Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02610372 2007-11-26
PCT/EP2006/004909
based on DE 10 2005 024 207.3
Ashland-Sudchemie-Kernfest GmbH
Our Ref.: K2021
Method for drying water-based washes
The present invention relates to a method for drying water-based coating
compounds (water-
based washes) without the use of drying aggregates. In particular, the present
invention relates
to a method for drying water-based coating compounds, comprising applying a
flammable
liquid to a previously applied, optionally matted layer of a water-based
coating compound and
completely burning off the flammable liquid.
Molten materials are transformed into geometrically shaped articles with
certain workpiece
properties by means of casting. Most products in the iron and steel industries
as well as the
materials used in the non-ferrous metal industry undergo casting processes for
primary
shaping. One prerequisite for the production of castings is, inter alia, the
manufacture of
sometimes very complicated casting molds for accepting the molten material.
Casting molds
can be subdivided into break molds, which usually consist of a mineral, fire-
resistant, granular
basic material with a binder and often different further additives, e.g. for
achieving high-quality
casting surfaces, and which are destroyed after every casting, and permanent
molds, which can
be used for producing a large number of castings. Most of the time, the fire-
resistant, granular
basic substance used in break molds is washed, classified quartz sand, in
specific cases,
chromite, zircon and olivine sands are used as well. In addition, forming
materials based on
fireclays, magnesite, sillimanite, corundum, and the like, are used. The
binders for the molding
sands can be of inorganic or organic nature. Break molds are usually produced
from bentonite-
bonded forming materials for smaller molds or synthetic resin-bonded forming
materials for
larger molds, which have to be mechanically compacted in order to obtain
sufficient stability.
Depending on the field of application, cast iron in particular as well as
unalloyed and alloyed
steels, but also copper, aluminum, graphite, sintered metals and ceramic
materials have been
proven to be suitable forming materials for permanent molds. Permanent-mold
casting
processes include gravity die casting, pressure casting, centrifugal casting
and continuous
casting.
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A process for the production of casting molds and cores from synthetic resin-
bonded molding
sands comprises for example the production of a basic mold or a basic core
from the molding
sand and the application of a coating compound comprising fire-resistant
inorganic
components (mold coating), which is also referred to as "wash", at least on
those surfaces of
the basic mold and/or the basic core which come into contact with the cast
metal. On the one
hand, the coating compounds serve the purpose of influencing the surface of
the molding,
improving the appearance of the casting, influencing the casting in terms of
metallurgy and/or
preventing casting defects. Furthermore, the function of these coatings or
washes is to
chemically isolate the mold from the molten metal during casting, thus
preventing any
adherence and allowing or facilitating the separation of mold and casting
later on. In addition,
the wash guarantees a thermal separation of the mold and the casting. If this
function is not
fulfilled, a metal mold for example is subjected to such high thermal loads in
the course of
subsequent casting processes that it is destroyed prematurely. The heat
transfer can be made
use of for the purpose of controlling the cooling of the casting. In the
production of metal
parts, for example made from cast iron, heavy casting and centrifugal casting
processes are
used amongst others. During the casting of large parts, the mold and core are
subjected to
enormous metallostatic pressure as well as to a long-term thermal load. This
is why the wash
in that particular process assumes a very important protective function in
order to prevent the
metal from entering the molding sand (penetration), the cores from ripping
(leaf ribs), or a
reaction with the molding sand (metallization). During centrifugal casting,
the casting metal is
filled into a pipe- or ring-shaped gravity die rotating around its own axis,
where it is formed for
example into cans, rings and pipes by the centrifugal forces. It is absolutely
necessary that the
casting be completely solidified before it is removed from the mold so that
quite a long period
of contact between the mold and the casting is required. In this case, the
mold is coated with
an isolating wash in the form of a single-layer or multi-layer coating.
Commonly used washes comprise e.g. clays, quartz, diatomite, cristobalite,
tridymite,
aluminum silicate, zirconium silicate, mica, fireclay, as well as coke and
graphite as basic
materials. These basic materials constitute the functional portion of the
washes which cover
the surface of the mold and close the pores to prevent penetration of the cast
metal.
The carrier liquid component (solvent component) of coating compounds or
washes usually
comprises water or one or more volatile, preferably aliphatic, alcohols or
mixtures of alcohol(s)
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3
and water, and optionally one or more organic volatile solvents different than
the alcohols as an
additional component. Washes whose carrier liquid consists mainly of water are
commonly
referred to as "water-based washes". Washes whose carrier liquid consists
mainly of alcohol or
alcohol mixtures are referred to as "alcohol-based washes". Alcohol-based
washes often
comprise isopropanol as carrier liquid for the coating components.
The use of washes in foundry technology always requires drying the applied
wash layer.
Applied alcohol-based washes are usually dried by flash-off or by burning off
the alcohol used
as carrier liquid. Applied water-based washes are usually dried by the
application of heat
and/or an air stream. In the case of smaller cores or molds, the acquisition
of a drying
aggregate for drying water-based washes is economically justifiable. For very
large molds
whose production requires a long time, there is no choice but to air-dry the
water-based
washes. In the case of the most common medium-sized molds and cores used in
hand mold
casting, the only option is the use of alcohol-based washes which then have to
dried by flash-
off or are ignited in order to dry the wash. However, this frequently leads to
conflicts with
VOC (volatile organic compounds) emission limits often set by the legislator.
