EP1982140A1

EP1982140A1 - Method for measuring the thickness of multi-layer films

Info

Publication number: EP1982140A1
Application number: EP06806158A
Authority: EP
Inventors: Albert Keller; Markus Hänggli; Philipp Weber; Peter Stuker
Original assignee: HCH Kuendig and Cie AG
Current assignee: HCH Kuendig and Cie AG
Priority date: 2005-10-28
Filing date: 2006-10-02
Publication date: 2008-10-22
Also published as: US20100141274A1; EP1780499A1; WO2007048499A1

Abstract

The invention relates to a method for determining the thickness of multi-layer films (13) comprising layers consisting of various non-conductive materials. According to said method, the thickness of the multi-layer film (13) is measured by a first sensor (17) and a second sensor (16) and optionally additional sensors, whereby all the sensors take a measurement at the same location under the same conditions if possible. The first sensor (17) and the second (16) or additional sensors generate different measured values for layers of the multi-layer film (13) of the same thickness consisting of the same material (13). The measured signals of the sensors (16, 17) are fed to a computer (18), which determines the total thickness of the multi-layer film (13) and/or the thickness of the individual layers of the multi-layer film (13) from the different measured values of the first sensor (17) and the second (16) or additional sensors.

Description

Method for measuring the thickness of multilayer films

The invention relates to a method for determining the thickness of multilayer films according to the preamble of independent claim 1. Multilayer films of plastics are constructed from a plurality of layers of different materials. Commonly used thermoplastics are polyethylene (PE), polypropylene (PP), polyamide (PA), ethylene-vinyl alcohol copolymers (EVOH), and others.

The multilayer films are produced from the different thermoplastics with coextrusion or multilayer extrusion. Known such extrusion processes are blown extrusion and flat extrusion. In the

Blasextrusion be produced so-called blown films. The melt is extruded from a ring die in blown extrusion and formed into a tube. In The hose is blown air to expand this. The tube is then laid flat, in many cases cut into two or more sheets and wound up. In flat film extrusion, the melt is extruded from a slot die.

In the production of multilayer films, various thermoplastics with different properties are simultaneously extruded through multiple nozzles and combined to form the multilayer film. In many cases it is necessary to introduce so-called adhesion promoters (HV) between individual layers of the multilayer film. The adhesion promoters have to improve the bond between layers of the multilayer film.

Multilayer films are used for the packaging of foods in large quantities. This is called barrier plastics. These multilayer films have layers, e.g. are poorly permeable to oxygen, moisture or other substances, which leads to better shelf life of the food. For food packaging, multi-layer films are also used as shrink films, cooking bags, germ-free packaging for dairy products, and the like. Typical barrier films have e.g. a construction

PE or PP HV

COATING LAYER (PA, EVOH) HV

PE or PP

Further details about multilayer films, the materials used for them and their properties, as well as their preparation can be found, for example, in an easily understandable form in the book "Plastic Films, Fabrication, Properties, Application" by Joachim Nentwig, Carl Hanser Verlag Munich Vienna, 1994. In the production of films in general, but especially in the production of multilayer films, the thickness of the films is monitored, and in case of deviations, for example, in the production of flat films, the width of the slot of extrusion dies changed to produce as possible films of the same thickness. In blown film extrusion, the temperature of the melt or the cooling air or the amount of cooling air is locally changed.

With e.g. the quality of a blown film is equal on the whole size, thickness on the whole size should be as uniform as possible. A uniform thickness is, among other things, necessary to be in the other

Processing of the film e.g. to allow a uniform printing. To monitor a uniform thickness in production or by

To control adjusting elements in the blow head, the thickness profile of the film must be measured.

For example, the following sensor types for measuring the thickness of films are known:

Capacitive sensors which by the dielectric constant and / or the

Damping factor of the film can be influenced. Capacitive sensors can measure reflectively or transmissively.

Sensors that work and measure with ionizing radiation, with backscatter or with absorption.

Sensors that work and measure transmissively with infrared absorption.

Sensors which optically work and measure with interference methods. Sensors which mechanically or pneumatically thickness after

Measure flattening.

Sensors that measure thermally.

Sensors that use ultrasound and measure transit times, damping, reflection and / or phase shifts.

