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Hyperbranched urethane-acrylates
Hyperbranched urethane-acrylates based on alkoxylated
hydroxy acrylates combine high molecular weight, excellent
reactivity and good coating properties, such as hardness,
flexibility and chemical resistance. These compounds are
good candidates for a range of UV-curing applications.
Branko Dunjic, Srba Tasic, Branislav Bozic.
Hyperbranched
polymers
are
highly
branched
macromolecules with a large number of end groups. Their
tree-like structure has been shown to result in some unique
properties that are very different from conventional linear or
slightly branched polymers [1]. Hyperbranched polymers
have high solubility and low melt and solution viscosity
compared to linear polymers of similar molecular mass.
These properties make them attractive in many application
fields, especially for coatings, where low viscosity in
combination with high functionality and high molecular
weight can give enhanced coating properties [2].
The use of hyperbranched polymers in UV-curable
applications has been described in several papers [3, 4].
Acrylated hyperbranched polyesters based on bis-methylol
propionic acid have lower viscosity compared to linear
UV-curable resins of similar molecular weight and cure very
rapidly (even without the photoinitiator). They have good
chemical resistance, scratch resistance and low shrinkage
upon curing. Due to their high functionality, the films
obtained are very hard but brittle, so they have poor
flexibility.
To the authors' knowledge, no papers have been published
about urethane-acrylates based on hyperbranched
polymers. In previous work the preparation of
urethane-acrylates based on hyperbranched aliphatic
polyesters partially modified with short chain saturated fatty
acids was presented [5]. In the present work the synthesis of
new hyperbranched urethane-acrylates (HBUA) based on
alkoxylated hydroxy functional (meth)acrylate monomers is
investigated. Introduction of a flexible alkoxylated spacer
between a compact hyperbranched core and crosslinkable
groups reduces steric hindrance by moving the unsaturated
groups away from hyperbranched core and increase its
reactivity. At the same time, higher molecular weight
oligomers give good performance of cured coatings.
reaction of the NCO groups, which was confirmed by FTIR
analysis - the disappearance of the peak at 2267cm-1. After
evaporation of the solvent (THF) a clear viscous liquid was
obtained.
Measuring the cured oligomers
The complex dynamic viscosity (h*) of oligomers diluted with
20wt.% HDDA were measured with a Rheometrics
mechanical spectrometer "RMS-605" operating in rate
sweep mode, using a cone and plate geometry at 30°C.
Dynamic mechanical properties of cured oligomers were
analysed using Rheometrics "RMS-605" in the temperature
sweep mode (at frequency of 1Hz).
Differential scanning calorimetry (DSC) was performed
using a Perkin Elmer "Pyris 6 DSC" analyser at a heating
rate of 10°C/min under nitrogen.
HBUA oligomers were mixed with HDDA (20wt.%) and
"Irgacure 184" (4wt.%) and drawn on metal plates at a film
thickness of 40 ± 5µm. The films were cured using 2" UVPS
metal halide lamp (80W/cm). Hardness of coatings was
determined by Persoz pendulum. The flexibility of the
coatings was determined by measuring the Erichsen
indentation.
Materials used
The purchased materials used in HBUA synthesis were:
isophorone diisocyanate (IPDI, Hüls); 2-hydroxyethyl
acrylate (2-HEA), polyethyleneglycol(6) monoacrylate
(PEA6), polypropyleneglycol(6) monoacrylate (PPA6) and
polypropyleneglycol(5) monomethacrylate (PPM5S, Laporte
Performance Chemicals); hexanediol diacrylate (HDDA,
BASF); 2,2-bis(methylol) propionic acid (Bis-MPA) and
ditrimethylolpropane (DITMP, Perstorp AB); dibutyltin
dilaurate
(DBTDL,
Merck),
and
1-hydroxy-cyclohexyl-phenyl-ketone ("Irgacure 184", Ciba).
All chemicals were used as received.
