GB1585217A

GB1585217A - Coating device suitable for coating webs of material

Info

Publication number: GB1585217A
Application number: GB35260/77A
Authority: GB
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1976-03-19
Filing date: 1977-08-23
Publication date: 1981-02-25
Also published as: NL186066B; NL7709026A; DE2611625C3; FR2399876A1; NL186066C; CH628532A5; DE2611625B2; DE2611625A1; FR2399876B1

Description

(54) COATING DEVICE SUITABLE FOR COATING WEBS OF MATERIAL (71) We, HOECHST AKTIENGESELL SCHAFT, a Body Corporate organised according to the laws of the Federal Republic of Germany, of 6230 Frankfurt/Main 80, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a coating device suitable for the continuous coating of webs of material.

In the paper and film finishing industry, many different methods for coating travelling webs have found acceptance. In one of these methods, a so-called doctor roll applicator, as described in German Offenlegungsschriften Nos. 1,577,647 and 2,034,044, is employed.

Figure 2 of the first-mentioned Offenlegungsschrift illustrates very well the mode of action of sucn processes. Thus, an excess of coating agent is first applied, by means of a feed roll which rotates in the direction of, or counter to, the moving web and dips into an applicator trough filled with coating substance, to the substrate to be coated, for example a web of paper, film or metal. The necessary adjustment to give the particular desired coating weight is then effected immediately afterwards by a downstream doctor roll, usually a rotating circular rod which may be smooth or may sometimes be wound with wires of very diverse thicknesses, this rod, in the latter case, also being known in the art under the English specialist tenn wire-bar.

Devices, for example those disclosed in German Offenlegungsschrift No. 1,577, 647 referred to above, which employ freely mounted rods, the diameter of which is of the order of approximately 40 mm, however, suffer from disadvantages, predominantly due to problems of processing technology. Thus, for example, contamination of the doctor rod by coating material which adheres and dries out is encountered, resulting in impairment of the quality of the coating as a result of the formation of streaks and structures.

As the technique has progressed, the devices which are customarily used at the present time, such as those described in German Offenlegungsschriften Nos. 2,034,004 and 2,307,404, have been developed. Characteristic features of these systems are substantially reduced doctor rod diameters, the mounting of the doctor rod in a doctor bed which supports it over its entire length and in part surrounds it, and devices for keeping the rotating rod clean, the devices being in the form of wiping edges and flushing grooves.

In spite of the undoubted improvements provided by the last-mentioned devices, these devices still exhibit substantial shortcomings which hinder their universal application. A particular disadvantage is the use with the new metering device of the conventional feed system in the form of dip rolls, transfer rolls and/or applicator rolls. This is a particularly troublesome factor in high speed coating using coating substances which are sensitive to external influences, for example polyvinylidene dispersions which are sensitive to shear and tend to coagulate and foam. At the desired web speeds of 300 to 400 m/minute, the feed roll, which the machinery manufacturers traditionally like to make of large size, for example with diameters of d=200 mm and above, must of course also run at high speed in order to ensure that the desired amount of material is applied. The consequences are, in a very short time, mountains of foam in the applicator trough and, resulting therefrom, a blistered coating of inferior quality.

Obvious counter-measures in the form of a reduction of the diameter of the feed rolls, however, have an effect exactly opposite to that desired. Smaller rolls must, since they have a smaller contact surface, be operated at correspondingly higher speeds of revolution in order to obtain application weights comparable to those obtained using a large roll. Furthermore, modifying the roll surface by grooving, using a screen-type surface or equivalent methods, which might be expected to lead to a higher feed rate, do not produce the required effect of lowering the speed of rotation of the rolls.

To overcome the problems described above, an emergency solution has been adopted in most cases, namely constant and repeated circulation of the coating substance by pumping. However, this step undoubtedly amounts to an additional harmful treatment of the coating agent, even if the pumping of the material is effected relatively gently by means of suitable conveying units such as membrane pumps.

In addition to the disadvantages described above, the actual doctor roll system also suffers from disadvantages. The doctor beds are preferably made of plastics, such as, for example, polyethylene, polypropylene, polyamide and polytetrafluoroethylene, or of rubber-like materials. The doctor bed profiles, which are subject to wear and hence can only be employed for a limited time, are either extruded as profiles, or cast, or produced from solid material by mechanical machining processes such as milling, planing and drilling, and are produced in separate manufacturing operations. These manufacturing methods are cost-intensive, and in view of the fact that the worn doctors have to be thrown away, the methods are not economical, particularly since this situation influences the total operating costs for coating, and since down-times have to be accepted.

