U.S. patent application number 15/272604 was filed with the patent office on 2017-01-12 for shape memory products and method for making them.
This patent application is currently assigned to Tyco Electronics UK Ltd.. The applicant listed for this patent is Tyco Electronics UK Ltd.. Invention is credited to Sreeni Kurup.
Application Number | 20170009040 15/272604 |
Document ID | / |
Family ID | 50391206 |
Filed Date | 2017-01-12 |
United States Patent
Application |
20170009040 |
Kind Code |
A1 |
Kurup; Sreeni |
January 12, 2017 |
Shape Memory Products and Method For Making Them
Abstract
A method for producing a heat-shrinkable product is provided.
First, a polymer composition containing a polymer, a crosslinking
agent and a micro-encapsulated foaming agent uniformly dispensed
therein is melt mixed. The foaming agent has a peak activation
temperature which is higher than a temperature of the melt mixing.
Next, the polymer composition is injection molded into a molded
product. This carried out at the peak activation temperature to
activate the foaming agent. Then, the molded product is crosslinked
within the mold.
Inventors: |
Kurup; Sreeni; (Wiltshire,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tyco Electronics UK Ltd. |
Wiltshire |
|
GB |
|
|
Assignee: |
Tyco Electronics UK Ltd.
Wiltshire
GB
|
Family ID: |
50391206 |
Appl. No.: |
15/272604 |
Filed: |
September 22, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/GB2014/050916 |
Mar 24, 2014 |
|
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15272604 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29C 35/0805 20130101;
B29K 2023/0641 20130101; C08J 2203/22 20130101; C08J 2423/08
20130101; B29K 2105/0076 20130101; B29K 2223/083 20130101; B29C
44/3415 20130101; B29K 2023/083 20130101; B29K 2995/0077 20130101;
C08J 2201/026 20130101; B29K 2995/0002 20130101; C08J 9/0061
20130101; C08J 9/0095 20130101; B29K 2105/24 20130101; B29K
2105/0026 20130101; C08J 2323/06 20130101; B29C 2035/0877 20130101;
C08J 9/0023 20130101; B29K 2995/0015 20130101; C08J 9/32 20130101;
B29C 44/42 20130101; B29K 2105/048 20130101 |
International
Class: |
C08J 9/32 20060101
C08J009/32; B29C 35/08 20060101 B29C035/08; B29C 44/34 20060101
B29C044/34; C08J 9/00 20060101 C08J009/00 |
Claims
1. A method for producing a shape memory product comprising the
steps of: (i) melt mixing a polymer composition containing a
polymer, a crosslinking agent and a micro-encapsulated foaming
agent uniformly dispensed therein, the micro-encapsulated foaming
agent having a peak activation temperature which is higher than a
temperature of the melt mixing, (ii) injection molding the polymer
composition into a molded product, the injection molding carried
out at the peak activation temperature to activate the
micro-encapsulated foaming agent; and (iii) crosslinking the molded
product within a mold.
2. The method according to claim 1, wherein the micro-encapsulated
foaming agent is a blowing agent.
3. The method according to claim 2, wherein the polymer composition
includes a polyolefin.
4. The method according to claim 3, wherein the polymer composition
further includes polyethylene.
5. The method according to claim 4, wherein the polymer composition
further includes ethylene-vinyl acetate copolymer (EVA).
6. The method according to claim 1, wherein the polymer composition
includes 40 to 60% by weight of polyethylene and 10 to 20% by
weight of EVA.
7. The method according to claim 1, wherein injection molding is
carried out at a temperature in a range of 120 to 160.degree.
C.
8. The method according to claim 7, wherein the mold is heated to a
temperature in a range of 170 to 210.degree. C.
9. The method according to claim 8, wherein the injection molding
is carried out at a pressure in a range of 10 to 20 MPa.
10. The method according to claim 1, wherein the crosslinking agent
includes an organic peroxide compound.
11. The method according to claim 10, wherein the polymer
composition includes a radiation crosslinking promoter.
12. The method according to claim 11, wherein molded product is
crosslinked using radiation.
13. The method according to claim 12, wherein the polymer
composition further includes a flame retardant.
