U.S. patent application number 09/742911 was filed with the patent office on 2002-06-20 for continuous preparation of a liquid silicone rubber composition.
Invention is credited to Boudreau, Bruce, Grocela-Rocha, Teresa, Jeram, Edward M., Liermann, August O., Rocha-Galicia, Gerardo, Williams, David A..
Application Number | 20020074685 09/742911 |
Document ID | / |
Family ID | 24986745 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074685 |
Kind Code |
A1 |
Boudreau, Bruce ; et
al. |
June 20, 2002 |
Continuous preparation of a liquid silicone rubber composition
Abstract
A continuous process provides a devolatilized liquid injection
moldable silicone composition. In the process, a filler, treating
agent and silicone polymer are introduced into an extruder having a
length to diameter ratio of at least greater than 50. The filler,
treating agent and silicone elastomer are continuously compounded
in the extruder into a devolatilized liquid injection moldable
silicone composition. A system for preparing a liquid silicone
rubber composition comprises a mixer to prepare a concentrate of
filler and silicone polymer, a long extruder having an L/D ratio of
greater than 50, connected to the mixer to receive the concentrate
from the mixer and to compound and devolatilize the concentrate, a
treating agent and silicone polymer into a liquid silicone rubber
composition containing volatiles and a cooler to receive the liquid
silicone rubber composition to cool, homogenize and further
devolatilized the composition.
Inventors: |
Boudreau, Bruce; (Waterford,
NY) ; Grocela-Rocha, Teresa; (Waterford, NY) ;
Jeram, Edward M.; (Burnt Hills, NY) ; Liermann,
August O.; (Schenectady, NY) ; Rocha-Galicia,
Gerardo; (Waterford, NY) ; Williams, David A.;
(Ganesvoort, NY) |
Correspondence
Address: |
Robert T. Barker
GE Plastics
One Plastics Avenue
Pittsfield
MA
01201
US
|
Family ID: |
24986745 |
Appl. No.: |
09/742911 |
Filed: |
December 20, 2000 |
Current U.S.
Class: |
264/176.1 |
Current CPC
Class: |
B29B 7/48 20130101; B29C
48/39 20190201; C08J 3/201 20130101; B29K 2105/0038 20130101; B29C
48/05 20190201; B29C 48/405 20190201; B29C 48/41 20190201; B29C
48/385 20190201; B29K 2083/00 20130101; C08J 2383/04 20130101; B29B
7/44 20130101; B29B 7/7461 20130101; B29K 2105/16 20130101; C08J
2483/00 20130101; C08J 3/226 20130101; B29B 7/7495 20130101; B29C
48/365 20190201 |
Class at
Publication: |
264/176.1 |
International
Class: |
B29C 047/00 |
Claims
What is claimed is:
1. A process of preparing a liquid silicone rubber composition,
comprising: introducing a filler, treating agent and silicone
polymer into an extruder having a length to diameter ratio of at
least greater than 50; and continuously compounding and
devolatilizing said filler, treating agent and silicone polymer
into said liquid silicone rubber composition in said extruder.
2. The process of claim 1, wherein said extruder is a co-rotating,
intermeshing double screw extruder.
3. The process of claim 1, comprising densifying filler with an
initial amount of silicone polymer prior to introduction of said
filler into said extruder.
4. The process of claim 1, further comprising discharging said
liquid rubber composition from said extruder to a cooler.
5. The process of claim 4, wherein said cooler comprises a counter
rotating double screw extruder.
6. The process of claim 4, wherein said liquid silicone rubber
composition is cooled, homogenized and further devolatilized in
said cooler.
7. The process of claim 6, wherein said liquid silicone rubber is
in residence in said cooler for a period greater than residence in
said extruder.
8. The process of claim 1, further comprising dividing said liquid
silicone rubber composition into a first stream and a second
stream.
9. The process of claim 8, wherein a platinum catalyst is added to
said first stream to produce a component mixture A and a
crosslinker is added to said second stream to produce a component
mixture B.
10. The process of claim 9, wherein said component mixture A and
said component mixture B are injected into a mold where they are
cured to produce a rubber part.
11. The process of claim 1, wherein said filler is a raw, untreated
silica.
12. The process of claim 1, wherein said filler is a pretreated
filler with treating agent.
