Biomedical Engineering Reference
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et al ., 1998). The polyol content, isocyanate:hydroxyl group (NCO:OH) ratio, and water
content were found to affect the foaming and water absorption of the PU foams. The foaming
rates increased with increasing moisture contents of liquefied wood. Moisture content of 7%
resulted in a more than 30 times increase of foaming rate comparing with moisture content
of 0.6%. But an increase in water content may result in a decrease in cross-links between
wood polyol and isocyanate, because the NCO:OH ratio is constant. Increasing moisture
contents have significantly decreased the density of PU foams. The optimum water content
should be about 2.5% or less in this preparation condition.
Polyols for PU foam production are required to have hydroxyl levels in the range of 300
to 500. Many biomass feedstocks have a hydroxyl value greater than 15 000. For PU foam
production, liquefaction conditions must allow the preparation of polyols having hydroxyl
values in the range of 300 to 500. Research has found that liquefying agents, temperature
and processing time have a strong influence on the hydroxyl values of polyols. Sometimes,
the usability of PU foams may be determined by their rigidity/flexibility and hydrophilic
water absorbing capability (Yao et al ., 1995 ).
Since biopolyols can act as reactive adhesives, blending solid materials and liquefied
materials with or without copolymerization may present another way of making useful
materials from liquefied biomass. Lin and co-workers (1996) blended soy protein isolate
(SPI) and polyether polyols to make water-blown rigid PU foam. Foams containing SPI
exhibited thermal conductivity values similar to, or slightly higher than, foams containing
no SPI. The density, compressive strength, compressive modulus, and dimensional stability
of foams with or without SPI decreased as the initial water content increased. An idea
inspired by this research is to incorporate DDGS (as filler) into liquefied biomass to produce
PU foams (Yu et al ., 2008 ).
Sellers and co-workers (2000) used a high-pressure flow-molding press to make flat and/
or molded panels from recycled powdered thermoplastics (polyethylene and polystyrene)
and various lignocellulosic materials (wood and kenaf). The blended materials had a ratio of
1:1, and the panels had a relative density range from 600 to 900 kg/m 3 . The modulus of
elasticity values for the polystyrene/wood panels were significantly higher compared to the
polyethylene/wood panels. All plastic/fiber panels had low thickness swell values. The
reactive adhesiveness of liquefied biomass makes liquefied biomass a perfect candidate for
making high density materials by mixing them with solid particles, such as lignocellulosic
meals or DDGS meals, and then molding the mixture to high strength panels or other forms
of products. Some copolymers may be added to the mixture to enhance the performance of
the materials.
Fast pyrolysis
Fast pyrolysis processes to convert cellulosic materials to produce value-added chemicals or
materials (such as sugars, acids, phenol, etc.) generally referred to as bio-oil, have been
developed. The fast pyrolysis process requires the reduction of the biomass fuel to
approximately sawdust size. Particles are heated to between 400 and 550 °C very rapidly in
the absence of oxygen followed by cooling to condense the pyrolysis product. This treatment
fractures the molecular bonds converting the biomass to the final bio-oil. The charcoal
by-product of the process is used as fuel to produce the required high pyrolyzation
temperatures, so that the process is nearly energy neutral. The yield of bio-oil is relatively
high, at about 60% dry weight basis or higher depending on the production process
(Bridgwater et al ., 1999 ).
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