Apparatuses and methods for studying material properties

The present invention relates to the determining of the electrical properties of a base material, particularly paper. It has areas of application in, among other things, paper technology, particularly in the printing industry. In particular, the invention relates to a device and method for detecting the electrical surface potential or charge of a base material. In such a device, there are means for altering the surface potential of the base material and means for measuring the surface potential of the base material. The invention also relates to a new measuring device, which is suitable for use in the device according to the invention, and to a new use.

In electro-photography, the colour agent, i.e. toner, is transferred to the paper primarily by exploiting an electric field that is induced through the paper. Due to the electric field, the charged toner particles move to the paper. However, as a non-homogeneous material the paper alters an electric field that runs through it. This affects, among other things, the spreading of the toner on the paper and in turn the print quality. Besides digital printing process, in traditional printing methods too, and generally in paper processing, there are several areas of application, in which the electrical properties of the base material are exploited, or in which the electrical properties of the base material affect the processes used.

Some commercial measuring devices built for research use exist for the analysis of the properties changing the electric field of paper. In the known devices, the surface of the paper can be charged and, after a set delay, the surface potential, or the time taken for the charge to discharge, among other properties, can be measured. Such a measurement method is subject to a delay, and thus imprecise for use, for example, in on-line printing processes. Another kind of method with a delay is disclosed in JP publication 01314977. In it, the surface of a laminated paper is charged with a corona charger, after which a surface potentiometer is used to scan a potential map of the surface in two dimensions. This method is only suitable for surface-laminated papers, or plastic films.

WO publication 00/68758 discloses an automated system for measuring the electrical properties of transfer rollers or paper used in electro-photography. With its aid, the discharge of the charges of the paper can be investigated by charging the paper using a

corona charger, after which a sensor not in contact with the surface can be used to measure the residual charge on the surface of the paper.

US publication 4244562 discloses a method for measuring the surface conductivity of paper. According to it, the paper is charged strongly with the aid of a voltage source, after which the electric field caused by the charges is measured from above the surface of the paper.

The known methods and devices do not provide a sufficient spatial-resolution capability, for example, for analysing electro-photographically implemented toner transfer. The known methods also include delays, which further reduces their accuracy, due to the temporal equalization of the charges. In particular, to ensure the evenness of the toner transfer and thus also of the impression, there is a need for new measurement methods of the electrical properties of paper, or other non-conductive bases.

The invention is intended to create a new method and device for the more precise measurement of the electrical properties of a base material.

In the method according to the invention for measuring the electrical properties of a planar base material, the surface potential of the base material is altered and detected locally. The detection of the surface potential of the base material takes place with the aid of a measuring element that comprises a measuring edge and a sensor electrode and counter electrode, located at a distance from the measuring edge. According to the invention, the sensor electrode is at least partly surrounded by a dielectric material with a high permittivity.

The device according to the invention comprises means for altering the surface potential of the base materially locally, and means for measuring the surface potential of the base material. The means for measuring the surface potential of the base material comprise a measuring head, in which there is a measuring edge, a sensor electrode located at a distance from the measuring edge, and a counter electrode further located at a distance from the sensor electrode. The sensor electrode is at least partly coated with a dielectric material with a high permittivity.

In practice, a precise measurement made utilizing the invention is performed in such a way that the measuring edge of the measuring head comes substantially into contact with the base material, i.e. no air gap is left between the measuring edge and the base material. When this feature is combined with the high permittivity of the measuring head, the important advantage is achieved that the share of the potential drop taking place in the actual base material increases, and the measurement becomes sensitive to field changes arising from small non-homogeneities in the base material, and a higher spatial resolution is achieved.

A particular intention of the invention is to permit a new method and device for measuring the electrical properties of a base material without a delay, which further improves the measurement and the spatial and temporal resolution, relative to known solutions. Delay- free measurement is achieved by using the measuring head according to the invention, in such a way that the alteration and observation of the surface potential of the base material take place essentially simultaneously at a specific point in the base material, in the vicinity of the measuring edge of the measuring head. Thus the measuring head and the means for altering the surface potential of the base material are located essentially opposite to each other, in order to permit the essentially simultaneous alteration and measurement of the surface potential at a specific point in the base material, in the vicinity of the measuring edge. Thus the changes caused by the base material in an electric field permeating it can, according to one embodiment of the invention, be measured in real time, for example, in printing processes based on electro-photography.

