Author: Robin Fisher, John Desmond Ltd.
- Properties and types of brasses
- The oxidisation and corrosion processes on brass
- Welding and creating joints in brass
- Clear coating and protective lacquers for brass
- Cleaning and maintenance of brass
- Decorative metal coatings to produce a brass finish
Considerations for architects and designers when specifying brass architectural hardware for bathrooms
In addition to their aesthetic appeal brass fixtures have a long service life thanks to their excellent resistance to corrosion. Furthermore, copper is bacteriostatic and its content in brasses has the effect of restricting the growth of micro-organisms on brass surfaces. Tests on door knobs and finger plates have shown that those made of clean brass are far less likely than other materials to encourage the growth of the organisms causing nosocomial infections. However, is brass really a suitable material to specify for bathrooms in view of its constant exposure to water and humidity? Are there coatings on brass that can help avoid patina over time? Are certain types of brasses more suitable for bathrooms or are there any suitable alternative materials?
1. Properties and types of brasses
Brasses are copper-based alloys in which the major alloying addition is zinc. The proportions of zinc and copper can vary to create different types of brass alloys with varying mechanical and electrical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses. The density of brass is 8.4 to 8.73 grams per cubic centimetre (0.303 to 0.315 lb/cu in). From this basic binary combination of copper and zinc different categories of brass products have been developed:
These are also called ‘cold working brasses’ (because only one solid solution (alpha) is present) contain a minimum of 63% of copper. These alloys are ductile at room temperature and ideal for cold working. The best-known material in this group contains 30% zinc and is often know as ‘70/30’ or ‘cartridge’ brass CuZn30 – due to the ease with which the alloy can be deep drawn for the manufacture of cartridge cases. Substantial quantities of alpha alloys are also used for metal connectors such as wood screws, rivets and zip fasteners.
These are also called ‘hot working brasses’ and usually contain between 38% and 42% of zinc. Brasses with tensile strengths in excess of more than 35 per cent are called alpha- beta or duplex alloys and are ideal for hot working. In contrast to the alloys of this first group, their ability to be deformed at room temperature is more limited. They are however, significantly more workable than the alpha brasses at high temperature to be extruded or forged into complex shapes.
High Tensile Brasses
Apart from zinc, small additions (less than 5%) of other alloying elements can be added to modify the properties so that the resulting material is more suitable for specific end-uses. The addition of small quantities of manganese, iron and aluminium in various proportions gives five high tensile brasses (CW721R, CW722R, CW705R, CW713R and CW720R) with tensile strengths of 500 MN/m² but also increased corrosion and wear resistance. High tensile brasses, some of which are known as manganese brasses, are particularly suitable for architectural applications because they can be patinated to a range of durable brown and bronze finishes.
Architectural bronze or manganese brass CW72OR (CZ136) contains 38% zinc with 2% manganese, 1% lead and 0.5% each of tin and iron is especially resistant to atmospheric corrosion and has an unlimited life span. It develops a durable and tenacious oxide film, which is an attractive chocolate brown colour, and has the added advantage of self-healing over superficial scratches making it very competitive against some of the more expensive colour coating systems used on other metals. For indoor service wax polish is sometimes used.
Aluminium brasses which are included in EN 12167 as CW624N (CZ130) have a bright yellow colour with a distinctive silvery sheen with good tarnish resistance. Aluminium also makes brass stronger and more corrosion-resistant. Aluminium also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self-healing.
Gilding metals and cap copper
Alpha brasses with high copper contents (80 to 90%) which closely match gold in their colour, are known as ‘gilding metals’, CuZn10, CuZn15, CuZn20 (CU501L, CU503L, CZ101-CZ103). They are used for the manufacture of decorative metal ware and roll-formed sections for architectural applications. They are sometimes known as ‘architectural bronzes’ which can cause confusion with the high tensile brass extrusions normally made for architectural purposes (the manganese brasses).
Dezincification Resistant Brass
The addition of a small amount of arsenic to alpha brass alloys produces a dezincification resistant brass, frequently used for water fittings. DZR or DR brasses, sometimes referred to as CR (corrosion resistant) brasses, are used where there is a large corrosion risk and where normal brasses do not meet the standards. Applications with high water temperatures, chlorides present, or deviating water qualities (soft water) play a role. The brass designated CW602N (CZ132) was therefore developed to be easily hot stamped as a duplex brass but to be converted to a mainly alpha structure by a subsequent anneal at 500°C. Free machinability is retained by a lead addition. Water fittings of this material which pass the British Standard dezincification- resistance test, are marked with the special symbol ‘CR’. Only if the product is assured to pass the stringent EN ISO 6509 test, is the special ‘CR’ mark used. DZR-brass is excellent in water boiler systems.
