Further Water-Based Polyurethane (PUD) Shoe Adhesives

The earliest adhesives, dating to 4000 BCE, were derived from natural sources such as tree resins and were used in the manufacture of clay vessels. Following natural adhesives, the first synthetic adhesive emerged in 1942 with Dr. Harry Coover's discovery of cyanoacrylate. This was subsequently followed by various adhesive systems over time.

Today, adhesives in use can be categorized as follows:

•Solvent-Based Adhesives •Solvent-Free Two-Component Adhesives •Solvent-Free One-Component Adhesives •Solvent-Free One-Component Solution Adhesives (Hot Melt) •Water-Based Adhesives Examining adhesive applications specific to the footwear sector, we see that solvent-based adhesives are predominant. These are obtained by dissolving polymeric materials such as polychloroprene, polyurethane, and styrene copolymers in suitable solvents. These products achieve adhesion through the evaporation of their solvent content. Taking the example of bonding a shoe upper to the sole: adhesive is applied separately to both the upper and sole. Subsequently, the assembly is passed through an oven at 140-180°C for drying. Upon exiting the oven, while the adhesive on both the upper and sole is still warm, they are immediately bonded together and pressed. As the adhesive cools, it forms a strong bond. Examining this process technically: as with all coating and adhesive systems, solvents with at least two different evaporation rates are used to ensure the gradual removal of solvent from the surface. Additionally, these adhesive applications are carried out manually. Therefore, regardless of investment in worker health, there is always a degree of risk present in the workplace. As PURİN Polyurethane, beginning in 2018 we started manufacturing water-based polyurethanes (PUD) in Turkey. With each passing day, we add new products to our PUD product range and increase our production volume in this field. As we view PUD technology as the future of polyurethane, we initiated research and development work in this area well in advance and began domestic production through targeted investments.

To explain PUD production technology simply:

Polyurethane synthesis faces its greatest challenge from water content in raw materials. For example, synthesis cannot proceed above moisture levels of 300-400 ppm, and drying agents must be used to remove moisture from the environment. Otherwise, due to reactions between isocyanates and water, the polymer molecular weight building process will not progress and will terminate. We can directly assess polymer molecular weight as product performance. Therefore, if we cannot build the molecule, the product becomes waste. On the other hand, when we synthesize the product under suitable conditions, contact with water in the application environment becomes a separate issue. Prepolymer-based products will experience blooming and low performance due to CO2 gas upon contact with water. In non-reactive polyurethane materials, contact with water causes coagulation in the products, resulting in precipitation. PUD technology addresses the question of how to conduct synthesis in water when water has such a negative effect on polyurethane synthesis and subsequent application.

With PUD technology, product synthesis is carried out in 5 distinct steps:

Prepolymer Synthesis Stage:

In this stage, prepolymer synthesis is performed using polyol, isocyanate, chain extender, hydrophilic monomer, and auxiliary solvent (for example, acetone) in appropriate stoichiometric ratios. Of course, the raw materials to be used depend entirely on the product's application, desired properties, and performance. For example, when hydrolysis resistance is required, such as non-degradation in aqueous environments, polyether-based or polycarbonate-based polyols should be used instead of polyester-based polyols. Where high elasticity is desired, higher molecular weight polyols and long-chain isocyanates should be employed.

Neutralization Stage:

The hydrophilic monomer used in the prepolymer stage is the most critical raw material in PUD technology. This monomer provides compatibility between water and the polyurethane chain. However, this monomer has an acidic structure and will not work directly as used in synthesis. To activate this monomer, it must be neutralized using an appropriate base.

Dispersion Stage:

After neutralization, the polyurethane chain can now interface with water. Water is added to the prepolymer under agitation, converting the product into a water-dissolved prepolymer form. After this critical step, synthesis will continue in an aqueous environment.

Chain Extension Stage:

In this stage, molecular weight building of the polyurethane prepolymer in water is performed using diamine compounds. After reaching the planned molecular weight, the product is complete, but auxiliary solvent must be removed from the system.

Solvent Recovery Stage:

As the final stage, the auxiliary solvent (acetone) in the completed product is removed under vacuum and heat. The final product is produced in this manner. In this way, water-based polyurethane resin synthesis is accomplished with PUD technology. Prior to resin synthesis, appropriate raw material selections for application and performance are made based on our experience and technical research. Subsequently, product analyses guide our optimized production flow. Following PUD resin synthesis conducted under the control of the Synthesis R&D unit, the Application R&D unit applies necessary modifications to the obtained resin and conducts work for sectorial applications.

Using PUD adhesive will:

•Provide an odor-free, clean, and healthy work environment, •Deliver superior adhesive performance, •Enable easier application with lower viscosity, •Reduce your energy costs, •Shorten your work processes, •Increase your production speed.

As PURİN, in our PUD product range as in all our product groups, we will provide you service well above industry standards:

•Develop projects technically and present you with the most suitable product, •Provide technical support throughout all processes (product selection, application, and beyond) with an R&D-trained expert team specialized in your product, •Fast supply to all of Turkey and regional countries through our Kocaeli facility, •Provide cost advantage through the most competitive pricing as a domestic PUD producer. In light of all these efforts, adhesives developed through business development initiatives for the footwear sector will reduce your work processes and increase your production speed. İbrahim Kecin Senior Chemist General Manager PURİN Polyurethane   Ayşenur Dede R&D Manager PURİN Polyurethane