Patent Application
20230256645
The present disclosure relates generally to processing of naturally-occurring wood, and more particularly, to forming and use of modified wood and/or transparent wood composites.
Embodiments of the disclosed subject matter provide modified wood and transparent wood composites, and methods for forming and use thereof. In some embodiments, a contiguous wood block is subjected to a chemical treatment such that natural sections therein experience different degrees of lignin removal. For example, the contiguous wood block can be a softwood, and the earlywood sections thereof can be delignified while the latewood sections thereof can retain substantial amounts of lignin after the chemical treatment. Subsequent infiltration of the chemically-treated wood block with an index-matching polymer converts the delignified sections to be substantially transparent while other sections remain opaque or translucent with respect to wavelengths in the visible light spectrum. The resulting wood composite can thus exhibit a natural pattern defined by the arrangement of transparent earlywood sections and translucent or opaque latewood sections.
In some embodiments, a contiguous wood block is subjected to a UV-assisted photocatalytic oxidation treatment to in situ modify lignin therein, thereby converting a color of the wood to white. For example, the contiguous wood block can be infiltrated with a liquid oxidation agent, such as hydrogen peroxide, and then subsequently exposed to UV radiation to cause a chromophore of lignin within the wood block to be removed therefrom while otherwise retaining the lignin within the microstructure of the wood. In some embodiments, the application of liquid oxidation agent to a surface of the wood block and/or the exposure of the wood block to UV light can form a pattern, which confines the in situ modification to particular sections of the wood block. Subsequent infiltration of the wood block with an index-matching polymer converts the in situ modified sections to be substantially transparent while other sections remain opaque or translucent with respect to wavelengths in the visible light spectrum. The resulting wood composite can thus exhibit a predetermined pattern defined by the application of oxidation agent and UV light and independent of any underlying natural patterns in the wood.
In a representative embodiment, a material comprises a contiguous block of chemically-modified wood infiltrated with polymer. The chemically-modified wood can retain a cellulose-based microstructure of the wood in its natural state. The polymer can have a refractive index substantially matching a refractive index of cellulose and filling open spaces within the microstructure. The contiguous block can have a first section and a second section adjacent to the first section. At least one of the first and second sections have been chemically modified such that a lignin characteristic of the first section is different than a lignin characteristic of the second section. The first section can be substantially transparent to light having a wavelength of 600 nm, and the second section can be translucent or opaque to the light having a wavelength of 600 nm.
In another representative embodiment, a material comprises a section of wood chemically-modified such that chromophores of lignin within the wood in its natural state are altered or removed. The section can retain at least 70% of the lignin of the wood in its natural state. The section can also retain a cellulose-based microstructure of the wood in its natural state.
In another representative embodiment, a method comprises subjecting a contiguous block of wood to a chemical treatment for a first time so as to remove lignin from first and second sections within the contiguous block while substantially retaining a cellulose-based microstructure of the wood. The first section can be adjacent to the second section. The first time can be selected such that at least 90% of the lignin of the wood in the first section is removed while less than 75% (e.g., no more than 65%, or no more than 50%) of the lignin in the second section is removed. The method can further comprise infiltrating the contiguous block with a polymer so as to fill open spaces within the retained cellulose-based microstructure of the first and second sections. The polymer can have a refractive index substantially matching a refractive index of cellulose. After the infiltrating, the first section can be substantially transparent to light having a wavelength of 600 nm, and the second section can be translucent to the light having a wavelength of 600 nm.
In another representative embodiment, a method comprises applying a first volume of a liquid oxidation agent to an external surface of a section of a contiguous block of wood, and, during or after the applying, exposing the section of the contiguous block of wood to ultra-violet (UV) radiation. Chromophores of lignin within said section can be chemically oxidized and removed in situ by the UV exposure in the presence of the liquid oxidation agent. After the exposing, at least 70% of the lignin in said section prior to the applying retained. After the exposing, the section can also retain a cellulose-based microstructure of the wood prior to the applying.
In another representative embodiment, a method comprises photocatalytically oxidizing a section of a contiguous block of wood so as to in situ chemically modify native lignin within the section to remove chromophores thereof while preserving its bulk aromatic skeleton.
Any of the various innovations of this disclosure can be used in combination or separately. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The foregoing and other objects, features, and advantages of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
Embodiments will hereinafter be described with reference to the accompanying drawings, which have not necessarily been drawn to scale. Where applicable, some elements may be simplified or otherwise not illustrated in order to assist in the illustration and description of underlying features. Throughout the figures, like reference numerals denote like elements.
For purposes of this description, certain aspects, advantages, and novel features of the embodiments of this disclosure are described herein. The disclosed methods and systems should not be construed as being limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The methods and systems are not limited to any specific aspect or feature or combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present, or problems be solved. The technologies from any embodiment or example can be combined with the technologies described in any one or more of the other embodiments or examples. In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are exemplary only and should not be taken as limiting the scope of the disclosed technology.
Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed methods can be used in conjunction with other methods. Additionally, the description sometimes uses terms like “provide” or “achieve” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms may vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.
The disclosure of numerical ranges should be understood as referring to each discrete point within the range, inclusive of endpoints, unless otherwise noted. Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods, as known to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited. Whenever “substantially,” “approximately,” “about,” or similar language is explicitly used in combination with a specific value, variations up to and including 10% of that value are intended, unless explicitly stated otherwise.
Directions and other relative references may be used to facilitate discussion of the drawings and principles herein, but are not intended to be limiting. For example, certain terms may be used such as “inner,” “outer,”, “upper,” “lower,” “top,” “bottom,” “interior,” “exterior,” “left,” right,” “front,” “back,” “rear,” and the like. Such terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” part can become a “lower” part simply by turning the object over. Nevertheless, it is still the same part and the object remains the same.
As used herein, “comprising” means “including,” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.
Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order, unless stated otherwise. Unless stated otherwise, any of the groups defined below can be substituted or unsubstituted.
Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Features of the presently disclosed subject matter will be apparent from the following detailed description and the appended claims.
The following explanations of specific terms and abbreviations are provided to facilitate the description of various aspects of the disclosed subject matter and to guide those of ordinary skill in the art in the practice of the disclosed subject matter.
Contiguous piece: A single continuous piece of wood taken from a single tree and subject to processing, as contrasted with a single piece formed by joining or combining multiple subpieces (e.g., laminate). In some embodiments, the processing forms sections or regions within the contiguous piece of wood with different lignin characteristics.
Lignin characteristics: In some embodiments, lignin characteristics refers to a content of naturally-occurring or native lignin in a wood section. Different lignin characteristics can thus refer to the native lignin content of one wood section being less than that of an adjacent wood section after processing (e.g., such that an earlywood region is substantially delignified while an adjacent latewood region retains a majority or at least some native lignin). Alternatively or additionally, in some embodiments, lignin characteristics refers to a naturally-occurring or native form of lignin in a wood section. Different lignin characteristics can thus refer to the native lignin of one wood section being in situ modified (e.g., by chemical oxidation) to alter or remove a chromophore of the lignin without otherwise removing the lignin, while an adjacent wood section retains the native form of lignin after processing.
Delignified: A wood section having at least 90% of naturally-occurring lignin originally therein removed therefrom. In some embodiments, a lignin content of a delignified wood section is no more than 3 wt %, for example, less than 1 wt %. Lignin content within the cellulose-based material before and after delignification can be assessed using known techniques in the art, for example, Laboratory Analytical Procedure (LAP) TP-510-42618 for “Determination of Structural Carbohydrates and Lignin in Biomass,” Version 08-03-2012, published by National Renewable Energy Laboratory (NREL), and ASTM E1758-01(2020) for “Standard Test Method for Determination of Carbohydrates in Biomass by High Performance Liquid Chromatography,” published by ASTM International, both of which are incorporated herein by reference
Longitudinal growth direction: A direction along which a plant grows from its roots or from a trunk thereof, with cellulose nanofibers forming cell walls of the plant being generally aligned with the longitudinal growth direction. In some cases, the longitudinal growth direction may be generally vertical or correspond to a direction of its water transpiration stream. This is in contrast to the radial direction, which extends from a center portion of the plant outward and may be generally horizontal.
