Amtec Guide To Coating Types and Their Uses
This chapter covers the major types of coatings that are currently available for use on vessels, general information on the composition of coatings and a glossary of common coating terms. It is intended to give basic information on coatings and is not a comprehensive guide to paint selection. The coating manufacturer is to be consulted if information on a specific product or coatings for particular areas on the vessel are required.
Coatings for ships are often divided into two broad categories; products for application at New Building and products for Maintenance and Repair situations, which would include both major refurbishment either at sea or in port and On Board Maintenance (OBM). The types of paint used for OBM are often single pack products as this avoids the difficulties of measuring and mixing small quantities of two pack products, although small packs are available from paint manufacturers.
In general, paints are either targeted for specific vessel areas and for specific functions for best performance, or universal coatings are available for all areas, with a compromise in performance. In both cases, a balance between cost, performance and difficulty of maintenance has to be achieved. For example, anti-corrosive coatings used on the outside of the accommodation area have different performance requirements from paints used in sea water ballast tanks as the corrosion stress placed on the latter is far greater. Ballast tanks are also much more difficult to maintain due to access difficulties and therefore the use of a highly effective coating is required to keep the steel in good condition.
In contrast, the holds of bulk carriers suffer from abrasion damage due to cargo impact and grab damage, which often leads to corrosion. Cargo holds used as ballast tanks during heavy weather can be particularly susceptible to corrosion at damage sites and a different coating is sometimes used for this cargo hold. Similarly, cargo tanks for oil carriers with a Notation of Clean Products, where any cargo tank may be used for heavy weather ballast.
Major types of paints.
Paint can be described as a liquid material capable of being applied or spread over a solid surface on which it subsequently dries or hardens to form a continuous, adherent, film.
Paints basically consist of three major components and many additives which are included in minor quantities. The major components are:
• Binder (also called: vehicle, medium, resin, film, polymer)
• Pigment and extender
Of these, only the first two form the final dry paint film. Solvent is necessary only to aid paint application and the initial film formation, but inevitably, some solvent is always retained in practice depending upon the level of ventilation.
Binders are the film forming components of paint which determine the principle characteristics of the coating, both physical and chemical. Paints are generally named after their binder component (e.g. epoxy paints, chlorinated rubber paints, alkyd paints, etc.). The binder forms a permanent continuous film which is responsible for adhesion to the surface and which will contribute to the overall resistance of the coating to the environment.
Binders used in the manufacture of paints fall into two classes, Thermoset and Thermoplastic. A Thermoset coating when dry will be chemically quite different from the paint in the can. Thermoset coatings are not affected by solvents, once cured. With a Thermoplastic coating, the dry film and the wet paint differ only in solvent content and chemically, these remain essentially the same. If the solvent originally used is applied to a thermoplastic coating, it will soften can be re-dissolved in that solvent.
Cross Linked (Thermoset) Coatings
These paints are usually supplied in two separate packs which are mixed together immediately before application. In liquid paints where solvent is involved, drying is considered a two stage process. Both stages actually occur together but at different rates.
Stage One: Solvent is lost from the film by evaporation and the film becomes dry to touch.
Stage Two: The film progressively becomes more chemically complex by one of the following four methods:
1) Reaction with atmospheric oxygen, known as oxidation.
2) Reaction with an added chemical curing agent.
3) Reaction with water (moisture in the atmosphere).
4) Artificial heating.
This transformation in the paint is known as drying or curing. The films formed by the above methods are chemically different from the original binders and are not affected by their own solvents.
These resins are particularly important and their development for use as binders was one of the most significant advances in paint technology.
The rate of cross-linking or curing is dependent on temperature. Below 5oC the curing rate of standard epoxies is considerably reduced and to obtain optimal film properties, full cure is essential. Epoxies with special curing agents will cure or set at temperatures down to –50C. It is essential that the coating manufacturers latest recommendations on application temperatures are strictly followed, to ensure that the coatings are effective in service.
The choice of curing agent is very important as with the base, this determines the properties of the film. There is a wide choice of both resins and curing agents which allows for formulation of products to suit most applications.
Epoxies are used in both under water and above water situations and show good resistance to many marine environments such as cathodic protection, but they have a tendency to chalk in sunlight. This process occurs when the binder is degraded by ultra violet light to produce a loose and friable surface, with the pigment particles remaining on the surface.
These are polymers formed by reaction between hydroxy compounds and compounds containing isocyanates.
In two-pack systems a special polyether or polyester resin with free hydroxyl groups is reacted with a high molecular weight isocyanate curing agent. A possible problem with these materials is their water sensitivity on storage and on application. Transport and storage should be in strict compliance with the manufacturers recommendations.
Polyurethane resins have excellent chemical and solvent resistance and are superior to standard epoxies in acid resistance. Epoxies are generally more resistant to alkaline conditions than polyurethanes. Polyurethane finish coats are very hard and have extremely good gloss, gloss retention, and can be formulated to be non-yellowing. However in some instances, they can be difficult to overcoat after ageing and require very clean surfaces for optimum adhesion. As with all spray applications, there is a potential health hazard when sprayed, but which can be overcome with the appropriate protective equipment.
They are formed by the reaction between a particular organic acid (e.g. Phthalic acid), a particular alcohol (e.g. Glycerol or Pentaerythritol) and a vegetable oil or its fatty acids. The final properties of the alkyd depend on the percentage of oil (termed ‘oil length’) and also on the alcohol and organic acid used
Alkyds are not resistant to acids or alkalis and many of the modifications are aimed at improving this weakness, however, none provide complete resistance. Alkyd resins can be further modified with different resins for specific purposes.
These types comprise the silicates which are almost always used in conjunction with zinc dust. There are water based inorganic silicates based on lithium, potassium, or sodium silicate and solvent based inorganic silicates normally based on ethyl silicate. Coatings based on these resins are very hard, corrosion resistant and temperature resistant. They require a good standard of surface preparation and are often repaired using organic coatings, which are more tolerant.
The zinc in the inorganic resins can dissolve under acid or alkali conditions, but the coatings perform well under neutral pH and are often used as tank coatings.
These types of paint binders are simple solutions of various resins or polymers dissolved in suitable solvent(s) and are usually supplied as one pack products, making them especially suitable for maintenance work. Drying is simply effected by the loss of the solvent by evaporation. This is termed physical drying as no chemical change takes place. The resulting film is therefore always readily soluble in the original solvent and can also be softened by heat. Since these coatings, by definition, require the presence of significant amounts of solvent, they are disappearing from markets where volatile organic content is regulated, particularly the USA and the EU. Generic types of binders in this category include:
• Chlorinated Rubber Resins
• Vinyl Resins
• Bituminous Binders
Chlorinated Rubber Resins.
Chlorinated rubber resins to have good acid and water resistance on well prepared surfaces. Their temperature sensitivity can lead to various film defects when used in very hot climates. In addition, white and pale colours have a pronounced tendency to yellow when exposed to bright sunlight. Chlorinated rubber paints will dry at low temperatures and give good intercoat adhesion in both freshly applied and aged systems, making them suitable for maintenance purposes.
These are based on film forming polymers consisting of varying ratios of, for example, polyvinyl chloride, polyvinyl acetate, and polyvinyl alcohol. Plasticiser types used are tricresyl phosphate or dioctyl phthalate.
Higher volume solid materials can be produced by blending the vinyl resin with other materials such as acrylic resins. Generally the film properties and weathering characteristics also show good low temperature drying and intercoat adhesion characteristics. In general, vinyl resins have a high VOC content, which restricts their use in some parts of the world.
Pigments and Extenders
Pigments and extenders are used in paints in the form of fine powders. These are dispersed into the binder to particle sizes of <20 microns for finishing paints and <70 microns for primers.
These materials can be divided into the following types:
Anticorrosive pigments To prevent corrosion of metals by chemical and electrochemical means, in above water areas.
Barrier pigments To increase impermeability of the paint film.
Colouring pigments To give permanent colour.
Extending pigments To help give film properties required.
Metallic zinc is widely used in primers giving resistance to corrosion of steel. Initial protection is by galvanic action. However, as the coating is exposed to the atmosphere, a progressive build up of zinc corrosion products occurs, producing an impermeable barrier with little or no galvanic protection. To give good galvanic and barrier protection, high levels of zinc are required, about 85% of zinc in the dry film by weight is typical.
Resins which may be considered are epoxies and silicates. Obviously, for the zinc to function correctly, it has to be in intimate contact with the steel substrate and therefore good surface cleanliness prior to application is essential.
Metallic aluminium flake is commonly used as an anti-corrosive pigment and is thought to act as an anti-corrosive by producing a tortuous pathway for water and ions around the lamellar flakes, as well as absorbing oxygen to give aluminium oxides, which block pores in the coating. Where the aluminium is in contact with steel, a limited cathodic protection mechanism will also operate, although when used on tankers and product carriers, the aluminium content in the dry film must not exceed 10% to avoid possible spark hazards where inflammatory gases build up.
This is also a widely used anticorrosive pigment but it is believed that under normal exposure condition protection is also afforded by a barrier effect, since high pigmentation levels are needed to give adequate anti-corrosion protection. Zinc phosphate can be incorporated into almost any binder and because of its low opacity or transparent nature, paints of any colour can be produced.
The most common types of these pigments are aluminium (leafing aluminium) and micaceous iron oxide (M.I.O.). Both have particle shapes which are termed lamellar (plate-like). These materials can be used in combination, the aluminium lightening the almost black shade of M.I.O.
M.I.O. pigmented films have durability, but to achieve this, high levels of M.I.O. are necessary in the order of 80% of the total pigment. Aluminium has been used for many years as the principle pigment in paints for use underwater, the lamellar shape helping to make the film more water impermeable. Glass flake is also used as a barrier pigment.
These pigments provide both colour and opacity and can be divided into either inorganic or organic types. The most common colouring pigment is titanium dioxide, which is white. In paint, all pigments are normally dispersed to a very fine particle size in order to give maximum colour and opacity (hiding power). Traditionally, bright colours were obtained using lead and chrome pigments. However due to health and safety concerns, these are less common in some parts of the world. Now organic pigments are used instead but the opacity is not as high with these products.
As the name suggests, they basically adjust or “extend” the pigmentation of the paint until the required pigment volume concentration (PVC) is achieved. They are all inorganic powders with various particle shapes and sizes. Although making little or no contribution to the colour opacity of the paint, they can have significant influence on physical properties. These include flow, degree of gloss, anti-settling properties, sprayability, water and chemical resistance, mechanical strength and hardness, firm build (volume solids, hold up thixotropy). Mixtures of extenders are often used to obtain the desired properties. They are relatively inexpensive when compared to resins, anticorrosive pigments and colouring pigments.
Solvents are used in paints principally to facilitate application. Their function is to dissolve the binder and reduce the viscosity of the paint to a level which is suitable for the various methods of application, i.e. brush, roller, conventional spray, airless spray, etc. After application, the solvent evaporates and plays no further part in the final paint film. Liquids used as solvents in paints can be described in one of three ways:
A liquid which will dissolve the binder and is completely compatible with it.
A liquid which is not a true solvent. However, when mixed with a true solvent, the mix has stronger dissolving properties than the true solvent alone.
A liquid which is not a true solvent. Normally used as a blend with true solvent/latent solvent mixes to reduce the cost. Binders will only tolerate a limited quantity of diluent.
There are numerous solvents used in the paint industry and this is partly due to the number of different properties which have to be considered when selecting a solvent or solvent mixture. In addition to commercial factors such as price and availability, these include toxicity, volatility, flammability, odour, compatibility, and suitability.1
1 In certain countries certain types of solvents are not allowed. This is especially true in the USA, where the Hazardous Air Pollutant Substances Act, (HAPS) dictates a time line for removing many solvents from coatings. Application properties, dry times, and overcoat windows will most likely be affected as this act is implemented.
With few exceptions (such as anti-fouling paints, cosmetic effects, fire retardants, etc) the majority of the coatings applied to a vessel are used for anti-corrosion protection. There are many types of anti-corrosion coatings, but epoxy paints generally cover the greatest area on a vessel, particularly when they are used in sea water ballast tanks. In recent years there has been debate about the terminology used for epoxy coatings and the following are in common usage.
Pure epoxy coatings are generally regarded as paints which contain only epoxy polymers, the cross linking agent, pigments, extenders and solvents. The coatings contain high levels of epoxy binder and are therefore expected to provide the maximum possible performance from a coating, in terms of anti-corrosion protection, long life and low maintenance. In addition, some products also claim abrasion resistance properties.
Other pigments such as aluminium can be added to pure epoxy coatings to provide additional anti-corrosion performance.
Also known as epoxy mastic, tar free epoxy and bleached tar epoxies, this group covers a wide range of products and anti-corrosion performance capabilities. In service, modified epoxies can be effective, however, as there are many possible modified epoxy formulations, it is not possible to generalise on their anti-corrosion performance.
Modified epoxies can contain non epoxy materials which are capable of cross linking into the final film. They may also contain non reactive materials either solid or liquid, foe example hydrocarbon resins, which do not take part in film formation, but remain like pigments or extenders in the final coating. However, if these materials are water (or cargo) soluble, they can leach out over an extended time period leaving a porous or brittle film with reduced anti-corrosive properties.
