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The manner in which paint films are formed determines many of the key properties of a paint film. Therefore, paints are often classified according to their film-formation characteristics. In general terms, paints can be divided into two categories:
Physically drying paints and chemically curing paints.
In the physically drying paints, the binder molecules in the dry paint film are already present in the wet paint. There is no change in binder composition or molecule structure and size. The paint film is formed entirely by evaporation of solvents (a physical process), thus leaving the binder molecules as chains coiled up and intertwined in the coating.
In chemically curing paints, the final binder molecules in the dried/cured paint film are not present in the wet film. The smaller "wet" molecules react chemically during and after application thus creating new, larger binder molecules. Thus, the film is formed by crosslinking of the molecules (polymerisation), and often by the evaporation of its solvents as well.
Tar and Bitumen
Both are natural products. Tar comes from the distillation of coal (coal tar), bitumen is most often distilled residues from crude oil. The texture varies from liquid to solid matter.
Paints based on these binders have the following characteristics:
- good water resistance
- good penetration (that is, they adhere well even on poorly cleaned surfaces)
- poor resistance to sunlight exposure (causes cracking of the surface because of evaporation of low-molecular oils)
- black or brownish colour only (contain tiny, black carbon particles)
- low cost
Chlorinated Rubber
Chlorinated rubber is manufactured by adding approximately 65% chlorine to isoprene polymers, which come from the oil industry. A paint based solely on chlorinated rubber will form a hard and brittle film when it is dry. Consequently, a plasticizing agent is always added to the binder phase.
Paints based on chlorinated rubber have the following characteristics:
- good water resistance
- good chemical resistance
- yellowing (due to the chlorine content)
- chalking, i.e. a thin top layer of the surface disintegrates on exposure to sunlight, thereby reducing the gloss.
Acrylics
Acrylic paints consist of a copolymer made up of various acrylic monomers, which together define properties of the individual binder. Acrylics often contain a plasticizer.
Acrylic paints have the following characteristics:
- good water resistance
- good colour retention
- good gloss retention
- relatively poor wetting.
Vinyl
Vinyl binders for paint are often made from a copolymer of vinyl chloride and vinyl acetate, although other monomers may be present. All vinyl copolymers require a plasticizer in order to be used in paint.
Vinyl paints have the following characteristics:
- good chemical resistance, including resistance to weak solvents
- may yellow (because of the chlorine content)
- low solids content
- relatively poor wetting.
Solvent-borne vinyl-based paints are being used less and less, mainly because of the low content of solids and the high price.
Alkyds
When slowdrying oils (e.g. linseed oils) are combined with hydroxyl-compounds and acids, they are converted to faster-drying alkyds. Alkyds can be divided into three main groups: short-, medium-, and long-oil alkyds, depending on the content of oil. They cure by reacting with oxygen from the air.
Alkyd-based paints are known for:
- reasonably good colour and gloss retention
- good penetration
- relatively low cost
- non-resistance to alkali (and are therefore never specified for use on concrete or galvanised steel)
- non-resistance to long-term water exposure (and therefore never specified for use below the waterline).
Epoxy
Epoxy resin reacts with a curing agent (e.g. polyamine or polyamide or compounds) to form a dense three-dimensional network. The curing process can take place at temperatures down to -10ºC.
Epoxy-based paints have the following characteristics:
- chemical resistance
- good mechanical resistance
- chalking, i.e. a thin top layer of the surface degrades on exposure to sunlight, thereby reducing the gloss.
- are delivered as two-pack products with limitations on time after mixing of components (pot-life restrictions)
Polyurethane
Polyurethanes are polymers formed by reaction with isocyanates and polyalcohols. The main reaction occurs on-site when the two components are mixed just prior to use. The curing process can take place at temperatures down to -10ºC.
Polyurethane paints have the following characteristic properties:
- good gloss retention
- excellent colour retention
- good chemical and mechanical resistance
- moisture sensitive during production and application
- are delivered as two pack products with limitations on time after mixing of components (pot-life restrictions).
Zinc silicate
Zinc (ethyl) silicates are two-pack products in which metallic zinc dust is added to a solvent-borne silicate solution prior to use. They cure by absorption of water from the ambient air.
Zinc silicates have the following properties:
- excellent anti-corrosive properties
- excellent wearability and impact resistance
- excellent solvent resistance
- good chemical resistance within pH 6 - 9
- withstand temperatures up to 400°C.
Silicone
Silicone paints are silica-based resins which are liquid at room temperature but cure at elevated temperatures. At 200°C they react to form a dense network through crosslinking between the individual molecules.
Silicone paints have the following properties:
- temperature resistant up to 550 °C (pigmented with aluminium)
- relatively high cost.
