1. Field of the Invention
This invention relates to thermal insulation. More specifically, this invention relates to thermal insulation containing infrared radiation (“IR”) absorbing and scattering material, which reduces radiative heat transfer through the thermal insulation.
2. Description of Related Art
Heat passes between two surfaces having different temperatures by three mechanisms: convection, conduction and radiation. These heat transfer mechanisms are combined in a quantitative measure of heat transfer known as “apparent thermal conductivity.”
Insertion of glass fiber thermal insulation in the gap between two surfaces reduces convection as a heat transport mechanism because the insulation slows or stops the circulation of air. Heat transfer by conduction through the glass fiber of the insulation is also minimal. However, many glass compositions used in glass fiber insulation products are transparent in portions of the infrared spectrum. Thus, even when the gap between surfaces has been filled with glass fiber thermal insulation, radiation remains as a significant heat transfer mechanism. Typically, radiation can account for 10 to 40% of the heat transferred between surfaces at room (e.g., 24° C.) temperature.
Fiber to fiber radiative heat transfer is due to absorption, emission and scattering. The amount of radiative heat transfer between fibers due to emission and absorption is dependent on the difference in fiber temperatures, with each fiber temperature taken to the fourth power.
To reduce radiative heat loss through thermal insulation, a number of approaches have been considered.
U.S. Pat. No. 2,134,340 discloses that multiple reflections of infrared radiation from a powder of an infrared transparent salt, such as calcium fluoride, added to glass fiber insulation can prevent the infrared radiation from penetrating any substantial distance into the insulation.
U.S. Pat. No. 5,633,077 discloses that an insulating material combining certain chiral polymers with fibers can block the passage of infrared radiation through the insulating material.
U.S. Pat. No. 5,932,449 discloses that glass fiber compositions displaying decreased far infrared radiation transmission may be produced from soda-lime borosilicate glasses having a high boron oxide content and a low concentration of alkaline earth metal oxides.
There remains a need for a cost effective thermal insulation product that can reduce radiative heat loss.
A thermal insulation product is provided in which an IR absorbing and scattering material is dispersed on fibers arranged in a porous structure. The IR absorbing and scattering material can be applied to the fibers before or after the fibers are formed into the porous structure. The IR absorbing and scattering material substantially reduces the radiative heat loss through the thermal insulation. Inclusion of the IR absorbing and scattering material improves the effective wavelength range over which the porous structure absorbs infrared radiation and improves its overall extinction efficiency. The IR absorbing and scattering material is about as effective as glass fiber in reducing radiative heat loss through a porous fiber structure, but can be much less expensive than glass fiber. Hence, the IR absorbing and scattering material can provide a cost-effective means of improving thermal insulation.
The preferred embodiments of the invention will be described in detail, with reference to the following figures, wherein:
The present invention reduces the radiant transmission of heat through a fiber based thermal insulation product by dispersing an IR absorbing and scattering material onto the fibers. Because the IR absorbing and scattering material can be less expensive than the fiber, the substitution of the IR absorbing and scattering material for some of the fiber can lead to a significant cost reduction in thermal insulation.
A suitable IR absorbing and scattering material absorbs and scatters infrared radiation with a wavelength in the 4 to 40 μm range. Preferably, the IR absorbing and scattering material absorbs 6-8 μm (1667-1250 cm−1) infrared radiation. The IR absorbing and scattering material can include borate compounds, carbonate compounds, alumina compounds, nitrate compounds and nitrite compounds. These compounds can be alkali metal salts or alkaline earth metal salts. Borate compounds, carbonate compounds and alumina compounds are preferred. Suitable borates include lithium borate, sodium borate, potassium borate, magnesium borate, calcium borate, strontium borate and barium borate. Preferably, the borate is sodium borate (i.e., borax, Na2B4O5(OH)4.8H2O or Na2B4O7.10H2O) or colemanite (Ca2B6O11.5H2O). Suitable carbonates include lithium carbonate, sodium carbonate, potassium carbonate, calcium carbonate (i.e., calcite, CaCO3), dolomite (CaMg(CO3)2), magnesium carbonate (i.e., magnesite, MgCO3), strontium carbonate and barium carbonate. Preferably, the carbonate is calcium carbonate, dolomite, or magnesite. Suitable alumina compounds include hydrated alumina (Al2O3.3H2O or Al(OH)3) and alumina (Al2O3). ALCOA produces HYDRAL and B-303 particles of hydrated alumina.
