Epoxy resin formulations

Abstract

Epoxy formulations comprising a polyfunctional epoxy resin, a solid bisphenol A epoxy resin, a phenolic end capped bisphenol A resin, a rubber epoxy adduct and a curative are provided as is their use in prepregs and as matrix materials in laminates of metal foil and fiber reinforced epoxy resins.

Claims

1. A formulation comprising: a) a polyfunctional epoxy resin b) a solid bis phenol A epoxy resin c) a rubber epoxy adduct d) a phenolic end capped bis phenol A resin e) a curative system.

2. A formulation according to claim 1 that is liquid at ambient temperature.

3. A formulation according to claim 1 in which the polyfunctional epoxy resin has a functionality of 3 or 4.

4. A formulation according to claim 1 in which the end capped bisphenol A resin is brominated.

5. A formulation according to claim 1 having a viscosity in the range of from 1000 to 100,000 Pa.Math.s at 23 C.

6. A formulation according to claim 1 having a storage modulus G of from 310.sup.5 Pa to 110.sup.8 Pa and a loss modulus G of from 210.sup.6 Pa to 110.sup.8 Pa at 20 C.

7. A prepreg comprising fiber selected from glass fiber, carbon fiber and/or aramid fiber and a formulation according to claim 1.

8. A laminar structure comprising a layer of metal foil in combination with a prepreg according to claim 7.

9. An adhesive comprising a formulation according to claim 1.

Description

(1) The preferred process for producing prepregs is a continuous process involving the passage of many thousands of fibres through a series of stages, typically guided by rollers. The point where the fibres meet the epoxy resin formulation of the invention, is the start of the impregnation stage. Before the fibres are contacted with the resin formulation and reach the impregnation zone, they are typically arranged in a plurality of tows, each tow comprising many thousands of filaments, e.g. 12,000. These tows are mounted on bobbins and are fed initially to a combing unit to ensure even separation of the fibres. It has been found that unusually low fibre tensions just after the bobbin feed position provide further improvement to the disruption of the fibres in the eventual prepreg. Thus, the tension per filament at this position is preferably from 0.0007 to 0.025 g, preferably from 0.01 to 0.015 g.

(2) In the process a second layer of the resin formulation maybe brought into contact with the other face of the fibres typically at the same time as the first layer, compressing the first and second layers of resin such that resin enters the interstices of the fibres. Such a process is considered to be a one-stage process because, although each face of the fibres is contacted with one resin layer, all the resin in the eventual prepreg is impregnated in one stage.

(3) Resin impregnation typically involves passing the resin formulation and fibres over rollers, which may be arranged in a variety of ways. Two primary arrangements are the simple nip arrangement and the S-wrap arrangement.

(4) An S-wrap stage is wherein the resin formulation and fibres, both in sheet form pass around two separated rotating rollers in the shape of the letter 5, known as S-wrap rollers. Alternative roller arrangements include the widely used nip wherein the fibre and the resin formulation are pinched, or nipped, together as they pass between the pinch point between two adjacent rotating rollers. Nip stages may also be used, provided the pressures are kept low, e.g. by control over the gap between adjacent rollers.

(5) It has been found that although large pressures in theory provide excellent resin impregnation by the resin formulation, they can be detrimental to the outcome of the prepreg in the one-stage process.

(6) Thus, it is preferred that the pressure exerted onto the fibres and the resin formulation preferably using nip rollers does not exceed 35 kg per centimeter of width of the fibre layer, more preferably does not exceed 30 kg per centimeter.

(7) For example, when in S-wrap arrangement, two rollers are preferably spaced apart to provide a gap between the centres of them of from 250 to 600 mm, preferably from 280 to 360 mm, most preferably from 300 to 340 mm, e.g. 320 mm.

(8) Two adjacent pairs of S-wrap rollers are preferably separated between the centres of respective rollers of from 200 to 1200 mm, preferably from 300 to 900 mm, most preferably from 700 to 900 mm, e.g. 800 mm.

(9) The impregnation rollers may rotate in a variety of ways. They may be freely rotating or driven. If driven, they are conventionally driven so that there is no difference between the speed of rotation and the speed of passage of the resin formulation and fibres over the rollers. Sometimes it may be desirable to apply a slight increased speed or decreased speed relative to the passage of the resin formulation and the fibres. Such a difference is referred to in the art as trim.

(10) Following impregnation of the resin formulation into the fibres, often there is a cooling stage and further treatment stages such as laminating, slitting and separating.

(11) The moulding material or structure of the invention may be characterized by the resin content of the resin formulation and/or its fibre volume and resin formulation volume and/or its degree of impregnation as measured by the water up take test.

(12) Resin formulation and fibre content of uncured moulding materials or structures are determined in accordance with ISO 11667 (method A) for moulding materials or structures which contain fibrous material which does not comprise unidirectional carbon. Resin and fibre content of uncured moulding materials or structures which contain unidirectional carbon fibrous material are determined in accordance with DIN EN 2559 A (code A). Resin formulation and fibre content of cured moulding materials or structures which contain carbon fibrous material are determined in accordance with DIN EN 2564 A.

(13) The fibre and resin formulation volume % of a prepreg moulding material or structure can be determined from the weight % of fibre and resin by dividing the weight % by the respective density of the resin formulation and carbon fibre.

(14) The % of impregnation of a tow or fibrous material which is impregnated with resin formulation is measured by means of a water pick up test.

(15) The water pick up test is conducted as follows. Six strips of prepreg are cut to size 100 (+/2) mm100 (+/2) mm. Any backing sheet material is removed. The samples are weighed near the nearest 0.001 g (W1). The strips are located between PTFE backed aluminium plates so that 15 mm of the prepreg strip protrudes from the assembly of PTFE backed plates on one end and whereby the fibre orientation of the prepreg is extends along the protruding part. A clamp is placed on the opposite end, and 5 mm of the protruding part is immersed in water having a temperature of 23 C., relative air humidity of 50%+/35%, and at an ambient temperature of 23 C. After 5 minutes of immersion the sample is removed from the water and any exterior water is removed with blotting paper. The sample is then weighed again W2. The percentage of water uptake WPU (%) is then calculated by averaging the measured weights for the six samples as follows: WPU (%)=[(<W2><W1>)/<W1>)100. The WPU (%) is indicative of the Degree of Resin Impregnation (DRI).

(16) Typically, the values for the resin formulation content by weight for the uncured prepreg of the invention are in the ranges of from 15 to 70% by weight of the prepreg, from 18 to 68% by weight of the prepreg, from 20 to 65% by weight of the prepreg, from 25 to 60% by weight of the prepreg, from 25 to 55% by weight of the prepreg, from 25 to 50% by weight of the prepreg, from 25 to 45% by weight of the prepreg, from 25 to 40% by weight of the prepreg, from 25 to 35% by weight of the prepreg, from 25 to 30% by weight of the prepreg, from 30 to 55% by weight of the prepreg, from 35 to 50% by weight of the prepreg and/or combinations of the aforesaid ranges.

(17) At room temperature (23 C.), the resin formulation preferably has a relatively high viscosity, typically in the range of from 1000 to 100,000 Pa.Math.s, more typically in the range of from 5000 Pa.Math.s to 500,000 Pa.Math.s. Also, the resin formulation may be tacky. Tack is a measure of the adhesion of a prepreg to a tool surface or to other prepreg plies in an assembly. Tack may be measured in relation to the resin itself or in relation to the prepreg in accordance with the method as disclosed in Experimental analysis of prepreg tack, Dubois et al, (LaMI)UBP/IFMA, 5 Mar. 2009. This publication discloses that tack can be measured objectively and repeatably by using the equipment as described therein and by measuring the maximum debonding force for a probe which is brought in contact with the resin or prepreg at an initial pressure of 30N at a constant temperature of 30 C. and which is subsequently displaced at a rate of 5 mm/min. For these probe contact parameters, the tack F/F.sub.ref for the resin is in the range of from 0.1 to 0.6 where F.sub.ref=28.19N and F is the maximum debonding force. For a prepreg, the tack F/F.sub.ref is in the range of from 0.1 to 0.45 for F/F.sub.ref, preferably from 0.3 to 0.40 F/F.sub.ref, more preferably from 0.32 to 0.39 F/F.sub.ref where F.sub.ref=28.19N and F is the maximum debonding force. However, a fibrous support web, grid or scrim may also be located on at least one exterior surface of the fibrous reinforcement to further enhance the integrity of the material or structure during handling, storage and processing.

(18) The epoxy resin formulation of the invention which is used as the matrix resin material in the prepreg preferably has a storage modulus G of from 310.sup.5 Pa to 110.sup.8 Pa and a loss modulus G of from 210.sup.6 Pa to 110.sup.8 Pa at room temperature (20 C.).

(19) Preferably, the resin formulation has a storage modulus G of from 110.sup.6 Pa to 110.sup.7 Pa, more preferably from 210.sup.6 Pa to 410.sup.6 Pa at room temperature (20 C.).

(20) Preferably, the resin formulation has a loss modulus G of from 510.sup.6 Pa to 110.sup.7 Pa, more preferably from 710.sup.6 Pa to 910.sup.6 Pa at room temperature (20 C.).

(21) Preferably, the resin formulation has a complex viscosity of from 510.sup.5 Pa to 110.sup.7 Pa.Math.s, more preferably from 7.510.sup.5 Pa to 510.sup.6 Pa.Math.s at room temperature (20 C.).

(22) Preferably, the resin formulation has a complex viscosity of from 110.sup.6 Pa to 210.sup.6 Pa.Math.s. more preferably from 5 to 30 Pa.Math.s at 80 C. Preferably, the resin formulation has a viscosity of from 10 to 25 Pa.Math.s at 80 C.

(23) Preferably, the prepreg moulding material is elongate in a longitudinal direction thereof and the fibrous reinforcement is unidirectional along the longitudinal direction of the prepreg.

(24) The behaviour of thermosetting prepreg materials is highly viscoelastic at the typical lay-up temperatures used. The elastic solid portion stores deformation energy as recoverable elastic potential, whereas a viscous liquid flows irreversibly under the action of external forces.

(25) This complex viscosity is obtained using a rheometer to apply an oscillation experiment. From this the complex modulus G* is derived as the complex oscillation which is applied to the material is known (Principles of Polymerization, John Wiley & Sons, New York, 1981).

(26) In viscoelastic materials the stress and strain will be out of phase by an angle delta. The individual contributions making the complex viscosity are defined as G (Storage Modulus)=G*cos (delta); G (Loss Modulus)=G*sin(delta). This relationship is shown in FIG. 8 of WO 2009/118536.

(27) G* is the complex modulus. G relates to how elastic the material is and defines its stiffness.

(28) G relates to how viscous a material is and defines the damping, and liquid non recoverable flow response of the material.

(29) For a purely elastic solid (glassy or rubbery), G=0 and the phase angle delta is 0, and for a purely viscous liquid, G=0 and the phase angle delta is 90.

(30) The loss modulus G indicates the irreversible flow behaviour and a material with a high loss modulus G is also desirable to prevent the early creep-like flow and maintain an open air path for longer. Therefore the resin formulation of the present invention has a high storage modulus and a high loss modulus, and correspondingly a high complex modulus, at a temperature corresponding to a typical lay-up temperature, such as room temperature (21 C.).

