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Preparation and Foaming Mechanism of Pyrocarbon Foams Controlled by Activated Carbon as the Transplantation Core Xuemei Li, Shouqing Liu, Yuanbo Huang, Yunwu Zheng, David P. Harper, and Zhifeng Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03826 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018
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ACS Sustainable Chemistry & Engineering
Preparation and Foaming Mechanism of Pyrocarbon Foams Controlled by Activated Carbon as the Transplantation Core Xuemei Lia,#, Shouqing Liua,#, Yuanbo Huanga, Yunwu Zhenga, David P. Harperb*, Zhifeng Zhenga,c*
a
National Joint Engineering Research Center for Highly-Efficient Utilization
Technology of Forest Biomass Resources; College of Materials Science and Engineering, Southwest Forestry University, Kunming 650224, PR China b
Center for Renewable Carbon, University of Tennessee, 2506 Jacob Dr., Knoxville
37996 USA c
Fujian Engineering and Research Center of Clean and High-valued Technologies for
Biomass; College of Energy, Xiamen University, Xiamen 361102, PR China
*
[email protected];
[email protected] KEYWORDS: Carbon foams, Transplantation core foaming, Foaming mechanism, Activated carbon, Biomass-based phenolic resins
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ABSTRACT: Carbon foams were prepared with walnut shell liquefaction resin (WLP-PF) as precursor and hexane/activated carbon (H/AC) as foaming core. The effect of the particle size of H/AC and the additive amount of H/AC on the microstructure and properties of carbon foams was investigated. Results showed that when the particle size of H/AC was in the range of 75 – 150 µm, the bulk density of carbon foams was in the range of 0.115-0.218g/cm3, the compressive strength (4.79MPa) reached maximum value, and the specific surface area of BET was 721m2/g. Use of foaming core of H/AC made resin with lower viscosity foam. Foaming core of H/AC more uniformly dispersed the blowing agent in WLP-PF resin, improved the microstructure of carbon foams, while effectively reducing the micropores in the ligaments and joints. Further, the volume shrinkage rate of carbon foams was reduced by using H/AC for foaming. The foaming mechanism using H/AC was considered to be implantation core foaming, accompanied by interfacial nucleation foaming. This led to uniform control of the size and distribution of foam cells.
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INTRODUCTION
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Foaming plays an important role in preparation of carbon foams with resin as raw materials,
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due to its low cost and simple processing1-2. Generally, foams form by bubble nucleating, growing,
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and then stabilizing in a resin matrix3-4. Bubble nucleation mainly depends on the gasification of
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blowing agent that is dispersed as small droplets in a polymer matrix. However, low solubility of
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blowing agents in a resin and large interfacial tension hinders the blowing agent dispersion in the
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resin. Homogeneous dispersion of small droplets of the blowing agent is necessary for
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homogeneous structure in the final carbon foams. Additionally, the viscosity of resin matrix
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should be more than 2000mPa·sfor the foaming process5-6. For phenol formaldehyde based foams,
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a large amount of energy is consumed to achieve the required high viscosity by means of
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dehydration under vacuum. This results in an increase in the cost to produce carbon foams.
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Additionally, high viscosity resins are not conducive to the homogeneous dispersion of blowing
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agents, which results in non-uniform cell morphologies. The foaming technology needs changed
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to obtain carbon foams with uniform, reproducible cell structure and excellent properties at low
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cost for commodity applications.
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Transplantation core foaming method was used by Guo7to study the foaming of
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plastic/wood-fiber composites. Water vapor in wood fiber acted as a blowing agent to foam
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high-density polyethylene matrix in wood-plastic composites. This foaming method decreases the
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bubble nucleation energy barrier, decreases the surface tension around the nucleation sites, and
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disperses the blowing agent evenly in the solid blown core. The result is a foam with uniform cell
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structure. Based on this notion, a low boiling point liquid absorbed on porous media can be
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dispersed uniformly in a resin matrix as a blowing agent. The resulting foam structure should be
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uniform such as those observed by Guo7. Although the preparation of carbon foams through
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transplantation core foaming method has not been reported yet, there is related research on the
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addition of the solid particles8-10, such as carbon nanofibers11, activated carbon12, hollow ceramic
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microspheres13 to enhance the compressive strength of carbon foams. Also, these studies
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demonstrate resins with low viscosity (usually less than 350 mPa•s) have improved foaming after
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adding solid particles.
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Among many factors affecting the structure and properties of carbon foams, the resin curing
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rate is very important. If the resin curing rate is too fast, it is difficult to disperse foaming additives 3
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(blowing agent, curing agent, and surfactant) homogeneously in the resin matrix, which affects the
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cell size and distribution in carbon foams. By comparison, liquefaction products phenolic resin
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(LP-PF resin) can be synthesized by using lignin or byproducts containing large amounts of lignin
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as raw materials. Because of the higher activation energy and the lower reactivity in the curing
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process14-16, the LP-PF resin has the long curing time necessary to disperse the blowing agent.
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Research on preparing carbon foams using LP-PF resin as a precursor has been reported17-18.
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However, process and foam structure relationships still need to be established.
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In this paper, walnut shell liquefaction product phenolic resin is used as a precursor for
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carbon foams. Activated carbon loaded with n-hexane (H/AC) serves as a blowing core for
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foaming. The relationships between structure and properties of carbon foams composited with
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activated carbon at different particle size and varied activate carbon amounts loaded with the
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foaming agent were investigated.
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EXPERIMENT SECTION
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Preparation of foaming core H/AC
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Activated carbon from wood with particle sizes of 830-1700µm， 75-150µm and