In Switzerland,
for example, these emissions are subject to a high tax. However, the use of
drying plants for
medium size molds and cores is not economically justifiable and considerably
slows down the
production flowline. For water-based washes, the following drying processes
are known:
Drying with warm air, drying with dehumidified air, drying by means of
infrared radiation and
drying by means of a combination of infrared heating and vacuum for smaller
cores.
At this time, only alcohol-based washes are used in the case of the most
commonly used
medium size molds and cores which are then dried by flash-off or ignited in
order to remove
the alcoholic carrier liquid. Compared to water-based washes, the use of an
alcohol-based
wash poses a considerable potential hazard for workers in particular during
application and
necessitates special workplace designs with respect to legal guidelines,
keeping within MAK
values when employees are permanently exposed and special fire protection
equipments.
Alcohol-based washes often use isopropanol as carrier liquid for the coating
components. Due
to its low flash point of +12 C, isopropanol is classified as a highly
flammable liquid, and
many countries have strict regulations in place regarding its use. Isopropanol
has a tendency to
burn vigorously and with a high flame so that, if an alcohol-based wash is
applied to a mold or
a core and ignited, the flames quickly spread from the point of ignition over
the surface of the
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mold or core. Consequently, burning off the alcohol in an alcohol-based wash
is very difficult
to control. Due to the higher costs of organic solvents such as alcohols,
alcohol-based washes
lead to higher production costs than water-based washes. Thus, the necessary
burning off is a
very high-cost factor in all casting processes known today. The high energy
requirements
contribute to the costs, furthermore, the necessary and cost-intensive
investments have to be
kept in mind, and the burning off has to be carried out very carefully and in
turn leads to an
increase in the costs for work hours. Since burning off of alcohol-based
washes is very hard to
control, small mistakes caused, e. g., by burning off the alcohol-based wash
too rapidly or
cooling the burned-off wash too quickly, can augment the small initial cracks
and micro cracks
caused during burning to such an extent that in the end the mold is useless.
This increases the
number of rejects which in turn increases production costs. Since the main
component of
carrier liquids of alcohol-based washes is alcohol, large amounts of alcohol
have to be in stock
which usually requires special storage areas. Many countries have strict
regulations regarding
the storage and use of such liquids. Furthermore, many countries have measures
in place to
reduce VOC emissions. In Switzerland, for example, VOC emissions are subject
to a high tax.
It is an object of the present invention to replace alcohol-based washes,
which are used in
particular for medium size molds and cores, with water-based washes and thus
to reduce the
content of VOC to be removed. It is another object of the present invention to
provide a
method for drying water-based washes which does not require drying plants.
One aspect of the present invention relates to a method for drying water-based
coating
compounds, comprising
(a) providing a water-based coating compound,
(b) applying the water-based coating compound to at least one part of the
surface of a
casting mold,
(c) optionally partially or completely rendering the applied water-based
coating compound
mat, with a part of the carrier liquid (preferably water) of the coating
compound passing
into the substrate depending on the time elapsed,
(d) applying a flammable liquid to the optionally matted layer of the water-
based coating
compound, and
(e) completely burning off the flammable liquid.
CA 02610372 2007-11-26
In an embodiment of the present invention, the water-based coating compound is
applied to at
least one part of the surface of an uncoated casting mold. In another
embodiment, the water-
based coating compound is applied to at least one part of the surface of the
undercoat which
has previously been applied to at least one part of the surface of an uncoated
casting mold.
Also, molds can be used where the undercoat has only been applied to certain
parts of the
surface. Another embodiment of the present invention relates to the use of
such casting molds
wherein the water-based coating compound is both applied to at least one part
of the uncoated
surface of the casting mold and to at least one part of the surface of the
undercoat.
The main component of the carrier liquid of the water-based coating compound
according to
the present invention is water. As other main components, the water-based
coating compound
comprises particle-shaped fire-resistant materials as are commonly used in
foundry technology
for the production of washes, and commonly used binders. In a preferred
embodiment, the
water-based coating compound comprises 20 to 79.9 wt.-% water, 79.9 to 20 wt.-
% particle-
shaped fire-resistant materials, 0.1 to 5 wt.-% binder and 0 to 18 wt.-% of
further additives such
as for example additional particle-shaped materials, wetting agents, defoaming
agents, dyes and
pigments, biocides and dispersing agents, based on all the components of the
water-based
coating compound.
In the present invention, flammable liquids with a flash point of -60 C to 100
C, preferably
-20 C to 60 C are typically used as flammable liquids. The flammable liquids
are selected
from commonly used flammable solvents, preferably from flammable, preferably
aliphatic,
alcohols, aliphatic, cycloaliphatic and aromatic hydrocarbons, esters of
aliphatic carboxylic
acids, ethers, ketones and mixtures thereof. The flammable liquid is
preferably applied to the
casting mold by spraying. In a preferred embodiment, the flammable liquid is
ignited
immediately after application and then burned off completely. In another
embodiment, more
flammable liquid is additionally sprayed on during the burning off.
In another embodiment of the present invention, the water-based coating
compound can be
applied in several layers. In that case, the individually applied layers can
be dried individually
by applying and completely burning off a flammable liquid according to the
present invention
before another layer of a water-based coating compound is applied. In another
embodiment of
the present invention, several different water-based coating compounds can be
applied to an
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uncoated surface of a casting mold or a surface of a casting mold provided
with an undercoat.
