DE2048916A1 - Method and device for the treatment of carbon fibers - Google Patents

Description

PATENT LAWYERS DR.-ING. BY KREISLER DR.-ING. SCHÖNWALD DR.-ING. TH. MEYER DR. FUES DIPL-CHEM. ALEK VON KREISLER DIPL.-CHEM. CAROLA KELLER DR.-ING. KLDPSCH COLOGNE 1, DEICHMANNHAUS

Cologne, October 2nd, 1970 Ke / Ax

Morgan! Te Research and Development Limited, Battersea Church Road, London, SW11 (England).

Method and device for treating carbon fibers

The invention relates to a method for treating carbon fibers to improve the strength of the joint between the fibers and investment materials in composite materials and a device for carrying out this treatment.

The term "carbon fiber" is to be understood as a paser, which consists essentially of carbon and may contain graphitic components. The term includes also carbon fibers impregnated with boron, for example.

Composite materials made of high elastic modulus and strength carbon fibers have recently become very important as advanced materials of construction. They are lighter than most metals and stronger and stiffer for the same weight or mass. For example, typical fibers with a more or less round cross-section and a diameter of 5 to 15 Ά (in particular 7 to 10 u) have a density of 1.6 to 2.0 g / cm ⁵ (in particular 1.7 to 1, 95 g / ov?) _T one

b 2

Young's modulus along the longitudinal axis from 1.4 to 7 x 10 kg / cm 109817/2259

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and a tensile strength along the axis of 4900 "to

35200 kg / cm. These strong carbon fibers are produced by controlled pyrolysis of organic fibers, with certain stages of pyrolysis being carried out under tension. For example, they can be produced by heating polyacrylonitrile fibers (PAN) under controlled oxidation conditions and under tension to 150 "to 300 ^° C. and then further heating them in an inert atmosphere" up to temperatures of 700 to 3000 ^{° C.}

In order to utilize the aforementioned properties for technical purposes, it is necessary to convert the individual fibers into to incorporate an investment that sets the material to a workable and usable composite material. As with glass fibers, a composite material is also typical and particularly useful, in which the fibers are bound in a plastic that is itself strong but lent, e.g. in a resin. One of the factors that When making the composites, however, is the fact that the overall strength of the composite may be limited by the strength of the connection between the fibers and the plastic. This is especially the case when manufacturing Schiohtstoffe or laminates in which the resin can detach from the fiber surface before the fiber breaks under tension. Pure resins have shear strengths on the order of 8.4 to 11.5 kg / cm, and composites are necessary with similar shear strengths when the fibers are used at maximum efficiency should. You therefore need carbon fibers to form a solid connection between fibers and investment material are "wetted" by the resin or the other investment materials.

The invention relates to a method for surface treatment of carbon fibers, through which the fibers adhere firmly to the embedding structure. The invention also includes one Device for carrying out this treatment,

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8AÖ ORIGINAL

It has been found that the adhesion between carbon fibers and investment material can be improved by the pasers be subjected to electrolysis treatment.

According to the invention, the strength of the adhesion between carbon fibers and investment material during processing the fibers are increased to a composite material by electrolytic treatment of the fibers in an aqueous

electrolytic bath containing an electrolyte contains the strength of the bond between carbon fibers and investment material in a composite material improved, with the carbon fibers being the anode and the fibers and the clamp on which the anode current pulls- ™

will be moved relative to each other in the longitudinal direction.

In the method according to the invention, the carbon fibers form the positive electrode (anode) of an electrolytic treatment bath. An electric current is passed through the bath to carry out the treatment.

The term "aqueous electrolytic bath" does not include only solutions that contain a high proportion of water, but also electrolytes that contain a low proportion of water contain.

A number of different electrolytes have been used by the applicant used. It has been found that the following electrolytes improve the strength of the adhesion between fibers and Effectively improve investment material if the treated fibers are incorporated into a composite material!

