U.S. patent application number 10/001499 was filed with the patent office on 2002-06-27 for self-adhesive prepreg face sheets for sandwich panels.
Invention is credited to Christou, Philippe D., Hedges, Winston L., Zhou, Ligui.
Application Number | 20020079052 10/001499 |
Document ID | / |
Family ID | 27076189 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079052 |
Kind Code |
A1 |
Zhou, Ligui ; et
al. |
June 27, 2002 |
Self-adhesive prepreg face sheets for sandwich panels
Abstract
Resin compositions are provided which are used in combination
with fibers to form self-adhesive prepreg sheets that are applied
to core materials to form sandwich panels. The prepreg resin
includes a thermoset resin, a curing agent and a viscosity control
agent. The prepreg resin further includes certain thermoplastic
particles which are used to control the flow characteristics of the
prepreg resin and the formation of fillets during bonding of the
prepreg to the core material.
Inventors: |
Zhou, Ligui; (Dublin,
CA) ; Christou, Philippe D.; (De La Tour, FR)
; Hedges, Winston L.; (Livermore, CA) |
Correspondence
Address: |
David J. Oldenkamp, Esq.
Shapiro, Borenstein & Dupont LLP
Suite 700
233 Wilshire Boulevard
Santa Monica
CA
90401
US
|
Family ID: |
27076189 |
Appl. No.: |
10/001499 |
Filed: |
November 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10001499 |
Nov 14, 2001 |
|
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|
09795177 |
Feb 27, 2001 |
|
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|
09795177 |
Feb 27, 2001 |
|
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09573760 |
May 18, 2000 |
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Current U.S.
Class: |
156/307.7 ;
156/210 |
Current CPC
Class: |
Y10T 156/1025 20150115;
Y10T 442/30 20150401; B32B 3/12 20130101; B29D 24/005 20130101;
B32B 37/146 20130101 |
Class at
Publication: |
156/307.7 ;
156/210 |
International
Class: |
B31F 001/20 |
Claims
What is claimed is:
1. A self-adhesive prepreg for bonding to a honeycomb, said
self-adhesive prepreg comprising: at least one fiber layer; a resin
which has been combined with said fiber layer to form a prepreg
which includes a bonding surface that is adapted to be bonded
directly to said honeycomb, said resin comprising a thermoset
resin, a curing agent and a sufficient amount of a viscosity
control agent to provide a resin having a viscosity which is
sufficient to allow said resin to be combined with said fiber layer
to form said prepreg; and thermoplastic fillet forming particles
which are incorporated into said resin in an amount sufficient to
form a prepreg resin which is self-adhesive and wherein said fillet
forming particles are not dissolved to a substantial degree in said
prepreg resin.
2. A self-adhesive prepreg according to claim 1 wherein said
thermoset resin is selected from the group consisting of epoxy and
cyanate ester resins.
3. A self-adhesive prepreg according to claim 1 wherein said
thermoplastic fillet forming particles are selected from the group
consisting of densified and micronized thermoplastic particles
which have a glass transition temperature that is above 200.degree.
C.
4. A self-adhesive prepreg according to claim 1 wherein said
thermoplastic fillet forming particles are selected from the group
consisting of densified polyether sulfone, micronized polyether
sulfone and densified polyetherimide.
5. A self-adhesive prepreg according to claim 3 wherein said
thermoplastic fillet forming particles have particle sizes ranging
from 1 to 100 microns.
6. A self-adhesive prepreg according to claim 1 wherein said
prepreg resin comprises an epoxy thermoset resin, a polyetherimide
or polyethersulfone viscosity control agent and densified polyether
sulfone fillet forming particles.
7. A self-adhesive prepreg according to claim 1 wherein the minimum
viscosity of said prepreg resin over the curing temperature range
of said prepreg resin is between 150 to 1500 poise.
8. A self-adhesive prepreg according to claim 1 wherein the minimum
viscosity of said prepreg resin over the curing temperature range
of said prepreg resin is between 300 to 1200 poise.
9. A self-adhesive prepreg according to claim 1 wherein said
thermoplastic fillet forming particles are located substantially at
said bonding surface of said prepreg.
10. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 1 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
11. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 3 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
12. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 5 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
13. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 6 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
14. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 7 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
15. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 8 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
16. A cured honeycomb panel comprising a core having at least one
face to which a self-adhesive prepreg according to claim 9 is
bonded and wherein said thermoplastic fillet forming particles are
substantially dissolved in said prepreg resin.
17. A method for adhesively bonding a prepreg face sheet to a
honeycomb comprising the steps of: forming a self-adhesive prepreg
comprising providing at least one fiber layer and a prepreg resin
wherein said prepreg resin is combined with said fiber layer to
form a prepreg resin layer comprising a bonding surface which is
adapted to be bonded directly to said honeycomb, said prepreg resin
comprising a thermoset resin, a curing agent and a sufficient
amount of a viscosity control agent so that said prepreg resin has
a viscosity which is sufficient to allow said prepreg resin to be
combined with said fiber layer to form said prepreg resin layer,
said step of forming a self-adhesive prepreg further including the
step of incorporating thermoplastic fillet forming particles into
said prepreg resin in an amount sufficient to form a prepreg layer
which is self-adhesive and wherein said fillet forming particles
are not dissolved to a substantial degree in said prepreg resin;
bonding said self-adhesive prepreg to said honeycomb wherein said
bonding comprises curing said self-adhesive prepreg for a
sufficient time and at a sufficient temperature to substantially
dissolve said fillet forming particles.