The problem to be solved with the invention will be explained with reference to the combination of capacitive sensors with sensors incorporating the Determine film thickness based on the backscatter of ionizing radiation. Multi-layer film is not just about ensuring that the entire thickness of a film over the entire film is as equal as possible. It is also necessary that the individual layers are as constant as possible thick.

The measuring signals of capacitive sensors are dependent on the dielectric constant of the material to be measured. The measurement signals from capacitive sensors operating according to the reflection principle are practically directly proportional to the thickness of a film and to the dielectric constant of the material of the film. The dielectric constants of certain materials are temperature dependent.

Sensors operating capacitively according to the reflection principle are used e.g. used with advantage for measuring the film thickness at the film bubble of blown film extrusion lines. In order to record the thickness profile of a film bubble online, a sensor is guided around the film bubble on an annular construction. One cycle typically takes 1-2 minutes. The sensor is pressed against the foil bubble with even pressure. This allows a very good and accurate online detection of the thickness profile of e.g. PE films.

When measuring multilayer films which consist of several layers of thermoplastics, sometimes with very different dielectric constants, the measurement of thickness and of thickness profiles can be faulty. Since the sensor can not recognize that, for example, at the same time the thickness of the entire film and also the thickness of one or more of the layers of the multilayer film simultaneously change so that the error, which is due to the thickness of the film and the measurement error caused by the Thickness of a layer of the multilayer film is due to partially or completely compensate each other. The capacitive sensor detects no or a wrong, too much or too little change in the thickness. In US 3,635,620, the combination of a mechanical measurement of total thickness and a capacitive measurement, which depends on the different dielectric constants of the film layers, is used to measure and control the average thickness of the two material layers. But since the thickness is measured only at a single point on the circumference, no thickness profile can be created.

US 202/0057096 makes use of the fact that the dielectric constant of the barrier materials PA and EVOH is strongly temperature-dependent. In addition to a first capacitive measurement at high temperature, a second capacitive measurement at much lower temperature is used to measure the thickness of two different material layers. However, this lower temperature is generally only reached after flattening, but certainly several meters after the first measuring point. This makes it difficult or impossible to measure the exact same film locations and the measurement results are greatly distorted by the thickness variations in the direction of production of the film.

The object of the invention is to provide a method which makes it possible to obtain profiles of the thickness of individual layers of multilayer films, or profiles of the thickness of groups of layers of multilayer films, such as e.g. of all barrier layers together as accurately as possible.

According to the invention, the method has the features of the characterizing part of independent claim 1. The dependent claims relate to advantageous embodiments of the invention.

The determination of the thickness and of the thickness profiles of multilayer films according to the new method utilizes, for example, the different sensitivity for the dielectric constant of capacitive reflective and, for example, of sensors based on ionizing radiation. The measured values of capacitive thickness sensors which work according to the reflection principle are Product of the thickness of the film and the relative dielectric constant ε _r . The measured values of thickness sensors, which measure for example on the basis of ionizing radiation, are practically only dependent on the thickness and the specific weight of the material. The measured values of sensors operating with an optical interference method are just as dependent on the dielectric constant.

Sensors that work with ionizing reflection or with an optical interference method can be arranged in the direction of the film directly in front of, behind or next to the sensor, which operates capacitive reflective.

It is advantageous if both sensors measure the exact same line on the film as precisely as possible so that variations in the thickness in the direction of production are simultaneously included in the measurement for both sensors. If both sensors are arranged one above the other or next to each other at a specific distance, they measure on two parallel spiral paths on the foil. The horizontal distance of these spiral tracks should be at least by a factor of 2 smaller than the corresponding distance of the control elements in the blow head. With sensors arranged one above the other, the spacing of these spiral tracks becomes small when the take-off speed is high, but it increases proportionally with the vertical distance between the two systems and greater reversing speed of the measuring device. The above condition is achieved for typical applications when the vertical distance of the two sensors is less than about 0.5 m. If the sensors are arranged next to each other, appropriate conditions apply.

If the sensors always rotate in the same direction around the bubbles, in contrast to the reversing operation, the sensors, which are mounted one above the other, can be displaced laterally just as much as is necessary to compensate for the offset caused by the combination of

Discharge speed and rotation speed, to compensate. In addition, the rotational speed of the take-off speed could be adjusted.

By the mentioned or similar measures can be achieved that measure both measuring systems as closely as possible in the same place. As a result, they also measure under the same thermal conditions.

