Abstract:
Bundled carbon nanotubes are disentangled and dispersed using the principles of extreme pressure reduction of fluids carrying the bundled nanotubes. They are added to a high pressure fluid upstream of a chamber operated at much lower pressure. These high-low pressure ratios are preferably at least 100:1. As the high pressure fluid enters the lower pressure chamber it violently expands causing separation and disentanglement of the bundled carbon fibers. To further assist in this disentanglement a nozzle may be used at the inlet to the lower pressure chamber to direct the high pressure fluid against a hardened anvil in the chamber. This impact further aids disentanglement. Coating the nanotubes with a dispersant also improves disentanglement.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with U.S. Government support under contract No. HR0011-06-C-01 awarded by DARPA (NGSCF). The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Disentangling nanotube bundles is important to the efficient use of nanotubes, particularly carbon nanotubes (CNTs). Nanotubes have numerous commercial applications. The unique electrical properties of single wall carbon nanotubes, particularly in the axial direction (excellent electrical conductivity) have opened up uses and potential uses in computers, for example, where significant increases in computing power with decreases in physical size, are being developed. Similarly, improved flexible displays for televisions and computers are being developed with the incorporation of nanotube materials. In medical applications, they have been proposed for use as medical delivery vehicles. However, it is the unusually high mechanical properties (strength and modulus) that have attracted the most interest for nanotube applications. Someday ultra strong carbon nanotubes may be the foundation of a space elevator while their use as reinforcements in composites promises to revolutionize the properties of composite materials. Already, the incorporation of nanotubes in composite systems has lead to significant improvements in composite toughness, strength, stiffness, and conductivity in many laboratories. Commercially, these composite materials have already found limited use in sporting goods (tennis rackets, bicycles, golf clubs), automotive (fuel lines, body parts), and aerospace applications. The difficulty in achieving adequate disentanglement and dispersion currently limits their use in additional applications. 
         [0003]    As in any composite material, in a nanotube reinforced composite at least one constituent serves the purpose of providing the reinforcement (nanotubes) while another constituent (e.g. polymeric matrix) serves the purpose of transferring the load between individual reinforcing entities. In order to achieve this reinforcement, it is necessary to maximize the amount of nanotube surface area in direct contact with the material it is reinforcing and to disperse the nanotubes as uniformly as possible throughout the matrix. The nanotube reinforced composites (“NRC”) may include matrix resins such as epoxies, polyesters, polyimides, polyamides, and the like. 
         [0004]    The greater the surface area in contact with the material being reinforced and the more uniform the distribution of nanotubes within the material the stronger the composite. However, nanotubes are typically produced in a manner whereby they are not single strands but rather tangled bundles. In the case of single wall nanotubes (SWNTs), the nanotubes are produced as bundles of ropes, caused in part by very strong van der Waals forces. In addition, the high aspect ratios of the carbon nanotubes make it difficult to separate them into individual ropes or tubes. As produced in bundles, the nanotubes offer lowered surface area per unit of weight available for adherence to the material being reinforced. In addition, highly entangled bundles can lead to the nanotubes acting as stress concentrators instead of reinforcements, thus degrading the mechanical properties of the NRC. In other applications, e.g. where nanotubes are incorporated to produce a conductive polymer, adequate dispersibility is required to obtain the continuous and uniform conductivity required throughout the composite. Thus, there is a need for methods to disentangle nanotube bundles into deagglomerated ropes or individual nanotubes. 
         [0005]    As discussed in more detail in the next section applicant has found that large pressure differentials create an environment that will separate the bundled nanotubes. More particularly, applicant has found that placing the entangled nanotubes in a high pressure waterjet that is allowed to expand into a zone of lower pressures introduces enough force to disentangle the nanotube bundles without adversely affecting their structure. 