13. The process of claim 12, wherein said treating agent is a
silanol-reacting treating agent.
14. The process of claim 1, wherein said filler contains silanol
groups and said treating agent comprises silanol-stopped
polydimethylsiloxane, octamethylcyclotetrasiloxane (D4) or
hexamethyldisilazane (HMDZ).
15. The process of claim 14, further comprising maintaining an
elevated pressure in a section of said extruder while compounding
said filler, treating agent and silicone fluid to enhance
reaction.
16. The process of claim 15, comprising maintaining a pressure
between about 16 and about 400 psi.
17. The process of claim 15, comprising maintaining a pressure
between about 40 to about 300 psi.
18 The process of claim 15, comprising maintaining a pressure
between about 100 and about 200 psi.
19. The process of claim 1, comprising controlling temperature in
said extruder in a feed section (L/D<9) to less than 150.degree.
C. and then from said feed section to discharge between about 120
and about 240.degree. C.
20. The process of claim 1, comprising controlling temperature in
said extruder in a feed section (L/D<9) to less than 120.degree.
C. and then from said feed section to discharge between about 130
and about 230.degree. C.
21. The process of claim 1, wherein said length to diameter ratio
is at least greater than 60.
22. The process of claim 1, wherein said length to diameter ratio
is at least greater than 70.
23. The process of claim 1, wherein said silicone polymer comprises
a vinyl-terminated polydimethylsiloxane represented by recurring
units of Formula I: 2wherein, R.sup.1 independently at each
occurrence represents C.sub.1-4 alkyl, or C.sub.2-4 alkylene;
R.sup.2 independently at each occurrence represents C.sub.1-4
alkyl, C.sub.1-C.sub.4 haloalkyl or C.sub.2-4 alkylene; R.sup.3
independently at each occurrence represents H, C.sub.1-10 alkyl,
C.sub.2-4 alkylene, C.sub.4-6 cycloalkyl, OH or C.sub.1-C.sub.4
haloalkyl; and n is a value of about 100 to less than 1000.
24. The process of claim 23, wherein R.sup.1 independently at each
occurrence represents, CH.sub.3 or CH.dbd.CH.sub.2; R.sup.2
independently at each occurrence represents, CH.sub.3,
CH.dbd.CH.sub.2 or CH.sub.2CH.sub.2CF.sub.3; and R.sup.3
independently at each occurrence represents CH.sub.3,
CH.dbd.CH.sub.2, OH or CH.sub.2CH.sub.2CF.sub.3.
25. A process of preparing a silicone composition, comprising:
introducing a filler, treating agent and silicone polymer into an
extruder having a length to diameter ratio of at least greater than
50; continuously compounding and devolatilizing said filler,
treating agent and silicone polymer into said silicone composition
in said extruder; and discharging said silicone composition from
said extruder to a cooler.
26. The process of claim 25, wherein said cooler comprises a
counter rotating double screw extruder.
27. The process of claim 25, wherein said silicone composition is
cooled, homogenized and further devolatilized in said cooler.
28. The process of claim 27, wherein said silicone composition is
in residence in said cooler for a period greater than residence in
said extruder.
29. The process of claim 25, wherein said silicone polymer
comprises a vinyl-terminated polydimethylsiloxane represented by
recurring units of Formula I: 3wherein, R.sup.1 independently at
each occurrence represents C.sub.1-4 alkyl, or C.sub.2-4 alkylene;
R.sup.2 independently at each occurrence represents C.sub.1-4
alkyl, C.sub.1-C.sub.4 haloalkyl or C.sub.2-4 alkylene; R.sup.3
independently at each occurrence represents H, C.sub.1-10 alkyl,
C.sub.2-4 alkylene, C.sub.4-6 cycloalkyl, OH or C.sub.1-C.sub.4
haloalkyl; and n represents an integer such that viscosity of the
silicone polymer is between about 100 to about 2,000,000 cps at
25.degree. C.
30. The process of claim 29, wherein R.sup.1 independently at each
occurrence represents, CH.sub.3 or CH.dbd.CH.sub.2; R.sup.2
independently at each occurrence represents, CH.sub.3,
CH.dbd.CH.sub.2 or CH.sub.2CH.sub.2CF.sub.3; R.sup.3 independently
at each occurrence represents CH.sub.3, CH.dbd.CH.sub.2, OH or
CH.sub.2CH.sub.2CF.sub.3; and n represents an integer from about
4,000 to about 10,000.