14. A shape memory product comprising: a base polymer in an amount
of 50-90 wt %; a micro encapsulated foaming agent in an amount of
0.5 to 3 wt %; an organic peroxide crosslinking agent in an amount
of 0.1 to 1 wt %; and a flame retardant in an amount of 15 to 30 wt
%.
15. The shape memory product according to claim 14, wherein the
base polymer includes polyethylene in an amount of 30 to 70wt
%.
16. The shape memory product according to claim 15, wherein the
base polymer includes polyethylene in an amount of 40 to 60wt
%.
17. The shape memory product according to claim 15, wherein the
base polymer further includes ethylene/vinyl acetate copolymer
(EVA) in an amount of 10 to 25wt %.
18. The shape memory product according to claim 17, wherein the
base polymer further includes ethylene/vinyl acetate copolymer
(EVA) in an amount of 10 to 20wt %.
19. The shape memory product according to claim 14, wherein the
micro encapsulated foaming agent is a blowing agent.
20. The shape memory product according to claim 19, wherein the
blowing agent includes a plurality of polymeric shells
encapsulating a heat-activated chemical compound.
21. The shape memory product according to claim 20, wherein the
plurality of polymeric shells have an unexpanded diameter between 6
.mu.m to 40 .mu.m, and an expanded diameter between 20 .mu.m to 150
.mu.m.
22. The shape memory product according to claim 14, wherein the
organic peroxide crosslinking agent is 2,5-bis
(t-butylperoxy)-2,5-dimethylhexane.
23. The shape memory product according to claim 14, further
comprising a radiation crosslinking promoter in an amount of 0.2 to
1.5 wt %.
24. The shape memory product according to claim 14, wherein the
flame retardant is selected from a group consisting of
polybrominated aromatics, alumina trihydrate, and phosphorus-based
retardants.
25. A method for producing a heat-shrinkable product comprising the
steps of: (i) melt mixing a polymer composition having: a base
polymer in an amount of 50-90 wt %; a micro encapsulated foaming
agent in an amount of 0.5 to 3 wt %; and an organic peroxide
crosslinking agent in an amount of 0.1 to 1 wt %; (ii) injection
molding the polymer composition into a molded product, the
injection molding carried out at a peak activation temperature to
activate the micro-encapsulated foaming agent; and (iii)
crosslinking the molded product.
26. The method according to claim 25, wherein the base polymer
includes polyethylene in an amount of 40 to 60wt %.
27. The method according to claim 26, wherein the base polymer
further includes ethylene/vinyl acetate copolymer (EVA) in an
amount 10 to 20wt %.
28. The method according to claim 27, wherein the blowing agent
includes a plurality of polymeric shells encapsulating a
heat-activated chemical compound.
29. The method according to claim 28, wherein the plurality of
polymeric shells have an unexpanded diameter between 6 .mu.m to 40
and an expanded diameter between 20 .mu.m to 150 .mu.m.
30. The method according to claim 29, wherein the polymer
composition further includes a crosslinking agent in an amount of
0.2 to 1.5 wt %.
31. The method according to claim 30, wherein the polymer
composition further includes a flame retardant in an amount of 15
to 30 wt %.
32. The method according to claim 24, wherein injection molding is
carried out at a temperature in a range from 120 to 160.degree.
C.
33. The method according to claim 32, wherein the molded product is
heated in a mold at a temperature in a range from 170 to
210.degree. C.
34. The method according to claim 33, wherein the injection molding
is carried out at a pressure in a range from 10 to 20 MPa.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of PCT International
Application No. PCT/GB2014/050916, filed Mar. 24, 2014.
FIELD OF THE INVENTION
[0002] The invention relates to shape memory products and, more
particularly, shape memory products for use for example as
insulating materials.
BACKGROUND
[0003] Lightweight heat shrinkable products are increasingly
important in a number of industries, notably the aerospace and
automotive industries, where weight reduction is a major
consideration. One way of making heat-recoverable products is
through a melt composition having a base polymer, such as ethylene
vinyl acetate copolymer, and a blowing agent made from at least one
heat-activated chemical compound encapsulated by a plurality of
polymeric shells. The melt composition may also include a
crosslinking promoter or other additives. The blowing agent has an
activation temperature and is activated through heat during the
manufacturing process. For instance, U.S. Pat. No. 3,615,972A
discloses known heat-activated liquid blowing agents encapsulated
in expansible thermoplastic microspheres.
SUMMARY
[0004] A method for producing a heat-shrinkable product is
provided. First, a polymer composition containing a polymer, a
crosslinking agent and a micro-encapsulated foaming agent uniformly
dispensed therein is melt mixed. The foaming agent has a peak
activation temperature which is higher than a temperature of the
melt mixing. Next, the polymer composition is injection molded into
a molded product. This carried out at the peak activation
temperature to activate the foaming agent. Then, the molded product
is crosslinked within the mold.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a graph showing a change in specific gravity of a
foamable composition according to the invention with different
addition levels of micro-encapsulated foaming agents;
[0006] FIG. 2 is a micrograph of a foamable body according to the
invention prior to expansion; and
[0007] FIG. 3 is a micrograph showing the foamable body of FIG. 2
after expansion.
DETAILED DESCRIPTION OF THE EMBODIMENT(S)
[0008] The invention provides a method for producing a
heat-shrinkable product which includes the following major steps
of: [0009] (i) melt mixing a polymer composition; and [0010] (ii)
injection molding the polymer composition.
[0011] In an embodiment of the invention, the polymer composition
according to the invention includes a cross linkable polymer, a
crosslinking agent and a micro-encapsulated foaming agent uniformly
dispersed therein. Specifically, the foaming agent, such as a
surfactant or a blowing agent, has a peak activation temperature
higher than the melt mixing temperature.
[0012] After mixing, the polymer composition is injection molded
under conditions of temperature and pressure such that expansion of
the composition by the foaming agent and subsequent crosslinking of
the molded product take place within the mold.
[0013] In addition to the foaming agent and crosslinking agent, the
polymer composition may include optional fillers such as
antioxidants, heat stabilizers, colorants, flame retardants and the
like.
[0014] The base polymer material of the composition is preferably
olefin-based and is chosen such that it will melt easily, withstand
the heat and pressure of injection molding without degrading and
mix homogeneously with the blowing agent.
[0015] Preferred polyolefins include polyethylenes which may be of
any suitable density from very low to high density, medium density
being preferred, metallocene polymerized ethylene; C4-C10
olefin-containing ethylene copolymers; copolymers and terpolymers
of polyethylene with vinyl acetate, alkyl acetate, acrylic acid,
maleic anhydride or carboxylic acid; polypropylene; ethylene
propylene diene rubbers (EPDM); styrene butadiene rubbers, and
rubber blends with polyolefins.
[0016] It is often desirable to mix a polymer having a relatively
high level of crystallinity with a polymer having a relatively low
level of crystallinity in order to achieve specific
heat-recoverable properties. Preferred base polymers include medium
density polyethylene and ethylene/vinyl acetate copolymer
(EVA).
[0017] The base polymer is preferably present at 50% by weight to
90% by weight of the melt composition, and in another embodiment
60% by weight to 75% by weight of the melt composition.
[0018] In an exemplary embodiment of the invention the base polymer
includes 30 to 70% by weight of polyethylene and 10 to 25% by
weight of EVA, and in another embodiment 40 to 60% by weight of
polyethylene and 10 to 20% by weight of EVA, based on the total
composition.
[0019] The invention utilizes a blowing agent in the form of a
plurality of polymeric shells encapsulating a heat-activated
chemical compound. These capsules are often called microballoons.
The chemical compound chosen is preferably a liquid at room
temperature, and has a relatively low boiling point, i.e. less than
50.degree. C. The specific chemical compound and polymeric shell
materials are chosen such that the polymeric shells remain intact
throughout the rigors of injection molding, more specifically, a
polymer shell material is chosen such that the shell will not
degrade or melt under the heat and pressure used in the mixing
process, and will not rupture during the applied forces of the
injection molding process. The shells should furthermore have high
chemical resistance and be compatible with the polymer matrix.