The individually applied layers of the water-based coating compounds can be
dried separately
with the same or different flammable liquids according to the process of the
present invention
before another layer is applied. The layers of water-based washes can also be
applied without
drying every individual layer, wherein the layer laminate can afterwards be
dried by applying
and then burning off a flammable liquid.
The dried coating compound layer or layers obtained after the complete burn-
off of the
flammable liquid preferably has/have a residual moisture of less than 8%,
preferably less than
6%, more preferred less than 5%, even more preferred less than 2%, and most
preferred less than
1%, determined according to the CM by Riedel de Hden, Seelze.
Water-based coating compounds
The water-based coating compounds suitable for use in the present invention
comprise as main
components a solid component and a carrier liquid component. The main
component of the
solid component are particle-shaped, fire-resistant materials, the main
component of the carrier
liquid component is water.
As main component, the solid component of the water-based coating compound
comprises
particle-shaped, fire-resistant materials which are selected from fire-
resistant materials
commonly used in water-based washes. Examples of particle-shaped, fire-
resistant materials
which can be used in the water-based coating compounds in the present
invention include
diatomite, kaolins, calcinated kaolins, kaolinite, metakaolinite,
pyrophyllite, mica, zirconia,
zirconium silicate (zircon flour, zircon sand), alumina, andalusite,
fireclays, iron oxides,
kyanite, bauxite, olivine, quartzes, talc, graphites and carbon blacks, as
well as clays and
mineral substances comprising these materials. In a preferred embodiment of
the coating
compound, zirconium silicate (zircon flour), bauxite, metakaolinite,
pyrophyllite, graphite,
hectorite, bentonite and attapulgite are used. The water-based coating
compound usually
comprises 20 to 79.9 wt.%, preferably 30 to 75 wt.-%, more preferred 40 to 70
wt.-%, even
more preferred 50 to 70 wt.-%, and most preferred 60 to 70 wt.%, of particle-
shaped fire-
resistant materials.
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The coating compounds which are used in the present invention can optionally
furthermore
comprise additional particle-shaped materials such as commonly used two-layer
silicates and
three-layer silicates, such as e.g. attapulgit, serpentines, smectites, such
as saponite,
montmorillonite, beidellite and nontronite, vermiculite, illite and mica, in
smaller amounts, e.g.
0.5 to 4.0 wt.-%, preferably 1.0 to 2.0 wt.-%.
Coating compounds that can be used can optionally furthermore comprise one or
more
additional components, such as e.g. binders, wetting agents, defoaming agents,
pigments, dyes,
biocides and dispersing agents.
The purpose of a binder is mainly to enable bonding of the ingredients of the
wash composition
after the wash applied to the casting mold has been dried. Preferably, the
binder cures
irreversibly and thus provides an abrasion-resistant coating on the casting
mold. Abrasion
resistance is of great importance to the finished coating since a lack of
abrasion resistance can
result in damage to the coating. In particular, the binder should not re-
soften when exposed to
atmospheric moisture. In preferred embodiments, the binder is cured in a
manner known in the
art. For example, in acrylate systems curing can be carried out using radical
formers which form
radicals when e.g. exposed to UV light. In the present invention, all binders
can be used which
are conventionally used in aqueous systems. For example, starch, dextrine,
lignin sulfates,
peptides, polyvinyl alcohol, polyvinyl acetate copolymers, polyacrylic acid,
polystyrene-,
polyvinyl acetate-polyacrylate dispersions and mixtures thereof can be used as
binders. In a
preferred embodiment of the invention, the binder consists of a dispersion of
an alkyd resin
which is soluble in water. Examples of alkyd resins include unmodified water-
dispersible
alkyd resins based on a natural oil or fatty acids thereof with polyalcohols,
as for example
described in US 3,442,835, or isocyanate-modified alkyd resins, as for example
described in
US 3,639,315 and which are preferred, or epoxide-urethane-modified alkyd
resins according to
DE 43 08 188. For example, products from the Necowel series from the company
ASK
GmbH, 40721 Hilden, Germany, can be used. Other preferred binders are
polyvinyl alcohols
and polyvinyl acetate copolymers. The binders are preferably used in an amount
of 0.1 to 5
wt.-%, more preferred 0.5 to 2 wt.-%, based on all the components of the water-
based coating
compound.
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Anionic and non-ionic surfactants having a medium and high polarity (HSB value
of 7 and
higher) known to the person skilled in the art are preferably used as wetting
agents. A wetting
agent that can be used in the present invention is for example
dioctylsulfosuccinate disodium.
The wetting agents are preferably used in an amount of 0.01 to 1 wt.-%, more
preferred 0.05 to
0.3 wt.-%, based on all the components of the water-based coating compound.
Defoamers or defoaming agents are used to prevent the formation of foam during
the
production of the coating material and its application. The formation of foam
during the
application of the wash composition can lead to an irregular layer thickness
and to holes in the
coating. Silicone or mineral oils can for example be used as defoamers. In the
present
invention, defoamers are used in an amount of 0.01 to 1 wt.-%, more preferred
0.05 to 0.3 wt.-
Optionally, the water-based coating compound can comprise commonly used
pigments and
dyes. They are optionally added to create a contrast between e.g. different
layers, or a more
pronounced separation effect between the wash and the casting. Examples of
pigments are red
and yellow iron oxide and graphite. Examples of dyes include commercially
available dyes
such as the Luconyl series from BASF. The dyes and pigments are usually used
in an amount
of 0.01 to 10 wt.-%, preferably 0.1 to 5 wt.-%.
