequently increased to 10,000 tons (1980), 37,500 tons (1990) and 70,000 tons (2007).
composition
Powder coatings consist of binders , additives , pigments and fillers . The absence of solvents is characteristic of powder coatings . The chemical differences between the raw materials used in powder coatings and conventional coatings are not great. The cross-linking mechanisms of the powder coating film are similar to those of a stoving enamel, in which two reaction partners enter into a chemical bond under the influence of temperature by forming an organic network. Almost all raw materials for powder coatings are available as powders.
binder
The main component of any paint system are the binders . These form the paint film, i.e. the basis that envelops all solid particles in the paint. It determines the basic properties such as surface properties, hardness and stability of the paint film. Binders consist of long-chain, mostly organic compounds that contain reactive groups . Synthetic resins are used for powder coatings, which can either crosslink with one another or via a hardener to form branched macromolecules.
Above all, epoxy resins , polyesters containing carboxy and hydroxyl groups , OH and GMA acrylate resins , and modified resins for special areas of application are used. The selection of the binder essentially determines the physical properties of the powder coating and thus its area of application.
The hardener is also part of the binder. Important substances used as hardeners are phenols and dicyandiamide for epoxy resins. Triglycidyl isocyanurate (TGIC) and hydroxylalkylamide for polyester resins, as well as dodecanedioic acid (DDDA from the English term dodecanoic diacid).
Other resins are used for the original form of the powder coatings, which are present as thermoplastics due to the fluidized bed sintering process used for coating . In these systems, film formers are predominantly polyamide , polyethylene , polyvinyl chloride and polyvinylidene chloride . These binders produce a comparatively soft film. The higher layer thickness results from the coating process.
Additives
Additives are auxiliaries. They influence important parameters such as B. the surface quality, i.e. course or structure, gloss, surface hardness and processing conditions. The addition of additives is unavoidable in every paint system. Additives make a decisive contribution to the quality of the individual powder coatings.
Polyacrylates are used as leveling agents. When added in small amounts in the powder coating film, they ensure reduced surface tension and thus a smooth and crater-free flow.
Degassing additives deaerate the paint film so that reaction gases and underground outgassing can be discharged via the paint surface. The paint surface remains free of pinholes . The substance most commonly used for this purpose is benzoin .
Waxes are added to influence the surface properties. In the melting phase of the hardening process, they swim to the surface of the lacquer film, which becomes smoother and less sensitive to scratches (slip effect). Such waxes can also be used for degassing and matting. Waxes help in the production of powder coatings by positively influencing the wetting of the pigments and fillers during extrusion. During the coating process, waxes can help prevent sintering in the powder hoses of the processor. The disadvantage of using surface-active waxes is that they cannot be painted over. On a wax-laden surface, the surface tension is reduced to such an extent that the subsequent coating does not hold. In the worst case, there is a loss of adhesion, which shows as peeling of the top coat.
The addition of texturing agents enables surface textures that range from a coarse-pored, wavy to a velvety fine structure. Another group of additives protects the powder coating against external influences such as overburning or baking in a directly heated gas oven.
Colorants
Pigments and dyes are used in all powder coatings with the exception of clear coats . Pigments give the coating an opaque impression and adjust the color . Due to the processing conditions in the extruder and the high stoving temperatures during curing, the choice of pigments is limited to those with high temperature stability. This choice is heavily dependent on the rest of the paint formulation. In spite of the limited range of pigments, almost all color tones can be represented that are possible in solvent-based paints.
Handling inorganic pigments is easier. Metal oxides and mixed phase oxide pigments of the rutile and spinel types are predominantly used . This group of pigments is characterized by a coarser particle size and thus easier dispersibility . The color strength of this group is usually less than that of organic pigments, but they have a greater hiding power and a predominantly greater temperature resistance . Compared to organic pigments, inorganic pigmentations produce a more cloudy hue. The most important inorganic pigments for powder coating applications are titanium dioxide (white), iron oxide pigments (yellow, red, black), chrome oxide green , bismuth vanadate (yellow) and cobalt oxide (blue, green).
