There are several advantages of powder coating over conventional liquid coatings:
1. Environmental aspects
VOC: Powder coatings emit zero or near zero volatile organic compounds.
- Environmentally friendly and energy and material efficient.
- Powder coating production lines produce less hazardous waste than conventional liquid coatings. No paint sludge is produced, as arises from water-wash booths used to trap over-sprayed liquid paint.
- Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.
- As Powder Coatings are solvent-free, no oven is required for the flash-off of solvents before curing.
2. Application and aspect:
- Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
- One-coat application: a thicker, more uniform coating can be obtained by a single coating than can be achieved with wet paint.
- A beautiful and uniform quality finish without paint runs.
- A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes.
- The electrostatic powder process ensures complete coverage, even on complex shapes (e.g., on castings).
- Choice of finishes – clear, plain color, metallic or textured.
- Exceptional gloss and color retention.
3. Technical aspects:
- Corrosion resistance.
- Robust mechanical and chemical performance – good at resisting abrasion, acids, solvents, etc.
- Good electrical insulation capabilities.
- Applicable to steel, zinc, brass and aluminum.
4. Productivity and Costs:
- Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.
- Cost-efficient on small batches or single items, thus ideal for job coaters.
- Quick turnaround times.
- Ready to use: no stirring, mixing or thinning is required as it may be with liquid paints. Some application equipment enables the powder to be fed directly from the box
- Fewer rejects: The ease of use of powder, both manually and with automatic application plant, gives a lower reject rate compared with wet paint. Rejects caused by damage after coating are also reduced due to the toughness of powder coatings
5. Capital equipment and operating costs
- Capex for a powder line is generally lower than for conventional liquid lines.
- Space requirement: less space is required than for liquid coatings equipments.
- Ease of use: powder is far easier to apply than wet paint and less operator training is necessary
While powder coatings have many advantages over other coating processes, there are limitations to the technology. While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more and moreorange peeled in texture due to the particle size and Tg (glass transition temperature) of the powder.
For optimum material handling and ease of application, most powder coatings have a particle size in the range of 30 to 50 μm and a Tg > 40°C.
For such powder coatings, film build-ups of greater than 50 μm may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to show fingerprints.
There are very specialized operations where powder coatings of less than 30 µm or with a Tg < 40°C are used in order to produce smooth thin films.
Some of the more obvious advantages of using thermosetting powders are:
Ready for use – Powders are immediately ready for use. They do not have to be mixed with any other ingredients such as solvents or catalysts. This saves on shop floor time and also eliminates a variable which it is vital to control with liquid paints to obtain satisfactory film properties.
Reduction in the fire risk as no solvent required – As no solvent is required with powders, there is a reduction in fire risk which gives cost savings on statutory safety features in plants, reduces insurance premiums and savings can be made on manpower time.
No costly wastage of solvent – environmental cleanliness – There is no costly wastage of solvents. These solvents, which volatilise during application and stoving are usually not recoverable and legislation introduced in various parts of the world has prohibited their discharge to atmosphere and in some cases, therefore, after-burners have had to be installed to eliminate solvents with an attendant increase in costs.
No effluent disposal problems – With liquid paint systems, water wash spray booths are commonly employed. The overspray is usually emulsified in the water, which in some cases is put directly to drain or in other cases, allowed to settle out in sludge tanks. Powders do not give rise to these problems and extra costs.
No air pollution – Oversprayed powder is recoverable and no powder need escape into the atmosphere.
Reduced health hazard to operators – There is a reduction in health hazards to operators. As powders do not contain solvent there is a marked reduction in nose, mouth and throat irritation as is sometimes noticeable particularly during hot weather in liquid paint shops. Any liquid paint which comes into contact with an operator’s skin needs to be washed off with solvent, and then removed by emulsifying with soap in hot water. In difficult cases special industrial hand cleaners have to be employed. These remove essential oils from the skin and in some cases can cause skin irritation. In general powder does not cause skin irritation though in rare cases individuals may react to certain types of powder. The powder can be removed from the skin easily by washing with warm water.
Considerable improvements have also been made in reducing the heavy metal content of powders, in particular lead. Lead-free products are now achievable for the vast majority of shades.
Processing time reduced – Powder processing times are generally shorter than those used for wet stoving paints since, as there is no solvent, no flash off period is required; the powder coated articles can pass directly into the oven. This gives substantial saving in space and time.
Economy in reduced energy requirements – There is a cost saving during stoving because no energy is required to evaporate solvent and evacuate it from the oven.
Superior film properties – With solvent based systems, the solvent balance must be carefully adjusted to the polymeric type, application and curing conditions, as many film defects can be traced directly to incorrect solvent balance. With powders no such condition exists and in general superior film properties such as adhesion and corrosion resistance are obtained with powders compared to polymers deposited from liquid systems.
Damaged parts easily rectified – Damaged or poorly coated areas can be easily rectified before baking by blowing off the powder and re-coating.
Air requirements reduced: cost savings – Air extraction in a powder spray booth is very much less than for solvent based paints and this leads to economies in ventilation and consequential heating of work areas. As less warm air has to be replaced in workshops, less dust is attracted and there is less air-draught for the operators to contend with.
95% powder utilisation – Material loss can be kept to less than 5% by powder overspray recovery. The powder recovered can be blended with virgin powder to provide up to 95% utilisation.
Controlled film thickness – A controlled more uniform and, if required, a higher film thickness can be obtained with powders than with conventional paint systems in one application. Improvements in the appearance of thin films has also made this a viable option for certain applications.
Wide range of coatings available – A broad variety of decorative and functional coatings are achievable using powder. These range from matt to high gloss, smooth to textured finish and include metallic and other speciality effects. Choice of polymer type can optimise properties such as corrosion resistance and exterior durability.
Minimal operator training required for application – Operators require much less training to apply powders than solvent based systems.
Surface pretreatment – As with most coated metallic surfaces, preparation and pretreatment are important to maximise corrosion and environmental resistance.
Powder cleaner to use – Applying powder is much cleaner than applying wet paint. A spray booth can be cleaned down by use of a rubber squeegee whilst the normal air extraction in the booth is operating. Brushes or an air hose should not be used for this purpose. Any spillages of powder outside the booth should be removed with an industrial vacuum cleaner which is fitted with an air-driven or dust-tight motor.
Powder application plant less costly – Powder application plant, either manual or automatic, is extremely simple to operate and less costly.
Lower cost of packaging for coated articles – A much higher order of physical and chemical resistance is obtained with thermosetting powder polymers than with their liquid counterparts. This leads to a lowering of the cost of protecting the work during transportation to the user.
Non-metallic surfaces can be coated – Surfaces such as glass and certain thermosetting mouldings which can withstand the stoving temperatures involved can be powder coated. The scope of materials which can be used is ever-increasing as advancements in lower bake technologies are made.
Less storage space required for the powder – Less storage space is required for powder compared to paint which means cost saving on factory floor space.