MANUFACTURING METHOD OF ULTRA THIN HIGH TEMPERATURE RESISTANT POLYPROPYLENE DIELECTRIC FILM FOR CAPACITOR

Technical Field The present invention relates to a method of manufacturing a ultra-thin heat- resistant polypropylene dielectric film for a capacitor, and more particularly to a method

of manufacturing an ultra-thin heat-resistant polypropylene dielectric film of up to 4 #m,

which is superior in heat-resistance and electric characteristics and has low loss-value, capable of being used to manufacture the capacitor requiring high capacity, low weight and miniaturization in machines such as a hybrid automobile or a notebook computer. Background Art

A hybrid automobile refers to an environment-friendly automobile which is designed to be driven in 2 modes including electric driving mode and internal- combustion engine driving mode. The Such hybrid automobile can reduce air pollution to 70 to 90% and reduce a quantity of fuel used for the automobile at least 30% in Korea where crude oil is absolutely insufficient. Electricity-related components such as capacitor are required to be developed in order to realize the hybrid automobile.

A capacitor is an element of electric circuit made for the purpose of storing charge, and is basically consisted of electrodes and a dielectric sandwiched between electrodes. Electrode drawing terminals are connected to the electrodes and all of them are put in adequate mold or resin- molded. A value representing how much the charge can be stored refers to electrostatic capacitance of the capacitor.

The capacitor has such structure as shown in FIG. 1. FIG. IA shows a laminated capacitor, in which conductive layers 1 and insulating layers 2 are alternatively laminated and electrode-drawing terminals 3 are connected to the conductive layers 1. FIG. IB shows roller-type capacitor, in which long conductive layers 1 and insulating layers 2 are alternatively laminated and then totally rolled and electrode-drawing terminals (not shown) are connected to the conductive layers 1.

The electrostatic capacitance of the capacitor is proportional to a width of the electrode and a dielectric constant of the dielectric inserted between electrodes and reversely proportional to a distance between electrodes. Therefore, in order to increase the electrostatic capacitance per unit volume, methods such as selection of materials having high dielectric constant, use of thinner dielectric, and larger area of the electrode from study (laminated structure, concave-convex of the electrode surface) for the

structure are used.

If a direct voltage is applied to the capacitor, the charge is instantly stored in each electrode in accordance with polarity of the voltage applied and in proportion to the voltage and the electrostatic capacitance to be provided as current from power source. Therefore, a transient current is flowed into the circuit, but normal current is not flowed. Meanwhile, if an alternative voltage is applied to the capacitor, the current is continuously flowed into the electrodes of the capacitor since the instant storage phenomenon is changed by polarity change of the electrode. As a result, as the electrostatic capacitance becomes larger and a switching rate, i.e., a frequency, of the

electrode becomes higher, an amount of the charge which enters and exits the electrodes per a unit time becomes larger proportionally.

The capacitor developed and utilized as components for use in electric or electronic machine is used with various dielectric materials and generally manufactured as structure maintaining characteristic of the used materials. If categorizing the capacitor in terms of structure, there are 4 types including a roller-type capacitor in which belt type dielectric and electrode are overlapped and then rolled like a sheet capacitor, a plate-type capacitor in which silver electrodes are connected to both sides of magnetic dielectric of round plate-type or cylinder-type, a laminated capacitor in which the dielectrics and the electrodes are alternatively laminated like mica capacitor, and an electrolysis capacitor in which capacitance per unit volume is made larger by generating the dielectric film chemically. In addition, variable capacitor having a structure of variable capacitance is allowed to change the electrostatic capacitance continuously across a certain range. Up to now, since only film for use in general-purpose capacitor is produced considering ash contents within general polypropylene material, it is impossible to satisfy a physical property required for the ultra-thin heat-resistant polypropylene dielectric film of high performance for use in machines such as the hybrid automobile.

Further, according to prior method for manufacturing the dielectric film, a

minimum thickness of a polypropylene biaxial-oriented film is limited to 4.5 β"

(0.0045mm). Particularly, in a case of prior polypropylene dielectric film, there was a problem that can not be used as polypropylene dielectric film for manufacturing the

capacitor for use in the hybrid automobile due to lack of thermal stability. Since a thinner polypropylene dielectric film can be used without causing changes of the structure of the capacitor itself, a variety of studies and developments concerning it have been proceeded in that existing manufacturing facilities can be used as it were. Therefore, there is a need for developing the ultra-thin heat-resistant polypropylene film.

