SOLAR REFLECTIVE FIBERS WITH PARTICLE-IN-VOID PORES AND PREPARATION METHOD FOR FABRIC THEREOF

Abstract

A polymer-based composite fiber includes a matrix including one or more polymers, a plurality of voids formed in the matrix, and a plurality of solid particles, each particle of the plurality of solid particles being disposed in a respective void of the plurality of voids. The fiber may have an aspect ratio of at least 10, the fiber aspect ratio being defined as the length of the fiber divided by the diameter of the fiber. Each void of the plurality of voids may have a respective long axis, each long axis being a line that extends through a longest dimension of the void, and each long axis may extend in a direction parallel to the length of the fiber.

Claims

1. A polymer-based composite fiber comprising: a matrix comprising one or more polymers; a plurality of voids formed in the matrix; and a plurality of solid particles, each particle of the plurality of solid particles being disposed in a respective void of the plurality of voids; wherein each void of the plurality of voids has a respective long axis, each long axis corresponding to a line that extends through a longest dimension of the void; and wherein each long axis extends in a direction parallel to a length of the fiber.

2. The polymer-based composite fiber of claim 1, wherein each void of the plurality of voids has an interior space, each interior space being isolated from an ambient environment.

3. The polymer-based composite fiber of claim 1, wherein each void of the plurality of voids has a void interior surface, and wherein each particle of the plurality of solid particles is completely encapsulated by a respective void interior surface.

4. The polymer-based composite fiber of claim 1, wherein each void has an aspect ratio of at least 3, the void aspect ratio being defined as the longest dimension of a respective void divided by a width of the void.

5. The polymer-based composite fiber of claim 4, wherein for each respective void of the plurality of voids, the width of the void is less than or equal to an effective diameter of the particle disposed within the void.

6. The polymer-based composite fiber of claim 5, wherein for each respective void, the particle is disposed in the void such that the particle divides the void into two separate sub-voids.

7. The polymer-based composite fiber of claim 1, wherein the diameter of the fiber is selected from the range of 100 nm to 1000 microns.

8. The polymer-based composite fiber of claim 1 having a ratio of a weight of the plurality of particles to a weight of the matrix of 5% to 50%.

9. The polymer-based composite fiber of claim 1 having a porosity in the range of 10% to 50%.

10. The polymer-based composite fiber of claim 1, wherein the plurality of solid particles comprises polyolefin, aluminium oxide (Al2O3), silicon dioxide (SiO2), barium sulfate (BaSO4), zinc oxide (ZnO), boron nitride (BN), yttrium oxide (Y2O3), zirconium oxide (ZrO2), titanium dioxide (TiO2), zinc sulfide (ZnS), magnesium oxide (MgO), and/or ytterbium oxide (Yb2O3).

11. The polymer-based composite fiber of claim 1, wherein the plurality of solid particles comprises polyolefin particles having an effective diameter in the range of 30 to 5000 m.

12. The polymer-based composite fiber of claim 1, wherein the plurality of solid particles comprises particles having an effective diameter in the range of 0.1 to 100 m.

13. The polymer-based composite fiber of claim 1, wherein the linear density of the fiber is in the range of 1 denier to 200 denier and the diameter of the fiber is in the range of 10 to 1000 m.

14. The polymer-based composite fiber of claim 1, wherein the fiber has an aspect ratio of at least 10, the fiber aspect ratio being defined as the length of the fiber divided by a diameter of the fiber.

15. The polymer-based composite fiber of claim 1, wherein the polymer-based composite fiber is part of a fabric.

16. A method of producing a solar reflective, polymer-based composite fiber, the method comprising: mixing a plurality of particles and a polymer material at a preset ratio to prepare a composite master batch; heating the composite master batch to melt the polymer material of the composite master batch without melting the plurality of particles; forming the melted composite master batch into a precursor composite fiber, the precursor composite fiber comprising at least some of the plurality of particles surrounded by a matrix of the polymer material; and drawing the precursor composite fiber, thereby forming the solar reflective, polymer-based composite fiber, wherein the drawing comprises: elongating the precursor composite fiber such that a plurality of voids are formed in the fiber via the particles in the fiber.

17. The method of claim 16, wherein the forming step comprises melt spinning the melted composite master batch to form the precursor composite fiber.

