WO1992007661A1

WO1992007661A1 - Apparatus and method for coating fibers using abrupt pressure change

Info

Publication number: WO1992007661A1
Application number: PCT/US1991/008163
Authority: WO
Inventors: Bruce K. Redding, Jr.
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-10-31
Filing date: 1991-10-31
Publication date: 1992-05-14
Anticipated expiration: 1993-04-30
Also published as: MX9101891A; AU9041791A

Abstract

An abrupt pressure change applied to a fiber (2) immersed in a liquid medium, comprising a coating material (1) coats the material onto the fiber (2). The method and apparatus of this invention permit a wider range of fibers and coating materials to be utilized than was possible with prior art methods, including materials which previously have not been usable as coating materials for fibers. By controlling the intensity of the abrupt pressure change, as well as the time of exposure to repetitive abrupt pressure changes, the thickness and hardness of the resulting coating on the fiber can be adjusted. The coated fibers can be made with several coats to increase strength or to improve other characteristics. In the preferred embodiment, ultrasound is used to generate the abrupt pressure change. Fibers may be coated by the method and apparatus of the present invention in a small fraction of the time required by the methods of the prior art.

Description

I

APPARATUS AND METHOD FOR COATING FIBERS USING ABRUPT PRESSURE CHANGE

Technical Field

This invention relates to the field of coating fibers. Coated fibers have many applications, such as in filament coatings in electrical applications, soil resistant fibers for clothing, and anti-static fibers for use in carpets. Examples of processes for coating fibers are given in the following United States Patents:

4,374,161 4,338,357 4,171,403

4,507,324 3,831,551 4,141,315 4,505,223 4,774,135 4,749,614

4,561,377 4,561,378

Background Art

The above-mentioned references describe several liquid-phase methods of coating fibers whereby a fiber is passed through a liquid bath. The liquid in the bath is either itself a coating material or is a carrier medium which has dissolved or suspension within it a material which will coat onto the fibers. The fiber may be passed through the liquid bath either in bulk process or as a single filament. As the fiber passes through the bath, it becomes coated by either the bath material or the resins or other materials suspended or dissolved within the bath. The bath material may be deposited directly onto the surface of the fiber or may be absorbed into the structure of the fiber if the fiber is porous.

The coating processes described above have many disadvantages. For instance, it is difficult to achieve precise control of the thickness of the coating. Inadequate agitation of the mixture frequently produces fibers which have coatings which are either too thick or too thin, or which are often beyond the size range suitable for the desired end application of the fiber. Solidification of the coating material after it has been applied to the fiber is also a problem. A hardened coating is often essential to enhance the shear impact resistance of the coated fiber and to enable the coated fiber to withstand higher temperatures. Many liquid-phase methods of fiber coating are also inefficient in the sense that many of the resulting fibers have incompletely coated surfaces. Such situations are often referred to as "Spotted Coatings. " A method which uniformly and completely coats fibers is desired for many industrial applications. Li addition to the disadvantages discussed above, the prior art coating processes are very time-consuming. The time required to coat the fibers thus adds significantly to the cost of their manufacture. Conventional liquid-phase methods of coating fibers are also limited by the types of materials which can be applied to the surface of the fiber. In addition, it is sometimes the case that the fiber material reacts with the coating material or to various cross-linking agents employed in the liquid bath containing the coating material. Thus, the structure of the fiber is compromised.

There is presently a great demand for coated fibers which can be inexpensively manufactured, and which are suitable for various industrial applications. The present invention provides a method and apparatus for making coated fibers with hardened and solidified coatings which can be produced in a small fraction of the time required by conventional methods. The present invention also permits the accurate adjustment of the thickness of the coating layer.