Biocides are usually added to coating compounds whose carrier liquid mainly
consists of
water, so-called water-based washes, in order to prevent a bacterial attack
and thus to prevent a
negative influence on the rheology and bonding strength of the binders.
Examples of suitable
biocides include formaldehyde, 2-methyl-4-isothiazolin-3 -one (MIT), 5-chloro-
2-methyl-4-
isothiazolin-3-one (CIT) and 1,2-benzisothiazolin-3-one (BIT). Preferably,
MIT, BIT or a
mixture thereof is used. The biocides are usually used in an amount of 0.01 to
0,5 wt.-% or 10
to 1,000 ppm, preferably 50 to 500 ppm.
Optionally, commonly used dispersing agents can be added to the water-based
coating
compound. The dispersing agents are preferably used in an amount of 0.01 to
1.5 wt.-%, more
preferred 0.02 to 0.5 wt.-%. Examples of suitable dispersing agents include
hexasodium
tetrapolyphosphate or sodium polyacrylate.
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The carrier liquid component of the water-based coating compound suitable for
use in the
present invention comprises water as the main component. The water is
preferably present in
an amount of at least 80 wt.%, more preferred at least 90 wt.-%, even more
preferred at least
95 wt.-% and most preferred at least 97 wt.-%, based on all the components of
the carrier liquid
component. In a preferred embodiment, the carrier liquid only consists of
water. In a preferred
embodiment, the coating compound comprises 20 to 79.9 wt.-%, more preferred 25
to
70 wt.-%, even more preferred 30 to 50 wt.-% and most preferred 30 to 40 wt.-%
water, based
on all the components of the water-based coating compound. As another
component, the
carrier liquid component of the coating compound can furthermore optionally
comprise one or
more organic volatile solvents such as volatile, preferably aliphatic,
alcohols, aliphatic,
cycloaliphatic or aromatic hydrocarbons, esters, ethers and ketones, which are
commonly used
as solvents, for example in order to facilitate the dissolution of dispersing
agents, binders,
wetting agents, defoaming agents, dyes and biocides. Ethanol, isopropanol,
methoxypropanol,
ethylene glycol, as well as aliphatic, isoaliphatic, cycloaliphatic and
aromatic hydrocarbons are
preferably used for this purpose. The carrier liquid component comprises less
than 20 wt.-%,
preferably less than 10 wt.-%, more preferred less than 5 wt.-% and most
preferred less than
3 wt.-% of organic volatile solvents. However, the water-based coating
compound preferably
comprises less than 5 wt.-%, more preferred less than 3 wt.-%, even more
preferred less than
2 wt.-% and most preferred less than 1 wt.% or no organic solvents, based on
all the
components of the water-based coating compound.
In a preferred embodiment, a water-based coating compound comprises the
following
additional components in addition to the particle-shaped, fire-resistant
materials:
0.02 to 0.5 wt.-% dispersing agent,
0.5 to 3.0 wt.-% binder,
0.01 to 0.5 wt.-% wetting agent,
0.01 to 0.5 wt.-% defoaming agent,
0 to 5.0 wt.-% pigments,
0 to 5.0 wt.-% dyes,
0.1 to 0.5 wt.-% biocides,
based on all the components of the water-based coating compound.
CA 02610372 2007-11-26
In another preferred embodiment of the present invention, water-based coating
compounds
(wash compositions) can be used as are described in the German patent
application 10 2004
060 649.8.
An example of a preferred heavily loaded water-based wash comprises the
following
components:
Zirconium silicate (particle size < 90 m) 40.0 to 60.0 wt.-%
Graphite (particle size < 90 m) 0.0 to 20.0 wt.-%
Attapulgite 0.0 to 2.0 wt.-% (preferably 0.2 to 2.0 wt.-%)
Hectorite 0.0 to 1.0 wt.-% (preferably 0.1 to 1.0 wt.-%)
Bentonite 0.0 to 2.0 wt.-% (preferably 0.2 to 2.0 wt.-%)
Kaolinite 0.0 to 5.0 wt.-% (preferably 0.5 to 5 wt.-%)
Biocides 0.1 to 0.5 wt.-%
Dispersing agents 0.02 to 0.1 wt.-%
Binders 0.5 to 1.5 wt. %
Water remainder to total 100 wt.-%
The water-based washes are produced by means of common processes. For example,
a water-
based wash is produced by providing a majority of the total amount of the
carrier liquid
component, preferably the entire amount of water, and decomposing therein clay
minerals such
as bentonite and hectorite, using a stirrer with a high shearing speed (e.g.
400 to 2,000 rpm).
