Impressive Surfaces with Coil Coatings

A rapidly developing market, particularly in the Asia-Pacific region:

Coil coatings are known for delivering high quality at low total cost to the end user in the market [1]. Transfer efficiency in roller coater applications is one of the primary reasons for their popularity. In recent years, the Asia-Pacific region has surpassed Europe as a major consumer in the coil coating sector [2]. Coating pre-treated aluminum and steel coils using roller application to obtain colored products without the need for a paint shop is an extremely economical approach. Products for architecture (façades, roofs) as well as consumer goods (furniture, washing machines) can be cited as principal examples of this application, alongside tinplate food packaging. Demand for coil coatings is expected to show annual growth exceeding 7% in the Asia-Pacific region [2]. There are three fundamental reasons for this growth, which exceeds the combined annual growth rate:• The roller application process is highly efficient. Since overspray does not occur, very little paint is wasted. High-speed line application provides high product output during production. • Thermal incineration systems are a technological marvel for solvent-containing coatings. Solvents that cause air pollution are not released into the environment, and the combustion energy is used to heat the curing oven in roller application. This system delivers high production volume combined with an application process that enables efficient resource use and is environmentally friendly. As a result, a general reduction in total emissions is observed in work areas. • After the coating is applied and cured, the coated layers are wound into coils, stored for extended periods, or sent to customers. When necessary, coated surfaces can be wrapped, cut, drilled, bent to the desired shape, and divided into desired quantities. Thanks to these characteristics, coil coating offers a much more flexible production model compared to the problematic process of spraying only finished cast iron surfaces and curing with numerous paint-related issues such as drying. All these factors lead to lower production speeds compared to the roller coating process. However, the roller coating process can efficiently serve hundreds of metal processing facilities by being added to the beginning of the economic value chain of a single coating facility.

Rising Quality Demands in Coil Coatings

Since long-term warranties are provided for durable products such as roof panels, roof systems, and windows, coil coating manufacturers face rising quality requirements, which also involve improving the optical quality of the coated surfaces. The high belt speeds of these industrial serial coating processes and the deformation of coated sheets over time demand extraordinary requirements regarding the optical appearance of coil coatings. These coatings must have perfect surface flow, with no foam or air bubbles remaining on the surface. They must also demonstrate surface properties such as smoothness, scratch resistance, and wear resistance.

Perfect optical appearance with flow additives

Polyacrylates are the best-known flow additives for coil coatings. Due to their chemical structure, they are resistant to the high curing temperatures in the coil coating process. They demonstrate smooth surface properties immediately after coating application. Since the surface energy of the dried coating does not decrease, the wetting properties of the next layer are not critical, making them extremely suitable for multi-layer coatings [3].

Table 1: Impact of flow properties on optical appearance

*Polyester/melamine-based test system

Two new silicone-free flow and levelling additives have been specially developed by Evonik for use in solvent-based tinplate and coil coatings. The most important characteristic of both products is that they provide perfect coat-on-coat application in primer/topcoat systems or lithographic images thanks to their unique and reliable polyacrylate chemistry. To meet current and future health and environmental requirements, the products do not contain aromatic solvents such as xylene and toluene or higher aromatics. Additionally, they contain no halogens, formaldehyde, or BPA. The first additive, Tego® Flow 375, is suitable for general use in coil and tinplate coatings and is compatible with a wide range of binders. This characteristic provides an extraordinary, crater-free optical appearance in the cured system, particularly in transparent applications. You can see this effect in the following laboratory results:

Image 1: Perfect flow achieved by adding 0.4% Tego® Flow 375 to the formulation, applied with a spin coater

The second additive, Tego® Flow 460 N, in addition to improving flow, also contributes to air release. Due to its highly water-repellent nature, it prevents foam retention during application. It was developed specifically for use in pigmented systems. Achieving these two important characteristics in tinplate and coil coatings reduces complexity. You can see this effect in the following laboratory results:

Graph 1: Improved anti-foam properties in the formulation*

* Polyester/melamine-based test system [4]

Image 2: Improved anti-foam properties achieved by adding 0.4% Tego® Flow 460 N to the formulation, resulting in perfect optical appearance, applied with a spin coater

Both of these new flow additives have no impact on pigment stabilization in solvent-based tinplate and coil coatings during production, storage, or application.

Appearance: Further increase in optical requirements

Surface appearance demands for oven-cured paints (enamels) are steadily increasing. The future goal is to create first-class surfaces through the roller coating process. Currently, first-class surfaces can only be created through spray cone or spray gun application. For this reason, high-gloss surfaces for premium products (automobiles or consumer electronics) cannot be achieved through the roller coating process. Beyond the orientation of metallic or special effect pigment particles, the most important factor for first-class surfaces is the visual appearance of the coated surface. Only smooth surfaces with extremely low short-wave and long-wave haze appearance provide suitable surface coating for first-class products. Both newly developed additives can be used to optimize long/short-wave haze appearance by up to 80%. This allows coating manufacturers to follow the global trend toward first-class optical appearance.

Comprehensive Listing and Global Registration

Due to the global demands of coating manufacturers and applicators, it was decided in the early stages of product development that global listing is extremely necessary to support customer and end-user needs. Accordingly, both additives have been listed in major chemical inventories for general use (EINECS, TSCA, DSL, ENCS, AICS, ECL, PICCS, IECSC, TSCI, NZIoC) and comply with 21 CFR FDA regulation 175.300. Additionally, all components are listed in Annex 6 of the list of substances assessed (A) or not assessed (B) in accordance with Swiss Ordinance (Swiss Ordinance SR 817.023.21). These international registrations minimize the complexity experienced by coating manufacturers in usage, logistics, and formulation, and enable their use in various applications. Thus, coating manufacturers can use the same additive for more diverse end uses such as roof coatings or food storage tinplate.

Summary

End customer decisions are ultimately determined by high demands regarding quality and perfect visual appearance. New surface additives significantly improve application and visual appearance. In solvent-based tinplate and coil coatings, polyacrylates specially developed for this purpose provide excellent appearance. The composition of the products, their comprehensive listing and registration, permit general applications. Since these additives do not affect the surface energy of cured films, perfect coat-on-coat application is possible. Both additives comply with 21 CFR FDA regulations 175.300 and all components are listed in Annex 6 of the list of substances assessed (A) or not assessed (B) in accordance with Swiss Ordinance (Swiss Ordinance SR 817.023.21). • Tego® Flow 375 demonstrates broad compatibility with all binders and provides high optical clarity. • Tego® Flow 460 N provides air release in addition to flow improvement. • Both products are free from hazardous substances such as BPA, aromatics, and halogens. The perfect optical appearance they provide is attracting interest from tinplate and coil coating customers worldwide. Marco Heuer Evonik Paint Additives Industrial Paints Application Technology Department Director       Kirstin Schulz Evonik Paint Additives Industrial Paints Europe, Middle East and Africa Marketing Manager     Translation: Duygu Özgün Evonik Paint Additives Department Business Manager    

Sources: [1] 2015 IHS Chemical Economics Handbook | Paint and Coatings Industry Overview. [2] KNG 2014 Global Paint & Coatings 2013-2018. [3] http://www.tego.de/sites/dc/Downloadcenter/Evonik/Product/Tego/en/Technical-Background/slip-and-flow-additives,-radiation-curing-additives.pdf. [4] http://www.dynapol.com/sites/lists/RE/DocumentsAC/DYNAPOL-for-can-coating-EN.pdf.