18. A method according to claim 17 wherein said thermoset is
selected from the group consisting of epoxy and cyanate ester
resins.
19. A method according to claim 17 wherein said thermoplastic
fillet forming particles are selected from the group consisting of
densified and micronized thermoplastic particles which have a glass
transition temperature that is above 200.degree. C.
20. A method according to claim 17 wherein said thermoplastic
fillet forming particles are selected from the group consisting of
densified polyether sulfone, micronized polyether sulfone and
densified polyetherimide.
21. A method according to claim 18 wherein said thermoplastic
fillet forming particles have particle sizes ranging from 1 to 100
microns.
22. A method according to claim 17 wherein said prepreg resin
comprises an epoxy thermoset resin, a polyetherimide or
polyethersulfone viscosity control agent and densified polyether
sulfone fillet forming particles.
23. A cured honeycomb sandwich panel comprising a core having at
least one face to which a self-adhesive prepreg according to claim
1 is bonded and wherein said thermoplastic fillet forming particles
are substantially dissolved in said prepreg resin and wherein said
honeycomb exhibits a core crush of less than 5%.
24. A cured honeycomb sandwich panel according to claim 23 wherein
said fabric layer comprises 6K or 12K carbon fabric.
25. A cured honeycomb sandwich panel according to claim 24 wherein
said fabric layer comprises 6K or 12 K carbon fabric and said
honeycomb exhibits a core crush which is essentially 0%.
Description
[0001] This is a continuation-in-part application of copending
application Ser. No. 09/795,177, filed Feb. 27, 2001, which is a
continuation-in-part of copending application Ser. No. 09/573,760
filed May 18, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to sandwich panels
and other related structural composite materials. Sandwich panels
are typically made up of face sheets which are adhesively bonded to
opposite sides of a core material to form the sandwich panel. In
particular, the present invention relates to sandwich panels in
which the face sheets are self-adhesive so that a separate adhesive
film is not required for bonding to the core.
[0004] 2. Description of Related Art
[0005] Sandwich panels are used in a wide variety of applications
where high strength and light weight are required. The cores which
are used in most sandwich panels are either lightweight honeycomb,
rigid foam, paper or wood. Honeycomb is a popular core material
because of its high strength to weight ratio and resistance to
fatigue failures. Honeycomb cores are made from a wide variety of
products including metals and composite materials.
[0006] The face sheets which are bonded to each side of the
sandwich core have also been made from a wide variety of materials
which also include metals and composites. An important
consideration in the formation of any sandwich panel is the
adhesive which is used to bond the face sheets to the core. The
adhesive must rigidly attach the facings to the core in order for
loads to be transmitted from one facing to the other and to permit
the structure to fulfill all the assumptions implied in the
acceptance of the commonly used stress calculation methods. If the
adhesive fails, the strength of the panel is severely compromised.
The adhesive layer is especially critical in sandwich panels which
use honeycomb cores because of the relatively small surface area
over which the honeycomb edges contact the face sheets.
[0007] Honeycomb sandwich panels are used in many applications
where stiffness and structural strength of the panel are primary
considerations. Additionally, honeycomb sandwich panels are also
widely used in the aerospace industry where the weight of the panel
is also of primary importance. As a result, there has been and
continues to be a concerted effort to reduce the weight of the
honeycomb sandwich panels without sacrificing structural strength.
One area which has been investigated to reduce weight is the
elimination of separate adhesive layers. This has been accomplished
by making the face sheets from composite materials which are
self-adhesive. Exemplary self-adhesive face sheets are described in
published European Patent Application Nos. EP0927737 A1 and
EP0819723 A1.
[0008] One procedure for applying face sheets to honeycomb involves
forming a prepreg sheet which includes at least one fabric or fiber
layer and an uncured prepreg resin. Prepreg is a term of art used
in the composite materials industry to identify mat, fabric,
nonwoven material or roving which has been preimpregnated with
resin and which is ready for final curing. An adhesive is typically
applied to the prepreg and it is then bonded to the honeycomb by
curing of both the prepreg resin and adhesive resin at elevated
temperature.
[0009] In these instances where the prepreg is bonded without using
a separate adhesive (i.e., the prepreg is self-adhesive), the
prepreg resin must meet the dual requirements of providing suitable
structural strength while still providing adequate adhesion to the
honeycomb. There is a present and continuing need to identify and
develop prepreg resins which are suitable for use in self-adhesive
prepregs to provide lightweight yet structurally strong sandwich
panels.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, prepreg face
sheets have been developed that are useful as self-adhesive face
sheets which are bonded to honeycomb to form sandwich panels that
are lightweight, structurally strong and exhibit many other
desirable properties. Sandwich panels incorporating the
self-adhesive prepregs of the present invention exhibit high peel
strength, good hot/wet properties, low solvent absorption, high
resistance to core crushing and other properties that are desirable
for aerospace applications.
