U.S. patent application number 12/225280 was filed with the patent office on 2010-09-09 for thermoplastic toughening material and related method.
This patent application is currently assigned to Hexcel Composites Limited. Invention is credited to Stephen Mortimer.
Application Number | 20100228001 12/225280 |
Document ID | / |
Family ID | 36384211 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100228001 |
Kind Code |
A1 |
Mortimer; Stephen |
September 9, 2010 |
Thermoplastic Toughening Material And Related Method
Abstract
A non-fibrous, apertured membrane comprises at least one
thermoplastic polymeric material and has a discrete porous
structure. The membrane is soluble in the thermoset matrix polymer
of a composite material.
Inventors: |
Mortimer; Stephen;
(Cambridgeshire, GB) |
Correspondence
Address: |
HEXCEL CORPORATION
11711 DUBLIN BOULEVARD
DUBLIN
CA
94568
US
|
Assignee: |
Hexcel Composites Limited
Duxford, Cambridge
GB
|
Family ID: |
36384211 |
Appl. No.: |
12/225280 |
Filed: |
March 23, 2007 |
PCT Filed: |
March 23, 2007 |
PCT NO: |
PCT/GB2007/001079 |
371 Date: |
September 17, 2008 |
Current U.S.
Class: |
528/322 ;
528/421 |
Current CPC
Class: |
B29C 70/025 20130101;
B29C 70/086 20130101; C08J 5/24 20130101 |
Class at
Publication: |
528/322 ;
528/421 |
International
Class: |
C08G 73/10 20060101
C08G073/10; C08G 65/04 20060101 C08G065/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2006 |
GB |
0606045.3 |
Claims
1. A method for locating insoluble moieties between the fibrous
layers of a composite material wherein said composite material
comprises a plurality of fibrous layers and a thermosetting resin,
said method comprising the steps of: providing a membrane
comprising a thermoplastic polymer that dissolves in said
thermosetting resin during curing of said resin and one or more
insoluble moieties that do not dissolve in said thermosetting resin
during curing of said resin; and locating said membrane between
said fibrous layers prior to curing of said thermosetting
resin.
2. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 1 wherein said
membrane comprises a plurality of apertures extending through said
membrane.
3-46. (canceled)
47. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 1 wherein said
thermosetting resin is selected from the group consisting of epoxy
resins and bismaleimide resins.
48. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 47 wherein said
thermoplastic polymer is polyethersulfone.
49. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 1 wherein said
insoluble moieties are selected from the group consisting of
toughening particles, intumescents, pigments, mould release agents,
nano-sized particles and conducting particles.
50. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 49 wherein said
toughening particles comprise a polymer selected from the group
consisting of polyamide and polyetherimide.
51. A method for locating insoluble moieties between the fibrous
layers of a composite material according to claim 2 wherein from 5
to 50 percent of said membrane comprises said apertures.
52. A method for making a composite material that comprises a
plurality of fibrous layers and a thermosetting resin, said method
comprising the steps of: providing a plurality of fibrous layers;
forming an assembly wherein a membrane is located between said
fibrous layers, said membrane comprising a thermoplastic polymer
that dissolves in said thermosetting resin during curing of said
resin and one or more insoluble moieties that do not dissolve in
said thermosetting resin during curing of said resin; incorporating
said thermosetting resin into said assembly to form a curable
composite material.
53. A method for making a composite material according to claim 52
wherein said step of incorporating said thermosetting resin into
said assembly comprises the step of adding said thermosetting resin
to said fibrous layers prior to formation of said assembly.
54. A method for making a composite material according to claim 52
wherein said step of incorporating said thermosetting resin into
said assembly comprises the step of adding said thermosetting resin
to said fibrous layers after forming said assembly.
55. A method for making a composite material according to claim 52
which includes the additional step of curing said curable composite
material.
56. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material wherein said composite
material comprises a plurality of fibrous layers and a
thermosetting resin, said membrane comprising: a thermoplastic
polymer that dissolves in said thermosetting resin during curing of
said resin and one or more insoluble moieties that do not dissolve
in said thermosetting resin during curing of said resin.
57. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 56
wherein said membrane has a discrete porous structure.
58. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 56
wherein said membrane comprises a plurality of apertures extending
through said membrane.
59. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 57
wherein said membrane comprises a plurality of apertures extending
through said membrane.
60. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 57
wherein said discrete porous structure comprises discrete pores
having an average pore size of about 40 microns.
61. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 58
wherein from 5 to 50 percent of said membrane comprises said
apertures.
62. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 56
wherein said insoluble moieties are selected from the group
consisting of toughening particles, intumescents, pigments, mould
release agents, nano-sized particles and conducting particles
63. A membrane for use in locating insoluble moieties between the
fibrous layers of a composite material according to claim 62
wherein toughening particles comprise a polymer selected from the
group consisting of polyamide and polyetherimide and said
thermoplastic polymer is polyether sulfone.
64. A composite material comprising a plurality of fibrous layers
and a thermosetting resin, said composite material further
comprising a membrane according to claim 56 located between said
fibrous layers.
65. A cured composite material according to claim 60 wherein said
thermosetting resin has been cured.
Description
[0001] The present invention relates to a thermoplastic toughening
material for use in composite assemblies.
