Abstract:
An electro-fusion drainage system coupling has a tubular body with outer and inner substantially parallel surfaces. The inner surface defines a passageway from end to end. A resistive heating element is disposed in the passageway. The predetermined distance represents a desired insertion distance of the pipe into the coupling body passageway. The electro-fusion coupling may further include a stop positioned in the passageway to contact the end of the thermoplastic pipe inserted into the passageway. Other implementations include an electro-fusion coupling with a fitting integrally formed on a second end and a thermoplastic piping system presented in a kit. Methods of the invention are also described.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to electro-fusion couplings and fittings for thermoplastic piping.  
       BACKGROUND  
       [0002]     Thermoplastic piping has been used to convey corrosive waste discharge from research laboratories since the late 1960s. Prior to that time, borosilicate glass piping was the standard material of choice for the aboveground portions of the piping systems located within buildings. For the underground portions of the systems, high-silicon alloy iron (Duriron) was the material of choice.  
         [0003]     In the early 1960s, attempts were made to replace glass and Duriron with low temperature, inexpensive thermoplastics such as polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene copolymer (ABS), as well as polyethylene. However, PVC and ABS both proved to lack the necessary chemical resistance to common laboratory solvents and environmental stress crack resistance to common disinfectant ingredients such as non-ionic surfactants. Further, PVC, ABS and polyethylene alike all proved to lack the necessary stiffness at temperatures greater than 140° F. Since mixtures of laboratory wastes causing exothermic chemical reactions and building temperature changes during construction can both result in thermal expansion, piping materials which soften at 140° F. are more likely to sag and twist between supports, resulting in the development of sagging and back-pitch (back-pitch refers to a situation where the pipes that are supposed to be “pitched” at a constant slope downward, instead bow upwards in places thereby preventing gravity from causing the fluid to flow freely). In drainage systems, back-pitch is highly undesirable as this can result in improper drainage and the possibility of fluid back-up through the system.  
         [0004]     Thus, it was determined early on that one material having an ideal mix of chemical resistance to a wide range of acids, bases and organic and inorganic solvents, and a higher glass transition temperature than PVC, ABS, and PE was polypropylene (PP). Upon its introduction in the late 1960s, polypropylene quickly grew to become the standard in the industry. One limitation of polypropylene however, is that due to its resistance to solvents, joining by solvent cementing is not a ready possibility. The material is instead joined either by some form of heat fusion, or by mechanical connection. Most forms of heat fusion that existed up to that point were limited to heat element socket fusion or heat element butt fusion, both of which have always been considered cumbersome, especially in fitting-intensive piping arrangements such as laboratory waste piping. To address this problem, and to facilitate easier installation of polypropylene into laboratory facilities, a new joining process referred to as “electro-fusion” was developed, e.g., as described in Blumencranz, U.S. Pat. Nos. 3,378,672; 3,465,126; and 3,506,519. This process, involving a wire coil imbedded within the plastic at joint locations, and through which electricity is later passed to create heat and fusion (with pressure applied via external clamping) was developed to make it easier to install polypropylene.  
         [0005]     In addition to electro-fusion, once polypropylene corrosive waste systems proved to be a commercial success, mechanical methods of connection were also established as an alternative method to join pipes. Mechanical methods, which involved fittings and couplings of a completely different design from that of the electro-fusion variety, soon gained popularity in aboveground installations, particularly in the under-sink plumbing.  
         [0006]     After the historic MGM Hotel fire in Las Vegas twenty-five years ago, new building standards were enacted to require that building materials for air handling areas and areas classified as return air plenums satisfy certain flame and smoke density requirements (namely flame spread value of less than 25 and smoke density rating of less than 50 according to ASTM E-84). Polypropylene, while being excellent chemically, unfortunately burns readily and produces relatively dense smoke, and as such is typically unable to meet these required values. Therefore, for a period of time, in those areas classified as fire rated, users were forced to transition to borosilicate glass or duriron through some means of mechanical connection. However, in the early 1990s, a special formulation of polyvinylidene fluoride (PVDF), a fluoropolymer thermoplastic material known for its fire resistance as well as its excellent chemical resistance, was able to pass the required flame spread and smoke density requirements (when tested to ASTM E-84) of most building codes. As a result, PVDF has since become the standard corrosive waste material of choice in those portions of buildings designated as fire rated areas.  