Dielectric properties of plastics such as thermoplastics used for multilayer film are e.g. in book "The plastics and their properties", Hans Domininghaus, Springer, 1998 to find. On page 128 are e.g. the dielectric constants ε and the dielectric loss factor tan δ for plastics used for multilayer films are plotted as a function of the temperature. It can thus be seen that in many cases it contributes to further increasing the measuring accuracy of the sensors, even if the temperature of the film is measured and included in the determination of the correction values and values of the thickness. Since the dielectric constants are much greater at higher temperatures for the frequently used barrier layer materials PA and EVOH, it is also advantageous to carry out the method according to the invention at a measuring point at which the film is still very hot.

The invention will be explained in more detail with reference to the schematic drawing. The sole Figure 1 shows the schematic diagram of a multilayer film blown extrusion line to which the thickness is measured and monitored by the method of the present invention.

The production of foils takes place in the blown film extrusion line 1 as follows: From the extruder with a multi-ring die (not shown), the exiting melt of the various thermoplastics is formed into a tube. This film tube is withdrawn at a rate that is greater than the exit velocity of the melt. By a connection for compressed air in the blowing head 11 with the mold 12, the hose is inflated to the film bubble 13. At the end of the flattening 14, the film bubble with two Squeezing rollers 14 'squeezed. The flattened film tube 13 'is then passed to the winding device (not shown) (arrow) and wound into a roll.

The thickness of the film is applied to the film bladder 13 with a first, e.g. Capacitive sensor 17, measured, which operates on the reflection principle. A second sensor, e.g. a sensor with ionizing radiation 16, measures as far as possible at the same point or on the same line in the production direction. Both sensors run together on a web 17 'reversing around the film bubble 13 around and back. The sensors can also run continuously around the film bubble 13. The reversing process or the circulation time takes about half to several minutes.

After the squeezing rollers 14 ', the film tube is guided over the turning bars 15 to the fixed roller 15' and from there to a winder (not shown).

The measured values of the two sensors 16 and 17 are fed to the computer. The computer 18 then determines the profiles of the total thickness and the thickness of individual layers according to the equations in Tables 2 and 3. On the screen of the computer 18, the values determined for the thickness of the individual layers or groups of layers and the total thickness can be, for example graphically and / or numerically.

For multilayer film, the assumption made by the new method is that the multilayer film consists of two types of thermoplastics. First of the base material, which is usually a polyethylene (PE) and barrier material such as polyamide (PA) or ethylene vinyl alcohol (EVOH), which have a much higher dielectric constant ε _r from 4 to 15. Although multilayer films are made up of 5 and more different layers, this simplistic assumption is acceptable because the PE layers are below and the PA layers below have comparably similar practically identical values for the dielectric constant ε _r . Here and in the following, therefore, the barrier layers are referred to simply as PA. Mathematically, therefore, the measured values of the two sensors 16 and 17 for the thickness of the entire multilayer film can essentially be used to deduce the thickness of the PE layers, the thickness of the PA layers and of course also the total thickness of the multilayer film.

The profile of the total thickness and / or the thickness of sub-layers calculated by the computer 18 are fed to the console 19 with which the system 1, i. We controlled and regulated the extrusion process and where the data could also be displayed on a screen. Finally, provision can also be made for the console 19 to regulate and control the thickness of individual layers and the entire multilayer film on the basis of the thickness values determined by the method in the computer 18, as previously explained.

The invention relates to the measurement and regulation of the thickness of the entire multilayer film but also to the measurement of the thickness of layers of different materials, usually thermoplastics.

The following is an example of how the thickness of the PA layers and those of the PE layers can be determined by the method. Reference is also made to FIG. 1 reference.

TABLE 1 / THE TERMS / MEANINGS USED

It is based on the following assumptions and assumptions.

• All high ε _r barrier layers (PA, EVOH, etc.) are considered and grouped together as a single layer.

• All layers with PE-like ε _r are combined and considered as a single layer.

• The sensor 17, which measures capacitive reflective, is calibrated to the mean value of the total thickness at PE.

The sensor 16, which measures, for example, on the basis of ionizing radiation, is calibrated to the mean value of the total thickness of the multilayer film.

• The measuring sensitivity of the two measuring systems, ie the two sensors 16 and 17 concerning ε _r is known.