Disclosure of the Invention

It is an object of the present invention, to solve the above problems, to provide a method of manufacturing a ultra-thin heat-resistant polypropylene dielectric film of up

to 4 /λH which is superior in heat-resistance and electric characteristics and has low loss-

value, capable of being used to manufacture the capacitor requiring high capacity, low weight and miniaturization in machines such as a hybrid automobile or a notebook computer.

To achieve the object of the invention, a method of manufacturing an ultra-thin heat-resistant polypropylene dielectric film for use in capacitor according to the present invention comprises the steps of: (1) an extrusion-molding step of heating a polypropylene chip of raw material through at least 2 heating stages along its traveling

direction at temperature range of 170 to 250 ^° C in order to melt and extrusion-mold it,

thereby being molded as a sheet, wherein the temperature set at each of the stages has a

deviation of ±0.3 °C ; (2) a cooling step of cooling the sheet molded by the extrusion-

molding step at temperature range of 40 to 100°C ; (3) a preheating step of preheating the

sheet cooled by the cooling step through at least 2 preheating stages along its traveling

direction at temperature range of 110 to 140 ^° C, wherein the temperature set at each of

the stages has a deviation of ±0.3 ^° C ; and (4) a drawing step of drawing the sheet

preheated by the preheating step to 40 to 50 times at temperature range of 135 to 190°C

by means of successive biaxial drawing. Hereinafter, specific embodiments of the present invention will be described referring to accompanying drawings.

The method of manufacturing an ultra-thin heat-resistant polypropylene dielectric film for use in capacitor according to the present invention comprises the steps of: (1) an extrusion-molding step of heating a polypropylene chip of raw material through at least 2 heating stages along its traveling direction at temperature range of 170

to 250 ^° C in order to melt and extrusion-mold it, thereby being molded as a sheet,

wherein the temperature set at each of the stages has a deviation of ±0.3 ^° C ; (2) a cooling

step of cooling the sheet molded by the extrusion-molding step at temperature range of

40 to 100 ^° C ; (3) a preheating step of preheating the sheet cooled by the cooling step

through at least 2 preheating stages along its traveling direction at temperature range of

110 to 140 ^° C, wherein the temperature set at each of the stages has a deviation of

±0.3 ⁰ C ; and (4) a drawing step of drawing the sheet preheated by the preheating step to

40 to 50 times at temperature range of 135 to 190 ^° C by means of successive biaxial

drawing. The method for manufacturing the ultra-thin heat-resistant polypropylene dielectric film for use in capacitor according to the present invention is allowed to make

thinner a thickness of the dielectric film as one scheme for manufacturing the capacitor of high performance. According to the present invention, the polypropylene having high heat-resistance and high dielectric constant is processed to make a sheet and then drawn, preferably drawn by biaxial drawing, whereby it is possible to make the ultra-

thin film of 2 to 4 pn.

The polypropylene may preferably have the isotactic index of 98 to 99.5%. The related prior art uses the polypropylene having the isotactic index of 95 to 97% for manufacturing the dielectric film, whereas the present invention uses the polypropylene having the isotactic index of at least 98%, thereby producing an ultra-thin heat-resistant

polypropylene dielectric film. If the isotactic index is less than 98%, the material is too weak to resist the heat, as well as the mechanical property of the material is weak, and thus it is difficult to function as the capacitor when being used at high temperature. In the meantime, if it is more than 99.5%, it is impossible to produce the raw material itself, as well as the film. The polypropylene chip can preferably have ash contents of up to 50 ppm.

The ash contents of the polypropylene chip required for manufacturing the dielectric film are at least 100 ppm in a case of the prior art, whereas the present invention reduces the ash contents up to 50 ppm, thereby decreasing the dielectric constant and to increase charging degrees by making narrow the distance between the electrodes. Particularly, it is possible to manufacture the ultra-thin dielectric film with higher yield.

In the above-mentioned extrusion-molding step (1), the polypropylene chip of the raw material is heated, melted and extrusion-molded through 2 or more heating

stages along its traveling direction at temperature range of 170 to 250° to mold a sheet.

In this time, the temperature set in each of the stages has a deviation of ±0.3 °C .