         [0007]     PVDF, like polypropylene, is highly solvent resistant, and able to be joined by the same methods inherently used on PP (e.g. electro-fusion and other heat fusion methods, and mechanical joining methods).  
         [0008]     While the electro-fusion method has appeal, it has not been without its problems as there are many subtleties that can result in problems in joining polypropylene and PVDF by electro-fusion. Additionally, subtleties in joint design, coupled with inadequate joining, can lead to failures occurring in service after a prolonged period of time. For this reason, many projects have suffered through severe installation difficulties, while others have seen after installation failures occur due to chemical attack mechanisms.  
         [0009]     Generally, contractors tend to prefer the labor savings and ease of joining offered by mechanical joining methods, whereas engineers and code officials tend to prefer the use of fused joints in areas that are inaccessible, such as behind walls and in underground locations. Most engineers tend to view fused joints as having less likelihood of leaks over time. Conversely, most engineers and code officials tend to view mechanical methods as having a distinct possibility of loosening over time, whereby leaks could result. As a result, most mechanical joints are limited to installations where there is access to repair the joints, if needed.  
         [0010]     In prior art electro-fusion methods developed for corrosive waste systems, the methods involve putting a minimum amount of heat into the joints, and using a coupling design with a small mass of material. In order to create pressure between the coupling or fitting coupling portion and the pipe to be fused, external clamps are required during the electro-fusion process. The clamping force required is difficult to quantify. If the contractor does not apply a sufficient clamping force, or too much clamping force is applied, a poor joint can result. Since there are many human elements involved in the joining processes, and a minimum amount of heat is introduced into the joints, the results are often less than satisfactory. In the best-case scenario, when all steps of the fusion process are performed properly, the joints are typically rated for drainage pressures only. However, if one or more of the steps involved are not followed properly, or tolerances are less than ideal, the result may be a high rate of leaks encountered during the joining process. In any event, the requirement for clamping adds a significant amount of additional labor on sizeable projects.  
         [0011]     In prior art electro-fusion systems as shown in U.S. Pat. Nos. 6,450,544 and 6,250,686, a single coupling incorporates changeable sleeves to allow for joining by either electro-fusion or mechanical means using the same coupling. However, while providing some advantages in reducing the number of parts needed for manufacture and inventory, the prior art systems do not solve some of the fundamental problems described in the previous paragraphs. One of the problems with the prior art systems is the need to use manually-applied external clamping force during the electro-fusion cycle (accomplished by hand tightening of a nut). Additionally, in the prior art, when joining by either electro-fusion or mechanical method a short piece (“pup”) of pipe is required when making the fitting-to-fitting connections. Since the mechanical coupling utilizes an external threaded nut, this can loosen over time due to expansion and contraction, leading to a failure later in service. Further, with the electro-fusion coupling, the installer has no way to apply a clamping force, in the event a second or third fusion cycle is required if the first cycle did not create a pressure tight joint, because the threads may become fused tight. Since this type of joint uses a minimum amount of heat and depends on manual force for applying pressure during the joint, this problem occurs in many installations.  
         [0012]     Another disadvantage in prior art electro-fusion systems used in corrosive waste systems, as well as in many prior art mechanical joint systems, is that a fitting-to-fitting joint requires that a short pipe nipple be cut and prepared, resulting in additional labor and two distinct joints.  
         [0013]     Mechanical joints are satisfactory for installation in certain accessible areas, and electro-fusion is satisfactory underground and behind walls, but in applications involving both methods, the requirement for different fitting types is a disadvantage.  
       SUMMARY  
       [0014]     The present disclosure pertains to electro-fusion couplings and fittings for use with a thermoplastic piping system adapted for use in corrosive waste piping systems. The system incorporates use of plain end fittings with use of electro-fusion couplings and fittings. Plain end pipe and plain end fittings are capable of use with other prior art joining systems such as heat element butt fusion, heat element socket fusion, mechanical joint, solvent cementing or adhesive bonding. Therefore, the electro-fusion system of the present invention can be used individually on a given installation, or several of the listed methods can be combined on a given project, e.g., with different methods used in different portions of the system. Regardless of the method or methods used on a given installation, common fittings can be used interchangeably since the end configuration is always the same.  
