Chemical and physical principles of industrial coating systems

Fundamentals of industrial coating systems - from water-based to solvent-based coatings: trends, innovations and sustainability aspects of modern coating technology summarized in compact form.

Industrial coatings are more than just decorative surfaces. They protect materials against corrosion, weathering and mechanical stress and shape the visual effect of a product. Without modern coating systems, the high quality standards in the industry would hardly be achievable.

On the one hand, legislation and social requirements are pushing for the reduction of volatile organic compounds (VOCs), which is why water-based systems are increasingly being promoted. On the other hand, everyday industrial practice shows that the established solvent-based systems are still indispensable thanks to their robustness, process reliability and proven performance. As a specialist with almost 100 years of experience, Stoz provides information below on the current status and trends in industrial coatings, surface technology and corrosion protection.

Essentially, a distinction is made between one-component (1K) and two-component (2K) paint systems.

1K lacquers: All components are contained in a finished mixture. Curing takes place physically by evaporation of the solvent or chemically by reaction with atmospheric oxygen or moisture. Their application is uncomplicated, but they do not achieve the durability of more complex systems.

2K lacquers: The two components - resin and hardener - are mixed shortly before application. The subsequent chemical reaction creates a densely cross-linked film with high hardness, abrasion resistance and chemical resistance. This requires more discipline in processing, but is the first choice when maximum performance is required.

Stoving enamels: They only cure completely when exposed to heat in an oven. The cross-linking of polyester or acrylic resins with amino resins (usually melamine) creates a highly cross-linked, thermosetting film. This creates surfaces with exceptional hardness, chemical resistance and gloss retention as well as high process and end performance. However, the energy required for the oven process must be taken into account in the ecological assessment.

Solvent-based stoving enamels have been the standard in automotive OEM painting and coil coating for decades. The water-based form, such as cathodic dip coating (CDC), is used today on practically every vehicle body shell.

The development of water-based coating systems has made considerable progress. One-component coatings based on acrylic or polyurethane dispersions are widely used today. They dry quickly, are elastic and non-yellowing. They are used, for example, in the furniture industry or in interior design. However, they also have weaknesses: Block and scratch resistance are limited, they react sensitively to climatic conditions and drying times are significantly longer in high humidity or low temperatures.

Water-based two-component coatings, such as those based on polyurethane and epoxy resins, reduce many weaknesses. Thanks to their high hardness, good resistance to chemicals and stable abrasion resistance, they are increasingly being used for demanding industrial floors, corrosion protection and high-quality furniture surfaces. However, processing is challenging due to exact mixing ratios, short pot lives and increased sensitivity to climatic fluctuations.

The ecological advantage of water-based systems must be viewed in a differentiated manner. Although VOC emissions are lower, the evaporation of water requires longer or more energy-intensive drying processes. In addition, binders in water have to be stabilized with additives - substances that are not always harmless. The actual ecological balance of water-based coatings is therefore de facto dependent on the respective process design and, above all, on the durability of the coating.

Water-based stoving enamels are a special case, above all the KTL coating. It is considered the standard in automotive construction as it provides an even, full-surface corrosion protection primer. Its strength: seamless protection of complex geometries. Its weaknesses: high energy requirement of the curing process and limited decorative function.

Solvent-based paint systems are still regarded as the benchmark for technical stability and process reliability. 1K systems such as classic alkyd or acrylic paints impress with their very good flow, excellent adhesion and reliable workability, even under less than ideal climatic conditions. Acrylic resins are known for their high UV stability and clarity, alkyd resins for their elasticity and robustness, even if they tend to yellow.

Solvent-based stoving enamels, typically polyester or acrylic resins in combination with melamine resins, are particularly well established. They cure at temperatures of around 140-180 °C. The result is highly cross-linked, thermosetting films with exceptional hardness, chemical resistance and gloss retention. They are still indispensable today, especially in automotive OEM painting and coil coating. Their process reliability and proven performance over decades make them a standard that is difficult to replace with modern alternatives.

The strength of solvent-based systems is even more evident in 2K coatings. Polyurethane coatings are universal high-performance coatings with a unique combination of hardness, elasticity and chemical resistance. Acrylic-PUR systems combine brilliance and lightfastness with the resistance of polyurethane crosslinking and are irreplaceable in vehicle construction, aviation and high-quality mechanical engineering. Epoxy resin coatings, on the other hand, are the gold standard in heavy-duty corrosion protection, tank interior coatings and pipeline construction, as they offer unparalleled adhesion and resistance to aggressive media.

In terms of ecological assessment, it should be noted that modern solvents are increasingly being produced from renewable raw materials. Considering the often longer service life plus fewer recoatings, the environmental balance of solvent-based systems is quite positive.

The aim is to combine sustainability with the requirements of industrial practice. To this end, the coatings industry has developed high-solids systems that contain less solvent thanks to a higher solids content. This reduces VOC emissions without sacrificing the strengths of solvent-based chemistry. Hybrid solutions are also becoming increasingly important. They combine different types of binders and thus enable customized properties. At the same time, the importance of bio-based resins and sustainable raw materials is growing.

At present, these systems do not yet offer the same processing stability or endurance as the established technologies. Although water-based systems are also gaining market share, they are reaching their limits in industrial reality. The development towards sustainability is being driven forward. With its expertise in research and development, Stoz tests innovations at an early stage and offers customized solutions for economical, high-quality and sustainable industrial coatings.

Industrial coating systems reflect the balance between innovation, regulation and technical reality. Water-based systems make an important contribution to reducing emissions. However, on closer inspection, their ecological advantage is not always as clear as it seems at first glance and their performance remains limited in certain fields of application.

Solvent-based systems, on the other hand, have proven their strengths for decades and remain the benchmark for maximum performance, particularly in their two-component variants. Their broad base of experience, the long service life of the coatings and the increasing possibility of using bio-based solvents will continue to make them an indispensable component of industrial coating technology in the future. (OM-11/25)