18. The method of claim 16, wherein the heating step comprises heating the polymer to a temperature in the range of of 100 to 400 C.

19. The method of claim 16, wherein the drawing step comprises drawing the precursor composite fiber at a draw ratio in the range of of 2 to 6.

20. The method of claim 16, wherein the solar reflective, polymer-based composite fiber has an aspect ratio of at least 10, the fiber aspect ratio being defined as a length of the fiber divided by a diameter of the fiber; wherein each void of the plurality of voids has a respective long axis, each long axis being a line that extends through a longest dimension of the void; and wherein each long axis extends in a direction parallel to the length of the fiber.

21. A method of fabricating a cooling fabric, the method comprising: producing a plurality of solar reflective, polymer-based composite fibers, wherein producing a respective solar reflective, polymer-based composite fiber of the plurality of solar reflective, polymer-based composite fibers comprises: mixing a plurality of particles and a polymer material at a preset ratio to prepare a composite master batch; heating the composite master batch to melt the polymer material of the composite master batch without melting the plurality of particles; forming the melted composite master batch into a precursor composite fiber, the precursor composite fiber comprising at least some of the plurality of particles surrounded by a matrix of the polymer material; and drawing the precursor composite fiber, thereby forming the solar reflective, polymer-based composite fiber, wherein the drawing comprises: elongating the precursor composite fiber such that a plurality of voids are formed in the fiber via the particles in the fiber; and weaving or knitting the solar reflective, polymer-based composite fibers into the cooling fabric.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting.

[0030] FIG. 1A shows a schematic diagram of the forces acting on a particle within a polymer matrix inside a fiber in accordance with the present disclosure.

[0031] FIG. 1B shows a schematic diagram of a self-formed eye-like air pore (void) inside a fiber of the present disclosure.

[0032] FIGS. 2A-2B show simulations of the pore-forming process. FIG. 2A: Fiber at the initial state. FIG. 2B: Drawn fiber with a draw ratio of 2, as can be seen a void is created via the particle during the drawing process.

[0033] FIGS. 3A-3D show the scattering efficiency of particle, integrated particle and air pore, and air pore obtained by FDTD simulations. FIGS. 3A-3C: Scattering efficiency spectra of particle, integrated particle and air pore, and air pore respectively. FIG. 3D: Averaged scattering efficiency of scatters with diameters from 0.2 m to 2 m.

[0034] FIGS. 4A-4B show the scattering cross section of (FIG. 4A) particle and (FIG. 4B) particle and air pore with diameters from 0.2 m to 2 m in the wavelength range of 0.3 to 2.5m.

[0035] FIGS. 5A-5B show SEM images of the PET/BaSO4 fiber. FIG. 5A: SEM image of the fiber before drawing. FIG. 5B: SEM image of the fiber after drawing.

[0036] FIGS. 6A-6D show elemental mappings of the as-spun fiber. FIGS. 6A-6D: The corresponding elemental mapping of C, O, Ba, and S, respectively.

[0037] FIGS. 7A-7B show tensile tests of porous PET/BaSO4 yarns. FIG. 7A: Tensile testing results of ten PET/BaSO4 yarns. FIG. 7B: Comparison of tensile testing results of yarn of the present disclosure vs those of commercial cotton and PET yarns.

[0038] FIG. 8 shows the reflectivity/emissivity spectrum of the PET/BaSO4 fabric in the UV-visible-IR range.

[0039] FIGS. 9A-9C show a photo and schematic of the radiative cooling tests. FIG. 9A: Photo of the test setup. FIG. 9B: Schematic of the sub-ambient cooling performance test. FIG. 9C: Schematic of the skin cooling performance test.

[0040] FIGS. 10A-10B show (FIG. 10A) ambient temperature, PET/BaSO4 fabric temperature, solar irradiation, (FIG. 10B) relative humidity, and wind speed of the daytime sub-ambient cooling performance tests.

[0041] FIGS. 11A-11B show (FIG. 11A) ambient temperature, PET/BaSO4 fabric temperature, solar irradiation, (FIG. 11B) relative humidity, and wind speed of the nighttime sub-ambient cooling performance tests.

[0042] FIGS. 12A-12B show (FIG. 12A) ambient temperature, PET/BaSO4 fabric temperature, solar irradiation, (FIG. 12B) relative humidity, and wind speed of the nighttime sub-ambient cooling performance tests.

