Polyurethane is defined as a polymer containing urethane bonds being generated by polyaddition reaction of isocyanate (NCO) groups with hydroxyl (OH) groups.

Polyurethane coatings are already widely used as general-purpose products, and the development of polyurethane raw materials has steadily progressed in response to the demands of the age. Today, as social transformations towards achieving Sustainable Development Goals (SDGs) and Carbon Neutrality are ongoing, it is imperative for coatings industry to develop products and services that reflect on reduced environmental impact and triple bottom line (People, Planet, Profit). Additionally, there is a need to create new business models for transition from a linear economy of mass production, consumption, and disposal to a circular economy.

This article provides an overview of the history of polyurethane resin development for coatings, typical polyurethane raw materials, and considerations of its adaption to a circular economy.

History of Polyurethane Resin Development for Coatings

Coatings have been an essential material used by humanity for many centuries. Natural resins, oils and fats were used as the first raw materials for coatings. For instance, a natural resin called “Shellac” was utilized in India as a weather- resistant coatings for surfaces. Shellac, derived from the secretion of the lac insect was purified to produce coating resin and is considered the origin of the term "lacquer." Later it was discovered that lacquer resins could also be obtained from various other sources. For example, resin synthesis using oils extracted from wood became a common practice. In the early 20th century, the development of synthetic resins began and that led to improved and stabilized coating quality, resulting in a significant increase in coating production volume.

Polyurethane resin emerged in the 1930s when Heinrich Rinke developed 1,6-hexamethylene diisocyanate (HDI), and Otto Bayer invented polyurethane resin through the polyaddition reaction of isocyanates [1-3]. In the 1950s, polyurethane resin began to be adopted for coatings. Otto Bayer and his team discovered that the properties of alkyd resins could be enhanced by addition of diisocyanates. And then, the industrial use of polyurethane coatings began with the use of oligomers of toluene diisocyanate (TDI), which was the low-monomer containing polyisocyanate. However the applications of these aromatic polyurethane coatings were limited to indoor applications or primers due to tendency of yellowing on exposure to sunlight.

In the 1960s, Bayer AG introduced aliphatic polyisocyanate, HDI based curing agent, known as Desmodur® N, to the coating industry. This opened the door for outdoor applications. The two-component polyurethane coatings, consisting of polyisocyanate curing agents (Desmodur® N) and base resins (Desmophen®, polyols), were referred to as “DD coatings”. These coatings gradually replaced the traditional alkyd coatings. The initial adoption of DD coatings happened in applications of large vehicles such as aircraft, railway vehicles, and buses. The driving force behind these adoptions was fast drying without baking process, high weathering resistance, and excellent film properties.

In the 1970s, polyurethane coatings began to be adopted for automotive refinish. While alkyd coatings were predominant in automotive refinish at that time, it was discovered that the addition of isophorone diisocyanate (IPDI)-based curing agents into alkyd coatings improved significantly film hardness, recoatability, and gasoline resistance. Today, two-component polyurethane coatings have almost completely replaced the alkyd coatings in this segment.

Due to the unique performance of polyurethane resins, such as excellent film properties, high reactivity, durability, and diversity of formulation design, the adoption of polyurethane coatings has been expanding in various applications such as wood coatings, corrosion-protection coatings, architectural coatings, and textile coatings.

In the 1980s, polyurethane coatings were adopted for automotive OEM (Original Equipment Manufacturer) application. ??During?the transition of bumper materials from iron to urethane, flexible and impact resistant polyurethane coatings were utilized for the protection of automotive plastic materials. Additionally, in the late 1980s, acid rain became a global concern, leading to the replacement of acrylic melamine coatings, previously used for car bodies in Europe, with polyurethane coatings.