TABLE 2 / BASIC EQUATIONS

TABLE 3 / DETERMINATION OF THE THICKNESS PARTS OF PA, OF PE AND THE SUM OF THE TWO THICKNESSES

With the formulas according to Table 3, the total thickness of the multilayer film can be calculated correctly, although the measurements of each sensor alone have a measurement error which depends on the thickness of the PA layer in relation to the total thickness of the films.

As a first approximation, it suffices to use the values for k and εA / εE mentioned in the table. It is of course possible to further optimize the measurement and to determine even more precise thickness values. For example, with a system with k = 0 from Equation III, the correct ratio εA / εE and hence the εA valid for the current temperature can be determined if the setpoint is used for DA and the mean values over a whole profile for D1 and D2 become. If, in addition, the temperature profile of the film at the measuring point 16 and / or 17 is measured, the ratio εA / εE can be determined even more precisely for each individual measured value. However, in this example, it is only and above all a matter of showing the principle of determining the thickness values from the measured values which are determined by the two sensors 16 and 17.

Do not the assumed values for k and εA / εE exactly the reality and / or the sensor 16 and the sensor 17 are not accurate matched, the calculated K also contains a Kalibrierfehleranteil. The calculated proportions of PE and PA are then not accurate. However, it can be shown that, for example, with 5% calibration error of the sensor 17, the resulting profile error for the PA profile at 30% PA content is less than 0.5%.

In the method for determining the thickness of multilayer films 13 with layers of different non-conductive materials, the thickness of the multilayer film 13 is measured with a first sensor 17 and a second sensor 16 and possibly further sensors. The first sensor 17 and the one or more further sensors 16 generate different measured values for layers of the same thickness made from the same material of the multilayer film 13. The measurement signals of the sensors 16, 17 are fed to a computer 18, which determines the total thickness of the multilayer film 13 and / or the thickness of individual layers of the multilayer film 13 from the different measured values of the first sensor 17 and of the further sensor or sensors 16.

Claims

claims

1. A method for determining the thickness of multilayer films (13) with layers of different non-conductive materials with a first sensor (17), characterized by a further sensor (16) or more sensors, all sensors, the film as possible in the same place below measure the same conditions, the first sensor and the one or more other sensors for layers of the same thickness of the same material of the multilayer film (13) produce different measured values, and the measurement signals of the sensors (16, 17) are fed to a computer (18) consisting of the different measured values of the first sensor (17) and the one or more other sensors (16) determines the total thickness of the multilayer film (13) and / or the thickness of individual layers of the multilayer film (13).

2. The method of claim 1, for determining the thickness of multilayer films (13) of layers of materials with different dielectric constants ε _r and / or different dielectric loss factors tan δ.

3. Method according to one of claims 1 to 2, for determining the thickness of multilayer films (13) in which the first sensor (17) capacitively by the reflection method (reflective), the dielectric constant ε _r , and the second sensor (16) capacitively after the Reflection method (reflective) measures the loss factor tan δ of the multilayer film.

4. The method according to any one of claims 1 to 3, wherein a single sensor measures both the dielectric constant ε _r and the dielectric loss factor tan δ and thus serves as the first and second sensor.

5. The method according to any one of claims 1 to 4 wherein the first sensor operates with a measuring principle whose thickness measurements of the dielectric constant ε _{r of} the material of the film are strongly and preferably directly proportional dependent, in particular with a sensor of the capacitive reflective operating principle works and measures and the second sensor works with a measuring principle whose thickness measurements of the dielectric constant ε _{r of} the material of the film and the layers of the film are not or only weakly dependent, preferably, with a sensor of the thickness of the film with the backscatter ionizing radiation or a sensor that measures the thickness of the film with an optical interference method.

6. The method according to any one of claims 1 to 5, wherein the sensors, the thickness of the film at the same point of the film or at adjacent locations of the film, preferably on the film bubble or flattened

Measure film.

7. Method according to one of claims 1 to 6, in which the computer (18) to which the measured values of the two measuring devices, which react differently sensitive to the dielectric constants of the film materials, are supplied from these measured values the dielectric constant or the ratio of the dielectric constant of the determined different film materials.

8. Application of the method according to one of claims 1 to 7, in a multilayer film blown extrusion line or in a flat film extrusion line, for measuring, monitoring and / or controlling the total thickness, and / or the thickness of individual layers and / or the thickness of groups of layers, in particular the barrier layers in multilayer films (13).