Furthermore, it is not infrequently the case that temperature-conditioning, that is to say either cooling or heating, of the coating substance and of the equipment is a technological necessity. The poor heat conductivity of plastics doctor beds in such a situation, however, also prevents optimum operating temperatures being set up, which is a further disadvantage of the conventional applicator systems.

A further shortcoming of the known apparatuses is to be seen in the flushing circuits for keeping the doctor rods clean. Since, according to experience, complete sealing of these systems is impossible, it is possible that the flushing medium will come into contact with the coating substance. In the case of for example, polyvinylidene coating, contact with water, which is the preferred flushing agent, can lead to precipitation of the disdispersion. Obviously, the possibility of similar occurrences cannot be excluded in the case of reactive adhesive mixtures or solutions. The coagulate sludge thus produced blocks the flushing channels and can even be carried by the rotating doctor rod into the plastics bed, making the latter unusable.

The present invention provides a device suitable for coating a travelling web of material with a flowable coating agent, which device comprises an applicator trough and two channels which communicate with the trough, one of the channels being located upstream and the other downstream of the applicator trough, each of the channels guiding a doctor rod and the width of each channel being less than the diameter of the respective doctor rod, the channels being so disposed that the doctor rods are parallel to each other and communicate with the applicator trough.

More particularly, the present invention provides a device for coating travelling material webs with flowable coating agents, which device comprises an applicator trough and, viewed in the direction of travel of the web, two parallel channels, respectively in front and behind the applicator trough, which is closed at the side walls, the channels communicating with the said trough, the side walls of each channel guiding a respective doctor rod and the width of the channels being less than the diameter of the doctor rods which project beyond the side walls of the channels.

In effect, the doctor rods form at least part of opposite walls of the applicator trough.

Advantageously, the side walls of the channels are inclined relative to the top of the applicator trough. In this case it is preferred that the channels are so arranged relative to the applicator trough that the resultant of the parallelogram of forces derived from the magnitude and direction of the forces acting on the doctor rods and which result from the running of the web, is directed, at least approximately, to the centre of the channels; in other words, for e.g. channels of rectangular or square cross-section, the resultant is preferably substantially parallel to, and half way between, the side walls.

The forces acting on the doctor rods are dependent on the tensional stress and the selected wrap-round angle of the web. The fact that the walls of the channels are inclined has the effect that in the case of very rapid coating processes, or when using very viscous coating agents or doctor rods of fairly large diameter, the doctor rods are prevented from jumping out of the guides.

Although the channels can have any shape, as long as they fulfil the requirement that they guide the doctor rods (that is, that they maintain the doctor rods in the desired position), channels of rectangular or square cross-section have proved particularly advantageous in practice, since these are relatively simple to manufacture. Whatever the shape of the channels, the side walls of the channels may be matched to the radius of the doctor rods which they guide.

Although a wiping effect could be provided merely by the edges of the channels, it is nevertheless advantageous to insert doctor strips, for example of sheet metal and preferably of a springy material, into the channels, the strips preferably projecting by between 0 1 and 3 0 mm above the upper edge of the channels. Advantageously, the doctor strips are mounted on both sides over the entire length of the channels and are either curved profiles and appropriately matched to the channel cross-section or, if they are used as individual sheets, are pressed against the walls of the channels by a clamping strip. It is also possible to use a plurality of doctor strips side by side. The preferred thickness of the strips is from 0-02 to 0 5 mm.

The doctor strips ensure that the edges of the channels do not wear as a result of the rubbing action of the doctor rods; such wear would reduce the life of the apparatus. When the strips have become worn down to the height of the upper edge of the channels, they can easily be replaced, and the resulting material costs are only slight. The springy action of the doctor strips furthermore gives rise to a good and constant cleaning action on the doctor rods.

In order to achieve good running of the doctor rods, the latter are preferably driven; it is preferred that they are driven in opposite directions to each other so that any dirt which may adhere to the doctor rod is in every case scraped off outside the applicator trough and cannot enter the trough.

The diameter of the doctor rods is advantageously from 5 to 50 mm, preferably from 10 to 25 mm. For particularly good control of the amount of coating agent applied it has proved advantageous to have the doctor rods of different diameter, with the doctor rod which, viewed in the direction of travel of the web, is behind (downstream of) the applicator trough, having the larger diameter.

In practice, it has proved advantageous to accommodate the applicator trough and the channels in a base block so that the stability of the apparatus is increased and further technical measures can be carried out. Thus, for example, it has been found that when using viscous coating agents, heating is advantageous. For this purpose, bores can be machined in the base block, through which a heating medium, for example hot water, can be passed. This warms the coating agent and the resulting decrease in viscosity contributes to an improvement of the coating.

Alternatively, or in addition, the doctor rods can be provided with bores, to make it possible to pass a heating medium through them.

Advantageously, the doctor rods are in that case constructed as tubes.

The coating agent can be fed into the applicator trough by, for example, pouring in from above, or allowing it to run in continu ously. In a preferred embodiment, however, the coating agent is fed in from below, advantageously through bores in the base block, the bores terminating in the applicator trough; in the applicator trough, the feed is preferably constructed as a slit which extends lengthwise over the applicator trough. In order to ensure particularly uniform feed of the coating agent over the entire length of the applicator trough when the coating agent is fed through a bore in the base block, a pressure-balancing channel is preferably provided in the base block, which channel is in communication with the feed channel entering from below.

Since normally the rolls over which the web is led upstream and downstream of the coating device can only be moved in a vertical direction with great difficulty, it is advantageous to provide the base block with a height adjustment device. This allows an optimum setting of the tension relationships between the doctor rods and the web guide, and the entire device can be moved away from the web, for example in order to carry out repair work on the coating device, such as replacing the metal doctor strips. For some purposes, it has proved advantageous to wrap the doctor rods with wires.

Depending on the composition of the coating agent, it may be necessary to make the entire apparatus, or parts thereof from materials which are not corroded by the coating agent. Above all, plastics or stainless steels have proved particularly suitable for this purpose. This applies especially to the applicator trough and to the doctor rods and doctor strips.

The device according to the invention has proved excellent for coating, especially for coating webs travelling at high speeds of up to about 300a00 m/minute. It is possible by means of the device to coat all coatable webs, for example of paper or metal, but especially film webs of plastics or regenerated cellulose. Examples of coating agents that may be used include dispersions, emulsions, solutions and hot-melt adhesives.

Two embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through the simplest embodiment of the invention; Figure 2 is a cross-section through a preferred embodiment of the device of the invention, and Figure 3 shows a perspective view of the device of Figure 2.

Referring now to the drawings, in Figure 1, a web 15 passes under a roll 13 which holds down the web and is so positioned that the web travels upwards, at an angle to the horizontal, to the coating device. The webs then pass over metal doctor rods 5, which are held in channels 2 and which communicate directly with an applicator trough 7; the applicator trough 7 is also in communication with the channels 2. The holding-down roll 13 on the outlet is set so that the web travels downwardly towards it, at an angle to the horizontal, from the coating device. The coating agent is located in the applicator trough 7 and in particular comes up to the upper edge thereof.

As may be seen from Figure 2, the device according to the invention consists, in the preferred embodiment, of a prismatic base block 1 which, in order to avoid corrosion, is preferably made of stainless refined steel.

Two channels 2 of rectangular cross-section are machined in the base block 1, the channels being at an angle to the vertical so that each rod is centred precisely in its support; the angle is chosen such that the resultants of the parallelogram of forces derived from the magnitude and direction of the forces acting on the rod are directed to the centre of the channels. Narrow, thin metal doctor strips 4 are held in the channels 2 by clamping strips 3, which can be made either of steel or of plastics. With a view to avoiding corrosion, the possibility of which cannot be excluded, and to avoiding possible catalytic reactions with the coating agent, it is again advisable to use, in particular, refined steel sheet. The doctor strips 4 perform two functions; they must support the rotatable doctor rod 5 over its entire length and hold it in its prescribed position, and they must also keep it free from any solid particles.

To intensify the process of cleaning the doctor rods, the latter are, in addition, externally flushed. For this, the actual coating substance, for example a polyvinylidene dispersion,- is used as a flushing and wetting agent. This dispersion is fed to the applicator trough 7 which is delimited by the doctor rods 5 and by the side covers 6, which latter have to be matched to the coating width, and a part thereof passes via connecting bores 8 into the channels 2, through which it flows axially in the direction of two lateral outlet orifices. In this process, not only is the doctor rod wetted and under certain circumstances also temperature-controlled, but, moreover, particles of solid and of dirt scraped off the rod by the doctor strips, acting like a knife, are flushed away deliberately.

The device is charged with coating substance via a bore 9, which enters a pressure compensating channel 10 extending over the entire width of the base block 1. This channel communicates with the applicator trough 7 via a narrow slit 11. Temperature conditioning, that is to say heating or cooling, of the entire device is possible via bores 12.

The vertically adjustable holding-down rolls 13 serve to regulate the wrap-round angle on the doctor rods 5. Regulation of this angle is of course also possible, in the case of fixed-position rolls, by an adjustment of the entire coating unit which for this pur -pose is mounted on a height adjustment device 14.

In Figure 3, the side covers 6 and the drive means 16 for the doctor rods5 can also be seen. In Figure 3, the same numerals represent the same constructional parts as in Figure 2.

The coating operation using the device according to the invention, illustrated, by way of example only, with reference to Figures 2 and 3, proceeds as follows: The coating substance is fed via the feed bore 9 into the pressure compensating channel 10 by means having adjustable conveying capacity, for example a suitable pump or an accumulator vessel. From the channel 10, the coating substance uniformly fills the applicator trough 7 via the slit 11; lateral loss of the coating agent from the trough is prevented by the side covers 6. By lowering the rolls 13 into the working position shown, the web 15 to be coated, which now runs over the doctor rods 5, closes the only open side of the applicator trough 7. The coating substance which is in this way applied to the web is immediately reduced to the desired amount on the exit doctor rod 5 which is rotated in the direction of, or counter to, the web travel (as desired), by means of a variable-speed gearbox 16.

Only a small part of the coating substance passes from the applicator trough 7 via the bores 8 into the channels 2. When the material flows through the channels 2, on the one hand the doctor rods 5 are wetted, whereby the deposition of solid particles or dirt particles is substantially prevented, whilst on the other hand, the coating agent flushes foreign bodies scraped off the rods by the doctor strips 4 out of the channels. The coating substance which overflows is collected and returned to the coating process.

Since the coating substance may be obtained in a substantially bubble-free and foam-free condition-the substance is not churned up, even at the highest machine speeds, by any rotating components present in the applicator system-it is in principle possible to dispense with auxiliary units which in other cases are necessary, such as, for example, the foam settling vessels customary in the case of polyvinylidene coating.

Although the first doctor rod 5 exerts virtually no influence on the adjustment of the amount of the applied substance, and one might therefore be inclined to replace it by a rigid edge, it has nevertheless proved to be highly advantageous to allow this rod to rotate also. In this way the accumulation of dirt and dust originating from the travelling web of material, and the formation of scratches, is substantially avoided, in contrast to the apparatus described in German Auslegeschrift No. 1,652,402, in which the web of material to be coated runs over a fixed curved surface. The preferred directions of rotation are counter-clockwise for,the left-hand rod of Figure 2 and clockwise for the right-hand rod.

The drive means for the doctor rods advantageously comprises variable-speed gearboxes with reversible direction of rotation. In order to avoid any effect on the required precise rotation of the doctor rods and of the drive units, the use of elastic con .necting elements, for example couplings, cardan joints or universal joints, is preferred.

The application weights of coating sub stances achievable with the described mode of operation of the device are dependent on the size of the adjusting (doctor) rods employed, in addition to many other factors.

Customary coating weights of the order of magnitude of 3 to 6 g/m2 can, for example, be realised successfully with adjustment (doctor) rods of 8 to 12 mm diameter. Where larger amounts are to be applied, rods with diameters of up to 50 mm become necessary.

A particular advantage of the described device according to the invention is to be seen in the manner of application of the coating agent without any moving transfer elements, for example rolls, and in the adjustment of the amount, which follows directly thereafter. This provides the preconditions for processing liquid coating substances of all kinds which are difficult to handle.

WHAT WE CLAIM IS: 1. A device suitable for coating a travel ling web of material with a flowable coating agent, which device comprises an applicator trough and two channels which communi cate with the trough, one of the channels being located upstream and the other down stream of the applicator trough, each of the channels guiding a doctor rod and the width of each channel being less than the diameter of the respective doctor rod, the channels being so disposed that the doctor rods are parallel to each other and communicate with the applicator trough.

2. A device as claimed in claim 1, wherein the channels are inclined relative to the top of the applicator trough.

3. A device as claimed in claim 2, wherein the channels are so arranged relative to the applicator trough that the resultant of the parallelogram of forces derived from the magnitude and direction of the forces acting on the doctor rods, which forces result from the running of the web, is directed at least approximately to the centre of the channels.

4. A device as claimed in any of claims 1 to 3, wherein the channels have a rectangular cross-section.

5. A device as claimed in any of claims 1 to 4, wherein the channels are provided with doctor strips over their entire length, the doctor strips projecting by between 0-1 to 3 0 mm beyond the upper edge of the channels.

6. A device as claimed in any of claims 1 to 5, wherein both sides of the channels are provided with doctor strips.

7. A device as claimed in claim 5 or claim 6, wherein the doctor strips have a thickness of from 0-02 to 0 5 mm.

8. A device as claimed in any of claims 5 to 7, wherein the doctor strips comprise a springy material.

9. A device as claimed in any of claims 5 to 8, wherein each doctor strip is in the form of a profile corresponding to that of the respective channel.

10. A device as claimed in any of claims 5 to 8, wherein the doctor strips are individual sheets which are held in the channels by means of a clamping strip.

11. A device as claimed in any of claims 1 to 10, wherein the doctor rods can be driven.

12. A device as claimed in claim 11, wherein the doctor rods can be driven so as to rotate in opposite directions to each other.

13. A device as claimed in any of claims 1 to 12, wherein the doctor rods have a diameter of from 5 to 50 mm.

14. A device as claimed in any of claims 1 to 13, wherein the doctor rods have different diameters, with the doctor rod which is located behind the applicator trough, viewed in the direction of travel of the web, having the larger diameter.

15. A device as claimed in any of claims 1 to 14, wherein the applicator trough and the channels are formed in a base block.

16. A device as claimed in claim 15, wherein the base block includes bores which connect the channels to the applicator trough.

17. A device as claimed in claim 15 or claim 16, wherein the base block includes at least one bore for supplying the coating agent to the applicator trough.

18. A device as claimed in any of claims 15 to 17, wherein the base block includes a pressure equalizing channel for the supply of coating agent.

19. A device as claimed in any of claims 15 to 18, wherein the base block and/or the doctor rods is/or are provided with a bore through which a temperature conditioning medium can flow.

20. A device as claimed in any of claims 15 to 19, wherein the base block is provided with means for height adjustment.

21. A device as claimed in any of claims 1 to 20, wherein the doctor rods are wrapped with wire.

22. A device as claimed in any of claims 1 to 21, wherein the entire device or parts thereof are made from a non-corrosive material.

23. A coating device constructed substantially as described herein with reference to, and as illustrated by, Figure 1 or Figures 2 and 3 of the accompanying drawings.

24. A device for coating travelling material webs with flowable coating agents,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The drive means for the doctor rods advantageously comprises variable-speed gearboxes with reversible direction of rotation. In order to avoid any effect on the required precise rotation of the doctor rods and of the drive units, the use of elastic con .necting elements, for example couplings, cardan joints or universal joints, is preferred. The application weights of coating sub stances achievable with the described mode of operation of the device are dependent on the size of the adjusting (doctor) rods employed, in addition to many other factors. Customary coating weights of the order of magnitude of 3 to 6 g/m2 can, for example, be realised successfully with adjustment (doctor) rods of 8 to 12 mm diameter. Where larger amounts are to be applied, rods with diameters of up to 50 mm become necessary. A particular advantage of the described device according to the invention is to be seen in the manner of application of the coating agent without any moving transfer elements, for example rolls, and in the adjustment of the amount, which follows directly thereafter. This provides the preconditions for processing liquid coating substances of all kinds which are difficult to handle. WHAT WE CLAIM IS:

1. A device suitable for coating a travel ling web of material with a flowable coating agent, which device comprises an applicator trough and two channels which communi cate with the trough, one of the channels being located upstream and the other down stream of the applicator trough, each of the channels guiding a doctor rod and the width of each channel being less than the diameter of the respective doctor rod, the channels being so disposed that the doctor rods are parallel to each other and communicate with the applicator trough.

24. A device for coating travelling material webs with flowable coating agents,

which device comprises an applicator trough and, viewed in the direction of travel of the web, two parallel channels, respecitvely in 'front and behind the applicator trough, which is closed at the side walls, the channels communicating with the said trough, the side walls of each channel guiding a respective doctor rod and the width of the channels being less than the diameter of the doctor rods which project beyond the side walls of the channels.

25. A method for the continuous coating of driven material webs of paper, plastics and metal, by means of flowable coating agents, the ethe method being carried out using a device as claimed in any one of claims 1 to 24.

26. A method as claimed in claim 25 carried out substantially as described herein.