Preparation Of Microcellular Epoxy Foams Using Chemical Foaming Agent And Their Properties Research

Microcellular foaming is one of the effective ways to fabricate plastic products with lightweight and functionality while maintaining good mechanical properties.Since this concept was proposed in the 1980s,microcellular foams have been attracted extensive attention.Many researches of preparing microcellular foams were concerning on thermoplastic polymers at home and abroad,however,there were scare reports fouced on the preparation of thermosetting microcellula epoxy foams.Moreover,the preparation proceduce reported mainly adopted the supercritical CO₂ as blowing agent using batch foaming process,which was difficult to be applied in practice due to its long process time and the prepared products with small size.This paper chose epoxy-anhydride system using chemical foaming agent as the research object.The gel network of epoxy precursor was regulated by adding inorganic nanoparticles,changing epoxy components,as well as process parameters,and thus a novel process was introduced to prepare microcellular epoxy foam through free foaming.In addition,the underlying mechanism about tuning cell morphology was also investigated.In order to further decrease the cell size,epoxy foams were prepared through limited foaming process.The results showed that the bubbles size can be reduced by limited foaming process,and simitanously the glass transition temperature（T_g）of these foams was unaffected by postcuring temperature and time,which contradicted that indicated by the Time–Temperature–Transformation（TTT）cure diagram.After analyzing the molecular mechanism of this phenomenon,a novel two-step foaming method combined processes of limited-foaming and free-foaming process was established,which was confirmed to be an effective method to resolve the prombles of limited-foaming process.Conductive microcellular epoxy composite foams were prepared through introducing inorganic carbon（carbon nanotube）as conductive fillers.The effect of density and foaming methods on mechanical properties,thermal insulation perfanmance and electrical properties of epoxy foams was also investigated,which provided a guide for design and preparation of high-quality and functional microcellular epoxy composite foams.The main results of this research are as follows:Firstly,the effect of process parameters,inorganic nucleating agent,and resin composition on the foaming behavior of epoxy foam prepared through free foaming was studied.The mechanical and thermal insulation performances were also discussed to obtain the relationship between the properties and microstructures of the epoxy composite foams.The experimental results showed that an appropriate precuring extent was very important to obtain high-quality epoxy foams.The cell size increased with the increase of foaming temperature as well as the content of chemical foaming agent.In addition,the addition of inorganic nanoparticles can improve the cell morphology,although there were different results between different inorganic nanoparticles.The organic modified montmorillonite（OMMT）was proved to be more effective in improving the cell morphology than that of silicon dioxide（SiO₂）,and synergism effect was found when these two particles were used together as nucleating agent.The introduction of resin with high epoxy value can effectively increase the crosslink density of the precursor and thus reduce the the bubble size.However,epoxy foams with cell size less than 30μm was hard to prepare through free-foaming process.The mechanical properties tests showed that the compressive properties and the storage modulus at room temperature can be increased by adding organic modified montmorillonite（OMMT）as well as resins with high epoxy value which can enhance the crosslink density of the foams.Compared with epoxy syntactic foams which were obtained by employed hollow glass microspheres as lightweight filler in epoxy resin,the epoxy foams prepared by chemical foaming method showed better themal insulation performances whereas weak compressive properties.The bubbles size of epoxy foams decreased from 52μm to 14μm while the cell density increased from 3.94×10⁷ cells/cm³ to 9.66×10⁷ cells/cm³,when the the foaming process was conducted in a limited space that is smaller than the volume of a specimen with the same weight but foamed in a free space,resulting in the density of the foams increased from 0.279 g/cm³ to0.993 g/cm³.In addition,the T_g,which can not be tuned by postcuring temperature and time,and the initial decomposition temperature of epoxy foams prepared by limiting foaming process was much lower than that of epoxy foams prepared by free foaming process.This phenomenon,which was related to high gas pressure in the bubbles,contradicted that indicated by the TTT cure diagram.High bubble pressure promoted the movement of molecular chains under heating at low temperature and simultaneously suppressed the etherification cross-linking reaction during post-curing.In order to obtain lightweight epoxy foams with small bubble size,and simultaneously to eliminate the influence of compressive gas in the samples,a novel two-step foaming method which combined processes of the free foaming and limited foaming was developed using epoxy resin/carbon nanotubes（EP/CNTs）system as the research object.The effect of limited time,foaming space and foaming agent content on the foaming behavior was investigated.The electrical properties of the epoxy foams were also studied.Compared with that of epoxy foams prepared through the free-foaming method,the epoxy foams prepared by two-step method showed smaller cell size,while had similar crosslink density.The rules that affected the cell morphology during two-step foaming process were special and the electrical conductivity of the foams exhibited new regulation.CNTs can not only improve the quality of cell morphology,but also enhance the mechanical and electrical properties.However,the thermal conductivity of the composite foams was not significantly increased.The epoxy foams with different density were prepared through three different processes（Free-foaming,Limited-foaming,Two-step foaming）,and the mechanical properties of different foams were also investigated.The results showed that the foaming processes not only affected the cellular structure,but also affected the cross-linking networks of matrix and the conductive network structures.The electrical conductivity of epoxy foams prepared through free-foaming process was the highest,the thermal conductivity of epoxy foams prepared through limited-foaming process was the highest,while the mechanical properties of of epoxy foams prepared through two-step foaming process were the highest among these three kinds of foams prepared through different methods at the same density.