Paints Flashcards
Why might paint be used
-Prevent deterioration of materials
-Decorative
Easily maintainable surface
-Provide light/heat reflecting properties
-Prevent metallic corrosion
-Electrically conductive paints
-Intumescent paints of fire protection
5 Constituents of paint
- Vehicle
- Binder
- Solvent/Thinner
- Pigments
- Additives
Role of the binder
The part that holds together the pigments and other chemicals. Converts from a liquid coating to a hard elastic skin that provides resistance
Role of the solvent/thinner
The solvent dissolves the resinous constituents, whereas the thinner (a blend of different solvents) is used to dilute the already dissolved resin
Name 4 additives and their role
- Driers, added to oil based paints to give out oxygen and therefore increase the speed of drying
- Plasticisers, make the hardened film more flexible
- Light Stabilisers, make colour more stable under sunlight
- Insecticides/Fungicides, prevent attack by insects and the formation of mould
What 2 factors affect film formation and why
- Temperature, less than 4C reaction is slow and can impair film formation, above 35C water base paints may lose their water before solids have had a chance to coalesce
- Humidity, excessive humidity prevents evaporation of water, too low and it may prevent a chemical reaction that is dependent on water vapour
Components of typical paint systems
- Primer
- Under coat
- Top Coat`
What is the role of the primer
- Adheres to the substrate
- Can protect metal surfaces from corrosion
- Dries to matt finish to provide key for next layer of paint
What is the role of the undercoat
- Provides a flat even surface
- generally dries to a matt finish to promote bond with final coat
What is the role of the top coat
-Provides colour, texture and protection
What are the different ways paint can achieve an impervious barrier
- By adhesion of the binder to the substrate which stabilises the substrate
- Formation of a barrier between the substrate and the deleterious influences (light, water gases….)
- By chemical action, some pigments react with aggressive agencies that might attack the substrate
For paint protection to be effective what must the substrate be
- Firm
- Clean
- Doesn’t react chemically
- Such as excessive alkalinity of new cement
- Rapid oxidation of aluminium
2 types of opacity and examples
- Clear Coatings: varnishes and Stains
2. Pigmented: Oil based and thermoplastic
2 different types of bases for paints
- Solvent Based
2. Water Based
How should new timber be prepared for painting
- Preservative treatment
- Knotting
- Primer
- Stopping and Filling
- One or two undercoats
- One of two alkyd gloss or semi gloss top coats
How should old timber be prepared for painting
- Selotape will pull away any loose flakes
- Substandard timber must be replaced and sanded down so no edge are left proud
How should a cement based substrate be painted
- Alkali resisting primer
- Finish with alkyd, emulsion, masonry or cement paint
How should steel be prepared and painted
- Initial protection must be carried out less than 4 hours after cleaning or the paint is adhering to an oxide coating and not steel
- remove mill-scale by blast cleaning or pickling in acid
- Surface needs to be above the ambient dew point to avoid condensation
What is a sacrificial paint system
The paint corrodes in preference to the steel it is protecting
-Principles of electrolytic corrosion apply
What is an intumescent paint
- Used to provide fire resistance
- On heating components in the paint give off non flammable gases (CO2 and ammonia)
- Coating expands up to 100 fold, forming a rigid foam
What is the importance of the time left between coats
- If the minimum time between coats isn’t adhered to solvents in the upper coat may interfere with the drying of the lower coat
- If the maximum time between coats isn’t adhered to he first coat has dried too hard and the subsequent coat will dry before the solvents have had a chance to penetrate the now hard surface
Potential defects with coatings
-Adhesion failure
-Blistering
-Chalking
-Colour defects e.g. fading
-Cracking
Detachment of mill scale from steel
-Rust staining
-Organic growths
-Flaking
