Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Reexamination Certificate
2000-11-28
2003-06-24
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
C073S054040, C073S054390
Reexamination Certificate
active
06581440
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention provides a process for investigating and determining changes in liquid media, in particular coating agents and their components, due to shear stress and also a device for performing the process.
Liquid media which contain disperse constituents may experience irreversible changes or damage when they are subjected to shear stress.
Typical examples of liquid media which contain disperse constituents being subjected to shear stress can be the liquid lacquers passed through circular pipework systems for the purpose of supplying the spraying devices during mass production lacquering. The liquid lacquers are pumped from a storage tank into the circular pipework, some is removed via the spraying devices during the application of lacquer and the unused remainder is then returned to the storage tank and mixed with the lacquer material found therein. Whereas the liquid lacquer in the storage tank is under atmospheric pressure, it is under a pressure of, for example, up to 12 bar in the circular pipework. The liquid lacquer being circulated experiences pressure differences within the circular pipework which are produced, for example, upstream and downstream, with respect to the direction of flow, of components installed in the circular pipework, in particular for example upstream and downstream of pumps and valves. The shear stress then acting on the liquid lacquer can lead to damage of disperse binder constituents and/or pigments. The extent of damage may be such that the liquid lacquer can no longer be used. The damage may consist, for example, of an unwanted and irreversible change in the Theological behaviour, the production of specks in the lacquer and/or changes in the final finishing effect or colour. The stability to shear of a given liquid lacquer is not known exactly. In practice, testing for these types of changes takes place empirically, for example by taking a sample and using conventional application-oriented tests. The liquid lacquer samples taken have not been subjected to defined, reproducible shear stress.
In order to test the stability to shear or stability to circular pipework of liquid lacquers, it has been disclosed that a quantity of the liquid lacquer to be tested be stirred magnetically and tested for changes in the lacquer material by taking samples after a defined period. This method correlates with the shear stresses in a circular pipework installation to only a limited extent and suffers in particular from very poor reproducibility.
BRIEF SUMMARY OF THE INVENTION
The object of the invention is the provision of a reproducible method for testing the effects of shear stresses on liquid media, in particular liquid coating agents.
The object is achieved by repeatedly subjecting a given volume of a liquid medium to shear under defined conditions.
Thus, the invention provides a process for determining reversible or irreversible changes in a liquid medium during the exertion of a shear stress which is characterised in that a given volume of the liquid medium to be tested is allowed to pass repeatedly through a shear unit under reproducible conditions.
DETAILED DESCRIPTION OF THE INVENTION
In the process according to the invention, a given volume, for example 500 to 2000 ml, of a liquid medium to be subjected to a shear stress is allowed to pass repeatedly through a shear unit under defined conditions. In general, the shear stress is applied at temperatures of, for example, 20 to 50° C. It is expedient that as much as possible of the given volume of liquid medium is allowed to pass through the shear unit during each passage through the shear unit. The proportion of non-sheared material per passage through the shear unit is preferably less than 5%, in particular less than 1%.
The liquid medium passes repeatedly through the shear unit, i.e. a predetermined number of, for example, 50 to 2000 passes, and experiences a shear gradient of, for example, between 100 and 10
7
s
−1
when passing through the shear unit. The shear stress in the liquid medium is produced as a result of the prevailing shear gradient and the number of repeated passages through the shear unit. The shear gradient itself is produced in the given liquid medium by the pressure difference P
2
-P
1
prevailing upstream and downstream, with respect to the direction of flow, of the shear unit. Immediately upstream of the shear unit the liquid medium builds up under the initial pressure P
1
of, for example, 5 to 50 bar, and the rate of flow is low, during passage through the shear element the rate of flow is high, on leaving the shear unit the pressure in the liquid medium is reduced to the pressure P
2
<P
1
and the rate of flow is again low. P
2
generally corresponds to ambient atmospheric pressure, or slightly higher than that. For a given liquid medium, a given temperature and a given pressure P
2
in the liquid medium, the size of the shear gradient depends on the initial pressure P
1
upstream of the shear unit and on the cross-section of the shear unit. It is obvious to a person skilled in the art that, in the case of a non-ideal circular cross-section in the shear unit, there is an additional effect on P
1
and thus on the shear gradient.
A constriction in the cross-section, within a closed deaerated pipe system which contains a given volume of the liquid medium, is used as the shear unit. The deaerated status ensures that the liquid medium is present as an incompressible or virtually incompressible liquid. The pipe system itself has a cross-section which does not produce a significant shear effect on the liquid medium. The constriction in the cross-section being used as the shear unit, however, is of such a size that the liquid medium is subjected to shear while passing through the shear unit. For example, the cross-section in the pipe system is greater than 10 mm
2
, for example 10 to 10000 mm
2
, whereas the cross-section in the shear unit is, for example, 0.5 to 10 mm
2
, preferably less than 5 mm
2
. The constriction in the cross-section my have a fixed value, but it is preferably an adjustable constriction in the cross-section so that, for example, the shear gradient in a liquid medium subjected to a given initial pressure P
1
can be adjusted to a desired value. The shear unit may expediently be a gap which can be adjusted by means of a micrometer screw, for example to a gap width of 0.1 to 3 mm.
The pipe system may be designed as a closed circuit for the liquid medium. A single complete circuit executed therein by the entire volume of the liquid medium then corresponds to a single passage through the shear unit. The pipe system is preferably one in which the given volume of liquid medium can pass as completely as possible through the shear unit in a shuttle process with repeated changes in the direction of flow. In the process according to the invention, liquid media are subjected to a defined and reproducible shear stress, in particular in order to test their stability under shear stress. For example, it can be tested whether a change in or damage to a liquid medium occurs under shear stress, whether a change or damage occurring under shear stress is reversible or irreversible, how rapidly, i.e. after how many passages through the shear unit, a change or damage occurs in the liquid medium or how great is the extent of a change in or damage to the liquid medium after a predetermined amount of shear stress, i.e. after a predetermined number of passages through the shear unit. The process according to the invention enables the provision of correct answers to these questions. Finally, the entire volume, or at least approximately the entire volume, of the liquid medium is subjected to uniform shear stress during each individual passage through the shear unit; thus the shear stress as such is exerted in a defined and reproducible way.
In order to ensure the presence of a defined and reproducible shear stress, the shear gradient can be kept constant during one shear cycle consisting of a defined number of passages through the shear unit, an
Höffer Michael
Rupieper Paul
Voyé Christian
Cygan Michael
E. I. du Pont de Nemours and Company
Lerman Bart E.
Williams Hezron
LandOfFree
Process and device for determining changes in liquid media,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process and device for determining changes in liquid media,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process and device for determining changes in liquid media,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3119434