Moulding methods and moulded articles

A method of molding a composite material which includes a fiber reinforcement within a resin comprises laying alternately on a mold first and second layers of fiber material pre-impregnated with uncured resin, where the resin content of the first layer is different from that of the second layer and the combination of layers gives the desired overall total volume fraction of fibers and resin in the laminate, enclosing the layers within an impervious membrane, evacuating the membrane and applying heat to the layers to flow gellate and at least partially cure the resin and during the process to allow released gasses and vapors to flow along the layers of lower resin content towards the membrane for subsequent evaluation.

The present invention relates to a method of moulding and to moulded
 articles, especially to a method of moulding and to articles moulded from
 a composite material comprising reinforcing filaments or fibres within a
 resin matrix.
 There are currently many methods of moulding high performance fibre
 reinforced, resin based, composites all of which involve combining a
 liquid or semi-liquid or solid resin with a relatively stiff strong
 fibrous reinforcement. The combined materials can then be cured and
 converted into a consolidated structural composite article by the
 application of heat and pressure in a controlled process.
 One convenient method of combining the resin and fibres is by
 pre-impregnation of a formulated resin, that is a resin and a hardener, in
 a controlled fibre array to form a sheet form (prepreg) which is easily
 handled and placed in a mould and has a suitable shelf life at ambient
 temperature. Typically sheets of prepreg are laid on to a shaped former
 and sealed within a tough membrane. The application of heat and pressure
 is then used to cause the resin to flow and the individual layers to
 coalesce and consolidate prior to gellation of the resin and the formation
 of the fully cured composite article suitable for high performance
 structural applications.
 This is often achieved by the use of an autoclave, that is a pressure
 vessel in which prepregs are laid up on to a dimensionally accurate former
 or mould and are subject to pressures of typically 0.69 MPa (100 psi) and
 elevated temperatures between 120.degree. and 200.degree. C. Such
 conditions readily cause the prepreg layer to coalesce to form the moulded
 shape required. Sufficient pressure is applied to generate hydrostatic
 pressure within the resin mass causing reduction in size of internal
 voids, or to completely force them into solution. If pressure is
 maintained throughout gellation and cure a void free resin matrix is
 obtained. requiring high capital investment on equipment and high energy
 consumption during the cure process. Conventional prepregs require
 temperatures of 120-180.degree. C. to effect cure. As a consequence of the
 combined high pressure and temperatures required, the mould tool materials
 must also be capable of withstanding such pressure without failure and to
 be dimensionally stable at the moulding temperature. Thus, for large
 components and applications involving small numbers of parts, tooling
 costs are inevitably very high compared to the overall cost of the
 manufactured components. It is clear, therefore, that conventional
 prepregs are unsuitable for certain applications (especially those that
 are cost sensitive) despite their good handling characteristics and high
 laminate performance. The use of an autoclave also places serious
 constraints on the size of components that can be made.
 A cheaper alternative is vacuum bag processing, in which the laid up
 prepregs are placed on an impervious mould covered by an impermeable
 membrane sealed at its edges to the mould. The assembly is then evacuated
 and heated to a temperature typically between 120.degree. C. and
 180.degree. C. The combination of atmospheric pressure and elevated
 temperature provides the conditions necessary to promote resin flow and
 coalesce individual layers together, whilst the elevated temperature
 results in the gellation and cure of the resin.
 Conventional vacuum bag moulding is much cheaper than autoclave moulding
 but the moulded products are usually inferior in quality because of the
 occurrence of voids within the resin matrix. Typically the void level
 achieved by conventional vacuum bag moulding of normal prepegs exceeds 5%
 by volume, and the level may be very variable.
 It has been proposed that an improved vacuum bag process for high
 temperature curing resins (120.degree. C. or higher) utilises a
 semi-permeable membrane to assist extraction of entrapped air or volatiles
 prior to resin gellation. The semi-permeable membrane is placed in direct
 contact with the prepreg and vacuum is available over the total surface
 area of one side of any preformed assembly. This enables some extraction
 of entrapped volatiles through the thickness of the material, providing
 that such pathways exist in the particular architecture of the composite
 lay-up and the type of prepreg used. Moulding efficiency varies according
 to the complexity and thickness of the moulded article. It is recognized
 that when normal unidirectional prepreg is used there is very little or no
 through-the-thickness transmission of gaseous material. Additionally
 semi-porous membranes are not readily available and are expensive.
 Another form of prepreg used currently is that identified in U.K. Patent
 No. 2108038 in which a concept and application for low temperature curing
 prepreg is identified (LTM). Such materials have been found to be of
 significant advantage for many applications including for aircraft
 prototyping and production items. In such applications the impact
 resistance or toughness of the laminate is an important property but the
 current types of LTM prepreg available are not satisfactory in this
 respect.
 Furthermore, when used in a vacuum bag oven cured process, as is preferred
 for minimum cost manufacture, the existing prepregs do not reliably
 produce low void content laminates, values of 2-3% being common place,
 especially when unidirectional fibre constructions are required.
