Original polyols based on organosolv lignin and fatty acids: New bio-based building blocks for segmented polyurethane synthesis

Article abstract of DOI:10.1039/c4gc00596a

A macropolyol was successfully prepared by combining two different biomolecules from biomass, i.e. lignin and oleic acid, using a solvent-free and catalyst-free method. The chemical structures of this original lignin-fatty acid based polyol and its intermediates were determined by ¹H NMR and FT-IR analyses. A series of polyurethanes (PUs) were then prepared by a two-step procedure. Three linear isocyanate prepolymers were first synthesized from 4,4′-methylenebis(phenyl isocyanate) and poly(propylene)glycol of different molecular weights (PPG-425, 1000 and 2000 g mol^-1). These intermediates were used to obtain different PU macromolecular architectures by varying reaction parameters in the presence of the lignin-fatty acid based macropolyol. The final lignin-based polyurethanes were thoroughly chemically characterized by FT-IR studies, whereas the properties of these polymers were assessed by DSC, TGA, DMTA, and tensile test experiments. All these were performed in order to evaluate the influence of the NCO : OH molar ratio (from 0.2 to 1.0) as well as the influence of the PPG chain length. These new polymers with controlled architectures presented advanced properties. Their high biobased content (until 89%) depicts an important output for the valorisation of lignin and they may be an optimal alternative to conventional PUs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00596a

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Original polyols based on organosolv lignin and
fatty acids: new bio-based building blocks for
segmented polyurethane synthesis†
Cite this: DOI: 10.1039/c4gc00596a
Stéphanie Laurichesse,a,b Cédric Huilleta and Luc Avérous*a
A macropolyol was successfully prepared by combining two different biomolecules from biomass,
i.e. lignin and oleic acid, using a solvent-free and catalyst-free method. The chemical structures of this
original lignin–fatty acid based polyol and its intermediates were determined by 1H NMR and FT-IR
analyses. A series of polyurethanes (PUs) were then prepared by a two-step procedure. Three linear iso-
cyanate prepolymers were first synthesized from 4,4’-methylenebis(phenyl isocyanate) and poly(propyl-
ene)glycol of different molecular weights (PPG – 425, 1000 and 2000 g mol−1). These intermediates
were used to obtain different PU macromolecular architectures by varying reaction parameters in the
presence of the lignin–fatty acid based macropolyol. The final lignin-based polyurethanes were
thoroughly chemically characterized by FT-IR studies, whereas the properties of these polymers were
assessed by DSC, TGA, DMTA, and tensile test experiments. All these were performed in order to evaluate
the influence of the NCO : OH molar ratio (from 0.2 to 1.0) as well as the influence of the PPG
chain length. These new polymers with controlled architectures presented advanced properties. Their
high biobased content (until 89%) depicts an important output for the valorisation of lignin and they may
be an optimal alternative to conventional PUs.
Received 3rd April 2014,
Accepted 18th June 2014
DOI: 10.1039/c4gc00596a
www.rsc.org/greenchem
residues) by different extraction processes, but its chemical
structure still remains under investigation. It is an amorphous
1. Introduction
Renewable resources have recently gained interest as
a
polymer consisting of phenylpropane units, originating from
petroleum substitute in chemical and polymer applications.1 three aromatic alcohol precursors (monolignols), p-coumaryl,
Biomass can also be the starting point of innovative pathways coniferyl and sinapyl alcohols.5 Given its unattractive struc-
for designing new tailor-made building blocks and then the ture, lignin possesses therefore significant potential for being
original macromolecular architecture.2 Among the multitude a source of macromolecular materials due to its huge avail-
of resources which are extracted from biomass, vegetable ability and variety of functional groups. In fact, the numerous
oils and lignocellulosic components represent a great potential sites on the lignin backbone offer many opportunities
and have attracted a lot of researchers, notably to design for chemical modification. Several studies have tried to take
monomers (mainly polyols) for the synthesis of bio-based advantage of this aromatic structure by performing reactions
polymers.3
that would confer to lignin’s higher reactivity and suitability
Lignin, a lignocellulosic component, represents after cellu- for industrial applications, e.g. for the synthesis of polymers.6
lose the most abundant biopolymer on earth. It is a substi- In that frame, chemical modification of lignin can be used to
tuted hyperbranched polyphenol and constitutes a natural introduce reactive sites such as available hydroxyl groups and
aromatic polymer that accounts for up to 18–35% by weight of improve the capacity of hydrogen bonding with other com-
wood.4 Lignin can be extracted from lignocellulosic biomass ponents. The production of lignin-based polymers is, however,
(wood, annual plants such as wheat straw and agricultural limited by its hindered structure, i.e. its high crosslink density
and the highly polydisperse molecular weight of the molecule.
Development of new advanced lignin-derived polymers is thus
related to chemical modifications that can be carried out on
this macromolecule. Similar to thermoplastic cellulose ester
derivatives, one method adopted to improve the properties
aBioTeam/ICPEES-ECPM, UMR 7515, Université de Strasbourg, 25 rue Becquerel,
67087 Strasbourg, Cedex 2, France. E-mail: luc.averous@unistra.fr;
Fax: +333 68852716; Tel: +333 68852784
bSOPREMA, 14 rue de Saint Nazaire, CS 60121, 67025 Strasbourg Cedex, France
†Electronic supplementary information (ESI) available. See DOI:
using lignin as a thermoplastic polymer is esterification.