Disclosure of Invention Applicant has discovered an entirely new method and apparatus for coating fibers which increases dramatically the types of materials which can be coated onto fibers as well as the types of fibers which may be coated. The method and apparatus of this invention allow the use in coating processes of coating materials and fibers which could not be utilized with prior art processes. In addition, the time for coating the fibers is reduced by the method and apparatus of this invention to either a few minutes or a few seconds, as opposed to a maximum of several hours using the prior art processes. According to the method of this invention, the fibers are first immersed in a liquid medium. The liquid medium may comprise the coating material itself or have a coating material dissolved or dispersed within it. An abrupt pressure change then applied to the liquid medium containing the fiber causes the coating material to envelop and coat the fiber. The underlying physical mechanism by which an abrupt pressure change causes the coating of the fibers is not well understood. It is suggested that pressure shock waves, shear forces, and/or cavitation effects resulting from the abrupt pressure change may be the operative mechanism. However, there may be other mechanisms involved. Whatever the mechanism, the discovery that an abrupt pressure change applied to a fiber immersed in a coating medium results in coating of the fiber represents a significant advance in the art of fiber coating technology. In addition, the thickness and hardness of the fiber coat can be adjusted by regulating the magnitude of the abrupt pressure change and the period of time and number of abrupt pressure changes to which the fiber and coating material are subjected.

The coated fibers formed by the method of this invention may be recycled by using the method of the invention to alter the characteristics of the fiber coat. Thus, the fiber coat may be strengthened, made less porous, made more porous, or constructed of multiple coating materials quickly and easily by the method of this invention. In the preferred embodiment of the invention, ultrasound is applied to the liquid medium containing the coating material and fiber. It is believed that the expansion and collapse of the resulting cavitation bubbles produces a localized abrupt pressure change which causes the coating. In its simplest form, the horn of an ultrasonic transducer is introduced into a vessel which contains both the fiber and the coating material in a liquid medium. The fiber and liquid medium are then subjected to high intensity ultrasound. It is found that the abrupt pressure changes generated by the ultrasound coat the fibers almost instantaneously. The coated fibers can then either be withdrawn from the vessel before the next length of fiber is introduced, or subjected to repeated or prolonged exposure to additional ultrasound. Fibers may be coated in this manner either in a continuous or semi- continuous process. In a semi-continuous or batch process, the fibers are placed in the coating medium, exposed to ultrasound, and then removed. In the continuous process, a filament of the fiber is drawn continuously through a coating medium while being subjected to ultrasound.

In a variation of the process described above, the abrupt pressure change may be used to complete a coating process begun by one of the prior art conventional techniques. In such a combined process, the fibers are first coated according to prior art processes, then subjected to an abrupt pressure change while the fiber is still contained within the liquid medium containing the coating material.

Generally, it has been found that higher intensity ultrasound tends to produce thinner coatings, while low intensity ultrasound tends to yield thicker coatings. The thickness of the applied coat can be regulated by controlling the amount of time the fiber is exposed to the ultrasound during the coating process as well as the ultrasonic intensity. The method and apparatus of this invention can also be used to repair coated fibers, that is, to complete the layering of coating materials so that the coat has a given minimum thickness. In another variation of the invention, fibers previously coated by the method and apparatus of this invention may be recycled using the method of this invention to form additional coating layers over the original coated fiber. All this may be accomplished in a minimal amount of time. In yet another variation of the invention, fibers coated initially by the method of this invention with one coating material may have further coats added by the method of this invention by being cycled through liquid mediums containing different coating materials, each different coat being applied by the application of an abrupt pressure change. In this way, it is possible to produce coated fibers having multiple coating layers, each of a different material and each of a desired thickness.

Therefore, it is a first object of this invention to provide a fiber coating method which utilizes an abrupt pressure change applied to a liquid coating medium which contains the fiber.

It is a further object of this invention to provide a method and apparatus for coating fibers which produce the coated fibers in much less time than in prior art processes. It is another object of the present invention to provide a method of coating fibers, wherein the thickness of the coated fibers can be easily adjusted.