Transparent: Having a transmittance value (i.e., ratio of intensity of transmitted light to intensity of incident light) of at least 80% with respect to a particular wavelength of light or range of light wavelengths.
Translucent: Having a transmittance value of between 36% and 80% with respect to a particular wavelength of light or range of light wavelengths.
Opaque: Having a transmittance value less than 36% with respect to a particular wavelength of light or range of light wavelengths.
Transparent wood composites with improved mechanical properties can be formed by retaining some or all of the lignin that naturally occurs within the starting wood material. In prior transparent wood composites, removal of most or all (e.g., at least 90%) of the lignin in the starting wood material is necessary to yield high transparency (e.g., >80% for visible wavelengths). However, the removal of such significant amounts of lignin can compromise the integrity of the cellulose-based microstructure of the wood, thereby complicating subsequent fabrication steps (e.g., polymer infiltration) and the resulting mechanical strength of the composite material.
In some embodiments of the disclosed subject matter, a contiguous wood block can be subjected to a chemical treatment such that natural sections therein experience different degrees of lignin removal. For example, the contiguous wood block can be a softwood, and the earlywood sections thereof can be delignified while the latewood sections thereof can retain substantial amounts of lignin after the chemical treatment. Subsequent infiltration of the chemically-treated wood block with an index-matching polymer converts the delignified sections to be substantially transparent while other sections remain opaque or translucent with respect to wavelengths in the visible light spectrum. The resulting wood composite can thus exhibit a natural pattern defined by the arrangement of transparent earlywood sections and translucent or opaque latewood sections. Moreover, since the latewood sections retain substantial amounts of lignin, the overall mechanical strength of the material is improved as compared to completely delignified wood composites.
Alternatively or additionally, in some embodiments of the disclosed subject matter, contiguous wood block is subjected to a UV-assisted photocatalytic oxidation treatment to in situ modify lignin therein, thereby converting a color of the wood to white. For example, the contiguous wood block can be infiltrated with a liquid oxidation agent, such as hydrogen peroxide, and then subsequently exposed to UV radiation to cause a chromophore of lignin within the wood block to be removed therefrom while otherwise retaining the lignin within the microstructure of the wood. In some embodiments, the application of liquid oxidation agent to a surface of the wood block and/or the exposure of the wood block to UV light can form a pattern, which confines the in situ modification to particular sections of the wood block. Subsequent infiltration of the wood block with an index-matching polymer converts the in situ modified sections to be substantially transparent while other sections remain opaque or translucent with respect to wavelengths in the visible light spectrum. The resulting wood composite can thus exhibit a predetermined pattern defined by the application of oxidation agent and UV light and independent of any underlying natural patterns in the wood. Since no or only minimal of the lignin is removed by the photocatalytic oxidation treatment (e.g., less than 30% of the lignin in the original wood removed), the overall mechanical strength of the material is improved as compared to completely delignified wood composites.
In addition, prior transparent wood composites require substantial amounts of chemicals and extensive processing times for the delignification of wood, which may inhibit manufacturability. In contrast, in some embodiments, UV-assisted photocatalytic oxidation treatment is used to process wood to in situ modify lignin therein by surface application of a liquid oxidation agent. Thus, the treatment time and the amount of chemicals used can be reduced as compared to prior transparent wood processing. Moreover, as compared to delignification agents such as NaClO2 that can release toxic chlorine gas, the use of hydrogen peroxide (H2O2) as the liquid oxidation agent provides a more environmentally friendly process, since H2O2 only produces water or oxygen as byproducts.
In some embodiments, a naturally-patterned transparent wood composite (also referred to as aesthetic wood) is provided. The aesthetic wood can have aesthetic features (e.g., intact wood patterns), excellent optical properties (e.g., an average transmittance of ˜80% and a haze of ˜93%), good UV-blocking ability (e.g., a transmittance of ≤20%), and low thermal conductivity (0.24 W·m−1 K−1) based on a process of spatially-selective delignification and refractive-index-matched polymer (e.g., epoxy resin) infiltration. Moreover, the rapid fabrication process (e.g., chemical treatment of 2 hours or less) and mechanical robustness (e.g., a high longitudinal tensile strength of 91.95 MPa and toughness of 2.73 MJ·m−3) of the aesthetic wood can enable manufacturing at scale while saving large amounts of time and energy as compared to conventional complete delignification processes. For example, the aesthetic wood may be used in energy-efficient building applications, such as glass ceilings, rooftops, transparent decorations, and indoor panels.
In some embodiments, a modified wood (also referred to as in situ lignin modified wood, lignin-modified wood, or photonic wood) is provided. Lignin within natural wood can be modified using an in situ, rapid, and scalable process, in particular, by photocatalytic oxidation of native lignin in wood using a liquid oxidation agent (e.g., hydrogen peroxide) and UV light (e.g., solar radiation or artificial illumination in the UVA band). The photocatalytic oxidation reaction selectively eliminates chromophores of the lignin while leaving the aromatic skeleton of the lignin intact, thus modulating the optical properties of wood. The resulting photonic wood retains ˜80% of its original lignin content, which continues to serve as a strong binder and water-proofing agent. As a result, the photonic wood exhibits a much higher mechanical strength in a wet environment (e.g., 20-times higher tensile strength and 12-times greater compression resistance), significant scalability (e.g., ˜2-meter long sample), and largely reduced processing times (e.g., 1-6.5 hours versus 4-14 hours) as compared with delignification of wood. Moreover, the in-situ lignin-modified wood structure can be patterned using photocatalytic oxidation process, in particular, by selective application of the liquid oxidation agent or UV radiation to surfaces of the wood. This photocatalytic production of photonic wood can enable large-scale manufacturing of sustainable bio-sourced functional materials for a range of applications, including energy-efficient buildings, optical management, and fluidic, ionic, electronic, and optical devices.
In some embodiments, a transparent wood composite (also referred to as in situ lignin modified transparent wood composite, artificially patterned transparent wood composite, or simply, transparent wood) is provided. Lignin within natural wood can be modified using UV-assisted photocatalytic oxidation, similar to photonic wood. This preserves most of the native lignin to act as a binder, thereby providing a robust wood scaffold for polymer infiltration while greatly reducing the chemical and energy consumption as well as processing time. After polymer infiltration, the resulting transparent wood (e.g., ˜1 mm in thickness) can exhibit a high transmittance (e.g., >90%), high haze (e.g., >60%), and excellent light-guiding effect with respect to visible light wavelengths. Moreover, similar to photonic wood, patterns can be formed directly on the wood surfaces by selective application of the liquid oxidation agent (e.g., brushing or printing) or UV radiation (e.g., masking or laser illumination). Compared to delignified wood (e.g., tensile strength of 0.4 MPa), the lignin-modified wood has a substantially higher tensile strength (e.g., 20.6 MPa) due to the presence of the modified lignin binding with the well-oriented cellulose fibrils.
At process block 104, the contiguous piece of natural wood can be subject to one or more chemical-based treatments to modify lignin characteristics of at least one section of the wood piece. In some embodiments, the lignin characteristic modification is such that at least one section is formed having a different lignin property than that of an adjacent section. For example, in some embodiments, the lignin property is a lignin content of sections of the contiguous piece 107 of processed wood, and the chemical-based treatment can be such that the lignin content of one wood section 109 (e.g., formerly EW region 103) is less than that of an adjacent wood section 111 (e.g., formerly LW region 105), as described in further detail below with respect to
The method 100 can proceed to decision block 106, where it is determined if a transparent composite is desired. If it is determined that a transparent composite is not desired, for example, for use as photonic wood, then the method 100 can proceed from decision block 106 to process block 110. Otherwise, if it is determined that a transparent composite is desired, the method 100 can proceed from decision block 106 to process block 108, where the contiguous piece of modified wood is infiltrated with an index-matching polymer.
In some embodiments, the polymer infiltration of process block 108 can be accomplished by one or more vacuum-assisted infiltration sessions, for example, by immersing the modified wood in a container of liquid polymer or polymer precursors and applying a vacuum to chamber containing the container, or as otherwise described in International Publication No. WO-2017/136714, filed Feb. 3, 2017, which is incorporated herein by reference. The polymer can be any polymer having a refractive substantially matching that of cellulose (e.g., having a refractive index of ˜1.47) and capable of infiltration into the wood microstructure. For example, the infiltrated index-matching polymer can include any type of thermosetting polymer (e.g., epoxy resin), thermoplastic polymer (e.g., acrylic), cellulose derivative (e.g., cellulose acetate), and/or a functional index-matching material (e.g., liquid crystal or piezoelectric material). Non-limiting examples of polymers that can be infiltrated into the modified wood can include, but are not limited to, those described in International Publication No. WO-2017/136714 incorporated by reference above. In some embodiments, the polymer can be an epoxy resin (e.g., AeroMarine 300/21 epoxy).
In some embodiments, process block 110 can also include drying or allowing infiltrated precursors to polymerize. In some embodiments, the modified wood with infiltrated polymer is subjected to pressing during the drying or polymerization. For example, when a first section 109 has been delignified in process block 104 and a second section 111 retains lignin, the different mechanical strengths of the sections could lead to warping as the polymer dries or polymerizes in these sections. Accordingly, nominal pressure (e.g., without changing a thickness of the contiguous piece by more than 10%) can be applied during the drying or polymerization to prevent, or at least reduce, any warping.
The infiltration of the polymer via process block 110 can thus convert some or all of the wood sections to be substantially transparent. For example, when wood section 109 was substantially delignified or had chromophores removed via the chemical treatment at process block 104, the infiltrating polymer can convert section 109 into a substantially transparent section 115. Meanwhile, when wood section 111 retained lignin and its chromophore after the chemical treatment at process block, section 111 remains a translucent or opaque section 117 after polymer infiltration. Alternatively, in some embodiments, when the entire contiguous piece 107 was formed having the modified lignin property, then the entire piece 113 will be made transparent after polymer infiltration.
After the polymer infiltration of process block 108, or if no polymer infiltration was desired at decision block 106, the method 100 can proceed to process block 110, where the modified wood or transparent wood composite is used in a particular application or adapted for use in a particular application. For example, process block 110 can include machining, cutting, or otherwise forming the contiguous piece into a particular shape. The use of process block 110 can involve use of the contiguous piece of modified wood or transparent wood composite by itself or assembling it together with non-wood materials (e.g., metal, metal alloy, plastic, ceramic, composite, etc.) to form a heterogenous composite structure). In some embodiments, after the polymer infiltration of process block 108, the contiguous piece of transparent wood composite can be used as part of a building (e.g., a window or skylight). Alternatively, in some embodiments, when a transparent composite is not desired at decision block 106, the contiguous piece of modified wood can be used as an insulating structure or a visible light reflector. Other applications beyond those specifically listed are also possible for the modified wood and transparent wood composite structures fabricated according to the disclosed technology. Indeed, one of ordinary skill in the art will readily appreciate that the modified wood and transparent wood composite structures disclosed herein can be adapted to other applications based on the teachings of the present disclosure.
Although some of blocks 102-110 of method 100 have been described as being performed once, in some embodiments, multiple repetitions of a particular process block may be employed before proceeding to the next decision block or process block. In addition, although blocks 102-110 of method 100 have been separately illustrated and described, in some embodiments, process blocks may be combined and performed together (simultaneously or sequentially). Moreover, although
In some embodiments, the chemical treatment of process block 112 can be performed under vacuum, such that the solution associated with the treatment is encouraged to fully penetrate the cell walls and lumina of the contiguous piece of wood. Alternatively, in some embodiments, the chemical treatment of process block 112 can be performed under ambient pressure conditions or elevated pressure conditions (e.g., ˜6-8 bar). In some embodiments, the chemical treatment of process block 112 can be performed at any temperature between ambient (e.g., ˜23° C.) and an elevated temperature where the chemical solution is boiling (e.g., ˜70-160° C.). In some embodiments, the chemical solution is not agitated in order to avoid disruption to the cellulose-based microstructure of the wood. In some embodiments, the chemical solution can include sodium chlorite (NaClO2) alone or in combination with other chemicals (e.g., acetic acid). For example, in some embodiments, the chemical solution comprises a boiling solution of NaClO2.
In some embodiments, the immersion time can be less than 5 hours, for example, 2 hours or less. The amount of time of immersion within the chemical solution may be a function of amount of lignin to be removed, size of the piece, density of the EW section, temperature of the solution, pressure of the treatment, and/or agitation. For example, smaller amounts of lignin removal, smaller piece size, lower density of the EW section, higher solution temperature, higher treatment pressure, and agitation may be associated with shorter immersion times, while larger amounts of lignin removal, larger piece size, higher density of the EW section, lower solution temperature, lower treatment pressure, and no agitation may be associated with longer immersion times.
At decision block 114, it is determined if the treatment of process block 112 should continue. The treatment with the chemical solution can continue (or can be repeated with subsequent solutions) until a desired reduction in lignin content in the EW section is achieved, for example, to achieve a desired light transmittance after infiltration with index-matching polymer at process block 108. In some embodiments, the treatment of process block 112 continues until the lignin content in the EW section has been reduced by at least 90% (e.g., less than 10% of the lignin originally in the EW section is retained), which may correspond to a light transmittance of at least 80% for one or more wavelengths in the visible spectrum (e.g., 600 nm). For example, after the treatment of process block 112, the EW section can have a lignin content less than or equal to 3 wt %, such as less than or equal to 1 wt %. In some embodiments, the treatment of process block 112 can be effective to reduce the lignin content in the LW section by no more than 75% (e.g., greater than 25% of the lignin originally in the LW section is retained), for example, reduced by no more than 65%, or even by no more than 50%, which may correspond to a light transmittance of less than 70% for one or more wavelengths in the visible spectrum (e.g., 600 nm). For example, after the treatment of process block 112, the LW section can have a lignin content greater than or equal to 7.5 wt %, such as greater than or equal to 12.5 wt %.
Once sufficient lignin has been removed from the EW section, the sub-routine 104a can proceed from decision block 114 to process block 116, where the contiguous piece of modified wood is removed from the chemical solution in preparation for polymer infiltration at process block 108. In some embodiments, process block 116 can further include an optional rinsing step after the chemical treatment(s), for example, to remove residual chemicals or particulate resulting from the delignification process. For example, the contiguous block of modified wood can be partially or fully immersed in one or more rinsing solutions. The rinsing solution can be a solvent, such as but not limited to, de-ionized (DI) water, alcohol (e.g., ethanol, methanol, isopropanol, etc.), or any combination thereof. For example, the rinsing solution can be formed of water and ethanol. In some embodiments, the rinsing may be repeated multiple times (e.g., at least 3 times) using a fresh mixture rinsing solution for each iteration. In some embodiments, after the rinsing, the contiguous piece can be stored in an alcohol (e.g., ethanol). In some embodiments, after the storing, the contiguous piece can be immersed in another solvent (e.g., toluene) to exchange with the alcohol therein prior to polymer infiltration at process block 108.
Although some of blocks 112-116 of sub-routine 104a have been described as being performed once, in some embodiments, multiple repetitions of a particular process block may be employed before proceeding to the next decision block or process block. In addition, although blocks 112-116 of sub-routine 104a have been separately illustrated and described, in some embodiments, process blocks may be combined and performed together (simultaneously or sequentially). Moreover, although
The sub-routine 104b can proceed to optional process block 120, where a first volume of alkali in solution is applied to the upper exposed surface portion of the contiguous piece corresponding to the first section(s). For example, the alkali can be sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH4OH), calcium hydroxide (Ca(OH)2), or any combination thereof. In some embodiments, the alkali in solution has a concentration of at least 10 wt %. The application can be by brushing, spraying, rolling, printing, or any other controlled surface application technique. In some embodiments, the first volume can be much less than a corresponding volume of the liquid oxidation agent applied in the subsequent process block 122. For example, the first volume can be less than or equal to 20% of the volume of liquid oxidation agent. In some embodiments, the first volume is in a range of 1-3 ml, inclusive. By including a small quantity of alkali, the decomposition of liquid oxidation agent (e.g., H2O2) can be accelerated without otherwise causing substantial lignin removal from the wood. However, in some embodiments, the application of alkali to the contiguous piece of wood can be omitted.
