Patent Application
20160102182
Conventional methods of making fiber-reinforced articles include placing bare fibers in a mold for the part and then flowing in the liquid precursors of a thermoset polymer. Once the precursors have infused through the fibers and filled the mold, a curing stage (sometimes called a hardening stage) commences to polymerize the thermoset into a polymer matrix that surrounds the fibers. The fiber-reinforced composite may then be released from the mold and, if necessary, shaped, sanded, or otherwise processed into the final article.
The unhardened thermoset resins used to make the composite are generally inexpensive and efficiently wet the fibers at low processing temperatures. Unfortunately however, many of the resins off gas irritating and sometimes dangerous volatile organic compounds (VOCs). The outgassing of VOCs are of particular concern during curing, when the exothermic nature of many thermoset polymerization reactions raise the temperature of the composite and drive more VOCs into the gas phase. In many instances, it is necessary to cure large thermoset articles in facilities equipped with robust ventilation and air scrubbing equipment, increasing the overall production costs.
Thermoset articles are also difficult to repair or recycle. Hardened thermoset binders often have a high degree of crosslinking, making them prone to fractures and breaks. Because thermosets normally will not soften or melt under heat, they have to be replaced instead of repaired by welding. Compounding difficulties, the unrepairable thermoset part normally cannot be recycled into new articles, but must instead be landfilled at significant cost and adverse impact on the environment. The problems are particularly acute when large thermoset parts, such as automotive panels and wind turbine blades, need to be replaced.
Because of these and other difficulties, thermoplastic resin systems are being developed for fiber-reinforced articles that were once exclusively made using thermosets. Thermoplastics typically have higher fracture toughness and chemical resistance than thermosets. They also soften and melt at raised temperatures, allowing operators to heal cracks and weld together pieces instead of having to replace a damaged part. Perhaps most significantly, discarded thermoplastic parts can be broken down and recycled into new articles, reducing landfill costs and stress on the environment.
Unfortunately, many thermoplastics also have production challenges, including high flow viscosities that cause difficulties loading and wetting the thermoplastic resin into the fibers. In some instances the melted thermoplastic is raised to high temperature, pulled into the fibers under high pressure, and if necessary under high vacuum, to increase the infiltration rate. At a minimum, these techniques increase the complexity and cost of producing the fiber-reinforced article and often result in a thermoplastic matrix that is poorly bonded to the integrated fibers. Thus, there is a need to develop new thermoplastic resin formulations and new ways to combine thermoplastic resins with reinforcing fibers. These and other issues are addressed in the present application.
Methods of making and using prepregs in the construction of fiber-reinforced composite articles are described. The present prepregs include thermoplastic resin delivered to a fiber-containing substrate as a mixture of resin particles in a liquid medium. The resin particles may be pre-polymerized and/or partially-polymerized compounds such as thermoplastic monomors and/or oligomers. The resin particles may also include fully-polymerized thermoplastic polymers as a replacement for or complement to the monomers and oligomers.
The fiber-containing substrate coated with the resin mixture may be treated to form the prepreg. Treatment steps may include removing the liquid medium, for example by evaporation. They may also include heating the combination of substrate and resin particles, and in some instances melting them. They may further include partially-polymerizing a pre-polymerized resin through heat and/or catalysis.
The prepregs may be used to make thermoplastic fiber-reinforced articles such as automotive parts, airplane parts, and turbine blades, among other articles. Because the polymer resin is already present in the prepregs, less or no thermoplastic resin has to be injected into fiber-containing substrate, which mitigates a common problem thermoplastic resins have infiltrating and wetting substrate fibers.
An exemplary resin mixture may include resin particles of a cyclic alkylene terephthalate (e.g., cyclic butylene terephthalate) in an aqueous mixture. The resin particles are insoluble in water and may be dispersed in the aqueous medium, for example as a suspension. The resin mixture may also contain a polymerization catalyst, which is typically a metal salt (e.g., a tin or titanate salt).
An exemplary fiber-containing substrate is a woven fabric (e.g., woven carbon fiber, woven fiberglass, etc.). After the resin mixture of resin particles in a liquid medium is poured, dipped, sprayed, coated, etc., on the woven fabric, it may be heated to evaporate off the liquid and leave behind a coating of the resin particles. In some embodiments, the resin particles are coarse enough to remain close to the fabric surface, while in other embodiments the particles are fine enough to penetrate through the exposed surface of the fabric. In some embodiments, the amount of heat applied to the coated fabric may be enough to melt the resin particles and form a prepreg of melted resin particle as fabric. Additional embodiments include a prepreg of unmelted or partially-melted resin particles coated on the fabric.