         [0006]    The available abstract of Japanese Patent Application Publication No. 150541 (“541 Application”) entitled “Method for Rupturing Carbon Nanotube and [Resulting] Carbon Nanotube” discloses a method for “rupturing carbon nanotubes” but does not disclose disentanglement of nanotubes. The device disclosed in this abstract and, to the limited extent understood, the specification of the &#39;541 Application facilitates this rupturing or breakage of carbon nanotubes by directing multiple streams of water containing nanotubes at each other. These water/nanotube streams are directed through “complicated flow passages of fine tubes.” According to the abstract of the &#39;541 Application, the collision of these streams with each other as they exit the fine tubes and the boundaries of the chamber into which they flow ruptures or breaks the nanotubes. This object of the &#39;541 Application is unlike Applicant&#39;s object, namely to debundle and separate the nanotubes to improve their utility, particularly as a reinforcement in composite materials. 
         [0007]    A companion Japanese Patent Application Publication No. 2006-016222 (“222 Application”) discloses a device similar to that of the &#39;541 Application for “rupturing” or breaking nanotubes. The primary differences between these two Japanese Applications relate to the structure of the “complicated flow passages” within the devices carrying the water borne nanotubes before they exit the passages and collide.  FIG. 2  of the &#39;222 Application schematically illustrates multiple very high pressure (175,000 psi) water streams  14  and  16  which merge as stream  28  after collision and exit the device at discharge port  30 . To get the desired degree of rupturing the exit stream for port  30  is split and recycled 10-20 times. There is no disclosure in the &#39;222 Application of any process or means for disentangling nanotubes. 
       SUMMARY OF THE INVENTION 
       [0008]    Improved disentanglement and dispersibility of carbon nanotubes is achieved by this invention relative to other methods currently in use. This is achieved by introducing the bundled nanotubes needing separation into a high pressure fluid stream. This combination of the bundled nanotubes with the high pressure fluid, typically water, is then introduced into a closed chamber operating at a much lower pressure. The sudden reduction in pressure creates cavitation with the formation and sudden collapse of bubbles in the liquid. The collapse generates ultra high energy shock waves, which can then perform work on the water/nanotube mixture and also causes violent movement of the water/nanotube mixture. It has been shown that the energy generated by that work separates the bundled nanotubes. 
         [0009]    To further assist the disentanglement of nanotubes entering the lower pressure chamber, the high pressure fluid stream enters that chamber through a nozzle at the entrance to the chamber. This nozzle can then direct the entering stream against an anvil spaced in the chamber close to the nozzle. The high pressure stream entering the chamber is traveling at a very high speed as it enters the lower pressure in the chamber. It then impacts upon the anvil adding further force to disentangle the nanotubes. 
         [0010]    The disentangled nanotubes collect in the fluid, usually water, within the closed chamber, and exit through an outlet in the chamber for collection and use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a side elevational view in partial cutaway of the chamber used to disentangle nanotubes in accordance with this invention. 
           [0012]      FIG. 2  is a cross sectional view of the chamber along lines  2 - 2  of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The present invention is directed at processes to disentangle nanotube bundles. It does not have as an object the fracturing or breaking the nanotubes such as is the case with the aforementioned Japanese Published Applications. In fact, in many applications, e.g., reinforcement of composites, the fracturing (shortening) of nanotubes would be antithetical to the application. In any fiber reinforced composite, there is a critical length that the fibers must have to strengthen a material to their maximum ability. This critical length can be mathematically derived from the mechanical and physical properties of the fiber. If the reinforcing fiber (nanotube in this case) falls below this critical length, the reinforcing effect will be minimal or nonexistent. Coupled with this