31. A system for preparing a liquid silicone rubber composition,
comprising: a mixer to prepare a concentrate of filler and silicone
polymer; a long extruder having an L/D ratio of greater than 50,
connected to said mixer to receive said concentrate from said mixer
and to compound and devolatilize said concentrate, a treating agent
and silicone polymer into a liquid silicone rubber composition
containing volatiles; and a cooler to receive said liquid silicone
rubber composition to cool, homogenize and further devolatilized
said composition.
32. The system of claim 31, wherein said extruder is a co-rotating,
intermeshing double screw extruder.
33. The system of claim 31, wherein said cooler comprises a counter
rotating double screw extruder.
34. The system of claim 31, further comprising a mold to receive at
least a part of said composition for molding into a rubber
part.
35. The system of claim 31, wherein said long extruder has an L/D
ratio of greater than about 60.
36. The system of claim 31, wherein said long extruder has an L/D
ratio of greater than about 70.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a process of preparing a liquid
silicon rubber (LSR) composition.
[0002] An LSR composition also referred to as a LIM composition, is
a multi-component combination of a vinyl-containing
polydiorgansiloxane fluid, a hydrogen-containing
polydiorganosiloxane fluid, an effective amount of a platinum
catalyst and a reinforcing filler, such as a fumed silica and
additional additives. Typically, two component mixtures are mixed
and charged into a preheated mold where they are rapidly cured to
produce a rubber part. A first component mixture includes a
vinyl-containing polydiorgansilozane fluid, a silica filler and an
effective amount of a platinum catalyst and a second component
mixture includes a hydrogen containing polydiorganosiloxane fluid
in combination with other ingredients, including a vinyl containing
polyorganosiloxane fluid and a silica filler.
[0003] Typically, the LSR composition is produced by kneading a
polydiorganosiloxane, inorganic filler and additives by means of a
kneading machine such as a Banbury mixer, a turbulizer, a change
can mixer or a low intensity double arm dough mixer. In this
process, polydiorganosiloxane, inorganic filler, treating agents
and additives are batch mixed until desired properties are
obtained. The batch mixing process can take 12 to 30 hours per
batch. After mixing, the LSR composition is stripped of volatiles
and cooled. This final step requires additional time to complete
preparation of the LSR composition.
[0004] A batch process requires long residence times and large
amounts of energy. Non-homogeneous shear and extensional stresses
across a commercial sized batch can result in non-uniform size
distribution of filler that results in variations in properties.
Different physical properties may result form batch to batch.
Batches processed at different times may be characterized by
different physical properties. The batch process is labor, energy
and capital intensive and produces materials of only marginal
consistency.
[0005] There is a need for a continuous process that consistently
produces a devolatilized LSR composition from polymer, filler and
additives.
BRIEF SUMMARY OF THE INVENTION
[0006] The invention provides a continuous process that prepares a
devolatilized LSR composition. The process comprises introducing a
filler, treating agent and silicone polymer into an extruder having
a length to diameter ratio of at least greater than 50. The filler,
treating agent and silicone polymer are continuously compounded and
devolatilized in the extruder into a devolatilized LSR
composition.
[0007] In an embodiment, the invention provides a system for
preparing a liquid silicone rubber composition. The system
comprises a mixer to prepare a densified concentrate of filler and
silicone polymer, a long extruder having an L/D ratio of greater
than 50, connected to the mixer to receive the densified
concentrate from the mixer and to compound and devolatilize the
densified concentrate, a treating agent and silicone polymer into a
liquid silicone rubber composition containing volatiles and a
cooler to receive the liquid silicone rubber composition to cool,
homogenize and further devolatilized the composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic representation of a continuous LSR
composition process;
[0009] FIG. 2 is a schematic representation of an overall process
for preparing an LSR composition;
[0010] FIG. 3 is a table of processing conditions; and
[0011] FIG. 4 is a table of testing results including product
properties.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The invention reduces filler treatment reaction time and
provides an economical system for producing silicone compositions
including LSR compositions. Two parts of an LSR composition can be
made simultaneously. Inventories can be reduced and equipment
clean-out required between runs of a batch process can be
eliminated.