This procedure is an effective way to solubilise it in a common
10.1039/c4gc00596a
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solvent and to easily characterize it. Various studies have been 1000 and 2000 g mol−1. Final PU syntheses were performed
performed on lignin esterification using organic acids such as between the biobased macropolyol and the different isocyanate
acid anhydrides or acyl chlorides in the presence of suitable prepolymers. The properties of the resulting PUs were modu-
acids or basic catalysts (sulphuric acid or pyridine). Lignin lated by varying the NCO : OH molar ratio from 0.2 to 1.0, and
esterification using diacids has already been performed7,8 but the PPG chain length. The synthesized chemicals (intermedi-
the use of fatty acids has only been seen for the functionalisa- ates and lignin-based polyurethanes) were first analysed by
tion of wood to enhance its thermoplastic properties, flexibility FT-IR and DSC. To further investigate the thermo-mechanical
and melting temperature.9,10
properties and their thermal stability, TGA, DMTA and tensile
Recently, with the rapid development of green chemistry test experiments were performed to define the structure and
and engineering, a large quantity of polymeric materials determine the behaviour of the resulting polymers.
mainly derived from plant oil and its derivatives have been
reported.11–13 In the last few decades, plentiful literature has
been dedicated to the synthesis of polyol based on vegetable oil
2. Experimental section
2.1. Raw materials
functionalization through their double bonds.14,15 These latter
are generally performed by epoxidation and direct hydroformy-
lation reactions whereas the ester bonds can be derivatized by,
e.g., transesterification or amidation. Those functionalization
Organosolv lignin (Alcell) from hardwood was supplied by
Lignol Company (Canada) and used as received without any
further purification. Specific techniques for lignin characteriz-
processes require the epoxidation of double bonds followed by
the oxirane ring-opening by ozonolysis,16 hydroformylation17
ation have been carried out according to different methods
previously reported6 (Table 1). Priolene 6936 mainly based on
and more recently metathesis,18,19 and thiol–ene coupling
reactions20–22 offer a new and green approach to modify fatty
oleic acid (91% C18 : 1, 4% C18 : 2, 2% C16, 2% C18) was pur-
chased from Croda. It is a yellow transparent viscous liquid
acid derivatives. In this way, a great variety of polyols based on
with an acid number of 199 mg KOH g−1
.
vegetable oils have been synthesized and suggests interesting
intermediates for designing, e.g., polyurethanes (PUs) with
new macromolecular architectures.
Peracetic acid (35 wt% in an acetic acid solution), dichloro-
methane (DCM – 99.9%), methanol (99.6%), tetrahydrofuran
(THF – 99.8%), para-toluene sulfonic acid (PTSA) and dibutyl
tin dilaurate (DBTDL – 95%) were all obtained from Sigma
Aldrich. Poly(propylene)glycol polymers (PPG) of 425, 1000
and 2000 g mol−1 were obtained from Acros and were dried
under reduced pressure at 100 °C before using. All of the
solvents were distilled before their utilization for chemical
synthesis.
PUs represent one of the most important and versatile
classes of polymeric materials. They can vary from thermo-
plastic to thermosetting materials with broad applications such
as flexible or rigid foams, thermoplastic elastomers, adhesives
and coatings.23 The production of PUs is based on a polyaddi-
tion reaction between oligomeric polyol with terminal hydroxyl
groups and a diisocyanate (or polyisocyanate).24 Thanks to the
high variety of designs for oligomeric polyols, it is possible to
modulate the architecture and then the final properties of
PUs. Among them, we have mainly focused our investigations
on segmented polyurethanes (PUs), well-known for their versa-
tility with a wide range of behaviours and applications. They
are multi-phase copolymers structured with soft and hard seg-
ments. The soft block is built out of a polyol responsible for
the flexibility and elastomeric character of the PU whereas the
hard block, constructed from a chain extender and isocyanate,
gives its toughness and physical performance properties.
Following the original approach we have used vegetable oil
derivatives (triglycerides and fatty acids) as a raw material for
lignin chemical modification. This method depicts an innova-
tive way for designing new tailor-made bio-based building
blocks and then new macromolecular architectures.25 The aim
of this study is focused on the esterification of lignin with
chlorinated fatty acid (oleoyl chloride) in a solvent-free system
to develop a new polyol. The resulting biobased polyester, with
an original core–shell structure, can represent a promising pre-
cursor for polymer synthesis.