[0011] Self-adhesive prepregs in accordance with the present
invention include at least one fiber layer that is impregnated with
a prepreg resin to form a prepreg which has a prepreg resin layer
having a bonding surface which is bonded directly to the honeycomb
during sandwich panel formation. The prepreg resin includes a
thermoset resin, a curing agent and a sufficient amount of a
viscosity control agent to provide a prepreg resin having a
viscosity which is sufficient to allow the prepreg resin to be
combined with the fiber layer to form the prepreg resin layer. As a
feature of the present invention, it was discovered that certain
types and sizes of thermoplastic particles may be used as fillet
forming particles that can be incorporated into the prepreg resin
in amounts sufficient to make the prepreg self-adhesive while not
adversely affecting the viscosity or other properties of the resin
which are required for its use as a prepreg resin.
[0012] As a further feature of the present invention, it was found
that thermoplastic fillet forming particles, such as densified
polyethersulfone, are not dissolved to any substantial degree when
they are loaded into the prepreg resin. As a result, the prepreg
resin can be loaded with enough fillet forming particles to
substantially increase the bonding strength of the resin while at
the same time not increasing or decreasing the viscosity of the
prepreg resin to unacceptable levels. It was discovered that the
fillet forming particles dissolve during the curing process to
provide the resin with adhesive characteristics that enhance fillet
formation between the prepreg and honeycomb. Fillet size and shape
are known to be an important consideration in the bonding of face
sheets to honeycomb. In addition, the dissolved fillet forming
particles provide toughening of the resin which improves overall
bond strength.
[0013] The present invention involves not only the uncured
self-adhesive prepregs, but also includes the prepregs after they
have been attached to honeycomb and cured to form finished sandwich
panels. The invention also covers methods for bonding the
self-adhesive prepregs to honeycomb in order to form sandwich
panels. The methods involve forming a self-adhesive prepreg by
providing at least one fiber layer and a prepreg resin wherein the
prepreg resin is combined with the fiber layer to form a prepreg
having a bonding surface which is adapted to be bonded directly to
one or both faces of the honeycomb. The prepreg resin includes a
thermosetting resin, or a combination of thermosetting resins such
as epoxy, cyanate ester, bismaleimide, and the like, curing agents
and a sufficient amount of viscosity control agent so that the
prepreg resin has a viscosity which is sufficiently low to allow
the prepreg resin to be combined with the fiber layer to form the
prepreg and yet sufficiently high to be largely retained in the
fiber layer during curing.
[0014] In accordance with the present invention, the step of
forming a self-adhesive prepreg further includes the step of
incorporating thermoplastic fillet forming particles into the
prepreg resin in an amount sufficient to form a bonding surface
which is self-adhesive and wherein the fillet forming particles are
not dissolved to any substantial degree in the prepreg resin. As a
final step in the method, the self-adhesive prepreg is bonded to
said honeycomb wherein the bonding involves curing the
self-adhesive prepreg for a sufficient time and at a sufficient
temperature to substantially dissolve the fillet forming
particles.
[0015] The prepregs and finished sandwich panels made in accordance
with the present invention may be used in a wide variety of
situations where a light weight and structurally strong material is
needed. However, the invention is especially well-suited for use in
aerospace applications where a multitude of strict mechanical and
chemical requirements must be met while at the same time not
exceeding weight limitations. The above-described and many other
features and attendant advantages of the present invention will
become better understood by reference to the following detailed
description when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a perspective view of an exemplary honeycomb core
and two self-adhesive face sheets prior to bonding together to form
the sandwich panel.
[0017] FIG. 2 is a perspective view of an exemplary sandwich panel
which has been formed by bonding together the honeycomb core and
face sheets shown in FIG. 1
[0018] FIG. 3 is a side view of a portion of the sandwich panel
shown in FIG. 2.
[0019] FIG. 4 is a side schematic view showing fillet formation and
particle dissolution in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The prepregs of the present invention are preferably used to
form self-adhesive faces sheets which are attached to honeycomb
cores to form light weight structural panels for use in aerospace
applications. Referring to FIGS. 2 and 3, a honeycomb sandwich
panel is shown generally at 10. The sandwich panel 10 includes a
honeycomb core 12 which has first and second faces 14 and 16,
respectively. Uncured self-adhesive prepregs 17 and 19 (see FIG. 1)
are applied to the core 12 and then cured to form face sheets 18
and 20 of the finished panel 10 (see FIG. 2). As shown in FIG. 3,
the face sheets 18 and 20 are applied directly to the core 12
without a separate adhesive layer. Each of the face sheets 18 and
20 are made up of fibers 22 which are embedded in a prepreg resin
matrix 24. In FIG. 1, the face sheets are shown as prepregs 17 and
19 prior to application to the honeycomb core 12.
