JPS5824554B2 - Electrolytic treatment method for graphite fibers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides graphite fiber (graphite fiber)
r) regarding electrolytic surface treatment.

Composite materials such as graphite fiber-reinforced plastics are currently being produced as industrial materials and are finding widespread use in aircraft construction, underwater equipment, boat hulls, and other areas.

Composite materials exhibit many combinations of properties not available with previously known structural materials.

However, the strength properties and durability of the strength properties of composite materials are highly dependent on the bond strength between two sides of the material, i.e. between the graphite fibers and the composite resin matrix, especially in adverse environments. Depends on it.

The shear strength values of the alternating layers, which are a measure of the degree of bonding or adhesion of the reinforcement to the resin matrix, are low for graphite fiber reinforced composites, thus limiting the potential use of these materials.

By improving the adhesion properties of graphite fibers to resin matrix,
A number of methods have been developed to improve the shear strength of the alternating layers of composite material formed.

One of the main methods used in the past is electrolytic surface treatment of the fibers.

In such methods, graphite fibers are subjected to an electrolytic reaction in an aqueous electrolyte so that negative ions are attracted to the surface of the fibers, which acts as an anode, thus altering the fiber surface.

As a result of this surface modification, the bonding strength of the treated graphite fibers to the resin is improved to the extent that the shear force is increased by a factor of 1, with little or no loss of tension.

A number of electrolytes commonly used in the form of aqueous solutions are recognized in the prior art that can be used to surface treat graphite fibers.

Some of the electrolytes disclosed in the art for this purpose include sodium hydroxide, potassium hydroxide, phosphoric acid, nitric acid, sulfuric acid, and others.

One preferred electrolyte is aqueous sodium hydroxide.

This material has been considered desirable because it has high electrical conductivity and thus has the advantage that many fiber ends can be processed in an electrolytic bath at high speeds and low voltages.

The main drawback in using aqueous sodium hydroxide as the electrolyte is the difficulty in removing all the sulfur hydroxide electrolyte from the fibers.

Removal of the electrolyte is accomplished by rinsing the treated fiber with relatively pure water.

Contaminants in the water and residues on the fiber surface from the electrolyte, such as HaHCO3 or Na2CO3, often form voids in composites made from such fibers, identified by surface blistering, must be pure.

Such voids or blisters are particularly apparent when the composite is boiled or exposed to high humidity.

To avoid the above-mentioned disadvantages inherent in the use of electrolytes such as sodium hydroxide, aggressive washing conditions with pure water must be used to remove substantially all electrolyte residues.

Vigorous cleaning of graphite fibers often results in entanglement of the filaments and creates a network of loose filaments that is difficult to remove and poses quality control problems in the manufacture of graphite fiber reinforced composites. cause

Ammonium sulfate,
Known to ammonium nitrate and ammonia.

(Special Publication No. 47-29942). However, ammonium nitrate is potentially highly explosive as an electrolyte.

It is particularly dangerous when mixed with oxidizable materials, such as carbon fibers and their residues.

Therefore, we did not conduct an experiment using ammonium nitrate as the electrolyte.

In addition, ammonium is very volatile and contaminates the surrounding environment where it is treated, and due to volatilization, the concentration of ammonium hydroxide changes rapidly, making it extremely difficult to control the degree of surface treatment of carbon fibers. It is difficult to

Ammonium sulfate was 250% as shown in Comparative Example 1 below.
It does not decompose below °C and therefore remains on the surface of treated carbon fibers.

The present invention is a method for electrolytically surface treating graphite fibers such that no electrolyte remains after treatment, comprising organic and inorganic ammonium which substantially completely decomposes into gaseous products when heated to temperatures below about 250°C. An improved method is provided in which the compound is dissolved in water.

Organic and inorganic ammonium compounds that can be used in the process of the invention include ammonium carbonate (N)T4)2CO3,1
, ammonium bicarbonate [(NH4)HC03], ammonium carbamate [NH4NH2CO21 ammonium benzoate (NH4・C7H3O□]), ammonium dithionate ((NH4)2 S206), ammonium hydrosulfide [NH4H8], ammonium sulfite ((NH4)
2 so3・H20], ammonium thiosulfate [(NH
4) 2S203], ammonium tartrate ((NH4,)
2 C4H40a) and mixtures thereof.

In order to carry out the electrolytic process of the present invention efficiently, it must be carried out at a potential sufficient to dissociate oxygen on the graphite fiber (anode) surface.