2.2. General methods and analysis
1H NMR were recorded in CDCl3 on a Bruker UltraShield 300
spectrometer operating at 300 Hz. Chemical shifts (δ-ppm) are
reported in parts per million relative to the internal standard
tetramethylsilane (TMS, δ = 0.00 ppm). Polymer analysis was
performed with a size exclusion chromatography (SEC) system
from Shimadzu equipped with a pre-column PLGel 5µ, two
PLGel 5µ Mixed-C 300 mm columns, one PL 100 Å 300 mm
column and refractive index (RI) and ultra-violet (UV) detec-
tors. Chloroform (CHCl3) was used as the eluent at a flow rate
Table 1 Results of analysis performed on organosolv lignin
Moisture content (%)
SEC analysis
3.83
1000
2275
2.28
Mn (g mol−1
)
)
Mw (g mol−1
PDI (Mw/Mn)
1H NMR analysis
Total OH (mmol g−1
)
2.26
1.30
0.96
2.26
OH phenolic (mmol g−1
)
)
OH aliphatic (mmol g−1
Average hydroxyl groups/mol of lignin
This paper also reports the PU synthesis in a two-step
procedure with synthesized prepolymers. A total of three iso-
cyanate prepolymer intermediates were produced with different
molecular weights of poly(propylene)glycol (PPG), i.e. 425,
Tappi method
DSC analysis
Methoxy content (mmol g−1
)
2.07
76
Tg (°C) crude lignin
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of 1 mL min−1. All determinations of molar mass were per- dynamic storage modulus (G′) and dynamic loss modulus (G″)
formed relative to linear polystyrene standards from 580 to were measured as a function of the temperature ranging from
1 650 000 g mol−1
.
0 °C to 200 °C. A fixed strain was used to ensure that the
Fourier transformed infra-red spectroscopy was carried out measurements were taken well within the linear viscoelastic
with a Nicolet 380 FT-IR spectrometer working in attenuated range of the material investigated. Results of rheological experi-
total reflectance mode (FTIR-ATR). This analysis was used to ments are only presented in the ESI.†
detect specific chemical groups which provide direct evidence
of different reactions performed with lignin.
From uniaxial tensile tests, Young’s modulus (E), ultimate
strength (σmax) and strain at break (εmax) were obtained using a
Differential scanning calorimetry (DSC) experiments were Zwick/Roell machine. The experiments were performed at
carried out using a TA DSC Q200 calorimeter, under a nitrogen room temperature, using a crosshead speed of 20 mm min−1
atmosphere using a sample mass in the range 2–5 mg. The and a load cell of 200 N sensitivity. After adjusting the para-
samples were heated to 120 °C at a heating rate of 10 °C min−1 meters, experiments were carried out 5 times for each sample.
(first heating scan), then cooled to −80 °C at 10 °C min−1 and
finally re-heated to 120 °C at a heating rate of 10 °C min−1
(second heating scan). The glass transition temperature, Tg, is
2.3. Synthesis and characterization of lignin–fatty acid based
polyol
reported as the midpoint of the heat capacity change. These
values were determined in the second heating scan in order to
erase the previous thermal history of the samples during the
first scan.
Lignin–fatty acid based polyol (LOAP) is obtained in a multi-
step synthesis involving the esterification of lignin with oleic
acid, followed by the epoxidation of the fatty acid double bond
and then the oxirane ring-opening.
Thermogravimetric analyses (TGA) were performed on a
Hi-Res TGA 2950 apparatus from TA Instruments at a heating
rate of 10 °C min−1 up to 750 °C under a nitrogen atmosphere
(flow rate = 25 mL min−1).
2.3.1. Esterification of organosolv lignin. Lignin esterifica-
tion has been performed with oleic acid from sunflower oil
(Fig. 1). In order to improve the reactivity of the oleic acid, a
chlorination step of the compound has been carried out
before esterification. According to the procedure established
by Thiebaud et al.9,10 esterification has been performed by a
solvent-free and catalyst-free reaction using oleoyl chloride.
Chlorination step. The reaction took place in a three-necked
500 mL round-bottomed flask equipped with a magnetic
stirrer, a nitrogen-gas bubbling system and an outlet con-
nected to a wash-bottle containing an aqueous sodium hydrox-
ide solution to trap the hydrogen chloride formed. The
chlorination step is carried out with an excess of oxalyl chlor-
ide, oleic acid–oxalyl chloride (1 : 2). Oleic acid (40 g, 0.14 mol)
Dynamic mechanical thermal analysis (DMTA) was per-
formed on polymers (with dimensions around 23 × 4 × 1 mm3)
with RSA-II apparatus from TA Instruments equipped with a
liquid–nitrogen cooling system. Experiments were recorded on
PU films with traction mode at a maximum strain from
0.003% to 0.9% and a frequency of 1 Hz. The sample was
heated from −100 °C to 200 °C at a heating rate of 2 °C min−1
.
Dynamic rheological analyses (DRA) were performed using
a strain-controlled rheometer (ARES, Rheometric Scientific)
with parallel-plate geometry (25 mm in diameter). The
Fig. 1 Reaction schemes for the synthesis of the esterified lignin (EL).
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was first dissolved in ethyl acetate (300 mL) and then cooled to
0 °C under a nitrogen flow. An excess of oxalyl chloride
(36.02 g, 0.28 mol) was then added dropwise at 0 °C (15 min)
IR (cm−1): 3480 (OH), 1740 (COOCH3).
2.4. Polyurethane synthesis
and finally heated at 50 °C for 5 h. The solvent and the excess Before the PU synthesis, the synthesized lignin–fatty acid
of oxalyl chloride were then distilled out from the reaction
mixture.
based polyols have been characterized. A standard method has
been developed to evaluate the amount of hydroxyl groups
Oleoyl chloride (OC) was obtained as a yellow viscous available for the reaction with isocyanates. This parameter is
product.
defined as the hydroxyl number or as the hydroxyl index
1H NMR (300 MHz, CDCl3, δ (ppm)): 0.88 (3H, t, CH3), 2.87 (the procedure used for IOH determination is described in the
(2H, t, CH2–COCl), 5.32–5.38 (vinylic protons).
