Pre-treatment is the preparation of the substrate surface, by chemical and physical means, so that it becomes optimised to accept the powder coating finish. To do so, it is essential to ensure that the substrate is free of dirt, grease, oil and metal oxides, such as rust and mill scale. The wide range of techniques available for pretreatment fall into two broad categories, a)Chemical, b) Mechanical methods. The majority of powder coating applied is onto metals and the type of surface contaminant (e.g. oxidation, organic dirt or grease) will dictate the choice of pre-treatment. Consequently, surface cleaning is often followed by, or combined with the application of a chemical conversion coating which provides both protection and enhanced bonding of the powder film.

•   Removal of Oxide Scales and Surface Defects : Pickling



As discussed previously, ferrous metals are prone to surface oxidation, which in the case of hot - rolled steel in the form of a blue - black mill scale, formed as the steel cools. If the scale, which has a tri-laminate structure, remained intact, it would provide protection of the underlying steel. However, mill scale is very brittle and invariably cracked so that it has to be completely removed to powder coating. Pickling is commonly used to remove mill scale and involves immersing the substrate for around 30 minutes in a bath of an aqueous solution of hydrochloric acid (25-300C) or warm sulphuric acid (60-700C). The aim is to dislodge the oxide layers from the metal through promoting hydrogen gas evolution in the underlying layer. After pickling process care must be taken to ensure efficient rinsing.

•   Removal of Oxide Scales and Surface Defects : Mechanical (Abrasive) Cleaning Method



Chemical cleaning is divided into two distinct groups: a)Organic solvent based, b) Alkaline and acid aqueous method.



•   Removal of Oxide Scales and Surface Defects : Chemical Cleaning



Chemical cleaning is divided into two distinct groups: a)Organic solvent based, b) Alkaline and acid aqueous method.
Emulsifiable Solvent and Emulsion Cleaning:- The component is either sprayed or immersed in an organic solvent which contains emulsifying agents. After comprehensive coverage, the component is rinsed with water to emulsify the solvent together with contaminating oil or grease. Another advantage is that treatment is usually at ambient temperature, although cleaning efficiency is directly related to physical agitation over the component surface during the water rinsing stage.
Alkaline and Acid Cleaners:-Alkaline cleaners are the most extensively used chemical cleaners for substrate pre-treatment, primarily on grounds of economics, safety, and resistance of steels to attack. They are also commonly used before metal undergoes conversion coating. The degree of alkalinity is known to effect phosphate conversion coatings (particularly zinc), with higher the pH, coarser the resulting crystal structure. In general, a finer structure is preferred for improved mechanical strength of the phosphating and gloss of the applied powder coating. Acid cleaners have a relatively restricted application, limited to mainly light rust removal. They are generally inefficient for oil and grease removal, and if the component is soiled as well as rusty, then acid cleaning is usually a follow-on to solvent or alkaline.



•   Removal of Oxide Scales and Surface Defects : Conversion Coating



Chemical surface conversion after removal of dirt, oil, grease and loose or contaminating oxidative corrosion, is a common practice for metal components to be powder coated. Essentially, the process involves controlled modification of the freshly cleaned surface into thin, 1-10m, layer of a metal salt (typically phosphate or chromate), with a relatively brittle, crystalline structure. The major gain from use of conversion coatings is increased long - term corrosion resistance by the finished powder coated system.




•   Surface Oxidation



As most powder coating applications are onto metallic components, they can play major role in preventing substrate corrosion or oxidation and hence significantly improve the longevity of the final product. The two most common metal are steel and aluminum of which the former is most prone to degenerative corrosion and tends to require the most extensive pre-treatment. The choice of steel along with its final use (and environment), poses a complex problem to the applicator. As corrosion is essentially a wet oxidation process, ideally the paint film only needs to act as physical barrier between the atmosphere and the steel substrate and indeed, thick coatings such as those provided by powder, do present an excellent barrier to diffusion of water and oxygen. The corroded surface of unprotected mild steel exposed to an aggressive (polluted) atmosphere, generally also holds ferrous sulphate crystals. Under moist conditions these are formed by inter - action between air-borne sulphure dioxide (e.g. originating from fossil fuel combustion), oxygen and iron. As hygroscopic ferrous sulphate crystals can expand by almost three times when transformed into rust, they can be important nuclei for the breakdown of an applied coating. Pretreatment removal of ferrous sulphate is difficult and normally requires relatively stringent techniques, such as abrasive blasting. The attraction of aluminum lies in its own inherent self - protection against excessive corrosion. This is achieved by the natural build up of a thin surface layer of aluminum oxide, which acts as a barrier significantly to hinder further oxidisation.