[0013] In one step of the process, filler surface treatment is
effected by mixing and reacting filler, polydimethylsiloxane
polymer and treating agents. The mixture can then be stripped of
volatiles and cooled. According to the invention, filler treatment
is reduced from hours to a few minutes, e.g., 2-6. The is
continuously in-situ treated to produce a base, which can be
filtered and then catalyzed.
[0014] In one embodiment, the process comprises four steps; 1)
filler densification, 2) In-situ treatment, volatiles stripping and
filtering, 3) cooling, homogenization and de-airing and 4)
catalization. Prior to densification, the filler can be pre-mixed
with silicone polymer to facilitate feeding into a reactor. A
co-rotating twin screw extruder can be used as the reactor to mix
the densified filler with treating agents (HMDZ, water, vinyl
silazane) and additional polymer.
[0015] The subsequent reaction is effected at higher temperatures
and pressures than in a batch process. The in-situ treatment can
require a minimum residence time in the extruder, a temperature in
the range of 160-210.degree. C. and pressure between about 70 and
about 300 psi.
[0016] Stripping of volatiles can be effected toward the end of the
extruder while the material is hot thereby eliminating secondary
operations. Residual amounts of nitrogen containing by-products
measured as parts per million HMDZ can be at about the 100 ppm
level. Filtration can be done at the discharge of the extruder
while the material is hot, eliminating secondary operations.
Cooling and de-airing can be effected in a separate counter-
rotating mixer, which can provide back-mixing and a high residence
time to homogenize the material into a base.
[0017] Finally, the base can be divided in two components and
separately mixed in static mixers with catalyst and other additives
to produce two separate "A" and "B" components. The two components
can be made simultaneously to reduce inventories.
[0018] These and other features will become apparent from the
drawings and following detailed discussion, which by way of example
without limitation describe preferred embodiments of the present
invention.
[0019] In FIG. 1, a filler such as fumed silica is continuously in
situ treated and compounded with a silicone polymer such as a
vinyl-terminated polydimethylsiloxane. In a first step, fumed
silica 12, an initial amount of silicone polymer 14 and a treating
agent 16 are charged into a continuous mixer 18 such as a Bepex
Turbolizer to form a densified polymer/filler mass.
[0020] The filler that can be used in this invention is any
inorganic filler with silanol surface groups that can be compounded
with silicone polymers. Examples of inorganic fillers include a
reinforcing silica such as fumed silica or precipitated silica. A
single filler or a combination of fillers can be used to reinforce
the silicone polymer.
[0021] The silicone polymer used in the compositions of the present
invention is typically a vinyl-terminated polydimethylsiloxane
having a viscosity varying from 100 to 2,000,000 cps at 25.degree.
C. The polymer can be represented by recurring units of Formula I:
1
[0022] wherein, R.sup.1 independently at each occurrence represents
C.sub.1-4 alkyl, or C.sub.2-4 alkylene; R.sup.2 independently at
each occurrence represents C.sub.1-4 alkyl, C.sub.1-C.sub.4
haloalkyl or C.sub.2-4 alkylene; R.sup.3 independently at each
occurrence represents H, C.sub.1-10 alkyl, C.sub.2-4 alkylene,
C.sub.4-6 cycloalkyl, OH or C.sub.1-C.sub.4 haloalkyl; and n
represents an integer so that the viscosity of the silicone polymer
varies from 100 to 2,000,000 cps at 25.degree. C. Broadly, n can be
about 80 to about 3500, desirably, n is a value of about 100 to
less than 1000.
[0023] A further preferred composition comprises a silicone polymer
wherein, R.sup.1 independently at each occurrence represents,
CH.sub.3 or CH.dbd.CH.sub.2; R.sup.2 independently at each
occurrence represents, CH.sub.3, CH.dbd.CH.sub.2 or
CH.sub.2CH.sub.2CF.sub.3; R.sup.3 independently at each occurrence
represents CH.sub.3, CH.dbd.CH.sub.2, OH or
CH.sub.2CH.sub.2CF.sub.3; and n represents an integer from about
200 to about 900.
[0024] Another embodiment provides a composition wherein the vinyl
content of the silicone polymer ranges from about 0.05% to about
0.5% by weight of the silicone polymer.
