A Study On Silicone Sealants With Low Modulus And High Elongation

The silicone sealants with low modulus and high elongation possess high elasticity, excellent adhesion properties, which can be widely used in such areas like high-road, airstrip, high buildings and so forth.In this paper, the primary objective is to obtain specific silicone sealants with ultra-low modulus and high elongation by using introduction of chain-extender, proper fillers and the design of procedures and formulations.At first, a linear difunctional silylamide, bis-(N-methylacetamido) dimethyl silane (MADMS), was synthesized through direct and indiect methods (by the reaction) in the presence of dimethyldichlorosilane and N-methylacetamide. The chemical structure of the product was confirmed by IR, 1H NMR and elemental analysis. On the other hand, GPC and viscometer have revealed that both the molecular weight and the viscosity of the mixture increase significantly when MADMS was mixed with PDMS,-confirming the chain-extending effect of MADMS. Furthermore, it was found that the silicone sealant incorporated with chain-extender diacetamidosilane has a rapid curing speed. Ambient for 7 days, the sealant's elongation at break is more than 1100% and 100% tensile stress is less than 0.3MPa, which fulfils the requirements of low modulus.Due to the high costing and hardness of synthesizing, upon adding of chain-extender diketoximosilane, the curing speed become correspondingly slower. For this case, different modifiers, catalyst mixture and assisstant catalyst were added to improve curing speed. The results showed that silicone sealants with low modulus and high elongation also could be preparedThree kinds of modifiers with polar groups, i.e. modifier A, modifier B and modifier K were utilized to improve the curing speed of sealant. The results revealed that they could effectively reduce the surface-free time; however the modifier A is the best among them and the optimistic addition is 0.4 phr.Catalyst mixtures also have effects on the improvement of curing speed and reduction of surface-free time, with simultaneously the increase of viscosity. If theconcentation is too much higher, curing would occur during the process of sealants preparation.The assisstant catalyst of amino group containing can accelerate curing speed while not affect the mechanical properties of silicone sealants. It should be noted that the concentation should be carefully controlled to avoid the transfer of assisstant catalyst to the surface.Moreover, the influence of concentation of chain-extender and curing agent on the mechanical properties of sealant was also disscussed. The results revealed that too little of chain-extender and curing agent would result in the sealant curing in the cylinder; on the contrary, if too much chain-extender and curing agent were incorporated, the sealants would not cure. If the concentration of curing agent remains constant, the curing speed tends to reduce upon increasing the addition of chain-extender; on the other hand, simply increasing the concentration of curing agent will give rise to the increase of modulus and the decrease of elongation at break.Additionally, study on the influence of curing time on the mechanical properties of silicone sealants revealed that the elongation at break reduced and tensile stress increased with the curing time extended, It is found that after 10 days the data did not change so much. The sealant with diketoximosilane and additive A still had a good properties of high elongation (>800%) and low modulus (100% tensile stress <0.4 MPa), after curing for 21 days at room temperature.