1) Concentrated sulfuric acid

2) Concentrated sulfuric acid + chromium ions

3) Dilute sulfuric acid

4) hydrogen peroxide

5) Potassium permanganate solution

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6) hydrochloric acid + ferric chloride

7) saline solution

θ) sodium hypochlorite solution

Although all of these electrolysis have some effect, however, sodium hypochlorite is particularly preferred because it provides a particularly good improvement in adhesive strength between Pasern and investment material is achieved. Some of the remaining electrolytes (e.g. 1 and 2) are natural undesirable because of their corrosive properties, while others reduce the tensile strength of carbon fibers can worsen. In general, oxidizing agents appear to give good results. the Best results have been found by the applicant when using sodium hypochlorite.

It has been found that an aqueous sodium hypochlorite solution containing 12 to 16% by weight of available chlorine is very effective. Another preferred electrolyte, however, is a much weaker solution that has less than 1% by weight available Contains chlorine, and to which an acid, preferably hydrochloric acid, has been added.

The transmission of a current to the fibers is not a problem in itself, as this can be done using conventional methods, e.g. with With the help of clamps or by guiding the pasers through mercury or over a roll made of a metal or can happen from carbon or another conductive material (i.e. there is apparently no non-conductive "skin" present).

Since carbon fibers are preferably made in great lengths for making composite materials, it is expedient and economically advantageous to treat fiber lengths from 1 m up to 1000 m and even endless fibers treat continuously. An important limitation in any treatment of long strands or endless strands is

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but given by the fact that the Pasern a relatively have high resistance (e.g. 20 Ohm / m for fibers made of polyacrylonitrile and have a cross-sectional area of about 0.005 cm). In the case of a static arrangement, the Pasern that would connect to the connection or are closest to the power supply point, carry a larger current than the more distant pasers, and in the case of the electrolysis process, the effect would be stronger near the power supply point than at more distant ones Place. This is not desirable because the surface treatment of the fibers would not be uniform. In order to produce evenly treated fibers, therefore the power supply point is relative to the Paseracbse move. This can be done by moving the feed point relative to the stationary paser (for example when using a movable roller), however, this is much easier to achieve by using the Pasern is continuously moved over a fixed clamp through which the anode current is supplied.

The Pasern and the place where the anode current is supplied are preferably moved smoothly relative to each other.

For example, it is convenient to operate so that a length of the parsers are in electrically conductive connection of 'i is an electrically conductive roller which serves as an input terminal for the anode current, and is then pulled through the electrolyte solution containing the cathode.

Although the above description enables the construction of a device for the production of uniformly treated fibers, but no particularly economical Porm of a device is obtained. The reason this lies in the fact that the effect of the electrolysis is made uniform by the measures described, However, that does not address the problem of longitudinal potential drop affecting treatment time and costs

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the fiber axis is released. This drop in potential has the consequence that only the one closest to the supply terminal are exposed to the maximum current density for the electrolysis at any point in time. This can be Observe "good when using sodium hypochlorite as electrolyte", because with this electrolyte there is gas on the fiber surface is developed and the gas development in the vicinity of the terminal at which the anode current is supplied, is stronger.

According to a preferred embodiment of the device and the method, additional feed points for the anode current are therefore provided along the axis of the pasers intended. Their effect can be explained as follows: If you look at a clamp, for example, would be at a voltage drop along the fiber of 1 V in 1 cm when using a power source of 12 V is the limiting distance about 12 cm before the tension would reach zero. If, however, another electrode is placed after 12 cm that is connected to the same power source, this would have to compensate for the voltage drop so that the Voltage on the one hand along the Paser falls with increasing distance from the first input terminal, but on the other hand, increases with the approach to the next terminal, so that the voltage can be prevented from being increased falls to zero between any point on the fiber and the cathode.

The arrangement of a number of anode power clamps is conveniently done by placing the fiber over two conductive rollers leads, which represent feed points for the anode current (hereinafter referred to as anode rollers). This arrangement 'is shown in Fig.1.

