CAB-O-SIL® Hydrophobic Fumed Silicas Used in Polyurethane Polyols

Turkchem 04 Oct 2017 61 3 dk okuma

TURKCHEM

Introduction

Two-component (2K) liquid polyurethanes contain thermoset resins that are highly convenient and frequently utilized by adhesive and coating formulators. The two components in the system are distinctly different from one another and require mixing before use. One component contains isocyanate, while the other contains polyol. Polyol is an alcohol composed of multiple hydroxyl groups. These multiple hydroxyl groups enable the desired organic reaction in this two-component structure resulting from the mixture of polyols with isocyanates. Polyols found in 2K PUR systems are polyether, polyester, and hydroxyl-terminated polybutadiene.

Challenges Faced by Formulators

Formulators select thixotropic additives (such as fumed silica) for the following two reasons: • To provide optimal sag resistance required before curing during application, • To thicken the polyol, enabling more effective mixing of the two-component mixture. In this way, homogeneity is achieved in the mixture by bringing the viscosities of the components closer to each other. Unfortunately, these two conditions may not always provide the performance desired by the formulator. At this point, formulators need to balance the following properties: • Degree of thickening, • Ease of dispersion, • Clarity of dispersion. This article has been prepared to serve as a guide to formulators in achieving their desired chemical formulations through proper silica selection.

Case Studies

Polyether: Polyether polyols are the most frequently used polyols in PUR systems. Typically, this type of polyol has favorable pricing, desired alkali resistance, low viscosity, and good low-temperature flexibility properties. Following the combination of various polyether polyols with Cabosil fumed silicas, the graph containing viscosity values related to flow rate can be found in Figure 1a, and the effects of various fumed silica usage on properties can be found in Figure 1b. As seen in the graphs, CAB-O-SIL TS-720 provides excellent thickening, while CAB-O-SIL 610, CAB-OSIL 530, 610 and M5 provide less thickening (Figure 1a). The selection of fumed silica in polyurethane systems requires prioritizing and balancing application-specific properties. For example, if thixotropy and ease of dispersion are the most important parameters in the formulation, as can be seen in Figure 1b, TS-530 would be more suitable than TS-720 in terms of both providing adequate thickening and offering ease of mixing. If clarity is very important in the formulation, using TS-610 is an excellent option.

Polyester

Polyesters are among the basic polyols used in polyurethane applications. They are advantageous due to good adhesion properties, solvent resistance, and superior mechanical values. The viscosity values dependent on shear rate in polyester polyols using various CAB-O-SIL types are shown in Figure 2a. Accordingly, the highest viscosity gain was observed in CAB-O-SIL TS-720 and subsequently in CAB-O-SIL TS-530. As with polyethers, CAB-O-SIL TS-610 and CAB-OSIL M5 provided minimal thickening in polyesters as well. As seen in Figure 2b, when product selection is considered in terms of thixotropy and average dispersion time balance, the choice should favor CAB-O-SIL TS-530.

Polybutadiene

Polybutadienes are among the main polyol types used in polyurethane applications. Polybutadiene polyols are advantageous due to their flexibility, good adhesion, and low moisture absorption. The viscosity values dependent on shear rate in polybutadiene polyols using various CAB-O-SIL types are shown in Figure 3a. Accordingly, CAB-O-SIL TS-720 appears to be the most effective thixotropic agent. CAB-OSIL TS-530, TS-610 and M-5 types provide virtually no thickening with this type of polyol. If dispersion time and clarity are important criteria, as seen in Figure 3b, the use of CAB-O-SIL TS-610 or CAB-OSIL TS-530 is appropriate.

Summary

Good thickeners are needed to achieve the desired viscosity in all components of PUR, as this enables good mixing and consistent adhesive system application. Ease of dispersion provides formulators with the opportunity for shorter processing time and lower costs. On the other hand, clarity is important for those producing unfilled systems and can also benefit all formulators as it serves as an indicator of air release or bubble formation.