         [0015]     In the electro-fusion method, the system makes use of full integrity “pressure rated” electro-fusion technology using molded-in wire or post-molding imbedded wire and “clampless” designs for the coupling. The resulting installation in a gravity drainage system is pressure-testable and pressure rated to at least the rating of the component having the lowest pressure rating installed in the system. This is highly desirable for applications involving critical fluids that are to be drained down the systems by gravity due to the unprecedented level of afforded safety factor. The electro-fusion system of the present disclosure results in a relatively much higher level of fusion integrity, and one that is more repeatable for polypropylene, PVDF and other thermoplastic piping systems.  
         [0016]     As discussed above, advantages of the present system include full pressure integrity without exterior clamping of the joint during fusion. The coupling wall thickness of the present invention is predetermined to be sufficient to restrain the coupling from expanding during the heat fusion stage of coupling. The thickness of the coupling is equal to or greater than the wall thickness of the associated pipe, thereby providing a pressure rating equal to or greater than the pipe. Further advantages include use in a system of plain end fittings, thereby eliminating the need for short pipe nipples.  
         [0017]     Other advantages of the electro-fusion coupling of the present disclosure include the reduced space required for assembly of an electro-fusion coupling compared to a mechanical coupling. For example, a mechanical coupling requires additional space for manipulating a wrench to tighten the mechanical fasteners (bolt and nut) of the mechanical clamp, then slipping the electro-fusion coupling on and connecting the power supply wires to the coupling terminals.  
         [0018]     In one embodiment, an electro-fusion drainage system coupling has a tubular body defining an outer surface and an inner surface, the outer surface and the inner surface being substantially parallel, the inner surface defining a passageway from a first end to a second end. A first resistive heating element is disposed in the passageway. The resistive heating element may be disposed in the passageway, e.g., in a spiral groove cut in the interior surface of the body of the coupling with a resistive heating element wire laid down in the groove, or the groove may be a continuous spiral from a first end portion of the coupling to a second end portion. Alternatively, a first spiral groove may be inscribed in the surface of the first end portion to the central portion of the coupling and a second spiral may be inscribed from the central portion to the second end portion of the body, with a first resistive wire disposed in the first spiral groove and a second wire disposed in the second groove. The wires may be electrically connected. In yet another implementation, a tubular preformed sleeve has a resistive heating element comprising a spiral wound wire disposed on an outer surface of the sleeve. A tubular body is over-molded over the sleeve and heating element.  
         [0019]     The electro-fusion coupling may include a first radial opening from the inner surface to the outer surface of the body and a second radial opening from the inner surface to the outer surface of the first portion of the body. A first conductive terminal is disposed in the first radial opening and electrically connected to the resistive heating element and a second conductive terminal is disposed in the second radial opening and electrically connected to the resistive heating element.  
         [0020]     The resistive heating element of the electro-fusion coupling may be coated with fluoropolymer before being disposed in the groove of the coupling body or coated with fluoropolymer after being disposed in the groove of the coupling body.  
         [0021]     In some implementations, the electro-fusion coupling further includes a stop in the passageway of the body, the stop being positioned for contact with a first proximal end portion of first thermoplastic pipe inserted into the passageway of the coupling body. The stop may comprise a continuous circumferential ring of the same material as the body of the coupling and the ring is sized to be partially received in a radial groove milled in the inner surface of the body. The groove is milled in the inner surface before the resistive heating element is put in place.  
         [0022]     The electro-fusion coupling may include a pop-up fusion indicator having an annular depression on an outer surface of a portion of the fitting body. The annular depression extends into but not through the sidewall. An integral button of sidewall material is disposed to the center of the annular depression.  
         [0023]     The electro-fusion coupling may further comprise a fitting integrally formed on a second end of the coupling. The fitting may be a tee-fitting, elbow-fitting, wye-fitting, or other standard waste discharge system fitting.  
         [0024]     In some implementations, the thermoplastic piping system may be in the form of a kit having at least one mechanical coupling with a predetermined interior diameter and predetermined length configured to accept and couple a thermoplastic pipe of a specified outside diameter. The kit further includes at least one electro-fusion coupling having a predetermined interior diameter configured to accept and couple a thermoplastic pipe of the specified outside diameter of the pipe to be coupled by the mechanical coupling. The electro-fusion coupling has a longitudinal length substantially equivalent to the length of the mechanical coupling. The electro-fusion coupling has an internal working pressure equal to or greater than internal working pressure of the mechanical coupling.  
