Microcapsule Toughening Polymeric Composites

Cyanate ester (CE) and bismaleimide(BMI) resins, as two kinds of high performancematerials, have been used in aerospace and electronic industries, etc. owing to theirexcellent mechanical properties, good thermal stability and resistance to fatigue, etc.However, the cured CE and BMI resins show significant brittleness because of theirhigh crosslink densities, which limit their applications. Many researches have been doneto enhance the toughness of cured CE and BMI. Microcapsules filled with liquidmaterials have been embedded in polymeric matrix to toughen. The applications ofthese microcapsules to polymeric matrix are potential. The synthesis of microcapsules,which can be applied to polymeric matrix, and the preparation of polymer compositesembedded microcapsules have been paid more attention. In the present study,microcapsules with good thermal stability have been synthesized, and polymercomposites embedded microcapsules based on the CE and BMI resin matrixes havebeen prepared. The main research work can be described as following:A new series of poly(urea-formaldehyde)(PUF) microcapsules filled with epoxyresins(diglycidyl ether of bisphenol A: DGEBPA) (MCEs) were prepared by in situpolymerization technology with urea and formaldehyde as wall materials and DGEBPAas core material. The different processing parameters such as different prepolymers,weight ratios of urea to formaldehyde, the agitation rates, the concentration of surfactant,the adjusting time of pH and the ratio of raising temperature on the physical propertiesof MCEs were investigated. The chemical structure of MCEs was characterized usingFourier-transform infrared spectroscopy (FTIR). Morphologies of MCEs were observedusing optical microscopy (OM), metalloscope (MS) and scanning electron microscopy(SEM). The composition on the surface of microcapsule was analyzed by using energydispersive analysis of X-ray (EDAX). The wall shell thickness, diameter and sizedistribution of MCEs were determined by OM. The thermal properties of microcapsuleswere investigated using differential scanning calorimetry (DSC) and thermogravimetricanalysis (TGA), the permeability of MCEs in acetone was investigated. Additionally, the mechanical property of MCEs has been investigated. The results indicate that MCEscan be synthesized successfully by in situ polymerization technology, the size andmorphology of MCEs can be controlled by adjusting processing parameters. Thesynthesized MCEs exhibit good storage stability, thermal stability and excellent solventresistance and show a certain mechanical property.The MCEs were applied to 4,4"-dicyanate diphenylpropane (BADCy) resins. Theinfluences of MCEs on the reactivity of BADCy resins have been discussed. TheBADCy/MCEs and fibre reinforced BADCy/MCEs composites have been prepared.The influences of MCEs on the mechanical properties have been investigated, themorphologies of composites were characterized by SEM and the toughening mechanismof MCEs on the composites has been analyzed. The results indicate that MCEs candecrease the gel time of BADCy resins at lower temperature owing to the core materialdiffusion during longer heating process, and MCEs have no significant effect on the geltime at higher temperature. The addition of MCEs can decrease the reactivity ofBADCy resins. The proper addition of MCEs can improve the mechanical propertiesand hot-water resistance of BADCy matrix and fibre reinforced BADCy composites.But MCEs can reduce the initial thermal decomposition temperature(T_d) of BADCymatrix and the glass transition temperature(T_g) of fibre reinforced BADCy composites.The MCEs were applied to O,O'-diallylbisphenol A (BA) modified4,4"-bismaleimidodiphenylmethane(BMI) reisns (BMI/BA), and the influences ofMCEs on the reactivity have been investigated. The BMI/BA/MCEs and fibrereinforced BMI/BA/MCEs composites have been prepared. The mechanical propertiesof composites have been evaluated, the morphologies of composites are characterizedby SEM, and the toughening mechanism of MCEs on the composites has beeninvestigated. The results indicate that MCEs have no significant influence on thereactivity of BMI/BA system. The appropriate of MCEs can enhance the mechanicalproperties and hot-water resistance of BMI/BA system and fibre reinforced BMI/BAcomposites. The addition of MCEs may reduce slightly the Td of BMI/BA, but can notdecrease the Tg of fibre reinforced BMI/BA composites.The toughening mechanisms of MCEs on the BADCy and BMI/BA composites are similar to the rubber toughening thermosetting resins. The separated MCEs phase caninduce microcracking and shear yielding in the matrix under triaxial stress conditions,which consume larger energy, meanwhile, MCEs can inhibit or blunt the propagation ofcrack. Due to the different diameter of MCEs and a certain size of interspaces betweenfibres, the toughening mechanism of MCEs with different diameter on the fibrereinforced BADCy and BMI/BA composites may be different. For MCEs with largerdiameter, the interspaces of fibres are relative smaller than the size of MCEs, MCEs cannot been protected from the intactness and then fractured easily during the pressuringprocess of the composite fabrication. The released core materials may polymerize andthen separate for the matrix, forming two phase structure, which can toughen the matrix.Meanwhile, the released core material may improve the interface adhesion of fibre andmatrix and enhance the mechanical properties. As for MCEs with smaller diameter, theinterspaces can provide the natural site for protecting MCEs. The tougheningmechanism of MCEs on the composites is that MCEs can generate microcracking andshear yielding in the matrix, inhibit or blunt the propagation of cracks.