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Polymer InternationaI 41 (1996) 13-16
Cardanol-Lign in- Based Polyurethanes
Ton That Minh Tan
Polymer Research Center, HoChiMinh City University of Technology, 268 Ly Thuong Kiet, Dist. 10, HoChiMinh City, Vietnam
(Received 17 November 1995; revised version received 29 February 1996; accepted 12 March 1996)
Abstract: Polyurethane films were prepared from the reaction of cardanollignins (CL) with a different cardanolpignin ratio and toluene diisocyanate. The
crosslink density, the glass transition temperature and the mechanical properties
of the obtained films were also determined. The increase of cardanol content
used in CL improves the flexibility as well as the tensile strength and the glass
transition temperature of the polyurethane film.
K e y words: cardanol, lignin, polyurethane, swelling, DSC
INTRODUCTlON
has been paid much attention in polymer synthesis
during the last ten year~.’l-’~ The phenolation of lignin
with cardanol, and also products of cardanol-lignin
(CL) such as cardanol-lignin-formaldehyde resins and
cardanol-lignin-based
epoxy resins, have been
s t ~ d i e d . ~ ~The
, ’ ~results demonstrate that the use of
cardanol-lignin improves various properties of the
lignin-based polymers, especially the solvent compatibility as well as the heat transition and the elasticity,
because the long alkyl (C15) of cardanol can play a role
as a plasticizer/flexibilizer.’s~’9
The use of cardanol-lignin in the synthesis of the
polyurethanes, which can improve some properties of
the lignin-based polyurethanes, is the aim of this study.
The preliminary synthesis of the polyurethanes based
on CL as well as characterization of the films from them
are reported.
Lignin occurs in the plant cell wall as a polymer with
several attractive structural features, having macromolecular architecture and many types of reactive functional groups. The chemical structure of lignin is not
quite clear yet; nevertheless, the structure of lignin is
known to be a random three-dimensional network
polymer comprising phenylpropane units linked
together in different ways.’*3 Lignin is a potentially
useful raw material for the synthesis of various polymeric products ranging from wood adhesives to plastics,
such as epoxy, phenol and polyurethane.
Polyurethane (PU) is one of the most useful polymers
because various forms of P U can be obtained with
properties ranging from highly glassy to elastomeric at
room temperature. Therefore, there have been many
attempts to obtain lignin-based polyurethanes, such as
P U from hydroxyalkylated l i g n i n ~ , ~fractionated
~’~
kraft lignin,” solvolysis lignin,12 etc. In general, the
lignin-based polyurethanes have been high modulus and
brittle materials with poor solvent compatibility and
high glass transition temperature. To improve the
toughness and ultimate strength characteristics of the
PU, oligomeric glycols were used together with
lignin.13-16 Lignin with a single type of functionality
obtained by the reaction with ethylene or propylene
oxides has also been used to improve the solubility as
well as the glass transition of the PU resin^.'^*'^-^^
Cardanol, a natural alkyl phenol from cashew nut
shell liquid, which can take various reactions of phenol,
‘v4-’
EXPERIMENTAL
Materials
A softwood kraft lignin was obtained by precipitation of
an industrial kraft black liquor through the addition of
dilute sulphuric acid as described el~ewhere.~’
The
nuclear magnetic resonance (NMR) measurement of its
acetylated product provided a methoxy : aryl ratio of
1.47 and the molecular weight of C6-C3 units of lignin
was calculated to be about 180. Cardanol was obtained
by vacuum distillation of cashew nut shell liquid at
13
Polymer International 0959-8103/96/$09.000 1996 SCI. Printed in Great Britain
14
T . T. M . Tan
225-230°C, 2-6 mmHg. Chemicals used for this study
were laboratory grade and were obtained from Fluka.
under a dry nitrogen gas. The sample was first scanned
to 120°C, then cooled slowly (20"Cmin-') and recorded
a second time. T, was determined in the second scan.
