Polymer Problem Solver

Low-viscosity, plasticizer-free SPUR⁺prepolymer allows freedom in formulating sealants

Silane-terminated polyurethanes (SPUR⁺prepolymers) have become increasingly appealing to adhesives, sealants and coatings manufacturers due to the synergy between the silane-curing mechanism and the polyurethane backbone properties.

Formulations developed using this technology offer fast room-temperature cure and good durability, and the adhesives or sealants they produce are free of unreacted isocyanates, allowing them to have a less harmful impact on the environment. Benefits of the resulting products also include no bubbling during cure, and a broadening of the formulation latitude compared to conventional polyurethane technologies.

Figure 1. Curing Rates of SPUR⁺ Prepolymers

These prepolymers allow formulators to use a variety of additives and adhesion promoters to meet end users' performance needs, such as the following.

Good elasticity and durability
Primerless adhesion to both organic and inorganic, porous and non-porous substrates
Superior resistance to chemicals, such as automotive fluids (glycols, motor and transmission fluids)
Minimal shrinkage
Excellent weatherability
Immediate paintability
No staining on porous substrates

Table 1. Mechanical Properties of SPUR⁺ Y-15735 LM Prepolymer

The production of SPUR⁺prepolymers offering both low viscosity and a good balance of properties has been a primary challenge to resin producers. Many approaches to the preparation of these polymers have been reported. Most focused on amino alkoxysilanes reacting with a conventional isocyanate-terminated prepolymer, an approach that has been used by researchers to prepare hybrid polymers containing both difunctional and trifunctional organosilane endcappers.¹Many of these polymers were too brittle or offered curing profiles that were unsuitable for most sealant applications.

Reviewing GE's development of SPUR⁺ prepolymers, we published our first technical article 10 years ago.² The article contrasted different silane endcappers to the properties of resultant SPUR⁺ prepolymers. Among those potential endcappers (mercapto-silane, ureidosilane and aminosilanes), a secondary aminosilane, Silquest* Y-9669 (phenylaminopropyltrimethoxysilane), demonstrated the best balance of durability and curing rate. That was GE's first generation of SPUR⁺ prepolymer technology.

Figure 2. Shelf life of SPUR⁺ Y-15735 LM Prepolymer

We have since focused our efforts on continuing development of SPUR⁺polymer technology. In 1997, we reported on the superior adhesion of SPUR⁺based sealants to a range of plastic substrates.³We further improved the SPUR⁺process in 1999 with the introduction of a second-generation endcapper, Silquest A-Link* 15 silane, an aliphatic aminosilane designed to address the slow reactivity of Silquest Y-9669.⁴Silquest A-Link 15 silane reacts twice as fast as Y-9669 with -NCO terminus of the polyurethane prepolymer, thereby offering improved process efficiency.

Table 2. SPUR⁺ Sealant Formulation

Though the technology of the second-generation of SPUR⁺overcame many of the deficiencies associated with earlier versions, the aminosilane-capped SPUR⁺prepolymer's viscosity remained above 100K cP for a desired molecular length. For ease of handling, the prepolymers have often been diluted with a plasticizer. However, presence of the diluent has, in many instances, limited the use of these polymers.

Therefore, a low-viscosity, SPUR⁺ prepolymer without added plasticizers has become more important, as formulators seek to improve the performance of their products by selecting different plasticizers, or to optimize their properties by mixing thixotropic agents with the prepolymer before addition of the plasticizer. To meet the industry demand, one of the next-generation silylated polyurethane resins is now available: SPUR⁺ Y-15735 LM prepolymer, low modulus resin.

Table 3. Organofunctional Silane Adhesion Promoters

The Next Generation of SPUR⁺ Prepolymers

The SPUR⁺Y-15735 LM prepolymer, the first product in this line, is obtained from the reaction of a -OH terminated SPUR⁺prepolymer that has been capped by an isocyanato-silane.5 In comparison to SPUR⁺polymers obtained using amino-silane capping groups, resins prepared using isocyanate silanes have fewer isocyanate hard segments, thereby reducing the potential for the intermolecular hydrogen bonding that results in lower polymer viscosity.

The SPUR⁺ Y-15735 LM prepolymer is a clear liquid resin. It has a viscosity of ~50K cP and is free of any added plasticizer. The polymer is easily handled, as it is freely flowable at room temperature. Figure 1 shows that the Y-15735 resin offers faster tack-free performance than similar resins prepared using secondary aminosilane capping groups. When mixed with 1 wt% dibutyltin dilaurate in the absence of added fillers, Y-15735 LM is three times as fast in tack-free time, and offers a 50% enhancement in the rate of deep-section cure under ambient conditions.

Table 4. SPUR⁺ Sealant Properties Tailored by Amino Silane Adhesion Promoters

Generally, surface cure can be accelerated by the combination of a tin catalyst and co-catalyst(s). But the deep-section cure is more dependent on the nature of the prepolymers. Here, the SPUR⁺Y-15735 LM prepolymer demonstrates the ability to cure-through faster. That characteristic can be a benefit to many applications where a thick layer of adhesives or sealant is used.

