Note: Descriptions are shown in the official language in which they were submitted.
2 ~
Two-component water-based varnishes containin~ acrylic-urethane
polymers
~escription of the invention
The present invention relates to water-based varnishes prepared
by mixing, just before use, two basic components, i.e.
component A, consisting of an acrylic and urethane polymers
aqueous dispersion, and component B7 consisting of a liquid
polyisocyanate compound either as is or in the form of a
solution, e.g. in ethyl acetate.
The properties of the films obtained from the varnish as per
the invention - which envisages the use of polyisocyanate to
cross-link the acrylic-urethane polymeric material, are not
1~ those characteristic of thermoplastic non-cross-linked films,
but of two-component cross-linked polyurethane systems, which
are analogous to those of the known solvent-based systems.
In particular, their resistance to chemicals, to abrasion, etc.
is high, while their appearance is as good as that of the
traditional solvent-based varnish films.
The varnish as per the present inventi~n - whose basic
polymeric component (component A) is in the form of an aqueous
dispersion - has the advantage, arising from the absence of
solvents, that it does not cause any harm to the users' health
and does not create any atmospheric pollutio~ problem.
Po~meric component A in an aqueous dispersion essentially
consists of an acrylic polymer containing free hydroxy groups
(2% to L~X by wt. 0~ groups on dry basis) ~ixed with a lower
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amount of aliphatic polyurethane resin.
The acrylic resin/polyurethane resin weight ratio ranges
between 60/40 and 65/35. The acrylic polymer content ranges
from 15% to 18% by wt. of the total aqueous dispersion.
Polymeric dispersion A must act as an emulsifyirlg agent of
polyisocyanate (component B).
The polymer particles are 12 to 16 nm in diameter, which is the
optimum size for their being uniformly dispersed in the
reactive system.
The formulation oP polymeric dispersion A envisages the use of
some of the usual additives of water-based systems, e.g.
antifoam and thickening agents, surfactants, extenders,
pigments, etc.
The product viscosity is adjusted by water addition, if any, to
40 to 50 sec. (Ford 4 viscosity at 20 C).
Component B, which acts as a cross-linking agent, consists of a
low-volatility aliphatic polyisocyanate, in particular low
molecular weight isocyanate dimers and trimers. The free -NC0
groups content must range from 18.5% to 20.5% by wt. Component
B is used in the liquid form with a viscosity of 50 to 16,000
mPa.s (at 20 C); in particular, with a view to obtaining the
convenient viscosity it can be diluted with a solvent, e.g.
ethyl acetate,
The react:Lons taking place in the acrylic-urethane-isocyanate
polymer system as per the inventl~n may be schematically
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represented as follows.
During the first reaction step between isocyanate and water
(non-selective reaction) a carbamic acid derivative is formed
through water addition to the NCO groups: said acid easily
liberates CO2, being thus converted into NH2 group; this last
readily reacts with an isocyanic group with formation of a urea
polymer.
Il
-N=C=O ~ H20 ---> -NH-C-OH
O
-NH-C-OH ---> -NH2+CO2
o
-NH2+0=C=N- ---> -NH-C-NH-
The following scheme represents the reaction between the
hydro~y acrylic polymer and isocyanate, with formation of a
urethane bond:
-N-C=O ~ HO ---> -NH-C-O-
g
Varnish preparation
The varnish is prepared by mixing under stirring, just before
use, component A w.ith the isocyanic cross-linking agent B in
convenient amounts to obtain a molar ratio between the free
isocyanic groups of component B and the free hydroxy groups of
component A of 1 to 1.5.
The physicochemical properties of the varnish obtained are as
follows:
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density at 20 C 1.100 to 1.215
g/c~3
Ford 4 viscosity at 20 C 40 to 50 sec.
dry residue (2 hrs at 130 C) 35% to 45% by wt.
gelling time (time of use) at 20~C 60 to 150 min.
A typical formulation of a varnish according to the invention
is reported hereinbelow (% by wt.).
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COMPONENT A % ON THECONTENT OF DRY
WHOLE PRODUCT %
Acrylic resin dispersion with
hydroxy groups 51-435 30-32
Aliphatic polyurethane resin
dispersion 28.350 33-35
Polyethylene wax emulsion8.000 50-55
Silica 3.125 100
Water 6.874
Preservatives 0.001 15-20
Silicone-based antifoam agent 1.215 100
Silicone 1.000 100
100.000%
COMPONENT B
Low-volatility aliphatic
polyisocyanate 50.000% by wt.
~thyl acetate 50.000% by wt.
100.000% by wt.
Weight ratios between the components
COMPONENT A 100
COMPONE.NT B 10
Performance of the varnish according to the invention
The performance test.s carried out on the varnish in actual use
gave the results reported in the following table (according to
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BS 6250/3 stAndard for severe-duty horizontal surfaces -
excepting kitchen worktops). Application ti~e: 1 hr.
Humid heat ( C)Min. values requestedResults
BS 3962/2
24 5
23 5
22 5
Dry heat ( C3
BS 3962f3
21l 5
100 23 5
Resistance to staining
BS 3962L~
Min. values requested Results
Acetone 23 4
Ethyl/butyl acetate 23 4
Tea =5 5
Coffee =5 5
Ethyl alcohol (96X) ~3 4
Ethyl alcohol (48%) 24 5
Resistance to oil and grease
BS 3962/5 =5 5
The results obtained from the performance tests prove that the
film loses its thermoplastic properties and becomes a
themosetting material.
VARNISH FILM CROSS-LINKING
Film formation of the two-component water-based acrylic-
urethane polymeric system does not occur by coalescence as in
the case of the usual aqueous dispersions. Instead, after water
evaporation, a 100 to 200 hrs cross-linking reaction takes
place to form urethane and urea groups; after that, the
physicochemical resistance of the varnish film obtained reaches
the highest value.
~ se-E5~perties
On the basis of wide experimentation carried out in the
commercial plants producing usual solvent varnishes, the
application properties of the water-based varnish containing
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: .
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acrylic-urethane polymers were found to be good.
e coating film adhesiveness, perPormance and appearance were
found to be excellent.
Adhesion
According to the tests, adhesion to the following items was
excellent:
- wood oP various types
- wood particle boards
polyester substrates
- polyurethane substrates
- polyacrylic substrates
- polyepoxydic substrates
- metallic substrate (iron)
- substrate coated with a different water-based varnish
Excellent and identical adhesion values (crosshatch test,
standard 150-4624) were shown by tests on the various supports.
The MIR (Multiple Internal Reflectance) analysis of the product
applied to wood board showed the formation of urea and urethane
groups between 1600 and 1640 cm~l, which are the same groups
observed in the MIR plot of solvent polyurethane, This is a
proof that a reaction occurred between the NCO groups of the
cross-linking agent and the OH groups of the acrylic resin
aqueous dispersion.
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