In the 1990s, the reduction of volatile organic compounds (VOCs) became a challenge for air pollution control, urging active development in waterborne coatings. It initially seemed nonsense to use polyisocyanates in waterborne coatings since they are hydrophobic and react with water in the coatings. However, incorporating hydrophilicity to polyisocyanates made it possible to disperse them easily in water, enabling the formulation of waterborne two-component polyurethane coatings. As base resins of waterborne two-component polyurethane coatings, acrylic dispersions and polyester dispersions containing OH groups were also developed. Additionally, the development of high molecular weight polyurethane dispersions (PUD) with physical drying properties for one-components polyurethane coatings were also begun.

The global coatings consumption is estimated 49 million tons, 130 billion euros in the market value as of 2017. The corresponding coating resins consumption (supply form) amounts to 16 million tons, 44 billion euros. Polyurethane resins accounts for 1.6 million tons, 10% of the total resin consumption.?The main components of polyurethane resins for coatings are polyisocyanates, polyurethane dispersions, NCO-terminated prepolymers, polyols, and diamines [4]. The breakdown is estimated as aliphatic polyisocyanates (13%), aromatic polyisocyanates (8%), polyurethane dispersions (11%), and others co-reactant resins like polyols, amines, etc. (68%) [5].

The substitution of traditional technologies in coatings is ongoing with polyurethane technology, fulfilling technical requirements in various coating applications. The development history of polyurethane resin for coatings is illustrated in Figure 1.

Figure 1: History of polyurethane product line development for coatings.

There has been growth in the use of high solid, solvent-free, waterborne and radiation-curing formulations of one- and two-components polyurethane coatings in the world. Today, the development of next-generation polyurethane resin products composed of renewable feedstocks is ongoing, aligning with initiatives for Sustainable Development Goals (SDGs) and the transition to a carbon neutral society.

Diisocyanates

Polyurethane is synthesized through the polyaddition reaction of polyols and polyisocyanates. Polyisocyanates for coatings are synthesized through the oligomerization of monomeric diisocyanates. These diisocyanates are industrially produced through phosgenation reactions in the liquid or vapor phase from corresponding primary amine derivatives [6]. For specific aliphatic diisocyanates, manufacturing processes that do not involve phosgene have been developed. In this production process, urethane is produced by reacting amines with alcohols to form urethane. The generated urethane is then decomposed at high temperatures to produce isocyanates [7].

Polyisocyanates used in the field of coatings encompass derivatives of several aliphatic, aliphatic-aromatic, and aromatic diisocyanates. The diisocyanates which are industrially used in the coatings and adhesives applications are shown in Table 1.

Table 1: Industrial diisocyanates.

With the exception of MDI (Methylene Diphenyl Diisocyanate), most monomeric diisocyanates have high vapor pressures and volatility. Due to occupational health and safety considerations, monomeric diisocyanates are generally not used as raw materials for coatings and adhesives. To mitigate the harmful effects of monomeric diisocyanates, it is mandatory to reduce their volatility by transforming them into higher molecular weight polyisocyanates through modification (oligomerization).

Schematic Reaction Principals of Isocyanates

Isocyanates react with active hydrogen compounds, such as polyols, polyamines etc. The schematic reaction principals of isocyanate groups are shown in Table 2. Using these reactions, polyisocyanates (oligomers) as coating raw materials are manufactured. Particularly, polyisocyanate prepolymers, biurets, trimers, dimers, and allophanates are crucial as coating raw materials.

Table 2: Schematic reaction principles of isocyanates.

The urethane formation reaction rate varies significantly based on the type and structure of polyisocyanates and the choice of co-reactants. For example, the reactivity of aromatic polyisocyanates with alcohols is much higher than that of aliphatic polyisocyanates. In aliphatic polyisocyanates, the reactivity of primary isocyanate groups is highest and followed by secondary and then tertiary. It is also influenced by stereo-isomeric structures and catalytic effects. Appropriate catalysts must be selected for each application [8]. The reactivity of typical monomeric diisocyanates is shown in Table 3.

NCO groups have extremely higher reactivity with primary and secondary amines, rapidly resulting in polyureas even at room temperature, than that with polyols. Under elevated temperatures and in the presence of suitable catalysts, ureas and urethanes are formed by reacting excess isocyanate groups, yielding biurets and allophanates, respectively.