In the area of intense color tones (e.g. strong red and yellow), it is necessary to use strong and pure pigments. The inorganic pigments containing heavy metals and based on lead and cadmium compounds , which were previously used as standard, are rarely used because of their toxicological properties and have therefore largely been replaced by organic pigments. In most cases, these are considerably more expensive than inorganic pigments, which affects the price of the finished powder coating. In addition, organic pigments have a lower level of fastness . From a chemical point of view, pigment black (black), azo pigments (yellow, orange, red), phthalocyanines (blue, green), quinacridones (red), diketo-pyrrolo-pyrrole pigments (orange, red, ruby), dioxazine pigments (violet) and some less frequently used groups of polycyclic pigments are used.
By using effect pigments , a metallic or pearlescent effect can be obtained. The difficulty lies in the incorporation of the effect pigments, since their effect is based on the platelet shape. This is destroyed in the extruder and the desired effect is lost. Effect pigments are therefore added subsequently ( dry blend ), which harbors the risk of separation, or they are connected to the powder coating by bonding . The most important types used are metallic effect pigments , pearlescent pigments and interference pigments .
A special case are the functional pigments, which are not used to adjust the color tone and thus rather act as fillers. The most important functional pigments are anti-corrosive pigments, mostly zinc phosphates .
In contrast to pigments, dyes are soluble in the coating material. They are not present in the paint as solids, but rather in dissolved form . This results in a translucent , i.e. non-opaque color impression. Interesting optical effects can be displayed with dyes, but a flawless background is essential as it can be seen. A disadvantage of the use of dyes is the lower resistance compared to pigments, such as, for example, light and weathering stability , which are already exceeded by the less stable organic pigments.
Fillers
Like the pigments, fillers or extenders are present in the paint system as solids encased by the binder. In addition to the possibility of designing the recipe economically, the inexpensive fillers have other advantages. With their larger particle size compared to the pigments, they give the paint film volume, with the smaller pigment particles taking up the spaces between the filler particles and thus achieving an optimal packing density . Functionally, fillers in powder coatings act as matting agents and support corrosion protection.
The most commonly used fillers are calcium carbonates . These are felled or used as naturally occurring chalk . This type of filler is suitable as a matting agent because of its platelet-like structure. Talc , a naturally occurring magnesium silicate hydrate , which is also available in platelet form, is suitable for influencing flow properties. Spherical barium sulfate is ideally suited for controlling the packing density of a paint. Like chalk, barium sulfates are available naturally ( barite ) and like ( Blanc Fixe ). Advantages of the industrially produced type are the lighter color, fewer impurities and a defined grain size distribution.
Manufacturing
Powder coatings are produced in the following order: weighing, mixing, extruding , grinding, sieving and filling. The individual production steps are described below.
Weighing in
The raw materials are weighed according to the specifications of a recipe on which the raw material names, proportions and work instructions are noted. Almost without exception, it is produced discontinuously (batchwise). The batch size depends on the volume of the batch containers used. Recipes are usually designed for an optimized weighing sequence, as the sequence influences the subsequent mixing behavior.
Mix
After weighing in, the batch container is pushed under a mixer and clamped in place. Depending on the type of mixer , an overhead mixing process is also possible. The mixing process serves to homogenize the mixture for the subsequent dispersion. Proper mixing can support dispersion by introducing shear forces. After mixing, the mixture of paint raw materials is ready for extrusion , a process that achieves an intensive homogenization (dispersion) of the raw materials.
Extrusion
The effect of the extrusion is achieved through the melting of the resin particles combined with intense shear forces.
One aim of extrusion is the homogeneous distribution of the paint components in the powder paint. Pigments are also available in the form of so-called agglomerates , which are dissolved by the shear forces ( dispersion ). Ideally, the particle size of the primary grain would be achieved, but in practice there are aggregates and smaller agglomerates.
The exact and reproducible setting of the desired color shade can only be achieved through good dispersion. For several reasons, this represents a particular difficulty in powder coating production in contrast to liquid coating production. On the one hand, all pigments have to be dispersed simultaneously during extrusion. Furthermore, there is no possibility of several grinding passages. The reason for this is the preliminary reactions that occur with multiple extrusions (partial cross-linking of binder and hardener). This makes classic nuance , the gradual addition of tinting pigments, almost impossible.
Structure of an extruder
→ Main article: Extruder
Extruders used for powder coating production consist of a heated housing and one or more rotating screws. The raw material mixture is conveyed through the extruder towards the outlet nozzle via the intake area of the screw. The resins melt here, causing the mass to change its consistency into a highly viscous melt. In the further process, this melt is kneaded vigorously, depending on the extruder design through the special configuration of the screw or through resistance elements in the housing. At the end, the melt ( extrudate ) emerges. This is usually cooled and rolled out on a cooling belt. Finally, the solidified mass is broken up into small pieces ( chips ) by a shredder .
