DE102016107956A1 - Process for the impregnation of continuous fibers for the production of fiber-reinforced semi-finished products - Google Patents

Abstract

The present invention relates to a process for the production of fiber-reinforced semi-finished products in which a fiber mat is impregnated with a plastic consisting of the steps: A) encasing the fiber mat with the plastic and B) moving the plastic-coated fiber mat through one of a sealing surface of a first sonotrode and a gap formed by a counter tool, wherein during step B) the sonotrode is excited with an ultrasonic vibration.

Description

The present invention relates to a process for impregnating continuous fibers for the production of fiber-reinforced semi-finished products.

The importance of components made of fiber-reinforced plastics is steadily increasing. Fiber reinforced plastics are materials that consist of high strength / high stiffness fibers and a plastic matrix. The plastic matrix surrounds the high-strength fibers. As fiber-reinforced plastics generally have high specific rigidities and strengths despite their low weight, they are used in particular for lightweight construction applications. In order to produce fiber-reinforced plastic components, a fiber mat of "dry" reinforcing fibers, such as e.g. Glass fibers, carbon fibers, aramid fibers, basalt fibers, synthetic fibers or natural fibers, impregnated with the plastic. In this case, the fiber mat can consist of unidirectional material or it can be used as a textile structure, such as e.g. be designed as a scrim, woven, knitted or knitted fabrics.

When impregnating, care must be taken that each filament is surrounded by matrix material. Correspondingly impregnated or preimpregnated fiber mats are also referred to as "prepreg". Thermoplastic matrix materials are generally used as the plastic material since they are of low viscosity and easily penetrate the filaments. However, thermoplastic semi-finished products are also desirable, especially for large series applications, since they can be stored almost indefinitely and are easier to transport and handle. In addition, the processing times of thermoplastic semi-finished products are shorter.

However, thermoplastic melts have a much higher viscosity compared to thermosets and penetrate even thin bundles of reinforcing fibers, so-called rovings, very difficult and often only in the edge regions. In addition, the reinforcing fibers are usually cold, which further complicates the impregnation.

• 1. Bei der Schmelzimprägnierung wird eine Fasermatte mit einer Thermoplastschmelze umspritzt und durch eine beheizte Einwalzstrecke geführt. Danach wird das Material durch ein Kalandrierwalzenpaar geführt. Nachteil dieses Verfahrens ist es, dass es unter Umständen sehr schwierig ist, eine homogene Mischung der Komponenten zu erzielen. Zudem müssen relative hohe Imprägnierungstemperaturen verwendet werden, die die Lebensdauer der Prepregs reduzieren können.
• 2. Bei der Pulverimprägnierung wird das Thermoplast in Pulverform auf die Fasermatte gleichmäßig aufgebracht und durchläuft dann eine Heizstrecke, in der das Pulver aufschmelzt. Im Anschluss an die Heizstrecke wird die Matte durch Imprägnierwalzen geführt, die das geschmolzene thermoplastische Material in die Matte einarbeiten. Im Anschluss daran wird das Material durch ein Kalandrierwalzenpaar geführt.
• 3. Bei der Lösungsmittelimprägnierung wird das thermoplastische Material als Granulat in einem Lösemittel in einem Imprägnierbad vorgehalten durch das die Fasermatte geführt wird. Im Anschluss daran muss die Fasermatte eine Trockenstrecke durchlaufen, in der das Lösemittel verdunstet.
• 4. Bei der sogenannten Film-Stacking-Imprägnierung wird eine Fasermatte beidseitig mit einer Thermoplastfolie bedeckt und die dadurch entstehende Sandwichstruktur durch eine Doppelbandpresse geführt, in der zunächst eine Heiz- und dann eine Kühlstrecke durchlaufen wird.

For the impregnation of rovings with thermoplastics, essentially four different impregnation technologies are used.

• 1. In the melt impregnation, a fiber mat is encapsulated with a thermoplastic melt and passed through a heated Einwalzstrecke. Thereafter, the material is passed through a pair of calender rolls. Disadvantage of this method is that it may be very difficult to achieve a homogeneous mixture of the components. In addition, relatively high impregnation temperatures must be used, which can reduce the lifetime of the prepregs.
• 2. In powder impregnation, the thermoplastic powder is applied evenly to the fiber mat and then passes through a heating section in which the powder melts. Following the heating section, the mat is passed through impregnating rollers which incorporate the molten thermoplastic material into the mat. Following this, the material is passed through a pair of calender rolls.
• 3. In the solvent impregnation, the thermoplastic material is held as granules in a solvent in an impregnating bath through which the fiber mat is guided. Following this, the fiber mat must pass through a drying section in which the solvent evaporates.
• 4. In the so-called film-stacking impregnation, a fiber mat is covered on both sides with a thermoplastic film and the resulting sandwich structure is passed through a double belt press, in which first a heating and then a cooling section is passed.

All described methods are associated with significant thermal losses. Furthermore, due to the slow heat penetration of the material, the processor speed is limited. Although the transition of the thermoplastic material from the solid to the molten state is in principle reversible and therefore theoretically repeatable as often as desired, in practice, however, a reduction of the molecular weight is observed in each melting process.

