Note: Descriptions are shown in the official language in which they were submitted.
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UV-CURABLE COATING COMPOSITIONS CONTAINING URETHANE-
(METH)ACRYLATES WITH FREE ISOC'.YANATE GROUPS
BACKGROUND OF T:HE INVENTION
Field of the Invention
The present invention relates to coating compositions containing urethane-
(meth)-
acrylates having free isocyanate groups arid compounds containing isocyanate-
reactive groups and to a process for preparin~; coatings from these coating
composi-
tions by a dual cure mechanism.
Description of the Prior Art
The curing of coating compositions contaiining (meth)acryloyl groups by UV-
initiated free radical polymerization is known and used industrially. It is
one of the
fastest curing methods in coating technology. However, coatings cured
throughout
with UV have disadvantages, such as high volume shrinkage, which can lead to
adhesion problems. The high crosslinking density inherent to the system leads
to
brittleness, a lack of elasticity and a lack of irr~pact resistance. Also,
complete curing
of UV-curable coating compositions containing (meth)acryloyl groups requires
the
availability of an adequate radiation dose of appropriate wavelength. Poorly
exposed
regions cure to a reduced extent, which leads to considerable losses in the
surface
resistance, and non-exposed regions do not cure at a11.
Coatings based on polyisocyanates and compounds containing isocyanate-reactive
groups, preferably polyols or sterically hindered polyamines, can be
formulated to
obtain a wide range of properties, such as adhesion, elasticity, impact
resistance and
stability to weathering, based on the choice of reaction partners. However,
compared
with radiation-curing compositions, complete curing requires a long time and
prefer-
ably high temperatures, but does not require th.e presence of a special light
source.
Combinations of the two types of curing are also known, e.g., as dual cure
systems.
US-A 4,342,793 describes compositions containing a saturated polyol, a polyiso-
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cyanate and a radiation-curable reactive dilu.ent, i.e., low molecular weight
acrylic
acid esters. Curing is carried out by irradiation in order to polymerize the
reactive
diluent and thereafter by thermal curing to form a polyurethane from the
polyol and
the polyisocyanate.
The disadvantage of such systems is that under adverse irradiation, non-
incorporated
reactive diluent can remain in the cured paint film. Because of its potential
extractability, this can lead to physiological problems on skin contact with
the paint
film. In addition, it adversely influences properties of the paint film, such
as hardness
I O and physical and chemical resistance.
An object of the present invention is to provide a coating composition that
may be
rapidly cured by UV to allow handling of l:he coated substrates and that may
be
crosslinked by the reaction of NCO groups with compounds containing isocyanate-
reactive groups, preferably polyols and/or sterically hindered polyamines, to
allow
the properties to be adjusted according to the; intended use and to provide
adequate
curing even on non-exposed or poorly exposed regions.
Surprisingly, it has been found that this object may be achieved with the
coating
compositions according to the present invention, which contain polyols,
urethane-
(meth)acrylates having (meth)acryloyl groups and free NCO groups and
optionally
other polyisocyanates. After UV curing the rc;sulting coatings are dry, such
that they
can be handled, and may be crosslinked to connpletion by the reaction of NCO
groups
to obtain coatings having high chemical and mechanical resistance. In the
event of
poor exposure during UV cure, which causes a certain residual double bond
density,
crosslinking via reaction of the NCO groups guarantees the minimum required
resistance level.
These results are surprising since it was expected that the two reaction
processes
would hinder each other and that the rapid UV-initiated polymerization would
virtually "freeze" the significantly slower reaction of the NCO groups such
that they
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no longer react with isocyanate-reactive groups.
SUMMARY OF THE INVENTION
The present invention relates to coating compositions containing
a) a urethane-(meth)acrylate containing (meth)acryloyl groups and free iso-
cyanate groups,
b) optionally a polyisocyanate other than a),
c) a UV initiator which initiates free radical polymerization and
d) a compound containing isocyanate-reactive groups.
