Conventional electrostatic powder application leads to large quantities of negative charges being transferred to the object. If this large surplus of electrons cannot be efficiently earthed, the coated surface rapidly develops a powerful negative charge thus repelling the negatively charged powder from the spray gun. Consequently, without sufficient earthing it becomes impossible to build up a thick layer of powder with good flow and finish, rapidly and efficiently.
Powder application by friction charging results in powder leaving the spray gun positively charged; that is it will have a deficiency of electrons. To build up the powder layer on an object, it is therefore necessary to partially neutralize the powder by drawing electrons from earth. Should this not occur quickly enough, the coated surface would develop a strong positive charge that subsequently repels positively charged powder on leaving the gun. The resultant effect being the same as for high voltage equipment; an insufficient powder coating thickness.
Inadequate earthing can easily be recognized by little or no powder accumulation around the hanging point of the object being coated.
To ensure plant safely, it is absolutely essential to fully and effectively earth spray equipment, spray booths, and related equipment. This ultimately maximizes the avoidance of high voltage discharge and the possibility of resultant electrical sparks.
With friction charging equipment, good earthing of the spray gun is decisive for successful application. As the powder becomes positively charged, the electrons that are stripped away must be earthed. Without effectively earthing the spray gun, negative charges will accumulate and powder will then be allowed to pass through the gun without being charged.
Relative humidity within the working environment is of great significance for spraying efficiency. Ideal relative humidity being 45-55%. A quicker build-up of the powder layer and more even coating thickness are two of the advantages achieved by controlled humidity. Relative humidity can be measured with simple and inexpensive equipment.
Only clean, dry compressed air should be supplied to powder coating equipment. The quality of the compressed air can change the electrostatic charging and transporting properties of the powder. Impure compressed air may also cause visual defects in the coating.
Pressurized air must be free from oil, water and as dry as possible. Chillers are often used in addition to mechanical water/oil separators for removing moisture from compressed air. Air leaving the cooler/dryer at a temperature of 3°C or lower is an indication that it is dry and well suited for powder coating.
Spraying efficiency can be expressed as the percentage (%) of sprayed powder, which adheres to the object at the first application attempt. Maximum spraying efficiency provides several, advantages, both technical and economic.
The following are representative of some of the most important factors that influence spraying efficiency.
All powder coating processes necessitate the powder, suspended in its air flow, being as close as possible to the object. The force of electrostatic attraction between powder particles and the object decreases by the square of the distance between them (D2), and only when that distance is just a few centimeters will the powder be drawn towards the object. Careful positioning of the spray gun also assures that small and large particles are deposited on the object in the same proportions found in the virgin powder.
In order to increase spraying efficiency it remains advantageous to suspend objects as close together as possible along the conveyor line. This reduces the amount of powder that is recycled thus preventing an excess of finer particles returning to the powder reservoir. To achieve the same coating thickness on all objects, however, spacing must be adapted according to the size of the objects, as the following diagrams illustrate:
a. When the distance is too small, objects are not evenly coated:
b. By increasing the distance, the coating thickness is even on all objects:
c. A small object will produce a higher concentration of fields and subsequently receive a thicker coating than a larger object adjacent to it. It is advantageous to hang objects of equal size next to each other along the conveyor. Hanging objects correctly on the conveyor is of greater importance to successful conventional electrostatic spraying than it is to friction charged powder spraying.
A major advantage of powder coatings over liquid paint systems is the ability to reuse powder that does not adhere to an object (overspray). Air and powder are separated by either a cyclone or a filter system, as illustrated.
Powder that is recycled will be recollected in the powder hopper. Particles which are more easily charged adhere themselves to the object whilst the proportion being recycled are of another particle distribution size than that of virgin powder. The relationship between small and large powder particle size has significant meaning regarding spraying properties and consequently the end result.
Recycled powder will always have properties that differ from virgin powder. In order to maintain a constant composition it is always an advantage to keep the volume of reused powder to a minimum and continuously return any recycled powder to the powder hopper.
To further reduce variations in particle size distribution due to recycling, virgin powder should be added frequently to the powder hopper. Without the addition of virgin powder, the volume level of fine-grained particles will tend to increase to an extent, which ultimately makes particle charging and transporting within the spray equipment more difficult.
| ADVANTAGES |
DISADVANTAGES |
| Simple construction |
Effectiveness of separation depends to a large extent on operating conditions. Can produce considerable waste. |
| Simple Cleaning |
|
| ADVANTAGES |
DISADVANTAGES |
| All powder is recycled |
Accumulation of fine-grained particles. May produce problems with spraying process, particularly with friction charging. Extensive cleaning: filter change requirement between colors. |
Preparatory treatment is very often a theme of discussion between the powder supplier and the powder user. How necessary is pre-treatment and what type of pre-treatment should be adopted?