[0020] The microballoons have an activation temperature at which
the encapsulated liquid begins to boil and turn into a gas. At this
activation temperature, the polymer shells are soft enough to begin
to expand to allow for the increase in volume, as the chemical
compound enters a gaseous phase, while still effectively
encapsulating the chemical compound. This expansion of the
microballoons forms the voids in the base polymer in order to
create a foamed material. Generally, the activation temperature
includes a temperature range in order to accommodate differences in
microballoon size in a particular batch of the product. In
addition, encapsulated blowing agents are often defined in terms of
a minimum expansion temperature (or temperature range), i.e. the
temperature at which the encapsulant begins to expand, and a
maximum expansion temperature (or temperature range), i.e. the
temperature at which the encapsulant has completed expansion. The
activation temperature is generally somewhat lower than the maximum
expansion temperature.
[0021] The polymeric shell can include, without limitation,
polymers and copolymers of vinyl chloride, vinylidene chloride,
acrylonitrile, methacrylonitrile, styrene, or combinations thereof.
Preferably, the polymeric shell encapsulates a low-boiling
hydrocarbon-based liquid such as isopentane or isobutane.
[0022] In an embodiment of the invention, the unexpanded polymer
shells have a diameter ranging from 3 .mu.m to 60 .mu.m, and in
another embodiment from 6 .mu.m to 40 .mu.m. The density of the
unexpanded encapsulated blowing agent is generally less than 25
kg/m3. The unexpanded encapsulated blowing agent is used in an
amount of between 0.5% and 3% by weight of the melt composition,
and in another embodiment between about 1% and 2% of the melt
composition, although the precise amount of blowing agent used is a
function of the type of polymer, the type of blowing agent, and the
presence of optional fillers.
[0023] In an embodiment, the encapsulated blowing agent is
Expancel.TM. polymeric microballoons, available from Akzo Nobel. In
general, such microballoons have an unexpanded diameter between 6
.mu.m to 40 .mu.m, and an expanded diameter between 20 .mu.m to 150
.mu.m. Preferred encapsulated heat-activated blowing agents include
Expancel.TM.091-DU-80 or Expancel.TM.92-DU-120, or
Expancel.TM.950MB80(in masterbatch form), which have polymeric
shells comprising copolymers of acrylonitrile and
methacrylonitrile, and all of which encapsulate isopentane.
[0024] In order to create a uniform, stable foam, however, the
injection molding temperature and pressure are selected so that the
blowing agent microballoons do not begin to expand until the
polymer mixture is injected into the mold. Preferably, the blowing
agent will be chosen such that its activation temperature is higher
than that in the mixing temperature zone, but lower than that in
the mold. It is therefore preferable to choose a blowing agent with
a minimum expansion temperature above the melting temperature of
the base polymer, so that expansion does not occur while the base
polymer material is melting. And in another embodiment, the chosen
blowing agent has a minimum expansion temperature higher than the
desired mixing temperature of the melting step.
[0025] In one embodiment, the injection molding pressure is in a
range from 10 to 20 MPa, and in another embodiment 12 to 17 MPa,
and possibly about 15 MPa.
[0026] In order to produce a heat-recoverable molded product, the
polymer material is crosslinked. Crosslinking gives the polymer a
"memory" of its current shape, and gives the finished polymer
molded part the ability to shrink or otherwise change shape upon
heating. Crosslinking also increases the structural rigidity of the
foamed polymer and assures that the foam will not decompress, or
"go flat" when expanded or heat-shrunk. Crosslinking may be
achieved by radiation or chemical means, and the polymer mixture
may include crosslinking agents or promoters to increase the amount
of crosslinking between discrete polymer chains. There are two
general types of crosslinking promoters--chemical crosslinking
promoters and radiation crosslinking promoters. Either or both of
these types of crosslinking promoters may be used.
[0027] Preferred chemical crosslinking agents include peroxides,
preferably organic peroxides such as 2,5-bis
(t-butylperoxy)-2,5-dimethylhexane. A preferred content for a
chemical crosslinking agent is 0.1 to 1.0% by weight based on the
total composition.
[0028] A radiation crosslinking promoter acts as a catalyst to
polymer crosslinking when exposed to radiation, such as from a high
energy electron beam. The radiation crosslinking promoter may be
chosen from among those conventionally used to promote crosslinking
of polymers, including triallyl cyanurate (TAC), triallyl
isocyranurate (TAIC), triallyl trimellitate, triallyl trimesate,
tetrallyl pyromellitate, the diallyl ester of
1,1,3,-trimethyl-5-carboxy-3-(4-carboxyphenyl)indene,
trimethylolpropane trimellitate (TMPTM), pentaerythritol
trimethacrylate, tri(2-acryloxyethyl) isocyanurate,
tri(2-methacryloxyethyl) trimellitate, and the like and
combinations thereof. One preferred radiation crosslinking promoter
is TMPTM commercially available as SartomerTMSR350 from Sartomer
Company. Another is TAC. A preferred content for a radiation
crosslinking promoter is 0.2 to 1.5% by weight, based on the total
composition.
[0029] Flame retardants may also be added in an amount such as will
provide effective flame retardancy for the foamed product. Suitable
flame retardants typically include polybrominated aromatics, such
as decabromobiphenyl, in combination with inorganic materials, such
as antimony trioxide. Other possible flame retardants include
alumina trihydrate and phosphorus-based retardants. Other fillers
such as antioxidants, adhesion promoters, UV screeners,
plasticizers, colorants, heat stabilizers and other additives may
also be employed in conventional amounts. Such additives may be
chosen based on the final end use of the product, as is known to
those of skill in the art. The total content of flame retardants is
preferably in a range of 15 to 30% by weight based on the total
composition.
[0030] The quantity of optional fillers (which in this
specification includes both crosslinking promoters and flame
retardants, as well as the other fillers and additives referred to
above) present in the melt composition is dependent on the type and
quantity of base polymer and encapsulated blowing agent, as well as
the desired physical properties. In general, the quantity of
fillers is at most 35% by weight of the melt composition, and
preferably at most 20% by weight of the melt composition. In
another embodiment, the quantity of fillers is at most 10% by
weight of the melt composition, e.g. 0 to 19.9% by weight of the
melt composition.
[0031] The injection molding process for a foamable polymer
according to the invention is similar to that known to those of
skill in the art. The base polymer is first mixed with the blowing
agent and other foamable polymer composition elements. Mixing is
performed by any method known to those of skill in the art;
preferably, mixing occurs by a Banbury type mixer. A twin-screw
mixing device may also be successfully employed. Mixing occurs at
an elevated temperature selected to be high enough that the base
polymer will melt during mixing, but not high enough to activate
the other components of the polymer mix, such as the blowing agent
or any crosslinking promoter. Therefore, the mixing step takes
place at a temperature higher than the melt temperature of the
selected base polymer, but below the minimum activation
temperatures of the blowing agent.
[0032] The mixed polymer blend in a melt state is then injected
into a mold as is known in the art. The temperature of the polymer
blend after melting and mixing but before injection can be
independently set from the temperature at the time and point of
injection. Preferably, the post-mixing, pre-injection temperature
is set at a temperature equal to or higher than that of mixing but
less than the minimum activation temperatures of the components in
the polymer blend. The temperature of the mold is preferably set
well above the minimum activation temperature of at least the
blowing agent, i.e. at least 10.degree. C. above. By waiting until
just before injection to reach a temperature that activates the
blowing agent it has been found that a more stable, uniform foam is
created. At the time of activation, the liquid inside the polymer
capsules of the blowing agent changes phase into a gaseous state,
and the capsule expands to create a gas pocket. The melt
composition is thereby formed into the desired shape, and foamed at
approximately the same time.
[0033] The injection molding temperature is preferably in a range
of 120 to 160.degree. C., and in another embodiment 130 to
150.degree. C. The mold temperature is preferably in a range of 170
to 210.degree. C., and in another embodiment 180 to 200.degree.
C.
[0034] Once the molded body has undergone foaming, the blowing
agent microballoons are in a fully expanded state, and the base
polymer is in a melt, i.e. a highly viscous liquid-like state. As
the body begins to cool just after molding, the polymer begins to
solidify around the expanded microballoons. The microballoons are
then frozen in an expanded state, creating the voids in the
foam.
