RHEOMETER FOR HIGH SOLIDS SUSPENSIONS

Abstract: A rheometer (10) for measuring viscosity at near-zero shear rate, for predicting stiffness or slump resistance of a free-standing paste at rest with high suspended solids therein, is provided that has a moving rod (42) with a ball-shaped tip (40) fastened at the end thereof, travelling vertically downwards at a preselected constant speed via a linearly moveable slide member (16) mounted on a slide guide bearing rail (20, 22) into a pool of paste placed in a container (C), while a digital force gauge (38) attached to such a rod (42) monitors, measures and records the maximum force exerted on the ball tip (40) during its impingement and subsequent travel downward thru the paste pool at such preselected constant speed as maintained by a controller module (34), ball screw means (28) and stepper motor (30).

Description: RHEOMETER FOR HIGH SOLIDS SUSPENSIONS Field of the Invention This invention relates to a new and improved rheometer for predicting the slump resistance of high solids suspensions such as TV sealing frit paste. Background of the Invention Rheology of high-solids suspension has been both interesting and challenging. In order to control the ease of dispensing a paste having a high-degree of solids suspended therein through flow channels and a dispensing orifice, and to control the post-flow after the paste is positioned upon a substrate, accurate viscosity measurements at both regions of medium and near-zero shear rates are needed. Past viscosity at medium-to-high shear rates (which dictates the ease of dispensing the paste) can be satisfactorily measured with conventional viscometers such as a Brookfield. On the other hand, a simple and production-worthy rheometer for measuring viscosity at near-zero shear rate (which dictates the slump resistance of the paste at rest) has been illusive. The flow of most particle-filled suspensions is in a Non-Newtonian mode, wherein the viscosity is a function of the shear rate and/or the shear time. Such suspensions are predominately pseudoplastic, such that the viscosity decreases as the shear rate increases, and often they are thixotropic, wherein the viscosity decreases as the shear time increases. Pastes having a relatively high degree of solids suspended therein, and with an extremely high degree of pseudoplasticity, exhibit a yield stress in the form of a false body which resists external stresses of a finite magnitude. Only when the external stresses exceed this critical value (the yield stress) does any strain become possible. Since most available rheometers can only measure paste response, that is torque, force, displacement, angular motion, etc. in a steady-state fashion, this false body of the paste under test is often disrupted or even destroyed while it is being tested. Thus, an accurate measurement of the slump resistance of the paste is not obtainable. Conventional falling ball viscometers have been available for some time, and explicit shear analysis has been developed for Newtonian fluids. However, such analysis has all dealt with Newtonian flow and not the non-Newtonian flow exhibited by pastes having a high degree of solids suspended therein. Conventional falling ball rheometers have not been satisfactory for measuring the slump resistance of high solids suspensions such as TV frit paste. One obstacle lies in the fact that the paste is opaque, which renders visual examination of the falling ball impossible. Further, some of the pastes have significant yield stress, which prevent the ball from falling by its own weight through the paste. An attempt to increase the force behind the ball was deemed impractical since the ball would not fall with too small of a force, and fell too fast to discriminate ball times with too large of a force. Numerous measuring devices have been utilized in the past including U.S. patents nos. 1,232,782; 1,651,596; 1,748,512; 1,748,513; 5,144,832; and 5,327,778 all of which utilize a weight to force a plunger or ball downwardly into a test substrate, with the 1,748,512 and 1,748,513 patents also utilizing an oil bath and piston to control the downward movement of the weighted plunger or push rod. U.S. patents nos. 1,894,369; 2,625,034; and 2,747,399 all utilize the use of a balance or scale to measure the rates of penetration of a test sample. In addition, U.S. patent no. 2,638,779 measures the current to a magnetic coil to obtain a reading as to resistance to penetration of a sample. Finally, U.S. patent no. 5,357,786 utilizes an electromagnetic balance which provides a signal or reading relating to the vertical impingement of a plunger on a sample. However, none of the foregoing references suggests the utilization of a constant speed traveling stage to insert the ball into the test sample and measuring the maximum force on the ball as it travels through such sample. It thus has been an object of the invention to provide a new and improved falling ball rheometer wherein the ball is moved linearly at a constant predetermined speed into a test sample having a high concentration of solids suspended therein, and measuring the maximum force