It is another object of the present invention to provide a method of coating fibers wherein the thickness of the coat may be made substantially uniform. It is another object of the present invention to improve the fiber coating made by conventional liquid-phase fiber coating methods by applying an abrupt pressure change to complete or adjust the coating process. It is another object of the invention to provide an apparatus which may be used to apply an abrupt pressure change to a fiber immersed in a liquid coating medium.

It is another object of the invention to reduce substantially the cost of producing coated fibers.

It is a further object of the invention to provide a method for coating fibers in a continuous process.

Other objects and advantages of the method and apparatus of this invention will become apparent to those skilled in the art from the description of the preferred embodiment which follows:

Brief Description of the Drawings

Figure 1 is a schematic diagram showing an ultrasonic transducer applying an abrupt pressure change to a fiber contained within a liquid coating medium. Figure 2 is a schematic diagram showing an ultrasonic transducer applying an abrupt pressure change to a continuous fiber filament drawn through a liquid medium.

Detailed Description of the Invention And Best Mode for Carrying Out the Invention The methods and apparatus of the present invention are based upon the discovery that an abrupt pressure change applied to a liquid coating medium in which a fiber is immersed causes the fiber to be coated by the coating material. This phenomena is unknown in the prior art of fiber coating. As mentioned above, fibers coated by the method and apparatus of this invention may be recycled and coated with additional layers by the method and apparatus of this invention. Also, fibers coated by other prior art methods may be recoated by the method and apparatus of this invention. Coats of different coating materials may be applied by repetitive application of the method of this invention using a different coating medium each time. In both cases above, the thickness of the coats may be varied by controlling the intensity of the abrupt pressure change as well as the period of time during which the fiber is exposed to abrupt pressure changes. The coating material may consist of any of the commonly known coating materials used in prior art processes, including those which are said to form films upon dissolution in solvent, as well as those which are said to be colloidal in nature and bloom into gelatinous masses in the proper solvent. In addition, it is possible to use, as coating materials, those materials which are not suitable for use as coating materials by methods known in the prior art. As mentioned above, the coating material may be a polymer-based material which dissolves partly into a film, or it may be a gelatinous material which will swell into a manipulatable mass. The coating mixture or dispersion may be made by generating a micro-dispersed state through any available means, including batch mixers, static mixing devices, motionless mixers, and fluidization homogenation equipment. Table 1 lists some typical coating materials which may be used with the method and apparatus of the present invention. This table is meant to be representative, however, and not inclusive. TABLE 1

Some Fiber Coating Chemicals

Natural Polymers

Carboxymethylcellose Zein

Cellulose acetate phthalate Nitrocellulose

Ethylcellulose Propylhydroxylcellulose

Gelatin Shellac Gum Arabic Succinylated gelatin

Starch Waxes, paraffin

Natural Polymers Bark Proteins

Methylcellulose Kraft lignin

Arabinogelactan Natural Rubbe TABLE 1 (Cont'd) Some Fiber Coating Chemicals

Synthetic Polymers

Polyvinyl alcohol Polyvinylidene chloride

Polyethylene Polyvinyl chloride

Polypropylene Polyacrylate Polystyrene Polyacrylonitrile

Polyacrylamide Chlorinated polyethylene

Polyether Acetal copolymer

Polyester Polyurethane

Polyamide Polyvinylpyrrolidone Polyurea resins Poly (p-xylylene)

Epoxy resins Polymethyl methacrylate

Ethylene-vinyl acetate copolymer Polyhdroxyetyl methacrylate

Polyvinyl acetate Synthetic Elastomers

Polybutadiene Acrylonitrile Polyisoprene Nitrile Neoprene Butyl rubber Chloroprene Polysiloxane

Styrene-butadiene rubber Hydrin rubber Silicone rubber Ehtylene-propylene-diene copolymer

Other Chemicals

Amino silanes Phenolic resins Acetate resins Sulphides Methacrylate Latexes Mineral or Vegetable oils Fatty Amines Fatty Acids Lubricants Photo resistant chemicals Teflon Soil resistant coating Thermoplastic resins Anti-static coating

