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Description  |
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The invention relates to a process or method for the continuous processing
of two-component, commercially available varnishes or paints of differing
color shades, discharged from a ring conduit system.
The utilization of two-component varnishes or paints on a large technical
scale in paint shops or paint departments of, for example, automobile
factories will become of greater significance in the future. This
importance can be attributed primarily to an improved film quality.
Besides, the use of two-component varnishes on a large technical scale is
due to the fact that energy and expenses are reduced by this method.
Moreover, one can expect a considerably lower burden on the environment by
pollutants produced by the paint shops, as soon as the so-called
high-solid-content two-component varnishes, presently being developed, are
ready for use. These high-solids two-component varnishes are characterized
by a proportion of organic solvents which is considerably reduced as
compared to the conventional systems, so that the proportion of harmful
organic compounds in the emissions caused by the paint shops is greatly
lowered.
The two-component process is based on a polyaddition reaction of
polyisocyanates with the hydroxy groups of polyesters or polyethers, which
contain hydroxy groups, contained respectively in one of the two
components, the basic varnish or the hardener or cross-linking agent. In
general, the reaction takes place, with the use of unblocked
polyisocyanates, under a continuous rise in viscosity at room temperature.
The durability and processability of the blended varnish material is
characterized by the so-called pot life which frequently is less than one
hour. It is thus necessary, in the processing of such materials, to
conduct the combining of the components in maximally close proximity to
the processing apparatus, e.g. close to a spray gun, to avoid disturbances
in the processing operation and losses of material.
A prerequisite for a flawless crosslinking action and thus for the quality
of the varnish film (particularly, for example, hardness, luster, light
stability, weatherability, and resistance to chemicals) is that the
stoichiometric mixing ratio of the two components be maintained. The
mixing ratio can be rather varied for various products, but is indicated
by the manufacturer of the product in processing directives, including the
tolerance range. The tolerance range is very narrow in individual
products, so that exact metering must be carried out. Thus, the tolerance
range of systems utilized in the automobile sector for repairing the
enamel paint, for example, is .+-.10% of the hardener or cross-linking
agent proportion with a mixing ratio of basic varnish/hardener or
cross-linking agent of 2:1. It must be expected that in future systems the
tolerance ranges, with altered mixing ratios, become even substantially
more restricted.
When processing small amounts of two-component varnishes, the metering of
the components is accomplished under practical conditions by liquid
measure or weighing, and then the components are intermixed, for example
by stirring, in relatively small storage containers. When processing large
quantities of varnish, as, for example, in the series production painting
of automobiles, different processing methods are necessary, in principle,
ensuring a continuous operation of the paint supply, the combining of the
component, the metering, and the mixing step, over longer periods of time.
It is, therefore, an object of this invention to propose a process by which
large amounts of two-component varnishes can be processed continuously and
wherein the mixture ratios are maintained constant within narrow limits in
the existing framework of the operation over large periods of time.
This object has been attained by the invention by providing that, to
intermix the components, the differently colored basic varnishes are
withdrawn from different ring conduits and a colorless curing agent is
withdrawn from a further ring conduit; and that the prescribed mixing
ratio, which can be set by means of a metering element or device, is
maintained by controlling the feed amount of at least one component of the
two components, basic varnish or curing agent, to be fed to a
color-changing block. For this purpose, the mixing ratio of the two
components can be measured, after mixing in a mixing system, by means of a
measuring system, the output measuring signal of which is a function of
the mixing ratio, and the added amount of curing agent can be regulated by
way of a control device with the aid of a controllable metering element,
to have a predeterminable value. Furthermore, the entire flowthrough
quantity of the component mixture can be measured and thereafter the added
amount of hardener or cross-linking agent can be regulated.
Instead of regulating the curing agent, it is also possible to control one
or several of the basic varnish components.
In view of the fact that basic varnishes or the hardener or cross-linking
agent can vary already from the point where they are supplied, it is also
possible to effect, even prior to measuring the mixing ratio, a
measurement of the different basic varnishes or of the hardener or
cross-linking agent, in each case individually.
Various parameters are suitable for measuring the component mixture or the
hardener or cross-linking agent. Thus, the electric conductivity of the
component mixture can be measured, for example, utilizing the thus-derived
measured value for controlling the metering element. This measurement of
electric conductivity can be accomplished by means of an
alternating-current bridge which is balanced to zero with respect to the
resistance of a conductivity measuring cell filled with the component
mixture with the desired mixing ratio. The electric conductivity is
proportional to the dielectric constant of the component mixture, namely
R=1/.omega..multidot.c.multidot..epsilon..
In this connection, R is the a.c. resistance, .omega. is the angular
frequency of the alternating current, c is the capacitance of the
measuring arrangement, and .epsilon. is the dielectric constant, which
thus can also be utilized for determining a measured value for controlling
the metering element. For this purpose, the capacitance of a measuring
capacitor in a Wheatstone bridge can be balanced to zero, this measuring
capacitor being filled with the component mixture of the desired mixing
ratio. The phase of the bridge output signal can be utilized, in this
connection, relatively to the bridge input signal for deciding by
measuring technology on the direction of the occurring deviations from the
desired mixing ratio.
The invention will be explained in greater detail in the following
description with reference to a mixing and measuring arrangement and the
associated diagrams illustrated in the accompanying drawings, and further
details of the process can be derived from this description, wherein:
FIG. 1 is a block diagram of the process of the invention illustrating the
sequence of operations utilized to mix and measure the two-component
varnishes or paints of the invention prior to application;
FIG. 2 is a graph showing the linear relationship between the electrical
conductivity of the mixture of the two-component varnishes or paints and
proportion of hardener or cross-linking agent in this mixture;
FIG. 3 is a graph showing the linear relationship between the dielectric
constant and the proportion of hardener or cross-linking agent component
contained within the two-component varnish or paint; and
FIG. 4 is a graph illustrating the relationship between the bridge output
circuit of an arrangement for measuring the dielectric constant .epsilon.
over the range in which the hardener or cross-linking agent may be admixed
with the mixture of the two-component varnish or paint.
According to FIG. 1, the process of this invention is carried out by
feeding basic varnish to a conventional, commercially available
color-changing block 3 from ring conduits 1, each of which conveys a
varnish of a particular shade, via tap conduits 2. Reducing valves 4 in
the tap conduits 2 make it possible to preselect an amount of
throughflowing varnish in dependence on the color hue. This quantity can
be adjusted in accordance with the required covering capacity of the layer
thickness of a color hue. It is furthermore possible to feed hardener or
cross-linking agent from a ring conduit 5 via a tap conduit by way of a
metering element 6 to the color-changing block 3 wherein the basic varnish
component is also present in the required dosage. Thereafter, the mixture
of hardener or cross-linking agent and basic varnish is conveyed via any
conventional mixing system 7 to a measuring system or means 8 prior to
further processing; the measured quantity derived from this measuring
system is a function of the mixing ratio of the two components, i.e. the
varnish and hardener or cross-linking agent, and regulates, via a control
unit 10, the controllable metering element 6 so that it provides a
constant mixing ratio of both components, this ratio being preselectable
at the measuring system 8. From the measuring system 8, the mixed varnish,
which contains the hardener or cross-linking agent, is then conveyed to
the site of application by way of the spray gun 9.
Starting with the fact that materials, which are different from one
another, exhibit different, measurable data, and that the material data of
mixtures represent functions of the mixing ratio of the starting
components which can be determined by calculation or experiment, the
invention executes a measurement of a specific material property of the
mixture, for example, a measurement of the dielectric constant or a
measurement of the electric conductivity in the measuring means 8, and
utilizes such measurement for controlling the metering element 6 after
determining the dependence of the measuring signal from the mixture ratio.
A deviation from a specific value of the measuring signal, corresponding
to the required mixing ratio, triggers the control operation in accordance
with the invention, until the required value, which can be set at the
measuring system, has been reached at the measuring site.
For effecting the measuring process according to the method described
herein, all properties of a material can be utilized which render the
components different from one another and which can be detected by a
measuring technique and can be converted into a measuring signal suitable
for controlling a metering element. In addition to measuring the specific
electric conductivity or the dielectric constant or the dielectric loss
factor, still other parameters can be measured. Depending on the type of
the controllable metering element, it is possible to employ for its
control, for example, electric, pneumatic, or hydraulic signals derived
directly or indirectly from the measurement of the selected property of
the material.
The process described herein can be varied, in principle, by basing the
control procedure on the basic varnish rather than on the hardener or
cross-linking agent, as discussed above. It is also possible to introduce
additionally a control of the entire throughflow quantities.
Since furthermore basic varnishes or hardener or cross-linking agents can
display properties of the material which are different, dependent on the
manner in which they are fed, this aspect can also be taken into account
and included into the control operation by means of an additional
measuring instrument. Before combining the basic varnishes and the curing
agent, these components can be measured individually, and the
thus-measured data can be fed via a computer unit to the metering element
6 which also considers these data when metering the basic varnish or the
hardener or cross-linking agent.
Measurable properties of the material useful for controlling the metering
procedure are, in particular, the electric conductivity and the dielectric
constant of the material mixture. FIGS. 2 and 3 show the adequately linear
correlation between the electric conductivity .rho. and the dielectric
constant .epsilon., determined by experiments, and the proportion of the
hardener or cross-linking agent component in the mixture of materials.
All conventional methods for measuring the electric conductivity and the
dielectric constant are suitable for the practical detection of the
measuring variables by means of measuring devices, but especially
advantageous are compensating methods according to the measuring bridge
principle. The mixture of materials flows through a measuring cell
connected into an alternating-current bridge along the lines of a
Wheatstone bridge. The measuring cells employed are measuring cells for
determining the conductivity or the dielectric constant and are of a
conventional construction. The bridge can be tuned by making the bridge
output signal equal to zero once the electric conductivity .rho. or the
dielectric constant .epsilon. of the mixture of materials assumes the
value corresponding to a certain, desired mixing ratio of the two
components. A change in the mixing ratio results in a change in the
electric conductivity .rho. or the dielectric constant .epsilon., and thus
ensues a change of the bridge output signal which then becomes different
from zero. The decision on the direction in which the change in mixing
ratio is taking place can be detected, from the viewpoint of measuring
technology, for example, by measuring the relative phase position of input
signal and output signal, since this phase position is subject to a
rotation by 180.degree. with a suitable design of the bridge, in the range
of the bridge compensation or tuning step.
For a two-component varnish, FIG. 4 shows an example in a diagram, plotting
the bridge output signal of an arrangement for measuring the dielectric
constant .epsilon. over the hardener or cross-linking agent proportion of
the mixture. The processing directions require a mixing ratio of basic
varnish to hardener or cross-linking agent of 2:1 with a tolerance of
.+-.10% of the proportion of curing agent.
The output signal of the bridge, dependent on the mixing ratio of the two
components, is present in the form of an electric voltage and is suitable
directly or indirectly, after any desired conversion, for controlling a
regulatable metering element of any desired kind.
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