EP0328304A1

EP0328304A1 - A process and apparatus for coating wire mesh

Info

Publication number: EP0328304A1
Application number: EP89301022A
Authority: EP
Inventors: Peter Alan White
Original assignee: Tinsley Wire Sheffield Ltd
Current assignee: Tinsley Wire Sheffield Ltd
Priority date: 1988-02-04
Filing date: 1989-02-02
Publication date: 1989-08-16
Also published as: GB8802499D0

Abstract

The process for applying a protective coating to wire mesh comprising pretreating steel based wire mesh panels or continuous lengths for eventual use as fencing, reinforcement or other like products with a conversion coating, immersing the coated wire mesh in a bath of anti-corrosive primer coating which is suspended in water and subjected to an electric current, applying a powder coating exhibiting polar properties or characteristics and curing this over the primer coating. The invention also covers continuous plant apparatus for carrying out the process.

Description

This invention relates to a process and apparatus for applying a protective coating to wire mesh.
Various processes are known to coat wire mesh with a protective coating. However welded wire mesh panels or rolls of welded wire mesh protected with existing coatings have the disadvantage that they are subject to corrosion at the welded intersection of the mesh or at previously damaged areas where corrosion creeps beneath the protective coating.
The present invention aims to overcome this disadvantage by providing an anti-corrosion layer on the wire mesh prior to applying a further layer of coating material in order to give both excellent penetration into the weld area and high adhesion.
According to the present invention, there is provided a process for applying a protective coating to wire mesh comprising pretreating the wire mesh with a conversion coating, immersing the coated wire mesh in a bath of anti-corrosion primer coating which is suspended in water and charged, applying a powder coating exhibiting polar properties or characteristics and curing this over the primer coating.
Preferably, the conversion coating is a chromate or phosphate to enhance adhesion and wetting by the anti-corrosion primer coating.
Conveniently, the anti-corrosion primer coating is an electrophoretic deposited waterborne resin selected from one of the following: alkyds, acrylics, epoxide, esters, phenolics and polyesters.
The powder coating exhibiting polar properties or characteristics is preferably a low moisture-permeable weather-resistant coating that will crosslink and bond with the anti-corrosion primer coating when cured.
According to another aspect of the present invention there is provided a continuous plant apparatus for applying a protective coating to wire mesh using the process as set forth in the four preceding paragraphs, comprising means for transporting wire mesh into a pretreatment section including a plurality of baths for containing material to apply a conversion coating to the wire mesh, an electrocoating section including a plurality of baths for containing an anti-corrosion primer coating and then passing the pretreated wire mesh through a plant apparatus having means to apply a powder coating exhibiting polar properties or characteristics and means to cure this coating over the wire mesh pretreated with the conversion and primer coatings.
Preferably, the invention also extends to a wire mesh panel or a continuous length of wire mesh having a protective coating applied using the process according to the present invention.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic plan view of a continuous pretreatment plant for wire mesh;
Figure 2 is a cross-section taken along the line II-II of Figure 1;
Figure 3 is a diagrammatic plan view of a further continuous coating plant for wire mesh panels and continuous lengths; and
Figure 4 is an elevation of the plant shown in Figure 3.

The wire mesh used in the process is a steel based mesh for eventual use as fencing, reinforcement or other like products and is produced by weaving or by welding. Existing products of this kind are currently protected from the environment by either galvanising or powder coating or by a combination of both.
The woven mesh is produced by applying an anti-corrosion coating to the steel wire which is then woven and thereby a complete coating is applied to the mesh product. The steel wire used in the production of the mesh can either be galvanised or coated with a powder or both.
If, however, the mesh is produced by welding then the application of the anti-corrosion coating can follow a number of routes:

a) Galvanise only - The galvanised wire relies upon the sacrificial action of zinc to protect the welded area. Alternatively the galvanising can be applied after welding, but this process is considerably more expensive.
b) Power coating only - The powder coating is applied to the welded wire product either by spraying or using the fluid bed technique of dipping. In either case an extremely thick coating greater than 350 microns is required to obtain sufficient coverage so as to bridge the weld intersections and even then total penetration into the welded area cannot be guaranteed.
c) Galvanised and Powder Coat - A powder coating is applied to the welded mesh product either over pre-galvanised wire or over the galvanised welded mesh. In either case the sacrificial action of the zinc is used to protect those welds insufficiently protected by the powder coating. The galvanising in addition provides corrosion protection to those areas of the mesh where mechanical damage has occurred.