In the preferred embodiment, the abrupt pressure change is generated by use of ultrasound. The horn of an ultrasonic transducer is immersed in the coating solution, and the resulting ultrasound causes cavitation to occur within the coating medium. It is known that the expansion and collapse of cavitation bubbles produces significant abrupt pressure changes within the liquid medium. The actual mechanism by which an abrupt pressure change applied to a coating material within a liquid medium causes the coating material to coat fibers is not entirely understood. It is possible that the pressure shock waves created in the liquid medium by the abrupt pressure changes generated by cavitation are directly responsible for the coating of the fibers. However, it is also possible that the abrupt pressure changes and/or the pressure shock waves generated by the expanding and collapsing cavitation bubbles generate shear forces at the interface of the fiber and the surrounding coating material which cause the coating material to surround and coat the fibers. It is also possible that other unrecognized cavitation effects or their consequences induce the coating material to coat the fibers. Finally, it is possible that localized temperature changes and/or gradients caused by the abrupt pressure changes, pressure shock waves, shear forces, or cavitation actually induce the coating material to surround and coat the fibers. Whatever the precise mechanism of action may be by which the abrupt pressure change works on a coating material within the liquid medium at the interface between the coating material and the fiber, the coating of the fibers by such process represents a new and unanticipated advance in the art of fiber coating.

As noted above, in the preferred embodiment, an ultrasonic transducer is used to produce cavitation within the liquid medium, thereby causing the abrupt pressure changes. Figure 1 shows an ultrasonic apparatus which may be used to apply an abrupt pressure change by means of cavitation effects to the coating medium. A treatment container 7 is mounted on heater/stirrer 11 which is a combination heating element and magnetic stirrer. Magnetic stir bar 10 is located at the bottom of treatment container 7. The fiber 2 to be coated is held in place by two mounting blocks 6a and 6b. The container 7 is filled with a polymer coating material 1 which is suspended within fluid 3. Ultrasonic converter 5a is suspended so that horn 5b may be placed within the coating liquid in treatment container 7. Converter 5a is connected by power cable 8 to ultrasonic generator power supply 12. The horn of the ultrasonic transducer is placed directly above the fiber to be coated, and ultrasound is generated in the medium by the ultrasonic transducer. The ultrasound produces cavitation effects within the medium. The ultrasound sound waves are diagramically represented at 4. Magnetic stir bar 10 driven by heater stirrer 11 causes the suspended coating material 1 to circulate about the fiber during the time that the ultrasound is being applied. In these circumstances, the fiber is coated with the coating material. The abrupt pressure changes caused by the ultrasound are applied until the coating of the fiber is complete or until a desired thickness is achieved. The coating process continues as long as the ultrasound is applied. The frequency (or wave length) as well as the intensity level of the ultrasound may be adjusted as necessary to achieve optimum coating results with different coating mediums. The thickness of the coating may be similarly controlled. Any apparatus which produces abrupt pressure changes within the coating medium will serve to cause the coating material to surround and coat the fiber. Thus, it is possible to replace the ultrasonic transducer horn 5b with another source of ultrasonic emission. Such a source could include a method of vibrating treatment container 7 at sufficiently high frequencies. For instance, the ultrasonic transducer horn 5b could be mechanically connected to treatment container 7. The apparatus illustrated in Figure 1 is the most basic embodiment of the apparatus of the invention. Clearly, many fibers could be coated simultaneously with only a slight modification of this apparatus. In this apparatus the abrupt pressure changes are produced by the cavitation bubbles expanding and collapsing, the cavitation bubbles being generated by ultrasound being applied to the medium. Coating material may or may not be permeable to the fiber material or any other material to which a completed coated fiber may later be added. The abrupt pressure change causes the coating material to coat, penetrate, and seal a fiber, and then to harden into a final solidified form. It should be noted that the abrupt pressure changes generated occur over a very short time interval. In the preferred embodiment of the invention, the expansion and collapse of the cavitation bubbles produces the abrupt pressure change. As stated earlier, the exact physical mechanism by which the fibers are coated by the above process is not known. It is believed that the application of the abrupt pressure change produces a pressure shock wave which directly the causes the coating material to coat to the fiber. On the other hand, the shear forces generated by the abrupt pressure change, the pressure shock wave, or, perhaps, by local cavitation effects may be responsible for the coating of the fiber. Indeed, the fiber coating may even be achieved by thermal effects induced by the abrupt pressure change caused by the expansion and collapse of the cavitation bubbles. For purposes of this Application, the term "abrupt pressure change" is intended to encompass the actual physical mechanism by which the fiber is coated, including, but not limited to, pressure shock waves, shear forces, and other recognized or unrecognized cavitation effects. Ultrasound is usually considered to work its effect through the creation of cavitation bubbles in the medium, although pre-cavitation oscillation in the medium occurs. The collapse of cavitation bubbles is accompanied by localized abrupt pressure changes, which cause pressure shock waves, shear forces, and abrupt temperature spikes. Applicant is uncertain which of these postulated mechanisms, if any, is directly responsible for causing the coating of the material onto the fibers, but has discovered the effect. There are several variables which may be changed with the preferred ultrasonic apparatus. For instance, the intensity of the forces generated by the ultrasonic transducer is determined not only by the power supplied to the transducer, but also by both the geometry and the volume of the vessel in addition to the characteristics of the liquid medium. Further, to ensure adequate exposure of the coating material to the fiber in the presence of the ultrasound, continuous agitation throughout the period of ultrasonic treatment is required and is provided by the heater/stirrer 11 and magnetic stir bar 10. The intensity and frequency of ultrasound required to coat different fibers is dependant upon the materials in the coating mixture, the solids content of the coating mixture, and, finally, the temperature of the mixture or dispersion.

Sound transmissions through a slurry are effected not only by the viscosity of the fluid, but also by the temperature and density of that fluid. Paniculate matter dispersed within the coating fluid may also act to alter the transmission efficiency of the sound waves and the corresponding abrupt pressure changes associated with the expansion and collapse of the cavitation bubbles. In the preferred embodiment of the invention, high intensity ultrasound is applied for a short period of time. The advantage of using this method is that the time to coat is reduced significantly and coating occurs more rapidly. However, it is also possible to reduce the level of intensity of the ultrasound to lengthen the time to coat a given fiber. The optimum power level used will depend upon the physical characteristics of both the fiber and coating medium. Additionally, the power level may be regulated to vary the thickness of the coating.

As stated above, the intensity and frequency of the abrupt pressure change necessary to coat fibers depends on several factors, including the physical state of the liquid coating medium. These factors clearly effect the interval during which the abrupt pressure changes need to be maintained in order to achieve a coating of desired characteristics. Thus, in the preferred embodiment, the function of ultrasound intensity versus time will appear different for different coating solutions. Colloid materials generally require a higher intensity of abrupt pressure change or longer exposure times to the abrupt pressure change generated by the ultrasound to coat fibers than do polymeric materials. Through the variation of the intensity of the abrupt pressure change and the regulation of the time to which the fiber is exposed to the abrupt pressure changes in any given application, many materials may be induced to coat fibers. Generally, such coating materials include those that can be cast into a film state within the liquid medium or those that can be adequately dispersed within the liquid medium.

It is known from the prior art that hardening agents and catalytic compounds can be used with coating compounds for fibers. Such agents act to seal a particular coating into a final solid and hardened form. Alternatively, thermal treatment and/or cross-linking chemicals may be used to produce a hardened coating with a variety of coating materials. In the preferred embodiment of this invention, the heat produced by the ultrasonic energy applied to the liquid medium (by the expansion and collapse of the cavitation bubbles) also hardens the coating layer once it is in place upon the fiber. Thermal hardening is particularly useful when urea-formaldehyde resins are used as the coating material. The heat generated by the cavitation effects in the liquid medium causes such polymers to cross-link and solidify into a hardened coating about the fibers, thus completing the coating process. The method of this invention may be used not only to coat fibers, but also as an aid in the hardening process of fibers coated by conventional liquid phase methods.