The present invention also relates to a process for preparing a coating from
this
coating composition.
DETAILED DESCRIPTION OF THE INVENTION
The present coating composition is preferably prepared in two components,
compo-
nent I) containing components a)+b) and component II) containing components
c)+d).
To achieve stability to weathering
e) a UV absorber having an absorption range up to a maximum of 390 nm and
f) a HALS stabilizer
may optionally be added to component II). In addition,
g) conventional paint additives, such as flow control or deaerating agents and
h) catalysts to accelerate the reaction ofl\fC0
may optionally be added to component II).
For reduce the viscosity, components I) and ifI) can each contain a solvent
which is
inert to the polyisocyanate addition reaction.
Components I) and II are combined such that the equivalent ratio of NCO groups
to
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isocyanate-reactive groups is 2:1 to 0.8:1, preferably l.2:1 to 0.8:1 and more
pre-
ferably 1:1.
Compounds a) are prepared from monohydric alcohols containing (meth)acryloyl
groups and di- or polyisocyanates. Preparation processes for urethane-(meth)-
acrylates are known and described, e.g. in DlA 16 44 798, DE-A 21 15 373 (U.S.
Patent 3,782,961), DE-A 27 37 406 (U.S. Patent 4,225,695). To ensure that the
urethane-methacrylates according to the invention contain free isocyanate
groups, the
di- or polyisocyanates are reacted with the monohydric alcohols an equivalent
ratio
of NCO groups to OH groups of 1:0.2 to 1:0.8, preferably 1:0.3 to 1:0.6.
Monohydric alcohols containing (meth)acryloyl groups include esters containing
a
free hydroxyl group and based on the reaction. product of acrylic acid or
methacrylic
acid with dihydric alcohols, such as 2-hydrox:yethyl, 2- or 3-hydroxypropyl or
2-, 3-
or 4-hydroxybutyl (meth)acrylate, and any mixtures thereof. Also suitable are
mono-
hydric alcohols containing (meth)acryloyl groups or reaction products
containing
these alcohols, which are obtained by esterification of an n-hydric alcohol or
a
mixture of these alcohols with (meth)acrylic acid wherein n has a value of 2
to 4,
preferably 3. To prepare these alcohols, n - 0.8 to n - 1.2 moles, preferably
n - 1
moles of (meth)acrylic acid are reacted for mole of n-hydric alcohol.
Examples of these compounds include the reaction products of i) glycerol, tri-
methylolpropane, pentaerythritol, low molecular weight alkoxylation products
of
these alcohols (such as ethoxylated or propox.ylated trimethylolpropane or the
addi-
tion product of ethylene oxide onto trimethylolpropane, e.g., one having an OH
number 550), and mixtures of least trihydric alcohols with dihydric alcohols
(such as
ethylene glycol or propylene glycol), with ii) (meth)acrylic acid in the
previously
described molar ratios.
These compounds have a number average molecular weight (M~) of 116 to 1,000,
preferably 116 to 750 and more preferably 116 to 158.
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Suitable di- or polyisocyanates include the known di- or polyisocyanates,
preferably
those containing (cyclo)aliphatically-bound isocyanate groups, such as
butylene
diisocyanate, hexamethylene diisocyanate, 2,2,4- and/or 2,4,4-trimethyl-hexa-
methylene diisocyanate, neopentyl diisocyan.ate, isophorone diisocyanate,
dicyclo-
hexylmethane diisocyanate and derivatives thereof containing urethane,
allophanate,
isocyanurate, biuret and/or uretdione groups. Diisocyanates containing
urethane
groups obtained from diisocyanates and dihydric alcohols are also suitable.
The addition reaction to prepare component a) can be accelerated in known
manner
with suitable catalysts, such as tin octoate, dibutyltin dilaurate or tertiary
amines.
