Abstract:
An improved knit fabric system for protecting against electrical arch hazards and promoting garment visibility is provided. The inventive knit fabric is either a solid fabric substrate or of a mesh construction that absorbs the thermal energy of an electric arc and, especially for the mesh construction, absorbs thermal energy or heat flux to prevent garment combustion. The solid or mesh version of the inventive knit fabric incorporates conductive fibers for draining away and thereby dissipating accumulated electrical charges. Such conductive fibers are incorporated into the fabric of the invention, by either being knitted into the fabric as it is formed or by the process of yarn spinning.

Description:
[0001]     This application claims priority of Provisional Application No. 60/847,305 filed Sep. 26, 2006. The subject application is also related to the following applications:  
         [0000]     Knit Elastic Mesh Loop Pile Fabric for Orthopedic and other Devices  
         [0000]     60/847,186 filed Sep. 26, 2006; U.S. patent application Ser. No. ______ filed ______,  
         [0000]     Improved High Performance Fire Resistant Fabrics and the Garments Made Therewith  
         [0000]     60/847,002 filed Sep. 25, 2006; U.S. patent application Ser. No. ______ filed ______, and  
         [0000]     Under Body Armor Cooling Vest and Fabric Thereof  
         [0000]     60/847,307 filed Sep. 26, 2006; U.S. patent application Ser. No. ______ filed ______. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a fabric for protection against electrical arc hazards, and, more particularly, to a fabric that is both highly visible and which reduces the hazards of electrical arcs.  
         [0003]     A. Garment Visibility  
         [0004]     Personnel employed in all modes of traffic control, utility and survey work, emergency response, construction, equipment operation and vehicle roadway traffic are exposed to accident hazards due to insufficient conspicuity of ordinary workwear worn by them. These hazards are due to the workers&#39; low visibility, which are intensified by the often complex and varying backgrounds of the above mentioned occupations and job assignments.  
         [0005]     A major hazard issue involves situations in which objects can be visible, but are not consciously recognized by the vehicle driver within sufficient time to take corrective action in order to avoid an accident. This conscience recognition is often influenced by the level of task activities, varying daytime or nighttime lighting conditions, the complexity of backgrounds, vehicle speed and the visual performance of the operator. Thus, worker safety is compromised by insufficient decision/reaction time resulting from the use of workwear not designed to provide sufficient visibility. It is thus important that workers are readily perceived by drivers when, for example, directing traffic, operating equipment, digging roadside trenches and doing maintenance work.  
         [0006]     In order to reduce hazards to which the workers are exposed in performance of their tasks, special high visibility garments are available for their protection. These are covered by the requirements of both the American National Standard Institute (ANSI) and ISEA—The Safety Equipment Association. The garments may take the form of a coverall, jacket, vest, trousers, harness/sash belt and others, depending on the work performed by the wearer.  
         [0007]     Fluorescent dyed materials emit optical radiation at wavelengths longer than absorbed. They enhance daytime visibility, especially during dawn and dusk. Accordingly, garments may be provided with strips of retroreflective material placed in appropriate locations in order to enhance their conspicuity. Such retroreflective materials have the property of returning light to its source.  
         [0008]     Garments instead may be made with a fabric dyed with one of three approved fluorescent colors; such colors are intended to be highly conspicuous to ensure visibility against most backgrounds found in urban and rural situations. The three colors are: yellow-green, orange-red and red. The chromaticity (the x and y coordinates) and the minimum luminance factor are stipulated by ISEA standards (see TABLE 1 and 2), as are all the fabric parameters applicable to the fabric. Performance requirements of garments must be tested and verified for conformance with these standards by an accredited testing lab.  
         [0009]     Fabrics currently in use in high visibility garments are of the woven type. While such fabrics are adequate in performance, they leave much to be desired in wearing comfort, durability and economics. Particularly, woven fabrics are, by their nature, tightly configurated in their system of warp and filling threads. This limits the air permeability and hence the comfort factor, which is of a particular importance for workers exposed to the sun for prolonged periods of time.  