Table to show effect of additions of elements to brass:
|Lead||1 to 3 per cent||Machinability|
|0.75 to 2.5 per cent||Yield strength, Up to 500MN/m2|
|0.4 to 1.5 per cent||Corrosion resistance especially in sea water|
2. The oxidisation and corrosion processes on brass
One must distinguish here the oxidation process on brass surfaces from the corrosion process on brass components in water systems such as particularly faucets.
A safe, natural patina can develop as brass oxidizes, preventing further damage from exposure to the air. This patina can appear reddish, black to brown, or green to blue and will often form a thick crust.
Brass that is stored in heavily polluted environments, exposed to salt water, or mishandled can develop more serious corrosion that can damage the integrity of the metal. Harmful corrosion can leave pitting on the surface, commonly called “brass disease.” This is caused by chlorides, which are particularly aggressive corrosive agents (e.g. salt, which is sodium chloride).
For many years, brass has been the choice for plumbing equipment such as meters and valves. But even high-quality brass contains 8% lead. The addition of Pb or Bi is made to improve the machinability of the brass. The Pb content is typically between 2 to 8 % even if these can be marketed as “lead free”.
Envirobrass – lead-free metal alloys
New lead-free metal alloys have been created under the foundry term SeBiLOY, which reflects the addition of selenium and bismuth to replace the lead content. Bismuth acts very much like lead in many respects. It is lead’s neighbour on the Periodic Table of the Elements, and its melting point is 56 C (101 F) lower than that of lead, making its behaviour during casting and solidification similar to that of lead. Like lead, bismuth is nearly insoluble in copper and copper alloys. Furthermore, it causes machining chips to break up into small, easily removed particles, similar to lead. Unlike lead, bismuth is not known to be toxic to humans, except in cases of consumption of immense doses. In fact, its most common use for many years has been as the major ingredient in popular stomach medications. Selenium enhances the effect of bismuth in red brasses; therefore, it reduces the amount of bismuth needed to achieve the desired improvement in properties. From a health standpoint, selenium, like copper, is one of the essential nutrient elements. Animals, including humans, require a minimum intake of selenium (as well as copper). These lead-free alloys have been branded as Envirobrass. The EnviroBrass alloys, C89510, C89520 and C89550, were developed to provide the plumbing industry with environmentally friendly alloys that will perform as well or better than existing materials while meeting the strict requirements of current Water Supply Regulation and remove concerns of lead leaching into drinking water from plumbing components.
|CaptionBrass elbow joints for pipingChange title to Compositions of EnviroBrass I, EnviroBrass II and EnviroBrass III Alloys*|
|Elements||Range or maximum, %|
|EnviroBrass I (C89510)||EnviroBrass II (C89520)||EnviroBrass III (C89550)|
|Copper||86.0 – 88.0||85.0 – 87.0||58.0 – 64.0|
|Tin||4.0 – 6.0||5.0 – 6.0||1.2|
|Zinc||4.0 – 6.0||4.0 – 6.0||32.0 – 38.0|
|Bismuth||0.5 – 1.5**||1.6 – 2.2***||0.6 – 1.2|
|Selenium||0.35 – 0.75**||0.8 – 1.1***||0.01 – 0.1|
|Nickel (incl. Cobalt)||1.0||1.0||1.0|
|Aluminum||0.005||0.005||0.1 – 0.6|
* Cu + sum of named elements, 99.5% min.
** Experience favors Bi:Se ≥ 2:1.
*** Bi:Se ≥ 2:1
3. Welding and creating joints in brass
Brass can easily be joined to itself and all other copper alloys, by soft soldering and brazing. This makes the fabrication of intricate brass work much easier than with some other metals and also contributes to cleaner lines due to the omission of rivets, straps and brackets. The close tolerances to which brass components can be manufactured makes it ideal for joining by silver brazing which produces leak tight joints without melting and distorting the brasses to be joined. Modern MIG and TIG welding processes can successfully weld most brasses with the correct choice of filler alloy. The CuproBraze technology uses brazing instead of soldering to join copper and brass radiator components in the car industry.
