Katsumi Kamegawa (Senior Research Scientist), the Biomass Refining Technology Team (Leader: Takashi Endo), the Biomass Technology Research Center (Director: Kinya Sakanishi) of the National Institute of Advanced Industrial Science and Technology (AIST)  (President: Hiroyuki Yoshikawa), has developed a new method for producing ultra-lightweight hollow carbon fine particles (diameters ranging from several nanometers to several tens of micrometers) from lignin, which is a byproduct obtained in large quantities during the manufacture of paper or bio-ethanol, and inorganic salts.

Global warming and depletion of oil reserves are issues of global concern; hence, it is desirable to use biological resources in place of fossil resources such as oil.  It is noteworthy that almost 7 million tons of lignin, which is a biological resource (biomass), is produced as a byproduct of paper manufacturing in Japan every year, and is generally burnt as waste.

In the method developed in this study, lignin is mixed with an inorganic salt to form a complex; this complex is then pyrolyzed at 600 - 800°C, washed, and finally dried to yield sub-micrometer-sized hollow carbon particles or nanometer-sized hollow carbon particles whose diameters are in the range of 3–30 nm.  The sample in the 200-mL container is extremely light, and weighs less than 3 g, as shown in the photograph.

Rubber or plastics can be reinforced by these particles, instead of carbon black, and would be lightweight materials with improved properties.

The details of this method will be exhibited at "nano tech 2009," which will be held at Tokyo Big Sight on February 18–20, 2009.

Global warming and depletion of oil reserves are becoming issues of global concern, and hence, the use of biological resources in place of fossil resources such as oil is desired.  The carbon black, conventional carbon fine particles, is widely used as tire reinforcing materials and pigments, and about 10 million tons of carbon black is produced in the world every year.  Carbon black is usually produced by pyrolysis of fossil resources such as oil at high temperatures of ~1400°C.

In Japan, almost 7 million tons of lignin, a biomass, is produced every year as a byproduct of paper manufacturing.  Lignin is, however, burnt as waste due to its limited applications.  Large-scale mass production of bio-ethanol from wood etc. is planned, and thus the accumulation of a larger amount of lignin is expected in the near future.  Hence, it is desired  to develop new techniques that aid the effective use of lignin.

At AIST, various technologies are being developed for the use of biological resources as an alternative to fossil resources, including effective use technology for lignin.  Water soluble lignin is less than 10 nm in its molecular size.  Being a kind of thermoplastic resin, lignin melts to form carbon lumps upon heating.  In an attempt to prevent the formation of lignin lumps upon heating, we examined a complex of lignin with an inorganic salt that does not melt even at high temperatures.

This study was supported in part by the "Seeds Cultivation Experiment" of Japan Science and Technology Agency.

An aqueous solution of water-soluble lignin and an inorganic salt is first prepared, and then, small droplets of this solution are generated by spraying or ultrasonic atomization.  These droplets are then evaporated to obtain fine particles of the lignin-inorganic salt complex.  Hollow carbon particles (sub-micrometers to several ten micrometers in diameter) are obtained by pyrolysis of the complex particles at 600 - 800°C and subsequent washing and drying.

Fig. 1   Ultra-lightweight hollow carbon fine particles

The walls of these hollow carbon fine particles become thinner with an increase in the amount of certain inorganic salts, and hence, ultra-lightweight fine particles with a bulk density of less than 10 g/L can be produced.  Under specific fabrication conditions, hollow carbon fine particles with good elasticity are obtained; these particles shrink when subjected to a pressure of 4200 kg/cm² (roughly 4200 atm.) and regain their near-original shape when the pressure is released.  Figure 1 shows the scanning electron microscope image of ultra-lightweight hollow carbon fine particles in an atmospheric pressure after application of a pressure of 4200 kg/cm².

It is confirmed that by controlling the type and amount of added inorganic salts, we can obtain hollow carbon fine particles whose diameters are 3 - 30 nm; the thickness of the walls of these particles is 1 - 5 nm (Fig. 2).  We can also obtain carbon fine particles (diameter: 10–100 nm) that resemble carbon black.  Since the surface area of these fine particles is equal to or larger than that of activated carbon, they are expected to have functions related to surface, including absorbing function.

Fig. 2   Nanometer-sized hollow carbon particles

In the present study, we use commercially available lignin as the raw material.   Lignin produced as the byproduct in the paper industry (or pulp industry) is usually a waste fluid known as "black liquor."  Black liquor contains a large amount of other organic substances and inorganic salts.  We tried to separate lignin from black liquor by two methods: membrane separation, where components with relatively large molecular weights are separated using an ultra-filtration film, and precipitation separation, where carbon dioxide gas is absorbed into black liquor, and components with low solubility are precipitated under near-neutral conditions.  We could obtain hollow carbon fine particles from the waste lignin collected by the two methods.  Hence, we conclude that lignin in the form of black liquor can be used as the raw material for the production of the hollow carbon fine particles.

The lightweight hollow carbon fine particles developed in this study have sizes in the range of a few nanometers to several micrometers.  Since these hollow carbon particles have large surface areas and the fabrication conditions can be controlled to obtain carbon particles with good elasticity, we will conduct researches in applications to rubber reinforcing materials, lightweight filler, flexibility-imparting materials, heat insulators, electrical conductors, anti-static materials, adsorbents, and controlled-release materials.