U.S. patent application number 12/831701 was filed with the patent office on 2010-11-04 for method of forming structured sintered articles.
Invention is credited to Diane Kimberlie Guilfoyle, Paul John Shustack, Lung-Ming Wu.
Application Number | 20100276846 12/831701 |
Document ID | / |
Family ID | 40261375 |
Filed Date | 2010-11-04 |
United States Patent
Application |
20100276846 |
Kind Code |
A1 |
Guilfoyle; Diane Kimberlie ;
et al. |
November 4, 2010 |
METHOD OF FORMING STRUCTURED SINTERED ARTICLES
Abstract
Disclosed is a method of forming a structured sintered article
including providing a mixture comprising a sinterable particulate
material and a binder, the binder comprising, as a function of
total resin content of the binder, at least 50% by weight of a
thermoplastic binder material and at least 5% by weight of a
radiation-curable binder material; shaping the mixture with a mold
to form a structure; setting the structure by cooling the structure
or by allowing the structure to cool; separating the structure from
the mold; irradiating the structure so as to at least partially
cure the radiation-curable binder material, and debinding and
sintering the structure so as to form a structured sintered
article. Shaping may include forming a structure having one or more
open channels, and sintering may include sintering in together in
contact with at least one additional structure so as to cover or
enclose the channels.
Inventors: |
Guilfoyle; Diane Kimberlie;
(Painted Post, NY) ; Shustack; Paul John; (Elmira,
NY) ; Wu; Lung-Ming; (Horseheads, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
40261375 |
Appl. No.: |
12/831701 |
Filed: |
July 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11975414 |
Oct 19, 2007 |
7767135 |
|
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12831701 |
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Current U.S.
Class: |
264/494 |
Current CPC
Class: |
B81C 2201/034 20130101;
C03C 17/04 20130101; B81B 2201/058 20130101; C03C 17/28 20130101;
C03C 17/324 20130101; C03C 11/00 20130101; C03B 19/06 20130101;
C03C 23/0005 20130101; B81C 1/00071 20130101 |
Class at
Publication: |
264/494 |
International
Class: |
B29C 35/08 20060101
B29C035/08 |
Claims
1. A method of forming a structured sintered article, the method
comprising the steps of: providing a mixture comprising a
sinterable particulate material and a binder, the binder
comprising, as a function of total resin content of the binder, at
least 50% by weight of a thermoplastic binder material; shaping the
mixture with a mold so as to form a structure; setting the
structure by cooling the structure or by allowing the structure to
cool; separating the structure from the mold; and debinding and
sintering the structure so as to form a structured sintered
article; wherein the binder further comprises at least 5% by weight
of a radiation-curable binder material and the method further
comprises, after the step of separating the structure from the
mold, the step of irradiating the structure so as to at least
partially cure the radiation-curable binder material.
2. The method of claim 1 wherein the step of irradiating further
comprises irradiating the structure sufficiently such that the
yield stress of the mixture is at least 8 Pa at a temperature of
100.degree. C.
3. The method of claim 1 wherein the step of irradiating comprises
irradiating the structure sufficiently such that the yield stress
of the mixture is at least 100 Pa at temperatures in the range of
70 to 120.degree. C.
4. The method of claim 1 wherein the binder comprises in the range
of about 60 to about 90% by weight of a thermoplastic binder
material and in the range of about 20 to 40% by weight of a
radiation-curable binder material.
5. The method according to claim 1 wherein the thermoplastic binder
material comprises one or more hydrocarbon waxes, waxy alcohols, or
mixtures thereof.
6. The method according to claim 1 wherein the radiation-curable
binder material is a UV-curable material.
7. The method according to claim 6 wherein the UV-curable material
comprises one or more acrylates, methacrylates, a vinyls, epoxies,
thiols, styrenics, and combinations thereof.
8. The method of claim 6 wherein the UV-curable material comprises
an acrylated oligomer.
9. The method of claim 8 wherein the acrylated oligomer is a
polyester acrylate.
10. The method according to claim 1 wherein the step of molding the
mixture comprises molding the mixture in a silicone-containing
mold.
11. The method according to claim 1 wherein the step of providing a
mixture includes providing a mixture comprising a particulate
material transparent to radiation employed in the step of
irradiating.
12. The method according to claim 1 wherein the particulate
material comprises at least one of glass, ceramic, and
glass-ceramic.
13. The method according to claim 1 wherein the particulate
material is a glass.
14. The method according to claim 1 wherein the particulate
material comprises a metal.
15-19. (canceled)
Description
BACKGROUND
[0001] The present invention relates generally to methods of
forming structured sintered articles and particularly to methods of
forming structured sintered articles comprising one or more of
glass, ceramic, and glass-ceramic particles. The disclosed method
has been particularly developed for and may be particularly useful
in the manufacture of microfluidic devices and similar
structures.
[0002] Associates of the present inventors have previously
developed techniques for forming glass rib structures, as disclosed
for example in U.S. Pat. No. 5,853,446, and related methods for
producing microfluidic devices, as disclosed for example in U.S.
Pat. No. 6,769,444, both of which are assigned to the Assignee of
the present application. These previously developed methods
include, among other steps: providing a mixture of a binder and
sinterable particles, such as glass, glass-ceramic or ceramic
particles or mixtures thereof; molding the mixture to form a
desired structure; setting and demolding the structure; and
debinding and sintering the structure. Setting is performed by
heating the mixture for thermosetting binders, by cooling or
allowing the mixture to cool for thermoplastic binders, or by
irradiating the mixture for radiation-curable binders. In U.S. Pat.
No. 5,853,446 is disclosed that radiation-curable binders may take
the form of "hybrid" binders--mixtures of radiation-curable binder
materials and thermoplastic binder materials--providing fast
radiation-setting and at the same time achieving a more complete
debinding or "burnout" of the binder similar to that of
thermoplastic binders. In the purely UV or hybrid UV binder
embodiments, fast-setting by irradiation is used while the mixture
is in the disclosed roll mold and/or simultaneously with removal of
the mixture from the roll mold, in order to set the structure and
aid removal of the structure from the mold.
SUMMARY
[0003] For forming the rather complex structures that may be
desirable in a microfluidic device, flat molding processes similar
to that disclosed in U.S. Pat. No. 6,769,444 have proven more
reliable and adaptable than roll-molding such as disclosed in U.S.
Pat. No. 5,853,446. A disadvantage of the flat molding process,
however, is that the flat molding process generally requires the
use of absorbent material packed around and into the recesses of
the formed or structured mixture prior to debinding and sintering,
to assist in maintaining the shape of the structured mixture during
debinding and sintering. Alumina and more recently calcium
carbonate, which both can withstand high temperatures, have been
used by the inventors or their colleagues for this purpose. The use
of alumina or calcium carbonate is undesirable, however, in that it
must later be removed, adding to the number and expense of process
steps and providing a potential source of contamination.
Furthermore, etching, a typical process available to remove calcium
carbonate from glass, is not very environmentally friendly.
[0004] Without some absorbent material covering and packing the
structure, however, the shape of the structure is not always
retained to the degree that may be desired. In particular, without
the use of an absorbent material to help maintain the shape of the
green structure, as the structure is debinded and sintered, the
structure can soften or slump. Although radiation-curable binders
and thermo-setting binders can maintain the shape of the structure
to a greater degree than thermoplastic binders during debinding and
sintering, the use of radiation-curable binders for setting or
curing in contact with a mold can cause difficulty in separating
shaped structures from a mold, as well as difficulty in achieving
desired levels of curing or setting, and in achieving evenly
distributed curing or setting.
[0005] The present invention provides a method of forming a
structured sintered article including providing a mixture
comprising a sinterable particulate material and a binder, the
binder comprising, as a function of total resin content of the
binder, at least 50% by weight of a thermoplastic binder material
and at least 5% by weight of a radiation-curable binder material;
shaping the mixture with a mold to form a structure; setting the
structure by cooling the structure or by allowing the structure to
cool; separating the structure from the mold; irradiating the
structure so as to at least partially cure the radiation-curable
binder material, and debinding and sintering the structure so as to
form a structured sintered article. Shaping may include forming a
structure having one or more open channels, and sintering may
include sintering in together in contact with at least one
additional structure so as to cover or enclose the channels.
