DE2511395A1 - PROCESS FOR THE PRODUCTION OF A FINE GRAIN POWDER, IN PARTICULAR FOR THE ELECTROSTATIC COATING OF THE SURFACE OF AN OBJECT FROM CONDUCTIVE MATERIAL - Google Patents

Description

Dr. W. P. Radt

Dipl.-Ing. E. E. Finkener / Edward Ourran

Dipl.-Ing. W. Ernesti / Engineering Limited

Patent attorney website j Cardiff / Great Britain

463 Bochum

Heinridi-König-Strasse 12 Telephone 415 50, 4 23 27 Telegram address: Radtpatent Bochi ^ n

75 120
EEF / EP

Process for the production of a fine-grained powder, in particular for the electrostatic coating of the surface an object made of conductive material

The invention relates to a method of manufacture a fine-grained powder suitable for electrostatic coating of the surface of metallic objects is.

It is known that charged particles which are exposed to an electric field, in the field to an electrode or Surface of opposite polarity migrate; this effect is widespread in electrostatic coating exploited with powdery substances. If the particles of the powdery substance have low conductivity, i.e. have a high specific (ohmic) resistance, their charge is retained for a longer period of time and the particles form a surface layer on the object to be coated; at the same time a considerable one occurs Adhesion and a repulsion effect due to the thickness of the layer on the surface of the to be coated Object is automatically limited to a constant value. On the other hand, if the particles have high conductivity or have a low resistance, they lose their charge very quickly by dissipating to the surface, whereby the adhesion is lost and the particles as a result of the electrical forces acting on them " bounce off the surface. With particles whose conductivity is moderately high and which are hereinafter referred to as

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ORIGINAL INSPECTED

2b 1 1395

are referred to as "semiconducting", the charge on the surface is lost to an intermediate extent; In general, one can assume ₉ that the .Effekt is similar to that of conductive particles and the powder layer deposited on the surface does not adhere well enough, at least not for a sufficiently long time during the handling of the material during manufacture; the thickness of the layer is also not limited by itself «,

Those related to adhesion or adhesion In reality, problems are much more complicated than what the above suggests. Various forces work together, not all of which have been clearly recognized to this day, including the "Van der Waal's see forces", the meniscus forces, caused by moisture absorbed in the surface layers of the particles, the So-called electrical double-shift forces in the contact area between individual particles, soxcLe the electrostatic Forces between the loaded on particles and his reflection behind the metal layer. The adhesive force depends practically, among other factors, on the particle size, the strength of the electric field, as well as the applied voltage and current strength a.b.

In the context of the present invention, the term "semiconducting" used for substances with a resistance between 10 ° and 10 ohm meters Values, the material should be viewed here as non-conductive and, at higher values, as a real conductor. the Conductivity of the material designated as semiconducting

-7-10
lies between 10 ' and 10 S / cm.

The application of powdery layers of fabrics with low conductivity there are few problems.

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-D-

The non-conductive particles are charged and retained quickly their charge when in contact with the conductive surface to be coated. Considerable Difficulties arose when using semiconducting materials and conductive powdery substances to form a well-adhering powder layer, as mentioned above, with more uniform To obtain layer thickness, there are different Proposals have been made to overcome the difficulties, particularly the use of conductive metal powders; However, the known proposals are not applicable for semiconducting powders, e.g. ceramic or Glass enamel powder.

The object on which the invention is based is to the known method for the electrostatic coating of an object with a powder of a semiconducting Improve material; the invention also relates to a powdery material suitable for carrying out such processes Material with which the problems described can be at least partially solved.

In a method for producing a powder, in particular to find use in an electrostatic coating process, the invention consists in that small 'particles are coated from a semiconducting material or that their surface is treated to the to increase the specific resistance.

The invention also relates to the use of a powder produced by this method for application a layer of fine particles on a conductive surface in which the powdery particles are charged and are subjected to the action of an electric field, whereby the particles are moved towards the conductive surface and form an adhesive layer thereon.

