A lightweight, electroconductive foam comprising an effective amount of a conductive ionic salt, a polymer capable of complexing said conductive ionic salt and an effective amount of particulate conductive material such as carbon black or metal is disclosed. Additionally, a method of preparing a lightweight electroconductive foam having a surface resistivity less than about 10.sup.10 ohms per square by an extrusion process is disclosed. Foams of the present invention exhibit relatively high conductivities yet require only relatively low amounts of particulate conductive material.

BACKGROUND OF THE INVENTION 
This invention relates to foamed, lightweight, electrically conductive, 
polymeric materials. Electroconductive foams have widespread application 
in the packaging of electronic devices due in part, to the ability of such 
foams to dissipate static electricity. As electronic circuitry is 
miniaturized, it becomes increasingly susceptible to damage from 
electrostatic discharge (ESD) since the level of voltage which may 
permanently impair or destroy circuitry decreases as the physical size of 
circuitry is reduced. Thus, the range of voltages which may damage 
circuitry is now typically in the realm of voltages associated with ESD. 
Damage from ESD has been estimated to cost the electronics industry 
billions of dollars annually, and is expected to increase as further 
circuitry miniaturization occurs. 
There are primarily two mechanisms by which materials conduct electricity; 
ionic conduction and metallic conduction. Typical metallic conductors 
include metals (e.g. in the form of wire, films or fibers) and conductive 
carbon black. Metallic conduction requires the presence of an electrically 
conductive pathway through the material. Continuity is a critical factor 
in establishing metallic conduction. That is, physical contact or very 
near proximity between the conductive particles must occur for electrons 
to pass through the material. Thus, in a polymer matrix loaded with carbon 
black particles, the particles must touch or nearly touch one another in 
order to provide an electrically conductive pathway through the material. 
Prior artisans have utilized foams containing electrically conductive 
particles such as carbon black dispersed throughout the foam. However in 
order to obtain an adequately conducting foam, carbon black concentrations 
in the range of 10 to 25 percent by weight (based upon the total weight of 
the foam) are often required. Carbon black loadings up to 40 percent and 
higher have even been described as in U.S. Pat. Nos. 4,231,901 to Berbeco 
and 4,481,131 to Kawai et al. It is only at such high concentrations that 
the particles contact one another or are sufficiently close to one another 
to provide an electrically conductive pathway through the foam matrix. 
It is not desirable to have such high concentrations of conductive 
particles in foams for several reasons. First, the higher the 
concentration of particles in the foam, the greater the cost of materials 
and processing. Second, when attempting to foam polymeric resins 
containing such high particle concentrations, it is difficult to extrude 
the resin due to the resin's poor melt viscoelasticity and the tendency 
for particle agglomeration. Third, the resulting foams have relatively 
high densities rendering them undesirable for packaging and shipping 
applications. Fourth, the particles near the surface of these foams tend 
to slough from the foam surface during fabrication and handling, thereby 
increasing the risk of contamination of electronic devices if the foam is 
used for packaging or in the vicinity of sensitive components. 
The second mechanism by which a material may conduct electricity is ionic 
conduction. These systems rely on ionic charge carriers for electron 
transfer, and as such the charge carrier population, capacity, and 
velocity are critical factors which affect the conductivity of the 
material under consideration. Moreover, many of these factors are further 
dependent upon other criteria. For instance, the population or 
concentration of charge carriers depends upon the extent of dispersion, 
distribution and solubilization of the particular ionizable compound(s) in 
the host material. In addition to the complexity and unpredictability of 
ionic conduction, such systems are much slower than metallic systems since 
electron transfer occurs via ionic carriers as opposed to the near speed 
of light displacement of electrons along the conductive pathway in 
metallic conduction systems. 
An example of ionic conduction in polymeric materials is the application of 
topical treatments to the outer surface of the polymeric material, or the 
use of additives which migrate to the material surface to provide 
electrical conductivity on the surface or skin of the material. Examples 
of such surface active additives include quaternary ammonium salts, or 
other fatty amines, glycols, and sulfonates. For systems of this type, the 
conductivity properties as measured along the outer surface of the 
polymeric material are often very good. However, such surface active 
additives do not affect the volume resistivity of the material, i.e. the 
conductivity as measured across a cross section of the material. Moreover, 
foams having such surface active additives suffer from a variety of 
drawbacks such as; the conductivity of the foamed material tends to 
decrease over time, the conductivity is often significantly dependent upon 
humidity, the degree of conductivity is typically nonuniform, and the foam 
tends to be corrosive to sensitive electronics due to the presence of the 
additive(s). 