         [0010]     Woven fabrics that meet the ISEA performance requirements are relatively stiff and, therefore, to some extent, inhibit the garment wearer&#39;s freedom of movement. Also, woven fabrics are prone to ripping, tearing and fraying. This limits the useful life of the garment, which suffers much physical stress when worn at high rough work sites. Finally, there is the question of economics. Woven fabrics that meet the ISEA standards are relatively expensive due to the cost of suitable yarns and the involved processing cycles.  
         [0011]     B. Electrical and Thermal Discharge  
         [0012]     In addition to the problem of personnel workwear and insufficient conspicuity, workers attending to electrical utility lines and related equipment are also exposed to the risk of electrical arc flash hazards, against which such workwear must provide adequate protection.  
         [0013]     In particular, electrical utility linemen, industrial electricians, electrical contractors and electrical service personnel are routinely exposed to the momentary electric arc flash and its related thermal hazards. As a consequence, many workers have been electrocuted, burned or severely injured. In fact, recent U.S. Department of Labor statistics identified electrical workers as being the 3 rd  most dangerous profession.  
         [0014]     An arc flash is the explosive release of energy caused by the passage of electrical current between two electrodes through ionized gases or plasma, characterized by a temperature reaching several thousands degrees centigrade. As workers perform their tasks on or near energized wire systems or circuitry, an arc flash may occur as a result of their inadvertent movement, accidental contact or some equipment failure. The electrical energy supplied in the forming arc is converted into an explosive fireball-like phenomenon that is likely to impact or even envelop the worker. The resultant explosive effect of the arc produces intense thermal radiation, noise, melting and even vaporization of metal components of the equipment around the arc. Depending on the severity of the arc flash, burns will occur on bare or unprotected skin. Also, if the worker is wearing non-flame retardant clothing, the arc is likely to ignite it.  
         [0015]     Thus, to provide a safer workplace for the utility workers, the NFPA (National Fire Prevention Assn.) has issued a standard NFPA 70 E-2000 for electrical safety requirements. The standard calls for protective clothing to be tested and rated to the level of the arc flash energy hazards to which the electricity workers could be exposed.  
         [0016]     In addition to the dangers posed by electrical arc discharge, utility workers are also exposed to thermal hazards from the heat of the flash fires caused by ignited gas, combustible vapors, volatile solvents or chemical dust. Flash fires are defined as those lasting no more than three seconds.  
         [0017]     Thus, thermal performance of garments is covered by the AMTM (American Society for Testing Materials) test F 1950, which uses the electric arc to determine the number of calories required to create second degree burns in terms of calories per square cm. There is also an ASTM F 1506 standard for clothing worn around electric arc hazards.  
         [0018]     Yet another consideration in the production of utility workers&#39; protective garments is the hazard of static electricity spark discharge. Such discharge is likely to ignite a flash fire of the kind mentioned above. Static electricity charges of several thousands volts may be generated simply by rubbing one part of the garment against another or against a car seat or a plastic object. These charges can create a spark of sufficient length to ignite gas, fuel vapors, solvents, etc., thus causing a flash fire.  
         [0019]     Yet a further hazard to utility workers is when they come close to high tension equipment such as transformers, switchgear, overhead wires, etc.—the corona discharge. Such discharge occurs from electrodes with sharp points or angles. It is due to the ionization of the air surrounding these points, which makes possible the escape of electrical energy through the air. Corona discharge takes the form of luminous glow; the higher the voltage the more intense the corona discharge. Corona discharge can be hazardous to utility workers servicing high tension installations where the coronal discharge may induce dangerous levels of electrical energy flux in the workers apparel.  
         [0020]     At the present time, protective garments are made with densely woven, heavy fabrics using flame resistant fibers such as modacrylic, Kevlar, Nomex, PBI, FR rayon and others. These fibers not only must withstand the very high arc temperature for a brief span of time, but must also be resistant to melting and dripping, which can cause severe burns. A frequently used fiber in garments is modacrylic spun into medium count yarns. Modacrylics are the copolymers of acrylonitrile fibers, which are very difficult to ignite and have self extinguishing properties. These fibers also have good weathering properties, resistance to acids, alkalis and wide range of chemicals. Their dielectric strength exceeds 1500 volts per mil. of plastic film, which constitutes an important consideration in electrical applications.  
         [0021]     A distinct advantage of modacrylic yarns is their relatively moderate price in comparison with other types of yarns available on the market. Modacrylics feature also superior processability during manufacture.  
         [0022]     Nonetheless, woven fabrics incorporating flame resistant fibers and used in making electric arc protective garments are less than desirable. In the first place, since woven fabrics must be dense and tightly constructed in order to preserve their structural integrity. They have reduced porosity properties, resulting in reduced wearing comfort. In a warm environment, for example, garments made with such fabrics may feel excessively hot and clammy. The consequence being that some workers avoid wearing them altogether. Also, relative stiffness of woven fabrics and the lack of any “give” encumber the freedom of movement of the garment wearer.  
         [0023]     In addition, weaving is essentially a slow process and generally limited to narrow width fabrics. This causes wovens to be relatively expensive in comparison with other fabricating systems like warp and weft knitting.  
         [0024]     Furthermore, woven fabrics have a propensity to distort, rip and fray. Because the yarn components of warp and weft are held in the structure by frictional forces only, there is a tendency for them to slip on each other and distort the fabric in forming cracks and open areas on its face. In that regard, the peculiar geometry and interlacing of the yarn components of woven fabrics renders them susceptible to ripping. Thus, even a minor cut or puncture in the garment caused by a sharp part of the equipment can propagate itself into a long tear or rip, thereby destroying the garment.  
         [0025]     A related problem with woven garments is seam failure. This may be caused by the problem of fraying of the threads from a cut edge of the fabric. This may produce seam failure due to the individual fabric threads “combing out” from the seamed edge. Seam failure may have serious consequences in that it could allow the heat flux of the electric arc to penetrate inside a protective garment so as to cause burns to its wearer.  
         [0026]     Accordingly, it would be desirable to provide a fabric or garment which overcomes the above disadvantages.  
       SUMMARY OF THE INVENTION  
       [0027]     Generally speaking, in accordance with the invention, an improved knit fabric system for protecting against electrical arch hazards and promoting garment visibility is provided. The inventive knit fabric is either a solid fabric substrate or of a mesh construction and consists predominantly of high performance yarns that are resistant to melting, dripping and burning at high temperatures. The knit fabric absorbs the thermal energy of an electric arc and, especially for the mesh construction, absorbs thermal energy or heat flux to prevent garment combustion.  
         [0028]     In accordance with the invention, either the solid or mesh version of the inventive knit fabric incorporates conductive fibers for draining away and thereby dissipating accumulated electrical charges. Such conductive fibers are incorporated into the fabric of the invention by either being knitted into the fabric as it is formed or by the process of yarn spinning. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     For a fuller understanding of the invention, reference is now made to the following drawings in which:  
         [0030]      FIG. 1  depicts one form of the inventive fabric in which the conductive yarns are arrayed in a diamond-like configuration; and  
         [0031]      FIG. 2  depicts a second form of the inventive fabric comprising a mesh construction. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The fabric of the invention is a knitted fabric produced on either a warp or weft knit system. For warp knits, suitable equipment is either tricot or Raschel machine of a suitable gauge. For weft knits, suitable equipment is either a circular or flat knitting machine. The circular machines may be in 16-18 cut (needles per inch) with the interlock or double knit needle set out.  
         [0033]     In the embodiment with a mesh design, making mesh fabric utilizing circular weft knit machine involves pelerine transfer stitch technology, as is well known in the art; this creates eyelets through the transfer of sinker loops.  
         [0034]     The warp knit version of the inventive fabric is preferred due to its superior physical characteristics. This is because warp knit mesh fabrics are more stable and run-resistant that their weft knit counterparts. Warp knit fabrics can also be made in a much wider variety of mesh openings than is possible for weft knit fabrics. Further, warp knit mesh fabrics are, in general, easier and more economical to construct.  
         [0035]     The yarns used in the inventive fabric are high performance yarns such as those that are resistant to melting, dripping and burning at high temperature conditions (at least 700° F.). The high performance yarns are present in the inventive fabric in an amount between about 85% and 95% weight percent. The preferred high performance yarns are spun modacrylics. Modacrylics are polymers that have between 35 to 85% acrylonitrile units, and which may be modified by other chemicals such as vinyl chloride.  
         [0036]     The choice of modacrylic fibers or yarns for application in the fabric material of the invention is based on their excellent fire retardancy performance combined with their non-melt, non-drip and self-extinguishing properties. These are critically important attributes in many working environments. If sufficiently high temperatures are reached on exposure to fire or explosion, a garment made with the inventive fabric will just carbonize by forming a protective charred barrier. This prevents propagation of flames, thereby protecting the wearer from severe burn injuries.  
         [0037]     Modacrylics have a high so-called LOI value as compared with other fibers. The LOI represents the minimum oxygen concentration of an O 2 /N 2  mix required to sustain combustion of a material. The LOI is determined by the ASTM Test D 2862-77. Modacrylics have an LOI value preferably between about 28 and 33 while conventional polyesters have a much lower value of 20-22.  
         [0038]     Additionally, a very important aspect of wearing comfort is the so-called “moisture management” factor. This is often represented as the moisture vapor transport index of MVT, which reflects the efficiency in which a fabric moves perspiration away from the skin or underlying garment and causes it to evaporate into the ambient atmosphere. The MVT of the modacrylics used in the inventive fabric is approximately 2500 g/meter squared/24 hours ASTME96.  
         [0039]     Modacrylics are spun from an extensive range of copolymers of acrylonitrile. The types of modacrylic fibers that can be produced within this broad category are capable of wide variation in properties, depending on their composition. Some examples of commonly available modacrylics are: “Vere” by Eastman Corp., “Creslan” by Am Cyanamic Co., “Acrillan” by Mosanto Corp., “Kanecaron” by Kaneka Co. and “Orlon” by DuPont Co.  
         [0040]     Modacrylic fibers used in the inventive fabric preferably have a tenacity of up to 2.8 grams/denier, an elongation at break of between about 35 and 40%, and a fusing temperature of between about 371 and 410° F. The modacrylic fibers used in the inventive fabric also have a moisture regain (the amount of water by weight held by the fiber under controlled atmospheric conditions) of between about 0.4 and 4.0%.  
         [0041]     Modacrylic fibers and yarns are moderately priced as compared with other materials of good thermal performance. They are readily available in the industry; they have good knitting performance, ease of fabric processing and dyeing.  
         [0042]     A significant attribute of modacrylics is their charring on prolonged exposure to flames, rather than simply burning and dripping. The charred portions of the fabric protect the wearer from the effects of fire.  
         [0043]     Other high temperature resistant (high performance) fibers or yarns may also be used in the inventive fabric, either in combination with modacrylics or entirely on their own. One such fiber comprises aramid fibers, such as Kevlar and Nomex. Such fibers feature excellent thermal thermal stability and are virtually non-flammable. These fibers have a very high resistance to heat and are resistant to melting, dripping and burning at a temperature of at least 700° F. Moreover, their LOI value is preferably in the range of between about 28 and 30.  
         [0044]     Kevlar, made by Dupont Co., is a para-aramid fiber having a very high tenacity of between 28 and 32 grams/denier and outstanding heat resistance. Other para-aramid fibers suitable for the inventive fabric include Twaron by AKZO Co. and Technora by Teijin Co.  
         [0045]     Another type of aramid fiber suitable for the inventive fabric is “Nomex”, made by DuPont and “Conex” made by Teijin Co.  
         [0046]     Yet other types of flame resistant fibers are organic fibers composed of polybenzimidazole, such as PBI made by Celanese Corp. These fibers have an LOI of between about 35-40 and are resistant to melting, dripping and burning at a temperature of at least 750° F.  
         [0047]     Further high temperature resistant fibers or yarns may comprise certain polyester yarns that are resistant to melting, burning and dripping at a temperature of at least 700° F.  
         [0048]     In general, the high performance yarns used in the inventive fabric have a yarn count of between about 12/2 c.c. and 32/2 c.c. (two ply yarn).  
         [0049]     In accordance with the invention, the modacrylic or other high performance fibers are blended with from between about 3 and 5 weight percent of conductive fibers in order to impart anti-static properties to the fabric. Such fibers are available from several sources. The conductive yarn fibers are preferably intermixed with the high performance yarns; in other words, the conductive yarns are knitted together with the high performance yarns.  
         [0050]     One example of such conductive fiber is Negastat® produced by DuPont &amp; Co. This is a carbon fiber comprising a carbon core of conductive carbon surrounded by non-conductive polymer cover, either nylon or polyester. Another example is Resistat® made Shakespeare Conductive Fibers LLC. This is a fiber where the fine carbon particles are embossed on the surface of a nylon filament. The yarns of both such fibers are available in a denier of at least 40.  
         [0051]     Instead of conductive fabric fibers, one may use a very fine wire made of steel, copper or other metal. By way of example, a steel wire suitable for use in the inventive fabric is available under the names Bekinox and Bekitex from Bekaert S.A. in a diameter as small as 0.035 millimeter.  
         [0052]     A very effective conductive fiber that is suitable for the inventive fabric is the product X-static made by Noble Fiber Technologies. This is a nylon fiber coated with a metal layer, namely a silver layer; it provides excellent static draining performance as well as germicidical properties. The latter prevents development of objectionable odors. The X-static fibers are blended with modacrylics in the process of yarn spinning. A content of between about 3 and 5% of the X-static in the inventive fabric is sufficient to substantially control the static problem. The X-static fibers in the fabric must meet the standards of static control set forth by Noble Fiber Technologies, Inc.  
         [0053]     The conductive fibers may be introduced in the inventive fabric from warps or individual packages placed on a creel, the latter being the case with circular knitting system. For warp knits, one or two guide bars threaded with the conductive yarns may be employed. These bars could move in a zigzag or diamond configuration in order to provide optimum anti-static coverage.  
         [0054]     In the one preferred embodiment of the present invention, the conductive yarns are arrayed in a diamond-like configuration (See  FIG. 1 ). This provides optimum anti-static protection for the entire fabric surface.  
         [0055]     In a second preferred embodiment, the inventive knitted fabric is a knitted mesh. The advantages of such a mesh construction include increasing the permeability of air so as to enhance evaporation of perspiration. This significantly improves wearing comfort, especially in heat stressful applications. The presence of mesh openings in the inventive fabric also contributes to improved visibility by breaking up the fabric texture.  
         [0056]     The mesh openings should have a mesh count of between about 3 and 5 meshes/inch in width and between about 4 and 6 meshes/inch in length. Larger mesh openings will adversely affect the chromaticity and conscupicuity characteristics of the fabric. An embodiment of the inventive warp knit fabric having a mesh construction is shown in  FIG. 2 .  
       EXAMPLE 1  
       [0057]     One type of warp knit fabric according to the present invention contains the following yarns: 
        30/1&#39;s c. modracrylic (90%), polyester (10%) blend—to produce the ground fabric; and        
 
         [0059]     30/1&#39;s c. modacrylic (87%), polyester (8%), X-static (5%) blend—to produce the diamond overlay for anti-static protection.  
                                                                           KNITTING CONSTRUCTION DETAILS                Beam   Inches   Ends               Number   Rack   Total   Yarn                            1(front)   90″   234   30/1 Modacrylic (87%)           2   90″   234   30/1 Modacrylic (87%)           3   90″   2340   30/1 Modacrylic (90%)           4   156″    2340   30/1 Modacrylic (90%)           (back)                      
 
         [0060]     The threading chart for Example 1 is as follows:  
                                           Bar 1 (front)   ....................|   .........|   ....|.............   1                           .....       Bar 2   ............|.......|   .........|   ..................   0                       .....       Bar 3   ...........||||||||||   ||||||||||   ||||||||||..........   1                       .....               Bar 4   ...........||||||||||   ||||||||||   ||||||||||........   3                       .....          
 
         [0061]     Stitch construction for example, is as follows:  
                                                           BAR 1           BAR 4           (front)   BAR 2   BAR 3   (back)                           1-0   6-7   1-0   3-4           1-2   6-5   1-2   1-0           2-3   5-4           3-4   4-3           4-5   3-2           5-6   2-1           6-7   1-0           6-5   1-2           5-4   2-3           4-3   3-4           3-2   4-5           2-1   5-6                      
 
         [0062]     The finished fabric of Example 1 has a width of 2×60 inches, a fabric weight of 6.5 oz/square yard and a count of 27 courses/inch×18 wales/inch.  
         [0063]     In Example 1, the finished fabric is jet dyed to the desired color and then stabilized and set by tenter framing at a temperature of 250° F. at the speed of 15 yds/min.  
       EXAMPLE 2  
       [0064]     Another type of warp knit fabric according to the present invention contains the following yarns: 
        30/1&#39;s c. modacrylic (90%), polyester (10%); and     30/1&#39;s c. modacrylic (87%), polyester (8%), X-static (5%) blend.        
 
         [0067]     The fabric in Example 1 is produced on an 18-20 gauge, tricot or raschel machine.  
         [0068]     In constructing the fabric of Example 1, the threading is 5 in, 1 out for both back guide bars and 1 in, 5 out for both front guide bars.  
         [0069]     The threading chart for Example 2 is as follows:  
                                       Bar 1   ........||||.....|||   .|||||   .|||||.|||....           (back)               Bar 2   ........||||||.|||||   .|||||   .|||||.|||....               Bar 3   .............|.....|   .....|   ..............               Bar 4   ....................   .|....   .|............       (front)          
 
         [0070]     Stitch construction for Example 2, is as follows:  
                                                           BAR 1           BAR 4           (back)   BAR 2   BAR 3   (front)                           4-5   1-0   6-7   1-0           4-3   1-2   6-5   1-2           4-5   1-0   6-7   1-0           3-2   2-3   4-3   3-4           1-0   4-5   1-0   6-7           1-2   4-3   1-2   6-5           1-0   4-5   1-0   6-7           -3   3-2   -4   4-3                      
 
         [0071]     The finished fabric of Example 2 has a width of 60 inches and a weight of 6 oz/yd squared. Moreover, the finished fabric has a mesh count of 4 holes per inch in width and 5 holes per inch in length.  
         [0072]     In Example 2, the finished fabric is jet dyed to a desired color and then stabilized and set by tenter framing at a temperature of 335° F. at the speed of 15 yds/min.  
         [0073]     In general, the knitted fabric of the invention is advantageous over their prior art woven counterparts.  
         [0074]     For example, knit fabrics, but the virtue of their inherent stretch, elasticity and porosity, are more comfortable to wear than wovens. Both stretch and porosity parameters of knit fabrics may be engineered into the structure to the required degree. The stretch factor contributes to the freedom of movement and comfort of the protective suit wearer, an important consideration for workers. Knitted fabrics are also cheaper to produce than woven fabrics due to substantially higher manufacturing rates of the knitting equipment. Also, the wider width of knit fabrics improves processing economics and reduces the cutting waste in garment manufacture.  
         [0075]     Knit fabrics, by the virtue of their locked loop structure, do not rely on friction between the yarn members of the structure to preserve the fabric integrity. Consequently, there is not fraying from cut edges of the fabric, no ripping from holes or tears, and no distortion due to slippage of the yarn members.  
         [0076]     Furthermore, the so called “overlock” seam used on knit fabrics is superior to what is found in wovens. The “overlock” seam has a system of three sewing threads, which together securely encase the seam in a triple-laced thread system so as to hold it securely in place regardless of the stresses and strains imposed during the course of wearing the garment.  
         [0077]     Moreover, because knit structures are interlooped and therefore not subject to any slippage of constituent threads, mesh fabrics may be constructed with greater ease and economy on a knit basis, especially of the warp type. The latter allows for making the mesh openings into any desired size and shape.  
         [0078]     And by no means of least importance, knit fabrics, in contrast to woven fabrics, allow for the ready introduction of conductive yarns into the thread structure. This is because a weaving system involves the use of one type of yarn that is put up on the beam of a loom. Therefore, to introduce other yarns of different size and characteristics in the weaving process requires setting up a cumbersome external creel in order to accommodate the yarn packages. This in turn impairs the weaving process, thereby causing weaving defects. In contrast, for warp knitting, the different yarns are put up on separate beams and thus carried on appropriate guide bars.  
         [0079]     The scope of the invention will now be set forth in the following claims.