High-Solids Resin Design: An Eco-Friendly Approach

High-Solids Resin Design: An Environmentally Friendly Approach In the United States, according to the definition of the Environmental Protection Agency (EPA), volatile organic compounds (VOCs) mean any carbon compound, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates and ammonium carbonate, that participates in atmospheric photochemical reactions [1]. Essentially, volatile organic compounds (VOCs) are chemical substances with organic structure, low boiling points and easy evaporation. The release of VOCs into the atmosphere can trigger ozone formation, negatively affecting air quality in the troposphere. This situation can lead to increased tropospheric ozone levels and exceedance of air quality standards. Additionally, by contributing to the formation of secondary pollutants, it can cause widespread air pollution. Apart from environmental damage, exposure to high concentrations of VOCs through inhalation or skin contact can cause various problems for human health. In recent years, considering all harmful factors, strict legal restrictions regarding VOC limits have been introduced worldwide. These are primarily enforced through the EPA and the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) in the EU. The REACH regulation, which came into force in 2007 by the European Union, provides a framework for the registration, evaluation, authorization and restriction of chemical substances. This regulation aims to minimize environmental impacts and provide high standards for human health and environmental safety. Paints and coatings industry is subject to the obligation to register and declare chemical substances used in their products within this scope. At the same time, REACH supports environmental sustainability by restricting the use of potentially hazardous substances. The EPA implements various regulations to reduce environmental impacts. Through special regulations created for the paints and coatings sector, it controls VOC use and establishes standards limiting VOC emissions. Given the global legal restrictions on VOCs used in the paints and coatings sector and their effects on the environment and human health, the sector's orientation towards sustainable and environmentally friendly alternatives is of critical importance. These legal restrictions encourage collaboration across various stages of the industry—from production to consumption—to control environmental and human health impacts. This in turn increases the marketing of lower VOC content and environmentally compatible paints and coatings, supporting a sustainable future. Among the zero VOC or low VOC policies that companies implement in their systems, solvent-free powder coatings, water-based systems where organic solvents are completely or partially replaced by water, and UV-curable coatings are particularly noteworthy [2]. Another increasingly common method used to reduce organic solvent emissions is to increase the solids content of solvent-based coating and paint systems [3]. High-solids paints are solvent-based or solvent-free paints containing larger amounts of pigment and binder than traditional solvent-based paints. High-solids paints offer significant advantages in many important areas: their much lower solvent content compared to traditional materials aims to minimize solvent emissions, while also providing benefits such as low paint consumption, short curing time, low fire hazard, and cost improvement by reducing the amount of coating and paint layers on surfaces. With new technologies such as the use of high-solids resins, it is possible to reduce VOC levels by preserving the performance of traditional products and developing high-solids paint systems. Due to the low-solvent nature of high-solids binders, the most important criterion to be considered is low viscosity. For this reason, when implementing high-solids resin design, the parameters that strongly affect viscosity should be observed and modifications should be made to comply with requirements. The main factors affecting viscosity in a binder system include hydrogen bonds formed between polymer chains, the selection of monomers that will constitute the polymer structure, the molecular weight of the polymer structure, the structure of the polymer chain and the free volume created between chains, functional side groups on the chain and the selection of solvent to be used [5]. Hydrogen bonds are strong intermolecular forces between hydrogen atoms of groups known as hydrogen bond donors such as –OH, –SH and –NH and hydrogen bond acceptors such as carboxyl or ester groups. These strong bonds that can form between chains in the polymer structure cause viscosity increase in resins. The selection of monomers and other building blocks to be used in resin synthesis is also a potential approach in adjusting viscosity. In general, long-chain monomers provide lower viscosity because they create a larger distance between polar groups that can form hydrogen bonds within the polymer structure. Additionally, the solution viscosities of polymers prepared using monomers containing cycloaliphatic compounds