Additionally, the new-generation prepolymer provides balanced mechanical properties (see Table 1), thus making it an excellent material for the formulation of products for construction, industrial and consumer applications.

Table 5. SPUR⁺ Sealant Adhesion to Substrates

Shelf life is often a concern for a reactive polymer. Figure 2 shows that SPUR⁺prepolymer is quite stable in the absence of moisture. An accelerated aging test was conducted in an oven at 50°C for four weeks, which corresponds to a year at room temperature.

Table 6. Silquest Y-15710 Silane and Silquest Y-15713 Silane Adhesion to Various Substrates.

SPUR⁺Y-15735 LM Performance in Sealants

The SPUR⁺ strategic platform is built on technologies in both SPUR⁺ prepolymers and silane additives. By combining the SPUR⁺ Y-15735 LM prepolymer with desired silane adhesion promoters, sealant properties can be tailored for different applications. This is demonstrated through a generic formulation shown in Table 2.

Figure 3. Adhesion on Rough Concrete

Selected silane adhesion promoters are described in Table 3. All evaluations followed ASTM test methods (ASTM D 412, C 661, C 794) and European test methods (ISO 34, 37, 868, 4578, 9046, 7389). The adhesion data presented in Table 5 is the result of 7 days' immersion in water, unless stated otherwise.

For the construction market, sealant flexibility is a fundamental requirement for the ASTM C 920 standard. For the industrial and consumer sectors, modulus and tensile strength are emphasized. Nonyellowing is often important when formulating a white or clear sealant. As demonstrated, selection of the silane adhesion promoter has a dramatic impact on the sealant's performance characteristics.

Adhesion to wet concrete without primer, whether applied on wet concrete or tested after water immersion, is of concern to formulators. As a result of the high pH condition of the wet surface of concrete, the only reliable method to achieve good adhesion has been the initial application of a primer. To satisfy contractor needs, many formulators are striving to gain adhesion without primer.

Figure 4. Adhesion on Smooth Concrete

When used in combination with a SPUR⁺Y-15735 LM prepolymer-based sealant formulation, the new silane adhesion promoters Silquest* Y-15710 and Y-15713 offer primerless adhesion to wet concrete (see Table 3). Using only 0.58% of Silquest Y-15710 or Y-15713 in the formulation (see Table 2), the resultant SPUR⁺sealant demonstrates good adhesion to glass, aluminum and plastic (see Table 6), as well as improved adhesion to concrete, under all test conditions. The tests were carried out on different kinds of concrete - smooth and flat surfaced or rough and porous surfaced. The adhesion obtained with Y-15710 and Y-15713 is significantly improved, compared to a conventional silane, Silquest A-1110, without primer (see Figures 3-4).

Conclusion

The next-generation of SPUR⁺resin technology is a low-viscosity SPUR⁺prepolymer without added plasticizers, and is the flagship product in a new generation of SPUR⁺prepolymers. In addition to the advantages, the new SPUR⁺Y-15735 LM prepolymer provides additional features: non-yellowing, improved tack free, and deep section cure performance. These performance advantages provide formulators with more latitude to optimize their products for industry needs.

Combined with new silane technology, adhesives and sealants formulated with this new prepolymer can provide adhesion to many difficult substrates, including wet concrete. This new development in silane technology helps enable adhesion to wet concrete without use of a primer.

Acknowledgments

The authors gratefully acknowledge Dr. Brendan O'Keefe for his contribution to this project; as well as Mr. Jeries Nesheiwat, Mr. Rob Gorrell, Mr. Jenning Goldsmith, and Mr. Remy Gauthier for syntheses and the property tests reported in this paper. Throughout the development, we acknowledge the many managers and engineers who supported this project, including Dr. Shayne Landon, Dr. Kevin Bobbit, and Mr. John Banevicius.

For more information on SPUR⁺ prepolymers, contact GE, Advanced Materials, Silicones, www.gesilicones.com.

References

1 Saunder, J.H. Frisch, K.C. Polyurethanes: Chemistry and Technology, Vol. I, Part I, Interscience, NY (1962), p. 273 (b).

2 (a) Feng, T.M.; Waldman, B.A. "Silylated Urethane Polymers Enhance Properties of Construction Sealants," Adhesives Age, April 1995. (b) Landon, S.J., Guillet, A.; Johnston, R.R. "Silylated Urethane Polymers for Sealants," European Adhesives and Sealants; Dec. 1995.

3 Landon, S.J.; Dawkins, M.B.; Waldman, B.A.; Johnston, R.R. "The Adhesion of Hybrid Sealants to Plastic Substrates, Adhesives," Adhesives Age, April 1997.

4 Huang, M.W.; Handel, R. "New Developments in Silylated Polyurethane Technology," Adhesives Age, April 1999.

5 Johnston, R.R.; Lehmann, P. US 5,990,257.