Patent Description:
The use of composite materials including fiber reinforced composites has continued to increase in recent years in applications ranging from vehicles to bathroom fixtures. With the increased use of composite materials, the need for recycling of discarded composite has become more acute to keep the composite based materials from filling up landfills, and to conserve resources that are used to form these composites. In general, thermoplastic polymers such as polyethylene and polypropylene may be recycled through remelting/spinning of the polymers. However, for thermosetting composites such as fiber reinforced composites (FRCs), however, melt recycling is difficult because of the cross-linked nature of the resin.

A recycling method that has been tried to recycle FRC based materials is solvolysis that employs reactive solvents such as benzyl alcohol, diethylene glycol, diethylene glycol monomethyl ether in the presence of a tripotassium phosphate hydrate catalyst under an inert atmosphere and temperatures of <NUM> to break down ester bonds in polyester thermoset resins, while effectively keeping the properties of the reinforcing fibers. While this method as developed by Hitachi Chemical Co. proved effective in separating resin from glass fiber, the resulting glass fibers where damaged in the process, also the cost of solvents, and reaction times rendered the process impractical.

Other techniques to recycle FRCs were able to solubilize epoxy resin matrices containing carbon fibers or glass fibers using poly(ethylene glycols) in a sodium hydroxide solution. The carbon fibers and non-alkali fibers recovered after matrix solvolysis retained more than <NUM>% of their original strength; however, the method has met with limited acceptance owing to the specificity of the chemistry towards epoxy matrices and a low overall recycle efficiency.

Aminolysis of FRCs is attractive in yielding carbon fibers with <NUM>% purity. The method involves an elevated temperature solvent digestion in excess of ethanolamine in the presence of catalysts such as glacial acetic acid, sodium acetate or potassium sulfate; followed by washing the recovered carbon fibers in a boiling solvent such as methyl ethyl ketone (MEK), sonication and drying. While the aminolysis process is somewhat effective, the method has met with limited acceptance owing to the long process times up to <NUM>, as well as the cost and toxicity of the solvents.

Publication to <NPL>, disloses a a method to reclaim carbon fiber from a cured thermoset matrix that requires use of expensive polyethylene glycol/NaOH solvent.

Publication <CIT> discloses a method to reclaim carbon fiber from a cured thermoset matrix to free more than <NUM>% by weight of carbon fiber from the cured thermoset matrix. Publication <CIT> generally teaches to use high temperatures and long processing time. More especially in all examples disclosed in publication <CIT>, the processes include significant and non-constant temperatures swings, for example from <NUM> to <NUM> and back to <NUM> in example <NUM>. These sort of temperature swings are highly inefficient and require a great deal of time, making these processes inefficient, time consuming, and expensive.

Thus, there exists a need for a more efficient, less time consuming, cost effective, and environmentally friendly recycling process for breaking down a thermoset resin matrix to reclaim a high percentage of carbon fibers that are suitable for reusing carbon fibers.

A process is provided to reclaim carbon fiber from a cured thermoset matrix as defined in claim <NUM>. Cured thermoset matrix containing carbon fiber inclusions is optionally ground to form particles. The freed carbon fibers are washed and dried to reclaim carbon fiber reusable to reinforce a polymer to form a new FRC article. Solvents are chosen that are low cost and low toxicity. Processing is further facilitated by techniques such as solvent pre-swell of the particles, size reduction, microwave heating, and sonication to promote thermoset matrix digestion to free reinforcing carbon fibers.

The present invention is further detailed with respect to the following drawings. These drawings are intended to illustrate various aspects of the present invention, and not be a limitation on the practice thereof.

The present invention has utility in reclaiming reusable carbon fibers from thermoset polyester, vinyl ester, polyurethane or epoxy matrices. While the prior art has attempted to react a thermoset to create reactive monomers and oligomers from the thermoset matrix, the present invention is optimized for reclamation of carbon fiber from such a matrix. The carbon fiber as an inert inclusion in a thermoset matrix is considerably easier to reclaim in reusable from compared to matrix precursors. Additionally, with carbon fiber having a cost of approximately US$<NUM> per kilogram, there is an economical and environmental motivation to preclude such fibers from being discarded.

In certain embodiments of the present invention, the cost of materials and processing to reclaim carbon fibers is less than the cost of new carbon fiber upon consideration of the disposal cost of the spent thermoset article containing the carbon fiber. It is appreciated that use of certain solvents and catalysts, while effective to break covalent bonds in a thermoset matrix having inherent costs, handling hazards, or disposal costs that render the resulting process a mere academic curiosity. Exemplary of such processes are those detailed in the prior art. While processing time and energy inputs are factors in the efficiency of the inventive process, it has been found that the discovery of a low cost, low toxicity solvent in which to digest a thermoset matrix largely dictates whether a process is viable for industrial scale carbon fiber reclamation.

It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from <NUM> to <NUM> is intended to include <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>.

The input and output materials for an inventive process is shown schematically in <FIG> in the unit of pounds. In <FIG> Sheet molding composition (SMC) having a vinyl ester cured matrix containing <NUM>% by total weight of chopped carbon fiber, totaling <NUM> kilograms of carbon fiber is digested to with consumption of solvent and water to yield dry carbon fiber with better than <NUM>% yield by weight of carbon fibers in the SMC. It is appreciated that a cured matrix from which carbon fibers can be reclaimed according to the present invention includes vinyl esters, polyesters, and epoxies; regardless of whether such matrices also include glass fibers.

The present invention affords an economical and environmentally friendly recycling process for breaking down an SMC matrix to the extent needed to release reusable carbon fibers. It has been surprisingly found that matrix digestion to recover carbon fibers can be accomplished with lower inputs of glycolysis reagents, as compared to digestion to matrix monomers and oligomers suitable for repolymerization. Embodiments of the inventive carbon fiber recycling process utilize environmentally solvents that are lower in cost and less toxic than methyl ethyl ketone (MEK), acids, or amines. Polyol solvents suitable for the present invention have common attributes of hydroxyl groups suitable under reaction conditions of cleaving ester bonds in the SMC matrix. Solvents and solvent systems operative herein illustratively include: monosaccharides, disaccharides, such as a glucose-melt or sucrose dissolved in glycols; sugar alcohols such as erythritol, theitol, arabitol, xylitol, ribitol, mannitol, sorbitol, galactitol, fucitol, iditol, inositol, volemitol, isomalt, maltitol, lactitol, maltotritol, maltotetraitol, or combinations thereof; glycols such as ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol, <NUM>,<NUM>-propanediol, <NUM>,<NUM>-propanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-pentanediol, glycerol, or dipropylene glycol. It is appreciated that solvent systems are readily produced by dissolved sugars and sugar alcohols that are solids at standard temperature and pressure in glycols to adjust the amount of available hydroxyl groups, as well as the reaction temperature and economics. The solubility of sucrose in propylene glycols is exemplary of such solvent systems. It is appreciated that several glycols are known to form azeotropes with water than reduce solvent usage and afford a solvent system with a stable composition over a range of temperatures.

A solvent system that is more than <NUM>% total weight percent of diethylene glycol monomethyl ether is particularly well suited to provide acceptable solvent cost, reaction time and carbon fiber reclamation. A molten glucose bath also represents a solvent that affords an attractive balance of properties and an aqueous waste stream that requires minimal treatment prior to discharge to a public waste water treatment facility.

Embodiments of the inventive process may be conducted in a process as follows. The SMC cured thermoset matrix is optionally ground to a particle size that while increasing surface area limits shearing of carbon fibers. A typical length of a carbon fiber in an SMC matrix is between <NUM> and <NUM> millimeters (mm) in length. Grind particle sizes for an inventive process typically range from <NUM> to <NUM> in diameter for spherical particulate and a longest linear extent in a X-Y-Z coordinate system for anisotropic particles on average of between <NUM> and <NUM>. The SMC grind is combined with a glycolysis solvent in a ratio of SMC:solvent of <NUM>:<NUM>-<NUM> by weight with the proviso that the number of reactive hydroxyl groups in the solvent or solvent system is in a stoichiometric excess relative to the ester bonds in the SMC matrix. According to the present invention, a batch process is run involving at least <NUM> kilograms of carbon fiber to achieve economies of scale as to thermal management and reactor scale. Solvent is present in a ration sufficient to free more than <NUM>% by weight of the carbon fiber from the composite.

Variables that can be adjusted include the solvent, temperature and time, pressure, atmosphere - eliminate oxygen, provide an inert gas blanket, and the types of catalyst or co-reactant. Although any temperature can be used in digesting the polymer matrix of the particles, a temperature ranging between the melting point and the boiling point of the solvent is preferred. Inert gases operative herein illustratively include nitrogen, argon, carbon dioxide, carbon monoxide, helium and combinations thereof. Typical reaction temperatures range from <NUM> to <NUM> degrees Celsius. It is appreciated that the boiling point of the solvent can be increased with a higher than atmospheric pressure to the reaction mixture. Moreover, the reaction mixture exists under an air atmosphere or in an inert gas and either under atmospheric pressure (normal pressure), under reduced pressure, or under pressure. Other components such as surfactant, antifoaming agent, viscosity reducer, or boiling chip are readily added.