[0002] Composite materials made from a polymeric matrix and a
reinforcing fibrous material are used in many commercial
applications including aerospace, sports goods, transportation,
civil engineering and energy generation. Some of the most commonly
used composites are made from thermosetting resins with glass or
carbon-fibre reinforcement. Epoxy resins are the most extensively
used class of matrix polymers for high performance applications,
followed by phenolics, cyanate esters, bismaleimides, benzoxazines
and several less familiar chemistries. These combinations of fibre
and curable polymeric matrix resins are used in an increasing
variety of processes, according to the requirements of the specific
application. Particularly convenient ways of utilising composites
are prepreg, semi-preg, resin transfer moulding (RTM), vacuum
assisted resin transfer moulding (VaRTM), liquid resin infusion
(LRI), resin film infusion (RFI), pultrusion, pressure assisted
moulding and several variants thereof.
[0003] Improvements in the properties of composite materials have
come from various sources, and a major focus of development work
has been the improvement of toughness of composite parts. Epoxy
resins are inherently rather brittle, and they can be toughened by
including a rubber or thermoplastic in the resin formulation.
Thermoplastic additives are preferred because the glass transition
temperature and modulus of the toughened matrix can be maintained
by selecting a suitable thermoplastic. The thermoplastic can be
soluble or insoluble in the uncured resin composition. Insoluble
thermoplastic particles are therefore useful per se as toughening
agents. Soluble thermoplastics work by phase separating from the
matrix during the cure reaction, thereby forming a dispersed or
co-continuous morphology in the final cured part.
[0004] The choice of a specific type of composite process is
influenced by many factors including cost, convenience, complexity
of the part to be made, health and safety considerations and
mechanical performance requirements. In general, the highest
performance can be achieved by the judicious assembly of layers of
prepreg. This is a labour intensive operation and it is preferable
to use RTM or some other resin injection or infusion method. In
RTM, an infusible structure (or preform) is made from reinforcing
fibres and other additives including binders, and the preform is
injected or infused with liquid resin and the resin is cured at
elevated temperature to form a useable component.
[0005] The composite systems used today generally have very good
mechanical properties and can withstand high stresses from sources
such as impact, tension and shear. However, further improvement is
desirable in most cases. In particular, some specific processing
methods are known to pose exacting challenges for design engineers.
For example, it is very difficult to toughen RTM resins because of
a viscosity increase of the resin when high molecular mass
thermoplastic toughening agents are dissolved in the liquid
resin.
[0006] This can make it impossible to inject the resin into a large
part, because the resin begins to cure before the preform is
completely filled with resin. Conversely, if the thermoplastic or
rubber toughening agent is dispersed into the resin in the form of
undissolved particles, these particles are then filtered by the
fibre preform, resulting in a concentration gradient of the
toughener, or in fact completely blocking further
injection/infusion of resin.
[0007] In the area of pre-impregnated composite materials, there is
little ability to precisely locate components in a depth-wise
sense. Toughening particles can be blended with the resin but
during impregnation of the carbon fibre reinforcement these
particles are filtered by the reinforcement to create an interlayer
region in the composite that is rich in toughening particles.
However, this is far from ideal as the filtration process is
difficult to control and some of the particles are forced into
carbon fibre layers.
[0008] There have been several attempts to solve the problem of
toughening resin infused components. These include EP 1317501 which
discloses a soluble system for RTM whereby soluble polymeric fibres
(or modes of fibres such as multifilaments, ribbons or other
combinations thereof) are interwoven with carbon tows. This
approach requires the manufacture of thermoplastic fibres and
subsequent weaving or other processing into a fabric reinforcement
for use in composite assemblies. Such woven and spun-bonded fabrics
contain crossover points that can act as stress concentrators which
results in local deformation of reinforcing fibres.
[0009] EP0496518 discloses a porous polymeric membrane film for
incorporation into a composite assembly in which the polymer
dissolves during the curing process. Whilst useful, such films have
met with limited commercial success as they do not have the same
high tensile strength as non-porous films. This can lead to the
film breaking during processing.
[0010] U.S. Pat. No. 6,737,158 describes a composite containing a
porous polymeric membrane toughening layer. The layers of the
invention are made from expanded polytetrafluoroethylene (PIPE) and
remain insoluble throughout the cure process. This insolubility
gives rise to a weak resin-thermoplastic interface. This weak
interface between interlayer and matrix can cause poor resistance
to cracking between plies, especially when exposed to a moist
environment.
[0011] U.S. Pat. No. 5,288,547 discloses a process for the
preparation of a composite comprising reinforcing fibers embedded
in a thermosetting resin matrix. Prior to curing a porous membrane
film of thermoplastic material is placed between two layers of
fibres embedded in thermosetting resin. The porous membrane film is
defined as a porous polymeric film, the pores of which are
interconnected.
[0012] Such interconnected porous structures would create a very
large specific surface area in the membrane. A large surface area
of membrane tends to encourage the polymer membrane to dissolve
more rapidly in the resin matrix of a composite assembly. Whilst
this may be desirable in some applications, it is not the case, for
example, for resin infusion applications. Here, if the membrane
dissolves too quickly it leads to the undesirable effect of
increasing the resin viscosity to a level that slows down further
infusion of the resin through the reinforcing fibres.
[0013] Manufacture of composite assemblies falls into two broad
categories, namely direct and indirect. Direct manufacture allows a
cured composite assembly to be produced without intermediate
product being formed. In contrast, indirect manufacture produces an
intermediate prepreg which may be partially cured. The cured
composite assembly is then produced off-line.
[0014] Whilst prepregs remain important to the industry, there is a
general move towards direct manufacturing processes such as RTM and
RFI. Direct processes are generally preferred as the problems
associated with the storage of prepregs are eliminated.
Furthermore, direct processes reduce waste and reduce both
manufacturing times and costs as the need for an extra impregnation
step is eliminated. Direct processes also allow more complex
composite parts to be manufactured.
[0015] The present invention seeks to provide a membrane which
finds utility in both direct and indirect manufacturing processes,
wherein said membrane serves to toughen the composite assembly of
which it forms part.
[0016] According to a first aspect of the present invention there
is provided a non-fibrous, apertured membrane comprising at least
one soluble thermoplastic polymeric material said material being at
least partially soluble in a monomer and insoluble in a polymer
derived from the monomer, wherein the membrane has a discrete pore
structure.
[0017] In one embodiment the monomer in which the thermoplastic
material is soluble comprises one or more epoxy or bismaleimide
(BMI) materials. On polymerisation the monomer forms a
thermosetting polymer.
[0018] The invention has particular, but not exclusive application
in direct manufacturing processes for composites, such as RTM and
RFI.
[0019] Typically the membranes of the invention having a discrete
pore network, have a density of about 0.8 g/cm.sup.3. For the
purposes of the present invention a membrane having a density over
0.5 g/cm.sup.3 is deemed to have a discrete pore structure.
[0020] Typically a membrane having an interconnected porous
structure has a sub-micron average pore size whereas the membrane
of the invention, having a discrete porous structure, has an
average pore size of about 40 microns.
[0021] The membranes described herein are free from fibre
cross-over points that act as stress concentrators.
[0022] It is intended that the membranes described herein form part
of a curable composite assembly. Therefore, by soluble it is meant
at least partially soluble in a matrix resin which forms part of
the assembly. Furthermore, the membrane is soluble only during cure
of the assembly.
[0023] Therefore, the membranes described herein may be made from
any suitable thermoplastic polymer. The polymer is limited only by
its ability to dissolve in the matrix resin during curing of said
resin. Such matrix resins, as will be described hereinafter, are
typically thermosetting resins.
[0024] Suitable thermoplastic polymers for use in the membrane of
the present invention include any of the following either alone or
in combination: polyether sulfone (PES), polyetherethersulfone
(PEES), polyphenyl sulfone, polysulfone, polyimide, polyetherimide,
aramid, polyamide, polyester, polyketone, polyetheretherketone
(PEEK), polyurethane, polyurea, polyarylether, polyarylsulfides and
polycarbonates.
[0025] Although the terminal group of the chosen polymer is not
material to the invention, reactive terminal groups are preferable
in order to link the polymer membrane and the matrix resin upon
curing. Forming such a link enhances the toughness of the cured
composite assembly of which the membrane forms parts.
[0026] Preferably, the polymers of the present invention comprise
at least one linking member in order to link together the polymer
membrane and the matrix resin upon curing. The linking member may
be a particular terminal group or a side-chain functional group
present on the polymer.
[0027] Suitable terminal and/or side-chain groups include any of
the following either alone or in combination: hydroxyl, chloro,
amino, isocyanato, cyanato, glycidyl, carboxyl, nitro and
sulfato.
[0028] The polymer preferably has a number average molecular weight
in the range of from 100 to 10,000,000 daltons and most preferably
from 10,000 to 1,000,000 daltons.
[0029] The membranes described herein may comprise a plurality of
apertures of varying shapes so as to provide an openwork structure.
Advantageously, the shape and frequency of the apertures can be
tailored to the specific physical characteristics e.g. viscosity,
of the resin in order that the resin can flow through the apertures
and be evenly distributed through the composite assembly.
[0030] The presence of apertures eliminates the need to
mechanically perforate a continuous membrane in order to facilitate
the flow of resin therethrough. The membranes made in accordance
with the matrix and described herein are stronger than mechanically
perforated membranes such that the risk of tearing the film during
its production is minimised.
[0031] The apertures of the present invention may take a variety of
pattern formats. Non-limiting examples include, fabric (so-called
linen pattern), lattice, trellis, mesh, mat, net, stipple,
pyramidal, hexagonal, rhomboid, hammered, knurled, lozenge and
grid.
[0032] More than one pattern format may be present in a membrane in
order to achieve particular properties. For example, combining
pattern formats may result in a preferential strength or
infusibility in a particular region of the membrane.
[0033] These varying patterns allow a membrane to be tailored to a
particular system. The pattern selection is influenced by the
viscosity of the matrix resin of the composite assembly. Clearly a
lower viscosity resin can pass through a smaller sized aperture and
vice versa. In the case of a resin soluble membrane, the infusion
of a laminate containing a membrane described herein must be
performed at a temperature below the dissolution temperature of the
membrane, to avoid washing or viscosity build up.
[0034] Although the membranes described hereinbefore have a fabric
like appearance, such membranes are non-fibrous. It is this
non-woven nature which gives the membranes described herein a
significant advantage over prior art fibrous materials. The
membranes described herein do not contain fibre cross-over points,
as with woven fabrics, which cause local deformation of the
reinforcing fibres. Furthermore the membranes described herein are
of a uniform thickness. This is a particular advantage for vacuum
infusion processes as it provides reduced interlayer thickness and
enables higher volume fraction fibre composites to be produced, as
required by the aerospace industry.
[0035] The membranes described herein preferably have an areal
weight in the range of from 3 to 50 gm.sup.-2 and more preferably
in the range of from 5 to 25 gm.sup.-2.
[0036] The membranes described herein are preferably such that from
1 to 90% of the surface area of the membrane is apertured, so as to
be open. More preferably from 5 to 50% of the surface area of the
membrane is apertured so as to be open.
