Waterborne Polyurethanes (PUD)

Polyurethanes, as is well known, were discovered in 1937 by Otto Bayer and continue to grow, forming a specialized sector. Turkey ranks among countries showing rapid growth in the polyurethane sector and, due to its geographic location, is a preferred country for polyurethane supply by many nations. When we classify polyurethane product groups structurally, they can be divided into two categories: foam or non-foam. Hard foam products form sectors such as thermal insulation, while flexible foam products create sectors such as sponge systems. Of course, we can expand these product groups further. Non-foam products structurally contain no foam and are coatings, adhesives, elastomers and sealants, abbreviated as Polyurethane CASE. These CASE products have a much lower tonnage volume compared to foam products, but their applications and variations are extensive. In foam product groups, raw materials such as polyols, catalysts and stabilizers are generally mixed to form one component. The other component is isocyanate/isocyanate prepolymers. These two components are mixed in predetermined ratios using machinery for application; the actual reaction occurs at the application site. In non-foam products, the main synthesis takes place in reactors and follows a systematic approach. By changing the process from the same raw materials and identical percentage formulations, you can obtain different products. For this reason, reaction systematization becomes very important in this type of synthesis. The product formed in reactors simply takes on its form at the application site. For example, when a solvent-based product is synthesized, you remove the solvent at the application site to obtain the polyurethane resin within the product according to your application method. These types of products can be: two-component polyurethane resins containing 100% solids, one-component polyurethane resins dissolved in solvent and one-component polyurethane resins dissolved in water.

When we examine the polyurethane synthesis system in the CASE field, we see that it is a systematic synthesis using different stoichiometric calculations of polyols, chain extenders and isocyanates. We need to know that due to the different properties of each raw material used here, the final products will have different characteristics.

There are certain barriers in this type of synthesis. These are: temperature, stoichiometry and synthesis environment. Leaving aside temperature and stoichiometry and looking 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, it would not be correct to continue synthesis. You should reduce the moisture content with different chemical desiccants and continue synthesis; otherwise, tons of raw material in the reactor will convert to a non-recyclable form. Therefore, water is the most important barrier for polyurethane synthesis. When we consider liquid polyurethanes that have not completed their formation/solidification (not cured); • A product that has completed its reaction and is dissolved in solvent, • Two products that have not completed their reaction, meaning in prepolymer form (solvent-based/solvent-free). When we bring water into contact with polyurethanes that have completed their reaction, the polyurethane resin will coagulate and lose its original resin form. Prepolymer products that have not completed their reaction will react with water, and due to the CO₂ gas released by the reaction, the product will expand and take on a rigid form. As seen, water damages the structure of the products in both systems. R-N=C=O + H-O-H → [ R-NH-COOH ] → R-NH2 + CO2(g) While water is such a risky and unwanted raw material in polyurethane synthesis, how are water-based polyurethanes synthesized? And why is this method used? In water-based polyurethane resin synthesis, unlike traditional polyurethane resin syntheses, there are raw material and process changes. Briefly looking at the synthesis content and process; Different from general polyurethane syntheses, raw materials such as DMPA (dimethylol propionic acid) are used that enable the polymer structure to interact with water. 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 performed with polyol, isocyanate and two or three functional chain extenders. Here, as in every synthesis, raw material selection should be our first priority. We should select appropriate polyols and other raw materials according to desired properties in the final product such as flexibility, mechanical resistance, chemical resistance or hydrolysis resistance. 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, we must bind DMPA (dimethylol propionic acid), an important raw material that will increase the product's affinity for water, to the prepolymer chain.

Neutralization Stage:

After prepolymer synthesis is performed, the 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 amines used at this stage will affect the structure and performance of the final product.

Dispersion Stage:

We designed polyurethane base skeleton raw materials for the desired final product, bound DMPA to the polymer chain for interaction with water and neutralized it with tertiary amine. Finally, our system can come into contact with water. At this stage, we add water to the system to dissolve the product 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 reacted 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 like a science; it is very difficult to explain this briefly, but we have tried 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 feature is being a polymer material dissolved in water. Unlike solvent-based products, it does not emit odor. 2) It is a safe product; working with water provides advantages in many fields, whether occupational safety or food safety. 3) They are high-performance products; they have performance at least equal to solvent-based high molecular weight resins. 4) Low viscosity; one of its most important characteristics is high molecular weight with low viscosity. For example, consider a high molecular weight solvent-based product containing 35% solids with a viscosity of 150,000 cP. When you make the same performance product water-based, its viscosity will be around 100 cP. Working with low-viscosity water-based polyurethanes among products of equal performance is easier. 5) High solids content; it is possible to synthesize quite high solids products around 60% with low viscosities. In solvent-based systems, synthesizing equivalent performance products at 60% solids is both very difficult and impossible to use due to their 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 with equivalent products, but water-based polyurethanes dry faster under environmental 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 many advantages as shown above. With these advantages, it has established itself in the following fields as adhesives and coatings: • Furniture • Textiles • Marble/Ceramic • Wood • Automotive • Leather • Construction • Footwear • Plastics

Water-based polyurethane product groups we have developed over the past year in line with our polyurethane R&D experience and market needs identified are briefly as follows:

Ceramic and marble coating product; Applying coating to ceramic surfaces is one of the most difficult operations. Water testing is performed on products applied to ceramic after the cure process is completed. After remaining in water for 12/24 hours, the coating surface is strained to separate from the surface. While normal coatings are easily removed from the surface, the coating adhered to ceramics should not come off the surface after a 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 and then a quality prepolymer, inability to apply to wet/damp surfaces and high costs can be considered barriers to the product. Water-based polyurethanes are normally not as successful as prepolymers in adhesion. However, with our specially developed product, we have achieved very strong adhesion of our product to ceramic without any primer application. Moreover, without any distinction between wet/damp/dry surfaces. In this way, without drawing any solvent odor, without considering ground moisture, without applying primer, we have provided protection/water insulation 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 PVC and MDF to adhere and makes separation impossible under vapor 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 limited active use. We have made preparations for demands in this field. Currently, we continue to develop and diversify our water-based polyurethane products in line with customer needs and demands. We hope that water-based polyurethane dispersions, seen as the future of polyurethane, will see rapid increased usage in Turkey and regional countries. The two most important parameters to trigger this are first, being an environmentally friendly product, and second, being a product that will be prioritized for human health. İbrahim Kecin / Senior Chemist / General Manager / PURİN Polyurethane