Soldering is easily carried out using any of the lead/tin or lead-free solders to EN 29543 and either an active or non-active flux. Sudden heating of stressed parts in contact with molten solder can result in cracking of the material due to intergranular solder penetration. In such cases parts should be stress relieved before soldering. After soldering it is good practice to remove any flux residues in order to reduce the tendency for these to cause staining and corrosion.
All the brasses are readily joined by brazing alloys covered by EN 1044. When a flux is used it is likely to cause corrosion if allowed to remain in place on the component. It should be washed off as soon as practicable. This is easy if the component is still warm after brazing but the brass should be quenched directly from the brazing temperature or quench-cracks may be caused.
4. Clear coatings and protective lacquers for brass
To prevent tarnishing and to retain a polished finish, the use of protective lacquers and coatings is recommended. Brass can be protected from oxidation using a clear and durable coating on the different types of brass finishes. Lacquer does not affect the colour of the brass, and it eliminates the need for polishing. Lacquer can always be removed, without damaging the brass, by using paint remover or lacquer thinner. The brass can then be re-polished and re-lacquered.
Each lacquer has particular attributes and they may be used either singly or in combination with each other to obtain a coating with the required properties. Certain manufacturers are willing to offer a 20-year guarantee on their custom lacquering process for indoor applications. However, lacquer on objects exposed to moisture (such as bathroom faucets) will not hold up as long. That explains why certain specifiers prefer solid brass because it can always be re-polished and re-lacquered, no matter how oxidized it is.
The tables under Appendix 1, at the end of this article, list these different polymers and indicates their film properties as well as their typical applications. For instance, it is worth noting that modified epoxy is particularly suited for brass fixtures with heavy service such as bathroom taps. Each coating is defined by the constituent polymer or polymers, but the properties are also controlled by the way in which the coating is prepared and applied. There are two main types of coating: solvent-based coatings (air dried or stoved) or powder coatings.
Solvent-based coatings (or lacquers)
In order to apply a coating in liquid form the film-forming polymer (or mixture of polymers) must be dissolved or dispersed in a suitable liquid. This type of coating is frequently referred to as a “lacquer”. To be more environmentally friendly most lacquers today are water-based. The coating is applied to the surface, the solvent evaporates and the coating film forms. Some coatings may simply be allowed to air-dry, others require gentle heating to speed up the process. Some polymers require stoving to complete a cross-linking reaction.
Air-drying formulations have the advantage that no drying or baking equipment is required, but care must be taken to protect the wet coating from dust which will spoil its appearance. Humidity in the atmosphere may also cause defects. The coatings take some time to reach a fully hard state and whilst they are still soft they are vulnerable to damage.
Stoved coatings are generally harder and more wear resistant than air drying, non-crosslinked ones, but care must be taken to ensure that the metal surface does not discolour during the stoving process. It is wise to use as low a temperature and as short a time as possible. In some cases, the copper surface catalyses the cross-linking reaction thus enabling lower temperatures or shorter times to be used.
When a coating is being formulated it is generally necessary to include a number of ingredients other than the solvent and the basic film forming polymers. Many different types of additives may be used. They have a wide range of functions such as to improve film properties or to ensure that the coating is more easily applied. All the constituents of a coating solution must be considered for a particular application in order to ensure that optimum performance is obtained.
Additives can be added to solvents such as for instance:
- Ultra-violet absorbers which increase the coatings ability to prevent darkening of the
- Antioxidants to reduce the degradation of coatings during long and severe exposure
- Chelating agents to protect the metal against oxidants permeating through the coating. The most commonly used chelating agent is benzotriazole.
- Plasticisers when a particularly tough and resistant film is required
- Levelling agents ensuring a better surface appearance
Solvent-based coatings (or lacquers) – methods of application
Solvent-based coatings may be applied in a number of different ways from brushing, dipping and flow coating to electrostatic spray or electrophoretic coating. The latter also known as E-coat or Electro coating is a liquid form application method. Because it is a liquid application method it is easier to apply on complex forms and avoids blocking holes while providing a consistent thickness. Each method has its advantages and disadvantages. Under Annex 1 Table 2 gives details of the methods available and includes comments on each one.
Powder coating was invented around 1945. It is a dry finishing process, using finely ground particles of pigment and resin that are generally electrostatically charged and sprayed onto electrically grounded parts. Before coating, the parts to be coated are first pre-treated similarly to conventional liquid coated parts. The pre-treatment process is normally conducted in series with the coating and curing operations.
The film forming polymer (or mixture of polymers) is prepared in the form of a fine powder which is applied to the surface to be coated. The powder particles should generally be in the size range 20-40 microns. The coating is then cured in an oven which allows the coating to flow to form a ‘skin’ that is tougher than conventional paints. By heating the powder-coated surface the powder particles soften and flow out to form a smooth continuous surface film.
4. Clear coatings and protective lacquers for brass; Powder coating continued
Powder coating: Advantages
One of the key advantages of powder coatings is that they are an environmentally friendly solution: powder coatings contain no solvents and release little or no amount of Volatile Organic Compounds (VOC) into the atmosphere thus complying more easily with environmental regulations. Because powder coating does not have a liquid carrier, it can produce thicker coatings than conventional liquid coatings without running or sagging, and powder coating produces minimal appearance differences between horizontally coated surfaces and vertically coated surfaces.
Powder coating: Methods of application
Powder coatings are applied either by electrostatic spraying or fluidized bed dipping:
Powder coating: Fluidized bed dipping
The fluidized bed is the original powder coating technique. It is still the primary technique used for the application of thermoplastic powders. The fluidized bed is also used for the application of some thermoset powders where high film build is required. Thermoset powders designed for electrical insulation often use the fluidized bed technique. The parts are pre-heated to a temperature significantly higher than the melting point of the powder. The parts are then immersed into a “fluidized bed” of the coating powder where the plastic powder is melted onto the part.
Powder coating: Electrostatic spray
This is the most usual method for application of powders. Electrostatic spray is the primary technique used for thermoset powders. The particles of powder are given an electrical charge in the powder coating gun. The target part is attached to a fixture that is grounded. The electrically charged powder particles are attracted to the grounded part and attach themselves like little magnets to the part. The particles build-up on the surface of the part until it is covered with charged particles and the part surface is charged. At this point the oncoming particles are actually repelled by the charged particles on the part and the coating process stops. This provides an even film thickness.
Powder coating: Types
There are two main categories of powder coating: thermosets and thermoplastics. The type of polymer used determines whether the film is described as thermoplastic or thermosetting. In the former, the film hardens on cooling but could be re-softened simply by heating again. In the latter, a chemical reaction takes place during heating and causes the film to harden. A thermosetting film cannot be softened again just by heating to the curing temperature.
4. Clear coatings and protective lacquers for brass; Types of powder coatings continued
A thermoplastic powder coating is one that melts and flows when heat is applied, but continues to have the same chemical composition once it cools and solidifies. Thermoplastic powders exhibit excellent chemical resistance, toughness, and flexibility. They are applied mainly by the fluidized bed application technique, in which heated parts are dipped into a vat where the powders are fluidized by air, and are used in many thick film applications. They are generally applied to a surface that has been preheated to a temperature significantly higher than the melting point of the powder. As a thermoplastic powder material is applied to the hot surface it will melt and “fusion bond” to the surface and then “flow out” into a strong, continuous film. As the film cools it develops its physical properties. Nylon powder coating materials are the most commonly used thermoplastic powders.
Thermosetting powder coatings are based on lower molecular weight solid resins, and melt when exposed to heat. After they flow into a uniform thin layer, however, they chemically crosslink within themselves or with other reactive components to form a reaction product of much higher molecular weight. These newly formed materials are heat stable and, unlike the thermoplastic products after curing, will not soften back to the liquid phase when heated. Thermosetting powders are derived from three generic types of resins: epoxy, polyester and acrylic. From these resin types, several coating systems are derived. Resins used in thermosetting powders can be ground into fine particles necessary for spray application and a thin film finish. Most of the technological advancements in recent years have been with thermosetting powders.
The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.
Epoxy powder coatings exhibit inherent toughness, corrosion resistance, chemical resistance, flexibility, adhesion and abrasion resistance. Epoxy powder is normally used where a tough durable film is required and the product will not be exposed to direct sunlight for long periods of time. They are particularly suited for brass fixtures with heavy service such as bathroom taps. Polyurethane powder offers excellent gloss retention and long-term resistance to humidity and corrosion in thin film applications. Polyester powder is particularly suited to long-term exterior applications because of their durability and high performance mechanical properties. Acrylic powder is specified when resistance to ultraviolet rays from sunlight for a longer period of time is critical such as in the automotive industry.
5. Cleaning and maintenance of brass – effects of wear, water and cleaning on coatings
How to clean lacquered brass?
Lacquer is not permanent and can be chipped, scratched or broken down chemically leading to oxidisation in the atmosphere. Once the lacquer has been breached, steps must be taken to restrict exposure. That is why polish or abrasive cleaners should never be used whilst the lacquer is in good order. Regular dusting with a slightly moistened soft cloth and an occasional application of furniture wax will prolong the life of the lacquer. Like varnish or paint, lacquer may break down after exposure to atmospheric conditions.
To prolong the life of the lacquer it is advised that the item is regularly cleaned with warm soapy water. To do this the lacquered brass needs to be wiped with a clean wet cloth and dried thoroughly. If you do need to wipe something off, you should only use tepid water and mild washing up liquid. The occasional use of a good quality wax polish will also help to seal and chips and scratches in the lacquer itself. In exposed and external situations, more regular waxing is advised to keep it clean.
At some point it is possible that the lacquer will deteriorate beyond maintenance capabilities. When this happens, there are two possible options: either the lacquer is removed using a lacquer or varnish remover and then polished back to its original finish using a good quality brass cleaner such as Duraglit. Once the item’s lacquer has been removed, it is best to undertake periodical cleaning. Alternatively, if re-lacquering the item is the preferred option then it is best to use a car spray lacquer and not use a brush-applied lacquer as it will leave streaks once the lacquer is dry. However, the car lacquer should not be sprayed onto a cold metal as the lacquer will set almost immediately and bubbles can become trapped. Equally, excessive spraying drip marks will be evident. It is best to let the lacquered brass item warm through in a warm area such as an airing cupboard before spraying.
How to Maintain Unlacquered Brass
This option will be useful for in situations where an aged look to door knobs and handles is desired as the brass will form a natural patina and turn a coppery colour. The unlacquered brass item will come in a bright-mirrored finish but in time will oxidize to a soft brown patina. These products can be re-polished using a non-abrasive metal cleaner such as “Duraglit” wadding polish. However, unlacquered brass can tarnish quickly, so to keep the brass in top condition, it is best for it to be polished every few months.
6. Decorative metal coatings to produce a brass finish
Many manufacturers of bathroom hardware use the industrial processes described below to create a brass finish on another metal base. Brass plated items are usually made of steel or white metal (zinc) base to which molecules of brass are added. The two most widely used techniques to obtain a brass plating are electroplating and PVD (Physical Vapor Deposition).
This plating process was first invented by Italian chemist Luigi V. Brugnatelli in 1805. However, it wasn’t until 1840 that the first industrial patents appeared in Britain. Electroplating uses electrical current to coat an object with a thin layer of metal material. This deposition of a layer of metal material provides the desired properties of wear resistance, corrosion protection, and creates aesthetic properties that the surface originally lacked. Nickel, chrome, and copper are the most commonly electroplated metals.
The brass metal plating process involves creating an outer coating of brass to inhibit corrosion or to enhance the appearance of the parent metal. It is usually done by immersing the metal in an acid solution with an anode electric current and cathode. The material to be plated is made the cathode (negative electrode) of an electrolysis cell through which a direct electric current is passed. The solution or bath contains the required metal in an oxidized form. The anode is usually a bar of the metal being plated. During the electrolysis process, the metal is deposited on the work and the metal from the bar dissolves. The process is governed by Faraday’s law of electrolysis. The electroplating process causes the plating material to deposition on the parent metal.
For bright decorative work, most is done by flash plating brass over bright nickel or other suitable bright plate. Heavier brass plating is required where buffing or brushing is required, or where the work is to be coloured or antiqued, usually with subsequent brushing or highlighting. The brass alloy most often applied is a yellow colour and contains 70 to 80% copper, with the balance zinc. This is a ductile, stable alloy with only about one shade of colour variation over the entire range.
The important ratio in brass plating is the ratio of the sodium cyanide to zinc. With quite high ratios of sodium cyanide to zinc, rich low brass (85115) or even architectural bronze (90/10 alloy) are produced. Higher copper alloys such as so-called red brass (rich, low, or 85/15) alloy or architectural bronze (90/10 alloy) can be plated. The major change is to operate with a higher cyanide to zinc ratio.
Brass plating has many of the characteristics of cyanide zinc plating. It is slow to plate on cast and malleable iron. It is an excellent plate over zincated aluminium. On zinc castings the brass will diffuse rapidly into the zinc so that a substantial thickness of deposit is necessary. While conventional brass plating fills most of the needs, high speed brass plating is often necessary where heavier thicknesses are required and for uses such as strip plating where times are measured in seconds.
Because electroplating is a low energy form of plating, ions arrive at the substrate with relatively low energy and deposit on the surface. Large edge build-ups are common and uncontrollable in this process. The part geometry can also affect the deposit’s uniformity. Channels and crevices are very difficult to electroplate without receiving a large build-up on the outer edges.
Physical Vapor Deposition (PVD):
Physical Vapor Deposition (PVD) (also known as Thin Film Deposition) is a vacuum-based coating process to vaporise a solid metal to a plasma of atoms or molecules, vapor that can be deposited as a high-performance coating on wide variety of substrates. PVD is a relatively new finish to the architectural ironmongery market and is designed to remove the finish maintenance hassles associated with brass. This process involves using a vacuum chamber to deposit thin layers of film onto a particular surface. The PVD manufacturing process alters the chemical state of the first few microns of the brass. A wide variety of types of PVD coatings are available, including Zirconium Nitride (ZrN), Zirconium Carbon Nitride (ZrCN), Titanium Nitride (TiN), Titanium Carbon Nitride (TiCN), Chromium Nitride (CrN), Chromium Carbon Nitride (CrCN), and Chromium Nitride (CrN).
PVD finishes resist corrosion, tarnishing, and discoloration. Once this is done, the resultant finish is slightly more gilt than normal brass but depending on the supplier, generally has between a 20 and 25-year finish guarantee. PVD coatings are sometimes harder and more corrosion resistant than coatings applied by the electroplating process. For instance, a brass finish in Zirconium Nitride (ZrN) has a hardness of 2800 Vickers or HRc-80.
Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary. PVD coatings adhere best to chrome plated materials and stainless steel. Provided the material is thoroughly chrome plated with a Nickel/Chromium almost any material can be PVD coated. With Titanium alloys and Graphite PVD coating is applied directly to the substrate material.
However, the PVD coating will not level or fill like an electroplated finish so surface imperfection will still be visible after the coating process. Polished or mirror surfaces are used to produce PVD polished finishes and brushed or satin surfaces to produce satin or matt PVD finishes. The coating thickness is usually between 0.25 microns and 5 microns for decorative finishes. Thicker coatings can be applied for functional coatings.
The PVD process provides a more uniform deposit, improved adhesion up to six times greater in some cases, wider choice of materials to be deposited and there are no harmful chemicals to dispose of. Because PVD coating is more environmentally friendly and chemical disposal costs are minimal, the cost of PVD coating and electroplating is very close on some products. Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary The application of PVD surface coating technologies at large scale, high volume operations will result in the reduction of hazardous waste generated when compared to electroplating and other metal finishing processes that use large quantities of toxic and hazardous materials. PVD is a desirable alternative to electroplating and possibly some painting applications because it generates less hazardous waste and uses less hazardous materials (i.e., no plating baths).
Appendix 1 – Table 1 – Polymers used for clear coatings (lacquers)
|Polymer||Film Properties||Typical application|
|Acrylic||Available in air-drying or thermosetting compositions, acrylics are relatively high cost materials. The airdrying modifications are popular for exterior applications. The thermosetting types are useful for applications requiring high resistance to heat and abrasion. The addition of a chelating agent such asbenzotriazole gives good protection against tarnishing occurring under the lacquer.||Since the thermosetting coatings are not easily stripped off for re-coating, they are not normally suitable for major architectural applications. The copper roof of the Sports Palace in Mexico City is covered with Incralac, an inhibited air-drying acrylic lacquer formulated also with an ultra-violet absorber.|
|Modified acrylic||Acrylic resins can be modified with polyisocyanate, polyurethane, amino and other resins to produce crosslinked systems with good mechanical strength, abrasion resistance, flexibility and adhesion.||These lacquers are durable and have good resistance to chemicals.|
|Epoxy||Epoxy coatings have excellent resistance to wear and chemicals. They are relatively expensive and are available in thermosetting or two-part compositions, the latter having a relatively short pot life. They are good for severe indoor applications, but they darken in a few months of exterior service.||Outstanding adhesion and protection for copper surfaces used indoors.|
|Modified epoxy||The most important combination partners are phenoli or amino resins for improving elasticity, impact resistance, hardness and abrasion resistance.||Ideal for severe service such as bathroom taps.|
|Nitrocellulose||These are less expensive and the most common air drying coatings for interior service. They are modified with alkyd, acrylic, polyurethane and other resins. They do not have high resistance to chemicals, but they are fast drying and easy to use.||Mainly used for interior applications. They can be used outdoors, but they are usually stripped and replaced at intervals of less than one year.|
|Cellulose acetate butyrate and propionate||These coatings have a cost comparable with acrylics. They can be used alone or to modify acrylics or alkyds||Could be used for interior or exterior applications.|
|Polyurethane||Tough and flexible films with good adhesion. They have good abrasion resistance and are resistant to chemicals. Available in both single and two component formulations. Some forms are prone to yellow with time. They darken on exposure to elevated temperatures.||Good for all interior applications.|
|Vinyl||Vinyl films are flexible and resistant with goodadhesion. Stabilisation is required.||Very good protection for interior applications. Good for exterior applications provided they are well stabilised.|
|Silicone||Silicones provide the best potential for coatings which must operate at elevated temperatures. They have excellent resistance up to 250°C. Thin films of these high-cost coatings are sometimes used with protection by a second coat of a more durable, abrasion-resistant lacquer. They require extended curing at high temperatures, and this may cause discolouration of the copper surface.||High temperature applications|
|Alkyd||Slow drying or baking is required when applying the alkyd coatings. They can be modified with melamine or urea resins. They have a low-cost and are sufficiently durable for exterior applications, although yellowing may occur. Resistance to chemicals is usually good.||Domestic applications where high wear resistance is required.|
|Soluble fluoro polymer||Will cure to full hardness at ambient temperature or can be stoved to accelerate hardening. Resistant to weathering and ultra-violet light.||Excellent protection with 20 years life expectancy for exterior applications. Suitable for coil coating or on-site application.|
Typical application methods of solvent-based coating (lacquers)
|Brushing||A commonly used method for small scale applications. There is efficient use of the coating and capital costs are low, but it is labour intensive. The viscosity of the coating must be carefully controlled. Skill is required to produce films of uniform thickness free from runs.|
|Dipping||A traditional application method still useful with hand or mechanised techniques. The dip cycle must be slow in order to reduce air entrapment during immersion and to permit good drainage on extraction.|
|Flow coating||Similar in effect to dipping but the components are flooded with lacquer in an enclosed cabinet. Excess lacquer is recycled.|
|Roll coating||An automated process for coating strip either on one side or on both sides simultaneously. A high standard of quality is obtained.|
|Curtain coating||An automated process for coating sheet and strip. The metal passes through a “waterfall” of the coating liquid.|
|Compressed air spray||A conventional method of application using small, medium or large spray guns and nozzles. 1-2m2/min coverage is obtained. Automation is possible but otherwise skill is needed to obtain a uniform film thickness. Overspray leads to inefficient utilisation of the coating and fume extraction can be a problem.|
|Airless, or high pressure spray||Can achieve greater efficiency than with conventional compressed air spray. Coverage up to 6m2/min is possible. Inclusion of air bubbles and the occurrence of runs are less frequent.|
|Electrostatic spray||Electrostatically charged droplets of spray are attracted by charged components. The process gives good, even coverage to complex parts including those with sharp edges. It is easily automated and gives relatively efficient usage of coating mixture.|
|Electrophoretic coating||An aqueous dispersion of the coating mixture forms the eletrolyte of an electrolytic cell. The component is immersed in the cell and forms one of the electrodes. For coating copper and brass the component must form the cathode. The coating is quickly applied and is of uniform thickness, even over sharp edges and recessed areas, making the process very useful for articles with intricate designs and re-entrant areas. It is particularly suitable for continuous production. The articles do not need to be dried after cleaning. Investment in plant is repaid by low losses of coating mixture, good surface quality, low production costs and minimal fume problems. Skilled maintenance of the plant is essential and care must be taken to avoid contamination of the electrolyte.|