Synthesis of hyperbranched urethane-acrylates
Most urethane-acrylate oligomers are synthesised by
reaction of the linear polyols (polyester or polyether type)
with diisocyanate and hydroxyalkyl acrylate (usually
2-hydroxyethyl acrylate) and give flexible films upon curing.
New urethane-acrylate oligomers based on hyperbranched
polyesters and alkoxylated hydroxy functional (meth)acrylate
monomers have recently been demonstrated.
A hyperbranched polyester of the second generation
(HBP,G2) with ditrimethylolpropane as a core and
dimethylolpropionic acid as a branching unit was prepared
by a procedure described in reference [6]. The
hydroxy-functional hyperbranched polyester obtained was
used as a polyol core for synthesis of hyperbranched
urethane-acrylates.
The synthesis of urethane-acrylates can be divided into two
steps: synthesis of an adduct from IPDI and hydroxyalkyl
(meth)acrylate monomer as a first step, and in second step
modification of polyol core (HBP,G2) with IPDI based adduct
obtained in the first step. In this study a series of
urethane-acrylates were synthesised with different
hydroxy-functional (meth)acrylate monomers. The chemical
structures of the hydroxy-functional (meth)acrylate
monomers used are given in Table 1.
The reaction diagram of the synthesis of hyperbranched
urethane-acrylate based on PEA6 is presented in Figure 1.
All other polymers were obtained in the same way (by the
procedure already described), with a different hydroxyalkyl
(meth)acrylate.
The
composition
of
synthesised
urethane-acrylates is given in Table 2.
Synthesis of urethane-acrylate oligomers
An adduct of IPDI and hydroxyalkyl (meth)acrylate was
prepared by dropping an equimolar amount of hydroxyalkyl
(meth)acrylate into a 250ml 3-necked round-bottom flask
containing 0,1mol of IPDI and catalytic amount of DBTDL.
The flask was equipped with a mechanical stirrer, dropping
funnel, water condenser and a thermometer. The reaction
mixture was stirred at below 40°C over 2h. Then, an
appropriate amount of hyperbranched polyester (HBP,G2)
dissolved in THF (20wt.%) was added to the flask. The
reaction mixture was stirred at 70°C until the complete
Complex dynamic viscosities of hyperbranched
urethane-acrylates
The complex dynamic viscosities (h*) of urethane-acrylate
oligomers diluted with 20wt.% of HDDA determined at 30°C
are given in Figure 2. It can be seen that the type of
hydroxyalkyl (meth)acrylate monomer used for end-capping
of HBP,G2 have a considerable effect on the viscosity of
urethane-acrylate
oligomers.
The
hyperbranched
urethane-acrylate based on 2-HEA (H2(HEA)8) have the
highest viscosity (285Pas at 1Hz), much higher than
conventional urethane-acrylates. All the other oligomers
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have much lower viscosities and exhibit Newtonian
behaviour, i.e. no shear thinning. HBUA based on
alkoxylated
(meth)acrylate
monomers
have
lower
concentration of polar urethane groups, which reduce the
density of hydrogen bonding as well as the final viscosity of
the oligomers. The values of h* at 30°C and 1 Hz are given
in Table 2.
Dynamic mechanical analysis of cured oligomers
Viscoelastic properties of UV-cured oligomers, such as Tg
and shear modulus were determined by dynamic
mechanical analysis in a temperature-scanning mode. Glass
transition temperature, Tg, was determined as the
temperature of the maximum on tan δ peak. Figure 3 shows
the tan δ curves of UV-cured hyperbranched urethane
acrylate oligomers mixed with 20 wt.% HDDA. The type of
hydroxyalkyl (meth)acrylate used as well as the type of
unsaturated end groups (acrylate or methacrylate) has a
considerable effect on the Tg and the shape of tan δ curve.
Hyperbranched urethane acrylate based on 2-HEA (H2(HEA)
8) has the highest Tg (95°C). The broad tan δ curve also
indicates that the obtained network is very inhomogeneous,
i.e. there is a broad distribution of chain lengths between
crosslinks.
