Chemistry Reference
In-Depth Information
industries. Bio-mass is also rich in extractive compounds which have high
hydroxyl functionalities and can be extracted easily.
Although polyol structures vary greatly depending on the liquefaction con-
ditions, their phenolic structures impart thermal stability and fire-resistant
properties. Cheradame and co-workers have reported that lignin-based
liquids can react with hexamethylebe diisocyanate under rather mild con-
ditions.
36
Ionescu reported that cashew nut shell bio-mass liquefied with
polyethylene glycol has a high content of phenolic components. Due to
cashew nut shell liquid being rich in phenolic structures,
33
Mannish polyols
were successfully obtained from them, producing rigid PU foams with good
physical and mechanical properties. It was also reported that a high
aromatic content in PU foam results in low fire-retardant properties.
33
Bark-based polyols for PU foams were liquefied with glycol and poly-
ethylene glycol by Zhao and D'Souza.
34,35
Though some holocellulose was
converted into levulinate and formic esters at the temperature of 130-160 1C,
the lignin fraction can only be extracted above its soft point of 150-160 1C.
In addition, extractives from lignin, bark or waste nut shell underwent
condensation reactions with the liquefaction solvents.
6.3 Bio-based PU Foams with the Reinforcement
of Bio-mass
6.3.1 Wood Fibers
It is known that microclay and wood fiber particles both have nucleation
effects in foamed plastics.
65-67
Considering the hydroxyl groups in wood
fiber, wood fiber is expected to be a reactive polysaccharide in PU foams. Gu
and co-workers have reported that the presence of a small amount of wood
fiber furnished PU foams with higher decomposition temperatures,
3,6
lower
b-transition temperatures and higher glass-transition temperatures.
6
With
the increase in the amount of water, the content of hard segments in bio-
based PU foams increased, as indicated by the lower b-transition and higher
glass-transition temperatures [Figure 6.5(a) and (a
0
)]. As a rigid poly-
saccharide, the employment of wood fiber in the foams also offered lower
b-transition and higher glass-transition temperatures [Figure 6.5(b) and (b
0
)],
which means that some hard structures were generated. It was understood
that smaller fiber particles have a lower degree of fiber condensation, which
leads to a lower cross-linked density. Unlike increasing the amount of wood
fiber, smaller fiber particles resulted in PU foams with higher b-transition
and glass transition temperatures [Figure 6.5(c) and (c
0
)].
Because wood fiber has the ability to react with the NCO groups of iso-
cyanates, PU foams containing wood fibers have superior compressive and
tensile strengths compared with neat foams
5,6
and microclay-reinforced
foams with the increase of isocyanate index (Figure 6.6).
3
It was also reported