Electrical treatment levels of fibers are expressed as coulombs passing per inch of fiber length, i.e., to account for different combinations of current level, feed rate, and number of tows to be treated. , is standardized as the ratio of the total current per second to the raw material supply speed (inch 7 seconds).

The above definition of treatment level assumes that the degree of fiber treatment depends only on the amount of current passed per unit length of fiber during treatment.

This assumption was based on the NOL short beams (5h
or -tbeam) in agreement with the results of shear strength measurements.

According to the method of the present invention, about 0.5 to 15 colon/
A fiber electrical treatment level of inches is used.

A preferred electrical processing level is about 260 to about 6.0 corons per inch.

As used herein, the term "graphite fibers" refers to graphite fibers and This means that it includes both carbon fiber.

The following examples illustrate the method of the invention.

In the examples, parts and percentages are by weight unless otherwise specified.

Examples 1-7 Graphite fibers were electrolytically treated using the improved electrolyte of the present invention as follows.

The graphite fibers were passed through a series of anodes and a non-conductive idle roll (1 dl-error) mounted on a stainless steel shaft to wind the graphite fibers (1 dl-erroll) as shown in the drawings.
ler 5pool) I pulled out the drawer.

The empty roll is disposed intermittently apart from the anode and is immersed in an aqueous electrolyte.

A series of spaced apart graphite cathodes are also immersed in the electrolyte.

A direct current power supply was used to provide the necessary potential between the anode and cathode.

The anode side of the power supply was held at base potential to reduce electrical hazards and establish a reference point.

The graphite fibers acquire a negative potential when they come into contact with the anode, and then the surface-treated graphite fibers coming out of the outlet of the electrolytic cell are treated as anodes in an electrolyte bath.The electrical treatment level of the working fibers is shown in Table I. , passed through a hot air oven as illustrated in the drawings operating in the range from 93° C. at the inlet to 125° C. at the outlet.

The residence time of the fibers in the hot air oven is approximately 5 minutes.

The dried graphite fibers were collected on a winding roll under constant tension for further use and testing.

Graphite fibers were surface treated in the manner described above using the electrolyte of the present invention at various concentrations and using different electrical levels as shown in Table I.

It was used to make a composite using ERLA 2256 resin (manufactured and commercially available by Union Carbide Company).

Samples of the composite were prepared in the form of N0LIJ rings containing approximately 60% by volume treated graphite fibers.

In making the composite, the graphite fibers were passed through an epoxy resin system, passed through a tensioning device, and placed into a rotary mold.

The entire system was enclosed in a vacuum chamber to produce low porosity composite specimens.

The mold was removed from the NOL winder and placed into a curing open to cure the resin.

For the above resin system, the resin was cured at 125°C for 2 hours and then at 155°C for 4 hours.

The epoxy resin system used was ER, a mixture of 35% by weight bis-2,3-epoxycyclopentyl ether and 65% by weight diglycidyl ether of bisphenol A.
Consists of LA2256.

The curing agent or curing agent used in this resin system is a eutectic mixture of metaphenylene diamine and methylene dianiline.

A discussion of N0LIJ samples and how to make them can be found in Plastic Technology, November 1958, p.
p, 1017-1024, and Annual Technical Conference SPI Reinforced Plastics Division Bulletin No. 21.8-D, February 1966.

Composite samples prepared as described above were treated (1) without further processing, and (2) after boiling the samples in water for 72 hours.
Tested for short beam shear strength according to ASTM-2344.

The boil test is a rapid test to determine whether blisters or voids appear in composites made using treated graphite fibers.

The results of this test showed that there was no visible blistering in composites made using graphite fibers treated according to the present invention and subjected to an underwater boil test.

These results indicated that ammonium electrolyte and residual compounds were substantially completely removed from the electrolytically treated fibers as a result of evaporation of both the electrolyte and residual decomposition products during the post-electrolysis drying operation.

Shear strength retention was high. To further evaluate fibers surface treated using the electrolyte of the present invention, the treated fibers were dried and refluxed in distilled water using 20 parts of distilled water per part of treated fiber.

After this reflux, the iract conductivity of the water was measured.

The results shown in Table 1 indicate that the conductivity of the fibers treated without water washing is low.

The fibers from Examples 5, 6 and 7 were drum dried on a steam heated rotating drum with a surface temperature of about 120°C.

The contact time of the fibers with the heated surface is about 30 seconds to about 90 seconds depending on the feed rate used.