Some foams contain a hygroscopic antistatic additive which functions to 
reduce surface resistivity by migrating to the foam surface and attracting 
moisture from the surroundings. Antistatic properties of the foam skin are 
excellent, however the conductivity as measured across a cut surface of 
the foam is only marginal. Since moisture is one of the essential 
components in forming a thin electrolyte layer on the material outer 
surface, antistatic foams made with the additive may perform poorly at low 
relative humidity. Additionally, the additive may cause contamination of 
adjacent devices or materials and be incompatible with some polymeric 
resins. 
Prior artisans have attempted to avoid many of the problems encountered in 
the prior art associated with ionic conduction systems by utilizing 
complexes of ionizable salts and oxygen-containing polymeric materials to 
achieve electrical conductivity, such as described in U.S. Pat. Nos. 
4,617,325 and 4,618,630 to Knobel et al., assigned to the Dow Chemical Co. 
and 4,359,411 to Kim et al. Although such compositions generally provide 
improved electrical conductivity and moisture dependency, such 
compositions do not exhibit surface resistivities of less than 10.sup.10 
ohms per square. 
Thus, the need exists for a lightweight foam which has a surface 
resistivity less than 10.sup.10 ohms per square, and which has a 
relatively low concentration of conductive particles thereby avoiding the 
problems experienced with prior art compositions containing relatively 
high concentrations of conductive particles such as relatively high 
material and processing costs, difficult manufacturing aspects, relatively 
high densities even after foaming, and detrimental sloughing of conductive 
particles from the foam surface. 
Moreover, it has been found that it is difficult if not impossible to 
produce foams having large cross-sectional areas by extrusion processes if 
the resin contains a relatively high concentration of carbon black 
particles. Thus, the need exists for a method of producing an 
electroconductive foam having a surface resistivity of less than 10.sup.10 
ohms per square by an extrusion process. 
In addition, the need exists for an electroconductive foam which avoids 
many of the problems encountered by prior artisans when utilizing ionic 
conduction systems in foams such as decreasing conductivity over time, 
significant dependence of conductivity upon humidity, nonuniform 
conductivity, and corrosiveness of such foams due to the relatively high 
levels of additives in the foams. 
SUMMARY OF THE INVENTION 
The lightweight eletroconductive foam of the present invention comprises an 
effective amount of a conductive ionic salt, a polymer capable of forming 
a complex with the conductive ionic salt, and an effective amount of 
particulate conductive material. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, from about 5 to about 15 percent by weight of 
a conductive ionic salt and from about 5 to about 10 percent by weight of 
conductive carbon black, conductive metal or mixtures thereof are blended 
with a polymer capable of complexing the salt and optionally further 
blended with one or more additional polyolefins, and expanded to produce a 
lightweight, electroconductive foam. 
The preferred embodiment foam of the present invention has a density of 
between about 0.6 pcf (pounds per cubic foot, 9.61 Kg/m.sup.3) to about 
12.0 pcf (192.2 Kg/m.sup.3) and exhibits a surface resistivity of less 
than about 10.sup.10 ohms. The phrase "surface resistivity" as used herein 
refers to the resistance to the flow of electricity as measured between 
opposite sides of a square on the surface of a sample. The value when 
expressed in ohms is independent of the size of the square and the 
thickness of the surface film. The surface resistivity values as described 
herein are measured in accordance with ASTM test method D257. 
