EP2114111A1

EP2114111A1 - Amorphous electrostatic discharge brush

Info

Publication number: EP2114111A1
Application number: EP08155482A
Authority: EP
Inventors: Ben Vandenbroucke; Geert Braekevelt; Wim Van Vooren
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2008-04-30
Filing date: 2008-04-30
Publication date: 2009-11-04

Abstract

An ESD brush 10 comprises one or more ESD ribbons 16 of fully or partially amorphous metallic sheet material or metallic sheet material with a nanocrystalline microstructure. This ESD brush 10 has as advantages a good resistance against breaking and a good discharge behaviour. The amorphous material together with the ribbon form allows a cheap way of manufacturing.

Description

Technical Field

The invention relates to improved electrostatic discharge (= ESD) brushes, to their uses and to their way of manufacturing.

Background Art

Static electricity generated on a product during a process, e.g. static electricity on paper during photocopying, can be dramatic for the process reliability or for the performance of the process and must be removed from the product. Other examples, where static electricity is charged by rubbing or separating of two different materials, are plastic foil processing, printing, paper and bookbinding industries, semi-conductor and office appliances such as a copier, fax, scanner and printer.
US2004074410 describes the use of an ESD brush as part of a device for removing electrostatic charges from flat materials. ESD brushes are a self-discharge type of passive static dissipation. When the product passes through/under the brush, the charges are drawn from the product by contact or by non-contact, thus dissipating the charges through the machine frame to the ground via the electrodes of the brush.
ESD fibres or very thin wires or filaments such as micro-wires are commonly used in ESD brush applications. ESD fibres come in various compositions such as e.g. carbon, stainless steel, acrylic coated with Cu, carbon or with a metal. A disadvantage of fibres is that the fibre ends are so fine that they scratch the material to be discharged when coming into contact with it. This is especially the case in e.g. the plastic foil processing industry. Another disadvantage is that the fingers of the operator can get stabbed by the very fine tips of the fibres that act like needles. Moreover, fibres are easily bent or broken, causing decreasing discharge capacity over time.
US4352143 claims a device for discharging static electricity where bundles of stainless steel fibres are used as 'electrodes'. JP2005285684 describes a static removal brush with a focusing wire with several metal fine wires.