(31) In this specification, the viscoelastic properties, i.e. the storage modulus, loss modulus and complex viscosity, of the resin formulation used in the prepregs of the present invention were measured at application temperature (i.e. a lay-up temperature of 20 C.) by using a Bohlin VOR Oscillating Rheometer with disposable 25 mm diameter aluminium plates. The measurements were carried out with the following settings: an oscillation test at increasing temperature from 50 C. to 150 C. at 2 C./mm with a controlled frequency of 1.59 Hz and a gap of 500 micrometer.

(32) Typically, the stiffness of the viscoelastic prepreg is characterised by the resin formulation exhibiting a high elastic rheological response. The resin formulation rheology is characterised by a storage modulus G of the resin formulation at room temperature, preferably between 310.sup.5 Pa and 110.sup.8 Pa at 20 C., more preferably from 110.sup.6 Pa to 110.sup.7 Pa, yet more preferably from 210.sup.6 Pa to 410.sup.6 Pa. The higher the storage modulus at room temperature (20 C.), the greater the air transport properties of the prepreg stack. However, the upper limit of the storage modulus is limited because otherwise the prepreg would become too rigid and would develop a tendency to snap as the prepreg is being laminated.

(33) In the manufacture of a structural member using the prepreg moulding material or structure of the present invention, preferably the resin formulation has a high loss modulus G between 210.sup.6 Pa and 110.sup.8 Pa at 20 C., more preferably from 510.sup.6 Pa to 110.sup.7 Pa, yet more preferably from 710.sup.6 Pa to 910.sup.6 Pa.

(34) The resin formulation preferably has a high complex viscosity at 20 C. of from 510.sup.5 Pa to 110.sup.7 Pa.Math.s, more preferably from 7.510.sup.5 Pa to 510.sup.6 Pa.Math.s, yet more preferably from 110.sup.6 Pa to 210.sup.6 Pa.Math.s.

(35) In order to produce final laminates with substantially uniform mechanical properties it is important that the structural fibres and the epoxy resin formulation be mixed to provide a substantially homogenous prepreg. This requires uniform distribution of the structural fibres within the prepreg to provide a substantially continuous matrix of the resin formulation surrounding the fibres. It is therefore important to minimise the encapsulation of air bubbles within the resin formulation during application to the fibres. It is therefore preferred to use high viscosity resins. The prepregs should contain a low level of voids in order, and it is therefore preferred that each prepreg and the prepreg stack has a water pick-up value of less than 9%, more preferably less than 6%, most preferably less than 3%.

(36) Where the prepreg is to be used in laminates where the fibre reinforced epoxy layer is interspersed between metal foils, the metal foil is preferably steel or aluminium and aluminium foils of thickness 0.01 to 10 mm, preferably from 0.07 to 5 mm, more preferably from 0.1 to 2 mm, or from 0.2 to 0.5 mm are particularly preferred. Alternatively, the foil may comprise an organic material such as a fibre reinforced laminate. The fibre reinforced laminate may comprise a thermoset or a thermoplastic resin material.

(37) Once it is created in the mould, the prepreg, prepreg stack or prepreg and other (metal) layers may be bonded and cured by exposure to an externally applied elevated temperature in the range of from 70 C. to 150 C., preferably from 11 C. to 130 C. and more preferably from 120 C. to 125 C., and optionally elevated pressure, to produce a cured laminate.

(38) Curing is preferably conducted in an autoclave. Curing may also be achievable by the so-called vacuum bag technique. This involves placing the prepreg, prepreg stack or combination of layers of prepreg and layers of other materials in an air-tight bag and creating a vacuum on the inside of the bag, the bag being placed in a mould prior to or after creating the vacuum and the resin is then cured by externally applied heat to produce the moulded laminate. The use of the vacuum bag has the effect that the stack experiences a consolidation pressure of up to atmospheric pressure, depending on the degree of vacuum applied.