1 shows in section a device according to the invention. The Paser is drawn off from a Vorlaseapule (1) and via a first anode roller (2) attached to the positive pole

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ÖAD ORIGINAL

the battery (3) is connected, and then passed through an electrolyte bath (4). The cathode (5) is one flat plate made of a suitable conductive material, e.g. graphite. It is located under the fiber in the bathroom and connected to the negative pole of the battery (3). The fiber is guided by guides (6) essentially parallel to the Cathode held and when leaving the bath on a second anode roller (7), which is also with the positive pole of the battery is connected. The fiber is then washed in a further bath (8) to remove any adhering Liberate the electrolyte, and then wound it on a second spool (9).

It should be noted that the anode rollers for the functioning of the device with a certain distance from each other must be arranged. The distance only changes the efficiency. If the distance is too great, the In the middle there is a zone from which no effective current is drawn and which is useless. When the roles too stand close together, the maximum treatment zone is not used. The distance is preferably chosen so that that at the selected working voltage and the selected working current in a given electrolyte the potential of the fiber is essentially the same over the entire fiber length. This not only enables maximum utilization of the treatment bath, it also means that the fiber has a fairly constant treatment rate is subject during their passage through the bath, thereby simplifying the monitoring methods. This distance can be determined experimentally by actually measuring the current or the voltage drop along the Fiber axis or by using visible effects, e.g. gas development, as a rough guide will.

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When the problems of uneven power distribution once recognized, further modifications of the treatment result. There is no reason the number of anode rolls to two, rather a number of rollers may be used arranged to accommodate the Alternating between dipping pasern into the electrolyte and leaving it. The arrangement of more than two roles means that the introduction of the current into the fiber on both sides of the anode roll and not just on one side, like this is the case with the device shown in FIG.

For example, the device with a number of anode rollers (11) can be roughly similar to that shown in FIG Manner (the other reference numbers have the same meanings as in Fig. 1).

In Figure 2, four anode rolls (11) are provided. On two In places, the fiber exits and re-enters the electrolyte, contacting two anode rollers. Between the anode rolls, the fiber is kept immersed in the bath by suitable guides.

The device shown in Figure 2 is not useful because the length of the treatment bath increases with each additional role. For example, would be preferred a non-linear arrangement of the rollers to make more efficient use of the bathroom space. This can be done by appropriate Guides or rollers or other means can be provided by which the fibers can be guided around corners.

In a particularly preferred embodiment of the treatment method and the device for performing this treatment is repeated over the fiber over the same Anode roller guided without it touching the fiber that has previously been passed over the roller.

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The invention accordingly comprises an apparatus having a conductive roller connected to the positive pole of a power source can be connected and is arranged over a bath that receives an electrolyte liquid, one Cathode in this bath, which can be connected to the power source, and means for guiding one several times Carbon thread over the anode roller and for guiding loops of the thread under the surface of the electrolyte between adjacent points of contact with the anode roller O

In a preferred embodiment of the method according to the invention, the terminals for the supply i of the anode current consist of a conductive roll connected to the power source, the carbon fiber running several times over this anode without touching the fiber which was previously over the roll and the carbon fiber is immersed in the electrolyte bath between adjacent points of contact between the fiber and the anode.

This form of apparatus is shown in FIGS. 3 and 4 and is described in detail in the following example, in which the details of an experimental apparatus on a laboratory scale are given. This device wei3t an upper conductive roller (21) which is to the terminal for supplying the anode current, and a lower roller (22)%, where the fiber is wound from roll to roll around the two rollers (23) helically.

The lower roll and most of the helical one The fiber is immersed in a bath (24) that contains the electrolyte (25). The cage made by the fiber and the Rolls is formed is surrounded by the cathode (26), which is also immersed in the electrolyte and preferably is generally U-shaped to match the shape of the roll and fiber cage, although it is not It is essential that the axes of the two rollers are arranged at a slight angle to one another, 30 da.3 the fiber

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BATH

seeks to advance from one end of the role to the other. The fiber lies in the places where it touches the roller surface, on a semicircular circumference on this roller in a plane that is im is aligned at right angles to the roll axis, and leaves the roll essentially tangential to the next roll where they turn a semicircular path along the surface of the second roll in the right angle to the axis follows. Since the roles are slightly inclined to each other, the path is Fibers, of course, from roll to roll in the form of a helix.