The sub-routine 104b can proceed to process block 122, where a second volume of liquid oxidation agent is applied to the upper exposed surface portion of the contiguous piece corresponding to the first section(s). For example, the liquid oxidation agent can be H2O2 having a concentration of at least 30 wt %. In some embodiments, the liquid oxidation agent can be applied to the surface portion of the first section(s) without otherwise applying to the surface portion of the second section(s), thereby defining a pattern by virtue of the oxidation agent application. The application can be by brushing, spraying, rolling, printing, or any other controlled surface application technique. In some embodiments, part of the second volume can be applied to the upper exposed surface portion, and the remaining part of the second volume can be simultaneously or subsequently applied to a lower exposed surface portion on an opposite side of the contiguous piece.
In some embodiments, the second volume can be based on a surface area and/or thickness of the wood section to which the liquid oxidation agent is to be applied. For example, when the contiguous piece has a thickness (e.g., in a direction perpendicular to the upper exposed surface) of ˜0.6 mm, the applied second volume for the liquid oxidation agent can be at least 800 ml per square meter of surface area. When the contiguous piece has a thickness of ˜0.8 mm, the applied second volume for the liquid oxidation agent can be at least 1200 ml per square meter of surface area. When the contiguous piece has a thickness of ˜1 mm, the applied second volume for the liquid oxidation agent can be at least 2400 ml per square meter of surface area. Alternatively, the applied second volume for the liquid oxidation agent can be at least 125 ml per 0.1 mm thickness and per square meter of surface area. In some embodiments, the applied second volume for the liquid oxidation agent can be based on the volume of the wood section to which the liquid oxidation agent is to be applied. For example, the second volume can be at least equal to the volume of the wood section, or within a range of 1-5 times, inclusive, of the volume of the wood section. For example, the second volume can be 10-20 ml, inclusive.
The sub-routine 104b can proceed to process block 124, where the contiguous block is subjected to UV radiation from a natural light source (e.g., insolation at a UV index of 5 or greater) or artificial light source (e.g., 20 W of UVA band). In some embodiments, the entire upper surface can be exposed to the UV radiation for a time sufficient to in situ modify the lignin in the first section via the photocatalytic oxidation, in particular, to remove chromophores from the lignin. The exposure of process block 124 can continue via decision block 126 until the photocatalytic oxidation reaction in the first section has proceeded to completion, as evidenced by the first section turning completely white in color. In some embodiments, the exposure time may be less than or equal to 2 hours, for example, 1-2 hours.
After the exposure, the contiguous block can retain at least 80% of the lignin originally therein prior to processing (e.g., reduction of the lignin content of no more than 20%) in the now-white first section. Moreover, since the adjacent second section was not subject to photocatalytic oxidation (e.g., due to lack of application of the oxidation agent thereto), substantially all of the lignin originally therein should be retained. Accordingly, the second section may have a higher lignin content than the first section, albeit only slightly. For example, after the treatment of process block 124, both the first and second sections can have a lignin content greater than or equal to 15 wt %.
Although some of blocks 118-126 of sub-routine 104b have been described as being performed once, in some embodiments, multiple repetitions of a particular process block may be employed before proceeding to the next decision block or process block. For example, the application of liquid oxidation agent in process block 122 may be effected by multiple fractional applications (e.g., by brushing the same surface area more than once (e.g., 3 to greater than 10 times) to cumulatively apply the desired second volume). In addition, although blocks 118-126 of sub-routine 104b have been separately illustrated and described, in some embodiments, process blocks may be combined and performed together (simultaneously or sequentially). For example, the application of first volume of alkali in solution of process block 120 may be combined with the application of second volume of liquid oxidation agent in process block 122. Moreover, although
The sub-routine 104c can proceed to process block 132, where the contiguous block is subjected to UV radiation from a natural light source or artificial light source. In some embodiments, the UV radiation can be applied to the upper exposed surface portion of the first section(s) without otherwise applying to the upper exposed surface portion of the second section(s), thereby defining a pattern by virtue of the UV exposure. For example, the UV radiation from the light source can be passed through a photomask to screen off the second sections from exposure. Alternatively or additionally, the UV light source can be a UV laser or laser diode (e.g., Nd:YAG laser) configured and controlled to sequentially illuminate the upper exposed surface portions corresponding only to the first section(s). In some embodiments, the UV exposure of the upper surface portions of the first section may continue for a time sufficient to in situ modify the lignin in the first section via the photocatalytic oxidation, in particular, to remove chromophores from the lignin. The exposure of process block 132 can continue via decision block 134 until the photocatalytic oxidation reaction in the first section has proceeded to completion, as evidenced by the first section turning completely white in color. In some embodiments, the exposure time may be less than or equal to 2 hours, for example, 1-2 hours.
After the exposure, the contiguous block can retain at least 70% of the lignin originally therein prior to processing (e.g., reduction of the lignin content of no more than 30%) in the now-white first section. Moreover, since the adjacent second section was not subject to photocatalytic oxidation (e.g., due to the lack of UV radiation thereon), substantially all of the lignin originally therein should be retained. Accordingly, the second section may have a higher lignin content than the first section, albeit only slightly. For example, after the treatment of process block 132, both the first and second sections can have a lignin content greater than or equal to 15 wt %.
Although some of blocks 128-134 of sub-routine 104c have been described as being performed once, in some embodiments, multiple repetitions of a particular process block may be employed before proceeding to the next decision block or process block. For example, the application of liquid oxidation agent in process block 130 may be effected by multiple fractional applications (e.g., by brushing the same surface area more than once (e.g., 3 to greater than 10 times) to cumulatively apply the desired second volume). In addition, although blocks 128-134 of sub-routine 104c have been separately illustrated and described, in some embodiments, process blocks may be combined and performed together (simultaneously or sequentially). For example, the application of first volume of alkali in solution of process block 128 may be combined with the application of second volume of liquid oxidation agent in process block 130. Moreover, although
Walls of cells in the natural wood are primarily composed of cellulose (40 wt %˜50 wt %), hemicellulose (20 wt %˜30 wt %), and lignin (20 wt %˜30 wt % for hardwoods and 25 wt %˜35 wt % for softwoods), with the three components intertwining with each other to form a strong and rigid wall structure. Natural hardwood has a unique three-dimensional porous structure with multiple channels or lumina formed by longitudinal cells, including vessels 214 (e.g., having a maximum cross-sectional dimension, or diameter, in a plane perpendicular to a length thereof of 40-80 μm, inclusive) and fibers 216 (e.g., having a maximum cross-sectional dimension, or diameter, in a plane perpendicular to a length thereof of 10-30 μm, inclusive) extending in a direction of wood growth 210, as illustrated by the exemplary section 212 in
Softwoods are wood from gymnosperm trees such as pine (e.g., Eastern white pine, Lodgepole pine, Parana pine, Scots pine, Southern yellow pine, etc.), cedar (e.g., red cedar, etc.), spruce (e.g., European spruce, Sitka spruce, etc.), larch, and fir (e.g., Douglas fir). Natural softwood presents an intrinsic aesthetic pattern of annual growth rings with alternating structures at macroscopic and microscopic scales. From the macro perspective, the rings result from the alternating formation of EW 218 in spring and LW 220 in summer, as shown in
The piece of natural wood can be cut in any direction with respect to its longitudinal growth direction 210. Since the tracheids are naturally aligned with the growth direction, the cut direction will dictate the orientation of the cell lumina in the final structure, which orientation can affect the optical or mechanical properties of the final transparent wood composite. For example, in some embodiments, a piece of natural wood can be cut from a trunk 202 of tree 200 in a vertical or longitudinal direction (e.g., parallel to longitudinal wood growth direction 210) such that lumina of longitudinal cells are oriented substantially parallel to a major face (e.g., largest surface area) of the longitudinal-cut wood piece 206. Alternatively, in some embodiments, the piece of natural wood can be cut in a horizontal or radial direction (e.g., perpendicular to longitudinal wood growth direction 210, also referred to as a transverse cut) such that lumina of longitudinal cells are oriented substantially perpendicular to the major face of the radial-cut wood piece 204. Alternatively, in some embodiments, the piece of natural wood can be cut in a rotation direction (e.g., perpendicular to the longitudinal wood growth direction 210 and along a circumferential direction of the trunk 202) such that lumina of longitudinal cells are oriented substantially parallel to the major face of the rotary-cut wood piece 208. In some embodiments, the piece of natural wood can be cut at any other orientation between longitudinal, radial, and rotary cuts. For any of the cut directions, a thickness of the piece of natural wood can be measured in a direction perpendicular to the major face and may be 10 mm or less.