[0043] FIGS. 13A-13E show thermal images of (FIG. 13A) PET/BaSO4, (FIG. 13B) cotton, (FIG. 13C) CA, (FIG. 13D) PET, and (FIG. 13E) linen fabrics at solar irradiation of 800 W m-2 (15:00, 10 Jul. 2023). The average apparent temperatures are marked in the center of the images.

[0044] FIG. 14 shows the results of testing the durability of the solar reflecting property of the presently disclosed fibers against washing, abrasion, bending, and UV.

STATEMENTS REGARDING CHEMICAL COMPOUNDS AND NOMENCLATURE

[0045] In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

[0046] In an embodiment, a composition or compound of the invention, such as an alloy or precursor to an alloy, is isolated or substantially purified. In an embodiment, an isolated or purified compound is at least partially isolated or substantially purified as would be understood in the art. In an embodiment, a substantially purified composition, compound or formulation of the invention has a chemical purity of 95%, optionally for some applications 99%, optionally for some applications 99.9%, optionally for some applications 99.99%, and optionally for some applications 99.999% pure.

DETAILED DESCRIPTION

[0047] In the following description, numerous specific details of the devices, device components and methods of the present invention are set forth in order to provide a thorough explanation of the precise nature of the invention. It will be apparent, however, to those of skill in the art that the invention can be practiced without these specific details.

In one aspect, the present disclosure provides a polymer-based fiber with self-formed eye-like air pores and its preparation method for daytime passive radiative cooling fabric. The fabrication of the fiber in this invention follows the industrial production of commercial fibers. The pre-mixed master batch comprises one or more polymers and particulate (solid) additives. The mixture may be baked for more than 24 hours before spinning to completely remove moisture. After the melting spinning and lubricating process, partially oriented yarns, which have relatively low crystallinity and mechanical strength, are collected on the cone. Generally, partially oriented yarns are not well suited to be used for weaving or knitting directly. The partially oriented yarn may be drawn on differential speed rollers with heating functions to improve its crystallinity and mechanical strength. Thus, fully oriented yarns may be produced. After twisting, the yarns can be used to produce fabrics by knitting or weaving.
Air pores, or voids, may be self-formed around the solid particulate additives during the drawing process of the fibers. The composite structure of the fiber, including a continuous polymer matrix, dispersed solid particles, and air pores associated with the solid particles in the fibers produces a large scattering efficiency and scattering cross section in the solar wavelength range, which enables fibers to scatter incident sunlight effectively, even at a low concentration of solid particulate additive. As a result, the prepared fabric can have high solar reflectivity and emissivity in the atmospheric window, and realize all-day passive radiative cooling.

[0048] The fabrication of fibers of the present disclosure may include spinning, drawing, and twisting, followed by weaving or knitting to make fabrics. The pre-mixed master batch may include polymer and solid particulate additives. In the drawing process, polymers will transition from a hard and relatively brittle glassy state into a viscous or rubbery state at the temperature above their glass transition temperatures. When the tensile stress in the x-axis .sub.x overcomes the weak Van der Waals force, the interface between the matrix and the nanoparticle separates and pores form around the particle (FIGS. 1A-1B). If fibers are cooled down under tension, the porous structure can be preserved inside the fibers because molecular chains of the polymer matrix are frozen in place and behave like solid glass again.

[0049] The pore-forming process of this method may be simulated by physics simulation software such as COMSOL. A particle is placed in the center of a polymer fiber. The composite fiber is solid at the initial state (FIG. 2A). After stretching, the interface is separated and an eye-like structure is formed (FIG. 2B). As a result, the eye-like porous structure has larger scattering efficiency compared to air pores (FIG. 3D). Meanwhile, its scattering cross section is much larger than a single particle due to the existence of pores (FIGS. 4A-4B). As a result, the textile can scatter incident sunlight strongly and have high solar reflectivity (r.sub.0.3-2.5 m). Light will experience strong resonant absorption if the frequency of the light matches the vibrational frequency of the chemical bonds. Most polymers for fabric use have multiple chemical bonds, which can emit mid-infrared radiation. As a result, the disclosed fabrics generally have a high emissivity in the atmospheric window (.sub.8-13 m). Thus, the high solar reflectivity and mid-infrared emissivity of the fabric in this invention can enable it to achieve all-day passive radiative cooling.

Example 1Polyethylene Terephthalate/Barium Sulfate (PET/BaSO4) Porous Fabric

[0050] A polyethylene terephthalate/barium sulfate (PET/BaSO.sub.4) porous fabric for all-day passive radiative cooling is prepared following the procedures mentioned above. The pre-mixed master batch comprises PET and ceramic nanoparticles. A PET/BaSO.sub.4 composite master batch was prepared with a BaSO.sub.4 concentration of 60% in weight (35% in volume). The composite master batch includes PET at a mass ratio of 1 to 2. The mixture should be baked for more than 24 hours before spinning to completely remove moisture. After the melting, spinning and lubricating process, partially oriented yarns, which have relatively low crystallinity and mechanical strength, are collected on the cone. Generally, partially oriented yarns are poorly suited for weaving or knitting directly. The partially oriented yarn may be drawn on differential speed rollers with heating functions to improve its crystallinity and mechanical strength. Then fully oriented yarns are yielded. After twisting, the yarns can be used to produce fabrics by knitting or weaving.

[0051] The density measurement of the as-spun fiber and the drawn fiber is based on Archimedes' principle. First, some cut fibers were weighed to get the mass. Next, the fibers were submerged in distilled water. Thus, the increased volume of the fibers in the water as compared to the water alone can be obtained, wherein the increase in volume equals the volume of the submerged fibers. Then the density of these fibers can be calculated. The densities of PET and BaSO.sub.4 are 1.38 g cm.sup.3 and 4.5 g cm .sup.3 respectively. So the density of the as-spun fiber (.sub.solid) should be 1.55 g cm .sup.3 by calculation, which is very close to the measured value of 1.52 g cm.sup.3. The measured density of the drawn fiber (.sub.porous) is 1.31 g cm.sup.3. Then the porosity of the drawn fiber can be calculated by

[00001]$=1-\frac{{}_{\mathrm{porous}}}{{}_{\mathrm{solid}}},$

which is 15.6%.

[0052] The SEM images of melt-spinning fibers are shown in FIGS. 5A-5B. The core region and the surface of the fiber before drawing are both solid (FIG. 5A). From the EDS element mappings (FIGS. 6A-6D), the BaSO.sub.4 nanoparticles are dispersed very uniformly in the fiber. After the drawing process, it can be seen from FIG. 5B that the interface between the particle and polymer matrix is separated and air pores are self-generated.

[0053] Tensile tests of ten specimens were conducted by a universal testing machine. As shown in FIGS. 7A-7B, the tensile stress and the elongation of most specimens are in good agreement with each other. The ultimate stress of PET/BaSO.sub.4 yarns is similar to commercial cotton or PET yarns. Furthermore, the elongation of the yarns produced in accordance with the present disclosure is close to that of PET yarns, which is much higher than cotton.

[0054] Due to the internal structure of the fibers, including particles within voids within the fibers, the resulting fabric made from the fibers has a solar reflectivity of 92.0% (FIG. 8), which is much higher than commercial fabrics. The high reflectivity in the visible-IR region origins from the abundant scatters in fibers and enlarged scattering cross section during the drawing process. In the UV region, the reflectivity drops suddenly in the UV-B range (280-315 nm). Most incident light in this region is absorbed by PET and nearly zero light penetrates the fabric due to the existence of bonds in benzene rings. While this part of energy only accounts for 0.07% of the general solar energy on the earth, the absorbed heat is not a huge concern. Besides, PET is widely used in textile industries and its UV resistance can meet the demands of textile use.

[0055] Field tests were conducted to evaluate the cooling performance of the PET fabric using the same setup as in FIGS. 9A-9B. The tests were performed on summer days (July 2023) in Hong Kong (221650 N, 1141020 E), a tropical and coastal city with a total precipitable water (TPW) value of 45 mm. A weather station was used to monitor the ambient temperature, solar irradiation intensity, relative humidity (RH), and wind speed, near the samples in real-time. T-type thermocouples were attached on the backside of the fabrics, which were placed on the EPS foam (303030 cm.sup.3, density of 25 kg m.sup.3). Promisingly, sub-ambient cooling of above 1 C. was achieved under a solar irradiation of 700 W m.sup.2 (FIG. 10A). In the daytime, the disclosed fabrics can achieve a cooling temperature T of 3.4 C. relative to the ambient with RH of 70% and cloudage of 40% (FIGS. 10A-10B). High RH and cloudage are both hindrances to radiative cooling because water vapor has strong IR emissions, which can block thermal radiation to outer space. For example, at RH of 70%, the cooling power at a cloudage of 40% is around 25% lower than that on a sunny day (cloudage of 10%). During the nighttime, a large T of 4.5 C. was achieved with RH of 85% (FIGS. 11A-11B). These results validate the superior sub-ambient cooling performance of the disclosed fabrics even under weather conditions of high RH.

