Powder coating is a process of electroplating materials using powder paints instead of liquid paints. The powder which is electro-statically charged sticks to the surface layer of a grounded material. Powder coating is quickly gaining popularity due to its advantages over the traditional liquid paints. These advantages include:
- The process is less harmful to the environment
- Its cost effective in terms of lower energy requirements per square meter of the surface sprayed
- It guarantees uniform and aesthetically appealing finishes compared to liquid paints
There are two main types of powder coats
- Thermoplastic: – when heat is applied the coating powders melt and flow retaining their chemical composition even when the cool and solidify. They are mostly applied on surfaces pre-heated to temperatures relatively higher than powders melting point so they can melt and form a continuous strong layer.
- Thermoset: – when exposed to heat these coating powders melt altering their composition by mixing with other reactive components consequently the cured coating attains a different chemical structure. Thermosetting coatings do not melt back to liquid form when re-heated
The process of powder coating
There are 6 major steps involved in powder coating metallic materials described as follows:
Step 1: Cleaning
Cleaning is done to remove all inorganic and organic substances contaminating the surface of material. Oil and grease are the first contaminants to be removed by use of alkali cleaners, detergents or solvent emulsions.
Steel which is widely used in manufacturing and construction is often covered by an oil film after fabrication to prevent corrosion before use has to be cleaned using appropriate cleaning solutions. However cleaning galvanized steel is more intricate compared to steel since it has a protective zinc layer bonded on the steel surface.
Since aluminum is reactive to acidic and alkali solution the cleaning solutions have to be carefully picked and process heat controlled.
Step 2: Rinsing
High quality water rinsing is required whenever aqueous solutions are used in cleaning the materials. This is to wash off and neutralize the acidic and alkali solutions left on the surface. The quality and quantity of fresh water used in the cleaning has to be controlled to ensure optimum washing.
Step 3: conversion coating
Once the metal surface has been cleaned of the impurities it has to be condition for optimal adhesion with the powder coating. Acidic phosphate solution is often used thereby improving the corrosion resistance.
Step 4: Drying
Before powder coating, the material has to be dried and cooled. There are two method used for drying namely:
- i) dry- off method
- ii) blow-off method
Step 5: powder coat application
The most common used method of application is electrostatic spraying. The powder particles are electro-statically charged on emission from the spray gun to the grounded work piece. The electrostatic attraction causes the particles to be deposited and bonded on to the surface forming a strong continuous layer.
Step 6: Curing
The curing process is initiated immediately after the powder coat application by applying high heat for some time to polymerize the coating powders.