[0021] The honeycomb core 12 can be made from any of the materials
which are used to form honeycomb cores. Exemplary honeycomb
materials include aluminum, aramid, carbon or glass fiber composite
materials, resin impregnated papers and the like. A preferred
honeycomb material is an aramid-based substrate, for example,
NOMEX.RTM. which is available from E.I. DuPont de Nemours &
Company (Wilmington, Del.). The dimensions of the honeycomb can be
varied widely. Typically, the honeycomb cores will have 1/8 to 1/2
inch (3.2-12.7 mm) cells with the cores being 1/4 inch (6.4 mm) to
2 inches (50.8 mm) thick. In so far as the present invention
provides a "self adhesive" prepreg, "self-adhesive" is
self-defining in that the prepreg will form a suitable panel
without the use of an adhesive layer. As will be discussed
hereinafter, a peel test is a primary way of verifying that the
resultant panel is suitable. Preferably, "self-adhesive" prepregs
yield a peel strength (under ASTMD 1781) of 20 in-lb/3 in width or
higher (8 pound core), and more preferably yield a peel strength of
28 in-lb/3 in width or higher (8 pound core). However, peel
strength specifics are dependent on the type and size of honeycomb
used.
[0022] The fibers 22 which are used in the face sheets 18 and 20
can be any of the fiber materials which are used to form composite
laminates. Exemplary fiber materials include glass, aramid, carbon,
ceramic and hybrids thereof. The fibers may be woven,
unidirectional or in the form of random fiber mat. Preferred fiber
materials include 193 gsm plain weave carbon fabric with 3K fibers
which are commercially available.
[0023] The resins which are combined with the fiber layer to form
prepregs in accordance with the present invention include epoxy
and/or cyanate ester resins, curing agents, viscosity control
agents and thermoplastic fillet forming particles. Epoxy or cyanate
ester resins are first mixed with viscosity control agents to form
a resin mixture. If necessary, the mixture is heated to ensure that
viscosity control agents are completely dissolved. Curing agents
and fillet forming particles are then added to the resin mixture.
The final resin mixture is kept below the temperature at which the
fillet forming particles dissolve in the resin. As a result, the
fillet forming particles which at this stage are uniformly mixed
throughout the resin are not dissolved to a substantial degree and
therefore do not increase the resin viscosity to an unacceptable
level. The viscosity of the resin mixture is important because it
must be such that the resin can be impregnated into the fiber to
form the prepreg. For the purposes of this specification, particles
which retain at least 90 weight percent of their original particle
weight are considered to be not dissolved to a substantial degree.
Particles are considered to be substantially dissolved when less
than 10 percent by weight of the original particle remains intact
within the resin.
[0024] The viscosity of the final resin mixture, including fillet
forming particles, should be between 150 and 1500 poise. The
preferred viscosity is between 300 to 1200 poise. The preceding
viscosity ranges represent minimum viscosities for the final resin
mixture prior to making prepreg when said viscosity is measured by
Pheometric Dynamic Analysis (Rheometrics RDA2) at settings of
2.degree. C./min, 10 rads/sec and 0.8-1.0 mm gap. The viscosity of
the resin mixture gradually increases when the fillet forming
particles dissolve during the curing process.
[0025] During the prepregging process the fillet forming particles
tend to be concentrated toward the surface of the prepreg due to
inherent filtering of the particles by the fiber layer.
Alternatively, the fillet forming particles may be applied (e.g.,
by powder deposition) to the surface of the resin after it has been
formed into a prepreg film or after the resin has been impregnated
into the fiber layer. In this way, the fillet forming particles are
distributed substantially on the surface of the prepreg. In either
case, the resin temperature is maintained at a sufficiently low
level to prevent fillet forming particles from dissolving until the
prepreg is applied to the core material and cured.
[0026] During the curing process, the prepreg is heated to a
sufficient level to substantially dissolve the fillet forming
particles. The dissolving of the particles during the curing
process was found to control the flow of resin at the prepreg-core
interface to promote fillet formation. In addition, the dissolved
thermoplastic particles enhance the toughness of the bond. Fillet
size and shape have long been known to be an important aspect of
bond formation between honeycomb core and adhesively bonded face
sheets.
[0027] Referring to FIG. 4, the uncured face sheet 17 is shown in
position against the face 14 of one wall of the honeycomb 12. The
prepreg 17 includes a fiber layer 22 which has been combined with
the prepreg resin to form a prepreg resin layer 30 which includes a
bonding surface 32 which is adapted to be bonded directly to the
honeycomb 12 at face 14. The thermoplastic fillet forming particles
34 are shown being distributed preferentially on the outer faces of
the face sheet 17. It will be noted by those skilled in the art
that the fibers and particles are not shown to scale in FIG. 4.
Typically, the diameter of the fibers 22 will be much smaller than
the thermoplastic particles 34.