The present invention utilizes a polymer which is capable of complexing the 
conductive ionic salt. In particular, this polymer is one in which there 
is a polarity or charge separation across the molecule or portions of the 
molecule. Although not wishing to be bound to any particular theory, it is 
believed that when the conductive ionic salt is dissolved in a suitable 
medium containing a polymer capable of complexing the salt, the salt 
dissociates and the salt cations migrate toward and are retained by the 
portion or group of the polymer having a negative charge. The salt anions 
are then relatively free to function as charge carriers and transfer 
electrons from one location in the medium to another. By utilizing charge 
carriers, the concentration of the particulate conductive material may be 
significantly reduced while still achieving the same static electricity 
dissipation characteristics. The mobile charge carriers are believed to 
transfer electrons between the relatively stationary, conductive 
particles. In the absence of these charge carriers, much higher 
concentrations of particulate conductive materials are necessary so that 
the distances between neighboring particles are within the range of direct 
electron transfer between conductive particles. Thus, the function of the 
complexing polymer is at least twofold. First, the complexing polymer 
induces dissociation of the ionic salt. Secondly, once the salt has 
disassociated into its respective ions, the negatively charged groups or 
portions of the polymer attract the salt cations and form a relatively 
stable complex. In many instances and as described in greater detail 
below, the medium for dissolving the salt may be comprised of entirely the 
complexing polymer or blends of the complexing polymer and other polymeric 
materials. 
The preferred polymer for complexing the conductive ionic salt is a 
copolymer of ethylene and carbon monoxide (herein referred to as ECO). 
Typical amounts of the carbon monoxide group in the ECO copolymer may 
range from about 1 to about 45 mole percent and preferably from about 10 
to about 20 mole percent of the ECO copolymer. The ECO copolymer is 
preferred since it readily complexes with the ionic salt. Commercially 
available ECO copolymers are sold under the designations ECO XU 60766.02L 
(10 mole percent CO) by Dow Chemical Co. of Midland, Mich. and ECO 
E-36017-139 (15 percent CO) by Du Pont de Nemours, E. I. & Co. of 
Wilmington, Del. 
In the case of an ECO copolymer, the polarity of the polymer primarily 
results from the CO group of the polymer having a negative charge relative 
to the remaining portion of the molecule. When a conductive ionic salt is 
dissolved in the polymeric medium, the positively charged salt cation is 
attracted to one or more CO groups of the polymer thereby forming a 
complex. As a result of attraction between the oppositely charged species, 
the salt cation is generally retained by the CO group. Depending upon the 
salt, a complex between the salt cation and one or more CO groups may be 
formed, often involving CO groups from adjacent polymer molecules. The 
free salt anion is believed to function as a charge carrier and transfer 
electrons between neighboring particles of conductive material. These 
charge carriers in essence, provide a bridge or electrical pathway between 
the conductive particles. 
In addition to or in place of ECO other polymers containing polar groups 
such as ethers, esters, amides and urethanes which are capable of forming 
complexes with the ionic salts described herein are envisaged for use in 
the present invention. Polyvinyl chlorides and aldehydes may also be 
operable as ionic salt complexing polymers. In addition to a polymer which 
is capable of forming a complex with the ionic conductive salt, one or 
more polyolefins may be used in the resin to be foamed. Since these other 
polyolefins do not necessarily have to aid in complexing the salt, they 
may be selected in view of their properties and effect upon both the resin 
and resulting foam. Examples of polyolefins which may be used in 
conjunction with the polymer capable of complexing the ionic salt include 
low density polyethylene, medium and high density polyethylene, 
polypropylene, polybutene-1, a copolymer of ethylene or propylene and 
other copolymerizable monomer, for example, propyleneoctene-1-ethylene 
copolymer, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, 
ethyleneacrylic acid copolymer, ethylene-ethyl acrylate copolymer and 
ethylene-vinyl chloride copolymer. In addition ionomer resins (generally 
comprising a copolymer of ethylene and a vinyl monomer with an acid group) 
may be utilized. An example is SURLYN 8660, available from Du Pont. Other 
polyolefins may also prove useful in the preferred embodiment resins. 
One factor guiding the selection of the choice of polymer or polymers for 
use in the present invention is the melt index of the resulting polymeric 
resin. The "melt index" is the viscosity of a thermoplastic polymer at a 
specified temperature and pressure and is a function of the molecular 
weight of the polymer. Specifically, the melt index is defined as the 
number of grams of a particular polymer that can be forced through a 
0.0825 inch (0.209 cm) orifice in 10 minutes at 190.degree. C. by a 
pressure of 2160 grams. ASTM D1238 describes measuring the flow rate or 
melt index of a material. The preferred polymeric resin for use in the 
present invention should have an overall melt index of from about 0.5 to 
about 5.0 and preferably from about 1.0 to about 3.0. It has been found 
that such melt index values may generally be obtained by employing a blend 
of an ECO copolymer having a melt index of from about 0.3 to about 20.0, 
and one or more other polymers such as polyethylene, having a melt index 
of from about 0.3 to about 7.0. 
The ionic salt for use in the preferred embodiment may be nearly any 
conductive ionic salt that is compatible with the polymeric resin 
selected. The preferred ionic salt for use in the present invention is 
sodium tetraphenylboron (STPB) (also known as sodium tetraphenylborate), 
available from Aldrich Chemical Co., Inc. of Milwaukee, Wis. Other 
suitable salts include FC-124 FLUORAD and FC-98 FLUORAD, available from 
3M of St. Paul, Minn. FC-124 is a lithium perfluoroalkane sulfonate salt 
and FC-98 is a potassium perfluoroalkyl sulfonate salt. It may be 
desirable to utilize these salts in relatively high temperature 
applications as they are more stable than STPB at elevated temperatures. 
Representative examples of other ionic salts for use in the present 
invention include sodium thiocyanate, sodium trifluoromethanesulfonate and 
lithium trifluoromethanesulfonate. It is envisioned that other salts 
having lithium, potassium, sodium or other analogous cations may be 
utilized in the present invention so long as the salt selected has a bulky 
anion to complex with the ECO copolymer. 
The amount of ionic salt present in the foam of the preferred embodiment 
may be any amount which, when taken in conjunction with the concentration 
of CO units of the ECO copolymer and the particulate conductive material, 
renders the foam sufficiently conducting to thereby dissipate static 
electricity. This amount of ionic salt is referred to herein as "an 
effective amount". The preferred concentration range of the ionic salt 
added to the polyolefin resin may vary from about 0.5 to about 15 percent 
and most preferably from about 6 to about 10 percent by weight based upon 
the total foam weight. The amount of ionic salt added depends upon the 
physical and conductive properties desired for the foam product. Usually, 
increasing the concentration of the ionic salt decreases the amount of 
conductive carbon black particles required to achieve the same degree of 
conductivity. 
The particulate conductive material may be nearly any electrically 
conductive material, preferably in particulate form. The preferred 
conductive material is conductive carbon black of any suitable grade. The 
most preferred conductive carbon black is KETJEN Black 300J and 600J 
available from Akzo Chemie America of Chicago, Ill. Other suitable types 
of commercially available electrically conductive carbon black include 
VULCAN XC-72R available from Cabot Corp. of Boston, Mass. and CONDUCTEX SC 
from Columbian Carbon Co. of Atlanta, Ga. The amount of particulate 
conductive material present in the foam of the preferred embodiment may be 
any amount which, when taken in conjunction with the concentration of CO 
units of the ECO copolymer and ionic salt, renders the foam sufficiently 
conducting to thereby dissipate static electricity. This amount of 
particulate conductive material is referred to herein as "an effective 
amount." The amount of carbon black added to the polymeric resin is 
preferably from about 5 percent to about 10 percent by weight of the total 
foam weight. Greater or lesser amounts of conductive carbon black may be 
utilized depending upon the degree of conductivity desired for the foam 
and the amount of ionic salt used. 
Other conductive particles may be used in addition to or in place of the 
carbon black. Examples of such other conductive particles include finely 
divided metal particles, such as silver, aluminum and salts thereof such 
as aluminum silicate. Although it is preferred to utilize conductive 
carbon black in the form of finely divided particles, it is further 
envisaged that this conductive component could be in the form of strands, 
fibers, or flakes dispersed or distributed throughout the foam matrix. 
Accordingly, the same is envisaged for other conductive materials besides 
carbon black such as metal or salts thereof as described above. 
In addition to the above mentioned components, other components may be 
added to the foamable resin such as pigments, polymerizing agents, 
stabilizers, antioxidants, antimicrobials, flame retardants, fragrances, 
impact modifiers, lubricants, platicizers and colorants. Moreover, the 
present inventor envisages that conductivity enhancers such as KENAMIDE 
S180 (stearylstearamide), available from Humko Chemical Div., Witco 
Chemical Corp. of Memphis, Tenn., may be added to the polymeric resin in 
accordance with U.S. Pat. No. 4,431,575 to Fujie et al., assigned to Dow 
Chemical Co. 