The resulting urethane-(meth)acrylates containing free NCO groups must be
stabilized against premature polymerization by the addition of suitable
inhibitors and
antioxidants, such as phenols, hydroquinones and quinones, such as e.g. 2,5-di-
tert-
butylquinone. These inhibitors are added during or after preparation in an
amount of
0.001 to 0.3 wt.%, based on the weight of the reactants. The products can be
prepared
in solvents which are inert to the preparation reaction and to the subsequent
curing
reaction.
Optional polyisocyanates b) include polyisocyanates other than those set forth
under
a), i.e., those that do not contain (meth)acrylate groups. Preferably, they
are selected
from polyisocyanates that contain isocyanurat:e, allophanate, biuret and/or
uretdione
groups, more preferably polyisocyanates of this type that are prepared from
hexa-
methylene diisocyanate, 2,2,4- and/or 2,4,4-trimethyl-hexamethylene
diisocyanate or
isophorone diisocyanate.
UV initiators c) include 2-hydroxyphenyl ketones such as 1-hydroxy-cyclohexyl
phenyl ketone, benzyl ketals such as benzyl dimethyl ketal, acylphosphine
oxides
such as bis-(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, benzophenone and
deri-
vatives thereof and mixtures of the preceding t1V initiators.
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Compounds d), which contain isocyanate-reactive groups, include polyols, such
as
polyester, polyacrylate polyols and polyethf.r polyols, and compounds
containing
sterically hindered amino groups. The polyesters can be obtained by the
esterification
of di- or triols with dicarboxylic acids. The p~olyacrylate polyols may be
prepared by
the copolymerization of unsaturated monomers, preferably mixtures containing
(meth)acrylic acid esters, hydroxyalkyl(meth)acrylic acid esters and
optionally
styrene and/or other monomers, such as acrylonitrile. The polyether polyols
include
those obtained by the alkoxylation of diols and/or polyols. The compounds con-
taming sterically hindered amines are obtained by the addition of malefic acid
esters
onto (cyclo)aliphatic primary diamines as disclosed in U. S. Patent 5,126,170
and
5,236,741.
To stabilize the cured coating to weathering, a UV absorber having an
absorption
1 S range up to a maximum of 390 nm and a HAIJS stabilizer can preferably be
added to
component II). Suitable UV absorbers include those of the triphenyltriazine
type,
e.g., Tinuvin 400 (Ciba), or those of the oxalic acid dianilide type, e.g.,
Sanduvor
3206 (Clariant). The UV absorbers are preferably added in amounts of 0.5 to
3.5%,
based on resin solids.
Suitable HALS stabilizers include those that are commercially available, such
as
Tinuvin 292 or Tinuvin 123 (Ciba) or Sanduvor 3058 (Clariant). These HALS
stabilizers are preferably added in amounts of 0.5 to 2.5%, based on resin
solids.
Suitable additives g) include deaerating agents, such as polyacrylates, and
flow
control agents, such as polysiloxanes.
Suitable solvents are those which are inert towards isocyanate groups and C=C
double bonds, such as esters, ketones, ethers, ether-esters, alkanes, and
aromatic
solvents, e.g., xylenes or toluene.
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If all of the components of the coating composition according to the invention
are
formulated as a one-component composition, the system has a limited pot life.
A pre-
ferred formulation is a two-component composition in which component I)
contains
components a) and b) and component II) contains components c) and d) and op-
tionally components e) to h). The two-component compositions have a storage
stability that corresponds to the stability of tlhe individual components.
Components
I) and II) are mixed in the required ratio before application, or are applied
via so-
called two-component units.
T'he coating compositions according to the invention can preferably be applied
by a
spraying, pouring or rolling application. Complete curing can be carried out:
1. by optionally allowing added solvents to evaporate at room temperature or
at
elevated temperature, preferably up to 80~C;
2. by UV curing using, e.g., commercially available high or medium pressure
1 S mercury lamps, which can be doped with other elements and preferably have
an output of 80 to 240 W/cm lamp len,~gth; and
3. by reacting (crosslinking) the NCO-containing compounds with the
isocyanate-reactive components at room temperature or at elevated
temperatures, preferably up to 150~C.
