Abstract:
A fabric suitable for use as a firebreak is knitted from a yarn formed from a first synthetic material having high mechanical strength and high reflectivity and a second non-flammable, thermally stabilized synthetic material having a high reflectivity. Preferably the first synthetic material is an aramide filament, and preferably the second synthetic material is a pre-oxidized filament. Preferably the fibers are cracked whereby they exhibit highly reflective surfaces especially useful in the reflection of heat produced by a fire.

Description:
This application is a continuation-in-part of Ser. No. 07/616,386, filed Nov. 21, 1990, now abandoned, the disclosure of which is hereby incorporated in its entirety by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a new fabric for use as a firebreak. The invention is more particularly, but not exclusively, concerned with a fabric for use as a firebreak for forest fires. 
     2. Discussion of Background Material 
     It is known that forest fires are propagated mainly by radiation, the flame front temperature favoring the release of volatile substances contained in various tree species which are particularly flammable. The use of ceramic fiber-based fireproof fabrics as barriers to fight such fires has been considered. Devices of this kind have not yielded the desired results, and are additionally very costly, heavy and difficult to use. Finally, they merely constitute a simple mechanical screen. One object of the present invention is to provide a new lightweight fabric of reasonable cost which remedies the disadvantages of the known prior art technique. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a fabric suitable for use as a firebreak. The fabric is knitted from a yarn formed from a first synthetic material having high mechanical strength and high reflectivity, together with a second non-flammable, thermally stabilized synthetic material having a high reflectivity. 
     The first synthetic material is preferably a &#34;KEVLAR&#34; synthetic aramide filament. The second synthetic material is preferably a &#34;SIGRAFIL 0&#34; synthetic pre-oxidized filament. According to one feature of the invention, the first and second materials are cracked to provide fibers approximately 60 to 200 millimeters long, the resulting fibers being spun to produce a yarn which is then knitted to obtain the fabric. 
     The cracked fibers of the first and second synthetic materials, after cracking, have barbs or hairs forming a down on the surface of the yarn obtained after spinning. The distance between the barbs or hairs on the surface of a single filament is between one and a few tens of microns (e.g. 50  microns). The resulting structure forms an optical filter with cut-off wavelengths matching those of the emission spectrum of thermal radiation from a flame. 
     The yarn fibers of the first material, after cracking, spinning, and knitting, preferably form about 30% of the resulting fabric of the present invention. The yarn fibers of the second material, after cracking, spinning, and knitting, preferably form about 70% of the resulting fabric of the present invention. The result is a lightweight fabric that is easy to use and which once stretched and installed constitutes a firebreak barrier minimizing the radiation from a flame front. 
     The present invention also pertains to a method for protecting an object which can be damaged by exposure to a fire. The method comprises shielding the object with a firebreak, wherein the firebreak is a knitted fabric which comprises a yarn formed from a first synthetic material having high mechanical strength and high reflectivity and a second, non-flammable, thermally stabilized synthetic material having high reflectivity. 
     The present invention also pertains to a method for controlling the spreading of a fire. The method comprises placing a firebreak around at least a portion of the fire. The firebreak is a knitted fabric while comprises a yarn formed from a first synthetic material having high mechanical strength and high reflectivity and a second, non-flammable, thermally stabilized synthetic material having high reflectivity. 
     The present invention also pertains to a combination of an object to be preserved from damage from a fire in the proximity thereof, together with a firebreak, wherein the firebreak is a knitted fabric which comprises a yarn formed from a first .synthetic material having high mechanical strength and high reflectivity and a second, non-flammable, thermally stabilized synthetic material having high reflectivity. 
     The present invention also relates to a firebreak comprising a yarn formed from a first synthetic material having high mechanical strength and high reflectivity and a second, non-flammable, thermally stabilized synthetic material having high reflectivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exploded, enlarged view of the fabric of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The fabric of the present invention is formed from fibers made from an aramide synthetic filament commercially available under the trade name &#34;KEVLAR&#34; and a pre-oxidized synthetic filament commercially available under the trade name &#34;SIGRAFIL 0&#34;. Kevlar synthetic aramide filament is known for its mechanical strength and light weight. The pre-oxidized synthetic filament has the advantage of being thermally stabilized and non-flammable and has a high melting point. 
     Each of these filaments is produced in the form of a continuous filament, via an extrusion process in which the fiber is formed by extruding the appropriate polymer through a die in order to form a continuous filament. The resulting continuous filaments are cracked by stretching them until they break. In general, the length of the fibers obtained after cracking may be from about 60 mm to about 200 mm. The fibers obtained after cracking are most preferably mixed in the proportion of about 70% pre-oxidized synthetic filament fibers together with about 30% aramide synthetic filament fibers. 
     Preferably the Kevlar synthetic aramide fiber is of approximately 1.7 dtex. The extruded Kevlar strand is subjected to cracking such that the continuous filament becomes discontinuous (i.e. is broken into relatively short lengths) upon stretching. Preferably the resulting cracked Kevlar filament exhibits a maximum length of about 140 mm and a minimum length of about 100 mm, with the cracked Kevlar filaments exhibiting an average length of about 120 mm. 