Embodiments of the invention include methods of making a prepreg. The methods may include the steps of forming a fiber-containing substrate, and contacting the fiber-containing substrate with a resin mixture. The resin mixture may include particles of monomers or oligomers mixed in a liquid medium, and the particles may be coated on the fiber-containing substrate to form a coated substrate. The liquid medium may be removed from the coated substrate to form the prepreg.
Embodiments of the invention further include methods of making fiber-reinforced composite articles with the prepregs. The method may include the step of contacting a fiber-containing substrate with a resin mixture of resin particles dispersed in a liquid medium, where the resin particles comprise monomers, oligomers, or polymers. The resin particles may be dried and melted on the fiber-containing substrate to make a prepreg comprising resin and the fiber containing substrate. The prepreg may then be formed into the fiber-reinforced composite article.
Embodiments of the invention still further include method of forming a resin mixture. The methods include incorporating a cyclic alkylene terephthalate into an aqueous medium. The incorporated cyclic alkylene terephthalate is in the form of solid particles in the aqueous medium.
Embodiments of the invention still further include prepregs that include resin particles coated on a fiber-containing substrate. The resin particles may be monomers or oligomers of a cyclic alkylene terephthalate that have been coated on the fiber-containing substrate from a resin mixture of the resin particles dispersed in a liquid medium.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components.
Methods are described for making exemplary resin mixtures that may be used to make exemplary pre-pregs, which in turn may be used to make exemplary fiber-reinforced composites. Also described are exemplary resin mixtures, pre-pregs, and fiber-reinforced composites themselves. The resin mixtures may include a particulate phase of the resin particles dispersed in a continuous phase of a liquid medium. The pre-pregs may include combinations of the resin with a fiber-reinforced substrate, such as a woven fabric made of carbon and or glass fibers. The pre-pregs may be shaped and arranged in a template, mold, etc., and treated to form the fiber-reinforced composites. Exemplary fiber-reinforced compasses may include turbine blades for windmills, wings for aircraft, and a variety of other types of fiber-reinforced composite parts.
As noted above, one exemplary class of thermoplastic resins that may be used to make the resin mixture is macrocyclic oligoesters such as cyclic alkylene terephthalates. One exemplary group of cyclic alkylene terephthalates is cyclic butylene terephthalate (CBT). An exemplary CBT, whose ring includes two butyl groups and two terephthalate groups, is illustrated below:
It should be appreciated that the present CBT may include additional butyl and/or terephthalate groups incorporated into the ring. It should also be appreciated that some exemplary CBT may have other moieties coupled to the CBT ring. CBT may comprise a plurality of dimers, trimers, tetramers, etc., of butylene terephthalate.
When the CBT monomers and/or oligomers are exposed to polymerization conditions such as elevated temperature (e.g., about 170° C. to about 250° C.) in the presence of a polymerization catalyst, the rings will open and react to create a linear polybutylene terephthalate (PBT) polymer. The polymerization reaction is reversible, and under certain conditions the PBT polymer can be converted back into cyclic monomers and oligomers of CBT. PBT polymers are sometimes referred to as the polymerized form of CBT or pCBT.
The method 100 also includes the step of providing a liquid medium 104 for the resin mixture. The liquid medium may be a room temperature liquid that can form a suspension of the resin particles without substantially dissolving the particles. For example, when the resin particles are made of water-insoluble thermoplastic monomers, oligomers, and/or polymers, the liquid medium may be water.
The liquid medium may include additional compounds such as polymerization catalysts, polymerization promoters, thickeners, dispersants, colorants, surfactants, flame retardants, ultraviolet stabilizers, and fillers including inorganic particles and carbon nanotubes, among other additional compounds. The polymerization catalyst may include a salt and/or acid that can be partially or fully dissolved, or dispsed, in the liquid medium. When the resin particles are monomers or oligomers of a cyclic alkylene terephthalate, the polymerization catalyst is selected to drive the polymerization of these types of macrocyclic oligoesters. Exemplary polymerization catalysts may include organometallic compounds such as organo-tin compounds and/or organo-titanate compounds. One specific polymerization catalyst for the CBT monomers and oligomers that may be butyltin chloride dihydroxide.
Alternatively, the polymerization catalysts may be incorporated onto the fibers (e.g., carbon fibers, glass fibers, etc.) in the fiber-containing substrate. For example, glass or carbon fibers may be treated with a polymerization catalyst composition (e.g., a sizing composition) that coats the fibers with the polymerization catalyst. When the resin material makes contact with the treated fibers at the polymerization temperature, the polymerization catalyst on the fibers facilitate the polymerization of the resin into a polymerized resin matrix. In some instances, application of the polymerization catalyst on the fibers of the fiber-containing substrate eliminate the need to incorporate the polymerization catalyst into the resin or the liquid medium of the resin mixture. This may be advantageous when the polymerization catalyst is not easily dissolved and/or dispersed in either the polymer resin or liquid medium. For example, the sizing/coating composition of the polymer catalyst may use a different solvent than the liquid medium, a solvent that would otherwise be undesirable to include in the resin mixture.
The polymerization catalyst may also be optionally accompanied by a polymerization promoter that accelerates the polymerization rate of the monomers and/or oligomers. When the resin particles include CBT, the polymerization promoter may by an alcohol and/or epoxide compound. Exemplary alcohols may include one or more hydroxyl groups, such as mono-alcohols (e.g., butanol), diols (e.g., ethylene glycol, 2-ethyl-1,3-hexanediol, bis(4-hydroxybutyl)terephthalate), triols, and other polyols. Exemplary epoxides may include one or more epoxide groups such as monoepoxide, diepoxide, and higher epoxides, such as bisphenol A diglycidylether. They may also include polyol and polyepoxides, such as poly(ethylene glycol).
The method 100 also includes incorporating the resin composition into the liquid medium 106 to form the resin mixture. When the resin composition is a thermoplastic monomer, oligomer, or polymer, it may be incorporated into the liquid medium as a liquid, a solid, or both. Introducing the resin composition as a liquid may include heating the resin to its melting temperature and pouring or injecting the melted resin into the liquid medium to form an emulsion. In many instances, melted resin is cooled on contact with the liquid medium, causing the resin to solidify.
In case of CBTs, the resins are typically solids at room temperature (e.g., about 20° C.), and begin to melt at around 120° C. At around 160° C., CBTs are generally fully melted with a liquid viscosity of about 150 centipoise (cP). As the molten CBTs are heated further, the viscosity may continue to drop, and in some instances may reach about 30 cP at about 190° C. However, the viscosity can start to climb as the CBT starts polymerizing to PBT. Temperature ranges for CBT polymerization are generally about 170° C. to about 250° C., with higher temperatures rapidly increasing the polymerization rate. The melting point of the polymerized PBT is typically around 225° C.
The CBT may be melted around 120-160° C. and introduced to an aqueous medium where the melted CBT rapidly cools and solidifies into a dispersion of CBT resin particles. In some instances a polymerization catalyst for the CBT may be added to the resin mixture after the resin particles form to minimize the extent CBT polymerization. However, because the CBT emulsion cools quickly in the aqueous medium a polymerization catalyst may be mixed with the water even before the emulsion is formed. In still other instances, a polymerization catalyst may be present in the melted CBT resin before forming the emulsion with the aqueous medium.
Additional techniques for incorporating the resin composition into the liquid medium include dispersing solid particles of the resin composition into the liquid medium. When the resin composition is a solid at room temperature, it may be ground, milled, or otherwise formed into dispersible particles that are added to the liquid medium. For example, commercial sources of CBT resin (such as CBT® made by Cyclics Corporation of Schenectady N.Y.) are commonly sold as pellets that can be ground into fine particles with average particle diameters of about 1 μm to about 50 μm. The CBT particles may then be dispersed into an aqueous medium to form the resin mixture.
The present methods of making the resin mixture may also include adding additional pre and post polymerized thermoplastics to the mixture. For example, an aqueous resin mixture of CBT particles described above may also include particles of PBT, as well as monomers, oligomers, and/or polymers of other thermoplastic resins, such as polyesters, polyalkylenes, polyamides, etc.
The resin mixtures may be used to form prepregs that are the starting materials of fiber-reinforced composites. The present prepregs are fiber-containing materials that have been pre-impregnated with thermoplastic monomers, oligomers, and/or polymers that contribute to the formation of the resin matrix in a fiber-reinforced composite made with the prepregs. In some examples the resin materials in the prepreg may be partially cured to produce a “B-stage” prepreg that has undergone some polymerization of the resin material, but requires additional curing to be fully polymerized. In other examples, the prepreg may be made from uncured (a.k.a., “A-stage”) thermoplastic monomers and/or oligomers, or fully-cured (a.k.a., “C-stage”) thermoplastic polymers.
