In the second of his series of six articles on coatings, Prof. A S Khanna, Retd. IIT Bombay, Chairman SSPC India, turns his attention to decorative and industrial coatings. Amongst others, this article indicates market sizes, discusses selection criteria for industrial paints, looks at coating solutions for various types of environment and outlines what is meant by coating durability.
Posted by David Sear
One of the easiest techniques of material protection is paint coating, which serves primarily two purposes: aesthetics and corrosion prevention. Based on this, paint coatings can be broadly categorised into two terminologies: decorative coatings and industrial coatings. The primary purpose of decorative coatings is to improve the aesthetics of a surface while also making it look attractive and colourful. In some instances, other advantages such as increased strength and durability may also be realised.
Buildings and concrete structures, as well as a number of items that require bright colours and glossy or matte finishes, such as automotive interior and exterior parts, consumer goods like cell phones, computers, and cameras, jewellery and watches, bathroom accessories, door handles, and eyewear, fall into the two main categories of decorative paints. Again, there are two kinds when it comes to buildings and structures: inside walls and external surfaces. Both have quite distinct requirements. Water proofing, a glossy finish, and anti-graffiti protection are some needs for indoor surfaces, whereas exterior walls and roofs need paint coatings that are long-lasting, anti-fungal, UV light-resistant, hydrophobic, anti-dust-pickup, and quickly changing colours.
Now let’s examine several criteria needed to select a decorative finish. It is crucial to realise that selecting paint only based on colour is not the greatest option. Instead, decorative paint should be selected first based on the substrate, which can be either, concrete, steel, plastic, or wood. The second crucial decision is whether to choose an inside wall or an external wall, depending on where the object is located. Even inside walls, it makes a difference if they are the main drawing room, bedroom, kitchen, or bathrooms (see Figures 1a, 1b and 1c below). In terms of colour, atmosphere, and durability, all may have a slightly different option. For example, bathrooms and kitchens may have more moist environments than other walls.
Types of decorative wall painting
There are numerous paint methods available for wall painting that can provide finishes and levels of durability depending on the type of resin used and whether the paint is solvent or water borne. Despite the fact that there are a number of straightforward cement- and chalk-based exterior wall coverings with extremely low durability and life, the following classification is straightforward and is based on the resin type:
- Latex paint or plastic: The most popular paint for interior walls is plastic or latex paint, which is known for being water-based. But, with a little alteration in composition, it can also occasionally be used on external walls. Also, it dries rapidly, can be cleaned, and is very simple to apply. Moreover, it can be divided into acrylic and vinyl, which each have three different finish options: matt, satin, and gloss.
Acrylic paint is more durable when used internally or outside. In addition to providing better wall waterproofing power, it is also completely mould and UV sunlight resistant. For most surfaces, with the exception of untreated hardwood surfaces, it has excellent adherence. Vinyl paint, on the other hand, has better adaptability and flexibility and is simpler to use.
- Synthetic enamel paints: These paints are far more resilient and durable than plastic paint since they are made of synthetic resins with solvent-based construction. Because it is more resistant to moisture and produces a smoother finish, it also keeps the gloss of the surface very well, even in outdoor environments. It is typically used on wooden or metal walls. However, it takes longer to dry and particular precautions must be taken when handling the paint’s inherent vapours.
- Paint in acrylic enamel: These are fully water soluble, odourless acrylic resins, with catalysts, and pigments that are frequently employed in creative decoration. Interior paint coating ranges from a straightforward distemper to numerous plastic paint types. There are various different kinds of plastic paints, with finishes ranging from fine to matte to smooth to very smooth, soft to extremely soft, touchable to not washable, and resistant to mould and mildew. Exterior paints start from simple cement base paints such as, snow-cem to various acrylic base paints to sophisticated silicon base paints. The requirements are long durability, saving from rain, fast color and damage from UV light.
- Paint primers: Pre-coating serves as a foundation for decorative paint and is a crucial step. In order to solidify the substrate and make it easier for the paint used to adorn and finish the wall to absorb, primer is utilised. Adherence increases significantly, diminishes flaws, and simultaneously lowers the price of finishing paint. On concrete surfaces, it aids in the balance of water coming from both within and outside the concrete.
- Varnishes: Are used to give any kind of paint colour or sheen in addition to protecting it. Often, a polyurethane basis is used to create them. They are renowned for their abrasion resistance, toughness, and waterproof qualities.
- Silicate paint: Because of their strong silicon to oxygen bonds (Si-O), silicate paints are the most resilient paints and, as a result, stay stable in terms of chalking, cracking, and colour fastness. They are now regarded as the greatest exterior paint systems. Decorative paints made with insoluble silicates and inorganic mineral colours, resemble lime paint in appearance, but they are better and more durable. These can be categorised as ecological paints because they are manufactured from renewable basic ingredients.