A preferred complete curing process is onc: in which the reaction of the NCO-
containing compounds proceeds partly during; UV curing at an elevated
temperature
(10~C to 50~C above reaction temperature) or during a longer UV irradiation
time
(exposure time) at an unchanged temperature.
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TiYAMpITiC
Example 1 - Preparation of a urethane-a.crylate containing isocyanate groups
according to the invention:
1,268.7 g of an HDI-isocyanurate having an lVCO content of 23%, 383.9 g of
meth-
oxypropyl acetate, 0.96 g of dibutyltin dilaurate and 1.92 g of 2,6-di-tert-
butylcresol
were weighed into a stirred tank equipped with a thermoelement, dropping
funnel,
reflux condenser, gas feed line and gas remo val line. While stirring, three
times the
tank volume of air per hour was passed through and six times the tank volume
of
nitrogen per hour was passed over the mixture, which was then heated to 50~C.
266.8 g of hydroxyethyl acrylate were added dropwise such that the temperature
was
increased to a maximum of 60~C due to the exothermic reaction. After 6 to 8
hours
when the addition of the hydroxyethyl acrylate was complete, the mixture was
subsequently stirred at 60~C for a further hour. A urethane-acrylate solution
having
an NCO content of 9.2% and a viscosity of l,(170 mPa.s (23~C) was formed.
Example 2 - Preparation, application and testing of a coating composition
according to the invention
A coating composition was prepared from components I) and II) at an equivalent
ratio of OH to NCO of 1.0:1Ø
Component 1: the urethane-acrylate solution from Example 1
Component 2: a polyacrylate polyol having an OH content of 3% and present as a
70% solution in butyl acetate (Desmophen A VP LS 2009/1, Bayer
AG), also containing 1%, based on resin solids, of a HALS
stabilizer (Sanduvor 30S8, Clariant); 2.5%, based on resin solids, of
a UV absorber (Sanduvor 3206, Clariant); and 5%, based on the
solids content of component I), of Irgacure 184 Photoinitiator
(Ciba)
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For application the mixture was diluted to a. solids content of 30% with
methoxy-
propyl acetate and applied in an amount sufficient to obtain a film thickness
of
approx. 120 g/mz using a 1.5 mm nozzle under 3 bar to a metal plate precoated
with a
pigmented 2-component polyurethane coating.
The wet films were cured by 3 different methods:
2a) 30 min, 130~C
2b) 10 min, 60~C; UV curing (lm/min belt speed, 1 high pressure Hg lamp
80 W/cm, 10 cm lamp distance); and 30 min, 130~C
2c) 10 min, 60~C; UV curing as in 2b); after-curing at room temperature
The resulting hardness of the coatings was tested by pendulum damping by the
Konig method, and the resistance to solvents and to 1 % sulphuric acid were
tested.
The following table sets forth the results.
Curing method 2a :?b 2c
Pendulum damping 71 l40 90 after
by 30
min
the Konig method, 110
sec after
1 d
120
after
2 d
140
after
7 d
storage
at
room
temperature
Solvent resistanceAction Action Action
time time time
1 min S min ll 5 min 1 min 5 min
min
Xylene 2 3 0 0 0* 0*
Methoxypropyl 2 3 0 0 0* 0*
acetate
Ethyl acetate 3 3 0 2 0* 0*
Acetone 3 3 1 3 1 * 2*
Resistance to
1%
HZS04, first damage
(brown edge) at 36C 4E6C 46C
* Condition after storage at room temperature for 7 d
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The numbers for solvent resistance denote:
0 = unchanged
1 = trace of a change
2 = slightly changed
3 = moderately changed
4 = greatly changed
5 = dissolvedldestroyed
Although the invention has been described in detail in the foregoing for the
purpose of
illustration, it is to be understood that such detail is solely for that
purpose and that
variations can be made therein by those skilled in the art without departing
from the
spirit and scope of the invention except as it may be limited by the claims.
