Abstract:
An electrically conductive polymeric housing or enclosure for process control equipment. The enclosure comprises a moldable thermoplastic polymer and carbon nanotubes. The carbon nanotubes are generally uniformly dispersed within said moldable thermoplastic polymer, such that the surface resistivity of the housing is less than 10 9  Ohms for Level 1 compliance.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to process control equipment, and more particularly to an electrically conductive polymeric housing for process control equipment.  
         BACKGROUND OF THE INVENTION  
         [0002]    In 1994, the European Union (EU) adopted the ATEX Directive 94/9/EC, which establishes the technical and legal requirements for products intended for use in potentially explosive environments. Compliance with the Directive becomes mandatory for all subject products within the EU as of Jul. 1, 2003. Any product placed on the market or put into use in hazardous environments governed by the Directive is required to meet the standards set out in the Directive. The standards vary depending on the classification of the hazardous area. The Directive describes three levels of escalating explosion potential within either “gas” or “dust” hazard environments. The levels are classified as 1, 2 or 3, with Level 1 being the most hazardous level.  
           [0003]    One of the standards for equipment in hazardous areas requires that polymer housings containing electrical equipment be electrically conductive. Specifically, polymer housings for use within Level 1 hazardous areas must have a surface resistivity of less than 10 9  Ohms. The concern that the Directive is addressing is the potential build up of static charges within the polymer housing. If a sufficient static charge is built up, then there is the potential for a spark, which in a gaseous or dusty environment presents a significant risk of explosion or fire.  
           [0004]    In addition to the compliance issue raised by the Directive and the potential for static charge build-up, the low conductivity of polymer housings presents a problem for containing electromagnetic radiation produced by electronic circuitry contained within the housings. The radiation can cause unwanted interference with equipment outside the housing. As a result, attempts have previously been made to increase the electrical conductivity of polymer housings in order to contain electromagnetic radiation.  
           [0005]    Most thermoplastic polymers have a surface resistivity in the range of 10 13  to 10 16  Ohms. There are some well-known methods of reducing the surface resistivity of thermoplastic polymers. These methods involve dispersing an electrically conductive material within the polymer. Electrically conductive materials typically used to reduce the resistivity of polymer housings include aluminum filaments or carbon black. These materials, in sufficient quantity, can dramatically reduce the resistivity of the polymer housing.  
           [0006]    The polymers used to form electrical housings for equipment used in hazardous areas are carefully chosen for their physical properties. Among the important properties are chemical resistance, cold impact strength, flame retardance and toughness. The typical polymers that are most often used include polypropylene; Kynar®, a polyvinylidene flouride; and Tefzel®, a flouropolymer resin.  
           [0007]    There are, however, significant drawbacks to the use of carbon black or aluminum filaments. These additives negatively impact the mechanical properties of the polymer, usually adding substantial stiffness and lowering the impact strength of an article constructed using the polymer. In addition, the additives have a dramatic impact on the melt index, a measure of the viscosity of the polymer. The melt index is an important property that indicates the ease with which a polymer can be poured into a mould. In some cases, additives like carbon black can lower the melt index of a polymer by a factor of 10.  
           [0008]    Considering the standards imposed by the Directive, the need to prevent static charge build-up and the problem of containing electromagnetic radiation, there remains a need for an electrically conductive polymeric housing for electrical equipment that does not sacrifice the advantageous physical properties of traditional polymeric housings, while providing the requisite electrical conductivity properties.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    Accordingly, the present invention comprises an electrically conductive polymeric housing for electronic equipment that is constructed of a moldable polymer that contains a dispersion of carbon nanotubes sufficient to make the surface resistivity of the housing less than 10 9  Ohms for Level 1 compliance.  
           [0010]    In a first aspect, the present invention provides an electrically conductive polymeric housing for process control equipment including a moldable thermoplastic polymer and carbon nanotubes dispersed within the moldable thermoplastic polymer, such that the surface resistivity of the housing is in the range of 10 9  Ohms.  
           [0011]    In a second aspect, the present invention provides a level detection apparatus including an electrically conductive polymeric housing comprising a moldable thermoplastic polymer and carbon nanotubes dispersed within the moldable thermoplastic polymer, and the electrically conductive polymeric housing having a surface resistivity which is less than 10 9  Ohms, a transducer mounted within the housing adapted to send a signal and receive a reflected signal, and an electronic circuit coupled to the transducer, and the electronic circuit controls the transducer and processes the reflected signal.  
           [0012]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Reference will now be made, by way of example, to the accompanying drawings which show a preferred embodiment of the present invention, and in which:  
         [0014]    [0014]FIG. 1 shows a perspective view of an electrically conductive polymer housing according to the present invention; and  
         [0015]    [0015]FIG. 2 is a schematic diagram of a level measurement device utilizing an electrically conductive polymeric housing according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Reference is first made to FIG. 1 which shows a level measuring device indicated generally by reference  10 . The level measuring device  10  provides non-contactive measurement, and is utilized to determine the distance to a reflective surface, such as the liquid level in a storage tank, by transmitting a signal and measuring the time for the signal to return. The level measuring device  10  may utilize ultrasonic pulses, capacitive signals or microwave signals. The level measurement device  10  finds application in a wide variety of process control situations and in various industries, such as the petroleum industry, water treatment, storage, and chemical industries.  
         [0017]    The level measurement device  10  shown in FIG. 1 comprises an ultrasonic transducer and includes a housing  20  with an emitter face  30 . The ultrasonic transducer  10  is coupled to a transceiver  100  (FIG. 2) through a conductor  40 . The conductor  40  may comprise a two wire arrangement which provides a link for receiving transmit pulses from the transceiver  100 , and a link for transmitting receive (i.e. echo) pulses to the transceiver  100 . The transducer  10  includes additional circuitry and elements, e.g. piezoelectric elements, and transformer (not shown), for generating the ultrasonic pulses and receiving the reflected pulses. The transceiver  100  includes electronic and programmable controlled circuitry for processing the echo signals and determining the level measurements, which techniques will be familiar to those skilled in the art, and as such are not part of the present invention.  
         [0018]    As shown in FIG. 2, the ultrasonic transducer  10  is mounted on an aperture  110  in the top of a storage tank  120 . The storage tank holds a material  130  having a level defined by a top surface indicated by reference  140 . The surface  140  of the material  130  serves to reflect the ultrasonic energy which is emitted by the transducer  10 . The ultrasonic transducer  10  may include a threaded collar  12  which is secured to the tank  120  by a nut  122  or other suitable fastener. Ultrasonic energy pulses are generated by the transducer  10  and transmitted through the emitter face  30  (e.g. a sealed rubber or stainless steel surface) towards the surface  140  of the material  130  contained inside the storage tank  120 . Reflected pulses from the surface  140  are sensed by the transducer  10  and transmitted to the transceiver  100  for further processing and to determine the level of the material  130  in the tank  120  by measuring the distance to the reflective surface  140 .  
         [0019]    The housing or enclosure  20  for the ultrasonic transducer  10  is preferably formed from a chemically resistant material such as Kynar™. When used in hazardous areas, such as in many petroleum industry applications, the ultrasonic transducer  10  or level measurement device must meet strict safety standards. In particular, the housing or enclosure  20  must not permit static charge to build up. In accordance with this aspect of the invention, the housing  10  comprises a polyvinylidene flouride, Kynar®, and a dispersion of carbon nanotubes. The carbon nanotubes reduce the surface resistivity of the housing  10 . With the reduced surface resistivity, the housing  20  provides a path to ground that prevents the build up of significant static charges and reduces the possibility of sparking. The surface resistivity of the housing  20  should be in the range of 10 9  Ohms, and preferably less than 10 9  Ohms for Level 1 Compliance.  
         [0020]    Advantageously, the small size of the nanotubes, which typically have a diameter of 10 to 20 nm, and their high aspect ratio, which can range from 5 to 1000, reduces the percentage by weight of the additive necessary to achieve the desired conductivity in the polymer. The reduced amount of additive necessary results in a lesser impact upon the desirable properties of the polymer. The nanotubes have almost no impact upon the strength and flexibility of the polymers and a less significant impact upon the melt index of the polymers. For instance, the melt index of Kynar®, which is normally 21, is reduced by a factor of about three, to 6.5, when prepared with a carbon nantotubes additive.  
         [0021]    Referring to FIG. 2, the installation for storage tank  120  and the transducer  10  may comprise a hazardous area  200  so designated because of the risk of explosion due to sparking. The transceiver  100  is located in a safe area  300  which is separated through an appropriate safety barrier  250 . If static, charge were to build up on the housing  20  for the transducer  10 , there is a possibility that the charge could arc causing a spark that may ignite any gases or dust in the surrounding environment. However, because the transducer  10  with the housing  20  according to the invention comprises a polymer with a dispersion of carbon nanotubes, the housing  20  has a reduced surface resistivity allowing any surface static charge to be dissipated through the storage tank  120  to ground, reducing the risk of sparking and explosion.  
         [0022]    While the present invention is described as a polymeric housing for process control equipment in the context of use in a hazardous area, the present invention has application to housing alternative kinds of equipment and to use in alternative areas. The range of equipment that may be housed and contexts in which the present invention may be used will be obvious to those skilled in the art. Additionally, different configurations of the level measurement device  10  and the enclosure or housing  20  and the internal components of the level measuring system will be understood by those skilled in the art.  
         [0023]    The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the above-discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.