Disclosure of Invention

It is an object of the invention to avoid the disadvantages of the prior art.
It is a further object of the invention to provide ESD brushes particularly but not exclusively suitable for contact discharge applications that are cost-effective and have excellent product performance.
It is yet a further object to provide ESD brushes that do not damage the product to be discharged.
The optimal solution to this technical problem is to provide ESD ribbon brushes of amorphous metal.
An amorphous metal is a metallic material with a disordered atomic-scale structure. Amorphous metals are commonly referred to as "metallic glasses" or "glassy metals". There are several ways in which amorphous metals can be produced. These ways include physical vapor deposition, solid-state reaction, ion irradiation, melt spinning, and mechanical alloying. Amorphous metals produced by these techniques are, strictly speaking, not glasses; however, amorphous alloys are commonly considered to be a single class of materials, regardless of how they are prepared.
In 1976, H. Liebermann and C. Graham developed a new method of manufacturing thin ribbons of amorphous metal on a super cooled fast-spinning wheel. This was an alloy of iron, nickel, phosphorus and boron. The material, known as Metglas used for low-loss power distribution transformers (Amorphous metal transformer).
Amorphous metal is an alloy rather than a pure metal. Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys.
Two aspects that are very advantageous for the ESD brushes of the present invention are the high strength and the resistance against breaking.
Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys. The fact that metallic glasses are not ductile at room temperature is not an issue, since the ESD brushes are not loaded under tension.
Amorphous fibres have a high tensile strength and are, as such, very advantageous for use in ESD brushes. However, the production of amorphous fibres is very difficult and the fibres are very expensive.
It has now been found that amorphous material in the form of thin sheet ribbons, which are much cheaper than the monofilament fibres mentioned above, have the overall advantage of not breaking and not damaging the material to be discharged.
The term "ribbon" refers to an elongated element with a rectangular-like or elliptic cross-section. The width of this cross-section is several times, i.e. at least five to ten times, the height or thickness of the cross-section. The relatively small thickness allows together with the amorphous material for flexibility and a high level of repeated bending before fracture. The relatively great width allows good contact with the product to be discharged.
The amorphous thin sheet ribbons can be obtained in mainly two ways. A first way involves obtaining continuous ribbons out of the melt. In a later production phase (see further in this description, the long continuous ribbon is cut into ribbons of smaller length for use in an ESD brush. A second way involves obtaining the thin sheet material in the form of small individual non-continuous pieces of sheet material. Sometimes these non-continuous pieces are also referred to as "fibres", although they are actually thin sheets of material. The term 'ribbon' both refers to the continuous thin sheet material as to the non-continuous thin sheet material.
If the amorphous ribbons are made out of the melt, the amorphous and nanocrystalline ribbons of the present invention are prepared by melt spinning, whereby metal is rapidly solidified by rapid cooling, resulting in very thin ribbons or metallic glass ribbons. Fully amorphous alloys are obtained after melt-spinning at higher speeds (>15 m/s). Ribbons melt-spun at lower speeds consisted of a mixture of amorphous and crystalline metal.
A high corrosion resistance may be obtained by an alloy of Co-Fe-Cr-Br.
The production of amorphous ribbons, and brushes comprising these, is much cheaper than amorphous fibres.
The ESD brushes of the present invention, comprising the above prepared amorphous ribbons, not only have enhanced mechanical but have also enhanced electrical properties.
The electrical properties of these alloys are very suitable for use in ESD brushes. The discharge capacity increases with increasing atomic content. The nanocrystalline alloy shows the lowest discharge capacity compared to the amorphous alloys. When used in ESD brushes, amorphous material has the advantage over carbon, or other known materials, that there is less to no breakage.
According to a first aspect of the invention, there is provided an ESD brush for removing electric charges from flat materials comprising fully amorphous metallic material, partially amorphous metallic material or metallic material with a nanocrystalline microstructure.
In a preferred embodiment, the ESD brush comprises one or more ESD ribbons of fully amorphous metallic sheet material, partially amorphous metallic sheet material or metallic sheet material with a nanocrystalline microstructure.
Within the context of the present invention, the terms 'fully amorphous metallic material' refer to a metallic material lacking any crystalline structure, or to a material with a hardly detectable crystallinity (crystal domain size < 5 nm).
The terms 'partially amorphous metallic material' refer to metallic material with only here and there a crystalline structure, the majority of the metallic material remaining amorphous.
The terms 'nanocrystalline structure' refer to a structure where the crystal domains have a maximum dimension of 100 nm.
In the context of this invention, the term 'amorphous' refers to fully amorphous metallic material, partially amorphous metallic material or to metallic material with a nanocrystalline structure.
As a matter of example only, amorphous metals with following compositions have been tested by the inventors: an alloy of 88,1 Fe 11,9 Si; an alloy of 48,6 Fe 39,5 Ni 10,1 Si 1,8 Cr; and an alloy of 63 Ni 13 Cr 12 B 8 Si 4 Fe (all weight percentages).
In one embodiment the ribbons have a width smaller than 2000 µm, and preferably ranging from 100 µm to 1500 µm.
In another embodiment the ribbons have a thickness smaller than 100 µm, and preferably ranging from 15 µm to 50 µm.
The ribbons are held in place e.g. by clamping in a metal bracket or by fixing to one or more strips, foils or tape by an adhesive.
In one embodiment said strips, foils or tape are conductive. In another embodiment they are non-conductive. The adhesive may be conductive or non-conductive. The conductive strips or foil may comprise aluminium or copper or may also be made of an amorphous alloy. Preferably the adhesive is conductive and the foil is copper. An alternative may be a metal wire, such as a copper wire which is enclosed by the adhesive tape.
Use.
According to a second aspect of the invention, the ESD brush is used in a contact discharge system, such as e.g. in a printing, photocopying, faxing, scanning and bookbinding/paper industry of plastic foil industry.
In another embodiment the ESD brush is used for a non-contact discharge system, such as e.g. in the printing, photocopying, faxing, scanning bookbinding/paper or plastic foil industry.
The ESD brush according to the invention may comprise means for holding or installing, such as e.g. a hole at each end of the strip to connect to the device comprising the flat material to be discharged.
According to a third aspect of the present invention, there is provided a method of manufacturing an ESD brush, comprising the steps of

(a) applying one or more conductive elongated elements on a drum surface in the direction of the drum axis;
(b) applying a continuous ribbon helically with small pitch on the drum,
(c) connecting said elongated elements to the ribbon; and
(d) cutting parallel to the one or more elongated elements to provide the ESD brush of cut ribbons attached to a conductive elongated element.

