The In Situ Polymerization Of SiO₂/PDMS Nanocomposite And Application Research On Liquid Silicone Rubber

Hydroxyl-terminated polydimethylsiloxane(PDMS)has many unique properties,such as electrical insulation,weather aging,ozone resistance,physical inertia,etc.And it can react with many functional groups due to highly active hydroxyl-termination.Therefore,it is widely used in silicone rubber,daily chemical coatings,adhesives and other fields,especially used as the base gum of condensation type liquid silicone rubber.However,the performance of silicone rubber is poor using pure PDMS,the addition of nano silica reinforcing agent is necessary.The traditional way to reinforce silicone rubber with nano silica is physical blending,but it inevitably results in the aggregation of nanoparticles.So we synthesize a series of SiO2/PDMS nanocomposite base gum via in-situ polymerization,and study the influence of performed time,nano silica content,nano silica type on the structure and properties of nanocomposite base gum.Silicone sealant,namely condensation type liquid silicone rubber,is prepared by using nanocomposite base gum and its performance including viscoelastic,cohesiveness,construction and mechanical properties,etc is studied.The main research contents and conclusions are as follows:1.The influence of performed time tp on the performance of SiO2/PDMS nanocomposite base gum.The polydimethylsiloxane(PDMS)synthesis process is investigated in order to determine the optimum reaction conditions.Two key parameters should be taken into consideration:catalysts and performed time.A series of SiO2/PDMS nanocomposite base gum are synthesized by varying the catalysts and performed time.The relationship between performed time tp and microstructure,the microstructure and rheological behavior are revealed.And the performance of nanocomposite base gum prepared by different approaches including physical blending and in-situ polymerization are compared.In terms of the catalysts,KOH has weaker catalytic activity,and is partially consumed by silica,tetramethyl ammonium hydroxide(TMAOH)has higher catalytic activity with shorter ring opening polymerization time.The performed time directly affects the molecular chain length of PDMS,leading to the unique rheological behavior of nanocomposite base gum.Compared to the physical blending,the in-situ synthesized nanocomposite base gum shows a narrow molecular weight distribution,lower viscosity and better thixotropy,effectively improving the processing performance.Meanwhile samples of 30SP and 60SP exhibit higher frequency sensitivity on molecular level and better dispersity than linear PDMS and classical binary SiO2/PDMS blends and 30SP sample begins to exhibit solid-like behavior when temperature is above 168 ?.2.The influence of silica content on the performance of SiO2/PDMS nanocomposite base gum.For nanocomposite base gum with KOH as catalyst,storage modulus(G')and loss modulus(G")gradually increase with the increase of silica content.For samples with a silica content of 10 phr,the complex viscosity,storage modulus(G')and loss modulus(G")increase as the temperature elevates.For nanocomposite base gum with TMAOH as catalyst,storage modulus(G')and loss modulus(G")also gradually increase with the increase of silica content.As silica content is as high as 6 phr,it still exhibits a Newtonian fluid behavior.For 8 phr and 10 phr samples,along with the rise of temperature,the storage modulus(G')and loss modulus(G")gradually increase,accompanied with the inversion of solid-liquid behavior.The in-situ addition of silica is favorable for the formation of the three-dimensional network structure.3.The influence of silica species on the performance of SiO2/PDMS nanocomposite base gum.The increase of specific surface area is conductive to improve the modulus.And the frequency-sensitivity on molecular level of nanocomposite base gum produced by using DNS series nano silica reinforcing agents is better than that of reinforced by R106.Nanocomposite base gum with RNS-D reinforcement has higher modulus and the peak of loss angle(tan?)is in the lower frequency.RNS-E reinforced nanocomposite base gum has lower modulus and the peak of loss angle(tan?)is in the higher frequency.Nanosilica with epoxy groups is much easier to bond with PDMS chain owing to the ring opening of epoxy and subsequent reaction with hydroxyl groups of PDMS4.SiO2/PDMS nanocomposite base gum in the application of silicone sealant.As silicone sealant,the performance of nanocomposite base gum is carefully investigated in respect of dynamic viscoelasticity,construction performance,adhesive property,thermal stability and mechanical property as well as the dispersity of nano-silica in silicone sealant.Research results show that adhesive performance of silicone sealant with in situ synthesis of nanocomposite base gum is better than that of using physical blending,exhibiting the cohesion damage.And it shows good extrusion property,easy for construction work.The dispersion of silica in the matrix is good,even the addition of silica content is up to 14 phr,all properties meet the requirements,whereas physical blending cannot accomodate 14 phr silica.At the same time,the hardness of in-situ prepared silicone sealant is higher than that of physical-blending silicone sealant,and the hardness increases with increasing of silica content.Concerning the elongation strength and breaking strength,silicon sealant made by in-situ process is inferior,which is mainly due to the unique star structure of nanocomposite base gum.