[0025] The amount of filler in the densification step can be from
about 20 to about 150 parts by weight, desirably from about 30 to
about 100 parts by weight and preferably from about 40 to about 70
parts by weight, per 100 parts by weight of the high viscosity
silicone polymer. During the compounding step, addition of further
silicone polymer reduces the proportion of silica in the final
silicone composition to about 10 to about 100 parts by weight,
desirably from about 15 to about 90 parts by weight and preferably
from about 25 to about 70 parts by weight, per 100 parts by weight
of the total high viscosity silicone polymer.
[0026] The treating agent is a silanol or methoxy stopped silicone
polymer that reacts with the filler surface hydroxyl or silanol
groups to prevent a condensation reaction among filler particles or
between filler and gum molecules that otherwise leads to stiffening
and loss of elastomeric properties. The treating agent reduces
filler silanol groups and reduces the time required for aging of
the silicone, to prevent crepe hardening and/or to regulate
plasticity. The treating agent can be an organosilane, an
organosilazane, a low-viscosity polyorganosiloxance or a silicone
resin, which has a silanol group and/or an alkoxy group having 1 to
6 carbon atoms. Examples include diphenylsilanediol,
dimethylsilanediol, methyltriethoxysilane and
phenyltrimethoxysilane. The low-viscosity polysiloxane may contain
one or more kinds of organic groups selected from a methyl group, a
phenyl group, a vinyl group and a 3,3,3-trifluoropropyl group.
Preferred silanol-reactant treating agents include silanol-stopped
polydimethylsiloxane, octamethylcyclotetrasiloxan- e (D4) and
hexamethyldisilazane (HMDZ). The viscosity of the polysiloxane
measured at 25.degree. C. is in the range of from about 1 to about
300 cP, preferably from about 5 to about 100 cP.
[0027] The treating agent can be added in the compounding step in a
weight proportion of about 0.1 to about 100 parts fluid to 100
parts of filler, desirably about 0.5 to about 75 parts fluid to 100
parts of filler and preferably about 1.0 to about 50 parts fluid to
100 parts of filler. The treating agent can react to reduce
available groups of the filler to a concentration of between about
8 to about 2 hydroxyl groups/(nanometer).sup.2 of filler,
preferably between about 7 to about 3 hydroxyl
groups/(nanometer).sup.2 of filler. In an embodiment, the treating
agent can be a combination of HMDZ and water. This combination can
comprise a weight ratio of HMDZ/water of between about 0.1 to 10,
desirably between 0.2 to 5 and preferably between 0.3 to 3.
[0028] In the extruder 22, a densified filler and polymer
concentrate 20 and additional silicone polymer 24, treating agent
and water are fed at barrels 1 to 2, further additional fluid 26 is
added downstream at about barrel 16 and volatiles are stripped in
two vacuum vents 28, 30 at barrels 18 to 19 and at 21 to 22. Water
32 is injected between vent sections to facilitate stripping.
According to the invention, the L/D ratio of the entire extruder is
at least greater than 50 to achieve proper filler treatment and
stripping of volatiles. Desirably, the L/D ratio is greater than 60
and preferably greater than 70. The exemplary extruder 22 of FIG. 1
has an L/D ratio of 78.
[0029] During the extruder step, addition of further silicone
polymer reduces the proportion of silica in the final silicone
composition to about 5 to about 100 parts by weight, desirably from
about 10 to about 40 parts by weight and preferably from about 15
to about 30 parts by weight, per 100 parts by weight of the total
high viscosity silicone polymer.
[0030] Reaction between silanol groups on the filler and the
treating agent takes place under pressure and at elevated
temperature in the extruder compounding step. The screws of the
treatment section (barrels 3 to 18), can be configured to provided
a pressure of about 16 and about 400 psi. Desirably, the pressure
is about 40 to about 300 psi and preferably about 100 and about 200
psi. Temperature in a feed section of the extruder (L/D<9) is
maintained at less than about 150.degree. C., preferably less than
about 120.degree. C. Temperature is controlled in the treatment
section (L/D=9 to discharge) between about 100 and 250.degree. C.,
desirably between 120 and 240.degree. C. and preferably between 130
and about 230.degree. C.
[0031] Compounding in extruder 22 produces a base silicone
composition 32 that can be used to produce LSR silicone composition
components.
