Fire resistant materials based on endothermic alumina-silica hydrate fibers

Abstract

An alumina-silica hydrate fiber for thermal protection in the most hazardous environments experienced by firefighters. The fiber has a combination of heat resistance at temperatures above 1,000° C. and an endothermic behavior when heated. Its endothermic condensation reaction releases up to 12% water upon heating, thereby contributing to the thermal insulating properties. The fibers are extremely soft to the touch and do not irritate the skin. They are also non-respirable in the deep lung, so they can be used safely without risk of inhalation. The fabric is sufficiently lightweight and flexible as well, allowing firefighters to move easily. These properties of alumina-silica hydrate fibers enable their use for apparel.

Claims

1. A material resistant to burning, comprising a plurality of textile fibers, in which the fibers themselves are formed of an inorganic endothermic compound, wherein the fibers consist of the inorganic endothermic compound consisting of alumina (Al.sub.2O.sub.3)-silicate (SiO.sub.2) hydrate (H.sub.2O) and a metal oxide, such that Al.sub.2O.sub.3, SiO.sub.2, and H.sub.2O combine to form Si—(OSi)3OH, wherein Si—(OSi)3OH is a condensation product resulting in endothermic behavior in response to elevating temperatures.

2. The material of claim 1, further comprising aramid polymer fibers.

3. The material of claim 1, wherein the plurality of textile fibers is woven together by a needle felt process.

4. The material of claim 1, wherein the plurality of textile fibers has a basis weight of 90-200 g/m.sup.2.

5. The material of claim 1, wherein the alumina-silicate hydrate and the metal oxide of the fibers contain: 70-95% SiO.sub.2 by weight; 1-20% Al.sub.2O.sub.3 by weight; and 5-15% H.sub.2O by weight.

6. The material of claim 1, wherein the alumina-silicate hydrate and the metal oxide of the fibers contain: 70-95% SiO.sub.2 by weight; 1-20% Al.sub.2O.sub.3 by weight; 1-10% TiO.sub.2 by weight; and 5-15% H.sub.2O by weight.

7. The material of claim 1, wherein the alumina-silicate hydrate and the metal oxide of the fibers contain: 70-95% SiO.sub.2 by weight; 1-20% Al.sub.2O.sub.3 by weight; 1-10% MgO by weight; and 5-15% H.sub.2O by weight.

8. The material of claim 5, wherein OH is chemically bound within the fibers as Si—(OSi)3OH.

9. The material of claim 1, wherein the condensation product results from release of hydroxide and water.

10. The material of claim 7, wherein OH is chemically bound within the fibers.

11. The material of claim 1, alumina-silicate hydrate and the metal oxide of the fibers contain: 70-95% SiO.sub.2 by weight; 1-20% Al.sub.2O.sub.3 by weight; and 5-15% H.sub.2O by weight; and wherein the fibers are formed with chemically bound hydroxyl groups.

12. The material of claim 1, wherein the alumina-silicate hydrate form a structural basis from which a chemically bound OH group is derived.

13. The material of claim 1, wherein the endothermic profile comprises a first endotherm and a second endotherm, wherein the first endotherm is irreversible and occurs between 100° C. and 150° C.

14. A material resistant to burning, comprising a plurality of textile fibers, whereby an inorganic endothermic compound consisting of alumina (Al.sub.2O.sub.3)-silicate (SiO.sub.2) hydrate (H.sub.2O) is integrated into the plurality of textile fibers and a condensation product chemically attached to the inorganic endothermic compound.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention, in accordance with one or more various embodiments, is described in detail with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict typical or example embodiments of the invention. These drawings are provided to facilitate the reader's understanding of the invention and shall not be considered limiting of the breadth, scope, or applicability of the invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

(2) FIG. 1 is a graph of a thermogravimetric analysis (TGA) of alumina-silica hydrate and aramid fibers.

(3) FIG. 2 is a graph of a differential scanning calorimetry (DSC) of alumina-silica hydrate fibers showing endothermic condensation reaction.