The abrupt pressure change required to be generated by the ultrasound, i.e. the intensity and frequency of the ultrasound, to practice the invention varies significantly from one formulation to another. The factors determining the efficiency of coating are:

1. The viscosity of the various coating materials.

2. The time interval during which ultrasound is applied to the system. This time interval determines the features and thickness of the coating on the fiber.

3. The intensity level of ultrasound employed. Low intensity ultrasound tends to lengthen the time required to complete the coating process. High intensity ultrasound tends to decrease the thickness of the coating and the time required to complete the coating process. This decrease in coating thickness probably occurs because the abrupt pressure change compresses the coating into a smaller volume or fractures the particles within the coating into a smaller more homogeneous form.

4. The ultrasonic responsiveness of the coating compound.

5. The temperature of the mixture being treated with the ultrasound. 6. The solids content of the mixture. Physical factors affecting the transmission and intensity of the abrupt pressure change, such as interference from other components of the mixture, may act to require a longer exposure time or a greater intensity level of the abrupt pressure change to complete the fiber coating. In a film state, certain coating material may be very responsive to the abrupt pressure change applied to the liquid mixture. The abrupt pressure change tends to force films within the liquid medium onto any surface which is also within the liquid medium. While fibers are specifically mentioned in this disclosure, it should be obvious to anyone skilled in the art that any other materials treated by the method of this invention will also become coated, regardless of their shape.

Examples of liquids which can be used as the liquid medium/solvent are water, hexane, toluene, cyclohexane, and alcohols.

Figure 2 illustrates a continuous fiber coating system. An ultrasonic transducer 57 is contained within a continuous flow cell housing 53, generating ultrasound 4 toward inlet opening 55 of the cell housing. A fiber 54 to be coated, which is immersed within a constantly replenished coating solution 59, enters cell 53 through inlet 55 and is directed onto a first spool 50, thence to a second spool 51, and finally to an outlet spool 52 from which the fiber exits through the output of the cell 56. The fiber 54 is led directly under the path of the ultrasound 4 by spools 50 and 51. The abrupt pressure change generated by ultrasound 4 acts to both coat and solidify the coating material within the coating solution onto the fiber as the fiber 54 is drawn through the processing cell 53.

The fiber 54 may be drawn through processing cell 53 by a mechanical or electric motor (not illustrated). Upon existing the processing cell, the fiber is coated 58. The speed of the fiber's flow through the processing cell, and the intensity of the ultrasound determines the thickness of the coating and the degree of coating solidification.

Variations of the design of a continuous or batch treatment system are possible employing variations of the other ultrasonic equipment including, but not limited to:

Tube Reactors Whistle Reactors Submersible Transducer Baths Cup-Horn Flow Cells Vibrating Baths

Such designs are noted in the text, "Sonochemistry " The Uses of

Ultrasound in Chemistry, T.J. Mason, Royal Society of Chemistry, 1990.

While the invention has been described with respect to specific embodiments, it is understood that many variations are possible. The examples given are intended to be illustrative and not limiting.

EXAMPLE 1

For this experiment, the apparatus illustrated in Figure 1 was employed.

A 1 liter glass beaker was placed atop a magnetic stirring/heating device. No heat was applied during these experiments.

A three inch long section of replacement clothing thread was placed between two blocks of glass which were located on the bottom of the larger beaker. A coating formulation was prepared consisting of 5.0 grams of Gelatin

(#300 bloom), 5.0 grams of Gum Arabic, 2.5 grams of Ethycellulose (#411 from Bermocel Corp.) and 2.5 grams of Karaya Gum for a total weight of 15.0 grams of natural polymer mix.