         [0025]     Polymeric waste discharge system pipe may be joined using the electro-fusion coupling of the present disclosure by the steps of: providing a first piece and a second piece of polymeric tubular pipe to be joined, each with a proximal terminal end and a proximal end portion adjacent to the proximal terminal end; and providing an electro-fusion coupling (or fitting with electro-fusion ends); inserting the proximal terminal end of the first piece of pipe to be joined in a first end portion of the electro-fusion coupling for a predetermined distance of insertion; inserting the proximal terminal end of the second piece of pipe to be joined in a second end portion of the electro-fusion coupling for a predetermined distance; and applying an electrical current to the resistive element to heat the resistive heating element to a temperature sufficient to fuse each end of the tubular pipe to the coupling absent any external support applied to the outside of the tubular body of the electro-fusion coupling. The method may also include the step of inserting the terminal proximal end of the pipe to be joined into the electro-fusion coupling until the terminal end contacts a stop disposed on the interior surface of the central portion of the coupling.  
         [0026]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.  
     
    
     DESCRIPTION OF DRAWINGS  
       [0027]      FIG. 1  is a perspective view of an electro-fusion coupling with a resistive heating element in an internal passageway;  
         [0028]      FIG. 2  is an end view of the electro-fusion coupling of  FIG. 1 ;  
         [0029]      FIG. 3A  is a cross-sectional side view of the electro-fusion coupling taken along section  3 A- 3 A of  FIG. 2 ;  
         [0030]      FIG. 3B  is a cross-sectional side view of an electro-fusion coupling taken along section  3 B- 3 B of  FIG. 2  with a resistive heating element configured differently than that of the implementation of  FIG. 3A ;  
         [0031]      FIG. 3C  is an enlarged partial cross-section of a portion of  FIGS. 3A and 3B  illustrating a pop-up fusion indicator and an electrical terminal connector;  
         [0032]      FIG. 3D  is an enlarged partial cross-section taken along section  3 D- 3 D of  FIGS. 3A and 3B  illustrating a pop-up fusion indicator and an electrical terminal connector;  
         [0033]      FIG. 4  is a partial cross-sectional side view of a polymeric pipe to polymeric pipe connection, using the electro-fusion coupling of  FIG. 1 ;  
         [0034]      FIG. 5  is a partial cross-sectional side view of a polymeric pipe to polymeric pipe connection, using a prior art mechanical coupling;  
         [0035]      FIG. 6  is a cross-sectional side view of a polymeric pipe to polymeric elbow connection joint, using the electro-fusion coupling of  FIG. 1  and a prior art plain end elbow;  
         [0036]      FIG. 7  is a partial cross-sectional side view of a polymeric pipe to polymeric elbow connection joint, using a prior art mechanical coupling and a prior art plain end elbow; and  
         [0037]      FIG. 8  is a cross-sectional view of elbow fitting with an integrally-formed electro-fusion coupling on at least one end of the elbow joined to a polymeric pipe. 
     
    
       [0038]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0039]     Referring now to  FIGS. 1, 2 ,  3 A and  4 , an electro-fusion coupling  100  for making electro-fusion (re: ASTM F 1073 and ASTM F 1290) connections between carrier pipe and/or other fittings (hereinafter referred to generally as “couplings”) of various configurations is designed to receive pipes  210  and  212  in opposing socket ends  110  and  112 . On the interior of the coupling  170 , there is a wire circuit  300 . The circuit  300  can consist of one circuit that runs from a near first end  110  of the coupling  100  to a second end  112 . Alternatively, the coupling  100  can contain two separate wire circuits, one for each of the opposing socket ends  110 ,  112 , ( FIG. 3B ). On the interior of the coupling, there may be an optional center stop  150 . In this particular implementation, the stop  150  is a full circumferential stop formed of the same material as the coupling body. This is desirable in drainage systems so a smooth interior surface is in the flow area of a gravity draining system. The stop  150  can be configured so that in cross-section, it is essentially “square” to match pipe and fitting ends that are squarely cut. Alternatively, the stop can be implemented with a smooth bevel (i.e., radius), e.g., having a radius of 3 mm or ⅛-inch, with all ends of pipes and fittings advantageously prepared with the same external bevel to match the contour of the bevel so that when the pipe or fitting ends are properly inserted, the ends are essentially flush with the stop resulting in a substantially smooth surface to be encountered by the fluid flow of the draining system.  