A particle crystallization of the polypropylene chip can influence resultant property, and a structure and a size of the crystalline lattice depend on practical processing condition. When the crystalline polymer is melted, the polymer particles may lose some regular crystalline structure and cause disorder motion. If such polymer is cooled, a free motion of the particles is hindered due to attraction within the polymer, whereby growing to crystalline structure with regularity.

The polymer particles changed to solid state move and vibrate a little in only limited positions above glass transition temperature Tg (micro browning motion), and stop such motion under glass transition temperature, whereby having property similar to glass. The solidified polypropylene material is melted in an extruder temperature of

170 to 250 ^° C and the polypropylene resin is extruded very constantly and uniformly.

This suggests that temperature setting and adjustment are important factors to control the process.

The present invention involves setting the heating step for melting into at least 2

stages, setting the temperature at each of the stages and making a deviation of ±0.3 ⁰ C

from the set temperature.

In a case of heating the raw material without dividing the temperature setting step in at least 2 heating stages, the material is rapidly heated and therefore the crystalline structure of the material is rapidly destroyed, which results in causing problem similar to using the raw material having low crystalline degree.

The present invention can preferably divide the temperature setting step into 2 to 8 stages. If the temperature setting step includes more than 8 stages, the temperature difference between stages is reduced, but the heating line for melting becomes too long, which results in deteriorating the raw material as well as lowering productivity. Each of the temperature setting steps can be set such that the temperature set in each of the stages may become higher as the raw material gradually travels in longitudinal direction. It will be appreciated by the person skilled in the related art that the temperature set in each of the stages depends on the physical property of the polypropylene material and the temperature can be set in accordance with the polypropylene material inputted.

If the polypropylene material is heated less than 17O ⁰ C , the polypropylene

material is not sufficiently melted, causing problem of not being able to be properly molded as the sheet in following molding step. In the meantime, if it is heated more

than 250 °C , high molecule is flamed or decomposed, causing problem of reducing the

thermal characteristics and mechanical property.

Particularly, the extrusion-molding step includes a melting step in which the polypropylene chip of raw material is heated and melted through at least 2 stages along

its traveling direction at temperature range of 200 to 250 ^° C - the temperature set at each

of the stages has a deviation of ±0.3 °C . - , and an extrusion step in which the raw

material melted by the melting step is heated through at least 2 stages along its traveling

direction at temperature range of 170 to 225 °C - the temperature set at each of the stages

has a deviation of ±0.3 ^° C . The present invention can preferably divide the melting step

into 2 to 4 heating stages. If the temperature setting step includes more than 4 stages, a temperature difference between the stages is reduced and heating line for melting becomes too long, which results in lowering the productivity. Each of the temperature setting stages can set the temperature at each of the stages to be higher, as the raw material gradually travels in longitudinal direction. It will be appreciated by the person skilled in the related art that the temperature set at each of the temperature setting stages depends on the physical property of the polypropylene material and the temperature can be set in accordance with the polypropylene material inputted.

Further, the present invention can preferably divide the temperature setting step in the extrusion step into 2 to 4 stages. If the temperature setting step includes more

than 4 stages, the temperature difference between stages is reduced and the heating line for melting becomes too long, which results in deteriorating the raw material as well as lowering productivity.

Each of the temperature setting stages can be set such that the temperature set at each of the stages be higher as the raw material gradually travels in longitudinal direction. It will be appreciated by the person skilled in the related art that the temperature set in each of the stages depends on the physical property of the polypropylene material and the set temperature can be properly set in accordance with the polypropylene material inputted.

The above cooling step (2) is performed by cooling the sheet molded by the

extrusion-molding step to 40 to 100°C .

In order to determine optimum crystallization degree of the polypropylene in accordance with molding conditions, the temperature setting for cooling among manufacturing conditions is important. A method for detecting the crystallization degree is performed by detecting a value such as specific volume, specific heat, and X- ray diffraction strength. Among them, a detection of the specific heat is generally used using Differential Scanning Calorimeter (DSC). However, the structure of the polypropylene can not be defined only by the crystallization degree itself. The noncrystalline resin has its rigidity to be rapidly decreased at a temperature more than a glass transition temperature, whereas the crystalline resin maintains considerable rigidity even between the glass transition temperature and a melting temperature Tm. This is due to a difference between molecule structures, and because denser the crystalline structure, stronger Van der wal's force, which results in consuming more energy to move high molecular chains. This means that more energy is required to move a high crystalline polymer since the polymer is in a state of high-level energy.

