Carbon fiber fabric cleaning and finishing

Abstract

A process to clean carbon fiber fabric of a pre-applied sizing, while simultaneously activating or preparing the fabric to receive a polymer resin is described. The cleaning process and chemistry combines an enzymatic cleaning solution with an oxidizing agent. The enzymatic solution strips the fibers of lubricants, surfactants, and other chemicals of the sizing which might otherwise interfere with interfacial properties and bonding of the fabric to the matrix resin. The inclusion of an oxidizing agent concurrently adds hydroxyl groups to the surface of the fabric allowing for the grafting of organic copolymers to the surface of the fabric, the copolymer being chosen based upon the desired polymer resin. This process provides a customized finished carbon fiber fabric to bond to a specific polymer resin, without interference resulting from an inadequate fiber sizing chemistry. In this way, a customizable finished fabric may be manufactured.

Claims

1. A process for finishing carbon fiber fabric comprising the steps of: providing a fabric made from carbon fibers; removing fiber sizing from said fabric by applying a solution to said fabric, said solution comprising an enzymatic cleanser and an oxidizing agent, whereby said enzymatic cleanser comprises cellulase, lipase, and phosphated alcohol surfactant; forming hydroxyl groups on the surface of said fabric, whereby said fabric is prepared to receive a functional group; determining a polymer resin to be used in a final composite; selecting a functional group based on said polymer resin of said final composite; attaching said functional group to said fabric whereby said fabric is prepared to receive said polymer resin.

2. The process of claim 1, further including the step of applying a polymer resin coating to said fabric.

3. The process of claim 1, whereby said cellulose and lipase are present in a range of about 0.1 to 10% by volume, and said phosphated alcohol surfactant is present in a range of about 0.05 to 5% by volume.

4. The process of claim 1, whereby said oxidizing agent is hydrogen peroxide.

5. The process of claim 1, whereby the ratio range of said oxidizing agent to said enzymatic cleanser is about 2:1 to 5:2.

6. The process of claim 1, whereby said oxidizing agent and said enzymatic cleanser are mixed in an aqueous solution and present in a concentration range of about 2-15% by volume.

7. The process of claim 1, whereby silane coupling agent is used to attach said functional group.

8. The process of claim 7, whereby said silane coupling agent is an epoxysilane.

9. A process for finishing carbon fiber fabric comprising the steps of: providing a fabric made from carbon fibers; removing the carbon fiber sizing by applying a solution to said fabric, said solution comprising hydrogen peroxide in a ratio range of about 2:1 to 5:2 with an enzymatic cleanser comprising cellulase, lipase, and phosphated alcohol surfactant; forming hydroxyl groups on the surface of said fabric whereby said fabric is prepared to receive an organic functional group; determining a polymer resin to be used in a final carbon fiber composite; selecting an organic functional group based on said polymer resin of said final carbon fiber composite; selecting an organofunctional silane based on said organic functional group; attaching said organic functional group to said fabric via said silane, whereby said fabric is prepared to receive said polymer resin.

10. The process of claim 9, whereby said cellulose and lipase are present in a range of about 0.1 to 10% by volume, and said phosphated alcohol surfactant is present in a range of about 0.05 to 5% by volume.

11. The process of claim 9, further including the step of applying a polymer resin coating to said fabric.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) Adhesion between matrix resin and carbon fiber is crucial in a reinforced composite. During the manufacture of carbon fiber, surface treatment is performed to enhance this adhesion. Producers use different treatments, but a common method involves pulling the fiber through an electrochemical or electrolytic bath that contains solutions, such as sodium hypochlorite or nitric acid. These materials etch or roughen the surface of each filament, which increases the surface area available for interfacial fiber/matrix bonding and adds reactive chemical groups, such as carboxylic acids. Next, a sizing is applied. At 0.5 to 5 percent of the weight of the carbon fiber, sizing protects the carbon fiber during handling and processing (e.g., weaving) into intermediate forms, such as dry fabric and prepreg. Sizing also holds filaments together in individual tows to reduce fuzz, improve processability and increase interfacial shear strength between the fiber and matrix resin.

