What is Painting or Surface Coating?
Paint is a surface coating that decorates and protects the surface on which it is applied. It can be defined as a homogeneous mixture of pigments, binders, solvents, additives. The success or failure of any coating is influenced by the following factor:
- Substrate Condition
- Surface condition and method of application
- Environmental condition at which it is applied and expected to withstand during service
- And last but not least the quality of paint used
- Decorates the surface on which it is applied.
- Protects the surface from rust
- Protects from micro-organism like fungi and algae maintaining its original body.
RUST NEVER SLEEPS!!!
Corrosion of Steel (Fig. 1)
- Environmental Corrosion: Unstable condition of steel
- Chemical Corrosion : Chemicals like acids, alkalies attack metal
- Corrosion Prevention: Isolation of metals from corrosive environment by using paint coatings
- Using Inhibitive Primers Zinc phosphates/Chromate forms a passive layer with adhered rust.
- Using Sacrificial Primer. Indirect catholic Protection by Zinc in Zinc-rich primers
- Barrier Coatings. High DFT Coating System Isolates Surface from corrosive Environment
Composition of Paint
Basic constituents of paints are
- Pigments 5 to 25%
- Binders 60 to 65%
- Solvents 15 to 25%
- Additives 1 to 5 %
Relative proposition of these ingredients can be varied to produce films with any desired physical and application characteristics
A finely divided powder which can disperse in media of various types to produce paints. It is insoluble in the medium. Important properties are
- Tinting Strength
- Fastness to light
- Resistance to heat
- Oil absorption of pigment
- Particle size: Hiding Power, Gloss or smoothness, Rate of settling of pigment
Binders are the heart of the paint system. Binders bind or cement the pigment particle into a coherence film which adheres to the substrate. Mechanical and resistive properties of the film are controlled very largely by binder.
The durability of the paint depends on the quality and quantity of binder used!!!!
- Convert the liquid coating on application to a solid dry film.
- Provide gloss to film
- Making coating adhere to the substrate
- Given elasticity to film
- Resistance to water, chemical, and abrasion
- Disperse the pigments and extenders
- Hold the pigment in suspension.
The Choice of Binder for Paint depends on the end use of the paint
Type of Binders
- Drying oils: Vegetable oils on exposure of air, convert from liquid to solid thru a process of oxidation. Can be a sole film former but most often mixed with resin
- Resins: Most surface coating contain a synthetic resin based film former. Most decorative paints are based on oil modified resins.
Few Important Binders/Resins
- Largest groups of synthetic resins.
- They are oil modified polyester
- Good exterior durability
- Low alkali and water resistance.
AMINO Resins: Melamine and urea-formaldehyde
- Has excellent adhesion, hardness, chemical, and corrosion resistance
- Can be used to do high build paint
- Poor exterior durability
Poly-amide Resins: Used as curing agents for epoxy resins.
- Good resistance to high temp, chemical and acid resistance, good resistance to various gases, alkali resistance.
- Low resistance to solvents like ketones, esters
- It is one pack thermoplastic.
- Have good Chemical resistance, good acid and alkali resistance.
- Can be applied as high build paint.
- Disadvantage: Poor resistance to high temp and solvent like ketones, aromatic HC
- Vinyl Resins:
- Cellulose Resins: Widely used in auto finishing
- Acrylic Resins: Possess good light fastness, good adhesion and excellent durability.
The primary function of the solvent is to dissolve film formers, thereby consistency suitable for the application. Choice solvent influences viscosity, drying and flow, and leveling.
Solvents are lost at atmosphere, so it is an economic loss.
Solvents, in isolation or combination, are used in making thinner for the paints. Examples of solvent:
Hydrocarbon Solvent: Aliphatic, aromatic, solvent Naptha, alcohols, ketones, esters, etc.
Used in a small amount to give a coating one or more desirable properties. The only difference between additive and other raw materials is that the amount of additive is very small. Properties that can be controlled thru additives are:
Viscosity Setting Drying
Gloss Opacity Bacterial action
Thickness Deodorants etc
Classification of Paints
Paints can be classified based on:
- The Physical state: Liquid Paint and Stiff Paint
- The Thinner Used: Water thinnable and solvent thinnable
- The End used: Decorative and protective.
- Modes of film formation: Thermosetting and Thermoplasts
- The order of application: Undercoat and topcoat
- The extent of gloss: Glossy, semi-glossy, egg-shell. matt
- Modes of Film Formation: Film formation is either by thermosetting or thermoplasts.
Thermoplastic (Non -convertibles):
In these coatings when the paint is applied on a surface, the solvent evaporates living resin to its original form spread over the surface. So change is only physical and can be reversed to its original form by using thinner. E.g Chlorinated Rubber
Chemical changes occur in the coating and dry film is different from its liquid state. Ex. Epoxy, alkyds, etc.
Surface preparation is the most important part of a coating system. The surface preparation of the coating system is what a foundation is to a building.
Surface Preparation of Steel
Some of various methods of steel are
- Hand tool cleaning
- Power tool cleaning
- Flame Cleaning
- Abrasive Blast Cleaning
- Wet Abrasive Blast Cleaning
International Standard of Blast Cleaning (Fig. 2)
Proper application of the protective coating is an important criterion in giving the paint system. Four main application procedures:
- Brush Application
- Roller Application
- Conventional Spray
- Airless Spray