(165f) Preparation of SiO2/CE/BMI Composite Using Microwave Curing

As a new curing method, microwave curing has a lot of advantages such as high heating rate, uniform curing, high utilization rate of thermal energy, no overheating at the surface and convenient process control. It has been widely used in the processing of polymers and polymer matrix composites.

Cyanate (CE) is one of the high performance resins, having a lot of advantages such as good heat resistance, low dielectric constant, very low dielectric loss, good mechanical properties and good fire resistance property. But CE also has the disadvantage of lacking toughness. If CE is modified by bismaleimide with active diluent agent (BMI), the toughness of CE can be increased and the other advantages can be kept. Furthermore, the modified system has the low viscosity, and it is beneficial to the dispersion of the filler.

In our present work, a new type of materials used for integrate circuit board-SiO₂/CE/BMI composite-was prepared using microwave assisted curing. The filler used is nano-sized and micro-sized SiO₂. Firstly, the nano-sized and micro-sized SiO₂ were dried in an oven at 150 ^oC for 1-2h, then were modified by g-(2,3-epoxypropoxy)propyl trimethoxysilane and dispersed using ultrasound for 30 minutes. Secondly, BMI was heated up to about 110 ^oC in a flask, and then CE was added into it. The mixture was stirred for 10-15 minutes to form the prepolymer. Then the modified SiO₂ was added into the prepolymer, and the new mixture was stirred for another 10-15 minutes. Then the mixture was poured into a PTFE mould, and the mould with the mixture was put into a vacuum drying chamber for 1-2h to degas, and then was put into a microwave oven to cure using microwave radiation. To prevent the mixture from reacting tempestuously at the beginning of the curing reaction, intermittent curing method was adopted and the radiation power of the microwave oven was relatively low. As the curing reaction progresses, the radiation power could be higher and the time for continuous radiation could be longer. Thirdly, after curing in the microwave oven for about half an hour, the sample was postcured using conventional thermal curing in an oven for 2h at 150 ^oC in order to cure it thoroughly. In order to compare the microwave assisted curing with conventional thermal curing, the sample of the same formulation was also prepared by conventional thermal curing method, The curing procedure was 150 ^oC/2h+180 ^oC/2h.

DSC was employed to study the curing kinetics of SiO₂/CE/BMI composite, and the impact strength and the dielectric properties of both microwave and thermal cured samples were measured. The conclusions are: (1) The composite can be cured well by using power-changing and intermittent curing method. And a lot of time can be saved by using microwave curing. (2)The curing kinetics parameters of SiO₂/CE/BMI were calculated by Kissinger and Ozawa method: the average activation energy is 6.692 kJ/mol, and the reaction order is 1.493. (3) For the composite with the same component, the impact strength of the microwave cured samples are higher than the impact strength of the thermal cured samples. The materials cured by microwave have good toughness. In the range of our experiment, the impact strength increase with the content of the filler. (4)For the composite with the same component, the dielectric constant and dielectric loss tangent of the microwave cured samples are lower the dielectric constant and dielectric loss tangent of the thermal cured samples in a large range of frequency. In the range of our experiment, the variety of dielectric constant and dielectric loss tangent with the content of the filler does not have uniform rule, there may be interaction between the content of nano-sized and micro-sized SiO₂.