Patent Application
20150148485
Thermoset plastics are favored for making many kinds of fiber-reinforced articles because of their ease of manufacture. Uncured thermosets are often low viscosity liquids at room temperature and easily wet a fabric of fibers. Once they have migrated through the fabric and surrounded its fibers, a curing stage (sometimes called a hardening stage) commences to polymerize the thermoset into a polymer matrix. Often, this wetting and curing takes place in a mold that defines the shape of the fiber-reinforced article.
The uncured thermoset resins used to make the composite are generally inexpensive, but often 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 resins 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 reinforcing 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 extruded fiber-resin compositions in the construction of fiber-reinforced composite articles are described. The present compositions include the combination of reactive resin compositions and reactive fibers. The reactive resin composition may be melted and combined with the reactive fibers in an extruder. The reactive fibers have been sized with one or more polymerization and/or coupling agents that promote the polymerization of the reactive resin composition and/or the bonding of the polymerized resin to the fibers. The low-viscosity reactive resin compositions are significantly easier to wet and mix with the fibers in an extruder compared to a melt of the polymerized thermoplastic resin.
Embodiments may include methods of making fiber-resin compositions. The methods may include providing a reactive resin composition to an extruder, where the reactive resin composition may include monomers, oligomers, or both, that are capable of polymerizing into a thermoplastic resin. The methods may further include combining the reactive resin composition with a plurality of reactive fibers that are also supplied to the extruder. The plurality of reactive fibers maybe sized with at least one polymerization agent and/or coupling agent. The fiber-resin composition may be extruded from the extruder, where the composition includes a thermoplastic resin in contact with the plurality of fibers that is formed by the polymerization of the monomers and/or oligomers of the reactive resin composition.
Embodiments may further include methods of making a fiber-reinforced composite article. The methods may include providing a reactive resin composition to an extruder, where the reactive resin composition may include monomers, oligomers, or both, that are capable of polymerizing into a thermoplastic resin. The methods may further include combining the reactive resin composition with a plurality of reactive fibers that are also supplied to the extruder. The plurality of reactive fibers are sized with at least one of a polymerization agent and/or coupling agent. The fiber-resin composition may be extruded from the extruder, where the composition includes a thermoplastic resin in contact with the plurality of fibers that is formed by the polymerization of the monomers and/or oligomers of the reactive resin composition. The extruded fiber-resin composition may be formed into the fiber-reinforced composite article using processes like injection molding or compression molding, among other processes.
Embodiments may yet further include exemplary fiber-resin compositions and fiber-reinforced composite articles made from the compositions. The exemplary fiber-resin compositions may be made from the reactive extrusion of a reactive resin and a plurality of reactive fibers. The fiber-reinforced composite articles may be made from fiber-resin compositions that form the article.
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.
The present application includes methods of making exemplary fiber-resin compositions from reactive resin compositions that include low-viscosity melts of monomers and/or oligomers that can polymerize into a thermoplastic resin that holds the adjacent fibers. The low-viscosity reactive resin compositions are significantly easier to wet and mix with the fibers in an extruder compared to a melt of the polymerized thermoplastic resin. The fiber-resin compositions extruded from the extruder may be formed into a fiber-reinforced composite article using a variety of thermoplastic molding techniques. Details about the methods and systems used to make the exemplary fiber-reinforced compositions are described below.
The extruder configuration and extrusion technique may be selected based on the size and type of fibers combined with the reactive resin composition in the extruder. For example, when the plurality of fibers are chopped, short glass fibers (e.g., less than 0.5 inches in length) a reactive extrusion technique may be used to produce the fiber-resin composition. When the plurality of fibers are glass rovings and/or continuous glass fibers, a direct-long fiber thermoplastic (D-LFT) extrusion technique may be used to produce the fiber-resin composition. Additional details about each of these extrusion techniques are provided as follows:
Reactive extrusion is a low-cost, versatile extrusion technique that involves the use of an extruder as a chemical reactor. Polymerization and other chemical reactions associated with reactive resins are carried out in situ while the extrusion process, including mixing of the reactive resin composition with fibers and other reinforcement material, is in progress. Therefore, reactive extrusion differs from conventional extrusion methods in which typically no polymerization or other chemical reactions occur during extrusion.
A reactive extrusion process may start by supplying short glass fibers and the reactive resin composition to the extruder. Once inside the extruder, the fibers and reactive resin composition mix under conditions that promote the in-situ polymerizaton of the monomers and/or oligomers in the composition. The low melt viscosity of monomers and/or oligomers of the reactive resin composition facilitates excellent mixing and wetting of the composition with the plurality of fibers, resulting in improved mechanical properties of composites.