[0006] In the methods according to the present invention, the
thermoplastic binder, desirably a low melting binder, remains the
primary binder and the binder that performs the function of setting
or initially curing the structure, allowing for mold separation and
any necessary initial handling. This provides for easily controlled
and complete curing by simple means--cooling or allowing the
structure to cool. The thermoplastic binder can also provide good
flow and lubrication properties during debinding, minimizing
cracking and other issues that might occur with a heavily
crosslinked polymer as the primary binder. A sufficient amount of
radiation-cured resin acts as a rheology modifier belonging to the
thermoset family; therefore it does not re-melt continues to be an
effective rheology modifier up to and possibly even beyond the
thermoplastic binder debinding temperature range, thus maintaining
the shape of the structures being debinded and sintered, without
the use of particulate material packed around and into the
structure.
[0007] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein, including the detailed description which follows, the
claims, as well as the appended drawings.
[0008] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated in and constitute a part of this specification.
The drawings illustrate various embodiments of the invention, and
together with the description, serve to explain the principles and
operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIGS. 1A-1E are cross sections of one instance of a
structured sintered article at various points in a process of being
formed according to one or more methods of the present
invention;
[0010] FIG. 2 is a cross section of an alternative structure useful
in the step depicted in FIG. 1D;
[0011] FIG. 3 is a cross section of another alternative structure
useful in the step depicted in FIG. 1D;
[0012] FIGS. 4A-B are cross sections of another instance of a
structured sintered article that may be produced by one or more
methods of the present invention;
[0013] FIG. 5 is a flowchart depicting an embodiment of the method
of the present invention;
[0014] FIG. 6 is a digital photograph of a structured article
produced by a comparative process showing some slumping after
debinding and before sintering;
[0015] FIG. 7 is a digital photograph of a structured article
produced by a process or method according to an embodiment of the
present invention showing shape retention after debinding and
before sintering in contrast to the article of FIG. 6;
[0016] FIG. 8 is a graph of structured feature height for three
structures before and after debinding, the first two being
comparative and the last being an example according the methods of
the present invention;
[0017] FIG. 9 is a graph of yield stress as a function of
temperature for a structure undergoing a process according to one
or more methods of the present invention, along with structures
undergoing two comparative processes.
DETAILED DESCRIPTION
[0018] Reference will now be made in detail to the presently
preferred embodiments of the invention, example of which are
illustrated in the accompanying drawings. Whenever possible, the
same reference numerals will be used throughout the drawings to
refer to the same or like parts.
[0019] FIGS. 1A-1E are cross sections of one instance of a
structured sintered article at various points in a process of being
formed according to one or more methods of the present invention.
With reference to FIG. 1A, the methods generally include providing
a mixture 20 comprising a sinterable particulate material and a
binder, the binder comprising, as a function of total resin content
of the binder, at least 50% by weight of a thermoplastic binder
material, and at least 5% by weight of a radiation-curable binder
material. The mixture 20 is shaped with a mold 22 so as to form a
structure 26.
[0020] In the example of FIG. 1, the mold 22 is used to shape the
mixture 20 while the mixture 20 is in contact with a substrate 24,
which in the embodiment of FIG. 1 forms part of the resulting
shaped structure 26 and the final structured sintered article 50.
The step of shaping the mixture 20 with a mold may beneficially be
used to form the particular type of structure shown in FIG. 1, a
structure 26 having one or more open channels 28. The mixture 20
may be molded at an elevated temperature as needed or desired, and
the thermal energy may be supplied in various ways, such as heating
the mold, pre-heating the mixture, and the like.
[0021] After the mixture 20 is shaped with the mold 22, the
resulting structure 26 is then set by cooling the structure 26 or
by allowing the structure 26 to cool. The structure 26 is then
separated from the mold 22, resulting in a structure 26 comprising
the shaped mixture 20 of FIG. 1B.