Then, the fire-resistant components, individually or as mixture, and the
pigments and dyes are
stirred in until a homogenous mixture is obtained. The order in which the
components are
added is of no or only secondary importance which then can easily be
determined by a person
skilled in the art. Wetting agents, antifoaming agents, biocides and binders
are stirred in at the
end. The wash compositions are prepared at a temperature of preferably 5 to 50
C, more
preferred 10 to 30 C, with a stirrer speed of preferably 400 to 2,000 rpm,
more preferred 1,000 to
1,500 rpm, and with a stirrer toothed disk of preferably d/D=0.3 - 0.7, more
preferred d/D=0.4 -
0.6. Ready-to-use coating compounds, e.g. ready-to-use water-based washes, can
be used in the
present invention as well. For preparing a ready-to-use wash from a water-
based wash
concentrate, the carrier liquid component (water) has to be added in an amount
suitable for
adjusting the necessary viscosity and density properties of the wash.
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Depending on the desired use of the water-based wash and the desired layer
thickness of the
water-based wash to be applied, additional characteristic parameters of the
wash composition
can be adjusted. For example, in a preferred embodiment, water-based washes
used to coat
molds and cores in foundry technology, have a viscosity of 12 to 25 s, more
preferred 14 to 16
s (determined according to DIN 53211; flow cup 4mm, Ford Cup). Preferred
density values of
a ready-to-use water-based wash are in the range of 20 to 70 Be, more
preferred 30 to 50 Be
(determined according to the Baume flotation weight loss method; DIN 12791).
Flammable liquids
The flammable liquid which can be used for drying the water-based coating
compound has a
flash point of -60 C to 100 C, preferably -20 C to 60 C, more preferred 10 C
to 55 C and
most preferred 10 C to 40 C. The flash point is the lowest temperature,
corrected to a
barometer reading of 101.3 kPa (760 Torr), at which the vapors of the sample
ignite under the
pre-set test conditions using an ignition flame. The flash point of flammable
liquids is
determined in the "closed cup" according to Pensky-Martens, DIN EN 22 719
(1993), ISO
2719 (1988). In the case of mineral oil products with a flash point of +5 to
65 C, the Abel-
Pensky apparatus, DIN 51 755 (1974), can be used as well. The flash points of
the most
common solvents can be inferred from the usual relevant literature (see e.g.
Ullmann's
Encyclopedia of Industrial Chemistry, 4th edition (1978), Vol. 16, pages 305
to 307).
Examples of flammable liquids suitable for use in the present invention
include flammable
alcohols, hydrocarbons, esters, ethers and ketones, which are also commonly
used as solvents.
In an embodiment of the present invention, flammable aliphatic alcohols having
preferably 1 to
carbon atoms as well as Cl-C4 alkyl derivatives thereof can be used as
flammable liquid.
Preferred examples of suitable alcohols include methanol, ethanol, n-propanol,
isopropanol, n-
butanol, isobutanol, sec-butanol, tert-butanol, amyl alcohols, pentanols and
hexanols as well as
mixtures thereof. Methanol, ethanol, n-propanol, isopropanol and mixtures of
these alcohols
are especially preferred. Mixtures of these alcohols with other commonly used
flammable
solvents, selected from hydrocarbons, ethers, esters and ketones, as described
below, can be
used as well. The alcohols are preferably used in an amount of 0 to 100 wt.-%,
preferably 20
to 100 wt.-%, more preferred 60 to 90 wt.%, based on all the components of the
flammable
liquid.
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In another embodiment of the present inventions, flammable solvents containing
hydrocarbons
can be used as flammable liquid. Preferably, flammable aliphatic,
cycloaliphatic and aromatic
hydrocarbons having 5 to 12 carbon atoms are used. Examples of suitable
hydrocarbons are
various special boiling-point gasolines having a flash point of -60 C to -5 C,
pentane, hexane
and various kinds of gasoline. Suitable gasolines include for example
petroleum ether [DIN
51 630 (11/1986)], special-boiling-point spirits [DIN 51 631 (01/1988)] of
type 1, type 2 or
type 3, and white spirits [DIN 51 632 (01/1988)] of the type 1, type 2, type
3, type 4, or type 5
boiling range. White spirits having a boiling point of 130 C to 220 C and a
flash point of
>21 C determined according to Abel-Pensky are especially preferred. Examples
of
cycloaliphatic hydrocarbons are cyclohexane and methylcyclohexane. The
hydrocarbons are
preferably used in the form of hydrocarbon mixtures. The hydrocarbons can also
be used in
admixture with alcohols, esters, ethers and ketones. The content of the
hydrocarbons in the
flammable liquid is 0 to 100%, preferably 10 to 40 wt.-%.
In another embodiment of the present invention, flammable esters can be used
as flammable
liquid. Preferably, flammable C,-C4 esters of aliphatic CZ-Clo carboxylic
acids are used.
Examples of suitable esters include methyl, ethyl, propyl, isopropyl and butyl
esters of acetic,
propionic, butyric and isobutyric acid, respectively, and mixtures thereof.
The esters of
aliphatic carboxylic acids are preferably used in an amount of 10 to 20 wt.-%,
based on all the
components of the flammable liquid. Mixtures of these esters with alcohols,
hydrocarbons,
ethers and ketones as described herein can also be used.