On the following pages we will attempt to provide a guide to assist the selection of pre-treatment systems, based on the stresses anticipated for the finished products
An absolute prerequisite for successful powder coating is that the surface to be coated must be clean and uncontaminated.
Contamination of the surface may give rise to aesthetic defects in the coatings, (bumps, craters, etc) and may cause poor adhesion between the coating, and the base. The most typical contaminants are oil and grease, surface corrosion, mill scale and what can collectively be called particles – both loose and fixed.
A clean surface will in many cases be insufficient to achieve the necessary or desirable corrosion protection. In such cases – where corrosion protection is a primary requirement – chemical pre-treatment should be chosen.
“Solvent degreasing” is very often employed as the sole pre-treatment prior to powder coating. To illustrate what can happen when such a system is exposed to a corrosive environment, salt spray chamber tests are suitable. In comparison, a system recommended for pre-treatment of higher corrosion classes can be used.
A series of factors will consequently affect the choice of pre-treatment method, some of which will be specific to the individual user.
The following factors should be identified:
- Metal type and quality
- Surface condition, i.e. degree of contamination and what contami nants to be removed
- The finished products and areas of application and its protective quality requirements
- Economic and environmental considerations (most often a question of various alternatives to similar or equivalent pre-treatment systems)
Whether you consider using cleaning/degreasing as the only pre-treatment or as an integral part of a more comprehensive treatment, the method employed should be based on the types of contamination to be removed, as well as the type of substrate (base) in question.
Solvents remove only “greasy” contaminants such as oil, fats, lubricants etc. The most common being tri and perchloroehtylene. Subsequent rinsing is not necessary after degreasing with solvent.
Degreasing with acidic, neutral or alkaline chemicals can also be employed. Such agents may also remove corrosion, mill scale and other oxides.
Mechanical methods are used both to remove stubborn contaminants such as welding flash, mill scale etc., and to provide better adhesion for the subsequent surface coating.
If blast cleaning is used (the abrasive agent to be chosen be determined by the base in question and the desired roughness), it is important to remember that grwasy contaminants should be removed in advance.
Treatment with iron phosphate (often called thin layer phosphating) provides very good adhesion properties and has no adverse effects in the mechanical properties of the powder coating. Iron phosphate provides good corrosion protection for exposure in the low and middle corrosion classes, though it cannot compete with zinc phosphate in this respect. Iron phosphate can be used in either spray or dip facilities. The number of steps in the process may vary from 2-7, depending on the basemetal and the requirement for protection. In relation to zinc phosphate treatment, the iron phosphate process is generally cheaper and simpler to accomplish The phosphate layer normally weighs between 0.3-1.0g/m2.
The zinc phosphate process deposits a thicker layer than iron phosphating, and is securely anchored to the base material. Zinc phosphate also has very favourable adhesion properties, though in some cases it may reduce the mechanical integrity (flexibility of the system. Zinc phosphate provides excellent corrosion protection and is recommended for pre-treatment of steel and galvanized steel for exposure in high corrosion classes. Zinc phosphate can be used in either spray or dip facilities. The number of steps in the process varies between 4-8.
Zinc phosphating is normally more expensive than iron phosphating, due to both higher plant costs and more expensive operation.
A series of different systems are available within the chromate group of treatments. The system selected depends on the type of metal or alloy, the type of object (method of manufacture: casr, extruded etc.) and of course, quality requirements.
Chromate treatment may be sub-divided into:
- Thin layer chromate treatment
- Green chromate treatment
- Yellow chromate trearment
The latter is the most common method for pre-treatment prior to powder coating. The number of steps in the process may vary, depending on how extensively the goods have to be prepared for chromating, for example by pickling, neutralization etc. and consequent rinsing steps.
Use of traditional chemical pre-treatment may involve chemicals subjected to strict regulations, resulting in high expenses on water/effluent treatment and waste disposal.
More environmentally friendly pre-treatment systems are continually being developed, examples being non-rinse processes and chrome-free systems. Some of these more novel pre-treatment processes also offer good prospects for use with powder coatings.
Reputed suppliers of pre-treatment chemicals will be able to give background, experience and documentation for their systems with powder coatings.
This is special kind of corrosion appearing mostly on aluminum. The phenomenon resembles a worm creeping under the coating,
always starting from a cut edge or a damage in the layer.
This is special kind of corrosion appearing mostly on aluminum. The phenomenon resembles a worm creeping under the coating,
always starting from a cut edge or a damage in the layer.
To minimize filiform corrosion it is advised to ensure a proper alkaline etching followed by an acidic wash prior to the chrome conversion coating.
An aluminum surface removal of 2g/m2 (minimum 1.5g/m2) is recommended.
Anodizing as pre-treatment for aluminum is a technology specially developed to prevent filiform corrosion.
A special anodization process is required when the thickness and porosity of the anodization layer is of vital importance.
As an aid in finding what requirements should be made for pre-treatment, we can define different classes of corrosion:
