Note: Descriptions are shown in the official language in which they were submitted.
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Docket No: 55912PCT
Express Mail No.: EL895436838US
INDUCTION HEATING USING DUAL SUSCEPTORS
FIELD OF THE INVENTION
The present invention relates to methods of rapid heating of material,
e.g., polymeric materials, by mixing combinations of susceptors of particular
compositions in the material to be heated. More specifically, the present
invention provides heating agents or susceptors that heat, under an
alternating
magnetic field, at a rate that is significantly faster than those heating
agents
that have been identified in the prior art. More specifically, the present
invention provides heating agents that heat at average heating rates greater
than 300~C/sec (575 ~F/sec).
BACKGROUND OF THE INVENTION
It is desirable to have highly efficient heating agents or susceptors for
use in the heating of plastic substrates and welding to a substrate. High
heating rates are desired, and sometimes demanded, for maximum production
efficiency, e.g., to reduce the time and cost of production, while maintaining
product quality. High heating rates are especially desired in the welding of
plastic closures for liquids and food where temperatures on the order of 180~C
must be attained in 250 to 300 milliseconds. Thus, it would be desirable to
have a method of rapid heating and melting of plastic substrates that can be
used on production lines for sealing and welding plastic components in
manufacturing facilities.
Present methods of induction heating include United States Patent
Number 4,969,968, issued Nov.13, 1990 to Leatherman, et al. This patent
describes the use of non-conductive, sub-micron ferric oxide (Fe20a)
particles,
which generate heat because of hysteresis losses, with micron-sized,
conductive, ferromagnetic ferrous (e.g., iron) particles, which generate heat
primarily because of eddy current losses. Leatherman requires the use of
integrated sub-micron-sized, non-conductive particles (e.g., Fe20s) and micron-
sized, conductive particles (e.g., iron), with each being a significant part
of the
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bonding agent by weight. Leatherman's process includes the application of RF
from 1.2 KHz to 7 MHz, with a preferred range of 1.8 to 4.8 MHz and 3.5 to 4
MHz being a typical range. The mixed particles of Leatherman form a
substantially greater percentage by weight than the inert resin carrier, e.g.,
polypropylene. The second plurality of particles constitutes about twice the
weight of the first plurality of particles. Leatherman teaches that, in
preferred
embodiments, the second particles constitute substantially 40 percent by
weight of the bonding layer and said first particles constitute substantially
25
percent by weight of the bonding layer. The second particles are larger than -
200 mesh (-- 75 Vim) and the first particles are less than 1.0 Vim. In
addition,
Leatherman teaches the use of very high coil current, i.e., 600 amps.
Leatherman teaches a maximum heating rate of 425°F/sec.
Thus, it would be desirable to have heating agents that are able to heat
thermoplastics faster than presently known methods. In addition, it would be
desirable to have a method of rapid heating that is more economical than the
presently known methods and that can attain rapid heating rates using
standard commercial equipment.
SUMMARY OF THE INVENTION
The present invention provides heating agents that heat, under an
alternating magnetic field, at a rate that is significantly faster than those
heating agents that have been identified in the prior art. More specifically,
the
invention provides heating agents that unexpectedly heat at average heating
rates greater than 300°C/sec (575 °F/sec).
The shortcomings of the prior art with respect to the heating efficiencies
of particulate heating agents are addressed by the present invention which
comprises heating agents composed of unique mixtures of particulate matter
incorporated in a resin matrix that provide exceptionally high heating rates
under an applied alternating magnetic field.
The invention relates to an agent for heating materials, e.g.,
thermoplastics, comprising dual susceptors. The dual susceptors comprise (a)
at least one plurality of electrically non-conductive, fernmagnetic susceptors
and (b) at least one plurality of electrically conductive susceptors.
Preferably
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the electrically non-conductive susceptors comprise micron-sized fernmagnetic
particles (e.g., magnetic oxides). Examples of the electrically non-conductive
particles useful in the present invention comprise iron oxides, hexagonal
ferrites, or magnetically soft ferrite particles. Examples of hexagonal
ferrites
include compounds that have the composition SrF, Mea-2W, Mea-2Y, and Mea
2Z, wherein 2W is Ba0:2Mea0:8FeaOs, 2Y is 2(BaO:Mea0:3FezOs), and 2Z is
3Ba0:2Mea0:12FeaOs, and wherein Mea is a divalent cation. Examples of the
magnetically soft ferrite particles have the composition lMebO:lFeaOs, where
MebO is a transition metal oxide. Mea comprises Mg, Co, Mn or Zn and Meb
comprises Ni, Co, Mn, or Zn.
The electrically conductive susceptors used in the present invention
comprise ferromagnetic particles or intrinsically conductive polymer (ICP)
particles. The electrically conductive ferromagnetic particles useful in the
present invention comprise elemental ferromagnetic particles or ferromagnetic
alloys. Examples of ferromagnetic, electrically conductive particles comprise
nickel, iron, and cobalt, and combinations thereof or of their alloys.
Preferably
the particles are ferromagnetic. Examples of ICPs include, but are not limited
to, polyaniline (PAni), polypyrrole (PPy), polythiophene (PTh),
polyethylenedioxythiophene, and polyp-phenylene vinylene). The particles,
either the electrically conductive particles and/or non-conductive particles,
may
be irregularly-shaped, spherically-shaped or in flake form. One of ordinary
skill
in the art can readily select the desired shape. In preferred embodiments the
ferrimagnetic particles have a size of from about l.OUm to about 50um and the
ferromagnetic particles have a size of from about Sum to about 100~m, .more
preferably, from about l0um to about 50~m.
The electrically non-conductive particles comprise from about 10/0 (20
W/o) to about 30/0 (58 W/o) of the heating agent. The electrically conductive
particles comprise from about 5~/o to about 15/0 of the heating agent.
The invention also relates to a welding agent comprising (a) a matrix
material and (b) an agent for heating the material, wherein the agent
comprises
dual susceptors. The dual susceptors comprise ( 1) at least one plurality of
electrically non-conductive, ferrimagnetic susceptors and (2) at least one
plurality of electrically conductive, ferromagnetic susceptors. The matrix can
be
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selected from any thermoplastic material or combinations of materials.
Examples of useful matrices include, but are not limited to, polyethylene,
polypropylene, polystyrene, PVC, polyacetal, acrylic (PMMA), polyamide (PA),
Nylon 6, Nylon 66, polycarbonate (PC), polysulfone (PSU), polyetherimide
(PEI),
polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyphenylene
sulfide (PPS), polyurethane (PU), polyphenylene oxide (PPO),
polytetrafluorethylene (PTFE), or combinations thereof. The dual susceptors
are
as described above.
The invention also relates to an article of manufacture comprising (a) a
matrix material and (b) an agent for heating the material, wherein the agent
comprises dual particles. The dual particles comprise (1) at least one
plurality
of electrically non-conductive, ferrimagnetic susceptors and (2) at least one
plurality of electrically conductive, ferromagnetic susceptors. Preferably the
electrically non-conductive susceptors comprise micron-sized ferrimagnetic
particles and the electrically conductive susceptors comprises ferromagnetic
particles or ICP particles. The susceptors are set forth above and further
described below. The matrix can be selected from any polymeric or ceramic
type of material or combinations of materials. Examples of polymeric materials
include, e.g., plastics, elastomers, adhesives, coatings and natural polymers,
such as rubbers. Some examples of useful matrix materials include, but are
not limited to, polyethylene, polypropylene, polystyrene, PVC, polyacetal,
acrylic
(PMMA), polyamide (PA), Nylon 6, Nylon 66, polycarbonate (PC), polysulfone
(PSU), polyetherimide (PEI), polyetheretherketone (PEEK),
polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), polyurethane (PU),
polyphenylene oxide (PPO), polytetrafluorethylene (PTFE), or combination
thereof. The particles can be positioned on a surface of the matrix material
or,
alternatively, embedded in the matrix material, as necessary for the desired
application. One of ordinary skill in the art can readily determine where the
particles should be positioned.
The invention also relates to a method of heating a material comprising
(a) providing at least one plurality of electrically non-conductive
susceptors, (b)
providing at least one plurality of electrically conductive susceptors,
wherein
the electrically non-conductive susceptors have a specific Curie temperature
(T~
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in the material, (c) applying an alternating magnetic field to the material,
wherein the susceptors in (a) generate heat due to hysteresis loss and the
susceptors in (b) generate heat due to eddy current flow.
The invention further relates to a method of rapid heating of a
thermoplastic material comprising (a) providing an agent for heating the
material, wherein the agent comprises ( 1 ) at least one plurality of
electrically
non-conductive, ferrimagnetic susceptors and (2) at least one plurality of
electrically conductive ferromagnetic susceptors, in a first thermoplastic
material, wherein the electrically non-conductive, ferromagnetic susceptors
have a specific Curie temperature (T~) in the first thermoplastic material,
(b)
applying an alternating magnetic field to the first thermoplastic material to
heat
the susceptors, and (c) ceasing the application of the alternating magnetic
field
when the susceptors reach the desired temperature.
In methods of the present invention, the applying comprises applying an
alternating magnetic field at about 2MHz to about 30 MHz, and in preferred
cases, the alternating magnetic field is applied at about 10 to about 15 MHZ.
In preferred methods, the method further comprises the step of
providing a second thermoplastic material in contact with the first
thermoplastic material before applying the alternating magnetic field. In yet
other embodiments, the method further comprises initially placing the first
thermoplastic material on an uncured or partially cured thermoset material and
bonding the thermoplastic material and the thermoset material while curing the
thermoset material. The method may also include initially juxtaposing the
first
thermoplastic material on the thermoset material, bonding the thermoplastic to
the thermoset while curing the thermoset material, and juxtaposing the bonded
assembly with the second material. Preferably, the second material is a second
thermoset material with a second thermoplastic material and wherein the
bonding comprises flowing and bonding the first and second thermoplastic
materials while curing the thermoset material. In other methods, the second
material is a second thermoplastic material. The second material may have the
same chemical composition as the first thermoplastic material or a different
chemical composition. The second thermoplastic material may have the
susceptors embedded therein. In such embodiments, the susceptors may be
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embedded in adjacent surfaces of the first and second thermoplastic materials.
The susceptors may be embedded in a surface of the first or second
thermoplastic material.
In preferred methods, T~ of the electrically non-conductive susceptors is
greater than the melting temperature of the thermoplastic material, and the
magnetic field is applied so that the susceptors melt the thermoplastic
material.
In other embodiments, T~ of the susceptors is less than the melting
temperature
of the thermoplastic material.
In certain methods and articles of the present invention, the amount of
zinc in the ferrimagnetic particles can be varied as to control the Curie
temperature of the particles.
In preferred methods, the method further comprises the step of
providing a second thermoplastic material in contact with the first
thermoplastic material before applying the alternating magnetic field. In yet
other embodiments, the method further comprises initially placing the first
thermoplastic material on an uncured or partially cured thermoset material and
bonding the thermoplastic material and the thermoset material while curing the
thermoset material. The method may also include initially juxtaposing the
first
thermoplastic material on the thermoset material, bonding the thermoplastic to
the thermoset while curing the thermoset material, and juxtaposing the bonded
assembly with the second material. Preferably, the second material is a second
thermoset material with a second thermoplastic material and wherein the
bonding comprises flowing and bonding the first and second thermoplastic
materials while curing the thermoset material. In other methods, the second
material is a second thermoplastic material. The second material may have the
same chemical composition as the first thermoplastic material or a different
chemical composition. The second thermoplastic material may have the
susceptors embedded therein. In such embodiments, the susceptors may be
embedded in adjacent surfaces of the first and second thermoplastic materials.
The susceptors may be embedded in a surface of the first or second
thermoplastic material.
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In preferred methods, T~ of the susceptors is greater than the melting
temperature of the thermoplastic material, and the magnetic field is applied
so
that the susceptors melt the first thermoplastic material.
The invention also relates to a sealable apparatus comprising a first
element having a shaped matrix and having a rim; a second element having an
annular area for bonding to the rim of the first element; at least one
plurality of
electrically non-conductive susceptors and at least one plurality of
electrically
conductive susceptors disposed in the rim of the first element or in the
annular
area of the second element, for heating the rim or the annular area to a
predetermined temperature upon application of an alternating magnetic field,
for bonding the first element and the second element together. In certain
embodiments the susceptors are disposed in both the rim and the annular area.
The matrix used in the sealable apparatus preferably comprises at least
one thermoplastic material and can be selected from any thermoplastic material
or combinations of materials. Examples of useful matrices include, but are not
limited to, polyethylene, polypropylene, polystyrene, PVC, polyacetal, acrylic
(PMMA), polyamide (PA), Nylon 6, Nylon 66, polycarbonate (PC), polysulfone
(PSU), polyetherimide (PEI), polyetheretherketone (PEEK),
polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), polyurethane (PU),
polyphenylene oxide (PPO), polytetrafluorethylene (PTFE), or combinations
thereof. The particles can be positioned on a surface of the matrix material
or,
alternatively, embedded in the matrix material, as necessary for the desired
application. One of ordinary skill in the art can readily determine where the
particles should be positioned.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a top view of a heating agent in sheet or tape form comprising a
mixture of electrically non-conductive, micron-sized ferrimagnetic (e.g.,
ferrite)
particles and electrically-conductive, micron-sized ferromagnetic particles
randomly dispersed in a thermoplastic matrix.
Fig. 2 is the heating curve (solid line) for 20/0 (36 W/o) strontium ferrite
and 13/0 (41 W/o) flake nickel in high density polyethylene (HDPE). Dashed
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curve marks when the generator power was turned on (t = 0) and then turned
off at t = 250msec. Heating Rate: 1120 °F/sec.
Fig. 3 is the heating curve (solid line) for 20°/° (36
W/°) MnZn ferrite and
13~/° (40 W/°) iron in high density polyethylene (HDPE). Dashed
curve marks
when the generator power was turned on (t = 0) and then turned off at t =
250msec. Heating Rate: 740 °F/sec.
Fig. 4 is the heating curve (solid line) for 20~/° (44.9 W/°)
MnZn ferrite and
5~/° (20.8 W/°) Ni-A1 flake in high density polyethylene (HDPE).
Dashed curve
marks when the generator power was turned on (t = 0) and then turned off at t
= 250msec. Heating Rate: 740 °F/sec.
Fig. 5 is the heating curve (solid line) for 20~/° (46.1 W/°)
strontium ferrite
and 5~/° (20.6 W/°) flake nickel in high density polyethylene
(HDPE). Dashed
curve marks when the generator power was turned on (t = 0)and then turned off
at t = 250msec. Heating Rate: 760 °F/sec.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides heating agents comprising combinations
of susceptors that heat, under an alternating magnetic field, at a rate that
is
surprisingly faster than those heating agents that have been identified in the
prior art. The heating agents of the present invention heat at average heating
rates greater than 300°C/sec (575 °F/sec).
The present invention uses a combination of at least two susceptors and
high frequency alternating magnetic fields to generate heat, which is used to
bond or weld plastic substrates. For example, the welding agent of the present
invention comprises multiple susceptors embedded in a plastic, e.g.,
thermoplastic matrix.
Both ferromagnetism in a ferromagnetic material and ferrimagnetism in a
non-conductive ferromagnetic material disappears at the Curie temperature as
thermal oscillations overcome the orientation due to exchange interaction,
resulting in a random grouping of the atomic particles. When a non-conductive
ferrimagnetic material is placed in an electromagnetic field, the hysteresis
losses in the material cause its temperature to rise, eventually reaching its
Curie temperature. Upon reaching its Curie temperature, the material crystal
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lattice undergoes a dimensional change, causing a reversible loss of magnetic
dipoles. Once the magnetic dipoles are lost, the ferrimagnetic properties
cease,
thus halting further heating. While not intending to be bound by theory, it is
believed that the rapid heating phenomenon seen in the methods and
compositions of the present invention are due to the combination of the non-
conductive susceptors and the second electrically conductive susceptors. The
addition of the second susceptor type helps to focus the magnetic field on the
non-conductive susceptors, enabling the temperature to continue to rise
rapidly.
Among the important parameters in this process are the following:
1) Size and Shape of The Ferrima~netic Hysteresis Loop: The size and
shape of the ferrimagnetic hysteresis loop are controlled by the choice of the
susceptor. For example, a magnetically hard ferrite exhibits a larger
hysteresis
loop than does a magnetically soft ferrite. The larger the hysteresis loop,
the
greater is the heat that can be generated per cycle. To take advantage of the
larger hysteresis loop, the strength of the applied, alternating magnetic
field
must be sufficiently large to permit the loop to be completely traversed in
each
cycle (e.g., for the susceptor to reach magnetic saturation).
2) Susceptor Loading: The amount of susceptor used is controlled and
optimized for the intended application. In the case of a thermoplastic weld
material, the volume fraction of the susceptor phase and the thickness of the
weld material play a direct role in the temperature achieved and the rate of
heating within the thermoplastic polymer.
3) Alternate Heating Mechanisms: The present invention takes
advantage of the effect of alternate heating mechanisms to provide additional
heat.
4) Particle Size: The particle size is controlled and optimized for the
intended application.. Particle size affects heat transfer to the
thermoplastic
weld material.
5) Particle Shape: The particle shape is controlled and optimized for the
intended application. Certain shapes may exhibit unique responses to the
induction field, and thus optimized heating for the application.
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By manipulating these parameters as described herein, the inventors
have found that the rate of heating can be increased substantially.
The term "susceptor" as used herein refers to a material that interacts
with a magnetic field to generate a response, e.g., eddy currents and/or
hysteretic losses. The methods and apparatus of the present invention are
based on the use of dual "susceptors" that can be used to heat a polymer
matrix. The susceptors are further described below.
As shown in Figure 1, the electrically non-conductive susceptors, e.g.,
micron-sized ferrimagnetic particles 2 and the electrically conductive
susceptors, e.g., micron sized ferromagnetic particles or ICP particles 3, are
dispersed in the thermoplastic host matrix 1. The susceptors can be dispersed
throughout the article that will be heated, e.g., if the article is a tape
that will be
used to bond two pieces of thermoplastic together. Or alternatively, a portion
of
the article to be welded or bonded to another article or portion of the
article,
e.g., a rim or annular area, can be manufactured to have the susceptors
embedded therein. One of ordinary skill in the art can readily determine where
the susceptors should be placed to maximize the rate of heating and sealing or
welding of the articles.
Preferential heating of the thermoplastic bond area during fusion is
achieved by induction heating of the susceptor materials, e.g., particles 2
and 3
placed in the bond interface. This technology is amenable to production line
manufacturing where rapid rates of production require rapid heating of
composite structures. It would also be useful in rapid field repair of
composite
structures, for example, and is more cost effective in initial fabrication
than
presently known methods of repair.
SUSCEPTORS
The invention relates to an agent for heating thermoplastic materials
comprising dual susceptors. The susceptors comprise (a) at least one plurality
of electrically non-conductive susceptors and (b) at least one plurality of
electrically conductive susceptors. The methods and compositions of the
present invention utilize the fact that magnetic induction heating occurs in
magnetic or electrically conductive materials when they are subject to an
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applied alternating magnetic field. The present invention specifically takes
advantage of the heating that occurs in the combination of susceptors
described
herein. When a current-carrying body, or coil, is placed near the susceptors
of
the present invention, the magnetic field caused by the current in the coil
induces a current in the susceptors. In the electrically conductive magnetic
susceptors of the present invention, heating occurs by both eddy current and
hysteresis losses. It is eddy currents losses that dominate. In the non-
conducting magnetic materials, heating occurs by hysteresis losses. In this
later case, the amount of energy available for heating is proportional to the
area
of flux vs. field intensity hysteresis curve (B vs. H) and frequency of the
alternating field. This mechanism exists as long as the temperature is kept
below the Curie point (T~) of the material. At the Curie point, the originally
magnetic material becomes non-ferromagnetic. Thus, at its T~, heating of the
magnetic material ceases. Thus, as aforesaid, it was surprisingly found that
the combination of these conductive and non-conductive susceptors as
described herein, produces a rapid rate of heating, e.g., greater than
300°C/sec.
The methods of the present invention enable the user to achieve high
rates of heating by selecting the appropriate combination of susceptors based
upon the desired application. For example, one of ordinary skill in the art
can
control the rate of heating by controlling the ratios of the susceptors.
The dual susceptors comprise electrically non-conductive susceptors and
electrically conductive susceptors. The electrically non-conductive susceptors
are preferably micron-sized fernmagnetic particles. Examples of the
electrically
non-conductive particles useful in the present invention, include, but are not
limited to, iron oxides, hexagonal ferrites, or magnetically soft ferrites.
Examples of hexagonal ferrites include compounds that have the composition
SrF, Mea-2W, Mea-2Y, and Mea-2Z, wherein 2W is Ba0:2Mea0:8Fe20s, 2Y is
2(BaO:Mea0:3Fe20s), and 2Z is 3Ba0:2Mea0: l2FeaOs, and wherein Mea is a
divalent cation. Examples of the magnetically soft ferrite particles have the
composition lMebO:lFeaOs, where MebO is a transition metal oxide. Mea
comprises Mg, Co, Mn or Zn and Meb comprises Ni, Co, Mn, or Zn. In preferred
embodiments the electrically non-conductive particles, e.g., ferrimagnetic
particles, have a size of from about l.OUm to about 50um. The electrically non-
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conductive particles comprises from about 10/0 (20 W/o) to about 30/0 (58 W/o)
of the composition.
Examples of useful hexagonal ferrites include, but are not limited to
those shown in Table 1:
Table 1
Me-2W Me-2Y Me-2Z
CozBa1Fe160z6 CozBazFelzOzz CozB~Fez4041
ColZnlBa1Fe160z6 ColZnlBazFelzOzzColZn1 Ba3Fez4041
M Ba1Fe16~26 M BazFelzOzz M Ba3Feza0al
M lZnlBa1Fe160zs M lZnlBazFelzOzzM lZn1 Ba3Fez4041
Mnz Ba1Fe16~z6 MnzBazFelzOzz Mnz Ba3Fez40a1
MnlZnlBa1Fe160z6 MnlZnlBazFelzOzzMnlZn1 BasFez404~
~
See L. L. Hench and J. K. West: "Principles of Electronic Ceramics" (John
Wiley
& Sons, 1990) pp. 321 - 325. The ferromagnetic hexagonal ferrites are also
known as hexagonal ferrimagnetic oxides. Examples of preferred ferrimagnetic
hexagonal ferrites include SrF, Co-2Y and Mg-2Y. A range of Curie
temperatures is preferred for the susceptors to be effective in bonding and
other
processing of a wide range of thermoplastic and thermoset composites.
