Diisocyanate free and melt polycondensation preparation of bio-based unsaturated poly(ester-urethane)s and their properties as UV curable coating materials

Article abstract of DOI:10.1039/c4ra08665a

This paper reported the synthesis of bio-based unsaturated poly(ester-urethane)s via a nonisocyanate route, by metal-catalyzed melt polycondensation of itaconic acid with urethanediols. Three novel types of bio-based unsaturated poly(ester-urethane)s, namely, poly(urethanediol 2-itaconic acid), poly(urethanediol 4-itaconic acid) and poly(urethanediol 6-itaconic acid) (poly(U2-IA), poly(U4-IA) and poly (U6-IA) for short code, respectively), were prepared by a green synthetic route. The urethane linkage was formed by the reaction of two equivalent of ethylene carbonate with 1,6-hexanediamine, 1,4-butanediamine and 1,2-ethanediamine to form urethanediols. The urethanediols underwent polymerization with itaconic acid (IA) in the presence of metal catalyst dibutyltin dilaurate (DBTL) to produce low-molecular-weight bio-based unsaturated polyurethanes. Then, these bio-based unsaturated poly(ester-urethane)s were formulated with free radical photoinitiator and curing promoter to prepare UV curable polyurethane systems. After UV curing, the tensile properties, thermal properties and general coating properties of the three UV-cured polyurethane films were similar to that of UV cured polyurethane films prepared by polyurethane-acrylate (PUA). The results suggested that the obtained bio-based unsaturated polyurethanes could serve as coating materials.

Full text of DOI:10.1039/c4ra08665a

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COMMUNICATION
Diisocyanate free and melt polycondensation
preparation of bio-based unsaturated poly(ester-
urethane)s and their properties as UV curable
coating materials†
Cite this: RSC Adv., 2014, 4, 49471
Received 14th August 2014
Accepted 26th September 2014
*
Lijing Han, Jinyue Dai, Lisheng Zhang, Songqi Ma, Jun Deng, Ruoyu Zhang
DOI: 10.1039/c4ra08665a
*
and Jin Zhu
www.rsc.org/advances
This paper reported the synthesis of bio-based unsaturated poly(ester- polyurethanes. However, diisocyanate are still not bio-based
urethane)s via a nonisocyanate route, by metal-catalyzed melt poly- and are considerably harmful for human health. From the
condensation of itaconic acid with urethanediols. Three novel types of standpoint of green chemistry, diisocyanate- and phosgene-free
bio-based unsaturated poly(ester-urethane)s, namely, poly- synthetic routes for the production of the next generation of
(urethanediol 2-itaconic acid), poly(urethanediol 4-itaconic acid) and polyurethanes are needed, especially, bio-based polyurethanes
poly(urethanediol 6-itaconic acid) (poly(U2-IA), poly(U4-IA) and poly derived from renewable resources.
(U6-IA) for short code, respectively), were prepared by a green
A new class of poly(ester-urethane) (PEU), synthesized by the
synthetic route. The urethane linkage was formed by the reaction of polyesterication of urethanediols with various diacids/diesters
two equivalent of ethylene carbonate with 1,6-hexanediamine, 1,4- using lipase, is starting to attract researchers. Since the
butanediamine and 1,2-ethanediamine to form urethanediols. The urethanediols were prepared by the reaction of diamines and
urethanediols underwent polymerization with itaconic acid (IA) in the cyclic carbonates, such as ethylene carbonate and trimethylene
presence of metal catalyst dibutyltin dilaurate (DBTL) to produce low- carbonate, from an ecological and economical point of view,3
molecular-weight bio-based unsaturated polyurethanes. Then, these this new PEU is much ‘greener’ than before. Interestingly, this
bio-based unsaturated poly(ester-urethane)s were formulated with kind of PEU has attracted strong interest in the elds of
free radical photoinitiator and curing promoter to prepare UV curable biomaterials and commodity plastics due in part to the fact that
polyurethane systems. After UV curing, the tensile properties, thermal it exhibited similar mechanical properties compared with
properties and general coating properties of the three UV-cured conventional polyurethanes.4,5
polyurethane films were similar to that of UV cured polyurethane
McCabe and Taylor reported the enzymatic synthesis using
films prepared by polyurethane-acrylate (PUA). The results suggested lipase of a novel polyester polyurethane with bis(hydroxyethyl)-
that the obtained bio-based unsaturated polyurethanes could serve as carbamates.6 Considering the desired physical properties of the
coating materials.
nished polyester polyurethane, they used urethanediol with
about 10% of the total diol. It has also been reported that
poly(ester-urethane)s were prepared by the lipase-catalyzed
direct polycondensation of urethane-containing diacids and
diols.7 In the recent study, to produce a high-molecular-weight
polyester-based polyurethane, the cyclic ester-urethane
oligomer was rstly prepared by the transesterication of
dimethyl adipate and urethanediol using Candida Antarctica
lipase (lipase CA) in dilute anisole solution, followed by the
ring-opening polymerization (ROP) of the cyclic ester-urethane
oligomer in a more concentrated solution.8,9 However, a large
amount of lipase was required in the process of the reaction so
that the cost of the production was relatively high. Furthermore,
the lipase-catalyzed polycondensation of a diol and diacid were
time-consuming due to its lower reaction rate compared with
metal-catalyzed polycondensation.
Introduction
Due to its high elasticity, abrasive resistance, and other
outstanding properties, polyurethane (PU) has been widely
accepted by the market for a number of decades. Conventional
polyurethane is mostly produced in the reaction of polyols with
toxic diisocyanate, which is derived from the even more toxic
phosgene. Recently, bio-based polyols, mainly synthesized from
vegetable oils1,2 have been used for the production of
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology
and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China. E-mail:
zhangruoy@nimte.ac.cn; jzhu@nimte.ac.cn
To the best of our knowledge, no reports has ever tried the
synthesis of bio-based unsaturated polyurethanes by the melt
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08665a
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polycondensation of urethanediols with diacid in the presence
1H NMR (400 MHz; d6-DMSO; Me4Si): 3.01 (4H, d, CH2), 3.51
of metal catalyst using an isocyanate free method. Besides, in (4H, t, J ¼ 5.2 Hz, CH2OH), 3.92 (4H, t, J ¼ 5.2 Hz, CHO), 4.67
order to develop PEU coating materials, double bonds were (2H, br, OH), 6.72 (0.4H, br, NH), 7.08 (1.6H, t, NH).
introduced into the main chain of PEU. The widely used
biomolecule “itaconic acid” (IA) possessing two carboxyl groups
and one carbon–carbon double bond was selected here, which
has been proved to be suitable for the synthesis of unsaturated
Synthesis of bio-based unsaturated poly(ester-urethane)s
polyesters.10 Then these bio-based unsaturated polyurethanes The reaction mixture with itaconic acid/urethanediol molar
were formulated with free radical photoinitiator and curing ratio of 1.2 was charged into a 250 mL four-necked ask,
promoter to prepare UV-curable polyurethane systems. Aer UV equipped with a stirrer, nitrogen inlet, and Barrett type receiver.
curing, the tensile and thermal properties of the cured lms MEHQ (0.5 wt% relative to total reactants) and p-toluene-
were evaluated using tensile, differential scanning calorimeter sulfonic acid monohydrate (0.5 mol% relative to itaconic acid)
(DSC), and thermogravimetry (TGA). In addition, coating were used as the free radical polymerization inhibitor and the
properties such as pencil hardness, exibility, and adhesion catalyst for pre-polymerization, respectively. The mixture was
ꢁ
were also investigated.
precondensated at normal pressure at 110 C for 4 h, with dry
nitrogen bubbling through the mixture. Then DBTL (0.5 wt%
relative to the total weights of reactants) was added as the
ꢁ
Experimental part
Synthesis of 1,2-bis(hydroxyethyloxycarbonylamino)ethane
polycondesation catalyst. Then the mixture was kept at 110 C
for 3 h under high vacuum. The synthetic route was shown in
Scheme 1.
(urethanediol 2)
The products were puried by dissolving in chloroform and
precipitating in cold water. The polymer was ltered and dried
in the oven at 35 ꢁC to a constant weight. The Mn values of
polymers were calculated from the acid value (AV) and hydroxyl
value (OHV) according to eqn (1)
Ethylene carbonate (26.4 g, 0.3 mol), 1,2-ethanediamine (9.0 g,
0.15 mol) and 20 mL of methylene chloride were stirred in a 250
mL round-bottomed ask at room temperature until the
absorption band at 1800 cmꢀ1 in the IR spectrum disappeared.
The white precipitate formed was ltered and further puried
by recrystallization from acetone to give a white solid. The
monomer urethanediol 2 was obtained aer drying under
reduced pressure (25.1 g, yield 71%).
Mn ¼ 56.1 ꢂ 1000 ꢂ f/(AV + OHV)
(1)
where f is the functionality of the polymer.
Scheme 1 Two-step synthesis of bio-based unsaturated poly(ester-urethane)s.
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segment of the polyurethane, were prepared by the reaction of two
equivalent of ethylene carbonate with 1,6-hexanediamine, 1,4-
butanediamine, 1,2-ethanediamine.11 The reaction formed
urethane bond quickly at room temperature in the absence of any
catalyst, and gave high yields (above 70%). In the second step, the
urethanediol underwent melt polycondensation with itaconic acid
in the presence of metal catalyst DBTL to produce low-molecular-
weight polyurethane. Fig. 1 represents the 1H NMR spectra of
UV curing system with bio-based unsaturated poly(ester-
urethane)s
The UV curable polyurethane system was formulated by mixing
the bio-based unsaturated poly(ester-urethane) with 3 wt%
photoinitiator DMPA and 2 wt% curing promoter triethanol
amine (on the base of the oligomer' weight). The solution with
20% solid content was obtained. The resulting UV curable
polyurethane system was poured into a stainless steel mould
(with the grooves' size of 80 mm ꢂ 8 mm ꢂ 0.5 mm), followed
by drying at 80 ꢁC to get a constant mass and cured by UV
irradiation for 30 min at room temperature (using a UV light
source with a high-pressure mercury lamp (500 W) at 366 nm).
The distance from lamp to the surface of samples was 15 cm.
Besides, the system was cast on the tinplate with a thickness of
about 40 mm by a drawdown bar. The UV-cured polyurethane
lms were stored in a dust free cabinet for 3 days before testing.
1
poly(U2-IA), poly(U4-IA) and poly(U6-IA). The H NMR spectra of
synthesized bio-based unsaturated poly(ester-urethane)s are
conrmed with signals of double bond at about 5.8 ppm and 6.3
ppm. (See detail analysis in ESI†) from the 1H NMR analysis we can
Characterization
The details of characterization of oligomers and UV-cured
polyurethane lms can be found in the ESI,† and only the
characterization of coating properties is briey introduced here.
The pencil hardness of coatings with the thickness of 40–50
mm on the tinplate was measured according to ASTM D 3363-00.
Coated tinplates were placed on a rm horizontal surface. The
pencil was held rmly against the lm at a 45ꢁ angle and
pushed away from the operator in a 6.5 mm stroke. The process
was started with the hardest pencil and continued down the
scale of hardness until the pencil will not scratch the lm.
The exibility of the coatings was measured by T-bend test
according to ASTM D4145-10. Coated tinplates were bent 180ꢁ
around progressively more thicknesses of metal, the end point
being when failures no longer occur. This test was a way of
evaluating the ability of a coating system to withstand the
stresses of fabrication.
The adhesion of the UV-cured lms on the tinplate was
evaluated using the ASTM D3359-09 crosshatch adhesion
method. A lattice pattern with eleven cuts in each direction was
made in the lm to the tinplate, pressure-sensitive tape was
applied over the lattice and then removed, and adhesion was
evaluated by comparison with descriptions.
Results and discussion
Three novel types of bio-based unsaturated poly(ester-urethane)s
containing ester linkages, namely poly(U2-IA), poly(U4-IA), and
poly(U6-IA), were designed and synthesized by a green synthetic
route without the use of diisocyanates (Scheme 1).
(The detailed synthesis conditions and the materials used
are illustrated in ESI.†) The urethane linkage was formed by the
reaction of diamines and ethylene carbonate that produced
urethane-containing diols excluding hazardous monomers
such as phosgene and isocyanates. Also, ester linkages and
double bonds were periodically introduced into the poly-
urethane chain.
Fig. 1 1H NMR spectra of the three bio-based unsaturated poly-
The synthetic procedure contained two steps as illustrated in
Scheme 1. In the rst step, a series of urethanediols, as the hard urethanes in D2O.
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Scheme 2 UV curing process of the three bio-based unsaturated polyurethanes.
conclude that the targeted bio-based unsaturated polyurethanes photoinitiator DMPA by hemolytic cleavage of C–C bonds. Then,
were successfully prepared. Their hydroxyl value (OHV) and the once initiated, the chain reaction will develop very much like in a
acid value (AV) were also measured by titration in order to further conventional thermal polymerization with the formation of insol-
quantify the chemical structures. All bio-based unsaturated uble crosslinked polymer.
poly(ester-urethane)s exhibited high acid value and relatively low
The gel content is proportional to the crosslinking density
hydroxyl value, which was in accordance with the feeding ratio of and can be used as a parameter to indicate the efficiency of
urethanediol and itaconic acid. (See Table S1 in ESI†) the Mn curing.14 In order to study the crosslinking properties of the
values of the bio-based unsaturated poly(ester-urethane)s calcu- three UV cured polyurethane lms, the gel content were
lated from the AV and OHV were 758 g molꢀ1 for poly(U2-IA), 1336 obtained from the difference in the weights of the sample
g molꢀ1 for poly(U4-IA) and 1111 g molꢀ1 for poly(U6-IA). It was before and aer acetone extraction. The un-crosslinked polymer
found that poly(U2-IA) exhibited the lowest Mn. It may be due to would dissolve in the acetone while the crosslinked polymer
the fact that urethanediol 4 and urethanediol 6 were slightly more remained. The gel contents of UV-cured poly(U2-IA), UV-cured
reactive toward itaconic acid than urethanediol 2.9 We could see poly(U4-IA) and UV-cured poly(U6-IA) were 78.90%, 86.68%,
that the average double bond contents within each bio-based and 88.35%, respectively (as shown in Table 1). These gel
unsaturated poly(ester-urethane) chain were about 2.3, 3.7 and content values may indicate that the UV curing system was
2.9 for poly(U2-IA), poly(U4-IA) and poly(U6-IA), respectively. Since highly, but not completely UV cured. Clearly, the UV-cured
the double bond contents were all higher than that of curable poly(U2-IA) exhibited the lowest gel content. This may be
PUA,12 the three bio-based unsaturated poly(ester-urethane)s could explained by the viscosity difference.15 The UV-curing system
be used for UV curable system. Therefore, environmental friendly with poly(U2-IA) exhibited the lowest viscosity because poly(U2-
UV curable polyurethane systems could be prepared by using the IA) possessed the lowest Mn. Consequently, oxygen could more
bio-based unsaturated poly(ester-urethane)s.
easily spread into the UV-curing system with poly(U2-IA) where
more and more initiator radicals are scavenged,16 and then the
odds of the light polymerization would be reduced leading to
not fully UV cured. On the other hand, this result could also be
affected by the fact that the vitrication occurred during curing
can restrict the diffusion of PEU chains.17 Therefore, the
residual double bonds in PEU were remained and could not be
further cured at room temperature.
Gel content properties
Aer UV curing, the three bio-based unsaturated polyurethanes
transformed into crosslinked polymer by a chain reaction initiated
by free radicals, which were produced by UV irradiation.13 The
kinetics of UV curing process was illustrated in Scheme 2. Firstly,
free radicals were generated up on UV irradiation of free radical
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Table 1 The tensile properties of the three UV-cured polyurethane films
Samples
Tensile strength (MPa)
Elongation at break (%)
Young's modulus (MPa)
Gel content (%)
UV-cured poly(U2-IA)
UV-cured poly(U4-IA)
UV-cured poly(U6-IA)
1.37
2.50
0.78
51
33
53
16.1
51.9
2.3
78.90
86.68
88.35
Tensile properties
The tensile experiment was carried out by an Instron 5567
Electric Universal Testing Machine with gauge length of 50 mm
at a cross-head speed of 5 mm minꢀ1, and their tensile strength,
elongation at break, and Young's modulus were summarized in
Table 1. In some cases, the crosslinking density largely affect
the elongation at break and tensile strength.18 It could be seen
in this work that the UV-cured poly(U4-IA) lm exhibited the
lowest value of elongation at break among the three samples.
This fact may be attributed to the reason that the elongation at
break for cured lms depends on not only the chemical struc-
ture of the prepolymers but also the crosslinking density of
networks. On the other side, the UV-cured poly(U4-IA) lm
possessed the highest tensile strength (2.50 MPa), following by
the UV-cured poly(U2-IA) lm (1.37 MPa), while the UV-cured
poly(U6-IA) lm had the lowest strength. The results could be
attributed to the fact that the aliphatic chain length of poly(U4-
IA) was shorter than that of poly(U6-IA) and the crosslinking
density of UV-cured poly(U4-IA) lm was higher than that of UV-
cured poly(U2-IA) lm.
Fig. 2 DSC curves of the three UV-cured polyurethane films obtained
from the second heating runs from ꢀ40 to 120 ꢁC at a heating rate of
10 ꢁC minꢀ1
.
In conclusion, the tensile properties of the three UV-cured
polyurethane lms were similar to that of UV-cured lms
prepared with PUA.15 Such data suggested that the obtained bio-
based unsaturated poly(ester-urethane)s prepared by a green
method that avoids the use of hazardous diisocyanates could be
used for UV curable polyurethane system, instead of poly-
urethane-acrylate.
about 150 ꢁC was thought to be mainly due to the loss of
humidity in the lms. Stage II showed onset temperatures from
ꢁ
ꢁ
255 to 273 C and close peak temperatures from 285 to 298 C
for the samples. Thermal degradation of the urethane linkages
and the formation of –NCO, primary amines, and secondary
amines take place in this stage, as found in a previous study.20
Stage III, which occurred at approximately 390 to 401 ꢁC and
exhibited peak temperatures from 434 to 454 ꢁC, may corre-
spond to the thermal degradation of so fragments, such as the
ester bonds of alkyl chains, in good agreement with previous
observation21 (see Table S2 in ESI†).
Thermal properties
The thermal behavior of each UV-cured polyurethane lm was
characterized by using DSC as shown in Fig. 2. It can been seen
that the Tg values of the three cured polyurethane lms were 28,
21 and 8 ꢁC for poly(U2-IA), poly(U4-IA) and poly(U6-IA),
respectively. Obviously, the Tg values decreased with the
increase of diamine's aliphatic chain length in the UV-cured
polyurethane lm, which could be attributed to the reduction
of the chain mobility with the increase of aliphatic chain length
in diamine, as supported by the results of Ubaghs et al.19 And in
this case, the effect of un-crosslinking components is minor.
Such result clearly indicated that the Tg values depended on the
chemical structure of the diamine used.
The TGA and DTG curves of the three UV-cured polyurethane
lms under nitrogen and air were showed in Fig. 3. The TGA
curves of all the three UV-cured polyurethane lms were similar
under nitrogen, and all of them can be divided into three stages
as shown in Fig. 3(a). In stage I, an insignicant weight loss at
The TGA curves of samples under air atmosphere were
also similar with each other, but four, instead of three, stages
of thermal degradation were observed (Fig. 3(b)). Stages I, II
and III can be corresponded to the decomposition behavior
under nitrogen condition. The appearance of stage IV, with
an onset temperature from ꢁ 550 to 566 ꢁC and a peak
temperature from 589 to 593 C, was observed, which could
be attributed to the dehydrogenation and depolycondensa-
tion of alkyl groups of the UV-cured polyurethane lms22 (see
Table S3 in ESI†). Based on these results, we concluded that
all UV-cured polyurethane lms showed a similar thermal
decomposition behavior and exhibited excellent thermal
stability.
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Fig. 3 TGA curves (heating rate: 10 ꢁC minꢀ1) and DTG curves of the three UV-cured polyurethane films under nitrogen (a and c) and air (b and d).
Table
polyurethanes
2 General coatings properties of the three UV-cured
Conclusion
The bio-based unsaturated poly(ester-urethane)s containing
two urethane linkages, two ester linkages and one double
bond in each repeating unit were designed and synthesized
by the metal-catalyzed melt polycondensation of itaconic
acid with urethanediols as a diisocyanate-free green method.
The average double bond contents within the three bio-based
unsaturated poly(ester-urethane)s were higher than that of
PUA (about 2.3, 3.7 and 2.9 for poly(U2-IA), poly(U4-IA) and
poly(U6-IA), respectively), so that they could be used for UV
curable polyurethane systems. Aer the completion of UV
curing, the tensile properties, thermal properties and general
coating properties of the three samples were found to be
comparable to that of UV-cured PUA lms. These results
suggested that the bio-based unsaturated polyurethanes
prepared by a green method that avoids the use of hazardous
diisocyanates could be used for UV curable polyurethane
systems, instead of polyurethane-acrylate. The UV-cured
poly(U4-IA) showed the best tensile strength, thermal prop-
erty, pencil hardness, exibility, and adhesion among the
three samples, which could be an potential candidate as
oligomers for UV-curable coatings, plastics, inks and
adhesives.
Samples
Pencil hardness
Flexibility
Adhesion
UV-cured poly(U2-IA)
UV-cured poly(U4-IA)
UV-cured poly(U6-IA)
H
2H
2B
0T
1T
0T
5B
4B
5B
General coating properties
A summary of the UV-cured coating properties was found in
Table 2. Aer the completion of UV curing, several lms prop-
erties such as pencil hardness, exibility and adhesion were
evaluated. The three UV-cured polyurethane lms showed the
pencil hardness in the order UV-cured poly(U4-IA) > UV-cured
poly(U2-IA) > UV-cured poly(U6-IA). It could be attributed to
the fact that the hardness of the coatings depends on not only
the chemical structure of the prepolymers but also the cross-
linking density of networks. The T-bend test and tape test
showed that all UV-cured polyurethane coatings on the tinplate
substrate exhibited excellent exibility and adhesion, similar to
that of UV-curing coatings with PUA. The results further
demonstrated that the obtained bio-based unsaturated
poly(ester-urethane)s based on isocyanate-free chemistry could
be used as supplementary for polyurethane-acrylate.
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Acknowledgements
The authors greatly thank the nancial support from the
National Natural Science Foundation of China (21204096).
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