Table 3: Reactivity of monomeric diisocyanates.

Polyisocyanates

Polyisocyanate products with an average functionality of more than two, manufactured through the oligomerization reaction of diisocyanates, react with active hydrogen compounds to form three-dimensional crosslinked polyurethane structures. The three-dimensional crosslinked structures in cured films ensure the durability and performances of the coating layers. Depending on the choice of diisocyanate, the characteristics of polyisocyanates are determined. For instance, aliphatic polyisocyanates show superior light resistance compared to aromatic ones. Cured films from polyisocyanates consisting of cycloaliphatic diisocyanates can be hard and brittle, but flexibility can be achieved by selecting the appropriate polyol as the co-reactant. Conversely, cured films from polyisocyanates consisting of linear diisocyanates like HDI are soft and flexible, and the film hardness can be tailored by the selection of appropriate polyols commercially available in the market. It is important not only to fulfill the film properties but also to have longer pot life in painting work. Adjustment of reactivity during film formation to have enough pot life is possible by selection of polyisocyanates. It should be considered that the pot life of coatings is influenced by the stereo-, electronic- effects, and NCO group functionality of the chosen polyisocyanate. For example, addition of IPDI-based polyisocyanate in HDI based polyisocyanate prolongs the pot life of two-component polyurethane coatings. Further adjustments can be possible by the selection of appropriate catalysts.

The characteristics and typical applications of polyisocyanates used as coating raw materials are shown in Table 4.

Table 4: Characteristics of Aliphatic/Aromatic polyisocyanates (selection).

The average molecular weight and molecular weight distribution of the polyisocyanate correlate with the modification degree (polymerization degree) of monomeric diisocyanates, allowing customization of product characteristics such as isocyanate equivalent weight, average functionality, and viscosity. In cases of high polymerization degree, polyisocyanate with higher average molecular weight are obtained, resulting in higher isocyanate average functionality, but also resulting in higher product viscosity. Conversely, low polymerization degree leads to lower average molecular weight oligomers with relatively lower average functionality but higher isocyanate group content, resulting in lower product viscosity. Consequently, products with high polymerization degree, in other word with higher average molecular weight of the oligomer improve the drying properties of the coating film. Conversely, products with low polymerization degree, as the average molecular weight of the oligomer decreases, have slow drying properties of the coating film. However, the low viscosity of the product enables formulation in high solids coatings or solvent-free coatings. Various polyisocyanate curing agents have been developed to meet application-specific requirements and the representative aliphatic and cycloaliphatic polyisocyanate curing agents are shown in Figure 2.

Figure 2: Typical Aliphatic PU hardeners.

Prepolymers

NCO-terminated prepolymers are produced through the reaction of a long-chain polyol with an excess of diisocyanate. Various polyols, including polyether polyols, polyester polyols, and polycarbonate diols, are utilized, and unreacted excess diisocyanate monomer is removed by distillation as required. The characteristics of these prepolymers can be freely designed based on the choice of hydroxyl-containing compounds (type, molecular weight, functionality, etc.) used. Polyisocyanates react with polyols to produce polyurethanes, and polyisocyanates also react with polyamines or water to produce polyureas. The resulting products of these reactions are straight or branched polyurethane or polyurea prepolymers.

NCO-terminated polyurethane prepolymers are widely adopted in various applications due to their high reactivity towards active hydrogen compounds (OH, NH, SH). Their common use includes not only as curing agents for two-component polyurethane or polyurea coatings but also for one-component moisture-curing polyurethane coatings.

Thermally Activated Polyurethane Curing Agents

Ideal coatings are those that exhibit good storage stability as a one-component system and form robust films through chemical bonding. However, it seems impossible to meet these requirements due to the inherent properties of polyisocyanates mentioned earlier. Nevertheless, by chemically masking the NCO groups of polyisocyanates with blocking agents, the masked polyisocyanates “thermally activated polyurethane curing agents” make their NCO groups inactive towards reactive partners such as polyols in coating formulation, enabling the formulation as one-component coatings.