(4) FIG. 3 is a comparison between nonwoven 60%/40% aramid and alumina-silica hydrate after vertical flame test with loading.

(5) FIG. 4 is a graph of the relative change in single filament tensile strength versus temperature.

(6) The figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration, and that the invention be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

(7) Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All patents, applications, published applications and other publications referred to herein are incorporated by reference in their entirety. If a definition set forth in this section is contrary to or otherwise inconsistent with a definition set forth in applications, published applications and other publications that are herein incorporated by reference, the definition set forth in this document prevails over the definition that is incorporated herein by reference.

(8) The present invention provides a unique fibrous construction that absorbs thermal energy upon heating. Inorganic oxide fibers are produced in a hydrated form so that hydroxyl groups are released upon heating. A range of oxide chemistries can be used to achieve this endothermic behavior upon heating.

(9) Unlike dispersed endothermic particulates as described in U.S. Pat. No. 6,341,384 the present invention claims fibers which are themselves endothermic, providing a much more robust system. The present invention is also significant improvement upon U.S. Pat. No. 7,259,117 in which the fibers are organic and subject to oxidation at temperatures above 350 C. The present invention is also distinguished from CN201997011U, which describes a layered structure using removable, yet conventional flame retardant fabrics.

(10) An advantage of the present invention is that the endothermic material itself is produced into a fibrous form, rather than sheets or powders. High temperature resistant fibers with endothermic behavior have never before been achieved.

(11) The present invention covers a wide variety of inorganic endothermic fibers including the following examples (percentages are by weight):
SiO2(70-95%).Al2O3(1-20%).H2O(5-15%)
SiO2(70-95%).Al2O3(1-20%).TiO2(1-10%).H2O(5-15%)
SiO2(70-95%).Al2O3(1-20%).MgO(1-10%).H2O(5-15%)

(12) In order to achieve these unique chemistries, oxide fibers are first produced by continuous drawing at high temperatures in excess of 1350 C. Fibers are drawn using compositions containing alkali oxides with alkali oxide content up to 35%. Following drawing, an ion exchange reaction is performed in an acidic solution thereby replacing the alkali oxides with hydroxyl groups. This ion exchange reaction results in a very unusual fiber with unique characteristics.

(13) Due to its unique chemistry in its as-produced state, the hydroxyl containing oxide fiber feels more like a polymeric fiber than a ceramic fiber, thereby meeting the requirement of comfort when made into a wearable garment. Similar to current aramid fibers, the alumina-silica hydrate fiber is both flexible and lightweight.

(14) Extensive analysis has been performed on fibers produced in the silica-alumina hydrate composition.

(15) Thermogravimetric analysis (TGA) was used to examine how the sample fibers changed when exposed to high temperatures. The comparison between the aramid fiber and the alumina-silica hydrate is shown in FIG. 1, demonstrating a marked difference in the high temperature behavior of the industry standard aramid fiber versus the alumina-silica hydrate fiber. The nonwoven aramid fiber shown is the most commonly used material for the inner thermal lining of firefighter turnout gear. It is a blend of 60% meta-aramid and 40% para-aramid. In the figure, the aramid material exhibits rapid oxidation above 300° C., with the sample appearing completely charred by 400° C.

(16) During isothermal heating, the meta-aramid fiber charred completely in 30 seconds at 350° C. When heated at 600° C., 80% of its mass was lost in 20 minutes. This mass loss reflects the ability of the meta-aramid fiber to char rather than melt or completely oxidize. This behavior is useful for the less extreme fire conditions outlined in FIG. 1. However, upon exposure to extreme or critical conditions, the aramid fibers rapidly oxidize. This excessive loss of mass at temperatures above 400° C. would therefore be catastrophic under flashover conditions.