[0028] As represented by arrow 36, the prepreg 30 is cured at
elevated temperature to form the single ply face sheet 18. As shown
in FIG. 4, the cured face sheet 18 includes a cured prepreg resin
matrix 38 in which the thermoplastic fillet forming particles are
substantially dissolved. As the fillet forming particles 34
dissolve during the curing process, the viscosity of the prepreg
resin increases so as to form fillets 40 and 42. The fillets 40 and
42 are preferably sized so that the "A" dimension (parallel to the
prepreg face sheet) is approximately equal to the "B" dimension
(perpendicular to the face sheet). Further, the length of
dimensions A and B are preferably maximized as much as possible in
order to achieve optimum bond strength for a given resin. As will
be appreciated, resins having a viscosity which is too low during
the curing process will produce fillets wherein the B dimension is
relatively large and the A dimension is very small. Such fillet
shapes do not provide good bonding. Alternatively, if the viscosity
of the prepreg during curing is too high, the B dimension of the
fillet is unacceptably small so that adequate bonding is not
achieved. In accordance with the present invention, it was
discovered that including the thermoplastic fillet forming
particles in the resin matrix provides the appropriate resin
viscosity during the curing process, such that fillets were formed
where dimensions A and B were equivalent and sufficiently large to
provide good bonding. Although not wishing to be bound by any
theory, it is believed that the gradual dissolving of the fillet
forming particles during the curing process provides a gradual
increase in resin viscosity which enhances fillet formation. The
gradually dissolving thermoplastic particles control the flow of
the resin at the honeycomb surface 14 so that fillets of the type
shown in FIG. 4 at 40 and 42 are formed. In addition, the prepreg
resin forming the fillets is preferentially enriched with
thermoplastic that is believed to also contribute to increased bond
strength.
[0029] Exemplary thermosetting resins which may be used to make the
prepreg resin include epoxy, cyanate ester and bismaleimide resins.
Exemplary epoxy and cyanate ester resins include glycidylamine type
epoxy resins, such as triglycidyl-p-aminophenol,
tetraglycidyldiaminodiphenyl-m- ethane; glycidyl ether type epoxy
resins, such as bisphenol A type epoxy resins, bisphenol F type
epoxy resins, bisphenol S type epoxy resins, phenol novolak type
epoxy resins, cresol novolak type epoxy resins and resorcinol type
epoxy resins; cyanate esters, such as 1,1'-bis(4-cyanatophenyl)
ethane (AroCy L-10, available from Vantico, Inc., Brewster, N.Y.),
1,3-Bis (4-cyanateophenyl-1-1-(1-methylethylidene) benzene (RTX366,
available from Vantico, Inc., Brewster, N.Y.). Epoxy resins are
preferred. Especially preferred epoxy blends include a mixture of
trifunctional epoxy and a difunctional bis-F epoxy.
[0030] Curing agents and viscosity control agents are also added to
the thermosetting resin to form the basic prepreg resin. The curing
agent is preferably an amine curing agent and the viscosity control
agent is preferably a thermoplastic material which dissolves in the
thermosetting resin.
[0031] Although the present invention contemplates the use of
thermoplastic fillet forming particles in a wide variety of prepreg
resins to enhance formation of fillets between the face sheets and
honeycomb core, prepreg resins based on epoxy and cyanate ester
formulations are preferred. The following examples and description
will be limited to epoxy formulations with it being understood that
the bonding between other prepreg face sheets and honeycomb core
may be improved by incorporating the thermoplastic fillet forming
particles of the present invention into the prepreg face sheet. In
addition, this description will be limited to a discussion of
sandwich panels which have two prepreg plies on each face of the
honeycomb. The two layers are preferably either two (0/90) plies or
two (.+-.45, 0/90) plies with warp direction aligned with the
lengthwise direction of the honeycomb. Those of ordinary skill in
the art will recognize that the present invention is also
applicable to multiple ply face sheets which include two or more
fiber layers.
[0032] Exemplary preferred prepreg resin formulations are as
follows:
[0033] 1 to 70 parts by weight of an epoxy;
[0034] 5 to 40 parts by weight of an amine curing agent;
[0035] 1 to 30 parts by weight of a viscosity control agent;
and
[0036] 5 to 50 parts by weight of thermoplastic fillet forming
particles.
[0037] 10 to 40 parts by weight of a trifunctional epoxy resin;
[0038] 10 to 40 parts by weight of a difunctional epoxy resin;
[0039] 11 to 25 parts by weight of an aromatic curing agent;
[0040] 0 to 3 parts by weight of a non-aromatic curing agent;
and
[0041] 5 to 15 parts by weight of a viscosity control agent
[0042] 8 to 30 parts by weight of thermoplastic fillet forming
particles.
[0043] The epoxy may be composed of trifunctional epoxy,
difunctional epoxy and a wide variety of combinations of
trifunctional and difunctional epoxies. Tetrafunctional epoxies may
also be used. Exemplary trifunctional epoxy include triglycidyl
p-aminophenol and N,N-Diglycidyl-4-glycidyloxyaniline (MY-0510 or
MY-0500 available from Vantico, Inc., Brewster, N.Y.). Exemplary
difunctional epoxies which may be used in the resin include Bis-F
epoxies, such as GY-281, LY-9703 and GY-285 which are available
from Vantico, Inc., Brewster, N.Y.). Bis-A epoxies, such as GY-6010
(Vantico, Inc., Brewster, N.Y.) and DER 331 (Dow Chemical, Midland,
M.I.) are suitable Bisphenol-A type epoxies and may also be used.
