Chemistry Reference
In-Depth Information
6.3.2 Lignin
Lignin is the second richest bio-resource after cellulose. Although lignin
undergoes significant structural changes when isolated from plant cell walls,
it still has abundant phenol, carboxyl and hydroxyl groups, which can react
with isocyanates to form urethane bonds (see Figure 6.10). Lignin acts as a
network former due to its higher functionality compared to conventional
vegetable-oil-based polyols. Due to lignin containing aromatic hydroxyl
groups, the majority of lignin, either organic solvent lignin or kraft lignin, is
chemically cross-linked with the isocyanates during the urethane reaction, not
just physically trapped in the foams. That is the reason lignin is used as filler
rather than a polyol precursor when replacing petroleum-based polyols.
12,68-71
Generally, the aromatic hydroxyl groups in lignin provide it with a higher
reactivity than the aliphatic hydroxyl groups in wood fiber and polyols.
72
According to a report by Yoshida and co-workers,
68
the effective contri-
bution of lignin to the formation of cross-linked PU resin is apparent at low
NCO/OH ratios (less than 1), where the aromatic hydroxyl groups in lignin
compete with the aliphatic hydroxyl groups in polyols. Variable PUs, from soft
to hard, can be prepared at low NCO/OH ratios (0.5-1.2) by combining the
effects of increasing cross-link density and chain stiffness with increasing
lignin content. Moreover, the introduction of lignin increased the glass-
transition temperature due to the increase in cross-link density.
70
In addition,
either flexible but weak, or tough PU materials can be obtained depending on
the NCO/OH ratio with low lignin content.
68
However, the cross-link density
of PU composites increased, as did their Young's moduli at various NCO/OH
ratios with increasing lignin content.
68-70
In addition, a high lignin content
resulted in hard and brittle PU materials regardless of the NCO/OH ratio
used, due to the combined effect of the increased cross-link density caused by
the high functionality of the lignin, and the increase in chain stiffness.
68,69
At
high levels of lignin (over 30%) and low NCO/OH ratios (less than 1.5), the
ultimate stress increased with the increase in lignin content (up to 35%).
68,70
Different lignins from different separation methods have different re-
inforcement behaviors in PU foams.
71
Nonetheless, rigid PU foams had
acceptable cell structures and compressive strengths with the addition of
lignin in the range of 20-30%. There is no doubt that the addition of lignin
to polyols can increase the viscosity of their blend.
71
As discussed above, the
ratio of NCO to OH affected the reinforcement behavior of lignin in PU
foams. PU foams had lower compressive strengths and foam densities,
compared to neat foams, when they had a high level of lignin content and
the ratio of NCO to OH was 1.1 and 1.3.
71
However, the presence of lignin can
increase the foam density, compressive and impact strength and modulus of
rigid PU foams when used with a low content (around 5%) at higher NCO/OH
ratios (1.4).
12
Nevertheless, the effects of lignin on PU foams are similar to
those seen by reinforcement with wood fibers, as mentioned above.
Lignin bio-mass reinforcement produced PU foams with bio-degradability.
It is known that lignin, as a polyphenolic material, has an intrinsically high