Introduction of a flexible spacer (polyethylene oxide or
polypropylene oxide) between a compact hyperbranched
core and crosslinkable groups increases the mobility in the
cured samples and decreases the Tg. Hyperbranched
urethane-acrylates based on PEA6 and PPA6 have lower Tg
(55
and
31°C
respectively)
as
compared
to
urethane-acrylates based on 2-HEA. This is a consequence
of not only increasing flexibility of the oligomers but also of
the decrease in both: concentration of crosslinkable acrylate
groups and concentration of polar urethane groups
compared to H2(HEA)8 oligomer (Table 2).
The type of functional groups (acrylate or methacrylate) will
also determine the properties of cured oligomers. It is known
that methacrylate functional oligomers generally exhibit a
higher Tg compared to acrylate functional oligomers due to a
stiffer structure of the methacrylate group. By comparing the
Tg of the cured oligomers based on PEA6 and PPM5S
(which have similar concentration of unsaturated groups) it
can be seen that PPM5S (methacrylate end groups) based
oligomer has higher Tg (73°C). The values of Tg for the
cured oligomers determined as (tan δ)max or (G")max are
given in Table 3.
Properties of UV-cured coatings
The mechanical properties of UV-cured oligomers were
determined by measuring the coating properties such as
pendulum hardness (Persoz hardness) and flexibility
(Erichsen cupping). The solvent resistance of cured coatings
was determined by MEK rub testing. All urethane-acrylate
oligimers were diluted with 20wt.% of HDDA and contained
4wt.% of photoinitiator "Irgacure 184". The coat films,
applied on metal plates at typical thickness of about 40µm,
were cured by 2, 5, 10 and 15 successive passes under the
UV-lamp at belt speed of 10m/min. The obtained films were
kept for 1 hour at room temperature before testing. The
mechanical properties of the cured coatings are summarized
in Table 4.
One of the phenomena that is a limiting factor for obtaining
good UV-curable coating properties is oxygen inhibition [7,
8]. Oxygen reacts with free radicals (obtained after
photoinitiator decomposition) giving peroxy-radicals which
are insufficiently reactive to continue the polymerization. As
a result, tacky surface of the film is obtained. This problem
can be overcome in different ways: curing under inert
atmosphere (N2 or CO2), using amine synergists or
UV-lamps of high intensity.
The film characteristics (surface tackiness) after 2 passes
under a UV-lamp in an air atmosphere were evaluated in
order to determine the reactivity of synthesised
urethane-acrylate oligomers. It can be seen that oxygen
inhibition (surface tackiness) influences the properties of
coatings
based
on
propoxylated
hyperbranched
urethane-acrylates (H2(PPA)8 and H2(PPM)8). The coating
based on H2(HEA)8 did not show oxygen inhibition effect.
This is probably due to several overlapping effects such as
high viscosity which prevent oxygen diffusion in deeper
surface layers and high acrylate concentration (see Table
1). Ethoxylated hyperbranched urethane-acrylate (H2(PEA)8
) is more reactive due to the presence of abstractable
hydrogens in an a-position to the ether links. The coating
based on H2(PEA)8 did not show an oxygen inhibition effect
even after only one pass under UV-lamp. This is probably
due to their high reactivity. However, the overall effect of
reactivity and the appropriate coating viscosity results in
good surface curing.
In case the coatings were affected by oxygen inhibition, the
tacky surface layer (about 1-5µm) was removed by wiping
with ethanol. In order to keep constant curing conditions, the
films that did not show oxygen inhibition were also wiped
with ethanol prior to further testing. After UV-exposure of
HBUA, highly crosslinked coatings were formed. As for the T
g, also the mechanical properties of the cured coatings
change with the structure and the chemical composition of
used urethane-acrylates. The influence of the number of
passes under the UV-lamp (i.e. dose) on the film hardness
is illustrated in Figure 4.