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The invention is particularly applicable to powders, the major portion of which has a particle size not exceeding 200 ^ w. For certain coating processes, the particle size can be between 5 and 25 μ- . Powdered glass masses (frits), as used for glass enamels, have a suitable conductivity which lies within the semiconducting range defined above. It is therefore extremely difficult to obtain an adherent powder layer without using liquid binders, which complicate the subsequent firing during enamelling. The method of the present invention enables a dry powder coating that has excellent adhesion and a uniform thickness typical of powders of non-conductive material.

The surface treatment or coating of the individual particles can be done by various means and with different Substances. It can be a physical coating or a chemical reaction act received layer. In all cases, the layer or coating of the surface should preferably not be hygroscopic, as the absorption of moisture significantly affects the resistance of the surface of the particles can.

In a preferred embodiment of the invention, the surface of the particles is initially hydrophilic and is through Removal of surface groups, e.g. hydroxyl groups, by chemical means and their replacement by hydrophobic ones Modified material. A preferred embodiment of the invention provides that the surface layer consists of a cross-linked silicone.

The surface layer or the treated layer is preferably such that the components burn out, burn

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steam or otherwise removed or removed from the surface be separated or form a chemical bond with other existing substances when the powder layer is then subjected to a thermal melting or ßrennprocess or a chemical treatment. at In many areas of application, e.g. in glass enamelling, it is also important that the combustion products do not affect the properties of the glass enamel.

According to a particularly preferred embodiment of the invention, the powder contains fine-grained particles of a Enamel material, e.g. an alkali silicate, whereby the surfaces of the individual particles increase the resistance are treated with an organohalosilane or an organic substance that hydrolyzes on the surface and create a water-repellent effect. The invention also provides that the surface of the particles with a Organochlorosilane, in particular an ürganotrichlorosilane, is treated.

Embodiments of the invention are illustrated with the aid of the drawing described. Show it:

Figure 1 is a schematic longitudinal section through a device for treating powders,

characters

2 to 5 the assumed chemical structure of the particle surface in successive stages of treatment,

FIG. 6 shows a schematic representation of the suspected molecular structure of the surface layer a glass particle and the molecular changes that occur in another embodiment the invention,

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• - 6 -

FIG. 7 the final molecular structure of the surface in a schematic representation,

Figure 8 is a graph showing the surface resistance of powders that have been treated with various substances and

FIG. 9 shows a schematic longitudinal section through a device for carrying out the electrostatic Coating process.

The first embodiment shows the application of the invention in the electrostatic application of a layer from a powdered glass mass (frit) onto the surface a metallic object, e.g. a steel bath or a cooking utensil, which are glass-enamelled target. The first stage of the process comprises the treatment of the pulverulent glass mass to obtain a surface with increased resistance, the next stage is the firing of the metallic material provided with the adhesive powder layer Object in the usual WSise for the production of the glass-enamelled Surface.

The pulverulent glass mass is produced in three stages called pre-treatment, encapsulation and post-treatment.

During the pretreatment, the powdery glass mass is used first carefully dried, for example in a dry, warm air stream. To this end, a Fluidized bed can be used. Figure 1 shows a device which consists of a chamber 10 with a porous bottom 11, to which the powder 12 is applied and which contains the means to circulate the heated, dry air through the Introduce line 13 below the bottom 11, upwards

to flow through the particles and withdraw at the upper portion of the chamber through conduit 14 ·.

The glass mass can have different chemical structures; The example described is an alkali borosilicate which contains, for example, sodium, potassium or lithium in addition to boron, as well as other constituents such as fluorine, cobalt, nickel and manganese present in smaller quantities. The particle size is preferably between 5 ρ and 25 μ, with about 25% of the particles being larger than 2J μ and 25% being smaller than 13 μ. After drying

11, the ohmic resistance of the powder is around 5 x 10 Ohmmeter.