When the short glass fibers have been sized with additional reactive compounds such as polymerization agents and/or coupling agents, the conditions in the extruder also promote the reaction of the fibers with the monomers and/or oligomers, nascent thermoplastic resin, or both. For example, a polymerization initiator and/or polymerization catalyst on the fibers may initiate and/or promote polymerization of the monomers and/or oligomers and increase the polymerization rate. A coupling agent may covalently bond the thermoplastic resin to the fibers, improving the mechanical properties of the fiber-reinforced article made with the reactively extruded fiber-resin composition. In some examples, the function of a polymerization agent and coupling agent may be combined into a single compound, such as a coupler-initiator (CI) compound.
Direct long fiber thermoplastic (D-LFT) molding is a technology where thermoplastic resin is directly compounded with long glass fibers and then molded in one operation. Different from a conventional extrusion process in which chopped fibers are used, in a D-LFT process continuous roving strands are fed into extruder. The advantage of D-LFT is the ability to produce significantly longer glass fibers in the final composite materials. In comparison to a standard LFT process based on long fiber pellets, the D-LFT process doesn't produce semi-finished material. When D-LFT is used in compression or injection molding, a melted resin-fiber composition may be transferred into a molding tool located in a compression press or directly injected into the mold.
Additional LFT processes may form pellets as a fiber-resin composition. The pellets have a typical length of ½ inch to up to 2 inches and are produced by impregnating in a cross head tie. The reactive resin composition may be combined with fibers typically at the end of an extruder and then further polymerized by applying heat prior to the chopping step. The pellets are semi-finished materials that can be molded in a separate step, such as a compression step using a plasticator or in injection molding.
In both LFT and D-LFT processes the resulting composites contain longer glass fibers of ½″ (12 mm) up to 2″ (50 mm) in length. Longer fiber length combined with excellent wet-out can provide improved mechanical properties such as higher stiffness and strength compared to short fiber-reinforced composites made in a conventional extrusion process using chopped fibers. Long-fiber reinforced thermoplastic composites produced in LFT and D-LFT processes are of great interest to many industries including automotive, due to their excellent mechanical properties and high stiffness-to-weight ratio.
The fibers may be one or more types of fibers chosen from glass fibers, ceramic fibers, carbon fibers, metal fibers, and organic polymer fibers, among other kinds of fibers. Exemplary glass fibers may include “E-glass”, “A-glass”, “C-glass”, “S-glass”, “ECR-glass” (corrosion resistant glass), “T-glass”, and fluorine and/or boron-free derivatives thereof. Exemplary ceramic fibers may include aluminum oxide, silicon carbide, silicon nitride, silicon carbide, and basalt fibers, among others. Exemplary carbon fibers may include graphite, semi-crystalline carbon, and carbon nano tubes, among other types of carbon fibers. Exemplary metal fibers may include aluminum, steel, and tungsten, among other types of metal fibers. Exemplary organic polymer fibers may include poly aramid fibers, polyester fibers, and polyamide fibers, among other types of organic polymer fibers.
The fiber length may range from short-to-intermediate chopped fibers (e.g., about 0.5 inches or less in length) to long fibers (e.g., more than about 0.5 inches in length), including continuous fibers, rovings, and wound fibers, among others. In some instances, the plurality of fibers may be treated with a sizing composition that can enhance the fibers' physical characteristics in a number of ways including increased hardness, increased mechanical strength, greater wettability, and increased adhesion between the fibers and resin. The sizing composition may also enhance the chemical reactivity of the fibers by providing them with reactive agents that initiate and/or promote the polymerization of the resin composition that comes in contact with the “reactive” fibers. The reactive agents may include coupler-initiator compounds that include a silicon-containing moiety that forms a covalent bond with an exposed surface of the glass fiber, and an initiator moiety that initiates a polymerization reaction in the resin composition that comes in contact with the coupler-initiator compound bound to the glass fiber. In some examples, this initiator moiety is a caprolactam blocked isocyanate moiety that initiates a ring-opening polymerization reaction when the reactive fibers come in contact with caprolactam monomers in the resin composition. Exemplary reactive glass fibers are described in co-assigned U.S. patent application Ser. Nos. 13/335,690; 13/335,761; 13/335,793; and 13/335,813, all filed Dec. 22, 2011, and U.S. patent application Ser. No. 13/788,857, filed Mar. 7, 2013. The entire contents of all the applications are herein incorporated by reference for all purposes.
