Water Based Polyurethanes (PUD)

Polyurethanes, as is well known, were discovered by Otto Bayer in 1937 and continue to form an expanding and specialized sector. Turkey ranks among countries showing rapid growth in the polyurethane field, and due to its geographical position, it is a preferred country for polyurethane supply to many nations. When we classify polyurethane product groups, we can structurally divide them into two categories: foam or non-foam. Rigid foam products serve thermal insulation, while flexible foams form systems such as sponges and similar applications. Of course, we could expand the product groups here further. Non-foam products are structurally foam-free and comprise coatings, adhesives, elastomers and sealants, abbreviated as Polyurethane CASE. These CASE products have much lower tonnage volumes compared to foam products, but their application areas and variations are extensive. In foam product groups, a mixture of raw materials such as polyols, catalysts, stabilizers, etc. generally forms one component. The other component is isocyanate/isocyanate prepolymers. These two components are mixed in predetermined ratios using machinery for application, meaning the actual reaction occurs at the point of application. In non-foam products, the actual synthesis occurs in reactors and follows a systematic approach. By changing the process with the same raw materials and identical percentage formulations, you can obtain different products. For this reason, reaction systematics become very important in this type of synthesis. The product formed in reactors merely takes its structure at the point of application. For example, when a solvent-based product is synthesized, you remove the solvent at the application point to obtain the polyurethane resin from the product according to your application method. These types of products can be two-component polyurethane resins containing 100% solids, single-component polyurethane resins dissolved in solvent, and single-component polyurethane resins dissolved in water. Looking at the polyurethane synthesis system in the CASE field, we see a systematic synthesis conducted with different stoichiometric calculations of polyols, chain extenders and isocyanates. We need to know that each raw material used will have different final product properties due to their different characteristics. There are certain barriers in this type of synthesis. These are: temperature, stoichiometry, and synthesis environment. Setting temperature and stoichiometry aside, when we look at the synthesis environment, water is a major problem for polyurethane systems. For example, if you measure the moisture content in your system and it exceeds 500 ppm, continuing synthesis would not be appropriate. The moisture content should be reduced using different chemical desiccants before synthesis continues; otherwise, tons of raw material in the reactor converts to a non-recyclable form. Therefore, water is the most critical barrier for polyurethane syntheses. When considering liquid polyurethanes that have not completed formation / solidification (uncured): • A product that has completed reaction and is dissolved in solvent, • Two products in prepolymer form (solvent-containing/solvent-free) that have not completed reaction. When we activate water with fully reacted polyurethanes, the polyurethane resin will coagulate and lose its original resin form. Unreacted, that is prepolymer products, when in contact with water, will react with water and, due to the CO₂ gas released as a result of the reaction, the product will expand and take a rigid form. As can be seen, in both systems water destroys the structure of the products. R-N=C=O + H-O-H → [ R-NH-COOH ] → R-NH2 + CO2(g) Water is such a risky and unwanted raw material in polyurethane syntheses, yet how are water-based polyurethanes synthesized? And why is this method used? In water-based polyurethane resin synthesis, unlike conventional polyurethane resin syntheses, there are raw material and process modifications. Briefly looking at the synthesis content and process: Unlike general polyurethane syntheses, raw materials such as DMPA (dimethylol propionic acid) that enable the polymer structure to interact with water are used. The process occurs in five steps: prepolymer synthesis, neutralization, dispersion, chain extension and acetone recovery. Let us briefly examine these steps.

Prepolymer Synthesis Stage:

This is prepolymer synthesis conducted with polyol, isocyanate and bi- or tri-functional chain extenders. As with every synthesis, raw material selection should be our first priority. Based on desired properties in the final product such as flexibility, mechanical strength, chemical resistance or hydrolysis resistance, we should select appropriate polyols and other raw materials. The raw material difference in water-based polyurethane is required precisely at this stage. Since we will bring our product into contact with water in other process steps, in the prepolymer synthesis stage we must bind DMPA (dimethylol propionic acid), an important raw material that will increase the product's affinity for water, onto the prepolymer chain.

Neutralization Stage:

After prepolymer synthesis is completed, our aim at this stage is to continue preparing the polymer chain for water. We do this by neutralizing the acid portion in the prepolymer chain with a tertiary amine. The types of tertiary amine used at this stage will affect the structure and performance of the final product.

Dispersion Stage:

We designed the polyurethane backbone raw materials for the desired final product, bound DMPA to the polymer chain for water interaction, and neutralized it with tertiary amine. Finally, our system can encounter water. At this stage, we add water to the system to enable the product to dissolve in water. We added the necessary water to the system according to the targeted solids content, but our product is not yet complete.

Chain Extension Stage:

At this stage, the prepolymer dissolved in water is subjected to reaction with diamine compounds to extend its chain and increase molecular weight.   Acetone Recovery: As the final stage, acetone in the system must be recovered. Polyurethane synthesis is a science; it is very difficult to explain this briefly, but we have attempted to explain simply how water-based polyurethanes can be obtained through a process.

So why has there been a need for water-based polyurethanes?

1) It is an environmentally friendly product; its most important characteristic is that it is a polymer material dissolved in water. Unlike solvent-based products, it does not emit odor. 2) It is a safe product; whether occupational safety or food safety, working with water provides advantages in many fields. 3) They are high-performance products; they have performance equivalent to solvent-based high molecular weight resins. 4) Low viscosity; one of its most important characteristics is having high molecular weight with low viscosity. For example, consider a high molecular weight solvent-based product containing 35% solids with viscosity of 150,000 cP. When you make the same performance product water-based, its viscosity will be around 100 cP. Among products with the same performance, working with low viscosity water-based polyurethanes is easier. 5) High solids content; it is possible to synthesize quite high solids content products of up to 60% with low viscosities. In solvent-based systems, synthesizing the same performance products at 60% solids is both very difficult and their use is impossible due to high viscosities. 6) Short drying time; it is very difficult to dry products made with high polarity solvents such as DMF and DMSO outside an oven. Water has similar polarity to these products, but water-based polyurethanes dry faster under ambient conditions. 7) Stable; it is stable over a wide temperature range. Water-based polyurethane dispersions (PUD), obtained from the need for a safer product, have brought us numerous advantages as shown above. With these advantages, they have established their position as adhesives and coatings in the following areas: • Furniture • Textiles • Marble / Ceramic • Wood • Automotive • Leather • Construction • Footwear • Plastics The water-based polyurethane product groups we have developed over the past year based on our polyurethane R&D experience and market-identified needs are briefly as follows:

Ceramic and Marble Coating Product:

Applying coating to ceramic surfaces is one of the most difficult tasks. Coatings applied to ceramic surfaces undergo a water test after completing the cure process. After remaining in water for 12/24 hours, the coating surface is tested for separation. While normal coatings come off easily, coating adhering to ceramic should not come off the surface after the water test. This system demonstrates the product's performance under the most difficult conditions. To achieve this, a special primer is typically applied to the ceramic surface before application. While it seems possible to overcome this problem with a primer followed by a quality prepolymer, inability to apply to wet/moist surfaces and high costs are barriers we can consider for the product. Water-based polyurethanes are normally not as successful in adhesion as prepolymers. However, with our specially developed product, we achieved very strong adhesion of our product to the ceramic surface without applying any primer. Moreover, without distinction between wet/moist/dry surfaces. This way, without drawing any solvent odor, without considering ground moisture, without primer application, we have provided protection / water sealing with high mechanical and chemical resistance. Wood Coating Product; Water-based polyurethane dispersions to be applied as a protective layer on wood products with high mechanical and chemical resistance, Textile Coating Products; Water-based polyurethane dispersions with different flexibility values for technical textile applications, Membrane Press Adhesive; A water-based polyurethane dispersion adhesive that enables the bonding of PVC and MDF and also makes separation impossible under steam and heat conditions, Flexible Packaging Lamination Adhesive Product; In the flexible packaging sector, solvent-free and solvent-based adhesives are used. Water-based polyurethane adhesives are products found in the market but with low active use. We have made our preparations for demands in this field. Currently, in line with customer needs and demands, we continue to develop and diversify our water-based polyurethane products. We hope that 'water-based polyurethane dispersions', seen as the future of polyurethane, will see rapidly increasing use in Turkey and regional countries. The two most important parameters that will drive this are first that it is an environmentally friendly product and second that it is a product to be prioritized for human health. İbrahim Kecin / Senior Chemist / General Manager / PURİN Polyurethane