An exemplary tetrafunctional epoxy is tetraglycidyl diaminodiphenyl
methane (MY-721, MY-720 and MY-9512 available from Vantico, Inc.,
Brewster, N.Y.). Other suitable epoxies include phenol novolak type
epoxy, cresol novolak epoxy and resorcinol type epoxy. Preferred
bis-F epoxies include GY281 and GY285 which are available from
Vantico, Inc., Brewster, N.Y.
[0044] Exemplary curative agents include dicyandiamide,
3,3-diaminodiphenylsulfone (3,3-DDS), amino or glycidyl-silanes
such as 3-amino propyltriethoxysilane, CuAcAc/Nonylphenol (1/0.1),
4,4'-diaminodiphenylsulfone (4,4'-DDS),
4,4'-methylenebis(2-isopropyl-6-m- ethylaniline), e.g., Lonzacure
M-MIPA (Lonza Corporation, Fair Lawn, N.J.),
4,4'-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure M-DIPA
(Lonza Corp., Fair Lawn, N.J.). Dicyandiamide and 3,3-DDS are
preferred curative agents. Especially preferred are combinations of
3,3-DDS and dicyandiamide.
[0045] Exemplary viscosity control agents include thermoplastic
polyetherimides such as ULTEM.RTM. 1000P which is available from
General Electric (Pittsfield, Mass.); micronized polyethersulfone
such as 5003P, which is available from Sumitomo Chemical Co., Ltd.
(Osaka, Japan); HRI-1, which is available from Hexcel Corp.
(Dublin, Calif.); and polyimide MATRIMID.RTM. 9725, which is
available from Vantico, Inc. (Brewster, N.Y.). ULTEM.RTM. 1000P and
micronized PES are preferred. Micronized PES is especially
preferred. The amount and type of viscosity control agent which is
added to the epoxy resin mixture may be varied provided that the
minimum viscosity of the final resin mixture is maintained between
150 and 1500 poise when said viscosity is measured by Rheometric
Dynamic Analysis (Rheometrics RDA2) at settings of 2.degree.
C./mni, 10 rads/sec and 0.8-1.0 mm gap. As previously mentioned,
mixtures with minimum viscosities of between 300 to 1200 poise are
preferred. The viscosity of the prepreg resin prior to addition of
the fillet forming particles should be between about 50 poise and
2000 poise at room temperature. The preferred viscosity range is
100 poise to 1500 poise at room temperature.
[0046] Densified polyethersulfone (PES) and densified
polyetherimide particles may be used as suitable fillet forming
particles. Densified PES particles are preferred. The densified
polyethersulfone (PES) particles are preferably made in accordance
with the teachings of U.S. Pat. No. 4,945,154, the contents of
which is hereby incorporated by reference. The average particle
size of the PES particles range from 1 to 150 microns. Average
particle sizes of 1 to 50 microns are preferred and average
particle sizes of 10 to 25 microns are particularly preferred. The
microspheres are generally spherical in shape and are classified by
passing the densified microsphere powder through a micron sieve. It
is preferred that the glass transition temperature (Tg) for the
particles be above 200.degree. C.
[0047] In an alternative embodiment, the PES is "micronized."
Micronized PES refers to PES particles which have a rough surface
configuration which is produced by grinding the particles or other
abrasive techniques of manufacture which are known in the art.
Micronized PES particles may also be made by spraying and drying
procedures which are also known in the art. Micronized PES
particles are preferably less than 120 microns in size. Especially
preferred are particles under 50 microns in size with a range of 10
to 25 microns being particularly preferred.
[0048] The prepreg resin is made by first mixing the epoxy
components together and then slowly adding the polyetherimide or
micronized PES viscosity control agents. The resulting mixture is
heated to around 130.degree. C. and mixed for a sufficient time to
dissolve the polyetherimide/PES particles. Once the
polyetherimide/PES is dissolved, the mixture is cooled to around
75.degree. C. The aromatic amine curing agent and the fillet
forming densified PES particles are then added to the mixture. The
resin should be kept at temperatures below about 70.degree.
C.-75.degree. C. while the curative agent and densified PES
particles are being mixed into the resin. The final resin has a
minimum viscosity of between 150 to 1500 poise when said viscosity
is measured by Rheometric Dynamic Analysis (Rheometrics RDA2) at
settings of 2.degree. C./min, 10 rads/sec and 0.8-1.0 mm gap. The
preferred viscosity range is 300 to 1200 poise.
[0049] The finished resin is applied to the desired fabric to form
a prepreg. The resin content of the prepreg may be varied depending
upon a number of different parameters in order to achieve desired
mechanical and structural properties for the sandwich panel. It is
preferred that the prepreg have a resin content of 35-45 weight
percent.