The electrical conductivity values reported in Table 1 were approximately equivalent to those of graphite fibers subjected to an electrolytic process in which sodium hydroxide was used as the electrolyte and the fibers were thoroughly and repeatedly washed with pure water.

A proportional example illustrating its use.

Example 8-17 The electrolytically surface treated graphite fiber method described in Example 1-1 was repeated using 10% sodium hydroxide as the electrolyte.

Electrical processing levels for these examples varied from about 2.0 to about 6.0 coulombs per inch.

The surface-treated graphite fibers emerging from the outlet of the electrolytic cell were washed with water to remove residues containing sodium hydroxide from the surface of the fibers.

Washing was repeated until the water extract had a conductivity level of less than about 50 μmho/CTL.

In all examples the water extract conductivity was measured according to British Standard 3266:1969 entitled "Method for Determination of Conductivity, pH 1 Water Soluble Substances, Chlorides and Sulphates in Aqueous Extracts of Textile Materials". The rate was measured.

The washed graphite fibers were then used to prepare composite swatches according to the methods set forth in Examples 1-7.

Examples 12, 14 and 16 were carried out respectively except that the fibers used in Examples 13, 15 and 17 were further washed with water to remove all traces of sodium hydroxide and residual products from the surface of the fibers after electrolysis. Examples 13.15 and 17 were repeated.

The prepared composites were tested according to the methods used in Examples 1-7.

The test results are shown in Table ■. From the data in Table 1, it is clear that even if the electrolytically surface-treated fibers, in which the electrolyte is sodium hydroxide, are washed with water multiple times, the blistering rate of the composites as a result of the 72-hour water boiling test is high.

When using the improved electrolytes of this invention in the methods of this invention, there are a number of factors that must be weighed to determine the preferred processing conditions.

Thus, it is known that the electrical resistance of graphite fibers produced using, for example, different precursor materials, pretreatments and different methods varies substantially.

Also, the electrical resistance of various electrolytes suitable for use in the method of the invention will vary over a wide range.

Electrolytes such as ammonium carbonate and ammonium bicarbonate are preferred from the standpoint of low electricity consumption when carrying out the method of the invention.

Another factor that must be considered in the electrolytic process is the residence time in the electrolytic cell at a given electrolyte concentration and operating voltage.

These factors can be considered for each fiber type and cell system to achieve optimal performance of electrolytic surface treatment using the improved electrolyte of the present invention.

Comparative Example 1 Carbon fibers were electrolytically treated to produce surface-treated carbon fibers using an IN solution of ammonium carbonate and ammonium sulfate in deionized water as electrolytes.

The electrolytic treatment level in the electrolytic surface treatment of the fibers was 2.6 Cron/inch.

The surface treated fibers were collected directly from the electrolytic bath on two separate polyethylene coated reels.

That fiber. were then rolled onto separate glass beakers and placed in a preheated high-air oven at 250° C. for 15 minutes, removed and cooled to room temperature.

Each of the fibers was then extracted with boiling deionized water for 1 hour using a Soxhlet extractor.

The fibers were preweighed in the extractor and 20 cc of deionized water was used per 12 dry fibers for extraction.

After cooling to room temperature, the extract was measured for conductivity using a model 31YSI (Yellow Springs Instruments conductivity bridge).

The results are shown in Table ■.

Publishers Inc., Sunduski, Ohio In this case the ammonium sulfate remains largely in the treated carbon fibers, so that the conductivity of the water extract is very high.

Over a long period of time, the presence of such conductive impurities will cause
It adversely affects the bond stability of carbon fibers and causes blistering of epoxy laminates exposed to moisture.

[Brief explanation of drawings]

The figure is a schematic diagram of an apparatus for carrying out the method of the invention.

Claims (1)

Claims: 1. A method of surface treating graphite fibers by electrolysis in an electrolytic bath, the graphite fibers being in direct contact with and forming part of an anode. in which the level of electrical treatment is sufficient to dissociate oxygen at the surface of the graphite fibers: as an electrolyte in an aqueous solution, when heated at temperatures below 250° C., substantially decomposes to produce gaseous products; Ammonium carbonate, ammonium bicarbonate, power. using an ammonium compound selected from the group consisting of ammonium rubamate, ammonium benzoate, ammonium nitionate, ammonium hydrosulfide, ammonium sulfite, ammonium thiosulfate, ammonium tartrate, and mixtures thereof; A method characterized in that the electrolytically treated fibers are heat treated at a temperature of 250° C. or lower to remove residual electrolyte from the fibers.