Steven Vandebril – KANEKA BELGIUM N.V.
Moisture-curing sealants and adhesives have played for years a very important part in several applications. One of the well-known base resins are the Kaneka MS PolymerTM, a hybrid technology combining all the benefits of alternatives such as polyurethanes and silicones without combining the disadvantages. Moreover, label-free formulations can be developed, implying the ecofriendly and user-friendly character of these silane-terminated polymers. Although this technology is widely being recognized by formulators, applicators and engineers, their intrinsic high elongation could lead to insufficient tensile strengths for specific applications, such as vehicle construction.
Moisture-curable polyurethane adhesives fulfill these challenging requirements with regard to strength and elongation but are subject of very stringent regulations. As a result, isocyanate-free adhesives are gaining in importance and hence, a novel acryl-modified MS PolymerTM has been developed as binder for flexible and high-performance adhesives. In combination with advanced fillers and additives, a perfect balance between high elongation and high strength can be achieved, to supplement or even replace traditional high strength bonding techniques such as rivets and metal welds.
High strength silane-terminated polyacrylates
The moisture-curable silane-terminated polyether technology, or so-called Kaneka MS PolymerTM, has been introduced about 40 years ago as an ecofriendly alternative for silicones and polyurethanes. Driven by superior characteristics such as durability, reactivity and stability, the first generation of this isocyanate- and solvent-free polymer was mainly used in low modulus construction sealants with high flexibility. By finetuning the functionalized polyether backbone length and structure, the polymer platform could grow significantly and adhesives with higher tensile strengths and limited elongation could be formulated, as illustrated in figure1.
Fig. 1
Applications of Kaneka MS PolymerTM
This broad range of grades resulted in an increased usage of Kaneka MS PolymerTM for several applications such as construction, industrial and do-it-yourself.
To improve properties such as UV-weatherability, enhanced durability, excellent adhesion onto plastics and tensile strength, silane-terminated polyacrylates have been developed to broaden the polymer portfolio even more. These acryl-modified MS PolymerTM are formed by combining randomly silane-functionalized acrylic polymers with standard polyethers. Moreover, by differentiating the degree of silane-functionalization and specific selection of acrylic monomers, the compatibility and glass temperature can be easily controlled providing outstanding performances. This has resulted in the development of novel high strength acryl-modified MS PolymerTM resins to combine the superior elastic behavior of sealants and the improved tensile strength of adhesives. An incompatibility between the polyacrylate and polyether will occur during curing and homogeneously dispersed acrylic domains will contribute to the final strength (> 6 MPa), while the polyether phase ensures the high flexibility (> 500%), enabling the development of high-performance adhesives as shown in figure 1.
High strength – high elongation
A crucial factor in the development of high-performance sealants and adhesives based on high strength acryl-modified MS PolymerTM is the selection of compatible fillers and additives. Engineered calcium carbonate fillers are one of the most frequently used fillers in silane-terminated polymer formulations, although they have their limits regarding strength and elongation. By selecting functional fillers with a specific surface treatment, particle size distribution and particle shape, the mechanical properties of high-performance adhesives can be further enhanced, as shown in figure 2. The reference material has a tensile strength of 3 MPa and 250% elongation while sample A has even outstanding mechanical properties as both tensile strength (7.5 MPa) and elongation (550%) are remarkably improved.
Fig. 2
Tensile strength and elongation values of formulations based on Kaneka MS PolymerTM
An even better balance between strength and elongation is achieved by selecting a specific curing catalyst (sample B), as also shown in figure 2. Whereas the elongation reaches a value of 800%, the formulation is still very strong as the tensile strength is increased to a level of more than 8.5 MPa. These results indicate that the mechanical properties can be easily finetuned by adding the right fillers and crosslinkers in combination with our novel high strength acryl-modified Kaneka MS PolymerTM. In some cases, it is desirable to increase the tensile strength even more (> 10 MPa) while maintaining a high flexibility. Figure 2 clearly illustrates that sample C, consisting of a unique combination of fillers, additives and specific acryl-modified polymer, can lead to outstanding mechanical properties, with a tensile strength of almost 11 MPa and an elongation of 300%.
Improved lap shear strength
Although mixtures of silane-functional polyacrylates and polyethers are compatible, fully cured systems consist of polyacrylate domains surrounded by a polyether matrix phase. Depending on the acrylic monomer structure, type of polyether and polymer ratio, extremely flexible adhesives with a high lap shear strength value can be formulated, as illustrated in figure 3 (sample D).
Fig. 3
Lap shear strength values on various surfaces. Sample D consists of an high strength acryl-modified MS PolymerTM and reinforcing fillers
Conclusion
Various applications are requi-ring challenging properties such as a high elongation, high tensile strength and excellent, primer-less adhesion.
Although polyurethane-based adhesives could offer a solution, there is an increased demand for alternative technologies due to the recent restriction on diisocyanates. Therefore, moisture-curable high strength acryl-modified Kaneka MS PolymerTM mixed with functional fillers and specific crosslinking catalysts can result in highly flexible adhesives with a perfect balance between elongation and strength.
Tensile strength values of 6 MPa and even higher, combined with an excellent elongation of more than 500% can easily be obtained. In the context of the European Green Deal strategy towards a toxic-free environment, these high strength acryl-modified Kaneka MS PolymerTM will gain more market share as an ecofriendly resin for high-performance adhesives.