ESI†).
The general procedure followed for polyurethane (PU) syn-
IR (cm−1): 3005 (CvC–H), 1799 (CvO chloride group).
Esterification step. The esterification procedure was carried
out in a 500 mL reactor having a mechanical stirrer, a nitro-
gen-gas bubbling system and an outlet connected to a wash-
bottle containing an aqueous sodium hydroxide solution.
Organosolv dried lignin (30 g, 67.8 mmol OH) and OC (40.7 g,
135.6 mmol) were successively introduced into the reactor.
The reaction was then conducted at 130 °C under agitation
and a continuous nitrogen stream for 15 h. At the end of the
reaction a brown viscous but homogeneous product was
obtained. The esterified lignin (EL) was washed with cold
methanol in order to remove the excess acid chloride.
thesis is based on a two-step procedure. Firstly, three isocya-
nate prepolymers have been synthesized by varying the diol
(PPG) molar mass while the molar ratio of isocyanate and
hydroxyl groups remains constant. Then in a second step,
different PUs are synthesised with the bio-based polyol build-
ing blocks and the prepolymers (Fig. 3).
2.4.1. Prepolymer synthesis. A three-necked 250 mL
round-bottomed flask was fitted with a nitrogen inlet tube and
a mechanical stirrer. Poly(propylene glycol) and 4,4′-MDI
( f = 2) were added into the flask and stirred at 70 °C for
5 hours to yield an isocyanate prepolymer with a 1.8 molar
ratio of NCO : OH. The ensuing isocyanate prepolymers were
coded as P425, P1000 and P2000 for syntheses with PPG with
molecular weights of 425, 1000 and 2000 g mol−1, respectively.
The exact concentration of free isocyanate functions on the
prepolymer was then determined.
The esterification reaction was confirmed by NMR and IR
analyses and the remaining fatty acid has been quantified.
1H NMR (300 MHz, CDCl3, δ (ppm)): 0.88 (3H, t, CH3), 2.29
(2H, t, CH2–COOCH3), 2.54 (2H, t, CH2–COO–lignin), 3.88 (3H,
OCH3 lignin methoxy), 5.32–5.38 (vinylic protons).
IR (cm−1): 3005 (CvC–H), 1761 and 1737 (ester band).
2.3.2. Epoxidation of oleic acid double bond. A 500 mL
NCO content determination. This method allows determining
the amount of isocyanate that reacts with one equivalent of
round-bottomed flask was charged with esterified lignin (EL) N-dibutylamine, and gives NCO content by weight per cent.
(45 g, 72 mmol of double bonds) dissolved in DCM (250 mL)
and stirred until a homogenous solution is obtained. Peracetic
acid (10.95 g, 144 mmol) was added in small portions. The
ðVblank ꢀ VsÞ ꢁ 42 ꢁ 0:1
%NCO ¼
ð1Þ
WP
reaction mixture was stirred at room temperature for 5 h. The
organic layer was washed three times with brine and dried
over sodium sulfate. The solvent was evaporated under
reduced pressure and then dried under vacuum at 35 °C over-
night to obtain the epoxy lignin oleate (ELO).
where Vblank (mL) is the hydrochloric acid volume required for
blank titration, Vs (mL) is the HCl volume required for polyol
sample titration and WP (g) is the polyol weight.
2.4.2. Polymer synthesis. The typical procedure used is as
follows:
a three-necked 100 mL round-bottomed flask
1H NMR (300 MHz, CDCl3, δ (ppm)): 0.87 (3H, t, CH3),
1.20–1.70 (methylene protons), 2.29 (2H, t, CH2–COOCH3),
2.54 (2H, t, CH2–COO–lignin), 2.89 (2H, epoxy ring proton).
IR (cm−1): 1740 (COOCH3 – ester bond), 841 (epoxy).
2.3.3. Oxirane ring-opening synthesis. A 500 mL round-
bottomed flask was charged with epoxy lignin oleate (ELO),
methanol, and PTSA (2 wt% vs. ELO) as the catalyst. The reac-
tion mixture was stirred under reflux and a nitrogen flow for
15 h. After completion of the reaction, the methanol was dis-
tilled out under reduced pressure. The product was then dis-
solved in DCM and washed three times with brine to remove
the catalyst. The organic layer was dried over sodium sulfate,
filtered and evaporated under reduced pressure to obtain the
lignin–oleic acid polyol (LOAP).
equipped with a magnetic stirrer, a nitrogen inlet and a con-
denser was charged with LOAP and around 40 mL of anhy-
drous THF. The solution was stirred at 70 °C under a nitrogen
flow until the complete dissolution of polyol. Then, the iso-
cyanate prepolymers previously dissolved in THF (P425, P1000 or
P
2000) and DBTDL (5 wt% based on lignin monomers) were
added. The NCO : OH molar ratios, also known as isocyanate
index, were calculated on the basis of the hydroxyl index of the
polyol and NCO content from the prepolymers. Five different
molar ratios were used for the syntheses: 0.2, 0.4, 0.6, 0.8 and
1.0. The reaction mixture was stirred at 70 °C for 4 h. After
completion of the reaction, THF was evaporated and the PU
was dried overnight at 35 °C under vacuum. It may be noticed
from Table 2 that in PU series, up to 89 wt% of lignin/oleic
acid based macropoyol (LOAP) could be incorporated, high-
lighting the high content of biobased products in PU formu-
lations. The resulting PUs were characterized to determine
their structures and their physicochemical and mechanical
1H NMR (300 MHz, CDCl3, δ (ppm)): 0.88 (3H, t, CH3),
120–1.70 (methylene protons), 2.29 (2H, t, CH2–COOCH3), 2.54
(2H, t, CH2–COO–lignin), 2.99 (1H, dd, CH–OCH3), 3.41 (3H, s,
OCH3), 3.49 (1H, m, CH–OH).
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Table 2 Synthesis of the different PUs with the corresponding designations and formulations
PU formulation
Samples
Isocyanate prepolymer
P425
NCO : OH molar ratio
% LOAPa (w/w)
OH of LOAP (mol)
PPG (mol)
NCO (mol)
LPU-1
LPU-2
LPU-3
LPU-4
LPU-5
0.2
0.4
0.6
0.8
1.0
89
81
74
68
63
3.67
1.83
1.22
0.92
0.73
1
1
1
1
1
1.8
1.8
1.8
1.8
1.8
LPU-6
LPU-7
LPU-8
LPU-9
LPU-10
P1000
0.2
0.4
0.6
0.8
1.0
82
69
60
53
47
4.02
2.02
1.35
1.00
0.80
1
1
1
1
1
1.8
1.8
1.8
1.8
1.8
LPU-11
LPU-12
LPU-13
LPU-14
LPU-15
P2000
0.2
0.4
0.6
0.8
1.0
78
64
54
47
42
5.35
2.70
1.80
1.35
1.10
1
1
1
1
1
1.8
1.8
1.8
1.8
1.8
a % of biobased content (LOAP is composed of lignin and oleic acid).
properties by IR, DSC, DMTA, DRA and uniaxial tensile tests. the reaction and the properties of the resulting materials are
Rheological and mechanical tests were performed on PUs pro- discussed subsequently. The influence of reaction parameters
cessed into films obtained by compression moulding between such as a NCO : OH molar ratio and PPG molar mass on PU
two plates, at 100–120 °C.
materials is further investigated. In the last section, thermal
stability of PU is also studied by TGA experiments.
3.1. Lignin characterization
3. Results and discussion
The lignin was produced from an ethanol–water pulping
The results are presented in six sections. First, lignin has been (Alcell process) as already described elsewhere.29 The main
characterized to clearly define its chemical properties in view characteristics of this lignin have been reported in Table 1.
of chemical modification. Afterwards, the polyol building These data were found to be in the same range as that of
block and PU were analysed by FT-IR and H NMR to confirm others reported in the literature30 and were also in good agree-
1
Fig. 2 Synthetic pathway to macropolyol – LOAP (lignin oleic acid polyol) – from lignin and oleic acid. Schematic representation of lignin–fatty acid
based polyol.
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Fig. 3 Synthesis of lignin based PU (LPU).26–28
Fig. 5 1H NMR spectra of intermediates based on lignin and oleic acid.
ment with the values given by the supplier. The chemical data
constitute a key point since the hydroxyl functions were func-
tionalized. Total and phenolic hydroxyl contents were quanti-
fied according to different techniques including chemical
titration and 1H NMR based on acetylated lignin and SEC in
Fig. 4 FTIR spectra of intermediates based on lignin and oleic acid.
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chloroform.31–33 As can be observed from Table 1, results band from 1708 to 1798 cm−1 due to the formation of a more
found for Alcell lignin showed a low hydroxyl content of withdrawing chloride group and disappearance of the hydroxyl
2.26 mmol g−1 with a low number average molecular weight band in the region of 3100–3500 cm−1
.
(Mn = 1000 g mol−1), compared to kraft lignin and lignosulfo-
The esterification of lignin with OC gave EL as the main
nate. Therefore, SEC results should be taken as “relative product along with oleic acid as the remaining product from
values”. This lignin is well known to present a high purity (low the corresponding synthesis. EL was characterized by FT-IR
ash and sugar content) and also presents a good solubility in and 1H NMR. Esterification reaction resulted in a drastic
most solvents, particularly in THF and pyridine, which must reduction of the OH absorption band along with the appear-
be considered as a great advantage for chemical modifications. ance of the ester bands at 1761 and 1737 cm−1, corresponding
to the esterification of aliphatic and phenolic hydroxyl groups,
respectively. 1H NMR spectrum of EL is presented in Fig. 5.
3.2. Synthesis and characterization of macropolyol
The appearance of peaks at 3.60 ppm confirms the formation
of ester protons whereas a peak at 2.34 ppm assigned to CH2
to which the carboxylic group is attached is still remaining and
assigned to the presence of oleic acid in the final product. The
amount of oleic acid chain present in this mixture was esti-
mated by NMR integration at 80% of fatty acid grafted onto
lignin whereas 20% are free chains. EL is a very viscous
product where residual free oleic acid chains are partially
imbedded due to a challenging incomplete extraction of the
oleic acid in excess.