The term electrical properties of the base material refers not only to the surface potential of the base material or its accumulated electrical charge at a specific moment, but also to properties of the base that affect the shape and/or strength of the electric field, typically electrical permittivity and resistivity, and which can be determined by measured the surface potential of the base, or the charge accumulated in the base, according to the invention. The invention is particularly well suited for use with non-conductive, i.e. mainly strongly insulated base materials, or with lossy insulators, such as paper and board. In measurement with a delay, the surface potential/charge distribution of the base material must be equalized/discharged sufficiently slowly, in order to ensure a good spatial resolution. Delay-free measurement, however, is very suitable also for bases, in which the charge distribution relaxes more rapidly.

The invention is suitable for use in the regulation of the process parameters of, for example, paper-technology post-processing processes, such as printing processes.

More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The device according to the invention is, in turn, characterized by what is stated in the characterizing portion of Claim 17.

The measuring head according to the invention is characterized by what is stated in the characterizing portion of Claim 33.

The use according to the invention is characterized by what is stated in Claim 34.

Considerable advantages are gained with the aid of the invention. With the aid of the invention it is possible to measure the effect of local non-homogeneities in paper, plastic, or other non-conductive materials, typically used as a printing base, on an electric field permeating the paper and thus in turn on the transfer of toner, for example. Thus the fluctuations of the electric field experiences by the toner can be measured.

The measurement geometry of the device corresponds to the geometry generally used in electro-photography, so that it is well suited to several different environments. The paper is charged from the side opposite to the measuring head and in the measuring situation the paper is in contact with the measuring head. Because the device corresponds in its geometry to the geometry of the toner-transfer geometry, and because also electrically the measuring head according to the invention is built to track the toner layer, it gives a very true depiction of the electric field experiences by the toner. The measurement can be performed either before the transfer of the toner, in order to calculate the optimal transfer parameters, or after the toner transfer, in order to detect the properties of the electrical properties of the base that have changed due to the toner. In other applications too, similar advantages are achieved. It can be used, not only in many electro-photography applications, but also in research work and, for example, as a quality-control device as part of paper manufacture. The device will also detect, advantageously without a delay,

mechanical defects in the base, such as hole, tears, and thickness variations, as well as moisture, for instance. Thus it permits defective products to be removed from the process. Despite the fact that the measured strengths of the electric field do not necessarily correspond to the voltages in actual toner transfer, the device can also be used to investigate the strength of an electric field by comparative measurements.

The device is suitable for several different base materials. It can be used to detect not only the changes in an electric field caused by fibre materials and plastic films, but also the effects on the electrical properties of the material of surface impurities in surfacing, coating, or print layers arranged on their surfaces.

As stated above, according to a preferred embodiment, unlike known measuring devices, there is no delay between a change in the surface potential (or charging) of the base and the measurement, but instead the measurement takes place simultaneously with the change. Even a small temporal delay between charging and measurement will mean that the charge distribution in the base material will have time to change, though the strength of the phenomenon will of course always depend on the properties of the base material. In that case, the measurement will no longer reliably tell of the surface charge caused by non- homogeneities in the base, particularly as the equalization of the charges is not yet known, or cannot be modelled very precisely.

In the following, various embodiments of the invention are examined with reference to the accompanying drawings, in which

Figure 1 shows a schematic diagram the apparatus according to one embodiment, as a side cross-section,

Figure 2 shows a more detailed image of the measurement geometry in the vicinity of the measuring head and charger, according to one embodiment, as a side cross-section,

Figure 3 shows the capacitor-like measurement geometry according to one embodiment, as a side cross-section, and

Figure 4 shows graphically an example of a measurement performed with the aid of the

device.

In the following description, the terras 'upper' and 'lower', as well as their derivatives, refer to the directions that are apparent from the accompanying drawings. However, it will be obvious to one skilled in the art that the operation of the device is not dependent on its orientation.