[0037] The membranes described herein may comprise one or more
additional components which are useful in the cured composite of
which the membrane forms a part. Such components include, but are
not limited to, any of the following either alone or in
combination: toughening particles, fillers, intumescent agents,
flame retardants, pigments, conducting particles, short fibres,
resins and curing agents.
[0038] Toughening particles may include any of the following either
alone or in combination: polyamides, copolyamides, polyimides,
aramids, polyketones, polyetheretherketones, polyeesters,
polyurethanes, polysulfones, high performance hydrocarbon polymers,
liquid crystal polymers, PTFE, elastomers and segmented
elastomers.
[0039] Preferably, toughening particles constitute from 0.1% to 80%
by weight of the total weight of the membrane and most preferably
from 1% to 50% by weight of the total weight of the membrane.
[0040] Suitable fillers may include any of the following either
alone or in combination: silicas, aluminas, titania, glass, calcium
carbonate and calcium oxide.
[0041] Preferably, fillers constitute from 0.1% to 30% by weight of
the total weight of the membrane.
[0042] Suitable conducting particles include any of the following
either alone or in combination: silver, copper, gold, aluminium,
nickel, conducting grades of carbon, bucminsterfullerene, carbon
nanotubes and carbon nanofibres. Metal coated fillers may also be
used, for example nickel coated carbon particles and silver coated
copper particles.
[0043] Preferably the conducting particles constitute from 0.1% to
98% by weight of the total membrane weight and more preferably from
10% to 90% by weight of the total membrane weight.
[0044] Advantageously, the membranes described herein may be used
as carriers for the precise delivery of moieties to a particular
location in an assembly. This is particularly beneficial for the
location of moieties in an assembly in a depth wise sense.
[0045] During cure of a composite assembly the polymeric material
for the membrane dissolves leaving the insoluble moieties precisely
located, in a depth wise sense, with the cured assembly.
[0046] Suitable insoluble moieties for inclusion in the membrane
include any of the following either alone or in combination:
intumescents, pigments, mould release agents, nano sized particles
and conducting particles.
[0047] According to a second aspect of the present invention there
is provided the use of an at least partially soluble, non-fibrous,
apertured membrane comprising at least one soluble thermoplastic
polymeric material for delivering at least one insoluble moiety to
a precise location within a cured composite assembly and wherein
said membrane has a discrete porous structure.
[0048] The present invention further seeks to provide a composite
assembly having excellent toughness properties whereby it is
possible to control the location of toughening agent within the
assembly.
[0049] According to a further aspect of the present invention there
is provided a curable composite assembly comprising a polymeric
matrix resin, a fibrous reinforcement material and at least one
non-fibrous, at least partially soluble, apertured, membrane
wherein said membrane comprises at least one thermoplastic
polymeric material soluble in the matrix resin and wherein said
membrane has a discrete porous structure.
[0050] The assemblies of the present invention may be prepared by
either direct or indirect processes. That is, the assemblies of the
present invention may be prepreg, an indirect process, prepared by
incorporating the layers into the assembly during the prepregging.
Alternatively they may be assemblies prepared by direct processes,
such as RTM, VaRTM or RFI.
[0051] For prepreg made by a solvent process, the membrane would be
interleaved or consolidated onto the reinforcement just prior to
final wind-up of the product. For hot melt prepreg processes, the
membrane can either be laminated onto the reinforcement prior to
final wind-up or it could be positioned on or into the
reinforcement prior to combining the reinforcement with the matrix
film. Such techniques are well known to those skilled in the
art.
[0052] In the case of RTM, VaRTM and RFI processes, assemblies are
prepared by applying the membranes described herein to the dry
fibrous material of the preforms. There are some reinforcement
materials such as multiaxial textiles where it is possible to
locate the membrane inbetween individual layers comprising the
textile. The matrix resin is of a viscosity such that, during the
resin injection stage, the resin passes through the membrane into
the fibrous material. These technologies are described in chapter 9
of "Manufacturing Processes for Advanced Composites", F. C.
Campbell, Elseveir, 2004.
[0053] The preferred thermoset matrices for RTM processes are epoxy
or bismaleimide (BMI) with suitable epoxy examples being
HexFlow.RTM. RTM 6 or RTM 120. A typical BMI matrix is HexFlow.RTM.
RTM 651. HexFlow.RTM. VRM 34 may be used for Vacuum-assisted Resin
Transfer Moulding (VaRTM) applications. All of the above materials
are available from Hexcel Composites, Duxford, UK.
[0054] The reinforcement fibres can be selected from any of the
following commercially available high performance fibres which may
be used alone or in combination:--aramid (e.g. Kevlar.TM.), glass,
carbon, ceramic, hemp, or polyolefin. Carbon fibres are the
preferred material, particularly standard or intermediate modulus
fibres of between 3000-24000 filaments per fibre tow. The desirable
reinforcement form is a woven or non-crimped textile structure of
between 150-1000 gm.sup.-2 fibre areal weight. Typical weave styles
include plain, satin and twill weaves. Non-crimped or multiaxial
reinforcements can have a number of plies and fibre orientations
such as +45/-45; 0/+45/-45; 0/+45/-45/90. Such styles are well
known in the composite reinforcement field and are available from a
number of companies including Hexcel Reinforcements, Villeurbanne,
France.
[0055] The present invention also provides a method by which the
membranes described herein can be made.
[0056] Therefore, according to a still further aspect of the
present invention there is provided a method for the preparation of
a non-fibrous, at least partially soluble, apertured, porous
membrane comprising at least one soluble thermoplastic polymeric
material said method comprising the steps of: [0057] a) preparing a
polymer dope solution comprising said thermoplastic polymeric
material in solvent; [0058] b) casting said dope solution; [0059]
c) bringing the cast dope solution into contact with a coagulation
means so as to form a membrane; [0060] d) removing at least some of
the solvent from the membrane; and [0061] e) drying the
membrane.
[0062] The method of the present invention is also preferable to
prior art casting processes as these do not allow for the formation
of precisely defined microporous structures.
[0063] The polymer dope solution is prepared by dissolving the
polymeric material in a solvent or mixture of solvents. Any solvent
conventionally used for preparing polymer solution may be used.
However, preferred solvents are those which are substantially
entirely miscible with water. More preferably suitable solvents are
aprotic solvents and most preferably suitable solvents are polar
aprotic solvents.
[0064] Therefore, suitable solvents for use in the method of the
present invention include any of the following either alone or in
combination: dimethyl sufoxide, dimethyl formamide,
dimethylacetamide, 1-methylpyrrolidone, tetramethylurea,
gamma-butyrolactone, propylene carbonate and ethylene
carbonate.
[0065] The solvent or solvent mixture referred to above may also
comprise a co-solvent in order to modify the solution
characteristics of the solvent. Suitable co-solvents include any
solvent which is completely miscible with any of the aforementioned
solvents. Suitable examples include glycol ethers and alcohols.
[0066] The solvent for use with the present invention may contain
other soluble additives to aid in the coagulation and washing
stages of the process. For example, lithium chloride, potassium
chloride, calcium chloride, solium acetate, surfactants such as
sodium dodecylsulfate and the like.
[0067] The polymer dope solution of the present invention
preferably comprises from 5% to 90% by weight of polymer, more
preferably from 5% to 50% by weight of polymer and most preferably
from 10% to 35% by weight of polymer.
[0068] The polymer dope solution may also comprise one or more
additional components useful in a cured composite. Such additives
include, but are not limited to, any of the following either alone
or in combination: toughening particles, fillers, intumescent
agents, flame retardants, pigments, conducting particles, short
fibres, resins and curing agents. In fact any material that is
insoluble in the coagulation and washing liquids can be added to
the polymer dope solution.
[0069] Examples of suitable additional components are described
above in respect of the membrane per se.
[0070] Chemically reactive components may also be included in the
polymer dope solution. Suitable reactive components are preferably
insoluble in the coagulation and washing liquids used in the
process for the present invention. In practice, most epoxy resins
cyanate ester resins and most curing agents used in thermosetting
systems are very insoluble in water and are therefore suitable for
use with the present invention.
[0071] The polymer dope solution may be cast onto a substrate or
directly onto a patterned roller such as a gravure roller such as
those used in printing processes.
[0072] Suitable substrates include sheet material comprising
polyethylene and copolymers of ethylene, oriented polypropylene,
polypropylene and copolymers of propylene, polyamide, polyester and
similar materials. The substrates may be untreated or may be
surface coated. Suitable surface coating materials include silicone
release compounds with polyethylene and low density polyethylene
being particularly preferred.
[0073] Alternatively, the substrate may be porous and in the form
of one or more continuous belts. These substrates are well known in
the papermaking industry and may comprise a wire mesh, fibre mesh,
felt or similar. Such an arrangement allows for greater washing
efficiency and more rapid manufacture with reduced tendency for the
film material to break.
[0074] In order to provide an apertured membrane of a particular
open work pattern the substrate is suitably embossed in order to
create said pattern.
[0075] Following casting the material is contacted by a coagulation
means in order to form a film. Said coagulation means may be a
coagulation bath comprising a coagulation liquid. The coagulation
liquid may be water or another liquid such as methanol, ethanol,
propanols, acetone, and their aqueous mixtures. During coagulation
the polymer separates into a membrane having a microporous
structure.
[0076] After coagulation the polymeric membrane is passed through
at least one wash bath in order to remove any coagulation liquid
from the membrane. The wash bath comprises water which optionally
comprises additives and solvents such as alcohols to increase the
rate of washing, enzymes or other agents to accelerate the
degradation of extracted solvents, wetting agents, antifoaming
agents and the like. The washing stage may advantageously be
conducted using warm water (up to 50.degree. C.) and/or agitation
to further increase the rate of solvent removal.
[0077] Other methods for increasing the rate of solvent removal may
be employed, for example ultrasonics.
[0078] For the manufacture of a membrane having an interconnected
microporous structure it is preferable to remove the majority of
the solvent from the membrane during the washing stage. Following
washing it is preferable that the membrane comprises a maximum of
10% solvent with respect to the total membrane weight and it is
more preferable that the membrane comprises a maximum of 5% solvent
with respect to the total weight of the membrane. Further solvent
may be removed during the drying process.
[0079] For the manufacture of a membrane having a substantially
discrete microporous structure it is preferable to remove up to 90%
of solvent from the membrane. If too much solvent is removed a
membrane having a discrete pore network cannot be produced. In this
case, during the subsequent drying process the interconnecting
sub-micron pores formed during coagulation are converted to micron
sized discrete pores.
[0080] Following washing the membrane is dried. Drying is
preferably achieved using one or move ovens. That said the membrane
can be dried at room temperature i.e. 20-25.degree. C.