 Another form of prepreg used is where the resin is not fully impregnated
 into the fibrous reinforcement. The dry portions of the fibrous
 reinforcement can then act as paths for the extraction of air and
 volatiles under vacuum prior to resin flow and gellation. However, this
 technique cannot be applied satisfactorily to purely unidirectional
 reinforcement, the form most desirable for high performance applications,
 such as aerospace structures. If the partial impregnation option is
 adopted for purely unidirectional material the resulting prepreg is of
 relatively poor quality and is prone to producing puckers or kinks in the
 fibre array which can degrade the mechanical properties obtained from the
 cured laminate.
 The only method of applying the partial impregnation technique to
 unidirectional fibre arrays is to use stitching or chemical bonding to
 hold the fibres together, both of which are unsatisfactory for high
 performance, high quality aerospace applications. Apart from the effect of
 the stitching or bonding materials being incorporated into the layup, the
 bulk factor of the partially dry fibrous reinforcement leads to problems
 during layup which again affect the quality and performance of the
 resulting laminate and structure.
 A high degree of resistance to impact (toughness) is critically important
 if the use of the moulded article is in applications such as aircraft
 structures. Achieving toughness in 120.degree. C. and 180.degree. C.
 curing resin formulations is difficult. Achieving similar levels of
 toughness in prepregs cured initially at temperatures less than 80.degree.
 C. is even more difficult, due to the tendency of the toughening agents
 used to increase resin viscosity, and hence restrict resin flow.
 Layers of prepreg laid on the former have, in certain instances in the
 past, been identical that is they include reinforcing filaments of fibres
 of the same type and resin of the same type. In certain exceptions to this
 arrangement the reinforcing filaments or fibres of some of the layers, for
 example each alternate layer, have differed from those of the other
 layers.
 It has been considered inappropriate that the resin in each layer could be
 anything other than constant in composition, structure etc. throughout the
 arrangement of layers.
 The result of this has been that the moulded article has had
 characteristics which are dependent on the one hand on the reinforcement
 and on the other, on the resin which is uniformly present throughout the
 article.
 This is disadvantageous in certain circumstances. Different resin
 compositions can give different characteristics to the moulded article.
 For example certain resin compositions can provide toughness, others for
 example high temperature resistance, and others high mechanical
 performance. It has always been an objective of the prepreg resin
 formulator to provide a resin with the best possible characteristics in
 all these, and other respects. However, there are often conflicts in the
 requirements which lead to compromises being made and inferior properties
 compared to the ideal being accepted as a limitation of existing
 technology.
 According to the present invention there is provided a method of moulding a
 composite material which includes a fibre reinforcement within a resin,
 comprising laying alternately on a mould first and second layers of fibre
 material pre-impregnated with uncured resin, the resin content of said
 first layers being substantially different from the resin content of said
 second layers, enclosing the laid up layers within a membrane, evacuating
 the membrane and applying heat to flow, gellate and at least partially
 cure the resin to harden the material.
 Preferably the resin content of the first layer or layers differs from that
 of the second layer or layers in respect of the ratio of resin to fibre,
 with the ratio of resin to fibre of the first layer being greater than
 that of the second layer.
 Alternatively the resin content of the first layer or layers differs from
 that of the second layer or layers in respect of the viscosity of the
 resin.
 As a further alternative the resin content of the first layer or layers
 differs from that of the second layer or layers in respect of the
 composition of the resin, each resin composition being compatible with the
 other in the moulding operation and in use, but conferring different
 properties to the moulded laminate.
 Further according to the present invention there is provided a method of
 moulding a composite material which includes a fibre reinforcement within
 a resin, comprising laying alternately on a mould first and second layers
 of fibre material pre-impregnated with uncured resin, the resin content of
 said first layers being substantially different from the resin content of
 said second layers, such that the combination gives the desired overall
 total volume fraction of fibres and resin in the laminate whilst allowing
 clear pathways via the lower resin content layers for the extraction of
 gaseous material through the edges of the laminate, enclosing the laid up
 layers within a membrane, evacuating the membrane and applying heat to
 flow, gellate and at least partially cure the resin to harden the
 material.
 Further according to the present invention there is provided a method of
 moulding a composite material which includes a fibre reinforcement within
 a resin, comprising laying alternately on a mould first and second layers
 of fibre material pre-impregnated with uncured resin, the resin content of
 said first layers being substantially different from the resin content of
 said second layers, such that the combination gives the desired overall
 total volume fraction of fibres and resin in the laminate whilst providing
 in the cured material a combination of toughness, heat distortion
 temperature (H.D.T.) value and relevant mechanical properties which are
 more advantageous than that exhibited by the first and second layers
 individually, enclosing the laid up layers within a membrane, evacuating
 the membrane and applying heat to flow, gellate and at least partially
 cure the resin to harden the material.
 Still further according to the present invention there is provided a method
 of moulding an article from a composite material which includes a fibre
 reinforcement within a resin, comprising laying alternately on a mould at
 least one first layer of a fibre material pre-impregnated with uncured
 resin and at least one second layer of fibre material pre-impregnated with
 an uncured resin the composition of which differs from the resin of the
 first layer and which is compatible in the moulding operation and in use
 with the first layer, the laid up layers being subjected thereafter to
 resin curing conditions.
