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1. z Materials Science inc. Nanomaterials & Polymers
Reinstating Structural Stability of Castor Oil based Flexible
Polyurethane Foam using Glycerol
Aabid Hussain Shaik,[a]
Rajan Jain,[a]
Sindhu Manchikanti,[a]
Karthik Krishnamoorthy,[a]
Dharmendra Kumar Bal,[a]
Ariful Rahaman,[b]
Snehalata Agashe,[c]
and
Mohammed Rehaan Chandan*[a]
Use of castor oil as a renewable polyol in the polyurethane
foams has been creating an attractive research interest for
many researchers since last 5 decades. In this article, we
examine the structural stability of flexible polyurethane foam
produced using castor oil-glycerol blend by complete replace-
ment of synthetic polyol. Addition of castor oil in the foaming
blend as a complete substitute of synthetic polyol results in
instability of foam. However, addition of glycerol as a cross-
linking agent in the blend helps in overcoming this instability.
Cellular morphology, segmented phase morphology and bulk
properties of foams were investigated using scanning electron
microscope, Fourier transform infrared spectroscopy respec-
tively. Castor oil-glycerol blend significantly improves the
foaming process at low concentrations (till 15 wt% glycerol)
whereas, at higher concentrations volumetric expanding liquid
undergoes faster cross-linking leading to retarded foam growth
(above 20 wt% glycerol). Strut thickness shows a sharp decline
at 25 wt% glycerol. Polymer phase morphology shows absence
of H-bonded urea resulting in discrete hard segmented
morphology whereas urea domains undergo agglomeration
without glycerol. Foams were also characterized for thermal
and bulk mechanical properties.
Introduction
Polyurethane (PU) foams are versatile polymer which can
broadly be classified into two types-flexible polyurethane foam
and rigid polyurethane foam. Flexible polyurethane foam has
multiple applications in everyday world ranging from comfort
furnishings to intensive medical applications. Traditionally, PU
foams are synthesized using petroleum derived polyols which
are dwindling natural resource seeking viable replacement.
Plant-based oils[1]
such as palm oil,[2–6]
soybean oil,[7–9]
rapeseed
oil,[10–13]
sesame seed oil,[13]
cardanol oil,[14]
pumpkin seed oil,[13]
lignin,[15–17]
sorbitol,[18]
and castor oil[19–20]
were potential sub-
stitutes for the petroleum-based polyether polyols. Most of
these oils need pretreatment in order to incorporate hydroxyl
groups into the molecule. However, castor oil contains inherent
hydroxyl groups which make it suitable to be used as an
alternate polyol without any modification. While most of the
vegetable oils exhibit partial replacement capability of syn-
thetic polyol in the PU foam system,[19]
castor oil is a potential
candidate to replace conventional polyol completely without
any chemical modifications.[21]
It is observed from the literature
that flexible PU foam undergoes structural instability upon
addition of castor oil leading to foam collapse.[19]
This instability
can be attributed to low molecular weight of castor oil
molecules as compared to the relatively heavier petroleum-
based polyether polyol molecules, low transient viscosity
development of the foaming blend, slower reaction rate of
castor oil with isocyanate and agglomeration of hard urea
domains in the soft urethane matrix.[19]
However, only one
method has been reported in literature reinstating collapsing
foam structure upon use of castor oil. Sharma et al., (2016)
shows the introduction of Li+
ions in the formulation by
reacting castor oil with butyl lithium. Li+
ions are not only
smaller in size, but also highly electropositive in nature.[20]
This
helps in disrupting the hydrogen bonding between the hard
urea monodentate segments, a primary reason for hard domain
aggregation.[22]
In this article, various weight percent of glycerol (as a cross-
linker[23]
) in tandem with full replacement of petroleum-based
polyether polyol by castor oil were used in polyurethane foam
synthesis. Glycerol having a short chain and three hydroxyl
groups on the carbon chain was considered to be an ideal
cross-linker.[24]
Stability of the produced PU foam was analyzed
along with the thermo-mechanical properties. The novelty of
this study is to provide a basis for incorporating the renewable
content (castor oil) in the large scale synthesis of polyurethane
foam by completely replacing synthetic polyols. Moreover, bio
based glycerol or crude glycerol from industries can be used
instead of synthetic glycerol.
