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Modern 2K PU Water-Based Coatings: Efficient, High-Performance Eco-Friendly Technology

Turkchem 09 Jun 2017 19 8 dk okuma

TURKCHEM

Today's world is generating new demands. Environmentally friendly technology is one of the responsibility challenges facing a company on the path toward sustainable business practices. A constantly growing share of our customers have already recognized this reality. Analysts, investors and the general public are also calling for it. As a result, people and the media are turning environmentally friendly technology into an intense trend—a mega trend—in the global market.

Introduction

Today's world generates new demands. Environment-friendly technology and sustainable business practices represent one of the responsibility imperatives for a company.

An increasingly growing segment of our customers has become aware of this reality. Analysts, investors and the general public are also calling for it. People and media are thus making environment-friendly technology an intensive trend, a mega trend in the global marketplace

[1]. Years ago, the economy had already recognized the growing importance of both ecology and environmental protection responsibility principles. This is reflected succinctly in Bayer's current international slogan: "Bayer: Science for A Better Life". Research began on the opportunities offered by 2K PU (polyurethane) technology to reduce solvent content and thereby improve environmental compatibility. This led to the development of 2K PU very high solids (polyaspartic / Desmophen® NH) coatings that form an environment-friendly alternative to existing conventional systems and similarly low-emission water-based 2K PU systems

[2]. Water-based 2K polyurethane coating systems based on Bayhydrol®, Bayhydur® and Desmodur® offer users and coating manufacturers numerous options to meet the environmental compatibility, resistance and long service life requirements of coated objects. Another positive characteristic of water-based 2K systems is that the water present in dispersions and water-based 2K PU spray coating systems returns completely to the ecosystem naturally, namely through evaporation during the drying process after rain. In other words, nature only lends us water and then returns it to us. Furthermore, in the case of appropriately low VOC content water-based systems, the subsequent thermal afterburning process can be eliminated depending on the application area and maximum annual solvent consumption. This reduces CO2 emissions as well as operating and energy costs since additional firing with natural gas is no longer required.

Internal activation reduces energy costs and cuts CO2 emissions

A new generation of raw-material-based water-based 2K polyurethane coatings now offers improved performance. These are equivalent in level to solvent-based systems in terms of film appearance and resistance, are more environment-friendly due to their low solvent content and significantly reduce greenhouse gas emissions while also providing time and energy savings during drying due to lower oven temperatures, thus offering another cost advantage to the end user. This is achieved through the use of a feature known as "internal activation". What is this? When users want high productivity during the coating stage and therefore short hardening / drying times, conventional 2K PU coating systems typically contain DBTL or other similar external catalysts to accelerate cross-linking / drying. However, these have a major disadvantage: They significantly shorten pot life. Moreover, organic tin compounds are considered ecologically harmful. Internally activated Bayhydrol® A 2601 dispersion presents a completely different picture. In this case, harmless activation centers are incorporated almost entirely directly into the dispersion. Compared to conventional catalysts, they provide a significant advantage in that despite substantially accelerating drying, they do not shorten pot life. [1]

Depending on the application and substrate, oven temperature can therefore be significantly reduced compared to a solvent-based coating system, thus reducing both the end user's energy costs and CO2 emissions. Due to their good compatibility, internally activated Bayhydrol® A 2601 dispersion can be combined with PAC dispersions (for example, Bayhydrol® A 2470 or Bayhydrol® A 2695) and PU dispersions (for example; Bayhydrol® U XP 2766 or Bayhydrol® U XP 2750), allowing specific adjustment of the desired drying speed. The fact that this is not a physical drying but involves a chemical reaction is demonstrated using infrared spectroscopy, whereby residual free NCO groups from the low viscosity aliphatic polyisocyanate hardener (Desmodur® N 3900) are measured following the forced drying process (30 minutes at 60°C). In a comparison between a standard internally activated dispersion-based water-based 2K PU system and a coating based on internally activated PAC dispersion Bayhydrol® A 2601, IR analysis showed that 70% of NCO groups in the coating based on standard PAC dispersion remain uncross-linked. In other words, resistance properties to aggressive environments or water have not yet developed sufficiently over time at this point. In contrast to the coating based on internally activated PAC dispersion Bayhydrol® A 2601, all available OH groups for cross-linking are already fully cross-linked with the aliphatic polyisocyanate hardener Desmodur® N 3900, and only 30% of NCO groups remain uncross-linked. These result from 30% cross-linking of the coating system (NCO:OH ratio 1.3:1) and during the process they fully react with moisture in the air to form polyurea. [Figure 1]

Self-healing effect eliminates micro-scratches

New and environment-friendly manufacturing processes can be used to produce OH-bearing polyurethane dispersions with almost no solvent content. With appropriate selection of raw materials for dispersion design, these dispersions produce coating films with very advantageous properties. One possibility is, for example, a "self-healing" effect. Following scratching, the coating film exhibits a "self-regenerating" property, meaning a surface previously damaged by micro-scratches repairs itself by eliminating the micro-scratches and regaining its original gloss to approximately 98% as a function of ambient temperature and time (for example, 2 hours at 60°C starting without exposure to sunlight) [Figure 2].

Figure 2. Self-healing comparison test

Moreover, while such coating films are hard, they are quite flexible even at very low temperatures. This extraordinary series of properties makes the use of this polyurethane dispersion Bayhydrol® U XP 2750 particularly attractive for applications such as smartphone coatings, laptop and tablet PCs. Among owners of such luxury and accordingly expensive devices, interest in a coating with self-healing properties is very high, because the coating preserves a device's elegant appearance and thus helps maintain its value. Another application is automotive and transport vehicle coatings, where micro-scratches created during car washing no longer need to concern you. When a vehicle is cleaned at a car wash, micro-scratches can develop during drying with accumulated grit or very fine sand particles on brushes or dirty cloths, fabrics, sponges or chamois. Coatings can be applied to vehicle parts made of either metal or plastic. Self-healing is achieved through a targeted combination of a densely cross-linked network (necessary to provide the required resistance to environmental impacts such as tree resin, acid rain, insect secretions and aggressive bird droppings) with a high percentage of hydrogen bridges in the cross-linked film and flexible hard / soft segments between connection points. These allow the coating film to flex plastically (micro-scratching) when hydrogen bridge bonds are released, causing the film to creep elastically. When heated to a moderate temperature or a temperature between 30°C and 60°C (surfaces of motorized vehicles reach these temperatures as a result of sun exposure even in temperate climates), this process reverses at a molecular level; hydrogen bridges reform and micro-scratches repair themselves. This process can be triggered or accelerated by holding a common household hair dryer or infrared lamp to the scratched area for just a few minutes.

High gloss and high resistance

Modifying the molecular design of polyurethane dispersion not only results in very rapid hardening and drying of the coating film but also enables the production of another polyurethane dispersion Bayhydrol® U XP 2766 that exhibits excellent resistance to color fading due to prolonged sun exposure and heat. Another noteworthy property is that such varnish coatings and topcoats have extraordinarily high gloss due to the films' excellent pigment wetting and leveling properties. Due to its excellent compatibility, this new polyurethane dispersion Bayhydrol® U XP 2766 is also well suited as a combination partner for polyacrylate dispersions. This significantly increases the gloss of PAC dispersion, for example Bayhydrol® A 2695 [Figure 3] based coating systems and provides good gloss retention over the short term and in outdoor exposure conditions [Figures 4 and 5][3].

[caption id="" align="alignnone"] Xenon Test[/caption]

Figure 3. Water-based 2K topcoat gloss, white] Figure 4. Xenon test, water-based 2K varnish coating ]

Figure 5. Outdoor weathering, water-based 2K topcoat]. Since Bayhydrol® U XP 2766 appropriately formulated varnish layers and topcoats also provides high chemical resistance, its application area spans a wide range from classic metal coating to wood and furniture coating. For example, we tested the 16-hour resistance of a water-based 2K varnish layer formulated with Bayhydrol® U XP 2766 and air-dried at room temperature against red wine, coffee, ketchup, mustard and carrot puree (New mothers are well aware of the staining effect of carrot puree.) [Table 1]. Table 1. 16-hour resistance test, water-based two-component PU varnish layer

Summary

Water-based 2K polyurethane coatings based on a new generation of nearly solvent-free, OH-functional polyurethane dispersions Bayhydrol® U XP 2766, Bayhydrol® U XP 2750 and internally activated polyacrylate dispersion Bayhydrol® A 2601 are ideally suited for use in varnish layers and topcoats in transportation (for example trains, trucks, buses and the ACE market) and as well as for metal, plastic and wood surfaces.

Depending on the coating formulation, these have proven to be hard but sufficiently flexible even in cold conditions. Other noteworthy additional properties include the fact that the coatings in question are characterized by high gloss, weather resistance, high chemical resistance and, depending on the dispersion used, a pronounced self-healing effect following scratching, as well as very rapid drying, meaning that oven temperature for drying coated materials can be reduced to lower energy costs and CO2 emissions. These new polyurethane and PAC dispersions are already being used in transportation and general industrial sectors, as well as in wood and furniture coating applications.

The dispersions presented here have made it possible to use water-based products to meet and even exceed the requirements imposed in market segments previously dominated by conventional coating systems, and due to their low VOC content they are very environment-friendly. We expect significant advances in the use of water-based polyurethane coatings in the medium term. These new building blocks will therefore add a new chapter to the success story of aliphatic 2K polyurethane coatings.