Next-Generation Silane-Terminated Polyethers

First-generation Silane-Terminated Polyethers (STPE) were developed in Japan in 1978 as a construction sealant material and were named MS PolymerTM to symbolize modified silicone. They combine the positive properties of silicone and polyurethanes (PU). It distinguished itself from these technologies by consisting of a polyether backbone functionalized with silane-terminated groups without the need for harmful isocyanates and/or solvents. Chemical Structure Despite the name being derived from modified silicone, its backbone differs from that of silicones. After curing, STPE or MS PolymerTM consists of 2 siloxane bonds separated only by a polyether, providing higher flexibility, while the silicone backbone consists of multiple siloxane bonds. The PU backbone can be very similar to STPE, but it contains urethane bonds. More detailed structures are provided in Figure 1.

MS PolymerTM

 

Performance Comparison

STPEs combine the positive properties of silicones and polyurethanes as shown in Table 1. Adhesives and sealants based on this technology exhibit continuous elastic behavior even below zero degrees Celsius with durability comparable to polyurethanes. Table 1. Comparison of properties of MS Polymer, PU and silicone

High-Strength MS PolymerTM

The MS PolymerTM polymer backbone, initially developed only for sealant materials, has been modified over the years to expand its application field to adhesives. A recent modification in its design has enabled the development of high-strength polymers. There are two technologies based on original Kaneka silane acrylic modified polymers or modified polyether backbone. In high-strength silane acrylic modified polymers, polymers with different structures and thus different glass transition temperatures (Tg) are blended to control compatibility. During curing, compatibility will change and phase separation will occur between the cured polyacrylate and polyether phases. This process results in acrylic and ether phases homogeneously distributed, referred to as sea-island structure. The acrylic phases provide high strength and good adhesion to plastics, while the ether phases provide elasticity to the cured system. The other option is the modified polyether backbone, which allows for an increase in branching points at the backbone and thus results in more entanglement and a denser polymer matrix after curing. The increase in the number of siloxane bonds provides higher strength. When blended with silane acrylic modified polymers, strength can be raised to over 10 MPa (tensile strength at break). Figure 2 presents an overview showing the final strength and elasticity of the end product formulated based on different polymers. The excellent compatibility of MS PolymerTM allows it to be blended with epoxies to act as a plasticizer. High-strength MS Polymers provide general improvement compared to standard MS PolymerTM, particularly in mechanical properties. A direct proportional comparison of mechanical properties in the formulated product, demonstrating that high-strength polymers enable higher quality products, is shown in Figure 3. High-strength polymers are applicable in many adhesive applications with high demand, such as D4 wood adhesive, semi-structural adhesive, adhesive for dissimilar substrates, plastic adhesive, do-it-yourself repair adhesive, and coating materials with low water absorption. Figure 3. Proportional comparison of percentages of different mid-modulus MS Polymer technologies

Acrylic-Modified MS PolymerTM for Better UV Resistance

Acrylic-modified MS PolymerTM not only improves adhesion strength but also enhances other properties such as UV resistance and weathering resistance. The weathering resistance of acrylic-modified MS PolymerTM was measured in a formulated product by performing cyclic testing with a combination of carbon arc lamp and water spray (SWOM test). As seen in Table 2, cracks begin to form after 1500 hours of exposure with a standard DMS-MS PolymerTM. With acrylic-modified MS PolymerTM, crack formation is observed only after 4500 hours (27 weeks) of exposure, which is 3 times longer compared to standard MS PolymerTM exposure. It is clear that using new acrylic-modified polymers significantly increases weathering resistance and will result in improved heat/water resistance. Table 2. Comparison of weathering resistance test for standard MS PolymerTM and acrylic-modified MS PolymerTM

General Conclusion

New-generation high-strength Kaneka MS PolymerTM grades have been developed and have been shown to improve the overall mechanical properties of sealants and adhesives compared to standard STPE. High strength and modulus levels up to those found in polyurethanes can be achieved. Furthermore, other properties such as heat/moisture resistance, elasticity, adhesion to plastics, etc., have also been improved. These polymers provide new opportunities such as repair adhesives, (semi-) structural adhesives, etc.     Jo Indesteege Director MS Polymer Division EMEA Region