Preparation of cardanol-lignin (CL)
CL was prepared by the reaction of cardanol and lignin
in the presence of sulphuric acid as a catalyst as
described in the previous s t ~ d i e s . ' ~ *The
' ~ appearance
of a new signal at 155.7ppm in 13C NMR spectra,
assigned to the carbon adjacent to the OH group in
bound cardan01,~ was observed for the CL samples.
1932
Preparation of cardanol-lignin-based polyurethane
(CLPU) films
CLPUs were synthesized by reacting CLs with 2,4toluene diisocyanate (TDI) in tetrahydrofuran solution
in the presence of stannous octoate as a catalyst (0.1%
of total sample weight) at 50°C. In all reactions, the
molar ratio between NCO groups and total OH groups
was kept constant at 1 : 1. The composition of CLPUs is
listed in Table 1. The films were cast from the reaction
solution onto a glass plate and the residual solvent was
removed by drying in a vacuum. Curing was generally
done at 100°C for 6 h followed by post-curing at 120°C
for 2h. The disappearance of the absorbance at
2275cm-' assigned to the NCO group2' and the presence of the distinctive absorbance at 1720cm- assigned to the carbonyl group of ret thane^',^^ in the
infra-red (IR) spectra of all the obtained films indicate
the formation of urethane linkages. The absorbance of
the carbonyl group of urea at 1645cm-' was also
observed.
'
Mechanical properties. The tensile strength and Young's
modulus of all films were measured according to ASTM
D638M-89 at room temperature. The crosshead speed
was set at 5 mm min- '.
RESULTS AND DISCUSSION
Crosslink density
According to the chemistry of polyurethane, the polar
groups such as urethane, urea, biuret, etc., can be found
in the obtained films. This is the reason for the polar
solvent absorption of PU film but it is also strongly
dependent on the crosslink density of the film. The
influence of cardanol content in CLPU (or bound cardanol in CL) on the crosslink density of the PU film is
shown in Fig. 1. The increase of cardanol content causes
a rapid increase in crosslink density of the samples from
CLPUl to CLPU4, but a decrease in crosslink density
of the samples from CLPU4 to CLPUS. This can be
explained by the fact that the higher the bound cardanol content in CL, the higher the degree of functionality and consequently the higher the degree of
crosslinking in the CLPU film as seen in samples
CLPUl to CLPU3. At this point, it also should be
Crosslink density
XIO' (mollcm')
Characterization of the polyurethane films
Swelling test. The effective crosslink density of the polyurethane networks from equilibrium swelling data in
dimethylformamide was determined by the FloryRhener method as described in the l i t e r a t ~ r e . ~ ~ . ~ '
Glass transition temperature (TJ. T, measurement of the
obtained films was performed on a Perkin-Elmer differential scanning calorimeter DSC-7 (Norwalk, Ct., USA).
The specimens were scanned at a rate of 10"Cmin-'
Cardanol
content 1%)
15
25
35
45
Fig. 1. Influence of cardanol content on crosslink density of
the films.
TABLE 1. Composition of CLPU films
CL sample
Bound cardanol/lignin unit
ratio (mol/mol)
CL1
c L2
c L3
c L4
c L5
C L6
0.28
0.41
0.53
0.65
0.79
0.95
~
CLPU sample
NCO/OH ratio
(mol/mol)
Calculated cardanol content
in CLPU by weight (%)
CLPUl
CLPU2
CLPU3
CLPU4
CLPU5
CLPU6
1*I
1.I
1.I
1.I
1. I
1.I
20.3
25.9
29.9
33.2
36.3
39.1
~~
POLYMER INTERNATIONAL VOL. 41, NO. 1, 1996
15
Cardanol-lignin-based polyurethanes
-20
1
.
1
Carciano1
content (%)
15
25
35
45
Fig. 2. Influence of cardanol content on T, of the films.
mentioned that in the final stages of the crosslinking
reaction the mobility of the chains is severely restricted
because of the large molecular weight as well as the
high concentration of polar groups (such as urethane
group, urea group, biuret groups, etc.), preventing
further crosslinking and hence there is only a slight
increase in crosslink density from samples CLPU3 to
CLPU4. On the contrary, the presence of the long alkyl
chain C15 of cardanol may cause a steric effect on the
final curing reaction as well as the structural density of
the network. Furthermore, the long hydrocarbon chain
can give the network a better solvent compatibility.
