Note: Descriptions are shown in the official language in which they were submitted.
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Mo4062
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RAPIDLY, PHYSICALLY DRYING BINDER MIXTURES
AND THEIR USE FOR COATING WOOD SUBSTRATES
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a novel binder composition suitable for the
production of rapidly physically drying coatings based on a polyiso-
cyanate component and a special, hydroxy-functional polyacrylate
component, and to the use of the binder composition as a binder in
rapidly physically drying coating compositions for wood and derived
timber products.
Description of the Prior Art
Two-component polyurethane coating compositions based on
lacquer polyisocyanates and hydroxy-functional polyacrylate resins are
known. For example, DE-OS 2,460,329 describes polyacrylates which
may be crosslinked with polyisocyanates. These low molecular weight
products have a defined, number average molecular wei~ht (Mn~ of 500
to 2,000 and polydispersity of 0.~ to 1.5. A major disadvantage of these ~ ~-
compositions is their physical drying behavior, which is inadequate for
industrial applications. Rapid physical drying, i.e., the rapid production of
a tack-free coating long before complete chemical crosslinking is a
fundamental requirement for the suitability of the coating compositions in
large scale industrial lacquer coatings, particularly for wood and derived
timber products.
In contrast to the above coatings, the two-component polyurethane
coatings based on hydroxy-functional polyacrylate resins described in
EP-A 0,068,383 do exhibit distinctly improved drying behavior, but still do
not fulfil the stringent requirements in this regard, particularly for industrial
wood and furniture coatings.
Due to these problems, in commercial applications rapidly
physically drying binders such as cellulose acetate butyrate (CAB) or
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nitrocellulose ~NC) are always added to wood and furniture coatings
based on organic polyisocyanates and known polyacrylate resirls to
achieve the required drying speeds. Apart from the use of an additional
binder component, frequently observed disadvantages of such coating
cornposit~ons are excessive yellowing of the lacquer coating (NC) or poor
compatibility with the polyisocyanate component, particularly with
aromatic polyisocyanates (CAB).
An object of the present invention is to provide a two-component
binder compositions based on organic polyisocyanates and hydroxy-
functional polyacrylate resins, which do not have the stated
disadvantages of the prior art, i.e. they are suitable for the production of
rapidly physically drying coatings without the addition of the rapidly
physically drying binders such as NC and/or CAB.
This object may be achieved with the binder compositions
according to the invention which are described in greater detail below.
SUMMARY OF THE INVENTION
The presen~ invention relates to a binder composition for the
production of rapidly physically drying coating compositions containing
a) a polyisocyanate component having an NCO content of 10 to 30
wt.% and containing one or rnore organic polyisocyanates and
b) a hydroxy-functional polyacrylate component containing one or
more hydroxy-functional polyacrylate resins having a number
average molecular weight (Mn) of at least 10,000 and a viscosity
as a 40 wt.% solution in n-butyl acetate at 23C of at least
2,000 mPa.s,
wherein components a) and b) are present in amounts sufficient to
provide an NCO/OH equivalent ratio of 0.5:1 to 2.0:1.
The present invention also relates to the a solvent-containing, two-
component polyurethane coating composition containing this binder
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mixture as the binder, in particular for coating wood or derived timber
products.
pETAlLED DESCRIPTION OF THE INVENTION
Binder component a) according to the invention is selected from
known lacquer polyisocyanates, i.e., in particular, polyisocyanates having
biuret, urethane or isocyanurate groups and an average NCO
functionality of more than 2, preferably 2.~ to 6. These polyisocyanates
may have aromatically, cycloaliphatically or aliphatically bound isocyanate
groups. Polyisocyanates with aliphatically and/or cycloaliphatically bound
isocyanate groups are preferred for lighffast coatings.
The lacquer polyisocyanates to be used according to the invention
are produced in known manner by the reaction of monomeric organic
diisocyanates to form polyisocyanates containing biuret, urethane or
isocyanurate groups. After their preparation, any excess monomeric
starting isocyanate is removed, preferably by distillation, such that a
maximum of 0.7 wt.%, preferably a maximum of 0.5 wt.%, of excess
diisocyanate is present in the lacquer polyisocyanates according to the
invention. The lacquer polyisocyanates generally have an NCO content,
based on solids, of 10 to 30 wt.%, preferably 15 to 25 wt.%.