[0022] After the structure 26 is separated from the mold 22, the
structure 26 is irradiated with radiation 30, as depicted
diagrammatically in the cross-section of FIG. 1C, so as to at least
partially cure the radiation-curable binder material in the mixture
20. Desirably, the structure 26 is irradiated sufficiently such
that the resulting yield stress of the mixture 20 is at least 8 Pa
(Pascal) at a temperature of 100.degree. C. The irradiation lightly
crosslinks the irradiation-curable binder material to provide
shaped objects having dimensional stability above the critical
shear yield stress of 8 Pa. High yield stress allows molded objects
to maintain shapes and avoid distortion and slumping as temperature
is raised up to the temperature at which debinding occurs. The
crosslinking network acts to lock the particles in place and allows
the thermoplastic binder material to melt and vaporize or burn out
without the structure slumping. The degree of binder crosslinking
is controllable by adjusting the radiation-curable thermoplastic
ratio and/or the dose of radiation.
[0023] The methods and materials described in the examples below
have been shown to be capable of producing yield stresses of the
mixture, after irradiation, of 100 Pa or greater at temperatures in
the range of 70 to 120.degree. C. The radiation 30 need not be
parallel, and need not come only from the direction depicted in the
figure, but can be delivered in any suitable fashion, including
through the substrate 24 (in the upward direction in the
orientation of the figure).
[0024] The curing radiation 30 is desirably UV radiation, but other
radiation such as visible, electron beam, and others may also be
adapted for use in the present invention. The radiation-curable
binder material may be a UV-curable material, such as one or more
acrylates, methacrylates, a vinyls, epoxies, thiols, styrenics, and
combinations thereof. Particularly good performance in the examples
below was found using acrylated oligomers, particularly a polyester
acrylated oligomer. Although the curable resin composition
currently most preferred is cured using free radical initiation,
cationically curable compositions are possible.
[0025] Desirably, the sinterable particulate material used in the
mixture 20 is a particulate material transparent to the radiation
30 employed in the step of irradiating, such as transparent to UV
radiation if that is the type employed. Presently preferred
materials are glasses, particularly those transparent to UV
radiation, such as borosilicate glasses, alumina-containing
borosilicate glasses, glass ceramics, ceramics, and mixtures
thereof, and presently most preferred is glass. Useful applications
may extend, however, to sinterable particles not generally
transparent to polymerizing radiation, such as metallic particles,
where the desired structure's shape and thickness permit.
[0026] After the structure 26 is irradiated as depicted in FIG. 1C
the structure 26 is then debinded and sintered to form a structured
sintered article 50, as shown in the cross section of FIG. 1E. In
the case of the article 50 of FIG. 1, an optional step is included,
depicted in FIG. 1D, of placing at least one additional structure
32 in contact with the structure 26. In the embodiment shown, the
optional additional structure 32 includes a mixture 20A having the
same or similar constituents as the mixture 20, and a substrate
24A. The sintering step thus optionally includes sintering the
structure 26 in contact with at least one additional structure 32.
A weight or other means of applying a force may be applied in the
direction across the contact of the structures 26 and 32 during
sintering. The result in this particular embodiment is to cover or
enclose the channels 28, so as to form a structured sintered 50
article having covered channels 34 formed therein. In the
particular embodiment of FIG. 1, the depicted channels 34 form some
of the fluidic channels of a microfluidic device. Additional layers
of substrates and fluidic channels may also be formed as part of
the same device, in the same or subsequent sintering steps, as
desired.
[0027] FIG. 2 is a cross section of an alternative structure 32
useful in the step depicted in FIG. 1D. The structure 32 includes a
formed mixture 20A on a substrate 24A. With the particular form of
the mixture 20A shown, the structure 32 also includes open channels
28A. These may or may not line up exactly with the channels 28 of
structure 26, and can be used to form more complex enclosed or
covered channels within a microfluidic device. Another alternative
for structure 32 is shown in FIG. 3. A plain substrate 24A or other
suitable structure without a mixture formed thereon may be
used.
[0028] Aspects of another alternative process according to the
methods of the present invention are depicted n FIGS. 4A-B. As
shown in FIG. 4A, the mold used in the methods of the present
invention may take the form of mold halves 22a and 22b that
cooperate to form the mixture 20 into a structure 26 without a
substrate. The molding process may be press-molding, casting,
injection-molding, vacuum-assisted molding, or other suitable
process. The structured sintered device 50 resulting after
irradiation and debinding and sintering may be a container or
crucible. More complex or intricately shaped parts are possible,
particularly by forming with injection molding.