Benefits of ornamental painting on walls
Every decorative painting has unique advantages. Some offer waterproof qualities, some can produce metallic or rust finishes, some can be used both inside and outside, etc. It is vital to assess the requirements of each coating in advance for this reason. Obviously, a bathroom wall that is continually exposed to dampness is not the same as a wall in a bedroom. The following four criteria are used to choose between different decorative paints:
- Preserve the newly applied coating with a variety of surface effects and colours,
- a versatile application method,
- ease of use, and
- the option of waterproof and anti-slip finishes.
Market for decorative paint
It has been discovered that the decorative paint market is significantly bigger than the related industrial sector on a global scale. According to Figure 2 from a 2018 industry analysis, the decorative paint market accounts for around 68% of the overall 158 USD billion paint market. This proportion is now estimated to be between 75 and 78 percent by various surveys . According to this assessment, Figure 3 shows the decorative market revenues from 2016 to 2021.
Protective coatings explained
This is the second of six articles on coatings, kindly prepared for Valve World India & Middle East by Prof. A S Khanna. The complete series will discuss the following topics:
- Protective coatings – an overview
- Decorative coatings vis-a-vis industrial coatings
- Functional coatings
- Smart coatings
- Energy saving coatings
- Dedicated coatings for valves & actuators: internal & external
Missed an article? Contact email@example.com for copies.
Industrial coatings split-up
According to the coatings market, there was a USD 158 billion paint market in 2018, with 68% of it being decorative and the remaining 32% being industrial. The break-up of Industrial coating was as follows: Transportation, which includes the automotive industry, accounts for 19% of the market. General industry, which essentially implies painting steel structure in any industry, accounts for 16% of the market. Maintenance coating (14%) includes the application of surface-tolerant coatings, coatings for moist and wet surfaces, etc. Powder coating and automobile refinishing are the next (both at 9%). Marine still accounts for 5% of the market and mostly consists of naval constructions, ships, jetties, and offshore structures. In a significant portion of the market (5%) for coil coating, steel is directly coated with various paints on thin foils. The packaging industry has a much smaller market share of approximately 3%. According to the current market research estimates, the global paint market has reached USD 184 billion with a CAGR of 4.6% in the present, and consequently, changes in the other industrial sector would be comparable .
As was already said, the first criteria used to select industrial paints is the type of resin used as the binder, which is classified into alkyd, acrylic, polyester, epoxy, and polyurethane based resins. For each, there are hundreds of formulations available to satisfy the functional requirements of the particular industrial challenge. The various paint coatings are separated into conventional and high performance coatings based on the modifications they have undergone. The latter category includes paint systems that may produce large coating thicknesses, such as several solvent-free paint systems that have been strengthened with unique pigments like MIO (mica iron oxide), glass flakes, and fibre, among many others. Such high performances coatings can be used in harsh conditions with high humidity levels and impart greater strength in addition to prolonged endurance. Isophthalic ester-based polyester glass-flake coating is one such paint system that has gained a lot of popularity. It has the advantages of quick drying (due to radical-based mechanism), toughness (because to glass flakes), and increased thickness (due to lack of solvent). Such coatings are used on the splash zones of the piles of offshore constructions, which are the most corrosive locations in the C5M environment. (C1 to C5 are classifications of environments as defined by ISO 12944-2. For example, C3 means urban, C4 is coastal, C5 is offshore and C5M is offshore below the sea-level.)
The under-water coating, which employs the idea of water repellence during application, is another illustration of a high-performance coating. A basic epoxy and an amine hardener with several amine groups, such as cycloaliphatic amine, are used to create this coating. See Figures 4a and 4b. .
The carbon to fluorine (C-F) bond provides the foundation for high performance coatings that can withstand environments that are strongly acidic, alkaline, or sewage-related. Resins with a fluoropolymer basis are necessary to create coatings that can withstand very acidic environments. The most corrosion-resistant coatings are those made of PVDF. Aluminium-Composite Panel (ACP) facades, which have a lifespan of 20 years or more, are used on the majority of multi-story structures have a thin 25 micron PVDF coating that protects the building from UV rays and acid rain. With the proper selection and blending of epoxy resin and nanoparticles, a high performance coating that can withstand all acids and be applied on storage tanks for acids and alkalis is also possible. Very small nanoparticles distributed in a two pack epoxy system were used to provide a highly acid resistant covering for battery pit areas in ships and submarines. Such a covering can withstand temperatures of 250 oC while also resisting all acids .
Using the silicon-oxygen (Si-O) bond is another way to provide a high performance coating, particularly for high temperature applications. From 300 °C and above, silicon base paint begins to resist temperature. Nevertheless, coatings made of poly-siloxane can withstand temperatures of up to 650 °C .