[0081] Typically, drying takes place at a temperature within the
range of 70.degree. C. to 200.degree. C. However, the precise
temperature is dependent upon the nature of the polymer from which
the membrane is made and the substrate etc. Clearly drying the
membrane at an elevated temperature accelerates the drying
procedure.
[0082] In a further embodiment of the invention the membrane is
made by casting the polymer dope onto a pre-existing veil.
[0083] According to a further aspect of the present invention there
is provided an at least partially soluble, apertured membrane,
comprising at least one soluble thermoplastic material secured to a
substrate, the thermoplastic material having a discrete porous
structure.
[0084] The preferably insoluble non-woven substrate provides
strength and enables low weights of soluble polymer to be readily
introduced into a composite. The resin interface between the
substrate veil material and resin matrix is toughened, thereby
imparting additional toughness to the final composite part.
[0085] According to a still further aspect of the present invention
there is provided a method of making an at least partially soluble
apertured membrane, comprising at least one soluble thermoplastic
material secured to a substrate, wherein the thermoplastic material
has a discrete porous structure, the method comprising the steps of
[0086] a) preparing a polymer dope solution comprising said
thermoplastic polymeric material in solvent; [0087] b) providing a
substrate; [0088] c) coating the substrate with the polymer dope
solution; [0089] d) co-agulating the polymer dope solution so as to
form a membrane comprising the polymer and substrate; [0090] e)
removing at least some of the solvent from the membrane; and [0091]
f) drying said membrane.
[0092] In one embodiment of the invention the substrate comprises
non-woven material. The substrate would ideally comprise material
used for reinforcement fibres in composite materials. Hence the
substrate ideally comprises any of the following either alone or in
combination:--aramid, glass, ceramic, hemp, polyamide or
polyolefin. Polyamide or carbon fibres are particularly useful for
forming the substrate. One suitable substrate is commercially
available from Protechnic of France and is of 5 gsm weight.
[0093] This so-called hybrid membrane ideally has a weight per unit
area in the range from 1 to 25 and/or a thickness in the range from
5 .mu.m to 25 .mu.m.
[0094] The hybrid membrane can be applied to a prepreg as a
separate layer, either next to the prepreg's reinforcement or on
the resin surface or otherwise used in a direct process such as
within an RTM preform. The hybrid can be applied prior, during or
after the impregnation process for standard film impregnation of
woven or unidirectional fibres.
[0095] A preferred, but not exclusive, application is in direct
composite manufacturing processes, particularly those comprising
unidirectional reinforcements.
[0096] As previously, in this embodiment of the invention the
theremoplastic material is ideally heat treated in order to reduce
its speed of dissolution in the matrix resin.
[0097] The thermoplastic material may be applied to the substrate
in any desired pattern.
[0098] The present invention will now be described by way of
example only and with reference to the following drawings in
which:
[0099] FIG. 1 is a diagrammatic representation of one embodiment of
the process of the present invention;
[0100] FIG. 2 is a diagrammatic representation of a second
embodiment of the process of the present invention;
[0101] FIG. 3 is a diagrammatic representation of a third
embodiment of the process of the present invention:
[0102] FIGS. 4a-4d are diagrammatic representations of four
embodiments of membranes of the present invention having different
aperture arrangements;
[0103] FIG. 5 is a photomicrograph of a composite assembly wherein
insoluble polyamide (Orgasol.TM.) particles are precisely located
therein;
[0104] FIG. 6 is a photomicrograph of a composite assembly made
from a membrane comprising discrete pores and wherein polyamide
(Orgasol.TM.) particles are precisely located therein;
[0105] FIG. 7 is a photomicrograph of a composite assembly made
from a membrane comprising discrete pores;
[0106] FIG. 8 is a diagrammatic representation of a further
embodiment of the process of the invention;
[0107] FIG. 9 is a micrograph of the non-woven material prior to
coating with thermoplastic PES;
[0108] FIG. 10 is a micrograph of the non-woven material of FIG. 9
as coated with thermoplastic PES; and
[0109] FIG. 11 is a micrograph of a further membrane in accordance
with the invention.
[0110] FIG. 1 shows processing equipment whereby a substrate passes
over a roller 2. The substrate is suitably embossed so as to
provide a membrane having a particular pore arrangement. Polymer
dope contained in vessel 3 is applied on to the substrate 1 via a
coating head 4. A doctor blade 5 controls the thickness of the dope
coating applied to the substrate thereby affording a substrate
coated with a membrane of the present invention. The coated
substrate 6 passes into a coagulation bath 7 and subsequently
through a wash bath 8 via a plurality of rollers 9. Air knives 10
remove water from the surface of the coated substrate. The coated
substrate then passes through a drier 11 before being wound onto a
reel 12. The membrane remains on the substrate for ease of use.
[0111] FIG. 2 shows a variation of the processing equipment shown
in FIG. 1. Here a substrate 1 passes over a roller 2. Polymer dope
contained in vessel 3 is applied on to the substrate 1 via a
coating head 4. A doctor blade 5 controls the thickness of the
coating applied to the substrate thereby affording a substrate
coated with a membrane of the present invention. The coated
substrate 6 passes through a coagulation bath 7. Next, the coated
substrate passes through a wash bath 8 via a plurality of rollers
9. Air knives 10 remove water from the surface of the coated
substrate 6. The substrate 1 and the coating 11 are separated. The
substrate 1 is wound onto a reel 12. The coating is passed through
driers 13 and wound onto another reel 14. The coating may be
applied to a second substrate, which may be the same or different
to the original substrate 1, before or at roller 14.
[0112] FIG. 3 shows a gravure roller 15 to which is applied polymer
dope contained in vessel 3 by way of a coating head 4. The surface
of the gravure roller is such that the membrane produced has the
desired pore arrangement. A doctor blade 5 controls the thickness
of the dope coating applied to the roller 15. Following application
to the roller, the dope is coagulated by the application of a
coagulant via applicator 16. An air knife 17 is provided to dry the
roller. The cast dope passes through a coagulation bath 7, then
through a wash bath 8 by way of a plurality of rollers 9 thereby
affording a membrane according to the present invention. Air knives
10 remove superficial water from the surface of the membrane. The
membrane then passes through a drier 11 before being wound onto a
reel 12.
[0113] With regard to the apertured structure of the membranes
described herein, possible aperture arrangements are shown in FIGS.
4a to 4d.
[0114] FIG. 4a shows an elliptical aperture arrangement.
[0115] FIG. 4b shows a hexagonal aperture arrangement.
[0116] FIG. 4c shows a triangular aperture arrangement.
[0117] FIG. 4d shows a square aperture arrangement.
[0118] FIG. 5 shows a cured composite assembly 18 having a carbon
fibre reinforcement 19 and a resin rich zone 20. Insoluble
polyamide (Orgasol.TM.) particles 21 are precisely located between
said reinforcement 19 and resin zone 20.
[0119] FIG. 6 shows a cured composite assembly 22 having carbon
fibre reinforcement 23 and 24 wherein the fibres of the
reinforcement are located perpendicular to each other. A resin rich
interlayer 25 is observed comprising the solubilised and phase
separated membrane, and polyamide particles.
[0120] FIG. 7 is the cured composite assembly 22 at a higher
magnification. The base matrix 27 can also be seen.
[0121] FIG. 8 shows processing equipment 30 whereby a 5 gsm veil 31
of non-woven polyamide material is fed from a roll 32 over a
coating roller 33 which coats the veil 31 with a polymer dope from
a polymer dope reservoir 34. The polymer dope comprises a mixture
of DMSO (dimethyl sulfoxide), PES (polyether sulfone) and
Orgasol.TM. polyamide. Support is provided for the veil via upper
and lower polythene webs 35,36. The coated veil is fed through a
coagulation bath 37 and subsequently through two wash baths 38,39
charged with water. The wet coated substrate then passes to a
storage roller 40. The roll 40 is subsequently passed through
vertical ovens (not shown) to provide a finished dried product
attached to the polythene webs which are easily removable.
[0122] The final product, not including the polythene webs, weighed
approximately 6 to 7 gsm, and contained about 2-3 gsm of PES.
[0123] FIG. 9 shows the polyamide veil material used in the process
illustrated in FIG. 8. Here veil fibrous elements can be seen.
Comparing this with FIG. 10, showing a micrograph of the coated
veil, solidified PES can be observed as the transparent material
between the veil fibrous elements. Likewise undissolved Orgasol,
which provides additional toughening to the cured laminate, can be
observed as small dark dots.
[0124] FIG. 10 shows the dried PES thermoplastic material coated on
the veil. This exhibits a discrete pore arrangement,
[0125] The present invention will also be described further by way
of example only and with reference to the following examples.
[0126] Examples 1 to 3 describe the preparation of low porosity
membranes i.e. membranes exhibiting discrete pores. An example of
their use as toughening elements in RTM laminates is also
described.
[0127] Examples 4 and 5 describe the preparation of porous
membranes, with various additives in the polymer dope.
[0128] Example 6 describes a hybrid membrane comprising a porous
thermoplastic material coated on a substrate.
EXAMPLE 1
[0129] 10 g of lithium chloride were dissolved in water and made up
to 40 ml. This solution was added to 1960 ml of
N-methylpyrrolidinone (NMP). To this was added 500 g of
polyethersulfone (Sumikaexel, Sumitomo, Japan). The mixture was
stirred at room temperature until the PES had dissolved. Orgasol
1002DNatl (Arkema), 50 g, was added to the mixture and stirred.
Using a pilot printing line, the thus formed dope was cast from a
pan by means of a lightly etched gravure roll onto a 30 cm wide
embossed polythene substrate with a hexagonal pattern, excess dope
being removed with a doctor blade. The web was passed through a
20.degree. C. water tank at a speed of 7 metres/minute to coagulate
the dope and remove part of the NMP solvent. Coagulation occurred
within 10 cm. The wet web was wound up and subsequently, the
thus-formed PES membrane was removed from the substrate and lengths
of the grid allowed to equilibrate under ambient conditions for 15
hours. The initially white grid was nearly transparent at this
stage. Finally, traces of remaining solvent were removed from the
membrane by washing in water for 24 hours and drying the grid.
EXAMPLE 2
[0130] A PES dope was prepared as in Example 1 using 1600 g of PES,
7480 ml of dimethylsulfoxide in place of NMP, and 160 ml of a 25%
solution of calcium chloride in water in place of the lithium
chloride. 688 g of Orgasol 1002 DNat1 were stirred into the
mixture. The resulting dope was cast into the form of a grid using
a similar procedure to that described with reference to example 1
except that a preliminary drying was carried out by passing the
partially washed grid through ovens heated to 90.degree. C. to
106.degree. C. at a 1 metre/minute web speed. The grid was finally
washed free of residual solvent by immersing in water and drying.
The resulting web was semi-transparent.
EXAMPLE 3
[0131] A low porosity PES membrane was prepared in the same way as
in Example 2 except in that the oven temperatures were 100.degree.
C.-106.degree. C. and the line speed was 0.5 metre/minute. The
final measured solvent content was 2.8% and the final washing stage
was omitted in this case. The resulting membrane was
semi-transparent.
Preparation of Laminates by Resin Transfer Moulding.
[0132] A 16 ply carbon fibre preform (600.times.300 mm) was
assembled in a quasi-isotropic lay-up from Hexcel AS7 dry UD fibre
tape of 268 gsm. Between each ply, a PES membrane of approximately
15-20 gsm formed by the processes just described, was added as
tabulated below.
[0133] The preform was then placed in a closed metal tool of a
cavity size 4.0 mm. The mould tool was then heated to a temperature
of 90.degree. C. and injected with an epoxy RTM resin (RTM 6
available from Hexcel Composites Ltd., Dagneux, France). After
injection of the resin, the temperature of the tool was raised to
180.degree. C. and the laminate was cured for 2 hrs at 180.degree.
C. The finished laminate was c-scanned and the resulting scan
showed that the laminate had been successfully infused and that no
porosity was present. The laminate was then cut into test specimens
for CAI (compression after impact), OHC (open hole compression) and
bearing strength test. These tests were performed according to the
following Airbus Industries test procedures; CAI (AITM 1.0010); OHC
(AITM 1.0008); and bearing strength (AITM 1.0009). For the CAI
tests, all impacts were made with an impact energy of 30 J. A
similar reference laminate was made without the PES membrane and
tested in the same way. The results are shown in the table below
for the reference laminate and a number of PES membrane materials
and are quoted as actual results and as results normalised to a
fibre volume fraction of 60%.
TABLE-US-00001 CAI, MPa OHC, MPa Bearing, MPa Example Laminate
actual normalised actual normalised actual normalised Control RTM 6
169 169 286 286 859 859 1 RTM 6, 206 214 283 298 887 879 PES
membrane + 10% Orgasol 2 RTM 6, 207 222 PES membrane + 10% Orgasol
3 RTM 6, 200 212 287 302 838 874 PES membrane + 30% Orgasol 4 RTM
6, 204 210 280 293 829 859 PES membrane + 20% Orgasol, 2.8%
solvent
[0134] The results of the mechanical tests show an increase in CAI
for the PES membrane laminates, demonstrating the toughening effect
of the grid materials. Additionally, the results from the OHC and
bearing strength tests show that these properties are not reduced
by the presence of the grid. Furthermore, the presence of up to
about 3% residual solvent in the grid had no significant effect on
the mechanical properties.
EXAMPLE 4
[0135] A solution of 0.5 grams of lithium chloride in 1.5 grams of
water was added to 98 ml of 1-methylpyrrolidinone and to this was
added powdered polyethersulfone (25 grams) with stirring. A clear
colourless solution resulted. To this solution, 12.5 grams of Ultem
5000 polyimide particles (GE Plastics, MA, USA) (15 microns average
diameter) were added.
[0136] The mixture was stirred to yield a homogeneous dispersion
and then cast on to a flat, stippled stainless steel surface with a
pattern shown in FIG. 4a, with a pattern repeat of 2 mm. A flexible
doctor blade was used to produce a uniform, thin coating on the
steel surface and then the dope layer was contacted against water
for 5 minutes. After drying, a membrane resulted with an area
weight of 4 grams per square metre.
EXAMPLE 5
[0137] A polymer dope was prepared from 98 grams of
dimethylsulfoxide, 2 ml of 10% aqueous sodium acetate solution and
25 grams of polyethersulfone. 0.72 g of tetraglycidyl derivative of
diaminodiphenylmethane (Araldite MY721, product of Huntsman, Basel,
Switzerland) and 0.53 g of
4,4-methylenebis(2-methyl-5-isopropylaniline) (product of Lonza,
Basel, Switzerland) were incorporated into 10 ml of this dope and
cast on to a stippled surface and then precipitated from water.
When washed and dried, the layer comprised a white, strong veil
which when heated at 180.degree. C. for two hours gave a clear,
flexible, tough epoxy-reinforced PES film.
EXAMPLE 6
[0138] The hybrid membrane made in accordance with the process
described with reference to FIG. 8 was incorporated into a laminate
in resin transfer moulding. In this moulding Hexylow.RTM. RTM6
(from Hexcel Corporation) thermoset epoxy resin was used as the
matrix resin. Preforms were assembled consisting of 16 plies of 268
g aramid unidirectional fibre with a layer of non-woven material
between each ply. This preform was then placed into a heated mould
cavity (4 mm thickness) and Hexylow.RTM. RTM6 was injected at a
temperature of 100.degree. C. After resin injection the temperature
of the mould was raised to 180.degree. C. and the laminate cured
for 2 hours at 180.degree. C. before cooling and demoulding.
[0139] The laminates provided were tested for comparison after
impact (CAI) and bearing strength and the results are shown
below
TABLE-US-00002 Laminate type CA1 (MPa) Bearing Strength (MPa) RTM6
thermoset epoxy 169 859 RTM6 thermoset and 206 897 PES/aramid
hybrid membrane
[0140] The incorporation of the PES coated aramid hybrid membrane
into the composite material having the Hexylow.RTM. RTM6 thermoset
resin matrix resulted in improvements in both CAI and bearing
strength.
[0141] It is of course to be understood that the invention is not
intended to be restricted to the details of the above embodiments
which are described by way of example only.
* * * * *