     Preferably the continuous preoxidized synthetic filament is of approximately 1.5 dtex. Upon cracking the continuous preoxidized filament, preferably the cracked preoxidized filament possesses an average length of about 150 mm, with a maximum length of about 180 mm, and a minimum length of about 120 mm. 
     Cracking is a very important step in the process of making the fabric of the present invention. Cracking is important because it enables the production of a &#34;fluff&#34; (i.e. &#34;down&#34;) on the surface of the thread, thereby forming a network of reflector elements. 
     The cracked aramide filaments and cracked pre-oxidized filaments are mixed (preferably to a degree of substantial uniformity, and preferably in the proportions of 70 percent by weight preoxidized fiber, and 30 percent by weight aramide fiber) and thereafter spun to produce a yarn which is knitted to obtain the firebreak fabric of the present invention. Preferably the yarn which is spun from the mixture of filaments has a torque of 600 revolutions Z, and most preferably the yarn has a thickness of 20 tex (tex being equal to the weight in grams for a length of 10 meters). However, more generally the yarn thickness may vary from about 19.2 tex to about 20.8 tex. 
     In order to ensure that the knitted material provides great safety, each stitch of the fabric is formed of two threads. The knitted material, i.e. a jersey, is preferably manufactured on a circular knitting machine, and preferably the machine has thirty 18-gauge frame handles (i.e. 18 needles per frame). The circular knitting machine is preferably of the double bed type, i.e. the machine has needles on the cylinder as well as on the plate, so that the knitted material has rows of stitches on the front side thereof (i.e. by the cylinder needles) as well as on the reverse side (i.e. by the plate needles). Preferably the knitting machine has 24 drops thereon (i.e. 24 raised fiber suppliers), so that the machine forms 24 raised stitches per revolution. In each drop, each cylinder needle forms a stitch, whereas one of two plate needles forms a stitch (see FIG. 1). 
     FIG. 1 illustrates long cylindrical knitting needles (1) and short cylindrical knitting needles (2) which pass through loops of paired yarns (paired yarn 3 and paired yarn 4) undergoing knitting at respective positions drop no. 1 and drop no. 2. FIG. 1 illustrates the very substantial plate height preferred in the process of making the yarn utilized in the fabric of the present invention. A gap (5) between needles determines the distance between the stitches in the resulting fabric. The two threads shown at each needle drop position preferably have 50 filaments making up each thread. 
     It is important that the plate be at a very substantial height so that the stitch in-between is substantial. This enables a knitted material to be obtained with substantial breadthwise extension. Preferably the maximum height of the plate with respect to the cylinder is from about 2 millimeters to about 2.5 millimeters. 
     The tubular knitted material obtained on the circular frame preferably has a weight of about 200 grams per square meter. Furthermore, the knitted material tends to enlarge itself, given the stitch, and this tendency automatically brings about an opening of the stitch to create holes that imprison air. It is very important that each loop forming the stitch have a substantial dimension such that the knitted material obtained has a multiplicity of small holes therein. Thus the fabric of the invention not only has substantial breadthwise extension, furthermore the holes imprison some air that constitutes a natural thermal insulation. The holes are also necessary to slow the conduction of heat through the fabric. 
     The resulting fabric has some elasticity and the yarn obtained after cracking and spinning has the advantage of being both mechanically strong and heat resistant. The yarn has a downy surface. 
     The cracked synthetic fibers have an extremely shiny surface with a polished surface appearance such that the hairs and barbs which constitute the down on the surface of the yarn form an array of reflective elements spaced from each other by from about 0.1 micron to about 1 micron. The resulting structure forms an optical filter whose cut-off wavelengths match those of the emission spectrum of thermal radiation from a flame. 
     The fabric is stretched in front of a surface to be protected and thereby constitutes a dispersion, diffusion or diffraction filter whose reflection factor is very high for the wavelength concerned. Experiments have shown that for radiation temperatures between 30° and 950° C. it is possible to arrest approximately 90% of the heat flow. 
     The fabric may be used in the form of strips sewn together to constitute long lengths, the width of the strips being as much as six meters. The weight of a strip one hundred meters long by six meters high is approximately 60 kg. If the fabric is manufactured on a circular frame, it may be used in sock form with internally housed tensioning members. 
     The fabric in accordance with the invention has the advantage that it is not totally opaque, with the result that it enables visual verification of the action of firefighting equipment and allows hazards such as falling trees to be seen therethrough. The fabric can also be used to protect buildings. 
     In the very large number of trials that have been carried out, the best results were obtained with the new fabric as described and defined above. If the fabric of the present invention is used for applications in which the radiation temperatures are lower, the pre-oxidized synthetic filament might be replaced with polyamide fibers, polyester fibers, and/or acrylic fibers, each of which would be treated in the same way as the pre-oxidized synthetic filament. 
     Finally, although the invention has been described with reference of particular means, materials and embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims.