Biomedical Engineering Reference
In-Depth Information
process, in which the wood and starch were added at the same time, was independent of the
biomass composition (wood:starch ratio) but dependent on the liquid ratio and the catalyst
concentration. When the liquid ratio was decreased close to one (no matter what the
wood:starch ratio was), about 40% of the wood became difficult to liquefy. Based on these
findings, a stepwise liquefaction procedure was proposed in which the wood could be
preliquefied alone at a relatively large liquid ratio followed by the addition and liquefaction
of the starch. By this procedure, a large biomass-content liquid was prepared with a relatively
small amount of unliquefied residue. Similar results were observed by Cho and co-workers
(1998), who, taking a similar approach, studied the effect of addition of liquefied starch to a
corn stover liquefaction system and found that the liquefaction process was promoted and a
higher corn stover to liquefying reagent ratio could be used.
It appears that for each type of biomass, liquefying conditions must be developed.
These conditions may include liquefying reagents, catalysts, temperature and time.
Different types of biomass contain different functional groups, which determine the
potential use for the liquefied biomass. Currently, most research has been centered on how
effective the liquefaction process is in terms of yield, time and energy use. Little effort has
been made to understand the processes and the chemical profiles of the liquefied biomass,
not to mention controlling the processes to produce liquefied biomass with desirable
chemical profiles.
Liquefied biomass may contain low molecular weight polymers. These low molecular
weight polymers can be directly used as final products such as adhesive (glue) (Chen and
Chen, 1996). They also can be copolymerized to produce other polymeric materials such as
polyurethane, polyester, fibers, and so on. If they are blended with other biomass particles,
such as lignocellulosic particles or distiller's dried grains with solubles (DDGS), high
density panels or molded articles can be produced.
Pu and Shiraishi (1994) reported that water-soluble adhesives were prepared from a
phenol liquefied wood solution. The adhesives revealed satisfactory waterproof quality.
Addition of alkylresorcinol as a cure promoter to the adhesives resulted in a medium
temperature, curable, aqueous phenol resin adhesive.
The large quantity of hydroxyl groups in liquefied biomass is perfect for the production
of polyurethane (PU) through copolymerization. The use of PU foams continues to grow at
a rapid pace throughout the world, attributed to their light weight, excellent strength:weight
ratio, energy absorbing performance, and comfort features. Currently, PU foams are
produced primarily from non-renewable, fossil-origin chemicals. Interest in making PU
from renewable resources is increasing. Kurimoto and co-workers (2001) prepared
polyurethane (PU) films by solution-casting through copolymerization between liquefied
wood and polymeric methylene diphenylene diisocyanate (PMDI) at [NCO]:[OH] ratios of
1.0 and 1.2. The PU films prepared from liquefied wood with high viscosity were found to
be more rigid than the films prepared from the liquefied wood with low viscosity. The
increase in the viscosity contributed to increases in the cross-link density of the PU films.
Varying the viscosity is a way to control the mechanical properties of PU films at a constant
[NCO]:[OH] ratio. In a separate study, Kurimoto and co-workers found that the properties
of PU films were affected by the isocyanate:hydroxyl group ([NCO]:[OH]) ratio and wood
content in PU film. The increase of wood content at a [NCO]:[OH] ratio of 1.0 significantly
enhanced the Young's modulus and reduced the maximum elongation of PU film. The rigid
mechanical properties were due to the increase of dissolved wood fragments.
PU foams can also be prepared from liquefied woods or starch, diphenylmethane
diisocyanate (MDI), catalyst, foaming stabilizer, and viscosity aids (Yao et al ., 1995 ; Cho
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