         [0040]     Several methods of manufacture will be discussed later in this specification. For couplings manufactured by a wire inlaying method, the interior pipe stop can be manufactured using a separate continuous polypropylene ring, which is later assembled into a counterbore  152  on the inside surface  170  of the coupling. This method allows for a full circumferential stop to be used on the inside diameter of the coupling, which is otherwise difficult to achieve with a coupling where the wire is inlaid by this method. A full circumferential stop, or nearly full circumferential stop, is desirable in gravity-flow drainage applications to prevent fluid flow from slowing down, and creating the possibility of fluid back-up.  
         [0041]     There are several methods for manufacture of pipe couplings with the wire imbedded beneath the surface, e.g., by molding-in the wire using a preform (U.S. Pat. No. 4,224,505), or by the wire inlaying process (U.S. Pat. Nos. 6,751,840 and 6,530,139). In one implementation of the present disclosure a spiral groove  600  is inscribed on the inner surface  170  of a preformed coupling. It will be understood that the spiral groove may be of a continuous consistent pitch from beginning to end or the pitch of the spiral may change one or more times from the beginning of a groove to the end of the groove. A resistive heating element comprising a wire  176  is disposed in the spiral groove. First and second radial openings  182  and  184  extend from the inner surface  170  to the outer surface  180  of the coupling body. A first conductive terminal  500  is disposed in the first opening and electrically connected to a first end region of the resistive heating element  176  and a second conductive terminal  500  is electrically connected to a second end region of the resistive heating element  176 . The terminal connectors  500  can be of any diameter, but it is preferentially one of several standard diameters, including 4.0 mm, 4.7 mm, or other standard pin sizes. Surrounding the terminal connection, the molded coupling body includes a cylindrical terminal protector comprising a protective sleeve  510  formed around the terminal connector  500 , so that when the connector is affixed to the heating element leads, it is shielded by the terminal protector, thereby minimizing exposure of an installer to a terminal and reducing any chances for injury due to electrocution. Such terminal protectors may be used with other implementations of the electro-fusion couplings of the present invention.  
         [0042]     As illustrated in  FIGS. 3A, 3B  and  3 C on the exterior of the coupling  100 , there can be affixed “pop-up” fusion indicators  550 . Pop-up fusion indicators have an annular depression  552  into the surface  170 ,  470  and penetrating into the tubular body sidewall  172 ,  472 . Integral button  554  of sidewall material is left in the center of the annular depression  552 . In the preferred embodiment, the annular depression  552  is molded integrally into the sidewall  172 ,  472  leaving the central indicator button  554  in the center of the annular depression. Alternatively, the annular depression  552  may be drilled in the sidewall  172  or  472 . Prior art fusion indicators typically consist of a button and spring inserted into a depression in the sidewall of a coupling. The present design presents a significant cost savings over the prior art because the button is formed integral to the body. Additionally, since the button is integral to the body it will not detach from the coupling and become lost prior to the fusion process. The indicators are forced to rise by molten fusion material in the wall of the coupling, thereby indicating that fusion has taken place. Pop-up-fusion indicators may be used with other implementations of the electro-fusion coupling of the present invention.  
         [0043]     Referring to  FIG. 3B , in another implementation, the electro-fusion drainage system coupling  400  is similar to the first implementation. The coupling includes a tubular body having: a first end portion proximal to a first end  410 ; a second end portion proximal to a second end  412 ; a central portion  414  between the two end portions; and an outer surface  480  and an inner surface  470 . The outer surface  480  and the inner surface  470  are substantially parallel with the inner surface defining a passageway from the first end  410  to the second end  412 . A first spiral groove  610  is inscribed in the inner surface  470  extending from the central portion  414  of the body. The inscribing tool changes the pitch of inscribing in the central portion  414  leaving a space for the stop ring  150  in the central portion and a second spiral groove  612  is inscribed in the inner surface  470  extending from the central portion  414  of the body the second portion  412  of the body. A first resistive heating element comprising a wire  310  is disposed in the first spiral groove  610  and a second resistive heating element  312  comprising a wire is disposed in the second spiral groove  612 . The first and second heating elements may be electrically connected. Alternatively, a single wire may be laid down from the first portion  410  across the center portion  414  to the second portion  412 . First and second radial openings  182  and  184  extend from the inner surface  470  to the outer surface  480  of the coupling body. A first conductive terminal  510  is disposed in the first opening and electrically connected to the resistive heating element and a second conductive terminal  510  is electrically connected to the resistive heating element. The coupling  400  may also include a stop  150  and fusion indicators  550  as illustrated and discussed in the first implementation disclosed in  FIG. 3A .  
         [0044]     Referring now to  FIG. 4 , the electro-fusion coupling  100  of the present disclosure joins two pieces of pipe  210  and  212  with outside diameter D 1 . The inside diameter of the coupling is substantially equal to the outside diameter D 1  of the pipe to be joined. The outside diameter of the coupling is denoted as D 2 . The overall axial length of the coupling  100  is illustrated as L T . Referring in comparison to  FIG. 5 , a prior art mechanical coupling  1000  (manufactured by the assignee of this application) joins a pipe of outside diameter D 1  having the same overall longitudinal length L T . The prior art mechanical coupling  1000  includes a polymeric sleeve  1010  placed in contact with the pipes  210  and  212  to be joined and an exterior metal sleeve  1020  with ears  1030  and conventional fasteners  1040 . The common longitudinal length L T  permits interchangeability of the mechanical and electro-fusion couplings on the same construction site. Applicant believes the longitudinal length L T  is shorter than any commercially available electro-fusion coupling of the same pressure rating. Approximate internal diameter, external diameter and total length of one embodiment of the electro-fusion coupling is in the Table 1 below.  
                                                                               TABLE 1                                       Coupling                    Nominal Size   3″   4″   6″   8″                            D 1     4.315   5.450   7.70   9.7           D 2     3.524   4.303   6.236   7.768           L T     8.500   8.500   9.500   9.500                      
 
         [0045]     It will be understood that the dimensions in Table 1 are for illustrative purposes and not limiting on the scope of the present invention.  
         [0046]     Referring to  FIGS. 6 and 7 , therein is illustrated a polymeric elbow  220  joining to a polymeric pipe  210  with a polymeric coupling  100  ( FIG. 6 ) and with a standard prior art mechanical coupling  1000  ( FIG. 7 ). Again, the prior art clamp has total longitudinal length L T  equal to the longitudinal length L T  of the electro-fusion coupling, which permits interchangeability of the mechanical and electro-fusion couplings on the same construction site. Referring again to  FIG. 3 , the wall thickness W 2  of the coupling  100  is predetermined to be of thickness sufficient to restrain the coupling from expanding during the heat fusion stage of the coupling process. The thickness of the pipe coupling W 2  is equal to or greater than the wall thickness W 1  of the pipe to be joined.  
         [0047]     The electro-fusion couplings  100 ,  400  and  700  are designed with an internal working pressure rating equal to or greater than that of an equivalent mechanical coupling  1000 .  
         [0048]     Referring now to  FIG. 8 , an elbow fitting  700  has an integral coupling  790  formed on one end. The external diameter of the electro-fusion coupling end of the elbow fitting is D2, which is the same outside diameter (D2) of the electro-fusion fitting  100 ,  400  and  800 .  
         [0049]     In some implementations, it is desirable to protect the wire resistive heating elements  600 ,  610 ,  612  with a tough, high temperature corrosion resistant coating, e.g., fluoropolymers such as PFA (one of the grades of Teflon®) or polyamide-imide (PAI), on the wire. This is especially desirable when using chemically-reactive wire substances such as copper. Copper is beneficial due to its electrical properties in helping to minimize the required voltages for fusion, but it can react readily when in contact with corrosive acids and caustic solutions. Copper, when used with PP systems, may also induce stress cracks in polypropylene. In particular, copper ions, freed during reaction with acids and caustic solutions, can function as a stress-cracking reagent, which, if stress is present in the system, can lead to complete fracturing of the joints. Thus, it is desirable to protect the wire, e.g., by means of a coating, as a conservative measure. It is further beneficial to protect the wire with a coating that also serves a dielectric function so that if wires migrate during the fusion, the dielectric characteristic helps to prevent the wires from burning out or shorting out.  
         [0050]     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.