Suitable diisocyanates for the production of the lacquer diiso-
cyanates include 2,4- and/or 2,6-diisocyanatotoluene, 2,4'-diisocyanato-
dicyclohexylmethane, 4,4'-diisocyanatodicyclohexylmethane, hexa-
methylene diisocyanate and 1-isocyanato-3,3,5-trimethyl-5-isocyanato-
methylcyclohexane (IPDI). Particularly preferred lacquer polyisocyanates
include polyisocyanates which contain biuret groups and are prepared
from hexamethylene diisocyanate, for example, those according to U.S.
patent 3,124,605; polyisocyanates which contain urethane groups and
are prepared from 2,4-diisocyanatotoluene, trimethyolpropane and
optionally low molecular weight alkane diols, for example those according
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to U.S. patent 3,183,~12; polyisocyanates which contain isocyanurate
groups and are prepared from 2,4-diisocyanatotoluene, hexamethylene
diisocyanate, IPDI, mixtures of 2,4-diisocyanatotoluene with hexa-
methylene diisocyanate or mixtures of hexamethylene diisocyanate with
5 IPDI.
At a solids content of 50 wt.%, the hydroxy-functional polyacrylate
resins b) have solution viscosities which are no longer measurable using
the method described below. Any further viscosity values stated below
thus relat~ to 40 wt.% solutions in n-butyl acetate measured at 23C.
The solution viscosities of the polyacrylates to be used according
to the invention, measured as 40% solutions in n-butyl acetate at 23C in
a rotational viscometer, are at least 2,000 mPa.s, preferably 2,000 to
30,000 mPa.s and more preferably 2,000 ~o ~5,000 mPa.s. The poly-
acrylate stated in EP 0 068 383 (example 1) has a viscosity of
15 530 mPa.s under identical conditions (solvent, solids content,
tem,oerature).
The polyacrylate resins b) generally have a number average
molecular weight (Mn~ determined by gel permeation chromatography) of
at least 10,00û, preferably 10,000 to 120,000 and more preferably 10,000
20 to 100,000. These and all subsequent molecular weights of compon~nt b)
are determined by the molecular weight determination method described
by Z. Grabisic, P. Rempp and H. Benoit in J. Polymer Sc., part 8, Polym.
Lett. 5 (1967), p. 753, i.e., using gel permeation chromatography.
The disclosure in US-Patent 4 442 145 (EP 0 068 383) with respect to the molecular
25 weight of the polyhydroxy polyacrylates seems to be erroneous because a repelition
of example 1 of said patent leads to a polyacrylate resin having molecular weights
determined in accordance with the above method which are clearly lower than
47.700 (Mw = 24.000 resp. Mn = 6.100). In analogy thereto the repetition of
example 2 of said patent leads to a polyacrylate resin having a molecular weight Mw
of 27.100 and Mn of 5.200.
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The monomers used to prepare copolymers b) are selected from
2.5 to 30, preferably 2.5 to 20 wt.%, of monomers A), 30 to 90, ~:
preferably 55 to ~0 wt.% of monomer B), 0 to 50, preferably 0 to 30 wt.%
of monomer C), 0 to 2 wt.% of monomers D) and 0 to 50, preferably 0 to
30 wt.% of monomers E).
Monomers A) are selected from monomers containing hydroxyl -
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groups, in particular hydroxyalkyl esters of a,~-unsaturated carboxylic
acids, preferably acrylic acid or methacrylic acid with 2 to 12, preferably 2
to 6 carbon atoms in the hydroxyalkyl residue. Examples include 2-
hydroxyethyl acrylate, the isomeric hydroxypropyl acrylates (in particular,
those obtained by the addition of propylene oxide onto acrylic acid), the
isomeric hydroxybutyl acrylates, the isomeric hydroxypentyl acrylates, the
isomeric hydroxyhexyl acrylates, the methacrylates corresponding to
these acrylates and mixtures of any of these acrylates and/or meth-
acrylates. Preferred monomers A) are 2-hydroxyethyl (meth)acrylates
and/or the isomeric hydroxypropyl (meth)acrylates.
As may be seen from the examples of suitable (meth)acrylates,
the hydroxyalkyl esters containing hydroxyl groups may have both
primary and secondary hydroxyl groups. Monomers A) are used in
amounts, within the above-stated ranges, that the resulting acrylate
resins have a hydroxyl group content of 0.5 to 5, preferably 1 to 3 wt.%,
based on solid resin.
Methyl methacrylate is used as monomer B). Styrene is used as
optional monomer C). Optional monomer D) is selected from free acids,
such as itaconic acid, crotonic acid, semiesters of maleic or fumaric acid,
and preferably acrylic acid or methacrylicacid.
Optional monomers E) are selected from any desired olefinically
unsaturated monomers, other than those used as monomers A) to D).