The molecular weight represents a measure of the number of monomer repeating units in the chain. A reduction of the molecular weight therefore means a shortening of the polymer chains. This is essentially due to oxidation caused by contact with either atmospheric oxygen or oxygen dissolved in the polymer itself. Therefore, the longer the melting state lasts, the greater the negative influence of the oxidation process on the plastic material.

It is therefore an object of the present invention to provide a method for the production of fiber-reinforced semi-finished products, which heats the plastic for only a short time, but nevertheless ensures a reliable melting of the plastics involved.

• A) Umspritzen der Fasermatte mit dem Kunststoff und
• B) Bewegen der mit Kunststoff umspritzten Fasermatte durch einen von einer Siegelfläche einer ersten Sonotrode und einem Gegenwerkzeug gebildeten Spalt, wobei während Schritt B) die Sonotrode mit einer Ultraschallschwingung angeregt wird.

This object is achieved by a method according to claim 1, which comprises the steps:

• A) Injecting the fiber mat with the plastic and
• B) moving the plastic-coated fiber mat through one of a sealing surface of a first sonotrode and a counter-tool formed gap, wherein during step B) the sonotrode is excited with an ultrasonic vibration.

By the inventive processing of the plastic-coated fiber mat with an ultrasonically acted sonotrode the heated Einwalzstrecke can be shortened or even completely dispensed with in the known Schmelzimprägnierverfahren.

Processing by means of ultrasound makes it possible to purposefully generate heat at the contact surfaces between the fiber mat on the one hand and overmoulded plastic on the other hand, so that the overmolded structure is effectively locally melted with very low energy, which leads to a good impregnation.

In a preferred embodiment, a roller is used as a counter tool. The use of a roller has, inter alia, the advantage that the fiber mat overmolded with fiber material, which is still almost liquid immediately after encapsulation, can easily be moved between the sonotrode and the counter tool. The speed of the roller corresponds to the material speed.

In a particularly preferred embodiment, the roller has a cooling device, i. To accelerate the setting of the submitted semi-finished products, the counter-tool is cooled. When the counter tool is cooled, it is recommended to start the ultrasonic processing, preferably at the beginning of the contact interval, i. the time interval during which the over-molded fiber mat contacts the counter tool is performed. The ultrasound treatment causes local heating. The necessary heating is greater, the lower the temperature of the over-coated fiber mat. Therefore, the fiber mat should not have cooled down too much yet. In addition, the heat supplied by the sonotrode must be removed, so the ultrasonically treated fiber mat should remain in contact with the cooled counter tool for as long as possible.

For example, a sonotrode with a rectangular sealing surface can be used as the first sonotrode. The sealing surface should have a longitudinal extent that is greater than or equal to the desired material width. When very wide widths of material are desired, it may be advantageous to have multiple sonotrodes with rectangular sealing surfaces side by side, i. in a direction transverse to the direction of material feed.

In an alternative embodiment it is provided that a roller sonotrode with a cylinder jacket-shaped sealing surface is used as the first sonotrode.

In particular, in combination with a roller-shaped counter-tool, the sonotrode counter-tool pair can then also be used to clamp the web, i. to transport the over-molded fiber mat.

In a further preferred embodiment, it is provided that a second sonotrode with a sealing surface is used as the counter-tool, wherein the second sonotrode is also excited with an ultrasonic vibration. Basically, the plastic-coated fiber mat is passed between two sonotrodes and processed.

In a particularly preferred embodiment, the sealing surfaces of the first sonotrode and the second sonotrode are the same length at least in a direction perpendicular to the direction of movement of the over-coated fiber mat in step B), wherein preferably the two sealing surfaces in the direction perpendicular to the direction of movement of the over-coated fiber mat in step B) are offset from each other, wherein best the two sealing surfaces are offset by a quarter wavelength of the ultrasonic vibration to each other.

Basically, in the dimensioning of sonotrodes, the aim is that the ultrasonic vibration of the sealing surface is as homogeneous as possible. However, this is only limited achievable. In practice, regions of greater amplitude of vibration and regions of smaller amplitude of vibration often form on the sealing surface. The regions of greater amplitude of vibration are spaced by a distance of λ / 2, i. one-half wavelength of the ultrasonic vibration spaced from each other. The described measure of the offset by λ / 4 ensures that an area on the sealing surface of the first sonotrode with increased oscillation amplitude coincides with a region of the sealing surface of the second sonotrode with reduced oscillation amplitude, so that along the length of the sealing surface the introduced energy is homogenized ,

In a further preferred embodiment, a roller sonotrode with a cylinder jacket-shaped sealing surface is used in each case as first and second sonotrode.

In a further preferred embodiment, after step B), the processed semi-finished product is passed over a roller, preferably via a roller with a cooling device.

In a further preferred embodiment, it is provided that the thickness of the semi-finished product to be produced by adjusting the gap width, i. the distance between the sealing surface of the first sonotrode and the counter tool is determined. If appropriate, the regulation of the feed rate of material may additionally be advantageous here.

This measure makes it possible to dispense with a calendering tool even in applications which place great demands on the homogeneity of the material thickness.

In an alternative embodiment, it is provided that the pressure which the sonotrode counter-tool pair exerts on the over-coated fiber mat is kept constant. In practice, a control device that measures the applied force, compares it to a desired force value and adjusts the force accordingly, may be used.

For some applications, it may also be advantageous if the first and possibly also the second sonotrode is cooled. By processing the still hot material, there is a significant heating of the sonotrode and all associated with this components. The resulting thermal expansion must be compensated consuming. In addition, it can come at too high temperatures to a deterioration of the piezo elements that are used for vibrational excitation come. Therefore, cooling the sonotrode may be beneficial.

Further advantages, features and possible applications of the present invention will become apparent from the following description of two embodiments of the invention and the associated figures. Show it:

1 A schematic representation of a first embodiment of the invention and

2 a schematic representation of a second embodiment of the invention.

In 1 an apparatus for realizing a first embodiment of the method is shown. Here are from a run gate 1 the reinforcing fibers 2 peeled off and into a device 3 led to the formation of the fiber template. Into the device 3 is about an extruder 4 Plastic, preferably supplied thermoplastic plastic, so that the thermoplastic material completely surrounds the fibers. About a corresponding nozzle 5 the fiber material coated in this way is dispensed and thereby directly onto a roller 6 given up. This so-called "chill roll" serves to cool the melt.

With the help of a sonotrode 7 an ultrasonic vibration in the over-molded fiber mat 8th brought in. For this purpose, a gap between the sonotrode 7 or their sealing surface and the counter tool, the chill roll 6 , educated.

Ultrasound processing results in a very effective impregnation of the fiber mat with thermoplastic material. In the further course of the web, this is further cooled due to their contact with the cooled roller, so that the solidified material by means of the roller 11 can be wound up.

In 2 an alternative embodiment has been shown, wherein the reference numerals have been used in the same way to designate identical parts, as far as possible.

This in 2 The procedure shown differs from that in 1 shown method essentially in that the semi-finished fiber 8th after the nozzle 5 left, no longer directly on the cooled roller 6 is given, but between a first sonotrode 9 and a second sonotrode 10 , is passed through. Between the first sonotrode 9 and the second sonotrode 10 a corresponding gap is formed in which it comes to ultrasonic processing. The otherwise identical roller sonotrodes are offset in the longitudinal direction, ie in a direction perpendicular to the plane of the paper, by a ¼ ultrasonic wavelength.

After processing the fiber half tool 8th this will be on the chill roll 6 guided and then as in 1 on the roll 11 wound.

LIST OF REFERENCE NUMBERS

1

 creel

2

 reinforcing fibers

3

 contraption

4

 extruder

5

 jet

6

 Roll / chill roll

7

 sonotrode

8th

 overmoulded fiber mat / semifinished fiber product

9

 first sonotrode

10

 two sonotrode

11

 role

Claims (12)

Process for the production of fiber-reinforced semi-finished products in which a fiber mat is impregnated with a plastic consisting of the steps: A) encapsulating the fiber mat with the plastic and B) moving the plastic-coated fiber mat through a gap formed by a sealing surface of a first sonotrode and a counter tool, wherein during step B) the sonotrode is excited with an ultrasonic oscillation. A method according to claim 1, characterized in that as a counter tool, a roller, preferably a roller with cooling device is used. A method according to claim 1 or 2, characterized in that a sonotrode with a rectangular sealing surface is used as the first sonotrode. A method according to claim 1 or 2, characterized in that a roller sonotrode with a cylinder jacket-shaped sealing surface is used as the first sonotrode. Method according to one of claims 1 to 4, characterized in that as a counter tool a second sonotrode is used with a sealing surface, wherein the second sonotrode is excited with an ultrasonic vibration. A method according to claim 5, characterized in that the sealing surfaces of the first sonotrode and the second sonotrode in a direction perpendicular to the direction of movement of the over-coated fiber mat in step B) are the same length, preferably wherein the two sealing surfaces in the direction perpendicular to the direction of movement of the over-coated fiber mat in Step B) are offset from each other, wherein best the two sealing surface are offset by a ¼ wavelength of the ultrasonic vibration to each other. A method according to claim 5 or 6, characterized in that as the first and second sonotrode each a roller sonotrode is used with cylinder jacket-shaped sealing surface. A method according to claim 7, characterized in that after step B) the processed semifinished product is passed over a roller, preferably via a roller with cooling device. Method according to one of claims 1 to 8, characterized in that the gap width is regulated or controlled and thereby the thickness of the semi-finished product to be produced is determined. Method according to one of claims 1 to 8, characterized in that a constant pressure of the sonotrode and the counter-tool is barbed on the over-coated fiber mat, wherein preferably the pressure is controlled. Method according to one of claims 1 to 10, characterized in that the first sonotrode is cooled. Method according to one of claims 1 to 11, characterized in that a thermoplastic is used as plastic.

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