250 ml of distilled water was added to the beaker along with the coating formulation. The beaker was placed atop the magnetic stirrer/heater and a magnetic stir bar was placed in the beaker, directly under the suspended thread fiber. The stirrer was activated and the stir bar caused to rotate, generating mixing turbulence within the coating medium.

An ultrasonic transducer horn was placed within the liquid medium in the beaker at a distance of approximately 2.0 inches above the suspended fiber and directly above the magnetic stir bar. The ultrasonic device used was a Model #600 High Intensity Ultrasonic Processor system produced by Somes and Materials, Inc. The device was set to generate 25 watts/cm² intensity of ultrasound for a 10 minute exposure time using continuous, not pulsed radiation. The system was activated and allowed to run the full 10 minute time interval.

Sonic transmission waves were visible in the liquid medium which continued to rotate under the transducer. The mixture of coating formulation and water homogenized and changed to a lighter color. A temperature rise of nearly 15 C^° was recorded at the end of the 10 minute exposure interval. After the 10 minute treatment period had ended, the power to the transducer was turned off.

The fiber was then removed from the liquid medium and examined under a microscope. A coating was observed which was well defined into a membrane layer surrounding the fiber strands. As a control, identical fibers were then placed in a beaker of water and stirred for 10 minutes. A second comparison sample was provided by a dry untreated fiber. In microphotographs taken of these samples, the coated sample was clearly visible as being effectively coated with a discrete polymer membrane around the strands of the thread.

EXAMPLE 2: USE OF UREA-FORMALDEHYDE RESIN AND ULTRASOUND

EXAMPLE 1 was repeated, but instead of using 15 grams of natural polymers as the coating formulation, 53 grams of urea-formaldehyde resin, known as URAC-180 from American Cyanamid, was used. This aminoplast polymer resin is often used as a coating material for microencapsulation applications and was tested to determine its effectiveness as a fiber coating. URAC-180 is known to cross-link either by action of a catalyst or by thermal treatment.

Example 1 was repeated using 25 watts/cm² intensity ultrasound for ten minutes. The resulting coated fibers were again observed under microscope and then allowed to air dry. A thin coating layer was observed upon microscopic examination after one full treatment. Another identical fiber was placed in the beaker and stirred with the URAC-180 mixture, but no ultrasound was applied. The stirring was continued for a 10 minute interval. This second fiber was a control sample to determine if agitation alone in the URAC-180 medium was responsible for the observed coating rather than the use of ultrasound. A comparison of the two treated fibers revealed that the ultrasonically treated fiber had a more effective and complete coating layer over the entire fiber surface. The fiber stirred without exposure to ultrasound did possess a coating but the coating layer was incomplete and spotty in its construction.

The ultrasonically treated fiber had a far superior coating to the stirred fiber. It also possessed a more uniform coating layer deposited on the surface of the fiber.

EXAMPLE 3; MULTIPLE SHELL LAYER USING ULTRASOUND In this Example, the coated fibers made in Example 1 were used as a substrate for the application of a second coat of a dissimilar polymer as an outer coating material. The natural polymer coated fiber of Example 1 was placed into a beaker containing 53 grams of urea-formaldehyde resin, URAC-180, and the process as outlined in Example 2 was followed. The coated fiber was again exposed to ultrasound at 25 watts/cm² intensity for 10 minutes under constant stirring of the resin mixture.

After the second ultrasound treatment, the coated fibers was filtered and examined under a microscope. The resultant coated fibers were observed to have two distinct coating layers. General observations were:

PASS - 1 P - 2

The coated fiber was well-formed, but the natural polymer coated fiber of Example 1 had not been chemically hardened. Additional exposure to ultrasound, in the second pass, produced an even harder coating than had been accomplished when either the natural or the aminoplast coat was used alone.

EXAMPLE 4: HIGHER INTENSITY ULTRASOUND TREATMENT TO PRODUCE THINNER COATINGS Example 2 was repeated except that the intensity of the ultrasound was set for 100 watt/cm² over the 10 minute exposure period.