Grinding, sieving and filling
The chips are finally converted into the final processing form, the powder, by grinding. Exact grain size distributions are achieved using rotor classifier mills. An air stream conveys the chips through a channel into the grinding chamber. In the grinding chamber, the chips are gripped by a rapidly rotating rotor. The powder coating particles are thrown against the wall of the grinding chamber (impact lining). The air flow pulls the particles further in the direction of the outlet opening. On this way they pass the rotating classifier. Only sufficiently ground particles get through the so-called sifter window . Coarser particles are captured by the classifier and thrown back into the grinding chamber for further grinding.
At this point, the fluidization and charging behavior of the powder can be influenced by adding additives.
Once the fine particles have passed the classifier area, they pass through a pipeline into a cyclone separator . There, too fine grain is separated ( fine grain fraction ). The powder-air mixture enters the cyclone tangentially, causing it to rotate. The finest particles are removed centrally with the air flow through a dip tube. The powder itself is slowed down due to its inertia, collects at the bottom of the cyclone separator and can be fed to the filling station through a pressure lock ( rotary valve ). The desired grain distribution can be set in a defined manner by coordinating the rotor and classifier speeds, the volume flow and the cyclone settings.
After a final protective sieving and filling into the appropriate containers (PE bags, containers , big bags ), the powder is ready for dispatch and ready for use.
Special features when adding effect pigments
Effect pigments are mostly platelet-shaped and cannot be processed in the extruder because they are destroyed there and lose their effect. These pigments must therefore be added subsequently. The two usual processes are the dry blend process and the bonding process. In the dry blend process, the effect pigments are simply mixed in. The pigments are present in the product without binding to the powder coating particles. In the bonding process, the added effect pigments are mechanically bonded to the powder coating particles and thus cannot separate from the actual powder coating during application.
Properties of different types of powder coatings
Every powder coating processor is faced with the choice of choosing the right type of powder coating for his or her application. The main criterion for a coating company to find the right type of powder coating for its object to be coated is largely determined by the technological side. However, increasing price pressure is increasingly forcing people to reconsider. The development of raw material costs is increasingly shifting the market to polyester qualities that are resistant to the outside, as the previously inexpensive hybrid powders are partially losing their price advantage.
The systems most widely used in Europe are epoxy, TGIC-free polyester and hybrid powder coatings. In addition to these types of powder coating, there are other, less common systems such as polyurethane and acrylate powder coatings.
Epoxy powder coatings
Pure epoxy qualities are only suitable for indoor use due to the lack of light resistance. The UV component of sunlight breaks down the resin structure. This is followed by a breakdown of the binder, which over time mattifies the coating and causes the colors to fade, creating a typical chalking effect. A reduced protective effect can also be the result. Epoxy powder coatings are ideally suited for protection against corrosion by means of a multi-layer structure. In general, their area of application is primarily the primer. Pure epoxy powder coatings are characterized by excellent chemical resistance. They also have a high insulation effect against electrical current.
Polyester powder coatings
Pure polyester powder coatings meet the requirements for weather resistance. They are more resistant to UV radiation and therefore offer long-term protection outdoors. Special polyester qualities meet the requirements of the GSB and Qualicoat quality associations . Among other things, these certify powder coatings for facade applications that offer excellent coating quality.
Classic polyester systems contain TGIC ( triglycidyl isocyanurate ) and are very popular on the market because of their universal properties. Since these powder coatings have had to be labeled as toxic (T) since 1998 , they are almost no longer used in Europe. These systems continue to enjoy great popularity globally.
Two different networking mechanisms dominate the replacement systems.
The systems which crosslink on the basis of a polycondensation with hydroxylalkylamide and have many years of market experience are characterized by low stoving temperatures and a smooth course. A disadvantage is the pronounced tendency to pinholes at higher layer thicknesses, which results from the splitting off of water during crosslinking.
The alternative to this are direct successors to TGIC. Like products containing TGIC, these crosslink with the polyester resin by means of a polyaddition . In this way, no by-products are released, which can cause needle sticks. Disadvantages here, however, are the somewhat higher prices, a poorer course (which requires further optimization steps) and labeling of the powder as irritating (Xi) above a certain hardener concentration in the paint.