[0050] The prepreg is bonded to the faces of the honeycomb core
using vacuum and/or pressure and heat to cure the prepreg and form
face sheets which are securely bonded to the honeycomb. The amount
of vacuum, pressure and heat required to cure and bond the prepreg
to the honeycomb may be varied depending upon the particular resin
formulation and the amount of resin in the prepreg. In general,
sufficient pressure must be applied to the prepreg to ensure that
the resin flows into the honeycomb cells a sufficient amount to
provide adequate fillet formation and bonding.
[0051] The temperature and other curing conditions are selected
such that the densified PES particles are substantially dissolved
during the curing process. It has also been found that panels made
with the inventive self-adhesive provide excellent resistance to
core crush. For example, panels of the type set forth in Examples 1
and 2 exhibit essentially 0% core crush. Other conventional
aviation panels utilizing 3 K carbon fabric prepregs in accordance
with the present invention will also show improved resistance to
core crushing. Even panels made utilizing 12 K carbon fabric
prepreg in accordance with the present invention will produce
panels which exhibit only 5% core crush.
[0052] Examples of practice are as follows:
EXAMPLE 1
[0053] Resin was prepared having the following formulation:
[0054] 23 weight percent MY-0510
(N,N-Diglycidyl-4-glycidyloxyaniline)
[0055] 25 weight percent GY281 (bis-F epoxy)
[0056] 19 weight percent 3,3-Diaminodiphenylsulfone (3,3-DDS)
[0057] 7 weight percent ULTEM.RTM. 1000P (polyetherimide)
[0058] 26 weight percent densified PES
[0059] The densified PES was made from PES 5003P which is available
from Sumitomo Chemical Co. Ltd. (Osaka, Japan). The PES was
densified in accordance with U.S. Pat. No. 4,945,154. MY0510 and
GY281 were first mixed in a mixing vessel, heated to 70.degree. C.
for approximately 10 minutes. The ULTEM.RTM. 1000P particles were
then added and the resulting mixture heated to 130.degree. C. with
mixing for approximately 75 minutes to fully dissolve the
ULTEM.RTM. 1000P particles. The mixture was then cooled to
75.degree. C. and the 3,3-DDS was mixed in for about 15 minutes.
Then, the densified PES was slowly added and mixed in for
approximately 10 minutes to provide the final resin mixture. The
viscosity of the homogeneous resin was measured over the entire
curing temperature range (i.e., 20.degree. C. to 177.degree. C.)
using Rheometric Dynamic Analysis as previously described. The
resin had a minimum viscosity of 900 poise.
[0060] Panels were prepared by first forming a prepreg of 193 gsm
3K PW fabric containing 138 grams of resin square meter. The
prepreg was formed as follows:
[0061] The resin was coated on release paper by reverse roller at
about 175.degree. F. (79.degree. C.) to form a film containing 69
g/m.sup.2. Two resin films were impregnated into the carbon fiber
with an areal weight of 193 g/m.sup.2.
[0062] The prepreg was applied to HRH.RTM. 10 core having 1/8 inch
(0.31 cm) cells and being 1/2 inch (1.27 cm) thick under vacuum at
22 inches (56 cm) Hg and cured for 2 hours at 177.degree. C. with a
pressure of 45 psi, venting at 20 psi and ramp cooling at a rate of
2.degree. C. per minute.
[0063] The resulting specimens were subjected to peel test
according to ASTM D 1781. The face sheets all had peel strengths
above 29 in-lb/3 in width. The dimensions A and B for
representative fillets were measured and found to be approximately
of equal length.
EXAMPLE 2
[0064] Resin was prepared in the same manner as Example 1 except
that the ingredients used to make the resin were as follows:
[0065] 21 parts by weight MY-0510
[0066] 21 parts by weight AcroCy.RTM. L-10
[0067] 21 parts by weight GY281
[0068] 9 parts by weight ULTEM.RTM. 1000P
[0069] 1.5 parts by weight CuAcAc/Nonylphenol (1/0.1)
[0070] 26.5 parts by weight densified PES
[0071] The minimum viscosity of the homogeneous resin mixture was
found to be about 500 poise. The viscosity of the final resin
mixture was measured as set forth in Example 1. The final resin
mixture was used to form a prepreg and applied to HRH.RTM. 10 core
in the same manner as Example 1. The peel strength of the resulting
face sheet was 26 in-lb/3 in width.
EXAMPLE 3
[0072] Resin was prepared having the following formulation:
[0073] 27.0 weight percent MY-0510
(N,N-Diglycidyl-4-glycidyloxyaniline)
[0074] 24.9 weight percent GY285 (bis-F epoxy)
[0075] 15.8 weight percent 3,3'-Diaminodiphenylsulfone
[0076] 1.3 weight percent Dicyandiamide
[0077] 13.5 weight percent micronized Polyethersulfone (PES)
[0078] 17.5 weight percent densified Polyethersulfone (PES)
[0079] Resin formulations in accordance with this example may also
be made wherein the amounts of MY-510, GY281 and 3,3-DDS are varied
by up to +15%. Also, the amounts of both types of PES may be varied
by as much as .+-.40%. The amount of dicyandiamide may be varied by
up to .+-.50%.