 Preferably the resin in each of the first and second layers is as fully
 impregnated into the fibre as is necessary to produce a layer which is
 handleable without distortion or resin transfer and amenable to laying up
 on the mould.
 Preferably the material is partially cured at a temperature not exceeding
 120.degree. C. in one embodiment and not excluding 80.degree. C. or
 60.degree. C. in another embodiment and may be removed from the mould and
 finally cured at an elevated temperature while unsupported by the mould.
 The further cure may be effected at the temperature required for the
 specific application involved.
 Alternatively the required cure of the material is completed while
 supported on the mould.
 Preferably high temperature stable thermoplastic resins either in solution
 in the resin or in the form of finely divided particles or other
 toughening agents are incorporated in the resin prior to placing the
 layers on the mould.
 Preferably said high temperature stable thermoplastic resins or other
 toughening agents are added to the second layer. Alternatively or
 additionally they may be added to the first layer.
 Examples of high temperature stable thermoplastic resins which can be
 chosen are polysulphones, polyether sulphones, polyetherimides,
 polycarbonates, polyethylene terepthialate, polyether-etherketone,
 polyimides, polyamides. Other high temperature stable thermoplastic resins
 or toughening agents may be employed.
 Preferably the volume fraction of resin in the first layer is higher than
 that in the second layer.
 Alternatively the volume fraction of resin in the first layer is
 substantially equal to the volume fraction of resin in the second layer.
 Alternatively each of the first and second layers comprises a plurality of
 similar plies of resin impregnated fibre reinforcement.
 Strips of unimpregnated fibre material may be added around the perimeter
 and/or on the top and bottom surface of the laminate and remain
 substantially resin free during curing to provide a passage for gasses
 therethrough and after curing or partial curing of the layers, on removal
 from the mould, the strips are removed, or the edge of the laminate is
 removed to remove the excess material from the article being produced.
 Alternatively the strips of unimpregnated material may be laid up with any
 or all of the layers of the laminate as required.
 Further according to the present invention there is provided a composite
 material article made by a method as set out in the preceding paragraphs.
 A further aspect of the present invention provides a method of screening
 resin systems for their toughness properties by measuring loss in flexural
 modulus of composite material comprising impacting samples with a falling
 weight at a range of energy levels between 2 to 10 joules, adjusting said
 energy by changing the mass of the impact or whilst keeping the height and
 speed constant, measuring the flexural modulus before and after impact,
 producing graphs of loss of flexural modulus versus energy of impact and
 delamination damage area versus energy of impact.

The mould, tool or former 10 in the embodiment described takes the form of
 an aluminum plate with a glass reinforced polytetrafluroethylene fabric
 sheet 12 bonded thereon. First and second sheets 14,16 of prepreg having
 respectively high and low resin content are arranged with their respective
 fibre axes either at right angles to each other or parallel to each other.
 Typically sixteen plies or layers (only four of which are shown in the
 diagram for clarity) are laid together to form a laminate article of 2 mm
 total thickness. The assembly of layers is overlaid with an appropriate
 releasing micro-porous membrane 18 which is porous and assists in the
 removal of air or volatiles released from the layers during the moulding
 process but is non-porous to liquid resins. This is subsequently covered
 by a heavy-weight felt breather layer 20. The breather layer may be
 AIRBLEED 10 (Trade Mark) polyester felt available from AeroVac (Keighley)
 Limited. A nylon impermeable sheet 22 is then laid over the previously
 laid layers and attached at its edges to the mould 10 in air-tight manner
 using a proprietary tacky tape sealant 24, for example, SN5144 from
 AeroVac (Keighley) Limited. The sheet 22 is fitted with standard metal,
 through-the-bag vacuum sittings 26 so that a vacuum of up to 800 mm (28
 ins) mercury can be applied to the void defined by the mould 10 and the
 impermeable membrane 22.
 The surrounding temperature is then raised from room temperature to
 60.degree. C. at a rate of 0.5.degree. C./min and maintained at 60.degree.
 C. under full vacuum for 16 hours.
 During this process any gases within the layers 14,16 of prepregs are
 released and can readily flow along the first layers having a low resin
 impregnation to the edges of the layers, into the breather felt,
 subsequently to be exhausted from the enclosure at sittings 26. Other
 volatiles and gases may, subject to the composition of the layers of
 prepreg, migrate through the resinous material, also to be removed from
 the enclosure. During application of heat and pressure the individual
 layers of prepreg coalesce and consolidate together prior to gellation of
 the resin mass. The removal of gases is substantially complete prior to
 gellation, thus the resulting composite laminate article contains a very
 low level of internal voids (less than 1% by volume).
 In a modification the micro-porous membrane 18 can be replaced by a
 non-porous releasing membrane.
 The moulded article is now partially cured and is removed from the mould
 after the enclosing layers have been stripped off. A post cure is then
 applied to the article when in a free-standing condition within an air
 circulating oven according to the following cycle. The post cure may not
 achieve full cure of the resin.