In another embodiment of the present invention, ether and glycol ether can be
used as
flammable liquid. Suitable ethers include for example diethyl, diisopropyl and
dibutyl ether,
methyl-tert-butylether, tetrahydrofuran and dioxan. Suitable glycol ethers are
mono- and
dialkyl ethers of a polyvalent alcohol, such as the methyl, ethyl, propyl,
isopropyl and butyl
ethers of glycol and diglycol, respectively, preferably the monoalkyl ethers
of a polyvalent
alcohol, such as ethylene glycol monoethylether, diethylene glycol
monoethylether, triethylene
glycol monoethylether, ethylene glycol monomethylether and ethylene glycol
monobutylether.
The ethers can be used as ether mixtures and mixtures of ethers and the above-
mentioned
alcohols, hydrocarbons and esters.
CA 02610372 2007-11-26
13
In another embodiment of the present invention, ketones which are commonly
used as solvents
can be used as flammable liquid. Preferred ketones are acetone, methyl ethyl
ketone, methyl
propyl ketone, methyl isopropyl ketone, methyl butyl ketone, methyl isobutyl
ketone, diethyl
ketone, ethyl butyl ketone, diisopropyl ketone, diisobutyl ketone,
cyclohexanone, methyl
cyclohexanone and acetylacetone. The ketones can be used as ketone mixtures or
in admixture
with the above-mentioned alcohols, hydrocarbons, esters and ethers. The
ketones are used in
the flammable liquid in an amount of 0 to 100 wt.-%, preferably 10 to 40 wt.-
%.
In a preferred embodiment of the present invention, the flammable liquid
consists of a mixture
of at least one aliphatic C2-C3 alcohol and at least one aliphatic or
cycloaliphatic CS-C]z
hydrocarbon or hydrocarbon mixture. For this purpose, the alcohol is selected
from ethanol,
propanol and isopropanol. The C5-C12 hydrocarbon is preferably used in the
form of a
hydrocarbon mixture which is preferably selected from petroleum ether, light
and medium
gasolines and white spirits. A preferred mixture of a flammable liquid
preferably is composed
of 20 to 90 wt.-% of at least one alcohol, preferably a C2-C3 alcohol, 10 to
40 wt.-% of at least
one hydrocarbon, preferably an aliphatic or cycloaliphatic C5-C12 hydrocarbon,
or a mixture of
mainly C5-C12 hydrocarbons, and/or 10 to 40 wt.-% of at least one carboxylic
acid ester,
preferably a C1-C4 alkyl ester of aliphatic C2-C10 carboxylic acids, based on
all the components
of the flammable liquid.
Method for applyin and nd drying the water-based wash
The water-based washes can be used to coat casting molds. The term "casting
mold" as used
herein encompasses all kinds of bodies necessary for the production of a
casting, such as for
example cores, molds and gravity dies. The use of the water-based washes also
encompasses a
partial coating of casting molds. Preferably, those surfaces of a casting mold
are coated which
come into contact with the cast metal. The water-based washes are suitable for
all conceivable
applications wherein a coating of casting molds with washes is desired.
Examples of casting
molds, i.e. of foundry cores and molds, include sand cores bound with PUR
ColdBox, water
glass C02, MF resol, resol C02, furan resin, phenolic resin or water
glass/ester. Other
examples of preferred casting molds which can be coated with water-based
washes are for
example described in "Formstoffe und Formverfahren", Eckart Flemming and
Werner Tilch,
Wiley VCH, 1993, ISBN 3-527-30920-9.
CA 02610372 2007-11-26
14
A method for coating a casting mold with a water-based wash for example
comprises the steps:
(a) providing a basic casting mold (uncoated casting mold);
(b) providing a water-based wash;
(c) optionally applying and drying at least one layer of an undercoat
composition on at
least one part of the surface of the uncoated casting mold;
(d) applying at least one layer of a water-based wash to at least one part of
the surface of
the uncoated casting mold and/or the casting mold comprising an undercoat;
(e) drying the applied water-based wash; and
(f) optionally curing the dried wash.
In the case of casting molds for centrifugal casting processes, the wash is
usually applied in
several steps, i.e. layers, whereby the applied wash layer is usually
partially or completely
dried before the next layer is applied. Usually, 2 to 5 layers are applied. In
the casting of large
parts (heavy casting > 10 tons (t)), the wash composition is usually only
applied once, whereby
an undercoat layer can first be applied to critical areas. In light (casting
up to 2 tons) and
medium casting (up to 2 to 10 tons), usually no undercoat layer is applied.
The application of an undercoat to the uncoated casting mold, if desired, can
be carried out by
means of all the conventional application processes known in the art. The
undercoats can for
example comprise clays, talc, quartz, mica, zirconium silicate, alumina,
aluminum silicate,
bauxite, andalusites, fireclay and graphite as basic materials. These basic
materials constitute
the functional portion of the undercoat. They cover the casting mold surface,
close the sand
pores to penetration of the cast metal and, inter alia, also serve a thermal
insulation vis-a-vis
the casting mold. Commonly used application methods are immersing, flow
coating, spraying
and painting. The basic coating has a dry layer thickness of at least 0.1 mm,
preferably at least
0.2 mm, more preferred at least 0.45 mm and most preferred a range of 0.3 mm
to 1.5 mm.
Both water-based and alcohol-based washes can be used as undercoats with water-
based
washes being preferred.
The application of a water-based wash to prepare an overcoat can be carried
out according to any
conventional application method known in the art. Examples of preferred
application methods
CA 02610372 2007-11-26
include immersing, flow coating, spraying and painting. Conventional
application methods are
for example described in "Formstoffe und Formverfahren", Eckart Flemming and
Werner
Tilch, Wiley VCH, 1993, ISBN 3-527-30920-9.