(17) In contrast, the alumina-silica hydrate fiber exhibited a gradual mass loss which began at 60-100° C. (FIG. 1) and continued through 700° C. During this temperature increase, the alumina-silica hydrate fiber lost approximately 12% mass. This mass loss corresponds to a condensation reaction in which hydroxyl ions are evolved, combining to form H2O. Minimal mass loss was observed from 800-1,200° C., upon completion of the condensation reaction and the resulting stabilization of the alumina-silica matrix. The resultant fiber sample was much stiffer than the initial as-produced fiber, but it could still be flexed without fracture and therefore remained intact even up to 1,200° C.

(18) One characteristic that sets apart the alumina-silica hydrate fiber is its soft feel. Other inorganic fibers that can endure temperatures of 1,000° C. are itchy, respirable into the deep lung, or both. Such ceramic fibers are unsuitable candidates for firefighter apparel. Therefore, the combination of durability and softness (even softer than meta-aramid or PBI) suggest that the alumina-silica hydrate fiber has great potential for use in fire-resistant protective apparel.

(19) Differential Scanning Calorimetry (DSC)

(20) Both alumina-silica hydrate and aramid fibers were tested using Differential Scanning calorimetry (DSC). FIG. 2 is a DSC graph of alumina-silica hydrate fibers showing endothermic condensation reaction. The aramid fibers exhibited exothermic behavior, as expected from any polymeric fiber that is oxidizing. In contrast, the alumina-silica hydrated fiber had a strong endotherm at 100-150° C. (FIG. 2). A smaller endotherm also was present between 400 to 1,000° C. A strong endotherm at 100-150° C. corresponds to the condensation reaction in which water is released. It is this unique ability to absorb heat when exposed to elevated temperature that provides extra benefit when applied as a heat resistant material. It is important to note, however, that this condensation reaction only occurs during the first heating of the material. It is an irreversible reaction, so subsequent heat exposure does not repeat this endotherm.

(21) Vertical Flame Test

(22) The vertical flame test of ASTM D6412 was used to compare alumina-silica hydrate fibers (on the right side of FIG. 3) to several different aramid woven and nonwoven materials which are currently used in turnout gear. After a 12 second exposure to a vertical methane flame at approximately 700° C., the aramid fabrics and nonwovens had a char length which ranged from 50-80 mm in length (on the left side of FIG. 3). At the most severe portion of the char, the fibers were so brittle that they tore under almost zero load. For the most common 60/40 meta-aramid/para-aramid quilted nonwoven, referring to FIG. 3, the char length was 21 mm with a 0.23 kg load. This charring behavior does prevent the aramid fiber from completely melting at low temperatures like other common polymeric materials. However, the fiber quickly becomes extremely weak, even losing its ability to maintain its distinct identity of individual filaments. This low tensile strength after heat exposure reveals its limitations in providing protection for more than 12 seconds.

(23) In contrast, referring to the right side of FIG. 3, both the woven and nonwoven alumina-silica hydrate materials showed no visible charring after enduring the same vertical flame test and remained completely white in color. Under the same loading of 0.23 kg, zero tearing was observed. Even under close inspection, no visible changes were observed in these materials after being exposed by the 700° C. vertical flame.

(24) Single Filament Tensile Test

(25) Tensile strengths were evaluated on meta-aramid and alumina-silica hydrate fibers as shown in FIG. 4, highlighting the drastic chemical and physical differences between the materials. FIG. 4 is a graph of the relative change in single filament tensile strength of each fiber upon heating. The meta-aramid fibers exhibited loss of strength with increasing temperature, mainly due to its rapid char formation and oxidation. Strength loss accelerated dramatically above 200° C., with fiber strength becoming negligible by 300° C.

(26) In contrast, the alumina-silica hydrate fiber strength actually increased with temperature. In fact, the fiber strength increased by 40% of its original strength by 500° C., and by 50% after reaching 600° C. This remarkable behavior occurs because of the condensation reaction and stabilization of the alumina-silica matrix. The fiber transforms to more of a ceramic fiber which is both stiffer and stronger.