 The temperature is raised from room temperature to 175.degree.
 C..+-.5.degree. C. at 20.degree. C. per hour. The moulded article is then
 maintained at the high temperature for two hours. This converts the
 article to an essentially fully cured condition and produces a Glassy
 Transition Temperature greater than 180.degree. C. if an appropriate
 prepreg resin formulation is selected.
 In a further embodiment of the invention different resin formulations may
 be used in the second low resin content prepreg layers and the first high
 resin content prepreg layers to obtain other advantages, such as increased
 toughness or different expansion coefficient, or different secondary
 properties such as the dielectric constant or loss tangent.
 Other resin content levels have also been found to perform satisfactorily
 ranging from the combination of 45% and 75% by volume fibre to 55% and 65%
 by volume fibre. Even other variations, and other combinations of
 different fibre volume materials are possible, and may be preferred in
 some applications.
 This provides a method of achieving the high degree of resistance to impact
 (toughness) critically important for the use of the moulded article in
 applications such as aircraft structures. Achieving toughness in
 120.degree. C. and 180.degree. C. curing resin formulations is difficult.
 Achieving similar levels of toughness in prepregs cured initially at
 temperatures less than 80.degree. C. is even more difficult, due to the
 tendency of the toughening agents used to increase resin viscosity, and
 hence restrict resin flow.
 EXAMPLE 1
 Materials selected
 1. TENAX HTA-5131 AKZ0 FASER AG, carbon fibre unidirectional tape--145
 g/m.sup.2 fibre weight Advanced Composite Group Ltd/(ACG Ltd)-LTM45EL low
 temperature curing epoxy resin at 22.7% W/W resin content (equivalent to
 70% fibre volume fraction--NOMINAL).
 2. TENAX HTA-5131 AKZ0 FASER AG carbon fibre unidirectional tape--145
 g/m.sup.2 fibre weight Advanced Composite Group Ltd/(ACG Ltd)-LTM43EL low
 temperature curing epoxy resin at 40.6% W/W resin content (equivalent to
 50% fibre volume fraction--NOMINAL).
 The dimension of each prepreg layer is 150.times.150 mm.
 All plies laid with the fibre axis in the same direction and the high and
 low resin content prepregs were alternated as indicated below.
 H=High resin content prepreg, L=Low resin content prepreg.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
 H/L/H/L/H/L/H/L L/H/L/H/L/H/L/H
 The preformed laminate was cured at 60.degree. C. in accordance with the
 foregoing description as illustrated in FIG. 1 and postcured to
 175.degree. C.
 The resultant voidage within the composite moulded article was measured by
 Image Analysis at 0.35% voidage by volume.
 EXAMPLE 2
 Materials selected
 1. TENAX HTA-5131 AKZ0 FASER, AG carbon fibre unidirectional tape--145
 g/m.sup.2 fibre weight/ACG Ltd. LTM45EL low temperature curing epoxy resin
 at 27.0% W/W resin content (equivalent to 70% fibre volume
 fraction--NOMINAL).
 2. TEN.AX HTA-5131 AKZ0 FASER AG, carbon fibre unidirectional tape--145
 g/m.sup.2 fibre weight/ACG Ltd. LTM45EL low temperature curing epoxy resin
 at 42.5% W/W resin content (equivalent to 48% fibre volume
 fraction--NOMINAL).
 The dimension of each prepreg layer is 150.times.150 mm.
 The plies were laid with alternating plies laid at 90.degree. to the
 previous layer, to produce a composite laminate with a 0/90.degree.
 construction.
 Additionally the high and low resin content prepregs were alternated as
 indicated below.
 H=High resin content prepreg, L=Low resin content prepreg.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
 H/L/H/L/H/L/H/L L/H/L/H/L/H/L/H
 The preformed laminate was cured at 60.degree. C. in accordance with the
 foregoing description as illustrated in FIG. 1 and finally postcured at
 175.degree. C.
 The resultant voidage within the composite moulded article was measured by
 Image Analysis at 0.30% voidage by volume.
 EXAMPLE 3
 In this example a large laminate was prepared with prepreg layers
 dimensioned 1220.times.1220 mm to demonstrate that the benefits produced
 by the invention are independent of size. The prepregs were selected, as
 in the previous examples 1 and 2 to have alternating high and low resin
 contents. The majority of the laminate comprised sixteen plies of prepreg
 in which the axis of the fibres was arranged alternately at 0.degree. and
 90.degree. throughout in successive plies.
 A central thicker section was added to form a strip running centrally down
 the full length of the composite laminate. This central strip comprised 64
 further alternating plies of high and low resin content prepreg arranged
 also in alternate 0.degree. and 90.degree. directions to each other. FIG.
 2 shows a schematic plan view of this laminate as an aid to understanding.
 Materials selected
 1. TENAX HTA-5131 AKZ0 FASER AG, carbon fibre unidirectional
 tape--145g/m.sup.2 fibre weight/ACG Ltd. LTM45EL low temperature curing
 epoxy resin at 40.6% W/W resin content (equivalent to 50% fibre volume
 fraction).