In the next stage of the process, encapsulation, the powdery material in a fluidized bed with a dry air flow, which is passed through the bed, moves. A fluidized bed of the usual type can be used, as in of the previous process stage can be used. The the Chamber 10 containing the fluidized bed is provided with a mechanical rotary agitator 15 which is located above the porous bottom 11 is arranged. During the movement, dimethyldichlorosilane becomes (DDS) introduced in vapor phase into the air stream through line 16 and comes with the flowing powder in contact. The required amount of DDS is about 0.1 percent by weight, based on the total weight of the powdered glass material; the amount can be less for certain particle sizes. The DDS reacts with the surface of the Particles, as described in detail below; the treatment continues until the DDS is in the air stream can be detected, which is withdrawn from the chamber above the fluidized bed through line 14-. At this stage it can be assumed that the reaction is practically complete. Measurements have shown

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that the resistance of the powder is quite considerable to 3 χ 10 Ohmmeter was improved. However, the effect is not yet stable because the surface layers of the 'feilchen are still are hygroscopic and the resistance decreases very quickly when moisture is present.

In the third process stage, the aftertreatment, the powder is left in the presence of steam for about three hours long or less, for example heated to a temperature of about 215 ° C. This can be done in the same fluidized bed take place, this also for the preceding process steps was used. For this purpose the chamber 10 has one Inlet pipe 17 for the water vapor with a valve. at This treatment causes a chemical change in the surface of the powdery particles, whereby the resistance

15 stood the surface area up to a value of about 2 χ 10 Ohmmeter is improved. The treatment also leads to a stable, non-hygroscopic surface, provided of course, that the powdery material is not exposed to excessive moisture.

By way of comparison, it should be noted that the resistance

12 the surface area of nylon is on the order of 10 ohm meters and that of epoxy resins is on the order of 10 ^J ohm meters. This shows that the surface treatment according to the present invention leads to a glass powder which is comparable in terms of resistance to these practically non-conductive plastics.

It has been found that with a surface resistance,

12th

which is greater than 10 ohmmeters, the powder can be easily sprayed on, with the strength of the depressed Layer limited by itself. This effect is common to many surface coatings, especially when Glass enamelling, an advantage, since an excessively strong powder

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layer also leads to an undesirably thick glass enamel. The encapsulation also improves the flow behavior and the handling of the powder.

The chemical 'reactions that take place in surface conditioning ' stratification of the particles can be explained as follows:

In the schematic representation in FIG. 2, the surface of the glass mass is denoted by X, while the molecular Structure is represented by a chain made up of borosilicon and aluminum atoms with chained oxygen bonds consists. On the atoms near the surface there are hydroxyl groups (OH groups), the can form as a result of the oxygen taking up hydrogen from the moisture contained in the atmosphere. Normally, these hydroxyl groups are likely to lead to the low surface resistance of the original powdered glass material.

If vaporous DDS during the encapsulation stage is initiated, one of the chlorine atoms of the DDS is believed to combine with the hydrogen of a hydroxyl group and a condensation reaction occurs in which HGl is released as G-as. Between the oxygen atom and A chemical bond occurs with the silicon, in which a SiCl bond is retained, as shown in FIG. 3 is shown schematically.

During the aftertreatment, the moisture contained in the atmosphere reacts with the free SiCl bonds of two neighboring molecules and hydrogen chloride is released, as is shown schematically in Figure 4-. Through this a Si-0-Si chain is formed in the form of a silicone group, as shown in Figure ρ.

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This creates an impermeable molecular layer. It is important that the CH, groups burn out, when the glass enamel is fired in the presence of air at a temperature below the melting temperature the glass mass lies. The enamel is therefore not destroyed. The CH ^ -G groups react with oxygen, so that CGp and KpO are formed and SiOp remains as a residue. The composition of the enamel is not significantly changed by the small amount of additional SiOp.

In the second embodiment of the invention, the powder is produced as in the first example in three stages, namely pre-treatment, encapsulation and post-treatment.

The pre-treatment is the same by adding the powdery Glass mass is carefully dried in a fluid bed. The subsequent encapsulation is also similar; the dried glass powder is moved in the same fluidized bed with a stream of dry air and a Organohalosilane introduced into the air stream in vapor form, that with the flowing powder in contact and reacts with the surface of the particles.

In this second embodiment of the invention takes place treatment with vaporous trichlorosilane, preferably trichloromethylsilane

Cl Cl

Si
Cl CH,

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The three chlorine groups react as shown in Figure 6 results, with the surface of the powdered glass mass. At first they are attached to the atoms on the surface of the particles the above hydroxyl groups; when reacting with the steam, it is believed that one of the chlorine atony is dissolved with the hydrogen of a hydroxyl group and a chemical condensation occurs, in which HCl as G-as becomes free. This leaves two chloride groups and one Diethyl group unchanged.

-In the aftertreatment, the powder is in the presence of Steam heated in the same fluidized bed as described in connection with the first example. Han assumes that at this stage of the process moisture present in the atmosphere interferes with the free chloride r groups of two neighboring silane molecules reacts, whereby HGl is released again and a Si-ü-oi chain is formed. As can be seen from FIGS. 6 and 7, this chain is continuous and uninterrupted. On the actual In the area of the individual particles, there is thus a three-dimensional double cross-linking, the one very represents an effective continuous surface layer which is practically impermeable to water. At the final Firing To produce the glass enamel, the CH., Groups burn below the melting temperature of the glass mass and the enamel will not be contaminated.

The o "coff, which is used for steaming the powder can be of various kinds. As already mentioned, trichlorosilane is particularly suitable, as a result the networking effect. Other possibilities for substituting the organic component are paraffins and aromatic series, e.g. trichloroethylsilane, trichlorooutylsilane and the equivalent vinyl and phenyl compounds.

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Dichlorosilane mentioned in the first embodiment can also be used, although probably not to quite the same extent. Other possible forms of this substance are dichlorodiethylsilane or the equivalent compounds of butyl, vinyl or phenyl. Dichloroethylvinylsilanes can also be used or other modifications can be used.

There may be other halogens instead of the chlorine in the Compound are substituted, especially bromine; Fluorine and iodine are probably eliminated for reasons of cost. A typical bromine containing compound would be tribromomethylsilane.

Silane (SiH ^) is usually a monosilane; it can but also disilanes can be used. Instead of silicon it is theoretically possible to use other elements, to form the basis of the bond, for example carbon or germanium. For various reasons, including Due to the high cost and contamination of the enamel, these elements cannot be compared with the silane. A typical marriage Carbon containing compound would be trichloromethyl methane.

Another possibility is to coat the surfaces of the powdery glass mass with a purely organic To coat a layer that creates a hydrolyzing effect on the surface and is impermeable to water or repellent layer.

It is possible to use acetyl chloride and benzoyl chloride or their anhydrides. Another possibility consists in coating the particles with a layer of nylon or polytetrafluoroethylene (PTFE). Included However, physical difficulties can arise

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Applying such layers result. Preferably should the substance must always be selected in such a way that it is in vapor form can be applied in the fluidized bed. In the case of using PTFE, the only one seems tangible Possibility of being the substance in the form of a colloidal suspension of tiny particles in a liquid apply, which is then evaporated and leaves a layer on the glass particles.

Figure 8 is a graph showing the measured surface resistivity of the glass particles when they are different Treatments are exposed based on the time during which the '.particle air with a are exposed to relative humidity of 60%.

Curve A shows the treatment of a powdery one Glass mass with 'urichloromethylsilane, curve B relates to the treatment with dichlorodimethylsilane and curve C on treatment with chlorotrimethylsilane. It is to be noted that curve A shows a real increase in resistance over a considerable period of time which is a considerable advantage in practice, since the treated powder, before it is used, is often exposed to a humid atmosphere for some time. A treatment with the equivalent dichloro compound gives sufficient resistance, if not quite to the same extent, and the improvement lasts not during the same period of time. But there is no drop in resistance, as is the case with the Curve C is the F_ll.

In the exemplary embodiments described, the encapsulation is carried out in a fluidized bed. It is also possible, and in some cases advantageous, this treatment in a ball mill during the final stages of the normal size reduction. In this way

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agglomeration is avoided. The following procedural steps In this case, you can also do it in the ball mill or you can transfer the material to a fluidized bed to carry out the final treatments.

When sprayed electrostatically with a conventional electrostatic spray gun or applied by means of a device with a fluidized bed, as described in connection with FIG. 9, the powder produced produces excellent adhesive layers, the thickness of which is self-regulating. A glass enamel layer produced in this way has a thickness of approximately 0.06 to 0.075 nm. It's practically the ideal thickness. The normal firing process usually takes place at a temperature of around ψ

instead of.

approx. 780 to 860 ° G for a period of approx. 4 minutes.

An apparatus for electrostatic coating is shown in FIG. It consists of a hollow one box-shaped housing 30 made of insulating rigid plastic, the lower portion of which is used as a ventilation inlet chamber 31 is formed with an inlet line 32. About that there is a plate 33 made of porous material, which consists for example of porous polyethylene plastic. The pretreated powder is poured in above the plate 33 via a chute 34. Located above the plate an electrode grid 35, cLas with a high voltage squelIe 36 is connected. The object 37 to be coated with the powder, for example a steel pan, becomes above the electrode 35 with the help of a magnetic Holding device 38 held, the cable 39, on which the device is suspended, contains a conductor which serves to ground the object 37.

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The ule grid consists of a large number from K-orona points 4-5, which are connected by soldering or brazing are attached to an open grid of intersecting metal bars. That way one becomes practical uniform electrical .space charge is generated, which allows the free passage of the! partial cloud between the corona points enables.

The invention is also applicable to the treatment of semiconducting powders with the purpose of improving their flow behavior or their handling, or if one does not hygroscopic surface should be created, in particular for jetted beds or devices for treatment of bulk material.

Claims

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Claims (11)

- 16 claims 1. Process for the production of a Fulver, in particular for use in an electrostatic coating process, characterized in that that small particles of a semiconducting material is coated or their surface is treated to the to increase the specific resistance. 2. The method according to claim 1, characterized in that the particle size does not exceed 200 JLL. 3. The method according to claims 1 and 2, characterized in that that the particles consist of a glass enamel material, e.g. a glass mass (frit). 4. Process according to Claims 1 to 3> characterized in that the surface layer or the coating of the particles is not hygroscopic. 5. Process according to Claims 1 to 4, characterized in that the surface of the particles is initially hydrophilic and by removing hydroxyl groups on the surface and replacing them with hydrophobic ones Material is modified. 6. The method according to claims 1 to 5 *, characterized in that that the surface layer of the individual particles contains a cross-linked silicone material. 7. The method according to claims 1 to 6, characterized in that the surface layer or the treated Layer of the particles is so composed that the constituents burn off, evaporate or in some other way be removed or form a chemical bond when the powder layer undergoes a subsequent thermal Is subjected to melting or firing treatment. 609848/0703 8. The method according to claims 1 to 7, characterized in that the powder particles with a treatment exposed to an organohalosilane. 9. The method according to claim 8, characterized in that that the particles are treated with an organochlorosilane ". 10. The method according to claim 9, characterized in that the particles are treated with an organotrichlorosilane will. 11. Use of a powder prepared by the method according to claims 1 to 10 for applying a Layer of fine particles on a conductive surface in which the powder is charged and the action of an electrical Field, whereby the particles are moved towards the conductive surface and onto it form an adhesive layer. 509848/0709