Thermally expandable microspheres are synthesized by suspension polymerization using three ethoxylated trimethylolpropane triacrylates with different molecular weights as crosslinking agents. The polymeric shell is made of acrylonitrile and methacrylonitrile and iso-octane is encapsulated as blowing agent. As the crosslinking of shell increases, the shape of the microspheres changes from spherical to bumpy due to different local crosslinking density. The expansion behavior was influenced by types and amounts of crosslinking agents. Average size and thermal stability of expandable microspheres are increased by increasing the amount of crosslinking agent. The microspheres prepared with ethoxylated (6) trimethylolpropane triacrylate to achieve optimum crosslinking results in an excellent expansion performance.

자량이 다른 세가지의 ethoxylated trimethylolpropane triacrylate를 가교제로 사용한 현탁 중합을 통하여 열팽창 캡슐을 제조하였다. 고분자 쉘은 아크릴로나이트릴과 메타크릴로나이트릴로 구성되었으며 발포제인 아이소옥탄을 캡슐화하였다. 쉘의 가교정도가 증가함에 따라 국부의 가교밀도 차이에 의하여 캡슐의 형태는 구형에서 울퉁불퉁하게 변화하였다. 팽창 특성은 가교제의 종류와 함량에 의해 영향을 받았다. 캡슐의 입경은 가교제의 양이 증가함에 따라 증가하였다. 최적의 가교도를 지닌 ethoxylated (6) trimethylolpropane triacrylate를 사용한 캡슐의 열팽창 특성이 가장 우수하였다.

Keywords: thermally expandable microspheres, suspension polymerization, trimethylolpropane triacrylate, crosslinking agent

Thermally expandable microspheres (TEMs) represent the core-shell polymeric capsules which have an expansion characteristic upon heating.1 Since the manufacturing method of thermally expandable microspheres was originally developed by Dow Chemical Co., in the early 1970’s, the preparation method, composition of the polymer shell, and applications of the microspheres have been widely investigated.1-3 Thermally expandable microspheres are applied to variety of fields to achieve many industrial purposes such as weight reduction, bulk increase, and printing ink to improve the surface properties. Specifically, in the automotive industry, they are used to reduce noise and weight by underbody coatings and to improve corrosion resistance to protect metal materials.^4,5 The thermally expandable core-shell polymeric particles encapsulate volatile liquid hydrocarbons as blowing agents. The thermoplastic shell of expandable microspheres is polymerized using polyethylene, polyamide, and poly(vinyl chloride), etc. and crosslinking agent can be added to improve expansion properties. The synthesized microspheres typically have the diameter of 5-50 μm and the shell thickness of 3- 7 μm.⁶ The thermoplastic polymer shell is softened at around the glass transition temperature during heating, subsequently the liquid hydrocarbon that was contained inside the capsules is vaporized and erupted by internal pressure. Accordingly, the thermally treated microspheres expand to 50-100 times larger volume like microballoons and their density is decreased from 1100 to 30 kg/m.^3,7,8 In order to achieve excellent expandable microspheres, the conditions that the balancing of the melt viscosity between the shell polymer and the softening point, the choice of a gas-proof polymeric shell composition, and the boiling point and the molecular structure of the hydrocarbon as a blowing agent should be carefully considered.^9,10 During heating, intensity, durability, and ductility of the polymeric shell are also important factors in mechanical and chemical properties. Suspension polymerization method has been widely selected as the manufacturing technique of expandable microspheres, in which the monomer as a dispersion phase is suspended into water as a continuous phase containing stabilizing agent and monomer-soluble initiator by stirring.^11,12 Polymerization takes place within the monomer droplets and the polymer sediment is separated from the monomer droplets because of the lack of solubility, and finally polymers in shape of microspheres are obtained with high conversion.^13,14
It has been known that the degree of crosslinking in heterogenous polymerization causes changes in morphology of the resulting particles. For the synthesis of TEMs with good performances, a proper choice of crosslinking agents and degree of crosslinking causes the stability of shell upon heating, onset of the expansion, and morphology of the particles. However, only a few publications reports the importance of crosslinking.^1,2,15 In this study, in order to produce stable microspheres and improve expansion property of polymeric shell, a series of ethoxylated trimethylolpropane triacrylates (TMPTAs) having tri-vinyl groups and different lengths of ethoxy groups was employed. In conclusion, TMPTAs significantly contributed to enhance the formation of stable microspheres and improved thermal expansion property.