 2. TENAX HTA-5131AKZ0 FASER AG, carbon fibre unidirectional
 tape--145g/m.sup.2 fibre weight/ACG Ltd. LTM45EL low temperature curing
 epoxy resin at 22.7% W/W resin content (equivalent to 70% fibre volume
 fraction).
 This laminate was prepared in accordance with the foregoing example as
 illustrated in FIG. 1, and cured at 60.degree. C. Final postcure was
 completed at 175.degree. C.
 Eleven separate microsections were cut using a diamond edged rotary saw
 prior to potting in an acrylic resin medium and finally polished in
 preparation for microanalysis.
 The individual void contents were determined by Image Analysis and are
 presented below. In all cases including the 80 layer central section, void
 content was found to be less than 1%.

1 2 3 4 5 6 7 8 9
 10 11
 0.35% 0.87% 0.47% 0.12% 0.30% 0.63% 0.42% 0.15% 0.27%
 0.63% 0.52%
 Mean Value 0 0.43% voids
 EXAMPLE 4
 The embodiment of the present invention now to be described utilises
 a) a high resin content first prepreg layer made with normal or relatively
 low viscosity resin which may be a normal 120.degree. C. or 180.degree. C.
 curing resin or an LTM prepreg resin formulation.
 b) a low resin content second prepreg layer made with relatively highly
 toughened, high viscosity resin similar to conventional highly toughened
 120.degree. C. or 180.degree. C. cure aircraft structural prepregs. This
 resin may also be a conventional high temperature curing resin or an LTM
 resin. With the correct selection of resin formulations and properties
 this construction was found to give the highly desirable characteristics
 of being both relatively tough compared to purely LTM based resin systems
 available currently and having a high Tg and hot/wet compression strength
 required in many aerospace and other high performance structural
 applications.
 A further advantage of Example 4 is that the relatively high viscosity
 toughened resin in the second low resin content layers or plies will
 remain open to gasflow during the early stages of the moulding process,
 and this wil assist in removing all air, moisture, and other volatiles
 from the laminate as the temperature is increased. The void content of the
 resulting laminate is therefore milisnised and may often be reduced to
 substantially zero.
 The viscosity values referred to in the preceding Example relate to
 viscosity values at the appropriate time during the material manufacturing
 and moulding stages of conversion from raw materials to finished product.
 It should be realised that the viscosities vary during the process. For
 example the viscosity during the impregnation process is preferably
 relatively low to allow good impregnation of resin into the fibre array.
 Elevated temperatures are often used With hot melt resin formulations, or
 solution coating may be employed. Viscosity will also alter during the
 exposure of the prepreg to storage and/or transportation conditions, or
 during its lay-up period, as well as during the laminate curing process
 prior to resin gellation.
 EXAMPLE 5
 A plurality of first and second sheets 14,16 of prepreg having the same
 volumetric resin content but differing but compatible resin compositions
 are arranged with their respective fibre axes either at right angles to
 each other or parallel to each other. Typically sixteen plies or layers
 are laid together to form a laminate article of 2 mm total thickness. The
 assembly of layers is overlaid with an appropriate releasing micro-porous
 membrane 18 which is porous and assists in the removal of air or volatiles
 released from the layers during the moulding process but is non-porous to
 liquid resins. This is subsequently covered by a heavy-weight felt
 breather layer 20. The breather layer may be AIRBLEED 10 (Trade Mark)
 polyester felt available from AeroVac (Keighley) Limited. A nylon
 impermeable sheet 22 is then laid over the previously laid layers and
 attached at its edges to the mould 10 in air-tight manner using a
 proprietary tacky tape sealant 24, for example, SN5144 from AeroVac
 (Keighley) Limited. The sheet 22 is fitted with standard metal,
 through-the-bag vacuum fittings 26 so that a vacuum of up to 800 mim (28
 ins) mercury can be applied to the void defined by the mould 10 and the
 impermeable membrane 22.
 The surounding temperature is then raised from room temperature to
 60.degree. C. at a rate of 0.5.degree. C./min and maintained at 60
 .degree. C. under full vacuum for 16 hours.
 The moulded article is now partially cured and is removed from the mould
 after the enclosing layers have been stripped off. A second cure or
 postcure is then applied to the article when in a free-standing condition
 within an air circulating oven according to the following cycle.
 The temperature is raised from room temperature to 175.degree.
 C..+-.5.degree. C. at 20.degree. C. per hour. The moulded article is then
 maintained at the high temperature for two hours. This converts the
 article to a fully cured condition and produces a Heat Distortion
 Temperature (H.D.T.) greater than 180.degree. C.
 In a further modification the second low resin content layers are
 pre-impregnated with a high viscosity low curing temperature resin and the
 first high resin content lay ers are pre-impregnated with a low viscosity
 low curing temperature resin.
 In following the moulding methods described above with reference to FIG. 1
 a synergistic effect was found.
 Another advantage is that the tougher resin formulation in the second low
 resin content prepreg layers can make it easier to handle at the
 relatively low resin contents used, than is the case with less viscous
 resins used in the other layers, or in normal LTML prepregs.
