Tài liệu PROTECTIVE COATINGS doc

3. CLASSIFICATION AND PROPERTIES OF PAINT 3.1 Some basic chemistry 3.2 Classification 3.2.1 Physically drying 3.2.1.1 Solvent-borne paints 3.2.1.2 Waterborne paints 3.2.2 Chemically curing 3.2.2.1 Oxidative curing 3.2.2.2 Two-component curing 3.2.2.3 Other curing mechanisms 3.3 Classification Diagram 3.4 Properties of Individual Binder Types 3.4.1 Physically drying 3.4.1.1 Solvent-borne paints • Tars and Bitumen • Chlorinated rubbers • Acrylics • Vinyls 3.4.1.2 Waterborne paints 3.4.2 Chemically curing 3.4.2.1 Oxidative curing • Alkyds • Modified Alkyds 3.4.2.2 Two-component curing • Epoxy • Polyurethane 3.1 PROTECTIVE COATINGS TRAINING 3.4.2.3 Other Film-forming Mechanisms • Humidity curing • Zinc Silicates (ethyl) • Polyurethane (one-pack) • Carbon Dioxide curing • Zinc Silicates (alkali) • Heat curing • Silicone 3.1 Some basic chemistry The properties of the binder in the paint mainly determine the film-forming properties of a paint (ref. 2.2.1). A binder is always built up of small units (monomers) to a larger unit, the polymer. If the polymer consists of more than one type of monomer, it is called a copolymer. If the polymer is built up of the same monomer, it is called a homopolymer. The number of monomers in a polymer can vary from 5 to as much as 3500, and in connection with paint the number is often quite high. In a dried and/or cured paint film the binder is either found as a bunch of long chains (the polymer chains) or as closely knitted networks (the polymer chains are connected by cross-linking). polymer chains polymer chains with cross-linking The size and shape of the binder polymer will influence the properties of the final paint film. Longer chain molecules and denser network structures will increase toughness and resistance in the resulting binder, while smaller molecules will result in the best penetration into the substrate. 3.2 PROTECTIVE COATINGS TRAINING 3.2 Classification, based on type of binder The manner in which paint films are formed is determining for many of the key properties of a paint film. This is the reason why paints are often classified according to their film formation principles. Film formation can take place in one of two ways: • physically drying • chemically curing 3.2.1 Physically drying In 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), leaving the binder molecules as chains coiled up and intertwined in the coating. Binders of importance to the paint industry are: 3.2.1.1 Solvent-borne paints The binders are dissolved in solvents, both the natural binders such as tar and bitumen and the synthetic types like chlorinated rubber, acrylic and vinyl. Film formation Solvents evaporate. Binder molecules are intertwined and attracted to each other and to the substrate. 3.3 PROTECTIVE COATINGS TRAINING 3.2.1.2 Waterborne paints (dispersions) Water-borne paints are dispersions of small binder particles in water. Very large binder molecules can be incorporated in the binder phase by utilising the dispersion technology. Used in a solvent-borne paint the same size of molecules would result in too high viscosity, or if diluted to the same viscosity, in a very low solid content. Film formation Water evaporates. The binder particles deform and coalesce (or melt together) assisted by coalescing agents (powerful solvent for the binder particles) - until they form a continuous film, attached to the surface. Properties common to physically drying paints: Ò Solvent borne • Reversible, ie the coating, even months or years after application, is soluble in its own or more powerful solvents. Solvent molecules penetrate in between the binder molecules, push them apart and dissolve the binder. Solvent sensitive; consequent to being reversible, these paints are not resistant to their own or stronger solvents. • Temperature independent for film formation, since no chemical reactions are involved in the film formation. • Thermoplastic, ie physically drying coatings become soft at elevated temperatures. Like other molecules, the polymers become more mobile at higher temperatures and, as they are not 3.4 PROTECTIVE COATINGS TRAINING cross-linked to each other, they have a certain freedom of move - ment, resulting in a softening of the coating. • Excellent recoatability. Ò Waterborne • Reversible to a certain degree, as the coalescing agent or solvents of the same or stronger dissolution power will be able to re-dissolve the paint film; however, just adding water will not result in a re-dispersion of the paint film. Re-dispersion is never possible. • Solvent sensitive; consequent to the above, solvents similar or stronger in dissolution power to the coalescing agent in question will be able to attack the coating. • Temperature dependent for film formation, the softening point where the binder particles are able to melt together, is normally 5°C or slightly higher, minimum application temperature is 10°C. • Thermoplastic, as for solvent-borne paints. • Excellent recoatability. 3.2.2 Chemically curing In chemically curing paints the final binder molecules in the dry/cured paint film are not present in the wet film. The smaller "wet" molecules will during and after application take part in a chemical reaction whereby new, larger binder molecules are formed. The film is formed by the cross-linking of the molecules (polymerization) and often by the evaporation of solvents also. Binders of importance to the paint industry are: 3.5 PROTECTIVE COATINGS TRAINING 3.2.2.1 Oxidative curing Binders which are most often dissolved in solvents and which form the final binder with oxygen in the air, eg oleoresins, alkyds, modified alkyds. Film formation 3.2.2.2 Two-component curing Binders which are often dissolved in solvents and consist of two components that react with each other after mixing, forming the final binder, eg epoxy, polyurethane, unsaturated polyester. Film formation 3.6 PROTECTIVE COATINGS TRAINING 3.2.2.3 Other curing mechanisms Binders can be formed by other chemical reactions or polymerisation processes, eg • Reaction between binder molecules and water present as humidity in the air like zinc ethyl silicate and one-pack polyurethane. • Reaction between binder molecules and carbon dioxide in the air like sodium, potassium and lithium zinc silicates (alkali zinc silicates). • Reaction triggered off by high temperature as in the case of silicone by curing for a few hours at 200°C. Properties common to chemically curing paints: • Irreversible, ie the cured coating is not dissolvable. • Resistant to solvents (consequence to being irreversible). • Temperature dependent on film formation as the rate of the chemical reaction taking place during curing is directly related to temperature, ie there is a temperature limit below which film formation will not take place. • Non-thermoplastic. The binder molecules in the tight cross-linking are not able to vibrate or move even at high temperature, ie the paint film does not become softer at higher temperatures. • Critical recoating interval. Recoating must take place before complete curing has taken place. If curing is completed, the surface must be mechanically roughened before application of new paint. 3.7 PROTECTIVE COATINGS TRAINING 3.3 Classification Diagram (most important binder types only) Polyurethane Zinc Ethyl Silicates Zinc Alkali Silicates Silicones Others Epoxies Polyurethane Polyesters Two-pack Oleoresinous Alkyds Modified Alkyds Oxidation Chemically curing Acrylics Vinyl acetate copolymer Vinyl chloride copolymer Styrene-butadiene copolymer Waterborne Chlorinated Rubbers Acrylics Vinyl Tars Bitumen Solvent borne Physically drying P A I N T P A I N T P A I N T or as presented in the family tree: 3.8 PROTECTIVE COATINGS TRAINING 3.4 Properties of Individual Binder Types The following is a survey of some of the more important binders used in coatings for the protection of marine and industrial structures. The properties listed are general and may, for the actual coating, vary somewhat according to formulation and possible modification. 3.4.1 Physically drying 3.4.1.1 Solvent-borne paints Tars and Bitumen Coal tars are obtained by distillation from coal, formerly from gasworks, but today, mainly from steel works. They consist of a mixture of liquid tar oils and solid coal tar pitches. Bitumen is more often than not the heavy residues from oil distillations though it may still be found in nature. As opposed to most other binders used for protective coatings, coal tars and bitumen are often used without pigmentation. Their high content of carbon make them virtually opaque and black. Tars and bitumen are low cost barrier coatings, often specified behind linings and as maintenance coating in ballast tanks. General advantages and limitations of tars and bitumen are: • Excellent water resistance • Poor weathering, will crack when exposed to sunlight (due to evaporation of low boiling oils) • Fair chemical resistance • Poor solvent resistance • Excellent penetration and adhesion • Low cost • Give rise to bleeding; ie small molecules (the low boiling tar oils) will migrate to the surface and cause discoloration of any non-black topcoat • Black or very dark colours only 3.9 PROTECTIVE COATINGS TRAINING Due to the excellent water resistance of coal tar, it is often used in combination with other binders, epoxy, vinyl and polyurethane. Chlorinated Rubbers As the name indicates chlorinated rubber resin is formed by adding chlorine to rubber. In the early days only isoprene from natural rubber was utilised but, nowadays, most of the isoprene is extracted in the oil industry. After application and the evaporation of its solvent, chlorinated rubber leaves a dense but very brittle film. A softening agent, a plasticizer, is therefore always incorporated in a chlorinated rubber-based paint. Chlorinated rubber-based paints are used extensively as anticorrosive coatings, coatings for concrete and other alkaline substrates. General advantages and limitations of chlorinated rubber coatings are: • Good water resistance • Relatively good weathering • May yellow and chalk (chlorine content causes yellowing) • Fair chemical resistance • Good alkali resistance • Poor resistance to animal and vegetable oils and fats • Contain plasticizers (and many properties may depend upon the type of plasticizer used) • Contain chlorine (hydrochloric acid is formed at elevated tempera- tures such as when welding, burning, cutting, etc.) Acrylics Acrylic resins are produced by polymerising different acrylic mono- mers. Through the use of different monomers the properties of the acrylic resin can be varied to a very large extent. Properties are also influenced by the often incorporated plasticizer. Used on exterior steel work above the waterline. General advantages and limitations of acrylic coatings are: 3.10 PROTECTIVE COATINGS TRAINING [...]... properties to a high content of silicon-oxide in their composition In the protective coatings field, silicate binders are exclusively pigmented with zinc as zinc silicates As such they offer the best corrosion protection obtainable with a paint Lithium, potassium and sodium silicates dry by the loss of water and cure by: 3.20 PROTECTIVE COATINGS TRAINING a) b) a reaction between zinc and silicate, and a... resistance (ie the ultraviolet radiation in sunlight); they are frequently recommended for exterior finishing coats 3.18 PROTECTIVE COATINGS TRAINING Like epoxies, polyurethane may be produced in a wide range of molecular sizes, some small enough to allow their use in solvent-free or solvent-less coatings Polyurethane is used as a finishing coat above the water line in epoxy systems, heavy duty coating, floor... of acrylic binders can be very different, but in general they have • • • • Good water resistance Good colour retention Good gloss retention Good adhesion to a variety of different substrates 3.13 PROTECTIVE COATINGS TRAINING Polyurethane The reaction product between isocyanate and the hydroxyl containing binder, ie the polyurethane (ref 3.4.2.2), is present as complete binder particles in the dispersion... areas not exposed to sunlight Special properties are: • Good water resistance • Gets brittle and yellow when exposed in UV-light (sunlight) and oxygen from the air • Comparatively low cost 3.14 PROTECTIVE COATINGS TRAINING 3.4.2 Chemically curing 3.4.2.1 Oxidative curing The key component of paints forming film by an oxidation process, is a drying oil Drying oils are natural products, eg linseed oil,... spirits, a comparatively weak solvent Good penetration and adhesion Modified Alkyds Alkyds may be modified with a wide range of other binders to provide new binders with specific properties 3.15 PROTECTIVE COATINGS TRAINING Typical modifications are: • Styrenated alkyd Copolymerising alkyd with styrene gives an alkyd which is faster drying, more chemical and water resistant, but less tolerant of poor... comparatively expensive Epoxy esters exhibit good adhesion and good anticorrosive properties; often used for primers required to withstand rough handling or forced drying at elevated temperatures 3.16 PROTECTIVE COATINGS TRAINING 3.4.2.2 Two-component curing Binders in this group form their films through a polymerising reaction between two components, usually referred to as base and curing agent or hardener,... weight epoxies, used for solvent-free and solvent-less epoxies Epoxy-based paints are specified for the exterior and interior of steel work, in tanks, as heavy duty and high-build coatings General advantages and limitations of epoxy coatings are: • Good to excellent water resistance • Good weathering (apart from chalking) • Tendency to chalking • Good physical properties, such as toughness, flexibility and... solvent-free versions) • Good heat resistance, up to 120°C in continuous dry service Pre-reacted amines/poly amides with minor parts of epoxy still leaving reactive groups in the hardener 1 3.17 PROTECTIVE COATINGS TRAINING • Good to excellent chemical resistance (best for amine cured epoxies) • Good adhesion to a wide range of materials • Two-pack (attention to mixing and pot life) • Curing is temperature...3.11 PROTECTIVE COATINGS TRAINING • • • • • • • Relatively good water resistance Good weathering Good colour retention Low to fair resistance to animal and vegetable oils and fats Usually only fair wetting properties... resistance • Difficult application (low relative humidity and trained painters required) • Low temperature curing • Two components (attention to mixing and pot life) • Comparatively expensive 3.19 PROTECTIVE COATINGS TRAINING 3.4.2.3 Other Film-forming Mechanisms Humidity curing Humidity curing binders polymerise and form films through a reaction with water, usually in the form of vapour contained in . tree: 3.8 PROTECTIVE COATINGS TRAINING 3.4 Properties of Individual Binder Types The following is a survey of some of the more important binders used in coatings. as anticorrosive coatings, coatings for concrete and other alkaline substrates. General advantages and limitations of chlorinated rubber coatings are: •