This leads to an increase in solvent absorption or, in
other words, a decrease in crosslink-density value. For
the samples with high cardanol content, from CLPU4
to CLPU6, the negative effect on the crosslink density
seems to be more dominant; therefore, there is a slight
decrease of crosslink-density values in these samples.
Glass transition temperature
The analysis of PU films by differential scanning calorimetry (DSC) indicates that all samples were stable at
temperatures up to 105°C and unstable at higher temperatures, probably owing to the further curing. In the
first scan the samples were, therefore, heated up to
105°C. In contrast to previous studies on lignin-based
PU modified with soft segments, such as polyethylene
no melting
glycol and polypropylene glycol,’*’ 3-’
endotherm appeared for any CLPU samples. This may
be due to the fact that the presence in the film of the
TABLE 2. Mechanical properties of the CLPU films
Sample
Tensile strength
(MPa)
Young’s modulus
(GPa)
CLPUl
CLPU2
CLPU3
CLPU4
CLPU5
CLPU6
85.2
102.6
114.9
122.3
128.1
130.2
1618
1524
1427
1 309
1196
1037
POLYMER INTERNATIONAL VOL. 41, NO. 1, 1996
long alkyl chain of cardanol, with several double bonds,
prevents crystallization. However, a strong endothermic
peak overlapping the glass transition, which is due to
enthalpy r e l a x a t i ~ n , ’ ~was
* ~ ~observed. In the second
scan only the glass transition appeared. Figure 2 presents the influence of cardanol content in CLPU on the
glass transition temperature of the polyurethane films
obtained. In all cases, T, values are related directly to
the cardanol content in CLPU because it affects the
molecular structure as well as the crosslink density of
the obtained network. The long alkyl chain in cardanol
in the
can play a role as a plasticizer/flexibili~er~*~~~
crosslinked polymers ; therefore, the T , decreases with
the increase of cardanol content. However, as discussed
above, the increase of the bound cardanol in CL (or the
cardanol content in CLPU) increases the functionality
of CL and hence improves the crosslink density of the
CLPU network. The high crosslink density inhibits
thermal movements of some segments in the network
and causes the increase of T,. This may be the reason
for the slow decrease of T, of the samples at high cardanol contents.
It is obvious that at room temperature the films from
CLPU4 to CLPU6 are glassy polymers while the films
from CLPUl to CLPU3 are amorphous.
Mechanical properties
The tensile strength and Young’s modulus of the films
shown in Table 2 indicates that the mechanical properties of the films depend on the cardanol content. The
tensile strength increases with the increase of cardanol
content in CL, as expected. This is mainly due to the
improvement in crosslink density as the cardanol
content increases. However, the increasing rate of
tensile strength decreases with the cardanol content.
In contrast to tensile strength, the Young’s modulus
decreases with the increase of cardanol content. The
main reason may be that the presence of a long alkyl
chain of cardanol gives the films more flexibility and
hence decreases the Young’s modulus.
It should also be mentioned that the mechanical tests
were performed at room temperature (20°C); therefore,
some samples were tested above their T, (CLPU1 to
PCL3) and others below (CLPU4 to CLPU6). Around
the glass transition, the strength properties of a material
depend largely upon the test temperature, and the
modulus of a crosslinked polymer increases with temperature above T, while it shows hardly any change at
This would be an additemperatures below q.36*37
tional explanation for the decrease in Young’s modulus
of the samples.
CONCLUSION
From the results obtained it is concluded that elastomeric polyurethanes can be produced from cardanol-
16
lignin and TDI. The crosslink density as well as glass
transition temperature and mechanical properties of the
polyurethanes depend strongly on the cardanol content
used in the cardanol-lignin. The phenolation of lignin
by cardanol not only increases the functionality of
lignin, which strongly affects the chemical reactions of
lignin, but also improves the flexibility of the final
lignin-based products. Topics for further investigation
include characterization of the polyurethanes based on
cardanol-lignin oxyalkylated with ethylene or propylene oxide.
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