 In a further modification of the invention, either the first layer or the
 second layer, or both may incorporate different reinforcing fibres by
 co-weaving, co-impregnating or co-mingling in order to increase the
 resistance of the laminate to impact, or to tailor the mechanical,
 physical, thermal, optical, electrical or other secondary characteristics
 to meet specific requirements. Furthermore, additives to the resin
 formulation can be used to modify the secondary properties appropriately.
 Any form of fibrous reinforcement can be used in the method of the present
 invention but the most common are unidirectional, woven cloth, or
 stitched/bonded multiaxial fabrics. The fibre may comprise carbon, glass
 aramid or other fibre arrays in the form of unidirectional sheet or woven
 fabric form, needled felts, orientated discontinuous fibre tapes,
 intermingled hybrid fibre tows etc.
 The methods are most useful with purely unidirectional prepreg, which is
 the form used most widely in aerospace applications, where the maximum
 mechanical performance is required. This is also the form with which it is
 most difficult to obtain void free laminates by vacuum bag moulding.
 Cloth or unidirectional laminates can be made void free under vacuum bag
 processing by using a dry resin formulation which allows air to escape
 before the resin flows. These exhibit certain drawbacks, for example, the
 laminates have no tack and poor drape unless heated during layup.
 Stitched/bonded unidirectional prepreg could be made partially impregnated
 and still remain handleable, albeit with a large bulkl factor.
 The fibre layers may also be hybrid fibre layers prepared from intermingled
 tows comprising carbon and polyether etherketone (PEEK) fibres, or any
 other suitable thermoplastic polymer or toughening fibre.
 The mould for use with the method of the present invention could be made
 from one or more of the following materials, wood, plaster, foamed resin,
 glass fibre. Any material which is castable, formable, machinable at room
 temperature may be utilised.
 It is particularly desirable that the method is especially applicable for
 use with low temperature cure prepregs, and that it allows a convenient
 way of toughening such prepregs, which are currently inherently relatively
 poor in impact resistance.
 Furthermore, other additives can be added to the resin formulation to
 appropriately modify its properties. The resin of either layer may include
 additional high temperature stable thermoplastic resins or rubber
 toughening agents either in solution in the resin or in the form of finely
 divided particles. Examples of high temperature stable thermoplastics
 resins which may be selected are polysulphones, polyether sulphones,
 polyetherimides, polycarbonates, polyethylene terepthalate, polyether
 etherketone, thermoplastic polyirides, polyamides, or any other suitable
 material.
 In a still further modification the moulded article is made oversize and
 trimmed back to size after curing has taken place. In this modification
 strips of, for example, dry woven glass tape can be incorporated at the
 edges of the layup extending from the prepreg and the prepreg layup
 itself. These strips remain partially resin-free during resin flow so that
 they do not close up during cure thus leaving a flow path for gases
 released during cure. They are also useful for gripping the article when
 it is being removed from the mould in the initially semi-cured state. The
 article may be relatively fragile at this time and could otherwise suffer
 from careless handling.
 In further modifications the unimpregnated strips are laid on the top
 and/or bottom surfaces of the article or, if the edges are to be trimmed
 off subsequent to removal from the mould, they may be laid interspaced
 with the first/second layers.
 In a modification of the method described in Example 5 the first and second
 layer each comprises a plurality of plies each of which comprises a fibre
 material pre-impregnated with uncured resin. This gives effectively a
 composite article comprising two laminated components one of which
 comprises a plurality of first plies of similar composition and the other
 a plurality of second plies of similar composition.
 A number of laminates made of different materials were produced using the
 methods described above to demonstrate the validity of the invention in
 terms of its ability to toughen relatively brittle LTM resin systems,
 whilst retaining their high Tg and good mechanical properties in hot/wet
 conditions. Various cure temperatures were used in both autoclave and
 vacuum bag/oven moulding processes, together with postcure conditions
 suitable for the resin systems used to produce either a controlled level
 of partial cure or complete cure.
 The laminates were cut up into various samples for different tests as
 follows:
 1) Barely Visible Impact Damage (BVID) Level Determination
 A simple 20 mm diameter ball-nosed falling weight impact or was used to
 test the BVID level of the laminate samples, the onset of visible damage
 on the rear (non-impacted) face of the laminate being used as the test
 criterion in this case.
 2) Flexural Modulus Before and After Impact (FMBAI)
 A test method was used to demonstrate the validity of the toughening method
 described above.
 In this test a simple 20 mm diameter ball-nosed falling weight is used to
 impact laminate samples supported by a plate with a 38 mm hole at a range
 of energy levels usually between 2 and 10 Joules, the energy being
 adjusted by changing the mass of the impact or, whilst keeping the height,
 and therefore the speed of the impact or, constant.
 The flexural modulus of each test specimen (80.times.50 mm.times.2 mm) is
 measured before being impacted, and after impact. The loss in flexural
 modulus is a measure of the degree of damage caused. A graph of loss of
 flexural modulus versus energy of impact and delamination damage are then
 drawn from which various parameters can be measured.