The method 200 also includes providing a resin mixture 204. The resin mixture may be made according to the method 100 described above, and may include a combination of resin particles dispersed in a liquid medium.
The fiber-containing substrate may be contacted with the resin mixture and the resin-contacted substrate may be treated to form the prepreg 206. Techniques for contacting the fiber-containing substrate with the resin may include applying the resin mixture to the substrate by spraying, curtain coating, spin coating, blade coating, dip coating, and/or roll coating, among other techniques. The resin-coated substrate may then be treated to remove some or all of the liquid medium from the resin mixture and/or melt and partially cure the resin particles in the mixture.
The treatment step 206 may include heating the resin-contacted substrate under conditions conducive to evaporating the liquid medium and leaving a coating of the resin particles on the fiber substrate. In some examples, the heating temperature is set high enough to both evaporate the liquid medium and melt the resin particles. For example, if the resin mixture is an aqueous mixture of CBT particles, the heating temperature may be set somewhere in the range of about 120-200° C., which is high enough to both evaporate off substantially all the liquid water and melt the CBT particles on the substrate to form a prepreg of CBT resin coating the substrate. In further examples, the heating temperature may be set high enough to start polymerizing the resin to a B-stage where the prepreg is partially cured. The treatment step 206 may also include techniques used in addition to or in lieu of heating to partially polymerize the resin, such as exposure to ultraviolet light.
The method 200 may also include optional steps (not shown) of introducing additional compounds to the substrate and/or resin mixture. For example, while many resin mixtures may be one-part systems that include a polymerization catalyst in the mixture, it may be advantageous in some instances to keep the pre-polymerized resin separated from the polymerization catalyst until contacting the fiber-containing substrate. Thus, separate streams of the resin mixture and catalyst mixture or solution may be independently introduced to the substrate. It may also be desirable to introduce dry resin particles directly on the substrate before and/or after the substrate is contacted by the resin mixture. These dry resin particles may be the same or different from the resin particles in the resin mixture. For example, dry resin particles of polyethylene or polyester may be sprinkled onto the substrate before, during or after a resin mixture of CBT particles contact the substrate.
In some examples (not shown), the pre-pregs may be stacked into a plurality of adjacent layers. Embodiments may include a stacked plurality of pre-preg layers bonded to each other by the application of adhesive between adjacent layers. In additional embodiments, the stacked layer of pre-pregs may be bonded by the resin present in each of the individual pre-preg layers without the aid of adhesives.
The prepregs may be used in methods of making a fiber-reinforced article like the method 400 illustrated in
The resulting prepreg may be formed into a fiber-reinforced composite article 408 through a variety of techniques. For example a single layer or multiple layers of the prepreg may be compression molded into the fiber-reinforced article. When the prepreg includes pre-polymerized and/or partially-polymerized resin, the compression molding process may include a heating step (e.g., hot pressing) to fully polymerize the resin. Heat may also be used in the compression molding of fully-polymerized prepregs to melt and mold the prepreg into the shape of the final article.
The prepregs may also be used to in conjunction with other fibers and resin materials to make the final composite article. For example, the prepreg may be placed in selected sections of a tool or mold to reinforce the article and/or provide material in places that are difficult to reach for thermoset and/or thermoplastic resins. For example, the prepregs may be applied to sharp corners and other highly structured areas of a mold or layup used in reactive injection molding processes (RIM), structural reactive injective molding processes (SRIM), resin transfer molding processes (RTM), vacuum-assisted resin transfer molding processes (VTRM), spray-up forming processes, filament winding processes, long-fiber injection molding processes, and pultrusion, among others.
Prepregs are made from pre-polymerized or partially-polymerized CBT monomers and/or oligomers can be converted to a fully-polymerized fiber-reinforced article under isothermal processing conditions. As noted above, the CBT monomers and oligomers have melting points that start as low as 120° C. and significant polymerization rates starting at about 170° C. Because polymerized PBT has a higher melting point of around 225° C., the CBT can be melted and polymerized into a solid PBT matrix at the same temperature without a cooling stage prior to demolding. The isothermal processing of the prepreg (e.g., processing at a temperature between about 170° C. and 200° C.) can significantly speed production of the fiber-reinforced article, especially for larger volume articles that normally require longer cooling periods for the melted thermoplastic.
Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.
As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth.
Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.