[0029] When forming devices with covered or enclosed channels 32 as
shown in FIG. 1, it may be desirable to debind and/or pre-sinter
the articles, so that most if not all binder is removed, prior to
stacking as in FIG. 1D and sintering or final sintering. This
allows the gases produced by volatizing the binder material to
escape easily without having to follow long paths through the
resulting covered or enclosed channels 32.
[0030] The thermoplastic binder material is desirably one that is
easily vaporized or volatized without leaving a significant carbon
residue, and one that promotes mold release. Presently preferred
are one or more hydrocarbon waxes, waxy alcohols, or mixtures
thereof, including simple linear single waxy alcohols or blends,
paraffins, microcrystalline waxes, and so forth. The thermoplastic
binder material desirably comprises in the range of about 60 to
about 90% by weight of the resin content of the binder, with in the
range of about 20 to 40% by weight of a radiation-curable binder
material. Levels of irradiation-curable binder material greater
than 50% typically result in problems with mold separation and in
reduced green strength. The binder also typically includes a
photoinitiator appropriate to the radiation-curable system
employed, and may optionally include a dispersant.
[0031] Silicone or silicone-containing molds have been found useful
in producing the structures used in microfluidic devices, so it is
desirable in embodiments of the present invention used for
producing microfluidic devices that the binder components be
compatible with silicone. For this purpose it is also desirable
that the mixture have a viscosity in the range of 25 to about 50
Pasec (Pascal-seconds) at a shear rate of about 2.36/second at
about 75.degree. C., for appropriate formability in the molding
process.
[0032] The steps of a method according to an embodiment of the
present invention may be represented in a flowchart such as the
flowchart 100 of FIG. 5. In step 102 a mixture is provided
comprising a sinterable particulate material and a binder, the
binder comprising, as a function of total resin content of the
binder, at least 50% by weight of a thermoplastic binder material
and at least 5% by weight of a radiation-curable binder material.
In step 104 the mixture is shaped with a mold so as to form a
structure. The mixture may be shaped or formed against or on a
substrate or other object, as in the embodiments of FIGS. 1-3, or
solely within a mold structure, as in the embodiment of FIG. 4, or
even with objects included within the mold and/or within the
mixture, if desired. In step 106 the structure is set by cooling
the structure or by allowing the structure to cool. After the
structure is set, the structure is separated from the mold in step
108, and then irradiated so as to at least partially cure the
radiation-curable binder material in step 110. Finally in step 112,
the structure is debinded and sintered to form a structured
sintered article.
Experimental
[0033] As shown in Table I below, starting with system of glass
particles in a wax-based binder, experiments performed with various
ratios of binder to inorganic particles (comparative examples
C1-C6), with a special (bimodal) particle size distribution
(comparative examples C6 and C7), and with non-radiation curable
multi-component binders (comparative examples C8-C13), as well as
with materials useful in the inventive process, including a
thermoplastic binder material in the form of a wax-based binder and
a radiation-curable binder material in the form of a UV binder
(examples I1-I5). The materials were processed according to the
methods generally described above in connection with FIG. 1 above,
but ending at a debinding step, and without stacking or sintering
with additional elements. Viscosity of the mixtures at typical
molding temperatures were measured as shown in the table, and yield
stress of the set and mold-separated structures, or of the set,
mold-separated and then irradiated structures, were measured at
100.degree. C., a temperature at which slumping occurs in the
wax-based systems used. Results of these measurements are listed in
the table. The general shape retention of the structures through
debinding and/or pre-sintering was also observed.
[0034] In examples C1-C5, the shape of molded structures was
generally not well-preserved through debinding. FIG. 6 is a digital
photograph of a rectangular ridge formed using the described
processes with material corresponding to comparative example C4
after debinding. Softening of the previously sharply rectangular
shape can be detected in the figure.
[0035] In examples C6 and C7, a bimodal particle distribution was
employed. Replication of mold features was not optimal using the
mixture in C6, probably due in part to the higher viscosity, but
yield stresses were increased, and shape retention was good, even
at the modestly increased yield stress of 8 Pa at 100.degree. C. in
example C7. The desired viscosity however, at least for the flat
molding useful in making microfluidic devices, is in the range of
25 to about 50 Pasec at a shear rate of about 2.36/second at about
75.degree. C. Viscosity in these examples was thus outside the
preferred limits.