Examples of suitable monomers include (meth)acrylic acid alkyl esters
and/or cycloalkyl esters with 1 to 18, preferably 1 to 8 carbon atoms, in . ~ -~
the alkyl or cycloalkyl residue, such as the methyl, ethyl, n-propyl, n- - ~
butyl, isopropyl, isobutyl, t-butyl, the isomeric pentyl, hexyl, octyl, ~ :
dodecyl, hexadecyl, octadecyl, 3,5,5-trimethylcyclohexyl, isobornyl and/or
cyclohexyl esters of these acids, with the exception of methyl ~ -
methacrylate. Additional suitable monomers include acrylonitrile, vinyl
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ethyl ether, methacrylonitrile, vinyl acetate, vinyl chloride, vinyl toluene,
anhydride-functional, unsaturated monomers such as itaconic anhydride
or maleic anhydride and any desired mixtures of the preceding
monomers.
The copolymerization reaction is preferably conducted in an
organic solution~ optional~y in the presence of inert solvents. Examples
include toluene, xylene, chlorobenzene, n-butyl acetate, ethyl acetate,
ethylene glycol acetate, the isomeric pentyl acetates, hexyl acetates,
methoxypropyl acetate, tetrahydrofuran, dioxane, acetone, methyl ethyl
ketone, higher substituted aromatics (such as solvent naphtha, crude
benzene, Solvesso solvents, Shellsol solvents), higher-boiling aliphatic
and cycloaliphatic hydrocarbons (such as gasoline, mineral spirits, Isopar
solvents, Nappar solvents, tetralin and decalin) and mixtures of these
solvents.
n-Butyl acetate, methoxypropyl acetate, xylene and mixtures of
these solvents are the preferably used as solvents. In the copolymer-
ization reaction, the solvents are generally used in quantities of 50 to 70
wt.%, based on the total weight of the reaction mixture.
The viscosity of polyacrylate resins b), which is essential to the
invention, is achieved not only by the above-stated selection of suitable
monomers, but also by a suitable combination of polymerization
temperature with the type and quantity of initiator.
In general, the polymerization is conducted at a temperature of 80
to 150C, preferably 90 to 120C and more preferably 100 and 115C.
Suitable initiators are those with a half-life of between 5 minutes
and 2 hours at the stated temperature range. Examples include dilauroyl
peroxide, tert.-butyl peroxy-2-ethylhexanoate, tert.-butyl peroxydiethyl-
acetate, dibenzoyl peroxide, tert.-butyl peroxyisobutyrate, 1,1-di-tert.-
butylperoxy-3,3,5-trimethylcyclohexane, 1,1-di-tert.-butylperoxycyc!o-
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hexane, tert.-butyl peroxy-3,3,s-trimethylhexanoate, tert.-butyl peroxy-
isopropylcarbonate, 2,2-di-tert.-butylperoxybutane and tert.-butyl peroxy-
stearyl-carbonate.
The initiators are generally used in an amount of 0.5 to 5,
S preferably 1 to 3 wt.%, based on the weight of monomers A) to E). This
amount is based on solvent-free initiators and solvent-free monomers.
Polyacrylate resins b) are most preferably produced using n-butyl
acetate as the solvent and tert.-butyl peroxy-2-ethylhexanoate as the
initiator at temperatures of 100-115C.
The binder mixtures according to the invention are produced by
blending components a) and b), preferably with the addition of the
preceding solvents, such that 30 to 50 wt.% solutions of the stated binder :
components are produced. The amounts are also selected such that an~ ~ ~
NCO/OH equivalent ratio of 0.5:1 to 2:1, preferably 0.5:1 to 1.5:1 is ~:
1 5 obtained.
The binder mixtures according to the invention are used for -; ~ ~ -
coating any desired substrates, preferably for coating wood or derived
timber products and more preferably for coating furniture. -
Apart frorn the binder mixtures according to the invention, the
20 coating compositions may also contain known additives. Examples
include flow-control agents such as oligoalkyl acrylates; pigments and .
extenders; viscosity-controlling additives such as bentonites and silicic -
acid esters; flatting agents such as silica, aluminum silicates and high ;
molecular weight waxes; and catalysts for the isocyanate addition
2~ reaction such as for example tin(ll) octoate or dimethylbenzylamine.
The coating compositions containing the binder mixtures according ~ -
to the invention may be applied to the substrate to be coated using any
known methods such as spraying, brushing, dipping, pouring or rolling. -
Wood and derived timber products (furniture) are particularly preferred
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substrates.
The coatings produced with the coating compositions according to
the invention are generally tack-free within a period of 30 to 70 minutes
at 23C (physicai drying).
The following examples are intended to illustrate the subject
matter of the invention in greater detail. All parts and percentages are by
weight, uniess otherwise indicated.