Using the naturally-occurring patterns of alternating EW 218 and LW 220 regions illustrated in
In some embodiments, the naturally patterned transparent wood composite can be fabricated using a batch fabrication process. For example,
In some embodiments, the contiguous piece is removed from the fluid chamber 404 of the first stage 402 and inserted into the chamber 416 of the second stage 414. Alternatively, in some embodiments, the fluid chamber 404 and the chamber 416 are the same, and the transition from the first stage 402 to the second stage 414 is effected by replacing the delignifying solution 406 with the liquid polymer or precursors 418. Although not specifically discussed above, it should be appreciated that batch fabrication setup 400 can further include one or more rinsing stages (not shown).
In some embodiments, the batch fabrication setup can optionally include a drying or polymerization stage 426 that employs a pressing setup (e.g., a hydraulic press). For example, the pressing setup can have a top platen 430 and a bottom platen 428, with one or both platens capable of moving toward the other to apply pressure to the wood composite 432 held therebetween. For example, the pressing setup can apply nominal pressure to maintain a thickness and/or the planarity of upper and lower surfaces of the composite 432 as the polymer therein hardens. In some embodiments, the pressing stage 426 can include heating of the composite 432 while being pressed, for example, by heating of one or both platens 428, 430.
In some embodiments, the naturally patterned transparent wood composite can be fabricated using a semi-continuous fabrication process. For example,
After delignification station 460, the modified sliced layer 458 can continue to be conveyed to the next sequential station, e.g., rinsing station 464. Rinsing station may contain one or more solvents 466 (e.g., water, alcohol, etc.), and optionally one or more agitators, designed to remove any remnants of the delignifying chemicals 462 within the modified layer. Although only a single rinsing station is illustrated in
After rinsing station 464, the modified sliced layer 458 can continue to be conveyed to the next sequential station, e.g., polymer station 468, which includes liquid polymer or polymer precursors 472 within a chamber (e.g., a vacuum chamber). Infiltration into the cellulose-based microstructure of the modified EW and LW sections forms the transparent wood composite 480 with fully transparent sections alternating with translucent or opaque sections.
In some embodiments, fabrication setup 450 can optionally include a drying or polymerization station 474 that employs complementary rollers 476, 478. In some embodiments, the upper roller 476 and lower roller 478 remain at a fixed distance from each other that substantially equal to or slightly less than a thickness of composite wood 480, thereby applying a nominal pressing force that discourages warping during drying or polymerization. In some embodiments, one or both rollers 476, 478 can be heated so as to raise a temperature of the composite 480 above room temperature, for example, to encourage solidification or polymerization of the polymer. Alternatively or additionally, the rollers 476, 478 may be unheated, but a separate heating mechanism may be provided, or an environment containing or following the station 474 may be heated.
In situ modification of lignin in hardwood or softwood is accomplished by a photocatalytic oxidation mechanism resulting from simultaneous exposure 501 of the naturally- occurring forms 503 of lignin to both an oxidation agent 505 (e.g., hydrogen peroxide) and UV-radiation 507, as shown in
In some embodiments, a patterned transparent wood composite can be fabricated using a batch fabrication process. For example,
At a third stage 516, the entire upper surface 504 can be exposed to UV radiation 518 from a natural or artificial light source. Alternatively, in some embodiments, only the first section 510 may be exposed to UV radiation 518 (e.g., in a manner similar to UV exposure stage 540 in
Alternatively, in some embodiments, the modified wood 522 is subject to further processing at final stage 528, for example, to infiltrate the wood 522 with an index-matching polymer to form a patterned transparent wood composite 530 with a transparent first section 532 and an adjacent translucent or opaque second section 534. For example, polymer can be infiltrated into the microstructure of the modified wood 522 by immersing it in a liquid polymer or polymer precursors and applying a vacuum for a period of time, for example, as discussed above with respect to
The combination of oxidation agent and UV radiation in the first section 548, but not in the second section 550, causes photocatalytic oxidation to occur only in the first section 548. The resulting modified wood 553 (e.g., photonic wood) at stage 552 has a first section 554, which is in situ lignin-modified and thus substantially white in color, and a second section 556, which retains its natural lignin and thus is substantially non-white in color (e.g., natural wood color). In some embodiments, the modified wood 553 at stage 552 can be used as is, for example, as insulation or optically reflecting material.
Alternatively, in some embodiments, the modified wood 553 is subject to further processing at final stage 558, for example, to infiltrate the wood 553 with an index-matching polymer to form a patterned transparent wood composite 560 with a transparent first section 562 and an adjacent translucent or opaque second section 564. For example, polymer can be infiltrated into the microstructure of the modified wood 553 by immersing it in a liquid polymer or polymer precursors and applying a vacuum for a period of time, for example, as discussed above with respect to
In some embodiments, transparent wood composite (whether patterned or unpatterned) can be fabricated using a semi-continuous or continuous fabrication process. For example,
After station 608, the wood layer 606 can be conveyed to the next sequential station e.g., UV exposure station 614. UV exposure station 614 can include an artificial light source 616 and one or more optical elements 620 (e.g., reflector) designed to illuminate the wood with a substantially uniform light beam 618. Alternatively, in some embodiments, UV exposure station 614 takes advantage of natural insolation rather than using an artificial light source. As with the above described examples, the combination of UV exposure and oxidation agent within the wood results in photocatalytic oxidation that in situ modifies the lignin in the wood, in particular, by removing chromophores thereof without substantially decreasing lignin content. In some embodiments, the size of the station 614 and the speed of conveyance of the wood layer 606 through the station 614 may correspond to the desired UV exposure time (e.g., 1-2 hours). Thus, a time from when a portion of the layer 606 enters station 614 to when it leaves for polymer infiltration station 624 would correspond to exposure time for the in situ lignin modification.
In some embodiments, the modified wood 622 resulting from station 614 can be used without further processing. Alternatively, in some embodiments, the modified wood 622 is further conveyed to the next sequential station, e.g., polymer station 624, which includes liquid polymer or polymer precursors 626 within a chamber 628 (e.g., a vacuum chamber). Infiltration into the cellulose-based microstructure of the modified wood 622 forms the transparent wood composite 630.
Naturally-patterned transparent wood composites (also referred to as aesthetic wood) were fabricated based on two different cuts of wood, in particular, a radial (R) cut where the aligned channels of the cellulose-based microstructure extend in a direction perpendicular to a primary surface (e.g., a surface exposed to incident light to be transmitted) and a longitudinal (L) cut where the aligned channels of the cellulose-based microstructure extend in a direction parallel to the primary surface. Douglas fir was chosen in view of its a pronounced contrast of both color and density between earlywood (EW) and latewood (LW) sections. As shown in
R-cut contiguous blocks of Douglas fir, each with dimensions of 60 mm×60 mm×2 mm, were used to analyze the effect of delignification. A solution of acidic NaClO2 (80%) was employed to remove the colored components (primarily lignin, along with extractives) from the bulk wood. The solution was prepared by dissolving NaClO2 powder in de-ionized (DI) water, and then adding acetic acid to adjust the pH value (˜4.6). Each wood sample was placed in the boiling NaClO2 solution for a time period (for example, 2 hours) until the EW sections became white. Afterwards, the delignified wood samples were rinsed with DI water at least three times and then stored in ethanol until further processing. To form naturally-patterned wood composite, epoxy resin (e.g., AeroMarine 300/21 epoxy, a clear, low viscosity cycloaliphatic epoxy system Aeromarine Products, Inc., San Diego, Calif.) was infiltrated into the processed wood samples. The epoxy resin was allowed to solidify for approximately 24 hours, resulting the naturally-patterned wood composite.
