Note: Descriptions are shown in the official language in which they were submitted.
Mo4082
LeA 29,809
METHOD AND DEVICE FOR COATING
A BODY ROTATING ABOUT AN AXIS
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for coating
a body rotating about an axis, whereby a relative movement takes place
in the direction of the axis of rotation between the body and a nozzle
which ejects a reaction mixture forming a polyurethane, so that
application is effected in helical convolutions.
The coating of rotating bodies according to this method is
generally known. The method can be used for the production of hollow
bodies, in particular of pipes, by the coating of a removable core or
mandrel. Difficulties arise depending on parameters such as
circumferential speed and rate of feed of the body or the nozzle. In
particular there is a risk of air blisters and inadequate contact of
individual convolutions. Hence attempts have already been made in the
production of pipes by the coating of a core to apply the reaction mixture
to a carrier tape and to wind the latter about a core, whereby the gap
that is present between the individual convolutions is filled up with
reaction mixture (see, e.g., European Patent 523,509). Due to this
additional filling of the gap, the expenditure on process technology and
machinery is considerable. In addition, this method is not suitable for the
coating of bodies, since it is not possible to dispense with the carrier
tape.
The object of the present invention was to improve the method in
such a way that flawless coatings can be produced economically.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a longitudinal view of a device for external coating of
pipes.
Figure 2 is a front view of the device of Figure 1.
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Figure 3 is a longitudinal view of a device for internal coating of
pipes.
Figure 4 is a front view of the device of Figure 3.
DESCRIPTION OF THE INVENTION
The above object is achieved by making use of a sheet die disposed
parallel to the axis of rotation at an angle a, whereby the relative movement
and the rate of reaction of the reaction mixture are synchronized with the
circumferential speed of the rotating body in such a way that the successive
convolutions overlap in the form of scales and connect together seamlessly.
A sheet die herein shall mean a die with an outlet in the form of a slit so
that the reaction mixture is expelled in the form of a sheet-like stream. The
slit may have a length to width ratio of 10 to 300, preferably 100 to 250.
The new method is particularly suitable for coating rollers (such as
are used in the steel, materials-handling, transport and paper industries)
with a polyurethane elastomer. In addition, pipes with an external coating
can be produced according to the present method for the industrial and off-
shore sectors, as well as pipes with an internal coating for the hydraulic
conveying of abrasive goods. Where necessary, the surfaces to be coated
can be provided beforehand with an adhesion promoter.
Pipes or other hollow bodies can also be manufactured in accord-
ance with the new method by coating a removable core. In this case. either
a release agent is applied to the core or the core must be surrounded with
a release foil. Finally, the new method can also be used for providing pipes
with a heat-insulation jacket consisting of rigid polyurethane foam. In com-
parison with the known methods, the convolutions are applied not so as to __
lie side by side but so as to overlap. In fact, in using the present
invention,
formation of air blisters and lack of adhesion between individual convolut-
ions would have been expected. It is all the more surprising that there is no
evidence of air blisters and that the convolutions merge together with one
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another without forming a crust or seam and form a sufficiently smooth
coating surface. The new method is not only suitable for internal and
external coating of axially symmetrical bodies but can also be used to coat
bodies which exhibit variable diameters across the length and/or cross-
section. The method can be implemented by computer control. In the case
of irregularly shaped bodies it is necessary to guide the sheet die so that it
is always at the same distance from, and at the same inclination to, the
surface of the body to be coated. Due to the width of the sheet die, there
are limits as regards the uneven geometry of bodies. It is also possible to
achieve variable coating thicknesses by changing the angle a at desired
places.
An important parameter of the method is the viscosity of the reaction
mixture when it is discharged from the sheet die. In order to obtain the
desired coating thickness, it is frequently necessary to change the geometry
of the nozzle. For this reason it may be necessary to exchange the sheet
die for one of different geometry or to design the height of the slit and the
width of the slit to be adjustable, which might prove to be extremely
difficult,
since it would also be necessary to adapt the inside of the nozzle
correspondingly. '
It is particularly advantageous to make use of sheet dies which
ensure that the discharge speed of the mixture from the slit of the nozzle is
the same everywhere and that the mixture which emerges is of the same
age at each point of the slit.
In general, in the case of low viscosities, the height of the slit can be
smaller, whereas at higher viscosities, it has to be larger. In this way he
pressure gradient in the nozzle can be kept relatively low, so that when the
reaction components are dosed there is no need for high-pressure mixing
heads, and so, as a rule, the less expensive low-pressure mixing heads
satisfy the requirements. Of course, high pressure mixing heads can also
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be used since they have the advantage over low-pressure mixing heads of
being self-cleaning by means of a discharge piston.
The overlapping convolutions combine both physically and, at least in
part, chemically. In the case of external coatings the shrinkage stress
causes a contraction and acts to counter the thermal expansion brought
about by the exothermic nature of the reaction. As a result, shear stresses
arise in such a way that neither a crust nor a seam is formed.
The method is suitable for coating or producing bodies having the
most varied diameter. In the case of large diameters, use is made of
t o systems which react more slowly than those for smaller diameters. For the
internal coating of bodies the exothermic reaction should be adjusted so
that excessive contraction stresses do not arise - i.e., products should be
used which have been pre-reacted to as high a degree as possible. In
other words, isocyanate prepolymers should be used.
With the new method, it is also possible to process systems which
contain fillers, preferably in the form of glass microspheres, hollow glass
microspheres and glass fibers up to about 6 mm in length. Coarse-meshed
fabric tapes, glass-fiber rovings, wires and the like can also be worked into
the coating.
The angle a is preferably adapted to the angle of the shoulder that is
formed during coating. This means that, since the angle of the shoulder
can be calculated or determined empirically, it is known before-hand. The
angle is set at the same angle at which the sheet die is set with respect to
the surface to be coated. In this way, apart from the period from start-up
until the desired thickness has been obtained, a uniform distance of the __
aperture of the nozzle from the point of application is achieved. This
distance suitably amounts to 1 to 10 mm. Preferably the angle a is
between 5 and 40°, particularly preferred is an angle a of below
25°.
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According to a particularly advantageous way of implementing the
new method, the sheet die is aligned in a plane parallel to the axis of
rotation so that the film coating when discharged from the nozzle is
deflected in accordance with the winding angle. With this measure, a high-
quality coating can be achieved which is free from air blisters.
In this connection the sheet die is suitably disposed, in the case of
external coating, in the upper quarter rotating upwards and, in the case of
internal coating, in the lower quarter rotating upwards. This particularly pro-
motes the interconnection of the convolutions.
The circumferential speed at the largest diameter of the coating to be
applied is preferably adjusted to be less than the speed at which the react-
ion mixture emerges from the sheed die. By using of this measure, the
formation of blisters at the edges of the applied coating is avoided.
Reaction mixtures are preferably processed with a pouring-time of
t5 0.3 seconds to 10 minutes. Reaction mixtures with this pouring-time can be
processed particularly well, and in connection with the other process para-
meters they ensure a coating which has a sufficiently smooth surface. This
means that the individual, scale-like overlapped convolutions combine and
merge together with one another in such a way as to form a homogeneous
coating.
At the start of the application operation, the coating thickness first
has to build up. With roller coatings a coating width is therefore always
required which protrudes beyond the subsequent useful width of the coating
to such an extent that the regions of increasing and decreasing coating
thickness lie outside this useful width.
The new device for coating a body rotating about an axis with a
reaction mixture forming a polyurethane requires a bearing and a rotary
drive for the body as well as a sheet die, whereby either the sheet die is
provided with a feed drive acting parallel to the axis of rotation of the body
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or a feed drive acting in the direction parallel to the axis of rotation is
assigned to the body.
The novel aspect consists in that the sheet die together with the
surface of the body to be coated encloses an acute angle a opening out in
a direction opposite to the direction of feed. This results in the advantage-
ous effects described in connection with the new method.
Reference will now be made to the drawings which show an embodi-
ment for external coating of pipes and an embodiment for internal coating of
pipes.
1o In Figures 1 and 2, a pipe 1 is mounted in a rotary device which is
not illustrated. In the direction of the longitudinal axis of the pipe 1, a
mixing head 2 with a sheet die 3 is arranged so as to be displaceable
lengthwise. Feeding is effected at a constant rate. The sheet die is aligned
in the longitudinal direction of the pipe and together with the outer wall of
the pipe forms an angle a of 15°. It is arranged above the quarter of
the
pipe 1 rotating upwards at an angle ~i of 85° before the zenith. The
layer of
mixture applied is designated as 4.
In Figures 3 and 4, a pipe 31 is mounted on rollers 32 and is set in
rotation by rollers 33. A jib 35 mounted on driven rollers 34 is pushed by
2o the pipe 31, whereby there is attached to said jib a mixing head 36 with a
sheet die 37. The sheet die 37 is aligned on the lowest part of the pipe 31
in the longitudinal direction of the pipe, and together with the inner wall of
the pipe form an angle a of 8°. The layer of mixture applied is
designated
as 38.
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.
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EXAMPLES
Example 1
A steel roller with a core diameter of 240 mm is sand-blasted and
then coated with about 75 g/mZ of a two-component adhesion promoter
by rolling. The path length of the roller amounts to 1400 mm.
A polyurethane elastomer layer with a thickness of 23 mm is
applied by application of a reaction mixture corresponding to the
formulation given below by means of a 125 mm x 0.6 mm sheet die. The
nozzle is clamped in such a way parallel to the roller in its longitudinal
direction and inclined to the vertical by 15° that the lower edge is
suspended about 5 mm above the surface of the roller. The point of
impact of the reaction mixture is located at about 85° in relation to
the
horizontal axis of the roller. The rotary speed amounts to 13 miri', the
feed 208 mm/min and the total output 4270 g/min.
The reaction mixture was made from two components. Component
A consisted of a mixture of 85 parts by weight of a 35 OH number
polyether consisting of trimethylpropane, propylene oxide (85% by
weight) and ethylene oxide (15% by weight), 15 parts by weight
diethyltoluene diamine (isomeric mixture), and 0.05 parts by weight
diazabicyclooctane. Component A had a viscosity at 25°C of 1050 mPas.
Component B consisted of the reaction product of 100 parts by
weight of a 56 OH number polyether consisting of 1,2-propylene glycol
and propylene oxide, 79 parts by weight of diphenylmethane diisocyanate
(70°~ by weight 4,4'-MDI and 30% by weight 2,4'-MDI). The reaction
product had an NCO content of 12.2% by weight and a viscosity at 25°C
of 1600 mPas.
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Mixing ratio:
100 parts by weight of Component A
84 parts by weight of Component B
Pot life: 5 seconds
Properties of the elastomer:
Hardness (Shore A) 90
Tensile strength (MPa) 20
Elongation at tear (%) 450
Example 2
A steel roller with a core diameter of 260 mm is provided with a
coating with a thickness of 22 mm. The rotary speed of the roller is
about 28 min'' and the feed of the sheet die is about 182 mm/min. The
angle of incidence a of the sheet die was set at 17°.
The reaction mixture was made from three components.
Component A consisted of 100 parks by weight of a prepolymer having
an NCO content of 9.8% by weight (prepared from 4,4'-diphenylmethane
diisocyanate and a 2000 molecular weight polyether carbonate).
Component B consisted of 19.5 parts by weight of a diamine 3,5-bis-
thiomethyltoluene diamine (predominantly 2,4-diamino compound) .
Component C consisted of 5 parts by weight of a mixture consisting of
1.5 parts by weight diethyltoluene diamine (with an NH-number of 630)
and 3.5 parts by weight of a 2000 molecular weight polyether carbonate.
In this process Component A was dosed at a temperature of 90°C
with an output of 3000 glmin, Component B at a temperature of 25°C
with an output of 585 glmin, and Component C at a temperature of 50°C
and with an output of 150 g/min.
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Example 3
This example was undertaken in order to provide a steel pipe
having a nominal width of 360 mm and a length of 5000 mm with a
wear-protection inner coating having a thickness of 15 mm. The same
formulation was used as in Example 1.
Onto the freshly sand-blasted surface, an adhesion promoter was
applied in a quantity of 80 g/m2 by spraying and/or rolling. After the
prescribed air drying-time of one hour, a mechanically-operated mixing
head [which is attached to a support and equipped with a sheet die
(100 x 0.6 mm) arranged parallel to the pipe] is inclined at an angle of
10° and drawn through the pipe in the horizontal direction with a
clearance from the lower edge of 5 mm, measured from the inner surface
of the pipe, at a rate of feed of 300 mmlmin. The pipe is clamped into a
rotating device and is rotated at a rotary speed of 36 miri'. In the mixing
chamber of the mixing head there are dosed Component A with an
output of 2860 g/min and Component B with an output of 2400 g/min and
these are mixed continuously by stirring. The coating operation is
discontinued after 17 minutes.
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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