[a] Dr. A. Hussain Shaik, R. Jain, S. Manchikanti, K. Krishnamoorthy,
Dr. D. Kumar Bal, Dr. M. Rehaan Chandan
Colloids and Polymers Research Group, School of Chemical Engineering,
Vellore Institute of Technology, Vellore, Tamilnadu 632014, India
E-mail: chandan1816@gmail.com
[b] Dr. A. Rahaman
Manufacturing Engineering Department, School of Mechanical Engineer-
ing, Vellore Institute of Technology, Vellore, Tamilnadu 632014, India
[c] Dr. S. Agashe
Indian Polyurethane Association Technical Centre, Pune, Maharashtra
411088, India
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/slct.202000784
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2. Results and Discussion
Bulk foam properties and residual oligomer
Bulk density is an important parameter for PU foam to be used
for specific application. Bulk density of a typical flexible
polyurethane foam lies in between 0.04 to 0.08 g/cm3
.[25]
Figure 1 shows the bulk densities of the foams prepared with
varying glycerol content, which lies in the range of 0.04–0.08 g/
cm3
. It is also noted that with the increase of glycerol weight %
in the foam samples, the density increases till 15% and
decreases upon further addition of glycerol. This is due to low
degree of crosslinking at 5% and 10% glycerol. At 15, 20 and
25% glycerol composition, cross linking between polymer
chains increases; resulting in higher densities as compared to
low percent glycerol foams (5% and 10%). 15% glycerol foam
sample shows highest density of around 0.081 g/cm3
. This
result can be further verified by the cell diameters and the cell
structures as observed from cellular morphology, wherein 5%
and 10% glycerol foam samples have a more open cell
structure as compared to 15%, 20% and 25% glycerol foam
samples. It is also important to note that foam densities vary
with the NCO to OH molar ratio, wherein a higher ratio signifies
lower foam density due to increased OH conversion.[24]
Hence it
can be inferred that the densities of 15%, 20% and 25% are
higher than that of 5% and 10% samples due to higher molar
quantities of OH resulting from higher percentages of glycerol.
However, bulk density of 100% castor oil based foam was not
measured because of the collapsed foam.
Sol fraction studies were conducted to evaluate residual
oligomer or the extractable contents from the foam samples. It
was observed from figure 2 that PU foam synthesized solely
with castor oil has the highest sol fraction (around 36%). This
indicates the incomplete reaction between castor oil and
isocyanate leading to high amount of unreacted molecules.
Moreover, sol fraction shows a drastic decrease upon addition
of glycerol (5%), indicating improved reactivity and connectiv-
ity among the molecules. With the increase in glycerol (>15%),
increase in sol fraction is possibly due to unreacted secondary
hydroxyl group chains. This unreacted oligomer can be reduced
by adjusting the isocyanate index.
Foam cellular morphology and Polymer phase morphology
Figure 3 shows the scanning electron micrographs of different
foam specimens. It can be observed that the micrographs show
open cellular structure. Various parameters were obtained after
performing image analysis of the foams using ImageJ software.
It is found that mean cell diameter (MCD) of the foam increases
with increasing amounts of glycerol until 15%, beyond which
there is a decrease in the cell diameter. This is possibly due to
the slower gelling reaction at 5% and 10% glycerol foam,
thereby leading to higher cell growth. At 15% glycerol, the
gelation process is fastest resulting in lower cell diameter.
Beyond this point, the gelation process again slows down
resulting in decreasing cell diameter for 20% and 25% glycerol
samples. This result can also be confirmed with the bulk
densities obtained for the foam samples, since slower gelling
process signifies a more open cell structure which leads to
more flexible foams. The mean strut thickness (MST) of 5%,
10%, 15% and 20% glycerol samples are found to be
comparable around 800 μm. However, the MST of 25% glycerol
is very low. This can be attributed to the presence of large
volume of castor oil in transient state of modulus development,
leading to faster drainage of polymer mass. Higher strut
thickness signifies lower drainage rate of polymer while
foaming possibly due to faster modulus development.[26]
Cell
number density (CND) is defined as number of cells present in
a unit volume of foam structure. CND of 25% glycerol foam is
found to have the maximum CND in the range of 6051 cells per
micrometer cube of the foam structure, while CND of 15%
glycerol foam sample is found to have the lowest CND in the
range of 2514 cells per micrometer cube of the foam structure.
The MCD, MST and CND of typical commercial polyether polyol
flexible foam are 128 μm, 67.7 μm and 3630 per micrometer
cube of the foam structure respectively.[19]
When compared to
the results obtained with the castor oil-glycerol foam samples,
Figure 1. Bulk Density of PU foams.
Figure 2. Sol Fraction of PU foams.
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3. it can be inferred that the MCD of 20% glycerol foam sample is
comparable to that of synthetic flexible polyurethane foam.
The MST of the castor oil-glycerol foam samples are signifi-
cantly larger than the MST of the polyether polyol foam, and
the CND of the castor oil-glycerol foam sample with 10%
glycerol has CND comparable to that of the synthetic flexible
polyurethane foam.
Phase morphology plays a significant role in deciding the
properties of the polyurethane foams. Urea hard domains get
dispersed in soft urethane matrix while foaming and the nature
of dispersion decides the stability of polymer structure.[28]
FTIR
was performed on foam samples to probe the segmented
phase morphology of polymeric domains. Figures 4 shows the
FTIR spectra of the foam samples in the wave number range
1600–1800 cm 1
representing the region of carbonyl stretching.
The peak at 1725 cm 1
signifies the free urethane bonds and
peak at around 1700 cm 1
indicates free urea. Increasing the
concentration of castor oil usually leads to severe decrease in
Figure 3. Scanning Electron Micrographs of (a) 5%, (b) 10%, (c) 15%, (d) 20% and (e) 25% glycerol based foam sample.