Differential (e.g., spatially-selective) delignification between the EW and LW sections (e.g., based at least in part on the density differences between EW and LW) can be achieved in as little as 2 hours of immersion in the boiling in the NaClO2 solution. After the 2 hour treatment, the EW section becomes almost completely white, whereas the LW section retains color due to the residual lignin and other colored components. The main contributor to spatially-selective delignification is the inherent structural differences between EW and LW sections, which accordingly leads to a faster solution diffusion in the EW section than in the adjacent LW section. After 2 hours of immersion in the boiling NaClO2 solution, the weight of each wood sample had decreased by about 13.5%. However, nano-scale and macros-scale features of the native wood (e.g., the cellulose-based microstructure) is substantially preserved. In order to convert the LW sections to a completely white color, much longer treatment times are required (e.g., about 10 hours) with a corresponding increase in the amount of weight lost (e.g., weight decreased by about 35%). With these longer treatment times, the integrity of the delignified wood structure may not be well-maintained, resulting in poor mechanical properties in part due to the distinct density difference between the EW section (e.g., ˜284.6 kg·m−3) and LW section (e.g., ˜846 kg·m−3) and the lack of lignin in both the EW and LW sections.
To assess the distribution of lignin in the softwood scaffold after spatially-selective delignification, Raman spectroscopy imaging was used in combination with Vertex Component Analysis (VCA).
Following the same procedure described above, L-cut contiguous blocks of Douglas fir (using the quarter slice cutting of
The L-cut naturally-patterned transparent wood composite exhibits massive aligned dense microchannels along the wood growth direction after successful infiltration. In a cross-sectional view, although lumen in the LW section is much smaller than that of EW section, the lumina are all densely packed. Additionally, the channels and apertures in each section are fully filled with polymer (e.g., epoxy resin), which acts as a glue to create strong interaction between the cellulosic cell wall and that polymer itself. Raman spectroscopy imaging was further performed to identify the distribution of the impregnating polymer in the obtained wood cell including cell corner (CC), compound middle lamella (CML), cell wall (CW), and lumen. According to the corresponding Raman spectrum in
The hierarchical cellular structure of the spatially-delignified wood leads to unique anisotropic mechanical features. For example, naturally-patterned transparent wood composite formed from R-cut wood exhibits a dramatically improved tensile strength (e.g., 21.56 MPa) as compared to natural R-cut wood (e.g., 6.24 MPa), and naturally-patterned transparent wood composite formed from L-cut wood exhibits an even higher tensile strength (e.g., 91.95 MPa) The toughness of naturally-patterned transparent wood composite formed from R-cut wood and L-cut wood, respectively, is 0.523 MJ m−3 and 2.73 MJ m−3.
In addition, the inhomogeneous distribution of lignin and cell structures between EW and LW sections in the naturally-patterned transparent wood composite can result in non-uniform transmittance. As shown in
Moreover, the retention of lignin in the LW sections imbues the naturally-patterned transparent wood composite with unique UV-blocking capabilities (e.g., with respect to wavelengths in a range of 200-400 nm, inclusive), which can be tuned depending on the timing of the delignification treatment. For example, when contiguous wood blocks having thickness of 2 mm was subjected to delignification treatment times under 2 hours, the subsequent transparent wood composite was able to shield almost 100% of the UVC (200-275 nm) and UVB (275-320 nm) spectra and most of the UVA (320-400 nm) spectrum. If, however, the delignification treatment time is prolonged, for example, to 9 hours, the UVA blocking capability of the resulting transparent wood composite was remarkably decreased, as shown in
The naturally-patterned transparent wood composites also demonstrate anti-glare and light guiding capabilities. For example, the naturally-patterned transparent wood composites largely scatters the light forward, leading to a high optical haze of ˜93%, as shown in
The pattern of the transparent wood composite is defined by the natural pattern of EW and LW sections in the contiguous piece of original wood. However, other types of patterns can be realized by stacking multiple layers 700 of transparent wood composites together, each of which may be from the same tree, from the same wood species (e.g., both Douglas fir but from different trees), or from different softwoods (e.g., one is fir and the other is pine). For example, as shown in
At the same time, aesthetic wood can also improve energy efficiency due to its excellent thermal insulation properties compared to glass. For example, the naturally patterned transparent wood composite exhibits a thermal conductivity of 0.24 W·m−1 K−1 in the radial direction (e.g., perpendicular to the longitudinal growth direction), which is lower than in the longitudinal growth direction (e.g., ˜0.41 W·m−1 K−1) and lower than the isotropic thermal conductivity of common window glass (e.g., ˜1 W·m−1 K−1). The anisotropic thermal transport of the transparent wood composite in combination with such low thermal conductivities can be useful for replacing glass in energy-efficient buildings.
To demonstrate the use of the naturally-patterned transparent wood composite as a building material with high transparency and high haze, model houses were constructed using glass and the transparent wood composite as a skylight. Using an external white light source directed at the skylight, light intensities at various points within each model house were detected and compared. In the model house employing the glass skylight, the maximum light intensity (e.g., 56.8 mW·cm−2) was about 17 times higher than the minimum light intensity (e.g., 3.4 mW·cm−2), which yielded a non-uniform illumination. In contrast, the model house employing the transparent wood composite skylight experienced a diffused light distribution that was more uniform, with a maximum light intensity of 48.2 mW·cm−2 and a minimum light intensity of 20.9 mW·cm−2.
The weathering stability of the naturally-patterned transparent wood composite was also evaluated by exposing the materials outdoors for 3 weeks and measuring optical and mechanical properties. For the transparent wood composite formed from R-cut wood, the transmittance of the composite after exposure decreased slightly as compared to before the exposure, while the haze increased from ˜93% to ˜98% in the wavelength range of 400-800 nm. The transparent wood composite formed from L-cut wood experienced a similar change in transmittance and haze properties after exposure. Similar trends in the transmittance and haze occurred in aesthetic wood-L as well. However, the exposure did not impact the mechanical properties of either cut of the transparent wood composite. Rather, there was no significant degradation in the strength of the transparent wood composite by the exposure, suggesting that the composite enjoys at least short term weathering capability.
In the examples above, softwood was used to take advantage of the spatially-selective delignification based on the native sections within the contiguous wood piece. Although both hardwood and softwood are in principal suitable, hardwood possesses a significantly different structure consisting of vessels and fibers, while softwood mainly consists of tracheids. For example, basswood, a type of hardwood, has a substantially uniform cell wall thickness of ˜5.8 μm, which is much thinner than the cell wall thickness of the LW section in Douglas fir. Moreover, the vessel channels in basswood are larger in terms of lumen diameter than the relatively narrow tracheids of Douglas fir, and the vessel channels exhibit a bimodal pore-size distribution. As a result, the reactions in the EW and LW sections of basswood proceed substantially in sync, such that almost no apparent wood patterns are preserved after 2 hours of treatment. Similar results were obtained for balsa wood (another type of hardwood possessing bimodal pores and therefore substantially uniform solution diffusion). However, hardwoods otherwise exhibit substantial differences in density, porosity, cell wall thickness, lumina cross-sectional dimensions, or any combination of the foregoing and that results in different solution diffusion or reaction efficiency between different naturally-occurring sections therein can be used to form a naturally-patterned transparent wood composite according to embodiments of the disclosed subject matter.
The native lignin within contiguous blocks of wood was chemically modified in situ by UV-assisted photocatalytic oxidation method to fabricate modified wood (also referred to as in situ lignin modified wood, lignin-modified wood, or photonic wood). During the UV-assisted photocatalytic oxidation process, conjugated double bonds are cleaved so as to remove the chromophores of lignin while preserving the bulk aromatic skeleton of lignin, which continues to provide mechanical strength. The modified wood thus retained most of the lignin within the original wood (e.g., ≥80%), while the removal of chromophores imbues the modified wood with unique optical properties. In particular, the modified wood exhibits a high optical whiteness (e.g., ≥90% reflectivity for light having a wavelength in the range of 400-800 nm, inclusive), an intact cellulose-based microstructure, improved mechanical strength (e.g., ˜20 MPa wet tensile strength), remarkable water stability, and improved scalability (e.g., as much as 2 meter) when compared with prior delignification techniques used to generate optical properties in wood.
In the natural wood, the vertically-aligned wood channels enable H2O2 and UV light to penetrate efficiently into the wood structure for fast and in-depth de-coloration that can be achieved in less than 7 hours (e.g., 1-6.5 hours, depending on thickness of the contiguous piece of wood in a direction perpendicular to a surface of the wood piece upon which the UV light is incident). Furthermore, the wood can be selectively decolored by H2O2 printing in combination with UV light radiation (e.g., using a paper board carving as a mold for application of H2O2 to the wood surface), which makes it possible to directly generate custom, predetermined patterns of regions with different optical properties within the contiguous piece of wood.