In an alternative way of manufacturing the ESD brush, the method comprises the steps of:

(a) providing one or more parallel conductive elongated elements;
(b) applying a ribbon to the one or more parallel conductive elongated elements;
(c) connecting the elongated elements to the ribbon;
(d) cutting parallel to the ribbons to provide the ESD brush of cut ribbons attached to a conductive elongated element.

In yet another way of manufacturing an ESD brush according to the invention, the ESD brush is made out of one single piece of amorphous sheet metal. A wide strip of amorphous sheet metal is provided as starting material. This wide strip is cut one side into smaller ribbons. This way of manufacturing is cost-effective and allows fast production. No extra conductive strip or adhesive is needed. Moreover, the conductive elongated element or strip is amorphous thus adding strength, elasticity and discharge properties to the ESD brush.
The wide strip of amorphous sheet metal may be cut at regular intervals so as to obtain a ribbon in between each cut.
Alternatively, intermediate ribbons can be left out in between other ribbons.
Still another way is to cut out a gap out of the wide strip so as to obtain two ribbons at a predetermined gapped distance from each other.
Depending on the application of use, the ESD brush of the present invention is suitable for contact or non-contact type ESD brushes. For example, in plastic foil production, the ESD brush may not contact the plastic foil to avoid scratching of the electrode against the plastic foil. A distance of 3 to 5 mm is recommended between the ESD brush and the plastic foil to be discharged.
However, for the paper industry, such as e.g. in fax, photocopiers and printers, a contact type ESD brush may be provided.
In a specific embodiment, an ESD brush is provided optimising both contact and non-contact types. The ESD brush comprises ribbons of different length. As such there is less opportunity for scratching due to less contact points, yet the discharging ability is the same or even better. In one embodiment there is provided an ESD brush having one or more ribbons that make contact with the surface of the flat material to be discharged, and one or more ribbons that are placed for non-contact discharge, i.e. at 3-5 mm from the flat surface. The few ribbons contacting the flat surface optimise discharging, while minimising the risk of scratches. The non-contact ribbons further optimise discharging capacity over the entire length of the ESD brush. The percentage ratio contact/non-contact ribbons may be 50-50, 40-60, 30-70, 20-80, 10-90.
The amorphous ribbon ESD brushes of the present invention have the following advantages: they are strong (no breakage, straightness), they are elastic (no deformation/high resilience), they are soft, they are anti-scratch (due to the flat ribbon tip), they have a good discharge capacity, they have a high conductivity, they are corrosion-resistant and they are cost-effective.

Brief Description of Figures in the Drawings

Figure 1 shows a cross-section of an ESD brush according to the invention.
Figure 2 shows a method of manufacturing an ESD brush according to the invention.
Figure 3a shows a picture in front and Figure 3b shows a back view of an ESD brush according to the invention.
Figure 4 shows an ESD brush with ribbons of different length.

Mode(s) for Carrying Out the Invention

Figure 1 illustrates an embodiment of an ESD brush 10. Aluminium strips 12, 14 are fixed by means of an adhesive 15 to ribbons 16. At one side a paper strip 17 is attached to the aluminium strip 14 by means of a glue 18. By removing the paper strip 17, the entire ESD brush 10 can be glued to the frame of e.g. an electrical appliance.
The length of the ribbons and the brush is dependent on the application. An ESD brush for photocopying e.g. has a typical ribbon length of 15 mm and a brush length of 30 cm.
Example 1: Manufacture of ESD brushes comprising amorphous ribbons
ESD brushes as shown in Figure 1 may be manufactured according to Figure 2. Al-strips 12 with a non-conductive adhesive 15 are applied on a drum surface 20 in the direction of the drum axis 22. Continuous ribbons 16 are wound helically with a small pitch onto the Al-strips 12 along the rotating circumference of the drum. Second non-conductive adhesive Al-strips 14 are applied onto the ribbons 16 (not shown). Finally the brushes obtained in this manner are cut from the drum surface
In an alternative embodiment additional paper strips 17 can be applied, e.g. by means of a glue 18, onto the Al-strips 14.
Typically the ESD brush comprised ribbons having a width of 1 mm and a thickness 25 µm. The length of the brush was 25 mm.
Figure 3a and Figure 3b show a picture of an embodiment of an ESD brush 30 according to the invention.
Figure 4 shows an alternative embodiment of an ESD brush 40 according to the invention. The ESD brush 40 comprises ribbons 16' of relatively long length for contact discharge alternated with ribbons 16" of relatively short length for non-contact discharge.
Example 2: ESD brushes comprising amorphous ribbons

Table 1 lists the different brush types that were manufactured and tested having different distance between subsequent ribbons and different length of the ribbons.

Table 1

Brush Id	strip or foil	adhesive	ribbon free length	distance between ribbons
1.1	Copper	Conductive	1,5 cm	0,7 mm
1.2	Copper	Conductive	1,5 cm	0,7 mm
2.1	Copper	Conductive	1,5 cm	1 mm
2.2	Copper	Conductive	1,5 cm	1 mm
3.1	Aluminium	non-conductive	1,5 cm	1 mm
4.1	aluminium	non-conductive	1 cm	1 mm

Example 3: Testing discharge behaviour of ESD brushes
The discharge behaviour of each brush was measured for five calibrated input paper voltages on two positions along the width of the paper (one third left and one third right). The average of the two positions was calculated. An electrostatic charge was applied on the paper by the corona effect using a microwire. The speed of the paper was 6 m/min. The electrostatic charge on the paper was measured by a ground capacity sensor. The Al-strip making direct contact with the ribbons was grounded.

Table 2 represents the test results. The foil material, the adhesive conductivity, the ribbon free length and the distance between the ribbons do not influence the discharge behaviour. The discharge behaviour of the ribbon ESD brushes is very good since the "standard" requires a discharge from above 5 kV to below 1 kV. All ribbon ESD brushes lead to a discharge from 5,2 kV to 0,6 - 0,65 kV.

Table 2

Before brush	Average after brush
Before brush	Brush 1.1	Brush 1.2	Brush 2.1	Brush 2.2	Brush 3.1	Brush 4.1
(kV)	(kV)	(kV)	(kV)	(kV)	(kV)	(kV)
5,2	0,65	0,65	0,63	0,65	0,63	0,6
6,6	0,85	0,83	0,9	0,9	0,9	0,85
7,6	1,05	1,05	1,08	1,1	1,05	1
8,6	1,25	1,25	1,25	1,25	1,18	1,18
9,6	1,35	1,35	1,4	1,38	1,35	1,35

Example 4: Mechanical Properties of ESD Brushes
Fatigue tests have been performed whereby after 400.000 hits no breakage nor deformation was detected on the amorphous ribbons.

Claims

An electro-static discharge (ESD) brush comprising fully amorphous metallic material, partially amorphous metallic material or metallic material with a nanocrystalline microstructure.
An ESD brush according to claim 1, comprising one or more ESD ribbons, said ribbons comprising fully amorphous metallic sheet material, partially amorphous metallic sheet material or metallic sheet material with a nanocrystalline microstructure.
An ESD brush according to claim 2, wherein said ribbons have a width smaller than 2000 µm, and preferably ranging from 50 µm to 1500 µm.
An ESD brush according to claim 2 or 3, wherein said ribbons have a thickness smaller than 100 µm, and preferably ranging from 10 µm to 50 µm.
An ESD brush according to any of claims 2 to 4, wherein said ribbons are held in place by clamping in a metal bracket.
An ESD brush according to any of claims 2 to 4, wherein said ribbons are held in place by fixing to one or more strips or foils or tape an adhesive.
Use of an ESD brush according to any of the previous claims, in a contact discharge system.
Use according to claim 7, wherein said contact discharge system is comprised within a printer, copier, fax, scanner and/or bookbinder/paper brush.
Use of an ESD brush according to any of claims 1 to 6, in a non-contact discharge system.
Use according to claim 9, wherein said non-contact discharge system is comprised within a plastic foil production machine.
ESD brush according to any of the previous claims, further comprising means for holding or installing, such as e.g. a hole at each end of the strip.
Method of manufacturing an ESD brush according to any of claims 2 to 6, said method comprising the steps of
(a) applying one or more conductive elongated elements on a surface of a drum in the direction of the drum axis;

(b) applying a continuous ribbon helically with a small pitch on said drum when rotating;

(c) connecting said elongated elements to said ribbon; and (d) cutting parallel to said one or more elongated elements to provide said ESD brush of cut strips attached to a conductive elongated element.
Method of manufacturing an ESD brush according to any of claims 2 to 6, said method comprising the steps of
a) providing one or more parallel conductive elongated elements;

b) applying a ribbon to said one or more parallel conductive elongated elements;

c) connecting said elongated elements to said ribbon;

d) cutting parallel to said ribbons to provide said ESD brush of cut strips attached to a conductive elongated element.