One-component polyurethane coatings using thermally activated polyurethane curing agents undergo a process of dissociation of the blocking agents and then NCO groups are reactivated during baking period in an oven. These reactivated NCO groups then react with the reactive partner, typically polyols, to form a robust polyurethane coating. As an exception, the curing behavior of thermally activated polyurethane curing agents using malonic acid ester blocking agents are different, i.e., the crosslinking does not occur through dissociation of blocking agents but via transesterification reactions to form the coating film [9].

One-component polyurethane coatings are easy to handle compared to two-components polyurethane coatings since they do not require a mixing equipment, accurate measurement of main component/hardener ratio, and can be used without regard to pot life. However, to obtain desired coating film properties, a high-temperature baking process is mandatory. The baking temperature varies depending on the type of blocking agent. An appropriate thermally activated polyurethane curing agent must be selected with consideration of the factors such as the desired coating film properties, reactivity, resistance to overbake yellowing, and cost. In coating industry, blocking agents such as methyl ethyl ketoxime, dimethyl pyrazole, ε-caprolactam are commonly used with aliphatic, and aromatic polyisocyanates.

Thermally activated polyurethane curing agents, when used alone, have a dissociation temperature of the blocking agent within the range of decomposition temperature of urethane bond (approximately 240°C) and maintain good thermal stability. However, in the presence of a reactive partner, i.e., when formulating a one-component coating with polyols, the blocking agent dissociates at lower temperature which is determined by the type of blocking agent, resulting in a cured coating film. The use of catalysts allows for a reduction in the baking temperature to some extent [10]. Typical thermally activated polyurethane curing agents is shown in figure 3.

Figure 3: Typical thermally activated PU hardeners.

Hydrophilic Polyisocyanates

Hydrophilic polyisocyanates enable the use of waterborne two-component polyurethane coatings as an alternative to solventborne coatings in various applications, providing a low VOC coating option. In waterborne two-component polyurethane coatings, careful consideration must be given to the selection of waterborne polyol dispersions, polyisocyanate curing agents, mixing methods, choice of cosolvents, application conditions, and pot life, as these factors can influence the final coating properties.

Regarding the choice of curing agents for waterborne two-component polyurethane coatings, there are two options. One is the use of hydrophobic polyisocyanates, preferably with low viscosity, which are usually employed in solventborne two-component polyurethane coatings. The other option is hydrophilic self-emulsifying polyisocyanates.

When using low-viscous hydrophobic polyisocyanates as curing agents in waterborne two-component polyurethane coatings, they need to be dispersed in base component by applying high shear force. On the other hand, hydrophilic polyisocyanates can be uniformly dispersed in base one by hand stirring. However, in both cases, dilution of the curing agents with appropriate solvents tends to achieve more uniform dispersion, resulting in a glossy and smooth coating film.

There are several methods for the production of hydrophilic polyisocyanates. Besides using external emulsifying agents, active emulsifying compounds are incorporated into the hydrophobic polyisocyanate. For instance, introducing a small amount of monofunctional polyethylene oxide (polyether chain) into aliphatic polyisocyanates, e.g., HDI-based or IPDI-based polyisocyanates, enables the synthesis of nonionic water-dispersible polyisocyanates. However, the introduction of hydrophilicity by polyether chains can reduce the average functionality of the hydrophilic polyisocyanate curing agents. As a result, the crosslinking density of the cured film may decrease, leading to a deterioration in coatings film properties such as chemical resistance and weatherability [11]. This issue can be solved by allophanate modification, i.e., further reaction of NCO group of other polyisocyanate with the urethane bond in the polyether-modified water-dispersible polyisocyanates. This allophanate-modified hydrophilic polyisocyanate (second generation) increases the average functionality of NCO groups, resulting in higher crosslinking density and improved film properties. While these polyether-modified polyisocyanates are widely adopted as curing agents in waterborne two-component polyurethane coatings, their nonionic polyisocyanate structure may induce drawbacks in film properties, such as reduced drying performance, poor water resistance and durability due to the permanent hydrophilicity.

To overcome the drawbacks of polyether-modified polyisocyanates, a third generation of hydrophilic polyisocyanates has been developed. In this process, aliphatic polyisocyanates are reacted with the ionically active amino sulfonic acid, e.g., 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) in the presence of quaternary neutralizing amines. The resulting CAPS-modified polyisocyanate is ionic but remains clear, exhibiting excellent storage stability and dispersibility in water. This third-generation hydrophilic polyisocyanate is applicable to meet high-quality coating performance requirements, such as those needed for automotive coatings, large vehicle coatings and anti-graffiti coatings [12]. Additionally, low-viscosity variants of the third-generation products, can be easily dispersed in water without solvent dilution, making them suitable for environmentally friendly zero-VOC coatings. By using these chemically bonded hydrophilic polyisocyanates, there is no adverse impact on the film quality caused by migration of free emulsifying agents like in case of external emulsifier. Typical hydrophilic polyisocyanate curing agents are shown in Figure 4.

Figure 4: Typical hydrophilic PU hardeners.

Polyurethane Dispersions

Aqueous dispersions have gained significant importance for use in waterborne two-component as well as waterborne one-component polyurethane technology. Polyurethane Dispersions (PUD) consist of polyurethane or polyurethane/polyurea polymer particles dispersed in water or a water/organic solvent mixture as a continuous phase. Generally, PUD polymers have a high molecular weight and a linear structure, resulting in excellent physical and chemical properties. Unlike solventborne systems, the viscosity of PUDs is not influenced by the molecular weight of polymers therefore it is applicable for spray applications even its very high molecular weight.

PUDs comprise of OH-containing resins, isocyanates, amines, and hydrophilizing agent, as key components. As for OH-containing resins, polyester polyols, polyether polyols, polycarbonate polyols, and diols like 1,6-hexanediol, neopentyl glycol, butanediol, and ethylene glycol are often used. As for isocyanates, aliphatic diisocyanates like HDI, IPDI, H₁₂-MDI are used for coating applications. As for amines like hydrazine hydrate, ethylene diamine, diethylene triamine are used as chain extenders for increasing the molecular weight of PUD. On the other hand, monofunctional amines like diethyl amine or monoalcohols like butanol, amino alcohols like ethanolamine, diethanolamine act as chain terminators to control the molecular weight of PUD in chain extension reaction. Tri-functional amines or triols are used to introduce branched structures to improve the physical and mechanical properties of the coating film. Since polyurethane polymers are inherently hydrophobic, it is mandatory to incorporate hydrophilizing agent like dimethylolpropionic acid, 2-[(2-aminoethyl)amino]ethane sulfonic acid sodium salt in polymer backbone for better dispersibility and stability of PUDs in water.

PUDs are used in various coatings and adhesives application fields. Non-functional PUDs, which has higher molecule weight, are often used as one-component polyurethane coatings e.g., for wood, plastic coatings, and textile coatings. OH-containing PUDs, which have relatively lower molecule weight, are used for two-component polyurethane coatings. Crystalline PUDs are particularly important in heat-activated adhesives for automotive, furniture, and footwear applications [13].

PUDs provide flexibility, hardness, excellent mechanical properties, durability, soft tactile sensation qualities, and appearance in coating films. To utilize proper PUDs for targeted applications, it is essential to understand the complex relationship between polyurethane structure and performance, as well as morphology. For example, important and distinctive factors such as crystallinity, Tg (glass transition temperature), branching degree, and balance of soft-hard segments need to be considered. Soft and hard segments in the polymer backbone create soft and hard domains during film formation. By controlling the morphology balance of these domains, the physical and mechanical properties of the film are influenced, resulting in different stress-strain relationship, which directly correlate to physical and mechanical properties [14]. The other unique property of PUD is the hydrogen bonding between urea-urethane linkages in the polymer backbone. With the secondary crosslinking through hydrogen bonding, the physical and mechanical properties such as hardness, chemical resistance, impact resistance, and abrasion resistance etc. can be enhanced.