Once the components are added together and uniformly mixed, the composition 
may be foamed by conventional methods to produce either an open or a 
closed cell foam. For instance, there can be employed a continuous 
extrusion method wherein the resin composition of the present invention is 
heated and melted, a blowing agent is blended into and admixed with the 
molten resin composition at an elevated temperature and the resulting 
foamable blend is extruded to a low pressure zone for foaming. 
Alternatively, a batch type method may be employed wherein a blowing agent 
is added to the resin composition at an elevated temperature under high 
pressure and the pressure is reduced for foaming. 
Extrusion foaming is preferred since such process generally allows 
formation of products having larger cross sections than other comparable 
processes. The present invention may in addition enable the practitioner 
to utilize particular extrusion foaming processes, many of which are not 
suitable with resins containing high concentrations of carbon black. The 
resulting electroconductive foams preferably have densities of from about 
0.6 pcf to about 12.0 pcf. The preferred cell size of the foams of the 
present invention is from about 0.7 to about 2.5 millimeters. 
The blowing agent for use in foaming the resin composition of the present 
invention is an ordinary chemical blowing agent or a volatile blowing 
agent. Preferably, a volatile organic blowing agent is recommended and 
there may be used any one or more having a boiling point lower than the 
melting point of the polymeric resin. Typical blowing agents include lower 
hydrocarbons such as propane, butane, isobutane, pentane, hexane, and 
halogenated hydrocarbons such as methylene chloride, methyl chloride, 
trichlorofluoromethane (CFC 11), chlorofluoromethane (CFC 22), 
dichlorofluoromethane (CFC 21), chlorodifluoromethane, tetrafluoromethane 
(CFC 14), chlorotrifluoromethane (CFC 13), dichlorodifluoromethane (CFC 
12), 1,1-difluoroethane (HFC 152a), 1-chloro-1,1-difluoroethane (HFC 
142b), 1,1,2-trichloro- 1,2,2-trifluoroethane (CFC 113), 
1,2-dichloro-1,1,,2,2-tetrafluoroethane (CFC 114) and 
monochloropentafluoroethane. A mixture of any of the above is also useful. 
As chemical blowing agents, representative examples include 
azodicarbonamide, paratoluenesulfonylhydrazide and the like. Also a 
combination of a chemical blowing agent and a volatile organic blowing 
agent can be used, if desired. 
A measure of an electroconductive material's ability to dissipate ESD, in 
addition to surface and volume resistivities, is the material's static 
decay rate. The static decay rate is the amount of time required for an 
electrically grounded sample of a material to dissipate a static charge 
induced on the surface of the sample. In regards to the present invention, 
the shorter the time required, the better the ability of the foam to 
dissipate the charge, and the more conductive the polymer. The static 
decay rate as described herein is measured according to Federal Test 
Method Standard No. 101C, Method 4046.1. 
Various enhancers may be added to the composition of the present invention, 
most particularly to improve the static decay rate. Examples of such 
enhancers may include POLYMEG 650 (polytetramethylene ether glycol) 
available from QO Chemicals, Inc. of Des Plaines, Ill., DBEEA (dibutoxy 
ethoxy ethyl adipate) available from CP Hall Co. of Chicago, Ill., and 
TEGMER 804 (tetraethylene glycol di-2-hyphenethylhexoate) available from 
CP Hall Co. 
The following foamed compositions were prepared and various measurements 
taken, thereby illustrating the benefits and advantages of the present 
invention. Examples 1-3 illustrate conventional foams not part of the 
present invention, comprising conductive carbon black and an absence of an 
ionic salt. One embodiment of the present invention is described in 
Example 4. Table I. below illustrates the respective electroconductive 
foam formulations of Examples 1-4 and their corresponding properties. 
TABLE I 
__________________________________________________________________________ 
ELECTROCONDUCTIVE FOAM FORMULATION 
KETJEN Density 
Static Decay 
Surface 
Open 
Black.sup.1 
SURLYN.sup.3 
ECO.sup.4 CFC 114.sup.5 
(cured) 
Rate Resistivity 
Cell 
Ex. 
600J PE4005.sup.2 
8660 XU 60766.02L 
STPB 
phr pcf Sec. ohm/sq. 
% 
__________________________________________________________________________ 
1 7.5 30 62.5 -- -- 25 2.80 fail 1.98 
.times. 10.sup.14 
76.6 
2 8.6 34.4 57 -- -- 25 4.07 0.01 1.38 
87.3es. 10.sup.1 
3 10 40 50 -- -- 25 3.72 0.01 6.85 .times. 10.sup.7 
92.8 
4 7.5 29.8 36.8 25.4 0.5 25 2.96 0.01 1.18 .times. 10.sup.8 
91.7 
__________________________________________________________________________ 
.sup.1 Conductive carbon black from Akzo Chemie America. 
.sup.2 Low density polyethylene available from Dow Chemical. 
.sup.3 Ionomer resin available from Du Pont Co. 
.sup.4 Ethylene carbon monoxide copolymer (10 percent CO) available from 
Dow Chemical. 
.sup.5 Blowing agent, 1,2dichloro-1,1,2,2-tetrafluoroethane.

EXAMPLE 1 
A foamable composition was prepared by mixing 7.5 percent (all component 
percentages herein are percent by weight of composition before addition of 
blowing agent) of KETJEN Black 600J conductive carbon black available from 
Akzo Chemie America; 30 percent low density polyethylene available from 
Dow Chemical Co. under PE4005; and 62.5 percent of an ionomer resin, 
SURLYN 8660 from Du Pont. A blowing agent, CFC 114, was added in an amount 
of 25 parts per hundred parts of composition, and the resulting mixture 
extruded using a 11/4 inch (3.175 cm) extruder to produce a foam having an 
open cell content of 76.6 percent. The density of the cured, extruded 
sample was 2.80 pcf (44.85 Kg/m.sup.3). The static decay rate of the 
sample was so slow it was unacceptable. The surface resistivity of the 
sample was 1.98.times.10.sup.14 ohms. 
EXAMPLE 2 
The carbon black, polyethylene, and ionomer of Example 1 were added 
together in respective amounts of 8.6, 34.4 and 57 percent. The same 
blowing agent was added in the same amount as in Example 1 producing an 
extruded foam having an open cell content of 87.3 percent, a density of 
4.07 pcf (65.19 Kg/m.sup.3), a static decay rate of 0.01 seconds and a 
surface resistivity of 1.38.times.10.sup.11 ohms. 
EXAMPLE 3 
The carbon black, polyethylene, and ionomer of Example 1 were added 
together in respective amounts of 10, 40 and 50 percent. The same blowing 
agent was added in the same amount as in Example 1 to produce an extruded 
foam having an open cell content of 92.8 percent, density of 3.72 pcf 
(59.59 Kg/m.sup.3), a static decay rate of 0.01 seconds, and a surface 
resistivity of 6.85.times.10.sup.7 ohms. 
EXAMPLE 4 
The carbon black, polyethylene, and ionomer of Example 1 were added 
together in respective amounts of 7.5, 29.8 and 36.8 percent. In addition, 
an ECO copolymer available from Dow Chemical of Midland, Mich. designated 
as ECO XU 60766.02L, was added in an amount of 25.4 percent. Sodium 
tetraphenylboron (STPB) was added in an amount of 0.5 percent. The same 
blowing agent was added in the same amount as in Example 1 to produce an 
extruded foam having an open cell content of 91.7 percent, a density of 
2.96 pcf (47.41 Kg/m.sup.3), a static decay rate of 0.01 seconds, and a 
surface resistivity of 1.18.times.10.sup.8 ohms. 
SIGNIFICANCE OF EXAMPLES 1-4 
Example 4 illustrates the effect of the addition of the ionic salt and ECO 
copolymer combination of the present invention to a polymeric resin having 
conductive carbon black particles. The use of only 0.5 percent of STPB and 
25.4 percent of an ECO copolymer in Example 4 produced a foam having 
nearly identical open cell content and surface resistivity as the foam in 
Example 3 having approximately a 33 percent higher concentration of carbon 
black and a 25 percent higher density. Comparing the foam of the present 
invention in Example 4 to the foams of Examples 1 and 2, it is apparent 
that dramatic increases in conductivity are achieved by the incorporation 
of an ionic salt such as STPB and an ECO copolymer according to the 
teachings of the present invention. 
Of course, it is understood that the foregoing is merely a preferred 
embodiment of the invention and that various changes and alterations can 
be made without departing from the spirit and broader aspects thereof as 
set forth in the appended claims, which are to be interpreted in 
accordance with the principles of patent law, including the Doctrine of 
Equivalents.