(2) Typically in the carbon fiber composite industry, there is no finishing process for carbon fiber fabric. Instead, it is the sizing applied to the carbon fibers that aids in the bonding of the fabric to the matrix resin; in other words, the sizing itself serves as the finishing. With the advancements in matrix resins now in demand for end-use applications, the current sizing chemistries are proving to be insufficient. Rather, it would be preferable to finish carbon fiber fabric in a way that is compatible with a particular customer's resin characteristics, as well as specific properties desired in the composite.

(3) The present invention provides an efficient way to clean carbon fiber fabric of a pre-applied sizing, while simultaneously activating or preparing the fabric to receive a polymer resin. The cleaning process and chemistry of the present invention combines an enzymatic cleaning solution further including an oxidizing agent. The enzymatic solution strips the fibers of lubricants, surfactants, and other chemicals of the sizing which might otherwise interfere with interfacial properties and bonding of the fabric to the matrix resin. The inclusion of an oxidizing agent concurrently adds hydroxyl groups to the surface of the fabric. The addition of these hydroxyl groups allow for the grafting of organic copolymers to the surface of the fabric, the copolymer being chosen based upon the desired polymer resin to be added in a later step.

(4) Cleaning Process

(5) In one preferred example, the scouring agent (or cleaning solution) contains an enzymatic agent, referred to herein as Enzyme A, and hydrogen peroxide. Enzyme A is a blend of cellulase and lipase in a phosphated alcohol surfactant. The cellulase and lipase are present in a preferred range of about 0.1 to 10% by volume, with phosphated alcohol in a preferred range of about 0.05 to 5% by volume, all mixed together in aqueous phase at room temperature. The preferred ratio range of peroxide to Enzyme A is about 2:1 to 5:2 by volume. The peroxide-enzyme mix is preferably at a concentration of about 2-15% by volume mixed with water, the percentage being chosen based on the strength and weight of the fabric. A stronger or heavier fabric may necessitate a higher percentage of peroxide-enzyme mix. A surfactant, such as tergitol, may be added at a preferred concentration of about 0.01-0.5% by volume to maintain dispersion and keep the mix in solution. Preferred surfactants or lubricants are the mono- or diesters of a fatty acid or oil reacted with polyethylene glycol, having hydrophilic and lipophilic areas. Enzyme A serves to break down the fatty acids esters and lubricants on the yarn, while the hydrogen peroxide oxidizes the carbon fiber fabric, adding hydroxyl groups to the fabric surface. In an alternative embodiment, plasma may be used in conjunction with oxygen, both serving the same purpose to clean the fibers of the sizing and oxidize the surface of the fabric.

(6) While the chemicals comprising Enzyme A are preferred for the present invention, it is to be noted that any desired combination of specific enzymes may be substituted in the enzymatic component of the scouring agent, and certain enzymes may be preferred over others dependent upon the chemistry of the sizing that is to be removed. Any person skilled in the art is aware of enzymes available for use as well as their corresponding enzymatic function(s). Additionally, the enzymatic cleanser of the present invention is defined in terms of use on carbon fiber fabric-to strip the specific sizing on the carbon fabric. However, it is contemplated that this cleaning process and chemistry may be applied to other types of fabrics as well, such as quartz or fiberglass; and, adjustments to the cleaning chemistry may be made as needed for these different types of fabric.

(7) In a preferred process, the fabric may be passed dipped, sprayed, or rolled in a bath containing the solution, after which the fabric is removed and squeezed to remove any excess solution. This process can be performed at room temperature and may be repeated preferably 3-4 times for at least 2-3 minutes. The dip and squeeze process may be performed stationary, such as in a jig, or may be a continuous process, such as in a range; other suitable processes may be used as well. After the fabric has undergone a dip and squeeze process, the fabric is dried preferably at 200-315 degrees F. This drying may be performed in a convection oven for anywhere from 30 seconds to 5 minutes, or until all moisture is removed. Additional drying methods such as infrared, microwave power, laser, or other methods can also be utilized to dry the fabric. In such cases, the drying time and temperature may be below or above the above mentioned ranges; however, it is preferable that the temperature does not exceed a range that may result in a loss of fabric strength.

(8) The cleaning process removes the sizing chemistry from the fabric and prepares the fabric for the finishing process, described below.

(9) Finishing Process

(10) The oxidation of the fabric during the cleaning process forms functional hydroxyl groups on the surface of the carbon fiber fabric. These hydroxyl groups ready the carbon fiber fabric to receive an organic copolymer that may be attached through the use of a silane coupling agent. At this point, the appropriate functional group(s) may be added based on the desired end-use composite. These functional groups (such as epoxy, amino, and/or vinyl, for example), are selected based on compatibility with the desired matrix resin. For example, if the desired polymer composite is an epoxy thermosetting resin, then an epoxy group would be the preferred functional group to attach to the fabric during the finishing process. An organic copolymer may be attached through the use of a silane coupling agent, whereby the silane bonds with the hydroxyl group on the carbon fiber fabric surface, leaving the organic functional group available for bonding to a polymer resin. The organic functional group of this finishing step is customizable and specifically chosen dependent upon the end-use composite.

(11) Silane coupling agents are frequently used to bond a polymer resin to a fabric substrate. U.S. application Ser. No. 14/610,458, incorporated herein by reference, discusses this method in detail. The silane coupling agent has two functional groups, an organic substituent capable of bonding with an organic substrate, and an inorganic hydrolyzable substituent capable of bonding with an inorganic substrate. The silanes of the reactive type can serve as coupling agents between the carbon fibers and the matrix resin. The reactive silanes commonly contain a silicone head(s) and a tail(s) containing a functional group or groups that can react with the polymer resin. These groups include primary, secondary, or tertiary amines, vinyl, styryl, alkynyl, methacryloyl, acryloxy, epoxy, thio, sulphide, ureido, isocyanate, oxime, ester, aldehyde, and hydroxy moieties in either unprotected or protected form. The silicone head can be substituted with groups such as ethoxy, methoxy, methyldimethoxy, methydiethoxy, isopropoxy, acetoxy, etc. When the oxidized carbon fiber fabric is treated with an aqueous solution containing a silane coupling agent, hydrolysis of the labile groups occurs, resulting in silane oligomers bonding with the fabric substrate. A final drying process results in a covalent linkage between the fabric and the silane, simultaneously leaving the organic radical of the silane free for bonding to a compatible organic substrate.

(12) In a preferred example, the fabric may be passed through a bath with a solution containing the organic polymer. The polymer may be present in an aqueous solution of about 1-25% by volume of polymer to water. A surfactant may also be added, preferably at about 0.01-0.5% by volume. After being dipped, sprayed, or rolled in a bath containing the solution, the fabric is removed and squeezed to remove any excess solution. This process can be performed at room temperature and may be repeated preferably 3-4 times for at least 2-3 minutes. As in the cleaning process, the dip and squeeze process may be performed stationary, such as in a jig, or may be a continuous process, such as in a range; other suitable processes may be used as well. After the fabric has undergone a dip and squeeze process, the fabric undergoes the same drying process whereby the fabric is dried preferably at 200-315 degrees F. This drying may be performed in a convection oven for anywhere from 30 seconds to 5 minutes, or until all moisture is removed. Additional drying methods such as infrared, microwave power, laser, or other methods can also be utilized to dry the fabric. In such cases, the drying time and temperature may be below or above the above mentioned ranges; however, it is preferable that the temperature does not exceed a range resulting in a loss of fabric strength.

(13) In one embodiment of the present invention, the resulting product is a resin-free carbon fiber fabric finished with specific functional groups attached, ready to receive a particular polymer resin. This process allows for the manufacturing of a finished carbon fiber fabric that may be sold to a customer, whereby the customer may then add the appropriate resin desired for the end-use product. Additionally, and perhaps most importantly, this process provides a carbon fiber fabric with improved and superior ability to bond to a polymer resin, without interference resulting from an inadequate fiber sizing chemistry. In this way, a customizable finished fabric may be manufactured. In an alternative embodiment, the process may continue through to the addition of a polymer resin, resulting in a completed composite product.

(14) Although the present invention is described above in specific terms, values, and ranges, it is to be known that suitable substitutes may be made without departing from the spirit and scope of the invention. One skilled in the art is capable of knowing, for example, which functional groups are compatible for specific end use resins, which silane coupling agents would be appropriate in combination, and what types of substitutions may be appropriate or suitable.