Patent Application
20100003877
Thermal insulation and cold protective materials are extensively used in various applications, such as building constructions, energy storage facilities, aircrafts as well as cold protective clothing for reducing the heat transfer between a medium and its environment. Among the various thermal insulation and cold protective materials, such as powder insulation, foam insulation, vacuum panels, fibrous insulation materials have the advantage in terms of high thermal insulation, light weight, good moisture permeability, and shock-absorbing ability, because of their extremely high porosity, generally 95% or above (Tseng and Kuo, Thermal radiative properties of phenolic foam insulation, Journal of Quantitative Spectroscopy & Radiative Transfer, 72, 349-359 (2002)). Within the fibrous insulation materials, heat transfer mechanisms mainly involve conduction and thermal radiation (Farnworth, Mechanisms of heat flow through clothing insulation, Textile Research Journal, 53(12), 717-725 (1983)). A significant amount of work, for example Farnworth (1983), Wu et al. (2007), Du et al. (2007), has shown that radiative flux can be a significant contributor to the total heat transfer within these highly porous fibrous insulations. To reduce radiative heat flux, one may increase fiber fractional volume (or reducing the porosity) of the fibrous insulation, see Farnworth (1983) and Wu et al. (2007), or introducing thin dense films as interlayers (Wu and Fan, Measurement of radiative thermal properties of thin polymer films by FTIR, Polymer Testing, 27: 122-128 (2008)), however this would lead to an increase of conductive heat flux and reduction of moisture permeability. In applications where moisture transmission takes place in the fibrous insulation, for example, cold protective clothing or sleeping bags, reduction of moisture permeability will induce increased moisture accumulation and condensation within the fibrous insulation and consequently reduce effectiveness in thermal insulation. It is therefore a challenge to reduce the radiative heat loss without increasing conductive heat loss and blocking the moisture transmission.
Developing fine fibers and metallic or metallized fibers are found to be the most efficient ways to reduce radiative heat transfer by increasing their surface area to volume ratio or improving their absorption and scatting to thermal radiation. Superfine fibrous films, when used as interlayers in the fibrous insulation, have great potential in blocking the radiative heat loss without increasing conductive heat loss and reducing the moisture permeability (Wu et al., Thermal radiative properties of electrospun superfine fibrous PVA films, Materials Letters, 62, 828-831 (2008)). This is because the resistance to radiative heat transfer is strongly related to the total surface area and surface properties of fibers. The high surface-to-volume ratio of superfine fibers can increase the absorption efficiency to thermal radiation and consequently improve the blocking of heat transfer by radiation. The blocking thermal radiation can be further improved by coating a reflective layer on the fiber surface and hence increasing the radiation extinction coefficient. Furthermore, because of the high porosity and very fine fiber diameter, such material was also found to be highly permeable to moisture transmission (Gibson et al., Transport properties of porous membranes based on electrospun nanofibers, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 187-188, 469-481 (2001)). Nevertheless, the potential problem of the superfine fibers is their weak strength if used alone. This invention is therefore envisaged to fabricate three-tier reflective nanofibrous structures for use as interlayers in the thermal insulation systems such as cold protective clothing and sleeping bags.
Although several radiation reflection coatings with aluminum (Al), silver (Ag), or gold (Au) on general woven or nonwoven substrate have been reported, the coating layer is generally dozens of micrometers thick so the weight of the coating is relatively heavy. If such existing coating fabrics are used as interlayers in insulation systems, there will be substantial unwanted weight increase of over 20%. As a result, the weight of the cold protective system may be noticeably increased after coating with radiation reflection materials, which is unwanted for most cold protective systems especially for clothing, sleeping bags, and aircrafts. Moreover, the thick coating with metals will also significantly reduce the permeability to water vapor. Condensation of water vapor will take place when they are used as cold protective clothing under extremely cold environment, and this will significantly increase the heat loss through the clothing.
It is an object of the present invention to overcome the disadvantages and problems in the prior art.
The present invention provides a three-tier reflective nanofibrous structure having a woven or nonwoven substrate, a polymeric nanofibers web on the substrate, and an infrared radiation reflection coating on the web. The present nanofibrous structure has good reflection to heat radiation in the near infrared, has good resistance to heat conduction, has good permeability to water vapor moisture, and is light weight. A method of fabricating both radiation rejective and nanofibrous structure is also presented.
These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings where:
a-d) shows various used for the present three-tier structure;
a-b) show field-emission SEM images of the spunbonded PP web supported electrospun PVA nanofibers and the Al coated PVA nanofibers;
c) is a TEM image of the cross-sectional specimen of the Al coated PVA nanofibers;
The following description of certain exemplary embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
Now, to
The woven or nonwoven substrate 105 can be selected from the group consisting of wook, silk, gunny, polyester, nylon, polypropylene, polyethylene, polystyrene, polyether, polyamide, polyimide, polyacrylonitrile, polyvinylchloride, and acrylic polymers.
As will be discussed later, various polymeric materials including synthetic and natural polymers may be used to form the nanofibers web 103 on a woven or nonwoven substrate by electrospinning. While the amount of the IR reflection material 101 is very small, it has a noticeable improvement on radiation reflection since the supporting nanofibers provide a great surface area to volume ratio. As a result, the coating of IR radiation reflection layer increases the thermal radiation extinction in IR and consequently improves the thermal insulation performance.
Suitable polymeric materials to be applied to the substrate can include synthetic polymers such as ethenic polymers and condensation polymers. Ethenic polymers are formed by polymerizing monomers containing the carbon to carbon double bond group. Important ethenic polymers for electrospinning nanofibers include polyethylene, vinyl chloride polymers and copolymers, and polystyrene. For condensation polymers, the monomers have at least two functional groups such as alcohol, amine, or carboxylic acid gropu instead of a carbon-carbon double bond group. Various condensation polymers, such as Nylon, polyacrylic acid, polyacrylonitrile, polycarbonate, poly(etherimide), poly(ethylene terephthalate), poly(urethane), poly (vinyl alcohol) may be used to form nanofibers on a woven or nonwoven substrate. For natural polymers, proteins, and polysaccharides are most polymers to be electrospun for forming nanofibers. Since these natural polymers have a distinct advantage over synthetic materials such as degradable by naturally occurring enzymes, which is advantageous for safe post-disposal of the fibrous insulation.
As will be discussed later, electrospinning is an approach using electrostatic forces to produce fine fiber. In the present invention, an electric field is generated between a charged polymer fluid and a collection screen. As the power is increased, the charged polymer solution is attracted to the fiber collection. Once the voltage reaches a critical value, the charge overcomes the surface tension of the polymer cone and superfine fibers are produced. As the charged fibers are sprayed, the solvent quickly evaporates and the fibers are accumulated randomly or aligned on the surface of the fiber collector.
The IR radiation reflection layer may be deposited onto the polymeric nanofibers with metal such as aluminum (Al), silver (Ag), and gold (Au), metal oxide such as aluminum oxide (Al2O3), titanium dioxide (TiO2), zinc oxide (ZnO), and cerium dioxide (CeO2), and metal oxides doped with a dopant selected from the group consisting of fluorine, boron, aluminum, gallium, thallium, copper, and iron. The coating of the IR reflection materials may be formed onto the polymeric nanofibers by sputtering, arc plasma deposition, chemical vapor deposition, and sol-gel method.
a-d) shows various uses for the present three-tier structure.
As shown in
b) shows a single structure 419/421/423 used directly to form cold protective clothing or sleeping bags under medium cold environments (between 10° C. to 20° C.) 424.
c) shows that the three-tier reflective nanofibrous structure may also be usable in other protective systems, such as building constructions and aircrafts 426, in a certain number of three-tier structure 427/429 added as interlayers. Such a structure is suitable in extreme cold environments (below 10° C.) 440.
In both
In the above embodiments, the thickness of the woven or nonwoven substrate may be selected in the range of from about 0.1 to about 10 mm in terms of its specialized application requirement and the electrospinning process conditions. The thickness, diameter, and porosity of the nanofibers web may be controlled to support the IR radiation reflection coating by altering the electrospinning process parameters such as electrostatic pressure distance between spinneret and substrate, and flow rate of polymer solution. The diameters of the electrospun nanofibers from most polymers are typically in the range 100 to 1000 nm. The thickness of the IR radiation reflection layer may be selected to control both the percent of the IR radiation reflection under particular application temperature and the weight increase of the thermal insulation through coating. Generally, the thickness in the range of 10 nm and 100 nm are suitable for efficiently reflecting radiation in IR without unacceptable weight increase.
A three-tier reflective nanofibrous structure comprising PP web/PVA nanofibers/AL coating is made in accordance with the present invention. At first, the PVA nanofibers are electrospun onto the spunbonded PP web of the thickness about 0.23 mm by using electrospinning technique to improve the strength of the nanofiber membrane; then the electrospun nanofibers was coated with AL by using sputtering deposition technique to extinct thermal radiation in advance.
a and 5b show the field-emission SEM images of the spundbonded PP web supported electrospun PVA nanofibers and the AL coated PVA nanofibers, respectively. As shown in
The spectral extinction of the three specimens is shown in
In order to quantitatively compare the extinction coefficients of the three samples with or without nanofiber or aluminum coating, an apparent Rosseland mean extinction coefficient (σe,R) was introduced. The apparent Rosseland mean extinction is determined by using the Rosseland approximation:
where eb,λ is the spectral black body emissive power,and T is the medium temperature.
The determined results of the apparent Rosseland mean extinction coefficients for the three different samples are illustrated in
The water vapor transmission rates of the three samples with or without nanofibers or metal coating are listed in Table 1 using the water vapor transmission dish method according to the British Standard BS7209 (1990). The PP/PVA/AL three-tier reflective structure presented in this invention has very similar water vapor transmission rate like the uncoated the PP web. This behaviour is expected because the metal coating was deposited on the nanofibers rather than directly on the PP substrate so that the porous structure of the system was unaffected. High moisture permeability is advantageous for applications in cold weather clothing, sleeping bags, building constructions, and aircraft because the accumulation of water vapor accumulation will result in great reduction of the thermal insulation performance. Concerning the significant improvement in extinction thermal radiation but the little increase in weight gain and water vapor block, this present invention provides a three-tier reflective nanofibrous structure may be expected to be widely applied high performance thermal resistant but moisture permeable systems under extremely or medium cold environments.
Having described embodiments of the present system with reference to the accompanying drawings, it is to be understood that the present system is not limited to the precise embodiments, and that various changes and modifications may be effected therein by one having ordinary skill in the art without departing from the scope or spirit as defined in the appended claims.
In interpreting the appended claims, it should be understood that:
a) the word “comprising” does not exclude the presence of other elements or acts than those listed in the given claim;
b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements;
c) any reference signs in the claims do not limit their scope;
d) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; and
e) no specific sequence of acts or steps is intended to be required unless specifically indicated.