Properties
of Paint
The manner in which paint films are formed determines many of the key properties of a paint film. Therefore, paints are often classified according to their film-formation characteristics. In general terms, paints can be divided into two categories:
Physically drying paints and chemically curing paints.
In the physically drying paints, the binder molecules in the dry paint film are already present in the wet paint. There is no change in binder composition or molecule structure and size. The paint film is formed entirely by evaporation of solvents (a physical process), thus leaving the binder molecules as chains coiled up and intertwined in the coating.
In chemically curing paints, the final binder molecules in the dried/cured paint film are not present in the wet film. The smaller "wet" molecules react chemically during and after application thus creating new, larger binder molecules. Thus, the film is formed by crosslinking of the molecules (polymerisation), and often by the evaporation of its solvents as well.
Tar and Bitumen
Both are natural products. Tar comes from the distillation of coal (coal tar), bitumen is most often distilled residues from crude oil. The texture varies from liquid to solid matter.
Paints based on these binders have the following characteristics:
- good water resistance
- good penetration (that is, they adhere well even on poorly cleaned surfaces)
- poor resistance to sunlight exposure (causes cracking of the surface because of evaporation of low-molecular oils)
- black or brownish colour only (contain tiny, black carbon particles)
- low cost
Chlorinated Rubber
Chlorinated rubber is manufactured by adding approximately 65% chlorine to isoprene polymers, which come from the oil industry. A paint based solely on chlorinated rubber will form a hard and brittle film when it is dry. Consequently, a plasticizing agent is always added to the binder phase.
Paints based on chlorinated rubber have the following characteristics:
- good water resistance
- good chemical resistance
- yellowing (due to the chlorine content)
- chalking, i.e. a thin top layer of the surface disintegrates on exposure to sunlight, thereby reducing the gloss.
Acrylics
Acrylic paints consist of a copolymer made up of various acrylic monomers, which together define properties of the individual binder. Acrylics often contain a plasticizer.
Acrylic paints have the following characteristics:
- good water resistance
- good colour retention
- good gloss retention
- relatively poor wetting.
Vinyl
Vinyl binders for paint are often made from a copolymer of vinyl chloride and vinyl acetate, although other monomers may be present. All vinyl copolymers require a plasticizer in order to be used in paint.
Vinyl paints have the following characteristics:
- good chemical resistance, including resistance to weak solvents
- may yellow (because of the chlorine content)
- low solids content
- relatively poor wetting.
Solvent-borne vinyl-based paints are being used less and less, mainly because of the low content of solids and the high price.
Alkyds
When slowdrying oils (e.g. linseed oils) are combined with hydroxyl-compounds and acids, they are converted to faster-drying alkyds. Alkyds can be divided into three main groups: short-, medium-, and long-oil alkyds, depending on the content of oil. They cure by reacting with oxygen from the air.
Alkyd-based paints are known for:
- reasonably good colour and gloss retention
- good penetration
- relatively low cost
- non-resistance to alkali (and are therefore never specified for use on concrete or galvanised steel)
- non-resistance to long-term water exposure (and therefore never specified for use below the waterline).
Epoxy
Epoxy resin reacts with a curing agent (e.g. polyamine or polyamide or compounds) to form a dense three-dimensional network. The curing process can take place at temperatures down to -10ºC.
Epoxy-based paints have the following characteristics:
- chemical resistance
- good mechanical resistance
- chalking, i.e. a thin top layer of the surface degrades on exposure to sunlight, thereby reducing the gloss.
- are delivered as two-pack products with limitations on time after mixing of components (pot-life restrictions)
Polyurethane
Polyurethanes are polymers formed by reaction with isocyanates and polyalcohols. The main reaction occurs on-site when the two components are mixed just prior to use. The curing process can take place at temperatures down to -10ºC.
Polyurethane paints have the following characteristic properties:
- good gloss retention
- excellent colour retention
- good chemical and mechanical resistance
- moisture sensitive during production and application
- are delivered as two pack products with limitations on time after mixing of components (pot-life restrictions).
Zinc silicate
Zinc (ethyl) silicates are two-pack products in which metallic zinc dust is added to a solvent-borne silicate solution prior to use. They cure by absorption of water from the ambient air.
Zinc silicates have the following properties:
- excellent anti-corrosive properties
- excellent wearability and impact resistance
- excellent solvent resistance
- good chemical resistance within pH 6 - 9
- withstand temperatures up to 400°C.
Silicone
Silicone paints are silica-based resins which are liquid at room temperature but cure at elevated temperatures. At 200°C they react to form a dense network through crosslinking between the individual molecules.
Silicone paints have the following properties:
- temperature resistant up to 550 °C (pigmented with aluminium)
- relatively high cost.
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