Chemistry Reference
In-Depth Information
cells in bio-based foam were covered by round window membranes, com-
pared with the porous cell structures in petroleum-based foam.
5
The round
window membranes are typical cell structures in bio-based foams due to the
presence of triglycerides. This cell morphology results in higher closed-cell
content (closed-cell content is the percentage of cells with closed structures in
whole foam cells; higher closed-cell content means a superior R-value) and
dimensional stability of bio-based foams.
It is known that primary hydroxyl groups react faster with isocyanates than
secondary hydroxyl groups.
48
According to a report from Biobased Technolo-
gies,
49
natural oil polyols are suitable for producing rigid PU foams as they
enable rigid networks to form, due to their high content of primary hydroxyl
groups (reported to be up to 70%).
47
In addition, their complete miscibility
with conventional polyether polyols and hydrocarbon blowing agents makes
natural oil polyols suitable for bio-based PU foammanufacturing. According to
a study by Gautam and co-workers,
50
polyester PU is more susceptible to bio-
degradation by bacteria under controlled laboratory conditions, compared to
polyether PU. It is known that bio-based polyols made from plant or vegetable
oils are typical polyether polyols, due to their high levels of triglycerides. As a
result of their higher renewable content, bio-based PUs will be broken down
more easily, through their labile chemical moieties, by fungal attack.
51
Similar
results were also reported by Mathur and Prasad,
52
who reported that bio-
based PU foams made with castor oil had a high resistance to thermal deg-
radation, and could be used for roof insulation at temperatures above 80 1C.
53
However, bio-based PU foams can still degrade under thermal conditions.
Benes and co-workers reported that PU materials made with a castor-oil-based
polyol were decomposed successfully at 180 1C by basic hydrolysis and the
transesterification of the triglycerides of castor oil.
27
According to a report by
Hou,
54
bio-based PU foams were degraded either under cyclic compressive
loading conditions or aging conditions under ASTM (American Society for
Testing and Materials) D3574, when they were exposed to 140 1Cfor22hwith
a relative humidity of 45%, or to 50 1C for 22 h at 95% relative humidity.
It is known that a higher isocyanate index produces polyisocyanurate-type
foams with higher thermal stability.
55
Although the triglyceride structures in
vegetable oils consist of unsaturated chains, which have poor oxidative
stability,
56
unsaturated structures in triglycerides at 3010 cm
1
and
1654 cm
1
disappeared after epoxidation and hydroxylation, and the
hydroxyl peak of bio-based polyols appeared at 3440 cm
1
, as indicated in
Figure 6.2.
5
According to Gu's study,
5
soybean-based PU foams have higher
glass-transition temperatures and inferior cryogenic properties compared to
petroleum-based foams. However, it has lower thermal degradation in the
degradation of urethane segments due to natural molecular chains with
lower thermal stability than petroleum skeletons. Also, bio-foams had better
thermal stability at a high-temperature level. Guo also reported that cyclo-
pentane blown-rigid PU foams produced using soybean-oil-based polyols
had high thermal stabilities.
57
The increase of thermal stability of bio-based
PU foams, results in excellent dimensional stability. Naturally occurring oils
contain triglycerides which can generate stable three-dimensional networks
after reacting with the NCO groups of isocyanates. A very small shrinkage