are considerably lower than those of polymers containing short side chains or aromatic compounds [6]. The linear proportionality between the average molecular weight of a polymer and its viscosity can be defined through the Mark-Houwink equation ([η] = k · Mα). However, generally, as molecular weight decreases, drying properties are affected and mechanical and chemical properties may also change proportionally. For this reason, molecular weight balance is among the most important points to be considered when designing high-solids resin synthesis. For low molecular weight polymer designs, optimization of the hydroxyl value of the polymer and thus balancing the higher cross-link density it will have can make it possible to preserve the final properties of the coating. The structural architecture of the polymer is of great importance in resin viscosity. To obtain lower viscosity, the main structure constituting the polymer should not have hydrogen bond donor groups and segments with high glass transition temperature (Tg). The parameters affecting the glass transition temperature in polymers are stated as the molecular weight and structural content of the polymer. Polymers with low molecular weight have short chain lengths and high chain end density in their structure. The high amount of chain ends in the polymer structure increases the free volume in the structure. For this reason, the mobility of polymer chains is higher than polymers with high molecular weight. Thus, the increase in molecular weight of polymers raises the glass transition temperature. This results in high viscosity. This trend is generally observed in all semicrystalline and amorphous polymers. All structural properties that reduce the flexibility of the polymer chain and restrict the mobility of chains in the polymer structure increase Tg. Another important factor causing high viscosity in resin solutions is that the main polymer chains of medium and high molecular weight resins have very long linear structure. This results in chain entanglement and prevents chain mobility. Resin design with a more spherical structure is one of the approaches aimed at reducing polymer chain entanglement. Such polymeric structures have a branched structure. Resin molecules with a higher degree of branching compared to conventional resins are called hyperbranched resins or dendrimers [7]. These types of polymers have lower resin viscosity. Therefore, less solvent is needed to achieve the application viscosity of the entire system. The selection of solvent to be used in resin design is very important to reduce the contribution of strong hydrogen bonding that may form between functional groups in the main chain structure of polymers and the polar structure of side groups on the chain. Due to the limited amount of solvent contained in high-solids resins, solvents should be selected from those that act as hydrogen bond donors or hydrogen bond acceptors, minimizing hydrogen bonding between chains in the structure and creating hydrogen bonds with themselves and the chains. Solvents evaporate during film formation and interactions between chains are re-established, thus contributing to physical drying and mechanical properties. The use of reactive diluents to reduce viscosity is another practical approach for high-solids resins. These are very low molecular weight and low viscosity compounds that can react with polymer chains and thus be incorporated into the cured film, possessing functional groups. Continuous developments in the paint industry are increasing the importance of environmental awareness and sustainability-focused work. In this context, the Polymer R&D unit within Kanat Boyacılık Tic. ve San. A.Ş. carries out studies on low VOC resin design and production. Additionally, it continues its work on water-based resin technologies. In this way, it contributes to the company fulfilling its environmental responsibility in line with its sustainability principles by reducing its carbon footprint.   References [1] Dinh, T.-V. et al. (2016) 'Volatile Organic Compounds (vocs) in surface coating materials: Their compositions and potential as an alternative fuel', Journal of Environmental Management, 168, pp. 157–164. doi:10.1016/j.jenvman.2015.11.059. [2] Pélissier, K. and Thierry, D. (2020) 'Powder and high-solid coatings as anticorrosive solutions for Marine and offshore applications? A Review', Coatings, 10(10), p. 916. doi:10.3390/coatings10100916. [3] Goldschmidt, A., & Streitberger, H.-J. (2018). BASF Handbook: Basics of Coating Technology. Vincentz Network. [4] Goldschmidt, A. and Streitberger, H.-J. (2018) BASF Handbook: Basics of Coating Technology. Hannover, Germany: Vincentz Network. [5] Wicks, Z.W. (2008) Organic coatings: Science and technology /. Chichester, West Sussex etc.: Wiley. [6] Carraher, C.E. and Seymour, R.B. (2008) Polymer chemistry. Boca Raton, FL: CRC Press. [7] Lu, Y.Y. et al. (2012) 'Intrinsic viscosity of polymers: From linear chains to Dendrimers', EPL (Europhysics Letters), 97(6), p. 64003. doi:10.1209/0295- 5075/97/64003.   Gülten Özkul Atlı Senior Researcher - Polymers Kanat Boyacılık Tic. ve San. A.Ş.