Balsa wood was used to prepare the modified wood because of its low density and hierarchical porous microstructure, but other hardwoods or softwoods could also be used. First, balsa wood samples were impregnated with H2O2, in particular, by immersing each contiguous sample block in a 30% H2O2 solution to which was added 10% NaOH solution. The small quantity of alkali can serve to accelerate the decomposition of H2O2 without causing substantial lignin removal from the balsa wood. Each H2O2-impregnated balsa wood sample was then exposed to UV radiation using an artificial light source (UVA band, 20 W power) until the samples became completely white. For example, the natural balsa wood changes from brown to completely white after about 2 hours of UV exposure (in combination with the H2O2 exposure). In contrast, the color of the wood turns yellow when using H2O2 without UV light. Note that lignin has many photolabile chromophore groups (e.g., quinone groups and conjugated double bonds), which is prone to absorb the energetic photons of UV light to generate chromophoric radicals. Therefore, the photo-excited chromophoric radicals can efficiently react with H2O2 to enable the photocatalytic oxidation degradation of chromophore groups, eliminating the brownish color of natural wood. Meanwhile, the color of the wood did not change significantly when applying only UV light, despite the fact that lignin is sensitive to UV radiation. Moreover, if H2O2 treatment is used any UV exposure, even 10 hours of exposure to H2O2 is insufficient to modify the natural balsa wood to have the white color achievable with the combination of H2O2 and UV exposure. This indicates that the chemical oxidant alone may not be enough to completely bleach the wood, or at least not on the same time scales as the combination of the chemical oxidant with the UV exposure.
The combination of H2O2 and UV exposure further allows the modification of optical properties of the wood without removing significant amounts of lignin therefrom, thereby enabling enhanced mechanical strength of the modified wood as compared to conventional delignification. The compositional content of the modified wood was measured using the acid hydrolysis method, and the acid-insoluble lignin (Klason lignin) was determined by gravimetric analysis.
The chemical structure of the modified wood was further analyzed using X-ray diffraction spectroscopy (XDS) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) patterns were collected using a Rigaku Ultima III equipped with a curved detector manufactured by Rigaku Americas Corp. (operating tube voltage of 40 kV, tube current at 30 mA, Cu Kα, λ=1.5406 Å). As shown in
The above noted results, in addition to change in color to white of the modified wood, confirms that the UV-assisted photocatalytic oxidation is effect to modify lignin in situ to its chromophore while preserving most lignin skeletons. Different from other ex situ lignin modification techniques, the disclosed technique allows for in situ modification of the native lignin, which is composed of phenyl skeletons and oxygen containing branches and bonded by a series of C—O and C—C linkages. Such an intact lignin structure facilitates the elimination of its chromophores while preserving as much of the lignin skeleton structure as possible during photocatalytic oxidation process.
As discussed above, natural wood can possess large vessel channels with diameters of several hundred micrometers (˜100-300 μm) as well as small fiber lumen with diameters of tens of micrometers (˜20-50 μm), which are decorated by different ranged sizes of pits (˜0.8-10 μm). These hierarchical and interconnected microstructures serve as an efficient path for the photocatalytic oxidation process, enhancing the H2O2 penetration and UV light capture, which leads to an efficient synergistic reaction to obtain the modified wood. Furthermore, the wood scaffold structure with its hierarchical pores can be maintained after the photocatalytic oxidation process. In particular, the thickness of cell walls in the modified wood (˜2.06 μm) are similar to the natural balsa wood starting material (˜2.09 μm). For comparison, delignified wood was fabricated by immersing the natural balsa wood in a boiling chemical solution (e.g., an aqueous 5 wt % NaClO2 solution with acetic acid added to adjust the pH to ˜4.6) until the wood became completely white. Such delignified wood has cells walls that become significantly thinner after treatment (˜1.46 μm) and with substantial changes to the underlying cellulose-based microstructure, as shown in
To explore the uniformity of the treatment with respect to a thickness of the modified wood, a cross-section of a contiguous block of modified wood (38 mm×30 mm×9 mm) was taken along the longitudinal growth direction and divided it into three sections (section “I” corresponding to the 3 mm adjacent a top surface of the block subjected to UV exposure, section “III” corresponding to the 3 mm adjacent a bottom surface of the block opposite the top surface, and section “II” corresponding to the 3 mm between sections I and III). No obvious visual differences were observed between the three sections; rather, all displayed the same level of whiteness, indicating that the wood was consistently decolorized by the UV-assisted photocatalytic oxidation. The microscopic structures of these three sections appeared similar as well, demonstrating an intact wood microstructure. FTIR analysis showed these three sections also feature the same components, retaining the bulk structure of lignin. Additionally, the reflectance spectrum shown in
Compared to delignification methods, the disclosed photocatalytic oxidation technique exhibits many superior characteristics. First, the use of H2O2, which a green oxidant that decomposes into water and oxygen without producing any toxic gas or liquid, may be considered more environmentally friendly than delignification methods (e.g., using NaClO2 solution, which can produce significant amounts of toxic chlorine gas). Second, the processing times required for photocatalytic oxidation may be substantially less than delignification. For example, the disclosed photocatalytic oxidation can decolor (convert the wood to white in color) a wood block of 5 mm thickness in as little as 3.8 hours, whereas delignification may require at least 6 hours to achieve a similar decoloring. Third, the photocatalytic oxidation process also better retains the lignin content compared to delignification (82% vs. 1.4%) by selectively removing the chromophoric group while retaining lignin's bulk aromatic structure. Indeed, by retaining the majority of lignin after processing, the modified wood can retain the original cellulose-based microstructure of the wood, as shown in
The preserved lignin of the modified wood serves as a mechanical binder that provides mechanical strength and prevent to its deconstruction. Wood samples were kept in ultrapure water for 20 minutes, and then the samples were subjected to mechanical tests after removing the excessive water on the sample surface. The tensile properties of the natural wood, modified wood, and delignified wood samples were measured using a Tinius Olsen H5KT tester. The dimensions for the tensile samples were approximately 50 mm×5 mm×1.5 mm. The samples were stretched at a constant test speed of 5 mm/min along the sample length direction until they fractured. The modified wood in these wet conditions exhibited a tensile strength (along the longitudinal growth direction) of 20 MPa, which is 20-times stronger than that of the totally delignified wood (1 MPa) and essentially the same as that of unmodified natural wood. In this case, the in situ modified lignin of the modified wood is able to hold the cellulose fibers together to enhance the tensile properties of the wood as compared to delignified wood.
The compressive properties of the photonic wood and delignified wood samples were measured using a Tinius Olsen H5KT tester. The dimensions for the tensile samples were approximately 20 mm×10 mm×10 mm. The samples were compressed at a constant test speed of 5 mm/min along the direction perpendicular and parallel to the tree growth direction. The modified wood exhibited a higher compressive strength due to the support of the stiff lignin, while pressure exerted on the delignified wood results in irreversible collapse of the cell walls. After releasing the pressure, the modified wood recovers without obvious deformation (the thickness change after compression Δh=1.5 mm). In contrast, the delignified wood cannot recover, resulting in high compressive deformation (Δh=8.4 mm). Compression tests of wood samples were also performed at well state along the direction parallel to the tree growth direction. At the same compressive displacement (1.6 mm), the delignified wood exhibited collapse of the cell walls, while the modified wood showed no structural damage or a significant decrease in compressive strength.
The in situ modified lignin can also act as a barrier to water and improve the water stability of the modified wood due to the hydrophobic property of the lignin's aromatic rings. Water stability tests were performed by placing blocks of the natural wood, modified wood, and delignified wood in water. The dimensions of the wood in this experiment were 4.5 cm×4.5 cm×0.45 cm. The natural wood, photonic wood, and delignified wood were placed in water at the same time and the thicknesses were recorded every minute. More water was absorbed into the delignified wood, leading to a larger change in mass than that of the modified wood. In this case, without the shield of the hydrophobic lignin, the loosened cellulose fibers of the hydrophilic delignified wood are more sensitive to water. Meanwhile, the high water absorbance by the delignified wood also leads to a more obvious change in the material's thickness. Water absorption rate of the samples were also measured, in which one end of the wood was placed in a methylene blue (MB) solution. The delignified wood featured the highest level of MB adsorption, followed by the modified wood, which again indicates that water penetrates the delignified wood more easily due to the lack of hydrophobic lignin.