The infrared absorbing and scattering material is useful in improving the thermal resistance of a porous thermal insulation product containing fibers. In particular, carbonate compounds and alumina compounds are useful in improving the thermal resistance of porous thermal insulation containing fibers at temperatures of 300° C. or more or even 400° C. or more.
The amount of IR absorbing and scattering material in the thermal insulation product can range from 1 to 40 wt %, preferably from 2 to 30 wt %, more preferably from 4 to 20 wt %. If the amount of IR absorbing and scattering material is less than 1 wt %, then the reduction in radiative heat loss is negligible. If the amount of IR absorbing material is in excess of 40 wt %, then the IR absorbing and scattering material forms an undesirable amount of dust in the thermal insulation product.
The fibers in the thermal insulation product can be organic or inorganic. Organic fibers include cellulose fibers; cellulosic polymer fibers, such as rayon; thermoplastic polymer fibers, such as polyester; animal fibers, such as wool; and vegetable fibers, such as cotton. Preferably, the fibers are inorganic. Inorganic fibers include rock wool and glass wool. Preferably, the inorganic fibers comprise a glass.
The fibers form a porous structure. The porous structure can be woven or nonwoven. Preferably, the porous structure is nonwoven. The nonwoven fibers can be in the form of a batt, mat or blanket. A preferred porous structure is that found in FIBERGLASS.
Along with the fibers and IR absorbing and scattering material, the thermal insulation product can include a binder to capture and hold the fibers and IR absorbing material together. The binder can be a thermosetting polymer, a thermoplastic polymer, or an inorganic bonding agent. Preferably, the thermosetting polymer is a phenolic resin, such as a phenol-formaldehyde resin. The thermoplastic polymer will soften or flow upon heating to capture the fibers and IR absorbing and scattering material, and upon cooling and hardening will hold the fibers and IR absorbing and scattering material together. In embodiments of the present invention, the IR absorbing and scattering material can itself bond fibers together and thus render the addition of a binder unnecessary. When binder is used in the thermal insulation product, the amount of binder can be from 1 to 35 wt %, preferably from 3 to 30 wt %, more preferably from 4 to 25 wt %.
The thermal insulation product of the present invention can be formed by dispersing the IR absorbing and scattering material on to the surface of fibers, and by forming the fibers into a porous structure. The dispersed IR absorbing and scattering material can be in the form of particles. The optimum particle size is around 4 μm. Preferably 99% of the particles are less than 10 μm in size. The infrared absorbing and scattering material can be dispersed on the fibers before or at the same time or after the fibers are formed into the porous structure. Methods of forming fibers into porous structures are well known to the skilled artisan and will not be repeated here in detail.
The following non-limiting examples will further illustrate the invention.
FIBERGLASS samples are prepared in a laboratory with either borax {Na2B4O7.10H2O} or calcium carbonate dispersed throughout as IR absorbing and scattering materials. The samples are 30.5 cm wide×30.5 cm long×2.5 cm thick. The IR absorbing materials are weighed and mixed in a solution of 30% isopropanol and 70% water. The borax is dissolved in the water using a mixer and a hot plate to form a borax solution. The calcium carbonate is mixed in the alcohol/water by hand to form a calcium carbonate suspension. The liquid mixtures containing the IR absorbing and scattering material are loaded onto the samples either by soaking or by spraying. The soaking is performed by pouring 240 ml of one of the liquid mixtures onto each sample and soaking the sample. The spraying is performed by using a spray bottle to spray 120 ml of one of the liquid mixtures onto each sample. The apparent thermal conductivity of each of the samples is measured before and after the IR absorbing material is added. The apparent thermal conductivities are shown in Table 1.
48%
24%
Table 1 shows that the addition of borax or calcium carbonate to FIBERGLASS results in a reduction in the apparent thermal conductivity of the insulation. For the samples with calcium carbonate, the percentage reduction in thermal conductivity is roughly proportional to the percentage of calcium carbonate applied to the FIBERGLASS.
Comparative samples showing the reduction in apparent thermal conductivity produced by adding glass fiber to insulation are provided by standard R11, R13 and R15 FIBERGLASS insulation, as shown in Table 2.
Two sets of FIBERGLASS samples of varying compositions in a fiberglass insulation manufacturing process are prepared. The first set of samples is maintained as a reference. To the second set of samples is added 12 wt % calcium carbonate. The apparent thermal conductivity at 24° C. mean temperature of each sample as a function of density is determined by ASTM test procedure C518 and shown in Table 4.
Using the data in Table 3, the reduction in apparent thermal conductivity resulting from the addition of calcium carbonate is compared with the reduction in apparent thermal conductivity resulting from an increase in glass density in the FIBERGLASS insulation. The results are shown in Table 4.
Table 4 shows that the addition of 12 wt % calcium carbonate to FIBERGLASS is approximately 73% as effective as a 12% increase in FIBERGLASS density in reducing the apparent thermal conductivity of FIBERGLASS thermal insulation. Thus, about 1.37 (=1/0.73) times as much calcium carbonate as glass fiber must be added to achieve the same reduction in apparent thermal conductivity.
However, the cost of calcium carbonate can be less than 50% of the cost of glass fiber. Thus, the cost for reducing the thermal conductivity of FIBERGLASS insulation with calcium carbonate can be 68% (=(100)(1.37)(0.50)) or less than that of the cost of the same thermal conductivity reduction with glass fiber. Thus, calcium carbonate is a more cost-effective additive to FIBERGLASS than glass fiber for reducing the apparent thermal conductivity of the thermal insulation.
A fiberglass insulation sample with 12 wt % calcium carbonate is prepared in a fiberglass manufacturing process. Table 5 shows the reduction in apparent thermal conductivity at various temperatures compared to a fiberglass insulation sample with no calcium carbonate.
FIBERGLASS samples are prepared in a laboratory using hydrated alumina dispersed throughout as an IR absorbing and scattering material. The hydrated alumina is dispersed throughout the samples by spraying. The hydrated alumina is produced by ALCOA in the form of 1 μm particles (HYDRAL H710), 2 μm particles (HYDRAL H716), and 3.8 μm particles (B-303). The samples are 61 cm wide×61 cm long×2.5 cm thick. The apparent thermal conductivity at room temperature of each of the samples is measured before and after the hydrated alumina is added. The results are shown in Table 6.
The results in Table 5 show that the addition of hydrated alumina particles to FIBERGLASS can reduce the room temperature thermal conductivity of the FIBERGLASS and thus improve the insulation properties of FIBERGLASS.
The thermal conductivity of FIBERGLASS samples with and without dispersed hydrated alumina in the form of 1 μm particles (HYDRAL H710) is measured at 300° C. The results are shown in Table 7. The data represents averaged values from eight samples having identical dimensions. One set of averaged values is from four of the samples containing dispersed hydrated alumina. The other set of averaged values is from four reference samples that do not include hydrated alumina particles.
Table 7 shows that show that the addition of hydrated alumina particles to FIBERGLASS can reduce the 300° C. thermal conductivity of the FIBERGLASS by about 2.1% and thus improve the high temperature insulation properties of the FIBERGLASS.
The disclosure of the priority document, U.S. application Ser. No. 09/858,471, filed May 17, 2001, is incorporated by reference herein in its entirety.
While the present invention has been described with respect to specific embodiments, it is not confined to the specific details set forth, but includes various changes and modifications that may suggest themselves to those skilled in the art, all falling within the scope of the invention as defined by the following claims.
Number | Date | Country | |
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Parent | 10477996 | Aug 2004 | US |
Child | 13079631 | US |
Number | Date | Country | |
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Parent | 09858471 | May 2001 | US |
Child | 10477996 | US |