The main objective of this part was to synthesize a macro-
polyol. The resulting LOAP macropolyol with secondary
hydroxyl groups was designed to react in a second time with
isocyanate functions. LOAP was synthesized in a four step
pathway (Fig. 1 and 2). The structures of all the intermediates
were analysed. The targeted chemical structures were con-
firmed by FT-IR and 1H NMR spectroscopy (Fig. 4 and 5).
In the first step, chlorination of oleic acid with oxalyl chlor-
ide yield OC. FT-IR of oleic acid shows a band at 1708 cm−1
linked to the carbonyl group (CvO) from a carboxylic acid
function and a broad band from 3100 to 3500 cm−1 corres-
ponding to the hydroxyl groups, while a CvC–H stretching
vibration band is observed at 3005 cm−1. The chlorination of
oleic acid was confirmed by the shift of the carbonyl group
In the third step, EL was treated with peracetic acid to
obtain epoxy lignin oleate (ELO). The FTIR spectrum of inter-
mediate ELO shows the absence of a band at 3005 cm−1 which
was linked to the CvC–H group. The corresponding molecular
Fig. 6 FTIR spectra of PUs synthesized with P425 and different NCO : OH molar ratios.
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Table 3 Chemicals and materials used in the PU synthesis with their corresponding properties
Designation
Description
Structural formula
IOH (mg KOH g−1
)
% NCO
Functionality
Tg (°C)
LOAP
P425
P1000
P2000
Polyol
Isocyanate prepolymer
60
—
—
—
—
—
2
2
−24
−11
−30
−48
7.9
4.7
3.7
2
DBTDL
Catalyst
—
—
—
—
Fig. 7 Schematic representation of PU networks with different NCO : OH molar ratios.
1
structure was also confirmed by H NMR with the appearance This value is in the range of the hydroxyl value for bio-based
of a new peak at 2.89 ppm due to the epoxy ring protons and polyester polyols usually used for PU synthesis (between
the disappearance of the vinylic protons at 5.32 ppm. The 30 and 200 mg KOH g−1).27,34
fourth and the last step of the reaction pathway consists in the
3.3. Synthesis and characterization of PUs
ring-opening of epoxide using methanol in the presence of an
acid catalyst (PTSA) to obtain a mixture of two regioisomers
(4/4′-LOAP) containing secondary hydroxyl groups. Further
onwards, only one isomer structure will be used for the simpli-
city of the reaction scheme (see Fig. 3). The FT-IR spectrum of
the resulting macropolyol shows the appearance of a band at
3432 cm−1 due to the hydroxyl groups and another band at
1097 cm−1 attributed to the ether bond. Moreover, the appear-
ance of three new significant peaks at 2.99, 3.41 and 3.49 ppm
in the 1H NMR spectrum confirmed the ring-opening of the
epoxides. These peaks are attributed to CH to which methoxy
groups are attached, and methoxy protons and CH to which
hydroxyl groups are attached, respectively. The resulting
macropolyol was then characterized by pH metric titration; the
hydroxyl value was estimated at 60 mg KOH g−1 of the sample.
The reaction between LOAP and isocyanate was followed, ana-
lysed and confirmed by FT-IR analyses. The isocyanate absorp-
tion band is assigned at approximately 2270 cm−1, and the
absence of this absorbance can be used to monitor and check
the isocyanate group conversion during the polymerization
reaction. In this study, the absorbance corresponding to the
NH stretching region, assigned at approximately 3300 cm−1
was used to follow and confirm the reaction between isocya-
nate and hydroxyls groups but also to qualitatively evaluate the
effect of the NCO : OH molar ratio. Fig. 6 shows the FT-IR
spectra of PU samples synthesized with P425 at five different
NCO : OH ratios. The signal attributed to the OH stretching
band at around 3500 cm−1 tends to decrease with increasing
NCO : OH ratios whereas the NH stretching band becomes
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more intense. The conversion rate can be controlled and
followed through the formation of urethane functions and the
disappearance of the hydroxyl groups from macropolyol.
Concerning the properties of the synthesized PUs, thermal
and mechanical behaviours should reflect the corresponding
structural chemical organization, e.g., the cross linking
efficiency (related to the evolution of the NCO : OH molar
ratio) and the chain prepolymer length in the flexible domain
of the PU architecture. These statements will be investigated in
the following sections.
3.4. Effect of the NCO : OH molar ratio
Polyaddition reactions between LOAP and P425, P1000 or P2000
were carried out at 70 °C in anhydrous THF in the presence of
DBTDL. The NCO : OH molar ratio was regulated based on the
hydroxyl value and the isocyanate index of the macropolyol
and prepolymers respectively (Table 3).
In comparison with linear TPU resulting from the reaction
between diols and diisocyanates molecules, we may obtain a
highly branched and three-dimensional polyurethane network
derived from the lignin macropolyols35 (as suggested in Fig. 7).
In these systems, lignin should act as a crosslinking point, co-
valently bonded with more or less flexible PPG-based chains.
Table 4 showed Tg data for synthesized PUs. Neither
melting nor crystallization transition temperatures were
observed by DSC, indicating the amorphous nature of these
PUs.