is the fact that the longer the nanotube the greater is the reinforcement potential for the nanotube because it has greater surface area per unit of length adhering to the composite material. In conjunction with the need to exceed a critical length, reinforcing elements (e.g. nanotubes) must be well and uniformly dispersed through the composite to realize the optimum properties. There are two general classes of carbon nanotube materials, single wall (SWNTs) and multi wall (MWNTs). In the case of SWNTs, the individual nanotubes are attracted to each other during formation by strong inter-wall interactions, leading to the formation of “ropes” of SWNTs. MWNTs do not form ropes, per se, but in both cases, the resultant nanotubes typically entangle during formation and the strong van der Waals forces make it very difficult to separate the ropes and/or nanotubes. 
         [0014]    Disentanglement is an important factor in obtaining greater surface area of the nanotube in whatever application they might be used. Most applications for nanotubes rely on and require the small size of nanotubes, so large bundles are counterproductive. Also, in order to realize the extremely high mechanical and electrical properties of CNTs in any application, high levels of disentanglement are required. For example, in medical applications under development, where nanotubes might be used for drug delivery to remote parts of the body, bundles of entangled CNTs will not work. In electronic applications, such as future computer or other displays, or for use in fuel cells or other energy storage devices, significantly higher electrical efficiencies will be realized by single disentangled CNTs. Disentanglement allows the nanotubes to individually disperse into the surrounding medium, whether a composite matrix or other application. As an added benefit, because nanotubes are currently so expensive to make, debundling and/or disentanglement can save money because fewer nanotubes are required in a particular application. 
         [0015]    Others have tried to achieve the difficult task of disentanglement using chemical surface treatment, for example, as described in U.S. Pat. No. 5,853,877. This approach is complicated and not always successful. It can also limit the use of the resulting nanotubes by causing surface degradation and fracture. Others have used ball milling to disentangle CNTs but this leads to nanotube fracture. Still others use sonication techniques but this is inefficient and time consuming. For example, Applicant&#39;s approach has shown significant time savings over sonication techniques. 
         [0016]    Applicant has taken a totally different approach to debundling/disentanglement. More specifically, the applicant utilizes the tremendous forces associated with high pressure mixing and depressurization (cavitation) to achieve the desired separation of the nanotubes, one from the other. One device for achieving this separation associated with depressurization is illustrated in  FIG. 1 . This disentanglement device  10  includes a sealed chamber  12  with fluid inlet  14  and outlet  16 . Chamber inlet  14  is preferably equipped with a 0.020-0.030 inch diameter nozzle opening to direct and control flow into chamber  12 . Chamber  12  is in fluid communication thru conduit  18 ,  19  with a source of fluid  20 , preferably water, under high pressure. Bundled nanotubes to be disentangled are drawn into mixing tube  24  and conduit  18 , at inlet  22  so that they admix with the high pressure fluid stream in mixing tube  24  and conduit  18 . The passage of the high pressure fluid through conduits  18 ,  19  creates a venturi type effect in mixing tube  24  which draws them into and thru conduit  18 , and then to inlet  14 . 
         [0017]    The mixing tube  24  operates as follows: the high pressure fluid in conduits  18 ,  19  is obtained using, for example, a KMT Waterjet System. The inside diameter of conduit  19  can be, for example, 0.010 inch contributing to the high pressure in that conduit. The fluid exits conduit  19  into mixing tube  24  where it mixes with the CNTs and enters conduit  18 , with an inside diameter, for example, of about 0.020 to 0.030 inches, before exiting nozzle  14  into chamber  12 . The high pressure in these conduits ranges from 30,000-40,000 psi for best results, although lower pressures (above ˜5000 psi) may be used. The best pressure for the fluid entering chamber  12  is, in part, a function of the pressure within chamber  12  because the pressure differential between the fluid exiting the nozzle in inlet  14  and that inside chamber  12  controls the disentanglement of the bundled nanotubes. Preferably, a high pressure differential, in conjunction with a high internal pressure in chamber  12 , will yield the highest degree of turbulence. Typically, this pressure ratio is over 300:1 but lower ratios, e.g. 80-100:1, can be used. Tables 1 and 2 below provide more detail on experimental pressure differences between the chamber  12  and the inlet  14 . Pressure within the chamber  12  typically ranges from 80-100 psi but higher pressures (300-400 psi or higher) can be used provided the necessary ratio of inlet pressure to chamber pressure is maintained and the chamber  12  material construction can withstand the pressures. 