[0080] The densified PES was the same as used in Examples 1 and 2.
Average particle size was 1025 microns with no more than 13 weight
percent smaller than 5 microns and no more than 4 weight percent
greater than 40 microns. 24.9 parts by weight of GY285 and 6.0
parts by weight of MY0510 were mixed in a resin kettle and heated,
with stirring, to 65.degree. C. Once this temperature is attained,
13.5 parts by weight micronized PES 5003P is added to the resin
kettle. The mixture is then heated to 128.+-.2.degree. C. and held
at this temperature for 75 minutes. At the end of 75 minutes,
heating is removed and 21 parts by weight of MY0510 are added to
the kettle. Stirring is continued as the mixture cools to
65.degree. C. 15.8 parts of 3,3-DDS is added and mixed for 15
minutes. 1.3 parts of dicyandiamide is then added and the mixture
stirred for 5 minutes at 65.degree. C. Finally, 17.5 parts of
densified PES is added and stirred in for 10 minutes. The minimum
viscosity of the resin was measuared as set forth in Example 1 and
found to be about 370 poise. Panels were prepared by first forming
a prepreg of 193 gsm 3K PW carbon fabric containing 70 grams of
resin per square meter. The prepreg was formed as follows:
[0081] The resin was coated on release paper by reverse-roll coater
at about 165.degree. F. (74.degree. C.) to form a film containing
70 g/m.sup.2. The resin film was impregnated into a carbon fiber
fabric having an areal weight of 193 g/m.sup.2. The prepreg was
then applied to HRH.RTM. 10 core and cured in the same manner as
Example 1. The peel strength was around 32 in-lb/3 in width on 3
pound core and around 31 in-lb/3 in width on 8 pound core.
Comparative Example 1
[0082] Resin was prepared as follows:
[0083] Add 12.5 parts MY-0510 and 37.5 parts GY281 to a mixing
vessel and heat to 70.degree. C. for about 10 minutes. Then add 7
parts ULTEM 1000P and heat the mixture to 130.degree. C. Mix for
about 75 minutes to fully dissolve the ULTEM.RTM. 1000P. Cool the
mixture to a temperature of 75.degree. C. and slowly add 19 parts
3,3'-DDS. Mix 15 minutes at 75.degree. C. Finally, slowly add 26
parts densified PES and mix the resulting final mixture for
approximately 10 minutes at 75.degree. C. The minimum viscosity of
the resin was measured as set forth in Example 1 and found to be
118 poise.
[0084] Prepregs and sandwich panels were prepared in the same
manner as in the preceding example. The peel strengths for the
resulting face sheets were 22 in-lb/3 in width. The viscosity of
the resin is believed to be responsible for the relatively low peel
strength (i.e., below 25 in-lb/3 in width).
Comparative Example 2
[0085] Resin was prepared following the same procedure as set forth
in Comparative Example 1 except that the ingredient amounts were as
follows:
[0086] 23 parts by weight MY-0510
[0087] 25 parts by weight GY281
[0088] 19 parts by weight 3,3-DDS
[0089] 4.5 parts by weight ULTEM.RTM. 1000p
[0090] 26 parts by weight densified PES
[0091] The minimum viscosity of the resin was measured as set forth
in Example 1 and found to be 123 poise.
[0092] Prepregs and sandwich panels were prepared in the same
manner as the preceding examples. The peel strength for the
resulting face sheets was 20 in-lb/3 in width.
Comparative Example 3
[0093] Resin was prepared following the same procedure as set forth
in the preceding Comparative Examples except that the ingredient
amounts were as follows:
[0094] 50 parts by weight MY-0510
[0095] 50 parts by weight GY281
[0096] 47.6 parts by weight 3,3-DDS
[0097] 0.0 parts by weight ULTEM.RTM. 1000p
[0098] 30 parts by weight non-densified PES
[0099] The minimum viscosity of the resin was measured as set forth
in Example 1 and found to be about 30 poise.
[0100] Prepregs and sandwich panels were prepared in accordance
with the preceding examples.
[0101] The peel strength was 13 in-lb/3 in width.
Comparative Example 4
[0102] Resin was prepared following the same procedure as the
previously described Comparative Examples except that the
ingredients were as follows:
[0103] 13.6 parts by weight MY721
[0104] 11.8 parts by weight MY-0510
[0105] 25 parts by weight GY281
[0106] 5 parts by weight Matrimide 9725
[0107] 20 parts by weight 3,3-DDS
[0108] 25 parts by weight densified PES
[0109] The minimum viscosity of this resin was measured as set
forth in Example 1 and found to be 3187 poise. The resulting
prepreg had low tack and poor draping properties because the
viscosity was too high.
[0110] Having thus described exemplary embodiments of the present
invention, it should be noted by those skilled in the art that the
within disclosures are exemplary only and that various other
alternatives, adaptations and modifications may be made within the
scope of the present invention. Accordingly, the present invention
is not limited by the above preferred embodiments, but is only
limited by the following claims.
* * * * *