The method 100 may include providing a reactive resin compostion to an extruder 102. The reactive resin composition may include monomers and/or oligomers capable of polymerizing into a polymerized resin matrix that binds the pluarlity of fibers. Exemplary reactive resin compositions may include caprolactam. Caprolactam is a cyclic amide of caproic acid with an emperical formula (CH2)5C(O)NH, which may be represented by the structural formula:
Caprolactam has a low melting point of approximately 68° C. and a melted viscosity (4-8 cP) that is close to water, making it well suited for wetting and mixing with glass fibers in an extruder. Typically, the caprolactam-containing reactive resin composition may be introduced to the plurality of fibers as a liquid melt.
Caprolactam-containing reactive resin compositions may also include polymerization agents such as a caprolactam polymerization catalyst. Exemplary catalysts may include a salt of a lactam, and the salt may be an alkali metal salt, an alkali-earth metal salt, and/or a Grignard salt of the caprolactam. For example the polymerization catalyst may be an alkali metal salt of caprolactam, such as sodium caprolactam. In another example, the polymerization catalyst may be a Grignard salt of the caprolactam, such as a magnesium bromide salt of the caprolactam. As noted in the discussion of reactive glass fibers above, polymerization agents may also be present on the fibers, and in some instances a polymerization agent may be present in both the reactive resin composition and on the fibers. Incorporating a polymerization agent on the reactive glass fibers can reduce or eliminate its presence in the reactive resin composition, which may increase the pot-life of the reactive resin composition prior to being applied to the fibers.
Exemplary reactive resin compositions may also include additional type of lactam compounds, such as laurolactam, a cyclic amide where the heterocyclic ring includes 12 carbon atoms (C12H23NO).
Exemplary reactive resin compositions may also include oligomers of a cyclic alkylene terephthalate, such as cyclic butylene terephthalate (CBT). An exemplary CBT, whose ring includes two butylene 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.
CBT 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. The CBT oligomers may be selected to have a melting temperature range of, for example, 120-190° C.
CBT-containing reactive resin compositions may be introduced to the plurality of fibers as a melt. The reactive resin composition 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. When the resin particles are oligomers of a CBT, 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 may be butyltin chloride dihydroxide.
The CBT-containing reactive resin composition may also include 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 reactive resin compositions may include a single type of monomer and/or oligomer such as caprolactam or CBT, or alternatively may include two or more types of monomers and/or oligomers. For example, the reactive resin composition may include both caprolactam and CBT. In some examples, the combination of monomers/oligomers may be selected to form a melt suspension of higher melting point monomers/oligomers in a liquid medium made from a lower melting point monomer/oligomer. For example, a combination of caprolactam and CBT may be selected with CBT monomer/oligomers having melting points significantly above the melting point of caprolactam. When this reactive resin combination is heated above the melting point of the caprolactam it forms a liquid medium in which the CBT particles are suspended. The application of this reactive resin suspension on a glass fiber substrate can create an inhomogeneous distribution of the two types of monomers/oligomers in the resin-fiber mixture.
Additional reactive resin compositions include combinations of first and second resin systems having different polymerization temperatures. This may allow the formation of a semi-reactive fiber-resin composition that contains a polymerized resin of the first resin system having a lower polymerization temperature, while also containing unpolymerized monomers/oligomers of the second resin system having a higher polymerization temperature. For example, a reactive resin combination of caprolactam and CBT may be selected such that the CBT has a higher polymerization temperature than the caprolactam. Alternatively, a reactive resin combination can be formulated of two different types of cyclic alkylene terephthalates and/or a bimodal molecular weight distribution of CBT oligomers having different polymerization temperatures.
Exemplary techniques for forming the fiber-resin composition into the fiber-reinforced composite articles may include injection molding and/or compression molding of the composition into the fiber-reinforced article. When the fiber-resin composition includes pre-polymerized and/or partially-polymerized resin, the article fabrication 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 a fully-polymerized fiber-resin composition to maintain the flowability of the composition as it is filling a mold or otherwise forming a shape of the final article.
As noted above, examples of the present fiber-resin compositions may include a thermoplastic resin of polymerized PA-6 and unpolymerized or partially polymerized CBT. The pre-polymerized or partially polymerized CBT can be converted to PBT and form a fully-polymerized fiber-reinforced article under isothermal processing conditions.
The fiber-resin composition extruded by the extruder 306 may be directly supplied to a molding machine 308 that forms the composition into the fiber-reinforced composite article. Exemplary molding machines 308 may include injection molding machines, and compression molding machines, among other types of molding machines. A heated conduit (not shown) may be used to maintain the fiber-resin composition in a molten/liquid state as it is transported from the extruder 306 to the molding machine 308. Alternatively, the fiber-resin composition may be cooling or cooled as it moves from the extruder 306 to the molding machine 308.
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 fiber” includes reference to one or more fibers 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.