         [0018]    The desired operating pressure in chamber  12  is controlled by an outlet flow control valve at outlet  16 . This valve can be a manual valve regulated to maintain a certain pressure in chamber  12  or an automatic valve set to maintain a certain pressure in chamber  12 . Pressure in chamber  12  could also be maintained at a certain pressure, for example, by controlling outlet  16  at a certain orifice diameter. 
         [0019]    Chamber  12  can be constructed of any materials that can withstand the operating pressures therein. The chamber  12  illustrated in  FIGS. 1 and 2  comprises a thick plastic wall tube  26  held between end pieces  28 ,  29 . The end pieces  28 ,  29  preferably have circular grooves and gaskets therein (not shown) for sealing engagement with the plastic tube  26 . The plastic tube  26  is preferably of a substantial wall thickness, for example, ¼ to ½ inch to withstand the pressures in, and entering, chamber  12 , up to the maximum working pressure of plastic tube  26 . For higher pressures, alternative materials of construction, for example aluminum or steel, for tube  26  can be used. 
         [0020]    In a preferred embodiment of the disentanglement device  10  an anvil  30  is adjustably mounted in chamber  12  so that it is aligned with the high pressure fluid containing bundled nanotubes exiting the nozzle in inlet  14 . The anvil  30  is preferably constructed to receive hardened inserts  31  that can be replaced as they get worn away due to the impact of the high pressure fluid impinging on the inserts. A carbide insert about ¾ to 1 inch square has been found to provide good wear resistance under the impact of the high pressure fluid exiting the nozzle in inlet  14 . 
         [0021]    The anvil  30  is preferably mounted on an adjustable support  32 . In one embodiment illustrated in  FIG. 1  this support  32  is a rod with screw threads  33  along all or part of its length which mate with corresponding screw threads  33  in end piece  29 . Rotation of support  32  enables adjustment of the distance between inlet  14  and anvil  30 . The degree of adjustment can be altered by changing the pitch and distance of the mating screw threads on support  32  and end piece  29 . 
         [0022]    The disentanglement device  10  operates as follows. Fluid under pressure (˜10-40,000 psi) is introduced into fluid conduit  20 . That fluid, typically water, passes through conduit  19  (typically ˜0.010 inch diameter), exiting into mixing tube  24 , and then entering conduit  18  on its way to inlet  14 . After exiting the (typically 0.020-0.030 inch diameter) nozzle at inlet  14  the fluid violently and rapidly expands creating vapor bubbles as it enters the lower pressure inside chamber  12 . As the water builds up in chamber  12  it slowly exits through a control valve or orifice at outlet  16 . Outlet  16  preferably is equipped with a valve to control the pressure in chamber  12  and the rate at which fluid exits chamber  12 . Back pressure of about 80-100 psi is maintained within chamber  12  by controlling the exit of fluid from the chamber  12  via the valve or orifice in outlet  16 . Once the desired pressure is achieved in chamber  12 , bundled nanotubes sought to be disentangled are delivered to conduit  22 , preferably in a liquid medium. These nanotubes are drawn into the chamber  12  via inlet  14  by the venturi effect of the high pressure fluid passing through conduits  19  and  18 . 