Co-reactants operative in the present invention illustratively include hydroxides, carbonates, hydrogen carbonates and phosphates of alkali metals such as sodium carbonate, sodium hydroxide, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate, potassium hydrogen carbonate, tripotassium phosphate, or a combination thereof. Typical loadings of catalyst range from <NUM> to <NUM> total weight percent.

After reaction, the separated carbon fibers are washed and optionally sonicated. The washing is conducted with water. As shown in <FIG>, <NUM> pounds of water is used for washing to produce clean carbon fibers. The resulting separated fibers are dried. In a specific embodiment, the separated fibers may undergo anti-static, coupling, film-former, or other sizing being applied as a coating on the outside of the dried, reclaimed carbon fibers.

There may be a closed loop process where the byproducts obtained during the fiber recovery are used in the formation of new FRC materials. Solvents in some cases are subjected to a recycling process of neutralization, filtering, distillation, or other conventional forms of purification, such as solvent extraction or membrane separation, for reuse in an inventive process or a separate usage.

In some cases, the reaction mixture is exposed to sonication prior to or during heating the reaction mixture to a desired temperature. Sonication appears to loosen the carbon fibers from the cured SMC matrix and cleans and detangles the fibers as well.

In still other cases, the reaction mixture is exposed to microwave radiation to speed the matrix digestion. Microwaves have frequencies of from <NUM> to <NUM>. <NUM> and <NUM> are conventional klystron output frequencies. Reaction vessels particularly well suited for use with microwave radiation have limited absorption of these wavelengths and illustratively include glass, ceramic or fluoropolymers. It is appreciated that a large reaction vessel is readily equipped with a window of quartz glass or heat-resistant glass operating as a microwave permeable portal. Alternatively, a metallic waveguide serves to allow deliver microwaves into a vessel absorptive of microwaves.

An exemplary reaction apparatus for this disclosure is shown generally at <NUM>. The hopper <NUM> includes pieces of SMC containing carbon fibers. A screw <NUM> conveys particles from the hopper <NUM> to the reaction vessel <NUM>. The vessel <NUM> is adapted to include the solvent and any catalysts and other material as the particles are metered into the vessel <NUM> by way of the screw <NUM>. A motor <NUM> drives a mechanical stirrer <NUM> to homogenize the reaction conditions. A vapor conduit <NUM> allows for the collection of volatile matrix breakdown products and solvent. A microwave generator or sonicator <NUM> is provided to impart energy to the reaction volume within the vessel <NUM>. A drain <NUM> is provided to facilitate liquid removal from the vessel <NUM>. A filter <NUM> allows for transmission and collection of size excluded substances such a filler particulate from the reaction mixture via a side siphon valve <NUM>. A heating source for the reaction vessel <NUM> is not shown and includes conventional resistive heating elements.

Upon matrix digestion to liberate carbon fibers, the fibers are either collected after transmission through a filter <NUM> or decanted from the reaction mixture and a settled precipitate. The collected carbon fibers are in some embodiments further filtered to remove various insoluble fillers that in typical SMC illustratively include glass fibers, calcium carbonate, thickeners, glass beads, glass microspheres, paint pigments, and natural fibers. The weight of water added to wash the resulting carbon fibers is typically <NUM> to <NUM> times the weight of the carbon fiber released from the SMC matrix.

The resulting reclaimed carbon fibers are readily dried in a conventional oven or vacuum oven to remove residual water from the carbon fibers. The resulting carbon fibers are used as if virgin carbon fibers or chemical treating to apply a sizing coating thereto. A sizing is used to modify fiber surface properties and as a result interactions with a new matrix in which the reclaimed carbon fibers will be embedded.

The collected filtrate or recovered organics consisting of oligomers, co-reactant, and solvent can be distilled to recover the excess solvent. The recovered solvent can be used for subsequent carbon fiber recovery reactions while maintaining a recovered carbon fiber purity of ><NUM>%. Conventional distillation can be used or the pressure can be reduced to lower the boiling point.

The co-reactant can be removed from the collected recovered organics by neutralization with a suitable acid such as hydrochloric, sulfuric, or acetic acid. Neutralization can be done before or after the solvent recovery step mentioned above.

The recovered and concentrated organic, in some inventive embodiments, after neutralization, can be incorporated into new polymer systems due to and depending upon the nature of the functional groups present. At loadings of <NUM>-<NUM> wt% recovered organics and <NUM>-<NUM> wt% virgin polymer the properties are similar to <NUM>% virgin systems. Alternately the recovered organics can be used as a fuel.