Fe during operation, however, the fiber has a low Tendency to deviate from the desired helical path, possibly even making adjacent loops of the Touches carbon fiber and causes a short circuit. It is useful to provide guides to guide the fiber on the chosen path to keep. These guides can be simple grooves in the circumference of the lower and / or upper Roll are cut.

The upper roller and lower roller advantageously have the same diameter and both are rotated at the same speed in the same direction, for example by a common drive, eg a chain or gear drive ψ by a motor (27). If the diameter of the rollers is different, the speeds of rotation must be different to compensate for this. As an alternative, one of the rollers can be positively driven and the other roller can be a free-running roller. Intermediate rollers could be inserted into the cage to increase the length of the fiber path, or the single anode roller could be replaced by two anode rollers so that the three rollers form an isosceles triangle, but these variations do not affect the principle of the invention. The upper anode roll can be made of any conductive material, but it should be borne in mind that with

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öad original

Electrolysis processes, especially those in which oxidizing solutions, e.g. chlorine-forming solutions, are used many metals can be attacked. Rolls made of inert metals, e.g. made of precious metals suitable, but the cost is a limiting factor. Therefore, roles from are preferred Carbon. The lower roller is preferably not made of a conductive material or with a non-conductive surface, otherwise it would become part of the anode.

The cathode may be, like the anode from any electrically conductive material prepared, etc.

however, it should preferably not be attacked by the solution and the electrolysis process. Otherwise it should they can be cheap to replace. A carbon cathode is preferred.

The distance between the upper roller and the lower one The role is important to ensure that the current effectively penetrates the fiber over its entire length. at Using a 12 V power source, the most effective distance is about 15 cm. Just like that shown in Fig.X simple device, the device shown in Fig. 3 and 4 works most effectively with the optimal distance, while space is wasted when the distance | is too big or too small.

The influences of the concentration of the electrolyte, the drive speed the rolls and the number of turns of the fiber and thus the total treatment time for any given fiber length, temperature, agitation, tension, etc. to the degree of surface treatment seems to have the usual relationship to the electrolytic effect as in other electrolysis processes, e.g. 3. in the deposition of metal layers, etc.,

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except that the surface treatment reaches a maximum » when the entire paser surface is saturated with occluded ions or surface layers of ions (or whatever process takes place on the surface). It's easy to spmit factors like tension, concentration and to vary the speed of the pair throughput and here the limit effects of temperature and others. disregard and in this way an appropriate treatment time and the size of the equipment.

Mechanical stirring or ultrasonic vibration can be used to ensure that the solution is acceptably hornogenic remains, but this is an optional refinement of minor importance.

At times it is desirable to have a knife close to the top Place a roll to cut off any loose ends on the carbon fiber. This knife cuts off the paser, so that tangling is prevented when loose ends interrupt smooth running of the apparatus.

Further modifications are possible as required. A wash bath and additional rollers can be provided to guide the paser through the bath to hold the electrolyte solution to wash off the paser rising from the lower roller, before the Paser runs over the anode roll again. This would make the selection with regard to the materials that could be used as a top roller, enlarged.

example 1
The device shown in Pig. 3 and 4 is used.

This device is provided with two rollers which have a diameter of 20 mm and a length of 120 mm and are inserted into a "Perspex" frame so that they

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have a distance of 150 mm from center to center. The top roller is made of graphite (Morganite EY9) and the lower roll made of hard polyvinylchloride. Both rolls are level, but the top roll is over 15 ° 28 'inclined in the horizontal plane about its center line to the axis of the lower roller. Form when viewed from above the two roles thus an X.

The carbon fiber is fed continuously between the rollers (pitch 8 mm) at a linear speed of up to 20 m / hour. guided by the rollers being driven synchronously at speeds of up to 50 rpm. Thereby, the fiber under a voltage of about 2 to 10 kg overall ä

brings.

The whole thing is used in a housing made of polyethylene, which contains the electrolyte, that about 150 mm Paser per Turn of the strand are immersed. The electrolyte consists of a sodium hypochlorite solution that contains 12 to 16% by weight of available chlorine.

The cathode used is U-shaped, surrounds the fiber windings and consists of graphite (Morganite EY9).

A potential difference of 6 to 12V is created between the Power supply point and the cathode adjusted so that the current used is about 1.5 A per fiber turn. ™

The strength of the electrolyte is determined by iodometric titration of an aliquot at regular intervals (e.g. every hour). When the content of available Chlorine has fallen to 7 wt .- #, the experiment is terminated, the electrolyte is discarded and fresh sodium hypochlorite introduced before continuing operations.

The fiber emerging from the electrolysis bath becomes with 5 to 10% hydrochloric acid and then washed with water and dried.

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Carbon fibers "treated by this method" could be properly embedded in a resin and showed improved properties. For example, a fiber produced at a high graphitization temperature (type I fiber) in an investment material made of a resin has a shear strength from 176 to 246 kg / cm. However, when the fiber has been treated according to this example, this value increases to 422 to 703 kg / cm ² .

A similar improvement can be seen in the properties of resin-bonded fibers that have not been exposed to these high graphitizing temperatures (type II fibers). In this pail, the shear strength increases from 422-492 kg / cm to a value of 703 to 1055 kg / cm ² .

Example 2

This experiment was carried out in the same way as the experiment described in Example 1 using the the same apparatus and the same carbon fiber. The electrolyte bath was much weaker and passed the end

2% by weight of sodium hypochlorite (12 to 16% by weight available Chlorine)

1 wt .- ^ ^v concentrated hydrochloric acid and 97 wt. -I »Was8 he

The solution is so diluted that it is not necessary to wash the carbon fiber after treatment. Composites, those made from the fiber and resin treated in this way had physical Properties that are completely comparable with the physical properties of the material obtained according to Example 1 was.

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Claims (9)

Claims 1) Method of treating carbon fibers for the purpose of increasing. increase the strength of the bond between the lines and an investment when the lines are in a composite material are incorporated, characterized in that the Paser an electrolytic treatment is subjected to an aqueous electrolyte, which increases the strength of the bond between the fibers and an embedding material in a composite material, the carbon fiber being used as the anode is switched and the Paser and the terminal for the supply of the anode current in the longitudinal direction is relative are moved towards each other. 2) Method according to claim 1, characterized in that an oxidizing agent is used as the electrolyte. 3) Method according to claim 2, characterized in that a solution of a metal hypochlorite, preferably one aqueous sodium hypochlorite solution, is used as the electrolyte. 4) Method according to claim 1 to 3 »characterized in that the flow of the fiber at several along the" Paser distributed contact points for the introduction of the anode current is fed. 5) Method according to claim 1 to 4, characterized in that the thread and the or each insertion point for the anode current can be moved smoothly relative to each other. 6) Method according to claim 1 to 5, characterized in that that the or each connection point for the supply of the anode current is held stationary and the thread moves will. 109817/2259 7) Method a & ah claims 1 to 6, characterized in that the; Anacfalussateeile for the supply of the Aßodenatroine consist of a conductive should * which is connected to the power source, whereby the carbon thread runs several times over the anode without touching the thread previously run over the roll, and between adjacent contact point *! Des. Bathing with the Aaode immersed in the electrolyte bath. Q) Method according to claim 1 to 7, characterized in that that the thread is washed after the electrolytic treatment. 9) Device for performing the method according to claim 1 to 8, characterized by a conductive roller (21) attached to the positive pole of a power source can be connected and over a bath (25) that receives an electrolyte liquid is arranged, a cathode (26) arranged in the bath (25), which is connected to the Power supply can be connected, and means that allow the carbon thread (23) several times over the Anode roller (21) and the loops of the thread under the electrolyte surface between adjacent contact points with the anode roller (21). 1 0 9 8 1 7/2 2 F> 9 Lee r side