         [0023]    When the bundled nanotubes in the high pressure fluid passing thru conduit  18  enter chamber  12  through the nozzle in inlet  14  the fluid rapidly expands because of the substantially lower pressure in chamber  12 . The pressure in chamber  12  is preferably 1% or less of the pressure at inlet  14 . This rapid and substantial reduction in pressure causes the water to begin to vaporize, resulting in the formation of vapor bubbles in the fluid in chamber  12 . The pressure and turbulence in chamber  12  then cause these bubbles to rapidly collapse or implode, resulting in the generation of strong and significant shock waves in chamber  12 . The shock waves can cause the velocity of the liquid to locally exceed the speed of sound, resulting in very high energies in chamber  12 . These energies then work to disentangle the bundled nanotubes. This disentanglement is also facilitated by the impact of the fluid stream exiting inlet  14  upon anvil  30  and more particularly upon the hardened insert  31 . As the nanotube bundles in this fluid stream hit the hardened insert  31  they separate one from the other to further assist in their disentanglement. 
         [0024]    To further assist disentanglement, the bundled nanotubes can be treated with a dispersing agent, for example, a solution of water and polyvinylpyrrolidone (PVP). Multiple passes through chamber  12  can be also used to enhance disentanglement. 
         [0025]    The invention will now be described in further detail with reference to the following non-limiting examples. 
       EXAMPLE 1 
       [0026]    Three groups of multiple wall nanotubes were prepared for disentanglement. Two were treated with a commercially available chemical dispersant and one without. Twelve samples were made from the three groups of materials. Six of the twelve samples were treated with the impact anvil in place and six without. Water pressures were 30,000 psi while back pressure in chamber  12  was maintained at 80-100 psi, leading to significant energy transfer to the nanotubes. 
         [0027]    The treated dispersions appeared to stay well disentangled after treatment in chamber  12 , the degree of disentanglement dependent on the exact processing conditions and feed materials. Table 1 summarizes the testing methodology and results. As shown in this Table, three starting nanotube samples were analyzed, all at an initial weight of 0.375 g. Two of those samples (samples R-20-179-1 and R-20-179-2) were pretreated with a dispersing agent prior to introduction into chamber  12 . The third sample (R20-179-3) was not pretreated. The Table shows the other variables (pressures, number of passes through chamber  12 , use of anvil) tested regarding their relative contribution to disentanglement. 
         [0028]    After processing, great care is taken to collect all of the nanotubes sample from each run. The degree of disentanglement achieved is determined by the volume of the nanotubes after processing. The larger their volume after processing, relative to their volume before processing, the greater is the disentanglement. This volume is measured by simply pouring the treated and control nanotubes into separate jars and observing the level of the nanotubes within the jar. The higher the level of the nanotubes in the jar the greater is the disentanglement of the nanotubes. This relative volume of treated nanotubes, as measured in the jar, is accorded a scale of 1-5 with 5 being the largest volume increase relative to the untreated nanotubes in water. This is not meant to represent a linear scale to compare increases in volume between samples but is meant to signify a semi-quantitative comparison in volume for untreated samples versus treated samples as well as providing a rough measure for samples processed in different ways. 
         [0029]    The data in Table 1 shows that with a single pass through chamber  12 , without using anvil  30  (Sample Run No. 1), the relative volume of disentangled nanotubes versus the control is “2,” i.e., the initial volume of untreated nanotubes was noticeably greater after treatment in chamber  12 . After another pass through chamber  12  the relative volume of Sample Run 1 increased noticeably again as shown in the line marked Sample Run No. 2. The use of the anvil  30  again increases the relative volume, and disentanglement, of the nanotubes as shown by the relative volume readings in the right column of Table 1 for Sample Run No. 8. 
         [0030]    The relative volume, and therefore degree of disentanglement, is also affected by treatment with a dispersing agent. The amount and effectiveness of the dispersing agent used will affect the relative volume change even before treatment in chamber  12  as illustrated by a comparison of the relative volumes (degree of disbursement) for Control Sample No. 1 (R20-179-1) with minimal dispersing agent versus Control Sample No. 2 (R20-179-2) with maximum dispersing agent. The latter produced larger relative volume than the former. Each of these Control Samples had a larger relative volume than Control Sample No. 3 (R20-179-3) which was not treated with a dispersing agent. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Starting Samples: 
               
