Polyurethane Injection Resins

Polyurethane injection material is a prepolymer substance defined as having reactive ends, containing N=C=O (NCO) bonds. When these materials encounter moisture or water, they undergo a chemical reaction that closes their active ends. Following the reaction, the polyurethane prepolymer material transforms from liquid form to solid form. This chemical reaction is shown in Figure 1.

Figure 1. Polyurethane prepolymer reaction

Polyurethane injection resins are materials used in negative-side water insulation applications. These resins are used in situations where positive-side application is not possible or has failed. Injection resins can be divided into two main groups: single-component and two-component. Single-component products are generally used for water cutoff. Two-component products are systems that provide permanent waterproofing following single-component applications. Polyurethane products have varying reaction times. Thus, the same product can be adjusted to different reaction start and finish times according to customer requirements. For optimal use, the product type should be selected based on the location where the application will be performed. Using the same product in every application is incorrect. Different products should be preferred for tunnel, joint filling, soil reinforcement and lifting, underground and other applications. One product should be selected for water cutoff during application, while a different product should be chosen to provide permanent water sealing. Products structurally have different physical forms including very rigid, semi-rigid, rigid, flexible and gel formats. The appropriate product should be selected according to the application area.

What Should the Ideal Water Ratio Be?

Special formulation design is performed so that the foam form created as a result of the polyurethane resin's reaction with water remains in a stable structure. However, just as everything has its limits, this structure also has certain conditions under which it can remain stable. If these conditions are exceeded, the structural condition and pore form will change.

Figure 2. Demonstration of ideal water ratio

As shown in Figure 2, as the amount of water increases, foam skeleton stability deteriorates. Excess CO2 created due to too much water causes the material to deform and the pore form to become irregular. Large non-homogeneous pores in the cell structure cause water leakage. This deformation and deterioration in the structure can lead to material shrinkage over time.

What Should the Ideal Foam Structure Be Like?

To ensure permanent water sealing, the pore structure of flexible polyurethane foam should be small and regular.

Figure 3. Demonstration of ideal foam structure

In Figure 3, our product is compared at 4% and 100% water ratios as required by the reaction. In this study, the difference between them is due to the amount of water. It should be noted that our product is much superior to competitors even in a 100% aqueous environment.

What Should the Ideal Expansion Ratio Be?

Single-component injection materials react with water when injected into water-flowing areas or cracks. As a result of the reaction, the CO2 gas produced moves away from the environment while carrying the polyurethane material upward and causing the polyurethane to expand. The most important point here is that the polyurethane material generates CO2 gas equal to the amount of water with which it reacts. When it reacts with excess water, polyurethane expands significantly, and when it reacts with less water, it expands minimally. When water is added at 1% and 4% by weight to an injection system containing the same amounts of resin and catalyst, the expansion ratio is as shown in Figure 4. In the foam form with excess water (4%) (ideal foam form), more expansion occurred because more CO2 was released as a result of the reaction.

Figure 4. Demonstration of optimum expansion ratio

How Should Application Be Performed and What Should Be Considered?

Polyurethane injection systems expand in direct proportion to water content and isolate water based on this expansion. Consequently, it is incorrect to provide insulation for cracks filled with a product having an excessively expanded and open-cell foam structure. The injection process should not be terminated when water is cut off. Product should continue to be injected from packer systems until it can no longer be injected. Polyurethane will expand significantly upon its first contact with too much water and its cellular structure will not be very good. In fact, this foam structure will be washed out with water. This product that comes out by expanding excessively with water is not in the desired ideal form. This initial structural condition that forms should not be taken into account. When injection continues, polyurethane will encounter less water. As a result, as described above, the expansion ratio will gradually decrease due to water and the foam structure will gradually tighten, taking on its ideal form. In areas near packers, the material will solidify without expanding. Intensive application of material to cracks in injection systems provides permanent insulation. Along with its elastomeric properties, there will be no leakage due to shrinkage over time, dissolution with water, or breakage due to structural movements. However, it is a fact that the injection process terminated by water cutoff will not provide permanent waterproofing. Water insulation work performed with single-component injection products should be completed with two-component injection resins that do not expand and have elastomeric properties. This procedure provides permanent waterproofing for these materials, which are non-foam structured and show tolerance by flexing in static movements.  

İbrahim Kecin

Senior Chemist General Manager PURİN Poliüretan