 A level of 20% loss of flexural modulus has been selected arbitrarily as a
 useful criterion to indicate relative levels of toughness in a laminate,
 and this has been found to have a reasonable correlation with the
 generally accepted Compression After Impact (CAI) test.
 Another criterion obtained from the plot of modulus loss versus impact
 energy is the initiation of a rapid increase in modulus loss with
 increasing energy of impact which is used as an indication of relative
 toughness or resistance to intiation of significant damage in a fibre
 reinforced composite structure.
 Brittle systems tend to show a steady rise in damage level with increasing
 energy of impact. Tough systems usually have a knee, or sharp increase in
 modulus loss with increasing impact energy at some point along the impact
 energy scale. Such systems have been shown to generally have a high
 threshold of resistance to low energy impacts, but at higher impact energy
 densities they behave very similarly to relatively untoughened systems.
 3) Damage Area
 The same test specimen and method is used as for the FMBAI test. However in
 this case the area of delamination damage visible on the rear face of the
 test piece is measured manually, and estimated. Again an arbitrary choice
 is made, using the energy to cause the visible damage area to extend to
 the circular edge of the supporting plate as the primary criterion.
 4) Heat Distortion Temperature (HDT)
 Conventional TMA measurements of the Glassy Transition Temperature (Tg) of
 the system were carried out to demonstrate that no reduction in effective
 Tg is apparent from this test. This is because the surface plies are the
 normal high Tg system, whilst the tougher, lower Tg system plies are
 beneath the surface plies.
 This arrangement of the different materials is also important because the
 better overall retention of elevated temperature properties will lead to
 better structural properties under hot/wet conditions which is often a
 major design criterion for aerospace structures.
 In this case a simple three point bend test is used to determine at what
 temperature the laminate starts to creep significantly under a static
 load. This point is indicated by the onset of a rapid change in deflection
 and is easily identified by plotting a graph of deflection against
 temperature.
 5) Mechanical Properties
 Short beam shear or interlaminar shear (ILSS), flexural strength (FS) and
 flexural modulus (FM) tests at ambient temperature, and then after 24 hr
 water boil were used to demonstrate the superior combination of properties
 obtained by using the method of the invention.
 Those skilled in the art will recognise that increased levels of mechanical
 property retention under these conditions will be reflected in improved
 hot/wet compression properties, a critical design requirement for many
 aerospace or other high performance structural applications.
 6) Void Content
 The void contents of the laminates produced were determined by cutting
 specimens, potting them in casting resin, and then polishing the cut
 laminate edges to allow examination under a microscope equipped with an
 image analysis device to measure the void contents.
 The following series of laminates were produced, all using Toray T800
 carbon fibre. The cure and postcure times were varied as appropriate to
 achieve either partial or complete cure as appropriate.

LTM45 EL/EF 13697 COMBINATION
 FMBAI
 Prepreg Type
 20%
 & Fibre Postcure BVIDN
 Loss
 Volume Cure Method Cure Temperature Temperature
 (Joules) (Joules)
 Fraction (V/o) Laminate Auto- Vac (.degree. C.) (.degree. C.)
 Postcure Postcure
 Condition Number clave Bag 60 80 120 120 175 120
 175 120 175
 LTM34 EL 4097 .check mark. .check mark.
 .check mark. 0.64 4.2
 AT 60 V/o 4089 .check mark. .check mark. .check
 mark. 0.70 5.5
 FIBRE 4052 .check mark. .check mark. NONE
 0.75 5.4
 4077 .check mark. .check mark.
 .check mark. 0.45 4.0
 4075 .check mark. .check mark.
 .check mark. 0.62 4.0
 LTM45 EL/ 4053 .check mark. .check mark.
 .check mark. 0.76 5.5
 EF13697 AT 4051 .check mark. .check mark. NONE
 0.72 0.68 6.4
 60:60 V/o
 FIBRE
 LTM45 EL/ 4056 .check mark. .check mark.
 .check mark. 0.83 0.77 6.1
 EF 13697 AT 4014 .check mark. .check mark. NONE
 0.84 0.73 7.2
 50:70 V/o 4060 .check mark. .check mark.
 .check mark. 0.61 4.8
 FIBRE 4072 .check mark. .check mark.
 .check mark. 0.48 5.5
 EF 13697 4055 .check mark. .check mark.
 .check mark. 0.83 5.2
 AT 60 V/o 4049 .check mark. .check mark. NONE
 &lt;0.24 0.75 2.1
 FIBRE
 LTM45 EL/ 4090 .check mark. .check mark. .check
 mark. 0.53 6.0
 EF 13697 4081 .check mark. .check mark.
 .check mark. 0.70 4.8
 `ALLOY` 4083 .check mark. .check mark.
 .check mark. 0.80 3.6
 AT
 60 V/o
 FIBRE
 All After 175.degree. C.
 Postcure Except Where
 Marked ()
 % of
 21.degree. C. Property
 FMBAI Energy At
 Relation At Test
 Prepreg Type Knee Above 1000 mm HDT
 Temperature After
 & Fibre 3 Joules Damage Area (.degree. C.) 24 Hour
 Water Boil
 Volume (Joules) (Joules) Void Wet Test
 Temperature
 Fraction (V/o) Postcure Postcure Content (24 Hr)
 120.degree. C. 21.degree. C. 21.degree. C.
 Condition 120 175 120 175 (%) Dry Boil) H.SS
 FS FM
 LTM34 EL NO 4.0 ZERO 188 145 63.4
 106.7 105.1
 AT 60 V/o NO 4.2 0.03 (136) (126) (43.9)
 (87.5) (101.8)
 FIBRE NO 4.0 ZERO 197 133 (53.7)
 (97.2) (90.0)
 NO 4.0 0.90 183 132 64.7
 110.5 102.2
 NO 3.2 2.45 180 128 78.3
 101.6 100.9
 LTM45 EL/ NO 4.1 ZERO 180 137 69.2
 96.9 92.9
 EF13697 AT NO 4.0 0.005 (110) (127) (49.2)
 (99.8) (91.1)
 60:60 V/o
 FIBRE
 LTM45 EL/ NO 4.4 ZERO 178 145 64.9
 86.6 95.8
 EF 13697 AT NO 4.1 ZERO (138) (123) (47.2)
 (92.9) (91.8)
 50:70 V/o NO 3.6 0.10 159 149 72.9
 100.0 97.8
 FIBRE NO 4.0 0.22 179 146 72.8
 99.4 96.9
 EF 13697 NO 5.7 ZERO 179 133 73.0
 94.5 92.7
 AT 60 V/o NO 1.8 0.01 Failed (116) 61.0
 98.4 90.4
 FIBRE 84
 LTM45 EL/ NO 4.8 0.07 (137) (117) 42.4
 104.4 95.8
 EF 13697 NO 3.8 0.03 210 139 74.4
 94.6 110.5
 `ALLOY` NO 3.6 0.44 190 140 78.1
 107.4 101.8
 AT
 60 V/o
 FIBRE
 Analysis of the results given above demonstrates that laminates made using
 alternating plies of two different resin systems, one a relatively low
 viscosity high Tg resin, and the other a relatively high viscosity,
 heavily toughened, lower Tg resin, provide an advantageous combination of
 high HDT, good toughness, high hot/wet mechanical properties, and lower
 void contents (especially when using vacuum bag curing) than laminates
 made using either of the different materials on its own, or an "alloy" of
 the two different resins made by mixing them together to make a single
 prepreg type.
 The lever of benefit is not just that which would be estimated from a
 simple "rule of mixtures" analysis based on the properties of the
 laminates made switch a single article.
 A synergistic effect is demonstrated, where most of the desirable
 properties of each material are obtained, whilst the disadvantageous
 properties are almost eliminated or at least much reduced. For example:
 a) The minced prepreg laminates are generally almost as tough (as shown by
 the test methods described above) as the tougher material at 100% and in
 some cases they give higher results than either of the individual
 materials on their own, or the "alloy".
 b) The effective HDT of the laminate is almost as high as the Tg of the
 more temperature resistant material under the same postcure conditions.
 c) The hot/wet mechanical properties are substantially higher than those of
 the lower HDT tough material and will generally be close to those of the
 100% high temperature material.
 d) The void content is lower than normal for vacuum bag laminates, even
 where the 60/60 volume % prepreg is used, although the 70/50 mixture gives
 a clear advantage in this respect under vacuum bag moulding conditions,
 showing essentially zero void levels, rather than the 2-3% often seen with
 this process using a single type of normal prepreg.
 The results of the `alloyed` resin systems demonstrate the advantages of
 using two different prepreg materials together, alternating the layers, as
 compared to the simple mixing of all the resin ingredients together to
 produce a single prepreg material. The `alloying` approach merely
 compromises the properties which are desirable with those which are
 undesirable, giving a relatively poor result overall.
 It will be realised that the simple alternating pattern of plies is not the
 only laminate design which will give the desirable combination of
 properties, particularly if the 60/60 volume fraction specification is
 used, or any other fibre resin volume in either of the plies which will
 independently of each other produce a good quality void free laminate
 under the moulding conditions used.
 Double, or multiple plies of the higher Tg/lower toughness material on the
 outside of the laminate is likely to improve the HDT and hot/wet
 mechanical performance issues whilst the inner layers of the
 second/tougher prepreg will still impart higher toughness to the laminate
 without decreasing its Tg too much.
 Similarly, the incorporation of alternative fibres into individual plies of
 the laminates may be used to provide specific secondary properties, such
 as reducing the radar cross-section of the laminate, or increasing its
 reflectivity to radiation or microwave energy.
 Any combination of resins, fibres, additives etc which give a desirable
 combination of properties and/or processing advantages may be used.
 Preferably the material combinations are initially cured under vacuum bag
 moulding conditions at 120.degree. C. or less.
 In many cases an initial cure temperature between 80.degree. and
 120.degree. C. is desirable.
 Further advantages are gained if the initial cure stage is carried out at a
 temperature between ambient (21.degree. C. approx) and 60.degree. C. as is
 known to those skilled in the art.