Note: Descriptions are shown in the official language in which they were submitted.
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LeA 2~,854-us
POLYOLEFIN TANKS INTERNALLY COATED WITH MOISTURE HARDENING
COATING COMPOSITIONS AND A PROCESS FOR THEIR PREPARATION
BACK~ROUND OF THE INVENTION
Field of_the Invention
This invention relates to a process for coating the inside
of polyolefin plastic tanks, in particular fuel tanks, with
moisture-hardening coating compositions based on organic
polyisocyanates and to the resulting coated polyolefin plastic
tanks.
Description of the Prior Art
The use of polyolefin plastics for the production of fuel
tanks is known. It is also known that such fuel containers
require a surface treatment on the interior of the wall for
producing extreme imperviousness, i.e., for preventing
diffusion of the fuel to the outside. Treatment of the surface
with fluorine or fluorine compounds has the effect of a
diffusion barrier, but also has the disadvantage of not being
sufficiently environmentally friendly because the emission of
hydrogen fluoride takes place when the synthetic resin is
thermally recycled.
This disadvantage can be overcome by coating the internal
surfaces of tanks with halogen-free coating compositions.
However, this alternative is also not satisfactory because 1)
conventional coating compositions emit noxious substances since
they contain organic solvents and 2) the bond strength of
conventional organic coatings on polyolefin surfaces is not
sufficient when the surfaces are continuously subjected to the
action of fuels, even if the surfaces have been pretreated,
e.g., by corona discharge, plasma irradiation, etc.
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lt has now been surprisingly found that the moisture-
hardening, solvent-free coating compositions according to the
invention, which are described in detail below and contain
certain polyisocyanates which are liquid at room temperature,
s are eminently suitable for coating the inside of polyolefin
plastic tanks used for liquid hydrocarbons, in particular
fuels. These coatings retain the necessary bonding strength
even when permanently subiected to such hydrocarbons. They
also provide the necessary resistance to hydrocarbons and,
o thus, prevent permeation of the liquid hydrocarbons, in
particular fuels, to the outside.
This observation is also surprising in view of the
teachings of DE-OS 2,845,514 since according to the this
publication it is necessary to use mixtures of lacquer
polyisocyanates with certain monoisocyanates as binders for
solvent-free, one-component polyurethane coating compositions.
In addition, this publication does not suggest the suitability
of the mixtures described therein for coating the inside of
tanks made of polyolefin hydrocarbons.
In a similar manner the teachings of EPA 0,085,309 relate
to coating PVC substrates, and not to coating the inside of
hydrocarbon tanks made of polyolefin plastics. Accordingly,
this publication does not suggest the solution of the problem
solved by the present invention. The coating of comparatively
polar PVC surfaces does not allow any conclusions to be drawn
to the possibility of coating comparatively apolar polyolefin
surfaces which are known to be difficult to coat.
SUMMARY OF THE INVENTION
The present invention relates to a process for coating the
inside of a polyolefin plastic tank for storing liquid
hydrocarbons by
I) coating the inside of the tank with a moisture-hardening,
coating composition having a viscosity at 23C of 60 to
2,500 mPa.s and containing
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a) at least one polyisocyanate containing biuret,
isocyanurate, uretdione and/or allophanate groups
which is liquid at room temperature, is prepared from
hexamethylene diisocyanate and has an isocyanate
content of at least 10% by weight and
b) O to 5% by weight, based on the weight of the coating
composition, of an organic solvent for polyiso-
cyanates which is inert towards isocyanate groups and
II) curing the coating under the influence of moisture.
o The present invention also relates the resulting coated
polyolefin plastic tanks for storing liquid hydrocarbons, in
particular gasoline or diesel fuel.
DETAILED DESCRIPTION OF THE INVENTION
The coating compositions to be used according to the
invention have a viscosity at 23C of 60 to 2,500 mPa.s,
preferably 100 to 1000 mPa.s. Since the coating compositions
to be used according to the invention are preferably
solvent-free systems, polyisocyanate component a) also
preferably conforms to these viscosity requirements.
Component a) is selected from "lacquer polyisocyanates,"
i.e., in particular (i) allophanate, (ii) biuret, (iii)
isocyanurate or (iv) isocyanurate and uretdione group
containing polyisocyanates, which are prepared from
hexamethylene diisocyanate. Mixtures of these polyisocyanates
may also be used. The polyisocyanates have an average
isocyanate functionality of greater than 2, preferably 2.1 to 5
and more preferably 2.5 to 4; an isocyanate content of at least
10% by weight, preferably 20 to 25% by weight; and a free
hexamethylene diisocyanate content of less than 0.5% by weight.
The polyisocyanates a) are prepared in known manner by the
reaction of hexamethylene diisocyanate to introduce biuret
groups, allophanate groups, isocyanurate groups or both
isocyanurate and uretdione groups. Any excess hexamethylene
diisocyanate present after the reaction is removed in known
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manner, preferably by distillation, so that the free
hexamethylene diisocyanate content is less than 0.5% by weight.
The preparation of polyisocyanates containing biuret
groups may be carried out in known manner using biuretizing
agents such as tert.-butanol or by the reaction of
hexamethylene diisocyanate with subequivalent quantities of
hexamethylene diamine to form biuret.
The preparation of polyisocyanates containing allophanate
groups may be carried out by the reaction of hexamethylene
diisocyanate with simple alcohols such as methanol, ethanol,
propanol, butanol, etc.. The reaction proceeds through an
intermediate where the polyisocyanates contain urethane groups
which are then converted to allophanate groups.
The preparation of polyisocyanates containing isocyanurate
groups is carried out in known manner using suitable
trimerization catalysts such as quaternary ammonium bases.
The preparation of polyisocyanates containing isocyanurate
and uretdione groups is carried out in known manner by the
oligomerization of hexamethylene diisocyanate in the presence
of phosphine catalysts.
The lacquer polyisocyanates which are most preferably used
as component a) are polyisocyanate containing both isocyanurate
and uretdione groups, wherein the molar ratio of isocyanurate
groups to uretdione groups is preferably from 80:20 to 20:80.
These polyisocyanates have a viscosity at 23C of 100 to 1,000
mPa.s.
The coating compositions to be used according to the
invention may, if necessary, contain small quantities of
isocyanate-inert solvents b) to adjust the viscosity,
preferably from 0 to 5% by weight thereof, based on the weight
of the coating composition. Examples of these solvents include
n-butyl acetate, methyl ethyl ketone, methyl isobutyl ketone,
2-methoxypropyl acetate, toluene, xylene and mixtures of these
solvents. However, the coating compositions to be used
according to the invention are preferably solvent-free systems.
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The coating compositions to be used according to the
invention may optionally contain inert auxiliary agents and
additives c), such as those conventionally used in coating
composition formulations, in particular those based on
polyisocyanates. Examples of these auxiliary agents and
additives include levelling agents, defoamants, catalysts for
the isocyanate-water reaction (in particular known organic tin
compounds, e.g., dibutyl tin dilaurate) and plasticizer oils
and resins which increase the elasticity. If desired or
o necessary, the coating compositions may also contain known
fillers or pigments, optionally together with known pigment
drying agents.
~ The plastics tanks which are to be internally coated in
accordance with the present invention are tanks prepared from
polyolefin plastics, in particular polymers and copolymers of
ethylene, propylene and/or the isomeric butenes, which may
optionally contain small quantities of monomers having
functional groups, such as acrylic or methacrylic acid,
incorporated by polymerization. It is particularly preferred
according to the invention to use substrates prepared from
ethylene-butene copolymers, for example, Vestolen AX 4013,
available from Huls AG.
Before the internal coating is applied, a suitable
pretreatment of the internal walls of the tank may be carr;ed
out to optimize adhesion of the coatings according to the
invention. Suitable pretreatment processes include corona
discharge and, preferably, plasma irradiation.
The coating compositions to be used according to the
invention may be applied by conventional methods but for the
use according to the invention application by the centrifugal
method is particularly preferred. In this process, a certain
quantity of the coating composition is introduced into the tank
and set in motion by a centrifugal movement so that every point
on the internal surface becomes completely wetted. In order to
ensure that areas of the internal surface of the tank which are
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more difficult to reach will also be wetted, sources of
infra-red irradiation may be applied to the outside of these
areas so that the viscosity of the coating agent is lowered and
the flowability improved. A similar effect may also be
produced by means of ultrasound.
The coating compositions to be used according to the
invention are stable in storage when atmospheric moisture is
excluded and are hardened by moisture to form tough elastic
films or coatings. The moisture required for the reaction may
be available as atmospheric moisture, but the necessary
moisture may also be introduced artificially as steam, e.g., in
continuous processing plants.
The coating compositions to be used according to the
invention are generally introduced in quantities resulting in
wet film thicknesses of 40 to 100 ~m.
The coatings are preferably be cured at room temperature,
but curing may a~so be accelerated by applying elevated temperatures
such as, for example, lO0 to 180C. In ~his case, it is advisable to supply steam to
increase the reaction velocity (drying and curing).
The invention is further illustrated but is not intended
to be limited by the following examples in which all parts and
percentages are by weight unless otherwise specified.
EXAMPLES
The following polyisocyanates are used in the examples:
PolYisocvanate A
A biuret polyisocyanate primarily containing
tris-(isocyanatohexyl)-biuret and prepared by the biuretization
of hexamethylene diisocyanate according to US-PS 3 903 127
followed by the removal of excess hexamethylenediisocyanate by
distillation.
NC0 content: 23.5% by weight.
Free hexamethylene diisocyanate content: <0.5% by weight
Average isocyanate functionality: >3
Viscosity: 2400 mPa.s/23C.
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sy~ate B
A polyisocyanate mixture primarily containing tris-(iso-
cyanatohexyl)-isocyanurate and bis-(isocyanatohexyl)-uretdione
which was prepared by the phosphine-catalyzed oligomerization
of hexamethylene diisocyanate followed by the removal of excess
hexamethylene diisocyanate by distillation.
NC0 content: 22.0% by weight
Free hexamethylene diisocyanate content: < 0.5% by weight
Average NC0 functionality: 2.7
Viscosity: 200 mPa.s/23C.
Example 1
37.7 parts by weight of polyisocyanate A, 37.7 parts by
weight of polyisocyanate B and 3.5 parts by weight of a
commercial pigment drying agent, i.e., tolyl sulphonyl
monoisocyanate (Additive TI, available from Bayer AG,
Leverkusen) were homogeneously stirred, using a stirrer with a
mixing disc.
0.4 parts by weight of a commercial silica derivative
(Aerosil 972, Degussa AG, Frankfurt) and 19.8 parts by weight
of a commercial talc were then stirred in.
To wet the inorganic components, the speed of rotation of
the mixing disc was increased to 10-15 m/sec and rotation was
continued at this speed for about 10 to 20 minutes. The
reaction mixture was then cooled to room temperature and 0.5
parts by weight of a commercial defoamant (Byk 351 of Byk in
Wesel) were added with slow stirring together with 0.4 parts by
weight of dibutyl tin dilaurate as catalyst. A storage stable,
moisture hardening, one-component coating composition was
obtained.
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For test purposes, test plates of Vestolen AX 4013 plastic
which had been pretreated by plasma irradiation were coated
with the coating composition to provide wet film thicknesses of
40 to 100 ~m. At room temperature and 50% relative humidity,
the films were dust-dry after 2 h and dry to the touch after
5 h. Their adherence to the pretreated substrate was very
good. No change could be detected after several days' storage
in super grade gasoline.
Example 2
Example 1 was repeated with the exception that only
polyisocyanate B was used as component a). ~ue to the lower
viscosity of polyisocyanate B, the viscosity of the whole
formulation was reduced. The time required to reach
dust-dryness of the applied films was increased to 3-4 hours
because of the lower reactivity of polyisocyanate B and the
time for complete drying ~i.e., dry to the touch) was increased
to about 6 hours.
Adherence to the polyolefin plastic was very good.
Permeation of fuel through the coated polyolefin plastic was
only 0.15 to 0.25% of the amount which permeated through a
corresponding uncoated plastic surface.
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.
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