If immersing is used as an application method, a casting mold optionally
provided with an
undercoat is immersed in a vessel filled with a ready-to-use water-based wash
for about 2
seconds to 2 minutes. After a sufficient draining time, the coated casting
mold is subjected to
drying. If spraying is used as an application method, commercially available
pressure pot
spraying equipment is used. Here, the diluted water-based wash is filled into
a pressure pot.
Via an adjustable overpressure, the wash can be pushed into a spray gun from
where it is
sprayed with the help of separately controllable atomizing air. The water-
based wash can be
applied in one or several steps. If several layers are applied, each
individual layer can be
partially or completely dried after application.
The wet layer thickness of the water-based wash for the preparation of an
overcoat is applied
depending on the desired dry layer thickness of the overcoat. The dry layer
thickness of the
overcoat is at least 0.1 mm, preferably at least 0.2 mm, more preferred at
least 0.3 mm, even
more preferred at least 0.45 mm, especially preferred at least 0.55 mm and is
most preferred in
a range from 0.3 to 1.5 mm. The dry layer thickness is the layer thickness of
the dried water-
based wash obtained after drying the wash and optionally subsequent curing.
The dry layer
thickness of the undercoat and the overcoat is for example determined by
measuring with a wet
layer thickness gage-comb type. For example, the layer thickness can be
determined with the
comb by scratching off the wash at the end marks of the comb such a long time
until the
subsurface is visible. The layer thickness can then be read off the teeth of
the comb.
Alternatively, it is also possible to measure the wet layer thickness in a
matted state, wherein
the dry layer thickness accounts for 70 to 80% of the thickness of the matted
layer.
After application, the applied water-based wash can optionally be rendered
partially or
completely mat, whereby part of the carrier liquid of the coating compound
passes (infiltrates)
into the substrate depending on the time elapsed. A' matted ' layer is a layer
that is no longer
flowable, in which the content of carrier liquid has been reduced to such an
extent that the
surface is no longer glossy.
CA 02610372 2007-11-26
16
Drying of the applied water-based wash, which in one embodiment of the present
invention has
been rendered partially or completely mat, is carried out by the application
of a flammable
liquid as described above and completely burning off the applied flammable
liquid. The
flammable liquid is preferably applied in an amount of 10 to 5,000 g/m2, more
preferred 20 to
2,000 g/m2, even more preferred 50 to 1,000 g/m2 and most preferred 80 to 500
g/m2. The
flammable liquid can be applied by flow coating, immersing or spraying.
Spraying the
flammable liquid is especially preferred. For example, the flammable liquid
can be applied by
means of a commercially available hand-held sprayer (e.g. Gloria garden
sprayer for spraying
pesticides). Here, the spray angle is preferably set to about 30 with the
adjustable nozzle and
the material pressure is set to about 2 bar (200 kPa). Other application
devices include
commercially available spray guns (e.g. Sicmo spray gun), whereby the
application is carried
out for example with a 0.7 mm nozzle, a material pressure of 2 bar (200 kPa)
and a spray
pressure of 0.5 bar (50 kPa). A metallic pressure container should be used for
applying and
burning off the flammable liquid in order to prevent the organic liquids from
undergoing static
electrification and prematurely igniting due to discharging and the
accompanying formation of
sparks.
After the application of the flammable liquid, it should be ignited as soon as
possible,
preferably immediately, in order to prevent evaporation as well as excessive
mixing with the
water of the water-based wash. Both reduce the burning action. The flammable
liquid is
preferably ignited with a burning flame (e.g. a lighter or a gas flame).
During burning the
flammable liquid serves as a heat source for removing the carrier liquid
(water) of the water-
based wash. A complete burn-off is necessary in order to make use of all of
the heat. In case
the temperature build-up is too high and the edges burn off (burning of the
resin binder in the
sand), the amount applied can be reduced or the amount of hydrocarbons and
esters can be
reduced. An increase in the amount of high-boiling components in the flammable
liquid
extends the burning time and reduces the burning temperature, while an
increase in the amount
of low-boiling components reduces the burning time but increases the burning
temperature.
The formation of soot, which is undesired for reasons of protection of labor,
can optionally be
suppressed by a reduction of the hydrocarbon content. An infiltration of the
liquid into the
water-based wash prior to burning is undesired. It is important that the
amount of thermal
energy provided by the amount and composition of the flammable liquid is
sufficient to
remove the water from the applied wash. Preferably, temperatures between 100
and 180 C
CA 02610372 2007-11-26
17
should be strived for. The drying process due to burning off can be supported
by a slight air
stream. The additional use of gases with a high oxygen content is possible to
improve or
control the burning off. The dried layer of the coating compound obtained
after burning off the
flammable liquid has a residual moisture of less than 8% (possible sufficient
for grey-iron
casting), preferably less than 6%, more preferred less than 5%, even more
preferred less than 2%,
and most preferred less than 1%. In particular for sensitive casting qualities
such as steel,
aluminum, spheroidal casting and aluminium bronze, the residual moisture
should be less than 1
to 2%. The residual moisture can be determined according to the carbide method
(CM) by
Riedel de Haen, Seelze. The carbide method is based on the principle that dry
carbide reacts
with water to form acetylene gas:
CaC2 + 2 H20 -> Ca(OH)2 + C2H2
The determination of moisture can be carried out in accordance with a known
process in a
laboratory or on site with a CM measuring device. If the desired residual
moisture value of the
applied water-based wash is not reached after the first burning off process,
the above-described
drying process of spraying on a flammable liquid and burning off the flammable
liquid can
optionally be repeated several times until the desired residual moisture value
is reached. In a
preferred embodiment, additional flammable liquid can be sprayed on the
burning mold surface
during the burning off. In another embodiment of the present invention,
different parts of a
mold can be treated with different amounts and/or different compositions of a
flammable
liquid. In the case of repeated application, the amount and composition of the
flammable
liquid to be applied can optionally be modified.