EXAMPLES
production of polvhvdroxvacrvlates b)
The solvent (n-butyl acetate in parts by weight) was introduced
into a glass flask and heated to 110C. The monomer mixtures (parts by
weight) set forth in Table I were then added over a period of 2 hours. - n
The initiator solution was added incrementally over a period of 3 hours in
parallel with the addition of the monomers. The reaction mixture W3S then
stirred for a further 2 hours at 100C. Viscosities and properties of the
products are set forth in Table ll.
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Table I
Examples 1 2 ~ 3 4 5
Butyl acetate 19611961 1920 1920 1920
l Monomer Composition
Methyl 11951002;6 1009.51214.8 1012.6
methacrylate l
, . I
¦ 2-Ethylhexyl 203.4 205.7
acrylate
l .
¦ Styrene _ l g o
¦ Hydroxyethyl 227.7231.4 232.4 232.8 233.2
I methacrylate .
¦ Acrylic acid 29.014.3 14.4 14.4 18.2
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¦ Initiator Solution
¦ tert.-Butyl peroxy-2-34.934.9 20.0 20.0 28.0
I ethylhexanoate ~.
-:
Butyl acetate 152.4152.4 198.0 198.0 198.0
I .
3600.03600.0 3600.0 3600.0 3600.0
Table ll compares the properties of the products of Examples 1-5
20 with the properties of those from Examples 1 and 2 of EP-A-0 068 383
(examples 6 and 7 in Table ll).
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Table 1
. . I
Comparison
Examples
Examples 1 2 3 4 5 6 7
Sollds content 40.5 40.0 39.741.0 40.2 40.0 40.0
. _
Viscosity 3740 2210 7820 33390 23150530 890
(mPa s) at . = l
~-
Solids (non-volatiies) content was determined to according to DIN
53 216.
** Viscosity was determined in a Contraves rotational viscosimeter.
All products were dissolved in butyl acetate. Measurements were
made according to DIN 53 019, part 1. (Measurement of
viscosities and flow curves with standard geometry rotational
viscosimeters) .
Comparison of the viscosities demonstrates the clear difference
between the polyacryiate resins according to the invention and those
according to EP-A 0,068,383.
There are also clear differences in molecular weight, as is
apparent from the following comparison between Examples 1 and 2
(according to the invention) and Examples 6 and 7 (comparison
examples). The molecular weight set forth in Table lll was determined
using method set forth above. It may readily be seen that the molecular
weights of the polyacrylate resins according to the invention were
distinctly higher than those of the polyacrylate resins according to
EP-A-0,068,383.
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Table lll
Mn Polydispersity
Polyacrylate from65100 26800 1.4
Example 1
,
Polyacrylate from58600 22500 1.6
Example 2
_
Polyacrylate from24000 6100 2.9
Example 6~
Polyacrylate from27100 5200 4.2
Examp!e 7*
Comparison example
Practical examPles
Several lacquers were prepared using the polyacrylates set forth in
Table IV. The polyacrylates were diluted with butyl acetate such that
after the addition of the cross-linking agent solution, a spray viscosity of
18 seconds (23C) in a DIN 4 cup was obtained. The cross-linking agent
solution was a 75% solution in methoxypropyl acetate/xylene (1:1) of a
polyisocyanate containing biuret groups, prepared from 1,6-diisocyanato-
hexane and having an NCO content (based on solution) of approximately
16.5%. The NCO/OH equivalent ratio was 1:1 in each case. Further
details may be found in table IV below wherein the components are set
forth in parts by weight.
After the addition of the curing agent, the polyacrylates were
applied to a sheet of glass with a coating knife (210 ,um). A thumb was .
firmly applied to the coated surface from time to time to test whether it
was still tacky, whether any impression was left behind or whether no
impression was visible on the film. The times set forth in Table IV
represent the period of time after which the thumb left no impression.
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Table IV
Lacquer ¦ A ¦ B ¦ C ¦ D ¦ E F ¦ G
Comparison
coatings
5 Polyacrylate 100
from example 1
Polyacrylate 100
from example 2
Polyacrylate 100
from example 3
Polyacrylate 100
from example 4
Poiyacrylate 100
from example 5
Polyacrylate 100 100
from examp!e 6 _
CAB 381-0.5* 15
Butyl acetate 72 51 140.5 182 164 70 328
Curing agent 12.1 12.6 13 13.6 12.5 14.9 22.6
solution
Hand drying (in 48 54 55 44 56 ~480 79
minutes)
A commercial cellulose acetate butyrate from Eastman Kodak.
As demonstrated in Table IV, comparison coating F had a hand
drying time of over 8 hours. It is only after addition of cellulose acetate
butyrate that the rapid hand drying time required in commercial use is
achieved (comparison coating G). To the contrary, coatings A to E
30 according to the invention exhibit the drying times required in commercial
use even without cellulose acetate butyrate.
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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
5 the invention except as it may be limited by the claims.
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