•   Corona Charging



Corona charging works through the application of a high static electrical charge to a corona charging electrode, and powder is charged when passing through the area close to the electrode, by picking up free electrodes from the electrostatic field.
Air consists mainly of oxygen, nitrogen and carbon dioxide molecules, which in turn are comprised of atoms (two or more) with central nuclei of positive charges, surrounded by an equal number of orbiting negatively charged electrons. With equal numbers of positive and negative charges, the atoms are electrically neutral or balanced. However, if an atom loses an electron, it is no longer electrically balanced, but has a net positive charge. Conversely, if it gains electrons, it acquires a net negative charge. An atom with extra electrons or with a deficiency of electrons, resulting in its being electrically charged is called an ion. An ion with extra electrons is negatively charged and is called an anion; an ion deficient in electrons is positively charged and is called a cation.
The corona discharge of a powder-coating gun is obtained by applying a high voltage of up to 100KV to a sharp needlepoint electrode at the end of the spray gun. The high voltage electric field which results, "breaks down" the air in the immediate vicinity of the electrode, creating ions (normally negative), which are then free to attach themselves to nearest object or surface. Powder particles passing through this field therefore become charged and in turn are attached to an earthed substrate.
The main disadvantage of the system is that a relatively small number of the ions generated by the spray gun, attach themselves to powder particles, leaving a high proportion of free ions within the stream of particles and tend to build up on the surface, particularly in deep recesses or sharp, deep internal corners. This build up of free ions will eventually repel powder particles approaching the recesses, making it difficult to coat these areas which are commonly known as "Faraday cage effect". Electrons, which have not attached themselves to powder particles, also travel in the air stream and along the electrostatic field lines to the item being coated. This is because it is grounded. The unattached electrons are attracted to the component together with the powder particles where they build up, because of the isolating layer of the already deposited powder particles. If too many charges with the same polarity are deposited on the workpiece, the electrostatic force between the individual particles becomes so strong, that they repel each other and get pushed away from the surface. This is called back ionisation. Back ionisation causes discharges within the powder coat itself and also makes it difficult to coat complex shapes. Additionally, back ionisation causes the powder coat to be uneven and when stoved, the finished surface looks like orange peel.



•   Triboelectric Charging



The second type of charging is through the Tribo charge gun, which produces its electrostatic effect by passing powder through the gun. This has a "friction body" with a complex pattern of turbulators through which the powder passes. All powder-guiding parts are lined with insulting materials. Tribo guns charge positively and this implies that the powders have to be specifically formulated for Tribo charging. Since there is no charging electrode, there is much reduced Faraday cage formation, making Tribo coating the optimum choice for recessed areas and undercuts, e.g. Painting radiators. As the powder is charged inside the gun, no free ions are produced and therefore problems of recess penetration, re-coating and producing heavier coatings are overcome. Since there is no strong electrostatic filed (lines of force) between gun and substrate, powder flow from gun tends to be very soft and the "wrap around" effect associated with electrostatic application is not so evident. However, where the particular product presents exceptional problems in coating, due to complex geometry, deep recesses, very acute angles or the need to apply one heavy single coat without surface disruption, then, providing that suitable powder is available, the Tribo system would be a better option to Corona System.



•   Powder Delivery during Coating



The best powder performance can be achieved through fluidizing with compressed air. The resultant mixture of air and powder can then be readily transported through a closed system, with the powder flow controlled simply by the air rate. With the transporting air switched ON, the air/powder mixture will flow quite evenly through a hose system, but when the air is switched OFF, the powder in the system drops to the bottom of the hose or gun barrel. When switched ON again, fresh powder is fed into the system, but this must now pick up and integrate with the inert powder lying in the hose. This tends to cause a slight back-pressure in the system resulting in a puff or surge of powder from the guns.



•   Powder Application Spray Guns : Manual Spray Guns



Important points to consider in manual electrostatic spray systems are fluidisation stirring or vibration for powder in the hopper, together with low air consumption and smooth powder feed. Additionally, the system must readily link into a recovery cycle with uniform coating achieved through the continual supply of new powder through a replenishing hopper. For most applicators, the ability to allow rapid colour changes is also vital.



•   Powder Application Spray Guns : Automatic Spray Guns



A system for automatic spraying may involve up to 10 gun control modules. If more are needed, several cabinets can be interconnected, the number determined by the product shape and size, and conveyor speed. The system may be fed by powder, injected from hoppers, equipped with a fluidised bed. A module should control powder replenishment and stabilise levels through a sensor to ensure a uniform powder delivery rate. Gun movement can be coordinated through reciprocating control strokes, which are adjusted to the substrate geometry through keyboard entries (by this method several pre-programmed applications can be stored and called up when required. The most common method of gun movement is vertical reciprocation, particularly with substrates with a uniform geometry that generally presents a parallel surface to the guns, such as flat sheets, shelving aluminum extrusions etc. The guns on most reciprocators are fitted horizontally so that each gun travels the full height of the set stroke, although there is a growing tendency to mount the guns vertically with a short reciprocating stroke set to cover only a proportion of the passing components. The factors that determine the correct speed of the traverse to ensure even coating without striping are based on:



•    a) Number of guns
•   b) Distance between guns
•    c)Distance from nozzle to substrate surface
•    d)Substrate height
•    e)Conveyor speed

Metallic powder coatings are those that contain pigments based on metallic flake, mica, or other material that creates a colour change. Metallics are a popular choice for powder coatings because they provide a wide variety of appearances. Despite their popularity, metallics bring a new set of potential application problems when compared with solid colours.