The crystalline structure of the polypropylene is considered that a series of crystallization is surrounded by non-crystalline material. One molecule of the polypropylene can be one part of various crystalline structures and chains not participated in the crystallization have non-crystalline structure. The polymer crystallization of the polypropylene is consisted of plate-shaped lattices (lamella), and a plurality of such plate-shaped lattices is overlapped to form the crystalline lattice having a thickness of 100 to 500 A .

Generally, one molecular chain participating in one-hand crystallization part may participate in another-hand part so that plate-shaped lattices are connected with each other by polypropylene high-molecular chains. Such chains in connecting portion are irregular so that they may not form the crystallization. When the polypropylene is drawn, such non-crystalline molecules are oriented to change the physical property. In case of rapid cooling, a mobility of the molecular chains is suppressed to hinder the crystallization growth so that the crystallization degree becomes low. Therefore, the molecular chain requires time and temperature enough to form high-mobility crystallization in order to promote crystallization growth. That is, the crystallization degree becomes higher on slow-cooling at near the melting point. Moreover, when being extruded in high pressure, the mobility of the molecular chains is suppressed by the pressure to reduce the crystallization degree, which results in lowering the density (shrink ratio).

If the polypropylene is cooled, the crystallization is initiated from crystalline nuclei, in which materials serving as the crystalline nuclei are additives or clotted chain molecules. The crystalline nuclei initiating the crystallization growth is consisted of plate-shaped lattices, and grown in each direction to compose spherical-shaped

crystalline lattice (spheroid) having a size of 1 β® to several ywn. The size and shape of

the crystalline lattice largely influences the physical property and the roughness of the polypropylene, in which the growth of the crystallization is initiated from the crystalline nuclei when the polypropylene is cooled at near the melting point and the growth of the crystalline lattice is continued until the polymer is cooled so that its mobility is vanished.

The velocity of the crystallization influences the size of the crystalline lattice and depends on changes of the temperature. The growth velocity of the crystalline nuclei increase as the temperature becomes low, whereas the generation velocity of the crystalline lattice increases and then decreases as the temperature becomes low. In a case of raw material having slow crystallization velocity such as isotactic polypropylene (iPP), even though the crystallization can be suppressed by water cooling, it must be slowly cooled in order to improve characteristics of the internal voltage and the insulator, to reduce a size of β-crystallization and to make large amount of β-crystallization.

The cooling process is to make the β-crystallization by causing the polypropylene resin flowed out from the extrusion process to pass through cooling

medium of 40 to 100 ^° C In this process, a thickness of the sheet, a temperature of the polypropylene resin, and a temperature of the cooling medium are most important, in which the sheet has a thickness of 0.1 to 2mm, the temperature of the polypropylene

resin is 200 to 300 ^° Q and the cooling medium utilizes a method such as air cooling method, water cooling method and contact cooling method using metal roll. Particularly, the β-crystallization formed when the polypropylene resin is melted and then cooled is a key point which ensures giving the roughness to the surface constantly in order to protect the metal film on depositing and improve the internal voltage of the polypropylene film while giving only characteristics of the polypropylene film. If the polypropylene film is used as the dielectric of the capacitor, it is subject to changes due to heat until completion of the capacitor and mechanical forces caused by the depositing process and the winding process. Particularly, the ultra-thin

polypropylene biaxial-oriented film for use in the capacitor in the hybrid automobile must be not changed due to heat and mechanical force. Further, the capacitor in the hybrid automobile must be ensured to stabilize heat from outside and heat from the capacitor itself and make safe the electric characteristics and loss due to the storage of electricity (loss of the electrostatic capacitance). It is necessary to use the heat-resistant polypropylene material in order to improve the thermal characteristics and the mechanical strength. The β-crystallization is not generated if the sheet is cooled in rapid cooling at temperature less than 40 ^° Q whereas the size of the β-crystallization becomes larger and the surface roughness becomes higher above standard value if the

sheet is cooled at temperature more than 100 ^° C which results in a problem of lowering

the internal voltage of the capacitor.

Particularly, the cooling step is performed by dividing a top surface and a bottom surface of the sheet, in which the bottom surface is cooled at temperature range

of 60 to 100 ^° Cby contact with roll and the top surface is cooled at temperature range of

40 to 80 ^° Cby supply of heated wind.

In the above preheating step (3), the sheet cooled in the cooling step is

preheated at temperature range of 110 to 140 ^° Qhrough at least 2 preheating stages along

its traveling direction, in which the temperature set in each of the stages has a deviation

of ±0.3 °C The sheet is allowed to be drawn more evenly by controlling the temperature of the sheet in following drawing step, after preheating the sheet evenly in the preheating step.

The resulting film has a problem of having a irregular thickness in drawing

process if the sheet is preheated at temperature less than 110 "Q whereas it has a problem that it is difficult to generate the β-crystallization so that the crystallization is broken if

the sheet is preheated at temperature more than 140 ^° C

The present invention can preferably divide the temperature setting step in the preheating step into 2 to 4 stages. If the temperature setting step includes more than 4 stages, a temperature difference between the stages is reduced and the heating line for melting becomes too long, which results in lowering the productivity as well as optimum quality for the capacitor film.

In the above drawing step (4), the sheet preheated by the preheating step is

drawn 40 to 50 times by successive biaxial drawing at temperature range of 135 to 190 ^° C

The drawing is for the purpose of achieving high physical property and good optical property. FIG. 2 through FIG. 4 show variations of the oriented state of the molecule due to the biaxial drawing.

The drawing of the polypropylene means solid-state orientation, and is mainly performed after being melted and molded. It is advantageous to orientation, since it easy to form a tie molecule between plate-shaped lattices of the crystallization as an amount of molecule of the polypropylene becomes higher. The variation of the physical property caused by the orientation leads to improvements of stiffness, strength, internal voltage characteristics, and insulating property. As shown in FIG. 2 to FIG.4, the drawing is to stretch the film in desired direction before and after the melting point of the polypropylene, in which an draw ratio and an drawing temperature depend on molecular weight and distribution in molecular

weight of the polypropylene and the crystallization degree (%). The molecular orientation is realized by which the crystalline plate-shaped lattices are primarily connected by arranging them in parallel in tensile direction and then the crystallization is secondly obtained.

Particularly, the drawing step comprises a drawing performed in longitudinal

direction by the roll at temperature range of 135 to 141 ^" Qand a biaxial oriented drawing

performed in width direction by the wind pressure at temperature range of 155 to 190°C

If the temperature in the drawing of the longitudinal direction is less than 135 ^° Q there can be a problem of the drawing not being performed in the longitudinal direction. Contrary, if it is more than 141 ^" C the sheet becomes too soft or melted to be drawn.

Further, if the temperature in the drawing of the width direction is less than 155 ^° C there

can be a problem of the drawing not being performed in the width direction. Contrary,

if it is more than 190 ^° Q there can be a problem of the drawing being not performed regularly and thus thickness variance in width direction and quality variance in film being caused.

A Table 1 shown below shows the mechanical property and the internal voltage characteristics depending on the properties and the draw ratio of the raw material.

[Table 1]

\ . Process ability | S | >" | \ N,

The polypropylene chip has preferably the isotactic index of 98 to 99.5%. The present invention is allowed to use the polypropylene chip having the isotactic index of 98 to 99.5% as compared to the prior art using the isotactic index of less than 98%, and to manufacture the ultra-thin dielectric film for use in capacitor composed of the polypropylene which is superior in heat-resistance by controlling the temperature finely during the extrusion-molding step and the drawing step. This is not achieved by using the raw material of high isotactic index, but achieved by controlling the heat applied during the extrusion-molding step and the drawing step finely, preferably at the set

temperature±0.3 ^° C

The polypropylene chip has preferably ash contents of up to 50ppm. The present invention is allowed to use the polypropylene chip having the ash contents of up to 50ppm as compared to the prior art using the ash contents of less than 100 ppm, thereby manufacturing the ultra-thin dielectric film of high yield. The method for manufacturing the ultra-thin heat-resistant polypropylene dielectric film for use in capacitor according to the present invention further comprises a winding step of winding the ultra-thin polypropylene dielectric film to form air-layer containing air of 5 to 10% between films. Due to formation of the air-layer, it is possible to prevent the heat-resistant polypropylene dielectric film from being deformed by force such as deformation force applied.

Moreover, the method for manufacturing the ultra-thin heat-resistant

polypropylene dielectric film for use in capacitor according to the present invention further comprises an aging step of aging the ultra-thin polypropylene dielectric film

wound by the winding step by leaving it at a room temperature of 35±2 ^° Cfor 72 to 80

hours. The heat-resistant polypropylene dielectric film manufactured by the aging step can have its molecular structure stabilized so that it may not be deformed during the process or after the process.

Brief Description of the Drawings

FIG. 1 is a schematic diagram showing a structure of typical capacitor; FIG. 2 to FIG. 4 are schematic diagrams showing variances of the orientation

state of molecule caused by biaxial oriented drawing of the polypropylene film: FIG. 2 shows a non-drawn state; FIG. 3 shows a mono-axially oriented drawn state; and FIG.4 shows a biaxially oriented drawn state.

FIG. 5 is a photograph showing a shape of crystallization, which is to image a surface of the polypropylene film by means of electron microscopy. FIG. 6 is a photograph showing a shape of β-crystallization, which is to magnify the photograph of FIG. 5 to 50 times.

Best Mode for Carrying Out the Invention

Hereinafter, preferred embodiments and comparative embodiments of the present invention are illustrative as shown in the following Examples. However, it will be appreciated that those skilled in the art, on consideration of this disclosure, may make modifications and improvements within the spirit and scope of the present invention.

Embodiment 1

(1) The raw materials in Table 2 shown below are used as the film of 3 βn and

4 μm on which the melting process in Table 3 shown below is performed.

More specifically, the polypropylene chip of raw material is heated through 3 heating stages along its traveling direction at temperature range of 170 to 250 °C so that it may be melted and extrusion-molded to mold the sheet, in which the temperatures set at each of the stages has a deviation of ±0.3 ^° C .

[Table 2]

(Raw material)

[Table 3] (Melting process)

(2) The sheet molded by the extrusion-molding step is cooled at temperature of air cooling and roll contact cooling in Table 4 shown below.

[Table 4] (Cooling process)

(3) The sheet cooled by the cooling step is preheated through 3 stages preheating stages along its traveling direction at temperature range of 110 to 140 ^° C as shown in Table 5, in which the temperatures set at each of the stages has a deviation of ±0.3 ⁰ C .

[Table 5]

(Preheating process of the sheet)

(4) The sheet preheated by the preheating step is drawn 40 to 50 times by means of the successive biaxial drawing at temperature range of 135 to 190 ^° C, thereby obtaining the ultra-thin heat-resistant polypropylene dielectric film.

A Table 6 shown below illustrates comparison results of thermal shrink ratio test between the ultra-thin heat-resistant polypropylene dielectric film obtained in the first

embodiment and the prior polypropylene dielectric film (4.5 m existing film) obtained

by using the polypropylene chip similarly to the present invention.

[Table 6]

(KSC2374 6.3.4 thermal shrink ratio test)

Further, a Table 7 shown below illustrates comparison results of various the

physical property between the film of 4 m in the first embodiment and the existing

polypropylene dielectric film (existing film of 4.5 m).

[Table 7]

As shown in each of the above Tables, it will be appreciated that the ultra-thin heat-resistant polypropylene dielectric film manufactured according to the present

invention is thinner in thickness, and considerably improving in terms of tensile strength, thermal shrink, surface roughness, insulation defects and isotactic index, as compared with the existing dielectric film. It is expected that thinner the thickness of the dielectric in the capacitor, higher the electrostatic characteristic. Subsequently, the capacitor using the dielectric film according to the present invention can have higher electrostatic capacitance as compared with the capacitor using the existing insulating film, even though other factors are all same.

FIG. 5 (surface photograph) and FIG. 6 (50 times magnified photograph of it) show the surface of the polypropylene sheet (film) manufactured by the cooling process according to the present invention by means of electron microscopy, by which it can be confirmed that the crystalline structure, particularly β-crystallization is well formed, thereby improving thermal characteristics.

Industrial Applicability

According to the present invention, it is possible to provide the method for

manufacturing the ultra-thin heat-resistant polypropylene dielectric film of up to 4 μm

which is superior in heat resistance and electric characteristics and has a low loss-value.

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.