Hybrid powder coatings
A middle ground can be found with the use of hybrid powder coatings. In these, epoxy and polyester resins are cross-linked. This combination has a wide range of applications. The weather resistance is better than pure epoxy systems and the resistance to chemicals is sufficient in many cases. Furthermore, all gloss and almost all texture settings can be easily implemented. The variety of colors is almost unlimited. So far, the low price has often been a decisive factor in the decision for hybrid powder coatings. However, the steady rise in the price of epoxy resins could negate this advantage. In this case, the choice could fall on a higher quality and nevertheless inexpensive polyester powder.
Polyurethane powder coatings
Powder coatings based on polyurethane are very popular in the USA and Japan. They offer very good flow and excellent weather and chemical resistance. Due to this resistance, polyurethane powder coatings are often used in the area of anti- graffiti coatings. The disadvantage is the high price and high energy costs due to higher crosslinking temperatures. The majority of the highly weather-resistant powder coatings (Superdurable) are based on polyurethane.
Acrylate powder coatings
The technology based on glycidyl-functional acrylate resins ( glycidyl methacrylate , GMA) is a niche application. Acrylate powder coatings have so far failed to be widely launched due to their high price, the comparatively poor mechanical properties and the high level of incompatibility with conventional powder coatings. Even small amounts of an acrylate-based powder coating cause severe cratering in conventional systems. Here the investment in a spatially separated, separate production or painting line is inevitable.
The advantages of these systems are the extremely good flow and very good weather resistance. These properties meet the high requirements of the automotive industry. The applications of this technology are therefore to be classified in this area. Energy costs can also be reduced thanks to the lower network temperatures.
A compromise is made with another hybrid system, the so-called polyester-acrylate powder coatings. These systems have improved mechanical behavior and, above all, better compatibility with conventional powder coatings. On the other hand, flow and weather resistance are worse than with GMA systems.
New developments
New networking technologies are also being developed, such as B. UV-curing powder coatings. These allow use on temperature-sensitive substrates such as MDF, for example, and thus allow the energy consumption to be further reduced. It remains to be seen until these new developments are finally ready for the market.
The goals of the development are therefore lower baking temperatures, improved application at higher throughput rates, an increase in the rate of color change and lower layer thicknesses.
Processing of powder coatings
→ Main article: powder coating
The application of powder coatings is based on a physical principle: electrical charges collect on the surface of a non-conductive body. Due to the difference in charge, such bodies adhere to an earthed workpiece for up to a few hours until the charge is equalized.
Charging
In (classic) powder coating technology, electrical charging can be practiced in two ways.
The standard method is the corona application, in which the powder particles are charged with an electrode at the gun tip and are thus not only transported by the air flow, but also by the electric field between the earthed workpiece and the electrode.
With tribo-charging, the particles are not charged by an external voltage, but by frictional charging in a Teflon- coated plastic tube. Such pistols can often be recognized by their longer design compared to Corona pistols or by the fanned-out spray organs. Both methods guarantee extensive contact between the powder particles and the pipe wall, which ensures good charging. A specially adjusted (so-called tribo-capable) powder material is required for the tribo application. Not all varieties meet this criterion.
Since only the powder particles themselves are charged, no unbound ions can interfere with the application with the tribo-application. As a result, the course of these paint surfaces often looks more relaxed. Without an external voltage applied, only a weak electric field builds up; the Faraday cage is practically irrelevant here. This is why the tribo application is the first choice when coating complexly shaped workpieces with pronounced cavities.
In contrast, with the Tribo process, the powder throughput is lower, so that the area performance is reduced compared to the corona application, and it may be necessary to run at a reduced belt speed. Increased wear can also occur, which is due to the higher air throughput. Due to the lower electrical field, there is no wraparound and the increased edge structure is no longer necessary.
Spray booths
Powder coatings are applied in special booths, in which an air flow ensures that no sprayed material leaves this room and contaminates the environment. The flow speed should be selected so that the powder is not pulled away from the object or foreign particles are torn into the cabin. This essentially depends on the sizes of the various openings in a cabin. The cabins are made of galvanized steel, stainless steel, glass or plastic. Plastic booths have the property of hardly accepting powder paint. This means that the booth is less soiled and more powder material reaches the object, which increases the efficiency of the initial application.
Since not all of the sprayed powder adheres to the object ( overspray ), there are several methods of collecting this material. It is either deposited as waste (drive to loss) or processed and returned to use (drive to recovery). During recovery, the powder coating is sieved and fed back into the cycle. Since recovered powder is finer than fresh powder, the latter must be added in sufficient proportions before being reused.
If the variety of colors is too large and color changes are frequent, often combined with a small number of painted parts, the risk is lost because the mechanical and cleaning effort for recovery is much greater. A recovery device is used for large numbers of items and especially when using a so-called house color.
The powder is separated from the transport air in the cabin by means of a filter or cyclone with a downstream fine dust filter.
The powder coating is metered according to the same scheme for all types of booth: The powder must be prepared for conveying through hose lines, which is done via fluidization using compressed air. This compressed air is either fed directly into the powder container (container with a fluid base), or the powder is conveyed from the delivered bags into a storage container, the base of which is made of an air-permeable material through which the compressed air flows evenly through the powder. The powder coating is loosened and fluidized here. It can be pumped like a liquid.
Powder coating test
Tests on the manufactured powder
Initial tests of a coating powder are carried out before coating. There are several factors that influence the processing and thus the result of a powder coating, essentially these are the grain size distribution and the fluidization behavior.
Grain size distribution
The grain size distribution influences the sprayability of the powder. When grinding the powder by means of rotor classifier mills, the paint manufacturer tries to keep the particle size distribution within a narrow, defined spectrum. It would be ideal if 100% of the powder reached the desired grain size. This is not possible due to the grinding process. For example, grist samples are taken during powder production and the particle size spectrum is measured using sieve analysis or laser diffraction .
When measuring the grain size distribution by laser diffraction, the physical principle of light refraction is used. Different sized particles deflect a beam of light to different degrees. Measured values are obtained that represent the percentage of the different fractions of the powder coating. It has proven itself in practice to pay attention to special grain sizes, for example the values at 10 µm, 32 µm, 64 µm, 90 µm and 150 µm. The median of the grain sizes (D 50 value) provides further information . This indicates the grain size at which 50% of the particles are finer and 50% coarser than the specified value. The proportions of the particles below 10 µm and above 90 µm are of particular importance. If the proportions in these areas are too high, processing difficulties are to be expected.
Fluidizability
The grain size distribution has a direct influence on the fluidizability of the coating powder. Fluidizability is one of the few properties of powder coatings that can still be adjusted after production, but only to a limited extent. There is the possibility of making improvements by adding a fluidizing aid. Too little fluidizability leads to (partial or complete) clogging of the spray gun during application, which results in spitting, i.e. a paint defect.
To measure the fluidizability, a defined amount of compressed air is introduced into a measuring apparatus, which creates a fluidized bed . In the fluidized bed, the powder coating behaves like a liquid, which is desirable for the application. After a stable fluidizing height has been established, this is measured. In the second step, a small hole at the lower end of the fluidizing vessel is opened and the amount of powder coating that flows out of the vessel in a defined period of time is determined gravimetrically. The height difference between the fluidized and non-fluidized state is referred to as fluidizability . The product of fluidisability and application rate is called pourability . The test of fluidizability gives only relatively imprecise results. However, it is the only simple method to determine this property and is therefore widely used.
Networking behavior
A statement about the crosslinking behavior can be made in advance on the powder coating by determining the gel time. In this test, a defined amount of powder coating is first melted on a heating plate . Constant stirring determines the point in time at which crosslinking begins. This is a rough method that depends heavily on the auditor's way of working. However, relative deviations in the cross-linking behavior are shown precisely enough.
Dynamic differential calorimetry (Differential Scanning Calorimeter, DSC) is used for more precise information . In a comparative measurement process, energy absorption and output are determined ( endothermic , exothermic ). The glass transition temperature (T G ) and the energy requirement are determined. This allows the crosslinking behavior of a powder coating to be assessed very precisely.
Density determination
The density determination provides further information about the coating powder . It can be determined using an air comparison pycnometer . The measured values are usually given in g / cm³. By specifying the density and the desired layer thickness, the formula
Spreading rate = 1 / (density * (layer thickness / 1000)) [layer thickness in µm, density in g / cm³, spreading rate in m² / kg]
calculate the theoretical coverage of the powder coating in m² / kg.
