Abstract:
The present invention is a method for coating a plurality of coating compositions onto a moving support while minimizing the time required to switch from one coating composition to a different coating composition. The method involves supplying a first coating composition to a hopper at a first flowrate. When the switch is made to an alternate coating composition, the alternate coating composition is supplied to the hopper at a second flowrate while coating composition is removed from the hopper at a third flowrate equal to the first flowrate subtracted from the second flowrate. After sufficient pumping the alternate coating composition is supplied to the hopper at the first flowrate and no coating composition is removed from the hopper.

Description:
FIELD OF THE INVENTION 
     The present invention is a method of maximizing the number of coating composition variations that can be applied to a film or paper web within a time period. In addition, the present invention improves the fluid delivery of a coating to a web in an experimental operation. 
     BACKGROUND OF THE INVENTION 
     In the development of new photographic or other coated products, many coating events must be run in order to determine the optimum coating formulation. Each coating event or run can be thought of as a &#34;widget of knowledge&#34; about an experimental photographic or other coating system. To achieve accelerated research and development, more knowledge must be acquired in less time. The present invention achieves this accelerated research and development knowledge in a novel manner. 
     In addition, two other factors impact research and development productivity, precision and experimental design. Greater precision allows the researcher to make valid decisions with fewer replicates. Good experimental design maximizes the informational value of each variation. The present invention also allows greater precision and improved experimental design. 
     The typical mode of operation for a photographic research and development person is to have one coating &#34;slot&#34; per week on a particular coating machine. During this coating period, 25 to 35 coatings can be made. Each of these coatings yields a &#34;widget of knowledge&#34;. If the individual researcher could make more coatings with greater precision, his or her productivity would be increased. 
     However, certain constraints must be met in order to ensure a series of coating experiments is successful. The first is that a constant volumetric flowrate of fluid must be delivered to the web or support at all times. This includes both during the good coating interval as well as the transition interval while switching from one coating composition to the next. By delivering a constant volumetric flow to the web, dryer equilibrium is maintained. This translates to a constant drying profile for the web. A constant drying profile is desirable because the researcher wishes to maximize his ability to detect photographic differences caused by the composition changes in the coating, not from the drying profile induced differences. The drying profile is typically assumed to be constant. 
     Another constraint that faces the researcher trying to run a number of coating compositions is that the coating composition must continuously be applied to the web. If the operator lifts the hopper off of the web, not only is the dryer equilibrium disturbed, time is required to reestablish the coating bead when the hopper is put back in communication with the web. This applies to both a bead coating and curtain coating operation. If the operator leaves the hopper in communication with the web and pumps at purge rates, i.e. high flow rates, the coating machine dryer will become fouled. The coating composition would run off the edge of the web and the web would not be dried during windup. If the hopper was purged with water, the water would also run off the edge of the web. If the total flow during purges were redirected to waste, the coating bead or curtain would be broken requiring restart time. Air would inevitably get into the hopper making restart even more difficult. All the alternative hopper-in methods of composition changeover destroy dryer equilibrium. The present invention presents a method which meets the above constraints and allows the researcher to maximize the number of coating compositions coated onto a web in a minimum amount of time. 
     SUMMARY OF THE INVENTION 
     The present invention is a method of switching from a first coating composition to a second coating composition by providing a moving substrate; providing a coating hopper having a cavity, a slot in fluid communication with the cavity, inlet means in fluid communication with the cavity and outlet means in fluid communication with the cavity wherein the coating composition is capable of flowing through the slot and being deposited on the substrate. The first coating composition is supplied to the inlet means at a first predetermined volumetric flowrate, the switch to the second coating composition is accomplished by supplying the second coating composition to the inlet means for a predetermined time at a second predetermined volumetric flowrate while discharging from the outlet means coating composition at a third predetermined volumetric flowrate, the third predetermined flowrate being equal to the first predetermined flowrate subtracted from the second predetermined flowrate. The second coating composition is then supplied to the inlet means at the first predetermined flowrate while preventing flow out of the outlet means. 
     In a preferred embodiment of the present method, the predetermined time is such that at least three system volumes are passed through the cavity of the hopper. The system volume includes the internal volume of the coating hopper and inlet means. 
     In an alternate embodiment of the method of the present invention a plurality of coating compositions is coated by providing a moving support; providing a coating hopper having a cavity, a slot in fluid communication with the cavity, an inlet means in fluid communication with the cavity and an outlet means in fluid communication with the cavity wherein coating composition flows through the slot and is deposited on the substrate. One of the plurality of coating compositions is supplied to the inlet means at a first predetermined volumetric flowrate. An alternate coating composition is then supplied to the inlet means when switching to the alternate coating composition, the alternate coating composition is supplied for a predetermined time at a second predetermined volumetric flowrate while discharging from said outlet means coating composition from the cavity at a third predetermined flowrate, the third predetermined flowrate being equal to the first predetermined flowrate subtracted from the second predetermined flowrate. The alternate coating composition is then supplied to the inlet means at a first predetermined flowrate and these steps are repeated for each of the plurality of coating compositions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a schematic diagram of the fluid delivery system with rapid melt switching capability of the present invention. 
     FIG. 2 shows the average laydown using the balloon method to supply coating to the hopper. 
     FIG. 3 shows the average coating laydown using piston pumps. 
    
    
     For a better understanding of the present invention, together with other advantages and capabilities thereof, reference is made to the following detailed description and appended claims in connection with the preceding drawings and description of some aspects of the present invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a method which allows a coating operator to switch from one melt to the next without introducing air into the delivery system. In addition, cross-contamination from one melt to the next is minimized. The system used in the present invention is shown in FIG. 1. 
     Two so-called &#34;suck wands&#34; 11 (stainless steel tubes) are used alternately to draw in a coating composition. The coating composition is held in vessels 12. While one wand is sucking in the coating composition, the other wand is being washed in the suck wand wash station shown as 13 in FIG. 1. The inside of the wand is simultaneously flushed with water or gel solution. Each wand 11 is moved by pneumatic cylinders between either the wash station or the coating composition. Vessels 12 are held at 40° C. and magnetically stirred during coating. Microswitch or IR sensors are used in the system to insure that a vessel 12 is present before the suck wand 11 is inserted. The system accommodates most types of vessels. After coating, the vessels are pushed into a plastic bag for delivery to a building washing machine (not shown). Alternatively, the vessels can be dumped and washed prior to being pushed into a plastic bag. 
     The coating composition from vessel 12 is pumped through pump P1 and delivered to the hopper 30 at the normal coating flow rate, of for example 30 cc/min. For this example, pumps P3 and P4 which are connected to hardener vessels and other additive vessels, i.e. chemical addenda, are not active. The coating composition delivered to the hopper 30 is then applied to the web 31. At this time valves V5 and V6 are closed and all of the coating composition delivered to the hopper 30 is subsequently coated on the web 31. 
     When the switchover to the next coating composition is initiated, pump P1A starts pumping at a rate of, for example, 200 cc/min. Pump P1 is switched to the next coating composition and continues pumping at 30 cc/min. The total flow going into the hopper then becomes 230 cc/min, as pump P1 has not stopped pumping or changed speed. Valves V5 and V6 are opened when pump 1A begins pumping. Pump 2 is started simultaneously with pump 1A. The result is that 200 cc/min is sucked out the ends of the hopper while 30 cc/min continues to be delivered to the web. Therefore, the bead is never broken. No human intervention is required. After a predetermined volume of fluid has passed through the system, a volume judged to be sufficient for purging, pump P1A stops valves V5 and V6 close off and pump P2 continues to pump flush water to drain at a slow rate. Pump P1 never changes speed through all of these sequences. It continues to deliver the normal coating flow. 
     When pumps P3 and P4 are used with this system, their flow during purging will be maintained at a constant ratio to the stream being delivered by pumps P1 and P1A. 
     The purge volume is conveniently expressed in terms of system volumes. One system volume is the volume of the tubing, the pump, the valves, the mixer and the hopper. This is defined as the volume of the inlet means and the volume of the hopper. Usually an acceptable purge can be achieved by passing three system volumes through the hopper. In a preferred embodiment of the present invention the system as shown in FIG. 1 is controlled by a computer control system (not shown). All the timing, valve switching and calibration functions are controlled by the computer control system. In addition, all of the components, both computer and pumps, reside on a portable cart. This portability yields two important benefits. It facilitates delivery system construction without disrupting ongoing coating operations and it allows the system to be tested on a variety of coating machines. For a given experiment the operator enters the aim flow rate (cc/min), the number of coatings in the experiment, the number of &#34;good&#34; feet of the coating he wants to produce, etc. After these parameters are entered, the operator initiates the system and feeds the melt vessels to the delivery system and applies labels to the web when prompted by the computer controls. The hopper remains in the coating position at all times. 
     FIG. 1 also includes a calibration line 21 leading to a weigh station 22 for calibrating pumps P1, P1-A, P3 and P4. During calibration valve V7 directs flow through line 21 to the weigh station 22. The pumps can be calibrated with this configuration. 
     The pumps, P1, P1A, P2, P3, P4 used are reciprocating piston pumps manufactured by Fluid Metering Inc. These pumps use ceramic pistons inside of ceramic cylinders and have dialable strokes. The pump sizes available have strokes of 0.01 to 0.05 cc/revolution, 0.01 to 0.10 cc/revolution and 0.02 to 0.32 cc/revolution. These pumps deliver linear fluid flow over the range of 0 to 2500 rpm and are rated to 100 psi. 
     The stepper motors used to control the pumps are available from Seiberco Motors. The pump motor combination was tested over the 50 to 2500 rpm range. It was found to have a standard fluid delivery error of approximately ±0.2%. Although these were the pumps used with the present system, other pumps and motors can be substituted. 
     The mixing chamber 23 used is a visco-coupled mixer element that operates at approximately 800 rpm. One of the concerns in the present system was the use of reciprocating piston pumps. The concern was that cross-lines might appear on the coating. The tests run have shown that cross-lines disappear when the single stroke volumes are small and the stroke frequency is high. In tests using the pumps of the present invention, cross-lines disappeared when the pulse frequency was above approximately 275 pulses/min. This corresponds to a 10 cc/ft 2  laydown at 30 ft/min web speed. The example below gives the predicted crossline intervals for three cases. The objective was to make a 4 inch wide coating at three web speeds, 10, 30 and 90 ft/min. One pump was used to deliver the total flow. 
     
         ______________________________________CASE 1:Web speed              10      fpmWet laydown            10      cc/ft.sup.2Required flow rate     33.33   cc/min.FMI pump head is dialed to deliver                  0.01333 cc/rev.Pump speed             2500    rpmPredicted cross-line interval                  0.048   inchesCASE 2:Web speed              30      fpmWet laydown            8       cc/ft.sup.2Required flow rate     80.0    cc/min.FMI pump head is dialed to deliver                  0.032   cc/rev.Pump speed             2500    rpmPredicted cross-line interval                  0.144   inchesCASE 3:Web speed              90      fpmWet laydown            6       cc/ft.sup.2Required flow rate     180     cc/min.FMI pump head is dialed to deliver                  0.072   cc/rev.Pump speed             2500    rpmPredicted cross-line interval                  0.432   inches______________________________________ 
    
     None of the above cases produced detectable cross-lines. When multiple pumps are used, for example having the hardener and addenda pumps in use, higher pulsation frequencies result which smooth fluid flow even further. The high frequency pulses are readily dampened by the rubber delivery lines. 
     Shown in FIG. 2 is the average laydown of a coating when using conventional (balloon method) pumps. This is compared with the piston pump method of the present invention which is shown in FIG. 3. As can be seen from a comparison of FIGS. 2 and 3, significantly improved fluid delivery precision was achieved. In addition, no cross-lines were detected and rapid melt changeovers were achieved while the coating bead was essentially undisturbed during the purging operation. 
     Although the invention has been described as using a purge mode wherein the flow rate is greater than the flow rate that is used during normal coating operations, it is also possible to purge at the same flow rate, that is the flow rate equal to the coating flow rate. This is not the preferred procedure. Actual procedures may vary depending upon the coating machine. 
     Shown in Table I is a predicted increase in productivity when using the present invention. Examples 1 through 5 show the number of feet of a good coating required, the number of coatings produced per hour using conventional methods and the coatings per hour and percent productivity gain that can occur using the method of the present invention. As can be seen from Table I, productivity increases of 200 to 1200% are possible when using the method of the present invention. 
     
                                           TABLE I__________________________________________________________________________       Tubing I.D. 0.125 Inches                    Tubing I.D. 0.0625 Inches    Final  Present       Potentl            Potentl Percent                    Potentl                         Potentl PercentExmpl    Feet  Ctgs/hr       Ctgs/hr            Prdctvy Gain                    Ctgs/hr                         Prdctvy Gain__________________________________________________________________________1   15 20   179  895%    246  1230%2   15 30   179  597%    246  820%3    9 60   --   --      339  565%4   30 30   120  400%    146  487%5   30 60   120  200%    146  243%__________________________________________________________________________ 
    
     Ex. 1-3 had web speed of 10 ft/min, wet coverage of 10 cc/ft 2 . 
     Ex. 4 had web speed of 30 ft/min, wet coverage of 8 cc/ft 2 . 
     Ex. 5 had web speed of 90 ft/min, wet coverage of 6 cc/ft 2 . 
     While there has been shown and described what are at present considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes, alterations and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.