The resulting coated fiber was observed to possess a thinner coating than was observed on the fiber of Example 2. The coating in this Example was approximately 1.0 to 3.0 microns in thickness. The coating material was solidified by the ultrasound into a complete enclosure about the fiber.

These examples indicate the following features of the method and apparatus of this invention:

1. Abrupt pressure changes generated by ultrasonic treatment can form microcoated fibers with stable shell membranes. 2. Coated fiber can be formed using an abrupt pressure change with both natural and synthetic polymer coating materials

3. Higher intensity abrupt pressure changes can form coated fibers with a thinner coating.

4. Longer duration or repeat exposure to abrupt pressure changes can form coated fibers with both a thinner overall coating and also with a harder coating than when exposed only once. 5. Heat generated by the process of cavitation can provide a faster and more complete hardening of the coating material by effecting a cross- linking or solidification of the shell.

6. By using repeated exposures, it is possible to provide multiple coating layers on a fiber. Also, the various coating layers can be composed of dissimilar materials.

Claims

We claim:

1. A method for coating fiber comprising the following steps: a) immersing the fiber in a liquid medium containing the coating material; and b) applying an abrupt pressure change to the liquid medium in which the fiber is immersed, the abrupt pressure change being applied for a sufficient time and at a sufficient intensity to cause a coating to form on the fiber.

2. The method of Claim 1 wherein the abrupt pressure change is generated by ultrasound.

3. The method of Claim 1 wherein the abrupt pressure change generates a pressure shock wave.

4. The method of Claim 1 wherein the abrupt pressure change generates shear forces.

5. The method of Claim 1 wherein the abrupt pressure change generates cavitation within the liquid medium.

6. The method of Claim 1 further comprising the additional step, after immersing the fiber and before applying the abrupt pressure change, of agitating the liquid medium until a partial coat begins to form on the fiber.

7. The method of Claim 1 or Claim 6 further comprising the additional step of hardening the fiber coating by heating caused by the abrupt pressure change acting on the liquid medium.

8. The method of Claim 1 further comprising the additional steps of: a) immersing the previously coated fiber in a liquid medium containing the same coating material; and b) applying an abrupt pressure change to the liquid medium in which the fiber is immersed, the abrupt pressure change being applied for a sufficient time and at a sufficient intensity to cause an additional a coating layer to form on the fiber; and c) repeating steps (a) and (b) above to form as many coats as desired.

9. The method of Claim 1 further comprising the additional steps of: a) immersing the previously coated fiber in a liquid medium containing a coating material different from the coating material used to form the previous coat; and b) applying an abrupt pressure change to the liquid medium in which the fiber is immersed, the abrupt pressure change being applied for a sufficient time and at a sufficient intensity to cause an additional coating layer to form on the fiber; and c) repeating steps (a) and (b) above to form as many coats of different materials as desired.

10. The method of claim 1 further comprising the additional step of precoating the fiber by any prior art process before immersing the fiber.

11. A method of hardening fiber coatings comprising the following steps: a) immersing the coated fiber in a liquid medium; and b) apply an abrupt pressure change to the liquid medium in which the fiber is immersed, the abrupt pressure change being applied for a sufficient time and at a sufficient intensity to cause the coating to harden.

12. An apparatus for coating fiber comprising: a) a means for immersing the fiber in a liquid medium containing the coating material; b) a means for applying an abrupt pressure change to the liquid medium in which the fiber is immersed wherein said means applies an abrupt pressure change for a sufficient time and at a sufficient intensity to cause a coating to form on the fiber.

13. The apparatus of Claim 12 wherein the means for applying an abrupt pressure change is an ultrasonic transducer.

14. A coated fiber formed by: a) immersing the fiber in a liquid medium containing the coating material; and b) applying an abrupt pressure change to the liquid medium in which the fiber is immersed, the abrupt pressure change being applied for a sufficient time and at a sufficient intensity to cause a coating to form on the fiber.