According to a preferred embodiment of the invention, in the device there is a dielectric measuring edge and a conductive sensor electrode essentially parallel to this, which is arranged at a distance from the measuring edge and is embedded in a solid insulator material. The counter electrode is located in the device in such a way that an electric field, which travels through the measuring edge to the counter electrode, in the vicinity of the sensor electrode, is created through the insulator material. Typically, the surface area of the counter electrode, seen from the direction of the measuring edge, is considerably greater than the surface area of the sensor electrode. The device can be manufactured in several geometries that differ from each other, which are described hereinafter.

An insulator material with a high permittivity is located at least in the area between the sensor electrode and the measuring edge, typically in the whole area between the measuring edge and the counter electrode, in the immediate vicinity of the sensor electrode. A material with a high dielectric permittivity refers mainly to materials, the relative dielectric permittivity of which is in the range 2 ... 20 000, preferably at least 5, typically in the range 200 ... 10 000. The relative permittivity of the material being examined is typically in the range 2 ... 100, for example in paper typically about 2.5. Besides the geometry, the permittivity of the measuring head has great significance for its sensitivity.

According to one embodiment, the sensor electrode, counter electrode, and measuring edge are elongated and essentially parallel, i.e. the distance between them relative to each other is essentially constant. The sensor electrode has a typically point-like cross-section, but the counter electrode has typically a considerable width in the direction of the surface of the base material, in the direction of travel of the base material, as will become apparent from later examples. The measuring edge of the measuring head preferably narrows towards the material being examined and typically has a circular cross-section shape. Thus it can be

linearly symmetrical relative to the sensor electrode. Such a wide measuring-head construction is suitable, for example, to examining a paper web once over the entire width of the web, so that thickness or moisture variations in the longitudinal direction of the web, for example, will be sensitivity detected. The measuring-head construction is preferably rigid, so that it is particularly suited to the analysis of planar base materials, such as paper and board sheets and web, and to many existing apparatuses.

According to one embodiment, the insulator-material layer surrounding the sensor electrode is cylindrical and arranged as a roller, which can be rotated along the surface of the base material. In such a device, the edge of the roller, which is in the vicinity of the base material at the time, acts as the measuring edge. Thus friction between the measuring edge and the base material and static electricity are avoided, which can be advantageous in some applications. On the other hand, frictional electricity can also be exploited, as described later.

According to one embodiment, the measuring head is 'pen-like', so that it is well suited to the detection of local variations in the base. The measuring edge is then point-like. The more precise profile of the measuring edge can narrow towards the base, for example, it can be hemispherical (circularly symmetrical). The end of a conductor, which is surrounded with a layer of insulator material, and set normal to the base material can, for example, act as the sensor electrode. The use of a construction scanning in two dimensions, or the use of several such measuring heads in parallel, can achieve a good spatial resolution also in the width direction of the web or sheet.

According to one embodiment, the measuring edge of the device is moved over the surface of the material being examined, in which case the movement will create frictional electricity (triboelectricity). Due to the abrasion, the charges separate on the surfaces. Thus the surface potential of the base material changes, which appears as a change in the electric field between the measuring edge of the device and the counter electrode. If non- homogeneities occur in the electrical properties of the base, they will appear as an uneven charge and in turn as fluctuations in the measured voltage. In this embodiment, the material of the measuring edge is preferably selected from strongly triboelectric materials, which are, for example, often ceramic materials. Such a measuring head can easily be installed in any device at all that transports paper, on the line of travel of the web or sheet, but the

device can also be implemented as a scanning construction.

According to a second embodiment, an electric field is created through the base material with the aid of a separate electrode. This electrode will be referred to hereinafter as the 'third electrode'. The third electrode is preferably placed at the location of the measuring head, but on the other side of the base. Typically (in the rectangular geometry) both the measuring head and the third electrode in the normal plane of the base material, essentially arranged next to each other, in such a way that the planar base material can be brought between them and an electric field permeating the basic can be induced between the third electrode and the counter electrode of the measuring head. The base material changes the electric field (and thus the surface potential of the base material) depending on the properties of the base, which is detected with the aid of the apparatus, as described above, without delay as the voltage of the counter electrode.

According to a third embodiment, during the measurement the base material is also simultaneously charged electrically with the aid of a charger. Charging preferably takes place from the side of the lower surface of the base material, so that the charger is typically located on the opposite side of the base to the measuring head. Thus the measuring geometry is similar to that in an electro-photographic printing device based on direct corona transfer. The actual measuring event is similar to that in the embodiments described above. Generally known chargers can be used in the device. Examples of these include corona-type chargers and roller chargers. According to one embodiment, the aforementioned third electrode can act as a corona charger. The charge accumulated in the lower surface of the base can be utilized to 'raise' the base onto the measuring edge at the measuring head. If the measuring head is kept, for example, charged like the base, an electrostatic force will arise between the base and the measuring head.

Measurement according to the various embodiments can be easily added to be part of processes handling moving web-like surfaces. The lack of delay in the method means that on-line information on the base material is obtained. Printing processes are one important area of application.

A schematic diagram of a complete measuring apparatus according to one embodiment is shown in Figure 1. The apparatus consists of a measuring head 12 manufactured from a

suitable dielectric material, the necessary conductors 19, an electrometer 17 measuring voltage, conveyor apparatuses 18, 18' for the base material, and the necessary apparatuses 14, 15, 16 for the charging of the paper and the discharging of the charge.

Figure 2 shows an example of the construction of the measuring head in greater detail. The measuring head 200 comprises a body structure 23, which in this example also acts as the counter electrode, a sensor electrode 21, which a thin conductor acts as in this example, and a dielectric material 22 surrounding the sensor electrode 21. The sensor electrode 21 is typically located inside the dielectric material layer 22. The material 22 can be in direct contact with the body structure 23. The lower surface of the material layer 22, or in some applications of the separate coating layer arranged on top of this, forms the measuring edge 29 of the measuring head 200. Preferably, as shown in Figure 2, the measuring edge 29 of the measuring head 200 is shaped to be narrowing or curving in the transverse plane of the conductor 21, in such a way that the material being measured has only local contact with it in its direction of travel. The distance between the sensor electrode 21 and the lower surface of the measuring head (the lowest point of the measuring head 29) can be, for example, 0.025 - 5 mm. The counter electrode 23 is of a conductive metal, for example aluminium, steel, or copper, and its distance from the sensor electrode is typically 0.025 - 5 mm. By using a thin single-terminal conductor as the sensor electrode 21, the thickness of the insulating coating can be reduced to even less than 1 millimetre, preferably about 0.05 - 1 millimetre. Insulated conductors can be used.

Figure 3 shows the capacitor-like measuring geometry. In it there is a planar third electrode 35, which is located on one side of the material 30 being examined. On the other side of the material 30 being examined is a planar measuring head 300. In it there is a planar counter electrode 33 and a planar layer of insulator material 32. The sensor electrode is marked with the reference number 31. Such a measuring head can also include second sensor electrodes 31'. When an electric field is induced between the third electrode 35 and the counter electrode 32 with the aid of the charge created, the surface potential of the base material changes, which is detected as a change in the potential of the sensor electrode 31.

The selection of the insulator material 22, 32 can be used to affect the functionality of the device. By selecting a material with a sufficiently high permittivity, the measuring device will become sensitive to precisely changes in the base material 20, 30 being measured. In

_{

particular, if the permittivity is at least 100-times, typically at least 500-times, even 1000- times that of the base material, electrical changes in the base will be easily detected. Correspondingly, a material with a lower permittivity will reduce the sensitivity to changes in the paper, but will correspond on the other hand better to electro-photographic processes. Barium titanate (BaTiO3), for example, can be used as a material with a very high permittivity. Commercial toner, for example, can be used as a material with a lower permittivity. The material can be used as such, or for example as a mixture with epoxy or some other binder, so that the composition of the material is made suitable and it can be attached to th body structure 23, 33. The use of barium titanate and epoxy as a mixture with a high permittivity is described in greater detail in a later example.

Other materials with a high permittivity are many ceramics, at least titanates, such as lead- zircon titanate, as well as for example lead methaniobate, lithium sulphate, quartz, and other silicon derivatives, and lithium niobate. Ceramic elements can be manufactured from powder by pressing and sintering, or by mixing with epoxy to form a homogeneous mixture.