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4. the free urea concentration and a corresponding increase in
the hydrogen-bonded urea at around 1660 cm 1
(as seen in
Figure 4 for 0% glycerol sample). This decrease in free urea
concentration indicates hard domain aggregation.[19]
Absence
of discernable peak in all samples with glycerol confirms urea
domains are well dispersed. Hence, it can be deduced that the
addition of glycerol increases the cross linking leading to
distinct hard phase segregation thereby producing stable
foams.
Thermal degradation and bulk mechanical properties
The three peaks in the central abscissa range of the data curve
(Figure 5) depict three degradation zones. Each peak refers to
the breakage of a certain type of bond present in the
polyurethane foam. The first peak signifies the breakage of C H
bonds and polyurethane foam segments which is in the range
of 250°C–320°C. At this temperature, the foam starts disinte-
grating into smaller fragments from the parent body. The
second peak signifies the breakage of the ( C=O) bond in the
urea segment and the (N H) linkages of the polymer matrix.
This occurs at the temperature range of 350°C - 380°C. The
final peak depicts the breakage of the ( C C ) bonds in the
polymer, which leads to the total collapse of the polymer. This
occurs at temperatures beyond 415°C. This type of polyur-
ethane foam can possibly find use in low-medium heat
applications such as heat resistant gloves for insulation
purposes, insulation and packing material for general applica-
tions.
A significant result obtained from thermogravimetric analy-
sis is the derivative weight loss. The curve (Figure 5(a)) sheds
light upon the temperature ranges where considerable appa-
rent weight loss due to thermal degradation occurred. A
material is considered thermally stable as long as its weight
does not change considerably as the temperature increases. By
inferring from the data table (Table 2), it can be said that the
foam sample is thermally stable up to the temperature 310°C–
325°C. There is total thermal disintegration beyond 460°C.
The data obtained from the tensile strength tests was
compiled and visually mapped in the form of a scatter-point
chart (Figure 6) as shown below. The chart maps the elongation
as percentage and the tensile strength (in MPa) for the five
polyurethane foam samples that were tested.
From figure 6, it is evident that the tensile strength of the
foam samples initially decreases and then increases with
increasing quantities of glycerol percentage in the foam
samples. 15% glycerol foam sample is found to have the lowest
tensile strength of 0.0435 MPa. The highest tensile strength is
found at 5% glycerol (0.094 MPa) followed by 10% glycerol
sample with a tensile strength of 0.083 MPa. Beyond 15%
glycerol, there is a marginal increase in the tensile strength up
to 0.055 MPa for 25% glycerol foam sample. This may be due
to the fact that at lower percentages of glycerol, there is a
higher degree of hard segment agglomeration and this might
be the reason for higher tensile strength. With further addition
of glycerol, the tensile strength decreases, reflecting minor
Figure 4. FTIR spectra of PU foams.
Figure 5. (a) Derivative weight percentage vs temperature and (b) TGA curve depicting weight vs temperature of different wt % glycerol based PU foams.
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5. changes in the tensile strength beyond 15% glycerol. This can
also be associated with the increase in cross linking with
additional glycerol which thereby reduced the hard segment
agglomeration, resulting in lower tensile strengths. Commer-
cially available flexible polyurethane foam has a tensile
strength of 0.048 MPa and elongation at break is 79%,[25]
hence
the foams samples with 15%, 20% and 25% glycerol are
comparable with regard to tensile strength. Whereas, elonga-
tion of castor oil based foam with varying amount of glycerol is
lower than that of synthetic flexible polyurethane foam, with
the highest being 41.65% for 15% glycerol. This reduction in
elongation can be associated with short chain length of castor
oil, which results in lesser elongation since percentage
elongation is directly proportional to the molecular weight of
the polymer chain.[12]
Conclusion
Flexible polyurethane foam was successfully synthesized using
castor oil and varying weight percentages of glycerol. The
prepared foams were characterized for its physical and
chemical properties, cellular morphological properties and
thermo-mechanical properties. The results obtained indicate
that the density of foams prepared is comparable to the
commercial flexible polyurethane foam. Cellular morphology
shows open cellular nature of foams and phase morphology
shows dispersed hard urea domains in the soft urethane matrix.
The tensile strength of 15%, 20% and 25% glycerol foam are
comparable to the synthetic flexible foams, whereas the
elongation of the castor oil-glycerol foams was significantly
lower due to short chain length of castor oil. Thermal analysis
reveals that the foams prepared are thermally stable up to
300°C.
Supporting Information Summary
The detailed information of materials used and polyurethane
foam synthesis procedure presented in this work, their
characterization, can be found in the supporting information.
Acknowledgement
Authors acknowledge Vellore Institute of Technology, Vellore for
providing the research facilities. We would also like to thank
Huntsman Polyurethanes Indian Ltd. for generously providing
foam chemicals.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Castor oil · Polyurethane foam · Glycerol · Block
copolymers · Cellular morphology · Mechanical Properties
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Submitted: February 25, 2020
Accepted: March 19, 2020
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