GB2162542A

GB2162542A - Electro painting apparatus; degassifying electropainting bath

Info

Publication number: GB2162542A
Application number: GB08414338A
Authority: GB
Inventors: David John Gunn
Original assignee: CARRS PAINTS Ltd
Current assignee: CARRS PAINTS Ltd
Priority date: 1984-06-05
Filing date: 1984-06-05
Publication date: 1986-02-05
Also published as: GB8414338D0

Abstract

The apparatus comprises a tank (6) which is normally filled with an aqueous dispersion of paint particles, and is provided at one end with a supplementary chamber (10), into which liquid may flow from the tank (6) over a weir (8), recirculating pumps (12) being provided to continuously pump liquid from the supplementary chamber back into the bottom of the tank(6) through sparge pipes (14), and located in the supplementary chamber (10), slightly below the level of liquid therein, is a degasification means 16, comprising e.g. a mass of woven metal or plastics mesh or discrete elements, enclosed between upper and lower apertured elements, the degasification means being such as to provide a sufficiently high surface area as to permit gas bubbles e.g. of hydrogen being produced during the electrolysis of the water to adhere thereto. <IMAGE>

Description

SPECIFICATION Improvements relating to surface coating processes Description of Invention This invention is concerned with improvements relating to surface coating processes, particularly involving the electro-deposition of a coating material (such as paint) onto a workpiece.

A convenient process for coating large workpieces (such as of metal) with a paint involves the immersion ofthe workpiece in a tank containing an aqueous dispersion ofthe paint. In one such process, known as the "Anodic" process, a positive potential is applied to the workpiece, whilst a negative charge is induced on the paint particles (known as "Micells").

To ensure homogeneityofthe paint, the aqueous dispersion is advantageously continuously circulated.

In the Anodic process, a problem which is encountered is the production atthe cathode of negativelycharged hydrogen bubbles, due to electrolysis of the water. These bubbles become attached to the workpiece, both during lowering oftheworkpiece into the liquid, and subsequently by electrical attraction between the workpiece and the bubbles, and the presence of these bubbles on the surface of the workpiece deleteriously affectthe deposition of the paint particles on the workpiece.

The effect is believed to betwo4old. Firstly, a localised reduction in the thickness ofthe paint coating is achieved due to the physical presence ofthe bubbles: this may appear as a line of reduction as a bubble slowly travels upwardlythrough the aqueous disersion towards the surface. Secondly however, such bubblestendto accumulate atthe edges ofthe workpiece, and in large numbers they can reduce the electrical potential ofthe edge portions of the workpiece, reducing the rate at which paint particles are caused to adhere to these areas.

A similar problem is encountered in the cathodic process, in which bubbles of positively-charged oxygen are produced.

This problem is not only disadvantageous in itself, but moreover tends to impose restrictions on paint formulations which may be used in these painting processes.

According to this invention, there is provided apparatus for the electro-deposition of a coating material onto a workpiece comprising a tank adapted to contain a coating liquid and into which the workpiece may be positioned, a weir over which coating liquid may flow from the tank into a supplementary chamber, and means to pump liquid from the lower part ofthe supplementary chamber into the tank, wherein there is located inthesupplementary chamber belowthe normal level of liquid therein degasification means which is characterised by providing a large surface area on which gas bubbles may accummulate.

Conveniently used is a mass of woven mesh material, the aperture size of which is in the order of imam to 10mm,convenientlyabout4mmto6mm.

Alternatively, a large of number of discrete elements of particular shape may be utilised, such as those known as"Raschig rings".

The material which is selected forthe degasification means will be one which does not on immersion into the coating liquid adopt a sufficiently high charge of a polarity which will cause particles of the coating material to be deposited thereon, but which allows bubbles to be deposited on the surface, such as by surface tension forces.

In the application of the invention to the electrodeposition of paint involving the use of an aqueous dispersion of paint particles in an Anodic process, the degasification element is conveniently metal, stainless steel or copper being particularly advantageous.

A particularly advantageous material is sold under the trade mark"KNITMESH".

It is believed that the small negative charge accumulating on such a degasification element from immersion in an aqueous mediumwilltendto repel the negatively-charged paint particles, but thatthe surface tension at the interface between the metal surface and the gas bubbles is such as to allowthe gas readily to adhere to the surface. The gas bubbles will grow, and when sufficiently large become detached from the surface of the metal and floatto the surface of the supplementary chamber.

Alternatively in the cathodic process, it is believed the gasification element of a plastics material, such as polypropylene, similarly in the form of woven mesh, or discrete elements, would be advantageous.

In this way, much ofthe gas may be removed from the coating liquid priorto its recirculation back to the tank, reducing the deleterious effect on the workpiece being coated, and allowing the use of a wider range of coating formulations.