               
                 R20-179-1 —MWNT and water with minimal dispersing agent 
               
               
                 R20-179-2 —MWNT and water with maximum dispersing agent 
               
               
                 R20-179-3 —MWNT and water w/o dispersing agent 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                 Relative Volume 
               
               
                   
                 MWNT 
                 Feed 
                   
                 Number 
                   
                 Change After 
               
               
                   
                 Feedstock 
                 Water 
                 Chamber 12 
                 Passes 
                   
                 Treatment 
               
               
                 Sample 
                 Desig. 
                 Pressure 
                 Back 
                 Through 
                 Anvil Used 
                 (0 = least; 
               
               
                 Run # 
                 (R20-) 
                 (ksi) 
                 Pressure (psi) 
                 Chamber 
                 Cell ? 
                 5 = greatest) 
               
               
                   
               
               
                 Control 
                 179-1 
                 none 
                 — 
                 — 
                 — 
                 1 
               
               
                 No. 1 
               
               
                 Control 
                 179-2 
                 none 
                 — 
                 — 
                 — 
                 2 
               
               
                 No. 2 
               
               
                 Control 
                 179-3 
                 none 
                 — 
                 — 
                 — 
                 0 
               
               
                 No. 3 
               
               
                 1 
                 179-1 
                 30 
                 ~80-100 
                 1 
                 No 
                 2 
               
               
                 2 
                 Sample 1 
                 30 
                 ~80-100 
                 2 
                 No 
                 3 
               
               
                 3 
                 179-2 
                 30 
                 ~80-100 
                 1 
                 Yes; anvil ⅞″ 
                 5 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 4 
                 Sample 3 
                 30 
                 ~80-100 
                 2 
                 Yes; anvil ⅞″ 
                 5 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 5 
                 179-3 
                 30 
                 ~80-100 
                 1 
                 Yes; anvil ⅞″ 
                 2 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 6 
                 Sample 5 
                 30 
                 ~80-100 
                 2 
                 Yes, anvil ⅞″ 
                 3 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 7 
                 179-1 
                 30 
                 ~80-100 
                 1 
                 Yes; anvil ⅞″ 
                 3 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 8 
                 Sample 7 
                 30 
                 ~80-100 
                 2 
                 Yes; anvil ⅞″ 
                 4 
               
               
                   
                   
                   
                   
                   
                 from nozzle 14 
               
               
                 9 
                 179-2 
                 30 
                 ~80-100 
                 1 
                 No 
                 5 
               
               
                 10  
                 Sample 9 
                 30 
                 ~80-100 
                 2 
                 No 
                 5 
               
               
                 11  
                 179-3 
                 30 
                 ~80-100 
                 1 
                 No 
                 1 
               
               
                 12  
                 Sample 11 
                 30 
                 ~80-100 
                 2 
                 No 
                 2 
               
               
                   
               
             
          
         
       
     
       EXAMPLE 2 
       [0031]    Table 2 contains the processing conditions and relative volumes of three samples of single wall nanotubes (SWNTs) that were prepared for disentanglement. This Table illustrates the effects of different feed pressures and number of passes on SWNT dispersion. Evaluation of the resulting products was done visually, i.e. volume change after settling in water after the various treatments. Volume change correlates to CNT disentanglement as the disentangled nanotubes can no longer pack as efficiently in a given volume once the bundles separate. 
         [0032]    All three samples were treated with the impact anvil in place. Table 2 shows the relative volume change of 5 grams of SWNTs after treatment at 30 ksi water feed pressure and passes through the chamber compared to an untreated sample. The samples were allowed to settle for over three days after transfer to glass jars. There has been no indication that the treated samples will settle any more over time. 
         [0033]    This Table also illustrates the effect of pressure differentials on disentanglement. The Table shows that Sample 53-2, using a higher initial feed pressure than Sample 53-1, occupies more volume than the latter, indicating a greater degree of disentanglement of the SWNTs. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Relative Volume 
               
               
                   
                   
                 Feed 
                 Chamber 
                 Number 
                   
                 Change After 
               
               
                 Sample 
                 Weight 
                 Water 
                 12 Back 
                 Passes 
                   
                 Treatment 
               
               
                 Designation 
                 SWNTs 
                 Pressure 
                 Pressure 
                 Through 
                 Anvil 
                 (0 = least; 
               
               
                 R21- 
                 (g) 
                 (ksi) 
                 (psi) 
                 Chamber 
                 Used? 
                 5 = greatest) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 53-8 
                 5 
                 none 
                 — 
                 none 
                 — 
                 0 
               
               
                   
                   
                 (control) 
               
               
                 53-4E 
                 5 
                 30 
                 110-140 
                 5 
                 yes 
                 5 
               
               
                 53-1 
                 10 
                  8 
                 110-150 
                 1 
                 yes 
                 3 
               
               
                 53-2 
                 10 
                 30 
                 120-150 
                 1 
                 yes 
                 5 
               
               
                   
               
             
          
         
       
     
         [0034]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.