The electrically conductive susceptors useful in the present invention
include ferromagnetic particles and ICP particles. The electrically conductive
ferromagnetic particles can be elemental ferromagnetic particles or
ferromagnetic alloys. Examples of electrically conductive particles comprise
nickel, iron, and cobalt and combinations thereof and of their alloys.
Preferred
ferromagnetic particles have a size of from about 5~um to about 100um, more
preferably, from about l0um to about 50~m.
ICPs are organic polymers that conduct electric currents while retaining
the other typical properties commonly associated with a conventional polymer.
ICPs are different from so-called "conducting polymers" that are merely a
physical mixture of a non-conducting polymer with a conducting material such
as metal or carbon powder. In addition to the generation of heat by hysteresis
losses in the ferrimagnetic particles, eddy current losses within the
electrically
conductive polymer contribute additional heating to enhance the rate of
heating
of the heating agent. Since ICPs tend to lose their electrical conductivity at
temperatures above about 200~C, heating agents utilizing ICPs are preferably
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used in applications in which the maximum process welding temperature is
below 200~C. Examples of ICPs include, but are not limited to, polyaniline,
polypyrrole, polythiophene, polyethylenedioxythiophene, and poly (p-phenylene
vinylene) .
The electrically conductive particles preferably have a size of from about
5~m to about 100~m, more preferably, from about 10~m to about 50~m and
comprise from about 5~/o to about 15/0 of the composition.
In certain embodiments of the present invention, the Curie temperature
of the fernmagnetic particle changes in response to changing the proportion of
zinc in the particle, such as Zn/Mg-2Y and Zn/Co-2Y. For example, T~ may be
lowered by the partial substitution of Zn++ for the divalent ions in the
strontium
ferrite (SrF), Mg-2Y, and Co-2Y. The substitution of Zn++ for Mg++ and Co++ on
"a" sites in the lattice reduces the strength of a-b interactions and
decreases T~.
Preferably, sufficient zinc is added to the magnetically hard hexagonal
ferrite to
lower its T~ significantly while still retaining its hexagonal structure and
hard
magnetic properties. One of ordinary skill in the art can readily determine
the
amount of zinc to be added and the methods for adding it.
The addition of Zn to hexagonal ferrites decreases their Curie
temperatures. As shown in co-pending application No. 09/847055, when Co-
2Y was doped with 5, 10, and 15% Zn, each of the Zn additions lowered the
Curie temperature of Co-2Y. The addition of 15% Zn to Co-2Y decreased Tc
from 340 ~C to approximately 300 ~C. The x-ray diffraction patterns of the Zn-
doped materials show that even with the addition of 15% Zn, the hexagonal
structure of Co-2Y is retained. At 15% Zn, T~ decreased from 340~C to 300 ~C.
It appears that the zinc additions did not significantly affect the hysteresis
behavior.
The addition of Zn to Mg-2Y also reduces its Curie temperature. When
Mg-2Y was synthesized with zinc atoms substituting for half the magnesium
(Formula: MglZnlBazFeizOzz), the Zn/Mg-2Y ferrite exhibits a Curie
temperature of 175 °C. The addition of zinc to Mg-2Y reduces its Curie
temperature from 260 to 175°C.
Other non-conducting particles comprise magnetically soft ferrite
particles having the structure lMeO:lFezOs, where Me0 is a transition metal
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oxide. Examples of Me include Ni, Co, Mn, and Zn. Preferred particles include,
but are not limited to: (Mn,ZnO)FeaOs and (Ni,ZnO)Fe20s, also referred to as
MnZn and NiZn ferrites, respectively. Even though "soft" ferrites have a
narrower hysteresis loop than the "hard" ferrites, efficient heating with
"soft"
ferrites is achievable under proper processing conditions, e.g., power level
and
frequency, that utilize the total hysteresis loop area.
Examples of dual susceptor formulations include, but are not limited to
Strontium Ferrite/Flake Nickel; Mn-Zn Ferrite/Flake 97Ni-3A1 ; Mn-Zn
Ferrite/Iron. Examples are shown in Table 2:
Table 2: Dual Susceptor Formulations with HDPE Matrix
Strontium Ferrite (HM 181 ) and Nickel
Volume Percent (Weight Percent)
HM 181 Nickel HDPE
10 28.3 5 25.4 85 46.3
10 20.8 13 48.4 77 30.8
15 38.1 5 22.8 80 39.1
15 28.8 13 44.6 72 26.6
46.1 * 5 20.6 * 75 33.3
20 35.6 * 13 41.5 * 67 22.9
58.3 5 17.4 65 24.3
30 (46.7) 13 (36.2) ~ 57 ( 17.
l~
* Tested
Mn-Zn Ferrite (FP215) and Nickel
Volume Percent (Weight Percent)
FP215 Nickel HDPE
10 27.2 5 25.8 85 47.0
10 19.9 13 49.0 77 31.1
10 18.6 15 53.0 75 28.4
15 36.8 5 23.2 80 40.0
15 27.6 13 45.4 72 27.0
20 44.7 * 5 21.2 * 75 34.1
20 34.3 13 42.3 67 23.4
56.9 5 18.0 65 25.1
30 (45.3) 13 (37.2) 57 ( 17.5)
~ ~
* Tested
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Mn-Zn Ferrite (FP215) - Iron
Volume Percent (Weight Percent)
FP215 Iron HDPE
28.0 5 23.5 85 48.5
10 21.1 13 45.9 77 33.0
37.8 5 21.1 80 41.1
15 29.2 13 42.3 72 28.5
45.8 5 19.2 75 35.0
20 36.1 * 13 39.3 * 67 24
.6
58.1 5 16.2 65 _
25.7
_
30 (47.3) 13 (34.4) 57 ( 18.3)
~
5
* Tested
Both the non-conductive susceptors, i.e., the ferrimagnetic particles, and
certain of the conducting susceptors, e.g., ferromagnetic metal particles,
have a
Tc. Thus, in certain embodiments one can utilize the T~ of either the
10 ferrimagnetic particles and/or the ferromagnetic particles to obtain the
desired
temperature and rate of heating depending on the matrix that is selected.
MATRICES
For certain embodiments of the present invention the matrix material
15 preferably comprises any thermoplastic known in the art. Examples of
polymeric materials include, e.g., plastics, elastomers, adhesives, coatings
and
natural polymers, such as rubbers. The plastics can comprise either
thermoplastic or thermoset materials. Examples of thermoplastics (TPs)
include, but are not limited to: ethenic (vinyls, polyolefins, fluorocarbons,
20 styrenes, acrylics), polyamides, polyesters, cellulosics, acetals,
polycarbonates,
polyimides, and polyethers. Specific examples include, but are not limited to,
polyethylene, e.g., high density polyethylene (HDPE) and low density
polyethylene (LDPE), polypropylene, polystyrene, PVC, polyacetal, acrylic
(PMMA), , Nylon 6, Nylon 66, polycarbonate (PC), polysulfone (PSU),
25 polyetherimide (PEI), e.g. GE Ultem 1000, PEEK (polyetheretherketone),
polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), polyurethane (PU),
polyphenylene oxide (PPO), polytetrafluorethylene (PTFE) or combinations
thereof. Examples of thermoset materials include, but are not limited to,
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phenolics, unsaturated polyesters, urethanes, silicones, ureas, melamines,
epoxides.
Examples of susceptor/polymer systems include, but are not limited to
Strontium Ferrite/Flake Nickel in HDPE; Mn-Zn Ferrite/Flake 97Ni-3A1 in
HDPE; Mn-Zn Ferrite/Iron in HDPE; Mn-Zn Ferrite/Flake Nickel in HDPE;
Fes04/Flake Nickel in HDPE; Fe304/Iron in HDPE; Fe203/Flake Ni in HDPE;
Fe203/Iron in HDPE. In addition, the polymers can be combined with
ferrimagnetic particles such as Zn/SrF, Zn/Co-2Y, Zn/Mg-2Y and mixtures of
the hexagonal ferrites, and other combinations described herein and further
combined with ferromagnetic particles and determined by one of ordinary skill
in the art.
One aspect of the invention relates to an agent for heating a matrix, e.g.,
thermoplastic materials, comprising (a) at least one plurality of electrically
non-
conductive particles and (b) at least one plurality of electrically conductive
particles. The particles may be present on a surface of the matrix, or
alternatively, embedded in the matrix, depending on the desired use. For
example, if two surfaces of particular articles are being bonded or welded
together, then it may be desirable to have the susceptor particles embedded on
only the surface of the article that is to be bonded.
Alternatively, as described herein, the susceptors may also be dispersed
in a matrix to form a welding or bonding agent and applied to the surface of
one
or both thermoplastic articles to be welded, sealed or bonded. The welding
agent can be in any desirable form, e.g., tape, spray, liquid, sheet, tube or
paste, depending on the desired use. Upon application of the magnetic field,
when the particles heat up, the carrier or matrix may be melted or evaporated
away. Alternatively, if the entire article is to be heated according to the
present
methods, it would be desirable to disperse the susceptors throughout the
matrix of the article. One of ordinary skill in the art can readily determine
where the susceptors should be placed in order to maximize the efficiency and
efficacy of the controlled temperature heating of the susceptors.
The thermoplastics containing the susceptors as described herein, can
be shaped or molded into articles by methods known in the art, e.g., by
extrusion, compression molding, injecting molding or film casting. The article
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may be fabricated by a number of different methods well known in the art.
These methods include but are not limited to: (a) solution casting of the
article
as film or sheet, (b) extrusion compounding the article directly into film,
sheet
or tape form, (c) extrusion compounding the components of the article into
pellets followed by compression molding the pellets into sheets or other
shapes
suitable for the intended application, and (d) mixing the susceptor(s) and
matrix
in a mixer such as the Brabender Mixer (C. W. Babender; South Hackensack,
NJ) or the Haake Rheomix Mixer (Haake USA; Paramus, NJ) and compression
molding the mixture into sheets or other shapes suitable for the intended
application.
In other embodiments, the matrix comprises a ceramic type of material.
Examples of useful ceramics include single oxides (e.g., alumina, chromium
oxide, zirconia, titania, magnesium oxide, silica),. mixed oxides (e.g.,
kaolinite),
carbides (e.g., vandadium carbide, tantalum carbide, tungsten carbide,
titanium carbide, silicon carbide, chromium carbide, boron carbide), sulfides
(e.g., molybdenum disulfide, tungsten disulfide), and nitrides (e.g., boron
nitride, silicon nitride).
The susceptors may be added to the matrix in any order. For example,
the non-conducting susceptors can first be added to the thermoplastic mixture
and then the electrically conducting susceptors can be added. Or the
susceptors can be added in reverse order. While the susceptors can be first
mixed and then added to the thermoplastic matrix, it is in fact preferred to
add
the particles separately because it eliminates the step of mixing the
particles
together.
INDUCTION COIL DESIGN AND MAGNETIC FIELD PATTERNS
The compositions and methods of the present invention enable the use of
standard coil constructions and the use of commercially available induction
generators, e.g., solid state equipment from Ameritherm. The present invention
enables the use of lower coil current and higher frequencies than the prior
art.
The coil current used in the present invention ranges from about 50 to about
150 amps. Certain prior art inventions utilize very high coil currents, e.g.,
600
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amps to get the heating rates seen in the prior art. The methods of the
present
invention unexpectedly produce rapid heating rates at lower coil currents.
Depending on the susceptors used and the application, based on the
teachings herein, one of ordinary skill in the art can readily determine the
frequency and strength of the magnetic field used to induce heating in the
present methods and apparatuses. Preferably the useful frequency range is
from about 2 MHz to about 30 MHz and the preferred power ranges from about
1 KW to about 7.5 KW. Where the desired temperature is higher, e.g., bonding,
welding or sealing applications, the frequency and power will be at the higher
end of the range, e.g., from about 10 MHz to about 15 MHz. One of ordinary
skill in the art can select the appropriate power and frequency depending on
the
susceptor and thermoplastic selected and for the desired application, i.e.,
heating or bonding/welding/sealing.
Depending on the susceptors used, the field generated by the induction
coil influences the heating patterns of the susceptors and the field is a
function
of the coil geometry. Examples of coil design include solenoid, pancake,
conical
and Helmholtz. While these coil types are among those commonly used by
industry, certain embodiments of invention may require specialized coils. For
example, in certain embodiments solenoid coils are preferred because solenoid
coil geometry produces a very strong magnetic field. In other embodiments,
pancake coils are used. Pancake coils have been found to produce a non-
uniform field with its maximum at the center. One of ordinary skill in the art
can readily select the type of coil based on the teachings in the art and set
forth
herein.
Magnetic field strength increases with increasing number of coil turns,
increasing coil current and decreasing coil-work piece separation. The factors
can be readily manipulated by one of ordinary skill in the art to select
combinations of these factors to obtain the desired magnetic field strength.
Solenoid coil geometry produces the strongest field of all the possible
geometries. Pancake coils are most common in one-sided heating applications.
Changing the coil parameters (e.g., spacing between turns or the number of
turns) can change the field values, but the pattern is generally the same.
Magnetic field strength increases if the coil-part separation is reduced. If
the
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part is placed very close to the coil, one may see the heating dictated by
each
turn of the coil.
APPLICATIONS:
The present invention has many potential applications, especially where
very rapid rates of heating are required. One example of such a use is in high
velocity production lines, where thermoplastic materials need to be sealed,
welded, or bonded in a very short time period. For example, the heating agents
of the present invention reach 180°C within 300 msec. Such rapid rates
of
heating enables one to heat (e.g., seal, bond or weld) thermoplastic articles
very
quickly. The potential applications for the methods and compositions of the
present invention are innumerable, spanning both military and commercial
markets.
Examples of military uses include fabrication and repair of aircraft
structures, as well as fabrication and repair of shipboard structures.
Additionally, the present invention is not limited to fusion bonding of
thermoset-based composites, but also could be applied to consolidation and
repair of thermoplastic composites or elevated-temperature curing of thermoset
adhesives, thereby reducing repair time and increasing performance.
The commercial sector could enjoy similar benefits with respect to the
fabrication and repair of composite structures. For example, this technique
can
be used to repair aging metal structures with composite reinforcements or new
bonding techniques developed for commodity resins such as polyethylene.
The compositions and methods of the present invention are useful for
any application in which it is desirable to melt the matrix material, e.g.,
welding, sealing and/or bonding of thermoplastic materials. In such
applications, T~ of the non-electrically conductive particles is greater than
the
melting temperature of the thermoplastic material. The susceptor particles can
readily be selected based upon the teachings described herein.
The compositions and methods of the present invention may be used in
the packaging industry, specifically for closure systems. The broad
temperature range covered by the susceptors allows for use in a wide range of
commercial applications, e.g., in the food packaging industry, automotive
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assembly lines, etc. For example, induction heating may be used in the food
industry to seal lids without the use of the aluminum peel-away that is
commonly used in many packages. The advantages of replacing foil with a
direct polymer seal include lower cost, improved recyclability and the ability
to
control the bonding conditions, including temperature, of complex seal shapes,
such as a thin ring on the rim of a beverage container, or a lid on a food
tray.
This technology can also be used for sealing bags or other similar containers
for
foods, including prepared foods, instant foods or ingredients.
As one example of the sealing method, a cup containing a food product
may be sealed with a lid by inductively heating the dual susceptors uniformly
distributed throughout or concentrated in a rim of the cup or in an annular
area of the lid or both. Inductively heating the dual particles at the annular
seal area while pressing the cup rim and lid together, for example with an
induction heating horn, fuses and co-cures the plastic material of the cup and
lid. This method can be used for any sealing application, e.g., sealing boxes
or
containers enclosing any type of materials. Examples of such materials include
prepared foods, foodstuffs, ingredients, liquids as well as non-edible
products
and liquids. For example, the sealing technology can be used to seal
cartridges
and filters of different types, e.g., water filters, oil filters, and medical
devices.
One of ordinary skill in the art can readily apply the methods of the present
invention to any application that requires sealing or bonding of
thermoplastics.
The rapid rate of heating enables the manufacture of a high volume of these
products in a very short time period, thus decreasing production time,
reducing
costs, and increasing productivity.
In sealing or welding methods of the present invention, it may be useful
to apply pressure to the two parts to be welded or bonded together. If such
pressure is desirable one of ordinary skill in the art can readily determine
the
necessary pressure based on the application and polymer used.
Another example of a preferable use is in manufacturing aviation, auto
and marine structural components: specifically, fabricated structures that
comprise one polymer component welded to another polymer component. For
example, the methods of the present invention can be used on production lines
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in the automotive industry, for sealing or welding polymer components, e.g.,
tail
lights, etc.
The susceptors and methods of using the susceptors described herein
can be applied to either one or both of the components and inductively heated
to weld or seal the components together. Another use is in the repair of
structures that comprise one polymer component welded to another polymer
component.
In yet another embodiment, the methods of induction bonding are used
to weld the seams of structures made of thermoplastic materials, for use in
the
field, e.g., by military forces. One example is useful for joining
polyurethane
skin to itself. In one embodiment, filler particles (i.e., the susceptor
particles of
the present invention) are dispersed into a thermoplastic matrix that heat up
in
the presence of a magnetic field. These particles are designed to thermally
match the softening point of a variety of thermoplastic resins, into which
they
can be compounded.
The present invention is further illustrated by the following Examples.
The Examples are provided to aid in the understanding of the invention and are
not construed as a limitation thereof.
EXAMPLES
Example 1:
High density polyethylene (HDPE) pellets were placed in a Haake
Rheomix Mixer and mixed until the pellets melted, at which time strontium
ferrite particles (HM181) (particle size: 1.4~m; Supplier: Steward Ferrite;
Chattanooga, TN) and fine leaf nickel flake (diameter: 10-20um, thickness:
0.5~m; Supplier: Novamet; Wycoff, NJ) were added slowly to high density
polyethylene in the Rheomix mixer until the entire quantity of both susceptors
have been added such that the strontium fernte was at 36 percent by weight
(W/o) of the total mix and the flake nickel was at 41 percent by weight (W/o)
of the
total mix and thorough mixing has taken place. The mixture was then removed
from the Rheomix mixer and compression molded into sheets 10 to 20 mils
thick. Small sections approximately 1 x 1-in were cut from the sheet and
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mounted on glass slides. These samples were then placed inside a 5-turn, 2-in
long, oval-shaped (2 x 1/2-in) solenoid coil and subjected to an 11.8MHz
alternating magnetic field. The Nova Star 1M solid state 1.0 KW induction
generator (Ameritherm, Inc.; Scottville, NY) was used as the power source.
Coil
current was approximately 80 amps. An Ircon 06F05 IR pyrometer (Ircon, Inc.;
Niles, IL) with a response time of lOms and a temperature range of 200 to
600°F
(93 °- 315 °C) was used to measure and record temperature.
Because the spot
size of the pyrometer slightly impinged on the coil, the true temperature and
true rate of heating were higher than the measured values. A trigger was used
to mark time zero when the power was turned on. The pyrometer starts
measurements at 200°F. The initial ambient temperature of the samples
prior
to the start of heating was 70°F.
As can be seen from Table 3, heating rates ranging from 1050 to
1120°F/ sec. were achieved. One of the heating curves for 20% Strontium
Fernte and 13% Flake Nickel in High Density Polyethylene is shown in Figure 2.
The heating rates achieved by the present invention were approximately 2.5
times as great as that reported by Leatherman (United States Patent Number
4,969,968), at a significantly lower coil current (80 vs 600 amps).
Example 2:
Heating agents having HDPE as the matrix or host and containing the
following combinations (a), (b) or (c) were fabricated in the same manner as
described in Example 1:
(a) 46.0 W/o (20 ~/o) strontium ferrite (1.4 Vim) and
20.6 W/o (5.0 ~/°) Novamet flake nickel (D: 65-95~m; t: 0.5~m);
(b) 44.9 W/o (20 ~/°) Mn-Zn (PowderZ'ech FP215; Particle Size 14~m)
and 20.8 W/° (5.0 ~/o) Novamet flake 97Ni-3A1 alloy powder (D: 10-
20~m; t: 0.5um);
(c) 36.0 W/o (20 ~/o) Mn-Zn (Powdeffech FP215; Particle Size l4~rm)
and 40.0W/° (13 ~/o) Iron [-325 mesh (<44um) ].
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Heating tests similarly were conducted on similar size samples as described in
Example 1. The heating rates achieved for the test samples of Example 2 are
presented in Table 3 and the actual heating curves are shown in Figures 3 to
5.
The rates of heating (680 - 760~F/ sec) achieved by the present invention are
higher than those reported in the prior art (e.g., Patent # 4,969,968 (142 -
425
~F/sec)). The methods of the present invention use coil currents, which were
significantly lower than in Patent # 4,969,968.
Table 3. Results of Heating Tests
Test Conditions: (Frequency: 11.8MHz, Power:1.0 KW, Coil: 5-turn oval solenoid
(2 x 1/2-in), Length: 2-in, Coil Current: 80 amps, Matrix: High Density
Polyethylene (HDPE)).
Heating Agents Heating Rate
~F sec
36 W/o (20 ~/o) Strontium Ferrite 1050 - 1120
- 1.4~m
41W/o (13 ~/o) Flake Nickel - D: 10-20~m
t: 0.5~m
46 W/o (20 ~/o) Strontium Fernte - 690 - 760
1.4~m
20.6W/a (5 ~/o) Wflake Nickel - D:
65-95~m
t: 0.5~m
44.9 W/o (20 ~/o) Mn-Zn Fernte - 14~m 680 - 740
20.8W/o (5 ~/o) Flake 97Ni-3A1 - D:
10-20~m
t: 0.5~m
36 W/o (20 ~/o) Mn-Zn Ferrite - 14~m 680 - 740
40W o 13 ~ o Iron < 44um -325 mesh
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Example 3:
Heating agents having HDPE as the matrix and containing from 10 ~/o to
30 ~/o micron-sized, non-conducting, ferrimagnetic particles and 13 ~/o micron-
sized, electrically conducting ICP particles, are fabricated into films,
sheets or
other shapes suitable for the intended application by the method described in
Example 1. The said heating agents also can be fabricated by solution casting,
extrusion compounding, extrusion compounding followed by compression
injection molding or by a number of other methods known by those well versed
in the technology. Both the non-conducting and conducting particles can be
irregular or spherical in shape. These non-conducting susceptors also can be
in the form of fibers or flakes.
The invention has been described in detail with particular references to
the preferred embodiments thereof. However, it will be appreciated that
modifications and improvements within the spirit and scope of this invention
may be made by those skilled in the art upon considering the present
disclosure.
All references cited are incorporated herein by reference.
24