The insulator material surrounding the sensor electrode can be unified and of a single material from the surface of the conductor to the measuring edge. Such a layer has preferably a homogenous permittivity. There can also be several 'shells' in the insulator material layer. Such a situation arises, for example, when using as the sensor electrode a ready-insulated conductor, which is embedded in an insulator-material layer with a high permittivity. In addition, one such 'shell' can be form on the surface of the insulator- material layer, at least in the area of the measuring edge, an applied coating, with the aid of which, for example, the wear or friction of the measuring head can be reduced, or the triboelectricity of the measuring head can be adjusted.

It is preferable for it to be possible to transport the base material between the third electrode and the measuring head, essentially at right angles relative to the direction of the electric field between them. Thus, in a printing press suitable for electrical printing-ink transfer, the measuring head can be located in the space of the actual printing unit, so that the paper conveyor and charging apparatus used in the measurement and the possible charge discharging apparatus will be ready in the printing press.

Referring again to Figures 1 and 2, the possible charging of the base material is preferably implemented using a corona charge 14, 24, which is connected to a high- voltage supply. The corona wire 15, 25 of the charger thus acts as a field electrode, the second field electrode being the counter electrode of the measuring head, typically the earthed body structure 23. A discharge corona 16, the charges produced by which neutralize the charge of the paper, operating on alternating current, can also be connected to the apparatus. A belt or other corresponding solution 18, 18' can be used to transport the base material.

The base material is typically paper, but other insulating materials, such as board, cardboard, and various plastic films can be used. The device and method described are particularly well suited to the analysis of continuous webs, but they can also be used for the analysis of products in sheet form. Special long measuring heads can be made for the analysis of entire paper webs.

The control parameters of the apparatus are, for example, the speed of the conveyor apparatus, the voltage and polarity of the third electrode/charger, and the distance of the measuring head from the charger.

In practice, the measurement can be implemented as follows. With the aid of the conveyor apparatus 18, 18' the paper 10 is brought to the measuring area. Once the paper has reached the location of the corona charger 14, the power is connected to the charger 14. A difference in potential arises between the charger 14 and the measuring head 12, the body 23 of which is in the earth potential. Due to the force of the charges induced in the lower surface of the paper by the charger, the effect of the electrostatic pressure causes the paper to rise to make contact with the measuring head. The voltage differences detected in the sensor electrode 12 of the measuring head are measured using a electrometer 17 connected to it. Once the paper has been transported past the measuring head, the charger's power is switched off. The electric field and the changes in it can be determined by calculation from the voltage and the voltage differences by calculation.

The electrometer (or other voltmeter) 17 does not substantially affect the actual measuring event. The input impedance of the electrometer is preferably at least 500 Mohm, typically even considerably greater, even 10 - 500 Tohm. The impedance of the entire circuit is preferably the same or substantially the same as the impedance of the electrometer. More

specifically, the impedance of the paper typically forms at most 15 %, advantageously at most 5 %, and preferably less than 1 % of the impedance of the entire circuit.

The magnitude of the voltage measured can be influenced with the aid of the dimensioning and geometry of the measuring head. One significant parameter is the ratio of the thickness of the dielectric layer 22 to the distance of the sensor electrode 21 and the counter electrode 23 from each other. If the ratio in question is, for example, 5 mm : 2 mm, the division ratio of the surface potential being measured will be 2/5. In particular by reducing the distance between the sensor electrode and the measuring edge, it is possible to improve the spatial resolution of the device. At a very high permittivity of the measuring head (> 100), the decisive factor in terms of resolution will become mainly the distant of the measuring edge from the sensor electrode, which it is therefore advantageous to keep small.

With the aid of the measuring head construction described, it is possible to achieve a spatial resolution of about 0.05 - 10 mm in the direction of travel of the base material, depending on the geometry and materials. Thus, it is possible to monitor variations in the permittivity of the paper at even the scale of its fibres. The spatial resolution improves especially by reducing the dimensioning of the conductor wire and measuring head and by altering the geometry of the measuring head. The spatial resolution of the device can be improved particularly by reducing the thickness of the coating layer 22. The electrical fields that prevail during the measurement, and especially the electrical non-homogeneity of the base material, can be calculated, if the voltage of the charger, the measured sensor voltage, and the measuring geometry are known.

In research work, the measuring head is connected to the electrometer and the voltage measured by the electrometer is recorded, for example, with a computer. In on-line processes, the measuring head and/or the electrometer are connected, for example, to the control unit of the printing press, in which the information (the sensor-electrode voltage) it produces can be used to set the printing parameters. In addition to printing processes, the invention is also suitable for regulating other (for example, paper technology) sheet or web processes according to the properties of the base material.

When using a corona charger, the typical voltage of the charging corona is 2500 - 5000 V,

but the apparatus will also function at clearly higher or lower voltages. If the charging voltage is too low, the electrostatic force will not be sufficient to press the paper onto the measuring head and the measurement will fail due to the large air gap. To avoid this, it is of course possible to manufacture devices, in which the paper is guided onto the measuring head mechanically. However, to achieve good usability, a sensor geometry is required, in which the travel of the paper is not substantially hindered. By means of the sensor geometry, it is also possible to improve the sensitivity of the device and how faithfully the transfer geometry reproduces the actual transfer event.

Though the present document describes in detail a single-channel apparatus, one skilled hi the art will understand that the spatial resolution in the direction at right angles to the direction of travel of the base material can be unproved by adding measuring channels.

Example

In this example, a sensor developed for this purpose, which contains an aluminium body coated with barium titanate (BaTiO3), is used in place of the photo-conductor known in electro-photography. A conductor measuring the electric field is embedded inside the barium titanate. Barium titanate is selected as the coating, on account of its very high relative permittivity. The permittivity of barium titanate is about 1000 times that of plastic, paper, or toner. By using a thin conductor and a highly permittive coating, the variations in the properties of a sheet can be measured more realistically.

The measuring head comprised a rectangular aluminium strip 16 mm x 16 mm x 440 mm and three measuring conductors in a barium-titanate coating. The strip forms the rigid body for the measuring head. The thickness of the barium-titanate layer is 5 mm. A coaxial cable (diameter 1.2 mm) was embedded in it. The measuring head used differed from an ideal head in that, unlike a photo-conductor, it does not have a round surface. More specifically, the coating layer consisted of barium titanate and polyester resin in a 50/50 mixture by volume. In order to achieve an even permittivity, the mixture was well mixed. The relative density of the polyester was about 1.1 and that of the barium titanate 6. The density of the finished coated mixture was in the order of 2 kg/dm ³ .

The conductor was a Filotex 50 VMTX cable, in which the diameter of the actual

,

14 conductor is 170 micrometres. The input impedance of the electrometer used in the measurements was 200 Tohm. The ends of the conductors were estimated to be about 1 - 2 mm from the counter electrode, which in the present experiment was the aforementioned aluminium strip.

Figure 4 shows the measurement result for a material, on which conductive lines were made using a pencil. As can be seen from the figure, such lines with a conductivity that is only greater than that of paper are very clearly detected by the device. In the prototype device, the measuring head was manufactured from barium titanate, i.e. it is sensitive to changes in paper. It can be seen from the figure that the lines pass the measuring head in this case at about 0.7, 1.0, and 1.3 seconds. Depending on the settings of the discharge corona, either a clear rise or a clear fall in the voltage is seen at these points. If a detaching corona is not used, the pencil lines will cause an increase in the potential level, i.e. the electric field. If a detaching corona is used, the pencil lines will be detected as drops in the electric field. A normal pencil contains conductive graphite.

In Figure 4, a small rise in potential can also be detected at about 1.7 s. This is due to a small hole made in the paper, which can thus also be detected using the present apparatus and method.

In this example, a 1.2-mm coaxial cable was used as the sensor electrode. A so-called guard voltage can be connected to the outer jacket of the coaxial cable, in which case the leak current caused by the insulation of the conductor can be minimized. However, the guard voltage is often so small, that a single-terminal insulated conductor will also produce reliable results and permit the manufacture of a smaller, and thus more accurate measuring head.