Moving Towards Circular Economy

"Sustainable development" is succinctly defined in the Brundtland Report (1987) as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [15]. The Paris Agreement in 2015 established goals for climate action to prevent global warming, and in 2016, the United Nations adopted the Sustainable Development Goals (SDGs) with targets for achieving a sustainable society by 2030.

In addition to the primary roles of coatings such as beautiful decoratability, protection, and functionality, it is essential today to meet SDGs and carbon neutrality targets. Especially the transition from a linear economy to a circular economy with focus on the 3Rs (Reduce, Reuse, Recycle) is crucial for realizing a sustainable society. The circular economy involves concepts of the extension of product lifespans and the recycle of materials from the final stages of a product's life cycle.

In the design of coatings, it is important to consider not only the required performances but also the product's life cycle. This consideration involves four steps shown in Figure 5:

? Alternative raw materials, ? Efficient and Safety processing, ? Use phase performance, and ? Enabling circularity.

Figure 5: PU Solutions for Sustainable Life Cycle in Coatings.

Alternative Raw Materials

For the transition to a circular economy, it will be imperative that products formulated in coatings are manufactured from renewable feedstock in future. In the realm of polyurethane-related products, derivatives of 1,5-pentamethylenediisocyanate (PDI), manufactured from bio-based feedstock, instead of fossil-based one, are already on the market.

For instance, the partially bio-based PDI derivative” Desmodur® CQ ultra N7300” contains 71% renewable carbon and has been approved as a curing agent for clear coats in automotive applications, replacing standard fossil-based HDI based curing agents [16]. Additionally, Covestro has also introduced new products such as hydrophilic polyisocyanate curing agents and thermally activated polyurethane curing agents based on partially bio-based PDI derivatives on the market.

As shown above mentioned examples, today chemical industry focuses on development of bio-based products for the realization of circular economy. However there are issues in relation to stable supply and high price of renewable feedstocks. Thus, there are risks about the difficulty of securing the renewable feedstocks for future demand expansion. Additionally, the optimization of manufacturing cost through bioprocessing is also a current challenge. As an immediate measure, it is recommended to use products manufactured through the mass balance method. The mass balance is a method to account for the mass of substances involved in the process. It involves tracking the inputs, outputs, and accumulation of mass from bio-based and fossil-based raw materials within a system to ensure that mass is conserved. The allocation of renewable content to the final product is determined based on the input of bio-based raw materials.? This method allows for the use of existing facilities without changing production processes or methods, maintaining the quality of the final bio-attributed products at a level equivalent to conventional fossil-based products. Therefore, users can use these mass balanced products in the same way as the conventional ones. The mass balance method for MDI is shown in Figure 6. Covestro has already produced MDI and TDI using the mass balance method, and the businesses such as mattresses and adhesives are emerging utilizing these mass balanced products.

Figure 6: Mass balance MDI production.

Efficient and Safety Processing

For occupational health reasons, monomeric diisocyanates are not used as raw materials for coatings. To avoid the hazardous potential with monomeric diisocyanates, they must be converted into oligomers with below 0.1% monomer contents especially to comply with European REACH regulations in coatings and Covestro has introduced ultra-grade polyisocyanate curing agents that meet the regulatory requirements for coatings in Europe.

The high reactivity of polyurethane/polyurea coatings make the construction period shortened and thus it brings economic benefits by saving labor cost, especially for field work applications. For instance, aliphatic polyurea system (Pasquick®) consisting of polyaspartic acid ester and aliphatic polyisocyanate allows for quick-drying properties, excellent weather resistance, high solids formulations, and reduction of construction period.

DirectCoatings with polyurethane coating system is a new process for producing functionally integrated and decorated plastic parts. Solvent-free two-component polyurethane coating is applied with reaction injection mold (RIM) technology after a plastic part is produced in a mold. This contributes to high cost-effectiveness, short cycle times and a simple process removing cleaning the plastic parts and delivering plastic parts to paint shop.

