U.S. patent application number 10/625261 was filed with the patent office on 2004-11-18 for finish for glass fabrics used for reinforcing epoxy structures.
Invention is credited to Adams, Richard G., Carter, H. Landis, Murari, Shobha.
Application Number | 20040228778 10/625261 |
Document ID | / |
Family ID | 26924782 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040228778 |
Kind Code |
A1 |
Murari, Shobha ; et
al. |
November 18, 2004 |
Finish for glass fabrics used for reinforcing epoxy structures
Abstract
A composition and method for selectively increasing the resin
flow and gel time of an epoxy resin that fills the capillary region
between filaments in fiberglass yarns making up a woven fabric. The
composition and method are used in the lamination of fiberglass
reinforced composites such as copper clad laminates for circuit
boards and reduces the occurrence of voids in the capillary
region.
Inventors: |
Murari, Shobha; (Greenville,
SC) ; Adams, Richard G.; (Lavellette, NJ) ;
Carter, H. Landis; (Greer, SC) |
Correspondence
Address: |
McNair Law Firm, P.A.
P.O. Box 10827
Greenville
SC
29603-0827
US
|
Family ID: |
26924782 |
Appl. No.: |
10/625261 |
Filed: |
July 23, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10625261 |
Jul 23, 2003 |
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09873401 |
Jun 4, 2001 |
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6720080 |
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60231054 |
Sep 8, 2000 |
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Current U.S.
Class: |
422/180 ;
422/175; 428/417; 428/418; 428/447 |
Current CPC
Class: |
H05K 2201/0239 20130101;
Y10T 428/31529 20150401; C08J 5/08 20130101; C08J 2363/00 20130101;
Y10T 428/249969 20150401; Y10T 428/31612 20150401; Y10T 428/249948
20150401; Y10T 428/31525 20150401; Y10T 428/31663 20150401; Y10T
442/2992 20150401; Y10T 428/31511 20150401; C03C 25/40 20130101;
H05K 1/0366 20130101 |
Class at
Publication: |
422/180 ;
428/447; 428/417; 428/418; 422/175 |
International
Class: |
B32B 017/10 |
Claims
1. (Cancelled)
2. (Cancelled)
3. (Cancelled)
4. (Cancelled)
5. (Cancelled)
6. (Cancelled)
7. (Cancelled)
8. (Cancelled)
9. (Cancelled)
10. (Cancelled)
11. (Cancelled)
12. (Cancelled)
13. The method of claim 23 wherein the glass fabric that is used
for reinforcing an epoxy resin article is prepared by the steps of:
a) providing fabric woven from glass fiber; b) cleaning said fabric
to remove sizing and other deposits on the surface of the filaments
in the glass fiber; c) preparing a bath for said fabric comprising
water and at least one cationic amino-silane coupling agent. d)
immersing said cleaned fabric in said bath; e) removing the fabric
from the bath and removing excess bath solution; f) drying the
fabric; g) immersing the finish and dried fabric in a solution of a
weak acid; h) drying the fabric.
14. The method of claim 16 wherein the bath comprises at least one
additional silane coupling agent, namely, a chloro-silane coupling
agent and the boric acid is present in the range from about 0.1% to
5% of the combined weight of said coupling agents.
15. (Cancelled)
16. The method of claim 13 wherein said weak acid is boric
acid.
17. (Cancelled)
18. (Cancelled)
19. (Cancelled)
20. (Cancelled)
21. (Cancelled)
22. (Cancelled)
23. In the method of making an epoxy resin article reinforced by
multifilament, fiberglass fabric, the improvement which comprises
providing a latent catalyst inhibitor on the surfaces of the
filaments of the fiberglass fabric.
24. (Cancelled)
25. (Cancelled)
26. An epoxy resin article made by the method of claim 23 and
having a substantially reduced number of voids in the space between
filaments in the multifilament fiberglass fabric.
27. The epoxy resin article of claim 26 wherein the article is a
circuit board.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based on and claims priority from
provisional application Ser. No. 60/231,054 filed Sep. 8, 2000.
FIELD OF THE INVENTION
[0002] This invention relates to finishes for glass fabrics that
are used to reinforce structures formed from epoxies and like
materials. In particular, the invention relates to finishes
employing silane based coupling agents for woven glass fibers. More
specifically, the invention relates to glass fiber fabric
reinforced circuit board laminates.
BACKGROUND OF THE INVENTION
[0003] The need for coupling agents was first recognized in 1940
when glass fibers began to be used as reinforcement in organic
resin composite structures. Specific strength to weight ratios of
early glass fiber resin composites were higher than those of
aluminum or steel, but they lost much of their strength during
prolonged exposure to moisture. The interface between such
dissimilar materials as an organic polymer and an inorganic glass
fiber did not allow the formation of a water resistant bond. A
variety of materials have since been developed in an attempt to
provide a stable interface under a varying number of adverse
environments. These coupling agents can generally be described as
molecules which possess two different kinds of reactivity. The
siloxane portion of these molecules has reactivity with the glass,
while the organic portion of the molecule is tailored to react with
organic thermosetting resins used in composite manufacturing. The
main function of the coupling agent is to provide a stable bond
between two dissimilar surfaces.
[0004] The majority of such coupling agents have the general
formula:
R CH.sub.2 CH.sub.2 CH.sub.2 Si (OCH.sub.3).sub.3
[0005] Where R is a reactive organic group tailored to match the
reactivity of the resin system with which it will be used.
[0006] The siloxane portion will react with the glass surface as:
1
[0007] More than one SiOH group may react with the glass surface,
or alternatively with other silane molecules to form siloxane
oligomers or polymers, which can still provide a link between glass
and resin.
[0008] Epoxy resins have commonly been used in the manufacture of
multilayered laminates for various applications in the electronic,
recreation, marine, and aerospace industries. The most common epoxy
is formed from epichlorohydrin and bisphenol A. The resin is
usually provided in the form of a low molecular weight oligomer,
which can be cross-linked with a bifunctional curing agent to
result in a solid thermoset polymer. Catalysts are often added to
accelerate the reaction with the curing agent. Multi functional
epoxies are sometimes added to the resin mix to improve the high
temperature resistance of the cured resin. Straight chain
polymerization of epoxy can result in a solid material, which is
thermoplastic and can be melted. Cross-linking with the curing
agent provides a thermoset solid, which does not melt.
[0009] To make multilayered laminates, prepregs are first made by
pulling glass cloth through a solution of the particular resin
system chosen. The glass cloth is impregnated with the resin mix
and then proceeds to a heated tower where the solvent is driven off
and the resin is partially polymerized to a "B" stage. It is
important that little, or no cross-links occur, before the resin
can melt and flow in the laminating process. The prepreg process is
tightly controlled to provide an optimum melt viscosity for
lamination.
[0010] Prepregs are tested for melt viscosity, resin gel time and
resin content. The gel time measurement is widely used in
determining the potential reactivity of the prepreg material, as
well as, the time available for resin flow in press lamination.
Controlling these parameters has been thought to be critical if a
void free cured laminate is to be obtained. Gel time is also an
indicator of the rate of increase of melt viscosity in press
lamination.
[0011] A common problem for multilayered laminates, such as circuit
board is delamination during wave soldering. The most common cause
of delamination during soldering is moisture absorption. The
thermal energy imparted to the board in contact with 550.degree. F.
solder vaporizes any absorbed moisture and the resulting steam
pressure forces the laminations apart at the line of the weakest
bond. Moisture, which accumulates in even minute voids, is
especially likely to produce blistering during the soldering
process.
[0012] Accordingly, it is one objective of the present invention is
to eliminate voids and the resulting entrapped moisture in
laminated fiberglass/epoxy composites, so that rupture of the
structures will not occur due to the thermal shock of the soldering
process.
[0013] In the preparation of woven glass fabrics for use in a
composite epoxy structure for circuit boards, organofunctional
alkoxysilane finishes are applied to heat cleaned fiberglass
fabrics from dilute aqueous solutions. The finish content of the
dried fabric is typically 0.075% to 0.30% of the fabric weight.
[0014] Epoxy resins are usually formulated with difunctional or
multi functional curing agents, which provide cross-linking thereby
resulting in a thermoset polymer after curing. A catalyst to
accelerate the curing is often added to the epoxy formulation. In
some cases epoxy resin, curing agent, and catalyst are applied to a
glass fiber substrate in solution, and dried at an elevated
temperature to remove the solvent. In some applications, it is
advantageous to continue heating impregnated substrates to increase
the resin molecular weight, and thus its melt viscosity. This is
known as a "B" stage prepreg. Any significant cross-linking in the
B stage prepreg will prevent the resin from melting and flowing in
a heated press during consolidation and curing of laminates. The
typical epoxy resin formulation for making circuit board prepreg
comprises epoxy resin dissolved in an organic solvent,
dicyandiamide (dicy) curing agent, and an imidazole accelerating
catalyst.
[0015] Electrical grade laminates for circuit boards are made by
curing layers of epoxy/fiberglass prepreg between copper foil
surface sheets. In the laminating process, multiple sheets of
epoxy/fiberglass prepreg are placed between copper foil surface
sheets. These lay ups are placed between metal laminating plates. A
number of these assemblies are stacked to form a book, and each
book is placed between heating platens in a multi-opening press.
Two competing processes occur as the prepreg is heated in the
press. First, the epoxy resin melts, and its viscosity is reduced
with increasing temperature. As the temperature rises, the resin
begins to polymerize and increase in viscosity. Finally the resin
is sufficiently cross-linked that it gels and can no longer flow.
Consolidation of the laminate must be completed before the resin
gels. Complete cure is achieved with additional time in the press
and increased temperature. The two processes must be carefully
balanced to insure a void free laminate with good thickness
control, and minimize resin loss at the laminate edges. If the
resin gels too soon, there may not be sufficient flow to remove
solvent or air trapped in the capillaries between individual
filaments in a fiber bundle. Minute voids in the capillary spaces
of the cured laminate are often referred to as silver streaks.
[0016] In circuit board manufacturing, the copper clad boards are
coated with a photosensitive acid resist. The desired circuitry is
then photo printed on the copper. The board is then subjected to a
hot acid bath to remove the unwanted copper. Holes are drilled for
mounting surface components, or for establishing electrically
conductive connections between circuits on both sides of the
board.
[0017] The holes are then electroplated. Finally, the board with
its assembled components is floated across a 550.degree. F. molten
solder bath. Any moisture, which has been absorbed into a void or
silver streak during the wet processing of the board, will cause it
to blister. If a void stretches between two adjacent through holes,
it may cause a short circuit in the finished assembly.
[0018] As with most manufacturing processes, it is desirable to
maximize the productivity of capital equipment. For some
applications, a high laminate glass transition temperature is
required. These objectives can be achieved by increasing the
catalyst, the curing agent, or the processing temperatures,
individually or in combination. These speeded up processes are
difficult to control, and have a very narrow processing window for
quality production. Any premature gelling of the resin in the
prepreg "B" stage or in lamination will result in scrap laminates.
Capillary voids cannot be seen until the copper foil is etched.
Even then, they are often hard to see. If silver streaks are
undetected in the laminates in early stages of circuit fabrication,
the cost of scrap escalates. As a quality check, acid is used to
remove all of the copper foil from one or more laminates of a
production lot to check for silver streaks. This does not preclude
that some laminates in the lot may have silver streaks.
Understandably, circuit board laminators, and their customers, are
anxious to have the assurance of materials and processes, which
eliminate all silver streaks.
[0019] One method of increasing the size of the processing window,
is to add a weak acid, such as boric acid, succinic acid, citric
acid, benzoic acid or maleic acid, to the epoxy resin mix to
complex with the catalyst, and make it latent until a desired
curing temperature in the range of 190-210.degree. C. is reached in
the press, at which time the cure is very rapid and complete. Such
a process is described in U.S. Pat. No. 5,308,895 and U.S. Pat. No.
5,314,702. The objective of this process, is to have all of the
resin in the prepreg gel at the same time and temperature and cure
very rapidly.
[0020] Silver streaks have been an occasional quality problem in
electrical laminates in the past, but the move to faster curing
resin systems has greatly increased the severity and cost of the
problem. While the latent catalyst approach of the before mentioned
patents sufficiently controls the gel time of free resin on the
surface of the prepreg layers and between yarns, there may be
insufficient gel time or too rapid an increase in melt viscosity
for the inherently slower capillary flow within the yam bundles to
eliminate capillary voids or silver streaks. Lengthening the
overall gel time of the resin to accommodate greater capillary flow
will result in excessive edge loss and lack of thickness control.
Since the glass fiber fabric is typically only 30% to somewhat less
than 50% of the volume of typical electrical prepreg, ensuring
sufficient flow in the yarn capillaries to eliminate voids without
excessive outflow of the free resin presents a serious problem.
[0021] Furthermore, some of the best finishes for obtaining
superior laminate properties contain silanes with amine functional
groups. These amino-functional silanes have primary or secondary
amine groups, which can react with, or catalyze, epoxy resins at a
lower than desired press temperatures. Amine functional silanes are
advantageous for finishing fiberglass fabrics. Because of their
cationic nature, they are attracted to the anionic glass fiber
surface, and provide a more evenly distributed finish.
[0022] Accordingly, a broad objective of the present invention is
to eliminate the silver streak problem.
[0023] One specific objective of the present invention is to
selectively control the gel time of resin in the capillaries
between the filaments of the yarn bundles making up the fiberglass
fabric.
[0024] A second objective of the invention is to provide a coupling
agent finish for fiberglass fabrics to independently control the
gel time of resin in the capillaries and melt viscosity.
[0025] Still another objective of the invention is to provide a
simple test method to predict, in the laboratory, the degree
capillary gel time control provided by any given fabric finish.
[0026] These and other objectives of the invention are achieved by
the present invention, which will be better understood by the
summary of the invention and detailed description, which follows
below:
SUMMARY OF THE INVENTION
[0027] It has surprisingly been found that the foregoing objective
can be achieved by incorporating a catalyst inhibitor in the finish
applied to glass fibers and glass fabric, so that the gel time of a
polymeric resin is selectively lengthened as the resin flows around
individual glass filaments and the capillary spaces between them.
The longer time to gel within the yarn bundles allows the resin to
flow completely around filaments eliminating capillary voids or
silver streaks, while the free resin retains its faster gel time
preventing excessive edge resin flow and maintaining thickness
control. This benefit is surprisingly achieved without any increase
in cure time in the press.
[0028] Accordingly, in one primary aspect the invention is a
composition and method for selectively controlling the gel time of
an epoxy resin in the capillary regions of a glass fabric. Another
aspect of this invention is a novel finish comprising an epoxy
reactive organosilane coupling agent and catalyst inhibitor capable
of selectively lengthening the flow time within the capillaries of
a glass fabric. Still another aspect of this invention is a
laboratory test to determine the relative degree of lengthening of
said gel time.
[0029] A further aspect of the invention, is a method of
selectively lengthening the gel time of an epoxy resin within the
yarn bundles of a fiberglass fabric comprising the steps of
providing cleaned woven glass fabric, providing a silane finish
bath for the fabric, adding boric acid or other weak acid to the
bath, immersing the fabric in the bath, drying the fabric, and
subsequently impregnating the fabric with an epoxy resin mix
whereby the gel time within the yarns is selectively
lengthened.
[0030] In another aspect the finish of the invention includes at
least one cationic amino-functional alkoxysilane coupling agent and
a weak acid that can complex with the amino function to provide a
latent reactivity with epoxy resin, whereby during lamination under
heat and pressure the finish selectively provides a longer resin
flow time and a slower increase in resin viscosity. The finish may
include an additional alkoxysilane coupling agent having a critical
surface tension greater than that of the epoxy resin to promote
resin wetting of the finished fabric. The weak acid in the finish
may be predetermined to complex with the catalysts in selected
resin formulations.
[0031] The invention also includes the method of making glass fiber
reinforced articles of polymeric materials including circuit boards
and the like products where it is important that the polymeric
material substantially fills the voids between fiber filaments.
DESCRIPTION OF THE DRAWINGS
[0032] In the drawings which are appended hereto and made a part of
this disclosure:
[0033] FIG. 1 is a scanning electron photomicrograph of two cross
sections of fiberglass fabric reinforced epoxy composites namely, a
circuit board laminate, magnified 200:1, showing a "good" cross
section having no voids within the yarns and a "bad" cross section
showing several capillary voids within two yarns as represented by
the dark spots. The white halos around the voids are an artifact of
electron micrography.
[0034] FIG. 2 is an electron micrograph showing the voids at 500:1
magnification; and,
[0035] FIG. 3 is an electron micrograph of a void at 5000:1
magnification where some resin has been chipped away from the
fibers in sample preparation. This shows that resin has coated the
fibers, but there was insufficient resin flow to fill the capillary
void.
DETAILED DESCRIPTION
[0036] In an effort to solve and find the reason for capillary
voids in epoxy/fiberglass circuit board laminates, many
organofunctional silanes and combinations thereof have been
evaluated as finishes, but none have proved satisfactory with
today's faster curing resin systems. Trials, which involve
finishing large quantities of fiberglass fabric followed by making
prepreg and pressing laminates in a manufacturing facility, are
very costly, and often yield little insight into the cause of the
voids. Thus a model system to learn more about the cause was
developed. Glass micro beads were used as a model for the
fiberglass surface. The micro beads were treated with various
silane finishes, and dried as in normal production. A formulated
epoxy resin mix was mixed with the micro beads and heated to remove
the solvent. The mixture was then placed on a plate heated to
350.degree. F., and the time for the resin to gel was recorded.
Untreated micro beads were used as a control. Micro beads treated
with some silane finishes were found to yield a shorter gel time
than the control, indicating a catalytic effect on the resin.
Surprisingly this was the case even when the resin was formulated
with boric acid to give latency to the imidazole catalyst.
[0037] It has been assumed, that the reaction between resin and
silane finished glass fibers is limited to a layer only a few
molecules thick at the immediate surface. It has also been assumed,
that silver streaks were due to poor wetting of the fiberglass
yarns. It now appears, that some silanes may have a catalytic
effect on epoxies extending considerably further into the
impregnated resin than previously thought. This catalytic effect is
sufficient to cause premature resin gel, and trap voids in yarn
capillaries, while free resin in the inter-laminar surfaces and
between yarns flows much faster and gels later. In manufacturing
practice, the resin gel time is controlled to minimize resin flow
at the laminate edges and to maintain a specified thickness
[0038] It has been discovered, that the addition of certain weak
acids to an amine functional silane finishing bath will provide a
finished glass fiber fabric, which overcomes the problem of
capillary voids or silver streaks. To solve this problem a slower
rate of melt viscosity increase, as indicated by a longer gel time,
is required for the resin within the yarns because of slower
capillary flow. For this reason the addition of a weak acid to an
amine functional silane coupling agent finish is desirable to
lengthen the gel time of catalyzed epoxy resin, which infiltrates
the glass fibers. It is also desirable in the case where the finish
contains an amine functional silane, which is catalytic to epoxies.
Thus, the invention is not limited to finishes for use with epoxy
resin formulations containing latent catalysts. The acid should be
chosen from those that do not volatilize during drying of the
finish or in prepreg and laminate processing. Such acids should
complex with the amine function of the finish, but dissociate at a
temperature at, or below, the final curing temperature in the
press, so as not to hinder a rapid laminate cure.
[0039] We have discovered, that finishes comprising at least one
cationic aminosilane, chloroalkylsilane, and boric acid give the
best results. Chloropropylsilane finished fiberglass has been shown
to have critical surface tension greater than that of typical epoxy
resins, and is thus easily wet by epoxy resin. The preparation of a
finish illustrating the invention begins with the preparation of a
bath for the glass fiber, as in the following example: Three
commercially available silanes are mixed together namely:
1 TABLE 1 Reactive group Manufacturer Manufacturer's Code Amino +
Vinyl Dow Corning Z 6032.sup.1 Amino Dow Corning Z 6020.sup.2
Chloro OSI Z6026.sup.3
.sup.1N-2-(Vinylbenzylamino)-ethyl-3aminopro- pyltrimethoxysilane
monohydrogen chloride N-(2-aminoethyl)-3-amin-
opropyltrimethonysilane .sup.33-chloropropyltrimethoxy silane
[0040] The foregoing silanes are mixed together based on their
organosilane content. The bath volume is 100 gallons of water to
which acetic acid is added to maintain a pH level in the 3 to 5
range. About 0.1 to 5% of the silane mixture are added followed by
the addition of 0.05% by weight of cationic surfactant, then boric
acid is added in the range of 0.1 to 5.0% of the weight of the
silanes, as required to adjust the capillary gel time for a given
resin formulation. Fiberglass fabric, which has been heat cleaned
to remove yarn and weaving sizes, is dipped in the finish bath,
squeezed between rollers to remove the excess then dried in an
oven. The dried finish concentration is typically between 0.075 and
0.30% by weight of the dried fabric, but may vary depending on the
catalyst used in the epoxy resin. In an alternate finishing method,
the boric acid can be applied as an after treatment on the silane
finished fabric. The now finished fabric is ready for use as the
reinforcing material in a fiberglass/epoxy composite laminate for
circuit boards or other applications. This finish exhibits superior
wetting by the epoxy resin during impregnation, and the boric acid
will allow additional time for the resin to flow within the
interstitial space between filaments that form capillaries of the
yarns before the resin gels. In addition, the longer gel time
allows a slower build up of resin melt viscosity, further enhancing
capillary resin flow.
[0041] Referring now to FIG. 1, typical circuit board laminates in
magnified images are presented side by side cross sections labeled
good and bad. The bundles of filaments are shown surrounded by
epoxy. These bundles are the yarn from which the glass fabric is
woven. In the side labeled "bad", the dark spots surrounded by
light halos are capillary voids. FIG. 2 is a greater magnification
of part of FIG. 1. FIG. 3 is an electron micrograph of a void at
5000:1 magnification, where some resin has been chipped away from
the fibers in sample preparation. This shows that resin has coated
the fibers, but there was insufficient resin flow to fill the void.
Returning now to the glass bead test described above, the effect of
variations of the finish according to the present invention is
demonstrated below. Epoxy resin formulated with dicyandiamide
curing agent, an imidazole catalyst, and boric acid catalyst
inhibitor was used. This formula duplicates one used in industry.
This test is to predict which finishes will be best able to
eliminate the capillary voids without actually having to carry the
process through all the manufacturing steps to a finished
laminate.
[0042] The time the epoxy resin takes to gel at 350.degree. F.,
when mixed with untreated beads was assigned a value of 1.0 in a
"gel time index", and is the control. In comparing finished beads,
the numerator is the time to gel with the specified finish and the
divisor is the time to gel with the unfinished control and the
quotient is the gel time index. An index number more or less than
one, indicates that the finish is causing acceleration of resin
gelling and will probably cause capillary voids or silver streaks
in actual laminates. An index number greater than one, indicates
that the finish is lengthening the gel time and is less likely to
cause capillary voids in an actual laminate.
[0043] The following results were obtained:
2 Finish Description Gel Time Index None 1.0 Three silanes (Table
1) 1.35 Three silanes + 1% boric acid 1.54 Three silanes + 2% boric
acid 1.62 One half the amount of the three 1.79 Silanes + 2% boric
acid
[0044] While the example above utilizes an epoxy resin system
containing an imidazole catalyst, boric acid and dicyandiamide as
curing agent, the finishes of this invention have wide spread
additional applications. We have determined that cationic
amino-silane finishes on fiberglass fabrics have a catalytic effect
on the cure of epoxy resin systems and, that this effect is
additive to the effect of a catalyst in the resin system. We have
also determined, that it is advantageous to selectively extend the
gel time of epoxy resin in the capillaries between fiberglass
filaments in a multi-filament yarn to insure sufficient flow to
eliminate voids. Therefore the invention has the following
applications:
[0045] In epoxy resin systems containing only epoxy resin and
curing agent, to latently block the catalytic effect of the
amino-silane in the finish to provide sufficient capillary resin
flow to achieve a void free laminate in epoxy resin systems
containing a catalyst in addition to a curing agent, to block the
catalytic effect of the amino-silane and to latently inhibit the
resin catalyst and lengthen the time of capillary flow. Other epoxy
curing agents and catalysts are within the scope of this invention.
For example; anhydride curing agents with tertiary amine
catalyst.
[0046] These resin systems are used in molding structural parts for
many applications in addition to electrical laminates. End use
applications include aerospace, boats, machine components, and
general structural applications. Molding methods include press,
autoclave and, resin injection molding.
[0047] It is understood that variations and additional embodiments
within the scope of our invention may occur to those skilled in the
art after reading our above specification but our invention is
limited only by the claims which follow:
* * * * *