produced on the ball as it travels through the test sample. Summary of the Invention The present invention sets forth both method and apparatus for measuring the maximum force exerted on a plunger or ball as it travels at a constant speed through a portion of a test sample. More particularly, the method and apparatus of the present invention has utilization for predicting the slump resistance of a sample having a high concentration of solids suspended therein, such as TV frit pastes . In order to accomplish meaningful desired readings between various test samples, a moving rod with a ball- shaped tip is provided upon a traveling stage which moves linearly at a desired constant speed into a pool of the paste to be tested, while a force gauge attached to the rod monitors and records the maximum force exerted thereon during its travel through a portion of the test sample. In order to have consistency, the linear or downward motion of the ball is controlled by a motorized DC mdexer connected to a miniature PCU and under the control of ICL (Intelli-Command-Language) software. By utilizing a predetermined constant speed immersion of the ball into the paste, and by measuring the maximum force on the ball as it travels through the paste, a meaningful comparison of stiffness or slump resistance is obtainable between one paste to another, where the pastes are tested with the same diameter ball and the same diameter cup retaining the paste. Brief Description of the Drawings Fig. 1 is a somewhat schematic side elevational view of an improved rheometer of the present invention. Fig. 2 is a front elevational view thereof. Fig. 3 is a top plan view thereof. Description of the Preferred Embodiments Referring now to the drawings, an improved rheometer 10 of the present invention is shown comprising an upright slide stand or test stand 12 having a base plate or sample retaining portion 14 and a linearly moveable slide member 16 mounted thereon. The upright test stand 12 has a supporting back plate 18 and a pair of slide guide rails 20, 22 secured to the backplate 18 and between an upper cover member 24 and a bottom member 26 which is secured to the sample retaining portion 14. The slide member 16 is provided with a ball screw arrangement 28 and complimentary rails so as to move linearly up and down along guide rails 20, 22. A stepper motor 30 is mounted on the test stand 12 and is operatively connected to the ball screw through connector housing 32 so as to drive the ball screw means and move the slide member 16 linearly along the slide guide rails 20, 22. A controller module 34, described in more detail hereinafter, controls the operation of stepper motor 30 and accordingly the movement of the slide member 16 along guide rails 20, 22 of upright stand 12. An adapter member 36 is connected to the slide member 16 and has a digital force gauge 38 secured to the outer end thereof. A ball tip 40 is attached to a rod 42, which is screwed into the force gauge 38. The force gauge has a digital read out face 44 for recording test measurements. As shown in Fig. 1, upper and lower limit switches 46, 48 may be provided to limit the vertical extent of the slide 16 along the rails 20, 22 of the upright slide or test stand 12. The rheometer 10 incorporates the use of a moving rod 42 with a ball-shaped tip 40 traveling vertically downwards at a preselected speed on slide guide bearing rails 20, 22 into a pool of paste contained withm a container C shown in phantom lines of Fig. 1, while a force gauge 38, having the rod 42 attached thereto, monitors and records the maximum force exerted on the ball during its downward movement withm the paste contained in container C. The downward motion of the ball 40 is produced by the downward movement of the slide member 16 through the ball screw means 28, which in turn is controlled by the stepper motor 30. However, motor 30 is connected to a miniature Central Processing Unit (CPU) under an Intelli-Command-Language (ICL) software, which is set forth hereinafter. The force gauge 38, located above the falling ball rod 42, is a digital force gauge capable of recording and displaying the maximum compression force exerted thereon through the ball tip 40, as the tip moves through the test sample withm container C. Such force gauge may be a model CI-FGE2 force gauge obtainable from Controls International of Chicago, Illinois. The CPU operates the linearly moveable slide 16, carrying the force gauge 38 and ball tip 40, as a constant speed traveling stage, to insert the ball mto the test sample or paste contained in container C. The CPU in the drive unit utilizes the ICL software to drive the DC stepper motor at a predetermined constant speed so as to move the ball tip 40 into a pool of paste or test sample at a constant speed. As the ball tip is moving within the test material, the force gauge 38 is programmed to capture the maximum force exerted thereon through the ball tip. Preferably, the CPU controls the stepper motor in half steps so as to obtain a smooth flowing movement of the slide 16 along the stand 12 without any harmonic vibrations. If desired, the CPU may be programmed to move the ball rapidly downwardly to just above the sample, and then the constant predetermined speed is utilized to move the ball mto the sample and obtain a peak or maximum force reading on the force gauge as the ball moves withm the sample itself. The test stand may be a VersaTest model EP-59880-54 produced by Cole-Parmer Instrument Company of Vernon Hills, Illinois, and the CPU may be an American Precision Packaged Microstep System, Catalog No. P/N P261X-M232 obtainable from Hughes Industrial Products Inc., of Clarence, New York. The rheometer of the present mvention is m fact a relative measurement instrument, and therefore it is necessary that the diameter of the ball tip 40, and the diameter of the cup C retaining the test sample or paste, be kept constant in order to compare the stiffness or slump resistance of one paste to another. In operation, a test sample or paste is preferable shaken for three minutes on a standard shaker such as a Red Devil pail shaker. The sample is then positioned withm container C on the sample retaining portion 14 of the test stand 12. The test is started with the ball tip 40 positioned just above the paste surface, and not in contact therewith. The controller module is actuated to move the motorized stage (including the slide 16, adapter member 36, force gauge 38, ball 40 and rod 42), downwardly along slide rails 20, 22 of upright stand 12 at a controlled constant speed such as 0.02 inches per second. The ball tip 40 is preferable inserted into the sample such that the periphery thereof is between 1.5" and 2" from the edge of the container C. The depth of the ball tip drop is preferably terminated at 1" above the bottom of the container C. As the ball tip travels through the paste, the maximum force exerted thereon is recorded by the digital force gauge 38. Preferably three tests are performed on each sample, with the ball tip 40 being cleaned between each test. Further, it is desirable to rotate the container C 90° between each test. A force gauge reading is recorded for each of the three tests, and an average of the three tests is then calculated to provide the final result. The test results obtained from the rheometer are utilized to predict the slump resistance of high solids suspensions, such as TV sealing frit paste. Meaningful test results are obtainable with the present mvention due to the fact that the ball tip is driven by a motorized stage or slide unit at a controlled predetermined constant speed, while the maximum force experienced by the ball tip is recorded by the digital force gauge. As previously mentioned, the CPU utilized to drive the stepper motor is provided with an ICL program that produces the desired results, and accordingly the following program has been found to provide those results. "PWRUP" : 0:V0=100 1:V1=0 2:V2=V1*V0 3:V3=0 4:V4~V3*V0 5:V5=0 6:V7=10 7:V6=V5*V7 8:MR=5000 9:UR=1 10:A=500 11:D=500 12:B=100 13:H=100000 14:M=50000 15: J=10000 16:MUNITS 17:LA=00000000 18:G=0 19:Q=10 20:R=10 21:S=10 22:U=0 23:T=4 24:SC=20 25:SEND-1|"BS@" Λ TURN OFF SCROLL 26:SEND-1|"1BE" Λ CLEAR SCREEN ON T-120 27:SEND-1|"07" Λ CAUSE BUZZER TO BEEP 28:SEND-1|"1BPT" Λ MAKE LED1 FLASH 29: CURSOR 0 0 30:WAIT 100 31:SEND -1 "SELECT JOG OR SETUP" 32:QUIT "SETUP": 0:WAIT 100 1:SEND -1|"1BPD" Λ LED1 OFF 2:SEND -1|"1BE" Λ CLEAR SCREEN 3:CURSOR 0 0 4:WAIT 100 5:SEND -1 "SETUP MODE SELECTED" 6:SURSOR 1 0 7: PROMPT "X.XX START TEST POINT" VI 8:SEND -1|"1BE" Λ CLEAR SCREEN 9:WAIT 100 10:CURSOR 0 0 11:PROMPT "X.XX TEST MODE" V3 12:SEND -1| "IBE" Λ CLEAR SCREEN 13:WAIT 100 14: CURSOR 0 0 15:PROMPT ".XXX TEST SPEED IN IPS" V5 16:SEND -1|"1BE Λ CLEAR SCREEN 17:WAIT 100 18:CURSOR 0 0 19:SEND -1 "PRESS Fl TO RUN TEST" 20:CALL START 21:QUIT "START" 0: (START) SK-0 l.WAIT 100 2:B=10 3: IF (SK=1) GOOD 4: JUMP START 5: (GOOD) SK=0 6:V2=V1*V0 7:MOVE V2 8:WAIT 100 9:V6=V5*V7 10:M=V6 ll:V4=V3*VO 12:MOVE V4 13:WAIT 200 14:M-50000 15:B=5000 16:- HOME 17: JUMP START 18:QUIT Although the now preferred embodiments of the invention have been disclosed it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

Claims: We Claim: 1. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate which comprises: an upright stand having a base portion, a motorized stage linearly movable along said upright stand, a rod having a ball tip connected to said force gauge, means for retaining a sample to be measured below said ball tip, means for controlling the downward movement of said motorized stage so that said ball tip moves through a desired portion of said sample at a constant predetermined speed, and means for measuring the maximum force experienced by the bdll traveling at said constant speed within the sample. 2. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 1 wherein said motorized stage includes α slide member linearly moveable along slide guide rails on said upright stand, and ball screw means for moving said slide member. 3. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 2 wherein said ball screw means for moving said slide member along said upright stand is driven by a stepper motor. 4. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 3 wherein said means for controlling the downward movement of said motorized stage includes α controller module having α central processing unit operatively connected to said stepper motor for driving said ball screw means to move said slide member vertically along said slide guide rails on said upright stand. 5. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 4 wherein said controller module also includes operating software for said central processing unit programmed to control the downward movement of said slide ao that said ball moves through a desired portion of a sample positioned therebelow at a predetermined constant speed. 6. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 5 wherein said means for measuring said maximum force includes a force gauge connected to said slide member, and said force gauge having a setting to record the maximum force exerted on said ball tip as it moves through a portion of the sample at said predetermined constant speed. 7. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 2 wherein said slide member has means for connecting said maximum force measuring means thereto for movement therewith along said upright stand. 8. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 1 wherein said means for retaining α sample to be measured includes a container for said sample retained on said base portion. 9. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined m claim 1 wherein said means for controlling the downward movement of said motorized stage includes a controller module attached to said upright stand, and said controller module having a miniature central processing unit operatively connected to a stepper motor for driving ball screw means to move said stage linearly along said upright stand. 10. Apparatus for measuring the viscosity of a high solids suspension sample at near zero shear rate as defined in claim 9 wherein said controller module also includes software for operating said central processing unit such that said stepper motor is driven at a controlled speed to move said stage linearly downwardly along said upright stand at a constant desired rate of decent. 11. Apparatus for measuring the resistance to movement of a paste-like sample having a high solids suspension comprising: a vertical stand having a base portion, a slide member vertically moveable on said stand, a force gauge connected to said slide member, a rod having a ball tip connected to said force gauge, ball screw means and slide rails for moving said slide member on said stand, electric motor means for driving said ball screw means, said slide member and connected force gauge with said ball-tipped rod forming a motorized stage, means for retaining a sample to be measured on said base portion, a central processing unit operatively connected to said motor means and driven by software for moving said motorized stage downwardly at a controlled speed so as to move said ball tip within sample at a predetermined constant speed, and means on said force gauge for recording the maximum force on the ball tip as it moves at a constant speed through a portion of said sample to be measured on said base portion. 12. Apparatus for measuring the resistance to movement of a paste-like sample having a high solids suspension as defined in claim 11 wherein said electric motor means is a stepper motor connected to said ball screw means. 13. Apparatus for measuring the resistance to movement of a paste-like sample having a high solids suspension as defined in claim 12 wherein said central processing unit includes means for controlling said stepper motor in half steps to obtain a smooth flowing movement of said motorized stage along said vertical stand. 14. A method of measuring the viscosity of a sample comprising: positioning a sample to be tested within a container, positioning a force gauge, having a rod with a ball tip connected thereto, above said sample, moving said force gauge with said ball-tipped rod downwardly at a controlled speed, and moving the ball within said sample at a predetermined constant speed, and recording the maximum force exerted on said ball as it moves at such constant speed within said sample. 15. A method of measuring the viscosity of a sample as defined m claim 14, including the step of moving said force gauge and ball-tipped rod downwardly as a motorized stage and controlling said motorized stage through a central processing unit. 16. A method of measuring the viscosity of a sample in claim 15, including the step of providing software for operating the central processing unit to control the constant downward speed of the ball within the sample. 17. A method of measuring the viscosity of a sample as defined in claim 14, including the step of maintaining a constant ball diameter and container diameter during the measuring of various samples.

Inventors: DOHN PAUL B, SMITH NORMAN C, WU LUNG-MING, DOHN, PAUL, B., SMITH, NORMAN, C., WU, LUNG-MING
Applicants: CORNING INC, DOHN PAUL B, SMITH NORMAN C, WU LUNG MING, CORNING INCORPORATED, DOHN, PAUL, B., SMITH, NORMAN, C., WU, LUNG-MING
Application year: 1997
Main classification: G01N3
IPC codes: C03C8, G01N3, G01N11