The endurance of the coating is one of the factors considered while choosing industrial coatings. Many industrial applications have requirements of corrosion protection in the range from a few years to 25 years or even longer. What contributes to the coating’s increased durability? The coating system, which essentially consists of surface preparation, primer application, intermediate coat, and final top coat, must be examined in order to fully comprehend that. The longevity of paint is determined by a proper combination of four along with appropriate thicknesses.
Let’s use the straightforward example of an external steel structure in a C3 environment that has a zinc base primer coat, epoxy as an intermediate coat, and aliphatic polyurethane as the top coat to demonstrate this. The choice of zinc primers can provide a life of between 1-2 years to 25 years for a set thickness of top aliphatic polyurethane (50 microns) and intermediate epoxy coating (100 microns).
A composite coating that comprises of a priming layer that is mostly a zinc-based coating and provides the coating with cathodic protection determines how long a painted structure will last. The next step is to apply an intermediate coat of epoxy coating, which serves primarily as a barrier against the elements, moisture, and contaminants such chlorides, carbon dioxide, and sulphur dioxide. The top polyurethane layer shields the intermediate epoxy coat from environmental elements like UV rays from the sun, rain, and snow. Figure 5 depicts this schematically.
Now let’s change the primer layer by switching from one primer to another in a way that changes the amount of zinc in the coating, which essentially increases the coating’s cathodic protection of steel and increases the coating’s longevity. Table 1, column 2 lists the corresponding zinc levels for each zinc-based primer, and the last column lists the coating’s durability. Thus, it is abundantly clear that a thermally sprayed zinc coating that provides 150 microns of 100% zinc or hot-dip galvanised steel, where a chemical layer of zinc is formed, can provide a life as high as 25 years or even longer than an inhibited epoxy coating with a zinc concentration of just less than 10-15%.
Now let’s talk about a specific coating system, such as an epoxy intermediate coat of 100 microns and a top polyurethane coating (PU) of 50 microns that is appropriate for a C3 environment and has a life expectancy of 6–8 years. Now, if the environment changes from C3 to C4 or C5, all you need to do to get a life of 6–8 years is increase the coating thickness of the intermediate epoxy coat to 200 microns for C4 and 250–300 microns for C5. Table 2 provides an overview of the durability attained in different systems with coating thickness and type for C3-C5 settings for externally exposed utilities and internally exposed utilities .
The choice of coating for various internal and immersion systems as well as for underground pipelines is similarly shown in Table 3. If the fluid is more corrosive, like sewage water or glycol, a high-performance epoxy coating with glass flake is more durable. Nevertheless, for immersion service, such as the storage tank for oil, crude, and process water, a simple solvent-less epoxy coating of thickness 500 microns is sufficient. As was already said, a sturdy 3 LPE system is preferable for subterranean pipelines while a polyester glass flake is more reliable for splash zone applications. As a result, practically all industrial systems have solutions in Tables 2 and 3 .
Paint coating is integrated into our daily lives. In addition to safeguarding our resources and industrial infrastructure, it also makes our homes, offices, and public buildings more attractive, shiny, and colourful. Since it covers our personal living spaces, work spaces, as well as a significant number of consumer durables, such as gazettes, we use in our daily lives, decorative paint dominates the coating market. The industrial market is separated into various segments, each with its own needs and requirements. The automotive industry is quite vast, and in addition to corrosion protection, some ornamental elements are also necessary. The main goal of new research and novel formulations is industrial application, which is extremely difficult. Because of this, a number of new terminologies are being used in industrial paints. The very basic recommendations in this article might help you visualise the wide range of paint finishes.
 A.S.Khanna & Swati Gaur, Indian Patent No. 266579 on “ Fast Cure Underwater Coatings” 2011, IIT Bombay.
 Banti Singh & A.S.Khanna, M.Tech Thesis,”Development of Acid Resistant Coating for
Battery pits of Naval Submarines” June 2018, IIT Bombay.
 A.S.Khnana, Editor, Woodhead Publ., CRC Press, ISBN 978-1-4200-7969-2, in “High Performance Organic Coatings” chapter 5, p94-96, 200
About the Author
Dr. A S Khanna, a retired Professor from Indian Institute of Technology, Bombay, after 27 years of teaching and research, guiding 27 Ph.D’s, 125 Masters and creating research expertise in High Temperature Corrosion, Coatings, Surface Engineering, Corrosion of concrete structures and nano-coatings. He has won several International Awards such as Humboldt Fellowship from Germany, Royal Norwegian Fellowship from Norway, Fellow of Japan Key Centre and worked as visiting Professor in Germany and France. He has written three books and edited four books. He has published more than 300 papers in several International and National Journals with more than 4500 citations. He is a coating expert and is associated with Hindustan Zinc for several assignments related to galvanization and its applications, Sterling & Wilson for selection of sites for solar power plants, oil & gas industry for consolation on coatings for underground pipelines and offshore structures.
He can be reached on: Anandkh52@gmail.com
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