If desired, the steps of applying a water-based wash and the drying of the
water-based wash by
applying and burning off a flammable liquid can be repeated. Optionally, the
used water-based
coating compound, the used flammable liquid, or both, can be different than
the water-based
coating compound applied the first time and/or the flammable liquid used the
first time,
respectively.
Optionally, afterdrying can be carried out by means of an air stream, a hot
blast, a gas flame or
an infrared radiator. After drying of the water-based wash, the dried wash can
optionally be
cured further. All known curing processes can be used. Added curing agents can
be activated
CA 02610372 2007-11-26
18
by heat or electromagnetic radiation. Depending on the temperatures generated
during burning
off the flammable liquids, drying and curing of the wash can be carried out in
one step or in
separate steps. Free-radical and ionic curing processes are conceivable as
well.
The inventive method for drying water-based coating compounds as described
above can be
used for all kinds of casting molds and casting cores. Casting molds which
comprise an
overcoat prepared according to the method of the present invention are inter
alia used in the
production of pipes, cylinder liners, engine and motor components, machine
bases, turbines
and general machine components.
The invention is described in more detail in the example below.
Example
Six complete molds made from quartz sand H32 (Halterner Quartzsand Werke
GmbH), 1.2 %
phenolic resin (Cavenaghi, SPA) and 0.4% curing agent (Cavenaghi, SPA), i.e.
twelve mold
parts from the block forming plant, were coated with a water-based wash
(SOLITEC W 3 A).
The water-based wash had the following composition:
Raw material wt.-%
Water 31.25
Biocides 00.20
Starch 00.50
Thickening agent 00.05
Attapulgite 01.00
Iron oxide 01.50
Zirconium silicate 50.00
Graphite 10.00
Mineral hollow spheres 05.00
Wetting agents 00.50
CA 02610372 2007-11-26
19
Biocide common biocide from Thor Ltd., Margate Kent CT9 4JZ, U.K.
Wetting agent common wetting agent from the company Henkel AG, 40589
Dusseldorf, Germany
Thickening agent common thickening agent from CP Kelco, 4623 Lille Skensved,
Denmark
Attapulgite common antisettling agent from Engelhardt Corporation, Iselin
NJ 08830, U.S.A.
Iron oxide common iron oxide from Lanxess GmbH, 51369 Leverkusen,
Germany
Zirconium silicate common zirconium silicate from Possehl Erzkontor GmbH,
23552 Lubeck, Germany
Graphite common graphite from Georg H. Luh GmbH, 65393 Walluf,
Germany
Mineral hollow spheres common hollow spheres, as described in WO 94/26440
Starch corn starch from Cerestar Germany, Krefeld
The biocides, the starch and the thickening agent were added to the provided
total amount of
water. Then, it was stirred for 15 minutes at 1,000 rpm with a toothed disk
(d/D = 0.5).
Attapulgite and iron oxide were added to the mixture. Then, it was stirred for
15 minutes at
1,000 rpm with a toothed disk (d/D = 0.5). Zirconium silicate (particle size <
45gm) and
graphite were added to the resulting mixture. Then, it was stirred for 15
minutes at 500 rpm
with a toothed disk (d/D = 0.5). To this mixture, mineral hollow spheres and
the wetting agent
were added. Then, it was stirred for 15 minutes at 500 rpm with a toothed disk
(d/D = 0.5).
The flow time of the obtained water-based wash was adjusted to 12.6 seconds,
determined
according to DIN 53211, flow cup 4 mm, Ford Cup. The molds were flow-coated in
a metal
flow coating basin. After the water-based wash was drained off, the average
wet layer
thickness was about 125 m, measured with a wet layer thickness gage-comb
type.
After flow-coating, the wash curtaining and accumulations were removed with a
brush and
some water. The molds were then positioned vertically, i.e. edgewise, into a
sand bed. Then a
mixture of 20% white spirit (boiling point about 160 C) and 80% isopropanol
was sprayed
onto the mold with a small pressure container. The amount sprayed on was about
400 g/mz.
Immediately after the spraying, the mixture of white spirit and isopropanol
was ignited with a
CA 02610372 2007-11-26
gas flame. The mixture burned off quickly and without the formation of soot.
Afterwards, the
molds exhibited a dry surface. The castings obtained from these molds showed a
surface
structure superior to that of castings from similar molds which had been
coated with an
alcohol-based wash.
Instead of the above-mentioned mixture of 20% white spirit and 80%
isopropanol, a mixture of
20% white spirit and 80% ethanol can be used for drying the water-based wash
as well.
