Basic Properties of Paint
Properties required for ideal paint vary greatly according to the particular end use. The requirements for an automotive topcoat, for example, will be very different to those for a decorative ceiling paint.
The application methods also need to be borne in mind by paint producers. Numerous methods are used including: brush, roller, dipping, flowcoating, spraying, hot spraying, electrostatic spraying, airless spraying, electrodeposition, powder coating, vacuum impregnation and immersion.
Producers of prepainted metal and of the coatings which are used in this product are continually evolving what they offer. In this respect, there are three main fields of innovation in prepainted metal products:
1. Innovation in basic properties
2. Additional functionality
3. Environment Issues
Innovations in basic Properties
Basic properties of prepainted metal coatings include:
• Protect the metal substrate
• Durablility against the effects of weathering
• Flexiblility for forming
• Hardness and robustness
• Provide decorative appeal
• Ease of application
• Food flow out of application marks (e.g. brush-marking)
• Forming a continuous protective film
• High opacity
• Quick drying
• Corrosion resistance
• Water resistance
• Colour stability (i.e. against visible and ultraviolet radiation)
• Abrasion and scratch resistance
There are naturally trade-offs between these factors and of course also against cost, but prepainted metal products are constantly evolving to remove these barriers and provide products which meet the ever-improving requirements of users.
The durability and protection of the latest coatings is shown by the length of performance expectations and even long guarantees for prepainted metal used in buildings. Particularly in appliances, the improvements in flexibility and robustness is seen in the latest products.
Probably the most noticable of innovations is in appearance. Metallic-effect colours are now common, while two-tone colours are also available. Tinted clear-coats can give interesting metallic effects and high-gloss or high-matt finishes open up new possibilities for designers.
Recently, prepainted metal has become available with additional functionality included into the coating. Some of the functions now available include:
These added functions give added value in many applications, but the coatings of tomorrow may include even more such as self-healing coatings or the ability to capture the energy of the sun.
Paint Producers are increasingly cognizant of environment issues:
Lead compounds are no longer used in decorative paints and automotive paints. The quantity of lead compounds still being used in specialised industrial paints has been greatly reduced and eventually alternatives will be found. This also applies to chromates which, although they perform well and in the past have been extensively used on motor vehicles, are very toxic.
Because volatile hydrocarbons can lead to pollution in the troposphere, coatings with lower organic solvent content are required. The routes to achieve this include:
• water-based polymers (emulsion paints)
• higher solids content polymers (using less solvent)
• powder coatings
Water-based gloss paints are now available but the initial gloss of the finish is usually not as high as organic solvent based paints. The customer`s choice is between a high performance product and a more environmentally friendly one. Intense research effort continues to improve these paints.
High solids paints (which are solvent-based) are now available but not without compromises in cost and performance. The relative molecular masses of the polymer resins are reduced to a maximum of ca 1000 compared to 5000 in conventional paints. This allows the proportion of the polymer to be increased from 20-30% to 40%, hence the term high solids. The main problem is the need to maintain a low viscosity. As the amount of solids increases so does the viscosity, reaching a point at which the paint cannot be applied properly. The lower proportion of solvent tends to slow down the drying and film hardening process, so changes are made to the structure of the polymer - increased branching tends to reduce viscosity for the same molecular mass. The application of the paint is more difficult. If applied by aerosol, the paint has to be under pressure. Sometimes the paint is applied hot. It is difficult to get as good a finish in appearance using a high solids paint.
Powder coatings are used in particular for goods such as bicycles and white goods (refrigerators, washing machines). The powder is made up of a resin (often an epoxy resin), pigments, a catalyst to promote cross-linking when the powder is heated, and additives. The powder is sprayed on to the article using an electrostatic spray gun and is then heat cured to produce a hard coating. Recently acrylic powder coatings have been introduced as clear-coats on car bodies. Although an ideal solution for many applications, curing is achieved at high temperature in an oven and is therefore not universally applicable (e.g. painting of wood and plastics).