Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02262075 1999-02-23
Production of skins from polyurethane dispersions
Description
The present invention relates to processes for producing skins
from polyurethane dispersions, which comprise
I. producing on a mold a sheetlike layer of an aqueous
dispersion comprising a polyurethane (A), the mold surface
being free of any electrolyte which would cause the aqueous
dispersion comprising a polyurethane (A) to coagulate,
II. drying the film, produced in step (I), of the aqueous
dispersion comprising a polyurethane (A) to obtain a
single-layered skin,
III'optionally producing a skin composed of a plurality of skin
layers by producing on the mold, obtained in step II, coated
with a single-layered skin a further sheetlike layer of an
aqueous dispersion comprising a polyurethane (A),
IV. drying the new film, formed in step (III), of the aqueous
dispersion comprising a polyurethane (A) to form a further
skin layer, and
V. optionally repeating steps (III) and (IV) from 5 to 50 times.
Skins from polyurethane dispersion are known from JP-A
S63-256409, for example. The skins are produced therein by
dipping a salt wetted mold into a polyurethane dispersion and
subsequently subjecting the film formed to thermal curing. The
skins produced in this way can be backfoamed and be used for
example for the interior of automobiles. The disadvantage of this
process is that the mold has to be coated with a salt prior to
every processing step. This process is relatively complicated,
since the mold has to be coated with a salt in a separate
operating step.
DE-A-19708451, unpublished at the priority date of the present
invention, discloses the production of thin-walled elastic
articles by producing on the surface of a mold a layer of an
electrolyte, dipping the mold into a polyurethane dispersion,
coagulating the polyurethane dispersion on the surface,
optionally washing and curing the resulting layer of coagulated
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polyurethane particles, and removing the articles formed from the
mold.
EP-A-0257225 discloses heat coagulating heat sensitive
polyurethane dispersions to produce hollow articles. These
dispersions consist for example of a polyurethane rendered
hydrophilic by ionic groups, water-soluble polyether
polyurethanes as described for example in DE-A 2516979, 2534304
and 3330197 and an electrolyte, for example a metal salt.
It is an object of the present invention to provide a process for
producing skins from aqueous dispersions comprising a
polyurethane without the disadvantages of prior art processes.
We have found that this object is achieved by the process
described at the beginning.
The molds on which the sheetlike layer of an aqueous dispersion
of polyurethane (A) is produced in step (I) are molds of the type'
generally customary for producing automotive interior parts such
as dashboards, door linings and column linings. Such molds are
described in WO 93/23237 and JP-A-S63-256409, for example. The
molds preferably have surface structures whereby the resulting
skins have surface structures resembling those of leather (e. g.,
grain). The molds further preferably have convex and/or concave
surface profiles, so that the skins formed therein have surface
profiles of the type customary with automotive interior parts,
such as dashboards, door linings or column linings.
The surfaces of the molds to which the aqueous dispersion of
polyurethane (A) is applied are free of any coating with a salt
or electrolyte which would cause the aqueous dispersion of
polyurethane (A) to coagulate.
In general, there is no need for a specific pretreatment of the
surfaces to remove traces of adhering electrolyte quantities,
since traces of electrolytes are not sufficient to coagulate
p°lyurethane dispersions. It suffices for example to clean the
surfaces of the molds prior to their first use in the way which
is commonly customary with the generally known reaction injection
molding (RIM) process employing a conventional 2-component
polyurethane system. For example, the surface of the mold is
washed prior to its 1st use with an aqueous solution which
contains a customary detergent, rinsed off with tap water and
dried. After the first skin has been produced by the process of
the invention and removed from the mold, there is generally no
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need for any cleaning before the mold is recoated with the
polyurethane dispersion to produce the next skin.
To facilitate the removal of the finished skin from the mold, the
mold surface is wetted with a commercially available demolding
agent, for example a dispersion of waxes in base oils with and
without silicone additives, prior to the coating with the
polyurethane dispersion.
Any known process is suitable for applying the polyurethane
dispersion, for example dipping. Preferably, the aqueous
dispersion of polyurethane (A) is sprayed onto the mold. Suitable
for this purpose are the generally known spray guns used for
example in the application of paints.
Step (I) can be carried out particularly efficiently by heating
the aqueous dispersion of polyurethane (A) directly before the
dispersion is spray dispensed, for example by spray dispensing
the dispersion with pressurized hot steam. Suitable apparatus is~
described in EP-A-0291850, for example. This method makes it
possible to shorten the time required for drying the dispersion
film applied to the mold in this way.
The dispersion film produced in step (I) is preferably dried by
heating the mold surface to 20-150~C, preferably 50-80~C. At these
temperatures, drying times from 0.5 to 5 min are sufficient.
Drying is generally complete when the skin layer formed has a
water content of less than 1~ by weight.
The sheetlike application of the aqueous dispersion of
polyurethane (A) is customarily effected in such amounts that a
single performance of steps (I) and (II) will produce a
single-layered skin from 10 dun to 2 mm in thickness.
If thicker skins are desired especially, the first coating and
drying step (steps I and II) can be followed by further ones to
produce a skin built up from a plurality of skin layers.
This is accomplished by
III.producing a skin composed of a plurality of skin layers by
producing on the mold, obtained in step II, coated with a
single-layer skin a further sheetlike layer of an aqueous
dispersion comprising a polyurethane (A), and
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IV. drying the new film, formed in step (III), of the aqueous
dispersion comprising a polyurethane (A) to form a further
skin layer, and
V. optionally repeating steps (III) and (IV) alternatingly until
the desired skin thickness has been reached.
It is generally sufficient for steps (III) and (IV) to be
repeated from 5 to 50, preferably from 10 to 50, times in order
that a skin about 1 mm in thickness may be produced in a
particularly efficient way.
As to how said steps (III) and (IV) are carried out, the remarks
made in relation to steps (I) and (II) apply mutatis mutandis.
The skins preferably have a surface hardness from Shore A 55 to
Shore D 80, determined in accordance with German standard
specification DIN 5305.
Particularly suitable aqueous dispersions for producing the skins
comprise a polyurethane (A) polymerized from
a) diisocyanates having from 4 to 30 carbon atoms,
b) diols of which
bl) from 10 to 100 mold, based on total diols (b), have a
molecular weight from 500 to 5000, and
b2) from 0 to 90 mold, based on total diols (b), have a
molecular weight from 60 to 500 g/mol,
c) monomers, other than monomers (a) and (b), which contain at
least one isocyanate group or at least one isocyanate
reactive group and which in addition contain at least one
hydrophilic group or a potentially hydrophilic group to
render the polyurethanes water-dispersible,
d) optionally further polyfunctional compounds, other than
monomers (a) to (c), having reactive groups comprising
alcoholic hydroxyl groups, primary or secondary amino groups
or isocyanate groups, and
CA 02262075 1999-02-23
e) optionally monofunctional compounds, other than monomers (a)
to (d), having a reactive group comprising an alcoholic
hydroxyl group, a primary or secondary amino group or an
isocyanate group.
5
Suitable monomers (a) include the aliphatic or aromatic
diisocyanates customarily used in polyurethane chemistry.
Preference is given to the monomers (a), or mixtures thereof,
which are mentioned as monomers (a) in DE-A-19521500, too.
Monomers (b) and (d) are preferably the monomers (b) and (d)
mentioned in DE-A-19521500.
Monomers bl include for example polyesterdiols or polyetherdiols.
Monomers b2 are for example aliphatic diols having from 2 to 12
carbon atoms, e.g., 1,4-butanediol or 1,6-hexanediol.
Monomers (d) are for example aliphatic amines having from 2 to 12~
carbon atoms and from 2 to 4 groups selected from primary or
secondary amino groups. Examples are ethylenediamine,
isophoronediamine and diethylenetriamine.
To ensure that the polyurethanes are water-dispersible, the
polyurethanes are polymerized not only from the components (a),
(b) and (d) but also from monomers (c) which differ from said
components (a), (b) and (d) and which contain at least one
lsocyanate group or at least one isocyanate reactive group and in
addition at least one hydrophilic group or a group which is
convertible into hydrophilic groups. In what follows, the
expression "hydrophilic groups or potentially hydrophilic groups"
is abbreviated to "(potentially) hydrophilic groups". The
(potentially) hydrophilic groups react significantly slower with
isocyanates than the functional groups of the monomers which are
used for forming the polymer main chain.
Preferred monomers (c) are again the monomers (c) identified in
DE-A-19521500.
The proportion of components (a), (b), (c), (d) and (e) accounted
for by components having (potentially) hydrophilic groups is
generally determined in such a way that the molar quantity of the
(potentially) hydrophilic groups is from 80 to 1200, preferably
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140 to 1000, particularly preferably from 200 to 800, mmol/kg,
based on the weight quantity of all monomers (a) to (e}.
The (potentially) hydrophilic groups can be nonionic groups, for
5 example polyethylene oxide groups, or preferably (potentially)
ionic hydrophilic groups, for example carboxylate or sulfonate
groups. Preference is given to operating without effective
amounts of nonionic groups.
The level of nonionic hydrophilic groups, if such groups are
incorporated, is generally up to 5, preferably up to 3,
particularly preferably up to 1, ~ by weight, based on the weight
quantity of all monomers (a) to (e).
Monomers (e), the use of which is optional, are monoisocyanates,
monoalcohols and monoprimary and secondary amines. In general,
their proportion does not exceed 10 mold, based on the total
molar quantity of the monomers. These monofunctional compounds
customarily bear further functional grougs such as carbonyl .
groups and are used for incorporating into the polyurethane
functional groups which render the dispersion or crosslinking or
further polymer-analogous reaction of the polyurethane possible.
It is common knowledge in the field of polyurethane chemistry how
the molecular weight of the polyurethanes can be adjusted through
choice of the proportions of mutually reactive monomers and the
arithmetic mean of the number of reactive functional groups per
molecule.
The components (a) to (e) and their respective molar quantities
are normally chosen so that the ratio A : B where
A) is the molar quantity of isocyanate groups, and
B) is the sum total of the molar quantity of the hydroxyl groups
and the molar quantity of the functional groups capable of
reacting with isocyanates~in an addition reaction
is within the range from 0.5 . 1 to 2 . 1, preferably within the
range from 0.8 . 1 to 1.5, particularly preferably within the
range from 0.9 : 1 to 1.2 . I. The A : B ratio is most preferably
very close to 1 . 1.
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Furthermore, the proportion of monomers (a) is preferably chosen
so that the proportion of monomers (a) to (e) which is accounted
for by monomers (a) is within the range from 20 to 70~ by weight.
The monomers (a) to (e) used on average bear customarily from 1.5
to 2.5, preferably from 1.9 to 2.1, particularly preferably 2.0,
isocyanate groups or functional groups capable of reacting with
isocyanates in an addition reaction.
The polyaddition of components (a) to (e) is generally effected
at reaction temperatures from 20 to 180~C, preferably from 50 to
150~C, under atmospheric pressure or under autogenous pressure.
The reaction times required can range from a few minutes to
several hours. In the field of polyurethane chemistry it is known
how the reaction time is influenced by a multiplicity of
parameters such as temperature, concentration of the monomers,
reactivity of the monomers.
To speed up the reaction of the diisocyanates it is possible to
use the customary catalysts, such as dibutyltin dilaurate,
tin(II) octoate or diazabicyclo(2.2.2)octane.
Suitable polymerization apparatus includes stirred tanks or the
otherwise customary polymerization apparatus.
Preferred solvents are miscible with water in any proportion,
have an atmospheric-pressure boiling point within the range from
to 100~C, and react only slowly with the monomers, if at all.
The dispersions are usually prepared according to one of the
following processes:
35 The "acetone process" comprises preparing an ionic polyurethane
from the components (a) to (c) in a water-miscible solvent which
has an atmospheric-pressure boiling point below 100~C. Sufficient
water is added to form a dispersion in which water is the
coherent phase.
The "prepolymer mixing process" differs from the acetone process
in that initially a prepolymer bearing isocyanate groups is
produced and not a fully reacted (potentially) ionic
polyurethane. The components here are chosen so that the defined
ratio of A:B is within the range from greater than l.O to 3,
preferably within the range from 1.05 to 1.5. The prepolymer is
first.dispexsed in water and then optionally crosslinked or
. CA 02262075 1999-02-23
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branched by reaction of the isocyanate groups with amines bearing
more than 2 isocyanate reactive amino groups or extended by
reaction with amines bearing 2 isocyanate reactive amino groups.
Chain extension takes place even when no amine is added. In this
case, isocyanate groups are hydrolyzed to amine groups which
react with any remaining isocyanate groups on the prepolymers,
resulting in chain extension.
Customarily, if a solvent was used in the production of the
polyurethane, the bulk of the solvent is removed from the
dispersion, for example by distillation under reduced pressure.
The dispersions preferably have a solvent content of less than
10~ by weight and are particularly preferably free from solvents.
Particularly preferred dispersions are described for example in
DE-A-2645779 and DE-A-2651506 and also EP-A-0300335. Preferred
commercial products are Astacin~ Finish PUD or PUM from BASF
Aktiengesellschaft.
The dispersions preferably contain no substances with which the
dispersions are customarily modified into heat coagulable
dispersions, i.e., substances as described in EP-A-0257225. The
dispersions used in the process of the invention thus preferably
contain no effective amounts of water-soluble polyether
polyurethanes as described for example in DE-A 2516979, 2534304
and 3330197 or the electrolytes (metal salts) mentioned in
EP-A-0257225.
The dispersions of the invention may contain commercially
available auxiliaries and additives such as wetting agents,
defoamers, delustrants, emulsifiers, thickeners and
thixotropicizers, colorants such as dyes and pigments.
The skins of the invention are customarily backfoamed.
Backfoaming can take place in the mold in which the skins were
produced or the skins are removed from this mold and placed for
this operation into another mold which is suitable for the
expansion of polyurethane.
Customarily, the polyurethane foam (B) is applied to that side of
the skin which was not in contact with the mold.
The production of foams is common knowledge.
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The polyurethane foam (B) is advantageously applied by spraying
or casting a polyurethane reaction mixture comprising a blowing
agent onto the skin surface.
The polyurethane foam (B) generally is from 1 to 10, preferably
from 1 to 7, particularly preferably from 2 to 3, mm in
thickness. The polyurethane foam (B) is preferably an open-celled
semirigid foam.
IO
The isocyanate component is for example an isocyanate component
also useful as monomer (a) for the production of the aqueous
dispersion comprising a polyurethane (A). Particular preference
is given to polymeric MDI (methylene diphenyl isocyanate) (e. g.,
Lupranat~ M20 from BASF Aktiengesellschaft).
The polyol component is likewise selected from those which are
useful as monomers (a) for the production of the aqueous
dispersion comprising a polyurethane (A).
The blowing agent can be water.
Particular preference is given to water-blown polyether systems
from 50 to 180 kg/m3 in density. A particularly suitable system
for producing the polyurethane foam is a commercially available
system for producing semirigid foams such as, for example the
Elastoflex~ E systems (commercial product from Elastogran GmbH,
Lemforde).
Molds and application techniques suitable for the backfoaming of
the skins are described for example in WO 93/23237 and
JP A-S63-256409.
Optionally, the skins, especially when the polyurethane (A) is
predominantly derived from aromatic isocyanates as monomers (a),
are coated with an IMP coating for stabilization against W
radiation or with another customary coating. IMP coatings are
commercially available 2-component systems. Component 1 contains
for example a solution of a polyurethane resin which, as well as
alcohol, contains customary organic solvents such as, for
example, aromatic hydrocarbons and esters (e. g., xylenes, n-butyl
acetate or 1-methoxy-2-propyl acetate). Component 2 (curing agent
component) can be an aliphatic isocyanate, for example. IMP
coatings are available for example from ISL-Chemie GmbH, Kiirten,
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as a 2-component system under the tradenames of Isothan-TMP-2K
coating and Isothan curing agent.
The skins produced in this way are especially useful as
automotive interior parts, such as dashboards, door linings or
column linings.
Experimental part
Example A:
First the open mold was prepared by using a heating unit to raise
the mold surface to a temperature of 75 ~ S~C. The instrument
panel surface to be sprayed was then provided with a wax
dispersion in a base oil as demolding agent, Acmosil~ 56-4566
(from Acmos Chemie, Bremen) or Bomix~ 6389/8 (from Bottler,
Telgte).
Astacin Finish PUM from BASF AG, a polyurethane dispersion
comprising an anionic-functional polyurethane derived from
polyesterdiol and aliphatic isocyanates, was admixed with 10~ of
color, BASF Lepton Blue, and introduced into a spray gun pressure
vessel. The sprayer of the type HS-25 is from Krautzberger GmbH,
Eltville a.R., with a mini extension 90~C - for VD nozzles - it
was used to spray the undercuts repeatedly at low settings, about
10-20 m~, per spraying operation. The larger areas were sprayed
using a sprayer of the type Perfekt HS-25, A 10, at a higher
setting, about 30 - 50 m~, per spraying operation. These two
spraying operations were repeated until a skin of about 0.9
0.3 mm had been formed. Each spraying operation was followed by
an interval, during which all the water evaporated, until the
next spraying operation was carried out. The finished skin was
conditioned for 5 min by surface heat (conditioning oven), then
demolded and appropriately stored for further processing.
The properties of the skins produced in this way are reported in
Table 1.
Example B:
The PU skin was placed into an expansion mold with lid and pulled
flat against the wall by means of a vacuum. The lid was provided
with a wax-based demolding agent (Acmos), and thereafter an
insert of plastic or metal was attached in such a way that the
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later closed-lid foaming operation would cause the foam to form a
unit with skin and insert.
The open mold was entered, with the aid of a Puromat -
two-component machine - from Elastogran Dosiertechnik, with an
energy-absorbing, semirigid foam of the type Elastoflex~ E 3320.
Elastoflex~ E is a two-component water-blown MDI system from the
automotive and specialties division of Elastogran GmbH (foam
properties see Table 2). The closed lid caused the foam in the
mold to be compressed to a wall thickness of 2 cm, and after
3 min the foam was removed as a complete structural component
from the opened mold (foam properties see Table 2).
Table 1
Typical properties of sprayed skin:
Skin A1 Skin A2
Density kg/m3 1010 990
Tensile strength kPa 14440 11360.
Elongation % 780 710
Tear strength N/mm 37 36
Fogging - . mg 2 . 2 1. 5
Heat test 120C O.K. O.K.
Table 2
Typical properties of foam B:
Density kg/m3 110
Tensile strength kPa 320
Elongation % 45
Compression hardnesskPa
at 20% 38
at 40% 61
at 60% ~ 120