According to this invention, there is also provided a method of removing bubbles from a coating liquid, involving passing the liquid downwardly th rough a degasification means which provides a large surface area upon which gas bubbles may adhere.

An example ofthe invention will now be described with reference to the accompanying drawings, wherein: FIGURES 1,2 and 3 are respectively side elevation, plan and end elevational views of an electro-painting apparatus; and FIGURE 4 is an enlarged view illustrating schematicallythe degasification means ofthe apparatus.

The electro-painting tank illustrated in the accompanying drawings, and its use, have been selected for the purposes of illustrating the invention byway of example only, comprises a tank6 which is normally filled with an aqueous dispersion containing 1% to 2% water miscible solvent, and 10% dispersed paint particles, to the level L1, the capacity ofthe tank being in the order of 10,000 litres.

Atone end ofthetankthere is provided a supplementary chamber 10, into which liquid may flow from the tank over a weir8, recirculating pumps 12 being provided to continuously pump liquid from the supplementarychamber back intothe bottom of the tank 6, through sparge pipes 14.

The electro-painting process which is carried out by the illustrated apparatus is specifcally an Anodic process, the aqueous dispersion having typically a pH of 7.5 to 8.5, and a conductivity of 800 to 1 000us/cm at 25"C. The circulation rate is typically eightto ten tank volumes per hour,andthetemperature is maintained between 22"C and 28"C.

The bulk ofthe liquid flowing into the supplementary chamber is surface liquid from the tank, in which many ofthe bubbles created by the electrolytic action on the water will have accumulated, and these bubbles will be taken into the supplementary chamber.

Located in the supplementary chamber, slightly below the level L2 ofthe liquid therein, is a degasification means, comprising a mass 18 ofwoven metal mesh, enclosed between upper and lowerapertured elements 20. The woven mesh is such as to provide a sufficiently high surface area, as to permit hydrogen bubbles produced during the electrolysis ofthe water to adhere thereto, grow and subsequently release themselves from the metal, and float to the surface, whilstthe mass 18 presents minimum restriction to the passage of dispersed paint particlesthrough the mass 18, for recirculation backto the tank 6.

It has been found that by the use ofthe invention above described, a significant improvement in the quality of surface coating may be obtained, with minimal inconvenience.

It has been found that in a process as described above, a pad of"KNITMESH" having a volume in the order of 0.05 to 0.5 cubic metres serves satisfactorily to remove a significant proportion of gas bubbles.

Whilstthe invention has been described above in relation to the electo-deposition of paint onto a metal workpiece, it isto be appreciated that the invention may be utilised in the electo-deposition of other materials, where similar oranalogous problems arise.

Claims

1. Apparatus for the electro-deposition of a coating material onto a workpiece, comprising a tank adapted to contain a coating liquid and into which the workpiece may be positioned, a weir over which coating liquid mayflowfrornthetank into a sup- plementary chamber, and means to pump liquid from the lower part of the supplementary chamber into the tank, wherein there is located in the supplementary chamber belowthe normal level of liquid therein degasification means which is characterised by providing a large surface area on which gas bubbles may accumulated.

2. Apparatus according to Claim 1 whereinthe degasification means comprises woven material.

3. Apparatus accordingto one of Claims 1 and 2 wherein the degasification means comprises mesh material.

4. Apparatus according to one of Claims 2 and 3 wherein the aperture size ofthe material is in the order oflmmto 10mm.

5. Apparatus according to claim 4whereinthe aperture size is about 4mm to 6mm.

6. Apparatus according to Claim 1 wherein the degasification means comprises a large number of discrete elements.

7. Apparatus according to any one ofthe preceding claims wherein the material which is selected for the degasification means is one which does not on immersion into the coating liquid adopt a sufficiently high charge of a polarity which will cause particles of the coating material to be deposited thereon, but which allows bubbles to be deposited on the surface thereon.

8. Apparatus according to anyone ofthe preceding claims, for use in the electro-deposition of paint involving the use of an aquous dispersion of paint particles in an Anodic process, wherein the degasification means is metal.

9. Apparatus according to Claim 8 wherein the degasification means comprises stainless steel.

10. Apparatus according to Claim 8 wherein the degasification means comprises copper.

11. Apparatus according to any one of claims 1 to 7, for use in the electro-deposition of paint in a cathodic process, wherein the degasification means comprises plastics material.

12. Apparatus forthe electro-deposition of a coating material onto aworkpiece, constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

13. A method of removing bubbles from a coating liquid, involving passing the liquid downwardly through a degasification means which provides a large surface area upon which gas bubbles may adhere.

14. A method of removing bubbles from a coating liquid, substantially as hereinbefore described with reference to the accompanying drawings.

15. Any novel feature or novel combination of features as hereinbefore described andlorshown in the accompanying drawings.