The present invention is further detailed with respect to the following examples. These examples are intended to illustrate specific aspects of the present invention and should not be construed as a limitation on the scope of the appended claims.

<NUM> grams of carbon fiber SMC material that is <NUM>% carbon fiber and <NUM>% polymer is recycled with a <NUM>:<NUM> ratio of solvent of diethylenetriamine (DETA) to SMC. After <NUM> reaction time in boiling DETA, carbon fiber was filtered from solution, washed with methyl ethyl ketone, and dried. Fiber purity measured by mass retention at <NUM> via TGA was <NUM> wt %. The collected organic phase can be used as a curing agent, for example with epoxies, to produce second generation materials.

<NUM> grams of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monomethyl ether and <NUM> grams of sodium hydroxide for three hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, sonicated for <NUM> in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monomethyl ether and <NUM> grams of sodium hydroxide for three hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monomethyl ether and <NUM> grams of sodium hydroxide for two hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> kilograms of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> kilograms of diethylene glycol monomethyl ether and <NUM> kilograms of sodium hydroxide for three hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol and <NUM> grams of sodium hydroxide for twenty-three hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, sonicated for <NUM> in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of vinyl ester carbon fiber SMC included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monoethyl ether and <NUM> grams of sodium hydroxide for three hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, sonicated for <NUM> in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of carbon fiber reinforced epoxy-amine included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol and <NUM> grams of sodium hydroxide for twenty-four hours at a temperature of <NUM>. The carbon fibers are then filtered, washed with <NUM> of boiling water, rinsed in water, sonicated for <NUM> in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of carbon fiber reinforced epoxy-amine included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monoethyl ether and <NUM> grams of sodium hydroxide for twenty hours at a temperature of <NUM>. The carbon fibers are then filtered, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of carbon fiber reinforced epoxy-amine included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monoethyl ether and <NUM> grams of sodium hydroxide for <NUM> hours at a temperature of <NUM>. The carbon fibers are then filtered, washed in boiling water, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of carbon fiber reinforced epoxy-amine included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monoethyl ether, <NUM> grams of recycled diethylene glycol monoethyl ether from the distillation of previous reaction, and <NUM> grams of sodium hydroxide for twenty-one hours at a temperature of <NUM>. The carbon fibers are then filtered, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of carbon fiber reinforced epoxy-amine included <NUM> wt % carbon fiber and <NUM> wt % polymer is added to <NUM> grams of diethylene glycol monoethyl ether, <NUM> grams of diethylene glycol, and <NUM> grams of sodium hydroxide for sixteen hours at reflux. The carbon fibers are then filtered, washed in boiling water, rinsed in water, and dried. Carbon fiber purity is <NUM> wt %.

<NUM> grams of recovered organics collected from two batches of digested carbon fiber epoxy composite, as described in Example <NUM>, was neutralized by adding dropwise HCl diluted to the same molar concentration as the NaOH in the recovered organics. Resulting pH was <NUM>. Water is then removed via distillation followed by removing <NUM>-<NUM>% of the starting amount of diethylene glycol monoethyl ether solvent.

<NUM> grams of recovered organics collected from a batch of digested carbon fiber epoxy composite, as described in Example <NUM>, was neutralized by adding dropwise H<NUM>SO<NUM> (<NUM> vol% in water) to achieve a pH of <NUM>. Sodium sulfate precipitate was filtered from solution and washed with acetone. Water is then removed from the neutralized recovered organics via distillation followed by removing <NUM>-<NUM>% of the starting amount of diethylene glycol monoethyl ether solvent.

Claim 1:
A process to reclaim carbon fiber from a cured thermoset matrix comprising:
adding the cured thermoset polyester or vinyl ester or polyurethane or epoxy matrix to a polyol solvent composition under conditions to free more than <NUM>% by weight of the carbon fiber from the cured thermoset matrix, wherein the polyol solvent composition is selected from a group consisting of monosaccharides, disaccharides, sugar alcohols, ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol, <NUM>,<NUM>-propanediol, <NUM>,<NUM>-propanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-pentanediol, glycerol, or dipropylene glycol; and wherein said conditions comprise an inert gas blanket and/or comprise adding co-reactants selected from a group consisting of hydroxides, carbonates, hydrogen carbonates and phosphates of alkali metals such as sodium carbonate, sodium hydroxide, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate, potassium hydrogen carbonate, tripotassium phosphate, or a combination thereof;
washing the carbon fiber with water; and
drying the carbon fiber to reclaim the carbon fiber and wherein more than <NUM>% by weight of the carbon fiber is reclaimed;
and wherein said conditions comprise a constant temperature of between <NUM> and <NUM> degrees Celsius.