Unfortunately, because of the chemistry of the galvanising process, the adhesion of the powder coating to the galvanised surface is generally poor, resulting in a reduction of performance and life.
There are certain requirements of a good anti-corrosion coating system for welded mesh products. These are:-

a) That they give a good adhesion over cold rolled steel or galvanised wire;
b) The coating completely penetrates and continuously coats weld intersections including weld projections;
c) The coating is flexible but will withstand impact;
d) It will not be affected by U.V. (Ultra Violet) light and is available in a range of colours;
e) The coating must be carried out at a low process temperature to avoid deterioration of the galvanising;
f) When subjected to the equivalent of 20 years weathering, it will exhibit minimum corrosion at the weld intersections and any previously damaged area will show minimal creep of corrosion beneath the coating.

These requirements must fulfil a number of standard physical tests.
The process of the present invention was developed to meet all of the above referred to criteria and comprises the following steps:-

a) Surface Pretreatment - The wire mesh surface was cleaned and pretreated with a conversion coating such as chromate or phosphate to enhance adhesion and wetting by the anti-corrosion primer coating.
b) Anti-corrosion Primer Coating This coating results from the electrophoretic deposition of waterborne resins such as, alkyds, acrylics, epoxide, esters, phenolics, polyesters and produces a thin anti-corrosion coating of high adhesion which gives both total penetration into the weld area and coverage of weld projections and forms an excellent bond to the powder coating which exhibits polar properties or characteristics.
c) Power Coating Exhibiting Polar Properties or Characteristics - This coating which is applied as a top coat has excellent dielectric properties, low moisture permeability, excellent weather resistance and when applied over the anti-corrosion primer coat, will crosslink to form a flexible impact resistant three layer bonded system.

The resulting three layer system meets the above test criteria and gives a finish which exhibits no rust after a simulated 10 to 20 years life cycle.
Referring now to the drawings in detail, Figures 1 and 2 illustrate a continuous pretreatment plant comprising a loading station 1 for wire mesh in either roll form 2 or panels 3. The wire mesh is transported by a mechanical handling system 4 and dipped into a series of baths. Five rolls measuring up to 2.4 metres wide and 750mm diameter (25 or 50 metres long) can be accommodated in each bath at the same time. The panels 3 will normally measure 5 by 3.3 metres.
The transport is controlled from an electric control console 5 by an operator 6 standing on a platform 7.
The wire mesh is immersed in succession in a pretreatment section to apply a conversion coating comprising a bath 8 of alkali cleaner for five minutes at 70°C; two baths 9 and 10 of cold water rinse for periods of one minute at ambient temperature; a bath 11 of zinc phosphate for five minutes at 60°C; a bath 12 of cold water rinse for one minute at ambient temperature; a bath 13 containing chromic seal for one minute at ambient temperature; and a bath 14 containing a demineralised water rinse for one minute at ambient temperature. The pretreated wire mesh is then passed through an electrocoating section 15 to apply an anti-corrosive primer coating for a period of two minutes at 21°C and then to a post paint rinse section, comprising two baths 16 and 17 of ultrafiltrate rinse for periods of fifteen seconds at ambient temperature and finally to a bath 18 of demineralised water rinse for fifteen seconds at ambient temperature.
The wire mesh is then conveyed by an overhead conveyor 19 of the mechanical handling system 4 on a continuous path through a paint stoving oven 20 for a period of 25 minutes at 200°C. The pretreated wire mesh is then transported back to an unloading station 21 at the beginning of the continuous plant conveyor. The overhead conveyor 19 can carry 7.2 loads per hour.
Located at the rear of the operator platform 7 is an effluent treatment plant 22.
The pretreated wire mesh rolls or panels from the apparatus of Figures 1 and 2 are then coated with a powder coating exhibiting polar properties or characteristics in a further continuous coating plant illustrated in Figures 3 and 4. The pretreated wire mesh rolls 2 are mounted on a roll supporting car 23 driven by an A.C. geared motor 24 to a snubber roll 25 which pays out wire mesh under the control of a pneumatic disc brake 26. The wire mesh passes through a circular saw 27 where it may be cut to lengths as required, and through a joining press 28 to an accumulator 29, driven by a D.C. geared motor 30 and an accumulator drive A.C. torque control geared motor 31.
The continuous length of wire mesh is transported to a bridle 32 driven by a D.C. geared motor 33 and is treated in a spray booth 34 by applying a powder coating exhibiting polar properties or characteristics over the conversion coating and the anti-corrosion primer coating. Alternatively, the pretreated rolls 2 of wire mesh may be dipped into a fluidised bed containing the said powder coating. This powder coating is preferably a low moisture-permeable weather-resistant coating that will crosslink and bond with the anti-corrosion primer coating when cured.
The wire mesh with the protective coating, according to the invention, is then passed through a bridle 37 driven by a D.C. geared motor 38 and into an exit accumulator 39 driven by a D.C. geared motor 40 and an accumulator drive A.C. torque controlled geared motor 41. The wire mesh passes through a second circular saw 42 and onto a roll mandrel 43 driven by an A.C. geared motor 44.
The continuous plant illustrated in Figures 3 and 4 can also be used to apply a protective coating to panels 3 of wire mesh which have been pretreated in the plant of Figures 1 and 2, which panels are transported through the above described processing plant by an overhead monorail system 46. The panels 3 are carried from the pretreatment plant and stacked at a location 47, for uncoated panels. These panels are then carried by the overhead monorail system 46 through the spray booth 34 or the fluidised bed and are unstacked at a location 48 for the coated and cured panels.
The speed of throughput of the panels or continuous sheets of wire mesh is normally 3 metres per minute. The curing of the powder coating may be by infra-red or other suitable means, and normally takes one minute.

Claims

1. A process for applying a protective coating to wire mesh comprising pretreating the wire mesh with a conversion coating, immersing the coated wire mesh in a bath of anti-corrosion primer coating which is suspended in water and charged, applying a powder coating exhibiting polar properties or characteristics and curing this over the primer coating.

2. A process as claimed in claim 1, wherein the conversion coating is a chromate or a phosphate.

3. A process as claimed in claim 1 or claim 2, wherein the anti-corrosion primer coating is an electrophoretic deposited waterborne resin.

4. A process as claimed in claim 3, wherein the electrophoretic deposited waterborne resin is an epoxide, alkyd, acrylic, ester, phenolic or polyester resin.

5. A process as claimed in any preceding claim, wherein the powder coating is a low moisture water resistant coating that will crosslink and bond with the anti-corrosion primer coating when cured.

6. Continuous plant apparatus for carrying out the process as claimed in any preceding claim, comprising means for transporting wire mesh into a pretreatment section including a plurality of baths for containing material to apply a conversion coating to the wire mesh, an electrocoating section, including a plurality of baths for containing an anti-corrosion primer coating, and a plant apparatus having means to apply a powder coating exhibiting polar properties or characteristics and means to cure this coating over the wire mesh pretreated with the conversion and primer coatings.

7. Continuous plant apparatus as claimed in claim 6, wherein the means for applying the powder coating is a spray booth to spray the coating in powder form or a fluidised bed to apply the coating in powder form.

8. Continuous plant apparatus as claimed in claim 6 or claim 7, wherein the means to cure the powder coating is by infra-red radiation.

9. Continuous plant apparatus as claimed in any one of claims 6 to 8, wherein the wire mesh is in roll form.

10. Continuous plant apparatus as claimed in any one of claims 6 to 9, wherein the plant apparatus includes a series of motor driven roller means to transport the continuous rolls of wire mesh.

11. Continuous plant apparatus as claimed in claim 10, wherein the plant apparatus includes means for cutting the wire mesh rolls into lengths.

12. Continuous plant apparatus as claimed in claim 11 wherein the cutting means comprise at least one circular saw.

13. Continuous plant apparatus as claimed in any one of claims 6 to 8, wherein the wire mesh is in the form of panels.

14. Continuous plant apparatus as claimed in claim 13, wherein the plant apparatus includes an overhead monorail system to transport pretreated panels from a first location, through the means for applying the powder coating exhibiting polar properties or characteristics to a second unstacking location for the coated and cured panels.

15. A continuous length of wire mesh having a protective coating applied according to the process claimed in any one of claims 1 to 5.

16. A wire mesh panel having a protective coating applied according to the process claimed in any one of claims 1 to 5.