TABLE-US-00001 TABLE I Glass Binder Materials Yield Stress Base UV
or Special other UV or (Pa) wax other Particle wax/ UV/ binder/
Other Vis- @ 100 C. binder binder Wt. Distri- total total total
Binder cosity w/o with Ex. # Wt. % Wt. % % bution resin resin resin
Material (Pa s) UV UV C1 20 0 80 NA 100 0 0 NA 12.sup.6 1.4 NA C2
18 0 82 NA 100 0 0 NA 15.sup.6 2.7 NA C3 16 0 84 NA 100 0 0 NA
26.sup.6 3.7 NA C4 15 0 85 NA 100 0 0 NA 48.sup.6 4 NA C5 14 0 86
NA 100 0 0 NA 51.sup.6 5 NA C6 17 0 80 Bimodal 100 0 0 NA 68.sup.6
16.7 NA C7 22 0 78 Bimodal 100 0 0 NA 11.sup.6 8 NA C8 13 5.0 82 NA
72 0 28 B723.sup.1 226.sup.6 14.1 NA C9 12 6.0 82 NA 67 0 33
2044.sup.2 216.sup.6 19.8 NA C10 13 5.0 82 NA 72 0 28 2045.sup.3
248.sup.6 25.9 NA C11 19.25 2.75 78 NA 88 0 12 B723.sup.1 74.sup.6
17 NA C12 12 4 84 NA 75 0 25 400.sup.4 20.sup.6 0.4 NA C13 12 4 84
NA 75 0 25 500.sup.5 59.sup.6 0.8 NA I1 13.5 1.5 85 NA 90 10 0
2271.sup.8 46.sup.7 4.1 >100 I2 12.75 2.25 85 NA 85 15 0
2271.sup.8 51.sup.7 4.3 >100 I3 12.6 2.4 85 NA 84 16 0
2271.sup.8 53.sup.7 4.6 >100 I4 10.5 4.5 85 NA 70 30 0
2271.sup.8 60.sup.7 4.9 Not tested I5 7.5 7.5 85 NA 50 50 0
2271.sup.8 70.sup.7 3.8 Not tested .sup.1Neocryl .RTM. B723 (DSM
NeoResins) .sup.2Elvacite .RTM. 2044 (Lucite International)
.sup.3Elvacite .RTM. 2045 (Lucite International) .sup.4Polywax 400
(Baker Petrolite) .sup.5Polywax 500 (Baker Petrolite) .sup.6@
2.36/sec, 80.degree. C. .sup.7@ 2.34/sec, 70.degree. C.
.sup.8Sartomer CN2271 (Sartomer)
[0036] For comparative examples with non-radiation curable
multi-component binders, some examples with methyl methacrylate
polymers were tested (C8-C11), and some with added higher molecular
weight waxes, resulting in a multi-component wax binder (C12 and
C13).
[0037] The addition of methyl methacrylate polymers to the binder
in comparative examples C8-C11 provided the desired levels of yield
stress--in excess of 8 Pa. The viscosity of these examples was,
however, undesirably high.
[0038] The use of multi-component wax binders in examples C12 and
C13 resulted in even lower yield stresses than in the base wax
binder examples in C1-05. Polywax 700 was also tested with similar
results.
[0039] For materials useful according to the methods of the
invention, experiments were performed using the wax MX 4462 (CERDEC
France) at 11.1% by weight of the total mixture, with glass
particles at 84.05%, a UV-curable resin at 4.8%, and a
photoinitiator (Irgacure 1800, Ciba Specialty Chemicals, Zurich) at
0.05% by weight. UV resins tested include Rahn 01-554 and 01-514
(Rahn AG, Zurich), and Sartomer CN2270, CN2271 and CN9001
(Sartomer, Exton Pa. USA). De-molded structures were irradiated
with UV light at a typical dose of 2 J/cm.sup.2 by exposure with a
"D" bulb in a Fusion F450 unit (Fusion UV Systems, Inc., member of
Spectris PLC, Surrey, England) with conveyor.
[0040] Sartomer CN9001, an aliphatic urethane acrylate, was found
to be incompatible with wax. Rahn 01-514, a polyether acrylate, was
found to release undesirable amount of VOCs when employed in the
process. Rahn 01-544, a specialty resin, may be useful in various
processes but is not most preferred for applications using silicone
molds, as the mixture was found to swell a silicone-based mold.
Sartomer CN2270, a polyester acrylate, was found to result in
undesirably low viscosity, although viscosity could potentially be
increased by varying the mixture ratios or by other means. Sartomer
CN2271, another polyester acrylate, is presently most preferred as
it was found to produce acceptable viscosity, while being
compatible in the mixture and the described process with a wax
thermoplastic binder--with wax MX 4462 and additionally with C-18
(1-octadecanol)--and with a silicone or silicone-based mold.
[0041] Specific experimental mixture preparation for the inventive
examples I1-I1 was as follows: (1) weigh out binders both UV and
wax, and photoinitiator at 1% of UV weight, in amounts appropriate
to a 200 gram total batch weight of the mixture; (2) add wax and UV
resin and photoinitiator to a 100.degree. C. pre-heated 1.5 liter
planetary mixer (Charles Ross and Son Company, Hauppauge, N.Y.,
USA) and mix 5 minutes on low speed setting; (3) add 100.degree. C.
pre-heated glass frit to mixture and mix 3 hrs on high speed
setting; (4) package mixture in UV-light-blocking package while
shielding from UV light to preserve desired viscosity.
[0042] With Sartomer CN2271, some slight shape slumping was
observed at 5% by weight UV resin/total resin, but this potentially
acceptable or may potentially be overcome by use of more
photoinitiator, thus 5% appears as the lower bound of
radiation-curable resin. As shown in inventive examples I1-I5 in
Table I, weight percentages of total resin of 10, 15, 16, and 50%
were also tested successfully, although the range of 10-16% is
currently most preferred in order to provide lower viscosities at
typical molding temperatures. As mentioned above, at more than 50%
UV resin, mold separation becomes difficult and strength of the
thermally set but not-yet irradiated structure decreases
undesirably.
[0043] FIG. 7 is a digital photograph of a rectangular ridge formed
using the described inventive processes, with material
corresponding to inventive examples I1-I5, after debinding. As can
be seen in the figure, the rectangular shape is still sharp after
debinding and before sintering. Height in microns of raised
features such as the rib of FIG. 7 were measured before and after
debinding for glass-wax mixtures as in comparative examples C1-05,
and for UV-resin containing mixtures as in inventive examples
I1-I5, both with the step of post-demolding irradiation and
without. The results are shown in the graph of FIG. 8. Heights are
shown in microns, before and after, for a glass-wax mixture 202, a
UV-resin containing mixture without irradiation 204, and a UV-resin
containing mixture with irradiation 206. It may be seen from the
graph that the UV-induced cross linking has a powerful effect in
maintaining the shape of structures, even through the step of
debinding. That the effect is maintained--and even potentially
strengthened through additional cross-linking during debinding--is
shown by the results of the graph of FIG. 9, which shows measured
yield stress in Pa (Pascal), as a function of temperature in
degrees Celsius, for structures formed of the same three mixtures:
glass-wax with no UV resin (diamonds), a glass-wax-UV resin but no
irradiation in the process (rectangles), and glass-wax-UV resin
with irradiation in the process (triangles). From these results it
is also seen that the methods of the present invention can provide
yield stresses in shaped but not yet sintered structures in excess
of 100 Pa at temperatures from 70 to 120.degree. C.
[0044] In the methods according to the present invention, the
thermoplastic binder, desirably a low melting wax, remains the
primary binder, including the binder that performs the function of
setting the structure, allowing for mold separation and any
necessary handling. This also provides for easily controlled and
complete cure by simple means--cooling or allowing the structure to
cool. The thermoplastic binder, particularly a low melting wax,
also has good flow and lubrication properties during debinding.
This minimizes cracking and other issues that might occur with a
heavily crosslinked polymer as the primary binder. A sufficient
amount of radiation-cured resin, desirably UV-cured resin, acts as
a rheology modifier belonging to the thermoset family; therefore it
does not re-melt continues to be an effective rheology modifier up
to and possibly even beyond the wax debinding temperature range, as
suggested by the results shown in FIG. 9.
[0045] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *