Polyols as solvents

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Polyols as solvents
A new route to NMP-free high-performing
PUR-dispersions for various applications.
Waterborne polyurethane dispersions (PUDs) combine
excellent performance with low solvent content and can offer
a wide range of properties. Until recently, almost all PUDs
contained N-methyl pyrrolidone (NMP) which will in future be
subject to labelling requirements. A new route to NMP-free
PUDs uses polyols which themselves have solvent
properties. These resins are shown to have excellent
properties.
Dirk Mestach.
Waterborne polyurethanes are one of the first choices of
binder for high quality and demanding applications. They
can be formulated into environmentally acceptable coatings
containing low amounts of volatile organic components
(VOC). The nature of the polymeric backbone makes them
both hard and flexible, and so ideal for use in scratch
resistant, hard-wearing coatings.
These properties are the result of inter- and intra-chain
interactions between polyurethane chains, attributable to
hydrogen bonding between the urethane and urea linkages
in the polymer backbone. Waterborne polyurethanes can be
used in both one- and two- component and radiation curing
coatings and may be either thermoplastic or (self-)
crosslinking.
Production processes for polyurethane dispersions
Regardless of the application, the general synthesis route
for a polyurethane dispersion is that a polymeric diol, for
example a polyester, polyether or polycarbonate diol, is
reacted with a hydrophilising diol and a diisocyanate.
Depending on the ratio between the diols and the
diisocyanate, the polyurethane obtained can be either
isocyanate (NCO) or hydroxy functional. Where the
polyurethane is isocyanate functional, it is often referred to
as a polyurethane prepolymer.
The polyurethane is then dispersed into water either directly
or by means of the phase-inversion emulsification process.
If the polyurethane is NCO-functional it can be chain
extended after dispersion using a diamine. Optionally, a
chain stopper can be used to react with the NCO
functionality. This chain stopper can be used to introduce
functional groups, for example for crosslinking of the
polyurethane after drying of the coating.
Solvent choice for PUD manufacture is very limited
In most waterborne polyurethanes used in the coatings
industry, at least part of the hydrophilising diol is dimethylol
propionic acid (DMPAc). This has a hindered carboxylic acid
group which is less reactive than most acid groups, and it
therefore reacts as a diol. The free acid group can be
neutralised with a base to make the polyurethane water
dispersible.
Until recently, waterborne polyurethanes generally
contained a certain amount of volatile organic solvents,
which are used in the synthesis to reduce the (pre-)polymer
viscosity and also to aid in dissolving DMPAc in the polyols
prior to the addition of the diisocyanate.
Not all solvents can be used for this as they have to be
aprotic (non-reactive with isocyanates) and hydrolytically
stable. This limits the choice to ketones, (cyclic) ethers and
some amines and amides. The solubility of DMPAc in these
solvents
varies
greatly,
and
until
recently
N-methyl-2-pyrrolidone (NMP) was the preferred solvent.
The trouble with NMP
A problem associated with NMP is the fact that it cannot be
removed by distillation after dispersing the polyurethane in
water. It therefore remains in the dispersion and functions as
coalescing aid. The classification of NMP as "toxicologically
questionable" is currently being discussed by the European
Union. It is proposed that products containing more than 5%
NMP will have to be labelled as being irritant and toxic (Xi:
R36/37/38, T: R 61).
In the United States, California's Proposition 65 also
requires special labelling of products containing NMP while
other states and countries may follow. There is therefore a
worldwide need to replace or eliminate NMP from
polyurethane dispersions.
Alternative production routes - advantages and
drawbacks
Several approaches have been used to replace NMP in the
manufacture of polyurethane dispersions. A straightforward
replacement by N-ethyl-pyrrolidone (NEP) is suggested by
some companies [1]. Even though it is claimed that NEP is a
useful alternative and that its toxicological profile is
favourable compared to that of NMP, this does not appear to
be a sustainable option.
Other manufacturers have modified the well-known acetone
process [2]. An important problem associated with the
acetone process is that the solubility of DMPAc in acetone is
virtually zero. Neutralising the carboxylic acid group of
DMPAc with triethylamine, however, raises its solubility
considerably [3].
A drawback of this process is that large amounts of acetone
have to be stripped from the polyurethane after it has been
dispersed. In order to obtain an economically feasible
process, the acetone has to be recycled and used again. As
acetone has a very low flashpoint, some producers prefer to
work with methyl ethyl ketone (MEK).
Another way to increase the solubility of DMPAc in the
reaction mixture is to cap the hydroxyl groups with ε
-caprolactone. This modification converts the crystalline
material into a soft waxy material with a low melting point
and an enhanced solubility [4]. A drawback of this route is
that
the
hard
diisocyanate-dimethylol
propionic
acid-diisocyanate segment now becomes a soft segment.
Therefore the polyurethane has to be completely redesigned
in order to obtain the desired coating properties.
A further way to produce solvent-free polyurethane
dispersions is to use an ethylenically unsaturated monomer
as a temporary solvent in the polyurethane synthesis. Esters
of methacrylic acid are most suitable for this. These
"solvents" are emulsion polymerised after the polyurethane
is dispersed into water by adding a suitable initiator such as
a persulphate or a hydroperoxide in combination with a
reducing agent.
Optionally, additional monomers can be added at this stage.
This synthesis route leads to hybrid urethane-acrylic
dispersions. This approach, however, does not solve the
DMPAc solubility problem completely, because methacrylic
esters are relatively poor solvents for DMPAc.
The new aproach: Polyols assist in dissolving DMPAc
A novel production route to NMP-free polyurethane
dispersions has now been developed, which uses only small
quantities of auxiliary process solvents. These solvents can
be almost completely removed after production of the
dispersion. The key to this process is the in-house
development of polyol building-blocks that assist in
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dissolving the DMPAc.
Using this new process, several types of polyurethane
dispersions were developed:
- Chain extended high molecular weight polyurethane
dispersions ("Setaqua PU-1")
- Low molecular weight fatty acid modified polyurethane
dispersions ("Setaqua PU-2");
- Self-crosslinking urethane-acrylic dispersions ("Setaqua
UA").
High molecular weight polyurethane dispersions are most
frequently used in combination with acrylic dispersions,
where their main purpose is to improve the mechanical
properties of the acrylics. Common applications can be
found in industrial wood coatings. The chain extended
dispersions, however, can also be used as the main binders
in both one- and two-component coatings. Applications
include parquet, furniture and plastic coatings. Cross-linkers
such as polyaziridines or water-dispersible polyisocyanates
can be used to enhance their performance.
Fatty acid modified polyurethane dispersions are a very
versatile class of binders that find application in both
decorative and industrial coatings. Because of their
relatively low molecular weight, they offer excellent flow and
levelling. In wood primers or sealers, they allow good
wetting and penetration of the wood. When used for
decorative applications, such as in do-it-yourself trim paints,
they offer good wet-edge and open time.
Because of the fatty acid modification, the hardness and
chemical resistance properties build up after drying of the
coating. Most of these dispersions still contain free hydroxyl
groups which can be used for reaction with, for example,
water-dispersible polyisocyanates. Once again this boosts
the performance of the coating.
Self-crosslinking
urethane-acrylic
dispersions
offer
synergistic properties compared to simple physical blends of
polyurethane and acrylic dispersions. Their applications are
again numerous, ranging from parquet finishes to decorative
enamels. Some application examples will now be given for
these novel binders.
Blends with acrylics produce good parquet finishes
Parquet floors are currently in fashion. In addition to factory
finished systems, many parquet floors are coated after
installation. The lacquers used must meet a number of
criteria: they must be non-yellowing and have a high
chemical resistance, as well as fair abrasion resistance.
From the application point of view, they must be fast drying
and have a low odour.
These lacquers can be based solely on a polyurethane, but
most commonly this is combined with an acrylic dispersion.
Not only does this result in the improvement of a number of
coating properties, but it also brings economic benefits.
In the trial one-component formulations, blends of the new
chain extended PUD were produced with a self-crosslinking,
surfactant-free acrylic ("Setaqua XL") and a thermoplastic
acrylic ("Setaqua TP") dispersion. Both acrylic dispersions
have a minimum film formation temperature of about 15°C.
Test lacquers were based either on pure polyurethane, a
blend with 30 % (w/w) of either of the acrylics and a blend
containing 70 % of the self-crosslinking acrylic. (The
corresponding blend with 70 % thermoplastic acrylic was
hazy and not compatible). The formulations used are given
in Table 1.
It should be noted that the levels of cosolvent used in the
formulations was not constant, but was based on the
minimum film formation temperature (MFFT) of the binder
combination. Films of the varnishes were applied onto glass
(dry film thickness ca. 30 µm). The films were allowed to dry
at ambient temperature (23°C) and the hardness was
measured after one and seven days. These results are
shown in Figure 1.
The coatings were also applied onto oak veneer (150 µm
wet layer thickness) and the dust-dry and tack-free times
were recorded. The results are shown in Figure 2.
It is quite surprising to see that the addition of 30%
thermoplastic acrylic does not affect the drying times, even
though the level of cosolvent in the blend is higher. Blends
with the self-crosslinking acrylic have longer drying times,
although still short enough for this application.
The chemical resistance properties of the dried varnishes
were determined (on oak veneer, two coats after drying for
seven days at ambient temperature). These results are
shown in Table 2.
The blend with the thermoplastic acrylic does not offer much
advantage with respect to chemical properties, and some
resistance properties even deteriorate. Blends with the
self-crosslinking acrylic, on the other hand, offer interesting
improvements in properties. The blend containing the higher
level of acrylic performs particularly well.
NMP-free metal primers
Air-drying, fatty acid modified polyurethane dispersions are
very suitable as industrially applied metal coatings. So far,
however, these types of coatings have often contained
NMP. A low VOC NMP-free metal primer formulation was
developed and its main properties were studied. Good
adhesion is crucial for a primer, and the tests showed that
adhesion was excellent on virtually all metal substrates.
Primers based on this resin, designated "Setaqua PU-2" can
easily be overcoated with both one- and two- component
water-borne and solvent-borne topcoats. Even the primer
alone shows very good salt spray resistance (Table 3).
Urethane-acrylic hybrids
Urethane-acrylic dispersions offer advantages over simple
blends of a polyurethane dispersion and an acrylic
dispersion. Because the acrylic part is polymerised in the
presence of the dispersed polyurethane, grafting reactions
occur, resulting in the formation of true hybrid particles
where the polyurethane and acrylic polymer chains are
present in one particle. This is clearly shown in Figure 3,
where atomic force microscopy picture of a film cast from
such a hybrid is shown.
The acrylic-urethane hybrid polymer is modified with
carbonyl groups in order to cross-link via the reaction with a
polyhydrazide component. As described in numerous
previous papers, it takes about one week at ambient
temperature for such a system to reach full conversion.
Dynamical Mechanical Thermal Analysis (DMTA) was used
to study the mechanical properties after crosslinking (see
Figure 4).
Even though only one type of particle can be seen using
AFM, the DMTA plot shows different transitions, suggesting
a core-shell like morphology.
A clear furniture lacquer can be formulated using the
formulation shown in Table 4. The coating, applied at a wet
layer thickness of 150 µm, showed a Persoz hardness of
150 s after only one day of drying at ambient temperature.
After one week, hardness had increased to 200 s, indicating
that full crosslinking had taken place. Chemical resistance
properties were tested after 2 and 7 days of drying. The
results are given in Table 5. As the table shows, resistance
properties are quite satisfactory after short drying times and
excellent after full cure has been obtained.
REFERENCES
[1] K. Ott et al, Patent Application WO2005/090447 A2 to
BASF AG, 2005.
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[2] The Bayer Scientific Magazine, issue 17, p 92, 2005
(Internet:
http://www.research.bayer.com/medien/pages/4003/wood_c
oating.pdf)
[3] H. Schurmann, J. Bung, H. VanAlsten, US Patent
4,096,127 to Akzona Inc., 1978.
[4] R. L. Scriven, W. Chang, US Patents 4,066,591 and
4,147,679 to PPG Industries Inc. 1979.
Results at a glance
- Waterborne polyurethanes (PUDs) combine excellent
performance with low solvent content and can be produced
with a wide range of properties.
- Until recently, almost all PUDs contained N-methyl
pyrrolidone (NMP) which will in future be subject to labelling
requirements.
- A new route to the production of NMP-free PUDs has been
developed, by using novel polyols which themselves act as
solvents.
- Test results on these resins and their blends with acrylic
dispersions show that a good range of properties can be
obtained in wood finishes and metal primers.
- Hybrid systems, in which acrylates are polymerised in the
presence of the PUD, can provide even better performance
than blends of PUD and acrylic resins.
The author:
-> Dr. Dirk Mestach obtained his doctorate in polymer
chemistry at the University of Gent (Belgium). In 1989 he
joined Akzo Nobel, first in Belgium in the coatings division
and later on with Akzo Nobel Resins, today Nuplex Resins,
in the Netherlands. He has been active in the development
of waterborne binders for the coatings and printing inks
industry. At present he is R&D manager at Nuplex Resins.
This paper was presented at the European Coatings
Conference "Polyurethanes for High Performance Coatings
IV", Berlin, 23/24 March 2006
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Figure 2: Dust-dry and tack-free times as well as co-solvent amounts of parquet
varnishes.
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Figure 3: Atomic force microscopy views of a waterborne urethane acrylic hybrid resin
("Setaqua UA"); left: topographic, right: tapping mode.
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Figure 4: Dynamic mechanical thermal analysis on the waterborne urethane acrylic
hybrid resin "Setaqua UA" (frequency 11 Hz) after drying at ambient temperature for 7
days.
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