Tests on the cross-linked powder coating film
As with all coating systems, the visual appearance of a powder coating is largely determined by the nature of its surface. In addition to the color impression, these are the degree of gloss and the gradient (or structure) that give a paint layer its typical appearance. In addition to the optical properties, the haptics , i.e. the tactile properties of a surface, are influenced. Another surface test measures an invisible property, the layer thickness .
An important prerequisite for all of these tests is an optimally prepared substrate so that the test results are not falsified by incorrect pretreatment. The use of standardized test panels is ideal for this.
In the rarest of cases, an absolute picture of the coating is obtained from the measurement results. However, the combination of these methods gives the possibility of a comparative control in order to achieve reproducible quality.
Layer thickness
→ Main article: Layer thickness (coating)
A property of paint films that cannot be visually perceived and yet influences the visible properties of the paint film is the layer thickness. This parameter, which is important for every coater, not only influences the cost- effectiveness of a coating, but also physical properties such as e.g. B. the protective effect, the mechanical strength or the accuracy of fit during the assembly of powder-coated elements.
Visually visible is the reduced coverage of the surface by the paint layer and the resulting change in color due to the shining through of the surface, the thinner the paint layer. If the paint layer is too thin, the flow will also deteriorate.
To ensure reliable testing of the other properties, it must be ensured that all samples in a series have a similar layer thickness.
The measurement of the layer thickness on metal substrates is usually done with portable measuring devices that represent a combination of two measuring methods. The underlying methods are the magnetic - inductive measurement for steel substrates and the measurement using eddy current methods for substrates made of other metals . These devices allow a non-destructive measurement that can be carried out very quickly and can therefore be used for quality assurance.
One measurement method for all surfaces is the use of an IG clock . With this method, a piece of the coating is removed down to the substrate and the difference in height between the paint surface and the substrate is determined mechanically. The main disadvantage of this method is the partial destruction of the paint layer, which is why it is used almost exclusively on non-metallic surfaces where induction or magnetic field measurements are not possible.
Ultrasonic measuring devices are occasionally used for the non-destructive detection of the layer thickness, especially on non-metallic substrates.
Optical properties
As with all other paint systems, the color tone is the outstanding visual property, plus gloss and flow.
Colorimetric measuring methods and visual evaluation are used to determine the color shade. This ensures a sufficiently precise assessment of the color sensation . Because of the complexity of the topic, please refer to the articles Colorimetry and Coloristics .
The gloss according to DIN 67530 the proportion of the facing surface reflection (ISO 2813). If a light beam hits a high-gloss surface at a certain angle, it is reflected at the same angle. If the surface has a microscopic roughness , the light is only reflected diffusely , so the surface appears matt to the observer. With modern reflectometer measuring devices, gloss levels are determined under three selectable angles . The measuring angle is 20 ° for high-gloss lacquers, 60 ° for medium degrees of gloss (silk-gloss, silk-matt) and 85 ° for matt lacquers. Higher-quality measuring devices also offer the option of measuring the haze on high-gloss surfaces at a 20 ° angle. This effect describes a slight optical clouding that can occur on high-gloss surfaces. Objects that are reflected in such a surface appear slightly blurred at their edges. In practice, the specification of the 60 ° angle (which does not conform to the standard) has become established for all gloss areas.
The course of a surface is perceived visually by humans and can subconsciously influence purchasing behavior. For example, a perfectly smooth paintwork on a car body is more appealing than a wavy, disturbed surface. It does not matter whether the technical properties are met just as well or not. In contrast to this, there is the deliberately created structure, which is often used to conceal unevenness in the subsurface. In mechanical engineering , for example, structured paints are often used to cover weld seams and grinding marks.
In order to express the course as a quantitative variable, a measuring method has proven itself that evaluates the reflection angle of a laser beam at different measuring points. The laser beam is reflected on the surface. Depending on the structure of the surface, the angle of reflection can differ from the theoretical angle of reflection. In this way, the roughness of the surface can be determined mathematically. Hand-held devices allow the portable use of this measurement method.
Mechanical properties
Tests that measure the properties of a powder coating in the event of mechanical impact are roughly divided into cupping tests, hardness tests and deformation tests.
The Erichsen depression according to ISO 1520 represents a slow deformation of a surface, brought about by a hemisphere with a known radius. This is pressed at a slow feed rate into a test sheet, the front of which is coated. The sheet metal is greatly stretched and with it the powder coating film. The result is the indentation in mm up to which a paint film shows no cracks.