[0032] FIG. 2 illustrates an overall process and system for
preparing an LSR composition. In FIG. 2, the system 40 includes
long extruder 42 having an L/D ratio of at least greater than 50.
In the process, filler is fed from feeder 44 to densifier 46, e.g.,
a Bepex turbolizer or a Drais mixer. where it is mixed with initial
silicone polymer 48 to form a concentrate of a filler and polymer
concentrate 50. The densified filler and polymer concentrate 50 is
crammed by means of screw feed 52, e.g., a Werner and Pfleiderer
ESBV crammer feeder, into the long extruder 42.
[0033] Additional silicone polymer 54 is combined in mixer 56 with
additives such as HMDZ treating agent 58, water 60 and silazane 62
and is then charged 64 into the long extruder 42 for compounding
with densified filler and polymer concentrate 50. Further
additional silicone polymer 66 can be mixed with vinyl polymer 68
and charged 70 to the long extruder 42 downstream from the charge
of concentrate 50.
[0034] The compounded and reacted components are devolatilized at
vents 72 and 74 with water 76 charged between the vents.
Devolitilization is important for at least two reasons. First,
devolitilization removes unreacted treating agents to "quench"
further reaction with filler silanol groups. Otherwise, further
reaction can diminish filler reinforcement properties in the
heat-vulcanizable silicone composition. Second, devolitilization
removes surplus liquids that adversely affect Theological
properties of the silicone composition.
[0035] The compounded, reacted and devolatilized LSR composition is
filtered 78 and pumped 80 to a cooler 82. The LSR composition is
cooled and back mixed in cooler 82 to de-air and to further
homogenize the composition. The cooler 82 can be under vacuum to
produce further devolatilization. Cooler 82 can be a
counter-rotating twin screw mixer such as a LIST ORP. The volatiles
84 from the cooler are combined with volatiles 86 from the extruder
42 and are exhausted by vacuum 88 to volatiles recovery 90.
[0036] Cooled LSR composition 92 is divided into two streams, A and
B. Stream A is pumped 94 with platinum catalyst and other additives
A 96 to static mixer 98 to produce component mixture A 100 and
stream B is pumped 102 with hydrides and other additives B 104 to
produce component mixture B 106. Component mixture A 100 and
component mixture B 106 can be charged to a mold (not shown) where
they can be rapidly cured to produce a rubber part.
[0037] These and other features will become apparent from the
following detailed discussion, which by way of example without
limitation describes preferred embodiments of the present
invention.
EXAMPLES
[0038] Filler was mixed with vinyl-terminated polydimethylsiloxane
(viscosity about 40,000 cps at 25.degree. C., n about 800) in a
Bepex Turbolizer to provide a densified filler concentrate. The
densified concentrate was fed into a 53 mm, Krupp Werner &
Pfleiderer co-rotating twin screw extruder with a 78 L/D ratio. In
the extruder, the filler was mixed with HMDZ, water, vinyl silazane
and additional vinyl-terminated polydimethylsiloxane at barrels 1
and 2. Downstream (barrel 6), the filler was mixed with more
vinyl-terminated polydimethylsiloxane and the mixture was
compounded under elevated temperature in a section of the extruder
that was sealed to maintain pressure. FIG. 3 shows conditions for
two runs of the extruding step.
[0039] Hot compounded base material was discharged from the
extruder to a LIST ORP jacketed mixer for cooling. The base was
subjected to back mixing in the mixer to even-out inconsistencies
to provide a homogenous product. The mixer was under a vacuum,
which served to further strip and de-air the base.
[0040] The base is divided into two parts (A and B), which are fed
via metering pumps into static mixers. Platinum catalyst and other
additives are introduced into part A base just prior to feed to the
mixer. Hydrides and other additives are introduced into part B base
before mixing.
[0041] Properties of two final formulations each containing
catalyst and hydride are shown in FIG. 4. The two rows of results
shown in FIG. 4 correspond to the two runs of FIG. 3.
[0042] These Examples show that a devolatilized LSR composition
with excellent properties can be continuously produced from
polymer, filler and additives according to the invention.
[0043] While preferred embodiments of the invention have been
described, the present invention is capable of variation and
modification and therefore should not be limited to the precise
details of the Examples. The invention includes changes and
alterations that fall within the purview of the following
claims.
* * * * *