•   Colour Deviations and Effect



To largely eliminate colour/ effect differences caused by the coating system an entire coating job must be processed on the same coating line, without parameter fluctuations, preferably without interruption and with consistent recycling percentages (guideline 30%). Manual coating is likely to produce variations of colour and/or effect due to inconsistent film thickness. Manual coating must therefore be adjusted to automatic processing with respect to colour and effect. Coating thickness is of importance, as variations will cause colour/ effect differences. Variations inherent in metallic coatings can be directly linked to the content of metallic pigments. Generally fined flakes of metallic pigment are used. Positioning of those flakes within the applied coat determines the metallic effect and colour. All parameters of application may influence the position of the flakes and thus also colour/effect. It is therefore important that throughout an entire coating job, all equipment is left at precisely the same settings. Coating one entire job with a variety of equipment should be avoided, or else considered only after exact adjustments and comparison produce identical test results with different equipment.



•   Reclaim



To achieve a consistent colour/ effect it is important for the coater to establish a ratio of virgin and reclaim powder and adhere to this ratio. 70 % virgin powder should not be exceeded. Since not all metallic effect powders are reclaim-consistent, the virgin powder percentage must be established via upper and lower tolerance samples. A final quality inspection for colour is still highly advisable.



•   Application Equipment



Different powder coating guns systems and spray parameters are often the cause for varying results. It is very important to only work with nozzles suitable for metallic powder application. Depending on the type of object to be coated, powder should be applied with a flat-spray type nozzle respectively with an aerated impact disk, in an even cloud pattern. Grounding and charging of the powder cloud must be constantly monitored. Interim cleaning of the powder hoses and removal of deposits from powder guns and booths is also part of a regular process control. Metallic powder coating should exclusively be done from fluidized powder containers. Since metallic powder coatings react more sensitively to differing reclaim ratios, the coating should from the very beginning be at approximately 30% reclaim.



•   Charging and Grounding



Generally very few metallic powder coatings are suitable for Tribo application. Suitability must be established prior to a coating job. Due to the differing changing characteristics of powder coating and metallic particles, not all metallic particles are transported to the part. This too can cause a variation in colour/effect. Changing from electrostatic to Tribo-static charging is not permissible. With metallic powder coatings, a particularly clean coating system is very important in order to avoid short-circuiting in the gun area from powder deposits. Once again the importance of constant control over the charging of the powder cloud is stressed. When working with metallic powder coatings proper grounding of the equipment as well as work piece is very important. This contributes to a high degree of colour/ effect consistency.



•   Coating Durability



Generally the durability is determined by the processing system - one or two coats. The durability of a metallic powder coating is product specific. Consulting the powder manufacturer prior to application, with particular reference to special requirements, such as wear and scratch resistance, cleaning recommendations, colourfastness and chemical resistance is recommended. The manufacturer needs complete information about all of the requirements that the powder coating is subjected to in a project/application, in order to offer a powder of suitable composition. If materials of unknown chemical influence are to be used, tests must be performed after consultation with the coating manufacturer. Cleaning of metallic powder coated materials must be performed at regular intervals and as quickly as possible after they get soiled. Dried and old dirt can only be removed by scouring, which means scratching of the powder-coated surface. Working with metallic powder coatings requires precision. All guidelines must be observed. Most important is proper communication between the coater and the customer, and also between the coater and coating manufacturer to assure that all provisions are given for a quality finish.

Thermoplastic powders require heat only to fuse the powder together into a continuous film. However, Thermosetting powders often require additional heat to cure the film on the product. There are four basic methods normally used in the curing of powder coated parts: convection, infrared, a combination of the two, and ultraviolet (UV) curing. Convection ovens can be either gas or electric. Hot air is circulated around the powder-coated parts, and the parts attain the temperature within the oven. UV curing is commonly used with heat sensitive substrates. Specifically formulated UV powders flow at very low temperatures (121 Deg. Cel.) and can be cured via UV radiation in a matter of seconds. Infrared ovens, using either gas or electricity as their energy source, emit radiation in the IR wavelength. The powder and the substrate immediately below the powder absorb the radiated energy, so the entire part need not be heated to the cure temperature. This allows a relatively rapid heat rise causing the powder to flow and cure when exposed for a sufficient time. Combination ovens generally use IR as the first zone to melt the powder quickly. This process is termed near infrared (NIR) cure, and powders are formulated specifically to take advantage of this process. The part then progresses into a second zone, which is a convection oven. The oven-heating unit must be sized well within its capacity to ensure that it will be able to maintain oven temperature under full load conditions. Proper sizing will also give rapid heat up time, aided by circulation fans, which should provide rapid heat transfer to the substrate together with an even oven temperature. Thermoset coatings have a finite point at which maximum cross-link density is achieved. At this point the coating is set. Time Vs Temperature graph is an important indicator.

•   Following parameters play a vital role in achieving quality results:-
•  Heat up time - usually 4 to 8 minutes
•   Elapsed time above specified temperature (e.g. 180°C)) - usually recommended is 10 minutes Total cycle time = Heat up time + Elapsed time above specified temperature - usually 14 to 18 minutes (depending upon the system)