Modified epoxies in ballast tanks tend to be thickness sensitive and cracking can result if the dry film thickness is excessive.
Coal Tar Epoxy.
Coal tar is a naturally occurring product. They are available in a wide range of types from liquid to solid. The inclusion of coal tars in a coating results in a very dark brown or black colour to the coating, which can be slightly lightened by the addition of aluminium flake pigment for lighter coloured paints, which are preferred in SOLAS Ch. II part A-1, reg3-1 (preference).
Certain constituents of the coating can leach out over long time periods, leaving a more brittle and less protective coating. Coal tar epoxies have a long track record in service and generally have performed well. Since the 1990’s they are being phased out of ballast tanks due to Health & Safety issues for the coating applicators and the recommendation for light coloured coatings to aid inspections in ballast tanks.
Solvent Free Epoxy.
Solvent free (sometimes referred to as 100% solids) paints are, as the name implies, formulated and applied without the need for solvents, thus overcoming the problems of retained solvents in the coating. The viscosity required to spray the paint is obtained from the selection of low molecular weight raw materials or by heating and the use of plural component systems. Typical applications include ballast and cargo tanks and they are sometimes used where VOC (volatile organic components) removal is difficult due to poor ventilation, although it should be noted that the VOC for solvent free systems is not necessarily zero.
Typical applications for solvent free coatings include the inside of pipe work, some tanks and other areas where adequate ventilation cannot be provided or for areas where stringent VOC (volatile organic content) controls are in force.
Impact & Abrasion Resistant Coatings
This type of coating is generally applied to the areas of ships which are most susceptible to damage, such as boottops and decks and are sometimes used for the holds of bulk carriers. The regions around suction pipe ends and bell mouths are occasionally coated with abrasion resistant coatings as these areas can be subjected to damage from the high flow rates of the cargo or ballast water and may suffer from erosion due to the presence of sand or small particles of debris in ballast water.
Coatings which are described as abrasion or damage resistant exhibit an increased resistance to cargo damage and of course will not be able to withstand the severe impact of grabs and hold cleaning equipment which result in deformation of the steel itself.
Shop primers, also referred to as pre-construction primers, are anticorrosive coatings designed for application in automated plants, to plates or profiles prior to assembly or construction at the New Building of vessels.
Shop primers must:
- Provide protection against corrosion during the construction period.
- Be spray applicable in a variety of automatic installations.
- Permit a very short time between application and being dry to handle.
- Not significantly influence the speed of welding or cutting.
- Not produce noxious or toxic fumes during the welding or cutting process.
- Not influence the strength of the welds or induce weld porosity.
- Be able to withstand comparatively rough handling during vessel construction.
- Form a suitable base for the widest possible range of coating systems.
- Be capable of remaining on the steel and be overcoated or may be partially removed prior to coating.
Shop primers possess properties not normally found in paints designed for other purposes. They are applied at low film thicknesses, (typically 15mm to 20mm) so as to cause minimal interference to the speed of cutting or welding. The most common type of shop primer is zinc silicate.
Inherent in the formulation of shop primers are fast drying and retarded flow properties. A side effect of this is low cohesive strength. Shop primers applied with excessive dry film thickness (DFT) have a pronounced tendency to crack and split when overcoated.
To achieve the desired protection and avoid immediate or subsequent cracking, the dry film thickness of the primer must be closely monitored and the manufacturer’s specification followed closely. This usually occurs in automated paint facilities.
The weathering characteristics of zinc silicate shop primers depend upon the type of binder and the level of zinc in the primer and the local weather conditions. Longer lifetimes are achieved with higher levels of zinc but the zinc salts caused by atmospheric corrosion must be removed from the surface before subsequent coatings are applied. Iron oxide epoxy shop primers are also used successfully in some parts of the world, but generally do not have as long a weathering period. The weathering time will depend upon the local climate.
Where a temporary protection to blast cleaned steel is required, as in a maintenance situation, a suitable anticorrosive, holding primer with a reasonably long re-coating interval is often applied at relatively low film thickness. Anticorrosive primers used for this purpose are referred to as holding primers. They are generally epoxy based materials capable of accepting other generic top coats.
Ships’ underwater hulls are painted to protect the building material, usually steel, and prevent undue roughness. The effect of roughness on the hull area is an increase in resistance to movement, resulting in reduced speed and/or increased fuel consumption and consequently, a higher operating cost.
The most severe hull roughness is that caused by fouling, the growth of various marine plants, animals and organisms. Micro-organisms are the first to settle. They form the primary bio-film or slime layer. The most significant ones are bacteria and unicellular algae. Macro-organisms such as enteromorpha and ectocarpus and hard bodied animals such as barnacles, tubeworms, bryozoans are the most common. Soft bodied animals such as are hydroids and tunicates may also be present.
Antifouling paint types.
The antifouling paints used today are based on physically drying binders. Most paints prevent fouling by releasing bioactive materials that interfere with the biological processes of the fouling organisms. Bioactive materials used today are mainly cuprous oxides, organo-tin compounds or organic biocides. The ability to register new biocides is based on the environmental profile of the new product. It is very difficult and expensive to register new biocides. Antifouling paints are subject to the most regulations of any paint and in some countries, they have to be registered and approved for use by the relevant regulatory bodies of that country. Organotin compounds which act as biocides in anti-fouling systems are currently being phased out under new IMO regulations and cannot be applied or re-applied to vessels from 1st January 2003. By 1st January 2008 ships will either be free from organotin anti-fouling paint or a barrier coat will be applied over any non-compliant anti-fouling systems.
New advances in anti-fouling technology have resulted in improved biocide release systems and in foul release coatings which do not use biocides to control the fouling and the fouling organisms cannot adhere effectively to the paint surface.
Self Polishing anti-fouling coatings.
In contact with sea water the binder dissolves at an even and predictable rate. As the anti-fouling paint is removed by a polishing action or frictional effect of the water, the bioactive material is released at an even rate and this also enables coatings to remain fouling free when the vessel is not moving for periods of time. The polishing nature of the coating results in a smooth finish to the hull, as the polishing rate is highest at the roughest points.
Insoluble Matrix (controlled depletion) coatings.
This is an old anti-fouling technology and is based on the use of rosin which is slightly soluble in sea water, but it is also brittle and slowly releases biocides. Rosin based paints need other film forming resins to provide their mechanical strength but these materials form an insoluble leach layer which progressively reduces the rate of release of the biocides. The load of bioactive material must be high enough for the particles of this material to be in contact with each other.
Foul Release Coatings.
This type of paint is a relatively new addition to the anti-fouling paint family. The mechanism for effective anti-fouling in this coating type is based on the low free surface energy of the coating surface. Fouling organisms find the surface unattractive on which to settle. Biocides are not used and therefore these coatings are not affected by legislation commonly affecting other biocide containing antifouling paints.
Many of the coatings used on vessels today are compatible with each other, providing that the overcoating times and conditions recommended by the paint manufacturers are followed. This is particularly true for epoxy coatings, where the time intervals between coats is critical for the performance of the paint.
Some paints are specifically designed as repair or maintenance products which are suitable for application to vessels in service using rollers or brushes. The types of coating onto which they can be applied and the surface preparation necessary will be specified by the coating manufacturers.
Glossary of Frequently Used Coating Terms
Air drying paints: Paints which dry and form a film when exposed to air, without any external heat being applied.
Airless spray: A method of paint spraying which does not use compressed air to atomise the paint. In this method, the paint is put under great pressure (up to 5000 psi - 360 kg/ cm2) by a compressed air driven pump and is atomised by being forced through a small nozzle. Airless spray is a very fast and efficient method of application since the paint is forced into the surface at very high speed, which assists in wetting the surface.
Alkyd: A synthetic resin made by reacting two chemicals in the presence of a natural or processed oil. Because of the wide variety of possible constituents, alkyds can be ‘tailor-made’ to meet conditions found in practice.
Anode: A piece of metal fixed to steel to provide cathodic protection. Anodes must be fixed so that they are in electrical contact with the steel they have to protect, and must not be greased or painted.
Antifouling: For underwater use on hulls. Contains biocides which are released and which prevent the adhesion and growth of organisms on the hull. See also foul release coatings.
Binder: The component in paint or varnish which binds the constituents to the surface. Paints are usually named after the binder type.
Bleached tar epoxy: See Modified epoxy.
Cathodic protection: A method of controlling the corrosion of steel either by attaching a sacrificial anode or by the use of impressed current. Coatings used with cathodic protection systems must be resistant to alkali.
Coal tar epoxy: A combination of epoxy resins, tar and a curing agent, which produces a very water resistant film.
Emulsion paints: Paints in which the binder is dispersed in water (emulsified) e.g. polyvinyl acetate (PVA), acrylics etc. The paints dry as soon as the water evaporates and the emulsified droplets of resin join together to form a solid film.
Epoxy: Epoxy resins which are cured by chemically reacting with a curing agent such as amines, amine adducts, and polyamides. Properties can be tailored to meet a wide range of needs.
Epoxy mastic: Originally a high build epoxy containing a high level of thixotropes. Now, see Modified epoxy.
Film thickness: The thickness of the paint or system. The recommended film thickness for each product are given in the Technical Data Sheets. Specialised equipment is available to measure the film thickness. The wet and dry film thicknesses differ in the volume solids, as the solvent is lost.
Flashpoint: The temperature at which the vapour of a material will be ignited by a spark or open flame. It is measured under standardised conditions.
Foul release coatings: Antifouling paints which do not contain biocides and which have a low free energy surface which is unattractive to fouling organisms.
Hard Coatings: A coating which chemically converts during its curing process, normally used for new construction, or non-convertible air drying coating which may be used for maintenance purposes. Hard coatings can be either inorganic or organic.
Induction time: The period of time which must elapse between mixing the paint and its application, for some two pack materials.
Latex: A resin used in emulsion paints.
Mix ratio: For a two pack system, the mix ratio is the relative quantity of each pack.
Modified epoxy: Also called epoxy mastic, tar free or bleached tar epoxy. An epoxy paint which also contains non-epoxy polymers which can be cross linked into the film. Non reactive polymers, either solid or liquid, may also be included to provide specific properties in the final film.
Pigments: Powders, dispersed in resins, which give the paint its colour, finish, and protective properties.
Polymer: A high molecular weight material created from lower molecular weight constituents by chemical reaction. Polymers with resinous characteristics are frequently used in paints.
Polyurethane: Paints based on polyurethane are usually two pack, are extremely hard wearing, and are generally resistant to chemicals. They may be formulated to be exceptionally colour stable and weather resistant.
Pot life: The time for which a two pack paint or varnish be applied by the method specified (usually spray) before it should be discarded. The paint should be used within this time, since the curing will be so far advanced by then that the paint will not behave in the normal manner. Paint must never be allowed to remain in spray equipment after the expiry of it pot life.
Pure epoxy: A paint where the binder is based on epoxy polymers only.
PVA paint: See Emulsion Paints
Resin: A material used as a binder constituent which forms a non-crystalline film when dried.
Semi-hard coating: A coating which dries in such a way that it stays flexible, but still hard enough to touch and walk upon. These coatings do not appreciably erode with the usual ballast water movement. Since 1st July 1998 semi-hard coatings have not been applied to ballast tanks of new vessels.
Shop primer: A rust preventing paint for temporary protection of blasted steel plates and profiles immediately after blasting, at new building. Shop primers can be welded and are used to protect the surface from corrosion during construction and until the final paint system is applied.
Soft Coatings: A coating that remains soft so that it is removed easily at low mechanical impact or when touched by hand, these coatings are generally used to give temporary protection to existing structures. Not applied to ballast tanks since 1st July 1998 unless this type of coating had been previously applied before that date.
Spreading rate: The area which is covered by one litre of paint.
Stripe coat: A layer of paint usually applied to welds and edges in tanks and holds to provide extra corrosion protection.
Surface Tolerant: Coatings which are able to withstand a lower level of surface preparation. Manufacturers data sheets will specify the type and maximum quantity of contamination.
Tar epoxy: see Coal Tar Epoxy.
Thermoplastic Paints: Paints which dry by evaporation of solvent only. The binder is unreactive.
Thixotropic paints: Have a semi-solid or gel consistency when undisturbed, but flow readily when stirred or shaken, or when being applied. The process is reversible, and a fluid paint reverts to a gel consistency when the disturbance ceases. When applied, thixotropic paints will flow easily as long as they are being worked, but quickly regain a gel consistency which assists in preventing runs and sags.
Two pack paints: Used to describe paints which are supplied in two separate containers and which have to be mixed together before use.
Zinc phosphate: A pigment with corrosion preventing properties.
Zinc silicate paints: Zinc-filled paints based on an inorganic binder. Zinc silicates are commonly used as shop primers.
Zinc-rich paints: Zinc filled paints based on a large proportion of metallic zinc in powder form. They usually contain (for example) more than 85% zinc in the dry film and provide very hard films which are resistant to solvents.