The hardness of the coatings and the Tg are known to be
related. They both depend on the crosslink density of the
cured film, but also on the structure of UV-curable resin. By
plotting the hardness values as a function of the Tg obtained
by DSC an almost linear relationship was found (Figure 5).
In the same Figure, the flexibility data were plotted against Tg
temperature. It can be seen that flexibility of the coatings
having Tg above room temperature is significantly reduced.
Only the H2(PEA)8 based coating showed a good
compromise between hardness and flexibility. This can be
attributed to the flexible structure of polyethyleneglycol
chains between crosslinks.
Hyperbranched urethane-acrylates - some further
possibilities
The hyperbranched urethane-acrylates presented here can
be further tailored to a match specific end user's needs. The
remaining OH groups could be used for further
modifications. In order to decrease the amount of
extractables present in cured coatings (photoinitiator
fragments or unreacted photoinitiator) some remaining OH
groups were modified with photosensitive compounds such
as xanthates [9] , thus combining the concept of controlled
radical polymerization and dendritic polymers. The
photoinitiator moieties attached to the hyperbranched core
give reactive oligomers and low extractable matter in
UV-cured coatings. This is one of the topics of current work
which will be published soon.
References
[1] Y. H. Kim, J. Polym. Sci., Part A: Polym. Chem., 36,
(1998), 1685
[2] B. Pettersson, Pigment Resin Technology, 25, No.4,
(1996), 4
[3] M. Johansson, A. Hult, J.Coat. Tech., 67, No. 849,
(1995), 35
[4] M. Johansson, T. Glauser, G. Rospo, A. Hult, J. Appl.
Polym. Sci. 75, (2000), 612
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[5] E. Dzunuzovic, S. Tasic, B. Bozic, D. Babic, B. Dunjic, J.
Serb. Chem. Soc., accepted for publication.
[6] E. Malmström, M. Johansson, A. Hult, Macromolecules,
28, (1995), 1698
[7] C. Decker, A. Jenkinks, Macromolecules, 18, (1985),
1241
[8] C. Decker, Polym. Int., 45, (1998), 133.
[9] International Patent Pending, PCT/GB2003/003239
Result at a glance
- The synthesis and UV-curing of multifunctional
urethane-acrylates based on hyperbranched polyesters
have been investigated.
- Hyperbranched urethane-acrylates based on flexible
alkoxylated hydroxy functional (meth)acrylate monomers
were found to be good candidates for UV-curing
applications.
- The UV-cured films of these oligomers combine a high
crosslinking density with flexible segments between
crosslinks, which results in a good compromise between
hardness and flexibility.
- The problems associated with UV-curing under air, such
as- tacky surface due to oxygen inhibition and related poor
mechanical properties, might be overcome by using these
high molecular weight hyperbranched urethane-acrylates.
The authors:
> Dr Branko Dunjic is Chief Scientific Officer of Duganova
Ltd, part of the biggest south-east European paint
manufacturer, Duga Paints, Belgrade, Serbia, which is in
charge of development and application of new materials and
technologies, specialized for radiation-cured coatings.
> Srba Tasic is researcher in Laboratory for Synthesis in
Duganova, responsible for development of hyperbranched
oligomers for radiation-cured coatings.
> Branislav Bozic is researcher in Laboratory for Application
in Duganova, responsible for application of new concepts of
polymer
synthesis,
such
as
controlled
radical
polymerization, CRP, in coatings industry.
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Figure 1: Synthesis of hyperbranched urethane-acrylate, H2(PEA)8.
Figure 2: Complex dynamic viscosities ofhyperbranched urethane-acrylates.
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Figure 3: tan delta curves of UV-cured hyperbranched urethane-acrylates.
Figure 4: Hardness of coatings as a function of number of passes under the UV-lamp.
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Figure 5: Hardness and flexibility as a function of Tg .
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Table 1: Chemical structure of hydroxy-functional (meth)acrylate monomers.
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