DMTA experiments were carried out in order to obtain
further information on the morphology of these PUs. Fig. 8
shows storage modulus as a function of temperature for PU
films based on different NCO : OH molar ratios, in a large
temperature range from −100 to 140 °C. With increasing temp-
erature, all polymer materials showed a similar trend. In
detail, E′ presents no variations in the glassy plateau from
−100 °C to around −50 °C and shows for some samples a very
Table 4 Dynamic mechanical and thermal properties for the
synthesized polyurethanes
DSC
DMTA
Isocyanate NCO : OH
Samples prepolymer molar ratio Tg (°C) Tα (°C) νe (mol m−3
)
Fig. 8 DMTA curves of the PUs (a: E’ of PU based on P425 systems; b: E’
of PU based on P1000 systems; c: E’ of PU based on P2000 systems).
LPU-1
LPU-2
LPU-3
LPU-4
LPU-5
P425
0.2
0.4
0.6
0.8
1.0
−7
−7
53
49
42
42
41
—
20.6
35.2
75.6
28.5
−10
−7
broad transition before reaching the rubbery state, or decreas-
ing for the others until the limit of the measurement. All
the samples with high NCO : OH molar ratios demonstrated
−6
LPU-6
LPU-7
LPU-8
LPU-9
LPU-10
P1000
0.2
0.4
0.6
0.8
1.0
−27
−19
−24
−19
−15
51
44
31
29
22
—
10.1
30.8
43.0
14.6
a
rubbery plateau confirming their high cross-linking
density36,37 while those with low NCO : OH molar ratios display
a decrease of storage modulus. The resulting data are summar-
ized in Table 4. As expected, for polymers presenting a rubbery
plateau, the cross-linking density increases with the NCO :
OH molar ratio,30 which confirms that a modulation of the
properties of these PU systems is possible.
LPU-11
LPU-12
LPU-13
LPU-14
LPU-15
P2000
0.2
0.4
0.6
0.8
1.0
−36
−39
−42
−41
−42
51
50
38
34
30
—
1.8
8.2
12.2
44.8
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Table 5 Mechanical properties from uniaxial tensile tests of different synthesized PUs at ambient temperature with a crosshead speed of 20 mm
min−1
Traction
Samples Isocyanate prepolymer NCO : OH molar ratio σ Maximum strength (MPa) ε Elongation at break (%) E Young’s modulus, (MPa)
LPU-1
LPU-2
LPU-3
LPU-4
LPU-5
P425
0.2
0.4
0.6
0.8
1.0
2.7 0.1
4.7 0.7
6.6 0.8
6.1 0.2
9.6 0.8
173 22
235 62
255 34
1.6 0.1
2.5 0.2
2.3 0.3
2.0 0.2
2.8 0.2
201
2
181 10
LPU-6
LPU-7
LPU-8
LPU-9
LPU-10
P1000
0.2
0.4
0.6
0.8
1.0
3.2 0.1
3.5 0.2
4.1 0.5
5.2 1.0
—
235 22
240 22
205 15
202 21
—
0.8 0.1
0.7 0.1
0.7 0.1
0.9 0.0
—
LPU-11
LPU-12
LPU-13
LPU-14
LPU-15
P2000
0.2
0.4
0.6
0.8
1.0
1.7 0.1
1.5 0.2
1.4 0.1
1.5 0.2
—
317 12
0.5 0.0
0.2 0.0
0.2 0.0
0.2 0.0
—
360
6
230 35
237
6
—
As widely reported in the literature on lignin and/or fatty urethane with phase separation. The soft phase is composed
acid based polymers and their corresponding morphological of PPG polymers while lignin and urea groups form the hard
behaviours,26,28,38,39 we should expect a phase separation on segment. This microphase separation is also responsible for
the segmented PU materials between soft and hard segments. the elastomeric properties of these polyurethanes as shown by
Transition temperatures of both types of segments were deter- ambient stress–strain curves.28 Table 5 presents the main
mined by DSC and DMTA experiments.
mechanical properties under uniaxial tensile tests including
tensile strength, Young Modulus and elongation at break,
The Tg of the prepolymers (i.e. P425, P1000 and P2000
)
measured by DSC (Table 3) was in the range of −48 to −10 °C. obtained from the corresponding stress–strain curves. At
These values were in good correlation with the first transition ambient temperature, these materials present rubbery behav-
observed in DSC for PUs (Table 4). This Tg can be attributed to iour with a high elongation at break from 170 to 360% and the
the relative chain’s motion of PPG soft segments in the PU corresponding quite low modulus, from 0.2 to 2.8 MPa. The
architecture. It is also worth mentioning that the Tg of the soft maximum strength significantly increased with the NCO : OH
segment in the PU material is related to the NCO : OH molar molar ratio. These results can be explained by the increasing
ratio, i.e. the presence of crosslinking points.
of cross-linking points between isocyanate and hydroxyl
A second transition (Tα) was observed at higher temperature groups causing the formation of a PU with strong bonds, thus
by DMTA from the peak maximum in tan δ. This transition giving a more rigid material. The highest value is obtained for
was also observed and then confirmed by DRA (ESI†) which PU synthesized with P425 and with the highest NCO : OH molar
exhibit a relaxation temperature (maximum tan δ) in the same ratio (i.e., NCO : OH = 1.0) and reached 9.6 MPa.
range than by DMTA. This second type of transition tempera-
3.5. Effect of the prepolymer molar mass
tures were above ambient temperature from 30 to 54 °C. This
relaxation temperature can be attributed to the transition
linked to the hard segments motion, mainly composed of
lignin-based crosslinking points and urethane bonds. More-
over, modulated DSC experiments carried out on crude
organosolv lignin showed a Tg at 76 °C (Table 1). This glass
transition can be associated with the relaxation temperature
obtained from DMTA experiments with PU samples. In
addition this transition comes closer to the Tg of crude lignin
as the lignin content increases in the PU.