[0056] Further, simulated human skin was used to evaluate the skin cooling performance of the fabric when worn on the body as clothing (FIG. 9C). The simulated human skin (r.sub.0.3-2.5 m=0.38, and .sub.8-13 m=0.95) has quite similar spectral characteristics within 0.3-16 m to real human skin (r.sub.0.3-2.5 m=0.38, and .sub.8-13 m=0.93). The input power of the silicone rubber heater was set to 100 W m.sup.2, which is to simulate the basal metabolic rate of the human body. Commercial fabrics (white color), such as PET fabric, cellulose acetate (CA) fabric, linen, and cotton, which are commonly used in summer clothes, were also tested as a comparison. During the test, the skin temperatures of all samples are higher than the ambient temperature, because the heating power of the simulated skin is far beyond the cooling power in tropical climates, even for some high-performance radiative cooling films. During the daytime measurement, the white cotton cloth has the lowest skin temperature among commercial fabrics because of its higher solar reflectivity. Compared to cotton fabric, the skin temperature of the PET/BaSO.sub.4 fabric is 1.4 C. to 9.1 C. lower in the daytime. The maximum T appeared at noontime with peak solar irradiation of 1050 W m.sup.2 (FIGS. 12A-12B). From the thermal images of the tested fabrics at 15:00 on 10 Jul. 2023, cotton, CA, and PET have close apparent temperatures. The apparent temperature of the PET/BaSO.sub.4 fabric is 4.8 C. and 10.6 C. lower than the apparent temperatures of the cotton and linen, respectively, which also verifies the outstanding cooling performance of our fabric (FIGS. 13A-13E). At nighttime, T of all fabrics is within 0.4 C. due to their quite similar .sub.8-13 m. The PET/BaSO.sub.4 has the highest temperature because its relatively larger thickness brings better thermal insulation performance.

[0057] Durability of the solar reflecting property of the presently disclosed fibers was tested against washing, abrasion, bending, and UV exposure. As shown by the before and after tests of FIG. 14, the solar reflectively of the fiber was not materially changed by exposure to any of washing, abrasion, bending, or UV. Accordingly the tests indicated a high degree of durability of the solar reflecting property of the presently disclosed fibers against washing, abrasion, bending, and UV exposure.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

[0058] All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

[0059] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

[0060] As used herein and in the appended claims, the singular forms a, an, and the include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to a cell includes a plurality of such cells and equivalents thereof known to those skilled in the art. As well, the terms a (or an), one or more and at least one can be used interchangeably herein. It is also to be noted that the terms comprising, including, and having can be used interchangeably. The expression of any of claims XX-YY (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and in some embodiments is interchangeable with the expression as in any one of claims XX-YY.

[0061] When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

[0062] Certain molecules disclosed herein may contain one or more ionizable groups [groups from which a proton can be removed (e.g., COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein. With regard to salts of the compounds herein, one of ordinary skill in the art can select from among a wide variety of available counterions those that are appropriate for preparation of salts of this invention for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt may result in increased or decreased solubility of that salt.

[0063] Every device, system, formulation, combination of components, or method described or exemplified herein can be used to practice the invention, unless otherwise stated.

[0064] Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.

[0065] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their publication or filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art. For example, when composition of matter are claimed, it should be understood that compounds known and available in the art prior to Applicant's invention, including compounds for which an enabling disclosure is provided in the references cited herein, are not intended to be included in the composition of matter claims herein.

[0066] As used herein, comprising is synonymous with including, containing, or characterized by, and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, consisting of excludes any element, step, or ingredient not specified in the claim element. As used herein, consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms comprising, consisting essentially of and consisting of may be replaced with either of the other two terms. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0067] One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.