After immersing the samples in water for three weeks to study their water stability, the delignified wood completely disintegrated into short fibers, while the modified wood maintained its shape without obvious change. Due to the lack of lignin, water is able to penetrate into the delignified wood and disrupt the accessible and loose cellulose hydrogen bonds, leading to its weak mechanical performance. Meanwhile, benefitting from the hydrophobic and binding role of lignin, which crosslinks the microfibers of photonic wood, water is unable to break the hydrogen bonding structure of the cellulose, resulting in the excellent water stability of the photonic wood in addition to its improved mechanical features.
Although the fabricated examples discussed above have resulted in modification of the entire contiguous block, the photocatalytic oxidation technique can be used to form adjacent sections within the contiguous block having different properties, for example, according to a predetermined two-dimensional or three-dimensional pattern that is independent of the natural patterns of the wood. In particular, since both UV light and the chemical oxidant (H2O2) are used to achieve de-coloration within a particular treatment time, the controlled application of both to specific portions of the wood can dictate the resulting properties thereof. For example, H2O2 can be printed (e.g., brushed, painted, sprayed, or otherwise applied to the surface without immersion of the entire contiguous block) on the surface of the natural wood in a particular pattern (e.g., using a carved paperboard mold), which surface is then illuminated by UV radiation. Sections of the wood that receive both UV and H2O2 will be modified while the sections of the wood that receive only one or neither will be unmodified. For example, a pattern was printed on a surface of a contiguous block of wood using 30% H2O2, and then the surface was exposed to UV light to form patterned modified wood.
The native lignin within contiguous blocks of wood was chemically modified in situ by UV-assisted photocatalytic oxidation method to fabricate modified wood, which was then infiltrated with an index-matching polymer to form a transparent wood composite (also referred to as in situ lignin modified transparent wood composite, artificially patterned transparent wood composite, or transparent wood). A balsa wood log was cut along the transverse and longitudinal directions to form wood slices (having thicknesses ranging from 0.6 mm to 3.5 mm, inclusive). For each balsa wood slice, a trace amount of NaOH (2-3 ml at a concentration of 10 wt %) was coated (e.g., brushed) on a top surface (perpendicular to thickness direction) before brushing H2O2 to improve the oxidation efficiency of the H2O2. Then, the top surface of each wood slice was brushed with H2O2 (≥15 ml at a concentration 30 wt % concentration, with volume depending on wood thickness), followed by illuminating the top surface until the samples became completely white. A UV lamp emitting wavelengths of 380-395 nm was used for UV irradiation of the modified wood. For example, ˜15 ml of H2O2 (concentration of 30 wt %) was brushed on a natural balsa wood sample having dimensions of 200 mm×10 mm×0.6 mm, followed by exposure to UV light for 1 hour until the natural wood color turned completely white. This process removes the chromophore in lignin, causing the color of the wood to change from brown to white. The treated wood pieces were then immersed in ethanol for 5 hours to remove any remaining chemicals, and then transferred to toluene so as to exchange the ethanol in the wood. Subsequently, each treated wood piece was impregnated with epoxy resin (e.g., AeroMarine 300/21 epoxy, a clear, low viscosity cycloaliphatic epoxy system Aeromarine Products, Inc., San Diego, Calif.) by vacuum infiltration for 1.5 hours. Finally, the epoxy-impregnated wood samples were stored at room temperature until the epoxy was completely cured.
In particular, transparent wood formed from radial/transverse (T) cut wood (e.g., having a size of 70 mm×30 mm×1.5 mm) and transparent wood formed from longitudinal (L) cut wood (e.g., having a size of 400 mm×110 mm×1 mm) both exhibit excellent optical properties.
Although balsa wood was used for the above described examples, transparent wood can be made from any type of hardwood or softwood. Indeed, transparent wood with excellent optical transparency was also be made from other wood species with different densities, in particular, oak and poplar, suggesting the universality of this approach. Additionally, the transparent wood preserves the aligned channels of the original wood microstructure, which allow light propagation to be guided along the channel direction and providing anisotropic optical transmittance.
As discussed above, the retention of lignin within the transparent wood composite can enhance the mechanical properties thereof. Mechanical properties of the natural wood and transparent wood at different tensile directions were measured. The tensile strengths of the natural wood along the L and T directions were 24.5 MPa and 0.7 MPa, respectively, while the tensile strengths of the L- and T-transparent wood samples were 46.2 MPa and 31.4 MPa, respectively (corresponding to an enhancement of 1.8-times and 44.8-times the strength of the respective natural wood cut). The L- and T-transparent wood also have a significantly improved toughness of 0.93 MJ m−3 and 1.64 MJ m−3 compared to the natural wood (L, 0.26 MJ m−3; T, 0.03 MJ m−3). The toughness of the L-transparent wood is lower than that of the T-transparent wood because of the smaller elongation at break of the L sample (3.4%<7.4%). Benefiting from the high mechanical strength, the transparent wood is also quite flexible, capable of being bent through an angle greater than 90° (e.g., as much as 180°) without breaking.
Conventional solution-based delignification methods generally involve immersing of the entirety of a wood block into a chemical solution, which makes it difficult to bleach selective areas of the material. In contrast, surface application of a liquid oxidation agent (e.g., brushing of H2O2 onto the wood) combined with UV light illumination can allow for selective in situ lignin modification of designated areas of the wood samples, thereby enabling the preparation of transparent wood composites with unique predetermined patterns, independent of any underlying natural patterns in the wood. In particular, a patterned transparent wood composite can be formed by selectively and precisely patterning to define lignin-modified regions and unmodified (e.g., natural lignin) regions in a contiguous block, and then infiltrating the contiguous block with polymer. Within the polymer-infiltrated contiguous block, the lignin-modified regions thus exhibit a relatively high optical transmittance (e.g., ˜90% for visible wavelengths) while the unmodified regions exhibit an optical transmittance similar to natural wood (e.g., ˜6% to ˜36% for visible wavelengths).
To form a patterned transparent wood composite, a desired pattern was first drawn with a brush on the surface of the natural wood sample using H2O2 as an “ink.” Then the wood surface was illuminated with UV light, which turns the regions underlying the brushed surface white Epoxy resin was then infiltrated into the microchannels of the lignin-modified wood to obtain the transparent wood composite with desired patterns. For example,
Although the above described examples employ an artificial UV light source, solar radiation can also be used as the UV light source. Of the UV light (100-400 nm) emitted by the sun, more than 95% of the wavelengths that reach the Earth's surface are in the UVA range (e.g., 315-400 nm), which wavelengths are effective to provide the desired photocatalytic effect. For example, using solar radiation (Global Solar UV index of 7-8), three large pieces of balsa wood (having a length of 1 m) were in situ lignin modified after only one hour of exposure. Subsequent polymer infiltration converted the white modified wood sections to high transparency wood composite sections.
In view of the above described implementations of the disclosed subject matter, this application discloses the additional examples in the clauses enumerated below. It should be noted that one feature of a clause in isolation, or more than one feature of the clause taken in combination, and, optionally, in combination with one or more features of one or more further clauses are further examples also falling within the disclosure of this application.
Clause 1. A material comprising:
Any of the features illustrated or described with respect to
In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosed technology. Rather, the scope is defined by the following claims. We therefore claim all that comes within the scope and spirit of these claims.
The present application claims the benefit of U.S. Provisional Application No. 63/050,484, filed Jul. 10, 2020, entitled “Patterned, Transparent Wood and Wood Composite Structures and Methods of Making and Using the Same,” and U.S. Provisional Application No. 63/134,936, filed Jan. 7, 2021, entitled “Patterned, Transparent Wood and Wood Composite Structures and Methods of Making and Using the Same,” both of which are incorporated by reference herein in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US21/41181 | 7/9/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63134936 | Jan 2021 | US | |
| 63050484 | Jul 2020 | US |