(39) Upon curing, the prepreg, prepreg stack or laminar structure becomes a composite laminate, suitable for use in a structural application, such as for example an automotive, marine vehicle or an aerospace structure or a wind turbine structure such as a shell for a blade or a spar. Such composite laminates can comprise structural fibres at a level of from 80% to 15% by volume, preferably from 58% to 65% by volume.

(40) The formulations are particularly useful in the production of prepregs based on glass or carbon fibre and particularly for those that are used as the matrix in the production of prepregs used in laminates with metal foils such as aluminium foils to produce glass laminate aluminium reinforced epoxy systems composed of several very thin layers of aluminium interspersed with layers of glass fibre reinforced cured prepreg. The epoxy resin formulation of this invention is particularly useful as the matrix composition, where it is preferred that the resin formulation has a viscosity at room temperature in the range of 100 Pa.Math.s to 1000 Pa.Math.s at 60 C., an onset of cure at from 100 C. to 160 C., preferably from 120 C. to 140 C. to produce a cured resin of Tg from 100 C. to 150 C.

(41) An Automated Tape Laying (ATL) machine is an apparatus which automatically deposits strips of composite material provided in the form of a tape. ATL machines rely on the inherent tack of a composite material to deposit a tape to the surface below. The formulations of the present invention are particularly suited for use as a resin in an ATL tape. The formulation exhibits suitable tack for ATL deposition when combined with a fibrous reinforcement. The tack of the formulation can also be increased by heating with an ATL apparatus. The present invention is particularly suited for controlling tack level by heating because of the content of solid bis-phenol A epoxy resin which reduces the formulations viscosity when heated. Alternatively, the tack can also be increased by replacing some of the solid-bisphenol A epoxy resin with a semi solid or liquid variant of bisphenol epoxy. Preferably between 100% and 1% of the solid-bisphenol A is replaced with a semi-solid or liquid bisphenol A, more preferably between 20 and 1%, most preferably between 10 and 4%. Preferably the solid Bisphenol A resin is replaced in part by a liquid bisphenol A epoxy resin with an EEW of from 160 to 500. An exemplary liquid bisphenol A resin is Epikote 828 by Momentive. The present invention is particularly suited for use as a matrix in an ATL tape that further comprises fibrous reinforcement and a metal foil. The present invention is also suited to being used in a tape that is capable of being deposited onto a metal foil and a metal foil being deposited on to it.

(42) The invention is illustrated but in no way limited by the following example in which the following formulation was prepared.

(43) TABLE-US-00002 TABLE 2 Example formulation Component % Araldite MY9512 (tetrafunctional epoxy EEW 125 g/eq) 20.5 Hypox RA 1340 (liquid rubber - epoxy adduct EEW 350 e/eq\) 31 Araldite GT6071 (solid bis A epoxy EEW 457 g/eq) 29 Omicure U-52M (MDI urone) 5.6 4,4 DDS (diamino diphenyl sulphone) 6.7 Araldite EP 820 (phenolic end capped bis A epoxy thermoplastic) 7.2

(44) The formulation was liquid at ambient temperature and was found to have an onset of cure as measured by DSC of 142 C. and a Tg upon curing of 118 C. The resin formulation was found to be useful as the matrix in glass fibre based prepregs used in lamination with aluminium foil.

(45) A prepreg was prepared containing this formulation as the epoxy resin matrix and 27 wt % based on the weight of the prepreg of the S2 glass fibres S2-463-AA-750. The prepreg was cured by heating at 125 C. for 75 minutes. The Tg of the cured resin was 117 C. and after storage under the moist conditions (full immersion in water or 98%/85% immersion in water) the Tg was found to be as shown in the following Table.

(46) TABLE-US-00003 TABLE 3 Prepreg properties Storage condition Tg Moisture Uptake % 2 weeks/70 C./full immersion 83.7 0.79 750 hrs/70 C./98% immersion 88.2 0.74 750 hrs/70 C./85% immersion 95.8 0.45