The impact test (German: impact cupping ) according to ASTM D2794 is similar to the cupping test according to Erichsen, but the deformation is brought about suddenly. A defined weight falls from a known height onto the test surface and leaves a deformation in the test sheet. This very stress-intensive process allows conclusions to be drawn about the flexibility of the coating. The tests are repeated at different drop heights and the highest drop height at which no cracks are visible is determined. The result is the product of the height of fall and weight at this height of fall. Here, the specification in inchpound has become commonplace from the Anglo-American area. The test can be tightened by applying and tearing off a defined adhesive tape on the stressed area. If parts of the flaked paint film stick to the torn adhesive tape, the adhesion of the paint layer to the substrate is insufficient. In addition to the quality of the paint, in the event of flaking, insufficient pretreatment of the substrate is another possible cause of the defect.
In the mandrel bending test according to ISO 1519, a test sheet is bent around a round metal mandrel with a defined radius, a smaller radius representing a greater load. This test can best be compared with the quick test mentioned at the beginning. The test fails if cracks form.
In the cross-cut test according to ISO 2409 the paint film is added to a defined injury. For this purpose, the paint surface is cut with parallel lines at a distance of 2 mm (1 mm for a layer below 60 µm) down to the substrate. The process is repeated rotated by 90 ° so that the cuts cross each other. Poorly adhering paint material shows flaking at the intersection points and the areas between the cuts. In the worst case scenario, the entire surface may be lost. The evaluation is based on a comparison graphic, from which the percentage loss of adhesion on the test surface is determined as a GT value (from 0-5). The test can be tightened by tearing off an adhesive tape.
When testing the indentation resistance according to Buchholz according to ISO 2815, commonly known as Buchholz hardness, a round, sharp-edged test piece with a contact force of 5 Newtons is placed on the test surface. After a loading time of 30-40 seconds, the test device is removed. The length of the indentation is measured during the evaluation. The resulting indentation resistance is tabulated and given as the result. The shorter the impression left, the higher the corresponding characteristic value. This means that the coating has a higher hardness in this case.
literature
- J. Pietschmann: Industrial powder coating . 2nd Edition. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-13380-5 .
Web links
Individual evidence
- ↑ a b c European Coatings Directory - Special Issue: Powder Coatings; 2008.
- ↑ C. Bangert: Increasingly Consolidated but Fairly Divers. In: European Coatings Journal. 12/2008, p. 13ff.
- ↑ Production statistics for paints and varnishes in 2008. In: Farbe und Lack. 06/2009, p. 12.
- ^ Arthur A. Tracton: Coatings Materials and Surface Coatings CRC Press, Boca Raton 2006, ISBN 1-4200-4404-4 .
- ↑ a b K. Dohnke: The paint story: 100 years of color between protection, beauty and the environment. Dölling and Galitz, Hamburg 2000, ISBN 3-933374-64-2 .
- ↑ a b c d A. Goldschmidt, H. Streitberger: BASF Handbook Painting Technology. Vincentz Verlag, Hannover 2002, ISBN 3-87870-324-4 .
- ↑ B. Müller, U. Poth: Paint formulation and paint recipe: The textbook for training and practice. Vincentz Network, 2006, ISBN 3-87870-170-5 .
- ↑ a b J. Bieleman: coating additives. Wiley & Sons, 1998, ISBN 3-527-28819-8 .
- ↑ a b c Article production on www.pulverlackforum.de
- ↑ J. Keller: Avoiding errors - know how. In: Journal for surface technology. 05/2009, p. 16.
- ↑ Website of the quality association for the piece coating of components (GSB), requested on August 22, 2009 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ website of Qualicoat International Switzerland, polled 22 August 2009
- ↑ New generation: UV powder coatings. In: paint and varnish. 1/2009, p. 33.
- ↑ ISO 8130-1
- ↑ ISO 8130-5
- ↑ F. Tragor: On the influence of grain size distribution and additives on the application properties of powder coatings in the household appliance industry. Thesis. University of Applied Sciences for Technology Esslingen, 2004.
- ↑ DIN 50981
- ↑ ISO 2813.
- ↑ ISO 1520.
- ↑ ASTM D 2794.
- ↑ ISO 1519.
- ↑ ISO 2409.
- ↑ ISO 2815.