This DirectCoatings technology allows for unique design features e.g., glossy- and matt- surface on one plastic part in one RIM shot, and film insert molding. These design features are not achievable with traditional painting methods and thus it has begun to be adopted in automotive industry.

Moreover, groundbreaking painting process with advanced polyurethane technology have begun in original equipment manufacturer (OEM) automotive painting at Nissan, contributing not only to improve production efficiency but also to reduce environmental impact. This is an integrated painting process i.e., car body (metal part) and bumper (plastic part) are coated with the same coatings and baked at a low temperature of 85?, reducing both CO₂ emissions and energy consumption. This marks a significant step toward carbon neutrality targets in the automotive industry.

Use Phase Performance

Generally, coatings are employed to protect product surfaces from corrosion, degradation, and mechanical damage, thereby the service life of the coated products is enhanced, and it contributes to resource conservation. In other word, the extension of maintenance periods contributes to “circular economy” and offers an economically effective and sustainable solution. Polyurethane coatings, known for their high durability and excellent protective performance, have proven to be effective solutions in corrosion protection coatings, marine coatings, automotive coatings, and architectural coatings etc., contributing to resource conservation.

From the perspective of reducing volatile organic compound (VOC) emissions, there has been a transition from solventborne coatings to high-solid coatings, powder coatings, and waterborne coatings. In the realm of polyurethane resins, products and system developments are actively taking place. For instance, waterborne two-component polyurethane coatings show nearly equivalent properties to solventborne counterparts, enabling them to be a viable substitution (refer to Table 5) [17]. However, it should be noted that facility modifications, such as changes to painting equipment, drying ovens, and wastewater treatment, are probably necessary to shift to waterborne coatings.

Table 5: Performance of Waterborne 2K PU coatings and Solventborne 2K PU Coatings.

Enabling Circularity

It is crucial to make a conceptual design considering for recycling of coating ingredients at drafting of coating formulations. This is to say, the ideas of recyclable coatings such as the use of single-source raw materials, easily separable structures, and formulations anticipating recycling possibility by chemical decomposition, smart pyrolysis, or biodegradability should be taken into account. In the polyurethane foam industry, research on chemical recycling and smart pyrolysis is advancing. Covestro in collaboration with the value chain players has developed an innovative process to chemically recycle soft polyurethane foam from discarded mattresses. This project called “PUReSMART” has successfully been in operation in Europe [18].

However it is nearly impossible to separate raw materials from coating formulation compounds and recycle the raw materials, since coatings are typically applied in multilayer-structures and composed of various materials such as different resins, hardeners, pigments, fillers, and additives and so on. As one of alternative solution, peelable body painting was developed by Toyota and has already launched as a car leasing service” KINTO”. The car owners can choose their favorite colors, or peel off the paint to restore the original. In another example, BMW's i Vision Circular Concept Car aims for a 100% usage rate of recycled materials and 100% recyclability to optimize the closed material cycles. The car's body is made of color-alumite treated aluminum instead of traditional coatings. While this move raises questions about durability aspects such as corrosion protection and chipping resistance, the option of above-mentioned coating-less surfaces might gain traction in societies prioritizing sustainability.

Conclusion

While there are globally continuing deteriorations in social situations such as crises in the Red Sea, Gaza, and Ukraine, climate change-induced disasters, the disruption in international logistics, soaring raw material and fuel prices, we must strive towards realizing a sustainable society. In the transition to a circular economy, polyurethane is obviously one of excellent materials that ensures the safety and health of workers in the painting work, shortens construction period, and extends product lifespans. And needless to say, polyurethane contributes to reducing CO₂ emissions for carbon neutrality.

Covestro is actively engaged in developing polyurethane products using renewable feedstocks today. Through collaboration with industry stakeholders of a circular value chain, we should create added value in new business models.

By innovation and diligently working towards the realization of a globally sustainable society, we make the world brighter place.

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