These different results may indicate phase separation be-
haviour of PU samples, which is in good correlation with those
reported in the literature. While the Tg attributed to the soft
segments is below the ambient temperature, the Tg of hard
domains is above. In summary, these synthesized PUs might
be at least partially described as amorphous segmented poly-
The stress–strain curves of all the polymer materials show no
decrease in the stress evolution vs. strain, until the rupture
occurred. This typical mechanical behaviour can be compared
to those of lightly crosslinked amorphous rubbers. The values
of elongation-at-break display a very interesting trend which
was linked to the evolution of the molar masses (Mn = 425,
1000 or 2000 g mol−1) of the prepolymers, P425, P1000 and
P
2000, respectively. The increasing of PPG molecular weight
results in the increasing of elongation at break for all PUs
series. In addition, longer soft segments are more able to
respond to tensile deformations leading to higher elongations
at break. Contrary to the NCO : OH molar ratio, the isocyanate
prepolymer chain length seems to impact the mechanical pro-
perties of PUs. As explained before, since the isocyanate pre-
polymers are incorporated into the PU network, the stretching
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distance between the macropolyol (LOAP) and isocyanate
groups is modified by variation of the molecular weight of
PPG. PPG chains provide flexibility to the resulting PUs and
act as a soft segment in the macromolecular architecture
where lignin macropolyol acts as a cross linking point. The
increasing of νe is linked to the decreasing of prepolymer
molecular weight. PPG with low molecular weight brings
stiffness and stability at high temperature (large rubbery
plateau) whereas PPG with higher molecular weight brings
flexibility at room temperature. However, it is relevant to
notice that LPU-11 to -13 flow up to 80–100 °C.
3.6. Thermogravimetric analysis
Segmented LPUs are mainly composed of two different struc-
tural blocks based on LOAP and isocyanate prepolymer,
respectively. Their relative compositions affect their thermal
degradation. Usually, PUs thermal decomposition is a multi-
stage process with, e.g., the decomposition of urethane bonds
and the degradation of the soft segments.40 Fig. 9(a–c) displays
the TGA and DTGA curves of PU samples under a nitrogen
atmosphere at 10 °C min−1. It can be observed that all of the
PU materials show quite a similar two-step degradation behav-
ior. The first degradation step occurs between 200 °C and
300 °C (T1max) and concerns decomposition of hard segments
attributed to the LOAP component. On the other hand the
second step is attributed to soft segments decomposition34
with T2max between 350 and 400 °C. In fact, the degradation of
isocyanate prepolymers (P425, P1000 and P2000) is based on
urethane bonds decomposition which begins at 240 °C and
reaches a maximum rate at 302–359 °C depending on the
molar mass of PPG used. While LPU based on P2000 and P1000
show a clear two-step degradation mechanism, LPU-1 to -5,
show a broader degradation with no distinctive peak on DTGA
curves similar to LOAP behavior. However, this LPU series
shows the best thermal resistance compared to the others.
This might be due to the high content of LOAP in the LPU
matrix (from 89 to 63% w/w). Fig. 9 also shows a char residue
at 750 °C for all LPU materials. This high char residue (almost
20% of weight) is due to the high thermal resistance of lignin
due to the large number of ether linkages and aromatic groups
in its chemical structure.41
4. Conclusion
Fig. 9 TGA/DTGA curves for PUs based on LOAP and isocyanate pre-
polymers (P425, P1000 and P2000). (a) LPU-1 to -5 and their respective
components (LOAP and P425). (b) LPU-6 to -10 and their respective com-
ponents (LOAP and P1000). (c) LPU-11 to -15 and their respective com-
ponents (LOAP and P2000).
The reaction conditions of lignin esterification and fatty acid
modification were investigated, leading to the synthesis of a
new original macropolyol based both on lignin and fatty acid.
This macropolyol was then characterized and subsequently
used as an efficient building block for the synthesis of PU
architectures. The macromolecular structure of the resulting
PUs were designed in series by varying the NCO : OH molar
ratio and the molecular weight of PPG soft segments. The
thermal, mechanical and rheological properties of the corres-
ponding LPUs were investigated by DSC, TGA, DMA and uni-
axial tensile tests.
For a high NCO : OH molar ratio, LPUs exhibit a tempera-
ture independent rubbery plateau (indicated by dynamic
mechanical thermal analysis) and a clear phase separation
based on DSC and DMA results. It has been concluded that
soft segments were composed of PPG polymer chains and hard
domains were mainly based on lignin crosslinking points.
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Each domain was characterized through their characteristic 12 D. V. Palaskar, A. Boyer, E. Cloutet, J. F. Le Meins,
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for soft and hard segments, respectively.
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Acknowledgements
The authors would like to thank Prof. Jean Pierre Pascault
(INSA Lyon, France) and Sébastien Gallet (ICPEES-Strasbourg,
France) for helpful discussions. We are grateful to Alsace
Region, Oséo and ANRT for their financial support. We also
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