[0035] The molded body then undergoes post processing, generally
including one step that involves exposure to heat. In a further
step, the foamed polymer body is cross-linked. Depending on the
type of crosslinking process used, the body is irradiated and/or
heated to the proper temperature to activate the chemical
crosslinking promoter. Irradiation occurs by any method known in
the art, such as high energy electron beam irradiation. The purpose
of crosslinking the polymer strands in the foam is to give the
molded part a "memory" of the current shape of the product as well
as structural stability above the melting point of the base
resin.
[0036] Now, exemplary embodiments of the invention will now be
described in detail, reference being made to the accompanying
drawings wherein:
[0037] As set out in Table 1, three exemplary formulations for
foamable compositions using an encapsulated blowing agent in
accordance with the invention are described, together with a
comparative example not using a blowing agent.
TABLE-US-00001 TABLE 1 Weight % Ingredient Reference Description
Ingredient 1 Example 1 Example 2 Example 3 Medium Clearflex- 50.39
49.88 49.62 49.36 Density RL50 Polyethylene Base Resin EVA Base
Elvax- 15.22 15.07 14.99 14.92 Resin 3190 LGA Foaming Expancel- 0
1.0 1.5 2.0 Agent 950MB80 Antioxidant Agerite- 2.03 2.01 2.0 1.99
Resin-D Pastilles Carbon Statex 3.04 3.01 3 2.98 Black N115 (CB)
Processing Zinc 1.52 1.51 1.5 1.49 Aid Stearate Flame Saytex 18.77
18.58 18.49 18.4 Retardant 8010 Flame Sica- 8.17 8.09 8.05 8.01
Retardant Extra- Neige Activator Triallyl 0.51 0.5 0.5 0.5
Cyanurate Peroxide Luperox 0.35 0.35 0.35 0.35 Crosslinking 101
Agent TOTAL 100 100 100 100
[0038] The four compositions in Table 1 were melt mixed at a
temperature of 135 oC and injected into a mold at 15 MPa using an
injection molding machine with six sections of which the first five
were heated to 135.+-.10.degree. C. and the last to
145.+-.10.degree. C. The measured composition temperature rose from
134.degree. C. in the first section to 145.degree. C. in the
last.
[0039] The mold was heated to 190.+-.5.degree. C., whereby
expansion of the compositions of the invention took place in the
mold.
[0040] The expanded compositions were then subjected to chemical or
radiation crosslinking to give the final products. The physical
properties of these are set out in Table 2.
TABLE-US-00002 TABLE 2 Property Reference 1 Example 1 Example 2
Example 3 Tensile strength at room temperature, 20.64 14.29 12.42
13.76 MPa Ultimate elongation % 629 475 459 454 Tensile strength
after heat shock at 18.68 13.54 12.57 12.26 215.degree. C. for 4
hours, MPa Ultimate elongation (%) after heat 584 417 457 391 shock
at 215.degree. C. for 4 hours Tensile strength after heat shock at
18.25 14.3 13.42 13.13 160.degree. C. for 168 hours, MPa Ultimate
elongation (%) after heat 490 438 417 376 shock at 160.degree. C.
for 168 hours Specific Gravity 1.13 0.95 0.84 0.85 Low temperature
flexibility (-75.degree. C.) Pass Pass Pass Pass
[0041] Based on the above results, a preferred content of
encapsulated blowing agent is the 1.5 weight per cent of example 2,
which gives the most favorable balance between specific gravity and
physical properties, a key feature for heat-shrink products. This
amount can, however, be varied according to the desired specific
gravity of the foamed polymer. FIG. 1 shows that the specific
gravity of the product can be reduced, approximately linearly from
1.13 with no foaming agent addition to 0.85 with an addition of 2
weight per cent of foaming agent.
[0042] In FIG. 2, a micrograph shows a section of an expandable
body of a polymer in accordance with the invention in which 1.5
weight per cent of blowing agent micro-capsules are distributed.
These show up as small dark spots.
[0043] FIG. 3 shows the expandable polymer body of FIG. 2 after
expansion at 190.degree. C.
[0044] The foregoing illustrates some of the possibilities for
practicing the invention. Many other embodiments and fields of use
for a shape memory product according to the invention are possible
and within the scope and spirit of the invention. It is, therefore,
intended that the foregoing description be regarded as illustrative
rather than limiting, and that the scope of the invention is given
by the appended claims together with their full range of
equivalents.
* * * * *